A donor-acceptor-type host material for solution-processed phosphorescent organic light-emitting devices showing high efficiency

Article abstract of DOI:10.1246/cl.140807

The solution-processable host material, 3,3′-[bis(9-phenyl-carbazol-3-yl)]benzophenone (BCzBP), containing donor-type phenylcarbazole units and an acceptor-type benzophenone unit was synthesized. BCzBP showed a high excited triplet energy level and a high

Full text of DOI:10.1246/cl.140807

Received: August 29, 2014 | Accepted: September 29, 2014 | Web Released: October 3, 2014
CL-140807
A Donor-Acceptor-type Host Material for Solution-processed Phosphorescent
Organic Light-emitting Devices Showing High Efficiency
Chang-Hwa Jun, Yong-Jin Pu,* Masahiro Igarashi, Takayuki Chiba, Hisahiro Sasabe, and Junji Kido*
Department of Organic Device Engineering, Research Center for Organic Electronics, Yamagata University,
4-3-16 Johnan, Yonezawa, Yamagata 992-8510
(E-mail: pu@yz.yamagata-u.ac.jp, kid@yz.yamagata-ua.c.jp)
OH
O
The solution-processable host material, 3,3¤-[bis(9-phenyl-
Br
Br
Br
CHO
n-BuLi
PCC
Br
Br
Br
Br
+
Et2O
CH2Cl2
carbazol-3-yl)]benzophenone (BCzBP), containing donor-type
phenylcarbazole units and an acceptor-type benzophenone unit
was synthesized. BCzBP showed a high excited triplet energy
level and a high photoluminescence quantum efficiency with
tris[2-(4-tolyl)pyridine]iridium ([Ir(mppy)3]) as a dopant, and
a high glass-transition temperature. Solution-processed green
phosphorescent OLEDs were fabricated with BCzBP. The
device with BCzBP as a host showed comparable efficiencies
to those of the corresponding device with 4,4¤-bis(N-carba-
zolyl)biphenyl (CBP) as a host. At the same current density, the
device with BCzBP showed a longer device lifetime than that
with CBP, due to the high thermal stability and the bipolar nature
of the host compound, BCzBP.
1 (42%)
2 (88%)
B(OH)2
N
N
N
O
[Pd2(dba)3], SPhos
1.35 M K3PO4 aq.
BCzBP (77%)
xylene, EtOH
Scheme 1. Synthetic route of BCzBP.
HOMO
LUMO
Solution-processed organic light-emitting devices (OLEDs)
are considered essential for the next generation of low-cost and
large-area flat-panel displays and lighting sources.1-6 Phospho-
rescent compounds are effective to improve the efficiency of the
device, compared with fluorescent materials.7,8 However, it is
rather difficult to achieve a long lifetime in phosphorescent
OLEDs, and the solution process makes it more difficult.9,10 The
host materials for solution-processed phosphorescent OLEDs
require high energy level of excited triplet state (T1) not to
quench triplet exciton of the phosphorescent emitter, charge-
transporting ability for low driving voltage, high glass-transition
temperature (Tg) for long lifetime and less vulnerable to heat
damage in OLEDs, bipolar property for electrochemical stability
of cation radical and anion radical states, and good solubility in
organic solvents for solution processability.11-14
In this study, we report a unique molecular design of com-
bining phenylcarbazol as electron-donor unit and benzophenone
as electron-acceptor unit to give a novel bipolar host material of
3,3¤-[bis(9-phenylcarbazol-3-yl)]benzophenone (BCzBP). The
compound exhibited high T1 and high Tg. Solution-processed
green phosphorescent OLEDs using an emitter of tris[2-(4-
tolyl)phenylpyridine]iridium ([Ir(mppy)3]) showed high external
Figure 1. Calculated spatial distributions (DFT, B3LYP/6-
311G+(d,p)// B3LYP/6-31G(d), Gaussian 09) of HOMO and LUMO
of BCzBP.
scanning calorimetry (DSC). The decomposition temperature
(Td, corresponding to 5% weight loss) was 487 °C, and Tg was
116 °C. The much higher Td and Tg at 373 °C and 62 °C of
4,4¤-bis(N-carbazolyl)biphenyl (CBP) is attributed to the high
molecular weight of BCzBP.15,16 This high thermal stability is
highly desirable for improving the lifetime of the OLEDs.
Density surfaces of the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO)
were calculated using density functional theory (DFT), shown in
Figure 1. The HOMO is located on the electron-rich phenyl-
carbazole unit, and its energy level is calculated at ¹5.61 eV.
Conversely, the LUMO is located on the benzophenone unit,
and its energy level is calculated at ¹2.00 eV. The T1 of BCzBP
is also calculated at ¹2.91 eV, using time-dependent DFT.
The ionization potential (Ip) corresponding to the HOMO
level of the BCzBP film was ¹5.87 eV, measured by photo-
electron yield spectroscopy. The energy gap (Eg) was ¹3.32 eV,
determined by the UV absorption edge of the film. The LUMO
level was estimated at ¹2.55 eV from the difference of the Ip
and the Eg. The photoluminescence (PL) spectrum of the film
was measure at room temperature and low temperature at 5 K
(Figure 2). The fluorescence spectrum was obtained at room
temperature with a maximum wavelength of 445 nm, while the
low-temperature measurement showed a red-shifted phospho-
rescence spectrum. From the edge of the phosphorescence
spectra, the T1 level of BCzBP was estimated at ¹2.65 eV. This
value is high enough to confine the triplet exciton of the green
phosphorescence of [Ir(mppy)3] (¹2.55 eV) in OLEDs. We
measured the photoluminescence quantum yield (PLQY) of
[Ir(mppy)3] (12 wt %) doped in BCzBP or CBP film by selective
quantum efficiency (EQE) of 9.8% and a long half-lifetime of
¹2
365 h at a practical brightness of 1000 cd m
.
The bipolar host material, BCzBP was readily synthesized
by the Pd(0)-catalyzed Suzuki-Miyaura cross-coupling reaction
of two equivalents of 9-phenylcarbazole-3-boronic acid with
3,3¤-dibromobenzophenone (Scheme 1). The chemical structure
was identified by 1H- and 13C NMR, mass spectrometry, and
elemental analysis. The compound was thoroughly purified
by train sublimation and the purity was higher than 99.9%,
determined by high-performance liquid chromatography (HPLC)
analysis. The thermal properties of BCzBP were investigated
using thermal gravimetric analysis (TGA) and differential
Chem. Lett. 2014, 43, 1935–1936 | doi:10.1246/cl.140807
© 2014 The Chemical Society of Japan | 1935

