Manipulation of connecting topology in carbazole/benzimidazole universal bipolar host materials for RGB and White PhOLEDs

Article abstract of DOI:10.1039/c3ra42067a

Two isomeric bipolar molecules (o-CPhBzIm and m-CPhBzIm) composed of a carbazole as central donor and two benzimidazoles as peripheral acceptors bridged by phenylene groups with different connection topology are reported. The molecular π-conjugation is only limitedly extended with a meta-conjugation in m-CPhBzIm and a highly twisted conformation in o-CPhBzIm, giving blue-shifted UV-vis and PL spectra and higher triplet energies as compared to those of their para-counterpart (p-CPhBzIm). o-CPhBzIm exhibited interesting photophysical and electrochemical behaviors due to significant intramolecular donor-acceptor interactions, which were confirmed by the results of theoretical analyses and the observation of its X-ray structure. The physical properties are subtly governed by the linkage topology, giving o-CPhBzIm and m-CPhBzIm promising properties for serving as universal bipolar PhOLEDs with a common device configuration. Especially, the high triplet energies of o-CPhBzIm and m-CPhBzIm assure efficient confinement of blue phosphor emission, rendering the realizations of high efficiency blue and white PhOLEDs feasible. Monochromatic low roll-off PhOLEDs using m-CPhBzIm as universal host gave high efficiencies of 16.7% for red, 21.4% for green, 11.9% for blue, and 19.3% for yellow. PhOLEDs hosted by o-CPhBzIm performed with excellent efficiencies of 19.4% for red, 18.9% for green, 15.6% for blue, and 14.8% for yellow. In addition, three-color-based single-host WOLEDs hosted by o-CPhBzIm with high efficiency (18.5%, 32.6 cd A^-1, 34.1 lm W^-1) and CRI (up to 79) were successfully achieved.

Full text of DOI:10.1039/c3ra42067a

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Manipulation of connecting topology in carbazole/
benzimidazole universal bipolar host materials for RGB
and White PhOLEDs3
Cite this: RSC Advances, 2013, 3,
13891
Shu-Hua Chou,a Wen-Yi Hung,*b Chih-Ming Chen,b Qu-Yuan Liu,b Yi-Hung Liua
and Ken-Tsung Wong*ac
Two isomeric bipolar molecules (o-CPhBzIm and m-CPhBzIm) composed of a carbazole as central donor
and two benzimidazoles as peripheral acceptors bridged by phenylene groups with different connection
topology are reported. The molecular p-conjugation is only limitedly extended with a meta-conjugation in
m-CPhBzIm and a highly twisted conformation in o-CPhBzIm, giving blue-shifted UV-vis and PL spectra
and higher triplet energies as compared to those of their para-counterpart (p-CPhBzIm). o-CPhBzIm
exhibited interesting photophysical and electrochemical behaviors due to significant intramolecular
donor–acceptor interactions, which were confirmed by the results of theoretical analyses and the
observation of its X-ray structure. The physical properties are subtly governed by the linkage topology,
giving o-CPhBzIm and m-CPhBzIm promising properties for serving as universal bipolar PhOLEDs with a
common device configuration. Especially, the high triplet energies of o-CPhBzIm and m-CPhBzIm assure
efficient confinement of blue phosphor emission, rendering the realizations of high efficiency blue and
white PhOLEDs feasible. Monochromatic low roll-off PhOLEDs using m-CPhBzIm as universal host gave
high efficiencies of 16.7% for red, 21.4% for green, 11.9% for blue, and 19.3% for yellow. PhOLEDs hosted
by o-CPhBzIm performed with excellent efficiencies of 19.4% for red, 18.9% for green, 15.6% for blue, and
14.8% for yellow. In addition, three-color-based single-host WOLEDs hosted by o-CPhBzIm with high
efficiency (18.5%, 32.6 cd A21, 34.1 lm W21) and CRI (up to 79) were successfully achieved.
Received 15th March 2013,
Accepted 13th May 2013
DOI: 10.1039/c3ra42067a
www.rsc.org/advances
good charge carrier injections, as well as balanced and high
carrier mobilities to give high charge flux in the emitting layer.
Introduction
For the last decade, great research endeavors have been
devoted to the development of phosphorescent organic light-
emitting diodes (PhOLEDs) because a unity internal quantum
efficiency can be feasibly achieved due to the better harvest of
electro-generated singlet and triplet excitons.1 To realize high
efficiency electrophosphorescence,2 the transition-metal
embedded phosphorescent complex needs to be homoge-
neously dispersed into an adequate host material, which
entails several requirements such as higher triplet energy (ET)
than that of the guest phosphorescent emitter to prevent
detrimental reverse energy transfer,3 good thermal and
morphological stabilities to retain good thin film quality,
suitable energy levels with adjacent functional layers to ensure
Recently, tailor-made bipolar host materials featuring elec-
tron-donating (D) and electron-accepting (A)4 substructures
can be adopted to realize balanced charge flux, boosting the
electroluminescence (EL) efficiency approaching to the theo-
retical upper limit.5 The selection of appropriate hole- and
electron-transporting moieties as well as the linker to connect
them governs the physical properties and subsequent applica-
tions of bipolar host materials. A particular challenge will be
the bipolar hosts that can be utilized for various emission
colours ranging from blue to red and even white with a
common device structure. It is noteworthy that the fusion of
hole- and electron-transporting moieties in one bipolar host
molecule may reduce the band gap by intramolecular charge
transfer (ICT), resulting in a low ET, which is insufficient to
confine the emissive excitons on the phosphors i.e. back
energy transfer to non-emissive host. To diminish the direct
intramolecular chromophore interaction, the introduction of a
saturated spacer to decouple the D and A interactions is the
most effective strategy.6 Alternatively, the use of a twisted
spacer governed by steric effects7 and/or meta-substitution on
the hexagonal aromatic ring have also been successfully
aDepartment of Chemistry, National Taiwan University, Taipei, 10617, Taiwan.
