Role of Polyelectrolyte for Layer-by-Layer Compact TiO2

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Role of Polyelectrolyte for Layer-by-Layer Compact TiO2 Films in Efficiency Enhanced Dye-Sensitized Solar Cells A. O. T. Patrocinio, L. G. Paterno, and N. Y. Murakami Iha* Laboratory of Photochemistry and Energy ConVersion, Instituto de Quı´mica, UniVersidade de Sa˜o Paulo, AV. Prof. Lineu Prestes, 748, 05508-900, Sa˜o Paulo, Brazil ReceiVed: May 24, 2010; ReVised Manuscript ReceiVed: August 25, 2010

Efficient compact TiO2 films using different polyelectrolytes are prepared by the layer-by-layer technique (LbL) and applied as an effective contact and blocking film in dye-sensitized solar cells (DSCs). The polyanion thermal stability plays a major role on the compact layers, which decreases back electron transfer processes and current losses at the FTO/TiO2 interface. FESEM images show that polyelectrolytes such as sodium sulfonated polystyrene (PSS) and sulfonated lignin (SL), in comparison to poly(acrylic acid) (PAA), ensure an adequate morphology for the LbL TiO2 layer deposited before the mesoporous film, even after the sintering step at 450 °C. The so treated photoanode in DSCs leads to a 30% improvement on the overall conversion efficiency. Electrochemical impedance spectroscopy (EIS) is employed to ascertain the role of the compact films with such polyelectrolytes. The significant increase in Voc of the solar cells with adequate polyelectrolytes in the LbL TiO2 films shows their pivotal role in decreasing the electron recombination at the FTO surface and enhancing the electrical contact of FTO with the mesoporous TiO2 layer. 1. Introduction Dye-sensitized solar cells, DSCs, have emerged as one of the most promising devices for sustainable photovoltaics due to their reduced cost, low environmental impact, and fair efficiency for conversion of solar energy into electricity.1-4 Research in the field has been intense, either with focus on new materials and components or on cell’s assemblies for development of more efficient and environmentally friendly devices.5-14 Special attention has been given to the electron recombination processes that limit the DSC efficiency.15-19 Experimental and theoretical studies have been carried out in order to better understand and control these processes,20-27 which are typical interface phenomena. Application of a compact layer onto the FTO substrate before the mesoporous oxide film can prevent electron recombination at the FTO/TiO2 interface. This so-called blocking layer physically avoids the contact of the electrolyte with the FTO surface, decreasing the occurrence of triiodine reduction by photoinjected electrons. Several authors have proposed strategies to prepare efficient blocking layers in DSCs by using different techniques, such as spray pyrolysis, sputtering or by immersion in oxide precursor solutions.27-32 In a previous contribution, we have reported the potentiality of efficient layer-by-layer TiO2 compact films as one of the most effective blocking layers to avoid recombination processes at FTO surface in DSCs.33 Although not previously reported as blocking layers, LbL metal oxide films have been applied in several devices,34-40 including DSCs.41-44 In our first contribution,33 an LbL film based on TiO2 nanoparticles and sodium sulfonated polystyrene, PSS, applied onto the FTO substrate before the mesoporous TiO2 layer improved the overall conversion efficiency of DSCs in 28%. Other complementary effects of the compact LbL TiO2 layer in DSCs and the role of the polyelectrolyte itself were still under investigation. * Corresponding author. E-mail: [email protected]. Fax: +55(11)38155579.

