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Growth of aligned ZnO nanorods array on ITO for dye sensitized solar cell
Current Applied Physics 11 (2011) S113eS116
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Growth of aligned ZnO nanorods array on ITO for dye sensitized solar cell Ashish Yengantiwar a, *, Ramakant Sharma a, Onkar Game b, Arun Banpurkar c a
Department of Physics, Fergusson College, FC Road, Pune, Maharashtra 411004, India Physical and Materials Chemistry Division, National Chemical Laboratory, Pune, India c Center for Advanced Studies in Materials Science and Condensed Matter Physics, Department of Physics, University of Pune, Pune, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 September 2010 Received in revised form 29 November 2010 Accepted 29 November 2010 Available online 7 December 2010
Aligned ZnO films of nanorods arrays were grown on Indium doped Tin oxide (ITO) glass substrate for the Dye Sensitized Solar Cells (DSSCs) applications. Two-step approach was employed for the deposition, which offers advantages such as excellent surface adhesion, large area deposition, high reproducibility and yield. Structural characterization using X-ray diffraction (XRD) shows a preferred c-axis (002) oriented growth. Scanning Electron Microscopy (SEM) images reveals uniformly distributed hexagonal ZnO nanorods with average diameter varying from 80 to 250 nm. Average growth rate of the films was estimated using Surface Profilometer which was found to be w1 mm/h. Optical characterizations were carried out using Photo-spectrometer and Raman spectroscopy. The DSSCs using these ZnO films of nanorods array as photo-electrodes show conversion efficiency ranging from 0.24 to 0.71 %, the maximum efficiency was obtained for films deposited for time duration 8 h. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Aligned ZnO nanorod growth Open aqueous solution deposition Dye sensitized solar cell
1. Introduction Increasing concerns about the global warming have fuelled the intensive search for green energy harvesting technologies [1]. To meet these requirements, solar energy, is abundant and totally free, can be converted into electrical energy with emerging photovoltaic options of which excitonic solar cells are the most promising candidates because of an employment of low-cost materials and high possibility of being fabricated by large-scale and inexpensive techniques. Excitonic solar cell includes organic solar cells, hybrid solar cells, Dye Sensitized Solar Cells (DSSCs), etc. Wherein absorption of photon takes place due to presence of organic semiconductors which further leads to the production of electron-hole pair and this is completely different from inorganic solar cells where absorption of light leads to formation of electron-hole pair in bulk material itself [2,3]. Amongst all variants DSSCs, have an advantage of being flexible, inexpensive and ease of large-scale fabrication than conventional single crystal silicon solar cell. It consists of monolayer of dyes, photoanode made up of thick films of TiO2 or ZnO nanostructures grown on transparent conducting oxide substrates and solid or liquid electrolyte and platinum electrode acting as a cathode [4]. The dye monolayer creates excitons, a tightly bound electron-hole pair which must be split for charge generation. An electron is injected into the conduction band of host oxides (TiO2
* Corresponding author. Tel.: þ91 09970058332; fax: þ91 020 25691684. E-mail address: [email protected] (A. Yengantiwar). 1567-1739/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.11.111
or ZnO) available as nanostructure film and holes leaving the opposite side of device by means of redox species in liquid or solidstate electrolyte [5,6]. The bottleneck for achieving high power conversion efficiency is the competition between the generation and recombination of photoexcited carriers. In DSSCs, dense arrays of 1-dimensional (1D) nanostructure are of immense importance as an electron transport in 1D structure is expected to be several orders of magnitude faster due to less trapping site than the random network of other nanostructures, which ensures the rapid collection of photo carriers generated throughout the device [4,5]. ZnO a direct bandgap semiconducting material, has a tendency of forming self assembled nanostructures of specific orientation which propelled its use in nanogenerators [6], rectifying diodes [7], pH meter [8], sensors [9], and electron emitter [10,11]. In the recent past ZnO film was used in DSSCs, although it has less power conversion efficiency than what was reported for TiO2 but photonto-current conversion efficiency of these two oxides is comparable. Further, it was concluded that it might be due to carrier injection efficiency and dye compatibility [12,13]. Since then ZnO is considered as a potential material for DSSCs. Growth of vertically aligned 1D ZnO nanorods array on different substrates was being carried out by different methods such as Hydrothermal (HT) [14], Metal Organic Chemical Vapour Deposition (MOCVD) [15], Chemical Bath Deposition (CBD) [16], Spray pyrolysis (SP)[17], sol-gel [18], Electrochemical Deposition (ECD) [19] etc. Among all of these methods, even though hydrothermal method offers advantages of being environmental friendly, large area deposition and low production cost [20], it has disadvantages
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nanorods. The substrates were rinsed ultrasonically 10 min each with acetone, methanol, and trichloroethylene as cleansing agents. The procedure was repeated three times before substrates were used for ZnO seeding. The seed solution is spin coated on cleaned ITO at rotation speed of 2500 rpm for 90 s and was repeated five times for uniform and dense seeding. It is then annealed at 300 C for 1 h for better adherence of ZnO nano-seeds to the ITO substrate. It was accepted that the hydrolysis of HMT proceeds slowly above 70 C to produce ammonia (Eq. (1)). Ammonia reacts with zinc ions to produce Zn(OH)2 (Eq. (2)), then coordinate with NH3 to form [Zn(NH3)4]2þ (Eq. (3)). After that, according to Eqs. (4) and (5), ZnO molecule is obtained.
