Highly twisted biphenyl-linked carbazole-benzimidazole hybrid bipolar host materials for efficient PhOLEDs

Article abstract of DOI:10.1039/c4tc01463d

Three new highly twisted bipolar host materials (PhczNBI, PhczCBI, and PhczDCBI) comprising of a biphenyl core linking a hole-transport 9-phenylcarbazole (donor) and an electron-transport N-phenylbenzimidazole (acceptor) with different topologies were synthesized, characterized, and applied as host materials for full-color phosphorescent OLEDs. The highly twisted conformations imparted through a 2,2′-biphenyl linkage effectively disrupt the π-conjugation and electronic coupling between the donor and the acceptor, thus giving the bipolar hosts promising properties such as bipolar charge transport features (μ_h≈ μ_e≈ 10^-6to 10^-4cm²V^-1s^-1) and high triplet energies (E_T= 2.55-2.71 eV). The PhOLED device gave a maximum external quantum efficiency (η_ext) of 19.6% for the PhczNBI host with a yellow phosphor (Bt)₂Ir(acac). A two-color, single-emitting-layer and all-phosphor WOLED hosted by PhczDCBI achieved high efficiencies (13.9%, 34.9 cd A^-1, and 24.8 lm W^-1) and a CRI up to 65.5.

Full text of DOI:10.1039/c4tc01463d

Journal of
Materials Chemistry C
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PAPER
Highly twisted biphenyl-linked carbazole–
benzimidazole hybrid bipolar host materials for
efficient PhOLEDs†
Cite this: J. Mater. Chem. C, 2014, 2,
554
8
a
b
b
a
Shuo-Hsien Cheng, Wen-Yi Hung,* Ming-Hung Cheng, Hsiao-Fan Chen,
a
a
a
ac
Atul Chaskar, Gene-Hsiang Lee, Shu-Hua Chou and Ken-Tsung Wong*
Three new highly twisted bipolar host materials (PhczNBI, PhczCBI, and PhczDCBI) comprising of a biphenyl
core linking a hole-transport 9-phenylcarbazole (donor) and an electron-transport N-phenylbenzimidazole
(acceptor) with different topologies were synthesized, characterized, and applied as host materials for full-
0
color phosphorescent OLEDs. The highly twisted conformations imparted through a 2,2 -biphenyl linkage
effectively disrupt the p-conjugation and electronic coupling between the donor and the acceptor, thus
ꢀ6
giving the bipolar hosts promising properties such as bipolar charge transport features (m
h
z m
¼ 2.55–2.71 eV). The PhOLED device gave a maximum
external quantum efficiency (hext) of 19.6% for the PhczNBI host with a yellow phosphor (Bt) Ir(acac). A
two-color, single-emitting-layer and all-phosphor WOLED hosted by PhczDCBI achieved high efficiencies
e
z 10 to
Received 8th July 2014
Accepted 20th August 2014
ꢀ4
2
ꢀ1 ꢀ1
10
cm
V s ) and high triplet energies (E
T
2
DOI: 10.1039/c4tc01463d
ꢀ1
ꢀ1
www.rsc.org/MaterialsC
(13.9%, 34.9 cd A , and 24.8 lm W ) and a CRI up to 65.5.
bipolar transport capabilities to dene the charge recombina-
tion zone within the emissive layer, and (iv) a high morpho-
Introduction
7
Phosphorescent OLEDs (PhOLEDs) have been deemed as the logical stability. Among them, the demands of a high E and
T
most promising candidates to realize full-colour at-panel good bipolar transport capability are generally conicting for
displays and solid state lighting applications due to their organic molecules because the introduction of electron-
intrinsic capability of harnessing both singlet and triplet exci- donating (D) and electron-withdrawing (A) moieties easily trig-
1,2
tons through an effective intersystem crossing (ISC). PhOLED gers an intramolecular charge transfer character, which lowers
devices can achieve nearly 100% internal quantum efficiency by the energy gap and thus gives a lower E
utilizing heavy metal-centred complexes as emitters. To way to minimize electronic coupling between D and A is always
T
. As a result, an efficient
3
suppress the detrimental effects, such as aggregation quench- required for the design of high E
ing and/or triplet–triplet annihilation, which oen are the main
Various strategies have been developed to achieve bipolar
issues to deteriorate the device efficiency, a host–guest strategy hosts with a high E by impeding electronic communication
T
bipolar hosts.
4
T
with triplet emitters (guest) homogeneously dispersed into a between D and A. For example, the introduction of a saturated
suitable organic matrix (host) is frequently used. Therefore, the bridging centre, such as C or Si atom, between the D and A
design of appropriate host materials is of great importance for segments can efficiently interrupt the through-bond electronic
highly efficient PhOLEDs, and it should meet the following communication and conne the p-conjugation within an indi-
8–12
criteria: (i) sufficiently high triplet energies (E ) for ensuring vidual component.
