Theses supervised by Prof. Dr. Abdullah Yıldız
14 theses · Ankara Yıldırım Beyazıt University, Sakarya University
Integration of sol-gel derived doped zinc oxide films and solution-processed metal nanoparticles for thin film photodiode and solar cell applications
The thesis work aimed at deposition, characterization, and optimization of sol-gel- derived undoped ZnO, Al-doped ZnO (AZO) and Ga-doped ZnO (GZO) thin films with emphasizing on GZO films. The films were successfully deposited on glass substrates using spin coating technique and their optical and electrical properties were optimized in order to be eligible for optoelectronic devices applications. By the deposition of GZO films on p-Si substrate, p-n heterojunctions (GZO/p-Si) were successfully formed and GZO/p-Si based optoelectronic devices were fabricated and optimized. The enhancement potential in the optical response of GZO/p-Si devices via metal nanoparticles (NPs) was investigated by spin-coating of solution-processed 100 nm size Ag and Au NPs on GZO layer. The structural, surface morphological, optical, and electrical properties of the deposited ZnO thin films were investigated using x-ray diffractometer (XRD) and scanning electron microscope (SEM), atomic force microscopy (AFM), UV-vis spectroscopy, and the four-point probe and Hall effect measurement systems, respectively. The ratio of the electrical conductivity to the absorption coefficient was used as a figure of merit (FOM) for evaluating the quality of films. FOM value of ~1.6x10-5 -1 was measured for the deposited AZO and GZO thin films. The performance of GZO/p-Si devices were analyzed from the current-voltage (I-V) characteristics measured in the dark and under AM1.5 and UV illumination conditions. Compared to literature, excellent rectification behavior, high photoresponsivity and competitive photovoltaic property were exhibited by the devices. The inclusion of Au and Ag metal NPs into the devices led to enhanced optical absorption and consequently improved the performance of the devices in terms of photoelectric properties.
Diferensiyel ve integral operatörlerin özdeğerlerinin yaklaşık hesabı
Anahtar kelimeler: Özdeğer, özfonksiyon, simetrik operatör, asimptotik davranıs. Özdeğer problemleri bütün Matematik, Fizik, Mühendislik ve diğer disiplinlerde ortaya çıkan önemli bir problemdir. Bazı problemlerde kritik kuvvet, bazı problemlerde potansiyel enerji, bazı problemlerde karakteristik frekans, v.b. tarzda önemli kavramları temsil etmektedir. Bu önemine rağmen hesaplanması bazı özel yapıdaki problemler dısında oldukça zordur. Bu nedenle nümerik çözümleri yapma zorunluluğu ortaya çıkar. Sonlu boyutlu problemlerde Matris özdeğer-özvektör problemi olarak ortaya çıkar ve simetrik olmayan matrisler için güçlükler kendini gösterir. Sonsuz boyutlu problemler için de aynı zorluklar görülür. Yaklasık çözüm teknikleri de daha önemli olan en küçük özdeğerleri ve özvektörleri hesaplamada ise yarar. Ardısık özdeğerlerin hesabı stabil olmamakta ve hesaplamalar yanlıs neticeler vermektedir. Biz bu çalısmada Varyasyonel metodlar kullanacağız ve operatörümüz Diferansiyel operatörle sınırlı kalacaktır. Örneklendirmeyi Ritz Metodu'nu ve Minimax Metodu'nu anlatarak yapacağız.
