Organic electronic devices such as organic light-emitting diodes (OLEDs) and organic thin film transistors (OTFTs) have attracted considerable attention because of their advantage properties, including easy synthesis and design of organic molecules, low cost, lightweight, simple mass production and compatibility with flexible substrate. However, the intrinsic vulnerability of active materials to moisture and oxygen is main obstacle for commercialization of organic electronic devices. Therefore, an effective gas permeation barrier is required to prevent the penetration of moisture and oxygen into the devices. In order to fabricate high performance gas barrier material, the understanding about the gas permeation mechanism and gas barrier technologies is essential. Stability issues of organic electronic devices and encapsulation methods as well as gas permeation mechanism are introduced in Chapter 1.
In Chapter 2, Al2O3 films obtained with plasma-enhanced atomic layer deposition (PEALD) have been tested as passivation layers because of their excellent gas barrier properties. However, amorphous Al2O3 films are significantly corroded by water. We examined the deformation of PEALD Al2O3 films when immersed in water and attempted to fabricate a corrosion-resistant passivation film by using a PEALD-based Al2O3/TiO2 nanolamination (NL) technique. Our Al2O3/TiO2 NL films were found to exhibit excellent water anticorrosion and low gas permeation and require only low-temperature processing (< 100 °C). Organic thin film transistors with excellent air-stability (52 days under 38 °C and 90 % relative humidity) were fabricated.
In Chapter 3, the effects of the oxide ratio on the water permeation barrier properties of PEALD Al2O3/TiO2 nanolaminate films were investigated. The optimized Al2O3/TiO2 NL films exhibited the lowest WVTR value of 9.16 x 10-5 g m-2 day-1. The NL films were successfully used as encapsulation layers in OLEDs without the need for additional layers. OLED devices prepared using the optimized NL passivation layers showed extended shelf-lives exceeding 2,000 hours under high temperature and humidity conditions (60 °C and 90% RH) without the formation of dark spots or edge shrinkage.
In Chapter 4, HfO2 has the highest metal-oxygen bond energy except rare earth metal containing oxide and has relatively low refractive index, leading to high chemical stability and high transparency, respectively. We examined the water permeation barrier properties of Al2O3/HfO2 mixed oxide film-coated polymer. We found that Al2O3/HfO2 films show high water stability, transparency (about 95% in visible range), and dense amorphous structure. 50 nm thick Al2O3/HfO2 films exhibited the lower WVTR value of 1.44 x 10-4 g m-2 day-1 than Al2O3 (3.26 x 10-4 g m-2 day-1) or HfO2 (6.75 x 10-2 g m-2 day-1) single layers at 60 °C and 90% RH accelerated condition. At mild condition (25 °C and 40% RH), Al2O3/HfO2 films displayed WVTR value of 2.63 x 10-6 g m-2 day-1 comparable with general glass encapsulation.