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Synthesis of TiO2 submicro-rings and their application in dye-sensitized solar cell
Applied Energy 88 (2011) 825–830
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Synthesis of TiO2 submicro-rings and their application in dye-sensitized solar cell Ming Li a,b, Yong Liu a,⇑, Hai Wang a, Hui Shen a a School of Physics and Engineering, Institute for Solar Energy Systems, State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University, Guangzhou 510275, China b School of Chemical and Biologic Engineering, Guilin University of Technology, Guilin 541004, China
a r t i c l e
i n f o
Article history: Received 2 March 2010 Received in revised form 30 August 2010 Accepted 19 September 2010 Available online 20 October 2010 Keywords: TiO2 submicro-rings Potentiostatic anodization Dye-sensitized Solar cell
a b s t r a c t In this paper, novel TiO2 submicro-rings were synthesized via potentiostatic anodization of titanium powder coated on transparent conducting oxide glass. The TiO2 submicro-rings film was characterized by SEM, XPS and 3D optical profiling. Accordingly, a possible growth mechanism of submicro-rings was discussed. The TiO2 submicro-rings based dye-sensitized solar cell (DSSC) with the film thickness of ca. 3.1 lm was assembled and a conversion efficiency of 1.36% was achieved under AM 1.5 illumination. The photoelectron transport properties of TiO2 submicro-rings based DSSC were also discussed according to the electron impedance spectroscopy. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Dye-sensitized solar cell (DSSC) has been an active research topic in recent years due to its low cost and high energy conversion efficiency combining with a facile fabrication process [1–3]. Selfassembled TiO2 nanostructures with various morphologies, such as nanoparticles [4], nanowires [5,6] and nanotubes [7,8], have attracted much attention because of their potential applications in DSSC. Various physical and chemical methods, such as hydrothermal synthesis [9,10], template method [11,12], electrodeposition [13] and potentiostatic anodization [14,15], have been established to synthesize these nanostructures [16]. Among all these methods, potentiostatic anodization is a potential technique for large-scale fabrication of TiO2 film since it is very simple and controllable [17]. However, the application of potentiostatic anodization in fabricating DSSCs has been limited to a backside illumination due to the in situ growth of TiO2 nanotubes arrays (TNAs) on the opaque titanium substrate [18–20]. It is a less than optimal approach for light-to-electric energy conversion since light is partially reflected by the counter electrode, and partially absorbed by the counter electrode and iodine in the electrolyte before striking TNAs [21,22]. It has been demonstrated that Grätzel’s pioneer DSSCs lost about 40% of their efficiency when illuminated from the backside [23,24]. The fabrication of TNAs based DSSC enabling frontside illumination has been reported recently. Transparent TNAs array was prepared by anodizing a RF sputter-deposited tita⇑ Corresponding author. Tel.: +86 020 39332863; fax: +86 020 39332866. E-mail address: [email protected] (Y. Liu). 0306-2619/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2010.09.017
nium film on transparent conducting oxide (TCO) glass, and a 4.7% efficiency had been achieved for this DSSC fabricated by using 3.6 lm long TNAs [21,25]. More recently, Grimes et al. described the fabrication of transparent TNAs film on TCO glass with lengths up to 33.0 lm, and DSSCs containing these TNAs yielded a power conversion efficiency of 6.9% [26]. We previously reported a frontside illuminated TNAs based DSSC using laser drilling microchannels [27]. However, the reported methods involving the RF sputtering or laser process would lead to a high fabrication cost. Park et al. described another solution of transferring TNAs film from titanium substrate to TCO glass [20], and yet this method was inconvenient to fabricate large area DSSC because of the broken film and the complicated transfer process. Here we report the application of potentiostatic anodization technique for fabricating TiO2 submicro-rings from titanium powder paste coated on TCO glass. As we know, TiO2 submicro-rings have not been reported until now. Furthermore, titanium powder paste could be easily coated on TCO substrate by screen-printing or doctor-blading method to obtain the required TiO2 film thickness. It is expected this method could solve the problem of synthesizing titania directly on TCO glass via anodization. 2. Experimental 2.1. Materials The micro-size (ca. 10 lm) and submicron-size (ca. 300 nm) titanium powder were acquired from Beijing YiTianhui Institute of Metallic Materials. Polyvinyl alcohol (PVA), ethylene glycol,
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Fig. 1. SEM images of TiO2 honeycomb-like particles (a–c) and submicro-rings (d–f) at three levels of magnification. TiO2 honeycomb-like particles and submicro-rings were obtained by anodizing microparticles and submicron particles, respectively.
