Carboxylic Acid-Assisted Synthesis of Tin(II) Iodide: Key for Stable Large-Area Lead-Free Perovskite Solar Cells

Despite significant progress in tin-based perovskites, the development of stable and high-performance tin-based perovskite solar cells (TPSCs) remains a challenge. In this pursuit, a multitude of strategies have been explored, encompassing the use of reducing agents, antioxidants, bulky cations, and customized solvent systems. We propose an improved approach for synthesizing SnI₂ from elemental tin and iodine. Here, we generate tin nanoparticles grafted with a carboxylic acid <i>in situ</i> from tin powder-carboxylic acid-assisted synthesis (CAAS). This methodology not only improves the synthesis process of SnI₂ but also enhances precursor stability against oxidation. We use ¹¹⁹Sn MAS NMR to study the atomic-level structure of the resulting FASnI₃ thin films and find that the CAAS approach leads to highly pure and unoxidized material. We report remarkable reproducibility in fabricating large-area (1 cm²) flexible TPSCs with significant improvement in open-circuit voltage leading to the champion device showing a power conversion efficiency of 8.35%.