- Email: [email protected]
A flexible carbon counter electrode for dye-sensitized solar cells
CARBON
4 7 ( 2 0 0 9 ) 2 7 0 4 –2 7 0 8
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
A flexible carbon counter electrode for dye-sensitized solar cells Jikun Chen, Kexin Li, Yanhong Luo, Xiaozhi Guo, Dongmei Li, Minghui Deng, Shuqing Huang, Qingbo Meng* Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A pure carbon counter electrode (CE) for dye-sensitized solar cells (DSCs), has been fabri-
Received 11 March 2009
cated using an industrial flexible graphite sheet as substrate and activated carbon as the
Accepted 22 May 2009
catalytic material. The CE shows very low series resistance (Rs) and charge-transfer resis-
Available online 31 May 2009
tance (Rct) by combining the high conductivity of the flexible graphite with the high catalytic property of activated carbon. The Rs and Rct for the CE are respectively only a quarter and two-thirds of those for a platinized fluorine-doped tin oxide glass (Pt/FTO). DSCs with cell areas of 0.15 and 1 cm2 fabricated with this CE show higher solar-to-electricity conversion efficiencies. The respective values are 6.46% and 5%, compared with 6.37% and 2.91% for the Pt/FTO based devices. 2009 Elsevier Ltd. All rights reserved.
1.
Introduction
The dye-sensitized solar cell (DSC) has received widespread attention due to its low cost, easy production and relatively high efficiency to convert solar energy into electricity [1–5]. Typically, a DSC comprises a dye-sensitized nanocrystalline titanium dioxide (TiO2) electrode, electrolyte solution usually with a dissolved iodide/triiodide redox couple between the electrodes, and a counter electrode (CE). The function of the CE is to transfer electrons arriving from the external circuit back to the redox electrolyte and to catalyze the reduction of the triiodide ion [6,7]. Usually, Pt is used as the catalytic material and fluorine-doped tin oxide (FTO) glass as the substrate for a CE [1–7]. Although Pt exhibits excellent catalytic activity for triiodide reduction and good electric conductivity, it is extremely expensive and has the problem of reserves for large scale application [8,9]. Meanwhile, the relatively high sheet resistance of FTO glass imposes performance limitation, especially in large area devices [10,11]. FTO glass also has the problem of high price. It is estimated that the cost
of conductive glass sheets is about 30% of the total material cost of the DSC [12]. In addition, the shape limitation and fragile feature will bring transport problem for the FTO glass based DSCs [13]. Future large solar electric conversion systems will prefer materials abundantly available and easily handled. Therefore, it is necessary to develop cheap materials for CEs which also exhibit high electrical conductivity, good chemical stability and good catalytic activity to the reduction of triiodide ion. So far, inexpensive carbonaceous materials such as graphite, carbon black, activated carbon, hard carbon sphere, carbon nanotube, fullerene and graphene, have been employed as the catalytic materials on FTO or ITO (indium tin oxide) glass for the CEs [8,9,14–20]. Roughness factor and conductivity have been considered as two important factors influencing the performance of the CEs [9,14,15]. Activated carbon with large surface area shows superior catalytic activity toward the triiodide reduction [14]. Meanwhile, novel substrates such as metals or plastic foils (using Pt as catalytic material) have been used to fabricate flexible CEs to achieve the requirement
* Corresponding author: Fax: +86 10 8264 9242. E-mail address: [email protected] (Q. Meng). 0008-6223/$ - see front matter 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.05.028
CARBON
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for portable electricity and high-throughput industrial roll-toroll production [13,21]. However, the efficiency of the DSCs with these substrates is low and the improvements of their long time stability are also imperative for future large scale production. Industrial flexible graphite, which is used mainly for gaskets, has very good electrical conductivity, high chemical stability, high temperature resistance, good flexibility and low coefficient of thermal expansion, which make it a good candidate to fabricate electrode. In the past, it has been found that flexible graphite can offer better electron transfer rate and lower capacitance, and has been used as electrode for the electrochemical system, such as fuel cells, lead-acid cells and sensors [22–25]. Here, we present the feasibility of flexible graphite for the CE in DSCs by developing a pure carbon CE. In this CE a flexible graphite sheet is used as the substrate and activated carbon is chosen to be the catalytic material. Both substrate and catalytic material are inexpensive carbonaceous materials, which give a good adhesion between the catalytic material and the substrate. The DSCs with this pure carbon CE show better performance than thermal deposited Pt/FTO glass based devices, especially for DSCs with larger area. Furthermore, the density of the flexible graphite is very low. The mass per unit area of the flexible graphite is only 6% of that for the FTO glass with thickness of 2 mm. This will benefit for lowering the transportation cost for large scale production.
2.
Experimental
2.1.