In order to understand the effects of morphology and chemical composition of LbL TiO2 films on the DSCs electrical performance, a systematic analysis of films produced with TiO2 nanoparticles and different polyelectrolytes was carried out. Atomic force (AFM) and field-emission scanning electron (FESEM) microscopies as well as photoelectron spectroscopy (XPS) were thorough explored for this purpose. Further, photoelectrochemical measurements and electrochemical impedance spectroscopy (EIS) were exploited to determine the influence of LbL TiO2 compact films on the electrical properties of DSCs. A correlation of DSCs performance and LbL TiO2 film properties corroborated to confirm the blocking effect of such films in dye-sensitized solar cells. 2. Experimental Section All chemicals, reagent or HPLC grade, were used as received, except 3-methyl-2-oxazolidinone (Aldrich), which was purified by distillation under reduced pressure. The N3 dye, cis[Ru(dcbH2)2(NCS)2], dcbH2 ) 4,4′-dicarboxylic acid-2,2′-bipyridine, was synthesized as previously reported45 and was used as a standard sensitizer. Three different polyanions were used: sodium sulfonated polystyrene, PSS, (Aldrich; Mw ) 70 000 g mol-1), sulfonated lignin, SL, (Melbar Lignin Products; Mw ) 3000 g mol-1) and poly(acrylic acid), PAA, (Aldrich; Mw ) 400 000 g mol-1). Preparation of titania nanoparticles for blocking layers (5-7 nm) and for the mesoporous film (20-40 nm) is described in detail in our previous works.33,46 TiO2 compact films used as blocking layers were deposited onto previously cleaned FTO substrates (Pilkington, TEC-15, 15 Ω 0-1) using the LbL technique as described elsewhere.33 FTO substrates were immersed alternately for 5 min in a 10 mg mL-1 TiO2 sol (pH 2) and in 1.0 mg mL-1 aqueous solutions of PSS (pH 5) or SL (pH 6.5). A 0.5 mg mL-1 solution (pH 5) was used for PAA as an optimized condition to produce homogeneous and reproducible films.

10.1021/jp104751g  2010 American Chemical Society Published on Web 09/20/2010

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Figure 1. Tapping AFM images (1 µm × 1 µm) of LbL TiO2 films with different polyelectrolytes before the sintering step.

The mesoporous TiO2 layer (Meso-TiO2) was deposited by painting, sintered at 450 °C, and sensitized with the N3 dye as described elsewhere.5,61 Solar cells were assembled in a sandwich-type arrangement using a sensitized TiO2 photoanode and a transparent Pt-covered FTO (Pilkington, TEC-15) as counterelectrode. A solution of 0.05 mol L-1 I2/ 0.5 mol L-1 LiI/0.5 mol L-1 pyridine in acetonitrile (Aldrich) and 3-methyl2-oxazolidinone (90:10 v/v) was used as electrolyte mediator. The TiO2 nanoparticles size and the film morphology were evaluated by field-emission scanning electron microscopy, FESEM, using a JSM 7401F (JEOL) microscope and by atomic force microscopy, AFM, using a Digital Nanoscope III (Veeco Instruments). AFM images were registered under the tapping mode at a scan rate of 1 Hz with a silicon nitride tip and a cantilever of spring constant in the range of 20 ( 1 N m-1. Film thicknesses were measured with an Alpha step 500 (KLA Tencor) perfilometer. Thermogravimetric analyses were conducted under synthetic air atmosphere by using a STA 409 (Netzsch) thermoanalyser. X-ray photoelectron spectroscopy, XPS, experiments was carried out in an Axis Ultra DLD X-ray photoelectron spectrometer (Kratos) using a monochromatic Al K source. Binding energies were corrected according to the C 1s peak at 284.6 eV. Electrochemical impedance spectra, EIS, of single photoanodes and whole DSCs were registered with an impedance analyzer (Solartron 1260A) over a frequency range of 106-10-2 Hz under a 10 mV AC amplitude signal. Data fits were performed by using the Z-view software (Scribner Associates). Photoelectrochemical characterization of DSCs was carried out by current-potential measurements using a PAR270 galvanostat/potenciostat (EG&G Instruments) system at a simulated AM 1.5 solar radiation (100 mW cm-2) provided by a solar simulator (Newport/Oriel) as previously described.46,47 All photoelectrochemical parameters are the average values measured in, at least, five reproducible individual cells of each type of photoanode. 3. Results and Discussion All three types of polyelectrolytes used in depositions provided LbL TiO2 films optically very similar. The substrate

