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.
KeywordsDye-sensitized solar cells TiO2 nanotube Micro-flowers Anodizing
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).
- Oregan B, Grätzel M: A low-cost, high-efficiency solar-cell based on dye-sensitized colloidal TiO2 films. Nature 1991, 353(6346):737–740. 10.1038/353737a0View ArticleGoogle Scholar
- Li L-L, Diau EW-G: Porphyrin-sensitized solar cells. Chem Soc Rev 2013, 42(1):291–304. 10.1039/c2cs35257eView ArticleGoogle Scholar
- Yella A, Lee H-W, Tsao HN, Yi C, Chandiran AK, Nazeeruddin MK, Diau EW-G, Yeh C-Y, Zakeeruddin SM, Grätzel M: Porphyrin-sensitized solar cells with cobalt (II/III)-based redox electrolyte exceed 12 percent efficiency. Science 2011, 334(6056):629–634. 10.1126/science.1209688View ArticleGoogle Scholar
- Zhu X, Tsuji H, Yella A, Chauvin A-S, Grätzel M, Nakamura E: New sensitizers for dye-sensitized solar cells featuring a carbon-bridged phenylenevinylene. Chem Commun 2013, 49(6):582–584. 10.1039/c2cc37124cView ArticleGoogle Scholar
- Marszalek M, Nagane S, Ichake A, Humphry-Baker R, Paul V, Zakeeruddin SM, Grätzel M: Structural variations of D-π-a dyes influence on the photovoltaic performance of dye-sensitized solar cells. RSC Adv 2013, 3: 7921–7927. 10.1039/c3ra22249gView ArticleGoogle Scholar
- Margulis GY, Lim B, Hardin BE, Unger EL, Yum J-H, Feckl JM, Fattakhova-Rohlfing D, Bein T, Grätzel M, Sellinger A: Highly soluble energy relay dyes for dye-sensitized solar cells. Phys Chem Chem Phys 2013, 15: 11306–11312. 10.1039/c3cp51018bView ArticleGoogle Scholar
- Wu Y, Marszalek M, Zakeeruddin SM, Zhang Q, Tian H, Grätzel M, Zhu W: High-conversion-efficiency organic dye-sensitized solar cells: molecular engineering on D–a–π-a featured organic indoline dyes. Energy Environ Sci 2012, 5(8):8261–8272. 10.1039/c2ee22108jView ArticleGoogle Scholar
- Tian H, Gabrielsson E, Lohse PW, Vlachopoulos N, Kloo L, Hagfeldt A, Sun L: Development of an organic redox couple and organic dyes for aqueous dye-sensitized solar cells. Energy Environ Sci 2012, 5(12):9752–9755. 10.1039/c2ee23263dView ArticleGoogle Scholar
- Marszalek M, Nagane S, Ichake A, Humphry-Baker R, Paul V, Zakeeruddin SM, Grätzel M: Tuning spectral properties of phenothiazine based donor–π–acceptor dyes for efficient dye-sensitized solar cells. J Mater Chem 2012, 22(3):889–894. 10.1039/c1jm14024hView ArticleGoogle Scholar
- Paek S, Choi H, Kim C, Cho N, So S, Song K, Nazeeruddin MK, Ko J: Efficient and stable panchromatic squaraine dyes for dye-sensitized solar cells. Chem Commun 2011, 47(10):2874–2876. 10.1039/c0cc05378cView ArticleGoogle Scholar
- Hardin BE, Yum J-H, Hoke ET, Jun YC, Péchy P, Torres T, Brongersma ML, Nazeeruddin MK, Grätzel M, McGehee MD: High excitation transfer efficiency from energy relay dyes in dye-sensitized solar cells. Nano Lett 2010, 10(8):3077–3083. 10.1021/nl1016688View ArticleGoogle Scholar
- Feldt SM, Gibson EA, Gabrielsson E, Sun L, Boschloo G, Hagfeldt A: Design of organic dyes and cobalt polypyridine redox mediators for high-efficiency dye-sensitized solar cells. J Am Chem Soc 2010, 132(46):16714–16724. 10.1021/ja1088869View ArticleGoogle Scholar
- Wang M, Bai J, Le Formal F, Moon S-J, Cevey-Ha L, Humphry-Baker R, Grätzel C, Zakeeruddin SM, Grätzel M: Solid-state dye-sensitized solar cells using ordered TiO2 nanorods on transparent conductive oxide as photoanodes. J Phys Chem C 2012, 116(5):3266–3273. 10.1021/jp209130xView ArticleGoogle Scholar
