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.