M-Terphenyl-modified carbazole host material for highly efficient blue and green PHOLEDS

Article abstract of DOI:10.1039/b908139a

A simple m-terphenyl modified carbazole derivative, 9-phenyl-3,6-bis-[1, 1′;3′1″]terphenyl-5′-yl-9H-carbazole (CzTP) was developed. By using CzTP as a host material, the PHOLEDs showed the maximum power efficiencies (η_{p, max}) of 55 lm W^-1<

Full text of DOI:10.1039/b908139a

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m-Terphenyl-modified carbazole host material for highly efficient blue
and green PHOLEDSw
Hisahiro Sasabe, Yong-Jin Pu, Ken-ichi Nakayama and Junji Kido*
Received (in Cambridge, UK) 23rd April 2009, Accepted 28th August 2009
First published as an Advance Article on the web 15th September 2009
DOI: 10.1039/b908139a
A simple m-terphenyl modified carbazole derivative, 9-phenyl-
3,6-bis-[1,10;30100]terphenyl-50-yl-9H-carbazole (CzTP) was
developed. By using CzTP as a host material, the PHOLEDs
showed the maximum power efficiencies (gp, max) of 55 lm Wꢀ1
for blue and 113 lm Wꢀ1 for green, respectively.
m-terphenyl group makes the Tg of carbazole higher,
compared with that of well known host materials, such as
3,5-bis(9-carbazolyl)benzene (mCP) and 4,40-N,N0-dicarbazolyl-
biphenyl (CBP). Therefore, CzTP is supposed to have both
high ET1 and thermal stability.
First, we conducted the density functional theory (DFT)
calculations of CzTP. The optimized structures were
calculated at the RB3LYP 6-31G(d) for the ground state,
and UB3LYP 6-31G(d) for the excited triplet state, respectively.
The single-point energies were calculated at the corresponding
6-311 + G(d,p) levels. The calculated highest occupied
molecular orbital (HOMO) energy of CzTP is estimated to
be 5.52 eV, and lowest unoccupied molecular orbital (LUMO)
energy and HOMO–LUMO energy gap (Eg) are 1.28 eV, and
4.24 eV respectively. Further, to estimate the ET1 energy of
CzTP, we calculated the ET1 energies of CzTP, mCP and CBP
by using the previously reported method.11 The calculated ET1
energies were 2.87 eV for CzTP, 3.32 eV for mCP, and 2.66 eV
for CBP. Thus, CzTP is expected to have higher ET1 than that
of CBP from the calculations.
Since the discovery of phosphorescent iridium complexes,
those achieve the internal efficiency of phosphorescent organic
light-emitting devices (PHOLED) up to 100%, wide-energy-gap
phosphorescent materials have gained considerable attention
to realize highly efficient PHOLEDs.1–3 Generally, for use of
phosphorescent emitter, the host materials play a critical role
to determine the device performance. The primary roles of
phosphorescent host materials are (i) the confinement of the
triplet excitons on the emitter, (ii) the suppression of dopant
aggregation, and (iii) the adjustment of the carrier balance of
holes and electrons in the emissive layer (EML).
To meet these requirements, (a) carbazole-based host
materials,4 (b) tetraphenylsilane-based host materials,5
(c) phosphine oxide-based host materials,6 and (d) triphenylamine-
based host materials7 have been reported in FIrpic-based blue
PHOLEDs and the devices with the maximum power
efficiencies (Zp, max) up to 30 lm Wꢀ1 have been developed.
Among them, the carbazole derivatives are one of the most
attractive candidates to reduce the driving voltage of
PHOLEDs because of (i) the small singlet–triplet exchange
energy (DEST), which is attributed to the energy difference
between the singlet excited energy (ES1) and the triplet one
(ET1),8 and (ii) the high chemical compatibility with dopants to
suppress the aggregation.9 From this point of view, we have
developed phosphorescent host materials and demonstrated
CzTP was prepared via a Suzuki–Miyaura coupling reaction
of 3,6-dibromo-9-phenylcarbazole 1 with m-terphenyl boronic
ester 2 in 60% yield (Scheme 1). The characterization of CzTP
was established on the basis of mass spectrometry, NMR
spectroscopy, and elemental analyses. (see ESI, Fig. S1w)
The compound was purified by train sublimation before
device fabrication. The thermal properties of CzTP were
estimated by differential scanning calorimetry (DSC). The
glass transition temperature (Tg) of CzTP was 135 1C,
which is ca. 2 times higher than that of mCP (60 1C) and
CBP (62 1C),4e and formed morphologically stable amorphous
films. The HOMO level was observed at 5.91 eV by atmospheric
photoelectron spectroscopy (AC-3, Riken Keiki Co.). While
the LUMO energy was calculated at 2.48 eV by subtraction of
the optical energy gap (Eg) from the HOMO energy.
highly efficient FIrpic-based PHOLEDs with Zp,
over
max
60 lm W .
