Proceedings of nanoGe Spring Meeting 2022 (NSM22)
DOI: https://doi.org/10.29363/nanoge.nsm.2022.323
Publication date: 7th February 2022
The fine-structured film morphology in the active layer of all-polymer solar cells is the key to their high performance. Polymeric non-fullerene acceptors offer the potential to restrict the self-aggregation that is typical for small molecule non-fullerene acceptors. But what limits the phase separation that otherwise dominates in polymer-polymer blends? In this study, we employed a blend of the polymeric acceptor PF5-Y5 and the donor polymer PBDB-T to investigate the molecular interactions in solution in a joint experimental-theoretical spectroscopy study. Solar cells prepared of this blend have reached power conversion efficiencies of over 14%.(1) From absorption spectroscopy of the PBDB-T:PF5-Y5 blend solutions at increasing temperatures, combined with concentration-dependent fluorescence spectroscopy and excitation spectroscopy, we could conclude that in addition to temperature-induced disaggregation of both donor and acceptor polymers, donor-acceptor complexes are formed in dilute blend solutions of PBDB-T and PF5-Y5. The formation of the donor-acceptor complexes competes with the donor and acceptor self-aggregation and the solvent environment is found to influence these interactions. Our results show also that the donor-acceptor polymer complexes are stabilized in more polar solvents. The near IR-region of the absorption spectrum could be matched with the calculated electronic excitations of donor-acceptor complexes of PBDB-T and PF5-Y5 oligomers. The results corroborate that van der Waals interaction between segments of the donor and acceptor polymer chains favours the formation of donor-acceptor charge transfer complexes, stabilized by hybridization of the molecular orbitals, which reduces the electronic energy. These pre-formed donor-acceptor complexes in solution can be expected to have important consequences on the resulting film morphology. These insights are also expected to direct the future design of compatible donor-acceptor polymer pairs for high-performance all-polymer solar cells.
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