Solution-processible carbazole dendrimers as host materials for highly efficient phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1002/adfm.201201326

A group of dendrimers with oligo-carbazole dendrons appended at 4,4′- positions of biphenyl core are synthesized for use as host materials for solution-processible phosphorescent organic light-emitting diodes (PHOLEDs). In comparison with the traditional

Full text of DOI:10.1002/adfm.201201326

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Solution-Processible Carbazole Dendrimers as Host
Materials for Highly Efficient Phosphorescent Organic
Light-Emitting Diodes
Jiuyan Li,* Ting Zhang, Yunjing Liang, and Ruixia Yang
to suppress the severe concentration
quenching and triplet-triplet annihila-
tion that otherwise exist in neat films, the
metal complex phosphors are typically
A group of dendrimers with oligo-carbazole dendrons appended at 4,4′-
positions of biphenyl core are synthesized for use as host materials for solu-
tion-processible phosphorescent organic light-emitting diodes (PHOLEDs).
In comparison with the traditional small molecular host 4,4′-N,N′-
dicarbazolebiphenyl (CBP), the dendritic conformation affords these materials
extra merits including amorphous nature with extremely high glass transi-
tion temperatures (ca. 376 °C) and solution-processibility, but inherent the
identical triplet energies (2.60–2.62 eV). In comparison with the widely-used
polymeric host polyvinylcarbazole (PVK), these dendrimers possess much
higher HOMO levels (–5.61 to –5.42 eV) that facilitate efficient hole injection
and are favorable for high power efficiency in OLEDs. The agreeable proper-
ties and the solution-processibility of these dendrimers makes it possible
to fabricate highly efficient PHOLEDs by spin coating with the dendimers
as phosphorescent hosts. The green PHOLED containing Ir(ppy)3 (Hppy =
2-phenyl-pyridine) dopant exhibits high peak efficiencies of 38.71 cd A−1 and
15.69 lm W−1 , which far exceed those of the control device with the PVK host
(27.70 cd A−1 and 9.6 lm W−1 ) and are among the best results for solution-
processed green PHOLEDs ever reported. The versatility of these dendrimer
hosts can be spread to orange PHOLEDs and high efficiencies of 32.22 cd A−1
and 20.23 lm W−1 are obtained, among the best ever reported for solution-
processed orange PHOLEDs.
doped in a certain host material or mix-
ture in PHOLED.[4,5] Therefore, the host
plays an essential role in determining the
device performance and even the produc-
tion cost. In principle, an ideal phospho-
rescent host material should possess the
following attributes: (1) a certain charge
transporting ability to facilitate electrical
conduction of the whole device; (2) appro-
priate highest occupied molecular orbital
(HOMO) and lowest unoccupied molec-
ular orbital (LUMO) levels to guarantee
efficient charge injection into this emit-
ting layer from the adjacent layers and,
thus, as low driving voltages as possible;
(3) a suitably high triplet energy to guar-
antee efficient forward energy transfer
from the host to the metal complex
dopant and to prevent reverse energy
transfer. Furthermore, the small molecular
or polymeric nature of the host material
is the primary factor that determines the
fabrication method of the emitting layer
and the entire OLED. Until now, many
1. Introduction
PHOLEDs have been fabricated by vacuum thermal co-evapora-
tion with phosphor emitter doped in a certain small molecular
host,[6,7] which requires complicated technological processes
and the unavoidable waste of a large amount of organic mate-
rials, leading to relatively high fabrication costs. It is generally
believed that solution-based processes, such as spin-coating
or ink-jet printing, will be required for the fabrication of low-
cost, large-area OLED display or lighting products.[8,9] To date,
solution-processed PHOLEDs have mainly been produced from
polymeric materials by incorporation of iridium complexes
either by physical blending in polymeric host materials, such
as polyvinylcarbazole (PVK)[10–12] and polyfluorene (PFO),[13] or
chemical bonding on polymer chains.[14,15] A large number of
PHOLEDs with PVK as a triplet host for iridium complex phos-
phors have been reported, many of them achieving good elec-
trophosphorescence performance.[10–12] However, the polymeric
materials have their intrinsic disadvantages, such as uncertain
molecular structure and difficulty in purification (the purity of
materials has a great influence on their electroluminescence
Phosphorescent organic light emitting diodes (PHOLEDs) con-
taining transition metal complexes as emitters have attracted
much research and industry attention since they are capable
to utilizing excitons in both singlet and triplet states and may
achieve nearly 100% internal quantum efficiency.[1–3] In order
Dr. J. Li, T. Zhang
State Key Laboratory of Fine Chemicals
School of Chemical Engineering
Dalian University of Technology
2 Linggong Road, Dalian 116024, China
E-mail: jiuyanli@dlut.edu.cn
Y. Liang, R. Yang
School of Chemistry
Dalian University of Technology
2 Linggong Road, Dalian 116024, China
DOI: 10.1002/adfm.201201326
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performance). In particular, the most widely-used phosphores-
cent host PVK has a low HOMO level of –5.8 eV,[16] which has a
large hole barrier with the ITO (work function = –4.8 eV) anode or
the typical hole injecting layer poly(3,4-ethylenedioxythiophene):
poly(styrenesulfonate) (PEDOT:PSS) (HOMO = –5.2 eV).[17]
Consequently the PVK-based devices usually have relatively
high turn-on voltages and driving voltages, which are not favo-
rable for high power efficiencies of the devices.[18] Most recently,
there have been several reports exploring small molecular mate-
rials as triplet hosts to fabricate PHOLEDs by spin coating.[19,20]
Although acceptable performance was obtained, the device sta-
bility is still a concern because of the comparatively lower glass
transition temperatures and inferior film forming ability of the
solution techniques, especially when large area application is
taken into account.