For example, Yang and Ma et al. recently
T
exothermic energy transfer from the host material to the dopant reported new types of bipolar host materials in which a Si atom
5
and efficient exciton connement, (ii) suitable energy level was introduced to bridge an arylamino donor and benzimid-
match-ups with adjacent layers to reduce interfacial energy azole or oxadiazole acceptors. These materials were shown to
6
9,10
barriers and to increase charge balance, (iii) high and balanced possess high E
in a range of 2.69–2.73 eV.
Interestingly,
T
these new host materials were found to be capable of serving as
universal hosts for blue, green and orange phosphors to give
Department of Chemistry, National Taiwan University, 10617, Taipei, Taiwan. their corresponding devices an external quantum efficiency
a
E-mail: kenwong@ntu.edu.tw
(hext) as high as 16.1%, 22.7%, and 20.5%, respectively. More-
over, uorene has also been adopted as an effective non-
b
Institute of Optoelectronic Sciences, National Taiwan Ocean University, 20224,
Keelung, Taiwan. E-mail: wenhung@mail.ntou.edu.tw
3
11
conjugated spacer through its sp -hybridized C9 atom. Our
previous research also demonstrated the successful utilization
of uorene as an effective non-conjugated bridge for D and A at
c
Institute of Atomic and Molecular Sciences, Academia Sinica, 10617, Taipei, Taiwan
†
Electronic supplementary information (ESI) available. CCDC 1007528 and
007529. For ESI and crystallographic data in CIF or other electronic format see
1
12
T
the C9- and C3-positions, respectively. The E for this series of
DOI: 10.1039/c4tc01463d
8554 | J. Mater. Chem. C, 2014, 2, 8554–8563
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bipolar hosts can be as high as 2.86 eV, rendering a remarkable coupling of 1 and N-phenylcarbazole-3-boronic ester (2) to
of 15.1%, 17.9%, and 20% for blue, green, and red PhO- afford 3 in 70% yield. Both PhczCBI and PhczNBI were
h
ext
LEDs, respectively. Another strategy to reduce electronic synthesized in 62% yields by Suzuki–Miyaura cross-coupling of
coupling between D and A is to incorporate a linker with a 3 and N-biphenylbenzimidazole boronic esters, 4 and 5,
highly twisted conformation to minimize the effective respectively. PhczDCBI was synthesized by Suzuki–Miyaura
p-conjugation. This can be achieved by introducing multiple cross-coupling of the boronic ester intermediate (7) and 2-(2-
ortho-connections between D and A to impart severe steric bromophenyl)-1-phenyl-1H-benzimidazole (8) in 73% yield, in
hindrances, which largely twist the molecule into non-coplanar which 7 was obtained in an overall yield of 76% by successive
manners. Such twisted molecular conformation is also bene- Suzuki–Miyaura cross-coupling starting from 2 and 1-bromo-2-
cial to the formation of amorphous thin lms with enhanced iodobenzene. Detailed synthetic procedures and the character-
morphological stabilities because of less accessible p-planes for ization data are summarized in the electronic supplementary
the formation of intermolecular p-stacking. For instance, Ma information (see ESI† in detail).
and Yang et al. recently reported a series of highly twisted
bipolar hosts based on a carbazole–phosphine oxide hybrid and Crystal structure
0
a 2,2 -biphenyl bridge. The twisted conformation renders an E
T
The crystal structures of PhczNBI and PhczCBI were obtained by
X-ray diffraction analysis of their single crystals obtained from
the slow evaporation of a CH Cl –hexane mixture. Unfortu-
2 2
as high as 3.01 eV, which is suitable for hosting blue phosphors.
A maximum hext of 19.5% was achieved for deep blue
PhOLEDs.
In this work, we adopted the advantages of the highly twisted
molecular conformation to design three new bipolar host
materials, PhczNBI, PhczCBI, and PhczDCBI, by introducing
proper D and A moieties onto the 2,2 -positions of a biphenyl
linker. Hole-transport carbazole and electron-transport benz-
imidazole have been judiciously selected as the D and A
moieties, respectively, due to their excellent carrier transport
properties and high E . The regioisomeric PhczNBI and
PhczCBI were designed with different linking topologies (N- and
C-connectivity of the benzimidazole, respectively) to explore
their structure–property–performance relationship with
different effective conjugations and E
have shown that the N-connected isomers possessed higher E
than those of their C-connected analogues. Moreover, the
analogous PhczDCBI was designed based on direct connectivity
of the benzimidazole to the biphenyl linker without the phenyl
13
nately, we were unable to grow a single crystal for PhczDCBI.
17
The relevant crystal data are summarized in Table S1 (ESI†).
