Analysis and optimization of light outcoupling in OLEDs with external hierarchical textures

Hierarchical textures (combining 2D periodic large and small micro textures) as an external outcoupling solution for OLEDs have been researched, both experimentally and by optical simulations. For the case of a red bottom emitting OLED, different hierarchical textures were fabricated using laser-based methods and a replication step and applied to the OLED substrate, resulting in an increased light outcoupling. Laboratory-size OLED devices with applied textured foils show a smaller increase in efficiency compared to the final large area devices. The results show that the full exploitation of textured foils in laboratory-size samples is mainly limited by the lateral size of the thin film stack area and by limited light collection area of the measuring equipment. Modeling and simulations are used to further evaluate the full prospective of hierarchical textures in large area OLED devices. Optimization of hierarchical textures is done by simultaneously changing the aspect ratios of the small and large textures and a potential of 57% improvement in EQE compared to devices without applied textures is predicted by simulations. Optimized hierarchical textures show similar outcoupling efficiencies compared to optimized single textures, while on the other hand hierarchical textures require less pronounced features, lower aspect ratios, compared to single textures to achieve the same efficiencies. Hierarchical textures also help in eliminating flat parts that limit outcoupling efficiency. Finally, the limiting factors that prevent higher outcoupling are addressed. We show that the dominant factor is non-ideal reflection from the organic thin film stack due to parasitic absorption. In addition, possible ways to further increase the outcoupling from a thick substrate are indicated.Fil: Kovacic, Milan. University of Ljubljana; EsloveniaFil: Samigullina, Dinara. University of Ljubljana; EsloveniaFil: Bouchard, Felix. Technische Universität Dresden; AlemaniaFil: Krc, Janez. University of Ljubljana; EsloveniaFil: Lipovšek, Benjamin. Technische Universität Dresden; AlemaniaFil: Soldera, Marcos Maximiliano. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigación y Desarrollo en Ingeniería de Procesos, Biotecnología y Energías Alternativas. Universidad Nacional del Comahue. Instituto de Investigación y Desarrollo en Ingeniería de Procesos, Biotecnología y Energías Alternativas; ArgentinaFil: Lasagni, Andres Fabian. Technische Universität Dresden; AlemaniaFil: Reineke, Sebastian. University of Ljubljana; EsloveniaFil: Topic, Marko. University of Ljubljana; Esloveni

유기발광소자에 대한 다양한 광추출 구조와 배향 분극 분자의 효과

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 재료공학부, 2020. 8. 김장주.Since the first electroluminescence (EL) of organic compound was observed in the anthracene single crystal in 1965, organic light-emitting diodes (OLEDs) have undergone numerous developments, and have recently become the mainstream of small-sized displays. In addition, as OLEDs will have a modest substrate dependency, it has the potential to receive even more attention as the next generation display like flexible or transparent displays. The remaining challenges for OLEDs are high efficiency, and operational stability. In general, because organic materials have a wide emission spectrum, OLED can be a good lighting with a high color rendering index (CRI). It can also be used as a display with high color purity by optimizing the cavity length. However, more than half of the photons generated in OLEDs are dissipated due to total internal reflection by high-refractive-index organic layers and the substrate. Therefore, light extraction technology is required to increase the efficiency and reduce