Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 139 (2016) 64 – 68
MRS Singapore – ICMAT Symposia Proceedings 8th International Conference on Materials for Advanced Technologies
Influence of Bath Temperature on CBD-CdS Thin Films W. G. C. Kumaragea,b, L. B. D. R. P. Wijesundarac, V. A. Seneviratnea,b, C. P. Jayalatha,b and B. S. Dassanayakea,b* a
Postgraduate Institute of Science, University of Peradeniya, Peradeniya, Sri Lanka
b
Department of Physics, Faculty of Science, University of Peradeniya, Peradeniya, Sri Lanka c
Department of Physics, Faculty of Science, University of Kelaniya, Kelaniya, Sri Lanka
Abstract CdS thin films were grown on conducting glass substrates at different bath temperatures in order to investigate their influence on opto-electrical properties of the chemical bath deposited (CBD) CdS thin films. The CBD-CdS process was carried out with 0.001 M CdSO4, 0.002 M CS(NH2)2 and NH3 solution at different bath temperatures from 40 to 80 0C for one hour. Fabricated films were characterized using UV-Vis spectrometry, SEM, PEC cell and Profilometer. The optical transmittances of the films decreased with increasing bath temperature due to the increase of the film thickness. CdS cluster size of the thin films grown in the bath at a temperature of 60 0C was considerably larger compared to the films fabricated at other bath temperatures. The variation of optical band gap (Eg) was found to be in good agreement with cluster size variation while electrical characterizations reveal considerably high Voc and Isc values for the films fabricated at a bath temperature of 80 0 C. © by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license © 2016 2015The TheAuthors. Authors.Published Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT Keywords: CBD; CdS;bath temperature;thin films
* B. S. Dassanayake. Tel.: +94 812394580. E-mail address:
[email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT
doi:10.1016/j.proeng.2015.09.215
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1. Introduction Owing to its interesting opto-electronic properties, CdS thin films have grabbed the attention of researchers’ since early 90’s [1]. There are different deposition technologies involved in the fabrication process of CdS thin films such as chemical bath deposition (CBD), thermal evaporation, close-spaced sublimation, chemical pyrolysis deposition, and metal organic chemical-vapor deposition [2,3,4]. Comparatively CdS deposition by CBD method is known to be a low cost and simple process to fabricate high quality large area thin films [1,5,6]. It is known that the bath temperature has a significant influence on the morphological and structural properties of the CBD-CdS thin films [7,8]. The CBD-CdS process is based on the chemical reaction between the Cd2+ and S2- ions in the alkaline medium which results in heterogeneous precipitation of CdS on the substrate [1,9]. In this study, influence of bath temperature on opto-electronic properties of the CdS thin films and their structural and morphological changes are discussed. 2. Experimental CdS thin films were deposited on fluorine doped tin oxide (FTO, TEC 10, ~10 Ω/□) coated conducting glass which were cleaned with detergent, diluted HCl, de-ionized water, ultra sound sonication and organic solvents. CdS deposition by CBD method was carried out in a reaction bath consist of 0.001 M cadmium sulfate (3CdSO4.8H2O, 99%), 0.002 M thiourea (CS(NH2)2, 99%,) which was titrated throughout the experiment and 1.1 ml of ammonia solution (NH3, 35% w/w) in 140 ml of de-ionized water. The glass substrates were placed inside the beaker at an angle of 450. The deposition was carried out at five different bath departures from 40 to 80 0C for one hour. Fabricated films were rinsed with running de-ionized water followed by ultrasound sonication to remove loosely adhered CdS particles. In order to conduct the optical and electrical analysis, CdS was removed from one side of the substrate by chemical etching using diluted HCl. Then the films were rinsed with running de-ionized water and dried with nitrogen gas before the annealing at 200 0C for one hour in an oven (Büchi To-51, Switzerland).