Sınır integrali metodu ve uygulamaları
Uygulamalı Matematik ve mühendislikte nümerik hesaplama ve benzetme artan bir önemle, önemli bir rol oynamaktadır. Hesaplama kolaylığı ve kalitesi, deneylerin yapılamaması veya pahalılığı karşısında vazgeçilmez olmaktadır. Nümerik işlemlerin etkinlik, doğruluk ve güvenilirlik açısından güven verici olmaları önemlidir.Bu nümerik hesaba sınır elemanları yöntemi bir örnektir. Jaswon ve Symm 1963 yılında potansiyel problemler için sınır elemanları metodunu (BİEM) Green özdeşliği kullanarak geliştirdi. 1976'da Lachat&Watson, 1977'de Butterfield ve Brebbia çeşitli uygulama alanlarında gelişmelere sebep oldular.Bu metotta en önemli iş verilen diferansiyel denklem için Temel Çözüm dediğimiz noktasal yük kaynağı altında serbest uzay çözümünü bulmaktır.Bu tezde biz bazı örneklerle bu Temel Çözümün nasıl bulunduğunu ve nasıl kullanıldığını verdik ve bazı bilgisayar uygulamalarını gösterdik.Anahtar kelimeler: Temel Çözüm, Green Fonksiyonu, Sınır İntegrali Metodu
Rijit zemin üzerine oturmuş öngerilmeli plağın zorlanmış titreşimlerine karşılık gelen sınır değer problemleri
Bu çalışmada, rijit zemin üzerine oturmuş sonlu uzunluğa sahip öngerilmeli şerit-plağın zorlanmış titreşimine karşılık gelen sınır-değer problemleri gerek bir katmanlı hal gerekse iki katmanlı hal için ele alınmıştır. Ele alınan problemlerin matematik modellemesi öngerilmeli cisimlerde üç-boyutlu doğrusallaştırılmış elastik dalga teorisi çerçevesinde verilmiştir.İlk olarak, rijit zemin üzerine oturmuş tek katmanlı şerit-plağa ait sınır-değer problemi ele alınarak matematik modeli kurulmuştur. Analitik çözümü olmayan bu modelin sonlu elemanlar yöntemi ile sayısal çözümü yapılmıştır. Kurulan sonlu eleman modeli belirli parametreler ile test edilmiş ve modelin geçerliliği sonsuz uzunluğa sahip bölgeler için yapılan çalışmalara uygun olması ile sınanmıştır. Modellemedeki tüm parametre değişimlerinin ele alınan problemdeki sisteme etkisi ortaya konulmuştur.İkinci olarak, rijit zemin üzerine oturmuş iki katmanlı şerit-plağın zorlanmış titreşimine ait sınır-değer problemi ele alınmıştır. İki katmanlı hal için matematik model kurulmuş ve sonlu eleman formülasyonu yapılmıştır. Gerek katmanlar arasındaki ara yüzeyde gerekse zemin ile şerit-plak arasındaki yüzeyde gerilme dağılımı incelenmiş ve plakların uzunluklarının değişmesinin gerilme dağılımına etkisi gösterilmiştir.Diğer yandan, ele alınan her iki problem için katmanlardaki öngerilmenin ve zamana göre harmonik yükün frekansının sisteme etkisi incelenmiş ve elde edilen sonuçlar ortaya konulmuştur.
Lineer olmayan operatörlü denklemler için Newton metodu
Lineer olmayan denklemlerin çözümü için ardışık yaklaşımlar yöntemi ilk kez İngiliz matematikçi Isac Newton tarafından reel değerli ve reel değişkenli fonksiyonlar için ortaya atıldı. Bu metot denklemin, araştırılan kökünün yeterince yakın komşuluğundan seçilen bir başlangıç yaklaşım ile bu noktada fonksiyona teğet olan doğrunun denklemi yazılır ve bu denklemin x eksenini kestiği nokta ki bu ikinci yaklaşım olacaktır bulunur işlemi bu şekilde devam ettirmekle bir ardışık yaklaşımlar dizisi oluşturulur ve bu dizinin verilmiş fonksiyonun birinci ve ikinci türevlerinin varlığı, sürekli olması, aynı işaretli ve sıfırdan farklı olması halinde denklemin tek bir kökünün var olduğu dolayısı ile dizisinin yakınsak olduğu ispat edilir.Bu metot Rus matematikçi L. V. Kantoroviç tarafından lineer olmayan operatör denklemlerin çözümü için genelleştirilmiştir. Operatörün tanımlı olduğu bölgede frechet türevlenebilir ve türevlenmiş operatörün tersinin varlığı ve bunların bazı koşulları sağlaması halinde operatörün tanım bölgesini kendisine dönüştüren ve bu bölgede bir daralma operatörü olduğunun gösterilmesi ile operatör denklemin tanım bölgesinde tek bir çözümü olduğu ispat edilir.Anahtar kelimeler: Lineer olmayan, Operatör, Ardışık yaklaşımlar, Newton metodu.
