Transport properties of CuGaSe(2)-based thin-film solar cells as a function of absorber composition

The transport properties of thin-film solar cells based on wide-gap CuGaSe(2) absorbers have been investigated as a function of the bulk [Ga]/[Cu] ratio ranging from 1.01 to 1.33. We find that (i) the recombination processes in devices prepared from absorbers with a composition close to stoichiometry ([Ga]/[Cu] = 1.01) are strongly tunnelling assisted resulting in low recombination activation energies (E(a)) of approx. 0.95 eV in the dark and 1.36 eV under illumination. (ii) With an increasing [Ga]/[Cu] ratio, the transport mechanism changes to be dominated by thermally activated Shockley-Read-Hall recombination with similar E(a) values of approx. 1.52-1.57 eV for bulk [Ga]/[Cu] ratios of 1.12-1.33. The dominant recombination processes take place at the interface between CdS buffer and CuGaSe(2) absorber independently from the absorber composition. The increase of E(a) with the [Ga]/[Cu] ratio correlates with the open circuit voltage and explains the better performance of corresponding solar cells.

Approximate Analytical Model for the Squeeze-Film Lubrication of the Human Ankle Joint with Synovial Fluid Filtrated by Articular Cartilage

The aim of this article is to propose an analytical approximate squeeze-film lubrication model of the human ankle joint for a quick assessment of the synovial pressure field and the load carrying due to the squeeze motion. The model starts from the theory of boosted lubrication for the human articular joints lubrication (Walker et al., Rheum Dis 27:512–520, 1968; Maroudas, Lubrication and wear in joints. Sector, London, 1969) and takes into account the fluid transport across the articular cartilage using Darcy’s equation to depict the synovial fluid motion through a porous cartilage matrix. The human ankle joint is assumed to be cylindrical enabling motion in the sagittal plane only. The proposed model is based on a modified Reynolds equation; its integration allows to obtain a quick assessment on the synovial pressure field showing a good agreement with those obtained numerically (Hlavacek, J Biomech 33:1415–1422, 2000). The analytical integration allows the closed form description of the synovial fluid film force and the calculation of the unsteady gap thickness.

First principles study of Bi dopen CdTe thin film solar cells: electronic and optical properties

Nowadays, efficiency improvement of solar cells is one of the most important issues in photovoltaic systems and CdTe is one of the most promising thin film photovoltaic materials we can found. CdTe reported efficiencies in solar energy conversion have been as good as that found in polycrystalline Si thin film cell [1], besides CdTe can be easily produced at industrial scale.

Characterization of thin film PV modules under standard test conditions: Results of indoor and outdoor measurements and the effects of sunlight exposure

Photovoltaic modules based on thin film technology are gaining importance in the photovoltaic market, and module installers and plant owners have increasingly begun to request methods of performing module quality control. These modules pose additional problems for measuring power under standard test conditions (STC), beyond problems caused by the temperature of the module and the ambient variables. The main difficulty is that the modules’ power rates may vary depending both on the amount of time they have been exposed to the sun during recent hours and on their history of sunlight exposure. In order to assess the current state of the module, it is necessary to know its sunlight exposure history. Thus, an easily accomplishable testing method that ensures the repeatability of the measurements of the power generated is needed. This paper examines different tests performed on commercial thin film PV modules of CIS, a-Si and CdTe technologies in order to find the best way to obtain measurements. A method for obtaining indoor measurements of these technologies that takes into account periods of sunlight exposure is proposed. Special attention is paid to CdTe as a fast growing technology in the market.

Selective ablation with UV lasers of a-Si:H thin film solar cells in direct scribing configuration

Monolithical series connection of silicon thin-film solar cells modules performed by laser scribing plays a very important role in the entire production of these devices. In the current laser process interconnection the two last steps are developed for a configuration of modules where the glass is essential as transparent substrate. In addition, the change of wavelength in the employed laser sources is sometimes enforced due to the nature of the different materials of the multilayer structure which make up the device. The aim of this work is to characterize the laser patterning involved in the monolithic interconnection process in a different configurations of processing than the usually performed with visible laser sources. To carry out this study, we use nanosecond and picosecond laser sources working at 355nm of wavelength in order to achieve the selective ablation of the material from the film side. To assess this selective removal of material has been used EDX (energy dispersive using X-ray) analysis

