Synthesis and Characterization of New Low-Bandgap Diketopyrrolopyrrole-Based Copolymers

Eleven new low bandgap diketopyrrolopyrrole-based copolymers have been prepared by Suzuki or Stille cross-coupling polycondensation reactions. Comonomers derived from thiophene, carbazoles, fluorelle, dibenzosilole, and dithienylsilole have been investigated. The structural, thermal, optical, and electrochemical properties of all resulting copolymers have been characterized. These copolymers exhibit broad absorption extending into the near-infrared region with absorption maxima near 640-710 nm and optical band gaps ranging from 1.2 to 1.6 eV. HOMO energy levels of the copolymers vary between -5.6 and -5.2 eV whereas the LUMO energy levels are pinned between -3.9 and -3.8 eV. The combination of extending absorption into near-infrared region, optimal energy levels, and excellent mechanical and thermal properties makes this class of low bandgap copolymers very promising for photovoltaic applications.