Browsing by Author "Lakmal, A. A. I."

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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
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.
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.
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.