transmittance to the visible light is slightly reduced after the deposition of 30 bilayers, from 80% to 70%, as a result of the light reflection imposed by the oxide nanoparticles. Despite that, the overall photoanode transmittance in DSCs is determined by the mesoporous layer, which is much thicker and constituted by larger TiO2 particles. The LbL films present a regular and stepwise growth with a same amount of polyanions being adsorbed per each bilayer. Since the films are assembled via electrostatic interactions, the amount of TiO2 nanoparticles should also be constant at each deposition cycle to ensure the compensation of negative charges. The transmittance spectra of LbL films and the linear growth of the TiO2/PSS film can be visualized in the Supporting Information, SI-1. Film thicknesses after deposition of 30 bilayers were 115 ( 7, 117 ( 5, and 90 ( 8 nm, respectively for TiO2/PSS, TiO2/SL, and TiO2/PAA films. The morphology of these three types of films assessed by AFM images before the sintering step is presented in Figure 1. One can observe that all films are quite similar, composed by a continuous layer of densely packed nanoparticles and some aggregates. The rms roughness determined by the microscope software for TiO2/PSS, TiO2/SL, and TiO2/PAA films were 16, 14, and 13 nm, respectively, which can be considered experimentally equal. As pointed by different authors,48-50 the morphology of LbL films of polyelectrolytes and inorganic nanoparticles depends on the interplay of interactions among nanoparticles and polyelectrolytes and of the nanoparticles themselves. When the attraction force between polyelectrolytes and nanoparticles is comparable to the repulsion among the nanoparticles themselves, the resulting films are formed by dense-packed nanoparticles monolayers. On the other hand, if one of the above interactions dominates over the other, the film morphology is characterized by nanoparticle clustering or else by low particle density. Based on such a model picture, it is possible to infer the existence of a strong interaction between TiO2 nanoparticles and the polyanions, once the observed film morphology is characterized by dense-packed layers of nanoparticles. However, a significant interaction among nanoparticles is also existent, leading to the observation of few aggregates.

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Figure 2. FESEM micrographs of LbL TiO2 films taken before (a-c) and after (d-f) sintering at 450 °C using PSS (a and d) SL (b and e) or PAA (d and f) as polyanions.

Such a conclusion is supported by the ionization degree of electrolytes attained at the deposition conditions employed herein. At pH 5, PSS and PAA chains are completely deprotonated since the pKa of their respective acid groups are 0.7 and 4.3,51 as well as for SL at pH 6.5. In this way, positive charges at the TiO2 nanoparticles are effectively neutralized by negative charges provided by polyanions. The control of the polymer charge density, despite the molecular weight, is fundamental to produce homogeneous films as pointed out by Kim and Shiratori52 and also by Kniprath and co-workers.53 For weak electrolytes such as PAA, films of very poor quality were produced when polymer solutions at pH lower than its pKa were used. This occurs likely due to the coiled shape assumed by polymer chains at this condition and their lowered electrostatic attraction to TiO2 nanoparticles. FESEM micrographs of LbL TiO2 films show a very compact morphology before the sintering step, regardless on the polyanion type, Figure 2a-c. TiO2 particles are distributed all over the film surface as plates of nanoparticles embedded in a polymer matrix. Polyanion chains act as a “glue” that reduces the electrostatic repulsions among the particles and allow their packing onto the substrate surface. After the sintering step at 450 °C, Figure 2d-f, differences can be observed on the film morphologies as a function of the polymer employed. For TiO2/PSS and TiO2/SL films almost no changes are observed after sintering and the compact structure is maintained. On the other hand, the TiO2/PAA film morphology changes significantly after the thermal treatment. The compact arrangement is lost, Figure 2c, and nanoparticle aggregates are disrupted resulting in a more porous structure, Figure 2f. The TiO2/SL film exhibited a similar behavior to that of TiO2/PSS with only small changes on the morphology after sintering. Different degrees of morphological changes observed in FESEM micrographs after the sintering step can be correlated to the distinct thermal stability of polyelectrolytes. Thermogravimetric analyses for the three polyanions investigated indicate that PSS is the most stable at 450 °C followed by SL and PAA, which is the less stable one. At 450 °C, mass losses are approximately 15, 40, and 65%, respectively for PSS, SL, and PAA (see Figure SI-2 in the Supporting Information). The

TABLE 1: Surface Atomic Percentage of Elements of TiO2 LbL Films before and after Sintering at 450 °C Determined by XPS PSS

