Tuning The Optical, Charge Injection, and Charge Transport Properties of Organic Electronic Devices

Since the early 1900's, synthetic insulating polymers (plastics) have slowly taken over the role that traditional materials like wood or metal have had as basic components for construction, manufactured goods, and parts. Plastics allow for high throughput, low temperature processing, and control of bulk properties through molecular modifications. In the same way, pi-conjugated organic molecules are emerging as a possible substitute for inorganic materials due to their electronic properties. The semiconductive nature of pi-conjugated materials make them an attractive candidate to replace inorganic materials, primarily due to their promise for low cost and large-scale production of basic semiconducting devices such as light-emitting diodes, solar cells, and field-effect transistors.Before organic semiconductors can be realized as a commercial product, several hurdles must be cleared. The purpose of this dissertation is to address three distinct properties that dominate the functionality of devices harnessing these materials: (1) optical properties, (2) charge injection, and (3) charge transport. First, it is shown that the electron injection barrier in the emissive layer of polymer light-emitting diodes can be significantly reduced by processing of novel conjugated oligoelectrolytes or deoxyribonucleic acid atop the emissive layer. Next, the charge transport properties of several polymers could be modified by processing them from solvents containing small amounts of additives or by using regioregular and enantiopure chemical structures. It is then demonstrated that the optical and electronic properties of Lewis basic polymer structures can be readily modified by interactions with strongly electron-withdrawing Lewis acids. Through red-shifted absorption, photoluminescence, and electroluminescence, a single pi-conjugated backbone can be polychromatic. In addition, interaction with Lewis acids can remarkably p-dope the hole transport of the parent polymer, leading to a two-orders of magnitude increase in the hole mobility. Finally, the hole, electron, and double carrier transport in solar cell devices are studied in a bid to examine the correlations between bulk morphologies and free carrier recombination.The sum of these works help to create new pathways for the synthesis and design of new pi-conjugated materials and device architectures. All of this is in hopes of achieving higher performance and more stable devices to rival inorganic systems

유기발광소자를 위한 열 활성 지연형광 발광체 및 유기태양전지를 위한 전자 주개 물질과 은나노입자의 개발

학위논문(박사) -- 서울대학교대학원 : 자연과학대학 화학부, 2021.8. 홍종인.유기 발광소자(OLEDs)는 1987년 첫 발견 이후 꾸준한 관심을 받고 있으며 상용화에 성공하여 현재 대면적의 TV 부터 스마트폰, 그리고 스마트 시계 등 다양한 분야의 패널 디스플레이에 적용되고 있다. 기존의 형광 발광체를 기반으로 한 유기 발광소자는 최대 내부 양자효율이 25%로 제한되는 단점을 갖고 있지만 인광 발광체는 중금속의 스핀궤도 결합에 의해 삼중항의 여기자를 이용할 수 있어 최대 100%의 내부 양자효율을 취할 수 있다. 하지만 인광 발광체에 일반적으로 사용되는 고가의 백금이나 이리듐 계열의 중금속 물질은 상용화에 있어 경제력이 떨어지고, 따라서 새로운 대체물질의 도입이 필요하다. 중금속을 사용하지 않는 열 활성 지연 형광(TADF) 발광체 또한 단일항과 삼중항의 여기자를 모두 활용할 수 있기에 이는 인광 발광체를 대체할 수 있는 물질로 부상하고 있다. 열 활성 지연 형광 발광체는 역 계면 전이(RISC)에 의해 삼중항에 위치한 75%의 여기자를 단일항으로 이동시켜 모든 여기자를 발광에 활용하는 방법으로 특히 색 순도와 효율, 안정성이 적/녹색 발광체보다 낮은 청색의 발광체에 적용시키는 것이 주로 연구되고 있다. 이에 첫번째 파트에서는 유기 발광소자에 사용되는 청/녹색 열 활성 지연 형광 발광체를 개발 하고자 하였다. 유기 태양전지(OSCs)는 태양광이나 인공의 빛을 전기 에너지로 변환시키는 기술로서 매장량이 한정된 기존의 주 에너지원인 화석연료를 대체하기 위해 꾸준히 연구되고 있다. 유기 태양전지는 크게 두가지 종류로 분류 할 수 있는데 하나는 식물의 광합성에서 영감을 받은 염료 감응형 태양전지(DSCs)와 다른 하나는 광 기전 효과를 활용한 유기 광 기전 소자(OPVs) 이다. 기존의 루테늄 기반의 감응체를 사용한 염료 감응형 태양전지는 흡광 계수가 낮고, 가격이 비싸며 정제가 힘들다는 단점을 가지고 있다. 이를 보완하기 위해 유기물을 염료로 사용한 염료 감응형 태양전지가 개발되고 있다. 유기 광 기전 소자는 실리콘/무기 태양전지의 대체기술로서 높은 흡광 계수와 공정의 용이성, 경제적 이점에 있어 큰 장점을 보인다. 두번째 파트에서는 흡광 효율을 높이는 노력을 통한 고성능의 유기태양전지를 개발하고자 하였다. 고성능의 유기태양전지를 개발하기 위해서는 염두에 두어야 할 추가적인 요소들이 두가지 존재한다. 염료 감응형 태양전지의 액체 전해질 사용에 의한 낮은 안정성 문제와 여기자의 짧은 확산 거리로 인한 제한된 소자의 두께가 손꼽힌다. 