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Heat-resisting TCO films for PV cells
ARTICLE IN PRESS
Solar Energy Materials & Solar Cells 90 (2006) 3251–3260 www.elsevier.com/locate/solmat
Heat-resisting TCO films for PV cells Kenji Goto, Takuya Kawashima, Nobuo Tanabe Electronics Material Department, Material Technology Laboratory, Fujikura Ltd., 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan Received 24 May 2005; accepted 5 October 2005 Available online 30 August 2006
Abstract We developed new heat-resisting transparent conductive oxide (TCO) films with resistivity of 1.4 104 O cm, an optical transmittance of above 80% (at 550 nm) and heat-resisting temperature at above 600 1C. The TCO films consists of fluorine-doped tin oxide films coated on indium–tin oxide films. They were prepared by a spray pyrolysis deposition method on glass substrates. The 100 100 mm2 dye-sensitized solar cells (DSCs) were prepared with the TCO films. An energy conversion efficiency of the DSC was improved drastically in comparison to the case with conventional TCO films. r 2006 Elsevier B.V. All rights reserved. Keywords: FTO; ITO; Heat-resisting; High-transparency; Low-resistivity
1. Introduction Indium-tin-oxide (ITO) thin films are widely used as a transparent electrode in optoelectronics devices including liquid crystal displays, plasma display panels and solar cells. Although ITO films show high transparency and electrical conductivity at room temperature, the latter property is severely spoiled under high temperature. When ITO films are exposed to high temperature of 300 1C or higher, their electrical resistance increases more than three times. When these types of transparent conductive films are used to make dye-sensitized solar cells (DSCs) [1,2], the paste of fine oxide powders such as titanium oxide is coated on the surface of the ITO films. Then, the paste is calcinated at the temperature range of Corresponding author. Tel.: +81 3 5606 1067; fax +81 3 5606 1511.
E-mail address: [email protected] (K. Goto). 0927-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2006.06.048
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400–600 1C to form porous oxide semiconductor films. Unfortunately, as the conductivity of the ITO films decreases markedly during this process, the photoelectric conversion efficiency of the DSC also decreases. The object of this development is to provide transparent conductive films with the ITO films, used for example as the DSC, the electrical resistance does not increase even when it is exposed to high temperature of 300 1C or higher. In this work, the new transparent conductive films, in which fluorine-doped tin oxide (FTO) films are covered on ITO films (FTO/ITO films), were developed for DSC [3]. The reason of developing this doublelayered structure is because the heat resistance in more than 300 1C of FTO films is better than that of ITO films. FTO/ITO films were prepared by the spray pyrolysis deposition (SPD) method [3–9], which makes it possible for both ITO films and FTO films to form on a glass substrate. Characteristics of the films which were surfase morphorogy, transparency, surface resistivity, heat-resisting property and Hall coefficient were investigated. Then, the DSC were fabricated using these heat-resisting films and energy conversion efficiency of them were measured. 2. Experimental 2.1. Film formation Preparation of raw material compounds solution for ITO films and FTO films was as follows: InCl3 4H2O and SnCl2 2H2O were dissolved in ethanol. SnCl4 5H2O was dissolved in ethanol, and a saturated aqueous solution of NH4F was added. The mixture was placed in an ultrasonic washer to achieve complete dissolution. Deposition of FTO/ITO films was as follows: the raw material compound solution was atomized by compressed air. The atomized solution was transported onto a heated TEMPAX#8330 glass substrate (100 100 1.1 mm3). The substrate temperature was 350 1C in ITO and 400 1C in FTO, respectively. Through the above processes, doublelayered films were formed on a glass substrate; ITO films of the first layer and FTO films of the second layer. The characteristics of FTO/ITO films is indicated in Table 1. Film thickness was measured by a stylocontact method using a Slone Dectak 3030 after etching the film. The measurement of the concentration depth profile was also conducted by sequentially applying Auger electron spectroscopy (AES) with ion beam sputtering. The Table 1 Characteristics of FTO/ITO films Films Thickness Sheet resistance Resistivity Transmittance Haze ratio Carrier concentration Mobility Heat-resisting property
