Effect of Mixed Solvents on Fabrication of Polymer Light Emitting ...

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200
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Taiwan
Display

Conference

Effect of
Mixed Solvents
on Fabrication of Polymer Light
Emitting
Diodes

(PLED)

C. J. Huang
,
C
. C. Kang
1
,
K.

S. Shiau
1
,

Shih
-
Wei

Feng

and

Ming
-
Chang Shih
2

Department of Applied Physics, National University of Kaohsiung, Kaohs
iung, Taiwan

1
Department of Electr
ical

Engineering, Southern Taiwan University of Technology, Tainan, Taiwan

2
Department of Electr
ical

Engineering, National University of Kaohsiung, Kaohsiung, Taiwan

E
-
mail:
c
hien
@
nu
k
.edu.tw

ABSTRACT

Different

solvents including
chloroform, mixture of
chloroform and
cyclohexanone

and

mixture of chloroform
and

chlorobenzene

have been used in fabricating
1,1,4,4
-
tetraphenyl
-
1,3
-
butadiene (TPB)

doped
poly
(
N
-
vinylcarbazole)

(
PVK
)
blue
polymer light
-
emitting
diodes (PLEDs).
T
he optimum solvents for this
spin
-
coating process

were discussed
.
Turn
-
on voltage of
the PLEDs from mixed solvents is lower than
that of
chloroform
-
only.
A
nd
then
devices with
mixed solvents
show
the

higher current d
ensity than that of the
chloroform
-
only
.
I
n
addition
,
the devices

with

optimizing
the
mixture of chloroform and
cyclohexanone
,

and

mixture
of chloroform and

chlorobenzene

showed a current
efficiency as 0.468 and 0.816 cd/A, respectively.

Keywords
:
mixed so
lvent, polymer light
-
emitting diode,
blue

color

摘要

我們使用了氯仿
、氯仿和環己酮的混合溶劑及氯仿
和氯苯的混合溶劑去製備高分子
(PVK)
掺雜染料
(TPB)

藍光
高分子發光二極體,
在實驗中尋找最佳的溶劑



發現使用混合溶劑製備的元件,其起始電壓皆低於
使用單一氯仿為溶劑的元件
之起始電壓

而且在電流密
度的表現上亦是使用混合溶劑製備的元件有較大的電
流值

另外,本實驗中氯仿和環己酮混合溶

的最佳電
流亮度效率為
0.468 cd/A

氯仿和氯苯混合溶劑的
0.816
cd/A


關鍵詞

混合溶劑、高分子發光二極體
、藍光


1.

INTRODUCTION

During the

past several years, flats panel display
(FPD) has been intensively studied.

The main purpose of
these studies is to replace the traditional monitor, cathode
ray tube (CRT)
.
Recently,
organic light
-
emitting diodes

(OLEDs)

have received significant attentio
n for
FPD
applications
.

As we know,
the highly efficient OLEDs are
usually formed by an evaporation process, and the used
device structures are

multilayer. However, for large area
displays, it is generally

thought that solution processing
will have an adva
ntage over

the evaporation processing
techniques because in this case

the light
-
emitting lay
er can
be formed by simple spin

coating

and inkjet printing.

Using spin

coating, it is expected that the preparation
conditions

will have an impact

on the device
op
toelectronic properties
, such as
the solvents chosen to
dissolve the polymer, the polymer solution concentrations,
the spin speeds
, a
nd the casting temperature

[
1
-
5
]
.

However, the great
superiority

of PLED

is
due to
its
solution system
, t
herefore the choic
e of the solvent is
very

importance
.

J. Liu et al. report that in point of organic solvents,
t
hey can divide into two kinds
;

aromatic solvents, such as
p
-
xylene
,
cyclohexanone

and
chlorobenzene
,

and
nonaromatic solvents, such as
tetrahydrofuran

(THF)

and
c
hloroform
.

The device made with

n
onaromatic

solvents
such
that

THF shows a smaller injection current than

the
device made with
a
romatic solvents such
as
dichlorobenzene
(
DCB
)

at the same

magnitude of applied
voltage.

It was also found that the

light
-
emissi
on voltage
(
V
L
-
ON
)
of the device

made with THF is substantially
greater than

that of the device made with DCB
.

T
he
difference in the
turn
-
on voltage and
injection current

among
these
solvents
can be explained by
solvation

induced morphological

effects

on p
olymer/metal contacts

[
1
]
.

The device made with

n
onaromatic

solvent
,
such
that

chloroform

has
nonaromatic
(
NAR
)

conformation.

As a
result
, the
σ
-
bonded segments

of
nonconjugated

prevent
intimate

contacts between
π
-
conjugated

segments in the
polymer backbone

and the metal atoms
.
A higher

barrier
height will be expected
.

H
owever, the
σ
-
bonded segments

of
nonconjugated

as
SiO
2

as
buffer layer

is forme
d
between
ITO film and organic emission layer
.

B
ut
the
buffer layer

can alter through aromatic solvent.
T
herefore,
we
utilize

different mixed solvent to
change the buffer
layer
to
balance the combination of hole and electron
,

and
promote the current effici
ency of devices
.

