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Enhanced photovoltaic performance of dye sensitized solar cells using one dimensional ZnO nanorod decorated porous TiO2 film electrode
Applied Surface Science 292 (2014) 297–300
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Enhanced photovoltaic performance of dye sensitized solar cells using one dimensional ZnO nanorod decorated porous TiO2 film electrode Long Yang a , Qing-lan Ma a , Yungao Cai b , Yuan Ming Huang a,∗ a b
School of Mathematic and Physics, Changzhou University, Jiangsu 213164, China Department of Science and Technology, Baoshan University, Yunnan 678000, China
a r t i c l e
i n f o
Article history: Received 21 October 2013 Accepted 23 November 2013 Available online 1 December 2013 Keywords: Dye sensitized solar cells ZnO nanorod decorated porous TiO2 film Dye adsorption amount Energy barrier layer Power conversion efficiency
a b s t r a c t A low cost and effective working electrode with one dimensional ZnO nanorod grown on the porous TiO2 film is used to improve the power conversion efficiency of dye sensitized solar cells. The one dimensional ZnO nanorod is introduced into the porous TiO2 film by a simple and facile hydrothermal route, and the obtained composite film is characterized using the field-emission scan electron microscopy, X-ray diffractometer and photoluminescence spectroscopy. The photocurrent–voltage curves of fabricated dye sensitized solar cells are measured by a solar cell measurement system. Compared with the bare porous TiO2 film based dye sensitized solar cell, it is found that the power conversion efficiency of dye sensitized solar cell with ZnO nanorod decorated TiO2 porous film was improved by more than triple. It is mainly believed that the improved power conversion efficiency of dye sensitized solar cell is ascribed to the increased dye adsorption amount and formation of energy barrier between ZnO nanorod and porous TiO2 film. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Dye sensitized solar cells (DSSCs) based on nanocrystalline TiO2 have attracted extensive attention in academic research and industrial application due to its low costs and relative high conversion efficiency since O’Regan and Grätzel first reported their breakthrough in 1991 [1]. A conventional DSSC is composed of a dye adsorbed nanocrystalline TiO2 porous film on the transparent conductive glass, redox electrolyte and platinum coated counter electrode [2–4]. Through the literature survey, it is revealed that the dye adsorption amount on surface of working electrode and recombination of a photogenerated electron were two important limiting factors to future improve the power conversion efficiency () of DSSC [5–8]. Furthermore, the recombination of photogenerated electron is mainly occurred at the surface of electrode/electrolyte due to the absence of energy barrier layer [9–11]. At present, an effective method for reducing the recombination of photogenerated electron is used by coating the wide band gap nanosemiconductor on the TiO2 porous film, such as ZnO [12,13], MgO [14] and Nb2 O5 [15] and so on. In addition, the one dimensional nanostructure of wide band gap semiconductor oxide as the working electrode of DSSCs are attracted wide attention in recently due to the higher specific surface area than nanoparticles [16–18].
∗ Corresponding author. Tel.: +86 51986056701; fax: +86 51986056701. E-mail address: [email protected] (Y.M. Huang). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.11.134
ZnO as a wide band gap semiconductor oxide have great potential in photoelectric devices, and ZnO nanorod can be prepared using various method, such as electrochemical deposition [19], thermal evaporation [20], solvothermal [21] and hydrothermal method [22,23]. In this study, a low cost effective working electrode with one dimensional ZnO nanorod decorated TiO2 porous (ZNTP) film was prepared by a simple and facile hydrothermal method and applied in DSSCs to improve the . The ZNTP film working electrode not only increases the dye adsorption amount but also reduces the recombination of photogenerated electron. The fabrication and characterization of ZNTP film were detailed in paper, and the transfer process and recombination of photogenerated electron also were discussed. Compared with typical porous TiO2 film based DSSC, the of DSSC with ZNTP film electrode has apparently enhanced.