FTO/ITO FTO: 100 nm, ITO: 700 nm 1–2 O/& 1.4 104 O cm Above 80% (550 nm) 3–4% 1.3 1021/cm3 48 cm2/V s After H.T. at 600 1C 1h in the air reducing ratio: Resistivity: less than 10%, Transmittance: less than 2%
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AES data were obtained using primary electron beam energy of 5 keV when the etching rate was 10 nm/min (estimated with SiO2). The surface morphology and cross-section of the present films were observed with a field-emission-type scanning electron microscope (FE-SEM; HITACHI S-5200). Acceleration voltage was 5 kV. The transmittance in the 200–1200 nm regions was measured with a JASCO V-570 spectrophotometer. The crystal structure of the films was determined by X-ray diffraction (XRD) using Cu-Ka radiation (RIGAKU RAD-1C diffractometer). MITSUBISHI KAGAKU MCP-T600 four-probetype resistance meter measured sheet resistance of the films. The Hall effect was studied by the four probe method with an electromagnet having maximum field strength of 3.2 kG (Bio Rad HL5500) in Sizuoka University. 2.2. Solar cell fabrication TiO2 colloidal printing paste (Ti nanoxide-T, Solaro-nix SA) was coated on the FTO/ ITO films (100 100 mm2) using a doctor-blading technique. After drying the nanoporous electrode films on the FTO/ITO films, the films were sintering at 450 1C in the air for 1 h. The FTO/ITO films with TiO2 layer were immersed in 50 wt% butanol and 50 wt% acetonitrile mixed solution of 0.3 mM ruthenium(II) (2, 20 -bipyridyl-4,40 -dicarboxylate)2 (NCS)2 dye (called as N3 Dye) for 18 h at room temperature. A counter electrode was prepared with platinum sputtering deposition on a TCO (FTO films; 8 O/&). Thickness of platinum thin film was approximately 4.0 mm. Solar cells were made by placing the counter electrode on the dye-sensitized TiO2 electrode and then the gap of the two glass substrates was filled with electrolyte of the following composition: 0.1 M LiI, 0.05 M I2, 0.5 M 4-tertbutylpyridine (TBP) and 0.3 M 1,2-dimethyl-3-propylimidazolium iodide (DMPImI) in methoxyacetonitrile as solvent. For purposes of comparison, the same processes were used to make solar cells using the ITO films (100 100 mm2). I–V characteristics of the solar cells (Isc, Voc, FF and Z) were derived with an Schlumberger SI 1286. The solar cells under test were illuminated under a xenon lamp (YAMASHITA DENSO YSS-150A solar simulator, 100 mW/cm2 at room temperature). The YSS-150A solar simulator was set to AM 1.5 conditions. 3. Results and discussion Cross-sectional FE-SEM image of the FTO/ITO films is reproduced in Fig. 1. The double-layered films were observed to consist of the ITO in the first layer (thickness of 700 nm) and the FTO in the second layer (thickness of 100 nm). The surface morphologies of the FTO, ITO and FTO/ITO films are shown in Fig. 2. The average grain sizes of FTO and ITO films were approximately 50 and 150 nm, respectively. Since the grain grows with the increase of thickness of ITO films, based on our observation, it seems to be influenced by the fact that deference of grain size between ITO and FTO was corresponding to thickness of the films. It was found that the grain size of FTO films was not influenced by the grain size of under layers, the ITO films, as the result of the observation, the grain size of FTO films on the ITO films was the same as FTO films. Fig. 3 shows the transmittance spectra of the FTO, ITO and FTO/ITO films in comparison with the transmittance of the uncoated glass. The transmittance spectrum of the FTO/ITO films was similar to that of the ITO films and the value at 550 nm of wavelength was approximately 80%. The graph in Fig. 6 illustrates extreme high
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Fig. 1. Cross-sectional FE-SEM image of the double-layered films composed of ITO (first) and FTO (second).
Fig. 2. FE-SEM images showing the surface morphology of (a) FTO films, (b) ITO films and (c) FTO/ITO films.
