Iodine-free high efficient quasi solid-state dye-sensitized solar cell containing ionic liquid and polyaniline-loaded carbon black

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1 PAPER Journal of Materials Chemistry Iodine-free high efficient quasi solid-state dye-sensitized solar cell containing ionic liquid and polyaniline-loaded carbon black Chuan-Pei Lee, a Po-Yen Chen, a R. Vittal a and Kuo-Chuan Ho* ab Received 27th October 2009, Accepted 23rd December 2009 First published as an Advance Article on the web 2nd February 2010 DOI: /b922350a An incombustible and non-volatile paste with carbon black (CB), a conducting polymer (CP), and an ionic liquid (1-buty-3-methylimidazolium iodide, BMII or 1-methyl-3-propyl imidazolium iodide, PMII) was placed between the dye-sensitized porous TiO 2 and the Pt counter electrode to fabricate a quasi solid-state DSSC, without the addition of iodine. While the solar-to-electricity efficiencies (h) were measured to be 4.38% and 3.68% for the cells with PMII/CB and BMII/CB, respectively, the corresponding values without CB were 0.6% and 0.3%; indicating the remarkable role played by the carbon material in the electrolyte. When the CB was replaced with polyaniline-loaded carbon black (PACB), an efficiency of 5.81% was obtained, at 100 mw cm 2 AM1.5 illumination using PMII, the highest ever reported for a quasi solid-state DSSC made without iodine. High thermal stability up to 250 C for each component in the composite electrolytes was confirmed by thermogravimetric analyses (TGA). At-rest durability of the DSSC with PACB was studied both at room temperature and at 70 C and was found to be far superior to that of a cell with an organic solvent electrolyte. Electrochemical impedance spectroscopy (EIS) and dark current measurements were used to substantiate the results. 1. Introduction It is well known that a typical DSSC consists of three adjacent thin layers: a mesoporous TiO 2 supported on a transparent conducting glass, dye molecules, such as ruthenium bipyridyl derivatives which are sensitive to visible light in the solar spectrum, and an organic liquid electrolyte, essentially containing iodide and triiodide ions as a redox couple. These three layers are sandwiched together between two conducting glasses, one covered with a thin buffer layer of TiO 2 and the other with a platinum layer. However, presence of traditional organic liquid electrolytes in such devices is associated with problems such as hermetic sealing of the cell, precipitation of salts in the electrolyte at low temperature, evaporation of liquid electrolyte at high temperature, corrosion and lack of long-term stability of the cells. Manufacturing of multi-cell modules is also a difficult task with liquid electrolyte cells for obvious reasons, especially due to the corrosive nature of iodine, one of the components of a liquid electrolyte cell. Therefore, solidification and quasi-solidification of DSSCs have been intensely studied with various approaches, such as using of p-type inorganic semiconductors, 1 3 organic hole conducting materials, 4 7 ionic gel electrolytes having a polymer or a gelator, 8 11 and of ionic liquid (IL) electrolytes containing dispersed nano-components In these cases, imperfect filling of the dye-coated porous TiO 2 film by p-type inorganic semiconductors or polymers has resulted in poor efficiency for the cells. Moreover, the carrier diffusion length was limited in the case of conducting polymers due to their low conductivity. a Department of Chemical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei, 10617, Taiwan. kcho@ntu.edu.tw; Fax: ; Tel: b Institute of Polymer Science and Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei, 10617, Taiwan Although IL electrolytes solidified with nano-components can reduce leakages, it is not satisfactory, because they still contain some volatile components, e.g., 4-tert-butylpyridine, TBP and the sublimate, iodine. This study presents the fabrication of high efficient quasi-solid state DSSCs with the composite electrolytes containing carbon black (CB) or polyaniline-loaded carbon black (PACB) and 1-buty-3-methylimidazolium iodide (BMII) or 1-methyl- 3-propyl imidazolium iodide (PMII), without the addition of iodine. Wang et al. have reported a solid-state composite electrolyte devoid of iodine for a DSSC and achieved a light-toelectricity efficiency of 5.4%, their mention about the stability of this cell was that it retained 70% of its initial efficiency after one month operation at room temperature. 