Nanotech 2010

Hydrothermal Synthesis of Single Crystalline TiO2 Nanowire arrays on Arbitrary Substrate and its Application in Dye-Sensitized Solar Cells

A. Kumar
University of Southern California, US

Keywords: nanowires, dye solar cell, solid electrolyte


Photovoltaic devices have become one of the most important research directions in view of the ever increasing demand of energy. Among various technologies, excitonic solar cells including organic, hybrid organic-inorganic and dye sensitized cells (DSSCs) are the prime candidates for low-cost, highly-efficient solar cells. In DSSCs, upon exposure to light, excitons are created in the dye and due to the offset in the conduction band energies of dye and TiO2, photogenerated electron migrates to TiO2. These electrons, after traveling through the TiO2 layer, are subsequently collected at the anode. Recent efforts have been focused on replacing thin film of TiO2 with zero dimensional nanoparticles and one dimensional nanowires to improve upon the charge collection efficiencies. Because of several orders of magnitude difference in electron diffusion coefficient between the nanparticles and their single crystal bulk counterpart, nanowire arrays can significantly enhance the performance of dye soalr cell if used with optimized configuration. Towards this objective, use of TiO2 nanotubes and ZnO nanowires have been reported recently as a conduit for electrons. In this work, we report on the synthesis of vertical array of TiO2 nanowires directly on the transparent conductive electrode and its use in dye-sensitized solar cell. A mild hydrothermal synthesis protocol was developed to grow high quality TiO2 nanowires on virtually any substrate including FTO, ITO, Glass and Si (100). XRD and TEM analysis indicated that as synthesized nanowires are of rutile phase. Effect of various titanium precursors on the growth rate and nanowire morphology was studied. For increased growth time, a self-standing film of vertical TiO2 nanowires was obtained. Finally, vertical TiO2 nanowires on FTO were sensitized with standard dye molecules to obtain a solar cell with a power conversion efficiency of 3.0 %. Incorporation of a thin TiO2 barrier layer and TiCl4 treatment improved the efficiency. In order to make the DSSC stable for longer period of time, we have developed a vapor deposited solid state hole transport layer which can efficiently transport holes to the counter electrode, eliminating the need of using corrosive liquid electrolytes. Our preliminary data show extremely high fill factors suggesting elimination of recombination centers typically present in traditional dye solar cells.
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