Enhanced Spectral Sensitivity from Ruthenium(II) Polypyridyl Based Photovoltaic Devices

Abstract

Ruthenium polypyridyl compounds, cis-[4,4'-(CO2H)(2)-2,2'-bipyridine]Ru-2(X)(2) and cis-[5,5'-(CO2H)(2)-2,2'- bipyridine]Ru-2(X)(2) where X = Cl-, CN-, and SCN-, have been prepared, spectroscopically characterized, and anchored to high surface area TiO2 electrodes for the conversion of visible light into electricity. Vibrational studies reveal a surface ester linkage and indicate that the sensitizers bind to TiO2 through a distribution of interfacial interactions in a similar manner. When operating in a photoelectrochemical cell, these materials convert visible photons into electrons. Transition metal sensitizers based on the 5,5'-(CO2H)(2)-2,2'-bipyridine ligand generally enhance photon-to-current efficiencies at low photon energies. Consistent with a previous report, photoanodes based on cis-[4,4'-(CO2H)(2)-2,2'-bipyridine]Ru-2(NCS)(2) are the most efficient under simulated sunlight (Gratzel, M., et al. J. Am. Chem. Soc. 1993, 115, 6382). The observation of an electric field dependent luminescence indicates that radiative recombination of the dye competes with photocurrent production in some cases. A lower limit for the rate of interfacial electron injection is estimated to be k(et) similar to 5 x 10(7) s(-1). A lower photocurrent efficiency observed for sensitizers based on 5,5'-(CO2H)(2)-2,2'-bipyridine stems in part from less efficient electron transfer to the TiO2 surface.