微奈米結構透明電極之製作與有機發光元件應用

Abstract

過去為了要克服在有機發光二極體元件上的出光量子效率，有相當多種在元件內外部的結構都曾經被研究；如常用的外部出光增益結構：微透鏡、表面結構、形狀化基板等等都被用來導出已經進入基板的光。內部結構一般是製作在元件有機層和基板間，使更多的侷限在波導與表面電漿模態的光被導出，進而比傳統無結構平面元件有更高出光效率；結合有顯著折射率差異材料的光子晶體和周期性起伏光柵結構等微奈米結構便是具有代表性的兩種有機發光二極體內部出光結構。過去很多研究中這些微奈米光子結構使用不導電材料所以獨立於元件主動層外，若是這些結構可以直接內嵌於元件主動層中則應可達成更有效地光學萃取。 本論文提出一種新穎且簡單製程，結合高折射率的透明導電膜及低折射率的導電高分子聚合物製作出微/奈米結構複合電極，並研究其在增加有機發光二極體出光萃取上應用。除此之外，也使用以嚴謹電磁波理論為基礎的時域有限差分法建構三維模型，以探討這些元件的光學特性和機制。透過光學模擬提供了深入的觀點去了解微/奈米結構複合電極如何增進元件光學萃取並和傳統平面元件比較。實驗和模擬顯示，微奈米結構複合電極可比平面元件更萃取多的光進入基板。結合外部出光結構(如透鏡)後，我們成功在綠色磷光有機發光二極體元件上研究出分別達到46.8%(微米結構透明電極)和61.9%(奈米結構透明電極)的外部量子效率。

In order to overcome the limited optical out-coupling efficiency of OLEDs, various strategies of using internal and/or external light extraction structures have been investigated. External light extraction structures, such as microlens, surface textures, and shaped substrates etc., are typically constructed outside the device (i.e. on the substrate side having no OLED devices) and provide access/extraction of light already entering the substrates. Yet it is the internal light extraction structures typically fabricated between OLED layers and substrates that provide further access/light extraction of ITO/organic and SPP modes and thus potentially give higher extraction efficiencies than simple external extraction methods. Photonic nanostructures such as photonic crystals composed of materials having sufficient contrast in optical refractive indices or corrugation/grating structures are two representative types of internal light extraction structures for OLEDs. In many cases, these photonic crystal or corrugation/grating structures are formed by non-conducting materials and thus they are extra materials and layers outside the active OLED layers. It may be desired to embed the photonic nanostructures directly into active layers of OLEDs so that even closer and stronger optical interaction may be induced to benefit larger light extraction. In this thesis study, a novel fabrication of micro-/nano-structured composite electrodes consisting of the high-index ITO micro-/nano-mesh and the low-index conducting polymer PEDOT:PSS by the facile and convenient microsphere lithography was investigated in this dissertation. We then studied their use for enhancing light extraction of OLEDs. In addition, the rigorous electromagnetic simulation based on the 3D FDTD method was conducted to study optical properties and mechanisms in these devices. It provides a different but consistent viewpoint/insight of how the micro-/nano-structured electrodes enhance optical out-coupling of OLEDs, compared to that provided by ray optics simulation in previous works. Both experimental and simulation studies indicate such micro-/nano-structured electrodes effectively enhance coupling of internal radiation into the substrate, compared to devices with the typical planar ITO electrode. By combining such internal extraction structures and the external extraction scheme (e.g. by attaching extraction lens) to further extract radiation into the substrate, rather high external quantum efficiencies of 46.8% and 61.9% were achieved with micro- and nano-structured green phosphorescent OLED, respectively.