Formation of Cu-Rich and Sn-Poor CZTSSe via Cu3Sn(S,Se)4‑ZnS Solid-Solution as the Intermediate

In Cu2ZnSn­(S,Se)4 (CZTSSe) thin film solar cells, it is commonly accepted that a Cu-poor and Zn-rich composition is required for high-efficiency devices. However, the finding from our nanoparticle-derived CZTSSe photovoltaic (PV) devices challenges this belief. Despite starting with a Cu-poor and Zn-rich nanoparticle precursor film, STEM-EDS compositional analysis of the photoactive large-grain layer in our high-efficiency CZTSSe thin film PV devices revealed a surprising Cu-rich and Sn-poor composition. Multiple approaches were employed to confirm the Cu-rich and Sn-poor composition determined by STEM-EDS. In order to understand why the large-grain layer in our nanoparticle-derived CZTSSe thin films is Cu-rich and Sn-poor, a detailed investigation of the formation mechanism of the bilayer CZTSSe thin film was undertaken by tracking the evolution of the microstructure, crystalline phases, and composition of annealed precursor films ex situ. From this mechanistic study, we found an intermediate phase is formed on top of the precursor films at temperatures as low as 450 °C. STEM-EDS analysis of the intermediate phase reveals an interesting composition that appears to be a solid-solution between Cu3Sn­(S,Se)4 and ZnS, i.e. Cu3Sn­(S,Se)4-ZnS. We propose that during the high-temperature annealing step, the Cu3Sn­(S,Se)4-ZnS intermediates lead to the formation of the densely packed Cu-rich and Sn-poor CZTSSe large-grain layer.