High quantum efficiency in solution and vacuum processed blue phosphorescent organic light emitting diodes using a novel benzofuropyridine- based bipolar host material

Article abstract of DOI:10.1002/adma.201203180

High quantum efficiency in solution and vacuum processed blue phosphorescent organic light emitting diodes are achieved using a new benzofuropyridine based bipolar host material. High quantum efficiencies of 18.0% and 23.0% are obtained in soluble and vacuum evaporable blue devices. Copyright

Full text of DOI:10.1002/adma.201203180

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High Quantum Efficiency in Solution and Vacuum
Processed Blue Phosphorescent Organic Light Emitting
Diodes Using a Novel Benzofuropyridine-Based Bipolar
Host Material
Chil Won Lee and Jun Yeob Lee*
It is important to improve the device performances of vacuum
deposited and solution processed phosphorescent organic
light-emitting diodes (PHOLEDs) through material and device
structure development. In particular, the device performances
of blue PHOLEDs should be enhanced due to relatively low
quantum efficiency of blue PHOLEDs compared with red or
green PHOLEDs[1–6] although the external quantum efficiency
of sky blue PHOLED has been improved up to 26%.[7]
dibenzofuran and dibenzothiophene derivatives were also
developed as high triplet energy bipolar host materials, but
weak electron transport property of those units limited the use
of them as the core of high triplet energy bipolar host mate-
rials.[14–19] Therefore, the development of electron deficient core
structure derived from dibenzofuran may enhance electron
transport properties of dibenzofuran while keeping the high tri-
plet energy of dibenzofuran.
Several organic charge transport and emitting materials
are being used in blue PHOLEDs and the most critical mate-
rials for the device performances are host materials in emit-
ting layer.[1–7] The host materials manage holes and electron
balance in the emitting layer and determine the quantum
efficiency of PHOLEDs. There are several requirements for
the host materials of blue PHOLEDs. First, the host materials
should possess higher triplet energy than dopant materials
for efficient energy transfer to blue emitting phosphorescent
dopant.[8] For this purpose, host materials for blue PHOLEDs
have a chemical structure made up of high triplet energy moie-
ties such as carbazole,[7–13] dibenzofuran[14–17] and dibenzothi-
ophene.[18,19] Secondly, the host materials must exhibit bipolar
charge transport properties for balanced hole and electron
density in the emitting layer. Therefore, both hole and electron
transport units should be combined in the molecular struc-
ture. Additionally, highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) levels for
efficient charge injection are important for low driving voltage
and high efficiency.
In this work, an electron deficient high triplet energy core
structure, benzofuropyridine (BFP), was developed as the core
structure of high triplet energy host material and a high tri-
plet energy bipolar host material based on BFP and phenyl-
carbazole, 3-(3-(carbazole-9-yl)phenyl)benzofuro[2,3-b]pyridine
(PCz-BFP), was synthesized. High quantum efficiencies of
18.0% and 23.0% were demonstrated in solution and vacuum
processed blue PHOLEDs, respectively, using PCz-BFP host
material.
BFP was developed as an electron deficient high triplet energy
core structure of host materials for blue PHOLEDs. Compared
with common dibenzofuran, the introduction of pyridine unit
makes the core electron deficient and can improve electron
transport properties. As BFP was designed as an electron defi-
cient core, it was combined with a common hole transport type
carbazole to develop bipolar host materials.
Synthetic scheme of PCz-BFP is shown in Scheme 1. PCz-BFP
was prepared by Suzuki coupling reaction of 3-chloro[1]-
benzofuro[2,3-b]pyridine (ClBFP) with boronic acid of 9-phe-
nylcarbazole. The new core unit, ClBFP, was synthesized by
ring closing reaction of 5-chloro-3-(2-methoxyphenyl)pyridin-2-
amine prepared by Suzuki coupling reaction of 2-methoxyphe-
nylboronic acid and 5-chloro-3-iodopyridin-2-amine. Synthetic
yield of PCz-BFP was 65%. Detailed synthetic procedure is
described in the Supporting Information (SI).
