Regulation of photoinduced charge transfer in all-small-molecule organic solar cells through the synergistic effect of external electric field and solvent

The donor (BTR-Cl) and acceptor (BTP-FCl-FCl) have well-defined small molecule properties and excellent repeatability, and they can form charge transfer complexes with a wide spectral absorption range. Using density functional theory (DFT), we simulate the photoinduced charge transfer of bulk-heterojunction (BHJ) organic solar cell (OSC) materials modulated by the external electric field (Fext) at the microscopic level. The excited-state properties, reorganization energy (λ), Gibbs free energy (ΔG), electron coupling matrix elements (VDA) and intermolecular charge transfer (ICT) rate dependent on Fext are systematically analyzed. The results manifest that Fext has apparent positive regulation on the charge separation rate (KCS), and VDA is the main factor affecting KCS. The synergistic effect of solvent and Fext on charge transfer process is further investigated. It is found that the charge transfer rate in solvent is lower than that in solvent-free conditions, and the charge transfer rate in chlorobenzene (CB) solvent does not change significantly under the control of Fext. At the same time, Fext has a positive effect on the charge transfer rate in tetrahydrofuran (THF) solvent. Considering the charge transfer rate under different solvents and Fext intensities, it is found that the solvent and Fext play an essential role in determining the KCS and charge recombination rate (KCR).