Enhancing efficiency of dye-sensitized solar cells by combining use of TiO2 nanotubes and nanoparticles

Materials Chemistry and Physics 124 (2010) 179–183

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Enhancing efficiency of dye-sensitized solar cells by combining use of TiO2 nanotubes and nanoparticles X.D. Li a,b , D.W. Zhang a , S. Chen a , Z.A. Wang a , Z. Sun a , X.J. Yin b , S.M. Huang a,∗ a Engineering Research Center for Nanophotonics and Advanced Instrument, Ministry of Education, East China Normal University, North Zhongshan Rd. 3663, Shanghai 200062, PR China b Advanced Materials Technology Centre, Office of the Chief Technology Officer, Singapore Polytechnic, 500 Dover Rd., Singapore 139651, Singapore

a r t i c l e

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Article history: Received 12 November 2009 Received in revised form 29 April 2010 Accepted 4 June 2010 Keywords: Titanium dioxide nanotubes Internal resistances Electron lifetime Dye-sensitized solar cells

a b s t r a c t Titanium dioxide nanotubes (TiNTs) were fabricated from commercial P25 TiO2 powders via alkali hydrothermal transformation. Dye-sensitized solar cells (DSCs) were constructed by application of TiNTs and P25 nanoparticles with various weight percentages. The influence of the TiNT concentration on the performance of DSCs was investigated systematically. The electrochemical impedance spectroscopy (EIS) technique was employed to quantify the recombination resistance, electron lifetime and time constant in DSCs both under illumination and in the dark. The DSC based on TiNT/P25 hybrids showed a better photovoltaic performance than the cell purely made of TiO2 nanoparticles. The open-voltage (Voc ), fill factor (FF) and efficiency () continuously increased with the TiO2 nanotube concentration from 0 to 50 wt%, which was correlated with the suppression of the electron recombination as found out from EIS studies. Respectable photovoltaic performance of ca. 7.41% under the light intensity of 100 mW cm−2 (AM 1.5G) was achieved for DSCs using 90 wt% TiO2 nanotubes incorporated in TiO2 electrodes. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Dye-sensitized solar cells (DSCs) have attracted much attention as the next-generation solar cells with low production costs and good efficiency for energy conversion, reaching 11% and a module efficiency of 7% in some cases [1]. The active electrode in DSC is usually composed of a thick TiO2 mesoporous film which provides a large surface area for anchoring the light-harvesting dye molecules [2,3]. However, slow electron percolation through the interconnected nanoparticles with a randomly distributed 3D network and the charge recombination between injected electrons and electron acceptors (e.g. I3 − ions) in the electrolyte hinder the DSC performance [4,5]. In order to increase the efficiency of energy conversion, a photoanode comprised of an orderly and strongly interconnected architecture was proposed to improve charge transfer [6,7]. Extensive research has been specially conducted on one-dimensional (1D) oxide morphologies, such as nanowires [8], nanorods [9], and nanotubes [10–12]. These 1D nanomaterials have been considered for efficiency enhancement as facilitated electron transport can be expected from these structures due to the suppression of random walk phenomena. In the case of TiO2 nanotubes (TiNTs), DSCs based

∗ Corresponding author. E-mail address: [email protected] (S.M. Huang). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.06.015

on highly ordered nanotube arrays have been mainly investigated over the past few years [10–12]. For instance, Grimes and coworkers [10] demonstrated the use of highly ordered transparent TiO2 nanotube arrays in DSC with an efficiency of 2.9% for a 360 nm thick array. These nanotube arrays are generally prepared by anodic oxidation of titanium foils [10,13,14]. However, anodic oxidation process is not a cost effective method since a pure and expensive titanium foil was needed and consumed in the fabrication process. Some cost effective methods, such as sol–gel, electrodeposition and hydrothermal methods, were reported in the literature [15–18] on the synthesis of disordered TiO2 nanotubes, nanorods and nanowires. In 1998 and 1999, Kasuga et al. [16,17] reported the preparation of TiO2 -derived nanotubes by a hydrothermal treatment of TiO2 powder in a 10 M NaOH aqueous solution. This method does not require any templates and the obtained nanotubes have a small diameter of 10 nm and high crystallinity. In the first report, the application of disordered TiO2 nanotube electrodes to DSC also fell short of the expectation as ordered TiO2 nanotube arrays did and only achieved an efficiency of 3.0% [19]. Meanwhile the efficiencies are steadily increasing since the first disordered TiNT solar cell construction and rising up to ca. 7.6% under 100 mW cm−2 using a 350 W Xenon lamp equipped with IR, Schott KG3, and UV cut-off filters in our recent work [20]. A highly pure indoline dye (D102) was used as the sensitizer in this work. However, the current comprehensive work based on either purely ordered or disordered nanotubes shows clearly that the limiting factor in efficiency is