I
n the present work, we have studied
a host
-
guest
emission system
poly (
N
-
vinylcarbazole)

(
PVK
):

the
host
material
,

and
1,1,4,4
-
tetraphenyl
-
1,3
-
butadiene (TPB)
:
the
guest material

doped as blue dyes

in chloroform
, mixture
of chloroform and

cyclohexanone

and

mixture of
chloroform and

chlorobenzene
.
T
his was done in order to
study the effects of mixed solvents

and find the optim
al

solvents
.


2

2.

EXPERIMENT

In this study, the cell configuration and molecular
structures of the
materials

and solvents

are
shown in
Fig
ure
1
.

A
node indium tin oxide (ITO) glass
substrate

was cleaned by sonication in acetone solvent, then in
methanol solvent, followed by rinsing in deionized water
and let dry under a nitrogen flow. The
hole
-
injection layers

of
Poly

(3,4
-
et
hylene

dioxythiophene) poly

(styrene

sulfonate)

aqueous dispersion

(PEDOT
:PSS
) was
spin
-
coated onto the ITO coated glass and evacuated to
remove the solvent.
T
he TPB

doped
PVK blend
was
dissolved in chloroform, mixture of chloroform and
cyclohexanone, mixt
ure of chloroform and

chlorobenzene

and was
then spin
-
coated on
to

the PEDOT
:PSS

to be
an

emissive layer.

Finally, the
cathode
is obtained by vacuum
evaporation contains
1

nm of lithium fluoride,
10

nm of
calcium

and

100

nm of aluminum, which are obtained b
y
vacuum evaporation. The
current

density
-
voltage (
J
-
V
)
,
luminance
-
voltage (
L
-
V
)

and CIE coordination of the
devices were measured respectively by programmable
voltage
-
current source (Keithley 2400) and spectra scan
spectrometer (Photo Research
;

PR650). Th
e surface
morphology of polymer film was
analyzed

by optical
microscope

(
Eclipse ME600L
)
.

PL spectra was measured
by Spectrophotometer (H
itachi

F4500 FL)



Fig
.

1
. Device
configuration and molecular structures of
the
materials

and solvents
.

3.

RESULTS AND D
ISCUSSION

3.1

Mixture of
chloroform

and
cyclohexanone

In this
section, w
e

add different q
uantity

of
cyclohexanone into the emission layer

solution

from 0ml ~
2ml.

It is

found that t
he
mix
ed solution

will produce the
precipitate while adding the quantity

of cyc
lohexanone

greater than
1.5ml

in
to
the
emission layer

solution
.

F
igure
2

shows the surface morphology of a PVK
:TPB

film
spin
-
coated from the
mixed

solution (PVK:TPB

in 7ml

chloroform +
1.5m
cyclohexanone
).
T
he spin
-
coated film
has a roughness surface.



F
ig. 2. S
urface morphology of a PVK
:TPB

film
spin
-
coated from the
mixed

solution.


Fig. 3.
PL spectra at
different

q
uantity

of cyclohexanone
into the emission layer

solution
.


(a)


(b)

Fig. 4. (a)
current

density
-
voltage

and

(b)
luminance
-

v
oltage

curves

with different q
uantity

of cyclohexanone.



3

F
igure
3

shows
the PL spectra at
different

q
uantit
ies

of
cyclohexanone into the emission layer

solution

with 0ml,
0.5ml, 1ml and 1.5ml,
respectively.

The radiating intensity
of the emission solution with adding 1
.5ml cyclohexanone
is
the
weakest. It is suggested that adding 1.5ml
cyclohexanone into the emission layer

solution

behave as
chemical reaction
, leading to the conjugated polymer
become a kind of complex thing with non
-
radiating.
T
herefore, q
uantity of add
ing the cyclohexanone
is
take
n

less

than 1
ml

as the
base.

F
igure
4

shows
current

density
-
voltage

(J
-
V)
and

luminance
-
voltage (L
-
V)

curves

for
sandwich structure
light

emitting d
evices

under the same process condition

but fabricated with different q
uantit
ie
s

of cyclohexanone
into the emission layer

solution

with 0ml, 0.
1
ml,
0.3
ml
,
0.5ml

and 1ml,
respectively.

I
n
F
igure 4(a), i
t is observed
that the devices with emission layer

solution

of more
cyclohexanone

content

show a higher current density than
that of t
he
chloroform
-
only device at the same magnitude
of applied voltage.

F
or example, at 15 V, the
d
evice

(
b
)
reaches 14.5
mA/cm
2
, device (c) reaches 53.4
mA/
cm
2
,

device

(d) reaches 141
mA/
cm
2

and device

(
e
) reaches 190
mA/
cm
2
, while the device (a) reaches only

6.17
mA/
cm
2
.
It was also found that the
turn
-
on voltage

of devices made
with d
evice

(
b
)

~
d
evice

(e)
is

substantially
low
er than

that
of the
d
evice

(a) made with chloroform
-
only.
I
n

F
igure
4(b)
,

i
t was found that the

V
L
-
ON

of the device

made with
d
evice

(
a
) (
about

16

V
)

is substantially greater than

that of
the device made with
d
evice

(b) ~
d
evice

(e
) (< 14
V
)
.

