J. Phys. Chem. C 2007, 111, 849-854
849
Synchrotron-Induced Photoelectron Spectroscopy of the Dye-Sensitized Nanocrystalline TiO2/Electrolyte Interface: Band Gap States and Their Interaction with Dye and Solvent Molecules Konrad Schwanitz, Ulrich Weiler, Ralf Hunger, Thomas Mayer,* and Wolfram Jaegermann Institute of Materials Science, Darmstadt UniVersity of Technology, D-64287 Darmstadt, Germany ReceiVed: July 24, 2006; In Final Form: October 25, 2006
The adsorption of Ru dye N3 [RuII(2,2′-bipyridyl-4,4′-dicarboxylate)2(NCS)2] from ethanol solution and gasphase adsorption and coadsorption of the solvent acetonitrile on liquid nitrogen cooled nanocrystalline (nc) TiO2 films have been investigated using highly surface sensitive synchrotron-induced photoelectron spectroscopy. Resonantly excited Ti3+-related band gap states form a peak 2.2 eV above the leading edge of the valence band, and additional states up to the Fermi level are found. The intensities of the gap states and related Ti3+ 2p emissions of pristine samples depend strongly on sample preparation and history. Gap-staterelated emissions are partially quenched by dye as well as acetonitrile adsorption. Quenching of gap states due to acetonitrile is reversible. The maximum of the highest occupied molecular orbital (HOMO) emission of the dye is found 1.6 eV above the leading edge of the nc-TiO2 valence band. The binding energies of dye orbitals are increased upon acetonitrile coadsorption. The HOMO emission is shifted by 150 meV and that of N 1s of the dye NCS group by approximately 200 meV. These shifts are reversed upon acetonitrile desorption.
Introduction Wide-band-gap semiconductors can be sensitized to visible light by the adsorption of molecular dyes to their surface. Technological applications are found in dye-sensitized solar cells (DSSCs).1-3 The working principle of such devices is based upon the injection of an excited electron from the lowest unoccupied molecular orbital (LUMO) of the sensitizer dye into the conduction band of the semiconductor.4,5 Nanoporous oxide films are used specifically due to their high surface area available to dye sensitization. Films of sintered TiO2 anatase nanocrystallites, sensitized by the dye RuII(2,2′-bipyridyl-4,4′-dicarboxylate)2(NCS)2 (Ru dye N3), have yielded the most efficient cells reported to date.6,7 Most devices employ a liquid electrolyte that typically contains an iodide/triiodide redox couple dissolved in acetonitrile (CH3CN). The function of the redox couple is to rereduce the dye cation HOMO (highest occupied molecular orbital) hole state following electron injection and to transport the resulting positive charge to the counter electrode. The complex interplay of TiO2 bulk and surface states, dye HOMO-LUMO states, and redox-active electrolyte states in the solvent ambient is crucial for the photovoltaic efficiency and is the subject of ongoing research.8 The possible conversion efficiencies depend on the electronic coupling of dye states to the substrate, the relative energy positions of the LUMO and the conduction band for electron injection, the relative position of the HOMO state and the reduced state of the redox couple for electrochemical reduction, and the density of interface states in the TiO2 gap for recombination, charge carrier trapping, and transport.9 In general, gap states play a detrimental role in solar cells by increasing the recombination rate. The role of surface band gap states in DSSCs is under discussion, and some authors even consider them to be beneficial to the photovoltaic conversion efficiency in forming a conduction channel on the * To whom correspondence should be addressed.
