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    Characterization of Cu(In,Ga)(Te,S)2 thin films grown on stainless steel foil substrates
    (Iop Publishing Ltd, 2023) Karaca, Abdullah; Basol, Bulent M.; Olgar, M. Ali; Buyuklimanli, Temel; Tomakin, Murat; Kucukomeroglu, Tayfur; Bacaksiz, Emin
    In this study, Cu(In,Ga)(Te,S)(2) (CIGTS) thin films with [Ga]/([In] +/- [Ga]) atomic ratios in the ranges of 0.22-0.28 and 0.50-0.67 were fabricated using a two-stage technique. During the first stage of the technique, in one set of samples, Cu, In and Ga layers were deposited by electrodeposition on a Mo coated stainless steel (SS) foil substrate forming a SS/Mo/Cu/In/Ga precursor structure. For another set of samples, a Te layer was also deposited by e-beam evaporation on the SS/Mo/Cu/In/Ga structure forming a SS/Mo/Cu/In/Ga/Te precursor structure. During the second stage, SS/Mo/Cu/In/Ga and SS/Mo/Cu/In/Ga/Te stacks were reacted using rapid thermal annealing (RTA) for 5 min at 600 degrees C with or without presence of S vapors to produce CIGTS series thin films. SS/Mo/Cu/In/Ga stack under S atmosphere yielded CuInGaS2 with a Ga-In gradient across the thickness by RTA process. SS/Mo/Cu/In/Ga/Te stack reacted without S in the reaction atmosphere yielded the CuInGaTe2 compound. When S was present, the same stack with top Te layer yielded only CuInGaS2 compound. When, however, already formed CuInGaTe2 compound layers were heated in S environment at 400 degrees C, some Te could be retained in the films in the form of elemental Te. Gallium and In grading in various reacted films were evaluated by x-ray diffraction, secondary-ion mass spectrometry and EDS. CIGTS films showed highly (112) preferred oriented chalcopyrite phase and with the increase of Ga content, shifts were observed in the XRD peak positions demonstrating Ga inclusion in the lattice. Gibbs free energy calculations were used to explain the preferred reaction of S with metallic constituents when both S and Te were present for reaction.
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    Deposition of CdSeTe alloys using CdTe-CdSe mixed powder source material in a close-space sublimation process
    (Springer, 2021) Ciris, Ali; Basol, Bulent M.; Atasoy, Yavuz; Karaca, Abdullah; Kuckomeroglu, Tayfur; Tomakin, Murat; Bacaksiz, Emin
    CdSexTe1-x films were deposited using a close-space sublimation (CSS) method and CdSe + CdTe mixed powders as the source material. Composition of the source was changed to obtain films with varying x values, and the resulting films were characterized by XRD, SEM, photoluminescence, Raman and optical transmission measurements. All data agreed with the fact that as the Se content of the source material was increased, the composition parameter x also increased. GI-XRD measurements showed the films to be graded in composition, the surface region being more Se-rich. Band gap values obtained from optical measurements showed a minimum band gap of about 1.4 eV for the material that had the highest Se content of about 45% near its top surface. Energy gap vs composition data demonstrated the expected bowing effect in band gap values and a bowing parameter of 0.678 was determined.
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    Effect of CdS and CdSe pre-treatment on interdiffusion with CdTe in CdS/CdTe and CdSe/CdTe heterostructures
    (Elsevier Sci Ltd, 2021) Ciris, Ali; Basol, Bulent M.; Atasoy, Yavuz; Kucukomeroglu, Tayfur; Karaca, Abdullah; Tomakin, Murat; Bacaksiz, Emin
    High efficiency CdTe solar cell structure has the configuration of CdS/CdSe/CdTe. Depending on the deposition and post deposition techniques employed, this stack is often subjected to high temperatures, often in presence of CdCl2, which leads to various degrees of interdiffusion at the CdS/CdSe and CdSe/CdTe interfaces. Such interdiffusion greatly influences device performance. Therefore, understanding and controlling these interdiffusion processes are important. In this contribution, interdiffusion between CdS-CdTe and CdSe-CdTe pairs were studied using CdS/CdTe and CdSe/CdTe stacks annealed at 673 K. Effect of pre-treating the CdS and CdSe layers with CdCl2 before the CdTe deposition on this interdiffusion was investigated. CdS films were grown by CBD and CdSe and CdTe films were vacuum evaporated. CdTe thickness was intentionally kept at the low 150?200 nm range to more easily identify alloy phases formed. GA-XRD measurements demonstrated that in absence of any CdCl2 pretreatment, there was more interdiffusion between CdSe and CdTe compared to CdS and CdTe. In all cases CdCl2 pre-treatment of CdS or CdSe before the deposition of the CdTe film was found to reduce diffusion of S and Se into CdTe.
