A universal host material for high external quantum efficiency close to 25% and long lifetime in green fluorescent and phosphorescent OLEDs

Article abstract of DOI:10.1002/adma.201400347

High quantum efficiency close to 25% and long lifetime in green thermally activated delayed fluorescent and phosphorescent organic light emitting diodes are achieved using universal 3′,5′-di(carbazol-9-yl)-[1,1′- biphenyl]-3,5-dicarbonitrile host material

Full text of DOI:10.1002/adma.201400347

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A Universal Host Material for High External Quantum
Efficiency Close to 25% and Long Lifetime in Green
Fluorescent and Phosphorescent OLEDs
Yong Joo Cho, Kyoung Soo Yook, and Jun Yeob Lee*
Quantum efficiency is an important device performance of
organic light-emitting diodes (OLEDs) because low power con-
sumption can be obtained in the OLED panel using the high
quantum efficiency device. In general, the quantum efficiency
can be increased by using highly efficient emitting materials
and phosphorescent emitting materials have been typically
used to achieve high quantum efficiency in OLEDs.[1–9] Com-
pared to common fluorescent emitting materials which use
only singlet excited state for light emission, the phosphorescent
emitting materials utilize both singlet and triplet excited states
for light emission and can show four times higher quantum
efficiency than fluorescent emitting materials.[1] Therefore,
almost 100% internal quantum efficiency was achieved in red,
green and blue phosphorescent OLEDs.[3–8]
Another approach to obtain high quantum efficiency in
OLEDs is to apply thermally activated delayed fluorescence
(TADF) emitting materials.[9–17] The TADF emitting materials
emit light from singlet excited state, but triplet state can be
converted to singlet state by reverse intersystem crossing from
triplet state to singlet state. Therefore, both singlet and triplet
excited states contribute to light emission from singlet state
and 100% internal quantum efficiency can be realized using
the TADF materials theoretically although the 100% internal
quantum efficiency was not reported yet.
In order to achieve high quantum efficiency in TADF and
phosphorescent OLEDs, host materials which can maximize
the light-emitting performances of TADF and phosphorescent
emitting materials should be developed. There are common
requirements for the host materials of TADF and phospho-
rescent OLEDs. Firstly, singlet and triplet energies of the host
materials should be higher than those of emitting materials
for energy transfer and exciton blocking. In addition, the sin-
glet emission of the host materials should be overlapped with
absorption of emitters. Secondly, the host materials should
have bipolar charge transport properties for charge injection
and charge balance in the emitting layer. Hole transport and
electron transport units should be included in the molecular
structure for bipolar charge transport properties. Thirdly, the
highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) of the host materials
should be matched with those of the emitters. The HOMO
of the host materials should be deeper than that of emitters,
while the LUMO of the host materials should be shallower than
that of emitters. Particularly, the HOMO and LUMO should be
adjusted for energy transfer process from host to dopant rather
than for charge trapping because it is difficult to balance holes
and electrons if charge trapping dominates the light-emission
process.[18]
In general, tris[2-phenylpyridinato-C2,N] iridium(III)
(Ir(ppy)3)[1] and (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophtha-
lonitrile (4CzIPN)[9] have been used as representative green
phosphorescent and TADF emitting materials, respectively, due
to high light-emitting efficiency of the dopant materials. How-
ever, the two dopant materials have different ultraviolet-visible
(UV-Vis) absorption and the HOMO/LUMO levels, which
makes it difficult to develop high efficiency TADF and phospho-
rescent OLEDs using the same host material. The use of one
single host material for both phosphorescent and TADF OLEDs
would facilitate the development of high efficiency phospho-
rescent OLEDs and TADF OLEDs. In particular, the efficiency
of TADF devices should be improved by host materials with
proper HOMO/LUMO levels.
