Steric engineering of metal-halide perovskites with tunable optical band gaps

Owing to their high energy-conversion efficiency and inexpensive fabrication routes, solar cells based on metal-organic halide perovskites have rapidly gained prominence as a disruptive technology. An attractive feature of perovskite absorbers is the possibility of tailoring their properties by changing the elemental composition through the chemical precursors. In this context, rational in silico design represents a powerful tool for mapping the vast materials landscape and accelerating discovery. Here we show that the optical band gap of metal-halide perovskites, a key design parameter for solar cells, strongly correlates with a simple structural feature, the largest metal-halide-metal bond angle. Using this descriptor we suggest continuous tunability of the optical gap from the mid-infrared to the visible. Precise band gap engineering is achieved by controlling the bond angles through the steric size of the molecular cation. Based on these design principles we predict novel low-gap perovskites for optimum photovoltaic efficiency, and we demonstrate the concept of band gap modulation by synthesising and characterising novel mixed-cation perovskites.Comment: This manuscript was submitted for publication on March 6th, 2014. Many of the results presented in this manuscript were presented at the International Conference on Solution processed Semiconductor Solar Cells, held in Oxford, UK, on 10-12 September 2014. The manuscript is 37 pages long and contains 8 figure

Co-purification of a small RNA species with multicatalytic proteinase (proteasome) from rat liver

AbstractPrevious studies have come to different conclusions about the presence of RNA in particles known variously as prosomes, proteasomes or multicatalytic proteinase (MCP). To determine the reason for this, MCP was isolated from rat liver by 4 different purification protocols. One major band of RNA, about 80 nucleotides in length, co-purified in all preparations. The amount of RNA detected was less than one molecule per MCP particle suggesting that there may be more than one population of MCP in rat liver cells

Stoichiometry of a regulatory splicing complex revealed by single-molecule analyses

Splicing is regulated by complex interactions of numerous RNA-binding proteins. The molecular mechanisms involved remain elusive, in large part because of ignorance regarding the numbers of proteins in regulatory complexes. Polypyrimidine tract-binding protein (PTB), which regulates tissue-specific splicing, represses exon 3 of α-tropomyosin through distant pyrimidine-rich tracts in the flanking introns. Current models for repression involve either PTB-mediated looping or the propagation of complexes between tracts. To test these models, we used single-molecule approaches to count the number of bound PTB molecules both by counting the number of bleaching steps of GFP molecules linked to PTB within complexes and by analysing their total emissions. Both approaches showed that five or six PTB molecules assemble. Given the domain structures, this suggests that the molecules occupy primarily multiple overlapping potential sites in the polypyrimidine tracts, excluding propagation models. As an alternative to direct looping, we propose that repression involves a multistep process in which PTB binding forms small local loops, creating a platform for recruitment of other proteins that bring these loops into close proximity

To form a union without having a child. The lengthening of the initial period of life in union before parenthood. A study based on European FFS data