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100
Transmittance (%)
80
(b) 60 (c) 40
(a)
20
0 200
300
400
500 600 Wavelength (nm)
700
800
900
Fig. 3. Optical transmittance of (a) FTO films, (b) ITO films and (c) FTO/ITO films.
transmittance of FTO films, it seems to be caused by the thin thickness, 100 nm, as compared with that of ITO films, 700 nm. The measurements of depth profiles for individual element concentration were performed by sequentially applying AES with ion beam sputtering (Fig. 4). The change of the existence ratio of tin and indium was observed at the thickness of approximately 100 nm, therefore, the thickness of FTO films in the FTO/ITO films was confirmed as approximately 100 nm. Since the thickness was equivalent to estimated value by prepared amount of raw materials, it is assumed that there is no interface reaction between FTO and ITO in continuous deposition of both films. Fig. 5a shows the XRD spectra of the ITO films. All peaks were assigned to In2O3 and also showed the existence of various crystal faces. Fig. 5b shows the XRD spectra of the FTO films. Diffraction peaks from (1 1 0) and (2 0 0) planes of SnO2 crystals were observed. The XRD spectra of the FTO/ITO films are shown in Fig. 5c. They illustrate all diffraction peaks observed in the spectrum of single In2O3 (Fig. 5a), although the only peak from (2 0 0) plane of SnO2 crystals was observed. Since the individual crystal face has been reported to be controlled by combination of solvent and solution [10], it is assumed that the peaks of both crystals are affected by them. The resistivities of ITO, FTO and FTO/ITO films are indicated in Fig. 6 as a function of temperature (annealing for 1 h in the air). Before annealing, the resistivities of ITO, FTO and FTO/ITO films were 1.2 104, 6.5 104 and 1.4 104 O cm, respectively. After annealing, although the values of FTO and FTO/ITO films increased by less than 10% over all temperature, that of ITO films began to increase at temperature above 300 1C and showed a value 3 times as much, 5.0 104 O cm, at temperature above 400 1C.
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(622)
(440) (431)
(411) (110)
(b)
(200)
Intensity (a.u.)
(211)
(a)
(400)
(222)
Fig. 4. Depth profiles of Sn, In, O concentration in FTO/ITO films (etching rate of 10 nm/min, estimated with SiO2).
(c)
0
20
40
60
2θ/degree Fig. 5. X-ray diffraction patterns of (a) ITO films, (b) FTO films and (c) FTO/ITO films.
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Fig. 6. Dependence of resistivities on annealing temperature for 1 h in the air. (a) FTO films, (b) ITO films and (c) FTO/ITO films.
25
50
40
ITO Mobility →
30
15 FTO/ITO
20
10
5
0 200
Carrier Concentration
300
ITO
400 Temperature (°C)
Mobility (cm2/ V.s)
20
→
Carrier Concentration (1020/cm3)
FTO/ITO
10
500
0 600
Fig. 7. The carrier concentration and mobility of ITO, FTO and FTO/ITO films as a function of heat-treatment temperature.
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The carrier concentration and mobility of ITO, FTO and FTO/ITO films are indicated in Fig. 7 as a function of temperature. The measured values of them for FTO/ITO films hardly showed any deterioration of carrier concentration, however, for ITO films the values began to decrease at temperature above approximately 300 1C and showed one half as much at temperature above 400 1C. The tendency is similar to that of resistivity, it is assumed that the increase of resistivity at the high temperature was caused by decrease of carrier concentration, which was due to decrease of hole density for the ITO crystal in progressing of oxidization through heat treatment. As a result of these measurements, it becomes clear that FTO/ITO films maintain good conductivity after heat treatment at 600 1C by the protection of FTO films which have heat-resisting property. The transmittance properties of FTO/ITO films before and after heat treatment at 450 1C for an hour in the air are indicated in Fig. 8. After annealing at 450 1C, the property of FTO/ITO films hardly showed any shift. The graph in Fig. 9 illustrates those of ITO films after heat treatment at 300, 400 and 600 1C for 1 h. The property hardly showed any shift at less than 300 1C. But it began to increase on longer wave side, more than 800 nm, at temperature above 350 1C and became remarkable at higher temperature. As introduced in several reports [11], it is considered that the shift of transmittance property on longer wave side, more than 800 nm, is caused by the decrease of carrier concentration in ITO crystal. Although the decrease of carrier concentration after heat treatment at high temperature above 350 1C causes deterioration
100
before H.T.
