A rational molecular design on choosing suitable spacer for better host materials in highly efficient blue and white phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1016/j.orgel.2014.03.028

A series of host materials, 3,3′-linked carbazole-based molecules have been designed with phenyl and biphenyl spacers. Their optical and electrical properties can be fine-tuning by the spacers. Their HOMO energy levels depend on HOMO distributions within the range of -5.64 to -5.96 eV. On the other hand, the three compounds have similar LUMO energy levels and triplet energies. Their thermal, photophysical, electrochemical and carrier mobilities properties were also systematically investigated. The relationship between the molecular structures and optoelectronic properties are discussed. A blue PHOLED device incorporating PBCz achieved a maximum external quantum efficiency, current efficiency, and power efficiency of 19.5%, 45.5 cd/A and 43.8 lm/W, respectively. Moreover a two-color, all-phosphor and single-emitting-layer WOLED hosted by PBCz was also achieved with a maximum external quantum efficiency, current efficiency and power efficiency of 24.6%, 76.3 cd/A and 69.4 lm/W respectively. Furthermore, we also utilized this versatile host for three-component RGB white PHOLEDs and show excellent performance. For example, combination of PBCz with FIrpic, Ir(ppy)₂(acac) and Ir(MDQ) ₂(acac) in the active layer, the resulting WOLEDs showed three evenly separated peaks and gave a high efficiency of 49.2 cd/A. The efficient PHOLEDs demonstrated that the versatile host PBCz has great potential for applications in the solid-state lighting.

Full text of DOI:10.1016/j.orgel.2014.03.028

Organic Electronics 15 (2014) 1368–1377
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
A rational molecular design on choosing suitable spacer
for better host materials in highly efficient blue and white
phosphorescent organic light-emitting diodes
⇑
⇑
Lin-Song Cui, Yuan Liu, Qian Li, Zuo-Quan Jiang , Liang-Sheng Liao
Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Collaborative Innovation
Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu 215123, China
a r t i c l e i n f o
a b s t r a c t
A series of host materials, 3,30-linked carbazole-based molecules have been designed with
phenyl and biphenyl spacers. Their optical and electrical properties can be fine-tuning by
the spacers. Their HOMO energy levels depend on HOMO distributions within the range of
ꢀ5.64 to ꢀ5.96 eV. On the other hand, the three compounds have similar LUMO energy lev-
els and triplet energies. Their thermal, photophysical, electrochemical and carrier mobili-
ties properties were also systematically investigated. The relationship between the
molecular structures and optoelectronic properties are discussed. A blue PHOLED device
incorporating PBCz achieved a maximum external quantum efficiency, current efficiency,
and power efficiency of 19.5%, 45.5 cd/A and 43.8 lm/W, respectively. Moreover a two-
color, all-phosphor and single-emitting-layer WOLED hosted by PBCz was also achieved
with a maximum external quantum efficiency, current efficiency and power efficiency of
24.6%, 76.3 cd/A and 69.4 lm/W respectively. Furthermore, we also utilized this versatile
host for three-component RGB white PHOLEDs and show excellent performance. For exam-
ple, combination of PBCz with FIrpic, Ir(ppy)2(acac) and Ir(MDQ)2(acac) in the active layer,
the resulting WOLEDs showed three evenly separated peaks and gave a high efficiency of
49.2 cd/A. The efficient PHOLEDs demonstrated that the versatile host PBCz has great
potential for applications in the solid-state lighting.
Article history:
Received 25 January 2014
Received in revised form 26 February 2014
Accepted 18 March 2014
Available online 16 April 2014
Keywords:
Organic light-emitting diodes
White
Host materials
Carbazole
Spacer
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
solar cells (DSCs) etc. by using carbazole derivatives
[3–5]. One of the significant applications of carbazole is
With the emergence of organic semiconductor in the
prosperous optoelectronic research, carbazole, a tricycle
heteroaromatic compound, successfully catch the chem-
ists’ eyesight for its electron-active conjugated ring and
the electron-donating property [1,2]. To date, considerable
work has been studied on organic light-emitting diodes
(OLEDs), organic photovoltaics (OPVs) or dye-sensitized
designing host materials in phosphorescent OLEDs, espe-
cially when the PHOLEDs are considered as the solution
of low efficient fluorescent OLEDs, because it could achieve
theoretical 100% internal quantum efficiency by utilizing
both electrogenerated singlet and triplet excitons for emis-
sion [6–13].
The host materials, as the majority component, are
doped with phosphor emitters (usually heavy-metal com-
plexes, Ir, Pt or Os etc.) in the emitting layer of a prototyp-
ical PHOLED [14–16]. Moreover, the white PHOLEDs based
on a versatile host doped with different color dopants have
⇑
Corresponding authors. Tel.: +86 512 65880945; fax: +86 65882846.
E-mail addresses: zqjiang@suda.edu.cn (Z.-Q. Jiang), lsliao@suda.
edu.cn (L.-S. Liao).
http://dx.doi.org/10.1016/j.orgel.2014.03.028
1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

L.-S. Cui et al. / Organic Electronics 15 (2014) 1368–1377
1369
attracted great attention because of its relatively simple
and more cost effective fabrication processes [17–24]. On
the other hand, the host–guest system is indispensable to
avoid triplet polaron quenching (TPQ) and triplet–triplet
annihilation (TTA) and realize the improvement of light-
emitting quantum yields via efficient energy-transfer
[25–29]. In this research field, carbazole–based materials
can easily fulfill the basic requirements of suitable hosts,
such as: (1) higher triplet energy (ET), (2) matched
HOMO/LUMO levels, (3) good thermal stability, and (4)
good carrier mobility [30–37].
