Quantitative Analysis of Light-Harvesting Efficiency and Electron-Transfer Yield in Ruthenium-Dye-Sensitized Nanocrystalline TiO₂Solar Cells

We have conducted a quantitative analysis of light-harvesting efficiency and electron-transfer yield for solar cells employing ruthenium dye, (tetrabutylammonium) 2 cis -(2,2‘-bipyridyl-4-COOH, 4‘-COO-) 2 (NCS) 2 ruthenium(II), sensitized nanocrystalline TiO 2 films with a series of light-scattering magnitudes. The light-harvesting efficiency increases with the addition of relatively large particles to a transparent film, especially for near-infrared wavelengths. Excess addition, however, lowers the light-harvesting efficiency over the whole visible wavelengths owing to enhanced light reflection at the conducting glass/TiO 2 interface. Following a rigorous calculation using the results obtained from the light-harvesting efficiency and the short-circuit photocurrent measurements, we demonstrate for the first time that the electron-transfer yield markedly decreases with increasing optical thickness, that is, film light-scattering magnitude, by a maximum of ≈60%. The origins of the change in the electron-transfer yield can be complex, involving multiple excitation or many electron-transfer processes. The analytical results obtained in this study suggest that an appropriate light-scattering magnitude in the TiO 2 film originating from particle sizes, their distribution, and the film thickness is a key parameter in controlling the electron-transfer yield as well as the light-harvesting efficiency and thus the short-circuit photocurrent.