Barrier Layer Effect on the Electron Transport of the Dye-Sensitized

Article pubs.acs.org/JPCC

Barrier Layer Effect on the Electron Transport of the Dye-Sensitized Solar Cells Based on TiO2 Nanotube Arrays Chul Rho, Ji-Hye Min, and Jung Sang Suh* Nano-materials Laboratory, Department of Chemistry, Seoul National University, Kwanakro 599, Kwanakgu, Seoul 151-742, Republic of Korea ABSTRACT: We have studied the effect of the barrier layer on the electron transport of dye-sensitized solar cells (DSSCs) based on TiO2 nanotube arrays. The barrier layer of TiO2 nanotubes, corresponding to the underlayer of the detached TiO2 nanotube films, was ion-milled to reduce its thickness. The energy conversion efficiencies (measured by back-sideillumination) of DSSCs based on the TiO2 nanotube films whose barrier layers were ion-milled for 0, 10, 20, and 30 minwere 2.5, 2.7, 2.9, and 3.1%, respectively. The conversion efficiency increased significantly with reducing the thickness of the barrier layer without opening the bottom tips. The improvement of the conversion efficiency was mainly due to the improvement of the electron transfer efficiency by reducing the thickness of the barrier layer. Also, electrochemical impedance spectroscopy (EIS) analysis showed that the thickness of the barrier layer has a pronounced impact on the electron transport resistance of the cells. It was concluded that the electron transport is hindered considerably by the barrier layer.

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INTRODUCTION Dye-sensitized solar cells (DSSCs) have received much attention because of their low-cost and high energy conversion efficiency.1−4 In the fabrication of DSSCs, mesoporous TiO2 films and ruthenium sensitizers have been used as the main component materials. The power conversion efficiency of DSSCs is affected by several factors: molar absorption coefficiency, energetically suitable HOMO−LUMO levels, available surface area for dyes, transport kinetics of the electrons, regeneration by a redox couple, and losses of recombination and back reactions.5 TiO2 nanoparticles are mostly used as electron acceptors for DSSCs.2,6 However, nanoparticles are networked randomly and have grain boundary effects that are critical factors to electron transport. To improve electron transport, one-dimensional materials such as nanotubes, nanowires, and nanorods have been used.7−11 TiO2 nanotube arrays with a large internal surface area could be obtained through the anodization of Ti foil in nonaqueous electrolytes.12−18 Large-area free-standing crystallized TiO2 nanotube films were prepared by using a two-step anodization technique.16 In a front-side-illumination, the energy conversion efficiency of DSSCs based on opened-end TiO2 nanotube films is much higher than the efficiency based on closed-end ones.18 This means that the barrier layer, corresponding to the underlayer of the detached TiO2 nanotube films, has a marked effect on the conversion efficiency, since the barrier layer has been removed in an open-end TiO2 nanotube film, while it is left intact in a closed-end film. The barrier layer could affect the conversion efficiency by preventing the diffusion of materials such as electrolytes and dye molecules, by reducing the transmittance of light, and by hindering the electron transport. © 2012 American Chemical Society

The electrons generated by excitation of the dyes adsorbed on the TiO2 nanotubes could transfer to the electrode through TiO2 nanotubes. In this case, they must pass the barrier layer to reach the electrode. So far, the effect of the barrier layer on electron transport has not been studied in detail. Here, we prepared free-standing TiO2 nanotube arrays, controlled the thickness of the barrier layer by controlling the ion milling time, and then used them to fabricate DSSCs, as shown in Scheme 1. We measured the photovoltaic properties of the DSSCs by a back-illumination, and we studied the barrier layer effect to the electron transport of the dye-sensitized solar cells (DSSCs) based on TiO2 nanotube arrays. Also, we used electrochemical impedance spectroscopy (EIS) to quantitatively evaluate the variation in the electron transport resistance of the DSSCs with varying thickness of the barrier layer.

