Optical designs that improve the efficiency of Cu2ZnSn(S,Se)4 solar cells

We present an optical-design approach that improves the short-circuit current and efficiency of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells by tuning the thickness of the upper-device layers in order to maximize transmission into the CZTSSe absorber. The design approach combines optical modeling of idealized planar devices with a semi-empirical approach for treating the impact of surface roughness. Experimentally, we demonstrate that the new device architecture — which emphasizes thinner CdS and transparent-conducting layers — increases short-circuit current by about 10% in champion-caliber devices. These improvements are directly realized in the power-conversion efficiencies of CZTSSe devices, resulting in a certified improvement in the overall record power-conversion efficiency for CZTSSe from 11.1% to 12.0%. We also report comparable improvements for devices with band gaps in the range of 1.1–1.3 eV.