However,
Figure
5

shows the current efficiency
-
voltage
characteristic of PLED with various q
uantity

of
cyclohexanone into the emission layer

solutio
n
. We could
observe the best efficiency as the q
uantity

of
cyclohexanone was 0.1ml.
The maximum luminescence
i
s
592

cd/m
2

at 18V.

T
he maximum
current

efficiencies
is
0.468

cd
/
A
.

Therefore, the optimum
q
uantity

of
cyclohexanone was 0.1ml

in the PLEDs.



Fi
g
.

5.

T
he current efficiency
-
voltage characteristic of
PLED with various q
uantity

of cyclohexanone
.

3.2

Mixture of
chloroform

and chlorobenzene

We investigated the change in

electro
-
optical
properties

of
PVK:TPB
with solvent quality in

chloroform

and chloroben
zene

mixtures.

F
igure
6

shows
current

density
-
voltage

(J
-
V)
and

luminance
-
voltage (L
-
V)

curves

in different solvent mixtures ranging from

7ml

chloroform

to

2ml

chloroform

and
7ml

chlorobenzene
.

In
F
igure 6(a),

i
t is observed that

the turn
-
on
voltage in chl
oroform
-
only
is
greater

than that in chlorobenzene
-
only.

T
he mixed
solvent, not surprisingly, falls in between

chloroform
-
only
and chlorobenzene
-
only
.
I
n
addition
,
the devices with
mixed solvents and chlorobenzene
-
only

show a higher
current density than th
at of the
chloroform
-
only device
.

I
n

F
igure
6(b)
, i
t was found that the

V
L
-
ON

of the device

made
with
chloroform
-
only

(
about

16

V
)

is substantially greater
than

that of the device made with

mixed solvent and
chlorobenzene
-
only (< 12V)
. However,
Figure
7

sh
ows the
current efficiency
-
voltage characteristic of PLED
with
chloroform

and chlorobenzene

mixtures
.

It is

observe
d that

the best efficiency as the q
uantity

of
mixed solvent
i
s
chloroform 5ml
: chlorobenzene

2
ml.
The maximum
luminescence
is
1070

cd/m
2

at 1
7
V.

T
he maximum
current

efficiencies
is
0.
816

cd
/
A
.

Therefore, the optimum
q
uantity

of
mixed solvent
was
chloroform 5ml
:
chlorobenzene

2
ml

in the PLEDs.



(a)


(b)

Fig. 6. (a)
current

density
-
voltage

(J
-
V)
and

(b)
luminance
-

voltage (L
-
V)

curves

in diffe
rent solvent mixtures
.


4


Fig
.

7.

T
he current efficiency
-
voltage characteristic of
PLED
with
chloroform

and chlorobenzene

mixtures
.

4.

C
ONCLUSION


I
n summary,

i
t
has been observed that the
current

characteristics for PLEDs fabricated under the same
conditions
from PVK:TPB with different solvents are
different.
The device made with chloroform
-
only has
poor
current efficiency 0.393 cd/A
.
T
his is due to the
chloroform of nonaromatic resulted

higher

barrier
between
ITO film and organic emission layer
.
A
nd we change

the
ratio
of the chloroform

by aromatic solvent to alter the
barrier between
ITO film and organic emission layer
.
A
ctually, we demonstrate
mixture

of
chloroform

7ml

and
cyclohexanone

0.1ml

and

mixture of chloroform
5ml
and

chlorobenzene

2ml
have a higher
current efficiency as
0.468 and 0.816 cd/A, respectively.

5.

REFERENCES

[1]

J
. Liu, T
. F.

Guo, Y
.

Shi and Y
.

Yang
, "
Solvation
induced morphological effects on the polymer
/
metal
contacts
,"

J.
Appl. Phys
.
,
89
, pp.
3668
-
3673

(
2001
).

[2]

T.

Q. Nguyen, R.C. Kwong, M.E. Th
ompson

and

B.J.
Schwartz
, "
Improving the performance of conjugated
polymer
-
based devices

by control of interchain
interactions and polymer film morphology
,"

Appl.
Phys. Lett
,
76
, pp.
2454
-
2456

(
2000
)
.

[3]

J. Liu, Y. Shi, L. Ma

and

Y. Yang
, "
Device
performance and polymer morphology in polymer
light emitting diodes: The control of device electrical
properties and metal/polymer contact
,"

J.
Appl. Phys
.
,
8
8
, pp.
605
-
609

(
200
0
)
.

[4]

Y. Shi, J. Liu

and

Y. Yang
, "
Device performance and
polymer morpholo
gy in polymer light emitting

diodes: The control of thin film morphology and
device quantum efficiency
,"

J.
Appl. Phys
.
,
8
7
, pp.
4254
-
4263

(
200
0
)
.

[5]

Y. Wang, J.S. Shen

and

C.F. Long
, "
The effect of
casting
temperature

on morphology of poly(styrene
-

ethylene/
butylenes
-
styrene) triblock copolymer
,"

Polymer
,
42
, pp.
8443
-
8446

(
2001
)
.