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Öğe Dependence of CZTS thin film properties and photovoltaic performance on heating rate and sulfurization time(Elsevier Science Sa, 2023) Olgar, M. A.; Erkan, S.; Zan, R.In this study, effect of heating rate and sulfurization time on the properties of CZTS thin films and their photovoltaic performances was investigated. CZTS layers were prepared by a two-stage technique comprising deposition of precursor films by sputtering on Mo-foil substrates (Mo-foil/ZnS/CuSn/Cu) and a sulfurization process utilizing Rapid Thermal Annealing (RTA) approach. Heating rates and sulfurization times at 550 degrees C were changed to determine the best conditions. CZTS films were characterized by several techniques. All samples showed Cu-poor and Zn-rich chemical composition regardless of the heating rate and annealing time of the sulfurization process. XRD diffraction patterns showed that only kesterite CZTS phase was formed in all samples. Although Raman spectroscopy measurements confirmed formation of kesterite CZTS phase in all samples, forming of Cu2SnS3 (CTS) secondary phase was detected in all films except for the sample prepared utilizing a relatively low heating rate of 1 degrees C/s and 60 s sulfurization time (CZTS-I-60). The largest crystallite size and the less micro strain and dislocation density were also found in this sample. All CZTS thin films displayed dense and compact polycrystalline surface microstructure. Optical characterizations showed that the optical band gaps of the films were in the range of 1.48-1.58 eV. PL emission spectra of the films exhibited transition from conduction band to acceptor/defect levels instead of band-to-band transition. Solar cells were fabricated and the highest Voc and conversion efficiency (4.39%) were achieved employing the film obtained with ramp rate of 1 degrees C/s and the sulfurization time of 60 s, because of better structural properties and purer crystal structure determined for this sample.Öğe Eco-Friendly Synthesis and Characterization of Reduced Graphene Oxide(Iop Publishing Ltd, 2017) Ickecan, D.; Zan, R.; Nezir, S.Graphene is a single sheet of sp(2) bonded carbon having a two-dimensional (2D) layer. It has remarkable electronic, mechanical and thermal properties. In this paper, the graphene oxide (GO) was reduced by reducing chemicals such as ascorbic acid and hydrazine and then characterized by transmission electron microscopy (TEM), Raman spectroscopy and Fourier transform infrared spectroscopy. TEM results of the chemically reduced graphene were showed that the structure consists of a mixture of single and few layers of reduced graphene oxide (rGO).Öğe Fabrication of Cu-rich CZTS thin films by two-stage process: Effect of gas flow-rate in sulfurization process(Elsevier, 2021) Olgar, M. A.; Altuntepe, A.; Erkan, S.; Zan, R.In this study, CZTS thin films were prepared using a two-stage process consisting of sputter deposition of Cu, Sn and ZnS layers on the glass substrates to form CuSn/ZnS/Cu stacked precursor films followed by sulfurization process under various flow-rate (0, 20, 40, 60, 80, 100 sccm) of Ar+ H-2 mix gas in a tubular furnace. The EDX measurements of the precursor and reacted films are Cu-rich and Zn-rich composition. However, Zn and Sn loss was observed for samples annealed under the flow-rate of gas above the 60 sccm. The XRD pattern of the samples showed diffraction peaks of pure kesterite CZTS structure and the FWHM values extracted from their XRD patterns and W-H plot calculations demonstrated that CZTS-40 thin film had a more desired crystallite size. The Raman spectra of the samples confirmed the formation of CZTS phase for all samples and indicated the formation of some secondary phases such as Cu2SnS3, SnS2, Sn2S3 except for CZTS-40 thin film. All the samples displayed dense and polycrystalline microstructure according to their SEM images. The optical band gap values of CZTS samples showed variation between 1.40 and 1.48 eV. The room-temperature PL spectra of the samples revealed a broad band that had a peak value at around 1.36-1.40 eV that are close to the optical band gap values for prepared CZTS samples. (C) 2021 Elsevier B.V. All rights reserved.