Air-Stable Crystalline Polymer-Based Field-Effect Transistors Fabricated by a Thermal Gradient Process

Low-voltage-driven organic field-effect transistors (OFETs) with an organic polymer p-type semiconductor poly(2,5-bis(3-alkylthiophen-2-yl) thieno[3,2-b] thiophene) (PBTTT-C14) as the active layer were explored for the processing and crystallization of polymer semiconductors whose opto-electronic properties critically depend on the microstructure. Here, we report polymer crystallization from processed hexamethylbenzene (HMB)/PBTTT-C14 mixtures using a thermal gradient system to yield fiber-like crystals with up to 0.8 μm in width and to fabricate crystalline PBTTT-C14-based OFETs. In the thermal gradient system, the HMB separated from the HMB/PBTTT-C14 mixtures and crystallized along the samples' moving direction. The crystals' physical properties characterized by in situ atomic force microscopy and Raman spectroscopy at different temperatures revealed that the HMB-processed PBTTT-C14 thin film can improve the microstructure and achieve a directionally crystalline structure. These results combined with theoretical calculations show a high degree of π-stacking within the crystalline PBTTT-C14 (c-PBTTT-C14) crystal. c-PBTTT-C14 has good crystallinity, which enhances the intra- and intermolecular transmission of electrons. Whether in a nitrogen-filled glovebox or in the atmosphere, the electrical performances of the c-PBTTT-C14-based OFET had a remarkable increase compared with those of the spin-coated PBTTT-C14-based OFET. This phenomenon was also observed in the crystalline P3HT-based OFET fabricated by the same thermal gradient system.