Deposition of transparent TiO 2 nanotube-films via electrophoretic technique for photovoltaic applications

Transcription

1 SCIENCE CHINA Materials mater.scichina.com link.springer.com ARTICLES Published online 18 September 215 doi: 1.17/s Sci China Mater 215, 58: Deposition of transparent TiO 2 nanotube-films via electrophoretic technique for photovoltaic applications Jin Zhang 1*, Shijie Li 1, Pengfei Yang 1, Wenxiu Que 2 and Weiguo Liu 1 In this paper, semitransparent TiO2 nanotube-films were prepared on fluorine doped tin oxide (FTO) glass by the electrophoretic deposition (EPD) process and their properties were characterized. Furthermore, dye-sensitized solar cells (DSSCs) based on the as-prepared TiO2 nanotube-films were assembled, and effects of the film thickness and EPD voltage on performance of the DSSCs were investigated. Power conversion efficiency with the maximum value of 7.1% was successfully achieved, indicating the semitransparent TiO2 nanotube-films are very promising for high transmittance substrates and high efficiency photovoltaic electrodes. INTRODUCTION Titanium dioxide (TiO 2 ) nanotubes have attracted considerable interests in scientific and technological communities, due to their unique functional properties such as good chemical stability and high photoconversion efficiency [1 4]. Therefore, TiO 2 nanotube-film electrodes have been widely utilized in various applications including lithium ion battery [5], photocatalysts [6], solar cells [7], hydrogen sorption [8] and biomedical materials [9]. Recently, TiO 2 nanotubes have been widely applied in dye- sensitized solar cells (DSSCs) to overcome the drawback of carrier recombination by providing short and direct pathways for electron transport and collection [1 25]. For instance, Varghese et al. [1] used long vertically aligned TiO 2 nanotube arrays to greatly improve the efficiency of DSSCs; Zhong et al. [11] observed TiO 2 nanotube photoanode owned a longer electron diffusion length and a larger electron lifetime than the nanoparticle one in DSSCs. In addition, the incorporated TiO 2 nanotubes could enhance light-harvesting efficiency due to their large specific surface areas as well as the light scattering effect [26,27]. Usually, TiO 2 nanotubes with different microstructures and geometrical shapes can be synthesized by a wide va- riety of techniques such as electrodeposition [28], sol-gel synthesis [29], freeze-drying [3], chemical treatments of TiO 2 particles [31], and electrophoretic deposition (EPD) [32]. Especially, the EPD method allows for a simple fabrication of large scale and complex shape thin film, with controllable coating thickness, simple equipment required and low cost. Kim et al. [32] adopted TiO 2 nanoparticles to fabricate TiO 2 nanotubes which were deposited on fluorine doped tin oxide (FTO) by EPD method. Chiu et al. [33] and Chou et al. [34] employed EPD method to prepare TiO 2 nanoparticle thin films and applied them in DSSCs. But there are only few reports on fabricating TiO 2 nanotube-films via EPD method by using individual TiO 2 nanotube-powders, and not to mention further application in photovoltaic devices. In this paper, TiO 2 nanotube-films were deposited on FTO glass by an EPD process, and DSSCs based on the TiO 2 nanotube-films were fabricated. Effects of the film thickness and electrophoretic voltage on performance of the DSSCs were investigated. EXPERIMENTAL SECTION Materials Individual TiO 2 nanotube-powders were firstly prepared by a two-step process as our earlier report [35]. The TiO 2 nanotube-powders were synthesized by a rapid anodization process and then they were disaggregated into individual TiO 2 nanotubes under assistance of ultrasonic oscillation. Subsequently, the as-obtained TiO 2 nanotubes were deposited on FTO (F-SnO 2 coated glass) substrates by using the EPD method. The electrolyte was prepared by adding disaggregated TiO 2 nanotubes (.5 g) into 1 ml absolute anhydrous ethanol with a small addition of water (1 ml) and acetylacetone (1 ml). The electrophoretic 1 School of Optoelectronic Engineering, Xi an Technological University, Xi an 7132, China 2 School of Electronic and Information Engineering, Xi an Jiaotong University, Xi an 7149, China * Corresponding author ( jinzhang_postbox@163.com) October 215 Vol.58 No.1 785

