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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 Effect of reduced graphene oxide addition on cathode functional layer performance in solid oxide fuel cells(Pergamon-Elsevier Science Ltd, 2023) Timurkutluk, Cigdem; Zan, Recep; Timurkutluk, Bora; Toruntay, Furkan; Onbilgin, Sezer; Hasret, Onur; Altuntepe, AliSolid oxide fuel cells (SOFCs) operating at high temperatures are highly efficient electrochemical devices since they convert the chemical energy of a fuel directly into heat and electrical energy. The electrochemical performance of an SOFC is significantly influenced by the materials and microstructure of the electrodes since the electrochemical reactions in SOFCs take place at three/triple phase boundaries (TPBs) within the electrodes. In this study, graphene in the form of reduced graphene oxide (rGO) is added to cathode functional layer (CFL) to improve the cell performance by utilizing the high electrical properties of graphene. Various cells are prepared by varying the rGO content in CFL slurry (1-5 wt %), the number of screen printing (1-3) and the cathode sintering temperature (900-1100 degrees C). The electrochemical behavior of the cells is evaluated by electrochemical performance and impedance tests. It is observed that there is a similar to 26% increase in the peak performance of the cell coated with single layer CFL having 1 wt % graphene and 1050 degrees C sintering temperature, compared to that of the reference cell. (c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.Öğe Enhanced photovoltaic performance of silicon-based solar cell through optimization of Ga-doped ZnO layer(Iop Publishing Ltd, 2023) Erkan, Serkan; Altuntepe, Ali; Yazici, Duygu; Olgar, Mehmet Ali; Zan, RecepIn the present study, the impact of deposition pressure and substrate temperature of Ga-doped Zinc Oxide (GZO) thin film and the photovoltaic performance of this structure as a transparent conductive oxide (TCE) layer in silicon-based solar cell were investigated. Implementing a single target of GZO, the structural, optical, and electrical properties of 350 nm thick GZO thin films with various deposition pressure (5 mTorr, 10 mTorr, 15 mTorr and 20 mTorr) at room temperature (RT) and substrate temperature (RT, 150 degrees C, 200 degrees C, 250 degrees C) at 15 mTorr deposition pressure were fabricated using RF magnetron sputtering technique. The aim here was to find out the GZO films with the optimum pressure and substrate temperature to incorporate them into solar cell as a TCE layer. The X-ray diffraction (XRD) and atomic force microscopy (AFM) techniques were used to determine the structural properties of all samples. The optical transmission measurements were performed using spectroscopic Ellipsometer and the band gap values were calculated by Tauc plot using optical transmission data. In addition, the electrical characterization of the GZO samples were analyzed by the Van der Pauw method and Hall measurements. Finally, the most promising GZO thin film was determined based on the structural and optoelectrical characterization. The findings indicated that the XRD pattern of all the prepared films was dominated by (002) preferential orientation irrespective of the deposition pressure and substrate temperature. The AFM measurements showed that all the samples had a dense surface morphology regardless of the deposition pressures, but the surface morphology of the samples was clearly changed upon increasing substrate temperatures. The transmission values of the film did not significantly alter (similar to 82%) when the deposition pressures except for the substrate temperature of 200 degrees C (86%) were changed. The band gap values were calculated between 3.30 eV and 3.36 eV, which can be associated with enhancement of crystalline quality of the films. The lowest resistivity and the highest carrier concentration values belonged to the film fabricated