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.