Flexible counter electrodes based on metal sheet and polymer film for dye-sensitized solar cells

Thin Solid Films 472 (2005) 242 – 245 www.elsevier.com/locate/tsf

Flexible counter electrodes based on metal sheet and polymer film for dye-sensitized solar cells Xiaoming Fanga,b, Tingli Maa,*, Morito Akiyamaa, Guoqing Guana, Shuji Tsunematsua, Eiichi Abea a

Institute for Structural and Engineering Materials (AIST Kyushu), National Institute of Advanced, Industrial Science and Technology (AIST), Tosu, Saga 841-0052, Japan b College of Chemical Engineering, South China University of Technology, Guangzhou 510640, China Received 17 December 2003; received in revised form 15 July 2004; accepted 15 July 2004 Available online 30 October 2004

Abstract Several types of counter electrodes based on flexible metal and plastic substrates for dye-sensitized solar cells (DSCs) have been investigated. The DSCs composed of the counter electrodes based on a stainless steel substrate, a nickel sheet and a conducting plastic film, have obtained conversion efficiencies comparable to that based on the conducting glass. The counter electrode based on the stainless steel substrate has the merit of improving the fill factor and conversion efficiency of the DSC by reducing its internal resistance. The counter electrode based on a polyester film can be aimed at lower cost markets. These counter electrodes based on the metal and plastic substrates are applicable for flexible DSCs. D 2004 Elsevier B.V. All rights reserved. Keywords: Solar cell; Metals; Titanium oxide; Resistivity

1. Introduction Dye-sensitized solar cells (DSCs) are an interesting and promising alternative to conventional silicon solar cells, and have achieved certified conversion efficiencies of 10.4% and lifetime expectancies of at least 10 years for outdoor use in laboratories [1–5]. A DSC consists of a dye-sensitized nanocrystalline TiO2 photoanode, an electrolyte solution containing the iodide /triiodide couple and a counter electrode. The roles of the counter electrode are to transfer electrons arriving from the external circuit back to the redox electrolyte and to catalyze the reduction of the redox couple [6,7]. Counter electrodes widely used in DSCs are constructed of conducting glass substrates coated with Pt films, where the Pt serves as a catalyst. It is estimated that the conducting glass sheet is the most expensive part of a

* Corresponding author. Tel.: +81 942 81 3640; fax: +81 942 81 3690. E-mail address: [email protected] (T. Ma). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.07.083

DSC (about 60% of the total cost). In addition, glass has fragility and shape limitations. Therefore, in order to reduce the cost of DSCs and broaden their applications, it is necessary to develop counter electrodes based on substrates other than the conducting glass. Metal sheets or foils are flexible substrates with excellent electrical and thermal conductivities. The application of an inexpensive metal material, such as stainless steel, to the substrate of a solar cell not only can reduce the fabrication cost of the cell, but also contributes to reducing its internal resistance and provide it with a good heat radiation property [8]. The proven solid-state photovoltaic devices (such as CdTe solar cells, a-Si solar cells, etc.) based on lightweight and flexible metal substrates have been of considerable technological interests since the mid-1990s [9,10]. However, there has not been a report on the application of a metal sheet or foil as the substrate of a counter electrode for a DSC. Plastics substrates have advantages in terms of flexibility, weight, handling and transport. In recent years, there has been an increasing interest in flexible DSCs based on the

X. Fang et al. / Thin Solid Films 472 (2005) 242–245

introduction of plastic films as substrates instead of glass, and several attempts have been made to manufacture a flexible photoanode by depositing a nanostructured TiO2 layer on a plastic substrate [11–14]. However, less attention has been paid for flexible counter electrodes based on plastic films. In this paper, we report four types of counter electrodes based on a stainless steel sheet, a nickel sheet, a polyester film, and a conducting plastic film. The corrosion stabilities of the four substrates in the electrolyte solution that is generally used in DSCs have been examined. The performances of the DSCs composed of the counter electrodes based on the four substrates in comparison with that based on a conducting glass have been investigated.

