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[(NN)-N-boolean AND](2)[(NC)-C-boolean AND], where [(NN)-N-boolean AND] is a diimine ligand and [(NC)-C-boolean AND] is a cyclometalated ligand. The triphenylamino group is attached to the -para position of the ruthenium-carbon bond of the [(NC)-C-boolean AND] 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 (J(sc)) than cells sensitized with our prior sensitizers with phenylene linkers. Morever the (NN)-N-boolean AND ligands are systematically tuned from 2,2'-bipyridine (O3), to 1,10-phenanthroline (O13), and to bathophenanthroline (O17). Following the series, the conjugation of the NN ligand is increased, which results in the enhancement of extinction coefficient and the red shift of light absorption. However the solar cell sensitized with O3 still gives the largest J(sc) of 3.04 mA/cm(2). The large J(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 O3 solar cell giving the largest J(sc) is likely caused by the slow geminate charge recombination and efficient dye regeneration.