Inverse Opal Carbons for Counter Electrode of Dye-Sensitized Solar

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Inverse Opal Carbons for Counter Electrode of Dye-Sensitized Solar Cells Da-Young Kang, Youngshin Lee, Chang-Yeol Cho, and Jun Hyuk Moon* Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 121-742, South Korea S Supporting Information *

ABSTRACT: We investigated the fabrication of inverse opal carbon counter electrodes using a colloidal templating method for DSSCs. Specifically, bare inverse opal carbon, mesoporeincoporated inverse opal carbon, and graphitized inverse opal carbon were synthesized and stably dispersed in ethanol solution for spray coating on a FTO substrate. The thickness of the electrode was controlled by the number of coatings, and the average relative thickness was evaluated by measuring the transmittance spectrum. The effect of the counter electrode thickness on the photovoltaic performance of the DSSCs was investigated and analyzed by interfacial charge transfer resistance (RCT) under EIS measurement. The effect of the surface area and conductivity of the inverse opal was also investigated by considering the increase in surface area due to the mesopore in the inverse opal carbon and conductivity by graphitization of the carbon matrix. The results showed that the FF and thereby the efficiency of DSSCs were increased as the electrode thickness increased. Consequently, the larger FF and thereby the greater efficiency of the DSSCs were achieved for mIOC and gIOC compared to IOC, which was attributed to the lower RCT. Finally, compared to a conventional Pt counter electrode, the inverse opal-based carbon showed a comparable efficiency upon application to DSSCs.

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reduction of I3−.6,7 However, Pt is expensive and, moreover, the catalytic property is deactivated by the redox couple, creating a problem in the long-term stability of DSSCs. Recently, as an alternative, carbon-based materials have been widely studied due to their cost advantage and facile control of electrical properties. Various carbons such as active carbon,8 graphite,9 carbon black,10 carbon nanofibers,11 and carbon nanotubes12 have showed a comparable performance to Pt. Carbon nanofiber film of ca. 24 μm thickness has been applied as a counter electrode and achieved a photon-to-electric conversion efficiency of DSSCs of 79% of that one of the Ptcoated electrode.11 In the case of carbon nanotube electrode, the efficiency of 85% of the Pt-coated electrode was obtained with a few micrometers thickness of film.13 In most of the cases, the lower efficiency compared to the Pt was attributed to the lower fill factor upon applying carbon as the counter electrode. Since the fill factor is enhanced by lowering the resistance of components in DSSCs and the interfacial resistances, it is important to have a higher conductivity through the electrode and a higher reaction rate at the electrode interface regarding the counter electrode.14 In this study, for the first time, we employed inversed opal carbons for the counter electrode of DSSCs. Compared to the aforementioned carbon materials, highly interconnected

INTRODUCTION Dye-sensitized solar cells (DSSCs) have been studied as promising photovoltaic devices due to their simple and low-cost fabrication. Conventional DSSCs consist of nanocrystalline semiconductor oxide particulate substrates as the photoanode, platinized conducting substrate as the counter electrode, and inbetween a liquid electrolyte containing iodide (I−)/tri-iodide (I3−) redox couples.1 Under operation of DSSCs, the dye molecules capture photons of the incident light, and subsequently the electrons from the dye molecules in excited states are injected rapidly into the conduction band of the nanocrystalline semiconductor oxide particulate electrode. The electrons are extracted and transferred to the counter electrode through the external circuit. Meanwhile, the holes are captured by iodide ions and shuttled to the counter electrode in the form of the oxidized iodide, i.e., tri-iodide ions. Finally, at the counter electrode, recombination of electron and hole occurs, resulting in the regeneration of tri-iodide to iodide ions.2−4 Typically, at the counter electrode, two types of reaction occur: I3− + 2e− = 3I− and 3I2 + 2e− = 2I3−. The reaction rate of the former has been reported to be the determining factor in the photovoltaic performance of DSSCs. Thus, in order to facilitate this reaction, the counter electrode should have good electrocatalytic properties for the reduction and a high surface area and sufficient electrical conductivity for efficient transfer of the electron to the redox couples.5 Platinum (Pt)-coated counter electrodes have been frequently used in DSSC because of their superior efficient electrochemical catalyst for the © 2012 American Chemical Society

Received: February 14, 2012 Revised: March 31, 2012 Published: April 4, 2012 7033

