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Ordered crystalline TiO2 nanohexagon arrays for improving conversion efficiency of dye-sensitized solar cells
Journal of Alloys and Compounds 646 (2015) 106e111
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Ordered crystalline TiO2 nanohexagon arrays for improving conversion efficiency of dye-sensitized solar cells Hafiz Muhammad Asif Javed a, Wenxiu Que a, *, Xingtian Yin a, Yonglei Xing a, Xiaobin Liu a, Ali Asghar a, Jinyou Shao a, Ling Bing Kong b, ** a Electronic Materials Research Laboratory, International Center for Dielectric Research, Key Laboratory of the Ministry of Education, State Key Laboratory for Manufacturing Systems Engineering, Xi'an Jiaotong University, Xi'an, 710049, PR China b School of Materials Science and Engineering, Nanyang Technological University, Nanyang Avenue, Singapore, 639798, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 March 2015 Received in revised form 12 June 2015 Accepted 15 June 2015 Available online 17 June 2015
Anatase TiO2 nanohexagon arrays were grown by using an anodization process of Ti foil in fluoride containing electrolytes. Photoanode based on the as-grown anatase TiO2 nanohexagon arrays for DSSCs showed a power photoconversion efficiency of 4.01% and incident photon-to-current conversion efficiency of 68%, which are significantly higher than those of the device based on anatase TiO2 nanotube arrays. This improvement in power conversion efficiency should be attributed to the fact that the nanotubes with hexagonal structure have higher surface area to allow the uploading of more dye molecules for light harvesting. Also, the spacing introduced inside the hexagon might allow the dye molecules to cover the interior of the walls. In addition, it is believed that the photoconversion efficiency can be further increased by optimizing the hexagonal structure through the electrochemical conditions. © 2015 Elsevier B.V. All rights reserved.
Keywords: Anatase TiO2 nanohexagon arrays Anodization Dye molecules Dye-sensitized solar cells
1. Introduction Since the discovery of TiO2 based dye-sensitized solar cells (DSSCs) in 1991 [1], extensive research has been dedicated to enhance their power conversion efficiency. In 2001, Grimes et al. found that, as compared with the conventional TiO2 nanoparticles (NPs), self-organized TiO2 nanotubes (NTs) prepared by anodic oxidation provided higher surface area for dye uploading [2]. Frank et al. proved that the light harvesting efficiency of the DSSCs based on nanotubes was higher than that of those based on nanoparticles due to the strong internal light scattering effects [3]. Various factors have been shown to have an influence on the power conversion efficiency of the DSSCs, including (i) the surface area of the TiO2 nanostructured film which should be maximized for dye adsorption and thus increasing light harvesting efficiency, (ii) the straight path for efficient injection of the electrons from excited dye to the conduction band of TiO2 and (iii) the transport of the electrons through the nanostructures with minimized recombination rate
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (W. Que), [email protected] (L.B. Kong). http://dx.doi.org/10.1016/j.jallcom.2015.06.119 0925-8388/© 2015 Elsevier B.V. All rights reserved.
[4]. The advantage of ordered TiO2 NTs film is that the nanoscale morphology can be well controlled through modifying the electrochemical conditions [5,6]. Several attempts to improve the nanotube geometry for higher power conversion efficiency have been reported [7e9]. Here, we report a new nanostructure of TiO2 nanohexagon arrays for the photoanodes of DSSCs, which can substantially enhance the power conversion efficiency of the asfabricated devices. Furthermore, the mechanism of such an improvement has been discussed, that is, TiO2 nanohexagon arrays based photoanode for DSSCs provides a higher surface area for uploading dye molecules as compared to that based on the conventional nanotube arrays. 2. Experimental TiO2 nanohexagon arrays were prepared by using an electrochemical anodization of Ti foil in an electrolyte (pH ¼ 5.5) consisting of 0.25 wt% of NH4F and 0.8 Vol% of DI water in ethylene glycol. The counter electrode was also Ti foil. The double anodization was performed at a constant anodization voltage of 50 V at 3 C (ice bath) for 8 h. After anodization, the Ti foil with TiO2 nanohexagon film was merged into DI water until the film was detached from the Ti foil. The detached TiO2 nanohexagon film was then transferred onto FTO glass by using a drop of TiO2 sol containing
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107
Fig. 1. SEM images of the TiO2 nanohexagon arrays with different anodization times: (a) 8 h, (b) 4 h, (c) 15 h, (d) cross sectional view, (e) bottom view.
