Research on wide-bandgap intermediate band solar cells and development of experimental techniques for their characterization

El trabajo que ha dado lugar a esta Tesis Doctoral se enmarca en la invesitagación en células solares de banda intermedia (IBSCs, por sus siglas en inglés). Se trata de un nuevo concepto de célula solar que ofrece la posibilidad de alcanzar altas eficiencias de conversión fotovoltaica. Hasta ahora, se han demostrado de manera experimental los fundamentos de operación de las IBSCs; sin embargo, esto tan sólo has sido posible en condicines de baja temperatura. El concepto de banda intermedia (IB, por sus siglas en inglés) exige que haya desacoplamiento térmico entre la IB y las bandas de valencia y conducción (VB and CB, respectivamente, por sus siglas en inglés). Los materiales de IB actuales presentan un acoplamiento térmico demasiado fuerte entre la IB y una de las otras dos bandas, lo cual impide el correcto funcionamiento de las IBSCs a temperatura ambiente. En el caso particular de las IBSCs fabricadas con puntos cuánticos (QDs, por sus siglas en inglés) de InAs/GaAs - a día de hoy, la tecnología de IBSC más estudiada - , se produce un rápido intercambio de portadores entre la IB y la CB, por dos motivos: (1) una banda prohibida estrecha (< 0.2 eV) entre la IB y la CB, E^, y (2) la existencia de niveles electrónicos entre ellas. El motivo (1) implica, a su vez, que la máxima eficiencia alcanzable en estos dispositivos es inferior al límite teórico de la IBSC ideal, en la cual E^ = 0.71 eV. En este contexto, nuestro trabajo se centra en el estudio de IBSCs de alto gap (o banda prohibida) fabricadsas con QDs, o lo que es lo mismo, QD-IBSCs de alto gap. Hemos fabricado e investigado experimentalmente los primeros prototipos de QD-IBSC en los que se utiliza AlGaAs o InGaP para albergar QDs de InAs. En ellos demostramos une distribución de gaps mejorada con respecto al caso de InAs/GaAs. En concreto, hemos medido valores de E^ mayores que 0.4 eV. En los prototipos de InAs/AlGaAs, este incremento de E^ viene acompaado de un incremento, en más de 100 meV, de la energía de activación del escape térmico. Además, nuestros dispositivos de InAs/AlGaAs demuestran conversión a la alza de tensión; es decir, la producción de una tensión de circuito abierto mayor que la energía de los fotones (dividida por la carga del electrón) de un haz monocromático incidente, así como la preservación del voltaje a temperaura ambiente bajo iluminación de luz blanca concentrada. Asimismo, analizamos el potencial para detección infrarroja de los materiales de IB. Presentamos un nuevo concepto de fotodetector de infrarrojos, basado en la IB, que hemos llamado: fotodetector de infrarrojos activado ópticamente (OTIP, por sus siglas en inglés). Nuestro novedoso dispositivo se basa en un nuevo pricipio físico que permite que la detección de luz infrarroja sea conmutable (ON y OFF) mediante iluminación externa. Hemos fabricado un OTIP basado en QDs de InAs/AlGaAs con el que demostramos fotodetección, bajo incidencia normal, en el rango 2-6/xm, activada ópticamente por un diodoe emisor de luz de 590 nm. El estudio teórico del mecanismo de detección asistido por la IB en el OTIP nos lleva a poner en cuestión la asunción de quasi-niveles de Fermi planos en la zona de carga del espacio de una célula solar. Apoyados por simuaciones a nivel de dispositivo, demostramos y explicamos por qué esta asunción no es válida en condiciones de corto-circuito e iluminación. También llevamos a cabo estudios experimentales en QD-IBSCs de InAs/AlGaAs con la finalidad de ampliar el conocimiento sobre algunos aspectos de estos dispositivos que no han sido tratados aun. En particular, analizamos el impacto que tiene el uso de capas de disminución de campo (FDLs, por sus siglas en inglés), demostrando su eficiencia para evitar el escape por túnel de portadores desde el QD al material anfitrión. Analizamos la relación existente entre el escape por túnel y la preservación del voltaje, y proponemos las medidas de eficiencia cuántica en función de la tensión como una herramienta útil para evaluar la limitación del voltaje relacionada con el túnel en QD-IBSCs. Además, realizamos medidas de luminiscencia en función de la temperatura en muestras de InAs/GaAs y verificamos que los resltados obtenidos están en coherencia con la separación de los quasi-niveles de Fermi de la IB y la CB a baja temperatura. Con objeto de contribuir a la capacidad de fabricación y caracterización del Instituto de Energía Solar de