PEI based selective contact for different photovoltaic absorber materials

La investigación en tecnologías fotovoltaicas se ha basado mayoritariamente en la teoría de la unión PN pero, gracias a la interpretación mediante potenciales electroquímicos más que campos eléctricos, la visión de una célula solar como un absorbedor más dos filtros ha ganado peso en los últimos años. Por este motivo, la investigación en nuevos contactos selectivos es, actualmente, de gran relevancia. Sobre todo para aquellos que no incorporan procesos a altas temperaturas, ni materiales críticos o tóxicos. En el trabajo que se presenta a continuación se ha podido constatar los beneficios de utilizar una unión "Metal-Dipolo-Semiconductor" con Polietilenimina (PEI) como contacto selectivo de electrones. Con el fin de caracterizar la calidad del contacto se han realizado diferentes medidas eléctricas y ópticas, con los resultados de las cuales se ha elaborado un modelo teórico que explica el comportamiento detrás del contacto de PEI. Además, se han fabricado diferentes células solares a partir de varios absorbedores, encontrando un contacto libre de materiales críticos y de procesos altamente costosos con el que se han obtenido resultados bastante prometedores. Actualmente, las células fotovoltaicas basadas en silicio cristalino (c-Si) dominan ampliamente el mercado gracias a sus eficiencias comerciales superiores al 15%. Pero, debido a que la mayoría de estos dispositivos utilizan interficies dopadas y contactos metálicos directamente depositados, el coste energético vinculado a su fabricación es muy elevado. Depositando fácilmente una fina capa de PEI entre el semiconductor y el metal mediante la técnica de "spin-coating", somos capaces de fabricar un contacto de gran eficiencia sin necesidad de dopar por difusión térmica la interfície del absorbedor. Además, al estudiar este contacto orgánico sobre diferentes absorbedores de capa fina, donde el uso de materiales críticos y tóxicos es uno de sus máximos inconvenientes, hemos podido presentar un contacto con unas prestaciones muy similares a las del contacto de electrones clásico de CdS, pero sin involucrar a este.La recerca en tecnologies fotovoltaiques s'ha basat majoritàriament en la teoria de la unió PN però, gràcies a la interpretació mitjançant potencials electroquímics més que camps elèctrics, la visió d'una cèl·lula solar com a un absorbidor més dos filtres ha guanyat pes en els últims anys. Per aquest motiu, la investigació en nous contactes selectius és, actualment, de gran rellevància. Sobretot per aquells que no incorporen processos a altes temperatures, ni materials crítics o tòxics. En el treball que es presenta a continuació s'ha pogut constatar els beneficis d'utilitzar una unió "Metall-Dipol-Semiconductor" amb Polietilenimina (PEI) com a contacte selectiu d'electrons. Per tal de caracteritzar la qualitat del contacte s'han realitzat diferents mesures elèctriques i òptiques, amb els resultats de les quals s'ha elaborat un model teòric explicant el comportament darrera el contacte de PEI. A més, s'han fabricat diferents cèl·lules solars a partir de diversos absorbidors, trobant un contacte lliure de materials crítics i de processos altament costosos amb el qual s'han obtingut resultats força prometedors. Actualment, les cèl·lules fotovoltaiques basades en silici cristal·lí (c-Si) dominen àmpliament el mercat gràcies a les seves eficiències comercials superiors al 15%. Però, degut a que la majoria d'aquests dispositius utilitzen interfícies dopades i contactes metàl·lics directament dipositats, el cost energètic vinculat a la seva fabricació és molt elevat. Dipositant fàcilment una fina capa de PEI entre el semiconductor i el metall mitjançant la tècnica de "spin-coating", som capaços de fabricar un contacte de gran eficiència sense necessitat de dopar per difusió tèrmica la interfície de l'absorbidor. A més, al estudiar aquest contacte orgànic sobre diferents absorbidors de capa fina, on l'ús de materials crítics i tòxics és un dels seus màxims inconvenients, hem pogut presentar un contacte amb unes prestacions molt similars a les del contacte d'electrons clàssic de CdS, però sense involucrar aquest.Research in photovoltaic (PV) devices has usually focused on PN junction theory. However, with the new perspective on electronic transport driven by electrochemical potentials rather than electrical fields, the understanding of solar cells as the combination of an absorber and two selective contacts has gained strength. As a consequence, there has been a growing interest in researching new selective contacts to obtain high efficiency and low-temperature processed photovoltaic devices that can reduce their cost and ecological footprint. In this work, we report the promising performance obtained with a "Metal-Dipolar-Semiconductor" junction based on Polyethylenimine (PEI) as electron transport layer. Electrical and optical characterization have been performed to determine the contact quality over different absorbers, and a theoretical model explaining these results has been presented. Different photovoltaic devices have been fabricated and characterized, finding promising free-dopant and CRM-free architectures for different absorbers. Currently, the global photovoltaic market is dominated by crystalline silicon (c-Si) based technologies with heavily doped, directly metallized contacts, which implies high-temperature steps. With the simple deposition of PEI between the semiconductor and the metal through spin-coating technique, we were able to obtain high-performance contacts without doping. In addition, regarding the study over thin-film absorbers, where the use of critical raw materials (CRM) is one of their main drawbacks, we proved that a stack combining a zinc oxide buffer layer and the polymeric interlayer yields outputs comparable to the classical CdS selective contact