Ti(OBu)4 and polyethylene glycol for front side illumination mode of DSSCs. The anodized TiO2 nanohexagon film was amorphous and can be crystallized to anatase structure upon annealing in air at 500 C for 2 h at 5 C/min. After annealing, the samples were soaked in 0.2 M TiCL4 for 30 min at 70 C to deposit TiO2 nanoparticles on the surface of the nanohexagonal structure, so as to enhance the power conversion efficiency of the as-fabricated DSSCs. After this procedure, the samples were annealed at 500 C for 30 min again. Next, the samples were immersed into the solution of 0.5 mM N719 in acetonitrile and tert-butyl alcohol for 24 h at dark room [10]. After dye-sensitization, the samples were rinsed with acetonitrile to remove nonchemisorbed dye molecules. Platinized counter electrodes for DSSCs were fabricated on FTO glass by using DC sputtering. Thus, the dye sensitized TiO2 nanohexagon films were sandwiched with 60 mm thick spacer with Pt films by heating and pressing for 1 min. The electrolyte, which includes 0.03 M iodine, 0.6 M 1-Methyl-3-Propyllimidazolium iodide, 0.1 M guanidine thiocyanate, 0.5 M 4-tert-butylpyridine, as well as
acetonitrile and valeronitrile with the volume ratio of 17:3 [10], was introduced into the space between the sandwiched full cells. Morphological and crystal structural properties of the TiO2 nanohexagon films were characterized by using scanning electron microscopy (SEM, Quanta FEG 250) and power X-ray diffractometry (XRD), respectively. The amount of dye adsorbed (Aad) on TiO2 nanohexagon arrays was measured by a UVeVis spectrophotometer, that is, the dye sensitized electrode was separately immersed into a 0.1 M NaOH solution containing mixed solvent (water: ethanol ¼ 1:1). After adsorption of N719 dye, the absorbance of the resulting solution was measured by UVeVis spectrophotometer and thus the amount of dye adsorbed Aad was calculated by the molar extinction coefficient of 1.41 104 dm3 mol1 cm1 at 515 nm [11]. Current-voltage characteristics of the as-fabricated DSSCs were measured by using a Keithley 2400 source meter under simulated AM 1.5G illumination (100 mW cm2) provided by a solar simulator (Oriel Newport) to determine the open-circuit voltage (Voc), short-
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Fig. 2. SEM image of the TiO2 nanohexagon arrays with TiCl4 treatment.
circuit current density (Jsc), fill factor (FF), and conversion efficiency (h). Incident photon-to-current conversion efficiency (IPCE) spectra of the devices were measured from Crowntech system (USA), which were carried out with a set up consisting of a 150 W Xe lamp as an irradiation source. 3. Results and discussion The anodization process has two simultaneous reactions: (i) chemical dissolution of titanium under a potential induced complexation with fluoride ions, which is called anodization speed (velectro), and (ii) reaction based on dissolution of the ejected Ti(OH)4, which is called chemical etching speed (vdis) [12]. When vdis is higher than velectro, the free standing TiO2 nanotubes are developed, while when velectro is higher than vdis, the porous TiO2 nanotubes are fabricated and the length of the nanotubes increases with the anodization time. Also, a low temperature and low acidic concentration of the electrolyte and high voltage lead to the porous nanotubes [12]. Thus, by controlling the anodization parameters, it is possible to vary the packing density of the nanotubes or tube-to-
tube connectivity. Actually, maximum nanotube packing density can be achieved by using an ethylene glycol and the nanotube coordination number is usually six, that is to say, each nanotube is surrounded by six tubes [13,14]. Thus, when the anodization parameters are changed, free standing nanotubes are converted into porous nanotubes. Similarly, by optimizing the anodization parameters, porous nanotubes are transferred into nanohexagon due to the coordination number of six. Fig. 1(a) shows SEM image of the TiO2 nanohexagon arrays with anodization time of 8 h. It can be seen that every nanohexagon has three to four nanotubes inside, which are due to the etching effect. The anodization parameters towards the TiO2 nanohexagon morphology include as follows: (i) anodization time, (ii) anodization voltage, (iii) anodization temperature, (iv) concentration of NH4F and DI water, (v) pH value of electrolyte, (vi) cathode material, and (vii) distance between anode and cathode. SEM images of the TiO2 nanohexagon arrays obtained after anodization for 4 and 15 h are shown in Fig. 1b and c, respectively. It can be seen that after 4 h anodization, the porous tubes are converted to the nanohexagon, whereas after 15 h anodization, the nanohexagones are broken and fallen. These results indicate that chemical dissolution and electrochemical etching are critical factors for the growth of the nanotubes. The rate of etching process varies with the temperature of the electrolyte [15], and the pH value of the electrolyte also affects the electrochemical etching and the chemical dissolution. As the pH value increases, which is from strong acidity to weak acidity, the length of the nanotube also increases. It is also noted that at a given pH value, the length and the pore size of the nanotubes increase with an increase of the applied potential [16,17]. In the present study, the optimized pH value for the formation of the nanohexagon is 5.5. As reported in Ref. [18] that the length and diameter of the nanotubes increased with increasing anodization time. Here, the length and diameter of the nanohexagon is also observed to increase with increasing anodization time as shown in Fig. 1d and e. According to Chin et al., TiO2 nanotubes with thin wall could be fabricated at room temperature [19], hence, our nanohexagon arrays should have thinner walls at a lower temperature. By controlling the electrolyte chemistry and the anodization conditions, the morphology and dimension (length and pore diameter) of the nanotubes can be controlled [20,21]. An optimized concentration of the electrolyte for the nanohexagon under present conditions is 0.25 wt% of NH4F and 0.8 vol % of DI water in ethylene glycol. In addition, a double anodization can obtain a cleaner surface and
Fig. 3. EDAX analyses of the TiO2 nanohexagon arrays with TiCl4 treatment.
