Highly Improved Ion Diffusion through Mesoscopically Ordered Porous

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Highly Improved Ion Diffusion Through Mesoscopically Ordered Porous Photoelectrodes Su-Jin Ha, and Jun Hyuk Moon J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 17 May 2017 Downloaded from http://pubs.acs.org on May 21, 2017

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The Journal of Physical Chemistry

Highly Improved Ion Diffusion through Mesoscopically Ordered Porous Photoelectrodes Su-Jin Ha and Jun Hyuk Moon* Department of Chemical and Biomolecular Engineering, Sogang University, 35 Baekbeom-ro, Mapo-gu, Seoul, 04107, Republic of Korea

Corresponding author, E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

Photoelectrochemical devices rely on porous photoelectrodes because of the formation of heterojunctions with the electrolyte solution. We evaluate the electrolyte ion-diffusion transport in mesoscopic inverse opal (meso-IO) structures with a uniform pore network by comparison with the diffusion in the conventional random pore electrode. The ion diffusivity through porous photoelectrodes was obtained using the modified Fick’s law with the diffusion-limiting current in the current-voltage characteristic. We observe that the ion diffusivity of iodine-based electrolytes through the meso-IO electrode film was 1.5 times greater than that through the random pore structure. More importantly, the diffusion of larger ions, cobalt-based ions, was 58% more enhanced in the meso-IO structure than in the random pore structure. In practice, we confirmed the effect of electrolyte ion diffusion on the photovoltaic performance of the dye-sensitized solar cell, and about 11% higher efficiency at the meso-IO electrode compared to the random pore electrode when the electrolyte containing large cobalt ions. This study suggests that with bulky electrolyte ions and a highly viscous polymer gel matrix, a uniform pore electrode structure, such as the IO structure, can be advantageous to achieve the best photoelectrochemical performance.


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INTRODUCTION Photoelectrochemical devices rely on porous oxide films as photoelectrode materials because of the formation of heterojunctions with the electrolyte solution.1-3 Specifically, dye-sensitized solar cells (DSSCs) use a porous semiconductor film obtained by randomly packing nanoparticles as a photoelectrode.2-5 The DSSCs operate using the potential induced by electron-hole spatial separation upon light exposure. The photo-generated electrons are injected into the conduction bands of electrode materials and diffused to the charge-collecting substrate, whereas the holes are transported to the counter electrode by the redox ions in the electrolyte solution.6 Thus, the pore network that forms in the electrode material affects the diffusion of ions and electrons. Because the ion diffusion, specifically the diffusion in solid-state electrolytes is slower than the electron diffusion, the topology of the pore network from the ion-diffusion perspective is the one of critical factors determining the performance of photoelectrochemical devices.5, 7-11 The ion diffusion in photoelectrochemical devices is a combination of concentration gradient-induced diffusion, which can be described by Fick’s law, and that based on Grotthusstype charge exchange.8, 12-15 For example, in DSSCs that use iodine-based electrolytes, which are the most widely used for redox couples, the effective ion diffusion reflects the physical diffusion of iodide (I-) and triiodide ions (I3-) and the charge-exchange-based diffusion between these ions.8, 16-18 Although the physical- and charge-exchange-based diffusion are difficult to consider separately, in a conventional electrolyte system, which typically consists of a low ion concentration of approximately 1 M, physical diffusion is dominant.8, 17 Moreover, a polymer gel electrolyte is often used instead of a liquid electrolyte. In this case, the physical diffusion of ions is significantly delayed due to the high viscosity of the polymer gel.17,

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The device

performance is more strongly affected by the diffusion of redox ions. Thus, the physical ion


