SWCNT counter electrode for flexible dye-sensitized solar cells

Electrochimica Acta 56 (2011) 8545–8550

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Low temperature preparation of a high performance Pt/SWCNT counter electrode for flexible dye-sensitized solar cells Yaoming Xiao, Jihuai Wu ∗ , Gentian Yue, Jianmin Lin, Miaoliang Huang, Zhang Lan Engineering Research Center of Environment-Friendly Functional Materials, Ministry of Education, Institute of Materials Physical Chemistry, Huaqiao University, Quanzhou 362021, China

a r t i c l e

i n f o

Article history: Received 9 March 2011 Received in revised form 11 July 2011 Accepted 12 July 2011 Available online 23 July 2011 Keywords: Platinum Carbon nanotube Counter electrode Vacuum thermal decomposition Flexible dye-sensitized solar cells

a b s t r a c t A platinum/single-wall carbon nanotube (Pt/SWCNT) film was sprayed onto a flexible indium-doped tin oxide coated polyethylene naphthalate (ITO/PEN) substrate to form a counter electrode for use in a flexible dye-sensitized solar cell using a vacuum thermal decomposition method at low temperature (120 ◦ C). The obtained Pt/SWCNT electrode showed good chemical stability and light transmittance and had lower charge transfer resistance and higher electrocatalytic activity for the I3 − /I− redox reaction compared to the flexible Pt electrode or a commercial Pt/Ti electrode. The light-to-electric energy conversion efficiency of the flexible DSSC based on the Pt/SWCNT/ITO/PEN counter electrode and the TiO2 /Ti photoanode reached 5.96% under irradiation with a simulated solar light intensity of 100 mW cm−2 . The efficiency was increased by 25.74% compared to the flexible DSSC with an unmodified Pt counter electrode. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction Since the original fabrication of a dye-sensitized solar cell (DSSC) in 1991 by O’Regan and Gratzel, substantial research in this area of solar cell design has ensued because of the low associated cost and simple preparation procedure for DSSCs, leading to the production of a DSSC with a photoelectric conversion efficiency that is as high as 12% [1,2]. A typical DSSC consists of a dye-sensitized porous nanocrystalline TiO2 film electrode, a redox electrolyte, and a platinized counter electrode. The function of the counter electrode is to transfer electrons arriving from the external circuit back to the redox electrolyte and to catalyze the reduction of the triiodide ion [3,4]. Generally, DSSC design has focused on the fabrication of porous titania films on a rigid and conductive glass substrate; however, it has been shown that fabrication of a flexible DSSC is possible using substrates such as metal foils and polymers. The flexible DSSC has attracted wide interest due to several distinguishing features, including its light weight, high flexibility, impact resistance, and lower production cost [5,6]. Additionally, a flexible DSSC’s shape or surface can be designed and constructed using large-scale continuous production and rapid coating techniques, which further decreases their cost. The DSSC counter electrode is generally prepared by depositing a thin layer of platinum catalyst on a fluorine-doped tin oxide (FTO)

∗ Corresponding author. Tel.: +86 595 22693899; fax: +86 595 22692229. E-mail address: [email protected] (J. Wu). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.07.043

or indium tin oxide (ITO) substrate followed by heat treatment above 400 ◦ C. Although the platinum film has high conductivity and good catalytic activity for the reduction of the triiodide ion, problems associated with the dissolution of the platinum film in corrosive electrolyte solutions and the high-temperature heat treatment, which is costly and can restrict the choice of flexible substrate, clearly demonstrate the need for more stable and cost-effective counter-electrode materials [7–10]. Therefore, different kinds of carbonaceous materials, such as graphite, carbon black, activated carbon, hard carbon spheres, carbon nanotubes, fullerenes and graphene, have been studied as a cost-effective and stable counter electrode for catalyzing the triiodide reduction reaction in DSSCs [7,11–15]. In this paper, integrating the merits of carbon-based materials and platinum, we report a facile, low-temperature vacuum thermal decomposition method to prepare high performance platinum/single-wall carbon nanotube (Pt/SWCNT) counter electrodes on flexible ITO/PEN substrates. Based on the Pt/SWCNT electrode, a flexible DSSC with a lightto-electric energy conversion efficiency of 5.96% is achieved under irradiation with a simulated solar light intensity of 100 mW cm−2 . 2. Experimental 2.1. Materials H2 PtCl6 ·6H2 O, ethanol, isopropanol, n-butanol, iodine, lithium iodide, tetrabutyl ammonium iodide, 4-tert-butyl-pyridine (TBP), acetonitrile (AN), tetrabutyl titanate, nitric acid, PEG-20000 and