Fig. 1. X-ray diffraction pattern of ZnO films of nanorods deposited on ITO for time duration of 1e8 h. XRD of the pristine ITO substrate is shown in dotted line.
of having comparatively low yield and uneven deposition on substrates with large surface area. These disadvantages were partially overcome in an aqueous solution method [21,22]. This is two step process firstly transparent conductive oxide substrates were seeded with ZnO quantum dots either by electrodeposition or by spin coating technique which acts as nucleating sites for the growth. These substrates are then introduced to solution geometry either by mounting them in horizontal position with respect to bottom face of the container and seeded plane facing the bottom [21,22]. In the present study, we adopted a modified protocol for preparation of the seed solution and developed an Open Aqueous Solution Deposition (OASD) technique which allows a good control on the growth of the film, morphology, thickness and yield. 2. Experimental ZnO films of nano-rod are deposited using analytical grade precursor: Zinc acetate, Zinc Nitrate, Hexamethylene tetramine (HMT) and sodium hydroxide pellets all are procured from Merck Chemicals. Zinc acetate solution (5 mM) is prepared in methanol and kept stirring at 65 C for 45 min on temperature controlled hot plate with stirrer. The solution was allowed to cool until it reaches room temperature and then the sodium hydroxide solution (30 mM) was prepared in methanol and added drop wise till the solution turns slight milky which indicates onset of ZnO precipitation. This milky solution is then used for mother seed. ITO glasses (2.5 2.5 cm2) were used as substrates for growth of ZnO
C6H12N4 þ 6H2O / 6HCHO þ 4NH3
(1)
2Zn2þ þ 4NH3 þ 4H2O / 2Zn(OH)2 þ 4NHþ 4
(2)
2Zn(OH)2 þ 4NH3 / [Zn(NH3)4]2þ þ 2OH
(3)
Zn(NH3)4]2þ þ 4OH / ZnO2 2 þ 4NH3 þ 2H2O
(4)
ZnO2 2 þ H2O / ZnO þ 2OH
(5)
Growth of ZnO nanorod was takes place using equimolar (25 mM) aqueous solution of Zinc Nitrate and HMT separately prepared using demineralised (Milli Q water, r ¼ 18.2 mU). The solutions were then transferred into the crystallizing borosilicate container of 300 ml capacity. The container was kept on hot plate with stirrer and was allowed to stir slowly for obtaining the monophased and homogenous solution. The pre-seeded substrates are loaded on to a Teflon holder such that seeded surface was facing to bottom of container and holder was immersed into the reagent solution. The container was then covered using a commercially fabricated Teflon lead to arrest the evaporation. During the film growth, solution temperature was maintained at 95 5 C with simultaneous slow stirring. The deposition was carried out for time duration ranging from 15 min to 8 h. In one set of experiments the deposition is carried out without replacing reagent solution. In another set of experiments, after each hour of film deposition the reagent solution was replaced by a fresh stock solution and continued the deposition. The deposits were taken out after desired time intervals and rinsed in distilled water bath to wash out debris if any. Finally the deposits were annealed at 300 C for 1 h for the removal of moisture and improving the surface adhesion as well. X-ray diffraction (XRD) pattern were recorded at room temperature with (Bruker Axis D8 Advance) X-ray Diffractometer using CuKa radiation (l ¼ 1.5406 Å). The structural morphology of
Fig. 2. (a) and (b) shows SEM images for the ZnO film deposited for 1 h.
A. Yengantiwar et al. / Current Applied Physics 11 (2011) S113eS116
Fig. 3. Thickness of the ZnO nanorods film plotted against the deposition time for both sets one for replacing solution after every hour and another is without replacing the nutrient solution.
films was characterized using Field Emission Scanning Electron Microscopy (FESEM) operated at accelerating voltage range from 10 to 20 kV. Further ZnO phase formation is confirmed using Raman spectroscopy (Horiba Jobin Yvon LabRAM HR System). Optical characterization, UVeVis absorbance spectra were obtained using Scan UVeViseNIR V-670 (Jasco) Spectrophotometer. Solar cells were prepared by immersing the ZnO deposited films into a dye solution of 0.5 mM cisbis (isothiocyanato) bis (2,20 biipyridyl-4,4’-dicarboxylato)-ruthenium (II) bis-tetrabutylammonium (N719, Solaronix) in acetonitrile/tert-butanol (1:1) for 2 h. The sensitized electrodes were sandwiched together with thermally platinized FTO counter electrodes separated by 25 mm thick hotmelt spacers. The internal space of the cell was filled with an electrolyte solution (0.1 M LiI, 0.5 M 1,2-dimethyl-3-propylimidazolium iodide, 0.03 M I2, and 0.5 M tert-butylpyridine in acetonitrile). The area of active electrode was typically 0.25 cm2. Efficiencies for solar energy conversion were immediately evaluated under AM 1.5 simulated sunlight.
