High quantum efficiency and color stability in white phosphorescent organic light emitting diodes using a pyridine modified carbazole derivative

Article abstract of DOI:10.1016/j.dyepig.2013.11.020

Abstract A high triplet energy material derived from carbazole and pyridine, 9,9′-(5-(2-methylpyridin-3-yl)-1,3-phenylene)bis(9H-carbazole), was synthesized as the host material for blue and white phosphorescent organic light-emitting diodes. The 9,9′-(5-(2-methylpyridin-3-yl)-1,3-phenylene) bis(9H-carbazole) host showed a high triplet energy of 2.99 eV for efficient energy transfer to blue triplet emitter. High quantum efficiencies of 19.8% and 17.1% were achieved in blue and white phosphorescent organic light-emitting diodes using the 9,9′-(5-(2-methylpyridin-3-yl)-1,3-phenylene)bis(9H- carbazole) host material. In addition, the 9,9′-(5-(2-methylpyridin-3-yl)- 1,3-phenylene)bis(9H-carbazole) based white phosphorescent organic light-emitting diodes showed good color stability.

Full text of DOI:10.1016/j.dyepig.2013.11.020

Dyes and Pigments 103 (2014) 34e38
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
High quantum efficiency and color stability in white phosphorescent
organic light emitting diodes using a pyridine modified carbazole
derivative
*
Chil Won Lee, 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
A high triplet energy material derived from carbazole and pyridine, 9,90-(5-(2-methylpyridin-3-yl)-1,3-
phenylene)bis(9H-carbazole), was synthesized as the host material for blue and white phosphorescent
organic light-emitting diodes. The 9,90-(5-(2-methylpyridin-3-yl)-1,3-phenylene)bis(9H-carbazole) host
showed a high triplet energy of 2.99 eV for efficient energy transfer to blue triplet emitter. High quantum
efficiencies of 19.8% and 17.1% were achieved in blue and white phosphorescent organic light-emitting
diodes using the 9,90-(5-(2-methylpyridin-3-yl)-1,3-phenylene)bis(9H-carbazole) host material. In
addition, the 9,90-(5-(2-methylpyridin-3-yl)-1,3-phenylene)bis(9H-carbazole) based white phosphores-
cent organic light-emitting diodes showed good color stability.
Article history:
Received 12 September 2013
Received in revised form
18 November 2013
Accepted 19 November 2013
Available online 28 November 2013
Keywords:
Carbazole
High triplet energy
Blue device
Ó 2013 Elsevier Ltd. All rights reserved.
White device
Quantum efficiency
Color stability
1. Introduction
reported as the host materials for the PHWOLEDs to enhance the
quantum efficiency. Although high quantum efficiency was re-
Phosphorescent white organic light emitting diodes (PHWO-
LEDs) are promising as a next generation lighting device due to
high quantum efficiency and broad emission spectrum for high
color rendering index. In particular, the high quantum efficiency of
the PHWOLEDs is an attractive feature for a lighting device because
power consumption can be reduced.
In general, the PHWOLEDs are constructed by combining a blue
triplet emitter with a yellow triplet emitter, or red, green and blue
triplet emitters [1e10]. The energy of the blue triplet emitter is
transferred to low energy triplet emitters during the light emission
process and the device performance of the PHWOLEDs are mostly
dominated by the device performance of the blue phosphorescent
organic light-emitting diodes (PHOLEDs). Therefore, the develop-
ment of high efficiency blue PHOLEDs is important to improve the
device performances of PHWOLEDs. There are several ways to
improve the device performances of PHWOLEDs and one of the
most efficient methods is to develop high triplet energy host ma-
terials [11e15]. Several high triplet energy host materials were
ported using various host materials, it was difficult to obtain both
high quantum efficiency and color stability in the PHWOLEDs.
