研究計畫名稱：微奈光機電系統研究及其在國防與商業上之應用
論文名稱：結晶性電洞傳輸層對有機發光元件之影響
作者：許勝裕(Sheng-Yu Chiu)、張志豪(Chin-Hao Chang)
周協利(Hsieh-Li Chou)、魏培坤(Pei-Kuen Wei)
聯絡人：魏培坤
通訊地址：中央研究院
應用科學及工程研究所籌備處
電話：(02)27898000
傳真：(02)27826680
e-mail：pkwei@gate.sinica.edu.tw
1
Effects of Crystallized Hole-Transport-Layer on Organic Light Emitting
Devices
Sheng-Yu Chiu, Chin-Hao Chang, Hsieh-Li Chou and Pei-Kuen Wei
Institute of Applied Science and Engineering Research, Academia Sinica.
Section 2, 128, Academia Road, Nangang, Taipei 11529, TAIWAN
pkwei@gate.sinica.edu.tw; phone 886-2-27898000 ; fax 886-2-27826680
Abstract
The properties of crystallized naphthaphenylene benzidine (NPB) thin films were studied by a polarization
modulation near-field scanning optical microscope. The NPB films show mesoscale crystallizations when
deposited on ITO substrates at elevated temperatures. The NPB crystalline domains are formed with nanometer
stripes at 100oC-120oC ITO substrates and become submicron particles when ITO temperature above 140oC.
Organic light emitting devices consisting of the crystalline NPB thin films were made to examine the
crystallization effects. Compared to a non-crystalline NPB, nanometer crystallizations improve the efficiency and
luminescence. However, the submicron crystallizations show the best in the lifetime measurement.
Keywords: OLED, crystallization, near-field optical microscopy
1. Introduction
Organic light emitting devices (OLEDs)
are important electroluminescence (EL) devices
in modern flat displays. However, the device
substrate, the NPB would form crystallized thin
film. Once NPB film is crystallized, the
morphological change during operation will be
greatly reduced and thus increase the lifetime of
stability and lifetime are still of major concerns.
One main cause of the device degradation is the
formation of non-emissive dark spots.1-2 The
nucleation and growth of these non-emissive
spots have been attributed to the cathode
delamination,3 which leads to poor injection of
elections from the cathode. The delamination is
a result of the morphological change of the
organic layers, especially the hole-transporting
layer (HTL) of the diamine.4 Due to the low
device. They also found under certain elevated
temperature, the brightness and luminescence
efficiency are considerably improved as
compared to the conventional devices using
amorphous HTL. However, due to the very
small crystalline domains, the morphology of
the crystalline domain and the degree of
crystallization were not studied. In this paper,
we employed a polarization modulation
near-field
scanning
optical
microscope
glass transition temperature (Tg), the Joule
heating by high current density would cause
HTL to change its morphology and thus shorten
the lifetime of the EL devices. Recently, Gao et
al.5 have reported an idea by using crystalline NN-di(naphthalene-1-y1)-N,N’-diphenybenzidine
(NPB) (Tg ~98oC) film as the HTL in the
OLEDs. When depositing NPB on heated ITO
(PM-NSOM)6,7 to directly image the mesoscale
crystalline domains. The PM-NSOM combines
the advantages of the super-resolution of
near-field optical microscope and polarization
sensitivity in crystals. From the PM-NSOM
images, we confirmed the mesoscale NPB
crystalline domains on ITO substrates, and the
degree and types of the NPB crystallizations.
2
2. Experimental Results
Figures 1 show the measured topographic
and PM-NSOM images of the NPB films when
deposited on ITO substrates at different elevated
temperatures. The scan area was 2m x2m.
The temperatures at the ITO substrates were
80oC, 100oC, 120oC and 140oC, respectively.
The NPB films were 400Å thickness. They were
deposited by a thermal evaporator with
deposition rates ~2 Å/sec in ~4x10-6 torr vacuum
Fig.1 The measured topographic and dichroic images of the NPB films.
chamber. The topographic images show that the
sample surface was rough when ITO
temperature was above 120oC. From 80oC to
NPB layers, the I-V, L-V and lifetime were
tested for OLEDs with different crystalline NPB
layers. Two kinds of OLEDs were made in the
120oC, the surfaces show stripe-like patterns
with r.m.s. roughness ranging from 80Å ~120Å.
When temperature up to 140oC, the stripe-like
topography becomes particle-like, the surface
was very rough, which up to ~250Å r.m.s
roughness. The dichroic images show the
crystalline domains. The bright region is the
vacuum chamber. One was the crystallized
OLED and the other was the normal OLED. The
crystallized OLED was made by depositing
NPB films at an elevated ITO substrate and then
the NPB and ITO films were cooled down to the
room temperature. After then 300 Å Alq film
(deposition rate ~2 Å/s) and 400 Å Mg:Ag (10:1)
higher crystalline domain, and the darker, the
less crystallization. The dichroic images show
same patterns as in the topographic images. The
bright region was located at the topographic
higher region. This verified that the topographic
higher region is the crystalline region.
alloy (deposition rate ~4Å/s) were in sequence
deposited on the crystallized NPB film.
Different from the crystallized OLED, the
normal OLED using amorphous HTL was made
on the room temperature ITO substrate. Figure
2(a) shows the I-V curves for normal OLED and
Upper row, topography and lower row, dichroic image.
