12,811 research outputs found
Atomistic origins of high-performance in hybrid halide perovskite solar cells
The performance of organometallic perovskite solar cells has rapidly
surpassed that of both conventional dye-sensitised and organic photovoltaics.
High power conversion efficiency can be realised in both mesoporous and
thin-film device architectures. We address the origin of this success in the
context of the materials chemistry and physics of the bulk perovskite as
described by electronic structure calculations. In addition to the basic
optoelectronic properties essential for an efficient photovoltaic device
(spectrally suitable band gap, high optical absorption, low carrier effective
masses), the materials are structurally and compositionally flexible. As we
show, hybrid perovskites exhibit spontaneous electric polarisation; we also
suggest ways in which this can be tuned through judicious choice of the organic
cation. The presence of ferroelectric domains will result in internal junctions
that may aid separation of photoexcited electron and hole pairs, and reduction
of recombination through segregation of charge carriers. The combination of
high dielectric constant and low effective mass promotes both Wannier-Mott
exciton separation and effective ionisation of donor and acceptor defects. The
photoferroic effect could be exploited in nanostructured films to generate a
higher open circuit voltage and may contribute to the current-voltage
hysteresis observed in perovskite solar cells.Comment: 6 pages, 5 figure
Spin Signatures of Photogenerated Radical Anions in Polymer-[70]Fullerene Bulk Heterojunctions: High Frequency Pulsed EPR Spectroscopy
Charged polarons in thin films of polymer-fullerene composites are
investigated by light-induced electron paramagnetic resonance (EPR) at 9.5 GHz
(X-band) and 130 GHz (D-band). The materials studied were
poly(3-hexylthiophene) (PHT), [6,6]-phenyl-C61-butyric acid methyl ester
(C60-PCBM), and two different soluble C70-derivates: C70-PCBM and
diphenylmethano[70]fullerene oligoether (C70-DPM-OE). The first experimental
identification of the negative polaron localized on the C70-cage in
polymer-fullerene bulk heterojunctions has been obtained. When recorded at
conventional X-band EPR, this signal is overlapping with the signal of the
positive polaron, which does not allow for its direct experimental
identification. Owing to the superior spectral resolution of the high frequency
D-band EPR, we were able to separate light-induced signals from P+ and P- in
PHT-C70 bulk heterojunctions. Comparing signals from C70-derivatives with
different side-chains, we have obtained experimental proof that the polaron is
localized on the cage of the C70 molecule
Fast Generation of Random Spanning Trees and the Effective Resistance Metric
We present a new algorithm for generating a uniformly random spanning tree in
an undirected graph. Our algorithm samples such a tree in expected
time. This improves over the best previously known bound
of -- that follows from the work of
Kelner and M\k{a}dry [FOCS'09] and of Colbourn et al. [J. Algorithms'96] --
whenever the input graph is sufficiently sparse.
At a high level, our result stems from carefully exploiting the interplay of
random spanning trees, random walks, and the notion of effective resistance, as
well as from devising a way to algorithmically relate these concepts to the
combinatorial structure of the graph. This involves, in particular,
establishing a new connection between the effective resistance metric and the
cut structure of the underlying graph
12CaO.7Al2O3 ceramic: A review of the electronic and optoelectronic applications in display devices
The alumina-based compound, 12CaO.7Al2O3, is a ceramic material with a unique cage-like lattice. Such a structure has enabled scientists to extract various new characteristics from this compound, most of which were unknown until quite recently. This compound has the ability to incorporate different anionic species and even electrons to the empty space inside its cages, thereby changing from an insulator into a conductive oxide. The cage walls can also incorporate different rare earth phosphor elements producing an oxide-based phosphor. All these characteristics are obtained without a significant change in the structure of the lattice. It is, therefore, reasonable to expect that this compound will receive attention as a potential material for display applications. This review article presents recent investigations into the application of 12CaO.7Al2O3 ceramic in various display devices, the challenges, opportunities and possible areas of future investigation into the development of this naturally abundant and environmental friendly material in the field of display.LP Displays Ltd, Blackburn, UK for partial funding of the studentship at Queen Mary, University of London. Dr Lesley Hanna of Wolfson Centre for Materials Processing, Brunel University Londo
Molecular theory of solvation: Methodology summary and illustrations
Integral equation theory of molecular liquids based on statistical mechanics
is quite promising as an essential part of multiscale methodology for chemical
and biomolecular nanosystems in solution. Beginning with a molecular
interaction potential force field, it uses diagrammatic analysis of the
solvation free energy to derive integral equations for correlation functions
between molecules in solution in the statistical-mechanical ensemble. The
infinite chain of coupled integral equations for many-body correlation
functions is reduced to a tractable form for 2- or 3-body correlations by
applying the so-called closure relations. Solving these equations produces the
solvation structure with accuracy comparable to molecular simulations that have
converged but has a critical advantage of readily treating the effects and
processes spanning over a large space and slow time scales, by far not feasible
for explicit solvent molecular simulations. One of the versions of this
formalism, the three-dimensional reference interaction site model (3D-RISM)
integral equation complemented with the Kovalenko-Hirata (KH) closure
approximation, yields the solvation structure in terms of 3D maps of
correlation functions, including density distributions, of solvent interaction
sites around a solute (supra)molecule with full consistent account for the
effects of chemical functionalities of all species in the solution. The
solvation free energy and the subsequent thermodynamics are then obtained at
once as a simple integral of the 3D correlation functions by performing
thermodynamic integration analytically.Comment: 24 pages, 10 figures, Revie
Carbohydrate-derived amphiphilic macromolecules: a biophysical structural characterization and analysis of binding behaviors to model membranes.
