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Strong Interactions in Dye-Sensitized Interfaces P. Palmgren,*,† K. Nilson,‡ S. Yu,† F. Hennies,§ T. Angot,| C. I. Nlebedim,† J.-M. Layet,| G. Le Lay,⊥ and M. Go1 thelid† Materials Physics, MAP, ICT, Royal Institute of Technology, Electrum 229, SE-164 40 Stockholm, Sweden, Department of Physics, Uppsala UniVersity, Box 530, SE-75121 Uppsala, Sweden, MAX-Lab, UniVersity of Lund, Box 118, SE-22100 Lund, Sweden, Physique des Interactions Ioniques et Mole´ culaires, UMR CNRS-UniVersite´ de ProVence 6633, F-13397 Marseille, France, and CRMCN-CNRS, Campus de Luminy, case 913, F-13288 Marseille, France ReceiVed: NoVember 29, 2007; In Final Form: January 25, 2008
Phthalocyanines (Pcs) are capable of converting sunlight into electric energy when adsorbed on TiO2 in a dye-sensitized solar cell. Of special interest in this type of cell is the energy level alignment as well as how molecules adsorb on the surface as it determines the output of the cell. We investigated the FePc-TiO2(110) interface using scanning tunneling microscopy, synchrotron-based photoelectron spectroscopy, and X-ray absorption spectroscopy. We found a strong coupling of the first-layer FePc to the substrate resulting in an alteration of the electronic structure and charge transfer from the molecules. The FePc in the second layer is not severely affected by the bonding to the surface and has bulk-like electronic properties. The growth of FePc thin films proceeds in a layer plus island mode, and the molecular plane is parallel to the surface. The energy level alignment at the interface is determined, and the lowest unoccupied molecular orbital is found above the conduction band minimum of the oxide substrate.
Introduction Dye-sensitized solar cells1-3 are under intense investigation due to the possibility of solving the world’s demand for ever more electric energy. A variety of dyes have been tested with various results; the highest conversion efficiency to date is 10.5%.4 In this context, phthalocyanines (Pcs) may be used as dyes5-7 as their energy gap is around 2 eV, suitable for excitation by sunlight. They are moreover chemically and thermally stable and thus capable of many excitation cycles to inject electrons into the semiconductor conduction band. The phthalocyanines are ring-like molecules consisting of four isoindole groups (each consisting of a benzene ring joined with one side to a pyrrole group) connected in such a way so that a void is formed in the center of the ring in which, e.g., a metal ion can reside. The molecule is normally flat, but deviations occur if the center moiety is too large. Phthalocyanines are considered to be weakly interacting with substrates such as Au, HOPG, and III-V semiconductors8-10 since the properties of the interface layer molecules are not strongly altered upon adsorption. Whether this interaction actually is weak is under current debate since phthalocyanine adsorption can cause a change in the periodicity on Au(110), lifting the 1 × 2 missing row reconstruction in favor of a substrate surface with 1 × 3 periodicity, matching molecular dimensions.11,12 The signature of this interaction is small in photoelectron spectroscopy (PES); the shakeup transition in the C1s core level (CL) remains the same as in the bulk film,12 and * To whom correspondence should be addressed. Phone: +46 8 790 4162. Fax: +46 8 752 78 50. E-mail:
[email protected]. † Royal Institute of Technology. ‡ Uppsala University. § University of Lund. | UMR CNRS-Universite ´ de Provence 6633. ⊥ CRMCN-CNRS.
