An Analytical Review of Strategies for Enhancing the Efficiency of Cu ₂ ZnSn(S <i> _x </i> , Se _{1‐ <i>x</i>} ) ₄ Thin‐Film Solar Cells

This study provides a comprehensive review of recent scientific developments aimed at enhancing the power conversion efficiency of copper zinc tin sulfur‐selenium (CZTSSe)–based thin‐film solar cells. The research focuses on the effects of dopant incorporation, buffer layer optimization, and long‐term operational stability as key strategies for improving the structural, optical, and photoelectrical properties of kesterite absorbers. CZTSSe, a quaternary semiconductor composed of earth‐abundant and nontoxic elements, offers a tunable bandgap ranging from 1.0 to 1.5 eV and a high optical absorption coefficient (&gt;10 4 cm −1 ) and is therefore considered a promising alternative to conventional Cd‐ and In/Ga‐based absorber materials. The review highlights how various dopants—including Na, Li, K, Ge, Rb, Ag, and Mn—enhance grain growth, suppress detrimental recombination centers, and improve charge‐carrier transport. Co‐doping strategies such as Ag/H and Li/Na have led to power conversion efficiencies approaching 14%–15%. Additionally, optimization of heterostructures and buffer layers (e.g., CdS, ZnSnO, Zn 1‐ x Mg x O, TiO 2 , ZnO, ZnS, Zn(O,S), In 2 S 3 , and Sb 2 Se 3 ) through both experimental and numerical modeling approaches has been shown to improve energy band alignment, passivate interface defects, and enhance carrier collection efficiency. Despite significant advances, Cd‐free alternative buffers still exhibit efficiencies below 14%, underscoring the need for further optimization of the energetic alignment at the absorber/buffer interface. Furthermore, the review addresses the metastability and long‐term degradation behavior of CZTSSe devices, identifying Cu diffusion, Cu/Zn disorder, and interface recombination as primary factors limiting operational stability, while demonstrating that low‐temperature annealing and controlled illumination can partially restore device performance. The findings suggest that a synergistic combination of controlled doping, buffer layer engineering, and stability‐oriented interface management provides a robust methodological framework for mitigating recombination losses and improving the overall photovoltaic performance of CZTSSe‐based thin‐film solar cells.

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