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dye composite particles and their applications in dye-sensitized solar cell
Powder Technology 187 (2008) 181–189
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Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Preparation of TiO2/dye composite particles and their applications in dye-sensitized solar cell Chuen-Shii Chou a,⁎, Ru-Yuan Yang b, Min-Hang Weng c, Chun-Hung Yeh a a b c
Powder Technology Laboratory, Department of Mechanical Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan Department of Materials Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan National Nano Device Laboratory, Tainan, Taiwan
a r t i c l e
i n f o
Article history: Received 20 June 2007 Received in revised form 13 December 2007 Accepted 26 February 2008 Available online 7 March 2008 Keywords: Dye-sensitized solar cell Dry particle coating TiO2/dye composite particles
a b s t r a c t In this study a sandwich TiO2 thin-film electrode for a dye-sensitized solar cell was designed and fabricated. It contained a nanocrystalline TiO2 layer that was sandwiched between an ITO (indium tin oxide) substrate and a layer of TiO2/dye composite particles. Dry particle coating was performed to coat powdered CuPc dye (copper phthalocyanine C32H16CuN8) on the surface of TiO2 powder. A nanocrystalline TiO2 layer and a layer of TiO2/ dye composite particles were subsequently fabricated in that order on the ITO substrate. This layer of the TiO2/ dye composite particles markedly increased the short-circuit photocurrent from 0.2 μA(conventional DSSC) to 4 μA(DSSC with a layer of TiO2/dye composite particles) because of the improved coverage of the TiO2 surface by the powder of CuPc dye. Nevertheless, this layer of TiO2/dye composite particles slightly increased the open-circuit photovoltage from 0.25 V (conventional DSSC) to 0.28 V (DSSC with a layer of TiO2/dye composite particles). The effects of the mass ratio of TiO2 to the CuPc dye and the rate of rotation of the rotating chamber on the short-circuit photocurrent were investigated. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The dye-sensitized solar cell (DSSC) that was proposed by O'Regan and Grätzel [1] has attracted considerable interest since 1991 because it has attractive properties, such as low production cost and low environmental impact during fabrication. However, a comparison with conventional solid-sate junction devices made of crystalline silicon indicates that the DSSC has a relatively low conversion efficiency of 10.6% [2]. Recently, several methods have been adopted to modify the structure of the working electrode (TiO2 electrode) to improve the performance of the DSSC. For example, Hauch et al. produced a photoelectrochromic device by adding an electrochromic layer (WO3) to a DSSC [3]. Kang et al. added a thin buffer layer (TiO2–WO3) between a TCO (transparent conducting oxide) substrate and a TiO2 (P25) layer in the DSSC [4]. Adachi et al. applied the single-crystalline titania nanotube as a semiconductor thin-film electrode in the DSSC [5]. Jiu et al. presented a process (including a mixed template method) for fabricating a crack-free porous TiO2 electrode film of any thickness [6]. Richards et al. produced a highly porous TiO2 electrode film using APCVD [7]. Han et al. fabricated a hybrid TiO2 electrode with a sputter-
⁎ Corresponding author. E-mail address: [email protected] (C.-S. Chou). 0032-5910/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.02.010
deposited layer and a nanocrystalline layer [8]. Wei et al. used titania H2Ti3O7 multi-wall nanotubes as an electrode material in the DSSC [9]. Kim et al. fabricated the titanate nanotubes film on FTO (F–SnO2 coated glass) substrate using electrophoretic deposition (EPD) [10–12]. Additionally, Jun and Kang developed a DSSC with a flexible metal substrate [13]. Novel sensitizers were synthesized and applied in the DSSC to promote the absorption of the visible spectrum [14,15]. The novel solid electrolyte based on HPN and LiI [16] and the clay-like electrolyte with high nanoparticle content [17] have been proposed to prevent leakage. Kato and Hayase presented a quasi-solid dyesensitized solar cell with straight ion paths based on an anodic oxidation Al2O3 film, which was full of nanopores from one side of the film to the other [18]. The cited studies are concerned with only the dipping of the porous substrate into the dye solutions. Less attention has been paid to increasing the dye coverage ratio on the surface of TiO2. Bando et al. stained the porous TiO2 layer with organic dye (Eosin Y) under supercritical CO2 conditions [19], and Ogomi et al. stained the porous TiO2 layer with Ru dye under pressurized CO2 [20]. Therefore, increasing the dye coverage ratio on the surface of the nanosized TiO2 particles is one of the most important issues in increasing the conversion efficiency of DSSC, and is worthy of ongoing study. The purpose of this work is to modify physically the surface characteristics of nanosized TiO2 by coating the dye powder onto the surface of TiO2 in a Mechanofusion system, which is based on
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the use of a large amount of mechanical energy to dry-coat powders without the use of any liquid. In a dry coating process, tiny (guest) particles are coated onto larger (host) particles to generate value-added composite particulate materials [21]. Yokoyama et al. [21], Alonso et al. [22–24], Naito et al. [25], Kaga et al. [26], Mort and Riman [27] and Yoshino et al. [28] experimentally investigated dry coating. Chen et al. [29] theoretically studied the processing of Mechanofusion powder. Chen et al. [30] numerically simulated the Mecnanofusion system using discrete element modeling (DEM). The aforementioned references on dry coating concerned either the formation of composite particles, such as PMMA-TiO2, PS-carbon black, SiO2–TiO2, PMMA-PTFE, PMMA-magnetite, glass beads-TiO2, Cu–Al2O3 and polymer-carbon black-PMMA, or the forces on the powder bed, qualitative visualization of the particulate patterns and qualitative diagnostic analysis in terms of mean kinetic energy, rotational energy and collisional granular pressure in the Mechanofusion system. Little attention has been paid to coating the dye powder onto the surface of nano-sized TiO2 particles. The TiO2/dye composite particles were prepared using the dry coating process in a Mechanofusion system. Then, a sandwich TiO2 thin-film electrode, which contained a nanocrystalline TiO2 layer sandwiched between an ITO substrate and a layer of TiO2/dye composite particles (Fig. 1), was fabricated for use in a new type of dye-sensitized solar cell to promote the conversion efficiency of DSSC. 2. Experimental apparatus and procedure The experiments involved (1) preparing the TiO2/dye composite particles, and measuring their characteristics; (2) preparing the working electrode, and measuring its surface properties; (3) assembling the DSSC by fitting: the working electrode, the counter electrode, the electrolyte and the copper conductive tape, and (4) making I–V measurements of the DSSC at an energy intensity of 100 mW/cm2. Fig. 2 shows the procedure followed herein. 2.1. Preparing and measuring TiO2/dye composite particle The Mechanofusion system (Hosokawa Micron Corp. AMS-mini) with a motor power of 0.75 kW, a rotor speed of 1000–7000 rpm, and a capacity of 100 cc., was used to prepare the iO2/dye composite particles. Fig. 3 schematically depicts the Mechanofusion system, in which a fixed rounded press-head, a fixed scraper and a rotating Fig. 2. The research procedures.
