Fluorination and chlorination effects on the charge transport properties of the IDIC non-fullerene acceptor: an ab-initio investigation

Abstract

Fused-ring electron acceptors end-capped with electron withdrawing groups have contributed to the ever-increasing power conversion efficiency of organic solar cells. Adding π-extensions and halogenating the end groups are two popular strategies to boost performance even further. In this work, a typical non-fullerene acceptor molecule, IDIC, is used as a model system for investigating the impact of the halogenation approach at the molecular level. The two end groups are substituted by fluorinated and chlorinated counterparts and their electronic and optical properties are systematically probed using ab-initio calculations. In gas phase, halogenation lowers the HOMO and LUMO energy levels and narrows the energy gap, especially for the chlorinated compound. Moreover, chlorinated IDIC exhibits the largest redshift and the smallest reorganization energy. Finally, crystal structures of the three compounds are constructed, revealing an improved transfer integral and transfer rate for the halogenated variants. Specifically, the chlorination strategy leads to an increase of 60% in transfer rate, compared to halogen-free IDIC.