A surface ligand reaction strategy for robust and phase-pure Cs2SnI6 crystals

Dual-functional organosilyl iodide additive simultaneously passivates iodine vacancies and regulates crystallization. • Lewis acid-catalyzed in situ generation of TMSAc yields uniform octahedral Cs 2 SnI 6 crystals (2–5 μm). • Optimized TMSI concentration (2 %) completely suppresses CsSnI 3 impurity phase formation. • Phase-pure Cs 2 SnI 6 crystals retain structural integrity after 4 weeks in ambient air (∼39 % RH) and up to 250 °C. • Transient TMSAc ligand permanently locks uniform morphology during nucleation despite post-synthetic removal. The development of inorganic lead-free perovskite is essential for environmentally friendly optoelectronic applications. Among them, semiconductor Cs 2 SnI 6 has emerged as a promising material due to its intrinsic oxidation resistance and outstanding photovoltaic properties. However, conventional synthesis methods often yield non-uniform crystal size distributions and excessive grain boundaries, stemming from the inherent difficulty in controlling the crystallization process, as well as CsI impurities originating from surface iodine vacancies. In this work, we developed a two-step surface ligand reaction strategy based on the dual-functional surfactant iodotrimethylsilane (TMSI) to synthesize surface-passivated Cs 2 SnI 6 crystals. Experimental and analytical results reveal the dual roles of TMSI: (1) it creates an iodine-rich environment that suppresses surface I-vacancies, thereby improving crystallinity and achieving uniform crystal morphology; (2) it acts as a Lewis acid catalyst to regulate molecular assembly and inhibit crystal aggregation, resulting in reduced crystal dimensions. The resulting main Cs 2 SnI 6 crystal phase maintains phase integrity after 4 weeks of ambient storage (∼39% relative humidity, ∼25 °C), and withstand thermal treatment up to 250 °C. This study presents a simple and effective way to improve the crystallinity and structural stability of Sn-based perovskites by introducing a dual-functional organosilyl iodide precursor solution, offering new insights into surface defect passivation.