What is the band alignment of Cu2ZnSn(S,Se)4 solar cells?

The band alignment at the Cu2ZnSn(S,Se)(4)/CdS solar cell heterojunction is a controversial issue, as different measurements and calculations point to substantially different conduction band offsets (CBO). As the actual value of the CBO has profound implications on solar cell performance, the aim of this work is to separate genuine process-dependent variations in the CBO from errors in its experimental determination. We argue that the two most likely mechanisms responsible for real CBO variations are Fermi level pinning (which tends to decrease the CBO) and chemical interdiffusion (which tends to increase the CBO). The experimental and computational approaches employed so far to determine the band alignment are analyzed to point out possible limitations for each approach, with an emphasis on photoemission-based approaches. The influence of Fermi level pinning on the CBO should be captured correctly by all types of measurements, except for measurements performed under flat-band conditions. This may explain some particularly large values of the CEO that have been measured under flat-band conditions. On the other hand, the influence of interdiffusion is difficult to resolve completely by most measurement approaches. Interestingly, a rough correlation can be established between the CBO measured at the Cu2ZnSnS4/CdS interface by different groups and their corresponding solar cell efficiency: lower-efficiency cells often have a large "cliff-like" offset, whereas most high-efficiency cells have a "spike-like" or nearly flat offset. Control of interdiffusion can be a powerful way to engineer the optimal band alignment in Cu2ZnSnS4/CdS solar cells, but it can be detrimental in Cu2ZnSnSe4/CdS solar cells, as it may increase the CBO above the optimal range for maximum efficiency.