carbon black counter electrode for dye-sensitized solar cells

Electrochimica Acta 67 (2012) 113–118

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Low cost poly(3,4-ethylenedioxythiophene):polystyrenesulfonate/carbon black counter electrode for dye-sensitized solar cells Gentian Yue, Jihuai Wu ∗ , Yaoming Xiao, Jianming Lin, Miaoliang Huang Eng. Res. Center of Environment-Friendly Functional Materials, Ministry of Education, China; Institute of Material Physical Chemistry, Huaqiao University, Quanzhou 362021, China

a r t i c l e

i n f o

Article history: Received 25 October 2011 Received in revised form 20 December 2011 Accepted 4 February 2012 Available online 13 February 2012 Keywords: PEDOT:PSS Carbon Counter electrode Dye-sensitized solar cell

a b s t r a c t A poly(3,4-ethylenedioxythiophene):polystyrenesulfonate/carbon (PEDOT:PSS/C) counter electrode is prepare for using in dye-sensitized solar cells (DSSCs). The PEDOT:PSS/C counter electrode possesses good conductivity of 1.73 S/cm and low charge transfer resistance. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements indicate that the PEDOT:PSS/C electrode has higher catalytic activity than the conventional Pt electrode. Under the optimized conditions, the DSSC with PEDOT:PSS/C counter electrode achieves a high light-to-electric conversion efficiency of 7.01% under a simulated solar light irradiation with an intensity of 100 mW cm−2 . © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Harvesting energy directly from sunlight using photovoltaic technology is being increasingly recognized as an essential component of future global energy production. Dye-sensitized solar cells (DSSCs) have been widely recognized as a potential alternative to the conventional silicon solar cell for its low-cost and high conversion efficiency over 12% since Gratzel [1–4] had breakthrough progress on DSSC in 1991. A standard DSSC consists of a sandwich structure with a dyesensitized porous nanocrystalline TiO2 photoanode for absorbing visible light, an iodide/triiodide redox electrolyte, and a counter electrode which serves to collect electrons and catalyze I2 /I− redoxcoupled regeneration reaction in electrolyte. The counter electrode plays a crucial role in DSSC [4–6]. It is typically made of platinized F-doped SnO2 (FTO) conductive glass. However, platinum is expensive and the two current methods (sputtering and thermal decomposition) for preparing Pt counter electrodes are both high energy-consuming. Therefore, many functional materials have been studied to replace Pt as counter electrodes for more costeffective DSSCs, such as various carbon-based materials including in carbon black [7], activated carbon [8,9], graphite [9], carbon nanotubes [10,11], graphene [12] and fullerene (C60 ) [13], and other materials such as tungsten carbide (WC) [14], molybdenum carbide (MoC) [14], tungsten oxide (WO2 )[15], titanium nitride

∗ Corresponding author. Tel.: +86 595 22693899; fax: +86 595 22693999. E-mail address: [email protected] (J. Wu). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2012.02.009

[16], polyaniline (PANI) [17–19], and pyrrole [20]. These materials have low-cost, simple preparation and comparable catalytic activity with Pt for I− /I3 − , which have attracted widely interests and researches. Recently, conducting polymer poly(3,4-ethylenedioxythiophene: polystyrene sulfonate) (PEDOT:PSS) [21,22] attract more attention as promising candidates for a Pt counter electrode due to its excellent catalytic activities, low cost, high transparency, good conductivity, easy preparation and good environmental stability [22]. Here, the conductive polymer PEDOT:PSS is blended with graphite and carbon black to prepare a new of PEDOT:PSS/Carbon (PEDOT:PSS/C) counter electrode for DSSCs. The counter electrode has good conductivity, low cost, facile preparation, and good contact with substrate of FTO glass. It is expected that PEDOT:PSS/C counter electrode can replace the Pt counter electrode used in DSSCs. 2. Experimental 2.1. Materials Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, surface resistance (/sq) = 1 × 105 to 3 × 105 ) purchased from Shanghai Chunyuan Phytochemistry Co., Ltd., China. The organometallic sensitized dye N-719 [RuL2 (NCS)2 , L = 4,4 -dicarboxylate-2,2 -bipyridine] was from Solaronix SA (Switzerland). The anhydrous ethanol (ETOH), isopropanol, nitric acid (HNO3 ), acetic acid (HAc), dimethyl sulfoxide (DMSO),