SL

PAA

element

before

after

before

after

before

after

Ti O S C

14.86 43.45 0.36 41.33

19.71 55.81 0.25 24.22

13.19 39.22 0.40 47.20

21.68 52.12 0.21 25.99

11.76 36.54

20.33 52.97

51.70

26.48

relative different losses of polymeric material from the films were quantified by XPS analysis which provided a quantitative in situ profile of polymer loss after sintering. Surface atomic percentage of elements for each type of film, before and after sintering is given in Table 1 and reflects the differences on the polyelectrolytes’ thermal behavior. A significant loss of polyanions after the sintering step can be detected by the decrease of the C 1s signal. However, the C 1s signal is highly influenced by organic contaminants and in order to avoid a misinterpretation of data, the Ti4+ atomic percentage was selected as a reliable probe to analyze the influence of the thermal treatment on the film surface composition. Thus, all Ti-2p signals were assumed to originate from TiO2 nanoparticles. The Ti atomic concentration increases in all films after the sintering process. There are enhancements of 32, 64, and 75%, respectively, for TiO2/PSS, TiO2/SL, and TiO2/PAA that follow inversely to the thermal stability of the polymers themselves. Thus, the more pronounced changes in the TiO2/PAA film morphology are due to the thermal decomposition of PAA that turns the film structure more porous. In the TiO2/PSS film, the small decomposition observed for PSS did not affect substantially the film morphology and the compact structure was preserved. For the TiO2/SL film, the increase on the Ti atomic percentage after sintering is relatively high, even though FESEM did not indicate significant changes in morphology, Figure 2b,e. Hence, the sintering step decomposed only the first polyanion layers and left intact inner layers with the compact structure. Therefore, the maintenance of the compact structure on LbL TiO2 films is dependent on the thermal stability of the polyelectrolyte. It is well-known that a film designed to work as an

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Figure 3. Current-voltage curves under AM 1.5 radiation (100 mW cm-2) of DSCs without blocking layer (green line) or with TiO2/PSS (black line), TiO2/SL (red line), or TiO2/PAA (blue line) blocking layers.

efficient blocking layer should physically avoid the contact of the electrolyte with the surface of FTO, in order to reduce the electron recombination. Thus, the compact and less porous morphology is a fundamental requirement for an efficient blocking layer. LbL TiO2 films with different polyelectrolytes were applied in DSCs. Devices with 30 TiO2/PSS bilayers presented the highest conversion among DSCs with other films. Figure 3, Table 2. Despite the excellent performance of the TiO2/PSS film, TiO2/SL also enhanced the DSC conversion. The Voc is increased at about 20-50 mV by application of either TiO2/PSS or TiO2/ SL layers. Such an increase was previously predicted as a consequence of the blocking effect.26 On the other hand, the TiO2/PAA film did not significantly improve the performance of DSCs, which can be attributed to the loss of the compact morphology after the sintering step. TiO2/PSS and TiO2/SL films also increase Jsc. The compact LbL TiO2 films provide an intermediate layer between FTO and the mesoporous TiO2 layer replacing the more abrupt interface by a smoother one. Thus, there is an improvement of the electrical contact at this interface which is accompanied by an increase of the Jsc. This enhancement corroborates to the increase observed in IPCE values of DSCs with LbL TiO2 compact layer (Supporting Information SI-3). The improvement of both Jsc and Voc due to the compact films was previously observed by other authors using different oxides or deposition techniques.24,25,54-57 In order to achieve a better understanding of the effect that a compact LbL TiO2 film cause on the DSC performance electrochemical impedance spectroscopy of single electrodes (bare and coated with a TiO2/PSS film, 30 bilayers) was carried out. The electrodes were set as working electrodes and a platinized FTO substrate was used as counter-electrode. The measurements were performed in the presence of the DSC electrolyte at different applied bias. In Figure 4a is shown the Bode plot at -0.7 V applied bias for the bare FTO and the TiO2/PSS electrode. One can observe two phase angle peaks that correspond to two semicircles in

Figure 4. (a) Electrochemical impedance spectra (Bode plot) of FTO electrodes with (O) and without (∆) TiO2/PSS blocking layer at -0.7 V applied potential. Inset: Nyquist plots. The lines are the fits obtained using the equivalent circuit shown in (b).

