Faculty Publications
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Item Characterization of p-CdTe/n-CdS hetero-junctions(2009) Mahesha, M.G.; Bangera, K.V.; Shivakumar, G.K.Nano-crystalline CdTe/CdS thin film hetero-junctions have been grown on glass substrate by thermal evaporation technique. The growth conditions to get stoichiometric compound films have been optimized. The grown hetero-junctions have been characterized for their I-V characteristics. Analysis of I-V characteristics has been made to investigate the current conduction mechanism in p-CdTe/n-CdS hetero-junction. The band gap energy of cadmium telluride and cadmium sulfide films have been computed from the study of variation of resistance with temperature. Based on the study, band diagram for p-CdTe/n-CdS hetero-junction has been proposed.Item Effect of substrate temperature on the suitability of thermally deposited cadmium sulfide thin films as window layer in photovoltaic cells(Academic Press, 2018) Barman, B.; Bangera, K.V.; Shivakumar, G.K.Cadmium sulfide has been studied as an important material in solar energy research because of its energy band gap and attractive electrical characteristics. While thin films of cadmium sulfide have been found to be useful as window layer in a solar cell, the role of various deposition parameters is yet to be understood. In the current study, the role of substrate temperature on the characteristics of the CdS thin films is analyzed. Thin films of cadmium sulfide (?450 nm thick) were deposited at various substrate temperatures viz., 300 K, 323 K, 373 K, and 423 K onto clean glass substrates by vacuum thermal evaporation method. The structural, morphological, and opto-electrical properties of the deposited films were studied as a function of substrate temperature. X-ray diffraction (XRD) study revealed that the thin films are polycrystalline in nature and having a hexagonal wurtzite crystal structure along (002) plane. Scanning electron microscopy (SEM) along with energy dispersive spectroscopy (EDS) revealed that the grown films are homogeneous, uniform and pin-hole free. All the films deposited at various substrate temperature displayed high optical transmittance (>60%) in the visible range. The optical energy band gap of the films was estimated using Tauc's plot and was found to increase by a slight margin with an increase in the substrate temperature and decrease at higher substrate temperature. The photosensitivity was found to be highest for the CdS thin film grown at a substrate temperature of 373 K. © 2018 Elsevier LtdItem A comprehensive study on the structural, morphological, compositional and optical properties of ZnxCd1-xS thin films(Institute of Physics Publishing helen.craven@iop.org, 2019) Barman, B.; Bangera, K.V.; Shivakumar, G.K.The absorption loss in cadmium sulfide (CdS) thin films which are widely used as a window layer in a photovoltaic cell limits the efficiency of the device. This issue can be addressed by ZnxCd1-xS thin films due to its tunable band gap nature. Herein, the various composition of ZnxCd1-xS (x=0, 0.15, 0.30, 0.45, 0.70, 0.85, 1) thin films were grown by a vacuum thermal evaporation technique and the characteristics of the films were investigated by varying the composition 'x'. The x-ray diffraction (XRD) studies displayed that the as-deposited films consist of diffraction peaks from both CdS and ZnS lattice. The formation of ternary ZnxCd1-xS films was verified when the deposited films were subjected to an annealing treatment. The morphology of the films was analyzed using a scanning electron microscope (SEM) and it was observed that the films are uniform, homogeneous and free from any pin-holes and cracks. The presence of Zn, Cd and S elements were quantized using an energy dispersive spectroscopy. Optical studies showed a successful non-linear band gap engineering (2.42-3.49 eV) for the deposited ZnxCd1-xS thin films. All films showed a very high optical transmittance of above 70% in the visible wavelength region. © 2020 IOP Publishing Ltd.Item Investigation of third-order nonlinear optical properties of nanostructured Ni-doped CdS thin films under continuous wave laser illumination(Springer, 2019) Bairy, R.; Arasalike, A.; Shivakumar, G.K.; Radhakrishnan, K.; Bhat K, U.K.We report the third-order nonlinear optical (NLO) properties and optical limiting (OL) thresholds of pure CdS and Ni-doped CdS thin films have been investigated with the Z-scan technique under continuous wave laser excitation. Nanocrystalline CdS thin films with various doping concentrations of Ni (0%, 1%, 3%, 5% and 10 at. %) are prepared by spray-pyrolysis technique. XRD patterns reveal that all the prepared films are polycrystalline and the incorporation of Ni does not lead to major changes in the crystalline phase of Cd1-xNixS thin films. The surface morphology of the prepared films is impacted by the Ni-doping and is indicated by Field Emission Scanning Electron Microscopy (FESEM) images. With an increase in Ni-doping concentration, the energy band-gap value decreased from 2.48 eV to 2.23 eV. From the Z-scan data, it is observed that the material show strong two-photon absorption (2PA) and with an increase in Ni-doping concentrations from 0 to 10 at. %, the nonlinear absorption coefficient (?) are enhanced from 0.92 x 10-5 to 4.46 x 10-5 (cm W-1), nonlinear refractive index (n2) from 0.2967 x 10-9 to 0.1297 x 10-8 (cm2 W-1) and thereby the third-order NLO susceptibility (?