Hierarchically Porous TiO_2 Electrodes Fabricated by Dual Templating Methods for Dye-Sensitized Solar Cells

摘要

Dye-sensitized solar cells (DSSCs) are of great interest due to their projected cost-effectiveness, their relatively high photo electric conversion efficiency, and the unique advantages of transparent cells over conventional silicon photovoltaics.[1,2] In DSSCs, light is harvested by a TiO_2 layer. This layer performs three functions: it acts as a substrate on which the dye mole cules adsorb, it transfers the photogenerated electrons, and it serves as a diffusion pathway for ions in the electrolyte solu tion. The engineering of TiO_2 electrodes with regard to char acteristics, such as nanostructure, crystalline morphology, and surface properties, is therefore a crucial aspect in efforts to enhance the photoconversion efficiency. There have been many efforts to engineer the microstructure of TiO_2 electrodes. Engineered mesopores on the order of 10 nm, such as ordered pores and nanotubes, show faster transport of charge carriers than conventional nanocrystalline TiO_2 (nc-TiO_2) electrodes, in which random diffusion is the dominant mechanism.[3,4] In nanotube TiO_2 electrodes, the suppression of the recombina tion of photogenerated electrons was observed to be ten times higher than in conventional nanocrystalline TiO_2 (nc-TiO_2) electrodes, which resulted in an enhancement of the charge col lection efficiency.[4] The incorporation of macropores or parti cles that are several hundred nanometers in diameter induces Mie scattering of incident photons, thereby enhancing the light absorption, particularly at infrared wavelengths where the extinction coefficients of dyes are low. The introduction of these pores also improves the electrolyte diffusion efficiency in TiO_2 electrodes.[5,6] Moreover, control of the macropore morphology (i.e., the size of macroscale particles or pores, as well as their organization) can be advantageous for the application of solid state electrolytes with high viscosities and larger molecular volumes.[7]