Fig. 1. Schematic of the dye-sensitized solar cell with a sandwich TiO2 thin-film electrode.
chamber are installed. Although the clearance space between the rounded press-head and the inner wall of the rotating chamber is adjustable, the clearance herein is fixed at 2 mm. The host particles are TiO2 particles (P-25) of 30% rutile and 70% anatase, with average size of 21 nm. The guest particles herein are CuPc dye powder (copper phthalocyanine C32H16CuN8) with a flash point of 170 °C, a specific gravity of 160 kg/m3 at 20 °C, a specific conductivity (≤ 150 μΩ) and a particle size range of 50– 200 nm. A high-precision balance (EXCELL BH-150) with a measuring range of 0.005–150 g was utilized to weigh the TiO2 particulates and the CuPc dye powder. The TiO2-to-dye mass ratios (gram:gram) were (1) 9:1, (2) 9:2 and (3) 9:3. The rotor speeds (rpm) were (1) 3000, (2) 6000. The dry coating time was 30 min. Table 1 presents the test conditions under which the TiO2/dye composite particle was prepared. The micrographs of the TiO2 particles, the CuPc dye powder and the TiO2/dye composite particles were obtained using a field emission
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Fig. 3. Schematic of the Mechanofusion system.
scanning electron microscope (Hitachi S-4700) with a magnification of 100–500,000×. A powder X-ray diffractometer (Shimadzu XRD6000) was used to obtain the X-ray diffraction spectra of TiO2 particles, the CuPc dye powder and TiO2/dye composite particles. An energy dispersive spectrum (Horiba EX-200) was utilized to analyze the weight ratios of the elements in the TiO2 particles, the CuPc dye powder and the TiO2/dye composite particles, and to verify that the CuPc dye powder was coated on the surfaces of the TiO2 particles. A surface area and pore size analyzer (Beckman Coulter SA3100) was employed to measure the BET surface areas of the TiO2 particles, the CuPc dye powder and the TiO2/dye composite particles. 2.2. Preparing and measuring the working electrode A sandwich TiO2 thin-film electrode was designed and fabricated for use in a dye-sensitized solar cell. This sandwich TiO2 thinfilm electrode consists of (1) the first layer — nanocrystalline TiO2
Fig. 4. The morphology of the ITO surface.
Table 1 Test conditions of preparing composite particles
Test Test Test Test
P1 P2 P3 P4
Mass ratio of TiO2 to dye (g:g)
Rotation speed of rotating chamber (rpm)
Time of dry coating (min)
9:1 9:2 9:2 9:3
3000
30
6000
Table 2 Test conditions of preparing the working electrode First layer Sintering temperature (°C)
Sintering time (h)
TiO2
450
1
TiO2
450
1
Powder
Test Test Test Test Test
F1 F2 F3 F4 F5
Second layer Composite powder
Sintering temperature (°C)
P1 165 P2 P3 P4 Dipping TiO2 electrode into dye solution for 12 h
Sintering time (h) 1
Fig. 5. The schematic of assembling the DSSC.
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Fig. 6. FE-SEM micrographs and EDS analysis of dye, TiO2 and Test P3.
that was deposited on the top of an ITO substrate, and (2) the second layer — TiO2/dye composite particles, on the top of the first nanocrystalline TiO2 layer (Fig. 1). Table 2 presents the test conditions under which the working electrode was prepared. An atomic force microscope (Veeco CP-II) was adopted to obtain the rms roughness (Rq) and the morphology of 1 mm2 area of the ITO surface (Fig. 4). The rms roughness (Rq) of the ITO used herein is 0.401 nm. The procedure for fabricating the first layer of nanocrystalline TiO2 is as follows. (1) Dilute 0.1 mL of acetylacetone in 1 mL of pure water; (2) mix 3 g of TiO2 (P-25) with the diluted acetylacetone, stirring well
to form the colloid of TiO2; (3) adjust the glutinosity of the colloid of TiO2 by adding 4 mL of pure water and 0.1 mL of Triton-100, and then homogenizing the colloid of TiO2 for 30 min. in an ultrasonic homogenizer (Delta DC150H); (4) deposit the colloid of TiO2 on a cleaned ITO substrate (20 × 20 × 7 mm, 100 Ω/sq) using the doctor blade method; (5) dry the electrode at room temperature for 10 min., and then sinter it at 450 °C for 1 h in a high-temperature furnace (Thermolyne 46100). The procedure for fabricating the second layer of TiO2/dye composite particles is as follows. (1) Dilute 0.1 mL of acetylacetone in 1 mL of pure water; (2) mix 3 g of TiO2/dye composite particles with
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Fig. 7. BEI mapping of composite particles in Test P2 and Test P3. (a) Test P2 (× 600), (b) Test P2 (×4000), (c) Test P3 (× 600), (d) Test P3 (×4000).
the diluted acetylacetone, and stir well to make the colloid of TiO2/dye composite particles; (3) adjust the glutinosity of the colloid of TiO2/ dye composite particles by adding 4 mL of pure water and 0.1 mL of Triton-100, and then homogenize the colloid of TiO2/dye composite particles for 30 min. in an ultrasonic homogenizer; (4) deposit the colloid of TiO2/dye composite particles on the top of the first nanocrystalline TiO2 layer using the doctor blade method; (5) dry
the sandwich electrode at room temperature for 10 min., and then sinter the sandwich electrode at 165 °C for 1 h. in a high-temperature furnace. A conventional working electrode was fabricated by immersing the ITO substrate with a layer of nanocrystalline TiO2 into the dye solution with a concentration of 3 × 10−4 mol/L for 12 h at room temperature, to demonstrate the feasibility and advantages of a sandwiched TiO2 working electrode.
