Role of the conjugation length of the central linker in asymmetric perylene diimide dimeric acceptors

The investigation on exploring the asymmetric central core-linked perylene diimide (PDI) dimeric acceptors is still insufficient. To this end, the conjugation-extended 6-(thieno[3,2-b]thiophene-2-yl)benzo[b]thiophene (TT-BTh) core was explored in this study, and two asymmetric PDI dimers, T-BTh-(PDI-HD)₂ and TT-BTh-(PDI-HD)₂, were developed using 6-(thien-2-yl)benzo[b]thiophene (T-BTh) and TT-BTh as the central linkages, respectively, and two imide-position 2-hexyldecyl (HD) substituted PDI-HD units as double flanks to probe into the role of different conjugation lengths. When the central asymmetric linker was extended from T-BTh to TT-BTh, it was observed that the thermal stability dropped, absorption was weakened, molecular aggregation and stacking were enhanced, a better solution-state photo-stability was achieved, E_LUMO was slightly lowered and the dihedral angle was significantly decreased from 87.70° to 27.05° between two PDI-HD sub-planes. Furthermore, extending the conjugation length from T-BTh to TT-BTh resulted in a 0.03 V reduction in V_OC from 0.81 V to 0.78 V, a 26.59% reduction in J_SC from 7.56 mA cm^-2 to 5.55 mA cm^-2, a 10.65% increase in FF from 47.33% to 52.37%, and a 21.53% decrease in PCE from 2.88% to 2.26%. The decrease in PCE was mainly due to the slightly lowered E_LUMO, weakened absorption, and decreased electron mobility originating from the reduced miscibility and rough surface morphology. Interestingly, the asymmetric T-BTh-(PDI-HD)₂ afforded 0.02-0.03 V increased V_OC, 2.07%-10.58% elevated FF and thus 11.63%-58.24% elevated PCE in comparison with the symmetric core-linked T-(PDI-HD)₂ and 2T-(PDI-HD)₂. This finding indicated that constructing asymmetric T-BTh linking cores is an efficient method for adjusting the molecular geometry and optoelectronic properties, thereby improving the photovoltaic efficiency. However, it is necessary to be cautious while further prolonging the conjugation length of the central linker in the asymmetric PDI dimeric acceptors.