Fluorination-Induced Conversion of the Quasi- to the Normal Interchain Packing in Push-Pull Conjugated Polymer Aggregates

We investigated the chain-length and fluorination effects on photophysics within push-pull conjugated polymer (CP, M-w 40k/100k PBDB-T and M-w 100k PM6) aggregates using transient absorption spectroscopy. Our findings reveal that J-type excitons were more induced in relatively ordered domains than in disordered domains of PBDB-T. Interestingly, the ordered domains of M-w 100k PBDB-T aggregates induce interchain packing with localized chain cross-linked points, referred to as quasi-interchain packing, due to chain folding and increased intrachain rigidity. Two fluorination effects were observed in the PM6 aggregates. First, the intrachain rigidity decreased in the PM6 film compared to the PBDB-T films. Second, the formation of localized chain cross-linked points was suppressed in the ordered domains, resulting in the conversion of the quasi- to the normal interchain packing and the enhanced H-type excitonic couplings. Our findings provide new insights into the exciton dynamics study in CP aggregates and the bulk-heterojunction blends of polymer solar cells.