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Impact of Oligo(Ethylene Glycol) Side Chains on the Thermoelectric Properties of Naphthalenediimide–Dialkoxybithiazole Polymers
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  • Xuwen Yang
    Xuwen Yang
    Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, Groningen 9747 AG, The Netherlands
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  • Gang Ye*
    Gang Ye
    Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei Key Laboratory of Polymer Materials, School of Materials Science and Engineering, Hubei University, Wuhan, 430062, China
    State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, P. R. China
    Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, Groningen 9747 AG, The Netherlands
    *E-mail: [email protected]
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  • Karolina Tran
    Karolina Tran
    Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, Groningen 9747 AG, The Netherlands
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    Orcidhttps://orcid.org/0000-0001-5432-5204
  • Yuru Liu
    Yuru Liu
    Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, Groningen 9747 AG, The Netherlands
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    Orcidhttps://orcid.org/0000-0003-4909-7782
  • Jiamin Cao
    Jiamin Cao
    Key Laboratory of Theoretical Organic Chemistry and Functional Molecule of Ministry of Education, School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China
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    Orcidhttps://orcid.org/0000-0003-2069-6976
  • Jingjin Dong
    Jingjin Dong
    Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, Groningen 9747 AG, The Netherlands
    Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, Nanjing, Jiangsu 211816, China
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  • Giuseppe Portale
    Giuseppe Portale
    Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, Groningen 9747 AG, The Netherlands
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    Orcidhttps://orcid.org/0000-0002-4903-3159
  • Jian Liu
    Jian Liu
    State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, P. R. China
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    Orcidhttps://orcid.org/0000-0002-6704-3895
  • Ping Zhang
    Ping Zhang
    School of Electrical Engineering and Automation, Jiangxi University of Science and Technology, Ganzhou, Jiangxi 341000, China
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  • Maria Antonietta Loi
    Maria Antonietta Loi
    Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, Groningen 9747 AG, The Netherlands
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    Orcidhttps://orcid.org/0000-0002-7985-7431
  • Ryan C. Chiechi*
    Ryan C. Chiechi
    Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, Groningen 9747 AG, The Netherlands
    Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States
    *E-mail: [email protected]
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    Orcidhttps://orcid.org/0000-0002-0895-2095
  • L. Jan Anton Koster*
    L. Jan Anton Koster
    Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, Groningen 9747 AG, The Netherlands
    *E-mail: [email protected]
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    Orcidhttps://orcid.org/0000-0002-6558-5295
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ACS Materials Letters

Cite this: ACS Materials Lett. 2024, 6, 4, 1207–1215
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https://doi.org/10.1021/acsmaterialslett.4c00068
Published February 29, 2024

Copyright © 2024 The Authors. Published by American Chemical Society. This publication is licensed under

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Organic thermoelectric materials have garnered significant interest as promising candidates for energy harvesting applications. In recent years, ethylene-glycol side-chain engineering in organic semiconductors has gradually become an efficient approach to boost the performance of organic thermoelectrics. Although this strategy is widely utilized, the impact of their volume and branching structure remains unknown. This contribution describes a trade-off phenomenon between the oligo(ethylene glycol) (OEG) side chains and thermoelectric properties based on the n-type doped low-bandgap conjugated polymers, achieved through the modification of the volume and structure of side chains. Three conjugated polymers comprising a naphthalenediimide-dialkoxybithiazole backbone and different linear length or branched OEG side chains exhibit good host/dopant miscibility after doping. We find that, in the linear OEG side-chain-based polymers, the increased volume of side chains slightly influences the planarity of backbones, thereby leading to similar and satisfactory thermoelectric performances. The high fraction of side chains does not consistently yield enhanced performance, as the branched OEG side-chain introduces steric hindrance. Consequently, the accordingly conjugated backbones become less planar and rigid, resulting in critical molecular packing changes and low charge carrier mobility and doping efficiency and thus low thermoelectric performance. Our work provides a unique insight into the fundamental understanding of the relationship between molecular packing and thermoelectric properties and guides the future rational design of efficient n-type organic semiconductors.

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Organic thermoelectric (OTE) materials have attracted attention recently as potential energy generators because of their mechanical flexibility, low toxicity, low-cost energy generation, and printing techniques. (1−5) Thermoelectric efficiency is defined by the figure of merit (ZT = S2σT/κ, where S, σ, T, and κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively). (6,7) The power factor (PF = S2σ) is another metric used to evaluate thermoelectric performance in organic semiconductors, particularly when measuring κ is complicated or κ does not vary considerably when the other parameters are optimized. (8,9) Currently, the thermoelectric properties and mechanisms of p-type thermoelectric materials have been well investigated; (10−16) however, research into n-type thermoelectric materials still lags substantially behind that of p-type thermoelectric materials. Developing n-type doped semiconducting polymers with high thermoelectric performance is critical to ensuring further practical applications in thermoelectric generators.

Advances in molecular design and process engineering over the past decade have resulted in much improved n-type doping efficiency and thermoelectric performance. One key strategy for optimizing the PF is to modify conjugated backbones, which influence the electronic structures, frontier energy levels, and interchain interactions. The Bao group (17) proposed a breakthrough design concept in n-channel thermoelectric polymers in 2016 with the development of an acceptor–acceptor-type conjugated polymer rather than the conventional donor–acceptor (D–A) backbone. Following this trend, numerous acceptor–acceptor or acceptor–weak donor-type semiconducting conjugated polymers based on strong acceptor building blocks, such as naphthalenediimide (NDI), (18−25) diketopyrrolopyrrole (DPP), (26−28) benzodifurandione-based oligo(p-phenylenevinylene) (BDOPV), (29−33) bithiophene imide (BTI), (34−37) and double B←N bridged bipyridine (BNBP), (38) have been developed for thermoelectric applications. For example, Pei and coauthors (29) developed an acceptor–acceptor character BDOPV-based conjugated polymer for thermoelectrics, achieving a high n-type doping level, high σ (>90 S cm–1), and high PF value (106 μW m–1 K–2). The Fabiano group (18) proposed another cardinal molecular design principle for n-type thermoelectric polymers in 2018, in which fully ladder-conjugated polymers are employed because of their torsion-free backbone and large polaron delocalization length. Following this concept, highly n-doped thermoelectric materials with a fully fused conjugated backbone, such as polybenzimidazobenzophenanthroline (BBL), (39,40) poly(p-phenylenevinylene) derivatives (LPPV), (41) naphthalene–naphthalene (N–N), (42−44) and poly(benzodifurandione) (PBFDO), (45) were developed and achieved high performance.

In addition to the development of new conjugated backbones, side-chain engineering is extremely important in increasing host–dopant miscibility and charge transport properties. Conjugated polymer side chains not only enhance polymer solubility in organic solvents but also affect interchain packing and thin film morphology. For instance, Takimiya and coauthors (46) reported two acceptor–acceptor-type thermoelectric conjugated polymers with different alkyl side-chain branching positions. They found that the polymer with a one carbon atom farther branching position avoided the bulky effect and exhibited stronger molecular packing in both pristine and doped films, which is important for charge carrier transport. Zhang and coauthors (19) recently modulated the performance of n-doped thermoelectric materials by varying the ratio of linear/branching alkyl side chains to reduce the steric hindrance of the bulky side chain and promote interchain packing, thereby enhancing charge carrier mobility and thermoelectric performance. It was recently discovered (21,22,24,25,47−49) that host–dopant miscibility in n-doped thermoelectric materials can be enhanced simply by replacing traditional alkyl side chains with ethylene glycol-type side chains, which possess a higher dipole moment and polarity that promotes dopant solubility in the polymer matrix, thereby increasing doping efficiency and σ. Although oligo(ethylene glycol) (OEG) side chains are frequently employed to increase the polarity of polymers, the impact of their volume and branching has not yet been studied. Thus, a concise investigation of the influence of side chain volume and configuration in n-type OTE materials is required.

In this contribution, we report that n-type OTEs show a composed behavior by modifying the volume and structures of the OEG side chains. A series of conjugated polymers comprising a naphthalenediimide–dialkoxybithiazole (NDI-2Tz) backbone with OEG side chains having different volumes (PNDI2TEG-2Tz and PNDI2HexEG-2Tz) or a branched structure (PNDI2BTEG-2Tz) were designed and used as the host matrix, with 4-(2,3-dihydro-1,3-dimethyl-1H-benzoimidazol-2-yl)phenyl–N,N-dimethylbenzenamine (N-DMBI) used as the dopant. We found that all polymers exhibited good host–dopant miscibility without phase separation, owing to functionalization with the polar OEG side chains. Moreover, the thermoelectric parameters were maintained as the number of the OEG units in the linear side chains increased because of the excellent intermolecular packing. Conversely, the branched side chains hindered molecular packing due to the twisted backbones, causing reduced doping efficiency and a simultaneous decrease in conductivity and PF. This investigation offers unique insight into the fundamental understanding of molecular packing and provides guidance for the future rational design of efficient n-type organic semiconductors.

Figure 1 presents the chemical structures of the NDI-2Tz-based copolymers functionalized with different polar OEG side chains (PNDI2TEG-2Tz (P-3O), PNDI2HexEG-2Tz (P-6O), and PNDI2BTEG-2Tz (P-B3O)) and the n-type dopant, N-DMBI. The synthesis of P-3O (50) and P-6O (24) was reported in our previous work. The synthetic route and synthesis details for P-B3O and the corresponding monomer are in the Supporting Information (Schemes S1 and S2 and Figures S1–S3). The polymers were synthesized via a copper iodide-assisted palladium-catalyzed Stille cross coupling polycondensation of a dibromo-NDI-based monomer with a distannyl-alkoxybithiazole-based monomer after refluxing the polymerization mixture for 3 days. The polymers were purified using a Soxhlet extractor and continuous extraction with hot methanol, followed by hexane and chloroform, to remove impurities and low-molecular-weight fractions. Finally, the polymers with high molecular weight (P-3O with Mn = 46 kDa, P-6O with Mn = 21 kDa, and P-B3O with Mn = 72 kDa) were extracted using chloroform (Figure S4, Supporting Information). Then, the polymer was dissolved, precipitated into cold methanol, collected, and dried in vacuo. The structures were characterized using proton nuclear magnetic resonance (1HNMR) and Fourier-transform infrared (FT-IR) spectroscopy (Figures S5 and S6, Supporting Information). The relative molecular weights and dispersities were determined using gel permeation chromatography (GPC) against polystyrene standards with hexafluoroisopropyl alcohol (HFIP) as the eluent. The resulting data are listed in Figure S6 (Supporting Information).

Figure 1

Figure 1. Chemical structures of NDI-2Tz-based D–A copolymers with different substituted OEG side chains and n-type dopant N-DMBI.

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We performed thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) to evaluate the thermal properties of the three NDI-2Tz-based conjugated polymers (Figure S7, Supporting Information). The temperature at 5% weight loss was considered the onset point of decomposition (Td). All copolymers exhibited excellent thermal stability with Td values of 350 °C, 350 °C, and 347 °C for P-3O, P-6O, and P-B3O, respectively. These Td values indicated that all copolymers were sufficiently stable for thermoelectric device applications. No distinct transitions were observed in the DSC curves of the polymers, revealing the absence of significant degrees of crystallinity or phase transitions across the measured temperature range.

Figure 2 shows ultraviolet–visible-near-infrared (UV–vis-NIR) absorption spectroscopy for pristine P-3O, P-6O, and P-B3O in chloroform and thin films. All polymer pristine samples exhibited two characteristic neutral absorption bands in both solution and thin film states. We assigned the high-energy bands from 300 to 600 nm to the π–π* transition and the low-energy bands between 600 and 1200 nm to intramolecular charge transfer (ICT). The absorption maxima of P-3O, P-6O, and P-B3O in solution were located at 930, 967, and 807 nm, respectively. The P-3O, P-6O, and P-B3O solid thin films all showed red shifts at the absorption maxima (972, 972, and 914 nm, respectively) due to the enhanced interchain π–π stacking in the solid state. Compared with P-3O and P-6O with linear OEG side chains, the absorption maxima peak of P-B3O with branched OEG side chains displayed a slight bathochromic shift, indicating that the branched OEG side chains could have a steric effect, influencing the π-conjugation efficiency along the polymer backbone.

Figure 2

Figure 2. Normalized UV–vis–NIR absorption spectra of P-3O, P-6O, and P-B3O in dilute chloroform (CHCl3) (10–5 M) and in films (spin coated from CHCl3 solutions).

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Previous research (51) has demonstrated that the rigidity and coplanarity of the conjugated polymer backbone could be determined from the absorption properties. Conjugated polymers with flexible and nonplanar backbones will become more planar in the solid state due to interchain π–π stacking, leading to a red shift in the solid state absorption spectra. As shown in Figure S8 and Table 1, P-6O and P-3O did not exhibit a noticeable red shift, suggesting that the P-6O molecule has a rigid and coplanar backbone configuration in both the solution and solid states. Conversely, P-B3O exhibited an obvious red shift compared to those of P-3O and P-6O, suggesting that it has a relatively flexible and nonplanar backbone conformation. This observed outcome also accounts for the considerable red shift in P-B3O relative to those of P-6O and P-3O in the solution state. In addition, P-B3O showed stronger absorption in the π–π* transition region than P-3O and P-6O in both solution and thin film states, indicating that the ICT in P-B3O was not as efficient as in P-3O and P-6O, resulting from its nonplanar molecular backbone conformation. Consequently, differences in molecular structure were the primary reason for the observed outcomes in our case. Because P-3O, P-6O, and P-B3O share the same backbone molecular structure, the different absorption profiles of the polymers were the result of their different side chains. As previously reported, the incorporation of side chains alters the planarity of polymer molecules with identical backbones and thus the molecular order. (52−55) In our case, the branched OEG side chain may have caused disordered molecular packing in P-B3O, which did not occur in the linear OEG side chain-substituted polymers. The optical band gaps of P-3O, P-6O, and P-B3O calculated from the thin film absorption onsets were 1.03, 0.95, and 0.96 eV, respectively.

Table 1. Optical Properties, Electrochemical Properties, and Energy Levels of the NDI-2Tz-Based Conjugated Polymers
polymersλmaxsol. (nm)λmaxfilm (nm)λonsetfilm (nm)red-shift (nm)Egopt. (eV)Eredonset (eV)HOMO (eV)LUMO (eV)
P-3O9309721203421.03–0.80–5.33–4.30
P-6O967972130450.95–0.80–5.25–4.30
P-B3O80791412881070.96–0.85–5.21–4.25

Cyclic voltammetry (CV) measurements of P-3O, P-6O, and P-B3O versus ferrocene/ferrocenium (Fc/Fc+) using an Ag/AgCl pseudoreference were conducted on thin films to determine their electronic energy levels. The resulting plots from the first cycle are shown in Figure S10, and the corresponding data are summarized in Table 1. All three copolymers exhibited distinctive reduction peaks in an acetonitrile solution containing 0.1 M tetrabutylammonium hexafluorophosphate electrolyte, corresponding to the n-doping (reduction) of the strongly electron-deficient nature of the NDI-2Tz backbone. The lowest unoccupied molecular orbital (LUMO) energy levels of these three polymers were calculated from the onset reduction potentials (Eonsetred.) using the equation ELUMO = −(5.10 + Eonsetred.) eV. The Eonsetred values of P-3O, P-6O, and P-B3O were −0.80, –0.80, and −0.85 V, respectively, which corresponded to the estimated LUMO energies of −4.30, −4.30, and −4.25 eV. Based on the optical bandgap and LUMO levels, the highest occupied molecular orbital (HOMO) levels of P-3O, P-6O, and P-B3O were calculated to be −5.33, −5.25, and −5.21 eV. The deep LUMO levels verify the strong electron affinity of the NDI backbone. All copolymers were expected to be efficiently doped by N-DMBI because charges can be transferred from the singly occupied molecular orbital (−2.36 eV) of N-DMBI to the LUMO level of the host molecules upon doping activation by energetics. From the slight difference and measurement error of the CV measurements, we can exclude the influence of the side chains on the energetics.

To gain insight into how the volume and branching of the OEG side chains influence the molecular configuration and the electronic structure of the conjugated polymers, we performed density functional theory (DFT) calculations at the B3LYP/6-31G(d,p) level using Gaussian 16. To simplify the calculations, we considered one repeat unit of each polymer and truncated the side chains of the bithiazole units to the methoxyl group. As shown in Figure S11, all the optimized model molecules for the polymers exhibited planar backbone geometries, and the dihedral angle between the NDI and the adjacent thiazole subunit was quite small (<6°) due to reduced steric hindrance by sp2-N, whereas the dihedral angle between the two adjacent thiazole units was close to 0°. The fully planar structure may lead to strong coupling between adjacent building blocks and promote delocalization of the frontier molecular orbitals (HOMO and LUMO) along the entire model molecule backbone. Such delocalization, particularly full LUMO delocalization in n-type polymers, might facilitate the formation of extended excitons/polarons with long delocalized lengths, be beneficial for intra- and intermolecular hopping, and finally enhance charge carrier transport. These calculated results indicate that the volume and type of the OEG side chain had a negligible effect on the molecular configuration and electronic structure in monomers. This is attributed to the DFT calculations being performed under a gas-phase condition, in which the impact of bulky side chains is less pronounced. The HOMO/LUMO values were calculated to be −5.52/–3.50 eV for the P-3O monomer, −5.53/–3.51 eV for the P-6O monomer, and −5.44/–3.40 eV for the P-B3O monomer (Figure S12). The calculated LUMO level trend agreed well with the CV data.

To highlight how OEG side chains of the OEG interact with the conjugate backbone in the polymer, we then did DFT calculations to investigate the effect of side chains on the monocular geometry by using the method of local minima of geometry minimization which partially sacrifices the orbital energies. We chose trimers as the calculation model and used the ωB97X-D4 functional and def2-TZP basis set. The calculation results are shown in the Figure S13. P-3O trimer displays a perfectly planar backbone, and the OEG side chains do not interact strongly. The dihedral angle between the NDI and the adjacent thiazole subunit is 1.4°, 1.4°, and 1.6°. In the P-6O trimer, the OEG side chains interact with each other to create some torsional strain on the backbone, with the actual torsional angles remaining small (2.8°, 5.6°, and 8.0°). In the P-B3O trimer, the branched OEG side chains have more steric congestion that is causing the backbone twist. The dihedral angle in the P-B3O trimer is 1.7°, 12.6°, and 8.1°, much higher than that in P-3O and P-6O. This qualitative trend in the degree of twisting of the polymer backbone is consistent with the trends in the aforementioned absorption spectra experimental data.

Previous studies have demonstrated that N-DMBI is a good n-type dopant because of its strong n-doping ability and good solution processability. (21,22,24,25,47,48,56) Therefore, we employed N-DMBI to dope the three NDI-2Tz-based copolymers. Figure S9 shows the UV–vis–NIR absorption spectra for pristine and doped NDI-2Tz-based copolymer thin films with different dopant ratios. With the addition of the dopant, all NDI-2Tz-based polymers showed a substantial reduction in the neutral ICT transition absorbance associated with sub-band gap polaronic absorption bands in the range of 1000–2000 nm, indicating that all NDI-2Tz-based polymers were strongly n-doped with N-DMBI.

Figure 3a displays the σ values of the doped P-3O, P-6O, and P-B3O thin films at different doping concentrations (see the details in the Supporting Information, device fabrication and characterization sections). The σ values of the pristine polymer thin films were below the measurement limit. After molecular doping, the σ increased, indicating the generation of free charge carriers. The σ of the doped P-3O films gradually increased to 9 S cm–1 at a doping concentration of 3-wt % and then decreased at much higher dopant ratios. For P-6O, the optimized σ of 3.3 S cm–1 occurred at a doping concentration of 5-wt %, which was consistent with our previous work. (24) Conversely, P-B3O exhibited a maximum σ of 0.03 S cm–1 at a 5-wt % doping concentration, representing a reduction of more than 2 orders of magnitude over the doped P-3O and P-6O films. The σ results demonstrate that the charging behavior of NDI-based conjugated polymers can be modified by adjusting the OEG unit structure and density in the side chains.

Figure 3

Figure 3. (a) Electrical conductivities, (b) Seebeck coefficients, and (c) power factors recorded for P-3O, P-6O, and P-B3O as functions of the N-DMBI ratio employed.

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We then evaluated the S values of the doped films by imposing a temperature difference across the sample (Figure 3b). The negative S values of all of the doped polymer thin films suggest that the dominant charge carriers were electrons. The S values of all polymers decreased as the dopant ratio increased, indicating that more charge carriers were produced with doping. The absolute S values of the doped polymers showed a consistent trend at each doping concentration, with P-B3O exhibiting the highest S followed by P-3O and P-6O having an S value comparable to P-3O. For example, at a 5-wt % doping concentration, the S values were −238.9 μV K–1 for P-B3O, −114 μV K–1 for P-3O, and −114.5 μV K–1 for P-6O. This difference can be attributed to the more disordered configuration of P-B3O. Based on the σ and S values, we calculated the PFs of the doped films, which are summarized in Figure 3c. Owing to its low conductivity, the P-B3O film with a 5-wt % doping concentration showed a relatively low PF of 0.15 μW m–1 K–2. The P-6O film doped with 5-wt % N-DMBI showed an optimal PF of 4.3 μW m–1 K–2, which was consistent with our previous study. Remarkably, a slightly higher PF of 6.6 μW m–1 K–2 was achieved for the P-3O film with a 5-wt % doping concentration, and the highest PF of ∼15.3 μW m–1 K–2 was achieved with a 3-wt % N-DMBI doping concentration.

The contribution of charge carrier density (n) and charge carrier mobility (μ) to the σ in the doped film is described as σ = μnq, where q represents the charge carried by the electron. The charge carrier density of doped films is related to doping efficiency, which can be affected by the energy offset between the host and dopant, doping mechanism, and host–dopant miscibility, among other factors. The charge transport is mainly influenced by the film morphology and microstructures.

Previous studies have demonstrated that the performance of polymeric thermoelectric materials is limited by the extrinsic factor of poor miscibility between the host polymer and employed dopant. (22,23,57,58) To gain insight into this miscibility for our NDI-2Tz-based polymer systems, we utilized atomic force microscopy (AFM) to investigate the morphologies of the pristine and doped copolymer thin films (Figure S14). All of the pristine P-3O, P-6O, and P-B3O films showed a fibrous microstructure. The P-3O-, P-6O-, and P-B3O-doped film topologies showed no obvious change with increasing doping concentration, even in the highly doped films, revealing that all of the NDI-2Tz-based polymers had excellent dopant/polymer mixing due to the high-polarity glycol side chains. Therefore, the poor performance of P-B3O was not attributed to host–dopant miscibility, eliminating the miscibility between dopants and matrices as a factor for the thermoelectric performance.

Grazing-incidence wide-angle X-ray scattering (GIWAXS) was performed to further investigate the effect of the side chain on polymer film crystallinity and molecular packing and to correlate this with device performance. The two-dimensional (2D) patterns and corresponding line-cut profiles of P-3O, P-6O, and P-B3O films are displayed in Figures 4 and S18. Among the three neat films, P-3O exhibits greater crystallinity as evidenced by the stronger lamellar diffractions up to (300) qz = 0.75 Å–1 in the out-of-plane (OOP) direction and obvious (010) π–π scattering peak at qxy = 1.80 Å–1 in the in-plane (IP) directions, indicating the neat P-3O film preferentially packed with an edge-on orientation relative to the substrate, which is thought to be beneficial for lateral charge carrier transport. On the other hand, both neat P-6O and P-B3O films adopted a bimodal packing. The neat P-6O film showed lamellar diffractions (100) at qxy = 0.21 Å–1 in the IP direction with a π–π stacking peak at qz = 1.86 Å–1 for face-on orientation fraction and a (100) peak at qz = 0.18 Å–1 for the edge-on orientation fraction. The neat P-B3O film adopts a bimodal orientation with both face-on and edge-on fractions: a (100) peak at qxy = 0.28 Å–1 and (010) peaks at qz = 1.81 Å–1 in the OOP direction with an additional (100) peak at qz = 0.17 Å–1 in the OOP direction, lacking the π–π stacking peak in the IP direction. Thus, although the three polymers have the same NDI-2Tz backbone chain, increasing the OEG side chain density resulted in changes in the microstructure and molecular packing. The bimodal configuration in neat P-6O and P-B3O films reduces the proportion of the edge-on orientation, which is unfavorable for in-plane charge transport. The π–π stacking distances of neat films of P-3O, P-6O, and P-B3O were 3.47, 3.38, and 3.46 nm, respectively, as calculated from the (010) peaks. The smaller π–π stacking distances in neat P-6O film should facilitate charge transport, resulting in enhanced electron mobility. The crystal coherence lengths are 33.60, (59) 21.60, and 20.90 Å for neat films of P-3O, P-6O, and P-B3O, respectively. Thus, the unfavorable molecular packing and reduced crystallinity of the P-B3O film are the main contributing factors for poor charge carrier transport properties, which correlate with its low thermoelectric performance.

