Tin-Based Perovskite Solar Cells: Structural Fundamentals, Material Engineering, and Prospects for Lead-Free Photovoltaics.

Tin-based halide perovskites have emerged as one of the most promising classes of lead-free materials for next-generation photovoltaic technologies. Their favorable optoelectronic properties, narrow band gaps, and structural compatibility with the ABX3 perovskite framework position them as viable alternatives to conventional lead-based absorbers. This review summarizes the fundamental structural principles governing Sn-based perovskites, including the role of A-site cations, halide composition, tolerance factor, and octahedral distortion in determining phase stability and electronic structure. Recent advances in material engineering-such as additive-assisted crystallization, interface modification, precursor chemistry optimization, and two-dimensional/three-dimensional heterostructure formation-have significantly improved film quality, reduced defect densities, and enhanced device performance. Despite these achievements, challenges remain, particularly the spontaneous oxidation of Sn2+ to Sn4+, uncontrolled crystallization, and limited operational stability. Strategies aimed at stabilizing the Sn2+ oxidation state, suppressing self-doping, and improving charge transport are discussed in detail. The review also highlights the strategic importance of tin as a sustainable element for renewable energy technologies and provides an overview of global tin resources relevant to future photovoltaic deployment. Overall, tin-based perovskites represent a compelling pathway toward environmentally responsible and high-efficiency solar cells, with continued research expected to accelerate their transition from laboratory materials to commercially viable technologies.