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Carbon black counter electrode for dye-sensitized solar cells
Available online at www.sciencedirect.com
Solar Energy 83 (2009) 845–849 www.elsevier.com/locate/solener
High-performance and low platinum loading Pt/Carbon black counter electrode for dye-sensitized solar cells Pinjiang Li a,b, Jihuai Wu a,*, Jianming Lin a, Miaoliang Huang a, Yunfang Huang a, Qinghua Li a a
Institute of Materials Physical Chemistry, Huaqiao University, Quanzhou, Fujian 362021, China b Institute of Surface Micro and NanoMaterials, Xuchang University, Xuchang 461000, China
Received 11 June 2008; received in revised form 29 October 2008; accepted 29 November 2008 Available online 25 December 2008
Abstract Pt/Carbon black counter electrode for dye-sensitized solar cells (DSSCs) was prepared by reducing H2PtCl6 with NaBH4 in carbon black. The Pt/Carbon black electrode had a high electrocatalytic activity for iodide/triiodide redox reaction. Using the Pt/Carbon black counter electrode, DSSC achieved 6.72% energy conversion efficiency under one sun illumination. Pt/Carbon black electrode shows the same energy conversion efficiency and lower cost compared with Pt electrode, which makes it available in DSSCs practical applications. Ó 2008 Published by Elsevier Ltd. Keywords: Dye-sensitized solar cell; Counter electrode; Platinum; Carbon; Electrochemistry
1. Introduction Since the prototype of a dye-sensitized TiO2 nanocrystalline solar cell (DSSC) was reported in 1991 by O’Regan and Gratzel, O’Regan and Gratzel, 1991 it has aroused intensive scientific and technological interest and have evolved a potential alternative to traditional photovoltaic devices over the past decade due to its low cost and simple preparation procedure. O’Regan and Gratzel, 1991; Nazeeruddin et al., 1993; Gratzel, 2004; Wu et al., 2007a; Wu et al., 2007b. Generally, a DSSC consists of three main components: a dye-covered nanocrystalline TiO2 layer on a transparent conductive glass substrate, an electrolyte contained iodide/triiodide redox couple, and a platinized conductive glass substrate as a counter electrode. Counter electrode, as one important component in DSSCs, is usually constructed with a conducting glass substrate coated with *
Corresponding author. Tel.: +86 595 22693899; fax: +86 595 22693999. E-mail address: [email protected] (J. Wu). 0038-092X/$ - see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.solener.2008.11.012
platinum film. The roles of the counter electrode are to collect electrons from external circuit and reduce I3 to I in electrolyte. To keep a low overvoltage and lessen energy losses, the counter electrode should have low resistance and high electrocatalytic activity for iodide/triiodide redox reaction Macyk et al., 2007; Papageorgiou et al., 1996; Papageorgiou et al., 1997; Fang et al., 2004. Platinized counter electrode has low resistance and high electrocatalytic activity for iodide (I)/triiodide (I3) redox couple in DSSCs. But platinum is one of the costly precious metals. Hence, other materials are attempted to reduce production cost of DSSCs. Some carbonaceous materials such as carbon nanotubes, activated carbon, and graphite have been employed as catalysts for counter electrodes to replace the Pt electrode Suzuki et al., 2003; Kay and Gratzel, 1996; Lindstrom et al., 2001; Imoto et al., 2003. However, the conversion efficiency of these cells was relatively low owing to their poor catalytic activity for I3 reduction. Carbon materials are inexpensive and show low resistance and electrocatalytic activity for the reduction of triiodide Wroblowa and Saunders, 1973; Kinoshita, 1987;
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P. Li et al. / Solar Energy 83 (2009) 845–849
Kamau, 1988. The catalytic active sites of carbon materials usually locate on the edges of carbon crystal sheet. Consequently the carbon black with low crystalline and a great amount of edges may be more active than that the carbon materials with highly orientated such as graphite or carbon nanotubes Murakami et al., 2006. In this paper, a Pt/Carbon black counter electrode for DSSC was prepared. The properties of the counter electrode, including morphology, electrocatalytic activity and the effect of platinum loading on the performance of DSSC were investigated. 