Figure 4

Figure 4. 2D GIWAXS patterns of pristine P-3O (a), P-6O (b), and P-B3O (c) films.

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Although molecular doping had a minimal effect on the molecular orientations of the three polymers, its influence on crystallinity varies. The P-3O film exhibited only a slight increase in crystallinity when doped with 3-wt % N-DMBI. The 5-wt % N-DMBI-doped P-6O film showed slightly enhanced crystallinity, whereas the 5-wt % N-DMBI-doped P-B3O film showed reduced crystallinity. The reduced crystallinity of doped P-B3O was consistent with low charge transport properties.

To directly measure the charge carrier density and determine the doping levels of the doped copolymers, admittance spectroscopy on ion-gel-based metal–insulator–semiconductor (MIS) devices (Figure S15, Supporting Information) was employed to extract the free carrier density and understand the charge transport inside the doped polymer films. This technique was established in the weak accumulation region caused by carrier depletion under sweeping voltages and a particular frequency, which is reflected in the reduced capacitance. From the capacitive measurement in the depletion region, we determined the carrier density according to the Mott–Schottky equation (see the carrier density measurement section, Supporting Information). The CP–V curves for doped P-3O, P-6O, and P-B3O are displayed in Figure S15. As shown in Figure 5, under the same doping concentration (5-wt % N-DMBI), the free charge carrier density extracted for the doped P-B3O film reached a maximum of ∼1.0 × 1018 cm–3, whereas P-3O and P-6O achieved carrier densities of ∼2.9 × 1019 and 1.4 × 1019 cm–3, respectively. The superior carrier densities of the doped P-3O and P-6O films resulted in higher doping efficiencies (∼25.9% for P-3O and ∼12.5% for P-6O) than those of the doped P-B3O films (∼0.9%) at the same doping concentration. According to the formula σ = μnq, the bulk charge mobilities in the in-plane direction of the doped films were calculated as 1.40 cm2 V–1 s–1 for P-3O, 1.47 cm2 V–1 s–1 for P-6O, and 0.15 cm2 V–1 s–1 for P-B3O based on the measured σ and charge carrier density. Considering the previously observed high absolute S, it can be concluded that P-B3O exhibited low carrier mobility due to its highly disordered molecular conformation. Furthermore, the relatively low charge mobility and doping efficiency of P-B3O were responsible for the poor thermoelectric performance.

Figure 5

Figure 5. Charge carrier density (a), doping efficiency (b), and mobility (c) of 5-wt %-doped P-3O, P-6O, and P-B3O thin films.

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To further elucidate and understand the charge transport in undoped conjugated polymers with different OEG side chains, we fabricated bottom-gate/bottom-contact organic field-effect transistors (OFETs) with a device structure of doped silicon/neat polymer/Al. The corresponding output and transfer characteristics of the OFETs are displayed in Figures S16 and S17, respectively. Both P-3O and P-6O exhibited ambipolar transport properties, whereas P-B3O only showed p-type unipolar charge transport. The electron mobilities of P-3O and P-6O were 3.69 × 10–4 and 5.61 × 10–3 cm2 V–1 s–1. However, it was not possible to measure this parameter for P-B3O under our experimental conditions. The lower OFET/bulk charge mobility of P-B3O than that of P-3O and P-6O can be explained by the poor polymer chain packing of the former in the thin films, which was consistent with the GIWAXS data. The significant difference in charge mobility between OFETs and MIS devices can be attributed to the mobility in OFETs located at the interface between the polymer and the gate dielectric, which depends on the material and the device. In contrast, the mobility derived from the Mott–Schottky analysis represents a bulk property acquired through bulk doping.

The doping of organic semiconductors can be viewed as a two-step process. (60−62) First, integer or partial charge transfer occurs between the dopant and the semiconductor, forming a Coulombically bound charge transfer complex (CTC). In the second step, the CTC overcomes the Coulomb interactions, effectively generating free charge carriers. Generally, in the first step of CTC formation, the original absorption transitions of neat polymer films will bleach gradually as the doping level increases (Figure S9, Supporting Information). At a 5-wt % doping concentration, the original peak intensities of P-3O, P-6O, and P-B3O decrease by 26.1%, 17.5%, and 39.8%, respectively. These results suggest that P-B3O more easily reacts with the dopant to form the CTC, which may be because the branching OEG side chain provides more space for the dopant. However, the ease of forming the CTC does not necessarily translate into a larger number of free-charge carriers because the charges in the CTC must overcome the Coulomb interaction. As measured using MIS (see above), the doped P-B3O produced the lowest free charge carrier density at a 5-wt % doping concentration; thus, we infer that the low doping efficiency is because of the poor CTC dissociation. Because the bulky branching OEG side chain in P-B3O leads to a twisted backbone structure, disrupts the molecular packing, and limits the exciton/polaron delocalized length, the CTC dissociation into free charges is suppressed. (36)

Three n-type semiconducting conjugated polymers comprising naphthalenediimide and dialkoxybithiazole units but with different OEG side chains were synthesized, characterized, and evaluated as thermoelectric materials. Introducing OEG side chains into semiconductors endowed good host–dopant miscibility. Due to the excellent ordered packing, the increasing volume of ethylene glycol units in linear side chains maintains enhanced thermoelectric performances, even with a slight backbone twist. In contrast, the bulky branching OEG side chains introduce more steric congestion, causing the backbone to twist. This disruption also reflects on molecular packing in the thin film and adversely affects molecular doping and charge transport, consequently lowering the thermoelectric performance. These results suggest the existence of a compromise between the performance of the OTE and ethylene-glycol side-chain engineering. Our comprehensive study provides a unique insight into the fundamental understanding of molecular packing and sheds light on the rational design for the future development of high-performance n-type thermoelectric semiconducting polymers.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsmaterialslett.4c00068.

  • Additional synthesis details, materials, and characterization including 1H NMR spectra, 13C NMR spectra, HRMS spectra, GPC plots, TGA plots, IR spectra, DSC plots, UV–vis–NIR absorption spectra, cyclic voltammogram plots, DFT-optimized molecular geometries and orbitals, AFM image, Mott–Schottky analysis plots, OFET plots, and GIWAXS patterns and linecuts (PDF)

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Author Information

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  • Corresponding Authors
    • Gang Ye - Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei Key Laboratory of Polymer Materials, School of Materials Science and Engineering, Hubei University, Wuhan, 430062, China;  State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, P. R. China;  Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, Groningen 9747 AG, The Netherlands;  Email: [email protected]
    • Ryan C. Chiechi - Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, Groningen 9747 AG, The Netherlands;  Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States;  Orcidhttps://orcid.org/0000-0002-0895-2095;  Email: [email protected]
    • L. Jan Anton Koster - Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, Groningen 9747 AG, The Netherlands;  Orcidhttps://orcid.org/0000-0002-6558-5295;  Email: [email protected]
  • Authors
    • Xuwen Yang - Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, Groningen 9747 AG, The Netherlands
    • Karolina Tran - Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, Groningen 9747 AG, The Netherlands;  Orcidhttps://orcid.org/0000-0001-5432-5204
    • Yuru Liu - Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, Groningen 9747 AG, The Netherlands;  Orcidhttps://orcid.org/0000-0003-4909-7782
    • Jiamin Cao - Key Laboratory of Theoretical Organic Chemistry and Functional Molecule of Ministry of Education, School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China;  Orcidhttps://orcid.org/0000-0003-2069-6976
    • Jingjin Dong - Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, Groningen 9747 AG, The Netherlands;  Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, Nanjing, Jiangsu 211816, China
    • Giuseppe Portale - Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, Groningen 9747 AG, The Netherlands;  Orcidhttps://orcid.org/0000-0002-4903-3159
    • Jian Liu - State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, P. R. China;  Orcidhttps://orcid.org/0000-0002-6704-3895
    • Ping Zhang - School of Electrical Engineering and Automation, Jiangxi University of Science and Technology, Ganzhou, Jiangxi 341000, China
    • Maria Antonietta Loi - Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, Groningen 9747 AG, The Netherlands;  Orcidhttps://orcid.org/0000-0002-7985-7431
  • Author Contributions

    CRediT: Xuwen Yang data curation, formal analysis, methodology, software, writing-original draft, writing-review & editing; Gang Ye funding acquisition, supervision, writing-original draft, writing-review & editing; Karolina Tran data curation, software; Yuru Liu data curation, software; Jiamin Cao resources; Jingjin Dong data curation, software; Giuseppe Portale resources, writing-review & editing; Jian Liu funding acquisition, resources; Ping Zhang resources; Maria Antonietta Loi resources, writing-review & editing; Ryan C. Chiechi methodology, software, supervision, writing-review & editing; L. Jan Anton Koster methodology, project administration, resources, supervision, writing-original draft, writing-review & editing.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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X.Y. acknowledges the China Scholarship Council. G.Y. and J.L. acknowledge the financial support by the National Natural Science Foundation of China (No. 52273201). G.Y. also acknowledges the China Postdoctoral Science Foundation Funded Project (No. grant 2022M723077). J.D. is grateful to the National Natural Science Foundation of China (grant number 62205143). J.L. is thankful for the financial support from the Jilin Scientific and Technological Development Program (No. 20230402070GH) and a grant for Distinguished Young Scholars of the National Natural Science Foundation of China (Overseas). K.T. and M.A.L. would like to acknowledge the financial support of CogniGron - Groningen Cognitive Systems and Materials Center.

