Biaxially Extended Thiophene–Fused Thiophene Conjugated Copolymers for High Performance Field Effect Transistors

We report the synthesis, properties, and field-effect transistor characteristics of biaxially extended thiophene-fused thiophene conjugated copolymers, P4TDTT and P4TTT, consisting of 2′,5″-5,5‴-di(2-ethylhexyl)-[2,3′;5′,2″;4″,2‴] -quaterthiophene and fused thiophene moieties of dithieno[3,2-b:2′,3′-d]thiophene and thieno[3,2-b]thiophene, respectively. The effect of fused thiophene size on the molecular packing and charge transport characteristics was explored. The high boiling point solvent facilitated the formation of ordered nanofiber morphology and enhanced the optoelectronic properties. P4TDTT exhibited a smaller d-spacing and π–π stacking distance than P4TTT based and a denser nanofiber network. The optical band gaps of P4TDTT and P4TTT films spin-coated from trichlorobenezene (TCB) solvent were 1.84 and 1.86 eV, respectively. The highest hole mobilities of P4TDTT and P4TTT using TCB solvent were 0.610 and 0.396 cm2 V–1 s–1, respectively, with the on/off ratios of 105–106. P4TDTT had better charge-transporting characteristics than P4TTT due to the nonsymmetry side-chain arrangement and larger coplanar moieties, leading to an enhanced interchain interaction and denser molecular packing. Furthermore, these polymers showed relatively high air stability due to the relative low-lying HOMO levels. This study revealed that the biaxially extended fused ring containing conjugated polythiophenes had excellent charge-transporting characteristics for FET applications.