Conductivity-type anisotropy in molecular
solids
J. R. Ostrick, A. Dodabalapur, L. Torsi, A, J. Lovinger, E. W. Kwock,
T. M. Miller, M. Galvin, Magnus Berggren and H. E. Katz
Linköping University Post Print
N.B.: When citing this work, cite the original article.
Original Publication:
J. R. Ostrick, A. Dodabalapur, L. Torsi, A, J. Lovinger, E. W. Kwock, T. M. Miller, M.
Galvin, Magnus Berggren and H. E. Katz, Conductivity-type anisotropy in molecular solids,
1997, Journal of Applied Physics, (81), 10, 6804-6808.
http://dx.doi.org/10.1063/1.365238
Copyright: American Institute of Physics (AIP)
http://www.aip.org/
Postprint available at: Linköping University Electronic Press
http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-74920
Conductivity-type anisotropy in molecular solids
J. R. Ostrick,
a)
A. Dodabalapur,
b)
L. Torsi,
c)
A. J. Lovinger, E. W. Kwock, T. M. Miller,
d)
M. Galvin, M. Berggren, and H. E. Katz
Bell Laboratories, Lucent Technologies, 700 Mountain Avenue, Murray Hill, New Jersey 07974
~Received 16 October 1996; accepted for publication 12 February 1997!
Thin polycrystalline films of perylenetetracarboxylic dianyhydride ~PTCDA!, an organic molecular
solid, exhibits substantial anisotropies in its electronic transport properties. Only electrons transport
in the directions along molecular planes, while mainly holes transport in the direction normal to
molecular planes. A series of measurements on both field effect transistors with PTCDA active
layers and light emitting diodes with PTCDA transport layers documents the anisotropy seen in the
electronic transport in thin films of PTCDA. © 1997 American Institute of Physics.
@S0021-8979~97!05910-0#
I. INTRODUCTION
Sublimed molecular solids such as
a
-sexithiophene ~
a
-
6T! and perylenetetracarboxylic dianyhydride ~PTCDA!
have interesting transport and optical properties which stem
from their strong tendency to form polycrystalline films in
which the molecular planes orient in fixed directions with
respect to the substrate. For example,
a
-6T forms films in
which the individual molecules lie almost edge-on on most
substrates. The hole mobility in a direction parallel to the
substrate is relatively high (0.01–0.1 cm
2
/V s), and has led
to the demonstration of field effect transistor action.
1
Al-
though individual molecules in molecular solids interact
through weak Van der Waals forces, many such materials
exhibit significant anisotropies in the magnitude of charge
mobility with respect to transport direction along the
crystal.
2–5
Highly purified single crystals of molecular solids such
as anthracene and naphthalene generally transport both elec-
trons and holes but with different mobilities which depend
on the direction of transport.
4
On the other hand, thin poly-
crystalline films of many molecular solids ~such as
a
-6I!
appear to transport only one carrier type. Transport in such
films depends on a number of factors such as molecular ori-
entation, electronic overlap, molecular orbital energy levels,
traps, and sometimes the nature of the ambient. In this ar-
ticle, we evaluate some of characteristics of charge transport
in thin films of PTCDA, which is shown to possess anisotro-
pies in the species of charge that can be transported along
different directions. We show that PTCDA transports elec-
trons in the directions parallel to the molecular planes and
mainly holes in the direction perpendicular to the molecular
planes. The charge carriers are referred to as electrons and
holes for convenience although it is recognized that in many
such materials the carriers are polaronic in nature.
6
The an-
isotropy in carrier type was deduced from a series of device
based measurements. Field-effect transistor ~FET! structures
were used to study electronic transport along molecular
planes at different temperatures. A series of multilayer light
emitting diodes based on the well understood
8-hydroxyquinoline aluminum ~Alq!/triphenyl diamine
~TAD! material system
7
were employed to study transport
properties normal to the molecular planes.
