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Highly Efficient Organic Sensitizers for Solid-State Dye-Sensitized Solar Cells Soo-Jin Moon,† Jun-Ho Yum,† Robin Humphry-Baker,† Karl Martin Karlsson,‡ Daniel P. Hagberg,‡ Tannia Marinado,§ Anders Hagfeldt,§ Licheng Sun,*,‡ Michael Gra¨tzel, and Md K. Nazeeruddin*,† Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, School of Basic Sciences, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland, and Centers of Molecular DeVices, Organic Chemistry, Royal Institute of Technology (KTH) Teknikringen 30, 100 44 Stockholm, Sweden, and Department of Physical and Analytical Chemistry, Uppsala UniVersity, Box 259, 75105 Uppsala, Sweden ReceiVed: April 12, 2009; ReVised Manuscript ReceiVed: July 22, 2009
Organic sensitizers comprising of donor, electron-conducting, and anchoring groups are designed and developed for dye-sensitized solar cell applications. A solar cell employing 3-(5′-{4-[bis-(4-hexyloxy-phenyl)-amino]phenyl}-[2,2′]bithiophenyl-5-yl)-2-cyano-acrylic acid dye spiro-OMeTAD as a hole-transporting material exhibits a short circuit photocurrent density of 9.64 mA/cm2, an open-circuit voltage of 798 mV, and a fill factor of 0.57, corresponding to an overall conversion efficiency of 4.4% at standard AM 1.5 sunlight. Photoinduced absorption spectroscopy probes an efficient hole-transfer from dyes to the spiro-OMeTAD. Introduction The conversion of sunlight to electricity using dye-sensitized solar cells (DSCs) represents one of the most promising methods for future large-scale power production from renewable energy sources.1,2 The key components in DSC are a semiconductor film, sensitizer, and electrolyte composition. The total efficiency of the dye-sensitized solar cell depends on optimization and compatibility of each of these components. Among these, the sensitizer’s optical response and the molar extinction coefficient play a significant role for high power conversion efficiency, and in this respect, organic sensitizers have an advantage compared to the ruthenium sensitizers in that the former generally have higher molar extinction coefficients, are easier to synthesize, and have lower cost. The highest efficiency achieved using organic sensitizers with a liquid electrolyte is around 9%.3 An impressive device efficiency up to 4% has been achieved by incorporating an indoline dye coded D-102 in solid-state DSCs.4 Although solid-state DSCs have lower efficiency compared with liquid-type DSCs, the research on the solid-state DSCs5 has gained considerable traction because it is attractive for realizing flexible PV cells in a roll-to-roll production. The disadvantage of solid-state DSCs is the problem of pore filling in thicker films; therefore, thin films of 1.5-3 µm TiO2 are used.4,6 Since the film thickness in solid-state DSCs is thin, organic dyes with a high molar extinction coefficient are more promising materials for enhanced light harvesting. Herein, we report solid-state DSCs with new organic dyes (molecular structures in Figure 1) showing efficiency of 4.44% at full intensity (100 mW/cm2). * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +41-21-693-6124. Fax: +41-21-6934111. † Swiss Federal Institute of Technology. ‡ Center of Molecular Devices and Organic Chemistry, Royal Institute of Technology. § Center of Molecular Devices, Physical Chemistry, Department of Physical and Analytical Chemistry, Teknikringen 30, 100 44 Stockholm, Sweden.
Figure 1. Molecular structures of D5L6, D21L6, and D25L6.
