The identification and characterization of electronic defect bands in organic photovoltaic devices

In any microelectronic device, fundamental physical parameters must be well understood if electronic properties are to be successfully optimized. One such prominent parameter is energetic trap states, which are well-known to plague amorphous or otherwise impure semiconducting materials. Organic semiconductors are no strangers to such states and their electronic properties are evidently tied to these defects. This dissertation aims to elucidate these states in organic photovoltaic (OPV) devices. The literature to date is first reviewed and the author\u27s contributions are subsequently detailed. Within the community, several techniques have been leveraged to study these mid-gap states. Atop the list are optical, capacitance and current based measurements and each has provided important pieces to the overall defect profile. Piecing together the works to date, organic photovoltaic materials are depicted as disordered semiconductors with a seemingly continuous distribution of both energetically shallow and deep trap bands. Upon blending the pure materials to create the modern day bulk heterojunction - the currently preferred photovoltaic architecture - energetic disorder increases and new trap bands appear. These states have been shown to stem from both intrinsic (e.g. structural disorder) and extrinsic (e.g. oxygen and synthesis contaminates) sources and it is quite clear that such states can have profound effects on, if not completely control, the electronic properties and long term stability of OPV devices. Most prominently, these states are known to enhance trap-assisted recombination, induce Fermi-level pinning and generate space-charge effects. Though these mid-gap traps have a large negative impact, they also can give an advantageous inherent doping, improving conductivity and interfacial electric fields. Evidently, continued progress in understanding the nature, sources, affects and possible mitigation of these defects in both current and future materials will be crucial to the optimization of this promising technology. The primary work of this study is to build upon these reports and to further the current body of knowledge on the identification and characterization of defect states in OPV devices. Capacitance techniques are heavily employed herein. As such, the accurate capacitance characterization of OPV devices was first visited. It was found that, owing to thinner films and larger series resistance, the series based parasitics could not be neglected in the typical frequency range of interest or significant errors and misinterpretations were introduced. Armed with this more accurate model, deeper, previously unknown trap states were then identified using low frequency capacitance measurements coupled with a point by point differential of high frequency capacitance-voltage measurements. The discovered defects remain important as it is those states closer to the midgap which more efficiently contribute to recombination and can be detrimental to device performance. More generally, the presented technique gives a generic overview of the capacitance response of OPV devices - resolving anomalies and enabling others to better study the defect profile in their devices. Lastly, the pre-exponential factor of trap emission, also known as the attempt-to-escape frequency, was characterized. This parameter is essential if trap occupancy kinetics are to be accurately described - important for any measurement or model dependent on the detrapping dynamics. It was found that the polymer based devices studied herein have a prefactor within a similar range, yielding similar trap capture cross sections. Not only does this give guidance as to the proper value to be used in the detrapping measurements/models, but also indicates that the trapping mechanisms in these devices are likely similar

Microfluidics engineering: A lab-on-chip bioassay for in vivo nematode tesing

The nematode is a microscopic worm belonging to the phylum Nematoda. With over 2200 described genera in about 250 families, the nematode is among the most diverse of all pseudocelomates. These worms are the most ubiquitous multicellular organisms on earth and are crucial for maintaining soil nutrients and overall symbiotic relationships between plants and certain organisms. However, as many as 33% of the estimated 40,000 nematode species have been classified as parasitic. Of particular interest to the farming community, nematode parasites can infect plants (e.g. corn, soybean and wheat), animals (e.g. pigs, sheep, goats and cows) and even humans, causing illness and severe agricultural loss. Conventional control methods based on chemotherapy face a major challenge as nematodes are developing resistances to the known nematocides. As new resistant isolates emerge and new drugs are developed to control them, there is a great need for improved methods of screening resistance and determining dose response. In this thesis, a microfluidic platform for screening drugs and their dose response on the locomotive behavior of parasitic nematodes is presented. The system offers reduced experimental time, higher sensitivity, and, for the first time, real-time observation of drug effects at a single worm resolution. The presented lab-on-chip bioassay can be reliably used to identify changes in multiple locomotion parameters and to determine exposure effects as a function of time. Existing nematode motility and migration assays do not offer such a level of sophistication. The device comprises two principal components: (i) microchannels to study nematode motility during the pre- and post-exposure periods of the experiment and (ii) a drug well for administering the dose and studying drug effects at different exposure times. The drug screening experiment can be described by three main phases: (i) `pre-exposure study\u27 - worms are inserted into the microchannels and their locomotion is characterized, (ii) `dose exposure\u27 - worms are guided from the microchannels into the drug well and exposed to a dose for a predefined time and (iii) `post-exposure study\u27 - worms are guided back into the microchannels where their locomotion is characterized and compared to pre-exposure motility. We demonstrate the workability of the microfluidic platform on the parasitic Oesophagotomum dentatum (levamisole sensitive, SENS and levamisole resistant, LEVR) using levamisole as the test drug. The proposed scheme of drug screening on a microfluidic device is expected to significantly improve the resolution, sensitivity and throughput of in vivo nematode testing, while offering new details on the real-time exposure effects of new and existing anthelmintics. A second project, `the electrotactic nematode gate\u27, is presented as a byproduct of the aforementioned lab-on-chip bioassay. Current microfluidic methods for gating (i.e. opening or closing a certain pathway to) nematode movement use a pinch type or polymer membrane valve. Although effective, these valves are generally large static structures, lack the potential for automation and, in some cases, require a multistep molding process. The electrical gate presented in this thesis advantageously uses the electrotactic response of nematodes to generate a dynamic, microscale gate that can be easily programmed and integrated into an automated bioassay. The gate requires only two electrode ports that have a separation larger than the nematodes\u27 penetration depth (10-300 μm depending on species) and can therefore be fabricated as a single-mold microfluidic device. It is expected that the presented device will help to streamline new and existing bioassays, especially in the Caenorhabditis elegans community