at 15 mTorr@200 degrees C by 2.0 x 10-3 omega.cm and 1.6 x 1020 cm-3, respectively. Both increasing the deposition pressure (up to 15 mTorr) and substrate temperature (up to 200 degrees C) contributes to improving the crystallite size, widening the optical band gap, lowering the resistivity, and increasing the carrier concentration. In order to evaluate and compare the effect of both deposition pressure and substrate temperature, Silicon-based solar cells were fabricated using the most promising layers (15 mTorr@RT, 15 mTorr@200 degrees C). The cell performance with the GZO thin film as a TCE layer showed that varying both the pressure and substrate temperature of the GZO film contributed to enhancing the solar cell parameters. Thus, the conversion efficiency increased from 9.24% to 12.6% with the sequential optimization of pressure and temperature. It can be concluded that the pressure applied during the deposition and substrate temperature had a significant impact on the properties of GZO thin films and its photovoltaic performance of solar cell used as TCE layer.Öğe GRAFEN SENTEZİ İÇİN ALT-TAŞ TEMİZLEME YAKLAŞIMLARI(2019) Altuntepe, Ali; Öztürk, M.; Kartal, Emre; Seyhan, Ayşe; Zan, RecepSon yılların nanoteknoloji alanında en ilgi çekici ve en yoğun araştırılan malzemelerinden bir tanesi olangrafen; yüksek elektrik ve ısı iletkenliği, saydamlık ve yüksek dayanım gibi birçok üstün özellikleri ile optoelektronikten medikal uygulamalara kadar geniş bir yelpazede kullanım alanına sahiptir. Grafen sentezinde ensık tercih edilen yöntemlerden biri kimyasal buhar biriktirme tekniği olup, bu teknikte kullanılan gazlar, alt-taş,alt-taş temizliği, sıcaklık, büyütme süresi, tavlama süresi, soğutma hızı ve basınç gibi birçok parametre grafensentezinde etkilidir. Bu parametreler arasında sentez sürecini direkt olarak etkileyen en önemli faktörlerden birtanesi alt-taş seçimi ve bunun temizlenme sürecidir. Bu çalışmada alt-taş olarak bakır tercih edilmiş olup, yüzeytemizliğinin grafen sentezine olan etkisi detaylı olarak araştırılmıştır. Bu anlamda literatürde bilinen dört farklıyüzey temizleme işlemi, grafen sentezinde kullanılan bakır folyolar için uygulanmış ve elde edilen sonuçlarortaya konmuştur. Sonuç olarak alt-taş temizliğinde grafen sentezini kolaylaştıran ve iyileştiren en etkiliyöntemlerin HF ile standart temizleme yöntemi olduğu belirlenmiş ve bu yöntemlerle tek katlı homojen grafensentezi gerçekleştirilmiştir. Diğer temizleme yöntemleri sonrası yapılan sentezde genellikle çok katmanlı vehomojen olmayan grafen filmler elde edilmiştir.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 Hybrid transparent conductive electrode structure for solar cell application(Pergamon-Elsevier Science Ltd, 2021) Altuntepe, Ali; Olgar, Mehmet Ali; Erkan, Serkan; Hasret, Onur; Kececi, Ahmet Emin; Kokbudak, Gamze; Tomakin, MuratThis study draws on our experiences with graphene to perform a hybrid TCO structure composed of AZO and graphene. We first set out to enhance the electrical and optical properties of AZO to enable its use especially in the field of solar cell. Hence, in our study, we deposited various thicknesses of AZO thin films on glass substrates and transferred single layer graphene on them to realize the formation of hybrid TCO structure. Among the various AZO film thicknesses, the optimum one, 300 nm, was determined and then the graphene film was added on top of the AZO film. This hybrid structure was applied to the silicon based heterojunction solar cell with the idea of improving the cell performance. The cell performance fabricated using AZO film and AZO + graphene structure was analyzed using solar simulator. Our findings highlight the fact that the presence of graphene improved the cell efficiency by about 7%. Our research was further extended using ITO and ITO + graphene hybrid structure as TCO for silicon-based solar cell. We discovered that graphene incorporation increased the cell efficiency by almost 12% based on our results with ITO + graphene hybrid TCO structure on a similar cell. (c) 2021 Elsevier Ltd. All rights reserved.