Experimental Methods Synthesis of Sensitizers. The synthesis of sensitizers (E)-3(5-(5-(4-(diphenylamino)phenyl)thiophen-2-yl)thiophen-2-yl)2-cyanoacrylic acid (D5L6), (E)-3-(5-(5-(4-(bis(4-(hexyloxy)phenyl)amino)phenyl)thiophen-2-yl)thiophen-2-yl)-2-cyanoacrylic acid (D21L6), and (E)-3-(5-(5-(4-(bis(4-(dodecyloxy)phenyl)amino)phenyl)thiophen-2-yl)thiophen-2-yl)-2-cyanoacrylic acid (D25L6) are described in the Supporting Information and illustrated in Figure S1. Fabrication of Solid-State Dye Sensitized Solar Cells. Fluorine-doped SnO2 glass (15 Ω/sq, Pilkington) substrates were cleaned first with Helmanex solution, rinsed with acetone, and then ethanol. Next, a ∼100 nm compact layer of TiO2 was deposited by spray pyrolysis.7 A porous layer of 20 nm TiO2 particles (∼1.7 µm thick) was coated by the doctor-blading technique, followed by sintering at 500 °C under an oxygen flow. After cooling, the thin TiO2 films were impregnated in a 0.02 M aqueous TiCl4 solution for 15 h and then rinsed with deionized water. The TiCl4-treated TiO2 films were annealed at 450 °C for 30 min and then cooled to ∼80 °C before plunging into the dye solution for 3 h. After being soaked in dye solution, the substrates were rinsed in acetonitrile and then the holetransporting material 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spiro-bifluorene (Spiro-OMeTAD) solution (180 mg/mL, in chlorobenzene) with additives of tert-butyl pyridine
10.1021/jp9033722 CCC: $40.75 2009 American Chemical Society Published on Web 08/27/2009
Organic Sensitizers for Solid-State Dye-Sensitized Solar Cells (17 µL/mL) and Li[CF3SO2]2N (19.5 mM) was spin-coated at 2000 rpm on top of the TiO2 film.8 Finally, a 50 nm gold layer was evaporated on the top of the Spiro-OMeTAD. Measurement of Solar Cells. For photovoltaic measurements of the DSCs, the irradiation source was a 450 W xenon light source (Osram XBO 450) with a filter (Schott 113), whose power was regulated to the AM 1.5G solar standard by using a reference Si photodiode equipped with a color-matched filter (KG-3, Schott) in order to reduce the mismatch in the region of 350-750 nm between the simulated light and AM 1.5G to less than 4%. Various intensities (from 0.01 to 1.0 sun) can be provided with neutral wire mesh attenuators. The measurement of incident photon-to-current conversion efficiency (IPCE) was plotted as a function of excitation wavelength by using the incident light from a 300 W xenon lamp (ILC Technology), which was focused through a Gemini-180 double monochromator (Jobin Yvon, Ltd.). Photovoltage transients were observed by using a pump pulse generated by four red light emitting diodes controlled by a fast solid-state switch in the presence of a white light bias.9 Pulse widths of 1 ms were used, with a rise and fall time of e2 µs. The pulse of red light was incident on the photoanode side of the cell, and its intensity was controlled to keep the modulation voltage below 10 mV. The bias light was incident on the same side and the bias light intensity was varied by controlling the electrical power input to the white light emitting diodes. Photoinduced absorption (PIA) spectroscopy was used to probe the photogenerated charge species in dyed TiO2 films with and without spiro-OMeTAD.10,11 The probe light source in this pump/probe technique was provided by a 10 W halogen lamp which was focused onto the sample, with the transmitted light refocused onto the slits of a double monochromator (Gemini180). The light intensity on the sample was ∼65 µW/cm2. A cooled dual-color solid-state detector (Si/InGaAs) was mounted on the exit slits of the monochromator to monitor the optical signal. To obtain the PIA spectrum, the pump source was provided by a Lumiled 470 nm diode that was modulated using the internal reference frequency of the lock-in amplifier. The pump light from this diode was focused onto the same face of the sample as the probe source but 20° off axis, with an approximate intensity of 6 mW/cm2. A dual-phase lock-in amplifier (SR 830) was used to separate out the DC signal component from the AC signal coming from the pump source. The signal provided the change in transmission (∆T) as a function of wavelength. So the plotted PIA spectra are shown as ∆T /T, where ∆T is from the AC component, and T is derived from the DC signal. All the PIA measurements were performed in air. Results and Discussion The cyclic voltammograms of sensitizers were measured in acetonitrile containing 0.1 M tetrabutylammonium tetrafluoroborate (TBA(BF4)) with 0.1 V/s scan rate in Table 1 and Figure S2. The D5L6 shows a reversible couple at 1.13 V vs NHE due to oxidation of the triphenylamine moiety. We note that the D21L6 and D25L6 oxidation potential (0.98 and 0.96 V vs NHE) shifted cathodically when compared to the D5L6, which demonstrates the extent of destabilization of HOMO caused by the alkoxy groups. The excited-state oxidation potential of the dye plays an important role in the electroninjection process from dye to TiO2. Neglecting any entropy change during light absorption, the value can be derived from the ground-state oxidation couple and the zeroth-zeroth excitation energy E(0-0) derived from the equation: E(S+/S*) ) E(S+/S)
