Halide Double Perovskites: Insights into Structure, Defects, and Luminescence

Lead halide perovskites have turned out to be a promising class of materials due to their excellent photophysical and electronic transport properties. However, lead toxicity and weak operational stability of lead halide perovskites necessitate the search for environment-friendly perovskites that can serve as a good visible-light absorber or emitter. Halide double perovskites (HDPs) offer a promising alternative with enhanced stability, less toxicity, rich combinational chemistry, and attractive optoelectronic characteristics. The structural and optical scrutiny of HDPs focuses on optimizing parameters to address challenges such as indirect or large direct or parity-forbidden transitions, low electronic dimensionality, weak photoluminescence (PL), and defects for better device performance. In this review, we highlight the fascinating structural, optical, and electronic properties of HDPs with theoretical as well as experimental insight. This review presents doping in HDPs as a promising route to optimize the PL quantum yield (PLQY) to up to 100%. The broad PL spanning the UV-vis-IR spectrum, extended PL lifetimes, and high stability collectively provide an optimal framework for light harvesting applications. We further examine how defects affect stability and explore challenges in material and film processing, highlighting their impact on optoelectronic properties. Finally, we discuss promising defect passivation strategies, which are key to future advancements.