Functional Multilayered Transparent Conducting Oxide Thin Films for

J. Phys. Chem. C 2009, 113, 1083–1087

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Functional Multilayered Transparent Conducting Oxide Thin Films for Photovoltaic Devices Jun Hong Noh,† Sangwook Lee,† Jin Young Kim,‡ Jung-Kun Lee,§ Hyun Soo Han,† Chin Moo Cho,† In Sun Cho,† Hyun Suk Jung,*,| and Kug Sun Hong*,† Department of Materials Science & Engineering, Seoul National UniVersity, Seoul 151-742, Korea, Chemical and Biosciences Center, National Renewable Energy Laboratory, Golden, Colorado 80401, School of AdVanced Materials Engineering, Kookmin UniVersity, Jeongneung-dong, Seongbuk-gu, Seoul 136-702, Korea, and Department of Mechanical Engineering and Materials Science, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260 ReceiVed: September 17, 2008; ReVised Manuscript ReceiVed: NoVember 5, 2008

In this study, we present a thermally stable multilayered transparent conducting oxide (TCO) functionalized for dye-sensitized solar cells (DSSCs). Nb-doped TiO2 (NTO) layers deposited on conventional Sn-doped In2O3 (ITO) substrates using pulsed laser deposition (PLD) enhanced the optical-to-electrical conversion efficiency of the DSSCs by as much as 17% compared to that of bare ITO-based DSSCs. The electrical properties and J-V characteristics of the multilayered NTO/ITO films showed that the improved cell performance was due to the facilitated charge injection from TiO2 to ITO that resulted from the formation of an ohmic contact with ITO, as well as the conserved high conductivity of ITO after the oxidizing annealing process. Moreover, the NTO/ITO-based DSSC exhibited higher efficiency than a F-doped SnO2(FTO)-based one, which demonstrates that optimization of multilayered NTO-based TCOs is a realistic approach for achieving highly efficient photoenergy conversion devices. Introduction Furubayashi et al. recently reported that Nb-doped TiO2 (NTO) could be used as a new transparent conducting oxide (TCO) material.1 From the viewpoint of dye-sensitized solar cells (DSSCs) and quantum-dot-sensitized solar cells (QDSSCs) using a mesoporous TiO2 working electrode, NTO films are of considerable interest because they can potentially enhance charge transfer from the TiO2 nanoparticles or layer to the NTO substrate resulting from the formation of a homojunction between the Nb-doped TiO2 and the pure TiO2.2 However, an expensive manufacturing process and expensive substrate materials restrict the potential applications of NTO films; NTO with good electrical conductivity must be grown epitaxially as the anatase phase on a (001) SrTiO3 (STO) single-crystal substrate at a high temperature of 550 °C using pulsed laser deposition (PLD). Among existing TCO materials, Sn-doped In2O3 (ITO) has the highest transmittance in the visible range and the lowest resistivity. It has therefore been widely used as a conducting layer in optoelectronic devices including sensors,3 panel displays,4 and solar cells.5 However, ITO films are not suitable for use in DSSCs or QDSSCs because the electrical conductivity of ITO films deteriorates after the thermal annealing at temperatures over 400 °C that is required to remove organic components and to interconnect the TiO2 particles.6 The conductivity degradation of ITO results in the destruction of its charge collection properties, thereby decreasing the energy * Corresponding authors. E-mail: [email protected] (H.S.J.), [email protected] (K.S.H.). Tel.: +82-2-910-4817 (H.S.J.), +822-880-8024 (K.S.H.). Fax: +82-2-910-4320 (H.S.J.), +82-2-886-4156 (K.S.H.). † Seoul National University. ‡ National Renewable Energy Laboratory. § University of Pittsburgh. | Kookmin University.