E-mail: kenwong@ntu.edu.tw
bInstitute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung,
20224, Taiwan. E-mail: wenhung@mail.ntou.edu.tw
cInstitute of Atomic and Molecular Sciences, Academia Sinica, Taipei, 10617, Taiwan
Electronic supplementary information (ESI) available: Crystal data of
3
o-CPhBzIm (CCDC reference number 929130), computational data of o-CPhBzIm
and m-CPhBzIm, and CV of o-CPhBzIm and m-CPhBzIm. For ESI and
crystallographic data see DOI: 10.1039/c3ra42067a
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exploited to suppress the electronic interactions between D
and
eV) which is insufficient for confining blue phosphor emis-
sion. Nevertheless, due to the extended p-conjugation through
C3 and C6 substitution, CPhBzIm revealed high efficiency
deep-blue emission that can be applied to give blue electro-
luminescence. CPhBzIm was employed in dual roles as blue
emitter and bipolar host for the yellow-green dopant
(PBi)2Ir(acac) to realize two-color-based single-emissive-layer
WOLEDs.14 This strategy has been applied for the realization
of three-color based WOLEDs using other structurally similar
carbazole-benziimidazole derivatives.12h In this study, two
isomers of CPhBzIm, ortho-linked (o-CPhBzIm) and meta-
linked (m-CPhBzIm), were designed, synthesized, and char-
acterized by tuning the connecting topology between carbazole
core and the benzimidazole moieties, together with theoretical
A
components.8 For example, Ge et al.9 reported
triphenylamine/benzimidazole bipolar derivatives with twisted
conformations by implementing an ortho-methyl group to
interrupt the p-conjugation. Also, Tao et al.10 reported a series
of oxadiazole(OXD)/carbazole(Cz) bipolar hosts through meta-
and/or ortho-connections, exhibiting less ICT and higher ET as
compared to their para-substituted counterpart. Particularly, a
PhOLED utilizing the ortho-linked o-CzOXD as host gave the
best EL performance with maximum external quantum
efficiencies (gext) as high as 20.6% for green and 18.5% for
deep red. Son et al.11 investigated the structure-property
relationship of a series of bipolar hosts comprising phenylca-
bazole as core with phosphine-oxide substituents on different
positions of the phenylene group. A deep blue PhOLED with a
high gext of 20.4% was achieved with the para-substituted
phosphine oxide. However, most of these bipolar hosts were
only reported for the fabrication of a single-color PhOLED.
Bipolar universal hosts configured with twisted conformation
and/or meta-conjugation for blue, green, red, and white
PhOLEDs are relatively rare.12 Recently, one elegant example
was demonstrated by Chi et al., in which the strategy of
incorporating a twisted spacer between D and A did lead to a
high triplet energy for efficient accommodation of various RGB
phosphors.13 The bipolar host, m-cbtz, configured carbazole as
D and dipyridyl-triazole as A bridged by a meta-linked
phenylene ring exhibited a higher ET as compared to that of
its para-substituted analogue. PhOLEDs adopting m-cbtz as a
universal host achieved gext of 16.7% for red, 16.7% for green,
8.8% for blue, and 17.5% for yellow under the same device
configuration. In addition, a dual-emitter single-emission-
layer white device performed well with gext of 12.4%.
analyses, to achieve
a systematic investigation on the
structure–property–performance relationship. With such facile
adjustment in connecting topology, the new CPhBzIm isomers
(o- and m-CPhBzIm) exhibiting higher triplet energies (ET
=
2.63 eV) as compared to that of CPhBzIm are amenable to
confine blue phosphor emission and capable of hosting other
long-wavelength phosphorescent dopants for the fabrication
of monochromatic R, G, B, and Y devices as well as three-
emitter-based single-emitting-layer WOLEDs with rather high
electro-luminescence efficiencies.
Results and discussion
Synthesis
The synthesis of 3,6-disubstituted carbazole-based bipolar
hosts o-CPhBzIm and m-CPhBzIm is depicted in Scheme 1.
Suzuki coupling of carbazole-cored diboronic ester 1 with
ortho- and meta-bromo-1-phenyl-1H-benzo[d]imidazole gave
o-CPhBzIm and m-CPhBzIm, respectively, with excellent
isolated yields. The structural identities and puirties of
o-CPhBzIm and m-CPhBzIm were confirmed with satisfactory
In our previous work,
a bipolar molecule CPhBzIm
(Scheme 1) comprising of two N-phenylbenzimidazole units
bridged by phenylene groups to link the C3 and C6 positions
of N-phenylcarbazole, exhibited a low triplet energy (ET = 2.48
Scheme 1 Molecular structures of CPhBzIm and syntheses of o-CPhBzIm, and m-CPhBzIm.
