Browsing by Author "Atapattu, H. Y. R."

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An alternative sulfur precursor for chemical bath deposition of CdS thin film
(Gajanayake, G. K. U. P., De Silva, D. S. M and Atapattu, H. Y. R. (2019). An alternative sulfur precursor for chemical bath deposition of CdS thin film. 4th International Research Symposium on Pure and Applied Sciences, Faculty of Science, University of Kelaniya, Sri Lanka. p98, 2019) Gajanayake, G. K. U. P.; De Silva, D. S. M.; Atapattu, H. Y. R.
Solar energy is the most appropriate electrification method for a tropical county like Sri Lanka. At present, Sri Lanka generates more than 72% of its electricity needs at a high cost by burning coal and diesel. To overcome the major obstacle of high initial cost in installation of solar power plants, many research groups worldwide at present are focusing towards manufacture of low cost and highly efficient photovoltaic cells based on cadmium sulfide and cadmium telluride (CdS/CdTe) semiconductors. Among the range of methods available for fabrication of CdS window layer, the chemical bath deposition (CBD) is an ideal method due to its simplicity and low cost. In this study, CdS layers were deposited on the FTO glass substrate by CBD method, using an alternative sulfur precursor; ammonium thiosulfate ((NH4)2S2O3) against the well-established but costly precursor thiourea (CS(NH2)2). The CBD bath was prepared with 0.25 mol/L cadmium acetate (Cd(CH3COO)2), 1.00 mol/L ammonium acetate (NH4COOCH3), concentrated NH4OH (pH adjuster), and 0.50 mol/L (NH4)2S2O3. The best growth condition for CdS was identified by varying the parameters; Cd:S ratio, pH, deposition temperature, and deposition time while preserving a constant stirring speed. Uniform CdS layers rich in Cd, were observed in an alkaline electrolyte with Cd:S ratio of 2:5 at a temperature of 95 °C in 90 minutes. The spectrophotometric studies revealed the energy band gap of the material as 2.41 eV which is the typical value for CdS. Further, the X-ray diffractions observed at angles of 26°, 28°, 36°, and 53° representing the planes of (002), (101), (102), and (201) verified the cubic structure, while the scanning electron microscopic studies confirmed the uniform surface morphology of the material with average grains sized of 105 nm. However, the presence of pin-holes observed in the cross-sectional view implied the need of further optimization of parameters to obtain materials comparable to thiourea based chemical bath deposited CdS layers.
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.
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.
Fabrication of FTO/CBD-CdS/ED-CdTe/Cu/Au solar cells and boosting its performance by CdCl2 treatment
(Faculty of Science, University of Kelaniya, Sri Lanka, 2021) Gajanayake, G. K. U. P.; Silva, D. S. M. De; Atapattu, H. Y. R.; Lakmal, A. A. I.
The thin film CdS/CdTe solar cells are promising cost-effective clean energy generating devices against the global energy crisis. Chemical bath deposition (CBD) and electrodeposition (ED) were recognized as being simple and low-cost techniques over a range of growth techniques available for development of CdS and CdTe thin films respectively. The use of aforesaid two techniques successively in fabrication of glass/FTO/CBD-CdS/ED-CdTe solar cells was not reported. This attempt is to do so and moreover, to assess the effect of CdCl2 treatment in performance enhancement of the device produced. In preparation of thin CBD-CdS layers on FTO glass substrate, a bath consisted of Cd(CH3COO)2 (0.033 mol/L), CS(NH2)2 (0.667 mol/L), CH3CO2NH4 (1.0 mol/L) and NH4OH (25%) was employed at 90 ℃. Annealed (375 ℃ for 30 min) CBD-CdS samples were subjected to CdTe deposition by ED system equipped with a three electrodes system. Herein, the CdS thin films were specifically developed enabling them to withstand in a highly acidic bath during the ED process. The ED bath used consisted of CdSO4 (1.0 mol/L) and TeO2 (1.0 mmol/L) at pH 2.3 and 65 ℃. The potential of -0.650 mV was maintained between the reference and working electrodes during each deposition (3 hrs). Samples were sprayed with CdCl2 solution (1.0 mol/L) for 2 s and then annealed (390 ℃ for 15 min). Back contacts (Cu/Au) were deposited on the CdCl2 treated glass/FTO/CBD-CdS/ED-CdTe devices by thermal evaporation. The devices were characterized under the illumination of AM 1.5 (100 mW/cm2). The efficiencies of the CdCl2 treated devices were found to be higher (6.23%) than untreated ones (2.66%). A significant variation in Jsc, Voc, and FF values was observed in CdCl2 treated devices (24.68 mA/cm2, 664 mV, and 38.0%) over untreated devices (14.95 mA/cm2, 531 mV, and 33.5%). The SEM analysis revealed remarkable increment in CdTe grain sizes (~140 nm to ~591 nm) with less grain boundaries in the CdTe sample upon CdCl2 treatment, thus leading to improved photovoltaic performance. This work demonstrated that CdS and CdTe can be synthesized using cost effective methods of CBD and ED respectively and, the FTO/CBD- CdS/ED-CdTe/Cu/Au device efficiency can be significantly improved by the CdCl2 treatment.