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diffusion through the pore network of electrode materials must be evaluated to improve the performance of such photoelectrochemical devices.20 However, most previous reports have focused on the development of novel nanostructured electrodes such as nanotubes, nanowires, nanorods, and inverse opals, focusing on improving electron diffusion compared to the conventional nanoparticle electrodes.21-25 There have been a few report of quantitative analysis of the ion-diffusion properties through the nanostructure electrodes. 26-27 Here, we evaluate the ion diffusion in the nanostructured photoelectrode with ordered and uniform pore networks. Specifically, we prepare a mesoscopic porous inverse opal (meso-IO) photoelectrode that we first presented in the previous report.28-29 The meso-IO structure, which is templated by colloidal crystals, has a face-centered cubic pore network with a pore volume of up to approximately 55%, and the pores are fully connected throughout the structure. Previously, the IO photoelectrode exhibited comparable cell performance with the conventional, random mesopore electrode in its DSSC application which was analyzed as a fast electron transfer in the periodic skeleton of IOs.30-31 In this study, we compare the diffusion coefficients of iodine- and cobalt-based electrolyte ions of the meso-IO structures with those of random pore structures. The cobalt-based electrolytes are promising alternative electrolyte ions, especially, those with lower redox potential than iodine-based redox couples, enabling high photovoltaic performance.15, 30, 32 Briefly, the meso-IO structure exhibits a 1.5 times higher ion-diffusion coefficient than the conventional electrode with iodine-based electrolytes, and this diffusion coefficient increases by 2.4 times when the cobalt-based electrolyte system is used. In practice, we confirm the effect of electrolyte ion diffusion on the photovoltaic performance of DSSC, and about 11% higher efficiency at the meso-IO electrode compared to the random pore electrode when the electrolyte containing large cobalt ions. This study suggests that with bulky electrolyte ions and a highly


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viscous polymer gel matrix, a uniform and regular-pore electrode structure, such as the IO structure, can be advantageous to achieve the best photoelectrochemical performance.


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EXPERIMENTAL METHODS Fabrication of the meso-IO film. Polystyrene (PS) nanoparticles (99 nm in diameter, 10 wt% in water, Bangs laboratories, Inc.) in the form of colloidal crystals were prepared on a Pt-coated fluorine-doped tin oxide (FTO) substrate. The Pt-coated FTO was prepared using 0.5 mM H2PtCl6 solution in anhydrous ethanol. The TiO2 precursor was deposited onto the PS colloidal crystals using atomic layer deposition (ALD) and subsequent annealing at 500°C for 2 hours to remove the PS particles. For ALD, TiCl4 and deionized water were used as the precursors and sequentially fed to the reactor using N2 gas. The chamber pressure was approximately 1 torr, and the precursor exposure time was 30 s. The TiO2 deposition rate under these conditions was approximately 0.20 nm/cycle. Then, the TiO2-deposited PS colloidal crystals were heated at 500°C for 2 hr in air to remove the PS template. To fabricate a conventional random mesopore electrode film, a TiO2 paste purchased from Dyesol (DSL 18NR-T) was coated on the Pt-coated FTO and then heat treated at 500°C for 15 min. Cell fabrication for diffusion measurement. We used the previously described Pt-gel-Pt sandwich-type device.33 The counter electrode was prepared by coating 0.5mM H2PtCl6 solution in anhydrous ethanol ion onto an FTO substrate and subsequently heat treating it at 450°C for 30 min. The TiO2 electrode and counter electrode were sandwiched with a 60µm gap using a spacer film (Surlyn, Dupont). The iodine-based (I-/I3-) electrolyte solution was prepared by dissolving 50mM Lil (Aldrich), 25 mM I2 (Yakuri), 0.7M 1-buthyl-2-methylimidazolium iodide (BMII, Aldrich), and guanidinium thiocyanate (GuSCN) in 3-methoxypropionitrile (Aldrich). The cobalt-based electrolyte ([Co(bpy)3]2+/[Co(bpy)3]3+) solution was prepared by dissolving 0.2M Co(bpy)3(B(CN)4)2 (Dyenamo), 0.05M Co(bpy)3(B(CN4)3 (Dyenamo), 0.1M LiClO4 (Aldrich) and 0.2M 4-tert-butylpyridine (Aldrich) in 3-methoxypropionitrile (Aldrich). These electrolytes