For the green or red phosphorescence, many carbazole-
based hosts can qualify for good performance [38,39]. But
for blue phosphorescence, the selection for good hosts
becomes much more limited, and need more rational
molecular design on how to find the balance between the
high triplet energy (above 2.70 eV) and other require-
ments. Nevertheless, the combination of functional groups
PHOLEDs and show excellent performance. Upon combina-
tion of PBCz with FIrpic, Ir(ppy)2(acac) and Ir(MDQ)2(acac)
in the active layer, the resulting WOLEDs showed three
evenly separated peaks and gave a high efficiency of
49.2 cd/A (46.5 lm/W, 19.1%). The RGB WOLEDs demon-
strated high efficiency and proper CIE coordinates for so-
lid-state lighting.
2. Results and discussions
2.1. Synthesis and characterization
The synthetic routes and chemical structures of the
three hosts are shown in Scheme 1. The new host PBCz
was facilely prepared through classic Suzuki–Miyaura
cross coupling reaction from N-phenylcarbazole-3-boronic
acid with 1,3-dibromobenzene in high yields. The start
material 1,3-dibromobenzene is easier to attainable and
could reduce the process time and cost of PBCz. After-
wards, PBCz was further purified by repeated temperature
gradient vacuum sublimation. Its chemical structure was
fully characterized by 1H NMR and 13C NMR spectrosco-
pies, mass spectrometry and elemental analysis. The
detailed synthetic methods and analysis are described in
the Experiment Section.
often leads to
p-conjugation enlargement of host
materials, which would accordingly reduce their triplet
energies [40]. Hence, to obtain high triplet energies for
host materials, the
p-conjugation in the host molecules
must be minimized. The design strategies such as using
meta or ortho-linkage ways, non-conjugation building
blocks or steric groups to limit the p-conjugation are usu-
ally employed [41–47]. As to the factually basic building
block of N-phenyl-9H-carbazole (PCz), there are several
linking ways to expand the molecular system to be an
appropriate host material. Recently, we have done a series
of systematic work to disclose the relationship between
the molecular topology and physical properties of the host
material to develop better high triplet host materials for
blue and white PHOLEDs. The three simple isomers (CTP-
1, CTP-2 and CTP-3) by using one biphenyl group to sepa-
rate two PCz groups with different linkage ways [48]. Our
method is based on the fact that appending one more phe-
nyl ring from the 3-position of the carbazole will not sig-
nificantly reduce its triplet energy, and furthermore, it
may bring better hole-mobility because of the longer
HOMO distribution. Though, the CTP-1 is a good host
material as compared to its isomers and other traditional
carbazole-based hosts. We believe that it can be further
improved by carefully modifying the structure by contin-
uing our design rule.
In this paper, we would like to develop better host
material by reinvestigating the HOMO and LUMO distribu-
tion of CTP-1, and on this basis, we proposed that using
one phenyl ring to replace biphenyl ring may be a better
choice because of more suitable HOMO/LUMO level and
higher carrier mobility. The resulted new material
1,3-bis(9-phenyl-9H-carbazol-3-yl) benzene (PBCz), was
synthesized by one-step reaction and used in PHOLEDs as
host for blue and white emission. When using PBCz as host
and FIrpic and PO-01 as dopants, the maximum power effi-
ciencies can reach 43.8 and 69.4 lm/W in the blue and
warm white PHOLEDs, which are higher than those in the
corresponding BCzPh and CTP-1-based devices. Although
high in efficiency, these devices show low color rendering
index and poor coordinates, which is insufficient for illu-
mination sources. To solve this problem, we also utilized
this versatile host for three-component RGB white
2.2. Thermal properties
The thermal property of PBCz was investigated through
thermogravimetric analysis (TGA) and differential scan-
ning calorimetry (DSC) and the data of BCzPh and CTP-1
are also presented for comparison, which are summarized
in Table 1. As shown in Fig. 1, the PBCz exhibited clear
glass transition under heating at 115 °C, which is 13 °C
higher than that of BCzPh and 8 °C lower than that of
CTP-1. The decomposition temperature (Td) with 5% loss
is estimated to be 409 °C (Fig. S1). This suggests that PBCz
could form morphologically stable and uniform amor-
phous films by vacuum deposition for OLED fabrication.
2.3. Photophysical properties
Fig. 2 show the UV–Vis absorption and fluorescence (PL)
spectra of BCzPh, PBCz and CTP-1 in dilute CH2Cl2 solu-
tion, as well as the phosphorescence (Phos) spectra mea-
sured in
77 K. The absorption around the 270–300 nm of the three
materials are associated with the
pꢁ transitions of
a frozen 2-methyltetrahydrofuran matrix at
pꢀ
the carbazole units and the absorption in the range of
325–355 nm can be assigned to the n–pꢁ electron transi-
tion of the entire conjugated backbone. Upon photoexcita-
tion at the absorption maximums at room temperature,
the fluorescent spectra of PBCz and CTP-1 are nearly iden-
tical to each other, but different from the spectrum of
BCzPh. The emission peaks of PBCz and CTP-1 exhibit
ꢂ30 nm hypochromatic shift as compared to that of
BCzPh. This assignment implied that inserting one
phenyl or biphenyl can trail off the interaction between
the two PCz units. The ET of BCzPh and CTP-1 were
determined by the highest energy vibronic band of the

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L.-S. Cui et al. / Organic Electronics 15 (2014) 1368–1377
Scheme 1. Molecular structures and synthesis of BCzPh, PBCz and CTP-1ꢂ3.