2. Experimental 2.1. Materials Titanium (IV) iso-propoxide and 4-tert-butylpyrldine (TBP) were purchased from Fluka and used as received. Chloroplatinic acid (H2PtCl6), carbon black powder, acetonitrile, tetrapropylammonium iodide, potassium iodinate and iodine were all purchased from Shanghai Chemical Agent Ltd. China, and used without further purification. Organometallic dye cis-bis(isothiocyanato)bis(2,20 -bipyridyl-4,40 -dicarboxylato) ruthenium (II) [RuL2(NCS)2] was obtained from Solaronix SA (Switzerland), the other reagents came from Shanghai Chemical Agent Ltd. China. Conductive glass substrate (FTO glass, Fluorine doped tin oxide over-layer, sheet resistance 8 X cm2 from Hartford Glass Co., USA), was used as a substrate for precipitating TiO2 porous film, and was cut into 2 1.5 cm2 sheets. 2.2. Preparation of counter electrode A mixture of carbon black and platinum was prepared by reducing H2PtCl6 with a reducing agent (NaBH4) in carbon black. 300 mg of carbon black powder and 5 ml of isopropanol were added to twice distilled water in a beaker and ultrasonically stirred for 15 min. H2PtCl6 solution (7.72 mM) with a predetermined Pt/carbon black weight ratio was added and ultrasonically stirred for 30 min, followed by adding 0.05 mol/L of NaBH4 solution (NaBH4 was excess to reduce H2PtCl6 fully) and the ultrasonically stirring for 2 h. After filtering and washing for 3 times, the deposit substance was sintered at 250 °C for 1 h in the air. Thus, a mixture powder of Pt and carbon black was obtained. The mixture powder of Pt and carbon black was dispersed in a mixed solution with 2 ml distilled water and 2 ml ethanol. Then, the 30 mg of hydroxyethyl cellulose as an adhesive was dissolved in this dispersion to form a paste contained Pt and carbon black. The Pt/Carbon black counter electrode was prepared by coating the paste on FTO conductive glass sheet using doctor-blading technique. Then the electrode was sintered at 180 °C for 1 h and dried in vacuum oven at 120 °C overnight.
2.3. Preparation of TiO2 paste 0.05 mol of acetic acid was mixed with 0.05 mol of titanium iso-propoxide under stirring at room temperature. The mixed solution was rapidly poured into 120 ml distilled water with vigorous stirring for 30 min and a white precipitate was formed. Then, acetic acid (12 ml) and nitric acid solution (65 wt%, 1.2 ml) were added. Then the system was peptized at 80 °C for 12 h and was autoclaved at 200 °C for 12 h to form a white suspension with some precipitate. The resultant suspension was concentrated to 1/4 of its original volume, PEG-20000 (10 wt% TiO2) and a few drops of emulsification regent of Triton X-100 was added to the resultant colloidal solution with stirring. Then the colloidal solution was concentrated to form a TiO2 paste of suitable concentration. 2.4. Fabrication of DSSC A DSSC (active area of 0.25 cm2) was assembled according to the following procedure. Conducting glass sheet (FTO) was washed with ethanol and immersed in 50 mM TiCl4 aqueous solution for 12 h in order to make a good contact between the TiO2 layer and conducting glass substrate. A TiO2 electrode (TiO2 film thickness about 6 lm) was obtained by spreading the TiO2 paste on the conducting glass substrate using a ‘‘doctor blade method” and then sintered at 450 °C for 30 min in air. After cooling to 80 °C, the TiO2 electrode was dye-sensitized with an organometallic dye ([RuL2(NCS)2], 0.5 mM) absolute ethanol solution for 24 h at room temperature. Afterwards, the dye-sensitized TiO2 electrode was rinsed with absolute ethanol and dried in moisture-free air. A liquid electrolyte was prepared by blending 0.6 M tetrapropylammonium iodide, 0.1 M I2, 0.1 M KI, and 0.5 M TBP in acetonitrille solution. A dye-sensitized solar cell was assembled by dropping a drop of the liquid electrolyte on the dye-sensitized TiO2 porous film electrode. The Pt/Carbon black counter electrode was laid over. The two electrodes were clipped together and a cyanoacrylate adhesive was used as sealant to prevent the electrolyte solution from leaking. 2.5. Measurements The microstructure of sample was observed with a JEM2000EX transmission electron microscope (JEOL, Japan). The crystal structure of samples was investigated by Xray powder diffraction (XRD) on an X-ray diffractometer (D8 ADVANCE, Germany) with Cu Ka radiation. Cyclic voltammetry (CV) was carried out in a three electrode one compartment cell with a self-made Pt/Carbon working electrode, Pt foil counter electrode and an Ag/AgCl reference electrode dipped in an acetonitrile solution of 10 mM LiI, 1 mM I2, and 0.1 M LiClO4. CV performed using CHI660B electrochemical measurement system (sweep condition: 100 mV s1).