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particulate inverse opal powders with a particle size below 45 μm in diameter. The inverse opal particles were dispersed in ethanol under mild ultrasonication. The resulting homogeneous solution of inverse opal particles was sprayed on a FTO glass substrate with an airbrush with 30 psi of air pressure. Assembly of Dye-Sensitized Solar Cells. Conventional nanocrystalline TiO2 (Solaronix) was deposited onto a FTO substrate and applied as a photoanode. For the sensitization of the TiO2 elctrode, the TiO2 electrode was immersed in 0.5 mM N719 ethanol solution for 24 h. Then the counter electrodes were assembled with photoanode, and the gap was controlled by 100 μm polyimide tape. The liquid electrolyte 0.5 M LiI (Aldrich), 0.05 M I2 (Yakuri), and 0.5 M 4-tert-butylpyridine (Aldrich) in a solution of 10 mL of acetonitrile (Aldrich) was injected into the cells. Characterization. The surface morphologies were measured by SEM (Hitachi) and TEM (Hitachi). The graphitic phases were analyzed by X-ray diffraction (XRD) using Cu Kα radiation. The transmittance and the diffuse reflectance of the counter electrodes were determined from a UV−vis spectrophotometer (JASCO V550). The photocurrent and voltage of the DSSCs were measured using a Source Meter (Keithley Instruments) under simulated solar light, produced by a 150 W Xe lamp (Oriel) and AM 1.5G filters. The intensity was adjusted using a Si reference cell (BS-520, Bunko-Keiki) to a power density of 100 mW/cm2. The impedance was measured using an impedance analyzer (Versastat, AMETEK). The frequency range explored in the impedance was 105−0.1 Hz.

structures in three dimensions and well-defined pores of inverse opal carbons might provide fast electron transport as well as facile diffusion of electrolyte ions.15,16 Moreover, highly porous structures might be advantageous to apply solid or quasi-solid electrolytes, which possess a high viscosity and contain large polymeric molecules or particles. Here, we prepared a stable dispersion of inverse opal carbon to apply a spray coating; the thickness of IO carbon electrode was easily controlled by the number of coating. Specifically, a spray coating allowed a highthroughput and the large-scale fabrication of inverse opal-based electrodes. Moreover, we have fabricated three types of inverse opal-based carbon electrode: inverse opal carbon (IOC), inverse opal carbon with mesopores (mIOC), and graphitized inverse opal carbon (gIOC). mIOC was compared with IOC to evaluate the existence of mesopores, thereby enlarging the specific area of the inverse opal-based electrode on the photovoltaic performance upon application to a counter electrode for DSSCs. gIOC was compared with IOC to evaluate the increase in conductivity of the inverse opal-based carbon electrode on the photovoltaic performance. It was mainly the FF among photovoltaic parameters that affects the efficiency. Thus, electrochemical impedance of inverse opal coated electrodes was investigated to the charge-transfer resistance at the counter electrode/electrolyte interface and thereby the relative magnitude of FF. Briefly, the higher FF, and thereby higher efficiency was obtained for gIOC, which was attributed to the higher conductivity of gIOC film and lower interfacial resistance with electrolyte. The mIOC film showed higher efficiency due to higher specific area and ordered mesopores. Moroever, among these samples, the maximum efficiency of 5.5% was achieved for the mIOC electrode, which was comparable to the DSSCs with Pt-coated counter electrode; the efficiency of that with the Pt electrode was 95%.

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RESULTS AND DISCUSSION As shown in Scheme 1, three types of sample were prepared based on inverse opal carbons. First, inverse opal (IO) carbons Scheme 1. Formation of (a) Inverse Opal Carbons (IOC), (b) Inverse Opal Carbons with Mesopores (mIOC), and (c) Graphitized Inverse Opal Carbons (gIOC)

EXPERIMENTAL SECTION

Fabrication of IOC. The inversed opal carbon (IOC) was prepared by impregnation of resol solution into a PS colloidal crystal (or opal) template and the subsequent removal of the PS template and carbonization at a high temperature. Monodispersed polystyrene (PS) colloidal particles of 400 nm diameter were obtained by emulsion polymerization. Resol was prepared from phloroglucinol (C6H3(OH)3) and formaldehyde in a base-catalyzed process. In a typical process, 0.01 mol of phloroglucinol was dissolved in an ethanol−water mixture solution (50:50). The solution was mixed with a small amount of 20 wt % sodium hydroxide (NaOH) aqueous solution for 2 h. Then, 0.17 mol of 37 wt % formaldehyde was added to the solution dropwise, resulting in phloroglucinol/formaldehyde (PF) resol. The resol was poured onto the opal structure of dried PS particles. The solvent was removed by vacuum evaporation for 3 h. The resol that had infiltrated into the PS opals was cured at 100 °C for 12 h. Pyrolysis-induced carbonization and simultaneous removal of the PS opal templates were performed in a tubular furnace in a nitrogen atmosphere at 900 °C for 3 h, resulting in IOC. Fabrication of mIOC. In order to create IOC with mesopores (mIOC), the PF resol was mixed by block copolymers and used as a carbon precursor. Here, Pluronic F127 (EO106PO70EO106) was used and added to the PF resol. Fabrication of gIOC. The graphitic inversed opal carbon (gIOC) was prepared by catalytic graphitization of IOC by heat treatment at low temperature.17 1 g of IOC was soaked in 3 mM of a nickel nitrate−ethanol solution and then dried under vacuum. The nickeltreated IOC was heat-treated at 1000 °C under a nitrogen atmosphere for 3 h. Finally, the obtained products were washed with 10 wt % HCl solution to remove the nickel moieties on the surface. Formation of Inverse Opal-Based Counter Electrode. The inverse opal carbon samples were milled and sieved to obtain

