Impact of Terminal End-Group of Acceptor–Donor–Acceptor-type Small Molecules on Molecular Packing and Photovoltaic Properties

In this study, we synthesized two acceptor–donor–acceptor (A–D–A)-type small molecules (SMs) (P3T4–VCN and P3T4–INCN) with different terminal end-groups (dicyanovinyl (VCN) and 2-methylene-3-(1,1-dicyanomethylene)indanone (INCN)) based on the 1,4-bis(thiophenylphenylthiophene)-2,5-difluorophenylene (P3T4) core that possesses high coplanarity because of intrachain noncovalent Coulombic interactions. We investigated the influence of terminal end-groups on intermolecular packing and the resulting electrical and photovoltaic characteristics. A small change in the end-group structure of the SMs induces a significant variation in the torsional structures, molecular packing, and pristine/blend film morphology. It is noteworthy that the less crystalline P3T4–INCN with tilted conformation is highly sensitive to post-treatments (i.e., additives and annealing) such that it permits facile morphological modulation. However, the highly planar and crystalline P3T4–VCN exhibits a strong tolerance toward processing treatments. After morphology optimization, the fullerene-based bulk-heterojunction solar cell of tilted P3T4–INCN exhibits a power conversion efficiency (PCE) of 5.68%, which is significantly superior to that of P3T4–VCN:PC71BM (PCE = 1.29%). Our results demonstrate the importance of the terminal end-group for the design of A–D–A-type SMs and their sensitivity toward the postprocessing treatments in optimizing their performance