Las tobas cuaternarias del río Añamaza (provincia de Soria, Cordillera Ibérica): aproximación cronológica

High-gradient, stepped fluvial tufa systems with dammed areas existed in the River Añamaza valley (NW Iberian Ranges, Spain) during Quaternary times. Single deposits range from a few meters to about 70 m thick, in which prograding-aggrading wedges separated by erosional surfaces exist. Several episodes of tufa formation have been distinguished by means of U-series, Amino-acid racemization and radiocarbon techniques. These correlate to MIS 8, 7, 5 and 1. The presence of MIS 9 is uncertain, as chronological data may also correspond to older stages. Most tufas in this area formed in MIS 5. Distinct tufa episodes can also be distinguished in the Holocene. These are the first chronological data presented for one of the northernmost Quaternary tufa systems in the Iberian Ranges.

Análisis preliminar de las tobas cuaternarias del río Ebrón (Castielfabib, Valencia, Cordillera Ibérica)

In this paper the Alpine cleavage affecting the Permo-Triassic series of the Espadan Range (Castellón) is studied. Cleavage affects to argillites and sandstones in Saxonian and Buntsandstein facies. At cartographic scale it is linked with the Espadan box anticline with constant ONO-ESE trend. At microscoscopic scale it constitutes a “spaced cleavage” with a predominance of pressure solution and passive rotation mechanisms. At outcrop scale the cleavage characterizes by a sigmoidal geometry linked both the post-cleavage flexural slip as a cleavage-related flexural flow mechanism. The proposed kinematic model to explain its origin includes three main stages: 1) incipient development of cleavage linked to layer-parallel shortening, 2) buckling and increasing of cleavage penetrativity and 3) folfing amplification and layer-parallel shear. RESUMEN Se estudia la esquistosidad alpina que afecta a la serie Permo-Triásica de la Sierra de Espadán, (Castellón). La esquistosidad afecta a los tramos argilíticos y areniscosos en facies Saxoniense y Buntsandstein, con distinto grado de penetratividad. A escala cartográfica se asocia al anticlinal de Espadán con geometría en cofre y orientación ONO-ESE. A escala microestructural se clasifica como esquistosidad espaciada con predominio de los mecanismos de disolución por presión y rotación mecánica de filosilicatos. A escala de afloramiento destaca la geometría sigmoidal de las superficies de esquistosidad atribuida tanto a un mecanismo post-esquistoso de flexodeslizamiento en las capas competentes como a flexofluencia sin-esquistosa en capas incompetentes. El modelo cinemático para su génesis contempla tres estadios: 1) desarrollo incipiente de esquistosidad en relación a acortamiento paralelo a las capas, 2) buckling e incremento del grado de penetratividad y 3) amplificación de los pliegues y cizalla simple paralela a las capas

A numerical study of Bi-periodic binary diffraction gratings for solar cell applications

In this paper, a numerical study is made of simple bi-periodic binary diffraction gratings for solar cell applications. The gratings consist of hexagonal arrays of elliptical towers and wells etched directly into the solar cell substrate. The gratings are applied to two distinct solar cell technologies: a quantum dot intermediate band solar cell (QD-IBSC) and a crystalline silicon solar cell (SSC). In each case, the expected photocurrent increase due to the presence of the grating is calculated assuming AM1.5D illumination. For each technology, the grating period, well/tower depth and well/tower radii are optimised to maximise the photocurrent. The optimum parameters are presented. Results are presented for QD-IBSCs with a range of quantum dot layers and for SSCs with a range of thicknesses. For the QD-IBSC, it is found that the optimised grating leads to an absorption enhancement above that calculated for an ideally Lambertian scatterer for cells with less than 70 quantum dot layers. In a QD-IBSC with 50 quantum dot layers equipped with the optimum grating, the weak intermediate band to conduction band transition absorbs roughly half the photons in the corresponding sub-range of the AM1.5D spectrum. For the SSC, it is found that the optimised grating leads to an absorption enhancement above that calculated for an ideally Lambertian scatterer for cells with thicknesses of 10 ?m or greater. A 20um thick SSC equipped with the optimised grating leads to an absorption enhancement above that of a 200um thick SSC equipped with a planar back reflector.