NH4F and hexachloro platinic acid were obtained from Tianjin Chemical Reagent Co. Ltd. The dye, bis(tetrabutylammonium)-cis(dithiocyanato)-N,N-bis(4-carboxylato-4-carboxylic acid-2,2bipyridine) ruthenium(II) (N719) were purchased from Solaronix. N-methylbenzimidazole and tert-butylpyridine were acquired from Dalian HeptaChroma Solar Tech, Co., Ltd. TCO glass (14 X/ h) was supplied by Nippon Sheet Glass. Surlyn spacer (30 lm thick) was available from DuPont. 2.2. TiO2 submicro-rings synthesis and DSSC assembly Both micron-size and submicron-size titanium particles were used to prepare titanium pastes. In a typical synthesis, 0.1 g PVA was dissolved in 30 g distilled water. Subsequently, 0.3 g titanium powder was added to the solution followed by vigorous stirring. The contents in dispersion were concentrated by evaporator at 80 °C under atmospheric pressure. The resultant paste was coated on TCO glass substrates by the doctor-blading method, and then air-dried at room temperature. The prepared films were anodized at 40 V for 40 min in ethylene glycol solution containing 0.5 wt.% NH4F and 2 wt.% H2O to obtain the transparent TiO2 film. Thermal annealing was carried out at 525 °C for 3 h with heating rate of 1 °C/min. The samples were immersed in 5 104 M N719 in a t-butanol/acetonitrile (1:1, in vol.%) solution for 24 h. For preparation of Pt counter electrodes, isopropanol solution of 5 103 M hexachloro platinic acid was spread on TCO glass by the dip-coating process and then calcined at 380 °C for 20 min. A surlyn spacer was sandwiched between the photoanode and the counter electrode. The electrolyte, containing 0.5 M LiI, 0.05 M I2, 0.6 M N-methylbenzimidazole, and 0.5 M tert-butylpyridine in acetonitrile, was injected into the space between photoanode and counter electrode. A 0.25 cm2 active area was defined by a hole punched through the surlyn frame and was additionally masked from illumination by black electrical tape to the same size. 2.3. Characterization The morphology of anodized film was observed by field-emission scanning electron microscopy (FESEM, JSM-6330F). The X-
ray photoelectron spectroscopy (XPS) was measured with monochromatized Al Ka radiation in the energy range of 0–1103 eV at room temperature. The thickness and surface condition of the films were characterized by the optical profiling system (Wyko NT1100). The photocurrent–voltage (I–V) curves were measured by Keithley 2400 source meter under an illumination of a solar simulator (Newport Oriel 91192) at 100 mW cm2 (AM 1.5 was determined by a standard silicon solar cell) and in the dark, respectively. Electron impedance spectroscopy (EIS) measurements were also carried out by using an electrochemical workstation (CHI760C) under 100 mW cm2 illumination and in the dark to investigate the kinetics of charge transfer and recombination in DSSCs and were performed at open circuit voltage over the frequency range of 102–105 Hz.
3. Results and discussion Two kinds of TiO2 powder were used to synthesize TiO2 films on TCO glass. Fig. 1a–c show low and high magnification scanning electron microscopy (SEM) images of the TiO2 film obtained by anodization of the titanium microparticle paste. It was found that the films made of microparticles had poor substrate adhesion, peeling off seriously after anodization (Fig. 1a). TiO2 honeycomblike microparticles were obtained from the titanium microparticles, as shown in Fig. 1b. It can be seen from Fig. 1c that the edges of walls of the honeycomb-like particles were very coarse. To solve the problem of film peeling, we used submicron titanium powder to replace micro-powder. Fig. 1d–f are SEM images of TiO2 film obtained by anodizing submicron titanium powder. As shown in Fig. 1d, large scale submicro-rings were formed. It was inferred from Fig. 1e that typical outer and inner diameter of submicrorings were about 250 nm and 40 nm, respectively. The high-magnification SEM image (Fig. 1f) clearly shows the circular-shaped rings. The 3D images and the profile curves of the as-obtained film surfaces detected by the optical profiling system are shown in Fig. 2. Inferred from Fig. 2a, the TiO2 film fabricated by anodizing micro-powder did not cover the substrate completely. The smooth section of sample in right part of Fig. 2a is the exposed surface of
M. Li et al. / Applied Energy 88 (2011) 825–830
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Fig. 2. 3D profile measurement of films deposited on TCO substrate. (a) 3D image of anodized micro-powder film. (b) The profile curve of anodized micro-powder film. (c) 3D image of submicro-rings film. (d) The profile curve of submicro-rings film. The red lines marked in (a) and (c) were the route from which the line scans shown in (b) and (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. X-ray photoelectron spectroscopy patterns of (a) anodized micro-powder film and (b) submicro-rings film. Sn and Si, the composition of TCO substrate, were detected in anodized micro-powder sample. Ti2p3/2 peaks were at (c) 458.4 eV for anodized micro-powder sample and (d) 458.2 eV for submicro- rings sample.
TCO glass, indicating that the TiO2 film have peeled off the TCO glass. The film peeling phenomenon was also confirmed in Fig. 2b. The even profile curve at right hand revealed that TiO2 film at this area flaked away from substrate completely. However, as observed in Fig. 2c, the obtained TiO2 submicro-rings film almost covered the substrate, which indicated that submicro-rings film
was attached on the TCO substrate relatively firmly compared with the microparticles film. The average film thickness, estimated from Fig. 2d, reached appropriately 3.1 lm. The corresponding X-ray photoelectron spectroscopy (XPS) patterns recorded from the anodized micro-powder and submicron powder are shown in Fig. 3. As shown in Fig. 3a, Ti and O elements
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Fig. 4. Proposed growth process of TiO2 submicro-rings: (a) The constructed profile of the titanium particles on TCO glass before anodization. (b) Fine pits formed on the oxide surface. (c) The deeper pores in titanium particles. (d) The finished TiO2 submicro-rings.
