A time-resolved microwave conductivity study of the optoelectronic processes in TiO2∣In2S3∣CuInS2 heterojunctions

Abstract

Photoinduced interfacial charge carrier generation, separation, trapping, and recombination in TiO2∣In2S3∣CuInS2 cells have been studied with time-resolved microwave conductivity (TRMC). Single layer, double layer, and complete triple layer configurations have been studied. Selective electronic excitation in one of the components is accomplished by using monochromatic pulsed laser excitation. In bare CuInS2 films and in TiO2∣CuInS2 double layers, photoinduced charge carriers recombine on a subnanosecond time scale. This fast recombination slows down significantly when an In2S3 buffer layer is applied between TiO2 and CuInS2. In that case, the charge separation lifetime increases by more than one order of magnitude. A superlinear dependence of the TRMC signals on the incident laser intensity is observed for the triple layer configuration, which indicates saturation of electron traps in In2S3 or hole traps in CuInS2. Furthermore, TRMC signals from TiO2∣In2S3∣CuInS2 triple junctions and those from In2S3∣CuInS2 double layers are identical, which shows that charge carrier separation exclusively takes place at the In2S3∣CuInS2 interface.