nanocrystallite surfaces.10 As electrochemical techniques will not be able to give a complete analysis of the processes involved, photoelectron spectroscopy has also been applied despite the problems inherent in analyzing a “wet interface” under ultrahigh-vacuum (UHV) conditions.11-13 Gap states have been observed with photoelectron spectroscopy on pristine nanocrystalline (nc) TiO211,14 and with increased intensity upon Li intercalation.11,15 They have been assigned to Ti 3d states (from which the TiO2 conduction band is derived) occupied due to either oxygen vacancies or lattice distortions, forming stabilized Ti3+ 3d1 sites.16,17 In this study we demonstrate quenching of the pristine gap state emission in the presence of Ru dye N3 and the solvent CH3CN. We point out that for the discussion of gap states all species present on the nc-TiO2 surface in the DSSC device, dye, solvent, and redox couple (which will be investigated in the near future) should be taken into account. In addition, we specifically address the influence of the solvent on the alignment of the dye HOMO level versus the substrate valence band, demonstrating that the HOMO position of the adsorbed dye, previously determined with photoelectron spectroscopy,3,11,14 changes in the presence of the solvent. Experimental Section The experiments have been performed at beamlines TGM7 and U49/PGM2 of the BESSY storage ring with the experimental station SoLiAS specifically designed for the analysis of solid/liquid interfaces.18 The base pressure of the analysis chamber is in the 10-10 mbar range. We analyze original materials as used for the production of state-of-the-art photovoltaic devices delivered by Solaronix S. A. Switzerland. Nanocrystalline TiO2 films have been prepared by sintering a colloidal solution of TiO2 crystallites, approximately 13 nm in size, for 30 min at 450 °C on glass substrates coated with the transparent conductive oxide SnO2:F. Sintering ex situ in air, sintering in UHV, and sintering in UHV under defined partial
10.1021/jp064689r CCC: $37.00 © 2007 American Chemical Society Published on Web 12/14/2006
850 J. Phys. Chem. C, Vol. 111, No. 2, 2007
Figure 1. N 1s core level spectra of nc-TiO2 in the course of Ru dye N3 adsorption and CH3CN coadsorption and desorption. Emissions assigned to the dye NCS, the dye bipyridine groups, and CH3CN are indicated. All measurements were performed on LN2-cooled samples.
O2 pressure were applied, leading to different carbon and water contamination levels and to varied surface Ti3+ contents. The Ru dye N3 was adsorbed from an ethanol solution in a pure N2 atmosphere and within a UHV-integrated electrochemical cell that allows for the transfer of wet chemically prepared interfaces to vacuum without contacting ambient air. After the adsorption of the dye, the sample was rinsed with ethanol to obtain monolayer coverage. Acetonitrile was adsorbed at a base pressure of 10-10 mbar from the gas phase of a liquid reservoir containing 0.3 nm molecular sieves onto samples cooled by contact to a liquid nitrogen (LN2) reservoir. All photoemission spectra displayed in this paper taken from pristine, (co)adsorbed, or desorbed samples have been measured on LN2-cooled samples. The binding energies of all spectra are referenced to the Fermi level of a gold test sample. Results and Discussion Adsorption of dye and coadsorption of acetonitrile may be followed by the N 1s spectra displayed in Figure 1. The highest surface sensitivity is given by the excitation photon energy used, hν ) 450 eV (photoelectron kinetic energy Ekin ) 50 eV). A Shirley background has been subtracted, and a fit using two Gauss-Lorentz (Voigt) profiles has been performed. The Ru dye N3 is indicated by emissions at binding energies EB ) 398.0 eV and EB ) 400.1 eV that have been assigned to N on the negatively charged NCS groups and to N on the bipyridine groups, respectively.19 CH3CN coadsorption is indicated by an increasing N 1s emission at EB ) 400.0 eV, too close to the bipyridine position for a separate fit. The measured CH3CN N 1s binding energy compares well to that of a physisorbed CH3CN phase condensed onto Si(100) with EB ) 400.2 eV.20 No indication of a reacted species is given for CH3CN adsorbed to nc-TiO2 (not displayed) or coadsorbed to the dye N3 on nc-TiO2 (Figure 1). In contrast to the CH3CN (co)adsorption on polycrystalline in-situ-prepared TiO2, the intensity of the N 1s emission on nanoporous TiO2 does not scale with exposure to CH3CN. One explanation of the differences in the adsorption processes between nc-TiO2 and polycrystalline samples may be found in the different film
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Figure 2. Ti 2p core level spectra at hν ) 600 eV excitation energy (from bottom to top): nc-TiO2 as prepared, after adsorption of Ru dye from ethanol solution, after in situ coadsorption of acetonitrile, and after annealing at room temperature.