2. Experimental section The stainless steel sheet (SUS304, The Nilaco), the nickel sheet (The Nilaco), the polyester film (OHP, Sakurai), and the polyethylene naphthalate film coated with tin-doped indium oxide (ITO-PEN, Tobi) were used as the substrates of the counter electrodes for the DSCs, and their properties are summarized in Table 1. The corrosion stabilities of the four substrates in an electrolyte solution were tested as follows: Small pieces of the four substrates (2020 mm) were separately immersed in 10 ml of the electrolyte solution at room temperature for 3 months, and then the four pieces were removed for observations. The concentrations of Fe, Cr and Ni in the electrolyte solution that the stainless steel sheet had been immersed in were measured using a polarized zeeman atomic absorption spectrophotometer (Z-6100, Hitachi). The Ni concentration in the electrolyte solution that the nickel sheet had been immersed in was also measured. The detection limits of the AAS for Fe, Cr, and Ni are 0.004, 0.002, and 0.005, respectively. The electrolyte solution was composed of 0.1 M LiI, 0.1 M I2, 0.6 M 1,2-dimethyl-3-propylimidazolium iodine, and 0.5 M tert-butylpridine in methoxypropionitrile. Platinized counter electrodes were fabricated by depositing Pt particles onto the four substrates using a sputtering instrument (PMC-5000 Plasma multi coater); A platinized

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glass counter electrode based on an FTO glass sheet (Asahi Glass, fluorine-doped tin dioxide over layer, FTO) was fabricated under the same conditions for comparison with the fabricated counter electrodes based on the metal and plastic substrates. The thickness of the Pt film coated on the all substrates was ca. 10 nm, which was calculated by the deposition time and the deposition rate of the plasma multi coater. The sheet resistances (R s) of the substrates and the platinized counter electrodes were measured by the four-dot method (HL5500 PC Hall effect measurement system), and the results are shown in Table 1. Porous TiO2 films were prepared by coating the FTO glass using a screen-printing technique with a viscous paste of TiO2 powder (P25, Nippon Aerosil) according to the procedure described in our previous paper [15]. The TiO2 films of 45, 1010, 1515 and 2020 mm size were fabricated on one piece of the FTO glass at one time. The thicknesses of the obtained TiO2 films were determined to be approximately 19 Am. After drying at 50–60 8C for 15 min on a hot plate, the TiO2 films were sintered at 450 8C for 30 min. The annealed films were impregnated with a 0.05 M titanium tetrachloride solution in a hermetic glass bottle at 70 8C for 30 min, followed by a firing at 500 8C for 30 min. The obtained TiO2 films were immersed in an ethanol solution of cis-bis(isothiocyanato)bis(2,2V-bipyridyl-4,4V-dicarboxylato)-ruthenium(II)-bis-tetrabutylammonium (N719 dye, 510 4 M) for 12 h at 25 8C. Current–voltage curves of the DSCs were obtained by scanning a bias voltage while measuring the photocurrents under white light irradiation (1000 W/m2) using a sandwich solar cell constructed of the dye-sensitized TiO2 electrode and the platinized counter electrode. The Pt electrode was placed over the dye-coated electrode. The edges of the cell were sealed with the 50-Am-thick polymer film (Mitsuidenpo) that also served as a spacer. The electrolyte solution was introduced into the cell through one of two small holes drilled in the counter electrode. The edges of the cell were further sealed with Amosil 4 (Solaronix SA). The composition of the electrolyte solution was the same as that described above.

