One Electron Changes Everything. A Multispecies Copper Redox Shuttle for Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSCs)
are an established alternative
photovoltaic technology that offers numerous potential advantages
in solar energy applications. However, this technology has been limited
by the availability of molecular redox couples that are both noncorrosive/nontoxic
and do not diminish the performance of the device. In an effort to
overcome these shortcomings, a copper-containing redox shuttle derived
from 1,8-bis­(2′-pyridyl)-3,6-dithiaoctane (PDTO) ligand and
the common DSC additive 4-tert-butylpyridine (TBP)
was investigated. Electrochemical measurements, single-crystal X-ray
diffraction, and absorption and electron paramagnetic resonance spectroscopies
reveal that, upon removal of one metal-centered electron, PDTO-enshrouded
copper ions completely shed the tetradentate PDTO ligand and replace
it with four or more TBP ligands. Thus, the Cu­(I) and Cu­(II) forms
of the electron shuttle have completely different coordination spheres
and are characterized by widely differing Cu­(II/I) formal potentials
and reactivities for forward versus reverse electron transfer. Notably,
the coordination-sphere replacement process is fully reversed upon
converting Cu­(II) back to Cu­(I). In cells featuring an adsorbed organic
dye and a nano- and mesoparticulate, TiO2-based, photoelectrode,
the dual species redox shuttle system engenders performance superior
to that obtained with shuttles based on the (II/I) forms of either
of the coordination complexes in isolation.