Synthesis, Properties, and Photovoltaic Performances of Donor–Acceptor Copolymers Having Dioxocycloalkene-Annelated Thiophenes As Acceptor Monomer Units

New donor–acceptor-type copolymers containing dioxocycloalkene-annelated thiophenes as electron-accepting units have been designed and synthesized for application to p-type organic semiconducting materials in organic photovoltaics. The investigation of their photophysical and electrochemical properties revealed that these copolymers possessed low optical bandgaps (from 1.63 to 1.92 eV) and low-lying HOMO energy levels (from −5.41 to −5.33 eV). Organic field-effect transistor measurements revealed that these copolymers had hole-transporting characteristics with mobilities on the order of 10^–7–10^–4 cm² V^–1 s^–1. The bulk-heterojunction photovoltaic devices fabricated from blends of these copolymers with fullerene derivatives as acceptors showed high power conversion efficiencies of up to 4.87%, with an open-circuit voltage of 0.90 V, a short-circuit current of 11.46 mA cm^–2, and a fill factor of 0.48 under air mass 1.5 simulated solar illumination