Studies of the fine-grain sub-layer in the printed CZTSSe photovoltaic devices

Cu2ZnSn(S, Se)(4) (CZTSSe) is a promising low-cost and earth-abundant photovoltaic (PV) absorber material. Using binary and ternary chalcogenide nanoparticles as precursors, we have developed a chemical solution route to produce CZTSSe PV devices with the device efficiency as high as 8.8%. The cross-sectional view of the CZTSSe film shows an interesting bilayer microstructure which consists of an upper micrometer-sized polycrystalline (large-grain) layer and a lower layer with a dense, smooth bottom amorphous (fine-grain) morphology. In this paper, we present the composition and properties of the layers and our understanding on the fine-grain layer's impact on the device performance. Based on the observed optoelectronic properties, a numerical model for the CZTSSe-based PV device is developed, and the simulation results show that the fine-grain layer does not affect the device's performance. An experimental design of CZTSSe PV devices with different thicknesses of the fine-grain layers has also confirmed our findings.