Preparation of counter electrodes
To fabricate the pure carbon CE, flexible graphite sheets with thickness of 0.2 mm and area density of 0.03 g/cm2 were cleaned with distilled water, ethanol and then air-dried. The activated carbon paste was made by mixing 0.4 g activated carbon (the particle size is 1–10 lm and Brunauer–Emmett– Teller (BET) surface area 1958 m2 g1), 0.1 g carbon black (particle size is about 30 nm and BET surface area 77 m2 g1), 4 g terpineol and 0.1 g SnO2 nanoparticle (the particle size is 10 nm) together and ball-milling for 5 h. Then the activated carbon paste was coated onto flexible graphite sheet for preparing pure carbon CE or onto FTO glass (TEC-15, LOF) for carbon/FTO electrode by doctor-blading. The films were dried and sintered at given temperature for 60 min. For comparison, the Pt/FTO electrode was fabricated by thermal decomposition of H2PtCl6 (30 mM in isopropanol) on FTO glass at 385 C for 30 min [26]. The Pt loading was controlled to be 5 lg/cm2.
2.2.
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electrolyte consist of 0.6 M methylhexylimidazolium iodide, 0.05 M iodine, 0.1 M LiI and 0.5 M tert-butylpyridine in 3methoxypropionitrile. An open sandwich-type cell was fabricated in air by clamping the sensitized TiO2, a drop of electrolyte and a CE with two clips. A mask was also clipped on the TiO2 side to define the active area of the cell.
2.3.
Characterization and measurements
The scanning electron microscopy (SEM) measurements of the pure carbon CE were conducted on a LEO-1530 microscope (Germany). The sheet resistance of the flexible graphite and FTO glass was measured by using a four-point resistivity measurement system (RST-9, China). The photocurrent–voltage measurements were recorded by a potentiostat (Model 263A, Princeton Applied Research). A solar light simulator (91192, Oriel) was used to mimic one sun air mass 1.5 (AM 1.5) illumination on the surface of the solar cells. The intensity of incident light was measured with a radiant power/energy meter (70260, Oriel) before each experiment. The electrochemical impedance spectroscopy (EIS) measurements of the cells were performed with an electrochemical station (IM6ex, Zahner) with the frequency range being 100 mHz–100 kHz. The magnitude of the alternative signal was 10 mV.
3.
Results and discussion
Fig. 1 shows the SEM cross-section image of the pure carbon CE. From the SEM image, we can see that the large activated carbon particles are well enclosed by the small carbon black particles and the thick porous carbon film adheres well with the flexible graphite substrate. The measured average thickness of the carbon film is about 20 lm. The activated carbon particles with large surface area can offer more active sites for triiodide reduction and the carbon black can improve the conductivity of the film by filling the large pores between activated carbon particles [8,14]. Previous analyses of EIS and the equivalent circuit of DSCs have suggested that resistive effects in solar cells reduce the efficiency of the cells by dissipating power in internal resistance [27–30]. The key impact of internal resistance is to reduce fill factor (FF). The internal resistances of the DSCs are mainly related to the sheet resistance of the substrates, the charge-transfer processes occurring at the CEs, the electron
DSC fabrication
The nanocrystalline TiO2 films were fabricated on the FTO glass by screen-printing a TiO2 (Degussa P25) paste [2]. The thickness of the TiO2 film was about 15 lm. The electrode was heated to 450 C and sintered for 30 min, then cooled down to 80 C and immersed into 0.3 mM N3 (cis-dithiocyanato-N,N-bis(2,2-bipyridyl-4,4-dicarboxylic acid) ruthenium(II) dihydrate) (Dyesol) ethanol solutions overnight. The
Fig. 1 – SEM cross-section image of the as-prepared pure carbon CE.
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Fig. 2 – (a) Nyquist plots of electrochemical cells consisting of various electrodes; (b), (c) and (d) are the expanded range of the ordinate and abscissa from (a).