H.M.A. Javed et al. / Journal of Alloys and Compounds 646 (2015) 106e111
more uniform nanotubes as compared with single anodization [20]. In nonaqueous electrolyte, the dimensions of the TiO2 nanotube are also affected by temperature [22e24]. Similarly, the formation of the nanohexagon should be also affected by temperature, since the present electrolyte is nonaqueous. The optimized anodization voltage is 50 V for required wall thickness, diameter and length of the nanohexagon. Fig. 2 shows SEM image of the nanohexagon after a treatment with 0.2 M TiCl4. It can be seen that TiO2 nanoparticles are deposited on the surface of the TiO2 nanohexagon arrays, whose EDAX result is shown in Fig. 3. Fig. 4 shows XRD pattern of the TiO2 nanohexagon arrays, indicating that the as-anodized nanohexagon sample is amorphous. After annealing at 500 C for 2 h, the diffraction peaks can be clearly indexed to the anatase TiO2 (PCPDF card 211272), which is important for photovoltaic performance of the DSSCs. Fig. 5 shows IeV characteristics of the as-fabricated DSSCs based on the TiO2 nanotube arrays and the TiO2 nanohexagon arrays, respectively. Short circuit current densities, open circuit voltages, fill factors and efficiencies are also presented in Table 1. It can be seen that the DSSC based on the TiO2 nanohexagon arrays with TiCl4 treatment has the highest power conversion efficiency and its value is 4.01%. Since the open circuit voltages in all measured devices are comparatively close (0.70e0.75), the power conversion efficiency is mainly affected by the short circuit current density. It is commonly regarded that Jsc value is dependent on the absorption of light, which is directly related to the amount of the adsorbed dye molecules [25]. The amounts of the dye adsorption Ada were estimated by the absorbance spectra of the dye adsorption solutions as shown in Fig. 6. The origin of the absorption peaks at ~375 nm and 510 nm is ascribed to the N719 dye as the measured solution was only dye absorption solution, also all peaks have a good agreement with the previous data as reported in Refs. [26,27]. Results show that the Ada increases from 96 nmol/cm2 to 136 nmol/cm2 as seen in Table 1, the corresponding power conversion efficiency of the asfabricated dye-sensitized solar cell is also enhanced from 2.27% to
109
3.28%. Based on the amount of the dye adsorption of TiO2 nanohexagon arrays, by taking one dye molecule occupy an area of 1 nm2,1 for each 1 cm2 of geometric surface area, the calculated inner surface area of the dye adsorption, Sda, is 819 cm2, that is, 1 cm2 TiO2 nanohexagon film of 10 mm thickness has an inner surface area of 527 cm2. This value is higher than those for TiO2 nanotubes [28,29]. It is noted that the power conversion efficiency (PCE) of the as-obtained dye-sensitized solar cell by using pure TiO2 nanohexagon arrays as photoanode is comparatively low. The asobtained highest PCE is 3.28% when using pure TiO2 nanohexagon arrays as photoanode. The DSSC schematic structure based on the TiO2 nanohexagon arrays is shown in Fig. 7. However, the relatively high PCEs, which have been achieved in recent papers, are generally combined with a TiCl4 treatment [10,25,30,31]. Actually, the TiCl4 treatment has been widely used as a surface modification by introducing 3e10 nm TiO2 nanoparticles onto the
Fig. 5. IeV characteristics of the DSSCs based on the TiO2 nanotube arrays and the TiO2 nanohexagon arrays.
Fig. 4. XRD patterns of the TiO2 nanohexagon arrays.
Fig. 6. Absorbance spectra of the dye adsorption solutions.
Table 1 Photovoltaic parameters, amount of dye adsorption (Ada) and evaluated inner surface area (Sad) of TNT and TNH based photoanodes for dye-sensitized solar cells. Photoanodes
Jsc (mA/cm2)
Voc (V)
FF (%)
Efficiency (%)
Ada (nmol/cm2)
Sda (cm2)
TNT TNH TNH/TNP
4.37 7.18 9.29
0.73 0.75 0.70
70 61 62
2.27 3.28 4.01
96 136 221
578 819 1331
110
H.M.A. Javed et al. / Journal of Alloys and Compounds 646 (2015) 106e111
and the surface area of the photoanode [33]. The IPCE spectrum of the DSSC based on the TiO2 nanohexagon arrays shows a maximum efficiency of 65.15%, which is significantly higher than that of the one based on TiO2 nanotube arrays. The IPCE value of 68% was also achieved for the DSSC based on the TiO2 nanohexagon arrays deposited with TiO2 nanoparticles, which was brought by TiCl4 treatment. These results indicate that the photoanode with TiCl4 solution treatment can increase the surface area of the TiO2 nanohexagon arrays, and thus increases the dye-loading capacity of the photoanode. 4. Conclusions
Fig. 7. Schematic structure of the DSSC based on the TiO2 nanohexagon arrays.
nanotube to increase the surface area, which improves the dyeloading capacity of the photoanode [32]. Furthermore, it has been also observed that this treatment can eliminate the cracks on the nanotube surface and reduce the charge recombination [10,25]. Park et al. reported a 30% increase of the photocurrent density when using TiCl4-treated TiO2 nanotubes as photoanode [10]. Our result indicates that for the DSSC based on the photoanode of the TiO2 nanohexagon with TiCl4 treatment, its Jsc increases from 7.18 mA/cm2 to 9.29 mA/cm2 and power conversion efficiency enhances from 3.28 % to 4.01 %. The amount of the dye adsorption Ada is determined to be 221 nmol/cm2, the evaluated Sda is about 1331 cm2, the inner surface area of 857 cm2 for 10 mm thickness, which is higher than that of TiO2 nanoparticles photoanode (780 cm2 for film of 10 mm thickness) [1]. IPCE measurements of the devices were performed to analyze the details of the enhanced performance of the devices based on the TiO2 nanotube arrays and the TiO2 nanohexagon arrays as shown in Fig. 8. The IPCE or quantum efficiency is determined by quantum yield of the electron injection, light absorption efficiency of the dye molecules, and efficiency of collecting the injected electrons at the FTO glass, which is directly affected by the structure
Fig. 8. IPCE curves of the DSSCs based on the TiO2 nanotube arrays and the TiO2 nanohexagon arrays sensitized by N719.
