Iwamoto M., Kwon Y.-S., Lee T. (Eds.) Nanoscale Interface for Organic Electronics

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Interface Control of Vertical-Type Organic Transistors 29

high-current value of larger than 40 µA of the OSITs were achieved,

which demonstrate that it is a key factor for high performance OSITs to

optimize the electronic states such as the hole injection barrier and band

bending at the interface between the organic semiconductor layer and

the source electrode. The improved OSITs were applied to fabricate

the organic inverters which can operate at low-voltage. From the

measurement results of the inverter transfer characteristics, it was found

that the operational voltage of the organic inverters based on the

OSITs was from -2 to +2 V. In addition, the vertical-type OLETs, which

combined with the OSITs based on pentacene/CuPc and conventional

OLEDs based on Alq3 and NPD, were fabricated. The results show that

the current is controlled by relatively small gate voltage of -1 V and

typical OSITs characteristics are obtained for the OLETs. The luminance

also varies corresponding to the I-V characteristics.

2. Vertical-Type Organic Transistors

Based on the carrier transport direction, organic transistors can be

classified into two groups; lateral-type organic transistors, and vertical-

type organic transistors. Conventional OFETs is well-known as lateral-

type organic transistors, where the carrier flow is parallel to the substrate.

On the other hand, many kinds of vertical-type organic transistors have

also been reported, where the carrier flow is perpendicular to the

substrate. Figure 1 shows cross-sectional schematic illustrations of a

various vertical-type organic transistors such as (a) polymer grid triode

(PGT),

(b) organic static induction transistor (OSIT),

channel polymer field-effect transistor,

(d) vertical organic field-effect

transistor (VOFET),

(e) metal-base organic transistor (MBOT),

(f) OSIT using colloidal lithography,

(g) dual self-aligned vertical

multichannel organic transistor (DSA-VMCOT),

(h) three-dimensional

organic field-effect transistor (3D-OFET),

(i) step-edge vertical-type

channel organic field-effect transistor (SVC OFET).

Their operational

mechanisms are based on vertical-type inorganic transistors such as

metal-base transistor (MBT),

SIT

and permeable-base transistor

(PBT)

and so on. Here, we have focused on the vertical-type organic

transistors that operate as a SIT, that is, the OSITs (Fig. 1(b)).

Y. Watanabe and K. Kudo 30

The advantages of vertical-type OSITs over lateral-type OFETs are

the realization of low-voltage operation and ease of fabrication on

flexible substrates. In lateral-type OFETs, the current flows along the

channel formed in the lateral direction. In general, the channel length

between the source and drain electrodes is limited by the size of the

evaporation mask (10-20 µm). On the other hand, the current flows in the

vertical direction through the organic thin-film in a vertical-type OSIT

(Fig. 1(b)). The channel length corresponds to the thickness of the

organic semiconductors (0.1-0.2 µm). Therefore, the channel length of

(a) Polymer grid triode (PGT)

(b) Organic static induction

transistor (OSIT)

field-effect transistor

(d) Vertical organic field-effect

transistor (VOFET)

(e) Metal-base organic

transistor (MBOT)

(g) Dual self-aligned vertical

multichannel organic

transistor

(DSA

VMCOT)

(h) Three-dimensional organic

field-effect transistor

(

OFET

)

(i) Step-edge vertical-channel

organic field-effect transistor

(SVC OFET)

(f) OSIT using colloidal

lithography

Fig. 1. Various structures of vertical-type organic transistors.

Interface Control of Vertical-Type Organic Transistors 31

the OSIT is shorter than that of the OFET, which leads to lower

operational voltage. Furthermore, the effect of surface roughness on the

characteristics of OFETs may be significant, because the channel is

formed along the interface of the gate-insulator and the semiconductor

film. In contrast, the characteristics of OSITs may be less affected by

surface roughness, because the channel is formed in the vertical direction

through the thin-film. The lateral-type OFET was proposed as a

prototype for the thin-film transistor (TFT), where a highly doped silicon

(Si) substrate functions as the gate electrode.

However, in order to

fabricate flexible OFETs, not only should non-flexible Si wafers be

replaced with flexible substrates such as plastic, but the non-flexible SiO

gate-insulator should also be replaced with a flexible material. In contrast,

vertical-type OSITs consist of organic semiconductors and metal

electrodes. It should be noted that no gate-insulator is required for

fabrication of the OSITs; therefore, only the non-flexible glass substrate

requires replacement by a flexible substrate. This means that OSITs can

be fabricated on a flexible substrate in the same manner as those on

glass substrates. Considering this, a simple method such as a vacuum

evaporation technique could be applied to the fabrication of OSITs on

either glass or flexible plastic substrates. To confirm the advantages of

vertical-type FETs, OSITs using pentacene or CuPc evaporated films

were fabricated on glass substrates and the basic characteristics

were evaluated,

20,21

as detailed in Sec. 3. Furthermore, OSITs based on

pentacene evaporated film were fabricated on flexible substrates

and

the electrical characteristics under bending were measured.