The design and synthesis of enhanced membrane-intercalating biomaterials for drug delivery or vascular membrane targeting is currently challenged by the lack of screening and prediction tools. The present work demonstrates the generation of a Quantitative Structural Activity Relationship model (QSAR) to make a priori predictions. Amphiphilic macromolecules (AMs) "stealth lipids" built on aldaric and uronic acids frameworks attached to poly(ethylene glycol) (PEG) polymer tails were developed to form self-assembling micelles. In the present study, a defined set of novel AM structures were investigated in terms of their binding to lipid membrane bilayers using Quartz Crystal Microbalance with Dissipation (QCM-D) experiments coupled with computational coarse-grained molecular dynamics (CG MD) and all-atom MD (AA MD) simulations. The CG MD simulations capture the insertion dynamics of the AM lipophilic backbones into the lipid bilayer with the PEGylated tail directed into bulk water. QCM-D measurements with Voigt viscoelastic model analysis enabled the quantitation of the mass gain and rate of interaction between the AM and the lipid bilayer surface. Thus, this study yielded insights about variations in the functional activity of AM materials with minute compositional or stereochemical differences based on membrane binding, which has translational potential for transplanting these materials in vivo. More broadly, it demonstrates an integrated computational-experimental approach, which can offer a promising strategy for the in silico design and screening of therapeutic candidate materials
Synthesis and characterization of fluorine-containing C60 derivatives and their charge transfer photophysics in organic photovoltaics
2013 Fall.Includes bibliographical references.Transformative advances in the science of new materials and technological solutions for energy conversion and storage require focused efforts from scientists across different disciplines. One of the major frontiers for modern chemistry is the molecular design of advanced materials from earth-abundant elements with finely tuned chemical, photophysical, and electronic properties. In this work, several highly efficient and, in some cases, highly regioselective synthetic methodologies have been developed for the first time that resulted in a wide array of versatile fullerene-based organic electron acceptors with highly tunable electronic properties. The classes of these newly synthesized and characterized materials include mono-perfluorocarbocyclic C60 derivatives, highly functionalizable ω-X-perfluoroalkylfullerenes (X = SF5, Br, I, COOEt), twenty one new isomers of deca-trifluoromethyl[60]fullerenes, and several new isomers of octa- and hexa-trifluoromethyl[60]fullerenes. Improved synthetic and separation techniques yielding up to multi-gram amounts of difluoromethylene[60]fulleroid and several other classes of technologically important perfluoroalkylfullerenes have also been developed, which enabled several organic photovoltaic-relevant studies using state-of-the art facilities at the National Renewable Energy Laboratory. This included the first experimental determination of an optimal driving force for the relative yield of free carrier generation in a family of polyfluorene polymers by using a series of trifluoromethylfullerene acceptors with a large range of electron affinities synthesized by the author. In another study, a judiciously selected series of acceptors was applied for a time-resolved microwave conductivity (TRMC) study that provided the first compelling experimental evidence that the yield for uncorrelated free charge generation in organic photovoltaic (OPV) device-relevant blends of donor:acceptor active layers is a function of carrier mobility. Finally, a new fullerene acceptor rivaling one of the champion fullerene derivatives, phenyl-C61-butyric acid methyl ester (PCBM), in OPV performance was studied by TRMC and in OPV devices
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