Öğe Imaging Two Dimensional Materials and their Heterostructures(Iop Publishing Ltd, 2017) Zan, R.; Ramasse, Q. M.; Jalil, R.; Tu, J-S; Bangert, U.; Novoselov, K. S.Stacking different two-dimensional (2D) atomic layers is a feasible approach to create unique multilayered van der Waals heterostructures with desired properties. 2D materials, graphene, hexagonal boron nitride (h-BN), molybdenum disulphate (MoS2) and graphene based van der Waals heterostructures, such as h-BN/graphene and MoS2/graphene have been investigated by means of Scanning Transmission Electron Microscopy (STEM).Öğe Impact of in/ex situ annealing and reaction temperature on structural, optical and electrical properties of SnS thin films(Elsevier, 2021) Olgar, M. A.; Ciri, A.; Tomakin, M.; Zan, R.In this work, SnS thin films were prepared by in situ and ex situ annealing process of precursor films deposited by RF (Radio Frequency) magnetron sputtering employing binary SnS target. In situ annealing treatment was performed in sputtering chamber and ex situ annealing treatment was performed using RTP (Rapid Thermal Processing) system under Ar+H-2 mix gas employing 225, 300 and 375 degrees C as reaction temperatures in order to find out the best fabrication parameters of SnS thin film since it has been used as an absorber layer in the cell structure. The EDX (Energy Dispersive X-ray Spectroscopy) measurements showed that precursor and reacted films have almost stoichiometric composition except for in situ annealed sample at 375 degrees C. XRD (X-ray Diffraction) patterns of all samples revealed orthorhombic SnS phase regardless of annealing route and temperature. In addition to SnS phase, formation of SnS2 phase was observed in in situ annealed SnS samples at 225 and 300 degrees C. Moreover, in situ annealed samples displayed larger crystallite size and lower micro strain compared to ex situ annealed samples. Raman spectra of the samples confirmed formation of orthorhombic SnS phase and it was also seen that crystalline quality gave rise to shift in position of Raman bands for some samples. Only a SEM (Scanning Electron Microscopy) image of in situ annealed sample at 375 degrees C displayed distinct surface morphology. Optical band gap values of the samples showed variation between 1.35 and 1.66 eV. Electrical characterization of the films showed that resistivity values changed from 3.34x10(3) to 2.28x10(4) Omega-cm and carrier concentration values changed from 1.34x10(14) to 1.05x10(15) cm(-3). It was seen that in situ annealing at 375 degrees C exhibited more promising results for potential SnS based photovoltaic applications. (C) 2021 Elsevier B.V. All rights reserved.Öğe Impact of stacking order and annealing temperature on properties of CZTS thin films and solar cell performance(Pergamon-Elsevier Science Ltd, 2021) Olgar, M. A.; Sarp, A. O.; Seyhan, A.; Zan, R.In this study, Cu2ZnSnS4 (CZTS) thin films were synthesized by a two-stage process. In the first stage, CuSn/Zn/Cu (E-type) and CuSn/ZnS/Cu (B-type) stacked films were formed using the sputtering method. In the second stage, precursor films were annealed in sulfur atmosphere utilizing various annealing temperatures (500, 525, 550 and 575 degrees C) employing the Rapid Thermal Processing (RTP) method. The EDX measurements demonstrated that almost all the samples had Cu-poor and Zn-rich compositions, as targeted. The XRD patterns of all the CZTS samples were dominated by diffraction peaks of the kesterite CZTS phase. In addition to CZTS phase, Cu-S/Sn-S based secondary phases in all E-type CZTS thin films and some B-type CZTS samples annealed at lower temperatures (500 and 525 degrees C) were observed. The samples annealed at above 525 degrees C revealed purer crystal structure in terms of secondary phases and they have more promising crystallite size in both types of CZTS thin films. The Raman spectroscopy measurements confirmed the formation of kesterite CZTS phase and distinguished the formation of Cu2SnS3 (CTS) phase for some samples. The samples annealed at 550 degrees C presented purer structure for potential solar cell application. The SEM surface and cross-section images of all CZTS samples displayed dense and polycrystalline structures but samples annealed at 550 degrees C presented a larger-grained surface and crosssection structure in both types of CZTS films. PL spectra of the B-type CZTS samples exhibited a purer band structure with respect to E type CZTS samples according to their PL band values. The best solar cell performance was achieved with CZTS thin film prepared using CuSn/ZnS/Cu stack annealed at 550 degrees C temperature with 252 mV, 32 mA/cm(2), and 3.79% parameters. (C) 2021 Elsevier Ltd. All rights reserved.