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We present evidence that band gap narrowing at the heterointerface may be amajor cause of the large open circuit voltage deficit of Cu$_2$ZnSnS$_4$/CdSsolar cells. Band gap narrowing is caused by surface states that extend theCu$_2$ZnSnS$_4$ valence band into the forbidden gap. Those surface states areconsistently found in Cu$_2$ZnSnS$_4$, but not in Cu$_2$ZnSnSe$_4$, byfirst-principles calculations. They do not simply arise from defects atsurfaces but are an intrinsic feature of Cu$_2$ZnSnS$_4$ surfaces. By includingthose states in a device model, the outcome of previously publishedtemperature-dependent open circuit voltage measurements on Cu$_2$ZnSnS$_4$solar cells can be reproduced quantitatively without necessarily assuming acliff-like conduction band offset with the CdS buffer layer. Ourfirst-principles calculations indicate that Zn-based alternative buffer layersare advantageous due to the ability of Zn to passivate those surface states.Focusing future research on Zn-based buffers is expected to significantlyimprove the open circuit voltage and efficiency of pure-sulfide Cu$_2$ZnSnS$_4$solar cells.

Interface band gap narrowing behind open circuit voltage losses in Cu$_2$ZnSnS$_4$ solar cells