DFT investigation of fluorination sites effects on the photovoltaic performance of D-π-A donor materials.

CONTEXT: Fluorination is an effective strategy for precisely regulating the energy levels, molecular conformations, and optoelectronic properties of donor materials in organic solar cells (OSCs), thereby significantly impacting charge transport, light absorption, and device efficiency. In this work, three D-π-A-type fluorinated isomers -PBT-BDT-F, PffBT-BDT, and PffBT-BDT-FT- were theoretically investigated to elucidate the effects of different fluorine substitution positions on photovoltaic performance. The results demonstrate that fluorination on the donor unit enhances molecular planarity, light absorption, and charge transport to a greater extent than substitution on the acceptor unit or π-bridge, providing valuable guidance for optimizing fluorination sites in D-π-A type donor materials.

METHODS

Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed using Gaussian 09. Ground-state geometries were optimized at the B3P86/6-31G(d) level, and excited-state properties were calculated at the PBE1PBE/6-31G(d) level in CHCl₃ solvent with the CPCM solvation model. Molecular geometries, spectral properties, electrostatic potentials, exciton binding energies, reorganization energies, and charge transfer rates were systematically calculated to evaluate the photovoltaic performance of the three isomers. Molecular visualizations were generated using GaussView 6.0, VMD, and Multiwfn.