Highly air stable branched octithiophene oligomer for organic field effect transistor and pH sensor applications

We report the synthesis, morphology, and organic field effect transistor (OFET) characteristics of novel branched octithiophene oligomer: 5,5′″″-dihexyl-3′,5″″-bis(5-hexylthiophen-2- yl)-2,2′:5′,3″:5″,2″′:4″′, 2″″:4″″,2″″′-sexithiophene (8T) prepared by the Stille coupling reaction. The effects of the surface modification and film thickness on the crystallinity, charge transport and sensing characteristics were explored systematically. 8T exhibited two absorption peaks at 300 and 350 nm with an optical band gap of 2.88 eV. It showed a highly crystalline structure with a melting point at 65 °C and had a lamellar packing structure, confirmed by grazing incidence small-angle X-ray scattering (GISAXS). Among different vapor-deposition conditions, the 8T based OFETs showed the field effect mobility up to 2.12 × 10^-2 cm² V^-1 s^-1 and a high on/off ratio of 5.1 × 10⁶ on the bare substrate. The relatively low-lying HOMO energy level (-5.53 eV) of 8T led to the outstanding air stability of the OFET device. Also, the 8T sensor devices maintained the stable device performance under aqueous conditions, and the current ratio between pH values of 5 and 10 could be above 3 orders of magnitude. The above results demonstrated that the air-stable 8T could be as a promising candidate for high performance organic electronic devices.