Synthesis and application of low band gap broad absorption oligomers based on 2,5-bis(2-thienyl)-N-arylpyrrole for bulk heterojunction solar cells

Two new alternating oligomers, poly[5,5′-(1-(2,6-diisopropylphenyl)-2,5-di(2-thienyl)pyrrole)-alt-4,7-(2,1,3-benzothiadiazole)] (TPyTBz-1) and poly[5,5′-(1-(4-octylphenyl)-2,5-di(2-thienyl)pyrrole)-alt-4,7-(2,1,3-benzothiadiazole)] (TPyTBz-2), were synthesized and characterized. Thermogravimetric analysis (TGA) showed that the oligomers are thermally stable, with the 5% weight loss temperature of over 345 °C. The absorption spectra of the oligomers were found to be quite broad from 300 nm to 800 nm, and the optical band gaps were calculated from the onset wavelength of the optical absorption in film to be 1.49 eV and 1.60 eV, respectively. The electrochemical studies of the oligomers showed that the energy levels are suitable for bulk heterojunction (BHJ) solar cell applications. Under 100 mW/cm2 of AM 1.5 white-light illumination, the photovoltaic properties of BHJ solar cell devices containing an active layer of electron donor oligomer mixed with electron acceptor [6,6]-phenyl C70-butyric acid methyl ester (PC70BM) at different weight ratios were investigated. The solar cell device containing TPyTBz-1:PC70BM (1:5 wt%) as an active layer was found to show the best power conversion efficiency (PCE) of 1.30% with an open-circuit voltage of 0.45 V, a short-circuit current of 9.4 mA/cm2, and a fill factor of 0.31.

Low band gap oligomers utilizing electron rich 2,5-bis(2-thienyl)-N-arylpyrrole unit and electron deficient benzothiadiazole unit were prepared and applied to bulk heterojunction solar cells.Figure optionsDownload as PowerPoint slideHighlights
► Two new low-band gap oligomers were prepared.
► The absorption of the oligomers was in the range of 300–800 nm in film.
► The optical bad gaps of the oligomers were 1.49 eV and 1.60 eV, respectively.
► The oligomers were applied to bulk heterojunction solar cells.