18 Besides, a few reports have been made on DSSCs without iodine in their preparation; these most cells showed efficiencies less than 2.4% and the authors have not established the stabilities for these cells. Among these reports, a clay-like conductive composite which contained only PACB particles and an ethyleneoxide-substituted imidazolium iodide was used as composite electrolyte; the corresponding quasi solid-state DSSC showed a cell efficiency of 3.48% at 100 mw cm 2 irradiation. 22 With the background of these reports on iodine-free DSSCs, we report here about the iodine-free quasi solid-state DSSC, containing a conducting polymer (CP), an ionic liquid, 1-methyl- 3-propyl imidazolium iodide (PMII), and polyaniline-loaded carbon black (PACB), which exhibited a solar-to-electricity efficiency (h) of 5.81%, the highest ever reported for a quasi solidstate DSSC. Avoidance of iodine was intended to understand distinctively the function of CB or PACB in a quasi solid-state DSSC and also to maintain the electrolyte to be as solid as possible; the electrolyte in our study is a near-solid. Iodine-free electrolyte is desirable for flexible DSSCs where a metal 2356 J. Mater. 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2 substrate, such as titanium is prone to corrosion by iodine. The research also assumes importance due to its unfailing stability in its preliminary observations, because stability is one of the two essential criteria for a good DSSC, other being its solar-to-electricity efficiency (h). 2. Experimental Iodine (I 2 ), poly(ethylene glycol) (PEG, M.W. 20,000), 1-methyl- 3-propyl imidazolium iodide (PMII), and 1-buty-3-methylimidazolium iodide (BMII) were obtained from Merk; tert-butyl alcohol (tba, 96%) and 4-tert- butylpyridine (TBP, 99.5%) were purchased from Acros and Fluka, respectively; titanium(iv) isopropoxide (TTIP, 98%), acetonitrile (ACN, 99.99%), acetylacetone (AA, 99.5%), ethanol (99.5%), and isopropyl alcohol (IPA, 99.5%) were obtained from Aldrich. The polyaniline-loaded carbon black (PACB), which was supplied from Sigma-Aldrich Inc., contained 20 wt% of polyaniline emeraldine salt (half oxidized and hydrogenated polyaniline doped with organic sulfonic acid). The composite electrolyte was prepared by mixing a solid powder of PACB or carbon black (CB, Alfa Aesar, 99.9%, ca. 42 nm) and one of the ILs mentioned above in a weight ratio of 1 : 8. Carbon black, owing to its larger size than the pore size of TiO 2 film (about nm), is not expected to infilltrate into the film. At the same time, ACN was added to the composite to improve the mixing condition, and the contents were moved to a vacuum oven to obtain a well-mixed composite electrolyte. The thermal stability of each component in the composite electrolyte was measured by a thermogravimetric analyzer (TGA, TGA-7, Perkin-Elmer). As the organic solvent electrolyte, a mixture of 0.1 M LiI, 0.6 M PMII, 0.05 M I 2, and 0.5 M TBP in g-butyrolactone (GBL, Fluka) was used. The commercial titanium dioxide (ST-21, 50 m 2 g 1, 6 g, Ya Chung Industrial Co. Ltd., Taiwan) was thoroughly mixed with a solution of AA (500 ml) in DI-water (11 g). This was stirred for 3 days and 1.8 g of PEG was then added to the well-dispersed colloid solution. The final mixture was stirred for additional 2 days, and the TiO 2 paste was prepared. A fluorine-doped SnO 2 conducting glass (FTO, 15 U sq 1, Solaronix S.A., Aubonne, Switzerland) was first cleaned with a neutral cleaner, and then washed with DI-water, acetone, and IPA, sequentially. The conducting surface of the FTO was treated with a solution of TTIP (0.084 g) in ethanol (10 ml) for obtaining a good mechanical contact between the conducting glass and TiO 2 film. A 10 mm-thick film of TiO 2 was coated by doctor blade method onto the treated conducting