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Item Electrodeposited ZnS Thin Films for NO2 Gas Sensing Applications(19th Conference on Postgraduate Research, International Postgraduate Research Conference 2018, Faculty of Graduate Studies,University of Kelaniya, Sri Lanka, 2018) Wickramathilaka, P.A.K.Y.; Namawardana, D.G.K.K.; Atapattu, H.Y.R; De Silva, D.S.M.Sensors are becoming a consequential part in human’s daily life. Typically, they are classified based on the physical parameter sensed itself namely; thermal, mechanical, magnetic, chemical, and optical. Gas sensors are chemical sensors that can be fabricated as metal-oxide or metal-sulfide semiconductor materials viz.; TiO2, ZnO, CdS and ZnS etc. Among these materials ZnS is a highly abundant and non-toxic material and can be easily adopted for gas sensing applications. Electrodeposition can be identified as an ideal fabrication method owing to its simplicity and low cost in production amid various fabrication methods that have been employed for developing ZnS thin films. This study focuses on the growth of ZnS thin films for gas sensing applications using the technique of electrodeposition. A three electrode electrolytic system consists of an Ag/AgCl reference electrode, FTO glass substrate (1×3 cm2) working electrode and high purity carbon counter electrode was used in electrodepositing ZnS material in an aqueous electrolyte containing ZnCl2 (0.10 - 0.05 mol/L) and Na2S2O3 (0.01 - 0.05 mol/L) precursors. The ZnS depositions were carried out in the cathodic deposition potential (CDP) range of 0.70 - 1.10 V and pH range of 4.0 - 3.5 at temperature of 30 °C for 90 minutes. After deposition, samples were annealed at 300 °C for 10 minutes and characterized for their crystalline structure, surface morphology and elemental composition using the techniques of X-ray diffraction spectroscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy respectively. The sample grown at CDP of 1.05 V at pH of 3.7 were found to have notable material properties and shown 2 Ω average change in resistance with respect to the initial average resistance of 26.2 Ω while exposing to NO2 gas at 3× 104 Pa and 30 °C within a time interval of 2-3 minutesItem Effect of Concentration of Cd2+ on the Material Properties of CdTe Thin Films Electrodeposited via a Two Electrode Electrolytic Cell(Faculty of Graduate Studies, University of Kelaniya, Sri Lanka, 2016) Wedisinghe, K.C.; Atapattu, H.Y.R.; de Silva, D.S.M.Cadmium telluride (CdTe) is a vital semiconductor material which can absorb most of the electromagnetic radiation of solar energy spectrum to yield more efficient solar cells. Among the thin film fabrication methods, electrodeposition is an emerging technique to produce good quality CdTe materials using either two or three electrode system. This study was performed using the two electrode electrolytic system to investigate the effect of Cd2+ concentration on the material properties of CdTe thin films to enhance its photovoltaic activities. Electrodepositions of CdTe were carried out in an aqueous electrolyte which was prepared utilizing analytical grade CdSO4 as the cadmium and TeO2 as the tellurium precursors. Fluorine doped tin oxide coated conducting glass (1×3 cm2) and high purity carbon electrode were used as the working (cathode) and counter electrodes (anode) respectively in the electrolytic cell used for the depositions. The concentration of TeO2 was maintained around 1 mmol/L throughout while varying the CdSO4 concentration from 1.00 mol/L to 1.50 mol/L. pH of the electrolyte was initially adjusted to 2.2 and depositions were carried out in the cathodic potential range of (1.33 - 1.37) V at 65 °C while stirring at continuous stirring rate of 60 rpm. The resulting electrodeposited CdTe layers were heat treated for 10 minutes at 400 °C in air and subsequently, their optical, electrical morphological and structural properties were studied using UV-visible spectrophotometry, photo-electrochemical cell, scanning electron microscopy and X-ray diffraction spectroscopy. As elucidated by the results, at the deposition potentials of 1.34 V and 1.35 V, the short circuit current and open circuit voltage values were increased with the increasing of concentration of Cd2+ from 1.00 mol/L to 1.25 mol/L while the band gap energy reached to its theoretical value of 1.50 eV.Item Effect of Thermal Annealing on Electrodeposited CdS and CdS/CdTe Heterojunction(Faculty of Graduate Studies, University of Kelaniya, Sri Lanka, 2016) Atapattu, H.Y.R.; de Silva, D.S.M.; Pathiratne, K.A.S.At present CdS/CdTe based solar cells have a significant commercial impression due to its lowcost, scalability, manufacturability and simplicity. Nevertheless, it is essential to elevate the optoelectronic qualities of CdS and CdTe materials and the interface properties of CdS/CdTe heterojunction and ultimately the efficiency of the solar cells. In this regard thermal annealing is one of the key steps to be considered in order to enhance the material and heterojunction properties. Hence, in this study, the effect of thermal annealing on electrodeposited CdS and CdS/CdTe heterojunction was investigated. CdS and CdTe semiconductor layers were potentiostatically electrodeposited on bare fluorine doped tin oxide (FTO) glass substrates and FTO/CdS respectively using the typical three electrode electrolytic cell. For both layers, saturated calomel electrode and high purity (99%) graphite rod were used as reference and counter electrodes respectively. 