Role of the energy offset on the charge photogeneration and voltage loss of nonfullerene acceptor-based organic solar cells

The trade-off between the short-circuit current density (JSC) and open-circuit voltage (VOC) has been one of the largest challenges in improving the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Although energy offset between the optical bandgap and the charge transfer (CT) states should be maintained as small as possible to improve VOC, a very small energy offset typically leads to degradation of JSC, even when novel nonfullerene acceptors (NFAs), such as Y6, are used. Therefore, understanding the limit to what extent the energy offset can be minimized and the underlying physics behind the trade-off relationship is important to optimize the design of new materials and further improve the PCEs. This study provides a threshold energy that can ensure high charge photogeneration quantum efficiencies for Y-series NFA-based OSCs and discusses the role of the energy offset in device performances. We found that an insufficient energy offset led to not only slow hole transfer at the donor:acceptor interfaces, but also inefficient long-range spatial dissociation of the CT states and degradation of the fill factor (FF). This study also discusses the interplay of the energy levels of two NFAs that constitute ternary blend OSCs. We found that combining two NFAs enables to reduce the voltage loss, while maintaining a high charge photogeneration quantum efficiency. Our findings highlight the importance of overcoming the trade-off between FF and VOC for further improving the PCE.