The relationship between the substitution position of the diphenylphosphine oxide on the spirobifluorene and device performances of blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1016/j.orgel.2010.03.005

A high triplet energy host (SPPO11) with a diphenylphosphine oxide group attached to the 4 position of the spirobifluorene was synthesized and its device performances were compared with those of the triplet host with the diphenylphosphine oxide at the 2 position of the spirobifluorene (SPPO1). The thermal stability of the SPPO11 was much better than that of the SPPO1 and the efficiency of the blue PHOLED with the SPPO11 host was higher than that of the blue PHOLED with the SPPO1 host. The SPPO11 could be effectively used as the high triplet energy host in the blue PHOLED and a high quantum efficiency of 17.2%and a current efficiency of 35.3 cd/A were obtained in the blue PHOLED with the SPPO11 host.

Full text of DOI:10.1016/j.orgel.2010.03.005

Organic Electronics 11 (2010) 1059–1065
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
The relationship between the substitution position of the
diphenylphosphine oxide on the spirobifluorene and device
performances of blue phosphorescent organic light-emitting diodes
*
Sang Eok Jang, Chul Woong Joo, Soon Ok Jeon, Kyoung Soo Yook, Jun Yeob Lee
Department of Polymer Science and Engineering, Dankook University, Suji-gu, Jukjeon-dong, Yongin, Gyeonggi 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:
A high triplet energy host (SPPO11) with a diphenylphosphine oxide group attached to the
4 position of the spirobifluorene was synthesized and its device performances were com-
pared with those of the triplet host with the diphenylphosphine oxide at the 2 position of
the spirobifluorene (SPPO1). The thermal stability of the SPPO11 was much better than that
of the SPPO1 and the efficiency of the blue PHOLED with the SPPO11 host was higher than
that of the blue PHOLED with the SPPO1 host. The SPPO11 could be effectively used as the
high triplet energy host in the blue PHOLED and a high quantum efficiency of 17.2% and a
current efficiency of 35.3 cd/A were obtained in the blue PHOLED with the SPPO11 host.
Ó 2010 Elsevier B.V. All rights reserved.
Received 21 October 2009
Received in revised form 4 March 2010
Accepted 4 March 2010
Available online 17 March 2010
Keywords:
Phosphine oxide
Blue phosphorescent organic light-emitting
diodes
High efficiency
Spirobifluorene
1. Introduction
been generally used as the high triplet energy host
material for the blue PHOLEDs and silane type materials
Power consumption is one of the most important per-
formances of the organic light-emitting diodes (OLEDs)
and high efficiency is critical to the power consumption
of the OLEDs [1]. In particular, the efficiency of the blue de-
vice is low as fluorescent device is being used in the OLEDs.
The fluorescent OLEDs can exhibit only 25% of the effi-
ciency of the phosphorescent OLEDs (PHOLEDs) and the
development of the blue PHOLEDs is strongly required to
reduce the power consumption of the OLED panels.
There have been many developments of the blue PHOL-
EDs using various host materials [2–9]. In general, FIrpic
[7–9], FIr6 [3,10] and FCNIr [11,12] have been widely used
as dopant materials and many researches were focused on
synthesizing high triplet energy host materials. There are
several different classes of host materials for the blue
PHOLEDs, which include carbazole [2], silane [3–6] and
phosphine oxide [7–9]. Carbazole type host materials have
could give very high triplet energy in spite of poor charge
injection. The phosphine oxide type materials were also
effective as the high triplet energy host materials. The
phosphine oxide group was commonly bonded to the spi-
robifluorene [8], carbazole [7] or amine [9] type moieties.
