New phenothiazine-based dyes for efficient dye-sensitized solar cells: Positioning effect of a donor group on the cell performance

Two types of new phenothiazine-based dyes have been developed, in which Type 1 molecules are appended with a donor aryl group at the C(7) position and n-hexyl group at the N(10) of phenothiazine (PT1 and PT2), and Type 2 molecules are with the donor aryl group at the N(10) of phenothiazine (PT3 and PT4), together with a cyanoacrylate moiety at the C(3) position in both types of species. The structural features of a donor aryl group at the C(7) position of phenothiazine extends the π-conjugation of the chromophore, while the donor aryl group at N(10) significantly increases the steric hindrance of the dye due to its mutually perpendicular structural characteristics with either phenyl ring of bent phenothiazine. As a result, Type 1 dyes have better light harvesting properties in contact with TiO2films, and give much better dye-sensitized solar cell (DSSC) performance than Type 2 dyes. The PT1-sensitized DSSC shows a high open-circuit voltage (Voc) of 0.829 V and lead to a final power conversion efficiency of 6.72% based on PT1.