Yazar "Seyhan, Ayse" seçeneğine göre listele
Listeleniyor 1 - 8 / 8
Sayfa Başına Sonuç
Sıralama seçenekleri
Öğe Comparison of picosecond electron dynamics in isolated and clustered Si quantum dots deposited on a semiconductor surface(Amer Inst Physics, 2019) Fukumoto, Keiki; Seyhan, Ayse; Onda, Ken; Oda, Shunri; Koshihara, Shin-yaSemiconductor quantum dots (QDs) have been widely used in various optoelectronic devices. Extensive studies have been devoted to the application of Si QDs with the aim of realizing various optoelectronic functions based on the modified energy band structure in QDs compared with bulk crystals. Therefore, it is necessary to be able to directly probe the carrier dynamics in single Si QDs of nanoscale dimensions deposited on a SiO2/Si surface, where the environment is compatible with Si-based semiconductor devices. This letter reports the observation and comparison of the ultrafast electron dynamics just after the photoexcitation of isolated and clustered Si QDs on a SiO2/Si surface using time-resolved photoemission electron microscopy with spatial and temporal resolutions of 50nm and 100fs, respectively. The detailed structure of QDs was confirmed directly by scanning electron microscopy observations. The results obtained in the present study show that the carrier lifetime in isolated QDs is shorter than that in clustered QDs. This is consistent with the electron-hole interaction in nanospace, significantly modifying the carrier recombination rates. Published under license by AIP Publishing.Öğe Crystalline-silicon heterojunction solar cells with graphene incorporation(Elsevier, 2021) Zan, Recep; Altuntepe, Ali; Altan, Tolga; Seyhan, AyseAmongst the silicon (Si)-based photovoltaics, heterojunction solar cells are the most promising solar cells due to their low thermal coefficient, high efficiency, and compatibility with newly emerged materials, such as graphene and perovskite. Studies on Si-based solar cells with graphene have increased dramatically in recent years. Thus far, high power conversion efficiency has been achieved up to 15 % by integrating graphene into graphene/Si Schottky junction solar cells since graphene has excellent electrical and optical properties. © 2021 Elsevier Inc.Öğe Graphene for Si-based solar cells(Elsevier, 2020) Altuntepe, Ali; Seyhan, Ayse; Zan, RecepIn this paper, we report on the single layer graphene synthesis to establish the growth conditions and improve the opto-electronic properties that can be employed in silicon based heterojunction solar cells. To do this, the effect of hydrogen and methane flow on the graphene growth on copper foil in a CVD system was investigated. The analyses were conducted by changing either the hydrogen or the methane flow by keeping all the other growth parameters constant. Single layer graphene growth recipe was established in order to have the optimum optical transmission and sheet resistance values via amending the graphene growth conditions. It was found that the sheet resistance values of the single layer graphene should be lowered further to be used as transparent conductive electrode. However, the combination of graphene with indium tin oxide film functioned well as transparent conductive electrode in the silicon based the solar cells. Additionally, the cell efficiency increased by about 10% as a result of incorporating it with the single layer graphene. (C) 2019 Elsevier B.V. All rights reserved.Öğe Impact of Temperature Optimization of ITO Thin Film on Tandem Solar Cell Efficiency(Mdpi, 2024) Damgaci, Elif; Kartal, Emre; Gucluer, Furkan; Seyhan, Ayse; Kaplan, YukselThis study examined the impact of temperature optimization on indium tin oxide (ITO) films in monolithic HJT/perovskite tandem solar cells. ITO films were deposited using magnetron sputtering at temperatures ranging from room temperature (25 degrees C) to 250 degrees C. The sputtering target was ITO, with a mass ratio of In2O3 to SnO2 of 90% to 10%. The effects of temperature on the ITO film were analyzed using X-ray diffraction (XRD), spectroscopic ellipsometry, and sheet resistance measurements. Results showed that all ITO films exhibited a polycrystalline morphology, with diffraction peaks corresponding to planes (211), (222), (400), (440), and (622), indicating a cubic bixbyite crystal structure. The light transmittance exceeded 80%, and the sheet resistance was 75.1 Omega/sq for ITO deposited at 200 degrees C. The optical bandgap of deposited ITO films ranged between 3.90 eV and 3.93 eV. Structural and morphological characterization of the perovskite solar cell was performed using XRD and FE-SEM. Tandem solar cell performance was evaluated by analyzing current density-voltage characteristics under simulated sunlight. By optimizing the ITO deposition temperature, the tandem cell achieved a power conversion efficiency (PCE) of 16.74%, resulting in enhanced tandem cell efficiency.