Investigation of post-synthesis heat treatment of strontium gallium selenium:europium phosphor for improving quantum efficiency and suitable phosphor integration methods for enhancing the overall performance of white LEDs.

摘要

A high-quality white light source requires high luminous efficacy (lumens per input watt) and good color qualities.;Theoretically, in the "yellow-green" spectral region (with peak wavelength around 555 nm), the luminous efficiency (lumens per radiant watt) reaches a maximum based on the luminous efficiency function, V(lambda), and can potentially generate high luminous efficacy. Unfortunately, the light-emitting diode (LED) suffers from low external quantum efficiency in the "yellow-green" region, and therefore the luminous efficacy value becomes low. An alternative to generating green light is to use phosphor down-conversion by exciting a green-emission phosphor with a near-UV or blue LED of higher external quantum efficiency.;A simulation study was carried out to understand how the phosphor emission spectrum affects the phosphor-converted (PC) white LED performance, including both luminous efficacy and color properties. Based on the findings of the simulation study, an ideal emission spectrum of the PC white LED is proposed. The ideal emission spectrum includes: (1) a blue LED with excitation in the range from 440 to 470 nm; (2) a yellow-green phosphor emission with a peak at 555 nm and full-width at half-maximum (FWHM) of 60 nm; (3) a red phosphor emission with a peak at 640 nm and FWHM of 70 nm. To achieve a high luminous efficacy, the external quantum efficiency of the blue LED, the yellow-green phosphor and the red phosphor should be high.;As a start, a SrGa2Se4:Eu2+ phosphor with a peak at 555 nm and FWHM of approximately 60 nm was investigated in this study. The SrGa2Se4:Eu2+ phosphor was initially synthesized and followed by a systematic study of the post-synthesis annealing process. The purpose of this study was to investigate how the post-synthesis annealing conditions, including the annealing temperature, annealing duration, and the annealing ambient atmosphere, can affect the phosphor performance. The phosphor performance was characterized in terms of quantum efficiency and emission properties. The mechanism of how various annealing conditions affect the phosphor performance was analyzed, based on the analyses of the phosphor material and optical properties.;In the second part of this study, the optical properties of the phosphor were carefully analyzed to understand the behavior of the phosphor-converted photons. Different phosphor density, phosphor thickness, and geometric configurations of PC white LED packages were studied to maximize the photon extraction. Based on the simulation results, a secondary "red" phosphor is important for color properties improvement. The performance of a PC white LED, consisting of two types of phosphor ("yellow" and "red") was therefore investigated. It was found that the combined performance of "yellow" and "red" phosphors is influenced by their application as a mixture or stacked layers, the specific layer order, the phosphor densities, the phosphor external quantum efficiencies, the overall spectral power distribution (SPD), and the phosphor excitation and emission spectra and their efficiencies.