Cyclometalated Ruthenium Sensitizers Bearing a Triphenylamino Group for p‑Type NiO Dye-Sensitized Solar Cells

We report the synthesis, photophysical, and electrochemical studies of a series of cyclometalated ruthenium sensitizers carrying triphenylamino linkers for p-type NiO dye-sensitized solar cells (DSSCs). The general structure of these ruthenium sensitizers is Ru­[N^∧N]₂[N^∧C], where [N^∧N] is a diimine ligand and [N^∧C] is a cyclometalated ligand. The triphenylamino group is attached to the <i>-para</i> position of the ruthenium–carbon bond of the [N^∧C] ligand as a linker to bridge the ruthenium chromophore and the NiO surface and to enhance the electronic coupling for hole injection. As a result, cells made with these sensitizers generate higher short-circuit currents (<i>J</i>_sc) than cells sensitized with our prior sensitizers with phenylene linkers. Morever the N^∧N ligands are systematically tuned from 2,2′-bipyridine (<b>O3</b>), to 1,10-phenanthroline (<b>O13</b>), and to bathophenanthroline (<b>O17</b>). Following the series, the conjugation of the N^̂N ligand is increased, which results in the enhancement of extinction coefficient and the red shift of light absorption. However the solar cell sensitized with <b>O3</b> still gives the largest <i>J</i>_sc of 3.04 mA/cm². The large <i>J</i>_sc highlights the promising potential of using these cyclometalated ruthenium sensitizers for NiO DSSCs. In addition, the carrier dynamics of these solar cells has been systematically studied by intensity-modulated photovoltage spectroscopy (IMVS) and intensity-modulated photocurrent spectroscopy (IMPS). The results suggest that the <b>O3</b> solar cell giving the largest <i>J</i>_sc is likely caused by the slow geminate charge recombination and efficient dye regeneration