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    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.
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    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.
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    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.