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Preparation and electronic properties of Schottky devices utilizing an asymmetrically substituted metal-free phthalocyanine Langmuir-Blodgett film as barrier layers
ELSEVIER
Synthetic
Metals
71 (1995) 2249-2250
Preparation and electronic properties of Schottky devices utilizing an asymmetrically substituted metal-free phthalocyanine Langmuir-Blodgett film as barrier layers Y.Q. Liu, Y. Xu and D.B. Zhu Institute of Chemistry, Academia Sinica, Beijing 100080, China
Abstract Langmuir-Blodgett
films of an asymmetrically
substituted phthalocyanine, nitro-tri-t-butylphthalocyanine (NtBuPc) were fabricated The L-B filmlI1‘0 cell was prepared and its electronic properties were investigated. effect with a rectification ratio of 10 at f 2.0 V. The ideality factors were 1.3 in the low voltage region and
with a Z-type deposition
model.
cell showed a rectifying
An Al/NtBuPc
3 I in the high voltage region. respectively.
The origin of the rectifying properties
for such a cell is discussed.
order to make the Schottky device, about 3 mm width of longer
INTRODUCTION
edge of the IT0 was etched and this part was used for the Ag‘The incorporation Blodgett
considerable
attention thermal,
those works,
or
derivatives.
Langmuir-
of view,
years
because
and photochemical
the phthalocyanines
symmetrically
the orientation molecules
behaviour
asymmetrically
of
their
stability.
In
ETahalocyanineLayer
used were almost
substituted
The active area was 3 mm
X 12 mm square.
Top Electrode (Al)
phthalocyanine
original.
Our interesting metal-free
substituents
forming material
of donor-acceptor
is to employ an
phthalocyanine
as a promising
candidate
and to improve the electronic
deposition
substituted
in the L-B films would reveal the
substituted
donor-acceptor
fabrication
recent
paste contact with Al top electrode.
On the other hand, from the molecular rectification
phthalocyanine electrical
in
chemical
however,
unsubstituted
Z-type
derivative
(L-B) films into Schottky devices has been attracting
remarkably
point
of phthalocyanine
method.
In this
work,
with
for a film-
parameters
by a
we describe
the
of Schottky devices based on such a phthalocyanine
and discuss the electrical properties
Bottom Electrode (ITO)
of the resulting structures.
EXPERIMENTAL
Figure 1. Schematic diagram of Al/NtBuPc
The asymmetrically
substituted
phthalocyanine,
nitro-tri-t-
butylphthalocyanine condensation of
(NtBuPc) was synthesized by a mixed two corresponding 3substituted-l,
diiminoisoindolines
[I].
L-B films were prepared
on a KSV
L-B film/IT0
cell.
RESULTS AND DISCUSSION Fig.2 shows the current density-voltage
(J-V) curve from an
5000 system.
A given amount (- 200 pl ) of NtBuPc-CHC13
Al/50 layers of NtBuPc /IT0 cell. The turn-on voltage for such a diode is 1.l V, while no obvious breakdown voltage‘ is
solution
spreaded
observed
was
on a double-distilled
water
surface.
up to -3.0 V.
rectifying
behaviour
NtBuPc is 1.6 X low8 S/cm as calculated from the linear slope of the J-V curve on the assuming that the thickness (50 layers)
was firstly washed
of L-B films is 578 A (I 3.8 X 46.9156 X 50) [2].
water, methanol,
with detergent, and chloroform
then with freshly distilled in an ultrasonic
bath for 30
min each, dried. and immediately used. An aluminium contact was vacuum evaporated on top of the L-B films through a shadow mask, fomnng the device structure shown in Fig. 1. In 0379-6779/95/%09.50
0
1995 Elsevier
SSDI 0379-6779(94)03244-Z
Science
S.A.
All rights reserved
with a rectification
The cell exhibits
Monolayer was transferred onto a substrate by vertical dipping method at a surface pressure of 25 mN/m. The substrate, indium-tin oxide Nesa glass (ITO, 10 Rlsq.) cut into 2.5 X 5 cm
ratio of 10 at + 2.0 V The conductivity
for
Fig.3 shows the plot of 1nJ versus V for the Al/NtBuPc L-B films/IT0 cell. When a forward bias voltage is applied at the Al electrode the forward current is represented relationship [3]
by the following
Y.Q. Liu et al. I Synthetic Metals 71 (1995) 2249-2250
2250
organic molecule containing
the appropriate
group separated by a short sigma-bonded exhibit diode characteristics. obtain
molecular
reported.
