Flexible Counter Electrodes Based on Mesoporous Carbon Aerogel

ARTICLE pubs.acs.org/JPCC

Flexible Counter Electrodes Based on Mesoporous Carbon Aerogel for High-Performance Dye-Sensitized Solar Cells Bin Zhao,*,†,‡ Hui Huang,† Peng Jiang,† Huifang Zhao,† Xianwei Huang,† Ping Shen,† Dingcai Wu,§ Ruowen Fu,§ and Songting Tan‡ †

College of Chemistry and Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Xiangtan 411105, People's Republic of China ‡ Key Laboratory of Advanced Functional Polymeric Materials of College of Hunan Province, Xiangtan University, People's Republic of China § Materials Science Institute, PCFM Laboratory, Sun Yat-sen University, 135 Xingang West Road, Guangzhou, Guangdong 510275, People's Republic of China ABSTRACT:

Flexible counter electrodes (CEs) are designed and fabricated by depositing the composite film of carbon materials (carbon aerogel, carbon black, and active carbon) and poly(tetrafluoroethylene) on stainless steel mesh and applied in dye-sensitized solar cells (DSSCs). The effect of pore structure of the carbon materials on the electrocatalytic activity was investigated by cyclic voltammogram and electrochemical impedance spectroscopic analysis. The carbon aerogel with a mesopore structure showed the highest double-layer capacitance and best electrocatalytic activity. The DSSC, based on the carbon aerogel CE and a ruthenium dye (N719), showed a high power conversion efficiency of 9.06%, which is 99% of that based on a Pt CE (9.14%) under the same experimental conditions.

1. INTRODUCTION The prototype of a dye-sensitized TiO2 nanocrystalline solar cell was reported by O’Regan and Gr€atzel in 1991.1 Dyesensitized solar cells (DSSCs) have attracted considerable attention during the past years due to their respectable power conversion efficiency, facile fabrication, and potential low cost. The highest solar power conversion efficiency of DSSCs has reached 11% nowadays.2,3 In general, a DSSC typically comprises a porous nanocrystalline TiO2 film covered by a monolayer of dye molecules, an electrolyte solution consisting of the iodide/ triiodide redox couple between the electrodes, and a counter electrode (CE).4,5 The counter electrode is an indispensable component in DSSCs, which catalyzes the reduction of triiodide ions to iodide ions after electron injection.6,7 Usually, a layer of platinum coated on a transparent conducting oxide (TCO) substrate is used as a CE in a DSSC. Thick platinum layers are necessary to obtain the desired catalytic effect, which is not an economical way for mass production.8 A low-cost substitute for platinum film is carbon materials due to their high electrical r 2011 American Chemical Society

conductivity, corrosion resistance toward I2, and high electrocatalytic activity for triiodide reduction. Several varieties of carbonaceous materials, such as graphite, carbon black, activated carbon, hard carbon spheres, and carbon nanotubes and fullerenes, have been employed as the catalytic materials on fluorinedoped tin oxide (FTO) or indium tin oxide (ITO) glass for CEs.8
16 Kay and Gr€atzel first presented a carbon CE consisting of graphite and carbon black to replace the platinum CE.8 Recently, Jung et al. applied aligned carbon nanotubes on TCO glass as the counter electrode in DSSCs and achieved a remarkable power conversion efficiency (η) of 10.04%.14 However, it is really hard to fabricate a low-cost DSSC because of the relatively expensive FTO and ITO substrates.17 In addition, the DSSC based on FTO and ITO conductive glass substrates would suffer the relatively high sheet resistance and a transport problem for Received: June 27, 2011 Revised: September 19, 2011 Published: September 26, 2011 22615

