ARTICLE pubs.acs.org/JPCC
Efficiency Enhancement of Dye-Sensitized Solar Cell Using Pt Hollow Sphere Counter Electrode Van-Duong Dao,† Seong-Hoon Kim,† Ho-Suk Choi,*,† Jae-Ha Kim,‡ Han-Oh Park,‡ and Joong-Kee Lee§ †
Department of Chemical Engineering, Chungnam National University, 220 Gung-Dong, Yuseong-Gu, Daejeon 305-764, Korea Bioneer Co. 49-3, Munpyeong-Dong, Daedeok-Gu, Daejeon 306-220, Korea § Energy Storage Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea ‡
ABSTRACT: [Pt hollow spheres] electrodes and [Pt-sputtered + Pt hollow spheres] electrodes were fabricated on fluorine-doped tin oxide (FTO) glass as counter electrodes for dye-sensitized solar cells (DSCs), after sputtering platinum (Pt) on polystyrene microspheres and annealing them at 500 °C for 30 min. Electrochemical impedance measurement revealed that the charge transfer resistances of [Pt hollow spheres] electrodes and [Pt-sputtered + Pt hollow spheres] electrodes were 0.91 Ω cm2 and 0.48 Ω cm2, respectively, which were less than that of 1.28 Ω cm2 obtained from [Pt-sputtered] electrode. Furthermore, the DSCs with [Pt hollow spheres] and [Pt-sputtered + Pt hollow spheres] electrodes exhibited high energy-conversion efficiencies of 8.20((0.10)% and 8.53((0.08)%, respectively, which were both higher than the 7.89((0.05)% efficiency of the DSC with [Pt-sputtered] electrodes.
1. INTRODUCTION Gratzel first proposed dye-sensitized solar cells (DSCs) in 1991, and therefore, they are also called the Gratzel cells.1 Dyesensitized solar cells can achieve comparatively high conversion efficiency with low fabrication cost. A DSC consists primarily of a working electrode, an electrolyte, and a counter electrode. Although the working electrode has the greatest potential to improve the efficiency of DSCs, the counter electrode also has the important role of regenerating iodide from triiodide. Both high conductivity and excellent catalysis are required for this purpose. However, it is not easy to satisfy both requirements simultaneously. Generally, small particles, which provide high electrocatalytic activity because of their large surface area, have lower electron-transport efficiency because of their abundant grain boundaries and defects. Pt-coated fluorine-doped tin oxide (FTO) glass has been widely used in DSC counter electrodes because of its high conductivity and excellent catalysis.27 Methods for coating Pt onto a conductive substrate can be categorized into two methods, wet and dry. The wet method involves the pyrolysis of Pt precursors and electrochemical deposition. The activity of Pt clusters prepared through the thermocatalytic process was greater than that of Pt electrodeposited through thermal deposition or electrodeposition.2,3 In the deposition of Pt through polyol reduction,6,7 a surface with high active area was prepared, but the conductivity was decreased because of the polymer remaining in the substrate. Moreover, the charge-transfer resistance on Pt was as high as 2.28 Ω cm2. Dip coating8 provides an economical process with acceptable catalytic effect, but it is hard to improve its performance further because of the shielding effect of poly(4-vinyl)pyridine, which serves as a protective agent r 2011 American Chemical Society
surrounding the Pt nanoparticles. Moreover, the Pt deposit shows poor selectivity between the FTO surface and the glass substrate. Electroless deposition9 might overcome the poor selectivity between the FTO substrate and the glass substrate by grafting 3-(2-aminoethylamino)propylmethyldinethoxysilane (Me-EDA-Si) onto the FTO glass substrate. However, the conductivity of the substrate decreases with grafting of the Me-EDASi, resulting in a drastic decrease in cell efficiency. Problems also remain related to high charge-transfer resistance, for example 4.35 Ω cm2 for PVP-capped Pt (PVP/H2PtCl6 = 100/200 mg), 8.51 Ω cm2 for PVP-capped Pt (PVP/H2PtCl6 = 500/200 mg), 41.64 Ωcm2 for PVP-capped Pt (PVP/H2PtCl6 = 2000/ 200 mg),8 or 1.24 Ω cm2 for Pt prepared by electroless deposition (ELD).9 The major drawback of the wet method is the lack of reproducibility, which makes it difficult to control the thickness and uniformity of the substrate, resulting in low conductivity and high charge transfer resistance. The problem can be easily solved by using a dry method such as magnetron sputtering.2,4,10 The charge-transfer resistance depends on the thickness of the Pt layer,2 and the sheet resistance of the counter electrode decreases gradually with increasing deposition time. However, if the thickness of the Pt film is greater than 100 nm, further deposition gives no significant improvement in conductivity.4 The sheet resistance of Pt sputtered on FTO glass substrate is less than that of electrodeposited Pt electrodes.10 The major drawback of the dry method, however, is the limited active area. To overcome this drawback, an attempt has been made in Received: August 27, 2011 Revised: October 21, 2011 Published: November 15, 2011 25529
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Figure 1. Fabrication procedure for counter electrodes.
