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2008, 112, 17046–17050 Published on Web 10/11/2008
New Efficiency Records for Stable Dye-Sensitized Solar Cells with Low-Volatility and Ionic Liquid Electrolytes Dong Shi,† Nuttapol Pootrakulchote,‡ Renzhi Li,† Jin Guo,† Yuan Wang,† Shaik M. Zakeeruddin,‡ Michael Gra¨tzel,‡ and Peng Wang*,† State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China, and Laboratory for Photonics and Interfaces, Swiss Federal Institute of Technology, CH 1015, Lausanne, Switzerland ReceiVed: September 9, 2008; ReVised Manuscript ReceiVed: October 3, 2008
We report a high molar extinction coefficient heteroleptic polypyridyl ruthenium sensitizer, featuring an electronrich 3,4-ethylenedioxythiophene unit in its ancillary ligand. A nanocrystalline titania film stained with this sensitizer shows an improved optical absorption, which is highly desirable for practical dye-sensitized solar cells with a thin photoactive layer, facilitating the efficient charge collection. In conjunction with low-volatility and solvent-free electrolytes, we achieved 9.6-10.0% and 8.5-9.1% efficiencies under the air-mass 1.5 global solar illumination. These dye-sensitized solar cells retain over 90% of the initial performance after 1000 h full sunlight soaking at 60 °C. Continuous efforts in the research and development of lowcost dye-sensitized solar cells (DSCs), which utilize innovative light-harvesting dyes and electrolytes in conjunction with a highquality mesoporous semiconducting film show promise as a source for future renewable energy supplies, despite the looming depletion of fossil fuels.1 In combination with a volatile electrolyte, the DSC technology has reached respectable high efficiencies up to 11.0-11.3% measured under the standard air mass 1.5 global (AM 1.5G) sunlight.2-4 Although this efficiency is still less than half of what the best silicon solar cell achieved, a long-term outdoor comparative testing has proved the impressive total electricity output capacity of DSCs,5 benefiting from the almost stable power output over a moderate temperature range as well as the peculiar sensitive response to weak and diffusive lights. However, the use of toxic and volatile solvents such as acetonitrile in the high efficiency cells is a big hurdle for the large-scale application of this new photovoltaic technology. The costly hermetic sealing of volatile electrolytes will counteract the merit of DSCs as photovoltaic technologies with a high performance/price ratio. In the past several years, much work has been focused on the development of practical DSCs by using low-volatility electrolytes,6 solvent-free ionic liquids,7 or solid organic hole-transporters.8,9 Promoted by material designing together with device engineering, the highest efficiency of a stable device with a low-volatility electrolyte now stands at 9.1%,4 whereas that of its counterpart with a solvent-free electrolyte is 7.7%.10 Close insights on these practical devices in contrast to those with an acetonitrile electrolyte have revealed that the electron diffusion length is shorter in the former cases due to faster charge recombination, considerably affecting the charge col* Corresponding author. E-mail:
[email protected]. † Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. ‡ Swiss Federal Institute of Technology.
10.1021/jp808018h CCC: $40.75
lection yield if a standard film is used to meet the optical absorption requirement.4 Obviously, this issue could be addressed by suppressing the charge recombination at the nanoscopic interface, improving the electron diffusion coefficients in mesoporous films,11 or simply using a thinner nanocrystalline titania film. However, in the process of exploiting the latter route, we have noted that optical absorption cross sections of state-of the art nanocrystalline films in DSCs stained with the present best sensitizers are at least one order smaller than those of the active layers in other bulk-heterojunction excitonic solar cells.12-19 Thus, enhancing the optical absorptivity of a stained mesoporous film seems to be a feasible strategy,20 by employing high molar extinction coefficient sensitizers.21-24 However, as a preliminary requirement for high efficiency solar cells the photactive layer must have a good spectral match with the solar emission. In this letter, we report a new heteroleptic polypyridyl ruthenium sensitizer coded C103 (Figure 1A), featuring an electron-rich 3,4-ethylenedioxythiophene unit in its ancillary ligand, to address these issues simultaneously. Even with preliminary tests, we have already set new efficiency records for stable dye-sensitized solar cells. As presented in Figure 1B, the electronic absorption spectrum of the C103 dye has two intense absorption bands at 307 and 365 nm in the UV region and the characteristic metal-to-ligand charge-transfer transition (MLCT) absorption bands in the visible region like other heteroleptic polypyridyl ruthenium(II) complexes.4 In DMF, the low energy MLCT transition absorption peaks at 550 nm, which is 27 nm red-shifted compared to that of Z907 or its analogues.25 The measured peak molar extinction coefficient (ε) is 18.8 × 103 M-1 cm-1, which is significantly higher than the corresponding values for the standard Z907 (12.2 × 103 M-1 cm-1) and N719 (14.0 × 103 M-1 cm-1) sensitizers.25,26 As presented in the Supporting Information, by calculating the electronic states (Figure S1) of 2008 American Chemical Society
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Figure 1. (A) Molecular structures of the Z907 and C103 dyes. (B) Electronic absorption and emission spectra of the C103 dye dissolved in DMF. (C) Plot of optical absorption coefficient versus wavelength of a 7 µm transparent nanocrystalline TiO2 film coated with the Z907 and C103 dyes, respectively.
