Molecular engineering of donor–acceptor co-polymers for bulk heterojunction solar cells

We report a computational modeling, based on DFT and TD-DFT methodologies, on the structural, electronic, and optical properties of different donor–acceptor co-polymer system in bulk heterojunction solar cells. The donor moieties that were considered were the derivatives of thienocyclopentathiophene, fluorene, and thienobenzothiophene. We utilized for the acceptor groups the moieties of thieno[3,4-b]pyrrole-4,6-dione; thieno[3,4-b] thiophene-4,6-dione; tetrafluoro-1,3-dihydrothieno[3,4-c]thiophene and its non-fluorinated counterpart; various electron-donating substituents within the fused π-conjugated polymer system; and 3-fluoroselenophenothiophene. Among the donor–acceptor combination, the best in terms of molecular energy levels, energetic driving force, maximum absorption, calculated open-circuit voltage, reorganization energies, ionization potential, and electron affinity are poly[(bisthieno(bisthieno[3,2-b:2′,3′-d]thiophene)- alt-(5-alkyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione)] and poly[(4,8-dimethyl[1,3] oxazolo[5,4-f][1,3]benzoxazole)- alt-(thieno[3,4-d]pyrimidine)].