Single‐Crystal Red Phosphors: Enhanced Optical Efficiency and Improved Chemical Stability for wLEDs

Abstract Crystal phosphors have many unique advantages compared with powdery ones. Herein, room‐temperature‐grown millimeter‐sized single‐crystal phosphors of Cs 2 XF 6 :Mn 4+ (X = Ge, Si, and Ti) with remarkably higher external quantum efficiency than the corresponding powdery samples are reported. In addition, as compared with the powdery ones, the crystal samples exhibit much better stability toward water under different pH conditions. The red light‐emitting diodes (LEDs) based on Cs 2 XF 6 :Mn 4+ crystals show significantly improved luminous efficiency than those based on their corresponding powders. An assembled white LED device composed of the two layers of phosphors, i.e., the crystal (such as Cs 2 GeF 6 :Mn 4+ ) and commercial Y 3 Al 5 O 12 :Ce 3+ , on a blue chip exhibits intense warm white light with high luminous efficiency (up to 193 lm W −1 ), high color rendering indexes (88), and low correlated color temperatures (3107 K). Hence, these crystals with greatly improved efficiency and stability can be potentially applied in high‐quality LED backlighting display and white LED lighting, especially inside the micro‐LED devices.