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Physical Processes in Nanomaterials and Nanostructures

Multiexciton Solar Cells of CuInSe2 Nanocrystals
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† McKetta Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712, United States
‡ NovaCentrix, 200-B Parker Drive, Suite 580, Austin, Texas 78728, United States
§ Center for Nanoscale Materials, Argonne National Laboratories, Argonne, Illinios 60439, United States
∥ Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
*E-mail: [email protected]. Tel.: +1-512-471-5633. Fax: +1-512-471-7060.
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Cite this: J. Phys. Chem. Lett. 2014, 5, 2, 304–309
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https://doi.org/10.1021/jz402596v
Published December 26, 2013

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Abstract

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Peak external quantum efficiencies (EQEs) of just over 120% were observed in photovoltaic (PV) devices of CuInSe2 nanocrystals prepared with a photonic curing process. The extraction of more than one electron/hole pair as a result of the absorption of a single photon can occur if multiple excitons are generated and extracted. Multiexciton generation (MEG) in the nanocrystal films was substantiated by transient absorption spectroscopy. We propose that photonic curing leads to sufficient electronic coupling between nanocrystals to enable multiexciton extraction under typical solar illumination conditions. Under low light conditions, however, the EQE drops significantly, indicating that photonic curing-induced ligand desorption creates a significant amount of traps in the film that limit the overall power conversion efficiency of the device.

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A maximum of 34% of the energy available in sunlight can be converted to electricity by a single junction solar cell, known as the Shockley–Queisser limit. (1) The semiconductor in the device does not absorb photons with energy less than its band gap energy, and photon energy greater than the band gap is lost as heat due to the rapid relaxation of the photoexcited electron and hole to their band minima before they can be extracted as electrical current. One way to surpass the Shockley–Queisser limit is to use quantum dots that convert high-energy photons into multiple electron–hole pairs that can be extracted as photocurrent by the device. (2, 3) Colloidal nanocrystals provide a convenient source of quantum dots in which multiexciton generation (MEG) has been observed optically from a host of materials, including PbS, PbSe, PbTe, CdSe, InAs, and Si. (4-8) Extraction of more than one electron per absorbed photon as electrical current in devices has also been reported, (9-12) with a few instances of device quantum efficiencies (QEs) exceeding 100%, PbS (internal QE only), (13) PbSe (external QE, EQE) (14) nanocrystal solar cells, and an organic device exhibiting a related process of singlet fission. (15) Here, we report photovoltaic (PV) devices of CuInSe2 nanocrystals with peak EQEs of over 120% that result from multiexciton generation and extraction.

CuInSe2 is an important model semiconductor for PV devices that is closely related to Cu(InxGa1–x)Se2 (CIGS), which holds the record for highest device efficiency of all thin film semiconductors at just over 20%. (16) PV devices made from ink-deposited CuInSe2 nanocrystals have reached power conversion efficiencies of 3%, limited by poor charge transport. (17-19) Ink-deposited Cu(InxGa1–x)S2 nanocrystals can be sintered into polycrystalline films by heating (>500 °C) under selenium vapor (i.e., selenization) to achieve much higher efficiencies of just over 12%. (20, 21) To try to avoid the need for high temperature selenization, an alternative nanocrystal film processing technique called photonic curing is explored here to improve charge transport in the nanocrystal film. Photonic curing was carried out using a PulseForge 3300 (NovaCentrix) tool that uses pulsed light from a xenon flash lamp with spectrally broad blackbody radiation that can rapidly heat to very high temperature. Photonic curing can provide enough energy to sinter nanocrystals, (22) but in this study, relatively mild pulse conditions were used to remove organic ligands and bring nanocrystals into better electrical contact without destroying their nanoscale dimensions. Nanocrystal films processed in this way were found to yield PVs with peak EQEs exceeding 100%, indicating the possible occurrence of MEG and extraction from the devices. Transient absorption (TA) spectroscopy was employed to verify that MEG does indeed occur in the nanocrystal films.

PV devices were made by spray-depositing CuInSe2 nanocrystals from toluene dispersions on Au-coated soda lime glass substrates similar to Akhavan et al. (17) and then curing the nanocrystal films with the PulseForge tool (Figure 1) in a closed chamber with a quartz window using a single 160 μs light pulse with flux ranging from 2 to 3 J/cm2. The CdS buffer layer and ZnO/ITO top contacts were then added. Nanocrystal films pulsed with 2.2 J/cm2 light reach about 600 °C within 1 ms, which removes the oleylamine ligand but does not induce crystal grain growth (see Supporting Information Figures S1–S5). Figure 2 shows scanning electron microscope (SEM) images of CuInSe2 nanocrystal films before and after curing with 2.2 and >3 J/cm2 exposure. The nanocrystal grains remain small after 2.2 J/cm2 exposure but clearly grow into larger grains after >3 J/cm2 exposure. (See also the X-ray diffraction (XRD) peak width data in Figure S4 in the Supporting Information)

Figure 1

Figure 1. Photonic curing of nanocrystal films on Au-coated glass substrates. (a) Photonic curing can be used to remove oleylamine capping ligands from the CuInSe2 nanocrystal film without inducing nanocrystal grain growth. (b) When the capping ligands are present, they inhibit the collection of multiexcitons from the film, leading to electron–hole recombination by Auger recombination. (c) Without the ligand barrier between nanocrystals, multiexciton transport becomes much more probable.

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Figure 2

Figure 2. CuInSe2 nanocrystal layers before and after photonic curing and their PV device performance. Top-down and cross-sectional SEM images of an oleylamine-capped CuInSe2 (CIS) nanocrystal film on Au-coated glass (a, d) before and after photonic curing with (b, e) 2.2 and (c, f) and 3 J/cm2 pulse fluence. (g–i) Corresponding current–voltage measurements (the black curve is dark current; the red curve is measured under AM1.5G illumination (100 mW/cm2)) of devices made with the nanocrystal films provided below the SEM images.

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Although the nanocrystals could be grown into large grains by photonic curing, devices made from these sintered nanocrystals performed very poorly, as shown in Figure 2. Exposure of 3 J/cm2 sintered the nanocrystals but also led to dewetting by the formation of melt balls, leaving significant back contact exposed and devices with almost no short-circuit current. In contrast, devices made with nanocrystals cured using 2.2 J/cm2 exposure gave reasonable device response with a power conversion efficiency (PCE) of 1.25%, similar to the devices made with as-deposited nanocrystals (PCE = 1.19%). The biggest change in device response after photonic curing is a large increase in the short-ci,rcuit current (Jsc) and a drop in the open-circuit voltage (Voc); for example, in Figure 2g the Jsc and Voc changed from 5.65 to 18.65 mA/cm2 and 0.41 to 0.21 V, respectively.

EQE (also known as IPCE) measurements showed that most of the increased short-circuit current in the devices made with cured nanocrystals occurred in the short wavelength (<600 nm) range. Figure 3a shows a comparison of EQE spectra from PVs made with as-deposited CuInSe2 nanocrystals and nanocrystals that had been processed by photonic curing at 2.2 J/cm2. The measurements in Figure 3a are made under a white light bias of 50 mW/cm2. The as-deposited CuInSe2 nanocrystal device has a peak EQE of about 25%, whereas the peak EQE of the cured nanocrystal device is 123%. On the basis of the measured light absorption in the device, the peak internal quantum efficiency (IQE) was found to correspond to 143% (Supporting Information Figure S6). The application of a white light bias has little effect on the as-deposited CuInSe2 nanocrystal devices but had a significant influence on the EQE spectra of the cured nanocrystal devices (see Figure 3b, for example).

Figure 3

Figure 3. EQE enhancements resulting from photonic curing of the CuInSe2 nanocrystal layer used in PV devices. (a) EQE measurements taken under white light bias (50 mW/cm2) for CuInSe2 nanocrystal devices without photonic curing (black curve) compared to the device made with cured (2.2 J/cm2 pulse fluence) nanocrystals (red curve). The short-circuit currents determined from these data, of 4.95 and 14.29 mA/cm2, are consistent with the short-circuit currents measured under AM1.5 illumination (100 mW/cm2). (b) EQE measured under varying white light bias intensity (100, 50, 25, 10, and 0% of a 50 mW/cm2 bias light) with the same intensity of monochromated probe light. There was no change in EQE for the device made with as-deposited nanocrystals (inset), but the EQE decreased significantly for the cured device when the white light bias intensity was reduced to the amounts indicated.

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The substantial effect of white light bias on the EQE of cured nanocrystal devices indicates that the curing process introduces traps into the nanocrystal layer that hinder charge extraction under low light conditions. (25, 26) Because EQE measurements of solar cells are performed with a low-intensity monochromatic probe beam, the additional intense white light bias is required to mimic the near full sun conditions experienced by the device in the field, (23) and EQE measurements taken without white light bias can give anomalous results. (23-26) For example, traps in the CdS layer in CdTe/CdS devices usually filled under AM1.5 illumination remain empty under low light conditions, significantly reducing device currents and leading to artificially low EQE values if white light bias is not used. (23-26) CdTe and CIGS PV devices can also exhibit EQE variations with light bias intensity due to photoconductive CdS. (24-27) In our case, the CdS layer is the same for all devices, and the EQE of the as-deposited nanocrystal device is not affected by the white light bias intensity (Figure 3b, inset). However, most telling is that the EQE of the devices with peak EQE > 100% was found to decrease proportionally with the probe light intensity (Supporting Information Figure S7, Table S1), additionally ruling out possible contributions from photoconductive gain or anomalous currents due to trapped carrier extraction related to the bias illumination. The EQE also did not vary with probe beam chopping frequency (Supporting Information Figure S8), eliminating the likelihood of measurement artifacts due to slow carrier kinetics. Lastly, the measured Jsc values of the CuInSe2 nanocrystal devices in Figure 2 agree pretty well with those calculated from the EQE measurements in Figure 3. The measured Jsc from the as-deposited nanocrystal device was 5.65 mA/cm2, compared to 4.95 mA/cm2 calculated from EQE data. The cured nanocrystal device Jsc is 18.65 mA/cm2 (Figure 2h), compared to 14.29 mA/cm2 calculated from the EQE data. The lower calculated Jsc value for the cured nanocrystal device results from the fact that the white light bias intensity in our IPCE setup was limited to ∼50 mW/cm2, and because the EQE of these devices was sensitive to the bias intensity, the measured EQE under white light bias was still slightly lower than that under true AM1.5 illumination at 100 mW/cm2.

