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    Effect of Concentration of Cd2+ on the Material Properties of CdTe Thin Films Electrodeposited via a Two Electrode Electrolytic Cell
    (Faculty of Graduate Studies, University of Kelaniya, Sri Lanka, 2016) Wedisinghe, K.C.; Atapattu, H.Y.R.; de Silva, D.S.M.
    Cadmium telluride (CdTe) is a vital semiconductor material which can absorb most of the electromagnetic radiation of solar energy spectrum to yield more efficient solar cells. Among the thin film fabrication methods, electrodeposition is an emerging technique to produce good quality CdTe materials using either two or three electrode system. This study was performed using the two electrode electrolytic system to investigate the effect of Cd2+ concentration on the material properties of CdTe thin films to enhance its photovoltaic activities. Electrodepositions of CdTe were carried out in an aqueous electrolyte which was prepared utilizing analytical grade CdSO4 as the cadmium and TeO2 as the tellurium precursors. Fluorine doped tin oxide coated conducting glass (1×3 cm2) and high purity carbon electrode were used as the working (cathode) and counter electrodes (anode) respectively in the electrolytic cell used for the depositions. The concentration of TeO2 was maintained around 1 mmol/L throughout while varying the CdSO4 concentration from 1.00 mol/L to 1.50 mol/L. pH of the electrolyte was initially adjusted to 2.2 and depositions were carried out in the cathodic potential range of (1.33 - 1.37) V at 65 °C while stirring at continuous stirring rate of 60 rpm. The resulting electrodeposited CdTe layers were heat treated for 10 minutes at 400 °C in air and subsequently, their optical, electrical morphological and structural properties were studied using UV-visible spectrophotometry, photo-electrochemical cell, scanning electron microscopy and X-ray diffraction spectroscopy. As elucidated by the results, at the deposition potentials of 1.34 V and 1.35 V, the short circuit current and open circuit voltage values were increased with the increasing of concentration of Cd2+ from 1.00 mol/L to 1.25 mol/L while the band gap energy reached to its theoretical value of 1.50 eV.
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    Growth of CdS and CdTe thin film semiconductors and fabrication of CdS/CdTe solar cells
    (Faculty of Graduate Studies, University of Kelaniya, 2015) Kumarasinghe, K.D.M.S.P.K.; de Silva, D.S.M.; Pathiratne, K.A.S.; Dharmadasa, I.M.; Salim, H.I.; Abdul-Manaf, N.A.; Ravirajan, P.; Balashangar, K.
    Thin films of CdS and CdTe semiconductor materials were electrodeposited onto glass/fluorine doped tin oxide conducting glass surfaces using a potentiostat/galvanostat equipped with a three electrode cell. Aqueous electrolytic bath containing CdCl2 and (NH4)2S2O3 was used for the electrodeposition of CdS thin films. CdTe thin films were electrodeposited onto glass/FTO/CdS substrates from aqueous solution having high concentrations of CdSO4 and low concentrations of TeO2 and CdCl2. The glass/FTO/CdS/CdTe/Cu-Au solar cell devices were prepared by thermal evaporation of Cu and Au on CdTe surface. CdS films grown were annealed at ~400 °C for 15 minutes in air and photo-electro chemical (PEC) cell measurements were performed to identify the electrical conductivity type. Both as-deposited and annealed CdS layers were identified as n-type in electrical conduction. CdS thin films were shown enhanced PEC responses upon heat treatment. The respective band gap values for as-deposited and heat treated CdS were 2.35±0.05 eV and 2.40±0.05 eV which were close to the band gap of bulk CdS. XRD analysis of as-deposited CdS layers revealed the presence of hexagonal CdS materials with the major peak arising from (002) plane. Following the CdTe deposition on glass/FTO/CdS substrate, the surface of CdTe layers were coated with a 0.1% CdCl2 solution and structures were annealed at ~400°C for 10 minutes in air. Band gaps for CdTe layers were found to be 1.45±0.02 eV for both as-deposited and annealed samples which exhibited the band gap of bulk CdTe. There was a little improvement in cubic (220) and (311) peaks of XRD spectra of annealed CdTe layers compared to the as-deposited material, but annealing exhibited a small reduction of cubic phase preferential orientation (111). SEM images showed that CdS and CdTe layers were fairly uniform. The fabricated solar cell devices showed the efficiency of 2.1% with Voc ~330 mV, Jsc~20 mA cm-2 and FF~33% under the illumination of air mass (AM) 1.5 conditions (100 mW/cm2, 1 Sun).