Advances in Organic Displays and Lighting: Towards Planar Lithographic Integration of Organic Light-Emitting Diodes

This work focusses on the advances of organic light-emitting diodes (OLEDs) for large area display and solid-state-lighting applications. OLED technology has matured over the past two decades, aided by the rapid advances in development of the novel material and device concepts. State-of-the-art OLEDs reach internal efficiencies of 100% and device lifetimes acceptable for commercial display applications. However, further improvements in the blue emitter stability and the device performance at the high brightness are essential for OLED technology to secure its place in the lighting market. As the current passing through the device increases, a rapid decrease in OLED efficiency, so-called efficiency roll-off, takes place, which hinders the use of OLEDs wherever high brightness is required. In addition, white OLEDs comprising multiple emitter molecules suffer from the emission colour change as the operating conditions are varied or as the devices age. Despite side-by-side structuring of the monochrome OLEDs could in principle circumvent most of bespoke issues, the limitations imposed by the shadow mask technique, employed to structure vacuum deposited films, renders such approach impractical for fabrication of the devices on a large scale. In order to address these issues, photolithographic patterning of OLEDs is implemented. Highly efficient state-of-the-art devices are successfully structured down to tens of micrometers with the aid of orthogonal lithographic processing. The latter is shown to be a promising alternative for the shadow mask method in order to fabricate the full-colour RGB displays and solid-state-lighting panels. Photo-patterned devices exhibit a virtually identical performance to their shadow mask counterparts on a large scale. The high performance is replicated in the microscale OLEDs by a careful selection of functional layer sequence based on the investigation of the morphological stability and solubility of vacuum deposited films. Microstructured OLEDs, fabricated in several different configurations, are investigated and compared to their large area counterparts in order to account for the observed differences in charge transport, heat management and exciton recombination in bespoke devices. The role of the Joule heat leading to the quenching of the emissive exciton states in working devices is discussed. Structuring the active OLED area down to 20 micrometer is shown to improve the thermal dissipation in such devices, thus enabling the suppression of the efficiency roll-off at high brightness in white-emitting electroluminescent devices based on side-by-side patterned OLEDs.:List of Publications 1 1 Introduction 5 2 Organic Semiconductors 9 2.1 Molecular Bonding 9 2.1.1 Intramolecular Interactions 10 2.1.2 Intermolecular Interactions 17 2.2 Optical Properties of Organic Semiconductors 23 2.2.1 Excited State Dynamics 26 2.3 Energy Transfer in Organic Solids 27 2.3.1 Förster Energy Transfer 29 2.3.2 Dexter Energy Transfer 30 2.4 Charge Transport Phenomena 31 2.4.1 Polarization and Energetic Disorder 31 2.4.2 Charge Transport Models 33 3 Electromagnetic Field Propagation in Layered Media 35 3.1 Maxwell's Equations 35 3.1.1 Wave Character of Electromagnetic Field 37 3.1.2 Energy of Electromagnetic Field 38 3.1.3 Boundary Conditions of Electromagnetic Fields 39 3.2 Reflection and Refraction of Plane Waves 40 3.2.1 Total Internal Reflection 43 3.3 Guided Optical Waves 44 3.3.1 Modes of Planar Waveguide 45 3.3.2 Multilayer Waveguides 49 3.3.3 Mode Coupling 53 3.4 EM Field in Presence of Charges 55 3.4.1 Volume Plasmons 58 3.4.2 Surface Plasmon Polaritons 58 3.4.3 Localized Plasmons 62 4 Organic Light-Emitting Diodes 65 4.1 Principle of Operation 65 4.1.1 Electroluminescence Efficiency 66 4.1.2 Charge Injection and Transport 66 4.1.3 Radiative Efficiency 68 4.1.4 Excited State Formation 69 4.1.5 Organic Emitters 71 4.1.6 Light Extraction 73 4.1.7 Efficiency Loss Mechanisms 74 4.2 Applications of OLEDs 76 4.2.1 Information Displays 76 4.2.2 Solid-State Lighting 77 4.2.3 OLED Based Sensors 77 4.3 OLED Structuring 79 4.3.1 Shadow Mask Patterning 79 4.3.2 Serial Printing 80 4.3.3 Unconventional Patterning Techniques 80 4.3.4 Photolithographic Patterning of OLEDs 81 4.3.5 Orthogonal Processing of Organic Semiconductors 83 5 Materials and Methods 87 5.1 Organic Functional Materials . 87 5.1.1 Hole Injection/Transport Layers 87 5.1.2 Electron Blocking Materials 88 5.1.3 Hole Blockers and Electron Transport Materials 88 5.1.4 Emitter Systems 90 5.1.5 Substrate and Electrodes 90 5.2 Device Fabrication 92 5.2.1 Vacuum Deposition 92 5.2.2 Photolithographic Structuring 92 5.3 Measurements 94 5.3.1 OLED Characterisation 94 5.3.2 Optical and Morphological Inspection 95 5.3.3 Calcium Conductance Test 95 5.3.4 Time-of-flight Spectroscopy 96 6 Orthogonal Patterning of Organic Semiconductor Films and Devices 97 6.1 Patterned Organic Films 97 6.2 Patterned Alq3 Based OLEDs 100 6.2.1 Direct Emitter Patterning 100 6.2.2 Cathode as Protection Layer 102 6.2.3 Impact of O2 Plasma Treatment 104 6.3 Summary 107 7 Photolithographic Structuring of State-of-the-Art p-i-n OLEDs for Full-Colour RGB Displays 109 7.1 Studied OLED Structures 109 7.2 HFE Compatibility Study 110 7.2.1 HFE Immersion Study 110 7.2.2 LDI-TOF-MS Analysis 112 7.3 Large area OLEDs 114 7.4 Microscale Devices 118 7.5 Bilayer Processing on p-i-n OLEDs 122 7.6 Summary 126 8 White Light from Photo-structured OLED Arrays 129 8.1 Fabrication of Micro-OLED Array 129 8.1.1 Structuring Procedure 130 8.1.2 Optical Device Optimisation 130 8.1.3 Choice of Hole Blocking and Electron Transport Layers 134 8.2 Performance of Microstructured Devices 143 8.2.1 Colour Temperature Tuning 143 8.2.2 Compatibility with Photo-patterning 145 8.2.3 Colour Stability 150 8.3 Summary 154 9 Efficiency Roll-off and Emission Colour of Microstructured OLEDs 155 9.1 Photolithographic Control of the Subunit Dimension 155 9.2 Control of the Emission Colour 156 9.3 Suppression of Efficiency Roll-off in Microscale Devices 157 9.4 Thermal Management in OLEDs 159 9.5 Modelling Impact of Joule Heat on Roll-off Characteristics 162 9.6 Summary 164 10 Conclusions and Outlook 165 10.1 Conclusions 165 10.2 Outlook 167 List of Abbreviations 171 List of Figures 173 List of Tables 177 Acknowledgements 179 Bibliography 181Die vorliegende Arbeit beschäftigt sich mit den neusten