Studies on bundle like ZnO nanorods for solar cell applications

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ScienceDirect Solar Energy xxx (2014) xxx–xxx www.elsevier.com/locate/solener

Studies on bundle like ZnO nanorods for solar cell applications M. Raja a,⇑, N. Muthukumarasamy a, Dhayalan Velauthapillai b, R. Balasundaraprabhu c, S. Agilan a, T.S. Senthil d a

Department of Physics, Coimbatore Institute of Technology, Coimbatore, India b Department of Engineering, University College of Bergen, Bergen, Norway c Department of Physics, PSG College of Technology, Coimbatore, India d Department of Chemistry, College of Science, Yeungnam University, South Korea

Communicated by: Associate Editor Smagul Zh. Karazhanov

Abstract Bundle like structured ZnO nanorods and vertically aligned ZnO nanorods have been prepared by simple sol–gel dip coating method on ITO substrate. The X-ray diffraction pattern shows that the prepared ZnO samples are of hexagonal phase, the scanning electron microscope images reveal the formation of bundle like ZnO nanorods and aligned ZnO nanorods. The high resolution transmission electron microscope images also show the presence of nanorods. The average diameter and length of the nanorods in the bundle of ZnO nanorods and vertically aligned ZnO nanorods are 200 nm, 1.5 lm and 100 nm, 1.8 lm respectively. Dye sensitized solar cells have been assembled using N719 dye as sensitizer and the conversion efficiencies of the vertically aligned ZnO nanorods and bundle like ZnO nanorods thin film based solar cells have been found to be 0.74% and 0.83%, respectively. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Sol–gel method; ZnO nanorods; Dye sensitized solar cells

1. Introduction Among the various semiconductors, zinc oxide (ZnO) is one of the well studied material because of its wide band gap energy of 3.37 eV. One dimensional nanostructures of ZnO are used in the field of photodiodes (Lee et al., 2002), solar cells (Lu et al., 2010; Thambidurai et al., 2011, 2012, 2013), and gas sensors (Hsu et al., 2013). ZnO nanorods are generally grown by both physical and chemical methods like pulsed laser deposition (Yu et al., 2008), sputtering (Water and Chen, 2009), hydrothermal method (Jiaqiang et al., 2006) and sol gel method (Huanga

⇑ Corresponding author.

E-mail address: [email protected] (M. Raja).

et al., 2011; Thambidurai et al., 2013). Among the different techniques, sol–gel process is a simple and inexpensive method. Dye-sensitized solar cells (DSSCs), exhibit tunable absorption and impressive conversion efficiencies and are of low cost and have shown considerable potential as new generation solar cells. It is observed that the morphology of the nanomaterials affect the properties of nanomaterials and hence the characteristics of the solar cell fabricated using them. Wijeratne et al., have reported about the preparation of one dimensional SnO2 nanorods by hydrothermal method and they have reported an efficiency of 1.5% for N719 dye sensitized SnO2 based solar cells (Wijeratne et al., 2012). Irannejad et al., have reported an efficiency of 0.92% for ZnO–TiO2 core shell nanorod based solar cells sensitized using N719 dye (Irannejad et al., 2011). Zinc oxide nanostructures are of considerable interest for dye sensitized solar cell applications and the

http://dx.doi.org/10.1016/j.solener.2014.01.043 0038-092X/Ó 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Raja, M., et al. Studies on bundle like ZnO nanorods for solar cell applications. Sol. Energy (2014), http:// dx.doi.org/10.1016/j.solener.2014.01.043