Fig. 6. Nyquist plots of submicro-rings based DSSC under AM 1.5 condition and in the dark. Fig. 5. Photocurrent–voltage curves of TiO2 submicro-rings based solar cells sensitized (a) without N719, (b) with N719 measured under AM 1.5 condition and (c) with N719 measured in the dark. The active surface area was 0.25 cm2.
were detected in the anodized micro-powder sample. Besides, the Sn and Si elements, composition of TCO glass, were also detected, which illustrated that most of microparticles peeled off from the TCO substrate due to the poor adhesion with substrate. However, Sn and Si elements were not detected in anodized submicron powder sample (Fig. 3b), which confirmed that TiO2 submicro-rings had better substrate adhesion. The Ti2p3/2 peaks were found to be at 458.4 eV for anodized micro-powder (Fig. 3c) and 458.2 eV for anodized submicron powder (Fig. 3d), which were both corresponding to TiO2 reference 458.8 eV. The growth process of submicro-rings structure could be understood on the basis of the formation mechanism of TNAs [17], which is illustrated in Fig. 4. Fig. 4a shows the constructed profile of the titanium particles on TCO glass before anodization. As the anodization process began, the initial oxide layer was formed due to interaction of the surface Ti4+ ions with oxygen ions (O2) in the
electrolyte. Fine pits were formed on the oxide surface at local points of high energy owing to chemical and field-assisted dissolution of TiO2 in fluoride ions containing solution, and acted as points of pore nucleation (Fig. 4b). The preferential dissolution of the pore bottom due to the enhanced electric field allowed the pores to grow deep into Ti particles (Fig. 4c). Finally, the perforating pores were formed and as a result the submicro-rings were obtained (Fig. 4d). The 3.1 lm-thick TiO2 submicro-rings film was used to assemble the DSSC. The I–V characteristics of the submicro-rings based solar cells with and without N719 were tested, as shown in Fig. 5. The active surface area is 0.25 cm2. For the cell fabricated with bare TiO2 submicro-rings, the short circuit current density (JSC) and the open circuit potential (VOC) under 1 sun illumination were 0.27 mA cm2 and 0.41 V, respectively, resulting in a very low energy conversion efficiency (g) of 0.04%. After sensitization, JSC, VOC and g of solar cell increased greatly, reaching 3.72 mA cm2, 0.79 V and 1.36%, respectively. An efficiency of 1.26% for DSSC was obtained by fabricating with 5.8 lm thick TNAs film which was anodized in ethylene glycol
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M. Li et al. / Applied Energy 88 (2011) 825–830 Table 1 A summary of photoelectron transport properties of TiO2 submicro-rings based DSSC. Condition
xmax (Hz)
keff (s1)
seff (s)
Rk (X)
Rw (X)
L (lm)
Deff (cm2 s1)
AM 1.5 Dark
21.230 0.549
21.230 0.317
0.047 3.155
270.8 842.6
169.3 2674.9
3.1 3.1
3.3 106 9.6 109
solution containing 0.5 wt.% NH4F and 2 wt.% H2O at 40 V for 4 h [28]. The employed anodization condition including electrolyte and potential voltage was same to our process. Importantly, the anodization time for synthesizing submicro-rings was shortened to 40 min here, and the close efficiency was obtained. The shortened anodization time may be attributed to the larger surface area of titanium powder exposed to the electrolyte. It is very significant for applying anodization to fabricate TiO2 film in DSSCs with the shortened anodization time, which would improve production efficiency. Further work is expected to apply smaller titanium particles to synthesize TiO2 rings with larger surface area. Besides, the performance of submicro-rings based DSSCs could be further improved by TiCl4 treatment and optimization of film thickness. Relatively high dark current was observed, which attributed to some submicro-rings peeling off from the TCO glass during anodization, resulting in the exposed TCO surface to electrolyte. The white circle in Fig. 2c marked the spot of film peeling. This problem also existed in titanium film coated on TCO glass by using RF sputtering process, which also had poor substrate adhesion and peeled off when immersed in the electrolyte [17]. The methods of changing the composition of anodization electrolyte and applied voltage will be expected to solve this problem in future work. As shown in Fig. 6, electron impedance spectroscopy of the submicro-rings based DSSC under 1 sun illumination and in the dark were measured at open circuit potential. Some interior parameters of the devices can be derived by well fitting the impedance data of the Nyquist plots to expressions based on the models suggested by Adachi et al. [29]. In the dark condition, the relationship among the peak frequency of the semicircle (xmax), the first-order reaction rate constant for the loss of electrons (keff), the charge transfer resistance related to recombination of electron at the TiO2/electrolyte interface (Rk) and the electron transport resistance in TiO2 (Rw) is expressed by
pffiffiffi
xmax ¼ 3keff Rdc ¼ ðRw Rk Þ
1=2
ð1Þ ð2Þ
Rdc is the dc resistance at x = 0. Under 1 sun illumination, the relationship among parameters has changed and is expressed by
xmax ¼ keff
ð3Þ
Rdc ¼ Rw =3 þ Rk
ð4Þ
In both instances, the electron lifetime (seff) is given as
seff ¼ 1=keff
ð5Þ
and the diffusion constant of electrons in the trap state (Deff) is given as
Deff ¼ ðRk =Rw ÞL2 keff
ð6Þ