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were mixed in a 5 wt% poly(vinylidene fluoride-co-hexafluoropropylene (PVdF-HFP, Aldrich) solution to form a polymer gel electrolyte. The polymer gel electrolyte was injected into the sandwich cell by vacuum infiltration. The full infiltration of the polymer gel was confirmed as shown in Figure S1. Assembly of dye-sensitized solar cells. The TiO2 photoelectrode film was prepared on FTO substrate. The area of the TiO2 electrode was controlled by scraping to be approximately 8-10 mm2. We sensitized the TiO2 electrode with D205 dye; the film was soaked in a 0.5 mM D205 solution in a mixture of acetonitrile solution overnight. A Pt counter electrode was prepared by coating a 0.7 mM H2PtCl6 solution in anhydrous ethanol onto an FTO substrate. The TiO2 electrode/FTO substrate and the counter electrode were assembled, and the gap between the two electrodes was fixed using a 60-μm-thick polymeric film (Surlyn, DuPont). Finally, the polymer gel electrolyte solution was injected into the gap by vacuum infiltration. In the case of iodine/cobalt complex electrolytes, the solution was prepared by incorporating 5mM 5mM Co(bpy)3(B(CN4)3 into the iodine electrolyte ion. Characterization. The morphologies of the TiO2 electrodes were characterized using fieldemission scanning electron microscopy (FE-SEM: AURIGA, Carl Zeiss). The cyclic voltammetry of the sandwich cell was also evaluated (Versastat, Ametek) in a voltage window of approximately 0 V to 1 V and a scan rate of 50 mV/s. The J-V characteristics of the DSSCs were measured using a source meter (Keithley Instruments) under 1 sun illumination. The solar light was simulated using a xenon lamp (300 W, Oriel) and an AM 1.5 filter. The intensity of the simulated solar light was calibrated using Si standard solar cells (Bunko-Keiki BS-520). The electrical impedance spectra (EIS) were measured with a scan rate of 10 mV/s over the


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frequency range of 105 Hz to 0.1 Hz and were analyzed using Z-View software (Scribner Associates, Inc.)


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RESULTS AND DISCUSSION The meso-IO TiO2 electrode was fabricated using a colloidal-crystal template of 99-nm PS spheres. The cross-sectional SEM image reveals uniform and fully connected pores over the entire film, as shown in Figure 1a. The surface SEM image shows the ordered pores with an average pore size of approximately 70 nm, which were connected by window pores of approximately 30 nm in diameter, as shown in inset of Figure 1a. The conventional random mesopore TiO2 film shows a broad range of pore size, and has a pore size range of approximately 10 - 100 nm, as shown in Figure 1b. TEM images of meso-IO and random pore structures are shown in Figure S2. The pore size for the TiO2 films was also confirmed by measuring Brunauer-Emmett-Teller (BET) as shown in Figure S3. XRD data of these structures can also be observed in Figure S4, and TiO2 of these structures is a mixture of anatase and rutile structures. The N2 adsorption/desorption isotherm curves are represented the both TiO2 film had similar mesopore structures. The meso-IO pore size was larger than that of conventional electrodes. The diffusion is known to be independent of the pore size when the ratio of the molecular size to the pore size exceeds 0.1 because the mean free paths of molecules comparable to the molecular size are much shorter than the pore size.14 Thus, comparing the diffusion properties of meso-IO and the conventional electrode, we can assume that the pore size difference between these structures contributes little to the diffusivity but characterizes the effect of the pore network, i.e., orderly regular pores in the meso-IO and random mesopores in the conventional electrode. The I3- ion diffusion was almost identical in the 7-20 nm random mesoporous TiO2 electrodes.14 The diffusivity was measured using the limiting current method. Briefly, we prepared a sandwich cell of Pt-coated FTO/electrolyte-solution-soaked TiO2 electrode/electrolytesolution/Pt-coated FTO and obtained the current-voltage (J-V) curve (see Figure 1). We used


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PVdF-HFP gel electrolytes. When the applied voltage decreased, the current density decreased and subsequently became saturated, as shown in Figure 2c and Figure 2d. The lower current density at a negative bias implies that the diffusion of I3- ions through the porous TiO2 electrode to the Pt substrate was retarded. When the I-/I3- redox couple electrolyte was used, the current was generated from the electro-catalytic reduction reaction of I3- at the Pt surface. The concentration ratio of I2 to I- in the electrolyte solution was 0.03; thus, I3-, which was produced from I2 + I-, is the limiting species. The increase in current density with the increase in bias voltage indicates that a more activated charge-transfer process occurred. First, we compared the ion diffusivity of I-/I3--based electrolytes with meso-IO and random mesopore electrodes. The JV curve exhibits a monotonic increase in current density to -0.4 V and a plateau region at approximately -1.0 − -0.4 V. This saturated current is caused by the limitation of I3- diffusion and is ascribed to the limiting current. Thus, the ion diffusivity of I3- through the porous electrode can be estimated from the limiting current density using the following equation of Fick’s diffusion law:12

