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Novel Sol-gel Route to Synthesize Transparent Conductive Oxides for Front Contact of Solar Cells
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 2 (2015) 5145 – 5149
International Conference on Solid State Physics 2013 (ICSSP’13)
Novel sol-gel route to synthesize transparent conductive oxides for front contact of solar cells Ans Farooq a*, Nazar Abbasb, M. Kamranc a
Department of Mechanical Engineering, Mohammad Ali Jinnah University, Islamabad Campus. b Department of Physics, COMSATS Institute of Information Technology, Islamabad, Pakistan. c Department of Electrical Engineering, COMSATS Institute of Information Technology, Islamabad, Pakistan.
Abstract
The thin films of transparent conductive zinc oxide (ZnO) are synthesized by novel sol gel spin coating method. We deposit thin films as primary application of front contact of the thin film solar cells such as Cu2 (Zn,Sn)S4(CZTS). The thin films are deposited by varying the molar concentration of the zinc oxide solution with different molar concentrations. The effect of molar concentration and annealing are temperature dependent. The main orientation of the deposited ZnO are observed at (101) peak by X-Ray diffraction (XRD) micrographs. We observed the optical transmittance of our samples by optical spectroscopy and one of them showed the transmittance more than 90%. The deposited thin films show high transparency and are the potential candidate to be used as window layer in thin film based solar cells. © 2015 2015Elsevier ElsevierLtd. Ltd. rights reserved. © AllAll rights reserved. Selectionand andPeer-review Peer-review under responsibility of Committee the Committee Members of International Conference Solid State Physics Selection under responsibility of the Members of International Conference on Solidon State Physics 2013 2013(ICSSP’13) (ICSSP’13). Keywords: Sol-gel; Transparent conducting oxides; Solar cells; Zinc oxide
* Corresponding author. E-mail address: [email protected]
2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the Committee Members of International Conference on Solid State Physics 2013 (ICSSP’13) doi:10.1016/j.matpr.2015.11.135
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Ans Farooq et al. / Materials Today: Proceedings 2 (2015) 5145 – 5149
1. Introduction Transparent conductive oxides (TCOs) thin films are optically transparent and electrically conductive [1]. TCOs are the binary or tertiary compounds of Oxides. TCOs have vast applications in various fields of technology and have potential applications in LEDs, OLEDs, Solar Cells, Transparent Displays, Water Treatment and many more [1-3]. Zinc Oxide is the member of TCOs and it has the potential to replace the expensive and rare Indium Tin Oxide (ITO) in various applications especially in window layer of thin film based solar cells.Cu2ZnSnS4 (CZTS) is one of the most promising absorber layer materials for low-cost thin film solar cells. For this type of solar cells we need a cost effective front contact or window layer that is of Zinc Oxide [5]. Zinc Oxide thin films are highly transparent in UV/Vis region and also have very good electrical conductivity [6-7]. 2. Experimental procedure Many synthesis techniques are reported for the preparation of TCOs like Chemical vapor deposition (CVD) [1], Pulsed laser deposition (PLD) [2] and Physical sputtering but in this work we used novel sol-gel route to prepare ZnO thin films [4]. As sol-gel involves less complications and no vacuum is needed, cost effective and easy to understand the whole procedure.
Fig. 1. Synthesis Route to deposit ZnO Thin Films
Soda lime glass (SLG) used as a substrate. First of all the glass ware and glass slides are washed with detergent and then dipped in chromic acid for 15 minutes, after that the glass wares and substrate was washed with methanol and put them in oven at 200oC for 10 minutes for evaporation of wet residue and to remove containments on the substrate. Zinc acetate di hydrate (ZAD) is used as precursor. Firstly ZAD is dissolved in 2-Proponal [iPrOH, (CH3)2CHOH] and Mono-Ethanol Amine [MEA, (HOCH2CH2)NH2] added as a stabilizer, the MEA:Zn+2ratios is maintained at 0.75-1. The sol is than put on a magnetic stirrer at normal speed and with a temperature of 60oC until the ZAD is completely dissolved. Normally ZAD is dissolved with in 2 hr on magnetic stirring. We prepared 3 different solutions (I) 0.1 Molar ZAD in iso-Propanol (II) 0.3 Molar ZAD in Iso-Propanol (III) 0.5 Molar ZAD in iso-Propanol Filter the sol to filter out the bigger particles and toget a sol of same particle size, for this we use syringe filter (Sartorius-Stedim Minisart NML) of 20 um pore size, which allows only the particles smaller or equal to 20 micron to pass through the filter. Controlled Spin Coating (CSC) unit spins at 2000 RPM for 30 seconds. Deposited thin film is than put into oven for 10 min at 200oC to remove excess water and containments. This
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Ans Farooq et al. / Materials Today: Proceedings 2 (2015) 5145 – 5149
procedure is repeated 3 times to gain a smooth surface of thin film to deposit each and every corner, so no part will left behind un-deposited of thin film. The deposited thin films of ZnO are annealed at 4500C for 1 hr. 3. Results and Discussion X-Ray Diffraction is performed to confirm that the deposited thin films are of good quality of ZnO. To find the orientation of the deposited thin films are made according to the given formulas purposed by Lotgering [6].