P. Li et al. / Solar Energy 83 (2009) 845–849
The photovoltaic test of dye-sensitized TiO2 nanocrystalline solar cells was carried out by measuring the J–V characteristic curves under irradiation of white light from a 100 W xenon arc lamp (XQ-500 W, Shanghai Photoelectricity Device Company, China) under ambient atmosphere. The incident light intensity and the active cell area was 100 mW cm2 and 0.25 cm2, respectively.
tive pair is assigned to the redox reaction Eq. (1) and the positive pair is assigned to redox reaction Eq. (2) Imoto et al., 2003; Popov and Geske, 1958; Huang et al., 2007. I 3 þ 2e ¼ 3I 3I2 þ 2e ¼ 2I 3
3.1. Morphology characterization of the Pt/Carbon black Counter electrode Fig. 1a shows the XRD pattern of the mixture of Pt and carbon black (Pt 1.5 wt%). The peak at about 26.6° corresponds to graphite (002) in carbon black. Graphite in carbon black is available for good electrical conductivity for the counter electrode. Several peaks at 39.4°, 46.0°, 67.4°, and 81.1° were also observed in the XRD pattern, which correspond to (111), (200), (220), and (311) of the face-centered cubic (fcc) lattice of platinum, respectively Schmid, 1992; Teranishi et al., 1999. The widening of the bandwidths is due to their small particle size. Fig. 1b is a TEM image of the mixture of Pt and carbon black (Pt 1.5 wt%). It can be observed that the mean size of platinum particles is 20–30 nm, and platinum nanoparticles were successfully supported and homogeneously dispersed on the carbon black.
3.3. Photovoltaic performance of DSSCs based on Pt/ Carbon black counter electrode Table 1 summarizes the photoelectric performance of the DSSCs using Pt/Carbon black counter electrodes with different platinum loadings. When a carbon black electrode without platinum is used as counter electrode, the open-circuit photovoltage (Voc) of the DSSC is 724 mV, the shortcircuit photocurrent density (Jsc) is 8.53 mA/cm2, and the overall energy conversion efficiency (g) is only 3.76%. Such low conversion efficiency is due to the lower catalytic activity of the carbon black for I/I3 redox couple. On the other hand, when platinum loading is 1.5 wt%, the Voc and Isc of the DSSC increase to 753 mV and 14.46 mA/ cm2, and the overall energy conversion efficiency (g) is up to 6.72%. It indicates that the catalytic activity of the counter electrode is improved when the platinum is filled in the carbon black. Furthermore, we observe no apparent difference in Voc, Isc, and g of the DSSCs based on Pt/Carbon black Counter electrodes with the platinum loading between 1.5 and 4.0 wt% (shown as Table 1). Therefore, the 1.5 wt% platinum loading on the carbon black is suffi-
3.2. Cyclic voltammograms for the Pt/Carbon counter electrode Fig. 2 compares cyclic voltammograms in I2/I system for Pt plate electrode, carbon black electrode and Pt/Carbon electrode at a scan rate of 100 mV s1. The oxidation/reduction peaks were observed in all cases. It indicates that three materials all have electrocatalytic activity for I2/I system. There were two pairs of redox waves for the Pt electrode and Pt/C electrode. The relative nega-
a
b
Intensity / a.u.
39.4
26.6
46.0
10
20
30
40
50
2θ /
60
70
81.1
80
ð1Þ ð2Þ
Fig. 2 also shows a higher current density of the redox peak for the Pt/C electrode and carbon black electrode than for the Pt electrode. It could be explained that the carbon black usually has large specific surface area, which increases effective active area of catalysis, so that the current density is improved. The high current density also suggests that the reaction rate was fast, in other word, the charge-transfer resistance (RCT) for the I3/I redox reaction was low on the carbon black electrode Imoto et al., 2003.
3. Results and discussion
67.4
847
90
o
Fig. 1. (a) X-ray diffraction pattern (b) TEM image for Pt and carbon black mixture (Pt 1.5 wt%).