C103 using time-dependent desnsity functional theory (TDDFT) we detailed the origins (Figure S2 and Table S1) of these transitions. Extending the π conjugated system of the ancillary ligands in heteroleptic ruthenium complexes such as C103 improves significantly the light havesting capacity as is apparent from Figure 1C, which depicts the absorption spectra in the visible region of Z907 and C103 anchored on a transparent nanocrystalline TiO2 film. The C103 sensitizer confers an improved optical absorption coefficient to the stained mesoporous titania film, which is highly desirable as it allows using thinner nanocrystalline layers, facilitating charge collection in practical DSCs while maintaining the light harvesting capacity. Decreasing the film thickness also augments the open-circuit photovoltage of the cell due to the reduction of the dark current. Excitation of the low energy MLCT transition of the C103 sensitizer in DMF produces an emission centered at 800 nm. As shown in Figure S3, the measured negative offset of its LUMO (-0.81 V vs. NHE) relative to the conduction band edge (-0.50 V vs. NHE) of TiO2 provides the thermodynamic driving force for charge generation. The formal redox potential (φ0(S+/ S)) of the C103 sensitizer determined by ultramicroelectode square-wave voltammetry is 0.90 V vs. NHE, which is 0.50 V higher than that of the iodide/triidodide couple employed in the electrolyte, supplying ample driving force for efficient dye regeneration and thus net charge separation. Some preliminary photovoltaic experiments were conducted to evaluate the potential of employing this sensitizer in photovoltaic devices of practical relevance. A screen-printed double layer film of interconnected TiO2 particles was used as mesoporous negative electrode. A 7 µm thick film of 20-nmsized TiO2 particles was first printed on the fluorine-doped SnO2 conducting glass electrode and further coated by a 5 µm thick second layer of 400-nm-sized light scattering anatase particles. The detailed preparation procedures of TiO2 nanocrystals, pastes for screen-printing, and double-layer nanostructured TiO2 films have been reported in our previous paper.27 The TiO2 electrodes were stained by immersing them overnight in a dye solution
containing C103 sensitizer (300 µM) and 3R,7R-dihyroxy-5βcholic acid coadsorbent (300 µM) in a mixture of acetonitrile and tert-butanol (volume ratio: 1/1). After washing with acetonitrile and drying under laboratory air flow, the sensitized titania electrodes were laminated with thermally platinized conducting glass electrodes. The electrodes were separated by a 25 µm thick Surlyn hot-melt gasket and sealed up by heating. The internal space was filled with a liquid electrolyte using a vacuum back filling system. The hole for electrolyte-injection was produced with a sand-blasting drill on the counter electrode glass substrate and was sealed with a Bynel sheet and a thin glass cover by heating. The composition of electrolyte in device A is as follows: 1.0 M 1,3-dimethylimidazolium iodide (DMII), 0.15 M I2, 0.5 M N-butylbenzoimidazole (NBB), and 0.1 M guanidinium thiocyanate (GNCS) in 3-methoxypropionitrile (MPN);4 electrolyte in device B: DMII/1-ethyl-3-methylimidazolium iodide/1-ethyl-3-methylimidazolium tetracyanoborate/ I2/NBB/GNCS (molar ratio: 12/12/16/1.67/3.33/0.67).10 The photocurrent action spectrum of device A with a lowvolatility electrolyte is shown in Figure 2A. The incident photonto-collected electron conversion efficiencies (IPCE) exhibit a high plateau of over 70% from 410 to 670 nm, reaching a maximum of 91% at 570 nm. As shown in Figure 2B, under AM 1.5G condition (99.8 mW cm-2), the short-circuit photocurrent density (Jsc), open-circuit photovoltage (Voc), and fill factor (FF) of device A with a low-volatility, 3-methoxypropionitrile-based electrolyte are 17.51 mA cm-2, 771 mV, and 0.709, respectively, yielding an overall conversion efficiency (η) of 9.6%. The overlap intergral of the IPCE curve with the standard global AM 1.5G solar emission spectrum generates a short-circuit photocurrent density (Jsc) of 17.47 mA cm-2, indicating that there is very small spectral mismatch between our solar simulator and the standard AM 1.5G sunlight. At various lower incident light intensities, overall power conversion efficiencies (Table 1) are even higher, reaching up to 10.0%. More importantly, the photovoltaic parameters (Jsc, Voc, FF, and η) of device B with a solvent-free ionic liquid electrolyte under AM 1.5G sunlight (99.8 mW cm-2) are 15.93 mA cm-2, 710