transfer at the TiO2/dye/electrolyter interface, and the carrier transport by ions within the electrolyte [27,28]. In order to gain insight into the effect of sheet resistance and catalytic properties of the CEs and remove the impact of the photoanode, the EIS analysis was carried out with a symmetric cell [6,29]. The symmetric cell consisted of two identical electrodes placed face to face in a sandwich configuration with effective electrode area of 1 cm2. The inter-electrode space was filled with electrolyte which was same as the one used in fully functional DSC. Fig. 2 shows the Nyquist plots of the symmetric cells for various electrodes of bare flexible graphite, pure carbon CE, carbon/FTO and Pt/FTO. For high frequencies around 100 kHz, where the phase is zero, the ohmic series resistance (Rs) can be determined. The charge-transfer resistance (Rct) of the electrodes can be taken as half the value of the real semicircle at high frequency side, while the semicircles at low frequency side represent diffusion impedance of electrolyte [6,29]. The Rs describes mainly the resistance of the substrates and the Rct measures the electrode catalytic activity for reducing triiodide ion. From the Nyquist plots shown in Fig. 2 and the impedance values listed in Table 1, we can see that the Rs for CEs with flexible graphite substrate is much lower than that with FTO glass substrate. The value of 4.7 X cm2 obtained with bare flexible graphite is only 18% of 26.1 and 26.3 X cm2 for the carbon/FTO and Pt/FTO. Due to the low conductivity of activated carbon in the catalytic layer, the Rs increases a little and reaches to 6.7 X cm2 for the pure carbon CE. The sheet resistances of these two substrates were also investigated by the four-point resistivity measurement. It is found that the sheet resistance of flexible graphite is only 4.2 · 102 X/h, whereas
that of the FTO glass is 15 X/h, implying that Rs will increase with increasing the sheet resistance of substrates. Similar result was also seen in the literatures [27,28]. However, the Rct of pure flexible graphite is quite large and reaches to 280 X cm2. By adding 20 lm thick activated carbon layer onto the flexible graphite, the Rct dramatically decreases to 1.2 X cm2, a value that is one-third less than 1.8 X cm2 for Pt/FTO electrode. These results indicate that pure flexible graphite has low catalytic activity for the reduction of triiodide ion. The contribution of the high surface area of activated carbon layer can considerably improve the catalytic activity of the CEs. While the Rct of the carbon/FTO is almost same as that of the pure carbon CE and reaches to 0.95 X cm2, the higher sheet resistance or Rs would be the main factor to affect the performance of DSCs with the carbon/FTO electrodes. Fig. 3 shows the photocurrent–voltage curves of the DSCs with various CEs and the related data are listed in Table 2. It can be seen that DSCs using bare flexible graphite electrode without any catalyst performed poorly, the photovoltaic parameters being short circuit photocurrent density (Jsc) of
Table 1 – Impedance parameters of various electrodes estimated from EIS in Fig. 2. Electrode Flexible graphite Pure carbon CE Carbon/FTO Pt/FTO
Rs (X cm2)
Rct (X cm2)
4.7 6.7 26.1 26.3
280 1.2 0.9 1.8
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Fig. 3 – Photocurrent–voltage curves of the DSCs based on flexible graphite, pure carbon CE, carbon/FTO and Pt/FTO electrodes under AM 1.5–100 mW cm2 light irradiation.
11.7 mA cm2, open circuit photo-voltage (Voc) of 663 mV, FF of 37.3% and conversion efficiency (g) of 2.88%. The low FF is induced by the large Rct of the bare flexible graphite (Table 1). The necessity of depositing an effective catalyst onto the flexible graphite can easily be appreciated. As shown in Fig. 3 and Table 2, the catalytic property can be improved by coating a 20 lm thick activated carbon layer onto the flexible graphite sheet to fabricate a pure carbon CE. It can be seen that DSCs with the pure carbon CE exhibit a higher g of 6.46%. Compared with the performance of the DSCs with CEs based on bare flexible graphite, the most pronounced change of DSCs with the pure carbon CE is FF, which increases from 37.3% to 70.2%. When using the carbon/FTO and Pt/FTO electrodes, the FF and g of the DSCs become lower compared with the pure carbon CE based devices. The FF decreases to 67.3% and 66.1%, and the g to 6.17% and 6.37% for DSCs with the carbon/FTO and Pt/FTO electrodes, respectively. This fact agrees with the higher sheet resistance of FTO glass substrates mentioned above. The effect of sheet resistance for CE substrates on the cell performance becomes more significant when the active area of DSCs is larger. Fig. 4 shows the relation of the g to the active area of DSCs with pure carbon CEs and Pt/FTO electrodes. It can be seen that for all the cell size, the g of DSCs with the pure carbon CEs is higher than that of the Pt/FTO based devices. Upon increasing the active area, both g and FF decrease for DSCs with the pure carbon CEs or Pt/FTO CEs. However, the magnitude of the decrease in g for DSCs with the Pt/FTO is greater. When the active area increases from 0.15 to 1 cm2, the g of DSCs with the Pt/FTO decreased from 6.37% to 2.91%. The g of DSCs with the pure carbon CE decreased a little and reached to 5%, which is 1.8 times that of the Pt/FTO based devices. The main difference of the g comes
Fig. 4 – The plots of g against cell active area for DSCs with pure carbon CEs and Pt/FTO electrodes.
from the large difference of FF. The FF for the pure carbon CE device decreased from 70.1% to 50%, while the FF for the Pt/FTO device decreased from 66.1% to 31.4%. These results indicated further that the reduction of the sheet resistance is very important in improving the performance of the solar cells. Though the g for DSCs with the pure carbon CE are still higher than that for the Pt/FTO cells, it drops quickly when the active area is larger than 1 · 1 cm2. Considering the limitation width is 1 cm for individual cells with good performance [10], the sheet resistance of photoanode becomes the dominating factor to control the FF and g for larger cells. If using photoanode with lower sheet resistance, we believe the g of DSCs with the pure carbon CEs would become higher for cells larger than 1 · 1 cm2.
