Zn–Se–Cd–S Interlayer Formation at the CdS/Cu2ZnSnSe4 Thin-Film Solar Cell Interface

The chemical structure of the CdS/Cu₂ZnSnSe₄ (CZTSe) interface was studied by a combination of electron and X-ray spectroscopies with varying surface sensitivity. We find the CdS chemical bath deposition causes a “redistribution” of elements in the proximity of the CdS/CZTSe interface. In detail, our data suggest that Zn and Se from the Zn-terminated CZTSe absorber and Cd and S from the buffer layer form a Zn–Se–Cd–S interlayer. We find direct indications for the presence of Cd–S, Cd–Se, and Cd–Se–Zn bonds at the buffer/absorber interface. Thus, we propose the formation of a mixed Cd(S,Se)–(Cd,Zn)Se interlayer. We suggest the underlying chemical mechanism is an ion exchange mediated by the amine complexes present in the chemical bath.