‫=ܬ‬

ଶி஽೛ ஼೏ ௗ

(1)

where J is the limiting current density, F is Faraday constant (96485.3329sA/mol), Dp is the effective diffusivity of transport-limiting ion species through a porous electrode, d is the thickness of the electrode or the distance from the electrode surface to the Pt substrate (see the cell geometry in Figure 1), and Cd is the ion concentration at position d. In the following experiment, the cell structure is assumed to be ideal. Meanwhile, considering the continuity of ion flux, the following equation is derived: 12


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2‫ܦܨ‬௣

஼೏ ି஼బ ௗ

= 2‫ܦܨ‬௕

஼೗ ି஼೏

(2)

௟ିௗ

where Cl and C0 are the concentrations of transport-limiting ion species at the Pt and counter Pt substrates, respectively, and Db is the diffusivity of a transport-limiting ion species in the region from the electrode surface to the counter substrate, i.e., in the bulk electrolyte region. Here, C0 is approximately zero because the ions may be depleted in this condition. Moreover, the average ion concentration in the entire cell is equal to the ion concentration inside the porous electrode and bulk electrolyte region. This mass balance can thus be represented as follows:

ܿ௔௩௚ ሾ߳݀ + ሺ݈ − ݀ሻሿ = ߳

௖బ ା௖೏ ଶ

݀+

௖೏ ା௖೗ ଶ

ሺ݈ − ݀ሻ

(3)

where Cavg is the average concentration of the transport-limiting ion species, and ε is the porosity of the electrode, which is defined as the ratio between the pore volume and the total volume of the film. By combining the continuity and mass balance equations to eliminate Cl and Cd in the diffusion equation, the limiting current density is expressed as follows;

‫ = ܬ‬4‫ܥܨ‬௔௩௚ ஽

஽್ ஽೛ ሺఢௗା௟ିௗሻ మ మ ್ ఢௗ ାଶ஽್ ௗሺ௟ିௗሻା஽೛ ሺ௟ିௗሻ

(4)

We prepared electrodes of various thicknesses (d): 2.2 µm and 4.7 µm for meso-IO and 2.3 µm and 5.0 µm for the randomly porous electrode. l was approximately 40 µm on average. The ε values of the meso-IO and random mesopore electrode were 0.55 and 0.68, respectively, as obtained from the Brunauer-Emmett-Teller (BET) measurement. Cavg is the concentration of I3in the case of I-/I3--based electrolytes. The concentration of I3- is equivalent to the concentration of I2 since the concentration of I- is larger than that of I2, and thus the reaction I2 + I- → I3- is compete. In the experiment, the I3- was generated by the oxidation of I- at the TiO2/Pt electrode,


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as confirmed in Figure 3; the current was observed at zero I2 concentration. We counted the oxidative generation of I3- to obtain Cavg. The Cavg for the I-/I3- based electrolytes was approximately 16 mM. The absolute value of J was 27.69 mA/cm2 for the 2.2 µm-thick IO and 25.92 mA/cm2 for the 2.3 µm-thick mesoporous electrode as observed in Figure 2c; the absolute value of J for the 2.3 µm-thick IO was 10% higher than that of the random mesoporous electrode. Thus, we expect that the diffusion transport of I3- ions in the meso-IO structure was greater than that in the random porous structure. The values of J for two different thicknesses were used to obtain the Db and Dp for each electrode. Table 1 lists Db and Dp, which were obtained by fitting the J-V curves to the above equation using this information. The meso-IO and random mesopore electrode had similar Db, as expected. We also prepared bare Pt substrates, in which the limiting current density was directly related to the ion diffusivity. The value of Db obtained using the bare Pt substrate was equal to Db in the presence of TiO2 electrodes, as expected. Thus, we confirm that the estimation of Dp is reliable and that the Dp for each electrode film can be directly compared. Here, the meso-IO structure had approximately 1.5 times larger Dp than the random mesopore structure. Given that the pore size dependence on the diffusivity may be neglected because both structures had sufficiently large pore sizes relative to the ion molecule size, this result implies that the uniform and fully connected pore network in the IO structure improves the diffusion transport of electrolyte ions.14 Second, we evaluated the diffusion transport in the [Co(bpy)3]2+/[Co(bpy)3]3+-based electrolyte system in the meso-IO and random mesopore electrodes. The J-V curves of the [Co(bpy)3]2+/[Co(bpy)3]3+-based redox couples are also shown in Figure 2d. [Co(bpy)3]3+ was added at a much lower concentration and was the limiting species. In contrast to the I-/I3- based