Our group already reported that the substitution at 2
position of the spirobifluorene with the diphenylphos-
phine oxide was efficient to synthesize high triplet energy
host material [8]. A high quantum efficiency was achieved
using the 2-diphenylphosphine oxide-9,90-spirobifluorene
(SPPO1). We expanded our previous work and synthesized
the host material with the diphenylphosphine oxide at 4
position of the spirobifluorene, 4-diphenylphosphine
oxide-9,90-spirobifluorene (SPPO11). The physical proper-
ties and device performances of the SPPO11 were com-
pared with those of the SPPO1. It was demonstrated that
the substitution of the diphenylphosphine oxide at 4 posi-
tion of the spirobifluorene is better than the substitution at
2
position in terms of efficiency and morphological
* Corresponding author. Tel./fax: +82 31 8005 3585.
E-mail address: leej17@dankook.ac.kr (J.Y. Lee).
stability.
1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2010.03.005

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S.E. Jang et al. / Organic Electronics 11 (2010) 1059–1065
quenched by methanol (7 mL). The mixture was extracted
2. Experimental
with dichloromethane. The combined organic layers were
dried over magnesium sulfate, filtered, and evaporated un-
der reduced pressure. The white powdery product was ob-
tained to 0.25 g (19%).
2.1. Materials and measurements
9H-fluoren-9-one, n-butyllithium (n-BuLi), triethyl bo-
rate and chlorodiphenylphosphine (Aldrich Chem. Co.)
were used without further purification. 2,20-Dibromobi-
phenyl (TCI Chem. Co.), hydrogen peroxide (Duksan Sci.
Co.) and sodium hydrogen carbonate (Junsei Chem. Co.)
was used as received. Tetrahydrofuran was distilled over
sodium and calcium hydride.
2.1.3. 4-Diphenylphosphine oxide-spiro[fluorene-9,90-
fluorene] (SPPO11)
A solution of 4-diphenylphosphine oxide-9,90-spirobi-
fluorene (0.25 g, 0.49 mmol), dichloromethane (50 mL),
and hydrogen peroxide (10 mL) were stirred overnight at
room temperature. The organic layer was separated and
washed with dichloromethane and water. The extract
was evaporated to dryness affording a white solid, which
was further purified by column chromatography to yield
0.24 g of chemically pure SPPO11.
The 1H nuclear magnetic resonance (NMR) was re-
corded on a Varian 200 (200 MHz) spectrometer. The pho-
toluminescence (PL) spectra were recorded on
fluorescence spectrophotometer (HITACHI, F-7000) and
the ultraviolet–visible (UV–Vis) spectra were obtained
a
Yield: 96%. Tm: 314 °C. 1H NMR (200 MHz, CDCl3): d
8.68–8.64 (d, 1H), 7.85–7.71(M, 6H), 7.52–7.49 (m, 6H),
7.40–7.14 (m, 4H), 7.11–6.61 (m, 8H). 13C NMR (CDCl3), d
150.4, 150.2, 148.1, 147.7, 144.9, 141.2, 139.3, 133.5,
133.3, 132.9, 132.6, 132.2, 132.0, 130.9, 128.7, 128.5,
128.2, 127.8, 127.4, 126.8, 125.5, 125.4, 124.7, 123.7,
123.0, 122.2, 120.0, 118.9 ppm. MS (FAB) m/z 517
[(M + 1)+].
using
a UV–Vis spectrophotometer (Shimadzu, UV-
2501PC). The differential scanning calorimeter (DSC) mea-
surements were performed on a Mettler DSC 822e under
nitrogen at a heating rate of 10 °C/min. The low and high
resolution mass spectra were recorded using a JEOL, JMS-
AX505WA spectrometer in FAB mode. The energy levels
were measured with a cyclic voltametry.
