In Cu2ZnSn(S,Se)4(CZTSSe) thin film solarcells, it is commonly accepted that a Cu-poor and Zn-rich compositionis required for high-efficiency devices. However, the finding fromour nanoparticle-derived CZTSSe photovoltaic (PV) devices challengesthis belief. Despite starting with a Cu-poor and Zn-rich nanoparticleprecursor film, STEM-EDS compositional analysis of the photoactivelarge-grain layer in our high-efficiency CZTSSe thin film PV devicesrevealed a surprising Cu-rich and Sn-poor composition. Multiple approacheswere employed to confirm the Cu-rich and Sn-poor composition determinedby STEM-EDS. In order to understand why the large-grain layer in ournanoparticle-derived CZTSSe thin films is Cu-rich and Sn-poor, a detailedinvestigation of the formation mechanism of the bilayer CZTSSe thinfilm was undertaken by tracking the evolution of the microstructure,crystalline phases, and composition of annealed precursor films exsitu. From this mechanistic study, we found an intermediate phaseis formed on top of the precursor films at temperatures as low as450 °C. STEM-EDS analysis of the intermediate phase reveals aninteresting composition that appears to be a solid-solution betweenCu3Sn(S,Se)4and ZnS, i.e. Cu3Sn(S,Se)4-ZnS. We propose that during the high-temperature annealingstep, the Cu3Sn(S,Se)4-ZnS intermediates leadto the formation of the densely packed Cu-rich and Sn-poor CZTSSelarge-grain layer. [ABSTRACT FROM AUTHOR]