II. EXPERIMENTAL RESULTS AND DISCUSSION
A. Field effect transistors
The FETs were fabricated by vapor depositing vacuum
purified PTCDA from a baffled Mo crucible in a high
vacuum deposition chamber (, 73 10
2 6
Torr) on to prepat-
terned substrates as illustrated in the inset of Fig. 1. The light
emitting diode ~LED! structures were similarly made by
deposition of various organic materials from Mo crucibles in
a high vacuum deposition chamber. Commercially available
high purity Alq and TAD were used in the light emitting
diode structures.
The PTCDA molecules stack in planes nearly parallel to
the substrate
9,10
in both the LED and FET device structures.
The molecular orientation and its measurements are dis-
cussed in more detail later in this article. Field-effect transis-
tors measure electronic transport along the PTCDA active
layer/gate dielectric interface; therefore, the measured elec-
tronic transport represents transport parallel to the molecular
planes of PTCDA. Measurements in a vacuum probe station
were made on transistors of channel lengths 1.5, 4, 12, and
25
m
m with a channel width of 250
m
m, and about 50 nm
thickness of PTCDA on both SiO
2
/Si and MgF
2
/Si sub-
strates. In Fig. 1, the drain-source current versus drain-source
voltage for different gate voltages is shown for a device with
a 12-
m
m-long channel on a SiO
2
/Si substrate. The increasing
drain-source current with positive gate voltage is character-
istic of an n-channel FET. This indicates that an accumula-
tion layer of electrons forms in PTCDA, and these electrons
move along the PTCDA/gate dielectric interface. The mea-
sured carrier mobilities range from 10
2 5
to 10
2 4
cm
2
/V s.
The transistors do not exhibit any inversion-mode ~p chan-
nel! operation and a negative gate voltage results in further
depletion of carriers. Furthermore, a careful examination of
the current–voltage characteristics shows no evidence of
hole transport
11
even at electric fields of 2.53 10
5
V/cm.
This is significant because the inability to realize p-channel
a!
Present address: Department of Physics, University of California at San
Diego.
b!
Corresponding author. Electronic mail: ananth@physics.lucent.com
c!
Present address: Department of Chemistry, University of Bari, Bari ~Italy!.
d!
Present address: CPS Chemical Company, Old Bridge, NJ 08857.
6804 J. Appl. Phys. 81 (10), 15 May 1997 0021-8979/97/81(10)/6804/5/$10.00 © 1997 American Institute of Physics
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operation ~i.e., a hole accumulation layer! may be because of
difficulty in modulating the surface potential in a FET; how-
ever, the complete absence of a hole current points to a more
intrinsic phenomena. This conclusion was corroborated by
studying the characteristics of devices in which MgF
2
is used
in lieu of SiO
2
as a gate dielectric. Similar behavior is seen in
these devices. The FETs characteristics were also measured
at temperatures ranging from 4.2 to 340 K. PTCDA transis-
tors do not exhibit field effect behavior for temperatures
lower than 130 Kelvin. No evidence of any hole transport
was found at low temperatures as well. A study of the varia-
tion of the temperature dependence of the field-effect mobil-
ity indicates that the carriers move by hopping. The very low
mobilities are indicative of a very small electronic orbital
overlap for electron transport in the direction of transport.
The field-induced carrier in the FET are located near the
interface with the gate dielectric. Experimental work as well
as modeling has shown that, when such FETs are operated in
the accumulation mode, most of the field-induced carriers are
located within 5 nm of this interface. This is because of the
potential well created by the gate bias. The FET measure-
ments therefore probe transport in this thin layer along the
plane of the substrate.
It is important to note that the PTCDA FETs do not
operate in moist air; however, under vacuum or at atmo-
spheric pressure in an ambient of dry oxygen they exhibited
n-channel FET behavior with the mobilities and electrical
characteristics described above. The FETs did not function in
aN
2
ambient when the gas was bubbled through a water
bath, indicating that moisture adversely affects electron
transport in this material while oxygen does not appear to do
so. Many electron transporting organic materials such as
C
60
, naphthalenetetracarboxylic dianhydride are similarly
affected by moisture. Interestingly, after exposure to mois-
ture, the PTCDA FETs recover their electrical characteristics
when tested under vacuum. This behavior was repeatedly
reversible.
