Materials and interface engineering in organic light-emitting diodes

Among the various fields of research on organic electronics, one of the most successful areas is related to organic light-emitting diodes (OLEDs) where significant improvements, such as good efficiency, high brightness, and low drive voltage, have been achieved. These improvements have led to the realization of high-efficiency full-color and white-color OLEDs.1,2 It is now well known that efficient electron and hole injection/transport from cathodes and anodes is essential for high-efficiency OLED devices. That requires the electroluminescent (EL) materials to have a good luminescence property as well as good electron-and hole-transporting abilities, which is hard to be met by almost any of the current EL materials. One of the alternative approaches to overcome this problem is to fabricate multilayer OLEDs by using a hole-transport layer (HTL) or an electron-transport layer (ETL) in device fabrication.3-5 As a consequence, the commonly used OLED devices normally adopt a multilayer device structure including the transparent conducting indium tin oxide (ITO) anode, HTL, emissive layer (EML), ETL, and metallic electrode cathode (Figure 8.1). And it has been found that carrier transport in most OLED heterostructures is largely injection limited.6 Thus, interfacial phenomena have been an important subject of OLED science and technology and many interfacial modification efforts have been applied to cathode/organic, anode/organic, and organic/organic interfaces resulting in improved device response.3 In this chapter, we review our recent progress on the development of materials and their applications on interface engineering between organic layer and electrodes to optimize charge-injection, -transport, and -recombination in OLEDs.