682 research outputs found
Current-Voltage Characteristics of Polymer Light-Emitting Diodes
Conduction in pristine conjugated polymers (other than polyacetylene) is by
polaron hopping between sites corresponding to conjugation lengths. The strong
increase of current with voltage observed for both emission-limited and
ohmic contacts is due in large part to mobility increase as increasing field
makes it more possible to overcome internal barriers, such as energy
differences between sites. For emission-limited contacts an additional source
of nonlinear increase of with increasing is greater ability to escpe
return to the injecting electrode due to the image force. For ohmic contacts
additional nonlinearity comes from space charge effects. We are able to fit
vs. for electron or hole conduction in some poly(-phenylene vinylene),
PPV, derivatives with ohmic contacts for reasonable values of the parameters
involved.Comment: 9 pages, REVTeX, 1 figure is aviable upon request, to be published in
SPIE pro
Effect of interchain coupling on conducting polymer luminescence: excimers in derivatives of poly(phenylene vinylene)
Optical excitation of a chain in a polymer film may result in formation of an
excimer, a superposition of on-chain excitons and charge-transfer excitons on
the originally excited chain and a neighboring chain. The excimer emission is
red-shifted compared to that of an on-chain exciton by an amount depending on
the interchain coupling . Setting up the excimer wavefunction and
calculating the red shift, we determine average values, referred to a
monomer, of 0.52 eV and 0.16 eV for poly(2,5-hexyloxy -phenylene
cyanovinylene), CN-PPV, and poly[2-methoxy, 5-(2'-ethyl-hexyloxy)-1, 4
p-phenylene vinylene], MEH-PPV, respectively, and use them to determine the
effect of interchain distance on the emission.Comment: 10 pages, RevTeX, 1 PS figure, replaced version of cond-mat/9707095,
accepted for publication in Phys. Rev. B, Rapid Communicatio
Contact Injection into Polymer Light-Emitting Diodes
The variation of current I with voltage V for poly(phenylene vinylene) and
other polymer light-emitting diodes has been attributed to carriers tunneling
into broad conduction and valence bands. In actuality the electrons and holes
tunnel into polaron levels and transport is by hopping among these levels. We
show that for small injection the I-V characteristic is determined mainly by
the image force, for large injection by space charge effects, but in both cases
the strong variation of mobility with field due to disorder plays an important
role.Comment: 9 pages, two Postscript figures are aviable upon reques
Semiconductor saturable absorbers for ultrafast THz signals
We demonstrate saturable absorber behavior of n-type semiconductors GaAs, GaP
and Ge in THz frequency range at room temperature using nonlinear THz
spectroscopy. The saturation mechanism is based on a decrease in electron
conductivity of semiconductors at high electron momentum states, due to
conduction band nonparabolicity and scattering into satellite valleys in strong
THz fields. Saturable absorber parameters, such as linear and non-saturable
transmission, and saturation fluence, are extracted by fits to a classic
saturable absorber model. Further, we observe THz pulse shortening, and an
increase of the group refractive index of the samples at higher THz pulse peak
fields.Comment: Submitted to Appl. Phys. Lett
Correlated-electron description of the photophysics of thin films of -conjugated polymers
We extend Mulliken's theory of ground state charge transfer in a
donor-acceptor complex to excited state charge transfer between pairs of
identical -conjugated oligomers, one of which is in the optically excited
state and the other in the ground state, leading to the formation of a
charge-transfer exciton. Within our theory, optical absorptions from the
charge-transfer exciton should include a low energy intermolecular
charge-transfer excitation, as well as distinct intramolecular excitations from
both the neutral delocalized exciton component and the Coulombically bound
polaron-pair component of the charge-transfer exciton. We report high order
configuration-interaction calculations for pairs of oligomers of
poly-paraphenylenevinylene (PPV) that go beyond our previous single
configuration-interaction calculation and find all five excited state
absorptions predicted using heuristic arguments based on the Mulliken concept.
