A-site cation-pseudohalide co-design for tin halide perovskite transistors

Tin halide perovskites are promising lead-free semiconductors for next-generation electronics; however, their implementation is constrained by a fragile crystal framework, facile Sn²⁺ oxidation, and rapid crystallization kinetics. Here, we establish a molecular design principle for non-metallic thiocyanate (SCN^-) additives that stabilizes the formamidinium tin iodide (FASnI₃) lattice and enables high-performance, stable p-channel thin-film transistors (TFTs). Density functional theory calculations reveal that the Sn-X-Sn bond strength in the lattice and the halide migration barriers can be tuned by the size of the A-site cation and its hydrogen-bonding capability with SCN^- and I^-. In particular, we find that both the Sn-SCN-Sn bonding interaction and the iodine migration barrier are maximized when guanidinium thiocyanate (GuaSCN) is employed. Consistently, GuaSCN enhances the crystallinity and grain size of tens-of-nanometer-thick FASnI₃ films, prolongs carrier lifetimes, and suppresses Sn⁴⁺ formation, without altering the intrinsic bandgap. TFTs incorporating GuaSCN achieve a field-effect mobility above 88 cm² V^-1 s^-1, an I_on/off ratio exceeding 4 × 10⁹, negligible hysteresis, and superior operational and shelf stability across hundreds of devices.