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The Journal of Physical Chemistry C the shape limitation and their fragile feature.18 Anyhow, future large-scale solar cells would prefer low-cost and flexible materials as conductive substrates. In the past few years, a stainless steel sheet, Ti metal sheet, and flexible graphite sheet had been used as conductive substrates and obtained a power conversion efficiencies of 5.2,18 4.0,19 and 6.46%,20 respectively. It is notable that stainless steel substrates have been widely studied because of the higher corrosion stability. Murakami and Gr€atzel first presented a carbon CE consisting of carbon and SUS-316 and achieved a power conversion efficiency (η) of 9.15%.21 Subsequently, Calogero et al. exploited novel carbon CEs by using a nonpurified SWNT as a catalyst and stainless steel as substrates, and the CEs showed comparable results to Pt film on a stainless steel substrate.22 Obviously, the CEs based on low-cost stainless steel substrates have the potential application for high-performance large-scale DSSCs. Because of low mass densities, continuous porosities, great mesopore volume, and high electrical conductivity,23
25 carbon aerogel can be a favorable catalyst candidate for counter electrodes of DSSC. Simultaneously, stainless steel mesh is suited for use as the conductive substrate of a counter electrode for its flexibility and corrosion resistant. In this study, novel counter electrodes were fabricated by employing carbon aerogel as the catalytic material on flexible stainless steel mesh for highperformance DSSCs. To evaluate and compare the photovoltaic performances of the different carbon CEs, carbon black and active carbon were used also as catalytic materials. The effect of pore structure of the carbon materials on the electrocatalytic activity was investigated by cyclic voltammogram and electrochemical impedance spectroscopic analysis in detail. Additionally, the relationship between the electrocatalytic activity of carbon counter electrodes and the effective specific surface area of carbon materials was also discussed.

2. EXPERIMENTAL SECTION 2.1. Preparation of CEs. Carbon aerogel was supplied by the PCFM laboratory,26 and active carbon and carbon black were purchased from the Huaxian Active Carbon Factory. Stainless steel mesh (the thickness of the mesh is 140 μm; the side length of the square aperture is 94 μm.) was ultrasonically cleaned in isopropanol for 15 min, rinsed with ethanol, and finally dried before use. Diversified carbon powders were mixed into a 60% water PTFE suspension (quality ratio of carbon powders to PTFE is 9:1), followed by adding ethanol and ultrasonically dispersing for 15 min; the obtained homogeneous mixture was spread on stainless steel mesh and dried in vacuum at 50 °C for 48 h. Finally, dried electrodes were compressed under a pressure of about 20 MPa to result in 50 μm thick films. Compared with the methods reported in the literature,8
14 the method used in this research is very straightforward and convenient for large area preparation. The Pt CEs are thin platinum sheets that are planished and polished. 2.2. Preparation of the Mesoporous TiO2 Layer. A nanoporous TiO2 electrode was prepared according to the following process. The FTO was cleaned and immersed in 40 mmol L
1 TiCl4 aqueous solution at 70 °C for 30 min, then washed with water and ethanol, and sintered at 450 °C for 30 min. The 20 nm TiO2 colloid was prepared according to the second method in the literature.27 The TiO2 colloid was diluted by adding magnesium acetate solution (2.7 mL of magnesium acetate solution per 9 g of P25 powders), which was prepared by dissolving 258.3 mg of

