A-π-D-π-A small-molecule donors with different end alkyl chains obtain different morphologies in organic solar cells

For bulk-heterojunction organic solar cells, the morphology of the blend films highly influence the exciton dissociation and charge transport process. In this work, two novel A-π-D-π-A (A represents the acceptor unit and D represents the donor unit) backbone structure small molecular electron donors based on two dimensional conjugated naphtho[1,2-b:5,6-b']dithiophene (NDT) with different end alkyl chains, named as NDT-3T-EH and NDT-3T-O, have been designed and synthesized. The photovoltaic property of NDT-3T-O-based device is better than that of the NDT-3T-EH and the best efficiency reaches 7.06%, and the photovoltaic property of NDT-3T-EH reaches 6.11%. The difference in the performance should be attributed to the different morphology and phase separation resulted from the different crystallinity and aggregation ability of two donors. The results demonstrate that the optimized end alkyl chains can be used to design A-π-D-π-A backbone structure small molecular electron donors for small-molecule organic solar cells.