In this work, a universal host material for both TADF and
phosphorescent OLEDs was synthesized and the device perfor-
mances of the green TADF and phosphorescent OLEDs were
investigated. The universal host material, 3′,5′-di(carbazol-9-
yl)-[1,1′-biphenyl]-3,5-dicarbonitrile (DCzDCN), showed sin-
glet energy of 2.98 eV and triplet energy of 2.71 eV for effi-
cient energy transfer to Ir(ppy)3 and 4CzIPN, bipolar charge
transport properties and proper HOMO/LUMO levels of
−6.14 eV/−3.26 eV for exciton blocking. A high quantum effi-
ciency close to 25% was demonstrated in the TADF and phos-
phorescent OLEDs using the same DCzDCN host material. In
addition, the lifetime of the DCzDCN device was also greatly
improved compared to that of 4,4′-bis(N-carbazolyl)-1,1′-
biphenyl (CBP) device. This is the first work reporting a uni-
versal host material which can achieve high quantum efficiency
close to 25% in the both TADF and phosphorescent OLEDs.
In particular, the external quantum efficiency of 26.7% of the
TADF OLEDs is the best efficiency value reported in the TADF
devices using a single host material.
Although the host materials for the TADF and phosphores-
cent OLEDs have several common requirements, the TADF host
materials should also meet additional requirements such as
deep HOMO/LUMO level and higher singlet and triplet energy
than singlet energy of the TADF dopant. The HOMO/LUMO
level of the TADF dopant such as 4CzIPN is deeper than that of
phosphorescent dopant like Ir(ppy)3. Therefore, host materials
Y. J. Cho, K. S. Yook, Prof. J. Y. Lee
Department of Polymer Science and Engineering
Dankook University
126, Jukjeon-dong, Suji-gu, Yongin,
Gyeonggi 448-701, Korea
E-mail: leej17@dankook.ac.kr
DOI: 10.1002/adma.201400347
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with deep HOMO/LUMO level should be
developed. In addition, the singlet and triplet
energy of the host should be higher than the
singlet energy of the TADF emitter to sup-
press singlet exciton quenching by the TADF
host. However, high triplet energy leads
to high singlet energy and wide bandgap,
which increases the driving voltage of the
device. These requirements of the TADF host
prompted us to design narrow bandgap host
material with the deep HOMO/LUMO level,
high triplet energy and narrow bandgap.
The DCzDCN host was designed to show
bipolar charge transport properties and
narrow bandgap by connecting a weak hole
transport type carbazole unit and an strong
electron transport type phthalonitrile unit
Figure 1. HOMO and LUMO distribution of DCzDCN.
with a phenyl linkage. The donor-acceptor
structure of the DCzDCN can reduce the bandgap and singlet
energy while keeping the high triplet energy of the host mate-
rial. The reduced singlet energy is good for energy transfer to
the 4CzIPN dopant with ultraviolet-visible (UV-Vis) absorp-
tion peaks below 470 nm and to Ir(ppy)3 with metal to ligand
charge transfer absorption and ligand absorption below 500
nm. The reduced bandgap can lower the driving voltage of the
device due to low energy barrier for charge injection from hole
and electron transport layers. In addition, the strong electron
withdrawing phthalonitrile unit can deepen the LUMO level of
DCzDCN for efficient energy transfer to 4CzIPN with the deep
HOMO/LUMO level of −5.8/−3.4 eV. Small HOMO and LUMO
level difference between DCzDCN and 4CzIPN allows the
energy transfer to dominate light-emission process rather than
charge trapping. In addition, the high triplet energy can sup-
press exciton quenching of Ir(ppy)3 and 4CzIPN, and facilitate
energy transfer from the DCzDCN host to Ir(ppy)3 and 4CzIPN.
The DCzDCN host was synthesized by coupling reaction
purification by column chromatography and vacuum train sub-
limation. The chemical structure of DCzDCN was confirmed by
1H and 13C nuclear magnetic resonance spectra, mass spectra
and elemental analysis. Purity of the DCzDCN host was over
99% from high performance liquid chromatography analysis.