the molecular energy levels in the interface layer are very close to the ones in higher layers.11 The conducting electrode in a solar cell is most often a transparent oxide; these surfaces are more reactive, which affects the adsorbed organic layer and alters the electronic properties at the interface.13-15 The interface is of importance as the molecules closest to the electrode are responsible for electron injection; thicker layers or aggregates of molecules have a negative effect on cell performance.16-19 This also leads to the issue of interface order; as many molecules as possible should be attached to the electrode surface to give a high cell current, favored by a well-ordered system, but too close contact between the dye molecules gives unwanted channels for intermolecular recombination. The energy level alignment at the interface determines the cell output voltage, although the basic principles behind this alignment are not yet well established.20 A suitable alignment of the energy levels between dye and substrate at the interface is of fundamental importance; an absolute prerequisite is that the excited state of the molecule is above the substrate conduction band minimum (CBM) to allow electron injection. Detailed investigations of phthalocyanines deposited on welldefined oxide surfaces are scarce despite their technological importance. TiOPc, CuPc, H2Pc, and ZnPc have been deposited on single-crystal TiO2 and ZnO surfaces, and morphology, energy level alignment, and excited-state dynamics were investigated.5,13,21-23 The phthalocyanine used in the present work, FePc, has previously been deposited on HOPG and Au, where it self-assembles into an ordered layer.24,25 The photovoltaic properties of the FePc-TiO2 system have been investigated by Sharma et al.,26 and the occupied and unoccupied electronic states in thick FePc films have been probed by Åhlund et al.27 In this work, we investigate the initial stages of interface formation and the energy level alignment of FePc on TiO2(110)
10.1021/jp711311s CCC: $40.75 © 2008 American Chemical Society Published on Web 03/26/2008
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using scanning tunneling microscopy (STM), PES, and X-ray absorption spectroscopy (XAS). We find that the FePc molecules in the first monolayer (ML) mainly adsorb with their plane parallel to the surface; they are oxidized by the interaction with the substrate, and the shakeup transitions in the C1s core level are quenched. At higher coverage, islands with lying molecules form. The shakeup is visible in the C1s spectrum, indicative of bulk-like properties, and the highest occupied molecular orbital (HOMO) appears at 1.6 eV below the Fermi level. Thus, the first FePc layer is relatively strongly bound to the TiO2 surface and the electronic structure is altered, whereas molecules in the outer layers are less affected by the substrate, retaining their molecular properties. Experimental Section The electron spectroscopy experiments were performed at beam line I511 at MAX-Lab in Lund, Sweden. This is an undulator-based beam line designed for photon energies ranging from 50 to 1000 eV. The end station is equipped with a Scienta R4000 electron spectrometer rotatable around the direction of the incoming synchrotron light. It also houses low-energy electron diffraction (LEED) optics and sputter cleaning possibilities. Further details on the beam line are found in ref 28. Photoelectron spectra are collected in normal emission and grazing incidence of the photons. The Fermi level (EF) is measured on a Ta foil in electric contact with the sample for energy reference. Core level spectra are normalized to the background on the low-energy side of the peak, and the valence band (VB) spectra are normalized at 15 eV binding energy. The film thickness was determined from the integrated intensity of the adsorbate CL and the development of the line profiles. X-ray absorption spectra are collected in partial yield by counting the number of emitted electrons in a ∼20 eV kinetic energy interval around the nitrogen Auger line using the Scienta analyzer. As the photon energy is scanned, photoelectrons from the substrates valence band as well as the O2s and Ti3p levels enter and leave the kinetic energy window, giving rise to an uneven background, which also changes with adsorbate thickness. The settings in the analyzer are chosen so that the background at the onset of absorption is a straight line. Because of this, all absorption spectra presented are difference spectra: the spectra are first normalized to the intensity of the incoming light, measured on a gold grid situated upstream, and then the normalized background intensity measured on the clean substrate is subtracted. The photon energy calibration was done by measuring a substrate core level with both first- and secondorder light. STM measurements were performed with an Omicron VTSTM in constant current mode. All images are captured at room temperature, and positive bias corresponds to imaging empty states. This system further contains sample cleaning facilities, LEED optics, as well as the deposition source which is a homebuilt Knudsen-type cell consisting of a quartz tube wound with a resistively heated W filament. The FePc is supplied by Aldrich (90% purity) and used as received after a thorough degassing in front of a mass spectrometer to monitor the levels of contamination (mainly water). Samples of rutile TiO2(110) were supplied by the Surf. Prep. Lab., The Netherlands, and initially transparent, but annealing at ∼1000 K in vacuum for several hours introduced oxygen vacancies in the bulk, giving the crystal a blue color. The color darkened with increasing number of sputtering and annealing cycles. After sputtering, annealing was done at roughly 1000 K either under UHV or in 5 × 10-8 mBar O2 to remove
Figure 1. C1s spectra of FePc on TiO2(110) measured with 347 eV photon energy. The fitted components are shown in black for the firstlayer peak and in gray for the peak originating in the second layer; the fit parameters are 1.1 and 0.8-0.9 eV fwhm (convolution of Gaussian and Lorentzian widths), respectively.