1
0.9
0.8
0.7
0.6
0.5
1
0.8
0.6
0.4
0.2
0
1
0.8
0.6
0.4
0.2
0
40
80
Time/h
120
160
200
300
400
500
600
Wavelength/nm
Figure 4. Lifetime characteristics of the devices using CBP and
¹2
BCzBP, measured at 5 mA cm
.
Figure 2. UV-vis absorption spectrum (circle) and PL spectrum
(square) of the film of BCzBP, and phosphorescence spectrum
(triangle) of the film of the BCzBP at 5 K.
¹2
Device stabilities were investigated under a 5 mA cm
driving condition. The initial luminances were 1700 and
1500 cd m¹2 for BCzBP and CBP, respectively. The device with
BCzBP as a host showed much improved stability compared
with CBP (Figure 4). The half lifetime was 89 h for CBP, on the
other hand, it was 160 h for BCzBP. The longer lifetime of the
device with BCzBP is considered to be resulted from its high
thermal stability and bipolar nature.
In summary, we synthesized a thermally stable bipolar host,
BCzBP, consisting of electron-donating phenylcarbazole units
and an electron-accepting benzophenone unit. The Tg of BCzBP
was much higher than that of usual CBP. The solution-processed
green phosphorescence OLEDs showed higher efficiencies and
longer lifetimes, compared with the device with CBP.
102
(a)
(b)
10
8
101
100
10-1
10-2
10-3
10-4
10-5
6
4
2
0
0
2
4
6
8
10
0
20
40
60
80
100
Voltage/V
Current density/mA cm−2
Figure 3. (a) Current density-voltage characteristics and (b) EQE-
current density characteristics of the devices using CBP and BCzBP.
Supporting Information is available electronically on J-STAGE.
excitation of [Ir(mppy)3] at 390 nm, where BCzBP and CBP
have no absorption (Figure S1). The PLQY of [Ir(mppy)3] with
BCzBP and CBP are 65% and 69%, respectively. These high
PLQYs also support the efficient triplet exciton confinement of
the hosts.
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1936 | Chem. Lett. 2014, 43, 1935–1936 | doi:10.1246/cl.140807
© 2014 The Chemical Society of Japan