Table 1
Physical Properties of BCzPh, PBCz and CTP-1.
HOMO/LUMOf (eV)
Hole mobilityg (cm2 Vꢀ1 Sꢀ1
)
a
b
b
c
d
e
Compound
Tga(°C)
Td (°C)
kabs (nm)
kem (nm)
kph (nm)
Eg (eV)
ET (eV)
BCzPh
PBCz
CTP-1
100
115
123
340
409
448
303
289
289
393, 408
362, 381
365, 380
452
450
448
3.37
3.41
3.49
2.74
2.76
2.77
5.64/2.36
5.86/2.40
5.96/2.43
2.93 ꢃ 10ꢀ5
3.62 ꢃ 10ꢀ5
1.18 ꢃ 10ꢀ5
a
b
c
d
e
f
Tg: glass transition temperatures.
Measured in dichloromethane solution at room temperature.
Measured in 2-MeTHF glass matrix at 77 K.
Eg: The band gap energies were estimated from the optical absorption edges of UV–Vis absorption spectra.
ET: The triplet energy were estimated from the onset peak of the phosphorescence spectra.
HOMO levels were estimated from cyclic voltammetry, LUMO levels were calculated from HOMO and Eg.
Mobility was calculated from space charge limited current.
g
phosphorescence spectra as ca. 2.75 eV and 2.76 eV,
respectively, which is almost identical to that of BCzPh
(2.75 eV), implying that the introduction of a phenyl or
biphenyl between two PCz does not alter the triplet en-
ergy. This may due to the meta–meta configuration of the
entire molecules. That also means it is feasible to fix T1
and simultaneously tune S1 in such simple ways. The trip-
let energies of PBCz and CTP-1 are sufficiently high to
serve as host materials for blue and white PHOLEDs.
The absorption spectrum of the phosphorescent dopant
is compared in Fig. S2 with the solid emission spectra
(PL) of the three hosts. Iridium(III) bis[(4,6-difluoro-
the three hosts. However, The overlap between the PL
spectrum of the PBCz and CTP-1 and the absorption of
the FIrpic are larger than that of BCzPh. Hence, the energy
transfer from the PBCz and CTP-1 to the FIrpic should be
more efficient in the host–guest system. This indicates that
the hosts PBCz and CTP-1 maybe show better performance
than that of BCzPh in host–guest system PHOLEDs.
2.4. Energy levels and molecular design
The electrochemical behaviors of BCzPh, PBCz and
CTP-1 were probed by cyclic voltammetry in deoxygenated
CH2Cl2 with ferrocene as the internal reference (see Fig. 3),
and they all displayed irreversible oxidation process. The
2
0
phenyl)pyridinato-N, C ] picolinate (FIrpic, ET = 2,65 eV)
serves as the emitter, with a lower T1 level than that of

L.-S. Cui et al. / Organic Electronics 15 (2014) 1368–1377
1371
level of BPCz and CTP-1 were ꢀ5.86 eV and 5.96 eV,
respectively, which were 0.22/0.26 eV and 0.32/0.36 eV
lower than BCzPh (ꢀ5.64 eV)/ FIrpic (ꢀ5.60 eV). These val-
ues indicate that the BPCz and CTP-1 hosts can probably
enhance hole-trapping on the guest FIrpic and thus im-
prove the device performance [49]. The optical band gap
(Eg) of BCzPh and CTP-1 are 3.45 eV and 3.47 eV, which
are calculated from the threshold of the absorption spectra
in CH2Cl2 solution. The LUMO levels of PBCz and CTP-1
are determined as ꢀ2.40 eV and ꢀ2.43 eV, respectively,
according to the equation ELUMO = EHOMO + Eg. In compari-
son with BCzPh (LUMO = ꢀ2.36 eV), the LUMO energy
levels of three compounds are nearly the same. These will
be detail discussed in the following section.
Tg = 123 o
C
CTP-1
PBCz
Tg = 115 o
C
Tg=102 o
C
BCzPh
80
100
120
140
160
180
200
Temperture (ºC)
In order to better understand the effects of phenyl and
biphenyl spacer on the spatial distribution of their HOMO
and LUMO, density functional theory (DFT) calculations
were performed for PBCz and CTP-1 using B3LYP hybrid
functional theory with Gaussian 09. The structure of BCzPh
was also calculated for comparison under identical condi-
tions. Fig. 4 illustrates the HOMO and LUMO distributions
for BCzPh, PBCz and CTP-1. For BCzPh, both HOMO and
LUMO are mostly localized at the dicarbazole backbone
with little contribution from the peripheral phenyl moiety.
However, with insertion of an additional phenyl (PBCz) or
biphenyl group (CTP-1) in the BCzPh conjugated backbone,
the HOMOs are not only localized at the two PCz units but
also spread over the phenyl or biphenyl spacer. But the LU-
MOs still concentrated in the carbazole ring. The obvious
separation between HOMOs and LUMOs in PBCz and
CTP-1 indicate that the one-electron HOMO–LUMO
transition would become a typical charge-transfer (CT)
transition, which is preferable for efficient hole- and elec-
tron-transporting properties and the prevention of reverse
energy transfer. Interestingly, the fact that the HOMOs of
BCzPh, PBCz and CTP-1 molecules are localized on differ-
ent subunits and can present higher and lower HOMO en-
ergy levels. On the one hand, as the HOMO of the BCzPh
Fig. 1. TGA curves of BCzPh, PBCz and CTP-1.