Crystal structures shown in Fig. 1a and b demonstrate the
ortho-/ortho- biphenyl spacer successfully imposed a highly
twisted geometry to the molecules. The dihedral angles between
the N-phenylbenzimidazole/biphenyl linker and carbazole/
0
14
ꢁ
ꢁ
biphenyl linker were found to be as large as 45.6 and 55.2 for
ꢁ
ꢁ
PhczNBI, respectively, and 46.2 and 46.3 for PhczCBI,
respectively. The large dihedral angles between the phenyl rings
are benecial to reducing p-conjugation, as well as weakening
the electronic interaction, between D and A, which are impor-
T
tant factors to retain a high E . In addition, only weak inter-
T
T
. Our previous studies
molecular p/p stacking was observed in the crystals of
T
˚
PhczCBI (dp/p ¼ 3.5 A, Fig. 1d). No appreciable dispersive
15
interaction was observed for PhczNBI. The lack of intermolec-
ular interactions for PhczCBI and PhczNBI in the solid state
gives them the potential to render an amorphous character with
a high morphological stability in their vacuum-deposited lms.
spacer. PhczDCBI is expected to possess a higher E , which may
T
lead to much more efficient blue PhOLED devices. We investi-
gated the thermal, photophysical, electrochemical, and carrier-
transport properties of these three host materials with respect
to their molecular structures. Blue, green, yellow, and white
phosphorescent OLEDs were fabricated based on PhczNBI,
PhczCBI, and PhczDCBI. Among them, PhczDCBI revealed the
exceptionally high device efficiencies with low efficiency roll-off
with hext of 12.3%, 15.1%, 16.1%, and 13.9% for blue, green,
yellow, and white PhOLED devices, respectively. PhczNBI-hos-
ted yellow PhOLED also exhibited a remarkable hext of 19.6%.
These results strongly suggested the successful molecular
design strategy of bipolar hosts for highly efficient PhOLEDs.
Thermal properties
Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) were used to probe the decomposition
temperature (T , corresponding to 5% weight loss) and glass
d
transition temperature (T ) under a nitrogen atmosphere for
PhczCBI, PhczNBI and PhczDCBI (Table 1). All compounds
showed excellent morphological and thermal stabilities in
terms of their high T
and from 346 to 355 C, respectively. The high T
of these compounds indicate that they are capable of enduring
not only vacuum thermal sublimation, a general requirement
for PhOLED fabrication, but also the inevitable joule heating
that would occur during device operation.
g
ꢁ
g
ꢁ
and T
d
values ranging from 123 to 130 C
g
and T
d
values
Results and discussion
Synthesis
Photophysical properties
Scheme 1 shows the synthetic pathways and structures of the
highly twisted bipolar hosts, PhczCBI, PhczNBI, and PhczDCBI. Fig. 2 shows the UV-vis absorption and photoluminescence (PL)
0
The precursor 2-bromo-2 -iodobiphenyl (1) was prepared in 67% spectra of PhczNBI, PhczCBI, and PhczDCBI recorded at room
yield via aryl-aryl coupling based on the corresponding orga- temperature in CH Cl and vacuum-deposited neat lms along
2
2
16
nolithium reagents according to the literature. Then, N-phe- with their phosphorescence spectra in neat lms at 10 K, and
nylcarbazole was introduced by selective Suzuki–Miyaura cross- the data are summarized in Table 1. All compounds exhibited
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Scheme 1 Synthesis of PhczCBI, PhczNBI and PhczDCBI.
18
similar absorption proles in the absorption spectra. We charge transfer state (CT), respectively. Therefore, we attrib-
assigned the absorption peaks near 290 nm to the p–p* tran- uted the tendency of the lowest excited state, whether being
sition of the N-phenylcarbazole moiety, whereas the smaller located in LE or CT states, to the propensity of charge transfer,
hump near 340 nm is the characteristic n–p* transition of which directly relates to the effectiveness of electronic coupling
19
carbazole. For PL in solutions, PhczNBI and PhczCBI exhibited between D and A in the excited states. Since both PhczNBI and
much different emission characteristics. A structured emission PhczCBI exhibited highly twisted conformations in solution, as
spectrum centred at 360 nm was observed for PhczNBI, whereas well as in their crystal structures, the charge transfer is less
PhczCBI showed a red-shied structureless emission at 420 nm. likely to happen by a series of electron hopping through bonds.
Apparently, the different emission spectra between PhczNBI Therefore, the charge transfer process can only occur in a
and PhczCBI indicate different behaviors for their lowest energy through-space manner when the D and A are close to each
excited states. The similar photoluminescent features were also other. The crystal structures of PhczNBI and PhczCBI show that
reported in a series of bipolar compounds, in which the struc- the carbazole and benzimidazole moieties are close to each
tured and structureless emissions were assigned to the emis- other in different manners. In PhczCBI, the parallel alignment
sion from the locally (Franck–Condon) excited state (LE) and of the D and A planes indicates a better conguration with a
8556 | J. Mater. Chem. C, 2014, 2, 8554–8563
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Fig. 1 X-ray structures of (a) PhczNBI and (b) PhczCBI with their
respective intermolecular stacking [(c) and (d)]. The hydrogen atoms
are omitted for clarity.
