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Effect of the Ancillary Ligand on the Performance of Heteroleptic Cu(I) Diimine Complexes as Dyes in Dye-Sensitized Solar Cells
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  • Daniele Franchi
    Daniele Franchi
    Institute of Chemistry of Organometallic Compounds (CNR-ICCOM), Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy
    Division of Organic Chemistry, Centre of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden
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    Orcidhttps://orcid.org/0000-0003-2811-247X
  • Valentina Leandri
    Valentina Leandri
    Division of Applied Physical Chemistry, Centre of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden
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  • Angela Raffaella Pia Pizzichetti
    Angela Raffaella Pia Pizzichetti
    Division of Applied Physical Chemistry, Centre of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden
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  • Bo Xu
    Bo Xu
    Division of Physical Chemistry, Centre of Molecular Devices, Department of Chemistry, Ångström Laboratory, Uppsala University, Box 523, SE-75120 Uppsala, Sweden
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    Orcidhttps://orcid.org/0000-0002-4703-7340
  • Yan Hao
    Yan Hao
    Division of Applied Physical Chemistry, Centre of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden
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    Orcidhttps://orcid.org/0000-0002-0996-1794
  • Wei Zhang
    Wei Zhang
    Division of Applied Physical Chemistry, Centre of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden
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    Orcidhttps://orcid.org/0000-0003-0232-9937
  • Tamara Sloboda
    Tamara Sloboda
    Division of Applied Physical Chemistry, Centre of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden
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  • Sebastian Svanström
    Sebastian Svanström
    Division of X-ray Photon Science, Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden
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    Orcidhttps://orcid.org/0000-0001-7351-8183
  • Ute B. Cappel
    Ute B. Cappel
    Division of Applied Physical Chemistry, Centre of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden
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    Orcidhttps://orcid.org/0000-0002-9432-3112
  • Lars Kloo
    Lars Kloo
    Division of Applied Physical Chemistry, Centre of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden
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    Orcidhttps://orcid.org/0000-0002-0168-2942
  • Licheng Sun
    Licheng Sun
    Division of Organic Chemistry, Centre of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden
    Center of Artificial Photosynthesis for Solar Fuels, School of Science, Westlake University, Hangzhou 310024, China
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    Orcidhttps://orcid.org/0000-0002-4521-2870
  • James M. Gardner*
    James M. Gardner
    Division of Applied Physical Chemistry, Centre of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0002-4782-4969
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ACS Applied Energy Materials

Cite this: ACS Appl. Energy Mater. 2022, 5, 2, 1460–1470
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https://doi.org/10.1021/acsaem.1c02778
Published January 13, 2022

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Abstract

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A series of heteroleptic Cu(I) diimine complexes with different ancillary ligands and 6,6′-dimethyl-2,2′-bipyridine-4,4′-dibenzoic acid (dbda) as the anchoring ligand were self-assembled on TiO2 surfaces and used as dyes for dye-sensitized solar cells (DSSCs). The binding to the TiO2 surface was studied by hard X-ray photoelectron spectroscopy for a bromine-containing complex, confirming the complex formation. The performance of all complexes was assessed and rationalized on the basis of their respective ancillary ligand. The DSSC photocurrent–voltage characteristics, incident photon-to-current conversion efficiency (IPCE) spectra, and calculated lowest unoccupied molecular orbital (LUMO) distributions collectively show a push–pull structural dye design, in which the ancillary ligand exhibits an electron-donating effect that can lead to improved solar cell performance. By analyzing the optical properties of the dyes and their solar cell performance, we can conclude that the presence of ancillary ligands with bulky substituents protects the Cu(I) metal center from solvent coordination constituting a critical factor in the design of efficient Cu(I)-based dyes. Moreover, we have identified some components in the I–/I3–-based electrolyte that causes dissociation of the ancillary ligand, i.e., TiO2 photoelectrode bleaching. Finally, the detailed studies on one of the dyes revealed an electrolyte–dye interaction, leading to a dramatic change of the dye properties when adsorbed on the TiO2 surface.

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Introduction

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Inspired by the natural process of photosynthesis, dye-sensitized solar cells (DSSCs) have received great interest in the past 30 years. (1,2) One of the features that distinguish DSSCs from other photovoltaic technologies is the possibility to easily tune their color, paving the way for nonconventional applications involving building integration and indoor applications. (3,4) The reason for the simplicity of color tuning of DSSCs, as compared to other types of solar cells, resides in one of their key components: the molecular dye or photosensitizer.
In DSSCs, the dye is typically an organic, molecular compound, or a coordination complex, which is responsible for light harvesting and electron transfer into the conduction band (CB) of a semiconductor electrode (typically TiO2) to which it is chemically bonded. Organic dyes offer high molar extinction coefficients, important for light harvesting, and an extended structural variety that can be tuned to optimize the spectral absorption range. (5−11) Despite the appealing properties of the organic sensitizers, it is mostly coordination complexes that have offered record performances in DSSCs. (12) The first, and perhaps the most famous, class of metal complexes employed as dyes consists of ruthenium(II) polypyridyl complexes. (13) Some of these compounds have shown record photon-to-current conversion efficiencies (PCEs) above 10% for more than a decade. (14,15) Among them, the most well-known dyes are the [cis-(dithiocyanato)-Ru-bis(2,2′-bipyridine-4,4′-dicarboxylate)] complex, N3, and its doubly protonated tetrabutylammonium salt, N719. (16−18) Another class of metal complexes that have been widely studied as photosensitizers is metal-based porphyrin derivatives. (19,20) The Zn porphyrins YD2-o-C8, SM371, and SM315 are of particular interest, as they have shown superior performances to those of the Ru(II) complexes in DSSCs, exhibiting solar cell PCEs of 11.9, 12.0, and 13.0%, respectively. (21,22) More recently, Cu(I) diimine complexes have attracted special attention. (23−25) These complexes have been used for a variety of applications in DSSCs as hole-transporting materials (HTM), redox mediators, as well as dyes. (26−29) The potential for combining copper(I) dyes with a wide band gap semiconductor such as TiO2 for light harvesting and photoconversion was first demonstrated by Sauvage and co-workers in 1994. (30,31) Despite a growing interest in the photovoltaic application of bis(diimine)copper(I) complexes, little progress was made in the following years. (32) In 2008, Schaffner and co-workers prepared DSSCs based on homoleptic copper(I) complexes with 6,6′-disubstituted 2,2′-bipyridines as dyes, showing promising results. (33) The symmetrical nature of the homoleptic copper(I) complexes limits the efficiency of a preferred excited-state electron transfer into the TiO2 CB. Therefore, to maximize the electron injection process, heteroleptic complexes mimicking the push–pull design exploited in the design of organic dyes have been synthesized. (34−39) However, copper(I) complexes in solution are notoriously labile and the exchange of ligands rapidly occurs. Therefore, the synthesis and study of heteroleptic copper(I) compounds can be challenging. (40−42) One solution to this problem is to rely on bulky substituents, which prevent the formation of the corresponding homoleptic complexes, thus leading to stable heteroleptic copper(I) complexes. (43,44) This concept has been explored by Odobel and co-workers, who prepared heteroleptic Cu(I) dyes for DSSCs showing a PCE of up to 4.7%. (45,46) An alternative to this approach was reported by Housecroft and Constable, (47,48) who adopted a stepwise, on-surface, self-assembly of the heteroleptic complexes by soaking a TiO2 substrate first in a solution of an anchoring ligand L′, and subsequently in a solution containing the homoleptic complex of copper(I) with the ancillary ligand L″, [Cu(L″)2]+. This allowed them to generate, through ligand scrambling, the desired heteroleptic complex [Cu(L′)(L″)]+ anchored on the semiconductor surface. (34,36)
In our previous work, we have explored the optical and electrochemical properties of a series of heteroleptic Cu(I) diimine complexes with 6,6′-dimethyl-2,2′-bipyridine-4,4′-dibenzoic acid (dbda) as an anchoring ligand and different ancillary ligands: 2,9-dimethyl-1,10-phenanthroline (dmp); 5-bromo-2,9-dimethyl-1,10-phenanthroline (Br-dmp); 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (bcp); 2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline (dsbtmp); 2,2′-biquinoline (biq); and 2,9-dianisyl-1,10-phenanthroline (dap). (39) The complexes were self-assembled on a TiO2 surface using the aforementioned method. Here, we continue with an investigation of the complexes as dyes for DSSCs, and we focus on rationalizing the role of the ancillary ligand with respect to the photovoltaic performance of DSSCs.

Experimental Section

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General Information

All chemicals were purchased from Sigma-Aldrich and used as received unless noted otherwise. The ligands and the Cu(I) complexes in this study have been synthesized according to procedures reported in our previous work. (39)

Solar Cell Fabrication

Preparation of Working Electrodes: Pilkington TEC15 substrates were sequentially cleaned in an ultrasonic bath first with a detergent solution (RBS 25 from Fluka Analytical), then deionized water, and finally ethanol. After drying in air, the substrates were screen-printed (active area, 0.36 cm2) with a transparent TiO2 paste (two layers, GreatCell Solar 18NR-T) and then dried at 125 °C for 6 min before being screen-printed with a TiO2 scattering layer paste (two layers, Solaronix, WER2-O). The samples were heated gradually at 180 °C (10 min), 320 °C (10 min), 390 °C (10 min), and 450 °C (60 min) in an oven (Nabertherm Controller P320) under ambient air atmosphere. After sintering, the samples were immersed into a 40 mM aqueous TiCl4 solution at 70 °C for 60 min. Subsequently, the substrates (total thickness around 12–14 μm) were removed from the solution, rinsed with deionized water, dried, and exposed to a final heating step (500 °C for 30 min). The TiO2 layer was sensitized with the copper(I) complexes according to the procedure reported in the “Self-Assembly of the Complexes on TiO2” section. Preparation of Counter Electrodes: A predrilled one-hole Pilkington TEC8 glass substrate was cleaned following the same procedure as reported for the working electrodes and heated in air at 400 °C for 30 min to remove residual impurities. After cooling to room temperature, 10 μL cm–2 of a 4.8 mM H2PtCl6 solution in ethanol was deposited on the glass substrate, followed by heating in air at 400 °C for 30 min. Solar Cell Assembly: The working electrode was sealed in a sandwich structure with the counter electrode using a 25 μm thick thermoplastic Surlyn frame (Meltonix 1170-25 from Solaronix). The electrolyte was introduced into the sealed devices through the predrilled hole by a vacuum backfilling technique. Finally, the hole in the counter electrode was sealed with a thermoplastic Surlyn film with a glass coverslip on top by heating at 120 °C for 20 s.

Self-Assembly of the Complexes on TiO2

The TiO2 substrates were soaked in a 1.0 mM methanol solution of the ligand dbda for 24 h at room temperature. Each electrode was removed from the solution, washed with methanol, and dried with compressed air. Each functionalized electrode was thereafter soaked at room temperature for 24 h in a 1 mM acetonitrile solution of the desired homoleptic complexes [Cu(dmp)2]+, [Cu(Br-dmp)2]+, [Cu(bcp)2]+, [Cu(dsbtmp)2], and [Cu(dap)2]+, or, in an acetonitrile solution containing 1 mM [Cu(CH3CN)4]PF6 and 2 mM biq ligand, alternatively in a methanol solution containing 1 mM [Cu(CH3CN)4]PF6 and 2 mM of the dbda ligand. As a final step, the electrodes were removed from the dye-bath solution and washed with acetonitrile.

Film Thickness Characterization

The thickness of the TiO2 electrode layer was determined by means of a profilometer (Veeco Dektak 150).

Photovoltaic Device Characterization

Current–voltage (I–V) measurements were carried out with a Keithley 2400 source/meter and a Newport solar simulator (model 91160); the light intensity was calibrated using a certified reference solar cell (Fraunhofer ISE) to an intensity of 1000 W m–2 (AM 1.5G spectrum). The efficiencies reported have been recorded without using a mask. Incident photon-to-current conversion efficiency (IPCE) spectra were recorded by a computer-controlled setup comprising a xenon lamp (Spectral Products ASB-XE-175), a monochromator (Spectral Products CM110), and a Keithley multimeter (model 2700), calibrated by a certified reference solar cell (Fraunhofer ISE). For the IPCE spectra, a black mask with an aperture slightly smaller than the active area of the cell was applied on top of the cell (0.5 × 0.5 cm2).

Density Functional Theory (DFT) Calculations

The copper(I) complexes were studied using density functional theory (DFT) calculations. All calculations were carried out using the program package Gaussian 16 (rev. B.01). (49) The molecular structures were geometrically optimized using the cam-B3LYP hybrid functional. (50) 6-311G basis sets were used for all light elements (H, C, N, O). Small-core, effective-core potentials (MDF10) in combination with a double-ζ-quality valence space were used for Cu and Br. (51,52)

Hard X-ray Photoelectron Spectroscopy (HAXPES)

HAXPES measurements were carried out in the HIKE end station at the KMC-1 beamline of the BESSY II synchrotron facility at the Helmholtz-Zentrum Berlin. (53) The available photon energy range at this beamline is 2–12 keV. A thin film of the [Cu(Br-dmp)2]+ homoleptic complex on a fluorine-doped tin oxide (FTO) substrate and the [Cu(dbda)(Br-dmp)]+ heteroleptic complex on TiO2 was studied. As references, a pristine TiO2 film and a TiO2 film with the anchoring ligand adsorbed to the surface were investigated. Experiments were carried out under ultrahigh vacuum conditions in the analysis chamber with a photon energy of 3 keV, selected using a Si(111) double-crystal monochromator. A high-resolution hemispheric electron analyzer (VG Scienta R4000) was used for the detection of ejected electrons. Overview spectra were recorded for each sample with a pass energy of 500 eV (see Figure S1, Supporting Information). Following this, high-resolution, core level spectra were obtained from a fresh sample spot with a pass energy of 200 eV and measurements from the same regions were repeated several times to monitor sample charging and potential X-ray beam damage. A shift in peak positions was observed after the first scan of core level spectra due to sample charging caused by the incident X-ray radiation (see the Supporting Information). For this reason, internal references of the core levels of interest obtained before and after measurements were used for binding energy calibration when comparing different samples: either the Ti 2p3/2 level of the TiO2 substrate was set to 458.6 eV or the adventitious C 1s peak was set to 284.8 eV. (54) Both calibration methods yielded similar binding energies. The spectra were modeled by pseudo-Voigt functions to evaluate peak intensities used for normalization of the spectra. (55) Where appropriate, elemental ratios were determined from the peak intensities using the photoionization cross section calculated by Scofield et al. (56)

Results and Discussion

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In our previous work, (39) we followed the self-assembly method employed by Housecroft and Constable to prepare a series of Cu(I) complexes directly on the surface of the TiO2 substrate (Scheme 1). (47,48)

Scheme 1

Scheme 1. Molecular Structures of the Surface-Assembled Heteroleptic Cu(I) Complexes Investigated as Dyes for DSSCs in This Work
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The optical and electrochemical properties of the complexes were investigated as well, and based on those results, it appeared evident that the complexes anchored on the surface of TiO2 showed a potential to be employed as dyes for DSSCs. In Figure 1, the energy levels of the copper(I) complexes derived from cyclic voltammetry and optical analysis (39) are shown together with their alignment with respect to the CB edge of TiO2 and the redox potential of the redox couple I–/I3– used in this study.

Figure 1

Figure 1. Energy level alignments of the Cu(I) complexes in Scheme 1 with respect to the CB edge of TiO2 and the potential of the redox couple I–/I3–. The values are reported as potential vs normal hydrogen electrode (NHE).

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From a purely energetic perspective, the complex [Cu(dbda)(dmp)]+ shows the most negative lowest unoccupied molecular orbital (LUMO) energy (−1.45 eV vs NHE) within the series of complexes studied and thus the highest driving force for electron injection into the CB of TiO2. The complex [Cu(dbda)(biq)]+ instead shows the most positive highest occupied molecular orbital (HOMO) energy (0.96 eV vs NHE), which could favor the process of regeneration of the oxidized complex. This may be especially valid in the case of the I–/I3– redox couple employed, as it is well known for requiring a significant driving force for efficient regeneration of the oxidized dye. (57) However, we would like to point out that, given the labile nature of the Cu(I) complexes, (41) the energetic alignments may play a limited role in the determination of the photovoltaic properties. Instead, the binding affinity of the ancillary ligand to the copper center, as well as its ability to efficiently screen the copper center from solvent interactions, may play the most crucial role in the device performance.
We have investigated the formation of the heteroleptic complexes on the TiO2 surface by hard X-ray photoelectron spectroscopy using the [Cu(dbda)(Br-dmp)]+ complex as an example. The addition of Br to the ligand of the homoleptic complex helps us to distinguish the two ligands on the surface and therefore to follow the assembly onto the TiO2 surface. Figure 2 shows the core level spectra of the heteroleptic complex compared to those of the anchoring ligand adsorbed on TiO2 and of a pristine TiO2 surface normalized to the intensity of the Ti 2p3/2 peak determined for each sample. The main peaks for the O 1s spectra (530 eV) clearly overlap for the three samples and correspond to the oxygen atoms in TiO2. A small side shoulder at higher binding energies is observed for all samples, which could originate from molecules adsorbed to the surface and/or surface contamination. This shoulder is significantly more intense for the assembled copper complexes (discussed below). The N 1s spectra show a clear difference, confirming the formation of Cu complexes on the surface. For the sample based on TiO2 alone, almost no signal from nitrogen is observed, while a clear peak from nitrogen is observed after binding the dbda ligand to the TiO2 surface. Upon formation of the Cu complex, the nitrogen peak increases in the intensity and its position shifts relative to the substrate reference peaks. This reveals that the formation of the Cu complex leads to additional nitrogen on the surface, as well as to a change of the chemical environment of nitrogen in the dbda ligand. The nitrogen peak for [Cu(dbda)(Br-dmp)]+ is shifted to higher binding energies, in agreement with the presumed additional bond formation between copper and the nitrogen atoms in the dbda ligand. This suggests that the [Cu(dbda)(Br-dmp)]+ complex forms efficiently on the surface and that little or no unbound dbda ligand remains.

Figure 2

Figure 2. Photoelectron spectra of [Cu(dbda)(Br-dmp)]+ adsorbed on TiO2 as compared to the TiO2 substrate and the anchoring ligand on the substrate studied at a photon energy of 3000 eV. The energy is calibrated to the Ti 2p3/2 peak at 458.6 eV. The intensity is normalized to the Ti 2p3/2 peak intensity.

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To further investigate the stoichiometry and formal oxidation state of the heteroleptic complexes, we compared [Cu(dbda)(Br-dmp)]+ adsorbed on TiO2 to its homoleptic equivalent ([Cu(Br-dmp)2]+) deposited on a FTO substrate. The thin film of the [Cu(Br-dmp)2]+ homoleptic complex deposited on FTO only showed a weak signal from the substrate present. To compare peak positions and intensities, the spectra were energy calibrated against the adventitious C 1s peak at 284.8 eV and normalized to the N 1s peak intensity obtained through fitting to a Voigt function. Figure 3 shows the core level spectra relating to the complex itself (Cu 2p, N 1s, and Br 3p), while Figure 4 shows the core level spectra relating to the PF6– counterion (P 1s and F 1s).

Figure 3

Figure 3. Photoelectron spectra of [Cu(Br-dmp)2]+ and [Cu(dbda)(Br-dmp)]+ adsorbed on TiO2 determined at a photon energy of 3000 eV. The energy is calibrated to the C 1s peak at 284.8 eV. The intensity is normalized to the N 1s peak intensity.

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Figure 4

Figure 4. Photoelectron spectra of the counterion (PF6–) core levels of [Cu(Br-dmp)2]+ and [Cu(dbda)(Br-dmp)]+ adsorbed on FTO and TiO2 obtained for a photon energy of 3000 eV. The energy is calibrated to the C 1s peak at 284.8 eV. The intensity is normalized to the N 1s peak intensity.

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The N 1s peak is found at a similar position in both samples but is wider for the heteroleptic complex when adsorbed to TiO2. The Cu 2p core level reveals the presence of some Cu(II) for the homoleptic complex based on the presence of an extra intensity around 940 eV and a satellite feature at 936 eV (Figure S2). However, during X-ray irradiation, Cu(II) was quickly converted to Cu(I) and quantification of Cu(II) was therefore not possible. For the heteroleptic complex adsorbed to TiO2, only Cu(I) was observed, and the constant peak feature indicates that no change takes place during X-ray radiation (Figure S3). The comparison of the spectra of [Cu(Br-dmp)2]+ and [Cu(dbda)(Br-dmp)]+ reveals that the Cu 2p peak is wider and more intense in relation to the N 1s peak for the heteroleptic Cu complex adsorbed to TiO2. This is indicative of a difference in the number of nitrogen atoms per copper atom for the complex when adsorbed to the TiO2 surface in comparison with the homoleptic complex. This observation could have two nonexclusive explanations: (1) an excess of unbound ligands in the sample of the homoleptic compound and/or (2) incomplete formation of the heteroleptic complexes on the TiO2 surface. In the latter case, Cu(I) would bind to the dbda ligand on the surface but, to some extent, lack the Br-dmp ligand or would be deposited on the surface without coordinating ligands. Since we only observe one narrow nitrogen peak, it is unlikely that there is a significant excess of ligands in the homoleptic sample. Quantification of the N to Cu ratio based on photoionization cross sections described by Scofield et al. (56) gives a N/Cu ratio between 4.5 and 4 to 1 (Table S1). However, due to the difference in molecular orbital composition and energies (s and p), differences in kinetic energies, uncertainties in the cross section, and variations in the Cu 2p spectra, this quantification should be taken with some reservation.
A further indication of the ligands present in the heteroleptic complexes can be obtained from the Br signal intensity using the feature of the Br 3p3/2 peak (∼183 eV). This peak was chosen due to overlapping peaks for several other orbitals as explained in the Supporting Information (Figure S10). The spectra from the heteroleptic complex were multiplied by a factor of 2 to compensate for the difference in the stoichiometric ratios of N/Br in the homoleptic complex (2:1) and heteroleptic complex (4:1). If half of the ligands of the heteroleptic complex are Br-dmp, we would expect the same Br peak intensities in the Br 3p spectrum in Figure 3 as from the homoleptic compound. While the Br 3p intensity for the heteroleptic complex looks somewhat lower than that from the homoleptic complex, this is not confirmed by quantification based on modeling the spectra (Tables S1 and S2). The total Br/N ratio for [Cu(Br-dmp)2]+ is 0.5 and for [Cu(dbda)(Br-dmp)]+, it is 0.25, both as expected from the formal stoichiometry. This observation suggests an equal number of dbda and Br-dmp ligands in the complex adsorbed on the TiO2 surface. Combined with the differences in Cu peak intensities, it follows that some copper adsorbed to the surface is uncoordinated to either Br-dmp or dbda ligands.
The counterion of the homoleptic complex is PF6– and peaks from this species are observed in the P 1s and F 1s spectra (Figure 4). Quantification suggests an F/P ratio between 6 and 7 to 1. When absorbed to the TiO2 surface, a strong P 1s peak is observed, but it is shifted to lower binding energies by about 3 eV. The F 1s signal intensity decreases significantly, and two peaks are observed instead of one when adsorbed on the metal-oxide surface. This indicates that the PF6– counterion is therefore not present on the TiO2 surface. A possible mechanism for the loss of fluorine would be through a reaction with protons from the TiO2 surface generating HF, which could escape to the solution leaving phosphorous behind in a chemically different state. Furthermore, for this particular sample, another phosphorous-containing species seems to be present on the surface. The shift to lower binding energies is in agreement with the formation of oxide-containing phosphates. (58) An O 1s signal from such phosphates is expected at higher binding energies than O 1s from the metal oxides. (59) For the [Cu(dbda)(Br-dmp)]+ complex, a higher O 1s peak intensity is observed to the high binding energy side of the TiO2 O 1s peak as compared to the references (Figure 3), consistent with oxide-based phosphates. This, therefore, suggests that the PF6– counterion is converted to phosphate on the TiO2 surface.
To identify the optimal conditions for DSSC fabrication, we used the complex [Cu(dbda)(bcp)]+ as a test dye. While we report a part of the optimization work in the Supporting Information (Tables S3 and S4), we would like to highlight some of our findings as they reveal interesting aspects regarding the behavior and properties of these Cu(I) complexes. In a first attempt, we tried to compare the efficiency of DSSCs based on the complex [Cu(dbda)(bcp)]+ using two different electrolyte redox couples: I–/I3– and [Co(bpy)3]2+/3+ (Table S3). However, the electrolyte based on the cobalt redox system partly bleached the dyed TiO2 directly after injection, rendering DSSCs with lower performances. Given the instantaneous bleaching effect and the difficulties in formulating other Co-based electrolytes, we decided to use I–/I3– as the redox mediator system for the rest of our study. Interestingly, when attempting to optimize the electrolyte composition, we again observed a bleaching phenomenon shown in Figure 5. The detailed photovoltaic parameters of the DSSCs based on the electrolytes 1 and 3 (Figure 5) are shown in Table 1.

Figure 5

Figure 5. Complete DSSCs based on the dye [Cu(dbda)(bcp)]+ in which different iodide/triiodide electrolyte compositions have been injected. Electrolyte 1: 0.65 M 1-butyl-3-methylimidazolinium iodide, 0.025 M LiI, 0.04 I2, 0.28 M 4-tert-butylpyridine (TBP) in acetonitrile/valeronitrile (volume ratio: 85/15); (60) electrolyte 2: 1.0 M 1-butyl-3-methylimidazolinium iodide, 0.1 M LiI, 0.05 I2, 0.5 M TBP in acetonitrile; electrolyte 3: same composition as for electrolyte 2 but in acetonitrile/valeronitrile (volume ratio: 85/15).

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Table 1. Photovoltaic Details of DSSCs Based on the Electrolytes 1 and 3 shown in Figure 5a
electrolyteη (%)VOC (mV)JSC (mA cm–2)FF (%)
12.07 ± 0.09622 ± 54.693 ± 0.1071 ± 1
31.74 ± 0.10577 ± 54.192 ± 0.1172 ± 1
a

The average values reported are based on three devices 2 days after sealing. The efficiencies of the devices based on electrolyte 2 are extremely low (≈0.005%) and the photovoltaic parameters are therefore not included in the table.

We do not expect the anchoring ligand dbda to desorb from the TiO2 surface under the conditions employed, but we are aware of the lability of the Cu(I) complexes in terms of ligand scrambling. (61) Therefore, a reasonable conclusion is that one or more components in the electrolytes may in part exchange with the ancillary ligand that is coordinated to the metal center. From the electrolyte compositions investigated, we can make the following conclusions: (1) the use of pure acetonitrile as an electrolyte solvent causes significant bleaching regardless of the redox couple used (see electrolyte 2, Figure 5); (2) high concentrations of TBP and of all of the electrolyte components cause partial bleaching (see electrolytes 2 and 3, Figure 5); and (3) 3-methoxypropionitrile causes significant bleaching (Table S4). This is in agreement with a previous report, where 6,6′-dimethyl substituted 2,2′-bipyridine anchoring ligand provide heteroleptic copper complexes stable in electrolytes when TBP is not used and 3-methoxypropionitrile is used as a solvent instead of acetonitrile. (36) Therefore, we found that electrolyte 1 composition reported by Colombo et al. (60) offered DSSCs with the best photovoltaic performance. The current–density characteristics of DSSCs assembled with N719 and the copper(I) dyes in this study are shown in Figure 6, and the detailed photovoltaic parameters are reported in Table 2. The lower efficiency of the Cu-based solar cells with respect to the N719-based one was ascribed to lower light harvesting, in agreement with lower extinction coefficients (∼7500 and ∼15 000 M–1 cm–1 for Cu complexes and N719, respectively) and less broad absorption spectra (350–650 and 350–800 nm for Cu complexes and N719, respectively) as previously reported. (39)

Figure 6

Figure 6. Current–density characteristics of DSSCs based on the Cu(I) dyes shown in Scheme 1, under AM 1.5G, 1 sun illumination using the electrolyte composition 1 (Figure 5). The results refer to an average of three solar cells recorded two days after assembly.

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Table 2. Detailed Photovoltaic Parameters of the J–V Characteristics Shown in Figure 6 and Reference DSSCs Assembled Using the Dye N719a
dyeη (%)VOC (mV)JSC (mA cm–2)FF (%)
[Cu(dbda)2]+1.17 ± 0.09550 ± 102.87 ± 0.1074 ± 2
[Cu(dbda)(dmp)]+1.38 ± 0.10563 ± 53.31 ± 0.1274 ± 2
[Cu(dbda)(Br-dmp)] +1.23 ± 0.10555 ± 53.17 ± 0.1170 ± 1
[Cu(dbda)(dsbtmp)]+1.81 ± 0.12563 ± 54.79 ± 0.1168 ± 1
[Cu(dbda)(bcp)]+2.05 ± 0.08565 ± 104.79 ± 0.0773 ± 1
[Cu(dbda)(biq)]+1.24 ± 0.09553 ± 53.35 ± 0.0967 ± 2
[Cu(dbda)(dap)]+1.73 ± 0.09566 ± 54.16 ± 0.1072 ± 1
N7197.60 ± 0.21700 ± 517.81 ± 0.0961 ± 1
a

The values originate from three DSSCs of each type investigated 2 days after assembly. Electrolyte 1 (Figure 5) was used.

Going from the lowest to the highest efficiency of the DSSCs based on the series of the Cu(I) complex, we find the following trend: [Cu(dbda)2]+, [Cu(dbda)(Br-dmp)]+ ≈ [Cu(dbda)(biq)]+, [Cu(dbda)(dmp)]+, [Cu(dbda)(dap)]+, [Cu(dbda)(dsbtmp)]+, and [Cu(dbda)(bcp)]+. To get better insights into the performances of the dyes, the HOMO and LUMO energy level distributions in the complexes were calculated using DFT (Figure 7).

Figure 7

Figure 7. Calculated HOMO and LUMO distribution of the copper(I) complexes. Hydrogen atoms are shown in white, carbon atoms in gray, nitrogen atoms in light blue, and oxygen atoms in orange. The calculated ground-state geometries, as well as HOMO and LUMO electronic distribution, for all of the complexes are reported in Figure S11.