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morphology of ZnO nanorods play an important role in enhanced electron transfer and electron injection in dye-sensitized solar cells. It has also been reported that the branched nanorod structure significantly improves the cell efficiency when compared to the simple nanorod structure (Suh et al., 2007). Pawar et al., have shown that the morphology plays a decisive role in determining the efficiency of cells fabricated using new structures like disks, rods, spindles and flowers (Pawar et al., 2011). Thambidurai et al., have synthesized ZnO with garland like structure on ITO substrate and have fabricated solar cells with hibiscus rosasinensis extract dye as sensitizer and they have reported an efficiency of 0.61% (Thambidurai et al., 2013). Pawar et al., have synthesized ZnO with bottle brush like structure on FTO substrate and reported an efficiency of 0.7% for N3 dye sensitized, ZnO solar cells (Pawar et al., 2011). Qurashi et al., have fabricated ZnO dumbbellshaped nanorods and aligned ZnO nanorods by using sputtering and have reported an efficiency of 0.26% and 0.32% for N3 dye sensitized ZnO dumb bell shaped nanorod and aligned ZnO nanorod based solar cells (Qurashi et al., 2010). In the present work, aligned ZnO nanorods and bundle like ZnO nanorods have been prepared by simple sol gel dip coating method by catalyst free technique. Dye sensitized ZnO nanorod based solar cell have been fabricated and their characteristics have been studied.

2. Experimental Zinc oxide nanorods have been prepared by sol–gel dip coating method onto ITO substrate by a two step chemical method. Fig. 1 shows the flow chart of preparation of ZnO nanorods. In the first step, ZnO seed layer has been prepared by sol–gel dip coating method. Zinc acetate dihydrate [Zn(CH3COO)22H2O)] and mono ethanolamine [(C2H7NO)] were used as precursors in the ratio of 1:1 mol concentration. The solution was stirred at 60 °C for 1 h to yield a clear and homogeneous solution. The dip coating method has been used to prepare thin films of ZnO sol onto ITO coated glass substrates. The films were post annealed at 300 °C for 1 h and these films form the seed layer of ZnO. In the next step an aqueous solution was prepared using zinc nitrate [Zn(NO3)26H2O] and hexamine (C6H12N4). The ZnO seed layer coated substrates were vertically placed in the zinc nitrate-hexamine aqueous solution and was heated. The deposition of film was carried out for two different deposition times 4 h and 6 h respectively. By using the prepared ZnO aligned nanorods and bundle of ZnO nanorods, dye sensitized solar cells have been assembled. ZnO nanorod electrodes were immersed in a 3.0  104 M N719 dye solution at room temperature for 24 h and then the film was rinsed in anhydrous ethanol

Fig. 1. Flow chart showing the preparation of ZnO nanorod thin film.

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and dried. A Pt-coated ITO electrode was then placed over the dye-adsorbed aligned nanorod and bundle of nanorod electrodes, and the edges of the cell were sealed with a sealing sheet (PECHM-1, Mitsui-Dupont polychemical) by heating with hot plate at 100 °C for 2 min. A redox electrolyte of 0.5 mol KI, 0.05 mol I2, and 0.5 mol 4-tert-butylpyridine was injected into the drilled hole in the counter electrode. Finally, the hole was sealed using additional cello tape. The size of the electrodes used was 0.25 cm2. In the present work X-ray diffraction studies have been carried out using PANalytical X-ray diffractometer, surface morphology of the samples have been studied using FEI QUANTA 200 scanning electron microscope (SEM) and the composition of the prepared samples has been studied by energy dispersive X-ray analysis (JEOL Model JED2300). The optical properties have been studied using the absorbance spectrum recorded using spectrophotometer (JASCO V-570). The selected area electron diffraction (SAED) patterns and high-resolution transmission electron microscope images were obtained using JEOL JEM-2100F microscope. The photocurrent–voltage (J–V) curves were obtained using white light from a xenon lamp (max. 150 W) using a sun 2000 solar simulator (ABE technologies). Incident light intensity was 100 mW cm2 (one sun illumination).

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shows that when the growth period is extended to 6 h, the preferential growth of nanorods is disturbed. The diffraction peaks in the pattern correspond to the hexagonal phase and the lattice parameters have been calculated and are a = 0.3243 nm and c = 0.5197 nm and are in good agreement with the standard values (a = 0.3249 nm, c = 0.5206 nm, JCPDS No. 36-1451). Lattice parameters have been calculated using the formula  2 4 h2 þ hk þ k 2 l 2 ðd hkl Þ ¼ þ 3 c a2

a

3. Results and discussion Fig. 2 shows the X-ray diffraction pattern of ZnO nanorods synthesized using a deposition time of 4 h and 6 h respectively. The diffraction peaks have been indexed as (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0) and (1 0 3) planes of ZnO and no impurity peaks were noted, indicating that high purity ZnO is formed. From the pattern, it is seen that for the samples with aligned nanorod like structure the (0 0 2) diffraction peak is dominant than the other diffraction peaks (1 0 1) and (1 0 0), and this reveals the alignment of nanorods along the c-axis and the growth direction is perpendicular to the base surface. However, for the samples prepared using 6 h deposition time the (0 0 2) peak is less intense than the other peaks (1 0 1) and (1 0 0). This

