Theoretical maximum quantum efficiency in red phosphorescent organic light-emitting diodes at a low doping concentration using a spirobenzofluorene type triplet host material

Article abstract of DOI:10.1016/j.orgel.2010.02.003

Red phosphorescent organic light-emitting diodes (PHOLEDs) with a 100% internal quantum efficiency at a low doping concentration were fabricated using a spirobenzofluorene type phosphine oxide as the host material. A spirobenzofluorene type phosphine oxide compound, 2,7-bis(diphenylphosphoryl)spiro[fluorene-7,11′-benzofluorene] (SPPO21), was synthesized as the electron transport type host material for the red phosphorescent organic light-emitting diodes. The device performances of the SPPO21 based red PHOLEDs were optimized at a low doping concentration of 1-2% because of energy transfer, little concentration quenching and charge trapping. A high external quantum efficiency of 20% was achieved for the red PHOLEDs with the SPPO21 host.

Full text of DOI:10.1016/j.orgel.2010.02.003

Organic Electronics 11 (2010) 881–886
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Theoretical maximum quantum efficiency in red phosphorescent
organic light-emitting diodes at a low doping concentration using a
spirobenzofluorene type triplet host material
*
Soon Ok Jeon, Kyoung Soo Yook, Chul Woong Joo, Jun Yeob Lee
Department of Polymer Science and Engineering, Dankook University, Jukjeon-dong, Suji-gu, Yongin-si, Gyeonggi-do 448-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Red phosphorescent organic light-emitting diodes (PHOLEDs) with a 100% internal quan-
tum efficiency at a low doping concentration were fabricated using a spirobenzofluorene
type phosphine oxide as the host material. A spirobenzofluorene type phosphine oxide
compound, 2,7-bis(diphenylphosphoryl)spiro[fluorene-7,110-benzofluorene] (SPPO21),
was synthesized as the electron transport type host material for the red phosphorescent
organic light-emitting diodes. The device performances of the SPPO21 based red PHOLEDs
were optimized at a low doping concentration of 1–2% because of energy transfer, little
concentration quenching and charge trapping. A high external quantum efficiency of 20%
was achieved for the red PHOLEDs with the SPPO21 host.
Received 11 September 2009
Received in revised form 28 January 2010
Accepted 2 February 2010
Available online 10 February 2010
Keywords:
Red phosphorescent organic light-emitting
diodes
High efficiency
Low doping concentration
Energy transfer
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
whereas the PHOLEDs degrade at low doping concentra-
tions because of an incomplete energy transfer between
Phosphorescent organic light-emitting diodes (PHOL-
EDs) exhibit better quantum efficiencies than fluorescent
organic light-emitting diodes and many high quantum effi-
ciency PHOLEDs have been developed. In particular, red
PHOLEDs have been widely studied for commercial appli-
cations [1].
Typically, organometallic complexes, including Al based
organometallic complexes [2], Zn based organometallic
complexes [3] and Be based organometallic complexes
[4], are used as host materials for red PHOLEDs. Generally,
Ir and Pt based organometallic complexes have been ap-
plied as red emitting phosphorescent dopant materials
[5–8], and the device performances of red PHOLEDs are
optimized at a doping concentration between 5% and 20%
[6]. The device performance decreases at high doping con-
centrations because of the concentration quenching effect,
the host and dopant materials.
However, Kwon’s group reported that a high quantum
efficiency could be obtained at a low doping concentra-
tion of 1% for PHOLEDs with a Be derivative as the host
material [4]. The complete energy transfer between the
Be type host and the red dopant was responsible for the
low optimum doping concentration because of the small
bandgap difference of 0.2 eV between the singlet and trip-
let excitons.
In this work, a spirobenzofluorene type phosphine
oxide compound, 2,7-bis(diphenylphosphoryl)spiro[fluo-
rene-7,110-benzofluorene] (SPPO21), was synthesized as
the host material for the red phosphorescent dopant, and
the device performances of the red PHOLEDs were investi-
gated. A very low optimum doping concentration of 1–2%
and an external quantum efficiency of 20% were observed
for the red PHOLEDs with the SPPO21 host. Additionally,
the cause of the low optimum doping concentration for
the device was elucidated in this work.