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    Being emerging green energy materials, org. polymer thermoelec. (TE) composites have attracted much current interest and witnessed a rapid development due to their diverse advantages, such as low d., low thermal cond., high flexibility, and highly tunable mol. structure. By adjusting the polymer structure and fabrication of their composites, the TE performance can be significantly enhanced. In this review, recent advances in the nanostructures of polymers and their composites for TE applications are introduced. The TE performance of polymers and their composites can be greatly improved and highly tuned by controlled construction of nanostructures via both in situ polymn. and phys. mixing. The mechanism of the relation between nanostructures and TE performance is also discussed. Finally, the outlooks of future research are remarked.
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  13. 13
    Fan, Z.; Du, D.; Guan, X.; Ouyang, J. Nano Energy 2018, 51, 481– 488,  DOI: 10.1016/j.nanoen.2018.07.002
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    13
    Polymer films with ultrahigh thermoelectric properties arising from significant seebeck coefficient enhancement by ion accumulation on surface
    Fan, Zeng; Du, Donghe; Guan, Xin; Ouyang, Jianyong
    Nano Energy (2018), 51 (), 481-488CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
    Org. thermoelec. (TE) materials have drawn great interest because of their advantages including mech. flexibility, easy availability, non-toxicity and low thermal cond. TE materials with high dimensionless figure-of-merit ZT are required for highly efficient TE conversion. But the elec. cond. and Seebeck coeff. of TE materials are interdependent. The increase in Seebeck coeff. is usually at the cost of the decrease in elec. cond. In this work, we report a facile approach to significantly enhance the TE properties of poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) films by ion accumulation of an ionic liq. on the polymer surface. The ion accumulation can increase the Seebeck coeff. of the PEDOT:PSS films by 1.2-2 fold while it does not remarkably affect the elec. cond. The PEDOT:PSS films can exhibit an ultrahigh power factor of 754μW m-1 K-2, corresponding to a ZT value of 0.75. This ZT value is comparable to that of inorg. TE materials like bismuth telluride at 300 K.
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  14. 14
    Culebras, M.; Gómez, C. M.; Cantarero, A. J. Mater. Chem. A 2014, 2, 10109– 10115,  DOI: 10.1039/C4TA01012D
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    14
    Enhanced thermoelectric performance of PEDOT with different counter-ions optimized by chemical reduction
    Culebras, M.; Gomez, C. M.; Cantarero, A.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2014), 2 (26), 10109-10115CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    This work reports on the synthesis of the intrinsically conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) doped with several counterions, ClO4, PF6 and bis(trifluoromethylsulfonyl)imide (BTFMSI), by electro-polymn. and its thermoelec. properties. Depending on the counterion size, the thermoelec. efficiency of PEDOT can be increased up to two orders of magnitude. A further chem. redn. with hydrazine optimizes the power factor (PF). By changing the counterions, the authors were able to increase the elec. cond. (σ) of PEDOT by a factor of three, while the Seebeck coeff. remains at the same order of magnitude in the three polymers. The best thermoelec. efficiency was obsd. in PEDOT:BTFMSI. From the measurement of the Seebeck coeff. and σ, a PF of 147 μW m-1 K-2 was deduced, while the measured thermal cond. is κ = 0.19 W m-1 K-1, resulting in a ZT ∼ 0.22 at room temp., one of the highest values reported in the literature for polymers. The increase in σ with the change of the counterion is mainly due to the stretching of the polymer chains. The authors provide a chem. route to further improve ZT in polymers and demonstrate a method of synthesis based on the electro-polymn. on Au. After removing the Au layer, a very thin semiconducting polymer film can be isolated.
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  15. 15
    Kim, G. H.; Shao, L.; Zhang, K.; Pipe, K. P. Nat. Mater. 2013, 12, 719– 723,  DOI: 10.1038/nmat3635
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    15
    Engineered doping of organic semiconductors for enhanced thermoelectric efficiency
    Kim, G-H.; Shao, L.; Zhang, K.; Pipe, K. P.
    Nature Materials (2013), 12 (8), 719-723CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Significant improvements to the thermoelec. figure of merit ZT have emerged in recent years, primarily due to the engineering of material compn. and nanostructure in inorg. semiconductors (ISCs). However, many present high-ZT materials are based on low-abundance elements that pose challenges for scale-up, as they entail high material costs in addn. to brittleness and difficulty in large-area deposition. Here we demonstrate a strategy to improve ZT in conductive polymers and other org. semiconductors (OSCs) for which the base elements are earth-abundant. By minimizing total dopant vol., we show that all three parameters constituting ZT vary in a manner so that ZT increases; this stands in sharp contrast to ISCs, for which these parameters have trade-offs. Reducing dopant vol. is found to be as important as optimizing carrier concn. when maximizing ZT in OSCs. Implementing this strategy with the dopant poly(styrenesulfonate) in poly(3,4-ethylenedioxythiophene) (PEDOT:PSS), we achieve ZT = 0.42 at room temp.
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  16. 16
    Bubnova, O.; Khan, Z. U.; Malti, A.; Braun, S.; Fahlman, M.; Berggren, M.; Crispin, X. Nat. Mater. 2011, 10, 429– 433,  DOI: 10.1038/nmat3012
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    16
    Optimization of the thermoelectric figure of merit in the conducting polymer poly(3,4-ethylenedioxythiophene)
    Bubnova, Olga; Khan, Zia Ullah; Malti, Abdellah; Braun, Slawomir; Fahlman, Mats; Berggren, Magnus; Crispin, Xavier
    Nature Materials (2011), 10 (6), 429-433CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Thermoelec. generators (TEGs) transform a heat flow into electricity. Thermoelec. materials are being investigated for electricity prodn. from waste heat (co-generation) and natural heat sources. For temps. below 200°, the best com. available inorg. semiconductors are Bi2Te3-based alloys, which possess a figure of merit ZT close to one. Most of the recently discovered thermoelec. materials with ZT>2 exhibit one common property, namely their low lattice thermal conductivities. Nevertheless, a high ZT value is not enough to create a viable technol. platform for energy harvesting. To generate electricity from large vols. of warm fluids, heat exchangers must be functionalized with TEGs. This requires thermoelec. materials that are readily synthesized, air stable, environmentally friendly and soln. processable to create patterns on large areas. Conducting polymers might be capable of meeting these demands. The accurate control of the oxidn. level in poly(3,4-ethylenedioxythiophene) (PEDOT) combined with its low intrinsic thermal cond. (λ = 0.37 W m-1 K-1) yields a ZT = 0.25 at room temp. that approaches the values required for efficient devices.
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  17. 17
    Naab, B. D.; Gu, X.; Kurosawa, T.; To, J. W. F.; Salleo, A.; Bao, Z. Adv. Electron. Mater. 2016, 2, 1600004,  DOI: 10.1002/aelm.201600004
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    Wang, S.; Sun, H.; Erdmann, T.; Wang, G.; Fazzi, D.; Lappan, U.; Puttisong, Y.; Chen, Z.; Berggren, M.; Crispin, X.; Kiriy, A.; Voit, B.; Marks, T. J.; Fabiano, S.; Facchetti, A. Adv. Mater. 2018, 30, 1801898,  DOI: 10.1002/adma.201801898
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    Song, Y.; Ding, J.; Dai, X.; Li, C.; Di, C.-a.; Zhang, D. ACS Mater. Lett. 2022, 4, 521– 527,  DOI: 10.1021/acsmaterialslett.2c00026
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    19
    Enhancement of the Thermoelectric Performance of n-Type Naphthalene Diimide-Based Conjugated Polymer by Engineering of Side Alkyl Chains
    Song, Yilin; Ding, Jiamin; Dai, Xiaojuan; Li, Cheng; Di, Chong-an; Zhang, Deqing
    ACS Materials Letters (2022), 4 (4), 521-527CODEN: AMLCEF; ISSN:2639-4979. (American Chemical Society)
    Developing n-type doped semiconducting polymers with high thermoelec. (TE) performance still remains challenging. In this paper, we show a new strategy to enhance the TE performance of n-doped naphthalene diimide (NDI)-based conjugated donor-acceptor (D-A) polymer by introducing one linear alkyl chain for each NDI unit. Film of the PNDI2T-1, in which each NDI unit is attached with one linear and one branching alkyl chains, exhibits higher elec. cond. and a greater power factor (PF) than those of PNDI2T-2 with the same conjugated backbone and two branching side chains, after doping with either (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI) or decahydro-3a,6a,9a-triazaphenalene (TAM) under the same conditions. The optimal PF values of the doped films of PNDI2T-1 with N-DMBI and TAM can reach 1.6 and 3.5 μW m-1K-2, resp., which are among the highest reported TE performances for NDI-based semiconducting polymers after n-doping. The higher TE performance of the doped films of PNDI2T-1 is attributed to the enhancement of charge mobility and improvement of doping degree after side-chain modification. These results provide a new mol. design rationale for the future development of high-performance n-type thermoelec. semiconducting polymers.
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    Wang, Y.; Takimiya, K. Adv. Mater. 2020, 32, 2002060,  DOI: 10.1002/adma.202002060
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    20
    Naphthodithiophenediimide-Bithiopheneimide Copolymers for High-Performance n-Type Organic Thermoelectrics: Significant Impact of Backbone Orientation on Conductivity and Thermoelectric Performance
    Wang, Yang; Takimiya, Kazuo
    Advanced Materials (Weinheim, Germany) (2020), 32 (30), 2002060CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The development of n-type conjugated polymers with high elec. cond. (σ) has continued to pose a massive challenge in org. thermoelecs. (OTEs). New structural insights into the charge-carrier transport are necessitated for the realization of high-performance OTEs. In this study, three new n-type copolymers, named pNB, pNB-Tz, and pNB-TzDP, consisting of naphthodithiophenediimide (NDTI) and bithiopheneimide (BTI) units, are synthesized by direct arylation polymn. The backbone orientation is altered by incorporating thiazole units into the backbone and tuning the branching point of the side chain. The alteration of the backbone orientation from face-on to bimodal orientation with both face-on and edge-on fractions significantly impacts the σ and the power factors (PFs) of the polymers. As a result, pNB-TzDP, with the bimodal orientation, demonstrates a high σ of up to 11.6 S cm-1 and PF of up to 53.4μW m-1 K-2, which are among the highest in soln.-processed n-doped conjugated polymers reported so far. Further studies reveal that the bimodal orientation of pNB-TzDP introduces 3D conduction channels and leads to better accommodation of dopants, which should be the key factors for the excellent thermoelec. performance.
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  21. 21
    Liu, J.; Ye, G.; Zee, B. V.; Dong, J.; Qiu, X.; Liu, Y.; Portale, G.; Chiechi, R. C.; Koster, L. J. A. Adv. Mater. 2018, 30, e1804290,  DOI: 10.1002/adma.201804290
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    Liu, J.; Qiu, L.; Alessandri, R.; Qiu, X.; Portale, G.; Dong, J.; Talsma, W.; Ye, G.; Sengrian, A. A.; Souza, P. C. T.; Loi, M. A.; Chiechi, R. C.; Marrink, S. J.; Hummelen, J. C.; Koster, L. J. A. Adv. Mater. 2018, 30, 1704630,  DOI: 10.1002/adma.201704630
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    Kiefer, D.; Giovannitti, A.; Sun, H.; Biskup, T.; Hofmann, A.; Koopmans, M.; Cendra, C.; Weber, S.; Anton Koster, L. J.; Olsson, E.; Rivnay, J.; Fabiano, S.; McCulloch, I.; Müller, C. ACS Energy Lett. 2018, 3, 278– 285,  DOI: 10.1021/acsenergylett.7b01146
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    23
    Enhanced n-Doping Efficiency of a Naphthalenediimide-Based Copolymer through Polar Side Chains for Organic Thermoelectrics
    Kiefer, David; Giovannitti, Alexander; Sun, Hengda; Biskup, Till; Hofmann, Anna; Koopmans, Marten; Cendra, Camila; Weber, Stefan; Anton Koster, L. Jan; Olsson, Eva; Rivnay, Jonathan; Fabiano, Simone; McCulloch, Iain; Muller, Christian
    ACS Energy Letters (2018), 3 (2), 278-285CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    N-doping of conjugated polymers either requires a high dopant fraction or yields a low elec. cond. because of their poor compatibility with mol. dopants. The authors explore n-doping of the polar naphthalenediimide-bithiophene copolymer p(gNDI-gT2) that carries oligoethylene glycol-based side chains and show that the polymer displays superior miscibility with the benzimidazole-dimethylbenzenamine-based n-dopant N-DMBI. The good compatibility of p(gNDI-gT2) and N-DMBI results in a relatively high doping efficiency of 13% for n-dopants, which leads to a high elec. cond. of >10-1 S cm-1 for a dopant concn. of only 10 mol % when measured in an inert atm. The doped polymer is able to maintain its elec. cond. for ∼20 min when exposed to air and recovers rapidly when returned to a N atm. Overall, soln. coprocessing of p(gNDI-gT2) and N-DMBI results in a larger thermoelec. power factor of up to 0.4 μW K-2 m-1 compared to other NDI-based polymers.
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  24. 24
    Liu, J.; Ye, G.; Potgieser, H. G. O.; Koopmans, M.; Sami, S.; Nugraha, M. I.; Villalva, D. R.; Sun, H.; Dong, J.; Yang, X.; Qiu, X.; Yao, C.; Portale, G.; Fabiano, S.; Anthopoulos, T. D.; Baran, D.; Havenith, R. W. A.; Chiechi, R. C.; Koster, L. J. A. Adv. Mater. 2021, 33, e2006694,  DOI: 10.1002/adma.202006694
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    24
    Amphipathic Side Chain of a Conjugated Polymer Optimizes Dopant Location toward Efficient N-Type Organic Thermoelectrics
    Liu, Jian; Ye, Gang; Potgieser, Hinderikus G. O.; Koopmans, Marten; Sami, Selim; Nugraha, Mohamad Insan; Villalva, Diego Rosas; Sun, Hengda; Dong, Jingjin; Yang, Xuwen; Qiu, Xinkai; Yao, Chen; Portale, Giuseppe; Fabiano, Simone; Anthopoulos, Thomas D.; Baran, Derya; Havenith, Remco W. A.; Chiechi, Ryan C.; Koster, L. Jan Anton
    Advanced Materials (Weinheim, Germany) (2021), 33 (4), 2006694CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    There is no mol. strategy for selectively increasing the Seebeck coeff. without reducing the elec. cond. for org. thermoelecs. Here, it is reported that the use of amphipathic side chains in an n-type donor-acceptor copolymer can selectively increase the Seebeck coeff. and thus increase the power factor by a factor of ≈5. The amphipathic side chain contains an alkyl chain segment as a spacer between the polymer backbone and an ethylene glycol type chain segment. The use of this alkyl spacer does not only reduce the energetic disorder in the conjugated polymer film but can also properly control the dopant sites away from the backbone, which minimizes the adverse influence of counterions. As confirmed by kinetic Monte Carlo simulations with the host-dopant distance as the only variable, a reduced Coulombic interaction resulting from a larger host-dopant distance contributes to a higher Seebeck coeff. for a given elec. cond. Finally, an optimized power factor of 18μW m-1 K-2 is achieved in the doped polymer film. This work provides a facile mol. strategy for selectively improving the Seebeck coeff. and opens up a new route for optimizing the dopant location toward realizing better n-type polymeric thermoelecs.
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  25. 25
    Ye, G.; Liu, J.; Qiu, X.; Stater, S.; Qiu, L.; Liu, Y.; Yang, X.; Hildner, R.; Koster, L. J. A.; Chiechi, R. C. Macromolecules 2021, 54, 3886– 3896,  DOI: 10.1021/acs.macromol.1c00317
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    25
    Controlling n-Type Molecular Doping via Regiochemistry and Polarity of Pendant Groups on Low Band Gap Donor-Acceptor Copolymers
    Ye, Gang; Liu, Jian; Qiu, Xinkai; Stater, Sebastian; Qiu, Li; Liu, Yuru; Yang, Xuwen; Hildner, Richard; Koster, L. Jan Anton; Chiechi, Ryan C.
    Macromolecules (Washington, DC, United States) (2021), 54 (8), 3886-3896CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
    We demonstrate the impact of the type and position of pendant groups on the n-doping of low-band gap donor-acceptor (D-A) copolymers. Polar glycol ether groups simultaneously increase the electron affinities of D-A copolymers and improve the host/dopant miscibility compared to nonpolar alkyl groups, improving the doping efficiency by a factor of over 40. The bulk mobility of the doped films increases with the fraction of polar groups, leading to a best cond. of 0.08 S cm-1 and power factor (PF) of 0.24μW m-1 K-2 in the doped copolymer with the polar pendant groups on both the D and A moieties. We used spatially resolved absorption spectroscopy to relate commensurate morphol. changes to the dispersion of dopants and to the relative local doping efficiency, demonstrating a direct relationship between the morphol. of the polymer phase, the solvation of the mol. dopant, and the elec. properties of doped films. Our work offers fundamental new insights into the influence of the phys. properties of pendant chains on the mol. doping process, which should be generalizable to any molecularly doped polymer films.
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  26. 26
    Yang, C.-Y.; Jin, W.-L.; Wang, J.; Ding, Y.-F.; Nong, S.; Shi, K.; Lu, Y.; Dai, Y.-Z.; Zhuang, F.-D.; Lei, T.; Di, C.-A.; Zhu, D.; Wang, J.-Y.; Pei, J. Adv. Mater. 2018, 30, 1802850,  DOI: 10.1002/adma.201802850
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    Yan, X.; Xiong, M.; Li, J.-T.; Zhang, S.; Ahmad, Z.; Lu, Y.; Wang, Z.-Y.; Yao, Z.-F.; Wang, J.-Y.; Gu, X.; Lei, T. J. Am. Chem. Soc. 2019, 141, 20215– 20221,  DOI: 10.1021/jacs.9b10107
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    Pyrazine-flanked diketopyrrolopyrrole (DPP): A new polymer building block for high performance n-type organic thermoelectrics
    Yan, Xinwen; Xiong, Miao; Li, Jia-Tong; Zhang, Song; Ahmad, Zachary; Lu, Yang; Wang, Zi-Yuan; Yao, Ze-Fan; Wang, Jie-Yu; Gu, Xiaodan; Lei, Ting
    Journal of the American Chemical Society (2019), 141 (51), 20215-20221CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    N-Doped conjugated polymers usually show low elec. conductivities and low thermoelec. power factors, limiting their applications in n-type org. thermoelecs. Here, we report the synthesis of a new diketopyrrolopyrrole (DPP) deriv., pyrazine-flanked DPP (PzDPP), with the deepest LUMO level in all the reported DPP derivs. Based on PzDPP, a donor-acceptor copolymer, P(PzDPP-CT2), is synthesized. The polymer displays a deep LUMO energy level and strong interchain interaction with a short π-π stacking distance of 3.38 Å. When doped with n-dopant N-DMBI, P(PzDPP-CT2) exhibits high n-type elec. conductivities of up to 8.4 S cm-1 and power factors of up to 57.3μW m-1 K-2. These values are much higher than previously reported n-doped DPP polymers, and the power factor also ranks the highest in soln.-processable n-doped conjugated polymers. These results suggest that PzDPP is a promising high-performance building block for n-type org. thermoelecs. and also highlight that, without sacrificing polymer interchain interactions, efficient n-doping can be realized in conjugated polymers with careful mol. engineering.
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  28. 28
    Yan, X.; Xiong, M.; Deng, X.-Y.; Liu, K.-K.; Li, J.-T.; Wang, X.-Q.; Zhang, S.; Prine, N.; Zhang, Z.; Huang, W.; Wang, Y.; Wang, J.-Y.; Gu, X.; So, S. K.; Zhu, J.; Lei, T. Nat. Commun. 2021, 12, 5723,  DOI: 10.1038/s41467-021-26043-y
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    28
    Approaching disorder-tolerant semiconducting polymers
    Yan, Xinwen; Xiong, Miao; Deng, Xin-Yu; Liu, Kai-Kai; Li, Jia-Tong; Wang, Xue-Qing; Zhang, Song; Prine, Nathaniel; Zhang, Zhuoqiong; Huang, Wanying; Wang, Yishan; Wang, Jie-Yu; Gu, Xiaodan; So, Shu Kong; Zhu, Jia; Lei, Ting
    Nature Communications (2021), 12 (1), 5723CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    Doping has been widely used to control the charge carrier concn. in org. semiconductors. However, in conjugated polymers, n-doping is often limited by the tradeoff between doping efficiency and charge carrier mobilities, since dopants often randomly distribute within polymers, leading to significant structural and energetic disorder. Here, we screen a large no. of polymer building block combinations and explore the possibility of designing n-type conjugated polymers with good tolerance to dopant-induced disorder. We show that a carefully designed conjugated polymer with a single dominant planar backbone conformation, high torsional barrier at each dihedral angle, and zigzag backbone curvature is highly dopable and can tolerate dopant-induced disorder. With these features, the designed diketopyrrolopyrrole (DPP)-based polymer can be efficiently n-doped and exhibit high n-type elec. conductivities over 120 S cm-1, much higher than the ref. polymers with similar chem. structures. This work provides a polymer design concept for highly dopable and highly conductive polymeric semiconductors.
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  29. 29
    Lu, Y.; Yu, Z.-D.; Un, H.-I.; Yao, Z.-F.; You, H.-Y.; Jin, W.; Li, L.; Wang, Z.-Y.; Dong, B.-W.; Barlow, S.; Longhi, E.; Di, C.-a.; Zhu, D.; Wang, J.-Y.; Silva, C.; Marder, S. R.; Pei, J. Adv. Mater. 2021, 33, 2005946,  DOI: 10.1002/adma.202005946
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    29
    Persistent Conjugated Backbone and Disordered Lamellar Packing Impart Polymers with Efficient n-Doping and High Conductivities
    Lu, Yang; Yu, Zi-Di; Un, Hio-Ieng; Yao, Ze-Fan; You, Hao-Yang; Jin, Wenlong; Li, Liang; Wang, Zi-Yuan; Dong, Bo-Wei; Barlow, Stephen; Longhi, Elena; Di, Chong-an; Zhu, Daoben; Wang, Jie-Yu; Silva, Carlos; Marder, Seth R.; Pei, Jian
    Advanced Materials (Weinheim, Germany) (2021), 33 (2), 2005946CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Soln.-processable highly conductive polymers are of great interest in emerging electronic applications. For p-doped polymers, conductivities as high a nearly 105 S cm-1 have been reported. In the case of n-doped polymers, they often fall well short of the high values noted above, which might be achievable, if much higher charge-carrier mobilities detd. could be realized in combination with high charge-carrier densities. This is in part due to inefficient doping and dopant ions disturbing the ordering of polymers, limiting efficient charge transport and ultimately the achievable conductivities. Here, n-doped polymers that achieve a high cond. of more than 90 S cm-1 by a simple soln.-based co-deposition method are reported. Two conjugated polymers with rigid planar backbones, but with disordered cryst. structures, exhibit surprising structural tolerance to, and excellent miscibility with, commonly used n-dopants. These properties allow both high concns. and high mobility of the charge carriers to be realized simultaneously in n-doped polymers, resulting in excellent elec. cond. and thermoelec. performance.
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  30. 30
    Lu, Y.; Yu, Z.-D.; Liu, Y.; Ding, Y.-F.; Yang, C.-Y.; Yao, Z.-F.; Wang, Z.-Y.; You, H.-Y.; Cheng, X.-F.; Tang, B.; Wang, J.-Y.; Pei, J. J. Am. Chem. Soc. 2020, 142, 15340– 15348,  DOI: 10.1021/jacs.0c05699
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    30
    The Critical Role of Dopant Cations in Electrical Conductivity and Thermoelectric Performance of n-Doped Polymers
    Lu, Yang; Yu, Zi-Di; Liu, Yi; Ding, Yi-Fan; Yang, Chi-Yuan; Yao, Ze-Fan; Wang, Zi-Yuan; You, Hao-Yang; Cheng, Xiu-Fen; Tang, Bo; Wang, Jie-Yu; Pei, Jian