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graphite, tetramethylammonium hydroxide (TMAOH), carbon black, polyvinylpyrrolidone (PVP), ethyl cellulose (EC), polyethylene glycols with average molecular weights 20,000 and 400 (PEG-20000 and PEG-400), OP emulsification agent (Triton X-100), tetrabutyltitanate [Ti(OBu)4 ] and titanium tetrachloride (TiCl4 ) were analytical purity grade and were purchased from Shanghai Chemical Agent Ltd, China. All reagents were used without further treatment before using. The conductive glass plate (FTO glass, fluorine doped tin oxide over-layer, sheet resistance of 8  cm−2 , purchased from Hartford Glass Co., USA) was used as a substrate for precipitation of the TiO2 porous film and was cut into 1 × 2 cm2 sheets. 2.2. Preparation of PEDOT:PSS/C electrode 30 ◦ C

for PEDOT and PSS were mixed in an ultrasonic bath at 30 min to form a PEDOT:PSS suspension solution. A polar solvent DMSO was added in the suspension with a PEDOT:PSS/DMSO volume ratio of 2/9 and stirred at room temperature for 6 h to produce an even solution. Then graphite powder, carbon black, PVP, EC, and PEG-400 were added the even solution in sequence at room temperature under stirring to obtain a PEDOT:PSS/C viscous sol. The viscous sol was coated on a FTO conductive glass, after the conductive glass was irradiated and vacuum annealed at 80 ◦ C, a PEDOT:PSS/C counter electrode with thickness of 3–4 ␮m was obtained. 2.3. Fabrication of dye-sensitized solar cell A TiO2 nanoporous film was prepared by the following procedure [23,24]. Tetrabutyltitanate (10 ml) was rapidly added to mixed solution of distilled water (100 ml) and equal volume ETOH, a white precipitate was formed immediately. The precipitate was filtered using a glass frit and washed with distilled water. Under vigorous stirring, the filter cake was added to aqueous solution (150 ml) containing 1 ml HNO3 and 10 ml HAC at 80 ◦ C, until the slurry became a translucent blue-white liquid. The blue-white liquid was autoclaved at 200 ◦ C for 12 h to form milky white slurry. The resultant slurry was concentrated down to 1/4 of its original volume, then PEG-20000 (10 wt.% slurry) and a few drops of the emulsification agent of Triton X-100 were added to form a TiO2 colloid. To increase the reflection of sunlight and enhance light absorption, the large particle (150–250 nm) TiO2 was prepared using organic alkali TMAOH as peptize and sol-hydrothermal method for 12 h in the solution of pH 13.6. Similar to the TiO2 nanocrystals preparation, the original volume was concentrated to 1/4, then PEG-20000 (10 wt.% slurry) and a few drops of emulsification regent of Triton X-100 was added to form a TiO2 colloid. To reduce the recombination of the electrons on the conductive glass with the electrolyte, a thin TiO2 blocking layer was deposited on the FTO glass substrate by immersing the glass in 0.15 M TiCl4 isopropanol solution for 12 h, followed by sintering at 450 ◦ C for 30 min [25,26]. Subsequently, two TiO2 layers with a particle size of 10–20 nm and 150 nm, thickness of 10 ␮m was coated on the blocking layer by using a “doctor blade method”, then sintering at 450 ◦ C for 30 min in air. A dye was loaded by immersing the TiO2 film in a 0.3 mM dye N719 ethanol solution for 24 h. Thus a dyesensitized TiO2 film anode was obtained. A dye-sensitized solar cell was fabricated by injecting a liquid electrolyte (0.05 M I2 , 0.1 M LiI, 0.6 M tetrabutylammonium iodide and 0.5 M TBP in acetonitrile) in the aperture between the dye-sensitized TiO2 electrode and the PEDOT:PSS/C counter electrode. The two electrodes were clipped together and a cyanoacrylate adhesive was used as sealant. Epoxy resin was used for further sealing the cell. The detailed fabrication procedure for the nanocrystalline TiO2 photoanodes and the

