Hannes Klumbies

Electrical calcium test for moisture barrier evaluation for organic devices.

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.

Breakdown and Protection of ALD Moisture Barrier Thin Films.

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.

Barrier performance optimization of atomic layer deposited diffusion barriers for organic light emitting diodes using x-ray reflectivity investigations

The importance of O3 pulse duration for encapsulation of organic light emitting diodes (OLEDs) with ultra thin inorganic atomic layer deposited Al2O3 layers is demonstrated for deposition temperatures of 50 °C. X-ray reflectivity (XRR) measurements show that O3 pulse durations longer than 15 s produce dense and thin Al2O3 layers. Correspondingly, black spot growth is not observed in OLEDs encapsulated with such layers during 91 days of aging under ambient conditions. This implies that XRR can be used as a tool for process optimization of OLED encapsulation layers leading to devices with long lifetimes.

The influence of laterally inhomogeneous corrosion on electrical and optical calcium moisture barrier characterization

The reaction of calcium thin films with water - monitored optically or electrically - is widely used for evaluating ultrahigh barrier foils for the encapsulation of organic electronic devices. We studied the common optical and the electrical method and compared them with in situ atomic force microscope topography scans. All three methods were applied at the same sample in parallel in a typical test design containing a gas volume for water distribution next to the calcium layer of 60 and 1000 nm thickness, respectively. The common assumption for the interpretation of such measurement data is laterally homogeneous calcium consumption of the layer from top to bottom. In contrast, we observed a significant ratio of laterally inhomogeneous corrosion of the calcium on the micro-scale for both thicknesses. Some areas were strongly or completely corroded through the whole layer while others exhibited less or no corrosion. Furthermore, those corroded spots grew in lateral direction. As a consequence of lateral inhomogeneous calcium corrosion the electrical calcium measurement method underestimates the amount of calcium left; according to our results this does not affect the water vapor transmission rate (WVTR). Optical data evaluated by Lambert-Beer law underestimate the amount of calcium left as well and also underestimate the WVTR. If the data are evaluated, using a linear relationship between transmission and amount of calcium left, the both values are more precise. The scope of this study is to call attention to the existence of lateral inhomogeneity in calcium corrosion and its impact on the calcium permeation measurements. While more investigations would be needed to quantify the effect of this inhomogeneity on the electrical and optical method in general, the discussion sheds light on the way, calcium test data are influenced by lateral inhomogeneous calcium corrosion. Our observations highlight the need for careful interpretation of calcium test results, but also demonstrate its capabilities for precise ultrahigh barrier measurements.

Note: Influence of calcium corrosion on the performance of an adjacent permeation barrier

Organic devices have to be protected with a permeation barrier against ambient moisture since their performance is strongly reduced by contact with water. The corrosion of calcium is widely used to evaluate such permeation barriers. However, design details of the calcium corrosion test may have severe consequences for the validity and precision of the measurement results. In this work, we present such a measurement distortion caused by the direct contact between calcium and the inorganic permeation barrier. Fortunately, even a 20 nm thin interlayer of a soft material (i.e., C60) sandwiched between calcium and the barrier reestablishes the validity of the measurement. In addition, we present our latest and improved electrical calcium test layout used for this investigation.

Integration of molecular-layer-deposited aluminum alkoxide interlayers into inorganic nanolaminate barriers for encapsulation of organic electronics with improved stress resistance

Diffusion barrier stacks for the encapsulation of organic electronics made from inorganic nanolaminates of Al2O3 and TiO2 with aluminum alkoxide interlayers have been deposited by atomic layer deposition (ALD) and molecular layer deposition (MLD). As a part of the MLD process development, the deposition of aluminum alkoxide with low a density of about 1.7 g/cm3 was verified. The ALD/MLD diffusion barrier stack is meant to be deposited either on a polymer film, creating a flexible barrier substrate, or on top of a device on glass, creating a thin-film encapsulation. In order to measure the water vapor transmission rate (WVTR) through the barrier, the device is replaced by a calcium layer acting as a water sensor in an electrical calcium test. For the barrier stack applied as thin-film encapsulation on glass substrates, high resolution scanning electron microscopy investigations indicate that the inorganic nanolaminates without MLD interlayers are brittle as they crack easily upon the stress induced by the ...

Importance of Interface Diffusion and Climate in Defect Dominated Moisture Ultrabarrier Applications.

OLEDs and organic photovoltaic (OPV) devices require encapsulation from water vapor using a permeation barrier system. As a benchmark for barrier quality, often only a single number is provided as water vapor transmission rate. However, this value is highly dependent on the aging climate. So far, little scientific effort has been undertaken to characterize ultrahigh moisture barriers at different temperatures and relative humidities. We present Ca-test studies on sputtered Zinc-Tin-Oxide and atomic layer deposited AlOx barriers in extensively varied climates. Relative humidities are changed at constant temperatures, and temperatures are changed at constant absolute humidity. We find Henrys law to apply for sorption and discover a fundamental change of the diffusion regime with time related to the interface between the test and the barrier thin-film.

Note: Inhibiting bottleneck corrosion in electrical calcium tests for ultra-barrier measurements

A major failure mechanism is identified in electrical calcium corrosion tests for quality assessment of high-end application moisture barriers. Accelerated calcium corrosion is found at the calcium/electrode junction, leading to an electrical bottleneck. This causes test failure not related to overall calcium loss. The likely cause is a difference in electrochemical potential between the aluminum electrodes and the calcium sensor, resulting in a corrosion element. As a solution, a thin, full-area copper layer is introduced below the calcium, shifting the corrosion element to the calcium/copper junction and inhibiting bottleneck degradation. Using the copper layer improves the level of sensitivity for the water vapor transmission rate (WVTR) by over one order of magnitude. Thin-film encapsulated samples with 20 nm of atomic layer deposited alumina barriers this way exhibit WVTRs of 6 × 10(-5) g(H2O)/m(2)/d at 38 °C, 90% relative humidity.