전자의 경우 페로브스카이트 구조의 감응체와 고체 정공 이동물질의 조합을 통해 고체형 염료 감응형 태양전지(ssDSCs)를 개발함으로써 극복하였다. 후자는 유기 태양전지 내에 금속 나노입자를 도입하여 그것의 국부적 표면 플라즈몬 공명(LSPR) 효과를 활용, 소자의 두께를 유지하면서 광자 흡수율 증가를 이루는 방법이 해결책 중 하나로 제시되었다. 세번째 파트에서는 페로브스카이트 태양전지에 은 나노입자를 도입하여 효율을 증대 시키는 연구를 진행 하였다. Part I 에서는 고효율의 청/녹색 열 활성 지연 형광 물질의 개발에 중점을 두었다. 고효율의 유기 발광 소자를 얻기 위해서는 작은 값의 단일항-삼중항간의 전위차이(ΔEST) 와 원활한 분자내 전하 전달(ICT)이 중요하다. 견고한 분자구조의 설계로 전하의 비발광 감쇠율을 줄이고 길고 평평한 분자구조로 물질의 수평 배치율을 높여 전면 발광을 하는 구조를 유도하는 것 또한 고려해야 한다. 본 연구에서는 단일항-삼중항간의 전위차이, 고휘도에서의 효율감소율, 비발광 감쇠율을 줄이는 동시에 분자내 전하전달과 발광 효율을 증가시켜 높은 외부양자 효율을 보이는 유기 발광 소자 개발에 초점을 맞추었다. 두개의 전자주개와 보론 전자받개를 포함하며 분자내 전하전달을 최대화 시키기 위한 뒤틀어진 구조의 CCDMB, PCDMB 물질을 고안하였고 그중 강한 전자주개인 페녹사진기를 가진 PCDMB물질은 0.13 eV의 작은 단일항-삼중항 전위 차이 값을 보였으며 효율적인 역 계면 전이가 일어났음을 알 수 있었다. PCDMB를 발광체로 사용한 소자는 (0.21, 0.45)의 CIE 1931 색 좌표를 보였으며 22.3%의 높은 효율을 기록했다. 분자의 견고함을 극대화 시키기 위해 전자주개와 전자받개를 스피로 구조로 연결시킨 CBZANQ와 PXZANQ물질이 고안되었고 간단한 두 단계의 반응을 통해 합성되었다. 전자가 원활하게 이동하지 않는 스피로 구조의 도입으로 분자의 견고함뿐만 아니라 최고 준위 점유 전자궤도 (HOMO)와 최저 준위 비점유 전자궤도(LUMO)의 효율적인 공간적 전하 분리를 이루었다. PXZANQ 발광체를 이용한 유기 발광소자는 528 nm에서 빛을 냈으며 22.1%의 소자 효율을 보였고, 이는 지금까지 알려진 전자주개-스피로-전자받개 구조의 열 활성 지연 형광 물질 중 가장 높은 효율을 기록했다. Part II 에서는 두가지 종류의 유기태양전지인 염료 감응형 태양전지와 유기 광 기전 소자의 광 변환 효율을 증가 시키기 위한 일을 진행하였다. 유기 분자의 넓은 파장 영역대에서의 태양 스펙트럼 흡수는 단락전류(JSC)의 증가에 영향을 끼치고 분자의 에너지 준위와 소자의 구조는 개방전압(VOC)과 곡선인자(FF)에 영향을 주며 이 세가지 요인은 태양전지의 효율을 구성하는 주요 인자들이다. 본 연구에서는 이 세가지 인자들 중 단락전류의 증가에 집중하였으며 높은 광 흡수율을 통한 높은 단락전류를 보이는 분자를 설계하기 위하여 노력하였다. 태양전지에서 자주 도입되지 않은 평평한 퀴나크리돈 유도체를 이용한 긴 콘쥬게이션을 가진 염료 감응형 태양전지의 염료물질과 유기 광 기전 소자의 전자주개 물질을 고안하고 연구하였다. Part III 에서는 은 나노입자를 페로브스카이트 태양전지에 도입하는 연구가 진행되었다. 태양전지의 광 흡수율은 소자의 활성층의 넓이와 두께에 비례하지만 이것의 증가는 소자의 저항 또한 증가시켜 광 전환효율을 감소시키게 된다. 따라서 이 파트에서는 은 나노입자를 티타늄 산화물 층에 도입하여 활성층의 두께를 증가시키지 않으면서 국부적 표면 플라즈몬 공명에 의한 흡수의 증가를 꾀하였다. 또한 은 나노입자와 다공성 티타늄 산화물과의 집성 효과에 의한 표면의 거칠기를 완화시켜 원활한 전하전달을 도모하였다. 하지만 은 나노입자의 전위가 티타늄 산화물의 전도대와 페로브스카이트 물질의 전위보다 낮게 위치하여 전하가 갇히는 구간을 생성하여 효율의 감소를 초래하게 된다. 따라서 도입된 은 나노입자의 양에 따른 빛의 흡수율과 전하의 갇힘, 표면의 거칠기의 변화 사이의 상관관계를 분석하는 연구를 진행하였다. 핵심어: 유기 발광 소자, 열 활성 지연 형광, 유기 태양전지, 염료 감응형 태양전지, 유기 광 기전 소자, 페로브스카이트 태양전지, 금속 나노입자.Organic light-emitting diodes (OLEDs) have drawn considerable attention since the first report in 1987 and were successfully applied to commercialization in various fields of panel displays including large-screen televisions, smart phones, and smart watches. Conventional fluorescent dopant-based OLED emitters can only achieve a maximum internal quantum efficiency (IQE) of up to 25%. On the other hand, phosphorescence emitters which can harvest triplet excitons utilizing spin-orbit coupling of heavy metal atoms can reach a maximal IQE of up to 100%. However, the high cost of commonly used heavy metal atoms such as platinum or iridium increases the cost of commercial products and alternative materials are required accordingly. Metal-free thermally activated delayed fluorescence (TADF) emitters have been considered as alternatives to phosphorescence emitters because they can utilize not only singlet but also triplet excitons by converting 75% of excitons in the lowest triplet excited state (T1) to the lowest singlet excited state (S1) through reverse intersystem crossing (RISC). The first part of the thesis describes the development of efficient blue-green TADF emitting materials for OLEDs. Organic solar cells (OSCs) are capable of converting light energy (sunlight or artificial light) into electrical energy and thus have gained continuous recognition as a potential alternative to conventional energy sources such as fossil fuels. There are two kinds of organic solar cells, dye-sensitized solar cells (DSCs) and organic photovoltaic cells (OPVs). Conventional ruthenium-based sensitizers for DSCs have several drawbacks; low absorption coefficient, high material cost, and difficulty in purification. Therefore, metal-free organic sensitizers with high light-harvesting ability are continuously developed as an