Molecular topology tuning of bipolar host materials composed of fluorene-bridged benzimidazole and carbazole for highly efficient electrophosphorescence

Article abstract of DOI:10.1002/chem.201300738

Two new molecules, CzFCBI and CzFNBI, have been tailor-made to serve as bipolar host materials to realize high-efficiency electrophosphorescent devices. The molecular design is configured with carbazole as the hole-transporting block and N-phenylbenzimidazole as the electron-transporting block hybridized through the saturated bridge center (C9) and meta-conjugation site (C3) of fluorene, respectively. With structural topology tuning of the connecting manner between N-phenylbenzimidazole and the fluorene core, the resulting physical properties can be subtly modulated. Bipolar host CzFCBI with a C connectivity between phenylbenzimidazole and the fluorene bridge exhibited extended π conjugation; therefore, a low triplet energy of 2.52 eV was observed, which is insufficient to confine blue phosphorescence. However, the monochromatic devices indicate that the matched energy-level alignment allows CzFCBI to outperform its N-connected counterpart CzFNBI while employing other long-wavelength- emitting phosphorescent guests. In contrast, the high triplet energy (2.72 eV) of CzFNBI imparted by the N connectivity ensures its utilization as a universal bipolar host for blue-to-red phosphors. With a common device configuration, CzFNBI has been utilized to achieve highly efficient and low-roll-off devices with external quantum efficiency as high as 14 % blue, 17.8 % green, 16.6 % yellowish-green, 19.5 % yellow, and 18.6 % red. In addition, by combining yellowish-green with a sky-blue emitter and a red emitter, a CzFNBI-hosted single-emitting-layer all-phosphor three-color-based white electrophosphorescent device was successfully achieved with high efficiencies (18.4 %, 36.3 cd A ^-1, 28.3 lm W^-1) and highly stable chromaticity (CIE x=0.43-0.46 and CIE y=0.43) at an applied voltage of 8 to 12 V, and a high color-rendering index of 91.6. Copyright

Full text of DOI:10.1002/chem.201300738

FULL PAPER
DOI: 10.1002/chem.201300738
Molecular Topology Tuning of Bipolar Host Materials Composed of
Fluorene-Bridged Benzimidazole and Carbazole for Highly Efficient
Electrophosphorescence
Ejabul Mondal,[a] Wen-Yi Hung,*[b] Yang-Huei Chen,[b] Ming-Hung Cheng,[b] and
Ken-Tsung Wong*[a, c]
Dedicated to Professor Scott E. Denmark on the occasion of his 60th birthday
Abstract: Two new molecules, CzFCBI
and CzFNBI, have been tailor-made to
serve as bipolar host materials to real-
ize high-efficiency electrophosphores-
cent devices. The molecular design is
configured with carbazole as the hole-
transporting block and N-phenylbenz-
tion; therefore, a low triplet energy of
2.52 eV was observed, which is insuffi-
cient to confine blue phosphorescence.
However, the monochromatic devices
indicate that the matched energy-level
alignment allows CzFCBI to outper-
form its N-connected counterpart
CzFNBI while employing other long-
blue-to-red phosphors. With a common
device configuration, CzFNBI has been
utilized to achieve highly efficient and
low-roll-off devices with external quan-
tum efficiency as high as 14% blue,
17.8% green, 16.6% yellowish-green,
19.5% yellow, and 18.6% red. In addi-
tion, by combining yellowish-green
with a sky-blue emitter and a red emit-
ter, a CzFNBI-hosted single-emitting-
layer all-phosphor three-color-based
white electrophosphorescent device
was successfully achieved with high ef-
ACHTUNGTRENNUNGimidazole as the electron-transporting
block hybridized through the saturated
bridge center (C9) and meta-conjuga-
tion site (C3) of fluorene, respectively.
With structural topology tuning of
the connecting manner between N-phe-
nylbenzimidazole and the fluorene
core, the resulting physical properties
can be subtly modulated. Bipolar host
CzFCBI with a C connectivity between
phenylbenzimidazole and the fluorene
bridge exhibited extended p conjuga-
wavelength-emitting
phosphorescent
guests. In contrast, the high triplet
energy (2.72 eV) of CzFNBI imparted
by the N connectivity ensures its uti-
lization as a universal bipolar host for
ficiencies
(18.4%,
36.3 cdAꢀ1,
28.3 lmWꢀ1) and highly stable chroma-
ticity (CIEx=0.43–0.46 and CIEy=
0.43) at an applied voltage of 8 to 12 V,
and a high color-rendering index of
91.6.
Keywords: bipolar hosts · electro-
phosphorescence · luminescence ·
organic light-emitting devices · pho-
tochemistry
Introduction
as concentration quenching through triplet–triplet annihila-
tion.[2] Therefore, the development of suitable host materials
also plays an important role in PhOLED-related research.