Figure 5. Electrochemical impedance spectra of FTO electrodes with TiO2/PSS blocking layer at -0.7 (O), -0.6 (∆), -0.5 (0), and -0.4 (]) applied potentials.

the Nyquist plot (Figure 4a, inset). Based on previous detailed studies,58-61 the high frequency peak can be ascribed to the charge transfer processes at the counter-electrode, whereas the response in 103-101 Hz is regarded to the working electrode. At lower frequencies there is also a contribution from the impedance of the Nerstian diffusion within the electrolyte. The presence of the TiO2/PSS compact layer shifts the phase angle peak of the working electrode from 800 to 300 Hz indicating a lower exchange current rate. As the applied bias becomes more positive, Figure 5, the working electrode exhibits a more capacitive behavior. Similar behavior was observed by Cameron and Peter21 with TiO2 compact layers deposited by spray pyrolysis. The impedance data were fitted to the equivalent circuit shown in Figure 4b, where Rwe and Cwe are the working electrode resistance and capacitance respectively. Rce and Cce are the counter-electrode resistance and capacitance respectively,

TABLE 2: Photoelectrochemical Parameters of DSCs using Different Blocking Layers (30 Bilayers) under AM (1.5 Radiation; 100 mW cm-2) blocking layer

Voc (V)

Jsc (mA cm-2)

ff

η (%)

improvement (%)

TiO2/PSS TiO2/SL TiO2/PAA

0.66 ( 0.02 0.70 ( 0.01 0.68 ( 0.01 0.69 ( 0.01

13.0 ( 0.8 15.8 ( 0.3 15.0 ( 0.5 11.8 ( 0.3

0.66 ( 0.01 0.67 ( 0.01 0.69 ( 0.01 0.72 ( 0.04

5.6 ( 0.5 7.3 ( 0.1 6.9 ( 0.3 5.8 ( 0.3

30 23 3

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Figure 7. Impedance spectra of DSCs with (O) and without (∆) TiO2/ PSS blocking layer under Voc conditions and iluminated. The lines are the fits obtained using the equivalent circuit shown in the Figure 6b.

Figure 6. Semilogarithmic plots of Cwe (b) and Rwe (9) (a) and τwe (2) (b) as a function of the applied potential for the FTO electrodes with (black) and without (green) TiO2/PSS blocking layer.

and, finally, Wd is the Warburg component related to the redox species diffusion.59,60 The values of Rwe and Cwe determined for the bare FTO and for that coated with the TiO2/PSS compact layer varied with the applied bias as presented in Figure 6a. At 0 V bias, both electrodes are at the I3-/I- equilibrium potential and Rwe reaches the highest value. As the applied bias becomes more negative, Rwe of the bare FTO electrode decreases exponentially with small variations in Cwe, which is characteristic of a high conductivity degenerated semiconductor that behaves very similarly to a metal.59 As a consequence, the more negative the applied potential is, the lower τwe is, Figure 6b, which is the time constant associated to the electron exchange at the working electrode, and can be calculated as τwe ) CweRwe. On the other hand, Rwe and Cwe in TiO2/PSS coated electrode vary according to two different regimes. In the first, from 0 to -0.5 V, Rwe decreases almost at the same rate of the bare FTO, but Cwe increases faster reaching a maximum at -0.5 V. This behavior indicates a charge accumulation process in the TiO2/ PSS layer, which act as an insulator and prevents the electron transfer to the I3- ions. Thus τwe values are almost constant in this region and higher than those calculated for the bare FTO electrode, Figure 6b. At potentials more negative than -0.5 V, Cwe of the TiO2/PSS coated electrode decreases exponentially, whereas Rwe diminishes faster than in the bare FTO electrode. The change in the slope at potentials more negative than -0.5 V indicates that the compact TiO2/PSS film becomes sufficiently conductive at this range so that it makes easier the electron transfer from the electrode to I3- and consequently leads to a decrease in τwe, Figure 6b. The behaviors observed and discussed above agree quite well to results and predictions of previous reports21,59-61 and allow us to conclude that LbL TiO2 compact layers can prevent efficiently the reduction of I3- by photoinjected electrons at the FTO surface. In other words, the LbL TiO2 film works as a blocking layer. When full DSCs are illuminated and analyzed by EIS under open-circuit conditions, one can observe two

