ISRN NanotechnologyVolume 2012 (2012), Article ID 132797, 10 pagesdoi:10.5402/2012/132797

Research Article

Alumina Template Assistance in Titania Nanotubes Dye-Sensitized Solar Cell ( NT-DSSC) Device Fabrication

Kuang Hsuan Yang¹ and Chien Chon Chen²

¹Department of Materials Science and Engineering, Vanung University, Chung-Li 32061, Taiwan²Department of Energy Engineering, National United University, 2 Lienda, Miaoli 36003, Taiwan

Received 9 February 2012; Accepted 27 February 2012

Academic Editors: M. Fernández-García and T. Vartanyan

Copyright © 2012 Kuang Hsuan Yang and Chien Chon Chen. 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.

Abstract.

In our previous works, we have made alumina template films and used it for nanowire, nanowhisk, and nanosphere fabrication and molecular aggregation studies. In the present paper, we have combined Al₂O₃ template and titania (TiO₂) NT fabrication processes to achieve an Al₂O₃/TiO₂ NT dye-sensitized solar cell (DSSC) devices. The DSSC structure includes glass substrate, transparent conductive film of ITO, Pt particles serving as the counter electrode, Al₂O₃/TiO₂ NT film, dye, and ITO serving as the working electrode, and the electrolyte is injected into the counter-working interface. Al₂O₃ template was made by anodization and TiO₂ NT was made by sol-gel deposition into Al₂O₃ template. Al₂O₃ template has a light, transparence, large surface, good mechanical strength, and flexibility, making it a candidate material for DSSC electrode template. TiO₂ NT is a semiconductor with an energy gap that matches up very nicely with N3 sensitized dye.

1. Introduction

Due to increasing energy demands and concerns about global warming, scientists are looking for potential renewable energy sources. Because the sun is the most important inexhaustible and clean energy source, efficiently harvesting solar energy to generate electric power using photovoltaic technology beyond silicon systems has undergone rapid development over the past few years.

Presently there are several technical schemes for solar cell design, including monocrystalline/polycrystalline silicon solar cells, amorphous silicon solar cells, thin film solar cells, and wet type dye-sensitized solar cells (DSSCs). Of these, monocrystalline silicon solar cells currently have the leading position in the market due to their relatively high transformation efficiency (12–20%). However, since monocrystalline silicon wafers are expensive, manufacturing costs for these cells are high. DSSCs have gradually become more popular due to their lower cost and relatively simple manufacturing process. A DSSC consists of an anode, electrolytic solution, and a cathode. A semiconductor layer is formed on the surface of the anode and photosensitive dyes are absorbed therein.

Since the development of low-cost DSSC technology in 1991 by O’Regan and Grätzel [1], DSSC has been regarded as a promising candidate for next-generation solar cell design [2]. Traditionally, the electron-collecting layer (anode) of a DSSC is composed of randomly packed TiO₂ nanoparticles (NPs). With sunlight irradiated from the transparent anode (front illumination), the best photovoltaic power conversion efficiency (η) of an NP-DSSC device has reached ~11% [3–5]. The great advantage of NP-DSSC is the large surface area of the nanoporous TiO₂ films for dye adsorption. However, the trap-limited diffusion for electron transport in NP-DSSC is a limiting factor in achieving higher light-to-electricity conversion efficiency [6]. To improve charge-collection efficiency by promoting faster electron transport and slower charge recombination, several different methods have been established using TiO₂ films constructed of oriented one-dimensional (1D) nanostructures. For example, the best cell performance of DSSCs based on 1D TiO₂ nanowires (NWs) has reached η = 5.0% under front-side illumination [7, 8]. Also, 1D TiO₂ nanotubes (NTs) have been synthesized using sol-gel [9] and potentiostatic anodization [10] methods.

