Rare-Metal-Free Ultrabroadband Near-Infrared Phosphors.

Trivalent chromium (Cr3+) is an attractive near-infrared (NIR) emitter, but its ultrabroadband NIR emission is limited to host crystals containing large amounts of rare-metal elements and usually suffers from low internal quantum efficiency (IQE) and poor thermal stability. Here, a class of high-performance, rare-metal-free ultrabroadband NIR phosphors, are reported by revealing that weak-field Cr3+ centers featuring broadband NIR emission with near-unity IQEs are intrinsic, though in trace quantities, to Cr3+ doped MgAl2O4 spinel (MAS) and its derivatives well-known for their narrowband far-red emission. It is shown that such weak-field Cr3+ centers stem from cation inversion ubiquitous in spinel compounds, and their quantity can be increased simply by superstoichiometric Al2O3/Ga2O3. Then SiO2 is introduced into Al2O3-excess MAS to break the inversion symmetry of Cr3+ centers for greatly improving the probabilities of their otherwise parity-forbidden 3d-3d transitions. The as-fabricated phosphor-converted light-emitting diodes are capable of emitting ultrabroadband NIR light with high photoelectric efficiency (16.0%) and optical power (180.8 mW), and excellent spectral stability, which apparently outperforms existing state-of-the-art devices.