Dendrimers may be ideal altenatives to polymers, serving
as triplet hosts in solution-processed PHOLEDs. Dendritic
molecules possess many advantages[21,22] such as repeat-
able monodispersity and high levels of purity like small mol-
ecules, which are essential for ideal device performance. In
addition, they possess high molecular weight and good solu-
bility in common organic solvents like polymeric materials,
which make them suitable for solution methods to prepare
thin films and OLEDs. Furthermore, the dendritic molecules
are usually amorphous with high glass transition tempera-
tures (Tg) based on their large molecular weight and bulky
steric conformation, which is really a valuable merit for host
material if high device stability is desired. Therefore, dendritic
molecules with good solubility and film morphology for solu-
tion-processible phosphorescent light-emitting devices, either
as hosts or doped emitters, have been investigated.[23,24] The
first report of dendritic molecules used as efficient solution-
processible hosts for triplet emitters was published by Wang
et al.[23] They adopted carbazole-based conjugated dendrimers as
the host and blue-emitting iridium(III)bis(4,6-(difluorophenyl)
pyridinato-N,C2′)picolinate (FIrpic) as the guest to prepare the
emitting layer by spin coating, achieving a maximum power
efficiency of 15.4 lm W−1 (27.6 cd A−1 , 12.7%) for double layer
devices. The efficiency of the blue electrophosphorescent device
with the dendrimer host was 86% higher than the PVK-based
device. Dendritic host materials for green PHOLEDs, how-
ever, have received little attention in the literature. Yang et al.
reported solution-processed green PHOLEDs using a new car-
bazole dendrimer as the host in the emitting layer, which is the
only example of using a dendrimer host for green PHOLEDs
reported thus far.[24a] However, the maximum luminance effi-
ciency was reported to be only 16.8 cd A−1 .
to prepare PHOLEDs by vaccum techniques. In these den-
dritic CBP derivatives, the individual carbazole is selected as
the dendron and the whole dendrimer grows up to higher gen-
eration in such a way that the outer carbazole is attached via its
N-position directly on the 3- and 6-positions of each inner carba-
zole moiety. The experimental data revealed that such a dendron
design strategy does not obviously influence the triplet energy of
the resultant dendrimers.[28] In order to ensure good solubility
and film-forming ability using wet methods for the desired den-
drimers, their peripheral carbazoles were decorated by tert-butyl
groups. Electroluminescence investigation revealed that, similar
to CBP, these novel dendrimers serve as excellent triplet hosts
for green and orange emitting iridium complexes in PHOLEDs.
However, they are advantageous over CBP because they are suit-
able for solution-processing to make OLEDs. In comparison with
the traditional solution-processible PVK host, all these dendrimers
are superior in terms of their higher HOMO levels and easier
hole injection. The green and orange-emitting PHOLEDs using
G2 as the host exhibited the maximum luminance efficiency (ηL)
of 38.71 cd A−1 (corresponding to a maximum power efficiency
of 15.69 lm W−1 ) and 32.22 cd A−1 (20.23 lm W−1 ), respectively,
which far exceed those of the best devices with PVK hosts.
2. Results and Discussion
2.1. Synthesis
The synthetic routes for the carbazole-based dendritic mol-
ecules (G1, G2, G3) are shown in Scheme 1. The synthesis of
G1 was previously reported.[29] First, the important intermedi-
ates including the first- to third-generation dendrons, D1, D2,
and D3, were synthesized according to literature methods,[24c,30]
as described in Scheme 1. The target compounds, G1, G2, and
G3, were then prepared at moderate yields of 18–44% by the
Ullmann C–N coupling reaction between 4,4′-diiodobiphenyl
with an excess molar ratio of carbazole dendrons, D1, D2, and
D3, respectively. They have good solubility in common organic
solvents, such as dichloromethane, tetrahydrofuran and ethy-
lacetate, such that they can be easily isolated and purified by
column chromatography and recrystallization to reach high
purity for spectroscopy characterization and OLED application.
1
Their chemical structures were confirmed by H and 13C NMR
spectroscopy, matrix-assisted laser desorption ionization time-
of-flight (MALDI-TOF) mass spectrometry (Figures S1–3 in the
Supporting Information), and elemental analysis.
Inspired by the solution-processed PHOLED trend
and the potential benefits of dendritic host materials,
our interest is to explore novel dendrimer materials with
excellent properties for application in solution-processed
PHOLEDs. The dendrimers reported in present paper, G1,
G2, and G3 (Scheme 1), have the same molecular skeletons
as 4,4′-N,N′-dicarbazolebiphenyl (CBP), in which the twisted
biphenyl core and the bulky dendrons are expected to lead to
2.2. Thermal Properties
The thermal properties of the dendrimers were investigated
by means of thermogravimetric analysis (TGA) and differen-
tial scanning calorimetry (DSC) at a scanning rate of 10 °C
min−1 ; the data are listed in Table 1. As shown by the TGA
curves in Figure S4 in the Supporting Information, G2 and
G3 exhibit high thermal stability with decomposition tem-
peratures (Td) (corresponding to 5% weight loss) over 500 °C
when heated in N2, which are higher than that of G1 (478 °C).
a
nonplanar three-dimensional molecular conformation.[25]
It is well established that CBP is one of the most successful
triplet host materials used in PHOLEDs.[26,27] However, the
small molecular nature of CBP means that it is only possible
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I
I
I
I
Ts-Cl
DMF
KI, KIO3
AcOH
N
S
N
H
N
H
O
O
1
2
I
I
N
N
H
Boc
3
D1
N
N
KOH, DMF
CuI, K2CO3
PhNO2
N
N
D1 + 2
N
S O
O
N
H
D2
4
N
N
N
N
N
TFA
N
N
CuI
toluene
N
N
N
N
D2 +3
N
CH2Cl2
N
H
N
Boc
D3
5
Ullmann coupling reaction
I
G1, G2, G3
I
D1, D2, D3
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
G1
G2
G3
N
N
N
Scheme 1. Synthetic routes of the dendrimers: G1, G2, and G3.
