The potential application of single-crystalline organic semiconductors for high performance appliances has made many researchers develop several solution-based approaches to grow crystalline films of organic molecules on large areas such as dip coating, solution shearing, zone casting and meniscus guided coating techniques.
These mechanisms are dependent on the unidirectional displacement of a solution droplet of the semiconductor across the substrate’s surface. The meniscus formed along the receding edge of the droplet evaporates easily leading to precipitation of the organic molecules. In the meniscus-guided coating method, coating speed, solvent choice, and substrate temperature are critical in the crystal formation, which translates to the quality of the film formed. However, the growth through this mechanism results from complex phenomena, which take place at different length and time scales. So far, the importance of the different process parameters is not clearly understood.
IMEC researchers in Belgium proposed, for the first time, a model to predict the equilibrium front evaporation speed where the edge of a drop of solvent is receding under the influence of mass loss by evaporation. They demonstrated that slow processing at the obtained evaporation speed under negligible shear forces opens a successful process window that is compatible with all meniscus-guided methods. Their work is published in Advanced Materials.
The authors dropped an amount of pure solvent on the edge of the silicon holding piece. They determined the receding speed of the droplet by measuring the time taken by the receding meniscus to cover a predetermined distance on the silicon substrate. The authors observed that all tested solvents follow a universal trend with their respective boiling point, molar mass and density being the major parameters governing the evaporation behavior.
From that, the authors managed to develop a predictive model determining the equilibrium speed of the receding drop of solvent initiated by evaporation only. The obtained formula relied on only one fitting parameter and has been shown to accurately predict the correct coating speed for a large variety of solvents, temperatures, organic semiconductors and coating techniques.
The authors observed that when the coating speed was equal to the evaporation speed, films with varying substrate temperatures, semiconductors and solvents, showed excellent electrical attributes and superior morphology. Optimized organic transistors displayed efficient mobility of approximately 7cm2/Vs while maintaining low voltages.
This study offers a straightforward start point to pinpoint the optimum coating speed for numerous coating processes, which are operated with low shearing forces. This offers an advantage over the trial-and-error approach, hence eliminating a number of unknown variables in the complex dynamics surrounding the formation of single organic crystals at the edge of the meniscus.
Robby Janneck, Federico Vercesi, Paul Heremans, Jan Genoe, and Cedric Rolin. Predictive model for the meniscus-guided coating of high quality organic single-crystalline thin films. Advanced Materials 2016, volume 28, pages 8007–8013.Go To Advanced Materials