Density of state determination of two types of intra-gap traps in dye-sensitized solar cells and its influence on device performance

Abstract

The density of state (DOS) of intra-gap traps and the dynamics of electron transport of a dye-sensitized TiO₂ solar cell were investigated by means of time-resolved charge extraction (TRCE). The intrinsic chemical capacitance of the TiO₂ layer was separated from the parasitic capacitance of the FTO electrode, and was found to be dependent biexponentially on the photovoltage. It was shown that the shallow traps (>700 meV) differ from the deep ones (<350 meV) by the respective characteristic energy of 48 meV and 765 meV, and that the amount of shallow traps is more than an order of magnitude larger than that of the deep ones. Our results support the mechanism of shallow-trap dominant multiple-trap limited charge transport, and suggest a substantial margin for the short-circuit photocurrent density to reach its theoretical limit.