Large Deviations for Stochastic Volterra Equations

This is the publisher's version, also available electronically from http://www.jstor.org/stable/3318580?origin=crossref&seq=1#page_scan_tab_contents.No abstract is available for this item

Cristalls de spin

L'estudi dels cristalls de spin iniciat a partir dels anys setanta ha tingut darrerament grans avenços en termes matemàtics. En aquest article presentem, de manera divulgativa, els conceptes bàsics d'aquesta àrea, detallem el seu marc matemàtic, descrivim els models clàssics que considerem més importants i expliquem algunes de les tècniques que s'utilitzen per a estudiar-los.The study of spin glasses initiated from the seventies has had lately great advances in mathematical terms. In this paper we present in an informative way the basic concepts of this area, detailing its mathematical frame, describing the classical models that we consider more important and explaining some of the techniques that are used to study them

Probabilistic models for vortex filaments based on fractional Brownian motion

This is the published version, also available here: http://dx.doi.org/10.1214/aop/1068646369.We consider a vortex structure based on a three-dimensional fractional Brownian motion with Hurst parameter H>12. We show that the energy H\vspace*{-1pt} has moments of any order under suitable conditions. When H∈(12,13) we prove that the intersection energy Hxy can be decomposed into four terms, one of them being a weighted self-intersection local time of the fractional Brownian motion in R3

The catalytic power of enzymes: Conformational selection or transition state stabilization?

AbstractThe mechanism by which enzymes produce enormous rate enhancements in the reactions they catalyze remains unknown. Two viewpoints, selection of ground state conformations and stabilization of the transition state, are present in the literature in apparent opposition. To provide more insight into current discussion about enzyme efficiency, a two-state model of enzyme catalysis was developed. The model was designed to include both the pre-chemical (ground state conformations) and the chemical (transition state) components of the process for the substrate both in water and in the enzyme. Although the model is of general applicability, the chorismate to prephenate reaction catalyzed by chorismate mutase was chosen for illustrative purposes. The resulting kinetic equations show that the catalytic power of enzymes, quantified as the kcat/kuncat ratio, is the product of two terms: one including the equilibrium constants for the substrate conformational states and the other including the rate constants for the uncatalyzed and catalyzed chemical reactions. The model shows that these components are not mutually exclusive and can be simultaneously present in an enzymic system, being their relative contribution a property of the enzyme. The developed mathematical expressions reveal that the conformational and reaction components of the process perform differently for the translation of molecular efficiency (changes in energy levels) into observed enzymic efficiency (changes in kcat), being, in general, more productive the component involving the transition state

The Use of Virtual Reality in the Study of People's Responses to Violent Incidents

This paper reviews experimental methods for the study of the responses of people to violence in digital media, and in particular considers the issues of internal validity and ecological validity or generalisability of results to events in the real world. Experimental methods typically involve a significant level of abstraction from reality, with participants required to carry out tasks that are far removed from violence in real life, and hence their ecological validity is questionable. On the other hand studies based on field data, while having ecological validity, cannot control multiple confounding variables that may have an impact on observed results, so that their internal validity is questionable. It is argued that immersive virtual reality may provide a unification of these two approaches. Since people tend to respond realistically to situations and events that occur in virtual reality, and since virtual reality simulations can be completely controlled for experimental purposes, studies of responses to violence within virtual reality are likely to have both ecological and internal validity. This depends on a property that we call ‘plausibility’ – including the fidelity of the depicted situation with prior knowledge and expectations. We illustrate this with data from a previously published experiment, a virtual reprise of Stanley Milgram's 1960s obedience experiment, and also with pilot data from a new study being developed that looks at bystander responses to violent incidents