The ratio and topology effects of benzodithiophene donor–benzooxadiazole acceptor fragments on the optoelectronic properties of donor molecules toward solar cell materials

Abstract

A series of conjugated donor molecules (D_mA_nS_q where m = 1–4, n = 1–7 while D = benzodithiophene, A = benzooxadiazole and S denotes ethyne spacers between D and A or D and D fragments) with various ratios of D/A fragments and topologies have been designed and investigated for OPV applications. An increase in the ratio of the acceptor fragment with respect to the donor fragment decreases the LUMO energy level and narrows the E_g for the designed molecule. More vertically (C4 and C8 substituted phenyl ring positions) bonded acceptor fragments than linearly (C2 and C6 substituted thiophene ring positions) bonded fragments result in a significant red shift in the maximum absorption wavelength. While, linearly bonded fragments lead to stronger absorption bands. Molecules with D–A–D topology exhibit more significant optical and electronic characteristics than those with D–D topology. All donor molecules (m = 2–4) of the D–A–D type show lower λ_h values than those of 1 donor containing (DA_n) molecules. D–D type molecules show only lower λ_e values than DA_n molecules because of the presence of a second donor fragment. The charge transfer phenomenon is shape dependent. The branched or anisotropic X, H, π, n, and square shaped molecules display higher charge transfer rates than the corresponding linear isomers due to better dimensionality. On the basis of these results, we suggest that designed donor and corresponding matched acceptor molecules have potential to act as promising candidates in solar cell devices.