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 (Ra = 90 and R9 = 85) have been fabricated using Cs2NbOF5:Mn4+.