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Improvement of performance of dye-sensitized solar cells based on electrodeposited-platinum counter electrode
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Electrochimica Acta 53 (2008) 4161–4166
Improvement of performance of dye-sensitized solar cells based on electrodeposited-platinum counter electrode Pinjiang Li, Jihuai Wu ∗ , Jianming Lin, Miaoliang Huang, Zhan Lan, Qinghua Li The Key Laboratory for Functional Materials of Fujian Higher Education, Institute of Materials Physical Chemistry, Huaqiao University, Quanzhou 362021, China Received 10 December 2007; received in revised form 26 December 2007; accepted 29 December 2007 Available online 6 January 2008
Abstract Platinum nanoparticle was electrodeposited on FTO conducting glass substrate as counter electrode for application in dye-sensitized solar cells (DSSCs). Images of transmission electron microscope (TEM) and Scanning Electron Microscope (SEM) showed that platinum nanoparticle was with the mean size of 20–30 nm and was homogeneously distributed on the surface of the FTO conductive glass sheet. Using such a counter electrode, DSSC showed a 6.40% overall energy conversion efficiency under one sun illumination. It exhibited the same high-performance as the DSSC with a platinum counter electrode prepared by electroplating. Furthermore, the present preparation method for the platinum counter electrode has the advantage of low platinum loading and transparence. © 2008 Published by Elsevier Ltd. Keywords: Dye-sensitized solar cell; Counter electrode; Platinum nanoparticle; Electrodeposition
1. Introduction Dye-sensitized solar cells (DSSCs), which are the bases for energy conversion in the injection of electrons from a photoexcited state of the dye sensitizer into the conduction band of the TiO2 semiconductor upon absorption of light, have been attracting widespread scientific and technological interest and have evolved a potential alternative to traditional photovoltaic devices in the past decade because of their high efficiency for the conversion of solar energy to electric power and low-production cost [1–5]. Generally, a dye-sensitized solar cell consists of three main components: a dye-covered nanocrystalline TiO2 layer on a transparent conductive glass substrate, an iodide/triiodide redox couple in an organic solvent as an electrolyte, and a platinized conductive glass substrate as a counter electrode. The working principle of a DSSC can be summarized into the following five steps [6–10]: ∗
Corresponding author. Tel.: +86 595 22693899; fax: +86 595 22693999. E-mail address: [email protected] (J. Wu).
0013-4686/$ – see front matter © 2008 Published by Elsevier Ltd. doi:10.1016/j.electacta.2007.12.073
(1) Photoexcitation of dye molecules under the illumination to induce charge separation. (2) Charge (electron) injection into the conduction band of mesoporous TiO2 . (3) Charge passage through the external circuit via electronic load. (4) Reduction of dye to the ground state by the redox couple in the electrolyte, which is usually an organic solvent containing the iodide (I− )/triiodide (I3 − ) couple. (5) Redox couple reduction on the counter electrode by the charge coming from the external circuit, where the reaction is I3 − + 2e− = 3I− . The solar energy is converted to electric energy by the above photochemical circulation process. Counter electrodes, as one important component in DSSCs, are usually constructed of conducting glass substrates coated with platinum films due to its superior electrocatalytic activity for iodide (I− )/triiodide (I3 − ) redox couple. The roles of the counter electrode are to transfer electrons arriving from the external circuit back to the redox electrolyte and to catalyze the
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reduction of the redox couple in order to keep the low overvoltage to the minimum energy losses, where the platinum serves as an electrocatalyst [11–14]. Platinum is one of precious metals with expensive price. Conventional platinized counter electrodes of DSSCs were prepared by methods such as electrochemical deposition, sputtering and thermal decomposition [10,13–18]. But electrodes prepared by those methods had the higher platinum loadings which were not in accordance with the character of low costing of DSSCs. To reduce the fabrication cost of the counter electrodes, several other types of counter electrodes have been reported. Carbon materials based counter electrodes with sufficient conductivity have been used as an attractive low cost substitute for the platinum counter electrode [11,19,20]. And Saito et al. [21,22] used chemically polymerized poly(3,4-ethylenedioxythiophene) on a conductive glass as a counter electrode. However, the conversion efficiencies of the DSSCs based on carbon counter electrodes were lower than that based on the platinized counter electrode. In this paper, the platinum counter electrode for application in DSSCs was prepared by electrodepositing platinum nanoparticle on a FTO conductive glass sheet. The preparation method is simple and feasible. The prepared electrode has a character of transparence, low platinum loading and high catalytic performance. Using such a counter electrode, DSSC showed 6.40% overall energy conversion efficiency under one sun illumination (100 mW cm−2 ). 