Luminescence Properties of Gadolinium Strontium Aluminate Phosphor and Luminescence Enhancement by Co-doping Lithium Ion for White LEDs

Abstract

A series of trivalent cerium (Ce3+) ions doped gadolinium strontium aluminate (Gd1-xSr2AlO5:xCe3+ (GSA:Ce3+)) yellow phosphors were prepared by a high-energy ball milling process. The X-ray diffraction patterns confirmed the complex phase of GSA:Ce3+ phosphors with tetragonal structure of GdSr2AlO5 phase and GdSrAlO4 phase. The photoluminescence excitation spectra exhibited the broad band with dominant excitation wavelength of 438 nm due to 4f-5d transition of Ce3+ ion in GSA host lattice. The photoluminescence emission spectra under 438 nm excitation showed yellow emission corresponding to the 5d-4f transition of Ce3+ ion. Concentration quenching in the PL intensity was observed when the Ce3+ ion concentration was increased over 2 mol% (x = 0.02) in the GSA host lattice. The emission peak of GSA:Ce3+ was shifted to the red region due to the enlargement of crystal splitting for Ce3+ ions. The white LEDs which were fabricated by blue LED (460 nm) chip and GSA:Ce3+ phosphor unveiled the CIE 1931 chromaticity coordinates of (0.347, 0.311). In order to improve the luminescence efficiency of GSA:Ce3+ phosphor, Li+ ions were doped into the (Gd0.98Ce0.02)Sr2AlO5 (GSA:Ce3+, Li+) phosphors. The XRD patterns of the GSA:Ce3+, Li+ phosphors confirmed complex phase of with tetragonal structure of GdSr2AlO5 phase and GdSrAlO4 phase. Compared with GSA:Ce3+ phosphors, the yellow emission of GSA:Ce3+, Li+ was increased by a factor of 5.17 when doped with 10 mol% Li+ ions due to the local crystal field distortion in GdSr2AlO5:Ce3+ host lattice. White LEDs were fabricated by encapsulating the GSA:Ce3+, Li+ phosphors on blue LED chips. The enhancement of the yellow emission in GSA:Ce3+, Li+ phosphors induces the color rendering improvement of white LEDs The GSA:Ce3+, Li+ phosphors were synthesized by using sol-gel method for different Li+ concentration. The XRD patterns of the GSA:Ce3+, Li+ phosphors synthesized by sol-gel method confirmed that the crystallinity of impurity decreased compared with the high-energy ball milling. The FE-SEM images confirm that the particles of phosphors grew and aggregated each other, when the Li+ concentration in GSA:Ce3+, Li+ phosphors increased. By adding 2.5 mol% of Li+ in GSA:Ce3+, the emission intensity of GSA:Ce3+, Li+ was increased to 1.7 times higher than GSA:Ce3+ phosphors. In terms of the synthesis method, emission intensity of the GSA:Ce3+, Li+ phosphors fabricated by sol-gel method was 3 times lower than the other phosphors. However, the color purity of the GSA:Ce3+, Li+ phosphors fabricated by sol-gel method is better than the other phosphors due to the low quantity of the impurity phase.