To help verify that MEG occurs in the nanocrystal films that exhibit peak EQE > 100%, the recombination dynamics of photoexcited excitons were determined by TA spectroscopy with 400 and 800 nm pump light. Figure 4a,b shows the decay in bleach signal near the absorption edge (see Supporting Information Figure S9 for a spectrum of the bleach signal). (28, 29) Multiexcitons undergo Auger recombination (the inverse process to MEG) on very short time scales (typically ∼100 ps) compared to much longer lived single excitons. (28) With 800 nm pump light (Figure 4a), an individual photon does not have enough energy to induce MEG, and only one exciton per nanocrystal is generated at low pump fluence. Under these conditions, the kinetics curves (normalized at delay times >1 ns) overlap. When the 800 nm pump fluence is increased so that some nanocrystals absorb more than one photon per excitation pulse, multiexcitons form and Auger recombination dynamics arise. The 400 nm pump photons carry about three times the band gap energy; therefore, MEG from a single photon is possible, and Auger recombination dynamics can be observed even at low fluences. Figure 4b shows the bleach signal for two low-fluence TA kinetics with 400 nm pump wavelength as well as an average of the 3, 6, and 15 μJ/cm2 TA curves at 800 nm pump wavelength for comparison. The low-fluence TA kinetics at 400 nm show increased signal at short times compared to the low-fluence 800 nm pump TA kinetics, indicating the presence of Auger recombination and therefore MEG. The possibility of anomalous results due to photocharging was eliminated by rapidly translating the sample through the measurement area. (30) Negligible differences were observed between measurements of static and translating samples (Supporting Information Figure S10).

Figure 4

Figure 4. TA spectroscopy of CuInSe2 nanocrystal films after photonic curing. (a) TA kinetics normalized to −Δα = 1 at 1000 ps with an 800 nm pump wavelength and pump fluences of 300 (dark blue), 90 (green), 60 (pink), 30 (teal), 15 (blue), 6 (red), and 3 μJ/cm2 (black). (b) TA kinetics normalized to −Δα = 1 at 1000 ps with a 400 nm pump wavelength and pump fluences of 18 (red) and 9 μJ/cm2 (blue). The average low fluence background (average of 3, 6, 15, and 30 μJ/cm2 signals) at an 800 nm pump wavelength is also shown for comparison (black). (c) TA kinetics showing the Auger recombination rate. The single exciton TA kinetics background (average 800 nm wavelength low-fluence pump) is subtracted from the high-fluence TA kinetics at an 800 nm, 300 μJ/cm2 pump, which shows the creation of multiexcitons due to the absorption of multiple photons per nanocrystal. The kinetics are plotted on a log scale and can be fitted to a single exponential with a time constant of 92 ps. (d) TA kinetics showing Auger recombination at a 400 nm pump and low fluence. The single-exciton TA kinetics background (average 800 nm wavelength low-fluence pump) is subtracted from the TA kinetics at a 400 nm, 9 μJ/cm2 pump, which should only show Auger recombination if MEG is present. The kinetics are plotted on a log scale and can be fitted to a single exponential with a time constant of 74 ps.

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The average single exciton recombination kinetics at an 800 nm pump and low fluence was used as a baseline to determine the Auger recombination rate. In Figure 4c,d, the single-exciton recombination background kinetics were subtracted (time constant of ∼600 ps) from the TA kinetics at the 800 nm pump wavelength and 300 μJ/cm2 fluence (a high-power regime where multiple photons are present per absorbing nanocrystal) and at the 400 nm pump wavelength and 9 μJ/cm2 fluence (in the regime of less than one photon per nanocrystal). The curves in Figure 4c,d both fit single exponentials with similar time constants of 93 and 74 ps, respectively. The presence of Auger recombination at low fluences of 400 nm pump light supports the presence of MEG in the cured CuInSe2 nanocrystal films, and the estimated MEG quantum yield of 125% is close to the device peak IQE measurements of ∼143% (see Supporting Information Figures S6 and S11).

The influence of the trap states limiting multiexciton extraction under low light conditions on the exciton decay dynamics was tested by applying an intense white light bias during TA measurements, and there was little effect on the TA kinetics (see Supporting Information Figure S12). The traps seem to have a detrimental effect only on charge extraction. Perhaps these traps are related to unpassivated surface defects. (31-34) TEM shows that prior to photonic curing, the nanocrystals have a diameter of 8.1 ± 2.1 nm, which is smaller than the Bohr exciton radius for CuInSe2 (Supporting Information Figure S5). (35) The red shift of 60 meV in the peak wavelength of the absorption bleach in the TA spectrum (Supporting Information Figure S9) after curing is consistent with a slight loss of quantum confinement resulting from the loss of capping ligands. However, the fact that the reduction in optical gap is larger than this (0.12 eV, Supporting Information Figure S13) and that the TA spectrum exhibits an asymmetric broadening into the red part of the spectrum (Supporting Information Figure S9) indicate that trap-related defects are present after photonic curing. In order to extract multiexcitons from a device, the photogenerated multiple electron–hole pairs must separate before Auger recombination can occur. CdTe and PbS nanocrystals both show charge-transfer rates between nanocrystals of ∼100 ps, and biexcitons can be extracted from separate nanocrystals prior to Auger recombination. (9, 36-38) Charge transfer rates as fast as 50 fs have been observed in PbSe nanocrystals and reported as hot carrier extraction. (39) Our calculated biexciton decay time is similar to that of coupled PbSe quantum dot films (∼100 ps), which have also demonstrated MEG in devices. (14, 28) Enhanced coupling in films of PbSe nanocrystals has enabled efficient conversion of multiexcitons into free charge carriers compared to the competing Auger recombination process. (40) In films of CuInSe2 nanocrystals coated with oleylamine ligands, charge carrier separation is inefficient, and multiexcitons are lost to Auger recombination (see Figure 1b). The observation of peak QEs over 100% after photonic curing indicates that multiexciton dissociation and extraction (as individual charges or as excitons) becomes much more efficient (see Figure 1c).

Ink-deposited CuInSe2 nanocrystal PVs treated by photonic curing exhibited high short-circuit currents and peak EQEs of over 120%. TA measurements provide substantiation that the high EQE results from the extraction of multiexcitons. It appears that photonic curing brings the nanocrystals into better electrical contact and enables multiexciton extraction. Ligand removal, however, also seems to induce a significant amount of traps in the nanocrystal film, which reduces device performance, especially under low light conditions. Passivation of these surface traps could perhaps provide a route to high-efficiency devices that utilize MEG and extraction along with reasonably efficient charge extraction for electrons and holes photoexcited closer to the band gap energy.

Supporting Information

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Experimental details, including materials used, nanocrystal synthesis, film deposition methods, PV device fabrication, characterization, and PV device testing, additional data ans supplementary figures, including the confirmation of ligand loss during photonic curing of nanocrystal films by TGA and FTIR, the calculated substrate temperature during photonic curing process, the extent of nanocrystal sintering after photonic curing determined by XRD, the IQE and MEG quantum yields, the effect of light biasing on EQE measurements, and TA spectroscopy, and Table S1, showing the EQE and calculated Jsc for each probe beam intensity. This material is available free of charge via the Internet at http://pubs.acs.org.

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Author Information

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  • Corresponding Author
    • Brian A. Korgel - McKetta Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712, United States;  Email: [email protected]
  • Authors
    • C. Jackson Stolle - McKetta Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712, United States
    • Taylor B. Harvey - McKetta Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712, United States
    • Douglas R. Pernik - McKetta Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712, United States
    • Jarett I. Hibbert - McKetta Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712, United States
    • Jiang Du - McKetta Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712, United States
    • Dong Joon Rhee - McKetta Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712, United States
    • Vahid A. Akhavan - NovaCentrix, 200-B Parker Drive, Suite 580, Austin, Texas 78728, United States
    • Richard D. Schaller - Center for Nanoscale Materials, Argonne National Laboratories, Argonne, Illinios 60439, United States;  Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
  • Notes
    The authors declare no competing financial interest.

Acknowledgment

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Financial support of this work was provided by the Robert A. Welch Foundation (F-1464) and the National Science Foundation Industry/University Cooperative Research Center on Next Generation Photovoltaics (IIP-1134849). Financial support was also provided for C.J.S. and D.R.P. by the National Science Foundation Graduate Research Fellowship program under Grant No. DGE-1110007. Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The authors also thank Sayan Saha and Sanjay Banerjee for use of their QEX10 Solar Cell Spectral Response Measurement System.