2. Experimental 2.1. Materials Titanium (IV) isopropoxide and 4-tert-butylpyrldlne (TBP) were purchased from Fluka and used as received. Chloroplatinic acid (H2 PtCl6 ), sodium citrate (NaH2 Cyt), dodecanethiol, tetrapropylammonium iodide, acetonitrile, potassium iodinate and iodine were all purchased from Shanghai Chemical Agent Ltd. China, and used without further purification. Organometallic dye cis-bis(isothiocyanato)bis(2,2 -bipyridyl4,4 -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, and sheet resistance 8 cm−2 were purchased 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 platinum counter electrode H2 PtCl6 solution (4 ml, 7.72 mM) was added to 120 ml of twice distilled water in a round-bottom flask. The solution was heated up to boiling and kept 10 min. Then, 6 ml of 1 wt.% NaH2 Cyt solution was added and the boiling was kept for 15 min (the quantity of H2 PtCl6 used was calculated to provide an excess of NaH2 Cyt). The color of the solution was observed to change from pale yellow to bright brown. After 15 min, the flask was cooled rapidly to room temperature by immersion in
water to stop the reaction. Thus, a platinum nanoparticle colloid was prepared. The conductive glass sheet was immersed in dodecanethiol ethanol solution (10−3 mol l−1 ) about 10 min. After then, the platinum nanoparticle on the surface of the conductive glass was prepared by electrodepositing with the conductive glass sheet as a cathode, a platinum plate as a anode and a constant voltage of 2 V in the platinum nanoparticle solution. The direct current stabilized voltage supply (HB1730SB3A) from Shanghai HONBA Power Source Co. was used. After electrodepositing, the conductive glass sheet was sintered at 450 ◦ C for 30 min and cooled to ambient temperature. Based on the above processes, a platinum counter electrode was prepared. 2.3. Preparation of TiO2 paste 0.05 mol of acetic acid was added to 0.05 mol of titanium isopropoxide under stirring at room temperature. The mixture was rapidly poured into 120 ml distilled water with vigorous stirring and a white precipitate was formed immediately. After a half hour stirring, acetic acid (12 ml) and nitric acid solution (65 wt.%, 1.2 ml) were added to the mixture. Then the mixture was heated to 80 ◦ C and peptized for 12 h. The resultant mixture 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 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 DSSCs A dye-sensitized nanocrystalline solar cell (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 mechanical contact between the following printed TiO2 layer and conducting glass substrate. The TiO2 electrode (TiO2 film thickness about 6 m) 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 0.5 mM organometallic dye solution in absolute ethanol for 24 h at room temperature. Afterwards, the dye-sensitized TiO2 electrode was rinsed with absolute ethanol and dried in moisture-free air. The liquid electrolyte contained 0.6 M tetrapropylammonium iodide, 0.1 M I2 , 0.1 M KI, 0.5 M TBP in the solution of acetonitrile. A dye-sensitized solar cell was assembled by dropping a drop of liquid electrolyte above the dye-sensitized TiO2 porous film electrode. A platinum counter electrode was placed above it. The two electrodes were clipped together and a cyanoacrylate adhesive was used as sealant to prevent the electrolyte solution from leaking.
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2.5. Measurements
corresponding formation of CO2 , according to the following scheme:
The adsorption curves of the solution samples were determined by using a UV-3100 UV–vis spectrophotometer (Shimadzu Corporation, Japan). The platinum nanoparticle colloid was observed with a JEM-2000EX transmission electron microscope (JEOL, Japan). The SEM micrographs were obtained with a JSM-6700F field emission Scanning Electron Microscope (JEOL, Japan). Cyclic voltammetry (CV) was carried out in a three electrode one compartment cell with a self-made Pt/Carbon black 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 s−1 ). The sheet resistances of the platinized counter electrodes were measured by a fourdot method (SZ-82 digital type four-probe instrument, Suzhou Baishen Technology Co., Ltd., China). The photovoltaic test of dye-sensitized TiO2 nanocrystalline solar cells was carried out by measuring the J–V character 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 cm−2 and 0.25 cm2 , respectively.
