Low-cost and completely non-fused small-molecule acceptors based on tetrathiophene featuring 3,5-dialkylthiophene side chain enable efficient organic solar cells

Developing completely non-fused small-molecule acceptors (CNF-SMAs) to further reduce production cost is strongly desired for the industrialization of organic solar cells (OSCs). In this contribution, we have designed and developed four tetrathiophene-based CNF-SMAs, named 4TTh-IC, 4TTh-ICF, 4TThC-IC, and 4TThC-ICF, by side chain engineering (hexyl vs. H atom) and end-capping group engineering (IC vs. ICF). These acceptors feature identical 3,5-dialkylthiophene side chains and a significant advantage of low production cost. Theoretical calculations demonstrate that the introduction of large steric hindrance 3,5-dialkylthiophene side chains is beneficial for maintaing almost a coplanar tetrathiophene backbone. We find that the introduction of hexyl side chain into two outer thiophene units has limited effect on absorption spectra, whereas introducing fluorine atoms into the end-capping group can effectively regulate HOMO/LUMO levels, crystallinity, charge mobility, film morphology, and photovoltaic properties. Polymer donors PBDB-T and PM6 paired with these CNF-SMAs to fabricate OSCs. Results demonstrate that PM6-based solar cells deliver superior device performance to PBDB-Tbased ones. In particular, 4TThC-ICF with hexyl side chain into two outer thiophene units and fluorinated endcapping groups exhibits excellent miscibility with the polymer donor PM6, and an encouraging and comparable PCE of 11.05%, owing to improved exciton dissociation and charge transport ability, more balanced charge mobility, reduced bimolecular recombination, and superior active layer morpology. This work provides a useful and effective attempt for the molecular design of high -performance and low-cost CNF-SMAs with an emphasis on the functionalized conjugated side chains.