Errungenschaften von organischen Licht-emittierenden Dioden (OLEDs) für großflächige Beleuchtungs- und Displayanwendungen. Die OLED-Technologie hat sich in den letzten zwei Jahrzehnten, begünstigt von neuartigen Material- und Bauteilkonzepten, weit entwickelt. Im aktuellen Stand der Technik erreichen OLEDs sowohl interne Effizienzen von 100% als auch Lebensdauern die für die kommerzielle Nutzung in Displays ausreichend sind. Nichtsdestotrotz sind weitere Verbesserungen für die Stabilität blauer Emitter und die Leistungsfähigkeit bei hohen Leuchtstärken erforderlich, damit die OLED Technologie ihren Platz auf dem Markt behaupten kann. Mit steigender Stromstärke, die durch ein solches Bauteil fließt, sinkt die Effizienz rapide (der sogenannte Effizienz-Roll-Off), was die Nutzung von OLEDs verhindert, wann immer hohe Leuchtstärken erforderlich sind. Zusätzlich verändern weiße OLEDs ihre Farbkomposition durch die unterschiedliche Alterung der unterschiedlichen Emittermoleküle oder veränderte Einsatzbedingungen. Obwohl die laterale Strukturierung nebeneinander aufgebrachter, monochromer OLEDs diese Probleme umgehen könnte, ist diese Herangehensweise durch die aktuelle Schattenmasken-Technologie limitiert, welche zur Strukturierung vakuumprozessierter Dünnschichten eingesetzt wird, und somit unpraktikabel für die Massenproduktion. Um diese Problemstellungen zu umgehen, wird hier die photolithographische Strukturierung von OLEDs angewendet. Mithilfe der orthogonalen Lithographie können hocheffiziente Bauteile damit erfolgreich auf Größenordnungen von 10 Mikrometer strukturiert werden. Dies zeigt, dass die orthogonale Prozessierung eine vielversprechende Alternative für die Schattenmasken-Technologie darstellt und für die Herstellung von RGB-Displays und Beleuchtungspanelen geeignet ist. Photostrukturierte Bauteile zeigen dabei eine nahezu identische Leistungsfähigkeit zu solchen, die großffächig mittels Schattenmasken hergestellt wurden. Diese hohe Leistungsfähigkeit kann hierbei durch eine sorgfältige Auswahl der einzelnen funktionellen Schichten erreicht werden, welche auf Untersuchung von morphologischer Stabilität und Löslichkeit dieser Schichten basiert. Mikrostrukturierte OLEDs in verschiedenen Konfigurationen werden mit ihren großflächigen Gegenstücken verglichen, um beobachtete Abweichungen im Ladungstransport, der Wärmeverteilung, sowie der Exzitonenrekombination zu erklären. Die Rolle der Joule'schen Wärme, die zur Auslöschung der emittierenden Exzitonenzustände führt, wird hier diskutiert. Die thermische Dissipation kann dabei verbessert werden, indem die aktive Fläche der OLED auf 20 Mikrometer herunterstrukturiert wird. Folglich kann der Effizienz-Roll-Off bei hohen Leuchtstärken in lateral strukturierten weißen elektrolumineszenten Bauteilen unterdrückt werden.:List of Publications 1 1 Introduction 5 2 Organic Semiconductors 9 2.1 Molecular Bonding 9 2.1.1 Intramolecular Interactions 10 2.1.2 Intermolecular Interactions 17 2.2 Optical Properties of Organic Semiconductors 23 2.2.1 Excited State Dynamics 26 2.3 Energy Transfer in Organic Solids 27 2.3.1 Förster Energy Transfer 29 2.3.2 Dexter Energy Transfer 30 2.4 Charge Transport Phenomena 31 2.4.1 Polarization and Energetic Disorder 31 2.4.2 Charge Transport Models 33 3 Electromagnetic Field Propagation in Layered Media 35 3.1 Maxwell's Equations 35 3.1.1 Wave Character of Electromagnetic Field 37 3.1.2 Energy of Electromagnetic Field 38 3.1.3 Boundary Conditions of Electromagnetic Fields 39 3.2 Reflection and Refraction of Plane Waves 40 3.2.1 Total Internal Reflection 43 3.3 Guided Optical Waves 44 3.3.1 Modes of Planar Waveguide 45 3.3.2 Multilayer Waveguides 49 3.3.3 Mode Coupling 53 3.4 EM Field in Presence of Charges 55 3.4.1 Volume Plasmons 58 3.4.2 Surface Plasmon Polaritons 58 3.4.3 Localized Plasmons 62 4 Organic Light-Emitting Diodes 65 4.1 Principle of Operation 65 4.1.1 Electroluminescence Efficiency 66 4.1.2 Charge Injection and Transport 66 4.1.3 Radiative Efficiency 68 4.1.4 Excited State Formation 69 4.1.5 Organic Emitters 71 4.1.6 Light Extraction 73 4.1.7 Efficiency Loss Mechanisms 74 4.2 Applications of OLEDs 76 4.2.1 Information Displays 76 4.2.2 Solid-State Lighting 77 4.2.3 OLED Based Sensors 77 4.3 OLED Structuring 79 4.3.1 Shadow Mask Patterning 79 4.3.2 Serial Printing 80 4.3.3 Unconventional Patterning Techniques 80 4.3.4 Photolithographic Patterning of OLEDs 81 4.3.5 Orthogonal Processing of Organic Semiconductors 83 5 Materials and Methods 87 5.1 Organic Functional Materials . 87 5.1.1 Hole Injection/Transport Layers 87 5.1.2 Electron Blocking Materials 88 5.1.3 Hole Blockers and Electron Transport Materials 88 5.1.4 Emitter Systems 90 5.1.5 Substrate and Electrodes 90 5.2 Device Fabrication 92 5.2.1 Vacuum Deposition 92 5.2.2 Photolithographic Structuring 92 5.3 Measurements 94 5.3.1 OLED Characterisation 94 5.3.2 Optical and Morphological Inspection 95 5.3.3 Calcium Conductance Test 95 5.3.4 Time-of-flight Spectroscopy 96 6 Orthogonal Patterning of Organic Semiconductor Films and Devices 97 6.1 Patterned Organic Films 97 6.2 Patterned Alq3 Based OLEDs 100 6.2.1 Direct Emitter Patterning 100 6.2.2 Cathode as Protection Layer 102 6.2.3 Impact of O2 Plasma Treatment 104 6.3 Summary 107 7 Photolithographic Structuring of State-of-the-Art p-i-n OLEDs for Full-Colour RGB Displays 109 7.1 Studied OLED Structures 109 7.2 HFE Compatibility Study 110 7.2.1 HFE Immersion Study 110 7.2.2 LDI-TOF-MS Analysis 112 7.3 Large area OLEDs 114 7.4 Microscale Devices 118 7.5 Bilayer Processing on p-i-n OLEDs 122 7.6 Summary 126 8 White Light from Photo-structured OLED Arrays 129 8.1 Fabrication of Micro-OLED Array 129 8.1.1 Structuring Procedure 130 8.1.2 Optical Device Optimisation 130 8.1.3 Choice of Hole Blocking and Electron Transport Layers 134 8.2 Performance of Microstructured Devices 143 8.2.1 Colour Temperature Tuning 143 8.2.2 Compatibility with Photo-patterning 145 8.2.3 Colour Stability 150 8.3 Summary 154 9 Efficiency Roll-off and Emission Colour of Microstructured OLEDs 155 9.1 Photolithographic Control of the Subunit Dimension 155 9.2 Control of the Emission Colour 156 9.3 Suppression of Efficiency Roll-off in Microscale Devices 157 9.4 Thermal Management in OLEDs 159 9.5 Modelling Impact of Joule Heat on Roll-off Characteristics 162 9.6 Summary 164 10 Conclusions and Outlook 165 10.1 Conclusions 165 10.2 Outlook 167 List of Abbreviations 171 List of Figures 173 List of Tables 177 Acknowledgements 179 Bibliography 18