    Journal of the American Chemical Society (2020), 142 (36), 15340-15348CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The low n-doping efficiency of conjugated polymers with the mol. dopants limits their availability in elec. cond., thermoelecs., and other elec. applications. Recently, considerable efforts have focused on improving the ionization of dopants by modifying the structures of host polymers or n-dopants; however, the effect of ionized dopants on the elec. cond. and thermoelec. performance of the polymers is still in a puzzle. Herein, we try to reveal the role of mol. dopant cations on carrier transporting through the systematic comparison of two n-dopants, TAM and N-DMBI-H. These two n-dopants exhibit various doping features with the polymer due to their different chem. structure characteristics. For instance, while doping, TAM perturbs negligibly on the polymer backbone conformation and microstructural ordering; and then after ionization, TAM cations possess weak π-backbone affinity but strong intrinsic affinity with side chains, which enables the doped system to screen the Coulomb potential spatially. Such doping features lead to high carrierization capabilities for TAM-doped polymers and further result in the excellent cond. up to 22 ± 2.5 S cm-1 and the power factor over 80μW m-1 K-2, which are significantly higher than the state-of-the-art values of the common n-dopant N-DMBI-H. More importantly, this strategy has also proven to be widely applicable in other doped polymers. Our investigations inspire the vital role of dopant counterions on high elec. and thermoelec. performance polymers and also suggest that, without sacrificing Seebeck coeffs., highly conductivities can be realized with precisely regulating the interaction between the cations and the host.
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  31. 31
    Shi, K.; Zhang, F.; Di, C.-A.; Yan, T.-W.; Zou, Y.; Zhou, X.; Zhu, D.; Wang, J.-Y.; Pei, J. J. Am. Chem. Soc. 2015, 137, 6979– 6982,  DOI: 10.1021/jacs.5b00945
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    31
    Toward High Performance n-Type Thermoelectric Materials by Rational Modification of BDPPV Backbones
    Shi, Ke; Zhang, Fengjiao; Di, Chong-An; Yan, Tian-Wei; Zou, Ye; Zhou, Xu; Zhu, Daoben; Wang, Jie-Yu; Pei, Jian
    Journal of the American Chemical Society (2015), 137 (22), 6979-6982CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Three n-type polymers BDPPV, ClBDPPV, and FBDPPV which exhibit outstanding elec. conductivities when mixed with an n-type dopant, N-DMBI ((4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine), in soln. High electron mobility and an efficient doping process endow FBDPPV with the highest elec. conductivities of 14 S cm-1 and power factors up to 28 μW m-1 K-2, which is the highest thermoelec. (TE) power factor that is reported for soln. processable n-type conjugated polymers. The authors' studies reveal that introduction of halogen atoms to the polymer backbones has a dramatic influence on not only the electron mobilities but also the doping levels, both of which are crit. to the elec. conductivities. This work suggests the significance of rational modification of polymer structures and opens the gate for applying the rapidly developed org. semiconductors with high carrier mobilities to thermoelec. field.
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  32. 32
    Zhao, X.; Madan, D.; Cheng, Y.; Zhou, J.; Li, H.; Thon, S. M.; Bragg, A. E.; DeCoster, M. E.; Hopkins, P. E.; Katz, H. E. Adv. Mater. 2017, 29, 1606928,  DOI: 10.1002/adma.201606928
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    Han, J.; Fan, H.; Zhang, Q.; Hu, Q.; Russell, T. P.; Katz, H. E. Adv. Funct. Mater. 2021, 31, 2005901,  DOI: 10.1002/adfm.202005901
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    33
    Dichlorinated Dithienylethene-Based Copolymers for Air-Stable n-Type Conductivity and Thermoelectricity
    Han, Jinfeng; Fan, Huidong; Zhang, Qingyang; Hu, Qin; Russell, Thomas P.; Katz, Howard E.
    Advanced Functional Materials (2021), 31 (5), 2005901CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Two donor-acceptor (D-A) polymers are obtained by coupling difluoro- and dichloro-substituted forms of the electron-deficient unit BDOPV and the relatively weak donor moiety dichlorodithienylethene (ClTVT). The cond. and power factors of doped devices are different for the chlorinated and fluorinated BDOPV polymers. A high electron cond. of 38.3 and 16.1 S cm-1 are obtained from the chlorinated and fluorinated polymers with N-DMBI, resp., and 12.4 and 2.4 S cm-1 are obtained from the chlorinated and fluorinated polymers with CoCp2, resp., from drop-cast devices. The corresponding power factors are 22.7, 7.6, 39.5, and 8.0μW m-1 K-2, resp. Doping of PClClTVT with N-DMBI results in excellent air stability; the electron cond. of devices with 50 mol% N-DMBI as dopant remained up to 4.9 S m-1 after 222 days in the air, the longest for an n-doped polymer stored in air, with a thermoelec. power factor of 9.3μW m-1 K-2. However, the cond. of PFClTVT-based devices can hardly be measured after 103 days. These observations are consistent with morphologies detd. by grazing incidence wide angle X-ray scattering and at. force microscopy.
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    Feng, K.; Guo, H.; Wang, J.; Shi, Y.; Wu, Z.; Su, M.; Zhang, X.; Son, J. H.; Woo, H. Y.; Guo, X. J. Am. Chem. Soc. 2021, 143, 1539– 1552,  DOI: 10.1021/jacs.0c11608
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    34
    Cyano-Functionalized Bithiophene Imide-Based n-Type Polymer Semiconductors: Synthesis, Structure-Property Correlations, and Thermoelectric Performance
    Feng, Kui; Guo, Han; Wang, Junwei; Shi, Yongqiang; Wu, Ziang; Su, Mengyao; Zhang, Xianhe; Son, Jae Hoon; Woo, Han Young; Guo, Xugang
    Journal of the American Chemical Society (2021), 143 (3), 1539-1552CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    N-Type polymers with deep-positioned LUMO energy levels are essential for enabling n-type org. thin-film transistors (OTFTs) with high stability and n-type org. thermoelecs. (OTEs) with high doping efficiency and promising thermoelec. performance. Bithiophene imide (BTI) and its derivs. have been demonstrated as promising acceptor units for constructing high-performance n-type polymers. However, the electron-rich thiophene moiety in BTI leads to elevated LUMOs for the resultant polymers and hence limits their n-type performance and intrinsic stability. Herein, we addressed this issue by introducing strong electron-withdrawing cyano functionality on BTI and its derivs. We have successfully overcome the synthetic challenges and developed a series of novel acceptor building blocks, CNI, CNTI, and CNDTI, which show substantially higher electron deficiencies than does BTI. On the basis of these novel building blocks, acceptor-acceptor type homopolymers and copolymers were successfully synthesized and featured greatly suppressed LUMOs (-3.64 to -4.11 eV) vs. that (-3.48 eV) of the control polymer PBTI. Their deep-positioned LUMOs resulted in improved stability in OTFTs and more efficient n-doping in OTEs for the corresponding polymers with a highest elec. cond. of 23.3 S cm-1 and a power factor of ~ 10μW m-1 K-2. The cond. and power factor are among the highest values reported for soln.-processed molecularly n-doped polymers. The new CNI, CNTI, and CNDTI offer a remarkable platform for constructing n-type polymers, and this study demonstrates that cyano-functionalization of BTI is a very effective strategy for developing polymers with deep-lying LUMOs for high-performance n-type org. electronic devices.
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    Shi, Y.; Li, J.; Sun, H.; Li, Y.; Wang, Y.; Wu, Z.; Jeong, S. Y.; Woo, H. Y.; Fabiano, S.; Guo, X. Angew. Chem., Int. Ed. 2022, 61, e202214192,  DOI: 10.1002/anie.202214192
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    35
    Thiazole Imide-Based All-Acceptor Homopolymer with Branched Ethylene Glycol Side Chains for Organic Thermoelectrics
    Shi, Yongqiang; Li, Jianfeng; Sun, Hengda; Li, Yongchun; Wang, Yimei; Wu, Ziang; Jeong, Sang Young; Woo, Han Young; Fabiano, Simone; Guo, Xugang
    Angewandte Chemie, International Edition (2022), 61 (51), e202214192CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    N-Type semiconducting polymers with high thermoelec. performance remain challenging due to the scarcity of mol. design strategy, limiting their applications in org. thermoelec. (OTE) devices. Herein, we provide a new approach to enhance the OTE performance of n-doped polymers by introducing acceptor-acceptor (A-A) type backbone bearing branched ethylene glycol (EG) side chains. When doped with 4-(2,3-dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine (N-DMBI), the A-A homopolymer PDTzTI-TEG exhibits n-type elec. cond. (σ) up to 34 S cm-1 and power factor value of 15.7 μW m-1 K-2. The OTE performance of PDTzTI-TEG is far greater than that of homopolymer PBTI-TEG (σ=0.27 S cm-1), indicating that introducing electron-deficient thiazole units in the backbone further improves the n-doping efficiency. These results demonstrate that developing A-A type polymers with EG side chains is an effective strategy to enhance n-type OTE performance.
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  36. 36
    Liu, J.; Shi, Y.; Dong, J.; Nugraha, M. I.; Qiu, X.; Su, M.; Chiechi, R. C.; Baran, D.; Portale, G.; Guo, X.; Koster, L. J. A. ACS Energy Lett. 2019, 4, 1556– 1564,  DOI: 10.1021/acsenergylett.9b00977
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    36
    Overcoming Coulomb interaction improves free-charge generation and thermoelectric properties for n-doped conjugated polymers
    Liu, Jian; Shi, Yongqiang; Dong, Jingjin; Nugraha, Mohamad I.; Qiu, Xinkai; Su, Mengyao; Chiechi, Ryan C.; Baran, Derya; Portale, Giuseppe; Guo, Xugang; Koster, L. Jan Anton
    ACS Energy Letters (2019), 4 (7), 1556-1564CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    Mol. doping of org. semiconductors creates Coulombically bound charge and counterion pairs through a charge-transfer process. However, their Coulomb interactions and strategies to mitigate their effects have been rarely addressed. Here, the authors report that the no. of free charges and thermoelec. properties are greatly enhanced by overcoming the Coulomb interaction in an n-doped conjugated polymer. Poly(2,2'-bithiazolothienyl-4,4',10,10'-tetracarboxydiimide) (PDTzTI) and the benchmark N2200 are n-doped by tetrakis (dimethylamino) ethylene (TDAE) for thermoelecs. Doped PDTzTI exhibits ∼10 times higher free-charge d. and 500 times higher cond. than doped N2200, leading to a power factor of 7.6μW m-1 K-2 and ZT of 0.01 at room temp. Compared to N2200, PDTzTI features a better mol. ordering and two-dimensional charge delocalization, which help overcome the Coulomb interaction in the doped state. Consequently, free charges are more easily generated from charge-counterion pairs. This work provides a strategy for improving n-type thermoelecs. by tackling electrostatic interactions.
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  37. 37
    Guo, H.; Yang, C.-Y.; Zhang, X.; Motta, A.; Feng, K.; Xia, Y.; Shi, Y.; Wu, Z.; Yang, K.; Chen, J.; Liao, Q.; Tang, Y.; Sun, H.; Woo, H. Y.; Fabiano, S.; Facchetti, A.; Guo, X. Nature 2021, 599, 67– 73,  DOI: 10.1038/s41586-021-03942-0
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    Transition metal-catalysed molecular n-doping of organic semiconductors
    Guo, Han; Yang, Chi-Yuan; Zhang, Xianhe; Motta, Alessandro; Feng, Kui; Xia, Yu; Shi, Yongqiang; Wu, Ziang; Yang, Kun; Chen, Jianhua; Liao, Qiaogan; Tang, Yumin; Sun, Huiliang; Woo, Han Young; Fabiano, Simone; Facchetti, Antonio; Guo, Xugang
    Nature (London, United Kingdom) (2021), 599 (7883), 67-73CODEN: NATUAS; ISSN:1476-4687. (Nature Portfolio)
    Chem. doping is a key process for investigating charge transport in org. semiconductors and improving certain (opto)electronic devices1-9. N(electron)-doping is fundamentally more challenging than p(hole)-doping and typically achieves a very low doping efficiency (η) of less than 10%1,10. An efficient mol. n-dopant should simultaneously exhibit a high reducing power and air stability for broad applicability1,5,6,9,11, which is very challenging. Here we show a general concept of catalyzed n-doping of org. semiconductors using air-stable precursor-type mol. dopants. Incorporation of a transition metal (for example, Pt, Au, Pd) as vapor-deposited nanoparticles or soln.-processable organometallic complexes (for example, Pd2(dba)3) catalyzes the reaction, as assessed by exptl. and theor. evidence, enabling greatly increased η in a much shorter doping time and high elec. conductivities (above 100 S cm-1; ref. 12). This methodol. has technol. implications for realizing improved semiconductor devices and offers a broad exploration space of ternary systems comprising catalysts, mol. dopants and semiconductors, thus opening new opportunities in n-doping research and applications12, 13.
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  38. 38
    Dong, C.; Deng, S.; Meng, B.; Liu, J.; Wang, L. Angew. Chem., Int. Ed. 2021, 60, 16184– 16190,  DOI: 10.1002/anie.202105127
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    38
    A Distannylated Monomer of a Strong Electron-Accepting Organoboron Building Block: Enabling Acceptor-Acceptor-Type Conjugated Polymers for n-Type Thermoelectric Applications
    Dong, Changshuai; Deng, Sihui; Meng, Bin; Liu, Jun; Wang, Lixiang
    Angewandte Chemie, International Edition (2021), 60 (29), 16184-16190CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Acceptor-acceptor (A-A) copolymn. is an effective strategy to develop high-performance n-type conjugated polymers. However, the development of A-A type conjugated polymers is challenging due to the synthetic difficulty. Herein, a distannylated monomer of strong electron-deficient double B←N bridged bipyridine (BNBP) unit is readily synthesized and used to develop A-A type conjugated polymers by Stille polycondensation. The resulting polymers show ultralow LUMO energy levels of -4.4 eV, which is among the lowest value reported for organoboron polymers. After n-doping, the resulting polymers exhibit elec. cond. of 7.8 S cm-1 and power factor of 24.8μW m-1 K-2. This performance is among the best for n-type polymer thermoelec. materials. These results demonstrate the great potential of A-A type organoboron polymers for high-performance n-type thermoelecs.
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  39. 39
    Wang, S.; Sun, H.; Ail, U.; Vagin, M.; Persson, P. O. Å.; Andreasen, J. W.; Thiel, W.; Berggren, M.; Crispin, X.; Fazzi, D.; Fabiano, S. Adv. Mater. 2016, 28, 10764– 10771,  DOI: 10.1002/adma.201603731
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    39
    Thermoelectric Properties of Solution-Processed n-Doped Ladder-Type Conducting Polymers
    Wang, Suhao; Sun, Hengda; Ail, Ujwala; Vagin, Mikhail; Persson, Per O. A.; Andreasen, Jens W.; Thiel, Walter; Berggren, Magnus; Crispin, Xavier; Fazzi, Daniele; Fabiano, Simone
    Advanced Materials (Weinheim, Germany) (2016), 28 (48), 10764-10771CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    This paper describes about thermoelec. properties of soln.-processed n-doped ladder-type conducting polymers. This paper describes about relationship between backbone structure of polymer and polaron delocalization length, setting mol.-design conjugated polymers.
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  40. 40
    Yang, C.-Y.; Stoeckel, M.-A.; Ruoko, T.-P.; Wu, H.-Y.; Liu, X.; Kolhe, N. B.; Wu, Z.; Puttisong, Y.; Musumeci, C.; Massetti, M.; Sun, H.; Xu, K.; Tu, D.; Chen, W. M.; Woo, H. Y.; Fahlman, M.; Jenekhe, S. A.; Berggren, M.; Fabiano, S. Nat. Commun. 2021, 12, 2354,  DOI: 10.1038/s41467-021-22528-y
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    A high-conductivity n-type polymeric ink for printed electronics
    Yang, Chi-Yuan; Stoeckel, Marc-Antoine; Ruoko, Tero-Petri; Wu, Han-Yan; Liu, Xianjie; Kolhe, Nagesh B.; Wu, Ziang; Puttisong, Yuttapoom; Musumeci, Chiara; Massetti, Matteo; Sun, Hengda; Xu, Kai; Tu, Deyu; Chen, Weimin M.; Woo, Han Young; Fahlman, Mats; Jenekhe, Samson A.; Berggren, Magnus; Fabiano, Simone
    Nature Communications (2021), 12 (1), 2354CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    Conducting polymers, such as the p-doped poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), have enabled the development of an array of opto- and bio-electronics devices. However, to make these technologies truly pervasive, stable and easily processable, n-doped conducting polymers are also needed. Despite major efforts, no n-type equiv. to the benchmark PEDOT:PSS exist to date. Here, we report on the development of poly(benzimidazobenzophenanthroline):poly(ethyleneimine) (BBL:PEI) as an ethanol-based n-type conductive ink. BBL:PEI thin films yield an n-type elec. cond. reaching 8 S cm-1, along with excellent thermal, ambient, and solvent stability. This printable n-type mixed ion-electron conductor has several technol. implications for realizing high-performance org. electronic devices, as demonstrated for org. thermoelec. generators with record high power output and n-type org. electrochem. transistors with a unique depletion mode of operation. BBL:PEI inks hold promise for the development of next-generation bioelectronics and wearable devices, in particular targeting novel functionality, efficiency, and power performance.
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    Lu, Y.; Yu, Z.-D.; Zhang, R.-Z.; Yao, Z.-F.; You, H.-Y.; Jiang, L.; Un, H.-I.; Dong, B.-W.; Xiong, M.; Wang, J.-Y.; Pei, J. Angew. Chem., Int. Ed. 2019, 58, 11390– 11394,  DOI: 10.1002/anie.201905835
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    Rigid Coplanar Polymers for Stable n-Type Polymer Thermoelectrics
    Lu, Yang; Yu, Zi-Di; Zhang, Run-Zhi; Yao, Ze-Fan; You, Hao-Yang; Jiang, Li; Un, Hio-Ieng; Dong, Bo-Wei; Xiong, Miao; Wang, Jie-Yu; Pei, Jian
    Angewandte Chemie, International Edition (2019), 58 (33), 11390-11394CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Low n-doping efficiency and inferior stability restrict the thermoelec. performance of n-type conjugated polymers, making their performance lag far behind of their p-type counterparts. Reported here are two rigid coplanar poly(p-phenylene vinylene) (PPV) derivs., LPPV-1 and LPPV-2, which show nearly torsion-free backbones. The fused electron-deficient rigid structures endow the derivs. with less conformational disorder and low-lying LUMO levels, down to -4.49 eV. After doping, two polymers exhibited high n-doping efficiency and significantly improved air stability. LPPV-1 exhibited a high cond. of up to 1.1 S cm-1 and a power factor as high as 1.96 μW m-1 K-2. Importantly, the power factor of the doped LPPV-1 thick film degraded only 2 % after 7 day exposure to air. This work demonstrates a new strategy for designing conjugated polymers, with planar backbones and low LUMO levels, towards high-performance and potentially air-stable n-type polymer thermoelecs.
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    Großkopf, J.; Plaza, M.; Seitz, A.; Breitenlechner, S.; Storch, G.; Bach, T. J. Am. Chem. Soc. 2021, 143, 21241– 21245,  DOI: 10.1021/jacs.1c11266
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    Photochemical Deracemization at sp3-Hybridized Carbon Centers via a Reversible Hydrogen Atom Transfer
    Grosskopf, Johannes; Plaza, Manuel; Seitz, Antonia; Breitenlechner, Stefan; Storch, Golo; Bach, Thorsten
    Journal of the American Chemical Society (2021), 143 (50), 21241-21245CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A photochem. deracemization of 5-substituted 3-phenylimidazolidine-2,4-diones (hydantoins) is reported (27 examples, 69%-quant., 80-99% ee). The reaction is catalyzed by a chiral diarylketone which displays a two-point hydrogen bonding site. Mechanistic evidence (DFT calcns., radical clock expts., H/D labeling) suggests the reaction occurs by selective hydrogen atom transfer (HAT). Upon hydrogen binding, one substrate enantiomer displays the hydrogen atom at the stereogenic center to the photoexcited catalyst allowing for a HAT from the substrate and eventually for its conversion into the product enantiomer. The product enantiomer is not processed by the catalyst and is thus enriched in the photostationary state.
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    Alsufyani, M.; Stoeckel, M.-A.; Chen, X.; Thorley, K.; Hallani, R. K.; Puttisong, Y.; Ji, X.; Meli, D.; Paulsen, B. D.; Strzalka, J.; Regeta, K.; Combe, C.; Chen, H.; Tian, J.; Rivnay, J.; Fabiano, S.; McCulloch, I. Angew. Chem., Int. Ed. 2022, 61, e202113078,  DOI: 10.1002/anie.202113078
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    Lactone Backbone Density in Rigid Electron-Deficient Semiconducting Polymers Enabling High n-type Organic Thermoelectric Performance
    Alsufyani, Maryam; Stoeckel, Marc-Antoine; Chen, Xingxing; Thorley, Karl; Hallani, Rawad K.; Puttisong, Yuttapoom; Ji, Xudong; Meli, Dilara; Paulsen, Bryan D.; Strzalka, Joseph; Regeta, Khrystyna; Combe, Craig; Chen, Hu; Tian, Junfu; Rivnay, Jonathan; Fabiano, Simone; McCulloch, Iain
    Angewandte Chemie, International Edition (2022), 61 (7), e202113078CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Three lactone-based rigid semiconducting polymers were designed to overcome major limitations in the development of n-type org. thermoelecs., namely elec. cond. and air stability. Exptl. and theor. investigations demonstrated that increasing the lactone group d. by increasing the benzene content from 0% benzene (P-0), to 50% (P-50), and 75% (P-75) resulted in progressively larger electron affinities (up to 4.37 eV), suggesting a more favorable doping process, when employing (N-DMBI) as the dopant. Larger polaron delocalization was also evident, due to the more planarized conformation, which is proposed to lead to a lower hopping energy barrier. As a consequence, the elec. cond. increased by three orders of magnitude, to achieve values of up to 12 S cm and Power factors of 13.2μWm-1 K-2 were thereby enabled. These findings present new insights into material design guidelines for the future development of air stable n-type org. thermoelecs.
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    Marks, A.; Chen, X.; Wu, R.; Rashid, R. B.; Jin, W.; Paulsen, B. D.; Moser, M.; Ji, X.; Griggs, S.; Meli, D.; Wu, X.; Bristow, H.; Strzalka, J.; Gasparini, N.; Costantini, G.; Fabiano, S.; Rivnay, J.; McCulloch, I. J. Am. Chem. Soc. 2022, 144, 4642– 4656,  DOI: 10.1021/jacs.2c00735
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    Synthetic Nuances to Maximize n-Type Organic Electrochemical Transistor and Thermoelectric Performance in Fused Lactam Polymers
    Marks, Adam; Chen, Xingxing; Wu, Ruiheng; Rashid, Reem B.; Jin, Wenlong; Paulsen, Bryan D.; Moser, Maximilian; Ji, Xudong; Griggs, Sophie; Meli, Dilara; Wu, Xiaocui; Bristow, Helen; Strzalka, Joseph; Gasparini, Nicola; Costantini, Giovanni; Fabiano, Simone; Rivnay, Jonathan; McCulloch, Iain
    Journal of the American Chemical Society (2022), 144 (10), 4642-4656CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A series of fully fused n-type mixed conduction lactam polymers p(g7NCnN), systematically increasing the alkyl side chain content, are synthesized via an inexpensive, nontoxic, precious-metal-free aldol polycondensation. Employing these polymers as channel materials in org. electrochem. transistors (OECTs) affords state-of-the-art n-type performance with p(g7NC10N) recording an OECT electron mobility of 1.20 x 10-2 cm2 V-1 s-1 and a μC* figure of merit of 1.83 F cm-1 V-1 s-1. In parallel to high OECT performance, upon soln. doping with (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI), the highest thermoelec. performance is obsd. for p(g7NC4N), with a max. elec. cond. of 7.67 S cm-1 and a power factor of 10.4μW m-1 K-2. These results are among the highest reported for n-type polymers. Importantly, while this series of fused polylactam org. mixed ionic-electronic conductors (OMIECs) highlights that synthetic mol. design strategies to bolster OECT performance can be translated to also achieve high org. thermoelec. (OTE) performance, a nuanced synthetic approach must be used to optimize performance. Herein, we outline the performance metrics and provide new insights into the mol. design guidelines for the next generation of high-performance n-type materials for mixed conduction applications, presenting for the first time the results of a single polymer series within both OECT and OTE applications.