semicircles in the Nyquist plot, Figure 7, which can be also fitted to the equivalent circuit proposed in Figure 4b. The DSC with the TiO2/PSS film exhibits higher values of Rwe and Cwe than the DSC without it. As the only difference between the two solar cells is the presence of the compact layer, the changes in the impedance spectrum are due to prevention of the electron recombination at the FTO substrate under Voc where the blocking effect becomes significant. Additionally, under short-circuit conditions, TiO2/PSS and TiO2/SL compact films improve the electrical contact between FTO and the mesoporous layer and, thus, lead to an enhancement of current densities in DSCs. 4. Conclusions The role of polyelectrolyte on the efficiency of LbL TiO2 compact layers in DSCs was investigated by thorough characterization carried out by XPS, UV-vis, FESEM, and AFM. LbL films using polyelectrolytes with good thermal stability at the electrode sintering temperature (450 °C), such as sodium sulfonated polystyrene and sulfonated lignin, maintain the compact morphology, and act as effective contact and blocking layers in DSCs. TiO2 LbL films with poly(acrylic acid) as a polyanion presented similar morphology to that exhibited by TiO2/PSS and TiO2/SL films before sintering. The lower thermal stability of PAA is responsible for more than 60% of the polymer mass lost after the heating step. The resulting film is more porous and, therefore, unable to block the contact of electrolyte and the FTO substrate. The best performance so far achieved is through the use of the TiO2/PSS compact layer that increases the overall efficiency of DSCs in 30%, from 5.6 to 7.3%. The TiO2/SL films (23% improvement) can be a costeffective option if a commercial application is considered. Impedance spectroscopy has proven the effectiveness of LbL TiO2/PSS film to improve the DSCs performance. Whereas the working electrode resistance varies with the applied bias almost similarly in both bare and LbL TiO2/PSS coated FTO, electrode capacitance varies significantly. The presence of such a film shifts the phase angle peak of the working electrode to lower frequency, which indicates a lower exchange current rate. The time constant, τ, associated to the resistance and capacitance of the working electrode remains higher than for in the bare FTO electrode and fairly constant in the range of 0 to -0.5 V. Thus, the LbL TiO2/PSS film imposes a longer time for a charge exchange at the electrode surface and decrease the electron recombination. This investigation provides a better understanding of the role of LbL TiO2 films with thermally stable polyelectrolytes as efficient compact and blocking layers in DSCs.