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Triton X-100 were purchased from the Shanghai Chemical Agent Ltd., China (analytical grade). Sensitized-dye N719 [cis-di(thiocyanato)-N,N -bis (2,2 -bipyridyl-4-carboxylic acid-4tetrabutylammonium carboxylate) ruthenium (II)] was purchased from Solaronix SA, Switzerland. Single-wall carbon nanotubes (SWCNT, purity: 90% CNTs, 90% SWCNTs, main range of external diameter < 2 nm, length 2–5 ␮m, special surface area 500–700 m2 g−1 ) were purchased from the Chengdo Organic Chemicals Co., Ltd., Chinese Academy of Sciences. The above reagents were used without further purification. The ITO coated polyethylene naphthalate film (ITO/PEN, 12  cm2 ) polymer substrate was purchased from PECCELL Technologies, Inc., Yokohama, Japan. Titanium foil (0.03 mm thickness, purchased from Baoji Yunjie Metal Production Co., Ltd., China) was washed with mild detergent, rinsed with distilled water and was then immersed in saturated oxalic acid solution for 10 min and rinsed in distilled water again prior to use.

the TiO2 film to absorb the dye adequately. Thus, a dye-sensitized TiO2 flexible film electrode was obtained.

2.5. Assemblage of flexible DSSC The flexible DSSC was assembled by injection of a redox-active electrolyte into the aperture between the TiO2 film electrode (anode) and the Pt/SWCNT counter electrode. The two electrodes were clipped together and a cyanoacrylate adhesive was used as a sealant to prevent the electrolyte solution from leaking. Epoxy resin was used to further seal the cell in order to measure the cell stability. The redox electrolyte consisted of 0.60 M tetrabutyl ammonium iodide, 0.10 M LiI, 0.10 M I2 , and 0.50 M 4-tert-butyl-pyridine in acetonitrile.

2.2. Purification of the polymer substrate

2.6. Characterization

The ITO/PEN substrate was dipped in a 50% ethanol solution for 24 h and then cleaned with 95% ethanol solution several times. The cleaned substrate was stored in absolute ethanol. The substrate was blow dried prior to use. The ITO/PEN substrate cannot be cleaned with a strong acid or base, which result in the destruction of the substrate surface [16].

The surface features of the flexible Pt/SWCNT counter electrode were observed using a scanning electron microscope (SEM, Hitachi S-4800, Japan). The cyclic voltammetry (CV) profiles of samples was measured in a three-electrode electrochemical cell with an Electrochemical Workstation (CHI660C, Shanghai Chenhua Device Company, China) using the Pt/SWCNT as the working electrode, a Pt-foil as counter electrode and an Ag/AgCl cell as reference electrode dipped in an acetonitrile solution of 10 mM LiI, 1 mM I2 and 0.10 M LiClO4 . The CV measurements were performed using a CHI660 C electrochemical measurement system (scan rates: 25–200 mV s−1 ). Electrochemical impedance spectroscopy (EIS) measurements were carried out using a CHI660 C electrochemical measurement system at a constant temperature of 20 ◦ C, with an AC signal amplitude of 20 mV in the frequency range from 0.1 to 105 Hz at zero DC bias, in the dark. A sandwich cell consisting of two identical electrodes (0.32 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 IPCE (incident monochromatic photon-to-current conversion efficiency) curves were measured with a Grating 1200 Groovers/MM monochromator (Shimadzu Corporation, Kyoto, Japan).

2.3. Preparation of flexible Pt/SWCNT counter electrodes A typical Pt/SWCNT counter electrode was prepared using the series of steps outlined below. A solution of H2 PtCl6 ·6H2 O was prepared by dissolution in isopropanol and n-butanol (volume ratio of 1:1) at a concentration of 0.48 wt.% [7]. The SWCNTs were added to the H2 PtCl6 ·6H2 O solution at 0.06 wt.%. The mixture was stirred for 6 h at room temperature and then sprayed onto the transparent ITO/PEN substrate to give an electrode with an active area of 0.80 × 0.40 cm2 . After blow-drying at room temperature, the substrate was heated at 120 ◦ C for 2 h in a vacuum drying oven (Suzhou Jiangdong Precision Instrument Co., Ltd., China). The Pt/SWCNT layer was thus deposited onto the ITO/PEN substrate to form a Pt/SWCNT counter electrode. For comparison, the Pt counter electrode without SWCNT was also prepared. A commercial flexible Pt/Ti counter electrode was purchased from PECCELL Technologies, Inc., Yokohama, Japan.