This is an essential property for organic light-emitting mate-
rials especially when they are used at high temperature. The
thermal stability is suggested to benefit at least partially from
the presence of a large amount of carbazole groups, which are
known to impart excellent thermal and chemical stability.[31]
The morphological stability of these novel dendrimers was
T a b le 1 . Summary of the physical parameters of the dendrimers.
em
max
abs
max
Compound
Tg
[%]
Td
[°C]
HOMO
[eV]c)
Eg
[eV]d)
LUMO
[eV]
ET
[eV]e)
Φ
λ
[nm]
λ
[nm]
[%]a)
fi lm
CH2Cl2
396
fi lm
CH2Cl2 [lg ε]
G1
G2
G3
298(4.40),332(4.26)
298(4.99),331(4.63)
298(5.13),330(4.64)
299,337
298,336
299,335
403
408
407
80
17
14
175b)
300
478
502
517
–5.61
–5.45
–5.42
3.28
3.16
3.12
–2.33
–2.29
–2.30
2.62
2.60
2.61
401
400
376
a)Measured in CH2Cl2 solution with quinine sulfate as the standard (Φ = 0.55 in 0.1 N H2SO4) ; b)The glass-transition temperature was obtained from Ref.[29]
;
c)Calculated
with reference to ferrocene (4.8 eV); d)The optical band gap, calculated by the absorption edge technique; e)Measured in 2-MeTHF at 77 K.
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2.3. Photophysical Properties
2
1
1st heating
2nd heating
The photophysical properties of these dendrimers were inves-
tigated by means of electronic absorption and steady state
photoluminescence (PL) measurements for both dilute solu-
tions in dichloromethane and solid films on quartz plates. The
pertinent data are summarized in Table 1. Figure 2 shows the
absorption and PL spectra of G2 as an example. The absorption
spectrum of G2 in dilute solution exhibits two strong absorp-
tion bands in the ultraviolet region. The band peaked at 331 nm
corresponds to the π–π∗ electron transition of the entire con-
jugated backbone and the band at 298 nm could be assigned
to the π–π∗ local electron transition of the carbazole dendron
at the terminal ends. A red shift of 5 nm was detected in the
absorption spectrum of the G2 thin film in comparison with its
solution. Upon photoexcitation at the absorption maximum, the
G2 solution exhibits deep-blue fluorescence with an emission
peak at 401 nm. Similar to the absorption, a red shift of 7 nm
was detected in the PL spectra of the thin film. The absorption
spectra of G1 and G3 (Figure S5, Supporting Information) have
the similar features with those of G2. They exhibit deep-blue PL
with emission peak at 396 nm in solution and 403 nm in the
thin film for G1, and at 400 nm in solution and 407 nm in the
thin film for G3. The bathochromic effect in both absorption
G3
Tg= 376
0
-1
-2
-3
G2
Tg= 300
50 100 150 200 250 300 350 400
Temperature
(
)
Figure 1. DSC traces of the G2 and G3 dendrimers (at a heating rate of
10 °C min−1 ).
monitored by the DSC. As shown by the thermograms of
G2 in Figure 1, only one endothermic peak due to the glass
transition was observed at 300 °C when a powder of G2 was
heated for the first run, implying that G2 was amorphous
even when it was obtained directly from organic solvents.[32]
No obvious phase transition was detected when the sample
was then cooled at a rate of either 50 or 10 °C min−1 . This
endothermic glass transition was detected again when the
sample was subsequently reheated. The thermograms of G3
have similar features to those of G2, only one endothermic
peak was observed at 376 °C, which is also assigned to the
glass transition process. It was reported in literature that G1
has a Tg of 175 °C.[29] Unfortunately, we did not detect such
a glass transition process for G1 under our own experimental
conditions. It is, nevertheless, clear that the Tg of these den-
drimers show a regular increasing trend with increasing den-
dron generation. The amorphous nature, even when obtained
from organic solvents, and the high Tg over 300 °C for G2 and
G3 indicate a high amorphous stability, which arises from
their three-dimensional, molecular conformation due to the
presence of the twisted biphenyl core and the bulky carbazole
dendrons. It should be noted that such high Tg of 300 °C and
376 °C for G2 and G3 are among the highest ever reported for
organic molecular materials.[33–35] By constructing vinyl-type
polynorbornene with CBP side groups, Lee et al. developed a
polymeric host for green PhOLEDs with a Tg of 361 °C, which
is among the highest for conjugated and side-chain polymeric
host materials.[33] Cheng et al. also reported three triptycene
derivatives with high Tg values of 178–238 °C.[34] Recently
Promarak and coworkers reported a carbazole dendronised
triphenylamine molecule with the fourth generation carbazole
dendrons possesses a Tg of as high as 401 °C, which was used
as hole transporting layer in fluorescent OLEDs.[35] Evidently,
the present G3 shows the highest Tg value among all the host
materials reported so far. Based on the excellent thermal and
amorphous stability of these dendritic compounds, the possi-
bility to prepare stable amorphous thin films from these com-
pounds by solution casting is highly promising for applications
in OLEDs.
