International Research Symposium on Pure and Applied Sciences (IRSPAS)

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Author: search.filters.author.Atapattu, H. Y. R.Subject: search.filters.subject.Electrodeposition

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    Electrodeposited metal sulfide thin films for gas sensing applications
    (4th International Research Symposium on Pure and Applied Sciences, Faculty of Science, University of Kelaniya, Sri Lanka, 2019) Namawardana, D. G. K. K.; Wickramathilaka, P. A. K. Y.; Atapattu, H. Y. R.; De Silva, D. S. M.
    Quantitative measurements of gases are based on a variety of physical or chemical principles. Among them semiconductor gas sensors are best candidates for the development of commercial gas sensors due to their higher specificity and sensitivity. They are mainly based on metal oxide and metal sulfide materials. Due to certain drawbacks of metal oxides, metal sulfides are extensively investigated as novel gas sensing materials. In this study ZnS and CdS were investigated for their gas sensing ability. Both types of thin films were fabricated by electrodeposition using a three electrode electrolytic system consisted of a fluorine doped tin oxide glass substrate (1×3 cm2) as working electrode and a high purity carbon as counter electrode. An aqueous electrolyte containing CdCl2 (0.10 mol/L) and Na2S2O3 (0.01 mol/L) precursors were used for the electrodeposition of CdS material and aqueous electrolyte containing ZnCl2 (0.10 - 0.05 mol/L) and Na2S2O3 (0.01 - 0.05 mol/L) precursors were used for electrodeposition of ZnS material. The CdS depositions were carried out in the cathodic deposition potential (CDP) range of 0.65 to 0.70 V vs. saturated calomel electrode and pH range of 1.5 to 2.0 at a temperature of 55 °C for 30 minutes. The ZnS depositions were carried out in the CDP range of 0.70 to 1.10 V vs. Ag/AgCl reference electrode and pH range of 4.0 to 3.5 at a temperature of 30 °C for 90 minutes. Both types of thin films were characterized for their crystalline structure, surface morphology, and elemental composition by using the techniques of X-ray diffraction spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy respectively and were exposed to various gases namely; NO2, H2S, and LPG. CdS thin films grown at CDP of 0.67 V and pH of 1.5 and ZnS thin films grown at CDP of 1.05 V and pH of 3.7 were found to have notable gas sensing properties. CdS has shown highest resistance variation of 1.2 Ω towards H2S with respect to the initial resistance of 36.0 Ω and ZnS has shown highest resistance variation of 2 Ω with respect to the initial resistance of 26.2 Ω when exposed to NO2 gas at 30 oC. Both CdS and ZnS thin films showed resistance variation of 1.1 Ω and 0.6 Ω towards LPG respectively at 30 oC.
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    Evaluation of solution parameters for CdCl2 solutions to be used in post-deposition treatments of CdTe thin films in CdS/CdTe solar cells.
    (International Research Symposium on Pure and Applied Sciences, 2017 Faculty of Science, University of Kelaniya, Sri Lanka., 2017) Atapattu, H. Y. R.; Silva, D. S. M. D.; Pathiratne, K. A. S.
    Owing to its high absorption coefficient and the near ideal band gap, CdTe has become one of the topmost solar energy materials available for conversion of solar energy into electricity. It exhibits excellent power conversion efficiencies, when coupled with the CdS window material to form CdS/CdTe heterojunction solar cells. Further, CdCl2 treatment has been identified as one of the promising post-deposition treatments available for achieving drastic improvements in the performance of CdTe material. However, no extensive investigations have yet been carried out to identify suitable solution parameters for the CdCl2 solutions used in the post-deposition treatment process. Hence, the present study was designed to investigate the effect of concentration and pH of CdCl2 solutions used for post-deposition treatments of CdTe material grown on glass/FTO/CdS surfaces. In this study, CdTe layers were potentiostatically electrodeposited on glass/FTO/CdS substrates in electrolytic baths containing 1.0 mol/L CdSO4 and 1.0 mmol/L TeO2 at pH 2.3. A cathodic deposition potential of 650 mV with respect to saturated calomel electrode and temperature of 65 °C at a continuous stirring rate of 60 rpm were maintained through the deposition. At the end of electrodeposition process, all the glass/FTO/CdS/CdTe samples were rinsed in de-ionized water and dried under a high purity nitrogen gas stream and conveyed for the CdCl2 treatment followed by air annealing at 390 °C for 15 min. For CdCl2 treatment, three different CdCl2 concentrations (1.0, 0.5 and 0.1 mol/L) were used. For each concentration, three different pHs; as prepared (5.6, 6.3 and 7.1 for 1.0, 0.5 and 0.1 mol/L CdCl2 solutions respectively), 2.0 and 6.5 at 25 °C were selected. Hence, nine sets of samples with two replicates in each were subjected to the CdCl2 treatment. Once the treatment process was over, samples were inspected for their optical, electrical and morphological properties using the techniques of optical absorption spectroscopy, photo-electrochemical cell studies and scanning electron microscopy. The results revealed that, two solutions; one with 1.0 mol/L CdCl2 solution at pH of 5.6 and the other with 0.1 mol/L CdCl2 solution at pH of 2.0 can be effectively used for the post-deposition treatment of CdTe material for improving its properties and eventually to produce power efficient CdS/CdTe based solar cells with ~80 % efficiency improvement compared to the untreated devices.