2 ARTICLES SCIENCE CHINA Materials cell contained two electrodes of FTO glass; one electrode was a cathode substrate and the other electrode served as a counter-electrode. These two electrodes were kept at a distance of 2 mm. The EPD process was thus performed using an optimized constant voltage (2 to 1 V) under room temperature for 2 to 8 min. After the EPD process, the FTO substrate was oven dried at 1 o C for 3 min and annealed at 5 o C for 3 min. Assembly of DSSCs The TiO 2 nanotube-films deposited on FTO substrates were dyed by soaking for 24 h at room temperature in.5 mmol L 1 solutions in absolute ethanol of the ruthenium complex RuL2(NCS)2:2TBA (L=2,2ʹ-bipyridyl-4,4ʹ-dicarboxylic acid) (commercially known as N719 dye). The Pt counter electrodes were obtained by sputtering Pt onto the FTO substrates. The DSSCs package processes were similar to those as reported previously [36]. The electrolytes consisted of.5 mol L 1 LiI,.5 mol L 1 I 2, and.5 mol L 1 tertbutylpyridine in acetonitrile. The active area of the resulting cell exposed in light was approximately.25 cm 2 (.5 cm.5 cm) which was limited by a mask. Characterization Morphological and structural properties of the anodized TiO 2 nanotube-powders and the EPD films were characterized by a field emission scanning electron microscope (FESEM, JSM-7F, JEOL Inc., Japan) and a transmission electron microscope (TEM, JEM21, JEOL Inc., Japan). The transmittance spectrum of the as-prepared TiO 2 nanotube-film was measured by a JASCO V-57 UV/vis/NIR spectrometer. The current-voltage (J-V) curves of DSSCs were recorded by a Keithley SMU 24 Source Measure unit under 1 mw cm 2, AM 1.5 conditions (Newport Solar Simulator). RESULTS AND DISCUSSION Fig. 1a shows a TEM image of the as-prepared anodized TiO 2 nanotube-powders. It can be seen that hundreds of TiO 2 nanotubes are aggregated together, which means they are not appropriate for the EPD. After ultrasonic oscillation, the bundled TiO 2 nanotubes are well disaggregated into individual TiO 2 nanotubes, as shown in Fig. 1b. These nanotubes are about 2 nm in outer diameter, while their tube-lengths are of multi-distribution. The nanoparticles in Fig. 1b can be ascribed to the collapse of partial TiO 2 nanotubes. These results are good in line with our earlier study in Ref. [35]. The TiO 2 nanotube-films were fabricated with different voltages. According to the Hamaker equation [37,38], the relationship between the deposited weight (w) and the electric field intensity (E) is t w AC Edt, (1) where μ is the electrophoretic mobility, A is the surface area of the electrode, C is the concentration of the suspension, and t is the time. The thickness of the TiO 2 nanotube-films were tested by FESEM and the deposition rate were calculated as shown in Table 1. Results show that the larger EPD voltage was applied, the faster deposition rate obtained. Increasing the EPD voltage could provide larger drag force, so the TiO 2 nanotubes could be deposited at a faster rate. Furthermore, DSSCs based on TiO 2 nanotube-films which were prepared with different EPD voltages were fabricated and investigated. According to the deposition rate, the thicknesses of these TiO 2 nanotube-films were specially deposited to about 4.5 μm. The performances of the DSSCs are showed in Fig. 2, and their data are summarized in Table 1. It is interesting that the conversion efficiency (FF) of the DSSC increases from 2.2% to 4.31% when the EPD a b Figure 1 Morphological and structural properties of the anodized TiO 2 nanotube-powders, (a) before ultrasonic oscillation, (b) after ultrasonic oscillation. 786 October 215 Vol.58 No.1

