Acceptor-acceptor′-acceptor type N-annulated perylenediimide dimeric acceptors at the outside-bay position: Synthesis and role of asymmetric 4,7-di(thien-2-yl)-5-fluorobenzothiadiazole linker

Acceptor-acceptor′-acceptor (A-A′-A) type N-annulation perylene diimide (PDI) dimeric acceptors possessed the fascinating merits but were ignored. Herein, two A-A′-A type N-annulated PDI dimers, termed as, DTBT-(PPDI-B)₂ and DTFBT-(PPDI-B)₂, in which the electron-deficient moieties 4,7-di(thien-2-yl)benzothiadiazole (DTBT) and monofluorinated 4,7-di(thien-2-yl)-5-fluorobenzothiadiazole (DTFBT) were selected as the central linkers and two N-annulation N-butyl-[1,12-b,c,d]pyrrole-N,Nʹ-bis(2-hexyldecyl)perylenediimide (PPDI-B) aromatic rings as the dual wings were explored. The annulation-free DTBT-(PDI-HD)₂ was chosen as reference material. Owing to electron-donating of the newly formed pyrrolic aromatic ring, these studied N-annulated acceptors possessed the improved absorption and up-shifted E_LUMO than those of the counterpart DTBT-(PDI-HD)₂. Both of them exhibited the wide absorption spectra spanning from 300 nm to 700 nm, the similar aggregation and good photostability in solution. Monofluorinating the central benzothiadiazole linker gave the enhanced absorption, the down-shifted E_HOMO and E_LUMO, the reduced the dihedral angle from 9.09° to 3.64° between two N-annulated PPDI sub-planes and the enhanced solid stacking and aggregation. As a result, the N-annulation modification achieved the 0.16 V increased V_OC, decreased J_SC and FF, and thus the reduced PCE of 0.90 %. Moreover, the 21.33 % increased J_SC of 3.64 mA cm⁻² and the 35.58 % raised FF up to 42.68 % were observed in the monofluorinated PTB7-Th:DTFBT-(PPDI-B)₂-based device, synergistically contributing to an 62.22 % elevated PCE of 1.46 % in spite of a 0.01 V dropped V_OC. This upgradation was predominantly profited from the increased absorption, the improved exciton dissociation, elevated electron mobility and improved microstructural morphology benefiting from introducing the asymmetric 4,7-dithienyl-5-fluorobenzothiadiazole. This work demonstrates that outside bay N-annulation strategy could tune the molecular structure, energy level and thus improve the voltage, and monofluorinating the central linker is an effective tactic for regulating the molecular configuration, aggregation, morphology and thus elevating the photovoltaic performance in the A-A′-A type N-annulated PDI dimeric acceptors.