P. Li et al. / Solar Energy 83 (2009) 845–849
Current density (mA/cm-2)
848 0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0
-0.2
-0.2
-0.4
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-0.6
-0.6 (a) Pt electrode
-0.8 0.5
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(c) Pt/C electrode
(b) C electrode
0.5
-1.0
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-1.0
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Voltage (V)
0.5
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Fig. 2. Cyclic voltammograms for Pt plate electrode (a) carbon black electrode (b) and Pt/Carbon black electrode (c) in 10 mM LiI, 1.0 mM I2 acetonitrile solution containing 0.1 M LiClO4 as the supporting electrolyte. [I]/[I2] = 10/1.
Table 1 Photoelectric parameters of dye-sensitized solar cells based on Pt/Carbon black counter electrodes with different platinum loadings. Platinum loading (%)
Jsc (mA/cm2)
Voc (mV)
FF
g (%)
0 1.0 1.5 2.0 4.0 100 (Pt electrode)
8.53 13.59 14.46 14.57 14.73 14.86
724 737 753 748 742 723
0.601 0.614 0.616 0.612 0.611 0.617
3.76 6.17 6.72 6.67 6.68 6.63
16
Current density (mA/cm2)
14
a b
12 10 8
ter electrodes exhibited no significant difference in the overall energy conversion efficiencies (g), which are 6.72% and 6.63%. 4. Conclusions In summary, Pt/Carbon black counter electrode for dyesensitized solar cells (DSSCs) was prepared by reducing H2PtCl6 with NaBH4 in carbon black. Pt/Carbon black electrode showed high electrocatalytic activity for iodide/ triiodide redox reaction. Using Pt/Carbon black (platinum 1.5 wt%) as counter electrode, DSSC achieved 6.72% overall energy conversion efficiency under one sun illumination (AM1.5, Pin of 100 mW cm2). Pt/Carbon black electrode shows the same energy conversion efficiency and lower cost compared with Pt electrode, which make it available in DSSCs practical applications. Acknowledgements
6 4 2 0 0.0
0.2
0.4
0.6
0.8
Voltage (V)
Fig. 3. Photocurrent-voltage curves of DSSCs with (a) platinized counter electrode and (b) Pt/Carbon black counter electrode. Under one sun illumination (AM1.5, Pin of 100 mW cm2).
cient for catalyzing the iodide (I)/triiodide (I3) redox couple in the electrolyte system. Fig. 3 gives photocurrent–voltage curves of DSSCs using a Pt/Carbon black electrode (Pt, 1.5 wt%) and a platinized electrode as counter electrode. The platinized electrode was prepared by electroplating Hao et al., 2006. The Jsc, Voc, FF, and g of the DSSCs are summarized in Table 1. It can be seen that DSSCs with the different coun-
The authors thank for jointly supporting by the National Natural Science Foundation of China (No. 50572030, 50842027), the Nano Functional Materials Special Program of Fujian Province (No. 2005HZ01-4), the Key Project of Chinese Ministry of Education.(No. 206074) and Specialized Research Fund for the Doctoral Program of Chinese Higher Education (No. 20060385001). References Fang, X., Ma, T., Guan, G., et al., 2004. J. Electroanal. Chem. 570, 257. Gratzel, M., 2004. J. Photochem. Photobio. A 164, 3. Hao, S., Wu, J., Lin, J., et al., 2006. Compos. Interface 13, 899. Huang, Z., Liu, X., Li, K., et al., 2007. Electrochem. Commun. 9, 596. Imoto, K., Takatashi, K., Yamaguchi, T., et al., 2003. Sol. Energy Mater. Sol. Cells 79, 459. Kamau, G.N., 1988. Anal. Chim. Acta 207, 1. Kay, A., Gratzel, M., 1996. Sol. Energy Mater. Sol. Cells 44, 99. Kinoshita, K., 1987. Carbon: Electrochemical and Physicochemical Properties. Wiley Interscience Publications, New York, p. 226–379.
P. Li et al. / Solar Energy 83 (2009) 845–849 Lindstrom, H., Holmberg, A., Magnusson, E., et al., 2001. Nano Lett. 1, 97. Macyk, W., Stochel, G., Szacilowski, K., 2007. Chem-Eur. J. 13, 5676. Murakami, T.N., Ito, S., Wang, Q., et al., 2006. J. Electrochem. Soc. A 153, 2255. Nazeeruddin, M.K., Kay, A., Rodicio, I., et al., 1993. J. Am. Chem. Soc. 115, 6382. O’Regan, B., Gratzel, M., 1991. Nature 353, 737. Papageorgiou, N., Athanassov, Y., Armand, M., et al., 1996. J. Electrochem. Soc. 143, 3099.