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  45. 45
    Tang, H.; Liang, Y.; Liu, C.; Hu, Z.; Deng, Y.; Guo, H.; Yu, Z.; Song, A.; Zhao, H.; Zhao, D.; Zhang, Y.; Guo, X.; Pei, J.; Ma, Y.; Cao, Y.; Huang, F. Nature 2022, 611, 271– 277,  DOI: 10.1038/s41586-022-05295-8
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    A solution-processed n-type conducting polymer with ultrahigh conductivity
    Tang, Haoran; Liang, Yuanying; Liu, Chunchen; Hu, Zhicheng; Deng, Yifei; Guo, Han; Yu, Zidi; Song, Ao; Zhao, Haiyang; Zhao, Duokai; Zhang, Yuanzhu; Guo, Xugang; Pei, Jian; Ma, Yuguang; Cao, Yong; Huang, Fei
    Nature (London, United Kingdom) (2022), 611 (7935), 271-277CODEN: NATUAS; ISSN:1476-4687. (Nature Portfolio)
    Conducting polymers (CPs) with high cond. and soln. processability have made great advances since the pioneering work on doped polyacetylene1-3, thus creating the new field of 'org. synthetic metals,4. Various high-performance CPs have been realized, which enable the applications of several org. electronic devices5,6. Nevertheless, most CPs exhibit hole-dominant (p-type) transport behavior7,8, whereas the development of n-type analogs lags far behind and only a few exhibit metallic state, typically limited by low doping efficiency and ambient instability. Here we present a facilely synthesized highly conductive n-type polymer poly(benzodifurandione) (PBFDO). The reaction combines oxidative polymn. and in situ reductive n-doping, greatly increasing the doping efficiency, and a doping level of almost 0.9 charges per repeating unit can be achieved. The resultant polymer exhibits a breakthrough cond. of more than 2,000 S cm-1 with excellent stability and an unexpected soln. processability without extra side chains or surfactants. Furthermore, detailed investigations on PBFDO show coherent charge-transport properties and existence of metallic state. The benchmark performances in electrochem. transistors and thermoelec. generators are further demonstrated, thus paving the way for application of the n-type CPs in org. electronics.
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  46. 46
    Wang, Y.; Nakano, M.; Michinobu, T.; Kiyota, Y.; Mori, T.; Takimiya, K. Macromolecules 2017, 50, 857– 864,  DOI: 10.1021/acs.macromol.6b02313
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    46
    Naphthodithiophenediimide-Benzobisthiadiazole-Based Polymers: Versatile n-Type Materials for Field-Effect Transistors and Thermoelectric Devices
    Wang, Yang; Nakano, Masahiro; Michinobu, Tsuyoshi; Kiyota, Yasuhiro; Mori, Takehiko; Takimiya, Kazuo
    Macromolecules (Washington, DC, United States) (2017), 50 (3), 857-864CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
    New π-conjugated polymers with strong electron affinity, PNDTI-BBTs, consisting of naphtho[2,3-b:6,7-b']dithiophenediimide (NDTI) and benzo[1,2-c:4,5-c']bis[1,2,5]thiadiazole (BBT) units, were synthesized. PNDTI-BBTs have low-lying LUMO energy levels (∼-4.4 eV), which is sufficiently low for air-stable electron transport in org. field-effect transistors and for being readily doped by a well-known n-dopant, N,N-dimethyl-2-phenyl-2,3-dihydro-1H-benzoimidazole (N-DMBI), affording doped polymer films with relatively high conductivities and Seebeck coeffs. Depending on the solubilizing alkyl groups (2-decyltetradecyl, PNDTI-BBT-DT, or 3-decylpentadecyl groups, PNDTI-BBT-DP), not only the electron mobility in the transistor devices with the pristine polymer thin films (PNDTI-BBT-DT: ∼0.096 cm2 V-1 s-1; PNDTI-BBT-DP: ∼0.31 cm2 V-1 s-1) but also the cond. and power factor of the doped thins films (PNDTI-BBT-DT: ∼0.18 S cm-1 and ∼0.6 μW m-1 K-2; PNDTI-BBT-DP: ∼5.0 S cm-1 and ∼14 μW m-1 K-2) were drastically changed. The differences in the elec. properties were primarily ascribed to the better cryst. nature of the PNDTI-BBT-DP than those of PNDTI-BBT-DT in the thin-film state. Furthermore, UV-vis and ESR spectra demonstrated that doping effectiveness was largely affected by the alkyl groups: the PNDTI-BBT-DP films with better cryst. order prevented overdoping, resulting in ca. 20 times higher cond. and power factors. From these results, it can be concluded that tuning the intermol. interaction and consequently obtaining the thin-film with well-ordered polymers by the alkyl side chains is a promising strategy for developing superior thermoelec. materials.
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    Liu, J.; Garman, M. P.; Dong, J.; van der Zee, B.; Qiu, L.; Portale, G.; Hummelen, J. C.; Koster, L. J. A. ACS Appl. Energy Mater. 2019, 2, 6664– 6671,  DOI: 10.1021/acsaem.9b01179
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    47
    Doping Engineering Enables Highly Conductive and Thermally Stable n-Type Organic Thermoelectrics with High Power Factor
    Liu, Jian; Garman, Matt P.; Dong, Jingjin; van der Zee, Bas; Qiu, Li; Portale, Giuseppe; Hummelen, Jan C.; Koster, L. Jan Anton
    ACS Applied Energy Materials (2019), 2 (9), 6664-6671CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
    This work exploits the scope of doping engineering as an enabler for better-performing and thermally stable n-type org. thermoelecs. A fullerene deriv. with polar triethylene glycol type side chain (PTEG-1) is doped either by "coprocessing doping" with n-type dopants such as n-DMBI and TBAF or by "sequential doping" through thermal deposition of Cs2CO3. Solid-state diffusion of Cs2CO3 appears to dope PTEG-1 in the strongest manner, leading to the highest elec. cond. of ∼7.5 S/cm and power factor of 32μW/(m K2). Moreover, the behavior of differently doped PTEG-1 films under thermal stress is examd. by elec. and spectroscopic means. Cs2CO3-doped films are most stable, likely due to a coordinating interaction between the polar side chain and Cs+-based species, which immobilizes the dopant. The high power factor and good thermal stability of Cs2CO3-doped PTEG-1 make it very promising for tangible thermoelec. applications.
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    Liu, J.; Qiu, L.; Portale, G.; Koopmans, M.; Ten Brink, G.; Hummelen, J. C.; Koster, L. J. A. Adv. Mater. 2017, 29, 1701641,  DOI: 10.1002/adma.201701641
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    Liu, J.; van der Zee, B.; Alessandri, R.; Sami, S.; Dong, J.; Nugraha, M. I.; Barker, A. J.; Rousseva, S.; Qiu, L.; Qiu, X.; Klasen, N.; Chiechi, R. C.; Baran, D.; Caironi, M.; Anthopoulos, T. D.; Portale, G.; Havenith, R. W. A.; Marrink, S. J.; Hummelen, J. C.; Koster, L. J. A. Nat. Commun. 2020, 11, 5694,  DOI: 10.1038/s41467-020-19537-8
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    N-type organic thermoelectrics: demonstration of ZT > 0.3
    Liu, Jian; van der Zee, Bas; Alessandri, Riccardo; Sami, Selim; Dong, Jingjin; Nugraha, Mohamad I.; Barker, Alex J.; Rousseva, Sylvia; Qiu, Li; Qiu, Xinkai; Klasen, Nathalie; Chiechi, Ryan C.; Baran, Derya; Caironi, Mario; Anthopoulos, Thomas D.; Portale, Giuseppe; Havenith, Remco W. A.; Marrink, Siewert J.; Hummelen, Jan C.; Koster, L. Jan Anton
    Nature Communications (2020), 11 (1), 5694CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    The 'phonon-glass electron-crystal' concept has triggered most of the progress that has been achieved in inorg. thermoelecs. in the past two decades. Org. thermoelec. materials, unlike their inorg. counterparts, exhibit mol. diversity, flexible mech. properties and easy fabrication, and are mostly 'phonon glasses'. However, the thermoelec. performances of these org. materials are largely limited by low mol. order and they are therefore far from being 'electron crystals'. Here, we report a molecularly n-doped fullerene deriv. with meticulous design of the side chain that approaches an org. 'PGEC' thermoelec. material. This thermoelec. material exhibits an excellent elec. cond. of >10 S cm-1 and an ultralow thermal cond. of <0.1 Wm-1K-1, leading to the best figure of merit ZT = 0.34 (at 120 °C) among all reported single-host n-type org. thermoelec. materials. The key factor to achieving the record performance is to use 'arm-shaped' double-triethylene-glycol-type side chains, which not only offer excellent doping efficiency (∼60%) but also induce a disorder-to-order transition upon thermal annealing. This study illustrates the vast potential of org. semiconductors as thermoelec. materials.
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    Zhang, Y.; van Doremaele, E. R. W.; Ye, G.; Stevens, T.; Song, J.; Chiechi, R. C.; van de Burgt, Y. Adv. Mater. 2022, 34, 2200393,  DOI: 10.1002/adma.202200393
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    50
    Adaptive Biosensing and Neuromorphic Classification Based on an Ambipolar Organic Mixed Ionic-Electronic Conductor
    Zhang, Yanxi; van Doremaele, Eveline R. W.; Ye, Gang; Stevens, Tim; Song, Jun; Chiechi, Ryan C.; van de Burgt, Yoeri
    Advanced Materials (Weinheim, Germany) (2022), 34 (20), 2200393CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Org. mixed ionic-electronic conductors (OMIECs) are central to bioelectronic applications such as biosensors, health-monitoring devices, and neural interfaces, and have facilitated efficient next-generation brain-inspired computing and biohybrid systems. Despite these examples, smart and adaptive circuits that can locally process and optimize biosignals have not yet been realized. Here, a tunable sensing circuit is shown that can locally modulate biol. relevant signals like electromyograms (EMGs) and electrocardiograms (ECGs), that is based on a complementary logic inverter combined with a neuromorphic memory element, and that is constructed from a single polymer mixed conductor. It is demonstrated that a small neuromorphic array based on this material effects high classification accuracy in heartbeat anomaly detection. This high-performance material allows for straightforward monolithic integration, which reduces fabrication complexity while also achieving high on/off ratios with excellent ambient p- and n-type stability in transistor performance. This material opens a route toward simple and straightforward fabrication and integration of more sophisticated adaptive circuits for future smart bioelectronics.
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    Mei, J.; Kim, D. H.; Ayzner, A. L.; Toney, M. F.; Bao, Z. J. Am. Chem. Soc. 2011, 133, 20130– 20133,  DOI: 10.1021/ja209328m
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    Siloxane-Terminated Solubilizing Side Chains: Bringing Conjugated Polymer Backbones Closer and Boosting Hole Mobilities in Thin-Film Transistors
    Mei, Jianguo; Kim, Do Hwan; Ayzner, Alexander L.; Toney, Michael F.; Bao, Zhenan
    Journal of the American Chemical Society (2011), 133 (50), 20130-20133CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The authors introduce a novel siloxane-terminated solubilizing group and demonstrate its effectiveness as a side chain in an isoindigo-based conjugated polymer. An av. hole mobility of 2.00 cm2 V-1 s-1 (with a max. mobility of 2.48 cm2 V-1 s-1), was obtained from soln.-processed thin-film transistors, one of the highest mobilities reported to date. In contrast, the ref. polymer with a branched alkyl side chain gave an av. hole mobility of 0.30 cm2 V-1 s-1 and a max. mobility of 0.57 cm2 V-1 s-1. This is largely explained by the polymer packing: the authors' new polymer exhibited a π-π stacking distance of 3.58 Å, while the ref. polymer showed a distance of 3.76 Å.
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    Razzell-Hollis, J.; Fleischli, F.; Jahnke, A. A.; Stingelin, N.; Seferos, D. S.; Kim, J.-S. J. Phys. Chem. C 2017, 121, 2088– 2098,  DOI: 10.1021/acs.jpcc.6b11675
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    Effects of Side-Chain Length and Shape on Polytellurophene Molecular Order and Blend Morphology
    Razzell-Hollis, Joseph; Fleischli, Franziska; Jahnke, Ashlee A.; Stingelin, Natalie; Seferos, Dwight S.; Kim, Ji-Seon
    Journal of Physical Chemistry C (2017), 121 (4), 2088-2098CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    We study the mol. order and thin film morphol. of the conjugated polymer polytellurophene, to understand how the tellurium atom and the choice of side-chain influence the conjugated polymer's backbone planarity and performance in org. transistors. We find that poly(3hexyltellurophene) (P3HTe) continues the trend from polythiophene (P3HT) to polyselenophene (P3HS): substitution with Tellurium leads to a more planar backbone, evident from the shifts of the C=C vibrational peak to lower wavenumbers (∼1389 cm-1) and a smaller optical band-gap (∼1.4 eV). Resonant Raman spectroscopy revealed that mol. order was highly dependent on the structure of the P3ATe alkyl side-chain: a longer chains introduces kinetic hindrance, reducing the fraction of ordered phase obtained at room temp., while a branched side-chain introduces steric hindrance, with intrinsic disorder present even when deposited at higher temps. When blended with the insulator HDPE, all three polymers exhibit little addnl. disorder and instead form phase-sepd. networks of high mol. order that are beneficial to percolated charge transport in transistors. We find that mol. order, as measured by Raman, correlates well with reported transistor mobilities and provides a greater understanding of the structure-property relationships that det. the performance of these novel organometallic polymers in electronic devices.
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    Madu, I. K.; Muller, E. W.; Kim, H.; Shaw, J.; Burney-Allen, A. A.; Zimmerman, P.; Jeffries-El, M.; Goodson, T., III J. Phys. Chem. C 2018, 122, 17049– 17066,  DOI: 10.1021/acs.jpcc.8b03914
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    53
    Heteroatom and Side Chain Effects on the Optical and Photophysical Properties: Ultrafast and Nonlinear Spectroscopy of New Naphtho[1,2-b:5,6-b']difuran Donor Polymers
    Madu, Ifeanyi K.; Muller, Evan W.; Kim, Hyungjun; Shaw, Jessica; Burney-Allen, Alfred A.; Zimmerman, Paul; Jeffries-El, Malika; Goodson, Theodore
    Journal of Physical Chemistry C (2018), 122 (30), 17049-17066CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The photophys. and electronic properties of four novel conjugated donor polymers were investigated to understand the influence of heteroatoms (based on the first two member chalcogens) in the polymer backbone. The side chains were varied as well to evaluate the effect of polymer soly. on the photophys. properties. The donor-acceptor polymer structure is based on naphtho[1,2-b:5,6-b']difuran as the donor moiety, and either 3,6-di(furan-2-yl)-1,4-diketopyrrolo[3,4-c]pyrrole or 3,6-di(thiophen-2-yl)-1,4-diketopyrrolo[3,4-c]pyrrole as the acceptor moiety. Steady-state absorption studies showed that the polymers with the furan moiety in the backbone displayed a favorable tendency of capturing more solar photons when used in a photovoltaic device. This is obsd. exptl. by the higher extinction coeff. in the visible and near-IR regions of these polymers relative to that of their thiophene counterparts. The excitonic lifetimes were monitored using ultrafast dynamics, and the results obtained show that the type of heteroatom π-linker used in the backbone affects the decay dynamics. Furthermore, the side chain also plays a role in detg. the fluorescence decay time. Quantum chem. simulations were performed to describe the absorption energies and transition characters. Two-photon absorption cross sections (TPA-δ) were analyzed with the simulations, illustrating the planarity of the backbone in relation to its torsional angles. Because of the planarity in the mol. backbone, the polymer with the furan π-linker showed a higher TPA-δ relative to that of its thiophene counterpart. This suggests that the furan compd. will display higher charge transfer (CT) tendencies in comparison to those of their thiophene analogs. The pump-probe transient absorption technique was employed to probe the nonemissive states (including the CT state) of the polymers, and unique activities were captured at 500 and 750 nm for all of the studied compds. Target and global analyses were performed to understand the dynamics of each peak and deduce the no. of components responsible for the transient behavior obsd. resp. The results obtained suggest that the furan π-linker component of a donor and acceptor moiety in a conjugated polymer might be a more suitable candidate compared with its more popular chalcogenic counterpart, thiophene, for use as donor materials in bulk heterojunction photovoltaic devices.
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    Sun, Y.; Zhang, C.; Dai, B.; Lin, B.; Yang, H.; Zhang, X.; Guo, L.; Liu, Y. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1915– 1926,  DOI: 10.1002/pola.27643
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    Side chain engineering and conjugation enhancement of benzodithiophene and phenanthrenequnioxaline based conjugated polymers for photovoltaic devices
    Sun, Ying; Zhang, Chao; Dai, Bin; Lin, Baoping; Yang, Hong; Zhang, Xueqin; Guo, Lingxiang; Liu, Yurong
    Journal of Polymer Science, Part A: Polymer Chemistry (2015), 53 (16), 1915-1926CODEN: JPACEC; ISSN:0887-624X. (John Wiley & Sons, Inc.)
    A series of donor-acceptor conjugated polymers incorporating benzodithiophene (BDT) as donor unit and phenanthrenequnioxaline as acceptor unit with different side chains have been designed and synthesized. For polymer P1 featuring the BDT unit and alkoxy chains substituted phenanthrenequnioxaline unit in the backbone, serious steric hindrance resulted in quite low mol. wt. The implementation of thiophene ring spacer in polymer P2 greatly suppressed the interannular twisting to extend the effective conjugation length and consequently gave rise to improved absorption property and device performance. In addn., utilizing the alkylthienyl side chains to replace the alkyl side chains at BDT unit in polymer P3 further enhanced the photovoltaic performance due to the increased conjugation length. For polymer P4, translating the alkoxy side chains at the phenanthrenequnioxaline ring into the alkyl side chains at thiophene linker group enhanced mol. planarity and strengthened π-π stacking. Consequently improved absorption property and increased hole mobility were achieved for polymer P4. Our results indicated that side chain engineering not only can influence the soly. of polymer but also can det. the polymer backbone planarity and hence the photovoltaic properties. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015.
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    Chen, S.; Pan, Y.; Chen, K.; Chen, P.; Shen, Q.; Sun, P.; Hu, W.; Fan, Q. Angew. Chem., Int. Ed. 2023, 62, e202215372,  DOI: 10.1002/anie.202215372
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    Increasing Molecular Planarity through Donor/Side-Chain Engineering for Improved NIR-IIa Fluorescence Imaging and NIR-II Photothermal Therapy under 1064 nm
    Chen, Shangyu Y.; Pan, Yonghui H.; Chen, Kai; Chen, Pengfei F.; Shen, Qingming M.; Sun, Pengfei F.; Hu, Wenbo B.; Fan, Quli L.
    Angewandte Chemie, International Edition (2023), 62 (6), e202215372CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Developing conjugated small mols. (CSM) with intense NIR-II (1000-1700 nm) absorption for phototheranostic is highly desirable but remains a tremendous challenge due to a lack of reliable design guidelines. This study reports a high-performance NIR-II CSM for phototheranostic by tailoring mol. planarity. A series of CSM show bathochromic absorption extended to the NIR-II region upon the increasing thiophene no., but an excessive no. of thiophene results in decreased NIR-IIa (1300-1400 nm) brightness and photothermal effects. Further introduction of terminal nonconjugated alkyl chain can enhance NIR-II absorption coeff., NIR-IIa brightness, and photothermal effects. Mechanism studies ascribe this overall enhancement to mol. planarity stemming from the collective contribution of donor/side-chain engineering. This finding directs the design of NIR-II CSM by rational manipulating mol. planarity to perform 1064 nm mediated phototheranostic at high efficiency.
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    Liu, J.; Qiu, L.; Portale, G.; Torabi, S.; Stuart, M. C. A.; Qiu, X.; Koopmans, M.; Chiechi, R. C.; Hummelen, J. C.; Anton Koster, L. J. Nano Energy 2018, 52, 183– 191,  DOI: 10.1016/j.nanoen.2018.07.056
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    Side-chain effects on N-type organic thermoelectrics: A case study of fullerene derivatives
    Liu, Jian; Qiu, Li; Portale, Giuseppe; Torabi, Solmaz; Stuart, Marc C. A.; Qiu, Xinkai; Koopmans, Marten; Chiechi, Ryan C.; Hummelen, Jan C.; Anton Koster, L. Jan
    Nano Energy (2018), 52 (), 183-191CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
    In this contribution, the two key parameters, the polarity and side chain length have been changed to study their effects on n-type org. thermoelecs. of a series of fullerene derivs. Fullerene derivs. bearing either an alkyl side chain or ethylene glycol (EG) side chains of different lengths are used as the host mols. for mol. doping. It is found that the polar EG side chains can enable better miscibility with the polar dopant than the alkyl side chain, which leads to more than 5-fold enhancement of doping efficiency. Beyond the doping efficiency, another crucial parameter of mol. doping, the mol. order, is readily acquired by simultaneous control of the polarity and the length of the side chain. A polar side chain with an appropriate chain length can contribute to increasing Seebeck coeffs. of doped fullerene derivs. more effectively than an alkyl side chain, likely due to the resultant good miscibility and high mol. order. As a result, an optimized power factor of 23.1μW m-1 K-2 is achieved in the fullerene deriv. with a tetraethylene glycol side chain. This represents one of the best n-type org. thermoelecs. Addnl., EG side chains can improve the air stability of n-doped fullerene derivs. films as compared to an alkyl side chain. Our work sheds light on the design of side-chains in efficient n-type small mols. thermoelec. materials and contributes to the understanding of their thermoelec. properties.
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    Jacobs, I. E.; Aasen, E. W.; Oliveira, J. L.; Fonseca, T. N.; Roehling, J. D.; Li, J.; Zhang, G.; Augustine, M. P.; Mascal, M.; Moulé, A. J. J. Mater. Chem. C 2016, 4, 3454– 3466,  DOI: 10.1039/C5TC04207K
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    Comparison of solution-mixed and sequentially processed P3HT:F4TCNQ films: effect of doping-induced aggregation on film morphology
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    Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2016), 4 (16), 3454-3466CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
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  1. Suryoday Prodhan, Alessandro Troisi. Effective Model Reduction Scheme for the Electronic Structure of Highly Doped Semiconducting Polymers. Journal of Chemical Theory and Computation 2024, 20 (22) , 10147-10157. https://doi.org/10.1021/acs.jctc.4c01131
  2. Xinyi Fan, Sihui Deng, Xu Cao, Bin Meng, Junli Hu, Jun Liu. Isomers of n-Type Poly(thiophene-alt-co-thiazole) for Organic Thermoelectrics. ACS Applied Materials & Interfaces 2024, 16 (35) , 46741-46749. https://doi.org/10.1021/acsami.4c08553
  3. Xiantao Peng, Gang Ye, Linlong Zhang, Yazhuo Kuang, Shuyan Shao, Jian Liu. Enhancing Charge Transport Performance of n-Doped Conjugated Polymers by Customizing Amphipathic Side Chains. Macromolecules 2024, 57 (15) , 7156-7164. https://doi.org/10.1021/acs.macromol.4c00837
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  • Abstract

