Molecular Order Control of Non-fullerene Acceptors for High-Efficiency Polymer Solar Cells

Adjusting molecular ordering, orientation, and nanoscale morphology within the photoactive layer of polymer:non-fullerene organic solar cells is crucial in achieving high power-conversion efficiency (PCE). Herein, we demonstrate that the molecular ordering and orientation of the n-type small-molecule acceptor COi8DFIC can be tuned from flat-on and edge-on lamellar crystalline to H- and J-type π-π stacking during the solution-casting process, resulting in broadened photon absorption and fine phase separation with the electron donor PTB7-Th. This favorable morphology with face-on π-π stacked electron donors and acceptors promotes efficient exciton dissociation at the donor/acceptor interface, together with enhanced and balanced carrier mobility. The enhanced short-circuit current density and fill factor lead to the achievement of a maximum PCE of 13.8% in binary, single-junction PTB7-Th:COi8DFIC non-fullerene polymer solar cells while also exhibiting superior stability.