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Item Electrodeposition and characterization of ZnO thin films for gas sensing(Faculty of Science, University of Kelaniya, Sri Lanka, 2020) Rathnaweera, U.M.C.; De Silva, D.S.M.; Atapattu, H.Y.R.The gas sensor is a sensing device that measures target gas molecules in a given atmosphere specially in the monitoring of environmental contaminants in air, water, and soil. Sensors based on semiconducting metal oxides are being widely used for gas or vapour sensing owing to their properties such as non-toxicity, biocompatibility, compact device structure, high sensitivity and stability and ease of syntheses. The most popular semiconducting metal oxides-based gas sensing materials are ZnO, SnO2, WO3 and TiO2. Among them, ZnO is attracted more due to its other properties such as chemical and photochemical stability and high-electron mobility. Hence, ZnO is one of the most propitious materials in developing sensors in electronic and optical technologies. In this study, characterization and fabrication of ZnO for gas sensing applications using a simple and cost-effective electrodeposition method is discussed. Aqueous electrolytic solutions of Zn (NO3)2 and ZnSO4 were used as the Zn precursors to find the best suited precursor to electrodeposit ZnO. The deposition was performed under a three-electrode electrochemical cell consisted of FTO coated glass (1×3 cm2 , 7 Ω/m2 ), graphite rod (99.995%) and a saturated Ag/AgCl electrode as the working, counter, and reference electrodes respectively. The gas sensing ability of the ZnO films, developed under different deposition parameters (cathodic deposition potential, pH of the electrolyte, precursor concentration), was studied. By obtaining the workable cathodic deposition potentials (CDP) by cyclic voltammetry, the k best suited bath pH and the temperature to develop uniform ZnO thin films were found to be 3.5 - 4.5 and 55 ℃ respectively. Subsequently, the heat treated (425 ℃ for 1 hour) samples were characterized with UV/Vis spectroscopy, X-ray diffraction, scanning electron microscopy and energy dispersive X-ray techniques to investigate the bandgap energy, crystal structure, surface morphology and the material’s composition respectively. The band gap energy of the material grown was fallen within 3.00 – 3.30 eV, while the crystals were found to be preferably grown along the [101] or [002] planes possessing hexagonal wurtzite structure in samples grown using two Zn precursors. SEM micrographs evidenced compact morphology with coral/rod-shaped appearance. According to the EDX analysis, Zn:O atomic ratio was revealed to be 1:1. The gas sensing ability of deposited films was examined against NO2 and H2S gases that causes due to electronic interactions between the crystallographic plane and the subjected gas molecules. The samples grown in 0.10 mol/L ZnSO4 at CDP 1.00 V in pH 4.50 at 55 ℃ was found to have an average sensitivity of 5% and 11% while the samples grown in 0.10 mol /L Zn (NO3)2 at CDP 1.10 V in pH 3.70 at 55 ℃ were found to have an average sensitivity of 2% and 5% after exposing to NO2 and H2S gases respectively for 5 minutes at 30 ℃.Item Effect of thermal annealing of CBD-CdS on the electrical properties of CBD-CdS/ED-CdTe solar cell(Faculty of Science, University of Kelaniya, Sri Lanka, 2020) Gajanayake, G.K.U.P.; De Silva, D.S.M.; Atapattu, H.Y.R.; Thivakarasarma, T.Chemical bath deposition, Electrodeposition, Thermal annealing, CdS/CdTe solar cell Thermal annealing is one of the key steps to enhance the optoelectronic properties of the CdS/CdTe solar cells. In this study, the effects of annealing temperature and annealing time of chemical bath deposited (CBD) CdS on the electrical properties of CBD-CdS/electrodeposited (ED) CdTe solar cells were investigated. CBD-CdS layers were prepared using pre-optimized deposition conditions (90 ℃, 55 min) on fluorine doped tin oxide (FTO) glass substrates utilizing a bath consisted of 0.033 mol/L Cd(CH3COO)2, 0.667 mol/L CS(NH2)2 as cadmium and sulfur precursors, respectively and therein, 1 mol/L CH3CO2NH4 and 0.735 mol/L NH4OH were used for pH adjustment. Thereafter, a set of CBD-CdS samples prepared was annealed at different temperatures (350, 375 and 400 ℃) by varying the annealing time (10, 20, 30, and 40 min). Consequently, CdTe thin films were electrodeposited on annealed CBD-CdS substrates using an ED-bath consisted of 1.0 mol/L CdSO4 and 1.0 mmol/L TeO2 at pH of 2.3, temperature of 65 ℃, and potential of -650 mV against a saturated calomel electrode. The prepared glass/FTO/CBDCdS/ED-CdTe samples were air annealed (400 ℃, 20 min) and Cu/Au back contacts were deposited using thermal evaporation technique. The electrical properties of the CBD-CdS samples were investigated by photo-electrochemical cell (PEC) study at the CBD-CdS/electrolyte junction. As per the PEC analysis, CBD-CdS sample annealed at 375 ℃, 30 min has shown the highest short circuit current density (Jsc) of 21.5 μA/cm2 , while the sample annealed at 400 ℃, 10 min shown the highest open circuit voltage (Voc) of 499 mV. The electrical properties of the CBD-CdS/ED-CdTe/Cu/Au devices were investigated under