Red emission generation through highly efficient energy transfer from Ce3+ to Mn2+ in CaO for warm white LEDs

Abstract

CaO:Ce³⁺,Mn²⁺ phosphors with various Mn²⁺ concentrations were synthesized by a solid state reaction method. Efficient energy transfer from Ce³⁺ to Mn²⁺ was observed and it allows the emission color of CaO:Ce³⁺,Mn²⁺ to be continuously tuned from yellow (contributed by Ce³⁺) to red (by Mn²⁺) with an increase in Mn²⁺ concentration and upon blue light excitation. The red emission becomes dominant when the Mn²⁺ concentration is ≥0.014 with an energy transfer efficiency higher than 87% which can reach as high as 94% for a Mn²⁺ concentration of only 0.02. A critical distance of 10.5 Å for the Ce³⁺–Mn²⁺ energy transfer was determined. A faster decrease of Ce³⁺ luminescence intensity in comparison with its lifetime was observed on increasing the Mn²⁺ concentration. The analysis of this feature reveals that the Ce³⁺ excitation energy can be completely transferred to Mn²⁺ if the Ce³⁺–Mn²⁺ distance is shorter than 7.6 Å. A warm white LED was fabricated through integrating an InGaN blue LED chip and a blend of two phosphors (YAG:Ce³⁺ yellow phosphor and CaO:0.007Ce³⁺,0.014Mn²⁺ red phosphor) into a single package, which has CIE chromaticity coordinates of (x = 0.37, y = 0.35), a correlated color temperature of 3973 K and a color rendering index of 83.1. The results indicate that CaO:Ce³⁺,Mn²⁺ may serve as a potential red phosphor for blue LED based warm white LEDs.