9[PtCl6 ]2− + 2H2 Cyt− + 10H2 O
3. Results and discussion 3.1. Formation of platinum nanoparticle The H2 PtCl6 solution before being reduced is pale yellow color and showed a absorption peak at 260 nm in its UV–vis spectrum due to the ligand-to-metal charge-transfer transition of the [PtCl6 ]2− ions [23], as shown in Fig. 1 (curve a). After reduction reaction, the absorption peak at 260 nm disappears (Fig. 1 curve b), indicating that the [PtCl6 ]2− ions are completely reduced. The formation of platinum nanoparticle results from the reduction of [PtCl6 ]2− by H2 Cyt− [24–26], with the
→ 9Pt0 + 12CO2 + 38H+ + 54Cl− The color of the solution turns from pale yellow into bright brown, and the absorption from the ultraviolet to the visible region increases, suggesting that the band structure of the platinum nanoparticle is formed. And the absence of any strong stabilizer (e.g. polymer) is the main advantage for this technique: the excess citrate or its reduction intermediate can be involved as the stabilizers [27,28]. Fig. 1 inserts a TEM image of the platinum nanoparticle colloid, in which the mean size of platinum nanoparticle prepared in this way is 20–30 nm. 3.2. SEM characterization of the platinum counter electrode The counter electrode prepared by platinum nanoparticle electrodepositing is optically transparent, showing a very low platinum loading. The surface morphologies of the platinum counter electrodes are shown in the SEM images illustrated in Fig. 2. As shown in Fig. 2a and b, the platinum nanoparticle mainly distribute in the gaps between SnO2 particles on the surface of FTO glass. The bare FTO glass has a rough scale-like surface structure due to the fluorine-doped SnO2 layer coating on the glass sheet, the structure is believed to be helpful for the adhesion of the platinum nanoparticle on the surface. As a result, platinum nanoparticle is anchored to the surface of FTO glass sheet. Compared with the platinum nanoparticle in the colloid (the insert in Fig. 1), the size of platinum particles have no remarkable change on the surface of FTO glass. Two platinized FTO counter electrodes were prepared by electroplating and thermal decomposition for comparison, respectively [13,18] (shown in Fig. 2c and d). The counter electrode prepared by electroplating is covered with a platinum light reflecting film on the surface of FTO conductive glass sheet and likes a mirror. In Fig. 2c, it is obvious that the FTO glass sheet is covered completely by platinum film (the SnO2 particles on the surface of FTO glass cannot been seen clearly in the SEM image). As shown in Fig. 2d, the platinum film of the electrode prepared by thermal decomposition has many surface damages and the size of platinum particles is uneven distribution. Furthermore, the platinum film was easy to be shed due to lack of adhesion to FTO substrate. It could pose a long-term stability risk. Compared with these images in Fig. 2, it can be seen that the amount of the platinum loading by platinum nanoparticle electrodepositing is much less than that by electroplating and thermal decomposition. 3.3. Cyclic voltammograms for the Pt counter electrode
Fig. 1. UV–vis spectra (a) H2 PtCl6 solution; (b) platinum nanoparticle colloid (inset: TEM image of platinum nanoparticle).
In the DSSC, electrons are injected from I− ions in electrolyte to photooxidized dye, and the produced I3 − are reduced on the counter electrode. Fig. 3 compares cyclic voltammo-
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Fig. 2. SEM images of platinum counter electrodes: (a) prepared by nanoparticle electrodepositing of 20 min; (b) prepared by nanoparticle electrodepositing of 40 min; (c) prepared by electroplating; (d) prepared by thermal decomposition.
grams in I2 /I− system for the platinum plate electrode and the electrodepositing platinum nanoparticle electrode at a scan rate of 50 mV s−1 . The oxidation/reduction peaks were observed in both cases. It indicates that two electrodes have good electrocatalytic activity for I2 /I− system. There were two pairs of redox waves for two electrodes. The relative negative pair is assigned to the redox reaction (Eq. (1)) and the positive pair is assigned to redox reaction (Eq. (2)) [20,29,30]. I3 − + 2e− = 3I−
(1)
3I2 + 2e− = 2I3 −
(2)
3.4. Photovoltaic performance of DSSCs based on platinum counter electrodes To investigate the effect of the platinum deposition amount of the counter electrode on the performance of the DSSCs, all the dye-sensitized TiO2 films used were prepared at one time, so that they could be considered to have the same microstructure. Table 1 shows the performance characteristics of the DSSCs
Fig. 3. Cyclic voltammograms for the platinum nanoparticle electrodes and the platinum plate electrode in 10 mM LiI, 1.0 mM I2 acetonitrile solution containing 0.1 M LiClO4 as the supporting electrolyte: [I− ]/[I2 ] = 10/1. (a) The platinum nanoparticle electrode; (b) the platinum plate electrode.
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Table 1 Performance characteristics of dye-sensitized solar cells based on platinum counter electrodes with different electrodepositing times Time of electrodepositing (min)
Jsc (mA cm−2 )
Voc (mV)
FF
η (%)
0 10 20 30 40
0.92 9.74 15.23 15.31 15.06
272 679 711 702 707
0.289 0.576 0.591 0.590 0.602
0.07 3.81 6.40 6.34 6.41
Fig. 5. Photocurrent–voltage curves of DSSCs with different platinum counter electrodes: (a) prepared by nanoparticle electrodepositing of 20 min; (b) prepared by electroplating; (c) prepared by thermal decomposition.