structures. Adsorbed CH3CN may penetrate the nanopores of the nc-TiO2 sample, thereby not contributing to the photoemission signal. Upon CH3CN coadorption the binding energy of the N 1s emission assigned to the NCS group of the dye increases by ∆EB ) 200 meV (at 10 L of CH3CN (1L ) 1.33 × 10-6 mbar × 1 s)) which will be discussed later together with a shift of ∆EB ) 150 meV observed for the HOMO in the valence band spectra (Figure 4b). Ti 2p core level emissions of nc-TiO2 as prepared and after treatment with Ru dye and with different amounts of coadsorbed CH3CN are shown in Figure 2. A Shirley background is subtracted, and a fit using two Gauss-Lorentz doublets has been performed. In addition to the main emission line centered at EB ) 459.9 eV, which is due to Ti4+ of stoichiometric bulk TiO2, the nc-TiO2 shows a line chemically shifted to lower binding energy by ∆EB ) -1.7 eV. In accordance with previous studies, we assigne this lowenergy line to Ti in the oxidation state 3+ generally assigned to O defects,21,22 but also induced upon Li intercalation.11,15,17 We have calculated a Ti3+/Ti4+ intensity ratio of approximately 0.3 for a full monolayer coverage at Ekin ) 140 eV (hν ) 600 eV). However, for the sample as prepared we measure a Ti3+/Ti4+ ratio of 0.14, indicating submonolayer concentration. From sample to sample the intensity ratio of Ti3+/Ti4+ emissions varies between 0.04 and 0.18 depending on the preparation method and the history of the sample prior to spectroscopic analysis in the UHV. The concentration of these states is stable under the experimental photoemission conditions and does not change with the time of exposure to the monochromatized undulator light. (However, we have observed that nc-TiO2 can be further reduced by exposure to zeroth-order light of the dipole monochromator TGM7 at BESSY.) The Ti3+/Ti4+ ratio is reduced by approximately 40% when the Ti 2p orbital is excited with 900 eV photons, and the Ti3+ 2p emission was completely invisible in accompanying XPS studies, clearly characterizing the Ti3+ emission as a surface component. Compared to previously published Ti 2p synchrotron-induced PES spectra on nc-TiO2,15 the samples used in the experiments described here show a higher concentration of Ti3+ states, which we
Dye-Sensitized TiO2/Electrolyte Interface
J. Phys. Chem. C, Vol. 111, No. 2, 2007 851
Figure 3. Ratio of the integrated intensities of Ti3+ to Ti4+ emissions in the course of CH3CN adsorption and desorption (circles) and Ru dye N3 adsorption and acetonitrile coadsorption and desorption (triangles). Quenching of the Ti3+ emission due to dye and acetonitrile (co)adsorption and the reversibility of the quenching due to acetonitrile are evident.
attribute to differences in the preparation and experimental procedure prior to UHV analysis. However, a higher concentration may help make the effects of the dye and acetonitrile (co)adsorption on the Ti3+ states more evident. Adsorption of the dye from the ethanol solution as well as (co)adsorption of acetonitrile is accompanied by quenching of most of the Ti3+ 2p emission, corroborating the assumption that this emission can be attributed to a surface component. As acetonitrile is a Lewis base, we do not expect electron uptake from Ti3+. Rather the potential depth of the trap states may be decreased in the presence of acetonitrile, leading to untrapping (delocalization). The quenching of the Ti3+ emission is even more pronounced in parallel experiments (not displayed) on polycrystalline TiO2 thin films prepared by in situ metal organic molecular beam deposition from titanium isopropoxide. The ratios of the integral emissions of Ti3+ to Ti4+ in the course of acetonitrile adsorption and desorption and Ru dye adsorption and acetonitrile coadsorption and desorption on nc-TiO2 are displayed in Figure 3. It is interesting to note that the quenching by CH3CN is reversible; i.e., after desorption of CH3CN at room temperature the Ti3+ emission is nearly completely restored. The slow time scale of acetonitrile desorption and regrowth of the Ti3+ emission may be attributed to the film structure, releasing acetonitrile slowly from the nanopores. The fact that not all Ti3+ emission can be quenched by the adsorption process indicates that at least a part of the defect levels cannot be reached by the solvent molecules. Evidently a certain percentage of related defect levels remain active in the nc-TiO2 films. Valence band spectra in the course of dye adsorption and CH3CN coadsorption measured at 50 eV photon energy (near the Ti 3p f 3d resonance at 47 eV23) are displayed in Figure 4a and detailed spectra of the gap region in Figure 4b. At 50 eV excitation energy, as used for practical reasons instead of 47 eV, the Ti 3d emission is close to the maximum resonant intensity. Ghost or second-order excitation was not observed as confirmed by changing the excitation energy. The as-prepared samples show additional emission intensity in the band gap region with a maximum at EB ) 1.35 eV and states just below the Fermi level.