Comunicació presentada a l'European Population Conference: "Migration and Migrants in Europe"(Sessió 53). Organitzat per l'European Association for Population Studies (EAPS); Centre d'Estudis Demogràfics. Barcelona, del 9 al 12 de juliol de 2008.The authors of the Second Demographic Transition scheme single out the postponement of the age at first childbearing as the main effect of the changes in habits of young adults associated with this transition. This postponement is accompanied by an increase in the length of the initial period of life in partnership when the couple has no plan yet to have children. This change is made possible by the use of contraceptive means by people living in partnerships in order to delay first childbearing. This is in sharp contrast with the First Demographic Transition, which was also characterized by the extension of the use of contraceptive means, but only after the birth of children. So contraception was used then to control fertility, when it is used nowadays to extend the period of life when no irreversible decisions, like having a child, have been made yet. In this work, we study various dimensions of this postponement of childbearing by couples. First, we try to quantify the magnitude of the increase in the duration of this initial period, when the couple delays parenthood. Second we have a look at the way this change in fertility behaviours in the first years of union change the duration model that was typical at the end of the First Demographic Transition. Finally, we investigate the possible determinants of the increase of this initial period using data on time spent by women studying and working, and of the transition from cohabitation to marriage. We use data from Fertility and Families Surveys for 17 countries and apply life table techniques and proportional hazard modelling.Un dels principals canvis associats a la Segona Transició Demogràfica és el retard en l'edat de tenir el primer fill, allargant-se el període d'unió sense plans de tenir-los. El retard és possible gràcies a la utilització dels mitjans anticonceptius, fet que contrasta amb la Primera Transició Demogràfica, a on els mitjans anticonceptius s'empraven després del naixement dels fills. En aquest estudi s'analitzen les diverses dimensions d'aquest ajornament. En primer lloc, es quantifica l'augment temporal d'aquest període inicial sense fills; en segon lloc s'analitza la forma en què aquest canvi modifica el model de la Primera Transició Demogràfica; finalment, s'apunten possibles determinants, emprant dades sobre el temps dedicat per les dones a estudiar i a treballar, i de la transició de la cohabitació al matrimoni. La font bàsica d'informació és la Fertility and Families Surveys , per a 17 països.Uno de los principales cambios asociados a la Segunda Transición Demográfica, es el aplazamiento en la edad de tener el primer hijo, ampliándose el período de unión sin planes de tenerlos. La demora es posible gracias a la utilización de los medios anticonceptivos, hecho que contrasta con la Primera Transición Demográfica, donde los medios anticonceptivos se utilizaban después del nacimiento de los hijos. En este estudio se analizan las dimensiones de este aplazamiento. En primer lugar, se cuantifica el aumento temporal de esta etapa inicial sin hijos; en segundo lugar, se analiza la forma en que este cambio modifica el modelo de la Primera Transición Demográfica; finalmente, se apuntan posibles determinantes utilizando datos sobre el tiempo dedicado por las mujeres a estudiar y a trabajar, y de la transición de la cohabitación al matrimonio. La fuente básica de información es la Fertility and Families Surveys, para 17 países

Strong cosmic censorship in de Sitter space

Recent work indicates that the strong cosmic censorship hypothesis is violated by nearly extremal Reissner-Nordstrom-de Sitter black holes. It was argued that perturbations of such a black hole decay sufficiently rapidly that the perturbed spacetime can be extended across the Cauchy horizon as a weak solution of the equations of motion. In this paper we consider the case of Kerr-de Sitter black holes. We find that, for any non-extremal value of the black hole parameters, there are quasinormal modes which decay sufficiently slowly to ensure that strong cosmic censorship is respected. Our analysis covers both scalar field and linearized gravitational perturbations

Relaxed Current Matching Requirements in Highly Luminescent Perovskite Tandem Solar Cells and Their Fundamental Efficiency Limits

Perovskite-based tandem solar cells are of increasing interest as they approach commercialization. Here we use experimental parameters from optical spectroscopy measurements to calculate the limiting efficiency of perovskite–silicon and all-perovskite two-terminal tandems, employing currently available bandgap materials, as 42.0% and 40.8%, respectively. We show luminescence coupling between subcells (the optical transfer of photons from the high-bandgap to low-bandgap subcell) relaxes current matching when the high-bandgap subcell is a luminescent perovskite. We calculate that luminescence coupling becomes important at charge trapping rates (≤106 s–1) already being achieved in relevant halide perovskites. Luminescence coupling increases flexibility in subcell thicknesses and tolerance to different spectral conditions. For maximal benefit, the high-bandgap subcell should have the higher short-circuit current under average spectral conditions. This can be achieved by reducing the bandgap of the high-bandgap subcell, allowing wider, unstable bandgap compositions to be avoided. Lastly, we visualize luminescence coupling in an all-perovskite tandem through cross-section luminescence imaging