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 p-p 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.