this research to combine the sputtering technique with a micro/ nanopatterning technique using a self-assembled colloidal monolayer. A variety of macroporous materials such as powders, films, and patterned substrates have been successfully fabricated using polymer microspheres as a sacrificial template.11 In this study, attempts were made to apply the micropatterning techniques using PS microspheres to the fabrication of DSC counter electrodes to enlarge the limited active area formed through sputtering Pt on the surface of FTO glass and thus eventually to improve the efficiency of DSCs. Furthermore, an effort was made to understand how the enlarged surface area affects the charge-transfer resistance and how the contact resistance between FTO glass and sputtered Pt affects the efficiency of DSCs.
2. EXPERIMENTAL SECTION 2.1. Materials. FTO glass as a conductive transparency electrode was purchased from Pilkington (∼8 Ohm/0). These substrates were used after cleaning by sonic treatment in acetone (Fluka). Nonporous TiO2 paste and ruthenium based-dye (N719) were purchased from Solaronix, Switzerland. The dye was adsorbed from a 0.3 mM solution in a mixed solvent of acetonitrile (Sigma-Aldrich) and tert-butyl alcohol (Aldrich) with a volume ratio of 1:1. The electrolyte was a solution of 0.60 M 1-methyl-3-butylimidazolium iodide (Sigma-Aldrich), 0.03 M I2 (Sigma-Aldrich), 0.10 M guanidinium thiocyanate (Sigma-Aldrich), and 0.50 M 4-tert-butylpyridine (Aldrich) in a mixed solvent of acetonitrile (Sigma-Aldrich), and valeronitrile, with a volume ratio of 85:15. 2.2. Preparation of Counter Electrodes. Polystyrene (PS) spheres were first prepared by the same procedure as in a previous study.12 A droplet of PS-sphere suspension was dropped onto FTO glass substrate (the drop coating process, Figure 1) and dried at 120 °C for 5 min to form a self-assembled monolayer on the substrate. The resulting PS monolayer-coated substrate was DCsputtered with Pt at 10 mA and 2 103 Torr for 5 min. Then the PS layer was removed by annealing it at 500 °C for 30 min, leaving hollow Pt spheres on the FTO glass substrate. The result was called the [Pt hollow spheres] electrode. A conventional Pt counter electrode sputtered onto FTO glass was also prepared as a control and is referred to here as the [Pt-sputtered] electrode. Additional Pt hollow spheres were fabricated on the [Pt-sputtered] electrode by
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following the procedure to construct the [Pt hollow spheres] electrodes. The result of this process is called the [Pt-sputtered + Pt hollow spheres] electrode. To observe the morphology of each counter electrode, scanning electron microscopy (SEM) (JEOL JSM-7000F) was used. 2.3. Preparation of TiO2 Working Electrode. TiO2 paste was purchased from Solaronix, Switzerland. After screenprinting (200 T mesh) the transparent film of 20 nm TiO2 particles with the thickness of 12 μm on the FTO-glass substrate, we also coated 4 μm thick scattering layer of TiO2 paste (DSL 18 NR-AO, Dyesol-Timo, Australia). The electrode was sequentially sintered at 325 °C for 5 min, at 375 °C for 5 min, at 450 °C for 15 min, and finally at 500 °C for 15 min under ambient condition. Before screen printing, the FTO glass was immersed in 40 mM of TiCl4 solution at 70 °C for 30 min and the same treatment was also done after sintering of the TiO2 layer. For dye adsorption, the layers were immersed into the dye solution (0.3 mM N719 dye solution of acetonitrile and tert-butyl alcohol with the volume ratio of 1:1) for 24 h at room temperature, immediately after reheating the layers at 500 °C for 30 min and cooling to 80 °C.13 2.4. Electrochemical Catalysis of Counter Electrodes. A cyclic voltammogram (CV) was used to measure the electrochemical catalysis of the Pt layer. The experiments were performed in a three-electrode cell with a potentiostat (IVIUMSTAT). The [Pt-sputtered], [Pt hollow spheres], and [Ptsputtered + Pt hollow spheres] electrodes were used as working electrodes, a Pt mesh was used as a counter electrode, and a saturated calomel electrode (SCE) was as a reference electrode. The electrolyte consisted of 10 mmol L1 LiI, 1 mmol L1 I2, and 1 mmol L1 LiClO4. The data were recorded from 600 to 300 mV with a scan rate of 50 mV s1. 