PL
1.0
0.8
0.6
0.4
0.2
0.0
BCzPh
UV
BPCz
CTP-1
Phos
300
400
500
600
700
Wavelength (nm)
Fig. 2. Room-temperature UV–Vis absorption and fluorescence (PL)
spectra of BCzPh, PBCz and CTP-1 in CH2Cl2 solution and phosphorescence
(Phos) spectra measured in a frozen 2-methyltetrahydrofuran matrix at
77 K.
consists primarily of the
p orbital of the dicarbazole unit.
In order to obtain more information about the HOMOs
for BCzPh, PBCz and CTP-1, the molecular orbital composi-
tions of these molecules were calculated [50,51]. The elec-
tron density of the HOMO is mainly localized on the
carbazoles with dominating contributions (72.75–
91.06%), suggesting that the HOMOs of the three molecules
are determined mostly by the carbazole unit. In other
words, the additional phenyl/ biphenyl unit of PBCz/CTP-
1 also have some contributions in PBCz and CTP-1 mole-
cules. For the two molecules, the contributions of the
carbazole groups to the HOMO are smaller than that in
corresponding BCzPh, with a decrease about 18%, leading
to higher HOMOs for the BCzPh than that for the PBCz
and CTP-1. Hence, the HOMO energy levels increase in
the order: CTP-1(ꢀ5.21 eV) < PBCz (ꢀ5.17 eV) < BCzPh
(ꢀ4.96 eV). The result is predicted to lower the HOMO lev-
els, which was consistent with the cyclic voltammetry
measurements and UPS data (see the Supporting Informa-
tion). Furthermore, the LUMO energies of the three mole-
cules are close because the LUMO shows similar
distributions. The electron density in the LUMOs of BCzPh,
PBCz and CTP-1 is primarily located on the biphenyl of two
BCzPh
PBCz
CTP-1
-0.5
0.0
0.5
1.0
1.5
2.0
Potential vs Fc/Fc+
Fig. 3. Cyclic voltammograms of BCzPh, PBCz and CTP-1 in dichloro-
methane solution for oxidation.
HOMO energy levels were estimated from the onset of oxi-
dation potentials relative to the vacuum level. The HOMO

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L.-S. Cui et al. / Organic Electronics 15 (2014) 1368–1377
Fig. 4. Frontier molecular orbitals(FMOs) distribution (above) and spin density distributions of the T1 values (below) for BCzPh, PBCz and CTP-1 calculated
with DFT at the B3LYP/6-31G(d) level.
carbazole function groups. Therefore, there is little change
in the LUMO levels upon by inserting a phenyl or biphenyl
between two PCz units of the BCzPh molecule. From the
calculated and experiment results, the point to emphasize
is that the HOMOs of the PBCz and CTP-1, are lower than
that of BCzPh, which would make efficient hole trap when
used as host materials. The HOMOs of PBCz and CTP-1 can
be independently turned by chemical modification of phe-
nyl/biphenyl spacer with little effect LUMO energy levels,
and this will greatly facilitate the molecular design and
property control of these materials.
As mentioned in introduction, efficient PHOLEDs need
to suppress the back energy transfer from guest to host
so that the triplet energy of the host must be higher than
that of the guest. The calculated triplet energies of the
three hosts are collected in Table S1. The theoretical results
are in good agreement with experiment and confirm that
the phenyl and biphenyl spacer do not alter the triplet en-
2.5. Hole transport properties
We also studied the carrier-transport properties of the
three materials using hole-only device with the structure
of ITO/MoO3 (20 nm)/host (100 nm)/MoO3 (20 nm)/Al
(100 nm). It can be assumed that only holes are injected
and transported in the devices because the work function
of MoO3 is high enough to block electron injection. Fig. 5
shows the current density versus voltage characteristics
of the hole-only devices. Here, the BCzPh-based and
PBCz-based hole-only device shows the more efficient hole
injection and transport than BCzPh-based device. The
slopes, referring to the trap-free space-charge-limited-cur-
rent region [52], are about 2.2 for the three hole-only de-
2
vices. According to the Mott–Gurney law JSCLC
¼
9 e0erlv
,
3
the mobilities of the three hosts are estimated and8summda-
rized in Table 1.
ergy of BCzPh molecule. This may be due to the p-conjuga-
103
tion are not extended because of the meta-configuration
and the meta-substitution. A further confirmation comes
from the NTO (natural transition orbital) analyses, carried
out on the basis of the optimized T1-state geometries of
the three hosts and illustrated in Fig. 6 below. The hole-
particle NTOs for PBCz and CTP-1 were interrupted by phe-
nyl and biphenyl spacer. For PBCz it is divided into two
parts: biphenyl and phenyl segment. For CTP-1 it is com-
prised of two biphenyl segments. Obviously, the hole-par-
ticle NTOs for BCzPh are localized on a biphenyl segment.
Resultantly, the three hosts are found to have similar trip-
let energy. However, phenyl and biphenyl spacer hardly af-
fected the triplet energy due to the similar nature of the
triplet state. On the other hand, in the cases of PBCz and
CTP-1, a phenyl or biphenyl group is inserted between
the two PCz groups and actually serves as a buffer to keep
the high triplet energy.
BCzPh
102
PBCz
CTP-1
101
100
10-1
Organic Layer
(100 nm)
10-2
10-3
ITO/MoO (10nm)
3
10-4
0.1
1
10
V (V)
Fig. 5. Current density versus voltage characteristics of the hole-only
devices consisting of BCzPh, PBCz and CTP-1 layers.

