53 research outputs found
A quantum dot heterojunction photodetector
No full textThesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2005.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 113-119).This thesis presents a new device architecture for photodetectors utilizing colloidally grown quantum dots as the principle photo-active component. We implement a thin film of cadmium selenide (CdSe) quantum dot sensitizers, sandwiched between an electron-transporting titanium dioxide (TiO2) layer and a hole-transporting N,N' diphenyl-N,N' bis(3-ethylphenyl)-(1,1'-biphenyl)- 4,4'-diamine (TPD) organic small molecule layer. The wide band gap TiO2 and TPD layers are found to block charge injection under reverse bias, yet serve as transport layers for photo-excited charge generated in the CdSe. The internal quantum efficiency is approximately 1% at zero bias and saturates at 3% at -1V. Current-voltage sweeps yield low dark current in reverse bias and significant hysteresis under illumination. We speculate that the hysteresis and low quantum efficiency are due to charge accumulation at the TiO2/CdSe interface.by Alexi Cosmos Arango.S.M
High open-circuit voltage in heterojunction photovoltaics containing a printed colloidal quantum-dot photosensitive layer
No full textThesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2010.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student submitted PDF version of thesis.Includes bibliographical references (p. 227-241).Within four to seven years, electricity generated from solar cells will cost less than grid electricity, making it the cleanest, cheapest, and most abundant energy source on the planet. The rise of solar energy, however, could come to an untimely end if current solar cell technologies fail to meet the staggering manufacturing volumes needed to sustain current growth rates. Nanostructured donor/acceptor photovoltaics utilizing small molecule organics or conjugated polymers offer processing advantages that might enable high-throughput, large-area production. However, power conversion efficiencies of these structures have remained low, due in large part to low open-circuit voltages (VOC). Using printing methods, we deposit a layer of colloidal cadmium selenide (CdSe) quantum dots (QDs) onto a wide band-gap organic hole-transporting thin film of N,N'-bis(3-methylphenyl)-N,N'-bis-(phenyl)-9,9-spirobiuorene (spiro-TPD) in order to form a unique planar heterojunction photovoltaic device. This structure is found to produce much higher VOC than previously predicted for donor/acceptor heterojunction photovoltaics. Absorption and charge generation occur primarily in the QD layer and indium tin oxide (ITO) provides the top contact, allowing for exceptional device stability and full transparency below the QD bandgap of 2.0 eV. Overall power conversion efficiencies remain low at 0.03% because only a small percentage of the incident light is absorbed (4% at the rst QD excitonic peak of 2.1 eV) and ll factors are near 0.4, yet VOC is 1.3V.(cont.) The high VOC is remarkable for an architecture with symmetric electrodes and exceeds the offset between the highest occupied molecular orbital (HOMO) of the acceptor (near 5.2 eV) and the lowest unoccupied molecular (LUMO) orbital of the QDs (near 4.6 eV). The internal quantum efficiency (IQE) exhibits a strong dependence on QD lm thickness and reaches a maximum of 30% at a thickness of 3-4 monolayers, indicating that transport losses dominate photocurrent generation for QD thicknesses above 4-5 monolayers. From the bias-dependence of quantum efficiency, we identify an intensity-independent compensation voltage V0 of 1.5 V that represents the maximum attainable VOC. Investigation of the bias-dependence of the photocurrent decay transients identifies charge diffusion as the dominant mechanism responsible for photocurrent generation and reveals a vast discrepancy between the time constant associated with charge extraction (0.6 s, measured at 0V) and that of recombination (0.4 [mu]s, measured at 2 V). An alternative model for VOC is presented that considers the dark current in forward bias as the critical mechanism determining VOC. We conclude that suppression of recombination across the spiro-TPD heterojunction interface forces recombination to occur predominantly in the QD lm. Electroluminescence from the QD layer recombination that hole injection from spiro-TPD into the QD layer and recombination in the QD layer is, in part, responsible for current ow in forward bias. Because the device architecture is straightforward and the fabrication techniques are simple, QD tandem cells are easily attained, furthering the prospect for high conversion efficiencies coupled with the potential for scaleable manufacturability.by Alexi Cosmos Arango.Ph.D
Mapping daily evapotranspiration at field to continental scales using geostationary and polar orbiting satellite imagery
No full textThermal infrared (TIR) remote sensing of land-surface temperature (LST) provides valuable information about the sub-surface moisture status required for estimating evapotranspiration (ET) and detecting the onset and severity of drought. While empirical indices measuring anomalies in LST and vegetation amount (e.g., as quantified by the Normalized Difference Vegetation Index; NDVI) have demonstrated utility in monitoring ET and drought conditions over large areas, they may provide ambiguous results when other factors (e.g., air temperature, advection) are affecting plant functioning. A more physically based interpretation of LST and NDVI and their relationship to sub-surface moisture conditions can be obtained with a surface energy balance model driven by TIR remote sensing. The Atmosphere-Land Exchange Inverse (ALEXI) model is a multi-sensor TIR approach to ET mapping, coupling a two-source (soil + canopy) land-surface model with an atmospheric boundary layer model in time-differencing mode to routinely and