Journal of the Korean Physical Society, Vol. 38, No. 1, January 2001, pp. L1∼L3
Letters
Transient Electroluminescence Study of Enhanced Recombination
Mobility in a Bilayer Organic Light-Emitting Diode
Jae Won Jang, Dong Keun Oh, Chang Hoon Lee and Cheol Eui Lee∗
Department of Physics, Korea University, Seoul 136-701
Kyungkon Kim and Jung-Il Jin
Division of Chemistry and Molecular Engineering and Center for Electro- and Photo-Responsive Molecules,
Korea University, Seoul 136-701
(Received 20 September 2000)
Transient electroluminescence (EL) measurements were carried out on indium-tin-oxide
(ITO)/poly[2-(2’-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH-PPV)/tris(8-hy droxyquinoline)aluminum (Alq3 )/Al bilayer and ITO/MEH-PPV/Al monolayer organic light-emitting
diode devices. Significantly enhanced recombination mobility and EL lifetime were obtained in the
bilayer device, explaining the greater EL efficiency in comparison to that of the monolayer device.
trode with a thickness of 100 nm was used for the holeinjecting contact. To remove the solvent, we dried the
spin-coated MEH-PPV layer at 150 ◦ C for 2 hours, and
Alq3 (280 nm) was thermally evaporated on the MEHPPV layer for the bilayer device. The Al (100 nm) electrode was deposited on top of the MEH-PPV/Alq3 layer
by using thermal evaporation at a pressure about 10−6
Torr. The active area of the devices was 1.54 mm2 . The
devices were mounted onto a strip transmission line with
a copper board, and indium solder was used to make
contacts between the device electrodes and the strip line
[8]. One side of the strip line had a 50-Ω termination
to prevent input pulse distortion, and the other side was
connected to the pulse generator.
For the transient EL measurements, voltage pulses
with 1-µs widths from a Hewlett-Packard 214B pulse
generator were applied to the device, and the emitted
light was collected by a photomultiplier tube (Hama-
In the last ten years, π-conjugation organic polymers
have attracted much attention as light-emitting materials. The organic light-emitting diodes (OLEDs) are
very attractive for development of large-area flat panel
displays and possess tunable electrical and optical characteristics, as well as flexibility [1,2]. Since the electroluminescence (EL) properties of poly(p-phenylenevinylene)
(PPV) were reported, many researchers have made efforts to improve the optical and the electrical properties.
The greatest obstacle to the better EL efficiency is the
mobility imbalance of charge carriers [3,4]. PPV and
most of its derivatives have hole mobilities superior to
the electron mobilities, and charge-carrier mobility balancing is a very important factor for better EL efficiency.
Thus, many attempts have been made to improve mobility balancing by using low work-function metals for the
cathode [5], blending the light-emitting polymer with an
electron transport material [6], and fabricating bilayer
device structures [7].
In this letter, we report transient EL measurements of the charge-carrier recombination mobility
and the EL lifetime in an ITO/MEH-PPV/tris(8hydroxyquinoline)aluminum (Alq3 )/Al bilayer device
with an Alq3 electron-transport layer in comparison to
those in an ITO/MEH-PPV/Al monolayer device where
the poly(p-phenylenevinylene) (PPV) derivative, MEHPPV, was used as the light-emitting polymer. The structures of MEH-PPV and Alq3 are shown in Fig. 1.
The MEH-PPV (62 nm) was spin coated on indiumtin-oxide (ITO) glass in an Ar atmosphere. An ITO elec∗
E-mail: rscel@korea.ac.kr Fax: +82-2-927-3292
Fig. 1. The structures of the the MEH-PPV and Alq3 .
-L1-
-L2-
Journal of the Korean Physical Society, Vol. 38, No. 1, January 2001
Fig. 4. Transient EL decays for the MEH-PPV/Alq3 bilayer and the MEH-PPV monolayer devices, which are well
fitted by a double-exponential function.
to occur. Then, the drift mobility, or the recombination
mobility, is given by
Fig. 2. Typical transient EL signals for the MEH-PPV
monolayer and the MEH-PPV/Alq3 bilayer devices. The inset shows how the delayed response time tr is defined.
matsu R928 PMT). The input pulse shape was monitored by a digital oscilloscope (HP 54600B), and the
PMT signal was recorded by a communication signal analyzer (Tektronix CSA 803A).
