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Birlikte buharlaştırma ve hibrit buhar çözeltisi sıralı yöntemi ile metal oksit delik (Hole) taşıma katmanlı düzlemsel perovskit güneş hücreleri
The recent success in achieving beyond 25% efficiency with perovskite solar cells (PSC) has called for further upgrading the fabrication techniques to meet the scalability requirements of the photovoltaic (PV) industry. Co-evaporation and a hybrid vapor-solution technique have been shown to produce uniform and efficient planar PSCs. Therefore, in this study, co-evaporation method was optimized studying the partial pressure of the organic precursor. Later electron transport layers in n-i-p and p-i-n structures were addressed achieving 13.0% and 16.1% efficiencies, respectively. Besides, mixed-halide perovskites were fabricated following a hybrid sequential method focusing on the deposition rate of PbI2 and a solution of methylammonium-halides to control the crystallization and morphology of the perovskite layer. This conferred efficiencies up to 19.8% in the case of MAPbI3-X-YBrXClY with 90 hours of operational stability. This is an important measure towards scalability due to the uniform deposition of the first inorganic layer by vacuum-deposition. As another step towards the scalability of perovskite PV, radio frequency (RF) magnetron sputtering was devised to deposit NiOX as a hole transport layer with wide bandgap, matched band structure with perovskite, and stability. The effect of Ar-partial pressure, deposition rate on the optoelectronic properties of the sputtered NiOX was investigated. The passivation of NiOX using organic (Poly-TPD) and inorganic (CuO) materials boosted the overall efficiency of the PSCs by 2.2% and 1.2%, respectively. Finally, Cu doping NiOX via co-sputtering enhanced the efficiency of the PSCs by 3%. This thesis provides a benchmark for applying scalable methods (from evaporation to sputtering) towards efficient PSCs.
Paraksiyal olmayan skalar kırılma teorisinin kapsamının genişletilmesi
The modelling of light scattering from rough textured surfaces is important to assess the light trapping performances of thin film solar cells. In this regard, Harvey-Shack scalar scattering theory is a method of choice established in the solar cell community. It can be used to calculate the angular intensity distribution both in reflection and in transmission, by using the Fourier transform of the optical phase light accumulates while traversing the rough surface texture to evaluate a far-field approximation of the Rayleigh-Sommerfeld scalar diffraction integral, observed on a hemisphere centered around the sample aperture. In this work, different versions of the Harvey-Shack scalar scattering theory are implemented, and their results are compared to actual angular intensity measurements, using a purpose-built high-resolution goniometric instrument. These comparisons generally show remarkable quantitave predictions, which validate the overall approach. However, differences with the measurements suggest that the optical phase accumulation could benefit from an additional correction factor for rough surfaces containing lateral feature sizes on the order of the wavelength, which can be attributed to effective medium effects. Moreover, secondary interactions within the surface topography are shown to be a mechanism that partly redistributes scattered power, affecting angular intensity distribution results. These mechanisms emerge as the two main limitations of the generalized nonparaxial Harvey-Shack theory in the far-field. When applied to the scattering into an optically denser medium, this model predicts polar angle regions where no scattering should accur, regardless of the angle of incidence or the roughness of the texture. This prediction points at a limitation of light trapping using rough textured interfaces. Furthermore, the near-aperture and near-field terms of the Rayleigh-Sommerfeld scalar diffraction integral were investigated using a modification to the generalized Harvey-Shack theory computational algorithm. Within the restrictions of its scalar nature, this novel method can be an important tool for the characterization of near-field diffraction for thin film solar cells and many other problems.
RF katot püskürtme metodu ile üretilmiş üst-çevrim yapan iterbiyum-erbiyum silikate ince filmlerin fabrikasyonu ve karakterizasyonu
Upconversion is a growing research topic as it can be utilized in various applications extending from traditional fields, such as solar cells, and infrared sensing to novel fields, including bioimaging and 3D displays. However, heavy halides, which are frequently used as upconversion host matrices in the literature, exhibit unstable chemical, mechanical, and thermal properties. Oxides with high stability can be used as an alternative to heavy halides, but their high phonon energy reduces upconversion efficiency. In this study, we aimed to enhance the efficiency by increasing the number of luminescence centers (i.e. erbium ions) in the host matrix by utilizing erbium ions as constituents of the compound instead of doping ions. This thesis is dedicated to the improvement of upconversion properties of erbium-ytterbium disilicate (ErxYb2-xSi2O7) compounds as an upconversion material with stable properties. Herein, sputtering was preferred as the fabrication method not only for its uniform deposition capability but also for its compatibility with the silicon technology. In this thesis, we demonstrated erbium-ytterbium disilicate thin films that convert infrared photons with a wavelength from around 1540 nm into a wavelength of around 980 nm. Moreover, the effects of annealing temperature and, erbium and ytterbium concentrations on the upconversion efficiency were investigated. Noteworthy, we present upconversion from erbium-ytterbium silicates as thin as 110 nm. All films fabricated in this work show pure NIR-NIR upconversion, which is considered to be more favorable in various applications where precision is more requisite than high conversion efficiency in the literature, such as bioimaging and fingerprint detection. They can also be the sought-after material for new applications, for instance, coupling them with a cheap silicon detector to detect photons beyond 1.1 μm. In addition, we enhance a method to measure PL lifetimes of samples. This method combines square wave excitation and a lock-in amplifier and allows real-time lifetime measurements.