J. Phys. Chem. C, Vol. 113, No. 38, 2009 16817 TABLE 1: Absorption/Emission Spectra Data and Electrochemical Properties of D5L6, D21L6, and D25L6 dye D5L6 D21L6 D25L6
abs emission extinction maxa maxa coefficients [nm] [nm] [M-1 cm-1] 438 458 460
611 623 626
34 000 37 000 36 000
E
b (S+/S)
[V]
1.13 0.98 0.96
E [eV]
c (0-0)
2.40 2.33 2.31
E (S+/S*)
d
[V]
–1.27 –1.35 –1.35
a Absorption and emission spectra were measured in ethanol at 25 °C. b The oxidation potential of the dyes measured under the following conditions: working electrode, glassy carbon; electrolyte, 0.1 M tetrabutylammonium tetrafluoroborate, TBA(BF4) in acetonitrile; scan rate, 0.1 V/s. Potentials measured vs Fc+/Fc were converted to NHE by addition of +0.69 V. c The zero-zero excitation energies, E(0-0) are estimated from the intercept of the normalized absorption and emission spectra. d The excited state oxidation potentials were derived from the equation: E(S+/S*) ) E(S+/S) - E(0-0).
- E(0-0). From the absorption/emission spectra, E(0-0) energies of 2.40, 2.33, and 2.31 eV were extracted for D5L6, D21L6, and D25L6, respectively. The excited-state oxidation potentials of these dyes are higher than -1.27 V vs NHE, which are notably more negative than the TiO2 conduction band potential.12 The first oxidation potential of spiro-OMeTAD is 0.81 V vs NHE, which is more positive than that of the I-/I3- redox couple (≈0.4 V vs NHE).13,14 The difference between the oxidation potentials of the dyes and the oxidation potential of spiroOMeTAD is sufficient enough to drive the dye regeneration process to compete efficiently with recapture of the injected electrons by the dye cation radical. These organic dyes show around 3 times higher molar extinction coefficient (see Table 1 and Figure 2a) compared to the ruthenium dyes (Z907, ε ) 12 200 M-1 cm-1), which is an important issue for solid-state devices, where the film thickness is required to be thin (∼2 µm) in order to obtain a good pore filling. A device with a thinner film absorbs a smaller fraction of the incident light because of a lower effective surface area. Thicker nanoporous TiO2 layers can absorb more light but they lead to the problem of pore filling and the charge carrier mobility in organic hole conductors decreases within thicker films. Hence, applying a sensitizer, which has a high molar extinction coefficient is useful for fabricating devices with thinner TiO2 layers but also to achieve a higher efficiency compared to devices with thicker TiO2 layers due to enhanced light harvesting. Figure 2b shows that the absorption spectra of the dyes adsorbed onto 3 µm thick TiO2 electrodes are similar to those of the corresponding solution spectra but exhibit a red-shift due to the interaction of the anchoring groups with the surface titanium ions and scattering effect of light in mesoporous TiO2. The light-harvesting efficiency (LHE) can be expressed as 1 10-εΓ ) 1 - 10-A, where ε is the molar extinction coefficient of sensitizer, Γ is the dye loading per projected surface area of the film, and A is the absorption optical density of sensitizer stained film, which equals to product of ε and Γ.2 The A values of the dyes are ∼1.7, which means that almost unity of LHE can be expected (a reflection from the glass substrate is not taken into account). These high LHEs are attributed to the high extinction coefficient of the dyes in thin film. Figure 3a shows the incident monochromatic photon-tocurrent conversion efficiency (IPCE) of solid-state DSCs incorporating D5L6, D21L6, and D25L6 organic dyes. The IPCE spectra of D21L6 and D25L6 that contain long alkoxy chains exhibit a 30 nm red-shift, which is consistent with solution absorption spectra. The red-shift contributes to enhanced current collection of D21L6 and D25L6 when compared with
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Figure 2. Normalized absorption (solid line)/emission (dashed line) spectra (a) and absorption spectra (dotted line) on 3 µm thick TiO2 electrodes (b) of D5L6 (black), D21L6 (red), and D25L6 (blue).
