Low-Band-Gap Polymer-Based Ambipolar Transistors and Inverters Fabricated Using a Flow-Coating Method

The performances of organic thin film transistors (OTFTs) produced by polymer solution casting are tightly correlated with the morphology and chain-ordering of semiconducting polymer layers, which depends on the processing conditions applied. The slow evaporation of a high boiling point (bp) solvent permits sufficient time for the assembly of polymer chains during the process, resulting in improving the film crystallinity and inducing favorable polymer chain orientations for charge transport. The use of high bp solvents, however, often results in dewetting of thin films formed on hydrophobic surfaces, such as the commonly used octadecyltrichlorosilane (ODTS)-treated SiO₂ gate dielectric. Dewetting hampers the formation of uniform and highly crystalline semiconducting active channel layers. In this manuscript, we demonstrated the formation of highly crystalline dithienothienyl diketopyrrolopyrrole (TT-DPP)-based polymer films using a flow-coating method to enable the fabrication of ambipolar transistors and inverters. Importantly, unlike conventional spin-coating methods, the flow-coating method allowed us to use high bp solvents, even on a hydrophobic surface, and minimized the polymer solution waste. The crystalline orientations of the TT-DPP-based polymers were tuned depending on the solvent used (four different bp solvents were tested) and the employment of a thermal annealing step. The use of high bp solvents and thermal annealing of the polymer films significantly enhanced the crystalline microstructures in the flow-coated films, resulting in considerable carrier mobility increase in the OTFTs compared to the spin-coated films. Our simple, inexpensive, and scalable flow-coating method, for the first time employed in printing semiconducting polymers, presents a significant step toward optimizing the electrical performances of organic ambipolar transistors through organic semiconducting layer film crystallinity engineering.