Regardless of the various emitting colors, to obtain high effi-
ciency in PhOLEDs the host material should have 1) appro-
priate highest occupied/lowest unoccupied molecular orbital
(HOMO/LUMO) energy levels matching with the adjacent
layers for efficient charge injection; 2) good and balanced
charge-transporting properties to lead to efficient charge re-
combination occurring on the phosphorescent emitter; 3) a
triplet energy (ET) higher than that of the phosphorescent
emitter to prevent reverse energy transfer from guest to
host, and effective confinement of the triplet excitons on the
guest emitter;[3] and 4) high thermal and morphological sta-
bility for the thin-film stability of the emitting layer. In this
regard, bipolar host materials incorporating hole- and elec-
tron-transporting moieties provide the necessary physical
characteristics required for these purposes.[4]
Phosphorescent organic light-emitting devices (PhOLEDs)
have currently attracted intensive research activities in both
academia and industry because of their potential applica-
tions in full-color flat-panel displays and solid-state light-
ing.[1] To achieve 100% internal quantum efficiency in
PhOLEDs, heavy-metal-centered phosphorescent emitters
must typically be homogeneously dispersed into an appro-
priate host material to suppress detrimental processes, such
[a] Dr. E. Mondal, Prof. K.-T. Wong
Department of Chemistry, National Taiwan University
Taipei 10617 (Taiwan)
E-mail: kenwong@ntu.edu.tw
[b] Prof. W.-Y. Hung, Y.-H. Chen, M.-H. Cheng
Institute of Optoelectronic Sciences
National Taiwan Ocean University
Keelung 202 (Taiwan)
For more cost-effective production, a particular challenge
will be the development of universal bipolar hosts suitable
for blue, green, and red phosphorescent emitters with a
common device configuration. Furthermore, the blue host
E-mail: wenhung@mail.ntou.edu.tw
[c] Prof. K.-T. Wong
Institute of Atomic and Molecular Sciences
Academia Sinica, Taipei 10617 (Taiwan)
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10563

needs a higher ET (ꢁ2.70 eV) than that of blue phosphores-
cent emitters.[5] To preserve such a high ET, the bipolar hosts
usually comprise large-bandgap hole- and electron-trans-
porting building blocks. However, such a molecular configu-
ration may lead to ill-matched HOMO/LUMO levels rela-
tive to those of neighboring functional layers, which leads to
high operation voltage and thus low efficiency. Therefore,
an essential compromise between the triplet energy ET and
HOMO/LUMO levels of the bipolar host materials is inevi-
table for designing universal host materials. With judicious
selection of molecular building blocks, universal bipolar host
materials suitable for high-efficiency red/green/blue (RGB)
and even white OLEDs (WOLEDs) using a common device
structure can be reasonably achieved. For designing bipolar
hosts, carbazole is commonly used as the hole-transporting
(HT) component, since carbazole-based materials have been
widely used in PhOLEDs because of their high triplet
energy and HT properties.[6] On the other hand, benzimida-
zole is an important component that is utilized widely as a
functional group for facilitating the injection and transport
of electrons.[6g,7] We have previously achieved an interesting
universal bipolar host, CNBzIm, which incorporates two
electron-transporting (ET) N-phenylbenzimidazole units
onto the C3 and C6 positions of HT N-phenylcarbazole.[8]
CNBzIm has been used to realize a high-efficiency RGB
device and a two-color, all-phosphor single-emitting-layer
WOLED. Another important issue for designing bipolar
host materials is the molecular structural feature linking the
HT and ET blocks. In this regard, a saturated spacer has
been successfully utilized for blocking the direct electronic
coupling between the HT and ET blocks, thus allowing the
resultant bipolar host to have a high ET. One excellent ex-
ample was recently reported by Yang et al., in which a sili-
con-bridged bipolar compound, p-BISiTPA, which combines
triphenylamine and benzimida-
coupling between the HT and ET moieties can be effectively
blocked, and the limited extension of molecular conjugation
by a meta substitution can be anticipated to retain a reason-
ably high ET. In addition, the rigidity of fluorene will ensure
the resulting bipolar hosts exhibit a sufficiently high Tg,
hence giving
a morphologically stable emitting layer.
Through the structural topology tuning of the manner of
connectivity between N-phenylbenzimidazole and the fluo-
rene core, the physical properties of the bipolar hosts can be
subtly modulated, to give the new host CzFNBI with an ET
of 2.72 eV and suitable energy levels to realize PhOLEDs
with low roll-off and high EQE for blue (14%, [FIr
pic=picolinic acid), green (17.8%, [Ir(acac)(PPy)2]; acac=
acetylacetonate, Py=pyridine), yellowish green (16.6%, [Ir-
(acac)(m-tpm)2]; m-tpm=meta-tolylpyrimidine), yellow
(19.5%, [Ir(acac)(bt)2]; bt=2-phenyl benzothiazolate), and
red (18.6%, [Os(bpftz)2A(PPhMe2)2]; bpftz=3-(trifluoro-
ACHTUNGTRENNUNG(pic)];
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
CHTUNGTRENNUNG
methyl)-5-(4-tert-butylpyridyl)-1,2,4-triazolate), respectively
(see below for details of complexes). In contrast, a slightly
extended p conjugation of the C-connected counterpart
CzFCBI gives a lower ET of 2.52 eV, which is unable to con-
fine blue phosphor emission, but superior EQEs were
reached for green (20%), yellowish green (18%), yellow
(19.4%), and red (18.1%). More importantly, CzFNBI was
utilized to realize a high-efficiency (18.4%, 36.3 cdAꢀ1
,
28.3 lmWꢀ1) single-emitting-layer WOLED showing three
evenly separated RGB emission peaks with highly stable
chromaticity (CIEx=0.43–0.46 and CIEy=0.43) at applied
voltages of 8 to 12 V, and a high color-rendering index
(CRI) of 91.6. This result represents one of the best exam-
ples of efficient all-phosphor three-color-based WOLEDs.[10]
We attributed the high color stabilities to the broad charge
recombination zone in the emitting layer incorporated with
this novel bipolar host.
zole moieties, exhibited a high
ET of 2.69 eV and a glass transi-
tion temperature Tg of 1028C.[9]
Devices with p-BISiTPA as the
host gave external quantum ef-
ficiencies (EQEs) as high as
16.1% for blue, 22.7% for
green, 20.5% for orange, and
19.1% for all-phosphor two-
color-based WOLEDs employ-
ing a common device structure.
Herein, we utilized fluorene
as a bridge to make two bipolar
host materials (CzFCBI and
CzFNBI, Scheme 1) with carba-
zole as the HT block and
N-phenylbenzimidazole as the
ET block attached to the satu-
rated bridge center (C9) and
meta-conjugation site (C3) of
fluorene, respectively. In this
molecular design, the electronic Scheme 1. Synthesis and chemical structures of CzFCBI and CzFNBI.
