Impact of Fluorination on Photovoltaic Performance in High Thermo- and Photo-Stability Perylene Diimide-Based Nonfullerene Small Molecular Acceptors

Fluorinations play the momentous impact in tuning the optical absorption, energy levels, aggregation and microstructurally morphology. Two acceptor-donor-acceptor (A-D-A) non-fullerene small molecular acceptors (SMAs), namely, 2T-(PDI-HD)(2) and F2T-(PDI-HD)(2), by incorporating two fluorine atoms into the central 2,2'-bithiophene linked core, were synthesized. The resultant PDI-based SMAs had excellent thermo- and photostability, wide absorption ranging from 300 to 700 nm, and E(LUMO)s of -3.71 to -3.84 eV. It was demonstrated that the enhanced molar absorption coefficient, elevated E-LUMO, slightly strengthened film aggregation and lowered miscibility with donor PTB7-Th were observed after incorporating fluorine atom onto the SMAs. Further surface electrostatic potential (ESP) calculation elucidated that the fluorination could increase molecular polarity and maximize the ESP difference between PTB7-Th and SMAs, facilitating exciton dissociation and the charge transport. Accordingly, an 39.01% improved PCE from 1.82% to 2.53% was yielded in PTB7-Th:F2T-(PDI-HD)(2) based device, mainly benefitting from 26.17% enhanced J(SC) and 22.82% elevated FF even if the adverse condition of decreased V-OC. This improvement was profited from the increased absorption coefficient, enhanced electron mobility as well as favorable microstructurally morphology. The results demonstrated that inserting fluorine atoms into the central linked core was an effective strategy to improve the device performance.