Large process-dependent variations in band alignment and interface band gaps of Cu2ZnSnS4/CdS solar cells

Electron-hole recombination at the Cu2ZnSnS4/CdS interface is believed to play a major role in limiting the efficiency of Cu2ZnSnS4 solar cells. In this work, we experimentally determine detailed Cu2ZnSnS4/CdS interface band diagrams as a function of process conditions, and correlate them to chemical processes occurring during interface formation and subsequent post-annealing. The newly devised experimental method involves a combination of photoemission spectroscopy and spectroscopic ellipsometry. Our measurements reveal that, under most process conditions, the band gaps of both Cu2ZnSnS4 and CdS decrease by several hundred meV near the interface. Furthermore, interface band bending and conduction band offsets are highly process-dependent and roughly correlated to the amount of chemical interdiffusion. The interface electronic properties are found to be unfavorable under all process conditions studied in this work, either due to a cliff-like conduction band offset, or to substantial band gap narrowing in Cu2ZnSnS4, or to both effects. According to the present study, the least harmful process conditions for the interface electronic properties are a low CdS deposition temperature without post-annealing. Even in such a case, a minimum open circuit voltage loss of 230 mV is expected due to interface- or near-interface recombination.