Optical properties of two-dimensional tin nanosheets epitaxially grown on graphene

Heterostacks formed by combining two-dimensional materials show novel properties which are of great interest for new applications in electronics, photonics and even twistronics, the new emerging field born after the outstanding discoveries on twisted graphene. Here, we report the direct growth of tin nanosheets at the two-dimensional limit via molecular beam epitaxy on chemical vapor deposited graphene on Al2O3(0001). The mutual interaction between the tin nanosheets and graphene is evidenced by structural and chemical investigations. On the one hand, Raman spectroscopy indicates that graphene undergoes compressive strain after the tin growth, while no charge transfer is observed. On the other hand, chemical analysis shows that tin nanosheets interaction with sapphire is mediated by graphene avoiding the tin oxidation occurring in the direct growth on this substrate. Remarkably, optical measurements show that the absorption of tin nanosheets show a graphene-like behavior with a strong absorption in the ultraviolet photon energy range, therein resulting in a different optical response compared to tin nanosheets on bare sapphire. The optical properties of tin nanosheets therefore represent an open and flexible playground for the absorption of light in a broad range of the electromagnetic spectrum and technologically relevant applications for photon harvesting and sensors.Comment: 14 pages, 7 figure

Mapping domain junctions using 4D-STEM: toward controlled properties of epitaxially grown transition metal dichalcogenide monolayers

Epitaxial growth has become a promising route to achieve highly crystalline continuous two-dimensional layers. However, high-quality layer production with expected electrical properties is still challenging due to the defects induced by the coalescence between imperfectly aligned domains. In order to control their intrinsic properties at the device scale, the synthesized materials should be described as a patchwork of coalesced domains. Here, we report multi-scale and multistructural analysis on highly oriented epitaxial WS$_2$ and WSe$_2$ monolayers using scanning transmission electron microscopy (STEM) techniques. Characteristic domain junctions are first identified and classified based on the detailed atomic structure analysis using aberration corrected STEM imaging. Mapping orientation, polar direction and phase at the micrometer scale using four-dimensional STEM enabled to access the density and the distribution of the specific domain junctions. Our results validate a readily applicable process for the study of highly oriented epitaxial transition metal dichalcogenides, providing an overview of synthesized materials from large scale down to atomic scale with multiple structural information.Comment: 22 pages, 6 figures and Supplementary Informatio

Finite p-groups all of whose maximal subgroups, except one, have its derived subgroup of order ≤ p

Let G be a finite p-group which has exactly one maximal subgroup H such that |H\u27|>p. Then we have d(G)=2, p=2, H\u27 is a four-group, G\u27 is abelian of order 8 and type (4,2), G is of class 3 and the structure of G is completely determined. This solves the problem Nr. 1800 stated by Y. Berkovich in [3]

Deep blue oxadiazole-containing thermally activated delayed fluorescence emitters for organic light-emitting diodes

EZ-C thanks the University of St Andrews for support. The authors are grateful to the EPSRC for financial support (grants EP/P010482/1, EP/J01771X and EP/J00916). IDWS is a Royal Society Wolfson Research Merit Award Holder. CM acknowledges funding by the European Commission through a Marie Skłodowska Curie Individual Fellowship (No. 703387).A series of four novel deep blue to sky blue thermally activated delayed fluorescence (TADF) emitters (2CzdOXDMe, 2CzdOXD4MeOPh, 2CzdOXDPh and 2CzdOXD4CF3Ph) have been synthesized and characterized. These oxadiazole-based emitters demonstrated bluer emission compared with reference emitter 2CzPN thanks to the weaker acceptor strength of oxadiazole moieties. In particular, the oxadiazole compounds doped in hosts (mCP and PPT) emitted from 435-474 nm with photoluminescence quantum yields ranging from 14%-55%. The emitters possess relatively large singlet-triplet excited state energy gaps (ΔEST) from 0.25-0.46 eV resulting in long lifetimes of the delayed components ranging from 4.8 to 25.8 ms. Electroluminescent devices using these oxadiazole emitters as dopant emitters gave emission maxima (λEL) from 446 nm to 474 nm with maximum external quantum efficiencies ranging from 4.2-11.2%.PostprintPeer reviewe

