Review on recent trends in NIR-emitting trivalent chromium (Cr ³⁺ ) ion-doped phosphors for applications in pc-LEDs

Designing the NIR pc-LED based on blue chip has now become a thrust research area. Among various NIR-emitting phosphors, those based on the Cr3+ activator are most adequate due to the wide excitation bands showing good overlap with blue LED emission, high quantum efficiency, wavelength selectivity over a wide range by a suitable choice of a host, broad emission bands and low cost. Among the host materials for Cr3+-doped activator, halides, phosphate, silicate, oxides and garnets are widely explored owing to their stable structure and excellent chemical, optical and thermal properties. Such phosphors, which were used to fabricate NIR pc-LEDs, are reviewed here. The review starts with a brief description of diverse applications of NIR-emitting phosphors, followed by luminescence characteristics of Cr3+ ions related to broad-band NIR emission. In the next sections, results on various host materials, including halides, borates, phosphates, silicates, garnets and oxides with Cr3+ as an activator, are reviewed with an emphasis on materials for actual LED fabrication. The data are critically examined and evaluated to highlight the best phosphors for designing the targeted LEDs. The review concludes with a section on limitations, allied challenges and future directions in this important and expanding field.