4.
Conclusions
We have fabricated a pure carbon CE for DSCs using an industrial flexible graphite sheet as substrate and activated carbon as catalytic layer. Main advantage of the pure carbon CE over electrode with FTO substrate lies in low sheet resistance, leading to improved FF and g for the DSCs. With active cell area of 0.15 cm2, the FF and g of the DSCs with pure carbon CEs reached to 70.2% and 6.46%, which are higher than those of the thermal deposited Pt/FTO CE based devices. The efficiency difference between DSCs with the pure carbon CE and the Pt/FTO CE becomes larger as the active area is increasing. When the active cell area is increased to 1 cm2, the g of the DSCs with pure carbon CEs is almost twice that of the cells with Pt/FTO CEs. Furthermore, the flexibility, low weight and low price of the pure carbon CE would be a significant advantage to build a DSC with reducing production and transportation cost.
Table 2 – Detailed photovoltaic parameters for DSCs based on various CEs. CE type Flexible graphite Pure carbon CE Carbon/FTO Pt/FTO
Jsc (mA cm2) 11.7 13.1 13.3 14.0
Voc (mV)
FF (%)
g (%)
663 703 689 688
37.3 70.2 67.3 66.1
2.88 6.46 6.17 6.37
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Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 20725311, 20673141, 20703063, 20721140647 and 20873178), the Ministry of Science and Technology of China (973, Project No. 2006CB202606 and 863, Project No. 2006AA03Z341) and the 100-Talents Project of Chinese Academy of Sciences.
R E F E R E N C E S
[1] O’Regan B, Gra¨tzel M. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 1991;353:737–40. [2] Nazeerudin MK, Kay A, Rodicio I, Huxnpbry-Baker R, Miiller E, Liska P, et al. Conversion of light to electricity by cisX2bis(2,2 0 -bipyridyl-4,4 0 -dicarboxylate) ruthenium(II) chargetransfer sensitizers (X = C1, Br, I, CN, and SCN) on nanocrystalline TiO2 electrodes. J Am Chem Soc 1993;115:6382–90. [3] Chiba Y, Islam A, Watanabe Y, Komiya R, Koide N, Han LY. Dye-sensitized solar cells with conversion efficiency of 11.1%. Jpn J Appl Phys 2006;45:L638–40. [4] Frank AJ, Kopidakis N, van de Lagemaat J. Electrons in nanostructured TiO2 solar cells: transport, recombination and photovoltaic properties. Coor Chem Rev 2004;248:1165–79. [5] Jayaweera PVV, Perera AGU, Tennakone K. Why Gratzel’s cell works so well. Inorg Chim Acta 2008;361:707–11. [6] Papageorgiou N, Maier WF, Gra¨tzel M. An iodine/triiodide reduction electrocatalyst for aqueous and organic media. J Electrochem Soc 1997;144:876–84. [7] Papageorgiou N. Counter-electrode function in nanocrystalline photoelectrochemical cell configurations. Coor Chem Rev 2004;248:1421–46. [8] Kay A, Gra¨tzel M. Low cost photovoltaic modules based on dye sensitized nanocrystalline titanium dioxide and carbon powder. Sol Energy Mater Sol Cell 1996;44:99–117. [9] Murakami TN, Ito S, Wang Q, Nazzeruddin MK, Bessho T, Caser I, et al. High efficient dye-sensitized solar cells based on carbon black counter electrodes. J Electrochem Soc 2006;153:A2255–61. [10] Spa¨th M, Sommeling PM, van Roosmalen JAM, Smit HJP, van der Burg NPG, Mahieu DR, et al. Reproducible manufacturing of dye-sensitized solar cells on a semi-automated baseline. Prog Photovolt Res Appl 2003;11:207–20. [11] Okada K, Matsui H, Kawashima T, Ezure T, Tanabe N. 100 mm · 100 mm Large-sized dye sensitized solar cells. J Photochem Photobiol A 2004;164:193–8. [12] Kroon JM, Bakker NJ, Smit HJP, Liska P, Thampi KR, Wang P, et al. Nanocrystalline dye-sensitized solar cells having maximum performance. Prog Photovolt Res Appl 2007;15:1–18.
