One-pot homopolymerization of thiophene-fused isoindigo for ambient-stable ambipolar organic field-effect transistors

Abstract

Donor–acceptor (D–A) type conjugated polymers have been extensively studied for organic field-effect transistors (OFETs), while the homopolymers of acceptors were much less investigated. However, conjugated homopolymers are structurally simpler, so their solid-phase behavior such as packing pattern, long-range order and crystallinity are more predictable, so are their charge-transport properties. Here we report the synthesis of the homopolymer of thiophene-fused isoindigo (TII), which was directly obtained using a “one-pot” polymerization method either via Suzuki condensation polymerization (denoted as PTII-1) or Stille condensation polymerization (denoted as PTII-2), respectively. To further elucidate the structure–property relationship of the PTII, TII was also copolymerized with II to give P(TII-co-II) for comparison. Encouragingly, solution-processed OFETs based on PTII-2 exhibited ambipolar transport behavior with balanced hole and electron mobilities (0.065 and 0.15 cm² V⁻¹ s⁻¹, respectively) after annealing, which is two orders higher than that of P(TII-co-II). 2D grazing incidence X-ray diffraction (2D-GIXRD) analysis explicitly showed that PTII films have much better well-ordered structure and crystallinity than P(TII-co-II), which accounted for their better OFET performance. To the best of our knowledge, PTII-2 is among the best homopolymers of A units with high ambipolar charge-carrier mobility, indicating that the design of such homopolymers might be an important strategy toward high performance polymer OFETs. In addition, the “one-pot” homopolymerization of dibromo-monomers via Suzuki or Stille condensation also provides a more convenient way to construct conjugated homopolymers, which deserves a closer study.