  There is evidence that interface recombination in Cu2ZnSnS4 solar cells
contributes to the open-circuit voltage deficit. Our hybrid density functional
theory calculations suggest that electron-hole recombination at the
Cu2ZnSnS4/CdS interface is caused by a deeper conduction band that slows
electron extraction. In contrast, the bandgap is not narrowed for the
Cu2ZnSnSe4/CdS interface, consistent with a lower open-circuit voltage deficit.
