Determination of Electron Diffusion Coefficient and Lifetime in Dye-Sensitized Solar Cells by Electrochemical Impedance Spectroscopy at High Fermi Level Conditions

The electron diffusion coefficient D-n and lifetime tau(n) of a dye-sensitized solar cell (DSC) are determined from the electrochemical impedance spectra at potentials beyond the open-circuit potential. At these potentials, the Warburg feature associated with electron diffusion is observable from the impedance spectra. The fitting result shows that D-n and tau(n) remain nearly constant within this potential range, and possible reasons for this phenomenon are given. The constant D-n and tau(n) are used approximately as their conduction band values in a steady-state model to yield a short-circuit electron Fermi level profile. It is shown that direct recombination through surface trap states must be considered to accurately interpret the short-circuit characteristics of DSCs.