  The Cu2ZnSn(S, Se)4 (CZTSSe) emerging inorganic solar cell is highly
promising for accelerating the large-scale and low-cost applications of
thin-film photovoltaics. It possesses distinct advantages such as abundant and
non-toxic constituent elements, high material stability, and excellent
compatibility with industrial processes. However, CZTSSe solar cells still face
challenges related to complex defects and charge losses. To overcome these
limitations and improve the efficiency of CZTSSe solar cells, it is crucial to
experimentally identify and mitigate deep defects. In this study, we reveal
that the dominant deep defect in CZTSSe materials exhibits donor
characteristics. We propose that incomplete cation exchange during the
multi-step crystallization reactions of CZTSSe is the kinetics mechanism
responsible for the defect formation. To address this issue, we introduce an
elemental synergistic alloying approach aimed at weakening the metal-chalcogen
bond strength and the stability of intermediate phases. This alloying strategy
has facilitated the kinetics of cation exchange, leading to a significant
reduction in charge losses within the CZTSSe absorber. As a result, we have
achieved a cell efficiency of over 14.5%. These results represent a significant
advancement for emerging inorganic solar cells and additionally bring more
opportunities for the precise identification and regulation of defects in a
wider range of multinary inorganic compounds.