conversion efficiency of DSSCs. This degradation is an inevitable problem in cation-doped TCO systems, such as ITO, Aldoped ZnO, and Sb-doped SnO2. F-doped SnO2 (FTO), an anion-doped TCO, has been considered as a promising TCO material for use in DSSCs because of its good thermal stability; however, FTO films exhibit inferior conductivity and optical transmittance, indicating that new thermally stable cation-doped TCO materials are required to achieve high-performance DSSCs.7 Multilayered TCOs have been exploited to improve efficiencies of end devices.8,9 Kawashima et al. reported a multilayered FTO/ITO with improved thermal stability.6 Furthermore, because NTO films form a homojunction with TiO2 films, the use of NTO-based TCO in DSSCs is expected to produce good photovoltaic performance. Since the fabrication of epitaxially grown NTO films is not cost-effective, we fabricated multilayered NTO/ITO transparent conducting oxide. The NTO overlayer conserves the conductivity of ITO during air annealing and yields an ohmic contact with ITO. In addition, the photovoltaic properties of DSSCs employing the multilayered NTO/ITO were investigated, and the improved energy conversion efficiency was explained in terms of enhanced electron transport in the NTO/ITO-based photoelectrode. Experiments Undoped and Nb-doped (6 atom %) TiO2 ceramic targets were prepared using high-purity (99.99% pure) TiO2 and (99.99% pure) Nb2O5 powders by the conventional powder method. Undoped and Nb-doped (6 atom %) TiO2 are denoted 0NTO and 6NTO, respectively. 0NTO and 6NTO layers were deposited on commercial ITO substrates or fused silica substrates by PLD at room temperature and 5.0 × 10-4 Torr of oxygen pressure using a KrF (248 nm) excimer laser with a pulse energy density of 2 J/cm2 and a repetition rate of 5 Hz.

10.1021/jp808279j CCC: $40.75  2009 American Chemical Society Published on Web 12/29/2008

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Figure 1. (a) FESEM and (b) HRTEM images of the 6NTO/ITO/ glass multilayer structure.

Noh et al. Field-emission scanning electron microscopy (FESEM, model JSM-6330F, JEOL, Tokyo, Japan) and high-resolution transmission electron microscopy (HRTEM, model JEM-3000F, JEOL) were used to obtain cross-sectional views of the films. The crystalline phases and electronic structures of the thin films were analyzed by Raman spectroscopy (T64000, HORIABA Jobin Yvon, Villeneuve d’Ascq, France) and X-ray photoelectron spectroscopy (XPS, model Sigma Probe, ThermoVG, West Sussex, U.K.), respectively. Electrical properties of the films were measured using the van der Pauw method (HL5500PC, Bio-Rad, Hemel Hempstead, U.K.) and by a four-point probe (SR1000N, AIT, Gyeonggi, Korea). Optical transmittance was measured by a UV-vis spectrophotometer (model Lambda 35, Perkin-Elmer, Wellesley, MA). Photovoltaic properties and electrochemical impedance spectra were measured using a potentiostat (model CHI 608C, CH Instruments, Austin, TX) and a solar simulator (AM 1.5, 100 mW/cm2) (model PECL11, Peccell, Yokohama, Japan). I-V characteristics of the NTO/ITO contact were also measured using the potentiostat. Results and Discussion

Figure 2. Sheet resistance of as-received ITO, 0NTO/ITO, and 6NTO/ ITO films as a function of air-annealing temperature.

Figure 3. (a) Raman and (b) XPS spectra of 6NTO/ITO multilayer before and after 450 °C air annealing.

The as-deposited NTO/ITO multilayered specimens were annealed for 1 h under air atmosphere at temperatures from 30 to 500 °C. DSSCs were fabricated according to the process described in our previous report.10 The thickness and area of the TiO2 nanocrystalline electrodes in the fabricated DSSCs were 12 µm and 5 × 5 mm, respectively.

