Rare‐Metal‐Free Ultrabroadband Near‐Infrared Phosphors

Abstract 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.