Sonlu elemanlar yöntemi
Bu çalışmada, bir ve iki boyutlu uzaylarda sonlu eleman analizi yapılmıştır. Öncelikle; bir boyutlu uzaylarda sonlu elemanlar yöntemi incelenmiştir. Yaklaşım teorisindeki varyasyonel metodlar incelenmiş, zamana bağlı problemler hakkında bilgi verilmiştir. Model örnek olarak Poisson problemi alınmıştır. Bir boyutlu uzayda Poisson probleminin varyasyonel formülasyonu yapılarak ayrık denklemler elde edilmiştir. Bununla birlikte, çeşitli normlarda hata analizi yapılmıştır. Bölüm sonunda çözülmüş problemler verilmiştir. İkinci olarak; iki boyutlu uzaylarda sonlu elemanlar yöntemi incelenmiştir, sonlu eleman aynklaştrrması esnasında elemanlara ait ayrık denklemler oluşturulmuştur. Elemanlara ait bu niceliklerden problemin global denklem sistemi elde edilmiştir. Sınır koşullarına bağlı olarak denklemlerde ne gibi değişiklikler meydana geleceği incelenmiştir. Bölüm sonunda çözümlü örnekler verilmiştir. Anahtar Kelimeler: Sonlu eleman, klasik çözüm, minimizasyon prensibi, zayıf çözüm, varyasyonel problem, eliptik problem.
Investigation of indoor applications of luminescent solar concentrators under different intensities
A luminescent solar concentrator (LSC) is an optical device that uses light to generate energy. Total internal reflection (TIR) allows it to focus light into certain regions while also collecting direct and scattered light, absorbing photons and reemitting at longer wavelengths. At the locations where the light is focused on the luminophore, one or more solar cells are positioned. As a result, electrical power can be generated. LSCs are created by combining a luminophore such as polymer or glass with fluorescent dye and/or quantum dots. The most common dye in thin film doped LSC applications is Lumogen F Red 305. In this work, an alternate method for creating luminophores is to dope a poly (methyl methacrylate) (PMMA) waveguide with a combination of the dyes Safranin O and Eosin Y. The edges of the luminophore have four monocrystalline silicon solar cells attached in sequence. This thesis work investigates indoor uses. Halogen light sources at twelve various intensity levels, ranging from 90 W/m2 to 350 W/m2, are used to monitor current-voltage (I-V characteristics). According to measurements, the mix luminophore, which is a combination of the dyes Safranin O and Eosin Y, produces the greatest results.
Design of induction generator for wind turbine
The electromagnetic induction method is usually used for this purpose. Electromagnetic induction is carried out by moving a wire file around a fixed rod with an electric field, either by a magnet or an electric magnet. The induction generator depends on the principle of electromagnetic induction. This phenomenon was discovered by Michael Faraday, and it is reported that the current is produced in the conductor when it is moved through the magnetic field. The resulting output voltage is called the electromotive force (emf). This process can also be reversed. In other words if the electric charge is moving, it will generate a magnetic field. The Faraday-Maxwell induction code determines the relationship in which the value of emf versus the number of cycles (for a file) indicates the rate of change in the magnetic flow. The reason for the minus sign is that the direction of the wattage direction is opposed to the change in flow that produces the voltage. Electromagnetic induction has many applications. For example, in credit cards (magnetic strips on the back of the card) or in order to generate voltage in the motor. Another one is very important because the generator is an "inverted" engine. The objective of this study is to design a small wind turbine which is optimized for the constraints that have residential use. The design process includes the selection of the wind turbine type and the determination of the blade airfoil pitch angle and chord length distribution along the radius. The pitch angle and chord length distributions are optimized based on conservation of angular momentum and theory of aerodynamic forces on an airfoil. Blade Element Momentum (BEM) theory is first derived and then used to conduct a parametric study that will determine if the optimized values of blade pitch and chord length create the most efficient blade geometry. Finally, two different airfoils are analyzed to determine which one creates the most efficient wind turbine blade. The study includes a discussion of the most important parameters in wind turbine blade designed to maximize the efficiency.