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still the amount of dye that can be deposited per unit length of such TiO2 nanotubes [21]. For nanoparticulate layers, to increase the surface area of the TiO2 solar cells, approaches to decorate the structure with an extra layer of TiO2 nanoparticles is frequently attempted. In the present work, we investigate the effect of incorporating TiO2 nanoparticles in the TiO2 nanotubes and the resulting effect on solar cell performance. TiO2 nanotubes were fabricated by a hydrothermal process from commercial P25 TiO2 powder via alkali hydrothermal transformation. The nanoporous TiO2 films was fabricated by screen-printing method. The TiO2 screen-printing pastes were prepared from ethylcellulose, P25 TiO2 powders and TiO2 nanotubes as additives with sufficient dispersion. The photoelectrochemical properties of the DSCs were analyzed by electrochemical impedance spectroscopy (EIS). The optimal conditions for the fabrication of the DSCs with TiO2 nanotubes/nanoparticles and the electron transport properties in the TiO2 films were evaluated and explored. Finally, the DSC based on TiO2 nanotubes with the addition of 10 wt% TiO2 nanoparticles produced a respectable conversion efficiency of 7.41% under 100 mW cm−2 (AM 1.5G). 2. Experimental 2.1. TiO2 nanotube synthesis TiNTs were prepared using a hydrothermal process described in our previous work [20]. TiO2 source used for the TiNTs was commercial-grade TiO2 powder P25 (Degussa AG, Germany) with crystalline structure of ca. 20% rutile and ca. 80% anatase and primary particle size of ca. 20 nm. In a typical preparation, 2 g of the TiO2 powder was mixed with 200 ml of 10 M NaOH solution in a 500 ml Teflon flask. The mixture was kept at 110 ◦ C for 24 h in an oil bath. After the hydrothermal reaction, the precipitate was separated by filtration and washed with HCl solution and distilled water. The acid washed TiNT samples were dried in a vacuum oven at 60 ◦ C for more than 3 h and stored in glass bottles until used.

Fig. 1. Scheme of electron transfer processes in TiO2 electrodes made of (a) P25 nanoparticles and (b) TiNT/P25 hybrid.

tonitrile) was introduced into the cell via vacuum backfilling. Finally, the hole was sealed using the same Surlyn film and a cover glass with a thickness of 0.7 mm. 2.4. Measurements The prepared TiO2 nanoporous films were examined by SEM analysis. Photocurrent–voltage (I–V) measurements were performed using an AM 1.5 solar simulator (Newport, USA). The solar simulator was calibrated by using a standard Silicon cell (Newport, USA). The light intensity was 100 mW cm−2 on the surface of the test cell. Current–voltage curves were measured with a mask with an aperture slightly bigger than the active area of the cell using a computer-controlled IV tracer (VS-6810, Industrial Vision Technology (S) Pte Ltd., Singapore). The area of the solar cells is 0.196 cm2 . It is very important to use a standard AM 1.5 solar simulator and apply a mask covering the solar cell to do the I–V measurements. Otherwise, the efficiency of the measured cell will be over estimated for up to 30% [24]. EIS measurements of the DSCs were recorded with a potentiostat/falvanostat (PG30.FRA2, Autolab, Eco Chemie B. V Utrecht, the Netherlands) both in the dark and under illumination 100 mW cm−2 . The frequency range was 0.1 Hz to 100 kHz. The applied bias and ac amplitude were set at open-circuit voltage (Voc ) of the DSCs and 10 mV between the Pt counter electrode and the TiO2 working electrode, respectively. The obtained spectra were fitted with Z-View software (V3.10) in terms of appropriate equivalent circuits.

