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Open Access Publications from the University of California

Study of Polymer-filler composites Based High Performance Diffuse Optical Reflectors for Optoelectronic Device Applications

  • Author(s): Shao, Yue
  • Advisor(s): Shi, Frank G.
  • et al.

The packaging materials are critical for optoelectronic device packaging in terms of optical and thermal performance. Continuous innovation and improvement of packaging materials have been introduced to improve the overall efficiency of LEDs, but there are still some critical issues need to be addressed for their application in general lighting and LCD backlight displays. In this work, dual-functional silicone-filler composites are investigated and introduced to LEDs and LCD backlight units to enhance their optical and thermal management.

In the first part of this work, we optimized the reflectance of polymer-filler reflectors by controlling filler particle size, filler volume fraction and thickness of reflectors. A simple analytical model was developed to investigate the critical parameters of inorganic fillers required to obtain the highest reflectance, which is expected to accelerate the design and optimization of polymer-filler composite reflectors. Our results demonstrate for the first time that for inorganic fillers, the effect of filler size on reflectance is non-monotonic, and a critical filler size ranging from one submicron to several microns provides the maximum reflectance. The existence of a critical thickness at which the optical reflectance of a polymer-filler composite reaches its maximum value is demonstrated.

In the second part of this work, the apparent radiation cooling effect of silicone-based composite coatings was investigated for its dependence on coating thickness and filler size. It is established, contrary to prior reports, that the effective passive radiation cooling does not exhibit a significant filler size dependence. We found the apparent cooling effect is filler type dependent and the optimal thickness is around 70 µm regardless of filler type.

In the third part of this work, dual-functional polymer-filler composites with high optical diffusive reflectance and passive cooling ability is investigated for optoelectronic device applications. It was demonstrated the addition of inorganic fillers to a volume fraction higher than 0.2 can effectively improve the apparent cooling effect and light reflectance of silicone composites. The new materials we developed can be used as diffuse reflector to provide better overall optical aesthetics for displays. Meanwhile, it can be used as passive cooling coatings in advanced general LED lighting, LCD display backlighting, solar cells and other energy related applications.

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