Hyeonmin Choi, Joonha Jung, Young‐Kwang Jung, Seok Woo Lee, Taehyun Kong, Y. L. Kim, Youjin Reo, Jinwoo Sim, Yeeun Kim, Jaeyong Woo, Sunggyu Ryoo, Jaeyoon Cho, Youcheng Zhang, Stefano Pecorario, Bo Ram Lee, H. Sirringhaus, Yong‐Young Noh, Samuel D. Stranks, Takhee Lee, Keehoon Kang
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 Sn 2 + sites. The stronger Lewis base TMPPO stabilises Sn 2 + , yielding a twofold increase in hole mobility (up to 2.2 cm 2 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.