The Photocatalytic Activity and Compact Layer Characteristics of TiO2 Films Prepared Using Radio Frequency Magnetron Sputtering

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Research Article

The Photocatalytic Activity and Compact Layer

Characteristics of TiO

₂

Films Prepared Using Radio

Frequency Magnetron Sputtering

H. C. Chang,

1

H. H. Huang,

2

C. Y. Wu,

2

R. Q. Hsu,

1

and C. Y. Hsu

2

1_{Department of Mechanical Engineering, National Chiao Tung University, 1001 University Road, Hsinchu 30010, Taiwan}

2_{Department of Mechanical Engineering, Lunghwa University of Science and Technology, No. 300, Sec. 1, Wanshou Road, Guishan,}

Taoyuan 33306, Taiwan

Correspondence should be addressed to C. Y. Hsu; [email protected]

Received 7 March 2014; Revised 18 April 2014; Accepted 18 April 2014; Published 22 May 2014 Academic Editor: Ho Chang

Copyright © 2014 H. C. Chang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

TiO₂compact layers are used in dye-sensitized solar cells (DSSCs) to prevent charge recombination between the electrolyte and

the transparent conductive substrate (indium tin oxide, ITO; fluorine-doped tin oxide, FTO). Thin TiO₂ compact layers are

deposited onto ITO/glass by means of radio frequency (rf) magnetron sputtering, using deposition parameters that ensure greater photocatalytic activity and increased DSSC conversion efficiency. The photoinduced decomposition of methylene blue (MB) and

the photoinduced hydrophilicity of the TiO₂thin films are also investigated. The photocatalytic performance characteristics for the

deposition of TiO₂films are improved by using the Grey-Taguchi method. The average transmittance in the visible region exceeds

85% for all samples. The XRD patterns of the TiO₂films, for sol-gel with spin coating of porous TiO₂/TiO₂compact/ITO/glass,

show a good crystalline structure. In contrast, without the TiO₂compact layer (only porous TiO₂), the peak intensity of the anatase

(101) plane in the XRD patterns for the TiO₂film has a lower value, which demonstrates inferior crystalline quality. With a TiO₂

compact layer to prevent charge recombination, a higher short-circuit current density is obtained. The DSSC with the FTO/glass and Pt counter electrode demonstrates the energy conversion efficiency increased.

1. Introduction

Dye-sensitized solar cells (DSSCs) have been extensively studied as a promising alternative to conventional solar cells that use a p-n junction because of their reasonable conversion efficiency, low cost, environmentally friendly components, use of a flexible cell design, and simple fabrication process, when compared to silicon solar cells [1]. DSSCs are the next-generation solar cells [2]. If low cost and highly efficient DSSCs can be developed, it will be an important new direc-tion for the development of solar cells. A typical DSSC consists of dye molecules that act as sensitizers, a nanoporous metal oxide film (TiO₂semiconductor material), a transpar-ent conducting oxide (indium tin oxide, ITO), an electrolyte charge carrier, and a counter electrode (Pt or carbon) [3]. The dye and metal oxide, which are used for the sensitizer and

the electrode, respectively, are important to the photoelectric conversion efficiency of DSSCs [4].

TiO₂is one of the most popular photocatalytic materials, so it has many commercial applications, such as antibacterial applications, waste purification, self-cleaning, and sensors [5]. It is also used for photoelectrodes and in high perfor-mance DSSC applications because it has an adequate photore-sponse and effective electron transport [6]. A high incident photon to current conversion efficiency is expected for TiO₂ films that have a better phase structure and crystallinity and a higher specific surface area [7]. The control of the TiO₂ nanostructures is very important for the photovoltaic performance of a DSSC [8]. In order to improve the con-version efficiency of DSSCs, several studies have focused on the structural design, material development, photovoltaic characterization, and analysis of the mechanism of TiO₂

Volume 2014, Article ID 786165, 8 pages http://dx.doi.org/10.1155/2014/786165

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Table 1: The factor and level settings for sputter deposition of TiO2compact layers.