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Table 1 Physical properties of CPhBzIm, m-CPhBzIm, and o-CPhBzIm
OX
b
d
Tg/Td/Tm
(uC)
E1/2
E1/2
/
HOMO/
LUMO/Eg (eV)
ET
(eV)
Abs lmax
PL lmax
mh/me
REV
a
c
(V)
Sol./Film (nm) Sol./Film (nm) WPL (%) (cm2 V21 s21
)
CPhBzIm
170/480/n.d. 1.24/22.06
25.49/22.37/3.12 2.45 317/322
25.52/22.02/3.5 2.63 299/303
412/426
392/402
412/410
98
70
29
9.6 6 1026/2.8 6 1025
1.8 6 1026/7.1 6 1026
4.4 6 1025/2.4 6 1026
m-CPhBzIm 153/427/n.d. 1.38/22.18
o-CPhBzIm
n.d./395/321 1.02e/22.15e 25.60/22.14/3.46 2.63 288/291
a
Determined through Riken AC-2 measurements; LUMO = HOMO + Eg; the value of Eg was calculated from the absorption onset of the solid
b
c
film. Measured in neat film at 77 K. PL quantum yield (WPL) determined using an integrating sphere (Hamamatsu C9920) in CH2Cl2.
d
e
Under an electric field (E) of 4.9 6 105 V cm21
. Determined by the onset point of the potentials.
spectroscopic and elemental analyses. The high molecular
weights of these two bipolar molecules are favorable for the
thermal stability as manifested by their high thermal decom-
position temperatures (Td, corresponding to a 5% weight loss
by thermogravimetric analysis) of 427 uC for m-CPhBzIm and
395 uC for o-CPhBzIm (Table 1), which are lower than that of
their para-counterpart (CPhBzIm). By differential scanning
calorimetry (DSC) analysis, m-CPhBzIm exhibited a high glass
transition temperature (Tg) of 153 uC, whereas the highly
twisted o-CPhBzIm only had a high melting temperature (Tm)
that appeared at 321 uC.
observed with lmax centred at 288 and 299 nm, respectively,
which were assigned to the p–p* transition of the C3 and C6
substituted carbazole. This result obviously indicates that the
p-conjugations of o-CPhBzIm and m-CPhBzIm are effectively
localized. Consequentially, o-CPhBzIm and m-CPhBzIm exhib-
ited blue-shifted emission spectra as compared to that of
CPhBzIm. The blue-shifted PL spectra could be ascribed to the
meta-conjugation existing in m-CPhBzIm and the remarkable
twisted conformation between the peripheral phenylbenzimi-
dazoles and carbazole core which disrupt the p-conjugation in
o-CPhBzIm. However, the possibility of ICT character upon
excitation should not be excluded since D and A components
coexist in m-CPhBzIm and o-CPhBzIm. The degree of ICT can
be easily probed by the solvatochromism of PL spectra, which
is strongly related to the excited state relaxation and mainly
dependent on the nature of the local dielectric environment.
As depicted in Fig. 2a, limited solvatochromic shifts with
intact vibronic features in the emission spectra of m-CPhBzIm
were observed as the solvent changed from cyclohexane to
acetonitrile. This result implies that m-CPhBzIm is relatively
inert to the dielectric environment, showing a rather small
change in the transition dipole moments upon photo-excita-
tion. In contrast, a pronounced positive PL solvatochromism15
of o-CPhBzIm was observed, as shown in Fig. 2b. Interestingly,
the solvent-dependent PL spectra of o-CPhBzIm consist of a
solvent-independent short wevelength emission region and
solvent polarity sensitive broad/structureless peaks in the long
wavelength region, which is different from the previous case of
parent molecule CPhBzIm.14 We assigned the short weve-
length emission to be the local excited (LE) state of
o-CPhBzIm. Then, the broad long wavelength emission peaks
of o-CPhBzIm in polar solvents can be attributed to the ICT
state.16 The disrupted p-conjugation in m-CPhBzIm and
o-CPhBzIm concurrently leads to their blue-shifted phosphor-
escence spectra, where the high energy onsets were utilized to
determine the triplet energies. Apparently, m-CPhBzIm and
o-CPhBzIm exhibit a higher ET (2.63 eV) as compared to that of
CPhBzIm (ET = 2.45 eV), giving a better chance to accom-
modate higher energy phosphors.
Photophysical properties
The electronic absorption (UV-vis) and photoluminescence
(PL) spectra at room temperature in dilute solutions (CH2Cl2)
and neat films on quartz, and phosphorescence (Phos.) spectra
at 77 K in neat films of o-CPhBzIm and m-CPhBzIm as
compared to those of CPhBzIm are shown in Fig. 1, and the
photophysical data are summarized in Table 1. The strategy of
tuning the substitution pattern of phenylene bridge subtly
governs the photophysical properties of o-CPhBzIm and
m-CPhBzIm. Blue-shifted absorption spectra of o-CPhBzIm
and m-CPhBzIm, as compared to that of CPhBzIm, were
Theoretical calculations
The molecular geometries and photophysical properties of
o-CPhBzIm and m-CPhBzIm at the molecular level were
analyzed by Gaussian 09 program, in which the conformations
of ground states (S0) and the electron density distributions
(Fig. 3) of HOMO/LUMO orbitals were optimized by the density
Fig. 1 Room-temperature normalized UV-Vis absorption and PL emission
spectra of CPhBzIm, m-CPhBzIm, and o-CPhBzIm in CH2Cl2 solutions (dotted
lines) and neat films (solid lines); phosphorescence spectra in neat films at 77 K.
