Effect of Hole Transport Layer and Electron
Transport Layer on the Performance of a Single
Emissive Layer Organic Light Emitting Diode
AriefUdhiarto, Yefa Sister, Sandia Rini, Muhammad
Badrul Munir
Asvial
Department of Metallurgy & Materials Engineering,
Electrical Engineering Department, Faculty of Engineering
Faculty of Engineering
Universitas Indonesia
Universitas Indonesia
Depok, Indonesia
Depok, Indonesia
arief@ee.ui.ac.id
Ahstract- In this paper we study the effect of Hole Transport
(N,N'-diphenyl-N,N'-his(l-naphtyl)-J,J'-hiphenyl-4,4"­
diamine) and Electron Transport Layer on the performance of a
·3 . 0
Layer NPB
single
layer
of
Blue
Organic
Light
Emitting
Diode.
eV
·3 , 6
BFE
Three
different structures are analyzed and are compared to the
PEDOT·PSS
reference structure. SimOLED is used to simulate and to analyze
·5,1
eV
LiF·AI
5nm
I
eV
·S . 7
both electrical and optical characteristics. We found that the
eV
addition of 1 nm hole transport layer significantly improve the
luminance by more than 1.5 times which is ascribed to the
Fig. I Band diagram of the reference device structure.
increase of the hole injection leading to the balance charge in the
emissive layer. On the other hand, the application of 1 nm
electron transport layer provide lower operating voltage which
-2.4 eV
become more significant when both hole transport layer and
electron transport layer are combined. The proposed structure
2
reaches a luminance of 49,543 cd/m at 10 V.
-3.0 eV
·3.6 eV
LiF-AI
BFE
I
Keywords- OLED; hole transport layer; electron transport
layer; emissive layer; luminance; current efficiency
I
PEDOT·PSS
-5,1eV
-5.5eV
·S.7eV
I.
(a)
INTRODUCTION
In the past decade, organic light emitting diodes (OLED)
-3.0 eV
have gained a lot of attention due to their potential application
for flat panel display as well as for lighting [1], [2].
I
simplest OLED structure consists only a single emissive layer
(EML)
sandwiched
Cathode.
In
between
order
to
two
enhance
electrodes,
the
Anode
�1--I
BFE
The
PEDOT·PSS
·3.6 eV
LiF·AI
I
·5.1eV-
and
-5.7eV
electroluminescence
efficiency, several methods have been proposed [3]-[11]. The
application of hole transport layer (HTL) as well as electron
(b)
transport layer (ETL) between the electrodes and the EML is
considered
as
the
easiest
way
to
enhance
-2.4 eV
the
-3.0eV
electroluminescence efficiency. In this paper we study the
SFE
effect of HTL and ETL in a blue single emissive layer based
-5,1 eV
optical and electrical characteristic of the OLED before and
after the insertion of HTL and ETL.
-S.SeV
-3,6eV
liF-AI
I PEDOT·PSS I
OLED. The SimOLED from Sirn4tec is used to analyze both
II.
-3.3eV
-S.7eV
-6.3eV
METHODOLOGY
(c)
We design and simulate a single emissive layer of blue
Fig. 2 Three evaluated devices structure; (a) device with HTL, (b) device
with ETL, (c) Device with HTL and ETL
OLED structure which will be used as a reference. A blue
fluorescent emitter (BFE) as EML is sandwiched between a
poly-(3,4-ethylenedioxidythiophene)-poly(styrene
978- 1-4799-655 1-9115/$3 1.00 ©20 15 IEEE
sulfonate)
137
20 15 International Conference on Quality in Research
Aluminum/Lithium Fluoride
1.8x10·
(AII LiF), as Cathode. The device structure is shown in Fig. l.
1,6x 1OS
(PEDOT-PSS) as Anode, and
We varied the thickness of BFE from 1 - lO
run
in order to
N,N'­
diphenyl-N,N'-bis(J -naphtyl)-l, 1 '-biphenyl-4,4"-diamine
� 8.0x10'
� 6.0x10,
run.
