A steric hindrance strategy facilitates direct arylation polymerization for the low-cost synthesis of polymer PBDBT-2F and its application in organic solar cells

Abstract

PBDBT-2F (PM6), a prominent member of the donor–acceptor (D–A) conjugated polymer family, has attracted considerable interest for application in organic solar cells (OSCs) due to its high efficiency and excellent universality. Traditionally, PM6 is synthesized via Stille coupling polymerization, which requires tedious pre-functionalization of monomers. In this study, we report an improved catalytic system for synthesizing PM6 samples through palladium-catalyzed direct arylation polymerization (DArP). Optimization of the reaction conditions revealed that introduction of the appropriately sterically hindered additive 2,2-diethylhexanoic acid (DEHA) is necessary to prepare polymer P-14, which exhibits the highest M_n, the lowest PDI, and the strongest aggregation. However, analysis of the polymer structure indicates that a small amount of branching defects is present in P-14, leading to weaker π–π stacking and a smaller phase separation domain. As a result, the P-14:IT-4F device achieved a moderate PCE of 8.90% with a higher V_oc of 0.893 V compared to the PM6-S-based device. Importantly, the development of catalytic system for DArP reactions without using amine ligands, further reduce preparation costs. This work confirmed the potential of the steric bulk strategy in improving molecular weight and suppressing defects while also revealing the importance of optimal catalytic conditions for preparing well-defined conjugated polymers.