Düz yapılar ve saçıcı ortamlar ile güneş hücrelerinde ışık yönetimi
Photovoltaics is a prime interest for not only developed but also developing countries considering its historically-low efficiency/cost ratio. Further development of solar cells necessities the maximum utilization of available light, which has to be done by taking the multi-physics nature of the solar cells and the fabrication constraints to maintain its competitive price into account. In this thesis, we explore light management strategies to improve the overall performances of various solar cells. First, light management is exploited to eliminate the reflection, and at the same time to increase the trapping of the non-reflected light within the active layer in the thin-film solar cells. Using optical simulation methods, transfer matrix method (TMM), finite-difference time-domain approach (FDTD), and ray-tracing, as well as analytical approaches, material independent optical optimization guidelines are provided for achieving higher optical absorption under ideal and practical conditions. Simple and effective empirical algorithms are developed to replace simulations. Developed algorithms are applied to various thin-film solar cell technologies; particularly, perovskite, organic, and CdTe solar cells. Besides, optoelectronic effects of transparent conductive oxides in thin-film solar cells are discussed. Using the simultaneous optimization of optoelectronic properties of cell geometries, we develop a roadmap for the perovskite solar cells that can increase their efficiency and at the same time decrease the fabrication costs. Secondly, we have presented a low-cost rear reflector material for silicon solar cells to eliminate the metal parasitic absorption. Using a combination of three different optical simulations methods (TMM, Ray Tracing, and FDTD), optical interaction mechanisms are quantized. Lastly, using a combination of thin-film and silicon solar cell technologies, we provide a detailed investigation of CdZnTe-Si tandem solar cells. We present theoretical limits of CdZnTe-Si tandem solar cells using a combined optical and electrical simulations environment. We investigate the effect of the top cell absorber bandgap on the optimum optical and electrical trends. Light management strategies provided in this thesis can be beneficial for researchers developing various types of solar cells, not necessarily skilled in optics.
Investigation of perfluoropentacene thin films on substrates with different chemical structures
Due to their promising properties in electronic device applications, organic semiconductors (OSs) are being studied heavily. In this study, we aimed to investigate the structural properties of PerFluoroPentacene (PFP) thin films on flat and vicinal Au(111), and Ag(111) surfaces grown by the Supersonic Beam Deposition(SMBD) technique as a function of film thickness, metal surface step density, the effect of chemical and electronic properties of substrate, molecular flux (deposition rate), and energy during film growth which will enable us to determine the growth parameters that yield the highest quality PFP films. This study consists of two stages. In the first stage, we used conventional techniques such as Atomic Force Microscopy (AFM), Contact Angle measurements (CAs) and X-Ray Photoelectron Spectroscopy (XPS) for PFP film characterization. In the second stage, we aimed to present the design and construction of a new He diffraction system and the investigation of crystallographic properties of PFP films with this setup. The results indicate that PFP molecules arrange in standing-up orientation in the first layer and upper layers on all surfaces. The first layer of PFP thin films tends to wet the substrate surfaces, and after the first layer is almost completely covered, the second and other layers start to grow with needle-like grains which indicate a layer plus island (Stranski–Krastanov) growth mode. Substrate surface hydrophobicity and deposition rate directly affect the mean grain size and dendriticy of the first layer grains. The film formation is observed to be faster when the PFP molecules were seeded in lighter carrier gas resulting in higher kinetic energy PFP molecules. Construction of the new helium diffraction system was completed, and the performance of all parts of the system was tested. After a final optimization and minor modifications, the system is ready to collect diffraction data.