Fig. 4. Time dependence of platinum nanoparticle electrodepositing on overall light to electric energy conversion efficiency of DSSCs.
composed of counter electrodes with platinum nanoparticle of different electrodepositing time, and the electrodepositing timeoverall energy conversion efficiency of the DSSCs curve is shown in Fig. 4. The platinum deposition amount on the FTO conductive glass sheet increased with increasing electrodepositing time (comparison with the images of Fig. 2a and b). When a FTO conductive glass sheet without platinum nanoparticle was used as the counter electrode, the Voc was only 272 mV, the Jsc was 0.92 mA cm−2 , and the η achieved was only 0.07%, as shown in Table 1. Such low conversion efficiency is due to the lack of catalytic activity of the bare FTO conductive glass for the I− /I3 − redox couple. On the contrary, when platinum nanoparticle was electrodepositing on a FTO glass sheet after 20 min, the Voc and Isc of the DSSC were strikingly increased to 711 mV and 15.23 mA cm−2 respectively, and the overall energy conversion efficiency achieved was up to 6.40%. This
result means that the electron transfer rate at the electrolytecounter electrode interface is increased, due to the catalytic activity of the platinum nanoparticle electrodeposited on the surface of the FTO conductive glass sheet. It indicates that the catalytic activity of the platinum nanoparticle is a key factor for the performance of the DSSCs. On the other hand, we observed no apparent difference in Voc , Jsc and η of the DSSCs based on counter electrodes with the electrodepositing time between 20 and 40 min (shown as Fig. 4). The results obtained indicate that the electrodepositing time of platinum nanoparticle on FTO conductive glass has no significant influence on the performance of the DSSC for the electrodepositing time between 20 and 40 min. Therefore, the electrodepositing time of platinum nanoparticle as short as 20 min is sufficient for catalyzing the iodide (I− )/triiodide (I3 − ) redox couple in the electrolyte. Fig. 5 and Table 2 present the performance characteristics of DSSCs with different counter electrodes under one sun illumination condition (AM1.5, Pin of 100 mW cm−2 ). The data show that the performance properties of DSSCs with the platinum nanoparticle electrodeposition counter electrode and the electroplating platinum counter electrode are no significant differences. The results obtained indicate that the low loading of platinum nanoparticle on an FTO glass sheet has no significant influence on the performance of the DSSC. However, the photovoltaic performance of DSSC based on the platinum counter electrode prepared by thermal decomposition is lowest among three counter electrodes under the same conditions.
Table 2 Performance comparison of dye-sensitized solar cells based on different platinum counter electrodes Preparation methods of counter electrodes
Sheet resistance ( cm−2 )
Jsc (mA cm−2 )
Voc (mV)
FF
η (%)
Pt loada (g cm−2 )
Electrodepositing Electroplating Thermal decompositing
9.3 3.2 8.6
15.23 14.86 13.74
711 723 692
0.591 0.617 0.587
6.40 6.63 5.58
<10 >80 >30
a
The platinum loads on FTO glass sheets were estimated by weighing method.
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4. Conclusions A novel method to fabricating the platinum counter electrode was introduced for application in DSSCs. The platinum counter electrode was prepared by electrodepositing platinum nanoparticle on a FTO conductive glass sheet, and its application in DSSCs has been studied. The preparation method is simple and feasible. The present electrode had the characters of transparence, low platinum loading and high catalytic performance. Because light may be irradiated from both sides of the TiO2 electrode and the counter electrode, the transparent counter electrode could improve the light absorption of the solar cells. And low platinum loading of the counter electrode could reduce the production cost of DSSCs. Using such a counter electrode, DSSC showed 6.40% overall energy conversion efficiency under one sun illumination (AM1.5, Pin of 100 mW cm−2 ). Acknowledgements The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Nos. 50572030, 50372022), the Nano Functional Materials Special Program of Fujian Province (No. 2005HZ01-4) and the Key Science Technology Program by the Ministry of Education, China (No. 206074). References [1] B. O’Regan, M. Gr¨atzel, Nature 353 (1991) 737. [2] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Mueller, P. Liska, N. Vlachopoulos, M. Gr¨atzel, J. Am. Chem. Soc. 115 (1993) 6382. [3] M. Gr¨atzel, J. Photochem. Photobiol. A 164 (2004) 3. [4] Z. Xu, X. Zou, X. Zhou, B. Zhao, C. Wang, Y. Hamakawa, J. Appl. Phys. 75 (1994) 588. [5] M.K. Nazeeruddin, R. Humphry-Baker, P. Liska, M. Gr¨atzel, J. Phys. Chem. B 107 (2003) 8981.
