Preparation of TiO2 nanotube/nanoparticle composite particles and their applications in dye-sensitized solar cells
© Lee et al; licensee Springer. 2012
Received: 9 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
Efficiency of dye-sensitized solar cells [DSSCs] was enhanced by combining the use of TiO2 nanotubes [TNTs] and nanoparticles. TNTs were fabricated by a sol-gel method, and TiO2 powders were produced through an alkali hydrothermal transformation. DSSCs were constructed using TNTs and TiO2 nanoparticles at various weight percentages. TNTs and TiO2 nanoparticles were coated onto FTO glass by the screen printing method. The DSSCs were fabricated using ruthenium(II) (N-719) and electrolyte (I3/I3-) dyes. The crystalline structure and morphology were characterized by X-ray diffraction and using a scanning electron microscope. The absorption spectra were measured using an UV-Vis spectrometer. The incident photocurrent conversion efficiency was measured using a solar simulator (100 mW/cm2). The DSSCs based on TNT/TiO2 nanoparticle hybrids showed better photovoltaic performance than cells made purely of TiO2 nanoparticles.
Dye-sensitized solar cells [DSSCs] have been intensively studied following their discovery in 1991. DSSCs have been extensively researched over the past decades due to their high energy-conversion efficiency and especially their low production cost as cheaper alternatives to silicon solar cells [1–3]. A DSSC is composed of a dye-adsorbed nanoporous TiO2 layer on a fluorine-doped tin oxide [FTO] glass substrate, redox electrolytes, and a counter electrode. A unidirectional charge flow with no electron leakage at the interfaces is essential for high energy-conversion efficiency . The energy-conversion efficiency is likely to be dependent on the morphology and structure of the dye-adsorbed TiO2 film. Ito et al. introduced mesoporous TiO2 particular films as photoanodes to enhance the effective surface area, to absorb more dye molecules, and thus, to achieve more light absorption and greater efficiency [5, 6]. The high conversion efficiency achieved by the DSSC may be attributed to its uniquely porous titania film, which is usually prepared with titania nanoparticles. Sol-gel processing of titanium dioxide has been extensively investigated, and modern processes have been developed to refine and control the stability as well as the phase formation of the colloidal precursors . However, because the mesoporous TiO2 particles are randomly connected, this will unavoidably lead to the recombination of electron-hole pairs, decreasing efficiency. Subsequently, researchers started to explore the use of ordinal TiO2 in DSSCs; this includes TiO2 nanowires, nanorods, and TiO2 nanotubes [TNTs]. The preparation of TNTs by a hydrothermal treatment of TiO2 powder in a 10-M NaOH aqueous solution has been reported [8, 9]. The use of oxide semiconductors in the form of nanorods, nanowires, and nanotubes may be an interesting approach to improve electron transport through the film. Because of the one-dimensional nature of these nanostructures, their morphology facilitates electron transfer up to the collecting electrode, decreasing the ohmic loss through the TNTs [10–13]. To improve electron transport, provide a large surface area to adsorb the sensitized dye, and enhance incident light harvest, the use of TNTs in DSSCs has been explored [14, 15]. In the present work, the effect of combining TiO2 nanoparticles with TNTs and the resulting effect on solar cell performance have been investigated. DSSCs were constructed by the application of TNTs and TiO2 nanoparticles at various weight ratios. TNTs were fabricated by a hydrothermal-temperature process using the sol-gel method. TiO2 powder was produced through alkali hydrothermal transformation. The introduction of TNTs, with a much more open structure, enables the electrolyte to penetrate easily inside the film, increasing the interfacial contact between the nanotubes, the dye, and the electrolyte. In addition, a high level of dye adsorption on TiO2 in the form of nanorods and nanotubes is expected because of the high surface area of these nanostructures. It is expected that the photoelectrical performance of the DSSC can be further improved.
Preparation of TiO2 nanoparticles and nanotubes
The TiO2 main layer was prepared using the sol-gel method. Nano-TiO2 was synthesized using titanium(IV) isopropoxide [TTIP] (Aldrich Chemical, Sigma-Aldrich Corporation, St. Louis, MO, USA), nitric acid, ethyl alcohol, and distilled water. The TTIP was mixed with ethanol, and distilled water was added drop by drop under vigorous stirring for 1 h. This solution was then peptized using nitric acid and heated under reflux at 80°C for 8 h. After this period, a TiO2 sol was prepared. The prepared sol was dried to yield a TiO2 powder. The TiO2 particles were calcined in air at 450°C for 1 h using a programmable furnace to obtain the desired TiO2 stoichiometry and crystallinity. TNTs were prepared using a hydrothermal process described in the authors' previous work. Then, 5 g of TiO2 particles prepared by the sol-gel method were mixed with 500 ml of a 10-M NaOH aqueous solution, followed by hydrothermal treatment at 150°C (TNTs) in a Teflon-lined autoclave for 12 h. After the hydrothermal reaction, the treated powders were washed thoroughly with distilled water and 0.1 M HCl and subsequently filtered and dried at 80°C for 1 day. To achieve the desired TNT size and crystallinity, the powders were calcined in air at 500°C for 1 h .