AM 1.5 light source and therein, CBD-CdS sample annealed at 375 ℃, 30 min scored the highest Jsc (14.12 mA/cm2 ) and the one annealed at 400 ℃, 10 min displayed the highest Voc (616 mV). Also, the device annealed at 375 ℃, 30 min showed the lowest series resistance (205 Ω) while the one annealed at 400 ℃, 10 min demonstrated the highest shunt resistance (1401 Ω). Accordingly, the 375 ℃, 30 min and 400 ℃, 10 min were found to be the effective conditions for annealing CBD-CdS that can result in materials with better electrical properties for CBDCdS/ED-CdTe/Cu/Au device fabrication.Item Electrodeposition of CdTe thin films using a two electrode system(Faculty of Science, University of Kelaniya, Sri Lanka, 2016) Wedisinghe, K.C.; Atapattu, H.Y.R.; de Silva, D.S.M.Cadmium telluride (CdTe) is a promising material for thin film solar cell applications due to its ideal band gap of ~1.5 eV which has the ability to absorb the maximum of the solar spectrum and higher conversion efficiency of sun light. Among the various deposition techniques available to produce CdTe semiconductor material in commercial quantities, electrodeposition has drawn more attention due to its simplicity, scalability and easy control of the material properties through growth parameters; applied potential, temperature, pH and the composition of the bath etc. Since the reference electrode could be a potential impurity source in the conventional three electrode electrolytic system, this study was mainly focused on the use of two electrode electrolytic system to determine suitable deposition potential and pH ranges for growth of CdTe thin film while avoiding the influence of impurities. The two electrode electrolytic cell consisted of, fluorine doped tin oxide coated glass substrate as the working electrode and 99.99% pure carbon electrode as counter electrode was used for the deposition of CdTe thin films. The electrolyte contained analytical grade reagents of 1.25 mol/L CdSO4 and 1.0 mmol/L TeO2 as cadmium and tellurium precursors respectively. Prior to electrodepositions, pH of the electrolytic baths were varied from 2.0 to 2.4 at 25 °C. While changing the cathodic deposition potentials in the range of (1.30 - 1.37) V, the CdTe depositions were carried out stirring the bath at 60 rpm and at the temperature of 65 °C. Following the heat treatment of the samples for 10 minutes at 400 °C in air, the characterization of CdTe thin films was carried out based on optical absorption, photo-electrochemical cell, X-ray diffraction and scanning electron microscopic studies. The results of the study indicate that, CdTe thin films can be successfully grown in the cathodic potential range of (1.34 -1.35) V and at a pH of 2.2 using two electrode electrolytic system.Item Electrodeposition of well-adhered CdTe thin films for solar cell applications(Faculty of Science, University of Kelaniya, Sri Lanka, 2016) Atapattu, H.Y.R.; de Silva, D.S.M.; Pathiratne, K.A.S.Among the second generation thin film photovoltaics, CdS/CdTe based solar cell device is one of the leading contenders for large scale commercialization. Since the CdTe is the crucial absorber material of the foregoing device, it is essential to maintain a well-adhered CdTe layer to obtain high photovoltaic activities. If not, loosened CdTe layers with numerous pinholes can reduce the electrical, optical, structural and morphological properties of the material and hence extinguish the entire activities of CdS/CdTe solar cells. In the present study, an electrodeposition procedure was developed to fabricate welladherent CdTe layers to the substrate using the typical three electrode electrolytic cell. A fluorine doped tin oxide conducting glass substrate (7Ω/sq.) with dimensions of (1×3) cm2 was used as the working electrode in the cell. A saturated calomel electrode and a high purity graphite rod served as reference and counter electrodes respectively. All the electrodepositions were carried out using an aqueous solution containing 1.0 mol/L CdSO4, 1.0 mmol/L TeO2 and 5.5 mmol/L CdCl2. Based on the cyclic voltammetry studies and the stoichiometry of the proposed chemical reaction which forms CdTe material, the possible cathodic deposition potential (CDP) and pH ranges were identified to be in the ranges of 550-710 mV and 1.4-2.4 respectively. Henceforth, CdTe layers were electrodeposited at above mentioned conditions at temperature of 65 °C and subsequently annealed in air at 400 °C for 10 min. Thereafter, by considering the physical appearance of deposited CdTe layers and their adhesiveness upon a high pressure N2 flow, the feasible values for CDP and pH were found to be in the ranges of 590-660 mV and 2.0-2.4 respectively. To further fine-tune the values for CDP and pH, a series of CdTe layers were deposited at above feasible growth conditions and inspected for their electrical, optical, structural and morphological properties using the methods of photo-electrochemical cell, optical absorption spectroscopy, X-ray diffraction and scanning electron microscopy respectively. Results revealed that, the optimum CDP is in the range of 620-660 mV and pH is in the range of 2.1-2.3 to exhibit good photovoltaic qualities.