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(a)
1.0
0.8
0.6
0.4
0.2
0.0
Solution
Film
250 300 350 400 450 500 550 600
Wavelength (nm)
1.0
0.8
0.6
0.4
0.2
0.0
(b)
350
400
450
500
550
600
650
Wavelength (nm)
Figure 2. a) Absorption and fluorescence spectra of G2 in dilute dichlo-
romethane solution and in the solid state. b) Phosphorescence spectra
of G2 in 2-MeTHF at 77 K.
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and PL spectra are frequently observed due to intermolecular
interaction in solid state. However, the small red shift of only
5–7 nm in both absorption and PL spectra of these dendrimers
reflects that the introduction of carbazole dendrons to the
4,4′-positions of the twisted biphenyl core further render the
molecule in a nonplanar three-dimensional structure. Such a
molecular conformation can effectively inhibit unwanted inter-
molecular interaction. The phosphorescence emission spectra
of these dendrimers were measured in 2-MeTHF at 77 K; the
phosphorescence spectrum of G2 is illustrated in Figure 2b
as an example. The peak wavelength of the highest energy 0-0
band was used to calculate the triplet excited state energy (ET),
giving the values of 2.62, 2.60, and 2.61 eV for G1, G2, and G3,
respectively. It is obvious that these dendritic CBP derivatives
have almost the same triplet energies as the parent CBP (meas-
ured as 2.67 eV under the same conditions),[36] indicating that
such a molecular design strategy, by attaching higher genera-
tioin carbazoles via their N-position to the 3,6-positions of inner
carbazole moiety, does not significantly influence on the triplet
energy of the whole dendrimer molecules. The triplet energies
of these dendrimers are significantly higher than the values of
the commonly used green-emitting phosphor Ir(ppy)3 (2.41 eV,
Hppy = 2-phenylpyridine)[33] and other longer-wavelength emit-
ters. A higher value of ET for the host material is a necessary
provision for effective confinement of the triplet excitons on the
guest by preventing back energy transfer from guest to the host
molecules. In this case, these newly synthesized dendrimers
with high triplet energy levels are expected to serve as appro-
priate hosts for green- and even longer-wavelength emitting
dopants.
G1
G2
G3
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Potential
(
V vs Fc/Fc+
)
Figure 3. Cyclic voltammograms of G1, G2 and G3 measured in CH2Cl2
at a scan rate of 100 mV s−1 .
function of the widely-used hole injecting material PEDOT:PSS
(–5.2 eV),[17] implying that efficient hole injection into the emit-
ting layer could be expected when these dendrimers are used as
host materials in the emitting layer of OLEDs. The energy band
gaps were determined by the absorption edge technique,[37] and
the LUMO levels were calculated by subtracting the gap from
the energy of the HOMO. Detailed electrochemical and elec-
tronic data of these molecules are listed in Table 1.
2.4. Electrochemical Properties
2.5. Electroluminescent Properties of OLEDs
Cyclic voltammetry (CV) measurements were performed using
a conventional three-electrode cell setup with 0.1 M tetra-n-
butylammonium hexafluorophosphate (Bu4NPF6) as a sup-
porting electrolyte in CH2Cl2, and ferrocene as the internal
standard, to investigate the redox behavior of the dendrimers
and to estimate their frontier molecular orbital levels. The cyclic
voltamograms are shown in Figure 3. Upon the anodic sweep,
G1, G2, and G3 exhibit one, two, and three reversible oxidation
processes, respectively. No reduction signal was detected for
any of the dendrimers during the cathodic scan. As shown in
Figure 3, the first oxidation waves of these dendrimers shift to
less positive potentials, gradually with increasing dendrimer
generation from G1 to G3. This implies that the removal of
the first electron from these dendritic molecules become easier
with increasing dendron generation. This is consistent with
the previous observation that the electron density increases on
the outmost carbazole rings with increasing carbazole dendron
generation.[30] The half-potential (E1/2) of the first oxidation
wave are in the range of 0.62–-0.81 V relative to ferrocene for
these dendrimers. The HOMO energy levels were estimated
from the values of E1/2 to be –5.61, –5.45, and –5.42 eV for G1,
G2, and G3 respectively, according to the equation of HOMO
(eV) = −(E1/2+ 4.8 eV).[37] Apparently the HOMO levels of these
carbazole-based dendrimers are higher than those of both
CBP (–5.9 eV) and PVK (–5.8 eV) and quite close to the work
In order to evaluate the performance of the dendrimers when
used as hosts for phosphorescent emitters, three types of
PHOLEDs (A, B, and C) were fabricated by doping the green
or orange iridium complexes in the dendrimers as the emitting
layer. Based on the good solubility and film-forming abilities of
the dendrimers, the emitting layers of all the OLEDs in present
study were made by spin coating the mixed solution of these
dendrimers and a certain dopant. The green emitting Ir(ppy)3
was first selected as the dopant to prepare devices of type A,
which have the configuration of ITO/PEDOT:PSS (40 nm)/den-
drimer: 6 wt% Ir(ppy)3 (40 nm)/TPBI (40 nm)/LiF (1 nm)/Al
(100 nm). In these devices, PEDOT:PSS was used as the hole-
injecting layer, and TPBI is 1,3,5-tris[N-(phenyl)benzimidazole]-
benzene and was deposited by vacuum thermal evaporation
on top of the emitting layer to act as the electron-transporting
and hole-blocking layer. Devices A1, A2, and A3 contained G1,
G2, and G3 as host materials in emitting layer, respectively. All
these devices showed typical green emission at around 510 nm
originating from the guest Ir(ppy)3, as show by the EL spectra in
Figure S6. The absence of any residual emission from the den-
drimer hosts or TPBI layer even at high current densities sug-
gests a complete energy transfer from these dendrimer hosts
to the iridium dopant and the effective hole-blocking function
of the TPBI layer in addition to its electron-transporting role.