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    Figure 1

    Figure 1. Chemical structures of NDI-2Tz-based D–A copolymers with different substituted OEG side chains and n-type dopant N-DMBI.

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    Figure 2

    Figure 2. Normalized UV–vis–NIR absorption spectra of P-3O, P-6O, and P-B3O in dilute chloroform (CHCl3) (10–5 M) and in films (spin coated from CHCl3 solutions).

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    Figure 3

    Figure 3. (a) Electrical conductivities, (b) Seebeck coefficients, and (c) power factors recorded for P-3O, P-6O, and P-B3O as functions of the N-DMBI ratio employed.

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    Figure 4

    Figure 4. 2D GIWAXS patterns of pristine P-3O (a), P-6O (b), and P-B3O (c) films.

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    Figure 5

    Figure 5. Charge carrier density (a), doping efficiency (b), and mobility (c) of 5-wt %-doped P-3O, P-6O, and P-B3O thin films.

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      Mechanically Robust and Flexible Films of Ionic Liquid-Modulated Polymer Thermoelectric Composites
      Li, Zhipeng; Deng, Liang; Lv, Haicai; Liang, Lirong; Deng, Wenjiang; Zhang, Yichuan; Chen, Guangming
      Advanced Functional Materials (2021), 31 (42), 2104836CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      In the recent decade, polymer thermoelec. (TE) composite has witnessed explosive achievements to address energy generation and utilization. Besides the significant progress in enhancement of TE performance, the high mech. property has received increasing attention, being important for practical applications in complex environments. However, the mech. performance has always been improved at the sacrifice of TE performance, and vice versa, which poses a great challenge. Here, ionic liq. (IL)-assisted fabrication of flexible films of polymer TE composites with simultaneously high TE and mech. performances based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), polyvinyl alc. (PVA), and single-walled carbon nanotubes (SWCNTs) are reported. The resultant composite shows a high TE performance with a power factor of 106.1 ± 8.2μW m-1 K-2 at room temp., and strong mech. robustness with a tensile modulus of 4.2 ± 0.5 GPa and fracture strength of 136.5 ± 10.6 MPa. It is the most mech. robust TE composite known with such a high power factor in the available literature. The present study provides a promising way to help address the longstanding and intractable issue of inferior mech. performance of TE composites without compromising TE performance.
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    11. 11
      Villalva, D. R.; Haque, M. A.; Nugraha, M. I.; Baran, D. ACS Appl. Energy Mater. 2020, 3, 9126– 9132,  DOI: 10.1021/acsaem.0c01511
      11
      Enhanced Thermoelectric Performance and Lifetime in Acid-Doped PEDOT:PSS Films Via Work Function Modification
      Villalva, Diego Rosas; Haque, Md Azimul; Nugraha, Mohamad Insan; Baran, Derya
      ACS Applied Energy Materials (2020), 3 (9), 9126-9132CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
      In recent years, most of the works on p-type org. thermoelecs. have focused on improving the thermoelec. properties of poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) through a sequential doping-dedoping process. However, the air-stability of thermoelec. parameters of these systems, which is essential for the realization of reliable devices, remains largely unexplored. In this study, poly(ethyleneimine)-ethoxylate (PEIE) acts as a work function modification agent and an encapsulation layer to improve the thermoelec. performance and air-stability of nitric acid (HNO3)-doped PEDOT:PSS films. The evapn. of HNO3 is responsible for a simultaneous decrease in elec. cond. and an increase in the Seebeck coeff. leading to the degrdn. of the power factor. PEIE reduces the evapn. of HNO3 from PEDOT:PSS and increases the power factor from 72 to 168 μW m-1 K-2. After a week of exposure to air, the films show a power factor of 124 μW m-1 K-2, retaining 74% of its initial thermoelec. merits. These results underscore the importance of PEIE as a material for enhancing thermoelec. performance and air-stability in the development of polymer-based thermoelecs.
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    12. 12
      Xue, Y.; Gao, C.; Liang, L.; Wang, X.; Chen, G. J. Mater.Chem. A 2018, 6, 22381– 22390,  DOI: 10.1039/C8TA09656B
      12
      Nanostructure controlled construction of high-performance thermoelectric materials of polymers and their composites
      Xue, Yufeng; Gao, Chunmei; Liang, Lirong; Wang, Xin; Chen, Guangming
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2018), 6 (45), 22381-22390CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Being emerging green energy materials, org. polymer thermoelec. (TE) composites have attracted much current interest and witnessed a rapid development due to their diverse advantages, such as low d., low thermal cond., high flexibility, and highly tunable mol. structure. By adjusting the polymer structure and fabrication of their composites, the TE performance can be significantly enhanced. In this review, recent advances in the nanostructures of polymers and their composites for TE applications are introduced. The TE performance of polymers and their composites can be greatly improved and highly tuned by controlled construction of nanostructures via both in situ polymn. and phys. mixing. The mechanism of the relation between nanostructures and TE performance is also discussed. Finally, the outlooks of future research are remarked.
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    13. 13
      Fan, Z.; Du, D.; Guan, X.; Ouyang, J. Nano Energy 2018, 51, 481– 488,  DOI: 10.1016/j.nanoen.2018.07.002
      13
      Polymer films with ultrahigh thermoelectric properties arising from significant seebeck coefficient enhancement by ion accumulation on surface
      Fan, Zeng; Du, Donghe; Guan, Xin; Ouyang, Jianyong
      Nano Energy (2018), 51 (), 481-488CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
      Org. thermoelec. (TE) materials have drawn great interest because of their advantages including mech. flexibility, easy availability, non-toxicity and low thermal cond. TE materials with high dimensionless figure-of-merit ZT are required for highly efficient TE conversion. But the elec. cond. and Seebeck coeff. of TE materials are interdependent. The increase in Seebeck coeff. is usually at the cost of the decrease in elec. cond. In this work, we report a facile approach to significantly enhance the TE properties of poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) films by ion accumulation of an ionic liq. on the polymer surface. The ion accumulation can increase the Seebeck coeff. of the PEDOT:PSS films by 1.2-2 fold while it does not remarkably affect the elec. cond. The PEDOT:PSS films can exhibit an ultrahigh power factor of 754μW m-1 K-2, corresponding to a ZT value of 0.75. This ZT value is comparable to that of inorg. TE materials like bismuth telluride at 300 K.
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    14. 14
      Culebras, M.; Gómez, C. M.; Cantarero, A. J. Mater. Chem. A 2014, 2, 10109– 10115,  DOI: 10.1039/C4TA01012D
      14
      Enhanced thermoelectric performance of PEDOT with different counter-ions optimized by chemical reduction
      Culebras, M.; Gomez, C. M.; Cantarero, A.
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2014), 2 (26), 10109-10115CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      This work reports on the synthesis of the intrinsically conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) doped with several counterions, ClO4, PF6 and bis(trifluoromethylsulfonyl)imide (BTFMSI), by electro-polymn. and its thermoelec. properties. Depending on the counterion size, the thermoelec. efficiency of PEDOT can be increased up to two orders of magnitude. A further chem. redn. with hydrazine optimizes the power factor (PF). By changing the counterions, the authors were able to increase the elec. cond. (σ) of PEDOT by a factor of three, while the Seebeck coeff. remains at the same order of magnitude in the three polymers. The best thermoelec. efficiency was obsd. in PEDOT:BTFMSI. From the measurement of the Seebeck coeff. and σ, a PF of 147 μW m-1 K-2 was deduced, while the measured thermal cond. is κ = 0.19 W m-1 K-1, resulting in a ZT ∼ 0.22 at room temp., one of the highest values reported in the literature for polymers. The increase in σ with the change of the counterion is mainly due to the stretching of the polymer chains. The authors provide a chem. route to further improve ZT in polymers and demonstrate a method of synthesis based on the electro-polymn. on Au. After removing the Au layer, a very thin semiconducting polymer film can be isolated.
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    15. 15
      Kim, G. H.; Shao, L.; Zhang, K.; Pipe, K. P. Nat. Mater. 2013, 12, 719– 723,  DOI: 10.1038/nmat3635
      15
      Engineered doping of organic semiconductors for enhanced thermoelectric efficiency
      Kim, G-H.; Shao, L.; Zhang, K.; Pipe, K. P.
      Nature Materials (2013), 12 (8), 719-723CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      Significant improvements to the thermoelec. figure of merit ZT have emerged in recent years, primarily due to the engineering of material compn. and nanostructure in inorg. semiconductors (ISCs). However, many present high-ZT materials are based on low-abundance elements that pose challenges for scale-up, as they entail high material costs in addn. to brittleness and difficulty in large-area deposition. Here we demonstrate a strategy to improve ZT in conductive polymers and other org. semiconductors (OSCs) for which the base elements are earth-abundant. By minimizing total dopant vol., we show that all three parameters constituting ZT vary in a manner so that ZT increases; this stands in sharp contrast to ISCs, for which these parameters have trade-offs. Reducing dopant vol. is found to be as important as optimizing carrier concn. when maximizing ZT in OSCs. Implementing this strategy with the dopant poly(styrenesulfonate) in poly(3,4-ethylenedioxythiophene) (PEDOT:PSS), we achieve ZT = 0.42 at room temp.
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    16. 16
      Bubnova, O.; Khan, Z. U.; Malti, A.; Braun, S.; Fahlman, M.; Berggren, M.; Crispin, X. Nat. Mater. 2011, 10, 429– 433,  DOI: 10.1038/nmat3012
      16
      Optimization of the thermoelectric figure of merit in the conducting polymer poly(3,4-ethylenedioxythiophene)
      Bubnova, Olga; Khan, Zia Ullah; Malti, Abdellah; Braun, Slawomir; Fahlman, Mats; Berggren, Magnus; Crispin, Xavier
      Nature Materials (2011), 10 (6), 429-433CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      Thermoelec. generators (TEGs) transform a heat flow into electricity. Thermoelec. materials are being investigated for electricity prodn. from waste heat (co-generation) and natural heat sources. For temps. below 200°, the best com. available inorg. semiconductors are Bi2Te3-based alloys, which possess a figure of merit ZT close to one. Most of the recently discovered thermoelec. materials with ZT>2 exhibit one common property, namely their low lattice thermal conductivities. Nevertheless, a high ZT value is not enough to create a viable technol. platform for energy harvesting. To generate electricity from large vols. of warm fluids, heat exchangers must be functionalized with TEGs. This requires thermoelec. materials that are readily synthesized, air stable, environmentally friendly and soln. processable to create patterns on large areas. Conducting polymers might be capable of meeting these demands. The accurate control of the oxidn. level in poly(3,4-ethylenedioxythiophene) (PEDOT) combined with its low intrinsic thermal cond. (λ = 0.37 W m-1 K-1) yields a ZT = 0.25 at room temp. that approaches the values required for efficient devices.
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    17. 17
      Naab, B. D.; Gu, X.; Kurosawa, T.; To, J. W. F.; Salleo, A.; Bao, Z. Adv. Electron. Mater. 2016, 2, 1600004,  DOI: 10.1002/aelm.201600004
      There is no corresponding record for this reference.
    18. 18
      Wang, S.; Sun, H.; Erdmann, T.; Wang, G.; Fazzi, D.; Lappan, U.; Puttisong, Y.; Chen, Z.; Berggren, M.; Crispin, X.; Kiriy, A.; Voit, B.; Marks, T. J.; Fabiano, S.; Facchetti, A. Adv. Mater. 2018, 30, 1801898,  DOI: 10.1002/adma.201801898
      There is no corresponding record for this reference.
    19. 19
      Song, Y.; Ding, J.; Dai, X.; Li, C.; Di, C.-a.; Zhang, D. ACS Mater. Lett. 2022, 4, 521– 527,  DOI: 10.1021/acsmaterialslett.2c00026
      19
      Enhancement of the Thermoelectric Performance of n-Type Naphthalene Diimide-Based Conjugated Polymer by Engineering of Side Alkyl Chains
      Song, Yilin; Ding, Jiamin; Dai, Xiaojuan; Li, Cheng; Di, Chong-an; Zhang, Deqing
      ACS Materials Letters (2022), 4 (4), 521-527CODEN: AMLCEF; ISSN:2639-4979. (American Chemical Society)
      Developing n-type doped semiconducting polymers with high thermoelec. (TE) performance still remains challenging. In this paper, we show a new strategy to enhance the TE performance of n-doped naphthalene diimide (NDI)-based conjugated donor-acceptor (D-A) polymer by introducing one linear alkyl chain for each NDI unit. Film of the PNDI2T-1, in which each NDI unit is attached with one linear and one branching alkyl chains, exhibits higher elec. cond. and a greater power factor (PF) than those of PNDI2T-2 with the same conjugated backbone and two branching side chains, after doping with either (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI) or decahydro-3a,6a,9a-triazaphenalene (TAM) under the same conditions. The optimal PF values of the doped films of PNDI2T-1 with N-DMBI and TAM can reach 1.6 and 3.5 μW m-1K-2, resp., which are among the highest reported TE performances for NDI-based semiconducting polymers after n-doping. The higher TE performance of the doped films of PNDI2T-1 is attributed to the enhancement of charge mobility and improvement of doping degree after side-chain modification. These results provide a new mol. design rationale for the future development of high-performance n-type thermoelec. semiconducting polymers.
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    20. 20
      Wang, Y.; Takimiya, K. Adv. Mater. 2020, 32, 2002060,  DOI: 10.1002/adma.202002060
      20
      Naphthodithiophenediimide-Bithiopheneimide Copolymers for High-Performance n-Type Organic Thermoelectrics: Significant Impact of Backbone Orientation on Conductivity and Thermoelectric Performance
      Wang, Yang; Takimiya, Kazuo
      Advanced Materials (Weinheim, Germany) (2020), 32 (30), 2002060CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The development of n-type conjugated polymers with high elec. cond. (σ) has continued to pose a massive challenge in org. thermoelecs. (OTEs). New structural insights into the charge-carrier transport are necessitated for the realization of high-performance OTEs. In this study, three new n-type copolymers, named pNB, pNB-Tz, and pNB-TzDP, consisting of naphthodithiophenediimide (NDTI) and bithiopheneimide (BTI) units, are synthesized by direct arylation polymn. The backbone orientation is altered by incorporating thiazole units into the backbone and tuning the branching point of the side chain. The alteration of the backbone orientation from face-on to bimodal orientation with both face-on and edge-on fractions significantly impacts the σ and the power factors (PFs) of the polymers. As a result, pNB-TzDP, with the bimodal orientation, demonstrates a high σ of up to 11.6 S cm-1 and PF of up to 53.4μW m-1 K-2, which are among the highest in soln.-processed n-doped conjugated polymers reported so far. Further studies reveal that the bimodal orientation of pNB-TzDP introduces 3D conduction channels and leads to better accommodation of dopants, which should be the key factors for the excellent thermoelec. performance.
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    21. 21
      Liu, J.; Ye, G.; Zee, B. V.; Dong, J.; Qiu, X.; Liu, Y.; Portale, G.; Chiechi, R. C.; Koster, L. J. A. Adv. Mater. 2018, 30, e1804290,  DOI: 10.1002/adma.201804290
      There is no corresponding record for this reference.
    22. 22
      Liu, J.; Qiu, L.; Alessandri, R.; Qiu, X.; Portale, G.; Dong, J.; Talsma, W.; Ye, G.; Sengrian, A. A.; Souza, P. C. T.; Loi, M. A.; Chiechi, R. C.; Marrink, S. J.; Hummelen, J. C.; Koster, L. J. A. Adv. Mater. 2018, 30, 1704630,  DOI: 10.1002/adma.201704630
      There is no corresponding record for this reference.
    23. 23
      Kiefer, D.; Giovannitti, A.; Sun, H.; Biskup, T.; Hofmann, A.; Koopmans, M.; Cendra, C.; Weber, S.; Anton Koster, L. J.; Olsson, E.; Rivnay, J.; Fabiano, S.; McCulloch, I.; Müller, C. ACS Energy Lett. 2018, 3, 278– 285,  DOI: 10.1021/acsenergylett.7b01146
      23
      Enhanced n-Doping Efficiency of a Naphthalenediimide-Based Copolymer through Polar Side Chains for Organic Thermoelectrics
      Kiefer, David; Giovannitti, Alexander; Sun, Hengda; Biskup, Till; Hofmann, Anna; Koopmans, Marten; Cendra, Camila; Weber, Stefan; Anton Koster, L. Jan; Olsson, Eva; Rivnay, Jonathan; Fabiano, Simone; McCulloch, Iain; Muller, Christian
      ACS Energy Letters (2018), 3 (2), 278-285CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      N-doping of conjugated polymers either requires a high dopant fraction or yields a low elec. cond. because of their poor compatibility with mol. dopants. The authors explore n-doping of the polar naphthalenediimide-bithiophene copolymer p(gNDI-gT2) that carries oligoethylene glycol-based side chains and show that the polymer displays superior miscibility with the benzimidazole-dimethylbenzenamine-based n-dopant N-DMBI. The good compatibility of p(gNDI-gT2) and N-DMBI results in a relatively high doping efficiency of 13% for n-dopants, which leads to a high elec. cond. of >10-1 S cm-1 for a dopant concn. of only 10 mol % when measured in an inert atm. The doped polymer is able to maintain its elec. cond. for ∼20 min when exposed to air and recovers rapidly when returned to a N atm. Overall, soln. coprocessing of p(gNDI-gT2) and N-DMBI results in a larger thermoelec. power factor of up to 0.4 μW K-2 m-1 compared to other NDI-based polymers.
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    24. 24
      Liu, J.; Ye, G.; Potgieser, H. G. O.; Koopmans, M.; Sami, S.; Nugraha, M. I.; Villalva, D. R.; Sun, H.; Dong, J.; Yang, X.; Qiu, X.; Yao, C.; Portale, G.; Fabiano, S.; Anthopoulos, T. D.; Baran, D.; Havenith, R. W. A.; Chiechi, R. C.; Koster, L. J. A. Adv. Mater. 2021, 33, e2006694,  DOI: 10.1002/adma.202006694
      24
      Amphipathic Side Chain of a Conjugated Polymer Optimizes Dopant Location toward Efficient N-Type Organic Thermoelectrics
      Liu, Jian; Ye, Gang; Potgieser, Hinderikus G. O.; Koopmans, Marten; Sami, Selim; Nugraha, Mohamad Insan; Villalva, Diego Rosas; Sun, Hengda; Dong, Jingjin; Yang, Xuwen; Qiu, Xinkai; Yao, Chen; Portale, Giuseppe; Fabiano, Simone; Anthopoulos, Thomas D.; Baran, Derya; Havenith, Remco W. A.; Chiechi, Ryan C.; Koster, L. Jan Anton
      Advanced Materials (Weinheim, Germany) (2021), 33 (4), 2006694CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      There is no mol. strategy for selectively increasing the Seebeck coeff. without reducing the elec. cond. for org. thermoelecs. Here, it is reported that the use of amphipathic side chains in an n-type donor-acceptor copolymer can selectively increase the Seebeck coeff. and thus increase the power factor by a factor of ≈5. The amphipathic side chain contains an alkyl chain segment as a spacer between the polymer backbone and an ethylene glycol type chain segment. The use of this alkyl spacer does not only reduce the energetic disorder in the conjugated polymer film but can also properly control the dopant sites away from the backbone, which minimizes the adverse influence of counterions. As confirmed by kinetic Monte Carlo simulations with the host-dopant distance as the only variable, a reduced Coulombic interaction resulting from a larger host-dopant distance contributes to a higher Seebeck coeff. for a given elec. cond. Finally, an optimized power factor of 18μW m-1 K-2 is achieved in the doped polymer film. This work provides a facile mol. strategy for selectively improving the Seebeck coeff. and opens up a new route for optimizing the dopant location toward realizing better n-type polymeric thermoelecs.
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    25. 25
      Ye, G.; Liu, J.; Qiu, X.; Stater, S.; Qiu, L.; Liu, Y.; Yang, X.; Hildner, R.; Koster, L. J. A.; Chiechi, R. C. Macromolecules 2021, 54, 3886– 3896,  DOI: 10.1021/acs.macromol.1c00317
      25
      Controlling n-Type Molecular Doping via Regiochemistry and Polarity of Pendant Groups on Low Band Gap Donor-Acceptor Copolymers
      Ye, Gang; Liu, Jian; Qiu, Xinkai; Stater, Sebastian; Qiu, Li; Liu, Yuru; Yang, Xuwen; Hildner, Richard; Koster, L. Jan Anton; Chiechi, Ryan C.
      Macromolecules (Washington, DC, United States) (2021), 54 (8), 3886-3896CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
      We demonstrate the impact of the type and position of pendant groups on the n-doping of low-band gap donor-acceptor (D-A) copolymers. Polar glycol ether groups simultaneously increase the electron affinities of D-A copolymers and improve the host/dopant miscibility compared to nonpolar alkyl groups, improving the doping efficiency by a factor of over 40. The bulk mobility of the doped films increases with the fraction of polar groups, leading to a best cond. of 0.08 S cm-1 and power factor (PF) of 0.24μW m-1 K-2 in the doped copolymer with the polar pendant groups on both the D and A moieties. We used spatially resolved absorption spectroscopy to relate commensurate morphol. changes to the dispersion of dopants and to the relative local doping efficiency, demonstrating a direct relationship between the morphol. of the polymer phase, the solvation of the mol. dopant, and the elec. properties of doped films. Our work offers fundamental new insights into the influence of the phys. properties of pendant chains on the mol. doping process, which should be generalizable to any molecularly doped polymer films.
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    26. 26
      Yang, C.-Y.; Jin, W.-L.; Wang, J.; Ding, Y.-F.; Nong, S.; Shi, K.; Lu, Y.; Dai, Y.-Z.; Zhuang, F.-D.; Lei, T.; Di, C.-A.; Zhu, D.; Wang, J.-Y.; Pei, J. Adv. Mater. 2018, 30, 1802850,  DOI: 10.1002/adma.201802850
      There is no corresponding record for this reference.
    27. 27
      Yan, X.; Xiong, M.; Li, J.-T.; Zhang, S.; Ahmad, Z.; Lu, Y.; Wang, Z.-Y.; Yao, Z.-F.; Wang, J.-Y.; Gu, X.; Lei, T. J. Am. Chem. Soc. 2019, 141, 20215– 20221,  DOI: 10.1021/jacs.9b10107
      27
      Pyrazine-flanked diketopyrrolopyrrole (DPP): A new polymer building block for high performance n-type organic thermoelectrics
      Yan, Xinwen; Xiong, Miao; Li, Jia-Tong; Zhang, Song; Ahmad, Zachary; Lu, Yang; Wang, Zi-Yuan; Yao, Ze-Fan; Wang, Jie-Yu; Gu, Xiaodan; Lei, Ting
      Journal of the American Chemical Society (2019), 141 (51), 20215-20221CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      N-Doped conjugated polymers usually show low elec. conductivities and low thermoelec. power factors, limiting their applications in n-type org. thermoelecs. Here, we report the synthesis of a new diketopyrrolopyrrole (DPP) deriv., pyrazine-flanked DPP (PzDPP), with the deepest LUMO level in all the reported DPP derivs. Based on PzDPP, a donor-acceptor copolymer, P(PzDPP-CT2), is synthesized. The polymer displays a deep LUMO energy level and strong interchain interaction with a short π-π stacking distance of 3.38 Å. When doped with n-dopant N-DMBI, P(PzDPP-CT2) exhibits high n-type elec. conductivities of up to 8.4 S cm-1 and power factors of up to 57.3μW m-1 K-2. These values are much higher than previously reported n-doped DPP polymers, and the power factor also ranks the highest in soln.-processable n-doped conjugated polymers. These results suggest that PzDPP is a promising high-performance building block for n-type org. thermoelecs. and also highlight that, without sacrificing polymer interchain interactions, efficient n-doping can be realized in conjugated polymers with careful mol. engineering.
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    28. 28
      Yan, X.; Xiong, M.; Deng, X.-Y.; Liu, K.-K.; Li, J.-T.; Wang, X.-Q.; Zhang, S.; Prine, N.; Zhang, Z.; Huang, W.; Wang, Y.; Wang, J.-Y.; Gu, X.; So, S. K.; Zhu, J.; Lei, T. Nat. Commun. 2021, 12, 5723,  DOI: 10.1038/s41467-021-26043-y
      28
      Approaching disorder-tolerant semiconducting polymers
      Yan, Xinwen; Xiong, Miao; Deng, Xin-Yu; Liu, Kai-Kai; Li, Jia-Tong; Wang, Xue-Qing; Zhang, Song; Prine, Nathaniel; Zhang, Zhuoqiong; Huang, Wanying; Wang, Yishan; Wang, Jie-Yu; Gu, Xiaodan; So, Shu Kong; Zhu, Jia; Lei, Ting
      Nature Communications (2021), 12 (1), 5723CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      Doping has been widely used to control the charge carrier concn. in org. semiconductors. However, in conjugated polymers, n-doping is often limited by the tradeoff between doping efficiency and charge carrier mobilities, since dopants often randomly distribute within polymers, leading to significant structural and energetic disorder. Here, we screen a large no. of polymer building block combinations and explore the possibility of designing n-type conjugated polymers with good tolerance to dopant-induced disorder. We show that a carefully designed conjugated polymer with a single dominant planar backbone conformation, high torsional barrier at each dihedral angle, and zigzag backbone curvature is highly dopable and can tolerate dopant-induced disorder. With these features, the designed diketopyrrolopyrrole (DPP)-based polymer can be efficiently n-doped and exhibit high n-type elec. conductivities over 120 S cm-1, much higher than the ref. polymers with similar chem. structures. This work provides a polymer design concept for highly dopable and highly conductive polymeric semiconductors.
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    29. 29
      Lu, Y.; Yu, Z.-D.; Un, H.-I.; Yao, Z.-F.; You, H.-Y.; Jin, W.; Li, L.; Wang, Z.-Y.; Dong, B.-W.; Barlow, S.; Longhi, E.; Di, C.-a.; Zhu, D.; Wang, J.-Y.; Silva, C.; Marder, S. R.; Pei, J. Adv. Mater. 2021, 33, 2005946,  DOI: 10.1002/adma.202005946
      29
      Persistent Conjugated Backbone and Disordered Lamellar Packing Impart Polymers with Efficient n-Doping and High Conductivities
      Lu, Yang; Yu, Zi-Di; Un, Hio-Ieng; Yao, Ze-Fan; You, Hao-Yang; Jin, Wenlong; Li, Liang; Wang, Zi-Yuan; Dong, Bo-Wei; Barlow, Stephen; Longhi, Elena; Di, Chong-an; Zhu, Daoben; Wang, Jie-Yu; Silva, Carlos; Marder, Seth R.; Pei, Jian
      Advanced Materials (Weinheim, Germany) (2021), 33 (2), 2005946CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Soln.-processable highly conductive polymers are of great interest in emerging electronic applications. For p-doped polymers, conductivities as high a nearly 105 S cm-1 have been reported. In the case of n-doped polymers, they often fall well short of the high values noted above, which might be achievable, if much higher charge-carrier mobilities detd. could be realized in combination with high charge-carrier densities. This is in part due to inefficient doping and dopant ions disturbing the ordering of polymers, limiting efficient charge transport and ultimately the achievable conductivities. Here, n-doped polymers that achieve a high cond. of more than 90 S cm-1 by a simple soln.-based co-deposition method are reported. Two conjugated polymers with rigid planar backbones, but with disordered cryst. structures, exhibit surprising structural tolerance to, and excellent miscibility with, commonly used n-dopants. These properties allow both high concns. and high mobility of the charge carriers to be realized simultaneously in n-doped polymers, resulting in excellent elec. cond. and thermoelec. performance.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXisVyis73L&md5=06263c1ba77fe49727624eeb020f5511
    30. 30
      Lu, Y.; Yu, Z.-D.; Liu, Y.; Ding, Y.-F.; Yang, C.-Y.; Yao, Z.-F.; Wang, Z.-Y.; You, H.-Y.; Cheng, X.-F.; Tang, B.; Wang, J.-Y.; Pei, J. J. Am. Chem. Soc. 2020, 142, 15340– 15348,  DOI: 10.1021/jacs.0c05699
      30