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In analogy to the ruthenium dyes, the HOMO electron density distribution is also located around the metal coordination center in all of the studied copper complexes. (62,63) The complex [Cu(dbda)2]+ has previously been investigated by Melchiorre et al., (60) and they used the complex for the fabrication of DSSCs that showed 3.0% power conversion efficiency (η). It is interesting to point out that, despite the difference in absolute efficiency between our device and their DSSC devices, all of the other complexes investigated in this work performed better than [Cu(dbda)2]+. Compared to the heteroleptic complexes, the worst performance of the DSSCs based on the complex [Cu(dbda)2]+ is mostly related to its lower photocurrent (Jsc). Being a homoleptic complex, the [Cu(dbda)2]+ structure does not promote a metal-to-ligand charge transfer (MLCT) selectively to the ligand anchored on the surface of TiO2. Instead, since the LUMO is equally distributed on both ligands (Figure 7), a reasonable assumption would be that the probability that the MLCT leads to an electron promoted to the anchoring ligand is ≈50%. Therefore, as a result, the process of electron injection into the semiconductor CB is expected to be statistically less likely, causing the lower Jsc and open-circuit voltage (Voc) observed. The DSSCs based on the complexes [Cu(dbda)(Br-dmp)]+ and [Cu(dbda)(biq)]+ show very similar efficiency and Voc (Table 2). Despite the LUMO being fully delocalized on the biq ligand (Figure 7), the complex [Cu(dbda)(biq)]+ yields a slightly higher photocurrent, however, counteracted by the lower fill factor (FF), suggesting a higher degree of carrier recombination loss. The devices based on the complex [Cu(dbda)(dmp)]+ as dyes perform slightly better as compared to those based on the brominated analog [Cu(dbda)(Br-dmp)]+. Both the complexes [Cu(dbda)(Br-dmp)]+ and [Cu(dbda)(dmp)]+ display LUMOs delocalized on the anchoring dbda ligand (Figure S11), and therefore, the difference in the solar cell performance cannot be simply explained in terms on differences in spatial HOMO/LUMO distributions. The sensitizer [Cu(dbda)(dap)]+ yields a good performance within the series, which is most likely due to the high Jsc. From a chemical perspective, the ligand dap contains bulky-donating anisyl substituents in the 2,9 positions of the 1,10-phenanthroline core. As a result, the structure of this heteroleptic Cu(I) complex strongly resembles the push–pull design commonly employed for organic sensitizers. (64−67) In this case, the donating dap ligand, being electron-rich, actively “pushes” the MLCT toward the anchoring dbda ligand and, in turn, into the TiO2 CB resulting in a higher Jsc. DSSCs based on the sensitizer [Cu(dbda)(dsbtmp)]+ showed the highest photocurrent within the series. This is quite interesting as the result cannot be trivially attributed to the different light absorption properties (Figure S12) or to the donating/withdrawing character of the ancillary ligand of the complex. The ligand dsbtmp was reported by McCusker and Castellano, (68) and despite showing a rather low molar extinction coefficient, its resulting homoleptic Cu(I) complex exhibited relatively high stability attributed to the ability of the sec-butyl groups to efficiently screen the metal center from solvent coordination, thus retarding detrimental ligand exchange reactions. Therefore, we speculate that the affinity constant of this ligand is higher than that for the other ligands, leading to a higher effective amount of this complex being assembled on the surface of TiO2, in turn resulting in a higher Jsc. Finally, devices based on the dye [Cu(dbda)(bcp)]+ offer photocurrent very similar to those based on the dye [Cu(dbda)(dsbtmp)]+, but the slightly higher efficiency recorded for the resulting DSSCs originates from a higher FF. The ligand bcp, like dap, is electron donating in character, thus able to push the MLCT toward the TiO2 surface as shown by its LUMO being delocalized on the dbda ligand (Figure 7). Moreover, the relatively high molar extinction coefficients of both the homoleptic complexes [Cu(dbda)2]+ and [Cu(bcp)2]+  (39,60,69) suggest that there is also a chance that the heteroleptic complex [Cu(dbda)(bcp)]+ may be a good light absorber. The reference N719 system (Table 2) shows a superior performance, which is determined by the significantly higher photocurrent around four times higher than that of the DSSCs based on the best Cu(I) complex in this study and can in part be attributed to the broader light absorption by N719 than from any of the Cu(I) complexes.
The IPCE spectra of the DSSC devices based on the Cu(I) complexes and the ruthenium dye N719 are shown in Figure 8. Within the series, from the highest to the lowest IPCE maxima, we find the order [Cu(dbda)(dsbtmp)]+ followed by [Cu(dbda)(bcp)]+, [Cu(dbda)(dap)]+, [Cu(dbda)(dmp)]+, [Cu(dbda)(biq)]+, [Cu(dbda)2]+, and [Cu(dbda)(Br-dmp)]+. As expected, this trend is coherent with the current densities obtained for the respective devices. Table 3 shows a comparison between the wavelengths associated with the absorption maxima of the dyes adsorbed on TiO2 (Figure S12) and the maximum intensity of the IPCE.

Figure 8

Figure 8. IPCE spectra of the DSSCs sensitized by Cu(I) complexes in this study and the reference N719 dye. Each curve represents the average of 3 DSSCs. The investigation is performed on the device 2 days after assembly.

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Table 3. Maximum Absorption Wavelengths of the Cu(I) Dyes Adsorbed on TiO2 (Figure S12) and Wavelengths of the IPCE Maxima Reported in Figure 8a
dyeλmax (nm)aλIPCEmax (nm)
[Cu(dbda)2]+474480
[Cu(dbda)(dmp)]+459474
[Cu(dbda)(Br-dmp)]+451473
[Cu(dbda)(dsbtmp)]+460473
[Cu(dbda)(bcp)]+471485
[Cu(dbda)(biq)]+531473
[Cu(dbda)(dap)]+ 473
Curiously, except for the benchmark N719 and despite the diverse intensities, most of the IPCE spectra show the same shape and λIPCEmax (473–474 nm). The sole exceptions to the general pattern are represented by the IPCE spectra related to the complexes [Cu(dbda)(bcp)]+ and [Cu(dbda)2]+, which display a slightly more red-shifted λIPCEmax as compared to the rest of the series (485 and 480 nm, respectively). Of particular interest is the difference between the maximum absorption and IPCE wavelengths related to the complex [Cu(dbda)(biq)]+, which is around 60 nm. As shown from Figure 7, a possible explanation is that the LUMO for the [Cu(dbda)(biq)]+ complex is located on the biq ligand, which is not likely to contribute to generating photocurrent. Also, it is surprising how a great change in the appearance of the sensitized TiO2 before and after electrolyte injection makes the sample [Cu(dbda)(biq)]+ look like all of the other samples (such evidence is further discussed in the Supporting Information after Figure S13). The intensity and the shape of the IPCE spectra hint at the cause for the observed performance of the DSSCs in this study. In particular, the shape of all of the Cu(I) complex IPCE spectra is characterized by a well-defined broad band as observed for the light absorption spectra of most of the complexes adsorbed on TiO2. This observation, combined with the relatively low intensity of the IPCE spectra and the high FF of the DSSCs (Table 2), suggests that the reason for the low overall performance may be related to low light-harvesting ability of the dyes themselves. As indicated from the calculations of the HOMO and LUMO energy levels (Figures 7 and S11), for the majority of the molecules, the electron density in the ground state is located on Cu(I), whereas for the LUMO, it is primarily positioned on the anchoring ligand dbda. The anchoring ligand dbda appears to dominate the electronic properties for the molecules and the ancillary ligands impose a more limited effect on photovoltaic performance by affecting the donation of electron density to the Cu(I) center. Finally, consistent with the results reported in Table 2, the IPCE spectrum related to the devices based on the dye N719 is significantly higher than those of the Cu(I) complexes.

Conclusions

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A series of Cu(I) diimine complexes were self-assembled on a TiO2 surface to investigate their efficacy as dyes in DSSCs. We closely studied the self-assembly of one of the complexes ([Cu(dbda)(Br-dmp)]+) on TiO2 by HAXPES. Our results suggest that the complex forms efficiently on TiO2 and that no significant amount of unbound dbda remains. However, some additional copper of unknown character not coordinated to either ligand might be deposited on the surface. In addition, the PF6– counterion is not stable on the TiO2 surface and is either replaced by OH– groups from the TiO2 surface or by phosphate ions formed through oxidation. The dyes were investigated in DSSCs, and their performance has been rationalized on the basis of the nature of their ancillary ligand. We have shown by theoretical calculations, absorption properties, and DSSC performances that the push–pull design is a key aspect for designing heteroleptic Cu(I) dyes with better performance: complexes including donating ancillary ligands display higher photocurrents that most likely can be attributed to a more efficient electron injection. Another important factor to consider is, not surprisingly, the molar extinction coefficients and spectral width of light absorption from the complexes. The results from the complex [Cu(dbda)(bcp)]+ suggest that, when designing a heteroleptic complex, it is important to use ligands for which the corresponding homoleptic Cu(I) complexes show high molar extinction coefficients. (39) The dyes [Cu(dbda)(dsbtmp)]+ and [Cu(dbda)(dap)]+ contain the ancillary ligands dsbtmp and dap, respectively, whose corresponding homoleptic Cu(I) complexes display blue-shifted absorption spectra and low molar extinction coefficients. However, the heteroleptic complexes show very similar device performance as the best performing dyes in the series [Cu(dbda)(bcp)]+. This clearly highlights that there are factors, other than the optical properties, affecting the capacity of the complexes to inject electrons into the semiconductor. The inclusion of ancillary ligands with bulky substituents in the 2,9 positions of the phenanthroline core may screen the Cu(I) coordination center and bind more strongly to it. This may constitute another critical aspect of good solar cell performance. The low performance of DSSCs based on the complexes [Cu(dbda)2]+, [Cu(dbda)(Br-dmp)]+, and [Cu(dbda)(biq)]+ have, respectively, shown that homoleptic complexes, ancillary ligands containing electron-withdrawing substituents, and ancillary ligands with low complex formation constants should be avoided. In particular, the complex [Cu(dbda)(biq)]+ has revealed a dye–electrolyte interaction that, to various degrees, may also be present for the other Cu(I) dyes, a phenomenon that deserves to be further investigated.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsaem.1c02778.

  • XPS spectra; detailed photovoltaic parameters of nonoptimized DSSCs with different electrolyte compositions; DFT-calculated ground-state geometries of the dyes; absorption spectra of the dyes on TiO2; and N719 DSSC lifetime (PDF)

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Author Information

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  • Corresponding Author
  • Authors
    • Daniele Franchi - Institute of Chemistry of Organometallic Compounds (CNR-ICCOM), Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy;  Division of Organic Chemistry, Centre of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden;  Orcidhttps://orcid.org/0000-0003-2811-247X
    • Valentina Leandri - Division of Applied Physical Chemistry, Centre of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden
    • Angela Raffaella Pia Pizzichetti - Division of Applied Physical Chemistry, Centre of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden
    • Bo Xu - Division of Physical Chemistry, Centre of Molecular Devices, Department of Chemistry, Ångström Laboratory, Uppsala University, Box 523, SE-75120 Uppsala, Sweden;  Orcidhttps://orcid.org/0000-0002-4703-7340
    • Yan Hao - Division of Applied Physical Chemistry, Centre of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden;  Orcidhttps://orcid.org/0000-0002-0996-1794
    • Wei Zhang - Division of Applied Physical Chemistry, Centre of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden;  Orcidhttps://orcid.org/0000-0003-0232-9937
    • Tamara Sloboda - Division of Applied Physical Chemistry, Centre of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden
    • Sebastian Svanström - Division of X-ray Photon Science, Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden;  Orcidhttps://orcid.org/0000-0001-7351-8183
    • Ute B. Cappel - Division of Applied Physical Chemistry, Centre of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden;  Orcidhttps://orcid.org/0000-0002-9432-3112
    • Lars Kloo - Division of Applied Physical Chemistry, Centre of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden;  Orcidhttps://orcid.org/0000-0002-0168-2942
    • Licheng Sun - Division of Organic Chemistry, Centre of Molecular Devices, Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden;  Center of Artificial Photosynthesis for Solar Fuels, School of Science, Westlake University, Hangzhou 310024, China;  Orcidhttps://orcid.org/0000-0002-4521-2870
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors would like to thank the Swedish Research Council, the Swedish Energy Agency, and the Swedish Foundation for Strategic Research (project no. RMA15-0130) for their financial support. The co-author W.Z. thank the China Scholarship Council (CSC) for financial support. J.M.G. gratefully acknowledges support from the Swedish Government through the strategic research area “STandUP for ENERGY”. The authors thank HZB for the allocation of synchrotron radiation beam time. The research leading to this result has been supported by the project CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. They would like to thank Dr. Haining Tian (Physical Chemistry, Uppsala University) for his synthetic insights in this work.