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Papageorgiou, N., Maier, W.F., Gratzel, M., 1997. J. Electrochem. Soc. 144, 876. Popov, A.I., Geske, D.H., 1958. J. Am. Chem. Soc. 80, 1340. Schmid, G., 1992. Chem. Rev. 92, 1709. Suzuki, K., Yamamoto, M., Kumagai, M., et al., 2003. Chem. Lett. 32, 28. Teranishi, T., Hosoe, M., Tanaka, T., et al., 1999. J. Phys. Chem. B 103, 3818. Wroblowa, H.S., Saunders, A., 1973. J. Electroanal. Chem. 42, 329. Wu, J.H., Hao, S.C., Lan, Z., et al., 2007a. Adv. Funct. Mater. 17, 2645. Wu, J., Lan, Z., Lin, J.M., et al., 2007b. Adv. Mater. 19, 4006.
Solar Energy 83 (2009) 845–849 www.elsevier.com/locate/solener
High-performance and low platinum loading Pt/Carbon black counter electrode for dye-sensitized solar cells Pinjiang Li a,b, Jihuai Wu a,*, Jianming Lin a, Miaoliang Huang a, Yunfang Huang a, Qinghua Li a a
Institute of Materials Physical Chemistry, Huaqiao University, Quanzhou, Fujian 362021, China b Institute of Surface Micro and NanoMaterials, Xuchang University, Xuchang 461000, China
Received 11 June 2008; received in revised form 29 October 2008; accepted 29 November 2008 Available online 25 December 2008
Abstract Pt/Carbon black counter electrode for dye-sensitized solar cells (DSSCs) was prepared by reducing H2PtCl6 with NaBH4 in carbon black. The Pt/Carbon black electrode had a high electrocatalytic activity for iodide/triiodide redox reaction. Using the Pt/Carbon black counter electrode, DSSC achieved 6.72% energy conversion efficiency under one sun illumination. Pt/Carbon black electrode shows the same energy conversion efficiency and lower cost compared with Pt electrode, which makes it available in DSSCs practical applications. Ó 2008 Published by Elsevier Ltd. Keywords: Dye-sensitized solar cell; Counter electrode; Platinum; Carbon; Electrochemistry
1. Introduction Since the prototype of a dye-sensitized TiO2 nanocrystalline solar cell (DSSC) was reported in 1991 by O’Regan and Gratzel, O’Regan and Gratzel, 1991 it has aroused intensive scientific and technological interest and have evolved a potential alternative to traditional photovoltaic devices over the past decade due to its low cost and simple preparation procedure. O’Regan and Gratzel, 1991; Nazeeruddin et al., 1993; Gratzel, 2004; Wu et al., 2007a; Wu et al., 2007b. Generally, a DSSC consists of three main components: a dye-covered nanocrystalline TiO2 layer on a transparent conductive glass substrate, an electrolyte contained iodide/triiodide redox couple, and a platinized conductive glass substrate as a counter electrode. Counter electrode, as one important component in DSSCs, is usually constructed with a conducting glass substrate coated with *
Corresponding author. Tel.: +86 595 22693899; fax: +86 595 22693999. E-mail address: [email protected] (J. Wu). 0038-092X/$ - see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.solener.2008.11.012
platinum film. The roles of the counter electrode are to collect electrons from external circuit and reduce I3 to I in electrolyte. To keep a low overvoltage and lessen energy losses, the counter electrode should have low resistance and high electrocatalytic activity for iodide/triiodide redox reaction Macyk et al., 2007; Papageorgiou et al., 1996; Papageorgiou et al., 1997; Fang et al., 2004. Platinized counter electrode has low resistance and high electrocatalytic activity for iodide (I)/triiodide (I3) redox couple in DSSCs. But platinum is one of the costly precious metals. Hence, other materials are attempted to reduce production cost of DSSCs. Some carbonaceous materials such as carbon nanotubes, activated carbon, and graphite have been employed as catalysts for counter electrodes to replace the Pt electrode Suzuki et al., 2003; Kay and Gratzel, 1996; Lindstrom et al., 2001; Imoto et al., 2003. However, the conversion efficiency of these cells was relatively low owing to their poor catalytic activity for I3 reduction. Carbon materials are inexpensive and show low resistance and electrocatalytic activity for the reduction of triiodide Wroblowa and Saunders, 1973; Kinoshita, 1987;
846
P. Li et al. / Solar Energy 83 (2009) 845–849