The DSSCs based on the TiO2 nanohexagon arrays show higher power efficiency than those based on the nanotubes arrays. The improvement in efficiency can be attributed to a higher surface area provided by the hexagon structure, which is helpful for the uploading of the dye molecules to achieve a maximum light harvesting. Also, the spacing introduced inside the hexagon allows more dye molecules to cover the interior of the nanohexagon walls. At present experimental conditions, the DSSC based on the TiO2 nanohexagon arrays as photoanode achieves a power conversion efficiency of 4.01% and IPCE of 68%. It can be also believed that the nanohexagon structure presented here can be further optimized by varying the electrochemical conditions to achieve even higher conversion efficiency. Acknowledgments This work was supported by the Research Fund for the Doctoral Program of Higher Education of China under Grant 20120201130004, the Science and Technology Developing Project of Shaanxi Province (2015 KW-001), partially by the National Natural Science Foundation of China Major Research Plan on Nanomanufacturing under Grant No. 91323303, the National Natural Science Foundation of China under Grant No. 61078058, and the 111 Project of China (B14040). The SEM work was done at International Centre for Dielectric Research, Xi'an Jiaotong University, Xi'an, China. The authors also thankful to Ms. Dai for her help in using SEM. References [1] B. O'Regan, M. Gr€ atzel, Nature 353 (1991) 737. [2] D. Gong, C.A. Grimes, O.K. Varghese, W.C. Hu, R.S. Singh, Z. Chen, E.C. Dickey, J. Mater. Res. 16 (2001) 3331. [3] K. Zhu, N.R. Neale, A. Miedaner, A.J. Frank, Nano Lett. 7 (2007) 69. [4] J. Halme, G. Boschloo, A. Hagfeldt, P. Lund, J. Phys. Chem. C 112 (2008) 5623. [5] J.M. Macak, P. Schmuki, Electrochim. Acta 52 (2006) 1258. [6] J.M. Macak, H. Tsuchiya, A. Ghicov, K. Yasuda, R. Hahn, S. Bauer, P. Schmuki, Curr. Opin. Solid State Mater. Sci. 11 (2007) 3. [7] D. Kim, A. Ghicov, S.P. Albu, P. Schmuki, J. Am. Chem. Soc. 130 (2008) 16454. [8] X. Luan, D. Guan, Y. Wang, J. Phys. Chem. C 116 (2012) 14257. [9] S.P. Albu, D. Kim, P. Schmuki, Angew. Chem. Int. Ed. 47 (2008) 1916. [10] J.H. Park, T.W. Lee, M.G. Kang, Chem. Commun. 25 (2008) 2867. [11] K. Fan, W. Zhang, T.Y. Peng, J.N. Chen, F. Yang, J. Phys. Chem. C 115 (2011) 17213. [12] D. Wang, Y. Liu, B. Yu, F. Zhou, W. Liu, Chem. Mater. 21 (2009) 1198. [13] M. Paulose, H.E. Prakasam, O.K. Varghese, L. Peng, K.C. Popat, G.K. Mor, T.A. Desai, C.A. Grimes, J. Phys. Chem. C 111 (2007) 14992. [14] J. Wang, Z. Lin, Chem. Mater. 20 (2008) 1257. [15] G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Nano Lett. 5 (2005) 191. [16] S. Sreekantan, Z. Lockman, R. Hazan, M. Tasbihi, L.K. Tang, A.R. Mohamed, J. Alloy. Compd. 485 (2009) 478. [17] C.A. Grimes, G.K. Mor, TiO2 Nanotube Arrays Synthesis, Properties, and Applications, 2009. [18] H. Xu, X. Liao, G. Yin, X. Ye, Y. Liu, Mater. Sci. 4 (2014) 22. [19] C.W. Lai, S. Sreekantan, Int. J. Photoenergy (2012), http://dx.doi.org/10.1155/ 2012/356943. [20] L.K. Tsui, T. Homma, G. Zangari, J. Phys. Chem. C 117 (2013) 6979.
