The key components of an OPV are the donor and acceptor polymers or molecules that are used. The donor (D)–acceptor (A) blend facilitates light absorption, exciton dissociation, and charge transport, and are crucial for obtaining high device efficiency. For about the first 15 years, OPV research focused on using fullerene derivatives, such as PC61BM and PC71BM, as the acceptor molecule. Innovative design in donor polymers allowed for big efficiency boosts during 2005–2012. However, a plateau in efficiencies was observed for a few years, as the field struggled to find good solutions to what appeared to be an efficiency bottleneck. This period also coincided with rapid development in perovskite solar cells. In roughly the last 5 years, nonfullerene acceptors (NFAs) have dominated the OPV efficiency chart. Similar to the efficiency boost between 2005 and 2012, which was caused by the introduction and optimization of alternating electron-rich donor (D) and electron-deficient acceptor (A) polymers (D–A type donor polymers), A–D-based NFAs have triggered a similar jump in performance. During the last 5–7 years, the efficiency of organic solar cells was raised from 10–12 % to nowadays 18–20 %. This special issue contains a variety of contributions, encompassing NFAs, all-polymer solar cells, new donor designs, and innovative and promising device concepts with the potential to further enhance performance beyond the 20 % milestone.