Material insights and challenges for non-fullerene organic solar cells based on small molecular acceptors

The field of non-fullerene organic solar cells has experienced rapid development during the past few years, mainly driven by the development of novel non-fullerene acceptors and matching donor semiconductors. However, organic solar cell material development has progressed via a trial-and-error approach with limited understanding of the materials' structure-property relationships and the underlying device physics of non-fullerene devices. In addition, the availability of hundreds of donor and acceptor semiconductors creates an extremely large pool of possible donor-acceptor combinations, which poses a daunting challenge for rational material screening and matching. This Review describes several important conceptual aspects of the emerging non-fullerene devices by highlighting key contributions that provided fundamental insights regarding rational material design, donor-acceptor pair matching, blend morphology control and the reduced voltage losses in non-fullerene organic solar cells. We also discuss the key challenges that need to be addressed to develop more-efficient non-fullerene organic solar cells.