the power consumption of OLEDs. In addition, as mentioned before, it is necessary to consider the lifetime of the device. There have been many studies on the short operation lifetime of OLEDs, and several mechanisms have been proposed, but there is no concrete description of the origin of degradation. Nonetheless, it is obvious that the stability of the layers constituting the OLED should be considered for high operational stability. This thesis concerns two research topics: (1) simple and efficient light extraction method for OLED lighting and display, and (2) improving operation lifetime of OLED using spontaneous orientation polarization molecule. In chapter 1, a brief introduction of OLEDs will be provided. In chapter 2, a facile and effective method for fabricating random organic microstructures for efficient light extraction from blue OLEDs is presented. Simple drop casting of a TCTA and B4PyMPM mixed solution followed by UV curing results in films with irregular-shaped microstructures (DACMs), ideal for light extraction without diffraction patterns. An external quantum efficiency (EQE) of 44.3% is realized by attaching DACMs formed on a polymer film to a blue phosphorescent OLED. The efficiency is improved by 35% compared to a planar device without the light extraction layer, greater than the 22% improvement obtained by using microlens arrays. The method is useful for OLED lighting and potentially in displays because of the simple fabrication method that is applicable to a large area on rigid or flexible substrates, the low material cost, the insolubility of the microstructure in alkyl halide solvents such as chloroform, and the controllability of the structure through the solution process. In chapter 3, we show the damageless light extraction structure for top-emittng organic light-emitting diodes (TEOLEDs). TEOLEDs are used in small displays, due to the high aperture ratios, unblurred imaging, and high color purity. However, the strong cavity structures responsible for these advantages result in a high optical loss within metal electrodes. Furthermore, as there is no substrate in the light path, it is rather complicated to form a structure on the emission surface without damage. Here, we present a facile and effective method for light extraction of TEOLEDs. When 1,5-diaminoanthraquinone (DAAQ) is deposited on the Ag electrode, crystallization occurs immediately and nanowire arrays are formed in the out-of-plane direction. The shape and distribution of nanowire arrays can be controlled by deposition thickness and evaporation rate. In addition, morphological changes affect the transmittance of DAAQ-deposited Ag thin films. DAAQ nanowire arrays were applied to a red phosphorescent inverted TEOLED, enhancing the external quantum efficiency (EQE) by 8.6% in a narrow full-width-at-half-maximum (FWHM) device, and by 10.6% in a wide FWHM device. The emission spectra of the out-coupled devices are similar to those of the reference devices, as the DAAQ layer is located outside the OLED stacks sandwiched between two highly reflective metal electrodes. The method is useful for OLED displays because it is simple, vacuum-processable, and does not compromise device lifetime or the emission spectrum. In chapter 4, we suggest the importance of patternability of internal light extraction layer. Integration of internal light extraction layers in OLED displays requires electrical connection between driving circuits in the backplane and an OLED electrode, therefore needs fabrication of a via hole. Generally, internal light extraction layers consist of two materials with different refractive indices; thus, good patternability may be difficult with the matched etch selectivities of these two materials. Both the patternability of the internal light