High efficiency Cu ₂ZnSn(S _xSe _1-x) ₄ thin film solar cells by thermal co-evaporation

We report on the device results of thermally evaporated high efficiency Cu 2ZnSn(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. © 2011 IEEE.