la Universidad Politécnica de Madrid (IES-UPM), hemos participado en la instalación y puesta en marcha de un reactor de epitaxia de haz molecular (MBE, por sus siglas en inglés) y el desarrollo de un equipo de caracterización de foto y electroluminiscencia. Utilizando dicho reactor MBE, hemos crecido, y posteriormente caracterizado, la primera QD-IBSC enteramente fabricada en el IES-UPM. ABSTRACT The constituent work of this Thesis is framed in the research on intermediate band solar cells (IBSCs). This concept offers the possibility of achieving devices with high photovoltaic-conversion efficiency. Up to now, the fundamentals of operation of IBSCs have been demonstrated experimentally; however, this has only been possible at low temperatures. The intermediate band (IB) concept demands thermal decoupling between the IB and the valence and conduction bands. Stateof- the-art IB materials exhibit a too strong thermal coupling between the IB and one of the other two bands, which prevents the proper operation of IBSCs at room temperature. In the particular case of InAs/GaAs quantum-dot (QD) IBSCs - as of today, the most widely studied IBSC technology - , there exist fast thermal carrier exchange between the IB and the conduction band (CB), for two reasons: (1) a narrow (< 0.2 eV) energy gap between the IB and the CB, EL, and (2) the existence of multiple electronic levels between them. Reason (1) also implies that maximum achievable efficiency is below the theoretical limit for the ideal IBSC, in which EL = 0.71 eV. In this context, our work focuses on the study of wide-bandgap QD-IBSCs. We have fabricated and experimentally investigated the first QD-IBSC prototypes in which AlGaAs or InGaP is the host material for the InAs QDs. We demonstrate an improved bandgap distribution, compared to the InAs/GaAs case, in our wide-bandgap devices. In particular, we have measured values of EL higher than 0.4 eV. In the case of the AlGaAs prototypes, the increase in EL comes with an increase of more than 100 meV of the activation energy of the thermal carrier escape. In addition, in our InAs/AlGaAs devices, we demonstrate voltage up-conversion; i. e., the production of an open-circuit voltage larger than the photon energy (divided by the electron charge) of the incident monochromatic beam, and the achievement of voltage preservation at room temperature under concentrated white-light illumination. We also analyze the potential of an IB material for infrared detection. We present a IB-based new concept of infrared photodetector that we have called the optically triggered infrared photodetector (OTIP). Our novel device is based on a new physical principle that allows the detection of infrared light to be switched ON and OFF by means of an external light. We have fabricated an OTIP based on InAs/AlGaAs QDs with which we demonstrate normal incidence photodetection in the 2-6 /xm range optically triggered by a 590 nm light-emitting diode. The theoretical study of the IB-assisted detection mechanism in the OTIP leads us to questioning the assumption of flat quasi-Fermi levels in the space-charge region of a solar cell. Based on device simulations, we prove and explain why this assumption is not valid under short-circuit and illumination conditions. We perform new experimental studies on InAs/GaAs QD-IBSC prototypes in order to gain knowledge on yet unexplored aspects of the performance of these devices. Specifically, we analyze the impact of the use of field-damping layers, and demonstrate this technique to be efficient for avoiding tunnel carrier escape from the QDs to the host material. We analyze the relationship between tunnel escape and voltage preservation, and propose voltage-dependent quantum efficiency measurements as an useful technique for assessing the tunneling-related limitation to the voltage preservation of QD-IBSC prototypes. Moreover, we perform temperature-dependent luminescence studies on InAs/GaAs samples and verify that the results are consistent with a split of the quasi-Fermi levels for the CB and the IB at low temperature. In order to contribute to the fabrication and characterization capabilities of the Solar Energy Institute of the Universidad Polite´cnica de Madrid (IES-UPM), we have participated in the installation and start-up of an molecular beam epitaxy (MBE) reactor and the development of a photo and electroluminescence characterization set-up. Using the MBE reactor, we have manufactured and characterized the first QD-IBSC fully fabricated at the IES-UPM.

