Transport and noise in organic field-effect devices
K. Morawetz,
1,2,3
S. Gemming,
1
R. Luschtinetz,
4
T. Kunze,
1
P. Lipavský,
5,6
L. M. Eng,
7
G. Seifert,
4
V. Pankoke,
1
and
P. Milde
7
1
Forschungszentrum Dresden-Rossendorf, PF 51 01 19, 01314 Dresden, Germany
2
International Center for Condensed Matter Physics, 70904-910 Brasília-DF, Brazil
3
Max-Planck-Institute for the Physics of Complex Systems, Nöthnitzer Strasse 38, 01187 Dresden, Germany
4
Institute of Physical Chemistry and Electrochemistry, TU Dresden, 01062 Dresden, Germany
5
Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 12116 Prague 2, Czech Republic
6
Institute of Physics, Academy of Sciences, Cukrovarnická 10, 16253 Prague 6, Czech Republic
7
Institute of Applied Photophysics, TU Dresden, 01062 Dresden, Germany
Received 2 October 2008; revised manuscript received 19 December 2008; published 4 February 2009
The transport and fluctuation properties of organic molecules ordered parallel between two Au contact leads
are investigated by the method of surface Green’s function. From first-principles simulation the relevant
hopping parameters are extracted and used to calculate nonlinear transport coefficients with respect to an
external bias voltage. A staggering of conductance is found in dependence on the number of molecules
squeezed in between the contacts. The thermal properties show an anomalous behavior whenever the voltage
reaches the values of the molecular energy levels active for transport. The thermoelectric figure of merit shows
a resonance allowing one to reach values even larger than one.
DOI: 10.1103/PhysRevB.79.085405 PACS numbers: 73.63.Fg, 85.85.j, 87.15.hj, 73.23.b
I. INTRODUCTION
The goal to develop low-cost storage and microelectronic
devices has triggered an enormous activity in the research of
organic field-effect transistors OFETs based on different
polymers
1–3
and small organic molecules.
4
It is desirable that
the molecular material possesses a high structural ordering
5,6
in order to reach high charge carrier mobilities and low re-
sistive losses. Among the most promising materials are olig-
othiophenes and their derivatives.
7
This is due to the variety
of intra- and intermolecular interactions, which originates
from the polarizability of the sulfur electrons and the embed-
ding aromatic -electron system.
8–10
The performance of OFETs based on oligothiophenes
3
shows characteristic features, e.g., the current starts at a cer-
tain threshold of gate voltage and reaches a saturation value
for certain drain voltages. In general, the charge transport
occurs in the direction perpendicular to the plane of the
thiophene rings due to good stacking. This suggests a con-
struction of devices with parallel ordered molecular rings.
This is the perpendicular direction to the usually considered
transport through various molecule classes ranging from
transport dependent on the thickness
11
to metallic behavior
independent of the thickness of molecules.
12
Thick-film de-
vices based on thiophenes were reported several years ago.
13
Recently an OFET structure has been built from ultrathin
self-assembled films made up from oligothiophenes, which
are arranged in a highly-order lamellar stacking perpendicu-
lar to the substrate surface
3
as illustrated in Figs. 1 and 2.
The energy gap between the lowest unoccupied molecular
orbital LUMO and the highest occupied molecular orbital
HOMO levels measures about 3 eV in quarterthiophene.
For the molecular structure, see Fig. 1. In contrast to the
conduction and valence band in semiconductors the transport
is due to localized states dominated by hopping
14,15
rather
than due to delocalized states limited by the scattering in
semiconductors.
If one considers the charge transport between the mol-
ecules one can think of two extreme mechanisms. On one
side one has either the ballistic transport through bonds or
the hopping transport through space. The diffusive transport
on the other side is present if the carriers meet scattering
partners. For the present situation of parallel stacked mol-
ecules the hopping transport by tunneling between the local-
ized HOMO or LUMO states is the relevant one. A related
mechanism of charge transfer due to the bending motion of
molecules by touching might lead to shuttling transport
16
which could be investigated by resonant spectroscopy.
17
Of
course, the most crucial question is the tilting of molecules
triggered by the metal-molecule contacts
18
which requires a
refined interface engineering.
19
FIG. 1. Color online The thiophene molecules between two Au
contacts.
PHYSICAL REVIEW B 79, 085405 2009
1098-0121/2009/798/08540512 ©2009 The American Physical Society 085405-1