Molecular simulation of PCz-BFP was carried out to study
the highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) distribution of PCz-BFP.
Figure 1(a) shows HOMO and LUMO distribution of PCz-BFP.
HOMO and LUMO were calculated using Gaussian 03 program
and the nonlocal density functional of Becke’s 3-parameters
employing Lee-Yang-Parr functional (B3LYP) with 6–31G∗
basis sets was used.[20] HOMO of PCz-BFP was dispersed over
9-phenylcarbazole, while LUMO of PCz-BFP was localized on
BFP. HOMO and LUMO of PCz-BFP were completely sepa-
rated from each other due to strong electron donating property
of carbazole and electron accepting property of BFP originated
Based on the essential requirements of host materials, many
high triplet energy bipolar host materials were developed and
most of them were derived from carbazole because of high
triplet energy of carbazole.[7–13] Typically, carbazole derivatives
were modified with electron transport type functional groups
due to strong hole transport property of carbazole and bipolar
carbazole compounds were effective to improve the quantum
efficiency of blue PHOLEDs.[11–13] Other than carbazole,
Prof. C. W. Lee, Prof. J. Y. Lee
Department of Polymer Science and Engineering
Dankook University
126, Jukjeon-dong, Suji-gu, Yongin,
Gyeonggi, 448-701, Korea
E-mail: leej17@dankook.ac.kr
DOI: 10.1002/adma.201203180
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H3C
H3C
H2N
O
O
NH2
N
O
N
Pd(PPh3)4, K2CO3
Toluene/Water
t-butyl nitrite
THF/AcOH
N
OH
OH
I
+
B
Cl
Cl
Cl
HO
HO
N
N
B
O
Cl
O
Pd2(dba)3, S-Phos, K3PO4
Toluene//H2O (10/1)
N
N
+
Scheme 1. Synthetic scheme of PCz-BFP.
from the introduction of electron accepting pyridine unit. This
indicates that HOMO will be dominated by carbazole, while
LUMO will be dominated by BFP unit.
It is important to secure low surface roughness after
spin coating to apply host material by solution process.
Therefore, surface morphology of PCz-BFP: iridium(III)
bis[(4,6-difluorophenyl)-pyridinato-N,C2]picolinate (FIrpic) film
fabricated by spin coating was analyzed using atomic force
microscope (AFM). AFM images of spin coated PCz-BFP:FIrpic
film is shown in Figure 2(a). Spin coated PCz-BFP film showed
smooth surface morphology and a low root mean square sur-
face roughness of 0.35 nm was obtained. The low surface
roughness of PCz-BFP:FIrpic film indicates that PCz-BFP is
suitable as a host for solution processed blue PHOLEDs. The
origin of smooth surface morphology of PCz-BFP can be found
in the asymmetric molecular structure and distortion of carba-
zole and BFP. The asymmetric molecular structure of PCz-BFP
hinders uniform packing of molecules and induces amorphous
film formation. Distortion of carbazole and BFP from phenyl
linkage also contributes to the amorphous film morphology of
PCz-BFP:FIrpic film.
Photophysical properties of PCz-BFP were analyzed using
ultraviolet-visible (UV-Vis) and photoluminescence (PL) spec-
trometer. UV–vis, solution PL and low temperature PL spectra
of PCz-BFP are shown in Figure 1(b). Strong absorption peaks
of BFP modified 9-phenylcarbazole were observed at 292 nm,
312 nm and 340 nm. Bandgap calculated from the absorption
edge of UV-Vis absorption spectrum was 3.58 eV. PL emission
of PCz-BFP was centered at 362 nm. Triplet energy was calcu-
lated from phosphorescent emission peak at 427 nm and was
2.89 eV. BFP did not degrade the triplet energy of PCz-BFP and
the triplet energy of PCz-BFP was high enough for use as a host
material for blue PHOLEDs.