In this work, a high triplet energy host material derived from
pyridine and carbazole, 9,90-(5-(2-methylpyridin-3-yl)-1,3-
phenylene)bis(9H-carbazole) (PPBC), was developed to improve
the quantum efficiency and color stability of PHWOLEDs. High
quantum efficiencies of 17.1% and good color stability with 0.01
change of color coordinate between 100 cd/m2 and 10,000 cd/m2
were demonstrated in PHWOLEDs using the PPBC host material in
addition to high quantum efficiency of 19.8% in blue PHOLEDs.
2. Experimental section
Reagents and solvents were purchased at reagent grade from TCI,
Aldrich, and P&H tech and used as received. All reactions were per-
formed under a positive pressure of high purity nitrogen. The 1H and
13C nuclear magnetic resonance (NMR) were obtained on Avance 500
(Bruker). The FT-IR spectra for all the samples were measured using a
Nicolet 380 FTIR spectrometer. High resolution mass spectra were
measured on JEOL, JMS-600W spectrometer in fast atom bom-
bardment(FAB) mode. Elemental analyses of carbon, hydrogen, and
nitrogen were performed on Flash2000 (Thermofisher). Differential
* Corresponding author. Tel./fax: þ82 31 8005 3585.
E-mail address: leej17@dankook.ac.kr (J.Y. Lee).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.11.020

C.W. Lee, J.Y. Lee / Dyes and Pigments 103 (2014) 34e38
35
scanning calorimetry (DSC) was performed on a Mettler DSC822e at a
heating rate of 10 ꢀC/min from 20 to 350 ꢀC under argon. The pho-
toluminescence (PL) spectra were recorded on a fluorescence spec-
trophotometer (HITACHI, F-7000) and the ultravioletevisible (UVe
Vis) spectra were obtained using a UVeVis spectrophotometer (Shi-
madzu, UV-2501PC). Low temperature PL measurement for triplet
energy analysis recorded on a PerkinElmer LS-55 in liquid nitrogen.
Cyclic voltammetry (CV) was carried out in acetonitrile solution
(oxidation scan) at room temperature with tetrabutylammonium
perchlorate (0.1 M) was used as the supporting electrolyte. The con-
ventional three-electrode configuration consists of a platinum
working electrode, a platinum wire auxiliary electrode, and an Ag
wire pseudoreference electrode with ferroceniumeferrocene (Fcþ/
Fc) as the internal standard. The synthetic route of PPBC is shown in
Scheme 1.
hexane) to give 9,90-(5-(3-methylpyridin-2-yl)-1,3-phenylene)
bis(9H-carbazole) (1.95 g, 46% yield). Additional purification by
sublimation (at 200 ꢀC and 10ꢁ5 mm Hg) resulted in 1.5 g of pure
compound (36% overall yield).
Light yellow powder, m.p.132 ꢀC, Tg 94 ꢀC, FT-IR (KBr) 3047,1590,
1450, 1334, 1311, 1229, 1155, 1120, 1026, 1002, 925, 806, 748, 710,
647 cmꢁ1 1H NMR (500 MHz, CDCl3):
. d 2.75 (s, 3H), 7.21 (t, 1H,
J ¼ 4.2 Hz), 7.30 (t, 4H, J ¼ 5.2 Hz), 7.44 (t, 4H, J ¼ 5.3 Hz), 7.58 (d, 4H,
J ¼ 4.0 Hz), 7.65 (s, 2H), 7.69 (d,1H, J ¼ 4.8 Hz), 7.85 (s,1H), 8.13 (d, 4H,
J ¼ 4.0 Hz), 8.55 (d,1H, J ¼ 3.3 Hz).13C NMR (125 MHz, CDCl3):
d 23.8,
109.5, 120.5, 121.3, 123.7, 124.1, 126.1, 126.2, 135.1, 137.1, 139.5, 140.5,
143.4, 148.9, 155.6. HRMS calcd for C36H25N3 (Mþ þ H) 500.2121,
found 500.2127. Element analysis Calcd. for C36H25N3 C(86.55%)
H(5.04%) N(8.41%); found C(86.04%) H(5.08%) N(8.52%).