L.-S. Cui et al. / Organic Electronics 15 (2014) 1368–1377
1373
Fig. 6. Energy levels of the materials employed in the devices.
2.6. Electroluminescence of OLEDs
were also fabricated. The best electroluminescence (EL)
performance was achieved with 5 wt.% FIrpic for the hosts.
Fig. 6 depicts the relative energy levels of the materials em-
ployed in the devices. All the devices performance is sum-
marized in Table 2.
In order to evaluate the potentials of PBCz and CTP-1 as
host materials for the sky blue phosphor FIrpic, the devices
were fabricated with typical sandwiched structures
by sequential vapor deposition of the materials onto
The current density–voltage–brightness (J–V–L) charac-
teristics and current efficiency, power efficiency and exter-
nal quantum efficiency versus current density
characteristics of the devices are shown in Fig. 7. All the
devices show similar EL spectra. The EL spectra of the
BCzPh, PBCz or CTP-1-based devices show identical pat-
terns to FIrpic emission without any emission from the
host and/or adjacent layer. The EL results indicate that
the perfect energy transfer from host to FIrpic generate
upon electrical excitation, and charge carriers and excitons
are completely confined in the emitting layer. Generally,
FIrpic is a sky-blue emitter with a typical maximum emis-
sion peak around 472 nm and a shoulder peak at 500 nm,
with a CIE (x, y) around (0.15, 0.37).
a
glass substrate coated with ITO. We utilized
strongly electron-deficient 1,4,5,8,9,11-hexaazatriphenylene-
hexacarbonitrile (HAT-CN) as hole-injection layer (HIL), 1,1-
bis[4-[N,N-di(p-tolyl)amino]phenyl]cyclohexane (TAPC) as
hole-transporting layer (HTL) and electron-blocking layer.
1,3,5-tri[(3-pyridyl)- phen-3-yl]benzene (TmPyPB) was
used as electron-transporting as well as hole-blocking
layer. 8-Hydroxyquinolinolato-lithium (Liq) served as elec-
tron-injecting layer and Al acted as cathode. The FIrpic-
doped devices were fabricated with a simple configuration
of ITO/HAT-CN (10 nm)/TAPC (45 nm)/Host: FIrpic (5 wt%,
20 nm)/TmPyPB
(50 nm)/Liq
(2 nm)/Al
(120 nm)
(Host = BCzPh: device BA; Host = PBCz: device BB;
Host = CTP-1: device BC). The emitting material layers are
the newly prepared FIrpic-doped PBCz or CTP-1. Mean-
while, the control devices using BCzPh (device BA) as hosts
As illustrated in Fig. 7, device BC hosted by CTP-1 exhib-
ited a maximum current efficiency(gc, max) of 40.5 cd/A, a
maximum power efficiency (gp, max) of 33.9 lm/W and a
Table 2
Electroluminescence characteristics of the devices.
Va (V)
gcb (cd/A)
gp (lm/W)
gext (%)
b
b
Device
Host/guest
BA
BB
BC
WA
WB
WC
WD
BCzPh/FIrpic
PBCz/FIrpic
CTP-1/FIrpic
BCzPh/FIrpic + PO-01
PBCz/FIrpic + PO-01
CTP-1/FIrpic + PO-01
PBCz/B + G + R
3.2
3.4
4.7
3.5
3.4
3.5
3.2
36.5, 35.7, 24.2
45.5, 45.4, 43.7
40.5, 40.4, 38.2
55.8, 55.8, 53.6
76.2, 76.2, 73.6
67.4, 67.4, 64.6
49.0, 48.5, 48.6
35.5, 6.5, 3.3
15.6, 15.3, 10.3
19.5, 19.4, 18.7
17.3, 17.3, 16.3
17.8, 17.8, 17.2
24.6, 24.6, 23.6
21.5, 21.5, 20.7
19.1, 18.9, 19.0
43.8, 43.7, 33.6
33.9, 17.2, 10.0
49.0, 49.0, 38.0
69.4, 69.4, 55.0
60.2, 60.2, 45.9
46.3, 46.3, 36.8
a
Voltage (V) at 100 cd/m2.
b
Current efficiency (gc), power efficiency (gp) or external quantum efficiency (g
ext) in the order of maximum, at 100 cd/m2 and at1000 cd/m2.

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L.-S. Cui et al. / Organic Electronics 15 (2014) 1368–1377
(a)
(b)
50
40
30
20
10
50
50
40
30
20
10
0
104
BCzPh
PBCz
BCzPh
PBCz
40
CTP-1
CTP-1
30
103
20
10
102
0
-10
101
3
4
5
6
7
8
9
0
10
20
30
40
50
Current Density (mA/cm2)
Current Density (mA/cm2)
(c)
(d)
50
40
30
20
10
0
50
40
30
20
10
0
1.0
0.8
0.6
0.4
0.2
0.0
BCzPh
PBCz
BCzPh
PBCz
CTP-1
CTP-1
@ 10 mA/cm2
-10
-20
-30
0
10
20
30
40
50
300
400
500
600
700
800
Current Density (mA/cm2)
Wavelength (nm)
Fig. 7. (a) Current density–voltage–luminance characteristics, (b) current efficiency and external quantum efficiency versus current density curves,
(c) power efficiency and external quantum efficiency versus current density curves and (d) the EL spectrum of devices BA–BC at10 mA/cm2.
