A carbazole-phenylbenzimidazole hybrid bipolar universal host for high efficiency RGB and white PhOLEDs with high chromatic stability

Article abstract of DOI:10.1039/c1jm14029a

A novel bipolar molecule CNBzIm comprised of electron-donating carbazole and electron-accepting phenylbenzimidazole is designed, synthesized and characterized for RGB phosphorescent organic light emitting diodes (PhOLEDs). CNBzIm exhibits promising proper

Full text of DOI:10.1039/c1jm14029a

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PAPER
A carbazole–phenylbenzimidazole hybrid bipolar universal host for high
efficiency RGB and white PhOLEDs with high chromatic stability
a
b
a
b
b
b
*
*
Wen-Yi Hung, Liang-Chen Chi, Wei-Jiun Chen, Ejabul Mondal, Shu-Hua Chou, Ken-Tsung Wong
and Yun Chic
Received 18th August 2011, Accepted 29th September 2011
DOI: 10.1039/c1jm14029a
A novel bipolar molecule CNBzIm comprised of electron-donating carbazole and electron-accepting
phenylbenzimidazole is designed, synthesized and characterized for RGB phosphorescent organic light
emitting diodes (PhOLEDs). CNBzIm exhibits promising properties such as high triplet energy (ET ¼
2.71 eV) due to disrupted p-conjugation, bipolar and balanced charge transport ability (mh z me z 10ꢀ5
cm2 Vꢀ1 sꢀ1), and high morphological stability (Tg ¼ 185 C). PhOLEDs adopted CNBzIm as a host
ꢁ
respectively doped with Os(bpftz)2(PPh2Me)2, (PPy)2Ir(acac) and FIrpic under the same device
structure show excellent performance with external quantum efficiencies (hext) as high as 19.1% for red,
17.8% for green, and 12.7% for blue. A two-color, all-phosphor and single-emitting-layer WOLED
hosted by CNBzIm was achieved with a maximum hext of 15.7%, current efficiency (hc) of 35 cd Aꢀ1, and
power efficiency (hp) of 36.6 lm Wꢀ1. Utilizing a bipolar host with balanced mobilities, WOLED can
effectively broaden its recombination zone and show high color stability. The Commission
Internationale de L’Eclairage (CIE) coordinates of the WOLED remain almost constant when the
brightness goes from 1700 cd mꢀ2 (0.32, 0.41) to 49 900 cd mꢀ2 (0.31, 0.41).
colors by adopting either fluorophore/phosphor or all-phosphor
systems. The fluorophore/phosphor system so-called hybrid
WOLED combines a blue fluorophore with complementary
phosphors,4 in which the blue fluorophore needs to have both
high photoluminescent quantum yield and higher triplet energy
(ET) than that of the complementary phosphors. In our previous
work, we reported that benzimidazole/carbazole hybrid bipolar
material (CPhBzIm) exhibits a blue fluorescent peak at 426 nm
with excellent solid state photoluminescence quantum yield
(FPL ¼ 69%), and suitable triplet energy (ET ¼ 2.48 eV), and
bipolar charge transport ability.5 Hybrid WOLEDs can be
reached by only one single emission layer made of the fluorescent
blue emitter doped with very small amounts of yellow–green
phosphorescent emitter. However, the limitation set by the
intrinsic photophysical property of a fluorescent blue emitter, the
WOLED, only exhibited a maximum hext of 7% (15.5 cd Aꢀ1) and
Introduction
Phosphorescent organic light-emitting diodes (PhOLEDs) have
become a subject of great research interest in recent years from
both academy and industry owing to their potential applications
in flat-panel displays and lighting sources.1 It is essential to
deliver a set of primary RGB emitters with sufficiently high
luminous efficiency for full-color OLED display applications.
Recently, the use of phosphorescent materials in which strong
spin–orbit coupling leads to singlet–triplet state mixing has
boosted the electroluminescence efficiency up to unity internal
quantum efficiency. To date, green and red PhOLEDs with 100%
internal quantum efficiency have been successfully achieved,2
while efficient and stable blue PhOLEDs still remain challenging.
On the other hand, white organic light-emitting diodes
(WOLEDs) have drawn intensive interests because of their
potential application in full-color flat-panel displays, backlights
for LCDs, and solid-state lighting.3 White emission is generally
achieved by mixing several materials that give rise to the three
primary colors (blue, green, and red) or two complementary
12.8 lm Wꢀ1
.
