Organic Optoelectronic Devices via Nano-Modification
Ten-Chin Wen
( Department of Chemical Engineering , National Cheng Kung University)
ABSTRACT
Organic optoelectronic devices involving the
active-matrix backplanes and light-emitting pixels
have attracted lots of researchers to devote the goal
of the flexible display/luminescence. In our labatory,
the nano-modification of devices for OFET and
OLED is proposed to improve both electrical and
electroluminescent properties. So far, there are some
satisfactory
results,
indicating
that
the
nano-modification is a good way for the future
plastic electronics.
INTRODUCTION
Organic optoelectronics has attracted a lot of
interest, especially for the prospect of flexible
displays. Basically, two types of devices are under
development: organic field-effect transistors
(OFETs), mainly for the active-matrix backplanes
controlling the display pixels; and organic
light-emitting diodes (OLEDs), the pixels
themselves.
OFETs are of interest for a variety of low-cost,
large area electronic applications, such as
active-matrix displays, chemical sensors, and flexible
microelectronic. Several groups have recently
demonstrated OFETs and integrated circuits, both on
rigid and flexible substrates and in many cases with
performance already sufficient for certain
applications [1-4]. These devices were fabricated
using
traditional
semiconductor
device
manufacturing techniques, including physical and
chemical vapor deposition, photolithography, wet
and dry etching, and lift-of.
However, performance of OFETs based on
active semi-conducting polymer should be not good
in comparison with traditional ones because it always
owns low mobility due to the randomly distributed
polymer chain. The device structure of OTFT is also
based on the design of Si transistors and generally
divided into two kinds of symmetric configuration,
top contact (TOC) and bottom contact (BOC). [4, 5]
Most researchers tried to improve the device
performance by increasing the transport mobility of
organic materials and reducing the contact resistance
of source/drain electrodes. [2, 5] Another way to
improve the mobility of charge carrier is
modification of chemical structure. One particularly
useful class of model compounds for this type of
investigation is the oligothiophenes, due to their
relatively straightforward synthesis and because of
the wide range of possible modifications in their
chemical structure. Carrier mobilities reported for α
-sexithiophene (α-6T) FETs have improved from
10-4 cm2/V s to greater than 0.01 cm2/V s [6, 7]. In
general, the performance of polymers stands roughly
one order of magnitude below of small molecules
and much lower than traditional FETs. This can be
understood from the fact that the solution processed
materials possess a poorer ordering than the
evaporated molecules. Brown and co-worker [8]
reported that the mobility of soluble polythiophene
can be increased by doping the conjugate polymer.
However, this increase is accompanied by an
equivalent increase of the conductivity. The
mobility-conductivity relationship can be described
by a simple power law, μ ∞ σδ, with δ around 0.7.
Chen et. Al. [9, 10] made the electrochemical
transistors by using PEDOT: PSS and reported that
the conductivity switching has been successfully
utilized in a low-voltage operating all-organic
electrochemical transistor and electrochemical
devices. According to their results, an additional
nano-scale polymer electrolyte can be used to
provide ions in/out conducting polymers and control
the electronic performance. From our new viewpoint,
we proposed two ways to improve the carrier
mobility. One is to use the alignment method (via ion
beam or photo) on a nano-scale film to control the
structural anisotropy of pentacene films, an active
semi-conducting layer, in thin-film transistors (TFTs)
with
conspicuously
anisotropic
electrical
characteristics. This design may achieve anisotropic
electrical characteristics of pentacene OTFTs and
enhance mobility. Another is to employ also a
nano-scale film (polymer electrolyte) to control
moving of ions in/out an active semi-conducting
layer, pentacene or conducting polymers, for
improving carrier mobility.
Electroluminescent (EL) polymers have a
particularly potential application as the active layer
in light emitting devices. Luminescence in this
system is achieved through the radiative
recombination of electron-hole pairs that exist as
excitions. Recently, there has been increasing in
interest in a more balanced recombination of the hole
and electron the emissive layer [11]. One problem
originates from the fact most EL polymers reported
to data allow dominant hole transport only in one
direction, the forward DC bias [12]. Particularly
useful methods to overcome this difficulty are (i) to
use a cathode with a low work function, such as Li,
Ca, etc., and (ii) to introduce a novel
charge-tranporting material with a high electron
affinity [13]. However, low work function metals are
very unstable in presence of moisture and oxygen.
Thermally stable bur high highly electron-affecting
- 32 -
poly(phenyl quinoxanline)(PPQ) has been used a an
electron-transporting and hole blocking material [13].
However, it is rather difficult to accommodate the
useful fabrication processing of this EL device, since
solvents for PPQ, such as chloroform, dichloroethane
and tetrachloroethane, tend to strongly dissolve most
EL polymer. Park et al [14, 15] has been reported
that the optical properties can be improved by using
ionomer as electron injection layer. Ion contents in
the polymer are controlled by the amount of acetyl
sulfate added during the sulfonation reaction.
