摘要

由於具有色溫穩定、色彩飽和度高、均勻度佳，製作成本低等優勢，UV激發螢光粉而混成白光的LED方式是目前白光LED發展的主要技術之ㄧ。然而，目前其光電轉換效率仍低於藍光激發黃色螢光粉混成白光之主流技術。導致其效率較低的主要因素有二，第一是紫外光晶粒外部量子效率很低；第二是近UV螢光粉轉換效率偏低。
此研究提出改善UV激發螢光粉而混成白光的LED之發光效率的方法。此方法是利用全方位反射膜來增加激發螢光粉的機率，來提升螢光粉轉換效率。並發展出一維近似之簡化模型來模擬此種螢光粉型LED之發光強度。從光吸收及轉換的能量平衡方程式出發，並假設光在螢光層之適當邊界條件，進而解出發光強度的理論公式，此公式之未知參數可由擬合實驗結果求出，此模型可用來預測該LED之理論發光強度並作最佳化之設計。
實驗結果顯示，用全方位反射膜的方式，確實能提升其發光效率。另外，擬合實驗結果成功的求出理論公式中的未知參數值，所預測之發光強度與實驗結果一致。而經由模型預測分析結果顯示，當螢光層厚度變薄、螢光層下邊界的紫外光反射率變高、螢光層中的矽樹脂對於光吸收變小時，可增強全方位反射膜對於發光效率之效果。使用此模型擬模最佳化的結果顯示其發光效率可提升2.62倍。

ABSTRACT

White LEDs based on UV LED pumping visible phosphors have the advantages of stable color temperature, high color rendition, good color uniformity, easy fabrication, and low cost. However, the electrical-to-optical conversion efficiency is lower than that of the mainstream white LEDs technology based on blue LEDs pumping yellow phosphor. The low efficiency is due to two factors. One is the low efficiency of current UV LED. The other is the low phosphor conversion efficiency.
This study proposes a way to improve the efficiency of UV pumped phosphor-converted LED using omni-directional reflector (ODR) film. A one-dimensional model to approximate the light extraction is derived to predict the light extraction and to optimize its design.
According to experimental result, the present method using ODR film can enhance the light extraction. Prediction of the proposed model is consistent with the experimental result. The LED using ODR films has higher efficiency with thinner phosphor layer, higher reflectance of bottom-boundary of phosphor layer for UV light, and smaller light absorption of silicone resin. A theoretical maximum improvement of 2.62X is predicted.

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