To realize high-efficiency WOLEDs, the more promising
alternative approach is to use all phosphor system that enable an
internal efficiency as high as 100%.6 Nevertheless, there are
challenges of using all phosphor WOLEDs, particularly difficult
to find highly efficient and stable blue host materials for signifi-
cantly reducing efficiency roll-off at the practical luminance. In
addition, the adaptation of two or multiple emitting layer (EML)
structures typically will result in higher complexity of device
fabrication, higher operation voltages and thus lower power
efficiency.7 Alternatively, the full-phosphor approach of
WOLEDs can be achieved by using only a single emitter layer
aInstitute of Optoelectronic Sciences, National Taiwan Ocean University,
Keelung 202, Taiwan. E-mail: wenhung@mail.ntou.edu.tw; Fax: +886 2
24634360; Tel: +886 2 24622192 ext. 6718
bDepartment of Chemistry, National Taiwan University, Taipei, 106,
Taiwan. E-mail: kenwong@ntu.edu.tw; Fax: +886 2 33661667; Tel:
+886 2 33661665
cDepartment of Chemistry, National Tsing Hua University, 300 Hsinchu,
Taiwan
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composed of a universal host doped with different color dopants
simultaneously. This approach could pave the way towards
relatively simple and more cost-effective fabrication processes for
WOLED with comparable output efficiencies. Along this line,
universal host materials for phosphorescent devices recently
caught great attention. For example, Shu et al. reported fluorene/
triarylamine hybrid hole-transport-type host material, tris[4-(9-
phenylfluoren-9-yl)phenyl]amine (TFTPA), with a high triplet
energy (ET) of 2.89 eV and a high glass transition temperature
(Tg) of 186 ꢁC.8 Devices with TFTPA as host show external
quantum efficiencies (hext) as high as 12.3% for blue, 11.8% for
green and 9.2% for red. Cheng et al. reported a hole transporting
host material, 2,6,14-tris-(diphenylamino)triptycene (TATP),
exhibiting a remarkably high ET of 2.94 eV and a Tg of 178 ꢁC.9
TATP can be employed as a general host for blue, green and red
electrophosphorescence devices providing hext of 7.8%, 11.5%
and 9.8%, respectively. Furthermore, Cheng and Chou reported
that a bipolar host material bis-(4-(N-carbazolyl)phenyl)phe-
nylphosphine oxide (BCPO) containing a phenylphosphine oxide
and two carbazole groups shows a high ET of 3.01 eV and a Tg of
137 ꢁC.10 BCPO can be applied as a universal host to blue, green
and red phosphorescent devices using simple device architec-
tures, giving extremely high hext of 23.5%, 21.6% and 17%,
respectively. Yang et al. reported a silicon-bridged compound
p-BISiTPA combining p-type triphenylamine and n-type benz-
imidazole moiety, giving a high ET of 2.69 eV, a Tg of 102 ꢁC and
bipolar transporting features.11 Devices with p-BISiTPA as host
show excellent performance, with hext as high as 16.1% for blue,
22.7% for green, 20.5% for orange and 19.1% for WOLED. In
addition, Kido et al. developed a bipolar host material
26DCzPPy by combining carbazoꢁle and pyridine into a single
Scheme 1 Synthetic pathways of CNBzIm and molecular structure of
CPhBzIm.
cd Aꢀ1 and power efficiency (hp) of 36.6 lm Wꢀ1. This WOLED
shows high color stability ascribed to a broad charge recombi-
nation zone in the emitting layer incorporating with this superior
bipolar host. The Commission Internationale de L’Eclairage
(CIE) coordinates of WOLED remain almost constant when the
brightness goes from 1700 cd mꢀ2 (0.32, 0.41) to 49 900 cd mꢀ2
(0.31, 0.41).
Results and discussion
molecule, exhibiting high Tg (102 C) and triplet energy (ET
¼
Scheme 1 depicts the synthetic routes for the bipolar host
material CNBzIm. The synthesis of 1-(40-bromophenyl)-2-
phenyl-1H-benzoimidazole (1)13 was carried out by reacting
N-(4-bromophenyl)-o-phenylenediamine with benzaldehyde in
acetic acid under refluxing condition gave a yield of 70%.
CNBzIm was obtained in 74% yield when benzimidazole deriv-
ative 1 was treated with carbazole-based diboronic ester 214 in the
presence of a catalytic amount of Pd(PPh3)4. CNBzIm was well
characterized with 1H NMR, 13C NMR, mass and elemental
analysis. CNBzIm exhibits high thermal stability with a decom-
position temperature (Td, corresponding to 5% weight loss) of
405 ꢁC by the thermogravimetric analysis (TGA). The glass
transition temperature (Tg ¼ 185 ꢁC) of CNBzIm was determined
by differential scanning calorimetry (DSC). We could use
vacuum deposition to prepare homogeneous, stable amorphous
thin films. The similar thermal stability as compared to that of
counterpart molecule CPhBzIm is presumably the result of the
same molecular weight (Table 1). CNBzIm exhibits excellent
thermal and morphological stability, which could reduce the
possibility of phase separation upon heating, thus can potentially
prolong the device operational lifetime.
2.71 eV).12 Excellent RGB phosphorescent devices employing
26DCzPPy as host with hext as high as 24.3% for blue, 26.9% for
green, 12.6% for red were achieved at a brightness of 100 cd mꢀ2
.