Since then, a lot of attempts
rectification
16-81
in
L-B
It is worth to point
structure
used by Ashwell
different
from that
donor and acceptor
bridge (D-o-A) should systems
have
to
been
out that the molecular
et al. was D-rc-A type; which was
of Aviram
and Ratner
model
(D-o-A).
Ashwell et al. have proved that Schottky barrier effects are not important by placing passive organic barriers .between the metal layers and the active molecules. same molecular deposition,
structure
the acceptor headgroup
IT0 electrode,
Voltage (V)
-10 t Figure 2. Current density-voltage film/IT0
cell,
deposition
The
curve from an Al/NtBuPc
L-B films
are
50 layers
and the aliphatic
aluminium electrode. molecular
with
L-B
Z-type
model.
rectifier
J is the current
density,
(1) current
J, is the saturation
density, q is the charge of an electron, V is an applied voltage, K is the Boltzmann
constant,
is the
factor.
dominates,
the
recombination ]4]
value
T is the absolute temperature and n When the ideal diffusion current
of n is equal
current dominates,
factors n are calculated
to
lines (Fig.3),
This indicates
that in the low-bias
cell behaved
well junction
dominates
the series
the
the ideality
properties
of NtBuPc
1.9 V, respectively.
voltage region the Schottky
resistance
the J-V characteristics.
low conductivity
when
to be 1.3 for the voltage less than - 1.9
V and 3.1 for the voltage higher than -
region
1, and
the value of n is equal to 2.
From the slope of the straight
voltage
contacts with the
r-butyl groups
contact
Such structure might be expected
properties.
with
to show
With the forward bias electron
would be to pass only from the Al anode to the NtBuPc cation moiety (DSt)
and from the NtBuPc anion moiety (As-) to the
the D(l+S)+-rc-A(l+s)-
ideality
For the ideal Z-type
intimately
IT0 cathode, while under reverse bias a high energy barrier to
.I = J, exp(qV/nkT) where
In our case, the NtBuPc has
of D-n-A type.
and in the high-bias
of the NtBuPc
L-B film
The current is limited by the
film as described
state limits the electron flow.
NtBuPc is also an optical second harmonic active molecule. SH intensity relationship
Unfortunately,
follows
diode behaviour
to the square of the
we failed to observed
[2], demonstrating
not as perfect as expected. dependence
aluminium
On the other hand, the forward bias
1nJ a V (Fig.3),
for Al/NtBuPc
observing
electrode.
which suggests
L-B Iihn/ITO
Very
by an alternative
metal/L-B film/metal
structures,
we
succeeded
of SH intensity
L-B deposition
would help us to identify the molecular
that the
cell was mainly
the organic layer and
recently,
a nearly quadratic dependence
film thickness
such a
that the Z-type L-B films were
the result of a Schottky barrier between the
(SHG)
For the ideally ordered Z-type L-B films, the
should increase proportional
film thickness.
Since the
generation
rectifying
in
on the
strategy.
It
properties
in
and these works are underway.
before (- 10-8
S/cm), which leads to a larger n value.
ACKNOWLEDGEMENT This project was supported Foundation
by the National. Natural
Science
of China.
REFERENCES 1. Y.Q. Liu, D.B. Zhu, T. Wada, A. Yamada and H. Sasabe, J.Hetercyclic
Chem., in press.
2. Y.Q. Liu, Y. Xu, D.B. Zhu, T. Wada, H. Sasabe, L. Liu and W. Wang, Thin Solid Films, 244 (1994) 943 3. S.M. Sze, Physcis ofSemiconductor
Devices. 2nd Ed., John
Wiley & Sons, New York, 1981, p.264.
I
I
I
I
0
1
2
3
Voltage (V) Figure 3. Plot of 1nJ versus V for the Al/NtBuPc
L-B film/IT0
cell. In 1974, Aviram and Ratner [5] predicted
that an asymmetric
4. C. Park, H.W. Nam, A.A. Ovchinikov Synth.Met., 55-57 (1993) 4065.
and Y.W. Park,
5. A. Aviram and M.A. Ratner, Chem.Phy&s.Lett., 29 (1974) 277. 6. R.M. Metzger and C.A. Panetta, New J.i!‘hem., 15 (1991) 209. 7. AS. Martin, J.R. Sambles and G.J. Ashwell, Physical Review Letters, 70 (1993) 2 18. 8. D.H. Waldeck and D.N. Beratan, ScScience.2C;f (1993) 576.
