Isomer Effects of Fullerene Derivatives on Organic Photovoltaics and Perovskite Solar Cells.

Solar energy conversion is one of the most important issues for creating and maintaining a future sustainable society. In this regard, photovoltaic technologies have attracted much attention because of their potential to solve energy and environmental issues. In particular, thin-film solar cells, such as organic photovoltaics (OPVs) and perovskite solar cells (PSCs), are highly promising owing to their flexibility, light weight, and low-cost production. One of the most important factors used to evaluate solar-cell performance is the power conversion efficiency (PCE), which is the ratio of the output electric power divided by the input light power. The PCEs of PSCs have become comparable to those of multicrystalline silicon solar cells in a laboratory level, but the PCEs of OPVs have yet to catch up with them and still need to be improved. The insufficient durability of PSCs and OPVs is also a challenge that needs to be addressed. Fullerene derivatives have been utilized as electron acceptors and electron-transport materials in OPVs and PSCs. However, the use of fullerene derivatives requires attention to their isomers if they are multiadducts or even monoadducts produced from fullerenes with low symmetry. Their nonuniform structures and electronic properties may exert a negative effect on photovoltaic properties. However, most researchers in the field of OPVs and PSCs have been unaware of the importance of the isomerism. Even the most prevalent, high-performance fullerene acceptor, [6,6]-phenyl-C