Now showing 1 - 10 of 14
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    Investigation on the structural, spectral, and optical properties of MAPbI3.H2O and MAPbI3 perovskite crystals for photovoltaic cells
    (01-05-2023)
    Sivakumar, N.
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    Saha, Subhashis
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    Madaka, Ramakrishna
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    Bandaru, Narendra
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    Organic–Inorganic perovskite solar cells (PSC) have reached efficiency near to c-Si cells due to their absence of grain boundaries, low trap density, and remarkable charge transport properties. However, they suffer from electronic degradation due to various environments. Our study is an attempt to address this issue by using crystalline perovskite absorber layers in solar cells. Methyl ammonium lead iodide (MAPbI3) crystalline perovskite materials were prepared by conventional solution synthesis process using hydrated and nonhydrated solvents. Synthesized perovskite crystals were subjected to single-crystal X-ray diffraction and powder X-ray diffraction studies to understand the crystal system and molecular arrangements. Single-crystal X-ray diffraction study confirmed the tetragonal and monoclinic crystal structure of MAPbI3.H2O and MAPbI3 materials, respectively. The interactions of Pb–I, C–H, and C–N in MAPbI3.H2O crystal were studied through their bond length and bond angles, and the results were compared with the reported MAPbI3 crystal structure. The unit cell parameters of MAPbI3.H2O single crystal were found to be a = 10.56(18) Å, b = 4.68(14) Å, c = 11.20(19) Å, α = γ = 90°, β = 101.17(6)°, and V = 544.03(15) Å3 and for the MAPbI3 crystal a = 8.91(9) Å, b = 8.91(9) Å, c = 12.60(19) Å, α = β = γ = 90°, and V = 998(11) Å3. FT-IR and FT-Raman spectroscopic studies were carried out to understand the presence of functional groups and the effect of water molecules in the crystals. Optical band gaps of these two perovskites were found to be 1.45 eV and 1.52 eV, respectively. The conductivity studies were performed on the as-grown perovskite single crystals using solar simulator experimental setup. Initial dark and AM 1.5 light I–V test on these samples provides the information about the suitability of these perovskite single crystals for the photovoltaic applications. The investigation of the solar cell device using these perovskites is in progress.
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    Publication
    Optical properties of SeO43- centres in NH4H2PO4 single crystals
    (01-01-1988) ;
    Radhakrishna, S.
    The optical absorption of the γ-irradiated SeO42- doped KDP crystal is studied. EPR and optical absorption studies are correlated to identify the optical absorption band of SeO43- radical. A strong coupling of the electronic states and the totally symmetry stretching mode of vibration is found to be present. © 1988.
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    Publication
    Tunnel recombination junction influence on the a-Si:H/SHJ tandem solar cell
    (01-01-2019)
    Madaka, Ramakrishna
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    Kumar, Dinesh
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    Singh, Ashish K.
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    Uddin, Seraj
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    An effective tunnelling recombination junction (TRJ) is the most important part in a series connected tandem solar cell, and it always plays a significant role to match the current densities of individual solar cells. The aim of this paper is to find a TRJ that shows more efficient ohmic behavior compared to that used in typical thin film tandem solar cells, due to high current nature. Silicon heterojunction (SHJ) based tandem solar cell structure (a-Si:H/SHJ) was studied in detail with simulations using Advanced Semiconductor Analysis (ASA) software. Trap assisted recombination tunnelling and field enhanced mobility mechanisms have been included in our model analysis. Moreover, we also have simulated individual device characteristics of each a-Si:H (n-i-p), SHJ cell (top and bottom subcells of an a-Si:H/SHJ tandem cell), and correlated with the performance of the a-Si:H/SHJ tandem solar cell. The tandem device with a TRJ, the type which used in a thin film silicon tandem cell, shows low open circuit voltage (Voc) = 1.12 V and poor fill factor (FF) = 0.29 with S-shape in light current density-voltage (J-V) characteristics due to charge transport issue at the interface of top and bottom cells. An appropriate TRJ gives efficient recombination and an excellent ohmic contact at the interface between the subcells to match the current of each device. With the implementation of the optimum TRJ at the junction of two cells, Voc and FF have improved significantly to 1.35 V and 0.79 respectively. Also, we observed that the Voc of the tandem solar cell is nearly equal to the sum of the corresponding individual subcells, confirming no trapping related field loss at the junction.
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    Publication
    Flexible CIGS, CdTe and a-Si:H based thin film solar cells: A review
    (01-05-2020)
    Ramanujam, Jeyakumar
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    Bishop, Douglas M.
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    Todorov, Teodor K.