NT-DSSCs using the TiO₂ NT arrays on Ti foil as working electrodes have three important intrinsic features that allow it to outperform the conventional NP-DSSCs. First, the charge-collection efficiencies of NT films have been proved to be much better than those of NP films because of the 1D nature of the former, with a much slower charge recombination rate [11, 12]. This intrinsic advantage of NT-DSSC promotes its cell performance significantly with tube lengths up to 20 μm, as reported by Grimes and coworkers [13]. Second, the light-harvesting efficiencies of the NT films are much better than those of the NP films because of the stronger light scattering effect of the former. In fact, for a traditional high-efficiency NP-DSSC, adding an additional TiO₂ layer with a larger particle size (~400 nm) is required to increase the light scattering effect, while this effect is a natural property for an NT-DSSC. Third, the anode fabrication of NT-DSSCs is much simpler and more cost-effective than that of NP-DSSCs. Direct anodization of a Ti foil in a one-step process produces the blank TiO₂ film ready to make a working electrode in NT-DSSCs, whereas making the blank TiO₂ film in NP-DSSCs requires a multiple coating process for at least two layers of TiO₂ NPs coated on a relatively expensive transparent conducting oxide (TCO) substrate.

The use of a longer NT in the device causes certain problems. For example, cracking and a compact layer are produced on the surface of the NT films, and the longer the TiO₂ NT, the weaker adhesion of NT on the Ti substrate. To reduce the cracks and eliminate the compact layer for a longer TiO₂ NT, Grimes [13] and coworkers used 0.1 M hydrochloric acid (HCl) to dissolve the compact layer; Zhu and coworkers [12] used a supercritical CO₂ drying technique to produce bundle-free and crack-free NT films. Chen et al. [14] used Al₂O₃ microparticles with the aid of ultrasonic vibration to remove the compact layer and to avoid crack formation. The mechanical strength of TiO₂ NT is not great, so a longer tube can crack easily, leading to the peeling off of the NT film from the Ti substrate. Therefore, we propose to use anodic aluminum oxide (AAO or Al₂O₃ template) as a template, followed by deposition of TiO₂ NT inside AAO to make a working electrode for NT-DSSC. AAO is fabricated by anodization process, and then the chemical etching method is used to achieve AAO nanotubes with open pores on both sides. Because AAO has better mechanical properties than TiO₂ NT, the AAO morphology can also be controlled more easily than can that of TiO₂ NT. For example, AAO pore diameter, length, and pore density can be controlled in the range of 10–500 nm, 0.1–200 μm, and 10⁸–10¹² tubes cm⁻², respectively.

According to reports from Imai [15–17], when TiF₄ solutions have a pH below 1.0 or a TiF₄ concentration below 0.03 M, neither precipitation nor film formation was observed. A large amount of precipitate was rapidly formed, and film was not deposited, on substrates above pH 3.1. Therefore, the concentration of TiF₄ should be controlled above 0.03 M, and the pH value should be controlled between 1 and 3 so that TiO₂ nanoparticles can present and be deposited on the substrate. For example, Chen et al. [18] used pH 1.8, 0.04 M TiF₄ solution at 60°C for deposition on a TiO₂ NT in AAO membrane.

In our previous work, we have made AAO film [19] and used this templates for the fabrication of nanowires [20–23], nanowhisks [24], and nanospheres [25–29] and the study of molecular aggregation [30, 31]. We have also made TiO₂ NT film [32–36] and used it for DSSC device fabrication, allowing the photovoltaic power conversion efficiency to reach η~7% [14, 37]. In this paper, we have combined our previous Al₂O₃ template, TiO₂ NT, and DSSC fabrication technologies to achieve an Al₂O₃/TiO₂ NT-DSSC device.

2. Experimental

2.1. Al₂O₃ Template Fabrication

Al₂O₃ templates with a pore size of 10 to 500 nm were generated by anodizing a commercial aluminum (Al) substrate (99.7%) in acid solutions of sulfuric acid (H₂SO₄), oxalic acid (COOH)₂, or phosphoric acid (H₃PO₄). The Al substrate was first ground to no. 1000 by SiC waterproof paper and then annealed in an air furnace at 550°C for 1 hr. The sample was then electropolished in a bath consisting of 15 vol.% perchloric acid (HClO₄, 70%), 70 vol.% ethanol (C₂H₆O, 99.5%), and 15 vol.% monobutyl ether ((CH₃(CH₂)₃OCH₂CH₂OH), 85%) with a charge of 42 volts (DC) applied for 10 min, used platinum plate as a counter.