Öğe Impact of sulfurization parameters on properties of CZTS thin films grown using quaternary target(Springer, 2020) Olgar, M. A.; Seyhan, A.; Sarp, A. O.; Zan, R.In this study, CZTS thin films were grown by annealing of sputtered films using quaternary single target employing various annealing parameters. The effects of the post-sulfurization treatment, reaction temperature (500, 525, 550 and 575 degrees C) and sulfurization time (60, 90, 120 and 150 s) on the properties of CZTS thin films were analyzed. The optimization of reaction temperature for 60 s dwell time was examined by annealing the precursor films with/without sulfur atmosphere. It was shown that annealing of the films under the sulfur atmosphere prevents Zn-loss in the samples for higher annealing temperatures (550 and 575 degrees C) and hindering the formation of secondary phases such as Cu2-xS, Cu2SnS3(CTS). The FWHM values of the sulfurized samples revealed that the sulfurization temperature of 550 degrees C is preferable for the fabrication of CZTS samples. Further optimization was performed at 550 degrees C for various sulfurization times. It was seen that all the samples have Cu-poor and Zn-rich composition. The XRD pattern of CZTS samples displayed formation of kesterite CZTS phase but SnS(2)phase formations were also observed for longer sulfurization time (> 120 s). It was also observed that the sulfurization time has more significant contribution on the crystallite size of the samples with respect to sulfurization temperature. The Raman spectra of the CZTS samples confirmed the formation of kesterite structures for all the films and appearance of secondary phase for films prepared using longer sulfurization time (> 120 s). All the samples displayed a dense and polycrystalline surface morphology, but the sulfurized sample for 120 s displayed more homogenous and prominent morphology. The room temperature PL measurements demonstrated a broad band which peaked at about 1.36-1.37 eV, which is very close to the band gap of kesterite CZTS structure. The electrical characterization of the samples showed that all the samples have p-type conductivity and the CZTS-S-550-120 sample has a more promising result considering both resistivity and carrier concentration.Öğe Impact of the ZnS layer position in a stacked precursor film on the properties of CZTS films grown on flexible molybdenum substrates(Elsevier, 2023) Yagmyrov, A.; Erkan, S.; Basol, B. M.; Zan, R.; Olgar, M. A.In the present study, CZTS thin films were prepared by annealing and reaction of Cu-Sn-ZnS precursor layers. First, sputter deposition was carried out on flexible molybdenum (Mo) foil to form Mo-foil/CuSn/ZnS/Cu, Mo-foil/CuSn/Cu/ZnS and Mo-foil/ZnS/CuSn/Cu stacked precursor structures. Annealing process was performed in sulfur atmosphere using Rapid Thermal Processing (RTP) method to obtain kesterite CZTS phase in the reacted layers. All prepared precursors and CZTS thin films displayed Cu-poor and Zn-rich chemical composition, as targeted. XRD patterns of CZTS samples showed that the kesterite phase was obtained in all samples regardless of the stacking order of the precursor films. However, the full width at half maximum (FWHM) values of the (112) preferential peaks extracted from the XRD patterns, and the corresponding structural parameters (crystallite size and microstrain), indicated that the Mo-foil/ZnS/CuSn/Cu precursor structure yielded more promising crystal-line quality. The occurrence of kesterite phase in all samples and existence of low amount of CTS phase were verified by Raman spectroscopy measurements. The CZTS sample prepared employing the Mo-foil/ZnS/CuSn/Cu precursor film structure presented more prominent, homogenous and compact surface microstructure as observed in SEM images. Optical band gap values were found to be in the range of 1.44-1.50 eV. The room temperature photoluminescence (PL) measurements showed that the transitions from the conduction band to intrinsic defect levels dominated the spectra instead of the band-to-band transitions. Electrical characterization of the films showed that Mo-foil/CuSn/ZnS/Cu and Mo-foil/ZnS/CuSn/Cu precursor films yielded lower elec-trical resistivity and higher carrier concentration due to better crystalline quality. Overall, it was seen that the CZTS thin films produced using the Mo-foil/ZnS/CuSn/Cu stacked precursor layers displayed better properties in terms of crystalline quality, surface microstructure, and optical and electrical properties, which are favorable for photovoltaic applications.