2.5. Measurement of PhotocurrentVoltage (IV) Curves and Impedance Spectra. Photocurrentvoltage characteristics were measured with an IVIUMSTAT under illumination from a Sun 3000 solar simulator composed of 1000 W mercury-based Xe arc lamps and AM 1.5-G filters. Light intensity was calibrated with a silicon photodiode. Impedance spectra were acquired with a computer-controlled potentiostat (IVIUMSTAT). The electrochemical impedance spectroscopy (EIS) of DSC was performed under constant light illumination (100 mW cm2) biased at open-circuit condition. The measured frequency range was 100 kHz to 100 mHz with perturbation amplitude of 10 mV.14 The obtained spectra were fitted using the Z-View software (v3.2c, Scribner Associates, Inc.) with reference to the proposed equivalent circuit.
3. RESULTS AND DISCUSSION The products of the experiments described above were examined using field-emission-scanning electron microscopy (FESEM) (JEOL JSM-7000F). Figure 2 shows PS spheres (a) before and (b) after Pt sputtering and cross-sectional views of (c) [Pt hollow spheres] and (d) [Pt-sputtered + Pt hollow spheres]. It can be seen that the [Pt hollow spheres] have a partially open hemispherical shape after annealing. This hollow structure of partially open hemispheres enables electrolytes to make contact easily with both the inside and the outside surfaces of the hollow spheres, which suggests that this structure can provide higher electrochemical activity with a larger surface area than conventional [Pt-sputtered] electrodes. The inside and outside of [Pt hollow spheres] can be used as a catalyst through 25530
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Figure 2. FESEM images of (a) PS spheres, (b) PS spheres covered with [Pt-sputtered] material, and a cross-section of (c) [Pt hollow spheres] and (d) [Pt-sputtered + Pt hollow spheres] electrodes.
contact with electrolytes; in the case of [Pt-sputtered + Pt hollow spheres], contact can be made with the inside surface, the outside surface, and also the bottom side surface which is covered by [Pt-sputtered] material. This suggests that the [Pt-sputtered + Pt hollow spheres] structure can improve the electrochemical activity with a larger surface area than that of the [Pt-sputtered] or [Pt hollow spheres] structures. To confirm this point, cyclic voltammetry of all three electrodes was carried out. The catalytic performances of the three electrodes were evaluated through comparative analysis of the cyclic voltammograms of the three electrodes in acetonitrile solution containing 10 mmol L1 LiI, 1 mmol L1 I2, and 1 mmol L1 LiClO4. For cyclic voltammetry, [Pt-sputtered], [Pt hollow spheres], and [Pt-sputtered + Pt hollow spheres] electrodes were used as working electrodes, the Pt mesh was a counter electrode, and Hg/Hg2+ in acetonitrile served as a reference electrode. The data were recorded from 600 to 300 mV with a scan rate of 50 mV s1. The inset of Figure 3a represents the configuration of the three electrodes set up for cyclic voltammetry. Both [Pt hollow spheres] and [Pt-sputtered + Pt hollow spheres] electrodes exhibited higher current density than the [Pt-sputtered] electrode, which can be confirmed from the higher peak current density of [Pt hollow spheres] and [Pt-sputtered + Pt hollow spheres] electrode curves in Figure 3a, and which is due to the larger active surface area of these structures.14 Figure 3b shows the effect of the interface resistance of the [Pt-sputtered], [Pt hollow spheres], and [Pt-sputtered + Pt hollow spheres] electrodes with the FTO glass substrate. Because electric current flows through FTO glass, the measured current density results from the interface resistance between the FTO glass and the Pt layer. Six electrodes were tested with the configurations shown in the insets of Figure 3b. Figure 3b shows that the interface resistances between FTO glass and [Ptsputtered], [Pt hollow spheres], [Pt-sputtered + Pt hollow spheres] layers definitely caused differences in the current density because the electrons should go through two interfaces between the FTO glass and the Pt layer. The inset images in Figure 3b show more detail of the electrode configurations. The inset images A, B, and C in
