Suppressing Defect Formation via Li2CO3-Assisted Synthesis Strategy: Toward High-Performance Al5BO9:Cr3+ Near-Infrared Phosphors.

The performance of near-infrared phosphor-converted light-emitting diodes (NIR pc-LEDs) is fundamentally limited by the phosphor, which tends to form defects during high-temperature air synthesis, and these defects degrade their luminous efficiency and thermal stability. Herein, we report a Li2CO3-assisted solid-state strategy that simultaneously enhances crystallinity and mitigates the undesired oxidation of Cr3+ to Cr4+ in Al5BO9:Cr3+. This is related to the unique decomposition chemical property of Li2CO3 at high temperatures, which creates favorable conditions for grain growth and the stabilization of Cr3+. The optimized phosphor exhibits broadband NIR emission spanning 650-900 nm under 400 nm excitation, with the internal quantum efficiency significantly increased from 55.4% to 80.5%. Meanwhile, the luminescence intensity retention at 423 K rises from 75.3% to 81.9%. A prototype NIR pc-LED fabricated with a 440 nm chip delivers an output power of 59.4 mW at a drive current of 100 mA and a photoconversion efficiency of 21%. In cultivation trials with butterhead lettuce, supplemental lighting from this NIR pc-LED significantly promotes plant growth, increasing fresh weight by 28.5% and dry weight by 18.9%. This work provides a practical and effective flux-assisted approach for preparing high-performance Cr3+-doped NIR phosphors under ambient atmosphere.