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Keywords:

  • atomic force microscopy (AFM);
  • conjugated polymers;
  • degree of polymerization (DP);
  • fullerenes;
  • gel permeation chromatography;
  • low bandgap;
  • polymer solar cells;
  • power conversion efficiency;
  • push–pull type copolymer;
  • quinoxaline

ABSTRACT

Low bandgap polymers with dithienylquinoxaline moieties based on 6H-phenanthro[1,10,9,8-cdefg]carbazole were synthesized via the Suzuki coupling reaction. Alkoxy groups were substituted at two different positions on the phenyl groups of the quinoxaline units of these polymers: in the para-position (PPQP) and in the meta-position (PPQM). The two polymers showed similar physical properties: broad absorption in the range of 400–700 nm, optical bandgaps of ∼1.8 eV, and the appropriate frontier orbital energy levels for efficient charge transfer/separation at polymer/PC71BM interfaces. However, the PPQM solar cell achieved a higher PCE due to its higher Jsc. Our investigation of the morphologies of the polymer:PC71BM blend films and theoretical calculations of the molecular conformations of the polymer chains showed that the polymer with the meta-positioned alkoxy group has better miscibility with PC71BM than the polymer with the para-positioned alkoxy group because the dihedral angle of its phenyl group with respect to the quinoxaline unit is higher. This higher miscibility resulted in a polymer:PC71BM blend film with a better morphology and thus in a higher PCE. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 796–803