Iwamoto M., Kwon Y.-S., Lee T. (Eds.) Nanoscale Interface for Organic Electronics
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Interface Control of Vertical-Type Organic Transistors 39
the inverter circuit schematic and the cross-sectional illustration of the
organic inverter based on OSITs acted as load transistor and drive
transistor, respectively.
Before the measurement results of OSITs characteristics, we explain
the operational mechanism of OSITs. I
DS
under applied V
DS
condition
was controlled by the applied V
GS
. When the applied V
GS
was negative,
the hole injected from the source electrode was enhanced by increasing
the electric field at the interface between organic semiconductor and
source electrode. When the applied V
GS
was positive, the hole carrier
flow between the source and drain electrodes was restricted by the
formation of depletion layer around Al gate electrode and by decreasing
the electric field at the interface between organic semiconductor and
source electrode. In both cases, the gate leak current was much smaller
than I
DS
due to the formation of the depletion layer around Al gate
electrode.
The fabrication process of the OSITs based on the pentacene
thin-film was as follows. The ITO/glass substrate was subjected to
Au electrode (D)
2
nd
pentacene (100 nm)
1
st
pentacene (100 nm)
ITO electrode (S)
Al electrode (G)
Ultra-thin CuPc (1 nm)
Substrate (Glass)
s
Output
Input
V
DD
Load OSITs
Drive OSITs
(a)
(b)
-3 -2 -1 0 1 2
-1.5
-1
-0.5
0
Input voltage (V)
Output voltage (V)
V
DD
= 1.0 V
V
DD
= 1.5 V
V
DD
= 0.5 V
(c)
Fig. 5. Inverter circuit schematic (a), cross-sectional illustration of
the organic inverter
based on OSITs with horizontally aligned structure (b) and t
ransfer characteristics of
organic inverter based on OSITs with horizontally aligned structure (c).
Y. Watanabe and K. Kudo 40
ultrasonic agitation in water, acetone and isopropanol at 70°C for 10 min,
respectively, and then expose to isopropanol vapor. In addition, the
substrate was exposed to UV-ozone treatment at 50°C for 5 min. First,
an ultra-thin CuPc with a thickness of 1 nm is deposited on the cleaned
ITO/glass substrate to improve in on/off ratio of the OSITs.
41,42
The
ultra-thin CuPc layers are evaporated under a vacuum of 2 × 10
-4
Pa and
the evaporation rate is 0.01 nm/s. Second, a 100 nm pentacene thin-film
is deposited on the ultra-thin CuPc layer. Third, a slit-type Al gate
electrode with a thickness of 25 nm is formed on the pentacene film.
Fourth, the Al gate electrode is covered with a second 100 nm pentacene
film. Finally, the drain Au with a thickness of 30 nm is fabricated on the
pentacene film. The pentacene films are evaporated under a vacuum of
2 × 10
-4
Pa and the evaporation rate is 0.1 nm/s. Further points of the
fabrication process are described in detail in Refs. 46 and 47. The
effective area of the source and drain electrodes of the OSIT is
approximately 1.76 mm
2
.
Electrical measurements were performed using a semiconductor
parameter analyzer (4156C, Agilent) in air at room temperature.
Measurements were carried out in the dark in order to obtain OSIT
characteristics without the photovoltaic effect. Prior to investigate the
voltage transfer characteristics of the inverters integrated two OSITs, the
static characteristics such as the source-drain current (I
DS
) as a function
of the source-drain voltage (V
DS
) of two OSITs were measured.
The organic inverters with horizontally aligned structure was
composed of two OSITs which interconnecting between the electrodes as
shown in Fig. 5(b). The inverter transfer characteristics were measured
under a variety of bias conditions. The supply voltage was varied in the
range from -0.5 to -1.5 V and the transfer characteristics measured in
each case as shown in Fig. 5(c). For low values of V
in
, the drive OSITs is
on and the load OSITs is off. As the input voltage is increased the drive
OSITs is gradually turned off. A further increase in input voltage causes
the drive OSITs to be turned off and the load OSITs to be turned on. The
voltage at the output is the supply voltage. From the results, it was found
that the operating voltage of the inverter based on OSITs was lower
(-2 to +2 V) than that of the inverters using the OFETs.
49-59
The slight
hysteresis at the switch-on and switch-off voltage was observed.
Interface Control of Vertical-Type Organic Transistors 41
Organic inverters which are vertically combined structure with
two OSITs were also fabricated and the device characteristics are
investigated. In this case, the OSITs acted as load transistor were
fabricated on the OSITs acted as drive transistor as shown schematically
in Fig. 6(a). This vertically combined structures have several advantages
as a simple fabrication process compared to that with horizontally
aligned structure. In addition, the effective electrode area of the organic
inverter with vertical stacked structure was smaller than that with
horizontally aligned structure as mentioned above. The inverter transfer
characteristics were measured under the same condition with the
horizontally aligned organic inverters, which was shown in Fig. 6(b).
Compared with the horizontally aligned organic inverters, the input
and output levels do not match at the output voltage level. In addition,
the vertically aligned organic inverters have a hysteresis around the
switching voltage range. These characteristics were seemed to be due to
the low on/off ratio of the drive OSITs and the load OSITs. However, it
is difficult to determine the reason of the levels mismatch and the
hysteresis because the organic inverter with vertically combined structure
cannot be divided in two OSITs acted as load transistor and drive
transistor to measure the static characteristics of the OSITs. For the
above reason, the horizontally aligned organic inverters composed of two
OSITs with low on/off ratio was fabricated to investigate the reason of
the levels mismatch and the hysteresis. The results showed a similar
s
-3 -2 -1 0 1 2
-1.5
-1
-0.5
0
Input voltage (V)
Output voltage (V)
V
DD
= 0.5 V
V
DD
= 1.0 V
V
DD
= 1.5 V
(b)
Fig. 6. Cross-sectional illustration (a) a
nd transfer characteristics (b) of the organic
inverter based on OSITs with vertically combined structure.
