Coating and stability of thin liquid films on chemically patterned substrates


In the manufacturing of large-area organic electronics, such as organic light-emitting diodes (OLEDs), thin films of the functional polymer are often applied via solution-based coating processes. In order to enable continuous manufacturing, multiple individual devices have to be delineated on a large substrate. The canonical method to achieve this delineation is to apply chemical patterning to the substrate, in which the device intended regions are wetting and the regions separating adjacent devices are partially wetting. As typical concentrations of functional material are on the order of 1%, the hydrodynamics of the solvent largely determine the thickness and uniformity of the applied films. As a consequence, the main focus of this research has been on the influence of the chemical patterning on the application of thin liquid films by means of coating processes and on the post-application liquid redistribution. Two strategies for liquid deposition onto patterned substrates are (1) applying the liquid selectively to the desired regions and (2) coating the entire substrate with a uniform film and relying on the subsequent liquid redistribution. The transition from process (1) to (2) occurs at a critical coating speed, which depends on the contact angle and the geometry of the partially wetting regions. In this research, process (1) has been studied both experimentally and numerically, whereas process (2) has been primarily investigated by means of numerical simulations. Using the well-studied cases of homogeneous substrates and partially wetting substrates with narrow wetting stripes as references, coating experiments have been conducted. The substrates studied contained compact patterns, such as squares and triangles. Two coating processes were considered: (1) dip-coating, in which the substrate is immersed in a large reservoir of liquid and subsequently withdrawn at constant speed and (2) die-coating, in which the substrate moves underneath a so-called die, which is kept at a small distance from the substrate. For both dip- and die-coating, it was found that the literature results provide a useful reference for the prediction of the entrained film thickness on more convoluted patterns. In order to investigate the post-coating redistribution dynamics, a finite element model has been set up. Using a small slope approximation (which is valid for partially wetting substrates with a relatively small contact angle), the liquid interface could be simulated as a function of position on the substrate. This enabled the 2D simulation of a 3D problem, which dramatically reduces computational demands. The chemical patterning has been implemented into the model by means of a disjoining pressure formalism. Using a one-dimensional simulation model, redistribution times and resulting liquid conformations have been identified as a function of liquid properties, process parameters and geometrical and chemical properties of the substrate. In addition, linear stability analysis of the interface evolution equations for homogeneous, partially wetting substrates has resulted in reliable reference values for the prediction of simulation results

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This paper was published in NARCIS .

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