Advances in Organic Displays and Lighting: Towards Planar Lithographic Integration of Organic Light-Emitting Diodes

This work focusses on the advances of organic light-emitting diodes (OLEDs) for large area display and solid-state-lighting applications. OLED technology has matured over the past two decades, aided by the rapid advances in development of the novel material and device concepts. State-of-the-art OLEDs reach internal efficiencies of 100% and device lifetimes acceptable for commercial display applications. However, further improvements in the blue emitter stability and the device performance at the high brightness are essential for OLED technology to secure its place in the lighting market. As the current passing through the device increases, a rapid decrease in OLED efficiency, so-called efficiency roll-off, takes place, which hinders the use of OLEDs wherever high brightness is required. In addition, white OLEDs comprising multiple emitter molecules suffer from the emission colour change as the operating conditions are varied or as the devices age. Despite side-by-side structuring of the monochrome OLEDs could in principle circumvent most of bespoke issues, the limitations imposed by the shadow mask technique, employed to structure vacuum deposited films, renders such approach impractical for fabrication of the devices on a large scale. In order to address these issues, photolithographic patterning of OLEDs is implemented. Highly efficient state-of-the-art devices are successfully structured down to tens of micrometers with the aid of orthogonal lithographic processing. The latter is shown to be a promising alternative for the shadow mask method in order to fabricate the full-colour RGB displays and solid-state-lighting panels. Photo-patterned devices exhibit a virtually identical performance to their shadow mask counterparts on a large scale. The high performance is replicated in the microscale OLEDs by a careful selection of functional layer sequence based on the investigation of the morphological stability and solubility of vacuum deposited films. Microstructured OLEDs, fabricated in several different configurations, are investigated and compared to their large area counterparts in order to account for the observed differences in charge transport, heat management and exciton recombination in bespoke devices. The role of the Joule heat leading to the quenching of the emissive exciton states in working devices is discussed. Structuring the active OLED area down to 20 micrometer is shown to improve the thermal dissipation in such devices, thus enabling the suppression of the efficiency roll-off at high brightness in white-emitting electroluminescent devices based on side-by-side patterned OLEDs.:List of Publications 1 1 Introduction 5 2 Organic Semiconductors 9 2.1 Molecular Bonding 9 2.1.1 Intramolecular Interactions 10 2.1.2 Intermolecular Interactions 17 2.2 Optical Properties of Organic Semiconductors 23 2.2.1 Excited State Dynamics 26 2.3 Energy Transfer in Organic Solids 27 2.3.1 Förster Energy Transfer 29 2.3.2 Dexter Energy Transfer 30 2.4 Charge Transport Phenomena 31 2.4.1 Polarization and Energetic Disorder 31 2.4.2 Charge Transport Models 33 3 Electromagnetic Field Propagation in Layered Media 35 3.1 Maxwell's Equations 35 3.1.1 Wave Character of Electromagnetic Field 37 3.1.2 Energy of Electromagnetic Field 38 3.1.3 Boundary Conditions of Electromagnetic Fields 39 3.2 Reflection and Refraction of Plane Waves 40 3.2.1 Total Internal Reflection 43 3.3 Guided Optical Waves 44 3.3.1 Modes of Planar Waveguide 45 3.3.2 Multilayer Waveguides 49 3.3.3 Mode Coupling 53 3.4 EM Field in Presence of Charges 55 3.4.1 Volume Plasmons 58 3.4.2 Surface Plasmon Polaritons 58 3.4.3 Localized Plasmons 62 4 Organic Light-Emitting Diodes 65 4.1 Principle of Operation 65 4.1.1 Electroluminescence Efficiency 66 4.1.2 Charge Injection and Transport 66 4.1.3 Radiative Efficiency 68 4.1.4 Excited State Formation 69 4.1.5 Organic Emitters 71 4.1.6 Light Extraction 73 4.1.7 Efficiency Loss Mechanisms 74 4.2 Applications of OLEDs 76 4.2.1 Information Displays 76 4.2.2 Solid-State Lighting 77 4.2.3 OLED Based Sensors 77 4.3 OLED Structuring 79 4.3.1 Shadow Mask Patterning 79 4.3.2 Serial Printing 80 4.3.3 Unconventional Patterning Techniques 80 4.3.4 Photolithographic Patterning of OLEDs 81 4.3.5 Orthogonal Processing of Organic Semiconductors 83 5 Materials and Methods 87 5.1 Organic Functional Materials . 87 5.1.1 Hole Injection/Transport Layers 87 5.1.2 Electron Blocking Materials 88 5.1.3 Hole Blockers and Electron Transport Materials 88 5.1.4 Emitter Systems 90 5.1.5 Substrate and Electrodes 90 5.2 Device Fabrication 92 5.2.1 Vacuum Deposition 92 5.2.2 Photolithographic Structuring 92 5.3 Measurements 94 5.3.1 OLED Characterisation 94 5.3.2 Optical and Morphological Inspection 95 5.3.3 Calcium Conductance Test 95 5.3.4 Time-of-flight Spectroscopy 96 6 Orthogonal Patterning of Organic Semiconductor Films and Devices 97 6.1 Patterned Organic Films 97 6.2 Patterned Alq3 Based OLEDs 100 6.2.1 Direct Emitter Patterning 100 6.2.2 Cathode as Protection Layer 102 6.2.3 Impact of O2 Plasma Treatment 104 6.3 Summary 107 7 Photolithographic Structuring of State-of-the-Art p-i-n OLEDs for Full-Colour RGB Displays 109 7.1 Studied OLED Structures 109 7.2 HFE Compatibility Study 110 7.2.1 HFE Immersion Study 110 7.2.2 LDI-TOF-MS Analysis 112 7.3 Large area OLEDs 114 7.4 Microscale Devices 118 7.5 Bilayer Processing on p-i-n OLEDs 122 7.6 Summary 126 8 White Light from Photo-structured OLED Arrays 129 8.1 Fabrication of Micro-OLED Array 129 8.1.1 Structuring Procedure 130 8.1.2 Optical Device Optimisation 130 8.1.3 Choice of Hole Blocking and Electron Transport Layers 134 8.2 Performance of Microstructured Devices 143 8.2.1 Colour Temperature Tuning 143 8.2.2 Compatibility with Photo-patterning 145 8.2.3 Colour Stability 150 8.3 Summary 154 9 Efficiency Roll-off and Emission Colour of Microstructured OLEDs 155 9.1 Photolithographic Control of the Subunit Dimension 155 9.2 Control of the Emission Colour 156 9.3 Suppression of Efficiency Roll-off in Microscale Devices 157 9.4 Thermal Management in OLEDs 159 9.5 Modelling Impact of Joule Heat on Roll-off Characteristics 162 9.6 Summary 164 10 Conclusions and Outlook 165 10.1 Conclusions 165 10.2 Outlook 167 List of Abbreviations 171 List of Figures 173 List of Tables 177 Acknowledgements 179 Bibliography 181Die vorliegende Arbeit beschäftigt sich mit den neusten