Fig. 1. A schematic diagram of the DSSC with PEDOT:PSS/C counter electrode and reflection layer.

assembly of DSSCs was described by us elsewhere [27,28]. The schematic diagram of the solar cell is showed in Fig. 1. 2.4. Measurements The conductivity of the PEDOT:PSS/C electrode was tested using an RTS-9 model, 4-point probes resistivity measurement system. Cyclic voltammetry (CV) of samples were measured in a three-electrode electrochemical cell with the electrochemical workstation (CHI660D, Shanghai Chenhua Device Company, China) using the PEDOT:PSS/C as the working electrode (2.2 × 2.7 cm2 ), a Pt foil as counter electrode and an Ag/AgCl (the concentration of KCl was 3 M) as reference electrode dipped in an acetonitrile solution of 10 mM LiI, 1 mM I2 , and 0.1 M LiClO4 (scan conditions: 40–200 mV s−1 ). The electrochemical impedance spectroscopy (EIS) was carried out using a CHI660D electrochemical measurement system at a constant temperature of 20 ◦ C with AC signal amplitude of 20 mV in the frequency range from 0.1 to 105 Hz at 0 V DC bias in the dark. A sandwich cell consisting of two identical electrodes (about 6 cm2 ), a spacer of 80 ␮m thick adhesive tape, and an electrolyte consisting of 0.60 M tetrabutyl ammonium iodide, 0.10 M LiI, 0.10 M I2 and 0.50 M 4-tert-butylpyridine in acetonitrile was used in the EIS measurements.The photovoltaic testing of the DSSCs was carried out by measuring photocurrent–photovoltage (J–V) characteristic curves under white light irradiation of a 100 mW cm−2 (AM 1.5) from a solar simulator and (XQ-500 W, Shanghai Photoelectricity Device Company, China) in an ambient atmosphere and using a computer controlled voltage current source-meter of the CHI660D electrochemical measurement system. The incident light intensity and the active cell area were 100 mW cm−2 and 0.5 cm2 , respectively. The fill factor (FF) and the overall light-to-electric energy conversion efficiency () of the solar cell were calculated according to the following equations [29]:  (%) = FF =

Vmax × Jmax VOC × JSC × FF × 100% = × 100% Pin Pin

Vmax × Jmax VOC × JSC

(1)

(2)

where JSC is the short-circuit current density (mA/cm2 ); VOC is the open-circuit voltage (V), Pin is the incident light power and Jmax (mA/cm2 ) and Vmax (V) are the current density and voltage at the point of maximum power output in the J–V curves, respectively. 3. Results and discussion 3.1. Electrochemical properties of PEDOT:PSS/C electrode Fig. 2 shows the CVs for the PEDOT:PSS/C and the Pt electrodes in I2 /I− system ([I2 ]/[I− ] = 1/10) at a scan rate of 50 mV s−1 . The more

G. Yue et al. / Electrochimica Acta 67 (2012) 113–118

PEDOT:PSS/C electrode

b

Pt electrode

0.4

0.4

Current density ( mA/cm2)

Current density ( mA/cm2)

a

115

0.0

-0.4

-0.8 -1.0

-0.5

0.0

0.5

0.0

-0.4

-0.8 -1.0

1.0

-0.5

0.0

0.5

1.0

Voltage (V)

Voltage (V)

Fig. 2. Cyclic voltammograms for the Pt electrode (a) and the PEDOT:PSS/C electrode (b) at a scan rate of 50 mV s−1 in a 10 mM LiI, 1 mM I2 acetonitrile solution containing 0.1 M LiClO4 as the supporting electrolyte.

negative pair is assigned to the reaction (3) and the more positive one is assigned to the reaction (4) [30]: I3 − + 2e− = 3I− 3I2 + 2e− = 2I3

(3)

−

(4)

Current density ( mA/cm

2

)

a

A

0.4 0.0 -0.4 A' -0.8

PEDOT:PSS/C electrode

-1.0

-0.5

0.0

Voltage (V)