Recent progress in research on tungsten materials for nuclear fusion applications in Europe

The current magnetic confinement nuclear fusion power reactor concepts going beyond ITER are based on assumptions about the availability of materials with extreme mechanical, heat, and neutron load capacity. In Europe, the development of such structural and armour materials together with the necessary production, machining, and fabrication technologies is pursued within the EFDA long-term fusion materials programme. This paper reviews the progress of work within the programme in the area of tungsten and tungsten alloys. Results, conclusions, and future projections are summarized for each of the programme´s main subtopics, which are: (1) fabrication, (2) structural W materials, (3) W armour materials, and (4) materials science and modelling. It gives a detailed overview of the latest results on materials research, fabrication processes, joining options, high heat flux testing, plasticity studies, modelling, and validation experiments.

Assessment of damage domains of the High-Temperature Engineering Test Reactor (HTTR)

This paper presents an assessment analysis of damage domains of the 30 MWth prototype High-Temperature Engineering Test Reactor (HTTR) operated by the Japan Atomic Energy Agency (JAEA). For this purpose, an in-house deterministic risk assessment computational tool was developed based on the Theory of Stimulated Dynamics (TSD). To illustrate the methodology and applicability of the developed modelling approach, assessment results of a control rod (CR) withdrawal accident during subcritical conditions are presented and compared with those obtained by the JAEA.

Metodología de simulación numérica de la transferencia de calor por ebullición en película en torno a superficies esféricas refrigeradas por líquido saturado en convección libre: aplicación a reactores nucleares de lecho de bolas (PBR)

En la presente Tesis se realizó un análisis numérico, usando el código comercial Ansys-Fluent, de la refrigeración de una bola de combustible de un reactor de lecho de bolas (PBR, por sus siglas en inglés), ante un escenario de emergencia en el cual el núcleo sea desensamblado y las bolas se dejen caer en una piscina de agua, donde el mecanismo de transferencia de calor inicialmente sería la ebullición en película, implicando la convección y la radiación al fluido. Previamente se realizaron pruebas de validación, comparando los resultados numéricos con datos experimentales disponibles en la literatura para tres geometrías diferentes, lo cual permitió seleccionar los esquemas y modelos numéricos con mejor precisión y menor costo computacional. Una vez identificada la metodología numérica, todas las pruebas de validación fueron ampliamente satisfactorias, encontrándose muy buena concordancia en el flujo de calor promedio con los datos experimentales. Durante estas pruebas de validación se lograron caracterizar numéricamente algunos parámetros importantes en la ebullición en película con los cuales existen ciertos niveles de incertidumbre, como son el factor de acoplamiento entre convección y radiación, y el factor de corrección del calor latente de vaporización. El análisis térmico de la refrigeración de la bola del reactor por ebullición en película mostró que la misma se enfría, a pesar del calor de decaimiento, con una temperatura superficial de la bola que desciende de forma oscilatoria, debido al comportamiento inestable de la película de vapor. Sin embargo, la temperatura de esta superficie tiene una buena uniformidad, notándose que las áreas mejor y peor refrigeradas están localizadas en la parte superior de la bola. Se observó la formación de múltiples domos de vapor en diferentes posiciones circunferenciales, lo cual causa que el área más caliente de la superficie se localice donde se forman los domos más grandes. La separación entre los domos de vapor fue consistente con la teoría hidrodinámica, con la adición de que la separación entre domos se reduce a medida que evolucionan y crecen, debido a la curvatura de la superficie. ABSTRACT A numerical cooling analysis of a PBR fuel pebble, after an emergency scenario in which the nucleus disassembly is made and the pebbles are dropped into a water pool, transmitting heat by film boiling, involving convection and radiation to the fluid, is carried out in this Thesis. First, were performed validation tests comparing the numerical results with experimental works available for three different geometries, which allowed the selection of numerical models and schemes with better precision and lower computational cost. Once identified the numerical methodology, all validation tests were widely satisfactory, finding very good agreement with experimental works in average heat flux. During these validation tests were achieved numerically characterize some important parameters in film boiling with which there are certain levels of uncertainty, such as the coupling factor between convection and radiation, and the correction factor of the latent heat of vaporization. The thermal analysis of pebble cooling by film boiling shows that despite its decay heat, cooling occurs, with pebble surface temperature descending from an oscillatory manner, due to the instability of the vapor film. However, the temperature of this surface has a good uniformity, noting that the best and worst refrigerated area is located at the top of the pebble. The formation of multiple vapor domes at different circumferential positions is observed, which cause that the hottest area of the surface was located where biggest vapor domes were formed. The separation between vapor domes was consistent with the hydrodynamic theory, with the addition that the separation is reduced as the vapor dome evolves and grows, due to the surface curvature.