Substrate Soda-lime glass, 20× 20 × 1 mm3

Target TiO2(99.99% purity)

Gas Ar and O₂(99.99% purity)

Base pressure 5.0× 10−6torr

Substrate-to-target distance 80 mm

Substrate temperature Room temperature

Substrate rotate vertical axis 10 rpm

Symbol Control factor Level 1 Level 2 Level 3

A rf power (W) 100 130 160

B Sputtering pressure (mtorr) 3 5 7

C O2/(Ar + O2) (%) 10 30 50

D Deposition time (hr) 2 3 4

nanoparticles [9]. Mesoporous TiO₂ is widely used as an electrode in DSSCs to produce a high surface area for the adsorption of a greater density of dye molecules, which produces a significant increase in the photocurrent [10]. However, the highly porous structure of the TiO₂ layer can cause an electrical shortage and recombination of the charge/electrons, which interferes with the unidirectional electron transport that takes place at the TiO₂ layer/dye molecule and ITO/TiO₂layer interfaces [11]. This leakage by electronic back transfer leads to a decrease in cell efficiency. To avoid this problem, the primary method used to prevent recombination is the use of a TiO₂compact layer (blocking layer) between the ITO and the porous TiO₂ layer [11]. This compact layer can be prepared using many growth techniques, such as sputter deposition, dip-coating, chemical vapor deposition, and spray pyrolysis.

This study determines the optical, structural, and surface properties of a TiO₂ compact layer that is grown by radio frequency (rf) magnetron sputtering on the ITO electrodes, as a function of the deposition parameters that ensure higher photocatalytic activity and greater DSSC conversion efficiency. The nanoporous TiO₂upper layer is coated using the sol-gel process and calcination at 450∘C. Moreover, the working electrode which is made of a dye-sensitized TiO₂ film that is immobilized onto a fluorine-doped tin oxide (FTO) substrate is also investigated.

The Taguchi method is a powerful tool for the design of high quality systems, which can be used to design low cost products, with improved quality [12]. To optimize the deposition process for TiO₂photocatalytic films, a statistical analysis of the signal-to-noise ratio (𝑆/𝑁) is performed, using an analysis of variance (ANOVA). The optimal deposition parameters are obtained by analyzing the results for various experimental permutations [13,14].Table 1shows the effect on the quality of the TiO₂photocatalytic films of four depo-sition parameters at three levels: the rf power, the sputtering pressure, the Ar-O₂ratio, and the deposition time. An L₉(34, with four columns and nine rows) orthogonal array is used.

2. Experimental

The TiO₂ photocatalytic thin films (compact layer) were coated onto ITO/glass substrates (and FTO/glass), using rf magnetron sputtering. The reactive and sputtering gases were O₂ (purity: 99.99%) and Ar (purity: 99.99%), respectively. The commercially available, hot pressed, and sintered ceramic target TiO₂ had a diameter of 50.8 mm and 99.99% purity (Elecmat, USA).

Prior to coating, the target was presputtered for 15 min, in order to remove any contamination, and the substrates were ultrasonically cleaned and degreased in acetone, rinsed in deionised water, and subsequently dried with nitrogen gas. A vacuum, of base pressure 5.0× 10−6Torr, was applied before deposition. The distance between the substrate and the target (80 mm) and the rotational speed of the substrate (10 rpm) were constant. By adjusting the experimental permutations, this study determined the effect of each deposition parameter on the deposition rate for TiO₂/ITO/glass, the methylene blue (MB) absorbance, the contact angle to a pure water droplet, the surface morphology, and the crystal structure.

