Efficient non-fullerene organic solar cells with low-temperature solution-processing ferrous oxides as hole transport layer

Non-fullerene organic solar cells (NF–OSCs) have recently attracted enormous attention due to the rapid advance of high-performance photoabsorbers. On the other hand, interfacial materials also play a crucial role in further increasing the device efficiency, but those materials in particular effective hole transporting ones for NF–OSCs are less developed. In this work, three low-temperature solution-processing ferrous oxide films (including CoOx, NiOx, and FeOx) are used as hole transporting layer (HTL) for NF–OSCs. By adding a surfactant and treating with the ultraviolet ozone (UVO), uniform ferrous oxide films with adjustable energy bands are achieved. The NF–OSCs based on PBDB-T-2Cl:IT-4F active layer and using CoOx, NiOx, and FeOx as the HTL afford power conversion efficiencies of 11.4%, 10.2% and 6.4%, respectively. The higher performance of NF–OSCs with the UVO-treated CoOx as the HTL is attributed to its more suitable energy level alignment and better hole transportation property relative to those of the other two counterparts.