Warm-white LED with ultra high luminous efficacy due to sensitisation of Eu3+ photoluminescence by the uranyl moiety in K4(UO2)Eu2(Ge2O7)2

Abstract

Narrow band red emitting phosphors have a great potential to boost the luminous efficacy of warm-white LEDs if their emission spectrum peaks between 610–630 nm and re-absorption of light emitted by the green to yellow LED phosphor, e.g. LuAG:Ce or YAG:Ce, is limited. Therefore, Mn⁴⁺ doped fluoride type materials dominate the chase for such novel phosphors. However, Eu³⁺ activated materials are of interest too, but their application is hampered by the very low absorption cross section in the near UV and particularly in the blue spectral range. Therefore, this work deals with the sensitisation of Eu³⁺ photoluminescence by uranyl cations in K₄(UO₂)Eu₂(Ge₂O₇)₂. It turned out that this approach yields a phosphor with a luminous efficacy of 260 lm W_opt⁻¹ with a CIE1931 colour point of x = 0.647 and y = 0.349. For the first time a phosphor converted 2700 K (In,Ga)N LED comprising a Eu³⁺ activated phosphor as the red emitter has been reported. K₄(UO₂)Eu₂(Ge₂O₇)₂ enables a CRI of at least 85 with an impressive luminous efficacy of around 360 lm W_opt⁻¹.