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EXPERIMENTAL SECTION TiO2 nanotube arrays were fabricated by anodizing thin Ti plates (99.7% purity, Alpha, 2.5 cm × 4.0 cm × 100 μm) in an electrolyte composed of 0.8 wt % of NH4F and 2 vol % of H2O in ethylene glycol at 20 °C and at a constant applied voltage of 60 V dc for 2 h.16 The TiO2 nanotubes formed by the Ti plate were annealed at 450 °C for 1 h under ambient air to improve their crystallinity. The oxide film of crystallized TiO2 nanotube arrays was detached from the Ti plate using a reported method.16 To detach the film, a second anodization was done Received: December 5, 2011 Revised: March 6, 2012 Published: March 7, 2012 7213

dx.doi.org/10.1021/jp211708y | J. Phys. Chem. C 2012, 116, 7213−7218

The Journal of Physical Chemistry C

Article

were set at the open circuit voltage (Voc) of the DSSCs and 10 mV, respectively. The impedance measurements were carried out at open-circuit potential under AM 1.5 one-sun light illumination. The electrochemical impedance spectra were analyzed using Z-View software (Solartron Analytical) with the aid of an appropriate equivalent circuit.

Scheme 1. Schematic Illustration of the Procedure for Fabricating DSSCs Based on Free-Standing TiO2 Nanotube Arrays: (a) First Anodization, and Then Annealing, (b) Second Anodization, and Then Detaching TiO2 Film, (c) Ion Milling, (d) Attaching on FTO Glass, Dye Adsorption, and Then Fabrication of a DSSC

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RESULTS AND DISCUSSION Figure 1 shows the SEM images of the top and side views of a TiO2 nanotube film, whose barrier layer was partially removed

at a constant applied voltage of 30 V dc for 10 min, and then the plate was dipped in the 10% H2O2 for 20 h. The barrier layer of TiO2 nanotubes was removed by ion milling with Ar+ bombardment for various time intervals.19 A TiO2 blocking layer was formed on FTO glass by spin-coating with 5 wt % of titanium di-isopropoxide bis(acetylacetonate) in butanol and then by heating at 500 °C for 30 min in air.20 A TiO2 nanoparticle viscous paste was prepared by the following procedures:21 titanium tetraisopropoxide was hydrolyzed in 0.1 M nitric acid at 220 °C for 20 h; water and polyethylene glycol were added; and then the contents were mixed well. The viscous paste was printed onto the FTO glass by the doctor blade technique. Then an ion-milled TiO2 nanotube film was attached to the FTO glass under a slight pressure and then annealed in air at 450 °C for 30 min. Dye molecules were adsorbed by immersing the nanotube film-attached FTO glass plate in an ethanol solution of 0.5 mM (Bu4N)2Ru(dobpyH)2(NCS)2 (N-719, Solaronix) at 50 °C over 8 h. The composition of the electrolyte was as follows: 0.7 M 1-butyl-3-methyl-imidazolium iodide (BMII), 0.03 M I2, 0.1 M guanidium thiocyanate (GSCN), and 0.5 M 4-tertbutylpyridine (TBP) in a mixture of acetonitrile and valeronitrile (85:15 v/v).22 The counter-electrode was prepared by sputtering Pt on FTO glass. The working electrode was further sandwiched with the Pt-sputtered FTO glass, separated by a 60-μm-thick hot-melt spacer. The morphology and thickness of the film of TiO2 nanotube arrays were analyzed by using a field emission scanning electron microscope (FE-SEM, JSM-6330F, JEOL Inc.). The current density−voltage (J−V) characteristics of the DSSCs were measured using an electrometer (KEITHLEY 2400) under AM 1.5 illumination (100 mW/cm2) provided by a solar simulator (1 KW xenon with AM 1.5 filter, PEC-L01, Peccel Technologies). The incident photon-to-current conversion efficiency (IPCE) was measured using McScience (model K3100) with reference to the calibrated diode. A 300 W xenon lamp was used as light source for generation of a monochromatic beam. The bias light was supplied by a 150 W halogen lamp. The EIS spectra were measured with a potentiostat (Solartron 1287) equipped with a frequency response analyzer (Solartron 1260), with the frequency ranging from 10−2 to 106 Hz. The applied bias voltage and ac amplitude