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Tuning of Bipolar Host Materials
FULL PAPER
Results and Discussion
of CzFCBI and CzFNBI were measured in EtOH at 77 K
and the triplet energy levels were estimated from the high-
est-energy vibronic sub-band of the phosphorescence spec-
tra. The high-energy phosphorescence emission peak of
CzFCBI is at 492 nm, corresponding to a triplet energy (ET)
of 2.52 eV. In contrast, CzFNBI exhibits a higher ET of
2.72 eV. The high ET of CzFNBI again confirms the disrup-
tion of p conjugation from C3-substituted fluorene to the N-
bridged benzimidazole moiety. Based on this observation,
CzFCBI can be a useful host material for green-to-red trip-
let emitters, whereas CzFNBI can be utilized in RGB and
white PhOLEDs.
Scheme 1 illustrates our synthetic route to new bipolar hosts
CzFCBI and CzFNBI. The starting material, 3-(3-bromo-9-
para-tolyl-9H-fluoren-9-yl)-9-phenyl-9H-carbazole (1), was
prepared according to our previously reported procedure.[10c]
The Suzuki coupling reaction of 1 and boronic esters 2[11]
and 3 in the presence of a palladium(0) catalyst gave
CzFCBI and CzFNBI in 74 and 70% yield, respectively.
1
CzFCBI and CzFNBI have been fully characterized with H
and 13C NMR spectroscopy, mass spectrometry, and elemen-
tal analysis (see the Experimental Section).
Figure 1 depicts the room-temperature UV/Vis absorption
and photoluminescence (PL) spectra of CzFCBI and
CzFNBI in dilute solution (CH2Cl2). The photophysical data
We used cyclic voltammetry to probe the electrochemical
properties of CzFCBI and CzFNBI, as shown in Figure 2.
These compounds have similar oxidation behavior with one
irreversible oxidation peak (onset at ca. 1.27 V vs. Ag/AgCl)
Figure 2. Cyclic voltammograms of CzFCBI and CzFNBI. The oxidation
potential was measured in CH2Cl2 with 0.1m tetra-n-butylammonium
hexafluorophosphate (TBAPF6) as the supporting electrolyte and reduc-
tion cyclic voltammetry was performed in DMF with 0.1m TBAP as the
supporting electrolyte.
Figure 1. Room-temperature absorption (Abs) and photoluminescence
(PL) spectra of CzFCBI and CzFNBI in CH2Cl2 and the corresponding
phosphorescence (Phos) spectra recorded in EtOH at 77 K.
are summarized in Table 1. Both CzFCBI and CzFNBI show
an absorption peak at approximately 300 nm, which can be
assigned to the p–p* transition of the carbazole moiety. An
evident absorption peak centered at 320 nm of CzFCBI is
attributed to the p–p* transition of the extended p conjuga-
tion from fluorene to the benzimidazole unit. In contrast,
CzFNBI shows an absorption peak around 270 nm, which
indicates the disruption of p conjugation between fluorene
and the benzimidazole unit due to the distorted N-bridged
linkage.[6c,7a,d,9a,b] The redshifted emission spectrum of
CzFCBI with lmax centered at 388 nm as compared to
357 nm for CzFNBI agrees with our assumption of the ex-
tended p conjugation in CzFCBI through the C connection
of benzimidazole to fluorene. The phosphorescence spectra
of the forward scan and a new peak appears at 0.98 V of the
backward scan. This result agrees with the typical signature
of a C3 and/or C6 unprotected carbazole moiety.[12] Notably,
the more p-conjugated chromophore in CzFCBI leads to a
reversible reduction potential (Er1ed) at ꢀ1.88 V (vs. Ag/
=
AgCl). In contrast, the higher 2 reduction potential of
CzFNBI is irreversible, which is a characteristic feature of
N-connected benzimidazole-containing materials.[6c,7a,d,9a,b]
For the practical applications of CzFCBI and CzFNBI in
their solid films, we determined the HOMO energy level of
CzFCBI and CzFNBI to be ꢀ5.60 and ꢀ5.80 eV, respective-
ly, by using a Riken AC-2 photoemission spectrometer.
From the equation LUMO=HOMO+DEg, in which DEg is
the optical bandgap determined from the absorption thresh-
Table 1. Physical properties of CzFCBI and CzFNBI.
[c]
Tg
[8C]
Td
[8C]
Eo1 x ½aꢂ
[V]
Er1ed ½aꢂ
[V]
HOMO[b]
[eV]
LUMO[c]
[eV]
DEg
ET
[eV]
labs
lPL
=
2
=
2
[eV]
[nm]
[nm]
sol./film
sol./film
CzFCBI
CzFNBI
175
181
415
441
1.30
1.27
ꢀ1.88
ꢀ2.22
ꢀ5.6
ꢀ5.8
ꢀ2.21
ꢀ2.17
3.39
3.63
2.52
2.72
300, 320/301, 321
270, 298/271, 300
370, 388/397
357, 373/361, 381
[a] Determined from onset oxidation/reduction potential (vs. Ag/AgCl). [b] HOMO determined by using photoelectron spectroscopy (AC-2).
[c] LUMO=HOMO+Eg, in which Eg was calculated from the absorption onset of the solution.
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10565

W.-Y. Hung, K.-T. Wong et al.
old, we estimated the LUMO energy level of CzFCBI and
CzFNBI to be ꢀ2.21 and ꢀ2.17 eV, respectively.
sient-photocurrent signals were still too weak and cannot be
used to determine the electron mobility.
To acquire the carrier mobility of CzFCBI and CzFNBI,
we first attempted to use a conventional time-of-flight
(TOF) transient-photocurrent technique. However, the tran-
sient-photocurrent signals were too weak due to the weak
absorption at 337 nm used for excitation. Instead, we intro-
duced an ambipolar terfluorene (T3)[13] as the charge-gener-
ation layer for TOF measurements[14] (see the Experimental
Section). Figure 3a and b display typical room-temperature
To verify the bipolar transport character of CzFCBI and
CzFNBI, we then fabricated hole-only devices having the
structure of indium tin oxide (ITO)/N,N’-diphenyl-N,N’-
bis[1-naphthyl-(1,1’-biphenyl)]-4,4’-diamine (NPB) (10 nm)/
test (30 nm)/NPB (10 nm)/LiF/Al and electron-only devices
having the configuration of ITO/2,9-dimethyl-4,7-diphenyl-
1,10-phenanthroline (10 nm)/test (30 nm)/BCP (10 nm)/LiF/
Al. The introduction of NPB and BCP layers is to prevent
electron and hole injection from the cathode and anode, re-
spectively. The current density versus voltage curves shown
in Figure 4 indicate the bipolar charge-transport nature, in
Figure 4. Current-density–voltage (I–V) characteristics of carrier-only de-
vices: hole-only device [ITO/NPB (10 nm)/test (30 nm)/NPB (10 nm)/
LiF/Al] and electron-only device [ITO/BCP (10 nm)/test (30 nm)/BCP
(10 nm)/LiF/Al].