Figure 3 represent partially sputtered electrodes of [Pt-sputtered], [Pt hollow spheres], and [Pt-sputtered + Pt hollow spheres], respectively, while insets D, E, and F represent fully sputtered electrodes of [Pt-sputtered], [Pt hollow spheres], and [Pt-sputtered + Pt hollow spheres], respectively. Thus, it was possible to investigate the effects of interface resistance between the FTO glass and the Pt layer for each electrode. As shown in Figure 3b, only B and E showed a difference in peak currents, but not A and D, or C and F. This suggests that the photocurrent densities of [Pt-sputtered] and [Pt-sputtered + Pt hollow spheres] electrodes are higher than that of [Pt hollow spheres] electrodes. To fabricate the DSC working electrode coated with TiO2 layer of 0.7 cm 0.7 cm on the FTO glass of 2 cm 2 cm, the procedure described in Ito et al.13 was used. The transmittance of TiO2 scattering layer has been known to be very poor.15 Figure 4 shows the transmittance of FTO glass before and after being coated with both transparent and scattering TiO2 layers. It becomes almost zero within the range of wavelength from 300 to 800 nm, as shown in Figure 4. Thus, the optical characteristics of hollow-sphere shaped Pt counter electrode can show a negligible effect on the efficiency of our DSC. Figure 5 shows a performance comparison of the three DSCs with different counter electrodes under one-Sun illumination from a Sun 3000 solar simulator composed of 1000-W mercury-based Xe arc lamps and AM 1.5-G filters. The DSCs with [Pt hollow spheres] and [Pt-sputtered + Pt hollow spheres] electrodes showed energy conversion efficiencies of 8.20((0.10)% and 8.53((0.08)%, respectively, which are both higher than the value of 7.89((0.05)% obtained for the DSC with [Pt-sputtered] electrode, as listed in Table 1. Under the same illumination conditions, the Voc values of the DSCs with [Pt hollow spheres] electrode and [Pt-sputtered + Pt hollow spheres] electrode were higher than that of the DSC with [Pt-sputtered] electrode. The Jsc value of the DSC with [Pt hollow spheres] electrode, however, was lower than that of the DSC with [Pt-sputtered] electrode, while the Jsc value of the DSC with [Pt-sputtered + Pt 25531
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Figure 4. Transmittance of FTO substrate before and after being coated with both transparent and scattering TiO2 layers.
Figure 3. Cyclic voltammograms of (a) three electrodes [Pt-sputtered], [Pt hollow spheres], and [Pt-sputtered + Pt hollow spheres], and (b) six electrodes (A) partially covered [Pt-sputtered], (B) partially covered [Pt hollow spheres], (C) partially covered [Pt-sputtered + Pt hollow spheres], (D) fully covered [Pt-sputtered], (E) fully covered [Pt hollow spheres], and (F) fully covered [Pt-sputtered + Pt hollow spheres] on FTO glass in 5 mM LiI + I2 acetonitrile solution containing 0.1 M LiClO4 as the supporting electrolyte. [I]/[I2] = 10/1, with Hg/Hg2+ reference electrode in acetonitrile.
hollow spheres] electrode was higher than that of the DSC with [Pt-sputtered] electrode. In a DSC, electrons are injected from I ions in an electrolyte into a photo-oxidized dye, and the produced I3 are reduced on the counter electrode. Figure 3 shows a much lower current density of the I3 reduction peak for the [Pt hollow spheres] electrode and [Pt-sputtered + Pt hollow spheres] electrode than for the [Pt-sputtered] electrode. This suggests that the reaction rate is very slow on the [Ptsputtered] electrode. In other words, the charge-transfer resistance Rct for the I3/I redox reaction was very large on the [Pt-sputtered] electrode. Therefore, the energy-conversion efficiency of the DSC with [Pt-sputtered] electrode was smaller than that of the DSCs with the [Pt hollow spheres] electrode and [Pt-sputtered + Pt hollow spheres] electrode, as shown in Figure 5 and Table 1.
