A highly efficient universal bipolar host for blue, green, and red phosphorescent OLEDs

Article abstract of DOI:10.1002/adma.201000061

The bipolar host material BCPO (bis-4(N-carbazolyl)phenylphosphine oxide) containing a phosphine oxide and two carbazole groups, synthesized in three steps, shows a high triplet energy gap of 3.01 eV. The material can be used as a universal host for blue,

Full text of DOI:10.1002/adma.201000061

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A Highly Efficient Universal Bipolar Host for Blue,
Green, and Red Phosphorescent OLEDs
By Ho-Hsiu Chou and Chien-Hong Cheng*
Organic electrophosphorescence devices have continued to
attract intense interest because their intrinsic efficiencies may
be up to four times higher as the fluorescence ones.[1] In the
design of highly efficient phosphorescent devices, the selection of
a proper material for each layer is of great importance. In
particular, the choice of host material for the phosphorescent
dopant in the emitting layer fundamentally affects the efficiency
of energy transfer from the host to the dopant. In general, to
prevent back energy transfer from a dopant to the host, the host
matrix should have a larger triplet energy gap (ET) than the
dopant, but a much larger ET of the host would induce charge
trapping by the dopant leading to a high driving voltage. High
carrier drift mobilities and balance of carrier fluxes are also
essential for the host in order to reduce the driving voltage and
increase the current and power efficiencies. In addition, a high
glass transition temperature (Tg) is desired for a host to increase
the thermal and morphological stability to lengthen the device
lifetime.
o-CzOXD[2] for both green and red phosphorescent devices. It
still is a great challenge to design and synthesize a bipolar material
with a high enough triplet energy gap that is suitable for use as a
blue phosphorescent dopant and with a balance of hole and electron
fluxes that can lead to excellent performance of the device.
Carbazole derivatives are widely used as the host materials
because of the high triplet energy gaps and excellent hole
transporting properties.[10] The most commonly used carbazo-
le-based material, 4,40-bis(N-carbazolyl)-1,10-biphenyl (CBP), has
been used as the host material for green and red phosphorescent
emitters.[14–20] However, high driving voltages were often
observed for the devices due to its low electron drift mobility.[11,12]
Other disadvantages are the low Tg (62 8C)[13] leading to easy
crystallization, especially when the dopant concentration is
low[14,15] and its low triplet energy ET of 2.56 eV even too low for
sky-blue phosphor, iridium(III) bis(4,6-(difluorophenyl)-
pyridinato-N,C20)picolinate (FIrpic).[16]
To increase the glass-transition temperature, triplet energy,
and the electron transporting ability of CBP, we proposed to
modify CBP by inserting a phosphine oxide moiety PhPO, known
to be a point of saturation and an electron-withdrawing group,
into the middle of the structure. The resulting compound
Recently, bipolar materials as hosts for organic electropho-
sphorescence devices have caught great attention. A great advantage
of using a bipolar host is the improvement of carrier mobilities and
balance of electron and hole fluxes in the emitting layer.[2] However,
a drawback is that the introduction of an electron-donating and
-withdrawing moiety to the host molecule would lead to an
intramolecular charge transfer resulting in the reduction of
energy gap of the molecule. Several bipolar hosts for sky-blue
phosphorescent devices were synthesized including a pyridine/
carbazole hybrid, 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy)
bis-4-(N-carbazolyl)phenyl)phenylphosphine
oxide
(BCPO,
Figure 1) is expected to have reduced conjugation, a lower
LUMO level, and bipolar properties, in addition to higher a ETand
Tg than CBP. BCPO was easily synthesized in three simple steps
from 4-bromophenylcarbazole (BrPCz) by treating with n-butyl
lithium, and then reaction with dichlorophenylphosphine to give
the corresponding phosphine product. Oxidation of the phos-
phine product with H2O2 (30%) afforded the desired product as a
white solid (see Supporting Information (S. I.), S. I. Scheme 1).
Further purification by sublimation at 325 8C gave BCPO in 61%
total yield. The 1H, 13C, and 31P NMR spectra, mass spectral data,
and elemental analysis of the product are in agreement with the
proposed structure.
The UV–vis absorption and photoluminescence spectra at
room temperature and 77 K of BCPO are shown in Figure 1, while
the peak maxima of these spectra are summarized in S. I. Table 1.