(1)
Where f(hkl) is the degree of (hkl) orientation,
(2)
(3)
I(hkl) is intensity of (hkl) peak , ΣI(hkl) is sum of intensities of all diffraction peaks in recorded data. Whereas Io(hkl) is intensity of (hkl) peak and ΣIo(hkl) is sum of intensities of all peaks in the recorded diffraction data of the standard card JCPDS 36-1451 It is observed that 0.5 Molar ZnO thin films annealed at 450 °C for 1 hour exhibited a higher degree of orientation of the (101) plane compared to the films with lower molar concentration. Intensity of (101) peak can also be analyzed using following relation:
(4)
(101) orientation of ZnO thin films is kinetically preferred. (101) plane possesses the higher grain-packing density and highest density of Zn atoms is found along this plane.
Fig. 2: XRD pattern of annealed (ann) and un-Annealed (un) samples of deposited ZnO thin films
The crystallinity of thin film improves as the sol concentration increases. We observed that crystal size increases with increase in molar concentration of ZnO solution in deposited annealed thin films. Scanning Electron
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Ans Farooq et al. / Materials Today: Proceedings 2 (2015) 5145 – 5149
Microscopy (SEM) is only done for annealed samples. A typical flake like morphology is observed in all deposited thin films.
Fig. 3. SEM micrograph of (a) 0.1 Molar (b) 0.3 Molar (c) 0.5 Molar concentration ZnO deposited thin films.
From SEM micrograph we can observe that particle size is uniform and surface morphology is smooth. High resistivity is the key for maximum optical transmittance. The most important part of this work is the Optical characteristics of deposited ZnO thin films, as we are finding a molar concentration of samples which is better for its use as window layer in thin film based solar cells; this can only happen if the sample can transmit the maximum of the incident light [5]. The reflection and absorbance must be minimal to gain the best results, so by the help of optical spectroscopy all the samples were characterized for optical transmittance. All the samples are observed in the visible range of electromagnetic spectrum from 300nm-700nm.
Ans Farooq et al. / Materials Today: Proceedings 2 (2015) 5145 – 5149
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Fig. 4. Optical Transmittance of annealed and un-annealed ZnO thin films.
In fig 4 we can observe that 0.5 M (dotted line) thin film shows the maximum optical transmittance (greater than 90%) and 0.1 M thin film is only at 70% optical transmittance. Absorption edge is at 375nm for almost all deposited thin films [7]. 4. Conclusions Highly oriented Zinc Oxide thin films prepared by sol-gel spin coating technique. All the deposited thin films are compact and dense in nature and crystallinity improves with increase in molar concentration of zinc oxide solution. Preferred peak of orientation is (101) which are observed by the help of XRD data. XRD analysis shows that all the deposited thin films have hexagonal wurtzite crystal and all are Nano-crystalline. It is observed that after annealing the deposited thin films features become more prominent. After annealing there was a considerable change in the crystallite size. 0.5 M thin film has the maximum optical transmittance of above 90% which is the good candidate for its use as window layer in CZTS based solar cells. References [1] W. Lin, R. Ma, J. Xue, B. Kang, Sol. Energy Mater. Sol. Cells 91 (2007) 1902–5. [2] J. Ebothe, I.V. Kityk, S. Benet, B. Claudet, K. J. Plucinski, K. Ozga , Opt. Commun. 268 (2006) 269–72. [3] K. Tanaka, M. Oonuki, N. Moritake, H. Uchiki, Sol. Energy Mater. Sol. Cells 93 (2009) 583–587 [4] K. Ellmer, A. Klein, ZnO and its Applications, Transparent Conducting Zinc Oxides: Basics and Aapplications in Thin Film Solar Cells, Springer series in Materials Science (2008). [5] J. Benny, K.G. Gopchandran, P.K. Manoj, P. Koshy, V.K. Vaidyan, Bull. Mater. Sci. 22(5) (1999)921–6. [6] F.K. Lotgering, J. Inorg. Nucl. Chem. 9 (1959) 113-23. [7] Z. Lamia, Mater. Sci. Eng. B 174 (2010) 18–30.
