Browsing by Author "Thivakarasarma, T."

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    Effect of thermal annealing of CBD-CdS on the electrical properties of CBD-CdS/ED-CdTe solar cell
    (Faculty of Science, University of Kelaniya, Sri Lanka, 2020) Gajanayake, G.K.U.P.; De Silva, D.S.M.; Atapattu, H.Y.R.; Thivakarasarma, T.
    Chemical bath deposition, Electrodeposition, Thermal annealing, CdS/CdTe solar cell Thermal annealing is one of the key steps to enhance the optoelectronic properties of the CdS/CdTe solar cells. In this study, the effects of annealing temperature and annealing time of chemical bath deposited (CBD) CdS on the electrical properties of CBD-CdS/electrodeposited (ED) CdTe solar cells were investigated. CBD-CdS layers were prepared using pre-optimized deposition conditions (90 ℃, 55 min) on fluorine doped tin oxide (FTO) glass substrates utilizing a bath consisted of 0.033 mol/L Cd(CH3COO)2, 0.667 mol/L CS(NH2)2 as cadmium and sulfur precursors, respectively and therein, 1 mol/L CH3CO2NH4 and 0.735 mol/L NH4OH were used for pH adjustment. Thereafter, a set of CBD-CdS samples prepared was annealed at different temperatures (350, 375 and 400 ℃) by varying the annealing time (10, 20, 30, and 40 min). Consequently, CdTe thin films were electrodeposited on annealed CBD-CdS substrates using an ED-bath consisted of 1.0 mol/L CdSO4 and 1.0 mmol/L TeO2 at pH of 2.3, temperature of 65 ℃, and potential of -650 mV against a saturated calomel electrode. The prepared glass/FTO/CBDCdS/ED-CdTe samples were air annealed (400 ℃, 20 min) and Cu/Au back contacts were deposited using thermal evaporation technique. The electrical properties of the CBD-CdS samples were investigated by photo-electrochemical cell (PEC) study at the CBD-CdS/electrolyte junction. As per the PEC analysis, CBD-CdS sample annealed at 375 ℃, 30 min has shown the highest short circuit current density (Jsc) of 21.5 μA/cm2 , while the sample annealed at 400 ℃, 10 min shown the highest open circuit voltage (Voc) of 499 mV. The electrical properties of the CBD-CdS/ED-CdTe/Cu/Au devices were investigated under AM 1.5 light source and therein, CBD-CdS sample annealed at 375 ℃, 30 min scored the highest Jsc (14.12 mA/cm2 ) and the one annealed at 400 ℃, 10 min displayed the highest Voc (616 mV). Also, the device annealed at 375 ℃, 30 min showed the lowest series resistance (205 Ω) while the one annealed at 400 ℃, 10 min demonstrated the highest shunt resistance (1401 Ω). Accordingly, the 375 ℃, 30 min and 400 ℃, 10 min were found to be the effective conditions for annealing CBD-CdS that can result in materials with better electrical properties for CBDCdS/ED-CdTe/Cu/Au device fabrication.
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    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.