      The Critical Role of Dopant Cations in Electrical Conductivity and Thermoelectric Performance of n-Doped Polymers
      Lu, Yang; Yu, Zi-Di; Liu, Yi; Ding, Yi-Fan; Yang, Chi-Yuan; Yao, Ze-Fan; Wang, Zi-Yuan; You, Hao-Yang; Cheng, Xiu-Fen; Tang, Bo; Wang, Jie-Yu; Pei, Jian
      Journal of the American Chemical Society (2020), 142 (36), 15340-15348CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The low n-doping efficiency of conjugated polymers with the mol. dopants limits their availability in elec. cond., thermoelecs., and other elec. applications. Recently, considerable efforts have focused on improving the ionization of dopants by modifying the structures of host polymers or n-dopants; however, the effect of ionized dopants on the elec. cond. and thermoelec. performance of the polymers is still in a puzzle. Herein, we try to reveal the role of mol. dopant cations on carrier transporting through the systematic comparison of two n-dopants, TAM and N-DMBI-H. These two n-dopants exhibit various doping features with the polymer due to their different chem. structure characteristics. For instance, while doping, TAM perturbs negligibly on the polymer backbone conformation and microstructural ordering; and then after ionization, TAM cations possess weak π-backbone affinity but strong intrinsic affinity with side chains, which enables the doped system to screen the Coulomb potential spatially. Such doping features lead to high carrierization capabilities for TAM-doped polymers and further result in the excellent cond. up to 22 ± 2.5 S cm-1 and the power factor over 80μW m-1 K-2, which are significantly higher than the state-of-the-art values of the common n-dopant N-DMBI-H. More importantly, this strategy has also proven to be widely applicable in other doped polymers. Our investigations inspire the vital role of dopant counterions on high elec. and thermoelec. performance polymers and also suggest that, without sacrificing Seebeck coeffs., highly conductivities can be realized with precisely regulating the interaction between the cations and the host.
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    31. 31
      Shi, K.; Zhang, F.; Di, C.-A.; Yan, T.-W.; Zou, Y.; Zhou, X.; Zhu, D.; Wang, J.-Y.; Pei, J. J. Am. Chem. Soc. 2015, 137, 6979– 6982,  DOI: 10.1021/jacs.5b00945
      31
      Toward High Performance n-Type Thermoelectric Materials by Rational Modification of BDPPV Backbones
      Shi, Ke; Zhang, Fengjiao; Di, Chong-An; Yan, Tian-Wei; Zou, Ye; Zhou, Xu; Zhu, Daoben; Wang, Jie-Yu; Pei, Jian
      Journal of the American Chemical Society (2015), 137 (22), 6979-6982CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Three n-type polymers BDPPV, ClBDPPV, and FBDPPV which exhibit outstanding elec. conductivities when mixed with an n-type dopant, N-DMBI ((4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine), in soln. High electron mobility and an efficient doping process endow FBDPPV with the highest elec. conductivities of 14 S cm-1 and power factors up to 28 μW m-1 K-2, which is the highest thermoelec. (TE) power factor that is reported for soln. processable n-type conjugated polymers. The authors' studies reveal that introduction of halogen atoms to the polymer backbones has a dramatic influence on not only the electron mobilities but also the doping levels, both of which are crit. to the elec. conductivities. This work suggests the significance of rational modification of polymer structures and opens the gate for applying the rapidly developed org. semiconductors with high carrier mobilities to thermoelec. field.
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    32. 32
      Zhao, X.; Madan, D.; Cheng, Y.; Zhou, J.; Li, H.; Thon, S. M.; Bragg, A. E.; DeCoster, M. E.; Hopkins, P. E.; Katz, H. E. Adv. Mater. 2017, 29, 1606928,  DOI: 10.1002/adma.201606928
      There is no corresponding record for this reference.
    33. 33
      Han, J.; Fan, H.; Zhang, Q.; Hu, Q.; Russell, T. P.; Katz, H. E. Adv. Funct. Mater. 2021, 31, 2005901,  DOI: 10.1002/adfm.202005901
      33
      Dichlorinated Dithienylethene-Based Copolymers for Air-Stable n-Type Conductivity and Thermoelectricity
      Han, Jinfeng; Fan, Huidong; Zhang, Qingyang; Hu, Qin; Russell, Thomas P.; Katz, Howard E.
      Advanced Functional Materials (2021), 31 (5), 2005901CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Two donor-acceptor (D-A) polymers are obtained by coupling difluoro- and dichloro-substituted forms of the electron-deficient unit BDOPV and the relatively weak donor moiety dichlorodithienylethene (ClTVT). The cond. and power factors of doped devices are different for the chlorinated and fluorinated BDOPV polymers. A high electron cond. of 38.3 and 16.1 S cm-1 are obtained from the chlorinated and fluorinated polymers with N-DMBI, resp., and 12.4 and 2.4 S cm-1 are obtained from the chlorinated and fluorinated polymers with CoCp2, resp., from drop-cast devices. The corresponding power factors are 22.7, 7.6, 39.5, and 8.0μW m-1 K-2, resp. Doping of PClClTVT with N-DMBI results in excellent air stability; the electron cond. of devices with 50 mol% N-DMBI as dopant remained up to 4.9 S m-1 after 222 days in the air, the longest for an n-doped polymer stored in air, with a thermoelec. power factor of 9.3μW m-1 K-2. However, the cond. of PFClTVT-based devices can hardly be measured after 103 days. These observations are consistent with morphologies detd. by grazing incidence wide angle X-ray scattering and at. force microscopy.
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    34. 34
      Feng, K.; Guo, H.; Wang, J.; Shi, Y.; Wu, Z.; Su, M.; Zhang, X.; Son, J. H.; Woo, H. Y.; Guo, X. J. Am. Chem. Soc. 2021, 143, 1539– 1552,  DOI: 10.1021/jacs.0c11608
      34
      Cyano-Functionalized Bithiophene Imide-Based n-Type Polymer Semiconductors: Synthesis, Structure-Property Correlations, and Thermoelectric Performance
      Feng, Kui; Guo, Han; Wang, Junwei; Shi, Yongqiang; Wu, Ziang; Su, Mengyao; Zhang, Xianhe; Son, Jae Hoon; Woo, Han Young; Guo, Xugang
      Journal of the American Chemical Society (2021), 143 (3), 1539-1552CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      N-Type polymers with deep-positioned LUMO energy levels are essential for enabling n-type org. thin-film transistors (OTFTs) with high stability and n-type org. thermoelecs. (OTEs) with high doping efficiency and promising thermoelec. performance. Bithiophene imide (BTI) and its derivs. have been demonstrated as promising acceptor units for constructing high-performance n-type polymers. However, the electron-rich thiophene moiety in BTI leads to elevated LUMOs for the resultant polymers and hence limits their n-type performance and intrinsic stability. Herein, we addressed this issue by introducing strong electron-withdrawing cyano functionality on BTI and its derivs. We have successfully overcome the synthetic challenges and developed a series of novel acceptor building blocks, CNI, CNTI, and CNDTI, which show substantially higher electron deficiencies than does BTI. On the basis of these novel building blocks, acceptor-acceptor type homopolymers and copolymers were successfully synthesized and featured greatly suppressed LUMOs (-3.64 to -4.11 eV) vs. that (-3.48 eV) of the control polymer PBTI. Their deep-positioned LUMOs resulted in improved stability in OTFTs and more efficient n-doping in OTEs for the corresponding polymers with a highest elec. cond. of 23.3 S cm-1 and a power factor of ~ 10μW m-1 K-2. The cond. and power factor are among the highest values reported for soln.-processed molecularly n-doped polymers. The new CNI, CNTI, and CNDTI offer a remarkable platform for constructing n-type polymers, and this study demonstrates that cyano-functionalization of BTI is a very effective strategy for developing polymers with deep-lying LUMOs for high-performance n-type org. electronic devices.
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    35. 35
      Shi, Y.; Li, J.; Sun, H.; Li, Y.; Wang, Y.; Wu, Z.; Jeong, S. Y.; Woo, H. Y.; Fabiano, S.; Guo, X. Angew. Chem., Int. Ed. 2022, 61, e202214192,  DOI: 10.1002/anie.202214192
      35
      Thiazole Imide-Based All-Acceptor Homopolymer with Branched Ethylene Glycol Side Chains for Organic Thermoelectrics
      Shi, Yongqiang; Li, Jianfeng; Sun, Hengda; Li, Yongchun; Wang, Yimei; Wu, Ziang; Jeong, Sang Young; Woo, Han Young; Fabiano, Simone; Guo, Xugang
      Angewandte Chemie, International Edition (2022), 61 (51), e202214192CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      N-Type semiconducting polymers with high thermoelec. performance remain challenging due to the scarcity of mol. design strategy, limiting their applications in org. thermoelec. (OTE) devices. Herein, we provide a new approach to enhance the OTE performance of n-doped polymers by introducing acceptor-acceptor (A-A) type backbone bearing branched ethylene glycol (EG) side chains. When doped with 4-(2,3-dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine (N-DMBI), the A-A homopolymer PDTzTI-TEG exhibits n-type elec. cond. (σ) up to 34 S cm-1 and power factor value of 15.7 μW m-1 K-2. The OTE performance of PDTzTI-TEG is far greater than that of homopolymer PBTI-TEG (σ=0.27 S cm-1), indicating that introducing electron-deficient thiazole units in the backbone further improves the n-doping efficiency. These results demonstrate that developing A-A type polymers with EG side chains is an effective strategy to enhance n-type OTE performance.
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    36. 36
      Liu, J.; Shi, Y.; Dong, J.; Nugraha, M. I.; Qiu, X.; Su, M.; Chiechi, R. C.; Baran, D.; Portale, G.; Guo, X.; Koster, L. J. A. ACS Energy Lett. 2019, 4, 1556– 1564,  DOI: 10.1021/acsenergylett.9b00977
      36
      Overcoming Coulomb interaction improves free-charge generation and thermoelectric properties for n-doped conjugated polymers
      Liu, Jian; Shi, Yongqiang; Dong, Jingjin; Nugraha, Mohamad I.; Qiu, Xinkai; Su, Mengyao; Chiechi, Ryan C.; Baran, Derya; Portale, Giuseppe; Guo, Xugang; Koster, L. Jan Anton
      ACS Energy Letters (2019), 4 (7), 1556-1564CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      Mol. doping of org. semiconductors creates Coulombically bound charge and counterion pairs through a charge-transfer process. However, their Coulomb interactions and strategies to mitigate their effects have been rarely addressed. Here, the authors report that the no. of free charges and thermoelec. properties are greatly enhanced by overcoming the Coulomb interaction in an n-doped conjugated polymer. Poly(2,2'-bithiazolothienyl-4,4',10,10'-tetracarboxydiimide) (PDTzTI) and the benchmark N2200 are n-doped by tetrakis (dimethylamino) ethylene (TDAE) for thermoelecs. Doped PDTzTI exhibits ∼10 times higher free-charge d. and 500 times higher cond. than doped N2200, leading to a power factor of 7.6μW m-1 K-2 and ZT of 0.01 at room temp. Compared to N2200, PDTzTI features a better mol. ordering and two-dimensional charge delocalization, which help overcome the Coulomb interaction in the doped state. Consequently, free charges are more easily generated from charge-counterion pairs. This work provides a strategy for improving n-type thermoelecs. by tackling electrostatic interactions.
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    37. 37
      Guo, H.; Yang, C.-Y.; Zhang, X.; Motta, A.; Feng, K.; Xia, Y.; Shi, Y.; Wu, Z.; Yang, K.; Chen, J.; Liao, Q.; Tang, Y.; Sun, H.; Woo, H. Y.; Fabiano, S.; Facchetti, A.; Guo, X. Nature 2021, 599, 67– 73,  DOI: 10.1038/s41586-021-03942-0
      37
      Transition metal-catalysed molecular n-doping of organic semiconductors
      Guo, Han; Yang, Chi-Yuan; Zhang, Xianhe; Motta, Alessandro; Feng, Kui; Xia, Yu; Shi, Yongqiang; Wu, Ziang; Yang, Kun; Chen, Jianhua; Liao, Qiaogan; Tang, Yumin; Sun, Huiliang; Woo, Han Young; Fabiano, Simone; Facchetti, Antonio; Guo, Xugang
      Nature (London, United Kingdom) (2021), 599 (7883), 67-73CODEN: NATUAS; ISSN:1476-4687. (Nature Portfolio)
      Chem. doping is a key process for investigating charge transport in org. semiconductors and improving certain (opto)electronic devices1-9. N(electron)-doping is fundamentally more challenging than p(hole)-doping and typically achieves a very low doping efficiency (η) of less than 10%1,10. An efficient mol. n-dopant should simultaneously exhibit a high reducing power and air stability for broad applicability1,5,6,9,11, which is very challenging. Here we show a general concept of catalyzed n-doping of org. semiconductors using air-stable precursor-type mol. dopants. Incorporation of a transition metal (for example, Pt, Au, Pd) as vapor-deposited nanoparticles or soln.-processable organometallic complexes (for example, Pd2(dba)3) catalyzes the reaction, as assessed by exptl. and theor. evidence, enabling greatly increased η in a much shorter doping time and high elec. conductivities (above 100 S cm-1; ref. 12). This methodol. has technol. implications for realizing improved semiconductor devices and offers a broad exploration space of ternary systems comprising catalysts, mol. dopants and semiconductors, thus opening new opportunities in n-doping research and applications12, 13.
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    38. 38
      Dong, C.; Deng, S.; Meng, B.; Liu, J.; Wang, L. Angew. Chem., Int. Ed. 2021, 60, 16184– 16190,  DOI: 10.1002/anie.202105127
      38
      A Distannylated Monomer of a Strong Electron-Accepting Organoboron Building Block: Enabling Acceptor-Acceptor-Type Conjugated Polymers for n-Type Thermoelectric Applications
      Dong, Changshuai; Deng, Sihui; Meng, Bin; Liu, Jun; Wang, Lixiang
      Angewandte Chemie, International Edition (2021), 60 (29), 16184-16190CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Acceptor-acceptor (A-A) copolymn. is an effective strategy to develop high-performance n-type conjugated polymers. However, the development of A-A type conjugated polymers is challenging due to the synthetic difficulty. Herein, a distannylated monomer of strong electron-deficient double B←N bridged bipyridine (BNBP) unit is readily synthesized and used to develop A-A type conjugated polymers by Stille polycondensation. The resulting polymers show ultralow LUMO energy levels of -4.4 eV, which is among the lowest value reported for organoboron polymers. After n-doping, the resulting polymers exhibit elec. cond. of 7.8 S cm-1 and power factor of 24.8μW m-1 K-2. This performance is among the best for n-type polymer thermoelec. materials. These results demonstrate the great potential of A-A type organoboron polymers for high-performance n-type thermoelecs.
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    39. 39
      Wang, S.; Sun, H.; Ail, U.; Vagin, M.; Persson, P. O. Å.; Andreasen, J. W.; Thiel, W.; Berggren, M.; Crispin, X.; Fazzi, D.; Fabiano, S. Adv. Mater. 2016, 28, 10764– 10771,  DOI: 10.1002/adma.201603731
      39
      Thermoelectric Properties of Solution-Processed n-Doped Ladder-Type Conducting Polymers
      Wang, Suhao; Sun, Hengda; Ail, Ujwala; Vagin, Mikhail; Persson, Per O. A.; Andreasen, Jens W.; Thiel, Walter; Berggren, Magnus; Crispin, Xavier; Fazzi, Daniele; Fabiano, Simone
      Advanced Materials (Weinheim, Germany) (2016), 28 (48), 10764-10771CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      This paper describes about thermoelec. properties of soln.-processed n-doped ladder-type conducting polymers. This paper describes about relationship between backbone structure of polymer and polaron delocalization length, setting mol.-design conjugated polymers.
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    40. 40
      Yang, C.-Y.; Stoeckel, M.-A.; Ruoko, T.-P.; Wu, H.-Y.; Liu, X.; Kolhe, N. B.; Wu, Z.; Puttisong, Y.; Musumeci, C.; Massetti, M.; Sun, H.; Xu, K.; Tu, D.; Chen, W. M.; Woo, H. Y.; Fahlman, M.; Jenekhe, S. A.; Berggren, M.; Fabiano, S. Nat. Commun. 2021, 12, 2354,  DOI: 10.1038/s41467-021-22528-y
      40
      A high-conductivity n-type polymeric ink for printed electronics
      Yang, Chi-Yuan; Stoeckel, Marc-Antoine; Ruoko, Tero-Petri; Wu, Han-Yan; Liu, Xianjie; Kolhe, Nagesh B.; Wu, Ziang; Puttisong, Yuttapoom; Musumeci, Chiara; Massetti, Matteo; Sun, Hengda; Xu, Kai; Tu, Deyu; Chen, Weimin M.; Woo, Han Young; Fahlman, Mats; Jenekhe, Samson A.; Berggren, Magnus; Fabiano, Simone
      Nature Communications (2021), 12 (1), 2354CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      Conducting polymers, such as the p-doped poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), have enabled the development of an array of opto- and bio-electronics devices. However, to make these technologies truly pervasive, stable and easily processable, n-doped conducting polymers are also needed. Despite major efforts, no n-type equiv. to the benchmark PEDOT:PSS exist to date. Here, we report on the development of poly(benzimidazobenzophenanthroline):poly(ethyleneimine) (BBL:PEI) as an ethanol-based n-type conductive ink. BBL:PEI thin films yield an n-type elec. cond. reaching 8 S cm-1, along with excellent thermal, ambient, and solvent stability. This printable n-type mixed ion-electron conductor has several technol. implications for realizing high-performance org. electronic devices, as demonstrated for org. thermoelec. generators with record high power output and n-type org. electrochem. transistors with a unique depletion mode of operation. BBL:PEI inks hold promise for the development of next-generation bioelectronics and wearable devices, in particular targeting novel functionality, efficiency, and power performance.
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    41. 41
      Lu, Y.; Yu, Z.-D.; Zhang, R.-Z.; Yao, Z.-F.; You, H.-Y.; Jiang, L.; Un, H.-I.; Dong, B.-W.; Xiong, M.; Wang, J.-Y.; Pei, J. Angew. Chem., Int. Ed. 2019, 58, 11390– 11394,  DOI: 10.1002/anie.201905835
      41
      Rigid Coplanar Polymers for Stable n-Type Polymer Thermoelectrics
      Lu, Yang; Yu, Zi-Di; Zhang, Run-Zhi; Yao, Ze-Fan; You, Hao-Yang; Jiang, Li; Un, Hio-Ieng; Dong, Bo-Wei; Xiong, Miao; Wang, Jie-Yu; Pei, Jian
      Angewandte Chemie, International Edition (2019), 58 (33), 11390-11394CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Low n-doping efficiency and inferior stability restrict the thermoelec. performance of n-type conjugated polymers, making their performance lag far behind of their p-type counterparts. Reported here are two rigid coplanar poly(p-phenylene vinylene) (PPV) derivs., LPPV-1 and LPPV-2, which show nearly torsion-free backbones. The fused electron-deficient rigid structures endow the derivs. with less conformational disorder and low-lying LUMO levels, down to -4.49 eV. After doping, two polymers exhibited high n-doping efficiency and significantly improved air stability. LPPV-1 exhibited a high cond. of up to 1.1 S cm-1 and a power factor as high as 1.96 μW m-1 K-2. Importantly, the power factor of the doped LPPV-1 thick film degraded only 2 % after 7 day exposure to air. This work demonstrates a new strategy for designing conjugated polymers, with planar backbones and low LUMO levels, towards high-performance and potentially air-stable n-type polymer thermoelecs.
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    42. 42
      Großkopf, J.; Plaza, M.; Seitz, A.; Breitenlechner, S.; Storch, G.; Bach, T. J. Am. Chem. Soc. 2021, 143, 21241– 21245,  DOI: 10.1021/jacs.1c11266
      42
      Photochemical Deracemization at sp3-Hybridized Carbon Centers via a Reversible Hydrogen Atom Transfer
      Grosskopf, Johannes; Plaza, Manuel; Seitz, Antonia; Breitenlechner, Stefan; Storch, Golo; Bach, Thorsten
      Journal of the American Chemical Society (2021), 143 (50), 21241-21245CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A photochem. deracemization of 5-substituted 3-phenylimidazolidine-2,4-diones (hydantoins) is reported (27 examples, 69%-quant., 80-99% ee). The reaction is catalyzed by a chiral diarylketone which displays a two-point hydrogen bonding site. Mechanistic evidence (DFT calcns., radical clock expts., H/D labeling) suggests the reaction occurs by selective hydrogen atom transfer (HAT). Upon hydrogen binding, one substrate enantiomer displays the hydrogen atom at the stereogenic center to the photoexcited catalyst allowing for a HAT from the substrate and eventually for its conversion into the product enantiomer. The product enantiomer is not processed by the catalyst and is thus enriched in the photostationary state.
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    43. 43
      Alsufyani, M.; Stoeckel, M.-A.; Chen, X.; Thorley, K.; Hallani, R. K.; Puttisong, Y.; Ji, X.; Meli, D.; Paulsen, B. D.; Strzalka, J.; Regeta, K.; Combe, C.; Chen, H.; Tian, J.; Rivnay, J.; Fabiano, S.; McCulloch, I. Angew. Chem., Int. Ed. 2022, 61, e202113078,  DOI: 10.1002/anie.202113078
      43
      Lactone Backbone Density in Rigid Electron-Deficient Semiconducting Polymers Enabling High n-type Organic Thermoelectric Performance
      Alsufyani, Maryam; Stoeckel, Marc-Antoine; Chen, Xingxing; Thorley, Karl; Hallani, Rawad K.; Puttisong, Yuttapoom; Ji, Xudong; Meli, Dilara; Paulsen, Bryan D.; Strzalka, Joseph; Regeta, Khrystyna; Combe, Craig; Chen, Hu; Tian, Junfu; Rivnay, Jonathan; Fabiano, Simone; McCulloch, Iain
      Angewandte Chemie, International Edition (2022), 61 (7), e202113078CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Three lactone-based rigid semiconducting polymers were designed to overcome major limitations in the development of n-type org. thermoelecs., namely elec. cond. and air stability. Exptl. and theor. investigations demonstrated that increasing the lactone group d. by increasing the benzene content from 0% benzene (P-0), to 50% (P-50), and 75% (P-75) resulted in progressively larger electron affinities (up to 4.37 eV), suggesting a more favorable doping process, when employing (N-DMBI) as the dopant. Larger polaron delocalization was also evident, due to the more planarized conformation, which is proposed to lead to a lower hopping energy barrier. As a consequence, the elec. cond. increased by three orders of magnitude, to achieve values of up to 12 S cm and Power factors of 13.2μWm-1 K-2 were thereby enabled. These findings present new insights into material design guidelines for the future development of air stable n-type org. thermoelecs.
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    44. 44
      Marks, A.; Chen, X.; Wu, R.; Rashid, R. B.; Jin, W.; Paulsen, B. D.; Moser, M.; Ji, X.; Griggs, S.; Meli, D.; Wu, X.; Bristow, H.; Strzalka, J.; Gasparini, N.; Costantini, G.; Fabiano, S.; Rivnay, J.; McCulloch, I. J. Am. Chem. Soc. 2022, 144, 4642– 4656,  DOI: 10.1021/jacs.2c00735
      44
      Synthetic Nuances to Maximize n-Type Organic Electrochemical Transistor and Thermoelectric Performance in Fused Lactam Polymers
      Marks, Adam; Chen, Xingxing; Wu, Ruiheng; Rashid, Reem B.; Jin, Wenlong; Paulsen, Bryan D.; Moser, Maximilian; Ji, Xudong; Griggs, Sophie; Meli, Dilara; Wu, Xiaocui; Bristow, Helen; Strzalka, Joseph; Gasparini, Nicola; Costantini, Giovanni; Fabiano, Simone; Rivnay, Jonathan; McCulloch, Iain
      Journal of the American Chemical Society (2022), 144 (10), 4642-4656CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A series of fully fused n-type mixed conduction lactam polymers p(g7NCnN), systematically increasing the alkyl side chain content, are synthesized via an inexpensive, nontoxic, precious-metal-free aldol polycondensation. Employing these polymers as channel materials in org. electrochem. transistors (OECTs) affords state-of-the-art n-type performance with p(g7NC10N) recording an OECT electron mobility of 1.20 x 10-2 cm2 V-1 s-1 and a μC* figure of merit of 1.83 F cm-1 V-1 s-1. In parallel to high OECT performance, upon soln. doping with (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI), the highest thermoelec. performance is obsd. for p(g7NC4N), with a max. elec. cond. of 7.67 S cm-1 and a power factor of 10.4μW m-1 K-2. These results are among the highest reported for n-type polymers. Importantly, while this series of fused polylactam org. mixed ionic-electronic conductors (OMIECs) highlights that synthetic mol. design strategies to bolster OECT performance can be translated to also achieve high org. thermoelec. (OTE) performance, a nuanced synthetic approach must be used to optimize performance. Herein, we outline the performance metrics and provide new insights into the mol. design guidelines for the next generation of high-performance n-type materials for mixed conduction applications, presenting for the first time the results of a single polymer series within both OECT and OTE applications.
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    45. 45
      Tang, H.; Liang, Y.; Liu, C.; Hu, Z.; Deng, Y.; Guo, H.; Yu, Z.; Song, A.; Zhao, H.; Zhao, D.; Zhang, Y.; Guo, X.; Pei, J.; Ma, Y.; Cao, Y.; Huang, F. Nature 2022, 611, 271– 277,  DOI: 10.1038/s41586-022-05295-8
      45
      A solution-processed n-type conducting polymer with ultrahigh conductivity
      Tang, Haoran; Liang, Yuanying; Liu, Chunchen; Hu, Zhicheng; Deng, Yifei; Guo, Han; Yu, Zidi; Song, Ao; Zhao, Haiyang; Zhao, Duokai; Zhang, Yuanzhu; Guo, Xugang; Pei, Jian; Ma, Yuguang; Cao, Yong; Huang, Fei
      Nature (London, United Kingdom) (2022), 611 (7935), 271-277CODEN: NATUAS; ISSN:1476-4687. (Nature Portfolio)
      Conducting polymers (CPs) with high cond. and soln. processability have made great advances since the pioneering work on doped polyacetylene1-3, thus creating the new field of 'org. synthetic metals,4. Various high-performance CPs have been realized, which enable the applications of several org. electronic devices5,6. Nevertheless, most CPs exhibit hole-dominant (p-type) transport behavior7,8, whereas the development of n-type analogs lags far behind and only a few exhibit metallic state, typically limited by low doping efficiency and ambient instability. Here we present a facilely synthesized highly conductive n-type polymer poly(benzodifurandione) (PBFDO). The reaction combines oxidative polymn. and in situ reductive n-doping, greatly increasing the doping efficiency, and a doping level of almost 0.9 charges per repeating unit can be achieved. The resultant polymer exhibits a breakthrough cond. of more than 2,000 S cm-1 with excellent stability and an unexpected soln. processability without extra side chains or surfactants. Furthermore, detailed investigations on PBFDO show coherent charge-transport properties and existence of metallic state. The benchmark performances in electrochem. transistors and thermoelec. generators are further demonstrated, thus paving the way for application of the n-type CPs in org. electronics.
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    46. 46
      Wang, Y.; Nakano, M.; Michinobu, T.; Kiyota, Y.; Mori, T.; Takimiya, K. Macromolecules 2017, 50, 857– 864,  DOI: 10.1021/acs.macromol.6b02313
      46
      Naphthodithiophenediimide-Benzobisthiadiazole-Based Polymers: Versatile n-Type Materials for Field-Effect Transistors and Thermoelectric Devices
      Wang, Yang; Nakano, Masahiro; Michinobu, Tsuyoshi; Kiyota, Yasuhiro; Mori, Takehiko; Takimiya, Kazuo
      Macromolecules (Washington, DC, United States) (2017), 50 (3), 857-864CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
      New π-conjugated polymers with strong electron affinity, PNDTI-BBTs, consisting of naphtho[2,3-b:6,7-b']dithiophenediimide (NDTI) and benzo[1,2-c:4,5-c']bis[1,2,5]thiadiazole (BBT) units, were synthesized. PNDTI-BBTs have low-lying LUMO energy levels (∼-4.4 eV), which is sufficiently low for air-stable electron transport in org. field-effect transistors and for being readily doped by a well-known n-dopant, N,N-dimethyl-2-phenyl-2,3-dihydro-1H-benzoimidazole (N-DMBI), affording doped polymer films with relatively high conductivities and Seebeck coeffs. Depending on the solubilizing alkyl groups (2-decyltetradecyl, PNDTI-BBT-DT, or 3-decylpentadecyl groups, PNDTI-BBT-DP), not only the electron mobility in the transistor devices with the pristine polymer thin films (PNDTI-BBT-DT: ∼0.096 cm2 V-1 s-1; PNDTI-BBT-DP: ∼0.31 cm2 V-1 s-1) but also the cond. and power factor of the doped thins films (PNDTI-BBT-DT: ∼0.18 S cm-1 and ∼0.6 μW m-1 K-2; PNDTI-BBT-DP: ∼5.0 S cm-1 and ∼14 μW m-1 K-2) were drastically changed. The differences in the elec. properties were primarily ascribed to the better cryst. nature of the PNDTI-BBT-DP than those of PNDTI-BBT-DT in the thin-film state. Furthermore, UV-vis and ESR spectra demonstrated that doping effectiveness was largely affected by the alkyl groups: the PNDTI-BBT-DP films with better cryst. order prevented overdoping, resulting in ca. 20 times higher cond. and power factors. From these results, it can be concluded that tuning the intermol. interaction and consequently obtaining the thin-film with well-ordered polymers by the alkyl side chains is a promising strategy for developing superior thermoelec. materials.
