Understanding and Overcoming the Fundamental Chemical and Electronic Challenges of SnBr₄ Impurities in Tin Perovskite Solar Cells

Tin perovskite solar cells (Sn-PSCs) have emerged as excellent candidates for nontoxic narrow bandgap PSCs. Nevertheless, the technology remains limited by both stability and suboptimal energetic alignment with conventional charge transport layers. Compositional tuning is central to high-performance Sn-PSCs, replacing substoichiometric iodide ions with bromide. However, incorporating SnBr₂ as the bromide source introduces SnBr₄ impurities, underscoring the need to understand the consequences of SnBr₄ on both performance and degradation chemistry. Presently, the absence of such understanding has engendered a reliance on organobromide salts, neglecting a critical opportunity to enhance stability via the reduction of unstable SnI₂. Herein, the influence of SnBr₄ impurities on the structural, optoelectronic, and electronic properties of Sn-perovskites is investigated. Removal of SnBr₄ impurities from SnBr₂ results in drastically improved morphology, a 40% lower trap density and enhanced device performance of 150%. Furthermore, both the fundamental chemistry and degradation pathways in SnI₄ and SnBr₄ are compared, demonstrating the latter does not decompose to the molecular halogen-a key weakness of iodine-based Sn-PSCs. The present findings offer critical chemical and electronic insights into the presence of SnBr₄, the importance of its removal and the opportunities afforded by using SnBr₂ to minimize unstable SnI₂ in Br-rich Sn-perovskite phases.