The porous TiO₂ film (p-TiO₂) was coated onto the TiO₂ compact/ITO/glass (and TiO₂ compact/FTO/glass) using a mixture of TiO₂ powders (P-25, particle size: <25 nm, 99.7%) with the TiO2 sol-gel component studied

in [15]. The TiO₂ sol-gel was mixed with 0.3 g of com-mercially available Degussa P-25, to avoid any cracking of the film. The TiO₂sol-gel was produced using spin coating and blade coating. The gels were predried for 15 min at 50∘C and then sintered in a box furnace at 450∘C (heat-ing rate 10∘C/min) for 30 min in air ambient, to produce the bare TiO₂ electrode used in this work to fabricate the DSSC. The porous TiO₂ films were immersed into the dye solution (0.4 mM N719 dye solution, Solaronix, Switzer-land, Di-tetrabutylammonium cis-bis(isothiocyanato)bis(2, 2󸀠-bipyridyl-4,4󸀠-dicarboxylato)-ruthenium(II); chemical formula C₅₈H₈₆N₈O₈RuS₂; Mol Wt: 1188.55) complex for 24 h at room temperature.

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Table 2: The experimental results and the𝑆/𝑁 ratios for the deposition rate, the contact angle, and the MB absorbance for the TiO₂compact layer coatings (the experiments were repeated twice).

Exp. Factors Deposition rate ( ˚A/min) 𝑆/𝑁 (dB) Water contact angle (degree) 𝑆/𝑁 (dB) MB absorbance 𝑆/𝑁 (dB)

𝐴 𝐵 𝐶 𝐷 𝑇1 𝑇2 𝑇1 𝑇2 𝑇1 𝑇2 1 1 1 1 1 3.35 3.00 9.9953 90.73 91.30 −39.182 1.328 1.332 −2.4770 2 1 2 2 2 2.73 3.04 9.1653 92.13 91.87 −39.275 1.327 1.325 −2.4508 3 1 3 3 3 2.19 2.23 6.8868 73.91 75.40 −37.461 1.296 1.293 −2.2420 4 2 1 2 3 4.85 4.97 13.8197 89.75 90.28 −39.086 1.198 1.187 −1.5292 5 2 2 3 1 4.35 3.95 12.3307 91.01 92.71 −39.262 1.284 1.283 −2.1679 6 2 3 1 2 4.84 4.76 13.6239 93.57 94.68 −39.474 1.205 1.200 −1.6017 7 3 1 3 2 6.38 6.11 15.9046 71.70 72.30 −37.146 1.287 1.284 −2.1814 8 3 2 1 3 7.48 7.25 17.3403 95.35 94.81 −39.561 1.077 1.074 −0.6322 9 3 3 2 1 4.73 4.82 13.5783 63.79 62.39 −35.999 1.031 1.030 −0.2609

Note:𝐴 = rf power (W), 𝐵 = process pressure (Pa), 𝐶 = O2/(Ar + O2) flow rate ratio (%), and𝐷 = deposition time (hr).

Glass ITO Pt/carbon Blade/spin coating Sputter coating ITO Glass

Figure 1: A schematic diagram of a DSSC with an rf-sputtered TiO₂

compact layer/ITO/glass on the ITO electrode.

The Pt counter electrode was coated onto ITO/glass (and FTO/glass) substrates using DC sputtering with pure Ar gas and a DC power of 30 W. The dye-adsorbed TiO₂working electrode and the counter electrode were assembled into a sandwich-type cell and sealed with a hot-melt sealant.

Figure 1shows a schematic diagram of a DSSC with an

rf-sputtered TiO₂ compact layer/ITO/glass on the ITO elec-trode. In order to prevent the leakage by electron transfer to the liquid electrolyte, dense TiO₂ passivating layers were used.