Neuro-2a hücrelerinin femtosaniye lazer ile işlenmis silikon yüzeylerinin üstündeki davranışları
Cells are known to interact with their physical environment and respond to cues such as substrate topography. Knowledge of the cell responses to topography may give in- formation about cell behavior in health and disease, as well as be exploited in order to exert control on cells for various purposes. Cell responses to topography are depen- dent on cell type, substrate material and topographical features. In the present study, Neuro-2a cell line was used as a versatile and widely available neuronal cell model. The substrates consisted of polished or laser-structured silicon. Structuring was per- formed using an ultrafast infrared pulsed laser, which generated topographies such as laser induced periodic surface structures (LIPSS) and microcolumns. The substrates were characterized with scanning electron microscopy (SEM). Cells were grown on control substrates (glass or plastic), polished silicon and laser-structured silicon of different topographies for 3 hours or 24 hours to evaluate various cell behaviors. Ini- tial cell adhesion and initial motility, as well as cell adhesion and shape after 24 hours were studied in different substrates using fluorescence microscopy and SEM. Initial cell adhesion was found to be strongest on the microcolumn topography, allowing for selective cell patterning on microcolumn regions. After 24 hours, cell adhesion was found to be equal in all topographies. Moreover, cell motility was found to be fastest in polished silicon, and slowest in the microcolumn topography. On the other hand, cell area and perimeter was found to be larger on polished silicon and LIPSS, com- pared to microcolumn topography or glass. No difference was found in the average cell circularity for all substrates. Finally, cell preference for microcolumn stripes was found to be more prominent when LIPSS was found between the stripes, compared to when thee stripes were separated by flat regions. In conclusion, different cell be- haviors related to spreading, migration and adhesion were found to be dependent on the substrate topography. Topography on silicon may be promising to control these behaviors in the Neuro-2A cell line.
Mikro ölçekli güç üretimi için MEMS tabanlı mikrolitre hacimli mikrobiyal yakıt pili
Fuel cells can be a part of the solution to energy problem in the world. They can supply power in both macro and micro scales. Especially, MEMS based microscale microbial fuel cells (μMFC) may hold the answer to manufacture easy, cheap, fast, and mobile power sources and sensors. μMFCs are electrochemical devices converting chemical energy into electrical energy utilizing microorganisms as biocatalyst, instead of precious metal catalysts used in conventional fuel cells. They can be integrated to power, for example, lab-on-a-chip systems, or they can be used as stand-alone biosensors for sensing applications. This study focused on the development of a compact microbial fuel cell with microliter volume fabricated using silicon MEMS technology. The aim was to have high power density and low start-up time to be integrated as a power source for small devices. Several µMFC systems were operated under different conditions throughout the study. Effects of external load, anolyte type, operating conditions, and chemical modification of gold anode surfaces were compared in terms of start-up time and power densities using Shewanella oneidensis MR-1. Performances were evaluated using polarization curves, Electrochemical Impedance Spectroscopy, and Scanning Electron Microcopy. The results showed that µMFCs modified with cysteamine self-assembled monolayers resulted in more than a 50% reduction in start-up times due to better bacterial attachment on the anode surface. The volumetric power density (330 µW/cm3) was found to be similar in cysteamine-modified and bare gold µMFCs and was comparable to results reported in similar studies in the literature.
Yüksek performanslı nBn kızılötesi fotodedektörler için bariyer mühendisliği
Despite intensive studies, for high-performance applications, lowering dark current is still a challenging problem for pn-type infrared (IR) photodetectors. Over the last two decades, barrier-type IR detectors have been proposed as a solution for obtaining high-performance and high operation temperature conditions. However, the valence band discontinuity limits the material alternatives to which the barrier detector architecture can be applied. In this thesis work, it has been numerically shown that some material limitations in the barrier detector architecture can be eliminated using bandgap engineering techniques. Herein, simulations and analyses were performed by using Synopsys Sentaurus technology computer-aided design (TCAD) commercial device simulator via calculations of the current, continuity, and Poisson's equations with high precision. In this study, delta-doped layers, together with compositionally grading, were utilized to get InGaAs and HgCdTe nBn type IR barrier detector configurations. For the shortwave IR (SWIR) band InGaAs nBn detector, lattice-matched InAlAs and lattice-mismatched InGaAs were used for the barrier material. At least 40 and 20 times improvement, respectively, were calculated in the dark current level by suppressing the surface leakage and generation-recombination (G-R) current mechanisms without compromising any photo-response when compared to the conventional pn junction. This method was also applied successfully for obtaining an extended SWIR (eSWIR)/SWIR InGaAs dual-band nBn detector structure. In the case of HgCdTe material systems, strong suppression of G-R and trap assisted tunneling (TAT) currents were numerically demonstrated with the designed nBn structures in the SWIR, midwave IR (MWIR), and longwave IR (LWIR) bands, which could be useful for the alternative substrate HgCdTe technology. The HgCdTe dual-band nBn detector configuration was also examined in MWIR/LWIR bands again by using compositionally graded and delta-doped layers. Thanks to the flexibility of this method, the length and thickness of the barrier can be adjusted, while zero valence band offset is achieved at the same time for the compositionally bandgap adjustable materials.