3. Results and discussion Table 1 Sheet resistances of the several substrates and the platinized counter electrodes Substrates

Stainless steel

Ni

OHP

ITO-PEN

FTO

Substrates thickness (Am) Sheet resistances of substrates (V/sq) Sheet resistances of CE (V/sq)

200

200

100

125

1100

3.810

3

3.510

4

1017

16.7

3.710

3

4.610

4

60.8

15.8

10

8.8

In order to examine the feasibility of applying the stainless steel sheet, the nickel sheet, the polyester polymer film and the ITO-PEN film to the substrates of the counter electrodes for the DSCs, we investigated the corrosion stabilities of the four substrates in the electrolyte solution. The immersion test results of the stainless steel sheet and the nickel sheet in the electrolyte solution are shown in Table 2. Because the main chemical composition of the stainless steel sheet is Fe, Cr and Ni, we measured the concentrations of Fe, Cr and Ni in the electrolyte solution, respectively. The measurement results showed that the electrolyte solution in which the stainless steel sheet had been immersed in for 3

244

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Table 2 The metal concentrations determined using atomic absorption spectrophotometer after immersion in the electrolyte solution for 3 months Substrates

Stainless steel

Chemical composition

Fe (N66 %)

Cr (ca. 19%)

Ni (ca. 9%)

Nickel Ni (99.7%)

Concentration. in the electrolyte solution (mg/l )

b0.004

b0.002

b0.005

36.8

months did not contain Fe, Cr or Ni, which suggested that the stainless steel sheet exhibited good stability in the electrolyte solution. Therefore, it is considered that the stainless steel sheet can be applied to the substrate of the counter electrodes for the DSCs. The Ni concentration of the electrolyte solution that the nickel sheet had been immersed in for 3 months was 36.8 mg/l. This indicated that the nickel sheet could slowly dissolve in the electrolyte solution. The weight loss of the nickel sheet was 0.092 mg/cm2. After being immersed in the electrolyte solution for 3 months, the polyester film and the ITO-PEN film did not show any change, indicating that they were stable in the electrolyte solution. It is known that the polyester films have an excellent balance of properties, such as mechanical strength, electrical properties, high and low temperature resistance and chemical resistance [16]. The ITO-PEN films possess better characteristics, including strength, electrical properties, and chemical resistance, etc., compared to the poly (ethylene terephthalate) films coated with tin-doped indium oxide, which films have been used to fabricate dyesensitized TiO2 electrodes. Therefore, the polyester film and the ITO-PEN film can be used as the substrates of the counter electrodes for the DSCs. To investigate exactly the performances of the DSCs composed of the counter electrodes based on the four substrates compared for that based on the FTO glass, all the dye-sensitized TiO2 films used were prepared at one time by the screen-printing method, so that they can be considered to have the same microstructure. Table 3 shows the performance characteristics of the DSCs consisting of the photoanodes with 45-mm TiO2 films and the counter electrodes based on the different substrates. The photocurrent–voltage (I–V) curves of the DSCs are shown in Fig. 1. The DSCs composed of the counter electrodes based on the stainless steel sheet and the nickel sheet had conversion efficiencies of 4.8% and 4.7%, respectively, which were Table 3 Performance of dye-sensitized solar cells composed of the counter electrode(CE) based on various substrates Substrates of CE

Voc (mV)

J sc (mA/cm2)

FF (%)

g (%)

Stainless steel Nickel Polyester ITO-PEN FTO

703 685 685 693 695

11.27 11.18 9.95 11.73 11.91

60 61 47 60 65

4.8 4.7 3.3 4.9 5.3

Fig. 1. Photocurrent–voltage curves of dye-sensitized solar cells composed of the counter electrodes based on the metal, plastic and FTO glass substrates.