ScienceDirect Materials Today: Proceedings 2 (2015) 5145 – 5149
International Conference on Solid State Physics 2013 (ICSSP’13)
Novel sol-gel route to synthesize transparent conductive oxides for front contact of solar cells Ans Farooq a*, Nazar Abbasb, M. Kamranc a
Department of Mechanical Engineering, Mohammad Ali Jinnah University, Islamabad Campus. b Department of Physics, COMSATS Institute of Information Technology, Islamabad, Pakistan. c Department of Electrical Engineering, COMSATS Institute of Information Technology, Islamabad, Pakistan.
Abstract
The thin films of transparent conductive zinc oxide (ZnO) are synthesized by novel sol gel spin coating method. We deposit thin films as primary application of front contact of the thin film solar cells such as Cu2 (Zn,Sn)S4(CZTS). The thin films are deposited by varying the molar concentration of the zinc oxide solution with different molar concentrations. The effect of molar concentration and annealing are temperature dependent. The main orientation of the deposited ZnO are observed at (101) peak by X-Ray diffraction (XRD) micrographs. We observed the optical transmittance of our samples by optical spectroscopy and one of them showed the transmittance more than 90%. The deposited thin films show high transparency and are the potential candidate to be used as window layer in thin film based solar cells. © 2015 2015Elsevier ElsevierLtd. Ltd. rights reserved. © AllAll rights reserved. Selectionand andPeer-review Peer-review under responsibility of Committee the Committee Members of International Conference Solid State Physics Selection under responsibility of the Members of International Conference on Solidon State Physics 2013 2013(ICSSP’13) (ICSSP’13). Keywords: Sol-gel; Transparent conducting oxides; Solar cells; Zinc oxide
* Corresponding author. E-mail address: [email protected]
2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the Committee Members of International Conference on Solid State Physics 2013 (ICSSP’13) doi:10.1016/j.matpr.2015.11.135
5146
Ans Farooq et al. / Materials Today: Proceedings 2 (2015) 5145 – 5149
1. Introduction Transparent conductive oxides (TCOs) thin films are optically transparent and electrically conductive [1]. TCOs are the binary or tertiary compounds of Oxides. TCOs have vast applications in various fields of technology and have potential applications in LEDs, OLEDs, Solar Cells, Transparent Displays, Water Treatment and many more [1-3]. Zinc Oxide is the member of TCOs and it has the potential to replace the expensive and rare Indium Tin Oxide (ITO) in various applications especially in window layer of thin film based solar cells.Cu2ZnSnS4 (CZTS) is one of the most promising absorber layer materials for low-cost thin film solar cells. For this type of solar cells we need a cost effective front contact or window layer that is of Zinc Oxide [5]. Zinc Oxide thin films are highly transparent in UV/Vis region and also have very good electrical conductivity [6-7]. 2. Experimental procedure Many synthesis techniques are reported for the preparation of TCOs like Chemical vapor deposition (CVD) [1], Pulsed laser deposition (PLD) [2] and Physical sputtering but in this work we used novel sol-gel route to prepare ZnO thin films [4]. As sol-gel involves less complications and no vacuum is needed, cost effective and easy to understand the whole procedure.
Fig. 1. Synthesis Route to deposit ZnO Thin Films
Soda lime glass (SLG) used as a substrate. First of all the glass ware and glass slides are washed with detergent and then dipped in chromic acid for 15 minutes, after that the glass wares and substrate was washed with methanol and put them in oven at 200oC for 10 minutes for evaporation of wet residue and to remove containments on the substrate. Zinc acetate di hydrate (ZAD) is used as precursor. Firstly ZAD is dissolved in 2-Proponal [iPrOH, (CH3)2CHOH] and Mono-Ethanol Amine [MEA, (HOCH2CH2)NH2] added as a stabilizer, the MEA:Zn+2ratios is maintained at 0.75-1. The sol is than put on a magnetic stirrer at normal speed and with a temperature of 60oC until the ZAD is completely dissolved. Normally ZAD is dissolved with in 2 hr on magnetic stirring. We prepared 3 different solutions (I) 0.1 Molar ZAD in iso-Propanol (II) 0.3 Molar ZAD in Iso-Propanol (III) 0.5 Molar ZAD in iso-Propanol Filter the sol to filter out the bigger particles and toget a sol of same particle size, for this we use syringe filter (Sartorius-Stedim Minisart NML) of 20 um pore size, which allows only the particles smaller or equal to 20 micron to pass through the filter. Controlled Spin Coating (CSC) unit spins at 2000 RPM for 30 seconds. Deposited thin film is than put into oven for 10 min at 200oC to remove excess water and containments. This
5147
Ans Farooq et al. / Materials Today: Proceedings 2 (2015) 5145 – 5149
procedure is repeated 3 times to gain a smooth surface of thin film to deposit each and every corner, so no part will left behind un-deposited of thin film. The deposited thin films of ZnO are annealed at 4500C for 1 hr. 3. Results and Discussion X-Ray Diffraction is performed to confirm that the deposited thin films are of good quality of ZnO. To find the orientation of the deposited thin films are made according to the given formulas purposed by Lotgering [6].