3. Result and Discussion The X-ray diffractograms (XRD, Siemens X-ray diffractometer D5000) show (Fig. 1) reflection peaks at the 2θ angels of 26.50, 30.80 and 43.90 corresponding to (111), (200) and (220) planes of cubic and 23.40 and 25.90 corresponding to (100) and (002) planes of hexagonal CdS. These results indicate that the fabricated films are in fact polycrystalline in nature [10]. Film thicknesses were estimated using XP-1 Profilometer by Ambios Technology and indicated in Table 1. The deposition rate (Fig. 2 and Table 1) suggests that lower bath temperatures have a lower rate of deposition compared to higher bath temperatures. This observation can potentially be due to the two different CdS deposition mechanisms present; ion by ion and cluster by cluster deposition. The ion by ion process is less thermally activated with low activation energy whereas cluster by cluster is believed to occur at relatively higher (> 60 0C) temperatures [7,11,12]. Optical transmittance and the absorptions of the fabricated samples were obtained using UV-Visible spectrometry (UV-1800 Shimadzu). The average transmittances of the fabricated films (Fig. 3) indicate that transmittance is decreasing with increasing bath temperature. This decrement can be explained by the incrementing trend of sample thickness. The optical band gap values (Eg) (Table 1) were calculated from the Stern relation [12]
hQ E A
n
D
g
hQ
2
(1)
where, α is the optical absorption coefficient, A is a constant, h is the Plank’s constant, Eg is the optical band gap and n is equal to 1 for direct band gap material such as CdS. The lowest Eg value was found to be at 60 0C while it increased beyond and below that temperature.
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Deposition rate (nm.min-1)
200
Counts (a.u)
Deposition rate
1.3
250
150
(111)
100
(002)
(200) (220)
(100)
50
1.2 1.1 1.0 0.9 0.8 0.7
0 10
20
30
40
50
2T
60
70
40
80
50
60
70
80
Bath temperature (0C)
o
Fig. 1: X-ray diffraction pattern obtained for the CBD-CdS samples fabricated at 80 0C.
Fig. 2: CdS deposition rate with respect to the bath temperature.
Table 1: Film deposition rate, optical band gap, Voc , Isc and Voc × Isc variation with respect to the bath temperature. Bath
Deposition rate
Eg
Voc
Isc
Voc × Isc
temperature (0C)
(nm.min-1)
(eV)
(mV)
(μA)
(nW)
40
0.717
2.55
198
2.23
441
50
0.987
2.36
180
1.20
216
60
1.09
2.27
141
0.54
76.1
70
1.13
2.25
208
1.88
391
80
1.28
2.34
318
1.95
620
700
Voc x Isc
Transmittance (avg)
96
600 500 92
Voc x Isc (nW)
Transmittance (avg) (%)
94
90 88
400 300
86
200
84
100
82 40
50
60
70
80
Bath temperature (0C) Fig. 3: Variation of average transmittance with respect to the bath temperature.
0 40
50
60
70
80
Bath temperature (0C) Fig. 4: The product of the Voc and Isc values of CdS thin films with respect to the bath temperature.
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Morphological analysis of the sample was conducted using scanning electron microscope (SEM) images by FEI Helios Nanolab 600 SEM. The films deposited at 60 0C bath (Fig. 5 (b)) show higher cluster sizes with respect to the films fabricated at other bath temperatures. The decrement of cluster sizes beyond and below 60 0C can be correlated with the deposition mechanisms. The trend of Eg variation is in good agreement with SEM images. The highest cluster size (at 60 0C) shows the lowest Eg whereas smaller cluster sizes show higher Eg values as seen in Table 1. .
(a)
(b)
(c)
0.5 μm
0.5 μm
0.5 μm
Fig. 5: SEM images of the CBD CdS deposited at (a) 40 0C (b) 60 0C (c) 80 0C of bath temperatures.
