可用於增進有機發光顯示器性能之有機發光材料及元件研究

Abstract

主動式有機發光二極體 (active-matrix organic light-emitting diode, AMOLED)因為其輕薄的結構、製程簡易和傑出的視覺表現(例如：反應速度快、飽和的色彩顯示、高對比以及廣視角)已成為主流的顯示技術之一，在高畫質的產品應用中如小尺寸顯示器或可攜式裝置，一般是採取上發光的OLED結構，此種結構由於其像素內的薄膜電晶體 (thin-film transistors)可以很好地成長在透明或不透明基板上以及埋在OLED疊層下方而不阻擋上發光式OLED的出光，使得高性能的驅動電路可以完好地整合在AMOLED中卻不降低顯示的畫質以及開口率 (filling factor, FF)。在產業應用上，常見的上發光OLED結構是將有機層夾在下反射陽極與上薄金屬半透明陰極之間，由於兩邊金屬電極的高反射率特性，上發光OLED會在其中產生足夠的微共振腔效應而極大地影響元件效率以及不同視角上的色彩表現，雖然現今的OLED可以透過使用載子複合較平衡的OLED結構以及高效率利用三重態 (triplet)的放光材料而達到近乎100%的內部量子效率 (internal quantum efficiencies, IQEs)，但是上發光OLED元件的內部發光仍僅少部分能萃取出來，波導模態、表面電漿模態及兩側金屬電極吸收導致的輻射能量流失構成了上發光元件提升其外部量子效率 (external quantum efficiencies, EQEs)的阻礙。此外，上發光OLED較強的微共振腔效應固然能使元件發光顏色更加地飽和，然而該效應也會使共振波長在不同方向上產生明顯的變化而導致隨角度變化帶來的嚴重的頻譜偏移以及色彩偏差。因此，現今OLED顯示器的低效率以及待改善的各角度色彩表現仍然是值得研究的關鍵議題。 為了進一步提升OLED出光，諸多光萃取技術被提出來或發表以增加出光效率，然而此些技術由於其製程不相容性、製造複雜度且可能破壞顯示器的影像品質而一般不太適合用於上發光OLED或AMOLED的像素結構中。因此，我們提出了一個簡單且實用的方式以同時解決以上的問題，其能夠在不影響影像清晰度及複雜化整個AMOLED的製程下，同時增加元件效率且增進色彩表現，此方法順勢利用傳統AMOLED的像素凹槽結構，將底面的下反射陽極延伸擴展到像素凹槽的邊坡之上以形成圍繞像素中心發光區域的三維反射面，之後再於像素的凹槽區域內充填高折射率填料 (filler)在OLED疊層之上，如此一來，OLED放光物質於發光層內部所產生的光其絕大部分皆能被萃取出而進入高折射率的填料區內，雖然起始入射角大過填料/空氣介面全反射臨界角的光起初仍會被困在填料區內而無法輻射到空氣，但其後透過周遭的反射面產生的多次反射與變向，最終可能順利脫離填料區而成功出光，並增加元件的出光效率。 本論文研究的第一部分詳細並依序地闡述了針對OLED三維光學結構而採用的多尺度光學模擬之工作原理及運算流程介紹、如何在裸玻璃基板上形成完整的反射式三維OLED像素凹槽結構以及如何準確地填充高折射率材料至每個像素內的實際製程(從基板規劃與反射式三維像素結構的具體設計介紹、精密金屬遮罩 (fine metal mask, FMM)的設計方案與製作細節、基板的製備、基板的預處理、OLED的蒸鍍、FMM/基板的對位流程、填料的沉積到最後的元件玻璃封裝)、多像素結構的上發光元件量測方法、所得的模擬/實驗結果之數據處理。在此研究中，我們更進一步地討論像素大小以及填料位置的對準情形對元件發光特性帶來的影響。實驗結果部分諸如：像素凹槽的(側)截面影像、元件的上視照、元件電性與效率、實驗發光場型與變角度頻譜、元件的色彩表現(CIE色座標與色偏量)皆呈現於本論文研究的結果與討論內，在採用反射式三維像素結構於上發光OLED之中後，可觀察到其在不影響影像品質下，具有較佳的色彩表現以及顯著的元件效率提升，使最小的反射式三維像素元件之效率增益高達1.28倍。實驗數據和模擬結果的一致性也共同驗證了反射式三維結構的元件效率增益會隨著像素縮小而隨之遞增。最重要的是，填料的對位情況並不會顯著地影響效率增益、造成各視角上的顏色變化以及破壞影像清晰度，這暗示著此結構對於填料對位誤差具有較高的容忍度，因此可放寬對位精確度的嚴格控制/要求以進一步地降低製程成本。總結來說，本項研究成果提供了一個簡單且低成本的方式以有效地降低現行OLED顯示產品的功耗，為AMOLED顯示技術的發展開拓出一條嶄新且前景廣闊的道路。 本論文的第二部分則為深藍磷光材料的光物理分析及元件研究，由於雙三牙配位銥金屬錯合物材料相比於三雙牙配位銥金屬錯合物具有較佳的剛性，我們合成出數種新穎且高效率的深藍雙三牙配位銥金屬錯合物磷光材料，並對其作光物理分析，再利用其中具有最高量子效率(~91%)的磷光材料製作出外部量子效率(EQE)高達20.7%且CIE (x, y)色座標位於(0.15, 0.17)的深藍磷光有機發光元件。