2.3. Device fabrication and measurements
2.1. Synthesis of 2-(3,5-dichlorophenyl)-3-methylpyridine
Device structure of blue PHOLEDs was indium tin oxide
(50 nm)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS, 60 nm)/4,40-cyclohexylidenebis[N,N-bis(4-methyl
phenyl)aniline] (TAPC, 20 nm)/1,3-bis(N-carbazolyl)benzene (mCP,
10 nm)/PPBC:iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2]
picolinate (FIrpic) (25 nm)/diphenylphosphine oxide-4-(triphenyl
silyl)phenyl (TSPO1, 35 nm)/LiF(1 nm)/Al(200 nm). Doping con-
centrations of FIrpic were 3, 5, and 10%. PHWOLEDs had the device
structure of ITO (50 nm)/PEDOT:PSS (60 nm)/TAPC (20 nm)/mCP
(10 nm)/PPBC:FIrpic (15 nm)/1,3,5-tris(N-phenylbenzimidazole-2-
yl)benzene (TPBI):iridium(III) bis(2-phenylquinoline) acetylaceto-
nate(Ir(pq)2acac) (15 nm, 5%)/TSPO1 (35 nm)/LiF (1 nm)/Al (200
nm). The doping concentrations of FIrpic were 3, 5 and 10%. The
doping concentration of FIrpic and Ir(pq)2acac was controlled by
managing the evaporation rate of host and dopant materials. Hole
only device with a device structure of ITO (50 nm)/PEDOT:PSS
(60 nm)/TAPC (20 nm)/mCP (10 nm)/PPBC (25 nm)/TAPC (10 nm)/
Al and electron only device with a device structure of ITO (50 nm)/
Ca (5 nm)/PPBC (25 nm)/TSPO1 (35 nm)/LiF (1 nm)/Al (200 nm)
were also prepared as single carrier devices. All devices were
fabricated by vacuum thermal evaporation and were encapsulated
with a glass lid after cathode deposition. The substrate and the
glass lid were sealed using epoxy adhesive and CaO desiccant was
inserted inside the device to capture moisture and oxygen. Current
densityevoltage characteristics of all devices were measured by
Keithley 2400 source measurement unit and luminance perfor-
mances were characterized by CS1000 spectroradiometer.
3-Bromo-2-methylpyridine (1.9 g, 11.04 mmol) and 3,5-
dichlorophenyl boronic acid (3.16 g, 16.57 mmol) dissolved in
degassed THF (60 mL) were placed into a 250 mL two necked round
bottom flask. A solution of potassium carbonate (3.05 g, 22.09 mmol)
in water (20 mL) was added slowly to the solution, and bubbled with
nitrogen for 30 min. Finally, tetrakis(triphenylphosphine)palla-
dium(0) (1.26 g,1.17 mmol) was added to the above reaction mixture,
then heated under reflux under nitrogen overnight. The solution was
cooled to room temperature and the organics were extracted using
ethyl acetate and washed with distilled water. The solution was dried
over magnesium sulfate and evaporated to remove solvent. The
product was further purified by flash chromatography on silica gel
using methylene chloride/n-hexane as an eluent to produce 2.0 g
(76% yield) of 2-(3,5-dichlorophenyl)-3-methylpyridine.
Yellow powder, m.p. 87 ꢀC, FT-IR (KBr) 3051, 1557, 1423, 1407,
1383, 1293, 1254, 1190, 1097, 1034, 884, 853, 793, 765, 736,
695 cmꢁ1 1H NMR (500 MHz, CDCl3):
. d 2.50 (s, 3H), 7.19 (t, 1H,
J ¼ 2.5 Hz), 7.21 (s, 2H), 7.39 (s, 1H), 7.47 (d, 1H, J ¼ 4.5 Hz), 8.54 (d,
1H, J ¼ 3.2 Hz). 13C NMR (125 MHz, CDCl3):
d 23.2,121.0,127.4,127.6,
134.3, 135.0, 136.8, 142.8, 148.8, 155.5. HRMS calcd for C12H9Cl3N
(Mþ þ H) 238.0185, found 238.0190.
