Solution-Shearing of Highly Smooth Ion-Gel Thin Films: Facilitating the Deposition of Organic Semiconductors for Ion-Gated Organic Field Effect Transistors

Abstract

A straightforward method is developed to produce ion-gels (IGs) with surface roughness at the nanometer level using a solution-shearing process, enabling the first successful growth of crystalline, small-molecule organic semiconductor (OSC) films directly on the IG layer. The effectiveness of this approach is demonstrated by fabricating top-contact electrolyte-gated organic field-effect transistors (EGOFETs) using thermal vapor deposition and solution-shearing. The gel matrix consists of polymethyl methacrylate (PMMA) or its blend with poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF:HFP), and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]) serves as ionic liquid. X-ray photoemission spectroscopy (XPS) reveals that the shearing speed controls the polymer phase separation in the blended gels, producing capacitance values of up to 10.1 µF cm⁻². The exceptional smoothness of the gel films permits vacuum deposition polycrystalline films of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophen (C8-BTBT), dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophen (DNTT), and 2,9-didecyldinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C10-DNTT), and solution-shearing of C8-BTBT and 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) on their surfaces. Grazing incidence wide-angle X-ray scattering (GIWAXS) can now be conducted directly on the OSC films without obstruction by the gel. EGOFETs with minimal hysteresis and mobilities up to 1.46 cm² V⁻¹ s⁻¹ are obtained for C10-DNTT. This study underscores the possibility of producing transistor-grade polycrystalline organic semiconductor films on top of IGs, making them attractive for surface characterization techniques and in situ measurements.