Preparation of TiO2 electrode films
TiO2 nanoparticles and TNTs prepared by the sol-gel and hydrothermal methods were mixed at various weight ratios (without TNT, 9:1 (10 wt.%), 8:2 (20 wt.%), 7:3 (30 wt.%), 5:5 (50 wt.%), and 100 wt.% TNTs; total weight 6 g) and ground in a mortar. Acetic acid (1 ml), distilled water (5 ml), and ethanol (30 ml) were added gradually drop by drop to disperse the TiO2 nanoparticles and nanotubes under continuous grinding. The TiO2 dispersions in the mortar were transferred with an excess of ethanol (100 ml) to a tall beaker and stirred with a 4-cm-long magnet tip at 300 rpm. Anhydrous terpineol (20 g) and ethyl celluloses (3 g) in ethanol were added, followed by further stirring. The dispersed contents were concentrated by evaporating the ethanol in a rotary evaporator. The pastes were finished by grinding in a three-roller mill . An optically transparent conducting glass (FTO, sheet resistance 8 Ω/sq) was washed in ethanol and deionized water in an ultrasonic bath for 10 min. The FTO glass was immersed in a 40-mm-deep TiCl4 aqueous solution at 70°C for 30 min to make good mechanical contact. A TiO2 film with a thickness of 12 to 15 μm was deposited onto the pretreated conducting glass using the screen printing technique and sintered again at 450°C for 15 min and at 500°C for 15 min in air.
Assembly of the DSSCs
The nanoporous TiO2 electrode films were immersed in the dye (N-719) complex for 24 h at room temperature. A counter electrode was prepared by spin-coating an H2PTCl6 solution onto the FTO glass and heating at 450°C for 30 min. The dye-adsorbed TiO2 electrode and the PT counter electrode were assembled into a sandwich-type cell and sealed with a hot-melt sealant of 50-μm thick. An electrolyte solution was introduced through a drilled hole in the counter electrode. The hole was then sealed using a cover glass.
The phase of the particles obtained at various hydrothermal temperatures was examined by X-ray diffraction [XRD] using a D/MAX-2200 diffractometer with CuKα radiation (Rigaku Corporation, Shibuya-ku, Tokyo, Japan). The morphology and thickness of the prepared TNT layers were investigated by field-emission scanning electron microscopy [FE-SEM] (model S-4700, Hitachi, Chiyoda-ku, Tokyo, Japan). The absorption spectra of the TiO2 electrode films were measured using a UV-Vis spectrometer (UV-Vis 8453, Agilent Technologies Inc., Santa Clara, CA, USA). The conversion efficiency of the fabricated DSSC was measured using an I-V solar simulator (McScience, Suwon-si, South Korea). The incident photocurrent conversion efficiency was measured using an IPCE Model Qex7 (PV Measurements, Inc., Boulder, CO, USA). The active area of the resulting cell exposed to light was approximately 0.25 cm2 (0.5 cm × 0.5 cm).
Results and discussions
Morphological characterization of TiO2 film
Influence of TNTs on dye adsorption
Photovoltaic performance of composite TiO2/TNT DSSCs
and Ps is the intensity of the incident light.
Jsc, Voc, FF, and efficiency
10 wt.% TNT
20 wt.% TNT
30 wt.% TNT
50 wt.% TNT
100 wt.% TNT
DSSCs were constructed with TiO2 films made of different weight percentages of TNTs and TiO2 nanoparticles. The anatase-phase crystal property was found to be at its best at a hydrothermal temperature of 150°C for 12 h. The size and structure of the TNTs were adjusted by varying the hydrothermal temperature. It was found that the conversion efficiency of the DSSCs was highly affected by the properties of the TNTs. A DSSC with a light-to-electric energy conversion efficiency of 4.56% was achieved under a simulated solar light irradiation of 100 mW/cm2 (AM 1.5). The DSSC based on a TiO2/TNT combination at the optimal weight percentage (10 wt.% TNT) showed better photovoltaic performance than the cell made purely of TiO2 nanoparticles.
This work was supported by a Human Resources Development grant from the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Korean government's Ministry of Knowledge Economy (No. 20104010100510).
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