attractive alternative. OPVs are a promising substitute to silicon/inorganic solar cells owing to their high absorption coefficient, ease of fabrication, and low manufacturing cost. One way to realize high-efficiency OSCs is to develop dyes having high light harvesting ability. The second part of the thesis is focused on designing organic dye materials for DSCs and electron donor materials for OPVs with high light-harvesting ability. There are two major problems to be solved to achieve highly efficient DSCs. One is instability of DSCs employing liquid electrolyte, and the other is insufficient light harvesting by limited thickness of the active layer of devices considering the short exciton diffusion length (~10 nm). The former can be overcome by adopting perovskite-structured sensitizers with solid hole transporting materials to form solid state DSCs (ssDSCs). The latter can be solved by introducing metallic nanoparticles (NPs) into OSCs which can increase the light absorption by the localized surface plasmon resonance (LSPR) without increasing the thickness of the device. The introduction of Ag-NPs into perovskite based ssDSCs is discussed in the third part of the thesis. Part I is focused on designing highly efficient blue-green TADF OLED emitters. To achieve high external quantum efficiency (EQE) of TADF OLED emitters, it is vital to fulfill a small singlet-triplet energy difference (ΔEST) value for efficient RISC and efficient intramolecular charge transfer (ICT) for increased emission. In addition, rigid structures are required to decrease non-radiative decay sites, and long, planar structures should be considered to increase the horizontal dipole orientation which can affect forward-emission and thus enhance light out-coupling efficiency. This part presents several ways to decrease ΔEST, efficiency roll-off, and non-radiative decay rate, but increase the ICT character and photoluminescence quantum yield (PLQY) for highly efficient TADF emitters. Two twisted donor (D)-acceptor (A) type structures (CCDMB and PCDMB) composed of double electron donors and a trivalent boron acceptor were designed to maximize the ICT character. PCDMB with a strong phenoxazine donor showed a small ΔEST value of 0.13 eV, resulting in effective RISC and triplet harvesting. PCDMB-based devices showed green emission with Commission Internationale de l’Éclairage (CIE) of (0.21, 0.45) and a high maximum EQE of 22.3%. Two rigid spiro-type D-σ-A TADF emitters (CBZANQ, PXZANQ) were designed and synthesized via a simple two-step route. To maximize the rigidity of the emitter structure, spiro-type D-σ-A structures have been exploited, resulting in efficient spatial separation of the HOMO and LUMO. The PXZANQ-based OLED devices with a DPEPO host exhibited the maximum emission at 528 nm with a high EQE of 22.1% and small roll-off. The EQE of PXZANQ-based devices was among the highest in D-spiro-A type TADF emitters Part II describes efforts toward increasing power conversion efficiency (PCE) of DSCs and OPVs by improving light-harvesting efficiency. One major way to increase short-circuit current (JSC) is to harvest a wide range of solar spectrum by planar and long conjugated molecules. Other ways to improve PCE is to increase open circuit voltage (VOC) and fill-factor (FF) by adjusting energy levels of layers. This part is focused on ways to obtain high JSC values by increasing the light-harvesting efficiency utilizing organic sensitizers for DSCs and electron donor materials for OPVs based on a planar quinacridone (QA) π-bridge in common. Part III investigates the effect of silver (Ag)-NPs on the PCE of perovskite solar cells. The light harvesting of solar cells highly depends on the width and thickness of the active layer. However, increasing those factors may be inevitably accompanied by enhanced device resistance which might decrease the device PCE. Ag-NPs were embedded