robustly map daily fluxes at continental scales and 5 to 10-km resolution using thermal band imagery and insolation estimates from geostationary satellites. A related algorithm (DisALEXI) spatially disaggregates ALEXI fluxes down to finer spatial scales using moderate resolution TIR imagery from polar orbiting satellites. An overview of this modeling approach is presented, along with strategies for fusing information from multiple satellite platforms and wavebands to map daily ET down to resolutions on the order of 10 m. The ALEXI/DisALEXI model has potential for global applications by integrating data from multiple geostationary meteorological satellite systems, such as the US Geostationary Operational Environmental Satellites, the European Meteosat satellites, the Chinese Fen-yung 2B series, and the Japanese Geostationary Meteorological Satellites. Work is underway to further evaluate multi-scale ALEXI implementations over the US, Europe, Africa and other continents with geostationary satellite coverage. © Author(s) 2011
Effect of Specific Exercise versus Bracing on Cobb Angle in Moderate Adolescent Idiopathic Scoliosis
No full textBackground/Purpose: The efficacy of the most common conservative treatments for adolescent idiopathic scoliosis (AIS) is controversial. The purpose of this paper is to determine whether scoliosis-specific exercise (SSE) is as effective as bracing on the outcomes of curve progression and reduction as measured by the Cobb angle.
Case Description: The patient was an 11 year old female prescribed a Boston brace and referred to physical therapy for treatment of AIS. Her clinical characteristics indicated that she was at high risk for curve progression and possible need for surgery. The patient was upset at having to wear a brace and seemed unlikely to fully comply with brace treatment, making it worthwhile to investigate whether SSE might be of benefit.
Outcomes: As this author was unable to undergo training in SSE while treating this patient, the patient was treated with exercises to strengthen the paraspinal musculature on the convex sides of the curves and to stretch the shortened paraspinal musculature on the concave sides of the curves in an attempt to stabilize or reduce the deformity, as well as additional exercises to address other impairments. The patient was close to meeting all physical therapy goals at the time of her final treatment by this author, however the effect of the intervention on Cobb angle was not able to be determined as radiographic evaluation of the patient was not indicated at this time.
Discussion: Strong evidence indicates that bracing is more effective than observation, and low level evidence indicates that SSE is likely more effective than observation. It is unclear at this time whether SSE or bracing is a more effective intervention. Given the preliminary evidence supporting SSE and the fact that the patient was unlikely to be fully compliant with bracing, it is likely that the patient would have benefitted from SSE if she proved compliant with an exercise intervention
Early Detection of Photovoltaic Panel Degradation through Artificial Neural Network
No full textIn this paper, an artificial neural network (ANN) is used for isolating faults and degradation phenomena occurring in photovoltaic (PV) panels. In the literature, it is well known that the values of the single diode model (SDM) associated to the PV source are strictly related to degradation phenomena and their variation is an indicator of panel degradation. On the other hand, the values of parameters that allow to identify the degraded conditions are not known a priori because they can be different from panel to panel and are strongly dependent on environmental conditions, PV technology and the manufacturing process. For these reasons, to correctly detect the presence of degradation, the effect of environmental conditions and fabrication processes must be properly filtered out. The approach proposed in this paper exploits the intrinsic capability of ANN to map in its architecture two effects: (1) the non-linear relations existing among the SDM parameters and the environmental conditions, and (2) the effect of the degradation phenomena on the I−V curves and, consequently, on the SDM parameters. The ANN architecture is composed of two stages that are trained separately: one for predicting the SDM parameters under the hypothesis of healthy operation and the other one for degraded condition. The variation of each parameter, calculated as the difference of the output of the two ANN stages, will give a direct identification of the type of degradation that is occurring on the PV panel. The method was initially tested by using the experimental I−V curves provided by the NREL database, where the degradation was introduced artificially, later tested by using some degraded experimental I−V curves.Photovoltaic Materials and Device
Social capital influence on sustainability of development (case study of Bulgaria)
No full textSocial capital is presented as feedback playing the role of a homeostatic mechanism of keeping the sustainability of economic systems in dynamics. Two basic measures of social capital are considered: the level of confidence (trust) among the members of society and the level of integration (consensus in ranking the social preferences). The author illustrates these assumptions in a case study of Bulgaria by means of a survey, which reflects the level of confidence and the level of integration in society. It is indicated that at present both levels are in a relatively good state in the micro aspect, but in the macro aspect the level of integration is not high, which creates problems with the consensus of ranking social preferences. The conclusion is that the need to improve the quality of social capital is a paramount task for the success of transition and building a democratic society. Copyright © 2005 John Wiley & Sons, Ltd and ERP Environment.