References

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    Colloidal Quantum-Dot Photodetectors Exploiting Multiexciton Generation
    Sukhovatkin, Vlad; Hinds, Sean; Brzozowski, Lukasz; Sargent, Edward H.
    Science (Washington, DC, United States) (2009), 324 (5934), 1542-1544CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Multiexciton generation (MEG) has been indirectly obsd. in colloidal quantum dots, both in soln. and the solid state, but has not yet been shown to enhance photocurrent in an optoelectronic device. Here, we report a class of soln.-processed photoconductive detectors, sensitive in the UV, visible, and the IR, in which the internal gain is dramatically enhanced for photon energies Ephoton greater than 2.7 times the quantum-confined bandgap Ebandgap. Three thin-film devices with different quantum-confined bandgaps (set by the size of their constituent lead sulfide nanoparticles) show enhancement detd. by the bandgap-normalized photon energy, Ephoton/Ebandgap, which is a clear signature of MEG. The findings point to a valuable role for MEG in enhancing the photocurrent in a solid-state optoelectronic device. We compare the conditions on carrier excitation, recombination, and transport for photoconductive vs. photovoltaic devices to benefit from MEG.
  10. 10
    Kim, S. J.; Kim, W. J.; Sahoo, Y.; Cartwright, A. N.; Prasad, P. N. Multiple Exciton Generation and Electrical Extraction from a PbSe Quantum Dot Photoconductor Appl. Phys. Lett. 2008, 92, 031107/1– 031107/3
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  11. 11
    Kim, S. J.; Kim, W. J.; Cartwright, A. N.; Prasad, P. N. Carrier Multiplication in a PbSe Nanocrystal and P3HT/PCBM Tandem Cell Appl. Phys. Lett. 2008, 92, 191107/1– 191107/3
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  12. 12
    Gabor, N. M.; Zhong, Z.; Bosnick, K.; Park, J.; McEuen, P. L. Extremely Efficient Multiple Electron–Hole Pair Generation in Carbon Nanotube Photodiodes Science 2009, 325, 1367– 1371
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    Sambur, J. B.; Novet, T.; Parkinson, B. A. Multiple Exciton Collection in a Sensitized Photovoltaic System Science 2010, 330, 63– 66
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    13
    Multiple Exciton Collection in a Sensitized Photovoltaic System
    Sambur, Justin B.; Novet, Thomas; Parkinson, B. A.
    Science (Washington, DC, United States) (2010), 330 (6000), 63-66CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Multiple exciton generation, the creation of two electron-hole pairs from one high-energy photon, is well established in bulk semiconductors, but assessments of the efficiency of this effect remain controversial in quantum-confined systems like semiconductor nanocrystals. We used a photoelectrochem. system composed of PbS nanocrystals chem. bound to TiO2 single crystals to demonstrate the collection of photocurrents with quantum yields greater than one electron per photon. The strong electronic coupling and favorable energy level alignment between PbS nanocrystals and bulk TiO2 facilitate extn. of multiple excitons more quickly than they recombine, as well as collection of hot electrons from higher quantum dot excited states. Our results have implications for increasing the efficiency of photovoltaic devices by avoiding losses resulting from the thermalization of photogenerated carriers.
  14. 14
    Semonin, O. E.; Luther, J. M.; Choi, S.; Chen, H.-Y.; Gao, J.; Nozik, A. J.; Beard, M. C. Peak External Photocurrent Quantum Efficiency Exceeding 100% via MEG in a Quantum Dot Solar Cell Science 2011, 334, 1530– 1533
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    14
    Peak External Photocurrent Quantum Efficiency Exceeding 100% via MEG in a Quantum Dot Solar Cell
    Semonin, Octavi E.; Luther, Joseph M.; Choi, Sukgeun; Chen, Hsiang-Yu; Gao, Jianbo; Nozik, Arthur J.; Beard, Matthew C.
    Science (Washington, DC, United States) (2011), 334 (6062), 1530-1533CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Multiple exciton generation (MEG) is a process that can occur in semiconductor nanocrystals, or quantum dots, whereby absorption of a photon bearing at least twice the bandgap energy produces two or more electron-hole pairs. Here, we report on photocurrent enhancement arising from MEG in PbSe quantum dot-based solar cells, as manifested by an external quantum efficiency (the spectrally resolved ratio of collected charge carriers to incident photons) that peaked at 114 ± 1% in the best device measured. The assocd. internal quantum efficiency (cor. for reflection and absorption losses) was 130%. We compare our results with transient absorption measurements of MEG in isolated PbSe quantum dots and find reasonable agreement. Our findings demonstrate that MEG charge carriers can be collected in suitably designed quantum dot solar cells, providing ample incentive to better understand MEG within isolated and coupled quantum dots as a research path to enhancing the efficiency of solar light harvesting technologies.
  15. 15
    Congreve, D. N.; Lee, J.; Thompson, N. J.; Hontz, E.; Yost, S. R.; Reusswig, P. D.; Bahlke, M. E.; Reineke, S.; Voorhis, T. V.; Baldo, M. A. External Quantum Efficiency Above 100% in a Singlet-Exciton-Fission-Based Organic Photovoltaic Cell Science 2013, 340, 334– 337
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    15
    External Quantum Efficiency Above 100% in a Singlet-Exciton-Fission-Based Organic Photovoltaic Cell
    Congreve, Daniel N.; Lee, Jiye; Thompson, Nicholas J.; Hontz, Eric; Yost, Shane R.; Reusswig, Philip D.; Bahlke, Matthias E.; Reineke, Sebastian; Van Voorhis, Troy; Baldo, Marc A.
    Science (Washington, DC, United States) (2013), 340 (6130), 334-337CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Singlet exciton fission transforms a mol. singlet excited state into 2 triplet states, each with half the energy of the original singlet. In solar cells, it could potentially double the photocurrent from high-energy photons. The authors demonstrate org. solar cells that exploit singlet exciton fission in pentacene to generate >1 electron per incident photon in a portion of the visible spectrum. Using a fullerene acceptor, a poly(3-hexylthiophene) exciton confinement layer, and a conventional optical trapping scheme, the authors show a peak external quantum efficiency of (109 ± 1)% at wavelength λ = 670 nm for a 15-nm-thick pentacene film. The corresponding internal quantum efficiency is (160 ± 10)%. Anal. of the magnetic field effect on photocurrent suggests that the triplet yield approaches 200% for pentacene films thicker than 5 nm.
  16. 16
    Jackson, P.; Hariskos, D.; Lotter, E.; Paetel, S.; Wuerz, R.; Menner, R.; Wischmann, W.; Powalla, M. New World Record Efficiency for Cu(In,Ga)Se2 Thin-Film Solar Cells Beyond 20% Prog. Photovoltaics Res. Appl. 2011, 19, 894– 897
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    16
    New world record efficiency for Cu(In,Ga)Se2 thin-film solar cells beyond 20%
    Jackson, Philip; Hariskos, Dimitrios; Lotter, Erwin; Paetel, Stefan; Wuerz, Roland; Menner, Richard; Wischmann, Wiltraud; Powalla, Michael
    Progress in Photovoltaics (2011), 19 (7), 894-897CODEN: PPHOED; ISSN:1062-7995. (John Wiley & Sons Ltd.)
    In this contribution, we present a new certified world record efficiency of 20.1 and 20.3% for Cu(In,Ga)Se2 thin-film solar cells. We analyze the characteristics of solar cells on such a performance level and demonstrate a high degree of reproducibility.
  17. 17
    Akhavan, V. A.; Panthani, M. G.; Goodfellow, B. W.; Reid, D. K.; Korgel, B. A. Thickness-Limited Performance of CuInSe2 Nanocrystal Photovoltaic Devices Opt. Express 2010, 18, A411– A420
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    Stolle, C. J.; Panthani, M. G.; Harvey, T. B.; Akhavan, V. A.; Korgel, B. A. Comparison of the Photovoltaic Response of Oleylamine and Inorganic Ligand-Capped CuInSe2 Nanocrystals ACS Appl. Mater. Interfaces 2012, 4, 2757– 2761
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    18
    Comparison of the Photovoltaic Response of Oleylamine and Inorganic Ligand-Capped CuInSe2 Nanocrystals
    Stolle, C. Jackson; Panthani, Matthew G.; Harvey, Taylor B.; Akhavan, Vahid A.; Korgel, Brian A.
    ACS Applied Materials & Interfaces (2012), 4 (5), 2757-2761CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Thin film photovoltaic devices (PVs) were fabricated with CuInSe2 (CIS) nanocrystals capped with either oleylamine, inorg. metal chalcogenide-hydrazinium complexes (MCC), or S2-, HS-, and OH-. A CIS nanocrystal layer deposited from solvent-based inks without high temp. processing served as the active light-absorbing material in the devices. The MCC ligand-capped CIS nanocrystal PVs exhibited power conversion efficiency under AM1.5 illumination (1.7%) comparable to the oleylamine-capped CIS nanocrystals (1.6%), but with significantly thinner absorber layers. S2--capped CIS nanocrystals could be deposited from aq. dispersions, but exhibited lower photovoltaic performance.
  19. 19
    Panthani, M. G.; Stolle, C. J.; Reid, D. K.; Rhee, D. J.; Harvey, T. B.; Akhavan, V. A.; Yu, Y.; Korgel, B. A. CuInSe2 Quantum Dot Solar Cells with High Open-Circuit Voltage J. Phys. Chem. Lett. 2013, 4, 2030– 2034
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    19
    CuInSe2 Quantum Dot Solar Cells with High Open-Circuit Voltage
    Panthani, Matthew G.; Stolle, C. Jackson; Reid, Dariya K.; Rhee, Dong Joon; Harvey, Taylor B.; Akhavan, Vahid A.; Yu, Yixuan; Korgel, Brian A.
    Journal of Physical Chemistry Letters (2013), 4 (12), 2030-2034CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    CuInSe2 (CISe) quantum dots (QDs) were synthesized with tunable size from <2 to 7 nm diam. Nanocrystals were made using a secondary phosphine selenide as the Se source, which, compared to tertiary phosphine selenide precursors, provides higher product yields and smaller nanocrystals that elicit quantum confinement with a size-dependent optical gap. Photovoltaic devices fabricated from spray-cast CISe QD films exhibited large, size-dependent, open-circuit voltages, up to 849 mV for absorber films with a 1.46 eV optical gap, suggesting that midgap trapping does not dominate the performance of these CISe QD solar cells.