Transmittance (%)
80
60 after 450°C 1h
40
20 FTO/ITO films 0 200
400
600 800 Wavelength (nm)
1000
1200
Fig. 8. Transmittance properties of FTO/ITO films before and after heat treatment at 450 1C for 1 h in the air.
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100 600°C 1h 80
Transmitttance (%)
400°C 1h
60 300°C 1h
40
20 ITO films
0 200
400
600 800 Wavelength (nm)
1000
1200
Fig. 9. Transmittance properties of ITO films after heat treatment at 300, 400 and 600 1C for 1 h.
of resistivity, the high transparency is taken. Therefore, it is necessary to develop TCO films controlled for transparency and resistivity corresponding to a design concept for individual solar cell. Fig. 10 shows I–V characteristics obtained by 100 100 mm2 sized (total photoelectrode area was 90 90 mm2) cells. For the DSC using the FTO/ITO films Jsc ¼ 8.47 mA/cm2, Voc ¼ 7.36 V, FF ¼ 0.589 and Z ¼ 3.7% was measured. Measurement of the reference cell using the ITO films resulted in values of Jsc ¼ 8.75 mA/cm2, Voc ¼ 6.89 V, FF ¼ 0.589 and Z ¼ 2.1%. These results prove that FTO/ITO films are effective in the electrode of the DSC. From the results, it becomes clear that using the heatresisting FTO/ITO films is a strong factor for effective large-sized DSC. 4. Conclusion New transparent conductive films, FTO/ITO films, were successfully deposited on a glass substrate by the SPD method. Several characteristics of the films were measured. The lowest resistivity of 1.4 104 O cm and an optical transmittance of more than 80% in the visible range of the spectrum were obtained. Heat resistance of the films was also measured. The electrical resistance increased by less than 10%, even when exposed to high temperatures of 300–600 1C for 1 h in air. The 100 100 mm DSC was prepared with the TCO films. The efficiency of the DSC was improved drastically in comparison to the case
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10 (b) = 3.7 %
Current density (mA/mm2)
8
6
(a) = 2.1 % 4
2
0 0
100
200
300
500 400 Voltage (mV)
600
700
800
Fig. 10. I–V characteristics of 100 100 mm2-sized cells using (a) ITO films and (b) FTO/ITO films.
with conventional TCO films. From the results, it is clear that using the heat-resisting FTO/ITO films is an important technology for large-sized DSC. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
B. O’Regan, M. Gra¨tzel, Nature 353 (1991) 737. H. Matsui, K. Okada, T. Kawashima, N. Tanabe, Fujikura Giho¯ 104 (2003) 37. T. Kawashima, H. Matsui, N. Tanabe, Thin Solid Films 445 (2003) 241. K.L. Chopra, R.C. Kainthla, D.K. Pandya, A.P. Thakoor, Phys. Thin Films 12 (1982) 167. M.S. Tomar, F.J. Garcia, Proc. Cryst. Growth Characteristics 4 (1981) 221. E. Shanthi, A. Banerjee, V. Dutta, K.L. Chopra, J. Appl. Phys. 53 (1982) 1615. K.L. Chopra, S. Major, D.K. Pandya, Thin Solid Films 102 (1983) 1. M. Fantini, I. Torriani, Thin Solid Films 138 (1986) 225. S. Kaneko, K. Nakajima, T. Kosugi, K. Murakami, Ceram. Trans. 100 (1999) 165. M. Okuya, T. Nakano, G.R.A. Kumara, S. Kaneko, J. Photochem. Photobiol. A. Chem., in press. Hamberg, C.G. Granqvist, J. Appl. Phys. 60 (11) (1986) 123.