- Liao J-Y, Lei B-X, Chen H-Y, Kuang D-B, Su C-Y: Oriented hierarchical single crystalline anatase TiO2 nanowire arrays on Ti-foil substrate for efficient flexible dye-sensitized solar cells. Energy Environ Sci 2012, 5(2):5750–5757. 10.1039/c1ee02766bView ArticleGoogle Scholar
- Law M, Greene LE, Johnson JC, Saykally R, Yang P: Nanowire dye-sensitized solar cells. Nat Mater 2005, 4(6):455–459. 10.1038/nmat1387View ArticleGoogle Scholar
- Kim W-R, Lee Y-J, Park H, Lee J-J, Choi W-Y: TiO2-nanotube-based dye-sensitized solar cells containing fluorescent material. J Nanosci Nanotechnol 2013, 13(5):3487–3490. 10.1166/jnn.2013.7290View ArticleGoogle Scholar
- Shao F, Sun J, Gao L, Yang S, Luo J: Forest-like TiO2 hierarchical structures for efficient dye-sensitized solar cells. J Mater Chem 2012, 22(14):6824–6830. 10.1039/c2jm15442kView ArticleGoogle Scholar
- Park H, Kim W-R, Yang C, Kim H-G, Choi W-Y: Effect of a fullerene derivative on the performance of TiO2-nanotube-based dye-sensitized solar cells. J Nanosci Nanotechnol 2012, 12(2):1535–1538. 10.1166/jnn.2012.4620View ArticleGoogle Scholar
- Park H, Yang C, Choi W-Y: Organic and inorganic surface passivations of TiO2 nantoube arrays for dye-sensitized photoelectrodes. J Power Sources 2012, 216(15):36–41.View ArticleGoogle Scholar
- Ko SH, Lee D, Kang HW, Nam KH, Yeo JY, Hong SJ, Grigoropoulos CP, Sung HJ: Nanoforest of hydrothermally grown hierarchical ZnO nanowires for a high efficiency dye-sensitized solar cell. Nano Lett 2011, 11(2):666–671. 10.1021/nl1037962View ArticleGoogle Scholar
- Yang D-J, Yang S-C, Hong J-M, Lee H, Kim I-D: Size-dependent photovoltaic property in hollow hemisphere array based dye-sensitized solar cells. J Electroceram 2010, 24(3):200–204. 10.1007/s10832-008-9555-6View ArticleGoogle Scholar
- Park H, Yang DJ, Yoo JS, Mun KS, Kim WR, Kim HG, Choi WY: Surface passivation of highly ordered TiO2 nanotube arrays and application to dye-sensitized solar cells using the concept of isoelectric point. J Ceram Soc Jpn 2009, 117(1365):596–599. 10.2109/jcersj2.117.596View ArticleGoogle Scholar
- Park H, Yang DJ, Kim HG, Cho SJ, Yang SC, Lee H, Choi WY: Fabrication of MgO-coated TiO2 nanotubes and application to dye-sensitized solar cells. J Electroceram 2009, 23(2–4):146–149.View ArticleGoogle Scholar
- Yang SC, Yang DJ, Kim J, Hong JM, Kim HG, Kim ID, Lee H: Hollow TiO2 hemispheres obtained by colloidal templating for application in dye-sensitized solar cells. Adv Mater 2008, 20(5):1059–1064. 10.1002/adma.200701808View ArticleGoogle Scholar
- Yang DJ, Park H, Cho SJ, Kim HG, Choi WY: TiO2-nanotube-based dye-sensitized solar cells fabricated by an efficient anodic oxidation for high surface area. J Phys Chem Solids 2008, 69(5–6):1272–1275.View ArticleGoogle Scholar
- Kang S, Choi S, Kang M, Kim J, Kim H, Hyeon T, Sung Y: Nanorod-based dye-sensitized solar cells with improved charge collection efficiency. Adv Mater 2008, 20(1):54–58. 10.1002/adma.200701819View ArticleGoogle Scholar
- Zhu K, Vinzant T, Neale N, Frank A: Removing structural disorder from oriented TiO2 nanotube arrays: reducing the dimensionality of transport and recombination in dye-sensitized solar cells. Nano Lett 2007, 7(12):3739–3746. 10.1021/nl072145aView ArticleGoogle Scholar
- Zhu K, Neale N, Miedaner A, Frank A: Enhanced charge-collection efficiencies and light scattering in dye-sensitized solar cells using oriented TiO2 nanotubes arrays. Nano Lett 2007, 7(1):69–74. 10.1021/nl062000oView ArticleGoogle Scholar
- Mor GK, Shankar K, Paulose M, Varghese OK, Grimes CA: Use of highly-ordered TiO2 nanotube arrays in dye-sensitized solar cells. Nano Lett 2006, 6(2):215–218. 10.1021/nl052099jView ArticleGoogle Scholar