These

results show that OSITs have potential for use as flexible organic

transistors with low operational voltages.

In application, optoelectronic elements using organic materials show

promise for low-cost, large-area, light-weight and flexible devices. In

particular, liquid crystals and OLEDs are expected to be used as display

components of mobile electronic devices, and OLEDs have been

developed with excellent stability and high efficiency.

24,25

On the other

hand, organic transistors have been greatly improved in recent years,

and all-organic displays are expected to be developed using organic

transistors.

26-29

However, for practicality, it is necessary to operate with a

Y. Watanabe and K. Kudo 32

drive voltage as low as a few volts and with sufficient reliability. The

vertical transistors, particularly OSITs, are suitable for flexible displays

and the basic characteristics of OSITs as driving elements for organic

display devices have already been reported.

In addition, OLETs

combined with OSITs and OLEDs,

and organic logic devices using

OSITs have also been reported.

3. Interface Control of Vertical-Type Organic Transistor

In OLEDs, it has been reported that the electron carrier injection

characteristics were improved by a thin insulating layer such as LiF

inserted between the cathode and organic layers. In addition, the

hole carrier injection characteristics were also improved by a thin copper

phthalocyanine (CuPc) inserted between the anode (typically indium-

tin-oxide (ITO) on glass) and organic layers.

32-34

These phenomena

demonstrate importance of the effect of the inserted layer between the

electrodes and the organic layer on the carrier injection in the organic

devices which is a key factor for designing organic devices. On the other

hand, in order to realize the high performance OFETs, many works have

been carried out to obtain the high field-effect mobility

35,36

and to

understanding the operation mechanism of OFETs.

37,38

Nevertheless, it

has been known to be difficult to operate OFETs at lower voltage

because of the low-carrier density and the low-carrier mobility of organic

semiconductors compared with inorganic semiconductors such as silicon.

Many works have been carried out to obtain the high field-effect

mobility by surface treatment on gate dielectric material.

OSITs have been studied for an attractive device in respect to the

realization of the high-speed and high-power operation at the lower

driving voltage.

4-6,22,23,39-42

The cross-sectional illustration and electrical

circuits of OSITs are shown in Fig. 2(a). The excellent characteristics

result from the vertical structure with very short channel length which

corresponds to the thickness of the organic semiconductors. Recently, we

have fabricated the OSITs using pentacene thin-film on ITO formed on

the flexible substrate as shown in Fig. 2(b) and have reported their

Interface Control of Vertical-Type Organic Transistors 33

basic electrical property

under the bending conditions.

The flexible

OSITs showed stable electrical characteristics under bending conditions.

However, several problems have remained in the OSITs with low on/off

ratio and low current density for use in the electronic circuits. The

schematic energy diagram of the OSITs was illustrated in Fig. 2(c). In

the OSITs, hole carriers were injected through the interface between

ITO source electrode and pentacene film and transported to Au drain

electrode. Therefore, we focus on the interface electronic states in

relation to the hole injection for improving the characteristics of the

OSITs.

Source (ITO)

Gate (Al)

Drain (Au)

Substrate (glass)

CuPc layer

Pentacene

< 0 V

< 0

> 0

CuPc layer was inserted

between source electrode

and pentacene film

Hole

Fig. 2. Cross-sectional illustration of device structure with electrical circuits for pentacene

OSITs with ultra-thin CuPc layer (a), photograph of flexible OSITs (b), and

energy band

diagram for pentacene-based OSITs (c). CuPc was deposited on ITO to

improve the

on/off ratio of OSITs.

Y. Watanabe and K. Kudo 34

In our recent research on the OSITs, an ultra-thin CuPc layer was

inserted between the ITO source electrode and the pentacene in order to

improve the carrier injection from the ITO source electrode. The CuPc

layer is well-known to the key materials to improve hole injection into

OLEDs.

32,33

As a result, the characteristics of OSITs were dramatically

improved.

In addition, the dependence of the CuPc thickness on the

static characteristics of the OSITs was investigated, which indicates that

the optimized CuPc thickness was from 1 to 3 nm.

There are many

reports on the research with regards to the CuPc/ITO systems in the

OLEDs. However, the pentacene/CuPc/ITO system has not been

reported.

In this report, we report that the effects of the electronic states of

CuPc at the interface between the pentacene and the ITO on the

characteristics of the OSITs.

In this experiment, the OSITs were also fabricated directly on an

ITO/glass substrate without the ultra-thin CuPc layer for comparison.