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electrolytes, we observed no current from the direct oxidation (see Figure 3). The Cavg for the [Co(bpy)3]2+/ [Co(bpy)3]3+-based electrolytes was confirmed to be the same as the concentration of [Co(bpy)3]3+ added to the solution. The J-V curve shows that the current increased to -0.7 V and then reached a plateau. The absolute values of J of the 2.2 µm-thick meso-IO electrode and 2.3 µm-thick mesoporous electrode were 2.60 mA/cm2 and 1.69 mA/cm2, respectively. The value of J of the 2.2 µm-thick IO electrode was approximately 58% greater than that of the randomly porous electrode. Similarly to the case of the I-/ I3- redox couple, this result implies that the diffusion transport of [Co(bpy)3]3+-based ions in the meso-IO structure was faster than that in the random porous structure. The Dp and Db values were obtained using the equation and J values for two different thicknesses and are listed in Table 1. The value of Db determined for the cobaltbased electrolytes was approximately 5.63*10-7 cm2/s in average, which is approximately 32 times lower than that of the iodine-based electrolytes. The [Co(bpy)3]3+ has a larger molecular size than I3-, which retarded the electrolyte transport. The molecular size of [Co(bpy)3]3+ ion is approximately 2 times larger than that of I3-, as depicted in Figure 2a and Figure 2b. The Dp of the meso-IO electrode was 2.4 times higher than that of the random mesopore electrode. Thus, the meso-IO pore structure provides a more favorable ion-diffusion pathway than the random pore structures. We compared the relative diffusivity, which is defined by the ratio of Dp/Db, for the iodine- and cobalt-based electrolytes, as observed in Table 1. In the meso-IO electrode, the Dp/Db values were 0.81 and 0.74 for the iodine- and cobalt-based electrolytes, respectively, and the cobalt-based electrolyte exhibited a 17% slower diffusivity. In the random pore electrode, the Dp/Db values were 0.52 and 0.32 for the iodine- and cobalt-based electrolytes, respectively, and the cobalt-based electrolyte exhibited a 28% slower diffusivity. Thus, it is noted that for larger


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ions, the diffusion transport was less retarded in the meso-IO structure than in the random pore electrode. In other words, the meso-IO electrode is superior to the random pore electrode when bulky electrolyte ions are used. Finally, we evaluated how the difference in electrolyte diffusion at these electrodes affects the photovoltaic properties of DSSCs. In particular, we prepared an iodine/cobalt complex electrolyte by mixing a cobalt redox electrolyte with an iodine electrolyte. A random pore and a meso-IO electrode DSSC were fabricated using a complex electrolyte and an iodinebased electrolyte. The photovoltaic parameters, including the photocurrent density (JSC), opencircuit voltage (VOC), and fill factor (FF) of these DSSCs are obtained from the J-V curve in Figure 4 and listed in Table 1. The photo-to-electric conversion efficiency (η) in Table 2 was calculated as JSC*VOC*FF (100 mW/cm2). When an iodine-based electrolyte is applied, the efficiency of the meso-IO electrode DSSCs is about 9% higher than that of the random pore electrode DSSCs. On the other hand, when an iodine/cobalt-based electrolyte is applied, the meso-IO DSSCs is about 13% higher as compared with the random pore DSSCs. These results show that the efficiency decreases due to the low ion diffusivity of cobalt ions when the electrolyte contains cobalt ions. More importantly, meso-IO DSSCs show less efficiency reduction than random pore DSSCs. This can be attributed to the meso-IO of uniform pores that are easier to diffuse with larger ions. The VOC of meso-IO DSSC is 5-6% lower than that of NP DSSC because the meso-IO has a relatively large average pore size, resulting in a small contact area with the charge collecting substrate and thus a large interface resistance. We evaluated the ion diffusion resistance (Rdiff) in DSSCs by EIS measurement as shown in Figure 5a. The Nyquist plots of meso-IO and random pore electrode DSSCs, including iodine and complex electrolytes, were analyzed by the equivalent circuit shown in the Figure 5b. RS, R1,


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and R2 indicate the ohmic series resistance, the interfacial resistance at pt/electrolytes, and the interfacial resistance at TiO2/dye/electrolytes, respectively. The high frequency semicircle in the Nyquist plot corresponds to the Rdiff. For random pore DSSCs, the Rdiff value is 21.0 for the iodine electrolyte and 40.0 ohm for the complex electrolyte. For meso-IO DSSCs, the Rdiff value is 21.9 ohm for the iodine electrolyte and 34.9 ohm for the complex electrolyte. Thus, when the cobalt ions are contained in the electrolyte, the diffusion resistance is increased, and the resistance of the meso-IO electrode is lower than that of the random pore electrode. These results are well matched to the efficiency comparison.


