We present a comparison between the electronic structures of inverse opal (IO) and nanoparticulate (NP)-TiO 2 electrodes. The electronic structure details were obtained from optical absorption, fluorescence, and valence band studies in order to clarify the nature of the higher photovoltaic performance observed in sensitized solar cells using IO-TiO 2 electrodes. We used photoacoustic (PA) and photoluminescence (PL) spectroscopy to characterize the optical absorption and fluorescence properties, respectively.Photoelectron yield (PY) spectroscopy was applied to characterize the position of the valence band maximum (VBM) of the IO-and NP-TiO 2 electrodes. The PA spectrum for IO-TiO 2 is different to that for NP-TiO 2 , indicating differences in the exciton-phonon interactions and the density of states in the conduction band. PL measurements showed that the curvature of the valence band structure of IO-TiO 2 is different to that of NP-TiO 2 . Also, PL measurements showed that the oxygen vacancy in IO-TiO 2 is different to that in NP-TiO 2 . Moreover, PY measurements showed VBM in IO-TiO 2 to be at a higher position than that in NP-TiO 2 , suggesting a correlation with the increased open circuit voltage (V oc ) in sensitized solar cells.

We describe the adsorption and growth of CdSe quantum dots (QDs) on single crystals of rutile TiO 2 with different crystal orientations. We used atomic force microscopy (AFM) to characterize the morphology of the QDs and photoacoustic (PA) spectroscopy to measure the optical absorption. Photoelectron yield (PY) spectroscopy was applied to characterize the valence band maximum (VBM) of the single crystal TiO 2 . The AFM images and the absorbance measurements showed that the number of CdSe QDs grown on the (111) surface was larger than those grown on the ( 110) and ( 001) surfaces. The absorbance measurements showed that the adsorption becomes linearly proportional to the adsorption time. However, the rate of adsorption is different for each crystal orientation. The crystals grow higher on (111) surfaces than on ( 110) and ( 001) surfaces. The position of the VBM for the (111) surface is higher than those for the (110) and (001) surfaces. Hence, the formation and growth of CdSe QDs on (111) surfaces is more active than on the other orientations. The increase in the average diameter of CdSe QDs with adsorption time is independent of the crystal orientation. Although the growth rate of CdSe QDs on (001) surfaces is lower than on (110) and (111) surfaces, the crystal quality is better on the former.

Semiconductor quantum dots (QDs) have many desirable characteristics for use as sensitizers, such as enabling tuning of the band gap on the basis of the quantum confinement effect, a higher extinction coefficient, and facilitating charge injection as a result of the large dipole moment. Despite these potential advantages, no major advance in the efficiency of quantum-dot-sensitized solar cells (QDSCs) has yet been reported. The poor efficiency can be attributed to electron-transfer (ET) reactions that compete with the ideal energy generation cycle in QDSCs. Despite the great technological significance, the interfacial ET between QDs and inorganic species remains poorly understood. In this paper, we describe the electronic structure and the interactions between multiple sized CdSe QDs and single crystal rutile TiO2 with (001), (110), and (111) orientations. Single crystal TiO2 is well characterized and is not only ideal for comparing the amount and the structure of the QDs but is also useful for studying ET reactions. The rate of adsorption of CdSe QDs depends on the crystal orientation, although the average increase in diameter of the QDs is independent of the crystal orientation. The highest occupied molecular orbital (HOMO) level is independent of the adsorption time. On the other hand, the value of the HOMO level depends on the crystal orientation of the R-TiO2 substrate. The ET rate constant increases as the change in free energy increases and depends on the crystal orientation. This suggests that the mixing of the wave functions between the conduction band in the R-TiO2 and the lowest unoccupied molecular orbital (LUMO) level in the CdSe QDs depends on the crystal orientation.

We describe the morphology and consider the optical absorption and the ground state energy level of CdSe quantum dots (QDs) on single crystal ZnO substrates with various crystal orientations. The morphologies and crystal growth of CdSe QDs were analyzed by atomic force microscopy (AFM), photoacoustic (PA) spectroscopy for the optical absorption measurements, and photoelectron yield (PY) spectroscopy to evaluate the ground state energy level. The adsorption and crystal growth of CdSe QDs depend on both the adsorption time and the surface orientation of the ZnO substrate. The change in average diameter of the QDs on ZnO with adsorption time is not related to the crystal surface orientation, and the growth is a two-step process, different from that on single crystal rutile-TiO2. This is characteristic of the difference between CdSe QDs grown on ZnO and those grown on rutile-TiO2. The crystal quality of the QDs on ZnO is poorer than those grown on rutile-TiO2, indicating that the reaction for crystal growth on ZnO is different from that on rutile-TiO2. The energy level of the valence band maximum (VBM) for the (0001) single crystal ZnO surface is higher than those for the (101̅0) and (112̅0) surfaces. The ground state energy level of CdSe QDs on single crystal ZnO depends on both the adsorption time and the surface orientation, which is indicative of the effect of the loss of solvation energy due to the dielectric polarization of the surroundings.

Quantum dots (QDs) provide an attractive alternative sensitizer to organic dyes. However, there have been few reports on QD-sensitized solar cells (QDSCs) that have photovoltaic conversion efficiencies exceeding those of dye-sensitized solar cells. This is because of the lack of fundamental studies of QDs on conventional nanocrystalline metal oxide electrodes which possess much amount of heterogeneity. An important first step is an investigation of the dependences of the optical absorption, the ground state energy level, and the interfacial electron transfer (IET) on the size of QDs deposited on well characterized single crystal oxides. The present study focuses on a system of CdSe QDs adsorbed on the (001), (110), and (111) surfaces of single crystal rutile-TiO 2 . The optical absorption spectra, characterized using photoacoustic spectroscopy, were found to be independent of the surface orientation concerning the optical absorption edge. The exponential optical absorption tail (Urbach tail) suggests that the disorder decreases with the increasing size of the QDs and is independent of the surface orientation. The ground state energy levels of the QDs were characterized using photoelectron yield spectroscopy. That on the (001) surface shifts upward, while that on the (110) surface shifts downward with increasing QD size. That on the (111) surface is independent of the QD size, indicating the difference of the influence of the surface orientation on adsorption of the QDs. The IET rate constant and the relaxation component were characterized. The IET rate constant was found to decrease as the size of the QDs increases and depends on the surface orientation, indicating the differences in the decrease of the free energy change and lower coupling between the excited state of CdSe QDs and the Ti 3d orbitals in rutile-TiO 2 . The relaxation component increases with increasing QD size and depends on the surface orientation, correlating with the density of states in the conduction band of rutile-TiO 2 .