glass and a portion of cm 2 was selected as the active area by removing the side portions by scraping. The TiO 2 film was gradually heated to 500 C in an oxygen atmosphere, and subsequently sintered at that temperature for 30 min. After sintering at 500 C and cooling to 80 C, the TiO 2 electrode was immersed in a M solution of N719 (Solaronix S.A., Aubonne, Switzerland) in ACN and tba (in the volume ratio of 1 : 1) at room temperature for 24 h. After dye-adsorption, a 25 mm-thick Surlyn (SX , Solaronix S.A., Aubonne, Switzerland) spacer was put on the dye-sensitized TiO 2 electrode and attached by heating. The composite electrolyte was then coated onto the dye-sensitized TiO 2 film at 80 C to ensure that the IL can penetrate well into the porous structure. The TiO 2 electrode with the composite electrolyte was assembled with a platinum-sputtered conducting glass electrode (ITO, 7 U sq 1, Ritek), and the edges were sealed by UV glue (Optocast K GEN2, Alexander Jewels Co., LTD.). The surface of the DSSC was illuminated by a class A quality solar simulator (PEC-L11, AM1.5G, Peccell Technologies, Inc., Japan). The incident light intensity (100 mw cm 2 ) was calibrated with a standard Si Cell (PECSI01, Peccell Technologies, Inc.). The photoelectrochemical characteristics of the DSSC were recorded with a potentiostat/galvanostat (PGSTAT 30, Autolab, Eco-Chemie, the Netherlands). The thickness of TiO 2 film was determined using a surface profilometer (Sloan Dektak 3030). EIS data were obtained by the above-mentioned potentiostat/galvanostat equipped with an FRA2 module under a constant light illumination of 100 mw cm 2. The frequency range explored was 10 mhz to 65 khz. The applied bias voltage and ac amplitude were set at the open-circuit voltage of the DSSCs and 10 mv, respectively, between the counter electrode and the FTO-TiO 2 -dye working electrode, starting from the short-circuit condition. The impedance spectra were analyzed by an equivalent circuit model. 23,24 3. Results and discussion An IL usually has favorable properties from the viewpoint of a DSSC, such as negligible vapor pressure, high thermal stability, wide electrochemical potential window, and high ionic conductivity Moreover, it was reported that the addition of IL improves the physical properties of conducting polymers. 30,31 Considering these aspects of ILs, two types of incombustible and non-volatile composite electrolytes containing PACB or CB and an IL (BMII or PMII) were incorporated into the DSSCs in the present research (Scheme 1). Our intention was that this IL would allow perfect contact between the surfaces of the dyecoated porous TiO 2 and the carbon material, i.e., PACB or CB, 16 and would function as a carrier mediator. The photograph of the composite electrolyte is shown as an inset of Fig. 1. The appearance indicates the near-solid-state of the electrolyte. Fig. 1 shows the thermogravimetric (TGA) traces for each component Scheme 1 Scheme showing charge-transport process in a DSSC with a quasi solid state composite electrolyte containing an IL and PACB. This journal is ª The Royal Society of Chemistry 2010 J. Mater. Chem., 2010, 20,

3 Table 2 Photovoltaic parameters of the DSSCs with different composite electrolytes and with bare IL electrolytes, measured at 100 mwcm 2 light intensity Electrolyte V OC /mv J SC /ma cm 2 h (%) FF Part A (Bare ILs) BMII PMII Part B (ILs/CB composite electrolytes) BMII/CB PMII/CB Part C (PMII/PACB composite electrolyte) PMII/PACB Fig. 1 Thermogravimetric traces of all components in the composite electrolyte. Inset shows the photograph of the composite electrolyte with PACB. of the composite electrolytes used in this study. All the traces indicate that there is no weight loss of any component for temperatures at least up to 250 C. This type of high thermal stability is beneficial for fabricating a durable DSSC. This fact will be proved in further discussions dealing with the stability of the DSSC at 70 C. We emphasize here that the thermal stability is not possible, if iodine is a component of the electrolyte, especially under the simple fabrication technique of a DSSC as used in this study. Two ILs, BMII