Kamau, 1988. The catalytic active sites of carbon materials usually locate on the edges of carbon crystal sheet. Consequently the carbon black with low crystalline and a great amount of edges may be more active than that the carbon materials with highly orientated such as graphite or carbon nanotubes Murakami et al., 2006. In this paper, a Pt/Carbon black counter electrode for DSSC was prepared. The properties of the counter electrode, including morphology, electrocatalytic activity and the effect of platinum loading on the performance of DSSC were investigated. 2. Experimental 2.1. Materials Titanium (IV) iso-propoxide and 4-tert-butylpyrldine (TBP) were purchased from Fluka and used as received. Chloroplatinic acid (H2PtCl6), carbon black powder, acetonitrile, tetrapropylammonium iodide, potassium iodinate and iodine were all purchased from Shanghai Chemical Agent Ltd. China, and used without further purification. Organometallic dye cis-bis(isothiocyanato)bis(2,20 -bipyridyl-4,40 -dicarboxylato) ruthenium (II) [RuL2(NCS)2] was obtained from Solaronix SA (Switzerland), the other reagents came from Shanghai Chemical Agent Ltd. China. Conductive glass substrate (FTO glass, Fluorine doped tin oxide over-layer, sheet resistance 8 X cm2 from Hartford Glass Co., USA), was used as a substrate for precipitating TiO2 porous film, and was cut into 2 1.5 cm2 sheets. 2.2. Preparation of counter electrode A mixture of carbon black and platinum was prepared by reducing H2PtCl6 with a reducing agent (NaBH4) in carbon black. 300 mg of carbon black powder and 5 ml of isopropanol were added to twice distilled water in a beaker and ultrasonically stirred for 15 min. H2PtCl6 solution (7.72 mM) with a predetermined Pt/carbon black weight ratio was added and ultrasonically stirred for 30 min, followed by adding 0.05 mol/L of NaBH4 solution (NaBH4 was excess to reduce H2PtCl6 fully) and the ultrasonically stirring for 2 h. After filtering and washing for 3 times, the deposit substance was sintered at 250 °C for 1 h in the air. Thus, a mixture powder of Pt and carbon black was obtained. The mixture powder of Pt and carbon black was dispersed in a mixed solution with 2 ml distilled water and 2 ml ethanol. Then, the 30 mg of hydroxyethyl cellulose as an adhesive was dissolved in this dispersion to form a paste contained Pt and carbon black. The Pt/Carbon black counter electrode was prepared by coating the paste on FTO conductive glass sheet using doctor-blading technique. Then the electrode was sintered at 180 °C for 1 h and dried in vacuum oven at 120 °C overnight.
2.3. Preparation of TiO2 paste 0.05 mol of acetic acid was mixed with 0.05 mol of titanium iso-propoxide under stirring at room temperature. The mixed solution was rapidly poured into 120 ml distilled water with vigorous stirring for 30 min and a white precipitate was formed. Then, acetic acid (12 ml) and nitric acid solution (65 wt%, 1.2 ml) were added. Then the system was peptized at 80 °C for 12 h and was autoclaved at 200 °C for 12 h to form a white suspension with some precipitate. The resultant suspension was concentrated to 1/4 of its original volume, PEG-20000 (10 wt% TiO2) and a few drops of emulsification regent of Triton X-100 was added to the resultant colloidal solution with stirring. Then the colloidal solution was concentrated to form a TiO2 paste of suitable concentration. 2.4. Fabrication of DSSC A DSSC (active area of 0.25 cm2) was assembled according to the following procedure. Conducting glass sheet (FTO) was washed with ethanol and immersed in 50 mM TiCl4 aqueous solution for 12 h in order to make a good contact between the TiO2 layer and conducting glass substrate. A TiO2 electrode (TiO2 film thickness about 6 lm) was obtained by spreading the TiO2 paste on the conducting glass substrate using a ‘‘doctor blade method” and then sintered at 450 °C for 30 min in air. After cooling to 80 °C, the TiO2 electrode was dye-sensitized with an organometallic dye ([RuL2(NCS)2], 0.5 mM) absolute ethanol solution for 24 h at room temperature. Afterwards, the dye-sensitized TiO2 electrode was rinsed with absolute ethanol and dried in moisture-free air. A liquid electrolyte was prepared by blending 0.6 M tetrapropylammonium iodide, 0.1 M I2, 0.1 M KI, and 0.5 M TBP in acetonitrille solution. A dye-sensitized solar cell was assembled by dropping a drop of the liquid electrolyte on the dye-sensitized TiO2 porous film electrode. The Pt/Carbon black counter electrode was laid over. The two electrodes were clipped together and a cyanoacrylate adhesive was used as sealant to prevent the electrolyte solution from leaking. 2.5. Measurements The microstructure of sample was observed with a JEM2000EX transmission electron microscope (JEOL, Japan). The crystal structure of samples was investigated by Xray powder diffraction (XRD) on an X-ray diffractometer (D8 ADVANCE, Germany) with Cu Ka radiation. Cyclic voltammetry (CV) was carried out in a three electrode one compartment cell with a self-made Pt/Carbon working electrode, Pt foil counter electrode and an Ag/AgCl reference electrode dipped in an acetonitrile solution of 10 mM LiI, 1 mM I2, and 0.1 M LiClO4. CV performed using CHI660B electrochemical measurement system (sweep condition: 100 mV s1).