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CONCLUSION Diffusion of electrolyte ions (i.e., diffusion of holes) in a photoelectrochemical device based on a heterojunction of an electrolyte/electrode is a crucial factor determining the performance of the device. In particular, with the recent use of high-conductive nanostructured electrodes and highviscosity electrolyte, the transport of electrolyte ions is more important. The diffusivity of the ion species was measured by the limiting current measurement and calculated by using a modified Fick’s law. We exhibited much improved ion diffusivity of meso-IO electrode compared to conventional random pore structure. More importantly, it was confirmed that the large the electrolyte ion, the more ordered and uniform pore structures are more advantageous to diffusion. In practice, we have analyzed the effect of ion diffusion on the photon-to-electric conversion efficiency by applying iodine- and cobalt-containing electrolytes to meso-IO and random pore electrode DSSCs. The result shows that the efficiency is reduced upon a low diffusivity ion is included, and that the efficiency reduction is low at uniform pore meso-IO electrodes. Our result suggests that with bulky electrolyte ions and a highly viscous polymer gel matrix, the use of uniform and regular pore electrode, such as the IO structure, should be considered for the best photoelectrochemical performance. This study can be extended to measure the ion diffusion coefficient in various types of photoelectrochemical devices, and can also be useful in applications such as microfluidics and bio-MEMS.


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ASSOCIATED CONTENT Supporting Information. The following data are available free of charge. The cross-sectional SEM of the TiO2 film including polymer gel (Figure S1), BET (Figure S2), HR-TEM (Figure S3) and XRD results of the TiO2 film. (Figure S4)

ACKNOWLEDGMENT This work was supported by grant from the National Research Foundation of Korea (NRF) (2011-0030253, 2016M3D3A1A01913254). The Korea Basic Science Institute is also acknowledged for the SEM measurements.


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REFERENCES (1) Oregan, B.; Gratzel, M., A Low-Cost, High-Effeiciency Solar-Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737-740. (2) Hamann, T. W.; Jensen, R. A.; Martinson, A. B. F.; Van Ryswyk, H.; Hupp, J. T., Advancing Beyond Current Generation Dye-Sensitized Solar Cells. Energ. Environ. Sci. 2008, 1, 66-78. (3) Bisquert, J.; Fabregat-Santiago, F.; Mora-Sero, I.; Garcia-Belmonte, G.; Barea, E. M.; Palomares, E., A Review of Recent Results on Electrochemical Determination of the Density of Electronic States of Nanostructured Metal-Oxide Semiconductors and Organic Hole Conductors. Inorg. Chim. Acta 2008, 361, 684-698. (4) Tiwana, P.; Docampo, P.; Johnston, M. B.; Snaith, H. J.; Herz, L. M., Electron Mobility and Injection Dynamics in Mesoporous ZnO, SnO2, and TiO2 Films Used in Dye-Sensitized Solar Cells. ACS Nano 2011, 5, 5158-5166. (5) Zhu, J.; Dai, S. Y.; Zhang, Y. H., Electron Transport and Recombination in Dye Sensitized Solar Cells. Prog Chem 2010, 22, 822-828. (6) Konenkamp, R., Carrier transport in nanoporous TiO2 films. Phys. Rev. B 2000, 61, 1105711064. (7) Tetreault, N.; Graetzel, M., Novel Nanostructures for Next Generation Dye-Sensitized Solar Cells. Energ. Environ. Sci. 2012, 5, 8506-8516. (8) Kawano, R.; Watanabe, M., Anomaly of charge transport of an iodide/tri-iodide redox couple in an ionic liquid and its importance in dye-sensitized solar cells. Chem. Commun. 2005, 21072109. (9) Snaith, H. J.; Schmidt-Mende, L., Advances in Liquid-Electrolyte and Solid-State DyeSensitized Solar Cells. Adv. Mater. 2007, 19, 3187-3200.