References

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    Franchi, D.; Calamante, M.; Reginato, G.; Zani, L.; Peruzzini, M.; Taddei, M.; Fabrizi De Biani, F.; Basosi, R.; Sinicropi, A.; Colonna, D.; Di Carlo, A.; Mordini, A. Two New Dyes with Carboxypyridinium Regioisomers as Anchoring Groups for Dye-Sensitized Solar Cells. Synlett 2015, 26, 2389– 2394,  DOI: 10.1055/s-0035-1560713
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    Dessì, A.; Sinicropi, A.; Mohammadpourasl, S.; Basosi, R.; Taddei, M.; Fabrizi de Biani, F.; Calamante, M.; Zani, L.; Mordini, A.; Bracq, P.; Franchi, D.; Reginato, G. New Blue Donor–Acceptor Pechmann Dyes: Synthesis, Spectroscopic, Electrochemical, and Computational Studies. ACS Omega 2019, 4, 7614– 7627,  DOI: 10.1021/acsomega.8b03560
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    New Blue Donor-Acceptor Pechmann Dyes: Synthesis, Spectroscopic, Electrochemical, and Computational Studies
    Dessi, Alessio; Sinicropi, Adalgisa; Mohammadpourasl, Sanaz; Basosi, Riccardo; Taddei, Maurizio; Fabrizi de Biani, Fabrizia; Calamante, Massimo; Zani, Lorenzo; Mordini, Alessandro; Bracq, Pamela; Franchi, Daniele; Reginato, Gianna
    ACS Omega (2019), 4 (4), 7614-7627CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
    The design, synthesis and characterization of a new class of blue-colored thiophene-substituted Pechmann dyes are reported. Due to a distinguishing blue coloration and the capability to absorb light in one of the most photon-dense regions of the solar spectrum, such compds. are of great interest for application as photoactive materials in org. optoelectronics, in particular in dye-sensitized solar cells (DSSCs). In order to achieve a fine tuning of the optical and electrochem. properties, the electron poor thiophene-bis(lactone) moiety has been decorated with donor (D) and acceptor groups (A), targeting fully conjugated D-A-π-A structures. The designed structures have been investigated by means of DFT and TD-DFT calcns. and the most promising dyes synthesized. These mols. represent the very first prepn. of unsym. Pechmann derivs. Optical and electrochem. properties of the new dyes have been studied by cyclic voltammetry, UV/Vis and fluorescence spectroscopy. In two cases, test cells were built proving that a photocurrent can indeed be generated when using electrolytes esp. formulated for narrow-band-gap dyes, although with a very low efficiency.
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    Błaszczyk, A. Strategies to Improve the Performance of Metal-Free Dye-Sensitized Solar Cells. Dyes Pigm. 2018, 149, 707– 718,  DOI: 10.1016/j.dyepig.2017.11.045
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    Strategies to improve the performance of metal-free dye-sensitized solar cells
    Blaszczyk, Alfred
    Dyes and Pigments (2018), 149 (), 707-718CODEN: DYPIDX; ISSN:0143-7208. (Elsevier Ltd.)
    A review. In recent years dye-sensitized solar cells (DSSCs) have attracted more and more attention due to many advantages such as high molar absorption coeff., facile mol. tailoring and simple fabrication process as compared to conventional silicon devices. High-energy conversion efficiency is one of the most important factors for the large-scale commercialization. However, the max. reported DSSCs efficiency of 12.3% is still short of the performance of silicon devices (> 25%) and other commercialized technologies. Even lower efficiency for metal-free org. dyes as sensitizers in DSSCs have been reported. Therefore much work is required to reach optimal efficiency. Improving performance of metal-free DSSCs is a great challenge to the academic research community and for industrial applications scientists. The purpose of this review is to highlight the recent progress in improving the performance of metal-free DSSCs based on the chem. design of the sensitizer and interface mol. engineering principles.
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    Carella, A.; Borbone, F.; Centore, R. Research Progress on Photosensitizers for DSSC. Front. Chem. 2018, 6, 481  DOI: 10.3389/fchem.2018.00481
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    Research progress on photosensitizers for DSSC
    Carella, Antonio; Borbone, Fabio; Centore, Roberto
    Frontiers in Chemistry (Lausanne, Switzerland) (2018), 6 (), 481CODEN: FCLSAA; ISSN:2296-2646. (Frontiers Media S.A.)
    A review. Dye sensitized solar cells (DSSC) are considered one of the most promising photovoltaic technologies as an alternative to traditional silicon-based solar cells, for their compatibility with low-cost prodn. methods, their peculiar optical and mech. properties and the high indoor efficiency. Photosensitizers represent one of the most important components of a DSSC device and probably the most thoroughly investigated in the last twenty years, with thousands of dyes that have been proposed and tested for this kind of application. In this review we aimed to provide an overview of the three main classes of DSSC photosensitizers, namely ruthenium(II) polypyridyl complexes, Zn-porphyrin derivs. and metal-free org. dyes. After a brief introduction about the architecture and operational principles of a DSSC and the state of the art of the other main components of this type of device, we focused our discussion on photosensitizers. We have defined the numerous requirements DSSC photosensitizers should satisfy and have provided an overview of their historical development over the years; by examg. specific dyes reported in the literature, we attempted to highlight the mol. design strategies that have been established for the optimization of their performance in real devices both in terms of efficiency (which recently reaches an outstanding 14.3%) and operational stability. Finally, we discussed, in the last section, the possible future developments of this intriguing technol.
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    Aghazada, S.; Nazeeruddin, M. Ruthenium Complexes as Sensitizers in Dye-Sensitized Solar Cells. Inorganics 2018, 6, 52  DOI: 10.3390/inorganics6020052
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    Ruthenium complexes as sensitizers in dye-sensitized solar cells
    Aghazada, Sadig; Nazeeruddin, Mohammad Khaja
    Inorganics (2018), 6 (2), 52/1-52/34CODEN: INORCW; ISSN:2304-6740. (MDPI AG)
    In this review, we discuss the main directions in which ruthenium complexes for dye-sensitized solar cells (DSCs) were developed. We critically discuss the implemented design principles. This review might be helpful at this moment when a breakthrough is needed for DSC technol. to prove its market value.
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    Albero, J.; Atienzar, P.; Corma, A.; Garcia, H. Efficiency Records in Mesoscopic Dye-Sensitized Solar Cells. Chem. Rec. 2015, 15, 803– 828,  DOI: 10.1002/tcr.201500007
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    Efficiency Records in Mesoscopic Dye-Sensitized Solar Cells
    Albero, Josep; Atienzar, Pedro; Corma, Avelino; Garcia, Hermenegildo
    Chemical Record (2015), 15 (4), 803-828CODEN: CRHEAK; ISSN:1528-0691. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The aim of the present review article is to show the progress achieved in the efficiency of dye-sensitized solar cells (DSSCs) by evolution in the structure and compn. of the dye. After an initial brief description of DSSCs and the operating mechanism the major part of the present article is organized according to the type of dye, trying to show the logic in the variation of the dye structure in order to achieve strong binding on the surface of the layer of nanoparticulate TiO2, efficient interfacial electron injection between the excited dye and the semiconductor, and minimization of the unwanted dark current processes. Besides metal complexes, including polypyridyls and nitrogenated macro rings, org. dyes and inorg. light harvesters such as quantum dots and perovskites have also been included in the review. The last section summarizes the current state of the art and provides an overview on future developments in the field.
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    Vougioukalakis, G. C.; Philippopoulos, A. I.; Stergiopoulos, T.; Falaras, P. Contributions to the Development of Ruthenium-Based Sensitizers for Dye-Sensitized Solar Cells. Coord. Chem. Rev. 2011, 2602– 2621,  DOI: 10.1016/j.ccr.2010.11.006
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    Contributions to the development of ruthenium-based sensitizers for dye-sensitized solar cells
    Vougioukalakis, Georgios C.; Philippopoulos, Athanassios I.; Stergiopoulos, Thomas; Falaras, Polycarpos
    Coordination Chemistry Reviews (2011), 255 (21-22), 2602-2621CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
    The present review article traces the discoveries that were instrumental to the evolution of DSSCs, taking a crit. look at the principles of the operation mechanism and insisting on the most important recent developments in the field. We draw our attention mainly to the dye (photosensitizer); the primary processes that take place inside a solar cell, and which are affected by the dye, are also reviewed along with some of the most significant recent information found in literature that one should keep in mind before designing a novel ruthenium sensitizer. Specific emphasis is given in trying to answer vital questions like defining the optimum no. of -COOH anchoring groups and protons that an efficient dye should carry, the correct choice of prepg. either a homoleptic or a heteroleptic complex, as well as which counterions are the most suitable. Phenomena such as the role of the adsorption geometry, elec. fields at the double layer, dye-redox couple interaction and recombination effects are also analyzed, in order to show how these factors influence the photoelectrochem. characteristics of the cell. Next, the contributions from our group in the direction of designing, synthesizing, characterizing and evaluating novel ruthenium(II)-based complexes, utilized as photosensitizers in DSSCs, are thoroughly discussed. Two types of Ru(II) complexes are examd.: one family mimicking the std. N719 dye, bearing bidentate ligands, and another, carrying tridendate ligands, resembling the "black dye".
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    Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Müller, E.; Liska, P.; Vlachopoulos, N.; Grätzel, M. Conversion of Light to Electricity by Cis-X2Bis (2,2′-Bipyridyl-4,4′-Dicarboxylate) Ruthenium (II) Charge-Transfer Sensitizers (X = Cl–, Br–, I–, CN–, and SCN−) on Nanocrystalline TiO2 Electrodes. J. Am. Chem. Soc. 1993, 115, 6382– 6390,  DOI: 10.1021/ja00067a063
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    16
    Conversion of light to electricity by cis-X2bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) charge-transfer sensitizers (X = Cl-, Br-, I-, CN-, and SCN-) on nanocrystalline titanium dioxide electrodes
    Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mueller, E.; Liska, P.; Vlachopoulos, N.; Graetzel, M.
    Journal of the American Chemical Society (1993), 115 (14), 6382-90CODEN: JACSAT; ISSN:0002-7863.
    Cis-X2bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) complexes (X = Cr-, Br-, I-, CN-, and SCN-) were prepd. and characterized with respect to their absorption, luminescence, and redox behavior. They act as efficient charge-transfer sensitizers for nanocryst. TiO2 films (thickness 8-12 μm) of very high internal surface area (roughness factor ∼1000), prepd. by sintering of 15-30-nm colloidal titania particles on a conducting glass support. The performance of cis-di(thiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) (I) was outstanding and is unmatched by any other known sensitizer. Nanocryst. TiO2 films coated with a monolayer of I harvest visible light very efficiently, their absorption threshold being ∼800 nm. Conversion of incident photons into elec. current is nearly quant. over a large spectral range. These films were incorporated in a thin-layer regenerative solar cell equipped with a light-reflecting counter electrode. Short-circuit photocurrents exceeding 17 mA/cm2 were obtained in simulated AM 1.5 sunlight using lithium iodide/triiodide in acetonitrile or acetonitrile/3-methyl-2-oxazolidinone mixts. as redox electrolyte. The open-circuit photovoltage was 0.38 V and increased to 0.72 V by treating the dye-covered film with 4-tert-butylpyridine. A solar-to-elec. energy conversion efficiency of 10% was attained with this system. The effect of temp. on the power output and long-term stability of the dye was also investigated. For the first time, a device based on a simple mol. light absorber attains a conversion efficiency commensurate with that of conventional silicon-based photovoltaic cells.
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    Nazeeruddin, M. K.; Humphry-Baker, R.; Liska, P.; Grätzel, M. Investigation of Sensitizer Adsorption and the Influence of Protons on Current and Voltage of a Dye-Sensitized Nanocrystalline TiO2 Solar Cell. J. Phys. Chem. B 2003, 107, 8981– 8987,  DOI: 10.1021/jp022656f
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    17
    Investigation of Sensitizer Adsorption and the Influence of Protons on Current and Voltage of a Dye-Sensitized Nanocrystalline TiO2 Solar Cell
    Nazeeruddin, Md. K.; Humphry-Baker, R.; Liska, P.; Graetzel, M.
    Journal of Physical Chemistry B (2003), 107 (34), 8981-8987CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    FTIR spectra of [Ru(dcbpyH2)2(NCS)2] (N 3), (Bu4N)2[Ru(dcbpyH)2(NCS)2] (N 719), and (Bu4N)4[Ru(dcbpy)2(NCS)2] (N 712) complexes measured as solid samples in photoacoustic mode display fine resoln. of IR bands and exhibit differences between the cis and the trans carboxylic acid groups. The interaction between N 3, N 719, and N 712 sensitizers with nanocryst. TiO2 film was studied by ATR-FTIR spectroscopy. These complexes are being anchored onto the TiO2 surface in bridging coordination mode using two out of their four carboxylic acid groups, which are trans to the NCS ligand. The effect of protons on both the short circuit photocurrent and the open circuit photovoltage of dye-sensitized nanocryst. solar cells was scrutinized. For the std. electrolyte formulation employed and TiCl4 treated mesoporous TiO2 films, the monoprotonated form of the N3 dye exhibited superior power conversion efficiency under AM 1.5 sun compared to the four, two, and zero proton sensitizers.
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    Nazeeruddin, M. K.; Péchy, P.; Grätzel, M. Efficient Panchromatic Sensitization of Nanocrystalline TiO2 Films by a Black Dye Based on a Trithiocyanato-Ruthenium Complex. Chem. Commun. 1997, 1, 1705– 1706,  DOI: 10.1039/a703277c
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    Min Park, J.; Lee, J. H.; Jang, W.-D. Applications of Porphyrins in Emerging Energy Conversion Technologies. Coord. Chem. Rev. 2020, 407, 213157  DOI: 10.1016/j.ccr.2019.213157
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    Song, H.; Liu, Q.; Xie, Y. Porphyrin-Sensitized Solar Cells: Systematic Molecular Optimization, Coadsorption and Cosensitization. Chem. Commun. 2018, 54, 1811– 1824,  DOI: 10.1039/C7CC09671B
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    Porphyrin-sensitized solar cells: systematic molecular optimization, coadsorption and cosensitization
    Song, Heli; Liu, Qingyun; Xie, Yongshu
    Chemical Communications (Cambridge, United Kingdom) (2018), 54 (15), 1811-1824CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    As a promising low-cost solar energy conversion technique, dye-sensitized solar cells have undergone spectacular development since 1991. For practical applications, improvement of power conversion efficiency has always been one of the major research topics. Porphyrins are outstanding sensitizers endowed with strong sunlight harvesting ability in the visible region and multiple reaction sites available for functionalization. However, judicious mol. design in consideration of light-harvest, energy levels, operational dynamics, adsorption geometry and suppression of back reactions is specifically required for achieving excellent photovoltaic performance. This feature article highlights some of the recently developed porphyrin sensitizers, esp. focusing on the systematic dye structure optimization approach in combination with coadsorption and cosensitization methods in pursuing higher efficiencies. Herein, we expect to provide more insights into the structure-performance correlation and mol. engineering strategies in a stepwise manner.
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    Yella, A.; Lee, H. W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.; Diau, E. W. G.; Yeh, C. Y.; Zakeeruddin, S. M.; Grätzel, M. Porphyrin-Sensitized Solar Cells with Cobalt (II/III)-Based Redox Electrolyte Exceed 12 Percent Efficiency. Science 2011, 334, 629– 634,  DOI: 10.1126/science.1209688
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    21
    Porphyrin-Sensitized Solar Cells with Cobalt (II/III)-Based Redox Electrolyte Exceed 12% Efficiency
    Yella, Aswani; Lee, Hsuan-Wei; Tsao, Hoi Nok; Yi, Chenyi; Chandiran, Aravind Kumar; Nazeeruddin, Md. Khaja; Diau, Eric Wei-Guang; Yeh, Chen-Yu; Zakeeruddin, Shaik M.; Graetzel, Michael
    Science (Washington, DC, United States) (2011), 334 (6056), 629-634CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    The iodide/triiodide redox shuttle has limited the efficiencies accessible in dye-sensitized solar cells. Here, the authors report mesoscopic solar cells that incorporate a Co(II/III)tris(bipyridyl)-based redox electrolyte in conjunction with a custom synthesized donor-π-bridge-acceptor zinc porphyrin dye as sensitizer (designated YD2-o-C8). The specific mol. design of YD2-o-C8 greatly retards the rate of interfacial back electron transfer from the conduction band of the nanocryst. titanium dioxide film to the oxidized cobalt mediator, which enables attainment of strikingly high photovoltages approaching 1 V. Because the YD2-o-C8 porphyrin harvests sunlight across the visible spectrum, large photocurrents are generated. Cosensitization of YD2-o-C8 with another org. dye further enhances the performance of the device, leading to a measured power conversion efficiency of 12.3% under simulated air mass 1.5 global sunlight.
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  22. 22
    Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod, B. F. E.; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin, M. K.; Grätzel, M. Dye-Sensitized Solar Cells with 13% Efficiency Achieved through the Molecular Engineering of Porphyrin Sensitizers. Nat. Chem. 2014, 6, 242– 247,  DOI: 10.1038/nchem.1861
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    22
    Dye-sensitized solar cells with 13% efficiency achieved through the molecular engineering of porphyrin sensitizers
    Mathew, Simon; Yella, Aswani; Gao, Peng; Humphry-Baker, Robin; Curchod, Basile F. E.; Ashari-Astani, Negar; Tavernelli, Ivano; Rothlisberger, Ursula; Nazeeruddin, Md. Khaja; Graetzel, Michael
    Nature Chemistry (2014), 6 (3), 242-247CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
    Dye-sensitized solar cells have gained widespread attention in recent years because of their low prodn. costs, ease of fabrication and tunable optical properties, such as color and transparency. Here, the authors report a molecularly engineered porphyrin dye, coded SM315, which features the prototypical structure of a donor-π-bridge-acceptor and both maximizes electrolyte compatibility and improves light-harvesting properties. Linear-response, time-dependent d. functional theory was used to study the perturbations in the electronic structure that lead to improved light harvesting. Using SM315 with the Co(II/III) redox shuttle resulted in dye-sensitized solar cells that exhibit a high open-circuit voltage VOC of 0.91 V, short-circuit c.d. JSC of 18.1 mA cm-2, fill factor of 0.78 and a power conversion efficiency of 13%.
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    Lu, X.; Wei, S.; Wu, C. M. L.; Li, S.; Guo, W. Can Polypyridyl Cu(I)-Based Complexes Provide Promising Sensitizers for Dye-Sensitized Solar Cells? A Theoretical Insight into Cu(I) versus Ru(II) Sensitizers. J. Phys. Chem. C 2011, 115, 3753– 3761,  DOI: 10.1021/jp111325y
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    23
    Can polypyridyl Cu(I)-based complexes provide promising sensitizers for dye-sensitized solar cells? A theoretical insight into Cu(I) versus Ru(II) sensitizers
    Lu, Xiaoqing; Wei, Shuxian; Wu, Chi-Man Lawrence; Li, Shaoren; Guo, Wenyue
    Journal of Physical Chemistry C (2011), 115 (9), 3753-3761CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Design of light-absorbent dyes with cheaper, safer, and more sustainable materials is one of the key issues for the future development of dye-sensitized solar cells (DSSCs). We report herein a theor. investigation on a series of polypyridyl Cu(I)-based complexes with general formula (L and L' represent bipyridyl ligands) by d. functional theory (DFT) and time-dependent DFT. Mol. geometries, electronic structures, and optical absorption spectra are predicted in both the gas phase and Me cyanide soln. Our results show that all the [CuLL']+ derivs. display Cu → bipyridine metal-to-ligand charge transfer absorption spectra in the range of 350-700 nm. Structural optimizations by enhancing π-conjugation and introducing heteroarom. groups on ancillary ligands lead to upshift of MO energies, increase in oscillator strength, and red shift of absorption spectra. Compared with Ru(II) sensitizers, polypridyl Cu(I)-based complexes show similar optical properties and improving trend of the DSSCs performance along with the optimizations of structures. The results of this work highlight the point that polypyridyl Cu(I)-based complexes could provide promising sensitizers for efficient next-generation DSSCs.
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    Magni, M.; Biagini, P.; Colombo, A.; Dragonetti, C.; Roberto, D.; Valore, A. Versatile Copper Complexes as a Convenient Springboard for Both Dyes and Redox Mediators in Dye Sensitized Solar Cells. Coord. Chem. Rev. 2016, 322, 69– 93,  DOI: 10.1016/j.ccr.2016.05.008
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    24
    Versatile copper complexes as a convenient springboard for both dyes and redox mediators in dye sensitized solar cells
    Magni, Mirko; Biagini, Paolo; Colombo, Alessia; Dragonetti, Claudia; Roberto, Dominique; Valore, Adriana
    Coordination Chemistry Reviews (2016), 322 (), 69-93CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
    In the early 1990s, dye-sensitized solar cells (DSSCs), fabricated by O'Regan and Graetzel, emerged as a realistic soln. for harnessing the energy of the sun and converting it into electricity. Since then, an impressive amt. of work has been carried out in order to improve the global photovoltaic efficiency of DSSCs, trying to optimize every components of the device in terms of material, structure and activity. Up to now, the best efficiencies have been reached with zinc-porphyrin and Ru(II) photosensitizers, but recently an exponential growth of articles showed the great potential of copper(I) coordination compds. as a convenient and cheap alternative. Similarly, it appeared that the use of copper complexes as electron transfer mediators for DSSCs can be an excellent way to solve the problems related to the more common I-3/I- redox couple.The goal of this review is to report on the promising use of versatile copper complexes as photosensitizers and electron shuttles in DSSCs which had a real boom in the last few years. The coverage, mainly from 2010 up to now, is not exhaustive, but allows the non-specialist reader to get into this specific field and to understand its potentiality.An important message that the review will convey is the mandatory role of comparing results on novel photosensitizers and/or on alternative redox mediators with well-established literature ref. systems such as N719 dye and I-3/I--based electrolyte, resp. In fact, reporting the performance of new compds. with respect to proper std. materials is the unique secure method to correctly and, firstly, constructively compare data from different studies.
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  25. 25
    Risi, G.; Becker, M.; Housecroft, C. E.; Constable, E. C. Are Alkynyl Spacers in Ancillary Ligands in Heteroleptic Bis(Diimine)Copper(I) Dyes Beneficial for Dye Performance in Dye-Sensitized Solar Cells?. Molecules 2020, 25, 1528  DOI: 10.3390/molecules25071528
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    25
    Are alkynyl spacers in ancillary ligands in heteroleptic bis(diimine)copper(I) dyes beneficial for dye performance in dye-sensitized solar cells?
    Risi, Guglielmo; Becker, Mariia; Housecroft, Catherine E.; Constable, Edwin C.
    Molecules (2020), 25 (7), 1528CODEN: MOLEFW; ISSN:1420-3049. (MDPI AG)
    The syntheses of 4,4'-bis(4-dimethylaminophenyl)-6,6'-dimethyl-2,2'-bipyridine (1), 4,4'-bis(4-dimethylaminophenylethynyl)-6,6'-dimethyl-2,2'-bipyridine (2), 4,4'-bis(4- diphenylaminophenyl)-6,6'-dimethyl-2,2'-bipyridine (3), and 4,4'-bis(4-diphenylaminophenylethynyl)- 6,6'-dimethyl-2,2'-bipyridine (4) are reported along with the prepns. and characterizations of their homoleptic copper(I) complexes [CuL2][PF6] (L = 1-4). The soln. absorption spectra of the complexes exhibit ligand-centered absorptions in addn. to absorptions in the visible region assigned to a combination of intra-ligand and metal-to-ligand charge-transfer. Heteroleptic [Cu(5)(Lancillary)]+ dyes in which 5 is the anchoring ligand ((6,6'-dimethyl-[2,2'-bipyridine]- 4,4'-diyl)bis(4,1-phenylene))bis(phosphonic acid) and Lancillary = 1-4 have been assembled on fluorine-doped tin oxide (FTO)-TiO2 electrodes in dye-sensitized solar cells (DSCs). Performance parameters and external quantum efficiency (EQE) spectra of the DSCs (four fully-masked cells for each dye) reveal that the best performing dyes are [Cu(5)(1)]+ and [Cu(5)(3)]+. The alkynyl spacers are not beneficial, leading to a decrease in the short-circuit c.d. (JSC), confirmed by lower values of EQEmax. Addn. of a co-absorbent (n-decylphosphonic acid) to [Cu(5)(1)]+ lead to no significant enhancement of performance for DSCs sensitized with [Cu(5)(1)]+. Electrochem. impedance spectroscopy (EIS) has been used to investigate the interfaces in DSCs; the anal. shows that more favorable electron injection into TiO2 is obsd. for sensitizers without the alkynyl spacer and confirms higher JSC values for [Cu(5)(1)]+.
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  26. 26
    Cao, Y.; Saygili, Y.; Ummadisingu, A.; Teuscher, J.; Luo, J.; Pellet, N.; Giordano, F.; Zakeeruddin, S. M.; Moser, J. E.; Freitag, M.; Hagfeldt, A.; Grätzel, M. 11% Efficiency Solid-State Dye-Sensitized Solar Cells with Copper(II/I) Hole Transport Materials. Nat. Commun. 2017, 8, 15390  DOI: 10.1038/ncomms15390
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    11% efficiency solid-state dye-sensitized solar cells with copper(II/I) hole transport materials
    Cao Yiming; Ummadisingu Amita; Luo Jingshan; Pellet Norman; Giordano Fabrizio; Zakeeruddin Shaik Mohammed; Gratzel Michael; Saygili Yasemin; Freitag Marina; Hagfeldt Anders; Teuscher Joel; Moser Jacques-E
    Nature communications (2017), 8 (), 15390 ISSN:.
    Solid-state dye-sensitized solar cells currently suffer from issues such as inadequate nanopore filling, low conductivity and crystallization of hole-transport materials infiltrated in the mesoscopic TiO2 scaffolds, leading to low performances. Here we report a record 11% stable solid-state dye-sensitized solar cell under standard air mass 1.5 global using a hole-transport material composed of a blend of [Cu (4,4',6,6'-tetramethyl-2,2'-bipyridine)2](bis(trifluoromethylsulfonyl)imide)2 and [Cu (4,4',6,6'-tetramethyl-2,2'-bipyridine)2](bis(trifluoromethylsulfonyl)imide). The amorphous Cu(II/I) conductors that conduct holes by rapid hopping infiltrated in a 6.5 μm-thick mesoscopic TiO2 scaffold are crucial for achieving such high efficiency. Using time-resolved laser photolysis, we determine the time constants for electron injection from the photoexcited sensitizers Y123 into the TiO2 and regeneration of the Y123 by Cu(I) to be 25 ps and 3.2 μs, respectively. Our work will foster the development of low-cost solid-state photovoltaic based on transition metal complexes as hole conductors.
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  27. 27
    Saygili, Y.; Söderberg, M.; Pellet, N.; Giordano, F.; Cao, Y.; Munoz-García, A. B.; Zakeeruddin, S. M.; Vlachopoulos, N.; Pavone, M.; Boschloo, G.; Kavan, L.; Moser, J. E.; Grätzel, M.; Hagfeldt, A.; Freitag, M. Copper Bipyridyl Redox Mediators for Dye-Sensitized Solar Cells with High Photovoltage. J. Am. Chem. Soc. 2016, 138, 15087– 15096,  DOI: 10.1021/jacs.6b10721
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    Copper Bipyridyl Redox Mediators for Dye-Sensitized Solar Cells with High Photovoltage
    Saygili, Yasemin; Soderberg, Magnus; Pellet, Norman; Giordano, Fabrizio; Cao, Yiming; Munoz-Garcia, Ana Belen; Zakeeruddin, Shaik M.; Vlachopoulos, Nick; Pavone, Michele; Boschloo, Gerrit; Kavan, Ladislav; Moser, Jacques-E.; Gratzel, Michael; Hagfeldt, Anders; Freitag, Marina
    Journal of the American Chemical Society (2016), 138 (45), 15087-15096CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Redox mediators play a major role detg. the photocurrent and the photovoltage in dye-sensitized solar cells (DSCs). To maintain the photocurrent, the redn. of oxidized dye by the redox mediator should be significantly faster than the electron back transfer between TiO2 and the oxidized dye. The driving force for dye regeneration with the redox mediator should be sufficiently low to provide high photovoltages. With the introduction of our new copper complexes as promising redox mediators in DSCs both criteria are satisfied to enhance power conversion efficiencies. In this study, two copper bipyridyl complexes, Cu(II/I)(dmby)2TFSI2/1 (0.97 V vs SHE, dmby = 6,6'-dimethyl-2,2'-bipyridine) and Cu(II/I)(tmby)2TFSI2/1 (0.87 V vs SHE, tmby = 4,4',6,6'-tetramethyl-2,2'-bipyridine), are presented as new redox couples for DSCs. They are compared to previously reported Cu(II/I)(dmp)2TFSI2/1 (0.93 V vs SHE, dmp = bis(2,9-dimethyl-1,10-phenanthroline)). Due to the small reorganization energy between Cu(I) and Cu(II) species, these copper complexes can sufficiently regenerate the oxidized dye mols. with close to unity yield at driving force potentials as low as 0.1 V. The high photovoltages of over 1.0 V were achieved by the series of copper complex based redox mediators without compromising photocurrent densities. Despite the small driving forces for dye regeneration, fast and efficient dye regeneration (2-3 μs) was obsd. for both complexes. As another advantage, the electron back transfer (recombination) rates were slower with Cu(II/I)(tmby)2TFSI2/1 as evidenced by longer lifetimes. The solar-to-elec. power conversion efficiencies for [Cu(tmby)2]2+/1+, [Cu(dmby)2]2+/1+, and [Cu(dmp)2]2+/1+ based electrolytes were 10.3%, 10.0%, and 10.3%, resp., using the org. Y123 dye under 1000 W m-2 AM1.5G illumination. The high photovoltaic performance of Cu-based redox mediators underlines the significant potential of the new redox mediators and points to a new research and development direction for DSCs.
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  28. 28
    Colombo, A.; Di Carlo, G.; Dragonetti, C.; Magni, M.; Orbelli Biroli, A.; Pizzotti, M.; Roberto, D.; Tessore, F.; Benazzi, E.; Bignozzi, C. A.; Casarin, L.; Caramori, S. Coupling of Zinc Porphyrin Dyes and Copper Electrolytes: A Springboard for Novel Sustainable Dye-Sensitized Solar Cells. Inorg. Chem. 2017, 56, 14189– 14197,  DOI: 10.1021/acs.inorgchem.7b02323
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    28
    Coupling of Zinc Porphyrin Dyes and Copper Electrolytes: A Springboard for Novel Sustainable Dye-Sensitized Solar Cells
    Colombo, Alessia; Di Carlo, Gabriele; Dragonetti, Claudia; Magni, Mirko; Orbelli Biroli, Alessio; Pizzotti, Maddalena; Roberto, Dominique; Tessore, Francesca; Benazzi, Elisabetta; Bignozzi, Carlo Alberto; Casarin, Laura; Caramori, Stefano
    Inorganic Chemistry (2017), 56 (22), 14189-14197CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    The combination of β-substituted Zn2+ porphyrin dyes and copper-based electrolytes represents a sustainable route for economic and environmentally friendly dye-sensitized solar cells. Remarkably, a new copper electrolyte, [Cu(2-mesityl-1,10-phenanthroline)2]+/2+, exceeds the performance reached by Co2+/3+ and I-/I3- ref. electrolytes.
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  29. 29
    Higashino, T.; Iiyama, H.; Nishimura, I.; Imahori, H. Exploration on the Combination of Push-Pull Porphyrin Dyes and Copper(I/II) Redox Shuttles toward High-Performance Dye-Sensitized Solar Cells. Chem. Lett. 2020, 49, 936– 939,  DOI: 10.1246/cl.200317
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    29
    Exploration on the Combination of Push-Pull Porphyrin Dyes and Copper(I/II) Redox Shuttles toward High-performance Dye-sensitized Solar Cells
    Higashino, Tomohiro; Iiyama, Hitomi; Nishimura, Issei; Imahori, Hiroshi
    Chemistry Letters (2020), 49 (8), 936-939CODEN: CMLTAG; ISSN:0366-7022. (Chemical Society of Japan)
    The effects of push-pull porphyrin dyes as well as copper(I/II) redox shuttles on photovoltaic properties have been examd. toward high-performance dye-sensitized solar cells (DSSCs). DSSC with a combination of porphyrin dye LG4 and bis(4,4',6,6'-tetramethyl-2,2'-bipyridyl)copper(I/II) exhibited the highest power conversion efficiency of 5.07% ever reported for porphyrin-based DSSCs using copper(I/II) redox shuttles.
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    Alonso-Vante, N.; Nierengarten, J.-F.; Sauvage, J.-P. Spectral Sensitization of Large-Band-Gap Semiconductors (Thin Films and Ceramics) by a Carboxylated Bis(1,10-Phenanthroline)Copper(I) Complex. J. Chem. Soc., Dalton Trans. 1994, 97, 1649,  DOI: 10.1039/dt9940001649
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    Huang, J.; Buyukcakir, O.; Mara, M. W.; Coskun, A.; Dimitrijevic, N. M.; Barin, G.; Kokhan, O.; Stickrath, A. B.; Ruppert, R.; Tiede, D. M.; Stoddart, J. F.; Sauvage, J. P.; Chen, L. X. Highly Efficient Ultrafast Electron Injection from the Singlet MLCT Excited State of Copper(I) Diimine Complexes to TiO2 Nanoparticles. Angew. Chem., Int. Ed. 2012, 51, 12711– 12715,  DOI: 10.1002/anie.201204341
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    Highly Efficient Ultrafast Electron Injection from the Singlet MLCT Excited State of Copper(I) Diimine Complexes to TiO2 Nanoparticles
    Huang, Jier; Buyukcakir, Onur; Mara, Michael W.; Coskun, Ali; Dimitrijevic, Nada M.; Barin, Gokhan; Kokhan, Oleksandr; Stickrath, Andrew B.; Ruppert, Romain; Tiede, David M.; Stoddart, J. Fraser; Sauvage, Jean-Pierre; Chen, Lin X.
    Angewandte Chemie, International Edition (2012), 51 (51), 12711-12715CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Ultrafast electron injection from surface bound excited 1MLCT state of [Cu(dppS)2]+ (dppS = disodium 2,9-bis(sulfonatediphenyl)-1,10-phenanthroline) to TiO2 nanoparticle was studied. Dynamics of charge sepn. and recombination was correlated with the mol. structure of [Cu(dppS)2]+. The ESR, transient absorption and PR studies confirmed formation of [Cu(dppS)2]2+ , resulting from an efficient charge-transfer process in [Cu(dppS)2]+/TiO2.
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    Sandroni, M.; Pellegrin, Y.; Odobel, F. Heteroleptic Bis-Diimine Copper(I) Complexes for Applications in Solar Energy Conversion. C. R. Chim. 2016, 19, 79– 93,  DOI: 10.1016/j.crci.2015.06.008
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    Heteroleptic bis-diimine copper(I) complexes for applications in solar energy conversion
    Sandroni, Martina; Pellegrin, Yann; Odobel, Fabrice
    Comptes Rendus Chimie (2016), 19 (1-2), 79-93CODEN: CRCOCR; ISSN:1631-0748. (Elsevier Masson SAS)
    A review is presented. The development of mol. materials for conversion of solar energy into electricity and fuels is one of the most active research areas, in which the light absorber plays a key role. In this article, we present a class of photo-sensitizers constituted by heteroleptic bis-diimine copper(I) complexes, whose features rely on their tunable MLCT excited states and earth abundant and environmentally friendly nature of copper. These properties make this class of dyes compatible with a sustainable development. Their synthesis through the HETPHEN strategy and the background of their photophys. properties as well as the recent advances in the fields of both dye-sensitized solar cells and photoinduced charge sepn. are presented. The presented examples prove that bis-diimine copper(I) complexes are valuable dyes to be used to build multicomponent arrays for long range electron transfer and for dye-sensitized solar cells. These examples illustrate that one major advantage of heteroleptic complexes is the possibility to finely tune their opto-electronic properties to a larger extent than homoleptic complexes.
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  33. 33
    Bessho, T.; Constable, E. C.; Graetzel, M.; Hernandez Redondo, A.; Housecroft, C. E.; Kylberg, W.; Nazeeruddin, M. K.; Neuburger, M.; Schaffner, S. An Element of Surprise: Efficient Copper-Functionalized Dye-Sensitized Solar Cells. Chem. Commun. 2008, 3717,  DOI: 10.1039/b808491b
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    An element of surprise-efficient copper-functionalized dye-sensitized solar cells
    Bessho, Takeru; Constable, Edwin C.; Graetzel, Michael; Hernandez Redondo, Ana; Housecroft, Catherine E.; Kylberg, William; Nazeeruddin, Md. K.; Neuburger, Markus; Schaffner, Silvia
    Chemical Communications (Cambridge, United Kingdom) (2008), (32), 3717-3719CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    Dye-sensitized solar cells with carboxylate-derivatized {CuIL2} complexes are surprisingly efficient and offer a long-term alternative approach to ruthenium-functionalized systems.
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  34. 34
    Bozic-Weber, B.; Brauchli, S. Y.; Constable, E. C.; Fürer, S. O.; Housecroft, C. E.; Malzner, F. J.; Wright, I. A.; Zampese, J. A. Improving the Photoresponse of Copper(i) Dyes in Dye-Sensitized Solar Cells by Tuning Ancillary and Anchoring Ligand Modules. Dalton Trans. 2013, 42, 12293– 12308,  DOI: 10.1039/c3dt51416a
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    34
    Improving the photoresponse of copper(I) dyes in dye-sensitized solar cells by tuning ancillary and anchoring ligand modules
    Bozic-Weber, Biljana; Brauchli, Sven Y.; Constable, Edwin C.; Fuerer, Sebastian O.; Housecroft, Catherine E.; Malzner, Frederik J.; Wright, Iain A.; Zampese, Jennifer A.
    Dalton Transactions (2013), 42 (34), 12293-12308CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    The syntheses of five homoleptic copper(I) complexes [CuL2][PF6] are described in which L is a 4,4'-di(4-bromophenyl)-6,6'-dialkyl-2,2'-bipyridine ligand (compds. 1-4 with Me, nbutyl, isobutyl and hexyl substituents, resp.) or 4,4'-di(4-bromophenyl)-6,6'-diphenyl-2,2'-bipyridine (5). The new ligands 2-5 and copper(I) complexes [CuL2][PF6] (L = 1-5) have been fully characterized. The single crystal structures of 2{[Cu(1)2][PF6]}·3Me2CO, [Cu(2)2][PF6], 2{[Cu(3)2][PF6]}·Et2O and [Cu(5)2][PF6]·CH2Cl2 have been detd. The first three structures show similar distorted tetrahedral environments for the Cu+ ions with angles between the least squares planes of the bpy domains of 85.6, 86.4 and 82.9°, resp.; in contrast, the Cu+ ion in [Cu(5)2][PF6]·CH2Cl2 is in a flattened coordinate environment due to intra-cation face-to-face π-interactions. The soln. absorption spectra of the complexes with ligands 1-4 are virtually identical with an MLCT band with values of λmax = 481-488 nm. In contrast, the absorption spectrum of [Cu(5)2][PF6] shows two broad bands in the visible region. Cyclic voltammetric data show that oxidn. of the copper(i) center occurs at a more pos. potential in [Cu(2)2][PF6], [Cu(3)2][PF6] and [Cu(4)2][PF6] than in [Cu(1)2][PF6] or [Cu(5)2][PF6] with the latter being oxidized at the lowest potential. The complexes have been used to prep. dye-sensitized solar cells (DSCs) incorporating heteroleptic dyes of type [Cu(L)(Lanchor)]+ where L is 1-5 and Lanchor is a 6,6'-dimethyl-2,2'-bipyridine functionalized in the 4- and 4'-positions with phosphonic acid groups with (Lanchor = 7) and without (Lanchor = 6) a spacer between the metal-binding and anchoring domains. The presence of the spacer results in enhanced performances of the dyes, and the highest energy conversion efficiencies are obsd. for the dyes [Cu(3)(7)]+ (η = 2.43% compared to 5.96% for std. dye N719) and [Cu(5)(7)]+ (η = 2.89% compared to 5.96% for N719). Measurements taken periodically over the course of a week indicate that the cells undergo a ripening process (most clearly seen for [Cu(5)(6)]+ and [Cu(5)(7)]+) before their optimum performances are achieved. IPCE (EQE) data are presented and confirm that, although the photo-to-current conversions are promising (37-49% for λmax ≈ 480 nm), the copper(i) dyes do not realize the broad spectral response exhibited by N719.
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    Malzner, F. J.; Brauchli, S. Y.; Constable, E. C.; Housecroft, C. E.; Neuburger, M. Halos Show the Path to Perfection: Peripheral Iodo-Substituents Improve the Efficiencies of Bis(Diimine)Copper(i) Dyes in DSCs. RSC Adv. 2014, 4, 48712– 48723,  DOI: 10.1039/C4RA06823H
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    Halos show the path to perfection: peripheral iodo-substituents improve the efficiencies of bis(diimine)copper(I) dyes in DSCs
    Malzner, Frederik J.; Brauchli, Sven Y.; Constable, Edwin C.; Housecroft, Catherine E.; Neuburger, Markus
    RSC Advances (2014), 4 (89), 48712-48723CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    The homoleptic copper(I) complexes [CuL2][PF6] (L = 4,4'-bis(4-halophenyl)-6,6'-dimethyl-2,2'-bipyridine with halogen = F (2) and Cl (3)) have been prepd. and characterized, and their absorption spectroscopic and electrochem. properties compared to that with L = 4,4'-bis(4-bromophenyl)-6,6'-dimethyl-2,2'-bipyridine (4). The synthesis of [CuL2][PF6] (L = 4,4'-bis(4-iodophenyl)-6,6'-dimethyl-2,2'-bipyridine, 5) resulted in a mixt. of [Cu(5)2][PF6] and [Cu(5)(MeCN)2][PF6]; variable temp. 1H NMR spectroscopy confirmed that the complexes are in equil. in CD3CN soln. The structure of [Cu(5)(MeCN)2][PF6] was detd. by single crystal X-ray crystallog., and confirms a distorted tetrahedral geometry for the copper(I) center. The heteroleptic dyes [Cu(1)(2)]+, [Cu(1)(3)]+, [Cu(1)(4)]+ and [Cu(1)(5)]+ (1 = ((6,6'-dimethyl-[2,2'-bipyridine]-4,4'-diyl)bis(4,1-phenylene))bis(phosphonic acid)) have been assembled by ligand exchange between [CuL2]+ and TiO2 functionalized with the anchoring ligand 1, and the performances of the dyes in fully masked dye-sensitized solar cells (DSCs) have been measured and compared. On the day of DSC fabrication, the trend for the global efficiencies, η, depends on the halo-substituent in the order I > F ≈ Br > Cl. Ripening of the DSCs occurs and after 7 days, the dependence of η on the halo-atom is in the order I > Cl ≈ F ≈ Br; the highest η is 3.16% for [Cu(1)(5)]+ compared to 7.63% for N719. Compared to the other halo-functionalized dyes, [Cu(1)(5)]+ shows an extended spectral response to longer wavelength, with enhanced electron injection. The results of DFT calcns. suggest that the better dye performance of [Cu(1)(5)]+ may be assocd. with improved electron transfer over the halogen of the aryl substituent from the reduced electrolyte. The assembly of anchored dye [Cu(1)(5)]+ by treating functionalized-TiO2 with a 1 : 1 mixt. of [Cu(MeCN)4]+ and 5, yields a dye which gives a DSC performance that matches that made by ligand exchange using [Cu(5)2][PF6] and [Cu(5)(MeCN)2][PF6].
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    Brauchli, S. Y.; Malzner, F. J.; Constable, E. C.; Housecroft, C. E. Copper(i)-Based Dye-Sensitized Solar Cells with Sterically Demanding Anchoring Ligands: Bigger Is Not Always Better. RSC Adv. 2015, 5, 48516– 48525,  DOI: 10.1039/C5RA07449E
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    36
    Copper(I)-based dye-sensitized solar cells with sterically demanding anchoring ligands: bigger is not always better
    Brauchli, Sven Y.; Malzner, Frederik J.; Constable, Edwin C.; Housecroft, Catherine E.
    RSC Advances (2015), 5 (60), 48516-48525CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    The synthesis and characterization of ((6,6'-diphenyl-[2,2'-bipyridine]-4,4'-diyl)bis(4,1-phenylene))bis(phosphonic acid), 2, are described. Compd. 2 has been incorporated as an anchoring ligand in copper(I)-contg. dyes in n-type dye-sensitized solar cells (DSCs), combined with 2,2'-bipyridine (bpy), 6-methyl-2,2'-bipyridine (6-Mebpy), 6,6'-dimethyl-2,2'-bipyridine (6,6'-Me2bpy), 4,4'-di(4-bromophenyl)-6,6'-dimethyl-2,2'-bipyridine (3) or 4,4'-di(4-bromophenyl)-6,6'-diphenyl-2,2'-bipyridine (4) as ancillary ligands (Lancillary). Dyes were assembled on mesoporous TiO2 using an on-surface assembly strategy which relies on ligand exchange between surface-anchored Lanchor and [Cu(Lancillary)2]+; 1H NMR spectroscopy was used to confirm that the bulky Ph substituents did not hinder ligand exchange. Comparison of values of the open-circuit voltages (VOC), short-circuit current densities (JSC) and external quantum efficiency (EQE) spectra for DSCs with model dyes [Cu(2)(bpy)]+, [Cu(2)(6-Mebpy)]+ and [Cu(2)(6,6'-Me2bpy)]+ confirm that methyl-substituents in Lancillary are beneficial. Performance data for DSCs with dyes [Cu(1)(3)]+, [Cu(1)(4)]+, [Cu(2)(3)]+ and [Cu(2)(4)]+ where 1 is the anchor ((6,6'-dimethyl-[2,2'-bipyridine]-4,4'-diyl)bis(4,1-phenylene))bis(phosphonic acid) show that dyes with anchor 2 (Ph substituents in the 6- and 6'-positions) give relative conversion efficiencies ≤10% with respect to std. dye N719 set at η = 100%; this compares with relative efficiencies of up to 34.5% for the dyes [Cu(1)(3)]+ and [Cu(1)(4)]+. The performance of [Cu(2)(3)]+ can be improved by the addn. of the co-adsorbent chenodeoxycholic acid. Although the Ph (vs. methyl) substituents lead to enhanced light absorption to lower energies, dyes with anchor 2 quickly bleach when exposed to the I-/I3- electrolyte; bleaching also occurs after soaking in solns. of LiI. The dye can be regenerated by treatment of a bleached electrode with Lancillary, or with [Cu(NCMe)4]+ followed by Lancillary.
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    Dragonetti, C.; Magni, M.; Colombo, A.; Melchiorre, F.; Biagini, P.; Roberto, D. Coupling of a Copper Dye with a Copper Electrolyte: A Fascinating Springboard for Sustainable Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2018, 1, 751– 756,  DOI: 10.1021/acsaem.7b00196