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It clearly reveals that the electron density would shift from
the carbazole core to the phenylbenzimidazole peripherals
upon excitation, leading to polarized excited states. The
properties of the high-energy excited-state may be of greater
importance than the ground state given the intrinsic roles of
host materials as energy-transferring media. Herein, we were
particularly interested in exploring the conformations and
molecular-orbital distributions for the first singlet excited
state (S1). The calculations conducted on the properties of the
excited state typically require more computational effort than
that used for the ground state. Therefore, the quantum-
mechanical calculations for excited-state research typically
adopt the configuration interaction with all singly excited
determinants (CIS) method17 that provides a cost-effective
alternative for the study of excited-state properties. To examine
the excited states, the molecular geometries of o-CPhBzIm and
m-CPhBzIm ground states obatined by B3LYP/6-31G(d) were
further calculated by CIS method with 6-31G(d) basis sets to
give the optimized conformation of S1. Then, time dependent
density functional theory (TD-DFT) is subsequently employed
to acquire the emission spectra18 together with the corre-
sponding oscillator strengths and compositions of excited
states in terms of singly excited Slater determinants. The
molecular geometries of o-CPhBzIm and m-CPhBzIm S0 and S1
states optimized by B3LYP/6-31G(d) and CIS/6-31G(d), respec-
tively, are shown in Fig. 4. The dihedral angles and bond
lengths of computed optimized S0 and S1 states are summar-
Fig. 2 The emission spectra of m-CPhBzIm and o-CPhBzIm in various solvents.
ized in Table S1, ESI , along with the atom-labeling numbers
3
shown in Fig. S1 (See supporting information ).
3
functional theory (DFT) using Beck’s three-parameterized Lee–
Yang–Parr exchange functional (B3LYP) with 6-31G(d) basis
sets. According to DFT calculations, the HOMO orbitals of
o-CPhBzIm and m-CPhBzIm are mainly located on the
electron-donating carbazole moiety with partial contribution
from the phenylene bridges at C3 and C6 positions, while the
LUMO orbitals are mainly dispersed on the electron-accepting
benzimidazole unit. The spatial separations of HOMO and
LUMO orbitals of o-CPhBzIm and m-CPhBzIm ensure the
preference for efficient electron- and hole-transporting proper-
ties.
It is noteworthy that the conformation of o-CPhBzIm S0 state
is evidently different from that of S1 state, where two different
intramolecular p–p interactions (ca. 3.0 and 3.3 Å) between the
central carbazole and the N-phenylbenzimidazole groups were
observed (Fig. 4b). Whereas, limited conformational variations
between S0 and S1 states were observed in m-CPhBzIm. It is
obvious that the ortho-substitution of central carbazole and
peripheral N-phenylbenzimidazole groups via phenylene
bridges leads to highly twisted conformations, which can also
Fig. 4 (a) Optimized S0 states of o-CPhBzIm and m-CPhBzIm computed by
B3LYP/6-31G(d). (b) Optimized S1 states computed by CIS/6-31G(d). All the
protons are excluded for clarity.
Fig. 3 Frontier molecular orbitals (HOMO and LUMO) of o-CPhBzIm and
m-CPhBzIm ground states (S0) calculated by DFT//B3LYP/6-31G(d).
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be confirmed by the X-ray diffraction analysis of o-CPhBzIm
(Fig. S2 and Table S2, ESI ). The calculations for the first
3
electronic transitions with their emission maxima (lem),
oscillator strengths (f), and the molecular orbital configura-
tions, as well as observed lem are summarized in Table 2. The
emission peak at 330.5 nm of o-CPhBzIm with a larger
oscillator strength could be attributed to its S1 state,19 in
which the ICT can be facillitated by the intramolecular p–p
interactions between the electron-donating carbazole and
electron-withdrawing benzimidazole. In contrast, a weaker
oscillator strength of the S1 A S0 transition indicates a low
propensity for efficient D/A charge transfer in m-CPhBzIm.
This result agrees with our observations in the PL solvato-
chromism.
Electrochemical property
Fig. 5 Typical transient photocurrent signals for m-CPhBzIm (1.76 mm thick) at E
= 7.1 6 105 V cm21 and o-CPhBzIm (1.42 mm thick) at E = 6.4 6 105 V cm21
:
The electrochemical properties of m-CPhBzIm and o-CPhBzIm
were investigated by cyclic voltammetry (CV) (See Fig. S3, ESI ).
(a)(c) holes; (b)(d) electrons. Insets are the double logarithmic plots of (a)–(d).
3
m-CPhBzIm exhibited a bipolar electrochemical character with
a quasi-reversible oxidation potential at 1.38 V and a quasi-
and o-CPhBzIm as compared to those of CPhBzIm that we
have previously reported.14 The hole mobilities varied within
the range from 1 6 1026 to 6.1 6 1025 cm2 V21 s21 for fields
varying from 1.4 6 105 to 7.1 6 105 V cm21 and the electron
mobilities fell within the range from 2.4 6 1026 to 4.2 6 1025
cm2 V21 s21 for fields varying from 4.2 6 105 to 7.1 6 105 V
cm21. The field-dependence of the carrier mobilities followed
reversible reduction potential at 22.18
V (vs. Ag/AgCl).
Interestingly, the electrochemical behavior of o-CPhBzIm
exhibited poor reversibility both for oxidation and reduction,
which can be possibly attributed to the interference from the
intramolecular interactions between central carbazole and
peripheral phenylbenzimidazole.20 For
a more practical
reference, we utilized a Riken AC-2 photoemission spectro-
meter to determine the HOMO energy levels of o-CPhBzIm
(25.60 eV) and m-CPhBzIm (25.52 eV), and the LUMO energy
levels were calculated by the the equation LUMO = HOMO +
DEg, where DEg is the optical band gap determined from the
absorption threshold. We estimated the LUMO energy levels of
o-CPhBzIm and m-CPhBzIm to be 22.14 and 22.02 eV,
respectively. The HOMO levels of m-CPhBzIm and
o-CPhBzIm are lower than that of CPhBzIm (25.49 eV),
agreeing with their inferred p-conjugations observed in the
photophysical measurements.
the nearly universal Poole-Frenkel relationship m 3 exp(bE1/2
)
typically observed for disordered organic systems, where b is
the Poole-Frenkel factor. Apparently, the connecting topology
has a crucial influence on the charge transporting behavior.
