Role of conjugation length for central linker in the asymmetric perylenediimide dimeric acceptors †

Abstract

The investigation of asymmetric central core linked perylene diimide (PDI) dimeric acceptors was still insufficient. For this end, the conjugation extended core 6-(thieno[3,2-b]thiophene-2-yl)benzo[b]thiophene (TT-BTh) was explored, and two asymmetric PDI dimers, T-BTh-(PDI-HD) 2 and TT-BTh-(PDI-HD) 2 , with 6-(thien-2-yl)benzo[b]thiophene (T-BTh) and TT-BTh as central linkages and two imide-position 2-hexyldecyl (HD) substituted PDI-HD units as double flanks, were developed to probe into the role of different conjugation length. When the central asymmetric linker was extended from T-BTh to TT-BTh, the dropped thermo-stability, weaken absorption, enhanced molecular stacking in film state, better solution state photo-stability, a slightly deepened E LUMO and significantly decreased dihedral angle from 87.70° to 27.05° between two PDI-HD sub-planes were found. Consequently, extending the conjugation length from T-BTh to TT-BTh suffered from a 0.03 V reduced V OC from 0.81 V to 0.78 V, a 26.59% reduced J SC from 7.56 mA cm -2 to 5.55 mA cm -2 but a 10.65% increased FF from 47.33% to 52.37%, resulting in the 21.53% decreased PCE from 2.88% to 2.26%. The decrease in PCE was mainly due to the slightly deepened E LUMO , weakened absorption, decreased electron mobility originating from deteriorated solubility and rough surface morphology. Interestingly enough, 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 symmetric cored T-(PDI-HD) 2 and 2T-(PDI-HD) 2 . This finding indicated that constructing asymmetric T-BTh linking core was an efficient method for adjusting the molecular geometry, optoelectronic property and thus improving the photovoltaic efficiency, however it should be cautious to further prolong the conjugation length of the central linker during these asymmetric PDI dimeric acceptors.2