Near-UV-excitable broadband green-emitting Ca2LaHf2GaAl2O12:Ce3+ garnet-type phosphors for high color rendering warm-white LEDs

Abstract

Efficient broadband green-emitting phosphors are promising for application in high color rendering white light-emitting diodes (LEDs). In this paper, we report on the preparation, crystal structure and photoluminescence properties of a novel green-emitting Ca2LaHf2GaAl2O12:Ce3+ (CLaHGA) garnet-type phosphor. All these CLaHGA:xCe3+ (x = 4, 6, 8, 10, and 12 mol%) phosphors have been synthesized through using the conventional high-temperature solid-state reaction method. It has been found that the optical property of phosphors critically depends on Ce3+ content, and the optimal doping concentration of Ce3+ ions is 6 mol%. The optimal CLaHGA:6%Ce3+ sample has a cubic structure with the space group Ia3‾d and lattice parameters of a = b = c = 12.62662 Å and V = 2013.08 Å3. The CLaHGA:xCe3+ phosphors exhibit an intense absorption band in the 300–485 nm range with a peak at 421 nm, due to the spin-allowed and parity-allowed 4f→5d transition of Ce3+ ions. Upon 421 nm excitation, CLaHGA:xCe3+ phosphors give rise to bright broad green emission bands in the 435–750 wavelength region with peak ranging from 519 to 531 nm, owing to the spin-allowed and parity-allowed 5d→4f transition of Ce3+ ions. For the optimal CLaHGA:6%Ce3+ sample, its CIE color coordinates and internal quantum efficiency are determined to be (0.3135, 0.4851) and 40.7%, respectively. The full width at half maximum of CLaHGA:6%Ce3+ is much larger than the commercial (Ba,Sr)2SiO4:Eu2+ green phosphor (124 nm versus 71 nm), while its emission intensity is only about 39% of that of (Ba,Sr)2SiO4:Eu2+. Besides, the thermal stability of CLaHGA:6%Ce3+ has also been studied. A white LED device is fabricated using the synthesized CLaHGA:6%Ce3+ green phosphors, showing a bright warm-white light emission with high color rendering index of Ra = 89.7 and low correlated color temperature of 4058 K.