X-ray and neutron reflectivity measurements of moisture transport through model multilayered barrier films for flexible displays

Abstract

One encapsulation approach to extend the lifetime of flexible organic light-emitting diode (OLED) devices uses inorganic Al2O3-polymer multilayer barrier films. However, a recent theoretical examination of multilayer barriers indicated that the barriers should not be effective for OLED applications, despite empirical evidence of success. It was suggested that a long-lived transient process in the transport of water molecules through multilayer films is responsible for its practical success, but has not been directly observed experimentally. X-ray reflectivity (XR) and neutron reflectivity measurements are used to measure permeation rates and structural changes in model barrier films upon exposure to water vapor. A film consisting of a stack of an undercured organic and the typical inorganic phases was found to barely swell [(7±5)Å] after an 11-d exposure to moisture [60 °C, 100% relative humidity (RH)]. Current measurements of ultralow moisture permeation assume that 10 d is sufficient for the equilibrium measurement, but XR data show that a stack of three dyad layers may require as many as 500 d (>12000h) to reach equilibrium. Barriers with a high number of defects in the inorganic phase reached equilibrium after 6 d of exposure to moisture (60 °C, 100% RH). Over this time scale, water breakthrough at each layer can be observed from XR. Neutron reflectivity measurements with deuterated water show an accumulation of water near the aluminum oxide∕polymer interface. This interface behaves similar to a desiccant, where the permeation of water through the barrier is retarded by the strong adsorption of water to aluminum oxide. This internal desiccant effect of the multilayered structure is clearly delineated and appears to be responsible for the long-term transient behavior of these barrier materials.