2. Experimental details 2.1. Materials Titanium tetrachloride (TiCl4 , ≥99%), butyl alcohol (≥99.5%), diethanolamine (DEA, ≥99%), hexamethylene tetramine (HMT, ≥99%) and acetonitrile (≥99%) were purchased from Shanghai Lingfeng Chemical Reagents Co., Ltd. Tetrabutyl titanate (≥98%), acetate acid (≥99.5%), polyethylene glycol (PEG-2000), zinc acetate dehydrate (≥99%) zinc nitrate hexahydrate (≥99%), potassium
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iodide (KI, ≥98.5%), ethanol and chloroplatinic acid hexahydrate (H2 PtCl6 ·6H2 O, Pt ≥ 37%) were purchased from Sinopharm Chemical Reagents Co., Ltd. The dye of di-tetrabutylammonium cis-bis (isothiocyanato) bis (2.2 -bipyridyl-4.4 -dicarboxylato) ruthenium (II) (N719), ethanolamine (EA, ≥99%) and iodide (I2 , ≥99.8%) were supplied by Shanghai MaterWin New Materials Co., Ltd., Tianjin Fuchen Chemical Reagents Factory and Shanghai Shisihewei Chemical Co., Ltd., respectively. Fluorine-doped tin oxide conductive glass (FTO, sheet resistance ∼14 /cm2 , thickness 2.2 mm) was used as electrode substrate and purchased from Wuhan Geao Co., Ltd. Except for tetrabutyl titanate and PEG-2000, all the other reagents were analytical purity and all reagents were used without further purification.
2.2. Preparation of ZNTP film The ZNTP film was prepared by a simple hydrothermal route. Briefly, FTO glass substrate (2.55 cm × 2.5 cm) was orderly cleaned in detergent, distilled water, acetone and ethanol by assistance of ultrasonic. After drying, the FTO substrate was immersed in an aqueous solution of TiCl4 (0.04 M) at 70 ◦ C for 30 min and sintered at 450 ◦ C for 30 min to form a compact layer. After that, TiO2 porous film was prepared by immersing the TiCl4 modified FTO substrate in TiO2 sol and the film was calcined at 450 ◦ C for 2 h. The TiO2 sol was synthesized by adding 4 mL of tetrabutyl titanate, 2 mL of DEA, 1.5 mL of distilled water, 1.2 mL of acetate acid and 0.045 g of PEG-2000 into the 60 mL of butyl alcohol under stirring and aged for 16 h. Afterward, the TiO2 coated FTO substrate was dipped into the ZnO transparent solution, which contains 3.3 g of zinc acetate dihydrate in 50 mL of ethanol and 1.8 mL of EA, and stirred at 60 ◦ C for 30 min. Then, the immersed substrate was dried at 300 ◦ C for 10 min to form the ZnO seed layer. Subsequently, the ZnO nanorod was prepared at 95 ◦ C for 5 h in a Teflon-lined stainless steel autoclave by immersing seeded substrate in the aqueous solution containing zinc nitrate hexahydrate (0.04 M) and HMT (0.04 M). Finally, the sample was rinsed with distilled water and dried in air. 2.3. Fabrication of DSSCs For fabrication of DSSC, the working electrode was prepared by dropping N719 (0.4 mM) dye solution in the ZNTP film. The N719 dye solution was synthesized by dissolving the N719 into ethanol. A clean FTO substrate was immersed into the ethanol solution of H2 PtCl6 (4 mM) and heated at 450 ◦ C for 2 h to form a platinum counter electrode. Finally, the working electrode and counter electrode were assembled into a sandwich structure and sealed using a sealing sheet and the internal space was filled with a redox liquid electrolyte, which contains 0.5 M KI, 0.05 M I2 in acetonitrile and distilled water.
2.4. Characterization and measurement The X-ray diffraction (XRD) patterns of samples were characterized by X-ray diffractometer (D/max 2500 PC, Rigaku, Japan) with copper target radiation. The surface morphological analysis of sample was performed with a field-emission scanning electron microscopy (FE-SEM, SUPRA55, Zeiss, Germany). For the performance characterization of DSSCs, an AM 1.5 solar simulator with a 500 W xenon lamp was employed to illuminate the DSSC, and the incident light power was calibrated to 100 mW/cm2 . A Keithley model 2400 digital source meter was used to measure the opencircuit photovoltage (Voc ) and short-circuit photocurrent (Jsc ) of DSSC.
Fig. 1. XRD patterns of pure TiO2 porous film (a), ZnO nanorod film (b) and ZNTP film (c).