extraction layer and high out-coupling efficiency are important so, it needs a proper etchant which arent lowering the extraction efficiency by demolishing its structure. Here, the patternability of light extraction layers is discussed and demonstrated experimentally. The random scattering layer (RSL) composed of SiOx nano scatterers and a TiO2 planarization layer was used in this study and it was patterned by photolithography and wet etching processes. For matching the etching selectivity of those two different materials, a mixture of buffered oxide etchant (BOE) and phosphoric acid (H3PO4), in a volume ratio of 0.5 of H3PO4 to BOE, shows the best results for forming electrical channels through the out-coupling layer. The OLEDs fabricated on this patterned substrate showed similar current density-voltage (J-V) characteristics to OLEDs on a glass substrate with low leakage levels. The device showed over 50% enhancement of external quantum efficiency (EQE; from 21.7% to 32.7%), similar to the device without via holes. Chapter 5 contains a method for improving the stability of electron transporting layer. After the alignment of permanent dipole moment (PDM) was found in Alq3, it was revealed that spontaneous orientation polarization (SOP) was observed in several electron transporting materials. Although studies on the electrical effects of the polarization have been reported, there are few reports on the effects of polarization on the lifetime of device. Here, we observed the SOP characteristics and the change in the lifetime of device when BAlq was doped in PO-T2T with different volume ratios. As the polarization increased, the operation lifetime also increased and the applied voltage change decreased. In addition, the lifetime enhancement was only observed when there was a polarized layer at the interface with the emtting layer (EML). This shows that hole blocking layer (HBL) can enhance the lifetime when it has a negative surface charge at the interface with EML, and also indicates that the SOP characteristics of the molecule should be considered for improving the lifetime.1965 년, 안트라센 단결정에서 유기물의 첫 전기 발광이 발견된 이후, 유기 발광소자는 많은 발전을 거듭해 최근 소형 디스플레이의 주류를 이뤘다. 또한 유기 발광소자는 적은 기판 의존성을 갖기 때문에 플렉시블 디스플레이나 투명 디스플레이와 같은 차세대 디스플레이로 활용될 수 있는 잠재력을 갖고 있다. 유기 발광소자에서 남은 과제는 높은 효율과 구동 안정성이다. 일반적으로 유기물은 넓은 발광스펙트럼을 보이기 때문에, 유기 발광소자는 연색성이 높은 조명으로 사용될 수 있다. 또한, 광학 공진 구조 최적화를 통해 높은 색순도의 디스플레이로도 활용될 수 있다. 하지만 유기 발광소자가 방출한 광자의 절반 이상은 높은 굴절률의 유기물 및 기판으로 인한 내부 전반사로 소멸된다. 따라서, 유기 발광소자의 효율을 올리고 전력 소모를 감소시키기 위해 광추출 기술은 필요하다. 이에 더불어, 위에서 언급한 바와 같이 소자의 구동시간에 대한 고려도 필요하다. 유기 발광소자의 짧은 구동 수명에 대한 연구는 많이 있고, 몇몇 메커니즘이 발표되었지만, 아직 소자 열화의 전반을 설명할 수 있는 이론은 전무하다. 그럼에도 불구하고, 높은 구동 안정성을 위해선 유기 발광소자를 구성하는 층의 안정성을 고려해야 하는 것은 자명하다. 본 논문은 2 가지 연구주제: (1) 단순하면서도 효율적인 조명용 그리고 디스플레이용 광추출 구조, 그리고 (2) 자발적 배향 분극 분자를 활용한 소자의 수명 향상에 대한 내용을 다루고 있다. 제1 장은 유기 발광소자에 대한 간략한 서론을 담고 있다. 제2 장은 파란색 하부 발광형 유기 발광소자의 외부 광추출 효율 향상을 위한 손쉬우면서도 효율적인 랜덤 유기 마이크로 구조 (DACM)에 대한 내용을 담고 있다. TCTA와 B4PyMPM 혼합 용액을 필름에 드롭 캐스팅 후 UV 경화 시키면 무작위 모양의 마이크로 구조물이 형성되었고, 회절 무늬가 없어 광추출로 적합했다. DACM 필름을 부착한 소자는 44.3%의 외부 광자 효율을 가졌고, 광추출 구조가 없는 소자 평판 소자 대비 35%의 향상을 보였다. 또한, 마이크로 렌즈 어레이 필름에 향상량인 22% 보다 높은 수치를 기록했다. 이 구조는 기판에 상관없이 대면적 공정이 가능하고, 저렴한 제작 공정, 클로로포름과 같은 알킬 할라이드 용매에 강했으며, 구조 컨트롤도 가능하기 때문에 조명용으로 매우 효율적이고, 디스플레이용으로 활용될 잠재력이 있다. 제3 장은 진공 열증착을 통해 손상 없이 형성한 상부 발광형 유기 발광소자용 광추출 구조에 대한 내용을 담고 있다. 상부 발광형 소자는 높은 픽셀 종횡비, 흐려짐 없는 이미징, 높은 색 순도로 인해 소형 디스플레이에 활용되고 있다. 하지만, 이러한 이점을 얻는 강공진 구조는 금속 전극에 의한 높은 광학 손실을 야기한다. 게다가 상부 발광형 소자에는 빛의 경로에 기판이 없는 까닭에 손상 없는 공정이 어렵다. 우리는 1,5-diaminoanthraquinone (DAAQ) 유기물의 진공 열증착을 통해 상부 발광형 소자의 은 (Ag) 박막 위에 손상 없이 광추출 구조를 형성했다. DAAQ는 은 박막 위에서 증착 즉시 결정화되었으며, 기판에 수직 방향으로 나노와이어를 형성했다. 나노와이어 어래이의 높이, 둘레, 그리고 주기는 증착 속도 및 두께로 조절 가능했다. 이를 상부 발광형 소자에 접목시켰고, 좁은 반치폭의 소자에서는 8.6%, 넓은 반치폭의 소자에서는 10.6%의 향상량을 얻었다. 