* Corresponding author. Tel.: +82 31 8005 3585; fax: +82 31 8005 3580.
E-mail address: leej17@dankook.ac.kr (J.Y. Lee).
1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2010.02.003

882
S.O. Jeon et al. / Organic Electronics 11 (2010) 881–886
methane (10 mL), and 2 mL of 30% hydrogen peroxide
2. Experimental
2.1. Materials
was stirred overnight at room temperature. The organic
layer was separated and washed with dichloromethane
and water. The extract was dried through evaporation to
produce a white solid, which was further purified using
column chromatography with a final yield of 0.74 g.
SPPO21 yield 83%. Tg 135 °C. 1H NMR (200 MHz, CDCl3):
d 8.75–8.71 (d, 1H), 8.34–7.30 (d, 1H), 7.95–7.84 (m, 3H),
7.70–7.41 (m, 15H), 7.35–7.20 (m, 12H), 7.13–7.06 (d,
2H) 6.73–6.69 (d, 2H). MS (FAB) m/z 767 [(M+1)+].
1-Iodo-2-bromobenzene, n-butyllithium (n-BuLi), chlo-
rodiphenylphosphine, N-bromosuccinimide, naphthalene-
1-boronic acid, tetrakis(triphenylphosphine)palladium(0),
potassium carbonate, 2,7-dibromo-9-fluorenone were pur-
chased from the Aldrich Chem. Co. and used without fur-
ther purification. Hydrogen peroxide was obtained from
the Duksan Sci. Co. and used without further purification.
Tetrahydrofuran (THF) was distilled over sodium and cal-
cium hydride.
2.5. Characterization
The 1H nuclear magnetic resonance (NMR) was ob-
tained using a Varian 200 (200 MHz) spectrometer. The
photoluminescence (PL) spectra were recorded on a fluo-
rescence spectrophotometer (HITACHI, F-7000), and the
ultraviolet–visible (UV–vis) spectra were obtained using a
UV–vis spectrophotometer (Shimadzu, UV-2501PC). The
transition temperature measurements for SPPO21 were
carried out using a differential scanning calorimeter (DSC,
Mettler DSC 822) at a heating rate of 10 °C/min under a
nitrogen atmosphere. The low and high resolution mass
spectra were recorded using a JEOL, JMS-AX505WA spec-
trometer in the FAB (fast atom bombardment) mode. The
energy levels were measured using cyclic voltammetry
(CV). Atomic force microscopic (AFM) measurements were
performed on the evaporated SPPO21 film in order to study
the morphological stability of the film.
2.2. Synthesis
The synthetic route of SPPO21 is shown in Scheme 1.
The synthesis of 1-(2-bromophenyl)naphthalene (1) and
2,7-dibromospiro[fluorene-7,110-benzofluorene] (2) was
described in a previous work [9].
2.3. 2,7-Bis(diphenylphosphino)spiro[fluorene-7,110-benzo-
fluorene] (3)
2,7-Dibromospiro[fluorene-7,110-benzofluorene] (0.97 g,
1.85 mmol) was dissolved in THF. The solution was cooled
to ꢀ78 °C and n-BuLi (10 M in hexane, 0.46 mL) was slowly
added dropwise. The solution was stirred for 3 h and then a
solution of chlorodiphenylphosphine (1.02 g, 4.62 mmol)
was added under an argon atmosphere. The resulting mix-
ture was gradually warmed to ambient temperature and
quenched with methanol. The mixture was extracted with
dichloromethane, and the combined organic layers were
dried over magnesium sulfate, filtered and evaporated at
a reduced pressure. The white powder that was obtained
was purified using column chromatography.