Öğe Integration of single layer graphene into CZTS thin film solar cells(Elsevier Science Sa, 2022) Erkan, S.; Yagmyrov, A.; Altuntepe, A.; Zan, R.; Olgar, M. A.In this study, CZTS samples were produced on Mo and graphene/Mo coated glass substrates using qua-ternary target. The CZTS thin films deposited by RF magnetron sputtering were annealed using rapid thermal processing (RTP) method in sulphur atmosphere at 500, 525, and 550 degrees C so as to obtain glass/Mo/ CZTS and glass/Mo/graphene/CZTS (g-CZTS) structures. The obtained CZTS and g-CZTS thin films were then characterized by several methods such as EDX, XRD, Raman spectroscopy, SEM etc. The EDX data demon-strated that all CZTS thin films had Cu poor composition regardless of the sulfurization temperature and increasing the temperature led to Sn loss from the films. Diffraction peaks of kesterite CZTS phase were observed in all the samples; additionally, SnS and CuS secondary phases were also observed in CZTS samples annealed at 500 degrees C. The crystallite size of the CZTS thin films were found to be increasing with both increasing annealing temperature and use of graphene film as an inter-layer. The creation of kesterite phase with a very small CTS phase in all the samples were verified by the Raman spectroscopy measurement. The SEM images of the samples indicated that using graphene improves the crystalline quality of the CZTS films and contributes to forming more compact, homogenous and larger crystal structure. The determined optical band gap values varied from 1.41 to 1.44 eV depending on the Sn-content of the samples. The produced solar cells selected from the more promising absorber layers showed that implementing graphene in CZTS cell structure enhanced the conversion efficiency from 2.40% to 3.52% due to improvement of crystalline quality of the absorber layer. (c) 2022 Elsevier B.V. All rights reserved.Öğe Permanent Boron Doped Graphene with high Homogeneity using Phenylboronic Acid(Elsevier, 2021) Altuntepe, A.; Zan, R.In this paper, we report on boron doped graphene synthesis using phenylboronic acid as a single source of carbon and boron. The growth conditions were established and optimized to improve the opto-electronic properties of a graphene film. The effects of the phenylboronic acid amount and growth time on the synthesis of graphene were investigated under similar growth conditions on polycrystalline copper foil in a three-zone CVD system. Based on Raman and XPS analysis, boron doped graphene was successfully synthesized under the given growth conditions in this study. It was also found that increasing the amount and growth times led to thicker graphene films. However, a single layer region was only found for a film grown using 0.5 gr phenylboronic acid along with 30-minute growth time, which is in contrast with the current literature. Additionally, carbon atoms were found to be substituted by the boron atoms in the honeycomb structure as revealed by the XPS measurements. Substitutional doping enables the doping to be stable for a long time, which is crucial for the doping to be employed in semiconducting technology particularly in optoelectronics. (C) 2020 Elsevier B.V. Allrightsreserved.