The revival of 4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) driven by low-cost and high-performance nonfused-ring electron acceptors

During the long history of organic solar cells (OSCs), many excellent building blocks have emerged for the construction of high-performance electron donor/acceptor materials. Among them, the 4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) unit was popular in the fullerene era, but declined in the early stage of the nonfullerene era. Recently, CPDT has experienced a revival with the emergence of low-cost nonfused-ring electron acceptors (NFREAs). This review aims to provide an overview of the structural isomers, photoelectric properties, and synthesis progress of the CPDT unit. Subsequently, the development of CPDT-based materials in the past few decades is summarized and discussed, including polymer donors for fullerene-based OSCs, NFREAs and polymerized NFREAs for nonfullerene-based OSCs.