2.6. Device fabrication
A basic device configuration of indium tin oxide (ITO,
150 nm)/N,N0-diphenyl-N,N0-bis-[4-(phenyl-m-tolyl-ami-
no)-phenyl]-biphenyl-4,40-diamine (DNTPD, 60 nm)/N,
N0-di(1-naphthyl)-N,N0-diphenylbenzidine (NPB, 20 nm)/
4,40,400-tris(N-carbazolyl)triphenylamine (TCTA, 10 nm)/
SPPO21: iridium(III) bis(2-phenylquinoline) acetylaceto-
nate (Ir(pq)2acac, 30 nm, ꢁ%)/2,9-dimethyl-4,7-diphe-
nyl-1,10-phenanthroline (BCP, 5 nm)/tris(8-hydroxy-
quinoline) aluminum (Alq3, 20 nm)/LiF(1 nm)/Al(200 nm)
was used for the device fabrication. Various doping
2.4. 2,7-Bis(diphenylphosphoryl)spiro[fluorene-7,110-
benzofluorene] (SPPO21)
A
mixture of 2,7-bis(diphenylphosphino)spiro[fluo-
rene-7,110-benzofluorene] (0.845 g, 1.14 mmol), dichloro-
O
Br
Br
HO
B
1-iodo-2-bromobenzene
ClH
CH COOH
n-BuLi
OH
HO
3
Li
Br
Br
Br
Br
Br
1
2
n-BuLi
Cl
P
CH Cl
2
2
O
O
P
P
P
P
H O
2
2
Li
Li
3
SPPO21
Scheme 1. Synthetic scheme of the SPPO21.

S.O. Jeon et al. / Organic Electronics 11 (2010) 881–886
883
tively managed by the two electron withdrawing
phosphine oxide units for efficient electron injection.
Fig. 2 shows the UV–vis and PL spectra of SPPO21. The
PL spectra were obtained at both room temperature and
77 K in order to measure the triplet bandgap of SPPO21.
SPPO21 exhibited strong absorption peaks at 297 and
340 nm, which corresponded to the absorption of the spi-
robenzofluorene unit. On the other hand, the diphenyl-
phosphine oxide group did not greatly contribute to the
optical absorption. A bandgap of 3.55 eV was calculated
for SPPO21 using the absorption edge of the UV–vis spec-
trum. A PL emission peak was observed at 417 nm because
of the wide bandgap of SPPO21. A low temperature PL
spectrum was also obtained at 77 K, and the triplet emis-
sion peak of SPPO21 was detected at 533 nm with a triplet
bandgap of 2.32 eV.
The CV measurements showed that SPPO21 had a
HOMO level of 6.12 eV, and a lowest unoccupied molecular
orbital (LUMO) of 2.57 eV as well a triplet bandgap of
2.32 eV, which was suitable for transferring energy to red
emitting Ir(pq)2acac (triplet bandgap of 2.2 eV). Therefore,
SPPO21 can be used as the host in the Ir(pq)2acac doped
device.
SPPO21 had a spirobenzofluorene core structure with a
twisted fluorene and benzofluorene backbone, which was
advantageous for amorphous and thermally stable film for-
mation. Additionally, the diphenylphosphine oxide unit
that was connected to the spirobenzofluorene backbone
structure also improved the thermal stability because of
the polar phosphine oxide group and the phenyl group that
was attached to the phosphine oxide unit. Therefore,
SPPO21 had a high glass transition temperature of 135 °C.
The surface morphology of the evaporated SPPO21 film
was analyzed using AFM in order to study the film stability
of SPPO21. The AFM measurements were carried out for a
pristine SPPO21 film and an SPPO21 film that was ther-
mally treated at 100 °C for 2 h. The AFM images of SPPO21
are shown in Fig. 3. SPPO21 formed a stable amorphous
film with a surface roughness of 0.12 nm. The surface mor-
phology of the SPPO21 film did not change even after ther-
mal treatment at 100 °C for 2 h, and the surface roughness
of the SPPO21 film baked at 100 °C was 0.14 nm. Therefore,
SPPO21 can effectively be used as a thermally stable host
material in organic light-emitting diodes. The stability of
the amorphous film was caused by the twisted structure
2.4
5.5
2.57
Al
2.8
3.0
5.8
3.0
5.2
Liq
Alq3
BCP
Al
4.3
SPPO21:Ir(pq)2acac
(30 nm)
ITO
4.9
NPB
5.1
PEDOT:PSS
ITO
6.12
6.1
Fig. 1. Device structures of the red PHOLEDs and energy level diagram of
the devices.