B. Light-emitting diodes
Pioneering work by Forrest and co-workers on contact
barrier diodes
11,12
and LEDs
13
have clearly shown that
PTCDA can transport holes normal to the molecular planes.
Forrest et al. have also documented the huge anisotropies in
the optical and dielectric properties of PTCDA along differ-
ent directions.
14
Additionally PTCDA has been shown to be
a very good optical waveguide material, possessing a large
refractive index ~2.0! and low optical loss at near-infrared
wavelengths along the molecular planes.
15
Similar findings
have been made subsequently by Ammermann and
co-workers
16
and Fuchigami et al.
17
We have fabricated a series of multilayer LED structures
in order to reconfirm hole transport normal to the molecular
FIG. 1. Room temperature PTCDA field-effect transistor drain current as a
function of drain-source voltage for a series of gate voltages. Positive gate
voltages induce electrons in the channel resulting in an increase of the drain
current with gate voltage. In the inset is shown the schematic structure of the
field-effect transistors.
FIG. 2. ~a! Layer structure and electroluminescence spectrum of an Alq ~60
nm! TAD ~60 nm! LED. The emitted light originates from the Alq and is
observed through the glass. ~b! In this structure, a 50 nm PTCDA layer is
placed between the TAD and Alq. The observed emitted light ~solid line!
still originates from the Alq but the spectrum is modified by partial absorp-
tion in the PTCDA layer. Also shown ~dotted line! is the calculated spec-
trum. It will be easily recognized that for Alq to emit light in this structure,
holes must be transported through the PTCDA. ~c! Similar device structure
as ~b! but with 100 nm of PTCDA. ~d! This LED has two Alq layers, one on
each side of the PTCDA layer. The emission spectrum is identical to that of
device ~c!.
6805J. Appl. Phys., Vol. 81, No. 10, 15 May 1997 Ostrick
et al.
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planes and further test the ability of PTCDA to transport
electrons in this direction. The LED structures are based on
the TAD/Alq material system @Fig. 2~a!# in which the TAD
solely transports holes
7
and the Alq functions as a light-
emitting layer. Our previous work with TAD/Alq LEDs, as
well as recent results
18
show that Alq has the ability to trans-
port holes in addition to electrons. The device structures
have been designed so as to provide a single PTCDA layer
the opportunity to transport~i! holes @Figs. 2~b! and 2~c!#, ~ii!
either electrons or holes @Fig. 2~d!#, and ~iii! electrons ~not
shown!. All of the LED electroluminescent spectra was mea-
sured in air. The absorption spectrum of PTCDA ~Fig. 3! has
a local maximum near 550 nm which will cause a double-
hump structure in the electroluminescence ~EL! spectra when
light from the Alq layer passes through the PTCDA layer. By
varying the thickness of the PTCDA layer, the EL spectra
will offer us a means to identify the source of light and to
test the transport properties of PTCDA.
The PTCDA absorption spectrum was determined with a
spectrophotometer and is consistent with other published
reports.
12
We also measured the photoluminescence spec-
trum of PTCDA, as shown in Fig. 3, with excitation from a
multiwavelength Ar laser. It is important to note that photo-
luminescence from PTCDA is spread over a substantially
different range of wavelengths than luminescence from Alq.
A layer of PTCDA is placed between Alq and TAD in
order to test the ability of PTCDA to transport holes. Since
TAD only transport holes, the Alq layer will emit light if the
PTCDA layer transports holes to the PTCDA/Alq interface.
The characteristic double hump structure, shown in the EL
spectrum of Figs. 2~b! and 2~c!, indicates that the Alq layer
emits light, and the PTCDA layer absorbs part of emitted
light. By varying the thickness of the PTCDA ~50 and 100
nm!, we can fit the measured EL spectra with the measured
absorption and Alq/TAD EL spectra. The fits ~which assume
that the top Al contact is reflecting! indicate that all of the
emitted light comes from the Alq layer. This demonstrates
that PTCDA transports holes in the direction normal to the
molecular planes.