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Fig. 8. X-ray diffraction spectra of TiO2, CuPc dye and the composite particles in Test P1.
The micrographs of the thin film on the ITO substrate of the working electrode were obtained using a field emission scanning electron microscope (Hitachi S-4700). An α-step (Tencor P-10) surface profiler was utilized to obtain the surface profile of the thin film on the ITO substrate of the working electrode, and to determine the average thickness of the thin film. 2.3. Assembling and testing the DSSC The thin film on the ITO substrate of the working electrode was fringed using the Scotch magic transparent tape (3 M). Then, the working electrode, the counter electrode (i.e. an ITO substrate) and the copper conductive tape (Ted Pella) were fitted together (Fig. 5), such that the space between the two electrodes was adjusted to approximately 25 μm for embarking the liquid electrolyte (Fig. 5). After sealing, the liquid electrolyte, comprising 20 mL of propylene carbonate, 0.254 g of iodine (I2) and 1.66 g of potassium iodine (KI), was injected into the cell through a hole on the cell that was prepared in advance. A digital multimeter (Brymen BM-817) was adopted to measure the open-circuit photovoltage and short-circuit photocurrent of the DSSC. A solar simulator with a 500 W halogen lamp and a light intensity of 100 mW/cm2, was employed to illuminate the DSSC. 3. Results and discussion 3.1. Characteristics of TiO2/dye composite particles Fig. 6 displays the micrographs and the weight ratios of the elements in the TiO2 particles, the CuPc dye powder and the TiO2/dye composite particles. It consists of three panels. (a) The top panel refers to the CuPc dye powder; (b) the central panel refers to the TiO2 particles, and (c) the bottom panel refers to the TiO2/dye composite particles in Test P3 (TiO2: CuPc dye = 9:2; 6000 rpm). Before the FESEM micrographs and the EDS analysis were obtained, the aurum (Au) was deposited on the TiO2 particles, the CuPc dye powder and the TiO2/dye composite particles. The dry coating process is as follows. When the rotating chamber, into which the TiO2 particulates and the CuPc dye powder are poured, rotates, the centrifugal force push both the TiO2 particulates and the
powder of CuPc dye to the inner wall of the rotating chamber. High shear-rate particle-particle and particle-wall collisions force the TiO2 particulates and the CuPc dye powder to adhere to each other and to the inner wall of the rotating chamber, while the TiO2 particulates and the CuPc dye powder are passing through the space between the rounded press-head and the inner wall of the rotating chamber. Then, the scraper blade is used to scrape the powder mixture off the inner wall of the rotating chamber. This sheared powder mixture continuously and repeatedly undergoes this procedure while the chamber is rotating (Fig. 3). The FE-SEM micrograph and the analysis of EDS, shown in the bottom panel of Fig. 6, demonstrate that the CuPc dye powder disperses and adheres to the surface of the TiO2 particles, since the element copper (Cu) is detected. This result probably follows from the fact that the TiO2 particles and the CuPc dye powder that are loaded into the rotating chamber periodically absorb a considerable amount of thermo-mechanical energy, bringing the CuPc dye powder into close contact with the TiO2 particles. Fig. 7 shows the BEI mapping of the TiO2/dye composite particles in Test P2 (TiO2: CuPc dye = 9:2; 3000 rpm) and Test P3 (TiO2: CuPc dye = 9:2; 6000 rpm), obtained using the energy dispersive spectrometer. It consists of four panels. (a) The top panel refers to Test P2 (×600); (b) the second panel from the top refers to Test P2 (×4000), (c) the third panel from the top refers to Test P3 (×600), and (d) the bottom panel refers to Test P3 (×4000). The black dots in Fig. 7 depict the dispersion of CuPc dye powder on the surface of TiO2 particles. As the rotation speed of the chamber increases from 3000 to 6000 rpm, the percentage of Cu on the TiO2 surface decreases from 1.82 to 1.73%. Therefore, increasing the rotational speed of the chamber makes the dispersion of CuPc dye powder on the TiO2 surface more uniform and extensive. Fig. 8 presents the X-ray diffraction spectra of the TiO2 particles, the CuPc dye powder and the TiO2/dye composite particles in Test P1. The diamonds, triangles and circles in Fig. 8 represent anatase, rutile, and CuPc, respectively. The peaks from the TiO2/dye composite particles, which comprise partial peaks of anatase (b101N and b211N), rutile (b110N and b211N) and CuPc b014N, also support the assertion that the CuPc dye powder disperses and adheres to the surface of the TiO2 particles. Table 3 presents the BET surface areas of the TiO2 particles, the CuPc dye powder and the TiO2/dye composite particles in Test P1 (9:1; 3000 rpm), Test P2 (9:2; 3000 rpm) and Test P3 (9:2; 6000 rpm). The BET surface area of the TiO2 particles (49.02 m2/g) significantly exceeds that of the CuPc dye powder (33.4 m2/g). This difference between the BET surface areas probably follows from the fact that the CuPc dye powder easily agglomerates at the standard temperature and pressure. However, the BET surface area of the TiO2/dye composite particles in Test P1 decreases to 43.71 m2/g because 1 g of the CuPc dye powder disperses and adheres to the surface of the TiO2 particles. For a fixed rotational speed of the rotating chamber, as the mass of the CuPc dye powder increases from 1 g (Test P1) to 2 g (Test P2), the BET surface area of the TiO2/ dye composite particles falls from 43.71 to 38.5 m2/g. Additionally, for a fixed mass of the CuPc dye powder, as the rate of rotation of the chamber increases from 3000 to 6000 rpm, the BET surface area
Table 3 BET surface areas of dye, TiO2 and composite particles BET surface area (m2/g) Dye TiO2 Test P1 Test P2 Test P3
33.40 49.02 43.71 38.50 39.39
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of the TiO2/dye composite particles increases slightly from 38.5 to 39.39 m2/g because the higher rate of rotation is responsible for the more extensive dispersion of the CuPc dye powder on the surface of the TiO2 particles. 3.2. Properties of working electrode Fig. 9 shows the FE-SEM micrographs of the surface and crosssection of the working electrode, and has six frames: (a) surface image (× 5k) in Test F5 (dipping TiO2 electrode into dye solution for 12 h), (b) surface image (× 5k) in Test F1 (in which the working
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electrode has a layer of TiO2 and a layer of TiO2/dye composite particles which were prepared in Test P1), (c) surface image (× 200k) in Test F5, (d) surface image (× 200k) in Test F1, (e) cross-section image (× 2k) in Test F5, and (f) cross-section image (× 2k) in Test F1. Table 4 also lists the average thicknesses of the thin films on the ITO substrate of the working electrodes in Test F1, Test F2, Test F3, Test F4 and Test F5. The porous TiO2 film on the ITO substrate of the working electrode in Test F5 (immersed in the solution of CuPc dye for 12 h) has numerous tiny cavities (Fig. 9(c)), which may delay the moving of electrons and reduce the short-circuit photocurrent below that in the
Fig. 9. FE-SEM micrographs of the surface and cross-section of the working electrode. (a) The surface image (×5k) in Test F5 (dipping TiO2 electrode into dye solution for 12 h.); (b) the surface image (×5k) in Test F1 (whose working electrode has a layer of TiO2 and a layer of TiO2/dye composite particles prepared in Test P1); (c) the surface image (×200k) in Test F5; (d) the surface image (×200k) in Test F1; (e) the cross-section image (×2k) in Test F5; (f) the cross-section image (×2k) in Test F1.