Y. Watanabe and K. Kudo 42
inverter transfer characteristics to that of the vertically aligned organic
inverters. These results demonstrate that the high on/off ratio of the
OSITs acted as load transistor and drive were needed to realization high
performance organic inverter with vertically combined structure.
We have reported the characteristics of the two types of the organic
inverters based on the OSITs. The inverter transfer characteristics were
observed at lower operational voltage compared to the organic inverters
based on OFETs as reported several authors. The obtained results as
mentioned above demonstrate the feasibility of the organic inverter based
on the OSITs.
5. Vertical-Type Organic Light-Emitting Transistor
OLETs which are vertically combined with the OSITs and OLEDs are
fabricated and the device characteristics are investigated. One of the
device structure of the OLETs is shown schematically in Fig. 7(a). The
OLET consists of the OSITs based on pentacene with CuPc layer and
conventional OLED based on α-NPD and a simple structure similar to
the OLEDs.
7,61
All layers were fabricated on ITO-coated glass substrate
using the vacuum evaporation technique at approximately 10
-4
Pa. The
substrate temperature was maintained at room temperature during
evaporation.
The OLET has a slit-type Al gate electrode in the organic
semiconductor layers of OSITs. The slit-type gate electrode was formed
using a shadow evaporation mask, as described in Secs. 2 and 3. Both
the line and space dimensions of the shadow mask are 20 µm. In the
OSIT, typical thicknesses for both the first and second pentacene layers
are 100 nm. On the other hand, in the OLED, typical thicknesses for
both the α-NPD layer used as an hole transport layer and tris-(8-
hydroxyquinoline) aluminum (Alq3) layer as an emitting layer are
approximately 50 nm. The effective electrode area of the OLED using
OSIT and the OLED is approximately 1.76 mm
2
. For fabrication of the
OLETs, the controllability of the slit-type Al gate electrode is important,
as with the fabrication of OSITs. The estimated gate gap, W, was
checked using optical microscopy, and W was less than 1 µm. Figure 7(b)
Interface Control of Vertical-Type Organic Transistors 43
shows the I-V characteristics of the OLET as a function of gate voltage
V
G
. The I-V curve indicates that the Al-slit-gate forms a Schottky barrier
to pentacene layer, and pentacene behaves as a p-type semiconductor.
The I
DS
at a constant V
DS
decreases with increasing V
G
. The current I
DS
is
controlled by relatively small V
G
(from -1 to +0.8 V) and typical OSIT
characteristics are also obtained for the OLETs. We confirmed that the
luminance also varies corresponding to the I-V characteristics. These
results are reasonable, because the potential barrier formed by the
Schottky gate restricts the current flow from the source to the drain
electrodes, and the emitted light is also obstructed by the Al gate
electrode (slit gate part in the device structure). Future developments of
OLETs operating with higher power and speed are expected from
optimization of the device structure, such as the thickness of the organic
layers and the dimensions of the gate electrode. In particular, the
modulation depth and operational frequency of the OLETs are strongly
dependent on the edge features of the gate electrode.
6. Summary
We have researched on vertical-type organic transistors that operate
as SITs and recently succeeded to improve the device performance.
Pentacene
Al electrode (G)
Al electrode (D)
Substrate (Glass)
ITO electrode (S)
CuPc (3 nm)
α-NPD
Alq3
OLED
OSIT
(a)
-10-50
-6
-4
-2
0
V
DS
[V]
I
DS
[ 10
-4
A/cm
2
]
V
G
= -1 V
V
G
= +0.8 V
0.2 V step
(b)
Fig. 7. Cross-sectional structures (a) and I-V characteristics (b) of OLET.
Y. Watanabe and K. Kudo 44
Measurement of the static characteristics revealed relatively high-current
and low-voltage operations for the OSITs due to a short length between
the source, drain, and gate electrodes. Typical SIT operation, with
non-saturating I
DS
versus V
DS
characteristics, was observed. It is also
expected that the device properties will be improved by controlling the
structure of the gate electrode, and by selecting appropriate organic
materials with improved electrical characteristics.
One of the important factors for improvement in OSIT performance is
the control of the electronic structure, i.e. the hole injection barrier and
band bending at the interface between the organic semiconductor layer
and the source electrode. Both high on/off ratio and high-current value
was achieved for the OSITs by inserting an ultra-thin CuPc layer. The
experimental results demonstrate that the effect of the ultra-thin CuPc
layer on the characteristics of OSITs is not only that the hole injection
barrier at the interface between pentacene and ITO is decreased, but also
that the pentacene film orientation may be controlled.
For the realization of flexible displays based on OSITs, flexible
transistors, logic devices, and light-emitting transistors based on OSITs
were fabricated. In our past experimental results showed that the flexible
OSITs exhibited stable electrical characteristics at compressive and
tensile strains up to a bending radius of 5 mm. Organic inverters using
two OSITs displayed low operational voltage compared to those based
on conventional OFETs. The device structures and performance of
OLETs combined with OSITs and OLEDs were also investigated.
Relatively high luminance modulation with relatively low gate voltage
(±1 V) was observed in the OLET by optimizing the gate electrode and
layer thicknesses. The results obtained here demonstrate that OSITs are a
suitable element for flexible sheet displays.
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