Figure 1a shows an FESEM cross-sectional image of 6NTO overcoated on ITO film, which illustrates that the multilayered film is composed of 90-nm-thick 6NTO and 190-nm-thick ITO. The HRTEM image shows that a periodic atomic lattice does not form in the as-grown 6NTO layer. This result demonstrates that the as-grown 6NTO layer is amorphous, which is consistent with the Raman results in Figure 3a (below). A multilayered 0NTO/ITO sample with a thickness identical to that of 6NTO/ITO was also successfully fabricated to compare its electrical properties with those of 6NTO/ITO. Figure 2 shows changes in sheet resistances of ITO, 0NTO/ITO, and 6NTO/ ITO as a function of air-annealing temperature. The ITO shows a dramatic increase in sheet resistance at annealing temperatures above 400 °C. However, the sheet resistances of both 0NTO/ ITO and 6NTO/ITO are rather stable up to an annealing temperature of 500 °C. These results indicate that the amorphous 0NTO and 6NTO overlayers preserve the conductivity of the ITO films during the air-annealing process. The carrier concentrations (n), mobilities (µ), and resistivities (F) of the ITO, 6NTO/ITO, and 6NTO films before and after air annealing at 450 °C are summarized in Table 1. As-received ITO exhibited a high n value of 1.1 × 1021 cm-3 and a low F value of 1.5 × 10-4 Ω cm. However, the electrical conductivity of the ITO deteriorated after air annealing at 450 °C for 1 h. The reduced carrier concentration of the ITO is responsible for the increased resistivity. As a result, the sheet resistance of the ITO also increased from 7.7 to 32 Ω/square (Ω/sq). The carrier concentration of degenerated ITO is determined by the concentrations of the donor (tin) and the interstitial oxygen, where the latter acts as an electron acceptor compensating for the donor effect of the tin.11 The degradation of ITO is associated with the formation of either interstitial oxygen (Oi) or interstitial-oxygenrelated defect complexes (such as Sn2*Oi) during air annealing.12 Therefore, the ITO should be protected from oxygen to prevent the generation of oxygen-related defects.

TABLE 1: Carrier Concentration (n), Hall Mobility (µ), and Resistivity (G) of the As-Received ITO, 6NTO/ITO/Glass, and 6NTO/Fused Silica before and after 450 °C Air Annealing before air annealing n (cm-3) ITO/glass 6NTO/ITO/glass 6NTO/fused silica

1.1 × 10 9.3 × 1020 6.0 × 1018 21

µ [cm2/(V s)] 35 31 0.3

after air annealing F (Ω cm) -4

1.5 × 10 2.2 × 10-4 3.3

n (cm-3)

µ [cm2/(V s)]

2.1 × 10 7.0 × 1020 4.4 × 1019

36 37 6.8

20

F (Ω cm) 8.0 × 10-4 2.4 × 10-4 2.1 × 10-2

Multilayered TCO Thin Films for Photovoltaic Devices

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Figure 5. Nyquist plots for 0NTO-DSSC and 6NTO-DSSC. R1 and R2 of these two DSSCs are summarized in the inset table. Figure 4. J-V characteristics of dye-sensitized cells. Inset shows transmittance of air-annealed ITO, 0NTO/ITO, and 6NTO/ITO films in the visible range.

TABLE 2: Summary of Sheet Resistance (RS) and Solar Cell Parameters, Namely, Short-Circuit Current (JSC), Open-Circuit Voltage (VOC), Fill Factor (FF), and Optical-to-Electrical Conversion Efficiency (η) RS (Ω/sq) JSC (mA/cm2) VOC (V) FF (%) η (%) ITO/glass 0NTO/ITO/glass 6NTO/ITO/glass

32 8.5 8.5

10.8 8.13 9.29

0.67 0.75 0.74

66 71 76

4.8 4.3 5.2

In contrast, the resistivity of the multilayered 6NTO/ITO film was not significantly changed after air annealing. Figure 3 shows Raman and XPS spectra of 6NTO/ITO films before and after 450 °C air annealing. The Raman spectra from the as-grown and 450 °C annealed 6NTO/ITO films illustrate that the amorphous NTO overlayer is crystallized into an anatase structure after the air-annealing process. In Figure 3b, the XPS spectrum from the as-grown 6NTO/ITO film indicates that the NTO overlayer contains Ti3+ ions, evidenced by the presence of a shoulder at lower binding energy (∼456.9 eV). The generation of Ti3+ is accompanied by the formation of an oxygen vacancy (VO), which is ascribed to the deposition process in a reducing atmosphere. As shown in the XPS spectrum of the 6NTO/ITO film annealed at 450 °C, Ti3+ ions are oxidized into Ti4+ ions, which indicates that the oxygen vacancies are also annihilated. Given the above Raman and XPS analysis, the NTO overlayer is crystallized while consuming the oxygen atoms and annihilating the Ti3+ and VO defects during the annealing process. These results imply that the NTO overlayer suppresses the formation of Oi in the ITO underlayer by self-consuming the oxygen during air annealing. This thermally stable conductivity, attributable to the NTO overlayer, could facilitate electron transport in photovoltaic applications. Figure 4 shows J-V curves of DSSCs employing bare ITO, 0NTO/ITO, and 6NTO/ITO TCO films. As summarized in Table 2, the multilayered 0NTO/ITO-based DSSC (0NTO-DSSC) and 6NTO/ITO-based DSSC (6NTO-DSSC) exhibit higher opencircuit voltages (VOC) and fill factors (FF) than the bare-ITObased DSSC (ITO-DSSC), which is probably due to blocked electron back-transfer from TCO to the electrolyte, as well as to conservation of the sheet resistance of ITO with TiO2 or an Nb-doped TiO2 overcoating.6,13 However, the short-circuit current densities (JSC) of NTO/ITO-based DSSCs are inferior