이 방법은 쉬우며, 소자에 손상을 주지 않아 열화 시키지 않으며, 패터닝 공정이 필요 없고, 진공공정이 가능한 까닭에 매우 유용하다. 제4 장은 내부 광추출 구조의 패터닝 특성의 중요성에 대해 서술한다. 내부 광추출 구조를 디스플레이에 활용하기 위해선 박막 트랜지스터 (TFT) 와 유기 발광소자 전극 사이에 신호를 주고받을 수 있는 통로, 즉 비아홀 (viahole) 이 필요하다. 내부 광추출 구조는 위치가 박막 트랜지스터와 유기 발광소자 사이에 위치한 까닭에, 구조 설계 시 viahole 패터닝을 고려해야 하지만, 지금까지 이런 고려는 적었다. 또한, 일반적인 내부 광추출구조는 고 굴절 물질 내에, 낮은 굴절률의 광 결정이 위치하는데, 이종의 물질은 식각 용액에 대해 서로 다른 에칭 선택비 (etching selectivity)를 갖기 때문에, 내부 광추출구조의 손상 없이 패터닝이 가능한 식각 용액을 찾는 것 또한 필요하다. 우리는 SiOx 스캐터와 TiO2 평탄층을 갖는 내부 광추출 구조의 비아홀 패터닝을 성공적으로 구현한 소자를 보고한다. SiOx와 TiO2에 대해 유사한 에칭 선택비를 갖는 buffered oxide etchant (BOE) 와 인산 (H3PO4) 의 혼합 식각 용액을 활용해 언더컷 (undercut) 문제가 해결된 비아홀 패터닝 공정을 확립했으며, 제작된 유기 발광소자의 낮은 누설 전류 및 손상 없는 광추출 효율을 통해 비아홀 패터닝이 성공적으로 되었음을 실험적으로 보였다. 제5장은 전자 전달층 (Electron transporting layer, ETL) 의 안정성을 향상시키기 위한 방법을 담고 있다. 영구 쌍극자 모멘트 (permanent dipole moment)의 정렬이 비정질의 Alq3 유기물에서도 발현됨을 발견한 이후, 여러 전자전달물질에서 이러한 특성 (배향 분극, orientation polarization)이 나타남이 밝혀졌다. 지금까진 배향 분극이 소자의 전기적 특성에 어떻게 영향을 끼치는지에 대한 연구가 주류를 이뤘다. 하지만 우리는 배향 분극이 유기 발광소자의 수명에 어떤 영향을 주는지 밝혔다. 소자의 에너지 전달 과정에 참여하지 않는 BAlq 분자를 활용해 전자 전달층과 발광층 (EML) 사이에 음의 표면 전하 밀도를 높혔을 때, 소자의 구동 수명이 증가함을 보였다. 또한 물질의 열화 정도를 파악할 수 있는 전압 변화가 낮아짐을 통해 BAlq에 의한 표면전하 변화가 전자전달층의 안정성을 올림을 알 수 있었다. 또한 분극된 층이 발광층과의 계면에 위치할 경우에서만 수명이 증가하는 결과를 통해 정공 차단층 (hole blocking layer, HBL) 이 음의 표면 전하를 발광층과의 계면에 가질 경우, 향상된 수명을 얻을 수 있다는 결론을 얻었으며, 이를 통해 분자의 배향 분극 특성도 소자의 수명향상을 위해 고려해야 함을 보였다.Chapter 1. Introduction 2 1.1 Brief history of Organic Light-Emitting Diodes (OLEDs) 2 1.2 Efficiency of OLEDs 10 1.3 Light extraction methods for OLEDs 16 1.4 Finite difference time domain (FDTD) method 24 1.5 Operational stability of OLEDs 26 1.6 Outline of the thesis 28 Chapter 2. Random Organic Nano-textured Microstructures by Photo-Induced Crosslinking for Light Extraction of Blue OLEDs 31 2.1 Introduction 31 2.2 Experimental section 34 2.3 Results and Discussion 36 2.4 Conclusion 57 Chapter 3. Random Nanowire Arrays Spontaneously Formed via Vacuum Deposition for Enhancing Light Extraction from Inverted Top-Emitting Organic Light-Emitting Diodes 59 3.1 Introduction 59 3.2 Experimental section 62 3.3 Results and Discussion 65 3.4 Conclusion 77 Chapter 4. Via Hole Patterning of Light Extraction Layer for Electrical Connection 78 4.1 Introduction 78 4.2 Experimental section 80 4.3 Results and Discussion 83 4.4 Conclusion 92 Chapter 5. Improving Operation Lifetime of OLEDs using Spontaneous Orientation Polarization 93 5.1 Introduction 93 5.2 Experimental section 97 5.3 Results and Discussion 99 5.4 Conclusion 124 Chapter 6. Summary and Conclusion 125 Bibliography 128 초 록 142 CURRICULUM VITAE 147 List of Publications 149 List of Presentations 151Docto

Efficiency Improvement of Organic Light-Emitting Diodes: Development of Novel Fabrication and Optical Evaluation Concepts

Organic Light-Emitting Diodes (OLEDs) currently advance in the display market due to their unique image quality. Innovations profit from further extraordinary properties such as mechanical flexibility, optical transparency and large area coverage. Despite considerable progress in development, high costs and moderate efficiencies hamper the entry into the lighting market. However, there still is enormous potential for efficiency improvement. Current problems are the work-intensive search for best material combinations and large amounts of trapped light in the flat OLED geometry. This thesis develops novel concepts for improving the efficiency of OLEDs: An optimized fabrication, a systematic evaluation of light outcoupling structures by proposing a new metric, and an examination of efficiency limitations with optical simulations to identify options for action. The optimization of OLEDs is closely related to the properties of the individual molecules, while the fabrication process is often