Current understanding of anomalous deep red / NIR emission bands in highly Mn4+, Mn2+, and Cr3+-doped phosphors: Exchange-coupled pair or trap-related luminescence?

Transition metal-activated phosphors belong to an important family of luminescent materials used as the next generation light sources for display, illumination systems, and other uses because of their small size, high luminescence efficacy, and long operation lifetime. Broad-band deep red or near infrared (NIR) emission has recently been observed in a wide variety of concentrated Mn⁴⁺, Mn²⁺, and Cr³⁺ phosphors as an additional anomalous emission band of the normal red or green emission band in such transition metal-activated phosphors. At present, the following two possibilities have been proposed as the cause of this anomalous broad emission band: (i) exchange-coupled transition-metal ion pair recombination and (ii) deep red or NIR-emitting traps activated by an efficient energy migration over the transition-metal sublattice to such trap centers. The present article presents current understanding of the anomalous deep red and NIR emission bands in highly Mn⁴⁺, Mn²⁺, and Cr³⁺-activated phosphors based on our current knowledge of the transition metal-activated phosphor physics.