into TiO2 layer to enhance the light absorption without increasing the thickness of the device via the localized surface plasmon resonance (LSPR) effect. Furthermore, the aggregation of Ag-NPs with mesoporous TiO2 induced the morphology change of TiO2 layer which is advantageous for efficient charge transfer. However, the energy level of Ag-NPs lower than both perovskite sensitizer and conduction band of TiO2 caused charge trapping site which hampered the efficient charge transfer. Consequently, it is important to optimize the loading levels of Ag-NPs for high-efficiency solar cell devices. The interrelations of optical absorption, charge trapping, and surface roughness of TiO2 layer of perovskite solar cells with different concentrations of neat Ag-NPs were demonstrated. Keyword : Organic light emitting diodes, thermally activated delayed fluorescence, organic solar cells, dye-sensitized solar cells, organic photovoltaics, perovskite solar cells, metallic nanoparticles.Part I. Thermally Activated Delayed Fluorescence Emitters for Organic Light Emitting Diodes １ 1.1. Study Background ２ 1.1.1. OLEDs in our life ２ 1.1.2. History of OLEDs ２ 1.1.3. Brief structure of OLED devices ３ 1.1.4. Efficiency of OLEDs ４ 1.1.5. Thermally activated delayed fluorescence (TADF) ５ 1.1.6. Outline for thesis １１ 1.2. The Effect of the Electron-Donor Ability on the OLED Efficiency of Twisted Donor-Acceptor Type Emitters １３ 1.2.1. Introduction １３ 1.2.2. Experimental Section １４ 1.2.3. Results and discussion １９ 1.2.4. Conclusion ３３ 1.3. Introduction of hydrogen-bonding on TADF emitting materials. ３４ 1.3.1. Introduction ３４ 1.3.2. Results and Discussion ３４ 1.4. Screenings of rigid and planar TADF materials. ３７ 1.4.1. Introduction ３７ 1.4.2. Experimental Section ３７ 1.4.3. Results and Discussion ４５ 1.5. SpiroType TADF Emitters Based On Acridine Donors and Anthracenone Acceptor. ５５ 1.5.1. Introduction ５５ 1.5.2. Experimental Section ５６ 1.5.3. Results and Discussion ５９ 1.5.4. Conclusion ６９ REFERENCES ７１ Part II. Highly Efficient Organic Materials for Organic Solar Cells ７６ 2.1. Introduction ７７ 2.1.1. Necessity of organic solar cells (OSCs). ７７ 2.1.2. Efficiency of OSCs ７８ 2.1.3. Outline for thesis ７９ 2.2. Organic sensitizers for dye-sensitized solar cells (DSCs) ８１ 2.2.1. Background ８１ 2.2.2. The novel organic sensitizers based on planarquinacridone πspacer for highly efficient dye-sensitized solar cells. ８４ 2.3. Electron donor materials for organic photovoltaic cells (OPVs) ９６ 2.3.1. Background ９６ 2.3.2. A solution processable quinacridone-based electron donor materials for bulk-heterojunction organic solar cells. １００ REFERENCES １１２ Part III. Metallic Nanoparticles in Solar cells. １１６ 3.1. Backgrounds １１７ 3.1.1. Introduction １１７ 3.1.2. Purpose of metallic NPs in solar cells １１８ 3.1.3. History of metallic NPs in solar cells. １２０ 3.2. Correlations of optical absorption, charge trapping, and surface roughness of TiO2 photoanode layer loaded with neat Ag-NPs for efficient perovskite solar cells １２２ 3.2.1. Abstract １２２ 3.2.2. Introduction １２２ 3.2.3. Experimental Section １２５ 3.2.4. Results and Discussion １２８ 3.2.5. Conclusion １３９ REFERENCES １４０ Abstract in Korean (국문초록) １４６ 감사의 인사 １４９박

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

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Organic Light-Emitting Diodes (OLEDs) and Optically-Detected Magnetic Resonance (ODMR) studies on organic materials

