Exploring the photovoltaic properties of promising non-fullerene acceptors with different degrees of asymmetry due to halogenations of terminal groups

[Display omitted] •Fluorinations exacerbate A-D-A type molecular asymmetry and can improve the light absorption.•Br-F2 and IM series have potential as novel asymmetric NFAs. Over the past few years, the strategy of asymmetric modification has become popular for designing new photovoltaic materials because it can effectively improve optoelectronic performance and morphology, therefore power conversion efficiency (PCE). However, how the halogenations (to further change asymmetry) of terminal groups (TGs) of an asymmetric small-molecule non-fullerene acceptor (Asy-SM-NFA) influence optoelectronic properties is still not very clear. In this work, we have selected a promising Asy-SM-NFA IDTBF (the OSC based on it has a PCE of 10.43 %), exacerbated the asymmetry through fluorinations of TGs, and finally designed six new molecules. Based on density functional theory (DFT) and time-dependent DFT calculations, we systematically examine how the changed asymmetry impacts the optoelectronic properties. We find that the halogenations of TGs may significantly affect the molecular planarity, dipole moment, electrostatic potential, exciton binding energy, energy loss, and absorption spectrum. And the results show that newly designed BR-F1 and IM-mF (m = 1,3, and 4) are potential Asy-SM-NFAs because they all have enhanced absorption spectra in the visible region. Therefore, we provide a meaningful direction for the design of asymmetric NFA.