Electron diffusion length L is equal to the TiO2 film thickness. Thus interior parameters of the TiO2 submicro-rings based DSSC were extracted and presented in Table 1. It is observed that Rk was much smaller under light than in the dark in spite of the same film potential, which was consistent with the phenomena reported by Wang et al. [30]. keff increased when xmax increased under illumination. Thus, the recombination rate increased, suggesting that seff was shortened. This can be ascribed to the local I3 concentration difference. Under illumination, I 3 was generated ‘‘in situ” at
the TiO2/electrolyte interface by dye regeneration, whereas in the dark, I 3 was formed at counter electrode and penetrated the TiO2 submicro-rings film by diffusion. 4. Conclusion In summary, the TiO2 submicro-rings were synthesized by anodizing titanium submicron powder for the first time. The in situ growth of submicro-rings film on TCO glass by potentiostatic anodization was used to fabricate DSSC, and a conversion efficiency of 1.36% was achieved. It is a promising method for application of anodization in fabricating DSSCs. Future work would be focused on controlling the film thickness and improving the adhesion properties of submicro-rings film on substrate. Acknowledgments This work was supported by a grant from the National Natural Science Foundation of China (No. 50702079) and the Fundamental Research Funds for the Central Universities. References [1] O’Regan B, Grätzel M. A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 films. Nature 1991;353:737–40. [2] Grätzel M. Photoelectrochemical cells. Nature 2001;414(6861):338–44. [3] Grätzel M. Solar energy conversion by dye-sensitized photovoltaic cells. Inorg Chem 2005;44(20):6841–51. [4] Ito S, Murakami TN, Comte P, Liska P, Grätzel C, Nazeeruddin MK, et al. Fabrication of thin film dye sensitized solar cells with solar to electric power conversion efficiency over 10%. Thin Solid Films 2008;516(14):4613–9. [5] Law M, Greene LE, Johnson JC, Saykally R, Yang PD. Nanowire dye-sensitized solar cells. Nat Mater 2005;4(6):455–9. [6] Feng XJ, Shankar K, Varghese OK, Paulose M, Latempa TJ, Grimes CA. Vertically aligned single crystal TiO2 nanowire arrays grown directly on transparent conducting oxide coated glass: synthesis details and applications. Nano Lett 2008;8(11):3781–6. [7] Macak JM, Tsuchiya H, Schmuki P. High-aspect-ratio TiO2 nanotubes by anodization of titanium. Angew Chem Int Ed 2005;44(14):2100–12. [8] Mor GK, Shankar K, Paulose M, Varghese OK, Grimes CA. Enhanced photocleavage of water using titania nanotube arrays. Nano Lett 2005;5(1):191–5. [9] Zhang DS, Yoshida T, Minoura H. Low-temperature fabrication of efficient porous titania photoelectrodes by hydrothermal crystallization at the solid/gas interface. Adv Mater 2003;15(10):814–7. [10] Wang H, Liu Y, Li M, Huang H, Zhong MY, Shen H. Hydrothermal growth of large-scale macroporous TiO2 nanowires and its application in 3D dyesensitized solar cells. Appl Phys A 2009;97(1):25–9. [11] Ren X, Gershon T, Iza DC, Munoz-Rojas D, Musselman K, MacManus-Driscoll JL. The selective fabrication of large-area highly ordered TiO2 nanorod and nanotube arrays on conductive transparent substrates via sol–gel electrophoresis. Nanotechnology 2009;20(36):365604. [12] Tan LK, Chong MAS, Gao H. Free-standing porous anodic alumina templates for atomic layer deposition of highly ordered TiO2 nanotube arrays on various substrates. J Phys Chem C 2008;112(1):69–73. [13] Tsai TY, Lu SY. A novel way of improving light harvesting in dye-sensitized solar cells – electrodeposition of titania. Electrochem Commun 2009;11(11):2180–3. [14] Chen QW, Xu DS. Large-scale, noncurling, and free-standing crystallized TiO2 nanotube arrays for dye-sensitized solar cells. J Phys Chem C 2009;113(15):6310–4. [15] Kang TS, Smith AP, Taylor BE, Durstock MF. Fabrication of highly-ordered TiO2 nanotube arrays and their use in dye-sensitized solar cells. Nano Lett 2009;9(2):601–6. [16] Chen X, Mao SS. Titanium dioxide nanomaterials: synthesis properties modifications and applications. Chem Rev 2007;7:2891–959. [17] Mor GK, Varghese OK, Paulose M, Shankar K, Grimes CA. A review on highly ordered, vertically oriented TiO2 nanotube arrays: fabrication, material
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M. Li et al. / Applied Energy 88 (2011) 825–830 properties, and solar energy applications. Solar Energy Mater Solar Cells 2006;90(14):2011–75. Paulose M, Shankar K, Varghese OK, Mor GK, Hardin B, Grimes CA. Backside illuminated dye-sensitized solar cells based on titania nanotube array electrodes. Nanotechnology 2006;17(5):1446–8. Zhu K, Neale NR, Miedaner A, Frank AJ. Enhanced charge-collection efficiencies and light scattering in dye-sensitized solar cells using oriented TiO2 nanotubes arrays. Nano Lett 2007;7(1):69–74. Park JH, Lee TW, Kang MG. Growth, detachment and transfer of highly-ordered TiO2 nanotube arrays: use in dye-sensitized solar cells. Chem Commun 2008:2867–9. 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):215–8. Snaith HJ, Schmidt-Mende L. Advances in liquid-electrolyte and solid-state dye-sensitized solar cells. Adv Mater 2007;19(20):3187–200. Stergiopoulos T, Ghicov A, Likodimos V, Tsoukleris DS, Kunze J, Schmuki P, et al. Dye-sensitized solar cells based on thick highly ordered TiO2 nanotubes produced by controlled anodic oxidation in non-aqueous electrolytic media. Nanotechnology 2008;19(23):235602.