ꢀ1 10 These results promise that carbazole derivatives
provide highly efficient PHOLEDs. In this communication, we
report a simple m-terphenyl modified carbazole derivative,
9-phenyl-3,6-bis-[1,10;30100]terphenyl-50-yl-9H-carbazole
(CzTP) as a host material for highly efficient blue and green
PHOLEDs (Fig. 1).
The solid state phosphorescent spectrum is often observed
as a broad spectrum with no clear peaks.10b Therefore, we
calculated the highest phosphorescent energy (ET1) of all
the materials from the onset of the phosphorescent spectrum.
CzTP was designed to possess both high ET1 and thermal
stability by introduction of two m-terphenyl moieties into the
3,6-position on the carbazole moiety. The ET1 of m-terphenyl
is 2.82 eV,1 which is applicable to blue PHOLED, and the
Department of Organic Device Engineering, Yamagata University,
4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan.
E-mail: kid@yz.yamagata-u.ac.jp; Fax: (+81)-238-26-3412;
Tel: (+81)-238-26-3052
w Electronic supplementary information (ESI) available: Experimental
details, spectroscopic and analytical data. See DOI: 10.1039/b908139a
Fig. 1 Chemical structure of CzTP.
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of 11 wt% FIrpic-doped CzTP film exhibited almost single-
exponential decay (98%) with the phosphorescence lifetime
(tp) of 1.34 ms at room temperature, indicating the effective
suppression of the FIrpic exciton quenching in the host
(see ESI, Fig. S3w). The triplet emission energy of CzTP
(2.70 eV), which is corresponding to the ET1, is 0.07 eV smaller
than that of FIrpic (2.77 eV). However, the triplet absorption
energy of CzTP is considered to be larger than the ET1
of FIrpic. This may be attributed to the large structural
relaxation energy of CzTP. From ZPL and tp, the radiative
Scheme 1 Synthesis of CzTP.
The ET1 energies of CzTP, mCP and CBP were observed at
2.70, 3.00 and 2.60 eV, respectively (see ESI, Fig. S1 and S2w).
On an average of these three ET1 energies, the experimental
values are 0.18 eV lower than the calculated values. The singlet
excited energy (ES1) of the materials were estimated from the
Eg, and the DEST is calculated from the ES1 and ET1 values.
The ET1 of CzTP is 0.07 eV smaller than that of FIrpic,10b
and 0.17 eV higher than that of Ir(ppy)3.12 The expanded
p-conjugation of the 3,6-position on the carbazole moiety
reduces the ET1 of CzTP compared with that of mCP.
decay rate (kr) of FIrpic in CzTP is evaluated to be 5.3 ꢃ 105 sꢀ1
,
which is slightly smaller than the kr in mCP. Similarly, ZPL of
8 wt% Ir(ppy)3-doped CzTP film showed 80 ꢂ 1%, that is
10% smaller than that of CBP/Ir(ppy)3 film (90 ꢂ 2%).3b The
transient PL decay curve of 8 wt% Ir(ppy)3-doped film
exhibited almost single-exponential decay (96%) with tp of
1.27 ms (see ESI, Fig. S3w). The kr of Ir(ppy)3 in CzTP is also
estimated to be 6.3 ꢃ 105 sꢀ1, which is good agreement with
the kr in CBP. Thus, the superior performances of blue and
green PHOLED can be realized using CzTP as a host material.
To investigate the function of CzTP as a host material, a
blue PHOLED with a structure of [ITO (110 nm)/poly(arylene
ether sulfone)-containing tetraphenylbenzidine (TPDPES)
doped with 10 wt% tris(4-bromophenyl)aminium hexachloro-
antimonate (TBPAH) layer14 (20 nm)/2,20-bis[300-(N,N0-ditolyl-
amino)phenyl]biphenyl (3DTAPBP)10a (20 nm)/FIrpic 11 wt%
doped CzTP (10 nm)/3,5,300,500-tetra-3-pyridyl-[1,10;30,100]terphenyl
(B3PyPB)10g (50 nm)/LiF (0.5 nm)/Al (100 nm)] were
fabricated. The ET1 of 3DTAPBP and B3PyPB are reported
to be 2.82 and 2.77 eV, respectively. Therefore, the triplet
exciton quenching of FIrpic at the EML/hole-transport layer
interface and/or the EML/electron-transport layer interface
can be minimized in this blue PHOLED. The EL spectrum is
illustrated as the inset in Fig. 3(b). The emission was only from
FIrpic with no emission from neighboring materials. The
current density–voltage–luminance (J–V–L) characteristics
are shown in Fig. 3(a). The turn-on voltage at 1 cd mꢀ2 was
3.0 V, and the applied voltages at 100 cd mꢀ2 and 1000 cd mꢀ2
were 3.4 V and 4.1 V for, respectively, those are lower than the
other FIrpic-based PHOLEDs. The power efficiency–luminance
and current efficiency–luminance characteristics are shown in
Fig. 3(b). The superior device performances were recordedꢀt1o
In OLEDs, the electrical carrier injection into the host
material takes place on its HOMO and LUMO, that is, holes
are injected into the HOMO, while electrons are injected into
the LUMO. Therefore, the host material with a smaller Eg,
which approximately corresponds to ES1, can make the driving
voltage of OLEDs lower.13 In addition, for the confinement of
the exciton on a blue phosphorescent emitter FIrpic, the host
material with high ET1 energy over 2.77 eV is also desirable.
