Simple Organic Donors Based on Halogenated Oligothiophenes for All Small Molecule Solar Cells with Efficiency Over 11

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.