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    Karpacheva, M.; Malzner, F. J.; Wobill, C.; Büttner, A.; Constable, E. C.; Housecroft, C. E. Cuprophilia: Dye-Sensitized Solar Cells with Copper(I) Dyes and Copper(I)/(II) Redox Shuttles. Dyes Pigm. 2018, 156, 410– 416,  DOI: 10.1016/j.dyepig.2018.04.033
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    Cuprophilia: Dye-sensitized solar cells with copper(I) dyes and copper(I)/(II) redox shuttles
    Karpacheva, Mariia; Malzner, Frederik J.; Wobill, Cedric; Buttner, Annika; Constable, Edwin C.; Housecroft, Catherine E.
    Dyes and Pigments (2018), 156 (), 410-416CODEN: DYPIDX; ISSN:0143-7208. (Elsevier Ltd.)
    With a goal of designing dye-sensitized solar cells (DSCs) contg. only Earth-abundant components to achieve sustainable energy conversion, DSCs with heteroleptic copper(I)-based dyes and homoleptic copper(I)/(II) redox shuttles were investigated. By using a phosphonic acid anchor, and 4,4'-dimethoxy-6,6'-dimethyl-2,2'-bipyridine as the ancillary ligand in the dye and in the electrolyte, a DSC photoconversion efficiency of 2.06% (38.1% relative to N719 set at 100%) was achieved. The results demonstrate the potential for all-copper-based DSCs, opening the way for further dye and electrolyte optimization.
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  39. 39
    Leandri, V.; Pizzichetti, A. R. P.; Xu, B.; Franchi, D.; Zhang, W.; Benesperi, I.; Freitag, M.; Sun, L.; Kloo, L.; Gardner, J. M. Exploring the Optical and Electrochemical Properties of Homoleptic versus Heteroleptic Diimine Copper(I) Complexes. Inorg. Chem. 2019, 58, 12167– 12177,  DOI: 10.1021/acs.inorgchem.9b01487
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    39
    Exploring the Optical and Electrochemical Properties of Homoleptic versus Heteroleptic Diimine Copper(I) Complexes
    Leandri, Valentina; Pizzichetti, Angela Raffaella Pia; Xu, Bo; Franchi, Daniele; Zhang, Wei; Benesperi, Iacopo; Freitag, Marina; Sun, Licheng; Kloo, Lars; Gardner, James M.
    Inorganic Chemistry (2019), 58 (18), 12167-12177CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Due to ligand scrambling, the synthesis and investigation of the properties of heteroleptic Cu(I) complexes can be a challenging task. In this work, we have studied the optical and electrochem. properties of a series of homoleptic complexes, such as [Cu(dbda)2]+, [Cu(dmp)2]+, [Cu(Br-dmp)2]+, [Cu(bcp)2]+, [Cu(dsbtmp)2]+, [Cu(biq)2]+, and [Cu(dap)2]+ in soln., and those of their heteroleptics [Cu(dbda)(dmp)]+, [Cu(dbda)(Br-dmp)]+, [Cu(dbda)(bcp)]+, [Cu(dbda)(dsbtmp)]+, [Cu(dbda)(biq)]+, [Cu(dbda)(dap)]+ adsorbed on the surface of anatase TiO2 (dbda = 6,6'-dimethyl-2,2'-bipyridine-4,4'-dibenzoic acid; dmp = 2,9-dimethyl-1,10-phenanthroline; Br-dmp = 5-bromo-2,9-dimethyl-1,10-phenanthroline; bcp = bathocuproine or 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline; dsbtmp = 2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline; biq = 2,2'-biquinoline; dap = 2,9-dianisyl-1,10-phenanthroline). We show that the max. absorption wavelengths of the heteroleptic complexes on TiO2 can be reasonably predicted from those of the homoleptic complexes in soln. through a simple linear relation, whereas the prediction of their redox properties is less trivial. In the latter case, two different linear patterns emerge: one including the ligands bcp, biq, and dap and another one including the ligands dmp, Br-dmp, and dsbtmp. We offer an interpretation of the data based on the chem. structure of the ligands. On one hand, ligands bcp, biq, and dap possess a more extended π-conjugated system, which gives a more prominent contribution to the overall redox properties of the ligand dbda. On the other hand, the ligands dmp, Br-dmp, and dsbtmp are all phenanthroline-based contg. alkyl substituents and contribute less than dbda to the overall redox properties.
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  40. 40
    Kaeser, A.; Delavaux-Nicot, B.; Duhayon, C.; Coppel, Y.; Nierengarten, J.-F. Heteroleptic Silver(I) Complexes Prepared from Phenanthroline and Bis-Phosphine Ligands. Inorg. Chem. 2013, 52, 14343– 14354,  DOI: 10.1021/ic402342y
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    Heteroleptic Silver(I) Complexes Prepared from Phenanthroline and Bis-phosphine Ligands
    Kaeser, Adrien; Delavaux-Nicot, Beatrice; Duhayon, Carine; Coppel, Yannick; Nierengarten, Jean-Francois
    Inorganic Chemistry (2013), 52 (24), 14343-14354CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    The heteroleptic coordination scenario of silver(I) with various phenanthroline ligands (NN) and different bis-phosphine (PP) derivs. was investigated. In addn. to the x-ray crystal structural characterization of the resulting mixed ligand Ag(I) complexes, detailed NMR studies were performed to disclose the behavior of the prepd. silver(I) complexes in soln. The results obtained with silver(I) were also systematically related to the one obtained for copper(I) with the same combination of PP and NN ligands. Starting from an equimolar mixt. of AgBF4, bis[(2-diphenylphosphino)phenyl] ether (POP), and 1,10-phenanthroline (phen), the mononuclear complex [Ag(POP)(phen)]+ was obtained as the tetrafluoroborate salt. By following the same exptl. procedure starting from bis(diphenylphosphino)methane (dppm) or 1,3-bis(diphenylphosphino)propane (dppp) as the PP ligand, dinuclear complexes with two bridging PP ligands, i.e., [Ag2(NN)2(μ-dppm)2]2+ and [Ag2(NN)2(μ-dppp)2]2+ with NN = phen or Bphen (bathophenanthroline), were isolated as the tetrafluoroborate salts. Surprisingly, by using an equimolar ratio of AgBF4, phen or Bphen, and 1,2-bis(diphenylphosphino)ethane (dppe), the corresponding monobridged diphosphine dinuclear complexes [Ag2(NN)2(μ-dppe)]2+ were obtained as the tetrafluoroborate salts. These compds. were also prepd. in excellent yield by using a more appropriate 2:1:2 (phen:dppe:Ag) stoichiometry. These results prompted the authors to also perform the reactions with dppm and dppp using a 1:2:2 (PP:NN:Ag) stoichiometry. Under these conditions, [Ag2(NN)2(μ-dppm)](BF4)2 (NN = phen or Bphen) and [Ag2(NN)2(μ-dppp)](BF4)2 (NN = phen or Bphen) were obtained upon crystn. When compared to their copper(I) analogs, the complexation scenario becomes more complex with silver(I) as the system tolerates also coordinatively frustrated metal ligand assemblies, i.e., with a trigonal coordination geometry. Depending on the stoichiometry or on the nature of the PP partner, silver(I) shows an adaptive capability leading to various complexes with different coordination geometries and compn. However, as in the case of copper(I), their soln. behavior is highly dependent on the relative thermodn. stability of the various possible complexes. In most of the cases, a single Ag(I) complex is obsd. in soln. and the NMR data are in a perfect agreement with their solid state structures. The dppp-contg. complexes are the only notable exception; both [Ag2(NN)2(μ-dppp)2](BF4)2 and [Ag2(NN)2(μ-dppp)](BF4)2 are stable in the solid state but a dynamic mixt. is obsd. as soon as these compds. are dissolved. Finally, whereas both dppe and dppp are chelating ligands for copper(I), it is not the case anymore with silver(I) for which a destabilization of species with chelating dppe and dppp ligands is clearly indicated.
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    Scaltrito, D. V.; Thompson, D. W.; O’Callaghan, J. A.; Meyer, G. J. MLCT Excited States of Cuprous Bis-Phenanthroline Coordination Compounds. Coord. Chem. Rev. 2000, 208, 243– 266,  DOI: 10.1016/S0010-8545(00)00309-X
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    MLCT excited states of cuprous bis-phenanthroline coordination compounds
    Scaltrito, Donald V.; Thompson, David W.; O'Callaghan, John A.; Meyer, Gerald J.
    Coordination Chemistry Reviews (2000), 208 (), 243-266CODEN: CCHRAM; ISSN:0010-8545. (Elsevier Science S.A.)
    A review with 79 refs. in which excited MLCT states of cuprous bis-phenanthroline compds. are discussed and contrasted with well-known MLCT behavior of (dπ)6 transition metal compds. Phenanthroline ligands coordinated to Cu(I) that are disubstituted in the 2- and 9-positions with alkyl or aryl groups, abbreviated Cu1(phen')2+, have long-lived excited states at room temp. The parent Cu1(phen)2+ compd. is non-emissive under the same conditions with a short excited state lifetime, τ < 10 ns. Disubstitution in the 2,9-positions stabilizes the Cu(I) state and increases the energy gap between the MLCT and the ground state. The prototypical and most well studied compd. is Cu1(dmp)2+, where dmp is 2,9-(CH3)2-1,10-phenanthroline. In dichloromethane soln. at room temp., Cu1(dmp)2+ displays broad MLCT absorption with λmax = 454 nm, a broad unstructured emission with λmax = 730 nm, and an excited state lifetime of 85 ns. The emission arises from two closely spaced MLCT excited states, sepd. in energy by 1800 cm-1, that behave as one state at room temp. Cu1(dmp)2+* excited states are quenched in the presence of Lewis bases and coordinating solvents. A 5-coordinate excited state complex, or exciplex, is proposed to account for temp. dependent quenching data. The substantial inner-sphere reorganizational energy changes that follow light excitation are novel features of these MLCT excited states.
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    Leandri, V.; Daniel, Q.; Chen, H.; Sun, L.; Gardner, J. M.; Kloo, L. Electronic and Structural Effects of Inner Sphere Coordination of Chloride to a Homoleptic Copper(II) Diimine Complex. Inorg. Chem. 2018, 57, 4556– 4562,  DOI: 10.1021/acs.inorgchem.8b00225
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    Electronic and Structural Effects of Inner Sphere Coordination of Chloride to a Homoleptic Copper(II) Diimine Complex
    Leandri, Valentina; Daniel, Quentin; Chen, Hong; Sun, Licheng; Gardner, James M.; Kloo, Lars
    Inorganic Chemistry (2018), 57 (8), 4556-4562CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    The reaction of CuCl2 with 2,9-dimethyl-1,10-phenanthroline (dmp) does not give [Cu(dmp)2](Cl)2 but instead to [Cu(dmp)2Cl]Cl, a 5-coordinated complex, in which one chloride is directly coordinated to the metal center. Attempts at removing the coordinated chloride by changing the counterion by metathesis were unsuccessful and resulted only in the exchange of the non-coordinated chloride, as confirmed from a crystal structure anal. Complex [Cu(dmp)2Cl]PF6 exhibits a reversible cyclic voltammogram characterized by a significant peak splitting between the reductive and oxidative waves (0.85 and 0.60 V vs. NHE, resp.), with a half-wave potential E1/2 = 0.73 V vs. NHE. When reduced electrochem., the complex does not convert into [Cu(dmp)2]+, as one may expect. Instead, [Cu(dmp)2]+ was isolated as a product when the redn. of [Cu(dmp)2Cl]PF6 was performed with L-ascorbic acid, as confirmed by electrochem., NMR spectroscopy, and diffractometry. [Cu(dmp)2]2+ complexes can be synthesized starting from Cu(II) salts with weakly and non-coordinating counterions, such as perchlorate. Growth of [Cu(dmp)2](ClO4)2 crystals in acetonitrile results in a 5-coordinated complex, [Cu(dmp)2(MeCN)](ClO4)2, in which a solvent mol. is coordinated to the metal center. However, solvent coordination is assocd. with a dynamic de-coordination-coordination behavior upon redn. and oxidn. Hence, the cyclic voltammogram of [Cu(dmp)2(MeCN)]2+ is identical to the one of [Cu(dmp)2]+, if the measurements were performed in acetonitrile. Halide ions in precursors to Cu(II) metal-org. coordination compd. synthesis, and most likely also other multivalent coordination centers, are not readily exchanged when exposed to presumed strongly binding and chelating ligand, and thus special care needs to be taken with respect to product characterization.
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    Schmittel, M.; Ganz, A. Stable Mixed Phenanthroline Copper(i) Complexes. Key Building Blocks for Supramolecular Coordination Chemistry. Chem. Commun. 1997, 999– 1000,  DOI: 10.1039/a701509g
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    New building blocks for sensors and supramolecular arrays. 4. Stable mixed phenanthroline copper(I) complexes. Key building blocks for supramolecular coordination chemistry
    Schmittel, Michael; Ganz, Andrea
    Chemical Communications (Cambridge) (1997), (11), 999-1000CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    A simple procedure is presented that was used to synthesize, for the first time, pure mixed phenanthroline copper(I) complexes that do not exchange ligands. One of the phenanthroline ligands is 2,9-diaryl deriv. and the 2nd is a 4,7-disubstituted deriv., the latter of which may also be linked by a polyether linkage to give macrocyclic ligands. Half-wave potentials are reported for several of the complexes.
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    Miller, M. T.; Gantzel, P. K.; Karpishin, T. B. A Highly Emissive Heteroleptic Copper(I) Bis(Phenanthroline) Complex: [Cu(Dbp)(Dmp)]+ (Dbp = 2,9-Di- Tert -Butyl-1,10-Phenanthroline; Dmp = 2,9-Dimethyl-1,10-Phenanthroline). J. Am. Chem. Soc. 1999, 121, 4292– 4293,  DOI: 10.1021/ja9901415
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    A Highly Emissive Heteroleptic Copper(I) Bis(phenanthroline) Complex: [Cu(dbp)(dmp)]+ (dbp = 2,9-Di-tert-butyl-1,10-phenanthroline; dmp = 2,9-Dimethyl-1,10-phenanthroline)
    Miller, Mark T.; Gantzel, Peter K.; Karpishin, Timothy B.
    Journal of the American Chemical Society (1999), 121 (17), 4292-4293CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Highly emissive [Cu(dbp)(dmp)](PF6) (1; dbp = 2,9-di-tert-butyl-1,10-phenanthroline; dmp = 2,9-dimethyl-1,10-phenanthroline) was prepd. and its excited state lifetime and emission spectrum recorded. The quantum yield of 1 is 1.19 ± 0.25 times higher than that of [Ru(bpy)3](PF6)2.
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    Sandroni, M.; Kayanuma, M.; Planchat, A.; Szuwarski, N.; Blart, E.; Pellegrin, Y.; Daniel, C.; Boujtita, M.; Odobel, F. First Application of the HETPHEN Concept to New Heteroleptic Bis(Diimine) Copper(i) Complexes as Sensitizers in Dye Sensitized Solar Cells. Dalton Trans. 2013, 42, 10818– 10827,  DOI: 10.1039/c3dt50852h
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    First application of the HETPHEN concept to new heteroleptic bis(diimine) copper(I) complexes as sensitizers in dye sensitized solar cells
    Sandroni, Martina; Kayanuma, Megumi; Planchat, Aurelien; Szuwarski, Nadine; Blart, Errol; Pellegrin, Yann; Daniel, Chantal; Boujtita, Mohammed; Odobel, Fabrice
    Dalton Transactions (2013), 42 (30), 10818-10827CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    We report here the synthesis and full chem. and phys. characterizations of the first stable heteroleptic copper(I)-bis(diimine) complexes designed for implementation in dye sensitized solar cells (DSC). Thanks to the HETPHEN concept, pure and stable heteroleptic copper(i) complexes were isolated. Anchorage of the sensitizers was provided by 2,2'-biquinoline-4,4'-dicarboxylic acid (dcbqH2), while sterically challenged ligands 2,9-dimesityl-1,10-phenanthroline (L0) and N-hexyl-2,9-dimesityl-1,10-phenanthroline-[a:b]imidazo-(4'-dianisylaminophenyl) (L1) were used to complete the copper(I) coordination sphere. The resulting heteroleptic complexes C1 and C2 exhibit a broad MLCT transition spreading over a wide wavelength domain, esp. when adsorbed onto nanoparticulate TiO2 photoanodes, providing a rather comprehensive visible light collection. The corresponding DSC were evaluated under AM 1.5 simulated solar light and rather weak performances were obtained owing to small Jsc and Voc. This is due to a combination of low extinction coeff. and poor driving forces for the various interfacial processes. However, significant improvements in the performances were monitored upon ageing in the dark, likely due to beneficial reorganization of the dye monolayers. The possibility to isolate stable asym. systems paves the way for structurally assisted photo-induced charge injection from the chemisorbed copper(I) based sensitizers into the conduction band of TiO2, through charge vectorialization.
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    Sandroni, M.; Favereau, L.; Planchat, A.; Akdas-Kilig, H.; Szuwarski, N.; Pellegrin, Y.; Blart, E.; Le Bozec, H.; Boujtita, M.; Odobel, F. Heteroleptic Copper(i)-Polypyridine Complexes as Efficient Sensitizers for Dye Sensitized Solar Cells. J. Mater. Chem. A 2014, 2, 9944– 9947,  DOI: 10.1039/c4ta01755b
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    Heteroleptic copper(I)-polypyridine complexes as efficient sensitizers for dye sensitized solar cells
    Sandroni, Martina; Favereau, Ludovic; Planchat, Aurelien; Akdas-Kilig, Huriye; Szuwarski, Nadine; Pellegrin, Yann; Blart, Errol; Le Bozec, Hubert; Boujtita, Mohammed; Odobel, Fabrice
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2014), 2 (26), 9944-9947CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    The synthesis and the physico-chem. characterization of HETPHEN based heteroleptic copper(I)-bis(diimine) complexes are reported. For TiO2 based dye sensitized solar cells (DSCs), the latter display impressive photoconversion efficiencies (PCEs), unprecedented for first row transition metal coordination complexes.
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    Kabehie, S.; Stieg, A. Z.; Xue, M.; Liong, M.; Wang, K. L.; Zink, J. I. Surface Immobilized Heteroleptic Copper Compounds as State Variables That Show Negative Differential Resistance. J. Phys. Chem. Lett. 2010, 1, 589– 593,  DOI: 10.1021/jz900324f
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    Surface Immobilized Heteroleptic Copper Compounds as State Variables that Show Negative Differential Resistance
    Kabehie, Sanaz; Stieg, Adam Z.; Xue, Mei; Liong, Monty; Wang, Kang L.; Zink, Jeffrey I.
    Journal of Physical Chemistry Letters (2010), 1 (3), 589-593CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Surface immobilized bidentate heteroleptic Cu(I) compds. are synthesized using a surface outward sequential synthesis and are characterized using solid-state NMR and at. force microscopy (AFM). Through use of chem. redox agents, the reversible switching characteristics of SiO2-immobilized Cu(I) compds. (tetrahedral) to Cu(II) (square planar) are verified via UV-visible absorption spectroscopy and ESR. Elec. properties of this system are characterized via prepn. of a sandwich-type device using p+ silicon and conductive AFM (cAFM). Current-Voltage (I-V) spectroscopy demonstrates that this system reproducibly switches between Cu(I) and Cu(II) states at approx. -0.8 and 2.3 V.
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    Housecroft, C. E.; Constable, E. C. The Emergence of Copper(I)-Based Dye Sensitized Solar Cells. Chem. Soc. Rev. 2015, 44, 8386– 8398,  DOI: 10.1039/c5cs00215j
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    The emergence of copper(I)-based dye sensitized solar cells
    Housecroft, Catherine E.; Constable, Edwin C.
    Chemical Society Reviews (2015), 44 (23), 8386-8398CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    Since the discovery of Gratzel-type dye sensitized solar cells (DSCs) in the early 1990s, there has been an exponential growth in the no. of publications dealing with their optimization and new design concepts. Conventional Gratzel DSCs use ruthenium(II) complexes as sensitizers, and the highest photon-to-elec. current conversion efficiency for a ruthenium dye is ≈12%. However, ruthenium is both rare and expensive, and replacement by cheaper and more sustainable metals is desirable. In this Tutorial Review, we describe strategies for assembling copper(I) complexes for use as dyes in DSCs, a research area that has been active since ≈2008. We demonstrate design principles for (i) ligands to anchor the complex to a semiconductor surface and promote electron transfer from dye to semiconductor, and (ii) ancillary ligands to tune the light absorption properties of the dye and facilitate electron transfer from electrolyte to dye in the DSC. We assess the progress made in terms of light-harvesting and overall photoconversion efficiencies of copper(I)-contg. DSCs and highlight areas that remain ripe for development and improvement.
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    Yanai, T.; Tew, D. P.; Handy, N. C. A New Hybrid Exchange-Correlation Functional Using the Coulomb-Attenuating Method (CAM-B3LYP). Chem. Phys. Lett 2004, 393, 51– 57,  DOI: 10.1016/j.cplett.2004.06.011
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    A new hybrid exchange-correlation functional using the Coulomb-attenuating method (CAM-B3LYP)
    Yanai, Takeshi; Tew, David P.; Handy, Nicholas C.
    Chemical Physics Letters (2004), 393 (1-3), 51-57CODEN: CHPLBC; ISSN:0009-2614. (Elsevier Science B.V.)
    A new hybrid exchange-correlation functional named CAM-B3LYP is proposed. It combines the hybrid qualities of B3LYP and the long-range correction presented by Tawada et al. [J. Chem. Phys., in press]. We demonstrate that CAM-B3LYP yields atomization energies of similar quality to those from B3LYP, while also performing well for charge transfer excitations in a dipeptide model, which B3LYP underestimates enormously. The CAM-B3LYP functional comprises of 0.19 Hartree-Fock (HF) plus 0.81 Becke 1988 (B88) exchange interaction at short-range, and 0.65 HF plus 0.35 B88 at long-range. The intermediate region is smoothly described through the std. error function with parameter 0.33.
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    Figgen, D.; Rauhut, G.; Dolg, M.; Stoll, H. Energy-Consistent Pseudopotentials for Group 11 and 12 Atoms: Adjustment to Multi-Configuration Dirac–Hartree–Fock Data. Chem. Phys. 2005, 311, 227– 244,  DOI: 10.1016/j.chemphys.2004.10.005
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    Energy-consistent pseudopotentials for group 11 and 12 atoms: adjustment to multi-configuration Dirac-Hartree-Fock data
    Figgen, Detlev; Rauhut, Guntram; Dolg, Michael; Stoll, Hermann
    Chemical Physics (2005), 311 (1-2), 227-244CODEN: CMPHC2; ISSN:0301-0104. (Elsevier B.V.)
    Two-component relativistic pseudopotentials (i.e., scalar-relativistic and spin-orbit (SO) potentials) of the energy-consistent variety have been adjusted for the group 11 and 12 atoms Cu, Zn; Ag, Cd; Au, Hg, replacing the 1s-2p; 1s-3d; and 1s-4f cores, resp. The adjustment has been done for the valence-energy spectrum of (near-)neutral atoms, to ref. data from numerical all-electron four-component multi-configuration Dirac-Hartree-Fock (MCDHF) calcns., including the two-electron Breit interaction. For use in mol. calcns., the potentials have been supplemented by energy-optimized (12s12p9d3f2g)/[6s6p4d3f2g] valence basis sets. First benchmark applications of the potentials and basis sets are presented for at. excitation energies and SO splittings at a correlated level, and for ground and excited state spectroscopic properties of group 11 monohalides and group 12 dimers.
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    Peterson, K. A. Systematically Convergent Basis Sets with Relativistic Pseudopotentials. I. Correlation Consistent Basis Sets for the Post- d Group 13–15 Elements. J. Chem. Phys. 2003, 119, 11099– 11112,  DOI: 10.1063/1.1622923
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    Systematically convergent basis sets with relativistic pseudopotentials. I. Correlation consistent basis sets for the post-d group 13-15 elements
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    Journal of Chemical Physics (2003), 119 (21), 11099-11112CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    New correlation consistent-like basis sets have been developed for the post-d group 13-15 elements (Ga-As, In-Sb, Tl-Bi) employing accurate, small-core relativistic pseudopotentials. The resulting basis sets, which are denoted cc-pVnZ-PP, are appropriate for valence electron correlation and range in size from (8s7p7d)/[4s3p2d] for the cc-pVDZ-PP to (16s13p12d3f2g1h)/[7s7p5d3f2g1h] for the cc-pV5Z-PP sets. Benchmark calcns. on selected diat. mols. (As2, Sb2, Bi2, AsN, SbN, BiN, GeO, SnO, PbO, GaCl, InCl, TlCl, GaH, InH, and TlH) are reported using these new basis sets at the coupled cluster level of theory. Much like their all-electron counterparts, the cc-pVnZ-PP basis sets yield systematic convergence of total energies and spectroscopic consts. In several cases all-electron benchmark calcns. were also carried out for comparison. The results from the pseudopotential and all-electron calcns. were nearly identical when scalar relativity was accurately included in the all-electron work. Diffuse-augmented basis sets, aug-cc-pVnZ-PP, have also been developed and have been used in calcns. of the at. electron affinities.
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    Schäfers, F. The Crystal Monochromator Beamline KMC-1 at BESSY II. J. Large-Scale Res. Facil. JLSRF 2016, 2, A96  DOI: 10.17815/jlsrf-2-92
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    Johansson, E. M. J.; Hedlund, M.; Siegbahn, H.; Rensmo, H. Electronic and Molecular Surface Structure of Ru(Tcterpy)(NCS)3 and Ru(Dcbpy)2(NCS)2 Adsorbed from Solution onto Nanostructured TiO2: A Photoelectron Spectroscopy Study. J. Phys. Chem. B 2005, 109, 22256– 22263,  DOI: 10.1021/jp0525282
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    Electronic and Molecular Surface Structure of Ru(tcterpy)(NCS)3 and Ru(dcbpy)2(NCS)2 Adsorbed from Solution onto Nanostructured TiO2: A Photoelectron Spectroscopy Study
    Johansson, E. M. J.; Hedlund, M.; Siegbahn, H.; Rensmo, H.
    Journal of Physical Chemistry B (2005), 109 (47), 22256-22263CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    The element specificity of photoelectron spectroscopy (PES) has been used to compare the electronic and mol. structure of the dyes Ru(tcterpy)(NCS)3 (BD) and Ru(dcbpy)2(NCS)2 adsorbed from soln. onto nanostructured TiO2. Ru(dcbpy)2(NCS)2 was investigated in its acid (N3) and in its 2-fold deprotonated form (N719) having tetrabutylammonium (TBA+) as counterions. A comparison of the O1s spectra for the dyes indicates that the interactions through the carboxylate groups with the TiO2 surface are very similar for the dyes. However, we observe that some of the dye mols. also interact through the NCS groups when adsorbed at the TiO2 surface. Comparing the N719 and the N3 mol., the fraction of NCS groups interacting through the sulfur atoms is smaller for N719 than for N3. We also note that the counterion TBA+ is coadsorbed with the N719 and BD mols. although the amt. was smaller than expected from the mol. formulas. Comparing the valence levels for the dyes adsorbed on TiO2, the position of the highest occupied electronic energy level is similar for N3 and N719, while that for BD is lower by 0.25 eV relative to that of the other complexes.
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    Ida, T.; Ando, M.; Toraya, H. Extended Pseudo-Voigt Function for Approximating the Voigt Profile. J. Appl. Crystallogr. 2000, 33, 1311– 1316,  DOI: 10.1107/S0021889800010219
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    Extended pseudo-Voigt function for approximating the Voigt profile
    Ida, T.; Ando, M.; Toraya, H.
    Journal of Applied Crystallography (2000), 33 (6), 1311-1316CODEN: JACGAR; ISSN:0021-8898. (Munksgaard International Publishers Ltd.)
    The formula of the pseudo-Voigt function expressed by a weighted sum of Gaussian and Lorentzian functions is extended by adding 2 other types of peak functions to improve the accuracy when approximating the Voigt profile. The full width at half-max. (FWHM) values and mixing parameters of the Gaussian, the Lorentzian and the other 2 component functions in the extended formula can be approximated by polynomials of a parameter ρ = ΓL/(ΓG +ΓL), where ΓG and ΓL are the FWHM values of the deconvoluted Gaussian and Lorentzian functions, resp. The max. deviation of the extended pseudo-Voigt function from the Voigt profile is within 0.12% relative to the peak height when 6th-order polynomial expansions are used. The systematic errors of the integrated intensity ΓG and ΓL, estd. by fitting the extended formula to Voigt profiles, are typically less than 1/10 of the errors arising from the application of the original formula of the pseudo-Voigt approxn. proposed by Thompson et al. (1987), while the time required for computation of the extended formula is only about 2.5 relative to the computation time required for the original formula.
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    Boschloo, G.; Hagfeldt, A. Characteristics of the Iodide/Triiodide Redox Mediator in Dye-Sensitized Solar Cells. Acc. Chem. Res. 2009, 42, 1819– 1826,  DOI: 10.1021/ar900138m
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    Characteristics of the Iodide/Triiodide Redox Mediator in Dye-Sensitized Solar Cells
    Boschloo, Gerrit; Hagfeldt, Anders
    Accounts of Chemical Research (2009), 42 (11), 1819-1826CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    Dye-sensitized solar cells (DSCs) are of interest because of their potential for low-cost solar energy conversion. Currently, the certified record efficiency of these solar cells is 11.1%, and measurements of their durability and stability suggest lifetimes exceeding 10 years under operational conditions. The DSC is a photoelectrochem. system: a monolayer of sensitizing dye is adsorbed onto a mesoporous TiO2 electrode, and the electrode is sandwiched together with a counter electrode. An electrolyte contg. a redox couple fills the gap between the electrodes. The redox couple is a key component of the DSC. The reduced part of the couple regenerates the photooxidized dye. The formed oxidized species diffuses to the counter electrode, where it is reduced. The photovoltage of the device depends on the redox couple because it sets the electrochem. potential at the counter electrode. The redox couple also affects the electrochem. potential of the TiO2 electrode through the recombination kinetics between electrons in TiO2 and oxidized redox species. The special properties of the I-/I3- redox couple in dye-sensitized solar cells were studied. It was the preferred redox couple since the beginning of DSC development and still yields the most stable and efficient DSCs. Overall, the iodide/triiodide couple has good soly., does not absorb too much light, has a suitable redox potential, and provides rapid dye regeneration. But what distinguishes I-/I3- from most redox mediators is the slow recombination kinetics between electrons in TiO2 and the oxidized part of the redox couple, triiodide. Certain dyes adsorbed at TiO2 catalyze this recombination reaction, presumably by binding I or triiodide. The std. potential of the iodide/triiodide redox couple is 0.35 V (vs. the normal H electrode, normal H electrode), and the oxidn. potential of the std. DSC-sensitizer (Ru(dcbpy)2(NCS)2) is 1.1 V. The driving force for redn. of oxidized dye is therefore as large as 0.75 V. This process leads to the largest internal potential loss in DSC devices. Overall efficiencies >15% might be achieved if half of this internal potential loss could be gained. The regeneration of oxidized dye with iodide gives the diiodide radical, I2-·. The redox potential of the I2-·/I- couple must therefore be considered when detg. the actual driving force for dye regeneration. The formed I2-· disproportionates to I3- and I-, which leads to a large loss in potential energy.
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    Franke, R.; Chassé, T.; Streubel, P.; Meisel, A. Auger Parameters and Relaxation Energies of Phosphorus in Solid Compounds. J. Electron Spectrosc. Relat. Phenom. 1991, 56, 381– 388,  DOI: 10.1016/0368-2048(91)85035-R
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    Auger parameters and relaxation energies of phosphorus in solid compounds
    Franke, R.; Chasse, T.; Streubel, P.; Meisel, A.
    Journal of Electron Spectroscopy and Related Phenomena (1991), 56 (4), 381-8CODEN: JESRAW; ISSN:0368-2048.
    The 1s, 2s, and 2p photoelectron binding energies and the KL2.3L2.3(1D2) Auger electron energies of P in 28 solid compds. were measured using a mixed x-ray source (Al-Ag). The modified Auger parameters, α, are presented in 2-dimensional chem. state plots. Extra-at. static relaxation energies were calcd. using Auger parameters ζ.
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    Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy: A Reference Book of Standard Spectra for Identification and Interpretation of XPS Data; Physical Electronics, Inc., 1992.
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    Colombo, A.; Dragonetti, C.; Roberto, D.; Valore, A.; Biagini, P.; Melchiorre, F. A Simple Copper(I) Complex and Its Application in Efficient Dye Sensitized Solar Cells. Inorg. Chim. Acta 2013, 407, 204– 209,  DOI: 10.1016/j.ica.2013.07.028
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    A simple copper(I) complex and its application in efficient dye sensitized solar cells
    Colombo, Alessia; Dragonetti, Claudia; Roberto, Dominique; Valore, Adriana; Biagini, Paolo; Melchiorre, Fabio
    Inorganica Chimica Acta (2013), 407 (), 204-209CODEN: ICHAA3; ISSN:0020-1693. (Elsevier B.V.)
    A novel copper(I) complex bearing two 6,6'-dimethyl-2,2'-bipyridine-4,4'-dibenzoic acid ligands was prepd. along with related deprotonated derivs. with Na+ or NBu4+ as counterion. Their performance as photosensitizer in dye sensitized solar cells was studied as a function of the anchoring group and the electrolyte compn., comparing it with that of the known copper(I) complex bearing two 6,6'-dimethyl-2,2'-bipyridine-4,4'-dicarboxylic acid ligands and the ruthenium benchmark N719. Interestingly, the novel protonated complex is characterized by the best overall power conversion efficiency (3.0%) reported up to now for a copper(I) complex.
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    Kaeser, A.; Delavaux-Nicot, B.; Duhayon, C.; Coppel, Y.; Nierengarten, J. F. Heteroleptic Silver(I) Complexes Prepared from Phenanthroline and Bis-Phosphine Ligands. Inorg. Chem. 2013, 52, 14343– 14354,  DOI: 10.1021/ic402342y
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    Heteroleptic Silver(I) Complexes Prepared from Phenanthroline and Bis-phosphine Ligands
    Kaeser, Adrien; Delavaux-Nicot, Beatrice; Duhayon, Carine; Coppel, Yannick; Nierengarten, Jean-Francois
    Inorganic Chemistry (2013), 52 (24), 14343-14354CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    The heteroleptic coordination scenario of silver(I) with various phenanthroline ligands (NN) and different bis-phosphine (PP) derivs. was investigated. In addn. to the x-ray crystal structural characterization of the resulting mixed ligand Ag(I) complexes, detailed NMR studies were performed to disclose the behavior of the prepd. silver(I) complexes in soln. The results obtained with silver(I) were also systematically related to the one obtained for copper(I) with the same combination of PP and NN ligands. Starting from an equimolar mixt. of AgBF4, bis[(2-diphenylphosphino)phenyl] ether (POP), and 1,10-phenanthroline (phen), the mononuclear complex [Ag(POP)(phen)]+ was obtained as the tetrafluoroborate salt. By following the same exptl. procedure starting from bis(diphenylphosphino)methane (dppm) or 1,3-bis(diphenylphosphino)propane (dppp) as the PP ligand, dinuclear complexes with two bridging PP ligands, i.e., [Ag2(NN)2(μ-dppm)2]2+ and [Ag2(NN)2(μ-dppp)2]2+ with NN = phen or Bphen (bathophenanthroline), were isolated as the tetrafluoroborate salts. Surprisingly, by using an equimolar ratio of AgBF4, phen or Bphen, and 1,2-bis(diphenylphosphino)ethane (dppe), the corresponding monobridged diphosphine dinuclear complexes [Ag2(NN)2(μ-dppe)]2+ were obtained as the tetrafluoroborate salts. These compds. were also prepd. in excellent yield by using a more appropriate 2:1:2 (phen:dppe:Ag) stoichiometry. These results prompted the authors to also perform the reactions with dppm and dppp using a 1:2:2 (PP:NN:Ag) stoichiometry. Under these conditions, [Ag2(NN)2(μ-dppm)](BF4)2 (NN = phen or Bphen) and [Ag2(NN)2(μ-dppp)](BF4)2 (NN = phen or Bphen) were obtained upon crystn. When compared to their copper(I) analogs, the complexation scenario becomes more complex with silver(I) as the system tolerates also coordinatively frustrated metal ligand assemblies, i.e., with a trigonal coordination geometry. Depending on the stoichiometry or on the nature of the PP partner, silver(I) shows an adaptive capability leading to various complexes with different coordination geometries and compn. However, as in the case of copper(I), their soln. behavior is highly dependent on the relative thermodn. stability of the various possible complexes. In most of the cases, a single Ag(I) complex is obsd. in soln. and the NMR data are in a perfect agreement with their solid state structures. The dppp-contg. complexes are the only notable exception; both [Ag2(NN)2(μ-dppp)2](BF4)2 and [Ag2(NN)2(μ-dppp)](BF4)2 are stable in the solid state but a dynamic mixt. is obsd. as soon as these compds. are dissolved. Finally, whereas both dppe and dppp are chelating ligands for copper(I), it is not the case anymore with silver(I) for which a destabilization of species with chelating dppe and dppp ligands is clearly indicated.
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    De Angelis, F.; Fantacci, S.; Mosconi, E.; Nazeeruddin, M. K.; Grätzel, M. Absorption Spectra and Excited State Energy Levels of the N719 Dye on TiO2 in Dye-Sensitized Solar Cell Models. J. Phys. Chem. C 2011, 115, 8825– 8831,  DOI: 10.1021/jp111949a
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    Absorption Spectra and Excited State Energy Levels of the N719 Dye on TiO2 in Dye-Sensitized Solar Cell Models
    De Angelis, Filippo; Fantacci, Simona; Mosconi, Edoardo; Nazeeruddin, Mohammad K.; Gratzel, Michael
    Journal of Physical Chemistry C (2011), 115 (17), 8825-8831CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    We have investigated the absorption spectrum and the alignment of ground and excited state energies for the prototypical N719 Ru(II) sensitizer adsorbed on an extended TiO2 model by means of high level DFT/TDDFT calcns. The calcd. and exptl. absorption spectra for the dye on TiO2 are in excellent agreement over the explored energy range, with an absorption max. deviation below 0.1 eV, allowing us to assign the underlying electronic transitions. We find the lowest optically active excited state to lie ∼0.3 eV above the lowest TiO2 state. This state has a sizable contribution from the dye π* orbitals, strongly mixed with unoccupied TiO2 states. A similarly strong coupling is calcd. for the higher-lying transitions constituting the visible absorption band centered at ∼530 nm in the combined system. An ultrafast, almost instantaneous, electron injection component can be predicted on the basis of the strong coupling and of the matching of the visible absorption spectrum and d. of TiO2 unoccupied states. Surprisingly, this almost direct injection mechanism, corresponding to excitation from the dye ground state to an excited state largely delocalized within the semiconductor, is found to give rise to almost exactly the same absorption profile as for the dye in soln., despite the drastically different nature of the underlying excited states. On the basis of our calcns. it seems therefore that no sizable lower bound to an injection time exists, rather the timings of electron injection are mainly ruled by electron dephasing in the semiconductor.
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    Murali, M. G.; Wang, X.; Wang, Q.; Valiyaveettil, S. Design and Synthesis of New Ruthenium Complex for Dye-Sensitized Solar Cells. RSC Adv. 2016, 6, 57872– 57879,  DOI: 10.1039/C6RA10881D
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    Design and synthesis of new ruthenium complex for dye-sensitized solar cells
    Murali, M. G.; Wang, Xingzhu; Wang, Qing; Valiyaveettil, Suresh
    RSC Advances (2016), 6 (63), 57872-57879CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    A series of ruthenium complexes (MC-1-MC-3), incorporated with carbazole, fluorene and phenothiazine units with dipyrido[3,2-a:2',3'-c]phenazine are synthesized, characterized and their optical, electrochem. and photovoltaic properties are investigated. The obsd. differences in light-harvesting ability of the sensitizers are assocd. with the electron donor strength of the ancillary ligand used for prepg. complexes. The dye-sensitized solar cell fabricated from complex MC-1 exhibited a power conversion efficiency of 6.18%. It is demonstrated that new mol. design and increase in molar absorption coeff. of the sensitizer improved the device performance.
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    Ji, J.-M.; Zhou, H.; Kim, H. K. Rational Design Criteria for D−π–A Structured Organic and Porphyrin Sensitizers for Highly Efficient Dye-Sensitized Solar Cells. J. Mater. Chem. A 2018, 6, 14518– 14545,  DOI: 10.1039/C8TA02281J
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    Rational design criteria for D-π-A structured organic and porphyrin sensitizers for highly efficient dye-sensitized solar cells
    Ji, Jung-Min; Zhou, Haoran; Kim, Hwan Kyu
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2018), 6 (30), 14518-14545CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Increasing energy consumption worldwide and environmental concerns about global warming have attracted great interest in the development of renewable and eco-friendly energy technologies. Dye-sensitized solar cells (DSSCs) have attracted considerable attention over the last 25 years since they offer possible low-cost conversion of photovoltaic energy. The sensitizer is the most important component of a DSSC, as it is largely responsible for light harvesting and charge sepn., as well as the dye regeneration process. As a result, there have been tremendous research efforts in developing sensitizers. However, many challenges remain, and a deeper understanding of the design rules of DSSC sensitizers is required to obtain efficient and long-term stable DSSCs. The purpose of this review is to discuss recent progress and the rational design criteria used in the structural design of org. dyes and porphyrin photosensitizers for use in DSSCs. The effects of mol. structural engineering on the photophys. and electrochem. properties, photovoltaic parameters, and efficiency of DSSCs are presented.
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    Yang, J.; Ganesan, P.; Teuscher, J.; Moehl, T.; Kim, Y. J.; Yi, C.; Comte, P.; Pei, K.; Holcombe, T. W.; Nazeeruddin, M. K.; Hua, J.; Zakeeruddin, S. M.; Tian, H.; Grätzel, M. Influence of the Donor Size in D-π-A Organic Dyes for Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2014, 136, 5722– 5730,  DOI: 10.1021/ja500280r
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    Influence of the Donor Size in D-π-A Organic Dyes for Dye-Sensitized Solar Cells
    Yang, Jiabao; Ganesan, Paramaguru; Teuscher, Joel; Moehl, Thomas; Kim, Yong Joo; Yi, Chenyi; Comte, Pascal; Pei, Kai; Holcombe, Thomas W.; Nazeeruddin, Mohammad Khaja; Hua, Jianli; Zakeeruddin, Shaik M.; Tian, He; Gratzel, Michael
    Journal of the American Chemical Society (2014), 136 (15), 5722-5730CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The authors report 2 new molecularly engineered push-pull dyes, i.e., YA421 and YA422, based on substituted quinoxaline as a π-conjugating linker and bulky-indoline moiety as donor and compared with reported IQ4 dye. Benefitting from increased steric hindrance with the introduction of bis(2,4-dihexyloxy)benzene substitution on the quinoxaline, the electron recombination between redox electrolyte and the TiO2 surface is reduced, esp. in redox electrolyte employing Co(II/III) complexes as redox shuttles. The open circuit photovoltages of IQ4, YA421, and YA422 devices with Co-based electrolyte are higher than those with iodide/triiodide electrolyte by 34, 62, and 135 mV, resp. Also, the cells employing graphene nanoplatelets on top of Au spattered film as a counter electrode (CE) show lower charge-transfer resistance compared to Pt as a CE. Consequently, YA422 devices deliver the best power conversion efficiency due to higher fill factor, reaching 10.65% at AM 1.5 simulated sunlight. Electrochem. impedance spectroscopy and transient absorption spectroscopy anal. were performed to understand the electrolyte influence on the device performances with different counter electrode materials and donor structures of donor-π-acceptor dyes. Laser flash photolysis expts. indicate that even though the dye regeneration of YA422 is slower than that of the other 2 dyes, the slower back electron transfer of YA422 contributes to the higher device performance.
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    Lu, J.; Liu, S.; Wang, M. Push-Pull Zinc Porphyrins as Light-Harvesters for Efficient Dye-Sensitized Solar Cells. Front. Chem. 2018, 6, 541  DOI: 10.3389/fchem.2018.00541
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    Push-pull zinc porphyrins as light-harvesters for efficient dye-sensitized solar cells
    Lu, Jianfeng; Liu, Shuangshuang; Wang, Mingkui
    Frontiers in Chemistry (Lausanne, Switzerland) (2018), 6 (), 541CODEN: FCLSAA; ISSN:2296-2646. (Frontiers Media S.A.)
    A review. Dye-sensitized solar cell (DSSC) has been attractive to scientific community due to its eco-friendliness, ease of fabrication, and vivid colorful property etc. Among various kinds of sensitizers, such as metal-free org. mols., metal-complex, natural dyes etc., porphyrin is one of the most promising sensitizers for DSSC. The first application of porphyrin for sensitization of nanocrystaline TiO2 can be traced back to 1993 by using [tetrakis(4-carboxyphenyl) porphyrinato] zinc(II) with an overall conversion efficiency of 2.6%. After 10 years efforts, Officer and Gr.ovrddot.atzel improved this value to 7.1%. Later in 2009, by constructing porphyrin sensitizer with an arylamine as donor and a benzoic acid as acceptor, Diau and Yeh demonstrated that this donor-acceptor framework porphyrins could attain remarkable photovoltaic performance. Now the highest efficiencies of DSSC are dominated by donor-acceptor porphyrins, reaching remarkable values around 13.0% with cobalt-based electrolytes. This achievement is largely contributed by the structural development of donor and acceptor groups within push-pull framework. In this review, we summarized and discussed the development of donor-acceptor porphyrin sensitizers and their applications in DSSC. A discussion of the correlation between mol. structure and the spectral and photovoltaic properties is the major target of this review. Deeply dicussion of the substitution group, esp. on porphyrin's meso-position were presented. Furthermore, the limitations of DSSC for commercialization, such as the long-term stability, sophisticated synthesis procedures for high efficiency dye etc., have also been discussed.
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    Risi, G.; Becker, M.; Housecroft, C. E.; Constable, E. C. Are Alkynyl Spacers in Ancillary Ligands in Heteroleptic Bis(Diimine)Copper(I) Dyes Beneficial for Dye Performance in Dye-Sensitized Solar Cells?. Molecules 2020, 25, 1528  DOI: 10.3390/molecules25071528
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  • Abstract

