Efficient As-Cast Polymer Solar Cells with High and Stabilized Fill Factor

Molecular ordering and miscibility of donor and acceptor materials play critical roles in developing high-performance as-cast polymer solar cells (PSCs). In this work, a highly crystalline nonfullerene small molecular acceptor, namely, C8-IT-4F, based on alkylated indacenodithieno[3,2-b]thiophene as the aromatic core and 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile moieties as end groups, is selected and synthesized. The π–π stacking distance in C8-IT-4F film can be tuned from 3.88 Å to a more compact (3.48 Å) state by a film-formation process and the confinement induced by the preaggregated polymer donor PM7, leading to broadened absorption and fine phase separation in the blend film. The optimal morphology with a framework of preaggregated polymer donor, J-type face-on π–π stacked acceptor, and appropriate donor/acceptor miscibility facilitates charge generation and transport and reduces charge recombination. As a result, the best PSC based on the as-cast PM7:C8-IT-4F blend film exhibits power conversion efficiency of 14.3%, with an open-circuit voltage of 0.82 V, a short-circuit current density of 22.7 mA cm⁻², a fill factor of 77.1%, and good photostability with a stabilized fill factor.