Öğe Hydrogen storage capacity of two-dimensional MoS2(Pergamon-Elsevier Science Ltd, 2024) Altuntepe, Ali; Erkan, Serkan; Olgar, Mehmet Ali; Celik, Selahattin; Zan, RecepHydrogen storage holds a crucial place for the future of the world in terms of green energy. Two-dimensional materials, in particular, are important in this regard. The aim of this study is to evaluate the performance of MoS2 which is one of the most popular member of two-dimensional materials family. To do this, bulk and exfoliated MoS2 were used. Firstly, the liquid phase exfoliation method was employed to obtain exfoliated MoS2 layers. Then, the hydrogen storage potential of the bulk and exfoliated MoS2 was addressed under 1, 3, 5, 7, and 9 bar hydrogen pressure. After the absorption process, all the samples were characterized using XRD, Raman Spectrometer, and BET ahead of evaluating their hydrogen storage potential. The XRD pattern showed that the peak positions of bulk and exfoliated MoS2 were not affected critically by hydrogen storage. In the Raman spectra, the A1g and E2g 1 peaks of all hydrogenated materials shifted to low wavelengths. Moreover, the BET measurements revealed that the specific surface area and the pore size of the bulk MoS2 were 12.31 m2/g and <= 67.46 nm respectively. Additionally, the specific surface area and the pore size of the exfoliated MoS2 were 23.16 m2/g and <= 69.28 nm, respectively. The hydrogen storage potential of the bulk and the exfoliated MoS2 was evaluated through an MFC in sccm unit. The bulk and exfoliated MoS2 stored 220 and 450 sccm hydrogen, respectively. Besides, the weight percent hydrogen storage for H-MoS2 and H -exfoliated MoS2 was determined to be 1.2 and 2.4 wt%, respectively. In summary, this study has shown that exfoliated MoS2, one of the twodimensional materials, can play a critical role for hydrogen storage due to their high specific surface area.Öğe İki boyutlu malzemeler, metal hidrürler ve kompozisyonlarının hidrojen depolama potansiyelinin araştırılması(Niğde Ömer Halisdemir Üniversitesi, 2024) Altuntepe, Ali; Çelik, Selahattin; Zan, RecepTez kapsamında, farklı basınçlarda yığın formda ve sıvı faz ayrıştırma yöntemiyle tabakalarına ayrıştırılmış iki boyutlu malzemeler ile kompozisyonlarının, 1, 3, 5, 7, 9 bar hidrojen basıncında hidrojen depolama özellikleri incelenmiştir. Aynı zamanda metal hidrür grubunda yer alan LiH, NaH, bilyalı öğütülmüş LiH ve NaH gibi malzemelerin hidrojen depolama özellikleri aynı basınçlar altında incelenmiştir. Bu bağlamda sıvı faz ayrıştırma yöntemiyle tabakalarına ayrıştırılmış iki boyutlu malzemelerin ve bilyalı öğütülmüş metal hidrürlerin hidrojen depolama yeteneklerinin arttığı belirlenmiştir. Ayrıca hidrojen basıncı artışıyla birlikte tüm malzeme grupları için hidrojen depolama kapasitelerinin arttığı belirlenmiştir. Kompozisyon oluşturma ile birlikte hidrojen depolama performansı MoS2+grafen için artmıştır. Çalışmada aynı zamanda çeşitli malzemelerin PEM yakıt hücresi uygulamalarındaki performanslarını incelemiştir. Bu çalışma, iki boyutlu malzemelerin ve kompozisyonlarının hidrojen depolama için potansiyel malzemeler olduğunu göstermektedir. Elde edilen sonuçlar, hidrojen depolama teknolojilerinin geliştirilmesine katkıda bulunabilecek niteliktedir.