Solar Energy Materials & Solar Cells 90 (2006) 3251–3260 www.elsevier.com/locate/solmat
Heat-resisting TCO films for PV cells Kenji Goto, Takuya Kawashima, Nobuo Tanabe Electronics Material Department, Material Technology Laboratory, Fujikura Ltd., 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan Received 24 May 2005; accepted 5 October 2005 Available online 30 August 2006
Abstract We developed new heat-resisting transparent conductive oxide (TCO) films with resistivity of 1.4 104 O cm, an optical transmittance of above 80% (at 550 nm) and heat-resisting temperature at above 600 1C. The TCO films consists of fluorine-doped tin oxide films coated on indium–tin oxide films. They were prepared by a spray pyrolysis deposition method on glass substrates. The 100 100 mm2 dye-sensitized solar cells (DSCs) were prepared with the TCO films. An energy conversion efficiency of the DSC was improved drastically in comparison to the case with conventional TCO films. r 2006 Elsevier B.V. All rights reserved. Keywords: FTO; ITO; Heat-resisting; High-transparency; Low-resistivity
1. Introduction Indium-tin-oxide (ITO) thin films are widely used as a transparent electrode in optoelectronics devices including liquid crystal displays, plasma display panels and solar cells. Although ITO films show high transparency and electrical conductivity at room temperature, the latter property is severely spoiled under high temperature. When ITO films are exposed to high temperature of 300 1C or higher, their electrical resistance increases more than three times. When these types of transparent conductive films are used to make dye-sensitized solar cells (DSCs) [1,2], the paste of fine oxide powders such as titanium oxide is coated on the surface of the ITO films. Then, the paste is calcinated at the temperature range of Corresponding author. Tel.: +81 3 5606 1067; fax +81 3 5606 1511.
E-mail address: [email protected] (K. Goto). 0927-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2006.06.048
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400–600 1C to form porous oxide semiconductor films. Unfortunately, as the conductivity of the ITO films decreases markedly during this process, the photoelectric conversion efficiency of the DSC also decreases. The object of this development is to provide transparent conductive films with the ITO films, used for example as the DSC, the electrical resistance does not increase even when it is exposed to high temperature of 300 1C or higher. In this work, the new transparent conductive films, in which fluorine-doped tin oxide (FTO) films are covered on ITO films (FTO/ITO films), were developed for DSC [3]. The reason of developing this doublelayered structure is because the heat resistance in more than 300 1C of FTO films is better than that of ITO films. FTO/ITO films were prepared by the spray pyrolysis deposition (SPD) method [3–9], which makes it possible for both ITO films and FTO films to form on a glass substrate. Characteristics of the films which were surfase morphorogy, transparency, surface resistivity, heat-resisting property and Hall coefficient were investigated. Then, the DSC were fabricated using these heat-resisting films and energy conversion efficiency of them were measured. 2. Experimental 2.1. Film formation Preparation of raw material compounds solution for ITO films and FTO films was as follows: InCl3 4H2O and SnCl2 2H2O were dissolved in ethanol. SnCl4 5H2O was dissolved in ethanol, and a saturated aqueous solution of NH4F was added. The mixture was placed in an ultrasonic washer to achieve complete dissolution. Deposition of FTO/ITO films was as follows: the raw material compound solution was atomized by compressed air. The atomized solution was transported onto a heated TEMPAX#8330 glass substrate (100 100 1.1 mm3). The substrate temperature was 350 1C in ITO and 400 1C in FTO, respectively. Through the above processes, doublelayered films were formed on a glass substrate; ITO films of the first layer and FTO films of the second layer. The characteristics of FTO/ITO films is indicated in Table 1. Film thickness was measured by a stylocontact method using a Slone Dectak 3030 after etching the film. The measurement of the concentration depth profile was also conducted by sequentially applying Auger electron spectroscopy (AES) with ion beam sputtering. The Table 1 Characteristics of FTO/ITO films Films Thickness Sheet resistance Resistivity Transmittance Haze ratio Carrier concentration Mobility Heat-resisting property
