Endowed with high charge transporting capabilities and formation of unique phase-separated microstructure, fullerenes and their derivatives have been playing a predominant role as electron acceptor in bulk hetero-junction devices. However, they also suffer from unconquerable drawbacks, such as poor absorption in visible light region, difficult modification and high cost, which limit the performance improvement and scalable application of organic solar cells. More interests are focused on the non-fullerene small-molecule acceptors. It is reasonable to regulate the molecular energy levels of non-fullerene materials through diversiform chemical approaches, especially for small organic molecules and their oligomers. And electron acceptors with specific aggregated-state morphologies and excellent properties could be accessible by virtue of diverse synthetic methods. In the review, the recent advances on non-fullerene acceptors are summarized with outstanding performance for solution-processed bulk-heterojunction solar cells. These non-fullerene acceptors mainly consist of perylenetetracarboxylic diimide acceptors including its monomers, dimers and quasi-3D-structured acceptors, diketopyrrolopyrrole acceptors, benzothiadiazole-based acceptors, as well as other miscellaneous high-performance small-molecule acceptors. The performance of these acceptors is analyzed from the view of molecular structures and their matching with the related donors. Finally, critical challenges that influence photovoltaic performance and the perspectives of small-molecule acceptors are discussed and addressed.

Contents
1 Introduction
2 High-performance small molecule acceptors for organic solar cells
2.1 Perylenetetracarboxylic diimide-based acceptors
2.2 Diketopyrrolopyrrole-based acceptors
2.3 Benzothiadiazole-based acceptors
2.4 Other small molecule acceptors
3 Conclusion and outlook