Based on this reported literature, an ideal universal host should
have (i) high triplet energy suitable for blue, green and red
phosphors, preventing from a reverse energy transfer, (ii)
appropriate HOMO/LUMO levels relative to neighboring
functional layers, allowing smooth charge carriers injection into
the emitting layer, and (iii) bipolar transporting characteristics,
rendering a broad exciton-formation zone and consequently
reducing the efficiency roll-off. In this report, we modified the
efficient green host CPhBzIm by changing the linking topology
from a C- into a N-bridge connecting the electron-accepting N-
phenylbenzimidazole to the electron-donating carbazole core.
This molecular design leads to a new benzimidazole/carbazole
hybrid compound, 3,6-bis(N-phenylbenzimidazol)-9-phenyl-
carbazole (CNBzIm, Scheme 1) with reduced p-conjugation and
thus an increased triplet energy (ET ¼ 2.71 eV); meanwhile, the
whole molecule would maintain suitable frontier orbitals energy,
bipolar charge transport (mh z me z 10ꢀ5 cm2 Vꢀ1sꢀ1) and
sufficient morphological stability (Tg ¼ 185 ꢁC). PhOLEDs
employing CNBzIm as a host doped with various phosphors
under the same device structure show excellent performance,
with external quantum efficiencies as high as 12.7% for blue,
17.8% for green, 19.1% for red. A two-color, all-phosphor and
single-emitting-layer WOLED hosted by CNBzIm was also
achieved with a maximum hext of 15.7%, current efficiency (hc) 35
Fig. 1 displays the room-temperature absorption and photo-
luminescence (PL) spectra of CNBzIm recorded from a dilute
dichloromethane solution and as a neat film on a quartz
substrate. A remarkable blue-shift (ca. 20 nm) in the spectra of
CNBzIm as compared with those obtained from CPhBzIm
indicating the efficient disruption of the p-conjugation between
the N-phenylbenzimidazole peripherals and 3,6-diarylcarbazole
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Table 1 Physical properties of CNBzIm and CPhBzIm
Tg/ꢁC
Td/ꢁC
E1/2 a/E1/2 b/V
HOMO/LUMO/Egc/eV
ET/eV
labs/nm [sol./film]
lPL/nm [sol./film]
OX
REV
CNBzIm
CPhBzIm
185
170
405
480
1.27/ꢀ2.13
1.24/ꢀ2.06
ꢀ5.57/ꢀ2.2/3.37
ꢀ5.49/ꢀ2.37/3.12
2.71
2.48
300/304
317/322
391/398
412/426
a
Oxidative CV was performed in 1.0 mM CH2Cl2 containing 0.1 M TBAF6. b Reductive CV was performed in 1.0 mM THF containing 0.1 M TBAP.
c
Determined through AC2 measurements; LUMO ¼ HOMO + Eg; the value of Eg was calculated from the absorption onset of the solid film.
phenylene rings connected to C3 and C6 positions with major
contribution from central carbazole moiety. On the other hand,
LUMO is mainly populated on carbazole and phenylene ring
connected to C3 position of carbazole core with minor contri-
bution from benzimidazole, implying that p-conjugation
between peripheral N-phenylbenzimidazole and carbazole core is
largely disrupted. The spatial distribution of CNBzIm frontier
orbitals is largely different from those of CPhBzIm, in which the
N-phenylbenzimidazole blocks made a significant contribution
for both HOMO and LUMO.5 These results indicate that the
manipulation of the spatial distributions of frontier orbitals,
energy levels, and triplet energies of benzimidazole/carbazole
hybrid bipolar materials can be easily achieved by a facile
strategy of altering the linking topologies (C- versus N-connec-
tion) between peripheral benzimidazole and central carbazole.
The triplet energy of CNBzIm (ET ¼ 2.71 eV) is sufficiently
higher than that of blue phosphor bis[4,6-(difluorophenyl)-pyr-
Fig. 1 Room-temperature UV-Vis absorption and PL spectra of
CNBzIm in dilute CH2Cl2 solution (dotted lines) and in neat film (solid
lines); Phosphorescence spectra of CNBzIm and CPhBzIm in EtOH
solution (symbol lines) at 77 K.
idinato-N,C2 ]picolinate (FIrpic, 2.62 eV)15 and other
0
long-wavelength phosphorescent dopants implying effective
confinement of the triplet excitons on the guest and prevention of
back energy transfer from the guest to the host are feasible.
The electrochemical properties of CNBzIm were investigated
by cyclic voltammetry (CV) as shown in Fig. 3. We observed one
reversible oxidation potential at 1.27 V and one quasi-reversible
reduction potential at ꢀ2.13 V (versus Ag/AgCl). Obviously,
there is no typical electropolymerization of carbazole upon
electrochemical oxidation since the electroactive sites (C3 and
C6) of carbazole were blocked with benzimidazole groups. The
HOMO energy level (ꢀ5.57 eV) of CNBzIm was estimated by
atmospheric photoelectron spectroscopy, and the LUMO energy
level (ꢀ2.2 eV) was calculated by subtraction of the optical
energy gap (3.37 eV) from the HOMO value. The electrochemical
results and HOMO/LUMO energy levels of CNBzIm are slightly
different from those of CPhBzIm (Table 1), indicating that the
core. Fig. 1 also depicts the phosphorescence spectra of CNBzIm
and CPhBzIm in EtOH solution at 77 K. The highest energy 0–0
phosphorescent emission of CNBzIm located at 457 nm was used
to calculate the triplet energy (ET ¼ 2.71 eV), which is higher
than that of CPhBzIm (ET ¼ 2.48 eV).