As revealed in Figure 1, the absorption and fluorescence of BCPO
are all within the UV range and phosphorescence spectra in the
near-UV and blue region. The fluorescence maximum of BCPO
in dichloromethane appears at 388 nm at 10ꢀ5 M and shows a
slight red-shift at 10ꢀ3 M and in the thin film state, suggesting
that there is no severe p–p stacking of the BCPO molecules in the
thin film state. The phosphorescence spectrum of BCPO at 77 K is
nearly the same as that of N-(4-bromophenyl)carbazole (BrPCz)
and some other N-phenyl carbazole-based molecules.[17,18] As a
result, the triplet energy gap of BCPO, estimated from the high
energy emission maximum of the phosphorescence spectrum of
by Kido et al.,[3]
a phosphine oxide/carbazole hybrid,
N-(4-diphenylphosphoryl phenyl) carbazole (MPO12) by Sapochak
and his coworkers and phosphine oxide/phenylamine bipolar
materials,[4,5] 2,7-bis(diphenylphosphineoxide)-9-(9-phenylcarbazol-
3-yl)-9-phenylfluorene (PCF) and 2,7-bis(diphenylphosphoryl)-
9-[4-(N,N-diphenylamino) phenyl]-9-phenylfluorene (POAPF) by
Shu and Chou et al.[6,7]. Bipolar hosts for green phosphorescent
devices were also known. Gao et al. reported a pyridine/carbazole
hybrid, 4,7-dicarbazol-9-yl-[1,10]-phenanthroline (BUPH1),[8] for
Ir(ppy)3-based green device, while Lee et al. used a non conjugated
silicon-cored spirobifluorene derivative (SBP-TS-PSB) as a host
material for the red phosphorescent Ir(III) complexes.[9] Recently,
Ma et al. reported a carbazole/oxadiazole hybrid molecule
[*] Prof. C.-H. Cheng, H.-H. Chou
Department of Chemistry
National Tsing Hua University
Hsinchu 30013 (Taiwan)
E-mail: chcheng@mx.nthu.edu.tw
DOI: 10.1002/adma.201000061
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HOMO and the phenylphosphine oxide group is responsible for
the LUMO of the BCPO molecule.
To find further support for the bipolar character of BCPO, a
hole-only and an electron-only device were fabricated. The hole-
only device consists of the following structure: ITO/NPB (10 nm)/
BCPO (30 nm)/NPB (10 nm)/Al (100 nm), while the electron-only
device contains the following layers: ITO/BCP (10 nm)/BCPO
(30 nm)/BCP (10 nm)/LiF (1 nm)/Al (100 nm). NPB (N,N0-bis-
(1-naphthyl)-N,N0-diphenyl,1,10-biphenyl-4,40-diamine) and BCP
(2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) layers were
used to prevent electron and hole injection from the cathode
and anode, respectively.[8] The current density versus
voltage curves of these two devices (see S. I. Figure 3) show
that the hole-only device gave slightly lower current density than
the electron-only device at applied voltage between 2–14 V. The
observations strongly support that BCPO is a bipolar material
capable of transporting electrons and holes.
The use of BCPO as the host material for phosphorescent
devices was examined next. Device B1 using FIrpic as the dopant
with device structure ITO/NPB (30 nm)/mCP (20 nm)/BCPO:
FIrpic (8%) (25 nm)/TAZ (50 nm)/LiF(1 nm)/Al (100 nm) was
thus fabricated. In this device, NPB acts as hole-transporting
material, mCP as a hole-transporting material and also as an
exciton blocker (ET, 2.9 eV) to prevent diffusion of exciton to the
NPB layer[20,21] and TAZ as the electron-transporting and hole
blocking layer.[10] The device performance, as summarized in
Table 1, Figure 2 and S. I. Figure 4, is extraordinary with a low
turn-on voltage of 2.8 V and a maximum external quantum
efficiency, current efficiency, and power efficiency of 23.5%,
45.1 cd Aꢀ1, and 40.6 lm Wꢀ1, respectively. At a brightness level of
1000 and 5000 cd mꢀ2, the external quantum efficiencies still retain
as high as 19.4% and 16.4%. The performance is comparable with
the previously reported highest values for FIrpic-base devices.[7,22]
To see the capability of BCPO as a host material for deeper blue
phosphorescent devices, FIr6[23] was chosen as the dopant for
device B2 having the same structure as B1 except the dopant. The
device shows a low turn-on voltage of 3.0 V and a maximum
external quantum efficiency, current efficiency, and power
efficiency of 19.8%, 36.8 cd Aꢀ1, and 33.1 lm Wꢀ1, respectively,
with CIE (Commission International de l’Eclairage) values of
(0.14, 0.25) at 6 V. These device efficiencies are much higher than
previously reported values for FIr6-based devices.[23–25]
Figure 1. The UV–vis absorption and fluorescence spectra in dichloro-
methane and in the thin film state of BCPO at room temperature and
phosphorescence spectra of BCPO and BrPCz in 2-methyltetrahydrofuran
at 77 K. Inset: the structure of BCPO.