A 10 nm pore diameter template was then fabricated by anodizing the polished-Al substrate at 18 V in 10 vol.% H₂SO₄ at 15°C for 20 min, which was the first anodization. In order to obtain an orderly pattern on the substrate for the second anodization, the first anodization film was removed in 1.8 wt.% chromic acid (CrO₃) + 6 vol.% H₃PO₄ solution at 60°C for 40 min. The resulting substrate, with a regular pattern on the surface, was used for the second anodization for several hours to form an AAO film with various thicknesses. After Al₂O₃ template formed, the Al substrate was removed by placing the sample in saturated copper chloride (CuCl₂) + 10 vol.% hydrochloric acid (HCl) for 30 min. Finally, the sample was put in 5 vol.% H₃PO₄ at 25°C for 5 to 20 min. The nanotubes were widened to an ordered array and a good quality Al₂O₃ template film with 10 to 50 nm pore size was formed. Similar to the above process, for 40 to 90 nm pore diameter Al₂O₃ template, the electrolyte was 3 vol.% (COOH)₂ at 25°C, and the applied voltage was 40 V. The time of pore widening was 10 to 90 min. For 180 to 500 nm pore diameter AAO template, the electrolyte was 1 vol.% H₃PO₄ at 0°C, and the applied voltage was 200 V. The time of pore widening was between 30 to 200 min [19, 20, 25, 38, 39].

2.2. DSSC Device Fabrication

Three key components are essential to construct a sandwich-type DSSC device: (1) a light-harvesting layer of TiO₂/Al₂O₃ template film with a TiO₂ and ITO coating to serve as a working electrode (anode); (2) a Pt-coated layer deposited on an ITO surface to serve as a counter electrode (cathode); (3) an iodine-based electrolyte filled into the space between the anode and the cathode to serve as a redox couple of the cell. The TiO₂ NT was prepared by immersing Al₂O₃ template in 0.02 M titanium fluoride (TiF₄) solution (pH = 3) at 25°C for 120 min, followed by annealing at 450°C for 1 hr to obtain anatase TiO₂ NT in the Al₂O₃ template. To prevent short-circuits, on the template bottom was deposited a 300 μm compact TiO₂ film by a 4-inch disk of Ti target (99.9% purity) sputtered in a gaseous mixture of 90% Ar and 10% O₂. Sputtering power, gas pressure, substrate temperature, and sputter time were kept constant at 50 W, , 50°C, and 80 min, respectively. Subsequently, a 120 μm ITO film was sputtered on TiO₂ film to form a conducting electrode by sputter-deposition. An RF magnetron sputtering system was used for ITO preparation. A 4 inch disk of ITO target (99.9% purity) was sputtered on the TiO₂ film. Sputtering power, gas pressure, substrate temperature, and sputter time were kept constant at 100 W,   torr, 30°C, and 30 min, respectively. The electrode fabrication process is shown in Figure 1. The electrode (Al₂O₃ template/TiO₂ NT/TiO₂ film/ITO film) was then soaked in ethanol containing  M RuL₂(NCS)₂ (N3 dye) for 7 hr to absorb N3 dye, forming the DSSC anode. The micromorphology of Al₂O₃ template, TiO₂ NT, TiO₂ compact film, and ITO film were determined by scanning electron microscope (SEM, JEOL 6500).

Figure 1: Schematic diagrams of DSSC anode-fabricated processing by used TiO₂ NT and Al₂O₃ templates. (a) purify Al (99.7%) through anodization forming and (b) Al₂O₃ template on the Al surface, (c) remove Al substrate by CuCl₂ + H₂O solution, (d) remove barrier layer by H₃PO₄ solution, (e) sputter TiO₂ as under layer and ITO as conducted layer forming conductor electrode, (f) deposit TiO₂ inside Al₂O₃ template by TiF₄ solution to form DSSC anode.