2.7. Photoelectrochemical measurements 2.4. Fabrication of flexible TiO2 film electrode Titanium foil was used as a substrate for the nanocrystalline TiO2 film to fabricate the flexible DSSC. Tetrabutyl titanate (20 mL) was rapidly added to distilled water (200 mL) leading to immediate formation of a white precipitate. The precipitate was filtered using a glass frit and washed three times with distilled water. The filter cake was added to an aqueous nitric acid solution (0.10 M, 200 mL) with vigorous stirring at 80 ◦ C until the slurry became a translucent, homogeneous, blue white solution. The resultant colloidal suspension was autoclaved at 200 ◦ C for 12 h to form a milky white slurry. The resultant slurry was concentrated to one-quarter of its original volume, then PEG-20000 (10 wt.% slurry) and a few drops of Triton X-100 emulsification regent were added to form a TiO2 colloid. The TiO2 colloid was coated on the Ti foil using a doctor-blading technique. The thickness of the TiO2 film was controlled by the thickness of the adhesive tape around the edge of the cleaned Ti foil [17–19]. After drying at room temperature, the TiO2 thin films were sintered at 450 ◦ C for 30 min in air to produce nanocrystalline TiO2 films. When the TiO2 electrode was cooled to 80 ◦ C, the TiO2 film was immersed in a 2.50 × 10−4 M solution of dye N719 in absolute ethanol for 24 h. This allowed sufficient time for

The photovoltaic performance test of the flexible DSSC was conducted by measuring the J–V character curves using a CHI660C electrochemical measurement system under irradiation with simulated solar light from a 100 W xenon arc lamp (XQ-500W, Shanghai Photoelectricity Device Company, China) under ambient conditions. The incident light intensity was 100 mW cm−2 (AM 1.5), and the active area of the flexible DSSC cell was 0.80 × 0.40 cm2 . The photovoltaic performance [i.e., fill factor (FF) and overall energy conversion efficiency ()] of the DSSC was calculated using the following equations [20]: FF =

Vmax × Jmax Voc × Jsc

 (%) =

Vmax × Jmax Voc × Jsc × FF × 100% = × 100% Pin Pin

(1)

(2)

where JSC is the short-circuit current density (mA cm−2 ), VOC is the open-circuit voltage (V), Pin is the incident light power, and Jmax (mA cm−2 ) and Vmax (V) are the current density and voltage in the J–V curves, respectively, at the point of maximum power output.

Y. Xiao et al. / Electrochimica Acta 56 (2011) 8545–8550

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Fig. 1. SEM images of (a) Pt electrode and (b) Pt/SWCNT electrode.

3. Results and discussion

1.5

3.1. Morphology and compositions of Pt/SWCNT counter electrode

1.0

A ox

3.2. Electrochemical properties of Pt/SWCNT counter electrode Fig. 2 shows cyclic voltammograms for Pt/Ti, Pt and Pt/SWCNT counter electrodes. The Pt and Pt/SWCNT counter electrodes show two pairs of oxidation and reduction peaks similar to those in the commercial Pt/Ti counter electrode, which suggests the obtained Pt and Pt/SWCNT counter electrodes have comparatively similar electrocatalytic activity for the I3 − /I− redox process. The oxidation and reduction pair (peaks Box and Bred ) on the right is attributed to the redox reaction of I3 − + 2e− → 3I− , which directly affects the DSSC performance; the redox pair (peaks Aox and Ared ) on the left results from the reaction of 3I2 + 2e− → 2I3 − , which has little effect on the DSSC performance [21,22]. The Pt/SWCNT counter electrode shows larger oxidation and reduction current density than either the Pt/Ti electrode or the Pt counter electrode, suggesting a fast rate of triiodide reduction. This indicates that the Pt/SWCNT counter electrode can be used as an efficient electrocatalyst counter electrode in DSSCs. Electrochemical impedance spectroscopy (EIS) was measured with the sandwich-type electrochemical cells (shown in Fig. 3a) comprising two identical electrodes, and the equivalent circuit diagram used to fit the impedance spectra is shown in Fig. 3b, where

0.5 -2

Current density ( mAcm )