3. Results and discussion We grew the ZnO films of nanorods using OASD technique with and without replacing the reagent solution. As we show in the subsequent section that deposition in a same solution bath shows saturation in the film thickness, whereas when the solution is replaced after an hour shows increase in thickness with deposition. Henceforth we present all the results for the ZnO films of nanorods deposited by replacing a reagent solution after every hour of reaction time.
Fig. 4. UVeVis absorption spectra of ZnO nanorod films grown for time duration 1e8 h.
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Fig. 5. Raman spectra for the ZnO film of nanorods on ITO deposited for 1 h.
Fig. 1 shows the XRD patterns of the films grown on ITO substrate for deposition time 1e8 h. The peak in the XRD pattern reveals an oriented growth of ZnO in all the films. We confirmed from the JCPDS data (card no. 80-0075) that diffraction from the planes corresponding to the 2q values were attributed to ZnO wurtzite structures. Moreover intensity of the especially (002) plane is higher than that for (101) plane, the 100% intense plane in JCPDS data, emphasizes nanorods growth preferentially oriented along the c-axis. The film deposited for 1 h shows maximum diffraction intension to (002) plane and least to the others, highlight the significant growth of nanorods is oriented along c-axis. This is not valid for the films deposited for time duration more than 1 h, diffraction from (100), (101), (102) and (103) planes is an indicative of the lateral growth together with c-axis oriented nanorods. Fig. 2(a) and (b) show a ZnO film of nanorod deposited for 1 h time duration. Figure shows a growth of dense nanorods of tip diameter between 60 and 80 nm. We also observed similar growth morphology for the films deposited for different time duration (data is not shown). The thickness of films of nanorods is measured using surface profilometer and variation in the average thickness is plotted against deposition time (see Fig. 3). As mentioned before, thickness of the films deposited without replacing a reagent solution is also shown in the plot. Deposition without replacing nutrient solution shows saturation in the growth after about 2 h. This is mainly due to the depletion of the free Znþþ in the solution. This discrepancy is overcome by replacing a nutrient solution by fresh one. The solution supply the nascent Znþþ for further growth on the previous nanorods and thus films of desired height is possible to grow on same substrate using this improved technique. Replacing the nutrient
Fig. 6. J-V characteristic of DSSCs using ZnO films of nanorods as photo-electrodes.
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Table 1 Performance of Solar cells using films of ZnO nanorods as photo-electrodes. Deposition Time (h)
Jsc (mA/cm2)
Isc (mA)
Voc (mV)
FF (%)
PCE h (%)
1 3 5 6 7 8
1.12 1.52 1.40 1.42 1.86 2.59
0.28 0.38 0.35 0.35 0.46 0.65
706 603 672 685 702 663
29.63 41 40.11 39.37 37.60 40.67
0.240 0.370 0.379 0.384 0.491 0.710
solution the growth rate w0.9 mm/h is achieved in study which is high in comparison with reported value [21]. The room temperature UVeVis absorbance spectra of the films of nanorods grown on the ITO substrate are shown in the Fig. 4. The figure shows thickness dependence absorption. Moreover, the absorption spectra replicates well defined exciton band at 374 nm indicates that optical properties of ZnO films of nanorods are similar to reported literature. The crystalline and structural quality of the deposited files is also analyzed using Raman technique. Fig. 5 shows the Raman absorption of the ZnO film deposited for 1 h. Raman spectra are sensitive to crystallization, structural disorder, and defects in micro-nanostructures. The appearance of a dominant, sharp, and strong peak at 438 cm1 is attributed to the Raman active optical phonon E2 (high) mode of the ZnO, confirming that the as-prepared nanorods have the wurtzite hexagonal phase. The peak at 329 cm1 is also observed, which can be attributed to the second-order Raman processes. The peak at 98 cm1 is attributed to the E2 (low) mode, which might be observed