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    Gunawan, Oki
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    Nekovei, Reza
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    Artegiani, Elisa
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    Romeo, Alessandro
    Flexible thin film solar cells such as CIGS, CdTe, and a-Si:H have received worldwide attention. Until now, Si solar cells dominate the photovoltaic market. Its production cost is a major concern since Si substrates account for the major cost. One way to reduce the module production cost is to use the low-cost flexible substrates. It reduces the installation and transportation charges also, thereby reducing the system price. Apart from metallic foils, plastic films and flexible glass, paper substrates such as cellulose papers, bank notes, security papers and plain white copying papers are also used as substrates for flexible solar cells. In this review, recent developments in flexible CIGS, CdTe and a-Si:H solar cells are reported. Progress on various flexible foils, fabrication and stability issues, current challenges and solutions to those challenges of using flexible foils, and industrial scenario are reviewed in detail. Encapsulation issues and solutions related to water vapor transmission rate are discussed.
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    Publication
    EPR studies of MoO 43− centres in KDP single crystals
    (01-01-1987) ;
    Radhakrishsa, S.
    γ‐irradiation of MoO 43− doped KDP crystals is shown to produce the paramagnetic centre MoO 43−. The g‐values of the MoO 43− centre suggest that its ground state is d z 2. This radical is used as a probe to detect the ferroelectric phase transition of the KDP crystal. The EPR spectrum of MoO 43− in the paraelectric phase shows a quintet proton hyperfine structure which changes to a triplet as the temperature is lowered below a critical temperature of 144 K. Copyright © 1987 WILEY‐VCH Verlag GmbH & Co. KGaA
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    Publication
    Silicon surface passivation of industrial n-type CZ Si (111) by Al2O3 layers deposited by thermal ALD process for application in carrier selective contact solar cells
    (01-05-2023)
    Dsouza, Namitha
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    Singh, Ashish K.
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    Maurya, Rajesh
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    Kanakala, Rajesh
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    Madaka, Ramakrishna
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    Bandaru, Narendra
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    Uddin, Md Seraj
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    c-Si based carrier selective contact solar cells are achieving high efficiency with a crucial step of silicon surface passivation, reducing the electronic recombination losses occurring at the interface of Si and the passivation layer. The ultrathin passivation layers of Al2O3 are deposited by atomic layer deposition (ALD), known for its conformance and homogeneity. ALD is an attractive technique for low-temperature deposition of layers required for the development of c-Si carrier selective contacts and passivation. The deposition temperature is fixed at 200 °C, which lies in the ALD window for Al2O3 films. In this study, deposition of Al2O3 films is done by thermal ALD process where the growth per cycle of the film is calculated to be 0.1 ± 0.01 nm using spectroscopic ellipsometry. Al2O3 films were deposited on n-type CZ Si (111) (2 Ω cm) wafers of thickness 170 μm after processing with saw damage removal, standard RCA clean, and HF dip. Post-deposition annealing was done in forming gas environments at various temperatures to probe the passivation quality. A monotonic improvement was obtained with annealing in such an environment, reaching an excellent lifetime of about 1.24 ms (measured by WCT-120 Sinton lifetime tester) at a minority carrier concentration of 1 × 1015 cm−3 for samples annealed in forming gas environment at 310 °C. This is an excellent value for an industrial-type CZ wafer with a measured bulk lifetime of only ~ 3 ms. The corresponding effective surface recombination velocity obtained is 3.7 cm/s. An implied open-circuit voltage (iV OC) of 0.704 V is achieved for the same. These results demonstrate that the passivation obtained here is of device quality for CZ Si wafers and facilitates the development of high-efficiency Si heterojunction (SHJ) solar cells.