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Figure 2. (A) Photocurrent action spectrum of device A employing the C103 sensitizer in conjugation with a low-volatility electrolyte. (B) J-V characteristics measured (a) in dark and (b-d) under an irradiance of AM 1.5G sunlight attenuated to different light intensities with various metal meshes. (b) 9.4 mW cm-2; (c) 51.3 mW cm-2; (d) 99.8 mW cm-2. The aperture area of metal mask: 0.158 cm2. A UV-absorbing antireflection film is covered on the cell during measurements.
TABLE 1: Detailed Photovoltaic Parameters of Devices A and B made with C103 as Sensitizer and 3r,7r-Dihyroxy-5β-cholic Acid as Coadsorbent Measured under Different Incident Light Intensitiesa device A B
Pin/mW cm-2
Jsc/mA cm-2
Voc/mV
Pmax/mW m-2
FF
η/%
9.4 51.3 99.8 9.4 51.3 99.8
1.70 9.11 17.51 1.64 8.55 15.93
700 760 771 645 688 710
0.912 5.13 9.57 0.855 4.57 8.45
0.766 0.741 0.709 0.808 0.777 0.747
9.7 10.0 9.6 9.1 8.9 8.5
a
The spectral distribution of our measurement system simulates AM 1.5G solar emission with a mismatch less than 2%. Incident power intensity: Pin; short-circuit photocurrent density: Jsc; open-circuit photovoltage: Voc; maximum electricity output power density: Pmax; fill factor: FF ) Pmax/Pin; total power conversion efficiency: η. cell area tested with a metal mask: 0.158 cm2.
mV, 0.747, and 8.5%, respectively. Under a low light intensity of 9.4 mW cm-2 attenuated by metal mesh, a very impressive efficiency of 9.1% has been reached. This new record efficiency for a solvent-free DSC will promote considerably the practical application of flexible devices, which are frequently operated under reduced light intensities and for which ionic liquids are the electrolytes of choice. Note that using the same tiania film and electrolyte, our previously reported cell with the Z907 dye gives a Jsc of 14.26 mA/cm2 under the AM 1.5G full sunlight,10 which proves the merit of better light harvesting capacity of this new C103 dye. In view of the preferential importance of using nonvolatile ionic liquid electrolytes in DSCs, it is of pivotal importance to understand the origins of their relatively low Voc and Jsc as shown above, shedding light on the further enhancement of device efficiency. We estimated the chemical capacitance Cµ at the titania/electrolyte interface by measuring the temporal relaxation of the Voc and electron concentration caused by a fast and small perturbation with red light-emitting diodes.28-30 Knowing that density of states (DOS) including surface and bulk traps is proportional to Cµ, we derived the DOS with an exponential distribution profile for both devices A and B as depicted in Figure S5. The density of states (DOS) are calculated by DOS ) 6.24 × 1018∆Q/[∆Vd(1 - P)], where ∆Q is the number of electron injected during the red light flash, ∆V is the peak of the photovoltage transient, and d and P are the thickness and porosity of titania films.4 Obviously, there is not a significant difference in the surface states of devices A and B with two different electrolytes. However, we note that the calculated electrolyte equilibrium potential of device A is 45
Figure 3. J-V characteristics of device B with the C103 sensitizer and a solvent-free ionic liquid electrolyte measured (a) in dark and (b-d) under an irradiance of AM 1.5G sunlight (100 mW cm-2). (b) 9.4 mW cm-2; (c) 51.3 mW cm-2; (d) 99.8 mW cm-2. The aperture area of metal mask: 0.158 cm2. A UV-absorbing antireflection film is covered on the cell during measurements.
