A low bandgap polymer based on isoindigo and bis(dialkylthienyl)benzodithiophene for organic photovoltaic applications

A new conjugated polymer PBDTT-ID based on N-alkylated isoindigo (ID) and bis(2,3-dialkylthienyl)-substituted benzo[1,2-b:4,5-b']dithiophene (BDTT) as repeating units was synthesized. It had an optical bandgap of 1.56 eV and a highest occupied molecular orbital (HOMO) energy level of -5.71 eV. The optical, electrochemical, and photovoltaic properties of new polymer were compared with previous reported polymer PBDT-ID, which was based on bis(alkoxy)-substituted benzo[1,2-b:4,5-b']dithiophene. The new polymer displayed lower HOMO energy level and better absorption properties than polymer PBDT-ID. The solar cells fabricated with PBDTT-ID/PC61BM (1:2, w/w) blends as active layers exhibited photoresponse in the range of 300-800 nm. A power conversion efficiency of 4.02% and an open circuit voltage (Voc) of 0.94 V were achieved in polymer solar cell device based on the new polymer. This was the highest Voc realized among the isoindigo-based polymers. The relatively high performances of new polymer in solar cell devices were interpreted in terms of material properties and morphologies of polymer/PCBM blends. (C) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013