高效率透明有機發光二極體研究

Abstract

本論文首先從模擬與實驗去探討透明下電極與半透明薄金屬上電極的厚度改變對於透明有機發光二極體的光學特性(例如：穿透頻譜、反射頻譜)與效率的影響。在所選定的上電極與下電極厚度變化範圍之內，元件所表現出來的電致發光效率總和(上發光與下發光加總)、電致發光頻譜與角度分布皆很一致，其中上發光與下發光的比值可由半透明薄金屬上電極來控制，而透明下電極的厚度變化對於元件內的微共振腔效應影響微弱，故此變因對於元件效率整體影響不大，但是對於穿透頻譜與反射頻譜的趨勢(波峰/波谷)會有所影響。同時，半透明薄金屬上電極對於元件的穿透/反射性質有關鍵性的影響，因此，透明有機發光二極體元件根據不同穿透/反射性質所顯現出來的視覺顏色便可透過改變上/下電極來控制其色調變化。在本論文中，我們實現了高效率透明綠色磷光有機發光二極體，穿透率可達81%，外部量子效率總和可達21-21.5%(其對應的電流效率總和為80-52 燭光/安培、能量效率總和為95-99 流明/瓦)，穿透率、效率都是目前世界上未添加出光結構的平面透明有機發光二極體最高的。透過變化透明上電極與半透明薄金屬下電極不但可決定不同的上下發光比例，也能夠在不需要犧牲電致發光效率與改變發光材料的前提下，控制透明元件在不發光時從自然光穿透/反射下顯現出來的視覺顏色，此種可調控的透明有機發光二極體光學特性或許能夠在未來找到許多有趣的應用。 進一步地，我們提出一種使用貼合上電極技術製作而成的高效率透明有機發光二極體的製程方法。這項研究成果使得利用導電高分子材料作為透明貼合膠成為可行的選項。我們將此方法應用於高效率有機小分子磷光發光材料，製作出外部量子效率達11.4%、電流效率達43.1燭光/安培的有機發光二極體，效率高於目前世界上發表過的其他使用貼合電極技術製作出來的有機發光二極體。此元件具有非常平衡的上/下側電致發光強度與頻譜、很寬的穿透頻譜帶以及達77%的最高穿透率。此研究提供了一種簡單又有效的製程方式來製作高效率透明有機發光二極體，此種方法未來也能夠應用於可撓式元件或是捲對卷製程的應用。

We conduct both simulation and experiment studies of impacts of simultaneously varying the thicknesses of transparent bottom electrodes and semi-transparent top thin metal electrodes on optical characteristics (e.g., transmission, reflection) and efficiencies of transparent organic light-emitting devices (OLEDs). For the thickness range of both electrodes studied, the total electroluminescent (EL) efficiencies (including both bottom and top emission), EL spectra, and emission patterns remain similar; yet the ratio of top to bottom emission would be modulated by the semi-transparent top metal electrode thickness. The thickness of the transparent bottom electrode has weak effects on the efficiencies of top/bottom/total emission, but it does have definite effects on optical transmission/reflection spectra (e.g. peak/valley wavelengths). Meanwhile, the thickness of semi-transparent top metal electrodes mainly affect magnitudes of the optical transmission/reflection. Transmissive/reflective hues and appearances of the transparent OLEDs can thus be tuned by the thicknesses of bottom/top electrodes. Overall, we demonstrated efficient transparent green phosphorescent OLEDs exhibiting a high peak transmittance of up to 81% and rather high total external quantum efficiencies of up to 21-21.5% (corresponding to a total current efficiency of 80-82 cd/A and total power efficiency of 95-99 lm/W), among the highest (if not the highest) for planar transparent OLEDs using no other optical out-coupling structures. By varying the thicknesses of transparent bottom electrodes and semi-transparent top metal electrodes, the ratio of top to bottom emission, and the transmissive or reflective hues/appearances of transparent OLEDs in the off state can be tuned, yet without sacrificing total EL efficiencies or changing their EL colors/patterns. Such tunable optical characteristics of transparent OLEDs may find some interesting applications. Furthermore, we report efficient transparent small-molecule organic light-emitting devices (OLEDs) with laminated top transparent electrode. This was made possible via the aid of the transparent conductive adhesive based on conducting polymers. With use of small-molecule materials and efficient phosphorescent emitters, rather high external quantum efficiency and current efficiency of up to (11.4%, 43.1cd/A), much higher than those in previously reported OLEDs using laminated top electrodes, were obtained. The transparent OLEDs exhibited rather balanced electroluminescent intensities and spectra from both sides of the devices, a broad optical transmission band, and a high optical transmittance of up to 77%. It provides a simple and yet effective approach for fabrication of efficient transparent OLEDs and can be extended to flexible devices or even roll-to-roll processes in future.