Influence of the Excited-State Charge-Transfer Character on the Exciton Dissociation in Donor–Acceptor Copolymers

We synthesize a polytriphenylamine homopolymer and two donor–acceptor copolymers (D–A-copolymers) based on triphenylamine (TPA) as donor in combination with two different acceptor moieties to study the effect of the acceptor unit on the excited-state charge-transfer characteristics (CT-characteristics) and charge separation. The two acceptor moieties are a dicyanovinyl group in the side chain and a thieno[3,4-b]thiophene carboxylate in the main chain. Absorption and photoluminescence studies show new CT-bands for both of the D–A-copolymers. Field-dependent charge extraction studies in bilayer solar cells indicate a stronger CT-character for the copolymer in which the acceptor group is less conjugated with the copolymer backbone. The D–A-copolymer carrying the acceptor unit in the main chain exhibits smaller excitonic CT-character and good conjugation leading to less-bound electron–hole pairs and a better charge separation. This fundamental study gives insight into the interdependence of conjugation, charge carrier mobility, and solar cell performance for two different D–A-copolymers.