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    Scheme 1

    Scheme 1. Molecular Structures of the Surface-Assembled Heteroleptic Cu(I) Complexes Investigated as Dyes for DSSCs in This Work
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    Figure 1

    Figure 1. Energy level alignments of the Cu(I) complexes in Scheme 1 with respect to the CB edge of TiO2 and the potential of the redox couple I–/I3–. The values are reported as potential vs normal hydrogen electrode (NHE).

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    Figure 2

    Figure 2. Photoelectron spectra of [Cu(dbda)(Br-dmp)]+ adsorbed on TiO2 as compared to the TiO2 substrate and the anchoring ligand on the substrate studied at a photon energy of 3000 eV. The energy is calibrated to the Ti 2p3/2 peak at 458.6 eV. The intensity is normalized to the Ti 2p3/2 peak intensity.

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    Figure 3

    Figure 3. Photoelectron spectra of [Cu(Br-dmp)2]+ and [Cu(dbda)(Br-dmp)]+ adsorbed on TiO2 determined at a photon energy of 3000 eV. The energy is calibrated to the C 1s peak at 284.8 eV. The intensity is normalized to the N 1s peak intensity.

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    Figure 4

    Figure 4. Photoelectron spectra of the counterion (PF6–) core levels of [Cu(Br-dmp)2]+ and [Cu(dbda)(Br-dmp)]+ adsorbed on FTO and TiO2 obtained for a photon energy of 3000 eV. The energy is calibrated to the C 1s peak at 284.8 eV. The intensity is normalized to the N 1s peak intensity.

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    Figure 5

    Figure 5. Complete DSSCs based on the dye [Cu(dbda)(bcp)]+ in which different iodide/triiodide electrolyte compositions have been injected. Electrolyte 1: 0.65 M 1-butyl-3-methylimidazolinium iodide, 0.025 M LiI, 0.04 I2, 0.28 M 4-tert-butylpyridine (TBP) in acetonitrile/valeronitrile (volume ratio: 85/15); (60) electrolyte 2: 1.0 M 1-butyl-3-methylimidazolinium iodide, 0.1 M LiI, 0.05 I2, 0.5 M TBP in acetonitrile; electrolyte 3: same composition as for electrolyte 2 but in acetonitrile/valeronitrile (volume ratio: 85/15).

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    Figure 6

    Figure 6. Current–density characteristics of DSSCs based on the Cu(I) dyes shown in Scheme 1, under AM 1.5G, 1 sun illumination using the electrolyte composition 1 (Figure 5). The results refer to an average of three solar cells recorded two days after assembly.

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    Figure 7

    Figure 7. Calculated HOMO and LUMO distribution of the copper(I) complexes. Hydrogen atoms are shown in white, carbon atoms in gray, nitrogen atoms in light blue, and oxygen atoms in orange. The calculated ground-state geometries, as well as HOMO and LUMO electronic distribution, for all of the complexes are reported in Figure S11.

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    Figure 8

    Figure 8. IPCE spectra of the DSSCs sensitized by Cu(I) complexes in this study and the reference N719 dye. Each curve represents the average of 3 DSSCs. The investigation is performed on the device 2 days after assembly.

