以分離式螢光粉波長轉換機制所形成的平面光源之研究

Abstract

本論文研製之螢光粉波長轉換機制所形成的平面光源技術，主要專注於解決目前LED發光二極體做為平面光源應用時在光學與色彩表現上不均勻的問題，以及進一步提升發光模組的光學效益之研究。雖然LED光源具有綠色環保以及節能減碳之技術優勢，但因LED屬於點光源，故傳統LED在形成面光源之後，在發光效益、光源特性以及發光品質仍有待進一步的改善。因此關於本論文的研究，從LED發光二極體的螢光粉封裝進行研討，再深入至LED發光模組的研究，最後再將相關研究結論應用於雙面顯示之系統。研究中發展了一種新的"螢光粉沿晶塗佈(Conformal Phosphor Coating, CPC)技術"以及另一種"分離式螢光粉塗層(Remote Phosphor Coating, RPC)技術"，並分別以該兩種技術研究改善LED光源的顏色不均以及發光不均等問題，並進一步提升LED面光源模組的發光效率。 由本論文研究結果顯示，使用脈衝式的CPC技術，螢光粉可以沿著LED晶片外緣形成一理想的螢光層塗層厚度，而有效降低了LED在不同視角之下的顏色不均勻問題，同時採用物理原理的脈衝式CPC技術又具有環保無化學污染的技術優勢。而採用RPC技術則具有高發光效益之特性，但該技術應用在平面光源模組之相關研究卻是非常稀少。研究結果顯示利用RPC技術經最佳化參數設計之後,可以兼具低色偏以及高發光效益之特性，同時RPC技術因為兼具波長轉換機制以及光擴散機制之二重效果，所以以RPC技術可以輕易的形成均勻發光之平面光源模組。研究結果顯示採用RPC技術的TFT-LCD背光源模組或是照明模組，其光學與色彩表現結果皆優於採用傳統白光LED的光源模組。 本論文除了研究以波長457 nm的藍光，激發分離式的黃色螢光粉塗層而形成平面光源之外，另採用波長254 nm的UV光源，激發R/G/B三色螢光粉混合塗層的平面光源技術。因為UV LED尚未發展成熟，本論文採用UV CCFL作為本研究的基礎光源。由於作為一次光源的UV光本身為不可見光，並不參與波長轉換之後的混光行為，當R/G/B螢光粉的激發頻譜經最佳化之後，以這樣的光源技術作為TFT-LCD背光源模組使用，可以達成高色彩飽和度之光學表現，同時並避免了混光不均所造成的色偏問題﹔若作為照明使用，又能達成高演色性的高品質光源需求。因為這樣的平面光源技術具有雙面對稱且均勻發光之光學特性，因此預期本技術所完成的雙面顯示系統將具有作為數位電子看板或是促進新應用之潛力。 本研究從LED元件的螢光粉封裝技術出發，再至LED發光模組的色彩分析與光學研究，最後結合熱傳模擬與熱輻射分析並應用於雙面顯示之數位電子看板系統。由樣品實測結果顯示，本研究具有低色偏、高發光效益且兼具薄型化之機構特性，同時又無化學污染等問題，因此本成果將完成一個新階段的LED光源系統之發展。

This study investigates planar lighting that is based on remote phosphor conversion to solve the problems of color deviation, non-uniform light distribution and low luminance efficiency of LED lighting. The LED is an emerging solid state light source that could potentially replace traditional light sources, and the interest in the use of LEDs for general illumination has been rapidly increasing. The point-type lighting of LEDs demands a focus on illumination performance, and understanding of the features of uniformity, light distribution and efficiency when LEDs are used in planar lighting. Conventional white LEDs is not simultaneously satisfactory in all these respects. This study focuses on LED devices using "conformal phosphor coating, CPC" and LED lighting module using "remote phosphor coating, RPC" method. Eventually, a dual-sided display system was produced to realize the color and optics performance as compared to conventional planar lighting systems. The results of this study reveal that the placement and arrangement of phosphors critically determine both the luminous efficiency and the color deviation. Since the phosphor in a conventional LED package is in contact with the LED die, a significant fraction of the blue light is backscattered by the phosphor and lost by absorption by the LED chips. Additionally, the high temperature of an operating wLEDs causes thermal quenching, reducing the light radiation efficiency of the phosphor and InGaN blue LEDs. Therefore, the placing of phosphor remote to the LED chip has been explored to analyze the luminous efficiency and color deviation of lighting module. The study of CPC by the pulsed spray (PS) method to an array of blue LED devices is investigated. PS can yield a wide range of color temperatures (Tc) from 2500 K to 9500 K with high color accuracy. Unlike other coating approaches, PS is a mechanical spray-based, environmentally friendly method that does not produce harmful ionic pollution in chemical reactions. In an investigation of RPC, a planar lighting module of blue LEDs was utilized to excite a yellow phosphor film. The phosphor film herein acts simultaneously as a wavelength converter and a light diffuser simultaneously. The proposed configuration yields a lower color deviation than a conventional wLEDs lighting system, and a uniform luminous distribution, from an ultra-slim structure. Most importantly, the configuration does not include a conventional diffuser plate or light guide plate (LGP). Accordingly, an RPC lighting system with excellent optical performance can be realized in a compact module. Hence, an ultraviolet (UV)-excited flat lighting (UFL) system, comprising a RPC is examined. An R/G/B trichromatic RPC is excited using a 254 nm UV light to achieve high color rendering and high luminous backlighting in a slim TFT-LCD display. A UFL system provides symmetrical dual-sided illumination without conventional optical reflectors. Furthermore, UFL exploits the thermal radiation mechanism to release the large amount of heat that is produced by the illumination system, preventing thermal accumulation. These characteristics of the UFL scheme were studied and compared with those of conventional lighting. A dual-sided display based on UFL lighting has potential for use in digital signage or public information displays, for example. This present investigation provides clear evidence of the low color deviation of LED devices. The color analysis and optics of an LED lighting module are studied. The findings are important since they suggest that a dual-sided display system with thermal radiation was low color deviation, high luminous efficiency and a compact mechanism design. The RPC scheme is environmentally friendly that does not involve chemical reactions that generate pollution. RPC lighting performs excellently in LED planar lighting systems.