The observed hole mobilities have the trend of ortho . para .
meta, whereas the electron mobilities have the trend of para .
meta . ortho. As compared to those of ortho- and meta-
counterparts, the para-substituted parent molecule exhibits
much more balanced electron/hole mobilities, which could be
ascribed to its more planar molecular structure and more
delocalized p-conjugation for inducing easier carrier hopping
ways among the adjacent molecules.13,22 However, since the
carrier mobilities of an amorphous thin film are mainly
governed by the energetic and positional orders of composing
Charge carrier mobility
We used time-of-flight (TOF) techniques to measure the carrier
mobility.21 Representative TOF transients for holes and
electrons of m-CPhBzIm and o-CPhBzIm are shown in Fig. 5.
The carrier-transit times (tT) needed for determining carrier
mobilities were evaluated from the intersection points of two
asymptotes from the double-logarithmic plots (Fig. 5, insets).
Fig. 6 depicts the obtained carrier mobilities, shown as a
function of the square root of the electric field, of m-CPhBzIm
Table 2 Calculated data acquired from TDDFT//B3LYP/6-31G(d) according to
the CIS/6-31G(d) optimized geometry of the first excited state
o-CPhBzIm
S1 A S0
m-CPhBzIm
S1 A S0
Electronic transition
Theoretical lem (nm)
f (Oscillator strength)
MO configuration
Coefficient
330.53
0.3495
HOMO A LUMO
0.68282
412
332.08
0.1080
HOMO A LUMO
0.64574
412
Fig. 6 Hole and electron mobilities of isomeric bipolar molecules (p-: CPhBzIm,
o-: o-CPhBzIm, and m-: m-CPhBzIm).
Experimental lem (nm)
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molecules,23 without detailed information on molecular
packing of thin films, it will be rather difficult to clearly
correlate the observed mobilities to the molecular structures.
OLED devices
To investigate m-CPhBzIm and o-CPhBzIm as bipolar host
materials, we fabricated PhOLED devices with four phosphors
(Scheme 2) including green bis(1,2-dipheny1-1H-benzoimidazole)
iridium(III) (acetylacetonate) [(PBi)2Ir(acac)],24 red bis[3-(tri-fluor-
omethyl)-5-(4-tert-butylpyridyl)-1,2,4-triazolate]osmium(II) bis-
(dimethylphenylphosphine) [Os(bpftz)2(PPhMe2)2, OS1],25 blue
[bis(4,6-difluorophenyl)-pyridinato-N,C29]iridium(III) picolinate
(FIrpic),26 and yellow bis(2-phenylbenzothiazolato)iridium(III)
(acetylacetonate) [(Bt)2Ir(acac)].27 The devices were fabricated
with a typical structure consisting of indium tin oxide (ITO)/
PEDOT:PSS (30 nm)/DTAF (25 nm)/host: 10 wt% dopant (25
nm)/DPPS (50 nm)/LiF (0.5 nm)/Al (100 nm). To improve the
hole injection from the anode, poly(3,4-ethylene-dioxythiophe-
ne):poly(styrene sulfonic acid) (PEDOT:PSS) was spun onto the
precleaned ITO substrate to form a polymer buffer layer. The
hole-transporting material 1,1-bis[(di-4-tolylamino)-phenyl]-
9,99-fluorene (DTAF),14,28 with a high triplet level (ET = 2.87
eV) and high hole mobility (mh in the order of 1023 cm2 V21 s21
)
that can efficiently confine the triplet energy of phosphors, was
selected as the hole-transporting layer (HTL). Next, 10 wt%
dopant was co-deposited with m-CPhBzIm or o-CPhBzIm to
form a 25 nm-thick EML. To further confine the holes or
generated excitons within the emissive layer, a 50 nm-thick
diphenyl-bis[4-(pyridin-3-yl)-phenyl]silane (DPPS),29 with a high
triplet level (ET: 2.7 eV, HOMO/LUMO: 6.5/2.5 eV) was selected
as the electron-transporting layer (ETL) to confine the excitons
within the EML. LiF and Al served as the electron-injection layer
and cathode, respectively.
Fig. 7 (a) Current density–voltage–luminance (J–V–L) characteristics, (b) external
quantum (gext) and power (gP) efficiencies as a function of brightness, and (c) EL
spectra for devices incorporating m-CPhBzIm.
Fig. 7 and 8 depict the current density–voltage–luminance
(J–V–L) characteristics, device efficiencies, and EL spectra of
the devices. The notation 1 and 2 indicates the devices
fabricated with host materials of m-CPhBzIm and o-CPhBzIm,
respectively. Table 3 summarizes the obtained electrolumines-
cence data. All devices displayed a low turn-on voltage at 2.5 V
and the difference in the J–V performance is presumably
because of the different emitting dopants that influence the
charge carrier transport behavior of the EML. m-CPhBzIm and
o-CPhBzIm exhibit a triplet energy (ET = 2.63 eV) that can
perform effective confinement of the FIrpic excitons.
Therefore, the performances of blue PhOLEDs using FIrpic
as dopant were examined. Device B1 achieved a maximum
brightness (Lmax) of 22 700 cd m22 at 11.5 V (700 mA cm22
)
with the CIE coordinates of (0.16, 0.36). The maximum
external quantum (gext), current (gc), and power (gp) efficien-
cies were 11.9%, 24.8 cd A21, and 26 lm W21, respectively.