Simulating the remote handling of the blanket segments in DEMO fusion reactor with thermo-mechanical meshfree multibody dynamics

The present study shows a first approach to the simulation of the remote handling oper- ation which takes into account the thermal and flexible behavior of the blanket segments and its implications on the remote handling equipment, in order to validate and improve its design.

The N-terminal domain of PsaF: Precise recognition site for binding and fast electron transfer from cytochrome c6 and plastocyanin to photosystem I of Chlamydomonas reinhardtii

The PsaF-deficient mutant 3bF of Chlamydomonas reinhardtii was used to modify PsaF by nuclear transformation and site-directed mutagenesis. Four lysine residues in the N-terminal domain of PsaF, which have been postulated to form the positively charged face of a putative amphipathic α-helical structure were altered to K12P, K16Q, K23Q, and K30Q. The interactions between plastocyanin (pc) or cytochrome c6 (cyt c6) and photosystem I (PSI) isolated from wild type and the different mutants were analyzed using crosslinking techniques and flash absorption spectroscopy. The K23Q change drastically affected crosslinking of pc to PSI and electron transfer from pc and cyt c6 to PSI. The corresponding second order rate constants for binding of pc and cyt c6 were reduced by a factor of 13 and 7, respectively. Smaller effects were observed for mutations K16Q and K30Q, whereas in K12P the binding was not changed relative to wild type. None of the mutations affected the half-life of the microsecond electron transfer performed within the intermolecular complex between the donors and PSI. The fact that these single amino acid changes within the N-terminal domain of PsaF have different effects on the electron transfer rate constants and dissociation constants for both electron donors suggests the existence of a rather precise recognition site for pc and cyt c6 that leads to the stabilization of the final electron transfer complex through electrostatic interactions.

Participation of the nuclear cap binding complex in pre-mRNA 3′ processing

Communication between the 5′ and 3′ ends is a common feature of several aspects of eukaryotic mRNA metabolism. In the nucleus, the pre-mRNA 5′ end is bound by the nuclear cap binding complex (CBC). This RNA–protein complex plays an active role in both splicing and RNA export. We provide evidence for participation of CBC in the processing of the 3′ end of the message. Depletion of CBC from HeLa cell nuclear extract strongly reduced the endonucleolytic cleavage step of the cleavage and polyadenylation process. Cleavage was restored by addition of recombinant CBC. CBC depletion was found to reduce the stability of poly(A) site cleavage complexes formed in nuclear extract. We also provide evidence that the communication between the 5′ and 3′ ends of the pre-mRNA during processing is mediated by the physical association of the CBC/cap complex with 3′ processing factors bound at the poly(A) site. These observations, along with previous data on the function of CBC in splicing, illustrate the key role played by CBC in pre-mRNA recognition and processing. The data provides further support for the hypothesis that pre-mRNAs and mRNAs may exist and be functional in the form of “closed-loops,” due to interactions between factors bound at their 5′ and 3′ ends.

