Preparation and properties of an organic light emitting diode with two emission colors
dependent on the voltage polarity
Tatsuo Mori, Kouji Obata, Kaname Imaizumi, and Teruyoshi Mizutani
Citation: Applied Physics Letters 69, 3309 (1996); doi: 10.1063/1.117289
View online: http://dx.doi.org/10.1063/1.117289
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Preparation and properties of an organic light emitting diode with two
emission colors dependent on the voltage polarity
Tatsuo Mori,a) Kouji Obata, Kaname Imaizumi, and Teruyoshi Mizutani
Department of Electrical Engineering, School of Engineering, Nagoya University, Furo-cho, Chikusa-Ku,
Nagoya 464-01 Japan
~Received 11 March 1996; accepted for publication 17 September 1996!
We developed an organic light emitting diode ~LED! that emits orange light at the forward bias
~ITO anode and Al cathode! and green light at the reverse bias ~ITO cathode and Al anode!. The
organic LED has a triple layer structure where an emission layer with different doped guest dye on
each side is interposed between two hole transport layers. Its brightness, maximum 20 cd/m2 was
not strong because electrons are blocked by the hole transport layer near the cathode. However, the
EL efficiency ~lm/W! for the reverse bias was 13 times higher than that for the forward bias.
© 1996 American Institute of Physics. @S0003-6951~96!03048-3#
The development of a color-tunable organic light emit- bias for the voltage polarity of the ITO anode and Al cathode
ting diode ~LED! using conducting polymers was reported and the reverse bias for the voltage polarity of the ITO cath-
by Berggren et al.1 The authors developed an organic LED ode and Al anode, respectively. The current–voltage charac-
with the squarylium dye-doped Alq3 emission layer whose teristics of an organic LED were measured with a program-
emission color changes with applied current.2 It was based mable electrometer with built-in current and voltage sources,
on the fact that the squarylium dye and Alq3 have different source measure unit, model 237 ~Keithley!. EL spectra were
EL efficiencies against applied current. In this letter, we measured with a photonic multichannel analyzer, PMA-10
demonstrate an organic LED whose color tuning is achieved ~Hamamatsu Photonics K.K.!. The brightness was measured
by the polarity of applied voltage. The actual emitting region with the brightness meter BM-8 ~TOPCON!, accompanied
in the organic LED with a hole transport layer and an emis- with current. The experiment was carried out in a vacuum at
sion layer was reported to be near the hole transport layer in room temperature.
the emission layer.3 If an emission layer is interposed be- Figure 1 shows the typical current density–voltage char-
tween two hole transport layers, the actual emitting region is acteristics of the device. The current at forward bias is about
alternately changed in the emission layer by the bias polarity. 100 times higher than that at reverse bias in the voltage re-
In addition, the polarity reversal is expected to be effective in gion above 10 V. The rectification ratio of current is about
dissipating space charge accumulated in organic layers under 103 for the conventional organic LED with a double layer
the forward bias and to improve the lifetime of an organic consisting of a hole transport layer and an emission layer.
LED.4 The driving voltage of the device is 7–10 V, which is higher
We used N, N8 –diphenyl–N,N8 – bis~3–methylphenyl!– than that of the ITO anode/TPD ~50 nm!/Alq3 ~70 nm!/Al
1, 18 diphenyl–4,48 -diamine ~TPD! as a carrier transport
material, 8–hydroxyquinoline aluminum ~Alq3! as a host
emitting material, and 4–dicyanomethylene–6–
( p-dimethylaminostyryl!–2–methyl–4H–pyran ~DCM! and
3–~2–benzothiazolyl!–7–diethylaminocoumarin ~C540! as
guest emitting materials. The EL device has three organic
layers. It consists of 30-nm-thick TPD thin film, 60-nm-thick
Alq3 thin film, and 30-nm-thick TPD thin film on indium–
tin–oxide ~ITO! electrode patterned on a glass substrate by
means of the vapor deposition method. The details of speci-
men fabrication are shown in a previous paper.2 An alumi-
num electrode was used as the top electrode. As TPD is an
excellent hole transport material, high electric field is re-
quired for the electron injection into TPD and the electron
transport in TPD.5 In addition, as the actual emitting region
shifted in the emission layer of the organic LED with the
third TPD layer with applied field,6 we used the 30-nm-thick
TPD layers. In order to realize two different emission colors
from an EL device, DCM and C540 were doped at 1 mol %
in the 20-nm-thick regions of Alq3 emission layer near the
ITO and Al electrodes, respectively. We called the forward FIG. 1. Typical current density–voltage characteristics of ITO/TPD ~30
nm!/Alq3 ~60 nm!/TPD ~30 nm!/Al: solid line, forward bias; broken line,
reverse bias; dash–dotted line, forward bias of ITO/TPD ~50 nm!/Alq3 ~70
a!
