J. Mater. Sci. Technol., 2012, 28(8), 713–722.
Solution Combustion Synthesis of TiO2 and Its Use for Fabrication of Photoelectrode for Dye-sensitized Solar Cell Shyan-Lung Chung1,2)† and Ching-Mei Wang1) 1) Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan, China 2) Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 70101, Taiwan, China [Manuscript received November 28, 2011, in revised form July 4, 2012]
Three different types of TiO2 nano powders were synthesized by a solution combustion synthesis (SCS) method using three different fuels and for comparison, another type of TiO2 nano powder was synthesized by calcination of titanyl hydroxide. These TiO2 nano powders were used to fabricate photoelectrodes for the dye-sensitized solar cell (DSSC) and their performance was compared to that of the DSSC fabricated with Degussa P25 TiO2 . The results showed that the SCS TiO2 could work well as photoelectrode for DSSC. The SCS TiO2 contained impurities of C and/or S, thus exhibiting visible light absorption and reduced band gap. The open circuit voltage and the fill factor both varied little among the various TiO2 and thus both had little effect on the photoelectrical conversion efficiency (η). However, the variation of η was seen to be in quite a good agreement with that of the short circuit current (Isc ), suggesting that η was dominated by Isc . Isc was found to be enhanced by light scattering effect due to the presence of large particles but reduced by high impurity content due to an increase in electron transfer resistance. In addition, the specific surface area of the powders was found to be an important factor affecting the Isc and thus the η. KEY WORDS: TiO2 ; Combustion synthesis; Dye-sensitized solar cells
1. Introduction Among several different types of solar cells, dyesensitized solar cell (DSSC) is one which has been considered to be of high potential applications due to its relatively simple fabrication process, relatively low production cost and fairly high photoelectrical conversion efficiency[1] . A DSSC is composed of three major parts: a photoelectrode which absorbs incident light, generates free electrons and transfers the electrons to the external circuit; a counter electrode which conducts the electrons from the external circuit back to the electrolyte of the cell; and the electrolyte which carries the electrons back to the photoelectrode to complete the electron circulation. In the photoelectrode of a typical type of DSSC[2] , dye molecules are adsorbed on a TiO2 thin film which † Corresponding author. Prof.; Tel.: +886 6 275 7575, Ext. 62654; Fax: +886 6 234 4496; E-mail address:
[email protected] (S.L. Chung).
is attached on a piece of transparent conducting oxide (TCO) glass. Free electrons are generated by the dye molecules upon absorption of incident light. The free electrons thus generated are quickly injected to the TiO2 film, and then transferred by the TiO2 film to the TCO glass, which conducts the electrons to the external circuit. Since the TiO2 film is designed to adsorb the dye molecules and to transfer the electrons out to the external circuit (through the TCO glass), its structure and morphology have significant effects on the photoelectrical conversion efficiency. A definite requirement on the TiO2 film is that it must be highly porous and nano-structured to create a high surface area so that a large number of dye molecules can be adsorbed, which can then absorb a large fraction of incident light, generating a large number of free electrons. Some additional requirements and desirable features of the TiO2 film can be summarized as follows: (1) It must possess a sufficient number of path ways for electron transfer[3] . (2) The pores must be in
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an optimum size range . If the pores are too small (TiO2 (G)>TiO2 (T)>TiO2 (U). Considering the particle sizes and impurity contents of these films, one may conclude that a higher impurity content and a larger particle size seem to reduce the dye adsorption capability. Although the proportional relationship between Isc and SA among the films is not held, the variation trend of Isc is roughly in agreement with that of SA with the exception of TiO2 (P), i.e., for Isc : TiO2 (P)>TiO2 (W)>TiO2 (G)>TiO2 (T)>TiO2 (U). This order is seen to be in accord with that of the measured IPCE (see Fig. 9). In the case of TiO2 (P) based DSSC, it exhibits the highest Isc (and IPCE )