Nuclear hourglass technique: An approach that detects electrically open nuclear pores in Xenopus laevis oocyte

Nuclear pore complexes (NPCs) mediate both active transport and passive diffusion across the nuclear envelope (NE). Determination of NE electrical conductance, however, has been confounded by the lack of an appropriate technical approach. The nuclear patch clamp technique is restricted to preparations with electrically closed NPCs, and microelectrode techniques fail to resolve the extremely low input resistance of large oocyte nuclei. To address the problem, we have developed an approach for measuring the NE electrical conductance of Xenopus laevis oocyte nuclei. The method uses a tapered glass tube, which narrows in its middle part to 2/3 of the diameter of the nucleus. The isolated nucleus is sucked into the narrow part of the capillary by gentle fluid movement, while the resulting change in electrical resistance is monitored. NE electrical conductance was unexpectedly large (7.9 ± 0.34 S/cm2). Evaluation of NPC density by atomic force microscopy showed that this conductance corresponded to 3.7 × 106 NPCs. In contrast to earlier conclusions drawn from nuclear patch clamp experiments, NPCs were in an electrically “open” state with a mean single NPC electrical conductance of 1.7 ± 0.07 nS. Enabling or blocking of active NPC transport (accomplished by the addition of cytosolic extracts or gp62-directed antibodies) revealed this large NPC conductance to be independent of the activation state of the transport machinery located in the center of NPCs. We conclude that peripheral channels, which are presumed to reside in the NPC subunits, establish a high ionic permeability that is virtually independent of the active protein transport mechanism.

Conversion of DNA gyrase into a conventional type II topoisomerase

DNA gyrase is unique among topoisomerases in its ability to introduce negative supercoils into closed-circular DNA. We have demonstrated that deletion of the C-terminal DNA-binding domain of the A subunit of gyrase gives rise to an enzyme that cannot supercoil DNA but relaxes DNA in an ATP-dependent manner. Novobiocin, a competitive inhibitor of ATP binding by gyrase, inhibits this reaction. The truncated enzyme, unlike gyrase, does not introduce a right-handed wrap when bound to DNA and stabilizes DNA crossovers; characteristics reminiscent of conventional type II topoisomerases. This new enzyme form can decatenate DNA circles with increased efficiency compared with intact gyrase and, as a result, can complement the temperature-sensitive phenotype of a parCts mutant. Thus these results suggest that the unique properties of DNA gyrase are attributable to the wrapping of DNA around the C-terminal DNA-binding domains of the A subunits and provide an insight into the mechanism of type II topoisomerases.

Retinoic acid induces sodium/iodide symporter gene expression and radioiodide uptake in the MCF-7 breast cancer cell line

The sodium/iodide symporter (NIS) stimulates iodide uptake in normal lactating breast, but is not known to be active in nonlactating breast or breast cancer. We studied NIS gene regulation and iodide uptake in MCF-7 cells, an estrogen receptor (ER)-positive human breast cancer cell line. All-trans retinoic acid (tRA) treatment stimulated iodide uptake in a time- and dose-dependent fashion up to ≈9.4-fold above baseline. Stimulation with selective retinoid compounds indicated that the induction of iodide uptake was mediated by retinoic acid receptor. Treatment with tRA markedly stimulated NIS mRNA and immunoreactive protein (≈68 kDa). tRA stimulated NIS gene transcription ≈4-fold, as shown by nuclear run-on assay. No induction of iodide uptake was observed with RA treatment of an ER-negative human breast cancer cell line, MDA-MB 231, or a normal human breast cell line, MCF-12A. The iodide efflux rate of tRA-treated MCF-7 cells was slow (t1/2 = 24 min), compared with that in FRTL-5 thyroid cells (t1/2 = 3.9 min), favoring iodide retention in MCF-7 cells. An in vitro clonogenic assay demonstrated selective cytotoxicity with 131I after tRA stimulation of MCF-7 cells. tRA up-regulates NIS gene expression and iodide uptake in an ER-positive breast cancer cell line. Stimulation of radioiodide uptake after systemic retinoid treatment may be useful for diagnosis and treatment of some differentiated breast cancers.