concentrations of 1% (device I), 2% (device II), 3% (device
III), 5% (device IV) and 10% (device V) were used for the
red emitting layer. The device structures of the red PHOL-
EDs are shown in Fig. 1. The photoluminescence (PL) mea-
surements were carried out for Ir(pq)2acac doped SPPO21
using
a fluorescence spectrophotometer. Ir(pq)2acac
doped solid films with the same doping concentration
as the red devices were fabricated for the PL measure-
ments. The current density–voltage–luminance and elec-
troluminescence (EL) characteristics of the red PHOLEDs
were measured using a Keithley 2400 source measure-
ment unit and a CS 1000 spectroradiometer. The external
quantum efficiency was calculated from the EL spectra,
luminance and current density. Lambertian distribution
of the light emission was assumed in the calculation of
the quantum efficiency and the light emission in the for-
ward direction was measured.
3. Results and discussion
SPPO21 was synthesized through the phosphonation
reaction of dibromospirobenzofluorene with chlorodiphen-
ylphosphine. Dibromospirobenzofluorene was effectively
prepared through the reaction of dibromofluorenone with
2-bromobiphenyl followed by lithiation and dehydration
[9]. The synthetic yield of SPPO21 was 83%, and after puri-
fication using column chromatography, the purity of
SPPO21 was over 99%. The synthetic scheme of SPPO21 is
shown in Scheme 1. SPPO21 was fully characterized using
1H NMR, high performance liquid chromatography, differ-
ential scanning calorimetry and high resolution mass
spectrometry.
SPPO21, containing a spirobenzofluorene unit and two
diphenylphophine units, was used as the electron trans-
porting host material for the red phosphorescent dopant.
The spirobenzofluorene moiety has good thermal and mor-
phological stability because of its rigid benzofluorene unit
[9]. Additionally, the spirobenzofluorene unit has a triplet
bandgap of 2.4 eV, and can be applied as a molecular back-
bone for the host in red PHOLEDs. The phosphine oxide
group isolates the core structure and has electron with-
drawing characteristics because of its polarity [10,11].
Therefore, the electron transport properties of the mole-
cule were enhanced by the phosphine oxide group and
SPPO21 was an efficient electron transport type host mate-
rial. Additionally, the energy levels of SPPO21 were effec-
UV-Vis (SPPO21)
PL (SPPO21)
PL 77K (SPPO21)
500
550
600
Wavelength (nm)
250
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 2. UV–vis and PL spectra of SPPO21.

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S.O. Jeon et al. / Organic Electronics 11 (2010) 881–886
Fig. 3. Atomic force microscopic pictures of the SPPO21 without thermal treatment and after thermal treatment at 100 °C.
of the spirofluorene–benzofluorene core and the two
diphenylphosphine oxide units.
The luminance of the SPPO21 red PHOLEDs was high at
a doping concentration of 3% despite the low current den-
sity, indicating a high recombination efficiency in device
III. The low luminance of devices I and II was caused by
their low current densities. Additionally the luminance of
devices IV and V was low because of the low recombina-
tion efficiency of the holes and electrons.
Fig. 5 shows the quantum efficiency and current effi-
ciency of the SPPO21 based red PHOLEDs. Device II exhib-
ited the highest quantum efficiency at high luminance
(>400 cd/m2), whereas the quantum efficiency of device I
The device performances of the red PHOLEDs were
studied with respect to the doping concentration of Ir(p-
q)2acac in order to evaluate the performance of SPPO21
as a host material for red PHOLEDs. Fig. 4 shows the cur-
rent density–voltage–luminance curves of the red PHOL-
EDs with different doping concentrations. The current
density of the red PHOLEDs increased as the doping con-
centration increased, and the highest current density was
obtained for the red PHOLED with the highest doping con-
centration. SPPO21 had a HOMO/LUMO level of 6.12/
2.57 eV, whereas the HOMO/LUMO of Ir(pq)2acac was
5.2/3.0 eV in the energy level diagram (Fig. 1), correspond-
ing to a HOMO/LUMO level difference of 0.92/0.43 eV be-
tween the SPPO21 and Ir(pq)2acac. The large HOMO level
gap between SPPO21 and Ir(pq)2acac induced significant
hole trapping by the dopant, and electrons were also
trapped by the dopant because of the LUMO level gap of
0.43 eV [12]. Therefore, the charge trapping was critical
to the current density, and the intermolecular hopping be-
tween the dopant materials dominated the charge trans-
port behavior of the SPPO21 device, resulting in high
current densities in the highly doped devices.