0.5

3.2. Electrochemical impedance analysis EIS measurements for a symmetrical cell fabricated with two identical PEDOT:PSS/C electrodes were carried out, and the equivalent circuit diagram used to fit the impedance spectrum is shown in Fig. 5, where Rs is Ohmic serial resistance, Rct is charge-transfer resistance of single electrode, CPE is double layer capacitance, and W is diffusion impedance [16,33]. Based on the EIS, the values of Rct and Rs for the PEDOT:PSS/C electrode were obtained and shown in Table 1, and the Rct and Rs values for the Pt electrode are shown in Table 3. It is found that Rs has a small change (13.42–14.42 /cm2 ) with the temperature rising (40–140 ◦ C), and the Rs is little smaller than

Current density ( mA/cm2)

The PEDOT:PSS/C electrode shows a larger peak current density (Fig. 2(b), 0.37 mA/cm2 at 0 V and −0.58 mA/cm2 at 0.52 V) for the I3 − oxidation and reduction than the Pt electrode (Fig. 2(a), 0.20 mA/cm2 at 0.053 V and −0.42 mA/cm2 at 0.83 V), which suggests that the PEDOT:PSS/C electrode has lower resistance and higher conductivity than that of the Pt electrode under the same conditions [8,31,32]. The smaller lower resistance and higher conductivity facilitates the enhancement of DSSC performance [33]. Fig. 3(a) shows four successive CV circles for the PEDOT:PSS/C electrode. On successive scans, the peak positions and current densities hardly change. This indicates that the PEDOT:PSS/C is coated tightly on the FTO glass surface. Both redox peak currents show a good linear relationship with the cycle times, as shown in Fig. 3(b). It thus indicates that the PEDOT:PSS/C electrode is uniform and homogeneous [34]. Fig. 4(a) shows cyclic voltammograms of the I2 /I− system on the PEDOT:PSS/C electrode with different scan rates. It can be seen that the peak currents densities increase with the increase in scan rate. With the scan rate increase from 20, 50, 100, 120, to 150 mV s−1 , the cathodic peak current density increase from −045, −0.57, −0.62, −0.71 to −0.83 mA/cm2 , and anodic peak current density increase from0.30, 0.37, 0.43, 0.47 to 0.55 mA/cm2 ,

respectively. The cathodic peak potential gradually shifts to the negative direction and the corresponding anodic peak potential shifts to the positive direction with increasing scan rate. Fig. 4(b) illustrates a relationship between the cathodic and anodic peak currents and the square root of the scan rate. The good linear relationship with various scan rates indicates that this redox reaction is a diffusion limited reaction on the PEDOT:PSS/C electrode, which might be connected with transport of iodide species out of the PEDOT:PSS/C surface [35,36]. This phenomenon shows that the adsorption of iodide species affects the redox reaction on the PEDOT:PSS/C surface, and suggests that no specific interaction between I2 /I− redox couple and the PEDOT:PSS/C counter electrode as the Pt electrode [35].

1.0

b

0.4 0.0

A

,

A

-0.4 -0.8 0

1

2 3 Circle times

4

5

Fig. 3. (a) Four consecutive cyclic voltammograms of I2 /I− system for PEDOT:PSS/C electrode in an acetonitrile solution containing 0.1 M LiClO4 as the supporting electrolyte and 10 mM LiI, 1 mM I2 as the redox couple, and the Pt foil as working electrode and v = 50 mV s−1 ; (b) the relationship between the cycle times and the maximum redox peak currents for the PEDOT:PSS/C electrode.

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0.8

0.8 0.4

Current density ( mA/cm2)

Current density ( mA/cm2)

B 0.0 -1

20 mV.s

-0.4

-1

50 mV.s

-0.8

-1

B'

100 mV.s

-1

120 mV.s

-1.2

-1

150 mV.s

-1.6 -1.2 -0.8 -0.4

0.0

0.4

b

a

0.8

1.2

0.4 0.0

B B'

-0.4 -0.8 -1.2 0.10

0.15

0.20

0.25

0.30

1/2

0.35

0.40

-1 1/2

(Scan rate) / (Vs )

Voltage (V)

Fig. 4. (a) Cyclic voltammograms for the PEDOT:PSS/C electrode in acetonitrile solution of 0.1 M LiClO4 , 10 mM LiI, 1 mM I2 with different scan rates (from inner to outer: 20, 50, 100, 120, and 150 mV s−1 , respectively). (b) relationship between all the redox peak currents and scan rates.