which the electron current density is slightly higher than the
hole current density in both CzFCBI and CzFNBI. The elec-
tron current density of CzFCBI is higher than that of
CzFNBI, whereas the hole current density of CzFCBI is
lower than that of CzFNBI. The hole currents clearly follow
the results observed by TOF measurements. The results
from single-carrier devices indicate that the electron-trans-
port behavior is better and the hole-transport property is
worse on altering the linking topology from C to N connec-
tivity of the benzimidazole unit, which is consistent with our
previous report of indolocarbazole–benzimidazole hybri-
dized materials (TICCBI and TICNBI).[15]
To evaluate the utilization of CzFCBI and CzFNBI as
host materials for different dopants (from blue to red), we
Figure 3. Representative TOF transients for a) CzFCBI (E=4.2ꢁ
105 Vcmꢀ1) and b) CzFNBI (E=4.1ꢁ105 Vcmꢀ1). Inset: Double-logarith-
mic plot. c) Hole mobilities of CzFCBI and CzFNBI plotted with respect
selected
N,C2’]picolinate iridium
phenylpyridinato)iridium
(PPy)2]),[17] yellow-green bis(meta-tolylpyrimidine)iridium-
(III) acetylacetonate ([Ir(acac)
(m-tpm)2]),[10c] yellow bis(2-
phenylbenzothiazolato)(acetylacetonato)iridium(III) ([Ir-
(acac)(bt)2]),[5b] and red bis[3-(trifluoromethyl)-5-(4-tert-
butylpyridyl)-1,2,4-triazolate]dimethylphenylphosphine os-
mium(II) ([Os(bpftz)2A
(PPhMe2)2], OS1)[18] as dopants
blue
[bis(4,6-difluorophenyl)pyridinato-
(III) ([FIr
(pic)]),[16] green bis(2-
(III) acetylacetonate ([Ir(acac)-
1
=
2
to E .
N
ACHTUNGTRENNUNG
N
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
TOF transient photocurrents under an applied electric field.
The transit time tT can be evaluated from the intersection
point of two asymptotes in the double-logarithmic represen-
tation of the TOF transient. Figure 3c shows that the field
dependence of the hole mobility follows the nearly universal
Poole–Frenkel relationship, and the values are in the range
of 2.3ꢁ10ꢀ6 to 1.8ꢁ10ꢀ5 cm2 Vꢀ1 sꢀ1 for CzFCBI at fields
varying from 1.6ꢁ105 to 4.2ꢁ105 Vcmꢀ1 and 6.9ꢁ10ꢀ6 to
2.8ꢁ10ꢀ5 cm2 Vꢀ1 sꢀ1 for CzFNBI at fields varying from
6.5ꢁ105 to 1.1ꢁ106 Vcmꢀ1. Unfortunately, the electron tran-
A
R
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
A
CHTUNGTRENNUNG
(Scheme 2). To improve the hole injection from the anode,
poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid)
(PEDOT/PSS) was spun onto the precleaned ITO substrate
to form a 30 nm thick polymer buffer layer. Two hole-trans-
port layers (HTLs), which consisted of a 20 nm thick layer
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Tuning of Bipolar Host Materials
FULL PAPER
Scheme 2. Molecular structures of the dopants used in this study and an
energy level diagram of the device.
of 9,9-di[4-(di-para-tolyl)aminophenyl]fluorine (DTAF)[6g,19]
and a 5 nm thick layer of 4,4’,4’’-tri(N-carbazolyl)triphenyla-
mine (TCTA)[20] were implemented. It is notable that
TCTA, with ET =2.76 eV and HOMO/LUMO levels of 5.7/
2.4 eV, also served as a triplet blocker to confine the exci-
tons in the emissive layer (EML, Scheme 2). Moreover, a
25 nm thick emissive layer consisted of 10 wt% of phos-
phors doped into the host material. Finally, a 50 nm thick
layer of diphenylbis[4-(pyridin-3-yl)phenyl]silane (DPPS)[21]
was used as the electron-transport layer (ETL), for which
DPPS (ET: 2.7 eV, HOMO/LUMO: 6.5/2.5 eV) could be
more effective in blocking the excitons within the emissive
layer. A cathode consisting of a 0.5 nm thick layer of LiF
and a 100 nm thick layer of Al was deposited through a
shadow mask.
Figure 5. a) Current-density–voltage–luminance (J–V–L) characteristics,
b) external quantum (hext) and power efficiencies (hp) as a function of
brightness, and c) EL spectra of devices with CzFCBI as host.
