Transition-Metal-Doped Halide Perovskites for Near-Infrared Emissions: Beyond Oxides

Transition-metal-doped oxide-based phosphors led to near-infrared (NIR) light-emitting diodes and lasers. Mostly, the first transition series (3d) metal ions, like Cr3+, are doped. Interestingly, heavier transition-metal ions, like Mo3+, Re4+, and Os4+, in their low oxidation states, can be doped in halide perovskites, unlike the oxide hosts that stabilize the higher oxidation state. Consequently, doping heavier transition metal ions in halide hosts opens up new avenues to tailor NIR-I/II emissions for both narrow and broad-band d–d transitions. Furthermore, characteristics such as (i) tunability of crystal field parameters by halide ions, (ii) host emissions, and (iii) colloidal nanocrystal synthesis add new NIR functionalities to transition-metal-doped halide perovskites compared to oxides. Here we provide insights on recent progresses on the NIR emitting transition-metal-doped halides, connecting synthetic materials chemistry, spectroscopy and device fabrication. The present challenges and future opportunities are discussed.