73 research outputs found
In situ determination of the energy dependence of the high-frequency mobility in polymers
The high-frequency mobility in disordered systems is governed by transport
properties on mesoscopic length scales, which makes it a sensitive probe for
the amount of local order. Here we present a method to measure the energy
dependence of the high frequency mobility by combining an electrochemically
gated transistor with in-situ quasi-optical measurements in the sub-terahertz
domain. We apply this method to
poly([2-methoxy-5-(3',7'-dimethylocyloxy)]-p-phenylene vinylene) (OC_1C_10-PPV)
and find a mobility at least as high as 0.1 cm^2V^-1s^-1.Comment: 3 pages (incl. 3 figures) in Appl. Phys. Let
Bias-dependent Contact Resistance in Rubrene Single-Crystal Field-Effect Transistors
We report a systematic study of the bias-dependent contact resistance in
rubrene single-crystal field-effect transistors with Ni, Co, Cu, Au, and Pt
electrodes. We show that the reproducibility in the values of contact
resistance strongly depends on the metal, ranging from a factor of two for Ni
to more than three orders of magnitude for Au. Surprisingly, FETs with Ni, Co,
and Cu contacts exhibits an unexpected reproducibility of the bias-dependent
differential conductance of the contacts, once this has been normalized to the
value measured at zero bias. This reproducibility may enable the study of
microscopic carrier injection processes into organic semiconductors.Comment: 4 pages, 4 figure
Doping, density of states and conductivity in polypyrrole and poly(p-phenylene vinylene)
The evolution of the density of states (DOS) and conductivity as function of
well controlled doping levels in OC_1C_10-poly(p-phenylene vinylene)
[OC_1C_10-PPV] doped by FeCl_3 and PF_6, and PF_6 doped polypyrrole (PPy-PF_6
have been investigated. At a doping level as high as 0.2 holes per monomer, the
former one remains non-metallic, while the latter crosses the metal-insulator
transition. In both systems a similar almost linear increase in DOS as function
of charges per unit volume c* has been observed from the electrochemical gated
transistor data. In PPy-PF_6, when compared to doped OC_1C_10-PPV, the energy
states filled at low doping are closer to the vacuum level; by the higher c* at
high doping more energy states are available, which apparently enables the
conduction to change to metallic. Although both systems on the insulating side
show log(sigma) proportional to T^-1/4 as in variable range hopping, for highly
doped PPy-PF_6 the usual interpretation of the hopping parameters leads to
seemingly too high values for the density of states.Comment: 4 pages (incl. 6 figures) in Phys. Rev.
Current saturation and Coulomb interactions in organic single-crystal transistors
Electronic transport through rubrene single-crystal field effect transistors
(FETs) is investigated experimentally in the high carrier density regime (n ~
0.1 carrier/molecule). In this regime, we find that the current does not
increase linearly with the density of charge carriers, and tends to saturate.
At the same time, the activation energy for transport unexpectedly increases
with increasing n. We perform a theoretical analysis in terms of a well-defined
microscopic model for interacting Frohlich polarons, that quantitatively
accounts for our experimental observations. This work is particularly
significant for our understanding of electronic transport through organic FETs.Comment: Extended version with 1 additional figure and an appendix explaining
the consistency of the theoretical calculatio
Wide energy-window view on the density of states and hole mobility of poly(p-phenylene vinylene)
Using an electrochemically gated transistor, we achieved controlled and
reversible doping of poly(p-phenylene vinylene) in a large concentration range.
Our data open a wide energy-window view on the density of states (DOS) and
show, for the first time, that the core of the DOS function is Gaussian, while
the low-energy tail has a more complex structure. The hole mobility increases
by more than four orders of magnitude when the electrochemical potential is
scanned through the DOS.Comment: 4 pages, 4 figure
Tunable Frohlich Polarons in Organic Single-Crystal Transistors
In organic field effect transistors (FETs), charges move near the surface of
an organic semiconductor, at the interface with a dielectric. In the past, the
nature of the microscopic motion of charge carriers -that determines the device
performance- has been related to the quality of the organic semiconductor.
Recently, it has been appreciated that also the nearby dielectric has an
unexpectedly strong influence. The mechanisms responsible for this influence
are not understood. To investigate these mechanisms we have studied transport
through organic single crystal FETs with different gate insulators. We find
that the temperature dependence of the mobility evolves from metallic-like to
insulating-like with increasing the dielectric constant of the insulator. The
phenomenon is accounted for by a two-dimensional Frohlich polaron model that
quantitatively describes our observations and shows that increasing the
dielectric polarizability results in a crossover from the weak to the strong
polaronic coupling regime
High Electron Mobility in Vacuum and Ambient for PDIF-CN2 Single-Crystal Transistors
We have investigated the electron mobility on field-effect transistors based
on PDIF-CN single crystals. The family of the small molecules
PDI8-CN has been chosen for the promising results obtained for
vapour-deposited thin film FETs. We used as gate dielectric a layer of PMMA
(spinned on top of the SiO), to reduce the possibility of electron
trapping by hydroxyl groups present at surface of the oxide. For these devices
we obtained a room temperature mobility of 6 cm/Vs in vacuum and 3
cm/Vs in air. Our measurements demonstrate the possibility to obtain
n-type OFETs with performances comparable to those of p-type devices.Comment: published online in JAC
PEROXIDASE ELECTROPHORETIC PATTERN OF WHEAT AND TOMATO SEEDLINGS GROWN ON BIOLOGICALLY ACTIVE COMPOUNDS-CONTAINING MEDIUM
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