Revisiting lifetimes from transient electrical characterization of thin film solar cells; a capacitive concern evaluated for silicon, organic and perovskite devices

Abstract

The lifetime of photogenerated charge carriers is one of the most important parameters in solar cells, as it rules the recombination rate that defines the open circuit voltage and the required minimum extraction time. It is therefore also one of the most discussed factors in all photovoltaic research fields. Lifetime evaluation of solar cells is frequently conducted via both optical and electrical means with the purpose of obtaining a deeper understanding of the dominant performance limiting recombination mechanisms. In many earlier recombination designations, performed via transient electrical means in novel thin film solar cells, the lifetime has been observed to be a decaying exponential function of the open circuit voltage. In this work we re-evaluate these previously assigned lifetimes as often being severely influenced by capacitive decay rates of spatially separated charge carriers. These “lifetimes” have thus very little in common with lifetimes relevant under steady state operational conditions of the solar cell. We show that the problem of lifetime determination via electrical means arises from that the relaxation of such charges, being associated with quasi-static capacitances of geometric type or from space-charge regions in the device, is also a decaying exponential function of the instantaneous open circuit voltage. This misconception hence also explains the often observed large discrepancy between optically and electrically determined lifetimes. We finally provide a simple expression outlining under what conditions relevant bulk recombination lifetimes are electrically accessible in thin film solar cells.