due to formation of defects such as zinc interstitials and oxygen vacancies. Therefore, the sharp and dominant E2 (high) mode with weak E2 (low) mode in the Raman spectrum indicates that the as-prepared ZnO nanorods are highly crystalline with less structural defects. As we can see, the remarkable E2 (low) and E2 (high) mode of ZnO are located at 98 and 438 cm1, respectively. The peak at 329 cm1 can be assigned to the secondorder Raman scattering arising from zone-boundary phonons 2-E2 (M) of ZnO. Fig. 6 shows photocurrentevoltage (JeV) characteristics for DSSCs using ZnO films of nanorods arrays deposited for different time duration. The cell performance were measured under 1 Sun AM 1.5 simulated sun light with an active area of 0.25 cm2. The fundamentals results for ZnO films have been shown in Table 1. Table 1 shows that power conversion efficiency increases with the film thickness, the best worked cell with maximum efficiency was obtained for the films deposited for 8 h which has maximum thickness (see Fig. 3). The power conversion efficiency h ¼ 0.71%, open circuit voltage (Voc) of 0.66 V, fill factor (FF) of 40.67% and short circuit photo-current density (Jsc) of 2.59 mA/cm2. The efficiency in the present study is less compared to the efficiency reported in the literature may be due to the thickness of ZnO films,
which need a further attention. The amount of dye loading into the films and etching of ZnO layer is also major factors affects the overall efficiency. 4. Conclusion The ZnO films of nanorods arrays were successfully grown on ITO substrates using aqueous solution method for DSSCs applications. The thickness of films was linearly increased with deposition time. These films were used as photo electrodes for DSSCs. The nanorods film deposited for 8 h gave maximum conversion efficiency of h ¼ 0.71%. It was observed that efficiency depends on film thickness, nature of the dye molecule and their adsorption on active film surface, which are currently under investigation. Acknowledgements The authors wish to thank Dr. Satishchandra B. Ogale, NCL, for supporting the facilities for solar cell characterisations. The authors also would like to acknowledge the financial support of BCUD, University of Pune. References [1] A.D. Pausquier, H. Chen, Y. Lu, Appl. Phys. Lett. 89 (2006) 253513. [2] R. Thitima, C. Patcharee, S. Takashi, Y. Susumu, Solid-State Electro. 53 (2009) 176. [3] I. Gonzalez-Valls, M. Lira-Cantu, Energy Environ. Sci. 2 (2009) 19. [4] H. Cheng, W. Chiu, C. Lee, S. Tsai, W. Hsieh, J. Phys, Chem. C 112 (42) (2008) 16359. [5] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P. Yang, Lett. Nat. Mat. 4 (2005) 455. [6] Z.L. Wang, J. Song, Science 312 (2006) 242. [7] C.S. Lao, J. Liu, P. Gao, L. Zhang, D. Davidovic, T. Rao, Z.L. Wang, Nano Lett. 6 (2006) 263. [8] B.S. Kang, F. Ren, Y.W. Heo, L.C. Tien, D.P. Norton, S.J. Pearton, Appl. Phys. Lett. 86 (2005) 112105. [9] C.S. Rout, S.H. Krishna, S.R.C. Vivekchand, A. Govindaraj, C.N.R. Rao, Chem. Phys. Lett. 418 (2006) 586. [10] A. Wei, X.W. Sun, C.X. Xu, Z.L. Dong, M.B. Yu, W. Huang, Appl. Phys. Lett. 88 (2006) 213102. [11] D. Pradhan, K.T. Leung, Langmuir 24 (17) (2008) 9707. [12] Y. Lee, Y. Zhang, S.L. Geok Ng, F.C. Kartawidjaja, J. Wang, J. Am, Ceram. Soc. 92 (9) (2009) 1940. [13] H. Rensmo, K. Keis, H. Lindstrom, S. Sodergren, A. Solbrand, A. Hagfeldt, S.E. Lindquist, L.N. Wang, M. Muhammed, J. Phys. Chem. B. 101 (14) (1997) 2598. [14] C.Y. Jiang, X.W. Sun, G.Q. Lo, D.L. Kwong, Appl. Phys. Lett. 90 (2007) 263501. [15] K.M. Coakley, Y. Liu, C. Goh, D. McGehee, Mater. Res. Bull. 30 (2005) 37. [16] W. Lee, S.K. Min, V. Dhas, S.B. Ogale, S. Han, Electrochem. Comm 11 (2009) 103. [17] N.O.V. Plank, M.E. Welland, J.L. MacManus-Driscoll, L. Schmidt-Mende, Thin Solid Films 516 (20) (2008) 7218. [18] S. Pang, T. Xie, Y. Zhang, X. Wei, M. Yang, D. Wang, Z. Du, J. Phys. Chem. C 111 (2007) 18417. [19] T. Yoshida, K. Terada, T. Sugiura, H. Minoura, Adv. Mater. 12 (16) (2000) 1214. [20] N.G. Ndifor-Angwafor, D. Riley, J. Phys. Stat. Sol 205 (10) (2008) 2351. [21] Y. Gao, M. Nagai, Langmuir 22 (2006) 3936. [22] R. Chander, A.K. Raichaudhuri, J. Mater. Sci. 41 (2006) 3623.
