Xanthene dye molecules form a chelate complex with the titanium species on the titania surface in dye−titania systems. The complex formation causes a fast electron injection into the titania conduction band. In this study, simple spectroscopic and photocurrent measurements of the xanthene dye-doped titania gels prepared by the sol−gel method were conducted in order to clarify the influence of a steam treatment on the dye−titania interaction and electron transfer. The photocurrent quantum efficiency of the fluorescein-doped electrode was remarkably increased by the steam treatment compared to that of the untreated electrode consisting of an amorphous titania gel. The photocurrent action spectrum was red-shifted, and the short circuit photocurrent and open circuit voltage values increased with the steam treatment time. The steam treatment promoted the dye−titania complex formation, a negative shift in the conduction band potential of the titania, and the electron injection from the dye to the titania.
The time-resolved fluorescence and photoelectrochemical properties of dyesensitized solar cells using crystalline titania electrodes coated with N3 dye-dispersing amorphous titania gel were investigated to clarify the influence of the dye−titania interaction and electron transfer on their photoelectric conversion performance. The photocurrent quantum efficiency of the electrodes was remarkably increased by a steam treatment due to the crystallization and densification of the amorphous titania layer compared to that of the untreated electrode. The electron injection from the dye to the crystalline titania foundation via the steam-treated titania dispersing the dye was confirmed to be more efficient than that in the conventional electrodes. The dye-dispersing titania layer prevented interaction between the dye molecules and back electron transfer from the titania to the electrolyte. The charge separation and photoelectric conversion performance of the dye-sensitized solar cells were improved by forming the specific dye-dispersing titania layer.
Amorphous dye-containing titania gel films were prepared on ITO electrodes coated with a crystalline titania foundation from titanium alkoxide sols containing a dye at room temperature. Photoinduced electron transport in the amorphous titania gel film was investigated by spectroscopic and photovoltaic measurements. Influences of the structure and morphology of the multilayered film on the photoelectron transport and electrically conductive properties were discussed. The photocurrent was observed from only the layer contacting the crystalline titania foundation. The electron transport from the amorphous upper layers was limited. Steam treatment of the electrodes improved the electron transport due to crystallization of the amorphous titania to anatase accompanied by enhancement of its electrical conductivity. The efficiency of the dye-sensitized electron transport in the steam-treated titania film was close to that of the anatase film prepared by heating at 773 K.The dye-containing titania layers functioned as efficient sensitizers.
Transient absorption spectroscopy is generally used to study the photoinduced electron transfer process in the dye–titania systems. Time-resolved fluorescence spectroscopy is more sensitive than transient absorption spectroscopy. Fluorescein-dispersing titania gel films were prepared by a sol–gel process and steam treatment using a titanium alkoxide solution containing fluorescein. The photoinduced electron transfer process in the films was investigated by steady state and time-resolved fluorescence measurements. The fluorescence quenching efficiency increased with an increase in the steam treatment time due to the dye–titania complex formation. The titanium species were coordinated to the carboxylate of the fluorescein species during the steam treatment based on an FTIR analysis. The dye–titania complex formation played an important role in the electron injection from the dye to the titania conduction band.
The dye-dispersing titania electrodes were prepared from the dye-containing titanium alkoxide sols by a room temperature sol−gel process and steam treatment at 110°C. The spectroscopic and photoelectric conversion properties of the electrodes were investigated in order to clarify the influences of the dye dispersion and the co-dispersion of the two dyes on the electron transfer process. The fluorescein and eosin Y molecules were dispersed into the titania as their monomers. The shapes of the photocurrent action spectra of the fluorescein and/or eosin Y-dispersing titania electrodes well corresponded to those of their absorption spectra because the excited electrons in the dyes were directly injected into the titania conduction band without any interaction between the dye molecules, such as energy transfer.This result indicated that the dye molecules were separately encapsulated in the pores between the titania nanoparticles and tightly adsorbed or bonded to the titania particle surface. The internal quantum efficiency of the photoelectric conversion was higher than that of the conventional dye-adsorbing titania electrodes in which the dye molecules were easily aggregated, thus deactivated by the energy transfer. The co-dispersion of the two dyes on the titania surface allowed to effectively extend the visible light region for the photoelectric conversion.3
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