2.2. Synthesis of 9,90-(5-(3-methylpyridin-2-yl)-1,3-phenylene)
bis(9H-carbazole)
2-(3,5-Dichlorophenyl)-3-methylpyridine (2.0 g, 8.40 mmol),
carbazole (3.51 g, 21.00 mmol), sodium tert-butoxide (4.04 g,
42.00 mmol), 2-dicyclohexylphosphino-20,60-dimethoxybiphenyl
(0.86 g, 2.10 mmol), tris(dibenzylideneacetone)dipalladium (0.48 g,
0.52 mmol) and xylene (140 mL) were combined and heated under
reflux under nitrogen for 36 h. The solution was cooled to room
temperature and the organics were extracted into ethyl acetate,
washed with distilled water, dried over magnesium sulfate, filtered.
The solvent was removed in vacuo, and the product was isolated by
column chromatography on silica gel (methylene chloride/n-
3. Results and discussion
The design of PPBC was based on carbazole and pyridine unit to
obtain bipolar charge transport properties from the PPBC host
material. The carbazole unit can play a role of a hole transport unit,
while the pyridine unit can act as an electron transport unit, which
can induce bipolar charge transport character in the PPBC host. In
addition, a methyl substituent was attached to the pyridine unit to
Scheme 1. Synthetic route of PPBC.

36
C.W. Lee, J.Y. Lee / Dyes and Pigments 103 (2014) 34e38
increase the triplet energy of the host material by reducing the
conjugation length through steric hindrance.
The PPBC host was synthesized by Pd catalyst mediated coupling
reaction between carbazole and 2-(3,5-dichlorophenyl)-3-
methylpyridine which was prepared by Suzuki coupling reaction
between (3,5-dichlorophenyl)boronic acid and 3-bromo-2-
methylpyridine. Synthetic yield of the PPBC host was 36%. The
PPBC host was purified by vacuum train sublimation after column
chromatography and high purity level over 99% was obtained from
high performance liquid chromatography. Chemical structure of the
PPBC host was confirmed by NMR spectroscopy, elemental analysis
and mass spectroscopy.
Photophysical properties of PPBC were analyzed using ultravio-
letevisible (UVeVis) absorption and photoluminescence (PL) spec-
trometers. Fig. 1 shows UVeVis absorption, solution PL and low
temperature PL spectra of the PPBC host. Strong UVeVis absorption
Fig. 2. HOMO and LUMO distribution of PPBC.
Hole only and electron only devices of PPBC were fabricated to
compare hole and electron densities in the emitting layer. Current
densityevoltage curves of the single carrier devices are shown in
Fig. 3. The current density of the electron only device was higher
than that of hole only device, indicating that PPBC host material
shows relatively strong electron transport properties. The electron
deficient 3-methylpyridine unit improved electron transport
properties of PPBC, resulting in relatively high electron density.
Blue PHOLEDs were fabricated using PPBC as the host material
for FIrpic dopant. Fig. 4 shows current densityevoltageeluminance
curves of the blue PHOLEDs with the PPBC host according to the
doping concentration of FIrpic. The current density and luminance
of the PPBC devices were increased according to the doping con-
centration of FIrpic because of better electron injection at high
doping concentration by electron trapping as reported in other
works [16].