(Fig.2 (b)). Finally we apply both HTL and ETL into the basic
2,Ox10'
structure as can be seen in Fig. 2(c). For each structures, we
current
efficiency,
V
:::J
() 4,Ox10'
2,OX10'
1
'"
/
1:
The same procedure is applied for device with ETL layer
2,5X10'�
/
�1.0x10·
thickness of BFE layer is kept constant based on the reference
luminance,
.€
�
(NPB) between Anode and EML as shown in Fig. 2(a). The
the
3,Ox10' .,-
<i: 1.2x10•
for the rest device structures. Next, we apply HTL,
investigate
- -luminance (appr. ) [cd/mIl
�1,4X10'
obtain the optimum luminance. The result is used as reference
results while the NPB thickness is varried from 1 - 10
�1 3 .5X10·
-;:::::::::;===�::;:::=�--�-:
J -e- C u rront do n si1y ImNcml]
1,5x10'
g
'"
c
1,OX10"�
....J
5,Ox10'
0,0 +-��!'-__-'l=-_�__�__�----l. 0,0
o
operating
2
4
6
8
10
Voltage (V)
voltage, electric field, and charge distribution along the device.
Finally we compare all the device structures and analyze for
Fig. 41-L-V of the reference device structure
the best perfonnance.
'
=
----------r
3.5x10 n==�=c==c=�,=",==;;
'
5x10
'
!f 3.0x1 0
' !f
'
4x10 �
:!i: 2.5x10
E
'
' <i
z: 2.0x1 0
3X10 �
'iii
c
� 1.5x10 ,
' '"
2x10
�
C
,
c
� 1.0x10
'E
'
():; 5.0x10'
1x10 .3
-II-Current density with HTL
III. RESULTS AND DISCUSSION
-!!-CU rrent density with out HTL
-II-Lum inance with HTL
A.
-!!!I-Luminance with o ut HTl
Reference Device Structure
We simulate both optical and electrical properties for the
reference device structure. Figure 3 shows the Luminance as
function of BFE thickness. We found that at the thickness
below 5
run,
thickness.
the luminance is increases by increasing BFE
The
luminance
optimum thickness at 5
run.
decreases
Five
run
after
reaching
the
is considered as the
optimum thickness for BFE and therefore is used as a
reference.
Current
density-Luminance-Voltage
(I-L-V)
characteristics of the single layer OLED BFE with thickness
of 5
run
Fig. 5 Comparison of the T-L-V of the reference device versus device with
HTL. Significant improvement for luminance is obtained by insertion of I
nm NPB thickness.
is shown in Fig. 4. The luminance and current density
increase by increasing applied voltage. The operating voltage
is 4 V. The maximum luminance and current density are
2
2
s
30,613 cd/m and 1.653x10 mA/cm respectively at the
applied voltage of lO V.
B.
luminance as shown in Fig.5. Although an addition of any
NPB thickness seems to improve the luminance, we found that
Device with Hole Transport Layer
Next, we add a NPB layer as HTL as schematically shown
1 run NPB to be the optimum thickness. The luminance of
2
49,543 cd/m is obtained at V
lO V. This is almost 60%
in Fig. 2(a). In order to obtain the optimum NPB thickness, we
larger than that of the reference device at the same applied
vary the NPB thickness from 1 to 1 °
voltage. As will be discussed later, the enhancement is caused
run.
=
We found that
by the improvement of both free holes and free electrons in the
addition of NPB layer significantly improved the device
emissive layer.
C.
30000
Device with Electron Transport Layer
Next, we simulate the device with Electron Transport
Material between EML and Cathode. Band diagram of the
device is schematically shown in Fig. 2(b). As can be seen
from Fig. 2 (b), ETL behave like staircase for electrons from
the Cathode to flow to the emissive layer. Since electrons are
Q)
g
ro
facing a lower barrier, we expect that luminance will also be
enhanced as in the case of device with HTL. However, it is
21000
surprising that the luminance is even lower than that for
.!: 18000
device
E
:::J
....J
without
ETL. The
maximum
luminance
for
this
structure is obtained for the ETL thickness of 1 run which is
2
only 30,031 cd/m • From the analysis we found that the
15000
number of free electrons decrease due to the addition of the
12000�-.���-.���-.�
o
2
3
4
5
6
7
8
9 10 11
ETM layer, while the number of holes relatively remain
unchanged compared to the reference device. These results
Fig. 3 Luminance as function of BFE thickness.
suggest that holes are playing the key role for the proposed
device. Although this device produce lower luminance, we
138
found that operating voltage is also lower which is a good
point.
