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 $I$ with voltage $V$ 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 $I$ with increasing $V$ 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 $I$ vs. $V$ for electron or hole conduction in some poly($p$-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 $t_\perp$. Setting up the excimer wavefunction and calculating the red shift, we determine average $t_\perp$ values, referred to a monomer, of 0.52 eV and 0.16 eV for poly(2,5-hexyloxy $p$-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 π\pi-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 $\pi$-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 nn-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 $n$-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

Evidence for Excimer Photoexcitations in an Ordered {\pi}-Conjugated Polymer Film

We report pressure-dependent transient picosecond and continuous-wave photomodulation studies of disordered and ordered films of 2-methoxy-5-(2-ethylhexyloxy) poly(para-phenylenevinylene). Photoinduced absorption (PA) bands in the disordered film exhibit very weak pressure dependence and are assigned to intrachain excitons and polarons. In contrast, the ordered film exhibits two additional transient PA bands in the midinfrared that blueshift dramatically with pressure. Based on high-order configuration interaction calculations we ascribe the PA bands in the ordered film to excimers. Our work brings insight to the exciton binding energy in ordered films versus disordered films and solutions. The reduced exciton binding energy in ordered films is due to new energy states appearing below the continuum band threshold of the single strand.Comment: 5.5 pages, 5 figure