View All Items
(21 - 26 of 26)
Pages
- Title
- Planar Organic Photovoltaic Devices.
- Creator
-
Alzubi, Feras, Khondaker, Saiful, Chow, Lee, Schelling, Patrick, Gesquiere, Andre, University of Central Florida
- Abstract / Description
-
Organic Photovoltaic devices (OPV) are considered to be attractive candidates for clean and renewable energy source because of their potential for low cost of fabrication, easy processing, and their mechanical flexibility. The device efficiency of OPV cells are limited by several factors. Among them are: (i) donor-acceptor interface, (ii) morphology of the materials, (iii) electrode-organic semiconductor (OSC) interface and (iv) device architecture such as active material thickness and...
Show moreOrganic Photovoltaic devices (OPV) are considered to be attractive candidates for clean and renewable energy source because of their potential for low cost of fabrication, easy processing, and their mechanical flexibility. The device efficiency of OPV cells are limited by several factors. Among them are: (i) donor-acceptor interface, (ii) morphology of the materials, (iii) electrode-organic semiconductor (OSC) interface and (iv) device architecture such as active material thickness and electrode separation. Although, the donor-acceptor interface has been studied in detail, the commonly prevalent vertical OPV device structure does not allow a good understanding of the other key issues as the vertical structure limits one of the electrode to be a transparent electrode as well as introducing inseparable relation between the electrodes separation and the active material thickness. In addition, it is also well known that the charge transport in OSC is anisotropic and the charge mobility is better in lateral direction rather than vertical direction. In order to address some of these issues, we fabricated OPV devices in a planar device structure where cathode and anode of dissimilar metals are in-plane with each other and their photovoltaic behaviors were studied. We used poly(3-hexylthiophene) and [6,6]-pheny1 C61-butyric acid methy1 ester (P3HT:PCBM) blend as an active material. In particular, we present a detailed study about the effects of the structural parameters such as the channel length, the active layer thickness, and the work function of the electrodes on the open circuit voltage (Voc), short circuit current (Isc), fill factor (FF) and the power conversion efficiency (PCE).In order to determine the suitable anode and cathode for the planar organic photovoltaic (P-OPV) structure, we first fabricated and measured organic field effect transistor (OFET) devices with different contacts and studied the effect of barrier height at the P3HT:PCBM/electrode interface on the device output and transport properties. The study showed a clear effect of varying the contact material on the charge injection mechanism and on the carriers mobilities. The results have also shown that Au with high hole mobility and on current in the p-channel can be used as an anode (holes extractor) in the P-OPV device while In, Cr, and Ti that showed a reasonable value of electron mobility can be good candidates for cathode (electron extractor). We also found that, Ag, Al, and Mg showed large barrier which resulted in large threshold voltage in the I-V curve making them undesired cathode materials in the P-OPV device. We then fabricated P-OPV devices with Au as an anode material and varied the cathode material to study the effect of the interface between the P3HT:PCBM layer and the cathode material. When Al, Mg, or Ag used as a cathode material no PV behavior was observed, while PV behavior was observed for In, Cr, and Ti cathode materials. The PV behavior and the characteristic parameters including Voc, Isc, FF and PCE were affected by varying the cathode material. The results have shown that the P-OPV device performance can be affected by the cathode material depending on the properties and the work function of the metal.We have also studied the effect of varying the P3HT:PCBM layer thickness at a fixed channel length for Cr and Ti cathode materials and Au as anode. While Voc and FF values do not change, Isc and PCE increase with increasing the layer thickness due to the increase of the light absorption and charges generation. Moreover, we studied the effect of varying the channel length at a fixed film thickness; and showed that the values of Isc and PCE increase with decreasing channel length while Voc and FF maintain the same value. In this thesis we will also present the results on experimentally defining and testing the illuminated area in the P-OPV device by using different measurement set-ups and different electrodes patterns. The results prove that the illuminated area in the P-OPV device is the area enclosed between the two electrodes. Lastly, we will present the effect of the P3HT:PCBM ratio on the P-OPV device performance. We show that 1:2 ratio is the optimized ratio for the P-OPV device. The detailed results in this thesis show a potential opportunity to help improving and understanding the design of OPV device by understanding the effects of the device structural parameters.
Show less - Date Issued
- 2013
- Identifier
- CFE0004804, ucf:49754
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004804
- Title
- Structure, stability, vibrational, thermodynamic, and catalytic properties of metal nanostructures: size, shape, support, and adsorbate effects.