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Electrochimica Acta 53 (2008) 4161–4166
Improvement of performance of dye-sensitized solar cells based on electrodeposited-platinum counter electrode Pinjiang Li, Jihuai Wu ∗ , Jianming Lin, Miaoliang Huang, Zhan Lan, Qinghua Li The Key Laboratory for Functional Materials of Fujian Higher Education, Institute of Materials Physical Chemistry, Huaqiao University, Quanzhou 362021, China Received 10 December 2007; received in revised form 26 December 2007; accepted 29 December 2007 Available online 6 January 2008
Abstract Platinum nanoparticle was electrodeposited on FTO conducting glass substrate as counter electrode for application in dye-sensitized solar cells (DSSCs). Images of transmission electron microscope (TEM) and Scanning Electron Microscope (SEM) showed that platinum nanoparticle was with the mean size of 20–30 nm and was homogeneously distributed on the surface of the FTO conductive glass sheet. Using such a counter electrode, DSSC showed a 6.40% overall energy conversion efficiency under one sun illumination. It exhibited the same high-performance as the DSSC with a platinum counter electrode prepared by electroplating. Furthermore, the present preparation method for the platinum counter electrode has the advantage of low platinum loading and transparence. © 2008 Published by Elsevier Ltd. Keywords: Dye-sensitized solar cell; Counter electrode; Platinum nanoparticle; Electrodeposition
1. Introduction Dye-sensitized solar cells (DSSCs), which are the bases for energy conversion in the injection of electrons from a photoexcited state of the dye sensitizer into the conduction band of the TiO2 semiconductor upon absorption of light, have been attracting widespread scientific and technological interest and have evolved a potential alternative to traditional photovoltaic devices in the past decade because of their high efficiency for the conversion of solar energy to electric power and low-production cost [1–5]. Generally, a dye-sensitized solar cell consists of three main components: a dye-covered nanocrystalline TiO2 layer on a transparent conductive glass substrate, an iodide/triiodide redox couple in an organic solvent as an electrolyte, and a platinized conductive glass substrate as a counter electrode. The working principle of a DSSC can be summarized into the following five steps [6–10]: ∗
Corresponding author. Tel.: +86 595 22693899; fax: +86 595 22693999. E-mail address: [email protected] (J. Wu).
0013-4686/$ – see front matter © 2008 Published by Elsevier Ltd. doi:10.1016/j.electacta.2007.12.073
(1) Photoexcitation of dye molecules under the illumination to induce charge separation. (2) Charge (electron) injection into the conduction band of mesoporous TiO2 . (3) Charge passage through the external circuit via electronic load. (4) Reduction of dye to the ground state by the redox couple in the electrolyte, which is usually an organic solvent containing the iodide (I− )/triiodide (I3 − ) couple. (5) Redox couple reduction on the counter electrode by the charge coming from the external circuit, where the reaction is I3 − + 2e− = 3I− . The solar energy is converted to electric energy by the above photochemical circulation process. Counter electrodes, as one important component in DSSCs, are usually constructed of conducting glass substrates coated with platinum films due to its superior electrocatalytic activity for iodide (I− )/triiodide (I3 − ) redox couple. The roles of the counter electrode are to transfer electrons arriving from the external circuit back to the redox electrolyte and to catalyze the
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reduction of the redox couple in order to keep the low overvoltage to the minimum energy losses, where the platinum serves as an electrocatalyst [11–14]. Platinum is one of precious metals with expensive price. Conventional platinized counter electrodes of DSSCs were prepared by methods such as electrochemical deposition, sputtering and thermal decomposition [10,13–18]. But electrodes prepared by those methods had the higher platinum loadings which were not in accordance with the character of low costing of DSSCs. To reduce the fabrication cost of the counter electrodes, several other types of counter electrodes have been reported. Carbon materials based counter electrodes with sufficient conductivity have been used as an attractive low cost substitute for the platinum counter electrode [11,19,20]. And Saito et al. [21,22] used chemically polymerized poly(3,4-ethylenedioxythiophene) on a conductive glass as a counter electrode. However, the conversion efficiencies of the DSSCs based on carbon counter electrodes were lower than that based on the platinized counter electrode. In this paper, the platinum counter electrode for application in DSSCs was prepared by electrodepositing platinum nanoparticle on a FTO conductive glass sheet. The preparation method is simple and feasible. The prepared electrode has a character of transparence, low platinum loading and high catalytic performance. Using such a counter electrode, DSSC showed 6.40% overall energy conversion efficiency under one sun illumination (100 mW cm−2 ). 