In transient EL measurements, the delayed response
time tr is defined as the time between the rising edge of
the voltage pulse and the occurrence of EL (Fig. 2). This
is the time for the radiative recombination of e-h pairs
Fig. 3. Recombination mobilities obtained from transient
EL measurements of the MEH-PPV/Alq3 bilayer and the
MEH-PPV monolayer devices.
µ=
d2
,
tr · (V − Von )
(1)
where d is the carrier transit distance, V is the applied
voltage, and Von = 0.6 V the is turn-on voltage, which
is the work-function difference between the ITO and the
Al [9]. Tunneling of carriers can take place when the
conduction and the valance bands are in the so-called
“flatband” condition, for which Von is necessary. For the
MEH-PPV monolayer device, the light-emission region
will be located near the cathode because of the mobility imbalance as the hole mobility is much greater than
the electron mobility. Thus, the transit distance d corresponds to the thickness of the polymer film. For the
MEH-PPV/Alq3 bilayer device, the light-emission region will be located near the MEH-PPV/Alq3 interface.
Because the HOMO (highest occupied molecular orbit)
level of Alq3 lies lower than that of MEH-PPV and in
Alq3 the electron mobility is superior to the hole mobility, it is difficult for holes to cross the Alq3 layer. In our
bilayer device, the Alq3 layer is thicker than the MEHPPV layer, so holes will wait near the MEH-PPV/Alq3
interface until electrons arrive. Thus, the time taken by
electrons dictates the time for the recombination process to occur, and d can be taken to be the thickness
of the Alq3 layer for the MEH-PPV/Alq3 bilayer device.
Figure 2 shows the EL signals for the MEH-PPV monolayer and the MEH-PPV/Alq3 bilayer devices. For the
bilayer device, a spike is noticed at the beginning of the
EL signal in contrast to the case of the monolayer device, and is ascribed to the charges accumulated at the
MEH-PPV/Alq3 interface [10].
The recombination mobilities of the e-h pairs in the
MEH-PPV monolayer and the MEH-PPV/Alq3 bilayer
devices obtained from our transient EL measurements
Transient Electroluminescence Study of Enhanced Recombination Mobility· · · – Jae Won Jang et al.
and Eq. (1) are shown in Fig. 3. It is noted that the bilayer device has an order of magnitude greater mobility
than that of the monolayer device, indicating that the
recombination process in the bilayer device is considerably faster than that in the monolayer device. The faster
recombination process arises from the electron mobility
being enhanced by the electron transport layer.
The effect of the mobility balancing is reflected in the
EL lifetime as well. Figure 4 shows the expansion plot of
the tails of the transient EL signals, which were well fitted by a double-exponential function. The shorter time
constant of the EL decay corresponds to the RC time
constant of the devices, and the longer part of the EL
decay corresponds to the EL lifetime of the e-h pairs.
The EL lifetime in the MEH-PPV/Alq3 bilayer device
('1.9 µs) is found to be much longer than that of the
MEH-PPV monolayer device ('0.3 µs), which is compatible with its EL efficiency being greater than that of the
monolayer device [11,12] due to the mobility balancing.
In this work, by using EL technique, we found the
recombination mobility and the EL lifetime of a MEHPPV/Alq3 bilayer device to be significantly greater than
those of a MEH-PPV monolayer device. Our results are
compatible with a greater EL efficiency in the bilayer
device, which can be attributed to the improved mobility balancing achieved by introducing an Alq3 electrontransport layer.
ACKNOWLEDGMENTS
This work was supported by the Korea Ministry of Science and Technology (National Research Laboratory and
through the Center for Photo- and Electro-Responsive
Molecules at Korea University) and by the Korea Re-
-L3-
search Foundation (D00064). Measurements at the Korea Basic Science Institute (KBSI), Seoul Branch, are
acknowledged.
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