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The improvement of the efficiency of Cu2ZnSn(S,Se)4 (CZTSSe)-based solar cells requires the formation of high-grain-sized pure CZTSSe throughout the film. We have successfully selenized precursor samples of Cu/SnS/ZnS/Mo/Soda lime glass in an almost sealed selenium furnace. Owing to the presence of confined and high-pressure Se vapor in the furnace, Se easily diffused into the precursor samples, and high-quality Se-rich CZTSSe absorbers were obtained. To understand the effect of the growth mechanism in our precursor and annealing system, this study examines the phase evolution and grain formation. Device parameters are discussed from the perspective of a material microstructure in order to improve performance. At a selenization temperature of 570 °C, a CZTSSe film showed fully developed grains with a size of around 2 Îm without noticeable pore development near the Mo back contact. Solar cells with up to 8.03% efficiency were obtained with a layer thickness of about 1.2 Îm. Detailed electrical analysis of the device indicated that the performance of the device is mainly associated with shunt resistance. [ABSTRACT FROM AUTHOR]