Fig. 2 Room-temperature absorption and emission (PL) spectra of
PhczNBI, PhczCBI and PhczDCBI in CH Cl solution and in neat films,
2 2
as well as the corresponding phosphorescence (Phos) spectra,
recorded from their neat films at 10 K.
shorter D–A distance, which benets the D–A interactions,
resulting in the evident CT state emission. The strong CT
behaviour in the PL of PhczCBI was further veried with evident
solvatochromism (Fig. S1†), in which the PL emission
maximum was red-shied from 397 nm in toluene to 439 nm in
acetonitrile along with a lower PL intensity. In contrast, in
addition to the highly twisted conformation, the perpendicular
arrangement of the carbazole and benzimidazole moieties
found in PhczNBI crystals impedes the charge transfer process.
Therefore, the lowest excited state of PhczNBI is purely a LE
state, which shows negligible solvent polarity-dependence as
shown in Fig. S1.† Interestingly, without the extra phenylene
linker between the biphenyl bridge and benzimidazole,
PhczDCBI exhibits an overlapped emission spectrum
comprising of a structured emission (ꢂ360 nm) and a broad
emission centred around 420 nm. The observation of dual
emissions from both the LE and CT states for PhczDCBI is
presumably due to a different molecular conformation, in
which the D–A alignment in PhczDCBI is speculated to lie at a
marginal boundary where the charge transfer is less efficient
than that of PhczCBI but more efficient than that of PhczNBI,
resulting in a reversible excited-state two-step reaction (i.e., LE
of PhczDCBI as shown in Fig. S1† also conrms the PL of
PhczDCBI has a combined LE and CT character, in which the LE
emission (ꢂ360 nm) is not solvent polarity-dependent, whereas
the CT emission shis to longer wavelengths with the increase
of solvent polarity. In thin lms, both PhczNBI and PhczDCBI
showed increased ratios (CT to LE) of their emission compo-
nents. The increased CT components in solid-state emissions
are attributed to the higher charge transfer propensity in the
excited state presumably due to the shorter intermolecular D–A
distances and restricted conformational change in the solid
states. The photoluminescence quantum yields (FPL
) of
PhczNBI, PhczCBI and PhczDCBI neat lms were measured
with an integration sphere to be 0.22, 0.67, and 0.15, respec-
tively. Since these new bipolar molecules are designed as host
materials for PhOLEDs, from a practical consideration, the
phosphorescent spectra of PhczNBI, PhczCBI and PhczDCBI
were recorded in neat lms at 10 K (Fig. 2) instead of measuring
in solution. As depicted in Fig. 2, the phosphorescence of
PhczCBI is much stronger than those of PhczNBI and
4
CT). This result is consistent with the observation of over-
lapped LE and CT emissions for molecules possessing a twisted
20
geometry reported by Ma et al. The solvatochromic behaviour
PhczDCBI, the triplet energies (E
T
) of PhczNBI, PhczCBI and
Table 1 Physical properties of PhczNBI, PhczCBI and PhczDCBI
a
b
c
g
d
f
T
(
g
ꢁ
/T
C)
/T
c m
T
( C)
d
ꢁ
E
(V)
ox/Ered
HOMO/LUMO
(eV)
HOMO/LUMO, DE
(eV)
E
T
l
abs (nm)
l
PL (nm)
F
PL
(eV)
sol./lm
sol./lm
lm
PhczNBI
PhczCBI
PhczDCBI
124/199/270 e
130/n.d./n.d.
123/n.d./n.d.
346
355
354
1.33/ꢀ2.10
1.28/ꢀ2.01
1.32/ꢀ2.20
ꢀ5.65/ꢀ2.16
ꢀ5.63/ꢀ2.28
ꢀ5.68/ꢀ2.10
ꢀ5.82/ꢀ2.40, 3.42
ꢀ5.75/ꢀ2.35, 3.40
ꢀ5.65/ꢀ2.21, 3.44
2.71
2.55
2.70
290/289
282/300
288/290
359, 375/362, 383
420/403
357, 376/364, 418
0.22
0.67
0.15
a
E
ox and Ered were estimated from the half-wave and onset potential in the oxidation and reduction scan of the cyclic voltammograms, respectively.
b
+
c
Calculated from the electrochemical results with respect to the Fc/Fc redox couple. HOMO was determined using photoelectron yield
d
spectroscopy (AC-2). LUMO ¼ HOMO + E
g g
, where E was calculated from the absorption onset of the solid lm. Estimated from onset of the
e
f
phosphorescence spectra of neat lm at 10 K. n.d.: not detected. Photoluminescent quantum efficiency of neat lm.
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PhczDCBI were determined to be 2.71, 2.55, and 2.70 eV,
respectively, estimated from the onset of the phosphorescence
spectra. The E values of PhczCBI, PhczNBI and PhczDCBI are
T
sufficiently high to host a wide range of phosphorescent emit-
ters as a result of the effective prevention of reverse triplet
energy transfer.
