Theoretical study of thieno-thiophene based low band gap copolymers and substituent effect on the optoelectronic properties of them

This paper studies donor-acceptor systems which incorporate benzodithiophene (BDT), benzodifuran (BDF) and benzodipyrrole (BDP) units as the electron-rich monomer with TT unit representing the electron-deficient monomer. This research is based on employing density functional theory (DFT) and time-dependent DFT (TD-DFT). The highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO), HOMO-LUMO gaps and dihedral-angles of these copolymers were calculated using oligomer extrapolation technique and periodic boundary condition (PBC) method. The optical band gaps and UV-vis absorption spectra of aforementioned copolymers were obtained by TD-DFT at the same level of theory. Based on the fair agreement between PBC-DFT calculated results and experimental data, the substituent effects of Cl, Br, CCH, COH, NO₂, OH, SH and NH₂ groups were investigated by PBC-DFT method. The difference between the ground and excited-states dipole moment (Δμ_ge) of all derivatives were also calculated. Taking these results into account, a better understanding of the substituent effects on the photo-physical properties of the copolymers under study was achieved. Due to the shift of HOMO and LUMO energy levels, smaller band gaps and higher Δμ_ge are observed in some derivatives. The calculation results demonstrate that the substitution of COH and NO₂ by fluorine in BDF-TT and BDP-TT leads to higher maximum theoretical efficiencies (η).