Browsing by Author "Ravirajan, P."

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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).
A simple solvothermal approach to synthesize Zn-doped TiO2 nanomaterials for dye sensitized solar cells
(Faculty of Science, University of Kelaniya, Sri Lanka, 2020) Gurunanthanan, V.; Rajaramanan, T.; Uthayaraj, S.; Velauthapillai, D.; Ravirajan, P.; Senthilnanthanan, M.
Dye sensitized solar cells (DSSC) are considered one of the most promising organic solar cells. It is found as an alternative to traditional silicon based solar cells due to low-cost, ease of fabrication, ability to work under low light conditions and environmentally friendly nature. In DSSCs, titanium dioxide (TiO2) is commonly used as a promising wide bandgap semiconductor photoanode and the light harvesting properties of the photoanode is a crucial factor that determines the overall efficiency of DSSCs. Doping could be used to improve the light harvesting properties of the photoanode by tuning the bandgap of the semiconductor. TiO2 photoanodes doped with elements such as alkali-earth metals, transition metals, rare-earth elements and nonmetals are found to improve the power conversion efficiency (PCE) of DSSCs. Among these elements, transition metal doped TiO2 photoanodes perform efficiently by suppressing the relaxation and recombination of charge carriers and improving the absorption of light in the visible region. This work reports the possibility of enhancing the PCE of DSSCs by employing Zn-doped TiO2 photoanodes since Zn is one of the promising n-type transition metals and Zndoped TiO2 improves the photocurrent in DSSCs. Zn-doped TiO2 nanomaterials were synthesized, using varied amounts of Zn precursors, by a facile solvothermal method using a reaction bottle instead of an autoclave and characterized by X-ray diffraction and UV-Visible spectroscopy. The X-ray diffraction studies confirmed the presence of anatase phase of TiO2 in the synthesized nanomaterials is unaffected by Zn-doping. The UV-Visible spectra of Zn-doped TiO2 showed a red shift which could be attributed to the reduced bandgap resulted by Zn doping. Subsequently, the DSSCs were fabricated by doctor-blade method with an effective area of 0.25 cm2 utilizing N719 dye, 𝐼 −/ 𝐼3 − redox couple and Pt electrode as the sensitizer, electrolyte and counter electrode respectively. Then, performances of the fabricated devices were investigated. Significant enhancement in PCE was observed with 1.0 mol% Zn-doped TiO2 based DSSC tested under simulated irradiation of intensity 100 mW/cm2 with AM 1.5 filter, which was 35 % greater than that of the control device fabricated with un-doped TiO2 photoanode. These improvements are attributed to the reduced band gap energy and the enhanced photocurrent due to Zn doping on TiO2.
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.