Figure 5. Photocurrentvoltage (IV) characteristics of DSCs fabricated with [Pt-sputtered], [Pt hollow spheres], and [Pt-sputtered + Pt hollow spheres] electrodes on FTO-glass substrate as counter electrodes.
Half-wave potentials in I (in Figure 3a) were 185 mV for the [Pt-sputtered] electrode, 165 mV for the [Pt hollow spheres] electrode, and 180 mV for the [Pt-sputtered + Pt hollow spheres] electrode. Therefore, the formal potential shifted more positively for the [Pt-sputtered] electrode and [Pt-sputtered + Pt hollow spheres] electrode.16,17 These positive shifts of I3/I redox energy level are directly related to the open-circuit voltage through eq 1 Voc ¼
Jinj KT Eref ln þ e qeket cox Nc e
ð1Þ
where Nc is the density of states in the TiO2 conduction band, Jinj is the current density of photogenerated electrons, q is the number of electrons involved in the charge-transfer process to the oxidized species of concentration cox with transfer rate ket and elementary charge e, K is the molar gas constant, T is the temperature, and Eref is the difference between the TiO2 25532
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Table 1. Photoelectric Performance of the Three Cells Shown in Figure 5 counter electrode
Jsc (mA cm2)
Voc (mV)
FF (%)
η (%)
[Pt-sputtered]
14.40 ( 0.23
786.3 ( 14.3
69.76 ( 0.47
7.89 ( 0.05
[Pt hollow spheres]
14.12 ( 0.28
815.0 ( 11.l
71.19 ( 0.19
8.20 ( 0.10
[Pt-sputtered + Pt hollow spheres]
15.13 ( 0.24
805.0 ( 7.07
70.10 ( 0.94
8.53 ( 0.08
Figure 6. Nyquist plots of DSCs with [Pt-sputtered], [Pt hollow spheres], and [Pt-sputtered + Pt hollow spheres] electrodes on FTOglass substrate. The top image shows the equivalent circuit diagram used to fit the observed impedance spectra in this figure. Abbreviations follow: Rh, ohmic serial resistance; Rct1, charge-transfer resistance of the counter electrode; CPE1, constant-phase element of the counter electrode; Rct2, charge-transfer resistance of the working electrode; CPE2, constant-phase element of the working electrode, and Ws, Warburg impedance.
conduction band edge (Ec) and the redox potential of the I3/I redox coupling electrolyte (Eref = Ec Eredox).1820 Therefore, the positive shift in the I3/I redox energy level results in the decrease of Voc in DSCs, which is shown in Figure 5. Moreover, the charge-transfer resistance Rct for the I3/I redox reaction on the [Pt-sputtered] electrode became larger than that on the [Pt hollow spheres] electrode and [Ptsputtered + Pt hollow spheres] electrode. The electrochemical impedance spectroscopy (EIS) in Figure 5 will further confirm this point. The spectra were fitted to the equivalent circuit using the Z-View software and are shown in the top image of Figure 6. Table 2 shows the estimated values of the charge-transfer resistances of both the counter electrode and the working electrode and also CPE parameters for the interfaces of both counter and working electrodes. For the purpose of comparing only counter electrodes, three working electrodes were fabricated using the same materials and method. As shown in Table 2, none of the parameters for the working-electrode interface showed any significant difference between the three DSCs. The Rct values of the [Pt hollow spheres] electrode and [Pt-sputtered + Pt hollow spheres] electrodes, however, were as small as 0.91 Ω cm2 and 0.48 Ω cm2, respectively, while that of the [Pt-sputtered] electrode was 1.28 Ω cm2. Figure 2 shows that the apparent reduction rates for the [Pt hollow spheres] electrode and [Pt-sputtered + Pt hollow spheres]