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H.M.A. Javed, W.X. Que, Z.L. He, J. Nanosci. Nanotechno 14 (2014) 1085. J. Wang, Z. Lin, J. Phys. Chem. C 113 (2009) 4026. N.K. Allam, C.A. Grimes, Sol. Energ. Mat. Sol. C 92 (2008) 1468. T.T. Isimjan, S. Rohani, A.K. Ray, Int. J. Hydrogen. Energy. 37 (2012) 103. B.X. Lei, J.Y. Liao, R. Zhang, J. Wang, C.Y. Su, D.B. Kuang, J. Phys. Chem. C 114 (2010) 15228. [26] D. Joly, L. Pelleja, S. Narbey, F. Oswald, J. Chiron, J.N. Clifford, E. Palomares, R. Demadrille, Sci. Rep. 4 (2014) 4033. [27] L. Zedler, F. Theil, A. Csaki, W. Fritzsche, S. Rau, M. Schmitt, J. Popp, B. Dietzek,
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RSC Adv. 2 (2012) 4463. [28] Z.G. Bao, J. Tao, L. Wang, X.T. He, Electro. Compon. Mater. 29 (2010) 64. [29] J. Zhang, S. Li, H. Ding, Q. Li, B. Wang, X. Wang, H. Wang, J. Power Sources 247 (2014) 807. [30] C.J. Lin, W.Y. Yu, S.H. Chien, J. Mater. Chem. 20 (2010) 1073. [31] Q. Zheng, H. Kang, J. Yun, J. Lee, J.H. Park, S. Baik, ACS Nano 5 (2011) 5088. [32] X. Pan, C.H. Chen, K. Zhu, Z.Y. Fan, Nanotechnology 22 (2011) 235402. [33] Y. Hao, J. Pei, Y. Wei, Y. Cao, S. Jiao, F. Zhu, J. Li, D. Xu, J. Phys. Chem. C 114 (2010) 8622.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Ordered crystalline TiO2 nanohexagon arrays for improving conversion efficiency of dye-sensitized solar cells Hafiz Muhammad Asif Javed a, Wenxiu Que a, *, Xingtian Yin a, Yonglei Xing a, Xiaobin Liu a, Ali Asghar a, Jinyou Shao a, Ling Bing Kong b, ** a Electronic Materials Research Laboratory, International Center for Dielectric Research, Key Laboratory of the Ministry of Education, State Key Laboratory for Manufacturing Systems Engineering, Xi'an Jiaotong University, Xi'an, 710049, PR China b School of Materials Science and Engineering, Nanyang Technological University, Nanyang Avenue, Singapore, 639798, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 March 2015 Received in revised form 12 June 2015 Accepted 15 June 2015 Available online 17 June 2015
Anatase TiO2 nanohexagon arrays were grown by using an anodization process of Ti foil in fluoride containing electrolytes. Photoanode based on the as-grown anatase TiO2 nanohexagon arrays for DSSCs showed a power photoconversion efficiency of 4.01% and incident photon-to-current conversion efficiency of 68%, which are significantly higher than those of the device based on anatase TiO2 nanotube arrays. This improvement in power conversion efficiency should be attributed to the fact that the nanotubes with hexagonal structure have higher surface area to allow the uploading of more dye molecules for light harvesting. Also, the spacing introduced inside the hexagon might allow the dye molecules to cover the interior of the walls. In addition, it is believed that the photoconversion efficiency can be further increased by optimizing the hexagonal structure through the electrochemical conditions. © 2015 Elsevier B.V. All rights reserved.
Keywords: Anatase TiO2 nanohexagon arrays Anodization Dye molecules Dye-sensitized solar cells
1. Introduction Since the discovery of TiO2 based dye-sensitized solar cells (DSSCs) in 1991 [1], extensive research has been dedicated to enhance their power conversion efficiency. In 2001, Grimes et al. found that, as compared with the conventional TiO2 nanoparticles (NPs), self-organized TiO2 nanotubes (NTs) prepared by anodic oxidation provided higher surface area for dye uploading [2]. Frank et al. proved that the light harvesting efficiency of the DSSCs based on nanotubes was higher than that of those based on nanoparticles due to the strong internal light scattering effects [3]. Various factors have been shown to have an influence on the power conversion efficiency of the DSSCs, including (i) the surface area of the TiO2 nanostructured film which should be maximized for dye adsorption and thus increasing light harvesting efficiency, (ii) the straight path for efficient injection of the electrons from excited dye to the conduction band of TiO2 and (iii) the transport of the electrons through the nanostructures with minimized recombination rate
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (W. Que), [email protected] (L.B. Kong). http://dx.doi.org/10.1016/j.jallcom.2015.06.119 0925-8388/© 2015 Elsevier B.V. All rights reserved.
[4]. The advantage of ordered TiO2 NTs film is that the nanoscale morphology can be well controlled through modifying the electrochemical conditions [5,6]. Several attempts to improve the nanotube geometry for higher power conversion efficiency have been reported [7e9]. Here, we report a new nanostructure of TiO2 nanohexagon arrays for the photoanodes of DSSCs, which can substantially enhance the power conversion efficiency of the asfabricated devices. Furthermore, the mechanism of such an improvement has been discussed, that is, TiO2 nanohexagon arrays based photoanode for DSSCs provides a higher surface area for uploading dye molecules as compared to that based on the conventional nanotube arrays. 2. Experimental TiO2 nanohexagon arrays were prepared by using an electrochemical anodization of Ti foil in an electrolyte (pH ¼ 5.5) consisting of 0.25 wt% of NH4F and 0.8 Vol% of DI water in ethylene glycol. The counter electrode was also Ti foil. The double anodization was performed at a constant anodization voltage of 50 V at 3 C (ice bath) for 8 h. After anodization, the Ti foil with TiO2 nanohexagon film was merged into DI water until the film was detached from the Ti foil. The detached TiO2 nanohexagon film was then transferred onto FTO glass by using a drop of TiO2 sol containing
H.M.A. Javed et al. / Journal of Alloys and Compounds 646 (2015) 106e111
107
Fig. 1. SEM images of the TiO2 nanohexagon arrays with different anodization times: (a) 8 h, (b) 4 h, (c) 15 h, (d) cross sectional view, (e) bottom view.
