Interplay Between Surface Preparation and Device Performance in CZTSSe Solar Cells: Effects of KCN and NH4OH Etching

Despite the many similarities between Cu₂ZnSn(S,Se)₄ (CZTSSe) and Cu(In,Ga)(S,Se)₂ materials and device architecture, open questions remain about the optimal surface preparation steps for CZTSSe absorbers, including whether differences exist for absorber layers deposited by different methods. In this work, we investigate KCN etching and NH₄OH treatment as surface preparation methods for the absorber/CdS interface for two-stage processed CZTSSe and co-evaporated CZTSe absorber layers. Ambient-exposed, thus oxidized and contaminated, thin film absorbers are utilized to examine the effectiveness of these surface preparation methods and to elucidate their effects on device performance. Topography and surface potential images simultaneously obtained by Kelvin probe force microscopy (KPFM) show the existence of an overlayer on the ambient-exposed absorbers. Moreover, KPFM results also demonstrate that although NH₄OH treatment removes much of the overlayer from the CZTSSe surface, KCN etching removes the overlayer completely. In addition, differences in the deposited CdS layer and depletion region width result depending on the surface preparation method, with the NH₄OH-treated solar cells having narrower depletion region. This is reflected in device results in which KCN-etched solar cells outperform their NH₄OH-treated counterparts due to increases in external quantum efficiency at long wavelengths and in open circuit voltage. KPFM measurements also demonstrate that grain boundaries (GBs) in the KCN-etched two-stage processed CZTSSe thin films are either negatively charged or neutral. On the other hand, KCN etching makes the surface almost equipotential in the co-evaporated CZTSe thin film by causing disappearance of positively charged GBs that existed before etching.