1,147 research outputs found
From spin coating to roll-to-roll: investigating the challenge of upscaling lead halide perovskite solar cells
Spin coating, typically used to achieve nanometre thick films, is the established method for depositing perovskite precursors at lab scale for use in solar cells. This study investigates the dynamics of spin coating perovskite. By combining experimental measurement with a semi-empirical model the evaporation rate of the dimethylformamide solvent during the spin coating of a mixed lead halide precursor is determined to be 1.2âĂâ10 â8â
m/s. When K-bar coating the same precursor the solvent does not significantly evaporate during the deposition process and when this film is crystallised on a hot plate a rough film results which gives a power conversion efficiency (PCE) of less than 2%. By increasing the airflow of the K-bar coated perovskite film during crystallisation to 2.7âĂâ10 â4â
m/s the PCE increases significantly to 8.5% through an improvement in short-circuit current and fill factor
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PERFLUOROCARBON GAS TRACER STUDIES TO SUPPORT RISK ASSESSMENT MODELING OF CRITICAL INFRASTRUCTURE SUBJECTED TO TERRORIST ATTACKS.
Development of real-time predictive modeling to identify the dispersion and/or source(s) of airborne weapons of mass destruction including chemical, biological, radiological, and nuclear material in urban environments is needed to improve response to potential releases of these materials via either terrorist or accidental means. These models will also prove useful in defining airborne pollution dispersion in urban environments for pollution management/abatement programs. Predicting gas flow in an urban setting on a scale of less than a few kilometers is a complicated and challenging task due to the irregular flow paths that occur along streets and alleys and around buildings of different sizes and shapes, i.e., ''urban canyons''. In addition, air exchange between the outside and buildings and subway areas further complicate the situation. Transport models that are used to predict dispersion of WMD/CBRN materials or to back track the source of the release require high-density data and need defensible parameterizations of urban processes. Errors in the data or any of the parameter inputs or assumptions will lead to misidentification of the airborne spread or source release location(s). The need for these models to provide output in a real-time fashion if they are to be useful for emergency response provides another challenge. To improve the ability of New York City's (NYC's) emergency management teams and first response personnel to protect the public during releases of hazardous materials, the New York City Urban Dispersion Program (UDP) has been initiated. This is a four year research program being conducted from 2004 through 2007. This paper will discuss ground level and subway Perfluorocarbon tracer (PFT) release studies conducted in New York City. The studies released multiple tracers to study ground level and vertical transport of contaminants. This paper will discuss the results from these tests and how these results can be used for improving transport models needed for risk assessment
Expansion for the solutions of the Bogomolny equations on the torus
We show that the solutions of the Bogomolny equations for the Abelian Higgs
model on a two-dimensional torus, can be expanded in powers of a quantity
epsilon measuring the departure of the area from the critical area. This allows
a precise determination of the shape of the solutions for all magnetic fluxes
and arbitrary position of the Higgs field zeroes. The expansion is carried out
to 51 orders for a couple of representative cases, including the unit flux
case. We analyse the behaviour of the expansion in the limit of large areas, in
which case the solutions approach those on the plane. Our results suggest
convergence all the way up to infinite area.Comment: 26 pages, 8 figures, slightly revised version as published in JHE
Influences of Non-fullerene Acceptor Fluorination on Three-Dimensional Morphology and Photovoltaic Properties of Organic Solar Cells
