Waterproof surface passivation of K2GeF6:Mn4+ by a dense Al2O3 layer via atomic layer deposition

Red-emitting Mn4+-doped fluoride phosphors are promising narrow-band and high efficiency down-conversion luminescent materials for use in high-color-rendition or wide-color-gamut white light emitting diodes. However, their intrinsic moisture-sensitive nature severely hinders their practical applications. Herein, we utilized the atomic layer deposition (ALD) technique to deposit a passive layer of Al2O3 on K2GeF6:Mn4+ (KGF) phosphor for enhancing its water resistance. The coated KGF phosphor can maintain 91% of its initial photoluminescence when immersed in water for 2 h, in comparison to 67% for the uncoated one. By the Al2O3 protective coating, the KGF phosphor particles can be well isolated from the water and their hydrolysis was then greatly suppressed. The Al2O3 coated phosphor enables the production of LEDs with much better reliability, which have a luminous flux of two times higher than that of the uncoated one when aged under 200 mA for 80 h. This demonstrates that the ALD approach provides a facile route for passivating the moisture-sensitive Mn4+-activated fluoride phosphors with enhanced chemical stability.