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

Abstract

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.