[Abstract]

2D layered tin halide perovskites are promising channel materials for field-effect transistors (FETs) owing to their high carrier mobility and lead-free composition, yet they suffer from severe defect sensitivity arising from facile Sn(II) oxidation. Here, we present a molecular design strategy that directly links passivator chemistry to device-level performance by synthesising a controlled pair of phosphine oxide Lewis bases—triphenylphosphine oxide (TPPO) and its methoxy-functionalised analogue (TMPPO)—to systematically tune Lewis basicity and coordination strength with undercoordinated Sn2+ sites. The stronger Lewis base TMPPO stabilises Sn2+, yielding a twofold increase in hole mobility (up to 2.2 cm2 V−1 s−1), negative threshold voltage shift, reduced hysteresis, and superior operational stability. These findings demonstrate that molecular basicity can be rationally translated into defect control and transistor performance, providing a general design principle for stable, high-performance, lead-free perovskite electronics.