<p>This thesis deals with synthesis and characterization of newoligothiophenes and derivatives thereof, for use as organicsemiconductors in optical and electronic applications, such asfield-effect transistors and light-emitting diodes. Much workis devoted to the development of new synthetic strategies forinteresting building blocks, to beused for synthesizing suchmaterials. One series of regio-defined oligothiophenes, up tothe octamer, has been prepared and evaluated. Photoluminescencequantum efficiencies of these were 22-31 % in solution, butdropped to 2-5 % in the solid state. Another project deals withthe development of oligothiophenes with in-chain chirality.These may find use in polarized lightemitting diodes. Finally,two oligothienyl-substituted porphyrins have been synthesizedand are currently evaluated for use in light-emitting diodesand possibly in solar cells.</p>

A polymer light-emitting electrochemical cell (LEC) is a solid-state polymer device operating according to in situ electrochemical doping and the formation of a light-emitting polymer p-n junction. This operating mechanism, however, has been the subject of much debate. Planar LECs with millimeter scale interelectrode spacings offer great advantages for directly observing the electrochemical doping process. Photoluminescence quenching and the formation of a light-emitting junction have been observed in planar polymer LECs, demonstrating the existence of electrochemical doping. The chemical potential difference between the p- and n-doped regions creates a built-in potential/electric field in the junction region, which can be probed by measuring the optical beam induced current (OBIC). This study utilizes a versatile and easy-to-use method of performing OBIC analysis. The OBIC and photoluminescence profiles of LECs have been simultaneously measured by scanning a focused light beam across large planar LECs that have been turned on and cooled to freeze the doping profile. The photoluminescence intensity undergoes a sharp transition between the p- and n-doped regions. The OBIC photocurrent is only observed in the transition region that is narrower than the width of the excitation beam, which is about 35 μm. The results depict a static planar polymer p-n junction with a built-in electric field pointing from n to p. The electrode interfaces do not produce a measurable photocurrent indicating ohmic contact. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2013-05-28 12:52:14.171

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Approved for public release, distribution is unlimited / Display technologies in the current market range from the simple and cheap incandescent bulb behind a graphic overlay to the upwardly expensive flat panel high definition plasma display. To provide a foundation of understanding for Light Emitting Polymers (LEP), samples were imaged in a scanning electron microscope. This was preformed to identify a potential method for answering questions on polymer charge mobility and diffusion mechanisms, which are currently unknown. Light Emitting Polymer (LEP) displays offer a viable alternative to the active matrix style, when an application calls for information to be sent in a simple visible format. By using the flexibility of the fabrication process, LEP displays can be applied to offer a low cost, lightweight, and durable means of communicating information during shipboard damage control and firefighting. A unique screen printing method was used in collaboration with Add-Vision, to produce a prototype that was designed, fabricated and tested for use in Naval shipboard firefighting evolutions. The application of the LEP technology to shipboard damage control was motivated by the experience gained from being both the Officer in Charge of a Naval Firefighting School and from time in the Fleet as a Damage Control Officer. / Lieutenant, United States Naval Reserve

A dual-wavelength mode-locked semiconductor vertical-external-cavity-surface-emitting laser is demonstrated. A semiconductor saturable absorber mirror allows for simultaneous mode locking of pulses centered at two center wavelengths with variable frequency spacing. The difference-frequency control is achieved with an intracavity etalon. Changing the finesse of the etalon enables the adjustment of the pulse duration between 6 and 35 ps. The emitted two-color pulses are modulated by a beat frequency in the terahertz range. Self-starting mode-locking with 0.8-W average output power is demonstrated.

In the past decade, GaN-based nitrides have had a considerable impact in solid state lighting and high speed high power devices. InGaN-based LEDs have been widely used for all types of displays in TVs, computers, cell phones, etc. More and more high power LEDs have also been introduced in general lighting market. Once widely used, such LEDs could lead to the decrease of worldwide electrical consumption for lighting by more than 50% and reduce total electricity consumption by > 10%. However, there are still challenges for current state-of-the art InGaN-based LEDs, including ‘efficiency droop’ issues that cause output power quenching at high current injection levels (> 100 A/cm2). In this dissertation, approaches were investigated to address the major issues related to state-of-the-art nitride LEDs, in particular related to (1) efficiency droop investigations on m-plane and c-plane LEDs: enhanced matrix elements in m-plane LEDs and smaller hole effective mass favors the hole transport across the active region so that m-plane LEDs exhibit 30% higher quantum efficiency and negligible efficiency droop at high injection levels compared to c-plane counterparts; (2) engineering of InGaN active layers for achieving high quantum efficiency and minimal efficiency droop: lower and thinner InGaN barrier enhance hole transport as well as improves the quantum efficiencies at injection levels; (3) double-heterostructure (DH) active regions: various thicknesses were also investigated in order to understand the electron and hole recombination mechanism. We also present that using multi-thin DH active regions is a superior approach to enhance the quantum efficiency compared with simply increasing the single DH thickness or the number of quantum wells (QWs, 2 nm-thick) in multi-QW (MQW) LED structures due to the better material quality and higher density of states. Additionally, increased thickness of stair-case electron injectors (SEIs) has been demonstrated to greatly mitigate electron overflow without sacrificing material quality of the active regions. Finally, approaches to enhance light extraction efficiency including using Ga doped ZnO as the p-GaN contact layer to improve light extraction as well as current spreading was introduced.

