Reliable Determination of Electron Diffusion Length and Charge Separation Efficiency in Dye-Sensitized Solar Cells

Reliable quantification of parameters influencing the efficiency of dye-sensitized solar cells (DSCs) is essential to guide device optimization and improve the fundamental understanding of device operation. The small-perturbation electron diffusion length (L-n) in DSCs has been determined by electrochemical impedance spectroscopy and by analysis of incident photon-to-collected-electron conversion efficiency (IPCE) spectra. When measurement conditions are chosen so that recombination can be treated as first-order in the free electron concentration, L-n values obtained by the two quite different techniques are found to be in good agreement. This result provides an important experimental validation of the simple diffusion-recombination model that is commonly used to explain DSC operation. Knowledge of L-n facilitates deconvolution of the charge separation efficiency (eta(sep)) from IPCE. eta(sep) is found to decrease significantly with increasing V-OC for all DSCs studied here. This phenomenon is likely to be caused by a decrease in sensitizer regeneration efficiency as electrons accumulate in the TiO2, and sensitizer regeneration by iodide can no longer effectively compete with electron transfer from the TiO2 to the oxidized sensitizer.