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Growth of aligned ZnO nanorods array on ITO for dye sensitized solar cell Ashish Yengantiwar a, *, Ramakant Sharma a, Onkar Game b, Arun Banpurkar c a
Department of Physics, Fergusson College, FC Road, Pune, Maharashtra 411004, India Physical and Materials Chemistry Division, National Chemical Laboratory, Pune, India c Center for Advanced Studies in Materials Science and Condensed Matter Physics, Department of Physics, University of Pune, Pune, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 September 2010 Received in revised form 29 November 2010 Accepted 29 November 2010 Available online 7 December 2010
Aligned ZnO films of nanorods arrays were grown on Indium doped Tin oxide (ITO) glass substrate for the Dye Sensitized Solar Cells (DSSCs) applications. Two-step approach was employed for the deposition, which offers advantages such as excellent surface adhesion, large area deposition, high reproducibility and yield. Structural characterization using X-ray diffraction (XRD) shows a preferred c-axis (002) oriented growth. Scanning Electron Microscopy (SEM) images reveals uniformly distributed hexagonal ZnO nanorods with average diameter varying from 80 to 250 nm. Average growth rate of the films was estimated using Surface Profilometer which was found to be w1 mm/h. Optical characterizations were carried out using Photo-spectrometer and Raman spectroscopy. The DSSCs using these ZnO films of nanorods array as photo-electrodes show conversion efficiency ranging from 0.24 to 0.71 %, the maximum efficiency was obtained for films deposited for time duration 8 h. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Aligned ZnO nanorod growth Open aqueous solution deposition Dye sensitized solar cell
1. Introduction Increasing concerns about the global warming have fuelled the intensive search for green energy harvesting technologies [1]. To meet these requirements, solar energy, is abundant and totally free, can be converted into electrical energy with emerging photovoltaic options of which excitonic solar cells are the most promising candidates because of an employment of low-cost materials and high possibility of being fabricated by large-scale and inexpensive techniques. Excitonic solar cell includes organic solar cells, hybrid solar cells, Dye Sensitized Solar Cells (DSSCs), etc. Wherein absorption of photon takes place due to presence of organic semiconductors which further leads to the production of electron-hole pair and this is completely different from inorganic solar cells where absorption of light leads to formation of electron-hole pair in bulk material itself [2,3]. Amongst all variants DSSCs, have an advantage of being flexible, inexpensive and ease of large-scale fabrication than conventional single crystal silicon solar cell. It consists of monolayer of dyes, photoanode made up of thick films of TiO2 or ZnO nanostructures grown on transparent conducting oxide substrates and solid or liquid electrolyte and platinum electrode acting as a cathode [4]. The dye monolayer creates excitons, a tightly bound electron-hole pair which must be split for charge generation. An electron is injected into the conduction band of host oxides (TiO2
* Corresponding author. Tel.: þ91 09970058332; fax: þ91 020 25691684. E-mail address: [email protected] (A. Yengantiwar). 1567-1739/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.11.111
or ZnO) available as nanostructure film and holes leaving the opposite side of device by means of redox species in liquid or solidstate electrolyte [5,6]. The bottleneck for achieving high power conversion efficiency is the competition between the generation and recombination of photoexcited carriers. In DSSCs, dense arrays of 1-dimensional (1D) nanostructure are of immense importance as an electron transport in 1D structure is expected to be several orders of magnitude faster due to less trapping site than the random network of other nanostructures, which ensures the rapid collection of photo carriers generated throughout the device [4,5]. ZnO a direct bandgap semiconducting material, has a tendency of forming self assembled nanostructures of specific orientation which propelled its use in nanogenerators [6], rectifying diodes [7], pH meter [8], sensors [9], and electron emitter [10,11]. In the recent past ZnO film was used in DSSCs, although it has less power conversion efficiency than what was reported for TiO2 but photonto-current conversion efficiency of these two oxides is comparable. Further, it was concluded that it might be due to carrier injection efficiency and dye compatibility [12,13]. Since then ZnO is considered as a potential material for DSSCs. Growth of vertically aligned 1D ZnO nanorods array on different substrates was being carried out by different methods such as Hydrothermal (HT) [14], Metal Organic Chemical Vapour Deposition (MOCVD) [15], Chemical Bath Deposition (CBD) [16], Spray pyrolysis (SP)[17], sol-gel [18], Electrochemical Deposition (ECD) [19] etc. Among all of these methods, even though hydrothermal method offers advantages of being environmental friendly, large area deposition and low production cost [20], it has disadvantages
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nanorods. The substrates were rinsed ultrasonically 10 min each with acetone, methanol, and trichloroethylene as cleansing agents. The procedure was repeated three times before substrates were used for ZnO seeding. The seed solution is spin coated on cleaned ITO at rotation speed of 2500 rpm for 90 s and was repeated five times for uniform and dense seeding. It is then annealed at 300 C for 1 h for better adherence of ZnO nano-seeds to the ITO substrate. It was accepted that the hydrolysis of HMT proceeds slowly above 70 C to produce ammonia (Eq. (1)). Ammonia reacts with zinc ions to produce Zn(OH)2 (Eq. (2)), then coordinate with NH3 to form [Zn(NH3)4]2þ (Eq. (3)). After that, according to Eqs. (4) and (5), ZnO molecule is obtained.