Quantum efficiencyecurrent density curves of the PPBC devices
are shown in Fig. 5. The quantum efficiency of the blue PHOLEDs
was optimized at a doping concentration of 5% and a maximum
quantum efficiency of 19.8% was obtained. The high quantum
efficiency of the PPBC blue PHOLEDs can be explained by the effi-
cient energy transfer and charge transport properties of PPBC. The
triplet energy of the PPBC was higher than that of FIrpic and the PL
emission of PPBC was overlapped with the UVeVis absorption of
FIrpic. Therefore, energy transfer from PPBC to FIrpic was efficient
and contributed to the high quantum efficiency of the PPBC blue
PHOLEDs. Charge balance in the emitting layer originated from the
charge transport properties of PPBC also enhanced the quantum
efficiency of the PPBC devices. As shown in the single carrier device
data, relatively large electron current density was observed in the
single carrier devices, which balanced holes and electrons in the
peaks were observed at 239 nm and 293 nm due to
of the PPBC backbone and weak absorption peaks appeared at
324 nm and 338 nm because of ne * transition of carbazole. UVeVis
pep* transition
p
absorption edge of PPBC was 345 nm, which corresponded to a
bandgap of 3.59 eV. Maximum solution PL emission peak of PPBC
was observed at 365 nm and solid PL emissionwas shifted to 388 nm
due to intermolecular interaction of the PPBC host caused by elec-
tron donating carbazole and electron deficient pyridine. Phospho-
rescent emission of PPBC was measured in liquid nitrogen by
applying delay time of 1 ms to exclude fluorescent emission. Peak
position of the first phosphorescent emission was 414 nm, which
corresponded to a triplet energy of 2.99 eV. The triplet energy of the
PPBC host was higher than thatof FIrpic (2.65 eV), indicating that the
PPBC host can be suitable as the host material for blue PHOLEDs.
The highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) of PPBC were simulated us-
ing Gaussian 09 program. Fig. 2 shows the HOMO and LUMO of
PPBC calculated using density functional of B3LYP with 6-31G*
basis sets. The HOMO of PPBC was distributed over carbazole of
PPBC because of electron donating character of carbazole, while the
LUMO was dispersed over pyridine of PPBC due to electron defi-
ciency of the pyridine unit. Therefore, the PPBC host can show
bipolar charge transport properties.
The HOMO level of PPBC was ꢁ6.11 eV from the oxidation curve
obtained by cyclic voltammetry measurement and the LUMO level
was ꢁ2.52 eV from the HOMO and bandgap from UVeVis absorp-
tion. The HOMO and LUMO levels of PPBC were suitable for hole
injection from mCP (HOMO: ꢁ6.10 eV) and electron injection from
TSPO1 (LUMO: ꢁ2.52 eV).
100
Hole only
90
80
70
60
50
40
30
20
10
0
Electron only
0
2
4
6
8
10
Voltage (V)
Fig. 1. UVeVis, PL and low temperature PL spectra of PPBC.
Fig. 3. Current densityevoltage curves of hole only and electron only devices of PPBC.

C.W. Lee, J.Y. Lee / Dyes and Pigments 103 (2014) 34e38
37
40
35
30
25
20
15
10
5
100000
10000
1000
100
4
3.5
3
1000
100
10
3%
5%
5%
10%
10%
2.5
2
10
1
1.5
1
1
0.1
0.01
0.1
0.5
0
0
0.01
0
2
4
6
8
10
0
2
4
6
8
10
Voltage (V)
Voltage (V)
Fig. 4. Current densityevoltageeluminance curves PPBC blue PHOLEDs according to
the doping concentration of FIrpic.
Fig. 6. Current densityevoltageeluminance curves PPBC PHWOLEDs according to the
doping concentration of FIrpic.
emitting layer. The relatively high electron current density of the
PPBC host is beneficial to balance holes and electrons in the emit-
ting layer because the electron current density is reduced by elec-
tron trapping of the FIrpic dopant in the emitting layer.
balance. Energy is transferred from the PPBC blue emitting layer to
the orange emitting layer and the high efficiency of the PPBC blue
emitting layer can boost the quantum efficiency of the PHWOLEDs.
Electroluminescence spectra of the PHWOLEDs are shown in
Fig. 8. The two devices exhibited two color white emission through
blue emission from FIrpic and orange emission from Ir(pq)2acac.
The blue emission was intensified at high FIrpic doping concen-
tration due to more electron injection from the orange emitting
layer to the blue emitting layer. As explained in the blue PHOLEDs
device data, electron density in the blue emitting layer is high at
10% doping concentration due to direct electron injection into the
FIrpic dopant. Therefore, recombination zone of the PHWOLEDs is
shifted from the orange emitting layer to the blue emitting layer,
resulting in strong blue emission at 10% doping concentration. The
color coordinates of the 5% and 10% FIrpic doped PHWOLEDs were
(0.40, 0.33) and (0.34, 0.31) at 1000 cd/m2, respectively.