and PMII with different viscosities were used as charge carrier mediators of the DSSCs. The presence of these redox-active ionic liquids is required, because they fill the mesoscopic interior of dye-adsorbed TiO 2, where no space is available for CB/PACB to occupy. The ILs were separately blended with CB to form slurry-like composite electrolytes. Table 1 shows the resistances of the two ILs and of the composites, PMII/CB and PMII/PACB, as determined by impedance measurements. It is very interesting to note that the composites PMII/CB and PMII/PACB show far smaller resistances than their corresponding ILs. It can also be noticed from the table that the less the viscosity, the less is the resistance. 15,32 The photocurrent-voltage (I V) characteristics of the DSSCs using different combinations of composite electrolytes and using bare ILs are shown in Table 2. Table 2 (Part B) shows that the cell efficiency of PMII/CB device is 4.38% (J SC : 8.89 ma cm 2, V OC : 726 mv, FF: 0.68), which is higher than that of BMII/CB Table 1 Viscosities of pure ILs (from references), and resistances of the ILs in combination with CB or PACB, determined by impedance measurements Electrolyte Viscosity (cp) Resistance (U) c BMII 1100 a PMII 880 b PMII/CB 9.25 PMII/PACB a From Ref. 25. b From Ref. 26. c A device with two platinum electrodes and a 60mm thick spacer with a 5.5 mm diameter hole was used. Fig. 2 Electrochemical impedance spectra of the DSSCs with PMII/CB and BMII/CB electrolytes, measured at 100 mw cm 2 light intensity under open-circuit voltage. Inset shows the equivalent circuit. device with an efficiency of 3.68% (J SC : 8.04 ma cm 2, V OC : 724 mv, FF: 0.63). Fig. 2 shows electrochemical impedance spectroscopy (EIS) data of the DSSCs with PMII/CB and BMII/CB composite electrolytes. The equivalent circuit is illustrated in the inset of the figure. In general, the EIS spectra of an FTO/TiO 2 /dye/ composite electrolyte/pt/ito device show three semicircles in the frequency range of 10 mhz to 65 khz. The ohmic serial resistance (R s ) corresponds to the overall series resistance. The first and second semicircles correspond to the charge-transfer resistances at the counter electrode (R ct1 ) and the TiO 2 /dye/carrier mediator (R ct2 ), respectively. The third circle represents the Warburg diffusion process (R diff ) of I /I 3 in the electrolyte. However, in our work the conventional diffusion resistance of the redox couple is apparently greatly retarded due to a shorter length for I and I 3 ions diffusion caused by the carbon material as the extended electron transfer material (EETM) (Scheme 1), and this is perceived as the reason for the absence of a distinct third semicircle (careful observation indicates an overlap of the third circle with second, which can be noticed at about 63 U on the Z 0 scale especially in the case of the larger second circle). As shown in Fig. 2, PMII/CB cell has smaller R ct2 than BMII/CB cell, because PMII has lower viscosity than BMII (Table 1). Unlike the platinum layer in a conventional DSSC, the carbon material in this study is perceived as extended well up to the TiO 2 surface from the counter electrode J. Mater. Chem., 2010, 20, This journal is ª The Royal Society of Chemistry 2010

4 Table 3 Photovoltaic performance of the DSSCs with PMII/CB composite electrolyte, containing various amounts of I 2 /wt%, measured at 100 mw cm 2 light intensity Iodine/wt (%) V OC /mv J SC /ma cm 2 h (%) FF Fig. 3 Electrochemical impedance spectra of the DSSCs with pure IL electrolytes measured at 100 mw cm 2 light intensity under open-circuit voltage. In order to study the role of CB, ILs/CB composite electrolytes were replaced with the corresponding two bare ILs, i.e., with BMII and PMII. It can be seen in Table 2 (Part A) that the cell efficiencies in both these cases are much less than 1%, due to significant decrease in both J SC and FF. This is further confirmed through the EIS analysis, shown in Fig. 3. The Warburg diffusion resistances (R diff ) for I and I 3 ions and the resistances of the charge-transfer at the counter electrodes (R ct1 ) have increased by about 2.5 and 5 times, respectively, as