Figure 4. (a, top) Valence band spectra of nc-TiO2 at hν ) 50 eV (from bottom to top): as prepared, after Ru dye N3 adsorption from solution, after coadsorption of 14 L of acetonitrile, and after annealing at room temperature. Inset: resonant (50 eV) and off-resonance (35 eV) excitation proves the Ti 3d character of the gap states as well as the states just below the Fermi level. (b, bottom) Gauss-Lorentz (Voigt) fits to the nc-TiO2 gap states, the states just below the Fermi level, and the Ru dye N3 HOMO. In the bottom curve the leading edge of the valence band emission at 3.6 eV is indicated. Sample conditions are as in (a).
The off-resonance spectra of the as-prepared sample taken at 35 eV excitation energy (upper inset of Figure 4a) look very similar to published spectra on nc-TiO2 taken off-resonance at hν ) 150 eV.11,15 At this energy, also a small continuous population of electronic energy states which level off in two steps was found throughout the energy gap. Since a similar electronic state distribution was observed for a single-crystal rutile (110) surface after argon ion bombardment and subsequent annealing in oxygen, the gap states have been interpreted as surface states due to lattice imperfections, in particular missing oxygen.11 At resonance (lower inset of Figure 4a), the step at around 1 eV binding energy changes to a peak with its maximum
852 J. Phys. Chem. C, Vol. 111, No. 2, 2007 at EB )1.35 eV. Interestingly, our spectra taken at resonance look similar to the off-resonance spectra after Li insertion11 which also show a peak and an increased step height at the Fermi level. Ti 3p f 3d resonance behavior is observed for the peak and the states just below the Fermi level, as clearly indicated by comparing the 50 eV resonant to the 35 eV offresonance detail spectra displayed in the two insets of Figure 4a. While the band gap of anatase TiO2 is 3.2 eV according to optical measurements,24 we measure the valence band edge (leading edge of O 2p emission, Figure 4) to be EB ) 3.6 eV with respect to the Fermi level (EB ) 0) as defined by reference measurements on a Au sample. The measured leading edge position compares well to previous measurements.11 In ref 11 the conduction band minimum is assigned to the measured onset of occupied sates and the valance band maximum is placed, according to the literature value, 3.2 eV lower, a position that is clearly above the leading edge of the valance band emission. In contrast, we assign the TiO2 valence band maximum EVBM to the measured position of the leading edge. As the stoichiometry of the surface deviates from TiO2, the surface electronic structure may be more complex, however. In experiments that create point defects by argon sputtering on vacuum-fractured single rutile crystals a shift of the valence band maximum EVBM to higher binding energy was observed.25 However, for a partial surface layer of TiO2-x we would still expect to observe the valence band maximum of the underlying stoichiometric TiO2. The leading edge of the valence band and the shape of the O 2p valence band emission do not change with variations of surface sensitivity in the measured excitation energy range (hν ) 21-90 eV). Therefore, no dispersion is observed, which justifies our choice to place the valence band maximum EVBM of nc-TiO2 at the actually measured position of the leading edge (EB ) 3.6 eV). As a consequence, the position of the Fermi level is situated 0.4 eV above the corresponding TiO2 conduction band minimum. Thus, in the rigid band model the occupied states just below the Fermi level would be attributed to the TiO2 conduction band (Figure 6a). As the rigid band model has to be questioned, the assignment of the states just below EF has to be discussed further. Detailed spectra of the gap region taken at 50 eV excitation energy are displayed in Figure 4b. The spectra have been normalized to the O 2p emission at 6.4 eV, which leads to a similar trend in the intensity ratios as observed in spectra taken at 90 eV that have been normalized to the Ti 3p intensity at EB ) 38 eV. Deconvolution of emissions in this region is not a simple task; however, we show a fit that quantitatively supports