HOMO of PCz-BFP was measured by cyclic voltammetry
from the onset of oxidation curve and was −6.10 eV. LUMO
level was calculated from the HOMO and optical bandgap
from UV-Vis absorption and was −2.52 eV. HOMO level of
PCz-BFP was suitable for hole injection from high triplet
energy hole transport material and the LUMO was appropriate
for electron injection from high triplet energy electron trans-
port material.
Hole only and electron only devices of PCz-BFP were fab-
ricated to compare hole and electron density in the emitting
layer. Figure 2(b) represents current density-voltage curves of
hole and electron only devices of PCz-BFP. The device struc-
ture of hole only device was indium tin oxide (ITO, 50 nm)/
(a)
(b)
1.2
1
UV-Vis
PL
Low Temp. PL
0.8
0.6
0.4
0.2
0
HOMO
250
300
350
400
450
500
550
Wavelength(nm)
LUMO
Figure 1. HOMO and LUMO distribution of PCz-BFP (a) and UV-Vis, PL and low temperature PL spectra of PCz-BFP (b).
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(a)
(b)
100
90
80
70
60
50
40
30
20
10
0
Hole only
Electrononly
0
3
6
9
12
Voltage(V)
Figure 2. AFM image of spin coated PCz-BFP film (a) and current density-voltage curves of hole only and electron only devices (b).
poly(3,4-ethylenedioxythiophene);poly(styrenesulfonate)
(PEDOT:PSS, 60 nm)/4,4′-cyclohexylidenebis[N,N-bis(4-
vacuum deposited device due to the high current density. Turn-
on voltage of two devices was the same (3.5 V) irrespective of
film forming process, but the driving voltage at 1000 cd m−2
was lower in vacuum deposited device (5.4 V) than in spin
coated device (6.8 V).
methylphenyl)aniline] (TAPC, 30 nm)/PCz-BFP (25 nm)/Al
and that of electron only device was ITO (50 nm)/Ca (5 nm)/
PCz-BFP (25 nm)/diphenylphosphine oxide-4-(triphenylsilyl)
phenyl (TSPO1, 30 nm)/LiF (1 nm)/Al (200 nm). Hole and
electron current densities were similar over all voltage range
measured, indicating similar hole and electron density in the
emitting layer by the bipolar charge transport properties of
PCz-BFP. The bipolar charge transport properties of PCz-BFP
were further confirmed by hole and electron only device with
only PCz-BFP host material (Figure S1 in SI). Single layer hole
and electron only devices also showed similar hole and electron
current density, proving bipolar charge transport properties of
PCz-BFP. The bipolar charge transport properties of PCz-BFP
are originated from strong electron transporting BFP unit. BFP
was designed to show strong electron transport properties due
to pyridine unit. Compared with dibenzofuran, the replacement
of one benzene with pyridine increases the electron transport
properties as reported in other works.[21,22] As carbazole plays
a role of strong hole transport unit and BFP works as a strong
electron transport unit, both holes and electrons are effectively
transported by PCz-BFP. Therefore, PCz-BFP can balance holes
and electrons in the emitting layer.
As PCz-BFP showed high triplet energy for energy transfer
to FIrpic and smooth film morphology after spin coating, blue
PHOLEDs were fabricated by spin coating and vacuum thermal
evaporation. Figure 3(a) shows current density–voltage–lumi-
nance curves of solution and vacuum processed blue PHOLEDs.
