提升有機發光二極體發光效率之研究

The Study to Improve the Luminance Efficiency of Organic Light Emitting Diodes

王舜熙(Shun-Hsi Wang)

指導教授 ： 莊賦祥

共同指導教授 ： 蔡裕勝

國立虎尾科技大學/電機資訊學院/光電與材料科技研究所/博士(2011年)

摘要

研究中利用旋轉塗佈搭配熱蒸鍍方式來製作電洞傳輸層以提升可撓式螢光與磷光OLED元件發光效率。是利用將電洞傳輸材料(α-NPD或NPB)溶解於有機溶劑(THF)中，再利用旋轉塗佈的方式將有機溶液旋塗在ITO陽極上以改善薄膜表面粗糙度並減少尖端突起。此外磷光OLED元件中分別以BCP或TPBi做為電洞/激子阻隔層並最佳化其厚度。研究結果顯示，可撓式螢光OLED元件最佳發光效率為3.87 cd/A，磷光OLED元件可達24.4 cd/A。由於利用利用旋轉塗佈方式製作電洞傳輸層，可有效改善表面粗糙度(及產生類似歐姆接觸的特性)，可增加電洞由陽極注入有機層的效果；此外以熱蒸鍍方式製作之電洞傳輸層，其薄膜有較佳的緻密性跟較少的缺陷，因此可有效改善電洞傳輸層與發光層的接觸特性。此外透過可撓式OLED元件壽命測試亦顯示，利用旋轉塗佈搭配熱蒸鍍沉積電洞傳輸層，可有效提升可撓式OLED元件壽命。
研究中亦利用將電子傳輸特性較佳之材料，與發光層部分共混合(partially-mixed)以製作藍色磷光OLED元件的複合式主發光體結構(composite host structure)，提昇電子注入的效果，並藉由控制元件中載子再結合區於發光層中間以擴大發光區域，進而大幅提升元件發光效率。利用將電子傳輸層(3TPYMB)與雙極性(bipolar)主發光材料(26DCzPPy)以1:1比例共摻雜，可降低電子由陰極注入到電子傳輸層的能障，並可以沒有能障的方式傳輸到發光層，進而提升發光層中電子的數量並降低驅動電壓。結果顯示，藍色磷光元件若撘配增亮膜，於發光亮度1000 cd/m2時，驅動電壓為6.5 V，功率效率為14.3 lm/W。接著，研究中以具較高電子移動率(7.0x10-4 cm2/Vs)之電子傳輸材料(TmPyPB)，與具電洞傳輸(1.6×10-4 cm2/Vs)特性材料(TCTA)共摻雜，並加入電洞注入層(EHI608)，提升電洞注入的效果，且利用CsF取代LiF且最佳化元件厚度。結果顯示，藍色磷光元件，於亮度1000 cd/m2時，驅動電壓可降至4.4 V，效率可提升至26 lm/W。
研究中利用高熱導係數銅來作為上發光OLED元件基板，並將UV膠旋塗在基板表面作為絕緣層，並達到改善表面粗糙度的效果。實驗結果顯示，銅基板之元件有較玻璃基板之元件佳的發光特性，其最大亮度可達14110 cd/m2、最大發光效率為7.14 cd/A，且元件可操作至更高的電壓及電流下而不會燒燬，發光效率衰退亦較緩慢。此外利用量測OLED元件基板表面與內部接面溫度，以探討散熱對OLED元件發光特性的影響。令研究中藉由比較接面溫度量測時因量測之元件類型不同，對量測參數之調整的影響。於表面及接面溫度量測量測結果發現，利用將上發光元件製作於銅基板上，元件會有較低的基板表面溫度(25.7 °C)和接面溫度(37 °C @ 5130 cd/m2)。另由元件熱流率計算，影響OLED元件熱流率的主要因素為基板及UV膠的熱導係數，其次則是基板與絕緣層的厚度。元件熱流率結果顯示，利用銅做為OLED元件基板，可有效增強元件熱導係數，進而改善元件發光效率及壽命(壽命可提升7倍以上)。此外於熱流率推導中發現，除使用高熱導係數材料做為基板，若能有效薄化基板厚度亦可提升元件熱流率，此經由實驗亦證實，當銅基板厚度由0.5 mm薄化至0.05 mm時，元件最大發光效率可由7.14 cd/A再提升至7.73 cd/A。