Synthesis of molybdenum carbide thin films for producti̇on of hydrogen via electrochemical water splitting
In this study, wet-chemical techniques were used to synthesize colloidal molybdenum nanoparticles (NPs). The synthesis of colloidal Molybdenum NPs was done by using pyrolysis of hexacarbonyl Molybdenum in 1-Octadecene under Argon atmosphere at 220 °C. Further, metallic Molybdenum and Molybdenum carbide catalysts were fabricated by spin coating of synthesized Molybdenum NPs onto graphite substrates and subsequent heat treatments at elevated temperatures under Argon atmosphere. For the first time, we achieved the fabrication of Molybdenum carbide thin films on graphite substrate from colloidal Molybdenum NPs without utilizing any reducing gas. In addition, Platinum NPs were synthesized by pyrolysis of platinum-bis(acetylacetonate) precursor and following subsequent heat treatments on graphite substrates. Prepared Platinum-based electrodes were used as a reference material towards hydrogen evolution reaction. Characterization of molybdenum, platinum and molybdenum carbide catalysts were carried out by using X-Ray Diffractometer, Transmission Electron Microscope, and Scanning Electron Microscope. Linear sweep voltammetry, cyclic voltammetry, chronoamperometry/chronopotentiometry and electrochemical impedance spectroscopy measurements were performed in a three-electrode cell in 0,5 molar sulfuric acid aqueous electrolyte solution (pH = 0). Molybdenum carbide thin films exhibit excellent catalytic activity and chemical stability for hydrogen evolution reactions.
Fabrication and optimization of homojunction photoanodes-based dye-sensitized solar cells
Today, when fossil resources are exhausted and energy needs increase day by day, interest in new energy sources and renewable energy resources has increased considerably. Furthermore, the growing global population, industrialization, and the negative effects of human hunger are manifesting themselves as an increase in greenhouse gas emissions, global warming, water pollution, and air pollution. Although the yields of existing solar cells are modest, the poisonous gases and production costs during production are a large dead end. At this point, third-generation solar cells grow. These batteries have a low recycling cost, and the cost is quite promising in terms of performance. Painted solar cells, one of the third-generation solar cell technologies, stand out with a low cost, low toxicity, and a simple production process. In the thesis study, the blocking layer and the key parameters affecting dye-sensitized solar cells efficiency, including porous TiO2 and electrolyte, were studied. A strategy has been developed to increase the open circuit voltage and short circuit current together. A blocking layer has been produced using TiCl3-based solutions optimized for production parameters. The efficiency is optimized with electrolytes used frequently in the literature for the final cell. Initially obtained, the battery's JSC value was approximately 10 percent, the VOC value was approximately 20 percent, and total cell performance was increased by approximately 80 percent to 8.03 percent. As a result, dye-sensitized solar cells are a promising renewable energy technology with great potential to reduce production costs and increase the efficiency of solar batteries. With ongoing research and development, dye-sensitized solar cells can eventually become the most cost-effective solar cells available on the market.
Fabrication and analysis of a dye sensitized solar cell: Hydrothermally grown hybrid photoanode and opaque pt counter electrode
A dye-sensitized solar cells is a low-cost solar cell with the advantages of a simple fabrication process and low toxicity. Titanium dioxide and Platinum are the leading actors as photoanode and counter electrodes that come to mind first when dye-sensitized solar cells is mentioned. In this study, different methods including chemical bath deposition, hydrothermal growing, and spin coating methods have been applied successfully to obtain counter electrode and photoanode components. Firstly, the Platinum counter electrode was coated with two different coating methods such as chemical bath and spin coating. The obtained efficiency values are 5.06 percent (Rct, 0.89 ohm.cm2) and 4.31 percent (Rct, 1.44 ohm.cm2), respectively. The difference can be attributed to increased catalytic activity of the Pt films. Then, a hybrid photoanode was prepared by hydrothermal reaction and spin coating. In this stage, the nanoparticles and hydrothermally grown metal oxide nanorod arrays were combined to improve the efficiency of dye-sensitized solar cells and called as hybrid device. The experimental results showed that the efficiency of a hybrid device (5.58 percent). consisting of nanoparticles and nanorods is higher than the one consisting only of nanoparticles (4.95 percent) and nanorods (1.04 percent). The efficiency of DSSCs is improved using hydrothermal techniques to maximize the amount of light absorption and also lower the recombination when light is incident on the cell. In conclusion, dye-sensitized solar cells are a promising renewable energy technology with great potential for reducing the production cost and increasing the efficiency of solar cells. With continued research and development, DSSCs may eventually become a candidate of the most cost-effective solar cells available on the market.