Fig. 1. X-ray diffraction pattern of ZnO films of nanorods deposited on ITO for time duration of 1e8 h. XRD of the pristine ITO substrate is shown in dotted line.
of having comparatively low yield and uneven deposition on substrates with large surface area. These disadvantages were partially overcome in an aqueous solution method [21,22]. This is two step process firstly transparent conductive oxide substrates were seeded with ZnO quantum dots either by electrodeposition or by spin coating technique which acts as nucleating sites for the growth. These substrates are then introduced to solution geometry either by mounting them in horizontal position with respect to bottom face of the container and seeded plane facing the bottom [21,22]. In the present study, we adopted a modified protocol for preparation of the seed solution and developed an Open Aqueous Solution Deposition (OASD) technique which allows a good control on the growth of the film, morphology, thickness and yield. 2. Experimental ZnO films of nano-rod are deposited using analytical grade precursor: Zinc acetate, Zinc Nitrate, Hexamethylene tetramine (HMT) and sodium hydroxide pellets all are procured from Merck Chemicals. Zinc acetate solution (5 mM) is prepared in methanol and kept stirring at 65 C for 45 min on temperature controlled hot plate with stirrer. The solution was allowed to cool until it reaches room temperature and then the sodium hydroxide solution (30 mM) was prepared in methanol and added drop wise till the solution turns slight milky which indicates onset of ZnO precipitation. This milky solution is then used for mother seed. ITO glasses (2.5 2.5 cm2) were used as substrates for growth of ZnO
C6H12N4 þ 6H2O / 6HCHO þ 4NH3
(1)
2Zn2þ þ 4NH3 þ 4H2O / 2Zn(OH)2 þ 4NHþ 4
(2)
2Zn(OH)2 þ 4NH3 / [Zn(NH3)4]2þ þ 2OH
(3)
Zn(NH3)4]2þ þ 4OH / ZnO2 2 þ 4NH3 þ 2H2O
(4)
ZnO2 2 þ H2O / ZnO þ 2OH
(5)
Growth of ZnO nanorod was takes place using equimolar (25 mM) aqueous solution of Zinc Nitrate and HMT separately prepared using demineralised (Milli Q water, r ¼ 18.2 mU). The solutions were then transferred into the crystallizing borosilicate container of 300 ml capacity. The container was kept on hot plate with stirrer and was allowed to stir slowly for obtaining the monophased and homogenous solution. The pre-seeded substrates are loaded on to a Teflon holder such that seeded surface was facing to bottom of container and holder was immersed into the reagent solution. The container was then covered using a commercially fabricated Teflon lead to arrest the evaporation. During the film growth, solution temperature was maintained at 95 5 C with simultaneous slow stirring. The deposition was carried out for time duration ranging from 15 min to 8 h. In one set of experiments the deposition is carried out without replacing reagent solution. In another set of experiments, after each hour of film deposition the reagent solution was replaced by a fresh stock solution and continued the deposition. The deposits were taken out after desired time intervals and rinsed in distilled water bath to wash out debris if any. Finally the deposits were annealed at 300 C for 1 h for the removal of moisture and improving the surface adhesion as well. X-ray diffraction (XRD) pattern were recorded at room temperature with (Bruker Axis D8 Advance) X-ray Diffractometer using CuKa radiation (l ¼ 1.5406 Å). The structural morphology of
Fig. 2. (a) and (b) shows SEM images for the ZnO film deposited for 1 h.
A. Yengantiwar et al. / Current Applied Physics 11 (2011) S113eS116
Fig. 3. Thickness of the ZnO nanorods film plotted against the deposition time for both sets one for replacing solution after every hour and another is without replacing the nutrient solution.
films was characterized using Field Emission Scanning Electron Microscopy (FESEM) operated at accelerating voltage range from 10 to 20 kV. Further ZnO phase formation is confirmed using Raman spectroscopy (Horiba Jobin Yvon LabRAM HR System). Optical characterization, UVeVis absorbance spectra were obtained using Scan UVeViseNIR V-670 (Jasco) Spectrophotometer. Solar cells were prepared by immersing the ZnO deposited films into a dye solution of 0.5 mM cisbis (isothiocyanato) bis (2,20 biipyridyl-4,4’-dicarboxylato)-ruthenium (II) bis-tetrabutylammonium (N719, Solaronix) in acetonitrile/tert-butanol (1:1) for 2 h. The sensitized electrodes were sandwiched together with thermally platinized FTO counter electrodes separated by 25 mm thick hotmelt spacers. The internal space of the cell was filled with an electrolyte solution (0.1 M LiI, 0.5 M 1,2-dimethyl-3-propylimidazolium iodide, 0.03 M I2, and 0.5 M tert-butylpyridine in acetonitrile). The area of active electrode was typically 0.25 cm2. Efficiencies for solar energy conversion were immediately evaluated under AM 1.5 simulated sunlight.
