Recent progress in small-molecule donors for non-fullerene all-small-molecule organic solar cells

All-small-molecule organic solar cells (all-SM OSCs), with p-type small molecule (p-SM) as donor and n-type small molecule (n-SM) as acceptor, have drawn intensive research interests in recent years. All-SM OSCs show advantages to their polymer counterparts, including well-defined chemical structure, easy purification, and better repeatability in devices. The highest power conversion efficiency (PCE) of all-SM OSCs has increased quickly to over 15% nowadays. In this review, we just summarize the progress of non-fullerene all-SM OSCs (NF all-SM OSCs) in recent five years, by dividing small-molecule donors into four categories based on the molecular segments of oligothiophene, benzo[1,2-b:4,5-b']dithiophene (BDT) trimer, typical BDT-central unit connected with pi-bridges of thiophene or electron deficient acceptor units. We aim to illustrate the relationship of molecular structures, properties, and device performance of small-molecule donors with those different molecular segments. Furthermore, the tendency of molecular design for photo-active materials are also depicted and summarized. Finally, we give our views on the future prospects of NF all-SM OSCs to design the next-generation OSCs.