Fig. 1a shows the SEM image of the Pt counter electrode, which was prepared using the same steps as for the preparation of the flexible Pt/SWCNT counter electrode. It is obvious that Pt particles have been dispersed onto the transparent ITO/PEN substrate with a size diameter of about 250–500 nm. When H2 PtCl6 was heated at 120 ◦ C for 2 h in a vacuum environment, the relevant chemical reactions are as follows: H2 PtCl6 ·6H2 O ↔ PtCl4 + 2HCl + 6H2 O; PtCl4 ↔ PtCl2 + Cl2 ; PtCl2 ↔ Pt + Cl2 . As we know, these three reversible reactions produce gaseous products, and the vacuum conditions promote the positive direction of reaction, which results in complete decomposition of H2 PtCl6 at low temperatures in a vacuum drying oven. Fig. 1b shows the SEM images of the Pt/SWCNT counter electrode. As shown, the Pt particles and SWCNTs have been produced on the flexible ITO/PEN substrates and the Pt particles are well connected by the SWCNTs. The surface of the electrode clearly shows that the Pt/SWCNT counter electrode has a large amount of void space, so the Pt/SWCNT counter electrode has good light transmittance. Based on comparing Fig. 1a and b, the dispersion of Pt particles on the Pt/SWCNT electrode is superior to the simple Pt electrode.

working electrode Pt/Ti Pt Pt/SWCNT B ox

0.0 -0.5 -1.0 -1.5 -2.0

A red

-2.5 -3.0

B red 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Potential (V vs Ag/AgCl) Fig. 2. Cyclic voltammograms for Pt/Ti, Pt and Pt/SWCNT electrodes at scan rate of 50 mV s−1 .

RS is the series resistance mainly related to the counter electrode, RCT (high-frequency semicircle) is the charge-transfer resistance, ZW (low-frequency arc) is the Nernst diffusion impedance of the electrolyte, and Cd is the capacitance of the electrical double layer [22,23]. The Nyquist plots were fit and the results are shown in the table accompanying Fig. 3. The charge transfer resistance (RCT ) of the Pt/SWCNT electrode was found to be 1.62 ± 0.04  cm2 , which is less than that of the Pt electrode (1.81 ± 0.05  cm2 ), indicating a higher electrocatalytic performance. The RCT of Pt/SWCNT electrode is higher than that of Pt/Ti electrode (1.40 ± 0.02  cm2 ) as well, but the series resistance (RS , 4.72 ± 0.03  cm2 ) of the Pt/SWCNT electrode is less than that of Pt/Ti electrode (4.83 ± 0.02  cm2 ). The Pt/Ti electrode, however, cannot be used as a counter electrode for the flexible DSSC in our system because the Pt/Ti counter electrode is light-proof. Fig. 4a shows 10 successive CV cycles of the Pt/SWCNT counter electrode. On consecutive scans, the peak positions and current densities hardly change. This indicates that the Pt particles and SWCNTs are tightly bound to the ITO/PEN surface. Both redox peak currents show a good linear relationship with the cycle time, as shown in Fig. 4b. This suggests that the Pt/SWCNT film has good chemical stability and is uniform [24]. Fig. 4c shows 200 successive CV cycles of the Pt/SWCNT counter electrode, showing similar results to the 10 scan experiment, indicating high stability for the Pt/SWCNT electrode. Fig. 5a shows CVs of the I3 − /I− system on the Pt/SWCNT counter electrode at different scan rates. This figure shows that the peak

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6.0 Rs (Ohm•cm 2)

RCT (Ohm•cm 2)

Pt/Ti4.83

± 0.02

1.40± 0.02

5.0

Pt4.84

± 0.04

1.81± 0.05

4.5

Pt/SWCNT4.72

± 0.03

1.62± 0.04

Counter electrode

working electrode Pt/Ti Pt Pt/SWCNT

4.0

(a)

2

-Z'' (Ohmcm )

5.5

Pt/Ti,

ITO

3.5

or Pt,

Electrolyte

or Pt/SWCNT

ITO

3.0 2.5

Electrochemical cell used for EIS measurements

2.0

(b)

2R

1.5 2R

Z C

1.0 Equivalent circuit

0.5 0.0 4

6

8

10

12

14

16

18

20

22

24

2

Z' (Ohm cm ) Fig. 3. Nyquist plots of the symmetrical Pt/Ti−Pt/Ti electrode, Pt−Pt electrode, or Pt/SWCNT−Pt/SWCNT electrode cell by assembling two identical electrodes on each side separated by a spacer filled with electrolyte. (a) Electrochemical cell, (b) equivalent circuit, RS : series resistance at the counter electrode, RCT : charge-transfer resistance, ZW : Nernst diffusion impedance, Cd : capacitance of electrical double layer.