[13] Ma TL, Fang XM, Akiyama M, Inoue K, Noma H, Abe E. Properties of several types of novel counter electrodes for dye-sensitized solar cells. J Electroanal Chem 2004;574:77–83. [14] Imoto K, Takahashi K, Yamaguchi T, Komura T, Nakamura J, Murata K. High-performance carbon counter electrode for dye-sensitized solar cells. Sol Energy Mater Sol Cells 2003;79:459–69. [15] Huang Z, Liu XZ, Li KX, Li DM, Luo YH, Li H, et al. Application of carbon materials as counter electrodes of dye-sensitized solar cells. Electrochem Commun 2007;9:596–8. [16] Li KX, Yu ZX, Luo YH, Li DM, Meng QB. Recent progress of counter electrodes in nanocrystalline dye-sensitized solar cells. J Mater Sci Technol 2007;23:577–82. [17] Suzuki K, Yamaguchi M, Kumagai M, Yanagida S. Application of carbon nanotubes to counter electrodes of dye-sensitized solar cells. Chem Lett 2003;32:28–9. [18] Trancik JE, Barton SC, Hone J. Transparent and catalytic carbon nanotube films. Nano Lett 2008;8:982–7. [19] Hino T, Ogawa Y, Kuramoto N. Preparation of functionalized and non-functionalized fullerene thin films on ITO glasses and the application to a counter electrode in a dye-sensitized solar cell. Carbon 2006;44:880–7. [20] Hong WJ, Xu YX, Lu GW, Li C, Shi GQ. Transparent graphene/ PEDOT–PSS composite films as counter electrodes of dyesensitized solar cells. Electrochem Commun 2008;10:1555–8. [21] Lindstro H, Holmberg A, Magnusson E, Lindquist S, Malmqvist L, Hagfeldt A. A new method for manufacturing nanostructured electrodes on plastic substrates. Nano Lett 2001;1:97–100. [22] Chung D. Flexible graphite for gasketing, adsorption, electromagnetic interference shielding, vibration damping, electrochemical applications and stress sensing. J Mater Eng Perform 2000;9:161–3. [23] Frysz C, Chung D. Electrochemical behavior of flexible graphite. Carbon 1997;35:858–60. [24] Hariprakash B, Gaffoor S. Lead-acid cells with lightweight, corrosion-protected, flexible-graphite grids. J Power Sources 2007;173:565–9. [25] Fu Y, Hou M, Liang D, Yan XQ, Fu YF, Shao ZG, et al. The electrical resistance of flexible graphite as flow field plate in proton exchange membrane fuel cells. Carbon 2008;46:19–23. [26] Wang G, Lin R, Lin Y, Li X, Zhou X, Xiao X. A novel highperformance counter electrode for dye-sensitized solar cells. Electrochim Acta 2005;50:5546–52. [27] Han L, Koide N, Chiba Y, Islam A, Komiya R, Fuke N, et al. Improvement of efficiency of dye-sensitized solar cells by reduction of internal resistance. Appl Phys Lett 2005;86:213501–3. [28] Koide N, Islam A, Chiba Y, Han L. Improvement of efficiency of dye-sensitized solar cells based on analysis of equivalent circuit. J Photochem Photobiol A 2006;182:296–305. [29] Hauch A, Georg A. Diffusion in the electrolyte and chargetransfer reaction at the platinum electrode in dye-sensitized solar cells. Electrochim Acta 2001;46:3457–66. [30] Wang Q, Moser JE, Gra¨tzel M. Electrochemical impedance spectroscopic analysis of dye-sensitized solar cells. J Phys Chem B 2005;109:14945–53.
4 7 ( 2 0 0 9 ) 2 7 0 4 –2 7 0 8
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
A flexible carbon counter electrode for dye-sensitized solar cells Jikun Chen, Kexin Li, Yanhong Luo, Xiaozhi Guo, Dongmei Li, Minghui Deng, Shuqing Huang, Qingbo Meng* Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A pure carbon counter electrode (CE) for dye-sensitized solar cells (DSCs), has been fabri-
Received 11 March 2009
cated using an industrial flexible graphite sheet as substrate and activated carbon as the
Accepted 22 May 2009
catalytic material. The CE shows very low series resistance (Rs) and charge-transfer resis-
Available online 31 May 2009
tance (Rct) by combining the high conductivity of the flexible graphite with the high catalytic property of activated carbon. The Rs and Rct for the CE are respectively only a quarter and two-thirds of those for a platinized fluorine-doped tin oxide glass (Pt/FTO). DSCs with cell areas of 0.15 and 1 cm2 fabricated with this CE show higher solar-to-electricity conversion efficiencies. The respective values are 6.46% and 5%, compared with 6.37% and 2.91% for the Pt/FTO based devices. 2009 Elsevier Ltd. All rights reserved.
1.