compared to the corresponding values from Fig. 2. Absence of iodine or carbon materials is obviously the reason for the great resistances in these cases. From these observations it is more clear that the presence of CB as the extended electron transfer material (EETM) facilitates electron transfer from counter electrode to I 3 ions; 16 a shorter length for I 3 ions and thereby for I ions is created which enables the redox couple to work more efficiently than they would in the absence of the carbon material. It is well-known that added iodine in the electrolyte gives rise to iodides in the form of polyiodides such as I 3 or I 5 (eqn (1)). An efficient transport of iodide and triiodide in the electrolyte is necessary for good performance of the conventional DSSC, because the oxidized state of the dye (dye + ) should be regenerated by I ions efficiently after the electrons from the excited state of the dye are injected into the conduction band of TiO 2 under illumination (eqn (2)). Moreover, the electrons accumulated at the counter electrode by the external circuit will lead to concentration overpotential for the electrolyte at the counter electrode and to loss of energy of the DSSC, if the electrons are not transferred to I 3 efficiently (eqn (3)). The reactions are as follows: electrolyte. From these observations it has become clear that the carbon material functions as a charge transfer mediator. In fact, we found that the incorporation of I 2 is not necessary, 22 and even detrimental in our case as can be seen in Table 3. Table 3 shows the photovoltaic performance of the DSSCs with PMII/CB composite electrolyte, containing various amounts of I 2 (wt%). These results demonstrate that the cell works best with an iodinefree composite electrolyte. We suggest that the iodide anion based IL can provide sufficient I for the regeneration of the Fig. 4 Dark currents of the DSSCs with PMII/CB composite electrolytes containing various amounts of I 2 (wt%). I þ I 2 4 I 3 ƒƒ!ƒƒ I 2 I5 ðin the electrolyteþ (1) 3I + 2dye + I 3 + 2dye (at dye-sensitized TiO 2 /carrier mediator) (2) I 3 +2e 3I (at EETM/carrier mediator) (3) We emphasize that the cell in our study works at high efficiency without the addition of iodine to the composite Fig. 5 Photocurrent density-voltage characteristics of the DSSCs with PMII/CB and PMII/PACB composite electrolytes, measured at 100 mw cm 2 light intensity. The inset shows the electrochemical impedance spectra of the corresponding DSSCs, measured at 100 mw cm 2 light intensity under open-circuit voltage. This journal is ª The Royal Society of Chemistry 2010 J. Mater. Chem., 2010, 20,

5 oxidized dye under illumination (eqn (2)); I in turn oxidizes to I 3, 30 which can be reduced back to I at the EETM (eqn (3)). Increasing content of I 2 can increase concentration of polyiodides in the porous dye-coated TiO 2 matrix. It can facilitate recombination of injected conduction band electrons with polyiodides, and increase the dark current as shown in Fig. 4. Furthermore, increasing content of I 2 also leads to enhanced light absorption even in the visible range by the carrier mediator existing in the porous dye-coated TiO 2 matrix. This decreases the light-harvesting of dye molecules. 19 Therefore, both V OC and J SC show decreases with the increases in the wt% of I 2. When CB was replaced with PACB, the highest cell efficiency of 5.81% was obtained. The results are shown in Fig. 5 and the corresponding parameters are listed in Table 2 (Part C). A smaller interfacial resistance was obtained for the DSSC with PMII/PACB composite electrolyte than that for the cell with Table 4 Photovoltaic performance of the DSSCs with PMII/PACB and PMII/CB composite electrolytes using bare ITO as counter electrode, measured at 100 mw cm 2 light intensity Electrolyte V OC /mv J SC /ma cm 2 h (%) FF PMII/PACB PMII/CB PMII/CB, as shown in the inset of Fig. 5. According to a recent report, polyaniline shows high reduction current for the reduction of I Based on this report, it is inferred that more chargetransfer paths were formed between PMII and the polymer chains of PACB than those between the PMII and bare CB. This can be the