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140
120
100
80
Ir(ppy)3
Device A1
Device A2
Device A3
(CF3-bt)2Ir(acac)
2.33
2.29 2.30
2.7
2.7
6.2
2.8
Ir
3.25
60
Al
40
3
4.3
Ir(ppy)3
20
ITO/
PEDOT
5.2
F3C
0
4
5.2
6
8
10
12
14
16
5.39
5.42
5.45
Voltage (V)
S
N
5.61
O
O
Ir
Figure 4. Current density-voltage (J–V) characteristics for devices A1, A2,
OXD-7
and A3.
2
6.5
(CF3-bt)2Ir(acac)
Furthermore, there is no voltage dependence of the EL spectra
over the whole applied voltage range. Figure 4 displays the cur-
rent density-voltage (J–V) characteristics of the three devices A1,
A2, and A3. They turn on (to deliver a luminance of 1 cd m−2 )
at 5.9, 5.1, and 4.8 V, respectively. The current density of these
devices exhibited a regular variation in spite of the identical
device configuration. With increasing dendrimer generation,
the current density of the corresponding device at a given
voltage increases regularly and the J–V curve shifts to lower
voltages. This likely results from the facilitated hole injection in
the devices containing the higher-generation dendrimer hosts.
According to the energy level diagram in Figure 5, the relatively
higher HOMO levels of G2 (–5.45 eV) and G3 (–5.42 eV) match
the work function of the PEDOT (–5.2 e V) layer better than that
of G1, resulting in a lower hole injection barrier at the PEDOT/
dendrimer interface and thus a much lower driving voltage.
The G2 based device, A2, exhibited a maximum luminance of
11 991 cd m−2 at 16 V, and a maximum luminance efficiency ηL
LiF
Figure 5. Left) Energy level diagram for the fabricated OLEDs. Right)
Chemical structures of Ir(ppy)3 and (CF3-bt)2Ir(acac).
of 23.76 cd A−1 , corresponding to a peak power efficiency ηp of
8.22 lm W−1 and a forward-viewing external quantum efficiency
ηext of 7.01%. The G3 based device, A3, achieved maximum
efficiencies of 20.75 cd A−1 , 8.69 lm W−1 and 6.12% (Figure S7,
Supporting Information). The efficiencies of the present three
devices far exceed the best data available in the literature
(15 cd A−1 ) for a partially solution-processed phosphorescent
device with a PVK:Ir(ppy)3 emitting layer.[38] The detailed per-
formances of these devices are summarized in Table 2.
Based on the good performance of device type A and the
hole-transporting feature of these carbazole based dendrimer
hosts, the electron-transporting component was introduced into
T a b le 2 . EL data of the different OLED devices based on different dendrimers hosts and dopants.
Devicea)
Host
Dopant
Von
Lmax
CIE (x, y) at 8 V
b)
c)
c)
c)
c)
ηL
ηp
ηext
[cd m−2 , V]
10930, 16
11991, 16
11300, 14
19220, 14
22020, 14
19820, 13
12820, 15
9745, 11
[cd A−1 , V]
18.25, 10
23.76, 10
20.75, 8
34.49, 9
38.71, 8
36.38, 7
27.70, 10
22.21, 5
32.22, 5
30.49, 5
[lm W−1 , V]
6.36, 8
[%]
[V]
5.9
5.1
4.8
5.0
4.5
3.8
5.0
4.1
3.9
3.8
A1
A2
A3
B1
B2
B3
B4
C1
C2
C3
G1
G2
G3
G1
G2
G3
PVK
G1
G2
G3
Ir(ppy)3
Ir(ppy)3
5.38
0.28, 0.62
0.28, 0.63
0.27, 0.62
0.27, 0.62
0.28, 0.63
0.27, 0.62
0.28, 0.63
0.51, 0.47
0.51, 0.48
0.51, 0.48
8.22, 8
7.01
Ir(ppy)3
8.69, 7
6.12
Ir(ppy)3
13.15, 8
15.69, 7
18.59, 6
9.58, 9
10.17
11.42
10.73
8.17
Ir(ppy)3
Ir(ppy)3
Ir(ppy)3
(CF3-bt)2Ir(acac)
(CF3-bt)2Ir(acac)
(CF3-bt)2Ir(acac)
13.94, 5
20.23, 5
19.15, 5
7.88
12050, 11
10810, 10
11.44
10.82
a)Devices configurations, A: ITO/PEDOT:PSS (40 nm)/dendrimer: 6 wt% Ir(ppy)3 (40 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm); B: ITO/PEDOT:PSS (40 nm)/dendrimer
or PVK: 30 wt% OXD-7: 6 wt% Ir(ppy)3 (40 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm); C: ITO/PEDOT:PSS (40 nm)/dendrimer: 30 wt% OXD-7: 8 wt% (CF3-bt)2Ir(acac)
(40 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm); b)Recorded at 1 cd/m2; c)Maximum values of the devices. The values after the comma are the voltages at which the data
are obtained.