2.2. Preparation of TiO2 electrodes

3. Results and discussion TiO2 nanocrystalline films were prepared by employing as-deposited TiO2 nanotubes and commercial P25. The screen pastes were prepared according to the procedures mentioned in literatures [22]. In order to break aggregates into separated nano-features, commercial TiO2 nanoparticles P25 and as-made nanotubes were mixed with different weight ratios and grounded in an agate mortar. Acetic acid (1 ml), distilled water (5 ml) and ethanol (30 ml) were added gradually drop by drop to disperse TiO2 nanoparticles and nanotubes under continuously grinding. The TiO2 dispersions in the mortar were transferred with excess of ethanol (80 ml) to a tall beaker and stirred with a 5 cm long magnet tip at 300 rpm. A Tihorn-equipped sonicator (Sonifier 250, Branson Ultrasonics Corporation) was used to perform the ultrasonic homogenisation. Anhydrous terpineol and ethyl celluloses in ethanol were added, followed by further stirring and sonication. The contents in dispersion were concentrated by evaporating ethanol with a rotary evaporator. The pastes were finalised by grinding in mortar again. The fluorine-doped SnO2 transparent conducting oxide (FTO) glass was first cleaned in detergent, washed with distilled water, and then, treated with 40 mM TiCl4 aqueous solution at 70 ◦ C for 30 min, in order to make a good mechanical contact between the following printed TiO2 layer and the conducting glass matrix. The first TiO2 film with a thickness of 12–14 ␮m was deposited on the pre-treated conducting glass using the screen-printing technique. The second 4–5 ␮m thick layer of TiO2 particles (ca. 300 nm in size) for the light-scattering was also deposited by screen-printing. The thickness of films was determined by a surface profiler (Dektak 6M). The screen-printed layers were gradually heated in air at 450 ◦ C for 15 min and 500 ◦ C for 15 min, to remove all organic components and to establish sufficient inter-contacts between nanoparticles and nanotubes. Finally, to improve photovoltaic performance, TiCl4 treatment [23] was conducted after the sintering of electrodes. The TiO2 electrode was treated with 40 mM TiCl4 solution, then washed with distilled water and ethanol, and sintered again at 500 ◦ C for 30 min in air. 2.3. Fabrication of dye-sensitized solar cells After sintering at 500 ◦ C and cooling to 80 ◦ C, the TiO2 electrodes were dyecoated by immersing into a 0.5 mM solution of dye N719 in acetonitrile and tert-butyl alcohol (volume ratio of 1:1) at room temperature for 20–24 h, and then assembled with thermally platinized conducting glass electrodes with a hole into sandwich-type cells. Then, the cell was sealed with a Surlyn 1702 (Dupont) gasket with a thickness of 25 ␮m. A drop of electrolyte solution (0.1 M LiI, 0.05 M I2 , 0.6 M 1-methyl-3-propylimidazolium iodide (MPII) and 0.5 M tert-buthylpyridine in ace-

