Benzooxadiazole-based donor/acceptor copolymers imparting bulk-heterojunction solar cells with high open-circuit voltages

In this study we used Suzuki cross-coupling to synthesize three new donor/acceptor copolymers—PFTBO, PAFTBO, and PCTBO—featuring soluble alkoxy-modified 2,1,3-benzooxadiazole (BO) moieties as acceptor units and electron-rich building blocks—dialkyl fluorene (F), alkylidene fluorene (AF), and carbazole (C), respectively—as donor units. These polymers, which we characterized using gel permeation chromatography, thermogravimetric analysis, NMR spectroscopy, UV–Vis absorption spectroscopy, and electrochemical cyclic voltammetry, exhibited good solubility, low-lying energy levels for their highest occupied molecular orbitals, excellent thermal stability, and air stability. Using these polymers, we fabricated bulk-heterojunction solar cell devices having the structure indium tin oxide/poly(3,4-ethylenedioxythiophene):polystyrenesulfonate/polymer:[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) (1:1, w/w)/Ca/Al. Under AM 1.5G illumination (100 mW cm−2), the solar cell incorporating PFTBO exhibited a high value of Voc of 1.04 V and that based on PCTBO provided a power conversion efficiency of 4.1% without the need for any post treatment.