[24] Ito S, Ha NL, Rothenberger G, Liska P, Comte P, Zakeeruddin SM, et al. Highefficiency (7.2%) flexible dye-sensitized solar cells with Ti-metal substrate for nanocrystalline-TiO2 photoanod. Chem Commun 2006(38):4004–6. [25] Paulose M, Shankar K, Varghese OK, Mor GK, Grimes CA. Application of highlyordered TiO2 nanotube-arrays in heterojunction dye-sensitized solar cells. J Phys D 2006;39(12):2498–503. [26] Varghese OK, Paulose M, Grimes CA. Long vertically aligned titania nanotubes on transparent conducting oxide for highly efficient solar cells. Nat Nanotechnol 2009;4(9):592–7. [27] Liu Y, Wang H, Li M, Hong RJ, Ye QH, Zheng JM, et al. Frontside illuminated TiO2 nanotube dye-sensitized solar cells using multifunctional microchannel array electrodes. Appl Phys Lett 2009;95(23):233505. [28] Xie ZB, Adams S, Blackwood DJ, Wang J. The effects of anodization parameters on titania nanotube arrays and dye sensitized solar cells. Nanotechnology 2008;19(40):405701. [29] Adachi MSM, Jiu J, Ogata Y, Isoda S. Determination of parameters of electron transport in dye-sensitized solar cells using electrochemical impedance spectroscopy. J Phys Chem B 2006;110:13872–80. [30] Wang Q, Moser JE, Gratzel M. Electrochemical impedance spectroscopic analysis of dye-sensitized solar cells. J Phys Chem B 2005;109(31):14945–53.
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Synthesis of TiO2 submicro-rings and their application in dye-sensitized solar cell Ming Li a,b, Yong Liu a,⇑, Hai Wang a, Hui Shen a a School of Physics and Engineering, Institute for Solar Energy Systems, State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University, Guangzhou 510275, China b School of Chemical and Biologic Engineering, Guilin University of Technology, Guilin 541004, China
a r t i c l e
i n f o
Article history: Received 2 March 2010 Received in revised form 30 August 2010 Accepted 19 September 2010 Available online 20 October 2010 Keywords: TiO2 submicro-rings Potentiostatic anodization Dye-sensitized Solar cell
a b s t r a c t In this paper, novel TiO2 submicro-rings were synthesized via potentiostatic anodization of titanium powder coated on transparent conducting oxide glass. The TiO2 submicro-rings film was characterized by SEM, XPS and 3D optical profiling. Accordingly, a possible growth mechanism of submicro-rings was discussed. The TiO2 submicro-rings based dye-sensitized solar cell (DSSC) with the film thickness of ca. 3.1 lm was assembled and a conversion efficiency of 1.36% was achieved under AM 1.5 illumination. The photoelectron transport properties of TiO2 submicro-rings based DSSC were also discussed according to the electron impedance spectroscopy. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Dye-sensitized solar cell (DSSC) has been an active research topic in recent years due to its low cost and high energy conversion efficiency combining with a facile fabrication process [1–3]. Selfassembled TiO2 nanostructures with various morphologies, such as nanoparticles [4], nanowires [5,6] and nanotubes [7,8], have attracted much attention because of their potential applications in DSSC. Various physical and chemical methods, such as hydrothermal synthesis [9,10], template method [11,12], electrodeposition [13] and potentiostatic anodization [14,15], have been established to synthesize these nanostructures [16]. Among all these methods, potentiostatic anodization is a potential technique for large-scale fabrication of TiO2 film since it is very simple and controllable [17]. However, the application of potentiostatic anodization in fabricating DSSCs has been limited to a backside illumination due to the in situ growth of TiO2 nanotubes arrays (TNAs) on the opaque titanium substrate [18–20]. It is a less than optimal approach for light-to-electric energy conversion since light is partially reflected by the counter electrode, and partially absorbed by the counter electrode and iodine in the electrolyte before striking TNAs [21,22]. It has been demonstrated that Grätzel’s pioneer DSSCs lost about 40% of their efficiency when illuminated from the backside [23,24]. The fabrication of TNAs based DSSC enabling frontside illumination has been reported recently. Transparent TNAs array was prepared by anodizing a RF sputter-deposited tita⇑ Corresponding author. Tel.: +86 020 39332863; fax: +86 020 39332866. E-mail address: [email protected] (Y. Liu). 0306-2619/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2010.09.017
nium film on transparent conducting oxide (TCO) glass, and a 4.7% efficiency had been achieved for this DSSC fabricated by using 3.6 lm long TNAs [21,25]. More recently, Grimes et al. described the fabrication of transparent TNAs film on TCO glass with lengths up to 33.0 lm, and DSSCs containing these TNAs yielded a power conversion efficiency of 6.9% [26]. We previously reported a frontside illuminated TNAs based DSSC using laser drilling microchannels [27]. However, the reported methods involving the RF sputtering or laser process would lead to a high fabrication cost. Park et al. described another solution of transferring TNAs film from titanium substrate to TCO glass [20], and yet this method was inconvenient to fabricate large area DSSC because of the broken film and the complicated transfer process. Here we report the application of potentiostatic anodization technique for fabricating TiO2 submicro-rings from titanium powder paste coated on TCO glass. As we know, TiO2 submicro-rings have not been reported until now. Furthermore, titanium powder paste could be easily coated on TCO substrate by screen-printing or doctor-blading method to obtain the required TiO2 film thickness. It is expected this method could solve the problem of synthesizing titania directly on TCO glass via anodization. 2. Experimental 2.1. Materials The micro-size (ca. 10 lm) and submicron-size (ca. 300 nm) titanium powder were acquired from Beijing YiTianhui Institute of Metallic Materials. Polyvinyl alcohol (PVA), ethylene glycol,
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M. Li et al. / Applied Energy 88 (2011) 825–830
Fig. 1. SEM images of TiO2 honeycomb-like particles (a–c) and submicro-rings (d–f) at three levels of magnification. TiO2 honeycomb-like particles and submicro-rings were obtained by anodizing microparticles and submicron particles, respectively.