3. Results and discussion 3.1. Morphology and structure Fig. 1 depicts the XRD spectra of samples with pure porous TiO2 film (a), ZnO nanorod film (b) and ZNTP film (c). As shown in Fig. 1(a), the observed diffraction peaks at 2 = 25.2◦ , 37.6◦ , 47.9◦ , 53.7◦ , 54.9◦ , 62.7◦ and 75.1◦ were assigned to the reflections from the (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4) and (2 1 5) planes of TiO2 anatase phase, according to the Joint Committee on Powder Diffraction Standard (JCPDS, PDF#21-1272). These diffraction peaks at 2 = 31.62◦ , 34.35◦ , 36.22◦ , 47.45◦ , 56.58◦ , 62.84◦ and 67.91◦ were observed in Fig. 1(b) and (c), and these peaks correspond to the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2) planes of ZnO wurtzite phase (JCPDS, PDF#36-1451). From Fig. 1(c), it can seen that the ZnO nanorod shows a sharp and strongest peak at 2 = 34.35◦ , indicating a preferential crystal growth and good crystalline along the c-axis direction. Aside from this, a small and broad diffraction peaks at 2 = 25.2◦ was found in Fig. 1(c) and this peak was assigned to the (1 0 1) plane of TiO2 anatase phase. For convenience, the magnified view of the peak is shown in the inset. Therefore, these results indicate that the ZNTP film can simultaneously possess the ZnO wurtzite phase and TiO2 anatase phase by hydrothermal method. Fig. 2 shows the FE-SEM image of ZNTP film. Fig. 2(a) and (b) shows the top view and cross-section of FE-SEM images of one dimensional ZnO nanorod, respectively. It can be clearly seen that the diameter of ZnO nanorod was about 50–80 nm and the length of ZnO nanorod was about 1.3 m, making the aspect ratio of the ZnO nanorod to be about 20. At the same time, it can observe that the ZnO nanorod was homogeneously grown on the substrate and hexagonal ZnO nanorod was obtained. Fig. 2(c) reveals the top view of porous TiO2 film prepared using sol–gel method. The crosssection of FE-SEM image of ZNTP film was shown in Fig. 2(d). In the FE-SEM image of Fig. 2(d), the ZNTP film clearly displays the trilayer structure with the bottom TiO2 compact layer, the middle TiO2 porous layer and the upper ZnO nanorod layer, and thicknesses of TiO2 compact layer and TiO2 porous layer are 250 nm and 650 nm, respectively. Therefore, it can demonstrate that the one dimensional ZnO nanorod was successfully grown on the porous TiO2 film by a simple hydrothermal method. Fig. 2(b) and (d) observes the different length of ZnO nanorod due to that Fig. 2(d) ZnO nanorod was broken during FE-SEM sample preparation.
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Fig. 4. J–V characterization curves of DSSCs with TiO2 porous film and ZNTP film electrodes under illumination intensity of 100 mW/cm2 . Fig. 2. FE-SEM images of ZNTP film: (a) top view of ZnO nanorod, (b) cross-section of ZnO nanorod, (c) top view of porous TiO2 film and (d) cross-section of ZNTP film.
Fig. 5. Energy level diagrams for working electrode of DSSCs: (a) bare porous TiO2 film and (b) ZNTP film. (For interpretation of the references to color in text, the reader is referred to the web version of the article.)
3.3. Photovoltaic performance of DSSCs Fig. 3. PL spectra of ZnO nanorod (a) and TiO2 porous film (b).
3.2. PL analysis Fig. 3 illustrates the photoluminescence (PL) spectra of ZnO nanorod and porous TiO2 film under room temperature. The PL spectrum of hydrothermal grown ZnO nanorod was shown in Fig. 3(a), and it can be decomposed into a weak ultraviolet luminescent band with it peak wavelength at 378 nm and a strong green luminescent band it peak located at 536 nm. It is commonly believed that the ultraviolet emission band is due to the recombination of free excitons between conductive band and valence band of ZnO and is called near band edge emission, while the green emission peak is considered to be the result of radiative recombination of photo-generated hole with an electron occupying the singly ionized oxygen vacancy [20,24]. Fig. 3(b) shows the PL spectrum of TiO2 porous film, and a weak and broad green emission band was observed. At present, PL spectrum of TiO2 anatase phase is interpreted as the emission from three physical origins: self trapped excitons, oxygen vacancies and surface states. The weak green emission band at about 542 nm is attributed to oxygen vacancy, and these oxygen vacancies are at the surface of TiO2 [25].