Electrochemical properties
The electrochemical behaviours of PhczCBI, PhczNBI, and
PhczDCBI were investigated by cyclic voltammetry (CV)
1/2
Fig. 4 Electron and hole mobilities versus E for PhczNBI, PhczCBI
measurements, in which tetra(n-butyl)ammonium hexa- and PhczDCBI.
uorophosphate (TBAPF ) in CH Cl and tetra(n-butyl)ammo-

6
2
2
nium perchlorate (TBAP) in DMF as supporting electrolytes
were used for the oxidation and reduction scans, respectively
energy levels obtained from the solid state are different as
compared to those determined from CV, presumably due to the
complicated intermolecular interactions.
(Fig. 3). Table 1 summarizes the measured redox potentials. All
three molecules exhibited one quasi-reversible oxidation and an
irreversible reduction, arising from their N-phenylcarbazole and
N-phenylbenzimidazole segments, respectively. The new peak
that appeared at around 1.0 V for the rst oxidation scan was Charge carrier mobility
due to the dimerization at the C6 position of the carbazole
To further understand the charge-carrier transport properties, we
used the time-of-ight (TOF) technique to evaluate the carrier
mobilities. Representative TOF transients for holes and electrons
of PhczNBI, PhczCBI, and PhczDCBI are shown in Fig. S2.† Fig. 4
shows the obtained carrier mobilities, shown as a function of the
square root of the electric eld. The hole mobilities varied within
21
moiety. Obviously, the reduction onset of PhczDCBI is at the
highest potential (ca. ꢀ2.20 V) as compared to those of PhczNBI
and PhczCBI, consistent with the reduced p-conjugation of the
benzimidazole moiety due to its highly twisted conformation.
Likewise, PhczNBI possesses slightly higher redox potentials
than those of PhczCBI due to the efficient interruption of
p-conjugation through the N-connection. Referring to the redox
22
ꢀ6
ꢀ5
2
ꢀ1 ꢀ1
the range from 4.4 ꢃ 10 to 7.8 ꢃ 10 cm V
s
for elds
5
6
ꢀ1
ꢀ6
varying from 3.8 ꢃ 10 to 1.2 ꢃ 10 V cm and the electron
+
of ferrocene/ferrocenium (Fc/Fc ), the HOMO and LUMO levels
ꢀ4
ꢀ1
2
ꢀ1 ꢀ1
mobilities fell within the range from 6.3 ꢃ 10 to 10 cm V
s
of PhczCBI, PhczNBI, and PhczDCBI were determined from the
half-wave and onset of the oxidation and reduction potentials,
respectively. The data are summarized in Table 1. However, for
the practical applications of PhczCBI, PhczNBI, and PhczDCBI
in their solid lms, we employed photoelectron yield spectros-
copy (Riken AC-2) to determine the HOMO energy levels of
PhczCBI, PhczNBI, and PhczDCBI lms to be ꢀ5.75, ꢀ5.82, and
5
5
for elds varying from 2.9 ꢃ 10 to 9 ꢃ 10 V cm . Apparently,
N-phenylbenzimidazole linking topologies, i.e., C- or N-
connections, with the ortho-substituted biphenyl linker have a
crucial inuence on the charge transport behavior. Both hole-
and electron-mobilities of PhczNBI are higher than that of
PhczCBI. In contrast, the transient photocurrent signals of
PhczDCBI for electrons were too weak for us to evaluate electron
mobility using the TOF technique. To assess the electron
transport properties, we also fabricated electron-only devices,
having a device structure of ITO/BCP (30 nm)/compounds
ꢀ
5.65 eV, respectively. The LUMO energy levels were calculated
from the HOMO energy levels using the equation of LUMO ¼
HOMO + E , where E is the optical energy gap determined from
g
g
the onset wavelength of the lm absorption band. The data are
summarized in Table 1. Obviously, the HOMO and LUMO
(50 nm)/TPBI (20 nm)/LiF/Al, as shown in Fig. 5. Here, low-
HOMO 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP, m
e
ꢀ
7
2
ꢀ1 ꢀ1
23
of 10 cm V
s
order) served as the hole-blocking layer to
Fig. 3 Cyclic voltammograms of PhczNBI, PhczCBI and PhczDCBI.
Oxidation scan was performed in CH
reduction scan was performed in DMF with 0.1 M of nBu
2
Cl
2
with 0.1 M of nBu
4
NPF
6
and
as a
4
NClO
4
supporting electrolyte. A glassy carbon electrode was used as the Fig. 5 Current density–voltage (J–V) characteristics of electron-only
ꢀ1
working electrode; scan rate 300 mV s
.
devices.
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impede the hole carriers in the electron-only devices and 1,3,5-
ꢀ
5
2
tris(N-phenylbenzimidazol-2-yl)benzene (TPBI, m of 10 cm
e
ꢀ1
ꢀ1
24
V
s
order) was used as the electron-transport layer. Except
for the energy barrier at the interface, the magnitude of the
current density is PhczNBI > PhczCBI > PhczDCBI, which clearly
follows the result observed by the TOF mobility.