NH4F and hexachloro platinic acid were obtained from Tianjin Chemical Reagent Co. Ltd. The dye, bis(tetrabutylammonium)-cis(dithiocyanato)-N,N-bis(4-carboxylato-4-carboxylic acid-2,2bipyridine) ruthenium(II) (N719) were purchased from Solaronix. N-methylbenzimidazole and tert-butylpyridine were acquired from Dalian HeptaChroma Solar Tech, Co., Ltd. TCO glass (14 X/ h) was supplied by Nippon Sheet Glass. Surlyn spacer (30 lm thick) was available from DuPont. 2.2. TiO2 submicro-rings synthesis and DSSC assembly Both micron-size and submicron-size titanium particles were used to prepare titanium pastes. In a typical synthesis, 0.1 g PVA was dissolved in 30 g distilled water. Subsequently, 0.3 g titanium powder was added to the solution followed by vigorous stirring. The contents in dispersion were concentrated by evaporator at 80 °C under atmospheric pressure. The resultant paste was coated on TCO glass substrates by the doctor-blading method, and then air-dried at room temperature. The prepared films were anodized at 40 V for 40 min in ethylene glycol solution containing 0.5 wt.% NH4F and 2 wt.% H2O to obtain the transparent TiO2 film. Thermal annealing was carried out at 525 °C for 3 h with heating rate of 1 °C/min. The samples were immersed in 5 104 M N719 in a t-butanol/acetonitrile (1:1, in vol.%) solution for 24 h. For preparation of Pt counter electrodes, isopropanol solution of 5 103 M hexachloro platinic acid was spread on TCO glass by the dip-coating process and then calcined at 380 °C for 20 min. A surlyn spacer was sandwiched between the photoanode and the counter electrode. The electrolyte, containing 0.5 M LiI, 0.05 M I2, 0.6 M N-methylbenzimidazole, and 0.5 M tert-butylpyridine in acetonitrile, was injected into the space between photoanode and counter electrode. A 0.25 cm2 active area was defined by a hole punched through the surlyn frame and was additionally masked from illumination by black electrical tape to the same size. 2.3. Characterization The morphology of anodized film was observed by field-emission scanning electron microscopy (FESEM, JSM-6330F). The X-
ray photoelectron spectroscopy (XPS) was measured with monochromatized Al Ka radiation in the energy range of 0–1103 eV at room temperature. The thickness and surface condition of the films were characterized by the optical profiling system (Wyko NT1100). The photocurrent–voltage (I–V) curves were measured by Keithley 2400 source meter under an illumination of a solar simulator (Newport Oriel 91192) at 100 mW cm2 (AM 1.5 was determined by a standard silicon solar cell) and in the dark, respectively. Electron impedance spectroscopy (EIS) measurements were also carried out by using an electrochemical workstation (CHI760C) under 100 mW cm2 illumination and in the dark to investigate the kinetics of charge transfer and recombination in DSSCs and were performed at open circuit voltage over the frequency range of 102–105 Hz.
3. Results and discussion Two kinds of TiO2 powder were used to synthesize TiO2 films on TCO glass. Fig. 1a–c show low and high magnification scanning electron microscopy (SEM) images of the TiO2 film obtained by anodization of the titanium microparticle paste. It was found that the films made of microparticles had poor substrate adhesion, peeling off seriously after anodization (Fig. 1a). TiO2 honeycomblike microparticles were obtained from the titanium microparticles, as shown in Fig. 1b. It can be seen from Fig. 1c that the edges of walls of the honeycomb-like particles were very coarse. To solve the problem of film peeling, we used submicron titanium powder to replace micro-powder. Fig. 1d–f are SEM images of TiO2 film obtained by anodizing submicron titanium powder. As shown in Fig. 1d, large scale submicro-rings were formed. It was inferred from Fig. 1e that typical outer and inner diameter of submicrorings were about 250 nm and 40 nm, respectively. The high-magnification SEM image (Fig. 1f) clearly shows the circular-shaped rings. The 3D images and the profile curves of the as-obtained film surfaces detected by the optical profiling system are shown in Fig. 2. Inferred from Fig. 2a, the TiO2 film fabricated by anodizing micro-powder did not cover the substrate completely. The smooth section of sample in right part of Fig. 2a is the exposed surface of
M. Li et al. / Applied Energy 88 (2011) 825–830
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Fig. 2. 3D profile measurement of films deposited on TCO substrate. (a) 3D image of anodized micro-powder film. (b) The profile curve of anodized micro-powder film. (c) 3D image of submicro-rings film. (d) The profile curve of submicro-rings film. The red lines marked in (a) and (c) were the route from which the line scans shown in (b) and (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. X-ray photoelectron spectroscopy patterns of (a) anodized micro-powder film and (b) submicro-rings film. Sn and Si, the composition of TCO substrate, were detected in anodized micro-powder sample. Ti2p3/2 peaks were at (c) 458.4 eV for anodized micro-powder sample and (d) 458.2 eV for submicro- rings sample.
