**5. Conclusions**

Narrow band gap polymers **P1-P64** developed by alternating donor (ca. fluorene, carbazole and thiophene) and acceptor (ca.benzothiadiazole, quinoxaline and diketopyrrolopyrrole) units in recent 4 years are summarized, with their fullerene blend-based BHJ OSCs contributing PCE over 3%. The design criteria for ideal polymer donors to achieve high efficiency OSCs is: (1) a narrow *Eg* (1.2-1.9eV) with broad absorption to match solar spectrum; (2) a HOMO energy level ranging from -5.2 to -5.8 eV and a LUMO level ranging from -3.7 to -4.0eV to ensure efficient charge separation while maximizing *Voc*; and (3) good hole mobility to allow adequate charge transport. Besides, device structure and morphology optimizations of polymer:fullerene blend film have been extensively demonstrated to be crucial for PCE improvement in OSCs. The current endeavors boosted OSCs PCEs up to 7% would encourage further efforts toward a next target of efficiency in excess of 10%.
