High moisture resistance of an efficient Mn⁴⁺-activated red phosphor Cs₂NbOF₅:Mn⁴⁺ for WLEDs

Mn⁴⁺-activated fluoride red phosphors, the most important red phosphors for warm white light emitting diodes (LEDs), usually suffer from inherent poor moisture resistance which is a major obstacle to their long-lasting outdoor applications in a high humidity environment. Surface modification of phosphors by coating with either organic or inorganic shells is an effective way to improve waterproof stability. However, the coating procedure usually has a negative impact on the luminous efficacy due to the increased passivation shell thickness. In this work, Mn⁴⁺-activated oxyfluoroniobate (Cs₂NbOF₅), a highly efficient phosphor with internal quantum efficiency of ca. 82%, has been successfully synthesized and it is interesting to note that Cs₂NbOF₅:Mn⁴⁺ can exhibit remarkably improved waterproof stability even without surface coating compared to well-accepted commercial fluoride red-emitting phosphor, K₂SiF₆:Mn⁴⁺. The results obtained indicate that Nb⁵⁺ ions inside red phosphor play a crucial role in improving the water-resistant performance of Mn⁴⁺, which provides a new concept for overcoming the downside of their waterproof in humid conditions and maintaining the luminescence efficiency. In the final phase white LEDs with a high luminous efficacy of 174 lm/W (higher than commercial fluoride red phosphors), low correlated color temperature (3164 K) and high color rendering index (R_a = 90 and R₉ = 85) have been fabricated using Cs₂NbOF₅:Mn⁴⁺.