Systematically investigating the influence of inserting alkylthiophene spacers on the aggregation, photo-stability and optoelectronic properties of copolymers from dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene and benzothiadiazole derivatives

Two random conjugated polymers (CPs), PDTBDT-SBT and PDTBDT-SFBT, which were derived from 5,10-bis(4,5-didecylthien-2-yl)dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (DTBDT-T) and 4-bromo-7-(4-octylthien-2-yl)-2,1,3-benzothiadiazole (SBT) or 4-bromo-7-(4-octylthien-2-yl)-5,6-difluoro-2,1,3-benzothiadiazole (SFBT), were synthesized and characterized. Optimal photovoltaic devices fabricated from the random polymers and the PC₇₁BM blend films with the configuration ITO/PFN/active layer/MoO₃/Ag, provided power conversion efficiencies (PCEs) of 7.51% for PDTBDT-SBT and 7.72% for PDTBDT-SFBT, respectively, under 100 mW cm⁻² illumination (AM 1.5 G). Following that, comparative investigation of the aggregation, photostability and optoelectronic properties of the random CPs and alternating CPs from DTBDT-T and benzothiadiazole derivatives of BT and FBT with two bilaterally symmetrical 3-octylthiophene spacers (PDTBDT-DTBT and PDTBDT-DTFBT) or without 3-octylthiophene spacers (PDTBDT-BT and PDTBDT-FBT) was performed. It has been found that the band gaps of the BT-based CPs in films were almost unchanged, and that the photostability and d-spacing distances of these first decreased and then increased alongside a gradual increase in the highest occupied molecular orbital (HOMO) energy levels and the π–π stacking interaction, when the number of the 3-octylthiophene spacers in each repeat unit of the CPs were varied from 0 to 1 and then up to 2. Using the same variation of the number of spacers for the FBT-based CPs, the insertion of the 3-octylthiophene spacers presented a similar effect on the photostability and aggregation characteristics of the FBT-based CPs, except the band gaps of the FBT-based CPs were gradually red-shifted, and the HOMO energy levels were first almost unchanged, and then increased remarkably in contrast with the BT-based CPs.