Fluorinated fused azobenzene boron-based polymer acceptors with 1000 nm absorbance edges for all-polymer solar cells

Fluorinated fused azobenzene boron (FBAz) is a novel electron-deficient building block for polymer electron acceptors in all-polymer solar cells (all- PSC). The B←N bridging units impart a fixed configuration and low-lying LUMO/HOMO energy. Three polymer acceptor materials (P2f, P3f and P5f) with different fluorine substitution positions by copolymerizing FBAz with indacenodithiophene (IDT), are synthesized and investigated to study the influence of fluorinated forms on the all-polymer solar cell performance. The FBAz units are synthesized in just three steps, facilitating the straightforward production of polymer acceptors P2f, P3f, and P5f. These acceptors exhibit strong light absorption in the visible to near-infrared range of 500–1000 nm and possess suitable LUMO/HOMO energy levels of -3.99/-5.66 eV which are very complementary to that (E_LUMO/HOMO = -3.59/-5.20 eV) of the widely-used polymer donor poly[(ethylhexylthiophenyl)-benzodithiophene-(ethylhexyl)-thienothiophene] (PTB7-Th). All-polymer solar cells (all-PSCs) with PTB7-Th as electron donor and P3f as electron acceptor exhibits highest power conversion efficiencies (PCE) 2.70 %. When PC₆₁ BM is added as the third component, the device efficiency can reach 5.36%. These preliminary results indicate that FBAz is a promising strong electron acceptor for the development of n-type polymer semiconductors, especially in organic photovoltaics (OPVs). To the best of our knowledge, this is the first example demonstrating the unique photovoltaic properties of the N=N double bond as an acceptor material.