3.01 eV (412 nm) is also very close to that of BrPCz of 3.02 eV
(410 nm) (see S. I. Table 1). The result indicates that the
phosphine oxide moiety does effectively block the electron
communication between the two N-phenyl carbazole groups
leading to much higher triplet energy of BCPO than CBP.
The electrochemical properties of BCPO and the related
compounds Ph3PO and BrPCz were investigated by cyclic
voltammetry (CV) and the cyclic oxidation voltammogram (S. I.
Figure 1) of BCPO is similar to that of BrPCz showing two
cathodic peaks characteristic of a carbazolyl group,[7,19] although
the oxidation potential of BCPO is larger than that of BrPCz by
0.17 eV (S. I. Table 1). On the other hand, the reduction
voltammogram of BCPO is like that of triphenylphosphine oxide,
but is more negative by 0.05 eV. The HOMO and LUMO levels of
BCPO calculated from its oxidation and reduction potentials are
5.76 and 2.19 eV, respectively, and the energy gap Es thus obtained
is 3.57 eV. It is interesting to note that the energy gap of BCPO
and BrPCz are identical within experimental error (see S. I. Table l).
To gain insight into the electronic states of BCPO, DFT
calculation was performed. The results show that its HOMO is
distributed mostly over the two carbazole groups and to a lesser
extent over the two phenylene groups (see S. I. Figure 2). On the
other hand, the LUMO is localized mainly on the two phenylene
groups and to a lesser degree on the phenyl group and the
nitrogen and oxide atoms. These results are in agreement with
the electrochemical studies that the carbazole groups control the
To our surprise, BCPO is also an excellent host for green and
red phosphorescent devices. The commonly used Ir(ppy)3 and
Ir(piq)3 were selected as the green and red dopant, respectively,
and a common structure ITO/NPB (10 nm)/TCTA (20 nm)/
BCPO:dopant (ꢁ7–8%, 30 nm)/BCP (10nm)/Alq (50 nm)/LiF
Table 1. Performances of BCPO–based OLEDs[a]
Device[b]
Von[c] [V]
L [cd mꢀ2, V]
hext [%, V]
hc [cd/A, V]
hp [lm/W, V]
CIE, (x,y) [6 V]
B1
B2
G
2.8
3.0
2.1
2.7
35677, 12.5
28718, 12.0
207839, 13.5
24529, 16.5
23.5, 3.5
19.8, 3.5
21.6, 3.0
17.0, 3.0
45.1, 3.5
36.8, 3.5
83.4, 3.0
19.4, 3.0
40.6, 3.5
33.1, 3.5
87.5, 3.0
20.4, 3.0
0.13, 0.30
0.14, 0.25
0.28, 0.65
0.67, 0.33
R
[a] The luminescence (L), external quantum efficiency (hext), current efficiency (hc) and power efficiency (hp) are the maximum values. [b] See text for the device structures. [c] The
applied voltage (Von) required for brightness of 1 cd mꢀ2
.
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giving extremely high device efficiencies. Further application of
this host material in the white light-emitting devices and
full-color displays is underway.
Experimental
Organic Light-emitting Diode (OLED) Fabrication and Measurements
[28]: Organic chemicals used for fabricating devices were generally purified
by high-vacuum, gradient temperature sublimation. The EL devices were
fabricated by vacuum deposition of the materials at 10ꢀ6 Torr onto a glass
precoated with a layer of indium tin oxide with a sheet resistance of 10 V
squareꢀ1. The rest of the procedure is essentially the same as reported [28].
As a reference, a reported device ITO/CuPc (10 nm)/NPB (30 nm)/mCP:
6 wt% FIrpic (30 nm)/BAlq (40nm)/LiF (1 nm)/Al (100 nm) was also
fabricated [17b]. The data produced from this device [28] are comparable
with those reported previously by Thompson et al. [17b].
Procedure for the Synthesis of bis-4-(N-carbazolyl)phenyl)phenylphosphine
oxide (BCPO): 4-Bromophenylcarbazole (0.644 g, 2.00 mmol) was dis-
solved in anhydrous THF (100 mL) under nitrogen and was cooled to
ꢀ78 8C in a dry ice/acetone bath. n-Butyl lithium (0.84 mL, 2.10 mmol,
2.5 M in hexane) was then added dropwise slowly to give a bright yellow
solution that was thickened to form a slurry. The temperature was not
allowed to rise above ꢀ70 8C during the addition. After stirring for 1 h at
ꢀ78 8C, dichlorophenylphosphine (0.179 g, 1.00 mmol) in THF (50 mL) of
was added to give a clear, pale yellow solution. The reaction solution was
returned to ambient temperature and was then stirred for 12 h before
quenching with 50 mL of water. The mixture was extracted with
1,2-dichloromethane (100 mL). The combined organic phases were dried
(MgSO4) and concentrated under reduced pressure to give a crude
compound. The crude material was purified by column chromatography
(Rf ¼ 0.70, SiO2, ethyl acetate/hexanes ¼ 1:9). The white material obtained
was dissolved in 1,2-dichloromethane (60 mL) and to the solution was
added 30% aqueous H2O2 (20 mL). The mixed solution was stirred for 1 h
at room temperature. The organic and aqueous portions in the mixed
solution were separated and the aqueous portion was extracted with
dichloromethane two times and the combined organic layer was dried over
MgSO4. The solvent was evaporated and the product was sublimed at
350 8C (1 ꢃ 10ꢀ5 Torr) to give the desired final product in 61% total yield.