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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXhsFKjt7vE&md5=337c34dcf46874ed724389ef827dae17
    9. 9
      Franchi, D.; Calamante, M.; Reginato, G.; Zani, L.; Peruzzini, M.; Taddei, M.; Fabrizi De Biani, F.; Basosi, R.; Sinicropi, A.; Colonna, D.; Di Carlo, A.; Mordini, A. Two New Dyes with Carboxypyridinium Regioisomers as Anchoring Groups for Dye-Sensitized Solar Cells. Synlett 2015, 26, 2389– 2394,  DOI: 10.1055/s-0035-1560713
      There is no corresponding record for this reference.
    10. 10
      Dessì, A.; Sinicropi, A.; Mohammadpourasl, S.; Basosi, R.; Taddei, M.; Fabrizi de Biani, F.; Calamante, M.; Zani, L.; Mordini, A.; Bracq, P.; Franchi, D.; Reginato, G. New Blue Donor–Acceptor Pechmann Dyes: Synthesis, Spectroscopic, Electrochemical, and Computational Studies. ACS Omega 2019, 4, 7614– 7627,  DOI: 10.1021/acsomega.8b03560
      10
      New Blue Donor-Acceptor Pechmann Dyes: Synthesis, Spectroscopic, Electrochemical, and Computational Studies
      Dessi, Alessio; Sinicropi, Adalgisa; Mohammadpourasl, Sanaz; Basosi, Riccardo; Taddei, Maurizio; Fabrizi de Biani, Fabrizia; Calamante, Massimo; Zani, Lorenzo; Mordini, Alessandro; Bracq, Pamela; Franchi, Daniele; Reginato, Gianna
      ACS Omega (2019), 4 (4), 7614-7627CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
      The design, synthesis and characterization of a new class of blue-colored thiophene-substituted Pechmann dyes are reported. Due to a distinguishing blue coloration and the capability to absorb light in one of the most photon-dense regions of the solar spectrum, such compds. are of great interest for application as photoactive materials in org. optoelectronics, in particular in dye-sensitized solar cells (DSSCs). In order to achieve a fine tuning of the optical and electrochem. properties, the electron poor thiophene-bis(lactone) moiety has been decorated with donor (D) and acceptor groups (A), targeting fully conjugated D-A-π-A structures. The designed structures have been investigated by means of DFT and TD-DFT calcns. and the most promising dyes synthesized. These mols. represent the very first prepn. of unsym. Pechmann derivs. Optical and electrochem. properties of the new dyes have been studied by cyclic voltammetry, UV/Vis and fluorescence spectroscopy. In two cases, test cells were built proving that a photocurrent can indeed be generated when using electrolytes esp. formulated for narrow-band-gap dyes, although with a very low efficiency.
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    11. 11
      Błaszczyk, A. Strategies to Improve the Performance of Metal-Free Dye-Sensitized Solar Cells. Dyes Pigm. 2018, 149, 707– 718,  DOI: 10.1016/j.dyepig.2017.11.045
      11
      Strategies to improve the performance of metal-free dye-sensitized solar cells
      Blaszczyk, Alfred
      Dyes and Pigments (2018), 149 (), 707-718CODEN: DYPIDX; ISSN:0143-7208. (Elsevier Ltd.)
      A review. In recent years dye-sensitized solar cells (DSSCs) have attracted more and more attention due to many advantages such as high molar absorption coeff., facile mol. tailoring and simple fabrication process as compared to conventional silicon devices. High-energy conversion efficiency is one of the most important factors for the large-scale commercialization. However, the max. reported DSSCs efficiency of 12.3% is still short of the performance of silicon devices (> 25%) and other commercialized technologies. Even lower efficiency for metal-free org. dyes as sensitizers in DSSCs have been reported. Therefore much work is required to reach optimal efficiency. Improving performance of metal-free DSSCs is a great challenge to the academic research community and for industrial applications scientists. The purpose of this review is to highlight the recent progress in improving the performance of metal-free DSSCs based on the chem. design of the sensitizer and interface mol. engineering principles.
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    12. 12
      Carella, A.; Borbone, F.; Centore, R. Research Progress on Photosensitizers for DSSC. Front. Chem. 2018, 6, 481  DOI: 10.3389/fchem.2018.00481
      12
      Research progress on photosensitizers for DSSC
      Carella, Antonio; Borbone, Fabio; Centore, Roberto
      Frontiers in Chemistry (Lausanne, Switzerland) (2018), 6 (), 481CODEN: FCLSAA; ISSN:2296-2646. (Frontiers Media S.A.)
      A review. Dye sensitized solar cells (DSSC) are considered one of the most promising photovoltaic technologies as an alternative to traditional silicon-based solar cells, for their compatibility with low-cost prodn. methods, their peculiar optical and mech. properties and the high indoor efficiency. Photosensitizers represent one of the most important components of a DSSC device and probably the most thoroughly investigated in the last twenty years, with thousands of dyes that have been proposed and tested for this kind of application. In this review we aimed to provide an overview of the three main classes of DSSC photosensitizers, namely ruthenium(II) polypyridyl complexes, Zn-porphyrin derivs. and metal-free org. dyes. After a brief introduction about the architecture and operational principles of a DSSC and the state of the art of the other main components of this type of device, we focused our discussion on photosensitizers. We have defined the numerous requirements DSSC photosensitizers should satisfy and have provided an overview of their historical development over the years; by examg. specific dyes reported in the literature, we attempted to highlight the mol. design strategies that have been established for the optimization of their performance in real devices both in terms of efficiency (which recently reaches an outstanding 14.3%) and operational stability. Finally, we discussed, in the last section, the possible future developments of this intriguing technol.
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    13. 13
      Aghazada, S.; Nazeeruddin, M. Ruthenium Complexes as Sensitizers in Dye-Sensitized Solar Cells. Inorganics 2018, 6, 52  DOI: 10.3390/inorganics6020052
      13
      Ruthenium complexes as sensitizers in dye-sensitized solar cells
      Aghazada, Sadig; Nazeeruddin, Mohammad Khaja
      Inorganics (2018), 6 (2), 52/1-52/34CODEN: INORCW; ISSN:2304-6740. (MDPI AG)
      In this review, we discuss the main directions in which ruthenium complexes for dye-sensitized solar cells (DSCs) were developed. We critically discuss the implemented design principles. This review might be helpful at this moment when a breakthrough is needed for DSC technol. to prove its market value.
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    14. 14
      Albero, J.; Atienzar, P.; Corma, A.; Garcia, H. Efficiency Records in Mesoscopic Dye-Sensitized Solar Cells. Chem. Rec. 2015, 15, 803– 828,  DOI: 10.1002/tcr.201500007
      14
      Efficiency Records in Mesoscopic Dye-Sensitized Solar Cells
      Albero, Josep; Atienzar, Pedro; Corma, Avelino; Garcia, Hermenegildo
      Chemical Record (2015), 15 (4), 803-828CODEN: CRHEAK; ISSN:1528-0691. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The aim of the present review article is to show the progress achieved in the efficiency of dye-sensitized solar cells (DSSCs) by evolution in the structure and compn. of the dye. After an initial brief description of DSSCs and the operating mechanism the major part of the present article is organized according to the type of dye, trying to show the logic in the variation of the dye structure in order to achieve strong binding on the surface of the layer of nanoparticulate TiO2, efficient interfacial electron injection between the excited dye and the semiconductor, and minimization of the unwanted dark current processes. Besides metal complexes, including polypyridyls and nitrogenated macro rings, org. dyes and inorg. light harvesters such as quantum dots and perovskites have also been included in the review. The last section summarizes the current state of the art and provides an overview on future developments in the field.
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    15. 15
      Vougioukalakis, G. C.; Philippopoulos, A. I.; Stergiopoulos, T.; Falaras, P. Contributions to the Development of Ruthenium-Based Sensitizers for Dye-Sensitized Solar Cells. Coord. Chem. Rev. 2011, 2602– 2621,  DOI: 10.1016/j.ccr.2010.11.006
      15
      Contributions to the development of ruthenium-based sensitizers for dye-sensitized solar cells
      Vougioukalakis, Georgios C.; Philippopoulos, Athanassios I.; Stergiopoulos, Thomas; Falaras, Polycarpos
      Coordination Chemistry Reviews (2011), 255 (21-22), 2602-2621CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
      The present review article traces the discoveries that were instrumental to the evolution of DSSCs, taking a crit. look at the principles of the operation mechanism and insisting on the most important recent developments in the field. We draw our attention mainly to the dye (photosensitizer); the primary processes that take place inside a solar cell, and which are affected by the dye, are also reviewed along with some of the most significant recent information found in literature that one should keep in mind before designing a novel ruthenium sensitizer. Specific emphasis is given in trying to answer vital questions like defining the optimum no. of -COOH anchoring groups and protons that an efficient dye should carry, the correct choice of prepg. either a homoleptic or a heteroleptic complex, as well as which counterions are the most suitable. Phenomena such as the role of the adsorption geometry, elec. fields at the double layer, dye-redox couple interaction and recombination effects are also analyzed, in order to show how these factors influence the photoelectrochem. characteristics of the cell. Next, the contributions from our group in the direction of designing, synthesizing, characterizing and evaluating novel ruthenium(II)-based complexes, utilized as photosensitizers in DSSCs, are thoroughly discussed. Two types of Ru(II) complexes are examd.: one family mimicking the std. N719 dye, bearing bidentate ligands, and another, carrying tridendate ligands, resembling the "black dye".
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    16. 16
      Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Müller, E.; Liska, P.; Vlachopoulos, N.; Grätzel, M. Conversion of Light to Electricity by Cis-X2Bis (2,2′-Bipyridyl-4,4′-Dicarboxylate) Ruthenium (II) Charge-Transfer Sensitizers (X = Cl–, Br–, I–, CN–, and SCN−) on Nanocrystalline TiO2 Electrodes. J. Am. Chem. Soc. 1993, 115, 6382– 6390,  DOI: 10.1021/ja00067a063
      16
      Conversion of light to electricity by cis-X2bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) charge-transfer sensitizers (X = Cl-, Br-, I-, CN-, and SCN-) on nanocrystalline titanium dioxide electrodes
      Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mueller, E.; Liska, P.; Vlachopoulos, N.; Graetzel, M.
      Journal of the American Chemical Society (1993), 115 (14), 6382-90CODEN: JACSAT; ISSN:0002-7863.
      Cis-X2bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) complexes (X = Cr-, Br-, I-, CN-, and SCN-) were prepd. and characterized with respect to their absorption, luminescence, and redox behavior. They act as efficient charge-transfer sensitizers for nanocryst. TiO2 films (thickness 8-12 μm) of very high internal surface area (roughness factor ∼1000), prepd. by sintering of 15-30-nm colloidal titania particles on a conducting glass support. The performance of cis-di(thiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) (I) was outstanding and is unmatched by any other known sensitizer. Nanocryst. TiO2 films coated with a monolayer of I harvest visible light very efficiently, their absorption threshold being ∼800 nm. Conversion of incident photons into elec. current is nearly quant. over a large spectral range. These films were incorporated in a thin-layer regenerative solar cell equipped with a light-reflecting counter electrode. Short-circuit photocurrents exceeding 17 mA/cm2 were obtained in simulated AM 1.5 sunlight using lithium iodide/triiodide in acetonitrile or acetonitrile/3-methyl-2-oxazolidinone mixts. as redox electrolyte. The open-circuit photovoltage was 0.38 V and increased to 0.72 V by treating the dye-covered film with 4-tert-butylpyridine. A solar-to-elec. energy conversion efficiency of 10% was attained with this system. The effect of temp. on the power output and long-term stability of the dye was also investigated. For the first time, a device based on a simple mol. light absorber attains a conversion efficiency commensurate with that of conventional silicon-based photovoltaic cells.
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    17. 17
      Nazeeruddin, M. K.; Humphry-Baker, R.; Liska, P.; Grätzel, M. Investigation of Sensitizer Adsorption and the Influence of Protons on Current and Voltage of a Dye-Sensitized Nanocrystalline TiO2 Solar Cell. J. Phys. Chem. B 2003, 107, 8981– 8987,  DOI: 10.1021/jp022656f
      17
      Investigation of Sensitizer Adsorption and the Influence of Protons on Current and Voltage of a Dye-Sensitized Nanocrystalline TiO2 Solar Cell
      Nazeeruddin, Md. K.; Humphry-Baker, R.; Liska, P.; Graetzel, M.
      Journal of Physical Chemistry B (2003), 107 (34), 8981-8987CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      FTIR spectra of [Ru(dcbpyH2)2(NCS)2] (N 3), (Bu4N)2[Ru(dcbpyH)2(NCS)2] (N 719), and (Bu4N)4[Ru(dcbpy)2(NCS)2] (N 712) complexes measured as solid samples in photoacoustic mode display fine resoln. of IR bands and exhibit differences between the cis and the trans carboxylic acid groups. The interaction between N 3, N 719, and N 712 sensitizers with nanocryst. TiO2 film was studied by ATR-FTIR spectroscopy. These complexes are being anchored onto the TiO2 surface in bridging coordination mode using two out of their four carboxylic acid groups, which are trans to the NCS ligand. The effect of protons on both the short circuit photocurrent and the open circuit photovoltage of dye-sensitized nanocryst. solar cells was scrutinized. For the std. electrolyte formulation employed and TiCl4 treated mesoporous TiO2 films, the monoprotonated form of the N3 dye exhibited superior power conversion efficiency under AM 1.5 sun compared to the four, two, and zero proton sensitizers.
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    18. 18
      Nazeeruddin, M. K.; Péchy, P.; Grätzel, M. Efficient Panchromatic Sensitization of Nanocrystalline TiO2 Films by a Black Dye Based on a Trithiocyanato-Ruthenium Complex. Chem. Commun. 1997, 1, 1705– 1706,  DOI: 10.1039/a703277c
      There is no corresponding record for this reference.
    19. 19
      Min Park, J.; Lee, J. H.; Jang, W.-D. Applications of Porphyrins in Emerging Energy Conversion Technologies. Coord. Chem. Rev. 2020, 407, 213157  DOI: 10.1016/j.ccr.2019.213157
      There is no corresponding record for this reference.
    20. 20
      Song, H.; Liu, Q.; Xie, Y. Porphyrin-Sensitized Solar Cells: Systematic Molecular Optimization, Coadsorption and Cosensitization. Chem. Commun. 2018, 54, 1811– 1824,  DOI: 10.1039/C7CC09671B
      20
      Porphyrin-sensitized solar cells: systematic molecular optimization, coadsorption and cosensitization
      Song, Heli; Liu, Qingyun; Xie, Yongshu
      Chemical Communications (Cambridge, United Kingdom) (2018), 54 (15), 1811-1824CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      As a promising low-cost solar energy conversion technique, dye-sensitized solar cells have undergone spectacular development since 1991. For practical applications, improvement of power conversion efficiency has always been one of the major research topics. Porphyrins are outstanding sensitizers endowed with strong sunlight harvesting ability in the visible region and multiple reaction sites available for functionalization. However, judicious mol. design in consideration of light-harvest, energy levels, operational dynamics, adsorption geometry and suppression of back reactions is specifically required for achieving excellent photovoltaic performance. This feature article highlights some of the recently developed porphyrin sensitizers, esp. focusing on the systematic dye structure optimization approach in combination with coadsorption and cosensitization methods in pursuing higher efficiencies. Herein, we expect to provide more insights into the structure-performance correlation and mol. engineering strategies in a stepwise manner.
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    21. 21
      Yella, A.; Lee, H. W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.; Diau, E. W. G.; Yeh, C. Y.; Zakeeruddin, S. M.; Grätzel, M. Porphyrin-Sensitized Solar Cells with Cobalt (II/III)-Based Redox Electrolyte Exceed 12 Percent Efficiency. Science 2011, 334, 629– 634,  DOI: 10.1126/science.1209688
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      Porphyrin-Sensitized Solar Cells with Cobalt (II/III)-Based Redox Electrolyte Exceed 12% Efficiency
      Yella, Aswani; Lee, Hsuan-Wei; Tsao, Hoi Nok; Yi, Chenyi; Chandiran, Aravind Kumar; Nazeeruddin, Md. Khaja; Diau, Eric Wei-Guang; Yeh, Chen-Yu; Zakeeruddin, Shaik M.; Graetzel, Michael
      Science (Washington, DC, United States) (2011), 334 (6056), 629-634CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      The iodide/triiodide redox shuttle has limited the efficiencies accessible in dye-sensitized solar cells. Here, the authors report mesoscopic solar cells that incorporate a Co(II/III)tris(bipyridyl)-based redox electrolyte in conjunction with a custom synthesized donor-π-bridge-acceptor zinc porphyrin dye as sensitizer (designated YD2-o-C8). The specific mol. design of YD2-o-C8 greatly retards the rate of interfacial back electron transfer from the conduction band of the nanocryst. titanium dioxide film to the oxidized cobalt mediator, which enables attainment of strikingly high photovoltages approaching 1 V. Because the YD2-o-C8 porphyrin harvests sunlight across the visible spectrum, large photocurrents are generated. Cosensitization of YD2-o-C8 with another org. dye further enhances the performance of the device, leading to a measured power conversion efficiency of 12.3% under simulated air mass 1.5 global sunlight.
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    22. 22
      Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod, B. F. E.; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin, M. K.; Grätzel, M. Dye-Sensitized Solar Cells with 13% Efficiency Achieved through the Molecular Engineering of Porphyrin Sensitizers. Nat. Chem. 2014, 6, 242– 247,  DOI: 10.1038/nchem.1861
      22
      Dye-sensitized solar cells with 13% efficiency achieved through the molecular engineering of porphyrin sensitizers
      Mathew, Simon; Yella, Aswani; Gao, Peng; Humphry-Baker, Robin; Curchod, Basile F. E.; Ashari-Astani, Negar; Tavernelli, Ivano; Rothlisberger, Ursula; Nazeeruddin, Md. Khaja; Graetzel, Michael
      Nature Chemistry (2014), 6 (3), 242-247CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
      Dye-sensitized solar cells have gained widespread attention in recent years because of their low prodn. costs, ease of fabrication and tunable optical properties, such as color and transparency. Here, the authors report a molecularly engineered porphyrin dye, coded SM315, which features the prototypical structure of a donor-π-bridge-acceptor and both maximizes electrolyte compatibility and improves light-harvesting properties. Linear-response, time-dependent d. functional theory was used to study the perturbations in the electronic structure that lead to improved light harvesting. Using SM315 with the Co(II/III) redox shuttle resulted in dye-sensitized solar cells that exhibit a high open-circuit voltage VOC of 0.91 V, short-circuit c.d. JSC of 18.1 mA cm-2, fill factor of 0.78 and a power conversion efficiency of 13%.
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    23. 23
      Lu, X.; Wei, S.; Wu, C. M. L.; Li, S.; Guo, W. Can Polypyridyl Cu(I)-Based Complexes Provide Promising Sensitizers for Dye-Sensitized Solar Cells? A Theoretical Insight into Cu(I) versus Ru(II) Sensitizers. J. Phys. Chem. C 2011, 115, 3753– 3761,  DOI: 10.1021/jp111325y
      23
      Can polypyridyl Cu(I)-based complexes provide promising sensitizers for dye-sensitized solar cells? A theoretical insight into Cu(I) versus Ru(II) sensitizers
      Lu, Xiaoqing; Wei, Shuxian; Wu, Chi-Man Lawrence; Li, Shaoren; Guo, Wenyue
      Journal of Physical Chemistry C (2011), 115 (9), 3753-3761CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Design of light-absorbent dyes with cheaper, safer, and more sustainable materials is one of the key issues for the future development of dye-sensitized solar cells (DSSCs). We report herein a theor. investigation on a series of polypyridyl Cu(I)-based complexes with general formula (L and L' represent bipyridyl ligands) by d. functional theory (DFT) and time-dependent DFT. Mol. geometries, electronic structures, and optical absorption spectra are predicted in both the gas phase and Me cyanide soln. Our results show that all the [CuLL']+ derivs. display Cu → bipyridine metal-to-ligand charge transfer absorption spectra in the range of 350-700 nm. Structural optimizations by enhancing π-conjugation and introducing heteroarom. groups on ancillary ligands lead to upshift of MO energies, increase in oscillator strength, and red shift of absorption spectra. Compared with Ru(II) sensitizers, polypridyl Cu(I)-based complexes show similar optical properties and improving trend of the DSSCs performance along with the optimizations of structures. The results of this work highlight the point that polypyridyl Cu(I)-based complexes could provide promising sensitizers for efficient next-generation DSSCs.
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    24. 24
      Magni, M.; Biagini, P.; Colombo, A.; Dragonetti, C.; Roberto, D.; Valore, A. Versatile Copper Complexes as a Convenient Springboard for Both Dyes and Redox Mediators in Dye Sensitized Solar Cells. Coord. Chem. Rev. 2016, 322, 69– 93,  DOI: 10.1016/j.ccr.2016.05.008
      24
      Versatile copper complexes as a convenient springboard for both dyes and redox mediators in dye sensitized solar cells
      Magni, Mirko; Biagini, Paolo; Colombo, Alessia; Dragonetti, Claudia; Roberto, Dominique; Valore, Adriana
      Coordination Chemistry Reviews (2016), 322 (), 69-93CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
      In the early 1990s, dye-sensitized solar cells (DSSCs), fabricated by O'Regan and Graetzel, emerged as a realistic soln. for harnessing the energy of the sun and converting it into electricity. Since then, an impressive amt. of work has been carried out in order to improve the global photovoltaic efficiency of DSSCs, trying to optimize every components of the device in terms of material, structure and activity. Up to now, the best efficiencies have been reached with zinc-porphyrin and Ru(II) photosensitizers, but recently an exponential growth of articles showed the great potential of copper(I) coordination compds. as a convenient and cheap alternative. Similarly, it appeared that the use of copper complexes as electron transfer mediators for DSSCs can be an excellent way to solve the problems related to the more common I-3/I- redox couple.The goal of this review is to report on the promising use of versatile copper complexes as photosensitizers and electron shuttles in DSSCs which had a real boom in the last few years. The coverage, mainly from 2010 up to now, is not exhaustive, but allows the non-specialist reader to get into this specific field and to understand its potentiality.An important message that the review will convey is the mandatory role of comparing results on novel photosensitizers and/or on alternative redox mediators with well-established literature ref. systems such as N719 dye and I-3/I--based electrolyte, resp. In fact, reporting the performance of new compds. with respect to proper std. materials is the unique secure method to correctly and, firstly, constructively compare data from different studies.
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    25. 25
      Risi, G.; Becker, M.; Housecroft, C. E.; Constable, E. C. Are Alkynyl Spacers in Ancillary Ligands in Heteroleptic Bis(Diimine)Copper(I) Dyes Beneficial for Dye Performance in Dye-Sensitized Solar Cells?. Molecules 2020, 25, 1528  DOI: 10.3390/molecules25071528
      25
      Are alkynyl spacers in ancillary ligands in heteroleptic bis(diimine)copper(I) dyes beneficial for dye performance in dye-sensitized solar cells?
      Risi, Guglielmo; Becker, Mariia; Housecroft, Catherine E.; Constable, Edwin C.
      Molecules (2020), 25 (7), 1528CODEN: MOLEFW; ISSN:1420-3049. (MDPI AG)
      The syntheses of 4,4'-bis(4-dimethylaminophenyl)-6,6'-dimethyl-2,2'-bipyridine (1), 4,4'-bis(4-dimethylaminophenylethynyl)-6,6'-dimethyl-2,2'-bipyridine (2), 4,4'-bis(4- diphenylaminophenyl)-6,6'-dimethyl-2,2'-bipyridine (3), and 4,4'-bis(4-diphenylaminophenylethynyl)- 6,6'-dimethyl-2,2'-bipyridine (4) are reported along with the prepns. and characterizations of their homoleptic copper(I) complexes [CuL2][PF6] (L = 1-4). The soln. absorption spectra of the complexes exhibit ligand-centered absorptions in addn. to absorptions in the visible region assigned to a combination of intra-ligand and metal-to-ligand charge-transfer. Heteroleptic [Cu(5)(Lancillary)]+ dyes in which 5 is the anchoring ligand ((6,6'-dimethyl-[2,2'-bipyridine]- 4,4'-diyl)bis(4,1-phenylene))bis(phosphonic acid) and Lancillary = 1-4 have been assembled on fluorine-doped tin oxide (FTO)-TiO2 electrodes in dye-sensitized solar cells (DSCs). Performance parameters and external quantum efficiency (EQE) spectra of the DSCs (four fully-masked cells for each dye) reveal that the best performing dyes are [Cu(5)(1)]+ and [Cu(5)(3)]+. The alkynyl spacers are not beneficial, leading to a decrease in the short-circuit c.d. (JSC), confirmed by lower values of EQEmax. Addn. of a co-absorbent (n-decylphosphonic acid) to [Cu(5)(1)]+ lead to no significant enhancement of performance for DSCs sensitized with [Cu(5)(1)]+. Electrochem. impedance spectroscopy (EIS) has been used to investigate the interfaces in DSCs; the anal. shows that more favorable electron injection into TiO2 is obsd. for sensitizers without the alkynyl spacer and confirms higher JSC values for [Cu(5)(1)]+.
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    26. 26
      Cao, Y.; Saygili, Y.; Ummadisingu, A.; Teuscher, J.; Luo, J.; Pellet, N.; Giordano, F.; Zakeeruddin, S. M.; Moser, J. E.; Freitag, M.; Hagfeldt, A.; Grätzel, M. 11% Efficiency Solid-State Dye-Sensitized Solar Cells with Copper(II/I) Hole Transport Materials. Nat. Commun. 2017, 8, 15390  DOI: 10.1038/ncomms15390
      26
      11% efficiency solid-state dye-sensitized solar cells with copper(II/I) hole transport materials
      Cao Yiming; Ummadisingu Amita; Luo Jingshan; Pellet Norman; Giordano Fabrizio; Zakeeruddin Shaik Mohammed; Gratzel Michael; Saygili Yasemin; Freitag Marina; Hagfeldt Anders; Teuscher Joel; Moser Jacques-E
      Nature communications (2017), 8 (), 15390 ISSN:.
      Solid-state dye-sensitized solar cells currently suffer from issues such as inadequate nanopore filling, low conductivity and crystallization of hole-transport materials infiltrated in the mesoscopic TiO2 scaffolds, leading to low performances. Here we report a record 11% stable solid-state dye-sensitized solar cell under standard air mass 1.5 global using a hole-transport material composed of a blend of [Cu (4,4',6,6'-tetramethyl-2,2'-bipyridine)2](bis(trifluoromethylsulfonyl)imide)2 and [Cu (4,4',6,6'-tetramethyl-2,2'-bipyridine)2](bis(trifluoromethylsulfonyl)imide). The amorphous Cu(II/I) conductors that conduct holes by rapid hopping infiltrated in a 6.5 μm-thick mesoscopic TiO2 scaffold are crucial for achieving such high efficiency. Using time-resolved laser photolysis, we determine the time constants for electron injection from the photoexcited sensitizers Y123 into the TiO2 and regeneration of the Y123 by Cu(I) to be 25 ps and 3.2 μs, respectively. Our work will foster the development of low-cost solid-state photovoltaic based on transition metal complexes as hole conductors.
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    27. 27
      Saygili, Y.; Söderberg, M.; Pellet, N.; Giordano, F.; Cao, Y.; Munoz-García, A. B.; Zakeeruddin, S. M.; Vlachopoulos, N.; Pavone, M.; Boschloo, G.; Kavan, L.; Moser, J. E.; Grätzel, M.; Hagfeldt, A.; Freitag, M. Copper Bipyridyl Redox Mediators for Dye-Sensitized Solar Cells with High Photovoltage. J. Am. Chem. Soc. 2016, 138, 15087– 15096,  DOI: 10.1021/jacs.6b10721
      27
      Copper Bipyridyl Redox Mediators for Dye-Sensitized Solar Cells with High Photovoltage
      Saygili, Yasemin; Soderberg, Magnus; Pellet, Norman; Giordano, Fabrizio; Cao, Yiming; Munoz-Garcia, Ana Belen; Zakeeruddin, Shaik M.; Vlachopoulos, Nick; Pavone, Michele; Boschloo, Gerrit; Kavan, Ladislav; Moser, Jacques-E.; Gratzel, Michael; Hagfeldt, Anders; Freitag, Marina
      Journal of the American Chemical Society (2016), 138 (45), 15087-15096CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Redox mediators play a major role detg. the photocurrent and the photovoltage in dye-sensitized solar cells (DSCs). To maintain the photocurrent, the redn. of oxidized dye by the redox mediator should be significantly faster than the electron back transfer between TiO2 and the oxidized dye. The driving force for dye regeneration with the redox mediator should be sufficiently low to provide high photovoltages. With the introduction of our new copper complexes as promising redox mediators in DSCs both criteria are satisfied to enhance power conversion efficiencies. In this study, two copper bipyridyl complexes, Cu(II/I)(dmby)2TFSI2/1 (0.97 V vs SHE, dmby = 6,6'-dimethyl-2,2'-bipyridine) and Cu(II/I)(tmby)2TFSI2/1 (0.87 V vs SHE, tmby = 4,4',6,6'-tetramethyl-2,2'-bipyridine), are presented as new redox couples for DSCs. They are compared to previously reported Cu(II/I)(dmp)2TFSI2/1 (0.93 V vs SHE, dmp = bis(2,9-dimethyl-1,10-phenanthroline)). Due to the small reorganization energy between Cu(I) and Cu(II) species, these copper complexes can sufficiently regenerate the oxidized dye mols. with close to unity yield at driving force potentials as low as 0.1 V. The high photovoltages of over 1.0 V were achieved by the series of copper complex based redox mediators without compromising photocurrent densities. Despite the small driving forces for dye regeneration, fast and efficient dye regeneration (2-3 μs) was obsd. for both complexes. As another advantage, the electron back transfer (recombination) rates were slower with Cu(II/I)(tmby)2TFSI2/1 as evidenced by longer lifetimes. The solar-to-elec. power conversion efficiencies for [Cu(tmby)2]2+/1+, [Cu(dmby)2]2+/1+, and [Cu(dmp)2]2+/1+ based electrolytes were 10.3%, 10.0%, and 10.3%, resp., using the org. Y123 dye under 1000 W m-2 AM1.5G illumination. The high photovoltaic performance of Cu-based redox mediators underlines the significant potential of the new redox mediators and points to a new research and development direction for DSCs.
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    28. 28
      Colombo, A.; Di Carlo, G.; Dragonetti, C.; Magni, M.; Orbelli Biroli, A.; Pizzotti, M.; Roberto, D.; Tessore, F.; Benazzi, E.; Bignozzi, C. A.; Casarin, L.; Caramori, S. Coupling of Zinc Porphyrin Dyes and Copper Electrolytes: A Springboard for Novel Sustainable Dye-Sensitized Solar Cells. Inorg. Chem. 2017, 56, 14189– 14197,  DOI: 10.1021/acs.inorgchem.7b02323
      28
      Coupling of Zinc Porphyrin Dyes and Copper Electrolytes: A Springboard for Novel Sustainable Dye-Sensitized Solar Cells
      Colombo, Alessia; Di Carlo, Gabriele; Dragonetti, Claudia; Magni, Mirko; Orbelli Biroli, Alessio; Pizzotti, Maddalena; Roberto, Dominique; Tessore, Francesca; Benazzi, Elisabetta; Bignozzi, Carlo Alberto; Casarin, Laura; Caramori, Stefano
      Inorganic Chemistry (2017), 56 (22), 14189-14197CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The combination of β-substituted Zn2+ porphyrin dyes and copper-based electrolytes represents a sustainable route for economic and environmentally friendly dye-sensitized solar cells. Remarkably, a new copper electrolyte, [Cu(2-mesityl-1,10-phenanthroline)2]+/2+, exceeds the performance reached by Co2+/3+ and I-/I3- ref. electrolytes.
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    29. 29
      Higashino, T.; Iiyama, H.; Nishimura, I.; Imahori, H. Exploration on the Combination of Push-Pull Porphyrin Dyes and Copper(I/II) Redox Shuttles toward High-Performance Dye-Sensitized Solar Cells. Chem. Lett. 2020, 49, 936– 939,  DOI: 10.1246/cl.200317
      29
      Exploration on the Combination of Push-Pull Porphyrin Dyes and Copper(I/II) Redox Shuttles toward High-performance Dye-sensitized Solar Cells
      Higashino, Tomohiro; Iiyama, Hitomi; Nishimura, Issei; Imahori, Hiroshi
      Chemistry Letters (2020), 49 (8), 936-939CODEN: CMLTAG; ISSN:0366-7022. (Chemical Society of Japan)