2. Experimental 2.1. Materials Titanium (IV) isopropoxide and 4-tert-butylpyrldlne (TBP) were purchased from Fluka and used as received. Chloroplatinic acid (H2 PtCl6 ), sodium citrate (NaH2 Cyt), dodecanethiol, tetrapropylammonium iodide, acetonitrile, potassium iodinate and iodine were all purchased from Shanghai Chemical Agent Ltd. China, and used without further purification. Organometallic dye cis-bis(isothiocyanato)bis(2,2 -bipyridyl4,4 -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, and sheet resistance 8 cm−2 were purchased 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 platinum counter electrode H2 PtCl6 solution (4 ml, 7.72 mM) was added to 120 ml of twice distilled water in a round-bottom flask. The solution was heated up to boiling and kept 10 min. Then, 6 ml of 1 wt.% NaH2 Cyt solution was added and the boiling was kept for 15 min (the quantity of H2 PtCl6 used was calculated to provide an excess of NaH2 Cyt). The color of the solution was observed to change from pale yellow to bright brown. After 15 min, the flask was cooled rapidly to room temperature by immersion in
water to stop the reaction. Thus, a platinum nanoparticle colloid was prepared. The conductive glass sheet was immersed in dodecanethiol ethanol solution (10−3 mol l−1 ) about 10 min. After then, the platinum nanoparticle on the surface of the conductive glass was prepared by electrodepositing with the conductive glass sheet as a cathode, a platinum plate as a anode and a constant voltage of 2 V in the platinum nanoparticle solution. The direct current stabilized voltage supply (HB1730SB3A) from Shanghai HONBA Power Source Co. was used. After electrodepositing, the conductive glass sheet was sintered at 450 ◦ C for 30 min and cooled to ambient temperature. Based on the above processes, a platinum counter electrode was prepared. 2.3. Preparation of TiO2 paste 0.05 mol of acetic acid was added to 0.05 mol of titanium isopropoxide under stirring at room temperature. The mixture was rapidly poured into 120 ml distilled water with vigorous stirring and a white precipitate was formed immediately. After a half hour stirring, acetic acid (12 ml) and nitric acid solution (65 wt.%, 1.2 ml) were added to the mixture. Then the mixture was heated to 80 ◦ C and peptized for 12 h. The resultant mixture 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 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 DSSCs A dye-sensitized nanocrystalline solar cell (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 mechanical contact between the following printed TiO2 layer and conducting glass substrate. The TiO2 electrode (TiO2 film thickness about 6 m) 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 0.5 mM organometallic dye solution in absolute ethanol for 24 h at room temperature. Afterwards, the dye-sensitized TiO2 electrode was rinsed with absolute ethanol and dried in moisture-free air. The liquid electrolyte contained 0.6 M tetrapropylammonium iodide, 0.1 M I2 , 0.1 M KI, 0.5 M TBP in the solution of acetonitrile. A dye-sensitized solar cell was assembled by dropping a drop of liquid electrolyte above the dye-sensitized TiO2 porous film electrode. A platinum counter electrode was placed above it. The two electrodes were clipped together and a cyanoacrylate adhesive was used as sealant to prevent the electrolyte solution from leaking.
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2.5. Measurements
corresponding formation of CO2 , according to the following scheme:
The adsorption curves of the solution samples were determined by using a UV-3100 UV–vis spectrophotometer (Shimadzu Corporation, Japan). The platinum nanoparticle colloid was observed with a JEM-2000EX transmission electron microscope (JEOL, Japan). The SEM micrographs were obtained with a JSM-6700F field emission Scanning Electron Microscope (JEOL, Japan). Cyclic voltammetry (CV) was carried out in a three electrode one compartment cell with a self-made Pt/Carbon black 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 s−1 ). The sheet resistances of the platinized counter electrodes were measured by a fourdot method (SZ-82 digital type four-probe instrument, Suzhou Baishen Technology Co., Ltd., China). The photovoltaic test of dye-sensitized TiO2 nanocrystalline solar cells was carried out by measuring the J–V character 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 cm−2 and 0.25 cm2 , respectively.