electrodes were faster than that for the [Pt-sputtered] electrode. Because the apparent reduction rate is directly related to the peak current, ip, and Rct = RT/nFip, the Rct values of the [Pt hollow spheres] electrode and the [Pt-sputtered + Pt hollow spheres] electrode become smaller than that of the [Ptsputtered] electrode. The constant-phase element (CPE1 = (CPE1-T)1(jw)(CPE1‑P)) of the counter electrode also confirmed that the active surface areas of the [Pt hollow spheres] electrode and the [Pt-sputtered + Pt hollow spheres] electrodes were larger than that of the [Pt-sputtered] electrode. Table 2 shows that the CPE1-T value was 46.9 105 F cm2 for the [Pt hollow spheres] electrode and 130 105 F cm2 for the [Pt-sputtered + Pt hollow spheres] electrode, which is much larger than the value of 5.86 105 F cm2 obtained for the [Pt-sputtered] electrode. The large CPE1-T value represents an increase in active surface area. Table 2 also shows that the porosity (CPE1-P) value was 0.77 for the [Pt hollow spheres] electrode and 0.68 for the [Pt-sputtered + Pt hollow spheres] electrode, which is much smaller than the value of 0.89 for the [Pt-sputtered] electrode. A smaller CPE1-P represents an increase in the porosity of the Pt layer. Table 2 also shows that the ohmic serial resistance (Rh) value of the [Pt-sputtered] electrode measured at high frequency range is 2.53 Ω cm2, which is lower than the value of 2.58 Ω cm2 for the [Pt hollow spheres] electrode, but higher than the value of 2.22 Ω cm2 for the [Pt-sputtered + Pt hollow spheres] electrode. Four-point probe measurements also showed that the sheet resistance of the [Pt-sputtered] electrode was 1.33 Ω/0, which is lower than the value of 1.52 Ω/ 0 for the [Pt hollow spheres] electrode, but higher than the value of 1.11 Ω/0 for the [Pt-sputtered + Pt hollow spheres] electrode. Since the series resistance, Rs, is a function of ohmic serial resistance of electrode,21 a decrease in sheet resistance results in an increase in short-circuit current density (Jsc), as described by eq 2:22,23 qðV þ IRs Þ V þ IRs I ¼ Iph Io exp ð2Þ 1 nKT Rsh Here, Rsh is the shunt resistance, which represents the recombination of electrodes.24 If all parameters are assumed constant except the series resistance Rs, current density should decrease with increasing Rs. This explains why the Jsc value of the DSC with the [Pt hollow spheres] electrode became smaller than that of the DSCs with the [Pt-sputtered] electrode and the [Pt-sputtered + Pt hollow spheres] electrode. These values were completely consistent with the existing interface resistance between the [Pt hollow spheres] electrode and the FTO glass substrate, as explained earlier. As mentioned before, the [Pt hollow spheres] electrode with a large active surface area can express a larger open-circuit voltage than the [Pt-sputtered] electrode, even if Jsc decreases due to the increase in series resistance. The low value of short-circuit current density was improved using a [Pt-sputtered + Pt hollow spheres] electrode. 25533
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Table 2. Impedance Parameters of Three DSCs with [Pt-Sputtered], [Pt Hollow Spheres], and [Pt-Sputtered + Pt Hollow Spheres] Electrodes on FTO-Glass Substrate As Estimated from the Impedance Spectra and Equivalent Circuit Shown in Figure 5 Ws counter electrode
2
2
2
2
Rh (Ω cm ) Rctl (Ω cm ) CPE1-T (Ω cm ) CPE1-P Rct2 (Ω em )
[Pt-sputtered]
2.53
1.28
5.86 105 4
[Pt hollow spheres]
2.58
0.91
4.69 10
[Pt-sputtered + Pt hollow spheres]
2.22
0.48
1.30 103
This caused an increase in the fill-factor component of DSC performance.
4. CONCLUSIONS In summary, a large active surface area has been demonstrated for counter electrodes of dye-sensitized solar cells using a [Pt hollow spheres] electrode and a [Pt-sputtered + Pt hollow spheres] electrode with 8.20% and 8.53% energyconversion efficiency, respectively. Moreover, these electrodes showed a low charge-transfer resistance because of their large surface area. There is still more potential for these electrodes to provide even higher energy-conversion efficiencies through the use of smaller PS nanospheres with diameters such as 500 or 200 nm. Furthermore, the short-circuit current density (Jsc) could be further improved, as shown in the demonstration of the [Pt-sputtered + Pt hollow spheres] electrode. ’ AUTHOR INFORMATION Corresponding Author
*To whom correspondence should be addressed. E-mail: hchoi@ cnu.ac.kr.