reason for the better performance of the cell wilth PACB than that with CB. Interestingly, the devices using bare ITO as counter electrode still have cell efficiencies of 3.65% and 2.67% for PMII/PACB and PMII/CB, respectively (Table 4). It is to be noted here that the maximum efficiency for a DSSC in absence of a platinum layer on the counter electrode was reported to be 0.12%, while it was as high as 4.90% with just 2 nm-thickness of platinum layer and no more than 5.18% with a platinum layer of 415 nm. 34 These statistics indicate the importance of platinum layer for a DSSC. The significance of these carbon materials (PACB or CB) as catalysts in the form of EETM (Scheme 1) for the reduction of I 3 ions to I ions at the counter electrode was univocally established from the fact that an efficiency as high as 3.65% could be achieved for our DSSC using bare ITO glass as the counter electrode. Thus it is established through the above observations that the carbon material in the composite electrolyte serves simultaneously as a charge transporter in the electrolyte and as a catalyst for electrochemical reduction of I 3 ions. As expected, the quasi solid-state DSSCs showed excellent durability compared with that of the cell with organic liquid electrolyte. Fig. 6 (a) shows the at-rest durability data of the DSSCs with PMII/PACB composite electrolyte and organic liquid electrolyte. In this experiment, all cells were sealed by Surlyn and UV glue, simultaneously. The cell efficiency was measured once per day after storing it in the dark at room temperature. Efficiencies were normalized to the average value of the preceding five days, because we had noticed that the cell efficiency reaches a stable value at room temperature in about 5 days. Although the overall power conversion efficiency of the quasi solid-state DSSC has increased by about 2.6% after more than 1000 h, the overall power conversion efficiency of the DSSC with organic liquid electrolyte has decreased by about 30%. Further, we assembled two fresh DSSCs, one with PMII/PACB composite electrolyte and the other with an organic liquid electrolyte, stored them in an oven at 70 C, and then measured their cell efficiencies at different periods of time. Efficiencies were normalized to those of the first day, because according to our experience the efficiencies usually reach stable values at 70 C within 1 day. As shown in Fig. 6 (b), the DSSC with the PMII/ PACB-composite electrolyte shows an extraordinary durability even through it was stored at the temperature of 70 C. On the contrary, the cell with organic liquid electrolyte lost its efficiency virtually in no time, notwithstanding the fact that the boiling point of the organic solvent, GBL is 204 C. Thus, these results proved the unfailing stability of the cell under our investigation for 1000 h both at room temperature and at 70 C. Conclusions Fig. 6 (a) At-rest durability data of the DSSCs (at room temperature) with PMII/PACB composite electrolyte and with an organic liquid electrolyte, (b) same as (a) at 70 C. Efficient quasi-solid state DSSC was developed using a PMII/ PACB composite electrolyte, without the addition of iodine to it. Each of the components of the composite electrolytes used in this study, i.e., PMII, BMII, CB, and PACB are found to be stable up 2360 J. Mater. Chem., 2010, 20, This journal is ª The Royal Society of Chemistry 2010

6 to the temperature of about 250 C. It is noted that the composite electrolytes PMII/CB and PMII/PACB show far lesser resistances than the bare PMII, which is also reflected in the photovoltaic performances of their DSSCs. The DSSCs with CB or PACB in their electrolytes showed efficiencies that are one order higher than those obtained with the corresponding bare ILs, i.e., in absence of CB or PACB. A high efficiency (5.81%) quasi solidstate DSSC was achieved with a composite electrolyte containing PMII (1-methyl-3-propylimidazolium iodine) and polyanilineloaded CB (PACB) under AM1.5 full sunlight, which is the highest ever reported efficiency for a DSSC without added iodine in the electrolyte. It is established that the carbon material in the composite electrolyte serves