Prior to the device fabrication, the photophysical properties
of 10 wt% FIrpic-doped host lms (100 nm) were evaluated. The
transient PL decay curve of FIrpic hosted by PhczNBI and
PhczDCBI lms exhibited almost a single-exponential decay
with a lifetime of 1.31 ms and 1.54 ms, respectively, which indi-
cates the effective connement of triplet excitons of FIrpic (see
ESI in Fig. S3†). In contrast, the PhczCBI-hosted FIrpic lm
exhibited a two-component decay, i.e., a fast (1.40 ms) decay
process and a slow (30 ms) decay process. The latter can be
attributed to thermally activated triplet–triplet energy transfer
between the host and the guest, such as that in the CBP/FIrpic
0
0
25
system (CBP is N,N -dicarbazolyl-4,4 -biphenyl).
Electroluminescent properties
To investigate PhczNBI, PhczCBI, and PhczDCBI as bipolar host
materials, we fabricated PhOLED devices with three different
dopants. We selected iridium(III)[bis(4,6-diuorophenyl)-pyr-
2
0
26
idinato-N,C ]picolinate (FIrpic),
dium(III) acetylacetonate [(PPy)
phenylbenzothiazolato) (acetylacetonate)iridium(III) [(Bt)
bis(2-phenylpyridinato)iri-
1b
2
Ir(acac)],
and bis(2-
2
-
27
Ir(acac)] as blue, green, and yellow dopants, respectively
Scheme 2). The devices were fabricated with a common struc-
(
Fig. 6 (a) Current density–voltage–luminance (J–V–L) characteris-
ture of ITO/PEDOT:PSS (30 nm)/DTAF (20 nm)/TCTA (5 nm)/ tics. (b) External quantum (hext) and power efficiencies (h ) as a func-
P
host: 10 wt% dopant (25 nm)/DPPS (50 nm)/LiF (0.5 nm)/Al tion of brightness. (c) EL spectra of PhczNBI-based devices.
(
100 nm). To improve the hole injection from the anode,
poly(3,4-ethylene-dioxythiophene):poly(styrene sulfonic acid)
T
PEDOT:PSS) was spun onto the precleaned ITO substrate to (DPPS) layer with a high triplet level (E : 2.7 eV, HOMO/LUMO:
3
f
(
form a polymer buffer layer. Two hole-transport layers (HTLs), 6.5/2.5 eV) was selected as the electron-transport layer (ETL) to
consisting of a 20 nm-thick layer of 9,9-di[4-(di-p-tolyl)amino- conne the excitons within the EML. LiF and Al served as the
28
phenyl]uorene (DTAF, 20 nm) and a 5 nm-thick layer of electron-injection layer and cathode, respectively.
0
00
29
4
,4 ,4 -tri(N-carbazolyl)triphenylamine (TCTA) were imple-
mented. Next, a 25 nm-thick emissive layer, which consisted of L) characteristics, device efficiencies, and EL spectra of the
0 wt% phosphors, was doped into the host material. To further devices. The notation 1, 2 and 3 indicates the devices fabricated
Fig. 6–8 depicts the current density–voltage–luminance (J–V–
1
conne the holes or generated excitons within the emissive with host materials of PhczNBI, PhczCBI, and PhczDCBI,
layer, a 50 nm-thick diphenylbis[4-(pyridin-3-yl)phenyl]silane respectively. FIrpic was incorporated as the blue phosphor to
Scheme 2 Molecular structures used in this study and an energy level diagram of the device.
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Paper
Fig. 8 (a) Current density–voltage–luminance (J–V–L) characteris-
tics. (b) External quantum (hext) and power efficiencies (h ) as a func-
P
tion of brightness. (c) EL spectra of PhczDCBI-based devices.
Fig. 7 (a) Current density–voltage–luminance (J–V–L) characteris-
tics. (b) External quantum (hext) and power efficiencies (h ) as a func-
P
tion of brightness. (c) EL spectra of PhczCBI-based devices.
fabricate the blue PhOLEDs named B1–B3. Analogously, G1–G3,
In contrast, PhczDCBI with a reduced phenyl linker exhibi-
Y1–Y3, and W1–W3 were the green, yellow, and white PhOLEDs, ted the best blue EL performance. Device B3 based on PhczDCBI
ꢀ2
respectively. Table 2 summarizes the obtained electrolumines- showed a maximum brightness (Lmax) of 25 800 cd m at 14.5 V
ꢀ
2
cence data. The J–V characteristics of the devices revealed (680 mA cm ) and a maximum external quantum efficiency
the electrical properties of the EMLs as a combination of hosts (hext) of 12.3%, corresponding to a current efficiency (h ) of 24.5
c
ꢀ
1
ꢀ1
and dopants that inuence the charge carrier transport cd A and a power efficiency (h ) of 20.6 lm W with CIE
p
ꢀ2
behaviour.
