Cu 2 ZnSnSe 4 single crystal solar cells with efficiencies up to 10% and V OC deficits comparable to champion thin film devices are demonstrated. The key differences in performance between high and low efficiency cells are analyzed and compared to state-of-the-art thin-film devices. Low temperature JV measurements suggest single crystals have shallow bulk defect states and do not exhibit the Fermi-level pinning found in champion thin films. Low-temperature voltages at 130 °C, in excess of 700 mV, are attributed to the lack of grain boundaries in single crystals. JV-T measurements suggest the best device is limited by bulk recombination due to an improvement in surface passivation from less efficient single crystal devices. High hole concentration and poor lifetimes are shown to limit collection at long wavelength which reduces current in single crystal devices. The differences in device behavior between single crystals and champion thin-films are compared via EQE, JV-T, CV and DLCP to better understand the nature of the CZTSe material system.