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 (V-OC) due, at least in part, to disorder induced band tailing. We present a method to increase V-OC 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. Model simulations indicating an increase in V-OC with decreasing absorber thickness are borne out by experiments with devices of varying thicknesses (0.7-2.0 mu m). We report V-OC increases of up to 49 mV for a 1 mu m thick absorber, with even greater increases up to 61 mV when the back CZTSSe surface is etched with bromine-methanol.