the observations that can also be deduced from close inspection of the original data (Figure 4a). An unstructured secondary electron background as displayed in Figure 4b was used for this fit. Gauss-Lorentz (Voigt) lines were fit to the nc-TiO2 gap states, the states just below the Fermi level, and the Ru dye N3 HOMO states despite the fact that a Gauss-Lorentz curve does not describe the Fermi edge correctly. The gap states at 1.35 eV EB have been discussed previously as localized Ti3+ 3d1 states.11 A similar emission has been observed to develop upon Li intercalation, and its intensity was related to the intensity of Ti3+ emission in Ti 2p core level spectra.15 As we observed no intensity change with monochromatized synchrotron light exposure time nor beam intensity, we rule out synchrotron-lightinduced formation of the gap states. In contrast, surface states have been induced by UV as observed in photocurrent action spectra.26 The gap states show quenching with dye and acetonitrile adsorption similar to that of the Ti3+ core level lines discussed above. According to the fit shown in Figure 4b, the
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Figure 5. Gap-state spectra of nc-TiO2 taken at hν ) 90 eV in the course of CH3CN adsorption and desorption. To correct for damping due to adsorption, the spectra have been normalized to the O 2s emission at 23 eV binding energy. Reversible quenching of the gap states at 1.35 eV is evident.
area of the gap state emission at EB ) 1.35 eV changes from 100% of the pristine surface to 90% in the presence of the dye, to 70% with CH3CN coadsorbed, and back to 90% after annealing. The quenching of the gap states due to CH3CN adsorption is even more evident in VB spectra taken in the course of CH3CN adsorption and desorption without the dye as shown in Figure 5. The spectra were taken at 90 eV excitation energy and have been normalized in intensity to the O 2s emission at EB ) 23 eV. The states just below the Fermi level have also been detected with increased intensity after electrochemical Li intercalation and were assigned to Ti sites interacting with Li+ or other cations stabilizing Ti 3d1 states.11 However, in our case no alkalimetal atoms are present. In a simple rigid band model this emission would be attributed to degenerate n-doping, shifting the Fermi level up into the conduction band. However, the rigid band model has been contested on several occasions, and an induced emission just below the Fermi level was related to interacting Ti3+ 3d1 states16,17 which for stoichiometric Ti2O3 give rise to occupied Ti 3d a1g states, forming the partially filled conduction band of metallic Ti2O3.25 Similar gap states have also been found on other d0 metal oxides with partially reduced M atoms due to intercalation or deficiency in the oxygen stoichiometry. For example, V2O5-δ or Li(Nax)V2O5 show similar d1-derived band gap states electronically separated by approximately 1.3 eV from the position of the Fermi level.27,28 After adsorption of the dye from the ethanol solution and transfer to UHV, the maximum of the adsorbed dye HOMO level emission is found 1.6 eV above the measured valence band edge corresponding to EB ) 2.0 eV (see Figure 4b). The difference between this and the published values for the HOMO positions of 1.4 eV14 (and also ref 11 when consistently referred to the measured leading valence band edge) and of 1.2 eV given in ref 3 is most likely due to different dye adsorption procedures especially considering that we have avoided contact to ambient air. With CH3CN coadsorption (which shows no additional emissions in the HOMO region; see Figure 5), the dye HOMO level, which extends mostly over the NCS group,3 is shifted to
Dye-Sensitized TiO2/Electrolyte Interface
Figure 6. Schematic of the photovoltaic-relevant valence states in the rigid band model (a) as measured on the as-prepared nanocrystalline TiO2 anatase film, (b) after dye adsorption from the ethanol solution with the HOMO position, and (c) after coadsorption of the solvent acetonitrile with the HOMO shifted by 150 meV to higher EB. Using the optical absorption maximum, the LUMO is found 0.17 eV above the Fermi level.