Green Infrastructure Systems Facing Fragmented Cities in Latin America - Case of Santiago, Chile
No full textAbstractReaching a balance between the protection of nature and the satisfaction of multiple social needs within highly dynamic metropolitan contexts - such as that of Santiago, Chile - is probably one of the main challenges facing territorial planning. This paper presents the progress made in the recognition of the green infrastructure system in Santiago, Chile and reflects on the opportunities and challenges for the development of a green infrastructure system to face socio-ecological fragmentation in Santiago
Coupled near-field and far-field exposure assessment framework for chemicals in consumer products
No full textAbstractHumans can be exposed to chemicals in consumer products through product use and environmental emissions over the product life cycle. Exposure pathways are often complex, where chemicals can transfer directly from products to humans during use or exchange between various indoor and outdoor compartments until sub-fractions reach humans. To consistently evaluate exposure pathways along product life cycles, a flexible mass balance-based assessment framework is presented structuring multimedia chemical transfers in a matrix of direct inter-compartmental transfer fractions. By matrix inversion, we quantify cumulative multimedia transfer fractions and exposure pathway-specific product intake fractions defined as chemical mass taken in by humans per unit mass of chemical in a product. Combining product intake fractions with chemical mass in the product yields intake estimates for use in life cycle impact assessment and chemical alternatives assessment, or daily intake doses for use in risk-based assessment and high-throughput screening. Two illustrative examples of chemicals used in personal care products and flooring materials demonstrate how this matrix-based framework offers a consistent and efficient way to rapidly compare exposure pathways for adult and child users and for the general population. This framework constitutes a user-friendly approach to develop, compare and interpret multiple human exposure scenarios in a coupled system of near-field (‘user’ environment), far-field and human intake compartments, and helps understand the contribution of individual pathways to overall human exposure in various product application contexts to inform decisions in different science-policy fields for which exposure quantification is relevant
Las tareas de la integración tecnológico-pedagógica en la gestión de la formación del estudiante universitario
No full textPresent it a conception of the tasks of integration offers postulate than from the steps of the formation of the professional in Education Superior, they permit understanding the technological pedagogic Integration like beginning than vertebra the scientific and technological in the aforementioned process; It is the same way that the scientific happens technological inasmuch as the process of steps is structured in different formative spaces ( University OACEs ) as from practical application of theoretic conceptual aspects that they make possible giving answers to the requirements of the formative process. A process has to do with steps itself that the typical contradictions of saying share in process in deeply felt general, but that what that the scientific advances, the technique may be let of side does is singularized, in the formation of the professional of Education Superior from the company like formative fundamental space, and of technology to be able to direct pertinently the educational practice that he integrate these aspects. The formation of the professional of Education Superior requires an especial attention from the perspective of all of the processes than in the ones that is implicated, at the edge of the ones that it cannot be the process of educational steps at the edge of. This result emerges like part of the systematization of investigating results that the author of the postulate directs of doctorate and of the project of technological pedagogic integration.La presente ponencia brinda una concepción de las tareas de integración que desde la gestión de la formación del profesional en la Educación Superior, permiten comprender la Integración tecnológico-pedagógica como principio que vertebra lo científico y tecnológico en dicho proceso; es así como lo científico deviene tecnológico en la medida en que el proceso de gestión se estructura en los diferentes espacios formativos (Carrera-OACEs) a partir de la aplicación práctica de los aspectos teóricos conceptuales que posibilitan dar respuestas a las exigencias del proceso formativo. Se trata un proceso de gestión que compartan las contradicciones típicas de dicho proceso en sentido general, pero que se singulariza, en la formación del profesional de la Educación Superior desde la empresa como espacio formativo fundamental, lo que hace que no pueda dejarse de lado los avances de la ciencia, la técnica y de la tecnología para poder dirigir pertinentemente la práctica educativa que integre estos aspectos. La formación del profesional de la Educación Superior requiere una especial atención desde la perspectiva de todos los procesos que en los que se implica, al margen de los cuales no puede quedar el proceso de gestión educacional. Este resultado emerge como parte de la sistematización de resultados investigativos de doctorado y del proyecto de integración tecnológico-pedagógica que dirige el autor de la ponencia
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