Method for Improving Molecular Crystallinity in Thin Films via Marginal Solvent Treatment

Technology for Improving Channel Crystallinity in Organic Transistors Using Marginal Solvent Post-Treatment

This technology is a non-thermal treatment method that enhances molecular crystallinity in the lower channel region of bottom-gate organic field-effect transistors. Immediately after spin-coating a poly(3-hexylthiophene) thin film to a thickness of approximately 48 nm, and before it fully solidifies, methylene chloride—a marginal solvent—is spin-cast onto the film.

Conjugated polymer thin films produced via solution processing often suffer from structural defects during formation, which limit charge transport and degrade device performance. Furthermore, in bottom-gate devices, the structure at the bottom of the thin film where the channel forms is difficult to analyze, creating limitations in verifying and optimizing treatment effects.

By leveraging the limited penetration depth of the solvent, this technology adjusts the film thickness so that the treatment effect reaches the bottom interface. The bottom surface of the film is then detached via epoxy transfer to directly observe the morphology using atomic force microscopy. Applicable to low-cost organic thin-film transistors and flexible electronics, this simple room-temperature process can increase charge carrier mobility by up to 4.3 times compared to untreated films.

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Key Features:

• Spin-coating a 0.5 wt% P3HT solution dissolved in chloroform onto the dielectric layer over the gate electrode to form a 48 nm thick polythiophene thin film
• Spin-casting methylene chloride, a marginal solvent, at room temperature to make direct contact with the thin film within 1–2 seconds after coating, before the film fully solidifies
• Curing embedding epoxy on the top surface of the thin film, then immersing it in liquid nitrogen to peel off the epoxy-P3HT composite and transfer the film
• Analyzing the morphology and roughness of the bottom of the separated polythiophene thin film using atomic force microscopy

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This invention was developed with support from the Ministry of Education for the analysis of charge transport mechanisms in π-electron molecular-based n-dimensional nanostructures.

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