Effect of Indium-Tin Oxide Surface Modifications on Hole Injection and Organic Light Emitting Diode Performance

Abstract

The aim of this work is to investigate the influence of various surface modifications on, in turn, ITO surface properties, hole injection efficiency, and finally device performance. This research is expected to provide important information on good understanding of hole injection mechanisms in OLED devices.

In this study, extensive work involving surface modifications of ITO was carried out. These included gas plasma treatments, electrochemical treatments, and insulating buffer layer. In order to understand the governing factors of ITO surface properties, ITO samples were treated with different types of plasma (i.e., H2, Ar, O2, and CF4) and characterized by in terms of surface morphology by AFM, surface chemical states by XPS, electron transfer kinetics by CV, and surface energy by contact angle measurements. Electrochemical process was first proposed as a new approach for ITO surface treatment. Similar to the plasma treatments, the electrochemically treated ITO surfaces were also characterized in surface properties. SiO2 buffer layers produced by thermals evaporation, self-assembled-monolayer, and sol-gel processes were applied on to ITO surfaces as well. The SiO2 buffered ITO surfaces were characterized by AFM and CV techniques. OLED devices based on the ITO electrodes modified by the different processes were fabricated and characterized in terms of L-I-V behaviour and EL efficiencies. More importantly, nucleation and initial growth of hole transport layer on the treated ITO surfaces were morphologically investigated to understand the influence of surface modification methods on interface property and therefore hole injection. Based on the results of surface properties and device performance, phenomenal interface models were proposed for discussion of hole injection mechanism and the influence of hole injection on EL efficiency.

The results show that oxidative plasma and electrochemical treatments change ITO surface chemical states by decontamination, oxidation and surface etching. The resulted polar species alter the surface energy, especially its polar component. OLED device performance is correlated to the surface polarities of the ITO electrodes, namely, the higher the surface polarity, the more effective the hole injection. The improved device performance is attributed to the improved ITO/HTL interface properties (i.e., the good contacts between ITO and hole transporting layer) by refining the HTL deposit and reducing voids and defects at the interface. In contrast, all the insulating buffer layers block hole injection by reducing the effective contact areas at the ITO/HTL interface. For the same coating process, thicker buffer layers block more holes. Being of the similar thickness, the denser coating blocks more holes than the porous coating. More importantly, the electrochemical treatment of ITO surface was found to be capable of increasing not only hole injection but also EL efficiency at the same time.