higher binding energy (lower distance to EVBM) by ∆EB ) 150 meV relative to the substrate emissions which are stable in binding energy. Also EB for the N 1s emission of the NCS group has been increased by ∆EB ) 200 meV after exposure to 10 L of CH3CN (Figure 1). These shifts are reversed after desorption of CH3CN at room temperature (all measurements taken at LN2 temperature). Shifts of dye N3 levels to higher binding energy versus TiO2 levels have also been observed in the course of CuI coadsorption and were attributed to NCS-CuI interaction.29 We have previously observed a solvation-induced shift to lower binding energy of the 2s emission of Na+ ions on WSe2 by H2O coadsorption, which could be related quantitatively to the oriented adsorption of H2O dipoles around Na+.30 A shift to higher binding energy could be induced by polar solvent molecules adsorbing with their positively charged tail oriented toward a negatively charged molecular group such as NCS. Also a change of the dipole between TiO2 and the dye molecules could be induced by coadsorbed solvent molecules. In such cases the binding energies of adsorbate states are not fixed versus substrate binding energy positions (see, e.g., ref 31). Similar dipolar potential shifts have also been proposed as the cause of the beneficial effect that the TiO2 alkali-metal treatment has on the photovoltaic efficiency.11 For the coadsorption of acetonitrile we instead expect the change of the preexisting surface dipole field established by proton transfer from the carboxylic acid anchoring groups of the ruthenium complex to the TiO2 surface.32 A decrease of the dipole field due to the high dielectric constant of acetonitrile ( ) 36.6 at 20 °C33) corresponds to increased binding energy of the dye states. The different values observed for the HOMO and the N 1s shifts of the NCS groups to which the HOMO expands are possibly related to different locations in the dipolar field across the dye molecule. Additional changes in the line shape of dye S and C core level spectra have been observed and preliminarily reported.34 A close inspection and interpretation of the dye core emission changes induced by CH3CN coadsorption will be given in a forthcoming paper. A schematic representation of the photovoltaic-relevant valence states as deduced in the simple rigid band model is displayed in Figure 6. Parts a-c show the measured occupied states for the untreated sample and the surface with adsorbed
J. Phys. Chem. C, Vol. 111, No. 2, 2007 853 dye and after acetonitrile coadsorption. The measured HOMO corresponds to the position of the lowest energy hole state created by the photoemission of an electron, i.e., the HOMO of the molecular cation. Except for a Franck-Condon shift of approximately 0.1 eV away from the Fermi level due to vibrational excitation in the photoemission process,35 this is the relevant energy position for rereduction by the redox system. For the electron injection process the alignment of the LUMO to the conduction band edge is crucial. The LUMO may be positioned by adding the energy of the optical absorption maximum (535 nm ) 2.32 eV) to the HOMO, but in general, the HOMO position in optical absorption may differ from the measured value in photoemission. This occurs because in optical absorption, a Frenkel exciton is created on the neutral molecule35 while photoemission tests the state of the cation. Due to the low dielectric constant in organic semiconductors, high values for the exciton binding energies are found in the range of 0.41.4 eV,35 which may shift the HOMO/LUMO positions accordingly.36 On the other hand, acetonitrile is a solvent with a high of 36.6 at 20 °C33 which is expected to drastically reduce the exciton binding energy that is proportional to 1/2 in the Bohr model. Therefore, we directly add the optical absorption energy to the HOMO as measured in the presence of CH3CN and find the LUMO 0.17 eV above the Fermi level. Summary, Conclusion, and Outlook Nanocrystalline films of anatase TiO2 and their interaction with Ru dye N3 and solvent molecules of acetonitrile have been examined by high-resolution photoelectron spectroscopy of valence and core level states. TiO2 sintered ex situ, in situ, or in situ under O2 partial pressure show different Ti3+ concentrations in Ti 2p core level spectra and band gap state concentration in valence band spectra. The dye was