9[PtCl6 ]2− + 2H2 Cyt− + 10H2 O
3. Results and discussion 3.1. Formation of platinum nanoparticle The H2 PtCl6 solution before being reduced is pale yellow color and showed a absorption peak at 260 nm in its UV–vis spectrum due to the ligand-to-metal charge-transfer transition of the [PtCl6 ]2− ions [23], as shown in Fig. 1 (curve a). After reduction reaction, the absorption peak at 260 nm disappears (Fig. 1 curve b), indicating that the [PtCl6 ]2− ions are completely reduced. The formation of platinum nanoparticle results from the reduction of [PtCl6 ]2− by H2 Cyt− [24–26], with the
→ 9Pt0 + 12CO2 + 38H+ + 54Cl− The color of the solution turns from pale yellow into bright brown, and the absorption from the ultraviolet to the visible region increases, suggesting that the band structure of the platinum nanoparticle is formed. And the absence of any strong stabilizer (e.g. polymer) is the main advantage for this technique: the excess citrate or its reduction intermediate can be involved as the stabilizers [27,28]. Fig. 1 inserts a TEM image of the platinum nanoparticle colloid, in which the mean size of platinum nanoparticle prepared in this way is 20–30 nm. 3.2. SEM characterization of the platinum counter electrode The counter electrode prepared by platinum nanoparticle electrodepositing is optically transparent, showing a very low platinum loading. The surface morphologies of the platinum counter electrodes are shown in the SEM images illustrated in Fig. 2. As shown in Fig. 2a and b, the platinum nanoparticle mainly distribute in the gaps between SnO2 particles on the surface of FTO glass. The bare FTO glass has a rough scale-like surface structure due to the fluorine-doped SnO2 layer coating on the glass sheet, the structure is believed to be helpful for the adhesion of the platinum nanoparticle on the surface. As a result, platinum nanoparticle is anchored to the surface of FTO glass sheet. Compared with the platinum nanoparticle in the colloid (the insert in Fig. 1), the size of platinum particles have no remarkable change on the surface of FTO glass. Two platinized FTO counter electrodes were prepared by electroplating and thermal decomposition for comparison, respectively [13,18] (shown in Fig. 2c and d). The counter electrode prepared by electroplating is covered with a platinum light reflecting film on the surface of FTO conductive glass sheet and likes a mirror. In Fig. 2c, it is obvious that the FTO glass sheet is covered completely by platinum film (the SnO2 particles on the surface of FTO glass cannot been seen clearly in the SEM image). As shown in Fig. 2d, the platinum film of the electrode prepared by thermal decomposition has many surface damages and the size of platinum particles is uneven distribution. Furthermore, the platinum film was easy to be shed due to lack of adhesion to FTO substrate. It could pose a long-term stability risk. Compared with these images in Fig. 2, it can be seen that the amount of the platinum loading by platinum nanoparticle electrodepositing is much less than that by electroplating and thermal decomposition. 3.3. Cyclic voltammograms for the Pt counter electrode
Fig. 1. UV–vis spectra (a) H2 PtCl6 solution; (b) platinum nanoparticle colloid (inset: TEM image of platinum nanoparticle).
In the DSSC, electrons are injected from I− ions in electrolyte to photooxidized dye, and the produced I3 − are reduced on the counter electrode. Fig. 3 compares cyclic voltammo-
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Fig. 2. SEM images of platinum counter electrodes: (a) prepared by nanoparticle electrodepositing of 20 min; (b) prepared by nanoparticle electrodepositing of 40 min; (c) prepared by electroplating; (d) prepared by thermal decomposition.
grams in I2 /I− system for the platinum plate electrode and the electrodepositing platinum nanoparticle electrode at a scan rate of 50 mV s−1 . The oxidation/reduction peaks were observed in both cases. It indicates that two electrodes have good electrocatalytic activity for I2 /I− system. There were two pairs of redox waves for two electrodes. The relative negative pair is assigned to the redox reaction (Eq. (1)) and the positive pair is assigned to redox reaction (Eq. (2)) [20,29,30]. I3 − + 2e− = 3I−
(1)
3I2 + 2e− = 2I3 −
(2)
3.4. Photovoltaic performance of DSSCs based on platinum counter electrodes To investigate the effect of the platinum deposition amount of the counter electrode on the performance of the DSSCs, all the dye-sensitized TiO2 films used were prepared at one time, so that they could be considered to have the same microstructure. Table 1 shows the performance characteristics of the DSSCs
Fig. 3. Cyclic voltammograms for the platinum nanoparticle electrodes and the platinum plate electrode in 10 mM LiI, 1.0 mM I2 acetonitrile solution containing 0.1 M LiClO4 as the supporting electrolyte: [I− ]/[I2 ] = 10/1. (a) The platinum nanoparticle electrode; (b) the platinum plate electrode.
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Table 1 Performance characteristics of dye-sensitized solar cells based on platinum counter electrodes with different electrodepositing times Time of electrodepositing (min)
Jsc (mA cm−2 )
Voc (mV)
FF
η (%)
0 10 20 30 40
0.92 9.74 15.23 15.31 15.06
272 679 711 702 707
0.289 0.576 0.591 0.590 0.602
0.07 3.81 6.40 6.34 6.41
Fig. 5. Photocurrent–voltage curves of DSSCs with different platinum counter electrodes: (a) prepared by nanoparticle electrodepositing of 20 min; (b) prepared by electroplating; (c) prepared by thermal decomposition.