X-ray diffraction and TEM measurement and analysis showed that only anatase (A) phase was observed in the fabricated TiO2 nanotubes and the crystalline size of the TiO2 nanotubes was smaller than that of TiO2 nanoparticles (P25) [20]. The nanotubes were up to ∼100 nm in length, with an inner diameter of ca. 4–6 nm, outer diameter of ca. 8–10 nm, and the wall thickness of ca. 2 nm. The surface area of the TiO2 nanotubes was 350.4 m2 g−1 [20]. This value is much higher than that (ca. 50 m2 g−1 ) of TiO2 nanoparticles (P25). The morphologies of mesoporous TiO2 electrodes made of TiNTs and P25 nanoparticles with various weight percentages were characterized by SEM. It was found that nanotubes aggregated more when the ratio of TiNT exceeded that of P25. The mesoporous TiO2 films made of P25 nanoparticles revealed a very uniformly porous structure and 3D network made of nanoparticles. The films made of TiNT/P25 hybrids (90 wt% TiNT: 10 wt% P25) showed a porous structure and 3D network made of titania nanotubes/nanoparticles and some TiNT aggregations. In a cell based on pure TiO2 nanoparticles, an electron, injected from dye molecules into TiO2 nanoparticles which composed a 10–15 ␮m thick mesoporous film, has to pass through more than 106 particles and grain boundaries until reaching the conductive substrate [25] as shown in Fig. 1(a). The as-deposited TiNT was with higher electron transport rate, electron diffusion coefficient and surface area than TiO2 nanoparticle [4,20]. TiO2 nanoparticles, however, have higher thermal stability and less aggregation than TiNTs. Therefore, application of TiNTs having higher aspect ratio by incorporating a suitable amount of TiO2 nanoparticles will be preferable as electron transporting structure. In this hybrid mesoporous film, enhanced surface area of nanotubes, combined with the much more open structure, allows for more dye molecules to

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Fig. 3. Impedance spectra of a DSC based on TiNT 50 wt % at Voc , 100 mW cm−2 and at 0.7 V in dark. (a) Bode phase plots and (b) Nyquist plots.

Fig. 2. Electrochemical impedance spectra of DSCs based on different TiNT concentrations measured at Voc , 100 mW cm−2 . (a) Nyquist plots. (b) Bode phase plot. The equivalent circuit is shown in the inset of (a).

be chemically adsorbed onto the surface, whilst simultaneously facilitating the penetration of the electrolyte, thereby improving the iodide diffusion inside the cell. The electrons were supposed to transport along the nanotubes, thus reducing electron scattering in the hybrid interface structure, and leading to the less recombination and higher fill factor shown in Fig. 1(b). The influence of the ratio of TiNT/TiO2 on the cell performance of the DSC was studied by EIS as shown in Fig. 2. The Nyquist plots of the electrochemical impedance spectra of the DSCs with different weight percentages of TiNT measured at open-circuit voltage (Voc ) and under 100 mW cm−2 are shown in Fig. 2(a), and the equivalent circuit is shown as its inset. Generally, all the spectra of DSCs exhibit three semicircles, which are assigned to electrochemical reaction at the Pt counter electrode, charge transfer process at the TiO2 /dye/electrolyte interface and Nernstian diffusion process of I− /I3 − within the electrolyte, respectively [26–30]. In our case, it was found that the recombination resistance at the TiO2 /dye/electrolyte interface (R3 ) decreased with the increasing of the TiNT concentration from 0 to 50 wt%, and then increased slightly with the further increasing of the TiNT percentage. The reduction of the R3 with the increase in the TiNT ratio suggested higher surface area of the TiNT/TiO2 electrode and more adequate pore sizes for facile transport of the redox couple in the TiO2 interface thereby reducing the corresponding resistance at the interface [31]. The TiNT had a higher surface area than the TiO2 nanoparticle [4,20]. However, nanotubes aggregated more in the formed TiNT/TiO2 electrode when the ratio of TiNT exceeded that of P25 according to