D5L6. The IPCE data of D21L6 plotted as a function of excitation wavelength show the highest value, 54% at 460 nm, whereas the D5L6 and D25L6 showed a slightly lower IPCE, 51%. Figure 3b shows the current-voltage (J-V) characteristics for solid-state solar cells incorporating different dyes. Under standard global AM 1.5 solar conditions, the D21L6-sensitized cell showed the highest Jsc of 9.64 mA/cm2, Voc of 798 mV, and ff of 0.57, corresponding to an overall efficiency, η, derived from Jsc × Voc × ff/light intensity of 4.44% (active area of 0.185 cm2) (see Table 2 and Figure S3). Under lower light intensities, 10% and 50% sun, the overall efficiencies of a DSC were higher than under 1 sun due to ohmic losses and faster recombination due to higher charge density, leading to a decrease in fill factor. However, Jsc obtained under lower intensities are relatively lower when normalized to 1 sun than Jsc measured under 1 sun condition. Figure 4 shows the current dynamics as function of light intensity which shows the individual photocurrents normalized to 1 sun Jsc. These currents are showing nonlinearity without achieving a plateau. At the low light intensities such as 0.019 and 0.095 sun, the normalized Jsc are only 66% and 79% of the Jsc at 1 sun. The light intensity for the IPCE measurement is measured at about these lower intensities and the low IPCEs when compared to the white light Jsc are ascribed to this nonlinearity of Jsc as function of light intensity. The IPCEs integrated from 350 to 750 nm are 6.1, 7.6, and 6.9 mA/cm2 which are lower than the Jsc obtained from the J-V characteristics. The lower observed photocurrent under low intensity could be attributed to aggregation of organic dyes on the TiO2
Moon et al.
Figure 3. Photocurrent action spectrum (a), and J-V characteristics (b) of solid-state DSCs incorporating D5L6 (black solid line), D21L6 (red dashed line), and D25L6 (blue dotted line) sensitizers. J-V performance was measured under simulated AM 1.5 solar illumination at an intensity of 100 mW/cm2.
TABLE 2: Photovoltaic Performance of Solid-State DSCs Incorporating D5L6, D21L6, and D25L6 under Various Intensity of Light dye a
D5L6
D21L6b D25L6a
a
light intensity
Jsc (mA/cm2)
Voc (mV)
ff
η (%)
9.3% sun 51.2% sun 99.4% sun 9.3% sun 51.3% sun 99.5% sun 9.3% sun 51.0% sun 99.5% sun
0.65 3.88 7.41 0.78 4.83 9.64 0.77 4.62 9.01
719 766 788 740 791 798 733 787 803
0.78 0.65 0.57 0.73 0.62 0.57 0.76 0.64 0.56
3.89 3.81 3.35 4.51 4.64 4.44 4.59 4.58 4.04
Active area of 0.2 cm2. b Active area of 0.185 cm2.
surface. The higher efficiency of the D21L6 and D25L6 compared to the D5L6 demonstrate the beneficial influence of alkoxy units that enhanced light harvesting resulting in a higher photocurrent. D25L6 showed a small decrease in photocurrent when compared to D21L6, although optical and electrochemical properties are similar. Figure 5 show that the relationship between electron lifetimes of solid-state sensitized solar cells of D21L6, and D25L6. As mentioned above, the electron lifetime decreases with increasing charge (electron) density, which means the recombination rate (the reciprocal electron lifetime) becomes faster. For a fixed charge (electron) density, D21L6 shows a longer electron lifetime than D25L6, leading to a slightly lower performance of the D25L6 solid-state solar
Organic Sensitizers for Solid-State Dye-Sensitized Solar Cells
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Figure 4. Current dynamics of solid-state DSCs incorporating D21L6 as various intensities of light, measured currents (solid), and their current, normalized to 1 sun (dashed line).
