Ultrawideband Emission of Bi ³⁺ Ions Spanning Visible to Near‐Infrared Spectral Regions (400 nm–1700 nm)

Although Bi³⁺ ions exhibit exceptional luminescent properties and spectral tunability, their potential as activators for near-infrared (NIR) ultrawideband emission remains underexplored, while transition metal ions such as Ni²⁺, Fe³⁺, and Cr^3+/4+ have long been the predominant candidates in this demanding research area. Herein, we demonstrate ultrawideband emission from 400 to 1700 nm in K₂MgGeO₄:Bi³⁺ (KMGO:Bi³⁺), achieving an internal quantum efficiency (IQE) of 88.02% and external quantum efficiency (EQE) of 66.41%. The emission spectrum features interconnected visible and NIR bands, peaking at 614 and 1125 nm, respectively. Notably, the full width at half-maximum (FWHM) of the NIR band exceeds 340 nm, significantly broader than most of the conventional phosphors doped with transition metal ions. Through a comprehensive combination of experimental investigations and crystal structure analysis, we elucidate the underlying mechanism of this ultra-broadband emission, attributing it to Bi³⁺ centers formed by the substitution of K⁺ and Mg²⁺ sites. This work expands the role of Bi³⁺ ions as activators in the second NIR (NIR-II) region, offering new insights into the design of ultra-broadband-emitting materials and introducing the only known phosphor capable of spanning the full 400 nm to 1700 nm spectrum, thereby filling a longstanding gap in this field.