FTO/ITO FTO: 100 nm, ITO: 700 nm 1–2 O/& 1.4 104 O cm Above 80% (550 nm) 3–4% 1.3 1021/cm3 48 cm2/V s After H.T. at 600 1C 1h in the air reducing ratio: Resistivity: less than 10%, Transmittance: less than 2%
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AES data were obtained using primary electron beam energy of 5 keV when the etching rate was 10 nm/min (estimated with SiO2). The surface morphology and cross-section of the present films were observed with a field-emission-type scanning electron microscope (FE-SEM; HITACHI S-5200). Acceleration voltage was 5 kV. The transmittance in the 200–1200 nm regions was measured with a JASCO V-570 spectrophotometer. The crystal structure of the films was determined by X-ray diffraction (XRD) using Cu-Ka radiation (RIGAKU RAD-1C diffractometer). MITSUBISHI KAGAKU MCP-T600 four-probetype resistance meter measured sheet resistance of the films. The Hall effect was studied by the four probe method with an electromagnet having maximum field strength of 3.2 kG (Bio Rad HL5500) in Sizuoka University. 2.2. Solar cell fabrication TiO2 colloidal printing paste (Ti nanoxide-T, Solaro-nix SA) was coated on the FTO/ ITO films (100 100 mm2) using a doctor-blading technique. After drying the nanoporous electrode films on the FTO/ITO films, the films were sintering at 450 1C in the air for 1 h. The FTO/ITO films with TiO2 layer were immersed in 50 wt% butanol and 50 wt% acetonitrile mixed solution of 0.3 mM ruthenium(II) (2, 20 -bipyridyl-4,40 -dicarboxylate)2 (NCS)2 dye (called as N3 Dye) for 18 h at room temperature. A counter electrode was prepared with platinum sputtering deposition on a TCO (FTO films; 8 O/&). Thickness of platinum thin film was approximately 4.0 mm. Solar cells were made by placing the counter electrode on the dye-sensitized TiO2 electrode and then the gap of the two glass substrates was filled with electrolyte of the following composition: 0.1 M LiI, 0.05 M I2, 0.5 M 4-tertbutylpyridine (TBP) and 0.3 M 1,2-dimethyl-3-propylimidazolium iodide (DMPImI) in methoxyacetonitrile as solvent. For purposes of comparison, the same processes were used to make solar cells using the ITO films (100 100 mm2). I–V characteristics of the solar cells (Isc, Voc, FF and Z) were derived with an Schlumberger SI 1286. The solar cells under test were illuminated under a xenon lamp (YAMASHITA DENSO YSS-150A solar simulator, 100 mW/cm2 at room temperature). The YSS-150A solar simulator was set to AM 1.5 conditions. 3. Results and discussion Cross-sectional FE-SEM image of the FTO/ITO films is reproduced in Fig. 1. The double-layered films were observed to consist of the ITO in the first layer (thickness of 700 nm) and the FTO in the second layer (thickness of 100 nm). The surface morphologies of the FTO, ITO and FTO/ITO films are shown in Fig. 2. The average grain sizes of FTO and ITO films were approximately 50 and 150 nm, respectively. Since the grain grows with the increase of thickness of ITO films, based on our observation, it seems to be influenced by the fact that deference of grain size between ITO and FTO was corresponding to thickness of the films. It was found that the grain size of FTO films was not influenced by the grain size of under layers, the ITO films, as the result of the observation, the grain size of FTO films on the ITO films was the same as FTO films. Fig. 3 shows the transmittance spectra of the FTO, ITO and FTO/ITO films in comparison with the transmittance of the uncoated glass. The transmittance spectrum of the FTO/ITO films was similar to that of the ITO films and the value at 550 nm of wavelength was approximately 80%. The graph in Fig. 6 illustrates extreme high
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Fig. 1. Cross-sectional FE-SEM image of the double-layered films composed of ITO (first) and FTO (second).
Fig. 2. FE-SEM images showing the surface morphology of (a) FTO films, (b) ITO films and (c) FTO/ITO films.