      The effects of push-pull porphyrin dyes as well as copper(I/II) redox shuttles on photovoltaic properties have been examd. toward high-performance dye-sensitized solar cells (DSSCs). DSSC with a combination of porphyrin dye LG4 and bis(4,4',6,6'-tetramethyl-2,2'-bipyridyl)copper(I/II) exhibited the highest power conversion efficiency of 5.07% ever reported for porphyrin-based DSSCs using copper(I/II) redox shuttles.
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    30. 30
      Alonso-Vante, N.; Nierengarten, J.-F.; Sauvage, J.-P. Spectral Sensitization of Large-Band-Gap Semiconductors (Thin Films and Ceramics) by a Carboxylated Bis(1,10-Phenanthroline)Copper(I) Complex. J. Chem. Soc., Dalton Trans. 1994, 97, 1649,  DOI: 10.1039/dt9940001649
      There is no corresponding record for this reference.
    31. 31
      Huang, J.; Buyukcakir, O.; Mara, M. W.; Coskun, A.; Dimitrijevic, N. M.; Barin, G.; Kokhan, O.; Stickrath, A. B.; Ruppert, R.; Tiede, D. M.; Stoddart, J. F.; Sauvage, J. P.; Chen, L. X. Highly Efficient Ultrafast Electron Injection from the Singlet MLCT Excited State of Copper(I) Diimine Complexes to TiO2 Nanoparticles. Angew. Chem., Int. Ed. 2012, 51, 12711– 12715,  DOI: 10.1002/anie.201204341
      31
      Highly Efficient Ultrafast Electron Injection from the Singlet MLCT Excited State of Copper(I) Diimine Complexes to TiO2 Nanoparticles
      Huang, Jier; Buyukcakir, Onur; Mara, Michael W.; Coskun, Ali; Dimitrijevic, Nada M.; Barin, Gokhan; Kokhan, Oleksandr; Stickrath, Andrew B.; Ruppert, Romain; Tiede, David M.; Stoddart, J. Fraser; Sauvage, Jean-Pierre; Chen, Lin X.
      Angewandte Chemie, International Edition (2012), 51 (51), 12711-12715CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Ultrafast electron injection from surface bound excited 1MLCT state of [Cu(dppS)2]+ (dppS = disodium 2,9-bis(sulfonatediphenyl)-1,10-phenanthroline) to TiO2 nanoparticle was studied. Dynamics of charge sepn. and recombination was correlated with the mol. structure of [Cu(dppS)2]+. The ESR, transient absorption and PR studies confirmed formation of [Cu(dppS)2]2+ , resulting from an efficient charge-transfer process in [Cu(dppS)2]+/TiO2.
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    32. 32
      Sandroni, M.; Pellegrin, Y.; Odobel, F. Heteroleptic Bis-Diimine Copper(I) Complexes for Applications in Solar Energy Conversion. C. R. Chim. 2016, 19, 79– 93,  DOI: 10.1016/j.crci.2015.06.008
      32
      Heteroleptic bis-diimine copper(I) complexes for applications in solar energy conversion
      Sandroni, Martina; Pellegrin, Yann; Odobel, Fabrice
      Comptes Rendus Chimie (2016), 19 (1-2), 79-93CODEN: CRCOCR; ISSN:1631-0748. (Elsevier Masson SAS)
      A review is presented. The development of mol. materials for conversion of solar energy into electricity and fuels is one of the most active research areas, in which the light absorber plays a key role. In this article, we present a class of photo-sensitizers constituted by heteroleptic bis-diimine copper(I) complexes, whose features rely on their tunable MLCT excited states and earth abundant and environmentally friendly nature of copper. These properties make this class of dyes compatible with a sustainable development. Their synthesis through the HETPHEN strategy and the background of their photophys. properties as well as the recent advances in the fields of both dye-sensitized solar cells and photoinduced charge sepn. are presented. The presented examples prove that bis-diimine copper(I) complexes are valuable dyes to be used to build multicomponent arrays for long range electron transfer and for dye-sensitized solar cells. These examples illustrate that one major advantage of heteroleptic complexes is the possibility to finely tune their opto-electronic properties to a larger extent than homoleptic complexes.
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    33. 33
      Bessho, T.; Constable, E. C.; Graetzel, M.; Hernandez Redondo, A.; Housecroft, C. E.; Kylberg, W.; Nazeeruddin, M. K.; Neuburger, M.; Schaffner, S. An Element of Surprise: Efficient Copper-Functionalized Dye-Sensitized Solar Cells. Chem. Commun. 2008, 3717,  DOI: 10.1039/b808491b
      33
      An element of surprise-efficient copper-functionalized dye-sensitized solar cells
      Bessho, Takeru; Constable, Edwin C.; Graetzel, Michael; Hernandez Redondo, Ana; Housecroft, Catherine E.; Kylberg, William; Nazeeruddin, Md. K.; Neuburger, Markus; Schaffner, Silvia
      Chemical Communications (Cambridge, United Kingdom) (2008), (32), 3717-3719CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Dye-sensitized solar cells with carboxylate-derivatized {CuIL2} complexes are surprisingly efficient and offer a long-term alternative approach to ruthenium-functionalized systems.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXpsVOhsrc%253D&md5=60f4c0ee566491d17e29e3ba02ea6178
    34. 34
      Bozic-Weber, B.; Brauchli, S. Y.; Constable, E. C.; Fürer, S. O.; Housecroft, C. E.; Malzner, F. J.; Wright, I. A.; Zampese, J. A. Improving the Photoresponse of Copper(i) Dyes in Dye-Sensitized Solar Cells by Tuning Ancillary and Anchoring Ligand Modules. Dalton Trans. 2013, 42, 12293– 12308,  DOI: 10.1039/c3dt51416a
      34
      Improving the photoresponse of copper(I) dyes in dye-sensitized solar cells by tuning ancillary and anchoring ligand modules
      Bozic-Weber, Biljana; Brauchli, Sven Y.; Constable, Edwin C.; Fuerer, Sebastian O.; Housecroft, Catherine E.; Malzner, Frederik J.; Wright, Iain A.; Zampese, Jennifer A.
      Dalton Transactions (2013), 42 (34), 12293-12308CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      The syntheses of five homoleptic copper(I) complexes [CuL2][PF6] are described in which L is a 4,4'-di(4-bromophenyl)-6,6'-dialkyl-2,2'-bipyridine ligand (compds. 1-4 with Me, nbutyl, isobutyl and hexyl substituents, resp.) or 4,4'-di(4-bromophenyl)-6,6'-diphenyl-2,2'-bipyridine (5). The new ligands 2-5 and copper(I) complexes [CuL2][PF6] (L = 1-5) have been fully characterized. The single crystal structures of 2{[Cu(1)2][PF6]}·3Me2CO, [Cu(2)2][PF6], 2{[Cu(3)2][PF6]}·Et2O and [Cu(5)2][PF6]·CH2Cl2 have been detd. The first three structures show similar distorted tetrahedral environments for the Cu+ ions with angles between the least squares planes of the bpy domains of 85.6, 86.4 and 82.9°, resp.; in contrast, the Cu+ ion in [Cu(5)2][PF6]·CH2Cl2 is in a flattened coordinate environment due to intra-cation face-to-face π-interactions. The soln. absorption spectra of the complexes with ligands 1-4 are virtually identical with an MLCT band with values of λmax = 481-488 nm. In contrast, the absorption spectrum of [Cu(5)2][PF6] shows two broad bands in the visible region. Cyclic voltammetric data show that oxidn. of the copper(i) center occurs at a more pos. potential in [Cu(2)2][PF6], [Cu(3)2][PF6] and [Cu(4)2][PF6] than in [Cu(1)2][PF6] or [Cu(5)2][PF6] with the latter being oxidized at the lowest potential. The complexes have been used to prep. dye-sensitized solar cells (DSCs) incorporating heteroleptic dyes of type [Cu(L)(Lanchor)]+ where L is 1-5 and Lanchor is a 6,6'-dimethyl-2,2'-bipyridine functionalized in the 4- and 4'-positions with phosphonic acid groups with (Lanchor = 7) and without (Lanchor = 6) a spacer between the metal-binding and anchoring domains. The presence of the spacer results in enhanced performances of the dyes, and the highest energy conversion efficiencies are obsd. for the dyes [Cu(3)(7)]+ (η = 2.43% compared to 5.96% for std. dye N719) and [Cu(5)(7)]+ (η = 2.89% compared to 5.96% for N719). Measurements taken periodically over the course of a week indicate that the cells undergo a ripening process (most clearly seen for [Cu(5)(6)]+ and [Cu(5)(7)]+) before their optimum performances are achieved. IPCE (EQE) data are presented and confirm that, although the photo-to-current conversions are promising (37-49% for λmax ≈ 480 nm), the copper(i) dyes do not realize the broad spectral response exhibited by N719.
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    35. 35
      Malzner, F. J.; Brauchli, S. Y.; Constable, E. C.; Housecroft, C. E.; Neuburger, M. Halos Show the Path to Perfection: Peripheral Iodo-Substituents Improve the Efficiencies of Bis(Diimine)Copper(i) Dyes in DSCs. RSC Adv. 2014, 4, 48712– 48723,  DOI: 10.1039/C4RA06823H
      35
      Halos show the path to perfection: peripheral iodo-substituents improve the efficiencies of bis(diimine)copper(I) dyes in DSCs
      Malzner, Frederik J.; Brauchli, Sven Y.; Constable, Edwin C.; Housecroft, Catherine E.; Neuburger, Markus
      RSC Advances (2014), 4 (89), 48712-48723CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      The homoleptic copper(I) complexes [CuL2][PF6] (L = 4,4'-bis(4-halophenyl)-6,6'-dimethyl-2,2'-bipyridine with halogen = F (2) and Cl (3)) have been prepd. and characterized, and their absorption spectroscopic and electrochem. properties compared to that with L = 4,4'-bis(4-bromophenyl)-6,6'-dimethyl-2,2'-bipyridine (4). The synthesis of [CuL2][PF6] (L = 4,4'-bis(4-iodophenyl)-6,6'-dimethyl-2,2'-bipyridine, 5) resulted in a mixt. of [Cu(5)2][PF6] and [Cu(5)(MeCN)2][PF6]; variable temp. 1H NMR spectroscopy confirmed that the complexes are in equil. in CD3CN soln. The structure of [Cu(5)(MeCN)2][PF6] was detd. by single crystal X-ray crystallog., and confirms a distorted tetrahedral geometry for the copper(I) center. The heteroleptic dyes [Cu(1)(2)]+, [Cu(1)(3)]+, [Cu(1)(4)]+ and [Cu(1)(5)]+ (1 = ((6,6'-dimethyl-[2,2'-bipyridine]-4,4'-diyl)bis(4,1-phenylene))bis(phosphonic acid)) have been assembled by ligand exchange between [CuL2]+ and TiO2 functionalized with the anchoring ligand 1, and the performances of the dyes in fully masked dye-sensitized solar cells (DSCs) have been measured and compared. On the day of DSC fabrication, the trend for the global efficiencies, η, depends on the halo-substituent in the order I > F ≈ Br > Cl. Ripening of the DSCs occurs and after 7 days, the dependence of η on the halo-atom is in the order I > Cl ≈ F ≈ Br; the highest η is 3.16% for [Cu(1)(5)]+ compared to 7.63% for N719. Compared to the other halo-functionalized dyes, [Cu(1)(5)]+ shows an extended spectral response to longer wavelength, with enhanced electron injection. The results of DFT calcns. suggest that the better dye performance of [Cu(1)(5)]+ may be assocd. with improved electron transfer over the halogen of the aryl substituent from the reduced electrolyte. The assembly of anchored dye [Cu(1)(5)]+ by treating functionalized-TiO2 with a 1 : 1 mixt. of [Cu(MeCN)4]+ and 5, yields a dye which gives a DSC performance that matches that made by ligand exchange using [Cu(5)2][PF6] and [Cu(5)(MeCN)2][PF6].
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    36. 36
      Brauchli, S. Y.; Malzner, F. J.; Constable, E. C.; Housecroft, C. E. Copper(i)-Based Dye-Sensitized Solar Cells with Sterically Demanding Anchoring Ligands: Bigger Is Not Always Better. RSC Adv. 2015, 5, 48516– 48525,  DOI: 10.1039/C5RA07449E
      36
      Copper(I)-based dye-sensitized solar cells with sterically demanding anchoring ligands: bigger is not always better
      Brauchli, Sven Y.; Malzner, Frederik J.; Constable, Edwin C.; Housecroft, Catherine E.
      RSC Advances (2015), 5 (60), 48516-48525CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      The synthesis and characterization of ((6,6'-diphenyl-[2,2'-bipyridine]-4,4'-diyl)bis(4,1-phenylene))bis(phosphonic acid), 2, are described. Compd. 2 has been incorporated as an anchoring ligand in copper(I)-contg. dyes in n-type dye-sensitized solar cells (DSCs), combined with 2,2'-bipyridine (bpy), 6-methyl-2,2'-bipyridine (6-Mebpy), 6,6'-dimethyl-2,2'-bipyridine (6,6'-Me2bpy), 4,4'-di(4-bromophenyl)-6,6'-dimethyl-2,2'-bipyridine (3) or 4,4'-di(4-bromophenyl)-6,6'-diphenyl-2,2'-bipyridine (4) as ancillary ligands (Lancillary). Dyes were assembled on mesoporous TiO2 using an on-surface assembly strategy which relies on ligand exchange between surface-anchored Lanchor and [Cu(Lancillary)2]+; 1H NMR spectroscopy was used to confirm that the bulky Ph substituents did not hinder ligand exchange. Comparison of values of the open-circuit voltages (VOC), short-circuit current densities (JSC) and external quantum efficiency (EQE) spectra for DSCs with model dyes [Cu(2)(bpy)]+, [Cu(2)(6-Mebpy)]+ and [Cu(2)(6,6'-Me2bpy)]+ confirm that methyl-substituents in Lancillary are beneficial. Performance data for DSCs with dyes [Cu(1)(3)]+, [Cu(1)(4)]+, [Cu(2)(3)]+ and [Cu(2)(4)]+ where 1 is the anchor ((6,6'-dimethyl-[2,2'-bipyridine]-4,4'-diyl)bis(4,1-phenylene))bis(phosphonic acid) show that dyes with anchor 2 (Ph substituents in the 6- and 6'-positions) give relative conversion efficiencies ≤10% with respect to std. dye N719 set at η = 100%; this compares with relative efficiencies of up to 34.5% for the dyes [Cu(1)(3)]+ and [Cu(1)(4)]+. The performance of [Cu(2)(3)]+ can be improved by the addn. of the co-adsorbent chenodeoxycholic acid. Although the Ph (vs. methyl) substituents lead to enhanced light absorption to lower energies, dyes with anchor 2 quickly bleach when exposed to the I-/I3- electrolyte; bleaching also occurs after soaking in solns. of LiI. The dye can be regenerated by treatment of a bleached electrode with Lancillary, or with [Cu(NCMe)4]+ followed by Lancillary.
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    37. 37
      Dragonetti, C.; Magni, M.; Colombo, A.; Melchiorre, F.; Biagini, P.; Roberto, D. Coupling of a Copper Dye with a Copper Electrolyte: A Fascinating Springboard for Sustainable Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2018, 1, 751– 756,  DOI: 10.1021/acsaem.7b00196
      There is no corresponding record for this reference.
    38. 38
      Karpacheva, M.; Malzner, F. J.; Wobill, C.; Büttner, A.; Constable, E. C.; Housecroft, C. E. Cuprophilia: Dye-Sensitized Solar Cells with Copper(I) Dyes and Copper(I)/(II) Redox Shuttles. Dyes Pigm. 2018, 156, 410– 416,  DOI: 10.1016/j.dyepig.2018.04.033
      38
      Cuprophilia: Dye-sensitized solar cells with copper(I) dyes and copper(I)/(II) redox shuttles
      Karpacheva, Mariia; Malzner, Frederik J.; Wobill, Cedric; Buttner, Annika; Constable, Edwin C.; Housecroft, Catherine E.
      Dyes and Pigments (2018), 156 (), 410-416CODEN: DYPIDX; ISSN:0143-7208. (Elsevier Ltd.)
      With a goal of designing dye-sensitized solar cells (DSCs) contg. only Earth-abundant components to achieve sustainable energy conversion, DSCs with heteroleptic copper(I)-based dyes and homoleptic copper(I)/(II) redox shuttles were investigated. By using a phosphonic acid anchor, and 4,4'-dimethoxy-6,6'-dimethyl-2,2'-bipyridine as the ancillary ligand in the dye and in the electrolyte, a DSC photoconversion efficiency of 2.06% (38.1% relative to N719 set at 100%) was achieved. The results demonstrate the potential for all-copper-based DSCs, opening the way for further dye and electrolyte optimization.
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    39. 39
      Leandri, V.; Pizzichetti, A. R. P.; Xu, B.; Franchi, D.; Zhang, W.; Benesperi, I.; Freitag, M.; Sun, L.; Kloo, L.; Gardner, J. M. Exploring the Optical and Electrochemical Properties of Homoleptic versus Heteroleptic Diimine Copper(I) Complexes. Inorg. Chem. 2019, 58, 12167– 12177,  DOI: 10.1021/acs.inorgchem.9b01487
      39
      Exploring the Optical and Electrochemical Properties of Homoleptic versus Heteroleptic Diimine Copper(I) Complexes
      Leandri, Valentina; Pizzichetti, Angela Raffaella Pia; Xu, Bo; Franchi, Daniele; Zhang, Wei; Benesperi, Iacopo; Freitag, Marina; Sun, Licheng; Kloo, Lars; Gardner, James M.
      Inorganic Chemistry (2019), 58 (18), 12167-12177CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Due to ligand scrambling, the synthesis and investigation of the properties of heteroleptic Cu(I) complexes can be a challenging task. In this work, we have studied the optical and electrochem. properties of a series of homoleptic complexes, such as [Cu(dbda)2]+, [Cu(dmp)2]+, [Cu(Br-dmp)2]+, [Cu(bcp)2]+, [Cu(dsbtmp)2]+, [Cu(biq)2]+, and [Cu(dap)2]+ in soln., and those of their heteroleptics [Cu(dbda)(dmp)]+, [Cu(dbda)(Br-dmp)]+, [Cu(dbda)(bcp)]+, [Cu(dbda)(dsbtmp)]+, [Cu(dbda)(biq)]+, [Cu(dbda)(dap)]+ adsorbed on the surface of anatase TiO2 (dbda = 6,6'-dimethyl-2,2'-bipyridine-4,4'-dibenzoic acid; dmp = 2,9-dimethyl-1,10-phenanthroline; Br-dmp = 5-bromo-2,9-dimethyl-1,10-phenanthroline; bcp = bathocuproine or 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline; dsbtmp = 2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline; biq = 2,2'-biquinoline; dap = 2,9-dianisyl-1,10-phenanthroline). We show that the max. absorption wavelengths of the heteroleptic complexes on TiO2 can be reasonably predicted from those of the homoleptic complexes in soln. through a simple linear relation, whereas the prediction of their redox properties is less trivial. In the latter case, two different linear patterns emerge: one including the ligands bcp, biq, and dap and another one including the ligands dmp, Br-dmp, and dsbtmp. We offer an interpretation of the data based on the chem. structure of the ligands. On one hand, ligands bcp, biq, and dap possess a more extended π-conjugated system, which gives a more prominent contribution to the overall redox properties of the ligand dbda. On the other hand, the ligands dmp, Br-dmp, and dsbtmp are all phenanthroline-based contg. alkyl substituents and contribute less than dbda to the overall redox properties.
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    40. 40
      Kaeser, A.; Delavaux-Nicot, B.; Duhayon, C.; Coppel, Y.; Nierengarten, J.-F. Heteroleptic Silver(I) Complexes Prepared from Phenanthroline and Bis-Phosphine Ligands. Inorg. Chem. 2013, 52, 14343– 14354,  DOI: 10.1021/ic402342y
      40
      Heteroleptic Silver(I) Complexes Prepared from Phenanthroline and Bis-phosphine Ligands
      Kaeser, Adrien; Delavaux-Nicot, Beatrice; Duhayon, Carine; Coppel, Yannick; Nierengarten, Jean-Francois
      Inorganic Chemistry (2013), 52 (24), 14343-14354CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The heteroleptic coordination scenario of silver(I) with various phenanthroline ligands (NN) and different bis-phosphine (PP) derivs. was investigated. In addn. to the x-ray crystal structural characterization of the resulting mixed ligand Ag(I) complexes, detailed NMR studies were performed to disclose the behavior of the prepd. silver(I) complexes in soln. The results obtained with silver(I) were also systematically related to the one obtained for copper(I) with the same combination of PP and NN ligands. Starting from an equimolar mixt. of AgBF4, bis[(2-diphenylphosphino)phenyl] ether (POP), and 1,10-phenanthroline (phen), the mononuclear complex [Ag(POP)(phen)]+ was obtained as the tetrafluoroborate salt. By following the same exptl. procedure starting from bis(diphenylphosphino)methane (dppm) or 1,3-bis(diphenylphosphino)propane (dppp) as the PP ligand, dinuclear complexes with two bridging PP ligands, i.e., [Ag2(NN)2(μ-dppm)2]2+ and [Ag2(NN)2(μ-dppp)2]2+ with NN = phen or Bphen (bathophenanthroline), were isolated as the tetrafluoroborate salts. Surprisingly, by using an equimolar ratio of AgBF4, phen or Bphen, and 1,2-bis(diphenylphosphino)ethane (dppe), the corresponding monobridged diphosphine dinuclear complexes [Ag2(NN)2(μ-dppe)]2+ were obtained as the tetrafluoroborate salts. These compds. were also prepd. in excellent yield by using a more appropriate 2:1:2 (phen:dppe:Ag) stoichiometry. These results prompted the authors to also perform the reactions with dppm and dppp using a 1:2:2 (PP:NN:Ag) stoichiometry. Under these conditions, [Ag2(NN)2(μ-dppm)](BF4)2 (NN = phen or Bphen) and [Ag2(NN)2(μ-dppp)](BF4)2 (NN = phen or Bphen) were obtained upon crystn. When compared to their copper(I) analogs, the complexation scenario becomes more complex with silver(I) as the system tolerates also coordinatively frustrated metal ligand assemblies, i.e., with a trigonal coordination geometry. Depending on the stoichiometry or on the nature of the PP partner, silver(I) shows an adaptive capability leading to various complexes with different coordination geometries and compn. However, as in the case of copper(I), their soln. behavior is highly dependent on the relative thermodn. stability of the various possible complexes. In most of the cases, a single Ag(I) complex is obsd. in soln. and the NMR data are in a perfect agreement with their solid state structures. The dppp-contg. complexes are the only notable exception; both [Ag2(NN)2(μ-dppp)2](BF4)2 and [Ag2(NN)2(μ-dppp)](BF4)2 are stable in the solid state but a dynamic mixt. is obsd. as soon as these compds. are dissolved. Finally, whereas both dppe and dppp are chelating ligands for copper(I), it is not the case anymore with silver(I) for which a destabilization of species with chelating dppe and dppp ligands is clearly indicated.
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    41. 41
      Scaltrito, D. V.; Thompson, D. W.; O’Callaghan, J. A.; Meyer, G. J. MLCT Excited States of Cuprous Bis-Phenanthroline Coordination Compounds. Coord. Chem. Rev. 2000, 208, 243– 266,  DOI: 10.1016/S0010-8545(00)00309-X
      41
      MLCT excited states of cuprous bis-phenanthroline coordination compounds
      Scaltrito, Donald V.; Thompson, David W.; O'Callaghan, John A.; Meyer, Gerald J.
      Coordination Chemistry Reviews (2000), 208 (), 243-266CODEN: CCHRAM; ISSN:0010-8545. (Elsevier Science S.A.)
      A review with 79 refs. in which excited MLCT states of cuprous bis-phenanthroline compds. are discussed and contrasted with well-known MLCT behavior of (dπ)6 transition metal compds. Phenanthroline ligands coordinated to Cu(I) that are disubstituted in the 2- and 9-positions with alkyl or aryl groups, abbreviated Cu1(phen')2+, have long-lived excited states at room temp. The parent Cu1(phen)2+ compd. is non-emissive under the same conditions with a short excited state lifetime, τ < 10 ns. Disubstitution in the 2,9-positions stabilizes the Cu(I) state and increases the energy gap between the MLCT and the ground state. The prototypical and most well studied compd. is Cu1(dmp)2+, where dmp is 2,9-(CH3)2-1,10-phenanthroline. In dichloromethane soln. at room temp., Cu1(dmp)2+ displays broad MLCT absorption with λmax = 454 nm, a broad unstructured emission with λmax = 730 nm, and an excited state lifetime of 85 ns. The emission arises from two closely spaced MLCT excited states, sepd. in energy by 1800 cm-1, that behave as one state at room temp. Cu1(dmp)2+* excited states are quenched in the presence of Lewis bases and coordinating solvents. A 5-coordinate excited state complex, or exciplex, is proposed to account for temp. dependent quenching data. The substantial inner-sphere reorganizational energy changes that follow light excitation are novel features of these MLCT excited states.
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    42. 42
      Leandri, V.; Daniel, Q.; Chen, H.; Sun, L.; Gardner, J. M.; Kloo, L. Electronic and Structural Effects of Inner Sphere Coordination of Chloride to a Homoleptic Copper(II) Diimine Complex. Inorg. Chem. 2018, 57, 4556– 4562,  DOI: 10.1021/acs.inorgchem.8b00225
      42
      Electronic and Structural Effects of Inner Sphere Coordination of Chloride to a Homoleptic Copper(II) Diimine Complex
      Leandri, Valentina; Daniel, Quentin; Chen, Hong; Sun, Licheng; Gardner, James M.; Kloo, Lars
      Inorganic Chemistry (2018), 57 (8), 4556-4562CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The reaction of CuCl2 with 2,9-dimethyl-1,10-phenanthroline (dmp) does not give [Cu(dmp)2](Cl)2 but instead to [Cu(dmp)2Cl]Cl, a 5-coordinated complex, in which one chloride is directly coordinated to the metal center. Attempts at removing the coordinated chloride by changing the counterion by metathesis were unsuccessful and resulted only in the exchange of the non-coordinated chloride, as confirmed from a crystal structure anal. Complex [Cu(dmp)2Cl]PF6 exhibits a reversible cyclic voltammogram characterized by a significant peak splitting between the reductive and oxidative waves (0.85 and 0.60 V vs. NHE, resp.), with a half-wave potential E1/2 = 0.73 V vs. NHE. When reduced electrochem., the complex does not convert into [Cu(dmp)2]+, as one may expect. Instead, [Cu(dmp)2]+ was isolated as a product when the redn. of [Cu(dmp)2Cl]PF6 was performed with L-ascorbic acid, as confirmed by electrochem., NMR spectroscopy, and diffractometry. [Cu(dmp)2]2+ complexes can be synthesized starting from Cu(II) salts with weakly and non-coordinating counterions, such as perchlorate. Growth of [Cu(dmp)2](ClO4)2 crystals in acetonitrile results in a 5-coordinated complex, [Cu(dmp)2(MeCN)](ClO4)2, in which a solvent mol. is coordinated to the metal center. However, solvent coordination is assocd. with a dynamic de-coordination-coordination behavior upon redn. and oxidn. Hence, the cyclic voltammogram of [Cu(dmp)2(MeCN)]2+ is identical to the one of [Cu(dmp)2]+, if the measurements were performed in acetonitrile. Halide ions in precursors to Cu(II) metal-org. coordination compd. synthesis, and most likely also other multivalent coordination centers, are not readily exchanged when exposed to presumed strongly binding and chelating ligand, and thus special care needs to be taken with respect to product characterization.
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    43. 43
      Schmittel, M.; Ganz, A. Stable Mixed Phenanthroline Copper(i) Complexes. Key Building Blocks for Supramolecular Coordination Chemistry. Chem. Commun. 1997, 999– 1000,  DOI: 10.1039/a701509g
      43
      New building blocks for sensors and supramolecular arrays. 4. Stable mixed phenanthroline copper(I) complexes. Key building blocks for supramolecular coordination chemistry
      Schmittel, Michael; Ganz, Andrea
      Chemical Communications (Cambridge) (1997), (11), 999-1000CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      A simple procedure is presented that was used to synthesize, for the first time, pure mixed phenanthroline copper(I) complexes that do not exchange ligands. One of the phenanthroline ligands is 2,9-diaryl deriv. and the 2nd is a 4,7-disubstituted deriv., the latter of which may also be linked by a polyether linkage to give macrocyclic ligands. Half-wave potentials are reported for several of the complexes.
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    44. 44
      Miller, M. T.; Gantzel, P. K.; Karpishin, T. B. A Highly Emissive Heteroleptic Copper(I) Bis(Phenanthroline) Complex: [Cu(Dbp)(Dmp)]+ (Dbp = 2,9-Di- Tert -Butyl-1,10-Phenanthroline; Dmp = 2,9-Dimethyl-1,10-Phenanthroline). J. Am. Chem. Soc. 1999, 121, 4292– 4293,  DOI: 10.1021/ja9901415
      44
      A Highly Emissive Heteroleptic Copper(I) Bis(phenanthroline) Complex: [Cu(dbp)(dmp)]+ (dbp = 2,9-Di-tert-butyl-1,10-phenanthroline; dmp = 2,9-Dimethyl-1,10-phenanthroline)
      Miller, Mark T.; Gantzel, Peter K.; Karpishin, Timothy B.
      Journal of the American Chemical Society (1999), 121 (17), 4292-4293CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Highly emissive [Cu(dbp)(dmp)](PF6) (1; dbp = 2,9-di-tert-butyl-1,10-phenanthroline; dmp = 2,9-dimethyl-1,10-phenanthroline) was prepd. and its excited state lifetime and emission spectrum recorded. The quantum yield of 1 is 1.19 ± 0.25 times higher than that of [Ru(bpy)3](PF6)2.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK1MXitlCmtL0%253D&md5=f0c7049e20074d3e15699b9918ed6255
    45. 45
      Sandroni, M.; Kayanuma, M.; Planchat, A.; Szuwarski, N.; Blart, E.; Pellegrin, Y.; Daniel, C.; Boujtita, M.; Odobel, F. First Application of the HETPHEN Concept to New Heteroleptic Bis(Diimine) Copper(i) Complexes as Sensitizers in Dye Sensitized Solar Cells. Dalton Trans. 2013, 42, 10818– 10827,  DOI: 10.1039/c3dt50852h
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      First application of the HETPHEN concept to new heteroleptic bis(diimine) copper(I) complexes as sensitizers in dye sensitized solar cells
      Sandroni, Martina; Kayanuma, Megumi; Planchat, Aurelien; Szuwarski, Nadine; Blart, Errol; Pellegrin, Yann; Daniel, Chantal; Boujtita, Mohammed; Odobel, Fabrice
      Dalton Transactions (2013), 42 (30), 10818-10827CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      We report here the synthesis and full chem. and phys. characterizations of the first stable heteroleptic copper(I)-bis(diimine) complexes designed for implementation in dye sensitized solar cells (DSC). Thanks to the HETPHEN concept, pure and stable heteroleptic copper(i) complexes were isolated. Anchorage of the sensitizers was provided by 2,2'-biquinoline-4,4'-dicarboxylic acid (dcbqH2), while sterically challenged ligands 2,9-dimesityl-1,10-phenanthroline (L0) and N-hexyl-2,9-dimesityl-1,10-phenanthroline-[a:b]imidazo-(4'-dianisylaminophenyl) (L1) were used to complete the copper(I) coordination sphere. The resulting heteroleptic complexes C1 and C2 exhibit a broad MLCT transition spreading over a wide wavelength domain, esp. when adsorbed onto nanoparticulate TiO2 photoanodes, providing a rather comprehensive visible light collection. The corresponding DSC were evaluated under AM 1.5 simulated solar light and rather weak performances were obtained owing to small Jsc and Voc. This is due to a combination of low extinction coeff. and poor driving forces for the various interfacial processes. However, significant improvements in the performances were monitored upon ageing in the dark, likely due to beneficial reorganization of the dye monolayers. The possibility to isolate stable asym. systems paves the way for structurally assisted photo-induced charge injection from the chemisorbed copper(I) based sensitizers into the conduction band of TiO2, through charge vectorialization.
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    46. 46
      Sandroni, M.; Favereau, L.; Planchat, A.; Akdas-Kilig, H.; Szuwarski, N.; Pellegrin, Y.; Blart, E.; Le Bozec, H.; Boujtita, M.; Odobel, F. Heteroleptic Copper(i)-Polypyridine Complexes as Efficient Sensitizers for Dye Sensitized Solar Cells. J. Mater. Chem. A 2014, 2, 9944– 9947,  DOI: 10.1039/c4ta01755b
      46
      Heteroleptic copper(I)-polypyridine complexes as efficient sensitizers for dye sensitized solar cells
      Sandroni, Martina; Favereau, Ludovic; Planchat, Aurelien; Akdas-Kilig, Huriye; Szuwarski, Nadine; Pellegrin, Yann; Blart, Errol; Le Bozec, Hubert; Boujtita, Mohammed; Odobel, Fabrice
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2014), 2 (26), 9944-9947CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      The synthesis and the physico-chem. characterization of HETPHEN based heteroleptic copper(I)-bis(diimine) complexes are reported. For TiO2 based dye sensitized solar cells (DSCs), the latter display impressive photoconversion efficiencies (PCEs), unprecedented for first row transition metal coordination complexes.
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    47. 47
      Kabehie, S.; Stieg, A. Z.; Xue, M.; Liong, M.; Wang, K. L.; Zink, J. I. Surface Immobilized Heteroleptic Copper Compounds as State Variables That Show Negative Differential Resistance. J. Phys. Chem. Lett. 2010, 1, 589– 593,  DOI: 10.1021/jz900324f
      47
      Surface Immobilized Heteroleptic Copper Compounds as State Variables that Show Negative Differential Resistance
      Kabehie, Sanaz; Stieg, Adam Z.; Xue, Mei; Liong, Monty; Wang, Kang L.; Zink, Jeffrey I.
      Journal of Physical Chemistry Letters (2010), 1 (3), 589-593CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Surface immobilized bidentate heteroleptic Cu(I) compds. are synthesized using a surface outward sequential synthesis and are characterized using solid-state NMR and at. force microscopy (AFM). Through use of chem. redox agents, the reversible switching characteristics of SiO2-immobilized Cu(I) compds. (tetrahedral) to Cu(II) (square planar) are verified via UV-visible absorption spectroscopy and ESR. Elec. properties of this system are characterized via prepn. of a sandwich-type device using p+ silicon and conductive AFM (cAFM). Current-Voltage (I-V) spectroscopy demonstrates that this system reproducibly switches between Cu(I) and Cu(II) states at approx. -0.8 and 2.3 V.
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    48. 48
      Housecroft, C. E.; Constable, E. C. The Emergence of Copper(I)-Based Dye Sensitized Solar Cells. Chem. Soc. Rev. 2015, 44, 8386– 8398,  DOI: 10.1039/c5cs00215j
      48
      The emergence of copper(I)-based dye sensitized solar cells
      Housecroft, Catherine E.; Constable, Edwin C.
      Chemical Society Reviews (2015), 44 (23), 8386-8398CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      Since the discovery of Gratzel-type dye sensitized solar cells (DSCs) in the early 1990s, there has been an exponential growth in the no. of publications dealing with their optimization and new design concepts. Conventional Gratzel DSCs use ruthenium(II) complexes as sensitizers, and the highest photon-to-elec. current conversion efficiency for a ruthenium dye is ≈12%. However, ruthenium is both rare and expensive, and replacement by cheaper and more sustainable metals is desirable. In this Tutorial Review, we describe strategies for assembling copper(I) complexes for use as dyes in DSCs, a research area that has been active since ≈2008. We demonstrate design principles for (i) ligands to anchor the complex to a semiconductor surface and promote electron transfer from dye to semiconductor, and (ii) ancillary ligands to tune the light absorption properties of the dye and facilitate electron transfer from electrolyte to dye in the DSC. We assess the progress made in terms of light-harvesting and overall photoconversion efficiencies of copper(I)-contg. DSCs and highlight areas that remain ripe for development and improvement.