2.1.1. Synthesis of 4-bromo-9,90-spirobifluorene (1)
2,20-Dibromobiphenyl (11.00 g, 35.25 mmol) was dis-
solved in THF (70 mL) in the 250 ml two-neck flask. The
solution was cooled to ꢀ78 °C and n-BuLi (10 M in hexane,
4.23 mL) was added dropwise slowly. The whole solution
was stirred at ꢀ78 °C for 3 h, followed by addition of a
solution of 9H-fluoren-9-one (7.62 g, 42.30 mmol) in THF
(40 mL) under argon atmosphere. The solution was gradu-
ally warmed to ambient temperature and quenched by sat-
urated, aqueous NaHCO3 (110 mL). The solution was
extracted with dichloromethane. The combined organic
layers were dried over magnesium sulfate, filtered, and
evaporated under reduced pressure. The white powdery
product was obtained and the crude product was placed
in other two-necked flask, and dissolved in acetic acid
(300 mL). Catalytic amount of aqueous HCl (30 mL) was
then added and the whole solution was refluxed for 12 h.
After cooling to ambient temperature, purification by col-
umn chromatography with a mixed solvent of dichloro-
methane and n-hexane as an eluent gave a white powder.
Yield 50%. Tg 66 °C. 1H NMR (200 MHz, CDCl3): d 8.68–
8.65 (d, 1H), 7.86–7.82(d, 2H), 7.53–7.34 (m, 3H), 7.26–
6.63 (m, 15H). 13C NMR (CDCl3), d 150.9, 148.3, 147.7,
141.1, 140.3, 139.2, 132.5, 131.6, 128.4, 127.8, 127.4,
126.8, 126.4, 123.8, 123.5, 122.9, 122.2, 122.0, 119.9,
118.9, 116.0 ppm. MS (FAB) m/z 395 [(M + 1)+].
2.1.4. OLED fabrication and measurement
A basic device configuration of indium thin oxide (ITO)/
poly-3,4-ethylenedioxythiophene:polystyrenesulfonate
(PEDOT:PSS)/N,N0-di(1-naphthyl)-N,N0-diphenylbenzidine
(NPB)
(20 nm)/N,N0-dicarbazolyl-3,5-benzene
(mCP)
(10 nm)/SPPO11:FIrpic (30 nm, 10%)/2,9-dimethyl-4,7-di-
phenyl-1,10-phenanthroline (BCP, 5 nm)/tris(8-hydroxy-
quinoline) aluminium (Alq3) (20 nm)/LiF (1 nm)/Al
(100 nm) was used to evaluate the SPPO11 as a host for
the blue PHOLEDs. The doping concentration of the FIrpic
was optimized at 10% and it was fixed in all devices. The
blue PHOLED with the SPPO11 as the electron transport
layer instead of the BCP/Alq3 was also fabricated and the
simple blue PHOLED without the BCP/Alq3 was prepared
as the simple device. In addition, the blue PHOLED
with the 2-diphenylphosphine oxide-9,90-spirobifluorene
(SPPO1) was fabricated to study the effect of the substitu-
tion position of the diphenylphosphine oxide on the device
performances of the blue PHOLEDs. Device structures of
the blue PHOLEDs are shown in Fig. 1. The devices were
encapsulated with CaO and a glass lid after cathode depo-
sition. Current density (J)–voltage (V)–luminance (L) char-
acteristics and electroluminescence (EL) spectra of the
devices were measured with Keithley 2400 source mea-
surement unit and Minolta CS 1000A spectroradiometer.
2.1.2. 9,90-Spirobifluorene-4-yldiphenylphosphine (2)
3. Results and discussion
The 4-bromo-9,90-spirobifluorene (1.00 g, 2.53 mmol)
was put into the THF (20 mL) solvent in the 100 ml two-
neck flask. The solution was cooled to ꢀ78 °C and n-BuLi
(10 M in hexane, 0.3 mL) was added dropwise slowly. The
whole solution was stirred for additional 3 h, followed by
addition of a solution of chlorodiphenylphosphine (0.67 g,
3.04 mmol) under argon atmosphere. The resulting mix-
ture was gradually warmed to ambient temperature and
The synthesis of the 4-bromospirobifluorene which
was an intermediate compound of the SPPO11 was diffi-
cult because the bromination of the spirobifluorene leads
to the substitution at 2 position of the spirobifluorene.
