L. Wengeler

Fluid-dynamic properties and wetting behavior of coating inks for roll-to-roll production of polymer-based solar cells

In the last decade, semiconducting and conducting materials were developed that can be processed by solvent-based deposition to form functional layers or complete electronic devices. These materials are typically synthesized in laboratory scale quantities and tested on small spin-coated substrates, whereas the final goal is to produce them on flexible substrates in a continuous roll-to-roll process. To enable a fast scale up and optimization, fluid-dynamic properties have to be known. Here, we present viscosity and surface tension data for typical material systems, applied in polymer-based solar cells. Materials presented include water-based polymer dispersions (hole-conducting and high-conductive PEDOT:PSS types), solvent-based anorganic nanoparticle dispersions (silver nanoparticle ink, hole-blocking ZnO nanoparticle ink), and dissolved organic molecules and polymers (P3HT:PCBM photoactive blend). Predictive models are proposed to approximate viscosity and surface tension for these materials at various compositions. As well, corona treatment is used to modify the surface energy of P3HT:PCBM and described as a function of web speed and corona power. The importance of material properties is demonstrated by predicting stable conditions for a slot-die coating process. A simple drying simulation highlights the possibility of using property models to investigate wetting problems.

Liquid film coating of small molecule OLEDs

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.