0.0

a Aox -2

0.5

b

-0.4

Current density (mAcm )

-2

Current density (mAcm )

1.0

0.0 -0.5 Box

-1.0 Ared

-1.5 -2.0

Bred

Ared Bred Aox Box

-1.2 -1.6 -2.0 -2.4

-2.5 -3.0

-0.8

-2.8 0.0

0.2

0.4

0.6

0.8

1.0

1.2

0

1

2

3

Potential (V vs Ag/AgCl) 1.0

5

6

7

8

9

10

11

Circle times

c Aox

0.5 -2

Current density (mAcm )

4

0.0

Box

-0.5

Ared -1.0

Bred -1.5 -2.0 -2.5

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Potential (V vs Ag/AgCl) Fig. 4. (a) A total of 10 consecutive cyclic voltammograms for the I2 /I− system using the Pt/SWCNT electrode at a scan rate of 50 m Vs−1 , (b) the relationship between the cycle time and the redox peak current for Pt/SWCNT electrode, (c) 200 consecutive cyclic voltammograms for the I2 /I− system using the Pt/SWCNT electrode at a scan rate of 50 m Vs−1 .

Y. Xiao et al. / Electrochimica Acta 56 (2011) 8545–8550

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Table 1 The photovoltaic performance of DSSCs with different H2 PtCl6 ·6H2 O and SWCNT contents. H2 PtCl6 ·6H2 O (%)

SWCNT (%)

Light transmittancea (%)

JSC (mA cm−2 )

VOC (V)

FF

 (%)

Vred b (V)

0.24 0.48 0.72 0.48 0.48 0.48

0 0 0 0.03 0.06 0.12

87 83 72 81 80 74

7.29 9.42 5.86 9.61 11.20 8.53

0.74 0.74 0.75 0.75 0.75 0.75

0.56 0.68 0.67 0.68 0.71 0.68

3.00 4.71 2.91 4.88 5.96 4.33

0.98 0.99 0.99 0.99 1.00 0.99

Light transmittance was measured by Radiometer (FZ-A, Photoelectric Inst. of Beijing Normal Univ., China). Vred is I− /I3 − redox potential against Ag/AgCl based on the cyclic voltammetry (CV) measurement.

a

3

Aox scan rate 200 100 50 25

-2

Current density ( mA cm )

2 1

Box 0 -1 -2 -3

Ared

-4

1 0

Ared Bred Box Aox

-1 -2 -3 -4

-5 -6

b

2 -2

3

Current density (mAcm )

a b

Bred -5 0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.15

0.20

0.25

0.30 1/2

Potential (V vs Ag/AgCl)

(Scan rate)

0.35

0.40

0.45

-1 1/2

[(V s

)

]

Fig. 5. (a) Cyclic voltammograms for the Pt/SWCNT electrode at different scan rates (from inner to outer: 25, 50, 100 and 200 mV s−1 , respectively), (b) relationship between all the redox peak currents and scan rates.

70 counter electrode Pt/SWCNT

60

Pt

50

3.3. Influence of H2 PtCl6 ·6H2 O and SWCNT content on the photovoltaic properties

40

IPCE (%)

electrode surface [25,26]. This phenomenon shows that the adsorption of iodide species is little affected by the redox reaction on the Pt/SWCNT counter electrode surface, suggesting no specific interaction between I3 − /I− redox couple and Pt/SWCNT counter electrode as is likewise the case for the Pt-ITO/PEN electrode [25].

30

20

10

0

-10 300

400

500

600

700

800

Wavelength (nm) Fig. 6. IPCE curves for the flexible DSSCs based on Pt/SWCNT and Pt counter electrodes.

current densities change with the scan rate. The cathodic peaks gradually and regularly shift negatively, and the corresponding anodic peaks shift positively with increasing scan rate. Fig. 5b illustrates the relationship between the cathodic and anodic peak currents and the square root of the scan rate. The linear relationship at various scan rates indicates that this redox reaction is diffusion limited at the Pt/SWCNT counter electrode, which may be a result of transport of iodide species off of the Pt/SWCNT counter

