Studies of surfaces and interfaces in organic electronic devices

Abstract

There has been much progress in organic electronic devices in the past two decades due to their potential applications in optical and electronic fields. The performance of these devices, such as organic photovoltaic devices (OPVs), organic thin film transistors (OTFTs), and organic light-emitting diodes (OLEDs), depends critically on the properties of the electrode/organic and organic/organic interfaces. In this work, X-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) have been used to investigate the electronic and chemical structures at interfaces of different organic electronic devices. For OPV devices, we mainly focus on the donor/acceptor interfaces and discussed the relationship between the interfaces and the performance of OPV devices, especially the open circuit voltage. We found that the substrate, interface middle layer, and doping can all result in changes of the electronic structures at the donor/acceptor interface, which determines the performance of OPV devices. For OTFTs, we studied pentacene/molybdenum oxide (MoO3) and copper phthalocyanine (CuPc)/MoO3 interfaces and discuss how the electronic structures of these interfaces determine performance of the corresponding OTFTs. Via analysis of the electronic structures of these interfaces, we found that the space charge region formed in organic layer leads to high conductivity and normally-on operation mode in the OTFTs. We also studied the electronic structure of the n-n isotype copper hexadecafluorophthalocyanine (F16CuPc)/phthalocyanatotin (IV) dichloride (SnCl2Pc) heterojunction by UPS and XPS. Due to charge transfer across the heterojunction, energy level bendings were observed at the two organic layers. Formation of space charge regions strongly determine performance of the transistor consisting of the heterojunction. For OLEDs, we mainly studied the interface between carbon nanotube (CNT) film as a novel anode material and N, N'-diphenyl-N, N'-bis(1-naphthyl)-(1, 1'-biphenyl)-4, 4'-diamine (NPB). Due to the lower work function of the CNT film comparing to that of indium tin oxide (ITO), the height of the hole injection barrier is high and leads to severe performance degradation. Reducing the height in a controlled way is of scientific and technological importance for replacing ITO with the CNT film as anode material. We found that a simple and widely-used UV-ozone treatment can dramatically enhance the work function of CNT film. The height of hole injection barrier at the interface between NPB and treated CNT film is remarkably lowered, which will lead to improving the performance of devices. Then, we systematically studied the effects of O2 and H2O exposures on the surfaces and interfaces of organic materials in a control OLED by using UPS to understand the mechanisms of device degradation. For different surfaces and interfaces, the effects of O2 and H2O exposure were studied and discussed.

Date of Award	15 Feb 2011
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chun Sing LEE (Supervisor)