Fig. 4 shows the photocurrent–voltage (J–V) characterization curves of DSSCs with pure porous TiO2 film and ZNTP film electrode under illumination intensity of 100 mW/cm2 . The photovoltaic performance parameters of DSSCs with Voc , Jsc , and fill factor (FF) were summarized and inserted in Fig. 4. Compared with bare porous TiO2 film based DSSC, it can be observed that the Jsc density of DSSC with ZNTP film was increased from 0.42 to 3.36 mA/cm2 . Through the further analysis, it found that the DSSC with ZNTP film electrode has achieved the for 0.76%, increased by more than triple as compared to that of bare porous TiO2 film DSSC. 3.4. Transport mechanism and recombination of photogenerated electron It is believed that the improved of DSSC can be attributed to the following three factors. Firstly, the formation of energy barrier layer between ZnO nanorod and porous TiO2 film. In order to analyze the electron transfer and charge recombination process in detail, the energy level diagrams of both the bare porous TiO2 film and ZNTP film working electrode are, respectively, shown in Fig. 5(a) and (b). The red solid arrows represent the advantage of electron transfer, while the red dashed arrows show the charge recombination process. In the energy level diagram of Fig. 5(a),
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the photoexcited electron is injected from the dye molecule to conduction band of TiO2 . At the same time, the one part injected electron is back transferred from the conduction band of TiO2 to electrolyte, and the recombination of photogenerated electron is occurred between electron and oxidized dye or tri-iodide in electrolyte. However, for the ZNTP film working electrode, the energy barrier layer existed between ZnO nanorod and porous TiO2 film can effectively suppress the injected electron from conduction band of TiO2 back transfer to the electrolyte. As a result, the recombination of photogenerated electron with oxidized dye or tri-iodide in the electrolyte would be suppressed, increased by the mount of photogenerated electron. Secondly, the high electron mobility of ZnO. The photogenerated electron can rapidly inject to conduction band of TiO2 due to the higher electron mobility of ZnO. In the end, the increased dye adsorption amount on the surface of working electrode. It is mainly due to that the one dimensional ZnO nanorod has a higher specific surface area than that of porous TiO2 film. However, Jsc as an important photovoltaic performance parameter, it is generally accepted that the generated photocurrent is directly depended on the amount of adsorbed dye on the surface of working electrode. Therefore, the increased dye adsorption amount also is an important reason for improved of DSSC. 4. Conclusion In summary, a conventional and effectively working electrode was used to improve the of DSSCs. The one dimensional ZnO nanorod was grown on the porous TiO2 film by a simple hydrothermal route, and the fabrication of ZnO nanorod with a high aspect ratio of 20 on TiO2 film can provide more adsorption sites for dye molecules, which contributed to increase of the photocurrent. Furthermore, the ZnO nanorod creates an energy barrier layer between ZnO nanorod and porous TiO2 film, making the
decreasing recombination of photogenerated electron. As expected, the DSSC based on ZNTP film shows a higher of 0.76%. Compared to the DSSC based on bare porous TiO2 film, the of DSSC with ZNTP film was improved by more than triple. References [1] B. O’Regan, M. Grätzel, Nature 353 (1991) 737–740. [2] S. Ito, T.N. Murakami, P. Comte, P. Liska, C. Grätzel, M.K. Nazeeruddin, M. Grätzel, Thin Solid Films 516 (2008) 4613–4619. [3] M. Grätzel, J. Photochem. Photobiol. C: Photochem. Rev. 4 (2003) 145–153. [4] E. Palomares, J.N. Clifford, S.A. Haque, T. Lutz, J.R. Durrant, J. Am. Chem. Soc. 125 (2003) 475–482. [5] U.O. Krasovec, M. Berginc, M. Hocevar, M. Topic, Sol. Energy Mater. Sol. Cells 93 (2009) 379–381. [6] S.S. Kim, S.I. Na, Y.C. Nah, Electrochim. Acta 58 (2011) 503–509. [7] V. Dhas, S. Muduli, S. Agarkar, A. Rana, B. Hannoyer, R. Banerjee, S. Ogale, Sol. Energy 85 (2011) 1213–1219. [8] J. Archana, M. Navaneethan, Y. Hayakawa, J. Power Sources 242 (2013) 803–810. [9] X.T. Yin, W.X. Que, D. Fei, H.X. Xie, Z.L. He, Electrochim. Acta 99 (2013) 204–210. [10] S.S. Kim, J.H. Yum, Y.E. Sung, Sol. Energy Mater. Sol. Cells 79 (2003) 495–505. [11] C.S. Chou, F.C. Chou, J.Y. Kang, Powder Technol. 