to that of ITO-DSSC, which indicates that there must be another factor that influences JSC in addition to the sheet resistance. As plotted in the inset of Figure 4, NTO/ITO substrates exhibit lower transmittances at wavelengths longer than 550 nm compared to the bare-ITO substrate, which leads to a decrease in the effective number of incident photons. The reduction in the number of photogenerated carriers thereby results in a decrease in the JSC of NTO/ITO-based DSSCs. Additional studies to improve the transmittance of NTO/ITO substrates for wavelengths longer than 550 nm are currently in progress. Although 0NTO contributes to the maintenance of the high conductivity of ITO and an improvement in the fill factor (FF) (71%), the reduction of JSC (∼8.13 mA/cm2) due to the low transmittance decreases the cell efficiency (4.3%). Unlike 0NTODSSC, however, 6NTO-DSSC shows improved JSC (∼9.29 mA/ cm2) and FF (76%) values, even though the 0NTO/ITO and 6NTO/ITO films having identical sheet resistance and transmittance values. The improvement in JSC and FF leads to higher efficiency (5.2%) in comparison with that (4.8%) of ITO-DSSC. This result indicates that there is another factor responsible for the improvements in JSC and FF in the 6NTO layer. Because all conditions in these two DSSCs are identical except whether Nb atoms are doped into the TiO2 overlayer, the improvements in JSC and FF are related to electron transport from TiO2 nanoparticles to ITO through the 0NTO or 6NTO layer. Electrochemical impedance spectroscopy (EIS) is a useful tool for investigating the electron-transport properties of DSSCs. Figure 5 shows Nyquist plots of 0NTO-DSSC and 6NTO-DSSC under open-circuit conditions with light illumination. The impedance components of the interfaces in the DSSCs are observed in the frequency ranges of 103-105 Hz (ω1 or ω2), 1-103 Hz (ω3), and 0.1-1 Hz (ω4), from left to right. These arcs are assigned to impedance at the conducting layer/TiO2 (ω1) or Pt/electrolyte (ω2) interface, impedance at the TiO2/ dye/electrolyte interface (ω3), and diffusion of the I3-/I- redox electrolyte (ω4).14 The arcs of ω1 and ω3 from 6NTO-DSSC are markedly decreased compared to those of 0NTO-DSSC. Resistances (R1 and R3) obtained by fitting the Nyquist plots are summarized in the inset table in Figure 5. Because thick TiO2 films in both the 6NTO- and 0NTO-DSSCs are coated onto the substrates in a similar manner, the difference in R3 is ascribed to the electron back-transfer from the conductive 6NTO film rather than the thick TiO2 film, indicating that the 0NTO layer blocks the leakage of electrons to electrolyte more effectively than the conductive 6NTO layer, thereby yielding a higher VOC for 0NTO-DSSC. Nevertheless, the significantly

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Figure 6. I-V characteristics of the Pt/Ti/sol-coated anatase/0NTO and 6NTO/ITO/Pt structures. Pt and Ti were deposited by sputter coating. The sol-coated anatase film was annealed at 450 °C for 1 h in air.