Ti(OBu)4 and polyethylene glycol for front side illumination mode of DSSCs. The anodized TiO2 nanohexagon film was amorphous and can be crystallized to anatase structure upon annealing in air at 500 C for 2 h at 5 C/min. After annealing, the samples were soaked in 0.2 M TiCL4 for 30 min at 70 C to deposit TiO2 nanoparticles on the surface of the nanohexagonal structure, so as to enhance the power conversion efficiency of the as-fabricated DSSCs. After this procedure, the samples were annealed at 500 C for 30 min again. Next, the samples were immersed into the solution of 0.5 mM N719 in acetonitrile and tert-butyl alcohol for 24 h at dark room [10]. After dye-sensitization, the samples were rinsed with acetonitrile to remove nonchemisorbed dye molecules. Platinized counter electrodes for DSSCs were fabricated on FTO glass by using DC sputtering. Thus, the dye sensitized TiO2 nanohexagon films were sandwiched with 60 mm thick spacer with Pt films by heating and pressing for 1 min. The electrolyte, which includes 0.03 M iodine, 0.6 M 1-Methyl-3-Propyllimidazolium iodide, 0.1 M guanidine thiocyanate, 0.5 M 4-tert-butylpyridine, as well as
acetonitrile and valeronitrile with the volume ratio of 17:3 [10], was introduced into the space between the sandwiched full cells. Morphological and crystal structural properties of the TiO2 nanohexagon films were characterized by using scanning electron microscopy (SEM, Quanta FEG 250) and power X-ray diffractometry (XRD), respectively. The amount of dye adsorbed (Aad) on TiO2 nanohexagon arrays was measured by a UVeVis spectrophotometer, that is, the dye sensitized electrode was separately immersed into a 0.1 M NaOH solution containing mixed solvent (water: ethanol ¼ 1:1). After adsorption of N719 dye, the absorbance of the resulting solution was measured by UVeVis spectrophotometer and thus the amount of dye adsorbed Aad was calculated by the molar extinction coefficient of 1.41 104 dm3 mol1 cm1 at 515 nm [11]. Current-voltage characteristics of the as-fabricated DSSCs were measured by using a Keithley 2400 source meter under simulated AM 1.5G illumination (100 mW cm2) provided by a solar simulator (Oriel Newport) to determine the open-circuit voltage (Voc), short-
108
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Fig. 2. SEM image of the TiO2 nanohexagon arrays with TiCl4 treatment.
circuit current density (Jsc), fill factor (FF), and conversion efficiency (h). Incident photon-to-current conversion efficiency (IPCE) spectra of the devices were measured from Crowntech system (USA), which were carried out with a set up consisting of a 150 W Xe lamp as an irradiation source. 3. Results and discussion The anodization process has two simultaneous reactions: (i) chemical dissolution of titanium under a potential induced complexation with fluoride ions, which is called anodization speed (velectro), and (ii) reaction based on dissolution of the ejected Ti(OH)4, which is called chemical etching speed (vdis) [12]. When vdis is higher than velectro, the free standing TiO2 nanotubes are developed, while when velectro is higher than vdis, the porous TiO2 nanotubes are fabricated and the length of the nanotubes increases with the anodization time. Also, a low temperature and low acidic concentration of the electrolyte and high voltage lead to the porous nanotubes [12]. Thus, by controlling the anodization parameters, it is possible to vary the packing density of the nanotubes or tube-to-
tube connectivity. Actually, maximum nanotube packing density can be achieved by using an ethylene glycol and the nanotube coordination number is usually six, that is to say, each nanotube is surrounded by six tubes [13,14]. Thus, when the anodization parameters are changed, free standing nanotubes are converted into porous nanotubes. Similarly, by optimizing the anodization parameters, porous nanotubes are transferred into nanohexagon due to the coordination number of six. Fig. 1(a) shows SEM image of the TiO2 nanohexagon arrays with anodization time of 8 h. It can be seen that every nanohexagon has three to four nanotubes inside, which are due to the etching effect. The anodization parameters towards the TiO2 nanohexagon morphology include as follows: (i) anodization time, (ii) anodization voltage, (iii) anodization temperature, (iv) concentration of NH4F and DI water, (v) pH value of electrolyte, (vi) cathode material, and (vii) distance between anode and cathode. SEM images of the TiO2 nanohexagon arrays obtained after anodization for 4 and 15 h are shown in Fig. 1b and c, respectively. It can be seen that after 4 h anodization, the porous tubes are converted to the nanohexagon, whereas after 15 h anodization, the nanohexagones are broken and fallen. These results indicate that chemical dissolution and electrochemical etching are critical factors for the growth of the nanotubes. The rate of etching process varies with the temperature of the electrolyte [15], and the pH value of the electrolyte also affects the electrochemical etching and the chemical dissolution. As the pH value increases, which is from strong acidity to weak acidity, the length of the nanotube also increases. It is also noted that at a given pH value, the length and the pore size of the nanotubes increase with an increase of the applied potential [16,17]. In the present study, the optimized pH value for the formation of the nanohexagon is 5.5. As reported in Ref. [18] that the length and diameter of the nanotubes increased with increasing anodization time. Here, the length and diameter of the nanohexagon is also observed to increase with increasing anodization time as shown in Fig. 1d and e. According to Chin et al., TiO2 nanotubes with thin wall could be fabricated at room temperature [19], hence, our nanohexagon arrays should have thinner walls at a lower temperature. By controlling the electrolyte chemistry and the anodization conditions, the morphology and dimension (length and pore diameter) of the nanotubes can be controlled [20,21]. An optimized concentration of the electrolyte for the nanohexagon under present conditions is 0.25 wt% of NH4F and 0.8 vol % of DI water in ethylene glycol. In addition, a double anodization can obtain a cleaner surface and
Fig. 3. EDAX analyses of the TiO2 nanohexagon arrays with TiCl4 treatment.
