10.3: Predicting the Lifetime of Flexible Permeation Barrier Layers for OLED Displays

Water diffusion through a single-layer barrier is measured with Secondary Ion Mass Spectrometry, electrical capacitance, and mechanical film stress. The time taken for one monolayer of water molecules permeate into the OLED is defined as the lifetime. Extrapolated to T = 38°C and 90% relative humidity, a 3∝m thick barrier is predicted to seal for 13 years. Author Keywords Permeation barrier; intrinsic barrier performance; diffusion coefficient; water solubility; Water Vapor Transmission Rate; lifetime; barrier evaluation; barrier characterization; SIMS; capacitance; stress. 1. Objective and Background Flexible displays and lighting panels with Organic Light Emitting Diodes (OLEDs) require high quality flexible protection in the form of encapsulation barrier layers. However, the ongoing intensive search for low-cost flexible barrier layer materials is made difficult by the lack of reliable techniques for predicting their operating lifetime. While the decisive inherent barrier material property is its bulk permeability, in laboratory tests barriers often fail by permeation along embedded particles or along their interfaces with the substrate. These failure modes prevent the methodical exploration and selection of barrier materials by the criterion of true bulk permeability. In addition, the long operating life expected from flexible barrier layers demands accelerated testing at elevated temperature (T) and relative humidity (RH). Yet no straightforward procedure has been identified to date for extrapolating high-T high-RH results to real-life operating conditions. Finally, no agreed-on criterion has been defined for barrier failure. Our objectives have been to (i) define such a criterion, (ii) work out a quantitative procedure for relating high-T high-RH results to typical OLED display operating conditions, and (iii) develop and apply fast-turnaround techniques for characterizing the diffusion of water through the bulk of a single-layer barrier. Meeting these three objectives requires the quantitative evaluation of barrier permeability, and the quantitative prediction the operating lifetime of a barrier layer. We expose barrier layers to water (H2O or D2O) vapor at temperatures from 65°C to 200°C. Under these conditions water diffuses into the barrier. We measure the concentration of H or D in the barrier material, and evaluate the water diffusion coefficient from either concentration-depth profiles or the total amount of water dissolved in the film. Concentration and concentration-depth profile of deuterium are measured by secondary ion mass spectrometry (SIMS) coupled with sputter profiling. This measurement is used to calibrate the water concentration in all samples. We introduce two fast-turnaround techniques that can be conducted in most laboratories. In one technique changes in water content are evaluated from changes in electrical capacitance. In the other technique changes in water content are evaluated from changes in mechanical stress of the barrier layer. The two techniques take advantage of the observation that both, electrical capacitance and film stress, vary linearly with the total number of water molecules dissolved in the layer. All three techniques are immune to point defects and particles, as confirmed by experimental results. In a single-layer barrier film that protects an OLED, water diffuses from the side of the barrier that is exposed to the humid atmosphere (x=0) toward the OLED side (x=h). This diffusion results in a concentration profile of water molecules n(x,t) in the barrier, where t is time. For short diffusion times t the number of in-diffused water molecules n(x,t) follows a complementary error function, n(x,t) = n(x=0) erfc[x/2√(Dt)], with the diffusion coefficient D. SIMS measures this erfc profile. On the other hand, the changes in electrical capacitance C and in mechanical film stress σ are proportional to the total number of water molecules N(t) that have penetrated into the layer. In a diffusion controlled process N(t) 2 is linear with time, t, and follows equation (1): t