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 Investigating surface area and hydrogen pressure effects on LiH and NaH(Academic Press Inc Elsevier Science, 2024) Altuntepe, Ali; Erkan, Serkan; Olgar, Mehmet Ali; Celik, Selahattin; Zan, RecepNaH and LiH are theoretically capable of storing hydrogen, but several challenges remain to be overcome before they can be widely used for hydrogen storage. In this study, LiH and NaH were ball-milled and the effect of surface area and hydrogen pressure on hydrogen storage capacity was investigated using the solid-state hydrogen storage method. XRD patterns and Raman spectra show significant shifts in main peak positions of LiH and NaH after hydrogen adsorption. BET analysis shows a significant increase in the specific surface area of LiH and NaH from 6.25 m(2)/g to 12.35 m(2)/g and from 1.34 m(2)/g to 2.33 m(2)/g respectively due to ball milling. The FTIR spectra showed more bonds in the 400-1200 cm(-1) fingerprint region after storing hydrogen in LiH and NaH. This suggests structural changes with enhanced bond bending due to hydrogen. At 9 bar pressure, LiH and NaH exhibited excellent hydrogen storage, with ball-milled LiH reaching about 3.55 wt% and 652 sccm, and NaH achieving approximately 1.58 wt% and 291 sccm. These results highlight the significant influence of surface area and hydrogen pressure on hydrogen storage potential. Incorporating the storage potential within the evaluation of PEM fuel cell performance, we suggest that an increased storage capacity directly corresponds to an augmented power density. The analysis of power density over time revealed that the hydrogen adsorbed ball-milled LiH exhibited the highest power density, peaking at 0.075 Wcm(-2) over the long term. In contrast, LiH displayed a lower power density (0.025 Wcm(-2)) while maintaining its long-term performance. The hydrogen adsorbed NaH and hydrogen adsorbed ball-milled NaH displayed power densities 0.050 Wcm(-2) and 0.073 Wcm(-2), respectively, but they showed short-term performance.Öğe Investigating the Impact of Growth Time and Methane Flow on Graphene Synthesis using Nickel Foil(Gazi Univ, 2021) Bozkaya, Meryem; Altuntepe, Ali; Ate, Hakan; Zan, RecepGraphene has been one of the most investigated nanomaterials in recent years due to its one atom thickness nature and its extraordinary properties such as high conductivity and excellent transmittance in visible range, bendability and high carrier mobility. Although there are many techniques to grow graphene, chemical vapor deposition technique is one of the best approach to synthesize homogeneous and commercial scale graphene in thin film form. Many parameters can affect the quality and homogeneity of the graphene film that is grown with this technique. In this study, two growth parameters which are methane flow rate and growth time were investigated to find out their effect on the quality of the graphene film, which is grown on nickel foil. Graphene film grown with different flow rates of methane (from 20 to 50 sccm) and growth time (50 to 20 mins.) were examined. We found that single layer graphene film could only be grown under 20 sccm methane flow in 20 mins. growth time as evidenced via Raman spectroscopy measurements. Furthermore, the single layer graphene film was found to be in high homogeneity as confirmed by Raman mapping. On the other hand, multi-layer graphene film was obtained by increasing both methane flow and growth time.Öğe Katkılı grafen sentezi ve karakterizasyonu(Niğde Ömer Halisdemir Üniversitesi / Fen Bilimleri Enstitüsü, 2019) Altuntepe, Ali; Çelik, Selahattin; Zan, RecepGrafen grafit tabakasının en ince halidir ve bu malzeme yüksek dayanım, yüksek ışık geçirgenliği, yüksek elektriksel iletkenlik gibi birçok üstün özelliğe sahiptir. Üstün özelliklere sahip bu malzeme tabaka direnci gibi bazı istenmeyen özelliklere sahiptir. Katkılama ile bu istenmeyen özelliklerin üstesinden gelinebilmektedir, bu nedenle piridin, amonyak gibi nitrojen kaynağı kullanılarak kalıcı katkılama işlemi bu tez kapsamında gerçekleştirilmiştir. Yapılan deneyler sonucunda, nitrojen katkılı grafen başarıyla sentezlenmiş ve Raman spektroskopisi, EDS ve XPS analizi ile doğrulanmıştır. Grafen tabakasının n tipi ve p tipi katkılamasının, sadece amonyak ile yapılan katkılı grafen sentezi ile karşılaştırıldığında hem karbon hem de azot kaynağı kullanılan piridinin tabakadaki nitrojen miktarı açısından çok daha başarılı olduğu belirlenmiştir. Bununla birlikte nitrojen katkılı tek katmanlı grafen az miktarda metan ile birlikte piridin kullanılarak başarılı bir şekilde sentezlenmiştir. Grafen filmin tabaka direncinin, katkısız grafene kıyasla azaldığı belirlenmiştir. Tabaka direncini azaltmak, optoelektronik cihazlarda, özellikle güneş hücrelerinde kullanılacak grafen için önemli bir gelişmedir.