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more uniform nanotubes as compared with single anodization [20]. In nonaqueous electrolyte, the dimensions of the TiO2 nanotube are also affected by temperature [22e24]. Similarly, the formation of the nanohexagon should be also affected by temperature, since the present electrolyte is nonaqueous. The optimized anodization voltage is 50 V for required wall thickness, diameter and length of the nanohexagon. Fig. 2 shows SEM image of the nanohexagon after a treatment with 0.2 M TiCl4. It can be seen that TiO2 nanoparticles are deposited on the surface of the TiO2 nanohexagon arrays, whose EDAX result is shown in Fig. 3. Fig. 4 shows XRD pattern of the TiO2 nanohexagon arrays, indicating that the as-anodized nanohexagon sample is amorphous. After annealing at 500 C for 2 h, the diffraction peaks can be clearly indexed to the anatase TiO2 (PCPDF card 211272), which is important for photovoltaic performance of the DSSCs. Fig. 5 shows IeV characteristics of the as-fabricated DSSCs based on the TiO2 nanotube arrays and the TiO2 nanohexagon arrays, respectively. Short circuit current densities, open circuit voltages, fill factors and efficiencies are also presented in Table 1. It can be seen that the DSSC based on the TiO2 nanohexagon arrays with TiCl4 treatment has the highest power conversion efficiency and its value is 4.01%. Since the open circuit voltages in all measured devices are comparatively close (0.70e0.75), the power conversion efficiency is mainly affected by the short circuit current density. It is commonly regarded that Jsc value is dependent on the absorption of light, which is directly related to the amount of the adsorbed dye molecules [25]. The amounts of the dye adsorption Ada were estimated by the absorbance spectra of the dye adsorption solutions as shown in Fig. 6. The origin of the absorption peaks at ~375 nm and 510 nm is ascribed to the N719 dye as the measured solution was only dye absorption solution, also all peaks have a good agreement with the previous data as reported in Refs. [26,27]. Results show that the Ada increases from 96 nmol/cm2 to 136 nmol/cm2 as seen in Table 1, the corresponding power conversion efficiency of the asfabricated dye-sensitized solar cell is also enhanced from 2.27% to
109
3.28%. Based on the amount of the dye adsorption of TiO2 nanohexagon arrays, by taking one dye molecule occupy an area of 1 nm2,1 for each 1 cm2 of geometric surface area, the calculated inner surface area of the dye adsorption, Sda, is 819 cm2, that is, 1 cm2 TiO2 nanohexagon film of 10 mm thickness has an inner surface area of 527 cm2. This value is higher than those for TiO2 nanotubes [28,29]. It is noted that the power conversion efficiency (PCE) of the as-obtained dye-sensitized solar cell by using pure TiO2 nanohexagon arrays as photoanode is comparatively low. The asobtained highest PCE is 3.28% when using pure TiO2 nanohexagon arrays as photoanode. The DSSC schematic structure based on the TiO2 nanohexagon arrays is shown in Fig. 7. However, the relatively high PCEs, which have been achieved in recent papers, are generally combined with a TiCl4 treatment [10,25,30,31]. Actually, the TiCl4 treatment has been widely used as a surface modification by introducing 3e10 nm TiO2 nanoparticles onto the
Fig. 5. IeV characteristics of the DSSCs based on the TiO2 nanotube arrays and the TiO2 nanohexagon arrays.
Fig. 4. XRD patterns of the TiO2 nanohexagon arrays.
Fig. 6. Absorbance spectra of the dye adsorption solutions.
Table 1 Photovoltaic parameters, amount of dye adsorption (Ada) and evaluated inner surface area (Sad) of TNT and TNH based photoanodes for dye-sensitized solar cells. Photoanodes
Jsc (mA/cm2)
Voc (V)
FF (%)
Efficiency (%)
Ada (nmol/cm2)
Sda (cm2)
TNT TNH TNH/TNP
4.37 7.18 9.29
0.73 0.75 0.70
70 61 62
2.27 3.28 4.01
96 136 221
578 819 1331
110
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and the surface area of the photoanode [33]. The IPCE spectrum of the DSSC based on the TiO2 nanohexagon arrays shows a maximum efficiency of 65.15%, which is significantly higher than that of the one based on TiO2 nanotube arrays. The IPCE value of 68% was also achieved for the DSSC based on the TiO2 nanohexagon arrays deposited with TiO2 nanoparticles, which was brought by TiCl4 treatment. These results indicate that the photoanode with TiCl4 solution treatment can increase the surface area of the TiO2 nanohexagon arrays, and thus increases the dye-loading capacity of the photoanode. 4. Conclusions
Fig. 7. Schematic structure of the DSSC based on the TiO2 nanohexagon arrays.