Organic semiconductors have evolved rapidly over the last decades and currently are considered as the next-generation technology for many applications, such as organic light-emitting diodes (OLEDs) in flat-panel displays (FPDs) and solid state lighting (SSL), and organic solar cells (OSCs) in clean renewable energy. This dissertation focuses mainly on OLEDs. Although the commercialization of the OLED technology in FPDs is growing and appears to be just around the corner for SSL, there are still several key issues that need to be addressed: (1) the cost of OLEDs is very high, largely due to the costly current manufacturing process; (2) the efficiency of OLEDs needs to be improved. This is vital to the success of OLEDs in the FPD and SSL industries; (3) the lifetime of OLEDs, especially blue OLEDs, is the biggest technical challenge. All these issues raise the demand for new organic materials, new device structures, and continued lower-cost fabrication methods. In an attempt to address these issues, we used solution-processing methods to fabricate highly efficient small molecule OLEDs (SMOLEDs); this approach is cost-effective in comparison to the more common thermal vacuum evaporation. We also successfully made efficient indium tin oxide (ITO)-free SMOLEDs to further improve the efficiency of the OLEDs. We employed the spin-dependent optically-detected magnetic resonance (ODMR) technique to study the luminescence quenching processes in OLEDs and organic materials in order to understand the intrinsic degradation mechanisms. We also fabricated polymer LEDs (PLEDs) based on a new electron-accepting blue-emitting polymer and studied the effect of molecular weight on the efficiency of PLEDs. All these studies helped us to better understand the underlying relationship between the organic semiconductor materials and the OLEDs\u27 performance, and will subsequently assist in further enhancing the efficiency of OLEDs. With strongly improved device performance (in addition to other OLEDs\u27 attributes such as mechanical flexibility and potential low cost), the OLED technology is promising to successfully compete with current technologies, such as LCDs and inorganic LEDs

Organic light-emitting diodes with doped charge transport layers

Organische Farbstoffe mit einem konjugierten pi-Elektronen System zeigen überwiegend ein halbleitendes Verhalten. Daher sind sie potentielle Materialien für elektronische und optoelektronische Anwendungen. Erste Anwendungen in Flachbildschirmen sind bereits in (noch) geringen Mengen auf dem Markt. Die kontrollierte Dotierung anorganischer Halbleiter bereitete die Basis für den Durchbruch der bekannten Halbleitertechnologie. Die Kontrolle des Leitungstypes und der Lage des Fermi-Niveaus erlaubte es, stabile pn-Übergänge herzustellen. LEDs können daher mit Betriebsspannungen nahe dem thermodynamischen Limit betrieben werden (ca. 2.5V für eine Emission im grünen Spektralbereich). Im Gegensatz dazu bestehen organische Leuchtdioden (OLEDs) typischerweise aus einer Folge intrinsischer Schichten. Diese weisen eine ineffiziente Injektion aus Kontakten und eine relative geringe Leitfähigkeit auf, welche mit hohen ohmschen Verlusten verbunden ist. Andererseits besitzen organische Materialien einige technologische Vorteile, wie geringe Herstellungskosten, große Vielfalt der chemischen Verbindungen und die Möglichkeit sie auf flexible große Substrate aufzubringen. Sie unterscheiden sich ebenso in einigen fundamentalen physikalischen Parametern wie Brechungsindex, Dielektrizitätskonstante, Absorptionskoeffizient und Stokes-Verschiebung der Emissionswellenlänge gegenüber der Absorption. Das Konzept der Dotierung wurde für organische Halbleiter bisher kaum untersucht und angewandt. Unser Ziel ist die Erniedrigung der Betriebsspannung herkömmlicher OLEDs durch den Einsatz der gezielten Dotierung der Transportschichten mit organischen Molekülen. Um die verbesserte Injektion aus der Anode in die dotierte Löchertransportschicht zu verstehen, wurden UPS/XPS Messungen durchgeführt (ultraviolette und Röntgen-Photoelektronenspektroskopie). Messungen wurden an mit F4-TCNQ dotiertem Zink-Phthalocyanin auf ITO und Gold-Kontakten durchgeführt. Die Schlussfolgerungen aus den Experimenten ist, das (i) die Fermi-Energie sich durch Dotierung dem Transportniveau (also dem HOMO im Falle der vorliegenden p-Dotierung) annähert, (ii) die