Terpolymers and additional π-bridges regulations for improving fill factors in efficient organic solar cells

Abstract

For efficient organic solar cells (OSCs), the fill factor (FF) plays a significant role in power conversion efficiency (PCE), which are closely related to molecular stacking and active layer morphology. For polymer design, a feasible strategy to solve this problem is constructing irregular conjugated backbones, such as random terpolymers. In this work, difluorinated benzothiadiazole (SZ) units are introduced into PM7 to synthesize terpolymers. By the SZ units and paired π-bridges, the favorable molecular stacking and active layer morphology can be obtained, which contributes to FF and PCE improving from 61% and 13% of PM7-based OSCs to 71% and 15% of the terpolymer-based OSCs, respectively. Quantum chemistry optimizations reveal that additional π-bridges can adjust the localized planar conformation between the donor and acceptor units. The reduced density gradient function and the independent gradient model approach suggest that different additional π-bridges can promote their effective stacking of SZ units by balancing intra-molecular attraction and repulsion and enhancing intermolecular interactions. This work demonstrates the practicality of terpolymer design and paves a way to improve molecular stacking and morphological properties for efficient OSCs.