Errungenschaften von organischen Licht-emittierenden Dioden (OLEDs) für großflächige Beleuchtungs- und Displayanwendungen. Die OLED-Technologie hat sich in den letzten zwei Jahrzehnten, begünstigt von neuartigen Material- und Bauteilkonzepten, weit entwickelt. Im aktuellen Stand der Technik erreichen OLEDs sowohl interne Effizienzen von 100% als auch Lebensdauern die für die kommerzielle Nutzung in Displays ausreichend sind. Nichtsdestotrotz sind weitere Verbesserungen für die Stabilität blauer Emitter und die Leistungsfähigkeit bei hohen Leuchtstärken erforderlich, damit die OLED Technologie ihren Platz auf dem Markt behaupten kann. Mit steigender Stromstärke, die durch ein solches Bauteil fließt, sinkt die Effizienz rapide (der sogenannte Effizienz-Roll-Off), was die Nutzung von OLEDs verhindert, wann immer hohe Leuchtstärken erforderlich sind. Zusätzlich verändern weiße OLEDs ihre Farbkomposition durch die unterschiedliche Alterung der unterschiedlichen Emittermoleküle oder veränderte Einsatzbedingungen. Obwohl die laterale Strukturierung nebeneinander aufgebrachter, monochromer OLEDs diese Probleme umgehen könnte, ist diese Herangehensweise durch die aktuelle Schattenmasken-Technologie limitiert, welche zur Strukturierung vakuumprozessierter Dünnschichten eingesetzt wird, und somit unpraktikabel für die Massenproduktion. Um diese Problemstellungen zu umgehen, wird hier die photolithographische Strukturierung von OLEDs angewendet. Mithilfe der orthogonalen Lithographie können hocheffiziente Bauteile damit erfolgreich auf Größenordnungen von 10 Mikrometer strukturiert werden. Dies zeigt, dass die orthogonale Prozessierung eine vielversprechende Alternative für die Schattenmasken-Technologie darstellt und für die Herstellung von RGB-Displays und Beleuchtungspanelen geeignet ist. Photostrukturierte Bauteile zeigen dabei eine nahezu identische Leistungsfähigkeit zu solchen, die großffächig mittels Schattenmasken hergestellt wurden. Diese hohe Leistungsfähigkeit kann hierbei durch eine sorgfältige Auswahl der einzelnen funktionellen Schichten erreicht werden, welche auf Untersuchung von morphologischer Stabilität und Löslichkeit dieser Schichten basiert. Mikrostrukturierte OLEDs in verschiedenen Konfigurationen werden mit ihren großflächigen Gegenstücken verglichen, um beobachtete Abweichungen im Ladungstransport, der Wärmeverteilung, sowie der Exzitonenrekombination zu erklären. Die Rolle der Joule'schen Wärme, die zur Auslöschung der emittierenden Exzitonenzustände führt, wird hier diskutiert. Die thermische Dissipation kann dabei verbessert werden, indem die aktive Fläche der OLED auf 20 Mikrometer herunterstrukturiert wird. Folglich kann der Effizienz-Roll-Off bei hohen Leuchtstärken in lateral strukturierten weißen elektrolumineszenten Bauteilen unterdrückt werden.:List of Publications 1 1 Introduction 5 2 Organic Semiconductors 9 2.1 Molecular Bonding 9 2.1.1 Intramolecular Interactions 10 2.1.2 Intermolecular Interactions 17 2.2 Optical Properties of Organic Semiconductors 23 2.2.1 Excited State Dynamics 26 2.3 Energy Transfer in Organic Solids 27 2.3.1 Förster Energy Transfer 29 2.3.2 Dexter Energy Transfer 30 2.4 Charge Transport Phenomena 31 2.4.1 Polarization and Energetic Disorder 31 2.4.2 Charge Transport Models 33 3 Electromagnetic Field Propagation in Layered Media 35 3.1 Maxwell's Equations 35 3.1.1 Wave Character of Electromagnetic Field 37 3.1.2 Energy of Electromagnetic Field 38 3.1.3 Boundary Conditions of Electromagnetic Fields 39 3.2 Reflection and Refraction of Plane Waves 40 3.2.1 Total Internal Reflection 43 3.3 Guided Optical Waves 44 3.3.1 Modes of Planar Waveguide 45 3.3.2 Multilayer Waveguides 49 3.3.3 Mode Coupling 53 3.4 EM Field in Presence of Charges 55 3.4.1 Volume Plasmons 58 3.4.2 Surface Plasmon Polaritons 58 3.4.3 Localized Plasmons 62 4 Organic Light-Emitting Diodes 65 4.1 Principle of Operation 65 4.1.1 Electroluminescence Efficiency 66 4.1.2 Charge Injection and Transport 66 4.1.3 Radiative Efficiency 68 4.1.4 Excited State Formation 69 4.1.5 Organic Emitters 71 4.1.6 Light Extraction 73 4.1.7 Efficiency Loss Mechanisms 74 4.2 Applications of OLEDs 76 4.2.1 Information Displays 76 4.2.2 Solid-State Lighting 77 4.2.3 OLED Based Sensors 77 4.3 OLED Structuring 79 4.3.1 Shadow Mask Patterning 79 4.3.2 Serial Printing 80 4.3.3 Unconventional Patterning Techniques 80 4.3.4 Photolithographic Patterning of OLEDs 81 4.3.5 Orthogonal Processing of Organic Semiconductors 83 5 Materials and Methods 87 5.1 Organic Functional Materials . 87 5.1.1 Hole Injection/Transport Layers 87 5.1.2 Electron Blocking Materials 88 5.1.3 Hole Blockers and Electron Transport Materials 88 5.1.4 Emitter Systems 90 5.1.5 Substrate and Electrodes 90 5.2 Device Fabrication 92 5.2.1 Vacuum Deposition 92 5.2.2 Photolithographic Structuring 92 5.3 Measurements 94 5.3.1 OLED Characterisation 94 5.3.2 Optical and Morphological Inspection 95 5.3.3 Calcium Conductance Test 95 5.3.4 Time-of-flight Spectroscopy 96 6 Orthogonal Patterning of Organic Semiconductor Films and Devices 97 6.1 Patterned Organic Films 97 6.2 Patterned Alq3 Based OLEDs 100 6.2.1 Direct Emitter Patterning 100 6.2.2 Cathode as Protection Layer 102 6.2.3 Impact of O2 Plasma Treatment 104 6.3 Summary 107 7 Photolithographic Structuring of State-of-the-Art p-i-n OLEDs for Full-Colour RGB Displays 109 7.1 Studied OLED Structures 109 7.2 HFE Compatibility Study 110 7.2.1 HFE Immersion Study 110 7.2.2 LDI-TOF-MS Analysis 112 7.3 Large area OLEDs 114 7.4 Microscale Devices 118 7.5 Bilayer Processing on p-i-n OLEDs 122 7.6 Summary 126 8 White Light from Photo-structured OLED Arrays 129 8.1 Fabrication of Micro-OLED Array 129 8.1.1 Structuring Procedure 130 8.1.2 Optical Device Optimisation 130 8.1.3 Choice of Hole Blocking and Electron Transport Layers 134 8.2 Performance of Microstructured Devices 143 8.2.1 Colour Temperature Tuning 143 8.2.2 Compatibility with Photo-patterning 145 8.2.3 Colour Stability 150 8.3 Summary 154 9 Efficiency Roll-off and Emission Colour of Microstructured OLEDs 155 9.1 Photolithographic Control of the Subunit Dimension 155 9.2 Control of the Emission Colour 156 9.3 Suppression of Efficiency Roll-off in Microscale Devices 157 9.4 Thermal Management in OLEDs 159 9.5 Modelling Impact of Joule Heat on Roll-off Characteristics 162 9.6 Summary 164 10 Conclusions and Outlook 165 10.1 Conclusions 165 10.2 Outlook 167 List of Abbreviations 171 List of Figures 173 List of Tables 177 Acknowledgements 179 Bibliography 18

