Intensity dependence of the electron diffusion length in dye-sensitised nanocrystalline TiO2 photovoltaic cells

The efficiency of dye-sensitised nanocrystalline solar cells is limited in part by the back reaction of photo-injected electrons with tri-iodide ions present in the electrolyte. Competition between this back reaction and the collection of electrons by diffusion to the substrate contact can be described in terms of the electron diffusion length L-n=(D(n)tau(n))(1/2), where D-n is the electron diffusion coefficient and tau(n) is the electron lifetime determined by the rate of reaction of electrons with tri-iodide. D-n and tau(n) have been determined over five orders of magnitude of illumination intensity using intensity-modulated photocurrent and intensity-modulated photovoltage spectroscopy. It has been found that tau(n) decreases with light intensity, whereas D-n increases, As a consequence, the electron diffusion coefficient L-n is only weakly intensity dependent, and the incident photon to current conversion efficiency (IPCE) is predicted to be almost independent of intensity. The experimental IPCE agrees well with the predicted values. The results suggest that the kinetics of the back reaction of electrons with tri-iodide couple may be second order in electron density. (C) 1999 Elsevier Science S.A. All rights reserved.