Single Crystals of Cs ₂ KAlF ₆ :Cr ³⁺ for High‐Power Near‐Infrared LEDs Applied in Vein Imaging and Advanced Wireless Communication

ABSTRACT Near‐infrared (NIR) phosphors with high luminescence efficiency and thermal stability are crucial components for NIR phosphor‐converted light‐emitting diodes (NIR pc‐LEDs), especially for high‐power NIR pc‐LEDs. In this study, a series of single‐crystalline Cs 2 KAlF 6 : x Cr 3+ ( x = 0–0.035) phosphors are synthesized using the room‐temperature solvent evaporation method. The optimized Cs 2 KAlF 6 :0.03Cr 3+ single crystal demonstrates an intense NIR emission 6.8‐fold stronger than its powder counterpart, yielding a high external quantum yield (EQY) of 40.8%. This phosphor retains 100% of its room‐temperature luminescence intensity even at 423 K. A high‐power NIR pc‐LED fabricated with this crystal phosphor and a 10 W blue light chip achieves a high photoelectric conversion efficiency (PCE) of 13.31% and an output power (OP) of 1283.01 mW at a driving current of 1.00 A. The NIR light from this device at a current of 1.00 A easily penetrates the tissue with a thickness of 5.1 cm, demonstrating its potential for practical vein imaging applications. Furthermore, the NIR LED enables reliable character signal transmission via pulsed frequencies and exhibits superior palm‐penetration capability compared to visible light, highlighting its potential for advanced optical wireless communication applications.