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 s 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. Sarah Schols was born on May 6, 1981, in Brugge, Belgium. She received her Master of Applied Science degree in electrical engineering (option nanotechnology) in July 2004 at the Katholieke Universiteit Leuven in Belgium. The topic of her master thesis was the fabrication of organic solar cells based on discotic liquid crystals. Inspired by the challenges in organic electronics, she started her doctoral research in October 2004 at the Interuniversity Microelectronics Center (imec) to pursue her PhD degree at the Department of Electrical Engineering (ESAT) of the Katholieke Universiteit Leuven. Her doctoral research was funded by the Belgian Fund for Scientific Research (FWO) and focused on the design of new device architectures and material concepts for organic light-emitting devices. Currently, she is a Senior Researcher in the Large Area Electronics Department of imec. Contents. List of symbols and abbreviations. 1 Introduction. 1.1 Organic semiconductors. 1.2 State of the art organic light-emitting devices. 1.3 Organic semiconductor lasers. 1.4 Book outline. 2 Materials and experimental techniques. 2.1 Sample fabrication. 2.2 Device characterization. 3 OLEDs with field-effect electron transport. 3.1 Device fabrication. 3.2 Device operation. 3.3 Device performance. 3.4 Improvement of the external quantum efficiency. 3.5 Summary and conclusions. 4 Devices based on diperfluorohexyl-quaterthiophene derivatives. 4.1 Characterization of DFH-4T and DFHCO-4T. 4.2 Devices based on DFH-4T. 4.3 Devices based on DFHCO-4T. 4.4 Comparison. 4.5 Summary and conclusions. 5 Control of the triplet concentration. 5.1 Pulsed excitation of OLEDs with field-effect transport. 5.2 Triplet excitation scavenging. 5.3 Summary and conclusions. 6 Triplet-emitter doped organic materials. 6.1 Triplet dynamics and charge carrier trapping. 6.2 Optical pumping of triplet-emitters. 6.3 Summary and conclusions. 7 Value of OLEDs with field-effect electron transport for lasing. 7.1 Potential as laser device configuration. 7.2 Optically pumped lasing experiments. 7.3 Device with integrated field-effect and photonic features. 7.4 Summary and conclusions. 8 General conclusions and future outlook. 8.1 Overview of the main results. 8.2 Suggestions for further research. Bibliography.