(3)) values also increased from 1.7075 x 10-6 to 7.4743 x 10-6 (esu). OL characteristics of the prepared films are studied at the experimental wavelength. The results propose that the Cd1-xNixS film is a capable material for nonlinear optical devices at 532 nm and optical power limiting applications. © Springer Science+Business Media, LLC, part of Springer Nature 2019.Item Effect of CuPc and PEDOT:PSS as hole transport layers in planar heterojunction CdS/CdTe solar cell(Institute of Physics Publishing helen.craven@iop.org, 2019) Varadharajaperumal, S.; Ilango, M.S.; Hegde, G.; Satyanarayan, M.N.This paper presents the fabrication and photovoltaic performance of new architecture based planar heterojunction CdS/CdTe thin film solar cells which were employed with two hole transport layers (PEDOT:PSS as HTL1 and CuPc as HTL2). The reported solar cells were fabricated through various deposition techniques such as sputtering, thermal evaporation, spin coating and characterized by FESEM, AFM, XPS, UPS and AM 1.5 solar simulator. The interfacial layer growth and chemical state identification of the deposited thin films were studied by cross-sectional FESEM and XPS techniques. The band bending occurs between absorbing and transporting layer helps to inject the excited charge carriers effectively into electrode that was explained using UPS analysis. The present work intends to explain the role of additional window layer (TiO2), buffer layer (CdS) and hole transporting layers (PEDOT:PSS and CuPc) in the novel device architecture. Further, these findings will offer new research directions to address the double hole transport (back contact) layers selection concept in CdS/CdTe heterojunction based solar cells. © 2019 IOP Publishing Ltd.Item Cadmium sulfide nanostructures: Influence of morphology on the photocatalytic degradation of erioglaucine and hydrogen generation(Elsevier B.V., 2019) Shenoy, S.; Jang, E.; Park, T.J.; Gopinath, C.S.; Sridharan, K.Size and shape of inorganic materials are known to have great effects on their physical and chemical properties. Here, for the first time we report the visible light driven photocatalytic degradation of erioglaucine – a stable organic dye molecule in the presence of chemically synthesized nanoscale CdS with 1D (nanorods), 2D (nanosheets) and 3D (hierarchical) morphology. Visible light driven photocatalytic degradation efficiency of both 1D and 3D CdS in the removal of erioglaucine are identical. Surprisingly, with 5 min of sonication, the highly crystalline 3D CdS stacked with many thin nanowires containing numerous active surface sites exhibited four-fold enhanced photodegradation efficiency in comparison to 1D and 2D CdS. Scavenger studies revealed that electrons and superoxide radicals are primary reactive species involved in the photodegradation of erioglaucine, while cyclic photodegradation studies revealed the good stability of 3D CdS against photocorrosion. Further, the photocatalytic hydrogen evolution studies also revealed the excellent activity of 3D CdS in comparison to 1D and 2D CdS. Thus, we find that the morphology indeed influences the photocatalytic activity. These results reveal that 3D CdS nanostructures investigated in the present work are efficient photocatalysts that could be fine-tuned for both environmental remediation and hydrogen generation applications. © 2019 Elsevier B.V.Item ZnxCd1-xS thin films: A study towards its application as a reliable photodetector(Academic Press, 2020) Barman, B.; Bangera, K.V.; Shivakumar, G.K.A reliable and stable photodetector shows enormous potential applications in health monitoring, intelligent wearable devices, and biological sensing. The noble opto-electrical properties of cadmium sulfide (CdS) makes it a favorable semiconductor for opto-electrical devices. The properties of CdS thin film can further be enhanced by alloying it with zinc sulfide (ZnS) to form ZnxCd1-xS compound semiconductor. Herein, a high performance, ZnxCd1-xS (x = 0, 0.15, 0.30, and 0.45) photodetector with good stability is designed and fabricated via a simple vacuum thermal evaporation technique. The various