The HOMO and LUMO distribution of DCzDCN was inves-
tigated using Gaussian 09 software with B3LYP/6-31G* basis
sets. Figure 1 shows the HOMO and LUMO distribution of
DCzDCN. The HOMO of DCzDCN was localized on the dicar-
bazolylphenyl unit, while the LUMO was dispersed over phtha-
lonitrile unit. The HOMO and LUMO were separated due to
electron donating carbazole unit and electron withdrawing
phthalonitrile unit. Therefore, the DCzDCN host can possess
bipolar charge transport properties and can exhibit good elec-
tron accepting properties as well as good hole accepting proper-
ties from charge transport layers.
The HOMO and LUMO of DCzDCN were measured by
cyclic voltammetry (CV) and CV curves are shown in Figure S1.
The HOMO and LUMO of DCzDCN can be calculated from
the oxidation and reduction potentials, which were −6.14 eV
and −3.26 eV, respectively. The HOMO of DCzDCN was sim-
ilar to that of common carbazole based host materials, but
the LUMO of DCzDCN was deepened compared to that of
other bipolar host materials. The strong
between
9,9′-(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
1,3-phenylene)bis(9H-carbazole) and 1,3,5-tribromobenzene
followed by CN substitution of 9,9′-(3′,5′-dibromo-[1,1′-
biphenyl]-3,5-diyl)bis(9H-carbazole) intermediate as shown in
Scheme 1. Synthetic yield of the DCzDCN host was 45% after
electron withdrawing dicyano substituent
stabilized the LUMO level and resulted in
deep LUMO level and narrow bandgap of
2.88 eV. Compared with the HOMO/LUMO
level of Ir(ppy)3 ( −5.6/−3.2 eV) and 4CzIPN
(−5.8/−3.4 eV), the HOMO and LUMO
levels of DCzDCN were suitable for energy
transfer and blocking exciton quenching.
Photophysical properties of DCzDCN were
analyzed using UV-Vis absorption and photo-
luminescence (PL) spectrometers. Figure 2(a)
presents UV-Vis, solid PL and low tempera-
ture PL spectra of DCzDCN. The UV-Vis
and solution PL spectra were obtained from
vacuum evaporated DCzDCN film. DCzDCN
showed strong absorption peaks at 236,
Scheme 1. Synthetic scheme of DCzDCN.
251, 280, and 291 nm which are assigned
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the DCzDCN host activate the delayed PL emission of 4CzIPN
without quenching the delayed PL emission and the delayed
emission of DCzDCN was transferred to 4CzIPN.
As the DCzDCN host possessed high singlet and triplet
energies for energy transfer to Ir(ppy)3 and 4CzIPN, extensive
spectral overlap with the emitters and HOMO/LUMO levels for
exciton blocking and charge confinement, it was used as the
host material for green phosphorescent and TADF OLEDs. The
device structure was indium tin oxide (ITO, 50 nm)/poly(3,4-
ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT:PSS,
60
nm)/4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)ani-
line] (TAPC, 20 nm)/1,3-bis(N-carbazolyl)benzene (mCP,
10 nm)/DCzDCN:Ir(ppy)3 or DCzDCN:4CzIPN (25 nm)/diphe-
nylphosphine oxide-4-(triphenylsilyl)phenyl (TSPO1, 35 nm)/
LiF (1 nm)/Al (200 nm). Doping concentrations of Ir(ppy)3
and 4CzIPN were 3% and 5%. CBP device was also fabricated
for comparison. Current density-voltage-luminance curves of
the green TADF and green phosphorescent OLEDs are shown
in Figure 3(a) and (b). Current density and luminance of the
DCzDCN TADF device were much higher than those of CBP
TADF device at the same driving voltage and turn-on voltage
of the DCzDCN device was reduced by 0.5 V compared to
that of CBP device. The turn-on voltages of DCzDCN:4CzIPN
and CBP:4CzIPN devices were 3.0 V and 3.5 V, respec-
tively. The driving voltage at 1000 cd/m2 was 5.1 V in the
DCzDCN:4CzIPN device, but the luminance of CBP:4CzIPN
device could not reach 1000 cd/m2. The DCzDCN host was
effective to lower driving voltage and increase luminance of
the device due to good electron injection and transport caused
by the deep LUMO level. The two CN units were effective to
deepen the LUMO level of DCzDCN and to reduce the driving
voltage. Similar results were obtained in the Ir(ppy)3 devices.
