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 (E_g^opt), while DTCPiTD and DTCPoTD exhibit much lower LUMO energy levels and reduced E_g^opt, 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 E_g^opt benefit the DTCPiTD- and DTCPoTD-based devices to have higher short current density (J_SC), while the DTCFiBT-, DTCFoBT-, and DTCF2BT-based devices benefited from the lower HOMO energy levels that lead to the higher open circuit voltage (V_OC). 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 V_OC of 0.94, J_SC of 11.3 mA/cm², fill factor (FF) of 0.65, and power conversion efficiency of 6.8%, which are attributed to the high V_OC as a result of the deeper HOMO level and the superior FF as a result of good exciton separation and charge carrier transport.