  20. 20
    Guo, Q.; Ford, G. M.; Agrawal, R.; Hillhouse, H. W. Ink Formulation and Low-Temperature Incorporation of Sodium to Yield 12% Efficient Cu(In,Ga)(S,Se)2 Solar Cells from Sulfide Nanocrystal Inks Prog. Photovoltaics Res. Appl. 2012, 21, 64– 71
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    There is no corresponding record for this reference.
  21. 21
    Akhavan, V. A.; Harvey, T. B.; Stolle, C. J.; Ostrowski, D. P.; Glaz, M. S.; Goodfellow, B. W.; Panthani, M. G.; Reid, D. K.; Vanden Bout, D. A.; Korgel, B. A. Influence of Composition on the Performance of Sintered Cu(In,Ga)Se2 Nanocrystal Thin-Film Photovoltaic Devices ChemSusChem 2013, 6, 481– 486
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    21
    Influence of Composition on the Performance of Sintered Cu(In,Ga)Se2 Nanocrystal Thin-Film Photovoltaic Devices
    Akhavan, Vahid A.; Harvey, Taylor B.; Stolle, C. Jackson; Ostrowski, David P.; Glaz, Micah S.; Goodfellow, Brian W.; Panthani, Matthew G.; Reid, Dariya K.; Vanden Bout, David A.; Korgel, Brian A.
    ChemSusChem (2013), 6 (3), 481-486CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Thin-film photovoltaic devices (PVs) were prepd. by selenization using oleylamine-capped Cu(In,Ga)Se2 (CIGS) nanocrystals sintered at >500° in Se vapor. The device performance varied significantly with [Ga]/[In+Ga] content in the nanocrystals. The highest power conversion efficiency (PCE) obsd. in the devices studied was 5.1% under AM 1.5 G illumination, obtained with [Ga]/[In+Ga] = 0.32. The variation in PCE with compn. is partly a result of bandgap tuning and optimization, but the main influence of nanocrystal compn. appeared to be on the quality of the sintered films. The [Cu]/[In+Ga] content is strongly influenced by the [Ga]/[In+Ga] concn., which appears to be correlated with the morphol. of the sintered film. For this reason, only small changes in the [Ga]/[In+Ga] content resulted in significant variations in device efficiency.
  22. 22
    Schroder, K. A. Mechanisms of Photonic Curing: Processing High Temperatures on Low Temperature Substrates Nanotech Conf. Expo 2011 2011, 2, 220– 223
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    22
    Mechanisms of photonic curing: processing high temperature films on low temperature substrates
    Schroder, K. A.
    Nanotech Conference & Expo 2011: An Interdisciplinary Integrative Forum on Nanotechnology, Biotechnology and Microtechnology, Boston, MA, United States, June 13-16, 2011 (2011), 2 (), 220-223CODEN: 69OMYV ISSN:. (CRC Press)
    Photonic Curing uses flashlamps to thermally process a thin film at high temp. on a low temp. substrate without damaging it. This has utility in printed electronics, where high temp. curing is generally equated to electronic performance and high speed processing is equated to low cost. Three significant processing advantages are realized over previous technologies: 1. Inexpensive (and flexible) polymer substrates can now be used in place of expensive, rigid substrates while achieving similar performance. 2. Thin films can be cured quickly enough to keep up with high speed printing processes in a small footprint, thus making it suitable for in-line placement with existing print systems. 3. The transient nature of the process has enabled the creation of new types of films on low temp. substrates, including those created by the photonic modulation of high temp. chem. reactions. In this paper we discuss the mechanisms of the process and model it using a thermal diffusion simulation. The simulation has been integrated into a 4th generation highspeed processing tool yielding predictive results.
  23. 23
    Sites, J. R.; Tavakolian, H.; Sasala, R. A. Analysis of Apparent Quantum Efficiency Sol. Cells 1990, 29, 39– 48
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    23
    Analysis of apparent quantum efficiency
    Sites, J. R.; Tavakolian, H.; Sasala, R. A.
    Solar Cells (1990), 29 (1), 39-48CODEN: SOCLD4; ISSN:0379-6787.
    Quantum efficiency measurements were made of cryst. Si and polycryst. thin film solar cells. The measurements are reliable input for photocurrent loss anal. when the forward current of the diode is small compared to the photogenerated current. All cells, however, exhibit major redns. in apparent quantum efficiency under forward bias. For the cryst. cells, the redn. is explained by the series resistances of the circuit and the cell. For polycryst. cells, the voltage dependence is addnl. affected by an increase in diode quality factor or a redn. in series resistance with radiation intensity. In no case does the voltage dependence of collection efficiency play a significant role.
  24. 24
    Hegedus, S. S. The Photoresponse of CdS/CuInSe2 Thin-Film Heterojunction Solar Cells IEEE Trans. Electron Devices 1984, 31, 629– 633
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    There is no corresponding record for this reference.
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    Hegedus, S.; Ryan, D.; Dobson, K.; McCandless, B.; Desai, D. Photoconductive CdS: How Does It Affect CdTe/CdS Solar Cell Performance? MRS Online Proc. Libr. 2003, 763, B9.5.1– B9.5.6
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    Gloeckler, M.; Sites, J. R. Apparent Quantum Efficiency Effects in CdTe Solar Cells J. Appl. Phys. 2004, 95, 4438– 4445
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    26
    Apparent quantum efficiency effects in CdTe solar cells
    Gloeckler, M.; Sites, J. R.
    Journal of Applied Physics (2004), 95 (8), 4438-4445CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    Quantum efficiency measurements of n-CdS/p-CdTe solar cells performed under nonstandard illumination, voltage bias, or both can be severely distorted by photogeneration and contact-barrier effects. In this work we discuss the effects that are typically obsd., the requirements needed to reproduce these effects with modeling tools, and the potential applications of apparent quantum efficiency anal. Recently published exptl. results are interpreted and reproduced using numerical simulation tools. The suggested model explains large neg. apparent quantum efficiencies (»100%) seen in the spectral range of 350-550 nm, modestly large neg. apparent quantum efficiencies (>100%) in the spectral range of 800-850 nm, enhanced pos. or neg. response obsd. under red, blue, and white light bias, and photocurrent gain significantly different from unity. Some of these effects originate from the photogeneration in the highly compensated CdS window layer, some from photogeneration within the CdTe, and some are further modified by the height of the CdTe back-contact barrier.
  27. 27
    Demtsu, S.; Albin, D.; Sites, J. Role of Copper in the Performance of CdS/CdTe Solar Cells. In Conference Record of the 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion; 2006; Vol. 1, pp 523– 526.
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  28. 28
    Luther, J. M.; Beard, M. C.; Song, Q.; Law, M.; Ellingson, R. J.; Nozik, A. J. Multiple Exciton Generation in Films of Electronically Coupled PbSe Quantum Dots Nano Lett. 2007, 7, 1779– 1784
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    28
    Multiple Exciton Generation in Films of Electronically Coupled PbSe Quantum Dots
    Luther, Joseph M.; Beard, Matthew C.; Song, Qing; Law, Matt; Ellingson, Randy J.; Nozik, Arthur J.
    Nano Letters (2007), 7 (6), 1779-1784CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Multiple exciton generation (MEG) was studied in electronically coupled films of PbSe quantum dots (QDs) employing ultrafast time-resolved transient absorption spectroscopy. The MEG efficiency in PbSe does not decrease when the QDs are treated with hydrazine, which was shown to greatly enhance carrier transport in PbSe QD films by decreasing the interdot distance. The quantum yield is measured and compared to previously reported values for electronically isolated QDs suspended in org. solvents at ∼4 and 4.5 times the effective band gap. A slightly modified anal. is applied to ext. the MEG efficiency and the absorption cross section of each sample at the pump wavelength. The absorption cross sections of the samples were compared to that of bulk PbSe. Both the biexciton lifetime and the absorption cross section increase in films relative to isolated QDs in soln.
  29. 29
    Stewart, J. T.; Padilha, L. A.; Qazilbash, M. M.; Pietryga, J. M.; Midgett, A. G.; Luther, J. M.; Beard, M. C.; Nozik, A. J.; Klimov, V. I. Comparison of Carrier Multiplication Yields in PbS and PbSe Nanocrystals: The Role of Competing Energy-Loss Processes Nano Lett. 2012, 12, 622– 628
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    29
    Comparison of Carrier Multiplication Yields in PbS and PbSe Nanocrystals: The Role of Competing Energy-Loss Processes
    Stewart, John T.; Padilha, Lazaro A.; Qazilbash, M. Mumtaz; Pietryga, Jeffrey M.; Midgett, Aaron G.; Luther, Joseph M.; Beard, Matthew C.; Nozik, Arthur J.; Klimov, Victor I.
    Nano Letters (2012), 12 (2), 622-628CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    IR band gap semiconductor nanocrystals are promising materials for exploring generation III photovoltaic concepts that rely on carrier multiplication or multiple exciton generation, the process in which a single high-energy photon generates >1 electron-hole pair. Measurements are presented of carrier multiplication yields and biexciton lifetimes for a large selection of PbS nanocrystals and compare these results to the well-studied PbSe nanocrystals. The similar bulk properties of PbS and PbSe make this an important comparison for discerning the pertinent properties that det. efficient carrier multiplication. PbS and PbSe have very similar biexciton lifetimes as a function of confinement energy. Together with the similar bulk properties, probably the rates of multiexciton generation, which is the inverse of Auger recombination, are also similar. The carrier multiplication yields in PbS nanocrystals are strikingly lower than those obsd. for PbSe nanocrystals. Probably this implies the rate of competing processes, such as phonon emission, is higher in PbS nanocrystals than in PbSe nanocrystals. The estns. for phonon emission mediated by the polar Froehlich-type interaction indicate that the corresponding energy-loss rate is approx. twice as large in PbS than in PbSe.
