High efficiency Cu2ZnSn(SxSe1−x)4 thin film solar cells by thermal co-evaporation

We report on the device results of thermally evaporated high efficiency Cu₂ZnSn(S_xSe_1-x)₄ (CZTSSe) thin film solar cells with power conversion efficiencies of 7.1% (x=1.0) and 7.5% (x=0.34). We have carried out extensive electrical and structural characterization of CZTSSe solar cells to identify major factors that limit the efficiency. Bias-dependent quantum efficiency measurements revealed ineffective collection of charge carriers photo-generated deep in the absorber layer suggesting a short minority carrier diffusion length, which was confirmed by time-resolved photoluminescence measurements. Temperature-dependence of the series resistance of the devices is consistent with the presence of a Schottky-type barrier in the back contact, likely caused by secondary phases near the CZTS/Mo interface and/or an interfacial MoS_x layer.