TCO glass, indicating that the TiO2 film have peeled off the TCO glass. The film peeling phenomenon was also confirmed in Fig. 2b. The even profile curve at right hand revealed that TiO2 film at this area flaked away from substrate completely. However, as observed in Fig. 2c, the obtained TiO2 submicro-rings film almost covered the substrate, which indicated that submicro-rings film
was attached on the TCO substrate relatively firmly compared with the microparticles film. The average film thickness, estimated from Fig. 2d, reached appropriately 3.1 lm. The corresponding X-ray photoelectron spectroscopy (XPS) patterns recorded from the anodized micro-powder and submicron powder are shown in Fig. 3. As shown in Fig. 3a, Ti and O elements
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Fig. 4. Proposed growth process of TiO2 submicro-rings: (a) The constructed profile of the titanium particles on TCO glass before anodization. (b) Fine pits formed on the oxide surface. (c) The deeper pores in titanium particles. (d) The finished TiO2 submicro-rings.
Fig. 6. Nyquist plots of submicro-rings based DSSC under AM 1.5 condition and in the dark. Fig. 5. Photocurrent–voltage curves of TiO2 submicro-rings based solar cells sensitized (a) without N719, (b) with N719 measured under AM 1.5 condition and (c) with N719 measured in the dark. The active surface area was 0.25 cm2.
were detected in the anodized micro-powder sample. Besides, the Sn and Si elements, composition of TCO glass, were also detected, which illustrated that most of microparticles peeled off from the TCO substrate due to the poor adhesion with substrate. However, Sn and Si elements were not detected in anodized submicron powder sample (Fig. 3b), which confirmed that TiO2 submicro-rings had better substrate adhesion. The Ti2p3/2 peaks were found to be at 458.4 eV for anodized micro-powder (Fig. 3c) and 458.2 eV for anodized submicron powder (Fig. 3d), which were both corresponding to TiO2 reference 458.8 eV. The growth process of submicro-rings structure could be understood on the basis of the formation mechanism of TNAs [17], which is illustrated in Fig. 4. Fig. 4a shows the constructed profile of the titanium particles on TCO glass before anodization. As the anodization process began, the initial oxide layer was formed due to interaction of the surface Ti4+ ions with oxygen ions (O2) in the
electrolyte. Fine pits were formed on the oxide surface at local points of high energy owing to chemical and field-assisted dissolution of TiO2 in fluoride ions containing solution, and acted as points of pore nucleation (Fig. 4b). The preferential dissolution of the pore bottom due to the enhanced electric field allowed the pores to grow deep into Ti particles (Fig. 4c). Finally, the perforating pores were formed and as a result the submicro-rings were obtained (Fig. 4d). The 3.1 lm-thick TiO2 submicro-rings film was used to assemble the DSSC. The I–V characteristics of the submicro-rings based solar cells with and without N719 were tested, as shown in Fig. 5. The active surface area is 0.25 cm2. For the cell fabricated with bare TiO2 submicro-rings, the short circuit current density (JSC) and the open circuit potential (VOC) under 1 sun illumination were 0.27 mA cm2 and 0.41 V, respectively, resulting in a very low energy conversion efficiency (g) of 0.04%. After sensitization, JSC, VOC and g of solar cell increased greatly, reaching 3.72 mA cm2, 0.79 V and 1.36%, respectively. An efficiency of 1.26% for DSSC was obtained by fabricating with 5.8 lm thick TNAs film which was anodized in ethylene glycol
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M. Li et al. / Applied Energy 88 (2011) 825–830 Table 1 A summary of photoelectron transport properties of TiO2 submicro-rings based DSSC. Condition
xmax (Hz)
keff (s1)
seff (s)
Rk (X)
Rw (X)
L (lm)
Deff (cm2 s1)
AM 1.5 Dark
21.230 0.549
21.230 0.317
0.047 3.155
270.8 842.6
169.3 2674.9
3.1 3.1
3.3 106 9.6 109
solution containing 0.5 wt.% NH4F and 2 wt.% H2O at 40 V for 4 h [28]. The employed anodization condition including electrolyte and potential voltage was same to our process. Importantly, the anodization time for synthesizing submicro-rings was shortened to 40 min here, and the close efficiency was obtained. The shortened anodization time may be attributed to the larger surface area of titanium powder exposed to the electrolyte. It is very significant for applying anodization to fabricate TiO2 film in DSSCs with the shortened anodization time, which would improve production efficiency. Further work is expected to apply smaller titanium particles to synthesize TiO2 rings with larger surface area. Besides, the performance of submicro-rings based DSSCs could be further improved by TiCl4 treatment and optimization of film thickness. Relatively high dark current was observed, which attributed to some submicro-rings peeling off from the TCO glass during anodization, resulting in the exposed TCO surface to electrolyte. The white circle in Fig. 2c marked the spot of film peeling. This problem also existed in titanium film coated on TCO glass by using RF sputtering process, which also had poor substrate adhesion and peeled off when immersed in the electrolyte [17]. The methods of changing the composition of anodization electrolyte and applied voltage will be expected to solve this problem in future work. As shown in Fig. 6, electron impedance spectroscopy of the submicro-rings based DSSC under 1 sun illumination and in the dark were measured at open circuit potential. Some interior parameters of the devices can be derived by well fitting the impedance data of the Nyquist plots to expressions based on the models suggested by Adachi et al. [29]. In the dark condition, the relationship among the peak frequency of the semicircle (xmax), the first-order reaction rate constant for the loss of electrons (keff), the charge transfer resistance related to recombination of electron at the TiO2/electrolyte interface (Rk) and the electron transport resistance in TiO2 (Rw) is expressed by