215–216 (2012) 38–45. [12] K.E. Kim, S.R. Jang, J. Park, R. Vittal, K.J. Kim, Sol. Energy Mater. Sol. Cells 91 (2007) 366–370. [13] R. Liu, W.D. Yang, L.S. Qiang, H.Y. Liu, J. Power Sources 220 (2012) 153–159. [14] H.S. Jung, J.K. Lee, M. Nastasi, Langmuir 21 (2005) 10332–10335. [15] J. Xia, N. Masaki, K.J. Jiang, S. Yanagida, J. Phys. Chem. C 111 (2007) 8092–8097. [16] J. Chung, J. Lee, S. Lim, Physica B 405 (2010) 2593–2598. [17] G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Nano Lett. 6 (2006) 215–218. [18] J.K. Oh, J.K. Lee, B. Han, S.J. Kim, K.W. Park, Mater. Lett. 68 (2012) 4–7. [19] O. Lupan, V.M. Guérin, I.M. Tiginyanu, V.V. Ursaki, L. Chow, H. Heinrich, T. Pauporté, J. Photochem. Photobiol. A: Chem. 211 (2010) 65–73. [20] Y.M. Huang, Q.L. Ma, B.G. Zhai, Mater. Lett. 93 (2013) 266–268. [21] Z. Chen, N. Zhang, Y.J. Xu, CrystEngComm 15 (2013) 3022–3030. [22] S. Hanna, S.L. Essi, P.P. Alexander, M.K. Juha, E. Alexander, H. Hironobu, L. Harri, I. Hiroshi, L. Helge, V.T. Nikolai, J. Phys. Chem. C 116 (2012) 2336–2343. [23] Z. Wang, B. Huang, X. Qin, X. Zhang, P. Wang, J. Wei, J. Zhan, X. Jing, H. Liu, Z. Xu, H. Cheng, X. Wang, Z. Zheng, Mater. Lett. 63 (2009) 130–132. [24] N. Kiomarsipour, R.S. Razavi, Superlattices Microstruct. 52 (2012) 704–710. [25] B.T. Liu, L.L. Peng, J. Alloys Compd. 571 (2013) 145–152.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Enhanced photovoltaic performance of dye sensitized solar cells using one dimensional ZnO nanorod decorated porous TiO2 film electrode Long Yang a , Qing-lan Ma a , Yungao Cai b , Yuan Ming Huang a,∗ a b
School of Mathematic and Physics, Changzhou University, Jiangsu 213164, China Department of Science and Technology, Baoshan University, Yunnan 678000, China
a r t i c l e
i n f o
Article history: Received 21 October 2013 Accepted 23 November 2013 Available online 1 December 2013 Keywords: Dye sensitized solar cells ZnO nanorod decorated porous TiO2 film Dye adsorption amount Energy barrier layer Power conversion efficiency
a b s t r a c t A low cost and effective working electrode with one dimensional ZnO nanorod grown on the porous TiO2 film is used to improve the power conversion efficiency of dye sensitized solar cells. The one dimensional ZnO nanorod is introduced into the porous TiO2 film by a simple and facile hydrothermal route, and the obtained composite film is characterized using the field-emission scan electron microscopy, X-ray diffractometer and photoluminescence spectroscopy. The photocurrent–voltage curves of fabricated dye sensitized solar cells are measured by a solar cell measurement system. Compared with the bare porous TiO2 film based dye sensitized solar cell, it is found that the power conversion efficiency of dye sensitized solar cell with ZnO nanorod decorated TiO2 porous film was improved by more than triple. It is mainly believed that the improved power conversion efficiency of dye sensitized solar cell is ascribed to the increased dye adsorption amount and formation of energy barrier between ZnO nanorod and porous TiO2 film. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Dye sensitized solar cells (DSSCs) based on nanocrystalline TiO2 have attracted extensive attention in academic research and industrial application due to its low costs and relative high conversion efficiency since O’Regan and Grätzel first reported their breakthrough in 1991 [1]. A conventional DSSC is composed of a dye adsorbed nanocrystalline TiO2 porous film on the transparent conductive glass, redox electrolyte and platinum coated counter electrode [2–4]. Through the literature survey, it is revealed that the dye adsorption amount on surface of working electrode and recombination of a photogenerated electron were two important limiting factors to future improve the power conversion efficiency () of DSSC [5–8]. Furthermore, the recombination of photogenerated electron is mainly occurred at the surface of electrode/electrolyte due to the absence of energy barrier layer [9–11]. At present, an effective method for reducing the recombination of photogenerated electron is used by coating the wide band gap nanosemiconductor on the TiO2 porous film, such as ZnO [12,13], MgO [14] and Nb2 O5 [15] and so on. In addition, the one dimensional nanostructure of wide band gap semiconductor oxide as the working electrode of DSSCs are attracted wide attention in recently due to the higher specific surface area than nanoparticles [16–18].
