Insight into Site Environment and Photoluminescence Properties of Cr3+-doped Gallogermanate Toward NIR pc-LEDs Applications

Near-infrared phosphor-converted light-emitting diodes (NIR pc-LEDs) are promising for applications in spectroscopy and bioimaging. Typical NIR phosphor designs feature isolated Cr ³⁺ ion centers in octahedral crystallographic sites. However, it is challenging to design broadband NIR phosphors based on tetrahedral crystallographic sites or Cr ³⁺ − Cr ³⁺ pairs. Herein, we explore a series of Cr ³⁺ -doped gallogermanate Ga ₆ Ge ₂ O ₁₃ (GGO) phosphors that feature both octahedral, tetrahedral crystal sites, as well as Cr ³⁺ − Cr ³⁺ pairs, and evaluate their structure-property relationships. Structural and spectroscopic analysis, including low-temperature (10 K) spectroscopy, X-ray absorption near-edge structure, and electron paramagnetic resonance, reveal multiple Cr ³⁺ emission centers in GGO — namely, octahedral, tetrahedral, and Cr ³⁺ − Cr ³⁺ pairs, resulting in NIR luminescence at approximately 650 − 1400 nm. Through Cr ³⁺ concentration engineering, the emission spectra of phosphors exhibit a remarkable redshift and an increase in their full width at half maximum. The optimized GGO:6% Cr ³⁺ exhibits satisfactory thermal stability (60% retention at 423 K) alongside outstanding absorption efficiency (69%) and external quantum efficiency (36%). The fabricated NIR-LED device delivers a high NIR output power of 55.59 mW at a driving current of 180 mA. This work provides deeper insight into the site environment of Cr ³⁺ ions in phosphors and, more specifically, demonstrates the potential of gallogermanates as promising host systems for NIR phosphors.