Our calculated excited state absorption spectrum exhibits strong qualitative
agreement with the complete wavelength-dependent ultrafast photoinduced
absorption in films of PPV derivatives, suggesting that a significant fraction
of the photoinduced absorption here is from the charge-transfer exciton. We
make detailed comparisons to experiments, and a testable experimental
prediction
Hot-electron effect in spin dephasing in -type GaAs quantum wells
We perform a study of the effect of the high in-plane electric field on the
spin precession and spin dephasing due to the D'yakonov-Perel' mechanism in
-type GaAs (100) quantum wells by constructing and numerically solving the
kinetic Bloch equations. We self-consistently include all of the scattering
such as electron-phonon, electron-non-magnetic impurity as well as the
electron-electron Coulomb scattering in our theory and systematically
investigate how the spin precession and spin dephasing are affected by the high
electric field under various conditions. The hot-electron distribution
functions and the spin correlations are calculated rigorously in our theory. It
is found that the D'yakonov-Perel' term in the electric field provides a
non-vanishing effective magnetic field that alters the spin precession period.
Moreover, spin dephasing is markedly affected by the electric field. The
important contribution of the electron-electron scattering to the spin
dephasing is also discussed.Comment: 11 pages, 11 figures, accepted for publication in Phys. Rev.
Integrated Optics: a Report on the 2nd OSA Topical Meeting
This report surveys the papers presented at the 2nd OSA Topical Meeting on Integrated Optics, which was held 21–24 January 1974 in New Orleans, La
Cooling Dynamics of Photoexcited Carriers in Si Studied by Using Optical Pump and Terahertz Probe Spectroscopy
We investigated the photoexcited carrier dynamics in Si by using optical pump
and terahertz probe spectroscopy in an energy range between 2 meV and 25 meV.
The formation dynamics of excitons from unbound e-h pairs was studied through
the emergence of the 1s-2p transition of excitons at 12 meV (3 THz). We
revealed the thermalization mechanism of the photo-injected hot carriers
(electrons and holes) in the low temperature lattice system by taking account
of the interband and intraband scattering of carriers with acoustic and optical
phonons. The overall cooling rate of electrons and holes was numerically
calculated on the basis of a microscopic analysis of the phonon scattering
processes, and the results well account for the experimentally observed carrier
cooling dynamics. The long formation time of excitons in Si after the above-gap
photoexcitation is reasonably accounted for by the thermalization process of
photoexcited carriers.Comment: 8 pages, 8 figures, to be published in Phys. Rev.
Electron-electron interaction effects on optical excitations in semiconducting single-walled carbon nanotubes
We report correlated-electron calculations of optically excited states in ten
semiconducting single-walled carbon nanotubes with a wide range of diameters.
Optical excitation occurs to excitons whose binding energies decrease with the
increasing nanotube diameter, and are smaller than the binding energy of an
isolated strand of poly-(paraphenylene vinylene). The ratio of the energy of
the second optical exciton polarized along the nanotube axis to that of the
lowest exciton is smaller than the value predicted within single-particle
theory. The experimentally observed weak photoluminescence is an intrinsic
feature of semiconducting nanotubes, and is consequence of dipole-forbidden
excitons occurring below the optical exciton.Comment: 5 pages, 3 figures, To appear in PR
Investigation of acceptor levels and hole scattering mechanisms in p-gallium selenide by means of transport measurements under pressure
The effect of pressure on acceptor levels and hole scattering mechanisms in
p-GaSe is investigated through Hall effect and resistivity measurements under
quasi-hydrostatic conditions up to 4 GPa. The pressure dependence of the hole
concentration is interpreted through a carrier statistics equation with a
single (nitrogen) or double (tin) acceptor whose ionization energies decrease
under pressure due to the dielectric constant increase. The pressure effect on
the hole mobility is also accounted for by considering the pressure
dependencies of both the phonon frequencies and the hole-phonon coupling
constants involved in the scattering rates.Comment: 13 pages, Latex, 4 ps figures. to appear in High Pressure Research 69
(1997
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