Figure 1. SEM images of a TiO2 nanotube array, whose barrier layer was partially removed, attached to FTO glass using TiO2 paste: (a) top view and (b) side view. In the side view, the layers from top to bottom are the TiO2 nanotube film, paste layer of TiO2 nanocrystals, TiO2 blocking layer, and FTO layer. The thicknesses are approximately 18 μm, 3 μm, 700 nm, and 2.2 mm, respectively.

by ion milling, attached on FTO glass using TiO2 paste. The tubes are relatively well ordered and are uniform in diameter. The inner diameter of the top tips is in the range of 80−100 nm. The length of the TiO2 nanotubes is approximately 18 μm. The size of the TiO2 films was 1.5 cm × 1.5 cm. In the side view, there are four layers. The layers from top to bottom are the TiO2 nanotube film, a paste layer of TiO2 nanocrystals, a TiO2 blocking layer, and a FTO layer. The thicknesses are approximately 18 μm, 3 μm, 700 nm, and 2.2 mm, respectively. In the side view, the glass part (which exists underneath the FTO layer) is not shown. Figure 2 shows the SEM images of the bottom surfaces of the films of TiO2 nanotube arrays peeled off from the Ti plates after ion milling for 0, 20, 30, and 90 min. Before ion milling, the closed tips of TiO2 nanotubes are clearly seen (see Figure 2a). The bottom surface is rough in appearance due to the hemispheres of the tips. However, the surface becomes smooth after ion milling. No bottom tips of TiO2 nanotubes were opened for milling times 30 min and less. After ion milling for 90 min, most of the bottom tips are opened (Figure 2d). The average inner diameter of the bottom tips opened by ion milling is approximately 30 nm. For our TiO2 nanotubes, the 7214



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Figure 2. SEM images of the bottom surfaces of the free-standing TiO2 films after ion milling for (a) 0, (b) 20, (c) 30, and (d) 90 min.

inner diameter of the top tips is in the range of 80−100 nm, while that of the bottom tips is about 30 nm. This means that the channels are shaped like a cone. A conical shape might be formed by pore widening during a relatively long anodization, since the part of the tubes made earlier had a longer widening time, resulting in greater widening. The inner diameter of our bottom tips is much smaller than the diameter of those opened by chemical etching.18 This may be due to the fact that pore widening could take place during chemical etching. It should be mentioned that we annealed the film itself at 450 °C before detachment. TiO2 films were fragile. However, the annealed TiO2 films were much less fragile than those that were not. Also, by annealing at 450 °C, the crystal structure of TiO2 nanotubes changes to anatase, which improves the performance of TiO2 nanotube-based DSSCs.16 Figure 3 presents the photocurrent−voltage curves measured by a back-side-illumination in air mass 1.5 sunlight (intensity of the solar simulator was 100 mW cm−2). Five different solar cells based on TiO2 nanotube films whose barrier layers were ion milled for 0, 10, 20, 30, and 90 min were fabricated. For comparison, we also fabricated the DSSC based on a TiO2 paste layer without attaching a TiO2 nanotube film. The values of a short circuit current (Jsc), open circuit voltage (Voc), fill factor ( f f), and energy conversion efficiency (η) for these cells measured by a back-side-illumination are summarized in Table 1. A DSSC based on a TiO2 nanotube film whose barrier layer was not ion milled exhibited a Jsc of 5.37 mA cm−2, a Voc of 0.76 V, a f f of 0.62, and a η of 2.5%. A 30 min ion milling resulted in greatly improved performance, with Jsc of 6.59 mA cm−2, Voc of 0.76 V, f f of 0.61, and η of 3.1%. The efficiency was significantly improved from 2.5% to 3.1%, corresponding to 24% improvement. The open circuit voltage was approximately 0.75. The fill factor was in the range of 0.61−0.63. In addition, the short circuit current and conversion efficiency increased substantially. Since Voc and f f were not changed significantly, the increase of the conversion efficiency might be due to the increase of Jsc with this increase in ion milling time. Most notably, the thickness of the barrier layer was reduced. Therefore, it is concluded that the conversion efficiency of DSSCs based on

Figure 3. Current density−voltage characteristics of DSSCs based on TiO2 nanotube films attached to FTO glass using a TiO2 paste measured by a back-side-illumination: the barrier layers of TiO2 nanotubes were ion milled for 90, 30, 20, 10, and 0 min, from top to bottom. The lowest one was measured from a DSSC based on only TiO2 paste, without attaching a TiO2 nanotube film.