with ca. 300 nm diameter pores were synthesized using PS colloidal templating and PF resol as the carbon precursors. Second, mesoporous inversed opal structured (mIO) carbons are fabricated by adding block copolymers as a template to introduce mesopores (pluronic F127, Aldrich) in PF resol. Third, graphitic inverse opal (gIO) carbons were obtained by catalytic graphitization with metal catalyst (nickel). Figures 1a and 1b show SEM images of the PS colloidal crystal template and their inverted carbon structure. Uniform PS colloids of 400 nm in diameter were applied to prepare the template. IOC synthesized from the PS template has regular arrays of interconnected macropores of 270 nm in diameter. During the heat-treatment for template removal and carbonization, linear shrinkage by 48% occurred. Figure 1c shows the SEM image of mIOC, and the inset TEM image clearly shows mesopores of around 5 nm in diameter. The gIOC in Figure 1d shows similar porous structures with IOC and mIOC. The XRD measurement for the gIOC in Figure 2 shows a sharp 7034

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Figure 3. SEM image of spray-coated carbon samples as a counter electrode of DSSCs. Inset image shows the inverse opal carbon powder dispersed ethanol solution.

Figure 1. SEM images of (a) PS colloidal crystals, (b) inverse opal carbons, (c) inverse opal carbons with mesopores, and (d) graphitized inverse opal carbons. Insets show the TEM image of the mesoporous structure in a carbon matrix.

Table 1. Photovoltaic Parameters of DSSCs Containing Designated Inverse Opal-Based Carbon Counter Electrode material

JSC (mA/cm2)

VOC (V)

FF

η (%)

IOC-1 IOC-2 IOC-3 gIOC mIOC Pt

10.30 10.64 11.51 11.18 11.04 12.38

0.76 0.77 0.76 0.76 0.77 0.72

0.358 0.497 0.592 0.612 0.643 0.650

2.808 4.087 5.184 5.206 5.493 5.808

graphitic phase of carbons or conductivity of carbon electrodes on the photovoltaic properties. First, in the case of the counter electrode thickness effect, the IO-based carbon electrodes were prepared with various thicknesses, which are controlled by the number of spray coatings; the maximum film thickness of 15, 18, and 25 μm was obtained by the number of coating by 1, 5, and 10 times, respectively. Specifically, it should be noted that as increasing the number of coating, both the surface density of IO carbons on the substrate and the thickness were increased. Thus, in order to consider the surface density as well as the thickness of IO carbon, the optical transmittance of the sample was used to estimate the relative thickness of the electrode. Figure 4 shows the transmittance spectrum in the visible light of the samples with different numbers of coating by 1, 5, and 10 times. By increasing the coating, the optical transmittance was decreased. Specifically, the carbon electrode of the spraying of IOC by once (IOC-1), 5 times (IOC-2), and 10 times (IOC-3) resulted in 25%, 10%, and 5% transmittance at a wavelength of 600 nm. Figure 5a shows the IV characteristics of DSSCs with these carbon counter electrodes. JSC, VOC, FF, and the efficiency extracted from the IV measurement are also listed in Table 1. By increasing the thickness of the counter electrodes, FF was improved, but JSC and VOC were almost unchanged. No substantial effect on JSC and VOC was attributed to the fact that these parameters were mainly affected by the property of photoanode (TiO2 electrode); JSC was mainly affected by the light absorption and transport properties of the photoanode, and VOC of DSSCs was determined by the energy level offset of the photoanode and redox potential of the electrolytes.18 It can be noticed that, compared to IOC-1, FF increased by 39% for IOC-2 and 65% for IOC-3.