TABLE!.
SUMMARY OF LUMINANCE, CURRENT DENSITY AND
CURRENT EFFICIENCY OF THE SIMULATED DEVICES
D. Device with Hole Transport Layer and Electron Transport
Layer
No
Device
Structures
Luminance
[cd/m2]
Current
density
[mA/cm2]
Current
Efficiency
[cd/AI
1
2
3
4
BFE
HTL+BFE
BFE+ETL
HTL+BFE
+ETL
30,613
49,543
30,031
39,770
1.653 x 10
3.055 x 10
1.249 x 10'
1.801 x 10'
0.01852
0.01622
0.02405
0.02209
Finally, we simulate the device with HTL and ETL coexist.
Band diagram of the device is schematically shown in Fig.
2(c). We found that both luminance and current density is
higher
than
that
of
the
reference
device;
however
the
improvement is lower than that of device with HTL only at the
operating voltage of 10 V. Although the luminance of this
structure is below the device with HTL only, we found that the
of the studied devices. From Tabel 1 we can see that addition
operating voltage of the devices is the lowest compared to the
of HTL and ETL not only reduce the operating voltage but
other structures. These results are ascribed due to the barrier
also improve the current efficiency by about 20 %.
lowering of both injected electrons and holes respectively.
Figure 6 shows the comparison of the luminance versus
E. Free Carries Distribution
voltage for all simulated devices. It can be seen that device
As have been already discussed, the addition of NPB to the
with PEDOT:PSSI NPB(lnm)/BFE(5run)/LiF-AI structure is
BFE provides the highest luminance. The reason behind this
the best structure with the highest luminance. This structure
improvement is found to be the balance of the number of free
also provides the lower operating voltage below 4V. Although
holes and electrons recombination in the emissive layer. As
the addition of HTL and ETL affect the luminance as well as
can be seen from Fig. 8, the balance is found for device with
the current density, the spectral radiance for all devices are
HTL as indicated by circle in the inset. The solid lines
maintained as shown in Fig. 7. Tabel 1 summaries the results
__
BFE
__ BFE with NPB
....... BFE with ETM
� BFE with NPB & ETM
O ����..������
��
o
2 3 4 5 6 7 8 9 10 11
Fig. 8 Carrier distribution of four different device structures. The
NPB/BFE device shows highest crossing point between electron and hole
as indicated in inset by black circle.
Fig. 6 Comparison of L-V characteristics of all studied devices.
correspond to the number of free electrons while the dot lines
are the number of free holes. The red lines which are belong to
device with HTL shows highest crossing point and located in
---BFE
the middle of the emissive layer suggesting that the number of
__ BFE with NPB
....... BFE with ETM
electrons - holes recombination is the highest.
-.- BFE with NPB & ETM
IV. CONCLUSSION
We study the effect of HTL and ETL in a blue single layer
OLED. We found that 1 nm HTL layer can significantly
improve the OLED performance due to the balance charge
between free holes and electrons in the emissive layer. The
addition of 1 nm of ETL, on the other hand, reduces the
operating voltage but provide less effect in the luminance. A
2
blue OLED structure with luminance of 49,543 cd/m is
400
450
500
550
attained by using PEDOT:PSSI NPB/BFE/LiF-AI structure.
600
Fig. 7 Spectral Radiance for all evaluated devices
139
layer in tandem organic light-emitting diodes," Appl. Phys. Lett., vol.
101,no. I,p. 013301,2012.
ACKNOWLEDGMENT
We acknowledge financial support by the Directorate
General of Higher Education (DGHE), Ministry of Education
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