Fluorination vs. chlorination: a case study on high performance organic photovoltaic materials

Halogenation is a very efficient chemical modification method to tune the molecular energy levels, absorption spectra and molecular packing of organic semiconductors. Recently, in the field of organic solar cells (OSCs), both fluorine- and chlorinesubstituted photovoltaic materials, including donors and acceptors, demonstrated their great potentials in achieving high power conversion efficiencies (PCEs), raising a question that how to make a decision between fluorination and chlorination when designing materials. Herein, we systemically studied the impact of fluorination and chlorination on the properties of resulting donors (PBDB-T-2F and PBDB-T-2Cl) and acceptors (IT-4F and IT-4Cl). The results suggest that all the OSCs based on different donor and acceptor combinations can deliver good PCEs around 13%-14%. Chlorination is more effective than fluorination in downshifting the molecular energy levels and broadening the absorption spectra. The influence of chlorination and fluorination on the crystallinity of the resulting materials is dependent on their introduction positions. As chlorination has the advantage of easy synthesis, it is more attractive in designing low-cost photovoltaic materials and therefore may have more potential in largescale applications.