Browsing by Author "Fernando, W. T. R. S."

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A Comparative Study: Sequential and Single-Step-Electrodeposited CZTS Thin Films
(Physica Status Solidi, 2022) Fernando, W. T. R. S.; Jayathilaka, K. M. D. C.; Wijesundera, R. P.; Siripala, W.
CZTS (Cu2ZnSnS4) is a relatively new and promising semiconductor material suitable for photovoltaic applications due to its favorable optoelectronic properties. Of the many techniques available for growing these films, a comparative study on sequential and single-step electrodeposition methods to grow CZTS films is carried out in this investigation to explore the possibility of improving the quality of the films using the inexpensive electrodeposition technique. Mainly in both methods, potentiostatic electrodeposition technique is adopted for growing CZTS thin films. In both methods, growth conditions of the CZTS films are optimized after measuring the photoresponses in a photoelectrochemical (PEC) cell of the films that resulted at the end of each deposition step. The observed structural and optoelectronic properties of the films reveal that, in general, structurally good and photoactive CZTS films can be prepared using both methods. Moreover, photoresponse and Mott–Schottky measurements on CZTS films in a PEC reveal that CZTS films prepared using the single-step electrodeposition have better photoactive properties and improved doping densities. This important finding shows that when developing CZTS-based solar cells using the inexpensive electrodeposition technique, single-step electrodeposition is more advantageous.
Growth of photoactive Cu2ZnSnS4 by single step electrodeposition
(Research Symposium on Pure and Applied Sciences, 2018 Faculty of Science, University of Kelaniya, Sri Lanka, 2018) Fernando, W. T. R. S.; Jayathileka, K. M. D. C.; Wijesundera, R. P.; Siripala, W.
Cu2ZnSnS4 (CZTS) is a promising candidate for application in low-cost and environmentally-friendly thin film solar cells due to its optoelectronics properties. It is a perfect absorber material for photovoltaic applications due to its high absorption coefficient (>10-4 cm-1) and direct optical bandgap (1.4 - 1.5 eV). Among the CZTS preparation techniques, single step electrodeposition is an attractive because of its simplicity, low cost and easy to control stoichiometry. In this study, CZTS thin films on Mo substrate were potentiostatically electrodeposited in a three electrode electrochemical cell containing 0.02 M copper (II) sulfate pentahydrate (CuSO4·5H2O), 0.01 M zinc sulfate heptahydrate (ZnSO4·7H2O), 0.02 M tin sulfate (SnSO4) and 0.02 M sodium thiosulfate (Na2S2O3) at room temperature. 0.2 M tri-sodium citrate (C6H5Na3O7:Na3-citrate) was used as complexing agent and tartaric acid (C4H6O6) was used as pH control solution. pH of the bath was maintained at 5.0 Ag/AgCl and platinum electrodes were used as reference and counter electrodes respectively. Mo substrate with a deposition area of 1×2 cm2 was used as the working electrode. Electrodeposition was carried out at -1.05 V vs Ag/AgCl using a Hokuto Denko model HZ-5000 Potentiostat/Galvanostat. CZTS samples were prepared using different deposition durations (5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55 min). Optimum bath conditions were explored using cyclic voltammetry. Samples were characterized using XRD, optical absorption, dark and light I-V measurements and spectral response measurements in a PEC containing 0.1 M sodium acetate. XRD measurements evidenced that the formation of single phase polycrystalline CZTS. Reflectance measurements has revealed that the band gap energy of the films is 1.5 eV and I-V measurements revealed that CZTS thin films were photoactive and p-type. To enhance the photoactive properties films were annealed at different temperatures (500, 550, 6000C) and durations (15, 30, 45 min) in H2S surrounding. As the results, photoactive performance of the films enhance with the annealing treatment in H2S. In conclusion, it can be mentioned that the highest photoactive p-CZTS thin films can be grown by annealing the 40 min deposited samples at 5500C for 30 min in H2S. The methodology developed in this study will be further investigated, in order to develop the material for wider applications.
Optimization of growth parameters of photoactive Cu2ZnSnS4.
(International Research Symposium on Pure and Applied Sciences, 2017 Faculty of Science, University of Kelaniya, Sri Lanka., 2017) Fernando, W. T. R. S.; Jayathilaka, K. M. D. C.; Wijesundera, R. P.; Siripala, W.