Conducting polymers were introduced to
enhance hole injection and to prevent oxygen
diffusion into the electroluminescent layer from
indium-tin oxide (ITO). Yang et al.[16] presented
that by using metallic emeraldine salt form of
polyaniline (PANI) doped with camphor sulfonic
acid (CSA) or a combination of PANI and ITO as the
transparent anode of a polymer light-emitting diode
with MEH-PPV as the active layer, device
performance can be significantly improved. The
operating voltage can be reduced by ~30 %-50 %
with respect to the devices using ITO alone as the
hole-injecting anode due to the barrier height at the
PANI/MEH-PPV interface was reduced half of that
at ITO/MEH-PPV interface. Similar research studies
have been done utilizing the higher surface area
PANI-CSA network electrodes [17], polypyrrole
(PPy)[18], polyethylene dioxythiophene-polystyrene
sulfonated (PEDOT-PSS)[19] instead of simple
PANI-CSA system. PANI-type polymeric units on
substitution with sulfonic acid groups show
significant improvement in properties over PANI
itself, specifically: stability, solubility, and redox
behavior [20]. Since benzenesulfonic acid is a strong
acid, sulfonic acid ring-substituted polyaniline
(SPAN) is capable of self-doping. Advincula et
al.[20] utilized SPAN to modify ITO substrate by
alternate polyelectrolyte deposition technique. The
result showed significant improvements in lifetime
and efficiency compared to bare ITO. Although
various reports on PANI toward this aspect are
available in literature, little attention has been paid to
substituted derivatives of PANI [21, 22].
In our view point, a nano-scale sulfonated
PDPA (SPDPA) was employed as hole injecting
material and polyurethane ionomer as electron
injection material to enhance the optical and
electrical properties of PLEDs.
pentacene or conducting polymer, for improving
carrier mobility. In this project, pentacene or a series
of conducting polymers, such as the derivatives of
PANI and P3HT will be patterned and manufactured.
The applications of nano-scale films including
polyimide for photoalignment/ion beam treatment
and polymer electrolyte for ion doped/dedoped
modification will be used to achieve the high
mobility.
 OLEDs
Polymer light-emitting diodes (PLEDs) based on
poly[2-methoxy,5-(2’-ethylhexyloxy)-1,4-phenylene
vinylene] (MEH-PPV) were fabricated as two
devices. (i) ITO/SPDPA/MEH-PPV/Ca/Al, (ii)
ITO/PEDOT/MEH-PPV/PUI/Al
and
(iii)
ITO/PEDOT/MEH-PPV/SPU/Al. The substrate
coated with PEDOT-PSS (AI 4083, Bayer) was then
heat to 120 0C on hot plate for 2 hours. As for
SPDPA and polyurethane ionomer coated films, the
procedure is similar to the above. Emitting layer was
spun-casting from 0.8% solution in toluene onto the
hole- injecting layer in glove box, subsequently,
vapor depositing 50 nm Ca followed by 100 nm Al
onto the MEH-PPV layer. The PLED devices were
fabricated with active area as 6 mm2.
Current-voltage (I-V) and luminance-voltage
(L-V) were measured with a current/voltage source
measurement unit (Keithley 2400) and a luminance
meter (Minolta LS-110). Atomic force microscopy
(AFM) was performed with a NanoScope IIIa
(Digital Instruments Inc.) run in tapping mode.
Stress-life testing was performed in an argon-filled
glove box (MERAUN LAB MASTER 100).
RESULTS AND DISCUSSION

OFETs
The alignment of pentacene was achieved by
using the photoaligned polyimide method which is
usually employed to align liquid crystal. The
structure and morphology of aligned pentacene
films were characterized using X-ray diffraction
and atomic force microscopy (Fig. 1).
(a)
(b)
EXPERIMENTAL
 OFETs
The enhancements of carrier mobility are proposed
by using two kinds of nano-scale films. One is use
the photoalignment method on a nano-scale film to
control the structural anisotropy of pentacene films,
an active semi-conducting layer, in the thin-film
transistors with conspicuously anisotropic electrical
characteristics. Another is to employ also a
nano-scale film (polymer electrolyte) to control
moving of ions in/out an active semi-conducting,
- 33 -
Fig. 1 AFM image of (a) photoalignment of PI
and (b) polarization of UV.
Saturation region
(i)
Sulfonated
conducting
polymer
as
hole
transpoting layer
mA
Fig. 4 shows the nearly identical
current-voltage behavior for both SPDPA and
PEDOT-PSS based PLEDs. The charge injection
occurred at bias around 1.7 in both devices. There is
small leakage current (< 10 -5 mA) until the bias scan
Linear region
was over 1.7 V where charge current start to inject.
