Near-Infrared Light from Chromium (III)–Based Phosphor-Converted Light-Emitting Diodes

Phosphor-converted near-infrared light-emitting diodes (pc-NIR-LEDs) are increasingly used as compact, energy-efficient light sources for spectroscopy, bioimaging, and food analysis. The phosphor determines the NIR optical properties through host and activator ion interactions. Cr 3+ is especially promising because its spin-allowed transition in weak crystal fields enables tunable emission under blue excitation. Despite this promise, achieving broadband emission, high efficiency, and thermal stability remains challenging. This review highlights recent advances in Cr 3+ -activated NIR phosphors, emphasizing multisite occupancy, crystal field tuning, and aliovalent cosubstitution strategies that collectively enable broadband emission from 700 to 1,100 nm. The review also explores temperature-dependent luminescence behavior of Cr 3+ phosphors, using Debye temperature, Huang–Rhys factor, and bandgap to understand and predict thermal quenching. The review concludes with application examples showing how Cr 3+ -based pc-NIR-LEDs enable heart rate monitoring and indoor agriculture. Linking host structure and performance, this review sets the stage for the development of next-generation Cr 3+ phosphors.