The phase identification of the particles produced using various deposition parameters was performed by X-ray diffraction (Rigaku-2000 spectrometer), using Cu-K𝛼 radi-ation (40 kV, 30 mA, and𝜆 = 0.1541 nm). The photoinduced hydrophilicity of the TiO₂ thin films was evaluated by measurement of the contact angle to pure water, using a contact angle meter (FACE CAVP150) that is accurate to less than 1∘. A black light (UVP UVL-225D) lamp with a principal wavelength of 365 nm (1.5 mW/cm2 at the film surface) was the UV light source. The decomposition of MB aqueous solution (10𝜇M) was photocatalyzed. An UV-Vis-NIR spectrometer (Jasco V-670) was used to measure the absorption spectra of the MB solution as a function of the UV irradiation time. The film thickness was measured, using

a surface profilometer (𝛼-step, AMBIOS XP-1). The surface morphology was analyzed using a field emission scanning electron microscope (FESEM, JEOL JSM-6500F). The crystal structure of the films was characterized by X-ray diffraction (Rigaku-2000 spectrometer), using Cu-K𝛼 radiation (40 kV, 30 mA, and𝜆 = 0.1541 nm), with a grazing incidence angle of 1∘. The scanning rate was 5∘/min.

The power used to test the prepared DSSC was a 150 W Xe lamp, which simulates sunlight (AM 1.5). Before the test, the distance between the light source and the sample was adjusted to allow a light source density of 100 mW/cm2. The cell performance parameters, including the short-circuit current density (𝐽sc), the open-circuit voltage (𝑉oc), the fill factor (FF),

and the photoelectronic conversion efficiency(𝜂(%) = 𝐽sc×

𝑉oc × FF/total incident energy × 100), were measured and

calculated using the𝐽-𝑉 characteristics of DSSC.

3. Results and Discussion

3.1. The Photocatalytic Activity of the TiO₂ Compact Films.

The TiO₂compact films were deposited onto ITO soda-lime glass substrates. The optimization of the parameter settings involved comparing the signal-noise (𝑆/𝑁) ratios, using the Taguchi method. In order to optimize the TiO₂compact films deposition parameters, the water contact angle and the MB absorbance had the smaller the better characteristics and the deposition rate had the larger the better characteristics. The respective𝑆/𝑁 ratios for the smaller the better characteristic and the larger the better characteristic are expressed as follows (Taguchi et al. [13]):

(𝑆 𝑁)𝑆= −10 log 1 𝑛 𝑛 ∑ 𝑖=1 𝑦2_𝑖 (𝑆 𝑁)𝐿= −10 log 1 𝑛 𝑛 ∑ 𝑖=1 1 𝑦2 𝑖 , (1)

where𝑛 is the number of iterations for the experiment and 𝑦𝑖 is the𝑖th average value of the characteristic measured.

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Table 3: The ANOVA results for the deposition rate, the water contact angle, and the MB absorbance.

Factor Degree of freedom Sum of square Variance Contribution (P %)

Deposition rate ( ˚A/min)

𝐴 2 74.4166 37.2083 84.45

𝐵 2 6.1132 3.0566 6.93

𝐶 2 6.1644 3.0822 7.00

𝐷 2 1.4211 0.7106 1.62

Total 8 88.1153 100

Water contact angle (degree)

𝐴 2 4.4559 2.22793 33.68 𝐵 2 4.4483 2.22417 33.62 𝐶 2 3.7790 1.88950 28.56 𝐷 2 0.5472 0.27359 4.14 Total 8 13.2304 100 MB absorbance 𝐴 2 2.80223 1.40111 54.36 𝐵 2 0.72560 0.36280 14.07 𝐶 2 1.03120 0.51560 20.00 𝐷 2 0.59634 0.29817 11.57 Total 8 5.15538 100

Using (1), the 𝑆/𝑁 ratio values were computed for depo-sition rate, water contact angle, and MB absorbance in the TiO₂ compact layers coatings, as shown in Table 2. The hydrophilicity of the TiO₂ films was determined by measuring the water contact angle. The change in the water contact angle is shown as a function of UV irradiation time for the TiO₂ films deposited with parameter sets in the orthogonal arrays (Table 2). When the TiO₂ film surface is irradiated by UV light for 12 min, the water contact angles of all of the films begin to decrease (less than 63∘, sample number 9), which indicates that the film surface becomes more hydrophilic. The absorption spectra for the MB aqueous solution degraded by TiO₂photocatalytic film after 240 min UV irradiation are shown for the orthogonal array settings

(Table 2). The TiO₂films deposited using the parameter sets

in the orthogonal arrays from number 1 to number 9 show MB absorbance between 1.33 and 1.03.

