β-Functionalized push-pull opp-dibenzoporphyrins as sensitizers for dye-sensitized solar cells: the role of the phenylethynyl bridge

A novel class of beta-functionalized push-pull zinc opp-dibenzoporphyrins was designed and synthesized as sensitizers for dye-sensitized solar cells. Dibenzoporphyrin YH7 displayed strong push-pull characteristics, which included clearly segregated HOMO/LUMO and fast charge injection from the singlet excited state of zinc porphyrin to the conduction band of TiO2. The incorporation of both the phenylethynyl bridge and amino group was critical to achieve the enhanced push-pull effect. The highest PCE value in this series obtained with the YH6 cell was 6.7%, which is comparable to that of the best ruthenium dye, N719 (eta = 7.4%), under similar conditions. The high PCE of the YH6 cell is ascribed to its high light harvesting ability and favorable cell kinetics, which is supported by electrochemical impedance spectroscopy studies and femtosecond transient absorption studies. An unusual illumination-induced photodegradation was observed for YH7, which was attributed as the major reason for the unexpected lower PCE of the YH7 cell (eta = 5.7%). This work demonstrates the applicability of pi-extended porphyrins as a new generation of sensitizers for high performance dye-sensitized solar cells.