increased JSC of 6NTO compared to 0NTO might improve the overall efficiency. Assuming that the numbers of light-induced electrons in 0NTO- and 6NTO-DSSCs are equivalent under identical conditions of transmittance and TiO2 working-electrode structure, JSC mainly depends on the electron-transport characteristics at the TCO/TiO2 working electrode interface. In Figure 5, a special feature between Nyquist plots of 0NTO and 6NTO is different size of ω1 arcs related to the resistance (R1) of the electrochemical interface between the TCO and the TiO2 working electrode. The small ω1 arc of 6NTO-DSSC is attributed to low resistance (R1) of the TCO/TiO2 interface. Given that 6NTO-DSSC has a lower R1 value, 3.88 Ω, than 0NTO-DSSC (8.51 Ω), the electrons in the TiO2 working electrode can be more effectively extracted to the multilayered 6NTO/ITO film than to 0NTO/ITO, resulting in the higher JSC of 6NTO-DSSC. To explore the origin of the difference in R1, the I-V characteristics of the 0NTO/ITO and 6NTO/ITO films were measured (Figure 6). The structure of measured samples is illustrated in the inset of Figure 6. The 6NTO/ITO film exhibits ohmic contact behavior, in contrast to the Schottky contact behavior of the 0NTO/ITO film, attributed to the larger work function (Φ) for ITO than for TiO2. Figure 7 shows schematic band diagrams of the 0NTO/ITO and 6NTO/ITO multilayer structures before and after contact, considering that the work function of ITO and the electron affinity of TiO2 are ∼4.5 eV and ∼4.0 eV, respectively.15 Tang et al. reported that a metallic transition occurs in anatase films with donor concentrations above 9 × 1018 cm-3, whereas anatase crystals with donor concentrations around 7 × 1017 cm-3 remain nondegenerate.16 Judging from their report, in this study, 6NTO with a carrier concentration of 4.4 × 1019 cm-3 (shown in Table 1) is degenerate, but 0NTO with a carrier concentration of 2.1 × 1017 cm-3 is a nondegenerate semiconductor. Therefore, the Fermi energy level of 6NTO is located above the conduction band edge shown in Figure 7a. After contact with ITO, the nondegenerate 0NTO forms a Schottky barrier due to a difference in Fermi energy levels as shown in Figure 7.15 In contrast, the degenerate 6NTO could form an ohmic contact with ITO as illustrated in Figure 7b because the electrons can tunnel through the existing Schottky barrier between degenerate

Noh et al.

Figure 7. Schematic diagrams of band alignment between ITO and 0NTO or 6NTO (a) before and (b) after contact.

Figure 8. J-V characteristics of as-received ITO, FTO, and 0NTO/ 6NTO/ITO DSSCs. Their solar cell parameters are summarized in the inset table.

semiconductors.17 The ohmic contact promotes carrier extraction from TiO2 nanoparticles to the ITO film in DSSC, which yields a lower R1 value and improves FF and JSC of the 6NTO-DSSC.18,19 We previously confirmed in an EIS study that the 0NTO layer is more effective in blocking back-transport of electrons than the 6NTO layer. To prevent electron back-transfer, an additional 0NTO layer (45 nm) was deposited on the 6NTO (45nm)/ITO film, denoted as 0NTO/6NTO/ITO. The 0NTO/6NTO/ITObased DSSC exhibits an enhanced JSC (∼9.72 mA/cm2) in comparison with that of 6NTO/ITO-based DSSC (shown in Figure 8). The efficiency of the 0NTO/6NTO/ITO-based DSSC is 5.5%, which is enhanced by as much as 17% compared to that of bare ITO-based DSSCs and is comparable to that of a conventional FTO-based DSSC (5.3%). Our results demonstrate that a multilayered NTO-based film is a promising TCO material for a highly efficient DSSC because of the good thermal stability of its electrical conductivity, the presence of a homojunction between the TiO2 particles and the NTO layer, and the ohmic contact it makes with the ITO. In addition, the improvement in transmittance of NTO is expected to further increase the DSSC efficiency. The aforementioned benefits of the multilayered

