JOURNAL OF APPLIED PHYSICS
VOLUME 87, NUMBER 6
15 MARCH 2000
Mechanism of enhanced quantum efficiency in light-emitting
diode based on a poly„p-phenylenevinylene… derivative
Jae Won Jang, Dong Keun Oh, Chang Hoon Lee, and Cheol Eui Leea)
Department of Physics, Korea University, Seoul 136-701, Korea
Dong Won Lee and Jung-Il Jin
Department of Chemistry, Korea University, Seoul 136-701, Korea
共Received 23 September 1999; accepted for publication 30 November 1999兲
Transient electroluminescence 共EL兲 measurements were made for a indium-tin–oxide 共ITO兲/
poly关共2-共4-biphenylyl兲-5-共4-tert-butylphenyl兲-1,3,4-oxadiazole兲-p-phenylenevinylene兴 共PPPDPV兲/
Al organic light-emitting diode 共OLED兲, where PPPDPV is a derivative of poly共pphenylenevinylene兲 共PPV兲 carrying the electron-transporting 2-共4-biphenyl兲-5-共4-t-butylphenyl-1,
3,4-oxadiazole兲 moieties directly attached to the PPV backbone. As a result, we were able to explain
the superior external quantum efficiency of a PPPDPV OLED device by a faster mobility as well as
a longer EL lifetime in comparison to those for a PPV device. © 2000 American Institute of
Physics. 关S0021-8979共00兲08105-6兴
1,3,4-oxadiazole兲-p-phenylenevinylene兴 共PPPDPV兲 in order
to understand further the carrier mobilities in the polymers.
Synthesis of PPV and the new PPV derivative, PPPDPV,
whose repeating unit is shown in Fig. 1, has been reported
elsewhere.12 Single layer EL device structures of indium–
tin–oxide 共ITO兲/PPPDPV/Al and ITO/PPV/Al were fabricated. An ITO electrode with a thickness of 1000 Å was used
for hole injecting contact. Polymer films with thicknesses of
350 Å 共PPV兲 and 500 Å 共PPPDPV兲 were fabricated onto the
ITO-coated glass electrode by spin coating in Ar atmosphere.
Al 共1000 Å兲 was deposited on top of the polymer films by
thermal evaporation at a pressure about 10⫺6 Torr. The device structures were designed to have small active areas of 7
mm2 for low capacitance. The devices were mounted onto a
strip transmission line with a copper board, and indium solder was used to make a contact between the device electrodes and the strip line.16 One side of the strip line had a 50
⍀ termination to prevent input pulse distortion, and the other
side was connected to the pulse generator.
For the transient EL measurements, voltage pulses of
1 ␮s width from a Hewlett–Packard 214B pulse generator
were applied to the device, and the emitted light was collected by a photomultiplier tube 共Hamamatsu R928 PMT兲.
The input pulse shape was monitored by a digital oscilloscope 共HP 54600B兲, and the PMT signal recorded by a communication signal analyzer 共Tektronix CSA 803A兲.
The EL spectra for both PPV and PPPDPV are shown in
Fig. 2共a兲. The EL spectrum of PPV is in good agreement
with previous works,12 whereas PPPDPV has a blueshifted
unresolved broad EL peak at 500 nm, which is attributed to
the disruption of the main chain ␲ conjugation by the PBD
side chain. This can be explained by a steric effect exerted by
the bulky pendent groups present in every repeating unit.12,17
Figure 2共b兲 shows the external quantum efficiency for
PPPDPV, which is about 10⫺3 %, is considerably greater
than that for PPV, which is about 10⫺4 %. 12 As mentioned
Over the last decade, organic light-emitting diodes
共OLEDs兲 have attracted much attention because of their
novel properties such as tunable electric and optical characteristics, flexibility,1 and their applicability in flat panel
display.2 Poly共p-phenylenevinylene兲 共PPV兲 is the best
known, typical light-emitting organic polymer,3 which has
the hole mobility superior to the electron mobility.4,5 Such an
imbalance of the mobilities acts to reduce the electroluminescence 共EL兲 efficiencies, and thus mobility balancing of
the electron–hole (e – h) pairs is very important in EL devices employing PPV and their derivatives.6 If the mobility
balancing of the e – h pairs can be improved via selective
control of the hole and/or electron mobilities,7,8 one can expect to obtain not only significantly increased e – h recombination rates but also the emission region removed away from
the polymer/Al interface where a larger number of defects
are located.9 As it will result in the enhancement of the EL
efficiency, many researchers have attempted to attain improved balancing of the e – h pairs via fabrication of
multilayer device structures,6,10 for example, by blending the
light-emitting polymer with an electron transport material.11
However, reasonable control of the mobility balancing has
yet to be achieved.
