Diffusion-limited recombination in dye-sensitized TiO2 solar cells

The effect of doping on the electron transport dynamics and recombination kinetics in dyesensitized solar cells was investigated. A simple electrochemical method was developed to dope TiO₂ nanoparticle films with Li. Increasing the doping levels is found to slow electron diffusion. The electron diffusion time exhibits a light intensity dependence at all doping levels consistent with a multiple electron-trapping model involving native and doping-induced traps. Importantly, the diffusion time and recombination lifetime of photocarriers are observed to increase in unison with increased doping. This is the first observation that electron diffusion limits recombination with the redox electrolyte under normal working conditions of the dye cell. A model is presented that accounts for the observation. The implications of this mechanism on cell performance are also discussed.