Ultra high frequency thin film saw devices

Durante los últimos años el flujo de datos en la transmisión que tiene lugar en los sistemas de comunicación ha aumentado considerablemente de forma que día a día se requieren más aplicaciones trabajando en un rango de frecuencias muy alto (3-30 GHz). Muchos de estos sistemas de comunicación incluyen dispositivos de onda acústica superficial (SAW) y por tanto se hace necesario el aumento de frecuencia a la que éstos trabajan. Pero este incremento de frecuencia de los dispositivos SAW no sólo es utilizado en los sistemas de comunicación, varios tipos de sensores, por ejemplo, aumentan su sensibilidad cuando la frecuencia a la que trabajan también lo hace. Tradicionalmente los dispositivos SAW se han fabricado sobre cuarzo, LiNbO3 y LiTaO3 principalmente. Sin embargo la principal limitación de estos materiales es su velocidad SAW. Además, debido a la alta temperatura a la que se depositan no pueden ser integrados en la tecnología de fabricación CMOS. El uso de la tecnología de capa delgada, en la que un material piezoeléctrico es depositado sobre un substrato, se está utilizando en las últimas décadas para incrementar la velocidad SAW de la estructura y poder obtener dispositivos trabajando en el rango de frecuencias requerido en la actualidad. Por otra parte, esta tecnología podría ser integrada en el proceso de fabricación CMOS. Durante esta tesis nos hemos centrado en la fabricación de dispositivos SAW trabajando a muy alta frecuencia. Para ello, utilizando la tecnología de capa delgada, hemos utilizado la estructura nitruro de aluminio (AlN) sobre diamante que permite conseguir velocidades SAW del sustrato que no se pueden alcanzar con otros materiales. El depósito de AlN se realizó mediante sputtering reactivo. Durante esta tesis se han realizado diferentes experimentos para optimizar dicho depósito de forma que se han obtenido los parámetros óptimos para los cuales se pueden obtener capas de AlN de alta calidad sobre cualquier tipo de sustrato. Además todo el proceso se realizó a baja temperatura para que el procesado de estos dispositivos pueda ser compatible con la tecnología CMOS. Una vez optimizada la estructura AlN/diamante, mediante litografía por haz de electrones se fabricaron resonadores SAW de tamaño nanométrico que sumado a la alta velocidad resultante de la combinación AlN/diamante nos ha permitido obtener dispositivos trabajando en el rango de 10-28 GHz con un alto factor de calidad y rechazo fuera de la banda. Estás frecuencias y prestaciones no han sido alcanzadas por el momento en resonadores de este tipo. Por otra parte, se han utilizado estos dispositivos para fabricar sensores de presión de alta sensibilidad. Estos dispositivos son afectados altamente por los cambios de temperatura. Se realizó también un exhaustivo estudio de cómo se comportan en temperatura estos resonadores, entre -250ºC y 250ºC (rango de temperaturas no estudiado hasta el momento) diferenciándose dos regiones una a muy baja temperatura en la que el dispositivo muestra un coeficiente de retraso en frecuencia (TCF) relativamente bajo y otra a partir de los -100ºC en la que el TCF es similar al observado en la bibliografía. Por tanto, durante esta tesis se ha optimizado el depósito de AlN sobre diamante para que sea compatible con la tecnología CMOS y permita el procesado de dispositivos trabajando a muy alta frecuencia con altas prestaciones para comunicaciones y sensores. ABSTRACT The increasing volume of information in data transmission systems results in a growing demand of applications working in the super-high-frequency band (3–30 GHz). Most of these systems work with surface acoustic wave (SAW) devices and thus there is a necessity of increasing their resonance frequency. Moreover, sensor application includes this kind of devices. The sensitivity of them is proportional with its frequency. Traditionally, quartz, LiNbO3 and LiTaO3 have been used in the fabrication of SAW devices. These materials suffer from a variety of limitations and in particular they have low SAW velocity as well as being incompatible with the CMOS technology. In order to overcome these problems, thin film technology, where a piezoelectric material is deposited on top of a substrate, has been used during the last decades. The piezoelectric/substrate structure allows to reach the frequencies required nowadays and could be compatible with the mass electronic production CMOS technology. This thesis work focuses on the fabrication of SAW devices working in the super-high-frequency range. Thin film technology has been used in order to get it, especially aluminum nitride (AlN) deposited by reactive sputtering on diamond has been used to increase the SAW velocity. Different experiments were carried out to optimize the parameters for the deposit of high quality AlN on any kind of substrates. In addition, the system was optimized under low temperature and thus this process is CMOS compatible. Once the AlN/diamond was optimized, thanks to the used e-beam lithography, nanometric SAW resonators were fabricated. The combination of the structure and the size of the devices allow the fabrication of devices working in the range of 10-28 GHz with a high quality factor and out of band rejection. These high performances and frequencies have not been reached so far for this kind of devices. Moreover, these devices have been used as high sensitivity pressure sensors. They are affected by temperature changes and thus a wide temperature range (-250ºC to 250ºC) study was done. From this study two regions were observed. At very low temperature, the temperature coefficient of frequency (TCF) is low. From -100ºC upwards the TCF is similar to the one appearing in the literature. Therefore, during this thesis work, the sputtering of AlN on diamond substrates was optimized for the CMOS compatible fabrication of high frequency and high performance SAW devices for communication and sensor application.