25
20
15
10
5
1%
2%
3%
6%
10%
(a)
0
0
1
100
10000
Luminance (cd/m 2)
100
80
60
40
20
0
100000
10000
1000
100
40
35
30
25
20
15
10
5
(b)
1%
2%
3%
6%
10%
1%
2%
3%
6%
10%
10
1
0.1
0.01
0.001
0
0
1
100
10000
0
2
4
6
8
10
Voltage (V)
Luminance (cd/m 2)
Fig. 4. Current density–voltage–luminance curves of the red PHOLEDs
with the SPPO21 host according to the doping concentration of the
Ir(pq)2acac.
Fig. 5. Quantum efficiency (a) and current efficiency (b) of the red
PHOLEDs with the SPPO21 host according to the doping concentration of
the Ir(pq)2acac.

S.O. Jeon et al. / Organic Electronics 11 (2010) 881–886
885
was higher than the other devices at low luminance
(<400 cd/m2). The maximum quantum efficiencies of de-
vices I and II were 20% and 18.3%, respectively, whereas
the quantum efficiencies at 1000 cd/m2 were 12.1% and
13.1%, respectively. The quantum efficiency of the SPPO21
device was higher than the SPPO2 that was reported earlier
[13]. The external quantum efficiency of device I was the
same as the theoretical external maximum quantum effi-
ciency of 20%. However, the quantum efficiency was signif-
icantly reduced in devices IV and V. The maximum
quantum efficiencies of devices IV and V were only 11.5%
and 5.4%, respectively. The quantum efficiency of the
SPPO21 based red PHOLEDs was optimized at a very low
doping concentration of 1% or 2%. The current efficiency
showed the same tendency. However, the reason for this
observation was unknown. Therefore, the PL spectra of
the Ir(pq)2acac doped SPPO21 films and the EL spectra of
the fabricated red PHOLEDs were examined at various dop-
ing concentrations.
estimated to be below 3 nm [16–18]. Therefore, the con-
centration quenching was serious above a doping concen-
tration of 3%, resulting in a decrease in the PL intensity. The
highest PL intensity was obtained at a doping concentra-
tion of 2% because of the small concentration quenching,
even though the SPPO21 emission was not completely
quenched by the dopant. The low PL intensity at 1% doping
was caused by the incomplete energy transfer, whereas the
low intensity at 3% doping was caused by the concentra-
tion quenching.
The PL intensity of the film was well correlated with the
quantum efficiency of the red PHOLEDs. The high efficiency
was obtained for device II at a 2% doping concentration,
and the efficiency decreased as the doping concentration
increased. Even though other factors affected the quantum
efficiency of the EL devices, these results proved that the PL
emission was critical to the quantum efficiency of the
SPPO21 based red PHOLEDs.
Fig. 7 shows the EL spectra of the SPPO21 based red
PHOLEDs. The red PHOLEDs exhibited main emission peaks
at around 600 nm and the emission peaks were red-shifted
at higher doping concentrations because of the intermolec-
ular dipole–dipole interaction that was caused by the short
distance between the dopant molecules at high doping
concentrations. The main emission peak shifted from
596 nm at 1% doping to 612 nm at 10% doping.
Comparing the PL spectra of the films and the EL spectra
of the devices, device I did not show any host emission in
the PL spectrum of the 1% doped film. All of the SPPO21
emissions were not absorbed by the 1% doped Ir(pq)2acac
in the PL excitation, but the host emission disappeared in
the EL emission because of the hole and electron trapping
by the Ir(pq)2acac in the energy level diagram of the host
and dopant. The excitons were generated not only in the
host but also in the dopant because of the charge trapping
caused by the large energy level difference between the
host and dopant materials. Both the excitons that were
formed in the host through the transfer of energy to the
dopant and the excitons that were generated in the dopant
directly emitted. Therefore, both the energy transfer and
the charge trapping were parts of the main mechanism
for the light emission in the SPPO21 based red PHOLEDs.
