Theoretical design of donor-acceptor conjugated copolymers based on furo-, thieno-, and selenopheno[3,4-c] thiophene-4,6-dione and benzodithiophene units for organic solar cells.

In this work, a series of donor-acceptor (D-A) copolymers (PBDTFPD(Pa1), PBDTTPD (Pa2) and PBDTSePD(Pa3)) were selected and theoretically investigated using O3LYP/6-31G(d), PBE0/6-31G(d), TD-O3LYP/6-31G(d)//O3LYP/6-31G(d) and periodic boundary conditions methods. The calculated results go well with the available experimental data of highest occupied and lowest unoccupied molecular orbital (HOMO/LUMO) energy levels and band gaps. A series of conjugated polymers (Pb1 ~ Pb3) comprised of electron-deficient benzodithiophene and electron-rich furo-, thieno-, and selenopheno[3,4-c]thiophene-4,6-dione were further designed and studied. Compared with Pa1-Pa3, the designed polymers of Pb1 ~ Pb3 show better performances with smaller band gaps, lower HOMO energy levels, red shift of absorption spectra, and larger open circuit voltage (Voc). For investigated polymers (Pa1, Pa2, Pa3, Pb1, Pb2, Pb3), the power conversion efficiencies (PCEs) of ~6.1 %, ~7.2 %, ~7.9 %, ~8.0 %, ~9.5 % and ~9.0 % are predicted by Scharber diagrams when they are used in combination with PC60BM as an acceptor. The results illustrate that these designed polymers which turn the electron-withdrawing capability in D-A conjugated polymers are expected to turn into highly efficient donor materials for organic solar cells.