∗ Corresponding author. Tel.: +86 51986056701; fax: +86 51986056701. E-mail address: [email protected] (Y.M. Huang). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.11.134
ZnO as a wide band gap semiconductor oxide have great potential in photoelectric devices, and ZnO nanorod can be prepared using various method, such as electrochemical deposition [19], thermal evaporation [20], solvothermal [21] and hydrothermal method [22,23]. In this study, a low cost effective working electrode with one dimensional ZnO nanorod decorated TiO2 porous (ZNTP) film was prepared by a simple and facile hydrothermal method and applied in DSSCs to improve the . The ZNTP film working electrode not only increases the dye adsorption amount but also reduces the recombination of photogenerated electron. The fabrication and characterization of ZNTP film were detailed in paper, and the transfer process and recombination of photogenerated electron also were discussed. Compared with typical porous TiO2 film based DSSC, the of DSSC with ZNTP film electrode has apparently enhanced.
2. Experimental details 2.1. Materials Titanium tetrachloride (TiCl4 , ≥99%), butyl alcohol (≥99.5%), diethanolamine (DEA, ≥99%), hexamethylene tetramine (HMT, ≥99%) and acetonitrile (≥99%) were purchased from Shanghai Lingfeng Chemical Reagents Co., Ltd. Tetrabutyl titanate (≥98%), acetate acid (≥99.5%), polyethylene glycol (PEG-2000), zinc acetate dehydrate (≥99%) zinc nitrate hexahydrate (≥99%), potassium
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iodide (KI, ≥98.5%), ethanol and chloroplatinic acid hexahydrate (H2 PtCl6 ·6H2 O, Pt ≥ 37%) were purchased from Sinopharm Chemical Reagents Co., Ltd. The dye of di-tetrabutylammonium cis-bis (isothiocyanato) bis (2.2 -bipyridyl-4.4 -dicarboxylato) ruthenium (II) (N719), ethanolamine (EA, ≥99%) and iodide (I2 , ≥99.8%) were supplied by Shanghai MaterWin New Materials Co., Ltd., Tianjin Fuchen Chemical Reagents Factory and Shanghai Shisihewei Chemical Co., Ltd., respectively. Fluorine-doped tin oxide conductive glass (FTO, sheet resistance ∼14 /cm2 , thickness 2.2 mm) was used as electrode substrate and purchased from Wuhan Geao Co., Ltd. Except for tetrabutyl titanate and PEG-2000, all the other reagents were analytical purity and all reagents were used without further purification.
2.2. Preparation of ZNTP film The ZNTP film was prepared by a simple hydrothermal route. Briefly, FTO glass substrate (2.55 cm × 2.5 cm) was orderly cleaned in detergent, distilled water, acetone and ethanol by assistance of ultrasonic. After drying, the FTO substrate was immersed in an aqueous solution of TiCl4 (0.04 M) at 70 ◦ C for 30 min and sintered at 450 ◦ C for 30 min to form a compact layer. After that, TiO2 porous film was prepared by immersing the TiCl4 modified FTO substrate in TiO2 sol and the film was calcined at 450 ◦ C for 2 h. The TiO2 sol was synthesized by adding 4 mL of tetrabutyl titanate, 2 mL of DEA, 1.5 mL of distilled water, 1.2 mL of acetate acid and 0.045 g of PEG-2000 into the 60 mL of butyl alcohol under stirring and aged for 16 h. Afterward, the TiO2 coated FTO substrate was dipped into the ZnO transparent solution, which contains 3.3 g of zinc acetate dihydrate in 50 mL of ethanol and 1.8 mL of EA, and stirred at 60 ◦ C for 30 min. Then, the immersed substrate was dried at 300 ◦ C for 10 min to form the ZnO seed layer. Subsequently, the ZnO nanorod was prepared at 95 ◦ C for 5 h in a Teflon-lined stainless steel autoclave by immersing seeded substrate in the aqueous solution containing zinc nitrate hexahydrate (0.04 M) and HMT (0.04 M). Finally, the sample was rinsed with distilled water and dried in air. 2.3. Fabrication of DSSCs For fabrication of DSSC, the working electrode was prepared by dropping N719 (0.4 mM) dye solution in the ZNTP film. The N719 dye solution was synthesized by dissolving the N719 into ethanol. A clean FTO substrate was immersed into the ethanol solution of H2 PtCl6 (4 mM) and heated at 450 ◦ C for 2 h to form a platinum counter electrode. Finally, the working electrode and counter electrode were assembled into a sandwich structure and sealed using a sealing sheet and the internal space was filled with a redox liquid electrolyte, which contains 0.5 M KI, 0.05 M I2 in acetonitrile and distilled water.