Design and simulation of a rooftop photovoltaic system in Turkish Standards Institution
In this study, by using the PV*SOL software, an off-grid rooftop PV system was designed for the Turkish Standards Institution building and it was analyzed how much of the energy that the system would produce could meet the real annual electricity consumption of TSE. The location of the institute was marked on the map in the software, and the temperature and radiation values of the region were taken. 3D design of the institute building was made and determined the areas where the PV modules will be placed. Domestic PV modules certified according to TS EN IEC 61215-2, TS EN IEC 61730-1 and TS EN IEC 61730-2 standards were selected. After calculating the minimum distances required so that these selected PV modules do not shade each other, the modules are placed on the roof. Inverters and batteries are selected according to the configuration automatically created by the software. After completing the design, the maximum possible PV energy of the system was calculated by subtracting losses such as configuration, inverter and radiation losses from the total global PV radiation. Finally, it has been calculated how much of the electricity consumption can be met from this system. According to the simulation results, designed system can produce 70283 kWh of electrical energy annually. The performance ratio of the system was calculated as 73.1 percent and the specific annual efficiency was calculated as 1112 kWp/kWh. The electrical energy produced by the designed system can meet approximately 30 percent of the annual real electricity consumption of the institute.
Fabrication and characterization of W-doped ZnO thin films for next generation photodetector device applications
In this study, tungsten (W)-doped ZnO thin films were successfully deposited on (100)-oriented p-type silicon (p-Si) substrates via the sol-gel spin-coating method at 500 °C, with W/Zn atomic ratios varying between 0% and 4%. The sample with 2% W doping exhibited the most favorable UV photodetector characteristics, displaying diode-like rectifying behavior in the dark with a rectification ratio of 5587 at ±5 V, a responsivity of 3.84 A/W, and a photosensitivity of 34 under 365 nm UV illumination at 0.5 mW/cm². The enhanced performance at this doping level is attributed to reduced carrier recombination and improved carrier separation in the depletion region, highlighting the critical role of W doping in device optimization. In the second stage, silver nanoparticles (Ag NPs) were deposited on the optimized 2% W-doped ZnO/p-Si photodetector. Top-view and cross-sectional SEM analysis confirmed a uniform Ag NP coating with a total film thickness of 167 nm. UV-Vis spectroscopy revealed that the Ag NP-coated film had an average optical transmittance of 79.5% and an optical bandgap of 3.17 eV. The incorporation of Ag NPs significantly improved the optoelectronic performance of the device, resulting in a responsivity of 4.83 A/W, a photosensitivity of 60.82, and a detectivity of 5.06 × 10¹² Jones. Overall, the findings demonstrate that the combination of optimal W doping and metal nanoparticle integration substantially enhances the UV detection performance of ZnO/Si-based heterojunction photodetectors.
Fabrication and characterization of W-doped ZnO thin films for next generation photodetector device applications
In this study, tungsten (W)-doped ZnO thin films were successfully deposited on (100)-oriented p-type silicon (p-Si) substrates via the sol-gel spin-coating method at 500 °C, with W/Zn atomic ratios varying between 0% and 4%. The sample with 2% W doping exhibited the most favorable UV photodetector characteristics, displaying diode-like rectifying behavior in the dark with a rectification ratio of 5587 at ±5 V, a responsivity of 3.84 A/W, and a photosensitivity of 34 under 365 nm UV illumination at 0.5 mW/cm². The enhanced performance at this doping level is attributed to reduced carrier recombination and improved carrier separation in the depletion region, highlighting the critical role of W doping in device optimization. In the second stage, silver nanoparticles (Ag NPs) were deposited on the optimized 2% W-doped ZnO/p-Si photodetector. Top-view and cross-sectional SEM analysis confirmed a uniform Ag NP coating with a total film thickness of 167 nm. UV-Vis spectroscopy revealed that the Ag NP-coated film had an average optical transmittance of 79.5% and an optical bandgap of 3.17 eV. The incorporation of Ag NPs significantly improved the optoelectronic performance of the device, resulting in a responsivity of 4.83 A/W, a photosensitivity of 60.82, and a detectivity of 5.06 × 10¹² Jones. Overall, the findings demonstrate that the combination of optimal W doping and metal nanoparticle integration substantially enhances the UV detection performance of ZnO/Si-based heterojunction photodetectors.