Surface analysis on organic light emitting diodes

Abstract

The main theme of this work is the application of various surface analysis techniques, including ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and atomic force microscopy (AFM), to the study of several issues related to the operating mechanisms of organic light emitting devices (OLEDs). Particular attention is paid to three areas: (1) the electronic properties and injection barrier at the organic/electrode interface; (2) the stability of organic thin films exposed to various environments; and, (3) the influence of processing parameters on the electronic and surface properties of organic thin films. Each of these topics is summarized in the following paragraphs. Properties of N'-bis-(1-naphthyl)-N,N'-diphenyl-1 ,1'-biphenyl-4,4'-diamine (NPB)/indium thin oxide (ITO) interface after different surface treatments were studied using UPS and AES. IT0 treated in situ by oxygen plasma possessed a work function of 5.2 eV and the NPB/ITO interface formed from this showed a hole injection barrier 0.5 eV lower than that of the untreated ITO. Insertion of an ultrathin SiO2 layer between the NPB and the ITO resulted in a similar reduction of the injection barrier. This improved hole injection favors the efficient operation of OLEDs, as manifested by the decreased operating voltage. The stability of organic thin films of aluminum 8-hydroxyquinoline chelate (Alq3) and NPB was studied by exposing the organic films to trace amounts of O2, H2O and ambient air and the application of thermal annealing. The vacuum energy level, the highest occupied state (HOS) and XPS core levels of the constituent elements in Alq3 shifted according to the type of vapor the sample was exposed to. A chemical reaction between O2 and Alq3 was observed upon exposure to O2. However the dominant influence on the electronic structures of Alq3 film upon exposure to air was from H2O. As for the thermal effect, Alq3/Si and NPB/Si were obviously changed by annealing at 90°C and 120°C, respectively. However, the electronic structures of Alq3/ITO and NPB/ITO interfaces did not change until the annealing temperature reached 150°C. XPS results indicated interfacial reactions readily took place at elevated temperatures for Alq3/Si and NPB/Si. No interfacial reaction was observed for either Alq3/ITO or NPB/ITO. Finally, the effects of Alq3 deposition rate on the morphology, chemistry and electroluminescence of Alq3 films was examined. As the Alq3 deposition rate was decreased from 1.33 Å s-1 to 0.55 Å s-1, the luminance efficiency of the fabricated OLED devices decreased from 4.75 cd A-1 to 2.0 cd A-1. AFM observations showed Alq3 films prepared at deposition rates of 1.33, 0.05, and 0.01 Å s-1 have root-mean-square roughness values of 12.0, 32.0, and 36.6 Å, respectively. Likewise, XPS measurements showed the film contained more N-containing species as the Alq3 deposition rate decreased from 1.33 to 0.01 A s-1. In conclusion, these changes in film morphology and chemistry are considered to be responsible for the changes in the electroluminescent performance of the devices.

Date of Award	16 Oct 2000
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chun Sing LEE (Supervisor)