Fluorination of conjugated molecules has been established as an effective structural modification strategy to influence properties and has attracted extensive attention in organic solar cells (OSCs). Here, we have investigated optoelectronic and photovoltaic property changes of OSCs made of polymer donors with the non-fullerene acceptors (NFAs) ITIC and IEICO and their fluorinated counterparts IT-4F and IEICO-4F. Device studies show that fluorinated NFAs lead to reduced Voc but increased Jsc and fill-factor (FF), and therefore, the ultimate influence to efficiency depends on the compensation of Voc loss and gains of Jsc and FF. Fluorination lowers energy levels of NFAs, reduces their electronic band gaps, and red-shifts the absorption spectra. The impact of fluorination on the molecular order depends on the specific NFA, and the conversion of ITIC to IT-4F reduces the structural order, which can be reversed after blending with the donor PBDB-T. Contrastingly, IEICO-4F presents stronger ÏâÏ stacking after fluorination from IEICO, and this is further strengthened after blending with the donor PTB7-Th. The photovoltaic blends universally present a donor-rich surface region which can promote charge transport and collection toward the anode in inverted OSCs. The fluorination of NFAs, however, reduces the fraction of donors in this donor-rich region, consequently encouraging the intermixing of donor/acceptor for efficient charge generation
Correlating three-dimensional morphology with function in PBDB-T:IT-M non-fullerene organic solar cells
In this work, the correlation between threeâdimensional morphology and device performance of PBDBâT:ITâM nonâfullerene organic solar cells is investigated. We found that a PBDBâTârich top layer formed when the PBDBâT:ITâM film is cast on PEDOT:PSS, indicating a vertical component distribution that will hinder electron transport toward the cathode in a conventional device. This PBDBâTârich top layer remained upon lowâtemperature annealing at 80â°C, but disappeared when the annealing temperature is raised, resulting in an optimum annealing temperature of 160â°C for conventional devices as the removal of this polymerârich layer facilitates electron transport toward the top cathode layer. The PBDBâTârich layer is also found in the surface region of the PBDBâT:ITâM film cast on a TiO2 substrate, but in this case it remained after thermal annealing at 80 or 160â°C, leading to a favorable vertical stratification for efficient charge collection in inverted devices. Although thermal annealing can enhance the crystallinity of PBDBâT:ITâM blend and lead to improved charge mobility, the correlation length of the PBDBâT component increased excessively under annealing at 160â°C. Further, the packing of ITâM clusters became loose upon high temperature annealing, an effect we believe results in more diffuse interfaces with PBDBâT that result in reduced charge separation efficiency, consequently reducing the shortâcircuit current in the inverted devices
Democratic cultural policy : democratic forms and policy consequences
The forms that are adopted to give practical meaning to democracy are assessed to identify what their implications are for the production of public policies in general and cultural policies in particular. A comparison of direct, representative, democratic elitist and deliberative versions of democracy identifies clear differences between them in terms of policy form and democratic practice. Further elaboration of these differences and their consequences are identified as areas for further research
Critical Dynamics of Magnets
We review our current understanding of the critical dynamics of magnets above
and below the transition temperature with focus on the effects due to the
dipole--dipole interaction present in all real magnets. Significant progress in
our understanding of real ferromagnets in the vicinity of the critical point
has been made in the last decade through improved experimental techniques and
theoretical advances in taking into account realistic spin-spin interactions.