  30. 30
    McGuire, J. A.; Sykora, M.; Joo, J.; Pietryga, J. M.; Klimov, V. I. Apparent versus True Carrier Multiplication Yields in Semiconductor Nanocrystals Nano Lett. 2010, 10, 2049– 2057
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    30
    Apparent versus true carrier multiplication yields in semiconductor nanocrystals
    McGuire, John A.; Sykora, Milan; Joo, Jin; Pietryga, Jeffrey M.; Klimov, Victor I.
    Nano Letters (2010), 10 (6), 2049-2057CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Generation of multiple electron-hole pairs (excitons) by single photons, known as carrier multiplication (CM), has the potential to appreciably improve the performance of solar photovoltaics. In semiconductor nanocrystals, this effect usually was detected using a distinct dynamical signature of multiexcitons assocd. with their fast Auger recombination. Here, we show that uncontrolled photocharging of the nanocrystal core can lead to exaggeration of the Auger decay component and, as a result, significant deviations of the apparent CM efficiencies from their true values. Specifically, we observe that for the same sample, apparent multiexciton yields can differ by a factor of ∼3 depending on whether the nanocrystal soln. is static or stirred. We show that this discrepancy is consistent with photoinduced charging of the nanocrystals in static solns., the effect of which is minimized in the stirred case where the charged nanocrystals are swept from the excitation vol. between sequential excitation pulses. Using side-by-side measurements of CM efficiencies and nanocrystal charging, we show that the CM results obtained under static conditions converge to the values measured for stirred solns. after we accurately account for the effects of photocharging. This study helps to clarify the recent controversy over CM in nanocrystals and highlights some of the issues that must be carefully considered in spectroscopic studies of this process.
  31. 31
    Ip, A. H.; Thon, S. M.; Hoogland, S.; Voznyy, O.; Zhitomirsky, D.; Debnath, R.; Levina, L.; Rollny, L. R.; Carey, G. H.; Fischer, A. Hybrid Passivated Colloidal Quantum Dot Solids Nat. Nanotechnol. 2012, 7, 577– 582
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    31
    Hybrid passivated colloidal quantum dot solids
    Ip, Alexander H.; Thon, Susanna M.; Hoogland, Sjoerd; Voznyy, Oleksandr; Zhitomirsky, David; Debnath, Ratan; Levina, Larissa; Rollny, Lisa R.; Carey, Graham H.; Fischer, Armin; Kemp, Kyle W.; Kramer, Illan J.; Ning, Zhijun; Labelle, Andre J.; Chou, Kang Wei; Amassian, Aram; Sargent, Edward H.
    Nature Nanotechnology (2012), 7 (9), 577-582CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    Colloidal quantum dot (CQD) films allow large-area soln. processing and bandgap tuning through the quantum size effect. However, the high ratio of surface area to vol. makes CQD films prone to high trap state densities if surfaces are imperfectly passivated, promoting recombination of charge carriers that is detrimental to device performance. Recent advances have replaced the long insulating ligands that enable colloidal stability following synthesis with shorter org. linkers or halide anions, leading to improved passivation and higher packing densities. Although this substitution has been performed using solid-state ligand exchange, a soln.-based approach is preferable because it enables increased control over the balance of charges on the surface of the quantum dot, which is essential for eliminating midgap trap states. Furthermore, the soln.-based approach leverages recent progress in metal:chalcogen chem. in the liq. phase. Here, we quantify the d. of midgap trap states in CQD solids and show that the performance of CQD-based photovoltaics is now limited by electron-hole recombination due to these states. Next, using d. functional theory and optoelectronic device modeling, we show that to improve this performance it is essential to bind a suitable ligand to each potential trap site on the surface of the quantum dot. We then develop a robust hybrid passivation scheme that involves introducing halide anions during the end stages of the synthesis process, which can passivate trap sites that are inaccessible to much larger org. ligands. An org. crosslinking strategy is then used to form the film. Finally, we use our hybrid passivated CQD solid to fabricate a solar cell with a certified efficiency of 7.0%, which is a record for a CQD photovoltaic device.
  32. 32
    Barkhouse, D. A. R.; Pattantyus-Abraham, A. G.; Levina, L.; Sargent, E. H. Thiols Passivate Recombination Centers in Colloidal Quantum Dots Leading to Enhanced Photovoltaic Device Efficiency ACS Nano 2008, 2, 2356– 2362
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    32
    Thiols Passivate Recombination Centers in Colloidal Quantum Dots Leading to Enhanced Photovoltaic Device Efficiency
    Barkhouse, D. Aaron R.; Pattantyus-Abraham, Andras G.; Levina, Larissa; Sargent, Edward H.
    ACS Nano (2008), 2 (11), 2356-2362CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    The use of thiol-terminated ligands has recently been reported to enhance 10-fold the power conversion efficiency of colloidal quantum dot photovoltaic devices. We find herein that, in a representative amine-capped PbS colloidal quantum dot materials system, improved mobility following thiol treatment accounts for only a 1.4-fold increase in power conversion efficiency. We then proceed to investigate the origins of the remainder of the quadrupling in power conversion efficiency following thiol treatment. We find through measurements of photoluminescence quantum efficiency that exposure to thiols dramatically enhances photoluminescence in colloidal quantum dot films. The same mols. increase open-circuit voltage from 0.28 to 0.43 V. Combined, these findings suggest that mid-gap states, which serve as recombination centers (lowering external quantum efficiency) and metal-semiconductor junction interface states (lowering open-circuit voltage), are substantially passivated using thiols. Through exposure to thiols, we improve external quantum efficiency from 5 to 22% and, combined with the improvement in open-circuit voltage, improve power conversion efficiency to 2.6% under 76 mW/cm2 at 1 μm wavelength. These findings are consistent with recent reports in photoconductive PbS colloidal quantum dot photodetectors that thiol exposure substantially removes deep (0.3 eV) electron traps, leaving only shallow (0.1 eV) traps.
  33. 33
    Konstantatos, G.; Levina, L.; Fischer, A.; Sargent, E. H. Engineering the Temporal Response of Photoconductive Photodetectors via Selective Introduction of Surface Trap States Nano Lett. 2008, 8, 1446– 1450
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    33
    Engineering the temporal response of photoconductive photodetectors via selective introduction of surface trap states
    Konstantatos, Gerasimos; Levina, Larissa; Fischer, Armin; Sargent, Edward H.
    Nano Letters (2008), 8 (5), 1446-1450CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Photoconductive photodetectors fabricated using simple soln.-processing have recently been shown to exhibit high gains (>1000) and outstanding sensitivities (D* > 1013 Jones). One ostensible disadvantage of exploiting photoconductive gain is that the temporal response is limited by the release of carriers from trap states. Here we show that it is possible to introduce specific chem. species onto the surfaces of colloidal quantum dots to produce only a single, desired trap state having a carefully selected lifetime. In this way we demonstrate a device that exhibits an attractive photoconductive gain (>10) combined with a response time (∼25 ms) useful in imaging. We achieve this by preserving a single surface species, lead sulfite, while eliminating lead sulfate and lead carboxylate. In doing so we preserve the outstanding sensitivity of these devices, achieving a specific detectivity of 1012 Jones in the visible, while generating a temporal response suited to imaging applications.
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    Nagpal, P.; Klimov, V. I. Role of Mid-gap States in Charge Transport and Photoconductivity in Semiconductor Nanocrystal Films Nat. Commun. 2011, 2, 486
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    34
    Role of mid-gap states in charge transport and photoconductivity in semiconductor nanocrystal films
    Nagpal Prashant; Klimov Victor I
    Nature communications (2011), 2 (), 486 ISSN:.
    Colloidal semiconductor nanocrystals have attracted significant interest for applications in solution-processable devices such as light-emitting diodes and solar cells. However, a poor understanding of charge transport in nanocrystal assemblies, specifically the relation between electrical conductance in dark and under light illumination, hinders their technological applicability. Here we simultaneously address the issues of 'dark' transport and photoconductivity in films of PbS nanocrystals, by incorporating them into optical field-effect transistors in which the channel conductance is controlled by both gate voltage and incident radiation. Spectrally resolved photoresponses of these devices reveal a weakly conductive mid-gap band that is responsible for charge transport in dark. The mechanism for conductance, however, changes under illumination when it becomes dominated by band-edge quantized states. In this case, the mid-gap band still has an important role as its occupancy (tuned by the gate voltage) controls the dynamics of band-edge charges.
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    Castro, S. L.; Bailey, S. G.; Raffaelle, R. P.; Banger, K. K.; Hepp, A. F. Nanocrystalline Chalcopyrite Materials (CuInS2 and CuInSe2) via Low-Temperature Pyrolysis of Molecular Single-Source Precursors Chem. Mater. 2003, 15, 3142– 3147
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    35
    Nanocrystalline Chalcopyrite Materials (CuInS2 and CuInSe2) via Low-Temperature Pyrolysis of Molecular Single-Source Precursors
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Cite this: J. Phys. Chem. Lett. 2014, 5, 2, 304–309
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  • Abstract