This small leakage current may cause by a flatten
ITO surface to reduce the roughness morphology and
minimize micro short across the device area using a
Fig. 2 ID-VD characteristics of the photoaligned conducting polymer with low conductivity. From Fig.
5, the device with SPDPA as hole injection layer
pentacene OTFTs.
gives a little higher brightness than the device
fabricated with PEDOT-PSS. This means efficiently
An enhanced field-effect mobility of 0.4－0.75
of hole and electron recombine to form exciton and
cm2/Vs have been achieved in the photoaligned
release energy to emit the light. Enhancement of hole
pentacene OTFTs (Fig. 2), in which the orientation
current is as good as PEDOT-PSS.
of pentacene is parallel to the direction of carriers
transport..An anisotropic ratio of 2.7 － 8.3 for
1000
100
improved mobility was achieved for the current
10
flow parallel and perpendicular to the orientation of
1
the pentacene films. The performance of OTFTs,
0.1
characterized by parameters such as field-effect
0.01
mobility, on/off current ratio, and threshold voltage,
1E-3
varies with the various aligned polyimides and the
1E-4
methods of alignment.
1E-5
-8E-5
1E-6
-8E-13
Structure:
ITO|PEDOT|MEH-PPV|Ca|Al
ITO|SPDPA|MEH-PPV|Ca|Al
1E-7
1E-8
0
2
4
6
V
Fig. 4. Current-voltage characteristics of (■)
ITO/PEDOT/MEH-PPV/Ca/Al
and
(●)
ITO/SPDPA/MEH-PPV/Ca/Al devices
2.0
ITO|PEDOT|MEH-PPV|Ca|Al
ITO|SPDPA|MEH-PPV|Ca|Al
1.5
cd/A
Another target is the modification in virtue of
nano-scale polymer electrolyte film that renders
active semi-conducting polymer doped/dedoped by
ions. The on/off ratio of polymer field-effect
transistors (PFETs) is a function of the doping level
of conducting polymers. The doping level of
conducting polymers might be due to in/out of ions
provided by polymer electrolyte. Fig. 3 shows the
ID-VD characteristics of PFETs with modification of
polymer electrolyte (PEO-LiClO4). In a normal
p-channel FETs, the current increase with applying
negative gate voltage. However, the applied gate
voltage reversed for polymer electrolyte modified
PFETs. Also, the leakage current is low in
comparison with drain current.
1.0
SiO2/PEO-LiClO4/PDMA (undoped)
0.5
Vg (from -20 V to + 80 V)
-4E-13
-4E-5
Ig (A)
Id (A)
0.0
0
4E-13
8E-13
0.0
-20.0
-40.0
Vd (V)
Fig. 3. Current-voltage characteristics of a PDMA
with polymer electrolyte (PEO-LiClO4) field-effect
transistor.

OLEDs
5
10
15
20
25
30
35
40
mA
Fig. 5. Luminance-current characteristics of (■)
ITO/PEDOT/MEH-PPV/Ca/Al
and
(●)
ITO/SPDPA/MEH-PPV/Ca/Al devices
0
4E-5
0
Device with SPDPA exhibits the better current
efficiency than that with PEDOT-PSS (Fig. 6).
Over range from 5 ~30 mA, it showed the uniform
display efficiency at about 1.4 cd/A, suggesting hole
current injection might be stable all over the
operating range. These behaviors are significantly
better than that obtained with ITO anode only. Since
- 34 -
4
256
224
voltage (V)
3
192
160
2
128
2
96
1
Considering from chemical structure of
SPDPA, we may attribute the high charge injection
ability to its high hole-carrier drift mobility of
arylamine moiety among polymer backbone which
also significantly reduce threshold voltage.
Numerous materials have been developed as hole
transport layers in organic EL device. Materials such
as TPD, NPD are the most prevalent triarylamine
derivatives, which have been widely used as hole
transport layer in OLED. [23]
Fig. 7 shows stress-life of device based in
SPDPA at a constant current of 1 mA, indicateing
that both voltage and brightness remain constant in
the stress-test measurement. The voltage increase
slowly from 2.6 V to 2.7 V and brightness decrease
from about 170 cd/m2 to 160 cd/m2. No black spots
were observed during this period. It was reported that
even in the absence of atmospheric oxygen and
moisture, oxidation of MEH-PPV occurs revealed as
aromatic aldehyde formation and loss of conjugation
via IR spectroscopy.[24] ITO anode serves as the
source of oxygen for carbonyl formation which
known to be a fluorescence quenching center. Using
a bare ITO as anode will be lead to device
degradation and eventually failure. However, this can
be overcome by using hole injection layer such as
SPDPA to block oxygen transfer. Besides blocking
of oxygen directly transfer from ITO surface into
polymer by SPDPA, it also smoothens the anode
surface prior to polymer casting. All advantages from
PEDOT-PSS in PLED are achieved by using
SPDPA.