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    47. 47
      Liu, J.; Garman, M. P.; Dong, J.; van der Zee, B.; Qiu, L.; Portale, G.; Hummelen, J. C.; Koster, L. J. A. ACS Appl. Energy Mater. 2019, 2, 6664– 6671,  DOI: 10.1021/acsaem.9b01179
      47
      Doping Engineering Enables Highly Conductive and Thermally Stable n-Type Organic Thermoelectrics with High Power Factor
      Liu, Jian; Garman, Matt P.; Dong, Jingjin; van der Zee, Bas; Qiu, Li; Portale, Giuseppe; Hummelen, Jan C.; Koster, L. Jan Anton
      ACS Applied Energy Materials (2019), 2 (9), 6664-6671CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
      This work exploits the scope of doping engineering as an enabler for better-performing and thermally stable n-type org. thermoelecs. A fullerene deriv. with polar triethylene glycol type side chain (PTEG-1) is doped either by "coprocessing doping" with n-type dopants such as n-DMBI and TBAF or by "sequential doping" through thermal deposition of Cs2CO3. Solid-state diffusion of Cs2CO3 appears to dope PTEG-1 in the strongest manner, leading to the highest elec. cond. of ∼7.5 S/cm and power factor of 32μW/(m K2). Moreover, the behavior of differently doped PTEG-1 films under thermal stress is examd. by elec. and spectroscopic means. Cs2CO3-doped films are most stable, likely due to a coordinating interaction between the polar side chain and Cs+-based species, which immobilizes the dopant. The high power factor and good thermal stability of Cs2CO3-doped PTEG-1 make it very promising for tangible thermoelec. applications.
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    48. 48
      Liu, J.; Qiu, L.; Portale, G.; Koopmans, M.; Ten Brink, G.; Hummelen, J. C.; Koster, L. J. A. Adv. Mater. 2017, 29, 1701641,  DOI: 10.1002/adma.201701641
      There is no corresponding record for this reference.
    49. 49
      Liu, J.; van der Zee, B.; Alessandri, R.; Sami, S.; Dong, J.; Nugraha, M. I.; Barker, A. J.; Rousseva, S.; Qiu, L.; Qiu, X.; Klasen, N.; Chiechi, R. C.; Baran, D.; Caironi, M.; Anthopoulos, T. D.; Portale, G.; Havenith, R. W. A.; Marrink, S. J.; Hummelen, J. C.; Koster, L. J. A. Nat. Commun. 2020, 11, 5694,  DOI: 10.1038/s41467-020-19537-8
      49
      N-type organic thermoelectrics: demonstration of ZT > 0.3
      Liu, Jian; van der Zee, Bas; Alessandri, Riccardo; Sami, Selim; Dong, Jingjin; Nugraha, Mohamad I.; Barker, Alex J.; Rousseva, Sylvia; Qiu, Li; Qiu, Xinkai; Klasen, Nathalie; Chiechi, Ryan C.; Baran, Derya; Caironi, Mario; Anthopoulos, Thomas D.; Portale, Giuseppe; Havenith, Remco W. A.; Marrink, Siewert J.; Hummelen, Jan C.; Koster, L. Jan Anton
      Nature Communications (2020), 11 (1), 5694CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      The 'phonon-glass electron-crystal' concept has triggered most of the progress that has been achieved in inorg. thermoelecs. in the past two decades. Org. thermoelec. materials, unlike their inorg. counterparts, exhibit mol. diversity, flexible mech. properties and easy fabrication, and are mostly 'phonon glasses'. However, the thermoelec. performances of these org. materials are largely limited by low mol. order and they are therefore far from being 'electron crystals'. Here, we report a molecularly n-doped fullerene deriv. with meticulous design of the side chain that approaches an org. 'PGEC' thermoelec. material. This thermoelec. material exhibits an excellent elec. cond. of >10 S cm-1 and an ultralow thermal cond. of <0.1 Wm-1K-1, leading to the best figure of merit ZT = 0.34 (at 120 °C) among all reported single-host n-type org. thermoelec. materials. The key factor to achieving the record performance is to use 'arm-shaped' double-triethylene-glycol-type side chains, which not only offer excellent doping efficiency (∼60%) but also induce a disorder-to-order transition upon thermal annealing. This study illustrates the vast potential of org. semiconductors as thermoelec. materials.
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    50. 50
      Zhang, Y.; van Doremaele, E. R. W.; Ye, G.; Stevens, T.; Song, J.; Chiechi, R. C.; van de Burgt, Y. Adv. Mater. 2022, 34, 2200393,  DOI: 10.1002/adma.202200393
      50
      Adaptive Biosensing and Neuromorphic Classification Based on an Ambipolar Organic Mixed Ionic-Electronic Conductor
      Zhang, Yanxi; van Doremaele, Eveline R. W.; Ye, Gang; Stevens, Tim; Song, Jun; Chiechi, Ryan C.; van de Burgt, Yoeri
      Advanced Materials (Weinheim, Germany) (2022), 34 (20), 2200393CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Org. mixed ionic-electronic conductors (OMIECs) are central to bioelectronic applications such as biosensors, health-monitoring devices, and neural interfaces, and have facilitated efficient next-generation brain-inspired computing and biohybrid systems. Despite these examples, smart and adaptive circuits that can locally process and optimize biosignals have not yet been realized. Here, a tunable sensing circuit is shown that can locally modulate biol. relevant signals like electromyograms (EMGs) and electrocardiograms (ECGs), that is based on a complementary logic inverter combined with a neuromorphic memory element, and that is constructed from a single polymer mixed conductor. It is demonstrated that a small neuromorphic array based on this material effects high classification accuracy in heartbeat anomaly detection. This high-performance material allows for straightforward monolithic integration, which reduces fabrication complexity while also achieving high on/off ratios with excellent ambient p- and n-type stability in transistor performance. This material opens a route toward simple and straightforward fabrication and integration of more sophisticated adaptive circuits for future smart bioelectronics.
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    51. 51
      Mei, J.; Kim, D. H.; Ayzner, A. L.; Toney, M. F.; Bao, Z. J. Am. Chem. Soc. 2011, 133, 20130– 20133,  DOI: 10.1021/ja209328m
      51
      Siloxane-Terminated Solubilizing Side Chains: Bringing Conjugated Polymer Backbones Closer and Boosting Hole Mobilities in Thin-Film Transistors
      Mei, Jianguo; Kim, Do Hwan; Ayzner, Alexander L.; Toney, Michael F.; Bao, Zhenan
      Journal of the American Chemical Society (2011), 133 (50), 20130-20133CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The authors introduce a novel siloxane-terminated solubilizing group and demonstrate its effectiveness as a side chain in an isoindigo-based conjugated polymer. An av. hole mobility of 2.00 cm2 V-1 s-1 (with a max. mobility of 2.48 cm2 V-1 s-1), was obtained from soln.-processed thin-film transistors, one of the highest mobilities reported to date. In contrast, the ref. polymer with a branched alkyl side chain gave an av. hole mobility of 0.30 cm2 V-1 s-1 and a max. mobility of 0.57 cm2 V-1 s-1. This is largely explained by the polymer packing: the authors' new polymer exhibited a π-π stacking distance of 3.58 Å, while the ref. polymer showed a distance of 3.76 Å.
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    52. 52
      Razzell-Hollis, J.; Fleischli, F.; Jahnke, A. A.; Stingelin, N.; Seferos, D. S.; Kim, J.-S. J. Phys. Chem. C 2017, 121, 2088– 2098,  DOI: 10.1021/acs.jpcc.6b11675
      52
      Effects of Side-Chain Length and Shape on Polytellurophene Molecular Order and Blend Morphology
      Razzell-Hollis, Joseph; Fleischli, Franziska; Jahnke, Ashlee A.; Stingelin, Natalie; Seferos, Dwight S.; Kim, Ji-Seon
      Journal of Physical Chemistry C (2017), 121 (4), 2088-2098CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      We study the mol. order and thin film morphol. of the conjugated polymer polytellurophene, to understand how the tellurium atom and the choice of side-chain influence the conjugated polymer's backbone planarity and performance in org. transistors. We find that poly(3hexyltellurophene) (P3HTe) continues the trend from polythiophene (P3HT) to polyselenophene (P3HS): substitution with Tellurium leads to a more planar backbone, evident from the shifts of the C=C vibrational peak to lower wavenumbers (∼1389 cm-1) and a smaller optical band-gap (∼1.4 eV). Resonant Raman spectroscopy revealed that mol. order was highly dependent on the structure of the P3ATe alkyl side-chain: a longer chains introduces kinetic hindrance, reducing the fraction of ordered phase obtained at room temp., while a branched side-chain introduces steric hindrance, with intrinsic disorder present even when deposited at higher temps. When blended with the insulator HDPE, all three polymers exhibit little addnl. disorder and instead form phase-sepd. networks of high mol. order that are beneficial to percolated charge transport in transistors. We find that mol. order, as measured by Raman, correlates well with reported transistor mobilities and provides a greater understanding of the structure-property relationships that det. the performance of these novel organometallic polymers in electronic devices.
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    53. 53
      Madu, I. K.; Muller, E. W.; Kim, H.; Shaw, J.; Burney-Allen, A. A.; Zimmerman, P.; Jeffries-El, M.; Goodson, T., III J. Phys. Chem. C 2018, 122, 17049– 17066,  DOI: 10.1021/acs.jpcc.8b03914
      53
      Heteroatom and Side Chain Effects on the Optical and Photophysical Properties: Ultrafast and Nonlinear Spectroscopy of New Naphtho[1,2-b:5,6-b']difuran Donor Polymers
      Madu, Ifeanyi K.; Muller, Evan W.; Kim, Hyungjun; Shaw, Jessica; Burney-Allen, Alfred A.; Zimmerman, Paul; Jeffries-El, Malika; Goodson, Theodore
      Journal of Physical Chemistry C (2018), 122 (30), 17049-17066CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The photophys. and electronic properties of four novel conjugated donor polymers were investigated to understand the influence of heteroatoms (based on the first two member chalcogens) in the polymer backbone. The side chains were varied as well to evaluate the effect of polymer soly. on the photophys. properties. The donor-acceptor polymer structure is based on naphtho[1,2-b:5,6-b']difuran as the donor moiety, and either 3,6-di(furan-2-yl)-1,4-diketopyrrolo[3,4-c]pyrrole or 3,6-di(thiophen-2-yl)-1,4-diketopyrrolo[3,4-c]pyrrole as the acceptor moiety. Steady-state absorption studies showed that the polymers with the furan moiety in the backbone displayed a favorable tendency of capturing more solar photons when used in a photovoltaic device. This is obsd. exptl. by the higher extinction coeff. in the visible and near-IR regions of these polymers relative to that of their thiophene counterparts. The excitonic lifetimes were monitored using ultrafast dynamics, and the results obtained show that the type of heteroatom π-linker used in the backbone affects the decay dynamics. Furthermore, the side chain also plays a role in detg. the fluorescence decay time. Quantum chem. simulations were performed to describe the absorption energies and transition characters. Two-photon absorption cross sections (TPA-δ) were analyzed with the simulations, illustrating the planarity of the backbone in relation to its torsional angles. Because of the planarity in the mol. backbone, the polymer with the furan π-linker showed a higher TPA-δ relative to that of its thiophene counterpart. This suggests that the furan compd. will display higher charge transfer (CT) tendencies in comparison to those of their thiophene analogs. The pump-probe transient absorption technique was employed to probe the nonemissive states (including the CT state) of the polymers, and unique activities were captured at 500 and 750 nm for all of the studied compds. Target and global analyses were performed to understand the dynamics of each peak and deduce the no. of components responsible for the transient behavior obsd. resp. The results obtained suggest that the furan π-linker component of a donor and acceptor moiety in a conjugated polymer might be a more suitable candidate compared with its more popular chalcogenic counterpart, thiophene, for use as donor materials in bulk heterojunction photovoltaic devices.
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    54. 54
      Sun, Y.; Zhang, C.; Dai, B.; Lin, B.; Yang, H.; Zhang, X.; Guo, L.; Liu, Y. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1915– 1926,  DOI: 10.1002/pola.27643
      54
      Side chain engineering and conjugation enhancement of benzodithiophene and phenanthrenequnioxaline based conjugated polymers for photovoltaic devices
      Sun, Ying; Zhang, Chao; Dai, Bin; Lin, Baoping; Yang, Hong; Zhang, Xueqin; Guo, Lingxiang; Liu, Yurong
      Journal of Polymer Science, Part A: Polymer Chemistry (2015), 53 (16), 1915-1926CODEN: JPACEC; ISSN:0887-624X. (John Wiley & Sons, Inc.)
      A series of donor-acceptor conjugated polymers incorporating benzodithiophene (BDT) as donor unit and phenanthrenequnioxaline as acceptor unit with different side chains have been designed and synthesized. For polymer P1 featuring the BDT unit and alkoxy chains substituted phenanthrenequnioxaline unit in the backbone, serious steric hindrance resulted in quite low mol. wt. The implementation of thiophene ring spacer in polymer P2 greatly suppressed the interannular twisting to extend the effective conjugation length and consequently gave rise to improved absorption property and device performance. In addn., utilizing the alkylthienyl side chains to replace the alkyl side chains at BDT unit in polymer P3 further enhanced the photovoltaic performance due to the increased conjugation length. For polymer P4, translating the alkoxy side chains at the phenanthrenequnioxaline ring into the alkyl side chains at thiophene linker group enhanced mol. planarity and strengthened π-π stacking. Consequently improved absorption property and increased hole mobility were achieved for polymer P4. Our results indicated that side chain engineering not only can influence the soly. of polymer but also can det. the polymer backbone planarity and hence the photovoltaic properties. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015.
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    55. 55
      Chen, S.; Pan, Y.; Chen, K.; Chen, P.; Shen, Q.; Sun, P.; Hu, W.; Fan, Q. Angew. Chem., Int. Ed. 2023, 62, e202215372,  DOI: 10.1002/anie.202215372
      55
      Increasing Molecular Planarity through Donor/Side-Chain Engineering for Improved NIR-IIa Fluorescence Imaging and NIR-II Photothermal Therapy under 1064 nm
      Chen, Shangyu Y.; Pan, Yonghui H.; Chen, Kai; Chen, Pengfei F.; Shen, Qingming M.; Sun, Pengfei F.; Hu, Wenbo B.; Fan, Quli L.
      Angewandte Chemie, International Edition (2023), 62 (6), e202215372CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Developing conjugated small mols. (CSM) with intense NIR-II (1000-1700 nm) absorption for phototheranostic is highly desirable but remains a tremendous challenge due to a lack of reliable design guidelines. This study reports a high-performance NIR-II CSM for phototheranostic by tailoring mol. planarity. A series of CSM show bathochromic absorption extended to the NIR-II region upon the increasing thiophene no., but an excessive no. of thiophene results in decreased NIR-IIa (1300-1400 nm) brightness and photothermal effects. Further introduction of terminal nonconjugated alkyl chain can enhance NIR-II absorption coeff., NIR-IIa brightness, and photothermal effects. Mechanism studies ascribe this overall enhancement to mol. planarity stemming from the collective contribution of donor/side-chain engineering. This finding directs the design of NIR-II CSM by rational manipulating mol. planarity to perform 1064 nm mediated phototheranostic at high efficiency.
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    56. 56
      Liu, J.; Qiu, L.; Portale, G.; Torabi, S.; Stuart, M. C. A.; Qiu, X.; Koopmans, M.; Chiechi, R. C.; Hummelen, J. C.; Anton Koster, L. J. Nano Energy 2018, 52, 183– 191,  DOI: 10.1016/j.nanoen.2018.07.056
      56
      Side-chain effects on N-type organic thermoelectrics: A case study of fullerene derivatives
      Liu, Jian; Qiu, Li; Portale, Giuseppe; Torabi, Solmaz; Stuart, Marc C. A.; Qiu, Xinkai; Koopmans, Marten; Chiechi, Ryan C.; Hummelen, Jan C.; Anton Koster, L. Jan
      Nano Energy (2018), 52 (), 183-191CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
      In this contribution, the two key parameters, the polarity and side chain length have been changed to study their effects on n-type org. thermoelecs. of a series of fullerene derivs. Fullerene derivs. bearing either an alkyl side chain or ethylene glycol (EG) side chains of different lengths are used as the host mols. for mol. doping. It is found that the polar EG side chains can enable better miscibility with the polar dopant than the alkyl side chain, which leads to more than 5-fold enhancement of doping efficiency. Beyond the doping efficiency, another crucial parameter of mol. doping, the mol. order, is readily acquired by simultaneous control of the polarity and the length of the side chain. A polar side chain with an appropriate chain length can contribute to increasing Seebeck coeffs. of doped fullerene derivs. more effectively than an alkyl side chain, likely due to the resultant good miscibility and high mol. order. As a result, an optimized power factor of 23.1μW m-1 K-2 is achieved in the fullerene deriv. with a tetraethylene glycol side chain. This represents one of the best n-type org. thermoelecs. Addnl., EG side chains can improve the air stability of n-doped fullerene derivs. films as compared to an alkyl side chain. Our work sheds light on the design of side-chains in efficient n-type small mols. thermoelec. materials and contributes to the understanding of their thermoelec. properties.
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    57. 57
      Jacobs, I. E.; Aasen, E. W.; Oliveira, J. L.; Fonseca, T. N.; Roehling, J. D.; Li, J.; Zhang, G.; Augustine, M. P.; Mascal, M.; Moulé, A. J. J. Mater. Chem. C 2016, 4, 3454– 3466,  DOI: 10.1039/C5TC04207K
      57
      Comparison of solution-mixed and sequentially processed P3HT:F4TCNQ films: effect of doping-induced aggregation on film morphology
      Jacobs, Ian E.; Aasen, Erik W.; Oliveira, Julia L.; Fonseca, Tayane N.; Roehling, John D.; Li, Jun; Zhang, Gwangwu; Augustine, Matthew P.; Mascal, Mark; Moule, Adam J.
      Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2016), 4 (16), 3454-3466CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
      Doping polymeric semiconductors often drastically reduces the soly. of the polymer, leading to difficulties in processing doped films. Here, we compare optical, elec., and morphol. properties of P3HT films doped with F4TCNQ, both from mixed solns. and using sequential soln. processing with orthogonal solvents. We demonstrate that sequential doping occurs rapidly (<1 s), and that the film doping level can be precisely controlled by varying the concn. of the doping soln. Furthermore, the choice of sequential doping solvent controls whether dopant anions are included or excluded from polymer crystallites. Atomic force microscopy (AFM) reveals that sequential doping produces significantly more uniform films on the nanoscale than the mixed-soln. method. In addn., we show that mixed-soln. doping induces the formation of aggregates even at low doping levels, resulting in drastic changes to film morphol. Sequentially coated films show 3-15 times higher conductivities at a given doping level than soln.-doped films, with sequentially doped films processed to exclude dopant anions from polymer crystallites showing the highest conductivities. We propose a mechanism for doping induced aggregation in which the shift of the polymer HOMO level upon aggregation couples ionization and solvation energies. To show that the methodol. is widely applicable, we demonstrate that several different polymer:dopant systems can be prepd. by sequential doping.
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    58. 58
      Euvrard, J.; Revaux, A.; Bayle, P.-A.; Bardet, M.; Vuillaume, D.; Kahn, A. Org. Electron. 2018, 53, 135– 140,  DOI: 10.1016/j.orgel.2017.11.020
      58
      The formation of polymer-dopant aggregates as a possible origin of limited doping efficiency at high dopant concentration
      Euvrard, Julie; Revaux, Amelie; Bayle, Pierre-Alain; Bardet, Michel; Vuillaume, Dominique; Kahn, Antoine
      Organic Electronics (2018), 53 (), 135-140CODEN: OERLAU; ISSN:1566-1199. (Elsevier B.V.)
      The polymer Poly[(4,8-bis-(2-ethylhexyloxy)-benzo(1,2-b:4,5-b)dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno [3,4-b]thiophene-)-2-6-diyl] (PBDTTT-c) p-doped with the mol. dopant tris[1-(trifluoroethanoyl)-2-(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd-COCF3)3) exhibits a decline in transport properties at high doping concns., which limits the performance attainable through org. semiconductor doping. SEM is used to correlate the evolution of hole cond. and hopping transport activation energy with the formation of aggregates in the layer. Transmission Electron Microscopy with energy-dispersive X-ray anal. along with liq.-state NMR expts. are carried out to det. the compn. of the aggregates. This study offers an explanation to the limited efficiency of doping at high dopant concns. and reinforces the need to increase doping efficiency in order to be able to reduce the dopant concn. and not neg. affect cond.
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    59. 59
      Zhang, Y.; Ye, G.; van der Pol, T. P. A.; Dong, J.; van Doremaele, E. R. W.; Krauhausen, I.; Liu, Y.; Gkoupidenis, P.; Portale, G.; Song, J.; Chiechi, R. C.; van de Burgt, Y. Adv. Funct. Mater. 2022, 32, 2201593,  DOI: 10.1002/adfm.202201593
      59
      High-Performance Organic Electrochemical Transistors and Neuromorphic Devices Comprising Naphthalenediimide-Dialkoxybithiazole Copolymers Bearing Glycol Ether Pendant Groups
      Zhang, Yanxi; Ye, Gang; van der Pol, Tom P. A.; Dong, Jingjing; van Doremaele, Eveline R. W.; Krauhausen, Imke; Liu, Yuru; Gkoupidenis, Paschalis; Portale, Giuseppe; Song, Jun; Chiechi, Ryan C.; van de Burgt, Yoeri
      Advanced Functional Materials (2022), 32 (27), 2201593CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Org. electrochem. transistors (OECTs) have emerged as building blocks for low power circuits, biosensors, and neuromorphic computing. While p-type polymer materials for OECTs are well developed, the choice of high-performance n-type polymers is limited, despite being essential for cation and metabolite biosensors, and crucial for constructing complementary circuits. N-type conjugated polymers that have efficient ion-to-electron transduction are highly desired for electrochem. applications. In this contribution, three non-fused, planar naphthalenediimide (NDI)-dialkoxybithiazole (2Tz) copolymers, which systematically increase the amt. of polar tri(ethylene glycol) (TEG) side chains: PNDI2OD-2Tz (0 TEG), PNDIODTEG-2Tz (1 TEG), PNDI2TEG-2Tz (2 TEG), are reported. It is demonstrated that the OECT performance increases with the no. of TEG side chains resulting from the progressively higher hydrophilicity and larger electron affinities. Benefiting from the high electron mobility, excellent ion conduction capability, efficient ion-to-electron transduction, and low-lying LUMO energy level, the 2 TEG polymer achieves close to 105 on-off ratio, fast switching, 1000 stable operation cycles in aq. electrolyte, and has a long shelf life. Moreover, the higher no. TEG chain substituted polymer exhibits good conductance state retention over two orders of magnitudes in electrochem. resistive random-access memory devices, highlighting its potential for neuromorphic computing.
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    60. 60
      Tietze, M. L.; Benduhn, J.; Pahner, P.; Nell, B.; Schwarze, M.; Kleemann, H.; Krammer, M.; Zojer, K.; Vandewal, K.; Leo, K. Nat. Commun. 2018, 9, 1182,  DOI: 10.1038/s41467-018-04275-9
      60
      Elementary steps in electrical doping of organic semiconductors
      Tietze Max L; Benduhn Johannes; Pahner Paul; Nell Bernhard; Schwarze Martin; Kleemann Hans; Vandewal Koen; Leo Karl; Tietze Max L; Tietze Max L; Krammer Markus; Zojer Karin; Vandewal Koen
      Nature communications (2018), 9 (1), 1182 ISSN:.
      Fermi level control by doping is established since decades in inorganic semiconductors and has been successfully introduced in organic semiconductors. Despite its commercial success in the multi-billion OLED display business, molecular doping is little understood, with its elementary steps controversially discussed and mostly-empirical-materials design. Particularly puzzling is the efficient carrier release, despite a presumably large Coulomb barrier. Here we quantitatively investigate doping as a two-step process, involving single-electron transfer from donor to acceptor molecules and subsequent dissociation of the ground-state integer-charge transfer complex (ICTC). We show that carrier release by ICTC dissociation has an activation energy of only a few tens of meV, despite a Coulomb binding of several 100 meV. We resolve this discrepancy by taking energetic disorder into account. The overall doping process is explained by an extended semiconductor model in which occupation of ICTCs causes the classically known reserve regime at device-relevant doping concentrations.
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    61. 61
      Méndez, H.; Heimel, G.; Winkler, S.; Frisch, J.; Opitz, A.; Sauer, K.; Wegner, B.; Oehzelt, M.; Röthel, C.; Duhm, S.; Többens, D.; Koch, N.; Salzmann, I. Nat. Commun. 2015, 6, 8560,  DOI: 10.1038/ncomms9560
      61
      Charge-transfer crystallites as molecular electrical dopants
      Mendez, Henry; Heimel, Georg; Winkler, Stefanie; Frisch, Johannes; Opitz, Andreas; Sauer, Katrein; Wegner, Berthold; Oehzelt, Martin; Roethel, Christian; Duhm, Steffen; Toebbens, Daniel; Koch, Norbert; Salzmann, Ingo
      Nature Communications (2015), 6 (), 8560CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      Ground-state integer charge transfer is commonly regarded as the basic mechanism of mol. elec. doping in both, conjugated polymers and oligomers. Here, we demonstrate that fundamentally different processes can occur in the two types of org. semiconductors instead. Using complementary exptl. techniques supported by theory, we contrast a polythiophene, where mol. p-doping leads to integer charge transfer reportedly localized to one quaterthiophene backbone segment, to the quaterthiophene oligomer itself. Despite a comparable relative increase in cond., we observe only partial charge transfer for the latter. In contrast to the parent polymer, pronounced intermol. frontier-orbital hybridization of oligomer and dopant in 1:1 mixed-stack co-crystallites leads to the emergence of empty electronic states within the energy gap of the surrounding quaterthiophene matrix. It is their Fermi-Dirac occupation that yields mobile charge carriers and, therefore, the co-crystallites-rather than individual acceptor mols.-should be regarded as the dopants in such systems.
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    62. 62
      Salzmann, I.; Heimel, G.; Duhm, S.; Oehzelt, M.; Pingel, P.; George, B. M.; Schnegg, A.; Lips, K.; Blum, R.-P.; Vollmer, A.; Koch, N. Phys. Rev. Lett. 2012, 108, 035502  DOI: 10.1103/PhysRevLett.108.035502
      62
      Intermolecular hybridization governs molecular electrical doping
      Salzmann, Ingo; Heimel, Georg; Duhm, Steffen; Oehzelt, Martin; Pingel, Patrick; George, Benjamin M.; Schnegg, Alexander; Lips, Klaus; Blum, Ralf-Peter; Vollmer, Antje; Koch, Norbert
      Physical Review Letters (2012), 108 (3), 035502/1-035502/5CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Current models for mol. elec. doping of org. semiconductors are found to be at odds with other well-established concepts in that field, like polaron formation. Addressing these inconsistencies for prototypical systems, we present exptl. and theor. evidence for intermol. hybridization of org. semiconductor and dopant frontier MOs. Common doping-related observations are attributed to this phenomenon, and controlling the degree of hybridization emerges as a strategy for overcoming the present limitations in the yield of doping-induced charge carriers.
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