The effective area of the source and drain electrodes of the OSIT is

approximately 1.76 mm

. The devices are fabricated by conventional

vacuum evaporation with the substrate temperature maintained at room

temperature during the vacuum deposition. Prior to the deposition

process, the ITO/glass substrate was cleaned because the work function

of ITO is extremely sensitive to the surface cleaning,

which affects the

characteristics of OSITs.

Here, we describe not only the method of

the surface cleaning but also the work function of ITO obtained by the

results of ultraviolet photoemission spectroscopy (UPS) measurements.

The ITO substrate was subjected to 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. The work functions of the ITO substrates

with and without the surface treatment were measured by UPS. From

these results, it was found that the work function of the ITO with the

surface treatment was estimated to be 5.3 eV, which was higher than the

ITO work function of 4.3 eV without the surface treatment.

In this study, the high work function ITO with the surface treatment

was used for fabricating the OSITs. The OSITs based on pentacene thin-

film with an ultra-thin inserting CuPc layer was fabricated as follows.

Interface Control of Vertical-Type Organic Transistors 35

The different process from our past research is that the CuPc layer with a

thickness of 1 nm is deposited on the ITO source electrode formed on the

glass substrate. On the other hand, the deposition conditions of pentacene,

Al gate electrode, Au drain electrode are the same as our past research,

which are described in detail in Refs. 22, 23, 41 and 42.

Here, we compare the static characteristics for the pentacene OSITs

between with and without the CuPc layer as shown in Fig. 3. In both

samples, source-drain current (I

) at a constant drain-source voltage

) decreased with increasing gate voltage (V

) and typical OSITs

characteristics were exhibited: the slope of the I

-V

curves increased

with increasing V

without current saturation. In this experiment, V

was changed continuously from 0 to -3 V while V

was changed from

-1 to +5 V in +0.2 V steps. I

was controlled by the V

. I

was found

to increase as V

decreased from 0 to -1 V, and decrease as V

increased

from 0 to +5 V. In the OSITs without the CuPc layer, the on/off ratio

of I

at V

= -3 V under applied V

from -1 V (on state) to +5 V (off

state) was 2. On the other hand, in the OSITs with CuPc layer, the on/off

ratio was 1080, which was two orders higher than that of the OSITs

without CuPc layer. In addition, the current value of the OSIT with CuPc

layer was also higher by three orders of magnitudes than those of the

OSIT without an CuPc layer. The obtained results also indicated that the

-3-2-10

-3

-2

-1

[ 10

-6

A/cm

]

[V]

= -1 V

= +5 V

0.2 V step

on/off ratio=2

(a) without ultra-thin CuPc layer

-3-2-10

-3

-2

-1

[ 10

-3

A/cm

]

[V]

= -1 V

= +5 V

0.2 V step

on/off ratio=1080

(b) with ultra-thin CuPc layer

Fig. 3. Static characteristics of pentacene OSITs without ultra-

thin CuPc layer (a) and

with ultra-thin CuPc layer (b). High-

current on/off ratio of OSITs was achieved by

inserting ultra-thin CuPc layer between ITO source electrode and pentacene film.

Y. Watanabe and K. Kudo 36

high on/off ratio arise from the increase in I

at the on state with no

variation in I

at the off state.

To investigate the effect of CuPc layer between the ITO and the

pentacene on the characteristics of the OSITs, the electronic structure of

pentacene/ITO and pentacene/CuPc/ITO was evaluated UPS. The UPS

measurement was carried out to clarify the influence of the CuPc layer

on the electronic states at the interface of the pentacene/CuPc/ITO.

Figures 4(a) and 4(b) show the UPS spectra of pentacene/ITO and

pentacene/CuPc/ITO systems as a function of the incremental deposition

of the pentacene overlayer in the secondary region and the HOMO

region. In secondary-cutoff region, the work function of ITO was

estimated approximately 5.3 eV. In the HOMO band region, the

hole injection barrier height of pentacene (1 nm, 10 nm)/ITO was

approximately 0.35 eV and the peak of the HOMO feature was 0.85 eV

as shown in Fig. 3(a). In this case, the hole injection barrier height is

defined as the energy difference between the low binding energy edge of

the HOMO feature and the Fermi level (E

sub

On the other hand, the hole injection barrier height of CuPc

(1 nm)/ITO was approximately 0.15 eV and that of pentacene/CuPc

(1 nm)/ITO was decreased from 0.35 to 0.15 eV with decreasing the

pentacene film thickness from 10 nm to 1 nm which originates a HOMO

band tailing located between the low binding energy edge of the HOMO

feature and the E

sub

. The HOMO feature peaking at 0.85 eV was also

seen in Fig. 4(b). From the results of the static characteristics of the

OSITs and the UPS spectra, the high on/off ratio was achieved when the

increases in I

at the on state which result from the decrease in the hole

injection barrier height by the CuPc layer and the prevention of the

increasing in I

at the off state result from the same peak of the HOMO

feature in the two samples.