Therefore, few compounds with a substituent at the 4
position of the spirobifluorene were synthesized [13,14].
One way to synthesize the 4-bromospirofluorene is to use

S.E. Jang et al. / Organic Electronics 11 (2010) 1059–1065
1061
LiF/Al
LiF/Al
LiF/Al
LiF/Al
Alq3(20 nm)
BCP(5 nm)
Alq3(20 nm)
BCP(5 nm)
SPPO11(25 nm)
SPPO11:FIrpic
(55 nm, 10 %)
SPPO1:FIrpic
(30 nm, 10 %)
SPPO11:FIrpic
(30 nm, 10 %)
SPPO11:FIrpic
(30 nm, 10 %)
mCP(10 nm)
NPB(20 nm)
mCP(10 nm)
NPB(20 nm)
mCP(10 nm)
NPB(20 nm)
mCP(10 nm)
NPB(20 nm)
PEDOT:PSS(60 nm)
PEDOT:PSS(60 nm)
PEDOT:PSS(60 nm)
PEDOT:PSS(60 nm)
ITO
ITO
ITO
ITO
Device II
Device III
Device IV
Device I
Fig. 1. Device structures of the blue PHOLEDs with the SPPO11 and SPPO1 hosts.
Br
O
Br
n-BuLi
THF
+
HO
Br
HCl
CH3COOH
O
P
P
Br
Cl
P
MC
n-BuLi
THF
+
H2O2
1
2
SPPO11
Scheme 1. Synthetic route of SPPO11.
Table 1
Physical properties of the SPPO11 and SPPO1.
Optical analysis
Absorption (nm)
Thermal analysis
Electrical analysis
Emission (nm)
Tg (°C)
Tm (°C)
HOMO (eV)
6.57
LUMO (eV)
2.82
Bandgap (eV)
3.75
ET (eV)
SPPO11
SPPO1
Solution
Solid
Solution
Solid
308, 322
310, 325
308, 316
309, 319
346
353
339
353
127
314
2.78
2.77
96
236
6.55
2.73
3.82
2,20-dibromobiphenyl. As shown in the Scheme 1, the 2,20-
dibromobiphenyl was reacted with n-BuLi followed by the
reaction with fluorenone. The use of 1 equivalent of n-BuLi
resulted in the lithiation of only one bromo unit of the two
bromo substituents in the 2,20-dibromobiphenyl. Therefore,
the reaction of the mono-lithiated 2,20-dibromobiphenyl
with the fluorenone followed by dehydration lead to the
synthesis of the 4-bromo-9,90-spirobifluorene. This synthetic
route is very simple to prepare the 4-bromo-9,90-spirobi-
fluorene. The 4-bromo-9,90-spirobifluorene was reacted
with chlorodiphenylphosphine through the lithiation and
then dehydrated by hydrogen peroxide, yielding the
SPPO11.
