In-situ analysis of organic thin film crystallinity and mass flow in meniscus through three-dimensional mathematical modeling

Abstract

Solution coating offers great potential for low-cost manufacturing of large-area and flexible electronics due to low temperature and pressure processability with high throughput. Among a variety of solution-based processing, meniscus-guided coating (e.g., dip-coating, zone casting, bar-coating, slot-die coating, and solution shearing) can fabricate large-area high-quality thin-film formation and be compatible with the roll-to-roll process, which leads to applications such as photovoltaics, metal organic frameworks (MOFs), sensors, and organic thin-film transistors (OTFTs). In particular, solution shearing, where the solution is inserted between the coating blade and the substrate, can precisely control solvent evaporation by controlling capillary force and viscous force. Unfortunately, in solution shearing, in-depth fundamental discussion of thin-film crystallization including nucleation, crystal growth, and film-formation has little direction. It is attributed to the fact that process parameters (e.g., temperature, coating speed, tilt angle, gap, concentration) are intricately interconnected and influence fluidic dynamics in solution is understood at a rudimentary level and limits the research to a trial-and-error approach. To define the fundamental kinetics of thin film crystallization, meniscus curvature is one of the most important parameters affecting solvent evaporation since it is closely related to various process parameters and is directly in contact with the liquid-solid boundary where crystallization. Here, a novel analysis of the effects of a three-dimensional meniscus curvature on solvent evaporation and crystallization by combining the top- and side-view in-situ microscopy and mathematical modeling was reported. Using in-situ analysis, we observe meniscus contact line is sinusoidal curve, not flat and using three-dimensional mathematical modeling, we demonstrate dendritic growth of OSCs is regulated by how meniscus curvature fluctuates along the liquid-solid boundaries. Dendritic growth hinders efficient charge transport owing to charge-carrier trapping at the prevalent grain boundaries. Reducing temperature and coating speed induces small fluctuation of meniscus curvature, which results in highly aligned TIPS-pentacene crystals without dendritic growth.