Table 1. Photovoltaic Properties of the DSSCs Based on TiO2 Nanotube Films, Whose Barrier Layers Were Ion Milled, Attached to FTO Glass Using TiO2 Paste, Measured by a Back-Side-Illumination ion milling time 0 min 10 min 20 min 30 min 90 min only TiO2 paste (without attaching a TiO2 film) 7215

Jsc (mA/cm2)

Voc (V)

ff

η (%)

5.37 5.69 6.17 6.59 7.85 2.16

0.76 0.75 0.76 0.76 0.75 0.77

0.62 0.63 0.61 0.61 0.62 0.61

2.5 2.7 2.9 3.1 3.7 1.0



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TiO2 nanotube films increases by reducing the thickness of the barrier layer. The efficiency of DSSC based on a TiO2 nanotube film whose barrier layer was ion milled for 90 min was 3.7%, which was significantly higher than the others (see Table 1). This might be due to the contribution of the TiO2 nanoparticles contained in the paste. The TiO2 nanotube films were attached to FTO glass using a TiO2 paste. Since the paste consists of TiO2 nanoparticles, it could contribute to the efficiency of our DSSCs. However, when the bottom tips of TiO2 nanotubes were not open, the nanoparticles could not contribute to the conversion efficiency under our experimental conditions. We attached a TiO2 nanotube film to FTO glass, and then we dipped the glass plate in a dye solution to adsorb dye molecules. In this case, the dye solution would be diffused into the pores of the nanotubes. However, the barrier layer might block the dye molecules from diffusing to the TiO 2 nanoparticles of the paste. Therefore, the TiO2 nanoparticles would not contribute directly to the conversion efficiency, since no dye molecules could be adsorbed on the surface. When the bottom tips of TiO2 nanotubes were open, the dye molecules could diffuse to the TiO2 nanoparticles of the paste. The bottom tips of the film ion milled for 90 min were open (see Figure 2d). Therefore, the TiO2 nanoparticles of the paste could contribute directly to the conversion efficiency. A DSSC based on only TiO2 paste, without attaching a TiO2 nanotube film, exhibited a η of 1.0% (see Table 1). However, the contribution of the TiO2 nanoparticles of the paste layer existing underneath the attached TiO2 nanotube film might be limited due to the difficulty of the electrolyte diffusion in the channels of TiO2 nanotubes and the decrease of the light intensity by passing through a TiO2 nanotube film in a backside-illumination. The thickness of the barrier layer could affect the transmittance of light. In a front-side-illumination, the light passes the barrier layer and then irradiates the dye molecules adsorbed on the TiO2 nanotubes. Therefore, the efficiency of the DSSCs will decrease when the intensity of light is reduced by the barrier layer.18 In a back-side-illumination, however, the light reaches the barrier layer after passing the dye molecules adsorbed on the TiO2 nanotubes, and the thickness of the barrier layer does not affect the conversion efficiency. This is supported by the results of the IPCE measurements. Figure 4 shows the IPCE spectra of the DSSCs based on TiO2 nanotube films attached to FTO glass using a TiO2 paste, measured by a back-side-illumination. The barrier layers of TiO2 nanotubes were ion milled for 90, 30, 20, 10, and 0 min, from top to bottom. The intensity of the IPCE spectra increases with increasing ion milling time. This supports the conclusion that the conversion efficiency of DSSCs based on TiO2 nanotube films increases by reducing the thickness of the barrier layer. Although their intensities differ, all spectra are very similar to each other. The normalized spectra were almost the same. Generally, the transmittance of light through a material is a function of wavelength. Therefore, if the barrier layer caused reduction of light intensity, the IPCE spectrum would be deformed upon decreasing the thickness of the barrier layer. However, all the spectra are almost the same, save for relative intensities. Therefore, it is concluded that in a backside-illumination, the improvement of the conversion efficiency with increasing ion milling time is not due to the transmittance increase of light via thickness reduction of the barrier layer.