3 SCIENCE CHINA Materials ARTICLES Current density (ma cm 2 ) Voltage (V) 1 V 8 V 6 V 4 V 2 V Figure 2 Performance of DSSCs based on TiO 2 nanotube-films which prepared with different EPD voltages. voltage decreases from 1 to 2 V. The main reason should be that the high EPD voltage leads to large drag force of the nanotubes during the EPD process, which means the film is more compact than that deposited by low voltage. That is to say, the nanotube-films deposited by high EPD voltage has less porous than that deposited by low EPD voltage, and the former one cannot adsorb enough dye to generate photocurrent. According to the results and considering the factor of deposition rate (deposition rate at 4 V is higher than that at 2 V), the EPD voltage of 4 V is fixed in the subsequent experiments. The thickness of TiO 2 nanotube-films is an important factor to influence the FF of DSSCs. In view of this, TiO 2 nanotube-films with different thickness were prepared by varying EPD time at 4 V. Fig. 3 shows the FESEM images of the top view and cross-section view of the TiO 2 nano- Table 1 Deposition rate of TiO2 nanotube-films prepared with different EPD voltages, and performance of DSSCs fabricated by the as-prepared TiO2 nanotube-films EPD voltage (V) Deposition rate (μm min 1 ) Thickness of TiO 2 thin film (μm) J sc (ma cm 2 ) V oc (V) FF η (%) 2.18 ~ ~ ~ ~ ~ a b c d e f Figure 3 SEM images of TiO 2 nanotube-films prepared with different EPD time, (a) top view, (b) (f) are cross section of TiO 2 nanotube-films prepared for 2, 3, 4, 6 and 8 min, respectively. The inset of (f) is the top view of TiO 2 nanotube-films prepared for 8 min. October 215 Vol.58 No.1 787

4 ARTICLES SCIENCE CHINA Materials tube-films. It can be seen that the individual TiO 2 nanotubes were randomly deposited on the FTO glass surface, forming a TiO 2 film as shown in Fig. 3a. Figs 3b f show that the thickness of the TiO 2 film increases from 5.34 to μm with the EPD time increasing from 2 to 8 min. However, when the EPD time reaches up to 8 min, the TiO 2 film will crack or detach from the FTO substrate as shown in inset of Fig. 3f, which implies that the film cannot bear the stress caused by the increasing of the film thickness. Fig. 4 shows the transmittance spectrum of the EPD TiO 2 nanotube-films prepared with different EPD time. It can be seen that the TiO 2 nanotube-films are semitransparent and the value of transmittance is high up to 7% when the film thickness is about 5 μm, indicating that the TiO 2 nanotubes disperse uniformly in the film without agglomerate. Actually, such kind of semitransparent TiO 2 nanotube-films can be utilized in many potential applications such as photoelectronic devices. Moreover, DSSCs based on the as-prepared TiO 2 nanotube-films with different EPD time were fabricated and investigated. The performances of the DSSCs are showed in Fig. 5, and their data are summarized in Table 2. It can be observed that a highest short circuit current density Transmittance (%) min 3 min 2 4 min 6 min FTO Wavelength (nm) Figure 4 Transmittance spectra of the EPD TiO 2 nanotube-films prepared with different EPD time. of ma cm 2 and maximum energy conversion efficiency (η) of 7.1 % can be obtained when the TiO 2 nanotube-films deposited for 4 min. With further increasing the EPD time of TiO 2 nanotube-films, the conversion efficiency of the DSSC decreases. A reasonable explanation is that the TiO 2 nanotube-films with appropriate thickness can adsorb enough dye to generate photocurrent, but with over-increased film thickness, the carrier recombination rate and transport resistance are also getting serious, which deteriorate the conversion efficiency. CONCLUSIONS Semitransparent TiO 2 nanotube-films on FTO substrates were successfully fabricated by the EPD method. In addition, the DSSCs based on the as-prepared TiO 2 nanotube-films were fabricated and investigated. The precursor TiO 2 for EPD is individual TiO 2 nanotubes derived from rapid anodization process and subsequent ultrasonic disaggregation. Results indicate the individual TiO 2 nanotubes can be randomly deposited on the FTO glass surface, forming a TiO 2 nanotube-film. The TiO 2 nanotube-films are suitable to severe as the photoanodes of DSSCs, the best conversion efficiency of the DSSCs achieves 7.1% when the TiO 2 nanotube-film EPD at 4 V for 4 min. Current density (ma cm 2 ) min 3 min 4 min 6 min Voltage (V) Figure 5 Performance of DSSCs based on TiO 2 nanotube-films prepared with different EPD time. Table 2 Thickness of TiO2 nanotube-films prepared with different EPD time, and performance of DSSCs fabricated by the as prepared TiO2 nanotube-films EPD time (min) Thickness of TiO 2 thin film (μm) J sc (ma cm 2 ) V oc (V) FF η (%) October 215 Vol.58 No.1