simultaneously as a charge transporter in the electrolyte and as the catalyst for electrochemical reduction of I 3. It was also verified that addition of even a small quantity of iodine (0.3 wt%) to the type of electrolyte used in this study has rather a detrimental effect on all the photovoltaic parameters of the DSSC. Finally, the stability of the DSSC with the PMII/PACB-composite electrolyte is proved to be unfailing and far superior to that with an organic solvent-based electrolyte, tested at-rest both at room temperature and at 70 C. Acknowledgements This work was supported in part by the National Taiwan University and the National Research Council of Taiwan. Some of the instruments used in this study were made available through the financial support of the Academia Sinica, Taipei, Taiwan, under grant AS-97-TP-A08. References 1 G. R. A. Kumara, A. Konno, K. Shiratshchi, J. Tsukahara and K. Tennakone, Chem. Mater., 2002, 14, V. P. S. P. Perera, K. D. D. P. Pitigala, P. V. V. Jayaweera, K. M. P. Bandaranayake and K. Tennakone, J. Phys. Chem. B, 2003, 107, K. Tennakone, G. R. R. A. Kumara, I. R. M. Kottegoda, K. G. U. Wijayantha and V. P. S. Perera, J. Phys. D: Appl. Phys., 1998, 31, U. Bach, D. Lupo, P. Comte, J. E. Moser, F. Weiss ortel, J. Salbeck, H. Spreizer and M. Gr atzel, Nature, 1998, 395, J. Kr uger, R. Plass, L. Cevey, M. Piccirelli and M. Gr atzel, Appl. Phys. Lett., 2001, 79, J.Kr uger, R. Plass and M. Gr atzel, Appl. Phys. Lett., 2002, 81, K. Peter, H. Wietasch, B. Peng and M. Thelakkat, Appl. Phys. A: Mater. Sci. Process., 2004, 79, J. Wu, S. Hao, Z. Lan, J. Lin, M. Huang, Y. Huang, L. Fang, S. Yin and T. Sato, Adv. Funct. Mater., 2007, 17, Y. Ying, C. H. Zhou, S. Xu, H. Hu, B. L. Chen, J. Zhang, S. J. Wu, W. Liu and X. Z. Zhao, J. Power Sources, 2008, 185, W. Kubo, K. Murakoshi, T. Kitamura, S. Yoshida, M. Haruki, K. Hanabusa, H. Shirai, Y. Wada and S. Yanagida, J. Phys. Chem. B, 2001, 105, W. Kubo, T. Kitamura, K. Hanabusa, Y. Wada and S. Yanagida, Chem. Commun., 2002, P. Wang, S. M. Zakeeruddin, P. Comte, I. Exnar and M. Gr atzel, J. Am. Chem. Soc., 2003, 125, H. Usui, H. Matsui, N. Tanabe and S. Yanagida, J. Photochem. Photobiol., A, 2004, 164, T. Katakabe, R. Kawano and M. Watanabe, Electrochem. Solid-State Lett., 2007, 10, F K. M. Lee, P. Y. Chen, C. P. Lee and K. C. Ho, J. Power Sources, 2009, 190, C. P. Lee, K. M. Lee, P. Y. Chen and K. C. Ho, Sol. Energy Mater. Sol. Cells, 2009, 93, Z. Chen, H. Yang, X. Li, F. Li, T. Yi and C. Huang, J. Mater. Chem., 2007, 17, H. Wang, H. Lee, B. Xue, Z. Wang, Q. Meng and L. Chen, J. Am. Chem. Soc., 2005, 127, H. Wang, X. Liu, Z. Wang, H. Li, D. Li, Q. Meng and L. Chen, J. Phys. Chem. B, 2006, 110, N. Ikeda and T. Miyasaka, Chem. Commun., 2005, N. Ikeda and T. Miyasaka, Chem. Lett., 2007, 36, N. Ikeda, K. Teshima and T. Miyasaka, Chem. Commun., 2006, L. Han, N. Koide, Y. Chiba and T. Mitate, Appl. Phys. Lett., 2004, 84, L. Han, N. Koide, Y. Chiba, A. Islam and T. Mitate, C.R. Chimie., 2006, 9, J. M. Pringle, J. Golding, C. M. Forsyth, G. B. Deacon, M. Forsyth and D. R. MacFarlane, J. Mater. Chem., 2002, 12, C. P. Fredlake, J. M. Crosthwaite, D. G. Hert, S. N. V. K. Aki and J. F. Brennecke, J. Chem. Eng. Data, 2004, 49, D. R. MacFarlane, S. A. Forsyth, J. Golding and G. B. Deacon, Green Chem., 2002, 4, J. G. Huddleston, A. E. Visser, W. M. Reichert, H. D. Willauer, G. A. Broker and R. D. Rogers, Green Chem., 2001, 3, P. Wang, S. M. Zakeeruddin, J. E. Moser, R. H. Baker and M. Gr atzel, J. Am. Chem. Soc., 2004, 126, W. Lu, A. G. Fadeev, B. Qi, E. Smela, B. R. Mattes, B. R. J. Ding, G. M. Spinks, J. Mazurkiewicz, J. D. Zhou, G. G. Wallace, D. R. MacFarlane, S. A. Forsyth and M. Forsyth, Science, 2002, 297, G. K. R. Senadeera, T. Kitamura, Y. Wada and S. Yanagida, Sol. Energy Mater. Sol. Cells, 2005, 88, D. Kuang, C. Klein, Z. Zhang, S. Ito, J.-E. Moser, S. M. Zakeeruddin and M. Gr atzel, Small, 2007, 3, Z. Li, B. Ye, X. Hu, X. Ma, X. Zhang and Y. Deng, Electrochem. Commun., 2009, 11, X. Fang, T. Ma, G. Guan, M. Akiyama, T. Kida and E. Abe, J. Electroanal. Chem., 2004, 570, 257. This journal is ª The Royal Society of Chemistry 2010 J. Mater. Chem., 2010, 20,