The theoretical calculations for HOMO/LUMO orbitals of
CNBzIm were performed by density functional theory (DFT)
using B3LYP with 6-31 + G(d,p) basis set. Fig. 2 shows the
HOMO of CNBzIm which is distributed over the carbazole and
Fig. 2 Frontier molecular orbitals HOMO (top) and LUMO (bottom)
of CNBzIm calculated with DFT on a B3LYP/6-31 + G(d,p).
Fig. 3 Cyclic voltammogram of CNBzIm.
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electron-accepting N-phenylbenzimidazole peripherals have
weaker electronic communications toward the carbazole core
upon using N-connectivity, which is in good agreement with the
results obtained by theoretical calculations.
in the range from 1.1 ꢂ 10ꢀ5 to 3.2 ꢂ 10ꢀ5 cm2 Vꢀ1 sꢀ1 for fields
varying from 5.6 ꢂ 105 to 7.2 ꢂ 105 V cmꢀ1. Fig. 4(c) shows the
carrier mobilities of CNBzIm and CPhBzIm plotted as a function
of the square root of the electric field. Apparently, N-phenyl-
benzimidazole linking topologies, i.e. C- or N-connection, with
the carbazolyl moiety do not have a pronounced effect on the
hole transporting ability but play a crucial role on electron-
transporting ability. The electron mobility of CPhBzIm is two
times of magnitude higher than that of CNBzIm. However, the
bipolar host CNBzIm had relatively balanced hole and electron
transporting ability as compared with that of CPhBzIm, which
will be highly beneficial for charge carrier balance in the emitting
layer to generate broader recombination zones.
To evaluate the bipolar character of CNBzIm, we used the
time-of-flight (TOF) transient-photocurrent technique at room
temperature to determine the carrier mobilities.16 Fig. 4(a) and
(b) show the representative TOF transient for holes and electrons
of CNBzIm.
The transient photocurrent signals were slightly dispersive,
suggesting the presence of carrier traps. The transit time, tT, can
be evaluated from the intersection point of two asymptotes in
the double-logarithmic representation of the TOF transient. The
carrier mobilities of CNBzIm were determined by using the
equation m ¼ d2/VtT, where d is the sample thickness and V is
the applied voltage. The hole mobility (mh) of CNBzIm lies in the
range from 4.3 ꢂ 10ꢀ6 to 1.2 ꢂ 10ꢀ5 cm2 Vꢀ1 sꢀ1 for fields varying
from 3.4 ꢂ 105 to 5.6 ꢂ 105 V cmꢀ1; the electron mobility (me) lies
In view of the high triplet energy (ET ¼ 2.71 eV) of CNBzIm, the
application of it as host material for RGB electrophosphorescent
devices was investigated by a simple three-layer configuration:
indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) : poly
(styrene sulfonate) (PEDOT : PSS) (30 nm)/DTAF (25 nm)/
emitter (25 nm)/TPBI (50 nm)/LiF (0.5 nm)/Al (100 nm). To
illustrate our concept, three phosphorescent dyes, osmium(II)
bis[3-(trifluoromethyl)-5-(4-tert-butylpyridyl)-1,2,4-triazolate]
diphenylmethylphosphine [Os(bpftz)2(PPh2Me)2] for red emis-
sion,17
[(PPy)2Ir(acac)] for green emission,18 and Iridium(III)[bis(4,6-
bis(2-phenylpyridinato)iridium(III)
acetylacetonate
difluorophenyl)-pyridinato-N,C2 ]picolinate (FIrpic) for blue
0
emission, are exploited (Scheme 2). The hole-transporting mate-
rial 1,1-bis[(di-4-tolylamino)phenyl]-9,90-fluorene (DTAF), with
a high triplet level (ET ¼ 2.87 eV) and wide bandgap (Eg ¼ 3.47 eV)
that can efficiently confine the triplet energy of phosphors, is
selected as the hole-transporting layer (HTL).5,19 To further
confine the holes or generated excitons within the emissive region,
1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene (TPBI)20 with
a high-energy gap is selected as the electron-transporting layer
(ETL). PEDOT : PSS and LiF served as hole- and electron-
injecting layer, respectively.
Fig. 5 shows the current density–voltage–brightness (J–V–L)
characteristics, EL efficiencies, and spectra of RGB devices using
CNBzIm doped with 10% Os(bpftz)2(PPh2Me)2 (red, R), 10%
(PPy)2Ir(acac) (green, G), and 12% FIrpic (blue, B), respectively.
The device performances are summarized in Table 2. All devices
displayed low turn-on voltage at 2.5 V. The blue emission device
B reveals a maximum brightness of 36 400 cd mꢀ2 at 12 V (900
mA cmꢀ2) with the CIE coordinates of (0.17, 0.39). The
maximum external quantum efficiency (hext) and power efficiency
(hp) were 12.7% (29 cd Aꢀ1) and 30.5 lm Wꢀ1, respectively.
