Density functional theory study of donor–acceptor conjugated polymers with substituent effect

In the field of organic photovoltaics, a full electron donor (D) and deficient electron acceptor (A) are used to prepare fragments of conjugated polymers called D–A-type polymers, the efficiency of which is fundamentally affected by the substituents. In this study, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed to elucidate the effects of substituents (methoxy (–OCH3) and fluorine (–F)) on an ethyl 4-(2-ethoxyacetyl)-5-(3-(2-ethylhexyl)-[2,2':5',2''-terthiophen]-5-yl)-2-(4-(2-ethylhexyl)thiophen-2-yl)benzoate polymer. Energy levels, optical absorption, electron–hole distribution, and charge transfer were then calculated on the basis of the optimized structure with different substituents, ranging from monomer to tetramer, and periodic boundary conditions. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels from DFT calculations were in good agreement with the experimental oxidation and reduction potentials. In conclusion, this study demonstrated that –OCH3 helps increase the HOMO energy level and induces a decrease in the band gap; moreover, it can enhance intramolecular charge transfer and the concentration of electrons. The functionalized polymer is suitable for use in the active layer of organic photovoltaics, and functionalization is an effective approach for improving electron transfer.