Synthesis, photophysics, and photovoltaic properties of low-band gap conjugated polymers based on thieno[3,4-c]pyrrole-4,6-dione: a combined experimental and computational study

Four novel donor-acceptor (D-A) alternating copolymers were designed and successfully synthesized by the palladium-catalyzed Stille coupling and Suzuki coupling reactions. Utilizing thieno[3,4-c]pyrrole-4,6-dione (TPD) as an acceptor comonomer coupled with dialkoxybithiophene or cyclopentadithiophene as the donor gave polymers PTBT and PTCT. Employing carbazole as the donor and the dithiophene-substituted TPD serving as the acceptor monomers yielded polymers PTC1 and PTC2. Owing to the various strengths of electronic coupling between the donors and the acceptor unit, the band gaps of these polymers can be adjusted from 1.57 to 1.90 eV. Due to the different electron-donor ability of dialkoxybithiophene, cyclopentadithiophene, and carbazole, the HOMO energy levels of polymers were tuned from -5.34 to -5.67 eV, while LUMO levels remained relatively unchanged. The theoretical calculations provided insight to the observed photophysical properties of these polymers. Theoretically estimated band gaps and oxidation potentials correlate well with the experimental data. Carrier mobility and photovoltaic properties of TPD polymers were also investigated for which 1.3% power conversion efficiency was obtained from a blend of PTCT:PC71BM (1 : 2) bulk-heterojunction device.