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Table 4 Open-circuit voltage and short-circuit current of DSSCs Working electrode Composition on the ITO Test Test Test Test Test
D1 D2 D3 D4 D5
Test F1 Test F2 Test F3 Test F4 Test F5
Open-circuit voltage (V)
Short-circuit current (μA)
0.21 0.25 0.28 0.26 0.25
2.3 3.6 4 3.2 0.2
Average thickness of film on the ITO (μm) 34.67 28.01 27.52 36.14 18.36
other four tests (Table 4). The average thicknesses of the TiO2 layer and the TiO2/dye composite particle layer in Test F1 (Fig. 9(f)) are 20.67 and 15.1 μm, respectively. 3.3. Open-circuit photovoltage and short-circuit photocurrent of DSSC Table 4 presents the open-circuit photovoltage and short-circuits photocurrent of the DSSC in Test D1, Test D2, Test D3, Test D4 and Test D5. In this investigation, an ITO was the counter electrode of the DSSC. The short-circuit photocurrent of the DSSC with a sandwich TiO2 thinfilm working electrode greatly exceeds that of the conventional DSSC (Test D5). For example, the short-circuit photocurrents of the DSSC in Test D3 and Test D5 are 4 and 0.2 μA, respectively. However, the opencircuit photovoltage slightly increases from 0.25 V in Test D5 (conventional DSSC) to 0.28 V in Test D3 (DSSC with a layer of TiO2/ dye composite particles). The short-circuit photocurrent of the DSSC increases with the mass ratio of TiO2 to CuPc dye. For instance, the short-circuit photocurrent of the DSSC increases from 2.3 to 3.6 μA with the mass ratio of TiO2 to CuPc dye from 9:1 to 9:2. The variation in the short-circuit photocurrent probably follows from the combination of the increase in mass of the CuPc dye and the increase in the coverage of the TiO2 surface by the CuPc dye powder upon dry coating, which not only increases the number of excited electrons from the CuPc dye, but also promotes the rapid injection of these excited electrons into the TiO2 particles. For a fixed mass ratio of TiO2 to CuPc dye, increasing the rate of rotation of the chamber may increase the dispersion of the CuPc dye powder onto the surfaces of the TiO2 particles, increasing the shortcircuit photocurrent of the DSSC. For example, the short-circuit photocurrent of the DSSC increases from 3.6 to 4.0 μA as the rate of rotation of the chamber increases from 3000 to 6000 rpm. This difference between the short-circuit photocurrents probably follows from the fact that, for a given duration of dry coating, faster rotation corresponds to greater dispersion and stronger bounding of the CuPc dye powder onto the surfaces of the TiO2 particles. For a given period of dry coating and rate of rotation of the chamber, as the mass ratio of TiO2 to CuPc dye increases above a certain value (such as 9:2), the dispersion of the CuPc dye powder onto the surfaces of the TiO2 particles becomes worse, reducing the shortcircuit photocurrent of the DSSC. For instance, the short-circuit photocurrent of the DSSC decreases from 4.0 to 3.2 μA as the mass ratio of TiO2 to CuPc dye increases from 9:2 to 9:3. The change in the short-circuit photocurrent probably follows from the fact that the redundant CuPc dye, which does not directly come into contact with the surfaces of the TiO2 particles, may intercept the electrons from the electrolyte and inhibit the recombination of charges in the CuPc dye that come into strong contact with the surface of the TiO2 particle. This shortcoming may be overcome by either raising the rate of rotation of the chamber or increasing the duration of dry coating. 4. Conclusion The TiO2/dye composite particles were prepared by dry coating. Then, the dye-sensitized solar cells, which contained a nanocrystalline
TiO2 layer sandwiched between an ITO substrate and a layer of TiO2/ dye composite particles, were fabricated. The short-circuit photocurrent of the DSSC with a sandwich TiO2 thin-film as an electrode greatly exceeded that of the conventional DSSC. Nevertheless, the open-circuit photovoltage of the DSSC with a sandwich TiO2 thin-film working electrode was very similar to that of the conventional DSSC. Most importantly, this study supports the application of TiO2/dye composite particles to improve the performance of a DSSC. However, the optimal process for fabricating a DSSC with a layer of TiO2/dye composite particles must be implemented to yield DSSC with a satisfactory conversion efficiency. Acknowledgement The authors would like to thank the National Science Council, R.O.C., Taiwan for financially supporting this research under Contract No. NSC 95-2211-E-020-037. 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Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Preparation of TiO2/dye composite particles and their applications in dye-sensitized solar cell Chuen-Shii Chou a,⁎, Ru-Yuan Yang b, Min-Hang Weng c, Chun-Hung Yeh a a b c
Powder Technology Laboratory, Department of Mechanical Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan Department of Materials Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan National Nano Device Laboratory, Tainan, Taiwan
a r t i c l e
i n f o
Article history: Received 20 June 2007 Received in revised form 13 December 2007 Accepted 26 February 2008 Available online 7 March 2008 Keywords: Dye-sensitized solar cell Dry particle coating TiO2/dye composite particles