pffiffiffi
xmax ¼ 3keff Rdc ¼ ðRw Rk Þ
1=2
ð1Þ ð2Þ
Rdc is the dc resistance at x = 0. Under 1 sun illumination, the relationship among parameters has changed and is expressed by
xmax ¼ keff
ð3Þ
Rdc ¼ Rw =3 þ Rk
ð4Þ
In both instances, the electron lifetime (seff) is given as
seff ¼ 1=keff
ð5Þ
and the diffusion constant of electrons in the trap state (Deff) is given as
Deff ¼ ðRk =Rw ÞL2 keff
ð6Þ
Electron diffusion length L is equal to the TiO2 film thickness. Thus interior parameters of the TiO2 submicro-rings based DSSC were extracted and presented in Table 1. It is observed that Rk was much smaller under light than in the dark in spite of the same film potential, which was consistent with the phenomena reported by Wang et al. [30]. keff increased when xmax increased under illumination. Thus, the recombination rate increased, suggesting that seff was shortened. This can be ascribed to the local I3 concentration difference. Under illumination, I 3 was generated ‘‘in situ” at
the TiO2/electrolyte interface by dye regeneration, whereas in the dark, I 3 was formed at counter electrode and penetrated the TiO2 submicro-rings film by diffusion. 4. Conclusion In summary, the TiO2 submicro-rings were synthesized by anodizing titanium submicron powder for the first time. The in situ growth of submicro-rings film on TCO glass by potentiostatic anodization was used to fabricate DSSC, and a conversion efficiency of 1.36% was achieved. It is a promising method for application of anodization in fabricating DSSCs. Future work would be focused on controlling the film thickness and improving the adhesion properties of submicro-rings film on substrate. Acknowledgments This work was supported by a grant from the National Natural Science Foundation of China (No. 50702079) and the Fundamental Research Funds for the Central Universities. References [1] O’Regan B, Grätzel M. A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 films. Nature 1991;353:737–40. [2] Grätzel M. Photoelectrochemical cells. Nature 2001;414(6861):338–44. [3] Grätzel M. Solar energy conversion by dye-sensitized photovoltaic cells. Inorg Chem 2005;44(20):6841–51. [4] Ito S, Murakami TN, Comte P, Liska P, Grätzel C, Nazeeruddin MK, et al. Fabrication of thin film dye sensitized solar cells with solar to electric power conversion efficiency over 10%. Thin Solid Films 2008;516(14):4613–9. [5] Law M, Greene LE, Johnson JC, Saykally R, Yang PD. Nanowire dye-sensitized solar cells. Nat Mater 2005;4(6):455–9. [6] Feng XJ, Shankar K, Varghese OK, Paulose M, Latempa TJ, Grimes CA. Vertically aligned single crystal TiO2 nanowire arrays grown directly on transparent conducting oxide coated glass: synthesis details and applications. Nano Lett 2008;8(11):3781–6. [7] Macak JM, Tsuchiya H, Schmuki P. High-aspect-ratio TiO2 nanotubes by anodization of titanium. Angew Chem Int Ed 2005;44(14):2100–12. [8] Mor GK, Shankar K, Paulose M, Varghese OK, Grimes CA. Enhanced photocleavage of water using titania nanotube arrays. Nano Lett 2005;5(1):191–5. [9] Zhang DS, Yoshida T, Minoura H. Low-temperature fabrication of efficient porous titania photoelectrodes by hydrothermal crystallization at the solid/gas interface. Adv Mater 2003;15(10):814–7. [10] Wang H, Liu Y, Li M, Huang H, Zhong MY, Shen H. Hydrothermal growth of large-scale macroporous TiO2 nanowires and its application in 3D dyesensitized solar cells. Appl Phys A 2009;97(1):25–9. [11] Ren X, Gershon T, Iza DC, Munoz-Rojas D, Musselman K, MacManus-Driscoll JL. The selective fabrication of large-area highly ordered TiO2 nanorod and nanotube arrays on conductive transparent substrates via sol–gel electrophoresis. Nanotechnology 2009;20(36):365604. [12] Tan LK, Chong MAS, Gao H. Free-standing porous anodic alumina templates for atomic layer deposition of highly ordered TiO2 nanotube arrays on various substrates. J Phys Chem C 2008;112(1):69–73. [13] Tsai TY, Lu SY. A novel way of improving light harvesting in dye-sensitized solar cells – electrodeposition of titania. Electrochem Commun 2009;11(11):2180–3. [14] Chen QW, Xu DS. Large-scale, noncurling, and free-standing crystallized TiO2 nanotube arrays for dye-sensitized solar cells. J Phys Chem C 2009;113(15):6310–4. [15] Kang TS, Smith AP, Taylor BE, Durstock MF. Fabrication of highly-ordered TiO2 nanotube arrays and their use in dye-sensitized solar cells. Nano Lett 2009;9(2):601–6. [16] Chen X, Mao SS. Titanium dioxide nanomaterials: synthesis properties modifications and applications. Chem Rev 2007;7:2891–959. [17] Mor GK, Varghese OK, Paulose M, Shankar K, Grimes CA. A review on highly ordered, vertically oriented TiO2 nanotube arrays: fabrication, material
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