Influence of the Temperature on the Photoluminescence Spectra of a Coating Based on Gd3Ga5O12 Luminophore for Visible Light-Emitting Diodes

Abstract

At present great attention in all countries is given to the development and manufacture of visible light-emitting diodes possessing much higher efficiency in comparison with the existing incandescent and luminescent lamps [1]. One of the photoluminophore (PL) types of domestic production possessing high quantum yield (exceeding 90%) and intended for transformation of dark blue crystal radiation with wavelength 430–490 nm into yellow-green broadband visible radiation with wavelength 530–640 nm is the powder based on gallium gadolinium garnet doped by cerium. The domestic industry produces such FLZh-7 (TS 2661-0016752722-2008) luminophores of several modifications determined by ratios of gallium, gadolinium, and cerium concentrations [2]. Investigations of the temperature dependences of the luminescence spectra and of the luminescence efficiency of visible light-emitting diodes are necessary for understanding of the processes excited in the luminophore by quanta with energy slightly exceeding the gap width. These investigations are important from the practical viewpoint, because the light-emitting diodes based on luminophores are heated during their operation, thereby changing their energy yield. In the present work, the temperature dependence of the photoluminescence spectra of the PLZh-7 luminophore representing gallium-gadolinium garnet Gd3Ga5O12 doped by cerium Ce3 is investigated. The examined samples were prepared by mixing of the luminophore with KO-859 silicon-organic varnish in the ratio 0.82: 0.18. After addition of distilled water to the mixture, it was dispersed in a magnetic stirrer, put on the substrate of the device to fix the sample, and dried for 24 h at room temperature. Five spectra were registered in the range 400–800 nm for each temperature value from the interval 24–89С, and the measurement results were averaged. In total, 11 spectra were registered. It was experimentally established (Fig. 1) that with increasing temperature, the intensity of the emission band decreased, and its half-width slightly decreased by 0.2 eV in the entire temperature interval. The band maximum was placed at 565–570 nm and did not change with increasing temperature. Since no considerable changes in the band profile were observed, the change of the luminescence power yield could be traced only from the intensity of the band maximum. Experimental points in the given temperature interval are well described by the dependence IT = I0[1–exp(–Ea/kT)] (Fig. 2), where IT is the band intensity at the given temperature, I0 is the band intensity at a temperature of 24С, k is the Boltzmann constant, and Ea is the activation energy of luminescence quenching. The estimated activation energy was 0.47 eV. This value was close to that of the Stokes losses (0.49 eV) retrieved from a comparison of the absorption and luminescence bands of the luminophore [3]. Therefore, this suggests that the activation energy for temperature quenching of the photoluminescence bands is determined by thermalization of electrons caused by their diffusion in gallium-gadolinium garnet.