a b s t r a c t In this study a sandwich TiO2 thin-film electrode for a dye-sensitized solar cell was designed and fabricated. It contained a nanocrystalline TiO2 layer that was sandwiched between an ITO (indium tin oxide) substrate and a layer of TiO2/dye composite particles. Dry particle coating was performed to coat powdered CuPc dye (copper phthalocyanine C32H16CuN8) on the surface of TiO2 powder. A nanocrystalline TiO2 layer and a layer of TiO2/ dye composite particles were subsequently fabricated in that order on the ITO substrate. This layer of the TiO2/ dye composite particles markedly increased the short-circuit photocurrent from 0.2 μA(conventional DSSC) to 4 μA(DSSC with a layer of TiO2/dye composite particles) because of the improved coverage of the TiO2 surface by the powder of CuPc dye. Nevertheless, this layer of TiO2/dye composite particles slightly increased the open-circuit photovoltage from 0.25 V (conventional DSSC) to 0.28 V (DSSC with a layer of TiO2/dye composite particles). The effects of the mass ratio of TiO2 to the CuPc dye and the rate of rotation of the rotating chamber on the short-circuit photocurrent were investigated. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The dye-sensitized solar cell (DSSC) that was proposed by O'Regan and Grätzel [1] has attracted considerable interest since 1991 because it has attractive properties, such as low production cost and low environmental impact during fabrication. However, a comparison with conventional solid-sate junction devices made of crystalline silicon indicates that the DSSC has a relatively low conversion efficiency of 10.6% [2]. Recently, several methods have been adopted to modify the structure of the working electrode (TiO2 electrode) to improve the performance of the DSSC. For example, Hauch et al. produced a photoelectrochromic device by adding an electrochromic layer (WO3) to a DSSC [3]. Kang et al. added a thin buffer layer (TiO2–WO3) between a TCO (transparent conducting oxide) substrate and a TiO2 (P25) layer in the DSSC [4]. Adachi et al. applied the single-crystalline titania nanotube as a semiconductor thin-film electrode in the DSSC [5]. Jiu et al. presented a process (including a mixed template method) for fabricating a crack-free porous TiO2 electrode film of any thickness [6]. Richards et al. produced a highly porous TiO2 electrode film using APCVD [7]. Han et al. fabricated a hybrid TiO2 electrode with a sputter-
⁎ Corresponding author. E-mail address: [email protected] (C.-S. Chou). 0032-5910/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.02.010
deposited layer and a nanocrystalline layer [8]. Wei et al. used titania H2Ti3O7 multi-wall nanotubes as an electrode material in the DSSC [9]. Kim et al. fabricated the titanate nanotubes film on FTO (F–SnO2 coated glass) substrate using electrophoretic deposition (EPD) [10–12]. Additionally, Jun and Kang developed a DSSC with a flexible metal substrate [13]. Novel sensitizers were synthesized and applied in the DSSC to promote the absorption of the visible spectrum [14,15]. The novel solid electrolyte based on HPN and LiI [16] and the clay-like electrolyte with high nanoparticle content [17] have been proposed to prevent leakage. Kato and Hayase presented a quasi-solid dyesensitized solar cell with straight ion paths based on an anodic oxidation Al2O3 film, which was full of nanopores from one side of the film to the other [18]. The cited studies are concerned with only the dipping of the porous substrate into the dye solutions. Less attention has been paid to increasing the dye coverage ratio on the surface of TiO2. Bando et al. stained the porous TiO2 layer with organic dye (Eosin Y) under supercritical CO2 conditions [19], and Ogomi et al. stained the porous TiO2 layer with Ru dye under pressurized CO2 [20]. Therefore, increasing the dye coverage ratio on the surface of the nanosized TiO2 particles is one of the most important issues in increasing the conversion efficiency of DSSC, and is worthy of ongoing study. The purpose of this work is to modify physically the surface characteristics of nanosized TiO2 by coating the dye powder onto the surface of TiO2 in a Mechanofusion system, which is based on
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the use of a large amount of mechanical energy to dry-coat powders without the use of any liquid. In a dry coating process, tiny (guest) particles are coated onto larger (host) particles to generate value-added composite particulate materials [21]. Yokoyama et al. [21], Alonso et al. [22–24], Naito et al. [25], Kaga et al. [26], Mort and Riman [27] and Yoshino et al. [28] experimentally investigated dry coating. Chen et al. [29] theoretically studied the processing of Mechanofusion powder. Chen et al. [30] numerically simulated the Mecnanofusion system using discrete element modeling (DEM). The aforementioned references on dry coating concerned either the formation of composite particles, such as PMMA-TiO2, PS-carbon black, SiO2–TiO2, PMMA-PTFE, PMMA-magnetite, glass beads-TiO2, Cu–Al2O3 and polymer-carbon black-PMMA, or the forces on the powder bed, qualitative visualization of the particulate patterns and qualitative diagnostic analysis in terms of mean kinetic energy, rotational energy and collisional granular pressure in the Mechanofusion system. Little attention has been paid to coating the dye powder onto the surface of nano-sized TiO2 particles. The TiO2/dye composite particles were prepared using the dry coating process in a Mechanofusion system. Then, a sandwich TiO2 thin-film electrode, which contained a nanocrystalline TiO2 layer sandwiched between an ITO substrate and a layer of TiO2/dye composite particles (Fig. 1), was fabricated for use in a new type of dye-sensitized solar cell to promote the conversion efficiency of DSSC. 2. Experimental apparatus and procedure The experiments involved (1) preparing the TiO2/dye composite particles, and measuring their characteristics; (2) preparing the working electrode, and measuring its surface properties; (3) assembling the DSSC by fitting: the working electrode, the counter electrode, the electrolyte and the copper conductive tape, and (4) making I–V measurements of the DSSC at an energy intensity of 100 mW/cm2. Fig. 2 shows the procedure followed herein. 2.1. Preparing and measuring TiO2/dye composite particle The Mechanofusion system (Hosokawa Micron Corp. AMS-mini) with a motor power of 0.75 kW, a rotor speed of 1000–7000 rpm, and a capacity of 100 cc., was used to prepare the iO2/dye composite particles. Fig. 3 schematically depicts the Mechanofusion system, in which a fixed rounded press-head, a fixed scraper and a rotating Fig. 2. The research procedures.
