Effects of monohalogenated terminal units of non-fullerene acceptors on molecular aggregation and photovoltaic performance

Three small molecule acceptors (SMAs), named ZF, ZC and ZB, have been designed and synthesized for organic solar cells (OSCs) by introducing single F, Cl and Br atom on each terminal unit, respectively. The effects of different halogen substituent on molecular aggregation, photophysical and photovoltaic performance have been systematically studied. Owing to its strong electronegativity of halogens, all the acceptors with monohalogenated terminal units possess red-shifted absorption spectra and deeper frontier energy levels compared to the corresponding acceptor without halogen substituents reported in the literature. Among the SMAs, ZB with brominated terminal units was found to show the higher molar absorption coefficient (2.31 × 10⁵ M⁻¹ cm⁻¹), more orderly face-on π-π stacking, higher electron mobility and more favorable morphology when blended with PM6. As a result, the PM6:ZB OSCs yielded a high power conversion efficiency (PCE) of up to 15.23% with a high short-circuit current density (J_sc) of 26.38 mA cm⁻², while the corresponding ZF- and ZC-based devices showed the relatively inferior PCEs of 13.36% and 14.71%, respectively. These results demonstrated that the modulation of electron-withdrawing halogen substituents on terminal group provides a promising strategy to design and synthesize efficient SMAs for fabricating high-performance OSCs.