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2θ(degrees) Fig. 2. X-ray diffraction pattern of the ZnO thin films prepared using (a) 4 h deposition time and (b) 6 h deposition time.

Fig. 3. Scanning electron microscope images of the ZnO thin films prepared using (a) 4 h deposition time and (b and c) 6 h deposition time.

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where a and c are the lattice parameters and d is the interplanar distance. Fig. 3(a and b) shows the scanning electron microscope (SEM) images of the zinc oxide samples prepared using 4 h and 6 h deposition time respectively. 3.1. Growth technique A simple chemical reaction mechanism is proposed for the formation of ZnO nanorods and bundle like ZnO nanorods. The growth process of ZnO can be explained by the following mechanism. C6 H12 N4 þ 6H2 Oƒƒƒ!6CH2 O þ 4NH3  NH3 þ H2 Oƒƒƒ!NHþ 4 þ OH

ZnðNO3 Þ2 ƒƒƒ!Zn2þ þ 2NO 3 2þ

Zn2þ þ 4NH3 ƒƒƒ!ZnðNH3 Þ4

Zn2þ þ 4OH ƒƒƒ!ZnðOHÞ2 4

in controlling the morphology of ZnO nanostructures (Guo et al., 2005). Huang et al. (2011) have reported that the photovoltaic performance of DSSCs are slightly improved on decrease in diameter and increase in length of nanorods. The chemical composition of the ZnO nanorods has been studied using energy dispersive X-ray (EDX) analysis and the EDAX pattern is shown in Fig. 4. The result shows the presence of 50.48 at.% of Zn and 49.52 at.% of O. Fig. 5 shows the HRTEM image of the ZnO samples prepared using the deposition time of 4 h and 6 h. The image shows lattice fringes and the d spacing is found to be 0.52 nm corresponding to the (1 0 0) plane. The inset SAED pattern shows the concentric rings corresponding to the different planes of hexagonal ZnO. Fig. 6 shows the absorbance spectra of ZnO nanorods. The absorption band edge of the aligned nanorods and bundle of nanorods are observed to be present at 360 nm and 375 nm respectively. The optical band gap energy has been calculated using the following equation

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ZnðOHÞ4 ƒƒƒ!ZnO þ H2 O þ 2OH The formation of nanorod and bundle like nanorod structures depends on the nucleation rate as well as the crystal growth rate. It is generally believed that crystal formation from solution has two stages namely crystal nucleation followed by crystal growth occurring from the crystal nuclei. In this chemical process (Zn(NO3)2 provides Zn2+ ions and H2O molecules in the solution provide –OH ions. During the growth period hexamethylenetetramine (C6H12N4) controls the concentration of –OH ions. Initially formed ZnO nuclei act as building blocks for the formation of bundle like nanorod structures. With increasing deposition time from 4 h to 6 h, the ZnO nuclei act as energetically favored sites for further deposition of incoming ZnO species, leading to the formation of bundle like ZnO nanorods in large quantity. For 6 h deposition time the rate of reaction decreases because of the depletion of the limiting reactants, thereby allowing the deposition of ZnO to follow more energetically favorable directions, resulting in anisotropic growth of agglomerates. Branches protrude from the surface of the agglomerates and extend in length, forming the observed bundle like nanorod structure. Fig. 3(a) shows the morphology of ZnO sample grown using 4 h deposition time. The ZnO nanorods are aligned vertically and the nanorods are typically about 200–250 nm in diameter with an average length of 1.5 lm. It is also observed that the nanorods uniformly cover the ITO substrate and have grown vertically on the ZnO seed layer. Fig. 3(b) shows the SEM image of the nanorods grown using 6 h deposition time. The 6 h deposition has favored the further deposition of seeds, leading to the formation of bundle like nanorods. Each bundle like ZnO nanorod contains more than fifty single nanorods and are arranged in a regular manner, and they are about 100 nm in diameter and the average length is 1.8 lm. Guo et al., have reported that growth time plays an important role

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Zn=49.65% O=50.35%

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Zn=49.38% O=50.62%

Fig. 4. EDAX pattern of ZnO thin film (a) 4 h deposition time and (b) 6 h deposition time.