The Pt counter electrodes were prepared by sputter-deposition. A DC magnetron sputtering system was used for the Pt particles. A 4-inch disk of Pt target (99.9% purity) was sputtered. Sputtering power, gas pressure, substrate temperature, and sputter time were kept constant at 10W, 50 ×10⁻³, 30°C, and 5 sec, respectively.

To prevent short circuits, the cell was assembled with both anode and cathode sealed by using a hot-melt spacer of 25 μm thickness; small holes on the corners of the cell were reserved for the entrance of the electrolyte and the exhaust of the air. A thin layer of electrolyte was sucked into the interelectrode space by capillary force, and the holes were sealed by epoxy resin. The typical electrolyte solution consisted of 0.5 M (lithium iodide) LiI and 0.05 M iodine (I₂) in acetonitrile (CH₃CN, 99.9%). The photocurrent was produced using an HP model 4140 B measuring unit. The photocurrent conversion efficiency was tested under an AM 1.5 (300 W, 91160 Oriel Solar Simulator, 100 mw cm⁻²) on a 0.28 cm² sample area.

3. Results and Discussion

The Al₂O₃ template was made by anodization, and the pore size was controlled by electrolyte and the pore-widening period. Figure 2 shows SEM images of Al₂O₃ template film with diameter of (a) 10 nm Al₂O₃ template, produced by 10 vol.% H₂SO₄ anodization and 5 vol.% H₃PO₄ pore widening for 5 min; (b) 50 and (c) 90 nm Al₂O₃ template produced by 3 wt.% C₂H₂O₄ anodization and 5 vol.% H₃PO₄ pore widening for 30 min and 90 min; and (d) 300 nm, (e) 400 nm, and (f) 500 nm Al₂O₃ template produced by 1 vol.% H₃PO₄ anodization and 5 vol.% H₃PO₄ pore widening for 90 min, 180 min, and 250 min, respectively. Using the various electrolytes and pore-widening periods, pore sizes of 10 to 500 nm were obtained.

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Figure 2: SEM images of Al₂O₃ template film with various pore sizes of (a) 10 nm, (b) 50 nm, (c) 90 nm, (d) 300 nm, (e) 400 nm, and (f) 500 nm.

A barrier layer, which is a dense Al₂O₃ film on the bottom of Al₂O₃ template, and the electronic insulator film of the barrier resist electron transport. To make the Al₂O₃ template as an electrode, the barrier layer should first be removed and then replaced with a coating of conductance film on the template bottom. In our previous results [18–21], the thickness of the barrier layer was proportional to the anodic applied voltage; the relationship between thickness and voltage can be denoted as nm/V. For example, the pore sizes of 10 to 50 nm, 40 to 90 nm, and 180 to 500 nm Al₂O₃ templates are anodized by 18, 40, and 200 V, producing barrier layers of 18, 40, and 200 nm, respectively. Therefore, the larger the pore sizes, the thicker the barrier layer and longer the pore-widening period needed. Figure 3 shows SEM images of the bottom of the Al₂O₃ template film: (a) a compact Al₂O₃ barrier layer covering the bottom of the Al₂O₃ template, (b) the barrier dissolved by 5% H₃PO₄ solution at 25°C for 10 min to etch grain boundaries, (c) 30 min, during which the barrier layer was partially dissolved, (d) 50 min, during which the barrier layer became a thin film, (e) 70 min, during which most of barrier layer was removed, and (f) 90 min, after which the barrier layer was removed completely.

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(b)

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Figure 3: SEM images of bottom Al₂O₃ template film. (a) A compact Al₂O₃ barrier layer cover on Al₂O₃ template bottom, (b) dissolving barrier by 5% H₃PO₄ solution at 25°C for 10 min during which grain boundaries were etched, (c) 30 min during which partial barrier layer was dissolved, (d) 50 min during which barrier layer has become thin film, (e) 70 min during which most of barrier layer was removed, and (f) 90 min during which barrier layer was removed, completely.