These values are moderate among FIrpic-based OLEDs
presumably because the ET of CNBzIm is not sufficiently high
enough relative to FIrpic for blocking the thermally activated
Fig. 4 Typical transient photocurrent signals for CNBzIm (3.2 mm thick)
at E ¼ 5.6 ꢂ 105 V cmꢀ1: (a) hole; (b) electron. Insets are the
double logarithmic plots of (a) and (b). (c) Electron and hole mobilities
versus E1/2, compared with CPhBzIm.
Scheme 2 Chemical structures of the heavy metal complex used in this
study.
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Fig. 6 (a) J–V–L characteristics and (b) plots of EL efficiency versus
brightness for white devices under different HTLs.
cd Aꢀ1) and 59.4 lm Wꢀ1, respectively. Remarkably, the device
shows rather low efficiency roll-off: at a brightness of 1000 cd
mꢀ2, hext is still as high as 17.7%. The values are similar to our
previously reported results using CPhBzIm doped with (PBi)2Ir
(acac), in which the value of hext reached 19.2%.5 In addition, the
red emission device R using Os(bpftz)2(PPh2Me)2 as the dopant
exhibits strong emission with the CIE coordinates of (0.61, 0.39).
For device R, the maximum values of hext and hp reach as high as
19.1% (33.8 cd Aꢀ1) and 27 lm Wꢀ1, respectively, with
a maximum brightness of 43 800 cd mꢀ2 at 11.5 V (1040 mA
cmꢀ2). The electroluminescence (EL) spectra consist only of
a phosphor emission without any residual emission from the host
and/or adjacent layers as shown in Fig. 5(c), even at high driving
currents—an indication of complete energy and/or charge
transfer from the CNBzIm host to the phosphors upon electrical
excitation. The EL spectra and CIE coordinates for the devices
exhibited no significant change on the variation of operating bias
voltages. More importantly, the excellent performances of these
RGB devices were obtained from the same device architecture
Fig. 5 (a) J–V–L characteristics, (b) plots of EL efficiency versus
brightness and (c) EL spectra for RGB devices under the same device
structure.
exciton diffusion completely. However, this is a worthy sacrifice
to acquire a compromise among the ETs of RGB phosphors.
Next we fabricated the green emission device G with the same
configuration as device B except for using (PPy)2Ir(acac) as the
dopant. The maximum brightness reaches 140 000 cd mꢀ2 at 10.5
V (1080 mA cmꢀ2) with the CIE coordinates of (0.35, 0.62) for
device G. The maximum values of hext and hp were 17.8% (66.8
Table 2 EL performances of devices
Device
Vona/V
At 1000 nitb/V, %
Lmax/cd mꢀ2
Imax/mA cmꢀ2
hext/%
hc/cd Aꢀ1
hp/lm Wꢀ1
B
G
R
W1
W2
2.5
2.5
2.5
2.5
2.5
5.5, 9.8
36 400 (12 V)
140 000 (10.5 V)
43 800 (11.5 V)
49 900 (14 V)
51 600 (13 V)
900
1080
1040
1240
1600
12.7
17.8
19.1
15.7
15.4
29
30.5
59.4
27
36.6
33.7
4.3, 17.7
5.2, 17.6
7.5, 12.4
5.2, 11.2
66.8
33.8
35
34.3
a
b
Turn-on voltage at which emission became detectable. The values of driving voltage and hext of device at 1000 cd mꢀ2
.
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incorporated with different phosphors. When the brightness
reaches to 1000 cd mꢀ2, the devices have similar driving voltages
(4.3–5.5 V) as well as the hext still remains 9.8% for blue, 17.7%
for green, and 17.6% for red, which make them very attractive for
commercial applications of full-color display.
emission mechanism in the device W1, device W2 with different
HTLs [4,40-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB,
20 nm)/4,40,400-tri(N-carbazolyl)triphenylamine (TCTA, 5 nm)]
to replace DTAF in device W1 structure was also fabricated and
tested. Fig. 7 displays the electroluminescence (EL) characteris-
tics of white devices W1 and W2; the data are summarized in
Table 2. The J–V characteristics reveal that the device W2
incorporating NPB/TCTA as HTL exhibits a lower driving
voltage relative to device W1 incorporating DTAF as HTL.
Compared with device W1, we note that device W2 gives a large
efficiency roll-off problem (the hext decreases to 11.2% at 1000 cd
mꢀ2) and a low color stability. The CIE coordinates (x, y)
changed from (0.35, 0.40) at 720 cd mꢀ2 to (0.31, 0.40) at 31 300
cd mꢀ2 as shown in Fig. 7(b). This could be attributed to the
recombination zone shifting in devices owing to the different
charge transporting abilities of HTL. We can conclude that
DTAF used as HTL can lead to a more balanced electron and
hole recombination in the emissive layer, and play a crucial role
in alleviating the efficiency roll-off problem and maintaining the
chromatic stability of WOLEDs.