Web Classroom Application. Digital technologies to study theory, history and criticism of architecture at Escuela Superior de Arquitectura UPMadrid. The search for some guides to direct the inquiry learning

xxx

Intercultural Approaches to Cities and Spaces in Literature, Film, and New Media: a review of new work by Manzanas and Benito and Lopez-Varela and Net

Intercultural Approaches to Cities and Spaces in Literature, Film, and New Media: a review of new work by Manzanas and Benito and Lopez-Varela and Net

Practical issues for testing thin film PV modules at standard test conditions.

Thin film photovoltaic (TF) modules have gained importance in the photovoltaic (PV) market. New PV plants increasingly use TF technologies. In order to have a reliable sample of a PV module population, a huge number of modules must be measured. There is a big variety of materials used in TF technology. Some of these modules are made of amorphous or microcrystalline silicon. Other are made of CIS or CdTe. Not all these materials respond the same under standard test conditions (STC) of power measurement. Power rates of the modules may vary depending on both the extent and the history of sunlight exposure. Thus, it is necessary a testing method adapted to each TF technology. This test must guarantee repeatability of measurements of generated power. This paper shows responses of different commercial TF PV modules to sunlight exposure. Several test procedures were performed in order to find the best methodology to obtain measurements of TF PV modules at STC in the easiest way. A methodology for indoor measurements adapted to these technologies is described.

Disjoining pressure and the film-height-dependent surface tension of thin liquid films: New insight from capillary wave fluctuations

In this paper we review simulation and experimental studies of thermal capillary wave fluctuations as an ideal means for probing the underlying disjoining pressure and surface tensions, and more generally, fine details of the Interfacial Hamiltonian Model. We discuss recent simulation results that reveal a film-height-dependent surface tension not accounted for in the classical Interfacial Hamiltonian Model. We show how this observation may be explained bottom-up from sound principles of statistical thermodynamics and discuss some of its implications

New strategies in laser processing of TCOs for light management improvement in thin-film silicon solar cells

Light confinement strategies play a crucial role in the performance of thin-film (TF) silicon solar cells. One way to reduce the optical losses is the texturing of the transparent conductive oxide (TCO) that acts as the front contact. Other losses arise from the mismatch between the incident light spectrum and the spectral properties of the absorbent material that imply that low energy photons (below the bandgap value) are not absorbed, and therefore can not generate photocurrent. Up-conversion techniques, in which two sub-bandgap photons are combined to give one photon with a better matching with the bandgap, were proposed to overcome this problem. In particular, this work studies two strategies to improve light management in thin film silicon solar cells using laser technology. The first one addresses the problem of TCO surface texturing using fully commercial fast and ultrafast solid state laser sources. Aluminum doped Zinc Oxide (AZO) samples were laser processed and the results were optically evaluated by measuring the haze factor of the treated samples. As a second strategy, laser annealing experiments of TCOs doped with rare earth ions are presented as a potential process to produce layers with up-conversion properties, opening the possibility of its potential use in high efficiency solar cells.

Analysis by finite element calculations of light scattering in laser-textured AZO films for PV thin-film solar cells

In the thin-film photovoltaic industry, to achieve a high light scattering in one or more of the cell interfaces is one of the strategies that allow an enhancement of light absorption inside the cell and, therefore, a better device behavior and efficiency. Although chemical etching is the standard method to texture surfaces for that scattering improvement, laser light has shown as a new way for texturizing different materials, maintaining a good control of the final topography with a unique, clean, and quite precise process. In this work AZO films with different texture parameters are fabricated. The typical parameters used to characterize them, as the root mean square roughness or the haze factor, are discussed and, for deeper understanding of the scattering mechanisms, the light behavior in the films is simulated using a finite element method code. This method gives information about the light intensity in each point of the system, allowing the precise characterization of the scattering behavior near the film surface, and it can be used as well to calculate a simulated haze factor that can be compared with experimental measurements. A discussion of the validation of the numerical code, based in a comprehensive comparison with experimental data is included.

Jacques Tati : vivienda experimental y espacio de trabajo a mediados del siglo XX

