Charge separation and recombination in radial $ZnO/In2S3/CuSCN$ heterojunction structures

A ZnO-nanorod/$In2S3/CuSCN$ radial heterostructure has recently shown promising photovoltaic conversion efficiencies. In this work, the charge separation and recombination in single $ZnO/In2S3$ and $In2S3/CuSCN$ interfaces as well as the complete $ZnO/In2S3/CuSCN$ structure were studied by time resolved microwave photoconductivity. Photoconductivity transients were measured for different thicknesses of the $In2S3$ light absorbing layer, under variation in the exciting light flux and before and after annealing of the ZnO nanorods at $450°C$⁠. Upon excitation with 532 nm light, a long lived (millisecond) charge separation at the $In2S3/ZnO$ interface was found, whereas no charge separation was present at the $In2S3/CuSCN$ interface. The presence of the CuSCN hole conductor increased the initial amplitude of the time resolved microwave conductivity signal of the $In2S3/ZnO$ interface by a factor of 8 for a 6 nm thick $In2S3$ layer, but the enhancement in amplitude dropped strongly for thicker films. The measurements show that the primary charge separation is located at the $In2S3/ZnO$ interface but the charge injection yield into ZnO depends critically on the presence of CuSCN.