Nanoscale Surface Potential Variation Correlates with Local S/Se Ratio in Solution-Processed CZTSSe Solar Cells

Thin film solar cells made from Cu, Zn, Sn, and S/Se can be processed from solution to yield high-performing kesterite (CZTS or CZTSSe) photovoltaics. We present a microstructural study of solution-deposited CZTSSe films prepared by nanocrystal-based ink approaches using scanning probe microscopy (SPM) and scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS). We correlate scanning Kelvin probe microscopy (SKPM) maps of local surface potential with SEM/EDS images of the exact same regions of the film, allowing us to relate observed variations in surface potential to local variations in stoichiometry. Specifically, we find a correlation between surface potential and the S/(S + Se) composition ratio. In particular, we find that regions with high S/(S + Se) ratios are often associated with regions of more negative surface potential and thus higher work function. The change in work function is larger than the expected change in the valence band position with these small changes in sulfur, and thus the data suggest an increase in acceptor-like defects with increasing sulfur. These findings provide new experimental insight into the microscopic relationships between composition, structure, and electronic properties in these promising photovoltaic materials.