3. Results and discussion We grew the ZnO films of nanorods using OASD technique with and without replacing the reagent solution. As we show in the subsequent section that deposition in a same solution bath shows saturation in the film thickness, whereas when the solution is replaced after an hour shows increase in thickness with deposition. Henceforth we present all the results for the ZnO films of nanorods deposited by replacing a reagent solution after every hour of reaction time.
Fig. 4. UVeVis absorption spectra of ZnO nanorod films grown for time duration 1e8 h.
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Fig. 5. Raman spectra for the ZnO film of nanorods on ITO deposited for 1 h.
Fig. 1 shows the XRD patterns of the films grown on ITO substrate for deposition time 1e8 h. The peak in the XRD pattern reveals an oriented growth of ZnO in all the films. We confirmed from the JCPDS data (card no. 80-0075) that diffraction from the planes corresponding to the 2q values were attributed to ZnO wurtzite structures. Moreover intensity of the especially (002) plane is higher than that for (101) plane, the 100% intense plane in JCPDS data, emphasizes nanorods growth preferentially oriented along the c-axis. The film deposited for 1 h shows maximum diffraction intension to (002) plane and least to the others, highlight the significant growth of nanorods is oriented along c-axis. This is not valid for the films deposited for time duration more than 1 h, diffraction from (100), (101), (102) and (103) planes is an indicative of the lateral growth together with c-axis oriented nanorods. Fig. 2(a) and (b) show a ZnO film of nanorod deposited for 1 h time duration. Figure shows a growth of dense nanorods of tip diameter between 60 and 80 nm. We also observed similar growth morphology for the films deposited for different time duration (data is not shown). The thickness of films of nanorods is measured using surface profilometer and variation in the average thickness is plotted against deposition time (see Fig. 3). As mentioned before, thickness of the films deposited without replacing a reagent solution is also shown in the plot. Deposition without replacing nutrient solution shows saturation in the growth after about 2 h. This is mainly due to the depletion of the free Znþþ in the solution. This discrepancy is overcome by replacing a nutrient solution by fresh one. The solution supply the nascent Znþþ for further growth on the previous nanorods and thus films of desired height is possible to grow on same substrate using this improved technique. Replacing the nutrient
Fig. 6. J-V characteristic of DSSCs using ZnO films of nanorods as photo-electrodes.
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Table 1 Performance of Solar cells using films of ZnO nanorods as photo-electrodes. Deposition Time (h)
Jsc (mA/cm2)
Isc (mA)
Voc (mV)
FF (%)
PCE h (%)
1 3 5 6 7 8
1.12 1.52 1.40 1.42 1.86 2.59
0.28 0.38 0.35 0.35 0.46 0.65
706 603 672 685 702 663
29.63 41 40.11 39.37 37.60 40.67
0.240 0.370 0.379 0.384 0.491 0.710
solution the growth rate w0.9 mm/h is achieved in study which is high in comparison with reported value [21]. The room temperature UVeVis absorbance spectra of the films of nanorods grown on the ITO substrate are shown in the Fig. 4. The figure shows thickness dependence absorption. Moreover, the absorption spectra replicates well defined exciton band at 374 nm indicates that optical properties of ZnO films of nanorods are similar to reported literature. The crystalline and structural quality of the deposited files is also analyzed using Raman technique. Fig. 5 shows the Raman absorption of the ZnO film deposited for 1 h. Raman spectra are sensitive to crystallization, structural disorder, and defects in micro-nanostructures. The appearance of a dominant, sharp, and strong peak at 438 cm1 is attributed to the Raman active optical phonon E2 (high) mode of the ZnO, confirming that the as-prepared nanorods have the wurtzite hexagonal phase. The peak at 329 cm1 is also observed, which can be attributed to the second-order Raman processes. The peak at 98 cm1 is attributed to the E2 (low) mode, which might be observed