Figure 5. Apparent electron lifetime of solid-state DSCs incorporating D21L6 (red circle) and D25L6 (blue square) sensitizers.
cell. The red-shift in IPCE is attributed to incorporating alkoxy functionality in D21L6 and D25L6. However, the long alkoxy chain in D25L6 does not give a further increase in photocurrent because of similar absorption behavior and the fast electron recombination when compared to D21L6. The reason for the shorter electron lifetime in D25L6 cell is not clear but is probably caused by issues with Spiro-OMeTAD due to the long chain because with a volatile electrolyte, D25L6 shows exactly the same electron lifetime as lifetime of D21L6 (see Figure S4). In our previous work,15 the beneficial influence of alkoxy units in organic sensitizers was introduced, and in this work, the effect with meticulous tuning of molecular structure was carried on for solid-state DSCs. In order to probe the photogenerated charge carriers, PIA spectroscopy was performed on thin TiO2 films stained by dye with and without spiro-OMeTAD. PIA is a quasi-continuouswave technique, which estimates the steady-state population of photoinduced species in dye-sensitized solar cells. Figure 6a shows the in-phase signal of the PIA spectrum of 2.5 µm thick TiO2 film stained by D25L6 on a glass substrate. The appearance of a new transient species is apparent due to a decreased transmission signal. The ∆T/T signal is configured to be displayed as a positive signal in Figure 6a. Therefore, the positive signal at 740 and 1300 nm indicates a strong absorption signal. To confirm the nature of the signal, the dye was chemically oxidized with a strong oxidizing agent, nitrosonium tetrafluoroborate (NOBF4). The oxidized spectrum of D25L6 in chlorobenzene is shown for comparison in Figure 6a. The ∆Asol (O.D of oxidized dye - O.D of dye) is consistent with
Figure 6. (a) Comparison of measured normalized photoinduced absorption spectrum of dyed TiO2 film with normalized difference of optical density after dye oxidation: ∆Asol ) O.D of oxidized dye O.D of dye. (b) Photoinduced absorption spectra of 2.5 µm TiO2 film stained by D25L6 with (red triangle) and without (black square) spiroOMeTAD.
measured PIA spectrum. At wavelengths longer than 1080 nm, the measured PIA shows a higher signal when compared to the oxidized spectrum. This was ascribed to the weak absorption of photoinduced electrons in the TiO2 film and a shift in the spectrum due to the different medium. In Figure 6b we compare PIA of dyed TiO2 film with and without spiro-OMeTAD. The spectrum in the absence of the spiro-OMeTAD shows the characteristic spectrum of the oxidized dye. When the dyed sample is infiltrated with the spiro-OMeTAD, changes in the PIA spectrum are observed. The absorption of the oxidized dye disappeared in the presence of spiro-OMeTAD as a result of hole-transfer from the oxidized dye to the spiro-OMeTAD. The signal at 740 nm is replaced by a small peak around 700 nm and is consistent with the oxidized spiro-OMeTAD. In addition, the IR peak due to the localization of the hole on the triaryl amine functionality of the spiro-OMeTAD shifts further to the red. This is unfortunately obscured to some extent due to the similar functionality in this series of dyes that localizes the positive charge. However, a more sophisticated mathematical treatment of the data involving analysis of the total spectra confirms the similar observations that have been reported.11,14,16 Cappel et al. found the red-shifted peak of oxidized spiroOMeTAD, especially in presence of dye and deduced it could be additionally affected by a localized electric field due to electrons in TiO2.14 Hence, an efficient hole-transfer from sensitizer to spiro-OMeTAD is deduced by PIA spectroscopy. D5L6 and D21L6 (data are not shown here) showed very similar
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signal with and without spiro-OMeTAD to D25L6, and here we observe an efficient hole-transfer process between organic dyes and spiro-OMeTAD. Conclusion In this study, we have demonstrated solid-state dye-sensitized solar cell efficiency of 4.4% with high molar extinction coefficient organic sensitizers on thin TiO2 film. The organic dyes incorporating alkoxy group as a donor moiety yielded very high overall conversion efficiency compared to the sensitizers without alkoxy groups. Photoinduced absorption spectroscopy probes an efficient hole-transfer from dyes to the spiro-OMeTAD. Acknowledgment. We gratefully acknowledge financial support by the Swedish research Council, Swedish Energy Agency, the Knut and Alice Wallenberg foundation, SOHYD and the Korea Foundation for International Cooperation of Science and Technology through the Global Research Lab. Mr. Lien-Hoa Tran (Stockholm University) for the HR-MS measurements. Supporting Information Available: Synthetic details. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. Gra¨tzel, M. J. Photochem. Photobiol., C 2003, 4, 145. Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Y. Jpn. J. Appl. Phys., 2 2006, 45, L638. Lagref, J. J.; Nazeeruddin, M. K.; Gra¨tzel, M. Inorg. Chim. Acta 2008, 361, 735. (2) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382.
