Surface and interface studies for organic electronic and electroluminescent devices

Abstract

Nowadays the field of organic electronics and optoelectronics are progressing rapidly. Many products using organic displays are already introduced into the marketplace, while there are many other opportunities for the application of organic semiconducting thin films. Actually it is difficult to predict when and what organic-based products will be introduced in the coming years. However, as a burgeoning technology, much remains to be done for the organic-based devices, including material synthesis, device design, and fundamental investigation. In the area of research and development, surface and interface characterization and analysis are indispensable to understanding the behaviors of the functional materials and improving the performance of the devices. This thesis focuses on the surface and interface studies for organic electronic and electroluminescent devices, covering the anode surfaces, interfaces between anode and hole-transporting layer (HTL) and interfaces between cathode and electron-transporting layer (ETL) in organic electroluminescent devices (OELDs), and the surfaces of ordered CuPc and pentacene thin films that are used as organic active layers in organic thin film transistors (OTFTs). The high-resolution electron energy loss spectrum (HREELS) of the as-received indium tin oxide (ITO) shows mainly CHx group adsorbed on the ITO surface. The oxygen-plasma treatment of the surface leads to the removal of the CHx group and additional oxidation of the ITO surface, which are responsible for the increase in the ITO work function after the treatment. We propose a simple chemical process associated with the oxygen-plasma treatment of ITO, indicating that the increase in the ITO work function is due to the changes of the surface dipole modes. HREELS measurements suggest a strong vibrational coupling between the ITO phonon mode at 71 meV and the NPB-derived vibrational mode at 65 meV; and annealing of the interface can result in the decoupling of the two vibrational modes. Ultra-violet photoelectron spectroscopy (UPS) measurements show that both the work function of ITO and the barrier height for hole injection from ITO into NPB exhibit a sharp change at the beginning of the NPB deposition. Both changes are attributed to a dipolar layer formed at the interface. Experimental results disclose the damage of Alq3 at the Al/ Alq3 interface, as well as the dissociation of the insulating materials in the Al/LiF/Alq3, MgF2/Alq3 and heated Al/LiF systems. In the Al/LiF/Alq3 case, the coexistence of Al, LiF and Alq3 leads to LiF dissociation, and the released Li atoms react with Alq3 molecules to form Alq3- anions. This process is responsible for the enhanced electron injection and better luminance efficiency in OELDs. In the Al/LiF case, the reaction between LiF and Al starts upon heating the system to above 70 °C. Heat treatment enhances the charge injection and electroluminescence in the Al/LiF/PFO device, presumably due to the presence of free Li atoms. Orderly growth of CuPc is observed on HOPG at high substrate temperatures. Scanning electron microscopy (SEM) image shows that the CuPc thin films have the morphology of sprinkling strip-like crystals with micron sizes. Vibrational and structural results suggest that CuPc molecules form a closely packed structure on the substrate, with a face-to-face columnar growth that maintains the flat-lying orientation throughout the growth process. In addition, the structural and morphological studies on the system of pentacene on native Si oxide surface are carried out. HREELS results indicate that the molecular orientation of pentacene in the first one or two monolayers is different from that in the upper layers. The pentacene molecules in the upper layers exhibit a flat-lying orientation with their molecular plane parallel to the substrate surface, but the molecules in the first one and two monolayers don’t. X-ray diffraction (XRD) results show the structure of the pentacene thin films is in accordance with the well-known thin film phase. However, the molecular orientation of pentacene in the thin film phase was believed to be nearly standing, as is contradictive to the HREELS results mentioned above. It is probable that the thin film phase is actually an undetermined phase with the flat-lying molecular geometry and the same XRD features. Atomic force microscopy (AFM) investigations reveal the pentacene molecules in the first two layers nucleate as islands and finally form a continuous film by edge growth. But the upper layers of the pentacene show a terrace structure, and a simple theoretical model is well established to describe the terrace formation.

Date of Award	15 Jul 2004
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Shuit Tong LEE (Supervisor)