maximum external quantum efficiency (
The same trend can be observed for devices B. Device BB
g
ext, max) of 17.3%.
lead to a broader recombination zone in PBCz-based device
compared to BCzPh-based device.
hosted by PBCz displays substantially higher efficiencies
In addition, to further evaluate their suitability for
white PHOLEDs, the all phosphor two-color white OLEDs
have been fabricated with the configuration of ITO/HAT-
CN (10 nm)/TAPC (45 nm)/Host: FIrpic: PO-01 (5%, 0.5%,
with gc,
of 45.5 cd/A, gp,
of 43.8 lm/W and gext,
max
max
of 19.5%. In comparison, the control device BA using
max
BCzPh as the host exhibits lower efficiencies (gc,
of
max
36.5 cd/A,
values are higher than that of the comparable blue device
using conventional host mCP, which achieves gc of
g
p, max of 35.1 lm/W and
g
ext, max of 15.6%). These
20 nm)/TmPyPB
(40 nm)/Liq
(2 nm)/Al
(120 nm)
(Host = BCzPh: device WA; Host = PBCz: device WB;
Host = CTP-1: device WC). FIrpic and PO-01 were co-doped
into the three hosts as emitting material layer. Fig. 8 shows
the curves of external quantum efficiency, power efficiency
and external quantum efficiency versus current density.
,
max
34.5 cd/A, gp
,
of 23.1 lm/W and gext
,
of 13.7%
max
max
(Fig. S6). On the other hand, the drive voltage (at the lumi-
nance of 100 cd/m2) of the device BA hosted by BCzPh was
3.3 V, which is lower than that of device BB by 0.2 V. The
current density of device BA is also higher than that of de-
vice BB at the same voltage. Interestingly, it is worth noting
that the devices BB and BC show low EQE roll-offs. Remark-
ably, when the brightness reached 1000 cd/m2, the EQEs
still remained as high as 18.7% and 16.4% for devices BB
and BC, respectively, while the EQE of device BA dropped
to 10.3%. It could be attributed the different HOMO energy
levels of the host materials. As depicted in Fig. 6, BCzPh
(ꢀ5.64 eV) has similar HOMO level with FIrpic
(ꢀ5.60 eV). In contrast, PBCz (ꢀ5.86 eV) is 0.25 eV lower
than FIrpic, which may be inclined to format dopant trap-
ping effect. For BCzPh-based device, holes are directly
transported on the host while for PBCz-based device, parts
of the holes are trapped on the dopant. Considering the
excellent electron transport property of FIrpic, this will
The devices WAꢀWC obtained the gc
,
of 55.8, 76.2,
max
67.4 cd/A, gp
,
of 49.0, 69.4, 60.2 lm/W and gext
,
of
max
max
17.8, 24.6, 21.5% respectively. Moreover, the emission color
of these devices exhibited stability and varied slightly from
(0.36, 0.48) at 4 V to (0.34, 0.47) at 8 V. Similar to the blue
PHOLEDs, the current/power efficiencies of the devices
hosted by these materials are in the order device WB > de-
vice WC > device WA, which also demonstrates the superi-
ority of PBCz. But, the lack of red peaks in the spectrum
leads to a poor CIE coordinates.
In this regards, we then studied the incorporation of
RGB three-color emitters in the white devices. The device
structure was ITO/HAT-CN (10 nm)/TAPC (45 nm)/PBCz:
FIrpic (5%, 19 nm)/PBCz: Ir(ppy)2(acac): Ir(MDQ)2(acac)
(3%, 1%, 1.5 nm)/ TmPyPB (40 nm)/Liq (2 nm)/Al
(120 nm), where 19 nm-thick PBCz doped with 5 wt.%

L.-S. Cui et al. / Organic Electronics 15 (2014) 1368–1377
1375
50
80
80
60
40
20
0
80
60
40
20
0
BCzPh
PBCz
CTP-1
BCzPh
PBCz
CTP-1
40
30
20
10
0
60
40
20
0
-20
0
10
20
30
40
50
0
10
20
30
40
50
Current Density (mA/cm2)
Current Density (mA/cm2)
Fig. 8. (a) current efficiency and external quantum efficiency versus current density curves, (b) power efficiency and external quantum efficiency versus
current density curves.
FIrpic served as the blue-emitting layer and 1.5 nm 3 wt.%
Ir(ppy)2(acac) and 1% Ir(MDQ)2(acac) were co-doped into
BPCz worked as the green–red-emitting layer. The EL char-
acteristics of device WD are shown in Fig. 9 and the respec-
tive data are summarized in Table 2. The resulting white
spectra exhibited three main peaks centered at 475 nm,
516 nm and 596 nm, which arose from the emission of
the FIrpic, Ir(ppy)2(acac) and Ir(MDQ)2(acac), respectively.
The spectra also demonstrated moderate color stability.
With the increase of driving voltage, the intensity of blue
and red peaks decreased slightly with the CIE coordinates
changing from (0.45, 0.42) at 3.0 V to (0.44, 0.43). The de-
with high efficiency and proper CIE coordinates for solid-
state lighting. On the other hand, the color rendering index
(CRI) of three-color white PHOLEDs (CRI = 76) are better
than that of two-color white PHOLEDs (CRI = 47).