Introduction
The dye-sensitized solar cell (DSC) has received widespread attention due to its low cost, easy production and relatively high efficiency to convert solar energy into electricity [1–5]. Typically, a DSC comprises a dye-sensitized nanocrystalline titanium dioxide (TiO2) electrode, electrolyte solution usually with a dissolved iodide/triiodide redox couple between the electrodes, and a counter electrode (CE). The function of the CE is to transfer electrons arriving from the external circuit back to the redox electrolyte and to catalyze the reduction of the triiodide ion [6,7]. Usually, Pt is used as the catalytic material and fluorine-doped tin oxide (FTO) glass as the substrate for a CE [1–7]. Although Pt exhibits excellent catalytic activity for triiodide reduction and good electric conductivity, it is extremely expensive and has the problem of reserves for large scale application [8,9]. Meanwhile, the relatively high sheet resistance of FTO glass imposes performance limitation, especially in large area devices [10,11]. FTO glass also has the problem of high price. It is estimated that the cost
of conductive glass sheets is about 30% of the total material cost of the DSC [12]. In addition, the shape limitation and fragile feature will bring transport problem for the FTO glass based DSCs [13]. Future large solar electric conversion systems will prefer materials abundantly available and easily handled. Therefore, it is necessary to develop cheap materials for CEs which also exhibit high electrical conductivity, good chemical stability and good catalytic activity to the reduction of triiodide ion. So far, inexpensive carbonaceous materials such as graphite, carbon black, activated carbon, hard carbon sphere, carbon nanotube, fullerene and graphene, have been employed as the catalytic materials on FTO or ITO (indium tin oxide) glass for the CEs [8,9,14–20]. Roughness factor and conductivity have been considered as two important factors influencing the performance of the CEs [9,14,15]. Activated carbon with large surface area shows superior catalytic activity toward the triiodide reduction [14]. Meanwhile, novel substrates such as metals or plastic foils (using Pt as catalytic material) have been used to fabricate flexible CEs to achieve the requirement
* Corresponding author: Fax: +86 10 8264 9242. E-mail address: [email protected] (Q. Meng). 0008-6223/$ - see front matter 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.05.028
CARBON
4 7 ( 20 0 9 ) 2 7 0 4–27 0 8
for portable electricity and high-throughput industrial roll-toroll production [13,21]. However, the efficiency of the DSCs with these substrates is low and the improvements of their long time stability are also imperative for future large scale production. Industrial flexible graphite, which is used mainly for gaskets, has very good electrical conductivity, high chemical stability, high temperature resistance, good flexibility and low coefficient of thermal expansion, which make it a good candidate to fabricate electrode. In the past, it has been found that flexible graphite can offer better electron transfer rate and lower capacitance, and has been used as electrode for the electrochemical system, such as fuel cells, lead-acid cells and sensors [22–25]. Here, we present the feasibility of flexible graphite for the CE in DSCs by developing a pure carbon CE. In this CE a flexible graphite sheet is used as the substrate and activated carbon is chosen to be the catalytic material. Both substrate and catalytic material are inexpensive carbonaceous materials, which give a good adhesion between the catalytic material and the substrate. The DSCs with this pure carbon CE show better performance than thermal deposited Pt/FTO glass based devices, especially for DSCs with larger area. Furthermore, the density of the flexible graphite is very low. The mass per unit area of the flexible graphite is only 6% of that for the FTO glass with thickness of 2 mm. This will benefit for lowering the transportation cost for large scale production.
2.
Experimental
2.1.
Preparation of counter electrodes
To fabricate the pure carbon CE, flexible graphite sheets with thickness of 0.2 mm and area density of 0.03 g/cm2 were cleaned with distilled water, ethanol and then air-dried. The activated carbon paste was made by mixing 0.4 g activated carbon (the particle size is 1–10 lm and Brunauer–Emmett– Teller (BET) surface area 1958 m2 g1), 0.1 g carbon black (particle size is about 30 nm and BET surface area 77 m2 g1), 4 g terpineol and 0.1 g SnO2 nanoparticle (the particle size is 10 nm) together and ball-milling for 5 h. Then the activated carbon paste was coated onto flexible graphite sheet for preparing pure carbon CE or onto FTO glass (TEC-15, LOF) for carbon/FTO electrode by doctor-blading. The films were dried and sintered at given temperature for 60 min. For comparison, the Pt/FTO electrode was fabricated by thermal decomposition of H2PtCl6 (30 mM in isopropanol) on FTO glass at 385 C for 30 min [26]. The Pt loading was controlled to be 5 lg/cm2.
2.2.
2705
electrolyte consist of 0.6 M methylhexylimidazolium iodide, 0.05 M iodine, 0.1 M LiI and 0.5 M tert-butylpyridine in 3methoxypropionitrile. An open sandwich-type cell was fabricated in air by clamping the sensitized TiO2, a drop of electrolyte and a CE with two clips. A mask was also clipped on the TiO2 side to define the active area of the cell.
2.3.
Characterization and measurements
The scanning electron microscopy (SEM) measurements of the pure carbon CE were conducted on a LEO-1530 microscope (Germany). The sheet resistance of the flexible graphite and FTO glass was measured by using a four-point resistivity measurement system (RST-9, China). The photocurrent–voltage measurements were recorded by a potentiostat (Model 263A, Princeton Applied Research). A solar light simulator (91192, Oriel) was used to mimic one sun air mass 1.5 (AM 1.5) illumination on the surface of the solar cells. The intensity of incident light was measured with a radiant power/energy meter (70260, Oriel) before each experiment. The electrochemical impedance spectroscopy (EIS) measurements of the cells were performed with an electrochemical station (IM6ex, Zahner) with the frequency range being 100 mHz–100 kHz. The magnitude of the alternative signal was 10 mV.