Advances in Organic Displays and Lighting: Towards Planar Lithographic Integration of Organic Light-Emitting Diodes

This work focusses on the advances of organic light-emitting diodes (OLEDs) for large area display and solid-state-lighting applications. OLED technology has matured over the past two decades, aided by the rapid advances in development of the novel material and device concepts. State-of-the-art OLEDs reach internal efficiencies of 100% and device lifetimes acceptable for commercial display applications. However, further improvements in the blue emitter stability and the device performance at the high brightness are essential for OLED technology to secure its place in the lighting market. As the current passing through the device increases, a rapid decrease in OLED efficiency, so-called efficiency roll-off, takes place, which hinders the use of OLEDs wherever high brightness is required. In addition, white OLEDs comprising multiple emitter molecules suffer from the emission colour change as the operating conditions are varied or as the devices age. Despite side-by-side structuring of the monochrome OLEDs could in principle circumvent most of bespoke issues, the limitations imposed by the shadow mask technique, employed to structure vacuum deposited films, renders such approach impractical for fabrication of the devices on a large scale. In order to address these issues, photolithographic patterning of OLEDs is implemented. Highly efficient state-of-the-art devices are successfully structured down to tens of micrometers with the aid of orthogonal lithographic processing. The latter is shown to be a promising alternative for the shadow mask method in order to fabricate the full-colour RGB displays and solid-state-lighting panels. Photo-patterned devices exhibit a virtually identical performance to their shadow mask counterparts on a large scale. The high performance is replicated in the microscale OLEDs by a careful selection of functional layer sequence based on the investigation of the morphological stability and solubility of vacuum deposited films. Microstructured OLEDs, fabricated in several different configurations, are investigated and compared to their large area counterparts in order to account for the observed differences in charge transport, heat management and exciton recombination in bespoke devices. The role of the Joule heat leading to the quenching of the emissive exciton states in working devices is discussed. Structuring the active OLED area down to 20 micrometer is shown to improve the thermal dissipation in such devices, thus enabling the suppression of the efficiency roll-off at high brightness in white-emitting electroluminescent devices based on side-by-side patterned OLEDs.:List of Publications 1 1 Introduction 5 2 Organic Semiconductors 9 2.1 Molecular Bonding 9 2.1.1 Intramolecular Interactions 10 2.1.2 Intermolecular Interactions 17 2.2 Optical Properties of Organic Semiconductors 23 2.2.1 Excited State Dynamics 26 2.3 Energy Transfer in Organic Solids 27 2.3.1 Förster Energy Transfer 29 2.3.2 Dexter Energy Transfer 30 2.4 Charge Transport Phenomena 31 2.4.1 Polarization and Energetic Disorder 31 2.4.2 Charge Transport Models 33 3 Electromagnetic Field Propagation in Layered Media 35 3.1 Maxwell's Equations 35 3.1.1 Wave Character of Electromagnetic Field 37 3.1.2 Energy of Electromagnetic Field 38 3.1.3 Boundary Conditions of Electromagnetic Fields 39 3.2 Reflection and Refraction of Plane Waves 40 3.2.1 Total Internal Reflection 43 3.3 Guided Optical Waves 44 3.3.1 Modes of Planar Waveguide 45 3.3.2 Multilayer Waveguides 49 3.3.3 Mode Coupling 53 3.4 EM Field in Presence of Charges 55 3.4.1 Volume Plasmons 58 3.4.2 Surface Plasmon Polaritons 58 3.4.3 Localized Plasmons 62 4 Organic Light-Emitting Diodes 65 4.1 Principle of Operation 65 4.1.1 Electroluminescence Efficiency 66 4.1.2 Charge Injection and Transport 66 4.1.3 Radiative Efficiency 68 4.1.4 Excited State Formation 69 4.1.5 Organic Emitters 71 4.1.6 Light Extraction 73 4.1.7 Efficiency Loss Mechanisms 74 4.2 Applications of OLEDs 76 4.2.1 Information Displays 76 4.2.2 Solid-State Lighting 77 4.2.3 OLED Based Sensors 77 4.3 OLED Structuring 79 4.3.1 Shadow Mask Patterning 79 4.3.2 Serial Printing 80 4.3.3 Unconventional Patterning Techniques 80 4.3.4 Photolithographic Patterning of OLEDs 81 4.3.5 Orthogonal Processing of Organic Semiconductors 83 5 Materials and Methods 87 5.1 Organic Functional Materials . 87 5.1.1 Hole Injection/Transport Layers 87 5.1.2 Electron Blocking Materials 88 5.1.3 Hole Blockers and Electron Transport Materials 88 5.1.4 Emitter Systems 90 5.1.5 Substrate and Electrodes 90 5.2 Device Fabrication 92 5.2.1 Vacuum Deposition 92 5.2.2 Photolithographic Structuring 92 5.3 Measurements 94 5.3.1 OLED Characterisation 94 5.3.2 Optical and Morphological Inspection 95 5.3.3 Calcium Conductance Test 95 5.3.4 Time-of-flight Spectroscopy 96 6 Orthogonal Patterning of Organic Semiconductor Films and Devices 97 6.1 Patterned Organic Films 97 6.2 Patterned Alq3 Based OLEDs 100 6.2.1 Direct Emitter Patterning 100 6.2.2 Cathode as Protection Layer 102 6.2.3 Impact of O2 Plasma Treatment 104 6.3 Summary 107 7 Photolithographic Structuring of State-of-the-Art p-i-n OLEDs for Full-Colour RGB Displays 109 7.1 Studied OLED Structures 109 7.2 HFE Compatibility Study 110 7.2.1 HFE Immersion Study 110 7.2.2 LDI-TOF-MS Analysis 112 7.3 Large area OLEDs 114 7.4 Microscale Devices 118 7.5 Bilayer Processing on p-i-n OLEDs 122 7.6 Summary 126 8 White Light from Photo-structured OLED Arrays 129 8.1 Fabrication of Micro-OLED Array 129 8.1.1 Structuring Procedure 130 8.1.2 Optical Device Optimisation 130 8.1.3 Choice of Hole Blocking and Electron Transport Layers 134 8.2 Performance of Microstructured Devices 143 8.2.1 Colour Temperature Tuning 143 8.2.2 Compatibility with Photo-patterning 145 8.2.3 Colour Stability 150 8.3 Summary 154 9 Efficiency Roll-off and Emission Colour of Microstructured OLEDs 155 9.1 Photolithographic Control of the Subunit Dimension 155 9.2 Control of the Emission Colour 156 9.3 Suppression of Efficiency Roll-off in Microscale Devices 157 9.4 Thermal Management in OLEDs 159 9.5 Modelling Impact of Joule Heat on Roll-off Characteristics 162 9.6 Summary 164 10 Conclusions and Outlook 165 10.1 Conclusions 165 10.2 Outlook 167 List of Abbreviations 171 List of Figures 173 List of Tables 177 Acknowledgements 179 Bibliography 181Die vorliegende Arbeit beschäftigt sich mit den neusten