A novel polymer donor based on dithieno[2,3-d:2′,3′-d′′]benzo[1,2-b:4,5-b′]dithiophene for highly efficient polymer solar cells

A novel polymer material, poly{5,10-bis(5-ethylhexyl-4-fluorothiophen-2-yl)dithieno[2,3-d:2',3'-d '']benzo [1,2-b:4,5-b']dithiophene-co-1,3-bis(thiophen-2-yl)-5,7-bis(2-ethylhexyl)-4H,8H-benzo[1,2-c:4,5-c'] dithiophene-4,8-dione} (PDBT-F) is designed, synthesized, and used as a donor in polymer solar cells. Dithieno[2,3-d:2',3'-d '']benzo[1,2-b:4,5-b']dithiophene (DTBDT) with an extended conjugation length shows better p-p stacking, higher charge-carrier mobility, and broader absorptions to boost the photovoltaic performance. Further introducing fluorine atoms in side groups effectively deepens the highest occupied molecular orbital (HOMO) level to enhance the open-circuit voltage (V-oc), promotes pi-pi stacking to increase the short-circuit current density (J(sc)), thus obtaining a power conversion efficiency (PCE) of 11.02% with IDIC. Moreover, ternary devices of PDBT-F:IDIC:PC71BM are constructed. As expected, the ternary device shows a higher J(sc) by virtue of the absorption complementarity of PDBT-F:IDIC:PC71BM, thus gaining extended absorption; a higher V-oc is caused by the deep lowest unoccupied molecular orbital (LUMO) level of PC71BM. The cascaded LUMO levels of PDBT-F, PC71BM and IDIC facilitate the charge transfer, finally achieving a PCE of 11.86% with a PC71BM content of 10%. The results we report show the best performances based on DTBDT materials. This work also demonstrates that it is a wise strategy to combine a fullerene derivative with non-fullerene acceptors to construct high-performance ternary solar cells.