Metal-oxide assisted surface treatment of polyimide gate insulators for high-performance organic thin-film transistors

Abstract

We developed a facile method for treating polyimide-based organic gate insulator (OGI) surfaces with self-assembled monolayers (SAMs) by introducing metal-oxide interlayers, called the metal-oxide assisted SAM treatment (MAST). To create sites for surface modification with SAM materials on polyimide-based OGI (KPI) surfaces, the metal-oxide interlayer, here amorphous alumina (α-Al₂O₃), was deposited on the KPI gate insulator using spin-coating via a rapid sol–gel reaction, providing an excellent template for the formation of a high-quality SAM with phosphonic acid anchor groups. The SAM of octadecylphosphonic acid (ODPA) was successfully treated by spin-coating onto the α-Al₂O₃-deposited KPI film. After the surface treatment by ODPA/α-Al₂O₃, the surface energy of the KPI thin film was remarkably decreased and the molecular compatibility of the film with an organic semiconductor (OSC), 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-C₁₀), was increased. Ph-BTBT-C₁₀ molecules were uniformly deposited on the treated gate insulator surface and grown with high crystallinity, as confirmed by atomic force microscopy (AFM) and X-ray diffraction (XRD) analysis. The mobility of Ph-BTBT-C₁₀ thin-film transistors (TFTs) was approximately doubled, from 0.56 ± 0.05 cm² V⁻¹ s⁻¹ to 1.26 ± 0.06 cm² V⁻¹ s⁻¹, after the surface treatment. The surface treatment of α-Al₂O₃ and ODPA significantly decreased the threshold voltage from −21.2 V to −8.3 V by reducing the trap sites in the OGI and improving the interfacial properties with the OSC. We suggest that the MAST method for OGIs can be applied to various OGI materials lacking reactive sites using SAMs. It may provide a new platform for the surface treatment of OGIs, similar to that of conventional SiO₂ gate insulators.