The heterojunctions formed between Cu 2ZnSn(S xSe 1-x) 4 (CZTSSe) and three Cd-free n-type buffers, ZnS, ZnO, and In 2S 3, were studied using femtosecond ultraviolet photoemission and photovoltage spectroscopy. The electronic properties including the Fermi level location at the interface, band bending in the CZTSSe substrate, and valence and conduction band offsets were determined and correlated with device properties. We also describe a method for determining the band bending in the buffer layer and demonstrate this for the In 2S 3/CZTSSe system. The chemical bath deposited In 2S 3 buffer is found to have near optimal conduction band offset (0.15 eV), enabling the demonstration of Cd-free In 2S 3/CZTSSe solar cells with 7.6 power conversion efficiency. © 2012 American Institute of Physics.