Aerogel Templated ZnO Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) based on nanocrystalline TiO2 have exhibited solar energy-conversion efficiencies of over 10% and remain one of the most promising candidates for cost-effective solar energy conversion devices. The most efficient DSSCs reported to date utilize a high surface area photoanode, which allows for good light harvesting with a moderate extinction dye, in contact with the I3 /I couple which acts as a redox mediator. The good performance of DSSCs is partially attributed to the slow dark reaction, i.e., electron transfer from TiO2 to I3 via a potentially multielectron process. The slow dark reaction kinetics allows for excellent charge collection despite the relatively slow (millisecond) transport through the nanoparticle network. The I3 /I couple, however, has several disadvantages, including limitations on the open-circuit voltage related to the redox potential of the mediator species. In order to push device performance beyond its current limits, a faster redox shuttle, which requires a smaller overpotential to reduce the oxidized dye, is likely necessary, either to increase the photovoltage (provided that the dark current in not enhanced) or to allow alternate dyes with increased spectral coverage to be used. Replacing iodide/ triiodide with faster one-electron, outer-sphere redox reagents such as ferrocene, however, has resulted in significantly worse overall performance. Although such fast redox species efficiently reduce the oxidized dye, rapid recombination between electrons in the TiO2 and the redox shuttle results in poor electron collection. Therefore, an alternate redox shuttle will concomitantly require faster charge transport through the metal oxide frameworks to allow for complete charge collection under cell operating conditions. Several interesting photoanode architectures have been fabricated with reduced dimensionality, a design feature that is