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anhydrous MgO in 100 mL of acetic acid and 150 mL of water. The dilute TiO2 colloid was homogeneously coated on the FTO glass by the doctor blade technique to obtain a TiO2 film of 10
12 μm in thickness. The 200 nm TiO2 colloid comprised 4.7% of 200 nm sized TiO2 and 9.5% ethyl cellulose in n-butanol. Subsequently, the TiO2 (200 nm) colloid was coated on the electrode by the same method, resulting in a TiO2 light-scattering layer of 4
6 μm in thickness. The double-layer TiO2 films were sintered at 450 °C for 30 min, then treated with TiCl4 aqueous solution and calcined at 450 °C for 30 min once again. After being cooled to room temperature, the nanoporous TiO2 electrode was obtained. 2.3. Assembling of DSSCs. For photosensitization, the calcined TiO2 electrodes were soaked in the di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,20 -bipyridyl-4,40 -dicarboxylato)ruthenium(II) (N719, Solaronix) dye solution with a concentration of 0.5 mmol L
1 in dry ethanol for 24 h. Subsequently, the dye-impregnated TiO2 electrode was rinsed with ethanol and dried at 50 °C. The counter electrode was clipped onto the top of the working electrode. The interelectrode space was filled with electrolyte that consists of 0.5 mol L
1 LiI, 0.05 mol L
1 I2, 0.6 mol L
1 1-methyl-3-hexylimidazolium iodide (MHII), and 0.5 mol L
1 4-tert-butylpyridine (TBP) in 3-methoxypropionitrile. A mask with a window of 0.196 cm2 was also clipped on the TiO2 side to define the active area of cells. All cells were produced under ambient conditions. 2.4. Measurement and Characterization. The morphology and microstructure of the counter electrodes were observed by using a scanning electron microscope (SEM) (JSM-6610LV, JEOL). The Brunauer
Emmett
Teller (BET) specific surface area (SBET), the pore volume, and pore size distribution were analyzed by Brunauer
Emmett
Teller (BET) theory and Barrett
Johner
Halendar (BJH) theory, respectively (Surface Area & Pore Size Analyzer NOVA 2200e, Quantachrome Corporation, Boynton Beach, FL). The photocurrent
voltage (J
V) characteristics were measured on a Keithley 2602 Source Meter. The photovoltaic performance of devices was recorded under 100 mW cm
2 simulated air mass (500 W xenon lamp, AM 1.5 G filter) solar light illumination. The light intensity of the illumination source was calibrated by using a standard silicon solar cell. The incident light intensity was checked by a power and energy meter (model FZ-A, Beijing). Electrochemical impedance spectroscopy (EIS) measurements of samples were performed with an electrochemical station (Zahner Z1.14) in an acetonitrile solution of 1 mmol L
1 I2, 10 mmol L
1 LiI, and 100 mmol L
1 LiClO4. The device was connected in a three-electrode system with carbon materials based on stainless steel mesh as the working electrode, a Pt foil as the CE, and a saturated calomel electrode as the reference electrode. The geometric area is 0.32 cm2. The distance between the working electrode and the counter electrode is 3.0 cm. EIS spectra were obtained by applying sinusoidal perturbations of 10 mV over the Voc at frequencies from 0.1 to 100 000 Hz in dark condition.

3. RESULTS AND DISCUSSION 3.1. Pore Structure of the Carbon Materials. The nitrogen adsorption
desorption isotherms of carbon black, active carbon, and carbon aerogel were measured and are shown in Figure 1. Active carbon is found to give a type-I nitrogen isotherm without any hysteresis loop, which indicates that the pore structure of active carbon is essentially microporous.28 Synchronously, the 22616

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Table 1. Pore Structure of the Carbon Materials and Specific Capacitance of Carbon Electrodes pore SBET

pore size

volume

Cm

(m2 g
1)

(nm)

(cm3 g
1)

(F/g)

carbon aerogel

489.5

18.1

0.859

64.52

active carbon

1829.0

1.4

0.638

3.91

carbon black

2.5

3.8

0.007

3.06

materials

Figure 1. N2 adsorption and desorption isotherms for carbon aerogel, active carbon, and carbon black.

Figure 3. SEM images of carbon counter electrodes of carbon black (a), active carbon (b), and carbon aerogel (c), and cross section of carbon aerogel counter electrode (d).

Figure 2. Pore size distribution of carbon black (a), active carbon (b), and carbon aerogel (c).

isotherm of the carbon black is similar to type-I with a hysteresis loop at high relative pressure,29 so the surface of the carbon black is composed of micropores and mesopores. Nevertheless, the isotherm of carbon aerogel exhibits a typical type-IV curve with an obvious hysteresis loop at high relative pressure, meaning that carbon aerogel is a type of mesoporous material.30 These results can be proved by the pore size distribution of the carbon materials calculated by BJH theory (see Figure 2). As shown in Figure 2, the pore volume of carbon black is obviously lower than those of active carbon and carbon aerogel. The pore size of active carbon is basically less than 2 nm. However, the pore size of carbon aerogel is mainly between 10 and 40 nm. The calculated results obtained by BET theory and BJH theory for the SBET,