Öğe Synthesis of reduced graphene oxide-fungal hyphae biochar/iron oxide composite: characterization, adsorption performance, and removal mechanisms(Springer, 2024) Deveci, E. U.; Madenli, O.; Akarsu, C.; Zan, R.In this study, a novel biochar, iron-impregnated reduced graphene oxide-fungal hyphae, named AOBC-rGO, was synthesized as an innovative approach for the removal of Cr(VI) ions from water. A scanning electron microscope showed the formation of large agglomerates on the surface of biochar samples while Raman analysis showed D and G bands at 1350 and 1580 cm(-1) respectively as the proof of polyaromatic hydrocarbons and graphitic material. The surface area of the AOBC-rGO was determined as 58.5650 m(2) g(-1). To evaluate the efficacy of AOBC-rGO, the effect of initial chromium concentration (25, 50, 100, 150 and 200 mg L-1), pH (2, 5, 7, 9 and 11), retention time (0, 5, 10, 20, 30, 40, 50, 60 and 90 min), and adsorbent dose (100, 300 and 500 mg L-1) was investigated in batch experiments for synthetic and real wastewater samples. Adsorption tests showed that the biochar adsorption capacity was 1116.01 mg g(-1). Furthermore, findings suggest that the chromium sorption on AOBC-rGO fits with the pseudo-second-order model and Freundlich isotherm model (R-2 > 0.93), indicating a heterogeneous surface with a dominant chemisorption mechanism. Thus, AOBC-rGO can be used as an effective adsorbent to remove heavy metals from wastewater.Öğe The choice of Zn or ZnS layer in the stacked precursors for preparation of Cu2ZnSnS4 (CZTS) thin films(Academic Press Ltd- Elsevier Science Ltd, 2020) Olgar, M. A.; Seyhan, A.; Sarp, A. O.; Zan, R.Cu2ZnSnS4 (CZTS) thin films are commonly used as an absorber layer in the thin film solar cell structure. In this study, CZTS thin films were produced by sulfurization of stacked precursor films that is prepared by deposition of Cu, Zn, SnS, ZnS films on glass substrate using sputtering method. The sequential sputter deposition was performed to obtain two distinct stacked precursors, Cu/SnS/Zn/Cu and Cu/SnS/ZnS/Cu respectively. Afterwards, annealing process was implemented at various reaction temperatures (500-575 degrees C) for 1 min utilizing rapid thermal processing (RTP). The EDX measurements revealed that all the prepared CZTS samples had Cu-poor and Zn-rich composition that are non-stoichiometric chemical composition. This non-stoichiometric composition is important for high efficient CZTS based solar cells. XRD measurements revealed that all patterns are dominated by diffraction planes of kesterite CZTS. The CuS, Cu2S and SnS2 secondary phases also were detected in the XRD pattern of some CZTS films. Raman spectroscopy measurements verified formation of kesterite CZTS phase for all films and picked out CTS phase for some samples prepared using Cu/SnS/Zn/Cu precursor films. The surface microstructure of the films that were obtained through SEM displayed polycrystalline surface structure. Room temperature PL emission spectra of the films showed broad peak at around 1.37-1.38 eV, which is near to the optical band gap of kesterite CZTS structure. Electrical characterization of the samples demonstrated that B-525 CZTS thin film has more suitable electrical resistivity and carrier concentration values for CZTS based solar cell applications.Öğe Under pressure: Control of strain, phonons and bandgap opening in rippled graphene(PERGAMON-ELSEVIER SCIENCE LTD, 2015) Monteverde, U.; Pal, J.; Migliorato, M. A.; Missous, M.; Bangert, U.; Zan, R.; Powell, D.Two-dimensional (2D) layers like graphene are subject to long-wavelength fluctuations that manifest themselves as strong height fluctuations (ripples). In order to control the ripples, their relationship with external strain needs to be established. We therefore perform molecular dynamics (MD) of suspended graphene, by the use of a newly developed force field model (MMP) that we prove to be extremely accurate for both C Diamond and Graphene. The MMP potential successfully reproduces the energy of the a-bonds in both sp(3) and sp(2) configuration. Our MD simulations and experimental electron microscopy analysis reveal that ordered and static ripples form spontaneously as a direct response to external pressure. Furthermore the morphology of graphene and strain response of the crystal bonds differ depending on the particular directions where external pressure is present. Different regions of the strained graphene sheet are then investigated by tight-binding. Localised bandgap opening is reported for specific strain combinations, which also results in particular signatures in the phonon spectrum. Such controllable morphological changes can therefore provide a means to practically control and tune the electronic and transport properties of graphene for applications as optoelectronic and nanoelectromechanical devices. (C) 2015 The Authors. Published by Elsevier Ltd.