The physical properties of the SPPO11 were compared
with those of the SPPO1 to investigate the effect of the
diphenylphosphine oxide substitution position on the
physical properties. Physical properties of the SPPO11
and SPPO1 are summarized in Table 1 and Fig. 2. The
UV–Vis absorption and PL emission peaks of the SPPO11
were similar to those of the SPPO1, but the red shift of

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S.E. Jang et al. / Organic Electronics 11 (2010) 1059–1065
The highest occupied molecular orbital (HOMO) and the
SPPO1 UV (solution)
SPPO11 UV (solution)
SPPO1 PL (solution)
SPPO11 PL (solution)
SPPO1 PL (77K)
(a)
lowest unoccupied molecular orbital (LUMO) distribution
of the SPPO11 and the SPPO1 were studied to understand
the physical properties of the SPPO1 and SPPO11 at molec-
ular level and they are shown in Fig. 3. Density functional
theory (DFT) calculations for host materials were carried
out using a suite of Gaussian 03 program to understand
the observed properties of the materials at the molecular
level. The nonlocal density functional of Becke’s 3-parame-
ters employing Lee–Yang–Parr functional (B3LYP) with 6-
31G* basis sets was used for the calculation [15]. The
HOMO was distributed over the spirobifluorene unit of
the SPPO11 and SPPO1, while the LUMO was localized in
the fluorene group connected to the diphenylphosphine
oxide. Comparing the HOMO distribution, both the SPPO11
and SPPO1 showed uniform orbital distribution, resulting
in similar HOMO level. There was only 0.02 eV HOMO level
difference between the SPPO1 and the SPPO11. In the
LUMO case, the LUMO of the SPPO11 was localized in the
fluorene unit connected to the diphenylphosphine oxide,
while the LUMO of the SPPO1 was rather dispersed. The
LUMO of the SPPO1 was distributed over the diphenyl unit
in the diphenylphosphine oxide as well as the fluorene
unit. The diphenyl unit in the diphenylphosphine oxide is
an isolated unit as the conjugation through the phosphine
oxide unit cannot be extended due to the tetrahedral struc-
ture of the phosphine oxide. Therefore, the electrons in the
diphenyl unit are isolated and the LUMO level is shifted
upward from the vacuum level due to lack of conjugation.
The phenyl group is less conjugated than the spirobifluo-
rene group, leading to the upward LUMO level shift. This
LUMO level shift results in the large bandgap in the SPPO1
compared with SPPO11. Even though there is steric hin-
drance in the SPPO11, the geometrical structure of the spi-
robifluorene core was not affected by the position of the
diphenylphosphine oxide.
SPPO11 PL (77K)
300
350
400
450
500
550
Wavelength (nm)
(b)
SPPO1 UV (solid)
SPPO11 UV (solid)
SPPO1 PL (solid)
SPPO11 PL (solid)
300
350
400
450
500
550
Wavelength (nm)
(c)
SPPO1
SPPO11
0
0.5
1
1.5
2
2.5
3
Voltage (V)
Fig. 2. UV–Vis, PL spectra and CV data of the SPPO1 and SPPO11 (a) UV–
The different substitution position of the diphenylphos-
phine oxide also affects thermal properties of the SPPO1
and SPPO11 and the Tg of the SPPO11 was improved signif-
icantly by the substitution of the diphenylphosphine oxide
at 4 position of the spirobifluorene. The Tg depends on the
molecular rigidity of the backbone structure and the steric
hindrance of the molecule. Comparing the molecular rigid-
ity of the molecule, the SPPO11 and the SPPO1 have the
same rigid spirobifluorene core structure with the diphen-
ylphosphine oxide unit. The difference between the
SPPO11 and the SPPO1 is the steric hindrance between
the diphenylphosphine oxide unit and the spirobifluorene
core. The rotation of the diphenylphosphine oxide in the
SPPO11 is limited due to the steric hindrance of the diphe-
nyl unit with the spirobifluorene unit as the diphenyl unit
and the fluorene unit is in plane. However, the rotation of
the diphenylphosphine oxide in the SPPO1 is not hindered
by the spirobifluorene group as the diphenylphosphine
oxide group is out of plane with the spirobifluorene group.
The steric hindrance suppressed the free rotation of the
diphenylphosphine oxide group, increasing the Tg of the
SPPO11.
Vis and PL in solution, (b) UV–Vis and PL in solid state, (c) CV.
the UV–Vis absorption and PL emission were observed. The
absorption peak around 308 nm was the same in the
SPPO1 and SPPO11, but the absorption peak around
320 nm was red shifted in the SPPO11 by about 6 nm.