Figure 4. IPCE spectra of the DSSCs based on TiO2 nanotube films attached to FTO glass using a TiO2 paste measured by a back-sideillumination: the barrier layers of TiO2 nanotubes were ion milled for 90, 30, 20, 10, and 0 min, from top to bottom.

The electrons generated by excitation of the dyes adsorbed on the TiO2 nanotubes would be transferred to the electrode through TiO2 nanotubes. In this process, the electrons must pass the barrier layer to reach the electrode. If the electron transfer is hindered by the barrier layer, the energy conversion efficiency will be affected by the barrier layer’s thickness. When the thickness of the layer was reduced by ion milling for 30 min, the efficiency was improved greatly from 2.5% to 3.1%, corresponding to 24% improvement. As mentioned previously, the barrier layer could affect the conversion efficiency by preventing the diffusion of materials, reducing the transmittance of light, and hindering the electron transfer. However, when the bottom tips of TiO2 nanotubes remained unopened, the conversion efficiency might not be affected by the blocking property of the barrier layer under our experimental conditions. Also, in a back-side-illumination, the transmittance reduction of light by the barrier layer does not affect the conversion efficiency. Therefore, it is concluded that the improvement of the conversion efficiency is due to the increase in electron transfer efficiency by reduction of the thickness of the barrier layer by increasing the ion milling time. This means that the electron transport is hindered considerably by the barrier layer. This is supported by the results of the EIS measurements. Figure 5 shows the characteristic EIS spectra for DSSCs based on TiO2 nanotube films whose barrier layers were ion milled for 0, 10, 20, and 30 min. Each spectrum contains two semicircles. The one at high frequency is assigned to the parallel combination of the resistance and capacitance at the PtFTO/electrolyte and the FTO/TiO2 interfaces, and the larger one at low frequency is associated with resistance and capacitance of the dye-absorbed TiO2/electrolyte interface and transport resistance in the TiO2 film.23,24 These data were analyzed using an equivalent circuit as shown in the inset of Figure 5 and a nonlinear least-squares fitting program. The fit parameters are listed in Table 2. With increasing the ion milling time, the resistances (Rs, R1, and R2) decrease while the capacitances (C1 and C2) increase. The Ohmic series resistance Rs is caused by the sheet resistance of FTO, current collector contacts, etc. It corresponds to the value in the x-axis where the first semicircle begins on the left-hand side. The value of Rs decreases very slightly with increasing ion milling time. This 7216



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unchanged with varying barrier layer thicknesses. The value of ωmax is approximately 7.9 Hz. By the impedance model for DSSCs,26 electrons are injected into the conduction band of TiO2 from the excited dye under illumination, and some of them are trapped at trap levels. Trapped electrons are lost by the recombination with I3−. The reaction rate constant for electron recombination kr is related to ωmax through27 k r = ωmax /2Ns

where Ns is the steady state electron density in the trap state. kr is proportional to ωmax. It is expected that they will have the same behavior with varying barrier thickness. Therefore, it is concluded that the reaction rate constant for electron recombination is almost unchanged with varying barrier thickness. Also, it is known that Voc depends logarithmically on 1/ωmax.28,29 By this relation, Voc will not change with the barrier thickness, since ωmax does change with barrier thickness. Almost no change in Voc (see Table 1) is indicative of no change in the reaction rate constant for electron recombination with varying barrier thickness. The charge collection efficiency affecting the photocurrent density is composed of time constants for electron transport and electron recombination. Therefore, the change in photocurrent density with barrier thickness is likely to be related to a change in electron transport rate. This means that the decrease of R2 with increasing ion-milling time could mainly be due to the decrease of the transport resistance in the TiO2 film. The length of TiO2 nanotubes was the same as approximately 18 μm, and only the thickness of the barrier layer was reduced with increasing ion milling time. Therefore, it is concluded that the decrease of the transport resistance of the cells with increasing ion-milling time is due to reduction of the thickness of the barrier layer. This means that the electron transport in the cells is greatly affected by the barrier layer even though its thickness is less than 100 nm. With increasing ion milling time, the thickness of the barrier layer reduced. Consequently, the thickness of the oxide layer on the surface of the FTO/TiO2 electrode was reduced with increasing ion milling time. Since the charge accumulation in a double layer increases with reducing the thickness of the oxide layer on conductors, the capacitance of the DSSC devices might be increased with reducing the thickness of the barrier layer by increasing ion milling time as shown in Table 2.

