Phase Evolution Extension of Cu₂ZnSn(S,Se)₄ Absorber Boosting the Efficiency of Kesterite Solar Cells to 14.99%

The presence of secondary phases and a high concentration of deep-level defects led to a large open-circuit voltage deficit (Voc,deficit) for Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Here we regulate the phase evolution from CZTS to CZTSSe in the initial selenization stage to obtain high-quality absorber with minimal defects and secondary phases. By adding the bidentate chelation structured mercaptopropionic acid (MPA) into the air-prepared 2-methoxyethanol (MOE) precursor solution, large CZTS colloidal particles and dense precursor films are prepared. During the initial selenization stage, the reduced nucleation sites can decrease selenium–molecule interactions and extend the phase evolution process. This extension makes the heterogeneous nucleation more controllable, fostering uniform element distribution and enhanced growth of permeating the large-grain layer. These benefits demonstrate a substantial increase in device efficiency up to 14.99% (certified at 14.38%) with a reduced Voc,deficit of 281.18 mV. The findings are of great significance for further efficiency leaps of kesterite solar cells.