Synthetic
Metals
71 (1995) 2249-2250
Preparation and electronic properties of Schottky devices utilizing an asymmetrically substituted metal-free phthalocyanine Langmuir-Blodgett film as barrier layers Y.Q. Liu, Y. Xu and D.B. Zhu Institute of Chemistry, Academia Sinica, Beijing 100080, China
Abstract Langmuir-Blodgett
films of an asymmetrically
substituted phthalocyanine, nitro-tri-t-butylphthalocyanine (NtBuPc) were fabricated The L-B filmlI1‘0 cell was prepared and its electronic properties were investigated. effect with a rectification ratio of 10 at f 2.0 V. The ideality factors were 1.3 in the low voltage region and
with a Z-type deposition
model.
cell showed a rectifying
An Al/NtBuPc
3 I in the high voltage region. respectively.
The origin of the rectifying properties
for such a cell is discussed.
order to make the Schottky device, about 3 mm width of longer
INTRODUCTION
edge of the IT0 was etched and this part was used for the Ag‘The incorporation Blodgett
considerable
attention thermal,
those works,
or
derivatives.
Langmuir-
of view,
years
because
and photochemical
the phthalocyanines
symmetrically
the orientation molecules
behaviour
asymmetrically
of
their
stability.
In
ETahalocyanineLayer
used were almost
substituted
The active area was 3 mm
X 12 mm square.
Top Electrode (Al)
phthalocyanine
original.
Our interesting metal-free
substituents
forming material
of donor-acceptor
is to employ an
phthalocyanine
as a promising
candidate
and to improve the electronic
deposition
substituted
in the L-B films would reveal the
substituted
donor-acceptor
fabrication
recent
paste contact with Al top electrode.
On the other hand, from the molecular rectification
phthalocyanine electrical
in
chemical
however,
unsubstituted
Z-type
derivative
(L-B) films into Schottky devices has been attracting
remarkably
point
of phthalocyanine
method.
In this
work,
with
for a film-
parameters
by a
we describe
the
of Schottky devices based on such a phthalocyanine
and discuss the electrical properties
Bottom Electrode (ITO)
of the resulting structures.
EXPERIMENTAL
Figure 1. Schematic diagram of Al/NtBuPc
The asymmetrically
substituted
phthalocyanine,
nitro-tri-t-
butylphthalocyanine condensation of
(NtBuPc) was synthesized by a mixed two corresponding 3substituted-l,
diiminoisoindolines
[I].
L-B films were prepared
on a KSV
L-B film/IT0
cell.
RESULTS AND DISCUSSION Fig.2 shows the current density-voltage
(J-V) curve from an
5000 system.
A given amount (- 200 pl ) of NtBuPc-CHC13
Al/50 layers of NtBuPc /IT0 cell. The turn-on voltage for such a diode is 1.l V, while no obvious breakdown voltage‘ is
solution
spreaded
observed
was
on a double-distilled
water
surface.
up to -3.0 V.
rectifying
behaviour
NtBuPc is 1.6 X low8 S/cm as calculated from the linear slope of the J-V curve on the assuming that the thickness (50 layers)
was firstly washed
of L-B films is 578 A (I 3.8 X 46.9156 X 50) [2].
water, methanol,
with detergent, and chloroform
then with freshly distilled in an ultrasonic
bath for 30
min each, dried. and immediately used. An aluminium contact was vacuum evaporated on top of the L-B films through a shadow mask, fomnng the device structure shown in Fig. 1. In 0379-6779/95/%09.50
0
1995 Elsevier
SSDI 0379-6779(94)03244-Z
Science
S.A.
All rights reserved
with a rectification
The cell exhibits
Monolayer was transferred onto a substrate by vertical dipping method at a surface pressure of 25 mN/m. The substrate, indium-tin oxide Nesa glass (ITO, 10 Rlsq.) cut into 2.5 X 5 cm
ratio of 10 at + 2.0 V The conductivity
for
Fig.3 shows the plot of 1nJ versus V for the Al/NtBuPc L-B films/IT0 cell. When a forward bias voltage is applied at the Al electrode the forward current is represented relationship [3]
by the following
Y.Q. Liu et al. I Synthetic Metals 71 (1995) 2249-2250
2250
organic molecule containing
the appropriate
group separated by a short sigma-bonded exhibit diode characteristics. obtain
molecular
reported.