La obra fílmica del director francés Jacques Tati podría considerarse como el perfecto reflejo del paradigma edificatorio de mediados del siglo XX en plena posguerra europea, una época ávida de transformaciones de las que el cine supo hacerse eco. Particularmente, el cine de Tati refleja las preocupaciones del ciudadano europeo de posguerra sobre las consecuencias de las masivas construcciones erigidas en sus devastados núcleos urbanos y la puesta en práctica de la ciudad funcional propuesta por la Carta de Atenas (1931). Pero, además, el análisis del cine de Jacques Tati permite un acercamiento a la modernidad desde diversos puntos de vista como la movilidad, el diseño urbano, las nuevas construcciones, los espacios de trabajo en los nuevos distritos terciarios, la vivienda -tradicional, moderna y experimental- o el diseño mobiliario en la posguerra. A través de su alter ego -Monsieur Hulot- Tati interacciona curioso con las nuevas construcciones geométricas de paños neutros y cuestiona su ruptura con la tradición edificatoria francesa, enfatizando la oposición entre el pasado nostálgico y la modernidad de las décadas de los 50 y 60, salpicadas por el consumismo feroz del recién estrenado estado de bienestar. La confrontación funcional, volumétrica, estética e incluso cromática entre ambos mundos construidos –el tradicional y el moderno- invita al espectador a un ejercicio de reflexión y crítica sobre la arquitectura moderna de este período en Europa. En particular, la mirada cinematográfica de Tati se centra en dos conceptos fundamentales. Por una parte, su atención se dirige a la famosa casa mecanicista Le Corbuseriana materializada en la ultra-moderna casa Arpel (Mon Oncle, 1958) y proyectada en la misma época en la que se desarrollaban importantes prototipos de vivienda experimental como la Casa de Futuro de Alison y Peter Smithson o las viviendas de Jean Prouvé. Debe ponerse de manifiesto que la crítica de Jacques Tati no se centraba en la arquitectura moderna en sí misma sino en el empleo erróneo que los usuarios pudieran hacer de ella. Por otro lado, Tati centra su atención en el prisma miesiano a través de los bloques de oficinas que conforman la ciudad de Tativille en Playtime (1967). Se trataba de una gran ciudad moderna construida explícitamente para el rodaje de la película y basada en los casi idénticos tejidos urbanos residenciales y terciarios ya en funcionamiento en las principales capitales europeas y norteamericanas en aquellos años. Tativille funcionaría como una ciudad autónoma disponiendo de diversas instalaciones y con el objetivo de integrarse y consolidarse en la trama urbana parisina. Lamentablemente, su destino al final del rodaje fue bien distinto. En definitiva, el análisis de la producción fílmica de Jacques Tati permite un acercamiento a la arquitectura y al urbanismo modernos de posguerra y al contexto socio-económico que favoreció su crecimiento y expansión. Por ello, su obra constituye una herramienta visual muy útil que aún hoy es consultada y mostrada por su claridad y humor y que invita a los ciudadanos –telespectadores- a participar en un ejercicio crítico arquitectónico hasta entonces reservado a los arquitectos. ABSTRACT The film work of French director Jacques Tati could be considered as the perfect reflection of the mid-20th century European post-war building paradigm, a period of time plenty of transformations perfectly echoed by cinema. In particular, Tati’s film work reflects the European post-war citizen’s concerns about the consequences of massive constructions built in their desvastated urban centres, as well as the development of functional cities proposed by the Athens Charter (1931). But, on top of that, an analysis of Jacques Tati’s cinematography allows for an approach to modernity from different perspectives, such as mobility, urban design, new buildings, working spaces in the new tertiary districts, housing -traditional, modern, and experimental-, or furniture design during the post-war period. Embodied by his alter-ego –Monsieur Hulot,- Tati curiously interacts with the new geometric constructions of neutral facades and questions the break with the French building tradition, highlighting the opposition between the nostalgic past and modernity of the 50s and 60s, affected by the fierce consumerism of the new welfare state. The functional, volumetric, aesthetic and even chromatic confrontation between both built worlds –traditional vs modern- invites the viewer to an exercise of meditation and criticism on the European modern architecture of that period. Tati’s film look is particularly focused on two basic concepts: on the one hand, his attention addresses Le Corbusier’s famous mechanistic house which is materialized in the ultra-modern Arpel house (Mon Oncle, 1958) and designed, in turn, when the development of other important experimental dwelling prototypes like Alison and Peter Smithson’s House of the Future or Jean Prouvé´s houses was taking place. It must be highlighted that Jacques Tati’s criticism was not addressed to modern architecture itself but to the wrong use that citizens could make of it. On the other hand, Tati focuses on the Miesian prism through the office buildings that shape the city of Tativille in Playtime (1967). It was a big, modern city built specifically for the film shooting, and based on the almost identical residential and tertiary urban fabrics already active in the main European and American capitals those years. Tativille would work as an autonomous city, having several facilities at its disposal and with the goal of getting integrated and consolidated into the Parisian urban weave. However, its final use was, unfortunately, quite different. In conclusion, an analysis of Jacques Tati’s film production allows for an approach to modern post-war architecture and urbanism, as well as to the socio-economic context that favoured its growth and expansion. As a result of this, Jacques Tati’s film production constitutes a suitable visual tool which, even nowadays, is consulted and shown due to its clarity and humour, and at the same time invites citizens –viewers- to participate in an architectural criticism exercise that, so far, had been reserved to architects.