Desarrollo de estructuras nanofotonicas de campo cercano para intensificacion localizada de luz en celulas solares de banda intermedia - Near-field nanophotonic structures for localized light enhancement in intermediate band solar cells

This doctoral thesis explores some of the possibilities that near-field optics can bring to photovoltaics, and in particular to quantum-dot intermediate band solar cells (QD-IBSCs). Our main focus is the analytical optimization of the electric field distribution produced in the vicinity of single scattering particles, in order to produce the highest possible absorption enhancement in the photovoltaic medium in their surroundings. Near-field scattering structures have also been fabricated in laboratory, allowing the application of the previously studied theoretical concepts to real devices. We start by looking into the electrostatic scattering regime, which is only applicable to sub-wavelength sized particles. In this regime it was found that metallic nano-spheroids can produce absorption enhancements of about two orders of magnitude on the material in their vicinity, due to their strong plasmonic resonance. The frequency of such resonance can be tuned with the shape of the particles, allowing us to match it with the optimal transition energies of the intermediate band material. Since these metallic nanoparticles (MNPs) are to be inserted inside the cell photovoltaic medium, they should be coated by a thin insulating layer to prevent electron-hole recombination at their surface. This analysis is then generalized, using an analytical separation-of-variables method implemented in Mathematica7.0, to compute scattering by spheroids of any size and material. This code allowed the study of the scattering properties of wavelengthsized particles (mesoscopic regime), and it was verified that in this regime dielectric spheroids perform better than metallic. The light intensity scattered from such dielectric spheroids can have more than two orders of magnitude than the incident intensity, and the focal region in front of the particle can be shaped in several ways by changing the particle geometry and/or material. Experimental work was also performed in this PhD to implement in practice the concepts studied in the analysis of sub-wavelength MNPs. A wet-coating method was developed to self-assemble regular arrays of colloidal MNPs on the surface of several materials, such as silicon wafers, amorphous silicon films, gallium arsenide and glass. A series of thermal and chemical tests have been performed showing what treatments the nanoparticles can withstand for their embedment in a photovoltaic medium. MNPs arrays are then inserted in an amorphous silicon medium to study the effect of their plasmonic near-field enhancement on the absorption spectrum of the material. The self-assembled arrays of MNPs constructed in these experiments inspired a new strategy for fabricating IBSCs using colloidal quantum dots (CQDs). Such CQDs can be deposited in self-assembled monolayers, using procedures similar to those developed for the patterning of colloidal MNPs. The use of CQDs to form the intermediate band presents several important practical and physical advantages relative to the conventional dots epitaxially grown by the Stranski-Krastanov method. Besides, this provides a fast and inexpensive method for patterning binary arrays of QDs and MNPs, envisioned in the theoretical part of this thesis, in which the MNPs act as antennas focusing the light in the QDs and therefore boosting their absorption

Near-field light focusing by wavelength-sized dielectric spheroids for photovoltaic applications

We explore the near-field concentration properties of dielectric spheroidal scatterers with sizes close to the wavelength, using an analytical separation-of-variables method. Such particles act as mesoscopic lenses whose physical parameters are optimized here for maximum scattered light enhancement in photovoltaic applications.

Radiative thermal escape in intermediate band solar cells

To achieve high efficiency, the intermediate band (IB) solar cell must generate photocurrent from sub-bandgap photons at a voltage higher than that of a single contributing sub-bandgap photon. To achieve the latter, it is necessary that the IB levels be properly isolated from the valence and conduction bands. We prove that this is not the case for IB cells formed with the confined levels of InAs quantum dots (QDs) in GaAs grown so far due to the strong density of internal thermal photons at the transition energies involved. To counteract this, the QD must be smaller.

Low-injection behaviour of a solar cell with a metallic grating back-reflector

Recent studies have dealt with the possibility of increasing light absorption by using the so-called electric field enhancement taking place within the grooves of metallic gratings. In order to evaluate the potential improvements derived from the absorption increase, we employ a simplified model to analyze the low-injection behaviour of a solar cell with a metallic grating back-reflector.

Descripción De Los Rasgos Petrográficos De Algunos Áridos De La Caldera De Los Frailes, En El Sureste De La Península Ibérica.

El trabajo que se presenta tiene la finalidad de ofrecer una descripción sistemática de algunas variedades de áridos naturales que yacen en la parte central de la Caldera de Los Frailes, en el Sureste de la Península Ibérica. Con el estudio detallado de 22 muestras, se identificaron las especies minerales formadoras de rocas, rasgos texturales, abundancia modal, alteraciones secundarias y petrogénesis. Los resultados obtenidos han permitido identificar varios tipos de áridos naturales en la Caldera de los Frailes, de acuerdo con sus propiedades petrográficas, representados, entre otros, por andesitas piroxénicas, dacitas y sus tobas, productos piroclásticos, zeolitas y bentonitas. El grado de conocimiento aportado por esta investigación la convierte en una guía práctica para orientar a los interesados en el uso efectivo y racional de estos áridos.

Influencia de la adición de zeolita en las propiedades micro y macroestructurales en pastas y morteros.