Diffusionspannung an der Grenzfläche durch Dotierung entsprechend verändert wird, und (iii) die Verarmungszone am Kontakt zum ITO sehr dünn wird. Der Kontakt aus organischem Material und leitfähigem Substrat verhält sich also ganz analog zum Fall der Dotierung anorganischer Halbleiter. Es entsteht ein stark dotierter Schottky-Kontakt dessen schmale Verarmungszone leicht durchtunnelt werden kann (quasi-ohmscher Kontakt). Die Leistungseffizienz von OLEDs mit dotierten Transportschichten konnte sukzessive erhöht werden, vom einfachen 2-Schicht Design mit dotiertem Phthalocyanine als Löchertransportschicht, über einen 3-Schicht-Aufbau mit einer Elektronen-Blockschicht bis zu OLEDs mit dotierten 'wide-gap' Löchertransport-Materialien, mit und ohne zusätzlicher Schicht zur Verbesserung der Elektroneninjektion. Sehr effiziente OLEDs mit immer noch niedriger Betriebsspannung wurden durch die Dotierung der Emissionsschicht mit Molekülen erhöhter Photolumineszenzquantenausbeute (Laser-Farbstoffe) erreicht. Eine optimierte LED-Struktur weist eine Betriebsspannung von 3.2-3.2V für eine Lichtemission von 100cd/m2 auf. Diese Resultate entsprechen den zur Zeit niedrigsten Betriebsspannungen für OLEDs mit ausschließlich im Vakuum aufgedampften Schichten. Die Stromeffizienz liegt bei ca. 10cd/A, was einer Leistungseffizienz bei 100cd/m2 von 10lm/W entspricht. Diese hohe Leistungseffizienz war nur möglich durch die Verwendung einer Blockschicht zwischen der dotierten Transportschicht und der Lichtemissions-Schicht. Im Rahmen der Arbeit konnte gezeigt werden, dass die Dotierung die Betriebsspannungen von OLEDs senken kann und damit die Leistungseffizienz erhöht wird. Zusammen mit einer sehr dünnen Blockschicht konnte einen niedrige Betriebsspannung bei gleichzeitig hoher Effizienz erreicht werden (Blockschicht-Konzept).Organic dyes with a conjugated pi-electron system usually exhibit semiconducting behavior. Hence, they are potential materials for electronic and optoelectronic devices. Nowadays, some applications are already commercial on small scales. Controlled doping of inorganic semiconductors was the key step for today's inorganic semiconductor technology. The control of the conduction type and Fermi-level is crucial for the realization of stable pn-junctions. This allows for optimized light emitting diode (LED) structures with operating voltages close to the optical limit (around 2.5V for a green emitting LED). Despite that, organic light emitting diodes (OLEDs) generally consist of a series of intrinsic layers based on organic molecules. These intrinsic organic charge transport layers suffer from non-ideal injection and noticeable ohmic losses. However, organic materials feature some technological advantages for device applications like low cost, an almost unlimited variety of materials, and possible preparation on large and flexible substrates. They also differ in some basic physical parameters, like the index of refraction in the visible wavelength region, the absorption coefficient and the Stokes-shift of the emission wavelength. Doping of organic semiconductors has only been scarcely addressed. Our aim is the lowering of the operating voltages of OLEDs by the use of doped organic charge transport layers. The present work is focused mainly on the p-type doping of weakly donor-type molecules with strong acceptor molecules by co-evaporation of the two types of molecules in a vacuum system. In order to understand the improved hole injection from a contact material into a p-type doped organic layer, ultraviolet photoelectron spectroscopy combined with X-ray photoelectron spectroscopy (UPS/XPS) was carried out. The experimental results of the UPS/XPS measurements on F4-TCNQ doped zinc-phthalocyanine (ZnPc) and their interpretation is given. Measurements were done on the typical transparent anode material for OLEDs, indium-tin-oxide (ITO) and on gold. The conclusion