Browsing by Author "Kumarasinghe, R. K. K. G. R. G."

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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
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.