Selective Tuning of Benzothiadiazole Functionality Enables High Crystallinity and Mobility in Regiorandom n-Type Polymers for Organic Field-Effect Transistors

We report three novel donor-acceptor (D-A) copolymers sharing a common fused donor unit (CDTT) but differing in the functionalization of the benzothiadiazole (BT) acceptor unit. Acceptors bearing two cyano groups (DCNBT) are compared to novel acceptors bearing one cyano and one fluorine group (FCNBT) or one nitro and one fluoro group (NO₂FBT). The choice of the acceptor has a significant effect on the optoelectronic properties of the resulting polymers. In organic field-effect transistor (OFET) devices, PCDTT-DCNBT exhibited moderate performance with an electron mobility of 0.031 cm² V^-1 s^-1, whereas PCDTT-FCNBT demonstrated significantly improved electron mobility (0.4 cm² V^-1 s^-1). The improved performance is attributed to increased backbone linearity combined with a more coplanar backbone and high thin-film crystallinity. In comparison, the presence of the nitro group is shown to have a detrimental impact, with a blue-shifted absorption and a 0.2 eV increase in band gap compared to the cyanated polymers. Steric effects are shown to limit the nitro group's π-accepting capability and result in reduced device performance, with an electron mobility of 0.024 cm² V^-1 s^-1. This study introduces a new BT building block and highlights that substituent tuning via cyano and fluorine groups is an effective approach for modulating polymer morphology and electron transport.