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100
Transmittance (%)
80
(b) 60 (c) 40
(a)
20
0 200
300
400
500 600 Wavelength (nm)
700
800
900
Fig. 3. Optical transmittance of (a) FTO films, (b) ITO films and (c) FTO/ITO films.
transmittance of FTO films, it seems to be caused by the thin thickness, 100 nm, as compared with that of ITO films, 700 nm. The measurements of depth profiles for individual element concentration were performed by sequentially applying AES with ion beam sputtering (Fig. 4). The change of the existence ratio of tin and indium was observed at the thickness of approximately 100 nm, therefore, the thickness of FTO films in the FTO/ITO films was confirmed as approximately 100 nm. Since the thickness was equivalent to estimated value by prepared amount of raw materials, it is assumed that there is no interface reaction between FTO and ITO in continuous deposition of both films. Fig. 5a shows the XRD spectra of the ITO films. All peaks were assigned to In2O3 and also showed the existence of various crystal faces. Fig. 5b shows the XRD spectra of the FTO films. Diffraction peaks from (1 1 0) and (2 0 0) planes of SnO2 crystals were observed. The XRD spectra of the FTO/ITO films are shown in Fig. 5c. They illustrate all diffraction peaks observed in the spectrum of single In2O3 (Fig. 5a), although the only peak from (2 0 0) plane of SnO2 crystals was observed. Since the individual crystal face has been reported to be controlled by combination of solvent and solution [10], it is assumed that the peaks of both crystals are affected by them. The resistivities of ITO, FTO and FTO/ITO films are indicated in Fig. 6 as a function of temperature (annealing for 1 h in the air). Before annealing, the resistivities of ITO, FTO and FTO/ITO films were 1.2 104, 6.5 104 and 1.4 104 O cm, respectively. After annealing, although the values of FTO and FTO/ITO films increased by less than 10% over all temperature, that of ITO films began to increase at temperature above 300 1C and showed a value 3 times as much, 5.0 104 O cm, at temperature above 400 1C.
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(622)
(440) (431)
(411) (110)
(b)
(200)
Intensity (a.u.)
(211)
(a)
(400)
(222)
Fig. 4. Depth profiles of Sn, In, O concentration in FTO/ITO films (etching rate of 10 nm/min, estimated with SiO2).
(c)
0
20
40
60
2θ/degree Fig. 5. X-ray diffraction patterns of (a) ITO films, (b) FTO films and (c) FTO/ITO films.
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Fig. 6. Dependence of resistivities on annealing temperature for 1 h in the air. (a) FTO films, (b) ITO films and (c) FTO/ITO films.
25
50
40
ITO Mobility →
30
15 FTO/ITO
20
10
5
0 200
Carrier Concentration
300
ITO
400 Temperature (°C)
Mobility (cm2/ V.s)
20
→
Carrier Concentration (1020/cm3)
FTO/ITO
10
500
0 600
Fig. 7. The carrier concentration and mobility of ITO, FTO and FTO/ITO films as a function of heat-treatment temperature.
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The carrier concentration and mobility of ITO, FTO and FTO/ITO films are indicated in Fig. 7 as a function of temperature. The measured values of them for FTO/ITO films hardly showed any deterioration of carrier concentration, however, for ITO films the values began to decrease at temperature above approximately 300 1C and showed one half as much at temperature above 400 1C. The tendency is similar to that of resistivity, it is assumed that the increase of resistivity at the high temperature was caused by decrease of carrier concentration, which was due to decrease of hole density for the ITO crystal in progressing of oxidization through heat treatment. As a result of these measurements, it becomes clear that FTO/ITO films maintain good conductivity after heat treatment at 600 1C by the protection of FTO films which have heat-resisting property. The transmittance properties of FTO/ITO films before and after heat treatment at 450 1C for an hour in the air are indicated in Fig. 8. After annealing at 450 1C, the property of FTO/ITO films hardly showed any shift. The graph in Fig. 9 illustrates those of ITO films after heat treatment at 300, 400 and 600 1C for 1 h. The property hardly showed any shift at less than 300 1C. But it began to increase on longer wave side, more than 800 nm, at temperature above 350 1C and became remarkable at higher temperature. As introduced in several reports [11], it is considered that the shift of transmittance property on longer wave side, more than 800 nm, is caused by the decrease of carrier concentration in ITO crystal. Although the decrease of carrier concentration after heat treatment at high temperature above 350 1C causes deterioration
100
before H.T.