coordinates of (0.15, 0.33). At a high brightness of 1000 cd m ,
The blue emission device B1 hosted by PhczNBI revealed a the hext of devices B1 and B3 are still as high as 7.9% and 11.6%,
ꢀ
2
maximum brightness (Lmax) of 210 00 cd m
(
at 12.5 V respectively, showing low efficiency roll-off (Table 2). However,
ꢀ2
690 mA cm ) with CIE coordinates of (0.17, 0.36). The the blue device performances are moderate among FIrpic-based
maximum external quantum (hext), current (h
c
), and power OLEDs presumably because the E
T
s of PhczNBI (2.70 eV) and
ꢀ1
ꢀ1
efficiencies (h ) were 12.2%, 24.0 cd A , and 20.3 lm W
,
PhczDCBI (2.71 eV) are not sufficiently high for blocking the
p
respectively, which are signicantly higher than those for the thermal activated exciton diffusion process. To further verify the
ꢀ
1
ꢀ1
PhczCBI based device B2 (4.5%, 9.9 cd A , and 7.2 lm W
with CIE coordinates of (0.16, 0.39). Apparently, PhczCBI lms with FIrpic (10 wt%) doped in PhczNBI, PhczDCBI and
possesses a relatively low triplet energy (E ow using an integrating
)
energy transfer behaviour of the host, FPL characteristics of
T
¼ 2.55 eV) that mCP were investigated under a N
2
leads to the inefficient connement of blue excitons, which sphere equipped with a multichannel spectrometer as the
was veried by the observation of a slow decay process in the optical detector (see ESI in Table S2†). The FPL of FIrpic/mCP
transient PL decay characteristics of a FIrpic (10 wt%) doped lm is 0.95, which is higher than those of PhczNBI (0.71) and
PhczCBI lm (Fig. S3†). In addition, the EL spectrum (Fig. 7c) PhczDCBI (0.73) hosted lms (see ESI in Table S2†). The hext of
of device B2 displayed a weak emission at ca. 400 nm, which device B3 was similar with that of device B1, which is consistent
was assigned to the emission from PhczCBI due to the highly with the observation of FPL
.
emissive character (FPL ¼ 0.67) and the incomplete energy
To further evaluate the suitability of these new compounds
as host materials for low energy triplet emitters, we also
transfer from PhczCBI to FIrpic.
8560 | J. Mater. Chem. C, 2014, 2, 8554–8563
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Table 2 EL performance of devices incorporating various hosts and emitter
a
V
[V]
on
At 1000 cd
L
max
I
max
h
ext
h
c
h
p
CIE
[x, y]
ꢀ
2
ꢀ2
ꢀ2
ꢀ1
ꢀ1
Host: 10% dopant
m
[V,%]
[cd m
]
[mA cm
]
[%]
[cd A
]
[lm W
]
B1
G1
Y1
W1
B2
G2
Y2
W2
B3
G3
Y3
PhczNBI:FIrpic
3.0
2.5
2.5
3.0
3.0
2.5
2.5
3.0
3.0
2.5
2.5
2.5
7, 10.0
6, 16.1
6.6, 18.7
7, 13.2
10, 1.8
6, 17.9
5.8, 17.5
9, 5
8.2, 11.6
6, 15.0
6.5, 15.7
6.8, 13.8
21 000 (12.5 V)
104 300 (14.5 V)
121 700 (15 V)
34 800 (14.5 V)
5700 (15 V)
158 600 (14 V)
131 900 (13.5 V)
11 500 (15.5 V)
25 800 (14.5 V)
166 000 (13.5 V)
100 200 (14.5 V)
57 100 (13.5 V)
690
690
810
650
720
970
1000
730
680
990
920
860
12.2
16.1
19.6
13.4
4.5
18.7
18.7
7.7
12.3
15.1
16.1
13.9
24.0
58.1
51
33.4
9.9
20.3
48.4
43.1
21
0.17, 0.36
0.33, 0.63
0.52, 0.47
0.35, 0.43
0.16, 0.39
0.33, 0.62
0.52, 0.47
0.34, 0.45
0.15, 0.33
0.33, 0.62
0.52, 0.48
0.36, 0.43
2
PhczNBI:PPy Iracac
PhczNBI:Bt Iracac
2
PhczNBI: 10%FIrpic: 0.3% Bt
PhczCBI:FIrpic
2
Iracac
7.2
PhczCBI:PPy
2
Iracac
69.2
47
66.3
42.2
12.6
20.6
43.2
34.9
24.8
PhczCBI:Bt Iracac
2
PhczCBI: 10%FIrpic: 0.2% Bt
PhczDCBI:FIrpic
2
Iracac
19.8
24.5
54.6
42.6
34.9
PhczDCBI:PPy
PhczDCBI:Bt Iracac
2
PhczDCBI: 10%FIrpic: 1% Bt Iracac
2
Iracac
2
W3
a
ꢀ2
ꢀ2
Turn-on voltage at which emission became detectable (10 cd m ). The notation 1, 2 and 3 indicate the devices fabricated with host materials of
PhczNBI, PhczCBI, and PhczDCBI, respectively, in the conguration: ITO/PEDOT:PSS/DTAF (20 nm)/TCTA (5 nm)/emitter (25 nm)/DPPS (50 nm)/
LiF/Al.
