Faculty Publications

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Morphology controlled n-type TiO2 and stoichiometry adjusted p-type Cu2ZnSnS4 thin films for photovoltaic applications
(American Chemical Society, 2017) Varadharajaperumal, S.; Sripan, C.; Ganesan, R.; Hegde, G.; Satyanarayana, M.N.
This paper presents the fabrication and characterization of stoichiometry adjusted Cu2Zn1.5Sn1.2S4.4 thin film (FTO/TiO2/CdS/CZTS/Au) photovoltaic (PV) devices. The PV devices were developed using the window layer of rutile TiO2 nanoarchitecture arrays, i.e., one-dimensional (1D) nanorods and three-dimensional (3D) combined/ hierarchical structures (nanorods with microspheres). Onedimensional (1D) nanorods and 3D combined structures of TiO2 window layers were synthesized by a hydrothermal method with different solvents without any assistance of surfactants and templates. We achieved two kinds of TiO2 nanostructures by tuning the precursor concentrations and volume by keeping a constant growth time and temperature. The detailed structural properties were studied using X-ray diffraction and high resolution transmission electron microscopy. Phase formation and chemical state of the prepared samples were examined by Raman spectroscopy and X-ray photoelectron spectroscopy. The surface morphology and luminescence studies of TiO2 nanostructures were analyzed using field emission scanning electron microscopy and cathodoluminescence techniques. The current-voltage performance of fabricated devices were measured under an AM 1.5 solar simulator. It is observed that combined structure PV device shows better efficiency (1.45%) than the nanorods alone structure (0.55%). Present work is a first attempt made to construct the inverted CZTS based solar cells. This study establishes the window layer of hierarchical TiO2 nanostructures based morphology that offers a great potential for the development of high-efficiency nonstoichiometric CZTS based solar cells. © 2017 American Chemical Society.
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.
Ultra-broadband fabrication-tolerant mode division (de)multiplexer on thin film Lithium niobate
(Elsevier B.V., 2020) Kaushalram, A.; T.r, Y.; Das, P.P.; Hegde, G.; Srinivas, S.
Mode division (de) multiplexers based on asymmetric directional coupler are designed and analysed on the emerging thin film lithium niobate platform in rib, buried and strip waveguide configurations. Rib configuration offers a large bandwidth of ?215 nm with a crosstalk level lower than ?10 dB, and provides a large fabrication tolerance of ±50 nm. Strip configuration exhibits a better crosstalk of ?19.7 dB at the design wavelength of 1550 nm, and has a reasonable fabrication tolerance of ±30 nm. Performance of (de) multiplexers on all three waveguide configurations are analysed in terms of modal crosstalk, mode conversion efficiency, bandwidth, along with fabrication tolerance to width and sidewall angle. The influence of electro-optic effect on the coupling length of lithium niobate mode multiplexers is also investigated. © 2020 Elsevier B.V.
Toxic-free surface level sulphur doped 1D Ti-Ox-Sy nanorods for superstrate heterojunction CZTS thin-film solar cells
(Elsevier Ltd, 2021) Varadharajaperumal, S.; Alagarasan, D.; Sripan, C.; Ganesan, R.; Satyanarayan, M.N.; Hegde, G.
Surface level sulphur (S) doped TiO2 nanorods (S-TNRs) were fabricated via toxic-free novel three-step processes such as low-temperature hydrothermal method followed by thermal evaporation (S layer) and post-annealing (350 °C, 450 °C and 550 °C) techniques. Present work focuses on the comprehensive studies of surface level doping, structure, morphology and compositional properties of different temperature annealed S-TNRs for CZTS thin-film (Au/CZTS/S-TNRs/TNRs/FTO) solar cells. The oxidation states of incorporated S atoms in the TiO2 matrix were identified from X-ray photoelector spectroscopy (XPS) analysis. A reduction in bandgap for 350 °C annealed S-TNRs film was observed from UV-Vis spectroscopy. The electrical characteristics showed the fabricated solar cells strongly depend on the S-TNRs annealing temperature. Proposed technique would be useful in effective and controlled (surface level) doping of S atoms into any desired nanostructured metal oxides for optoelectronic applications and, further useful in fabricating cadmium (Cd) free buffer layer in chalcogenide solar cells. © 2020 Elsevier Ltd

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