Nonfullerene small-molecule acceptors with perpendicular side-chains for fullerene-free solar cells

Organic photovoltaics (OPVs) represent one of the potential candidates of next-generation solar cells for converting green and sustainable solar energy into electrical power. An OPV cell utilizes a blend of electron donor (D) and electron acceptor (A) materials as photo-active layers, where photogenerated excitons are separated into mobile electrons and holes. Relative to fullerene acceptors, nonfullerene small-molecule acceptors (NF-SMAs) have several advantages such as synthesis-facile chemical modifications and straightforward tunability in absorptivity, spectral coverage, optical band gap, and Frontier molecular orbitals. In the recent 3 years, progress in design and synthesis of fused-ring NF-SMAs with perpendicular side-chains on an electron-rich core, and again, on design and synthesis of wide/medium/low band gap polymer donors have led to realizations of over 13% power conversion efficiencies (PCEs). These rapid advances require timely review articles. In this review article, we focus on this type of fused-ring NF-SMAs reported in the past 3 years, with special attention on their molecular structure design and structure-property relationships.