Gas Diffusion Barriers Using Atomic Layer Deposition: A New Calcium Test and Polymer Substrate Effects

摘要

The increasing demand on available energy resources has led to a desire for more energy efficient devices. The wide use of displays in consumer electronics, such as televisions, cell phones, cameras and computers makes them an ideal target for improvement. Organic light-emitting diodes (OLEDs) are a good candidate to replace traditional Si based devices. However, the low work function metals typically used as electrodes in OLEDs are very reactive with water and oxygen. Ultralow permeability gas diffusion barriers with water vapor transmission rates (WVTRs) as low as u3c10-6 g/(m2 *day) are required on the polymers used to fabricate organic electronic and thin film photovoltaic devices.Atomic Layer Deposition (ALD) uses self-limiting surface reactions to deposit thin conformal films. ALD is capable of depositing thin, conformal, high quality barriers. WVTR values as low as ≤ 5 x 10-5 g/( m2 *day) have been measured for Al2O3 ALD films at 38°C/85% RH using the Ca test with optical transmission probing. The Ca test is a technique with very high sensitivity to measure ultralow WVTRs. This test relies on measuring the oxidation of a Ca metal film by monitoring the change in its optical or electrical properties. However, glass lid control experiments have indicated that the WVTRs measured by the Ca test are limited by H2O permeability through the epoxy seals. Varying results have been reported in the literature using the electrical conductance of Ca to measure permeation.In this work, two approaches were applied to overcome the epoxy edge seal limitations. The first approach was to deposit Al2O3 ALD barriers directly on Ca metal. While the Al2O3 ALD barriers were successfully deposited, the measurement of an accurate WVTR was limited by barrier pinholes. The presence of pinholes in the Al2O3 ALD barrier on Ca results in the localized oxidation of the Ca sensor. Heterogeneous degradation of the Ca causes inaccuracies in the conductance of the film. As oxidation regions merge, large percolation paths are severed without complete Ca oxidation. To solve this problem, a new apparatus was developed that measures the electrical conductance of Ca films. This new apparatus does not rely on epoxy seals and separates the Ca metal from the barrier. Unfortunately, the electrical conductance of the Ca film versus Ca oxidation was found to be extremely nonlinear. This nonlinearity severely complicates the usual analysis to obtain WVTR values from the Ca test.The new apparatus was useful for the examination of PEN polymer substrates using the total lifetime of the Ca sensor. Polymer effects on the measurement of gas permeability for polymer/barrier systems have largely been ignored. Experiments were performed to determine the effect of the PEN polymer substrates on the WVTR measurement. The H2O permeation activation energy in the PEN polymer, the effect of water saturation of the PEN polymer and the dependence of the lifetime of the Ca sensor on the H2O flux on the PEN substrate were of particular interest. The experiments obtained H2O permeation activation energies in the PEN polymer of 12.4 kJ/mol. The Ca sensor lifetime was found to be linear with H2O flux. No difference in Ca sensor lifetime was observed between dry or H2O-saturated PEN polymer substrates.