關鍵字

有機發光二極體 ； 旋轉塗佈 ； 電荷載子注入 ； 電荷載子平衡 ； 散熱 ； 絕緣層 ； 接面溫度 ； 熱流率 ； 效率

並列摘要

The hole transport layers (HTL) made via spin-coating and evaporating were investigated to improve the luminance efficiency of flexible fluorescent and phosphorescent organic light emitting diodes (OLEDs). We dissolved hole transport materials (α-NPD or NPB) in THF solvent, and spin-coated the α-NPD+THF or NPB+THF solution onto ITO anode surface to improve the surface roughness and to reduce the spikes. Then the BCP and TPBi were employed as hole blocking layer (HBL) of phosphorescent device and its thickness was optimized. From the experimental results, the maximum efficiency is 3.87 cd/A of fluorescent device and 24.4 cd/A of phosphorescent device, respectively. Such an improvement in the device performance was attributed to the smoother surface (has like ohmic contact property), the hole were effectively injected from the anode into the organic layer. And the evaporated film has better compactness and fewer defects, thus improving the contact between the interface of evaporated HTL/emitting layer. In addition, the lifetime of flexible OLED with HTLs made via spin-coating and evaporating is longer than that for device with evaporating-only.
Performances improvement of blue phosphorescent OLEDs with composite host structure were investigated by partially-mixing hole and electron transport-type hosts in emitting layer. Improved charge carrier injection and confined triplet excitons within emitting layer, controlled the excitons generation zone within the middle of the EML and extended the recombination zone were achieved. First, the electron transport material (3TPYMB) which has low-lying LUMO level and high triplet energy was partially mixed into the bipolar host (26DCzPPy) to reduce the electron injection barrier and the driving voltage of blue PHOLED. From the experimental results, the blue PHOLED device (with an out-coupling enhancement film) showed the driving voltage of 6.5 V and the power efficiency of 14.3 lm/W (@ 1000 cd/m2). Next, the hole transport-type host material (TCTA) and electron transport material (TmPyPB) were partially-mixed as a composite host structure to improve the injection of charge carriers. Moreover, a hole injection layer (EHI608) was inserted to improve the hole injection and the electron injection layer (LiF) was replaced by CsF to enhance the electron injection. From the result, the blue PHOLED showed the driving voltage of 4.4 V and the power efficiency of 26 lm/W (@ 1000 cd/m2).
A high heat dissipation material (Cu) was employed as the substrate for top-emission OLEDs (TEOLEDs). UV glue was spin-coated onto the Cu substrate as the insulation layer to effectively improve surface roughness. From the experimental, the optimized device with the Cu substrate shows the maximum luminance of 14110 cd/m2 and efficiency of 7.14 cd/A. In addition, the surface and junction temperatures are measured to discuss the heat dissipating effect on performances. By comparing the junction temperatures, the effects of adjustment parameters were discussed. TEOLED fabricated on Cu substrate has lower junction (55.34 °C) and surface (25.7 °C) temperatures. From the calculations with the rate of heat flow, results shown that the major factors were the thermal conductivity and thickness of the substrate and UV glue. As the results, using Cu substrate could effectively enhance the thermal conductivity of TEOLEDs, and to further improve the luminance efficiency and lifetime of OLEDs (about 7 times). In addition, the calculating results of the device rate of heat flow had show, that besides use of substrate materials with high thermal conductive coefficients, the experiment also demonstrated if the substrate thickness could be effectively made thinner (from 0.5 to 0.05 mm), the thermal conductivity of a device was increased, and was also able to simultaneously increase the device efficiency (from 7.14 to 7.73 cd/A).

並列關鍵字

organic light-emitting diode ； spin-coating ； carrier injection ； charge carrier balance ； heat dissipation ； insulation ； junction temperature ； heat flow rate ； efficiency

參考文獻

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