Device B2 exhibited a Lmax of 42 800 cd m22 at 13.5 V (610 mA
cm22) with CIE coordinates of (0.16, 0.36) and attractive EL
efficiencies (15.6%, 31.7 cd A21, 22.7 lm W21) that are superior
to those of the device B1. At a high brightness of 1000 cd m22
,
the gext of devices B1 and B2 are still as high as 9.7% and
15.2%, respectively, showing low efficiency roll-off (Table 3).
To further verify the difference between m-CPhBzIm and
o-CPhBzIm as blue host, we measured the absolute photo-
luminescence quantum yield (WPL) of thin films of FIrpic
dispersed in m-CPhBzIm and o-CPhBzIm under N2 flow using
an integrating sphere excited at 337 nm with a multichannel
spectrometer as the optical detector. The better performances
of device B2 can be verified by the higher WPL of o-CPhBzIm/
FIrpic film (0.82) than that of m-CPhBzIm/FIrpic film (0.43).
Scheme 2 Molecular structures of the dopants used in this study.
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64.1 ml W21) with the CIE coordinates of (0.38, 0.59) while the
o-CPhBzIm hosted device G2 exhibited a Lmax of 133 800 cd
m
22 at 1110 mA cm22 (14.5 V) and respectable EL efficiencies
(18.9%, 64.4 cd A21, and 49.5 lm W21) with the CIE
coordinates of (0.38, 0.59). When the brightness reached
1000 cd m22, the value of gext are 21.3% and 18.9%, suggesting
that balanced injection and transport of holes and electrons in
the devices occurred even at high current densities.
Next, the yellow-emitting PhOLEDs were fabricated using
(Bt)2Ir(acac) as dopant (devices Y1 and Y2) with the CIE
coordinates of (0.52, 0.47–0.48). Excellent performance was
achieved for device Y1, which displayed substantially higher
efficiencies (19.3%, 49.7 cd A21, and 62.5 ml W21) than those
of device Y2 (14.8%, 36.4 cd A21, and 32.7 ml W21). Moreover,
we demonstrated red-emitting PhOLEDs using the emitter
Os(bpftz)2(PPhMe2)2, OS1 (device R1 and R2) with the CIE
coordinates of (0.66, 0.34). Device R1 exhibited a Lmax of
28 700 cd m22 at 12 V and respectable EL efficiencies (16.7%,
15.3 cd A21, and 14.8 lm W21) while the device R2 exhibited a
Lmax of 31 000 cd m22 at 13 V and EL efficiencies (19.4%, 18.4
cd A21, and 19.3 lm W21). All devices displayed relatively pure
emission from blue to red as shown in Fig. 7(c) and 8(c),
indicating that the electroluminescence can be completely
confined on the emissive dopants.
In addition to the evaluation for various monochromatic
phosphors, we also fabricated WOLEDs (device W1 and W2) by
adopting three primary color (R–G–B) phosphors within a
single host in distinct zones (B–G–R–B type). The single host
system can help reduce the structural heterogeneity of multi-
layer WOLEDs, thereby facilitating charge injection/transport
in the emission zone and broadening the exciton-formation
area.6e The WOLEDs have similar architecture to the afore-
mentioned monochromatic devices except for the emitting-
layer [host: 10% FIrpic (5 nm)/host: 10% (PBi)2Ir(acac) (5 nm)/
host: 10% OS1 (5 nm)/host: 10% FIrpic(10 nm)]. Device W1
achieved a Lmax of 84 000 cd m22 at 13 V (870 mA cm22), and
EL efficiencies of 17.0%, 35.3 cd A21, and 31.7 lm W21; device
W2 achieved a Lmax of 71 900 cd m22 at 16 V (930 mA cm22),
Fig. 8 (a) Current density–voltage–luminance (J–V–L) characteristics, (b) external
quantum (gext) and power (gP) efficiencies as a function of brightness, and (c) EL
spectra for devices incorporating o-CPhBzIm.
Besides blue PhOLEDs, m-CPhBzIm and o-CPhBzIm were
also applied in different dopants (from green to red). In green
devices using (PBi)2Ir(acac) as dopant, the m-CPhBzIm host
device G1 exhibited a Lmax of 153 600 cd m22 at 860 mA cm22
(12.5 V) and very good EL efficiencies (21.4%, 72.9 cd A21, and
and EL efficiencies of 18.5%, 32.6 cd A21, and 34.1 lm W21
.