Active-matrix organic light-emitting diode displays (AMOLEDs) have become a major display technology due to their slim structure, simple fabrication process, and outstanding visual performances such as fast response, saturated color, high contrast, and large viewing angle. In the general design of high resolution applications such as small size displays or portable devices, a top-emitting configuration is preferred since the pixel thin-film transistors could be grown on transparent or opaque substrates and be buried under OLED stacks. They would not obstruct the light out-coupling for top-emitting organic light emitting diode (TOLED). Hence, the high-performance driving circuits could be integrated well in AMOLEDs without further reducing the display resolution or filling factor (FF). In industry applications, TOLED has a general structure of organic active layers sandwiched between the bottom reflective anode and the top semi-transparent thin metal cathode. Due to the high reflectivity of two metal electrodes, the strong microcavity effect would occur in TOLED and greatly influence the efficiency and angle-dependent color performance of the device. Although current OLEDs can reach nearly 100% internal quantum efficiencies (IQEs) by employing carrier-balanced OLED structures and efficient triplet-harvesting emitters, waveguided modes, surface plasma modes, and corresponding energy losses from both metal electrodes still limit external quantum efficiencies (EQEs) of TOLEDs. While the stronger microcavity effect in TOLED could make device emission color more saturated, it would also cause the significant change of resonant wavelengths vs. viewing angles, leading to the severe angular spectra/color shift. Hence, the low efficiency and unsatisfactory angular color performance are still critical issues for current OLED displays. Numerous optical extraction techniques were proposed and reported to enhance OLED efficiency. However, they are generally not applicable in TOLEDs or AMOLED pixel configurations due to manufacturing incompatibility, process complexity, and defective display image quality. To simultaneously boost device efficiency and to improve the color performance yet without deteriorating image clarity and complicating the overall AMOLED manufacturing process, we proposed a simple and practical way to solve above issues altogether by taking advantage of the conventional concave pixel structure in AMOLED and extending the bottom reflective anode onto the pixel bank slope to from a 3-dimensional reflective surface surrounding the pixel emission region. After filling the high-index filler material onto OLED stack and into the pixel concave region, the majority of internally generated photons from OLED emitters could be extracted into the filler region. The light with an initial internal angle larger than the total-internal-reflection (TIR) critical angle of the filler-air interface would be firstly guided in the filler and could be redirected via surrounding reflective surface for out-coupling. It could make the confined light eventually escape out of the filler region to enhance light extraction and to improve device out-coupling efficiency. The working principle and calculating procedure of multi-scale simulation for this 3-dimensional OLED pixel structure, the manufacturing process of the entire reflective 3D concave OLED pixel configuration filled with patterned high-index material on a glass substrate (from the experimental layout for the device sample and the reflective 3D pixel, the design scheme and the fabrication details for the fine metal mask (FMM), substrate preparation, substrate preprocessing, OLED evaporation, FMM-sample alignment procedure, filler deposition, to final sample glass encapsulation), the measurement/characterization techniques for the fabricated top-emitting pixel devices, and the data processing for the collected simulated/experimental results are discussed in the first part of this dissertation. In this study, we further discuss the influences of the pixel dimension and the filler alignment condition on the emission characteristics of the device. The results, including cross-sectional/top-view device images, electrical characteristics, EQEs, emission pattern, angular spectra, and color performance such as CIE color coordinates and color shift, are presented. With the adoption of reflective 3D pixel configuration for TOLED, the maintained image quality with better color performance and substantially enhanced EQEs are observed, achieving a highest EQE gain of up to 1.28 for smallest 3D pixel device. With consistency between experiment and simulation, it was verified that the EQE gain of reflective 3D configuration would increase as device pixel dimension decreases. Moreover, the filler alignment condition would not substantially influence the EQE gain, color vs. viewing directions, and image clarity, indicating that the proposed configuration is rather tolerant of filler location variation/imperfection. The tight control/requirement of filler alignment accuracy could thus be relaxed to further reduce the production costs. In summary, results of this work provide a simple and low-cost way to effectively reduce power consumption of current OLED display products and shed light on the development of AMOLED display technology. In the second part of this dissertation, we focused on the photophysical analyses and device studies for deep-blue phosphorescent materials. Since Ir (III) metal complexes possessing two tridentate chelates (bis-tridentate) are known to be more robust compared to those with three bidentate chelates (tris-bidentate), the novel deep-blue-emitting bis-tridentate Ir (III) metal phosphors with high PLQYs were analyzed by photophysical characterization. A deep-blue phosphorescent organic light-emitting diode incorporating the phosphor with highest PLQY (~91%) was then fabricated and exhibited CIE (x, y) color coordinates of (0.15, 0.17) with maximum external quantum efficiency (EQE) up to 20.7%.