In this review, small-molecule donors for application in organic solar cells reported in the last
three years are highlighted. Especially, the effect of donor molecular structure on power conversion efficiency
of organic solar cells is reported in detail. Furthermore, the mechanism is proposed and discussed
for explaining the relationship between structure and power conversion efficiency. These results
and discussions draw some rules for rational donor molecular design, which is very important for further
improving the power conversion efficiency of organic solar cells based on the small-molecule donor.