Owing to the successful development of the high-efficiency
RGB PhOLEDs, we shifted to fabricate a two-color WOLED
(device W1) under the same device structure by utilizing 12 wt%
FIrpic and 0.3 wt% Os(bpftz)2(PPh2Me)2 co-doped into CNBzIm
as a single-emitting layer. Fig. 6 shows the J–V–L characteristics
and curves of external quantum efficiency and power efficiency
versus brightness. The white emission device W1 reveals a low
turn-on voltage of 2.5 V and a maximum brightness of 49 900 cd
mꢀ2 at 1240 mA cmꢀ2 at 14 V. The maximum values of effi-
ciencies (hext, hc, and hp) were 15.7%, 35 cd Aꢀ1, and 36.6 lm Wꢀ1
,
respectively. Even at a practical brightness of 1000 cd mꢀ2, the
hext remained at 12.4%. Notably, this WOLED W1 possesses
high color stability as shown in Fig. 7(a). When the brightness
changes from 1700 to 49 900 cd mꢀ2 (which corresponds to an
applied voltage of 6 to 12 V), the variation of the Commission
Internationale de L’Eclairage (CIE) coordinates is rather small,
CIE x ¼ 0.31–0.32 and CIE y ¼ 0.41, thus revealing superior
device chromatic stability.
Conclusions
In summary, a new bipolar host molecule CNBzIm comprised of
electron-donating carbazole core and electron-accepting N-phe-
nylbenzimidazole peripherals has been synthesized and chemi-
cally characterized. CNBzIm adopted a N-connectivity to link
benzimidazole and carbazole structures, rendering reduced p-
In general, WOLEDs suffer from low color stability (i.e., the
white spectra change with driving voltages), which is probably
due to the shifting of the recombination zone.21 The stable white
OLED device structure should be carefully tuned for better
charge carrier balance, adjusted the recombination zone, and
modulated energy transfer to achieve a balanced exciton distri-
bution in emissive layer.7b To gain further insight into the
conjugation and thus a relatively high triplet energy state (ET
¼
2.71 eV), balanced bipolar charge transport and sufficient
morphological stability. Due to the promising physical proper-
ties, CNBzIm can be utilized as a universal host material for
multicolor phosphors. We have successfully fabricated high-
efficiency RGB and white OLEDs incorporating various phos-
phors under the same device structure. The maximum external
quantum efficiencies obtained for the R, G, B, and W devices are
12.7%, 17.8%, 19.1%, and 15.7%, respectively. Under this device
structure, a WOLED can achieve a balanced exciton distribution
in its emissive layer to generate high color stability and low
efficiency roll-off. The CIE coordinates of the white device
remain almost invariable when the brightness goes from 1700 cd
mꢀ2 (0.32, 0.41) to 49 900 cd mꢀ2 (0.31, 0.41).
Experimental
Synthesis
Synthesis of bipolar material CNBzIm. A mixture of 3,6-bis
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9-phenyl-9H-car-
bazole (3)14 (2.97 g, 6.0 mmol), 1-(4-bromophenyl)-2-phenyl-1H-
benzo[d]imidazole (5.0 g, 14.3 mmol), Pd(PPh3)4 (0.35 g, 0.3
mmol), K3PO4 (24 mL, 2 M) and tri(tert-butyl)phosphine (12
mL, 0.05 M in toluene, 0.6 mmol) in 60 mL was refluxed for 2
days. The reaction mixture was cooled to room temperature, and
extracted with CH2Cl2 (100 mL ꢂ 2). The organic layer was
washed with brine and dried over MgSO4. The solution was
concentrated in vacuum to obtain a residue which was purified
by column chromatography on silica gel (SiO2) using ethyl
acetate/hexane (1 : 1) as eluent to give a white solid CNBzIm
(3.46 g, 74%). Mp 346–347 ꢁC; IR (KBr) n 3044, 2921, 2850,
Fig. 7 Electroluminescence spectra of the device with different HTLs:
(a) W1(DTAF) and (b) W2(NPB/TCTA), measured at various
brightness.
19254 | J. Mater. Chem., 2011, 21, 19249–19256
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1
1597, 1496, 1479, 1380, 1277, 809, 741, 696; H NMR (CDCl3,
intersection point of two asymptotes to the plateau and the tail
sections in double-logarithmic plots.
400 MHz) d 8.49 (d, J ¼ 1.6 Hz, 2H), 7.93 (d, J ¼ 7.2 Hz, 2H),
7.86 (d, J ¼ 8.4 Hz, 4H), 7.76 (dd, J ¼ 1.6, 8.8 Hz, 2H), 7.70–7.64
(m, 8H), 7.56–7.53 (m, 3H), 7.43 (d, J ¼ 8.4 Hz, 4H), 7.39–7.29
(m, 12H); 13C NMR (CDCl3, 100 MHz) d 152.2, 142.9, 141.8,
141.0, 137.1, 135.3, 132.0, 130.0, 129.9, 129.4, 129.3, 128.3, 128.2,
127.8, 127.6, 126.8, 125.5, 123.9, 123.2, 122.9, 119.8, 118.9, 110.4;
MS (m/z, FAB+) 780 (2.15); HRMS (m/z, FAB+) Calcd for
C56H37N5 779.3049, found 779.3052; Anal. Calcd. C, 86.24; H,
4.78; N, 8.98; found C, 86.50; H, 5.07; N, 9.12.