- Creator
-
Behafarid, Farzad, Roldan Cuenya, Beatriz, Chow, Lee, Heinrich, Helge, Kara, Abdelkader, Schoenfeld, Winston, University of Central Florida
- Abstract / Description
-
Recent advances in nanoscience and nanotechnology have provided the scientific community with exciting new opportunities to rationally design and fabricate materials at the nanometer scale with drastically different properties as compared to their bulk counterparts. In this dissertation, several challenges have been tackled in aspects related to nanoparticle (NP) synthesis and characterization, allowing us to make homogenous, size- and shape-selected NPs via the use of colloidal chemistry,...
Show moreRecent advances in nanoscience and nanotechnology have provided the scientific community with exciting new opportunities to rationally design and fabricate materials at the nanometer scale with drastically different properties as compared to their bulk counterparts. In this dissertation, several challenges have been tackled in aspects related to nanoparticle (NP) synthesis and characterization, allowing us to make homogenous, size- and shape-selected NPs via the use of colloidal chemistry, and to gain in depth understanding of their distinct physical and chemical properties via the synergistic use of a variety of ex situ, in situ, and operando experimental tools. A variety of phenomena relevant to nanosized materials were investigated, including the role of the NP size and shape in the thermodynamic and electronic properties of NPs, their thermal stability, NP-support interactions, coarsening phenomena, and the evolution of the NP structure and chemical state under different environments and reaction conditions.
Show less - Date Issued
- 2012
- Identifier
- CFE0004779, ucf:49796
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004779
- Title
- Characterization of gold black and its application in un-cooled infrared detectors.
- Creator
-
Panjwani, Deep, Peale, Robert, Chow, Lee, Del Barco, Enrique, Schoenfeld, Winston, University of Central Florida
- Abstract / Description
-
Gold black porous coatings were thermally evaporated in the chamber backfilled with inert gas pressure and their optical properties were studied in near-far-IR wavelengths. The porosities of coatings were found to be extremely high around ~ 99%. Different approaches of effective medium theories such as Maxwell-Garnett, Bruggeman, Landau-Lifshitz-Looyenga and Bergman Formalism were utilized to calculate refractive index (n) and extinction coefficient (k). The aging induced changes on...
Show moreGold black porous coatings were thermally evaporated in the chamber backfilled with inert gas pressure and their optical properties were studied in near-far-IR wavelengths. The porosities of coatings were found to be extremely high around ~ 99%. Different approaches of effective medium theories such as Maxwell-Garnett, Bruggeman, Landau-Lifshitz-Looyenga and Bergman Formalism were utilized to calculate refractive index (n) and extinction coefficient (k). The aging induced changes on electrical and optical properties were studied in regular laboratory conditions using transmission electron microscopy, Fourier transform infrared spectroscopy, and fore-probe electrical measurements. A significant decrease in electrical resistance in as deposited coating was found to be consistent with changes in the granular structure with aging at room temperature. Electrical relaxation model was applied to calculate structural relaxation time in the coatings prepared with different porosities. Interestingly, with aging, absorptance of the coatings improved, which is explained using conductivity form of Bergman Formulism. Underlying aim of this work was to utilize gold blacks to improve sensitivity in un-cooled IR sensors consist of pixel arrays. To achieve this, fragile gold blacks were patterned on sub-mm length scale areas using both stenciling and conventional photolithography. Infrared spectral imaging with sub-micron spatial resolution revealed the spatial distribution of absorption across the gold black patterns produced with both the methods. Initial experiments on VOx-Au bolometers showed that, gold black improved the responsivity by 42%. This work successfully establishes promising role of gold black coatings in commercial un-cooled infrared detectors.
Show less - Date Issued
- 2015
- Identifier
- CFE0005680, ucf:50197
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005680
- Title
- QUANTITATIVE SCANNING TRANSMISSION ELECTRON MICROSCOPY OF THICK SAMPLES AND OF GOLD AND SILVER NANOPARTICLES ON POLYMERIC SURFACES.
- Creator
-
Dutta, Aniruddha, Heinrich, Helge, Del Barco, Enrique, Chow, Lee, Chen, Bo, Kuebler, Stephen, University of Central Florida
- Abstract / Description
-
Transmission Electron Microscopy (TEM) is a reliable tool for chemical and structural studies of nanostructured systems. The shape, size and volumes of nanoparticles on surfaces play an important role in surface chemistry. As nanostructured surfaces become increasingly important for catalysis, protective coatings, optical properties, detection of specific molecules, and many other applications, different techniques of TEM can be used to characterize the properties of nanoparticles on surfaces...
