Matching Items (6)

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Organic optoelectronic devices employing small molecules

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Organic optoelectronic devices have remained a research topic of great interest over the past two decades, particularly in the development of efficient organic photovoltaics (OPV) and organic light emitting diodes (OLED). In order to improve the efficiency, stability, and materials

Organic optoelectronic devices have remained a research topic of great interest over the past two decades, particularly in the development of efficient organic photovoltaics (OPV) and organic light emitting diodes (OLED). In order to improve the efficiency, stability, and materials variety for organic optoelectronic devices a number of emitting materials, absorbing materials, and charge transport materials were developed and employed in a device setting. Optical, electrical, and photophysical studies of the organic materials and their corresponding devices were thoroughly carried out. Two major approaches were taken to enhance the efficiency of small molecule based OPVs: developing material with higher open circuit voltages or improved device structures which increased short circuit current. To explore the factors affecting the open circuit voltage (VOC) in OPVs, molecular structures were modified to bring VOC closer to the effective bandgap, ∆EDA, which allowed the achievement of 1V VOC for a heterojunction of a select Ir complex with estimated exciton energy of only 1.55eV. Furthermore, the development of anode interfacial layer for exciton blocking and molecular templating provide a general approach for enhancing the short circuit current. Ultimately, a 5.8% PCE was achieved in a single heterojunction of C60 and a ZnPc material prepared in a simple, one step, solvent free, synthesis. OLEDs employing newly developed deep blue emitters based on cyclometalated complexes were demonstrated. Ultimately, a peak EQE of 24.8% and nearly perfect blue emission of (0.148,0.079) was achieved from PtON7dtb, which approaches the maximum attainable performance from a blue OLED. Furthermore, utilizing the excimer formation properties of square-planar Pt complexes, highly efficient and stable white devices employing a single emissive material were demonstrated. A peak EQE of over 20% for pure white color (0.33,0.33) and 80 CRI was achieved with the tridentate Pt complex, Pt-16. Furthermore, the development of a series of tetradentate Pt complexes yielded highly efficient and stable single doped white devices due to their halogen free tetradentate design. In addition to these benchmark achievements, the systematic molecular modification of both emissive and absorbing materials provides valuable structure-property relationship information that should help guide further developments in the field.

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Date Created
2014

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Material properties of MBE Grown ZnTe, GaSb and their heterostructures for optoelectronic device applications

Description

Recently a new materials platform consisting of semiconductors grown on GaSb and InAs substrates with lattice constants close to 6.1 A was proposed by our group for various electronic and optoelectronic applications. This materials platform consists of both II-VI (MgZnCdHg)(SeTe)

Recently a new materials platform consisting of semiconductors grown on GaSb and InAs substrates with lattice constants close to 6.1 A was proposed by our group for various electronic and optoelectronic applications. This materials platform consists of both II-VI (MgZnCdHg)(SeTe) and III-V (InGaAl)(AsSb) compound semiconductors, which have direct bandgaps spanning the entire energy spectrum from far-IR (~0 eV) up to UV (~3.4 eV). The broad range of bandgaps and material properties make it very attractive for a wide range of applications in optoelectronics, such as solar cells, laser diodes, light emitting diodes, and photodetectors. Moreover, this novel materials system potentially offers unlimited degrees of freedom for integration of electronic and optoelectronic devices onto a single substrate while keeping the best possible materials quality with very low densities of misfit dislocations. This capability is not achievable with any other known lattice-matched semiconductors on any available substrate. In the 6.1-A materials system, the semiconductors ZnTe and GaSb are almost perfectly lattice-matched with a lattice mismatch of only 0.13%. Correspondingly, it is expected that high quality ZnTe/GaSb and GaSb/ZnTe heterostructures can be achieved with very few dislocations generated during growth. To fulfill the task, their MBE growth and material properties are carefully investigated. High quality ZnTe layers grown on various III-V substrates and GaSb grown on ZnTe are successfully achieved using MBE. It is also noticed that ZnTe and GaSb have a type-I band-edge alignment with large band offsets (delta_Ec=0.934 eV, delta_Ev=0.6 eV), which provides strong confinement for both electrons and holes. Furthermore, a large difference in refractive index is found between ZnTe and GaSb (2.7 and 3.9, respectively, at 0.7 eV), leading to excellent optical confinement of the guided optical modes in planar semiconductor lasers or distributed Bragg reflectors (DBR) for vertical-cavity surface-emitting lasers. Therefore, GaSb/ZnTe double-heterostructure and ZnTe/GaSb DBR structure are suitable for use in light emitting devices. In this thesis work, experimental demonstration of these structures with excellent structural and optical properties is reported. During the exploration on the properties of various ZnTe heterostructures, it is found that residual tensile strains exist in the thick ZnTe epilayers when they are grown on GaAs, InP, InAs and GaSb substrates. The presence of tensile strains is due to the difference in thermal expansion coefficients between the epilayers and the substrates. The defect densities in these ZnTe layers become lower as the ZnTe layer thickness increases. Growth of high quality GaSb on ZnTe can be achieved using a temperature ramp during growth. The influence of temperature ramps with different ramping rates in the optical properties of GaSb layer is studied, and the samples grown with a temperature ramp from 360 to 470 C at a rate of 33 C/min show the narrowest bound exciton emission peak with a full width at half maximum of 15 meV. ZnTe/GaSb DBR structures show excellent reflectivity properties in the mid-infrared range. A peak reflectance of 99% with a wide stopband of 480 nm centered at 2.5 um is measured from a ZnTe/GaSb DBR sample of only 7 quarter-wavelength pairs.