Figure 2. Powder X-ray diffraction patterns of inverse opal carbons and low-temperature graphitized inverse opal carbons.

peak at around 2theta = 26°, 43° which is assigned to (002), (100) diffraction of graphitic carbon. In contrast, in the case of IOC, the XRD pattern has a broad diffraction peak at 22°−25°, which means that the IO carbon has a disordered state.17 Here, in order to apply inverse opal-based carbons as counter electrodes for DSSCs, inverse opal powder was obtained and then dispersed in ethanol solution to apply spray coating (inset Figure of 3). A representative SEM image of the surface IOC in Figure 3 shows that the IOC powders, ranging from a few to tens of few micrometers, were coated onto the FTO-glass substrate. DSSCs were assembled using conventional nanoparticulate TiO2 photoelectrodes with a thickness of 10 μm (TSP, Solaronix), iodide/tri-iodide-based electrolytes (N719, Dyesol), and the inverse opal-based carbon counter electrode. Table 1 shows the photovoltaic parameters such as short-circuit photocurrent (JSC), open-circuit voltage (VOC), fill factor (FF), and the power conversion efficiency (η) calculated under AM 1.5 G and 1 sun illumination. In this study, we analyzed the effect of the thickness of IOC counter electrodes on the photovoltaic properties and compared IOC to mIOC to investigate the effect of surface area on the photovoltaic properties and IOC to gIOC to investigate the effect of the 7035

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spectra as shown in Figure 6, the highest frequency regime (>100 Hz) (around 100 kHz) represents an electrochemical

Figure 4. Transmittance of spray-coated inverse opal-based carbon by different spraying conditions (spraying once: (a) IOC-1; spraying 5 times: (b) IOC-2; spraying 10 times: (c) IOC-3, gIOC, mIOC). Inset images are camera pictures of IOC and IOC-3.

Figure 6. Nyquist plots of symmetric cells (a) coated with IOC of different thickness and (b) coated with various inverse opal-based carbons and Pt. Inset shows the equivalent circuit. RS: series resistance; RCT: charge-transfer resistance; CDL: double layer capacitance; ZD: Nernst diffusion.

charge transfer resistance (RCT) at the counter electrode/ electrolyte interface. By considering the equivalent circuit, the RCT of IOC-1, IOC-2, and IOC-3 was estimated to be 14.18, 6.22, and 2.46 Ω cm2, respectively.10,20 Lower charge transfer resistance at the interface of the electrolyte/electrode surface facilitates electron transport and electrochemical reaction. Thus, this result implies that by increasing the thickness of the IOC counter electrode, it provides enough surface area to regenerate redox couples, resulting in a decrease in RCT. Since series resistance lowers the FF of solar cells, the decrease in RCT with the thickness of IOC was attributed to the increase of FF. The conversion efficiency of IOC-3 was improved to over 80% that of the IOC-1. Second, the effect of mesopores on the photovoltaic performance was investigated by comparing IOC to mIOC. Here, the transmittance spectrum of mIOC with the same number of coatings showed an almost equal transmittance over the entire range as the case of IOC-3 (see Figure 4). As listed in Table 1, the result showed that the mIOC showed a 5.96% higher efficiency, and this was attributed to the higher FF than that of IOC. EIS analysis showed that the RCT of mIOC and IOC-3 was estimated to be 2.11 and 6.22 Ω cm2, respectively. Thus, as aforementioned, the lower RCT was attributed to the higher FF. Again, the lower RCT of mIOC was explained by the obviously large surface area of mesopore-incoporated IOC

Figure 5. Photovoltaic performance of DSSCs using (a) IOC with different thickness and (b) IOC, mIOC, gIOC, and Pt counter electrode with the same number of coatings (10 times).

In order to characterize the dependence of FF on the thickness, we evaluated these samples by using electrochemical impedance spectroscopy (EIS), which provide a useful information on the resistance of various material interfaces existing in DSSCs, such as the FTO/TiO2 electrode, TiO2/ dye/electrolyte, or electrolyte/counter electrode. Here, we measured the EIS for symmetrical cells of two identical counter electrodes, which can exclude interfaces in DSSCs other than the counter electrode interface.19 Specifically, in the impedance 7036

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both cases was attributed to the decrease in FF of the DSSCs. Finally, mIOC showed the highest efficiency of DSSCs of 5.49%. Compared to the conventional Pt electrode, the efficiency of DSSCs with mIOC was comparable to the Ptcoated one (95% of the DSSCs with the Pt electrode). We believe the inverse opal carbon can be one of the alternative counter electrodes for DSSCs, and the spray coating of inverse opal carbon also supports high-throughput processing.