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Adv. Funct. Mater. 2012,
DOI: 10.1002/adfm.201201326

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the data for devices with similar structures reported in the lit-
erature.[39,40] For example, Nakamura et al. reported that their
green PHOLEDs containing a PVK:OXD-7:Ir(ppy)3 active layer
achieved a maximum luminance efficiency of 24 and 31 cd A−1 ,
using Ca/Al and Cs/Al as the cathode material, respectively.[39]
Figure 6 clearly illustrates the performance evolution by opti-
mizing the device structure and utilizing the novel dendrimer
host to replace the traditional PVK host. Like in many OLEDs,
especially electrophosphorescent devices, efficiency roll-off
was observed in these device with increasing current density,
which can be typically attributed to a combination of triplet-
triplet annihilation and field-induced quenching effects.[10b,22]
Evidently, the G2 and G3 based devices show significantly
improved performance, with lower turn-on voltages, higher
brightness and emission efficiencies, as compared to those of
PVK. Moreover, the comprehensive performance of present
type B devices is superior to that of solution-processed green
PHOLEDs containing dendrimer hosts,[24a] and also close to
that of a solution-processed green device with dendritic iridium
phosphor doped as small molecular host.[24d] It should be noted
that the G3 based devices A3 and B3 have lower turn-on voltages
and higher power efficiencies than the corresponding devices
A2 and B2 containing G2. This is likely because the G3 host
has a slightly higher HOMO level than G2 and more efficient
hole injection occurs at the PEDOT/G3 interface. However, the
luminance efficiencies and external quantum efficiencies for
devices A3 and B3 were lower relative to those of devices A2
and B2. This is probably because the remarkably high hole cur-
rent in G3 based devices may lead to the imbalance of positive
and negative charge carriers in the emitting layer, which may
be unfavorable for a high charge-recombination efficiency. In
balance, the device performance improvement with increasing
dendrimer generation from G2 to G3 is not as great as from
G1 to G2. This indicates that G2 may be the most suitable den-
drimer generation for ideal device performance, considering
the comparatively complicated synthesis for higher generation
dendrimers.
160
(a)
10000
1000
100
10
Device A2
140
Device B2
Device B4
120
100
80
60
40
20
0
1
4
6
8
10
12
14
16
Voltage (V)
40
36
32
28
24
20
16
12
8
35
30
25
20
15
10
5
Device A2
Device B2
Device B4
(b)
4
0
0
0
20
40
60
80
(
100 120 140
Current density
mA/cm2
)
Figure 6. a) Voltage-current density-luminance (V-J-L) characteristics,
and b) luminance efficiency and power efficiency as a function of current
density for devices A2, B2, and B4.
the emitting layer of device type B to further facilitate charge
transport balance and to improve device performance. Devices
of type B have the structures ITO/PEDOT:PSS (40 nm)/den-
drimer: 30 wt% OXD-7: 6 wt% Ir(ppy)3 (40 nm)/TPBI (40 nm)/
LiF (1 nm)/Al (100 nm). OXD-7 is 2,2′-(1,3-phenylene)bis[5-
(4-tert-butylphenyl)-1,3,4-oxadiazole] and is incorporated into
the host matrix to facilitate electron-injection due to its lower-
lying LUMO level (–2.7 eV) than that of the dendrimers and
to enhance the charge balance due on its electron-transporting
capability. As shown in Figure 6, the G2 based device, B2,
exhibited a lower turn-on voltage and higher current density at
a given voltage than device A2. At the same time, the maximum
efficiencies of devices of type B are dramatically improved as
well in comparison with type A devices. For example, the G2
based device B2 achieved peak efficiencies of ηL = 38.71 cd A−1
and ηp = 15.69 lm W−1 , and the G3 based device B3 exhibited
similarly high values of 36.38 cd A−1 and 18.59 lm W−1 , much
higher than those of the corresponding device A. The detailed EL
data of these devices are summarized in Figure S8 and Table 2.
For comparison, we also prepared a control device employing
PVK as the host material with the same architecture of device
B. The control device exhibited a turn-on voltage of 5 V and a
maximum efficiency of 27.70 cd A−1 , which are comparable to
In order to explore the versatility of the dendrimers as host
materialsforvariouscolorphorphors,theorange-emittingiridium
complex,
bis(2-phenyl-6-trifluoromethylbenzothiozolato)-
iridium(acetylacetonate) [(CF3-bt)2Ir(acac)], was also selected as
a doped emitter to fabricate type C devices, with the configu-
ration ITO/PEDOT:PSS (40 nm)/dendrimer: 30 wt% OXD-7:
8 wt% (CF3-bt)2Ir(acac) (40 nm)/TPBI (40 nm)/LiF (1 nm)/Al
(100 nm). All the devices C1–3 show orange emission peaked
at 570 nm with CIE coordinates of (0.51, 0.48) exclusively from
(CF3-bt)2Ir(acac), as shown by the EL spectra in Figure S6 (Sup-
porting information). The current density-voltage-luminance
(J–V–L) characteristics and the efficiency curves of the orange
electrophosphorescence for the type C devices are illustrated in
Figure 7 and the data are listed in Table 2. The current density
and efficiencies of the type C devices follow a similar trend as
observed for the type A and B devices with changing the den-
drimer generation. The G2 and G3 based devices show max-
imum efficiency of 32.22 cd A−1 , 20.23 lm W−1 and 30.49 cd A−1 ,
19.15 lm W−1 , respectively. Orange-emitting devices fabricated
by spin-coating have rarely been reported in the literature.[41,42]
For example, Schubert et al. presented solution-processed
PhOLEDs using an orange-emitting crosslinkable iridium(III)
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DOI: 10.1002/adfm.201201326
2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
7

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thus low driving voltages and high power efficiency in OLEDs.
These novel dendrimer hosts are suitable for a wide color range
of dopants from green to red. The solution-processed green and
orange phosphorescent OLEDs containing these dendritic hosts
and iridium complex dopants exhibited excellent performances,
which greatly outweighed those containing the PVK host. Com-
parable device performances were obtained when increasing
the dendron generation from G2 to G3, despite the higher sta-
bilities of G3. This indicates that the second generation should
be the most preferred molecular dimension for these den-
drimer hosts considering the more complicated synthesis for
the higher generations. Given the ease of synthesis and some
performance advantages over the traditional small molecular
or polymeric phosphorescent hosts, these carbazole-based den-
drimers will be perfect alternatives as triplet host materials for
solution-processible OLEDs.