3. Conclusion
In summary, an evolution of carbazole-based host
materials is presented in this paper. After constructing a
series of carbazole-based compounds previously to reveal
the optimized linking position to modify required proper-
ties for blue host materials, we reach the concept of how
to control the T1 distribution to get sufficient triplet en-
ergy. A simple carbazole-based host material, namely
PBCz, for PHOLEDs has been designed and synthesized by
inserting one phenyl ring between the two PCz units. This
structure can endow it with sufficiently high thermal/mor-
phological stabilities though the corresponding Td/Tg is
slightly lower than that of CTP-1. Compared to CTP-1,
the subtraction from biphenyl group to one phenyl group
could effectively tune the HOMO energy level and improve
hole-mobility, while maintaining high triplet energy. Blue
and warm-white PHOLEDs were fabricated using PBCz as
hosts and FIrpic/PO-01 as dopants, which exhibited en-
hanced efficiencies with low roll-off. Specifically, the cur-
rent efficiencies of PBCz-based devices can reach 45.5 cd/
A and 76.2 cd/A at 100 cd/m2 in blue and white PHOLEDs,
which were superior to those of CTP-1-based devices.
Moreover, upon combination of PBCz with FIrpic,
Ir(ppy)2(acac) and Ir(MDQ)2(acac) in the active layer, the
three colors WOLEDs demonstrated a high EQE of 19.1%
and a proper CIE coordinates of (0.45, 0.42). This is a ra-
tional molecular design on how to investigate the more
suitable materials by considering position and spacer ef-
fects to tune the key factors for final device performances.
vice performances were achieved to be gc, max of 49.2 cd/A,
gp,
of 46.5 lm/W and gext of 19.1%, respectively.
,
max
max
When these WOLEDs are applied to solid-state lighting,
all of the photons should be taken into account for illumi-
nation because the originally trapped photons can be redi-
rected to the forward-viewing direction by applying light
out-coupling techniques such as by roughing the substrate,
patterning a micro-lens array or engineering the lighting
fixtures [53]. It has been experimentally demonstrate that
the efficiency can be improved by a factor of 1.2–2.3 if cer-
tain light out-coupling techniques are used. In this regards,
a maximal total current efficiency of ꢄ88.6 cd/A can be ob-
tained if a factor of 1.8 is adopted. The results indicated
that PBCz can be used as host material for white PHOLEDs
25
WD
20
15
3.0 V
7.0 V
10
4. Experimental section
5
400
500
600
700
800
Wavelength (nm)
4.1. Materials and methods
0
0
10
20
30
40
All chemicals i.e., (9-phenyl-9H-carbazol-3-yl) boronic
acid, [4-(9H-carbazol-9-yl) phenyl] boronic acid, [3-(9H-
carbazol-9-yl)phenyl] boronic acid, 1-bromo-3-iodoben-
zene, 3-Bromobenzeneboronic acid were purchased from
Current Density (mA/cm2)
Fig. 9. Current efficiency and external quantum efficiency versus current
density curves, The inset is the EL spectra of device WD at 3 V and 7 V.

1376
L.-S. Cui et al. / Organic Electronics 15 (2014) 1368–1377
Bepharm limited and Alfa Aesar. And all of materials used
without further purification. The important intermediates
3,30-dibromodiphenyl was prepared according to the litera-
ture methods[31]. THF was purified by PURE SOLV (Innova-
tive Technology) purification system. Chromatographic
separations were carried out by using silica gel (200–
300 nm). All other reagents were used as received from
commercial sources unless otherwise stated. 1H NMR and
13C NMR spectra were recorded on a Varian Unity Inova
400 spectrometer at room temperature. Mass spectra were
recorded on a Thermo ISQ mass spectrometer using a direct
exposure probe. UV–Vis absorption spectra were recorded
on a Perkin Elmer Lambda 750 spectrophotometer. PL spec-
tra and phosphorescent spectra were recorded on a Hitachi
F-4600 fluorescence spectrophotometer Transient PL de-
cays were measured by a single photon counting spectrom-
eter from HORIBA JOBIN YVON with a Nano LED pulse lamp
as the excitation source. Differential scanning calorimetry
(DSC) was performed on a TA DSC 2010 unit at a heating rate
of 10 °C/min under nitrogen. The glass transition tempera-
tures (Tg) were determined from the second heating scan.
Thermo gravimetric analysis (TGA) was performed on a TA
SDT 2960 instrument at a heating rate of 10 °C/min under
nitrogen. Temperature at 5% weight loss was used as the
decomposition temperature (Td). Cyclic voltammetry (CV)
was carried out on a CHI600 voltammetric analyzer at room
temperature with a conventional three-electrode configura-
tion consisting of a platinum disk working electrode, a plat-
inum wire auxiliary electrode, and an Ag wire pseudo-
reference electrode with ferrocenium–ferrocene (Fc+/Fc)
as the internal standard. Nitrogen-purged dichloromethane
was used as solvent for the oxidation scan and DMF for the
reduction scan with tetrabutylammonium hexafluorophos-
phate (TBAPF6) (0.1 M) as the supporting electrolyte. The
cyclic voltammograms were obtained at a scan rate of
100 mV/s.
of the devices were measured with a PHOTO RESEARCH
SpectraScan PR 655 photometer and a KEITHLEY 2400
SourceMeter constant current source at room temperature.
The EQE values were values were calculated according to
the previously reported methods [54].
9,90-diphenyl-9H,90H-3,30-bicarbazole (BCzPh). 3-bro-
mo-9-phenyl-9H-carbazole (1.50 g, 4.66 mmol), 9-phenyl-
9H-carbazol-3-ylboronic acid (1.47 g, 5.13 mmol) and
tetrakis(triphenylphosphine)palladium(0) (0.26 g, o.23 mm
ol) were dissolved in THF/2 M K2CO3 (3/1, v/v). The reac-
tion mixture was heated to 60 °C for 8 h under an argon
atmosphere. After cooling to room temperature, the organ-
ic layer was separated and evaporated to remove solvent.