Table 1 EIS parameters of the dummy cell assembled with the PEDOT:PSS/C electrodes. Temperature (◦ C)

Rs (/cm2 ) Rct (/cm2 )

40

60

80

100

120

140

13.92 ± 0.02 13.60 ± 0.02

14.42 ± 0.02 9.62 ± 0.02

13.75 ± 0.02 7.58 ± 0.02

13.75 ± 0.02 8.33 ± 0.02

13.42 ± 0.02 11.10 ± 0.02

14.09 ± 0.02 12.50 ± 0.02

Table 2 Influence of temperature on the conductivity of PEDOT:PSS/C electrode. Conductivity (S/cm)

Atmosphere Vacuum

Temperature (◦ C) 40

60

80

100

120

140

1.464 1.564

1.528 1.588

1.720 1.728

1.566 1.586

1.525 1.566

1.434 1.523

that of Pt electrode (14.76 /cm2 ). The Rct values decreases and then increases with temperature increases from 40 to 140 ◦ C, and has a minimum value (7.58 /cm2 ) at 80 ◦ C, which is a little higher than that of the Pt electrode (3.23 /cm2 ), indicating a good electrocatalytic performance as Pt electrode. It might be that the heat treatment improves the performance of the PEDOT:PSS/C electrode and the charge transfer resistance in the electrode/electrolyte interface for I3 − /I− redox reaction (Fig. 5).

Tafel polarization measurement is a powerful electrochemical characterization method. The symmetrical cells with the PEDOT:PSS/C and Pt electrodes for the Tafel measurement is similar to the one used in the EIS measurement, Fig. 6 shows the measured results. In the Tafel zone, the cathodic branch of the curve shows a large slope for PEDOT:PSS/C and the Pt electrodes, indicating a high exchange current density (J0 ) on the surface of PEDOT:PSS/C electrode. This means that PEDOT:PSS/C electrode has a similar effect as the Pt electrode on catalyze reducing triiodide to iodide. Besides, J0 is in inverse proportion to Rct according to the Eq. (5) [13,14]. With EIS results, J0 on PEDOT:PSS/C electrode surfaces has a higher current density at 80 ◦ C, which is in accordance with those obtained in Tafel-curve plots. J0 =

RT nFRct

(5)

where R is the gas constant, and Rct , T, n and F have their usual meaning.

Log j/log mA.cm-2

2 1 0 -1 -2 -0.6 Fig. 5. EIS of the dummy cells fabricated with two identical PEDOT:PSS/C electrodes at different annealed temperature in vacuum environment.

PEDOT:PSS electrode Pt electrode

-0.4

-0.2

0.0 U/V

0.2

0.4

0.6

Fig. 6. Tafel curves of symmetrical dummy cells fabricated with PEDOT:PSS/C and Pt electrodes.

G. Yue et al. / Electrochimica Acta 67 (2012) 113–118

117

Table 3 The electrochemical parameters of Pt and PEDOT:PSS/C electrodes and photovoltaic properties parameters of the DSSCs with Pt and PEDOT:PSS/C electrodes. Electrode

Rs (/cm2 )

Rct (/cm2 )

Conductivity (S/cm)

Voc (V)

Jsc (mA/cm2 )

FF

 (%)

Pt PEDOT:PSS/C

14.76 ± 0.02 13.75 ± 0.02

3.23 ± 0.02 7.58 ± 0.02

1.747 1.728

0.81 0.80

14.33 14.37

0.67 0.61

7.78 7.01

In addition, the Jlim values of the electrodes are of the same magnitude. This result indicates that there is a similar diffusion coefficient in the symmetrical dummy cell according to Eq. (6): D=

l J 2nFC lim

(6)

where D is the diffusion coefficient of the triiodide, l is the spacer thickness, C is the triiodide concentration. Combined with EIS results, the change tendency of Rct is in agreement with Tafel-polarization measurements on the whole. Briefly, the EIS and Tafel-polarization results well explain the photovoltaic performance of the DSSC with PEDOT:PSS/C electrode, and demonstrate that PEDOT:PSS/C electrode is potential alternative to the expensive Pt electrode for low-cost DSSCs. 3.3. Influence of temperature on the conductivity properties of PEDOT:PSS/C electrode The temperature and annealing environment has a major influence on the conductivity of the PEDOT:PSS/C electrode, as shown in Fig. 7 and Table 2. It can be seen the change trend of

conductivity (S/cm)

1.6

1.5

1.4

40

60

80 100 120 o Temperature/ C

140

Fig. 7. Influence of temperature and environment on the conductivity of PEDOT:PSS/C electrode.