An analysis of variance (ANOVA) was used to determine the effect of a change in the process parameters on the process response.Table 3shows the ANOVA results for the deposi-tion rate, the water contact angle, and the MB absorbance.

Table 3shows that the variables that most significantly affect

the deposition rate, the water contact angle, and the MB absorbance are the rf power (𝑃 = 84.45%, 33.68%, and 54.36%), the sputtering pressure (𝑃 = 6.93%, 33.62%, and 14.07%), and the argon-oxygen ratio (𝑃 = 7.00%, 28.56%, and 20.00%).

Grey relational analysis (GRA) provides an efficient solu-tion to difficult problems that involve multiple performance characteristics that are uncertain, have multiple inputs, and

Table 4: The Grey relational grade and its ranking for the TiO2

compact layer coatings.

Exp. Grey relational grade Rank

1 0.3595 8 2 0.3526 9 3 0.4252 6 4 0.4551 4 5 0.3914 7 6 0.4356 5 7 0.5698 3 8 0.7007 2 9 0.8329 1 300 400 500 600 700 800 55 60 65 70 75 80 85 90 95 T ra n smi tt ance (%) Wavelength (nm) Glass/ITO Glass/ITO/TiO₂annealed at₄₅₀∘C Glass/ITO/TiO₂as deposited

Figure 2: The optical transmittance spectra for TiO₂ compact

layer/ITO/glass.

generate discrete data. The objective of this study is to opti-mize the deposition parameters for the TiO₂compact films using GRA, which is used extensively in various industries [16].

The optimum combination does not yield suitable process parameters with a single performance characteristic (Taguchi method) for the TiO₂ compact films coated. In order to optimize the deposition parameters, the deposition rate, the contact angle, and the MB absorbance, multiple performance characteristics (grey relational analysis) must be analyzed. The calculated grey relational grade is taken as the inspected value in the Taguchi method.Table 4shows the grey rela-tional grade and its ranking for the TiO₂ compact layer coatings. A comparison of the experimental results for the orthogonal array (𝐴₃𝐵₃𝐶₂𝐷₁) and the photocatalytic activity optimal parameter set (𝐴₃𝐵₃𝐶₂𝐷₃) for TiO₂film deposition is shown inTable 5. The multiple performance characteristics for the deposition of TiO₂ thin films are greatly improved

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Table 5: The confirmation test results for the multiple performance characteristics, using the initial and the optimal process parameters.

Level Initial process parameters Optimal process parameters Improvement rate (%)

𝐴3𝐵3𝐶2𝐷1 𝐴3𝐵3𝐶2𝐷3

Deposition rate ( ˚A/min) 4.775 5.48 12.87

Contact angle (degree) 63.09 53.47 15.25

MB absorbance 1.0305 0.865 16.06

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Figure 3: (a) The SEM images for sputtered TiO₂ compact layer on ITO/glass, (b) the SEM images for porous TiO₂ onto TiO₂

compact/ITO/glass, produced using the sol-gel with spin coating method, and (c) the SEM images for porous TiO₂ onto TiO₂

compact/ITO/glass, produced using the sol-gel with blade coating method.

Sputter coating ITO Glass 1 𝜇m Spin coating (a) Sputter coating ITO 1 𝜇mGlass Blade coating (b)

Figure 4: The SEM cross-sectional image of TiO₂(a) corresponding toFigure 3(b)and (b) corresponding toFigure 3(c).

Table 6: The performance of a DSSC prepared using a photoelectrode with and without a TiO₂compact layer, using carbon and Pt counter

electrodes, and using ITO/glass and FTO/glass.