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    Publication
    Design and modeling of a planar 2D nanostructured intermediate layer for light management in a very-thin SHJ bottom cell based monolithic perovskite/silicon tandem solar cell
    (01-09-2023)
    Uddin, Md Seraj
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    We present an optical simulation for a very-thin (60 μm) SHJ (silicon heterojunction cell) bottom cell based planar monolithic perovskite/silicon tandem solar cell incorporated with an intermediate reflector layer (IRL) in between the top and the bottom sub-cell to study the light management. The simulation was performed in a commercial software called Ansys Lumerical FDTD Solver. In this study, the tandem solar cell was simulated by incorporating a TFOR (topologically flat but optically rough) IRL of different periods, a DBR (distributed Bragg reflector) IRL, and a combination of (TFOR + DBR) IRL in the intermediate region to check the optical absorptance enhancement in the top and the bottom sub-cells, and, also the reflectance spectrum of the tandem solar cell. The tandem solar cell incorporated with a TFOR IRL was simulated for a range of top cell thicknesses, keeping the bottom cell thickness 60 μm (fixed); aiming to use as a flexible tandem solar cell. This exhibits a current density (Jsc) enhancement in the bottom sub-cell due to the incorporation of TFOR IRL, with a maximum for 500 nm period TFOR. However, the current density of the top sub-cell remains unchanged. The closest Jsc of both the top and the bottom sub-cells reaching the current-match condition is found in the tandem solar cell incorporated with a 500 nm period TFOR IRL. For the PIN based perovskite (500 nm)/silicon (60 μm) tandem solar cells, the maximum current density enhancement in the bottom sub-cell resulting from the incorporation of a 500 nm period TFOR IRL is 0.43 mA/cm2 . However, a DBR IRL or (TFOR + DBR) IRL incorporation in the tandem solar cell could not improve the current density of the bottom sub-cells. Instead, it increases the current density in the top sub-cells. Moreover, a current density loss was also analyzed in the TFOR IRL which shows that the current density loss decreases by increasing the TFOR period and a maximum loss is at 200 nm period. Further, an electric field analysis was also done in the top and the bottom sub-cells at peak absorption wavelengths, which shows that the electric field is more concentrated in the bottom sub-cell due to TFOR IRL, confirming the forward scattering of light in the tandem solar cell incorporated with TFOR IRL.
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    Publication
    Sensitivity of germanium content on growth conditions of silicon-germanium nanoparticles prepared in nonthermal capacitively-coupled plasmas
    (01-08-2020)
    Kleider, Jean Paul
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    Johnson, Erik
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    Brüggemann, Rudolf
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    Uddin, Md Seraj
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    We report on the synthesis of Si1-x Ge x alloy nanocrystals by very-high-frequency plasma-enhanced chemical vapor deposition (VHF PECVD) technique at different silane to germane gas flow ratio (R) in a mixture of (H2+Ar) dilution gas and H2 dilution gas alone. TEM, SAED, EDS studies and HAADF-STEM mapping of the samples were done to investigate the NCs' size, crystallinity and distribution of Si and Ge in the Si1-x Ge x alloy NCs. The average estimated size of the NCs in all the samples are in the order of exciton Bohr radius of Ge (24.3 nm), thereby indicating the probability of good quantum confinement. The alloy nature of NCs was confirmed in Raman study. The content of Ge in SiGe NCs was evaluated from Raman spectra which show a direct correlation with the fraction of hydrogen flow in the dilution gas mixture.
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    The structural, optical, and electrical properties of thermal evaporation-deposited V2OX films for use in silicon heterojunction solar cells
    (01-04-2023)
    Bandaru, Narendra
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    Kanakala, Rajesh
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    Madaka, Ramakrishna
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    Dsouza, Namitha
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    Maurya, Rajesh
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    Vanadium suboxide (V2Ox) has been suggested as a promising transition metal oxide for the development of selective contacts on high-performance crystalline silicon heterojunction solar cells. In this study, V2Ox thin films were deposited on wet-cleaned wafers (Cz n-type c-Si (111) with a thickness of 160 μm; cleaning steps include SAW damage removal, RCA1, RCA2, and HF dip) and Corning (Eagle 2000) glass substrates utilizing the thermal evaporation technique at a pressure of 5 × 10−5 mbar. V2Ox thin films were deposited with various thicknesses from 5 to 20 nm. Ellipsometry, XRD, AFM, UV–Vis analysis, minority carrier lifetime measurements using SINTON photoconductivity decay (PCD) and lifetime mapping by SEMILAB µ-PCD were used to figure out the structural, optical, and electrical properties of deposited V2Ox films. The thickness of these films was measured by the ellipsometry technique. The amorphous nature of the as-deposited films is confirmed by the XRD patterns, and their optical transmittance is in the range of 93 to 76% in the visible range for 5 to 20 nm for V2Ox films. At 1 sun illumination, the minority carrier lifetime values range from 220.40 ± 1.26 µs to 293.27 ± 0.63 µs. These values are within the range of well-passivated wafers, they translate into implied-VOC values ranging from 642 to 652 mV, indicating a high degree of surface passivation. Surface passivation could be caused by a sub-stoichiometric SiOx interlayer that forms when silicon bonds with oxygen. We postulate that the fixed charges acquired by the silicon sub-oxide layers create the field effect passivation. According to our experimental findings, a 10 nm-thick V2Ox film has the best optoelectronic properties, including a minority life-time of 293.27 ± 0.63 µs with an implied Voc of 652 mV. The findings are critical for the fabrication of hetero-junction silicon solar cells.