from these experiments is that (i) the Fermi-energy comes closer to the transport energy (the HOMO for p-type doping), (ii) the built-in potential is changed accordingly, and (iii) the depletion layer becomes very thin because of the high space charge density in the doped layer. The junction between a doped organic layer and the conductive substrate behaves rather similar to a heavily doped Schottky junction, known from inorganic semicondcutor physics. This behavior favors charge injection from the contact into the organic semiconductor due to tunneling through a very small Schottky barrier (quasi-ohmic contact). The performance of OLEDs with doped charge transport layers improves successively from a simple two-layer design with doped phthalocyanine as hole transport layer over a three-layer design with an electron blocking layer until OLEDs with doped amorphous wide gap materials, with and without additional electron injection enhancement and electron blocking layers. Based on the experience with the first OLEDs featuring doped hole transport layers, an ideal device concept which is based on realistic material parameters is proposed (blocking layer concept). Very high efficient OLEDs with still low operating voltage have been prepared by using an additional emitter dopant molecule with very high photoluminescence quantum yield in the recombination zone of a conventional OLED. An OLED with an operating voltage of 3.2-3.2V for a brightness of 100cd/m2 could be demonstrated. These results represent the lowest ever reported operating voltage for LEDs consisting of exclusively vacuum sublimed molecular layers. The current efficiency for this device is above 10cd/A, hence, the power efficiency at 100cd/m2 is about 10lm/W. This high power efficiency could be achieved by the use of a blocking layer between the transport and the emission layer

VISUALIZING AND CONTROLLING CHARGE TRANSPORT IN CONJUGATED POLYMER NETWORKS AND FILMS

VISUALIZAING AND CONTROLLING CHARGE TRANSPORT IN CONJUGATED POLYMER NETWORKS AND FILMS MAY 2014 ANDREW R. DAVIS, B.S., UNIVERSITY OF VIRGINIA M.S., UNIVERSITY OF MASSACHUSETTS AMHERST Ph.D., UNIVERSITY OF MASSACHUSETTS AMHERST Directed by: Professor Kenneth R. Carter The desire for more commercially feasible flexible electronic plastics has led to the development of increasingly complex conjugated polymer architectures and device geometries. Through these efforts, tremendous advances have been made in the design and performance of electronic devices fabricated with solution-processable semiconducting polymers. However, none of these materials have yet reached commercial maturity, so the opportunity for their further exploration from both a fundamental science and an application-driven point of view motivates this dissertation. Chapter 1 presents a background introduction to many of the concepts, ideas, and existing research necessary to set the context of this dissertation’s work. The first component of this work (Chapters 2-6) investigates thiol-ene cross-linked conjugated polymer networks. By installing vinyl functionalities in poly(fluorene) molecules at chain ends (Chapters 2 and 3) and along the polymer backbone (Chapters 4 and 5), the polymers can be rapidly and efficiently cross-linked by photo-reaction with thiol cross-linkers into highly tunable semiconducting polymer networks. It is shown that the thiol-ene cross-linking reaction allows for a new molecular handle on modifying interchain electronic communication via network density, which is visualized using characteristic and unambiguous photoemission from low-energy fluorenone species. Light emitting diodes fabricated using these networks as an emissive layer show enhanced color stability compared to as-spun counterparts, and the robustness of the networks allows for solution processing of multiple stratified emissive layers for controlling color emission. Furthermore, the