potassium emerging from the bulk of the sample. These preparation conditions lead to a rather defect-rich surface with strands along the [001] direction and small terraces (about 50 nm in size). These strands are precursors to the 1 × 2 reconstruction; their quantity depends on the degree of reduction of the sample29-31 and increases with sample age. Annealing in a low oxygen pressure causes growth of the 1 × 2 phase mainly at the step edges,32 producing both regions of crosslinked 1 × 2 phase as well as large areas of 1 × 1 phase. Various defects on the surface result in Ti3d-derived states in the band gap,33 whose relative intensity is a measure of the degree of reduction.34 The LEED patterns obtained during our experiments showed 1 × 1 symmetry with weak streaks along the [11h0] direction due to the 1 × 2 precursors. Since the probe area in the photoelectron spectroscopy experiment is large, our results are dominated by the 1 × 1 phase. Results and Discussion In Figure 1 we present C1s CL spectra from selected preparations. To elucidate the molecule-substrate interaction, a submonolayer film is deposited. In the C1s signal from the 0.7 ML, the main peak found at 285.6 eV binding energy is due to emission from benzene-type carbon atoms and the shoulder at 287.3 eV is due to emission from pyrrole group carbon atoms. The relatively broad line profile is a consequence of several different adsorption sites on the surface. Also noticeable is the absence of the typical shakeup structure, normally found roughly 2 eV behind the main lines in the C1s spectra. The shakeup is an excitation of an electron across the molecular HOMO-LUMO (lowest unoccupied molecular orbital) energy gap by the escaping C1s photoelectron, which loses
5974 J. Phys. Chem. C, Vol. 112, No. 15, 2008 energy in the process. Due to the strong electronic coupling to the oxide surface, this excitation channel is clearly blocked or strongly reduced. New spectral features appear in the 1.4 ML spectrum; the C1s signal clearly has contributions originating in two types of FePc, assigned to first- and second-layer molecules. To separate them a numerical fit using Voigt functions is performed. The black area in the 1.4 ML spectrum represents the first-layer FePc; the line profile resembles that of the 0.7 ML film with the same two components: one originating in the benzene group carbon atoms at 285.7 eV binding energy and a second one arising in pyrrole group carbon atoms at 287.3 eV. The gray area in the 1.4 ML spectrum represents FePc molecules in the second layer; the C1s binding energy for the benzene-type carbon atoms is 284.5 eV, while it is 286.2 eV for the pyrrole-type carbon atoms. In addition, loss features appear at 286.5 eV for the benzene carbon-related shakeup and at 288.1 eV for the pyrrole carbon-related shakeup. Both binding energies and the line profile are similar to the thick film case.27 The two topmost spectra in Figure 1 are measured on 3 and 5 ML thick films, and a gradual change in line profile with coverage is clearly seen as the interface component becomes less significant and a clearly resolved shakeup at 288.2 eV binding energy emerges. The relative strength of the first-layer contribution to the thick film spectra suggests layer plus island growth of FePc on the TiO2 surface as this feature is slowly attenuated. Noticeable is also the change toward higher binding energy in the peak from the second and higher molecular layers; the benzene-type carbon peak goes from 284.5 eV for the second-layer FePc in the 1.4 ML film to about 284.8 eV for the 3 and 5 ML films. Evidently, there is a rather large binding energy shift of about 1.2 eV between the molecules in the first and second layer. The energy separation of the pyrrole and benzene components in the respective gray and black spectra is practically unaffected, while there is a shift of the whole spectrum. Core level shifts reflect the local electronic and chemical structure around the emitter. Even though it is often the ground-state properties that are of primary interest, one has to keep in mind that the shift cannot simply be translated into charge transfer or charge density since the excitation process itself leaves a core hole that induces final state shifts due to the ability of the system to effectively screen this core hole. Both the chemical, initial state effects and the final state effects can be of the same magnitude with shifts of a few electronvolts. To get an estimate in the present case we use the main benzene carbon line as a measure. The second-layer contribution appears at 284.5 eV, whereas it is found at 284.8 eV at 5 ML. This binding energy shift is essentially caused by the less efficient screening within the thicker film, since the chemical bonding between molecules can be expected to be very similar. Indeed, a similar value was found for thick FePc films.27 Interestingly, the same binding energy value has also been found in the Ag(110)-FePc monolayer,35 which is a metallically screened system. Thus, final state screening is not likely to dominate the observed shift in the present case, and we conclude that the first-layer molecules are oxidized as a consequence of the surface bond, manifested in the binding energy difference between the first- and secondlayer molecules as well as the loss of shakeup transitions for the first-layer molecules. The line shape becomes a bulk like with the advent of the shakeups as the coverage increases; the region strongly affected by adsorption is hence limited to the first layer. Photoemission spectra from the N1s CL are presented in Figure 2; the line profile associated with FePc has two features
Palmgren et al.