MEMS aygıtlarının disk seviyesinde paketleme işlemi için bağ malzemelerinin ve bağ oluşumunun incelenmesi
MEMS devices are microscale systems which are fabricated using micro fabrication techniques. MEMS systems suggest many different functional devices for both military applications and consumer products. The packaging of MEMS devices is one of the most challenging parts of MEMS commercialization. Each type of MEMS device may need special operation environment. Therefore, the packaging of MEMS devices must provide required hermeticity in order to keep the special environment throughout the device's lifetime. Also another aspect of the production of MEMS devices is the miniaturization of the devices. The die level packaging of the MEMS devices is very costly. The use of wafer-level-packaging (WLP) processes enables the packaging of the devices with a single bonding process before the wafer is diced to have individual devices. In this thesis work materials and material systems and processes which enables the wafer level packaging of MEMS devices will be investigated.
Dalga önü modülasyonu kullanılarak lidar ve makine vizyonu için 3 boyutlu görüntüleme
Autonomous vehicles have proven to be very efficient in daily routine jobs and their impact will continue to increase given the recent developments in artificial intelligence, boosted by increased computation capacity. These vehicles are generally equipped with 2D imaging sensors and asked to accomplish tasks in a 3D world, which hamper their functionalities. In this study, we experimentally investigate and develop 3D imaging technologies. We first demonstrate colorful 3D imaging via time of flight measurements in order to map the target scene to a 3D point cloud data and secondly perform triangulation method. For time of flight measurements, we employ a digital micro-mirror device with a resolution of 1920x1080 and a range finder in order to scan the target. We scan the target with a resolution of 8x2, 16x2, and 8x4 using our first method. Second, we employ structured light method to scan the target in 3D using again the digital micro-mirror device in conjunction with a white light source. Using triangulation, we were able to scan the target with a resolution 1920x1080. Our results indicate that in terms of resolution triangulation method gives better results. We can foresee that triangulation and time of flight measurement methods are combined a robust and high-resolution 3D vision can be developed which will increase vision capabilities of autonomous vehicles that shape our future.
Yüksek çözünürlüklü, değişken eşlenik yay sabitli titreşim tabanlı MEMS sıcaklık sensörü
This thesis presents the design, modelling, fabrication and characterization of a resonance based MEMS temperature sensor with improved sensitivity. The temperature sensor is composed of an electrostatically coupled double ended-tuning fork (DETF) MEMS resonator. The sensor utilizes the thermal expansion coefficient difference of the materials to detect the temperature change. The design consists of 2 resonator tines where each of them has two capacitive plates on each side. The capacitive plates facing each other on the inner side of the resonators are used for the electrostatic coupling, which is the crucial point of the study. The negative electrostatic coupling stiffness generated between these tines enables mode-ordering. By mode-ordering, the sensor can be operated closer to pull-in in order to achieve higher sensitivity for the out-of-phase mode. The outside capacitive plates are used for actuation and sensing, whose mechanisms are explained with the equations. The analytical model is presented with the thermo-electro-mechanical equations for the mode shapes and their corresponding natural frequencies. The model is verified by the Finite Element Analysis by comparing the resonance frequencies of the modes of interests. In FEM analysis, the effects of the electrostatic coupling are shown with the parametric sweep for the various proof mass voltage configurations with the thermal expansion physics node included. Having the thermal expansion, the effect of the temperature increase is shown in the modal analysis as the frequency shift in the mode of interests. The characterization tests are performed in a vacuum environment. The quality factor for the out-of-phase mode is about 25500 at a pressure of around 0.15mTorr. Resonance frequencies for the mode of interests of the resonator are close to the analytical model and the FEM simulation results. The effect of the electrostatic softening effect is investigated for the same, and the opposite sign proof mass voltage configurations. The frequency change for the opposite sign proof mass configuration from 6V to 20V is 3063Hz and 666Hz for out-of-phase and in-phase, respectively. For the same proof mass configuration, the resonance frequency shifts 579Hz. The out-of-phase mode and the in-phase mode frequency changes for the temperature increase from 25°C to 65°C are 1078Hz and 988 Hz for the VPM1=20V and VPM2=-20V. For the same proof mass configuration (VPM1=20V and VPM2=20V), the frequency change is 986Hz, still lower than the in-phase mode frequency of the opposite sign proof mass voltage configuration but very close as expected. The overall sensitivities are obtained using the maxima of the frequency response plots. For the opposite sign proof mass configuration, out-of-phase mode and in-phase mode temperature sensitivity are increased from 24.4Hz/K to 26.5Hz/K and 24Hz/K to 24.4Hz/K, respectively, with the voltage increase from 8V to 20V. This study shows that the temperature sensitivity can be increased by varying coupling stiffness by adjusting the proof mass voltages close to pull-in voltage and mode-ordering.
