TiO2 micro-flowers composed of nanotubes and their application to dye-sensitized solar cells
© Kim et al.; licensee Springer. 2014
Received: 24 January 2014
Accepted: 12 February 2014
Published: 24 February 2014
TiO2 micro-flowers were made to bloom on Ti foil by the anodic oxidation of Ti-protruding dots with a cylindrical shape. Arrays of the Ti-protruding dots were prepared by photolithography, which consisted of coating the photoresists, attaching a patterned mask, illuminating with UV light, etching the Ti surface by reactive ion etching (RIE), and stripping the photoresist on the Ti foil. The procedure for the blooming of the TiO2 micro-flowers was analyzed by field emission scanning electron microscopy (FESEM) as the anodizing time was increased. Photoelectrodes of dye-sensitized solar cells (DSCs) were fabricated using TiO2 micro-flowers. Bare TiO2 nanotube arrays were used for reference samples. The short-circuit current (Jsc) and the power conversion efficiency of the DSCs based on the TiO2 micro-flowers were 4.340 mA/cm2 and 1.517%, respectively. These values of DSCs based on TiO2 micro-flowers were higher than those of bare samples. The TiO2 micro-flowers had a larger surface area for dye adsorption compared to bare TiO2 nanotube arrays, resulting in improved Jsc characteristics. The structure of the TiO2 micro-flowers allowed it to adsorb dyes very effectively, also demonstrating the potential to achieve higher power conversion efficiency levels for DSCs compared to a bare TiO2 nanotube array structure and the conventional TiO2 nanoparticle structure.
Dye-sensitized solar cells (DSCs) have received much attention since Grätzel and O’Regan achieved a remarkable level of efficiency through their use of mesoporous TiO2 films as a photoanode for DSCs in 1991. DSCs have several advantages compared to Si or copper indium gallium selenide (CIGS) solar cells as follows: (a) DSCs can be fabricated with non-vacuum processes, as opposed to Si or CIGS solar cells. The use of non-vacuum equipment offers the possibility to reduce costs. (b) Wet etching processes such as saw damage etching and texturing, which are widely used in Si solar cells, are not required during the fabrication of DSCs. The fabrication of DSCs is thus simplified without a wet etching process. (c) Colorful DSCs can be easily fabricated because dyes have various colors according to their light absorption characteristics. Although DSCs have these merits, the relatively low power conversion efficiency has become the main cause which limits the commercialization of DSCs.
Several attempts to enhance the performance levels of dyes[2–12], photoelectrodes[13–30], counter cathodes[31–36], and electrolytes[3, 31, 37–41] have been attempted in an effort to obtain improved efficiency in DSCs. Among these efforts, increasing the surface area of the photoelectrodes and reducing the degree of charge recombination between the photoelectrodes and electrolytes have been shown to be critical factors when seeking to improve the power conversion efficiency of DSCs. The TiO2 nanoparticle structure has shown the best performance in DSCs. However, structural disorder, which exists at the contact point of TiO2 nanocrystalline particles, reportedly prohibits charge transport, resulting in limited photocurrents[27–29].
The effort to find alternative TiO2 nanostructures has been an important issue to researchers who attempt to increase the power conversion efficiency of DSCs. Various types of nanotechnologies have been applied to alternative TiO2 nanostructures such as nanorods, nanowires[14, 15], nanotubes[16, 18, 19, 22, 23, 25, 27–30, 42],, nanohemispheres[21, 24], and nanoforests[17, 20]. These structures were used to increase the surface area for dye adsorption and to facilitate charge transport through TiO2 films. Of these nanostructures, the TiO2 nanotube structure has the best potential to overcome the limitations of the TiO2 nanoparticle structure. A previous report showed that the electronic lifetimes of TiO2 nanotube-based DSCs were longer than those of TiO2 nanoparticle-based DSCs. Due to the one-dimensional structure of TiO2 nanotube arrays, charge percolation in TiO2 nanotube-based films is easier than it is in the TiO2 nanoparticle structure[27–29].
In this study, TiO2 micro-flowers composed of nanotubes were fabricated by means of dot patterning, Ti etching, and anodizing methods. The dot patterning and etching of Ti substrates increased the anodizing area to form TiO2 nanotubes. By controlling the anodizing time, beautiful TiO2 micro-flowers were successfully made to bloom on Ti substrates and were applied to the photoelectrodes of DSCs. To the best of our knowledge, this is the first study to report the fabrication of TiO2 micro-flowers and their application to DSCs. The TiO2 micro-flower structure is strongly expected to enhance the possibility to overcome the limitations of the TiO2 nanoparticle structure.
To fabricate the protruding dot patterns on a 0.5-mm-thick Ti foil (99%, Alfa Aesar Co., Ward Hill, MA, USA), 5-μm-thick negative photoresists (PR; L-300, Dongjin Co., Hwaseong-Si, South Korea) were coated on a flat layer of Ti foil using a spin coater (Mark-8 Track, TEL Co., Tokyo, Japan). The coated photoresists were softly baked at 120°C for 120 s and hardly baked at 110°C for 5 min. A dot-patterned photomask was used for PR, the patterning process via UV light exposure. UV light having an energy of 14.5 mJ/s was used for illumination for 5 s, and the PR were developed. The PR at areas not exposed to UV light were removed.
The PR-patterned Ti foil was dry-etched at 20°C for 30 min using reactive ion etching (RIE) equipment (ICP380, Oxford Co., Abingdon, Oxfordshire, UK). BCl3 and Cl2 were used as the etchant gas in the RIE process with a top power of 800 W and a bottom power of 150 W. The photoresists on the UV-exposed area served to protect the flat Ti surface during the RIE process. Only the Ti surface at the area not exposed to UV was etched out. The remaining photoresist after the RIE process was stripped at 250°C for 20 min using a photoresist stripper (TS-200, PSK Co., Hwaseong-si, South Korea). O2 and N2 gases were used to remove the photoresist at a power of 2,500 W.