Figure 2. N1s spectra recorded with a photon energy of 455 eV at the coverage indicated in the figure.
in the 0.7 ML spectrum: a peak at 400.4 eV and a shoulder at 399.1 eV. The small peak at 396.7 eV is not related to FePc as it was present in spectra from the clean surface prior to FePc exposure. It may be due to native nitrogen contamination in the crystal or introduced in the sputtering process. Its intensity varies with different preparations, but the binding energy remains the same, and as the coverage increases it becomes less visible. In the 1.4 ML spectrum, the component at 399.2 eV has increased, and we relate this to the second FePc layer. The N1s line profile becomes a bulk like in the two thicker films with a binding energy of 398.9 eV for the 3 ML film and 399.0 eV for the 5 ML film, in agreement with the result of Åhlund et al.27 The small binding energy shift is again related to less efficient screening in the organic multilayer film. In the 3 ML film the contribution from the first layer is still seen as a relatively strong emission at about 400.5 eV; the shakeup becomes visible and is clearly resolved at 401.0 eV binding energy in the 5 ML spectrum. The shakeup is also absent in the N1s spectrum from the first layer, further underlining that the HOMO-LUMO excitation channel is strongly disturbed by the chemical bond between FePc and the surface. Furthermore, the energy separation between the two main peaks is 1.2-1.3 eV, close to the C1s result and in line with oxidation of the first-layer molecules. The global shift indicates that the molecule as a whole is affected. The rather strong shoulder at 399.1 eV binding energy is puzzling as it coincides with emission from higher layer molecules, yet at this coverage this signal should be weak when comparing with C1s. It may instead be assigned to molecules adsorbed on the 1 × 2 strands with different adsorption geometry and leading to a difference in binding energy. This type of defect is relatively frequent on the surface and can therefore be expected to contribute to the line shape. The N1s spectra follow the development of the C1s spectra closely: the shift to higher binding energy is about the same in the two core levels, and the shakeup is also quenched only in the first-layer moleculed due to the relatively strong surface bond and the charge transfer from the molecule to the substrate.
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Figure 3. STM images of (a) the clean surface (700 × 370 Å2, +2.3 V, 0.24 nA) and after deposition of about 1.5 ML FePc (b; 180 × 100 Å2, +1.0 V, 0.11 nA) and (c; 150 × 95 Å2, +2.0 V, 0.20 nA). Nearly the same area is imaged in b and c; in both images the same molecule is encircled.
To visualize the situation at the interface, STM measurements were carried out, and the results are presented in Figure 3. The image displayed in Figure 3a is representative for the clean surface. The bright lines in the image running along the [001] directions are precursors to the 1 × 2 phase, predominately found at step edges, and the small bright dots are nuclei for these strands, but the main part of the surface exhibits the 1 × 1 phase. Figure 3b and c shows the surface after deposition of roughly 1.5 ML FePc imaged at two tunneling conditions; the same molecule in the first layer is encircled in both images: in b the FePc molecules are imaged at +1.0 V with a bright center, as commonly seen for FePc;24,25,36 in c, at +2.0 V the electrons tunnel into a state localized on the Pc ring, and the center of the molecule is not imaged bright. The distance between the molecular centers in the first layer is about 13.5 Å, in correspondence with FePc dimensions. The molecules in the second layer form tightly packed FePc islands (bright fields). These STM observations corroborate the layer plus island growth and also show that the first-layer FePc is intact on the surface. Measurement of the unoccupied states gives additional information about the FePc-TiO2 interface. Absorption is polarization dependent, and by aligning the E vector of the synchrotron light with the π orbitals of the FePc, their orientation on the surface can be determined. The unoccupied states measured in XAS are probed in the presence of a core hole, which may obstruct quantification of the unoccupied levels energy position. The topmost spectra in Figure 4 present the XA spectra from a 5 ML thick film deposited on TiO2(110). At the absorption edge in the vertical geometry two contiguous peaks are found at 398.5 (A) and 399.0 eV (B), and absorption into higher lying states is found at 400.7 (D) and 402.4 eV (E). In the horizontal geometry, two narrow absorption peaks are found at 399.1 (B) and 399.8 eV (C), a broader absorption peak at 402.5 eV (E), as well as a small shoulder at 398.5 (A) and 400.5 eV (D). The spectrum from the horizontal geometry in Figure 4 is very similar to XA spectra calculated for the absorption into states located in the molecular plane, while the spectrum from the vertical geometry strongly resembles the experimental spectra measured on a thick film and show some resemblance with calculated spectra corresponding to excitation into the π states of the FePc.27 The present results are hence representative of
Figure 4. Nitrogen K-edge spectra from FePc adsorbed on TiO2(110). The geometry for each spectrum is indicated. The horizontal geometry corresponds to the sample surface parallel to the E vector of the light.