The contribution of the charge trapping on the light emis-
sion was significant in the highly doped device, but the
concentration quenching had a negative effect on the
Fig. 6 shows the PL spectra of the Ir(pq)2acac doped
SPPO21 films at different doping concentrations of Ir(p-
q)2acac. The PL emission of the pure SPPO21 film was ob-
served at around 400 nm with
a maximum peak at
405 nm, and the Ir(pq)2acac PL emission was detected at
605 nm. The Ir(pq)2acac doped SPPO21 films exhibited PL
peaks for both SPPO21 and Ir(pq)2acac at doping concen-
trations below 6%, whereas the Ir(pq)2acac doped SPPO21
films exhibited only the Ir(pq)2acac emission at higher
doping concentrations above 6%. Therefore, the energy
transfer from the SPPO21 host to Ir(pq)2acac was not com-
plete at low doping concentrations. At 6% doping the
SPPO21 emission was completely absorbed by Ir(pq)2acac.
However, the PL intensity of Ir(pq)2acac was not as strong
in the 6% doped SPPO21 film as the others despite the com-
plete energy transfer from SPPO21 to Ir(pq)2acac. The PL
intensity of the emitter doped film is affected by the lumi-
nescence concentration quenching, which is governed by
the doping concentration [14–16]. The concentration
quenching rate constant was dependent on Rꢀ6, where R
is the average distance between dopant molecules. The
average distance between Ir(pq)2acac decreased according
to the following relationship, R = [(molecular density in
film) ꢁ (mol% of the dopant in a film)]ꢀ1/3 [16], and R de-
creased below 3.0 nm in the 3% doped film. The Förster ra-
dius of the phosphorescent emitter doped film was
1%
2%
3%
6%
10%
0%
1%
2%
3%
6%
10%
400 450 500 550
Wave le ngth (nm)
400
500
600
700
800
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 6. Photoluminescence spectra of the SPPO21: Ir(pq)2acac film
Fig. 7. Electroluminescence spectra of the red OLEDs with the SPPO21
according to the doping concentration of the Ir(pq)2acac.
host according to the doping concentration of Ir(pq)2acac.

886
S.O. Jeon et al. / Organic Electronics 11 (2010) 881–886
quantum efficiency. On the other hand, the energy transfer
dominated the light emission and the charge trapping
compensated for the incomplete energy transfer in the de-
vices with low doping concentrations. Therefore, a high
quantum efficiency over 20% was achieved in the SPPO21
based red PHOLEDs.
concentration because of the little concentration quench-
ing, energy transfer and charge trapping. SPPO21 exhibited
a high quantum efficiency and low optimum doping con-
centration making it an effective host material for red
PHOLEDs, and therefore, the SPPO21 host material exhib-
ited potential for use in future display and lighting
applications.
Even though SPPO21 did not exhibit an emission at
405 nm, a weak emission was detected at 465 nm in the
EL spectra of the red PHOLEDs at low doping concentra-
tions. The intensity of the emission weakened at higher
doping concentrations and was not observed in device V.
An expanded image of the emission peak is shown in
Fig. 7 as an inset. This emission peak was caused by the
exciplex of TCTA and SPPO21, which was confirmed by
the PL emission of the mixed film of TCTA and SPPO21.
