Synthesis and photovoltaic properties of low-bandgap 4,7-dithien-2-yl-2,1,3-benzothiadiazole-based poly(heteroarylenevinylene)s

Abstract

Three novel low-bandgap copolymers containing alkylated 4,7-dithien-2-yl-2,1,3-benzothiadiazole (HBT) and different electron-rich functional groups (dialkylfluorene (PFV-HBT), dialkyloxyphenylene (PPV-HBT) and dialkylthiophene (PTV-HBT)) were prepared by Horner polycondensation reactions and characterized by 1H NMR, gel permeation chromatography, and elemental analysis. The alkyl side chain brings these polymeric materials good solubility in common organic solvents, which is critical for the manufacture of solar cells in a cost-effective manner. The copolymers exhibit low optical bandgap from 1.48 to 1.83 eV. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the copolymers were measured by cyclic voltammetry. Theoretical calculations revealed that the variation laws of HOMO and the LUMO energy levels are well consistent with cyclic voltammetry measurement. The bulk heterojunction photovoltaic devices with the structure of ITO/PEDOT-PSS/polymer:PCBM/LiF/Al were fabricated by using the three copolymers as the donor and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor in the active layer. The device based on PTV-HBT:PCBM (1:4 w/w) achieved a power conversion efficiency of 1.05% under the illumination of AM 1.5, 100 mW/cm2. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011.