Las adiciones activas en los hormigones son cada día más usuales, no solo debido a razones económicas, sino porque los efectos que se desarrollan son beneficiosos para las prestaciones del hormigón, léase durabilidad y resistencias mecánicas. En Cuba ha sido frenada al no existir fuentes como las tradicionalmente conocidas y comercializadas como es cenizas volantes y las micro sílices (silica fume o fly ash). El desarrollo de estudios de algunos minerales industriales nacionales de génesis ígnea como los vidrios volcánicos, tobas vítreas o zeolitas han demostrado su actividad puzolánica. Es conocido que la zeolita tiene actividad puzolánica desde la época romana, y actualmente se utilizan en el mundo para la producción de cementos mezclados, sin embargo la experiencia cubana es el precedente de su uso como adición activa a hormigones. Se han realizado investigaciones a diferentes escalas del uso de adiciones de zeolita en tecnologías de prefabricado, premezclado y pretensado que han demostrado las mejoras en las prestaciones. El presente trabajo explica el aumento de las prestaciones antes demostradas mediante el estudio de los cambios microestructurales, tanto de composición química como en la morfología de los productos de hidratación formados, a partir de análisis por microscopia electrónica de barrido y microanálisis por espectroscopia de dispersión de energía de rayos X en pasta de cemento y cemento + zeolita comparativamente.

Understanding the operation of quantum dot intermediate band solar cells

In this paper, a model for intermediate band solar cells is built based on the generally understood physical concepts ruling semiconductor device operation, with special emphasis on the behavior at low temperature. The model is compared to JL-VOC measurements at concentrations up to about 1000 suns and at temperatures down to 20 K, as well as measurements of the radiative recombination obtained from electroluminescence. The agreement is reasonable. It is found that the main reason for the reduction of open circuit voltage is an operational reduction of the bandgap, but this effect disappears at high concentrations or at low temperatures.

Absorption coefficient for the intraband transitions in quantum dot materials

In this paper, we present calculations of the absorption coefficient for transitions between the bound states of quantum dots grown within a semiconductor and the extended states of the conduction band. For completeness, transitions among bound states are also presented. In the separation of variables, single band k·p model is used in which most elements may be expressed analytically. The analytical formulae are collected in the appendix of this paper. It is concluded that the transitions are strong enough to provide a quick path to the conduction band for electrons pumped from the valence to the intermediate band

Symmetry considerations in the empirical k.p Hamiltonian for the study of intermediate band solar cells

With the purpose of assessing the absorption coefficients of quantum dot solar cells, symmetry considerations are introduced into a Hamiltonian whose eigenvalues are empirical. In this way, the proper transformation from the Hamiltonian's diagonalized form to the form that relates it with Γ-point exact solutions through k.p envelope functions is built accounting for symmetry. Forbidden transitions are thus determined reducing the calculation burden and permitting a thoughtful discussion of the possible options for this transformation. The agreement of this model with the measured external quantum efficiency of a prototype solar cell is found to be excellent.

The influence of quantum dot size on the sub-bandgap intraband photocurrent in intermediate band solar cells

The effect of quantum dot (QD) size on the performance of quantum dot intermediate band solar cells is investigated. A numerical model is used to calculate the bound state energy levels and the absorption coefficient of transitions from the ground state to all other states in the conduction band. Comparing with the current state of the art, strong absorption enhancements are found for smaller quantum dots, as well as a better positioning of the energy levels, which is expected to reduce thermal carrier escape. It is concluded that reducing the quantum dot size can increase sub-bandgap photocurrent and improve voltage preservation.

Realistic performance prediction in nanostructured solar cells as a function of nanostructure dimensionality and density

The behavior of quantum dot, quantum wire, and quantum well InAs/GaAs solar cells is studied with a very simplified model based on experimental results in order to assess their performance as a function of the low bandgap material volume fraction fLOW. The efficiency of structured devices is found to exceed the efficiency of a non-structured GaAs cell, in particular under concentration, when fLOW is high; this condition is easier to achieve with quantum wells. If three different quasi Fermi levels appear with quantum dots the efficiency can be much higher.

Guide to intermediate band solar cell research

The intermediate band solar cell (IBSC) is a solar cell that, in order to increase its efficiency over that of single gap solar cells, takes advantage of the absorption of below-bandgap energy photons by means of an intermediate band (IB) located in the semiconductor bandgap. For this process to improve the solar cell performance, the belowbandgap photon absorption has to be effective and the IB cannot limit the open-circuit voltage of the cell. In this paper we provide a guide to the new researcher interested in the idea in order he can quickly become familiar with the concept and updated with the most relevant experimental results.