Terpolymers and Additional Π-Bridges Regulations for Improving Fill Factors in Efficiency Organic Solar Cells

For high-efficiency organic solar cells (OSCs), fill factors (FFs) play a significant role in power conversion efficiencies (PCEs), which are closely related to molecular stacking and the active layer morphology. For polymer design, a feasible strategy to solve this problem is constructing irregular conjugated backbones, such as generating random terpolymers. In this work, difluorinated benzothiadiazole (SZ) units are introduced into PM7 to synthesize terpolymers for facilitating the molecular stacking, which results in favorable active layer morphology and improved FF in OSCs. Terpolymers indicate improved molecular stacking and morphological properties. More importantly, the additional π-bridges along with SZ units can regulate the molecular stacking, which contributes to FF and PCE improving from 61% and 13% of PM7 to 71% and 15% of terpolymer, respectively. DFT optimizations reveal that additional π-bridges can adjust the planar conformation between the donor and acceptor units. The reduced density gradient function and the independent gradient model approach, which are used to show the intramolecular interactions and intermolecular interactions, respectively, suggest that different additional π-bridges can promote the effective stacking of SZ units by balancing intra-molecular attraction and repulsion, enhancing intermolecular interactions. This work demonstrates the excellent practicality of terpolymer design and paves a way to improve the molecular stacking and morphological properties for efficient OSCs