Theoretical Study on the Central Fused-Ring Modification of Nonfullerene Acceptors for Organic Solar Cells

Molecular structure modification of nonfullerene acceptors (NFAs) is an important approach to modulate optoelectronic properties and to improve power conversion efficiency of organic solar cells (OSCs). Herein, to deeply understand the influence of central-fused ring halogenation and extension on photoelectric properties and photovoltaic performances, we selected a series of NFAs with different central fused rings, including CH-6F, CH-20, CH-22, CH-23, CH-45, CH-BQ, CH-iBQ, and CH-BBQ, combined with electron donor PM6. Based on quantum chemical calculations, the geometric structures, electronic structures, excitation properties, and absorption spectra were systematically studied for the PM6 and NFA molecules, as well as the corresponding interface model complexes. Moreover, the rate constants of the electronic processes were discussed. The results indicate that the peripheral halogen substitution on the central fused ring can effectively enhance molecular backbone planarity, shrink dipole moment, lower the HOMO and LUMO energies with widening energy gap, induce blue shift of optical absorption, increase excitation energy and average electrostatic potential (ESP), and shorten excited-state lifetime. Extending conjugation of the central fused ring not only enhances molecular backbone planarity, lowers the HOMO and LUMO energies, and increases the average ESP and charge-transfer excitation energies but also reveals the importance of the fusing style for extending conjugation. This work unravels the trilateral relationship among molecular structures, properties, and photovoltaic performance of NFAs and provides foundations to design more efficient NFAs for OSCs.