Fig. 1. Schematic of the dye-sensitized solar cell with a sandwich TiO2 thin-film electrode.
chamber are installed. Although the clearance space between the rounded press-head and the inner wall of the rotating chamber is adjustable, the clearance herein is fixed at 2 mm. The host particles are TiO2 particles (P-25) of 30% rutile and 70% anatase, with average size of 21 nm. The guest particles herein are CuPc dye powder (copper phthalocyanine C32H16CuN8) with a flash point of 170 °C, a specific gravity of 160 kg/m3 at 20 °C, a specific conductivity (≤ 150 μΩ) and a particle size range of 50– 200 nm. A high-precision balance (EXCELL BH-150) with a measuring range of 0.005–150 g was utilized to weigh the TiO2 particulates and the CuPc dye powder. The TiO2-to-dye mass ratios (gram:gram) were (1) 9:1, (2) 9:2 and (3) 9:3. The rotor speeds (rpm) were (1) 3000, (2) 6000. The dry coating time was 30 min. Table 1 presents the test conditions under which the TiO2/dye composite particle was prepared. The micrographs of the TiO2 particles, the CuPc dye powder and the TiO2/dye composite particles were obtained using a field emission
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Fig. 3. Schematic of the Mechanofusion system.
scanning electron microscope (Hitachi S-4700) with a magnification of 100–500,000×. A powder X-ray diffractometer (Shimadzu XRD6000) was used to obtain the X-ray diffraction spectra of TiO2 particles, the CuPc dye powder and TiO2/dye composite particles. An energy dispersive spectrum (Horiba EX-200) was utilized to analyze the weight ratios of the elements in the TiO2 particles, the CuPc dye powder and the TiO2/dye composite particles, and to verify that the CuPc dye powder was coated on the surfaces of the TiO2 particles. A surface area and pore size analyzer (Beckman Coulter SA3100) was employed to measure the BET surface areas of the TiO2 particles, the CuPc dye powder and the TiO2/dye composite particles. 2.2. Preparing and measuring the working electrode A sandwich TiO2 thin-film electrode was designed and fabricated for use in a dye-sensitized solar cell. This sandwich TiO2 thinfilm electrode consists of (1) the first layer — nanocrystalline TiO2
Fig. 4. The morphology of the ITO surface.
Table 1 Test conditions of preparing composite particles
Test Test Test Test
P1 P2 P3 P4
Mass ratio of TiO2 to dye (g:g)
Rotation speed of rotating chamber (rpm)
Time of dry coating (min)
9:1 9:2 9:2 9:3
3000
30
6000
Table 2 Test conditions of preparing the working electrode First layer Sintering temperature (°C)
Sintering time (h)
TiO2
450
1
TiO2
450
1
Powder
Test Test Test Test Test
F1 F2 F3 F4 F5
Second layer Composite powder
Sintering temperature (°C)
P1 165 P2 P3 P4 Dipping TiO2 electrode into dye solution for 12 h
Sintering time (h) 1
Fig. 5. The schematic of assembling the DSSC.
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Fig. 6. FE-SEM micrographs and EDS analysis of dye, TiO2 and Test P3.
that was deposited on the top of an ITO substrate, and (2) the second layer — TiO2/dye composite particles, on the top of the first nanocrystalline TiO2 layer (Fig. 1). Table 2 presents the test conditions under which the working electrode was prepared. An atomic force microscope (Veeco CP-II) was adopted to obtain the rms roughness (Rq) and the morphology of 1 mm2 area of the ITO surface (Fig. 4). The rms roughness (Rq) of the ITO used herein is 0.401 nm. The procedure for fabricating the first layer of nanocrystalline TiO2 is as follows. (1) Dilute 0.1 mL of acetylacetone in 1 mL of pure water; (2) mix 3 g of TiO2 (P-25) with the diluted acetylacetone, stirring well
to form the colloid of TiO2; (3) adjust the glutinosity of the colloid of TiO2 by adding 4 mL of pure water and 0.1 mL of Triton-100, and then homogenizing the colloid of TiO2 for 30 min. in an ultrasonic homogenizer (Delta DC150H); (4) deposit the colloid of TiO2 on a cleaned ITO substrate (20 × 20 × 7 mm, 100 Ω/sq) using the doctor blade method; (5) dry the electrode at room temperature for 10 min., and then sinter it at 450 °C for 1 h in a high-temperature furnace (Thermolyne 46100). The procedure for fabricating the second layer of TiO2/dye composite particles is as follows. (1) Dilute 0.1 mL of acetylacetone in 1 mL of pure water; (2) mix 3 g of TiO2/dye composite particles with
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Fig. 7. BEI mapping of composite particles in Test P2 and Test P3. (a) Test P2 (× 600), (b) Test P2 (×4000), (c) Test P3 (× 600), (d) Test P3 (×4000).
the diluted acetylacetone, and stir well to make the colloid of TiO2/dye composite particles; (3) adjust the glutinosity of the colloid of TiO2/ dye composite particles by adding 4 mL of pure water and 0.1 mL of Triton-100, and then homogenize the colloid of TiO2/dye composite particles for 30 min. in an ultrasonic homogenizer; (4) deposit the colloid of TiO2/dye composite particles on the top of the first nanocrystalline TiO2 layer using the doctor blade method; (5) dry
the sandwich electrode at room temperature for 10 min., and then sinter the sandwich electrode at 165 °C for 1 h. in a high-temperature furnace. A conventional working electrode was fabricated by immersing the ITO substrate with a layer of nanocrystalline TiO2 into the dye solution with a concentration of 3 × 10−4 mol/L for 12 h at room temperature, to demonstrate the feasibility and advantages of a sandwiched TiO2 working electrode.