The Al₂O₃ template had a larger surface area and transparent film. Figure 4 shows images of transparent Al₂O₃ template film: (a) original 4-inch transparent Al₂O₃ template film and (b) the color changing to light red after N3 sensitized dye was absorbed. As can be seen, an Al₂O₃ template structure with discontinuous pores but continuous pore walls gives Al₂O₃ template a light, transparent, large surface, and good mechanical strength and flexibility, making it a candidate material for DSSC electrode template. Before adding a coating of a conductive film of ITO on the bottom of the Al₂O₃ template, a semiconductor film of TiO₂ should first be coated on the bottom of the template. A compact TiO₂ film, called an under layer, which serves as an electron transport film (from dye to the conductance electrode) but prevents short circuits (between anode and cathode) is thus formed. Figure 5 shows SEM images of (a) a side view of a 300 μm TiO₂ film on an Al₂O₃ template surface and (b) a top view of compact TiO₂ film, which can be an under layer on the DSSC anode, and the rough film surface will enhance the following adherence properties of ITO on TiO₂ film.

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Figure 4: Image of (a) original 4-inch transparent Al₂O₃ template film and (b) the color changing to light red after absorption in N3 sensitized dye.

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Figure 5: SEM images of (a) side and (b) top views of TiO₂ film on Al₂O₃ template surface.

Figure 6 shows side-view SEM images of Al₂O₃ template made with 10% H₂SO₄ solution and pore widening for 8 min with (a) a tube diameter of 20 nm, and (b) TiO₂ NT deposited inside Al₂O₃ template. Figure 7 also shows side-view SEM images of Al₂O₃ template made by 1% H₃PO₄ solution and pore widening for 80 min with (a) a tube diameter of 280 nm and (b) TiO₂ NT deposited inside Al₂O₃ template. The thickness of the TiO₂ NT pore wall depends on the deposition time. A longer deposition time causes a smaller TiO₂ NT pore size, and a TiO₂ rod presents after a long deposition time. However, the TiO₂ NT surface area is reduced, with increased NT walls. Figure 8 shows SEM top view images of the interiors of 280 nm Al₂O₃ template with TiO₂ NT deposited by 0.02 M TiF₄ solution at 25°C for (a) 15 min, (b) 30 min, (c) 60 min, and (d) 120 min. TiO₂ particles were deposited inside Al₂O₃ template and gradually formed a network film on Al₂O₃ template. Figure 9 shows a schematic diagram of electron (e⁻) and hole (h⁺) transport in the TiO₂ NT. The ITO film serves as a conductive film, and the TiO₂ compact film severs as an under layer that resists electrolyte contact with the ITO film. Electrons are transported through the TiO₂ NT to the TiO₂ compact film and then conducting electrode; on the other hand, holes are transported through the electrolyte to counter. In this one-dimensional channel for carrier transportation, the amount of recombination of electron-hole (e⁻/h⁺) is expected to be reduced. Figure 10 shows a schematic diagram of a DSSC device fabricated with Al₂O₃ and TiO₂ tubes. The structure includes glass, ITO, Pt particles serving as a counter electrode, and Al₂O₃/TiO₂ tubes film, dye, TiO₂, and ITO serving as a working electrode. The electrolyte is injected into the counter-working interface. Based on the structure in Figure 10, the current-voltage characteristics of Al₂O₃/TiO₂ NT-DSSC with N3 dye were measured as in Figure 11. The performance of the device was measured as having 2.7% conversion efficiency, 0.72 V of open circuit voltage, 5.5 mA cm⁻² of short circuit current density, and 0.69 of fill factor. An awkward sealing technology with thin film DSSC device may cause short circuit effects between anode and cathode. Also, the device performance had a lower of open circuit voltage fill factor and short current density.

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Figure 6: SEM side-view images of 20 nm Al₂O₃ template (a) before TiO₂ NT inside, and (b) after TiO₂ NT inside.

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Figure 7: SEM side-view images of 280 nm Al₂O₃ template (a) before TiO₂ NT inside, and (b) after TiO₂ NT inside.

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Figure 8: SEM top view images of 280 nm Al₂O₃ template inside with TiO₂ NT deposited by 0.02 M TiF₄ solution () at 25°C for (a) 15 min, (b) 30 min, (c) 60 min, and (a) 120 min.