Figure 5. Electrochemical impedance spectra of DSSCs based on TiO2 nanotube films whose barrier layers were ion milled for 0 min (filled squares), 10 min (filled circles), 20 min (triangles), and 30 min (squares).

Table 2. Fit Results of Impedance Spectra for DSSCs Based on TiO2 Nanotube Films Whose Barrier Layers Were Ion Milled for 0, 10, 20, and 30 min ion milling time

Rs (Ω)

R1 (Ω)

0 min 10 min 20 min 30 min

4.332 4.300 4.297 4.265

1.152 1.125 1.105 1.045

C1 (F) 3.324 3.338 3.522 4.266

× × × ×

10−5 10−5 10−5 10−5

R2 (Ω) 16.15 15.23 14.42 11.44

C2 (F) 7.28 7.76 8.08 9.68

× × × ×

10−3 10−3 10−3 10−3

may mean that the barrier layer might have very little effect on Rs. The resistance R1 is given by the sum of the interfacial resistance of FTO/TiO2 and Pt/electrolyte.24,25 The value of R1 decreases with increasing ion milling time. Since the same Pt-FTO counter electrode was used for all our DSSC devices, the decrease of R1 with increasing ion milling time might be caused by the change in FTO/TiO2 interfacial property with reducing the thickness of the barrier layer. The resistance R2 is given by the sum of the charge transfer resistance at the dyeabsorbed TiO2/electrolyte interface and the transport resistance in the TiO2 film. The value of R2 decreases significantly with increasing ion milling time. Dye molecules could be adsorbed on the inner surface of TiO2 nanotubes by diffusion through the pores. Therefore, the amount of dye molecules adsorbed might be not be affected by the thickness of the barrier layer whenever the bottom tips of nanotubes were not opened. No bottom tips of TiO2 nanotubes were opened for milling times 30 min and less (see Figure 2). Since the length of TiO2 nanotubes was the same as approximately 18 μm, the amount of dye adsorption might be the same. Therefore, the dye-coated TiO2/electrolyte interfacial property might be very similar in the DSSCs whose barrier layers were not removed completely. For all the spectra of Figure 5, the maximum imaginary resistance of the second semicircle is observed near the 12th data point from the right side. This means that the frequency at the maximum imaginary resistance of the second semicircle (ωmax) is almost the same for all the DSSC devices, since the spectra have been measured under the same experimental conditions. Therefore, it is concluded that ωmax is almost

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CONCLUSION We have studied the barrier layer effect on the electron transfer of DSSCs based on TiO2 nanotube arrays. The thickness of the barrier layer was controlled by modulating the ion milling time. The conversion efficiency measured by a back illumination increased greatly when reducing the thickness of the barrier layer by increasing the ion milling time. The improvement of the conversion efficiency by decreasing the thickness of the barrier layer is mainly due to the improvement of electron transfer efficiency. EIS measurements reveal that the thickness of the barrier layer has a pronounced impact on the transfer resistance of the cells. It is concluded that the electron transport is hindered significantly by the barrier layer of TiO2 nanotubes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 82-2-880-7763. Fax: 82-2875-6636. 7217



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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant, funded by the Ministry of Knowledge Economy, Republic of Korea, and by the BK21 program.