5 SCIENCE CHINA Materials ARTICLES Received 27 July 215; accepted 1 September 215; published online 18 September Mor GK, Varghese OK, Paulose M, et al. A review on highly ordered, vertically oriented TiO 2 nanotube arrays: fabrication, material properties, and solar energy applications. Sol Energy Mater Sol Cells, 26, 9: Liu R, Qiang LS, Yang WD, et al. Enhanced conversion efficiency of dye-sensitized solar cells using Sm 2 O 3 modified TiO 2 nanotubes. J Power Sources, 213, 223: Gao XF, Chen JH, Yuan C. Enhancing the performance of free-standing TiO 2 nanotube arrays based dye-sensitized solar cells via ultraprecise control of the nanotube wall thickness. J Power Sources, 213, 24: Wang KY, Liu GH, Hoivik N, et al. Electrochemical engineering of hollow nanoarchitectures: pulse/step anodization (Si, Al, Ti) and their applications. Chem Soc Rev, 214, 43: Panda SK, Yoon Y, Jung HS, et al. Nanoscale size effect of titania (anatase) nanotubes with uniform wall thickness as high performance anode for lithium-ion secondary battery. J Power Sources, 212, 24: Zhang Y, Xin Q, Cong Y, et al. Application of TiO 2 nanotubes with pulsed plasma for phenol degradation. Chem Eng, 213, : Lin LY, Ye MH, Tsai KW, et al. Highly ordered TiO 2 nanotube stamps on Ti foils: synthesis and application for all flexible dye-sensitized solar cells. Electrochem Commun, 213, 37: Chen S, Ostrom C, Chen A. Functionalization of TiO 2 nanotubes with palladium nanoparticles for hydrogen sorption and storage. Int J Hydrog Energy, 213, 38: Balakrishnan M, Narayanan R. Synthesis of anodic titania nanotubes in Na 2 SO 4 /NaF electrolyte: a comparison between anodization time and specimens with biomaterial based approaches. Thin Solid Films, 213, 54: Varghese OK, PauloseM, Grimes CA. Long vertically aligned titania nanotubes on transparent conducting oxide for highly efficient solar cells. Nat Nanotechnol, 29, 4: Zhong P, Liao YL, Que WX, et al. Enhanced electron collection in photoanode based on ultrafine TiO 2 nanotubes by a rapid anodization process. J Solid State Electrochem, 214, 18: Xu C, Shin PH, Cao L, et al. Ordered TiO 2 nanotube arrays on transparent conductive oxide for dye-sensitized solar cells. Chem Mater, 21, 22: Roy P, Albu SP, Schmuki P. TiO 2 nanotubes in dye-sensitized solar cells: higher efficiencies by well-defined tube tops. Electrochem Commun, 21, 12: Li LL, Tsai CY, Wu HP, et al. Fabrication of long TiO 2 nanotube arrays in a short time using a hybrid anodic method for highly efficient dye-sensitized solar cells. J Mater Chem, 21, 2: Lei BX, Liao JY, Zhang R, et al. Ordered crystalline TiO 2 nanotube arrays on transparent FTO glass for efficient dye-sensitized solar cells. J Phys Chem C, 21, 114: Zhuge F, Qiu J, Li X, et al. Toward hierarchical TiO 2 nanotube arrays for efficient dye-sensitized solar cells. Adv Mater, 211, 23: Pang Q, Leng L, Zhao L, et al. Dye sensitized solar cells using freestanding TiO 2 nanotube arrays on FTO substrate as photoanode. Mater Chem Phys, 211, 125: So S, Lee K, Schmuki P. High-aspect-ratio dye-sensitized solar cells based on robust, fast-growing TiO 2 nanotubes. Chem Eur J, 213, 19: Mir N, Lee K, Paramasivam I, et al. Optimizing TiO 2 nanotube top geometry for use in dye-sensitized solar cells. Chem Eur J, 212, 18: Liu N, Albu SP, Lee K, et al. Water annealing and other low temperature treatments of anodic TiO 2 nanotubes: a comparison of properties and efficiencies in dye sensitized solar cells and for water splitting. Electrochim Acta, 212, 82: Li KL, Xie ZB, Adams S. A reliable TiO 2 nanotube membrane transfer method and its application in photovoltaic devices. Electrochim Acta, 212, 62: Fan K, Chen J, Yang F, et al. Self-organized film of ultra-fine TiO 2 nanotubes and its application to dye-sensitized solar cells on a flexible Ti-foil substrate. J Mater Chem, 212, 22: Mirabolghasemi H, Liu N, Lee K, et al. Formation of single walled TiO 2 nanotubes with significantly enhanced electronic properties for higher efficiency dye-sensitized solar cells. Chem Commun, 213, 49: Lee K, Schmuki P. Bottom sealing and photoelectrochemical properties of different types of anodic TiO 2 nanotubes. Electrochim Acta, 213, 1: Hahn R, Stergiooulus T, Macak JM, et al. Efficient solar