Browsing by Author "Gajanayake, G. K. U. P."

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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.
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.
Introducing an ED-CdTe nucleation layer on the CBD-CdS layer in solar cell fabrication
(Faculty of Graduate Studies, University of Kelaniya Sri Lanka, 2022) Gajanayake, G. K. U. P.; Silva, D. S. M. D.; Thanihaichelvan, M.
Developing a cost-effective and efficient thin-film CdS/CdTe solar is vital in resolving the energy crisis in the world. In the fabrication of solar cells, the weak interlayer contact caused by the voids between the window and the absorber layer adversely affected the performance of the device. This study explores the effectiveness of introducing an intermediate ultra-thin CdTe nucleation layer on reducing the Te diffusion into CdS and recombination effect to mitigate layer mismatch between the window and the absorber layer. Herein, the CdS layer was grown on the glass/FTO substrates which were subjected to prior chemical and plasma cleaning processes. In CdS deposition, Cd(CH3COO)2 (0.033 mol/L), CS(NH2)2 (0.667 mol/L) were utilized as cadmium and sulfur precursors while CH3CO2NH4 (1.0 mol/L) and NH4OH (25%) were used to adjust the pH in the bath at 90 ℃. The deposited glass/FTO/CBD-CdS samples were sonicated and dried with N2 flow. Next, an ultrathin CdTe nucleation layer was electrodeposited in an electrolyte containing 1.0 mol/mL CdSO4 and 1.0 mmol/mL TeO2 in pH 2.3 at 65 ℃ for 40 s on the glass/FTO/CBD-CdS. Herein, the graphite sheet (counter electrode), saturated calomel electrode (reference electrode) and glass/FTO/CdS substrate (working electrode) were used at a cathodic deposition potential of -650 mV with respect to the SCE. The optical property analysis revealed that the energy band gap of the glass/FTO/CBD-CdS and glass/FTO/CBD-CdS/ED-CdTe declined from 2.39 eV to 2.37 eV and their optical transmittance is over 80% in the wavelength ranges of 520-900 nm and 535-900 nm, respectively, in the devices. Therefore, there is no adverse effect of the ED-CdTe nucleation layer on light absorption by the window layer. The miniature shrinkage of the band gap might arise due to Te diffusion into the CdS layer. The SEM cross-sectional analysis uncovered that the thickness of the CBD-CdS/ED-CdTe bilayer was ~102 nm, and further, there is no distinguishable boundary between the ED-CdTe layer and CBD-CdS layer. No drastic change in surface roughness was detected between CBD-CdS/ED-CdTe and CBD-CdS. The SEM and AFM imaging further evidenced the improved uniformity of the surface layer upon deposition of the nucleation layer. Hence, the well packed ultra-thin ED-CdTe layer developed on glass/FTO/CBD-CdS substrate facilitates the growth of the CdTe absorber layer while minimizing the possible degradation or contamination of the CdS layer underneath upon exposure to high-temperature deposition by the close-spaced sublimation of CdTe.
Introducing Spin‑coated ZnO Anti‑reflection Coating for CdS/CdTe Solar Cells
(The Minerals, Metals & Materials Society, 2024) Wijesingha, J. R.; Gajanayake, G. K. U. P.; Wickramasinghe, W. A. V. U.; Damayanthi, R. M. T.; De Silva, G. I. P.; De Silva, D. S. M.
Second-generation solar cells, commonly known as thin-film solar cells, have emerged as promising alternatives to traditional silicon-based first-generation photovoltaic cells. The superstrate configuration is the most widely used structure for constructing thin-film solar cells. Nevertheless, light reflection from the front cover glass surface significantly contributes to energy losses in thin-film solar cells. In this study, a ZnO anti-reflection (AR) coating was introduced using the spin coating technique on a glass/FTO/CdS/CdTe/Cu/Au substrate to improve the power conversion efficiency of the solar cell by reducing front-surface reflectance. The ZnO layer deposited at 3000 rpm in 15 s showed the minimum reflectance and higher transmittance over a wavelength range of 500–900 nm. Further, the thickness of the film under optimal conditions was 63.32 nm, which is compatible with the ideal theoretical AR coating thickness of 65 nm. Comparing the device performance of the CdS/CdTe solar cell with and without AR coating, all tested devices showed an average short-circuit current density improvement of 6.8% and overall enhancement in power conversion efficiency of 9.3%.
Investigation of electrically insulating and thermally conductive materials for a Peltier module with n-Cu2O and p-Cu2O electrodeposited semiconductors.
(International Research Symposium on Pure and Applied Sciences, 2017 Faculty of Science, University of Kelaniya, Sri Lanka., 2017) Gajanayake, G. K. U. P.; Abeywarna, U. K.; Wijesundera, L. B. D. R. P.
Thermoelectricity is a direct conversion of electrical energy into thermal energy and vice versa. Seebeck effect, Peltier effect and Thomson effect are thermoelectric effects observed for conductors during 1820-1920. After the development of semiconductors, a new era has begun in the field of thermoelectricity. Currently, the rare earth materials and their alloys are commonly used as semiconductors for constructing thermoelectric devices. In this research, copper based n-type Cu2O and p-type Cu2O semiconductors were used as thermoelectric materials. These semiconductors were deposited using potentiostatic electrodeposition technique in acetate bath. The study was carried out using two types of electrically insulating and thermally conductive material such as Thermal Heat Sink Transfer cooling pads (THST cooling pads) and mica. Multi stage Peltier modules were constructed using single stages assembling electrically in series and thermally in parallel. For the THST cooling pads, two stage Peltier module was assembled and a temperature gradient of 0.52 OC mm-1 at 2.0 V dc voltage was observed. However, the current through the module was higher and higher compared to the previous measurements when the data was repeated. It was found that the appearance of the semiconductor samples were changed when disassembling the module due to some fluid secreted from the THST cooling pads. The study was also done for both artificial and natural mica as electrically insulating and thermally conductive materials. For the artificial mica, a five stage Peltier module was built and it was observed 9.57 OC mm-1 temperature gradient for 2.0 V dc voltage. Having a three stage module of natural mica the temperature of the module decreases at the beginning and then increased while voltage was increased. A miniature temperature drop of 0.4 OC was able to achieve below the room temperature at 2.75-5.5 V range for the module with natural mica. In addition to that, a temperature gradient of 0.48 OC mm-1 was observed for this set up throughout the applied voltage. Present study demonstrated Peltier effect for both electrodeposited n-type Cu2O and p-type Cu2O semiconductors. The heating effect was always detected for the set ups with THST cooling pads and artificial mica as an insulating materials. However, the experimental data were not repeated for the semiconductor samples assembling with THST cooling pads due to some secretion of fluid. Therefore, THST cooling pads are not suitable for this kind of study. When considering artificial and natural mica as an insulating material, a higher temperature gradient was observed for artificial mica whereas a temperature drop was achieved using natural mica. Outcome of this study indicates that electrodeposited n-type and p-type Cu2O semiconductors along with mica as electrically insulating and thermally conductive materials can be used for a Peltier module.