Highly crystalline and uniform conjugated polymer thin films by a water-based biphasic dip-coating technique minimizing the use of halogenated solvents for transistor applications

Abstract

The commercialization of organic electronics will require minimizing the use of halogenated solvents used to solution-process organic semiconductors, which is a crucial step for large-area coating methods, such as the dip-coating method. Here, we report a novel biphasic dip-coating method which uses a water-based biphasic solution and produces a uniform, smooth, and crystalline conjugated polymer thin film in the presence of a solvent additive. We demonstrated that a solvent additive with a high boiling point and solubility parameter similar to that of the solution affected the solvent evaporation rate and improved the crystallinity of the dip-coated polymer thin film. The method used to add the solvent strongly influenced how the solvent additive diffused into the polymer solution, which affected the resulting film morphology. The crystallinity and morphology of the polymer films were correlated with the electrical characteristics, and the most crystalline film displayed a high hole field effect mobility of 0.0391 cm² V^-1 s^-1 when processed from the solvent mixture without post-treatment. Our findings provide a direction for the development of reliable and promising organic thin film transistor technologies.

Fig. 1. (a) Surface tensions and densities of various solvents. (b) Schematic diagram illustrating the biphasic dip-coating process. Photographs of biphasic systems comprising (c) CF (top)/THF (bottom), (d) CF (top)/DMF (bottom), and (e) CF (top)/DI water (bottom).

Fig. 2. (a) Normalized UV-vis absorption spectra of the P3HT thin films prepared from CF, CB, and DCB. (b) Schematic illustration of the solvent adding processes: the floating method and the mixing method. Normalized UV-vis absorption spectra of the P3HT thin films prepared from different solvent adding processes: (c) floating and (d) mixing method at a certain amount of additive solvent, 5 and 10 vol%. (e) The ratios of the intensities of the (0–0) to (0–1) peaks. The inset shows variations in the free exciton bandwidth W, calculated from the UV-vis absorption spectra of the dip-coated P3HT thin films.

Fig. 3. Photographs of P3HT films dip-coated from the homogeneous solvent, (a) CF, CB, and DCB, and from the mixed solvents prepared using different solvent addition processes: (b) floating and (c) mixing method, onto an OTS-treated silicon substrate. Optical microscopy images of the P3HT films dip-coated from the biphasic solvents with solvent additives CF:CB and CF:DCB for the two different solvent addition methods: (d) the floating method and (e) the mixing method.

Fig. 4. Tapping-mode AFM phase images of the P3HT films obtained from the biphasic P3HT solutions. AFM phase images of the films processed from the solvents (a) CF, CB, and DCB. AFM phase images of the films processed from the solvent mixtures CF:CB and CF:DCB prepared using different solvent addition methods: (b) floating method and (c) mixing method.

Fig. 5. Plots of the drain current versus the gate voltage at a fixed drain voltage of −80 V on both the linear (left axis) and logarithmic (right axis) scales for P3HT films dip-coated from the solvent mixtures CF:CB and CF:DCB, prepared by (a) the floating method and (b) the mixing method. (c) Average field effect mobilities of the P3HT films dip-coated from the various solvent mixtures using various solvent addition methods.

Fig. 6. Schematic drawing illustrating how the solvent addition method (floating and mixing) induced solvent intermixing and agglomeration in the solution state.