Fig. 4. Time dependence of platinum nanoparticle electrodepositing on overall light to electric energy conversion efficiency of DSSCs.
composed of counter electrodes with platinum nanoparticle of different electrodepositing time, and the electrodepositing timeoverall energy conversion efficiency of the DSSCs curve is shown in Fig. 4. The platinum deposition amount on the FTO conductive glass sheet increased with increasing electrodepositing time (comparison with the images of Fig. 2a and b). When a FTO conductive glass sheet without platinum nanoparticle was used as the counter electrode, the Voc was only 272 mV, the Jsc was 0.92 mA cm−2 , and the η achieved was only 0.07%, as shown in Table 1. Such low conversion efficiency is due to the lack of catalytic activity of the bare FTO conductive glass for the I− /I3 − redox couple. On the contrary, when platinum nanoparticle was electrodepositing on a FTO glass sheet after 20 min, the Voc and Isc of the DSSC were strikingly increased to 711 mV and 15.23 mA cm−2 respectively, and the overall energy conversion efficiency achieved was up to 6.40%. This
result means that the electron transfer rate at the electrolytecounter electrode interface is increased, due to the catalytic activity of the platinum nanoparticle electrodeposited on the surface of the FTO conductive glass sheet. It indicates that the catalytic activity of the platinum nanoparticle is a key factor for the performance of the DSSCs. On the other hand, we observed no apparent difference in Voc , Jsc and η of the DSSCs based on counter electrodes with the electrodepositing time between 20 and 40 min (shown as Fig. 4). The results obtained indicate that the electrodepositing time of platinum nanoparticle on FTO conductive glass has no significant influence on the performance of the DSSC for the electrodepositing time between 20 and 40 min. Therefore, the electrodepositing time of platinum nanoparticle as short as 20 min is sufficient for catalyzing the iodide (I− )/triiodide (I3 − ) redox couple in the electrolyte. Fig. 5 and Table 2 present the performance characteristics of DSSCs with different counter electrodes under one sun illumination condition (AM1.5, Pin of 100 mW cm−2 ). The data show that the performance properties of DSSCs with the platinum nanoparticle electrodeposition counter electrode and the electroplating platinum counter electrode are no significant differences. The results obtained indicate that the low loading of platinum nanoparticle on an FTO glass sheet has no significant influence on the performance of the DSSC. However, the photovoltaic performance of DSSC based on the platinum counter electrode prepared by thermal decomposition is lowest among three counter electrodes under the same conditions.
Table 2 Performance comparison of dye-sensitized solar cells based on different platinum counter electrodes Preparation methods of counter electrodes
Sheet resistance ( cm−2 )
Jsc (mA cm−2 )
Voc (mV)
FF
η (%)
Pt loada (g cm−2 )
Electrodepositing Electroplating Thermal decompositing
9.3 3.2 8.6
15.23 14.86 13.74
711 723 692
0.591 0.617 0.587
6.40 6.63 5.58
<10 >80 >30
a
The platinum loads on FTO glass sheets were estimated by weighing method.
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4. Conclusions A novel method to fabricating the platinum counter electrode was introduced for application in DSSCs. The platinum counter electrode was prepared by electrodepositing platinum nanoparticle on a FTO conductive glass sheet, and its application in DSSCs has been studied. The preparation method is simple and feasible. The present electrode had the characters of transparence, low platinum loading and high catalytic performance. Because light may be irradiated from both sides of the TiO2 electrode and the counter electrode, the transparent counter electrode could improve the light absorption of the solar cells. And low platinum loading of the counter electrode could reduce the production cost of DSSCs. Using such a counter electrode, DSSC showed 6.40% overall energy conversion efficiency under one sun illumination (AM1.5, Pin of 100 mW cm−2 ). Acknowledgements The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Nos. 50572030, 50372022), the Nano Functional Materials Special Program of Fujian Province (No. 2005HZ01-4) and the Key Science Technology Program by the Ministry of Education, China (No. 206074). References [1] B. O’Regan, M. Gr¨atzel, Nature 353 (1991) 737. [2] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Mueller, P. Liska, N. Vlachopoulos, M. Gr¨atzel, J. Am. Chem. Soc. 115 (1993) 6382. [3] M. Gr¨atzel, J. Photochem. Photobiol. A 164 (2004) 3. [4] Z. Xu, X. Zou, X. Zhou, B. Zhao, C. Wang, Y. Hamakawa, J. Appl. Phys. 75 (1994) 588. [5] M.K. Nazeeruddin, R. Humphry-Baker, P. Liska, M. Gr¨atzel, J. Phys. Chem. B 107 (2003) 8981.
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