The emission peak was also shifted to long wavelength in
the SPPO11. In the solid film case, the UV absorption peak
was similar to that of the solution except for the slight red
shift of the absorption peak, while the emission peak was
different. The emission peak of the SPPO11 was in the long
wavelength range in solution, but the peak position of the
emission peak in the solid film was similar. This is closely
related with the geometrical structure and molecular orbi-
tal distribution of the SPPO1 and SPPO11 in Fig. 3. The
molecular packing of the SPPO11 is difficult due to the ste-
ric hindrance of the diphenylphosphine oxide at 4 position
of the spirobifluorene, resulting in less molecular interac-
tion in solid state. Therefore, the peak position of the PL
emission of the SPPO1 and SPPO11 was similar. In addition,
the emission peak of the SPPO11 was narrower than that of
the SPPO1 due to the suppressed molecular interaction in
solid state.
The high Tg induced by the steric hindrance improved
the morphological stability of the SPPO11. Fig. 4 shows
the optical microscopic pictures of the SPPO11 and SPPO1

S.E. Jang et al. / Organic Electronics 11 (2010) 1059–1065
1063
Fig. 3. Molecular simulation results of the SPPO11 and SPPO1.
Fig. 4. Optical microscopic pictures of the SPPO11 and SPPO1 after thermal treatment at 80 °C for 10 min.
film after thermal treatment at 80 °C for 10 min on glass
20
16
12
8
substrate. The crystallization of the SPPO1 was serious
after thermal treatment and most of the film was crystal-
lized. However, the SPPO11 film maintained the amor-
phous film morphology of the thermally deposited film
even after thermal treatment. This indicates that the steric
hindrance of the SPPO11 suppress the crystallization.
Electron only devices of the SPPO11 and SPPO1 were
fabricated to compare the effect of substitution position
on the electron injection. Fig. 5 shows the current den-
sity–voltage curves of the electron only devices of the
SPPO11 and SPPO1. The device structure of the electron
only devices was ITO (150 nm)/SPPO1 or SPPO11
(100 nm)/LiF (1 nm)/Al (100 nm). The electron current
density of the SPPO11 was higher than that of the SPPO1,
SPPO11
SPPO1
4
0
0
2
4
6
8
Voltage (V)
Fig. 5. Current density–voltage curves of the electron only devices of the
SPPO11 and SPPO1.

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S.E. Jang et al. / Organic Electronics 11 (2010) 1059–1065
indicating that the SPPO11 is better than the SPPO1 in
terms of electron density. The electron density is deter-
mined by the LUMO level and electron mobility of the
materials. The LUMO of the SPPO11 was 2.82 eV, while
the LUMO of the SPPO1 was 2.73 eV. There was 0.09 eV en-
ergy level difference between the SPPO11 and the SPPO1.
The LUMO level of the SPPO11 is better for electron
injection than that of the SPPO1, leading to high electron
density. In addition, the high electron mobility of the
SPPO11 is also responsible for the high electron density
of the SPPO11. The electron mobility of the SPPO1 was
5 ꢁ 10ꢀ4 cm2/V s, while that of the SPPO11 was 7 ꢁ
10ꢀ4 cm2/V s.
20
16
12
8
50
40
30
20
10
0
Device I
Device II
Device I
Device II
4
0
0
1
10
100
1000
10000
Luminance (cd/m2)
The triplet bandgap of the SPPO11 was 2.78 eV and it
was similar to that of the SPPO1. The triplet bandgap of
2.77 eV is wide enough for energy transfer to the FIrpic
dopant and the SPPO11 was evaluated as the host for the
FIrpic. The device performances of the SPPO11 blue device
(device I) were optimized by changing the doping concen-
tration of the FIrpic and were compared with those of the
SPPO1 (device II). The doping concentration of the FIrpic
in the device I was optimized at 10%.
