Cyclometalated Ruthenium Sensitizers Bearing a Triphenylamino
Group for p‑Type NiO Dye-Sensitized Solar Cells
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Abstract
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<sup>∧</sup>N]<sub>2</sub>[N<sup>∧</sup>C], where
[N<sup>∧</sup>N] is a diimine ligand and [N<sup>∧</sup>C] is a cyclometalated ligand. The triphenylamino group is attached
to the <i>-para</i> position of the ruthenium–carbon
bond of the [N<sup>∧</sup>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><sub>sc</sub>) than cells sensitized with our prior sensitizers with phenylene
linkers. Morever the N<sup>∧</sup>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<sup>̂</sup>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><sub>sc</sub> of 3.04 mA/cm<sup>2</sup>. The large <i>J</i><sub>sc</sub> 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><sub>sc</sub> is likely caused by the slow geminate charge
recombination and efficient dye regeneration