The influence of substrates on the device performance of the TCO/CBD-CdS/ EDCdTe and TCO/CBD-CdS/CSS-CdTe solar cells
(Faculty of Science, University of Kelaniya Sri Lanka, 2023) Gajanayake, G. K. U. P.; Bandara, K. M. N. S.; De Silva, D. S. M.; Atapattu, H. Y. R.
The development of cost-effective efficient photovoltaic cells is crucial for generating electricity with the most abundant solar energy to eliminate the energy crisis globally. At present, there is a growing interest in CdS/CdTe solar cells due to minimal material cost and easy and cost-effective methods of thin film deposition. The aim of this work is to investigate the influence of different transparent conducting oxide (TCO) substrates in superstrate configuration (glass/ITO:5 Ω/sq, glass/FTO:13 Ω/sq, and glass/FTO:7 Ω/sq) on the device performance of CdS/CdTe solar cells. Herein, chemical bath deposited CdS (CBD-CdS) layers were grown using 0.0333 mol/L Cd(CH3COO)2, 0.0667 mol/L CS(NH2)2, concentrated NH4OH and 1.0 mol/L NH4(CH3COO) at 90 ℃ for 55 min. Subsequently, the CdTe layers were deposited using electrodeposition (ED) and close spaced sublimation (CSS) techniques as required. For electrodeposition of CdTe layers, CdSO4 (1.0 mol/L) and TeO2 (1.0 mmol/L) precursors were used at pH of 2.3 and 65 ℃ and deposition was run for 3 hrs. The CSS-CdTe layers were developed by maintaining the substrate and source temperature at 580 °C and 640 °C, respectively, and the deposition proceeded for 25 min. at 7.9 Torr. The glass/TCO/CBD-CdS/ED-CdTe samples were treated with CdCl2, and glass/TCO/CBD-CdS/CSS-CdTe were undergone NP etching as suitable post-deposition treatments. The device fabrication was completed with the back contact formation (Cu/Au). The devices; glass/TCO/CBD-CdS/ED-CdTe/Cu/Au and glass/TCO/CBD-CdS/CSS-CdTe/Cu/Au prepared with FTO:13 Ω/sq delivered the highest efficiency of 5.7% (JSC = 19.2 mA/cm2, VOC = 0.672 V, FF = 44%) and 8.6% (JSC = 30.3 mA/cm2, VOC = 0.606 V, FF = 47%), respectively while the cells prepared with glass/ITO:5 Ω/sq delivered the lowest efficiency. Hence, the glass/FTO:13 Ω/sq substrate was recognized as the most appropriate substrate for the fabrication of CBDCdS/ ED-CdTe and CBD-CdS/CSS-CdTe solar cells. The resultant optical transmittance (over 80%, above 500 nm) and surface roughness (RMS roughness of bare FTO:13 Ω/sq was 12.49 nm, and FTO:13 Ω/sq/CBD-CdS was 10.15 nm) of CBD-CdS further confirmed the suitability aptness of the glass/FTO:13 Ω/sq substrate in CdS/CdTe based solar cell fabrication.