The efficiency of device W2 is higher than that of device W1
due to a higher efficiency in blue as shown in Fig. 9(b). In
addition, the EL spectra of WOLEDs can clearly perform red,
Table 3 EL performance of devices incorporating various hosts and emitters
Vona/V
bAt 1000 cd m22/(V, %)
Lmax/cd m22
Imax/mA cm22
gext/%
gc/cd A21
gp/lm W21
CIE/(x,y)
B1a
G1
Y1
R1
W1
B2
G2
Y2
R2
W2
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
6.1, 9.7
22 700 (11.5 V)
153 600 (12.5 V)
73 100 (13 V)
28 700 (12 V)
84 000 (13 V)
42 800 (13.5 V)
133 800 (14.5 V)
110 100 (13 V)
31 000 (13 V)
71 900 (16 V)
700
860
760
920
870
610
1110
1320
890
930
11.9
21.4
19.3
16.7
17.0
15.6
18.9
14.8
19.4
18.5
24.8
72.9
49.7
15.3
35.3
31.7
64.4
36.4
18.4
32.6
26.0
64.1
62.5
14.8
31.7
22.7
49.5
32.7
19.3
34.1
0.16,0.36
0.38, 0.59
0.52, 0.48
0.66,0.34
0.46,0.45
0.16,0.36
0.38, 0.59
0.52, 0.47
0.66,0.34
0.40,0.42
4.5, 21.3
5.2, 16.1
6.0, 15.8
6.5, 15.0
6.1, 15.2
4.5, 18.9
4.9, 13.9
7.2, 16.0
8.5, 14.2
a
Turn-on voltage at which emission became detectable (1022 cd m22). The notation 1 and 2 indicates the devices fabricated with host
materials of m-CPhBzIm and o-CPhBzIm, respectively, in the configuration: ITO/PEDOT:PSS/DTAF(25 nm)/emitter (25 nm)/DPPS (50 nm)/LiF/
b
Al. The values of driving voltage and external quantum efficiency of device at 1000 cd m22
.
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green, and blue emissions to cover wavelengths from 450 to
800 nm as shown in Fig. 9, giving the color-rendering index
(CRI) of the WOLEDs of 82.9–84.1 for device W1 and 73.8–77.9
for device W2. As the operation voltage increased, the relative
intensity of blue and green emission slightly increased, which
led to a slight shift of the CIE coordinates. When the voltage
increases from 8 to 12 V, the CIE coordinates slightly vary from
(0.47, 0.45) to (0.45, 0.45) for device W1, and from (0.40, 0.41)
to (0.39, 0.42) for device W2.
showed the crucial role-play of D/A connection topology which
subtly governs the physical properties and consequential
applications as bipolar hosts in high efficiency RGB and
White PhOLEDs.
Experimental section
General information
1H and 13C NMR spectra were recorded on a Varian Mercury
Plus-400 using deuterochloroform (or other given solvents) as
an internal reference. High-resolution mass spectroscopy was
performed on a JMS-700 HRMS and IR spectra were obtained
on a Thermo Nicolet Avatar E.S.P. typed FT-IR. Melting points
were determined on a Fargo MP-1D melting point apparatus.
Differential scanning calorimetry (DSC) data were performed
on a TA Instrument 5100 thermal analysis system under
Conclusions
In summary, two carbazole/benzimidazole hybrid bipolar host
materials (m-CPhBzIm and o-CPhBzIm) have been successfully
developed. They showed blue-shifted UV-vis and PL spectra as
compared to those of their previously reported para counter-
part CPhBzIm, which can be ascribed to the meta-conjugation
existing in m-CPhBzIm and remarkable twisted conformation
between the peripheral phenylbenzimidazoles and carbazole
core which disrupts the p-conjugation in o-CPhBzIm. These
observations agree with the results obtained by DFT, CIS and
TDDFT calculations. The two new hosts exhibiting bipolar
charge-transporting character examined by TOF measure-
ments, together with higher triplet energies (ET = 2.63 eV)
can be adopted to realize efficient monochromatic PhOLED
devices with excellent performances. In addition, o-CPhBzIm-
hosted three-color-based single-host WOLED was also
achieved with high efficiencies (18.5%, 32.6 cd A21, 34.1 lm
nitrogen flowing condition at a heating rate of 10 uC min21
.
Material synthesis
All chemicals and solvents were reagent grade and purchased
commercially without further purification. Solvents for synth-
eses were freshly distilled before used. All reactions were
performed under an atmosphere of nitrogen or argon. 2-(2-
bromophenyl)-1-phenyl-1H-benzo[d]imidazole (2) and 2-(3-
bromo-phenyl)-1-phenyl-1H-benzo[d]imidazole
(3)
were
synthesized by
a
literature procedure.9 9-phenyl-3,6-bis-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole (1)
and CPhBzIm were obtained according to our previous
publications.14,30
W
21) and with CIE chromaticity (CIEx = 0.39–0.40 and CIEy =
Synthesis of 9-phenyl-3,6-bis(2-(1-phenyl-1H-benzo[d]-imida-
zol-2-yl)phenyl)-9H-carbazole (o-CPhBzIm). A mixture of 9-phe-
nyl-3,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
0.41–0.42) at an applied voltage of 8 to 12 V. Our results
9H-carbazole (1) (3.6 g, 7.2 mmol), 2-(2-bromophenyl)-1-
phenyl-1H-benzo[d]imidazole (2) (5.53 g, 15.84 mmol),
Pd(PPh3)4 (416 mg, 0.36 mmol), K3PO4 (2 M aqueous solution,
29 mL), and tributylphosphine (0.05 M toluene solution, 14.4
mL) were dissolved in dry toluene (75 mL) and the mixture was
heated to reflux under argon for 2 days. After cooling, water
was added and extracted twice with CH2Cl2. The combined
organic layers were dried over MgSO4 and the solvent was
removed by a rotary evaporator. The crude product was
purified by column chromatography on silica gel (eluent:
EtOAc/Hexane/CH2Cl2 = 1/4/1) to give o-CPhBzIm as a white
solid (5.2 g, 93%). M.p. 321–322 uC; IR (KBr) n 3053, 2922,
2851, 1597, 1500, 1462, 1450, 1379, 1325, 1262, 758, 742, 695;
1H NMR (CD2Cl2, 400 MHz) d 7.88–7.86 (m, 4H), 7.6–7.47 (m,
8H), 7.44–7.40 (m, 1H), 7.33–7.29 (m, 2H), 7.25 (dd, J = 1.2, 7.6
Hz, 2H), 7.17–7.13 (m, 2H), 7.08–7.03 (m, 6H), 6.95 (t, J = 7.2
Hz, 2H), 6.86 (t, J = 7.4 Hz, 4H), 6.77 (dd, J = 1.8, 8.6 Hz, 2H),
6.36 (d, J = 7.2 Hz, 4H); 13C NMR (CDCl3, 100 MHz) d 153.6,
143.3, 142.0, 140.0, 137.4, 135.5, 135.4, 132.4, 130.2, 130.0,
129.2, 128.7, 127.5, 127.1, 126.9, 126.7, 125.5, 123.6, 123.1,
122.7, 120.1, 120.0, 110.5, 109.4; HRMS (m/z, FAB+) Cacld. for
C
56H37N5 779.3049, found 779.3058; Anal. Calcd. (%) C, 86.24;
H, 4.78; N, 8.98, found C, 86.37; H, 4.99; N, 8.92.