FePc molecules with an electronic structure similar to the thick film case, inferring that the substrate influence is small in the 5 ML thick film. The observed peaks thus correspond to excitation into the LUMO and higher unoccupied levels of the unperturbed FePc. The orientation of the FePc in the higher layers can be deduced from the similarity with the calculated XA spectra27 and the signal ratio between the excitation into bound and continuum states in the two geometries. We find that the FePc molecules adsorb with the molecular plane parallel to the surface apart from a small tilt angle, evidenced by the small shoulder (A) at the absorption edge in the horizontal geometry. A similar adsorption mode is also found for phthalocyanines on weakly interacting substrates.37 The XA spectra from the horizontal geometry in the 1.4 ML thick film is dominated by three narrow peaks: B at 399.1 eV, C at 399.8 eV, and E at 402.1 eV. In the vertical geometry at the same coverage peaks B and C are found at the same position, but peak E is located at 401.9 eV; the line profile is severely broadened by shoulders A and D, which are related to excitation into states associated with FePc in the second layer. Shoulder A is also visible at the absorption edge in the horizontal geometry but with low intensity. The line profile in the horizontal geometry closely resembles that of the 5 ML spectrum, whereas the spectral line shape in the vertical geometry is not. The unoccupied energy levels in the first-layer molecules are altered, mainly in the orbitals perpendicular to the molecular plane due to the adsorption as these are probed in the vertical geometry. The XA spectra thus mirror the interaction between the adsorbate and substrate at the interface: the first FePc layer is affected by the bond to the surface, but already in the second-layer FePc has a bulk-like electronic structure. Comparing the signal strengths between the signal from excitation into bound and continuum states for the two geometries infers an orientation with the molecular plane mainly parallel to the surface in the first FePc layer. The occupied VB electronic states measured with 110 eV photon energy are presented in Figure 5. The spectrum from
5976 J. Phys. Chem. C, Vol. 112, No. 15, 2008
Figure 5. Valence band spectra measured with 110 eV photons. Vertical bars indicate the HOMO position in the thick film case. The arrow indicates a possible state related to the HOMO in the first-layer FePc molecules.
the clean substrate is dominated by emission from the gap states at 0.9 eV, and the valence band maximum (VBM) is found at 3.0 eV. The other spectra are scaled to the height of the gap state of the bare substrate to emphasize the line profile. The VBM appears to shift toward lower binding energy as the coverage of FePc increases, but this effect is due to emission from the HOMO-1 state. Deposition of the 1.4 ML film results in an increased emission between the gap states and the VBM. No distinct peak related to emission from the HOMO is clearly detected until reaching the 5 ML thickness, where it is found at 1.6 eV below EF (marked with a vertical bar). The strong coupling of the molecules to the surface and their oxidation results in an interface state (or states), located in the substrate energy gap. The molecular energy levels are clearly altered by adsorption: the chemical shifts in the C1s and N1s CL spectra as well as loss of the shakeups are indicative of charge transfer from the molecules. The former HOMO of the first-layer FePc molecules might be split into several components upon adsorption; the small foot at EF (marked with an arrow) in the 1.4 ML film could possibly be attributed to a part of the interface state; electrons transferred from the molecule ought to be found close to EF. However, emission from the first ML molecules can be obscured if any components coincide with the substrate gap states. Indeed, several interface states are formed upon adsorption of FePc on Ag(110).35 This is also the case for other organic molecules.38,39 On Au(110) the HOMO in the first ML has a slightly lower binding energy compared with the molecules in the upper layers.11 We now turn to the alignment of the energy levels at the interface. By extrapolating the VB leading edge, the