compared to the bare IOC. It has also been reported that wellordered mesopores form a highway network for more efficient transport of the reactants and products.21,22 Third, the effect of the graphitic properties of carbon on photovoltaic performance was investigated by comparing IOC to gIOC. The transmittance of gIOC formed by the same number of coatings was similar to that of IOC-3. As listed in Table 1 and shown in Figure 5b, the photovoltaic parameters showed that FF was further increased by 3.8% after graphitization. In previous reports, catalytic low-temperature graphitization induces the increase of conductivity by up to 20 times.17 The resistance of the counter electrode film was measured. The result showed that the resistance of the gIOC film was ca. 1 kΩ, which was 10 times lower than the IOC, ca. 10 kΩ. Meanwhile, in the EIS analysis shown in Figure 6b, the RCT of gIOC and IOC-3 was estimated to be 2.48 and 6.22 Ω cm2, respectively. Thus, the higher FF of gIOC can be explained by the fact that the higher conductivity of gIOC electrode facilitates the transport of electrons to the electrode− electrolyte interface, resulting in an enhancement of the electrocatalytic reaction or a lowering of RCT. Finally, we compared the efficiency of inverse opal-based electrodes (specifically, mIOC) to conventional Pt-coated electrodes as shown in Table 1. Briefly, an ethanol solution of 0.7 mM H2PtCl6 was spin-coated onto a FTO substrate and calcined at 450 °C for 30 min to obtain a Pt-coated counter electrode. The efficiency and FF of DSSCs with the Pt counter electrode was 5.8% and 0.650, respectively, as shown in Table 1. Previous results using 10 μm thickness TiO2 electrodes showed the efficiency of 6−7% and the FF of 0.65−0.70,23−25 which was similar to our results. Here, the mIOC showed comparable photovoltaic parameters, resulting in a similar efficiency of DSSCs; the efficiency of DSSCs with mIOC as a counter electrode was 95%. In a previous report that applied carbon nanotube counter electrodes, the conversion efficiency was 85% of the Pt electrode.13 Meanwhile, the JSC of DSSCs with a Pt electrode was 12% higher than that of the mIOC. This might be induced by the higher reflectivity of the Pt-coated electrode than the mIOC electrode, resulting in a higher absorption efficiency of light in the Pt electrode (see Figure S1).

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ASSOCIATED CONTENT

S Supporting Information *

Diffuse reflectance of Pt and mIOC coated counter electrode. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from the National Research Foundation of Korea (2010-0028961, 2011-0030253). The Korea Basic Science Institute is also acknowledged for the SEM measurements.

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CONCLUSION Inverse opal-based carbon was applied as a counter electrode for DSSCs. Bare IOC, mesopore-incorporated mIOC, and graphitized gIOC were prepared using a templating method. Here, the effect of IOC electrode thickness was controlled by the number of spray coatings; the relative thickness of the electrode was estimated by obtaining the transmittance of the electrode film. By increasing the thickness, it was mainly the FF of the DSSCs that was enhanced, resulting in an increase in the photon-to-electron conversion efficiency. The interfacial charge transfer resistance (RCT) at the electrode−electrolyte measured by EIS measurement showed that a lower RCT was observed with a thinner electrode. The lower RCT was attributed to the increase in FF by increasing the electrode thickness. Moreover, we characterized the effect incorporation of mesopores in carbon and the conductivity of the inverse opal carbon and the coated electrode by comparing IOC to mIOC and gIOC, respectively. The incorporation of mesopores increased the surface area of the counter electrode and thereby lowered RCT. The increase in conductivity of the IOC and also the coated film resulted in the decrease of RCT. Thus, the lowered RCT in 7037

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(21) Wang, Z. Y.; Li, F.; Ergang, N. S.; Stein, A. Chem. Mater. 2006, 18, 5543. (22) Ko, J.; Fang, B. Z.; Fan, S. Q.; Kim, J. H.; Kim, M. S.; Kim, M.; Chaudhari, N. K.; Yu, J. S. Langmuir 2010, 26, 11238. (23) Zhu, G.; Pan, L. K.; Lu, T.; Liu, X. J.; Lv, T.; Xu, T.; Sun, Z. Electrochim. Acta 2011, 56, 10288. (24) Wei, M. D.; Konishi, Y.; Zhou, H. S.; Yanagida, M.; Sugihara, H.; Arakawa, H. J. Mater. Chem. 2006, 16, 1287. (25) Hsiao, P. T.; Tung, Y. L.; Teng, H. S. J. Phys. Chem. C 2010, 114, 6762.

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