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Polythiophenes bearing electron-withdrawing groups in the side chain and their application to bulk heterojunction solar cells



Soluble polythiophenes bearing strong electron withdrawing groups, dicyanoethenyl [[BOND]CH[DOUBLE BOND]C(CN)2] (PTDCN) and cyano-methoxycarbonylethenyl [[BOND]CH[DOUBLE BOND]C(CO2Me)CN] (PTCNME), in the side chains have been prepared. Optical band gaps calculated from onset absorption were 1.70 eV and 1.73 eV for PTDCN and PTCNME, respectively. Highest occupied molecular orbital energy levels measured with a surface analyzer (AC-2) were −5.53 eV and −5.29 eV for PTDCN and PTCNME, respectively, which were much lower than that of poly(3-hexylthiophene) (−4.81 eV). To investigate photovoltaic properties, bulk heterojunction polymer solar cells based on PTDCN and PTCNME were fabricated with a structure of ITO/PEDOT:PSS/active layer/LiF/Al, where the active layer was a blend film of polymer and [6,6]-phenyl C61 butyric acid hexyl ester (PC61BH). Solar cell parameters were estimated from current density–voltage (JV) characteristics under the illumination of AM1.5 at 100 mW/cm2. The solar cell based on the blend film of PTCNME:PC61BH (1:1) showed power conversion efficiency (PCE) of 0.72% together with the open current voltage (Voc) of 0.61 V, the short current density (Jsc) of 3.90 mA/cm2, and the fill factor of 30.3%. The PCE of a solar cell fabricated from PTDCN in a similar way was 0.56%. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010