photodetector parameters of the ZnxCd1-xS films were investigated as a function of its composition. Among the various compositions of the ZnxCd1-xS thin films, the Zn0.15Cd0.85S films displayed excellent photosensitivity as high as 2.22 which is ~1.6 times higher than that of undoped CdS thin films. The research data suggests that a high-performance single layer ZnxCd1-xS photodetector with good stability and reproducibility can be fabricated using a thermal evaporation technique. © 2019 Elsevier LtdItem Enhanced photocatalytic efficiency of layered CdS/CdSe heterostructures: Insights from first principles electronic structure calculations(Institute of Physics Publishing helen.craven@iop.org, 2020) Shenoy, S.; Tarafder, K.Metal sulfides are emerging as an important class of materials for photocatalytic applications, because of their high photo responsive nature in the wide visible light range. In this class of materials, CdS with a direct band gap of 2.4 eV, has gained special attention due to the relative position of its conduction band minimum, which is very close to the energies of the reduced protons. However, the photogenerated holes in the valence band of CdS are prone to oxidation and destroy its structure during photocatalysis. Thus constructing a CdS based heterostructure would be an effective strategy for improving the photocatalytic performance. In this work we have done a detail theoretical investigation based on hybrid density functional theory calculation to get insight into the energy band structure, mobility and charge transfer across the CdS/CdSe heterojunction. The results indicate that CdS/CdSe forms type-II heterostructure that has several advantages in improving the photocatalytic efficiency under visible light irradiation. © 2020 IOP Publishing Ltd.Item Graphitic C3N4/CdS composite photocatalyst: Synthesis, characterization and photodegradation of methylene blue under visible light(Elsevier B.V., 2020) Shenoy, S.; Tarafder, K.; Sridharan, K.Design and development of heterojunction photocatalysts is one among the main strategies for improving the photocatalytic activity of semiconductor materials. Here, we report the synthesis of a heterojunction photocatalyst by the embedment of cadmium sulphide (CdS) nanoparticles on the surface of graphitic carbon nitride (g-C3N4) layers through hydrothermal approach. The g-C3N4/CdS heterojunction photocatalyst exhibited two-fold and three-fold enhancement in the photodegradation efficiency in comparison to pristine CdS and g-C3N4, respectively in the removal of 20 ppm methylene blue dye molecules under visible light irradiation. The enhanced photocatalytic activity can be attributed to the formation of heterojunction and the synergistic effect of g-C3N4 and CdS in the promotion of charge separation and charge mobility that was tracked through photoluminescence spectroscopy. © 2020 Elsevier B.V.Item Theoretical investigations of band alignments and SnSe BSF layer for low-cost, non-toxic, high-efficiency CZTSSe solar cell(Elsevier Ltd, 2021) Prabhu, S.; Pandey, S.K.; Chakrabarti, S.In this work, a numerical simulation approach is utilized using SCAPS-1D software to model, modify, optimize, and evaluate the CZTSSe solar cell structure. For the CZTSSe solar cell, one possible reason hindering the performance is improper band alignment between the absorber and the buffer layers. With conventional CdS as a buffer layer, having a fixed bandgap, tuning the band alignment is impossible. To overcome this issue, Cd-free zinc oxide-based compounds Zn(O1-xSx), Zn1-xSnxO, and Zn1-xMgxO are explored as buffer layers, and their performance is evaluated. Using their composition-dependent tunable bandgap as an advantage, suitable band alignment with the absorber layer is evaluated for equal or higher performance when compared to CdS. Further performance improvement is attempted by using SnSe as the back surface field (BSF) layer. Band alignment evaluation is also extended to the back contact (Mo)/SnSe interface, whereby an attempt is made to replace Mo with a suitable metal. The Ni is found as a good candidate to replace Mo to achieve high-efficiency solar cell. The same approach is repeated with the transparent conducting oxide layer, and aluminum doped zinc oxide (AZO) is found as a suitable material in place of ITO for optimized solar cell structure. A maximum power conversion efficiency of 17.55% is achieved with an optimized structure. It is also observed that the external quantum efficiency (EQE) of the solar cell is improved significantly in the blue photons region in comparison to the EQE of the champion solar cell. The optimized structure Ni/SnSe/CZT(S0.4Se0.6)/Zn(O0.3S0.7)/i-ZnO/AZO in this work will be very useful to fabricate low-cost and Cd-free high-efficiency kesterite solar cells. © 2021