Transmittance (%)
80
60 after 450°C 1h
40
20 FTO/ITO films 0 200
400
600 800 Wavelength (nm)
1000
1200
Fig. 8. Transmittance properties of FTO/ITO films before and after heat treatment at 450 1C for 1 h in the air.
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100 600°C 1h 80
Transmitttance (%)
400°C 1h
60 300°C 1h
40
20 ITO films
0 200
400
600 800 Wavelength (nm)
1000
1200
Fig. 9. Transmittance properties of ITO films after heat treatment at 300, 400 and 600 1C for 1 h.
of resistivity, the high transparency is taken. Therefore, it is necessary to develop TCO films controlled for transparency and resistivity corresponding to a design concept for individual solar cell. Fig. 10 shows I–V characteristics obtained by 100 100 mm2 sized (total photoelectrode area was 90 90 mm2) cells. For the DSC using the FTO/ITO films Jsc ¼ 8.47 mA/cm2, Voc ¼ 7.36 V, FF ¼ 0.589 and Z ¼ 3.7% was measured. Measurement of the reference cell using the ITO films resulted in values of Jsc ¼ 8.75 mA/cm2, Voc ¼ 6.89 V, FF ¼ 0.589 and Z ¼ 2.1%. These results prove that FTO/ITO films are effective in the electrode of the DSC. From the results, it becomes clear that using the heatresisting FTO/ITO films is a strong factor for effective large-sized DSC. 4. Conclusion New transparent conductive films, FTO/ITO films, were successfully deposited on a glass substrate by the SPD method. Several characteristics of the films were measured. The lowest resistivity of 1.4 104 O cm and an optical transmittance of more than 80% in the visible range of the spectrum were obtained. Heat resistance of the films was also measured. The electrical resistance increased by less than 10%, even when exposed to high temperatures of 300–600 1C for 1 h in air. The 100 100 mm DSC was prepared with the TCO films. The efficiency of the DSC was improved drastically in comparison to the case
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10 (b) = 3.7 %
Current density (mA/mm2)
8
6
(a) = 2.1 % 4
2
0 0
100
200
300
500 400 Voltage (mV)
600
700
800
Fig. 10. I–V characteristics of 100 100 mm2-sized cells using (a) ITO films and (b) FTO/ITO films.
with conventional TCO films. From the results, it is clear that using the heat-resisting FTO/ITO films is an important technology for large-sized DSC. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
B. O’Regan, M. Gra¨tzel, Nature 353 (1991) 737. H. Matsui, K. Okada, T. Kawashima, N. Tanabe, Fujikura Giho¯ 104 (2003) 37. T. Kawashima, H. Matsui, N. Tanabe, Thin Solid Films 445 (2003) 241. K.L. Chopra, R.C. Kainthla, D.K. Pandya, A.P. Thakoor, Phys. Thin Films 12 (1982) 167. M.S. Tomar, F.J. Garcia, Proc. Cryst. Growth Characteristics 4 (1981) 221. E. Shanthi, A. Banerjee, V. Dutta, K.L. Chopra, J. Appl. Phys. 53 (1982) 1615. K.L. Chopra, S. Major, D.K. Pandya, Thin Solid Films 102 (1983) 1. M. Fantini, I. Torriani, Thin Solid Films 138 (1986) 225. S. Kaneko, K. Nakajima, T. Kosugi, K. Murakami, Ceram. Trans. 100 (1999) 165. M. Okuya, T. Nakano, G.R.A. Kumara, S. Kaneko, J. Photochem. Photobiol. A. Chem., in press. Hamberg, C.G. Granqvist, J. Appl. Phys. 60 (11) (1986) 123.