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although its SA is not largest. As mentioned previously, the reflectance intensity of TiO2 (P) film in the visible region is highest among the five films (see Fig. 8), suggesting that the high Isc of TiO2 (P) film may be due to the large rutile particles contained in the film, which create light scattering effect, thus increasing the light absorption efficiency. In the case of using TiO2 (U), although it contains an even larger fraction of coarser particles than TiO2 (P), it suffers from a consequently low SA and a small amount of dye adsorbed, thus resulting in a low Isc . In the cases of TiO2 (G) and TiO2 (T), although they have SA comparable to TiO2 (P), their Isc is much lower than that of TiO2 (P). In addition to the lack of light scattering effect, the high impurity contents of TiO2 (G) and TiO2 (T) may cause a reduction in Isc by increasing the electron transfer resistance (or by acting as electron scattering centers), and this effect is seen to be greater for TiO2 (T) because it contains a higher impurity content than TiO2 (G). In addition to Eg, Voc , SA and Isc , Table 3 also shows the photoelectrical conversion efficiency (η) and the fill factor (FF ) of the DSSC based on the various TiO2 . (Note that the scale of each factor in this figure was determined according to its extent of effect on η.) As can be seen in Table 3, the open circuit voltages of the DSSC fabricated with the SCS TiO2 were not much different from those fabricated with TiO2 (W) and TiO2 (P). The variation of the Voc among these different TiO2 thus has little effect on that of the η. The variation of the FF is also seen to be small and thus has little effect on that of η. However, η is seen to vary in a similar trend and with a similar extent to that of Isc , suggesting that η is dominated by Isc . As mentioned previously, the variation of Isc agrees roughly with that of the SA of the film (with the exception of TiO2 (P)), which is determined by that of the powders, which is concluded that in addition to light scattering effect due to large particles, the SA of the powder is an important factor affecting η. 4. Conclusion Three types of TiO2 powder have been synthesized by solution combustion synthesis method and one type of TiO2 powder has been synthesized by calcination of titanyl hydroxide. These TiO2 powders as well as Degussa P25 TiO2 have been used to fabricate the photoelectrode for DSSC. The SCS TiO2 powders contain impurities (i.e. carbon and/or sulfur), showing visible light absorption and reduced band gap. The Voc of the DSSC assembled using the photoelectrodes fabricated with all these TiO2 is found to have little effect on the photoelectrical conversion efficiency. The fill factor is also found to be not much different among the DSSC based on the various TiO2 , thus having little effect on η. η is found to be dominated by Isc , which is found to be enhanced by light scattering effect due to the presence of large particles
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but reduced by a high impurity content due to an increase in electron transfer resistance. In addition, the specific surface area of the powders is found to be an important factor affecting Isc and thus η.
Acknowledgements Support of this research by the National Science Council of Taiwan under Grant No. NSC 100-2221-E-006-214 and partial support under Contract No. 101-D0204-6 by the Bureau of Energy, Ministry of Economic Affairs of Taiwan and the LED Lighting Research Center, NCKU are gratefully acknowledged. The authors also thank Professor Chien-Hsin Yang (National University of Kaohsiung) for his assistance in fabrication of the photoelectrodes and assembling of the DSSC. REFERENCES [1 ] B. O0 regan and M. Gr¨ atzel: Nature, 1991, 353, 737. [2 ] M. Gr¨ atzel: Nature, 2001, 414, 338. [3 ] C.Y. Huang, Y.C. Hsu, J.G. Chen, V. Suryanarayanan, K.M. Lee and K.C. Ho: Sol. Energy Mater. Sol. Cells, 2006, 90, 2391. [4 ] J. van de Lagemaat, K.D. Benkstein and A.J. Frank: J. Phys. Chem. B, 2001, 105, 12433. [5 ] C.J. Barbe, F. Arendse, P. Comte, M. Jirousek, F. Lenzmann, V. Shklover and M. Gr¨ atzel: J. Am. Ceram. Soc., 1997, 80, 3157. [6 ] K. Zhu, N.R. Neale, A. Miedaner and A.J. Frank: Nano Lett., 2007, 7, 69. [7 ] Z.S. Wang, H. Kawauchi, T. Kashima and H. Arakawa: Coord. Chem. Rev., 2004, 248, 1381. [8 ] F. Huang, D. Chen, X.L. Zhang, R.A. Caruso and Y.B. Cheng: Adv. Funct. Mater., 2010, 20, 1301. [9 ] K.C. Patil, S.T. Aruna and T. Mimani: Curr. Opin. Solid State Mater. Sci., 2002, 6, 507. [10] J.C. Kuang, C.R. Zhang, X.G. Zhou and S.Q. Wang: J. Cryst. Growth, 2003, 256, 288. [11] N. Papageorgiou, W.F. Maier and M. Gr¨ atzel: J. Electrochem. Soc., 1997, 144, 876. [12] M. Gr¨ atzel: Prog. Photovolt. Res. Appl., 2000, 8, 171. [13] A. Du Pasquier, H.H. Chen and Y.C. Lu: Appl. Phys. Lett., 2006, 89, 253513. [14] R. Debnath and J. Chaudhuri: J. Mater. Res., 1992, 7, 3348. [15] C.R. Hubbard, E.H. Evans and D.K. Smith: J. Appl. Cryst., 1976, 9, 169. [16] U. Diebold: Surf. Sci. Rep., 2003, 48, 53. [17] R.D. Shannon and J.A. Pask: J. Am. Ceram. Soc., 1965, 48, 391.
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