OLED device fabrications
All chemicals were purified through vacuum sublimation prior to
use. The OLEDs were fabricated through vacuum deposition of
the materials at 10ꢀ6 Torr onto ITO-coated glass substrates
having a sheet resistance of 15 U sqꢀ1. The ITO surface was
cleaned ultrasonically—sequentially with acetone, methanol, and
deionized water—and then it was treated with UV-ozone. The
ꢀ1
ꢀ
deposition rate of each organic material was ca. 1–2 A s
.
Photophysical measurements
ꢀ1
ꢀ
Subsequently, LiF was deposited at 0.1 A s and then capped
with Al (ca. 5 A sꢀ1) through shadow masking without breaking
ꢀ
Steady state spectroscopic measurements were conducted both in
solution and solid films prepared by vacuum (2 ꢂ 10ꢀ6 Torr)
deposition on a quartz plate (1.6 ꢂ 1.0 cm). Absorption spectra
were recorded with a U2800A spectrophotometer (Hitachi).
Fluorescence spectra at 300 K and phosphorescent spectra at 77
K were measured on a Hitachi F-4500 spectrophotometer upon
exciting at the absorption maxima. The experimental values of
HOMO levels were determined with a Riken AC-2 photoemis-
sion spectrometer (PES), and those of LUMO levels were esti-
mated by subtracting the optical energy gap from the measured
HOMO.
the vacuum. The J–V–L characteristics of the devices were
measured simultaneously in a glove box using a Keithley 6430
source meter and a Keithley 6487 picoammeter equipped with
a calibration Si-photodiode. EL spectra were measured using
a photodiode array (OTO SD1200).
Acknowledgements
We are grateful for the financial support from the National
Science Council (98-2119-M-002-007-MY3, NSC 100-2112-M-
019-002-MY3) and Ministry of Economic Affairs (99-EC-17-A-
08-S1-042) of Taiwan.
Cyclic voltammetry
Notes and references
The oxidation potential was determined by cyclic voltammetry
(CV) in CH2Cl2 solution (1.0 mM) containing 0.1 M tetra n-
butylammonium hexafluorophosphate (TBAPF6) as a support-
ing electrolyte at a scan rate of 100 mV sꢀ1. The reduction
potential was recorded in THF solution (1.0 mM) containing 0.1
M tetra-n-butylammonium perchlorate (TBAClO4) as a sup-
porting electrolyte at a scan rate of 100 mV sꢀ1. A glassy carbon
electrode and platinum wire were used as the working and
counter electrodes, respectively. All potentials were recorded
versus Ag/AgCl (saturated) as a reference electrode.
1 (a) S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer,
€
B. Lussem and K. Leo, Nature, 2009, 459, 234; (b) C.-H. Chang,
H.-C. Cheng, Y.-J. Lu, K.-C. Tien, H.-W. Lin, C.-L. Lin,
C.-J. Yang and C.-C. Wu, Org. Electron., 2010, 11, 247; (c)
C.-H. Chang, K.-C. Tien, C.-C. Chen, M.-S. Lin, H.-C. Cheng,
S.-H. Liu, C.-C. Wu, J.-Y. Hung, Y.-C. Chiu and Y. Chi, Org.
Electron., 2010, 11, 412.
2 (a) C. Adachi, M. A. Baldo, M. E. Thompson and S. R. Forrest, J.
Appl. Phys., 2001, 90, 5048; (b) H. Sasabe, T. Chiba, S.-J. Su,
Y.-J. Pu, K.-I. Nakayama and J. Kido, Chem. Commun., 2008,
5821; (c) Y. Z. Li, W. J. Xu, G. Z. Ran and G. G. Qin, Appl. Phys.
Lett., 2009, 95, 033307; (d) C.-H. Chien, F.-M. Hsu, C.-F. Shu and
Y. Chi, Org. Electron., 2009, 10, 871; (e) Y. Tao, Q. Wang,
C. Yang, C. Zhong, K. Zhang, J. Qin and D. Ma, Adv. Funct.
Mater., 2010, 20, 304; (f) Y. Tao, Q. Wang, L. Ao, C. Zhong,
J. Qin, C. Yang and D. Ma, J. Mater. Chem., 2010, 20, 1759.
3 (a) J. Kido, M. Kumara and K. Nagai, Science, 1995, 267, 1332; (b)
T. F. So, J. Kido and P. Burrows, MRS Bull., 2008, 33, 663.
4 (a) G. Schwartz, M. Pfeiffer, S. Reineke, K. Walzer and K. Leo, Adv.