Öğe KİMYASAL BUHAR BİRİKTİRME YÖNTEMİ İLE GRAFEN SENTEZİNE TAVLAMA VE BÜYÜTME SÜRELERİNİN ETKİSİ(2019) Zan, Recep; Utku, Gülcan; Altuntepe, AliSon yılların en ilgi çekici malzemelerinden biri olan grafen yüksek iletkenlik, yüksek dayanım vesaydamlık gibi birçok üstün özelliğe sahiptir. Grafenin kimyasal buhar biriktirme tekniği kullanılaraksentezlenmesinde, büyütme sıcaklığı, büyütme süresi, tavlama süresi, basınç, kullanılan alt-taş,kullanılan gazların saflıkları ve akış miktarları gibi birçok parametre etkilidir. Bu parametrelerdenbüyütme ve tavlama süreleri grafen sentezinde en önemli parametrelerdendir, bu nedenle bu çalışmadabüyütme ve tavlama sürelerinin tek katmanlı homojen grafen sentezine olan etkisi araştırılmıştır. Farklıbüyütme ve tavlama süreleri kullanılarak üretilen grafen filmler Raman Spektroskopisi tekniğikullanılarak karakterize edilmiş olup tek tabakalı homojen grafen sentezi için üretim süreci optimizeedilmiştir. Genel anlamda tavlama sürelerinin 20 dakikadan 40 dakikaya kadar arttırıldığı deneygruplarında grafen kalitesinde ciddi bir değişim gözlenmemiştir. Büyütme sürelerinin arttırıldığı deneygruplarında ise 40 dakikaya kadar olan süre artışlarıyla doğru orantılı olarak grafen kalitesinin iyileştiğitespit edilmiştir.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 Nitrogen doped single layer graphene for CZTS-based thin film solar cells(Elsevier, 2024) Olgar, Mehmet Ali; Erkan, Serkan; Altuntepe, Ali; Zan, RecepCdS thin films are commonly utilized as a buffer layer in chalcopyrite thin-film solar cells. However, due to the toxic nature of Cadmium (Cd), ongoing efforts are being directed towards exploring alternative options. In this contribution, doped graphene film with remarkable optical and electrical properties has been introduced for the first time as an alternative buffer layer, replacing CdS in CZTS thin-film solar cell application. In this study, nitrogen-doped graphene (N-doped graphene) film was utilized as a substitute buffer layer in the CZTS thin-film solar cell structure, replacing the conventional CdS thin film. For comparative analysis, CZTS/N-doped graphene and CZTS/CdS traditional solar cell structures were fabricated and separately characterized. The CZTS thin films produced were examined through EDX, XRD, SEM, Raman, optical transmission and PL spectroscopy measurements. According to performed analyses, the Cu-poor and Zn-rich kesterite CZTS thin films exhibited a uniform and dense polycrystalline microstructure as observed in surface and cross-sectional SEM images. XRD spectra of the kesterite CZTS thin film displayed predominant peaks corresponding to the (112), (220/204), and (312/116) diffraction planes of the kesterite CZTS phase. Raman spectra showed a dominant peak at similar to 336 cm (-1) associated with the kesterite CZTS phase. PL emission spectra indicated transitions from the conduction band to defect levels. The CVD-grown doped graphene film exhibited a 3.43 I-2D/I-G ratio and a 25 cm (-1) FWHM of the 2D peak, indicating a single -layer graphene according to Raman analysis. Permanent nitrogen doping with a 2% atomic concentration was confirmed by XPS measurement. The optical transmission measurement of the single layer doped graphene film showed a 95% transmittance value. Nitrogen doping was contributed to decrease the sheet resistance of the graphene film. The Glass/Mo/CZTS/N-doped graphene/i-ZnO/ITO/Al solar cell displayed a V-OC of 0.267 V, J(SC) of 24.6 mA/cm(2), FF of 36.46, and eta of 2.37%, showing higher FF and Jsc values but lower conversion efficiency compared to the Glass/Mo/CZTS/CdS/i-ZnO/ITO/Al traditional solar cell structure. Hence, the superior working function and transparency properties position the N-doped graphene film as a competitive buffer layer for use in CZTS-based thin-film solar cells.