Abstract
A very attractive property for many optoelectronic devices, such as solar cells and organic light emitting diodes (OLEDs), is light weight and mechanical flexibility. This will open new technological opportunities, such as thin flexible lighting, lightweight conformable solar cells, and rollable displays. Furthermore it allows for roll-to-roll production on cheap
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flexible substrates, which will greatly reduce their cost of production. An issue for devices made on cheap flexible substrates, such as plastics, is that permeation of oxygen and water vapor into their active layers can lead to quick deterioration of their performance. Thus, thin-film permeation barrier coatings are needed. When transparent barrier materials, such as oxides, nitrides and oxynitrides of metals and silicon are deposited as thin films, they inevitably contain pinholes, which tend to propagate through the entire layer. To prevent the propagation of defects through a barrier, the inorganic layers can be stacked with interposed organic layers, to decouple defects in the consecutive inorganic layers. A combination of silicon nitride (SiNx) and polymer, in our case poly(glycidyl methacrylate) (PGMA), is very suitable to create such a thin film encapsulation barrier. We deposited SiNx using hot wire chemical vapor deposition (HWCVD), a technique in which source gases are catalytically decomposed at heated wires. The created radicals react at the substrate, forming a film. The substrate temperature never exceeds 100°C, making the process compatible with our PGMA layers and delicate substrates and devices. The SiNx has a density of 2.07 g/cm3 and a N/Si ratio of 0.85, containing ~30 at.% hydrogen. Using initiated CVD (iCVD), in which an initiator molecule is dissociated into two radicals at a hot filament which starts the polymerization process with monomers on the substrate, we were able to deposit high molecular weight PGMA (105 g/mol) at a high deposition rate of 133 nm/min. The high molecular weight suggests that the PGMA is thermally stable. Also, we exposed the layers to an atomic hydrogen environment, mimicking SiNx deposition conditions. We found the PGMA layers deposited at lower filament temperatures to be more stable. We deposited a simple three layer stack, consisting of two 100 nm SiNx layers, with a 200 nm PGMA layer in between, which is pinhole free and has a water vapour transmission rate (WVTR) as low as 5*10−6 g/m2/day at 60°C and a relative humidity of 90%. This value meets the requirements for even the most sensitive flexible electronic device. It was shown that the robustness of our multilayers with respect to defect-creating dust particles improves when we increase the number of SiNx sublayers, while keeping the accumulative thickness of SiNx layers the same. Cross sectional scanning electron microscopy (SEM) images of a degraded sample reveal that the structural robustness of our multilayer is high, as is the adhesion between the organic and inorganic layers. The barrier layers investigated and developed in this thesis are excellent for the encapsulation of many new kinds of lightweight and flexible displays, solar cells, and lighting applications.
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