which possesses
a relatively low triplet energy (ET =
2.52 eV) that leads to inefficient confinement of emissive ex-
citons, gives poor device performance (device B1). To fur-
ther verify the exciton-confinement behavior of the host, the
transient PL decay characteristics of 10 wt% [FIr
doped CzFCBI and CzFNBI were investigated; the results
are shown in Figure 7. The CzFCBI/[FIr(pic)] film exhibits
ACHTUNGTRENNUNG(pic)]-
A
ACHTUNGTRENNUNG
two-component decay, that is, a fast decay process and a
slow decay process. The latter can be attributed to thermally
activated triplet–triplet energy transfer between the host
Figures 5 and 6 depict the current density–voltage–lumi-
nance (J–V–L) characteristics, device efficiencies, and elec-
troluminescence (EL) spectra of the devices incorporating
CzFCBI and CzFNBI host materials, respectively. Table 2
summarizes the EL data obtained. In the blue emission
device, the CzFNBI-based device B2 displayed a maximum
and the guest, such as in the [FIr
N,N’-dicarbazolyl-4,4’-biphenyl).[22] It may induce triplet ex-
citon quenching on CzFCBI/[FIr(pic)]-based blue PhOLEDs
and then a relatively poor device performance. However,
the emission of the [FIr(pic)]-doped CzFNBI film displays a
ACHTUNGERTN(NUNG pic)]/CBP system (CBP=
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
brightness (Lmax
)
as high as 43500 cdmꢀ2 at 13 V
clear single exponential decay with a lifetime of 1.73 ms and
no slow decay process. This can be understood from the rel-
atively high triplet energy level of 2.72 eV (CzFNBI), which
successfully suppresses triplet energy back-transfer and con-
(630 mAcmꢀ2), achieving a maximum EQE (hext) of 14.0%
corresponding to a current efficiency (hc) of 32.7 cdAꢀ1 and
power efficiency (hp) of 30 lmWꢀ1. In contrast, CzFCBI,
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W.-Y. Hung, K.-T. Wong et al.
Figure 7. Transient PL decay of CzFCBI and CzFNBI/10 wt% [FIrACHTUNGTRENNUNG(pic)]
under excitation by a nitrogen laser (l=337 nm, 10 Hz, 700 ps pulses).
ble), similar operating voltage (5.5–6.5 V), and rather low
efficiency roll-off at a brightness of 1000 cdmꢀ2 with differ-
ent dopants. In green phosphorescent devices with [Ir
ACHTUNGTRENNUNG(acac)-
AHCTUNGRTEGNUN(N PPy)2] as dopant, the device G1 using CzFCBI as host
showed a Lmax of 108500 cdmꢀ2 at 13.5 V (930 mAcmꢀ2),
and a maximum hext of 20% corresponding to an hc of
74.4 cdAꢀ1 and an hp of 75 lmWꢀ1, whereas device G2 using
CzFNBI as host achieved a Lmax of 149600 cdmꢀ2 at 13 V
(920 mAcmꢀ2), and a maximum hext of 17.8% corresponding
to an hc of 64.1 cdAꢀ1 and an hp of 46 lmWꢀ1. This can be
attributed to the more suitable energy-level alignments of
CzFCBI, which facilitate efficient hole injection to the emit-
ting layer. The same trend can also be observed for devices
with different long-wavelength-emitting dopants and
CzFCBI as host. Furthermore, yellowish-green (YG) PhO-
LEDs were fabricated by using [IrACHTUNTGRENN(GU acac)ACHTUTGNREN(NUGN m-tpm)2] as dopant
with an EL wavelength (lEL) of 547 nm; device YG1 hosted
by CzFCBI exhibited a relatively higher efficiency (18%,
62.9 cdAꢀ1, and 62.6 lmWꢀ1) than device YG2 hosted by
CzFNBI (16.6%, 57.5 cdAꢀ1, and 42.5 lmWꢀ1). With this
YG dopant, a broadband white spectrum with a uniform
emission intensity covering the whole visible spectrum can
be realized in a relatively simple three-emitter system.
Next, the yellow-emitting OLEDs were fabricated by
Figure 6. a) Current-density–voltage–luminance (J–V–L) characteristics,
b) external quantum (hext) and power efficiencies (hp) as a function of
brightness, and c) EL spectra of devices with CzFNBI as host.
fines the [FIrACHTUNGTRENNUNG(pic)] excitons in the emissive layer, thus lead-
ing to highly efficient blue PhOLEDs.
Besides blue PhOLEDs, CzFCBI and CzFNBI were also
applied with different dopants (from green to red). All of
the devices in this study exhibited a low turn-on voltage of
2.5 V (defined as the voltage at which EL became detecta-
using [IrACHTUNTGRNENG(U acac)(bt)2] as dopant. Excellent performance of
yellow electrophosphorescence can be achieved for devices
Y1 (19.4%, 48.7 cdAꢀ1, and 47.6 lmWꢀ1) and Y2 (19.5%,
Table 2. EL performances of devices.[a]
[b]
Device
Von
Lmax
Imax
[mAcmꢀ2
hext
hc
[cdAꢀ1
hp
[lmWꢀ1
At 1000 [cdmꢀ2],[c]
V [%]
CIE
[x,y]
[V]
G
R
]
[%]
G
]
G
]
ACHTUNGTRENNUNG
B1
3.0
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
1200 (16)
108500 (13.5)
1176000 (13)
135100 (12)
31800 (12.5)
43500 (13)
149600 (13)
109500 (13)
116100 (13.5)
29400 (13)
590
930
1000
1170
1000
630
920
800
950
820
4.0
20.0
18.0
19.4
18.1
14.0
17.8
16.6
19.5
18.6
18.4
8.5
5.3
75
62.6
47.6
17
30
46
42.5
36.5
13
15.0, 0.1
5.5, 18.6
5.9, 17.1
5.8, 19.0
6.5, 16.6
6.6, 13.0
5.5, 17.4
6.0, 16.3
6.5, 18.9
6.5, 17.4
9.5, 14.7
0.24,0.40
0.32,0.63
0.43,0.56
0.52,0.48
0.66,0.34
0.19,0.40
0.32,0.63
0.43,0.55
0.53,0.47
0.65,0.34
0.46,0.43
G1
YG1
Y1
R1
B2
G2
YG2
Y2
R2
W2
74.4
62.9
48.7
18.2
32.7
64.1
57.5
48.1
18.6
36.3
47500 (17)
480
28.3
[a] The notation 1 and 2 indicates the devices fabricated with host materials of CzFCBI and CzFNBI, respectively. [b] Turn-on voltage at which emission
became detectable. [c] hext and driving voltage of device at 1000 cdmꢀ2
.
10568
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Chem. Eur. J. 2013, 19, 10563 – 10572

Tuning of Bipolar Host Materials
FULL PAPER
48.1 cdAꢀ1, and 36.5 lmWꢀ1). Remarkably, device Y1 shows
rather low efficiency roll-off at a brightness of 1000 cdmꢀ2,
whereas the recorded hext is still maintained as high as 19%.
