Impact of Terminal Halogen and CN Substitutions on Photoelectric Properties of Asymmetric Y6-Based NFA with Terminal Groups in Different Orientations: A DFT/TDDFT Study.

Nonfullerene acceptors (NFAs) with an acceptor-donor-acceptor-donor-acceptor (A-DA'D-A) molecular framework have attracted much attention due to their excellent performance. However, the modifications of terminal units of asymmetric Y6-based NFA with terminal groups of different orientations are still few, and its effects on photoelectrical properties are still not clear. In this work, based on asymmetric IPC-BEH-IC2F (showing better performance than Y6 in experiment) with terminal groups in different orientations, we systematically designed six new NFAs via halogen and CN substitutions on terminal groups. The molecular planarity, dipole moments, electrostatic potential maps and their fluctuations, frontier molecular orbitals, exciton binding energy, UV-vis spectra, and energy difference between the first singlet and triplet states of these NFAs are predicted using reliable density functional theory (DFT) and time-dependent DFT (T-DFT) calculations. The results show that with respect to prototype CN-F, Br-F, CN-Br, and CN-Cl exhibit comparable energy levels of the lowest unoccupied molecular orbital (LUMO), reduced energy gap (by at least 0.026 eV), E_b (by at least 0.002 eV), and ΔE_ST (by at least 0.009 eV) values, red shifts (by at least 2 nm) in the wavelengths of the main absorption peaks, and enhanced absorption (by at least 0.05 in total oscillator strength) in the visible to near-infrared regions, indicating their potential as outstanding asymmetric NFAs. This study offers valuable insights into the future design and optimization of NFAs featuring asymmetric terminal groups.