The effect of combined glutamate receptor blockade in the NTS on the hypoxic ventilatory response in awake rats differs from the effect of individual glutamate receptor blockade.

Ventilatory acclimatization to hypoxia (VAH) increases the hypoxic ventilatory response (HVR) and causes persistent hyperventilation when normoxia is restored, which is consistent with the occurrence of synaptic plasticity in acclimatized animals. Recently, we demonstrated that antagonism of individual glutamate receptor types (GluRs) within the nucleus tractus solitarii (NTS) modifies this plasticity and VAH (J. Physiol. 592(8):1839-1856); however, the effects of combined GluR antagonism remain unknown in awake rats. To evaluate this, we exposed rats to room air or chronic sustained hypobaric hypoxia (CSH, PiO2 = 70 Torr) for 7-9 days. On the experimental day, we microinjected artificial cerebrospinal fluid (ACSF: sham) and then a "cocktail" of the GluR antagonists MK-801 and DNQX into the NTS. The location of injection sites in the NTS was confirmed by glutamate injections on a day before the experiment and with histology following the experiment. Ventilation was measured in awake, unrestrained rats breathing normoxia or acute hypoxia (10% O2) in 15-min intervals using barometric pressure plethysmography. In control (CON) rats, acute hypoxia increased ventilation; NTS microinjections of GluR antagonists, but not ACSF, significantly decreased ventilation and breathing frequency in acute hypoxia but not normoxia (P < 0.05). CSH increased ventilation in hypoxia and acute normoxia. In CSH-conditioned rats, GluR antagonists in the NTS significantly decreased ventilation in normoxia and breathing frequency in hypoxia. A persistent HVR after combined GluR blockade in the NTS contrasts with the effect of individual GluR blockade and also with results in anesthetized rats. Our findings support the hypotheses that GluRs in the NTS contribute to, but cannot completely explain, VAH in awake rats

Harbor Surging

[no abstract

A high resolution mid-infrared survey of water emission from protoplanetary disks

We present the largest survey of spectrally resolved mid-infrared water emission to date, with spectra for 11 disks obtained with the Michelle and TEXES spectrographs on Gemini North. Water emission is detected in 6 of 8 disks around classical T Tauri stars. Water emission is not detected in the transitional disks SR 24 N and SR 24 S, in spite of SR 24 S having pre-transitional disk properties like DoAr 44, which does show water emission (Salyk et al. 2015). With R~100,000, the TEXES water spectra have the highest spectral resolution possible at this time, and allow for detailed lineshape analysis. We find that the mid-IR water emission lines are similar to the "narrow component" in CO rovibrational emission (Banzatti & Pontoppidan 2015), consistent with disk radii of a few AU. The emission lines are either single peaked, or consistent with a double peak. Single-peaked emission lines cannot be produced with a Keplerian disk model, and may suggest that water participates in the disk winds proposed to explain single-peaked CO emission lines (Bast et al. 2011, Pontoppidan et al. 2011). Double-peaked emission lines can be used to determine the radius at which the line emission luminosity drops off. For HL Tau, the lower limit on this measured dropoff radius is consistent with the 13 AU dark ring (ALMA partnership et al. 2015). We also report variable line/continuum ratios from the disks around DR Tau and RW Aur, which we attribute to continuum changes and line flux changes, respectively. The reduction in RW Aur line flux corresponds with an observed dimming at visible wavelengths (Rodriguez et al. 2013).Comment: To appear in the Astrophysical Journa