Öğe Nitrogen doping of graphene by CVD(Elsevier, 2020) Zan, Recep; Altuntepe, AliGraphene, a single sheet of graphite, in situ doping was performed using nitrogen sources like ammonia and pyridine. Based on the conducted experiments, nitrogen doped graphene was successfully synthesized and this was confirmed by the Raman spectroscopy and EDS analysis. Nitrogen doping of the graphene layer was found to be much more successful in terms of nitrogen content and homogeneity when only pyridine was utilized in comparison to the doping only with ammonia. The pyridine was used both as carbon and nitrogen source. However, nitrogen-doped single layer graphene was only achieved using pyridine along with a small amount of methane. It was found that the sheet resistance of the graphene film decreased with the doping in comparison to the pristine graphene. Reducing the sheet resistance is a quiet important development for the graphene to be employed in the optoelectronic devices particularly in solar cells. (C) 2019 Elsevier B.V. All rights reserved.Öğe Performance of Si-based solar cell utilizing optimized Al-doped ZnO films as TCO layer(Springer, 2023) Altuntepe, Ali; Erkan, Serkan; Hasret, Onur; Yagmyrov, Atajan; Yazici, Duygu; Tomakin, Murat; Olgar, Mehmet AliAluminum-doped zinc oxide (AZO) is one of the most popular transparent conducting oxide layers that can be employed in many optoelectronic applications in particular in photovoltaic devices due to being a low-cost and nontoxic material. In this study, we report on the effect of deposition pressure and substrate temperature on the properties of AZO films and solar cell performance by employing the optimized films. This study consists of two stages, the first of which concerns the optimization deposition pressure while the second is the substrate temperature of AZO films by evaluating the structural, optical, and electrical properties of the films. The deposited AZO thin film under 10 mTorr deposition pressure exhibited high optical transmission (89.9%), low electrical resistivity (9.1 x 10(-2) omega.cm), and high carrier concentration (3.74 x 10(19) cm(-3)) among the others. The impact of substrate temperature was then investigated using this deposition pressure at room temperature, 150, 200, and 250 ?. The deposited AZO films at 150 ? temperature were found to possess the highest optical transmission (91.1%), lowest resistivity (9.9 x 10(-4) omega.cm), and highest carrier concentration (1.1 x 10(20) cm(-3)) values. Hence, the 10 mTorr deposition pressure and 150 ? substrate temperature were selected as the optimum growth parameters to obtain AZO films, which were then employed in the cell structure. It was, thus, revealed that utilizing AZO films in silicon-based solar cell using such parameters led to the enhancement in the cell efficiency.Öğe Potassium doping of sputtered MoS2 films by CVD method(Springer, 2024) Altuntepe, Ali; Erkan, Serkan; Olgar, Mehmet Ali; Toplu, Gueldoene; Zan, RecepDoping is an essential approach to enhance the electrical properties of 2D materials. In the present study, two-stage process was used to obtain potassium-doped (K-doped) MoS2. The MoS2 films were grown by magnetron sputtering technique and followed by doping process employing CVD method. The influence of KOH molarity and annealing time on the structural properties of the MoS2 films was investigated thoroughly. 