the SEM measurement. The main reason was because the diameter of the nanotube is only half size of P25. Moreover, when the TiNT ratio is over 50 wt%, the tubular structure collapse and shrinkage of TiNTs occurred during sintering TiNT/TiO2 at 500 ◦ C might play a role and result in a small decrease in the surface area of the electrode followed by the increase in the interconnection among TiO2 particles and nanotubes [32]. As a result, the surface area of the TiNT/TiO2 electrode (TiNT concentration >50 wt%) was a slight less than that of 50 wt% TiNT concentration but still higher than that of 30 wt% TiNT concentration. This trend can explain the dependence of R3 on the TiNT concentration observed from the Nyquist plots in Fig. 2(a). The characteristic frequency peak of TiO2 in intermediatefrequency regime of the corresponding Bode phase plot shifted to lower frequency with the increase in the TiNT concentration from 0 to 50 wt%, and then kept at an almost fixed position when further raising the TiNT concentration, as shown in Fig. 2(b). This result indicated that the electron lifetime in the TiO2 electrode increased with the increasing of the TiNT concentration from 0 to 50 wt%, and then, was kept at an almost constant value when further raising the TiNT concentration. Fig. 3 shows the impedance spectra of a DSC (TiNT 50 wt %) measured at Voc under 1 sun and under forward bias 0.7 V in the dark, respectively. From Fig. 3(a), the impedance due to electron transfer from the conduction band of the mesoscopic film to triiodide ions in the electrolyte, presented by the semicircle in intermediate- frequency regime, is much smaller under light than in the dark even though the potential of the film is the same. Correspondingly, the characteristic peak is at higher frequency under the illumination than in the dark, as shown in Fig. 3(b), suggesting that the electron lifetime is shortened in the former. This phenomenon can be attributed to the difference of the local I3 − concentration in the TiO2 electrode. In the dark under forward bias, electrons were injected in the conduction band of the nanoparticles. The electrons were transported through the mesoscopic TiO2 network and reacted with I3 − in the TiO2 electrode to form I− . Then, I− was transported

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Fig. 4. Impedance spectra of DSCs based on different TiNT concentrations measured at 0 .7 V in dark.

and oxidized to I3 − at the counter electrode. The generated I3 − at counter electrode transported to the TiO2 film by diffusion. However, under illumination, I3 − was formed in situ by dye regeneration at the TiO2 /dye/electrolyte interface. The higher local I3 − concentration produced within the porous network under illumination was suggested to accelerate the capture of electrons in conduction band and shortens their lifetime within the TiO2 film, leading to the higher characteristic frequency shown in Fig. 3(b). The result is consistent with the previously noted observation in Ref. [28]. Fig. 4 shows the Nyquist plots of electrochemical impedance spectra of DSCs based on different TiNT/TiO2 ratios at 0.7 V in dark. The fitted data of the electron transport resistance R3 and the chem-

Fig. 6. Photocurrent–voltage curves of the cells based on P25 nanoparticles (), 30 wt% TiNT (䊉), 50 wt% TiNT (), 70 wt% TiNT () and 90 wt% TiNT (). Cell area is 0.196 cm2 .

ical capacitance C at TiO2 /dye/electrolyte interface obtained from Fig. 4 are shown in Fig. 5(a) and (b), respectively [33]. The time constant  n can be calculated by Eq. (1): n = R3 C

(1)

It can be seen that the electron transport resistance R3 increased with the increase in the TiNT concentration from 0 to 50 wt%, and then decreased when further raising the TiNT concentration as shown in Fig. 5(a). The higher resistance R3 can lead to the less recombination and the higher fill factor (FF) and open-circuit voltage (Voc ) in a DSC. The chemical capacitance (C ) represents the equilibrium property. Compared to the trend of R3 with the TiNT concentration, the C shows an inverse dependence on the TiNT concentration, as shown in Fig. 5(a). As a result, the time constant  n increases with the increase in the TiNT ratio from 0 to 50 wt%, and then, decreased slightly when further increasing the TiNT ratio in Fig. 5(b). Comparing Fig. 5(a) and (b), the lowest capacitance C was at the TiNT percentage of 50 wt% while the highest time constant  n was at that ratio. This result confirmed that the cell based on 50 wt% TiNT had the largest time constant due to slowest recapture of electrons in the conducting band by I3 − . Fig. 6 shows the photovoltaic performance of DSCs based on 30 wt%, 50 wt%, 70 wt% and 90 wt% TiNT under AM 1.5 illumination (100 mW cm−2 ). More detailed information about the performance of DSCs based on different TiNT concentrations is provided in Table 1. From Table 1, both the fill factor and the open-circuit voltage initially increase and then decrease with the increase in the TiNT concentration from 0 to 90 wt%. Maximal FF and Voc values of 0.706 and 0.707 V, respectively, were obtained at a TiNT concentration of 50 wt%. This result is in agreement with our previous finding of the highest electron transport resistance R3 in the

Table 1 Cell performance of DSCs base on different TiNT concentrations measured under 100 mW cm−2 (AM 1.5G).

