Browsing by Author "Seneviratne, V. A."

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Effect of substrate temperature variation on opto-electronic properties of thermally evaporated CdS thin films
(4th International Research Symposium on Pure and Applied Sciences, Faculty of Science, University of Kelaniya, Sri Lanka, 2019) Lakmal, A. A. I.; Kumarasinghe, R. K. K. G. R. G.; Maddumage, D. C.; Kumarage, W. G. C.; Munasinghe, M. A. H. M.; Seneviratne, V. A.; Dassanayake, B. S.
Cadmium sulfide (CdS) is a II-VI group semiconducting material which has been thoroughly investigated due to its superior optical and electrical properties that can be applicable in wide range of semiconductor devices including photonic devices. Due to its direct and wide bandgap (~ 2.42 eV), it is vastly used as the window layer in heterojunction thin film solar cells. Compared to other deposition methods such as electrodeposition, spray pyrolysis, chemical bath deposition; thermal evaporation is an attractive method of deposition due to its high deposition rate, low cost of operation, low material consumption, minimum number of impurities and straight-line propagation of vapors. In the present study, CdS thin films were deposited on cleaned FTO glass substrates using thermal evaporation technique at substrate temperatures ranging from 50 to 250 °C at a pressure of 2×10-5 torr. Deposition was carried out using CdS powder (Sigma-Aldrich, 99.995%) using an alumina boat. Deposited samples were then annealed at 300 °C for 30 minutes in vacuum (pressure of 3×10-5 torr). Structural, optical and electrical properties of annealed CdS thin films were studied by employing X-ray diffraction, UV-Vis spectrometry, I-V measurements and capacitance vs. voltage measurements. All the electrical characterizations were carried out using a photoelectrochemical cell of (CdS/0.1 M Na2S2O3/Pt). The XRD analysis shows all the grown films are preferably oriented in the direction of (002) of hexagonal CdS. The optical band gap values were found to increase with increasing substrate temperature from 50 to 175 °C. ISC and VOC values of (CdS/0.1 M Na2S2O3/Pt) cell were also found to increase up to the substrate temperature of 175 °C. The observed highest ISC and VOC values were 37.24 μA and 314.9 mV respectively. Results indicate that the CdS thin films deposited at the substrate temperature of 175 °C has yielded the best optical and electrical properties compared to the films grown at other substrate temperatures
Growth and Characterization of Seed‑Assisted, EDTA‑Treated, Chemical Bath‑Deposited CdS
(Journal of Electronic Materials, 2021) Kumarage, W. G. C.; Wijesundera, R. P.; Seneviratne, V. A.; Jayalath, C. P.; Gunawardhana, N.; Kaur, N.; Comini, E.; Dassanayake, B. S.
A simple low-cost method to enhance the electrical properties including open-circuit voltage (VOC), flat-band potential (Vfb) and short-circuit current (ISC) in the photoelectrochemical (PEC) cell of cadmium sulfide (CdS) thin films is presented. The PEC cell properties were determined using the configuration Pt/0.1 M Na2S2O3/ CdS. Three different sets of CdS thin films were grown: (a) chemical bath-deposited CdS (CBD-CdS), (b) electrodeposited seed-assisted CBD-CdS (ED/CBD-CdS) and (c) ED/CBD-CdS deposited under the presence of ethylenediaminetetraacetic acid (EDTA) in a reaction solution of CBD (ED/(CBD+EDTA)-CdS). The FE-SEM images suggested the formation of clusters with spherical shape in the presence of a seed layer. All the samples grown with seed layers demonstrated improved ISC and VOC values in the PEC cell compared to the CBD-CdS films due to better contact between the substrate and CBD-CdS. Furthermore, the carrier concentration (ND) and Vfb were also found to improve due to the introduction of the seed layer. In the case of ED/(CBD+EDTA)-CdS, the cluster size was found to be smaller, giving rise to a larger effective surface area. The improved effective surface area, interparticle connections and adhesion of CdS to the FTO substrate resulted in superior electrical properties of ED/(CBD+EDTA)-CdS compared to ED/CBD-CdS and CBD-CdS films.
Investigation of the effect of source temperature on close-spaced sublimated CdTe thin films
(Faculty of Science, University of Kelaniya, Sri Lanka, 2021) Lakmal, A. A. I.; Kumarasinghe, R. K. K. G. R. G.; Seneviratne, V. A.; Dassanayake, B. S.