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M. Raja et al. / Solar Energy xxx (2014) xxx–xxx

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Fig. 5. High resolution transmission electron microscope images, SAED pattern of the ZnO thin films prepared using (a and b) 4 h deposition time and (c and d) 6 h deposition time.

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wavelength (nm) Fig. 6. Absorbance spectra of the ZnO thin films prepared using (a) 4 h deposition time and (b) 6 h deposition time.

ðahtÞ ¼ Aðht  Eg Þn where A is a constant, Eg is the band gap of material, t is the frequency of the incident radiation, h is the Planck’s constant and the exponent n is 2 for direct band allowed transitions. The optical band gap of the aligned ZnO nanorods and bundle of ZnO nanorods has been determined using the above equation. Fig. 7 shows the (aht)2 vs. ht plot of ZnO nanorods. The band gap values have been determined by extrapolating the linear portion of the curve to meet the ht axis. The band gap has been calculated and

Fig. 7. Plot of (aht)2 vs. photon energy of the ZnO thin films prepared using (a) 4 h deposition time and (b) 6 h deposition time.

is found to be 3.27 eV and 3.23 eV for aligned ZnO nanorods and bundle of ZnO nanorods respectively. The photocurrent density–voltage (J–V) characteristics of vertically aligned nanorods and bundle of nanorods based dye sensitized solar cells are shown in Fig. 8. A short circuit current density (Jsc) of 2.21 mA/cm2, an open circuit voltage (Voc) of 0.64 V, a fill factor (FF) of 0.58 and an overall power conversion efficiency (g) of 0.83% is obtained for the bundle like nanorod based dye sensitized solar cells. The aligned ZnO nanorods based dye sensitized solar cells shows a short circuit current density (Jsc) of 2.01 mA/cm2, an open circuit voltage of (Voc) 0.63 V, a fill factor (FF) of

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the reason for the better efficiency exhibited by bundle of nanorod based solar cells. This efficiency is higher than the efficiency of 0.46% achieved in pillar shaped ZnO based dye sensitized solar cells prepared by Suh et al. (2007) and the efficiency of 0.6% obtained in branch shaped ZnO based dye sensitized solar cells prepared by Cheng et al. (2008).

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0.63 and an overall power conversion efficiency (g) of 0.74%. It is clear that there is a significant enhancement of the short-circuit current density (from 2.01 to 2.21 mA/cm2) in the bundle like ZnO nanostructure based solar cell, as compared with the ZnO nanorod solar cells. The increase in the short-circuit current density is mainly due to the fact that, the surface area of the nanorods in the photoelectrode is increased. The power conversion efficiency strongly depends on dye adsorption volume and surface area of ZnO. Compared with nanorods, the bundle like structures have more dye adsorption area and adsorbs more dye molecules. This improved surface area due to the specific morphology improves light collecting efficiency and finally increases the overall power conversion efficiency. The amount of dye molecules adsorbed on the ZnO surface was found using spectroscopic measurement of the dye desorbed from the surface. The ZnO films with bundle like structures adsorb more dye molecules (8.9  108 mol/cm2) than the rod like structures, resulting in an increase in the generation of electron–hole pairs and therefore the short-circuit current density. The amount of dye adsorption for rod like structure is 6.20  108 mol/cm2. Another reason for increase in efficiency can be attributed to the random multiple scattering of the light within the bundle like nanorod network leading to photon localization, thereby increasing the probability of the interaction between the photons and dye molecules and reduced recombination loss at the ZnO/dye/electrolyte interface. The bundle like structures shows reduced fill factor than the rod like structures. It has been found that the low fill factor may be attributed to the recombination of charges at the interface between ZnO nanorods and I/I3 electrolyte. The recombination is also motivated by the presence of uncovered nanoparticles on the surface. Even though FF in the bundle like structure is reduced, adsorption of more dye molecules and the increased Jsc compensates for the overall power conversion efficiency. This may be