The J–V–L curves of the SPPO11 and SPPO1 blue PHOL-
EDs are shown in Fig. 6. The J and L of the SPPO11 device
were higher than those of the SPPO1 device. The high J
and L of the SPPO11 device are due to the efficient electron
injection as shown in the electron only device data in
Fig. 5. The electron density of the SPPO11 device was in-
creased, resulting in the increase of the J and L of the
SPPO11. The high L of the SPPO11 device indicates that
more electron injection generates more excitons in the
emitting layer.
The quantum efficiency and power efficiency of the
SPPO11 and SPPO1 devices are shown in Fig. 7. The quan-
tum efficiency of the SPPO11 device was higher than that
of the SPPO1 device and a maximum quantum efficiency
of 17.2% was obtained at 200 cd/m2 compared with 15.6%
of the SPPO1 device. The current efficiency was as high
as 35.3 cd/A in the SPPO11 device. The high quantum effi-
ciency of the SPPO11 device can be explained by the high
electron density in the emitting layer. The hole injection
from the ITO to the SPPO11 emitting layer is efficient as
the multilayer structure of PEDOT:PSS/NPB/mCP was used
Fig. 7. Quantum efficiency and power efficiency curves of the blue
PHOLEDs with SPPO11 and SPPO1 hosts.
as the hole transport layer. Therefore, holes can be effec-
tively injected from the ITO to the SPPO11 emitting layer
and more electron injection can balance the holes and elec-
trons in the emitting layer, improving the quantum effi-
ciency of the SPPO11 device. The high quantum efficiency
and low driving voltage of the SPPO11 device also en-
hanced the power efficiency and a high power efficiency
of 26.0 lm/W was obtained in the SPPO11 device.
The device performances of the SPPO11 device was fur-
ther optimized by changing the device structure of the
SPPO11 devices. A device with the SPPO11 as an electron
transport layer instead of the BCP/Alq3 (device III) and a
device with SPPO11:FIrpic emitting layer without any elec-
tron transport layer (device IV) were fabricated. Figs. 8
and 9 shows the J–V–L and efficiency–L curves of the de-
vice I, device III and device IV. The current density and
luminance were increased in the device III, while they
were reduced in the device IV compared with device I.
The high current density of the device III can be explained
by the reduced interfacial trap at the interface. Device III
had no interface between the emitting layer and the elec-
tron transport layer because the SPPO11 was used as both
the host and the electron transport layer. Therefore, there
was no interfacial energy barrier and interfacial traps,
increasing the electron density in the emitting layer. The
increased electron density in the emitting layer enhances
100
80
60
40
20
0
100000
10000
1000
100
100
80
60
40
20
0
100000
10000
1000
100
Device I
Device I
Device II
Device III
Device IV
Device I
Device III
Device IV
10
10
1
1
0.1
0.1
0.01
0.01
0
2
4
6
8
10
0
2
4
6
8
10
Voltage (V)
Voltage (V)
Fig. 6. Current density–voltage–luminance curves of the blue PHOLEDs
Fig. 8. Current density–voltage–luminance curves of the blue PHOLEDs
with SPPO11 and SPPO1 hosts.
with the SPPO11 as a host material according to the device structure.

S.E. Jang et al. / Organic Electronics 11 (2010) 1059–1065
1065
20
16
12
8
50
40
30
20
10
0
can be used as an intermediate compound for the synthesis
of thermally stable organic semiconductor materials for or-
ganic electronics applications.
Device I
Device III
Device IV
Device I
Device III
Device IV
Acknowledgement
This work was supported by 2009 Dankook university
project for funding RICT.
4
References
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1000
10000
Luminance (cd/m2)
[1] J.Y. Lee, J.H. Kwon, H.K. Chung, Org. Electron. 4 (2003) 143.
[2] R.J. Holmes, S.R. Forrest, Y.-J. Tung, R.C. Kwong, J.J. Brown, S. Garon,
M.E. Thompson, Appl. Phys. Lett. 82 (2003) 2422.