Errungenschaften von organischen Licht-emittierenden Dioden (OLEDs) für großflächige Beleuchtungs- und Displayanwendungen. Die OLED-Technologie hat sich in den letzten zwei Jahrzehnten, begünstigt von neuartigen Material- und Bauteilkonzepten, weit entwickelt. Im aktuellen Stand der Technik erreichen OLEDs sowohl interne Effizienzen von 100% als auch Lebensdauern die für die kommerzielle Nutzung in Displays ausreichend sind. Nichtsdestotrotz sind weitere Verbesserungen für die Stabilität blauer Emitter und die Leistungsfähigkeit bei hohen Leuchtstärken erforderlich, damit die OLED Technologie ihren Platz auf dem Markt behaupten kann. Mit steigender Stromstärke, die durch ein solches Bauteil fließt, sinkt die Effizienz rapide (der sogenannte Effizienz-Roll-Off), was die Nutzung von OLEDs verhindert, wann immer hohe Leuchtstärken erforderlich sind. Zusätzlich verändern weiße OLEDs ihre Farbkomposition durch die unterschiedliche Alterung der unterschiedlichen Emittermoleküle oder veränderte Einsatzbedingungen. Obwohl die laterale Strukturierung nebeneinander aufgebrachter, monochromer OLEDs diese Probleme umgehen könnte, ist diese Herangehensweise durch die aktuelle Schattenmasken-Technologie limitiert, welche zur Strukturierung vakuumprozessierter Dünnschichten eingesetzt wird, und somit unpraktikabel für die Massenproduktion. Um diese Problemstellungen zu umgehen, wird hier die photolithographische Strukturierung von OLEDs angewendet. Mithilfe der orthogonalen Lithographie können hocheffiziente Bauteile damit erfolgreich auf Größenordnungen von 10 Mikrometer strukturiert werden. Dies zeigt, dass die orthogonale Prozessierung eine vielversprechende Alternative für die Schattenmasken-Technologie darstellt und für die Herstellung von RGB-Displays und Beleuchtungspanelen geeignet ist. Photostrukturierte Bauteile zeigen dabei eine nahezu identische Leistungsfähigkeit zu solchen, die großffächig mittels Schattenmasken hergestellt wurden. Diese hohe Leistungsfähigkeit kann hierbei durch eine sorgfältige Auswahl der einzelnen funktionellen Schichten erreicht werden, welche auf Untersuchung von morphologischer Stabilität und Löslichkeit dieser Schichten basiert. Mikrostrukturierte OLEDs in verschiedenen Konfigurationen werden mit ihren großflächigen Gegenstücken verglichen, um beobachtete Abweichungen im Ladungstransport, der Wärmeverteilung, sowie der Exzitonenrekombination zu erklären. Die Rolle der Joule'schen Wärme, die zur Auslöschung der emittierenden Exzitonenzustände führt, wird hier diskutiert. Die thermische Dissipation kann dabei verbessert werden, indem die aktive Fläche der OLED auf 20 Mikrometer herunterstrukturiert wird. Folglich kann der Effizienz-Roll-Off bei hohen Leuchtstärken in lateral strukturierten weißen elektrolumineszenten Bauteilen unterdrückt werden.:List of Publications 1 1 Introduction 5 2 Organic Semiconductors 9 2.1 Molecular Bonding 9 2.1.1 Intramolecular Interactions 10 2.1.2 Intermolecular Interactions 17 2.2 Optical Properties of Organic Semiconductors 23 2.2.1 Excited State Dynamics 26 2.3 Energy Transfer in Organic Solids 27 2.3.1 Förster Energy Transfer 29 2.3.2 Dexter Energy Transfer 30 2.4 Charge Transport Phenomena 31 2.4.1 Polarization and Energetic Disorder 31 2.4.2 Charge Transport Models 33 3 Electromagnetic Field Propagation in Layered Media 35 3.1 Maxwell's Equations 35 3.1.1 Wave Character of Electromagnetic Field 37 3.1.2 Energy of Electromagnetic Field 38 3.1.3 Boundary Conditions of Electromagnetic Fields 39 3.2 Reflection and Refraction of Plane Waves 40 3.2.1 Total Internal Reflection 43 3.3 Guided Optical Waves 44 3.3.1 Modes of Planar Waveguide 45 3.3.2 Multilayer Waveguides 49 3.3.3 Mode Coupling 53 3.4 EM Field in Presence of Charges 55 3.4.1 Volume Plasmons 58 3.4.2 Surface Plasmon Polaritons 58 3.4.3 Localized Plasmons 62 4 Organic Light-Emitting Diodes 65 4.1 Principle of Operation 65 4.1.1 Electroluminescence Efficiency 66 4.1.2 Charge Injection and Transport 66 4.1.3 Radiative Efficiency 68 4.1.4 Excited State Formation 69 4.1.5 Organic Emitters 71 4.1.6 Light Extraction 73 4.1.7 Efficiency Loss Mechanisms 74 4.2 Applications of OLEDs 76 4.2.1 Information Displays 76 4.2.2 Solid-State Lighting 77 4.2.3 OLED Based Sensors 77 4.3 OLED Structuring 79 4.3.1 Shadow Mask Patterning 79 4.3.2 Serial Printing 80 4.3.3 Unconventional Patterning Techniques 80 4.3.4 Photolithographic Patterning of OLEDs 81 4.3.5 Orthogonal Processing of Organic Semiconductors 83 5 Materials and Methods 87 5.1 Organic Functional Materials . 87 5.1.1 Hole Injection/Transport Layers 87 5.1.2 Electron Blocking Materials 88 5.1.3 Hole Blockers and Electron Transport Materials 88 5.1.4 Emitter Systems 90 5.1.5 Substrate and Electrodes 90 5.2 Device Fabrication 92 5.2.1 Vacuum Deposition 92 5.2.2 Photolithographic Structuring 92 5.3 Measurements 94 5.3.1 OLED Characterisation 94 5.3.2 Optical and Morphological Inspection 95 5.3.3 Calcium Conductance Test 95 5.3.4 Time-of-flight Spectroscopy 96 6 Orthogonal Patterning of Organic Semiconductor Films and Devices 97 6.1 Patterned Organic Films 97 6.2 Patterned Alq3 Based OLEDs 100 6.2.1 Direct Emitter Patterning 100 6.2.2 Cathode as Protection Layer 102 6.2.3 Impact of O2 Plasma Treatment 104 6.3 Summary 107 7 Photolithographic Structuring of State-of-the-Art p-i-n OLEDs for Full-Colour RGB Displays 109 7.1 Studied OLED Structures 109 7.2 HFE Compatibility Study 110 7.2.1 HFE Immersion Study 110 7.2.2 LDI-TOF-MS Analysis 112 7.3 Large area OLEDs 114 7.4 Microscale Devices 118 7.5 Bilayer Processing on p-i-n OLEDs 122 7.6 Summary 126 8 White Light from Photo-structured OLED Arrays 129 8.1 Fabrication of Micro-OLED Array 129 8.1.1 Structuring Procedure 130 8.1.2 Optical Device Optimisation 130 8.1.3 Choice of Hole Blocking and Electron Transport Layers 134 8.2 Performance of Microstructured Devices 143 8.2.1 Colour Temperature Tuning 143 8.2.2 Compatibility with Photo-patterning 145 8.2.3 Colour Stability 150 8.3 Summary 154 9 Efficiency Roll-off and Emission Colour of Microstructured OLEDs 155 9.1 Photolithographic Control of the Subunit Dimension 155 9.2 Control of the Emission Colour 156 9.3 Suppression of Efficiency Roll-off in Microscale Devices 157 9.4 Thermal Management in OLEDs 159 9.5 Modelling Impact of Joule Heat on Roll-off Characteristics 162 9.6 Summary 164 10 Conclusions and Outlook 165 10.1 Conclusions 165 10.2 Outlook 167 List of Abbreviations 171 List of Figures 173 List of Tables 177 Acknowledgements 179 Bibliography 18