These values are comparable to those of the best yellow
PhOLEDs in the literature,[6g,10a,23] and may serve as the po-
tential emitting element for generating white emission in
combination with a complementary blue phosphor.[24] Final-
ly, we demonstrated red-emitting OLEDs by using the emit-
ter [OsACHTUNGTRENNUNG(bpftz)2ACHTUNGTRENNUNG(PPhMe2)2], OS1 (devices R1 and R2). Both
devices also exhibited high performance. For instance, devic-
e R1 exhibited a Lmax of 31800 cdmꢀ2 at 12.5 V and respect-
able EL efficiencies (18.1%, 18.2 cdAꢀ1, and 17 lm Wꢀ1),
whereas device R2 exhibited a Lmax of 29400 cdmꢀ2 at 13 V
and good EL efficiencies (18.6%, 18.6 cdAꢀ1, and
13 lmWꢀ1). Besides device B1, all devices displayed relative-
ly pure emission and there are no residual emissions from
the host and/or adjacent layers, as shown in Figures 5c and
6c, thus indicating that the EL originates solely from the
dopant and with complete energy transfer from host to
dopant. More importantly, excellent RGB EL performances
were obtained from CzFNBI-hosted devices with different
phosphors using a common device architecture. On reaching
a brightness of 1000 cdmꢀ2, the devices have similar driving
voltages (6.0–6.6 V) whereas the hext still remains at 13%
for blue, 16.3% for yellowish-green, and 17.6% for red,
which make them very attractive for commercial applica-
tions in full-color displays.
The excellent performance of the CzFNBI-based mono-
chromatic PhOLED prompted us to explore its application
in WOLEDs. The WOLED (device W2) adopted a similar
architecture to the aforementioned monochromatic devices
Figure 8. a) Current-density–voltage–luminance (J–V–L) characteristics,
b) external quantum (hext) and power efficiencies (hp) as a function of
brightness, and c) EL spectra at various voltages of white device W2.
except for the EML [CzFNBI/10% [FIr
CzFNBI:10% [Ir(acac)(m-tpm)2] (5 nm)/CzFNBI/10% OS1
(5 nm)/CzFNBI/10% [FIr(pic)] (10 nm)]. Herein, we fabri-
ACHTUNGTREN(NUNG pic)] (5 nm)/
G
ACHTUNGTRENNUNG
E
high efficiency (hext of 18.4%) and CRI value (91.6) are im-
pressive in device W2, which is superior to most currently
known all-phosphor three-color-based WOLEDs.[10]
cated WOLEDs by distributing the primary color emitters
with a B-G-R-B configuration in the EML, which can
reduce structural heterogeneity and facilitate charge injec-
tion and transport between different emissive centers. With
this strategy, device W2 exhibits rather high efficiency to-
gether with superior color rendition, particularly the impres-
sive color stability. Figure 8 shows the J–V–L characteristics,
EQE, and power efficiency versus brightness. The EL char-
acteristics of the white device are also summarized in
Table 2. Device W2 displayed a turn-on voltage of 2.5 V,
which is the same as that for the monochromatic devices, a
Lmax of 47500 cdmꢀ2 at 17 V (480 mAcmꢀ2), and a maxi-
mum hext of 18.4%, corresponding to an hc of 36.8 cdAꢀ1
and an hp of 28.3 lmWꢀ1. The EL spectra of this white
device have clearly distinguished red, yellowish-green, and
blue emissions to cover wavelengths from 450 to 800 nm as
shown in Figure 8c. Moreover, the CRI is calculated to
reach as high as (89.9–91.6) with low color temperatures
(2820–3052 K) close to incandescent lamps. On increasing
the driving voltage from 8 to 12 V, the variation of the Com-
mission Internationale de LꢂEclairage (CIE) coordinates is
rather small, CIEx=0.46–0.45 and CIEy=0.41, thereby re-
vealing superior device chromatic stability. The concomitant
Conclusion
Two bipolar host materials (CzFCBI and CzFNBI), com-
posed of electron-donating carbazole linked to the sp3-hybri-
dized C bridge of fluorene and electron-accepting benzimi-
dazole grafted onto the C3 position of fluorene, have been
synthesized and characterized. The designated molecular
configuration effectively decouples the direct electronic in-
teractions between donor and acceptor subunits to give high
triplet energy. These new materials show promising proper-
ties, such as high Tg values, suitable energy levels, and bipo-
lar charge transport, thus rendering them good candidates
for realizing highly efficient electrophosphorescent devices.
More importantly, the physical properties can be subtly
modified through the connecting pathways between the flu-
orene and benzimidazole units. Molecule CzFCBI exhibited
a lower triplet energy (2.52 eV) due to the extended p con-
jugation relative to that of the counterpart CzFNBI, which
led to an insufficient confinement of blue phosphorescence.
Chem. Eur. J. 2013, 19, 10563 – 10572
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10569

W.-Y. Hung, K.-T. Wong et al.
film thickness measurement (K-MAC ST2000). A pulsed nitrogen laser
(337 nm) was used as the excitation light source through the transparent
electrode (ITO) and 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 (Ag),
and then recorded with a digital storage oscilloscope. Depending on the
polarity of the applied bias, selected carriers (holes or electrons) were
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
However, the well-matched energy levels allow CzFCBI to
serve as a common host for green-to-red PhOLEDs with ex-
cellent EQEs as high as 18.1–20%. In contrast, CzFNBI ex-
hibited a high ET of 2.72 eV, was successfully utilized as a
universal bipolar host for various monochromic RGB PhO-
LEDs, and achieved high EQEs of 14–19.5% with a
common device configuration. In addition, white PhOLEDs
with evenly separated RGB peaks were achieved by em-
ploying CzFNBI as host. The WOLEDs exhibited a high
CRI of 91.6, high efficiencies (18.4%, 36.3 cdAꢀ1,
28.3 lmWꢀ1), and highly stable chromaticity (CIEx=
0.43–0.46 and CIEy=0.43) at an applied voltage of 8 to
12 V. Notably, all devices exhibit low-efficiency roll-off at
high luminance, which can be attributed to the matched
energy levels between the emitting layer and neighboring
layers for efficient charge-carrier injection, and balanced
charge fluxes for a broader recombination zone within the
emitting layer. These results clearly indicate the potential of
using tailor-made bipolar universal hosts for the realization
of efficient RGB and white PhOLEDs.
carrier mobility is then given by m=D/ACHTNUTRGNENUG ACHTUGNTREN(NUGN VtT), in which the car-
(tTE)=D2/
rier transit time tT can be extracted from the intersection of two asymp-
totes to the tail and plateau sections in double-logarithmic plots.