P. Li et al. / Solar Energy 83 (2009) 845–849
The photovoltaic test of dye-sensitized TiO2 nanocrystalline solar cells was carried out by measuring the J–V characteristic curves under irradiation of white light from a 100 W xenon arc lamp (XQ-500 W, Shanghai Photoelectricity Device Company, China) under ambient atmosphere. The incident light intensity and the active cell area was 100 mW cm2 and 0.25 cm2, respectively.
tive pair is assigned to the redox reaction Eq. (1) and the positive pair is assigned to redox reaction Eq. (2) Imoto et al., 2003; Popov and Geske, 1958; Huang et al., 2007. I 3 þ 2e ¼ 3I 3I2 þ 2e ¼ 2I 3
3.1. Morphology characterization of the Pt/Carbon black Counter electrode Fig. 1a shows the XRD pattern of the mixture of Pt and carbon black (Pt 1.5 wt%). The peak at about 26.6° corresponds to graphite (002) in carbon black. Graphite in carbon black is available for good electrical conductivity for the counter electrode. Several peaks at 39.4°, 46.0°, 67.4°, and 81.1° were also observed in the XRD pattern, which correspond to (111), (200), (220), and (311) of the face-centered cubic (fcc) lattice of platinum, respectively Schmid, 1992; Teranishi et al., 1999. The widening of the bandwidths is due to their small particle size. Fig. 1b is a TEM image of the mixture of Pt and carbon black (Pt 1.5 wt%). It can be observed that the mean size of platinum particles is 20–30 nm, and platinum nanoparticles were successfully supported and homogeneously dispersed on the carbon black.
3.3. Photovoltaic performance of DSSCs based on Pt/ Carbon black counter electrode Table 1 summarizes the photoelectric performance of the DSSCs using Pt/Carbon black counter electrodes with different platinum loadings. When a carbon black electrode without platinum is used as counter electrode, the open-circuit photovoltage (Voc) of the DSSC is 724 mV, the shortcircuit photocurrent density (Jsc) is 8.53 mA/cm2, and the overall energy conversion efficiency (g) is only 3.76%. Such low conversion efficiency is due to the lower catalytic activity of the carbon black for I/I3 redox couple. On the other hand, when platinum loading is 1.5 wt%, the Voc and Isc of the DSSC increase to 753 mV and 14.46 mA/ cm2, and the overall energy conversion efficiency (g) is up to 6.72%. It indicates that the catalytic activity of the counter electrode is improved when the platinum is filled in the carbon black. Furthermore, we observe no apparent difference in Voc, Isc, and g of the DSSCs based on Pt/Carbon black Counter electrodes with the platinum loading between 1.5 and 4.0 wt% (shown as Table 1). Therefore, the 1.5 wt% platinum loading on the carbon black is suffi-
3.2. Cyclic voltammograms for the Pt/Carbon counter electrode Fig. 2 compares cyclic voltammograms in I2/I system for Pt plate electrode, carbon black electrode and Pt/Carbon electrode at a scan rate of 100 mV s1. The oxidation/reduction peaks were observed in all cases. It indicates that three materials all have electrocatalytic activity for I2/I system. There were two pairs of redox waves for the Pt electrode and Pt/C electrode. The relative nega-
a
b
Intensity / a.u.
39.4
26.6
46.0
10
20
30
40
50
2θ /
60
70
81.1
80
ð1Þ ð2Þ
Fig. 2 also shows a higher current density of the redox peak for the Pt/C electrode and carbon black electrode than for the Pt electrode. It could be explained that the carbon black usually has large specific surface area, which increases effective active area of catalysis, so that the current density is improved. The high current density also suggests that the reaction rate was fast, in other word, the charge-transfer resistance (RCT) for the I3/I redox reaction was low on the carbon black electrode Imoto et al., 2003.
