Impact of Sn4+/Sn2+ ratio in the precursor solution on the performance of CZTSSe solar cell

Precursor design plays an important role in preparing high-quality Cu2ZnSn(S, Se)4 (CZTSSe) absorber layers via solution coating method. In this work, SnCl4 and SnCl2 with different molar ratios were utilized as Sn source in the precursor solution to investigate effects of Sn oxidation state on the film growth process and the related solar cell device performance. Sn4+-rich and Sn2+-rich precursors were found to follow different reaction paths in obtaining CZTSSe grains, thus resulting in quite different crystallinity and morphology of the absorber. For Sn4+-rich precursor system, the direct Se substitution reaction of macroscopically disordered CZTS precursor calls for relatively high temperature to eliminate the double-layer structure and form large-size CZTSSe grains. For Sn2+-dominated precursor solution, however, the reaction of binary compound phases with the catalysis of low-melting-point Sn(S, Se)2 prefers relatively low temperature during the selenization process. The work demonstrates that both types of precursors (or reaction paths) could actually lead to high-quality CZTSSe absorbers and solar cell devices with conversion efficiency higher than 8%, upon the proper selection of selenization conditions.