Based on the high quantum efficiency of the PPBC blue devices,
two color PHWOLEDs were developed by stacking an orange
emitting layer on the PPBC blue emitting layer. The doping con-
centration of the blue emitting layer was managed to control the
color of the PHWOLEDs because the FIrpic doping concentration
can change the electron injection from the orange emitting layer to
the blue emitting layer. The FIrpic doping concentrations were 5%
and 10%. Current densityevoltageeluminance curves of the
PHWOLEDs are shown in Fig. 6. The current density and luminance
of the PHWOLEDs was increased at high FIrpic doping concentra-
tion due to better electron injection into the blue emitting layer,
which agrees with the current density dependence on the doping
concentration of the PPBC blue devices.
Quantum efficiencyecurrent density curves of the PHWOLEDs
are presented in Fig. 7. The quantum efficiency of the PHWOLEDs
was similar at 5% and 10% doping concentrations and a maximum
quantum efficiency of 17.1% was obtained. The quantum efficiency
at 1000 cd/m2 was 14.9% and 15.3% at 5% and 10% doping concen-
trations, respectively. The high quantum efficiency of the PPBC blue
PHOLEDs and bipolar charge transport properties of the PPBC host
were effective to obtain high quantum efficiency through charge
Color stability of the PHWOLEDs was monitored from 100 cd/m2
to 10,000 cd/m2. Fig. 9 shows EL spectra of the PHWOLEDs doped
with 10% FIrpic according to the luminance of the devices. There
was little change of the EL spectra and the color coordinate was
changed from (0.34, 0.31) at 100 cd/m2 to (0.35, 0.31) at 10,000 cd/
m2. The good color stability of the PPBC PHWOLEDs is related with
balanced charge injection between the blue and orange emitting
layers. There is only 0.01 eV energy barrier for hole injection from
the blue emitting layer to the orange emitting layer, which leads to
25
20
15
18
16
14
12
10
8
10
6
3%
5%
4
5%
5
10%
2
10%
0
0
0.0001 0.001
0.01
0.1
1
10
100
0.001
0.01
0.1
1
10
Luminance (cd/m2)
Current density (mA/cm2)
Fig. 5. Quantum efficiencyecurrent density curves of PPBC blue PHOLEDs according to
Fig. 7. Quantum efficiencyecurrent density curves of the PPBC PHWOLEDs according
the doping concentration of FIrpic.
to the doping concentration of FIrpic.

38
C.W. Lee, J.Y. Lee / Dyes and Pigments 103 (2014) 34e38
4. Conclusions
5%
In conclusion, high efficiency blue and white PHOLEDs were
developed using PPBC bipolar host materials in the blue emitting
layer. The PPBC host showed high triplet energy of 2.99 eV for
efficient energy transfer to blue emitting FIrpic dopant and bipolar
charge transport properties for hole and electron balance in the
emitting layer. A high quantum efficiency of 19.8% in blue PHOLEDs
and 17.1% in PHWOLEDs were achieved using the PPBC host ma-
terial. In addition, the PPBC host was effective to obtain good color
stability in the PHWOLEDs.
10%
References
[1] Sun YR, Giebink NC, Kanno H, Ma B, Thompson ME, Forrest SR. Management of
singlet and triplet excitons for efficient white organic light-emitting devices.
Nature 2006;440:908e12.
[2] Schwartz G, Pfeiffer M, Reineke S, Walzer K, Leo K. Harvesting triplet excitons
from fluorescent blue emitters in white organic light-emitting diodes. Adv
Mater 2007;19:3672e6.
[3] Schwartz G, Reineke S, Rosenow TC, Walzer K, Leo K. Triplet harvesting in
hybrid white organic light-emitting diodes. Adv Funct Mater 2009;19:
1319e33.
