The development of A-DA'D-A type nonfullerene acceptors containing non-halogenated end groups

Compared with perovskite solar cells and silicon solar cells, the excessive voltage loss (V-loss) becomes a stubborn stone that seriously hinders the further improvement of organic photovoltaic (OPV). Thus, many researchers focus on finding an effective material system to achieve high-performance OPVs with low V-loss. In recent 5 years, acceptor-donor-acceptor'-donor-acceptor (A-DA'D-A) type non-fullerene acceptors (NFAs) have attracted great attention because of their promising photovoltaic performance. Among them, A-DA'D-A type NFAs containing non-halogenated end group (NHEG) exhibit the large potential to achieve high open-circuit voltage (V-OC) for the state-of-the-art OPVs, because of high-lying molecular energy levels and decreasing V-loss. In this review, we systematically summarize the recent development of A-DA'D-A type NHEG-NFAs and the impact of different NHEGs on the optoelectronic properties as well as the photovoltaic performance. In addition, we especially analyze the V-loss of NHEG-NFAs in the binary and ternary OPV devices. At last, we provide perspectives on the further molecular design and future challenges for this kind of materials as well as suggested solutions.