[3] R.J. Holmes, B.W. D’Andrade, S.R. Forrest, X. Ren, J. Li, M.E. Thompson,
Appl. Phys. Lett. 83 (2003) 3818.
Fig. 9. Quantum efficiency and power efficiency curves of the blue
PHOLEDs with the SPPO11 as a host material according to the device
structure.
[4] X. Ren, J. Li, R.J. Holmes, P.I. Djurovich, S.R. Forrest, M.E. Thompson,
Chem. Mater. 16 (2004) 4743.
the quantum efficiency of the device III. A quantum effi-
ciency of 17.5%, a current efficiency of 35.6 cd/A and a
power efficiency of 24.7 lm/W were achieved at 300 cd/
m2 in the device III.
[5] M.H. Tsai, H.W. Lin, H.C. Su, T.H. Ke, C.C. Wu, F.C. Fang, Y.L. Liao, K.T.
Wong, C.I. Wu, Adv. Mater. 18 (2006) 1216.
[6] M.F. Wu, S.J. Yeh, C.T. Chen, H. Murayama, T. Tsuboi, W. Li, I. Chao, S.
Liu, J.K. Wang, Adv. Funct. Mater. 17 (2007) 1887.
[7] A.B. Padmaperuma, L.S. Sapochak, P.E. Burrows, Chem. Mater. 18
(2006) 2389.
[8] S.O. Jeon, K.S. Yook, C.W. Joo, J.Y. Lee, Appl. Phys. Lett. 94 (2009)
013301.
[9] E. Polikarpov, J.S. Swensen, N. Chopra, F. So, A.B. Padmaperuma, Appl.
Phys. Lett. 94 (2009) 223304.
[10] J. Li, P.I. Djurovich, B.D. Alleyne, M. Yousufuddin, N.N. Ho, J.C.
Thomas, J.C. Peters, R. Bau, M.E. Thompson, Inorg. Chem. 44 (2005)
1713.
[11] S.H. Kim, J. Jang, S.J. Lee, J.Y. Lee, Thin Solid Films 517 (2008) 722.
[12] K.S. Yook, S.O. Jeon, C.W. Joo, J.Y. Lee, Org. Electron. 10 (2009) 170.
[13] X. Cheng, G.H. Hou, J.H. Xie, Q.L. Zhou, Org. Lett. 6 (2004) 2381.
[14] F. Steuber, J. Staudigel, M. Stossel, J. Simmerer, A. Winnacker, H.
Spreitzer, F. Weissortel, J. Salbeck, Adv. Mater. 12 (2000) 130.
[15] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R.
Cheeseman, J.A. Montgomery, T. Vreven Jr., K.N. Kudin, J.C. Burant,
J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi,
G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara,
K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross,
C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J.
Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K.
Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G.
Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K.
Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui,
A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A.
Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith,
M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W.
Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople,
Gaussian 03, Revision B05, Gaussian Inc., Pittsburgh, PA, 2003.
The current density was reduced in the device IV with-
out the SPPO11 electron transport layer. The reduction of
the current density is due to the electron trapping by the
FIrpic dopant material. The LUMO level of the FIrpic was
3.2 eV compared with 2.82 eV of the SPPO11. The LUMO le-
vel difference between the SPPO11 and FIrpic induces elec-
tron trap by the FIrpic dopant, decreasing the current
density of the device IV. The reduced electron density in
the device IV has negative effect on the charge balance in
the emitting layer, resulting in rather low quantum effi-
ciency and power efficiency compared with device I.
4. Conclusions
In conclusion, the substitution of the diphenylphos-
phine oxide at 4 position of the spirofluorene was better
than the substitution at 2 position in terms of thermal sta-
bility and electron injection. The SPPO11 could be effec-
tively synthesized through the simple synthetic route for
the synthesis of the 4-bromo-9,90-spirobifluorene and high
quantum efficiency was achieved in the device with the
SPPO11 host. Therefore, the 4-bromo-9,90-spirobifluorene