Figure 4. Cyclic voltammograms of different electrodes with a scan rate of 16 mV s
1. The electrolyte is 0.5 mol L
1 LiI, 0.05 mol L
1 I2, 0.6 mol L
1 MHII, and 0.5 mol L
1 TBP in 3-methoxypropionitrile.

pore size, and pore volume of these carbon materials are listed in Table 1. Evidently, the SBET and pore volume of carbon black are 2.5 m2 g
1 and 0.007 cm3 g
1, respectively, which are less than those of carbon aerogel and active carbon by a long way. Additionally, the pore volume of carbon aerogel is 0.859 cm3 g
1, which is more than that of active carbon. 3.2. Morphology Characterization of the CEs. The morphology of carbon CEs was studied by SEM and are shown in Figure 3. It is obvious that the surface of the carbon black CE is smooth, and there is little or no pores on the carbon black particles, as shown in Figure 3a. Figure 3b shows the surface of 22617

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the active carbon CE. It is clear that there are a few holes between active carbon particles, but small pores can hardly be seen on the surface of active carbon because its pore size is mainly less than 2 nm. Carbon aerogel is composed of abundant particles with a size diameter of 20 nm in the Figure 3c. The surface of the carbon aerogel CE clearly exhibits a great deal of pore space, which should benefit the diffusion of I
and I3
in the carbon aerogel CE. Contrarily, a homologous structure is absent in the carbon black CE and the active carbon CE due to their microporous structures, as shown Figure 3a,b. Figure 3d gives the cross section of the carbon aerogel CE based on stainless steel mesh. It can be seen that poly(tetrafluoroethylene) (PTFE) is filiform as a binder in the CE, which can enhance the conductivity of the CE because carbon aerogel particles are inseparably contacted to each other. 3.3. Cyclic Voltammograms of the CEs. Figure 4 displays cyclic voltammograms of the I3
/I
redox couple on different carbon CEs with a scan rate of 16 mV s
1. There are two pairs of redox waves for the Pt CE. The more negative pair is assigned to redox reaction 1, and the more positive one is assigned to redox reaction 2.31 I3
þ 2e ¼ 3I


ð1Þ

3I2 þ 2e ¼ 2I3


ð2Þ

However, there is only one pair of redox waves for the carbon CEs assigned to redox reaction 1, which indicates there is not redox reaction 2 on the carbon CEs. All the peak-to-peak separations (ΔEp's) are more than 30 mV, so the reaction of the I3
/I
redox couple at the CEs/electrolyte interface is a quasi-reversible electrode process. It can be seen that the redox peak current of I3
/I
at the Pt electrode is obviously peakier than those at carbon CEs, indicating a quicker redox reaction at Pt CE. On the other hand, the reduction peak potentials of I3
/ I
at carbon CEs are more negative, which implies that the reduction of I3
at carbon CEs is more irreversible. Because the carbon aerogel CE shows a higher redox peak current of I3
/I
than other carbon CEs, it possesses the best electrocatalytic activity among all the carbon CEs. Compared to other CEs, the carbon aerogel CE shows a large and irregular background current, which comes from doublelayer capacitance and surface Faradaic reactions. The doublelayer capacitance is demonstrated by the following equation32 Cm ¼ ic =mv

ð3Þ

where Cm is the specific capacitance (F g
1), ic is the charging current (A), m is the mass of carbon material on the electrode, and v is the scan rate (V s
1). The specific capacitance of the carbon aerogel electrode (64.52 F g
1) is larger by far than those of the other carbon electrodes, which could be attributed to the mesoporous structure of carbon aerogel.33 Usually, the bond lengths of I3
are 0.28 and 0.31 nm,34 and the radii of I
and I3
are 0.22 and 0.28 nm,35 so the length of I3
is more than 1 nm. As a result, partial I3
is hard to diffuse into the micropore on the active carbon because the micropore is too tiny (Figure 5). At the same time, the double-layer capacitance could not form on the interface of the active carbon electrode and electrolyte because the adsorptive capacity of active carbon is so little. However, the mesoporous structure of carbon aerogel may be of great

Figure 5. Theoretical diffusion and adsorption of I3
on the carbon electrodes.