Ka bant radar uygulamaları için GaN / algan HEMT yapılarında alaşımlı ve alaşımsız ohmik kontakların karşılaştırılması
Gallium Nitride (GaN) based High Electron Mobility Transistor (HEMT) is the most powerful alternative for high-power and high-frequency applications due to their unique material properties, such as high breakdown field, high electron drift velocity, high operation temperatures, high radiation resistance, and so on. A low ohmic contact resistance (Rc) is critical to enhancing the device performance at high frequency and high output power. Various metal stacks with different metal types and individual metal layer thicknesses have been reported. Electron beam evaporated Ti/Al/Ni/Au metal stacks have been widely used for ohmic contacts for GaN-based HEMT devices. In general, Ti-based ohmic contacts require higher annealing temperatures above 800 °C. Annealing metal stacks at high temperatures (above 800 °C) lead to a deterioration in the metal surface morphology and metal edge acuity which makes it difficult to carry out further operations to form the gate region. High annealing temperatures may also result in thermal degradation of the epitaxial heterostructure. It is essential to decrease the transistor dimensions including the gate length (Lg) and the source-drain spacing to operate the device at Ka-band (26.5GHz-40GHz) and higher frequencies. To realize these potentials, the transistor should have a proper epitaxial design along with lower on-resistance and smoother ohmic contact morphology. Not only ohmic contact resistance but also ohmic contact morphology and metal edge acuities of the down-scaled HEMT are very crucial since it affects the next coming device processing steps including gate alignment. Low-temperature annealing (<600oC) of metal stacks with recess etching is an alternative way of forming ohmic contacts to down-scaled GaN-based HEMTs. Annealing at lower temperatures is good for keeping metal surface morphology reasonable but this method may result in higher contact resistances which is not good for HEMT devices. Implantation could be another way of forming ohmic contacts for down-scaled HEMTs. Implanted species need to be annealed at high temperature (>1200oC) which may results in deteriorated epitaxial quality since the growth temperature of epitaxial layers is generally in the range of 900oC-1200oC. Regrown of GaN-based materials gathering great interest for forming an ohmic contact to HEMT which does not require annealing of metals which results in much better metal surface morphology compared to alloyed ohmic contacts. Both Molecular Beam Epitaxy (MBE) and Metal-Organic Chemical Vapor Deposition (MOCVD) methods are used to grown non-alloyed GaN-based material for forming ohmic contacts. To this end, this work describes the development of MOCVD regrown InGaN non-alloyed ohmic contacts for GaN-based HEMT devices designated for Ka-band application. To do the best of our knowledge, we are the first group in the world to successfully implement MOCVD regrown of InGaN layers for forming non-alloyed ohmic contacts to GaN-based HEMT devices for Ka-band applications. Entire microfabrication processes were performed at Coatings and Thin Films Laboratory at Middle East Technical University (METU) and AB MicroNano Company. MOCVD regrown InGaN material which was grown on patterned wafer exhibited high doping concentration above 1020cm-3 which is crucial for forming ohmic contact and the corresponding surface morphology of metal contacts was excellent. HEMT with alloyed ohmic contacts were also fabricated to do a performance comparison. The highlights of this study include improved contacts resistance of non-alloyed regrown InGaN ohmic contacts down to 0.3 W.mm compared to HEMT with alloyed ohmic contacts. An almost 7% improvement was observed in both drain-source current (Ids), transconductance (gm), and small-signal performance. Large-signal measurements showed that the output power of the HEMT with non-alloyed regrown InGaN ohmic contact was 3.07 W/mm which was 9% higher compared to HEMT with alloyed ohmic contacts. In this work, Atomic Layer Deposition (ALD) of Aluminum doped Zinc Oxide (AZO) films were also characterized. Al composition of AZO films was varying in 2-4% with varying growth conditions. AZO films exhibited degenerate doping which corresponds to the sheet resistance of about 10-3 .cm. Having similar electronic properties with MOCVD of InGaN thin films, ALD of AZO could be an alternative for non-alloyed ohmic contacts to GaN-based HEMT devices. In this respect, an investigation of material properties of ALD of AZO films performed. Initial trials did not yield expected results due to not optimized recess etching and post-plasma cleaning conditions.