2.4. Characterization and measurement The X-ray diffraction (XRD) patterns of samples were characterized by X-ray diffractometer (D/max 2500 PC, Rigaku, Japan) with copper target radiation. The surface morphological analysis of sample was performed with a field-emission scanning electron microscopy (FE-SEM, SUPRA55, Zeiss, Germany). For the performance characterization of DSSCs, an AM 1.5 solar simulator with a 500 W xenon lamp was employed to illuminate the DSSC, and the incident light power was calibrated to 100 mW/cm2 . A Keithley model 2400 digital source meter was used to measure the opencircuit photovoltage (Voc ) and short-circuit photocurrent (Jsc ) of DSSC.
Fig. 1. XRD patterns of pure TiO2 porous film (a), ZnO nanorod film (b) and ZNTP film (c).
3. Results and discussion 3.1. Morphology and structure Fig. 1 depicts the XRD spectra of samples with pure porous TiO2 film (a), ZnO nanorod film (b) and ZNTP film (c). As shown in Fig. 1(a), the observed diffraction peaks at 2 = 25.2◦ , 37.6◦ , 47.9◦ , 53.7◦ , 54.9◦ , 62.7◦ and 75.1◦ were assigned to the reflections from the (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4) and (2 1 5) planes of TiO2 anatase phase, according to the Joint Committee on Powder Diffraction Standard (JCPDS, PDF#21-1272). These diffraction peaks at 2 = 31.62◦ , 34.35◦ , 36.22◦ , 47.45◦ , 56.58◦ , 62.84◦ and 67.91◦ were observed in Fig. 1(b) and (c), and these peaks correspond to the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2) planes of ZnO wurtzite phase (JCPDS, PDF#36-1451). From Fig. 1(c), it can seen that the ZnO nanorod shows a sharp and strongest peak at 2 = 34.35◦ , indicating a preferential crystal growth and good crystalline along the c-axis direction. Aside from this, a small and broad diffraction peaks at 2 = 25.2◦ was found in Fig. 1(c) and this peak was assigned to the (1 0 1) plane of TiO2 anatase phase. For convenience, the magnified view of the peak is shown in the inset. Therefore, these results indicate that the ZNTP film can simultaneously possess the ZnO wurtzite phase and TiO2 anatase phase by hydrothermal method. Fig. 2 shows the FE-SEM image of ZNTP film. Fig. 2(a) and (b) shows the top view and cross-section of FE-SEM images of one dimensional ZnO nanorod, respectively. It can be clearly seen that the diameter of ZnO nanorod was about 50–80 nm and the length of ZnO nanorod was about 1.3 m, making the aspect ratio of the ZnO nanorod to be about 20. At the same time, it can observe that the ZnO nanorod was homogeneously grown on the substrate and hexagonal ZnO nanorod was obtained. Fig. 2(c) reveals the top view of porous TiO2 film prepared using sol–gel method. The crosssection of FE-SEM image of ZNTP film was shown in Fig. 2(d). In the FE-SEM image of Fig. 2(d), the ZNTP film clearly displays the trilayer structure with the bottom TiO2 compact layer, the middle TiO2 porous layer and the upper ZnO nanorod layer, and thicknesses of TiO2 compact layer and TiO2 porous layer are 250 nm and 650 nm, respectively. Therefore, it can demonstrate that the one dimensional ZnO nanorod was successfully grown on the porous TiO2 film by a simple hydrothermal method. Fig. 2(b) and (d) observes the different length of ZnO nanorod due to that Fig. 2(d) ZnO nanorod was broken during FE-SEM sample preparation.
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Fig. 4. J–V characterization curves of DSSCs with TiO2 porous film and ZNTP film electrodes under illumination intensity of 100 mW/cm2 . Fig. 2. FE-SEM images of ZNTP film: (a) top view of ZnO nanorod, (b) cross-section of ZnO nanorod, (c) top view of porous TiO2 film and (d) cross-section of ZNTP film.