High current density and high luminance were observed in the
DCzDCN device compared to CBP device.
Quantum efficiency-luminance and power efficiency-lumi-
nance curves of DCzDCN:4CzIPN and DCzDCN:Ir(ppy)3
devices are shown in Figure 3 (c) and (d). Maximum quantum
efficiencies of DCzDCN:4CzIPN and DCzDCN:Ir(ppy)3 devices
were 26.7% and 23.8% at 3% doping concentration compared
to 17.1% and 14.4% of CBP:4CzIPN and CBP:Ir(ppy)3 devices.
There was more than 50% improvement of the maximum
quantum efficiency using the DCzDCN host instead of CBP.
The high quantum efficiency at low luminance was main-
tained even at high luminance and the quantum efficiencies
at 1000 cd/m2 were 25.9% and 23.7% for DCzDCN:4CzIPN
and DCzDCN:Ir(ppy)3 devices, respectively. In particular, the
maximum quantum efficiency of the DCzDCN:4CzIPN device
was better than any other data reported in the TADF device.[9]
In previous work reported by Adachi et al., the maximum
quantum efficiency was 19.3% and the efficiency was sharply
dropped at high luminance. The maximum quantum efficiency
of 4CzIPN was enhanced by 38% and efficiency roll-off could
be suppressed using the DCzDCN host.
Figure 2. UV-Vis, solid PL and low temperature PL spectra (a) and tran-
sient PL decay curves of DCzDCN and DCzDCN:4CzIPN films (b).
to π−π* absorption and weak absorption peaks at 324 and
336 nm which are caused by n-π* absorption. Solid PL emis-
sion of DCzDCN was observed at 427 nm and phosphorescent
emission appeared from 430 nm. Singlet and triplet ener-
gies of DCzDCN were 2.88 eV and 2.71 eV from fluorescent
and phosphorescent emission peaks, respectively. The singlet
and triplet energies of DCzDCN were high enough for energy
transfer to Ir(ppy)3 and 4CzIPN. In particular, the triplet energy
of DCzDCN was higher than the singlet energy of 4CzIPN
(2.43 eV), which may suppress exciton quenching of 4CzIPN
by DCzDCN.
The solid PL emission of DCzDCN was compared with
UV-Vis absorption of Ir(ppy)3 and 4CzIPN to compare the
spectral overlap between DCzDCN and the emitting materials
(Figure S2 in supporting information). The solid PL emission
of DCzDCN was well overlapped with UV-Vis absorption of
4CzIPN and Ir(ppy)3, indicating efficient energy transfer from
DCzDCN to emitters.
Delayed emission of the 4CzIPN in the DCzDCN host
was further confirmed by transient PL measurement of the
4CzIPN doped DCzDCN film. Figure 2(b) shows transient
PL data of DCzDCN host and 4CzIPN doped DCzDCN films.
The DCzDCN host exhibited delayed PL emission with an
excited state lifetime of 0.55 µs and the DCzDCN:4CzIPN film
also showed delayed PL emission of 4CzIPN with the excited
state lifetime of 5.17 µs, which was similar to that of 4CzIPN
reported in other work.[9] Therefore, it can be confirmed that
The high quantum efficiency of the green TADF and green
phosphorescent OLEDs with the DCzDCN host can be mainly
explained by HOMO/LUMO and singlet/triplet energy for
energy transfer, bipolar charge transport properties and charge
confinement. The HOMO and LUMO levels of DCzDCN were
suitable for energy transfer rather than charge trapping, which
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Figure 3. Current density-voltage-luminance curves of DCzDCN:4CzIPN (a) and DCzDCN:Ir(ppy)3 (b). Quantum efficiency-luminance curves of
DCzDCN:4CzIPN (c) and DCzDCN:Ir(ppy)3 (d). Current density-voltage curves of hole and electron only devices of DCzDCN (e) and EL spectra of
DCzDCN:4CzIPN and DCzDCN:Ir(ppy)3 devices (f).
