Lead-free tin halide perovskites: progress, photophysics, and applications in next-generation optoelectronics.

Metal halide perovskites (MHPs) have transformed the landscape of photovoltaics and optoelectronics due to their remarkable optical and electronic properties. Although lead (Pb)based halide perovskites have shown exceptional power conversion efficiency (PCE) and superior photoluminescence (PL) properties, environmental concerns about Pb toxicity have prompted research into Pb-free alternatives. Fortunately, among the Pb-free candidates, tin (Sn)-based halide perovskites (Sn-HPs) have emerged as potential candidates. Sn-HPs have an optimal bandgap, high charge carrier mobility, and long-lived hot carriers. However, the inherent instability of Sn 2+ , which readily oxidizes to Sn 4+ , poses significant challenges, leading to unintentional p-type doping, thereby degrading PSCs performance. Despite these challenges, recent progress in the stabilization of Sn²⁺, improvement of thin-film quality, and advancement in device engineering has significantly enhanced the performance and reliability of Sn-HPs. In this review, we systematically discuss the fundamental photophysical properties and strategies to mitigate intrinsic defects in tin halide perovskites (Sn-HPs) across various structural modifications. Furthermore, advanced spectroscopic techniques, such as timeresolved spectroscopy, ultrafast transient absorption, and hot-carrier dynamics, enhance understanding of fundamental carrier dynamics and the potential of hot-carrier-based optoelectronic devices. Subsequently, we present recent advancements in achieving stable and efficient performance in LEDs, photodetectors, and photovoltaic technology, and provide perspectives on future developments.