Figure 4. Plots of (A) effective electron lifetime, (B) diffusion coefficient, and (C) diffusion length in devices A and B.
mV positive-shifted compared to that of device B. This along with the Jsc difference unambiguously explains the observed higher Voc of device A in contrast to B. We employed the intensity-modulated photovoltage spectroscopy31,32 (IMVS) to have an insight on the charge recombination at the titania/electrolyte interface. IMVS measures the periodic photovoltage response of a testing cell to a small sinusoidal perturbation of light superimposed on a large steady background level, providing information on electron lifetime under open-circuit conditions. Figure 4A presents the plot of electron lifetime versus Jsc which is linearly related to incident light intensity in our devices. Obviously, upon increasing the light intensity, the recombination becomes faster due to the higher electron concentration in the titania film as well as a larger driving force. At a given Jsc, device B with a solventfree ionic liquid electrolyte exhibits a much higher charge recombination rate compared with device A, although the former
Letters device has an even smaller driving force for charge recombination. Thus we believe that this difference is mainly caused by the higher triiodide concentration of ∼0.24 M used in device B as compared to 0.15 M in device A. Intensity-modulated photocurrent spectroscopy33,34 (IMPS) employs the same light perturbation but measures the periodic photocurrent response, detailing the dynamics of charge transport and back reaction under short-circuit conditions. The electron transport in mesoporous titania films can be described by the proposed multiple trapping-detrapping (MTD) model.35 As the light intensity or the resultant Jsc increases, more deep traps will be filled with photoinjected electrons and do not retard the electron transport any more. The detrapping of electrons from shallow traps is much faster, resulting in a higher effective electron diffusion coefficient (Dn) as shown in Figure 4B. The electron diffusion coefficients in these two devices are very similar, implying effective charge screening for electron transport in our ionic liquid device due to a very high cation concentration, even though there is a huge difference in the viscosity of the two electrolytes. The values of electron diffusion coefficient and lifetime can be used for the calculation of relative electron diffusion length, Ln ) (Dnτn)1/2, which is determined by the competition between electron transport and back reaction. Overall, the large electron diffusion length (Ln) directly related to a high charge collection yield is well consistent with the high Jsc measured for device A. For any new photovoltaic technology, passing a long-term stability test is very critical to assess its potential for largescale application. However, for every new system it is not practical to make a large solar panel for 10 or 20 years outdoor evaluation, because innovative materials and device concepts are now being actively pursued and an impressive improvement in the performance of low-cost excitonic solar cells have been seen in the past years.36,37 Hence, we submitted our cells covered with a UV absorbing polymer film to the 1000 h accelerated testing at 60 °C, in a solar simulator with a light intensity of 100 mW cm-2. Note that the total contribution of UV photons to the standard AM 1.5G solar emission is relatively small. UV light leads to direct band-edge excitation of titania nanocrystals, generating valence-band holes, which in turn may attack some organic materials in the device. Note that this reaction can be partially suppressed by the hole capture with iodide ions. In our light soaking experiments UV light is removed by antireflection coating employed. As presented in Figure 5, both devices with a low-volatility electrolyte and a solvent-free ionic liquid electrolyte show good stability, keeping 91% and 94% of their initial efficiencies of 9.6% and 8.4%, respectively. This is the first time such high efficiency DSCs have passed preliminary stability tests, which should considerably encourage the surging investigations on various nanostructured solar cells. Note that a cell showing a similar stability with the Z907 dye has a 7.7% initial efficiency.10 Overall, our systematic experiments have suggested that it is important to employ amphiphilic sensitizers like Z907 and C103 to achieve a stable DSC. While the photocurrents in both devices are very stable, the small attenuation of device efficiency is mainly due to a drop in open-circuit photovoltage. The latter is closely related to the augmentation of surface state of the mesoporous titania film as shown in Figure 6. The detailed chemical nature of these states needs to be clarified in further studies. We have demonstrated previously that the drop of photovoltage during aging can be attenuated by the cografting of alkylphosphonic acid with an amphiphilic ruthenium sensitizer.38
J. Phys. Chem. C, Vol. 112, No. 44, 2008 17049
Figure 5. Detailed photovoltaic parameters measured under the irradiance of AM 1.5G sunlight for devices A and B during successive full sunlight soaking at 60 °C.