The photovoltaic performance of flexible DSSCs with different H2 PtCl6 ·6H2 O and SWCNT content were measured and the results are summarized in Table 1 along with the values of the I− /I3 − redox potential (Vred ) versus Ag/AgCl based on the CV data. The Vred of the counter electrode with SWCNTs reaches 0.99 V, which is higher than that of the counter electrode without SWCNTs (0.98 V). Comparing the cells with SWCNT and the cells without SWCNT, the VOC for the former are higher than those for the latter, likely due to the larger values of the short-circuit current for the SWCNT-containing cells. Fig. 6 shows the IPCE curves of the flexible DSSCs with Pt/SWCNT and Pt counter electrodes. It is noticeable that the IPCE curves for the flexible DSSCs using the Pt/SWCNT counter electrode from 400 nm to 680 nm is higher than that for the Pt counter electrode, resulting in a difference in JSC . Comparing the cells with different H2 PtCl6 ·6H2 O and SWCNT content, the JSC and  values increase initially and then decrease with increasing H2 PtCl6 ·6H2 O or SWCNT content. This trend is due to the effect of scattering, electrocatalytic activity and light transmittance of the counter electrode. In other words, the proper amount of Pt particles and SWCNTs can enhance the scattering effect to increase the absorption and utilization of light, resulting in higher JSC . With an increase of H2 PtCl6 ·6H2 O or SWCNT on the counter electrode, the counter electrode becomes covered with more Pt particles and SWCNTs, resulting in a large

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I3 − /I− system revealed that the Pt/SWCNT counter electrode had a lower charge-transfer resistance and higher electrocatalytic activity for the I3 − /I− redox reaction than the Pt counter electrode alone, and they showed good chemical stability and good light transmittance. After optimization with 0.48% H2 PtCl6 ·6H2 O and 0.06% SWCNT content, the light-to-electric energy conversion efficiency of the flexible DSSC based on the Pt/SWCNT counter electrode reached 5.96% under irradiation with a simulated solar light intensity of 100 mW cm−2 . The efficiency was increased by 25.74% compared to the flexible DSSC with a Pt counter electrode. The low charge-transfer resistance, high electrocatalytic activity for the I3 − /I− redox reaction and facile preparation procedure at low temperature may allow Pt/SWCNT assemblies to be used in flexible DSSCs and in other fields.

12

counter electrode Pt/SWCNT ------- Pt

-2

Current density ( mA.cm )

10 8

JSC=9.42 mA.cm-2 6

VOC =0.74V

4

ff=0.68 η=4.74%

2

JSC =11.20 mA.cm-2 VOC =0.75V ff=0.71 η=5.96%

Light

0 -2 -4 0.0

Dark

Acknowledgments 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Photo voltage (V) Fig. 7. J–V curves for the flexible DSSCs based on Pt/SWCNT and Pt counter electrodes.

active surface area and excellent stability in the liquid electrolyte. It can also increase the rate of the I3 − /I− redox reaction at the Pt counter electrode [25,31], thereby enhancing the electrocatalytic activity of the counter electrode. However, the light transmittance decreases with the increase of Pt particles and SWCNTs, which results in a decrease in the incident light harvested and of the photocurrent. The higher FF for the flexible DSSCs based on Pt/SWCNT as compared with those fabricated using the Pt counter electrodes is due to the lower charge-transfer resistance at the interface between the electrolyte and the electrode for the I3 − /I− redox reaction, which is favorable for electron transport [27–30]. 3.4. Photovoltaic performance of DSSC with Pt/SWCNT counter electrode Based on the optimal preparation conditions of the Pt/SWCNT counter electrode, the flexible DSSCs were assembled using Pt/SWCNT (SWCNT content of 0.06 wt%) and Pt as counter electrodes, respectively. The photocurrent-voltage curves (Fig. 7) of the flexible DSSCs were measured under irradiation with a simulated solar light intensity of 100 mW cm−2 . The DSSC using Pt as the counter electrode had JSC of 9.42 mA cm−2 , VOC of 0.74 V, FF of 0.68, and  of 4.74%, whereas the DSSC using Pt/SWCNT as the counter electrode achieved JSC of 11.20 mA cm−2 , VOC of 0.75 V, FF of 0.71, and  of 5.96%. The light-to-electric energy conversion efficiency for the flexible DSSC using Pt/SWCNT as counter electrode is increased by 25.74% compared to the flexible DSSC using Pt as the counter electrode. 4. Conclusion In summary, a Pt/SWCNT film was sprayed on ITO/PEN substrates to form a flexible counter electrode using a vacuum thermal decomposition method at 120 ◦ C. Cyclic voltammograms of the

The authors thank for the joint support by the National High Technology Research and Development Program of China (No. 2009AA03Z217) and the National Natural Science Foundation of China (Nos. 90922028, 51002053). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]

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