Wide bandgap polymer donors for high efficiency non-fullerene acceptor based organic solar cells

In the past few years, the power conversion efficiency (PCE) of organic solar cells (OSCs) has improved rapidly with the milestone value exceeding 18%, primarily owing to the development of novel non-fullerene acceptors (NFAs) as well as matching polymer donors. The molecular structure of a polymer donor fundamentally determines its molecular packing (crystal structure and morphology) and optoelectronic properties, which influence the photovoltaic processes and the ultimate PCE of the OSC device. The structure-property-cell performance relationships of polymer donors with respect to the specific acceptor are very complex, involving numerous parameters, but are extremely important towards the development of high-performance polymer donors to achieve high PCE. This review provides a timely analysis of the top-performing wide bandgap (WBG) polymer donors that have been developed to match the three most representative narrow bandgap NFAs, ITIC, IT-4F, and Y6, in terms of their structural design, fine-tuning of their optoelectronic properties, and control of the morphology and crystallinity of their blends with NFAs. We hope that this article provides deeper insight into the structure-property-cell performance relationships of polymer donors and a collection of useful guidelines and strategies for the design and processing of novel polymer donors for matching with NFAs for achieving ultrahigh performance OSCs. Structural design and processing of wide bandgap (WBG) polymer donors for non-fullerene acceptors ITIC, IT-4F, and Y6 to achieve ultrahigh efficiency are discussed.