Harnessing the Inductive Effect To Design New Donor–Acceptor–Acceptor′-Configured Small-Molecule Donors for Vacuum-Processed Organic Photovoltaics

Three new donor–acceptor–acceptor′ (D–A–A′)-configured molecules, DTCFiBT, DTCFoBT, and DTCF2BT, with F-substituted benzothiadiazole (BT) as the A group, and two molecules, DTCPiTD and DTCPoTD, adopting pyridal[2,1,3]-thiadiazole (PTD) as the A group, were synthesized and characterized. The effects of the F-substitution number and the orientation of mono F-substituted BT and PTD relative to the D group on the physical properties and intermolecular interactions were examined, together with theoretical calculations to establish the structure–property relationship. In comparison to the parent molecule DTCPB, the inductive effects of F-substituted BTs lower both highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of DTCFiBT, DTCFoBT, and DTCF2BT and, thus, similar optical energy gaps (Egopt), while DTCPiTD and DTCPoTD exhibit much lower LUMO energy levels and reduced Egopt, ascribing to the electron-deficient character of PTD. The strong dipolar features of the D–A–A′-configured structure resulting in the antiparallel dimeric packing with different intermolecular interactions in their crystal structures were observed by X-ray analyses. Small-molecule organic solar cells (SMOSCs) with a bulk heterojunction active layer comprising new D–A–A′ donors and C70 were fabricated and characterized. The deeper LUMO levels and reduced Egopt benefit the DTCPiTD- and DTCPoTD-based devices to have higher short current density (JSC), while the DTCFiBT-, DTCFoBT-, and DTCF2BT-based devices benefited from the lower HOMO energy levels that lead to the higher open circuit voltage (VOC). Transient photoluminescence, atomic force microscopy, and incident-light-intensity-dependent device characteristics were examined to reveal the recombination issue for the inferior DTCFoBT-based device. Among these new donors, the DTCFiBT-based device shows the best performance, with VOC of 0.94, JSC of 11.3 mA/cm2, fill factor (FF) of 0.65, and power conversion efficiency of 6.8%, which are attributed to the high VOC as a result of the deeper HOMO level and the superior FF as a result of good exciton separation and charge carrier transport.