新穎有機發光元件光萃結構之光學模型與分析設計研究

Abstract

有機發光元件(Organic light-emitting devices, OLEDs)近年發展由於技術逐漸成熟，因此在顯示器以及照明市場愈趨成為一不可小覷之主流技術，然在光萃效率之提升方法上仍為眾多學者們所研究之棘手問題，本研究中不討論材料本身之效率問題，僅假設內部量子效率為100%條件下討論如何以微奈米結構以及在整個畫素結構設計下提升元件出光效率。 在進行OLED結構設計時，由於部分結構尺寸小於波長(例如奈米網格結構、有機膜層厚度等等)，同時也具有尺寸遠大於波長之結構 (例如微米網格結構、畫素結構以及填充層厚度等等)，因此在研究中使用一複合多尺度(multi-scale)光學模型進行計算，該模型利用基於波動光學之數值方法計算尺寸小於波長之結構並保留該波動特性，同時也利用幾何光學之數值方法如光線追跡法進行簡化計算。 研究中首先介紹複合電極網格結構，係利用ITO網格結構，並搭配填入高導電PEDOT:PSS材料所形成，除了能有效將內部模態萃取進入基板外，同時也作為導電膜層維持一定之電氣特性，實驗中分別可以在微米網格與奈米網格加入OLED元件後得到46.8%(微米網格)以及61.9%(奈米網格)之外部量子效率，而在光學模擬計算下其外部量子效率可達到57%(微米網格)與64%(奈米網格)，與實驗結果接近，並能使用所建立之光學模型分析在元件內部中之電場與模態分布以解釋其增益機制。 而針對OLED顯示器畫素結構，研究中設計一具有高反射斜面之畫素定義層形成反射杯結構，並搭配以區域選擇性方式再填入折射率匹配材料，在適當畫素定義層幾何結構設計並搭配材料及膜層厚度優化，其外部量子效率在數值計算上可以達到約80% (ITO上電極元件)之表現。

Due to the established fabrication process and technology of organic light-emitting devices (OLEDs) in recent years, OLEDs have posed as promising candidates for the next-generation display and lighting technologies. However, typical OLEDs still suffer from a poor outcoupling efficiency of internally generated photons. In this study, the micromesh and nanomesh structures and the pixel structures with OLEDs are investigated to improve the outcoupling efficiency. Since the proposed OLEDs contains structures of very different dimensional scales, i.e., nm-scale structures that are smaller than wavelengths (e.g., nanomesh structures, thicknesses of the OLED active layers) and μ-scale structures that are significantly larger than wavelengths (e.g., micromesh structures, pixel size, bank height, filler thickness etc.), the optical properties of the proposed structure are analyzed with a complex multi-scale optical simulation. It combines the rigorous and analytical electromagnetic wave-and dipole-based power dissipation model that is good for dealing detailed emission properties from nm-scale layered structures, with the geometric optics simulation based on Monte Carlo ray tracing that is good for dealing larger-scale structures. In this thesis, we report the composite electrode consisting of the ITO micro/nanomesh and high conductivity conducting polymer PEDOT:PSS for enhancing the outcoupling efficiency of OLEDs. They effectively enhance the outcoupling of internal radiation into the substrate, in addition to functioning as the current conductor/injector to maintain the electrical properties. By combining this internal outcoupling structure and the external outcoupling scheme (e.g. by attaching extraction lens), a very high EQE of nearly 48.8% (nmicromesh structures) and 62% (nanomesh structures) were achieved with a green phosphorescent OLED, respectively. Also in the optical modeling and analysis, the calculated EQE are 57% and 64% with familiar improvement as the experiment results, and the extraction mechanism are investigated by analyzing the field and mode distribution. And considering the OLED pixel structures, the pixel definition layer (PDL) are designed to be highly reflective as a reflective cup (concave) structure. Then an index-matching material is selectively deposited into/around the concave area. With proper designed geometrical parameters of the PDL and the optimized OLED stacks thickness, the calculated EQE of nearly 80% was achieved with ITO electrode device.