Novel conjugated alternating copolymer based on 2,7-carbazole and 2,1,3-benzoselenadiazole

A novel conjugated alternating copolymer (PCzDBSe) based on N-9′-heptadecanyl-2,7-carbazole and 5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzoselenadiazole) was synthesized by Suzuki polycondensation. The polymer reveals excellent thermal stabilities with the decomposition temperature (5% weight loss) of 390 °C and the glass-transition temperature of 140 °C. The absorption peaks of the polymer are located at 412 and 626 nm, respectively, while the absorption onset is extended to 716 nm, which is 56 nm red-shifted as compared with its analogue, polyN-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT). The HOMO and LUMO levels of the polymer were estimated to be −5.28 and −3.55 eV, respectively, with an optical bandgap of 1.73 eV. The hole mobility of PCzDBSe as deduced from a solution-processed organic field effect transistor (OFET) was found to be 3.9 × 10⁻⁴ cm² V⁻¹ s⁻¹. Polymer solar cells (PSCs) based on the blends of PCzDBSe and 6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) with a weight ratio of 1:4 were fabricated. Under AM 1.5 (AM, air mass), 100 mW cm⁻² illumination, the devices were found to have an open-circuit (V_oc) of 0.75 V, a short-circuit current density (J_sc) of 7.23 mA cm⁻², a fill factor (FF) of 45% and a power conversion efficiency (PCE) of 2.58%. The primary results indicate that 5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzoselenadiazole) is a promising unit for low bandgap polymer for polymer solar cells.