𝑉oc(V) 𝐽sc(mA/cm2) Fill factor Efficiency𝜂 (%)

ITO/sputter/spin coating/carbon 0.54 4.38 0.676 1.59 ITO/sputter/blade coating/carbon 0.53 4.08 0.666 1.44 ITO/spin coating/carbon 0.52 3.18 0.639 1.05 ITO/blade coating/carbon 0.52 3.07 0.572 0.90 ITO/sputter/spin coating/Pt 0.51 12.86 0.678 4.46 ITO/sputter/blade coating/Pt 0.49 10.73 0.675 3.51 ITO/spin coating/Pt 0.58 6.02 0.644 2.24 ITO/blade coating/Pt 0.61 5.10 0.649 2.01 FTO/sputter/spin coating/Pt 0.70 17.22 0.641 7.73 FTO/sputter/blade coating/Pt 0.77 12.79 0.750 7.36 FTO/spin coating/Pt 0.79 12.39 0.692 6.79 FTO/blade coating/Pt 0.79 10.80 0.771 6.53

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20 30 40 50 60 70 80 (211) (200) In te n si ty (a.u .) ITO/sputter/spin coating ITO/blade coating ITO/spin coating ITO/sputter/blade coating (101) (004) 2𝜃 (deg)

Figure 5: The XRD patterns for the TiO₂films after being annealed at 450∘C.

by using the Grey-Taguchi method. The improvement in the deposition rate is 12.87%, that in the water contact angle is 15.25%, and that in the MB absorbance is 16.06%.

The transmittance spectra are shown as a function of wavelengths in the range between 300 and 800 nm for TiO₂ compact layers inFigure 2. The average transmittance in the visible region exceeds 85% for all samples, but transmission in the UV-near visible region decreases abruptly. After annealing treatment, the optical transmittance of the film is increased.

3.2. DSSC Conversion Efficiency. SEM analysis was used

to determine the morphology of the sputtered TiO₂ com-pact layers (with photocatalytic activity optimal parame-ters, 𝐴₃𝐵₃𝐶₂𝐷₃) on the ITO substrate and the thick TiO₂ porous layer produced using the sol-gel method, as shown in

Figure 3. The uniform and smooth surfaces of the sputtered

compact accumulation film are well covered by spherical particles, which are densely coated with a small grain size

(Figure 3(a), TiO₂compact/ITO/glass). This is necessary to

prevent charge recombination between the ITO and the porous TiO₂ layer [17, 18]. The SEM images show the porous TiO₂film over the sputtered compact layer, produced using the sol-gel with spin coating method (Figure 3(b), porous TiO₂/TiO₂compact/ITO/glass) and the sol-gel with blade coating method (Figure 3(c), porous TiO₂/TiO₂ com-pact/ITO/glass). The porous TiO₂film structure is not dense and the crystallite size of the TiO₂ is increased. DSSC efficiency is improved by producing a TiO₂electrode with a large surface area and optimum pore structure [19,20].

The cross-section of the TiO₂films was observed by SEM.

Figure 4(a)corresponds toFigure 3(b)andFigure 4(b)

corre-sponds toFigure 3(c). The TiO₂compact/ITO films produced using the photocatalytic activity optimal deposition condi-tions (𝐴₃𝐵₃𝐶₂𝐷₃) are highly compacted and homogeneous and adhere perfectly to the glass substrate. These results

confirm a spongelike structure for the TiO₂layer (Figure 4), which is a prerequisite for a highly efficient DSSC [21]. The characteristics of the TiO₂ materials depend significantly upon the surface morphology, the crystal structure, and the crystallization.

Figure 5shows that the XRD patterns of the TiO₂films,

produced using the sol-gel with spin coating of porous TiO₂/TiO₂ compact/ITO/glass, show a good crystalline structure and anatase (101) diffraction peaks that demonstrate a higher crystallinity than the other films. In contrast, without the TiO₂compact layer (only porous TiO₂), the peak intensity of the anatase (101) plane in the XRD patterns for the TiO₂ film has a lower value, which demonstrates inferior crystalline quality.

