Device Architecture and Materials for Organic Light-Emitting Devices : Targeting High Current Densities and Control of the Triplet Concentration

Device Architecture and Materials for Organic Light-Emitting Devices focuses on the design of new device and material concepts for organic light-emitting devices, thereby targeting high current densities and an improved control of the triplet concentration. A new light-emitting device architecture, the OLED with field-effect electron transport, is demonstrated. This device is a hybrid between a diode and a field-effect transistor. Compared to conventional OLEDs, the metallic cathode is displaced by one to several micrometers from the light-emitting zone, reducing optical absorption losses. The electrons injected by the cathode accumulate at an organic heterojunction and are transported to the light-emission zone by field-effect. High mobilities for charge carriers are achieved in this way, enabling a high current density and a reduced number of charge carriers in the device. Pulsed excitation experiments show that pulses down to 1 μs can be applied to this structure without affecting the light intensity, suggesting that pulsed excitation might be useful to reduce the accumulation of triplets in the device. The combination of all these properties makes the OLED with field-effect electron transport particularly interesting for waveguide devices and future electrically pumped lasers. In addition, triplet-emitter doped organic materials, as well as the use of triplet scavengers in conjugated polymers are investigated. Device Architecture and Materials for Organic Light-Emitting Devices Preface Contents List of Symbols and Abbreviations Chapter 1: Introduction 1.1 Organic Semiconductors 1.1.1 Semiconducting Properties of Organic Materials 1.1.2 Charge Transport in Organic Materials Band Transport Model Multiple Trapping and Release Model Hopping Transport Model 1.1.3 Optical Transitions in Organic Materials 1.2 State of the Art Organic Light-Emitting Devices 1.2.1 Organic Light-Emitting Diodes 1.2.2 Organic Light-Emitting Transistors 1.3 Organic Semiconductor Lasers 1.3.1 General Aspects of Laser Action 1.3.2 Motivation for Plastic Lasers 1.3.3 Lasing in Organic Semiconductors Historical Overview of Lasing in Organic Materials Stimulated Emission and Gain in Organic Semiconductors Feedback Structures for Organic Semiconductor Lasers Lowering the Lasing Threshold with Energy Transfer 1.3.4 Prospects for Electrically Pumped Organic Lasers 1.4 Outline Chapter 2: Materials and Experimental Techniques 2.1 Sample Fabrication 2.1.1 Materials Used in this Work 2.1.2 Deposition Techniques Spin-Coating Organic Molecular Beam Deposition 2.1.3 Fabrication of Photonic Feedback Structures Fabrication of Linear Gratings Using Interference Lithography Fabrication of Linear Gratings Using Electron Beam Lithography 2.2 Device Characterization 2.2.1 Transistor Measurement and Parameter Extraction 2.2.2 Characterization of OLEDs 2.2.3 Time-Resolved Photoluminescence Measurements 2.2.4 Time-Resolved Pump-Probe Experiments 2.2.5 Amplified Spontaneous Emission and Loss Measurements 2.2.6 Determination of Thermally Stimulated Luminescence Chapter 3: Organic Light-Emitting Diodes with Field-Effect Electron Transport 3.1 Device Fabrication 3.2 Device Operation 3.3 Device Performance 3.3.1 Optical and Electrical Characterization 3.3.2 Analysis 3.4 Improvement of the External Quantum Efficiency 3.5 Summary and Conclusions Chapter 4: Devices Based on Diperfluorohexyl-quaterthiophene Derivatives 4.1 Characterization of the Electron-Transporting Materials DFH-4T and DFHCO-4T 4.2 Devices Based on DFH-4T 4.2.1 Transistors Based on DFH-4T 4.2.2 DFH-4T in OLEDs with Field-Effect Electron Transport 4.3 Devices Based on DFHCO-4T 4.3.1 High Performance DFHCO-4T Transistors 4.3.2 DFHCO-4T in OLEDs with Field-Effect Electron Transport 4.4 Comparison 4.5 Summary and Conclusions Chapter 5: Control of the Triplet Concentration in Organic Light-Emitting Devices 5.1 Pulsed Excitation of OLEDs with Field-Effect Electron Transport 5.1.1 Pulse-Width Dependence of Organic Light-Emitting Devices 5.1.2 Effects Influencing the Pulsed Excitation Behavior of OLEDs Charge Transport Charge Accumulation 5.2 Triplet Excitation Scavenging in Films of Conjugated Polymers 5.2.1 Triplet Scavenging in Liquid-State Organic Dye Lasers 5.2.2 Triplet Scavenging in Solid-State Organic Thin Films Triplet Scavenging by the Non-classical Triplet Acceptor COT Triplet Scavenging by the Classical Triplet Acceptor Anthracene 5.3 Summary and Conclusions Chapter 6: Triplet-Emitter Doped Organic Materials 6.1 Triplet Dynamics and Charge Carrier Trapping in Triplet-Emitter Doped Conjugated Polymers 6.1.1 Spectroscopic Characterization Pristine CNPPP Films Btp2Ir(acac) Doped into a PS Matrix CNPPP Films Doped with Btp2Ir(acac) 6.1.2 Thermally Stimulated Luminescence in CNPPP Doped with Btp2Ir(acac) 6.2 Optical Pumping of Triplet-Emitters 6.2.1 Attempts to Observe ASE Using Btp2Ir(acac) 6.2.2 Attempts to Observe ASE Using F5Ph and GDP16b 6.3 Summary and Conclusions Chapter 7: Value of OLEDs with Field-Effect Electron Transport for Lasing Applications 7.1 Potential of OLEDs with Field-Effect Electron Transport as Laser Device Configuration 7.1.1 High Current Densities 7.1.2 Reduced Absorption Losses 7.2 Optically Pumped Lasing Experiments 7.2.1 Stimulated Emission in Alq3:DCM2 7.2.2 Stimulated Emission in Stacks Comprising Alq3:DCM2 7.2.3 Discussion 7.3 Device with Integrated Field-Effect and Photonic Features 7.4 Summary and Conclusions Chapter 8: General Conclusions and Future Outlook 8.1 Overview of the Main Results 8.2 Suggestions for Further Research References