3.
Results and discussion
Fig. 1 shows the SEM cross-section image of the pure carbon CE. From the SEM image, we can see that the large activated carbon particles are well enclosed by the small carbon black particles and the thick porous carbon film adheres well with the flexible graphite substrate. The measured average thickness of the carbon film is about 20 lm. The activated carbon particles with large surface area can offer more active sites for triiodide reduction and the carbon black can improve the conductivity of the film by filling the large pores between activated carbon particles [8,14]. Previous analyses of EIS and the equivalent circuit of DSCs have suggested that resistive effects in solar cells reduce the efficiency of the cells by dissipating power in internal resistance [27–30]. The key impact of internal resistance is to reduce fill factor (FF). The internal resistances of the DSCs are mainly related to the sheet resistance of the substrates, the charge-transfer processes occurring at the CEs, the electron
DSC fabrication
The nanocrystalline TiO2 films were fabricated on the FTO glass by screen-printing a TiO2 (Degussa P25) paste [2]. The thickness of the TiO2 film was about 15 lm. The electrode was heated to 450 C and sintered for 30 min, then cooled down to 80 C and immersed into 0.3 mM N3 (cis-dithiocyanato-N,N-bis(2,2-bipyridyl-4,4-dicarboxylic acid) ruthenium(II) dihydrate) (Dyesol) ethanol solutions overnight. The
Fig. 1 – SEM cross-section image of the as-prepared pure carbon CE.
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4 7 ( 2 0 0 9 ) 2 7 0 4 –2 7 0 8
Fig. 2 – (a) Nyquist plots of electrochemical cells consisting of various electrodes; (b), (c) and (d) are the expanded range of the ordinate and abscissa from (a).
transfer at the TiO2/dye/electrolyter interface, and the carrier transport by ions within the electrolyte [27,28]. In order to gain insight into the effect of sheet resistance and catalytic properties of the CEs and remove the impact of the photoanode, the EIS analysis was carried out with a symmetric cell [6,29]. The symmetric cell consisted of two identical electrodes placed face to face in a sandwich configuration with effective electrode area of 1 cm2. The inter-electrode space was filled with electrolyte which was same as the one used in fully functional DSC. Fig. 2 shows the Nyquist plots of the symmetric cells for various electrodes of bare flexible graphite, pure carbon CE, carbon/FTO and Pt/FTO. For high frequencies around 100 kHz, where the phase is zero, the ohmic series resistance (Rs) can be determined. The charge-transfer resistance (Rct) of the electrodes can be taken as half the value of the real semicircle at high frequency side, while the semicircles at low frequency side represent diffusion impedance of electrolyte [6,29]. The Rs describes mainly the resistance of the substrates and the Rct measures the electrode catalytic activity for reducing triiodide ion. From the Nyquist plots shown in Fig. 2 and the impedance values listed in Table 1, we can see that the Rs for CEs with flexible graphite substrate is much lower than that with FTO glass substrate. The value of 4.7 X cm2 obtained with bare flexible graphite is only 18% of 26.1 and 26.3 X cm2 for the carbon/FTO and Pt/FTO. Due to the low conductivity of activated carbon in the catalytic layer, the Rs increases a little and reaches to 6.7 X cm2 for the pure carbon CE. The sheet resistances of these two substrates were also investigated by the four-point resistivity measurement. It is found that the sheet resistance of flexible graphite is only 4.2 · 102 X/h, whereas
that of the FTO glass is 15 X/h, implying that Rs will increase with increasing the sheet resistance of substrates. Similar result was also seen in the literatures [27,28]. However, the Rct of pure flexible graphite is quite large and reaches to 280 X cm2. By adding 20 lm thick activated carbon layer onto the flexible graphite, the Rct dramatically decreases to 1.2 X cm2, a value that is one-third less than 1.8 X cm2 for Pt/FTO electrode. These results indicate that pure flexible graphite has low catalytic activity for the reduction of triiodide ion. The contribution of the high surface area of activated carbon layer can considerably improve the catalytic activity of the CEs. While the Rct of the carbon/FTO is almost same as that of the pure carbon CE and reaches to 0.95 X cm2, the higher sheet resistance or Rs would be the main factor to affect the performance of DSCs with the carbon/FTO electrodes. Fig. 3 shows the photocurrent–voltage curves of the DSCs with various CEs and the related data are listed in Table 2. It can be seen that DSCs using bare flexible graphite electrode without any catalyst performed poorly, the photovoltaic parameters being short circuit photocurrent density (Jsc) of
Table 1 – Impedance parameters of various electrodes estimated from EIS in Fig. 2. Electrode Flexible graphite Pure carbon CE Carbon/FTO Pt/FTO
Rs (X cm2)
Rct (X cm2)
4.7 6.7 26.1 26.3
280 1.2 0.9 1.8
CARBON
2707
4 7 ( 20 0 9 ) 2 7 0 4–27 0 8
Fig. 3 – Photocurrent–voltage curves of the DSCs based on flexible graphite, pure carbon CE, carbon/FTO and Pt/FTO electrodes under AM 1.5–100 mW cm2 light irradiation.