1H NMR (400 Hz, CDCl3, d): 8.14 (d, J ¼ 8 Hz, 4H), 8.00 (m, 4H), 7.89 (m,
2H), 7.78 (d, J ¼ 7.2 Hz, 4H), 7.66–7.60 (m, 3H), 7.52 (d, J ¼ 8.0 Hz, 4H),
7.42 (t, J ¼ 7.6 Hz, 4H), 7.31 (t, J ¼ 7.2 Hz, 4H); 13C NMR (100 MHz,
CDCl3, d): 141.6 (C4), 140.1 (C4), 134.1 (C3), 132.5 (C3), 132.3 (C3), 129.0
(C3), 126.8 (C3), 126.2 (C3), 123.8 (C4), 120.6 (C3), 120.5 (C3), 109.8 (C3);
Figure 2. External quantum efficiency versus luminance for devices B1, B2,
G, and R. Inset: the corresponding EL spectra.
(1 nm)/Al (100 nm) was used for both devices. In these two
devices, we choose TCTA as the exciton blocker to prevent exciton
diffusion to the hole transporting layer. TCTA has been proven to
be an efficient exciton blocker for red phosphorescence devices,[9]
It is noteworthy that for devices B1 and B2, we used mCP instead
of TCTA as the exciton blocker, because the triplet energy
(ET ¼ 2.7 eV) of the latter is not large enough to effectively prevent
diffusion of blue excitons. The green device G (dopant ¼ Ir(ppy)3,
8%) reveals an extremely low turn-on voltage of 2.1 V and a
maximum external quantum efficiency, current efficiency, aꢀn1d
power efficiency of 21.6%, 83.4 cd Aꢀ1, and 87.5 lm W
,
respectively. Moreover, a maximum luminance of 207839 cd mꢀ2
at 13.5 V with CIE values of (0.28, 0.65) at 6 V was reached. At a
brightness level of 1000 and 5000 cd mꢀ2, the external quantum
efficiency of device G still retains as high as 19.1% and 17.6%.
Device R (dopant ¼ Ir(piq)3, 7%) exhibits a low turn-on voltage of
2.7 V, maximum external quantum efficiency, current efficiency,
and power efficiency of 17.0%, 19.4 cd Aꢀ1, and 20.4 lm Wꢀ1
,
respectively. The CIE values for this red device are (0.67, 0.33) at
6 V. All these BCPO-based blue, green and red devices revealed
very low turn-on voltage (ꢂ3 V) and extremely high external
quantum, current and power efficiencies. It is fascinating to
mention that by using BCPO as the common host, the
performance of these basic-color devices, as summarized in
Table 1, all appears to be among of the best ones that have been
reported. To the best of our knowledge, there are few reports
using a common host material for blue, green, and red
phosphorescent devices,[18,26] but the performance of these
devices are relatively low. However, as exhibited in Figure 2, the
efficiencies of these devices reduce at high brightness. This is
likely due to the enhanced triplet–triplet annihilation at high
density of triplet excitons at high brightness[27] or the unbalance
of holes and electrons in the device at high current density.
In summary, we have demonstrated that the bipolar host
material BCPO containing a phenylphosphine oxide and two
carbazole groups, synthesized conveniently in only three simple
steps, shows a high Tg of 137 8C and a high ET of 3.01 eV. This
material can be applied as a universal host to blue, green and red
phosphorescent devices using simple device architectures but
HRMS (FABþ): calcd for C42H29NOP, 608.2018; found, (Mþ) 608.2021. 31
P
NMR (CDCl3, 295 K): d 29.56. Anal. calcd. for C42H29NOP: C 82.88, H 4.80,
N 4.60; Found: C 82.64, H 4.85, N, 4.59.
Acknowledgements
We thank the Ministry of Economy of the Republic of China
(95-EC-17-A-08-S1-042) for support of this research. Supporting Informa-
tion is available online from Wiley InterScience or from the author.
Received: January 8, 2010
Revised: January 21, 2010
Published online: May 5, 2010
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