due to formation of defects such as zinc interstitials and oxygen vacancies. Therefore, the sharp and dominant E2 (high) mode with weak E2 (low) mode in the Raman spectrum indicates that the as-prepared ZnO nanorods are highly crystalline with less structural defects. As we can see, the remarkable E2 (low) and E2 (high) mode of ZnO are located at 98 and 438 cm1, respectively. The peak at 329 cm1 can be assigned to the secondorder Raman scattering arising from zone-boundary phonons 2-E2 (M) of ZnO. Fig. 6 shows photocurrentevoltage (JeV) characteristics for DSSCs using ZnO films of nanorods arrays deposited for different time duration. The cell performance were measured under 1 Sun AM 1.5 simulated sun light with an active area of 0.25 cm2. The fundamentals results for ZnO films have been shown in Table 1. Table 1 shows that power conversion efficiency increases with the film thickness, the best worked cell with maximum efficiency was obtained for the films deposited for 8 h which has maximum thickness (see Fig. 3). The power conversion efficiency h ¼ 0.71%, open circuit voltage (Voc) of 0.66 V, fill factor (FF) of 40.67% and short circuit photo-current density (Jsc) of 2.59 mA/cm2. The efficiency in the present study is less compared to the efficiency reported in the literature may be due to the thickness of ZnO films,
which need a further attention. The amount of dye loading into the films and etching of ZnO layer is also major factors affects the overall efficiency. 4. Conclusion The ZnO films of nanorods arrays were successfully grown on ITO substrates using aqueous solution method for DSSCs applications. The thickness of films was linearly increased with deposition time. These films were used as photo electrodes for DSSCs. The nanorods film deposited for 8 h gave maximum conversion efficiency of h ¼ 0.71%. It was observed that efficiency depends on film thickness, nature of the dye molecule and their adsorption on active film surface, which are currently under investigation. Acknowledgements The authors wish to thank Dr. Satishchandra B. Ogale, NCL, for supporting the facilities for solar cell characterisations. The authors also would like to acknowledge the financial support of BCUD, University of Pune. References [1] A.D. Pausquier, H. Chen, Y. Lu, Appl. Phys. Lett. 89 (2006) 253513. [2] R. Thitima, C. Patcharee, S. Takashi, Y. Susumu, Solid-State Electro. 53 (2009) 176. [3] I. Gonzalez-Valls, M. Lira-Cantu, Energy Environ. Sci. 2 (2009) 19. [4] H. Cheng, W. Chiu, C. Lee, S. Tsai, W. Hsieh, J. Phys, Chem. C 112 (42) (2008) 16359. [5] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P. Yang, Lett. Nat. Mat. 4 (2005) 455. [6] Z.L. Wang, J. Song, Science 312 (2006) 242. [7] C.S. Lao, J. Liu, P. Gao, L. Zhang, D. Davidovic, T. Rao, Z.L. Wang, Nano Lett. 6 (2006) 263. [8] B.S. Kang, F. Ren, Y.W. Heo, L.C. Tien, D.P. Norton, S.J. Pearton, Appl. Phys. Lett. 86 (2005) 112105. [9] C.S. Rout, S.H. Krishna, S.R.C. Vivekchand, A. Govindaraj, C.N.R. Rao, Chem. Phys. Lett. 418 (2006) 586. [10] A. Wei, X.W. Sun, C.X. Xu, Z.L. Dong, M.B. Yu, W. Huang, Appl. Phys. Lett. 88 (2006) 213102. [11] D. Pradhan, K.T. Leung, Langmuir 24 (17) (2008) 9707. [12] Y. Lee, Y. Zhang, S.L. Geok Ng, F.C. Kartawidjaja, J. Wang, J. Am, Ceram. Soc. 92 (9) (2009) 1940. [13] H. Rensmo, K. Keis, H. Lindstrom, S. Sodergren, A. Solbrand, A. Hagfeldt, S.E. Lindquist, L.N. Wang, M. Muhammed, J. Phys. Chem. B. 101 (14) (1997) 2598. [14] C.Y. Jiang, X.W. Sun, G.Q. Lo, D.L. Kwong, Appl. Phys. Lett. 90 (2007) 263501. [15] K.M. Coakley, Y. Liu, C. Goh, D. McGehee, Mater. Res. Bull. 30 (2005) 37. [16] W. Lee, S.K. Min, V. Dhas, S.B. Ogale, S. Han, Electrochem. Comm 11 (2009) 103. [17] N.O.V. Plank, M.E. Welland, J.L. MacManus-Driscoll, L. Schmidt-Mende, Thin Solid Films 516 (20) (2008) 7218. [18] S. Pang, T. Xie, Y. Zhang, X. Wei, M. Yang, D. Wang, Z. Du, J. Phys. Chem. C 111 (2007) 18417. [19] T. Yoshida, K. Terada, T. Sugiura, H. Minoura, Adv. Mater. 12 (16) (2000) 1214. [20] N.G. Ndifor-Angwafor, D. Riley, J. Phys. Stat. Sol 205 (10) (2008) 2351. [21] Y. Gao, M. Nagai, Langmuir 22 (2006) 3936. [22] R. Chander, A.K. Raichaudhuri, J. Mater. Sci. 41 (2006) 3623.