Figure 6. Electrochemical impedance spectra of DSSCs with different materials as the counter electrode measured at the frequency range of 0.1
100 000 Hz. The inset is the equivalent circuit of the devices.

Table 2. Photoelectric Performances of the Cells and the Charge-Transfer Resistance of the CEs materials platinum

a

Jsc (mA cm
2) Voc (V) 17.64 a

carbon aerogel

14.63

active carbon

(15.01)b 12.41

carbon black

11.32

0.74 0.84

η (%)

FF 0.70

a

0.74

a

9.14a

4.4

9.06 a

4.8

(0.84) b (0.75) b (9.43) b 0.75 0.72 6.74a 0.72

0.64

RCT (Ω cm2)

5.21a

5.7 7.1

The average value. b The maximum value.

advantage to diffusion and adsorption of I3
ions, so the double-layer capacitance can easily form on the interface of the carbon aerogel electrode and electrolyte. As we all know, the reaction process of the I3
/I
redox couple is controlled by the adsorption of I3
on CEs in the high I
concentration electrolyte.36 Therefore, it can be concluded that the reduction of the triiodide ion on the carbon aerogel CE is quicker than those on the other carbon CEs due to its effective adsorption, and the electrocatalytic activity of the active carbon CE decreases because the nonaccessible pores do not contribute to the catalytic reduction and the double-layer capacitance. 3.4. Electrochemistry Impedance Spectra of the CEs. To better understand the electrocatalytic activity of the CEs, the charge-transfer resistances (RCT) on the different electrodes were measured by EIS. Figure 6 shows Nyquist plots for the devices with carbon aerogel, active carbon, carbon black, and Pt CEs. The Ohmic serial resistance (RS) can be determined 22618

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Figure 7. Current density
voltage of DSSC curves with carbon aerogel, active carbon, carbon black, and platinum under one sun irradiation (AM 1.5, Pin = 100 mW cm
2). Active area of the device is 0.196 cm2.

according to the high frequency (around 100 000 Hz). In the middle-frequency range of 100
100 000 Hz, the impedance is dominated by the electrode/electrolyte interface, consisting of the charge-transfer resistance (RCT) and the double-layer capacitance (C). The impedance in the frequency range of 0.1
100 Hz should be ascribed to the Nernst diffusion impedance.12 The value of RCT can be obtained from Figure 6 and is listed in Table 2. The electrocatalytic activity of CEs for reducing the triiodide ion is calculated by the diameter of the semicircle illuminated in the spectra at the frequency range of 100
100 000 Hz. It is well accepted that RCT needs to be low in high-efficiency DSSCs.12 The Rct of the platinum sheet is lowest (4.4 Ω cm
2) because of its high electrocatalytic activity. However, the Rct of the carbon black CE (7.1 Ω cm
2) is maximal among these CEs. The result indicates that carbon black has the lowest electrocatalytic activity for its low surface area.21 Simultaneously, the Rct of the active carbon CE is 5.7 Ω cm
2, which is lower than that of the carbon black CE. It is well known that the electrocatalytic activity of the catalyst is strongly influenced by the roughness and the surface area of the catalyst because the high surface area facilitates the diffusion of the electrolyte and results in a lot of active sites for triiodide reduction.10,37 The surface area of active carbon is larger (SBET = 1829 m2 g
1) than that of carbon black (SBET = 2.5 m2 g
1) by a long way, so the electrocatalytic activity of the active carbon CE is higher than that of the carbon black CE. On the other hand, the Rct of the carbon aerogel CE is 4.8 Ω cm
2, which is lower than that of the active carbon CE, whereas the surface area of carbon aerogel is lower than that of active carbon by far. The unexpected result cannot be simply explained by the effect of the surface area of carbon materials. According to the aforementioned discussion, the mesoporous structure of carbon aerogel can facilitate the diffusion and adsorption of triiodide ions and results in more effective active sites for triiodide reduction, but the microporous structure of active carbon reduces the effective active sites because the nonaccessible pores do not contribute to the diffusion and adsorption of triiodide ions. Therefore, the carbon aerogel CE shows better electrocatalytic activity than the active carbon CE. 3.5. Photovoltaic Properties. The photocurrent
voltage (J
V) characteristics of DSSCs with carbon aerogel, carbon black, active carbon, and Pt CEs are shown in Figure 7, and the photovoltaic performances of these cells are listed in Table 2. As shown in Figure 7, the efficiency of the DSSC with the active

