Heat treatment method for polythiophene thin films via cooling rate control

Heat treatment of P3HT thin films to create nanoribbon crystals via sub-melting point isothermal heating and 0.6°C/min cooling

This technology is a heat treatment method that forms nanoribbon structures with a width of 20–40 nm by heating a P3HT thin film (melting point 223°C) at 200°C for 10–30 minutes, followed by slow cooling to room temperature at a constant rate of 0.6°C/min. By focusing on the fact that recrystallization occurs during the cooling process, the cooling rate is controlled as the key variable rather than the isothermal holding time.

Without specific treatment, P3HT thin films lack an ideal molecular structure, limiting their use in high-performance device fabrication. Furthermore, post-processing methods like thermal or solvent annealing often caused dewetting or morphological damage if conditions were too harsh. Existing research focused primarily on heating temperature and time, failing to systematically utilize the impact of cooling rates on crystallization.

This technology imparts molecular mobility at temperatures below the melting point and precisely lowers the cooling rate to enhance molecular orientation, alignment, and crystallinity, achieving transistors with an average field-effect mobility of 3.1×10⁻³ cm²V⁻¹s⁻¹. Applicable to the active layers of solution-processed organic field-effect transistors, organic photovoltaics, and organic light-emitting diodes, it allows for reproducible thin-film crystal quality by adjusting the cooling profile without the risk of dewetting.

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Key Features:

• Spin-coating a 10 mg/mL poly(3-hexylthiophene) solution dissolved in chloroform onto an HMDS-treated silicon wafer at 2500 rpm for 60 seconds
• Heating the coated thin film at a rate of 1.5°C/min to an isothermal temperature of 200°C, which is below the 223°C melting point, for 10–30 minutes
• Slowly cooling the heated thin film to room temperature at a rate of 0.6°C/min to form a nanoribbon structure morphology with a width of 20–40 nm
• Constructing a field-effect transistor using the heat-treated thin film that exhibits an average field-effect mobility of 3.1×10⁻³ cm²V⁻¹s⁻¹

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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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