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Fig. 8. X-ray diffraction spectra of TiO2, CuPc dye and the composite particles in Test P1.
The micrographs of the thin film on the ITO substrate of the working electrode were obtained using a field emission scanning electron microscope (Hitachi S-4700). An α-step (Tencor P-10) surface profiler was utilized to obtain the surface profile of the thin film on the ITO substrate of the working electrode, and to determine the average thickness of the thin film. 2.3. Assembling and testing the DSSC The thin film on the ITO substrate of the working electrode was fringed using the Scotch magic transparent tape (3 M). Then, the working electrode, the counter electrode (i.e. an ITO substrate) and the copper conductive tape (Ted Pella) were fitted together (Fig. 5), such that the space between the two electrodes was adjusted to approximately 25 μm for embarking the liquid electrolyte (Fig. 5). After sealing, the liquid electrolyte, comprising 20 mL of propylene carbonate, 0.254 g of iodine (I2) and 1.66 g of potassium iodine (KI), was injected into the cell through a hole on the cell that was prepared in advance. A digital multimeter (Brymen BM-817) was adopted to measure the open-circuit photovoltage and short-circuit photocurrent of the DSSC. A solar simulator with a 500 W halogen lamp and a light intensity of 100 mW/cm2, was employed to illuminate the DSSC. 3. Results and discussion 3.1. Characteristics of TiO2/dye composite particles Fig. 6 displays the micrographs and the weight ratios of the elements in the TiO2 particles, the CuPc dye powder and the TiO2/dye composite particles. It consists of three panels. (a) The top panel refers to the CuPc dye powder; (b) the central panel refers to the TiO2 particles, and (c) the bottom panel refers to the TiO2/dye composite particles in Test P3 (TiO2: CuPc dye = 9:2; 6000 rpm). Before the FESEM micrographs and the EDS analysis were obtained, the aurum (Au) was deposited on the TiO2 particles, the CuPc dye powder and the TiO2/dye composite particles. The dry coating process is as follows. When the rotating chamber, into which the TiO2 particulates and the CuPc dye powder are poured, rotates, the centrifugal force push both the TiO2 particulates and the
powder of CuPc dye to the inner wall of the rotating chamber. High shear-rate particle-particle and particle-wall collisions force the TiO2 particulates and the CuPc dye powder to adhere to each other and to the inner wall of the rotating chamber, while the TiO2 particulates and the CuPc dye powder are passing through the space between the rounded press-head and the inner wall of the rotating chamber. Then, the scraper blade is used to scrape the powder mixture off the inner wall of the rotating chamber. This sheared powder mixture continuously and repeatedly undergoes this procedure while the chamber is rotating (Fig. 3). The FE-SEM micrograph and the analysis of EDS, shown in the bottom panel of Fig. 6, demonstrate that the CuPc dye powder disperses and adheres to the surface of the TiO2 particles, since the element copper (Cu) is detected. This result probably follows from the fact that the TiO2 particles and the CuPc dye powder that are loaded into the rotating chamber periodically absorb a considerable amount of thermo-mechanical energy, bringing the CuPc dye powder into close contact with the TiO2 particles. Fig. 7 shows the BEI mapping of the TiO2/dye composite particles in Test P2 (TiO2: CuPc dye = 9:2; 3000 rpm) and Test P3 (TiO2: CuPc dye = 9:2; 6000 rpm), obtained using the energy dispersive spectrometer. It consists of four panels. (a) The top panel refers to Test P2 (×600); (b) the second panel from the top refers to Test P2 (×4000), (c) the third panel from the top refers to Test P3 (×600), and (d) the bottom panel refers to Test P3 (×4000). The black dots in Fig. 7 depict the dispersion of CuPc dye powder on the surface of TiO2 particles. As the rotation speed of the chamber increases from 3000 to 6000 rpm, the percentage of Cu on the TiO2 surface decreases from 1.82 to 1.73%. Therefore, increasing the rotational speed of the chamber makes the dispersion of CuPc dye powder on the TiO2 surface more uniform and extensive. Fig. 8 presents the X-ray diffraction spectra of the TiO2 particles, the CuPc dye powder and the TiO2/dye composite particles in Test P1. The diamonds, triangles and circles in Fig. 8 represent anatase, rutile, and CuPc, respectively. The peaks from the TiO2/dye composite particles, which comprise partial peaks of anatase (b101N and b211N), rutile (b110N and b211N) and CuPc b014N, also support the assertion that the CuPc dye powder disperses and adheres to the surface of the TiO2 particles. Table 3 presents the BET surface areas of the TiO2 particles, the CuPc dye powder and the TiO2/dye composite particles in Test P1 (9:1; 3000 rpm), Test P2 (9:2; 3000 rpm) and Test P3 (9:2; 6000 rpm). The BET surface area of the TiO2 particles (49.02 m2/g) significantly exceeds that of the CuPc dye powder (33.4 m2/g). This difference between the BET surface areas probably follows from the fact that the CuPc dye powder easily agglomerates at the standard temperature and pressure. However, the BET surface area of the TiO2/dye composite particles in Test P1 decreases to 43.71 m2/g because 1 g of the CuPc dye powder disperses and adheres to the surface of the TiO2 particles. For a fixed rotational speed of the rotating chamber, as the mass of the CuPc dye powder increases from 1 g (Test P1) to 2 g (Test P2), the BET surface area of the TiO2/ dye composite particles falls from 43.71 to 38.5 m2/g. Additionally, for a fixed mass of the CuPc dye powder, as the rate of rotation of the chamber increases from 3000 to 6000 rpm, the BET surface area
Table 3 BET surface areas of dye, TiO2 and composite particles BET surface area (m2/g) Dye TiO2 Test P1 Test P2 Test P3
33.40 49.02 43.71 38.50 39.39
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of the TiO2/dye composite particles increases slightly from 38.5 to 39.39 m2/g because the higher rate of rotation is responsible for the more extensive dispersion of the CuPc dye powder on the surface of the TiO2 particles. 3.2. Properties of working electrode Fig. 9 shows the FE-SEM micrographs of the surface and crosssection of the working electrode, and has six frames: (a) surface image (× 5k) in Test F5 (dipping TiO2 electrode into dye solution for 12 h), (b) surface image (× 5k) in Test F1 (in which the working
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electrode has a layer of TiO2 and a layer of TiO2/dye composite particles which were prepared in Test P1), (c) surface image (× 200k) in Test F5, (d) surface image (× 200k) in Test F1, (e) cross-section image (× 2k) in Test F5, and (f) cross-section image (× 2k) in Test F1. Table 4 also lists the average thicknesses of the thin films on the ITO substrate of the working electrodes in Test F1, Test F2, Test F3, Test F4 and Test F5. The porous TiO2 film on the ITO substrate of the working electrode in Test F5 (immersed in the solution of CuPc dye for 12 h) has numerous tiny cavities (Fig. 9(c)), which may delay the moving of electrons and reduce the short-circuit photocurrent below that in the
Fig. 9. FE-SEM micrographs of the surface and cross-section of the working electrode. (a) The surface image (×5k) in Test F5 (dipping TiO2 electrode into dye solution for 12 h.); (b) the surface image (×5k) in Test F1 (whose working electrode has a layer of TiO2 and a layer of TiO2/dye composite particles prepared in Test P1); (c) the surface image (×200k) in Test F5; (d) the surface image (×200k) in Test F1; (e) the cross-section image (×2k) in Test F5; (f) the cross-section image (×2k) in Test F1.