0.2-0.8-M KOH was used to obtain K-doped MoS2. The increase in the molarity of KOH caused a shift in the optical band gap from 1.98 to 1.81 eV. It was observed that increasing the KOH molarity resulted in the loss of homogeneity in the MoS2 films, the use of 0.2-M KOH for the growth of K-doped MoS2 exhibited the most promising results according to performed analyzes. In addition, annealing time also played a critical role in the growth of K-doped MoS2. The dwell times of 5, 10, and 15 min were also used and the effect of molarity and dwell times was investigated. The optical band gap was also shifted from 1.9 to 1.71 eV with increasing the dwell time of KOH. Longer annealing times resulted in the deterioration of the MoS2 film structure. Consequently, an annealing time of 5 min was found to be the optimum value for the growth of K-doped MoS2 film. Overall, this study demonstrates that successful growth of high-quality and homogeneous K-doped MoS2 films which can be employed for various optoelectronic applications.Öğe Substitutional boron doping of graphene using diborane in CVD(Elsevier, 2021) Zan, Recep; Altuntepe, Ali; Erkan, SerkanThis paper reports on a few layer boron doped graphene with high homogeneity, stability and size. To achieve this, we employed diborane to synthesize a boron doped graphene film in a CVD system. During synthesis, we investigated the effect of diborane flow and growth time on copper foil to optimize doped graphene growth conditions. Raman spectroscopy, XPS, EDXS and ellipsometer were employed for the characterization of the doped graphene films. The results of our study enabled the design of a recipe for thin film boron doped graphene growth with optimum optical transmission values. We further found that increasing the flow of diborane from 10 to 30 sccm and growth times from 10 to 30 min leads to the formation of thicker graphene films. However, we discovered that a few layer graphene film with high homogeneity could be obtained for the film that was grown using 10 sccm diborane along with 30-min growth time. The doping was confirmed by observing the shift in the Raman spectra peaks and XPS measurements in comparison to single layer pristine graphene. The study also revealed that boron atoms substituted carbon atoms in the honeycomb structure as confirmed by XPS measurements, which also provide the doping rate to be 2.4%. Our study has significant implications regarding substitutional doping which enables the doping to be stable for a long time, and this is crucial for the doped graphene to be employed in semiconducting technology particularly in optoelectronics.Öğe Synthesis of graphite/rGO-modified fungal hyphae for chromium (VI) bioremediation process(Taylor & Francis Ltd, 2024) Madenli, Ozgecan; Akarsu, Ceyhun; Adiguzel, Ali Osman; Altuntepe, Ali; Zan, Recep; Deveci, Ece UmmuBioremediation is a promising technology that can eliminate the drawbacks of conventional treatment methods in removing harmful toxic metals including chromium(VI). Therefore, in this study, fungal hyphae modified with graphite and reduced graphene oxide were synthesized and assessed for their potential to bioremediate heavy metals for the first time in the literature. The effects of the carbon-based materials on microbial structure were characterized using scanning electron microscopy analysis. Thermogravimetric, RAMAN, X-ray diffraction, and enzymatic analyzes were performed to determine the role of functional groups. In addition, batch adsorption experiments utilizing response surface methodology were conducted to optimize operating parameters such as time (1-11 h), chromium (10-50 mg/L), and graphite/reduced graphene oxide (0.1-1 g/L). The maximum adsorption capacity with the graphene fungal hyphae was determined to be 568 mg.g(-1), which is 9.7 times that of the crude fungal hyphae. The Cr(VI) removal for fungal hyphae-graphite and fungal hyphae-reduced graphene oxide biocomposites was 98.25% and 98.49%, respectively. The isothermal and kinetic results perfectly matched the 2nd order pseudo-model and Langmuir model in terms of the nature of the adsorption process. The laboratory scale test results indicate that fungal hyphae modified with graphite and reduced graphene oxide have a high adsorption capacity, suitable for the removal of chromium (VI) from wastewater.