Multilayered TCO Thin Films for Photovoltaic Devices NTO-based TCO will support its potential applications in other TiO2-film-based devices such as photocatalysts and watersplitting systems.16 Conclusions In the present study, a 6 atom % Nb-doped TiO2 (NTO) layer was deposited by pulsed laser deposition on ITO substrates for use as a transparent conducting oxide layer in DSSCs. The NTO overlayer was found to conserve the electrical conductivity of the ITO films after the binder-burnout process of the photoelectrode. The ohmic contact behavior of this film with ITO, as well as the formation of a homojunction with TiO2 particles, contributed to an improvement in the short-circuit current density (JSC) and fill factor (FF). The overall energy conversion efficiency of a DSSC employing the multilayered TiO2/NTO/ ITO film was 5.5%, which is enhanced by as much as 17% compared to that of bare ITO-based DSSCs (4.8%) and comparable to that of an FTO-based DSSC (5.3%). These results demonstrate that a multilayered NTO/ITO-based transparent conducting oxide is suitable for application in highly efficient energy conversion devices. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (R01-2007-000-11075-0). (RIAM) The Kookmin University portion was supported by the Korea Research Foundation Grant and the Korea Science and Engineering Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2007-313-D00345 & R11-2005-048-000000, ERC, CMPS). This work was also supported by the research program 2008 of Kookmin University.

J. Phys. Chem. C, Vol. 113, No. 3, 2009 1087 References and Notes (1) Furubayashi, Y.; Hitosugi, T.; Yamamoto, Y.; Inaba, K.; Kinoda, G.; Hirose, Y.; Shimada, T.; Hasegawa, T. Appl. Phys. Lett. 2005, 86, 252101. (2) Fortunato, E.; Ginley, D.; Hosono, H.; Paine, D. C. MRS Bull. 2007, 32, 242. (3) Lee, S. M.; Lee, Y. S.; Shim, C. H.; Choi, N. J.; Joo, B. S.; Song, K. D.; Huh, J. S.; Lee, D. D. Sens. Actuators B: Chem. 2003, 93, 31. (4) Ishibashi, S.; Higuchi, Y.; Ota, Y.; Nakamura, K. J. Vac. Sci. Technol. A 1990, 8, 1399. (5) Chopra, K. L.; Major, S.; Pandya, D. K. Thin Solid Films 1983, 102, 1. (6) Kawashima, T.; Ezure, T.; Okada, K.; Matsui, H.; Goto, K.; Tanabe, N. J. Photochem. Photobiol. A: Chem. 2004, 164, 199. (7) Rottkay, K. V.; Rubin, M. Mater. Res. Soc. Symp. Proc. 1996, 426, 449. (8) Ho, J J.; Chen, C Y.; Hsiao, R. Y.; Ho, O. L. J. Phys. Chem. C 2007, 111, 8372. (9) Yang, Y.; Wang, L.; Yan, H.; Jin, S.; Marks, T. J.; Li, S. Appl. Phys. Lett. 2006, 89, 051116. (10) Lee, S.; Kim, J. Y.; Youn, S. H.; Park, M.; Hong, K. S.; Jung, H. S.; Lee, J.-K.; Shin, H. Langmuir 2007, 23, 11907. (11) Mergel, D.; Qiao, Z. J. Appl. Phys. 2004, 95, 5608. (12) Gonza`lez, G. B.; Mason, T. O.; Quintana, J. P.; Warschkow, O.; Ellis, D. E.; Hwang, J.-H.; Hodges, J. P.; Jorgensen, J. D. J. Appl. Phys. 2004, 96, 3912. (13) Burke, A.; Ito, S.; Snaith, H.; Bach, U.; Kwiatkowski, J.; Gra¨tzel, M. Nano Lett. 2008, 8, 977. (14) Hoshikawa, T.; Yamada, M.; Kikuchi, R.; Eguchi, K. J. Electrochem. Soc. 2005, 152, E68. (15) Dai, W.; Wang, X.; Liu, P.; Xu, Y.; Li, G.; Fu, X. J. Phys. Chem. B 2006, 110, 13470. (16) Tang, H.; Prasad, K.; Sanjine`s, R.; Schmid, P. E.; Le´vy, F. J. Appl. Phys. 1994, 75, 2042. (17) Pierret, R. Semiconductor DeVice Fundamentals, 2nd ed.; Addison Wesley: Boston, 1996. (18) Kron, G.; Rau, U.; Werner, J. H. J. Phys. Chem. B 2003, 107, 13258. (19) Ru¨hle, S.; Cahen, D. J. Phys. Chem. B 2004, 108, 17946.

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