Abstract
The ionization energy (IE) offset of a donor-acceptor pair provides the driving force for hole transfer and subsequent free charge carrier generation in low-bandgap nonfullerene organic solar cells (OSCs). However, the interfacial energetic landscape in bulk heterojunction OSCs is determined by the materials' electronic structure and intermolecular interactions at the donor/acceptor interface, causing local energy-level shifts and disorder. Herein, the impact of the IE offset on the charge transfer efficiency and charge carrier dynamics is systematically evaluated by characterizing PM6/ITIC, PM6/IT-2Cl, and PM6/IT-4Cl planar heterojunction (PHJ) solar cells. Ultrafast spectroscopy and time-resolved charge carrier density measurements reveal that an IE offset of about approximate to 0.5 eV leads to efficient hole transfer and subsequent free charge generation. Furthermore, bimolecular charge recombination and consequently triplet generation are significantly reduced in systems with high IE offset. This work underlines the importance of sizeable donor-acceptor IE offsets in PHJ nonfullerene OSCs as critical for high-efficiency donor/acceptor material and device design.