Cadmium telluride (CdTe) is one of the most promising II-VI group semiconductors used to fabricate heterojunction thin-film solar cells. Close-spaced sublimation is one of the best techniques for the deposition of polycrystalline CdTe thin films. In this study, CdTe thin films were deposited on the cleaned FTO glass substrates using the close-spaced sublimation technique by varying the source temperature from 560 °C to 720 °C in steps of 20 °C. The temperature of the substrates, source to substrate separation, and deposition duration were maintained at 540 °C, 4 mm, and 5 minutes respectively. Ar(g) was introduced to the vacuum chamber, keeping the pressure at 7.9 Torr. The deposition was carried out using high purity CdTe powder placed in a graphite crucible. An almost transparent thin CdTe layer was observed at the source temperature of 560 °C. In comparison, a slightly decomposed layer was seen when the source temperature was 720 °C, which could be considered two boundary points in the temperature range selected. The CdTe layer deposited at source temperature 580 °C had a better thickness compared to 560 °C and pinholes could be visible to the naked eye. The average transmittance beyond the absorption edge was decreased with the increment of source temperature. The optical bandgaps of all samples were in the range of 1.48 - 1.50 eV. The crystallinity of the deposited thin films was shown an increasing trend with the increment of source temperature. According to the SEM analysis, the increment of source temperature has led to better grain enhancement. Based on the above characterizations, the optimum source temperature was determined as 660 °C. To further confirm this result, CdS/CdTe full cells were fabricated by depositing the above CdTe layers on thermally evaporated CdS films with back contacts in the order of Cu before Au. Among the CdS/CdTe/Cu/Au cells fabricated for electrical characterization, the highest efficiency was obtained for the source temperature of 660 °C.
A study on a natural rubber and coconut coir-based carbon black composite
(Faculty of Science, University of Kelaniya Sri Lanka, 2023) Fernando, H. S. R. R.; Seneviratne, V. A.
Carbon black (CB) is extensively employed as a reinforcing filler in various industrial applications. Widely available polymer-filler composites are synthetic. A main motive of this research was to synthesise and characterise an eco-friendly polymer composite for applications in bicycle tyre and tube industry. The study utilised coconut coir to produce CB and used it as the filler in natural rubber (NR). The coconut coir-based carbon black (CCCB) was prepared by the combustion of coconut coir at 200 0C in a nitrogen atmosphere. Instead of sulphur, pentane-1,5- diylidenediamine (PDD) with equal amounts of ammonia and glutaraldehyde was employed as a cross-linking agent at a low temperature to process NR. The NR and CCCB composites (NRCCCB) were synthesised incorporating 50 mL of NR latex, 6 ml of PDD solution, and different amounts of CCCB. The mixture was stirred at 45 0C for 3 hours, casted into the moulds and made NR-CCCB composites having different weight percentages of CCCB (0, 2, 4, and 6%). The structural properties and mechanical properties, including Young’s modulus, tensile strength, and elongation at break, were assessed to evaluate the influence of CCCB on the composite material. X-ray diffraction was conducted on CCCB, and scanning electron microscopy was conducted on both the composites and CCCB to understand the structural changes and morphological characteristics. The X-ray diffraction analysis indicated that the CCCB is inexhibit an amorphous phase. Morphological analysis revealed a uniform dispersion of CCCB within the rubber composite, which contributed to the enhanced mechanical properties observed in the composites. The particle size of CCCB was estimated to be 70-80 μm. The density of NR-CCCB increased from 887 kg m-3 to 1241 kg m-3 by 40% with the increase in filler loading. Universal testing machine was used to study the mechanical properties of NR-CCCB and the reference bicycle tube. The results demonstrate the increase of tensile strength and Young’s modulus of NR-CCCB 0.75×106 N m-2 to 1.32×106 N m-2 and from 0.72×106 N m-2 to 1.25×106 N m-2 respectively, with increasing filler content, resulting in increased hardness, stiffness, and strength of the rubber. However, the elongation at break of NR-CCCB decreased from 476% to 422% with increasing filler content, indicating a decrease in the flexibility of the NR composites. The density and Young’s modulus of the reference bicycle tube were measured as 1262 kg m-3 and 5.67×106 N m- 2 respectively.