Show moreTransmission Electron Microscopy (TEM) is a reliable tool for chemical and structural studies of nanostructured systems. The shape, size and volumes of nanoparticles on surfaces play an important role in surface chemistry. As nanostructured surfaces become increasingly important for catalysis, protective coatings, optical properties, detection of specific molecules, and many other applications, different techniques of TEM can be used to characterize the properties of nanoparticles on surfaces to provide a path for predictability and control of these systems.This dissertation aims to provide fundamental understanding of the surface chemistry of Electroless Metallization onto Polymeric Surfaces (EMPS) through characterization with TEM. The research focuses on a single EMPS system: deposition of Ag onto the cross-linked epoxide (")SU8("), where Au nanoparticles act as nucleation sites for the growth of Ag nanoparticles on the polymer surface. TEM cross sections were analyzed to investigate the morphology of the Au nanoparticles and to determine the thicknesses of the Ag nanoparticles and of the Ag layers. A method for the direct measurement of the volume and thickness of nanomaterials has been developed in the project using High-Angle Annular Dark-Field (HAADF) Scanning Transmission Electron Microscopy (STEM). The morphology of Au and Ag NPs has been studied to provide reliable statistics for 3-D characterization. Deposition rates have been obtained as a function of metallization conditions by measuring the composition and thickness of the metal for EMPS. In the present work a calibration method was used to quantify the sensitivity of the HAADF detector. For thin samples a linear relationship of the HAADF signal with the thickness of a material is found. Cross-sections of multilayered samples provided by Triquint Semiconductors, FL, were analyzed as calibration standards with known composition in a TECNAI F30 transmission electron microscope to study the dependence of the HAADF detector signal on sample thickness and temperature.Dynamical diffraction processes play an important role in electron scattering for larger sample thicknesses. The HAADF detector intensity is not linearly dependent on sample thicknesses for thick samples. This phenomenon involves several excitation processes including Thermal Diffuse Scattering (TDS) which depends on temperature-dependent absorption coefficients. Multislice simulations have been carried out by Python programming using the scattering parameters available in the literature. These simulations were compared with experimental results. Wedge-shaped Focused Ion Beam (FIB) samples were prepared for quantitative HAADF-STEM intensity measurements for several samples and compared with these simulations. The discrepancies between the simulated and experimental results were explained and new sets of absorptive parameters were calculated which correctly account for the HAADF-STEM contrasts. A database of several pure elements is compiled to illustrate the absorption coefficients and fractions of scattered electrons per nanometer of the sample.In addition, the wedge-shaped FIB samples were used for studying the HAADF-STEM contrasts at an interface of a high- and a low-density material. The use of thick samples reveals an increased signal at the interfaces of high- and low-density materials. This effect can be explained by the transfer of scattered electrons from the high density material across the interface into the less-absorbing low-density material. A ballistic scattering model is proposed here for the HAADF-STEM contrasts at interfaces of thick materials using Python. The simulated HAADF-STEM signal is compared with experimental data to showcase the above phenomenon. A detailed understanding of the atomic number contrast in thick samples is developed based on the combination of experimental quantitative HAADF-STEM and simulated scattering. This approach is used to describe the observed features for Ag deposition on SU-8 polymers.
Show less - Date Issued
- 2014
- Identifier
- CFE0005485, ucf:50333
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005485
- Title
- Study of Novel Power Semiconductor Devices for Performance and Reliability.
- Creator
-
Padmanabhan, Karthik, Yuan, Jiann-Shiun, Sundaram, Kalpathy, Atia, George, DeMara, Ronald, Chow, Lee, University of Central Florida
- Abstract / Description
-
Power Semiconductor Devices are crucial components in present day power electronic systems. The performance and efficiency of the devices have a direct correlation with the power system efficiency. This dissertation will examine some of the components that are commonly used in a power system, with emphasis on their performance characteristics and reliability. In recent times, there has a proliferation of charge balance devices in high voltage discrete power devices. We examine the same charge...