2

Current density ( mA/cm )

12

4

DSC with Pt electrode 2 JSC = 14.33 mA/cm VOC = 0.81 V FF = 0.67 η = 7.78%

Light current

0 Dark current

-4 -8 0.0

DSC with PEDOT:PSS/carbon electrode 2 JSC = 14.36 mA/cm VOC = 0.80 V FF = 0.61 η = 7.01 %

0.2

0.4

0.6

4. Conclusions In conclusion, a novel poly(3,4-ethylenedioxythio(PEDOT:PSS/C) counter phene):polystyrenesulfonate/Carbon electrode is prepared for using in DSSCs. The PEDOT:PSS/C counter electrode possesses good conductivity of 1.73 S/cm, low resistance and higher catalytic activity for I3 − /I− redox reaction. Using the PEDOT:PSS/C as counter electrode, adding barrier layer and reflection layer, a DSSC is assembled. The DSSC achieve a high light-to-electric conversion efficiency of 7.01% under a simulated solar light irradiation with an intensity of 100 mW cm−2 . Although the efficiency of the DSSC with a PEDOT:PSS/C counter electrode is little lower than that of the DSSC with Pt counter electrode (7.78%), the excellent photoelectric properties, facile preparation procedure and inexpensive cost allow the PEDOT:PSS/C electrode to be a credible alternative used in DSSC.

16

8

3.4. Photovoltaic performance of the DSSC with PEDOT:PSS/C electrode The photocurrent–voltage curves (Fig. 8) of the DSSCs with barrier layer and reflection layer and different electrodes were measured under irradiation of 100 mW cm−2 . The photovoltaic parameters of the DSSCs such as short-circuit photocurrent density (Jsc ), open circuit voltage (Voc ), fill factor (FF) and the overall energy conversion efficiency () are listed in Table 3. Compared to the DSSC with Pt electrode, the DSSC with PEDOT:PSS/C electrode has similar photovoltaic parameters. The overall energy conversion efficiency of the DSSC with PEDOT:PSS/C electrode reaches 7.01%, which is decreased by 9.9% compared with the DSSC with Pt electrode (7.78%). The reason is that the FF of the DSSC with the PEDOT:PSS/C electrode is smaller than that of Pt electrode, which come from the electrochemical parameters for PEDOT:PSS/C electrode: Rct (7.58 /cm2 ), Rs (13.75 /cm2 ) and conductivity (1.728 S/cm), and for Pt electrode: Rct (3.23 /cm2 ), Rs (14.76 /cm2 ) and conductivity (1.747 S/cm).

vacuum atmosphere

1.7

conductivity, although these values are only approximation using the four-electrode method. The conductivities of the electrodes increase with the increase of anneal temperature until 80 ◦ C, and then decrease, where we have a maximum conductivity values of 1.728 S/cm. The heat treatment will remove some additive and impurities on the electrode, and the conductivity of the electrode increase. However higher temperature treatment will damage conductive component on the electrode, which results in the decrease of conductivity of the electrode, this is the reason that the conductivities of the electrodes increase with the increase of anneal temperature and then decrease. Using the four-electrode method, the Pt electrode average conductivity of 1.747 S/cm was obtained and shown in Table 3, which is very close to the value of the PEDOT:PSS/C electrode. The conductivities of the PEDOT:PSS/C electrode annealed in vacuum are higher than that annealed in atmosphere, which is mainly due to the effect of H2 O and O2 in atmosphere.

0.8

1.0

Voltage (V) Fig. 8. Photocurrent-voltage curves of the DSSCs with PEDOT:PSS/C and Pt counter electrodes under 100 mW cm−2 light irradiation.

Acknowledgments This work was supported by the National High Technology Research and Development Program of China (no. 2009AA03Z217),

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