Fig. 5. Energy level diagrams for working electrode of DSSCs: (a) bare porous TiO2 film and (b) ZNTP film. (For interpretation of the references to color in text, the reader is referred to the web version of the article.)
3.3. Photovoltaic performance of DSSCs Fig. 3. PL spectra of ZnO nanorod (a) and TiO2 porous film (b).
3.2. PL analysis Fig. 3 illustrates the photoluminescence (PL) spectra of ZnO nanorod and porous TiO2 film under room temperature. The PL spectrum of hydrothermal grown ZnO nanorod was shown in Fig. 3(a), and it can be decomposed into a weak ultraviolet luminescent band with it peak wavelength at 378 nm and a strong green luminescent band it peak located at 536 nm. It is commonly believed that the ultraviolet emission band is due to the recombination of free excitons between conductive band and valence band of ZnO and is called near band edge emission, while the green emission peak is considered to be the result of radiative recombination of photo-generated hole with an electron occupying the singly ionized oxygen vacancy [20,24]. Fig. 3(b) shows the PL spectrum of TiO2 porous film, and a weak and broad green emission band was observed. At present, PL spectrum of TiO2 anatase phase is interpreted as the emission from three physical origins: self trapped excitons, oxygen vacancies and surface states. The weak green emission band at about 542 nm is attributed to oxygen vacancy, and these oxygen vacancies are at the surface of TiO2 [25].
Fig. 4 shows the photocurrent–voltage (J–V) characterization curves of DSSCs with pure porous TiO2 film and ZNTP film electrode under illumination intensity of 100 mW/cm2 . The photovoltaic performance parameters of DSSCs with Voc , Jsc , and fill factor (FF) were summarized and inserted in Fig. 4. Compared with bare porous TiO2 film based DSSC, it can be observed that the Jsc density of DSSC with ZNTP film was increased from 0.42 to 3.36 mA/cm2 . Through the further analysis, it found that the DSSC with ZNTP film electrode has achieved the for 0.76%, increased by more than triple as compared to that of bare porous TiO2 film DSSC. 3.4. Transport mechanism and recombination of photogenerated electron It is believed that the improved of DSSC can be attributed to the following three factors. Firstly, the formation of energy barrier layer between ZnO nanorod and porous TiO2 film. In order to analyze the electron transfer and charge recombination process in detail, the energy level diagrams of both the bare porous TiO2 film and ZNTP film working electrode are, respectively, shown in Fig. 5(a) and (b). The red solid arrows represent the advantage of electron transfer, while the red dashed arrows show the charge recombination process. In the energy level diagram of Fig. 5(a),
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the photoexcited electron is injected from the dye molecule to conduction band of TiO2 . At the same time, the one part injected electron is back transferred from the conduction band of TiO2 to electrolyte, and the recombination of photogenerated electron is occurred between electron and oxidized dye or tri-iodide in electrolyte. However, for the ZNTP film working electrode, the energy barrier layer existed between ZnO nanorod and porous TiO2 film can effectively suppress the injected electron from conduction band of TiO2 back transfer to the electrolyte. As a result, the recombination of photogenerated electron with oxidized dye or tri-iodide in the electrolyte would be suppressed, increased by the mount of photogenerated electron. Secondly, the high electron mobility of ZnO. The photogenerated electron can rapidly inject to conduction band of TiO2 due to the higher electron mobility of ZnO. In the end, the increased dye adsorption amount on the surface of working electrode. It is mainly due to that the one dimensional ZnO nanorod has a higher specific surface area than that of porous TiO2 film. However, Jsc as an important photovoltaic performance parameter, it is generally accepted that the generated photocurrent is directly depended on the amount of adsorbed dye on the surface of working electrode. Therefore, the increased dye adsorption amount also is an important reason for improved of DSSC. 4. Conclusion In summary, a conventional and effectively working electrode was used to improve the of DSSCs. The one dimensional ZnO nanorod was grown on the porous TiO2 film by a simple hydrothermal route, and the fabrication of ZnO nanorod with a high aspect ratio of 20 on TiO2 film can provide more adsorption sites for dye molecules, which contributed to increase of the photocurrent. Furthermore, the ZnO nanorod creates an energy barrier layer between ZnO nanorod and porous TiO2 film, making the
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