Moon et al. (3) Zhang, G. L.; Bala, H.; Cheng, Y. M.; Shi, D.; Lv, X. J.; Yu, Q. J.; Wang, P. Chem. Commun. 2009, 2198. (4) Schmidt-Mende, L.; Bach, U.; Humphry-Baker, R.; Horiuchi, T.; Miura, H.; Ito, S.; Uchida, S.; Gratzel, M. AdV. Mater. 2005, 17, 813. (5) Tennakone, K.; Kumara, G. R. R. A.; Kumarasinghe, A. R.; Wijayantha, K. G. U.; Sirimanne, P. M. Semicond. Sci. Technol. 1995, 10, 1689. Hagen, J.; Schaffrath, W.; Otschik, P.; Fink, R.; Bacher, A.; Schmidt, H. W.; Haarer, D. Synth. Met. 1997, 89, 215. Bachm, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weissortel, F.; Salbeck, J.; Spreitzer, H.; Gra¨tzel, M. Nature 1998, 395, 583. O’Regan, B.; Lenzmann, F.; Muis, R.; Wienke, J. Chem. Mater. 2002, 14, 5023. Schmidt-Mende, L.; Zakeeruddin, S. M.; Gra¨tzel, M. Appl. Phys. Lett. 2005, 86, 013504. Snaith, H. J.; Zakeeruddin, S. M.; Schmidt-Mende, L.; Klein, C.; Gra¨tzel, M. Angew. Chem., Int. Ed. 2005, 44, 6413. (6) Kru¨ger, J.; Plass, R.; Cevey, L.; Piccirelli, M.; Gra¨tzel, M.; Bach, U. Appl. Phys. Lett. 2001, 79, 2085. Schmidt-Mende, L.; Gra¨tzel, M. Thin Solid Films 2006, 500, 296. (7) Kavan, L.; Gra¨tzel, M. Electrochim. Acta 1995, 40, 643. (8) Kroeze, J. E.; Hirata, N.; Schmidt-Mende, L.; Orizu, C.; Ogier, S. D.; Carr, K.; Gra¨tzel, M.; Durrant, J. R. AdV. Func. Mater. 2006, 16, 1832. Snaith, H. J.; Gra¨tzel, M. Appl. Phys. Lett. 2006, 89, 262114. Snaith, H. J.; Schmidt-Mende, L.; Gra¨tzel, M.; Chiesa, M. Phys. ReV. B 2006, 74, 045306. (9) Zhang, Z. P.; Evans, N.; Zakeeruddin, S. M.; Humphry-Baker, R.; Gra¨tzel, M. J. Phys. Chem. C 2007, 111, 398. Yum, J. H.; Moon, S. J.; Humphry-Baker, R.; Walter, P.; Geiger, T.; Nuesch, F.; Gra¨tzel, M.; Nazeeruddin, M. D. K. Nanotech. 2008, 19, 424005. (10) Boschloo, G.; Hagfeldt, A. Chem. Phys. Lett. 2003, 370, 381. (11) Snaith, H. J.; Humphry-Baker, R.; Chen, P.; Cesar, I.; Zakeeruddin, S. M.; Gra¨tzel, M. Nanotechnology 2008, 19, 424003. (12) Nazeeruddin, M. K.; Gra¨tzel, M. Encyclopedia of Electrochemistry: Semiconductor Electrodes and Photoelectrochemistry; Wieley-VCH: Germany, 2002; Vol. 6. (13) Bach, U. Ph. D., EPFL, 2000. (14) Cappel, U. B.; Gibson, E. A.; Hagfeldt, A.; Boschloo, G. J. Phys. Chem. C 2009, 113, 6275. (15) Hagberg, D. P.; Yum, J. H.; Lee, H.; De Angelis, F.; Marinado, T.; Karlsson, K. M.; Humphry-Baker, R.; Sun, L. C.; Hagfeldt, A.; Gra¨tzel, M.; Nazeeruddin, M. K. J. Am. Chem. Soc. 2008, 130, 6259. (16) Boschloo, G.; Hagfeldt, A. Inorg. Chim. Acta 2008, 361, 729.
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