10000
1000
100
10
(a)
140
Device C1
Device C2
Device C3
120
100
80
60
40
20
0
1
4
6
8
10
12
Voltage (V)
35
30
25
20
15
10
5
Device C1
Device C2
Device C3
(b)
30
25
20
15
10
5
4. Experimental Section
General Information: Chemicals, reagents and solvents from
commercial sources were of analytical or spectroscopy grade and
1
used as received without further purification. H and 13C NMR spectra
were recorded on a Varian INOVA spectrometer (400 or 100 MHz).
Mass spectra were recorded on a GC-TOF MS (Micromass, UK) mass
spectrometer for TOF-MS-EI and a MALDI micro MX (Waters, USA)
for MALDI-TOF-MS. Elemental analyses were carried out on a Carlo-
Eriba 1106 elemental analyzer. Thermogravimetry analyses (TGA) and
differential scanning calorimetry (DSC) measurements were carried
out using a Perkin–Elmer thermogravimeter (Model TGA7) and a
Netzsch DSC 204 at a heating rate of 10 °C min−1 under a nitrogen
atmosphere, respectively. The fluorescence and UV-vis absorption
0
0
0
20
40
60
80
100 120 140
Current density
(
mA/cm2
)
spectra measurements were performed on
a Perkin-Elmer LS55
Figure 7. a) Voltage-current density-luminance (V-J-L) characteristics,
and b) luminance efficiency and power efficiency as a function of current
density for devices C1, C2, and C3.
fluorescence spectrometer and a Perkin-Elmer Lambda 35 UV-visible
spectrophotometer, respectively. The phosphorescence spectra of
the dendrimers were measured in 2-MeTHF glasses at 77 K on a
Horiba Jobin Yvon Fluoro Max-4 (TCSPC) instrument. Electrochemical
measurements were made by using a conventional three-electrode
configuration and an electrochemical workstation (BAS100B, USA) at a
scan rate of 100 mV s−1 . A glassy carbon working electrode, a Pt-wire
counter electrode, and a saturated calomel electrode (SCE) as the
reference electrode were used. All measurements were made at room
temperature on samples dissolved in dichloromethane, with 0.1 M tetra-
n-butylammonium hexafluorophosphate (Bu4NPF6) as the electrolyte
and ferrocene as the internal standard.
OLEDs Fabrication and Measurements: The precleaned ITO glass
substrates (30 Ω sq−1 ) were treated by UV-ozone for 20 min. PEDOT/PSS
(Bayer AG) was spin-coated on pretreated ITO substrates from aqueous
dispersion and baked at 120 °C for 1 h. Subsequently, a mixture of the
Ir complexes and the host was spin-coated from chlorobenzene solution
onto the PEDOT:PSS layer, the thickness of which was controlled as 40 nm
by adjusting the solution concentration and the spin rate. The substrate
was transferred into a vacuum chamber to deposit the TPBI layer with a
base pressure less than 10−6 Torr (1 Torr = 133.32 Pa). Finally, the device
fabrication was completed through thermal deposition of LiF (1 nm) and
then capping with Al metal (100 nm) as cathode. The emitting area of each
pixel is determined by overlapping of the two electrodes as 9 mm2. The
EL spectra, CIE coordinates, and current-voltage-luminance relationships
of devices were measured with computer-controlled Spectrascan PR 705
photometer and a Keithley 236 source-measure-unit. The forward viewing
external quantum efficiency (ηext) was calculated by using the luminance
efficiency, EL spectra and human photopic sensitivity. All measurements
were carried out at room temperature.
complex as the dopant, reaching a maximum luminance effi-
ciency of 18.4 cd A−1 (15.4 lm W−1 ) with CIE coordinates of (0.55,
0.44).[41] Phosphorescent OLEDs with orange-emitting iridium
complexes with efficiencies of 29.77 cd A−1 and 13.35 lm W−1
and CIE coordinates of (0.49, 0.50) were demonstrated by Wong
et al.[42] The EL performances of the devices we present here
are thus among the best ever reported for solution-processed
orange PHOLEDs.
3. Conclusion
We have demonstrated the successful evolution of the tradi-
tional small molecular phosphorescent host CBP to its dendritic
analogues, G1, G2, and G3, by incorporating oligo-carbazole
dendrons to the biphenyl skeleton. These dendrimer hosts
inherit similar HOMO and LUMO energy levels and identical
triplet energies to the parent CBP, but are superior in the sense
that they exhibit much higher thermal and amorphous stability
with extremely high glass transition temperatures (300 °C and
376 °C for G2 and G3), and that they are solution-processible.
In comparison with another widely-used phosphorescent host,
PVK, which is also suitable for wet methods but with a much
lower HOMO level, these dendritic hosts are improved because
their higher HOMO levels guarantee efficient hole injection, and
Compounds Synthesis: The carbazole dendrons D1, D2, D3 and their
intermidiates 1, 2, 3, 4, 5 were synthesized according to the literature
methods.[24c,30]
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2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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DOI: 10.1002/adfm.201201326

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G1: A mixture of compound D1 (1.51 g, 5.4 mmol), 4,4′-diiodobiphenyl
(1.0 g, 2.5 mmol), Cu-bronze (80 mg, 1.25 mmol), and K2CO3
(2.4 g, 17.5 mmol) were added to a 3-necked flask, and then 18-crown-6
(165 mg, 0.625 mmol) and o-dichlorobenzene (70 mL) were added under
a nitrogen atmosphere. After stirring for 36 h at 110 °C, the solvent was
removed under reduced pressure. Ammonia solution (30 mL) was added
and the mixture was stand for 2 h. Water was added and the mixture
was extracted with dichloromethane. The combined organic phases
were separated and washed with dilute HCl and brine, then dried over
anhydrous MgSO4. The solvent was removed to dryness and the residue
was purified by column chromatography over silica gel with petroleum
ether/dichloromethane (4:1) as the eluent to give G1 as a white solid
(700 mg, 40% yield).