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Date Created
2012

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Growth, optical properties, and optimization of infrared optoelectronic materials

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High-performance III-V semiconductors based on ternary alloys and superlattice systems are fabricated, studied, and compared for infrared optoelectronic applications. InAsBi is a ternary alloy near the GaSb lattice constant that is not as thoroughly investigated as other III-V alloys

High-performance III-V semiconductors based on ternary alloys and superlattice systems are fabricated, studied, and compared for infrared optoelectronic applications. InAsBi is a ternary alloy near the GaSb lattice constant that is not as thoroughly investigated as other III-V alloys and that is challenging to produce as Bi has a tendency to surface segregate and form droplets during growth rather than incorporate. A growth window is identified within which high-quality droplet-free bulk InAsBi is produced and Bi mole fractions up to 6.4% are obtained. Photoluminescence with high internal quantum efficiency is observed from InAs/InAsBi quantum wells. The high structural and optical quality of the InAsBi materials examined demonstrates that bulk, quantum well, and superlattice structures utilizing InAsBi are an important design option for efficient infrared coverage.

Another important infrared material system is InAsSb and the strain-balanced InAs/InAsSb superlattice on GaSb. Detailed examination of X-ray diffraction, photoluminescence, and spectroscopic ellipsometry data provides the temperature and composition dependent bandgap of bulk InAsSb. The unintentional incorporation of approximately 1% Sb into the InAs layers of the superlattice is measured and found to significantly impact the analysis of the InAs/InAsSb band alignment. In the analysis of the absorption spectra, the ground state absorption coefficient and transition strength of the superlattice are proportional to the square of the electron-hole wavefunction overlap; wavefunction overlap is therefore a major design parameter in terms of optimizing absorption in these materials. Furthermore in addition to improvements through design optimization, the optical quality of the materials studied is found to be positively enhanced with the use of Bi as a surfactant during molecular beam epitaxy growth.

A software tool is developed that calculates and optimizes the miniband structure of semiconductor superlattices, including bismide-based designs. The software has the capability to limit results to designs that can be produced with high structural and optical quality, and optimized designs in terms of maximizing absorption are identified for several infrared superlattice systems at the GaSb lattice constant. The accuracy of the software predictions are tested with the design and growth of an optimized mid-wave infrared InAs/InAsSb superlattice which exhibits superior optical and absorption properties.

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Date Created
2016

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Chemical vapor deposition of metastable germanium based semiconductors for optoelectronic applications

Description

Optoelectronic and microelectronic applications of germanium-based materials have received considerable research interest in recent years. A novel method for Ge on Si heteroepitaxy required for such applications was developed via molecular epitaxy of Ge5H12. Next, As(GeH3)3, As(SiH3)3, SbD3, S(GeH3)2 and

Optoelectronic and microelectronic applications of germanium-based materials have received considerable research interest in recent years. A novel method for Ge on Si heteroepitaxy required for such applications was developed via molecular epitaxy of Ge5H12. Next, As(GeH3)3, As(SiH3)3, SbD3, S(GeH3)2 and S(SiH3)2 molecular sources were utilized in degenerate n-type doping of Ge. The epitaxial Ge films produced in this work incorporate donor atoms at concentrations above the thermodynamic equilibrium limits. The donors are nearly fully activated, and led to films with lowest resistivity values thus far reported.

Band engineering of Ge was achieved by alloying with Sn. Epitaxy of the alloy layers was conducted on virtual Ge substrates, and made use of the germanium hydrides Ge2H6 and Ge3H8, and the Sn source SnD4. These films exhibit stronger emission than equivalent material deposited directly on Si, and the contributions from the direct and indirect edges can be separated. The indirect-direct crossover composition for Ge1-ySny alloys was determined by photoluminescence (PL). By n-type doping of the Ge1-ySny alloys via P(GeH3)3, P(SiH3)3 and As(SiH3)3, it was possible to enhance photoexcited emission by more than an order-of-magnitude.