comparable to the conversion efficiency of 5.3% obtained by that composed of the FTO glass counter electrode. This indicates that the counter electrodes based on metal substrates were feasible in DSCs, and possessed good properties. The DSC composed of a counter electrode based on the polyester film exhibited a relatively low fill factor. The polyester film is a nonconductor with a very high sheet resistance, as shown in Table 1. Although its sheet resistance drastically decreased from 1017 to 60.8 V/sq after a thin Pt film was coated on its surface, the sheet resistance of the counter electrode based on the polyester film was significantly higher than that based on the FTO glass. The sheet resistance of an electrode has an effect on the internal resistance of a solar cell, and consequently, influences the fill factor of the cell and its conversion efficiency [17]. Therefore, the conversion efficiency of 3.3% obtained by the DSC composed of the counter electrode based on the polyester film was lower than that based on the FTO glass. However, the DSC composed of the counter electrode based on the polyester film can be aimed at lower cost markets where a lower efficiency is acceptable, because the polyester films widely used for over-head slide projectors are very cheap polymer materials. The DSC composed of a counter electrode based on the ITO-PEN film obtained the high conversion efficiency of 4.9%, which was comparable to the conversion efficiency obtained by that based on the FTO glass. It is obvious that these counter electrodes based on the flexible metal substrates and plastic films can be used in flexible DSCs, which opens the way to attractive roll to roll production processes that are anticipated to allow the high speed and low cost manufacturing of DSC modules. Since metal substrates have excellent electrical conductivity, the sheet resistances of the stainless steel sheet and the nickel sheet are far lower than that of the FTO glass, as shown in Table 1. To elucidate how the counter electrodes based on metal substrates influence the performances of DSCs, we compared the performances of the DSCs composed of the counter electrodes based on the stainless

X. Fang et al. / Thin Solid Films 472 (2005) 242–245 Table 4 Performance of dye-sensitized solar cells composed of the counter electrodes based on stainless steel (SS) and FTO with different area TiO2 electrode area (mm)

1010

1515

2020

Substrates

SS

FTO

SS

FTO

SS

FTO

Voc (mV) J sc (mA cm 2) FF (%) g (%)

655 9.10 58 3.48

683 8.85 50 3.04

660 9.13 45 2.70

649 9.82 33 2.13

658 7.03 37 1.72

649 6.76 28 1.23

steel sheet with those based on the FTO glass under conditions using higher active areas. Table 4 shows the performance characteristics of the DSCs composed of the photoanodes constructed of TiO2 films with sizes of 1010, 1515 and 2020 mm. When the size of the TiO2 films was 1010 mm, the DSCs composed of the counter electrode based on the stainless steel sheet exhibited the larger fill factor of 58% compared to that based on the FTO glass. As a result, the conversion efficiency obtained by the DSC composed of the counter electrode based on the stainless steel substrate was higher than that based on the FTO glass. As the size of the TiO2 films increased to 1515 and 2020 mm, the DSCs composed of the counter electrodes based on the stainless steel sheet gained a more defined superiority in the fill factor and conversion efficiency over those based on the FTO glass. It was revealed that the counter electrode based on the stainless steel substrate could improve the fill factor and conversion efficiency of the DSC by reducing its internal resistance. These results are important for larger DSC modules. Therefore, the stainless steel sheet or foil can be considered to be a promising alternative to the FTO glass as the substrate of the counter electrodes for DSCs due to its low cost, excellent electric and thermal conductivities, and good stability in the electrolyte solution. Investigations on the stabilities of the DSCs composed of the stainless steel and ITO-PEN counter electrodes were performed under continuous illumination for 1000 h. The cells showed relatively good stabilities. The details have been discussed in a full paper.

4. Conclusions The stainless steel sheet, the polyester film and the ITOPEN film exhibited good stabilities in the electrolyte solution, and the nickel sheet showed a low weight loss. The conversion efficiencies obtained by the DSCs com-

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posed of the counter electrodes based on the metal substrates and the ITO-PEN film were comparable to that based on the FTO glass. The counter electrode based on the stainless steel substrate has the merit of improving the fill factor and conversion efficiency of the DSC by reducing its internal resistance. The DSC composed of the counter electrode based on the polyester film can be aimed at lower cost markets. These counter electrodes based on the flexible metal and plastic substrates are very useful for reducing the cost of DSCs, enlarging the size of the DSCs and developing flexible DSCs, and thus promoting the commercialization of DSCs.

Acknowledgments This research was supported by the Japan Science and Technology.

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