ZnO thin films with two different morphologies were successfully grown by simple sol–gel method. The X-ray diffraction pattern reveals the formation of ZnO thin films with hexagonal phase. The scanning electron microscope image showed the formation of ZnO nanorods. HRTEM image shows lattice fringes and the d spacing is found to be 0.52 nm corresponding to the (1 0 0) plane. Dye-sensitized solar cells made from vertically aligned ZnO nanorods showed photocurrent of 2.01 mA/cm2 and overall efficiency of 0.74%. Dye-sensitized solar cells made from bundle like ZnO nanorods showed photocurrent of 2.21 mA/cm2 and overall efficiency of 0.83%. References Cheng, An-Jen, Tzeng, Yonhua, Zhou, Yi, Park, Minseo, Tsung-hsueh, Wu, Shannon, Curtis, Wang, Dake, Lee, Wonwoo, 2008. Thermal chemical vapor deposition growth of zinc oxide nanostructures for dye-sensitized solar cell fabrication. Appl. Phys. Lett. 92, 092113. Guo, M., Diao, P., Cai, S., 2005. Hydrothermal growth of well-aligned ZnO nanorod arrays: dependence of morphology and alignment ordering upon preparing conditions. J. Solid State Chem. 178, 1864. Hsu, Cheng-Liang, Chen, Kuan-Chao, Tsai, Tsung-Ying, Hsueh, TingJen, 2013. Fabrication of gas sensor based on p-type ZnO nanoparticles and n-type ZnO nanowires. Sens. Actuators, B 182, 190–196. Huang, Qiuliu, Fang, Liang, Chen, Xia, Saleem, Muhammad, 2011. Effect of polyethyleneimine on the growth of ZnO nanorod arrays and their application in dye-sensitized solar cells. J. Alloys Compd. 509, 9456– 9459. Huanga, N., Zhub, M.W., Gaoa, L.J., Gonga, J., Suna, C., Jianga, X., 2011. A template-free sol–gel technique for controlled growth of ZnO nanorod arrays. Appl. Surf. Sci. 257, 6026–6033. Irannejad, A., Janghorban, K., Tan, O.K., Huang, H., Lim, C.K., Tan, X., Fang, P.Y., Chua, C.S., Maleksaeedi, S., Hejazi, S.M.H., Shahjamali, M.M., Ghaffari, M., 2011. Effect of the TiO2 shell thickness on the dye-sensitized solar cells with ZnO–TiO2 core–shell nanorod electrodes. Electrochim. Acta 58, 19–24. Jiaqiang, Xu, Yuping, Chen, Daoyong, Chen, Jianian, Shen, 2006. Hydrothermal synthesis and gas sensing characters of ZnO nanorods. Sens. Actuators, B 113, 526–531. Lee, J.Y., Choi, Y.S., Kim, J.H., Park, M.O., Im, S., 2002. Optimizing nZnO/p-Si heterojunctions for photodiode applications. Thin Solid Films 403, 553–557. Lu, Lanlan, Li, Renjie, Fan, Ke, Peng, Tianyou, 2010. Effects of annealing conditions on the photoelectrochemical properties of dye-sensitized solar cells made with ZnO nanoparticles. Sol. Energy 84, 844–853. Pawar, R.C., Shaikh, J.S., Shinde, P.S., Pati, P.S., 2011a. Dye sensitized solar cells based on zinc oxide bottle brush. Mater. Lett. 65, 2235– 2237. Pawar, R.C., Shaikh, J.S., Babar, A.A., Dhere, P.M., Patil, P.S., 2011b. Aqueous chemical growth of ZnO disks, rods, spindles and flowers: pH dependency and photoelectrochemical properties. Sol. Energy 85, 1119–1127.

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