We discuss the electrical calcium test--a method to measure very small rates of water vapor permeation through barrier films with high throughput. The sensitivity range for our design is found to be 10(-5) to 15 g/(m(2) d). Moreover, a closer look at the importance of electrodes series resistance is taken: We show that permeation rates are underestimated if it is neglected. Taking this series resistance and Fickian diffusion into account not only the steady, but also the transient state of the permeation curve can be fitted. Using this approach, permeation barriers with different permeabilities are evaluated leading to water vapor transmission rates well comparable to coulometric measurements. The calcium layer morphology is investigated by atomic force microscopy measurements indicating microscopical inhomogeneities during degradation. Variations of electrode material and calcium layer thickness are carried out to examine their influence on the measured permeation. Additionally, optical and electrical calcium tests are compared. Small differences in the time dependence are observed and discussed.
The water vapor barrier properties of low-temperature atomic layer deposited (ALD) AlOx thin-films are observed to be unstable if exposed directly to high or even ambient relative humidities. Upon exposure to humid atmospheres, their apparent barrier breaks down and their water vapor transmission rates (WVTR), measured by electrical calcium tests, deteriorate by several orders of magnitude. These changes are accompanied by surface roughening beyond the original thickness, observed by atomic force microscopy. X-ray reflectivity investigations show a strong decrease in density caused by only 5 min storage in a 38 °C, 90% relative humidity climate. We show that barrier stabilities required for device applications can be achieved by protection layers which prevent the direct contact of water condensing on the surface, i.e., the sensitive ALD barrier. Nine different protection layers of either ALD materials or polymers are tested on the barriers. Although ALD materials prove to be ineffective, applied polymers seem to provide good protection independent of thickness, surface free energy, and deposition technique. A glued-on PET foil stands out as a low-cost, easily processed, and especially stable solution. This way, 20 nm single layer ALD barriers for organic electronics are measured. They yield reliable WVTRs down to 2×10(-5) g(H2O) m(-2) day(-1) at 38 °C and 90% relative humidity, highlighting the great potential of ALD encapsulation.
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