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, NO, OH, SH and NH groups were investigated by PBC-DFT method. The difference between the ground and excited-states dipole moment (Δμ) 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 are observed in some derivatives. The calculation results demonstrate that the substitution of COH and NO by fluorine in BDF-TT and BDP-TT leads to higher maximum theoretical efficiencies (η).