Kemik doku mühendisliği için bor nitrür/çinko katkılı hidroksiapatit/polikaprolakton kompozit iskeleler
Bone diseases and disorders have been expected to increase mostly in time because of the bone diseases, aging, obesity and pysical activity problems. For this reason, bone tissue engineering has been focus point to design new biocompatible scaffolds and enhance bone tissue regeneration. Thus, the properties of biomaterial have to be comparable to natural bone tissue like porous, good mechanical strength and biocompatible. Therefore, Zn doped hydoxyapatite (HA), boron nitride Nanofibers (BNNFs) and Polycaprolactone (PCL) were chosen to construct 1D scaffolds with porous, biocompatible and good mechanical properties and use them in Bone Tissue Engineering.
Mesa ve planar yapılı kısa dalgaboyu kızılötesi dedektörlerin karanlık akım optimizasyon tasarımları
This thesis aims to report novel designs to achieve lower dark current values for modified pin heterojunction InGaAs/InP SWIR photodiodes for both mesa and planar type production methodologies. Mesa type studies cover novel passivation methodology based on a fully depleted thin p-InP layer. Mesa structured detectors are targeted due to their competitive advantages for applications such as multicolor/hyperspectral imaging. Test detector pixels with different perimeter/area ratios are fabricated with and without etching the thin InP passivation layer between pixels in order to comparatively examine passivating behavior. I-V characteristics of the test detectors are measured at room temperature. Based on the results from differently sized pixel groups, bulk and surface dark current components are separated. Results show that thin InP layer decreases dark current by a factor of 3 while increasing photo current due to higher carrier collection efficiency Planar type studies with developing novel epilayer structures, junction depth optimization and successful suppression of surface and bulk generation and recombination (GR) dark current components resulted in diffusion current limited high performance planar type SWIR detectors. For 15 μm pixel pitch, we obtained12 fA pixel dark current at room temperature (~26°C) and 1.08 A/W responsivity (at 1.55 μm) that is corresponding 86% quantum efficiency (QE). Dark current modeling analysis is performed based on bias dependency of diffusion, GR and tunneling current components. Results show excellent match with experimental data. Utilizing the developed planar type detectors with the state-of-the-art performance, post process optimizations also completed which result high performance megapixel SWIR imaging sensors with record level operability.
Bor yerleştirilmiş silisyum üzerindeki serigrafik ve buharlaşmış metal kontakların incelenmesi
Cihaz üretimindeki avantajlar ve düşük maliyet nedeni ile p-tipi dilimler üzerinde üretilen kristal silikon (c-Si) güneş hücreleri, fotovoltaik (PV) pazarına hakim olmaya devam ediyor. Öte yandan, n-tipi Czochralski (CZ) alttaşlar üzerinde yapılan çalışmalar, daha yüksek azınlık taşıyıcı ömrü, daha kolay yüzey pasivasyonu, ışık kaynaklı bozulmanın olmaması ve metalik safsızlıklara kara düşük hassasiyet gibi üstün malzeme ve proses avantajlarından dolayı karasal uygulamalar için p-tipi alttaşlara göre avantajlı olduklarını göstermiştir. Bu avantajlarla, n-tipi CZ tabanlı c-Si güneş hücreleri, geleceğin PV endüstrisinde büyük bir potansiyele sahiptir. Öte yandan, n-tipi hücre üretiminde bir çok zorluklar ve yükske maliyet söz konusudur. Bu zorlukları aşmak üzere alternatif prosesleri içeren çalışmalar yürütülmektedir. İyon ekme (Ion Implantation) katkılama işlemi içinkullanılan ve olldukça umut veren alternatif proseslerden bir tanesidir. Bu tezin amacı, elek baskı ve e-demeti buharlaştırma yöntemi ile oluşturulmuş metal kontaklarla boron ekilmiş silisyum katkılama aktivasyon sıcaklığını ve süresini optimize etmektir. İyon ekimin homojenliğini anlamak için tabaka direnci ölçümleri yapılmıştır. Al2O3/SiNx katmanlı yapısı sırasıyla pasivasyon ve yansıma önleyici kaplama yöntemleri için kullanılmıştır. Ayrıca, daha derin bor konsantrasyonunun daha düşük temas direncine yol açtığı gerçeği, İletim Hattı Ölçümü (TLM) kullanılarak e-demeti ve elek baskı yöntemi ile üretilmiş metal kontaklarla kontak direnci değerleri ile test edilmiştir. Bu tez, boron ekilmiş Si üzerine metal kontak oluşturma konusunda ayrıntılı deneysel çalışmaların sonuçlarını sunmaktadır.
