The driving forces for twisting or planarization of excited D–A (donor–acceptor)-type chromophore have been investigated.

Recent work has emphasized the pivotal role of halogen substituents in the development of polymer donors or small-molecule (SM) acceptors for efficient bulk-heterojunction (BHJ) solar cells. However, the application of –F or –Cl substitutions in the design of SM donors is yet to receive similar considerations. In this contribution, we report a set of oligothiophene derivatives (2F7T and 2Cl7T) with halogenation at the central thienyl moiety. Such halogenation is shown to induce a sufficient increase in ionization potential (IP; i.e. deeper HOMO) to modify the electron-donating and -transporting characteristics of the molecules. When combined with a low-bandgap SM acceptor (Y6), the devices with 2Cl7T achieved power conversion efficiencies (PCEs) of up to ca. 11.5% (vs. ca. 2.5% for non-halogenated donor DRCN7T based devices), representing the best photovoltaic performance of oligothiophene donors.

A simple oligothiophene donor (Tz6T) was developed for organic solar cells. The devices with Tz6T achieved power conversion efficiencies (PCEs) of up to ca. 15.4%, representing one of the crest values achieved by oligothiophene donors.