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Figure 8. Jsc versus radiant power for DSSCs based on different CEs.

carbon CE is 6.74%, which is higher than that of the DSSC with carbon black CE. According to the aforementioned discussion, the electrocatalytic activity of the active carbon CE is higher than that of the carbon black CE, so the DSSC with the active carbon CE shows a much higher short-circuit photocurrent (Jsc) and power conversion efficiency. Though the SBET of active carbon (1829.0 m2 g
1) is 3 times larger than that of carbon aerogel (489.5 m2 g
1), the DSSC with the carbon aerogel CE shows a higher Jsc and power conversion efficiency because the electrocatalytic activity of the carbon aerogel CE is higher than that with the active carbon CE. Simultaneously, the cell with the carbon aerogel CE shows a lower Jsc than the cell with the platinum CE under the same experimental conditions. It is readily understood that the RCT of the Pt CE is lower than that of the carbon aerogel CE, and the Pt foil also acts as a light reflector that can remarkably improve the Jsc. However, the open-circuit photovoltage (Voc) of the carbon aerogel CE based cell is 0.84 V, which is higher than that of the Pt CE (0.74 V). Additionally, the Voc values of the cells with active carbon and carbon black CEs are both close to 0.74 V. Few studies have focused on this unexpected phenomenon. As discussed before, the double-layer capacitor could form on the interface of the carbon aerogel electrode and electrolyte since the specific capacitance is larger by far than those of the other electrodes. When the DSSC with the carbon aerogel CE is irradiated, the capacitor would be charged by photocurrent and would become an energy storer. At the same time, extra charge at the capacitor would lead to a higher potential at the open-circuit condition. Therefore, the open-circuit photovoltage Voc of the cell with the carbon aerogel CE is higher than those with the other CEs. Another possible explanation is that the formal potential of the I
/I3
redox reaction on the carbon aerogel CE shifted more positively than that on the Pt CE.31 Therefore, the Voc value of the DSSC with the carbon aerogel CE is improved to 0.84 V. As a result, the cell with the carbon aerogel CE shows a maximum power conversion efficiency of 9.43%, which is superior to that of the cell (9.14%) with the Pt CE under the same experimental conditions. To better understand the effects of the diffusion and adsorption of I3
on electrocatalytic activity, the dependence of Jsc values and radiant power was investigated, as shown in Figure 8. It is found that the Jsc values of the DSSCs with Pt and carbon aerogel CEs show a nearly linear response to the radiant power, which means that the Jsc values are not limited by the diffusion and adsorption of I
/I3
on the surface of CEs.38 However, the 22619

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were sealed by EVA adhesive, and the results are shown in Figure 10. It can be seen that the η values of DSSCs are slightly degraded with the increase of light irradiation time, which would be attributed to slight evaporation of the liquid electrolyte solvent from our poor sealing technique. The decay rates of the efficiency of the DSSCs with the carbon aerogel CE and the Pt CE are similar by and large. The result suggests that the carbon aerogel CE based on stainless steel mesh could be a promising candidate to employ for DSSCs.

Figure 9. Relationships of the power conversion efficiencies and the active areas.