Isıl işlemin eriyik filament ile imalat yöntemiyle üretilmiş PLA parçaların mekanik özelliklerine etkisinin incelenmesi
Additive manufacturing has become a disruptive technology for the production of load bearing components in a wide range of applications. Fused filament fabrication (FFF) is among the most effective and economical techniques for the printing of polymeric parts. There are numerous thermoplastic materials suitable for FFF. Among these, Polylactic acid (PLA) is a renewable, sustainable and cost-effective alternative. For better utilization of PLA parts produced by FFF, there is a need to understand the structure-property relationships in this system. This thesis investigated the effect of annealing on the mechanical properties of FFF produced parts made of polylactic acid (PLA). The as-printed specimens exhibit an amorphous structure, whereas annealing at elevated temperatures, ranging from 80°C to 140°C for 24 hours resulted in the formation of different crystalline phases. 100°C annealing for 24 hours provided the best results in terms of ductility and strength improvement; the tensile strength increased by 6.5 % and elongation at break increased by 33%. A further increase in the annealing temperature resulted in a decrease in the strength and ductility. In the second part of the thesis, the effect of annealing for different infill ratios were investigated. The 60% infill ratio case with 100 °C annealing showed the best normalized mechanical properties. The findings of the thesis show the importance of the microstructure on the mechanical properties and demonstrate the feasibility of annealing for the process and property optimization of FFF-produced parts.
Yüksek kırılma tokluğuna sahip sert kaplamaların geliştirilmesi
Improving the machining performance of cutting tools is critical for increasing product quality and decreasing production cost and time. For this purpose, hard coatings on cutting tools are widely utilized in the industry. These hard coatings increase the lifetime of cutting tools by improving their wear resistance and allow higher cutting speeds. One of the primary drawbacks of these hard coatings is their brittle nature, which occasionally results in premature failure. This thesis investigated a solution to this problem through the development of new generation hard coatings. For this purpose, W and Mo alloying additions were made to conventional coating compositions such as AlTiN and AlCrN, based on the predictions of density functional theory calculations (DFT). The thesis study considered two different coating methods: magnetron sputtering (MS), suitable for lab-scale studies, and cathodic arc evaporation (CAE), suitable for industrial purposes. X-ray Diffraction, Scanning Electron Microscopy, and Energy Dispersive X-ray Spectroscopy characterized the coatings and nanoindentation measurements provided the hardness and fracture toughness values. In addition, new generation fracture toughness measurements based on microcantilever bending tests were employed for more accurate quantification of the fracture behavior. Lastly, selected coatings were subjected to drilling and ball-on disc tests to clarify their performance in actual working conditions. The results show that especially W additions provide at least an order of magnitude increase in fracture toughness combined with dramatic increases in tool lifetime. Therefore, the thesis results demonstrated an effective route to the development of new high-performance coatings for wear resistance applications. Future studies on optimizing the process parameters for cathodic arc evaporation will enable the wider utilization of the developed coatings in industry.
Minyatür soğutmasız kızılötesi mikrobolometre pikseli geliştirilmesi
This thesis reports the development of 12 µm pixel pitch single layer microbolometer structures for use in the 8-12 µm wavelength regions, following recent trends in the last decade in microbolometers. Various pixel structures are designed, simulated, fabricated, and characterized to obtain acceptable detector performance in these small pixels. The pixel structures are improved by reducing the pixel thermal conductance and by using planar type electrode structures where the active material is selected as VWOx (Vanadium-Tungsten Oxide). Predictive modeling with electro-thermal simulations has aided in attaining the best possible pixel structures. Besides that, absorptance is optimized using the cascaded transmission line (CTL) simulation method to improve the pixel absorption between 8-12 µm spectral region show that more than 85% level. Designed detector pixels are fabricated using a CMOS compatible process flow. Each fabrication step is optimized considering a possible array fabrication on CMOS wafers. Then, pixels are characterized electrically, thermally, and optically. These characterizations include temperature coefficient of resistance (TCR), noise, thermal conductance, absorption, responsivity, and thermal time constant measurements. The TCR of the active material, VWOx, is measured around -4 %/K with an optimum resistance value. The thermal conductance values of pixels are measured as low as 20 nW/K. Additionally, the corner frequency is measured 0.85 kHz at 10 μA bias current. Finally, a responsivity value of 37 kV/W at 473 µV is measured for the best-performed fabricated pixel with a low thermal time constant of less than 2 ms, suitable for applications that require high frame rates. Fabrication of 640x480 format FPAs is conducted in the framework of this thesis with the optimized detector structure. Among measured FPAs, the responsivity and operability are measured as high as 10 mV/K and >99%, respectively. Using these measurement results, the lowest NETD value of a 640x480 FPA measured minimum is 200 mK. The success of the developed detector structures is verified through the images obtained from the detectors that are monolithically built on the CMOS readout circuit. It should be noted that this thesis reports the first successfully fabricated 640x480 FPA array using a 12 µm pixel pitch single layer microbolometer in Turkey. The development in this small size FPA production may open a path to different applications to be developed and used in Turkey in the future.