We have developed that both the high on/off ratio and high-current

value of the OSITs was achieved by the CuPc layer. The obtained results

demonstrate that the CuPc layer plays an important role in the fabrication

of OSITs with high-current and high-current on/off ratio by decreasing

the hole injection barrier at the interface between pentacene and ITO.

The controllability of HOMO and LUMO level of pentacene films is

strongly depends on the crystal phase and grain orientation. In order to

Interface Control of Vertical-Type Organic Transistors 37

improve the performance of the OSITs, it is a key factor to optimize the

electronic structure such as the hole injection barrier and band bending at

the interface between the organic semiconductor layer and the source

electrode.

4. Organic Inverter Based on Vertical-Type Organic Transistors

Vertical-type OSITs have advantage with regard to the lower operational

voltages compared to the conventional lateral-type OFETs. For the above

reason, OSITs were applied to fabricate the organic inverters which can

operate at low-voltages. In this study, we have fabricated two types of

organic inverters with OSITs based on pentacene films. One is the

horizontally aligned structure, the other is vertical combined structure. In

particular, the organic inverter with vertically combined structure has

advantage of the effective device area compared to that with horizontally

aligned structure. From the measurement results of the inverter transfer

characteristics, it was found that the operational voltage of the organic

inverters based on the OSITs was from -2 to +2 V.

Fig. 4.

He I UPS spectra of pentacene/ITO (a) and pentacene/CuPc/ITO (b) system as a

function of deposition amount of pentacene overlayer in secondary region and HOMO

region. All of the spectra are measured at 295 K with a -

5 V bias applied to the sample to

observe the vacuum level. The EB scale refers to E

sub

00.511.52

ITO

HOMO region

ITO

Binding Energy from E

sub

[eV]

1 nm

10 nm

Pentacene

131415161718

Intensity [arb. units]

(a) He I UPS: Pentacene / ITO

secondary region

00.511.52

secondary region HOMO region

10 nm

CuPc (1 nm)

ITO

1 nm

10 nm

CuPc

(1 nm)

Pentacene

131415161718

Intensity [arb. units]

(b) He I UPS: Pentacene / CuPc / ITO

Binding Energy from E

sub

[eV]

ITO

(a) He I UPS: Pentacene/ITO (b) He I UPS: Pentacene/CuPc/ITO

Y. Watanabe and K. Kudo 38

Organic electronic devices such as organic light-emitting diodes

(OLEDs)

and organic thin-film transistors (OTFTs)

are very attractive

in terms of the light-weight and flexibility compared to the devices

based on inorganic semiconductors. The OLEDs contribute to the further

development of practical application for paper-like sheet displays.

the other hand, the OTFTs have been advanced the research of the

driving transistors for the OLEDs.

2,47

In addition, complementary metal-

oxide-semiconductor (CMOS) inverter using the organic semiconductors

have been carried out for flexible integrated circuits.

However, the

operating voltage of the CMOS based on the organic semiconductors

49-59

were higher than that of the CMOS using the inorganic semiconductors.

Because the CMOS using the organic semiconductors was composed

of the OTFTs with lateral-type structure which operate at higher

voltage arise from low-carrier density and low-mobility in organic

semiconductors.

To overcome the disadvantages of the OTFTs mentioned above,

we have researched on the OSITs with vertical-type structure.

4-7,20-23

Recently, other research group also reported on the OSITs.

Besides the

OSITs, various vertical-type organic transistors

8-16

have been proposed to

fabricate high performance organic thin-film transistors. The SIT based

on inorganic semiconductor was proposed by Nishizawa, and showed

high-speed and high-power operations and their excellent characteristics

mainly arise from the vertical structure which has a very short length

between the source, drain and gate electrodes.

In our past research, the

SITs have been applied to fabricate the organic transistors and were

also excellent characteristics with low driving voltage and high-speed

operation due to the short channel length corresponds to the thickness of

the organic semiconductor layer. We have succeeded to fabricate the

OSIT on the flexible substrate

22,23

and to improve in on/off ratio of the

OSITs with ultra-thin CuPc layer.

41,42

In this study, organic inverters based on the OSITs were fabricated

and their characteristics were investigated. Two types of device

structures for the organic inverters were proposed: one is the

conventional horizontally aligned structure, the other is the vertically

combined structure. We report their fabrication process and the basic

characteristics of inverters using the OSITs. Figures 5(a) and 5(b) show