InGaN heterostructures are at the core of blue light emitting diodes (LEDs) which are the basic building blocks for energy efficient and environment friendly modern white light generating sources. Through quantum confinement and electronic band structure tuning on the opposite end of the spectrum, Ge1−xSnx alloys have recently attracted significant interest due to its potential role as a silicon compatible infra-red (IR) optical material for photodetectors and LEDs owing to transition to direct bandgap with increasing Sn. This thesis is dedicated to establishing an understanding of the optical processes and carrier dynamics in InGaN heterostructures for achieving more efficient visible light emitters and terahertz generating nanocavities and in colloidal Ge1−xSnx quantum dots (QDs) for developing efficient silicon compatible optoelectronics. To alleviate the electron overflow, which through strong experimental evidence is revealed to be the dominating mechanism responsible for efficiency degradation at high injection in InGaN based blue LEDs, different strategies involving electron injectors and optimized active regions have been developed. Effectiveness of optimum electron injector (EI) layers in reducing electron overflow and increasing quantum efficiency of InGaN based LEDs was demonstrated by photoluminescence (PL) and electroluminescence spectroscopy along with numerical simulations. Increasing the two-layer EI thickness in double heterostructure LEDs substantially reduced the electron overflow and increased external quantum efficiency (EQE) by three fold. By incorporating δ p-doped InGaN barriers in multiple quantum well (MQW) LEDs, 20% enhancement in EQE was achieved due to improved hole injection without degrading the layer quality. Carrier diffusion length, an important physical parameter that directly affects the performance of optoelectronic devices, was measured in epitaxial GaN using PL spectroscopy. The obtained diffusion lengths at room temperature in p- and n-type GaN were 93±7 nm and 432±30 nm, respectively. Moreover, near field scanning optical microscopy was employed to investigate the spatial variations of extended defects and their effects on the optical quality of semipolar and InGaN heterostructures, which are promoted for higher efficiency light emitters owing to reduced internal polarization fields. The near-field PL from the c+ wings in heterostructures was found to be relatively strong and uniform across the sample but the emission from the c- wings was substantially weaker due to the presence of high density of threading dislocations and basal plane stacking faults. In case of heterostructures, striated regions had weaker PL intensities compared to other regions and the meeting fronts of different facets were characterized by higher Indium content due to the varying internal field. Apart from being the part and parcel of blue LEDs, InGaN heterostructures can be utilized in generation of coherent lattice vibrations at terahertz frequencies. In analogy to LASERs based on photon cavities where light intensity is amplified, acoustic nanocavity devices can be realized for sustaining terahertz phonon oscillations which could potentially be used in acoustic imaging at the nanoscale and ultrafast acousto-optic modulation. Using In0.03Ga0.97N/InxGa1-xN MQWs with varying x, coherent phonon oscillations at frequencies of 0.69-0.80 THz were generated, where changing the MQW period (11.5 nm -10 nm) provided frequency tuning. The magnitude of phonon oscillations was found to increase with indium content in quantum wells, as demonstrated by time resolved differential transmission spectroscopy. Design of an acoustic nanocavity structure was proposed based on the abovementioned experimental findings and also supported by full cavity simulations. Optical gap engineering and carrier dynamics in colloidal Ge1−xSnx QDs were investigated in order to explore their potential in optoelectronics. By changing the Sn content from 5% to 23% in 2 nm-QDs, band-gap tunability from 1.88 eV to 1.61 eV, respectively, was demonstrated at 15 K, consistent with theoretical calculations. At 15 K, time resolved PL spectroscopy revealed slow decay (3 − 27 μs) of luminescence, due to recombination of spin-forbidden dark excitons and effect of surface states. Increase in temperature to 295 K led to three orders of magnitude faster decay (9 − 28 ns) owing to the effects of thermal activation of bright excitons and carrier detrapping from surface states. These findings on the effect of Sn incorporation on optical properties and carrier relaxation and recombination processes are important for future design of efficient Ge1−xSnx QDs based optoelectronic devices. This thesis work represents a comprehensive optical study of InGaN heterostructures and colloidal Ge1−xSnx QDs which would pave the way for more efficient InGaN based LEDs, realization of terahertz generating nanocavities, and efficient Ge1−xSnx based silicon compatible optoelectronic devices.

