Realizing superior white LEDs with both high R9 and luminous efficacy by using dual red phosphors
The luminous efficiency of radiation (LER) and colour rendering indices (CRIs) are two key technological parameters for white light-emission diodes (WLEDs), but there is always a trade-off between them, especially between LER and R9 (the 9th CRI for red colour). It is therefore necessary to find ways to solve this problem by choosing appropriate red phosphors as well as the phosphor blend. In this work, we attempted to apply both a narrow-band (K2SiF6:Mn4+,KSF) and a broad-band ((Sr,Ca)AlSiN3:Eu2+, SCASN) red phosphors together with a green phosphor (Lu3Al5O12:Ce3+, LuAG) to minimize the trade-off. For a warm white LED with the correlated colour temperature of 3000 K, the combination of a single red phosphor (either KSF or SCASN) and LuAG led to high LERs (92.4 lm W−1 for KSF and 95 lm W−1 for SCASN) but low Ra (average of the first eight CRIs) and R9 (Ra = 60.9 and R9 = −28.2 for KSF; Ra = 83.7 and R9 = 16.6 for SCASN). On the other hand, by using dual red phosphors instead of one, CRIs and even LERs are significantly increased, which is dominantly ascribed to the narrow emission band and the absence of re-absorption of green light of KSF. A superior WLED with LER = 115.4 lm W−1, Ra = 96.9 and R9 = 95.8 was attained when pumping the phosphor blend of (KSF:SCASN:LuAG = 6 : 0.75 : 5.1 in weight ratio) by a blue InGaN LED, which enables it to be used broadly for high quality solid state lighting.