(1)
Where f(hkl) is the degree of (hkl) orientation,
(2)
(3)
I(hkl) is intensity of (hkl) peak , ΣI(hkl) is sum of intensities of all diffraction peaks in recorded data. Whereas Io(hkl) is intensity of (hkl) peak and ΣIo(hkl) is sum of intensities of all peaks in the recorded diffraction data of the standard card JCPDS 36-1451 It is observed that 0.5 Molar ZnO thin films annealed at 450 °C for 1 hour exhibited a higher degree of orientation of the (101) plane compared to the films with lower molar concentration. Intensity of (101) peak can also be analyzed using following relation:
(4)
(101) orientation of ZnO thin films is kinetically preferred. (101) plane possesses the higher grain-packing density and highest density of Zn atoms is found along this plane.
Fig. 2: XRD pattern of annealed (ann) and un-Annealed (un) samples of deposited ZnO thin films
The crystallinity of thin film improves as the sol concentration increases. We observed that crystal size increases with increase in molar concentration of ZnO solution in deposited annealed thin films. Scanning Electron
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Ans Farooq et al. / Materials Today: Proceedings 2 (2015) 5145 – 5149
Microscopy (SEM) is only done for annealed samples. A typical flake like morphology is observed in all deposited thin films.
Fig. 3. SEM micrograph of (a) 0.1 Molar (b) 0.3 Molar (c) 0.5 Molar concentration ZnO deposited thin films.
From SEM micrograph we can observe that particle size is uniform and surface morphology is smooth. High resistivity is the key for maximum optical transmittance. The most important part of this work is the Optical characteristics of deposited ZnO thin films, as we are finding a molar concentration of samples which is better for its use as window layer in thin film based solar cells; this can only happen if the sample can transmit the maximum of the incident light [5]. The reflection and absorbance must be minimal to gain the best results, so by the help of optical spectroscopy all the samples were characterized for optical transmittance. All the samples are observed in the visible range of electromagnetic spectrum from 300nm-700nm.
Ans Farooq et al. / Materials Today: Proceedings 2 (2015) 5145 – 5149
5149
Fig. 4. Optical Transmittance of annealed and un-annealed ZnO thin films.
In fig 4 we can observe that 0.5 M (dotted line) thin film shows the maximum optical transmittance (greater than 90%) and 0.1 M thin film is only at 70% optical transmittance. Absorption edge is at 375nm for almost all deposited thin films [7]. 4. Conclusions Highly oriented Zinc Oxide thin films prepared by sol-gel spin coating technique. All the deposited thin films are compact and dense in nature and crystallinity improves with increase in molar concentration of zinc oxide solution. Preferred peak of orientation is (101) which are observed by the help of XRD data. XRD analysis shows that all the deposited thin films have hexagonal wurtzite crystal and all are Nano-crystalline. It is observed that after annealing the deposited thin films features become more prominent. After annealing there was a considerable change in the crystallite size. 0.5 M thin film has the maximum optical transmittance of above 90% which is the good candidate for its use as window layer in CZTS based solar cells. References [1] W. Lin, R. Ma, J. Xue, B. Kang, Sol. Energy Mater. Sol. Cells 91 (2007) 1902–5. [2] J. Ebothe, I.V. Kityk, S. Benet, B. Claudet, K. J. Plucinski, K. Ozga , Opt. Commun. 268 (2006) 269–72. [3] K. Tanaka, M. Oonuki, N. Moritake, H. Uchiki, Sol. Energy Mater. Sol. Cells 93 (2009) 583–587 [4] K. Ellmer, A. Klein, ZnO and its Applications, Transparent Conducting Zinc Oxides: Basics and Aapplications in Thin Film Solar Cells, Springer series in Materials Science (2008). [5] J. Benny, K.G. Gopchandran, P.K. Manoj, P. Koshy, V.K. Vaidyan, Bull. Mater. Sci. 22(5) (1999)921–6. [6] F.K. Lotgering, J. Inorg. Nucl. Chem. 9 (1959) 113-23. [7] Z. Lamia, Mater. Sci. Eng. B 174 (2010) 18–30.