400
500
600
700
800
Wavelength (nm)
Fig. 8. Electroluminescence spectra of the PPBC PHWOLEDs according to the doping
concentration of FIrpic.
[4] Anzenbacher Jr P, Montes VA, Takizawa SY. High-purity white light from a
simple single dopant host-guest white organic light-emitting diode archi-
tecture. Appl Phys Lett 2008;93:163302e3.
100 cd/m2
[5] Ho CL, Wong WY, Wang Q, Ma D, Wang L, Lin Z. A multifunctional iridium-
carbazolyl orange phosphor for high-performance two-element WOLED
exploiting exciton-managed fluorescence/phosphorescence. Adv Funct Mater
2008;19:928e37.
1,000 cd/m2
10,000 cd/m2
[6] Wang Y, Hua Y, Wu X, Zhang L, Hou Q, Zhang N, et al. Application of mixed
interface in white-electrophosphorescent devices: an efficient approach to
adjust the distributions of carriers. Appl Phys Lett 2008;93:113302e3.
[7] Wang Y, Hua Y, Wu X, Zhang G, Hui J, Zhang L, et al. White electro-
phosphorescent devices based on tricolour emissive layers. J Phys D Appl Phys
2008;41:25104.
[8] Cheng G, Zhang Y, Zhao Y, Liu S, Ma Y. Improved efficiency for white organic
light-emitting devices based on phosphor sensitized fluorescence. Appl Phys
Lett 2006;88:83512e3.
[9] D’Andrade BW, Holmes RJ, Forrest SR. Efficient organic electrophosphorescent
white-light-emitting device with a triple doped emissive layer. Adv Mater
2004;16:624e8.
[10] Cheng G, Zhang Y, Zhao Y, Lin Y, Ruan C, Liu S, et al. White organic light-
emitting devices with a phosphorescent multiple emissive layer. Appl Phys
Lett 2006;89:43504e13.
[11] Han C, Xie G, Xu H, Zhang Z, Xie L, Zhao Y, et al. A single phosphine oxide
host for high-efficiency white organic light-emitting diodes with extremely
low operating voltages and reduced efficiency roll-off. Adv Mater 2011;23:
2491e6.
400
600
700
800
500
Wavelength (nm)
Fig. 9. Electroluminescence spectra of the PPBC PHWOLEDs doped with 10% FIrpic
according to the luminance.
[12] Jeon SO, Yook KS, Joo CW, Lee JY. Phenylcarbazole-based phosphine oxide
host materials for high efficiency in deep blue phosphorescent organic light-
emitting diodes. Adv Funct Mater 2009;19:3644e9.
[13] Su S, Gonmori E, Sasabe H, Kido J. Highly efficient organic blue-and white-
light-emitting devices having a carrier- and exciton-confining structure for
reduced efficiency roll-off. Adv Mater 2008;20:4189e94.
[14] Williams EL, Haavisto K, Li J, Jabbour GE. Excimer-based white phosphores-
cent organic light-emitting diodes with nearly 100% internal quantum effi-
ciency. Adv Mater 2007;19:197e202.
little field dependence of the hole injection on the driving voltage.
The electron injection is also little affected by the electric field
because the electron density is dominated by electron trapping of
the FIrpic dopant. As there is no energy barrier for the direct
electron injection from the TPBI emitting layer to the FIrpic dopant,
the electron injection is also little influenced by the electric field.
Therefore, the relative hole and electron density in the emitting
layer is maintained constant and good color stability is observed.
The use of the PPBC host was effective to obtain high quantum
efficiency and good color stability.
[15] Lee CW, Lee JY. High quantum efficiency in solution and vacuum processed
blue phosphorescent organic light emitting diodes using
a
novel
benzofuropyridine-based bipolar host material. Adv Mater 2013;25:596e600.
[16] Park HJ, Kim OY, Lee JY. Correlation of charge trapping and charge transport
properties of blue triplet emitters with device performances of blue phos-
phorescent organic light-emitting diodes. Synth Met 2013;176:47e50.