Synergistic Effects of Fluorination and Alkylthiolation on the Photovoltaic Performance of the Poly(benzodithiophene-benzothiadiazole) Copolymers

Atom functional modifications (e.g., fluorination and alkylthiolation) in conjugated building blocks, have attracted a lot of attention due to their promising potential in finely tuning optical-electronic properties. To have a better understanding of the influence of polymeric fluorinated and alkylthiolated effects on photovoltaic performance, here, four low band gap benzothiophene-benzothiadiazole (BDT-BT) copolymers, PB-BT, PB-FBT, PBS-BT, and PBS-FBT, featuring a step-by-step approach to using fluorination and alkylthiolation were designed and synthesized. Their optical and electronic properties, crystallinity, and the corresponding photovoltaic performance in bulk heterojunctions (BHJ) with fullerene as acceptor were systematically investigated. First, it is found that the fluorination in BT moieties offers several advantages including lowering the highest occupied molecular orbital (HOMO) levels, strengthening π-π stacking, and improving thermal stability. Second, alkylthiolation in the BDT moiety shows weak effects in modulating the polymeric HOMO levels and the crystallinity. Finally, it is interesting to note that the alkylthiolation added to fluoro-substituted copolymer synergistically strengthens the ordered π-π stacking and improves its crystallinity. Subsequently, superior photovoltaic performance was rendered in PBS-FBT based devices due to reasonable phase separation and enhanced fibrous morphology, which was characterized by utilizing a combination of atomic force microscopy (AFM), transmission electron microscope (TEM), grazing-incidence wide-angle X-ray scattering (GIWAXS), and other microscopy measurements. This work provides important insights into designing high-performance photovoltaic polymers by fluorinated and alkylthiolated modifications.