3.8x10
3
2.6x10
3
1.3x10
3
cd/m
2
ITO|PEDOT|MEH-PPV|Ca|Al
ITO|SPDPA|MEH-PPV|Ca|Al
64
0
5
10
200
400
600
800
1000
time (min)
Fig. 7. Light output vs. elapsed stress time of
ITO/SPDPA/MEH-PPV/Ca/Al device at a constant
current of 1 mA.
(ii) Polyurethane ionomers as electron injecting
and hole blocking layers
Light-emitting diodes (LEDs) based on
conjugated polymer have attracted much attention
because of their potential applicability to plat, large
area displays which can be operated at a relatively
low driving voltage. In order to enhance the light
output and luminous efficiency, one of the most
important challenges in the field of polymer LEDs is
to obtain a balance charge injection. Thus, a balanced
injection and confinement of an electron-hole pair
can significantly improve luminous efficiency. It had
been proposed that a new hetero-structured device
which consisted of the EL polymer overlaid with the
ionomer,
satisfying
two
major
criteria
simultaneously.
Fig. 8 shows the I-V-L characteristics of
ITO/PEDOT/MEH-PPV/Al,
ITO/PEDOT/MEH-PPV/PUI/Al
and
ITO/PEDOT/MEH-PPV/SPU/Al
with
different
coating speeds. It is found that the performance of
ITO / PEDOT / MEH-PPV / ionomer/Al devices is
significantly enhanced due to the excellent electron
injection and hole blocking by ionomer. The
luminance efficiency is enhanced by thirty-fold
compared with the corresponding single-layer
MEH-PPV device. Finally, in order to check our
materials using in PLED were stable, so the lifetime
were studied. From the lifetime behaviors, we got the
information that these devices made with PUI or
SPU as modified layer were stable and the curves of
time-luminance characteristics were different from
the general PLED. It is a very interesting
phenomenon which is worth to be studied further.
0.0
0
light intensity (cd/m )
these two devices own nearly identical I-V-L
characteristics,
the
barrier
height
of
SPDPA|MEH-PPV might be estimated as  ～0.1
eV in comparison with PEDOT-PSS. This small
barrier height suggests that the current will not be
injecting-limited. Similar results were obtained with
PAN-CSA as injecting layer. Hence, a very low
barrier for hole injecting was noticed for
ITO/SPDPA junction and indicates the stability as
hole injecting materials.
15
mA
Fig. 6. Display efficiency of the devices: (■)
ITO/PEDOT/MEH-PPV/Ca/Al
and
(●)
ITO/SPDPA/MEH-PPV/Ca/Al.
- 35 -
(a)
1000
100
5.
I. H. Campbell, J. D. Kress, R. L. Martin, D. L.
Smith, N. N. barashkov, and J. P. Ferraris, Appl.
Phys. Lett. 71, 3528 (1997).
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F. Garnier, X. Z. Peng, Synth. Met., 45 (1991)
163.
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H. E. Katz, L. Torsi, Science, 268 (1995) 270.
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A. R. Brown, D. M. Deleeuw, E. E. Havinga and
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10
current density (mA/cm2)
1
0.1
0.01
0.001
Al cathode
0.0001
2000 rpm
4000 rpm
1E-005
8000 rpm
1E-006
10000 rpm
1E-007
0
(b)
1
2
3
voltage (V)
4
5
6
1000
2000 rpm
4000 rpm
100
8000 rpm
10000 rpm
10
current density (mA/cm2)
Al cathode
1
10. M. Chen, D. Nisson, T. Kugler and M. Berggren,
Appl. Phys. Lett. 81 (2202) 2011.
0.1
0.01
0.001
11. I. D. Parker, J. Apply. Phys. 75 (1994) 1656.
0.0001
1E-005
12. S. A. Jenekhe, Chem. Mater., 9 (1997) 409.
1E-006
0
1
2
3
voltage (V)
4
5
6
Fig. 8. The I-V characteristics of the polymer
light-emitting devices using the different spin speeds
of PUI and SPU as an electron injection layer (EIL)
and the MEH-PPV single device in the positively
biased field.
CONCLUSION
The nano-modification techniques are potential
for improving optical and electrical properties of
optoelectronic devices.
Ion beam and photoalignment techniques on a
nano-scale film (such as polyimide) can be used to
control the order of pentacene to achieve high
carrier mobility. Also, a nano-scale polymer
electrolyte was employed for ion doped/dedoped
modification to achieve the high mobility.
Novel SPDPA can be easily prepared by
sulfonation of emeraldine PDPA and surprisingly
served as a better hole injection material than
commercial PEDOT. Single ion polyurethane
ionomer (PUI and SPU) can be as electron injection
and hole blocking materials to enhance the optical
properties of the device.
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- 36 -