Preparation Of Phosphor-in-Glass And Its Application In White Light-Emitting Diode Packaging

As a promising candidate for the next generation light source, high-power light-emitting diode (LED) has been widely applied into various illumination applications. The combination of blue LED chips and transparent polymer binders embedded with phosphor is the most commonly used method to obtain white LEDs. Due to the poor thermal stability of the epoxy resin or silicone, the application of LED packaging with organic polymer binder in high-power devices is very difficult. In order to enhance the optical performance and reliability of white LED package, a feasible scheme is proposed to produce phosphor-in-glass (PiG) by screen-printing and low-temperature sintering methods in this dissertation. The prepared PiG is placed into the high-power white LED package for evaluations. In order to analyze the performance of the LED package with PiG, we mainly foucus on three characteristics of LED packages, which include light extraction efficiency, white light quality and thermal reliability. The detail contents are described as follows:1) The PiG used in white LED package is prepared by screen-printing and low-temperature sintering methods. The effects of phosphor content and sintering temperature on the photoluminescence intensity of PiG are studied. The results indicate that the optimal sintering temperature is between 550 and 600?. The white LED devices are constructed by encapsulating the PiG plates. The influences of phosphor content, the position of the PiG layer and the thickness of glass substrate on the performance of LED package are analyzed. The results demonstrate that the overall optical properties of white LED are optimized when the glass-to-phosphor ratio is 8.3:1.7 by weight. Placing the PiG layer facing the chips or decreasing the thickness of glass substrate can effectively increase luminous efficiency of white LEDs.2) A textured PiG structure is proposed to enhance the light extraction efficiency of the LED package. Three different structures, including periodic microstructures, random microstructures and random nanostructures, are produced in experiments. The effects of different structures on the optical performance of LED package are investigated and the inherent mechanism is studied. The results of experiments reveal that the luminous flux of the LEDs with the periodic microstructures can be enhanced by over 9%. The view angle of the LEDs with the random microstructures is enlarged from 120.8° to 129.1°. The transmittance of the glass with random nanostructures is increased by 5%.3) A novel multilayer cone-shaped PiG is proposed to improve the light quality of LED package. Angular correlated color temperature (CCT) distributions of the LED packages with multilayer cone-shaped PiG are numerically simulated and experimentally measured. The results demonstrate that the three-layer cone-shaped PiG can significantly improve the angular color uniformity of the LED package. In order to achieve high color rendering properties, PiG color converters for LED package are fabricated by the mixture of yellow and red phosphors. By adjusting the mixing ratio of yellow and red phosphors, the facile control of the CIE chromaticity coordinates and correlated color temperature of the white LED following the Planckian locus has been achieved. In addition, the PiGs with dual-layered stacking structure and two patterned structures, including semicircle patterned structure and concentric circle patterned structure, are fabricated. The experimental results indicate that these strategies can efficiently overcome the spectral overlapping of the yellow and red phosphors.4) In order to analyze the thermal reliability of PiG, three-dimensional models with empirical particle size distribution and random particle position are used to calculate the effective thermal conductivity of PiG composites. The thermal conductivity of the PiG is also measured with a hot wire method for validation. The results demonstrate that the deviation of simulation and experiment is no more than 4%. Numerical simulations and experimental measurements are conducted to systematically study the effects of the thermal conductivity of the color converters on the optical performance of LED package. The results indicate that the temperature of PiG is lower than that of phosphor-in-silicone (PiS). Compared to the PiS based LEDs, the luminous flux of the PiG based LEDs is much higher, and the variation of color temperature is much lower. In accelerated thermal aging tests, the samples of PiG and PiS are aged at various temperatures. The results indicate that the optical properties of PiG can remain steady in the aging tests.