Before the anodizing process, Ti foil samples patterned with protruding dots were successively sonicated with acetone, ethanol, and deionized (DI) water to remove any residue on their surfaces. TiO2 micro-flowers, consisting of TiO2 nanotubes, were fabricated by the anodization of the Ti foil sheets which had been patterned with protruding dots in an ethylene glycol solution containing 0.5 wt% NH4F. A constant potential of 60 V with a ramping speed of 1 V/s was applied between the anode and the cathode. Pt metal was used as a counter cathode. The anodizing time was controlled for the successful blooming of the TiO2 micro-flowers. The as-anodized TiO2 nanotubes were rinsed with DI water and annealed at 500°C for 1 h. The morphologies of the TiO2 nanotubes and the micro-flowers were studied by field emission scanning electron microscopy (FESEM, Hitachi SU-70, Tokyo, Japan). The as-anodized and annealed TiO2 nanotubes were analyzed by X-ray diffraction (XRD; Rigaku D/MAX-RC, Cu Kα radiation, Rigaku Corporation, Tokyo, Japan) to confirm the crystallization characteristics.
Ti substrates based on TiO2 micro-flowers were used for the photoelectrodes of the DSCs. TiO2 photoelectrodes were immersed at room temperature for approximately 1 day in an ethanol solution containing 3 × 10-4 M cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) bis-tetrabutylammonium (N719) dye. The dye-adsorbed photoelectrodes were rinsed with an ethanol solution and dried at room temperature. Pt-coated fluorine-doped tin oxide (FTO) glass as a counter electrode was prepared by spin coating a 0.7 mM H2PtCl6 solution in 2-propanol at 500 rpm for 10 s followed by an annealing step at 380°C for 30 min. The dye-adsorbed photoelectrodes and the Pt-coated FTO glass samples were spaced using a 60-μm Surlyn® film (DuPont Co., Wilmington, DE, USA). The liquid electrolyte was prepared by dissolving 0.6 M 1-hexyl-2,3-dimethylimidazolium iodide (C6DMIm), 0.05 M iodine, 0.1 M lithium iodide, and 0.5 M 4-tert-butylpyridine in 3-methoxyacetonitrile. The J-V characteristics were measured under an AM 1.5 G condition (model 2400 source measure unit, Keithley Co., Cleveland, OH, USA). A 1,000-W Xenon lamp (91193, Oriel Co., Irvine, CA, USA) was used as a light source.
Results and discussion
When the anodization time was increased to 4 min, beautiful TiO2 micro-flowers started to bloom. The arrays of TiO2 micro-flowers are shown in Figure 4a. The thickness of each TiO2 nanotube is linearly correlated with the extent to which the TiO2 micro-flowers bloom. The blooming of the TiO2 micro-flowers is due to the severe cleavages of the TiO2 nanotubes between the top areas and the side walls of the protruding dots. As the anodization time was increased to 5 min, core bundles of nanotubes in TiO2 micro-flowers were slightly bent in random directions, as shown in Figure 5a,b,c,d. This occurred due to the difference in the growing speed of each TiO2 nanotube in the core bundles. The measured thickness of the TiO2 nanotubes in Figure 5d was 2 μm. As the anodization time was increased to 7 min, the center area of the core nanotube bundles in the TiO2 micro-flowers was removed, as shown in Figure 6a,b,c. Figure 6d shows the cleavage areas of the TiO2 micro-flowers. The structure of the TiO2 nanotubes in that area collapsed due to the additional etching by the fluorine ions in the anodizing solution.
J - V characteristics of DSCs based on bare TiO 2 nanotubes and TiO 2 micro-flowers
Thickness of the TiO2nanotubes (μm)
1.147 ± 0.167
1.187 ± 0.041
1.378 ± 0.092
1.517 ± 0.063
Ti-protruding dots with a cylindrical shape were fabricated by coating with a photoresist, illuminating with UV light, etching the Ti surface via the RIE method, and stripping the photoresist on Ti foil. When the Ti-protruding dots were anodized for over 3 min, beautiful arrays of TiO2 micro-flowers successfully bloomed on the Ti foil sheets. The blooming TiO2 micro-flowers were applied as the photoelectrodes of DSCs. The J-V characteristics of the DSCs based on the TiO2 micro-flowers were compared with those based on bare TiO2 nanotubes. The Jsc and power conversion efficiency values of DSCs based on TiO2 micro-flowers were higher than those of bare samples. TiO2 micro-flowers facilitated better dye adsorption, resulting in higher Jsc values. The TiO2 micro-flowers had a larger surface area for dye adsorption compared to that of bare TiO2 nanotubes. The efficiency of the DSCs based on the TiO2 micro-flowers was found to reach 1.517%. The efficiency levels of the DSCs based on the TiO2 micro-flowers were relatively low compared to those of conventional DSCs based on TiO2 nanoparticle structures, as the thickness of the TiO2 nanotubes in the micro-flowers was very small. To improve the efficiency of DSCs based on TiO2 micro-flowers, our future work will concentrate on controlling the characteristics of the dot patterns such as the dot diameter, the distance between adjacent dots, and the height of the protruding dots.
This research was financially supported by the Ministry of Education, Science, and Technology (MEST) and by the National Research Foundation of Korea (NRF) through the Human Resources Training Project for Regional Innovation (No. NRF-2012H1B8A2026009).
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