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    Figure 1

    Figure 1. Photonic curing of nanocrystal films on Au-coated glass substrates. (a) Photonic curing can be used to remove oleylamine capping ligands from the CuInSe2 nanocrystal film without inducing nanocrystal grain growth. (b) When the capping ligands are present, they inhibit the collection of multiexcitons from the film, leading to electron–hole recombination by Auger recombination. (c) Without the ligand barrier between nanocrystals, multiexciton transport becomes much more probable.

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    Figure 2

    Figure 2. CuInSe2 nanocrystal layers before and after photonic curing and their PV device performance. Top-down and cross-sectional SEM images of an oleylamine-capped CuInSe2 (CIS) nanocrystal film on Au-coated glass (a, d) before and after photonic curing with (b, e) 2.2 and (c, f) and 3 J/cm2 pulse fluence. (g–i) Corresponding current–voltage measurements (the black curve is dark current; the red curve is measured under AM1.5G illumination (100 mW/cm2)) of devices made with the nanocrystal films provided below the SEM images.

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    Figure 3

    Figure 3. EQE enhancements resulting from photonic curing of the CuInSe2 nanocrystal layer used in PV devices. (a) EQE measurements taken under white light bias (50 mW/cm2) for CuInSe2 nanocrystal devices without photonic curing (black curve) compared to the device made with cured (2.2 J/cm2 pulse fluence) nanocrystals (red curve). The short-circuit currents determined from these data, of 4.95 and 14.29 mA/cm2, are consistent with the short-circuit currents measured under AM1.5 illumination (100 mW/cm2). (b) EQE measured under varying white light bias intensity (100, 50, 25, 10, and 0% of a 50 mW/cm2 bias light) with the same intensity of monochromated probe light. There was no change in EQE for the device made with as-deposited nanocrystals (inset), but the EQE decreased significantly for the cured device when the white light bias intensity was reduced to the amounts indicated.

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    Figure 4

    Figure 4. TA spectroscopy of CuInSe2 nanocrystal films after photonic curing. (a) TA kinetics normalized to −Δα = 1 at 1000 ps with an 800 nm pump wavelength and pump fluences of 300 (dark blue), 90 (green), 60 (pink), 30 (teal), 15 (blue), 6 (red), and 3 μJ/cm2 (black). (b) TA kinetics normalized to −Δα = 1 at 1000 ps with a 400 nm pump wavelength and pump fluences of 18 (red) and 9 μJ/cm2 (blue). The average low fluence background (average of 3, 6, 15, and 30 μJ/cm2 signals) at an 800 nm pump wavelength is also shown for comparison (black). (c) TA kinetics showing the Auger recombination rate. The single exciton TA kinetics background (average 800 nm wavelength low-fluence pump) is subtracted from the high-fluence TA kinetics at an 800 nm, 300 μJ/cm2 pump, which shows the creation of multiexcitons due to the absorption of multiple photons per nanocrystal. The kinetics are plotted on a log scale and can be fitted to a single exponential with a time constant of 92 ps. (d) TA kinetics showing Auger recombination at a 400 nm pump and low fluence. The single-exciton TA kinetics background (average 800 nm wavelength low-fluence pump) is subtracted from the TA kinetics at a 400 nm, 9 μJ/cm2 pump, which should only show Auger recombination if MEG is present. The kinetics are plotted on a log scale and can be fitted to a single exponential with a time constant of 74 ps.