Synthesis of 9-phenyl-3,6-bis(3-(1-phenyl-1H-benzo[d]-imida-
Fig. 9 The normalized EL spectra of white devices hosted by (a) m-CPhBzIm and
(b) o-CPhBzIm at various voltages.
zol-2-yl)phenyl)-9H-carbazole (m-CPhBzIm).
A mixture of
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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 towards 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) 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 = D/(tTE) = D2/(VtT), in which the carrier transit
time, tT, can be extracted from the intersection of two
asymptotes to the tail and plateau sections in double-
logarithmic plots.
9-phenyl-3,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
9H-carbazole (1) (0.99 g, 2 mmol), 2-(3-bromophenyl)-1-phenyl-
1H-benzo[d]imidazole (3) (1.54 g, 4.4 mmol), Pd(PPh3)4 (115
mg, 0.1 mmol), K3PO4 (2 M aqueous solution, 8 mL), and
tributyl-phosphine (0.05 M toluene solution, 4 mL) were
dissolved in dry toluene (20 mL) and the mixture was heated
to reflux under argon for 1 day. After cooling, water was added
and extracted twice with CH2Cl2. The combined organic layers
were dried over MgSO4 and the solvent was removed by a
rotary evaporator. The crude product was purified by column
chromatography on silica gel (eluent: EtOAc/Hexane/CH2Cl2 =
1/2/1) to give m-CPhBzIm as a white solid (1.45 g, 93%). M.p.
203–204 uC; IR (KBr) n 3060, 1596, 1500, 1471, 1452, 1400,
OLED device fabrications
1
1372, 1325, 1284, 1264, 1233, 797, 761, 744, 714, 697; H NMR
All chemicals were purified through vacuum sublimation prior
to use. The OLEDs were fabricated through vacuum deposition
of the materials at 1026 torr onto ITO-coated glass substrates
having a sheet resistance of 15 V sq21. The ITO surface was
cleaned ultrasonically sequentially with acetone, methanol,
and deionized water and then it was treated with UV-ozone.
The film thicknesses were monitored by a calibrated quartz
crystal microbalance. The deposition rate of each organic
material was ca. 1–2 Å s21. Subsequently, LiF was deposited at
0.1 Å s21 and then capped with Al (ca. 5 Å s21) through shadow
masking without breaking the vacuum. The J–V–L character-
istics 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 (Ocean Optics
USB2000).
(CD2Cl2, 400 MHz) d 8.17 (d, J = 1.2 Hz, 2H), 7.98 (t, J = 1.8 Hz,
2H), 7.85 (d, J = 7.6 Hz, 2H), 7.8–7.77 (m, 2H), 7.69–7.65 (m,
9H), 7.63–7.60 (m, 3H), 7.54 (d, J = 7.2 Hz, 1H), 7.51–7.48 (m,
4H), 7.47–7.43 (m, 6H), 7.36–7.32 (m, 2H), 7.31–7.29 (m, 4H);
13C NMR (CD2Cl2, 100 MHz) d 152.6, 143.5, 141.9, 141.3, 137.9,
137.7, 132.9, 131.0, 130.4, 129.3, 129.1, 128.6, 128.3, 128.1,
127.3, 125.8, 124.3, 123.6, 123.2, 120.0, 119.1, 110.9, 110.6;
HRMS (m/z, FAB+) Cacld. for C56H37N5 779.3049, found
779.3054; Anal. Calcd. (%) C, 86.24; H, 4.78; N, 8.98, found
C, 85.97; H, 4.88; N, 9.24.
Cyclic voltammetry
Electrochemical properties were performed with
a CH
Instruments Inc. CH1619B in a three-electrode electrochemi-
cal cell with a glassy carbon working electrode, a platinum wire
counter electrode, and an Ag/AgCl reference electrode.
Oxidation potentials were measured in anhydrous CH2Cl2
(1.0 mM) with nBu4NPF6 (0.1 M) as a supporting electrolyte,
and the reduction potentials were investigated in a solution of
anhydrous DMF (1.0 mM) containing nBu4NClO4 (0.1 M) as a
supporting electrolyte, which were purged with nitrogen
before measurement. All potentials reported here were
calibrated with the ferrocene/ferrocenium redox couple (Fc/
Fc+) as an internal standard.
Acknowledgements
This study was supported financially from National Science
Council of Taiwan (NSC 100-2112-M-019-002-MY3, 101-2113-
M-002-009-MY3). We greatly appreciate the National Center for
High-Performance Computing for computer time and facil-
ities.
Photophysical measurements
The UV-Vis absorption spectra of the solutions were acquired
using
a HITACHI U2800A spectrophotometer. Emission
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