OLED device fabrication: 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 Wsqꢀ1. The ITO surface was cleaned ultra-
sonically—sequentially with acetone, methanol, and deionized water—
and then with UV/ozone. The deposition rate of each organic material
was approximately 1–2 ꢃsꢀ1. Subsequently, LiF was deposited at 0.1 ꢃsꢀ1
and then capped with Al (ca. 5 ꢃsꢀ1) through shadow masking without
breaking the vacuum. The J–V–L characteristics of the devices were
measured simultaneously in a glovebox by using a Keithley 6430 source
meter and a Keithley 6487 picoammeter equipped with a calibration Si
photodiode. EL spectra were measured by using a photodiode array
(Ocean Optics USB2000).
Synthesis: The starting material, 3-(3-bromo-9-para-tolyl-9H-fluoren-9-
yl)-9-phenyl-9H-carbazole (1), was synthesized by following our litera-
ture procedure.[10c]
Experimental Section
9-Phenyl-3-{3-[4-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl]-9-para-
tolyl-9H-fluoren-9-yl}-9H-carbazole (CzFCBI): Compound 1 (3.0 g,
5.22 mmol), 1-phenyl-2-[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
Material characterization: 1H and 13C NMR spectra were recorded in
CDCl3 by using a Varian (Unity Plus 400) spectrometer at 400 and
100 MHz, respectively. Low- and high-resolution mass spectra were re-
corded with a JEOL SX-102A spectrometer in fast atom bombardment
(FAB) mode. Elemental analyses were performed on a Perkin–Elmer
CHN-2400 or Elementar Vario EL-III analyzer. UV/Vis absorption spec-
tra were recorded on a JASCO V-670 instrument. PL spectra were re-
corded on a Hitachi F-4500 fluorescence spectrophotometer. Differential
scanning calorimetry (DSC) analyses were performed on a TA Instru-
ments DSC-2920 MDSC V2.6A low-temperature differential scanning
calorimeter at a heating rate of 108Cminꢀ1 from 0 to 3508C under nitro-
gen. Thermogravimetric analysis (TGA) was undertaken with a TGA
Q500 instrument. The thermal stability of the samples under a nitrogen
atmosphere was determined by measuring their 5% weight loss on heat-
ing at a rate of 108Cminꢀ1 from 0 to 8008C. Transient PL decays were
measured under excitation by a pulsed nitrogen laser (l=337 nm, 10 Hz,
700 ps pulses) combined with a photomultiplier tube (R928, Hamamatsu)
and a synchronous oscilloscope (500 MHz resolution).
phenyl]-1H-benzo[d]imidazole (2; 2.48 g, 6.26 mmol), [PdACTHUNTRGNE(UNG PPh3)4] (0.72 g,
0.62 mmol), and Na2CO3 (2.71 g, 25.58 mmol) were mixed in a flask. The
vessel was then vacuum-evacuated and filled with argon. Dry THF
(80 mL) and degassed H2O (15 mL) were added to the reaction mixture,
which was then heated at 858C for 12 h under an argon atmosphere.
After cooling, the solvent was evaporated under vacuum and the product
was extracted with CH2Cl2 (100 mLꢁ2). The organic solution was
washed with water and dried with MgSO4. Evaporation of the solvent
was followed by column chromatography on silica gel with CH2Cl2/
EtOAc (95:5) as eluent to give a white solid, CzFCBI (2.95 g, 74%).
1H NMR (400 MHz, CDCl3): d=8.00–7.93 (m, 4H; ArH), 7.85 (d, J=
7.6 Hz, 1H; ArH), 7.70 (d, J=8.4 Hz, 2H; ArH), 7.64 (d, J=8.8 Hz, 2H;
ArH), 7.59–7.50 (m, 10H; ArH), 7.45–7.28 (m, 12H; ArH), 7.22–7.18 (m,
3H; ArH), 7.07 (d, J=8.0 Hz, 2H; ArH), 2.3 ppm (s, 3H; CH3);
13C NMR (100 MHz, CDCl3): d=152.34, 152.03, 151.74, 143.38, 142.74,
142.26, 141.12, 140.82, 139.74, 139.71, 139.39, 137.67, 137.40, 137.24,
136.99, 136.25, 129.96, 129.86, 129.78, 128.98, 128.68, 128.50, 128.07,
127.98, 127.48, 127.41, 127.33, 127.00, 126.92, 126.80, 126.63, 126.52,
126.30, 125.86, 123.43, 123.27, 123.13, 123.10, 120.36, 120.22, 119.78,
119.72, 119.43, 118.71, 110.46, 109.70, 109.58, 65.09, 20.95 ppm; MS
(FAB+): m/z (%): 765.3562 (61) [M+], 766.3483 (100) [MH+]; HRMS
(FAB+): m/z calcd for C57H39N3: 765.3144 [M]+; found: 765.3159; ele-
mental analysis calcd (%) for C57H39N3: C 89.38, H 5.13, N 5.49; found:
C 89.52, H 4.96, N 5.08.
