Autor: Claudia López

The inability of a single-gap solar cell to absorb energies less than the band-gap energy is one of the intrinsic loss mechanisms which limit the conversion efficiency in photovoltaic devices. New approaches to “ultra-high” efficiency solar cells based on artificially engineered materials with designed optoelectronic properties, like semiconductor nanostructures such as multiple quantum wells (MQW) and superlattices (SL) systems in the intrinsic region of a p-i-n cell of wider band-gap energy (barrier or host) semiconductor. These configurations are intended to extend the absorption band beyond the single gap host cell semiconductor. Deviation from bulk behavior needs to be taken into account at the time of describing the device operation mechanisms, a requirement which involves theoretical approaches for the simulation. A theoretical model has been developed to study the performance of the strain-balanced GaAsP/InGaAs/GaAs MQW solar cell, and GaAs/GaInNAs MQW or SL solar cells. These approaches make very attractive for space applications or for triple-junction concentrator photovoltaic cell based on a GaAs/GaInNAs bottom cell that could reach 50% conversion efficiency.

In this talk we also present new types of photovoltaic device where Gaussian superlattice is inserted in the i-region of a GaAs/GaInNAs p-i-n solar cell. A theoretical model is developed to study the performances of these devices. We establish a new criterion to calculate miniband widths in superlattice heterostructures in the presence of electric field. By optimizing miniband width, the spectral response of the cell in the energy region below the absorption edge of host material is significantly enhanced. Our results show that these devices could reach higher conversion efficiencies than the single-gap solar cell.

We are exploring the substitution of superlattices for graphene layers because it has revealed huge promise for future electronics technology besides fundamental physics applications because of its linear energy-momentum dispersion relations that cross at Dirac point. We present a graphene-based multilayer device that greatly magnifies the vertical tunneling transport. The tunnel electrical properties of heterostructures formed between multiple n- and p-doped graphene sheets, with the same rotational alignment, are theoretically investigated. Expressions for the voltage dependence of the tunnel current that flows between the two-dimensional graphene electronic states are derived. At a voltage bias such that the Dirac points of the graphene sheets are aligned, a large resonant current peak is observed.

Transmission electron microscopy (TEM) provides information at atomic scale to study structural and chemical properties of novel materials such as metallic alloys, semiconductors, ceramics, etc. Understanding those materials’ behavior under external signals in now one of the most recent challenges in TEM. Physical measurements in situ TEM at individual nanostructures include mechanical (stress vs strain curves), electrical (I-V curves), phase transformations (cooling/heating), magnetization, and optical measurements.

New investigations in materials at nanoscale explored by in situ TEM will be presented. Particularly, the study of crystalline orientation maps using precession electron diffraction and off-axis electron holography will be covered to study magnetic properties in materials, including in situ magnetization of metallic nano-wires within the TEM column. In addition, new frontiers in crystalline analysis using fast electron diffraction, precession electron diffraction and electron dose-controlled for sensitive materials will be included in the seminar. Applications of the presented methods include the study of magnetic nano-structured materials and local magnetometry in TEM.

La ciencia por si misma, como la herramienta para acumular conocimiento de la humanidad, no es de gran utilidad; y no es hasta que se lleva ese conocimiento al desarrollo de satisfactores, que la ciencia toma un sentido de realidad para la sociedad. Ahí es donde la innovación toma lugar, y donde los grandes avances científicos son reconocidos al presentárseles como la base de nuevas tecnologías; como ejemplos están los discos duros de alta capacidad, la conectividad inalámbrica, los smartphones, los autos eléctricos, la refrigeración magnética, el internet de las cosas, entre otros. Y en todos estos, la ciencia de materiales ha jugado un papel de gran importancia. Veamos de qué manera.

La difracción de electrones implementada en la microscopia electrónica de transmisión es una técnica útil en la caracterización de materiales. En el estudio de nano-materiales, es de importancia caracterizar las partículas individualmente. Debido a las dimensiones de las nano-partículas es necesario obtener un haz paralelo de electrones, del orden de las partículas. Con esta técnica se puede caracterizar cada partícula o partes de una nano- partícula. Por ello, en la presente plática se mostrará la configuración de la óptica electrónica para la difracción de electrones con haz nano-métrico (NBD) en modo STEM, para diferentes microscopios electrónicos. También, se mostrarán ejemplos de los materiales estudiados; y se discutirá las perspectivas de esta técnica en el futuro.

Contenido:

1. Principios, ventajas y desventajas de Absorción Atómica
2. Principios, ventajas y desventajas de ICP
3. Principios, Ventajas y Desventajas de XRF
4. Introducción a ICP MS, principios y aplicaciones.