[Abstract]

Non-fullerene organic solar cells offer several advantages over conventional fullerene devices. Among them, one attractive feature of non-fullerene organic solar cells is the low voltage loss from the optical bandgap of the active layer to the open circuit voltage of the cell. In our research, we have developed material systems that can achieve highly efficient charge separation with a small energy offset between the donor acceptor materials. Such material systems can achieve low voltage loss of about 0.5 to 0.55 V, which, in theory, could enable non-fullerene organic solar cell near 20% efficiency. In the first aspect of our study, we use transient absorption spectroscopy to understand why non-fullerene organic solar cells can work well with small energy offsets. Our results show that the key factor is the long CT life-time that allows for efficient charge separation despite of a small energy offset. In the other aspect, we study structure-property relationship of high-performance donor and non-fullerene acceptor materials and reveal the key structure features that enable highly efficient non-fullerene organic solar cell devices with over 16% efficiency. With these understandings of mechanism and structure-property relationship, it is feasible to further increase the efficiency of organic solar cells to the range of 18 to 20% in near future.