Effect of fluorine substitution in organoboron electron acceptors for photovoltaic application

In this paper, we report the effect of fluorine substituents on the opto-electronic properties and photovoltaic device performances of organoboron electron acceptors. We synthesized two new organoboron electron acceptors with four fluorine substituents introduced to either the organoboron core unit or the endcapping groups. Owing to the unique electronic structures of organoboron electron acceptors, fluorine substituents at the organoboron core unit lead to a downshifted HOMO level and blue-shifted absorption spectrum, and fluorine substituents at the endcapping groups lead to a downshifted LUMO level and red-shifted absorption spectrum. In both cases, the fluorine substituents result in improved crystallinity of the organoboron electron acceptors. In polymer solar cell (PSC) devices, while the fluorine substituents at the organoboron core unit do not significantly affect the active layer morphology and the photovoltaic performance, the fluorine substituents at the endcapping groups result in large-size phase separation in the active layer and consequently decrease photovoltaic efficiency. These results suggest that the opto-electronic properties and device behaviors of organoboron compounds can be rationally tuned by fluorine substitution.