Thick-Film Organic Solar Cells Achieving over 11% Efficiency and Nearly 70% Fill Factor at Thickness over 400 nm

Thickness-insensitive small molecule acceptors (SMAs) are still a great challenge for developing thick-film organic solar cells (OSCs) towards practical use. Herein, two SMAs, MF1 and MF2, are designed and synthesized by employing a bifunctional end group with fluorine and methyl moieties. Combined with fused-ring cores with alkyl side chains, both MF1 and MF2 exhibit ordered pi-pi stacking and high charge carrier mobilities in neat and blend films. The champion devices based on PM7:MF1 and PM7:MF2 deliver high power conversion efficiencies (PCEs) of 12.4% and 13.7%, and high fill factors (FFs) of 78.3% and 74.5%, respectively. With increasing active layer thickness, the FFs of the OSCs decrease relatively slowly, demonstrating the preferrable properties of MF1 and MF2 in terms of their thickness insensitivity, especially for MF1. As a result, the two thick-film OSCs achieve over 11% PCEs at an active layer thickness over 400 nm (an FF close to 70% for PM7:MF1) and over 10% PCEs when the thickness is increased up to 500 nm. These are the highest PCEs among OSCs with such active layer thicknesses to date. This work reveals a molecular design strategy by reasonably combining fluorine and methyl together to simultaneously enhance charge carrier mobilities and fine-tune the morphology, which is beneficial to achieve high-performance thick-film OSCs.