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 (η sep ) from IPCE. η 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 TiO 2, and sensitizer regeneration by iodide can no longer effectively compete with electron transfer from the TiO 2 to the oxidized sensitizer.