An Effective Precursor‐Solutioned Strategy for Developing Cu₂ZnSn(S, Se)₄ Thin Film Toward High Efficiency Solar Cell

Abstract Enhancing the efficiency of Cu₂ZnSn (S, Se)₄ (CZTSSe) thin‐film solar cells requires the development of well‐crystallized light‐absorbing layers. A deep understanding of the role of precursor solution chemistry in film nucleation and crystal growth processes is essential. Insights into these processes enable the development of innovative strategies to enhance absorber quality, minimize detrimental bulk defects, and ultimately improve device performance. This study elucidates the condensation reactions between thiourea and metal cations, as well as the alcoholysis of 2‐methoxyethanol (MOE), at different concentrations of precursor solutions. The primary focus of this study is implementing a simple and environmentally friendly innovative spin‐coating strategy, aimed at optimizing Cu₂ZnSnS₄ (CZTS) precursor films and adjusting the Se content within the film bulk to promote grain growth during selenization. This strategy effectively improves absorber morphology while suppressing the formation of deep‐level defects, thereby enhancing carrier transport in both interfacial and bulk regions of the absorber layer. Consequently, CZTSSe absorbers with enhanced crystallinity and reduced defects are synthesized, resulting in a solar cell with an impressive efficiency of 14.10%. These findings underscore the potential for creating highly efficient kesterite CZTSSe solar cells through the manipulation of precursor solution chemistry using environmentally friendly solvents.