Fluorination with an enlarged dielectric constant prompts charge separation and reduces bimolecular recombination in non-fullerene organic solar cells with a high fill factor and efficiency > 13%

Fluorination of molecular donors and acceptors enables an effective tuning of the energetics in organic bulk heterojunctions, leading to enhanced power conversion efficiencies in organic solar cells with non-fullerene acceptors. To date, the impacts of fluorination on the dielectric properties and the key processes of charge dissociation and recombination are still poorly understood. Based on the fluorinated PBDBT-2F donor and IT-4F acceptor with a favorably enhanced dielectric constant in the blend films, we explore key factors attributing to the realized high fill factor (0.745) and efficiency (13.4%) in the solar cells. Through comparing the IT-4F solar cell to that with non-fluorinated ITIC acceptor, we identify distinct recombination profiles in the IT-4F based device that exhibits a highly efficient charge dissociation together with a small reduction factor (3.71 x 10(-2)) for bi-molecular recombination and a low recombination coefficient of 2.404 x 10(-13) cm(-3) s(-1). These particularities lead to an ultrafast carrier sweepout < 30 ns with mitigated recombination losses in the device with fluorinated acceptor. The modulated photophysical processes are correlated to the increased dielectric constant in the blend film, with which charge transfer exciton binding energies are likely to be reduced. This work enriches our fundamental insights into molecular engineering with fluorination toward high performance non-fullerene organic solar cells.