Asymmetrical Small Molecule Acceptor Enabling Nonfullerene Polymer Solar Cell with Fill Factor Approaching 79%

Relative to the increase of open-circuit voltage and short-circuit current, promoting fill factor (FF) of the polymer solar cells (PSCs) seems to be more challenging. Here, we designed and synthesized two asymmetrical small molecule acceptors (IDT6CN-M and IDT8CN-M) with large dipole moments. We find that the strong intermolecular interaction and favorable antiparallel packing induced by the dipole moment can effectively enhance both lamellar packing and π–π stacking. The PSCs based on PBDB-T:IDT6CN-M and PBDB-T:IDT8CN-M achieved FFs of up to 76.1% and 78.9%, corresponding to PCEs of 11.23% and 12.43%, respectively. To the best of our knowledge, 78.9% FF is a new record for nonfullerene PSCs. Overall, our work provides a simple and effective molecule-designing method to promote FF of nonfullerene PSCs.