Good performance for the counter electrode requires a low internal resistance and raw material cost. The best material for the counter electrode is Pt, which shows excellent electrochemical activity for I₃−reduction at film thicknesses of 2∼10 nm [22, 23]. Figure 6 shows the photo current-voltage (𝐼-𝑉) characteristics for the DSSC under AM1.5 solar irradiation with 100 mW/cm2 illumination, with and without the TiO₂compact layer.Figure 6(a)shows a carbon counter electrode and ITO/glass, Figure 6(b) shows a Pt counter electrode and ITO/glass, and Figure 6(c) shows a Pt counter electrode and FTO/glass [24]. The corresponding cell parameters are summarized inTable 6, which shows the performance of the DSSC. With a TiO₂ compact layer to prevent charge recombination, a higher𝐽scis obtained. The

energy conversion efficiency (𝜂) increases if a Pt counter electrode is used instead of a carbon counter electrode.

For the purposes of comparison, the energy conversion efficiency for the DSSC film deposited on FTO glass is also given. FTO substrates have good optoelectronic performance and higher energy conversion efficiency than ITO substrates.

Table 6 shows that a FTO/sputter/spin coating/PT setup

increases the conversion efficiency of the DSSC, with𝑉oc =

0.70 V, 𝐽sc = 17.22 mA/cm2, a fill factor = 0.641, and an energy

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0.0 0.1 0.2 0.3 0.4 0.5 0 1 2 3 4 Voltage (V) ITO/sputter/spin coating/carbon ITO/sputter/blade coating/carbon ITO/spin coating/carbon ITO/blade coating/carbon C u rr en t den si ty (mA/cm 2) (a) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0 2 4 6 8 10 12 14 ITO/sputter/spin coating/Pt ITO/sputter/blade coating/Pt ITO/spin coating/Pt ITO/blade coating/Pt Voltage (V) C u rr en t den si ty (mA/cm 2) (b) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 3 6 9 12 15 Voltage (V) FTO/sputter/spin coating/Pt FTO/sputter/blade coating/Pt FTO/spin coating/Pt FTO/blade coating/Pt 18 C u rr en t den si ty (mA/cm 2 ) (c)

Figure 6: The𝐼-𝑉 characteristics for DSSCs fabricated with and without a TiO₂compact layer, (a) using a carbon counter electrode and

ITO/glass, (b) using a Pt counter electrode and ITO/glass, and (c) using a Pt counter electrode and FTO/glass, under AM 1.5 solar irradiation

with a density of 100 mW/cm2.

4. Conclusion

TiO₂ films (compact layer) are coated onto ITO/glass sub-strates (and FTO/glass), using rf magnetron sputtering. The reactive and sputtering gases are O₂and Ar, respectively. The multiple performance characteristics for the deposited TiO₂ compact films’ photocatalytic activity are greatly improved by using the Grey-Taguchi method. The improvement in the deposition rate is 12.87%, that in the water contact angle is 15.25%, and that in the MB absorbance is 16.06%. The porous

TiO₂film that covers the sputtered compact layer produced by the sol-gel method has a structure which is not dense and a crystallite size that is increased. The XRD patterns for TiO₂films produced using sol-gel with spin coating of porous TiO₂/TiO₂ compact/ITO/glass result in a good crystalline structure and the anatase (101) diffraction peaks demonstrate a higher degree of crystallinity. The energy conversion effi-ciency (𝜂) for a Pt counter electrode is greater than that for a carbon counter electrode. The experimental results show that FTO/sputter/spin coating/PT setup increases

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the conversion efficiency of the DSSC, with 𝑉oc = 0.70 V,

𝐽sc = 17.22 mA/cm2, the fill factor = 0.641, and an energy

conversion efficiency as high as 7.73%.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors gratefully acknowledge the support of the Kung Chi Technology Co., Ltd., the Ministry of Education of Taiwan, through Grant nos. 102G-88-022 and 102 M-88-021, and the Chung-Shan Institute of Science & Technology (Armaments Bureau).

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