Electronic mail: tmori@nuee.nagoya-u.ac.jp mn!/Al.
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Appl. Phys. Lett. 69 (22), 25 November 1996 0003-6951/96/69(22)/3309/3/$10.00 © 1996 American Institute of Physics 3309
129.49.170.188 On: Fri, 19 Dec 2014 06:46:05
cathode for forward bias. The barrier heights of hole and
electron injection at the interface between ITO and TPD are
estimated to be about 1 and 2 eV, respectively. At the inter-
face between Al and TPD, the former and the latter are esti-
mated to be about 1.6 and 1.4 eV, respectively.6 It is easier to
inject holes from the TPD anode than from the Al anode.
The opposite is the case for the injection of electrons. At the
interface between TPD and Alq3, the hole injection from
Alq3 into TPD and the electron injection from TPD into
Alq3 are thought to be Ohmic. The barrier heights of the
hole injection from TPD into Alq3 and the electron injection
from Alq3 into TPD are estimated to be 0.26 and 0.73 eV,
respectively.6 In general, the current of an organic LED with
a hole transport layer is thought to be dominated by holes
since the work function of the anode metal dependence of
the current is much strong than that of cathode metal depen- FIG. 2. EL spectra as a function of applied voltage: EL spectra at forward
bias, solid line, 27.6 V; dotted line, 26.4 V; broken line, 24.5 V; dash–
dence. If the current of an organic LED is only due to the dotted line, 22 V; EL spectra at reverse bias, solid line, 26 V; dotted line,
thermal emission over the barrier height of hole injection, the 25.6 V; broken line, 24.4 V; dash–dotted line, 23.4 V.
rectification ratio ought to be over 1010. It suggests that the
electrical conduction of this device cannot be explained by
forward bias. The low brightness of the device is most likely
the thermal emission. In a previous paper,7 the electrical con-
caused by a poor electron transport in TPD between the cath-
duction in vapor-deposited organic semiconducting thin film
ode and the emission layer. The use of a bipolar carrier trans-
is thought to be dominated by such thermal emission such as
port material with better hole and electron transport ~i.e.,
Schottky injection at low electric fields, and by a tunneling
high mobility! can improve the brightness and lifetime of the
current at high electric fields of 1 MV/cm. Parker reported
device. Generally, although the use of the cathode metal with
carrier tunneling in detail for semiconducting polymer thin
lower work function reduces the threshold voltage of EL and
films.8 However, the barrier heights for hole injection for
increases the EL efficiency, the EL efficiency ~lm/W! of our
ITO/TPD and Al/TPD are much higher than that ~0.2 eV! for
device for a given current density is not much lower than that
the ITO/poly phenylen–vinylene derivative reported by
of the conventional organic LEDs in spite of the use of alu-
Parker. The barrier thickness of tunneling is described as the
minum with a higher work function and poorer electron in-
barrier height for injection divided by the electric field. Even
jection than magnesium. Figure 4 shows the EL efficiency as
if the electric field is 1 MV/cm, the barrier thicknesses for
a function of current density. The maximum EL efficiency at
hole tunneling for ITO/TPD and for Al/TPD are estimated to
reverse bias is 3.431022 lm/W and 2.731023 lm/W at for-
be 10 and 17 nm, respectively. This results shows that
ward bias. The former is 13 times higher than the latter. This
Fowler–Nordheim-type tunneling injection is quite unlikely
cannot be explained only by the difference of the photolumi-
to occur in our system. In addition, the typical conductivities
nescence ~PL! quantum yield of the guest dyes C540 and
of an organic LED at low electric field and high electric field
DCM. We have estimated the PL quantum yields of C540
are 10213210214 and 1027 210210 S/cm, respectively.
