89,238 research outputs found
Transient Optical Characterisation of Donor-Acceptor Block Copolymers for Use in Solar Cells
This thesis presents a study of photo-active, semiconducting block copolymers for use in molecular solar cells. Current state-of-the-art organic devices utilise blends of two (or more) materials that are co-deposited from a common solution; the resulting structures formed are determined by material properties and deposition conditions, but often result in configurations that are detrimental to device performance. An answer to this problem comes in the form of the block copolymer; using these materials, devices can be formed from a single material active layer. In addition, the counterbalance of forces within films of block copolymer can lead to nano-scale self-assembly that allows for a strong degree of control over layer equilibrium morphology. Such control will be an important step forward in the evolution of molecular solar cells. The main body of this work is concerned with the study of the photo-physics of photo-conductive block copolymers, especially the generation of free charge. First, an investigation is made into the inherent structure-function relationship in block copolymers. A varying chain length is seen to drastically affect the photoluminescence quenching and yield of long-lived charges. Photovoltaic devices made using these materials show a peak efficiency of 0.11% and correlate with the spectroscopic results, subject to a trade off between charge generation and transport/collection. In a second investigation, the effects of post-fabrication annealing on block copolymer films are considered; studies on annealed samples lead to the conclusion that domain crystallinity is a significant factor in determining the yields of long-lived charge carriers. It is found that these yields are comparable with those of a standard blend (that achieve 75% photon to electron conversion efficiency). Annealing leads to increases in photovoltaic device performance over unannealed samples, although additional control over active layer morphology is necessary for these materials to attain their potential. Following this, a comparative study is made between a block copolymer and a similarly composed blend formed from well studied polyfluorene copolymers. Further advantages of block copolymers are highlighted, including the stability of morphologies generated under different deposition conditions. Finally, a novel tool set is introduced using a block copolymer sample to emphasise the experiments potential with regard to studying interfacial photophysical effects
The 2D J_1-J_2 XY and XY-Ising Models
We consider the 2D classical XY model on a square lattice. In the
frustrated phase corresponding to , an Ising order parameter emerges
by an ``order due to disorder'' effect. This leads to a discrete symmetry plus
the O(2) global one. We formulate the problem in a Coulomb gas language and
show by a renormalization group analysis that only two phases are still
possible : a locked phase at low temperature and a disordered one at high
temperature. The transition is characterized by the loss of Ising and XY order
at the same point. This analysis suggests that the 2D XY model is in
the same universality class than XY-Ising models.Comment: 8 Pages, Latex, 1 ps figure, to be published in Europhysics Letter
The ellipticities of Galactic and LMC globular clusters
The globular clusters of the LMC are found to be significantly more
elliptical than Galactic globular clusters, but very similar in virtually all
other respects. The ellipticity of the LMC globular clusters is shown not be
correlated with the age or mass of those clusters. It is proposed that the
ellipticity differences are caused by the different strengths of the tidal
fields in the LMC and the Galaxy. The strong Galactic tidal field erases
initial velocity anisotropies and removes angular momentum from globular
clusters making them more spherical. The tidal field of the LMC is not strong
enough to perform these tasks and its globular clusters remain close to their
initial states.Comment: 3 pages LaTeX file with 3 figures incorporated accepted for
publication in MNRAS. Also available by e-mailing spg, or by ftp from
ftp://star-www.maps.susx.ac.uk/pub/papers/spg/ellip.ps.
The Ccontribution of normal, dim and dwarf galaxies to the local luminosity density
From the Hubble Deep Field catalog presented in Driver et al. (1998) we
derive the local (0.3 < z < 0.5) Bivariate Brightness Distribution (BBD) of
field galaxies within a 326 Mpc**3 Volume-Limited sample. The sample contains
47 galaxies which uniformally sample the underlying galaxy population within
the specified redshift, magnitude and surface brightness limits (0.3 < z < 0.5,
-21.3 < M_{B} < -13.7 mags, 18.0 < mu_{B} < 24.55 mags/sq arcsec). We conclude:
(i) A luminosity-surface brightness relation exists for both the field and
cluster galaxy populations, M_{B} ~ 1.5 mu_{e} - 50, (ii) Luminous low surface
brightness galaxies account for <10% of the L* population, (iii) Low luminosity
low surface brightness galaxies outnumber Hubble types by a factor of ~ 1.4,
however their space density is NOT sufficient to explain the faint blue excess
either by themselves or as faded remnants.
In terms of the local luminosity density and galaxy dynamical mass budget,
normal galaxies (i.e. Hubble tuning fork) contribute 88% and 72% respectively.
This compares to 7% and 12% for dim galaxies and 5% and 16% for dwarf galaxies
(within the above specified limits).Comment: Accepted for publication in Astrophysical Journal Letters. Nine pages
and two figure
The decade of galaxy formation: pitfalls in the path ahead
At the turn of the decade we arguably move from the era of precision
cosmology to the era of galaxy formation. One approach to this problem will be
via the construction of comprehensive galaxy samples. In this review I take the
opportunity to highlight a number of challenges which must be overcome before
we can use such data to construct a robust empirical blueprint of galaxy
evolution. The issues briefly highlighted here are: the Hubble tuning fork
versus galaxy components, the hierarchy of structure, the accuracy of
structural decompositions, galaxy photometry, incompleteness, cosmic variance,
photometric versus spectroscopic redshifts, wavelength bias, dust attenuation,
and the disconnect with theory. These concerns essentially form one of the key
motivations of the GAMA survey which, as one of its goals, will establish a
complete comprehensive kpc-resolution 3D multi-wavelength (UV-Opt-IR-Radio)
database of 250k galaxy systems to z <0.5.Comment: Review paper (12 pages, 11 figures) in "Hunting for the Dark: The
Hidden Side of Galaxy Formation", Malta, 19-23 Oct. 2009, eds. V.P.Debattista
& C.C.Popescu, AIP Conf. Ser., in pres
Nonperturbative results for the mass dependence of the QED fermion determinant
The fermion determinant in four-dimensional quantum electrodynamics in the
presence of O(2)XO(3) symmetric background gauge fields with a nonvanishing
global chiral anomaly is considered. It is shown that the leading mass
singularity of the determinant's nonperturbative part is fixed by the anomaly.
It is also shown that for a large class of such fields there is at least one
value of the fermion mass at which the determinant's nonperturbative part
reduces to its noninteracting value.Comment: This is an extended version of the author's paper in
Phys.Rev.D81(2010)10770
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