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(10) Zhang, K.; Chen, S.; Feng, Y.; Shan, Z.; Meng, S., Study of quasi-solid electrolyte in dyesensitized solar cells using surfactant as pore-forming materials in TiO2 photoelectrodes. J. Solid State Electrochem. 2016, 21, 715-724. (11) Lee, C. S.; Kim, J. K.; Park, J. T.; Kim, J. H., Well-organized mesoporous TiO2 film with high porosity made using alcohol-assisted EC-g-PMMA graft copolymer. Macromolecular Research 2016, 24, 573-576. (12) Kron, G.; Rau, U.; Dürr, M.; Miteva, T.; Nelles, G.; Yasuda, A.; Werner, J. H., Diffusion Limitations to I3-/I- Electrolyte Transport Through Nanoporous TiO2 Networks. Electrochem. Solid State Lett. 2003, 6, E11. (13) Durr, M.; Kron, G.; Rau, U.; Werner, J. H.; Yasuda, A.; Nelles, G., Diffusion-Limited Transport of I3- Through Nanoporous TiO2-Polymer Gel Networks. J. Chem. Phys. 2004, 121, 11374-11378. (14) Ma, Y.; Zhao, C. X.; Deng, L. L.; Yan, H.; Chen, S. S.; Xu, G., Transition of Pore-Size Dependence of Ion Diffusivity in Dye-Sensitized Solar Cells. Electrochim. Acta 2013, 102, 127132. (15) Trang Pham, T. T.; Koh, T. M.; Nonomura, K.; Lam, Y. M.; Mathews, N.; Mhaisalkar, S., Reducing Mass-Transport Limitations in Cobalt-Electrolyte-Based Dye-Sensitized Solar Cells by Photoanode Modification. ChemPhysChem 2014, 15, 1216-1221. (16) Thapa, R.; Park, N., First-Principles Identification of Iodine Exchange Mechanism in Iodide Ionic Liquid. J. Phys. Chem. Lett. 2012, 3, 3065-3069. (17) Hao, F.; Lin, H.; Zhang, J.; Li, J., Balance Between the Physical Diffusion and the Exchange Reaction on Binary Ionic Liquid Electrolyte for Dye-Sensitized Solar Cells. J. Power Sources 2011, 196, 1645-1650.


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(18) Yanagida, M., Charge Transport in Dye-Sensitized Solar Cell. Adv. Nat. Sci: Nanosci. Nanotechnol. 2014, 6, 015010. (19) Hwang, D.; Jo, S. M.; Kim, D. Y.; Armel, V.; MacFarlane, D. R.; Jang, S. Y., HighEfficiency, Solid-State, Dye-Sensitized Solar Cells Using Hierarchically Structured TiO2 Nanofibers. ACS Appl. Interf. Mater. 2011, 3, 1521-1527. (20) Snaith, H. J., Estimating the Maximum Attainable Efficiency in Dye-Sensitized Solar Cells. Adv. Func. Mater. 2010, 20, 13-19. (21) Zhang, Q.; Cao, G., Nanostructured photoelectrodes for dye-sensitized solar cells. Nano Today 2011, 6, 91-109. (22) Jennings, J. R.; Ghicov, A.; Peter, L. M.; Schmuki, P.; Walker, A. B., Dye-sensitized solar cells based on oriented TiO2 nanotube arrays: Transport, trapping, and transfer of electrons. J. Am. Chem. Soc. 2008, 130, 13364-13372. (23) Zhu, K.; Neale, N. R.; Miedaner, A.; Frank, A. J., Enhanced charge-collection efficiencies and light scattering in dye-sensitized solar cells using oriented TiO2 nanotubes arrays. Nano Lett. 2007, 7, 69-74. (24) Guldin, S.; Huttner, S.; Kolle, M.; Welland, M. E.; Muller-Buschbaum, P.; Friend, R. H.; Steiner, U.; Tetreault, N., Dye-Sensitized Solar Cell Based on a Three-Dimensional Photonic Crystal. Nano Lett. 2010, 10, 2303-2309. (25) Tetreault, N.; Arsenault, E.; Heiniger, L.-P.; Soheilnia, N.; Brillet, J.; Moehl, T.; Zakeeruddin, S.; Ozin, G. A.; Graetzel, M., High-Efficiency Dye-Sensitized Solar Cell with Three-Dimensional Photoanode. Nano Lett. 2011, 11, 4579-4584.