- Park H, Kim W-R, Jeong H-T, Lee J-J, Kim H-G, Choi W-Y: Fabrication of dye-sensitized solar cells by transplanting highly ordered TiO2 nanotube arrays. Sol Energy Mater Sol Cells 2011, 95(1):184–189. 10.1016/j.solmat.2010.02.017View ArticleGoogle Scholar
- Hauch A, Georg A: Diffusion in the electrolyte and charge-transfer reaction at the platinum electrode in dye-sensitized solar cells. Electrochim Acta 2001, 46(22):3457–3466. 10.1016/S0013-4686(01)00540-0View ArticleGoogle Scholar
- Xin X, He M, Han W, Jung J, Lin Z: Low-cost copper zinc tin sulfide counter electrodes for high-efficiency dye-sensitized solar cells. Angew Chem Int Ed 2011, 50(49):11739–11742. 10.1002/anie.201104786View ArticleGoogle Scholar
- Choi H, Kim H, Hwang S, Choi W, Jeon M: Dye-sensitized solar cells using graphene-based carbon nano composite as counter electrode. Sol Energy Mater Sol Cells 2011, 95(1):323–325. 10.1016/j.solmat.2010.04.044View ArticleGoogle Scholar
- Roy-Mayhew JD, Bozym DJ, Punckt C, Aksay IA: Functionalized graphene as a catalytic counter electrode in dye-sensitized solar cells. Acs Nano 2010, 4(10):6203–6211. 10.1021/nn1016428View ArticleGoogle Scholar
- Li G, Wang F, Jiang Q, Gao X, Shen P: Carbon nanotubes with titanium nitride as a low-cost counter-electrode material for dye-sensitized solar cells. Angew Chem Int Ed 2010, 49(21):3653–3656. 10.1002/anie.201000659View ArticleGoogle Scholar
- Han J, Kim H, Kim DY, Jo SM, Jang S-Y: Water-soluble polyelectrolyte-grafted multiwalled carbon nanotube thin films for efficient counter electrode of dye-sensitized solar cells. Acs Nano 2010, 4(6):3503–3509. 10.1021/nn100574gView ArticleGoogle Scholar
- Yum J-H, Baranoff E, Kessler F, Moehl T, Ahmad S, Bessho T, Marchioro A, Ghadiri E, Moser J-E, Yi C: A cobalt complex redox shuttle for dye-sensitized solar cells with high open-circuit potentials. Nat Commun 2012, 3: 631.View ArticleGoogle Scholar
- Bai Y, Cao Y, Zhang J, Wang M, Li R, Wang P, Zakeeruddin SM, Grätzel M: High-performance dye-sensitized solar cells based on solvent-free electrolytes produced from eutectic melts. Nat Mater 2008, 7(8):626–630. 10.1038/nmat2224View ArticleGoogle Scholar
- Snaith HJ, Schmidt‒Mende L: Advances in liquid electrolyte and solid-state dye-sensitized solar cells. Adv Mater 2007, 19(20):3187–3200. 10.1002/adma.200602903View ArticleGoogle Scholar
- Wang P, Zakeeruddin SM, Moser JE, Nazeeruddin MK, Sekiguchi T, Grätzel M: A stable quasi-solid-state dye-sensitized solar cell with an amphiphilic ruthenium sensitizer and polymer gel electrolyte. Nat Mater 2003, 2(6):402–407. 10.1038/nmat904View ArticleGoogle Scholar
- Nogueira AF, Durrant JR, De Paoli MA: Dye-sensitized nanocrystalline solar cells employing a polymer electrolyte. Adv Mater 2001, 13(11):826–830. 10.1002/1521-4095(200106)13:11<826::AID-ADMA826>3.0.CO;2-LView ArticleGoogle Scholar
- Park H, Kim H-G, Choi W-Y: Characterizations of highly ordered TiO2 nanotube arrays obtained by anodic oxidation. J Trans Electr Electron Mater 2010, 11: 112–115. 10.4313/TEEM.2010.11.3.112View ArticleGoogle Scholar
- Mor G, Varghese O, Paulose M, Shankar K, Grimes C: A review on highly ordered, vertically oriented TiO2 nanotube arrays: fabrication, material properties, and solar energy applications. Sol Energy Mater Sol Cells 2006, 90(14):2011–2075. 10.1016/j.solmat.2006.04.007View ArticleGoogle Scholar
- Yang D-J, Kim H-G, Cho S-J, Choi W-Y: Thickness-conversion ratio from titanium to TiO2 nanotube fabricated by anodization method. Mater Lett 2008, 62(4):775–779.View ArticleGoogle Scholar
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