To reduce the driving voltage of blue PHOLEDs, it is necessary
to adjust the ES1 energy as low as possible maintaining a high
ET1 energy. In other words, both small DEST and high
E
T1 energies are essentially required to design an effective host
material. In case of CzTP (DEST = 0.73 eV), the DEST is
0.11 eV smaller than that of CBP (DEST = 0.84 eV), and 0.24 eV
larger than that of mCP (DEST = 0.49 eV). Compared with the
non-carbazole materials such as TmPyPB (DEST = 1.17 eV)10c
and B3PyPB (DEST = 1.28 eV),10g the DEST is 0.34–0.58 eV
small (Fig. 2, see also ESI, Table S1w). Thus, the driving voltage
of PHOLED is expected to be reduced using CzTP as a host.
The photophysical properties of FIrpic and Ir(ppy)3-doped
CzTP films were evaluated. The PL quantum efficiency (ZPL
)
of 11 wt% FIrpic-doped CzTP film was measured under N2
flow using a integrating sphere excited at 331 nm with a
multichannel spectrometer as the optical detector. The doped
film showed high ZPL values of 71 ꢂ 1%, that is smaller than
that of mCP/FIrpic film (85 ꢂ 1%).3b Although CzTP has the
smaller ET1 than that of FIrpic, the transient PL decay curve
be 55 lm Wꢀ1 (52 cd Aꢀ1) at 1 cd mꢀ2, 43 lm Wꢀ1 (46 cd A
)
at 100 cd mꢀ2 and 29 lm Wꢀ1 (38 cd Aꢀ1) at 1000 cd mꢀ2
,
respectively.
On the other hand, the mCP/FIrpic device showed slightly
lower device performances than that of the CzTP/FIrpic
device, though the mCP/FIrpic film showed ca. 15% higher
ZPL than that of CzTP/FIrpic film. This probably results from
the better carrier balance in the CzTP/FIrpic device.
Further, we fabricated a green PHOLED with a structure of
[ITO (110 nm)/TPDPES: 10 wt% TBPAH (20 nm)/1,1-bis[4-
[N, N-di(p-tolyl)amino]phenyl]cyclohexane (TAPC) (30 nm)/
CzTP: 8 wt% Ir(ppy)3 (10 nm)/B3PyPB (50 nm)/LiF (0.5 nm)/Al
(100 nm)]. All the materials, TAPC, CzTP and B3PyPB, have
higher ET1 than that of Ir(ppy)3, thus the triplet exciton
quenching of Ir(ppy)3 can be completely suppressed in this
green PHOLED. The J–V–L characteristics and the EL spectrum
are shown in Fig. 4. The turn-on voltage at 1 cd mꢀ2 is 2.8 V,
Fig. 2 Singlet (ES1), triplet (ET1) and DEST (inset) energies of
materials.
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Fig.
3
(a) Current density–voltage and luminance–voltage
Fig. 4 (a) Current density–voltage and luminance–voltage character-
istics for CzTP/Ir(ppy)3-based green PHOLED. (b) Power efficiency-
luminance and current efficiency-luminance characteristics. Inset: EL
spectrum of the device.
characteristics for CzTP/FIrpic-based blue PHOLED. (b) Power
efficiency-luminance and current efficiency-luminance characteristics.
Inset: EL spectrum of the device.
and the applied voltages at 100 cd mꢀ2 and 1000 cd mꢀ2 are
3.6 V and 4.6 V, respectively. The green device perfomances were
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observed to be 113 lm Wꢀ1 (102 cd Aꢀ1) at 1 cd mꢀ2
,
89 lm Wꢀ1 (102 cd Aꢀ1) at 100 cd mꢀ2 and 57 lm Wꢀ1
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clearly indicate that CzTP functions as an effective host for
PHOLEDs.
In summary, we developed a simple m-terphenyl modified
carbazole phosphorescent host material, CzTP with high Tg of
135 1C and ET1 of 2.70 eV. By using CzTP as a host material
for PHOLEDs, the devices showed the Zp, max of 55 lm Wꢀ1
for blue and 113 lm Wꢀ1 for green without light outcoupling
enhancement, respectively.
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This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 6655–6657 | 6657