The residue was purified by column chromatography with
1:3 (v/v) dichloromethane/petroleum ether as the eluent
and recrystallized from dichloromethane/petroleum to
give a white crystalline powder (1.92 g, 85%). 1H NMR
(400 MHz, CDCl3) d (ppm): 8.46 (s, 2H) 8.25 (d, J = 8.0 Hz,
2H) 7.79 (d, J = 8.0 Hz, 2H) 7.67–7.59 (m, 8H) 7.54–7.44
(m, 8H) 7.36–7.28 (m, 1H). 13C NMR (100 MHz, CDCl3) d
(ppm): 143.5, 141.3, 140.4, 137.6, 133.6, 129.9, 125.6,
123.9, 120.4, 118.9, 110.0, 109.9. MS (EI): m/z 484.88
[M+]. Anal. calcd for C36H24N2 (%): C 89.18, H 4.89, N
5.93; found: C 89.23, H 4.99, N 5.78.
1,3-bis(9-phenyl-9H-carbazol-3-yl)benzene (PBCz):
PBCz was synthesized according to the same procedure
as for BCzPh by using 1,3–dibromophenyl (1.13 g,
4.81 mmol) and 9-phenyl-9H-carbazol-3-ylboronic acid
(3.03 g, 10.56 mmol). PBCz was afforded as a white solid
(2.23 g, 82.9%). 11H NMR (400 MHz, CDCl3) d (ppm): 8.45
(s, 2H) 8.23 (d, J = 8.0 Hz, 2H) 8.05 (s, 2H) 7.76 (d,
J = 8.0 Hz, 2H) 7.71 (d, J = 8.0 Hz, 2H) 7.66–7.55 (m, 9H)
7.54–7.38 (m, 8H), 7.35–7.27 (m, 2H). 13C NMR
(400 MHz, CDCl3) d (ppm): 142.6, 141.4, 140.5, 137.6,
133.4, 129.9, 129.3, 127.5, 127.1, 126.5, 126.1, 125.6,
123.8, 123.5, 120.4, 120.1, 119.0, 110.1, 109.9. MS (EI):
m/z 560.65 [M+]. Anal. calcd for C48H32N2 (%): C 89.97, H
5.03, N 5.00; found: C 90.00, H 5.08, N 4.92.
4.2. Computational methodology
3,30-bis(9-phenyl-9H-carbazol-3-yl)biphenyl (CTP-1):
CTP-1 was synthesized according to the same procedure
as for BCzPh by using 3,30–dibromodiphenyl (1.50 g,
4.81 mmol) and 9-phenyl-9H-carbazol-3-ylboronic acid
(3.03 g, 10.56 mmol). CTP-1 was afforded as a white solid
(2.61 g, 85.5%). 1H NMR (400 MHz, CDCl3) d (ppm): 8.43
(s, 2H) 8.21 (d, J = 8.0 Hz, 2H) 8.03 (s, 2H) 7.76–7.70 (m,
4H) 7.68 (d, J = 8.0 Hz, 2H) 7.64–7.55 (m, 10H) 7.51–7.37
(m, 8H), 7.33–7.26 (m, 2H). 13C NMR (400 MHz, CDCl3) d
(ppm): 142.6, 142.0, 141.4, 140.5, 137.6, 133.4, 129.9,
129.3, 129.9, 129.3, 127.5, 127.1, 126.5, 126.4, 126.2,
125.7, 125.6, 110.1, 109.9. MS (EI): m/z 636.52 [M+]. Anal.
calcd for C48H32N2 (%): C 90.54, H 5.07, N 4.40; found: C
90.54, H 5.50, N 4.46.
The geometrical and electronic properties of BCzPh,
PBCz and CTP-1 were performed with the Gaussian 09 pro-
gram package. The calculation was optimized by means of
the wb97xd (Becke three parameters hybrid functional
with Lee–Yang–Perdew correlation functionals) with the
6-311G(d) atomic basis set. Molecular orbitals were visual-
ized using Gaussview.
4.3. Fabrication of OLEDs
The OLEDs were fabricated through vacuum deposition
of the materials at ca. 2 ꢃ 10ꢀ6 Torr onto ITO-coated glass
substrates having a sheet resistance of ca. 30
X per square.
The ITO surface was cleaned ultrasonically – sequentially
with acetone, ethanol, and deionized water, then dried in
an oven, and finally exposed to UV-ozone for about
30 min. Organic layers were deposited at a rate of 2–3 Å/
s, subsequently, Liq was deposited at 0.2 Å/s and then
capped with Al (ca. 4 Å/s) through a shadow mask without
breaking the vacuum. For all the OLEDs, the emitting areas
were determined by the overlap of two electrodes as
0.09 cm2. The EL spectra, CIE coordinates and J–V–L curves
Acknowledgements
We are grateful to the assistance of Dr. Cheng Zhong in
molecular simulation. We acknowledge financial support
from the Natural Science Foundation of China (Nos.
21202114, 21161160446, 61036009, and 61177016), the
National High-Tech Research Development Program (No.
2011AA03A110), and the Natural Science Foundation of

L.-S. Cui et al. / Organic Electronics 15 (2014) 1368–1377
1377
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funded by the Priority Academic Program Development
of Jiangsu Higher Education Institutions (PAPD) and by
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Appendix A. Supplementary material
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
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