To examine the effects of the crystalline
400
9000
normal
o
80 C
o
100 C
o
120 C
o
140 C
2
luminance(cd/m )
7000
6000
2
5000
4000
3000
2000
1000
0
-1000
0
2
normal
o
80 C
o
100 C
o
120 C
o
140 C
350
current density(mA/cm )
8000
4
6
8
10
12
300
250
200
150
100
50
0
-50
0
voltage(V)
2
4
6
8
voltage(V)
Figure 2: The I-V curves (a) and L-V curves (b) for normal OLED and OLEDs with crystalline HTLs
3
10
12
OLEDs with crystalline HTLs. The 80oC and
100oC HTL samples have a little increase in
current density. For 120oC and 140oC samples,
there are large increases of current density.
Figure 2(b) shows the L-V curves, the
electroluminance was increased for 80oC and
100oC samples. On the other hand, the
luminance was drastically decreased when HTL
temperature over 120oC.
Fig. 3 shows the L-I curves for those
2
luminances(cd/m )
1
Fig. 4:
normal
o
80 C
o
100 C
o
120 C
o
140 C
2
luminance(cd/m)
5000
3000
2000
1000
0
-1000
0
50
100
150
200
250
300
350
400
2
current density(mA/cm )
Figure 3:
4
6
8
10
12
Lifetime tests for normal and crystallized OLEDs.
using amorphous HTL. On the other hand, when
the HTL was made on 120oC-140oC ITO
substrates, the crystallization domains were
comparatively large (submicron size) and stable.
As expected, the dark-spots were retarded and
OLEDs had a longer lifetime. However, from
the PM-NSOM images, we found the
topography was very rough for highly
crystallized NPB film. This is a great
disadvantage for the OLED device. The holes
4000
-50
2
substantially increase the brightness and
efficiency. From the images of PM-NSOM, we
know the topography roughness was small and
there was nanometer size crystallization for the
80oC-100oC samples. Because of the small
crystallizations, the morphology is not stable. At
ambient condition, the dark spots kept growth
and the lifetime was very close to that of OLED
9000
6000
0.01
time (hours)
OLED. The efficiency decreased to one half of
the conventional one when ITO was heated over
120oC.
7000
normal
o
80 C
o
100 C
o
120 C
o
140 c
0
different devices. It can be found that the
external efficiencies for 80oC and 100oC
samples were twice larger than that of normal
8000
0.1
The L-I curves for normal and crystallized OLEDs.
Figure 4 shows the lifetime measurement
for those devices. The samples were tested at
~1000cd/m2 under ambient condition. Obviously,
form the ITO anode will directly flow into the
metallic cathode from topographic peaks of
NPB films without being combined with
electrons. This results in a drastically increase of
current and decrease of electroluminescence.
The crystallization of NPB thin film can help in
elongating the lifetime and the performance of
OLED. On the other hand, the roughness of
the higher the temperature on the ITO substrate,
the longer the lifetime of the OLED. An
increase of three times of lifetime is obtained for
140oC sample.
3. Discussions and Conclusion
From the measurement of I-V and L-V
curves, we deduce that small crystallizations can
4
topography will decrease the OLED brightness.
Hence a good NPB layer should have both
crystallization and smooth surface. This
condition cannot be achieved by depositing NPB
at an elevated ITO substrate. However, the
crystallization of NPB layer can also be formed
when it was deposited on room temperature ITO
substrate and then annealed at a temperature
above 100oC. Under this annealing condition, it
is possible to obtain highly crystallized NPB
4. E. M. Han, L. M. Do, N. Yamamoto, and M.
Fujihira, ,Thin Solid Films. 273, 202 (1996)
5. Z. Q. Gao, W. Y. Lai, T. C. Wong, C. S. Lee, I.
Bello, and S. T. Lee, Appl. Phys. Lett. 74, 3269
(1999)
6. D. A. Higgins, D. A. Vanden Bout, J. Kerimo
and P. F. Barbara, J. Phys. Chem. 100, 13794
(1996)
7. Pei-Kuen Wei, Sheng-Yu Chiu and Wei-Lun
Chang, Rev. Sci. Instru. 73, 2624 (2002)
thin film with flat surface. The study of
morphology change and performance of OLED
with annealed NPB layer is now in progress.
結晶性電洞傳輸層對有機發光元件之
影響
In conclusions, we’ve verified that NPB
formed mesoscale crystallized domains on
heated ITO substrate. From the measurement
results of the PM-NSOM, the crystallized
domain size was found very small and was
partially crystallized when the elevated
temperature at the ITO substrates was near
許勝裕、張志豪、周協利、魏培坤
中央研究院 應用科學與工程研究所籌備處
摘要
我們利用偏光調制式近場光學顯微術研
究電洞傳輸層(NPB)之結晶性情形及其對有
機發光元件之影響。NPB 加熱之 ITO 基板上
會形成介尺度結晶性，在 100oC-120oC 基板
上，它的結晶為奈米大小的條狀結構，超過
140oC 時則形成次微米的小顆粒。比起非結晶
性的 NPB 層，奈米大小的 NPB 結晶有較好的
發光效率與啟動電壓，次微米結晶顆粒則有最
長的操作壽命。
100oC. Under such conditions, the I-V and L-V
curves show an increase in brightness and
efficiency. When the elevated temperature was
above 120oC, the crystallization domains were
submicron sizes with saturated crystallizations.
Devices with saturated crystallizations have a
great improvement in retarding the formation of
dark-spots. However, due to the large
topographic roughness, the brightness and
efficiency are greatly decreased.
關鍵詞：有機發光元件、結晶、近場光學顯微
術
4. References
1. M. Fujihira, L.M. Do, A. Koike, and E. M.
Han, Appl. Phys. Lett. 68, 1787 (1996)
2. S. F. Lim, L. Ke, W. Wang, and S. J. Chua,
Appl. Phys. Lett. 78, 2116 (2001)
3. D. M. Roitman, J. R. Sheats, and R. L. Moon,
Appl. Phys. Lett. 69, 6002 (1996)
5