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REFERENCES

(1) O’Regan, B.; Grätzel, M. Nature 1991, 353, 737. (2) Grätzel, M. Inorg. Chem. 2005, 44, 6841. (3) Wang, Z. S.; Kawauchi, H.; Kashima, T.; Arakawa, H. Coord. Chem. Rev. 2004, 248, 1381. (4) Kim, Y. J.; Lee, M. H.; Kim, H. J.; Lim, G.; Choi, Y. S.; Park, N. G.; Kim, K.; Lee, W. I. Adv. Mater. 2009, 21, 3668. (5) Ghicov, A.; Albu, S. P.; Hahn, R.; Kim, D.; Stergiopoulos, T.; Kunze, J.; Schiller, C.-A.; Falaras, P.; Schmuki, P. Chem. Asian J. 2009, 4, 520. (6) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Grätzel, M. J. Am. Chem. Soc. 2005, 127, 16835. (7) Pavasupree, S.; Ngamsinlapasathian, S.; Suzuki, Y.; Yoshikawa, S. J. Nanosci. Nanotechnol. 2006, 6, 3685. (8) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455. (9) Kang, S. H.; Choi, S. H.; Kang, M. S.; Kim, J. Y.; Kim, H. S.; Hyeon, T.; Sung, Y. E. Adv. Mater. 2008, 20, 54. (10) Nakayama, K.; Kubo, T.; Nishikitani, Y. Jpn. J. Appl. Phys. 2008, 47, 6610. (11) Rho, C.; Suh, J. S. Chem. Phys. Lett. 2011, 513, 108. (12) Roy, P.; Kim, D.; Lee, K.; Spiecker, E.; Schmuki, P. Nanoscale 2010, 2, 45. (13) Macák, J. M.; Tsuchiya, H.; Ghicov, A.; Schmuki, P. Electrochem. Commun. 2005, 7, 1133. (14) Zhu, K.; Vinzant, T. B.; Neale, N. R.; Frank, A. J. NanoLetters 2007, 7, 3739. (15) Park, J. H.; Lee, T. W.; Kang, M. G. Chem. Commun. 2008, 2867. (16) Chen, Q.; Xu, D. J. Phys. Chem. C. 2009, 113, 6310. (17) Albu, S. P.; Ghicov, A.; Macak, J. M.; Hahn, R.; Schmuki, P. NanoLetters 2007, 7, 1286. (18) Lin, C. J.; Yu, W. Y.; Chien, S. H. J. Mater. Chem. 2010, 20, 1073. (19) Jung, H. Y.; Jung, S. M.; Gu, G. H.; Suh, J. S. Appl. Phys. Lett. 2006, 89, 013121. (20) Park, J. H.; Choi, K. J.; Kim, J.; Kang, Y. S.; Lee, S. S. J. Power Sources 2007, 173, 1029. (21) Barbé, C. J.; Arendse, F.; Comte, P.; Jirousek, M.; Lenzmann, F.; Shklover, V.; Grätzel, M. J. Am. Ceram. Soc. 1997, 80, 3157. (22) Kwak, E. S.; Lee, W.; Park, N. G.; Kim, J.; Lee, H. Adv. Funct. Mater. 2009, 19, 1093. (23) Wang, Q.; Moser, J. E.; Grätzel, M. J. Phys. Chem. B 2005, 109, 14945. (24) Yoo, B.; Kim, K.; Lee, D. K.; Ko, M. J.; Lee, H.; Kim, Y. H.; Kim, W. M.; Park, N. G. J. Mater. Chem. 2010, 20, 4392. (25) Fabregat-Santiago, F.; Bisquert, J.; Palomares, E.; Otero, L.; Kuang, D.; Zakeeruddin, S. M.; Grätzel, M. J. Phys. Chem. C 2007, 111, 6550. (26) Kern, R.; Sastrawan, R.; Ferber, J.; Stangl, R.; Luther, J. Electrochim. Acta 2002, 47, 4213. (27) Adachi, M.; Sakamoto, M.; Jiu, J.; Ogata, Y.; Isoda, S. J. Phys. Chem. B 2006, 110, 13872. (28) Koelsch, M.; Cassaignon, S.; Thanh Minh, C.Ta; Guillemoles, J.F.; Jolivet, J.-P. Thin Solid Films 2004, 86, 451. (29) Lee, K.; Park, S. W.; Ko, M. J.; Kim, K.; Park, N.-G. Nat. Mater. 2009, 8, 665.

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Barrier Layer Effect on the Electron Transport of the Dye-Sensitized

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