neglected. However, literature has shown that vapor deposited organic glasses can exhibit extraordinary high stabilities when fabricated at the right conditions. The substrate temperature is therefore set to 85 % of the materials conventional glass transition temperature Tg while the deposition rates are kept below < 0.1 nm/s. This concept is adapted and the glass forming molecule TPBi is fabricated as stable host and electron transporter in a simple OLED. Efficiency and lifetime improvement could be achieved with four different phosphorescent emitters. For Ir(ppy)2(acac) the External Quantum Efficiency (EQE) at 100 cd/m² is enhanced from 19.4 to 24.0 % and the lifetime LT 70 at 1,000 cd/m² from 14.8 to 74.2 h, i.e. the time in which the initial luminance drops to 70 % is five times higher. The origin is found in improved radiative and electrical efficiency. This fabrication concept enables an additional optimization path besides material development. Next, the high refractive index of organic materials lead to light confinement through total internal reflection. Many light outcoupling strategies have been developed, but their direct comparison is difficult through the diversity of used structures and OLEDs. This thesis proposes a new efficiency metric, the Efficiency of Light Outcoupling Structures (ELOS), that states the effectiveness of a light outcoupling structure. It weights experimental efficiency enhancement to theoretically maximal gain that is simulated. It was found that a glass half-sphere extracts about 80 % photons from the OLED substrate, while a combination of a diffraction grating with the half-sphere extracts at best 43 % from the whole OLED. The corresponding EQEs are 32.5 % and 36.5 %. The ELOS promotes a systematic search for a universally efficient light outcoupling structures, because it removes misinterpretation through the specifically used OLED. Lastly, optical investigations have found the following statements for bottom-emitting OLEDs: Highest EQEs with external light outcoupling structures are reached for 3/4 λ cavities. EQEs of more than 90 % could be reached with a low refractive index electron transport layer in combination with a high refractive index substrate and ideal external light outcoupling. For top-emitting OLEDs, the thin film interference can be exploited to shift the emission color of sky-blue emitter towards blue. Commission Internationale de l’Éclairage (CIE) color coordinates of (0.127/0.168) could experimentally be achieved with an emitter that has (0.213/0.374). The color tuning reduces EQE, but therefore exploits higher lifetimes of sky-blue emitters. This opens an alternative to reach deep blue emission besides material development, which is a current challenge for displays.Organische Leuchtdioden (OLEDs) etablieren sich derzeit im Display-Markt aufgrund ihrer einzigartigen Bildqualität. Innovationen profitieren zudem von weiteren außergewöhnlichen Eigenschaften wie z.B. mechanische Flexibilität, optische Transparenz und Großflächigkeit. Obwohl es bereits beträchtliche Weiterentwicklungen gab, erschweren hohe Kosten und moderate Effizienzen den Markteintritt in den Beleuchtungssektor. Dabei gibt es noch enormes Potential für Effizienzsteigerungen. Derzeitige Probleme sind die aufwändige Suche nach den besten Material(-kombinationen) und große Verluste durch Licht, welches im flachen Bauteil verbleibt. In dieser Dissertation werden deshalb neuartige Konzepte entwickelt, um die Effizienzsteigerung voranzutreiben: Eine optimierte Herstellung, eine systematische Untersuchung von Lichtauskoppelstrukturen mittels einer neuen Metrik und die Untersuchung von Effizienzlimitierungen mithilfe von optischen Simulationen, um weitere Möglichkeiten zur Verbesserung zu identifizieren. Die Optimierung