Charge recombination and transport in dye sensitised TiO2 photovoltaic devices

Dye-sensitised solar cells are an important new class of photovoltaic device in which charge separation occurs at the junction between a porous, nanocrystalline metal oxide electrode and a hole conducting electrolyte. Material properties are believed to influence the rates of charge transfer and transport and hence device function. We have used electrical, electrochemical and transient absorption to study the influence of material properties on charge transport and recombination in nanocrystalline TiO₂ electrodes. We have established: (i) that the slow back recombination reaction which is responsible for the efficient charge separation in DSSC results from the trapping of photo-injected electrons; (ii) that charge recombination kinetics are extremely sensitive to the density of trapped electrons; and (iii) that observed behaviour can be explained in terms of a model of electron diffusion within an energetic distribution of trap states. We confirm our model by studying the effect of variations in material and chemical environment. We apply our model to cell performance at open circuit