Figure 6. DOS profiles of device B with a solvent-free ionic liquid electrolyte (A) before and (B) after 1000 h successive full sunlight soaking at 60 °C.
In summary, we have set new benchmarks for highperformance and practical dye-sensitized solar cells with lowvolatility and solvent-free electrolyte, which is realized by the rational design of a high molar extinction coefficient sensitizer to enhance the light-harvesting and charge-collection yields of stained mesoscopic semiconducting films synchronously. This great progress will foster the large-scale production and application of dye-sensitized solar cells. Further work on stability studies of a testing cell at 80 °C and long-term evaluations of a large solar panel with the newly developed systems are underway. Our device analysis has pointed out that it is pertinent
17050 J. Phys. Chem. C, Vol. 112, No. 44, 2008 to reduce the charge recombination and enhance the electron transport for the future efficiency improvement of DSCs with solvent-free ionic liquid electrolytes. Furthermore, our work has also implied that a robust or self-healing nanointerface needs to be realized to stabilize the Voc of dye-sensitized solar cells for long-term operation. Acknowledgment. The National Key Scientific ProgramNanoscience and Nanotechnology (No. 2007CB936700), the National Science Foundation of China (No. 50773078), and the “100-Talent Program” and the “Knowledge Invocation Program” of Chinese Academy of Sciences have supported this work. N.P, S.M.Z., and M.G. thank the U.S. Air Force (No. 8655-08-C4003) for financial support. Supporting Information Available: Experimental details and additional data. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Gra¨tzel, M. Nature 2001, 414, 338. (2) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Gra¨tzel, M. J. Am. Chem. Soc. 2005, 127, 16835. (3) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L Jpn. J. Appl. Phys. Part 2 2006, 45, L638. (4) Gao, F.; Wang, Y.; Shi, D.; Zhang, J.; Wang, M.; Jing, X.; Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; Gra¨tzel, M. J. Am. Chem. Soc. 2008, 130, 10720. (5) Toyoda, T.; Sano, T.; Nakajima, J.; Doi, S.; Fukumoto, S.; Ito, A.; Tohyama, T.; Yoshida, M.; Kanagawa, T.; Motohiro, T.; Shiga, T.; Higuchi, K.; Tanaka, H.; Takeda, Y.; Fukano, T.; Katoh, N.; Takeichi, A.; Takechi, K.; Shiozawa, M. J. Photochem. Photobiol. A 2004, 164, 203. (6) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Nazeeruddin, M. K.; Sekiguchi, T.; Gra¨tzel, M. Nat. Mater. 2003, 2, 402. (7) Wang, P.; Zakeeruddin, S. M.; Comte, P.; Exnar, I.; Gra¨tzel, M. J. Am. Chem. Soc. 2003, 125, 1166. (8) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weissortel, F.; Salbeck, J.; Spreitzer, H.; Gra¨tzel, M. Nature 1998, 395, 583. (9) Snaith, H. J.; Moule, A. J.; Klein, C.; Meerholz, K.; Friend, R. H.; Gra¨tzel, M. Nano Lett. 2007, 7, 3372. (10) Bai, Y.; Cao, Y.; Zhang, J.; Wang, M.; Li, R.; Wang, P.; Zakeeruddin, S. M.; Gra¨tzel, M. Nat. Mater. 2008, 7, 626. (11) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455. (12) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (13) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864.
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JP808018H
