Predictive model for meniscus-guided coating of high quality organic single-crystalline thin films

Significance Statement

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.

meniscus-guided coating of high quality organic single-crystalline thin films - advances in engineering

About The Author

Robby Janneck is a PhD researcher at the Large Area Electronics department in imec and the Electrical Engineering Department of the KU Leuven. He received his Master of Physics from TU Dresden in 2014 and subsequently joined IMEC as a PhD researcher in the Large Area Electronics department under supervision of Paul Heremans and Jan Genoe.

He is now working in the framework of the “Epos Crystalli” ERC project on high performance, single-crystalline organic thin-film transistors.

About The Author

Federico Vercesi is technology development engineer at STMicroelectronics. He received his B.S. and M.S. in Materials Engineering and Nanotechnology from the Politecnico di Milano in 2013 and 2015 respectively. In 2015 he conducted his master thesis within the Large Area Electronics department at imec under supervision of Robby Janneck and Cedric Rolin.

About The Author

Paul Heremans is an imec fellow and professor at the Electrical Engineering Department of the KU Leuven. He is also director of the Large Area Electronics department at imec, and manages the research program on “Organic and Oxide Transistors” of HOLST Centre, a collaborative research initiative by imec and TNO. He received the PhD degree in Electrical Engineering from the University of Leuven, Belgium, in 1990. From 1984 to 1990, he was a research assistant and senior research assistant at the Belgian Fund for Scientific Research (NFWO) working on hot-carrier degradation mechanisms in CMOS. In 1990, he joined the opto-electronics group of imec. He worked on optical interchip interconnects, and on high-efficiency III-V thin-film surface-textured light-emitting diodes. In 1998, he started the organic semiconductor activities at imec. The main focus today is oxide and organic electronics, including circuits, backplanes and memories, as well as organic photovoltaics.

About The Author

Jan Genoe is a Chief Scientist at imec and a part-time professor at the Electrical Engineering Department of the KU Leuven. He received an M.S. degree in Electrical Engineering and a PhD from KU Leuven in 1988 and 1994 respectively. Subsequently, he joined the High Magnetic Field Laboratory in Grenoble (France) as a Human Capital and Mobility Fellow of the European Community. In 1997, he became a lecturer at the KH Limburg in Diepenbeek, Belgium. In 2003 he joined the Large Area Electronics department in imec.

His major research interest is currently designs made using organic and oxide transistors as well as other emerging technologies such as organic photovoltaics and piezo-electric devices.

About The Author

Cedric Rolin is a senior researcher within the Large Area Electronics department at imec. He received his M.S. degree in materials science and the PhD degree from the Université Catholique de Louvain in 2004 and 2009 respectively. During his PhD, accomplished on the premises of imec, Leuven, Belgium, he developed a technology for the vapor phase growth of organic semiconductors. Afterwards, he joined the group of Prof. Steve Forrest at University of Michigan as a post-doctoral fellow, where he continued studying the growth of organic semiconductors. In 2013, he came back to imec as a senior researcher, to join the team of Paul Heremans working on the “Epos Crystalli” project funded by the ERC.

Reference

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.

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