Fig. 5. (a) Electron transport resistance and capacitance and (b) time constant of DSCs based on different TiNT concentrations obtained from impedance spectra measured at 0 .7 V in dark.

TiNT (wt%)

Jsc (mA cm−2 )

Voc (V)

FF

 (%)

0 10 30 50 70 90

14.513 14.065 14.129 14.296 14.94 15.546

0.696 0.696 0.698 0.707 0.704 0.693

0.583 0.644 0.677 0.706 0.686 0.688

5.89 6.29 6.68 7.13 7.21 7.41

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dark at the TiNT concentration of 50 wt% shown in Figs. 4 and 5. The highest resistance R3 induced the least recombination and the maximal FF and Voc values in the fabricated cells. The photocurrent density (Jsc ) of the DSC, however, continuously increases with the TiNT concentration from 0 to 90 wt%. This result could be attributed to the higher electron transport rate, electron diffusion coefficient and surface area of titania nanotubes compared to that of P25 [4,20]. The cell based on pure TiNTs, however, showed a poorer performance than that of the DSC based on pure P25. The finding can be associated with the obviously negative effects such as tubular structure collapse and shrinkage of TiNTs during the sintering process and nanotube aggregation at 100 wt% TiNT concentration. The DSCs based on TiNT/P25 hybrids from 10 to 90 wt% showed higher FF, Voc , Jsc and thereby higher efficiencies  than the cell based on pure TiO2 nanoparticles. The DSC fabricated with 90 wt% TiNT achieved conversion efficiency high up to 7.41%. In this study, higher efficiencies were achieved for DSCs based on TiNT/P25 hybrids from 10 to 90 wt% than the cell based on pure TiO2 nanoparticles or pure titanate nanotubes. These results were achieved by combining titanate nanotubes with TiO2 nanoparticles having relatively high thermal stability and low aggregation property. The efficiency of this hybrid DSC can be improved further by optimizing device architectures that enhance light absorption and facilitate electron transport by determining and designing appropriate dimensions of TiO2 nanotubes, by optimizing the cation concentration in the electrolyte solution for promoting electron injection yield from sensitizing dye molecules to hybrid TiO2 electrodes, or by synthesizing thermally stable titanate nanotube with a stable high surface area.

Acknowledgments This work was supported by National Natural Science Foundation of China (No. 10774046), Shanghai Municipal Science & Technology Committee (No. 09JC1404600, No. 0852nm06100 and No. 08230705400). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

4. Conclusions Titanium dioxide nanotubes were fabricated from commercial P25 TiO2 powder via alkali hydrothermal transformation. DSCs were constructed with TiO2 films made of different weight ratios of TiO2 nanotubes and P25 nanoparticles. The electron transport resistance, electron lifetime and time constant both under illumination and in the dark were evaluated in terms of electrochemical impedance spectra and photovoltaic characteristics of the cells. The DSC based on TiNT/P25 hybrids showed a better photovoltaic performance (higher fill factor, open-voltage and photocurrent density) than the cell purely made of TiO2 nanoparticles. The Voc , FF and  continuously increased with the TiO2 nanotube concentration from 0 to 50 wt%. EIS and SEM data and analyses supported the result of the conversion efficiency of the fabricated DSC. When the TiO2 nanotubes were incorporated at ca. 90 wt% in the TiO2 nanoparticle film, the conversion efficiency showed the best result. The DSC based on N719 dye yielded cell conversion efficiency high up to 7.41% under illumination of simulated AM 1.5 sunlight (100 mW cm−2 ).