Öğe Integration of graphene with GZO as TCO layer and its impact on solar cell performance(Pergamon-Elsevier Science Ltd, 2022) Zan, Recep; Olgar, Mehmet Ali; Altuntepe, Ali; Seyhan, Ayse; Turan, RasitIn this study, we investigated the impact of incorporating graphene with Ga-doped ZnO (GZO) when employing them as a TCO layer on Si-based solar cell. GZO thin films with various thicknesses (50-450 nm) were fabricated by the sputtering method using a single target. The aim here was to determine the GZO film with the optimum thickness to incorporate it with single layer graphene as TCO. This thickness was found to be 350 nm as that was the best crystalline quality found in the Opattern. Further, this sample had the lowest sheet resistance and highest transmission values as confirmed by electrical (sheet resistance), and optical characterizations (transmission). Topographic (SEM and AFM), electrical (resistivity and carrier concentration) measurements were also conducted on the same sample. The graphene film grown on copper in a CVD system was then transferred on top of this sample to fabricate the hybrid TCO structure. We found that graphene integrated GZO hybrid TCO film showed higher sheet resistance due to high sheet resistance of graphene and similar optical properties thanks to high optical transmission of graphene. Employing graphene-based TCO layer in the solar cell resulted in higher open-circuit voltage, consequently improving the conversion efficiency from 10.0% to 11.2%. (c) 2021 Elsevier Ltd. All rights reserved.Öğe Nitrogen Doped Graphene Film Synthesis and Characterization(Gazi Univ, 2022) Zan, Recep; Altuntepe, Ali; Erkan, Serkan; Seyhan, AyseHaving a single atom thickness formed by the combination of carbon atoms in a hexagonal mesh, graphene has been one of the most intensely researched areas in recent years. This is especially due to its wide range of uses thanks to its numerous superior properties such as high electrical and thermal conductivity, high light transmittance, high strength and large surface area. Despite these superior properties, graphene's high sheet resistance and lack of energy band gap limit its use in optoelectronic applications. Yet, these disadvantages can be overcome by implementing doping to the graphene structure. However, although the doping selection and synthesis method are very important in the doped graphene synthesis process, the permanence and homogeneity of the doping is even more critical. In this study, the synthesis and characterization of pure and nitrogen-doped graphene were carried out on copper foil using CVD system. Pyridine was used for the synthesis of the doped graphene, as a carbon and nitrogen source. We found out that the use of the CVD technique allows both homogeneous and permanent doping. In addition to pyridine, in the present study, the film thickness was also optimized by providing a low amount of methane gas flow to the CVD system during the synthesis. To enable the characterization of doped films, Raman spectroscopy, Energy Dispersive X-ray spectroscopy and Xray photo electron spectroscopy techniques were used, and the film quality, thickness, homogeneity, doping rate and type were determined.Öğe Optical, Electrical and Structural Properties of ITO/IZO and IZO/ITO Multilayer Transparent Conductive Oxide Films Deposited via Radiofrequency Magnetron Sputtering(Mdpi, 2023) Seyhan, Ayse; Kartal, EmreIn this study, we investigated the potential of multilayer TCO structures, specifically those made up of Indium Tin Oxide (ITO) and Indium Zinc Oxide (IZO), for crystalline silicon heterojunction solar cells (SHJ). We used the radiofrequency (RF) magnetron sputtering method to deposit various thin-film structures under various deposition temperatures and evaluated their electrical, optical, and morphological properties. The objective was to obtain films with lower sheet resistances and higher transmittances than those of single-layer thin films. Our results show that the ITO/IZO/ITO/IZO/ITO multilayer film structure deposited at 200 degrees C achieves the best sheet resistance of 18.5 Ohm/sq and a high optical transmittance of over 90% at a 550 nm wavelength. This indicates that multilayer TCO structures have the potential to be more optically and electrically efficient, and that they can improve the performance of optoelectronic devices. Finally, a power conversion efficiency of 17.46% was obtained for a silicon heterojunction (SHJ) solar cell fabricated using an ITO/IZO/ITO/IZO/ITO multilayer film structure deposited at 200 degrees C as a front TCO. Our study provides valuable insights into the field of TCOs and offers a promising avenue for future research.Öğe Sputtered Mo-bilayer thin films with reduced thickness and improved electrical resistivity(Iop Publishing Ltd, 2019) Keles, Filiz; Atasoy, Yavuz; Seyhan, AyseIn this study, Mo-bilayer film, the thickness of which was reduced to approximately 270 nm with a very low resistivity of 14 mu Omega.cm, was successfully grown by DC magnetron sputter. The Mo-bilayer, whose bottom and top layers were obtained by high pressure sputter (HPS) and low pressure sputter (LPS) respectively, demonstrates good adhesivity and crystalline properties, together with high reflectance. In order to obtain Mo-bilayer with these improved properties, we first determined the optimal growth temperature and pressure parameters by checking the structural and electrical properties respectively of Mo-single layers. As a result, we achieved a deposit of Mo-bilayer thin film that can be used as a good back contact layer in solar cell applications, both in terms of material cost saving and its superior properties, even at such low thickness.
