Advancing Earth-Abundant CZTSSe Solar Cells: Recent Progress in Efficiency and Defect Engineering

The earth-abundant, ecologically friendly structure of kesterite Cu₂ZnSn(S,Se)₄ (CZTSe) solar cells, with their advantageous optoelectronic characteristics, including a direct bandgap (1.0-1.5 eV) and a high optical absorption coefficient (>10⁴ cm^-1), have made them a very promising member of thin-film photovoltaics. However, the path toward commercialization has been slowed down by restraint such as high open-circuit voltage deficits, deep-level defect states, and compositional inhomogeneities that lead to charge recombination and efficiency loss. Despite these obstacles, very recent advances in material processing and device engineering have revitalized this technology. Incorporating elements like Ge, Ag, and Li; optimizing interface properties; and introducing methods like hydrogen-assisted selenization have all contributed to raising device efficiencies by around 15%. This review discusses recent progress and evaluates how far CZTSSe has come and what remains to be done to realize its commercial promise.