Available online at ScienceDirect. Procedia Engineering 139 (2016 ) MRS Singapore ICMAT Symposia Proceedings

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1 Available online at ScienceDirect Procedia Engineering 139 (2016 ) 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. Kumarage a,b, L. B. D. R. P. Wijesundara c, V. A. Seneviratne a,b, C. P. Jayalath a,b and B. S. Dassanayake a,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 M CdSO 4, M CS(NH 2 ) 2 and NH 3 solution at different bath temperatures from 40 to 80 0 C 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 0 C was considerably larger compared to the films fabricated at other bath temperatures. The variation of optical band gap (E g ) was found to be in good agreement with cluster size variation while electrical characterizations reveal considerably high V oc and I sc values for the films fabricated at a bath temperature of 80 0 C The Authors. Published by by Elsevier Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license ( 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.: address:buddhid@gmail.com The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT doi: /j.proeng

2 W.G.C. Kumarage et al. / Procedia Engineering 139 ( 2016 ) 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 Cd 2+ and S 2- 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 M cadmium sulfate (3CdSO 4.8H 2 O, 99%), M thiourea (CS(NH 2 ) 2, 99%,) which was titrated throughout the experiment and 1.1 ml of ammonia solution (NH 3, 35% w/w) in 140 ml of de-ionized water. The glass substrates were placed inside the beaker at an angle of The deposition was carried out at five different bath departures from 40 to 80 0 C 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 C 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 , and corresponding to (111), (200) and (220) planes of cubic and and 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 0 C) 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 (E g ) (Table 1) were calculated from the Stern relation [12] A h E h n 2 g where, α is the optical absorption coefficient, A is a constant, h is the Plank s constant, E g is the optical band gap and n is equal to 1 for direct band gap material such as CdS. The lowest E g value was found to be at 60 0 C while it increased beyond and below that temperature. (1)

3 66 W.G.C. Kumarage et al. / Procedia Engineering 139 ( 2016 ) Deposition rate Counts (a.u) (111) (002) (100) (200) (220) Deposition rate (nm.min -1 ) o Fig. 1: X-ray diffraction pattern obtained for the CBD-CdS samples fabricated at 80 0 C. 0.7 Fig. 2: CdS deposition rate with respect to the bath temperature. Table 1: Film deposition rate, optical band gap, V oc, I sc and V oc I sc variation with respect to the bath temperature. Bath Deposition rate E g V oc I sc V oc I sc temperature ( 0 C) (nm.min -1 ) (ev) (mv) (μa) (nw) Transmittance (avg) (%) 96 Transmittance (avg) Voc x Isc (nw) Voc x Isc 82 0 Fig. 3: Variation of average transmittance with respect to the bath temperature. Fig. 4: The product of the V oc and I sc values of CdS thin films with respect to the bath temperature.

4 W.G.C. Kumarage et al. / Procedia Engineering 139 ( 2016 ) Morphological analysis of the sample was conducted using scanning electron microscope (SEM) images by FEI Helios Nanolab 600 SEM. The films deposited at 60 0 C 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 0 C can be correlated with the deposition mechanisms. The trend of E g variation is in good agreement with SEM images. The highest cluster size (at 60 0 C) shows the lowest E g whereas smaller cluster sizes show higher E g 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 0 C (b) 60 0 C (c) 80 0 C of bath temperatures. Photo current-voltage measurements (CdS/0.1 M Na 2 S 2 O 3 /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 Na 2 S 2 O 3 ) junction was 0.25 cm 2. The photovoltaic product of V oc and I sc results of PEC characterization is shown in Fig. 4. The product values of the V oc and I sc have the similar trend of variation as in E g, V oc and I sc. The highest I-V product was given by the films fabricated at 80 0 C bath. The V oc variation with respect to the bath temperature can be concluded as due to the alteration of the E g of the fabricated CdS. Furthermore, I sc value was found to decrease with increasing bath temperature till 60 0 C 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 I sc values. 4. Conclusion Films fabricated at solution temperatures from 40 to 80 0 C 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 0 C bath are concluded to be the best along with the higher V oc, I sc 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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