Since then, a lot of attempts
rectification
16-81
in
L-B
It is worth to point
structure
used by Ashwell
different
from that
donor and acceptor
bridge (D-o-A) should systems
have
to
been
out that the molecular
et al. was D-rc-A type; which was
of Aviram
and Ratner
model
(D-o-A).
Ashwell et al. have proved that Schottky barrier effects are not important by placing passive organic barriers .between the metal layers and the active molecules. same molecular deposition,
structure
the acceptor headgroup
IT0 electrode,
Voltage (V)
-10 t Figure 2. Current density-voltage film/IT0
cell,
deposition
The
curve from an Al/NtBuPc
L-B films
are
50 layers
and the aliphatic
aluminium electrode. molecular
with
L-B
Z-type
model.
rectifier
J is the current
density,
(1) current
J, is the saturation
density, q is the charge of an electron, V is an applied voltage, K is the Boltzmann
constant,
is the
factor.
dominates,
the
recombination ]4]
value
T is the absolute temperature and n When the ideal diffusion current
of n is equal
current dominates,
factors n are calculated
to
lines (Fig.3),
This indicates
that in the low-bias
cell behaved
well junction
dominates
the series
the
the ideality
properties
of NtBuPc
1.9 V, respectively.
voltage region the Schottky
resistance
the J-V characteristics.
low conductivity
when
to be 1.3 for the voltage less than - 1.9
V and 3.1 for the voltage higher than -
region
1, and
the value of n is equal to 2.
From the slope of the straight
voltage
contacts with the
r-butyl groups
contact
Such structure might be expected
properties.
with
to show
With the forward bias electron
would be to pass only from the Al anode to the NtBuPc cation moiety (DSt)
and from the NtBuPc anion moiety (As-) to the
the D(l+S)+-rc-A(l+s)-
ideality
For the ideal Z-type
intimately
IT0 cathode, while under reverse bias a high energy barrier to
.I = J, exp(qV/nkT) where
In our case, the NtBuPc has
of D-n-A type.
and in the high-bias
of the NtBuPc
L-B film
The current is limited by the
film as described
state limits the electron flow.
NtBuPc is also an optical second harmonic active molecule. SH intensity relationship
Unfortunately,
follows
diode behaviour
to the square of the
we failed to observed
[2], demonstrating
not as perfect as expected. dependence
aluminium
On the other hand, the forward bias
1nJ a V (Fig.3),
for Al/NtBuPc
observing
electrode.
which suggests
L-B Iihn/ITO
Very
by an alternative
metal/L-B film/metal
structures,
we
succeeded
of SH intensity
L-B deposition
would help us to identify the molecular
that the
cell was mainly
the organic layer and
recently,
a nearly quadratic dependence
film thickness
such a
that the Z-type L-B films were
the result of a Schottky barrier between the
(SHG)
For the ideally ordered Z-type L-B films, the
should increase proportional
film thickness.
Since the
generation
rectifying
in
on the
strategy.
It
properties
in
and these works are underway.
before (- 10-8
S/cm), which leads to a larger n value.
ACKNOWLEDGEMENT This project was supported Foundation
by the National. Natural
Science
of China.
REFERENCES 1. Y.Q. Liu, D.B. Zhu, T. Wada, A. Yamada and H. Sasabe, J.Hetercyclic
Chem., in press.
2. Y.Q. Liu, Y. Xu, D.B. Zhu, T. Wada, H. Sasabe, L. Liu and W. Wang, Thin Solid Films, 244 (1994) 943 3. S.M. Sze, Physcis ofSemiconductor
Devices. 2nd Ed., John
Wiley & Sons, New York, 1981, p.264.
I
I
I
I
0
1
2
3
Voltage (V) Figure 3. Plot of 1nJ versus V for the Al/NtBuPc
L-B film/IT0
cell. In 1974, Aviram and Ratner [5] predicted
that an asymmetric
4. C. Park, H.W. Nam, A.A. Ovchinikov Synth.Met., 55-57 (1993) 4065.
and Y.W. Park,
5. A. Aviram and M.A. Ratner, Chem.Phy&s.Lett., 29 (1974) 277. 6. R.M. Metzger and C.A. Panetta, New J.i!‘hem., 15 (1991) 209. 7. AS. Martin, J.R. Sambles and G.J. Ashwell, Physical Review Letters, 70 (1993) 2 18. 8. D.H. Waldeck and D.N. Beratan, ScScience.2C;f (1993) 576.