The external quantum efficiency of Alq/TAD LEDs de-
pends on the cathode contact. Typically it is 0.3%–0.4%
~photons/electron! with Al cathodes and increases to more
than 0.8% with bilayer Li/Al cathodes. The quantum effi-
ciency of LEDs with a PTCDA hole transporting layer sand-
wiched between the Alq and the TAD is generally about an
order of magnitude lower than LEDs without PTCDA. This
decrease is qualitatively consistent with the findings of Bur-
rows et al.,
13
and may be partly explained by absorption in
the PTCDA layer. It is also possible that there are some
effects at the PTCDA/Alq interface which lower the quan-
tum yield relative to the TAD/Alq interface.
The device shown in Fig. 2~d! has two Alq layers ~one
on each side of the PTCDA!, and the EL spectrum of this
device also has the double hump structure. In fact, the EL
spectra of the previous structure with a single emitting layer
of Alq and same thickness ~100 nm! of PTCDA has been
superimposed to compare the two EL spectra. There is little
difference between the two EL spectra. This confirms that
thin films of Alq have the ability to transport electrons as
well as holes.
Electron transport in PTCDA normal to the plane of the
layers was evaluated by studying Al/PTCDA/Alq/TAD
LEDs in which the PTCDA is in direct contact with the
electron injecting Al cathode. For the Alq in this device to be
able to emit light, the PTCDA must transport electrons. The
EL intensity ~from Alq! and quantum efficiency were 2 to 3
orders of magnitude lower indicating that most of the in-
jected current is unipolar ~hole current!. Measurements under
vacuum ~identical to the conditions used to test the FETs!
also do not indicate any appreciable electron transport. This
is significant since moisture in the ambient may have im-
peded electron transport. The fact that no light is seen even
when the devices are tested under vacuum is evidence that
electron transport is not favored normal to the molecular
planes.
C. Film morphology
X-ray diffraction measurement were made to ensure that
the PTCDA molecules oriented parallel to the substrate in
both the FET and LED structures. Low angle x-ray diffrac-
tion data, shown in Fig. 4, of thin films of PTCDA in both
the FET and LED configuration indicate that the molecular
planes of PTCDA lie nearly parallel to the substrate. The
FIG. 3. The absorption and photoluminescence spectra of PTCDA.
FIG. 4. X-ray diffraction data from the FET structure, LED structure ~with-
out Alq and the Al cathode!, and PTCDA powder. Both the FET and LED
structures have an intensity peak at the ~102! plane indicating the PTCDA is
parallel to the substrate. The intensity peaks designated as A and B are due
to the polycrystalline ITO.
6806 J. Appl. Phys., Vol. 81, No. 10, 15 May 1997 Ostrick
et al.
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PTCDA powder gives rise to a spectrum with many peaks
corresponding to the different molecular planes. These peak
assignments were made with reference to earlier detailed
x-ray crystallography data published in Ref. 9. The spectrum
from the Si/SiO
2
/PTCDA sample, the configuration of a
FET, has one clear peak near 27.5 deg corresponding to the
102 plane. The x-ray spectrum from glass ITO/TAD/PTCDA
sample, the configuration in an LED, has a peak at 27.5 deg
~102 plane! as well as 21 and 30 deg ~due to the polycrys-
talline ITO!. The presence of the 102 peak and the absence
of other peaks in both of the thin film PTCDA samples in-
dicate that the molecules are oriented nearly parallel to the
substrate.
Additional characterization of thin film PTCDA mor-
phology was performed with electron diffraction on samples
in which PTCDA was deposited on amorphous carbon grids
in parallel with the samples used in the x-ray measurements.
This technique probes the samples from a direction roughly
perpendicular to the plane of the substrate which provides
complementary information to the x-ray diffraction data. The
electron diffraction data shows a strong signal from the 012
plane and no signal from the 102 plane. When the sample
was tilted at an angle 115 deg, the intensity of the diffrac-
tion pattern corresponding to the 012 plane reached a maxi-
mum, whereas a tilt of 215 deg produced a reduction in
intensity. The electron diffraction data corroborates evidence
from the x-ray diffraction measurements that the PTCDA
films orient with the molecular planes nearly parallel to the
plane of the substrate.