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(26) Tong, Z.; Hao, J.; Zhang, K.; Zhao, J.; Su, B.-L.; Li, Y., Improved electrochromic performance and lithium diffusion coefficient in three-dimensionally ordered macroporous V2O5 films. J. Mater. Chem. C 2014, 2, 3651-3658. (27) K. D. Benkstein; N. Kopidakis; J. van de Lagemaat; Frank, A. J., Influence of the Percolation Network Geometry on Electron Transport in Dye-Sensitized Titanium Dioxide Solar Cells. J. Phys. Chem. B 2003, 107, 7759-7769. (28) Lee, J. W.; Lee, J.; Kim, C.; Cho, C.-Y.; Moon, J. H., Facile fabrication of sub-100 nm mesoscale inverse opal films and their application in dye-sensitized solar cell electrodes. Sci. Rep. 2014, 4, 6804. (29) Lee, J. W.; Moon, J. H., Monolithic multiscale bilayer inverse opal electrodes for dyesensitized solar cell applications. Nanoscale 2015, 7, 5164-5168. (30) Lee, D. K.; Ahn, K.-S.; Thogiti, S.; Kim, J. H., Mass Transport Effect on The Photovoltaic Performance of Ruthenium-Based Quasi-Solid Dye Sensitized Solar Cells Using Cobalt Based Redox Couples. Dyes Pigm. 2015, 117, 83-91. (31) Andreas Stein; Benjamin E. Wilsona; Rudisilla, S. G., Design and Functionality of Colloidal-Crystal-Templated Materials-Chemical Applications of Inverse Opals. Chem. Soc. Rev. 2013, 42, 2763. (32) Cong, J.; Hao, Y.; Sun, L.; Kloo, L., Two Redox Couples are Better Than One: Improved Current and Fill Factor from Cobalt-Based Electrolytes in Dye-Sensitized Solar Cells. Adv. Energy Mater. 2014, 4, 1301273. (33) Rhee, S.-W.; Kwon, W., Key technological elements in dye-sensitized solar cells (DSC). Korean J. Chem. Eng. 2011, 28, 1481-1494.


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Figure 1. (up) Schematic diagram of the cell for diffusion measurement (down) The crosssectional SEM of (a) the meso-IO and (b) the conventional random mesopore TiO2 film. Inset shows the magnified SEM image of each TiO2 film.


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Figure 2. 3D-molecular model for (a) the I3- and (b) the [Co(bpy)3]3+ and the cyclic voltammetry measurements for the meso-IO and the conventional random mesoporous TiO2 film which were incorporated with (c) the iodine- and (d) the cobalt-based electrolytes, respectively.


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Figure 3. The limiting current at the various concentration of the I3- and the [Co(bpy)3]3+.


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Table 1. The parameters obtained from the results of the cyclic voltammetry.

Iodine-based electrolyte

Cobalt-based electrolytes

Avg. Db [cm2/s]

Avg. Dp [cm2/s]

Dp /Db

Avg. Db [cm2/s]

Avg. Dp [cm2/s]

Dp /Db

Meso-IO electrode

1.82*10-5

1.47*10-5

0.81

5.69*10-7

4.22*10-7

0.74

Random Mesoporous electrode

1.84*10-5

9.50*10-6

0.52

5.56*10-7

1.77*10-7

0.32


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Figure 4. Photocurrent density-voltage curves for the meso-IO and random pore electrode DSSCs with iodine-based and cobalt-containing electrolytes.


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Table 2. Photovoltaic parameters of the meso-IO and the random pore electrodes using iodinebased and cobalt-containing electrolytes.

Iodine-based electrolyte JSC [mA/cm2]

VOC [V]

FF

Iodine/Cobalt electrolyte η [%]

JSC [mA/cm2]

VOC [V]

FF

η [%]

MesoIO

16.2±0.1

0.77±0.10 0.65±0.20

8.1±0.1

16.3±0.1

0.76±0.10 0.63±0.10

7.8±0.1

Random pore

15.0±0.1

0.79±0.10 0.63±0.10

7.4±0.1

13.3±0.2

0.77±0.10 0.67±0.30

6.9±0.2


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Figure 5. (a) Nyquist plots of the meso-IO and the random pore electrode DSSCs using iodinebased and cobalt-containing electrolytes. (b) Equivalent circuit for fitting the plot.


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TOC Graphic


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Highly Improved Ion Diffusion through Mesoscopically Ordered Porous

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