nanotube to increase the surface area, which improves the dyeloading capacity of the photoanode [32]. Furthermore, it has been also observed that this treatment can eliminate the cracks on the nanotube surface and reduce the charge recombination [10,25]. Park et al. reported a 30% increase of the photocurrent density when using TiCl4-treated TiO2 nanotubes as photoanode [10]. Our result indicates that for the DSSC based on the photoanode of the TiO2 nanohexagon with TiCl4 treatment, its Jsc increases from 7.18 mA/cm2 to 9.29 mA/cm2 and power conversion efficiency enhances from 3.28 % to 4.01 %. The amount of the dye adsorption Ada is determined to be 221 nmol/cm2, the evaluated Sda is about 1331 cm2, the inner surface area of 857 cm2 for 10 mm thickness, which is higher than that of TiO2 nanoparticles photoanode (780 cm2 for film of 10 mm thickness) [1]. IPCE measurements of the devices were performed to analyze the details of the enhanced performance of the devices based on the TiO2 nanotube arrays and the TiO2 nanohexagon arrays as shown in Fig. 8. The IPCE or quantum efficiency is determined by quantum yield of the electron injection, light absorption efficiency of the dye molecules, and efficiency of collecting the injected electrons at the FTO glass, which is directly affected by the structure
Fig. 8. IPCE curves of the DSSCs based on the TiO2 nanotube arrays and the TiO2 nanohexagon arrays sensitized by N719.
The DSSCs based on the TiO2 nanohexagon arrays show higher power efficiency than those based on the nanotubes arrays. The improvement in efficiency can be attributed to a higher surface area provided by the hexagon structure, which is helpful for the uploading of the dye molecules to achieve a maximum light harvesting. Also, the spacing introduced inside the hexagon allows more dye molecules to cover the interior of the nanohexagon walls. At present experimental conditions, the DSSC based on the TiO2 nanohexagon arrays as photoanode achieves a power conversion efficiency of 4.01% and IPCE of 68%. It can be also believed that the nanohexagon structure presented here can be further optimized by varying the electrochemical conditions to achieve even higher conversion efficiency. Acknowledgments This work was supported by the Research Fund for the Doctoral Program of Higher Education of China under Grant 20120201130004, the Science and Technology Developing Project of Shaanxi Province (2015 KW-001), partially by the National Natural Science Foundation of China Major Research Plan on Nanomanufacturing under Grant No. 91323303, the National Natural Science Foundation of China under Grant No. 61078058, and the 111 Project of China (B14040). The SEM work was done at International Centre for Dielectric Research, Xi'an Jiaotong University, Xi'an, China. The authors also thankful to Ms. Dai for her help in using SEM. References [1] B. O'Regan, M. Gr€ atzel, Nature 353 (1991) 737. [2] D. Gong, C.A. Grimes, O.K. Varghese, W.C. Hu, R.S. Singh, Z. Chen, E.C. Dickey, J. Mater. Res. 16 (2001) 3331. [3] K. Zhu, N.R. Neale, A. Miedaner, A.J. Frank, Nano Lett. 7 (2007) 69. [4] J. Halme, G. Boschloo, A. Hagfeldt, P. Lund, J. Phys. Chem. C 112 (2008) 5623. [5] J.M. Macak, P. Schmuki, Electrochim. Acta 52 (2006) 1258. [6] J.M. Macak, H. Tsuchiya, A. Ghicov, K. Yasuda, R. Hahn, S. Bauer, P. Schmuki, Curr. Opin. Solid State Mater. Sci. 11 (2007) 3. [7] D. Kim, A. Ghicov, S.P. Albu, P. Schmuki, J. Am. Chem. Soc. 130 (2008) 16454. [8] X. Luan, D. Guan, Y. Wang, J. Phys. Chem. C 116 (2012) 14257. [9] S.P. Albu, D. Kim, P. Schmuki, Angew. Chem. Int. Ed. 47 (2008) 1916. [10] J.H. Park, T.W. Lee, M.G. Kang, Chem. Commun. 25 (2008) 2867. [11] K. Fan, W. Zhang, T.Y. Peng, J.N. Chen, F. Yang, J. Phys. Chem. C 115 (2011) 17213. [12] D. Wang, Y. Liu, B. Yu, F. Zhou, W. Liu, Chem. Mater. 21 (2009) 1198. [13] M. Paulose, H.E. Prakasam, O.K. Varghese, L. Peng, K.C. Popat, G.K. Mor, T.A. Desai, C.A. Grimes, J. Phys. Chem. C 111 (2007) 14992. [14] J. Wang, Z. Lin, Chem. Mater. 20 (2008) 1257. [15] G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Nano Lett. 5 (2005) 191. [16] S. Sreekantan, Z. Lockman, R. Hazan, M. Tasbihi, L.K. Tang, A.R. Mohamed, J. Alloy. Compd. 485 (2009) 478. [17] C.A. Grimes, G.K. Mor, TiO2 Nanotube Arrays Synthesis, Properties, and Applications, 2009. [18] H. Xu, X. Liao, G. Yin, X. Ye, Y. Liu, Mater. Sci. 4 (2014) 22. [19] C.W. Lai, S. Sreekantan, Int. J. Photoenergy (2012), http://dx.doi.org/10.1155/ 2012/356943. [20] L.K. Tsui, T. Homma, G. Zangari, J. Phys. Chem. C 117 (2013) 6979.
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