功能性光電透明薄膜應用於有機發光二極體之研究

Abstract

發光二極體 (Organic light-emitting diode, OLED) 是現今的一種相當重要的顯示技術，尤其因為它有很好的對比度與色彩表現，使得OLED在中小尺寸顯示器方面具有非常高的應用價值與競爭力。然而，OLED在戶外的應用與其自身效率仍然有改善空間；其中一個因素是因為紫外光會對OLED造成衰退，使得其壽命與效率會顯著降低。另一個原因則是雖然現今的OLED材料有著接近100 % 的內部量子效率 (internal quantum efficiencies, IQEs)，但是要如何有效地將OLED內部的光給萃取出來仍然是一個議題。在本論文研究第一部分，我們結合了分佈式布拉格反射器 (distributed Bragg reflector, DBR)與可以吸收紫外光的無機材料來研製適合應用於OLED的抗紫外線 (ultraviolet blocking, UV-blocking)結構。透過改善傳統的DBR常遇到的穿透度起伏問題，我們成功達成在可見光穿透度高達92 %與在高能可見光和紫外光穿透度接近為0 %的抗UV光學結構。透過IEC 60068-2-5的嚴格戶外測試條件，我們更進一步地驗證了我們所設計的抗UV光學結構的確可以有效地保護OLED 面板受紫外線的傷害。第二部分則是利用了浸鍍法(dip coating)的溶液製程來製作具有低折射率的氧化銦錫(indium tin oxide, ITO)之透明電極。透過可變角度頻譜式橢偏術(variable-angle spectroscopic ellipsometry, VASE)，我們利用溶液製程所製作的ITO薄膜具有奈米級結構之空間折射率分佈可以很清楚地被定義。利用了低折射率的電極在OLED中可以有效地抑制波導模態(waveguided mode)，並且結合了具有較水平取向的放光偶極材料(preferential horizontal emitting dipoles)，與結合了有效外部光萃取結構來提升總體的耦合效率。

Organic light-emitting diode (OLED) displays have become an important display technology, especially in small-to-medium size applications due to their high efficiency, and good color gamut. However, their extension to outdoor applications (e.g. wearable, vehicular, signage applications etc.) and efficiencies are still facing significant challenges. One of the major issues is their weak resistance against the UV-light induced degradation. In addition, although current OLEDs can offer nearly 100 % internal quantum efficiencies (IQEs), light extraction of internally generated emission in OLEDs is still an issue for achieving high external quantum efficiencies (EQEs). In the first part of this dissertation, we focus on developing effective UV-blocking structures that can prevent OLED displays from sunlight/UV degradation and still keep the OLED panels’ display performance. Modified distributed Bragg reflector (DBR) structures having UV-absorbing dielectric materials and adjusted layer/pair thicknesses were developed to realize effective UV blocking properties (nearly 0% transmittance below 400 nm), constantly high transmittance like glass in the visible range (~92%) required for display applications, and sharp transition in transmission between the UV and the visible ranges. Furthermore, under the rigorous IEC 60068-2-5 solar test condition, it was verified that the developed modified, UV-blocking DBR can effectively enhance the OLED panel’s resistance against UV/solar-induced degradation, effectively reducing voltage shifts of OLED devices after repeated solar test cycles. In the second part of this dissertation, we focus on developing solution-processed and low-refractive index transparent indium tin oxide (ITO) electrodes. Further combining the preferentially horizontal dipole emitters realized highly efficient thermally activated delayed fluorescent (TADF) OLEDs, since low-index electrodes in OLEDs can suppress waveguided modes. In this study, we adopt dip coating methods to develop solution-processed ITO (Sol-ITO), and we discuss the influences of different coating times, different pulling rates in Sol-ITO. With variable-angle spectroscopic ellipsometry (VASE), we further characterize the spatial distributions of nanostructures and the refractive index in Sol-ITO films with different coating conditions. With bi-layer (coating two times, n~1.5-1.7) Sol-ITO as low-index anodes for bottom-emitting OLEDs, overall coupling efficiencies of OLED internal radiation into substrates are substantially enhanced and very high EQEs can be achieved through effective extraction of radiation within substrates.