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      CuInSe2 Quantum Dot Solar Cells with High Open-Circuit Voltage
      Panthani, Matthew G.; Stolle, C. Jackson; Reid, Dariya K.; Rhee, Dong Joon; Harvey, Taylor B.; Akhavan, Vahid A.; Yu, Yixuan; Korgel, Brian A.
      Journal of Physical Chemistry Letters (2013), 4 (12), 2030-2034CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      CuInSe2 (CISe) quantum dots (QDs) were synthesized with tunable size from <2 to 7 nm diam. Nanocrystals were made using a secondary phosphine selenide as the Se source, which, compared to tertiary phosphine selenide precursors, provides higher product yields and smaller nanocrystals that elicit quantum confinement with a size-dependent optical gap. Photovoltaic devices fabricated from spray-cast CISe QD films exhibited large, size-dependent, open-circuit voltages, up to 849 mV for absorber films with a 1.46 eV optical gap, suggesting that midgap trapping does not dominate the performance of these CISe QD solar cells.
    20. 20
      Guo, Q.; Ford, G. M.; Agrawal, R.; Hillhouse, H. W. Ink Formulation and Low-Temperature Incorporation of Sodium to Yield 12% Efficient Cu(In,Ga)(S,Se)2 Solar Cells from Sulfide Nanocrystal Inks Prog. Photovoltaics Res. Appl. 2012, 21, 64– 71
      There is no corresponding record for this reference.
    21. 21
      Akhavan, V. A.; Harvey, T. B.; Stolle, C. J.; Ostrowski, D. P.; Glaz, M. S.; Goodfellow, B. W.; Panthani, M. G.; Reid, D. K.; Vanden Bout, D. A.; Korgel, B. A. Influence of Composition on the Performance of Sintered Cu(In,Ga)Se2 Nanocrystal Thin-Film Photovoltaic Devices ChemSusChem 2013, 6, 481– 486
      21
      Influence of Composition on the Performance of Sintered Cu(In,Ga)Se2 Nanocrystal Thin-Film Photovoltaic Devices
      Akhavan, Vahid A.; Harvey, Taylor B.; Stolle, C. Jackson; Ostrowski, David P.; Glaz, Micah S.; Goodfellow, Brian W.; Panthani, Matthew G.; Reid, Dariya K.; Vanden Bout, David A.; Korgel, Brian A.
      ChemSusChem (2013), 6 (3), 481-486CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Thin-film photovoltaic devices (PVs) were prepd. by selenization using oleylamine-capped Cu(In,Ga)Se2 (CIGS) nanocrystals sintered at >500° in Se vapor. The device performance varied significantly with [Ga]/[In+Ga] content in the nanocrystals. The highest power conversion efficiency (PCE) obsd. in the devices studied was 5.1% under AM 1.5 G illumination, obtained with [Ga]/[In+Ga] = 0.32. The variation in PCE with compn. is partly a result of bandgap tuning and optimization, but the main influence of nanocrystal compn. appeared to be on the quality of the sintered films. The [Cu]/[In+Ga] content is strongly influenced by the [Ga]/[In+Ga] concn., which appears to be correlated with the morphol. of the sintered film. For this reason, only small changes in the [Ga]/[In+Ga] content resulted in significant variations in device efficiency.
    22. 22
      Schroder, K. A. Mechanisms of Photonic Curing: Processing High Temperatures on Low Temperature Substrates Nanotech Conf. Expo 2011 2011, 2, 220– 223
      22
      Mechanisms of photonic curing: processing high temperature films on low temperature substrates
      Schroder, K. A.
      Nanotech Conference & Expo 2011: An Interdisciplinary Integrative Forum on Nanotechnology, Biotechnology and Microtechnology, Boston, MA, United States, June 13-16, 2011 (2011), 2 (), 220-223CODEN: 69OMYV ISSN:. (CRC Press)
      Photonic Curing uses flashlamps to thermally process a thin film at high temp. on a low temp. substrate without damaging it. This has utility in printed electronics, where high temp. curing is generally equated to electronic performance and high speed processing is equated to low cost. Three significant processing advantages are realized over previous technologies: 1. Inexpensive (and flexible) polymer substrates can now be used in place of expensive, rigid substrates while achieving similar performance. 2. Thin films can be cured quickly enough to keep up with high speed printing processes in a small footprint, thus making it suitable for in-line placement with existing print systems. 3. The transient nature of the process has enabled the creation of new types of films on low temp. substrates, including those created by the photonic modulation of high temp. chem. reactions. In this paper we discuss the mechanisms of the process and model it using a thermal diffusion simulation. The simulation has been integrated into a 4th generation highspeed processing tool yielding predictive results.
    23. 23
      Sites, J. R.; Tavakolian, H.; Sasala, R. A. Analysis of Apparent Quantum Efficiency Sol. Cells 1990, 29, 39– 48
      23
      Analysis of apparent quantum efficiency
      Sites, J. R.; Tavakolian, H.; Sasala, R. A.
      Solar Cells (1990), 29 (1), 39-48CODEN: SOCLD4; ISSN:0379-6787.
      Quantum efficiency measurements were made of cryst. Si and polycryst. thin film solar cells. The measurements are reliable input for photocurrent loss anal. when the forward current of the diode is small compared to the photogenerated current. All cells, however, exhibit major redns. in apparent quantum efficiency under forward bias. For the cryst. cells, the redn. is explained by the series resistances of the circuit and the cell. For polycryst. cells, the voltage dependence is addnl. affected by an increase in diode quality factor or a redn. in series resistance with radiation intensity. In no case does the voltage dependence of collection efficiency play a significant role.
    24. 24
      Hegedus, S. S. The Photoresponse of CdS/CuInSe2 Thin-Film Heterojunction Solar Cells IEEE Trans. Electron Devices 1984, 31, 629– 633
      There is no corresponding record for this reference.
    25. 25
      Hegedus, S.; Ryan, D.; Dobson, K.; McCandless, B.; Desai, D. Photoconductive CdS: How Does It Affect CdTe/CdS Solar Cell Performance? MRS Online Proc. Libr. 2003, 763, B9.5.1– B9.5.6
      There is no corresponding record for this reference.
    26. 26
      Gloeckler, M.; Sites, J. R. Apparent Quantum Efficiency Effects in CdTe Solar Cells J. Appl. Phys. 2004, 95, 4438– 4445
      26
      Apparent quantum efficiency effects in CdTe solar cells
      Gloeckler, M.; Sites, J. R.
      Journal of Applied Physics (2004), 95 (8), 4438-4445CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      Quantum efficiency measurements of n-CdS/p-CdTe solar cells performed under nonstandard illumination, voltage bias, or both can be severely distorted by photogeneration and contact-barrier effects. In this work we discuss the effects that are typically obsd., the requirements needed to reproduce these effects with modeling tools, and the potential applications of apparent quantum efficiency anal. Recently published exptl. results are interpreted and reproduced using numerical simulation tools. The suggested model explains large neg. apparent quantum efficiencies (»100%) seen in the spectral range of 350-550 nm, modestly large neg. apparent quantum efficiencies (>100%) in the spectral range of 800-850 nm, enhanced pos. or neg. response obsd. under red, blue, and white light bias, and photocurrent gain significantly different from unity. Some of these effects originate from the photogeneration in the highly compensated CdS window layer, some from photogeneration within the CdTe, and some are further modified by the height of the CdTe back-contact barrier.
    27. 27
      Demtsu, S.; Albin, D.; Sites, J. Role of Copper in the Performance of CdS/CdTe Solar Cells. In Conference Record of the 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion; 2006; Vol. 1, pp 523– 526.
      There is no corresponding record for this reference.
    28. 28
      Luther, J. M.; Beard, M. C.; Song, Q.; Law, M.; Ellingson, R. J.; Nozik, A. J. Multiple Exciton Generation in Films of Electronically Coupled PbSe Quantum Dots Nano Lett. 2007, 7, 1779– 1784
      28
      Multiple Exciton Generation in Films of Electronically Coupled PbSe Quantum Dots
      Luther, Joseph M.; Beard, Matthew C.; Song, Qing; Law, Matt; Ellingson, Randy J.; Nozik, Arthur J.
      Nano Letters (2007), 7 (6), 1779-1784CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Multiple exciton generation (MEG) was studied in electronically coupled films of PbSe quantum dots (QDs) employing ultrafast time-resolved transient absorption spectroscopy. The MEG efficiency in PbSe does not decrease when the QDs are treated with hydrazine, which was shown to greatly enhance carrier transport in PbSe QD films by decreasing the interdot distance. The quantum yield is measured and compared to previously reported values for electronically isolated QDs suspended in org. solvents at ∼4 and 4.5 times the effective band gap. A slightly modified anal. is applied to ext. the MEG efficiency and the absorption cross section of each sample at the pump wavelength. The absorption cross sections of the samples were compared to that of bulk PbSe. Both the biexciton lifetime and the absorption cross section increase in films relative to isolated QDs in soln.
    29. 29
      Stewart, J. T.; Padilha, L. A.; Qazilbash, M. M.; Pietryga, J. M.; Midgett, A. G.; Luther, J. M.; Beard, M. C.; Nozik, A. J.; Klimov, V. I. Comparison of Carrier Multiplication Yields in PbS and PbSe Nanocrystals: The Role of Competing Energy-Loss Processes Nano Lett. 2012, 12, 622– 628
      29
      Comparison of Carrier Multiplication Yields in PbS and PbSe Nanocrystals: The Role of Competing Energy-Loss Processes
      Stewart, John T.; Padilha, Lazaro A.; Qazilbash, M. Mumtaz; Pietryga, Jeffrey M.; Midgett, Aaron G.; Luther, Joseph M.; Beard, Matthew C.; Nozik, Arthur J.; Klimov, Victor I.
      Nano Letters (2012), 12 (2), 622-628CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      IR band gap semiconductor nanocrystals are promising materials for exploring generation III photovoltaic concepts that rely on carrier multiplication or multiple exciton generation, the process in which a single high-energy photon generates >1 electron-hole pair. Measurements are presented of carrier multiplication yields and biexciton lifetimes for a large selection of PbS nanocrystals and compare these results to the well-studied PbSe nanocrystals. The similar bulk properties of PbS and PbSe make this an important comparison for discerning the pertinent properties that det. efficient carrier multiplication. PbS and PbSe have very similar biexciton lifetimes as a function of confinement energy. Together with the similar bulk properties, probably the rates of multiexciton generation, which is the inverse of Auger recombination, are also similar. The carrier multiplication yields in PbS nanocrystals are strikingly lower than those obsd. for PbSe nanocrystals. Probably this implies the rate of competing processes, such as phonon emission, is higher in PbS nanocrystals than in PbSe nanocrystals. The estns. for phonon emission mediated by the polar Froehlich-type interaction indicate that the corresponding energy-loss rate is approx. twice as large in PbS than in PbSe.
    30. 30
      McGuire, J. A.; Sykora, M.; Joo, J.; Pietryga, J. M.; Klimov, V. I. Apparent versus True Carrier Multiplication Yields in Semiconductor Nanocrystals Nano Lett. 2010, 10, 2049– 2057
      30
      Apparent versus true carrier multiplication yields in semiconductor nanocrystals
      McGuire, John A.; Sykora, Milan; Joo, Jin; Pietryga, Jeffrey M.; Klimov, Victor I.
      Nano Letters (2010), 10 (6), 2049-2057CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Generation of multiple electron-hole pairs (excitons) by single photons, known as carrier multiplication (CM), has the potential to appreciably improve the performance of solar photovoltaics. In semiconductor nanocrystals, this effect usually was detected using a distinct dynamical signature of multiexcitons assocd. with their fast Auger recombination. Here, we show that uncontrolled photocharging of the nanocrystal core can lead to exaggeration of the Auger decay component and, as a result, significant deviations of the apparent CM efficiencies from their true values. Specifically, we observe that for the same sample, apparent multiexciton yields can differ by a factor of ∼3 depending on whether the nanocrystal soln. is static or stirred. We show that this discrepancy is consistent with photoinduced charging of the nanocrystals in static solns., the effect of which is minimized in the stirred case where the charged nanocrystals are swept from the excitation vol. between sequential excitation pulses. Using side-by-side measurements of CM efficiencies and nanocrystal charging, we show that the CM results obtained under static conditions converge to the values measured for stirred solns. after we accurately account for the effects of photocharging. This study helps to clarify the recent controversy over CM in nanocrystals and highlights some of the issues that must be carefully considered in spectroscopic studies of this process.
    31. 31
      Ip, A. H.; Thon, S. M.; Hoogland, S.; Voznyy, O.; Zhitomirsky, D.; Debnath, R.; Levina, L.; Rollny, L. R.; Carey, G. H.; Fischer, A. Hybrid Passivated Colloidal Quantum Dot Solids Nat. Nanotechnol. 2012, 7, 577– 582
      31
      Hybrid passivated colloidal quantum dot solids
      Ip, Alexander H.; Thon, Susanna M.; Hoogland, Sjoerd; Voznyy, Oleksandr; Zhitomirsky, David; Debnath, Ratan; Levina, Larissa; Rollny, Lisa R.; Carey, Graham H.; Fischer, Armin; Kemp, Kyle W.; Kramer, Illan J.; Ning, Zhijun; Labelle, Andre J.; Chou, Kang Wei; Amassian, Aram; Sargent, Edward H.
