Bisisoindigo–Benzothiadiazole Copolymers: Materials for Ambipolar and n-Channel OTFTs with Low Threshold Voltages

Abstract

The development of design strategies in both ambipolar and electron-conducting organic thin-film transistor (OTFT) materials is important for producing high-performance materials and devices in organic electronics. Isoindigo donor–acceptor polymers have been well studied as hole-conducting materials in OTFTs, while more electron-deficient, acceptor-rich isoindigo polymers have been underexplored. In this report, two common design strategies in isoindigo-based polymers, acceptor–acceptor polymers and core-expanded isoindigo structures, are combined to create copolymers of bisisoindigo, a core-expanded derivative of isoindigo, and electron-deficient benzothiadiazoles. These polymers exhibit the low energy lowest unoccupied molecular orbitals (LUMOs) required for electron transport and show ambipolar OTFT performance with hole and electron mobilities up to 4.0 × 10–3 and 1.4 × 10–3 cm2 V–1 s–1, respectively. Additionally, changing the source and drain contacts from Au to LiF/Al significantly lowered the threshold voltage due to improved alignment of the polymer and electrode energy levels. Comparisons of the optoelectronic properties between these polymers and previously reported bisisoindigo donor–acceptor polymers indicate that the increased acceptor strength lowers the frontier molecular orbital energies, while the dithienyl-benzothiadiazole unit impacts the film morphology by altering the shape of the polymer backbone.