Identification of a nuclear domain with deacetylase activity

Here, we describe the identification and characterization of a nuclear body (matrix-associated deacetylase body) whose formation and integrity depend on deacetylase activity. Typically, there are 20–40 0.5-μM bodies per nucleus, although the size and number can vary substantially. The structure appears to contain both class I and the recently described class II histone deacetylases (HDAC)5 and 7 along with the nuclear receptor corepressors SMRT (silencing mediator for retinoid and thyroid receptor) and N-CoR (nuclear receptor corepressor). Addition of the deacetylase inhibitors trichostatin A and sodium butyrate completely disrupt these nuclear bodies, providing a demonstration that the integrity of a nuclear body is enzyme dependent. We demonstrate that HDAC5 and 7 can associate with at least 12 distinct proteins, including several members of the NuRD and Sin3A repression complexes, and appear to define a new but related complex.

VIPP1, a nuclear gene of Arabidopsis thaliana essential for thylakoid membrane formation

The conversion of light to chemical energy by the process of photosynthesis is localized to the thylakoid membrane network in plant chloroplasts. Although several pathways have been described that target proteins into and across the thylakoids, little is known about the origin of this membrane system or how the lipid backbone of the thylakoids is transported and fused with the target membrane. Thylakoid biogenesis and maintenance seem to involve the flow of membrane elements via vesicular transport. Here we show by mutational analysis that deletion of a single gene called VIPP1 (vesicle-inducing protein in plastids 1) is deleterious to thylakoid membrane formation. Although VIPP1 is a hydrophilic protein it is found in both the inner envelope and the thylakoid membranes. In VIPP1 deletion mutants vesicle formation is abolished. We propose that VIPP1 is essential for the maintenance of thylakoids by a transport pathway not previously recognized.

The age of the universe from nuclear chronometers

An overview is presented of the current situation regarding radioactive dating of the matter of which our Galaxy is comprised. A firm lower bound on the age from nuclear chronometers of ≈9–10 Gyr is entirely consistent with age determinations from globular clusters and white dwarf cooling histories. The reasonable assumption of an approximately uniform nucleosynthesis rate yields an age for the Galaxy of 12.8 ± 3 Gyr, which again is consistent with current determinations from other methods.

Maize as a model for the evolution of plant nuclear genomes

The maize genome is replete with chromosomal duplications and repetitive DNA. The duplications resulted from an ancient polyploid event that occurred over 11 million years ago. Based on DNA sequence data, the polyploid event occurred after the divergence between sorghum and maize, and hence the polyploid event explains some of the difference in DNA content between these two species. Genomic rearrangement and diploidization followed the polyploid event. Most of the repetitive DNA in the maize genome is retrotransposable elements, and they comprise 50% of the genome. Retrotransposon multiplication has been relatively recent—within the last 5–6 million years—suggesting that the proliferation of retrotransposons has also contributed to differences in DNA content between sorghum and maize. There are still unanswered questions about repetitive DNA, including the distribution of repetitive DNA throughout the genome, the relative impacts of retrotransposons and chromosomal duplication in plant genome evolution, and the hypothesized correlation of duplication events with transposition. Population genetic processes also affect the evolution of genomes. We discuss how centromeric genes should, in theory, contain less genetic diversity than noncentromeric genes. In addition, studies of diversity in the wild relatives of maize indicate that different genes have different histories and also show that domestication and intensive breeding have had heterogeneous effects on genetic diversity across genes.

Direct conversion of an oligopeptide from a β-sheet to an α-helix: A model for amyloid formation

A 16-amino acid oligopeptide forms a stable β-sheet structure in water. In physiological solutions it is able to self-assemble to form a macroscopic matrix that stains with Congo red. On raising the temperature of the aqueous solution above 70°C, an abrupt structural transition occurs in the CD spectra from a β-sheet to a stable α-helix without a detectable random-coil intermediate. With cooling, it retained the α-helical form and took several weeks at room temperature to partially return to the β-sheet form. Slow formation of the stable β-sheet structure thus shows kinetic irreversibility. Such a formation of very stable β-sheet structures is found in the amyloid of a number of neurological diseases. This oligopeptide could be a model system for studying the protein conformational changes that occurs in scrapie or Alzheimer disease. The abrupt and direct conversion from a β-sheet to an α-helix may also be found in other processes, such as protein folding and protein–protein interaction. Furthermore, such drastic structure changes may also be exploited in biomaterials designed as sensors to detect environmental changes.