      Nature Nanotechnology (2012), 7 (9), 577-582CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Colloidal quantum dot (CQD) films allow large-area soln. processing and bandgap tuning through the quantum size effect. However, the high ratio of surface area to vol. makes CQD films prone to high trap state densities if surfaces are imperfectly passivated, promoting recombination of charge carriers that is detrimental to device performance. Recent advances have replaced the long insulating ligands that enable colloidal stability following synthesis with shorter org. linkers or halide anions, leading to improved passivation and higher packing densities. Although this substitution has been performed using solid-state ligand exchange, a soln.-based approach is preferable because it enables increased control over the balance of charges on the surface of the quantum dot, which is essential for eliminating midgap trap states. Furthermore, the soln.-based approach leverages recent progress in metal:chalcogen chem. in the liq. phase. Here, we quantify the d. of midgap trap states in CQD solids and show that the performance of CQD-based photovoltaics is now limited by electron-hole recombination due to these states. Next, using d. functional theory and optoelectronic device modeling, we show that to improve this performance it is essential to bind a suitable ligand to each potential trap site on the surface of the quantum dot. We then develop a robust hybrid passivation scheme that involves introducing halide anions during the end stages of the synthesis process, which can passivate trap sites that are inaccessible to much larger org. ligands. An org. crosslinking strategy is then used to form the film. Finally, we use our hybrid passivated CQD solid to fabricate a solar cell with a certified efficiency of 7.0%, which is a record for a CQD photovoltaic device.
    32. 32
      Barkhouse, D. A. R.; Pattantyus-Abraham, A. G.; Levina, L.; Sargent, E. H. Thiols Passivate Recombination Centers in Colloidal Quantum Dots Leading to Enhanced Photovoltaic Device Efficiency ACS Nano 2008, 2, 2356– 2362
      32
      Thiols Passivate Recombination Centers in Colloidal Quantum Dots Leading to Enhanced Photovoltaic Device Efficiency
      Barkhouse, D. Aaron R.; Pattantyus-Abraham, Andras G.; Levina, Larissa; Sargent, Edward H.
      ACS Nano (2008), 2 (11), 2356-2362CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      The use of thiol-terminated ligands has recently been reported to enhance 10-fold the power conversion efficiency of colloidal quantum dot photovoltaic devices. We find herein that, in a representative amine-capped PbS colloidal quantum dot materials system, improved mobility following thiol treatment accounts for only a 1.4-fold increase in power conversion efficiency. We then proceed to investigate the origins of the remainder of the quadrupling in power conversion efficiency following thiol treatment. We find through measurements of photoluminescence quantum efficiency that exposure to thiols dramatically enhances photoluminescence in colloidal quantum dot films. The same mols. increase open-circuit voltage from 0.28 to 0.43 V. Combined, these findings suggest that mid-gap states, which serve as recombination centers (lowering external quantum efficiency) and metal-semiconductor junction interface states (lowering open-circuit voltage), are substantially passivated using thiols. Through exposure to thiols, we improve external quantum efficiency from 5 to 22% and, combined with the improvement in open-circuit voltage, improve power conversion efficiency to 2.6% under 76 mW/cm2 at 1 μm wavelength. These findings are consistent with recent reports in photoconductive PbS colloidal quantum dot photodetectors that thiol exposure substantially removes deep (0.3 eV) electron traps, leaving only shallow (0.1 eV) traps.
    33. 33
      Konstantatos, G.; Levina, L.; Fischer, A.; Sargent, E. H. Engineering the Temporal Response of Photoconductive Photodetectors via Selective Introduction of Surface Trap States Nano Lett. 2008, 8, 1446– 1450
      33
      Engineering the temporal response of photoconductive photodetectors via selective introduction of surface trap states
      Konstantatos, Gerasimos; Levina, Larissa; Fischer, Armin; Sargent, Edward H.
      Nano Letters (2008), 8 (5), 1446-1450CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Photoconductive photodetectors fabricated using simple soln.-processing have recently been shown to exhibit high gains (>1000) and outstanding sensitivities (D* > 1013 Jones). One ostensible disadvantage of exploiting photoconductive gain is that the temporal response is limited by the release of carriers from trap states. Here we show that it is possible to introduce specific chem. species onto the surfaces of colloidal quantum dots to produce only a single, desired trap state having a carefully selected lifetime. In this way we demonstrate a device that exhibits an attractive photoconductive gain (>10) combined with a response time (∼25 ms) useful in imaging. We achieve this by preserving a single surface species, lead sulfite, while eliminating lead sulfate and lead carboxylate. In doing so we preserve the outstanding sensitivity of these devices, achieving a specific detectivity of 1012 Jones in the visible, while generating a temporal response suited to imaging applications.
    34. 34
      Nagpal, P.; Klimov, V. I. Role of Mid-gap States in Charge Transport and Photoconductivity in Semiconductor Nanocrystal Films Nat. Commun. 2011, 2, 486
      34
      Role of mid-gap states in charge transport and photoconductivity in semiconductor nanocrystal films
      Nagpal Prashant; Klimov Victor I
      Nature communications (2011), 2 (), 486 ISSN:.
      Colloidal semiconductor nanocrystals have attracted significant interest for applications in solution-processable devices such as light-emitting diodes and solar cells. However, a poor understanding of charge transport in nanocrystal assemblies, specifically the relation between electrical conductance in dark and under light illumination, hinders their technological applicability. Here we simultaneously address the issues of 'dark' transport and photoconductivity in films of PbS nanocrystals, by incorporating them into optical field-effect transistors in which the channel conductance is controlled by both gate voltage and incident radiation. Spectrally resolved photoresponses of these devices reveal a weakly conductive mid-gap band that is responsible for charge transport in dark. The mechanism for conductance, however, changes under illumination when it becomes dominated by band-edge quantized states. In this case, the mid-gap band still has an important role as its occupancy (tuned by the gate voltage) controls the dynamics of band-edge charges.
    35. 35
      Castro, S. L.; Bailey, S. G.; Raffaelle, R. P.; Banger, K. K.; Hepp, A. F. Nanocrystalline Chalcopyrite Materials (CuInS2 and CuInSe2) via Low-Temperature Pyrolysis of Molecular Single-Source Precursors Chem. Mater. 2003, 15, 3142– 3147
      35
      Nanocrystalline Chalcopyrite Materials (CuInS2 and CuInSe2) via Low-Temperature Pyrolysis of Molecular Single-Source Precursors
      Castro, Stephanie L.; Bailey, Sheila G.; Raffaelle, Ryne P.; Banger, Kulbinder K.; Hepp, Aloysius F.
      Chemistry of Materials (2003), 15 (16), 3142-3147CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Nanometer-sized particles of the chalcopyrite compds. CuInS2 and CuInSe2 were synthesized by thermal decompn. of mol. single-source precursors (PPh3)2CuIn(SEt)4 and (PPh3)2CuIn(SePh)4, resp., in the noncoordinating solvent dioctyl phthalate at 200-300°. The nanoparticles range in size from 3 to 30 nm and are aggregated to form roughly spherical clusters of ∼500 nm in diam. X-ray diffraction of the nanoparticle powders shows greatly broadened lines, indicative of very small particle sizes, which is confirmed by TEM. Peaks present in the XRD can be indexed to ref. patterns for the resp. chalcopyrite compds. Optical spectroscopy and elemental anal. by energy dispersive spectroscopy support the identification of the nanoparticles as chalcopyrites.
    36. 36
      Franzl, T.; Koktysh, D. S.; Klar, T. A.; Rogach, A. L.; Feldmann, J.; Gaponik, N. Fast Energy Transfer in Layer-By-Layer Assembled CdTe Nanocrystal Bilayers Appl. Phys. Lett. 2004, 84, 2904– 2906
      There is no corresponding record for this reference.
    37. 37
      Lazarenkova, O. L.; Balandin, A. A. Miniband Formation in a Quantum Dot Crystal J. Appl. Phys. 2001, 89, 5509– 5515
      37
      Miniband formation in a quantum dot crystal
      Lazarenkova, Olga L.; Balandin, Alexander A.
      Journal of Applied Physics (2001), 89 (10), 5509-5515CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      The authors analyze the carrier energy band structure in a three-dimensional regimented array of semiconductor quantum dots using an envelope function approxn. The coupling among quantum dots leads to a splitting of the quantized carrier energy levels of single dots and formation of three-dimensional minibands. By changing the size of quantum dots, interdot distances, barrier height, and regimentation, one can control the electronic band structure of this artificial quantum dot crystal. Results of simulations carried out for simple cubic and tetragonal quantum dot crystal show that the carrier d. of states, effective mass tensor and other properties are different from those of bulk and quantum well superlattices. Also the properties of artificial crystal are more sensitive to the dot regimentation rather then to the dot shape. The proposed engineering of three-dimensional mini bands in quantum dot crystals allows one to fine-tune electronic and optical properties of such nanostructures.
    38. 38
      Trinh, M. T.; Limpens, R.; Boer, W. D. A. M.; de Schins, J. M.; Siebbeles, L. D. A.; Gregorkiewicz, T. Direct Generation of Multiple Excitons in Adjacent Silicon Nanocrystals Revealed by Induced Absorption Nat. Photonics 2012, 6, 316– 321
      38
      Direct generation of multiple excitons in adjacent silicon nanocrystals revealed by induced absorption
      Trinh, M. Tuan; Limpens, Rens; de Boer, Wieteke D. A. M.; Schins, Juleon M.; Siebbeles, Laurens D. A.; Gregorkiewicz, Tom
      Nature Photonics (2012), 6 (5), 316-321CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
      The enhancement of carrier multiplication in semiconductor nanocrystals attracts a great deal of attention because of its potential in photovoltaic applications. Here, we present the results of investigations of a novel carrier multiplication mechanism recently proposed for closely spaced silicon nanocrystals in SiO2 on the basis of photoluminescence. Using ultrafast pump-probe spectroscopy rigorously calibrated for the no. of absorbed photons, we find that adjacent nanocrystals are excited directly upon absorption of a single high-energy photon. We demonstrate efficient carrier multiplication with an onset close to the energy conservation threshold of twice the bandgap, 2Eg. Moreover, with absorption of a single high-energy photon under low pump fluence conditions, it was found that carrier-carrier interaction was significantly suppressed, but the amplitude of the signal was enhanced. We show that these results are in excellent agreement with the dependence of photoluminescence quantum yield on excitation, as reported previously for similar materials.
    39. 39
      Tisdale, W. A.; Williams, K. J.; Timp, B. A.; Norris, D. J.; Aydil, E. S.; Zhu, X.-Y. Hot-Electron Transfer from Semiconductor Nanocrystals Science 2010, 328, 1543– 1547
      39
      Hot-Electron Transfer from Semiconductor Nanocrystals
      Tisdale, William A.; Williams, Kenrick J.; Timp, Brooke A.; Norris, David J.; Aydil, Eray S.; Zhu, X.-Y.
      Science (Washington, DC, United States) (2010), 328 (5985), 1543-1547CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      In typical semiconductor solar cells, photons with energies above the semiconductor bandgap generate hot charge carriers that quickly cool before all of their energy can be captured, a process that limits device efficiency. Although fabricating the semiconductor in a nanocryst. morphol. can slow this cooling, the transfer of hot carriers to electron and hole acceptors has not yet been thoroughly demonstrated. The authors used time-resolved optical 2nd harmonic generation to observe hot-electron transfer from colloidal lead selenide (PbSe) nanocrystals to a titanium dioxide (TiO2) electron acceptor. With appropriate chem. treatment of the nanocrystal surface, this transfer occurred much faster than expected. Also, the elec. field resulting from sub-50-fs charge sepn. across the PbSe-TiO2 interface excited coherent vibrations of the TiO2 surface atoms, whose motions could be followed in real time.
    40. 40
      Sandeep, C. S. S.; Cate, S.; ten Schins, J. M.; Savenije, T. J.; Liu, Y.; Law, M.; Kinge, S.; Houtepen, A. J.; Siebbeles, L. D. A. High Charge-Carrier Mobility Enables Exploitation of Carrier Multiplication in Quantum-Dot Films Nat. Commun. 2013, 4, 2360
      40
      High charge-carrier mobility enables exploitation of carrier multiplication in quantum-dot films
      Sandeep C S Suchand; ten Cate Sybren; Schins Juleon M; Savenije Tom J; Liu Yao; Law Matt; Kinge Sachin; Houtepen Arjan J; Siebbeles Laurens D A
      Nature communications (2013), 4 (), 2360 ISSN:.
      Carrier multiplication, the generation of multiple electron-hole pairs by a single photon, is of great interest for solar cells as it may enhance their photocurrent. This process has been shown to occur efficiently in colloidal quantum dots, however, harvesting of the generated multiple charges has proved difficult. Here we show that by tuning the charge-carrier mobility in quantum-dot films, carrier multiplication can be optimized and may show an efficiency as high as in colloidal dispersion. Our results are explained quantitatively by the competition between dissociation of multiple electron-hole pairs and Auger recombination. Above a mobility of ~1 cm(2) V(-1) s(-1), all charges escape Auger recombination and are quantitatively converted to free charges, offering the prospect of cheap quantum-dot solar cells with efficiencies in excess of the Shockley-Queisser limit. In addition, we show that the threshold energy for carrier multiplication is reduced to twice the band gap of the quantum dots.

  • Supporting Information

    Supporting Information

    Experimental details, including materials used, nanocrystal synthesis, film deposition methods, PV device fabrication, characterization, and PV device testing, additional data ans supplementary figures, including the confirmation of ligand loss during photonic curing of nanocrystal films by TGA and FTIR, the calculated substrate temperature during photonic curing process, the extent of nanocrystal sintering after photonic curing determined by XRD, the IQE and MEG quantum yields, the effect of light biasing on EQE measurements, and TA spectroscopy, and Table S1, showing the EQE and calculated Jsc for each probe beam intensity. This material is available free of charge via the Internet at http://pubs.acs.org.


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