Dye Sensitization of Nanocrystalline Titanium Dioxide with Osmium and Ruthenium Polypyridyl Complexes

A series of osmium polypyridyl complexes having various ground-state reduction potentials has been synthesized and used to sensitize nanoporous titanium dioxide electrodes to solar illumination. The spectral response and current vs potential properties of electrodes modified with these dyes have been compared with the behavior of their ruthenium analogues. The trends can be explained by the differences in absorption spectra and ground-state redox potentials. The osmium complexes appear to be promising candidates for further optimization in operating photoelectrochemical cells for solar energy conversion applications. Of the materials studied, all complexes having ground-state redox potentials in methanol more positive than ∼0.4 V vs aqueous SCE were able to sustain oxidation of I-/I_3- with a high steady-state quantum yield. For electrodes with very low dye coverages, the open-circuit voltage was mainly determined by the rate of reduction of I_2, whereas for high dye coverages, the open-circuit voltage depended on the nature of the complex and on the dye loading level

Theoretical and Experimental Upper Bounds on Interfacial Charge-Transfer Rate Constants between Semiconducting Solids and Outer-Sphere Redox Couples

Theoretical expressions for the charge-transfer rate constant at a semiconductor/liquid junction have been modified to include the effects of adiabaticity and the existence of a Helmholtz layer at the solid/liquid interface. These expressions have yielded an estimate of the maximum interfacial charge-transfer rate constant, at optimal exoergicity, for a semiconductor in contact with a random distribution of nonadsorbing, outer-sphere redox species. An experimental upper bound on this interfacial charge-transfer rate constant has been obtained through the determination of key energetic and kinetic properties for stable semiconductor electrodes in contact with outer-sphere redox species. For this purpose, n-Si/CH_3OH−dimethylferrocenium−dimethylferrocene, n-GaAs/CH_3CN−ferrocenium−ferrocene, and p-InP/CH_3CN−cobaltocenium−cobaltocene contacts were investigated using a combination of current density-potential and differential capacitance-potential methods. The upper limits for the interfacial charge-transfer rate constant at these semiconductor/liquid contacts were found to be consistent with the upper limits predicted by theory. The current density-potential behavior of n-InP and p-InP/Fe(CN)_6^(3-/4-)(aq) junctions was also examined in order to assess the validity of prior kinetic measurements on these interfaces

Theoretical and Experimental Upper Bounds on Interfacial Charge-Transfer Rate Constants between Semiconducting Solids and Outer-Sphere Redox Couples

Theoretical expressions for the charge-transfer rate constant at a semiconductor/liquid junction have been modified to include the effects of adiabaticity and the existence of a Helmholtz layer at the solid/liquid interface. These expressions have yielded an estimate of the maximum interfacial charge-transfer rate constant, at optimal exoergicity, for a semiconductor in contact with a random distribution of nonadsorbing, outer-sphere redox species. An experimental upper bound on this interfacial charge-transfer rate constant has been obtained through the determination of key energetic and kinetic properties for stable semiconductor electrodes in contact with outer-sphere redox species. For this purpose, n-Si/CH_3OH−dimethylferrocenium−dimethylferrocene, n-GaAs/CH_3CN−ferrocenium−ferrocene, and p-InP/CH_3CN−cobaltocenium−cobaltocene contacts were investigated using a combination of current density-potential and differential capacitance-potential methods. The upper limits for the interfacial charge-transfer rate constant at these semiconductor/liquid contacts were found to be consistent with the upper limits predicted by theory. The current density-potential behavior of n-InP and p-InP/Fe(CN)_6^(3-/4-)(aq) junctions was also examined in order to assess the validity of prior kinetic measurements on these interfaces

Limits on the Corrosion Rate of Si Surfaces in Contact with CH_3OH-Ferrocene^(+/0) and CH_3OH-1,1'-Dimethylferrocene^(+/0) Solutions

