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Langmuir 2006, 22, 3936-3940
Morphology Evolution of ZnO Thin Films from Aqueous Solutions and Their Application to Solar Cells Yanfeng Gao* and Masayuki Nagai AdVanced Research Laboratories, Musashi Institute of Technology, Tokyo 158-0082, Japan ReceiVed NoVember 11, 2005. In Final Form: February 8, 2006 This paper reports a reproducible low-temperature solution-based process for the preparation of ZnO films of nanorod arrays and their application to dye-sensitized solar cells (DSSCs). A two-step approach was employed for the epitaxial growth of ZnO. We began with the preparation of a (002)-oriented ZnO seed layer by the electrochemical deposition method. After the treatment the substrate was soaked in an aqueous solution containing ZnCl2 and complex agents. A large-scale fabrication of ZnO nanorod arrays on transparent conductive oxides has been achieved after soaking at 95 °C for 1-48 h. The as-deposited ZnO film has a large surface area, therefore permitting a great amount of dye loading. The individually separated nanorod forms a linear nanoroad which should show more effective electron transportation than that in the film derived from ZnO powders. The DSSCs using these ZnO films as photoelectrodes show a conversion efficiency of about 0.6% at AM1.5.
Introduction Zinc oxide (ZnO) is one of the most attractive functional materials with unique optical and electrical properties. It is wellknown that these properties of ZnO are dependent partly on the crystallinity, crystallographic orientation, crystallite size, and morphology.1-5 Control over these properties through either chemical1-4 or physical routes5 has attracted increasing interest. Chemical solution processes including sol-gel, hydrothermal synthesis, supersaturated-solution deposition, and electrochemical deposition have shown powerful ability to control the crystallization engineering of ZnO. One-dimensional (1D) ZnO nanostructures such as nanowires and nanorods have been attractive recently because of specific optoelectronic and field-emission characteristics originating from their unique heterogeneous crystallographical structures.4-6 Films with well-aligned ZnO nanorods or nanowires may exhibit much larger surface areas than ZnO films prepared from randomly oriented nanoparticles. Moreover, these nanorods are packed very densely, enabling the fast and effective transport of electrons. On the basis of the understanding of the crystallographic mechanism, these nanostructures are usually prepared by controlling the nucleation and growth behaviors so as that growth in the longitudinal direction is more favorable than that in the transverse one. Vapor transportation and condensation processes have been widely used for the preparation of ZnO nanorod array.4,5 This is a process that needs to treat raw materials at high temperatures, sometimes being catalyzed by using noble metals.5 Various low-cost solution-based deposition processes were also developed.7-9 Among these methods, the most attractive process * To whom correspondence should be addressed. E-mail: yfgao@ sc.musashi-tech.ac.jp. (1) Zhang, J.; Sun, L.-D.; Yin, J.-L.; Su, H.-L.; Liao, C.-S.; Yan, C.-H. Chem. Mater. 2002, 14, 4172. (2) (a) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Mcdermott, M. J.; Rodriguez, M. A.; Konishi, H.; Xu, H. Nature Mater. 2003, 2, 821. (b) Andelman, T.; Gong, Y.; Polking, M.; Yin, M.; Kuskovsky, I.; Neumark, G.; O’Brien, S. J. Phys. Chem. 2005, 109, 14314. (c) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Mcdermott, M. J. J. Am. Chem. Soc. 2002, 124, 12954. (3) Liu, B.; Zeng, H.-C. J. Am. Chem. Soc. 2004, 126, 16744. (4) Wang, Z.-L. Mater. Today 2004, 7, 26. (5) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (6) Greene, L. E.; Law, M.; Goldberger, J.; Kim, F.; Johnson, J. C.; Zhang, Y. F.; Saykally, R. J.; Yang, P. D. Angew. Chem. Int. Ed. 2003, 42 (26), 3031.
is that which employs pre-placed ZnO nanoparticles as a nucleus layer to grow ZnO nanorod array from an aqueous solution containing Zn2+ and a precipitation agent.6,7 This method enables the synthesis of well-aligned ZnO nanorod films at relatively low temperatures (
