Sol-gel anode fabrication and thin film encapsulation for OLEDs

Abstract

Organic light-emitting devices (OLEDs) are emerging from the stage of research curiosity to the levels of important commercial applications. They are gaining acceptance as a promising flat panel display technology in this new millennium. However, OLEDs require further research and development, in particularly high performance emitting materials and electrodes, and novel device architectures since LCDs have a higher efficiency and longer operational lifetimes at the moment. This work focuses on the optimization of OLEDs using novel anode fabrication techniques and new device architectures. Particular attention is paid to two areas: 1) a new fabrication technique of transparent conducting aluminum-doped zinc oxide (AZO), and 2) lifetime enhancement of OLEDs by multi-layer thin film encapsulation. Firstly, aluminum-doped zinc oxide (AZO) films were fabricated at a low substrate temperature (300 ℃) in air by KrF-excimer laser irradiation of sol-gel spin-coated films. Structural, electrical and optical properties of the laser-irradiated films at different laser energies were studied and compared with those prepared by conventionalhigh temperature prcoessing. The 200 nm laser-irradiated AZO film had a minimum resistivity of 44 × 10-3 Ω-cm and about 90 % optical transmittance in the visible range upon laser irradiation. The films were found to orient preferentially along [002] direction. These results are highly related to the crystal quality of films in terms of thermal and photoinduced effects which are induced by laser irradiation. Secondly, multi-alternative layers of fluorocarbon (CFx) and silicon nitride (Si3N4) films were applied as encapsulation layers on glass-based OLEDs. The general properties and electroluminescent (EL) performance of thin film encapsulated devices were investigated. The multi-layer thin film encapsulated layers: CFx/Si3N4 dramatically decreased the moisture and oxygen permeation into the cathode, and hence prolonged the operational stability of OLEDs. The SEM micrograph of the multi-layer thin film encapsulated layer was very smooth, uniform, and pin-hole free. Its hydrophobic nature as proved by the wettability measurements may be the main reason for the improved performance. Besides, the surface chemical composition of the polymerized fluorocarbon thin films was very uniform. Furthermore, the multi-layer thin film encapsulated layer is highly transparent making it a competitive candidate for encapsulation of TE-AMOLEDs (Top-Emitting Active-Matrix OLEDs). To evaluate the performance of the encapsulated OLEDs, we investigated the initial existence of dark spots in the emitting area of OLEDs and found they were almost suppressed by inserting a properly selected interlayer, CuPc, between the OLED and the thin film encapsulation layer. Also, the operation lifetime was increased from 200 hours to over 8,000 hours after the multi-layer thin film encapsulated layers were applied. The optimized lifetime is approximately 80 % of that of the control device with conventional metal cap packaging. The ultra thin encapsulation thickness promotes the application of slim and light weight display panel applications. In particular, it also improves the future prospects of fully flexible encapsulated OLEDs.

Date of Award	2 Oct 2008
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Shuit Tong LEE (Supervisor)