A novel isomer of pyrrolo[3,4-c]pyrrole-1,4 (2H,5H)-dione as a new building block for polymer solar cells

The synthesis of a novel electron-deficient derivatives of pyrrolo[3,4-c]pyrrole-1,4 (2H,5H)-dione, (DPP), 1,4-bis-(4-butyl-phenyl)-3,6-di- thiophen-2-yl-pyrrolo[3,2-b]pyrrole-2,5 - (1H,4H)-dione and 1,4-bis-(4- (pentyloxy)phenyl)-3,6-bis(thiophen-2-yl-pyrrolo[3,2-b]pyrrole-2,5-(1H,4H) -dione are reported. This new building block was copolymerized with bistannanes of thiophene and bithiophene and fluorene by using Stille or Suzuki polymerization, affording the corresponding copolymers (P1-P4, respectively). These exhibit small energy band gaps combined with low-lying HOMO energy levels. Energy band gaps of P1 and P2, calculated from absorption spectra, are 1.63 and 1.73 eV, respectively. The HOMO and LUMO energy levels of P1 and P2 are -5.12, -3.50, -5.09 and -3.50 eV respectively, as determined by cyclic voltammetry. The power conversion efficiency of P1:PC₆₁BM-based photovoltaic cells illuminated by AM 1.5G was 1.24%, without optimization of materials, significantly higher than for P2:PC₆₁BM, 0.33%. These results demonstrate that thiophene-pyrrolo[3,2-b]pyrrole-2,5-dione conjugated copolymers are promising materials for bulk heterojunction solar cell applications.