TiO2 VBM is found at 3.0 eV below EF, thus placing the CBM 0.1 eV above EF if a band gap of 3.1 eV is assumed. The alignment of the energy levels in the second and higher FePc layers can be deduced from the measured HOMO position at 1.6 eV and assuming an unaltered band gap. The HOMO-LUMO gap amounts to 2.6 eV,26 in agreement with an optical gap of 2.0 eV according to UV-vis measurements40,41 and an exciton binding energy of about 0.5 eV found for CuPc.42,43 This locates the LUMO 1.0 eV above CBM, a slightly underestimated value as the leading edge of the VB is more appropriate than the peak apex43,44 for determining the energy level position. However, a fit of the VB spectra would not give a reliable position for the
Palmgren et al. leading edge of the FePc HOMO given the complicated situation at the interface; the background is a function of coverage, and there is a reacted species underneath the second-layer FePc in which the peak position is not well known. Even so, the LUMO state for these molecules is located well above the CBM, and thus, the prerequisite for the function of the dye-sensitized cell is fulfilled. The situation for the first-layer FePc is rather different; the molecules appears to be intact at the surface, but adsorption leads to oxidation of the first-layer molecules and interface states both in the occupied and unoccupied energy levels. The electronic transition over the molecular energy gap by absorption of visible light thus ought to be severely affected by the molecular bond to the surface, which may be of great importance in the dye-sensitized solar cell application. Although the layer closest to the oxide electrode is affected by adsorption, electrons may tunnel from the second-layer molecules, through the first lying FePc ML, into the conduction band of the oxide. The second-layer molecules form a densely packed layer close to the conducting electrode, which could be considered advantageous. However, the interaction between the FePc opens up a channel for intermolecular recombination, which could be detrimental. Conclusion The first monolayer FePc molecules on TiO2(110) are adsorbed flat with their molecular plane parallel to the surface, thus in close contact with the substrate. The electronic properties are altered, the shakeup features in the C1s and N1s spectra are quenched, and only emission from pyrrole and benzene group carbon is observed. There is a large difference in the adsorbate core level binding energy between first- and second-layer molecules due to oxidation. All this points to a strong coupling of the FePc to the TiO2(110) surface. For thicker films, the molecules are also lying flat but a small tilt angle cannot be excluded. The interface exhibits a layer plus island growth mode observed in STM: where the second layer builds up in molecular islands. The energy level alignment at the interface is found to be favorable from the dye-sensitized solar cell point of view in that the lowest unoccupied state in the second and higher molecular layers is found above the CBM of the titanium dioxide, but the altered energy levels induced by adsorption of the oxidized first-layer FePc ought to be a significant disadvantage. Acknowledgment. We thank Dr. C. Puglia and Dr. J. Åhlund for supplying the FePc and also the staff at MAX-Lab. The Swedish Research Council (VR), Swedish Energy Agency (STEM), Go¨ran Gustafsson Foundation, and Carl Trygger Foundation are kindly acknowledged for financial support. References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Gra¨tzel, M. Nature 2001, 414, 338. (3) Gra¨tzel, M. Prog. PhotoVolt. Res. Appl. 2000, 8, 171. (4) Green, M. A.; Emery, K.; Hisikawa, Y.; Warta, W. Prog. PhotoVolt.: Res. Appl. 2007, 15, 425. (5) Yanagi, H.; Chen, S.; Lee, P. A.; Nebesny, K. W.; Armstrong, N. R.; Fujishima, A. J. Phys. Chem. 1996, 100, 5447. (6) He, J.; Hagfeldt, A.; Lindquist, S.-E.; Grennberg, H.; Korodi, F.; Sun, L.; Åkermark, B. Langmuir 2001, 17, 2743. (7) Lee, H.-J.; Kim, W.-S.; Park, S.-H.; Shin, W. S.; Jin, S.-H.; Lee, J.-K.; Han, S.-M.; Jung, K.-S.; Kim, M.-R. Macromol. Symp. 2006, 235, 230. (8) Peisert, H.; Knupfer, M.; Schwieger, T.; Auerhammer, J. M.; Golden, M. S.; Fink, J. J. Appl. Phys. 2002, 91, 4872.
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