Insights into constitutional isomeric effects on donor-acceptor intermolecular arrangements in non-fullerene organic solar cells

Donor-acceptor intermolecular interactions and/or arrangements are key factors determining the performance of organic solar cells (OSCs). Herein, two ITIC-based isomeric acceptors (o-F-ITIC and m-F-ITIC) with fluorine atoms at ortho and meta positions within the side chains were developed. The developed acceptors showed different interactions and/or arrangements with the donor counterpart (PBDB-T), which caused a significant variation in the power conversion efficiency (PCE) of OSCs. The PCE of the PBDB-T:o-F-ITIC device was 11.11% versus 8.90% of the PBDB-T:m-F-ITIC device. A detailed theoretical analysis of the intermolecular complexes formed between F-ITICs and PBDB-T allows a determination of how the two isomeric acceptors interact differently with PBDB-T. Relative to PBDB-T:m-F-ITIC with a preferential DCI-BDD complex, PBDB-T:o-F-ITIC showed a closer donor-acceptor interaction, with a tightly packed parallel-like DCI-BDT complex, resulting in larger interfacial areas. These behavioral characteristics explain in detail the experimental results obtained from the comparison of the two blend systems relating to electrical and morphological properties. Moreover, transient absorption spectroscopy of PBDB-T:o-F-ITIC revealed superior hole/electron transfer and separation processes, providing further evidence of the presence of beneficial intermolecular complexes. This study facilitates the understanding of the role of constitutional isomerism both in intermolecular interactions and in promoting the development of high-performance OSCs.