11.7 mA cm2, open circuit photo-voltage (Voc) of 663 mV, FF of 37.3% and conversion efficiency (g) of 2.88%. The low FF is induced by the large Rct of the bare flexible graphite (Table 1). The necessity of depositing an effective catalyst onto the flexible graphite can easily be appreciated. As shown in Fig. 3 and Table 2, the catalytic property can be improved by coating a 20 lm thick activated carbon layer onto the flexible graphite sheet to fabricate a pure carbon CE. It can be seen that DSCs with the pure carbon CE exhibit a higher g of 6.46%. Compared with the performance of the DSCs with CEs based on bare flexible graphite, the most pronounced change of DSCs with the pure carbon CE is FF, which increases from 37.3% to 70.2%. When using the carbon/FTO and Pt/FTO electrodes, the FF and g of the DSCs become lower compared with the pure carbon CE based devices. The FF decreases to 67.3% and 66.1%, and the g to 6.17% and 6.37% for DSCs with the carbon/FTO and Pt/FTO electrodes, respectively. This fact agrees with the higher sheet resistance of FTO glass substrates mentioned above. The effect of sheet resistance for CE substrates on the cell performance becomes more significant when the active area of DSCs is larger. Fig. 4 shows the relation of the g to the active area of DSCs with pure carbon CEs and Pt/FTO electrodes. It can be seen that for all the cell size, the g of DSCs with the pure carbon CEs is higher than that of the Pt/FTO based devices. Upon increasing the active area, both g and FF decrease for DSCs with the pure carbon CEs or Pt/FTO CEs. However, the magnitude of the decrease in g for DSCs with the Pt/FTO is greater. When the active area increases from 0.15 to 1 cm2, the g of DSCs with the Pt/FTO decreased from 6.37% to 2.91%. The g of DSCs with the pure carbon CE decreased a little and reached to 5%, which is 1.8 times that of the Pt/FTO based devices. The main difference of the g comes
Fig. 4 – The plots of g against cell active area for DSCs with pure carbon CEs and Pt/FTO electrodes.
from the large difference of FF. The FF for the pure carbon CE device decreased from 70.1% to 50%, while the FF for the Pt/FTO device decreased from 66.1% to 31.4%. These results indicated further that the reduction of the sheet resistance is very important in improving the performance of the solar cells. Though the g for DSCs with the pure carbon CE are still higher than that for the Pt/FTO cells, it drops quickly when the active area is larger than 1 · 1 cm2. Considering the limitation width is 1 cm for individual cells with good performance [10], the sheet resistance of photoanode becomes the dominating factor to control the FF and g for larger cells. If using photoanode with lower sheet resistance, we believe the g of DSCs with the pure carbon CEs would become higher for cells larger than 1 · 1 cm2.
4.
Conclusions
We have fabricated a pure carbon CE for DSCs using an industrial flexible graphite sheet as substrate and activated carbon as catalytic layer. Main advantage of the pure carbon CE over electrode with FTO substrate lies in low sheet resistance, leading to improved FF and g for the DSCs. With active cell area of 0.15 cm2, the FF and g of the DSCs with pure carbon CEs reached to 70.2% and 6.46%, which are higher than those of the thermal deposited Pt/FTO CE based devices. The efficiency difference between DSCs with the pure carbon CE and the Pt/FTO CE becomes larger as the active area is increasing. When the active cell area is increased to 1 cm2, the g of the DSCs with pure carbon CEs is almost twice that of the cells with Pt/FTO CEs. Furthermore, the flexibility, low weight and low price of the pure carbon CE would be a significant advantage to build a DSC with reducing production and transportation cost.
Table 2 – Detailed photovoltaic parameters for DSCs based on various CEs. CE type Flexible graphite Pure carbon CE Carbon/FTO Pt/FTO
Jsc (mA cm2) 11.7 13.1 13.3 14.0
Voc (mV)
FF (%)
g (%)
663 703 689 688
37.3 70.2 67.3 66.1
2.88 6.46 6.17 6.37
2708
CARBON
4 7 ( 2 0 0 9 ) 2 7 0 4 –2 7 0 8
Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 20725311, 20673141, 20703063, 20721140647 and 20873178), the Ministry of Science and Technology of China (973, Project No. 2006CB202606 and 863, Project No. 2006AA03Z341) and the 100-Talents Project of Chinese Academy of Sciences.
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