Intermediate inflation in light of the three-year WMAP observations

The three-year observations from the Wilkinson Microwave Anisotropy Probe have been hailed as giving the first clear indication of a spectral index n_s<1. We point out that the data are equally well explained by retaining the assumption n_s=1 and allowing the tensor-to-scalar ratio r to be non-zero. The combination n_s=1 and r>0 is given (within the slow-roll approximation) by a version of the intermediate inflation model with expansion rate H(t) \propto t^{-1/3}. We assess the status of this model in light of the WMAP3 data.Comment: 4 pages RevTeX4 with one figure. Minor changes to match PRD accepted versio

Detection of Water Vapor in the Thermal Spectrum of the Non-Transiting Hot Jupiter upsilon Andromedae b

The upsilon Andromedae system was the first multi-planet system discovered orbiting a main sequence star. We describe the detection of water vapor in the atmosphere of the innermost non-transiting gas giant ups~And~b by treating the star-planet system as a spectroscopic binary with high-resolution, ground-based spectroscopy. We resolve the signal of the planet's motion and break the mass-inclination degeneracy for this non-transiting planet via deep combined flux observations of the star and the planet. In total, seven epochs of Keck NIRSPEC $L$ band observations, three epochs of Keck NIRSPEC short wavelength $K$ band observations, and three epochs of Keck NIRSPEC long wavelength $K$ band observations of the ups~And~system were obtained. We perform a multi-epoch cross correlation of the full data set with an atmospheric model. We measure the radial projection of the Keplerian velocity ($K_P$ = 55 $\pm$ 9 km/s), true mass ($M_b$ = 1.7 $^{+0.33}_{-0.24}$ $M_J$), and orbital inclination \big($i_b$ = 24 $\pm$ 4$^{\circ}$\big), and determine that the planet's opacity structure is dominated by water vapor at the probed wavelengths. Dynamical simulations of the planets in the ups~And~system with these orbital elements for ups~And~b show that stable, long-term (100 Myr) orbital configurations exist. These measurements will inform future studies of the stability and evolution of the ups~And~system, as well as the atmospheric structure and composition of the hot Jupiter.Comment: Accepted to A

GUT-Scale Primordial Black Holes: Consequences and Constraints

A population of very light primordial black holes which evaporate before nucleosynthesis begins is unconstrained unless the decaying black holes leave stable relics. We show that gravitons Hawking radiated from these black holes would source a substantial stochastic background of high frequency gravititational waves ($10^{12}$ Hz or more) in the present universe. These black holes may lead to a transient period of matter dominated expansion. In this case the primordial universe could be temporarily dominated by large clusters of "Hawking stars" and the resulting gravitational wave spectrum is independent of the initial number density of primordial black holes.Comment: 4 pages; grey body factors included in graviton emission calculations, and a couple of references added, but the conclusions are unchanged. v3 Minor changes to references and wording; final versio

The Lightweight Integrated Solar Array and anTenna (LISA-T) Big Power for Small Spacecraft

NASA is developing a space power system using lightweight, flexible photovoltaic devices originally developed for use here on Earth to provide low cost power for spacecraft. The Lightweight Integrated Solar Array and anTenna (LISA-T) is a launch stowed, orbit deployed array on which thin-film photovoltaic and antenna elements are embedded. The LISA-T system is deployable, building upon NASA's expertise in developing thin-film deployable solar sails such the one being developed for the Near Earth Asteroid Scout project which will fly in 2018. One of the biggest challenges for the NEA Scout, and most other spacecraft, is power. There simply isn't enough of it available, thus limiting the range of operation of the spacecraft from the Sun (due to the small surface area available for using solar cells), the range of operation from the Earth (low available power with inherently small antenna sizes tightly constrain the bandwidth for communication), and the science (you can only power so many instruments with limited power). The LISA-T has the potential to mitigate each of these limitations, especially for small spacecraft. Inherently, small satellites are limited in surface area, volume, and mass allocation; driving competition between their need for power and robust communications with the requirements of the science or engineering payload they are developed to fly. LISA-T is addressing this issue, deploying large-area arrays from a reduced volume and mass envelope - greatly enhancing power generation and communications capabilities of small spacecraft and CubeSats. The problem is that these CubeSats can usually only generate between 7W and 50W of power. The power that can be generated by the LISA-T ranges from tens of watts to several hundred watts, at a much higher mass and stowage efficiency. A matrix of options are in development, including planar (pointed) and omnidirectional (non-pointed) arrays. The former is seeking the highest performance possible while the latter is seeking GN&C simplicity. Options for leveraging both high performance, 'typical cost' triple junction thin-film solar cells as well as moderate performance, low cost cells are being developed. Alongside, UHF (ultrahigh frequency), S-band, and X-band antennas are being integrated into the array to move their space claim away from the spacecraft and open the door for more capable multi-element antenna designs such as those needed for spherical coverage and electronically steered phase arrays