The above techniques for deposition of direct gap Ge1-ySny alloys and doping of Ge were combined with p-type doping methods for Ge1-ySny using B2H6 to fabricate pin heterostructure diodes with active layer compositions up to y=0.137. These represent the first direct gap light emitting diodes made from group IV materials. The effect of the single defected n-i¬ interface in a n-Ge/i-Ge1-ySny/p-Ge1-zSnz architecture on electroluminescence (EL) was studied. This led to lattice engineering of the n-type contact layer to produce diodes of n-Ge1-xSnx/i-Ge1-ySny/p-Ge1-zSnz architecture which are devoid of interface defects and therefore exhibit more efficient EL than the previous design. Finally, n-Ge1-ySny/p-Ge1-zSnz pn junction devices were synthesized with varying composition and doping parameters to investigate the effect of these properties on EL.

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Date Created
2016

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Small molecule organic optoelectronic devices

Description

Organic optoelectronics include a class of devices synthesized from carbon containing ‘small molecule’ thin films without long range order crystalline or polymer structure. Novel properties such as low modulus and flexibility as well as excellent device performance such as

Organic optoelectronics include a class of devices synthesized from carbon containing ‘small molecule’ thin films without long range order crystalline or polymer structure. Novel properties such as low modulus and flexibility as well as excellent device performance such as photon emission approaching 100% internal quantum efficiency have accelerated research in this area substantially. While optoelectronic organic light emitting devices have already realized commercial application, challenges to obtain extended lifetime for the high energy visible spectrum and the ability to reproduce natural white light with a simple architecture have limited the value of this technology for some display and lighting applications. In this research, novel materials discovered from a systematic analysis of empirical device data are shown to produce high quality white light through combination of monomer and excimer emission from a single molecule: platinum(II) bis(methyl-imidazolyl)toluene chloride (Pt-17). Illumination quality achieved Commission Internationale de L’Éclairage (CIE) chromaticity coordinates (x = 0.31, y = 0.38) and color rendering index (CRI) > 75. Further optimization of a device containing Pt-17 resulted in a maximum forward viewing power efficiency of 37.8 lm/W on a plain glass substrate. In addition, accelerated aging tests suggest high energy blue emission from a halogen-free cyclometalated platinum complex could demonstrate degradation rates comparable to known stable emitters. Finally, a buckling based metrology is applied to characterize the mechanical properties of small molecule organic thin films towards understanding the deposition kinetics responsible for an elastic modulus that is both temperature and thickness dependent. These results could contribute to the viability of organic electronic technology in potentially flexible display and lighting applications. The results also provide insight to organic film growth kinetics responsible for optical, mechanical, and water uptake properties relevant to engineering the next generation of optoelectronic devices.

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Date Created
2017

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Sensors and Their Applications for Connected Health and Environment

Description

Connected health is an emerging field of science and medicine that enables the collection and integration of personal biometrics and environment, contributing to more precise and accurate assessment of the person’s state. It has been proven to help to establish

Connected health is an emerging field of science and medicine that enables the collection and integration of personal biometrics and environment, contributing to more precise and accurate assessment of the person’s state. It has been proven to help to establish wellbeing as well as prevent, diagnose, and determine the prognosis of chronic diseases. The development of sensing devices for connected health is challenging because devices used in the field of medicine need to meet not only selectivity and sensitivity of detection, but also robustness and performance under hash usage conditions, typically by non-experts in analysis. In this work, the properties and fabrication process of sensors built for sensing devices capable of detection of a biomarker as well as pollutant levels in the environment are discussed. These sensing devices have been developed and perfected with the aim of overcoming the aforementioned challenges and contributing to the evolving connected health field. In the first part of this work, a wireless, solid-state, portable, and continuous ammonia (NH3) gas sensing device is introduced. This device determines the concentration of NH3 contained in a biological sample within five seconds and can wirelessly transmit data to other Bluetooth enabled devices. In this second part of the work, the use of a thermal-based flow meter to assess exhalation rate is evaluated. For this purpose, a mobile device named here mobile indirect calorimeter (MIC) was designed and used to measure resting metabolic rate (RMR) from subjects, which relies on the measure of O2 consumption rate (VO2) and CO2 generation rate (VCO2), and compared to a practical reference method in hospital. In the third part of the work, the sensing selectivity, stability and sensitivity of an aged molecularly imprinted polymer (MIP) selective to the adsorption of hydrocarbons were studied. The optimized material was integrated in tuning fork sensors to detect environmental hydrocarbons, and demonstrated the needed stability for field testing. Finally, the hydrocarbon sensing device was used in conjunction with a MIC to explore potential connections between hydrocarbon exposure level and resting metabolic rate of individuals. Both the hydrocarbon sensing device and the metabolic rate device were under field testing. The correlation between the hydrocarbons and the resting metabolic rate were investigated.

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Date Created
2018