Photo current-voltage measurements (CdS/0.1 M Na2S2O3/Pt) were conducted using photo electro chemical (PEC) cell (PEC Cell L01) where the counter electrode was a Pt rich glass plate. The effective area of solid/liquid (CdS/0.1 M Na2S2O3 ) junction was 0.25 cm2. The photovoltaic product of Voc and Isc results of PEC characterization is shown in Fig. 4. The product values of the Voc and Isc have the similar trend of variation as in Eg, Voc and Isc. The highest I-V product was given by the films fabricated at 80 0C bath. The Voc variation with respect to the bath temperature can be concluded as due to the alteration of the Eg of the fabricated CdS. Furthermore, Isc value was found to decrease with increasing bath temperature till 60 0C and increase beyond that. This variation can potentially be correlated with the cluster size variation. The presence of different cluster sizes results in films with different effective surface areas. Hence higher effective surface areas generate higher Isc values. 4. Conclusion Films fabricated at solution temperatures from 40 to 80 0C were studied for their optical and electrical properties. The results support the existence of the two different CdS deposition mechanisms at lower and higher bath temperatures. The films fabricated at 80 0C bath are concluded to be the best along with the higher Voc, Isc and appreciable optical band gap value. Acknowledgements Financial assistances from National Science Foundation - Sri Lanka (RG/12/BS/03), Higher Degree Committee - University of Peradeniya (AC/URF/2014/03), International Research Council - University of Peradeniya and Prof. S. Sivananthanthan of University of Illinois at Chicago (USA) are gratefully acknowledged. Prof. M.A.K.L. Dissanayake of the National Institute of Fundamental Studies, Sri Lanka and Dr. M. Nandasiri of Western Michigan University (USA) are acknowledged for their valuable suggestions and support.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
H. Khallaf, I. O. Oladeji, G. Chai and L. Chow, Characterization of CdS thin films grown by chemical bath deposition using four different cadmium sources, Thin Solid Films 516 (2008) 7306–7312. S. A. Tomás, O. Vigil, J. J. Alvarado-Gil, R. Lozada-Morales, O. Zelaya-Angel, H. Vargas, and A. F. da Silva, Influence of thermal annealing in different atmospheres on the band-gap shift and resistivity of CdS thin films, J. Appl. Phys 78 (1995). R. W. Birkmire, B. E. McCandless, and S. S. Hegedus, Analysis of stress-induced in CdS/CdTe sloar cells, Sol. Energy 1(1992). N. Amin, T. Isaka, T. Okamoto, A. Yamada, and M. Konagai, Doping effects of dimethyl1-tin-dichloride on material properties of CdS films and on formation of CdS/CdTe hetro structures, J. Appl. Phys 38, 1 (1999). R. Devi, P. Purkayastha, P. K. Kalita, R. Sarma, H. L Das and B. K Sarma, Photoelectric properties of CdS thin films prepared by chemical bath deposition, indian journal of pure and applied physics 45 (2007) 624-627. H. Metin and R. Esen, Annealing studies on CBD grown CdS thin films, Journal of Crystal Growth 258 (2003) 141–148. H. Moualkia, S. Hariech, M. S. Aida, N. Attaf and E. L. Laifa, Growth and physical properties of CdS thin films prepared by chemical bath deposition, Materials Science in Semiconductor Processing 21 (2014) 186–193. M. T. S. Nair, P. K. Nair and J. Campos, Effect of bath temperature on the optoelectronic characteristics of chemically deposited CdS thin films, Thin Solid Films 161(1988). J. N. Alexander, S. Higashiya, D. Caskey, H. Efstathiadisa and P. Haldar, Deposition and characterization of cadmium sulfide (CdS) by chemical bath deposition using an alternative chemistry cadmium precursor, Solar Energy Materials & Solar Cells 125 (2014). B. X. Hop, H. V. Trinh, K. Q. Dat and P. Q. Bao, Growth of CdS thin films by chemical bath deposition technique, VNU J. Science, Methematica - Physics 24 (2008) 119-123. Y. M. Reddy and M . N. V. Prasad, Properties of CdS Chemically Deposited thin films on the Effect of Ammonia Concentration, IOSR-JAP 4, 4 (2013) 01-07. H. Moualkia, S. Hariech and M. S. Aida, Structural and optical properties of CdS thin films grown by chemical bath deposition, Thin Solid Films 518, (2009).
