Electron Recombination Kinetics and the Analysis of Collection Efficiency and Diffusion Length Measurements in Dye Sensitized Solar Cells

A significant difference exists between two measured estimates of the electron diffusion length in dye sensitized solar cells. One method is based on spectral response at short circuit (L-1) and the other on small perturbations (L-n). Following a recent theoretical treatment of the diffusion length for non first order recombination, one can propose an effective diffusion length at short circuit (lambda((n) over bar)). Using simulation and measured values we show that L-1 approximate to lambda((n) over bar) for a wide range of recombination orders, despite the first order assumption made when deriving L-1. We also show L-n approximate to lambda((n) over bar) when L-n is derived from a time dependent full cell model incorporating non first order recombination. Thus the deviation of experimentally measured L-n from simulated L-n and L-1 cannot be explained simply by the incorporation of nonlinear recombination. Our results give support to the spectral response method, but do not give a theoretical basis for rejecting the small perturbation method.