Well-controllable Fabrication of Aligned ZnO Nanorods for Dye-sensitized Solar Cell Application
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Well-controllable Fabrication of Aligned ZnO Nanorods for Dye-sensitized Solar Cell Application Chaoyang Li Shengwen Hou 1 Center for Nanotechnology, Research Institute& School of System Engineering, Kochi University of Technology, 185 Miyanokuchi Tosayamada cho Kami, Kochi 782－8502, Japan. ABSTRACT ZnO nanorods were synthesized by recrystallization of ZnO thin films during multiannealing process. It was found that the obtained ZnO nanorods showed well-controlled grown direction. The periodical oxygen introducing between reducing annealing processes was effective to help on the oxidization reaction, result in the ZnO nanorods growth significantly. With controlling the annealing parameters, the morphologies of ZnO nanorods could be also controlled. The low-temperature (less than 420°C) initial reducing annealing process contributed to control the density of ZnO nanorods. The multi-annealing processes could reduce the ZnO thin film to produce ZnO nanorods efficiently. The structural, optical and electrical properties of the ZnO nanorods were investigated. Finally, the obtained ZnO nanorods used as photoelectrodes demonstrated in a dye-sensitized solar cell, the overall conversion efficiency of 3.65% was achieved. INTRODUCTION Zinc oxide (ZnO) is one of the most promising II-VI compound semiconductors for optoelectronic devices. Because its unique properties such as: a direct wide band gap of 3.37 eV, a large exciton binding energy of 60 meV at room temperature, high mechanical and thermal stabilities, etc [1]. Recent decade, 1D ZnO nanostructures including nanorods [2], nanowires [3], nanotubes [4], etc. are considerable interest for many research groups due to their importance in fundamental physics studies and potential applications in electronic and optoelectronic fields such as LED, nano-laser, solar cell, gas sensors, and flat panel displays [5-12]. Recent decade, ZnO nanostructures were expected to be used as photoelectrodes in dye-sensitized solar cell (DSSC) application [13-15]. Since the invention by O’Regan and Grätzel in 1991, DSSCs have been intensively studied and are currently undergoing rapid development for practical use [16]. To date, The DSSCs with the highest energy conversion efficiency of 15% was reported using the TiO2/SnO2 composites as the anode material combining with the black dye N719 [17] from Gratzel’s group. It is wellknown that the TiO2 nanocrystalline porous films is currently for commercial application, however, the performance is not good enough because the porous TiO2 film was not so high efficiency on the absorption of dye molecules. Most of the film surface was ineffective. Moreover, TiO2 is not easy to be textured or nanostructured, therefore the critical issue is how to increase the surface area to enhance the dye molecule absorption efficiency. This issue becomes a bottleneck for development of DSSCs. Comparing to traditional DSSC photoelectrode material TiO2, ZnO has not only similar wide band gap energy of 3.37eV, but also much higher electron mobility (up to 1000 cm2 V/s) [18
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