Back Contact Engineering for Increased Performance in Kesterite Solar Cells

The thin‐film photovoltaic absorber Cu2ZnSn(S,Se)4 (CZTSSe) holds considerable promise for large scale conversion of sunlight into electricity. CZTSSe is composed of Earth‐abundant elements that exhibit low‐toxicities, but improvements in device efficiency have been hampered by difficulties in increasing open circuit voltages (VOC) due, at least in part, to disorder induced band tailing. We present a method to increase VOC through direct modification of the back contact; our approach involves the separation of fully functioning devices from their Mo/glass substrate to reveal the back CZTSSe surface. Formation of a new back contact consisting of a thermally deposited high work function material (MoO3), together with a higly reflective (Au) capping layer, creates an electrostatic field that drives electrons to the front p‐n junction and leads to a decrease in electron‐hole recombination. M…