Structural Insights and Multi-center Luminescence in Cr ³⁺ -Doped Gallogermanate for High-Efficiency Near-Infrared pc-LEDs

Near-infrared phosphor-converted light-emitting diodes (NIR pc-LEDs) are promising for applications in spectroscopy and bioimaging. Typical NIR phosphor designs feature isolated Cr³⁺ ion centers in octahedral crystallographic sites. It is challenging to design broad-band NIR phosphors based on tetrahedral crystallographic sites or Cr³⁺ ion pairs. Herein, we explore an ultrabroadband NIR emission in Cr³⁺-doped gallogermanate Ga₆Ge₂O₁₃ (GGO) phosphors via multisite occupancy. Comprehensive analysis combining low-temperature spectroscopy, X-ray absorption near-edge structure, and electron paramagnetic resonance reveals the coexistence of three distinct luminescent centers: the octahedral site (650-850 nm), Cr³⁺ ion pairs (750-950 nm), and the tetrahedral site (850-1400 nm). This unique configuration enables a tunable and exceptionally broad emission from 650 to 1400 nm. Through Cr³⁺ concentration engineering, the emission spectra of phosphors exhibit a remarkable red shift. The optimized GGO:6% Cr³⁺ exhibits satisfactory thermal stability (60% at 423 K) alongside outstanding absorption efficiency (AE = 69%) and external quantum efficiency (EQE = 36%). The fabricated NIR-LED device delivers a high NIR output power of 55.59 mW at 180 mA, demonstrating promising performance in nondestructive testing and bioimaging. This work provides fundamental insights into multisite Cr³⁺ luminescence and offers a new design paradigm for advanced broadband NIR phosphors.