Therefore, we expect that the electrical conduction in our
organic LEDs is controlled by the carrier transport process in
the thin films. In fact, quadratic J2V behavior was observed
in our devices. However, we cannot discuss the conduction
mechanism in detail because of the poor reproducibility of
the I2V characteristics.
Figure 2 shows the bias voltage dependence of the EL
spectra. While the EL spectrum shows an emission peak of
590 nm attributed to DCM at forward bias, the EL with an
emission peak of 510 nm at reverse bias is due to C540. The
shape and position of the EL spectra, thus, depended on the
bias polarity, but not on the magnitude of the applied voltage
~current!. This suggests that the actual emitting region is lo-
cated in the doped Alq3 region near the anode at each bias
polarity.
Figure 3 shows the brightness–current density character-
istics of the device. The brightness at reverse bias is about
100 times higher than that at forward bias for a constant
applied current density. While brightnesses above 1000
cd/m2 have been observed at 100 mA/cm2 for conventional
FIG. 3. The brightness–current density characteristics of ITO/TPD ~30 nm!/
organic LEDs with hole transport layer and an Alq3 emission Alq3 ~60 nm!/TPD ~30 nm!/Al: open circles, forward bias; closed circles,
layer, the maximum brightness of our device is 20 cd/m2 at reverse bias.
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3310 Appl. Phys. Lett., Vol. 69, No. 22, 25 November 1996 Mori et al.
129.49.170.188 On: Fri, 19 Dec 2014 06:46:05
used, the ratio of the PL quantum yield is though to be at
most 4. Therefore, better EL efficiency at reverse bias must
be explained by additional factors, such as recombination
probability.
In summary, we have developed an organic LED with
different emission colors for forward and reverse biases on
the basis of previous experimental results. The use of a bi-
polar carrier transport material instead of TPD is expected to
improve the EL intensity of the LED. In spite of a large
applied voltage ~electric field! that might even lead to an
increase in dissociation of excitons, the EL efficiency at re-
verse bias was higher than that at forward bias.
1
M. Berggren, O. Inganäs, G. Gustafsson, J. Rasmusson, M. R. Andersson,
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C. W. Tang, S. A. VanSlyke, and C. H. Chen, J. Appl. Phys. 65, 3610
~1989!.
FIG. 4. The EL efficiency–current density characteristics of ITO/TPD ~30 4
T. Mori, T. Mizutani, and Minolta Camera Co., Ltd., Japan Patent No.
nm!/Alq3 ~60 nm!/TPD ~30 nm!/Al: solid line, forward bias; broken line,
3-110786 ~1991!.
reverse bias. 5
S. Miyake, T. Inagaki, T. Mori, and T. Mizutani, Trans. IEE Jpn. 115-A,
1257 ~1995! ~in Japanese!.
6
T. Mori, K. Miyachi, and T. Mizutani, J. Phys. D 28, 1461 ~1995!.
(1.431025 mol/l! and DCM (2.131025 mol/l! to be 0.80 7
T. Mori, E. Sugimura, T Kichime, and T. Mizutani, J. Phys. D 27, 1817
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(Ex5365 nm, in chloroform!. The ratio of PL quantum
8
I. D. Parker, J. Appl. Phys. 75, 1656 ~1994!.
9
J. N. Demas and G. A. Crosby, J. Phys. Chem. 75, 991 ~1971!.
yields of the two dyes is, thus, about 9. On the other hand, 10
Y. Watanabe and S. Egusa, Ext. Abstracts ~The 42nd spring Meeting,
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