In the field of bulk-heteroj unction organic photovoltaics (BHJ OPVs)

fullerenes and their derivatives are an important class of n-type electron acceptor materials. Nevertheless

they suffer from some disadvantages

such as weak absorption in the visible spectral region

limited spectral breadth

difficulty in variably tuning the band gap

and poor solubility

which severely limit the fullerene-based materials as an acceptor material more widely used in organic solar cells. Compared to the fullerene derivatives

non-fullerene organic small molecular acceptors (OSMAs) are much easier to be synthesized and purified with lower processing costs and more excellent solubility

and their band gaps could be finely adjusted by proper molecular design. Recent reports on OSMAs show great improvement and the power conversion efficiency has exceeded 4%. This article reviews recent developments in several classes of solution-processable non-fullerene acceptors for BHJ OPVs. Finally

critical challenges that determine photovoltaic performance and prospects for future directions are discussed.