energy conversion using TiO 2 nanotubes produced by rapid breakdown anodization-a comparison. Phys Status Solidi-Rapid Res Lett, 27, 1: Zhu K, Neale NR, Miedaner A, et al. Enhanced charge-collection efficiencies and light scattering in dye-sensitized solar cells using oriented TiO 2 nanotubes arrays. Nano Lett, 27, 7: Lee KS, Kwon J, Im JH, et al. Size-tunable, fast, and facile synthesis of titanium oxide nanotube powders for dye-sensitized solar cells. ACS Appl Mater Interfaces, 212, 4: Zhang Q, Gao L, Sun J, et al. Preparation of long TiO 2 nanotubes from ultrafine rutile nanocrystals. Chem Lett, 22, 2: Caruso RA, Schattka JH, Greiner A. Titanium dioxide tubes from sol gel coating of electrospun polymer fibers. Adv Mater, 21, 13: Ma DL, Schadler LS, Siegel RW, et al. Preparation and structure investigation of nanoparticle-assembled titanium dioxide microtubes. Appl Phys Lett, 23, 83: Kasuga T, Hiramatsu M, Hoson A, et al. Titania nanotubes prepared by chemical processing. Adv Mater, 1999, 11: Kim G, Seo H, Godble VP, et al. Electrophoretic deposition of titanate nanotubes from commercial titania nanoparticles: application to dye-sensitized solar cells. Electrochem Comm, 26, 8: Chiu WH, Lee KM, Hsieh WF. High efficiency flexible dye-sensitized solar cells by multiple electrophoretic depositions. J Power Sources, 211, 196: Chou JC, Lin SC, Liao YH, et al. The influence of electrophoretic deposition for fabricating dye-sensitized solar cell. J Nanomater, 214, Liao Y, Que WX, Zhang J, et al. A facile method for rapid preparation of individual titania nanotube powders by a two-step process. Mater Res Bull, 211, 46: Ito S, Murakami TN, Comte P, et al. Fabrication of thin film dye sensitized solar cells with solar to electric power conversion efficiency over 1%. Thin Solid Films, 28, 516: Gardeshzadeh AR, Raissi B, Marzbanrad E. Electrophoretic deposition of SnO 2 nanoparticles using low frequency AC electric fields. Mater Lett, 28, 62: Besra L, Liu M. A review on fundamentals and applications of electrophoretic deposition (EPD). Prog Mater Sci, 27, 52: 1 61 Acknowledgments This work was supported by the Dean Foundation of Optoelectronic Engineering School of Xi an Technological University. Author contributions Zhang J, Que W and Liu W jointly conceived the idea of this study. Zhang J assisted in designing the experiments and October 215 Vol.58 No.1 789

6 ARTICLES SCIENCE CHINA Materials analyzing the experimental data. Li S prepared the TiO 2 nanotube-films. Yang P assembled the DSSCs and performed the characterization experiments. Zhang J prepared the manuscript and Liu W revised it. Que W and Liu W co-supervised and coordinated this work. All authors discussed the results and commented on the manuscript. Conflict of interests The authors declare that they have no conflict of interest. Jin Zhang received his PhD degree majored in electronic science and technology from Xi an Jiaotong University, China in 211. He worked at Sunharmonics Co., LTD from 212 to 214 as a research team leader, and then became a lecturer in the School of Optoelectronic Engineering, Xi an Technological University. His current research interests are in fields of new optoelectronic semiconductor materials and their applications in photocatalysis and solar cells. Wenxiu Que received his PhD degree from Xi an Jiaotong University, China in He has been a professor in the School of Electronic and Information Engineering, Xi an Jiaotong University since 26. His current research interests include organic-inorganic hybrid materials based on organically modified silanes for photonic applications, TiO2 nanotubes/nanowire arrays and ZnO nanowire arrays sensitized with semiconductor quantum dots, etc. Weiguo Liu is a professor in the School of Optoelectronic Engineering, Xi an Technological University. In 1995, he received his PhD degree from Xi an Jiaotong University in microelectronics and solid state electronics. His research interests are in the fields of functional materials and devices for optical and photonic applications. 中文摘要本文采用电泳沉积法在透明导电玻璃 (FTO) 上制备了半透明的二氧化钛纳米管薄膜并对其进行了表征. 同时, 进一步采用二氧化钛纳米管薄膜作为光阳极组装了染料敏化太阳能电池, 研究了电泳电压及薄膜厚度对电池性能的影响. 电池的最终转换效率可达 7.1%. 这说明电泳法沉积二氧化钛纳米管薄膜在高产能和高效率光伏器件方面具有巨大的应用潜力. 79 October 215 Vol.58 No.1