highly efficient thiol-ene cross-linking reaction is shown to work as an effective means for grafting poly(fluorene)s onto functionalized surfaces. This work’s second component in Chapter 7 details the direct visualization of charge carrier density in polymeric thin film transistors using Modulation-Amplified Reflectance Spectroscopy (MARS) measurements. Owing to the unique changes in optical behavior following the formation of a charged state within the conjugated polymer film, optical spectroscopy coupled to a CCD camera offers a powerful visualization tool for observing and mapping charges across large areas as they interact with electrodes, defects, and film morphologies in active devices. The MARS technique illuminates the spatial distribution of carriers in electronic polymer films, allowing direct spatial visualization of charge density and film defects. Finally, a brief concluding comment on this work and an outlook for the field in general is presented in Chapter 8

2014 Status Report on Organic Light Emitting Diodes (OLED)

Organic light emitting diodes (OLED) are promising candidates for general illumination, since they offer the possibility to realize large area light sources which can even be transparent and flexible. The energy-saving potential of OLEDs is similar to that of LEDs, but the two technologies differ in a number of ways. The present report introduces the basics of the OLED technologies and its latest developments. It also describe the emerging markets, industry landscape and standardisation requirementsJRC.F.7-Renewables and Energy Efficienc

Synthesis and Characterization of Hole-Transporting and Electroluminescent Polymers

This thesis describes research on synthesis and characterization of electroluminescent and hole-transporting polymers for applications in organic light-emitting diodes (OLEDs). The first part of the project focuses on the synthesis of derivatives of the electroluminescent polymer poly(para-phenylenevinylene) (PPV) using ring-opening metathesis polymerization (ROMP) of substituted barrelenes. Barrelenes (a certain kind of bicyclic olefins) have been prepared through multi-step synthetic procedures, and the existing synthetic route was extended to barrelenes without electron-withdrawing groups. ROMP of barrelenes was explored because this polymerization can be living, which allows the preparation of well-defined polymeric products. A new version of a soluble PPV derivative was prepared via this route. The second part of the project focuses on hole-transporting (HT) polymers. A range of HT polymers were prepared via ROMP and anionic polymerization to explore the influence of different hole mobility and different ionization potential on the performance of a two-layer OLED. OLED devices were fabricated using spin-casting and vacuum vapor deposition, and were characterized in terms of current-voltage behavior and light output. A photo-crosslinkable hole transport layer was demonstrated. The HT polymers have been found to yield improved OLEDs by comparison to analogous small-molecule materials due to better film coverage and better film morphology. The device performance has been found to improve with increasing ionization potential of the hole transport polymer. An optimized device was fabricated, which showed 20 Lm/W efficiency. The best HT polymer was modified further to improve the operational stability of the device by improving the interfacial contact to the anode. A better adhesion to the conducting glass was achieved by preparing trimethoxysilane-containing copolymers via radical polymerization, and developing a procedure to cross-link these copolymers to the anode surface. The appendix describes a project unrelated to the general topic of materials for OLEDs. It presents a study on four chiral molybdenum-based ROMP-initiators with regard to their ability to yield highly stereoregular polymers.</p