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    49. 49
      Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A. V.; Bloino, J.; Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratchian, H. P.; Ortiz, J. V.; Izmaylov, A. F.; Sonnenberg, J. L.; Williams-Young, D.; Ding, F.; Lipparini, F.; Egidi, F.; Goings, J.; Peng, B.; Petrone, A.; Henderson, T.; Ranasinghe, D.; Zakrzewski, V. G.; Gao, J.; Rega, N.; Zheng, G.; Liang, W.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Throssell, K.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M. J.; Heyd, J. J.; Brothers, E. N.; Kudin, K. N.; Staroverov, V. N.; Keith, T. A.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Millam, J. M.; Klene, M.; Adamo, C.; Cammi, R.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Farkas, O.; Foresman, J. B.; Fox, D. J. Gaussian 16, revision B.01; Gaussian, Inc.: Wallingford CT, 2016.
      There is no corresponding record for this reference.
    50. 50
      Yanai, T.; Tew, D. P.; Handy, N. C. A New Hybrid Exchange-Correlation Functional Using the Coulomb-Attenuating Method (CAM-B3LYP). Chem. Phys. Lett 2004, 393, 51– 57,  DOI: 10.1016/j.cplett.2004.06.011
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      A new hybrid exchange-correlation functional using the Coulomb-attenuating method (CAM-B3LYP)
      Yanai, Takeshi; Tew, David P.; Handy, Nicholas C.
      Chemical Physics Letters (2004), 393 (1-3), 51-57CODEN: CHPLBC; ISSN:0009-2614. (Elsevier Science B.V.)
      A new hybrid exchange-correlation functional named CAM-B3LYP is proposed. It combines the hybrid qualities of B3LYP and the long-range correction presented by Tawada et al. [J. Chem. Phys., in press]. We demonstrate that CAM-B3LYP yields atomization energies of similar quality to those from B3LYP, while also performing well for charge transfer excitations in a dipeptide model, which B3LYP underestimates enormously. The CAM-B3LYP functional comprises of 0.19 Hartree-Fock (HF) plus 0.81 Becke 1988 (B88) exchange interaction at short-range, and 0.65 HF plus 0.35 B88 at long-range. The intermediate region is smoothly described through the std. error function with parameter 0.33.
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    51. 51
      Figgen, D.; Rauhut, G.; Dolg, M.; Stoll, H. Energy-Consistent Pseudopotentials for Group 11 and 12 Atoms: Adjustment to Multi-Configuration Dirac–Hartree–Fock Data. Chem. Phys. 2005, 311, 227– 244,  DOI: 10.1016/j.chemphys.2004.10.005
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      Energy-consistent pseudopotentials for group 11 and 12 atoms: adjustment to multi-configuration Dirac-Hartree-Fock data
      Figgen, Detlev; Rauhut, Guntram; Dolg, Michael; Stoll, Hermann
      Chemical Physics (2005), 311 (1-2), 227-244CODEN: CMPHC2; ISSN:0301-0104. (Elsevier B.V.)
      Two-component relativistic pseudopotentials (i.e., scalar-relativistic and spin-orbit (SO) potentials) of the energy-consistent variety have been adjusted for the group 11 and 12 atoms Cu, Zn; Ag, Cd; Au, Hg, replacing the 1s-2p; 1s-3d; and 1s-4f cores, resp. The adjustment has been done for the valence-energy spectrum of (near-)neutral atoms, to ref. data from numerical all-electron four-component multi-configuration Dirac-Hartree-Fock (MCDHF) calcns., including the two-electron Breit interaction. For use in mol. calcns., the potentials have been supplemented by energy-optimized (12s12p9d3f2g)/[6s6p4d3f2g] valence basis sets. First benchmark applications of the potentials and basis sets are presented for at. excitation energies and SO splittings at a correlated level, and for ground and excited state spectroscopic properties of group 11 monohalides and group 12 dimers.
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    52. 52
      Peterson, K. A. Systematically Convergent Basis Sets with Relativistic Pseudopotentials. I. Correlation Consistent Basis Sets for the Post- d Group 13–15 Elements. J. Chem. Phys. 2003, 119, 11099– 11112,  DOI: 10.1063/1.1622923
      52
      Systematically convergent basis sets with relativistic pseudopotentials. I. Correlation consistent basis sets for the post-d group 13-15 elements
      Peterson, Kirk A.
      Journal of Chemical Physics (2003), 119 (21), 11099-11112CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      New correlation consistent-like basis sets have been developed for the post-d group 13-15 elements (Ga-As, In-Sb, Tl-Bi) employing accurate, small-core relativistic pseudopotentials. The resulting basis sets, which are denoted cc-pVnZ-PP, are appropriate for valence electron correlation and range in size from (8s7p7d)/[4s3p2d] for the cc-pVDZ-PP to (16s13p12d3f2g1h)/[7s7p5d3f2g1h] for the cc-pV5Z-PP sets. Benchmark calcns. on selected diat. mols. (As2, Sb2, Bi2, AsN, SbN, BiN, GeO, SnO, PbO, GaCl, InCl, TlCl, GaH, InH, and TlH) are reported using these new basis sets at the coupled cluster level of theory. Much like their all-electron counterparts, the cc-pVnZ-PP basis sets yield systematic convergence of total energies and spectroscopic consts. In several cases all-electron benchmark calcns. were also carried out for comparison. The results from the pseudopotential and all-electron calcns. were nearly identical when scalar relativity was accurately included in the all-electron work. Diffuse-augmented basis sets, aug-cc-pVnZ-PP, have also been developed and have been used in calcns. of the at. electron affinities.
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    53. 53
      Schäfers, F. The Crystal Monochromator Beamline KMC-1 at BESSY II. J. Large-Scale Res. Facil. JLSRF 2016, 2, A96  DOI: 10.17815/jlsrf-2-92
      There is no corresponding record for this reference.
    54. 54
      Johansson, E. M. J.; Hedlund, M.; Siegbahn, H.; Rensmo, H. Electronic and Molecular Surface Structure of Ru(Tcterpy)(NCS)3 and Ru(Dcbpy)2(NCS)2 Adsorbed from Solution onto Nanostructured TiO2: A Photoelectron Spectroscopy Study. J. Phys. Chem. B 2005, 109, 22256– 22263,  DOI: 10.1021/jp0525282
      54
      Electronic and Molecular Surface Structure of Ru(tcterpy)(NCS)3 and Ru(dcbpy)2(NCS)2 Adsorbed from Solution onto Nanostructured TiO2: A Photoelectron Spectroscopy Study
      Johansson, E. M. J.; Hedlund, M.; Siegbahn, H.; Rensmo, H.
      Journal of Physical Chemistry B (2005), 109 (47), 22256-22263CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      The element specificity of photoelectron spectroscopy (PES) has been used to compare the electronic and mol. structure of the dyes Ru(tcterpy)(NCS)3 (BD) and Ru(dcbpy)2(NCS)2 adsorbed from soln. onto nanostructured TiO2. Ru(dcbpy)2(NCS)2 was investigated in its acid (N3) and in its 2-fold deprotonated form (N719) having tetrabutylammonium (TBA+) as counterions. A comparison of the O1s spectra for the dyes indicates that the interactions through the carboxylate groups with the TiO2 surface are very similar for the dyes. However, we observe that some of the dye mols. also interact through the NCS groups when adsorbed at the TiO2 surface. Comparing the N719 and the N3 mol., the fraction of NCS groups interacting through the sulfur atoms is smaller for N719 than for N3. We also note that the counterion TBA+ is coadsorbed with the N719 and BD mols. although the amt. was smaller than expected from the mol. formulas. Comparing the valence levels for the dyes adsorbed on TiO2, the position of the highest occupied electronic energy level is similar for N3 and N719, while that for BD is lower by 0.25 eV relative to that of the other complexes.
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    55. 55
      Ida, T.; Ando, M.; Toraya, H. Extended Pseudo-Voigt Function for Approximating the Voigt Profile. J. Appl. Crystallogr. 2000, 33, 1311– 1316,  DOI: 10.1107/S0021889800010219
      55
      Extended pseudo-Voigt function for approximating the Voigt profile
      Ida, T.; Ando, M.; Toraya, H.
      Journal of Applied Crystallography (2000), 33 (6), 1311-1316CODEN: JACGAR; ISSN:0021-8898. (Munksgaard International Publishers Ltd.)
      The formula of the pseudo-Voigt function expressed by a weighted sum of Gaussian and Lorentzian functions is extended by adding 2 other types of peak functions to improve the accuracy when approximating the Voigt profile. The full width at half-max. (FWHM) values and mixing parameters of the Gaussian, the Lorentzian and the other 2 component functions in the extended formula can be approximated by polynomials of a parameter ρ = ΓL/(ΓG +ΓL), where ΓG and ΓL are the FWHM values of the deconvoluted Gaussian and Lorentzian functions, resp. The max. deviation of the extended pseudo-Voigt function from the Voigt profile is within 0.12% relative to the peak height when 6th-order polynomial expansions are used. The systematic errors of the integrated intensity ΓG and ΓL, estd. by fitting the extended formula to Voigt profiles, are typically less than 1/10 of the errors arising from the application of the original formula of the pseudo-Voigt approxn. proposed by Thompson et al. (1987), while the time required for computation of the extended formula is only about 2.5 relative to the computation time required for the original formula.
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    56. 56
      Scofield, J. H. Theoretical photoionization cross sections from 1 to 1500 keV; California University, 1973; p 5– 6.
      There is no corresponding record for this reference.
    57. 57
      Boschloo, G.; Hagfeldt, A. Characteristics of the Iodide/Triiodide Redox Mediator in Dye-Sensitized Solar Cells. Acc. Chem. Res. 2009, 42, 1819– 1826,  DOI: 10.1021/ar900138m
      57
      Characteristics of the Iodide/Triiodide Redox Mediator in Dye-Sensitized Solar Cells
      Boschloo, Gerrit; Hagfeldt, Anders
      Accounts of Chemical Research (2009), 42 (11), 1819-1826CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      Dye-sensitized solar cells (DSCs) are of interest because of their potential for low-cost solar energy conversion. Currently, the certified record efficiency of these solar cells is 11.1%, and measurements of their durability and stability suggest lifetimes exceeding 10 years under operational conditions. The DSC is a photoelectrochem. system: a monolayer of sensitizing dye is adsorbed onto a mesoporous TiO2 electrode, and the electrode is sandwiched together with a counter electrode. An electrolyte contg. a redox couple fills the gap between the electrodes. The redox couple is a key component of the DSC. The reduced part of the couple regenerates the photooxidized dye. The formed oxidized species diffuses to the counter electrode, where it is reduced. The photovoltage of the device depends on the redox couple because it sets the electrochem. potential at the counter electrode. The redox couple also affects the electrochem. potential of the TiO2 electrode through the recombination kinetics between electrons in TiO2 and oxidized redox species. The special properties of the I-/I3- redox couple in dye-sensitized solar cells were studied. It was the preferred redox couple since the beginning of DSC development and still yields the most stable and efficient DSCs. Overall, the iodide/triiodide couple has good soly., does not absorb too much light, has a suitable redox potential, and provides rapid dye regeneration. But what distinguishes I-/I3- from most redox mediators is the slow recombination kinetics between electrons in TiO2 and the oxidized part of the redox couple, triiodide. Certain dyes adsorbed at TiO2 catalyze this recombination reaction, presumably by binding I or triiodide. The std. potential of the iodide/triiodide redox couple is 0.35 V (vs. the normal H electrode, normal H electrode), and the oxidn. potential of the std. DSC-sensitizer (Ru(dcbpy)2(NCS)2) is 1.1 V. The driving force for redn. of oxidized dye is therefore as large as 0.75 V. This process leads to the largest internal potential loss in DSC devices. Overall efficiencies >15% might be achieved if half of this internal potential loss could be gained. The regeneration of oxidized dye with iodide gives the diiodide radical, I2-·. The redox potential of the I2-·/I- couple must therefore be considered when detg. the actual driving force for dye regeneration. The formed I2-· disproportionates to I3- and I-, which leads to a large loss in potential energy.
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    58. 58
      Franke, R.; Chassé, T.; Streubel, P.; Meisel, A. Auger Parameters and Relaxation Energies of Phosphorus in Solid Compounds. J. Electron Spectrosc. Relat. Phenom. 1991, 56, 381– 388,  DOI: 10.1016/0368-2048(91)85035-R
      58
      Auger parameters and relaxation energies of phosphorus in solid compounds
      Franke, R.; Chasse, T.; Streubel, P.; Meisel, A.
      Journal of Electron Spectroscopy and Related Phenomena (1991), 56 (4), 381-8CODEN: JESRAW; ISSN:0368-2048.
      The 1s, 2s, and 2p photoelectron binding energies and the KL2.3L2.3(1D2) Auger electron energies of P in 28 solid compds. were measured using a mixed x-ray source (Al-Ag). The modified Auger parameters, α, are presented in 2-dimensional chem. state plots. Extra-at. static relaxation energies were calcd. using Auger parameters ζ.
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    59. 59
      Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy: A Reference Book of Standard Spectra for Identification and Interpretation of XPS Data; Physical Electronics, Inc., 1992.
      There is no corresponding record for this reference.
    60. 60
      Colombo, A.; Dragonetti, C.; Roberto, D.; Valore, A.; Biagini, P.; Melchiorre, F. A Simple Copper(I) Complex and Its Application in Efficient Dye Sensitized Solar Cells. Inorg. Chim. Acta 2013, 407, 204– 209,  DOI: 10.1016/j.ica.2013.07.028
      60
      A simple copper(I) complex and its application in efficient dye sensitized solar cells
      Colombo, Alessia; Dragonetti, Claudia; Roberto, Dominique; Valore, Adriana; Biagini, Paolo; Melchiorre, Fabio
      Inorganica Chimica Acta (2013), 407 (), 204-209CODEN: ICHAA3; ISSN:0020-1693. (Elsevier B.V.)
      A novel copper(I) complex bearing two 6,6'-dimethyl-2,2'-bipyridine-4,4'-dibenzoic acid ligands was prepd. along with related deprotonated derivs. with Na+ or NBu4+ as counterion. Their performance as photosensitizer in dye sensitized solar cells was studied as a function of the anchoring group and the electrolyte compn., comparing it with that of the known copper(I) complex bearing two 6,6'-dimethyl-2,2'-bipyridine-4,4'-dicarboxylic acid ligands and the ruthenium benchmark N719. Interestingly, the novel protonated complex is characterized by the best overall power conversion efficiency (3.0%) reported up to now for a copper(I) complex.
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    61. 61
      Kaeser, A.; Delavaux-Nicot, B.; Duhayon, C.; Coppel, Y.; Nierengarten, J. F. Heteroleptic Silver(I) Complexes Prepared from Phenanthroline and Bis-Phosphine Ligands. Inorg. Chem. 2013, 52, 14343– 14354,  DOI: 10.1021/ic402342y
      61
      Heteroleptic Silver(I) Complexes Prepared from Phenanthroline and Bis-phosphine Ligands
      Kaeser, Adrien; Delavaux-Nicot, Beatrice; Duhayon, Carine; Coppel, Yannick; Nierengarten, Jean-Francois
      Inorganic Chemistry (2013), 52 (24), 14343-14354CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The heteroleptic coordination scenario of silver(I) with various phenanthroline ligands (NN) and different bis-phosphine (PP) derivs. was investigated. In addn. to the x-ray crystal structural characterization of the resulting mixed ligand Ag(I) complexes, detailed NMR studies were performed to disclose the behavior of the prepd. silver(I) complexes in soln. The results obtained with silver(I) were also systematically related to the one obtained for copper(I) with the same combination of PP and NN ligands. Starting from an equimolar mixt. of AgBF4, bis[(2-diphenylphosphino)phenyl] ether (POP), and 1,10-phenanthroline (phen), the mononuclear complex [Ag(POP)(phen)]+ was obtained as the tetrafluoroborate salt. By following the same exptl. procedure starting from bis(diphenylphosphino)methane (dppm) or 1,3-bis(diphenylphosphino)propane (dppp) as the PP ligand, dinuclear complexes with two bridging PP ligands, i.e., [Ag2(NN)2(μ-dppm)2]2+ and [Ag2(NN)2(μ-dppp)2]2+ with NN = phen or Bphen (bathophenanthroline), were isolated as the tetrafluoroborate salts. Surprisingly, by using an equimolar ratio of AgBF4, phen or Bphen, and 1,2-bis(diphenylphosphino)ethane (dppe), the corresponding monobridged diphosphine dinuclear complexes [Ag2(NN)2(μ-dppe)]2+ were obtained as the tetrafluoroborate salts. These compds. were also prepd. in excellent yield by using a more appropriate 2:1:2 (phen:dppe:Ag) stoichiometry. These results prompted the authors to also perform the reactions with dppm and dppp using a 1:2:2 (PP:NN:Ag) stoichiometry. Under these conditions, [Ag2(NN)2(μ-dppm)](BF4)2 (NN = phen or Bphen) and [Ag2(NN)2(μ-dppp)](BF4)2 (NN = phen or Bphen) were obtained upon crystn. When compared to their copper(I) analogs, the complexation scenario becomes more complex with silver(I) as the system tolerates also coordinatively frustrated metal ligand assemblies, i.e., with a trigonal coordination geometry. Depending on the stoichiometry or on the nature of the PP partner, silver(I) shows an adaptive capability leading to various complexes with different coordination geometries and compn. However, as in the case of copper(I), their soln. behavior is highly dependent on the relative thermodn. stability of the various possible complexes. In most of the cases, a single Ag(I) complex is obsd. in soln. and the NMR data are in a perfect agreement with their solid state structures. The dppp-contg. complexes are the only notable exception; both [Ag2(NN)2(μ-dppp)2](BF4)2 and [Ag2(NN)2(μ-dppp)](BF4)2 are stable in the solid state but a dynamic mixt. is obsd. as soon as these compds. are dissolved. Finally, whereas both dppe and dppp are chelating ligands for copper(I), it is not the case anymore with silver(I) for which a destabilization of species with chelating dppe and dppp ligands is clearly indicated.
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    62. 62
      De Angelis, F.; Fantacci, S.; Mosconi, E.; Nazeeruddin, M. K.; Grätzel, M. Absorption Spectra and Excited State Energy Levels of the N719 Dye on TiO2 in Dye-Sensitized Solar Cell Models. J. Phys. Chem. C 2011, 115, 8825– 8831,  DOI: 10.1021/jp111949a
      62
      Absorption Spectra and Excited State Energy Levels of the N719 Dye on TiO2 in Dye-Sensitized Solar Cell Models
      De Angelis, Filippo; Fantacci, Simona; Mosconi, Edoardo; Nazeeruddin, Mohammad K.; Gratzel, Michael
      Journal of Physical Chemistry C (2011), 115 (17), 8825-8831CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      We have investigated the absorption spectrum and the alignment of ground and excited state energies for the prototypical N719 Ru(II) sensitizer adsorbed on an extended TiO2 model by means of high level DFT/TDDFT calcns. The calcd. and exptl. absorption spectra for the dye on TiO2 are in excellent agreement over the explored energy range, with an absorption max. deviation below 0.1 eV, allowing us to assign the underlying electronic transitions. We find the lowest optically active excited state to lie ∼0.3 eV above the lowest TiO2 state. This state has a sizable contribution from the dye π* orbitals, strongly mixed with unoccupied TiO2 states. A similarly strong coupling is calcd. for the higher-lying transitions constituting the visible absorption band centered at ∼530 nm in the combined system. An ultrafast, almost instantaneous, electron injection component can be predicted on the basis of the strong coupling and of the matching of the visible absorption spectrum and d. of TiO2 unoccupied states. Surprisingly, this almost direct injection mechanism, corresponding to excitation from the dye ground state to an excited state largely delocalized within the semiconductor, is found to give rise to almost exactly the same absorption profile as for the dye in soln., despite the drastically different nature of the underlying excited states. On the basis of our calcns. it seems therefore that no sizable lower bound to an injection time exists, rather the timings of electron injection are mainly ruled by electron dephasing in the semiconductor.
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    63. 63
      Murali, M. G.; Wang, X.; Wang, Q.; Valiyaveettil, S. Design and Synthesis of New Ruthenium Complex for Dye-Sensitized Solar Cells. RSC Adv. 2016, 6, 57872– 57879,  DOI: 10.1039/C6RA10881D
      63
      Design and synthesis of new ruthenium complex for dye-sensitized solar cells
      Murali, M. G.; Wang, Xingzhu; Wang, Qing; Valiyaveettil, Suresh
      RSC Advances (2016), 6 (63), 57872-57879CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      A series of ruthenium complexes (MC-1-MC-3), incorporated with carbazole, fluorene and phenothiazine units with dipyrido[3,2-a:2',3'-c]phenazine are synthesized, characterized and their optical, electrochem. and photovoltaic properties are investigated. The obsd. differences in light-harvesting ability of the sensitizers are assocd. with the electron donor strength of the ancillary ligand used for prepg. complexes. The dye-sensitized solar cell fabricated from complex MC-1 exhibited a power conversion efficiency of 6.18%. It is demonstrated that new mol. design and increase in molar absorption coeff. of the sensitizer improved the device performance.
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    64. 64
      Ji, J.-M.; Zhou, H.; Kim, H. K. Rational Design Criteria for D−π–A Structured Organic and Porphyrin Sensitizers for Highly Efficient Dye-Sensitized Solar Cells. J. Mater. Chem. A 2018, 6, 14518– 14545,  DOI: 10.1039/C8TA02281J
      64
      Rational design criteria for D-π-A structured organic and porphyrin sensitizers for highly efficient dye-sensitized solar cells
      Ji, Jung-Min; Zhou, Haoran; Kim, Hwan Kyu
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2018), 6 (30), 14518-14545CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Increasing energy consumption worldwide and environmental concerns about global warming have attracted great interest in the development of renewable and eco-friendly energy technologies. Dye-sensitized solar cells (DSSCs) have attracted considerable attention over the last 25 years since they offer possible low-cost conversion of photovoltaic energy. The sensitizer is the most important component of a DSSC, as it is largely responsible for light harvesting and charge sepn., as well as the dye regeneration process. As a result, there have been tremendous research efforts in developing sensitizers. However, many challenges remain, and a deeper understanding of the design rules of DSSC sensitizers is required to obtain efficient and long-term stable DSSCs. The purpose of this review is to discuss recent progress and the rational design criteria used in the structural design of org. dyes and porphyrin photosensitizers for use in DSSCs. The effects of mol. structural engineering on the photophys. and electrochem. properties, photovoltaic parameters, and efficiency of DSSCs are presented.
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    65. 65
      Yang, J.; Ganesan, P.; Teuscher, J.; Moehl, T.; Kim, Y. J.; Yi, C.; Comte, P.; Pei, K.; Holcombe, T. W.; Nazeeruddin, M. K.; Hua, J.; Zakeeruddin, S. M.; Tian, H.; Grätzel, M. Influence of the Donor Size in D-π-A Organic Dyes for Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2014, 136, 5722– 5730,  DOI: 10.1021/ja500280r
      65
      Influence of the Donor Size in D-π-A Organic Dyes for Dye-Sensitized Solar Cells
      Yang, Jiabao; Ganesan, Paramaguru; Teuscher, Joel; Moehl, Thomas; Kim, Yong Joo; Yi, Chenyi; Comte, Pascal; Pei, Kai; Holcombe, Thomas W.; Nazeeruddin, Mohammad Khaja; Hua, Jianli; Zakeeruddin, Shaik M.; Tian, He; Gratzel, Michael
      Journal of the American Chemical Society (2014), 136 (15), 5722-5730CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The authors report 2 new molecularly engineered push-pull dyes, i.e., YA421 and YA422, based on substituted quinoxaline as a π-conjugating linker and bulky-indoline moiety as donor and compared with reported IQ4 dye. Benefitting from increased steric hindrance with the introduction of bis(2,4-dihexyloxy)benzene substitution on the quinoxaline, the electron recombination between redox electrolyte and the TiO2 surface is reduced, esp. in redox electrolyte employing Co(II/III) complexes as redox shuttles. The open circuit photovoltages of IQ4, YA421, and YA422 devices with Co-based electrolyte are higher than those with iodide/triiodide electrolyte by 34, 62, and 135 mV, resp. Also, the cells employing graphene nanoplatelets on top of Au spattered film as a counter electrode (CE) show lower charge-transfer resistance compared to Pt as a CE. Consequently, YA422 devices deliver the best power conversion efficiency due to higher fill factor, reaching 10.65% at AM 1.5 simulated sunlight. Electrochem. impedance spectroscopy and transient absorption spectroscopy anal. were performed to understand the electrolyte influence on the device performances with different counter electrode materials and donor structures of donor-π-acceptor dyes. Laser flash photolysis expts. indicate that even though the dye regeneration of YA422 is slower than that of the other 2 dyes, the slower back electron transfer of YA422 contributes to the higher device performance.
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    66. 66
      Lu, J.; Liu, S.; Wang, M. Push-Pull Zinc Porphyrins as Light-Harvesters for Efficient Dye-Sensitized Solar Cells. Front. Chem. 2018, 6, 541  DOI: 10.3389/fchem.2018.00541
      66
      Push-pull zinc porphyrins as light-harvesters for efficient dye-sensitized solar cells
      Lu, Jianfeng; Liu, Shuangshuang; Wang, Mingkui
      Frontiers in Chemistry (Lausanne, Switzerland) (2018), 6 (), 541CODEN: FCLSAA; ISSN:2296-2646. (Frontiers Media S.A.)
      A review. Dye-sensitized solar cell (DSSC) has been attractive to scientific community due to its eco-friendliness, ease of fabrication, and vivid colorful property etc. Among various kinds of sensitizers, such as metal-free org. mols., metal-complex, natural dyes etc., porphyrin is one of the most promising sensitizers for DSSC. The first application of porphyrin for sensitization of nanocrystaline TiO2 can be traced back to 1993 by using [tetrakis(4-carboxyphenyl) porphyrinato] zinc(II) with an overall conversion efficiency of 2.6%. After 10 years efforts, Officer and Gr.ovrddot.atzel improved this value to 7.1%. Later in 2009, by constructing porphyrin sensitizer with an arylamine as donor and a benzoic acid as acceptor, Diau and Yeh demonstrated that this donor-acceptor framework porphyrins could attain remarkable photovoltaic performance. Now the highest efficiencies of DSSC are dominated by donor-acceptor porphyrins, reaching remarkable values around 13.0% with cobalt-based electrolytes. This achievement is largely contributed by the structural development of donor and acceptor groups within push-pull framework. In this review, we summarized and discussed the development of donor-acceptor porphyrin sensitizers and their applications in DSSC. A discussion of the correlation between mol. structure and the spectral and photovoltaic properties is the major target of this review. Deeply dicussion of the substitution group, esp. on porphyrin's meso-position were presented. Furthermore, the limitations of DSSC for commercialization, such as the long-term stability, sophisticated synthesis procedures for high efficiency dye etc., have also been discussed.
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    67. 67
      Risi, G.; Becker, M.; Housecroft, C. E.; Constable, E. C. Are Alkynyl Spacers in Ancillary Ligands in Heteroleptic Bis(Diimine)Copper(I) Dyes Beneficial for Dye Performance in Dye-Sensitized Solar Cells?. Molecules 2020, 25, 1528  DOI: 10.3390/molecules25071528
      67
      Are alkynyl spacers in ancillary ligands in heteroleptic bis(diimine)copper(I) dyes beneficial for dye performance in dye-sensitized solar cells?
      Risi, Guglielmo; Becker, Mariia; Housecroft, Catherine E.; Constable, Edwin C.
      Molecules (2020), 25 (7), 1528CODEN: MOLEFW; ISSN:1420-3049. (MDPI AG)
      The syntheses of 4,4'-bis(4-dimethylaminophenyl)-6,6'-dimethyl-2,2'-bipyridine (1), 4,4'-bis(4-dimethylaminophenylethynyl)-6,6'-dimethyl-2,2'-bipyridine (2), 4,4'-bis(4- diphenylaminophenyl)-6,6'-dimethyl-2,2'-bipyridine (3), and 4,4'-bis(4-diphenylaminophenylethynyl)- 6,6'-dimethyl-2,2'-bipyridine (4) are reported along with the prepns. and characterizations of their homoleptic copper(I) complexes [CuL2][PF6] (L = 1-4). The soln. absorption spectra of the complexes exhibit ligand-centered absorptions in addn. to absorptions in the visible region assigned to a combination of intra-ligand and metal-to-ligand charge-transfer. Heteroleptic [Cu(5)(Lancillary)]+ dyes in which 5 is the anchoring ligand ((6,6'-dimethyl-[2,2'-bipyridine]- 4,4'-diyl)bis(4,1-phenylene))bis(phosphonic acid) and Lancillary = 1-4 have been assembled on fluorine-doped tin oxide (FTO)-TiO2 electrodes in dye-sensitized solar cells (DSCs). Performance parameters and external quantum efficiency (EQE) spectra of the DSCs (four fully-masked cells for each dye) reveal that the best performing dyes are [Cu(5)(1)]+ and [Cu(5)(3)]+. The alkynyl spacers are not beneficial, leading to a decrease in the short-circuit c.d. (JSC), confirmed by lower values of EQEmax. Addn. of a co-absorbent (n-decylphosphonic acid) to [Cu(5)(1)]+ lead to no significant enhancement of performance for DSCs sensitized with [Cu(5)(1)]+. Electrochem. impedance spectroscopy (EIS) has been used to investigate the interfaces in DSCs; the anal. shows that more favorable electron injection into TiO2 is obsd. for sensitizers without the alkynyl spacer and confirms higher JSC values for [Cu(5)(1)]+.
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    68. 68
      McCusker, C. E.; Castellano, F. N. Design of a Long-Lifetime, Earth-Abundant, Aqueous Compatible Cu(I) Photosensitizer Using Cooperative Steric Effects. Inorg. Chem. 2013, 52, 8114– 8120,  DOI: 10.1021/ic401213p
      68
      Design of a Long-Lifetime, Earth-Abundant, Aqueous Compatible Cu(I) Photosensitizer Using Cooperative Steric Effects
      McCusker, Catherine E.; Castellano, Felix N.
      Inorganic Chemistry (2013), 52 (14), 8114-8120CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      A new homoleptic Cu(I) photosensitizer, [Cu(dsbtmp)2]+ (dsbtmp = 2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline), designed to exhibit cooperative steric hindrance, unexpectedly produced strong photoluminescence (Φ = 1.9-6.3%) and long excited state lifetimes (τ = 1.2-2.8 μs) in a broad range of coordinating and noncoordinating solvents. The combination of the 2,9-sec-Bu substituents with the neighboring 3,8-Me groups led to a Cu(I) complex with small degrees of ground and excited state distortion ultimately producing a mol. with robust metal-to-ligand charge transfer photophysics largely insulated from solvent interactions, reversible redox chem. serving as a strong excited state reductant, along with impressive thermodn. and photochem. stability in soln.
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    69. 69
      Ruthkosky, M.; Castellano, F. N.; Meyer, G. J. Photodriven Electron and Energy Transfer from Copper Phenanthroline Excited States. Inorg. Chem. 1996, 35, 6406– 6412,  DOI: 10.1021/ic960503z
      69
      Photodriven Electron and Energy Transfer from Copper Phenanthroline Excited States
      Ruthkosky, Mark; Castellano, Felix N.; Meyer, Gerald J.
      Inorganic Chemistry (1996), 35 (22), 6406-6412CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Electron and energy transfer from copper 1,10-phenanthroline excited states is obsd. at room temp. in org. solvents. The copper phenanthroline excited states are metal-to-ligand charge-transfer in nature and have lifetimes of ∼70-250 ns in dichloromethane soln. if Me or Ph substituents are placed in the 2- and 9-positions of the phenanthroline ligand. The unsubstituted cuprous compd. Cu(phen)2(PF6) is non-emissive under these conditions, and the excited state lifetime is <20 ns. The rate and efficiency of energy transfer to anthracene or electron transfer to viologens is reported. The cage escape efficiency of [Cu(dpp)22+, MV+•], where dpp is 2,9-diphenyl-1,10-phenanthroline, is close to unity within exptl. error. Back electron transfer to ground state products occurs at the diffusion limit, 2 × 1010 M-1 s-1.
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsaem.1c02778.

    • XPS spectra; detailed photovoltaic parameters of nonoptimized DSSCs with different electrolyte compositions; DFT-calculated ground-state geometries of the dyes; absorption spectra of the dyes on TiO2; and N719 DSSC lifetime (PDF)


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