Acknowledgements
We thank the National Natural Science Foundation of China
(21072026, 20923006), the Ministry of Education for the New Century
Excellent Talents in University (Grant NCET-08-0074), the NKBRSF
(2009CB220009), and the Fundamental Research Funds for the Central
Universities (DUT12ZD211) for financial support of this work.
Received: May 16, 2012
Revised: August 9, 2012
Published online:
1H NMR (400 MHz, CDCl3, δ): 8.15 (d, J = 8.4 Hz, 4H; ArH),
7.89 (d, J = 8.4 Hz, 4H; ArH), 7.69 (d, J = 8.8 Hz, 4H; ArH), 7.50-
7.41 (m, 8H; ArH), 1.47 (s, 36H; CH3); MALDI-TOF-MS (m/z): [M+]
calculated for C52H56N2, 708.4443; Found, 708.0338. Analytically
calculated for C52H56N2: C, 88.09; H, 7.96; N, 3.95. Found: C, 88.17;
H, 7.81; N, 3.86.
G2: A mixture of compound D2 (1.02 g, 1.4 mmol), 4,4′-diiodobiphenyl
(260 mg, 0.64 mmol), CuI (2.4 g, 0.128 mmol), K3PO4 (410 mg,
1.92 mmol) were added to a 2-necked flask, and then ( )-trans-1,2-
cyclohexanediamine (1.7 μL, 0.014 mmol) and toluene (60 mL) were
added under a nitrogen atmosphere. After stirring for 48 h at 110 °C,
the reaction mixture was cooled to room temperature. The solvent was
removed under reduced pressure, dichloromethane and water were
added. The organic layer was separated and washed with dilute HCl and
brine, then dried over anhydrous MgSO4. The solvent was removed to
dryness and the residue was purified by column chromatography over
silica gel with petroleum ether/dichloromethane (3:1) as the eluent and
then washed with petroleum ether to give G2 as a white solid (450 mg,
44% yield).
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1H NMR (400 MHz, CDCl3, δ): 8.28 (s, 4H; ArH), 8.18 (s, 8H; ArH),
8.07 (d, J = 8.4 Hz, 4H; ArH), 7.92 (d, J = 8.4 Hz, 4H; ArH), 7.77 (d, J =
8.8 Hz, 4H; ArH), 7.67-7.64 (m, 4H; ArH), 7.49-7.46 (m, 8H; ArH), 7.37
(d, J = 8.8 Hz, 8H; ArH), 1.47 (s, 72H; CH3); 13C NMR (400 MHz, CDCl3,
δ): 142.67, 140.39, 140.20, 139.83, 136.99, 131.13, 128.93, 127.68,
126.10, 124.18, 123.62, 123.22, 119.43, 116.29, 111.23, 109.14, 34.78,
32.09; MALDI-TOF-MS (m/z): [M+] calculated for C116H116N6, 1593.9295;
Found, 1593.8462. Analytically calculated for C116H116N6: C, 87.39; H,
7.33; N, 5.27. Found: C, 87.44; H, 7.21; N, 5.25.
G3:
A mixture of compound D3 (925 mg, 0.54 mmol),
4,4′-Diiodobiphenyl (100 mg, 0.25 mmol), CuI (9.5 mg, 0.05 mol), K2CO3
(240 mg, 1.75 mmol) in freshly distilled nitrobenzene (50 mL) was
stirred at reflux under a nitrogen atmosphere for 72 h. After evaporation
of the solvent under reduced pressure, water was added and the mixture
was extracted with dichloromethane. The combined organic phases
were washed with diluted HCl and brine, then dried over anhydrous
MgSO4. The solvent was removed to dryness and the residue was
purified by column chromatography over silica gel with petroleum
ether/dichloromethane (3:1) as the eluent and then recrystallized from
methanol to give G3 as a white solid (150 mg, 18% yield).
1H NMR (400 MHz, CDCl3, δ): 8.59 (s, 4H; ArH), 8.28 (s, 8H; ArH),
8.18 (s, 20H; ArH), 8.03 (d, J = 8.4 Hz, 4H; ArH), 7.94-7.88 (m, 8H;
ArH), 7.68-7.61 (m, 16H; ArH), 7.47-7.43 (m, 16H; ArH), 7.36 (d, J =
8.4 Hz, 16H; ArH), 1.46 (s, 144H; CH3); 13C NMR (400 MHz, CDCl3,
δ): 142.59, 141.39, 141.13, 140.23, 130.85, 130.28, 129.18, 127.89,
126.07, 124.35, 123.85, 123,57, 123,15, 119.47, 116.24, 111.07, 109.11,
34.74, 32.05; MALDI-TOF-MS (m/z): [M+] calculated for C244H236N14,
3363.8965; Found, 3364.0522. Analytically calculated for C244H236N14: C,
87.10; H, 7.07; N, 5.83. Found: C, 87.14; H, 6.98; N, 5.85.
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Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
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wileyonlinelibrary.com
Adv. Funct. Mater. 2012,
DOI: 10.1002/adfm.201201326
2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
9

www.afm-journal.de
www.MaterialsViews.com
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Adv. Funct. Mater. 2012,
DOI: 10.1002/adfm.201201326