4. CONCLUSIONS In summary, a novel flexible counter electrode has been fabricated by spreading carbon aerogel on stainless steel mesh for a DSSC. The mesoporous structure of carbon aerogel may be of great advantage to the diffusion and adsorption of I3
ions, so the electrocatalytic activity of the carbon aerogel CE is higher than those of the other carbon CEs. Furthermore, the mesoporous structure facilitates the formation of double-layer capacitance on the interface of the carbon aerogel electrode and electrolyte, which resulted in the high capacitance and high open-circuit photovoltage of the DSSC with carbon aerogel CE. Therefore, a high-performance DSSC with a power conversion efficiency of 9.06% (Jsc = 14.64 mA 3 cm
2, Voc = 0.84 V, FF = 0.74) was achieved. Our results indicate that the flexible counter electrode based on the composite of carbon aerogel and stainless steel mesh could be a potentially promising alternative to the Pt CE applied for high-performance DSSCs. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Figure 10. Efficiency variation of the DSSCs with the carbon aerogel CE and the Pt CE.

Jsc values of the DSSCs with the active carbon CE and the carbon black CE show a slightly nonlinear response at the high radiant power, so the reduction of I3
on the surface of the two CEs is slightly limited by the diffusion and adsorption of I
/I3
for their micropore structure. Therefore, we believe that the large pore size and volume of carbon aerogel should be the key factor that allows the excellent cell performance, and the microporous structure of active carbon would reduce the effective surface area for electrochemical reduction. Furthermore, to clarify how the size of the cell influences the performances of the DSSCs, we compared the power conversion efficiencies of DSSCs with four different active areas of 0.196, 0.25, 0.785, and 1 cm2, all of which were fabricated with carbon aerogel and Pt CEs, respectively. Figure 9 shows the relationship between the active area of illumination and the power conversion efficiencies. As shown in Figure 9, the η values are decreased with the increase of the active area of cells. It is notable that the η values of the cells with the carbon aerogel CE are reduced from 9.43% to 5.37% with the increase of the active area from 0.196 to 1.0 cm2. However, the η values of the cells with the Pt CE exhibit a larger decrease from 9.14% to 4.16% at the same conditions. The results indicate that the carbon aerogel CE would be more likely to prepare large-sized DSSCs. 3.6. Stability of DSSCs. To investigate the stability of the carbon aerogel counter electrode, a stability test for the DSSCs with the carbon aerogel CE and the Pt CE was carried out, which

’ ACKNOWLEDGMENT This work was supported by the National Nature Science Foundation of China (Nos. 50973092, 51003089) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (Nos. 20094301120005, 20104301110003). ’ REFERENCES (1) O’Regan, B.; Gr€atzel, M. Nature 1991, 353, 737. (2) Gao, F.; Wang, Y.; Shi, D.; Zhang, J.; Wang, M. K.; Jing, X. Y.; Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; Gr€atzel, M. J. Am. Chem. Soc. 2008, 130, 10720. (3) Bessho, T.; Zakeeruddin, S. M.; Yeh, C. Y.; Diau, E. W. G.; Gr€atzel, M. Angew. Chem., Int. Ed. 2010, 49, 6646. (4) Papageorgiou, N; Maier, W. F.; Gr€atzel, M J. Electrochem. Soc. 1997, 144, 876. (5) Papageorgiou, N. Coord. Chem. Rev. 2004, 248, 1421. (6) Kalyanasundaram, K.; Gr€atzel, M. Coord. Chem. Rev. 1998, 177, 347. (7) Papageorgiou, N.; Kay, A. L.; Gr€atzel, M. J. Electrochem. Soc. 1999, 146, 898. (8) Kay, A.; Gr€atzel, M. Sol. Energy Mater. Sol. Cells 1996, 44, 99. (9) Lee, W. J.; Ramasamya, E.; Lee, D. Y.; Song, J. S. Sol. Energy Mater. Sol. Cells 2008, 92, 814. (10) Hong, W. J.; Xu, Y. X.; Lu, G. W.; Li, C.; Shi, G. Q. Electrochem. Commun. 2008, 10, 1555. (11) Li, K. X.; Luo, Y. H.; Yu, Z. X.; Deng, M. H.; Li, D. M.; Meng, Q. B. Electrochem. Commun. 2009, 11, 1346. 22620

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dx.doi.org/10.1021/jp206043a |J. Phys. Chem. C 2011, 115, 22615–22621