Hücre zarı geçirgenliğinin sürekli bir termo-akışkan mikrokanallı sistemde farklı donmada koruyucu maddeler ile incelenmesi
Modeling cell membrane permeability in different solutions is a critical requirement in controlling the response of cells during preconcentration processes in biotechnological applications, such as drug delivery, fluorescence imaging, and cryopreservation . Current multi-step methods employed in loading cells with high concentrations of cryoprotectant agents (CPAs) prior to cryopreservation for long term storage affect cell viability as a result of extended exposure times associated with these methods. One of the objectives of this research to observe the response of different types of cells in a continous microfluidic system allowing for reduced exposure times to the CPAs and to model the transport of CPA through the cell membrane. In this study, a micro thermo-fluidic device designed for faster and continuous preconcentration of cells with CPAs is used to study cell membrane permeability. Cells are encapsulated in uniform aqueous droplets with a low CPA concentration, The concentration of the CPA in the droplet is increased along the microchannel by controlling the temperature, and thus the water solubility of the oil phase. Selective diffusion of water out of the droplet causes the droplet to shrink and get concentrated vi in CPAs. The response of the cell to the changes in the extracellular concentration is observed and analysed. The dynamic extracellular CPA concentration data is integrated in a permeability model to calculate the dynamic permeability of the cell membrane to a specific CPA. Two-phase flow conditions for droplet generation are optimized to determine the flow rate to achieve the desired droplet size. A linear relationship is found between flow rates and the size of the droplets, regardless of the size and geometry of the microfluidic device. The performance of encapsulation of cells in the droplets based on the flow rate is assessed. The undesired phenomena, such as the encapsulation of cells in multiples and the sedimentation inside the microchannels, are found to be associated with the low flow rates. Employing a continuous two-phase microfluidic system has proved valuable in observing the response of the cell to increasing CPA concentrations. Encapsulation in smaller droplets has yielded a higher increase in the intracellular CPA concentration. A conventional mathematical model for the permeability of the cell membrane is modified by using the dynamic extracellular concentrations obtained from the microfluidic system. Membrane permeability parameters are determined for MDA-MB231 cells using glycerol as the CPA, where concentration is increased from 1 M to 2.05 M in the aqueous droplets with heating to 40°C in the microfluidic channels.
Yeni yüzey yapısı ve geniş bant aralıklı taşıyıcı seçici katmanlarıyla silikon heteroeklem güneş hücresi performansının iyileştirilmesi
The photovoltaic (PV) industry is dominated by silicon-based solar cells owing to the abundance of silicon and its full-fledged technology. The main road for the PV industry points out to enhance the conversion efficiency of solar cells while decreasing production costs, which is crucial for improving renewable energy market share. The silicon heterojunction solar cells (SHJ) are receiving attention on this road map due to their higher conversion efficiencies, simple process flow, and low-temperature fabrication sequence. In order to further enhance the SHJ device performance, both electrical and optical properties should be improved simultaneously. In this Ph.D. thesis work, various aspects of SHJ solar cells, such as surface texturing, surface passivation, and material choices, were addressed. Firstly, surface texturing was studied to search for new approaches to reduce optical losses. Even though the well-established surface texturing method generating random pyramids on the surface reduces the reflection successfully, there is still room for improvement. A new and novel silicon surface texturing method based on copper-assisted chemical etching was developed for efficient light management on the surface. With this technique, tetragonal-star shaped inverted pyramids were formed, resulting in extremely low reflectance values. Secondly, the surface passivation of silicon was studied using different process conditions and material systems. The SHJ solar cell performance was significantly improved by understanding the chemical passivation kinetics and improving the surface passivation quality. Thirdly, wide band gap materials were integrated into the SHJ solar cell structure to decrease parasitic absorption losses. Furthermore, the free-carrier absorption losses were reduced significantly by tuning TCO's electrical and optical properties. Based on the theoretical and experimental explanations, the novel method for light trapping and integration of wide band gap materials to SHJ solar cell structure were shown to offer promising alternatives to existing technologies for future applications. With these new material systems and process improvements, we have achieved high-efficiency values of up to 21.2% in the SHJ solar cells fabricated at ODTÜ-GÜNAM