High-efficiency NIR luminescence of SrGe4O9:Cr3+ induced by charge compensation with lithium-ions and demonstration of its potential application

Broadband near-infrared (NIR) phosphors are considered key to smart NIR light sources for next-generation blue LEDs. The excitation modes of a few Cr 3+ -activated phosphors are mainly in the ultraviolet or red-light bands (possibly related to the coexistence of multiple valence states of Cr), and the low blue light absorption rate has a certain hindering effect on commercialization needs. In this work, Li  +  ion doping reduced the concentration of oxygen vacancies in the material and promoted the reduction of Cr 4+ ions to Cr 3+ , effectively enhancing the excitation intensity of SrGe 4 O 9 :Cr 3+ (I 1 /I 2 :0.59  →  1.19). Moreover, a small-scale shift occurred to the blue excitation peak (441 nm→462 nm), which is crucial for the efficient absorption of the blue light chip excitation light. The doping of Li  +  ions ultimately resulted in enhancement of the emission intensity of Cr 3+ by 17.2 times, obtaining a phosphor with extremely excellent luminescence efficiency (IQE = 99.6 %). In addition, the integral emission intensity of the phosphor at 423 K was 77.6 % of that at room temperature, demonstrating good temperature stability. The NIR pc-LEDs were fabricated by combining SrGe 4 O 9 :Cr 3+ , Li + phosphor with the blue LED, which can be applied in pigment penetration, blood vessel visualization and night vision, revealing its great potential for application.