Lanthanide‐Doped Lead‐Free Halide Double Perovskites: Optical Synergy, Performance Engineering, and Multifunctional Optoelectronics

ABSTRACT Lead‐free halide double perovskites (LHDPs) have emerged as robust, eco‐friendly semiconductors that overcome the intrinsic toxicity and instability of lead‐based perovskites for their superior physicochemical stability and structural tunability. This review systematically consolidates recent advancements in lanthanide (Ln 3+ )‐doped all‐inorganic LHDPs, elucidating the synergistic optical mechanisms between host‐lattice broadband self‐trapped exciton recombination and the characteristic sharp 4f–4f transitions of Ln 3+ activators. Comprehensive discussions concerning the diversity of crystal configurations, including charge‐ordered, vacancy‐ordered, and layered structures, and versatile synthesis methodologies ranging from hydrothermal growth to colloidal hot‐injection are provided. Meanwhile, the performance modulation strategies, such as ns 2 ‐ions sensitization, B‐site alloying, and defect engineering, designed to conquer the intrinsic parity‐forbidden absorption limitations of Ln 3+ , are critically analyzed. Furthermore, the review highlights the transformative potential of these materials in multifunctional applications, specifically single‐component phosphor‐converted white‐light‐emitting diodes, ultra‐broadband near‐infrared sources, multimodal optical anti‐counterfeiting, and high‐resolution X‐ray imaging. Concluding with a perspective on current limitations, the article addresses critical challenges in bridging the gap from photoluminescence to electroluminescence, enhancing quantum efficiency via host engineering, and achieving scalable, stable fabrication, thereby establishing a strategic framework for the future development of high‐performance, lead‐free optoelectronics.