Cyclic voltammetry: The oxidation/reduction potentials of CzFCBI and
CzFNBI were measured by cyclic voltammetry. The oxidation potential
was determined in CH2Cl2 (1.0 mm) containing 0.1m tetra-n-butylammo-
nium hexafluorophosphate (TBAPF6) as a supporting electrolyte at a
scan rate of 100 mVsꢀ1. The reduction potential was recorded in DMF
(1.0 mm)
containing
0.1m
tetra-n-butylammonium
perchlorate
(TBAClO4) as a supporting electrolyte at a scan rate of 100 mVsꢀ1. A
glassy carbon electrode and platinum wire were used as the working and
counter electrodes, respectively. The ferrocene/ferrocenium (Fc/Fc+)
redox couple in CH2Cl2/TBAPF6 occurred at E’o = +0.47 V for oxidation
and in DMF/TBAP occurred at E’o = ++0.51 V for reduction. All poten-
tials were recorded versus Ag/AgCl (saturated) as a reference electrode.
2-Phenyl-1-[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]-1H-
benzo[d]imidazole (3): A solution of 1-(4-bromophenyl)-2-phenyl-1H-
benzo[d]imidazole (6.0 g, 17.24 mmol), 4,4,4’,4’,5,5,5’,5’-octamethyl-2,2’-
bis(1,3,2-dioxaborolane)
(5.21 g,
20.51 mmol),
KOAc
(4.61 g,
47.23 mmol), and [Pd(dppf)Cl2] (70 mg; dppf=1,1’-bis(diphenylphosphi-
AHCTUNGTRENNUNG
Time-of-flight (TOF) mobility measurements: The terfluorene T3 was
used as the charge-generation layer in the TOF transient-photocurrent
technique. The TOF devices were configured as ITO glass/T3 (0.2 mm)/
CzFCBI (1.69 mm) or CzFNBI (1.74 mm)/Ag (150 nm). The thicknesses of
organic materials under test were set substantially larger than that of T3
so that the total transit time was larger than the time resolution of the
electronic system and the transit time across the T3 layer was negligible
in comparison with the total transit time. The thickness of the organic
film was monitored in situ with a quartz sensor and calibrated by a thin-
no)ferrocene) in anhydrous THF (120 mL) was deoxygenated by purging
with argon and then the reaction mixture was heated at 858C under
argon. After 12 h, the reaction mixture was cooled to room temperature,
mixed with water (50 mL), and extracted with CH2Cl2 (3ꢁ100 mL). The
combined organic extracts were dried over MgSO4 and concentrated
under reduced pressure. The crude product was purified through column
chromatography (n-hexane/EtOAc, 10:1) to give compound 3 (5.2 g,
76%) as a white solid. 1H NMR (400 MHz, CDCl3): d=7.92 (d, J=
10570
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Chem. Eur. J. 2013, 19, 10563 – 10572

Tuning of Bipolar Host Materials
FULL PAPER
8.0 Hz, 2H; ArH), 7.88 (d, J=8.0 Hz, 1H; ArH), 7.56 (d, J=7.2 Hz, 2H;
ArH), 7.35–7.24 (m, 8H; ArH), 1.36 ppm (s, 12H; 4ꢁCH3); 13C NMR
(100 MHz, CDCl3): d=152.24, 142.81, 139.35, 136.92, 136.24, 129.70,
128.35, 126.55, 123.42, 123.09, 119.80, 110.45, 84.22, 24.90 ppm; MS (ES+
TOF): m/z: 397 [MH+], 315 [M’B(OH)2 +H+]; HRMS (TOF, ES+) m/z
calcd for C25H26N2O2B: 397.2087 [MH+]; found: 397.2093.
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9-Phenyl-3-{3-[4-(2-phenyl-1H-benzo[d]imidazol-1-yl)phenyl]-9-para-
tolyl-9H-fluoren-9-yl}-9H-carbazole (CzFNBI): Compound 1 (2.1 g,
3.65 mmol), compound
3
(1.74 g, 4.4 mmol), [PdACHTNGUTERNNU(G PPh3)4] (0.5 g,
0.43 mmol), and Na2CO3 (1.9 g, 18 mmol) were mixed in a flask. The
vessel was then vacuum-evacuated and filled with argon. Dry THF
(70 mL) and H2O (10 mL) were added to the reaction mixture, which
was then heated at 858C for 12 h under an argon atmosphere. After cool-
ing, the solvent was evaporated under vacuum and the product was ex-
tracted with CH2Cl2. The CH2Cl2 solution was washed with water and
dried with MgSO4. Evaporation of the solvent was followed by column
chromatography on silica gel with CH2Cl2/EtOAc (95:5) as eluent to give
a white solid, CzFNBI (2.35 g, 70%). 1H NMR (400 MHz, CDCl3): d=
8.11 (brs, 1H; ArH), 7.99–7.91 (m, 3H; ArH), 7.87–7.83 (m, 3H; ArH),
7.67–7.52 (m, 9H; ArH), 7.49–7.29 (m, 15H; ArH), 7.23–7.20 (m, 3H;
ArH), 7.10 (d, J=8.0 Hz, 2H; ArH), 2.33 ppm (s, 3H; CH3); 13C NMR
(100 MHz, CDCl3): d=153.03, 152.99, 152.43, 143.95, 143.91, 142.07,
141.89, 141.58, 140.51, 140.33, 139.94, 138.26, 138.12, 138.02, 137.17,
136.88, 130.94, 130.49, 130.13, 129.98, 129.64, 129.58, 129.12, 128.67,
128.61, 128.40, 128.19, 128.06, 127.55, 127.45, 127.25, 127.17, 126.94,
126.58, 123.86, 123.79, 123.42, 120.99, 120.81, 120.45, 120.30, 119.98,
119.54, 111.14, 110.40, 110.22, 65.84, 21.26 ppm; MS (FAB+LR): m/z
(%): 523.1743 (21.29) [MPhCz+], 674.1599 (9.76) [MC6H4CH4+],
766.2271 (63.99) [M+]; HRMS (FAB+): m/z calcd for C57H39N3: 765.3144
[M+]; found: 765.3142; elemental analysis calcd (%) for C57H39N3: C
89.38, H 5.13, N 5.49; found: C 89.52, H 5.29, N 5.33.
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We gratefully acknowledge financial support from the National Science
Council of Taiwan (NSC 100-2112-M-019-002-MY3, 98-2119-M-002-007-
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