It is important to reiterate that the structural character-
ization described above was done on PTCDA films deposited
under the similar conditions to those used in the device stud-
ies. This is significant since early work by Forrest et al. have
shown that the preferential orientation of PTCDA depends
on the deposition conditions.
12
If this is indeed the case, we
have no way of knowing a priory if the structure of our
PTCDA films are similar to those reported in the literature.
Thus, our structural characterization assumes vital impor-
tance since the important conclusions depend on the molecu-
lar orientation in the two device structures ~FET and LED!
we employ. More recent structural characterization of
PTCDA films grown on Au substrates by Fenter et al. indi-
cate that PTCDA films can be grown quasiepitaxially under
the right conditions resulting in highly ordered films.
19
D. Energy levels
Let us now consider the relevant energy levels in the
materials considered above, and how these could affect
transport properties. The energy levels of the highest unoc-
cupied and the lowest unoccupied molecular orbitals
~HOMO and LUMO! are shown in Fig. 5. It is quite evident
from the data depicted in Fig. 5 that the barrier to electron
injection from the Al cathode ~F54.1 eV! to the PTCDA
~LUMO level ; 3.9–4.1 eV! is small. Despite this favorable
energy level alignment, there is very little electron transport
through the PTCDA layer as the LED data described above
indicate. In the case of the field-effect transistor, there is a
; 1 eV barrier between the Au source/drain ~S/D! electrodes
and the LUMO/HOMO of PTCDA. Unlike the case in or-
ganic LEDs, this does not present a serious problem because
the FET channel becomes highly conducting in the on state
because of the large induced charge density. C
60
FETs with
high mobility have been fabricated with Au S/D electrodes
despite the 1.3 eV barrier between the C
60
LUMO ~3.8 eV!
and the Au work function. We have also fabricated
a-sexithiophene ~HOMO energy level55.2 eV! FETs with
Al S/D electrodes ~F54.1 eV!. Thus, the energy barrier be-
tween the Au contacts and the PTCDA HOMO is unlikely to
be the cause of the absence of observable hole transport par-
allel to the molecular planes. However, the high value of the
HOMO ~. 6eV!makes it very likely that hole traps at a
lower energy prevent the formation of a hole accumulation
layer in the FETs.
III. SUMMARY
The above data and discussion illustrate the complexity
of transport in a molecular thin film. The transport properties
depend on a number of factors including molecular ordering
and orbital overlap, molecular orbital energy levels, and the
nature of the ambient. Nevertheless, it is apparent from our
study that hole transport dominates in the direction normal to
the molecular planes ~where the
p
orbital overlap is high!
while along the molecular planes PTCDA behaves as a
quasitwo-dimensional electron transporter ~in vacuum!. This
work also illustrates the utility of device measurements in
characterizing transport in such materials.
ACKNOWLEDGMENTS
The authors wish to thank R. C. Dynes and L. J. Roth-
berg for helpful discussions and I. Brener and D. Weissman
for the photoluminescence measurement.
1
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Science 265, 1684 ~1994!; A. Dodabalapur et al. Science 268, 270 ~1995!.
2
Semiconductors and Semimetals, edited by E. Conwell ~Academic, Bos-
ton, 1988!, Vol. 27.
3
H. Mohwald, D. Haarer, and G. Castro, Chem. Phys. Lett. 32, 433 ~1975!.
4
N. Karl and J. Ziegler, Chem. Phys. Lett. 32, 438 ~1975!.
5
D. Massa and N. Karl, Mol. Cyst. Liq. Cryst. 95,93~1989!.
6
L. Torsi, A. Dodabalapur, L. J. Rothberg, A. Fung, and H. E. Katz, Sci-
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FIG. 5. The HOMO and LUMO energy levels of PTCDA, Alq and TAD
and the workfunctions of Au and Al. The energy values have been as-
sembled from Ref. 20 ~PTCDA!, Ref. 21 ~Alq!, and Ref. 22 ~TAD!.
6807J. Appl. Phys., Vol. 81, No. 10, 15 May 1997 Ostrick
et al.
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