Although Si/CH^3OH contacts have been extensively investigated and reported to provide highly efficient photoelectrochemical energy conversion devices, a recent study using the scanning electrochemical microscope (SECM) has claimed that, in CH_3OH solutions, Si surfaces in contact with 4.57 mM ferrocenium (Fc^+) were etched in the dark at a mass-transport-limited rate. The reported etching rate constant of > 0.37 cm s^-1) at 4.57 mM ferrocenium corresponds to an equivalent corrosion current density of > 240 mA cm^(-2) and to a Si etch rate of > 75 nm s^(-1). The presence of such severe corrosion was inferred from an unexpectedly large feedback current in an SECM experiment. The present work describes a search for corrosion of Si in contact with CH_3OH-ferrocene^+/0) and CH_3OH-dimethylferrocene(Me2Fc)^(+/0) solutions through the use of very sensitive electrochemical, chemical, and physical methods. For CH_3OH - 1.0 M LiClO_4 - 100 mM Me_2Fc- 80 mM Me_2Fc^+ solutions, an upper limit on the etch rate of 6.6 x10^(-6) nm s^-1) has been established through direct experimental measurements; thus, a 400 pm thick Si photoelectrode in contact with the CH_3OH-Me_2-Fc^(+/0) electrolyte would require over 1500 years to corrode completely at room temperature. An alternative explanation for the SECM data, based on the documented existence of an inversion layer at the Si/liquid contact, is presented and shown to be consistent with the available data

Limits on the Corrosion Rate of Si Surfaces in Contact with CH_3OH-Ferrocene^(+/0) and CH_3OH-1,1'-Dimethylferrocene^(+/0) Solutions

Although Si/CH^3OH contacts have been extensively investigated and reported to provide highly efficient photoelectrochemical energy conversion devices, a recent study using the scanning electrochemical microscope (SECM) has claimed that, in CH_3OH solutions, Si surfaces in contact with 4.57 mM ferrocenium (Fc^+) were etched in the dark at a mass-transport-limited rate. The reported etching rate constant of > 0.37 cm s^-1) at 4.57 mM ferrocenium corresponds to an equivalent corrosion current density of > 240 mA cm^(-2) and to a Si etch rate of > 75 nm s^(-1). The presence of such severe corrosion was inferred from an unexpectedly large feedback current in an SECM experiment. The present work describes a search for corrosion of Si in contact with CH_3OH-ferrocene^+/0) and CH_3OH-dimethylferrocene(Me2Fc)^(+/0) solutions through the use of very sensitive electrochemical, chemical, and physical methods. For CH_3OH - 1.0 M LiClO_4 - 100 mM Me_2Fc- 80 mM Me_2Fc^+ solutions, an upper limit on the etch rate of 6.6 x10^(-6) nm s^-1) has been established through direct experimental measurements; thus, a 400 pm thick Si photoelectrode in contact with the CH_3OH-Me_2-Fc^(+/0) electrolyte would require over 1500 years to corrode completely at room temperature. An alternative explanation for the SECM data, based on the documented existence of an inversion layer at the Si/liquid contact, is presented and shown to be consistent with the available data

High Quantum Yield Sensitization of Nanocrystalline Titanium Dioxide Photoelectrodes withcis-Dicyanobis(4,4‘-dicarboxy-2,2‘-bipyridine)osmium(II) or Tris(4,4‘-dicarboxy-2,2‘-bipyridine)osmium(II) Complexes

Osmium polypyridyl complexes were used as sensitizers in solar cells that utilize nanocrystalline titanium dioxide photoelectrodes. Exposure of TiO_2 electrodes to sources of Os^(II)(H_2L‘)_2(CN)_2 (where L‘ is 4,4‘-dicarboxylato-2,2‘-bipyridine) or Os^(II)(H_2L‘)_3^(2+) extended the light absorption and spectral response of the cell to longer wavelengths than did exposure of TiO_2 to Ru(H_2L‘)_2(NCS)_2. The Os complexes also provided very high external quantum yields for photocurrent flow and produced open-circuit voltages similar to those of the Ru complex. The Os-based systems therefore offer promise in developing efficient dye-sensitized nanocrystalline TiO_2-based photoelectrochemical cells