von OLEDs ist eng verbunden mit der Materialsuche, wobei der Herstellungsprozess oft vernachlässigt wird. Allerdings konnte in der Literatur gezeigt werden, dass aufgedampfte organische Gläser außergewöhnlich stabil sein können, wenn die Herstellungsbedingungen optimiert werden. Dafür muss die Substrattemperatur auf etwa 85 % der gewöhnlichen Glasübergangstemperatur Tg gesetzt werden, während das Material mit niedrigen Raten von < 0,1 nm/s aufgetragen wird. Dieses Konzept wird übernommen für das Glasformer Molekül TPBi, welches als stabile Matrix und Elektronentransporter in einer einfachen OLED realisiert wird. Damit wird eine Effizienz- und Lebensdauerverbessung für vier phosphoreszente Emittermoleküle erreicht. Für Ir(ppy)2(acac) wird die externe Quanteneffizienz (EQE) bei 100 cd/m² von 19,4 auf 24,0 % erhöht und die Lebensdauer LT 70 bei 1000 cd/m² von 14,8 auf 74,2 h, d.h. die Zeit, bis die ursprüngliche Helligkeit auf 70 % fällt, wird verfünffacht. Ursache dafür ist eine verbesserte elektrische Effizienz und Strahlungseffizienz. Diese Herstellungsoptimierung eröffnet neben der Materialsuche eine weitere Möglichkeit für OLED Verbesserungen. Weiterhin führt der hohe Brechungsindex organischer Materialien zu Lichteinschluss im Bauteil durch totale interne Reflexion. Um zusätzliches Licht zu extrahieren, wurden viele Lichtauskoppelstrukturen entwickelt, welche sich jedoch aufgrund der Vielfalt der Strukturen und OLEDs nur schwer vergleichen lassen. Diese Arbeit schlägt eine neue Effizienzgröße vor, die sogenannte Effizienz von Lichtauskoppelstrukturen (ELOS), welche die Effektivität von den Strukturen angibt. Sie vergleicht die experimentell bestimmte mit der maximal erwartbaren Verbesserung, welche mit optischen Simulationen berechnet wird. Damit konnte gezeigt werden, dass eine Glashalbkugel etwa 80 % aller Photonen vom OLED Substrat extrahiert, während eine Kombination von Streugittern mit Glashalbkugel bestenfalls 43 % der verbleibenden Photonen in der gesamten OLED extrahiert. Die jeweiligen EQEs sind 32,5 % und 36,5 %. Die ELOS fördert eine systematische Suche nach den universell am besten funktionierenden Lichtauskoppelstrukturen, weil Missinterpretationen durch die jeweilig verwendeten OLEDs verringert werden. Letztendlich haben die optischen Untersuchungen folgende Aussagen für bottom emittierenden OLEDs gefunden: Die höchste EQE mit externen Lichtauskoppelstrukturen werden mit 3/4 λ Kavitäten erreicht. EQEs von mehr als 90 % könnten erreicht werden durch Kombination von niedrigbrechenden Elektronentransportschichten, hochbrechenden Substraten und idealen Lichtauskoppelstrukturen. Für top emittierende OLEDs kann Dünnschichtinterferenz ausgenutzt werden, um die Farbe von himmelblauen Emittern zu tiefblau zu verschieben. CIE Farbkoordinaten von (0.127/0.168) konnten experimentell erreicht werden mit einem Emitter von (0.213/0.374). Die Farbverschiebung verringert zwar die EQE, allerdings kann so die höhere Lebensdauer von himmelblauen Emittern ausgenutzt werden. Damit wird eine Alternative zur Materialsuche geschaffen, um tiefblaue Farbe zu erreichen, was eine derzeitige Herausforderung für Displays ist

5th International Symposium Technologies for Polymer Electronics - TPE 12: 22. - 24. May 2012, Rudolstadt/Germany ; Proceedings

Version notice: There are minor differences between the printed and online version

The Optical Outcoupling of Organic Light Emitting Diodes

OLEDs have seen a strong growth in development in recent years, however up to 80% of emitted light may be lost within the OLED stack and in the substrate layers. This thesis investigates the effects of the layer stack on the OLED properties and also studies a number of approaches to substrate structuring and treatment in order to couple light from the devices