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Table 4 Open-circuit voltage and short-circuit current of DSSCs Working electrode Composition on the ITO Test Test Test Test Test
D1 D2 D3 D4 D5
Test F1 Test F2 Test F3 Test F4 Test F5
Open-circuit voltage (V)
Short-circuit current (μA)
0.21 0.25 0.28 0.26 0.25
2.3 3.6 4 3.2 0.2
Average thickness of film on the ITO (μm) 34.67 28.01 27.52 36.14 18.36
other four tests (Table 4). The average thicknesses of the TiO2 layer and the TiO2/dye composite particle layer in Test F1 (Fig. 9(f)) are 20.67 and 15.1 μm, respectively. 3.3. Open-circuit photovoltage and short-circuit photocurrent of DSSC Table 4 presents the open-circuit photovoltage and short-circuits photocurrent of the DSSC in Test D1, Test D2, Test D3, Test D4 and Test D5. In this investigation, an ITO was the counter electrode of the DSSC. The short-circuit photocurrent of the DSSC with a sandwich TiO2 thinfilm working electrode greatly exceeds that of the conventional DSSC (Test D5). For example, the short-circuit photocurrents of the DSSC in Test D3 and Test D5 are 4 and 0.2 μA, respectively. However, the opencircuit photovoltage slightly increases from 0.25 V in Test D5 (conventional DSSC) to 0.28 V in Test D3 (DSSC with a layer of TiO2/ dye composite particles). The short-circuit photocurrent of the DSSC increases with the mass ratio of TiO2 to CuPc dye. For instance, the short-circuit photocurrent of the DSSC increases from 2.3 to 3.6 μA with the mass ratio of TiO2 to CuPc dye from 9:1 to 9:2. The variation in the short-circuit photocurrent probably follows from the combination of the increase in mass of the CuPc dye and the increase in the coverage of the TiO2 surface by the CuPc dye powder upon dry coating, which not only increases the number of excited electrons from the CuPc dye, but also promotes the rapid injection of these excited electrons into the TiO2 particles. For a fixed mass ratio of TiO2 to CuPc dye, increasing the rate of rotation of the chamber may increase the dispersion of the CuPc dye powder onto the surfaces of the TiO2 particles, increasing the shortcircuit photocurrent of the DSSC. For example, the short-circuit photocurrent of the DSSC increases from 3.6 to 4.0 μA as the rate of rotation of the chamber increases from 3000 to 6000 rpm. This difference between the short-circuit photocurrents probably follows from the fact that, for a given duration of dry coating, faster rotation corresponds to greater dispersion and stronger bounding of the CuPc dye powder onto the surfaces of the TiO2 particles. For a given period of dry coating and rate of rotation of the chamber, as the mass ratio of TiO2 to CuPc dye increases above a certain value (such as 9:2), the dispersion of the CuPc dye powder onto the surfaces of the TiO2 particles becomes worse, reducing the shortcircuit photocurrent of the DSSC. For instance, the short-circuit photocurrent of the DSSC decreases from 4.0 to 3.2 μA as the mass ratio of TiO2 to CuPc dye increases from 9:2 to 9:3. The change in the short-circuit photocurrent probably follows from the fact that the redundant CuPc dye, which does not directly come into contact with the surfaces of the TiO2 particles, may intercept the electrons from the electrolyte and inhibit the recombination of charges in the CuPc dye that come into strong contact with the surface of the TiO2 particle. This shortcoming may be overcome by either raising the rate of rotation of the chamber or increasing the duration of dry coating. 4. Conclusion The TiO2/dye composite particles were prepared by dry coating. Then, the dye-sensitized solar cells, which contained a nanocrystalline
TiO2 layer sandwiched between an ITO substrate and a layer of TiO2/ dye composite particles, were fabricated. The short-circuit photocurrent of the DSSC with a sandwich TiO2 thin-film as an electrode greatly exceeded that of the conventional DSSC. Nevertheless, the open-circuit photovoltage of the DSSC with a sandwich TiO2 thin-film working electrode was very similar to that of the conventional DSSC. Most importantly, this study supports the application of TiO2/dye composite particles to improve the performance of a DSSC. However, the optimal process for fabricating a DSSC with a layer of TiO2/dye composite particles must be implemented to yield DSSC with a satisfactory conversion efficiency. Acknowledgement The authors would like to thank the National Science Council, R.O.C., Taiwan for financially supporting this research under Contract No. NSC 95-2211-E-020-037. 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