Show morePower Semiconductor Devices are crucial components in present day power electronic systems. The performance and efficiency of the devices have a direct correlation with the power system efficiency. This dissertation will examine some of the components that are commonly used in a power system, with emphasis on their performance characteristics and reliability. In recent times, there has a proliferation of charge balance devices in high voltage discrete power devices. We examine the same charge balance concept in a fast recovery diode and a MOSFET. This is crucial in the extending system performance at compact dimensions. At smaller device and system sizes, the performance trade-off between the ON and OFF states becomes all the more critical. The focus on reducing the switching losses while maintaining system reliability increases. In a conventional planar technology, the technology places a limit on the switching performance owing to the larger die sizes. Using a charge balance structure helps achieve the improved trade-off, while working towards ultimately improving system reliability, size and cost.Chapter 1 introduces the basic power system based on an inductive switching circuit, and the various components that determine its efficiency. Chapter 2 presents a novel Trench Fast Recovery Diode (FRD) structure with injection control is proposed in this dissertation. The proposed structure achieves improved carrier profile without the need for excess lifetime control. This substantially improves the device performance, especially at extreme temperatures (-40oC to 175oC). The device maintains low leakage at high temperatures, and it's Qrr and Irm do not degrade as is the usual case in heavily electron radiated devices. A 1600 diode using this structure has been developed, with a low forward turn-on voltage and good reverse recovery properties. The experimental results show that the structure maintains its performance at high temperatures.In chapter 3, we develop a termination scheme for the previously mentioned diode. A major limitation on the performance of high voltage power semiconductor is the edge termination of the device. It is critical to maintain the breakdown voltage of the device without compromising the reliability of the device by controlling the surface electric field. A good termination structure is critical to the reliability of the power semiconductor device. The proposed termination uses a novel trench MOS with buried guard ring structure to completely eliminate high surface electric field in the silicon region of the termination. The termination scheme was applied towards a 1350 V fast recovery diode, and showed excellent results. It achieved 98% of parallel plane breakdown voltage, with low leakage and no shifts after High Temperature Reverse Bias testing due to mobile ion contamination from packaging mold compound.In chapter 4, we also investigate the device physics behind a superjunction MOSFET structure for improved robustness. The biggest issue with a completely charge balanced MOSFET is decreased robustness in an Unclamped Inductive Switching (UIS) Circuit. The equally charged P and N pillars result in a flat electric field profile, with the peak carrier density closer to the P-N junction at the surface. This results in an almost negligible positive dynamic Rds-on effect in the MOSFET. By changing the charge profile of the P-column, either by increasing it completely or by implementing a graded profile with the heavier P on top, we can change the field profile and shift the carrier density deeper into silicon, increasing the positive dynamic Rds-on effect. Simulation and experimental results are presented to support the theory and understanding.Chapter 5 summarizes all the theories presented and the contributions made by them in the field. It also seeks to highlight future work to be done in these areas.
Show less - Date Issued
- 2016
- Identifier
- CFE0006158, ucf:51148
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0006158
- Title
- Enhanced Hardware Security Using Charge-Based Emerging Device Technology.
- Creator
-
Bi, Yu, Yuan, Jiann-Shiun, Jin, Yier, DeMara, Ronald, Lin, Mingjie, Chow, Lee, University of Central Florida
- Abstract / Description
-
The emergence of hardware Trojans has largely reshaped the traditional view that the hardware layer can be blindly trusted. Hardware Trojans, which are often in the form of maliciously inserted circuitry, may impact the original design by data leakage or circuit malfunction. Hardware counterfeiting and IP piracy are another two serious issues costing the US economy more than $200 billion annually. A large amount of research and experimentation has been carried out on the design of these...
Show moreThe emergence of hardware Trojans has largely reshaped the traditional view that the hardware layer can be blindly trusted. Hardware Trojans, which are often in the form of maliciously inserted circuitry, may impact the original design by data leakage or circuit malfunction. Hardware counterfeiting and IP piracy are another two serious issues costing the US economy more than $200 billion annually. A large amount of research and experimentation has been carried out on the design of these primitives based on the currently prevailing CMOS technology.However, the security provided by these primitives comes at the cost of large overheads mostly in terms of area and power consumption. The development of emerging technologies provides hardware security researchers with opportunities to utilize some of the otherwise unusable properties of emerging technologies in security applications. In this dissertation, we will include the security consideration in the overall performance measurements to fully compare the emerging devices with CMOS technology.The first approach is to leverage two emerging devices (Silicon NanoWire and Graphene SymFET) for hardware security applications. Experimental results indicate that emerging device based solutions can provide high level circuit protection with relatively lower performance overhead compared to conventional CMOS counterpart. The second topic is to construct an energy-efficient DPA-resilient block cipher with ultra low-power Tunnel FET. Current-mode logic is adopted as a circuit-level solution to countermeasure differential power analysis attack, which is mostly used in the cryptographic system. The third investigation targets on potential security vulnerability of foundry insider's attack. Split manufacturing is adopted for the protection on radio-frequency (RF) circuit design.
Show less - Date Issued
- 2016
- Identifier
- CFE0006264, ucf:51041
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0006264
