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Item Effect of pH on the morphology of chemical bath deposited ZnO nanowires(Faculty of Science, University of Kelaniya Sri Lanka, 2023) Jayathilaka, A. P. S. P.; Abeykoon, Y. K.; Dassanayake, B. S.; Dasanayake, N. L.ZnO is a non-toxic chemical compound that forms nanostructures like nanoparticles, nanowires, nanotubes, and so on. Among these nanostructures, ZnO nanowires have shown the potential for use in applications like light-emitting diodes (LED), lasers, light detectors, chemical and biological sensors, photovoltaic cells, field-effect transistors (FET), nanogenerators, and so forth. ZnO nanowires can be synthesised using a variety of techniques, such as chemical vapour deposition (CVD), chemical bath deposition (CBD), and hydrothermal methods. Among these methods, CBD is a cost-effective, simple method to fabricate well-crystalline ZnO nanowires. The morphology of the ZnO nanostructures greatly varies with the alteration of growth parameters like growth temperature, growth time, precursor concentration, and pH level of the precursor. Herein, pH is an easily controllable parameter that has a significant effect on the morphology of ZnO nanowires as well. In this research, we synthesised ZnO nanowires using the CBD method and investigated the effect of the pH level of the precursor on the morphology of the assynthesised ZnO nanowires. The pH values 2, 4, 6, 6.5, 7, 8, and 10 were selected for the study, and the nanowires were grown on a seed layer for better crystallinity. First, the seed layer was deposited on a borosilicate glass slide by spraying the seed solution (3.69 g of Zn(CH3COO)2, 18.9 ml of Ethanol, 0.1 ml of Monoethanolamine, 100 ml of deionised water) using the spray pyrolysis technique. Then, the sample was annealed at 300 ̊C for 1 hour. Next, the precursor solution (3.83 g of Zn(NO3)2, 2.82 g of Hexamethylenetetramine, 200 ml of deionised water) was subjected to pH alteration using dil. HCl or dil. NH4OH and was then heated to 90 ̊C while magnetic stirring at 700 rpm. After that, the seed-layered glass slide was submerged in the solution for the chemical bath deposition for 2 hours. Finally, the sample was annealed at 300 ̊C for 30 minutes. The above-mentioned process was repeated for all the understudied pH values. Characteristics of the ZnO nanowire samples, such as average values of height, width, aspect ratio (height/width), surface-to-volume ratio, and wire density, were analysed using scanning electron microscopic (SEM) images. The outlook and the quantitative analysis of the SEM images showed that the most crystallised nanowires (hexagonal wurtzite shape) corresponded to pH 6.5, with the highest aspect ratio (10.308) and the lowest nanowire density (44 nanowires per μm2). The highest surface-to-volume ratio was reported for the nanowires grown at pH 7 (1.049), followed by pH 6.5 (1.037). In general, ZnO nanowires grown at pH 6.5 showed the highest crystallinity in the understudied pH range.Item Thermally evaporated copper iodide hole transport layer for CdS/CdTe thin film solar cells(Faculty of Science, University of Kelaniya, Sri Lanka, 2021) Thivakarasarma, T.; Lakmal, A. A. I.; Dassanayake, B. S.; Velauthappillai, D.; Ravirajan, P.CdS/CdTe thin-film solar cell is a cost effective and reliable photovoltaic device with reported power conversion efficiencies over 22%. Although large-scale thin-film solar panels with efficiency over 18 % are commercially available, it has been reported that the efficiency drops due to copper diffusion to the CdS/CdTe interface. To avoid the Cu diffusion in these devices, Cu-free back contacts have been introduced in the past with reasonable success. This work focuses on studying the photovoltaic performance of CdS/CdTe devices by replacing Cu with copper iodide (CuI). For the device fabrication, the n-CdS window layer was fabricated by the chemical bath deposition (CBD) method on a cleaned FTO substrate, and then the p-CdTe absorber layer was deposited by closed space sublimation (CSS) on top of the CdS layer at a substrate temperature and source temperature of 580 ˚C and 640 ˚C, respectively in argon gas medium for 25 minutes at 7.9 torr vacuum pressure. In order to study the effect of a CuI hole transport layer on photovoltaic performance of CdTe solar cells, CuI film of varying thicknesses from 5 nm to 30 nm were deposited on the CdTe films by thermal evaporation. After the CuI film deposition, Au layer of thickness 80 nm was thermally evaporated as a back electrode, and then the fabricated device was annealed at 200 °C for 10 min in an N2 environment. The UV-Visible spectroscopic studies confirmed that bandgap of thermally evaporated CuI hole transporter, chemically deposited n-CdS window layer and close spaced sublimated p-CdTe absorber layer are 3.0, 2.4 and 1.5 eV respectively. The XRD studies not only confirmed the presence of each layer but also confirmed the phase of thermally evaporated CuI film was hole-transporter (γ-CuI). AFM analysis confirmed the homogeneous well-adhered nature of each layer. Finally, photovoltaic performance of the devices with CuI film of thickness 5 nm to 30 nm was characterized under illuminations of 100 mW/cm2 (1 sun) with an Air Mass 1.5 filter. An optimized CdS/CdTe device with CuI thickness of 10 nm showed Power Conversion Efficiency of 6.92 % with JSC, VOC, and FF of 21.98 mA/cm2, 0.64 V, and 0.49 respectively.Item 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.Item 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