Recently, we introduced an idea of chemically attaching
an electron transport structure to the polymer backbone.12
The side chain structure makes the polymer organic
soluble,13 which is beneficial in device making. Here, a typical electron transporting side group, 2-共4-biphenyl兲-5-共4-tbutylphenyl兲-1,3,4-oxadiazole 共PBD兲,14,15 gives rise to an enhancement of the EL efficiency as well as an increase of
the electron mobility in the OLED devices. In this investigation we studied transient electroluminescence characteristics
of PPV and poly关共2-共4-biphenylyl兲-5-共4-tert-butylphenyl兲a兲
Author to whom correspondence should be addressed; electronic mail:
rscel@kuccnx.korea.ac.kr
0021-8979/2000/87(6)/3183/3/$17.00
3183
© 2000 American Institute of Physics
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3184
Jang et al.
J. Appl. Phys., Vol. 87, No. 6, 15 March 2000
FIG. 3. Typical transient EL spectrum for the PPPDPV device for a 24 V
pulse. The inset shows how the delayed response time t r is defined.
FIG. 1. The structure of the PPV derivative, PPPDPV, where the electrontransporting PBD moiety is attached to the PPV backbone.
where d is the thickness of the polymer film, V the applied
voltage, and V on the turn-on voltage,7,18 which is the work
function difference 共0.6 V兲 between ITO and Al. Tunneling
of carriers can take place when the conduction and the valence bands are in the so-called ‘‘flatband’’ condition,7,18 for
which V on is necessary. Employing our transient EL measurements and Eq. 共1兲,19 we were able to obtain the recombination mobility of the e – h pairs in PPV and PPPDPV, as
shown in Fig. 4. It is noted that PPPDPV has about two
times faster mobility than that of PPV. Here, the faster electron mobility in PPPDPV can give rise to the larger recombination rates of the e – h pairs, i.e., formation rates of excitons, as well as removal of the emission regions away from
the polymer/Al interface to the bulk regions. The latter
makes it possible to avoid the EL quenching centers in the
polymer/Al interface and gives rise to increase of the exciton
lifetime of PPPDPV.
Figure 5 shows the expansion plot of the tail part of the
PPPDPV transient EL spectrum in comparison to that of
PPV, which were well fitted by a double-exponential function. The shorter time constant of the transient EL decay for
the PPV and PPPDPV devices corresponds to the RC time
constant of the devices,16,20 as has been confirmed by direct
measurements using a LCR meter 共Quad Tech. 7600兲. On the
other hand, the longer part of the EL decay corresponds to
the EL lifetime of the e – h pairs,20,21 found to be ⯝0.7 ␮s
for PPV and ⯝2 ␮s for PPPDPV from our measurements,
respectively. The considerably longer EL lifetime for the PPPDPV device can readily account for the enhanced external
quantum efficiency. In recent measurements of the timeresolved photoluminescence, PPPDPV also showed a lifetime considerably longer than that for the similar polymer
FIG. 2. 共a兲 The cw EL spectra of the PPPDPV and PPV devices. 共b兲 The
external quantum efficiency of the PPPDPV and PPV EL devices.
FIG. 4. The recombination mobilities obtained from transient EL measurements of the PPPDPV and PPV devices.
above, the external quantum efficiency is influenced by the
recombination rates of the e – h pairs as well as the exciton
lifetimes. For the purpose of obtaining information on those
quantities, transient EL measurements were performed in
this work. In the transient EL spectrum, the delayed response
time t r is defined as the time between the rising edge of the
voltage pulse and the occurrence of the EL as depicted in
Fig. 3. It can directly provide information on the drift mobility ␮, which is given by
␮ ⫽d 2 /t r 共 V⫺V on兲 ,
共1兲
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Jang et al.
J. Appl. Phys., Vol. 87, No. 6, 15 March 2000
3185
ecules at Korea University兲, and by the Korea Research
Foundation 共Special Fund for University Research Institute
through the Advanced Materials Chemistry Center at Korea
University兲. Measurements at Korea Basic Science Institute
共KBSI兲-Seoul Branch are acknowledged.
1
FIG. 5. The enlarged view of the transient EL decay for the PPPDPV and
PPV devices, which are well fitted by a double-exponential function.
bearing the carbazole moiety instead of the PBD unit, which
yields a much smaller external quantum efficiency than
PPV.22
The results of our mobility and EL lifetime measurements can readily be used to explain the external quantum
efficiency for PPPDPV, which is significantly enhanced in
comparison to that for PPV. In other words, increase in both
the recombination rates and the exciton lifetime results in the
increase of the external quantum efficiency for PPPDPV.
In summary, a study of transient electroluminescence
has been performed on light-emitting diodes based on a synthesized organic-soluble poly共p-phenylenevinylene兲 derivative, PPPDPV, which carries the electron-transporting PBD
moiety and which has been reported to show much improved
external quantum efficiencies as a result. The recombination
process in PPPDPV was found to be faster than in PPV, as
evidenced by the enhanced electron mobility in PPPDPV.
The high external quantum efficiency in PPPDPV is explained by the lengthened EL lifetime arising from the enhanced electron mobility, which acts to move the radiative
region away from the interface between the polymer layer
and the metal cathode where quenching centers are abundant.
This work was supported by the Korea Science and Engineering Foundation 共Grant No. 981-0297-027-2 and
through the Center for Electro- and Photo-Responsive Mol-
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