Layer-by-Layer Compact TiO2 Films Acknowledgment. This work was supported by the Fundac¸a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo (FAPESP) and the Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico (CNPq). We also thank Pilkington Glass Company for supply the FTO glasses, the Chemistry Department of University of North Carolina at Chapel Hill (particularly Dr. M. Kyle Brennaman) for XPS analyses and the LME of the Escola Politecnica - USP (Prof. Fernando J. Fonseca) for providing the impedance analyzer. Supporting Information Available: Transmittance spectra of the electrodes (SI-1), thermogravimetric analyses of the polyanions employed (SI-2), and IPCE spectra of DSCs with and without TiO2/PSS compact layer (SI-3). 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 (6346), 737–740. (2) Gra¨tzel, M. J. Photochem. Photobiol. C: Photochem. ReV. 2003, 4 (2), 145–153. (3) Polo, A. S.; Itokazu, M. K.; Murakami Iha, N. Y. Coord. Chem. ReV. 2004, 248 (13-14), 1343–1361. (4) Argazzi, R.; Murakami Iha, N. Y.; Zabri, H.; Odobel, F.; Bignozzi, C. A. Coord. Chem. ReV. 2004, 248 (13-14), 1299–1316. (5) Garcia, C. G.; Polo, A. S.; Murakami Iha, N. Y. J. Photochem. Photobiol. A: Chem. 2003, 160 (1-2), 87–91. (6) Kroon, J. M.; Bakker, N. J.; Smit, H. J. P.; Liska, P.; Thampi, K. R.; Wang, P.; Zakeeruddin, S. M.; Gra¨tzel, M.; Hinsch, A.; Hore, S.; Wurfel, U.; Sastrawan, R.; Durrant, J. R.; Palomares, E.; Pettersson, H.; Gruszecki, T.; Walter, J.; Skupien, K.; Tulloch, G. E. Prog. PhotoVolt. 2007, 15 (1), 1–18. (7) Hwang, S.; Lee, J. H.; Park, C.; Lee, H.; Kim, C.; Park, C.; Lee, M. H.; Lee, W.; Park, J.; Kim, K.; Park, N. G.; Kim, C. Chem. Commun. 2007, (46), 4887–4889. (8) Prochazka, J.; Kavan, L.; Zukalova, M.; Frank, O.; Kalbac, M.; Zukal, A.; Klementova, M.; Carbone, D.; Gra¨tzel, M. Chem. Mater. 2009, 21 (8), 1457–1464. (9) Wang, Z. S.; Cui, Y.; Dan-Oh, Y.; Kasada, C.; Shinpo, A.; Hara, K. J. Phys. Chem. C 2008, 112 (43), 17011–17017. (10) Heng, L. P.; Wang, X. Y.; Yang, N. L.; Zhai, J.; Wan, M. X.; Jiang, L. AdV. Funct. Mater. 2010, 20 (2), 266–271. (11) Kwak, E. S.; Lee, W.; Park, N. G.; Kim, J.; Lee, H. AdV. Funct. Mater. 2009, 19 (7), 1093–1099. (12) Lu, X. J.; Mou, X. L.; Wu, J. J.; Zhang, D. W.; Zhang, L. L.; Huang, F. Q.; Xu, F. F.; Huang, S. M. AdV. Funct. Mater. 2010, 20 (3), 509–515. (13) Snaith, H. J. AdV. Funct. Mater. 2010, 20 (1), 13–19. (14) Zakeeruddin, S. M.; Gra¨tzel, M. AdV. Funct. Mater. 2009, 19 (14), 2187–2202. (15) Palomares, E.; Clifford, J. N.; Haque, S. A.; Lutz, T.; Durrant, J. R. J. Am. Chem. Soc. 2003, 125 (2), 475–482. (16) Frank, A. J.; Kopidakis, N.; van de Lagemaat, J. Coord. Chem. ReV. 2004, 248 (13-14), 1165–1179. (17) Wang, Z. S.; Yanagida, M.; Sayama, K.; Sugihara, H. Chem. Mater. 2006, 18 (12), 2912–2916. (18) Peter, L. M. J. Phys. Chem. C 2007, 111 (18), 6601–6612. (19) Zhao, Y.; Zhai, J.; He, J. L.; Chen, X.; Chen, L.; Zhang, L. B.; Tian, Y. X.; Jiang, L.; Zhu, D. B. Chem. Mater. 2008, 20 (19), 6022–6028. (20) Kruger, J.; Plass, R.; Gra¨tzel, M.; Cameron, P. J.; Peter, L. M. J. Phys. Chem. B 2003, 107 (31), 7536–7539. (21) Cameron, P. J.; Peter, L. M. J. Phys. Chem. B 2003, 107 (51), 14394–14400. (22) Cameron, P. J.; Peter, L. M.; Hore, S. J. Phys. Chem. B 2005, 109 (2), 930–936. (23) Cameron, P. J.; Peter, L. M. J. Phys. Chem. B 2005, 109 (15), 7392–7398. (24) Xia, J. B.; Masaki, N.; Jiang, K. J.; Wada, Y.; Yamagida, S. Chem. Lett. 2006, 35 (3), 252–253. (25) Hart, J. N.; Menzies, D.; Cheng, Y. B.; Simon, G. P.; Spiccia, L. C. R. Chim. 2006, 9 (5-6), 622–626. (26) Peter, L. J. Electroanal. Chem. 2007, 599 (2), 233–240.

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