Synthesis and characterization of 2,1,3-benzoselenadiazole-based conjugated polymers for organic photovoltaic cells

Low-band-gap organic semiconducting polymers comprising two dialkoxy-substituted benzoselenadiazole derivatives and two BDT derivatives as the electron acceptors and donors, respectively, were synthesized via Stille coupling for application as a donor material in organic photovoltaic cells. These polymers are soluble in common organic solvents, and can be spin-cast to form smooth, uniform thin films. The polymers were thermally stable, i.e., lost < 5% of their weight upon heating to ~300 °C. Intramolecular charge transfer between the electron-donating and -accepting blocks of the polymeric backbone induced a broad absorption from 300 to 750 nm. The optical band gap energies were 1.74–1.67 eV depending on the structure of the polymer. Solution-processed field-effect transistors fabricated using these polymers as the active layers showed p-type organic-thin-film transistor characteristics. Bulk hetero-junction photovoltaic devices using the polymers with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the electron acceptor were fabricated; one device showed a high power conversion efficiency of 3.57% with an open-circuit voltage of 0.77 V, a short-circuit current of 11.03 mA/cm2, and a fill factor of 0.42 under AM1.5G illumination (100 mW/cm2).