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Master of Science / Department of Architectural Engineering and Construction Science / Fred Hasler / This paper discusses the color rendition capabilities of light emitting diodes (LEDs) and their relationship with the current standard for color rendition quality. The current standard for judging light source color rendering properties, known as the color rendering index (CRI), has come under heavy scrutiny in recent years with the introduction of LED in commercial lighting applications. LEDs, depending on construction type, have highly structured spectral distributions which do not scale well under the color rendering index; moreover, CRI for LEDs has become disjointed with the subjective measurement of human color preference. Unfortunately, given the multidimensional nature of color, an all-encompassing scale with a single rated value for color rendition capabilities of a light source has proven difficult to establish. An analysis on the human visual system is first discussed, establishing how the visual system first detects color in the eye and subsequently encodes that color information through a color-opponent process, formulating conscious color appearance. The formation of color appearance leads into a discussion on human color vision and the creation of three dimensional color space, which is subsequently used for the measurement of color fidelity (CRI) of consumer light sources. An overview of how LED lamps create light and color is then discussed, showing that the highly structured spectral distribution of LED lamps is often the cause of discrepancy within the CRI system. Existing alternatives to the CRI system are then compared and contrasted to each other, and the existing CRI system. A final color preference study was conducted where four LED lamps where compared to a reference lamp of equal correlated color temperature. Observers were asked to rate the various test lamps against the reference lamp in terms of vividness, naturalness, overall preference, and individual color preference. It was found that no significant difference was found between the first three dimensions measured but significant trend lines existed for the preference of individual colors when illuminated by either LED lamps or the reference source. Recommendations are then made for how the lighting industry could move forward in terms of color metrics.

Master of Science / Department of Animal Sciences and Industry / Elizabeth A. E. Boyle / Light emitting diode (LED) and fluorescent (FLS) lighting effects on enhanced pork loin chops, beef longissimus dorsi and semimembranosus steaks, ground beef, and ground turkey displayed in two retail display cases set up with similar operational temperatures were evaluated using visual and instrumental color, Enterobacteriaceae (EB) and aerobic plate counts (APC), internal product and case temperatures, and thiobarbituric acid reactive substances (TBARS). Visual discoloration of the five meat products increased (P<0.05) as display time increased. Beef longissimus dorsi steaks, ground beef, and the superficial portion of beef semimembranosus steaks had less (P<0.05) visual discoloration under LED lighting than FLS. Compared to FLS, pork loin chops under LED lighting had higher (P<0.05) L* values and a lower (P<0.05) a/b ratio. The deep portion semimembranosus steak under LED was redder (P<0.05) and the superficial portion had a lower (P<0.05) a/b ratio; LED deep and superficial portion semimembranosus steaks had higher (P<0.05) saturation index values at 5.18 and 4.47, respectively, on d 0 than FLS. Pork chops under LED lighting had lower (P<0.05) APC populations than FLS by the end of display. Enterobacteriaceae populations fluctuated throughout display on ground turkey under FLS lighting while populations remained stable under LED. APC populations increased as display time increased for pork loin chops, ground beef and ground turkey, but not beef longissimus dorsi steaks possibly due to initial case-ready postmortem age. As display time increased, EB populations increased (P<0.05) for pork loin chops, ground beef and ground turkey. The internal temperature of all products, except beef longissimus dorsi steaks, was lower (P<0.05) in the LED case. FLS case temperatures were higher (P<0.05) by 0.56 to 1.11°C than LED over the duration of the study. Pork loin chops, ground turkey, and beef semimembranosus steaks had higher (P<0.05) TBARS values by 0.06 to 0.24 mg malonaldehyde/kg under LED lighting, but lighting type did not affect (P>0.05) lipid oxidation of beef longissimus dorsi steaks or ground beef. LED lighting results in lower display case temperatures, lower internal product temperatures, and extended color life; however, lipid oxidation was increased in some cuts under LED lighting.