Development of White Light Emitting Diodes by Multi-layered Red, Green, and Blue Phosphors Excited by Near-ultraviolet Light Emitting Diodes

Abstract

Cascade excitation process and cascade excitation loss (absorption loss) have been investigated by bilayer (BL-) LEDs. The cascade excitation loss was suppressed by multi-layered (ML-) LED structure, in which red phosphor is placed at the bottom. We have also developed white LEDs by using a ML-structure of red, green, and blue phosphors for the purpose of reducing the cascade excitation. Luminous flux of ML-white LEDs showed higher value under low correlation color temperature (CCT=2613K) and high color-rendering index (Ra=94) conditions than conventional phosphor mixed-white LEDs. The angular distribution of chromaticity in the ML-R/G/B white LED was comparable to that in mixed-RGB white LED and was very small.