Organic small molecules typically deposited by vacuum deposition are a promising material for efficient, low-cost, area light-emitting diodes (OLEDs). In this article we discuss methods to apply these materials by solvent-based large area coating methods. As a basis for the technical description of the coating and wetting process, we present material properties, such as viscosity and surface tension of SMOLED solutions and polar and disperse part for the surface energy of typical substrates or semiconducting organic layers. Whereas SM content has little effect on the ink properties, impurities were identified as an important factor for the wetting behavior. Based on the material properties and coating experiments with SMOLED solutions, the coating methods of blade and slot die coating are discussed with respect to film thickness and stability. We found that the film thickness of knife-coated films does depend on velocity, temperature, provided fluid volume, and composition. Calibration curves for commercial materials (NMP and Spiro-MeOTAD) are given. The stability of slot die coating was lower than expected from literature. However, homogeneous SMOLED layers could be produced by slot die coating at gap-to-film-thickness ratios of up to 50.
OLED Small molecule Roll to roll Coating Blade coating Slot die coating Dewetting Viscosity Surface tension
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The authors acknowledge financial support via the project PrintOLED of the Leading-Edge Cluster Forum Organic Electronics managed by InnovationLab GmbH within the High-Tech Strategy for Germany of the Federal Ministry of Education and Research and Philips Technologie GmbH. Special thanks goes to our cooperating partners BASF, Merck, and TSE Troller AG for the support. We would like to thank all involved mechanics, assistants, and our students for contributing to this work.
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