R
T
P
CPE2-T (Fern2) CPE2-P
0.89
3.21
3.11 0.34 0.5
5.63 103
0.95
0.77
3.33
2.03 0.26 0.5
4.69 103
0.93
0.68
2.96
1.82 0.32 0.5
5.39 103
0.95
(12) Zuo, X.; Anjie, W.; Kim, D. P. J. Mater. Chem. 2010, 20, 2853–2857. (13) Ito, S.; Murakami, T. N.; Comte, P.; Liska, P.; Gratzel, C.; Nazeeruddin, M. K.; Gratzel, M. Thin Solid Films 2008, 516, 4613. (14) Zhang, D. W.; Li, X. D.; Chen, S.; Tao, F.; Sun, Z.; Yin, X. J.; Huang, S. M. J. Solid-State Electrochem. 2010, 14, 1541. (15) Liau, L. C.-K.; Chung, Y.-C. PEA-AIT International Conference on Energy and Sustainable Development: Issues and Strategies (ESD 2010), 2010, Chiang Mai, Thailand. (16) Peter, L. M. Phys. Chem. Chem. Phys. 2007, 9, 2630. (17) Imoto, K.; Takahashi, K.; Yamaguchi, T.; Komura, T.; Nakamura, J.; Murata, K. Sol. Energy Mater. Sol. Cells 2003, 79, 459. (18) Huang, S. Y.; Schlichthorl, G.; Nozik, A. J.; Gratzel, M.; Frank, A. J. J. Phys. Chem. B 1997, 101, 2576. (19) Durr, M.; Yasuda, A.; Nelles, G. Appl. Phys. Lett. 2006, 89, 061110. (20) Murakami, T. N.; Ito, S.; Wang, Q.; Nazeeruddin, M. K.; Bessho, T.; Cesar, I.; Liska, P.; Humphry-Baker, R.; Comte, P.; Pechy, P.; Gratzel, M. J. Electrochem. Soc. 2006, 153, A2255. (21) Han, L.; Koide, N.; Chiba, Y.; Ashraful, I.; Komiya, R.; Fuke, N.; Fukui, A.; Yamanaka, R. Appl. Phys. Lett. 2005, 86, 213501. (22) Koide, N.; Islam, A.; Chiba, Y.; Han, L. J. Photochem. Photobiol., A 2006, 182, 296. (23) Lee, K. M.; Chiu, W. H.; Wei, H. Y.; Hu, C. W.; Suryanarayanan, V.; Hsieh, W. F.; Ho, K. C. Thin Solid Films 2010, 518, 1716. (24) Huang, Y.; Dai, S.; Chen, S.; Zhang, C.; Sui, Y.; Xiao, S.; Hu, L. Appl. Phys. Lett. 2009, 95, 243503.
’ ACKNOWLEDGMENT This work (Grant 00041548-1) was supported by the Business for Cooperative R&D between Industry, Academy, and Research Institute fund from the Korea Small and Medium Business Administration in 2010. The authors want to thank Prof. C.-G. Lee, Prof. D.-J. Kim, and Prof. D.-P. Kim for assistance with electrochemical analysis, Pt sputtering, and PS-sphere micropatterning, respectively. ’ REFERENCES (1) O’Regan, B.; Gratzel, M. Nature (London) 1991, 353, 737. (2) Hauch, A.; Georg, A. Electrochim. Acta 2001, 46, 3457. (3) Papageorgiou, N.; Maier, F.; Gratzel, M. J. Electrochem. Soc. 1997, 144, 876. (4) Fang, M.; Ma, L.; Guan, Q.; Akiyama, M.; Kida, T.; Abe, E. J. Electroanal. Chem. 2004, 570, 257. (5) Papageorgiou, N. Coord. Chem. Rev. 2004, 248, 1421. (6) Sun, K.; Fan, B.; Ouyang, J. J. Phys. Chem. C 2010, 114, 4237. (7) Cho, S. J.; Ouyang, J. J. Phys. Chem. C 2011, 115, 8519–8526. (8) Wei, T.; Wan, C.; Wang, Y.; Chen, C.; Shiu, H. Appl. Phys. Lett. 2006, 88, 103–122. (9) Lin, C.; Lin, J.; Lan, J.; Wei, T.; Wan, C. Electrochem. Solid-State Lett. 2010, 13, D77–D79. (10) Koo, B.; Lee, D.; Kim, H.; Lee, W.; Song, J.; Kim, H. J. Electroceram. 2006, 17, 79. (11) Li, L.; Zhai, T.; Zeng, H.; Fang, X.; Bando, Y.; Golberg, D. J. Mater. Chem. 2011, 21, 40. 25534
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