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    Effect of ultra-thin CdSexTe1-x interface layer on parameters of CdTe solar cells
    (Pergamon-Elsevier Science Ltd, 2022) Ciris, Ali; Basol, Bulent M.; Atasoy, Yavuz; Karaca, Abdullah; Tomakin, Murat; Kucukomeroglu, Tayfur; Bacaksiz, Emin
    Effects of an ultra-thin CdSexTe1-x junction interface layer on CdTe solar cell parameters were investigated employing a CdSexTe1-x/CdTe absorber structure. CdSexTe1-x thin films with varying composition were grown by vacuum evaporation and CdTe films were produced by the close spaced sublimation (CSS) method. XRD analysis showed that while the CdSexTe1-x layers with x values less than 0.39 crystallized in cubic structure, films that were richer in Se displayed a (cubic + hexagonal) mixed phase. SEM analysis demonstrated a morphology with compact grains for all films. However, the grain size decreased appreciably with increasing Se content. Optical measurements showed that the band gaps of the alloys reached the minimum value of 1.40 eV at x similar to 0.32. CdS/CdSexTe1-x/CdTe solar cells were fabricated employing 100 nm thick CdSexTe1-x interlayers. The Grazing Incidence (GI)-XRD spectra of CdSexTe1-x used in the device structure showed that these inter-layers had graded alloy composition. The average Se-concentration within the graded alloy films were found to agree with the values obtained by EDS. Conversion efficiencies of 9.59% 11.69% and 10.13%, were obtained for x values of 0.24, 0.32 and 0.39, respectively. Spectral response showed enhanced long wavelength response for all devices due to the presence of the CdSexTe1-x interlayer. It was concluded that using an ultra-thin CdSexTe1-x inter-layer with optimum properties between CdS (junction partner) and CdTe improves the cell performance by increasing the current density of the device.
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    Impact of CdSeTe and CdSe film deposition parameter on the properties of CdSeTe/CdTe absorber structure for solar cell applications
    (Iop Publishing Ltd, 2024) Ciris, Ali; Atasoy, Yavuz; Tomakin, Murat; Karaca, Abdullah; Kucukomeroglu, Tayfur; Bacaksiz, Emin
    In this study, the effect of depositing CdSeTe and CdTe layers at different substrate temperatures (STs) by evaporation in vacuum on the properties of the CdSeTe/CdTe stacks was investigated. First, CdSeTe layers in stack structure were grown at STs of 150 degrees C, 200 degrees C and 250 degrees C and then CdTe layers on the CdSeTe produced with the optimum temperature were coated at STs of 150 degrees C, 200 degrees C and 250 degrees C. The employing of STs up to 150 degrees C on both CdSeTe and CdTe films in CdSeTe/CdTe stacks demonstrated the presence of Te and/or oxide phases as well as the alloying, while more stable phase structures at higher temperatures. In the CdSeTe/CdTe stack, the increase in ST of CdSeTe promoted the alloying, while it weakened the alloy in which was applied in CdTe. It was concluded that under the applied experimental conditions, STs of 250 degrees C and 200 degrees C with the graded alloying structure, suitable absorption sites, more homogeneous surface morphology for potential solar cell applications would be more suitable for CdSeTe and CdTe, respectively. As a result, the application of ST to CdSeTe or CdTe in the stacks can be used as a tool to control the properties of the stack structure.
