Study of High-Efficiency and High-Color-Stability White Organic Light-Emitting Devices

本篇論文的核心是在如何設計適合且簡化之磷光觸發(第三章)以及高效率、高色穩定度(第四章)白光有機發光元件之發光層。除了基本的穩態量測分析，如亮度－電流－電壓以及效率表現，吾人亦使用暫態量測法去鑑定有機元件內載子動態與發光機制(第五章)。 第三章主要是在探討如何利用選擇性摻雜DCJTB於Ir(ppy)3之方法，製作一簡化之磷光觸發發光層。這遠離復合區域之共摻雜層可以在不影響載子傳輸特性的條件下，顯著地提高磷光觸發效率。此結構再搭配一高效率之藍色磷光發光體，即可製作出一於4.5V的驅動電壓(亮度107 cd/m2，CIE色座標(0.293, 0.427))下，可產生外部量子效率為5.51%、電流效率17.4 cd/A與電功效率12.2 lm/W之白光有機發光元件。兼之利用一分隔層來操控發光層內激子的擴散，該元件之色座標偏移量在亮度範圍1000-4000 cd/m2內為(-0.008, -0.006)。 在第四章前半的討論範圍內，吾人著重於探討藍光發光層對於色穩定度之影響。當大電壓降跨過藍光發光層時，會提高載子傳輸與注入對於外加電壓的敏感度。根據飛行時間量測法可知，mCP電子遷移率對於電場有很高的正相關性。因此，在高驅動電壓下，快速增加的mCP電子遷移率與下降之電子能障高度，將會使得具有較薄之藍光發光層元件之復合區域由藍光發光層延伸至綠光發光層。此復合區域之變化再加上薄藍光發光層所造成之較強的三重態-三重態激子淬熄，使得色穩定度更加惡化。在克服此兩個負面因素後，吾人可得一高色穩定度之藍綠光有機發光元件，其色座標隨著亮度由48.7 增加至12700 cd/m2僅些微由(0.256, 0.465)偏移至(0.259, 0.467)。如於此藍綠光結構內導入一紅色磷光發光體，即可製作出一高色穩定度之白光有機發光元件。該元件之色座標在常用之亮度範圍(1050-9120 cd/m2)內，呈現幾乎不變的狀態((0.310, 0.441)-(0.318, 0.446))。而其最大效率可達26.4 cd/A與19.8 lm/W。 於第四章後半，吾人使用與第四章前半所開發之元件架構，並利用選擇性摻雜之概念來設計藍光發光層，而開發出一超高色穩定度之白光有機發光元件。此白光元件之色座標在亮度145-12100 cd/m2變化範圍內，由(0.399, 0.483) 偏移至 (0.395, 0.479)。而在顯示器或照明的常用亮度範圍下(1240-4850 cd/m2)，色座標則由(0.401, 0.481) 偏移至 (0.400, 0.479)。除了在約略100倍的亮度變化下，元件表現出極為少量的色座標改變量((-0.004, -0.004))外，該元件之效率亦可達到34.1 cd/A，且在亮度小於2000 cd/m2的範圍內，效率皆大於30 cd/A。 在第五章的討論，吾人利用暫態電激發光量測法，經由分析不同Ir(ppy)3摻雜區域之有機發光元件，可發現Ir(ppy)3在mCP主體內表現出電洞傳導與電子注入之特性。在磷光觸發元件之暫態分析中，吾人觀察到，當關閉外加電壓脈衝後，一不尋常之DCJTB的強度增加。此不尋常之衰減的表現則可歸因為磷光觸發機制的存在。The core of this doctoral dissertation is based on how to design a suitable and simplified emitting layer (EML) in white organic light emitting devices (WOLED) for implementing phosphorescence-sensitization (chapter 3) and improving device efficiency and color stability (chapter 4). In addition to the steady state measurement such as B-I-V and efficiency performance, transient measurements will be utilized for further specifying the carrier dynamic and emission mechanism in OLEDs (chapter 5). In chapter 3, efficient phosphorescence-sensitization (PS) consisting of tris(phenylpyridine)iridium (Ir(ppy)3) sensitizers and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran) (DCJTB) exciton acceptors in a host material, N,N’-dicarbazolyl-3, 5-benzene (mCP), was implemented by a simplified emitting layer structure with selectively doped DCJTB in the Ir(ppy)3-doped region. This codoped region away from the recombination zone peak in the Ir(ppy)3-doped region significantly improved the PS efficiency without affecting the carrier transport characteristics. Coupled with an efficiently phosphorescent blue emitter, a WOLED with this PS-EML was demonstrated to have 5.51% external quantum efficiency, 17.4 cd/A and 12.2 lm/W. With a 3-nm spacer for manipulating the exciton diffusion in the emitting layer, a slight CIE coordinates variation of (-0.008, -0.006) was obtained at practical luminance levels of 1000 to 4000 cd/m2. In the first-half of chapter 4, we investigated the strong influence of the thickness of blue EML (B-EML) on color stability. The large voltage drop across the B-EML resulted in a higher sensitivity of the carrier transport and injection properties to the applied external voltage. According to carrier mobility measurements by the time-of-flight method, the electron mobility of the mCP exhibited a strong dependence on the electric field. Therefore, at a higher driving voltage, the more rapidly increasing electron mobility of the mCP and the decreasing energy barrier height on the electron transport path would extend the recombination zone from the B-EML to the Ir(ppy)3-doped mCP green emitting layer (G-EML) in devices with thinner B-EMLs. Coupled with the fluctuations of the recombination zone, stronger triplet-triplet exciton annihilation occurring in the thinner B-EMLs led to an even more evident deterioration of the color stability. After circumventing these two negative factors, a green-blue OLED with ultra-high color stability was demonstrated, with the CIE coordinates slightly shifted from (0.256, 0.465) to (0.259, 0.467) with increased luminance from 48.7 to 12700 cd/m2. Further adding a red phosphorescent dopant into this green-blue EML backbone, we successfully fabricated a WOLED with high color stability, which exhibited a nearly invariant CIE coordinate throughout the practical luminance range from 1050 ((0.310, 0.441)) to 9120 cd/m2 ((0.318, 0.446)) and maximum efficiencies of 26.4 cd/A and 19.8 lm/W. In the second-half of chapter 4, an ultra-high color stability of WOLED based on the same backbone was achieved by deliberately engineering B-EML with a selectively doping profile. The WOLED showed that CIE coordinate shifted from (0.399, 0.483) to (0.395, 0.479) as luminance increased from 145 to 12100 cd/m2 and from (0.401, 0.481) to (0.400, 0.479) as luminance from 1240 and 4850 cd/m2, the practical luminance range for display and lighting applications. In addition to the small CIE coordinate variation of (-0.004, -0.004) over wide luminance variation of about two order of magnitude, the device efficiency achieved a high value of 34.1 cd/A and kept larger than 30 cd/A below 2000 cd/m2. In chapter 5, by investigating OLED with different Ir(ppy)3-doped regions with transient EL measurement, carrier injection and transport in Ir(ppy)3 was found to show hole-transporting and electron-injection in mCP host. For PS-based OLEDs, an unusual increasing intensity of DCJTB after switching-off the applied voltage pulse indicated the occurring of PS process