Device structure of solution processed blue PHOLED was ITO
(50 nm)/PEDOT:PSS (60 nm)/poly(N-vinylcarbazole) (PVK,
15 nm)/PCz-BFP:FIrpic (20 nm, 5%)/TSPO1 (30 nm)/LiF
(1 nm)/Al (200 nm). Device architecture of vacuum processed
blue PHOLED was ITO (50 nm)/PEDOT:PSS (60 nm)/TAPC
(30 nm)/PCz-BFP:FIrpic (25 nm, 5%)/TSPO1 (30 nm)/LiF
(1 nm)/Al (200 nm). Doping concentration of FIrpic was 5%. Sim-
ilar device structure was used for solution and vacuum processed
devices except for the hole transport layer. TAPC could not be
used in solution processed device due to poor film quality of
solution coated TAPC film. The current density of PCz-BFP
device was high in vacuum deposited device because of high
hole mobility of TAPC.[23,24] The luminance was also high in
Quantum efficiency-luminance curves blue PHOLEDs are
shown in Figure 3(b). Maximum quantum efficiency of vacuum
deposited device was 23.0%, while quantum efficiency at
1,000 cd m−2 was 20.0%. The efficiency was a little degraded
in solution processed device and the maximum quantum effi-
ciency and quantum efficiency at 1,000 cd m−2 were 18.0%
and 17.6%, respectively. Although the quantum efficiency was
rather decreased in solution processed device, high quantum
efficiency was achieved both in solution and vacuum deposited
device using the PCz-BFP host material. The high quantum effi-
ciency of PCz-BFP devices can be explained by balanced charge
density and smooth amorphous film morphology of FIrpic
doped PCz-BFP film. As described above, PCz-BFP exhibited
similar hole and electron current density in the single charge
device, improving the recombination efficiency in the emitting
layer. The introduction of novel BFP moiety in the molecular
structure balanced holes and electrons in the emitting layer.
Smooth film morphology of spin coated PCz-BFP film also
contributed the high quantum efficiency of spin coated device.
Smooth film morphology implies little aggregation of FIrpic
dopant material and molecular interaction between dopant
materials is suppressed, resulting in high quantum efficiency.
Common 1,3-bis(N-carbazolyl)benzene could not work well as
the host material for soluble blue PHOLEDs due to crystalli-
zation caused by symmetrical molecular structure. The intro-
duction of BFP as the counter unit of carbazole improved the
film morphology after spin coating and bipolar charge trans-
port properties of PCz-BFP. Although the quantum efficiency
of PCz-BFP device was a little lower than that of state of the art
blue device,[7,25] the PCz-BFP worked well both in solution and
vacuum processed blue PHOLEDs.
Electroluminescence (EL) spectra of PCz-BFP are shown in
Figure 3(c). Both devices exhibited maximum emission peak at
468 nm from FIrpic emission and no other emission peak was
observed, indicating triplet exciton confinement inside the emit-
ting layer. The rather strong shoulder peak at 493 nm is due to
different film thicknesses of spin coated and vacuum deposited
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(a)
(b)
25
20
15
10
5
40
10000
1000
100
10
Solution
Vacuum
30
20
10
Solution
Vacuum
1
0.1
0
0
0
0.01
0.1
1
10
100
1000
10000
2
4
6
8
10
Luminance(cd/m2)
Voltage (V)
(c)
Solution
Vacuum
400
500
600
700
800
Wavelength (nm)
Figure 3. Current density-voltage-luminance (a), quantum efficiency–luminance (b) and electroluminescence (c) data of spin coated and vacuum
deposited blue PHOLEDs with PCz-BFP.
TSPO1 (30 nm)/LiF (1 nm)/Al (200 nm), respectively. Current density–
voltage–luminance characteristics of blue PHOLEDs were measured
using CS1000 spectroradiometer and Keithley 2400 source measurement
unit.
devices. Color coordinates of solution processed and vacuum
deposited devices were (0.15,0.33) and (0.14,0.33), respectively.
In conclusion, a novel benzofuropyridine based host material,
PCz-BFP, was synthesized as a high triplet energy bipolar host
material and high efficiency solution processed and vacuum
deposited blue PHOLEDs were developed using PCz-BFP.
High quantum efficiencies of 18.0% and 23.0% were achieved
in the solution processed and vacuum evaporated blue-emitting
devices using PCz-BFP:FIrpic emitting layer. PCz-BFP could
be effectively used as a universal host material irrespective of
film forming method. It was proved that BFP is efficient as an
electron deficient unit of bipolar host material and can be effec-
tively used as a building block of high triplet energy materials
for blue PHOLEDs.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Received: August 2, 2012
Revised: September 26, 2012
Published online: November 8, 2012
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Device Fabrication and Measurements: The basic device structures of
soluble and vacuum evporable PHOLEDs were ITO (50 nm)/PEDOT:PSS
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