Impact of fluorination of central thiophene linking core in thermostable perylene-diimide-based dimeric acceptors on molecular configuration and photovoltaic performance

Fluorination was an easy and effective strategy to adjust the absorption, electronic energy level, molecular configuration and aggregation as well as charge mobility, miscibility, morphology and photovoltaic property. Herein, two perylene-diimide-based non-fullerene dimeric acceptors, called T-(PDI-HD)2 and 2FT-(PDI-HD)2, were obtained via Stille coupling reaction between bistin compounds (TSn and 2FTSn) and monobromide PDI-HDBr. The dramatically reduced twisted angle from 81.80o to 19.78o and strengthened molecular stacking both in chlorobenzene solution and solid film states were found after fluorination, resulting in an elevated thermostability and slightly enhanced electron mobility. Furthermore, fluorination led to the down-shifted ELUMO, worse miscibility and undesired microstructural morphology with oversized aggregation which impeded the exciton dissociation and restricted the photocurrent. As a consequence, the slightly declined VOC, 18.52% down-shifted JSC but 12.38% increased FF were found, leading to a 9.69% decline in PCE from 2.58% to 2.33% after bisfluorinating the central thiophene linking core, which was imputed to the deepened ELUMO, worse light-harvesting capability and undesired morphology as a result of fluorination regardless of the slightly elevated electron mobility. Our results have demonstrated that it was cautious that incorporating fluorine substituents into central linking core by virtue of tuning the twisting degree boosted the photovoltaic property in PDI-based dimeric acceptors.