We start our review with a discussion of the theoretical results for the
critical dynamics based on recent renormalization group, mode coupling and spin
wave theories. A detailed comparison is made of the theory with experimental
results obtained by different measuring techniques, such as neutron scattering,
hyperfine interaction, muon--spin--resonance, electron--spin--resonance, and
magnetic relaxation, in various materials. Furthermore we discuss the effects
of dipolar interaction on the critical dynamics of three--dimensional isotropic
antiferromagnets and uniaxial ferromagnets. Special attention is also paid to a
discussion of the consequences of dipolar anisotropies on the existence of
magnetic order and the spin--wave spectrum in two--dimensional ferromagnets and
antiferromagnets. We close our review with a formulation of critical dynamics
in terms of nonlinear Langevin equations.Comment: Review article (154 pages, figures included
Exploring morphological correlations among H2CO, 12CO, MSX and continuum mappings
There are relatively few H2CO mappings of large-area giant molecular cloud
(GMCs). H2CO absorption lines are good tracers for low-temperature molecular
clouds towards star formation regions. Thus, the aim of the study was to
identify H2CO distributions in ambient molecular clouds. We investigated
morphologic relations among 6-cm continuum brightness temperature (CBT) data
and H2CO (111-110; Nanshan 25-m radio telescope), 12CO (1--0; 1.2-m CfA
telescope) and midcourse space experiment (MSX) data, and considered the impact
of background components on foreground clouds. We report simultaneous 6-cm H2CO
absorption lines and H110\alpha radio recombination line observations and give
several large-area mappings at 4.8 GHz toward W49 (50'\times50'), W3
(70'\times90'), DR21/W75 (60'\times90') and NGC2024/NGC2023 (50'\times100')
GMCs. By superimposing H2CO and 12CO contours onto the MSX color map, we can
compare correlations. The resolution for H2CO, 12CO and MSX data was about 10',
8' and 18.3", respectively. Comparison of H2CO and 12CO contours, 8.28-\mu m
MSX colorscale and CBT data revealed great morphological correlation in the
large area, although there are some discrepancies between 12CO and H2CO peaks
in small areas. The NGC2024/NGC2023 GMC is a large area of HII regions with a
high CBT, but a H2CO cloud to the north is possible against the cosmic
microwave background. A statistical diagram shows that 85.21% of H2CO
absorption lines are distributed in the intensity range from -1.0 to 0 Jy and
the \Delta V range from 1.206 to 5 km/s.Comment: 18 pages, 22 figures, 5 tables. Accepted to be published in
Astrophysics and Space Scienc
Radio emission and jets from microquasars
To some extent, all Galactic binary systems hosting a compact object are
potential `microquasars', so much as all galactic nuclei may have been quasars,
once upon a time. The necessary ingredients for a compact object of stellar
mass to qualify as a microquasar seem to be: accretion, rotation and magnetic
field. The presence of a black hole may help, but is not strictly required,
since neutron star X-ray binaries and dwarf novae can be powerful jet sources
as well. The above issues are broadly discussed throughout this Chapter, with a
a rather trivial question in mind: why do we care? In other words: are jets a
negligible phenomenon in terms of accretion power, or do they contribute
significantly to dissipating gravitational potential energy? How do they
influence their surroundings? The latter point is especially relevant in a
broader context, as there is mounting evidence that outflows powered by
super-massive black holes in external galaxies may play a crucial role in
regulating the evolution of cosmic structures. Microquasars can also be thought
of as a form of quasars for the impatient: what makes them appealing, despite
their low number statistics with respect to quasars, are the fast variability
time-scales. In the first approximation, the physics of the jet-accretion
coupling in the innermost regions should be set by the mass/size of the
accretor: stellar mass objects vary on 10^5-10^8 times shorter time-scales,
making it possible to study variable accretion modes and related ejection
phenomena over average Ph.D. time-scales. [Abridged]Comment: 28 pages, 13 figures, To appear in Belloni, T. (ed.): The Jet
Paradigm - From Microquasars to Quasars, Lect. Notes Phys. 794 (2009
Using social network sites in Higher Education: An experience in business studies
In the past 5 years the impact of Web 2.0 in new generations has been remarkably significant (Pew Research Center, 2010). This paper reports on an experience in the use of Social Network Sites (SNS) to support student involvement with the subject and to develop basic skills.
According to studentsâ opinion, the experience was deemed as positive. They considered that the experience contributed to a higher engagement with the subject and a deeper collaboration with other students and teaching staff. As a result, the majority of students would prefer the use of SNS as a first option if they had to enrol again in the subject.
Regarding the relationships between academic performance and use of the SNS, two different student profiles were identified based on usage patterns of the platform. Students with a more intensive use of the site showed a significantly better performance than students with a low usage profile.This work was partially supported by the Junta de AndalucĂa â FEDER (Proyectos de Excelencia: SEJ-02670
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