3. Results and discussion
67.4
847
90
o
Fig. 1. (a) X-ray diffraction pattern (b) TEM image for Pt and carbon black mixture (Pt 1.5 wt%).
P. Li et al. / Solar Energy 83 (2009) 845–849
Current density (mA/cm-2)
848 0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0
-0.2
-0.2
-0.4
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-0.6 (a) Pt electrode
-0.8 0.5
0.0
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(c) Pt/C electrode
(b) C electrode
0.5
-1.0
0.0
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Voltage (V)
Voltage (V)
0.5
0.0
-0.5
-0.8 -1.0
Voltage (V)
Fig. 2. Cyclic voltammograms for Pt plate electrode (a) carbon black electrode (b) and Pt/Carbon black electrode (c) in 10 mM LiI, 1.0 mM I2 acetonitrile solution containing 0.1 M LiClO4 as the supporting electrolyte. [I]/[I2] = 10/1.
Table 1 Photoelectric parameters of dye-sensitized solar cells based on Pt/Carbon black counter electrodes with different platinum loadings. Platinum loading (%)
Jsc (mA/cm2)
Voc (mV)
FF
g (%)
0 1.0 1.5 2.0 4.0 100 (Pt electrode)
8.53 13.59 14.46 14.57 14.73 14.86
724 737 753 748 742 723
0.601 0.614 0.616 0.612 0.611 0.617
3.76 6.17 6.72 6.67 6.68 6.63
16
Current density (mA/cm2)
14
a b
12 10 8
ter electrodes exhibited no significant difference in the overall energy conversion efficiencies (g), which are 6.72% and 6.63%. 4. Conclusions In summary, Pt/Carbon black counter electrode for dyesensitized solar cells (DSSCs) was prepared by reducing H2PtCl6 with NaBH4 in carbon black. Pt/Carbon black electrode showed high electrocatalytic activity for iodide/ triiodide redox reaction. Using Pt/Carbon black (platinum 1.5 wt%) as counter electrode, DSSC achieved 6.72% overall energy conversion efficiency under one sun illumination (AM1.5, Pin of 100 mW cm2). Pt/Carbon black electrode shows the same energy conversion efficiency and lower cost compared with Pt electrode, which make it available in DSSCs practical applications. Acknowledgements
6 4 2 0 0.0
0.2
0.4
0.6
0.8
Voltage (V)
Fig. 3. Photocurrent-voltage curves of DSSCs with (a) platinized counter electrode and (b) Pt/Carbon black counter electrode. Under one sun illumination (AM1.5, Pin of 100 mW cm2).
cient for catalyzing the iodide (I)/triiodide (I3) redox couple in the electrolyte system. Fig. 3 gives photocurrent–voltage curves of DSSCs using a Pt/Carbon black electrode (Pt, 1.5 wt%) and a platinized electrode as counter electrode. The platinized electrode was prepared by electroplating Hao et al., 2006. The Jsc, Voc, FF, and g of the DSSCs are summarized in Table 1. It can be seen that DSSCs with the different coun-
The authors thank for jointly supporting by the National Natural Science Foundation of China (No. 50572030, 50842027), the Nano Functional Materials Special Program of Fujian Province (No. 2005HZ01-4), the Key Project of Chinese Ministry of Education.(No. 206074) and Specialized Research Fund for the Doctoral Program of Chinese Higher Education (No. 20060385001). References Fang, X., Ma, T., Guan, G., et al., 2004. J. Electroanal. Chem. 570, 257. Gratzel, M., 2004. J. Photochem. Photobio. A 164, 3. Hao, S., Wu, J., Lin, J., et al., 2006. Compos. Interface 13, 899. Huang, Z., Liu, X., Li, K., et al., 2007. Electrochem. Commun. 9, 596. Imoto, K., Takatashi, K., Yamaguchi, T., et al., 2003. Sol. Energy Mater. Sol. Cells 79, 459. Kamau, G.N., 1988. Anal. Chim. Acta 207, 1. Kay, A., Gratzel, M., 1996. Sol. Energy Mater. Sol. Cells 44, 99. Kinoshita, K., 1987. Carbon: Electrochemical and Physicochemical Properties. Wiley Interscience Publications, New York, p. 226–379.
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