0.10 mol/L CdCl2 and 0.01 mol/L Na2S2O3 were used as Cd and S precursors respectively to produce CdS thin films while 1.35 mol/L CdSO4 and 1.0 mmol/L TeO2 were used as Cd and Te precursors respectively for CdTe. CdS layers were grown at cathodic deposition potential of 660 mV at pH 1.6 and temperature of 55 °C. Afterwards, one set of electrodeposited CdS samples was conveyed for fabrication of CdS/CdTe heterojunction. CdTe layers were grown on CdS layers at cathodic deposition potential of 650 mV at pH 2.3 and temperature of 65 °C. Subsequently, thermal annealing was carried out for both CdS and CdS/CdTe at three different temperatures; 390, 400 and 410 °C, for each annealing three different time periods; 10, 15, 20 min were considered. After the process of annealing all the samples were inspected for their optical, electrical and morphological properties using the techniques of optical absorption spectroscopy, photoelectrochemical cell and scanning electron microscopy respectively. According to the results, the optimum annealing conditions which yielded good optoelectronic qualities for CdS and CdS/CdTe were found to be 400 °C, 15 min and 390 °C, 15 min respectively.Item Optimization of three growth parameters for electrodeposition of CdS thin film semiconductor; pH, deposition temperature and deposition voltage in a stable electrolyte(Faculty of Graduate Studies, University of Kelaniya, 2015) Atapattu, H.Y.R.; De Silva, D.S.M.; Pathiratne, K.A.S.Cadmium sulfide has been identified as the most promising window material for fabrication of CdS/CdTe and CdS/CuInGaSe2 thin film solar cells. Among vast variety of commercially available CdS fabrication methods electrodeposition (ED) is a viable technique due to its low cost and simplicity. This study focuses a procedure followed for optimization of the three growth parameters; pH of the bath solution, deposition temperature and deposition voltage for ED-CdS thin films with high photovoltaic activities utilizing CdCl2 and Na2S2O3 as cadmium and sulfur precursors respectively. Based on the two initial leading experiments, feasible pH and deposition temperature ranges for a stable electrolyte which does not promote chemical bath formation of CdS were identified to be in the ranges of 1.5-2.0 and 50-70 °C respectively. Also, using cyclic voltammetry the feasible cathodic deposition voltage was identified to be in the range of 640- 720 mV with respect to saturated calomel electrode. Consequently, the technique of the design of experiment (DOE) was carried out to establish random combinations of levels of the three electrodeposition parameters amid the previously identified parameter ranges for deposition of CdS layers via the ED technique. Finally, the electrical, optical, structural and morphological properties of the CdS thin films electrodeposited under different combinations of parameter values were investigated using photo-electrochemical cell study, optical absorption spectroscopy, x-ray diffraction method and scanning electron microscopy respectively. The results indicated that, aqueous solutions in the pH range of 1.6 to 1.8 containing 0.10 M CdCl2 and 0.01 M Na2S2O3 at 55-65 °C can successfully be used for electrodeposition of thin film CdS semiconductor materials over a cathodic deposition voltage range of 650 to 680 mV with a deposition period of 20 to 40 min.Item Observation of interface modification of electrodeposited p-Cu2O thin films in an aqueous electrolyte(Faculty of Graduate Studies, University of Kelaniya, 2015) Kafi, F.S.B.; Jayathilekea, K.M.D.C.; Wijesundera, R.P.; Siripala, W.Interface engineering via modification of semiconductor surfaces of junction devices is a powerful technique to improve the performance of devices. In addition, semiconductor material Cu2O has recently gained a considerable attention as a low cost semiconductor material suitable for developing thin film solar cells, water splitting in photoelectrochemical cells and gas sensors. Indeed, the possibility of relative band edge shifts of Cu2O with suitable interfacing materials will pave the way for interface engineering to improve the efficiency of those devices. In this study we have investigated this possibility of using electrodeposited p-type cuprous oxide thin films deposited using a lactate bath containing 3 M sodium lactate and 0.4 M CuSO4 at various pH values. These Cu2O films were used in a photolectrochemical cell to form semiconductor/electrolyte junctions in a 0.1 M sodium acetate aqueous solution and then to measure the flat band potential variations with the pH of the Cu2O film deposition baths. It was observed that pH value of the Cu2O film deposition bath is very sensitive to the flat band potential. This result gives a direct evidence that the surface of Cu2O film is modified at the Cu2O/electrolyte interface producing a relative band edge shift yielding the observed flat band shifts. We observed a general trend of flat band potential shift of about 350 mV in the positive direction, as the pH of the deposition bath was changed from 7 to 12.5. The observed shift in the flat band potential in the positive direction is very useful for the water splitting reaction because the valence band edge of Cu2O is shifted positively relative to the oxygen redox potential. Our observation of highest photoresponse for Cu2O thin films prepared at pH 13.5 is a direct evidence for the positive shift of the band edges. The observation of the interface modification of Cu2O in aqueous electrolyte may be further extended to other suitable interfaces for developing Cu2O based junction devices.