Cu2ZnSnS4 (CZTS) is a promising candidate for application in low-cost and environmentally friendly thin film solar cells due to its optoelectronics properties. It is a perfect absorber material for photovoltaic applications due to its high absorption coefficient (>10-4 cm-1) and direct optical band gap (1.4 - 1.5 eV). Among the CZTS preparation techniques, electrodeposition of Cu, Sn and Zn stack layers followed by sulphurisation in H2S is an attractive technique because of its simplicity, low cost and easy to control stoichiometry. In this investigation, optimization of growth parameters in order to obtain photoactive CZTS thin films by sulphurisation of electrodeposited Cu, Sn and Zn stack layers has been investigated. Cu thin film was electrodeposited on Mo substrate at –0.89 V Vs Ag/AgCl electrode in an electrochemical cell containing 0.4 M CuSO4, 3 M lactic acid and NaOH at pH 11. Deposition of Sn thin film on Mo/Cu electrode was carried out at -1.2 V Vs Ag/AgCl in an electrochemical cell containing 0.055 M, 2.25 M NaOH and 8 ml of sorbitol. Zn thin film was electrodeposited on Mo/Cu/Sn at -1.2 V Vs Ag/AgCl in an electrochemical cell containing 0.2 M ZnSO4. Deposition parameters of Cu, Sn and Zn have been obtained by voltammograms. In order to grow CZTS, Mo/Cu/Sn/Zn thin film electrodes were annealed at 550 oC for 60 min in H2S. Sulphurisation process was carried out at different temperatures and durations using set of identical Mo/Cu/Sn/Zn thin film electrodes and thereby optimized temperature and duration of the sulpurisation. Atomic ratios of initial Cu, Sn and Zn layers could be crucial parameters in determining properties of CZTS thin films. Therefore, atomic ratios of Cu/Sn/Zn layers were optimized by changing Cu, Sn and Zn deposition duration. Various combinations of deposition durations were carried out and optimized by monitoring the dark and light I-V measurements in a PEC containing 0.1 M sodium acetate. Dark and light I-V characteristics revealed that the best photoactive CZTS films can be grown by depositing Cu for 20 min, Sn for 10 sec and Zn for 10 sec. Results further showed that photoconductivity of CZTS thin films is p-type. It is evident from reflectance measurements that the band gap of the CZTS films is 1.5 eV. In conclusion, it is found that the highest photoactive p-CZTS thin films can be grown by sulphurisation of electrodeposited Cu, Sn and Zn stack layers on Mo substrate using H2S at 550 oC for 60 min. Cu: Sn: Zn ratios of the stack layers are the crucial parameters in determining photoactive CZTS thin films. The methodology developed in this study will be further investigated in order to develop the materials for wider applications.
Structural analysis of LiNi1/3Mn1/3Co1/3O2, Li0.96 Na0.04Ni1/3Mn1/3Co1/3O2 and Li0.96K0.04Ni1/3Mn1/3Co1/3O2 materials synthesized by Pechini method
(Faculty of Graduate Studies, University of Kelaniya Sri Lanka, 2022) Fernando, W. T. R. S.; Amaraweera, T. H. N. G.; Wijayasinghe, A.
Layered tri-transition metal oxides, specially LiNi1/3Co1/3Mn1/3O2 (NMC 333), have become a promising alternative to LiCoO2 electrode material in the rechargeable Lithium-Ion Battery (LIB). The electrochemical performances of NMC 333 mainly depend on its crystallographic structural properties including lattice parameters, the unit-cell, c/a ratio, volume, crystallite size (D), dislocation density(δ), and lattice strain. This study aims to synthesize LiNi1/3Mn1/3Co1/3O2, Li0.96Na0.04Ni1/3Mn1/3Co1/3O2, and Li0.96K0.04Ni1/3Mn1/3Co1/3O2 materials and study their structural properties. The Pechini method was used for powder synthesis in this study. The synthesized materials were characterized using X-ray diffraction (XRD). X-ray characterization confirmed the formation of only the single-phase layered hexagonal lattice (α-NaFeO2-type) structure without any impurity phase for all these prepared materials. Interestingly, while confirming the formation of layered structures, a better splitting of the (006)/(102) and (108)/(110) peaks appeared for Li0.96K0.04Ni1/3Mn1/3Co1/3O2 than that of LiNi1/3Mn1/3Co1/3O2 and Li0.96Na0.04Ni1/3Mn1/3Co1/3O2 in the diffractograms. The lattice parameters, i.e. a, c, c/a, the unit-cell volume, the crystallite size (D), and dislocation density(δ) are 2.8641(Å)̇, 14.2143(Å)̇, 4.9629, 100.979(Å3)̇, 77.45 nm,1.666×1014 m−2, for LiNi1/3Mn1/3Co1/3O2. While they are 2.8675(Å)̇, 14.2317(Å), 4.9630, 101.347(Å3)̇, 85.06 nm, 1.382×1014 m−2 for Li0.96Na0.04Ni1/3Mn1/3Co1/3O2 and 2.869 (Å)̇, 14.2421(Å)̇, 4.9641, 101.528(Å3)̇, 128.38 nm, 0.606×1014 m−2 for Li0.96K0.04Ni1/3Mn1/3Co1/3O2, respectively. It is also observed that the lattice parameters, the unit-cell volume, c/a, and the crystallite size are increased with the substitution of Li+ by Na+ and K+. It may be due to the radii of Na+ and K+ are bigger than that of Li+ and that will pave the way for increasing the interlayer space of the substituted materials with the substitution of bigger ions. The c/a ratio constitutes a direct indication of the cation mixing. Li0.96Na0.04Ni1/3Mn1/3Co1/3O2 and Li0.96K0.04Ni1/3Mn1/3Co1/3O2 exhibit higher c/a values than LiNi1/3Mn1/3Co1/3O2, supporting the observation that the substituting bigger ions such as Na+ and K+ into LiNi1/3Mn1/3Co1/3O2 suppresses the cation mixing and forms a well-defined layered structure. The micro-strain calculated for the LiNi1/3Mn1/3Co1/3O2, Li0.96Na0.04Ni1/3Mn1/3Co1/3O2, and Li0.96K0.04Ni1/3Mn1/3Co1/3O2 are 1.38×10−3, 2.17×10−3 and 1.46×10−3, respectively. This implies a slight difference in the crystallinity of the materials, as the micro-strain was slightly affected by substituting Na+ and K+. Crystallite size (D) was 77.45 nm, 85.06 nm, and 128.38 nm for LiNi1/3Mn1/3Co1/3O2, Li0.96Na0.04Ni1/3Mn1/3Co1/3O2 and Li0.96K0.04Ni1/3Mn1/3Co1/3O2, respectively. It exhibits an increment of crystallite size, indicating a lowering of the dislocation density with the substitution of bigger ions. Altogether, this study reveals that substituting Li+ with bigger ions of Na+ and K+ is improving the structural stability of NMC 333.