Adjustable white-light emission from a photo-structured micro-OLED array

White organic light-emitting diodes (OLEDs) are promising candidates for future solid-state lighting applications and backplane illumination in large-area displays. One very specific feature of OLEDs, which is currently gaining momentum, is that they can enable tunable white light emission. This feature is conventionally realized either through the vertical stacking of independent OLEDs emitting different colors or in lateral arrangement of OLEDs. The vertical design is optically difficult to optimize and often results in efficiency compromises between the units. In contrast, the lateral concept introduces severe area losses to dark regions between the subunits, which requires a significantly larger overall device area to achieve equal brightness. Here we demonstrate a color-tunable, two-color OLED device realized by side-by-side alignment of yellow and blue p-i-n OLEDs structured down to 20 μm by a simple and up-scalable orthogonal photolithography technique. This layout eliminates the problems of conventional lateral approaches by utilizing all area for light emission. The corresponding emission of the photo-patterned two-unit OLED can be tuned over a wide range from yellow to white to blue colors. The independent control of the different units allows the desired overall spectrum to be set at any given brightness level. Operated as a white light source, the microstructured OLED reaches a luminous efficacy of 13 lm W−1 at 1000 cd m−2 without an additional light outcoupling enhancement and reaches a color rendering index of 68 when operated near the color point E. Finally, we demonstrate an improved device lifetime by means of size variation of the subunits