adsorbed from an ethanol solution under a N2 atmosphere in a wet chemical preparation cell integrated to the UHV system, and acetonitrile was coadsorbed in situ on LN2-cooled samples. Compared to insitu-prepared polycrystalline TiO2 films, we find peculiarities in adsorption and desorption kinetics which we attribute to the nanopore structure of the sintered nc-TiO2 films. The pores function as a sink for adsorbing CH3CN and as a source for desorbing molecules. Nanocrystalline films of anatase TiO2 show the leading edge of the valence band at EB ) 3.6 eV below the Fermi level compared to an optical gap of 3.2 eV. Gap states are found with a maximum at EB ) 1.35 eV, which is 2.2 eV above the measured valence band edge; additional states extend up to the Fermi level. The resonance behavior indicates Ti 3d orbital character for both emissions, which therefore are assigned to Ti ions in the 3+ oxidation state. Ti 2p core level spectra show an additional emission with a chemical shift of ∆EB ) -1.7 eV. This emission also indicates the presence of Ti3+ ions. The relative intensity of the Ti3+ 2p emission, and of the gap states, decreases with adsorption of the dye as well as with acetonitrile, thus indicating surface species. Obviously the Ti3+ 2p and the gap state intensity depend on coadsorbed species and are therefore strongly influenced by different sample treatments prior to UHV analysis in the respective studies. In accordance with the literature, we assign the gap states with peak emission at EB ) 1.35 eV to O vacancy defects of the nanocrystallite surfaces. For the states just below the Fermi level we discuss two models. For the discussion of energy alignment we use the rigid band model in which these states are attributed to the filling of the conduction band due to degenerate n-doping. Alternatively these states can be explained
854 J. Phys. Chem. C, Vol. 111, No. 2, 2007 by interacting Ti3+ defect states as high defect concentrations with interacting Ti3+ 3d orbitals give rise to states just below the Fermi level similar to the a1g band in metallic Ti2O3. The presence of Ti3+ surface ions in the submonolayer range is indicated by Ti 2p core level spectra. The Ti3+ states are quenched by dye as well as by acetonitrile adsorption and coadsorption. Since the gap states are involved in conduction and shunt recombination, quenching of these states may be directly related to the photovoltaic efficiency. The HOMO of the adsorbed sensitizer is found 1.6 eV above the measured VB edge (HOMO EB ) 2.0 eV). An important fact is that the dye binding energy positions are changed by coadsorption of acetonitrile. A dipole is induced, or an existing surface dipole is modified, shifting the HOMO by 150 meV and N 1s of the NCS group by approximately 200 meV to higher EB. In the presence of high CH3CN we directly add the energy value of the optical absorption to the measured HOMO position to find the LUMO maximum 0.17 eV above the Fermi level. In the rigid band model the Fermi level lies 0.4 eV above the TiO2 conduction band minimum. As the rigid band model has to be questioned, the states just below the Fermi level must be investigated further. In the near future, the electronic structure in the presence of the redox couple iodide/triiodide will be examined. Systematic comparisons with in-situ-prepared films of TiO2 using the single source precursor Ti(Opr)4 will also be performed. Acknowledgment. Funding of the project by DFG under Grant No. JA 859/3-3, of SoLiAS by BMBF under Project No. 05KSIRD1/0, and of BESSY beam time by BMBF under Grant No. 05 ES3XBA/5 is gratefully acknowledged. References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Moser, J. E.; Bonnote, P.; Gra¨tzel, M. Coord. Chem. ReV. 1998, 171, 245. (3) Hagfeldt, A.; Gratzel, M. Acc. Chem. Res. 2000, 33, 269. (4) Tachibana, Y.; Moser, J. E.; Gra¨tzel, M.; Klug, D. R.; Durrant, J. R. J. Phys. Chem. 1996, 100, 20056. (5) Hannappel, T.; Burfeindt, B.; Storck, W.; Willig, F. J. Phys. Chem. B 1997, 101, 6799. (6) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; Vonzelewsky, A. Coord. Chem. ReV. 1988, 84, 85. (7) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humpbry-Baker, R.; Miller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (8) Durrant, J. R.; Haque, S. A.; Palomares, E. Coord. Chem. ReV. 2004, 248, 1247.
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