The electron donating aromatic amine group in TCTA and
the strong electron withdrawing phosphine oxide group
combined to make the excited complex [19,20]. The TCTA
hole transport layer and the SPPO21 host formed the exci-
plex at the interface, which emitted a weak peak at
465 nm. The exciplex formation was facilitated by the elec-
tron accumulation and overflow at the interface between
the SPPO21 host and TCTA. Fewer electrons were trapped
by the dopant in the red PHOLEDs with lower doping con-
centrations, leading to better electron transport through
the SPPO21 emitting layer. Therefore, more electrons accu-
mulated at the interface and were injected into the TCTA
layer in device I, resulting in a relatively strong exciplex
emission. The exciplex emission was responsible for the
large efficiency roll-off in the red PHOLEDs with low dop-
ing concentrations in Fig. 5. A previous report showed that
the charge leakage is critical to the efficiency roll-off, and
confirming the results that were obtained in this work
[21]. The efficiency roll-off decreased as the intensity of
the 465 nm peak decreased.
Acknowledgement
The present research was conducted by the research
fund of Dankook University in 2009.
References
[1] J.Y. Lee, J.H. Kwon, H.K. Chung, Org. Electron. 4 (2003) 143.
[2] T. Tsuji, S. Kawami, S. Miyaguchi, T. Naijo, T. Yuki, S. Matsuo, H.
Miyazaki, SID Int. Symp. Digest Technol. Pap. 35 (2004) 900.
[3] H. Kanno, K. Ishikawa, Y. Nishio, A. Endo, C. Adachi, K. Shibata, Appl.
Phys. Lett. 90 (2007) 123509.
[4] W.S. Jeon, T.J. Park, S.Y. Kim, P. Pode, J. Jang, J.H. Kwon, Org. Electron.
10 (2009) 240.
[5] A. Tsuboyama, H. Iwawaki, M. Furugori, T. Mukaide, J. Kamatani, S.
Igawa, T. Moriyama, S. Miura, T. Takiguchi, S. Okada, M. Hoshino, K.
Ueno, J. Am. Chem. Soc. 125 (2003) 12971.
[6] M.A. Baldo, M.E. Thompson, S.R. Forrest, Pure Appl. Chem. 71 (1999)
2095.
[7] Y. Kawamura, S. Yanagida, S.R. Forrest, J. Appl. Phys. 92 (2002) 87.
[8] J.Y. Lee, Appl. Phys. Lett. 89 (2006) 223517.
[9] S.O. Jeon, Y.M. Jeon, J.W. Kim, C.W. Lee, M.S. Gong, Org. Electron. 9
(2008) 522.
[10] P.E. Burrows, A.B. Padmaperuma, L.S. Sapochak, P. Djurovich, M.E.
Thompson, Appl. Phys. Lett. 88 (2006) 183503.
[11] A.B. Padmaperuma, L.S. Sapochak, P.E. Burrows, Chem. Mater. 18
(2006) 2389.
[12] S. Lamansky, R.C. Kwong, M. Nugent, P.I. Djurovich, M.E. Thompson,
Org. Electron. 2 (2001) 53.
[13] S.O. Jeon, K.S. Yook, C.W. Joo, J.Y. Lee, Org. Electron. 10 (2009) 998.
[14] Y. Kawamura, K. Goushi, J. Brooks, J.J. Brown, H. Sasabe, C. Adachi,
Appl. Phys. Lett. 86 (2005) 071104.
[15] X. Gong, J.C. Ostrowski, D. Moses, G.C. Bazan, A.J. Heeger, Adv. Funct.
Mater. 13 (2003) 439.
[16] Y. Kawamura, J. Brooks, J.J. Brown, H. Sasabe, C. Adachi, Phys. Rev.
Lett. 96 (2006) 017404.
[17] I. Tanaka, Y. Tabata, S. Tokito, J. Appl. Phys. 99 (2006) 073501.
[18] G. Ramos-Ortiz, Y. Oki, B. Domercq, B. Kippelen, Phys. Chem. Chem.
Phys. 4 (2002) 4109.
[19] S.A. Jenekhe, J.A. Osaheni, Science 265 (1994) 765.
[20] Y. Yang, Q. Pei, Appl. Phys. Lett. 70 (1997) 1926.
[21] S.H. Kim, J. Jang, K.S. Yook, J.Y. Lee, Appl. Phys. Lett. 92 (2008)
023513.
4. Conclusions
The device performances of the red PHOLEDs with the
phosphine oxide based SPPO21 host were studied, and a
high external quantum efficiency of 20% was achieved.
The device performances were optimized at a low doping