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    Processing CdS- and CdSe-containing window layers for CdTe solar cells
    (Iop Publishing Ltd, 2021) Ciris, Ali; Basol, Bulent M.; Atasoy, Yavuz; Karaca, Abdullah; Tomakin, Murat; Kucukomeroglu, Tayfur; Bacaksiz, Emin
    The influence of heat treatment steps on the characteristics of (CdS, CdSe) junction partners and on solar cell performance was studied. CdS films were obtained by chemical bath deposition, and CdSe layers were evaporated. Structural and compositional properties of CdS/CdSe bilayer stacks did not change upon heat treatment at 400 degrees C up to 10 min, whereas heat treatment in the presence of CdCl2 for 10 min caused formation of a CdSSe alloy with a bandgap value of about 2.05 eV. Originally, the cubic structure of the stack was also transformed into a hexagonal structure during this treatment. CdSe-CdTe interdiffusion was also studied using CdS/CdSe/CdTe triple layer stacks. CdTe films were deposited using a close-spaced sublimation method. Limited CdSe-CdTe interdiffusion was seen when CdTe was deposited over the as-deposited CdSe layer at 580 degrees C. However, such interdiffusion was not detected for samples where CdTe deposition was carried out on CdS/CdSe stacks pre-annealed in the presence of CdCl2. This suggests that partial crystallization of the CdS/CdSe bilayer stack by CdCl2 reduced such an interaction. Solar cells with CdSe/CdTe, CdS/CdTe and CdS/CdSe/CdTe structures with efficiencies of 8.39%, 10.12% and 11.47% were fabricated using 4.5-5 mu m thick CdTe layers and a final CdCl2 treatment. Quantum efficiency measurements demonstrated the benefit of CdSe-CdTe alloying during the final CdCl2 treatment in improving the short circuit current values.
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    The effect of ZnCl2 and CdCl2 treatment on ZnS/CdS junction partner on CdTe cell performance
    (Elsevier Sci Ltd, 2022) Ciris, Ali; Atasoy, Yavuz; Karaca, Abdullah; Tomakin, Murat; Kucukomeroglu, Tayfur; Bacaksiz, Emin
    In this study, the impacts of ZnCl2 and CdCl2 treatments on the structural and optical properties of ZnS/CdS bilayers and on the parameters of CdTe solar cells with ZnS/CdS junction partners were investigated. CdS and ZnS thin films were grown by chemical bath deposition. In the as deposited ZnS/CdS sample, hexagonal CdS and Zn(S,O) phases were formed. After the application of CdCl2 treatment to the sample, ZnO and CdZnS alloys also appeared. In the ZnCl2 treated bilayer, it was observed that the crystal structure of the CdZnS alloy changed from hexagonal to cubic phase. While similar grain structure was observed in the as deposited and the ZnCl2 treated samples; it was seen that CdCl2 treatment significantly affected the grain form and size. CdCl2 treatment resulted in a large increase of Cd-ratio and a more balanced increase Zn-ratio in ZnCl2 treatment. It was determined that ZnCl2 and CdCl2 treatments caused a decrease in the transmittance of the samples. PL spectroscopy revealed the presence of many structural defects such as interstitial zinc, sulfur vacancies, surface states, cadmium vacancies in all bilayer samples. Solar cells with ZnS/CdS, ZnS/CdS (CdCl2-treated) and ZnS/CdS (ZnCl2-treated) junction partners achieved efficiencies of 4.56%, 5.64% and 5.20%, respectively. Solar cell parameters showed that ZnCl2 treatment increased the FF value, while CdCl2 treatment improved the Voc value. An efficiency of 6.01% was obtained from the ZnS/CdS (CdCl2-treated)/CdTe/CdCl2 cell produced by obtaining the highest efficiency with the ZnS/CdS/CdCl2 junction partner. This cell revealed that applying CdCl2 treatment on CdTe significantly increased Voc and Jsc while deteriorating FF.