Figure 9: Schematic diagram of electron (e⁻) and hole (h⁺) transport in the TiO₂ NT. Electron transports through TiO₂ NT to TiO₂ compact film and conducted electrode, hole transports through electrolyte to counter.

Figure 10: Schematic diagram of DSSC device fabricated by Al₂O₃ and TiO₂ tubes. The structure includes glass, ITO, and Pt particles as counter electrode, Al₂O₃/TiO₂ tubes film, dye, and ITO as working electrode, and the electrolyte is injected into counter-working interface.

Figure 11: Current-voltage characteristics of Al₂O₃ and TiO₂ tubes DSSC sensitized with N3 dye. The device was measured: 2.7% conversion efficient, 0.72 V open circuit voltage, 5.5 mA cm⁻² short circuit current density, and 0.69 fill factor.

4. Conclusions

We propose the use of anodic aluminum oxide as a template followed by deposition of TiO₂ NT inside the template to make a working electrode for flexible NT-DSSC. An alumina template was fabricated by anodization and chemical etching to achieve Al₂O₃ nanotubes with open pores on both sides. Al₂O₃ template with pore sizes of 10 to 50 nm, 40 to 90 nm, and 180 to 500 nm was anodized by 18, 40, and 200V. The thicknesses of the barrier layers were 18, 40, and 200 nm, respectively. Al₂O₃ template has better mechanical properties than TiO₂ NTs and the Al₂O₃ template morphology can also be controlled easily than TiO₂ NT. Based on the AAO and TiO₂ NT as an electron transport film, the Al₂O₃/TiO₂ NT-DSSC device was fabricated, and the performance of the device was measured at 2.7%.

Acknowledgments

Part of this study was supported by grants from the National Science Council, Taiwan (NSC-99-2221-E-239-025-). The authors would also like to thank Feng Chia University and National United University for financially supporting this work.

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J. S. Lin, Y. C. Chen, C. C. Chen, W. G. Diau, and T. F. Liu, “Aggregation of zinc protoporphyrin in anodized aluminum oxide (AAO) nanoporous environments,” Journal of the Chinese Chemical Society, vol. 53, no. 1, pp. 201–208, 2006.

C. C. Chen, W. C. Say, S. J. Hsieh, and E. W. G. Diau, “A mechanism for the formation of annealed compact oxide layers at the interface between anodic titania nanotube arrays and Ti foil,” Applied Physics A, vol. 95, no. 3, pp. 889–898, 2009.

W. C. Say, C. C. Chen, and Y. H. Shiu, “Monitoring the effects of growing titania nanotubes on titanium substrate by electrochemical impedance spectroscopy measurement,” Japanese Journal of Applied Physics, vol. 48, no. 3, Article ID 035004, 2009.

C. C. Chen, J. S. Lin, W. G. Diau, and T. F. Liu, “Self-cleaning characteristics on a thin-film surface with nanotube arrays of anodic titanium oxide,” Applied Physics A, vol. 92, no. 3, pp. 615–620, 2008.

C. C. Chen, J. H. Chen, C. G. Chao, and W. C. Say, “Electrochemical characteristics of surface of titanium formed by electrolytic polishing and anodizing,” Journal of Materials Science, vol. 40, no. 15, pp. 4053–4059, 2005.

C. C Chen and S. J. Hsieh, “Evaluation of Fluorine Ion Concentration in Titanium Oxide Nanotube (TiO₂ NT) Anodization Process,” Journal of The Electrochemical Society, vol. 156, pp. K125–K130, 2010.

C. C. Chen, W. D. Jehng, L. L. Li, and E. W. G. Diau, “Enhanced efficiency of dye-sensitized solar cells using anodic titanium oxide nanotube arrays,” Journal of the Electrochemical Society, vol. 156, no. 9, pp. C304–C312, 2009.

H. Masuda, H. Yamada, M. Satoh, H. Asoh, M. Nakao, and T. Tamamura, “Highly ordered nanochannel-array architecture in anodic alumina,” Applied Physics Letters, vol. 71, no. 19, pp. 2770–2772, 1997.

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