Semi-crystalline A1–D–A2-type copolymers for efficient polymer solar cells

Abstract

A series of acceptor1–donor–acceptor2 (A1–D–A2)-type copolymers was designed and synthesized using thiophene as an electron-rich unit and benzothiadiazole (BT) and benzotriazole (BTz) as electron-deficient moieties. A weaker acceptor, BTz, was incorporated as a solubilizing moiety with three tetradecyl (or tetradecyloxy) side chains, and a stronger acceptor, BT, was substituted with different numbers of fluorines to modulate the intramolecular charge transfer interaction and the resulting electronic structures. The similar molecular structures of BT and BTz did not disrupt the interchain organization significantly, and the absence of solubilizing alkyl substituents on the electron-rich thiophene did not increase the highest occupied molecular orbital of the resulting polymers. The polymers showed broad absorption in the range of 350–750 nm. Intra- and/or interchain non-covalent coulombic interactions (for example, dipole–dipole interactions via S···O, S···F) ensured a planar backbone and a semi-crystalline morphology in the film. Bulk heterojunction polymer solar cells were fabricated using blends of the polymers and phenyl-C71-butyric acid methyl ester (PC71BM). For a difluoro-BT containing polymer (BTzDT2FBT), a power conversion efficiency up to 7% was achieved with a short circuit current density of 14.64 mA cm−2, open circuit voltage of 0.75 V and fill factor of 0.64. Three types of A1–D–A2 conjugated polymers were synthesized based on thiophene as an electron-rich unit and two similarly structured electron-deficient moieties of benzothiadiazole (BT) and benzotriazole (BTz). The weaker acceptor, BTz, with three alkyl (or alkyloxy) substituents took on a solubilizing role without deteriorating the chain planarity. The absence of alkyl substituents on thiophene induced a deep valence band. Increasing the number of fluorine substituents on BT decreased the frontier orbitals and enhanced the interchain packing. The chain planarity and crystalline interchain organization were facilitated via intra- and/or interchain non-covalent S···O and S···F coulombic interactions. The best power conversion efficiency of 7% was measured for BTzDT2FBT:PC71BM.