6,203 research outputs found
A theory for magnetic-field effects of nonmagnetic organic semiconducting materials
A universal mechanism for strong magnetic-field effects of nonmagnetic
organic semiconductors is presented. A weak magnetic field (less than hundreds
mT) can substantially change the charge carrier hopping coefficient between two
neighboring organic molecules when the magnetic length is not too much longer
than the molecule-molecule separation and localization length of electronic
states involved. Under the illumination of lights or under a high electric
field, the change of hopping coefficients leads also to the change of polaron
density so that photocurrent, photoluminescence, electroluminescence,
magnetoresistance and electrical-injection current become sensitive to a weak
magnetic field. The present theory can not only explain all observed features,
but also provide a solid theoretical basis for the widely used empirical
fitting formulas.Comment: 4 pages, 2 figure
Quantum coherence and carriers mobility in organic semiconductors
We present a model of charge transport in organic molecular semiconductors
based on the effects of lattice fluctuations on the quantum coherence of the
electronic state of the charge carrier. Thermal intermolecular phonons and
librations tend to localize pure coherent states and to assist the motion of
less coherent ones. Decoherence is thus the primary mechanism by which
conduction occurs. It is driven by the coupling of the carrier to the molecular
lattice through polarization and transfer integral fluctuations as described by
the hamiltonian of Gosar and Choi. Localization effects in the quantum coherent
regime are modeled via the Anderson hamiltonian with correlated diagonal and
non-diagonal disorder leading to the determination of the carrier localization
length. This length defines the coherent extension of the ground state and
determines, in turn, the diffusion range in the incoherent regime and thus the
mobility. The transfer integral disorder of Troisi and Orlandi can also be
incorporated. This model, based on the idea of decoherence, allowed us to
predict the value and temperature dependence of the carrier mobility in
prototypical organic semiconductors that are in qualitative accord with
experiments
Gated nonlinear transport in organic polymer field effect transistors
We measure hole transport in poly(3-hexylthiophene) field effect transistors
with channel lengths from 3 m down to 200 nm, from room temperature down
to 10 K. Near room temperature effective mobilities inferred from linear regime
transconductance are strongly dependent on temperature, gate voltage, and
source-drain voltage. As is reduced below 200 K and at high source-drain
bias, we find transport becomes highly nonlinear and is very strongly modulated
by the gate. We consider whether this nonlinear transport is contact limited or
a bulk process by examining the length dependence of linear conduction to
extract contact and channel contributions to the source-drain resistance. The
results indicate that these devices are bulk-limited at room temperature, and
remain so as the temperature is lowered. The nonlinear conduction is consistent
with a model of Poole-Frenkel-like hopping mechanism in the space-charge
limited current regime. Further analysis within this model reveals consistency
with a strongly energy dependent density of (localized) valence band states,
and a crossover from thermally activated to nonthermal hopping below 30 K.Comment: 22 pages, 7 figures, accepted to J. Appl. Phy
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