Mater., 2007, 19, 3672; (b) G. Schwartz, S. Reineke, T. C. Rosenow,
K. Walzer and K. Leo, Adv. Funct. Mater., 2009, 19, 1319; (c)
M. E. Kondakova, J. C. Deaton, T. D. Pawlik, D. J. Giesen,
D. Y. Kondakov, R. H. Young, T. L. Royster, D. L. Comfort and
J. D. Shore, J. Appl. Phys., 2010, 107, 014515.
5 W.-Y. Hung, L.-C. Chi, W.-J. Chen, Y.-M. Chen, S.-H. Chou and
K.-T. Wong, J. Mater. Chem., 2010, 20, 10113.
6 Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson and
S. R. Forrest, Nature, 2006, 440, 908.
7 (a) S.-J. Su, E. Gonmori, H. Sasabe and J. Kido, Adv. Mater., 2008,
20, 4189; (b) Q. Wang, J. Ding, D. Ma, Y. Cheng, L. Wang and
F. Wang, Adv. Mater., 2009, 21, 2397.
8 P.-I. Shih, C.-H. Chien, F.-I. Wu and C.-F. Shu, Adv. Funct. Mater.,
2007, 17, 3514.
Time-of-flight (TOF) mobility measurements
Carrier-transport properties were studied in vapor-deposited
glasses of CNBzIm by the time-of-flight (TOF) transient photo-
current technique. The samples for the TOF measurement were
prepared by vacuum deposition using the structure: ITO glass/
CNBzIm (1.56 mm)/Al (150 nm), and then placed inside a cryo-
stat and kept under vacuum. The thickness of organic film was
monitored in situ with a quartz crystal sensor and calibrated by
a thin film thickness measurement (K-MAC ST2000). A pulsed
nitrogen laser was used as the excitation light source through the
transparent electrode (ITO) induced photogeneration of a thin
sheet of excess carriers. Under an applied dc bias, the transient
photocurrent was swept across the bulk of the organic film
toward the collection electrode (Al), and then recorded with
a digital storage oscilloscope. Depending on the polarity of the
applied bias, selected carriers (holes or electrons) are swept
across the sample with a transit time of tT. With the applied bias
V and the sample thickness D, the applied electric field E is V/D,
and the carrier mobility is then given by m ¼ D/(tTE) ¼ D2/(VtT),
in which the carrier transit time, tT, can be extracted from the
9 H.-H. Chou, H.-H. Shih and C.-H. Cheng, J. Mater. Chem., 2010, 20,
798.
10 H.-H. Chou and C.-H. Cheng, Adv. Mater., 2010, 22, 2468.
11 (a) S. Gong, Y. Chen, C. Yang, C. Zhong, J. Qin and D. Ma, Adv.
Mater., 2010, 22, 5370; (b) S. Gong, Y. Chen, J. Luo, C. Yang,
C. Zhong, J. Qin and D. Ma, Adv. Funct. Mater., 2011, 21, 1168.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 19249–19256 | 19255

View Article Online
12 (a) S.-J. Su, H. Sasabe, T. Takeda and J. Kido, Chem. Mater., 2008,
20, 1691; (b) S.-J. Su, C. Cai and J. Kido, Chem. Mater., 2011, 23,
274.
13 J. Ding, J. Gao, Y. Cheng, Z. Xie, L. Wang, D. Ma, X. Jing and
F. Wang, Adv. Funct. Mater., 2006, 16, 575.
14 K.-T. Wong, T.-H. Hung, T.-C. Chao and T.-I. Ho, Tetrahedron
Lett., 2005, 46, 855.
15 R. J. Holmes, S. R. Forrest, Y.-J. Tung, R. C. Kwong, J. J. Brown,
S. Garon and M. E. Thompson, Appl. Phys. Lett., 2003, 82,
2422.
17 P.-I. Shih, C.-F. Shu, Y.-L. Tung and Y. Chi, Appl. Phys. Lett., 2006,
88, 251110.
18 C. Adachi, M. A. Baldo, M. E. Thompson and S. R. Forrest, J. Appl.
Phys., 2001, 90, 5048.
19 (a) M.-T. Kao, W.-Y. Hung, Z.-H. Tsai, H.-W. You, H.-F. Chen,
Y. Chi and K.-T. Wong, J. Mater. Chem., 2011, 21, 1846; (b)
W.-Y. Hung, H.-W. You, F.-C. Fan, H.-F. Chen and K.-T. Wong,
Org. Electron., 2011, 12, 575.
20 S.-J. Yeh, M.-F. Wu, C.-T. Chen, Y.-H. Song, Y. Chi, M.-H. Ho,
S.-F. Hsu and C. H. Chen, Adv. Mater., 2005, 17, 285.
21 M. C. Gather, R. Alle, H. Becker and K. Meerholz, Adv. Mater.,
2007, 19, 4460.
16 P. M. Borsenberger and D. S. Weiss, Organic Photoreceptors for
Imaging Systems, Marcel Dekker, New York, 1993.
19256 | J. Mater. Chem., 2011, 21, 19249–19256
This journal is ª The Royal Society of Chemistry 2011