| Meniscus-guided coating techniques have acquired a lot of interests as a facile and low-cost solution-processing approach for large-area organic electronic device fabrication.Understanding how fluid dynamics in the meniscus affect the nucleation and development of organic molecular crystals is critical for controlling the deposition of organic semiconductor thin films and optimizing their crystalline quality.To that purpose,this research investigates the impacts of solution evaporation,flow,and heat transfer behaviors on the shape and crystalline quality of organic semiconductor thin films in the meniscus-guided coating process by using a computational fluid dynamics technique combined with experiments.The following are the specifics:1.The Effect of Solvent Composition in Meniscus-guided Coating on the Deposition of Acene-based Organic Semiconductor Thin FilmsWe develop a 2D transient simulation model for the physical processes in the meniscus using the multiphysics finite element approach,which included flow,heat transfer,mass transfer,and component transport.Using a combination of simulations and experiments,we study the flow of single and dual-solvent components during meniscusguided coating on 40℃ substrates,as well as their influence on the deposition of DiFTES-ADT acene-based organic semiconductor thin films.We discover that in a single mxylene solvent,the temperature-induced Marangoni flow always affects the flow at the meniscus,and the flow along the meniscus is a down-up motion.This flow causes the solute at the tip to "escape"the area where it should have been deposited,causing cracks and defects in the deposited film.The 1,2-dichlorobenzene/m-xylene two-solvent system with a volume fraction of 1:4 is then investigated.Within 0.02 seconds,the temperatureinduced Marangoni flow is still affecting the interior of the solution,causing fluid movement up along the meniscus.After 0.02 s,as additional vapor from the meta-xylene is evaporated,the shift in composition distribution intensifies,resulting in fluid at the front of the meniscus that is impacted by the Marangoni flow caused by the composition concentration and flows from top to bottom along with the meniscus.Temperature impacts are progressively being countered by components.The impact of temperature distribution on the flow is matched with the influence of component distribution at the time of 0.7 s.After 0.7 seconds,the flow is dominated by component distribution,and the flow at the front end of the meniscus is totally changed into Marangoni flow generated by component distribution,with fluid mostly flowing from top to bottom along with the meniscus.This top-down flow can aid the solute in the bulk solution refill the tip while also improving the mass transfer of organic semiconductor molecules to the contact line,allowing for the fast development of dense and uniform organic semiconductor crystalline thin films.We discover a mechanism to accurately regulate the internal flow of the solution by modifying the composition through the analysis of the meniscus model,which offers a theoretical basis for increasing the quality of thin-film deposition.2.Effect of Surface Microchannels on Nucleation and Growth of Thiophenebased Organic Semiconductor Crystals in Meniscus-guided CoatingWe create three-dimensional rectangular microchannel simulation models with various sizes,wettability,and structures,and combined experiments to systematically study the coating process of infiltration microchannels and the influence of the shape of the middle meniscus on C8-BTBT thiophene-based organic semiconductor deposition arrays using the VOF(Volume of Fluid)method.In terms of channel size,we create microchannel models with various widths(2 μm,3 μm,5 μm,10 μm)and thicknesses(0.5 μm,1 μm,2 μm,3 μm).We discover that decreasing the width and increasing the thickness of the microchannel can result in greater capillary action of the solution in the microchannel,as well as an excessively wide contact area with the air,which impairs crystallization uniformity.As a result,it is necessary to select a channel that is as wide and thin as possible.Wider channels and smaller thicknesses,on the other hand,increase the gravitational impact,making the meniscus front more slender and the deposited array thinner.As a result,the channel size must be modified in accordance with the crystal thickness necessary for the experiment.In terms of channel wettability,we first create several channels in the theoretical model with different contact angles(3.5°,50°,90°,and 120°),and discover that the bigger the curvature of the meniscus and the more evident the "sag," the more lyophilic channels generated.The solute is more likely to deposit on both sides of the lyophilic channel,according to the theory.The solute is more prone to deposit towards the channel’s center in the lyophobic channel,which matches the actual data.Finally,we devised a two-step dip-coating approach to investigate the impact of various channel structures.The initial step is to dip the microchannel in order to create a crystal array on both sides.On both sides of the channel,the array creates a new tiny channel structure.The second phase is repeating the dip-coating procedure.The simulation reveals that the tip of the meniscus created a thinner "stream" in the tiny channel in the second stage.We hypothesize that the second crystallization will be deposited inside the narrow channel,resulting in a double-layer stacking structure with the first deposited solute,which is in line with the experimental results.Overall,we can precisely control the crystalline growth of organic semiconductors in microchannels and successfully predict the crystallization of solutes in microchannels under the influence of different parameters by designing microchannels with different sizes,wettability,and structures.The immersion microchannel coating crystallization process is theoretically and experimentally guided by the location and quality. |
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