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[20] [21] [22] [23] [24]

[25] [26] [27] [28] [29] [30] [31] [32] [33]

M. Grätzel, Nature 414 (2001) 338. B. O’Regan, M. Grätzel, Nature 353 (1991) 737. M. Grätzel, J. Photochem. Photobiol. A Chem. 164 (2004) 3. K. Zhu, N.R. Neale, A. Miedaner, A.J. Frank, Nano Lett. 7 (2007) 69. G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Nano Lett. 5 (2005) 191. M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P.D. Yang, Nat. Mater. 4 (2005) 455. P.E. de Jongh, D. Vanmaekelbergh, Phys. Rev. Lett. 77 (1996) 3427. M. Adachi, Y. Murata, J. Takao, J. Jiu, M. Sakamoto, F. Wang, J. Am. Chem. Soc. 126 (2004) 14943. J.T. Jiu, S. Isoda, F.M. Wang, M. Adachi, J. Phys. Chem. B 110 (2006) 2087. G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Nano Lett. 6 (2006) 215. M. Paulose, K. Shankar, O.K. Varghese, G.K. Mor, B. Hardin, C.A. Grimes, Nanotechnology 17 (2006) 1446. G.K. Mor, O.K. Varghese, M. Paulose, K. Shankar, C.A. Grimes, Sol. Energy Mater. Sol. Cells 90 (2006) 2011. D. Gong, C.A. Grimes, O.K. Varghese, W.C. Hu, R.S. Singh, Z. Chen, E.C. Dickey, J. Mater. Res. 16 (2001) 3331. G.K. Mor, O.K. Varghese, M. Paulose, C.A. Grimes, Adv. Funct. Mater. 15 (2005) 1291. B.B. Lakshmi, P.K. Dorhout, C.R. Martin, Chem. Mater. 9 (1997) 857. T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Langmuir 14 (1998) 3160. T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Adv. Mater. 11 (1999) 1307. X.Y. Zhang, L.D. Zhang, W. Chen, G.W. Meng, M.J. Zheng, L.X. Zhao, Chem. Mater. 13 (2001) 2511. S. Uchida, R. Chiba, M. Tomiha, N. Masaki, M. Shirai, Electrochemistry 70 (2002) 418. X.D. Li, D.W. Zhang, Z. Sun, Y.W. Chen, S.M. Huang, Microelectron. J. 40 (2009) 108. A. Ghicov, S. Albu, R. Hahn, D. Kim, T. Stergiopoulos, J. Kunze, C.-A. Schiller, P. Falaras, P. Schmuki, Chem. Asian J. 4 (2009) 520. S. Ito, P. Chen, P. Comte, Prog. Photovolt: Res. Appl. 15 (2007) 603. M.K. Nazeerudin, A. Kay, I. Rodicio, J. Am. Chem. Soc. 115 (1993) 6382. S. Ito, M. Nazeeruddin, P. Liska, P. Comte, R. Charvet, P. Pechy, M. Jirousek, A. Kay, M. Shaik, M. Zakeeruddin, Gratzel, Prog. Photovolt: Res. Appl. 14 (2006) 589. K.D. Benkstein, N. Kopidakis, J. van de Lagemaat, A.J. Frank, J. Phys. Chem. B 107 (2003) 7759. C. Longo, J. Freitas, M.A. De Paoli, J. Photochem, Photobiol. A: Chem. 159 (2003) 33. L. Han, N. Koide, Y. Chiba, Appl. Phys. Lett. 84 (2004) 13. F. Fabregat-Santiago, J. Bisquert, G. Garcia-Belmonte, G. Boschloo, A. Hagfeldt, Sol. Energy Mater. Sol. Cell 87 (2005) 117. J. van de Lagemaat, N.-G. Park, A.J. Frank, J. Phys. Chem. B 104 (2000) 2044. F. Fabregat-Santiago, G. Garcia-Belmonte, J. Bisquert, A. Zaban, P. Salvador, J. Phys. Chem. B 106 (2002) 334. E. Hawlicka, R. Grabowski, B. Bunsenges, Phys. Chem. 94 (1990) 158. M. Wei, Y. Konishi, H. Zhou, H. Sugihara, H. Arakawa, J. Electrochem. Soc. 153 (2006) A1232. T. Hoshikawa, M. Yamada, R. Kikuchi, K. Eguchi, J. Electrochem. Soc. 152 (2005) E68.