attempted to evaluate green and yellow PhOLEDs using phos- when the voltage increases from 7 to 11 V, the CIE coordinates
phorescent dopants (PPy) Ir(acac) and (Bt) Ir(acac), respec- slightly vary from (0.37, 0.43) to (0.34, 0.42) for device W3, and
2
2
tively. These devices achieved better performances as compared the color-rendering indices (CRI) was shied from 64.8 to 65.5.
to those of blue devices (Table 2). All devices exhibited a high This is due to the low concentration of (Bt) Ir(acac) in the blend,
performance with a rather low efficiency roll-off at high which results in the partial saturation of emission from (Bt)
2
2
-
brightness. For instance, the green device G2 with PhczCBI as Ir(acac) and the easier formation of the blue emission at higher
ꢀ
2
31
the host achieved a Lmax of 158 600 cd m
at 14 V and voltages.
ꢀ
1
ꢀ1
respectable EL efficiencies (18.7%, 69.2 cd A , and 66.3 lm W
)
ꢀ
2
with a hext of 17.9% at a brightness of 1000 cd m ; however, the
Conclusion
yellow device Y1 with PhczNBI as the host achieved a L
21 700 cd m at 15 V and respectable EL efficiencies (19.6%, We have successfully designed and synthesized three bipolar
1 cd A , and 43.1 lm W ) with a hext of 18.7% at a brightness host materials PhczNBI, PhczCBI, and PhczDCBI, in which the
of 1000 cd m . The remarkably reduced efficiency roll-off is the donor 9-phenylcarbazole and the acceptor benzimidazole were
of
max
ꢀ
2
1
5
ꢀ1
ꢀ1
ꢀ
2
0
combined result of the characteristics of the hosts and suitable bridged through a 2,2 -biphenyl linker. The ortho–ortho linkage
device conguration, which leads to a balanced carrier injection imparts to these bipolar hosts highly twisted molecular
and transport in EMLs as well as facilitates the uniform distri- conformations, which not only benets the morphological
bution of carriers and excitons to reduce the eld-assisted stability but also endows sufficiently high triplet levels for
9,10,30
exciton dissociation.
All devices (except for device B2 hosting a wide range of phosphorescent dopants. Different
hosted by PhczCBI) displayed relatively pure emissions from linking topologies (C- or N-connectivity) between the benz-
blue to yellow as shown in Fig. 6c–8c, indicating that the elec- imidazole group and the biphenyl linker resulted in different
troluminescence can be completely conned on the emissive effective p-conjugations and different E . All three bipolar hosts
T
dopants.
We also fabricated two-emitter white OLEDs (WOLEDs) balanced carrier transport characteristics. Consequently, highly
devices W1–W3) utilizing 10 wt% FIrpic and 0.2–1 wt% (Bt)
efficient blue, green, yellow, and white PhOLEDs based on these
exhibited high morphological and thermal stabilities as well as
(
2
-
Ir(acac) co-doped into the hosts as a single emitting layer. The host materials were fabricated. Among the three host materials,
proportion of (Bt) Ir(acac) required to produce a balanced white PhczDCBI as a host gave the highest device efficiencies for the
2
ꢀ
1
ꢀ1
emission in these devices was relatively low because the yellow blue- and white-emitting devices (12.3%, 24.5 cd A , 20.6 lm W
ꢀ
1
ꢀ1
emission is either from efficient energy transfer of the blue for device B3 and 13.9%, 34.9 cd A , and 24.8 lm W for device
phosphor or the direct exciton formation through charge-trap- W3, respectively) with a low efficiency roll-off. We believe that
ping on the yellow dopant. Device W3 hosted by PhczDCBI the molecular design approach reported in this work with
exhibited the best performance, which achieved a Lmax of 57 100 tailored donor/acceptor and bridge moieties can lead to suitable
ꢀ
2
cd m at 13.5 V and respectable EL efficiencies (13.9%, 34.9 cd host materials, rendering high efficiency PhOLEDs feasible.
ꢀ1
ꢀ1
A
, and 24.8 lm W ). The efficiencies of device W3 were
higher than those of devices W1 and W2 because of a higher
ratio of blue emission in device W3. All WOLEDs exhibited a
Acknowledgements
voltage-dependent EL spectrum. As the operation voltage This study was supported nancially by the Ministry of Science
increased, the relative intensity of blue emission increased, and Technology, Taiwan (MOST 100-2112-M-019-002-MY3, 101-
which led to a slight shi of the CIE coordinates. For example, 2113-M-002-009-MY3).
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1
3 C. Fan, F. Zhao, P. Gan, S. Yang, T. Liu, C. Zhong, D. Ma,
J. Qin and C. Yang, Chem.–Eur. J., 2012, 18, 5510.
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