optimized the device performances at 3% doping concentra-
tion and enhanced the quantum efficiency of the green TADF
and phosphorescent OLEDs. Charge trapping is the dominant
light-emission mechanism in the CBP device, which makes
it difficult to manage the charge balance in the emitting layer
as reported in other work.[18] The high singlet and triplet ener-
gies of DCzDCN contributed to the high quantum efficiency
via efficient energy transfer and exciton blocking. As shown
in the UV-Vis absorption and PL emission spectra of the host
and dopant materials, extensive overlap of the PL emission
of DCzDCN with UV-Vis absorption of 4CzIPN and Ir(ppy)3
induced efficient energy transfer as can be confirmed in the
electroluminescence (EL) spectra in Figure 3(f). Energy transfer
efficiencies of DCzDCN:4CzIPN and DCzDCN:Ir(ppy)3 fi lms
were 95.0% and 94.8%. In the case of DCzDCN:4CzIPN, the
activation of delayed fluorescence by DCzDCN played a key role
of improving the quantum efficiency of the TADF device. As
confirmed by transient PL measurement, delayed fluorescence
of 4CzIPN was observed in the DCzDCN:4CzIPN film, which
increased the quantum efficiency of the TADF device. The
delayed fluorescence could be observed in the PL spectrum after
applying delay time of 1.0 µs. In addition, the high singlet/triplet
energies of DCzDCN compared to those of dopant materials
suppressed exciton quenching of dopant materials.
Bipolar charge transport properties of DCzDCN were also
critical to the high quantum efficiency of TADF and phos-
phorescent OLEDs. As can be seen in hole only and electron
only device data in Figure 3 (e), the hole current density and
electron current density were similar. This indicates that the
DCzDCN host material shows bipolar charge transport proper-
ties and is effective to balance holes and electrons in the emit-
ting layer. Charge confinement in the emitting layer is another
key factor for the high quantum efficiency. There are large
energy barriers of 0.86 eV and 0.65 eV for electron leakage
from DCzDCN to mCP and for hole leakage from DCzDCN
to TSPO1, respectively (energy level diagram in Figure S3).
Therefore, holes and electrons can be effectively confined
inside the emitting layer, contributing to the improvement of
quantum efficiency.
EL spectra of DCzDCN:4CzIPN and DCzDCN:Ir(ppy)3
are shown in Figure 3(f). Maximum emission peaks of
DCzDCN:4CzIPN and DCzDCN:Ir(ppy)3 devices were observed
at 497 nm and 512 nm without any emission from DCzDCN or
charge transport materials. Therefore, it can be confirmed that
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TADF device can replace current phosphores-
cent OLED from the viewpoint of lifetime and
quantum efficiency.
105
DCzDCN:4CzIPN
DCzDCN:Ir(ppy)3
CBP:4CzIPN
In conclusion, a universal host material
for both TADF and phosphorescent OLEDs,
DCzDCN, was synthesized as a bipolar host
material with high singlet/triplet energy,
narrow bandgap and proper HOMO/LUMO
levels. The DCzDCN host showed singlet
energy of 2.98 eV and triplet energy of 2.71 eV
for efficient energy transfer to Ir(ppy)3 and
4CzIPN, bipolar charge transport proper-
ties and proper HOMO/LUMO levels of
−6.14 eV/−3.26 eV for charge confinement.
High quantum efficiency close to 25% and
long lifetime both in green TADF and phos-
phorescent OLEDs were achieved using the
DCzDCN host material and this approach can
be useful for the development of the host mate-
rial for TADF and phosphorescent OLEDs.
100
95
CBP:Ir(ppy)3
90
85
0
50
100
150
200
Time (h)
Figure 4. Lifetime curves of DCzDCN:4CzIPN, DCzDCN:Ir(ppy)3, CBP:4CzIPN, and
CBP:Ir(ppy)3 devices. Lifetime was measured at constant luminance of 1000 cd/m2.
the energy transfer from the DCzDCN host to dopant materials
is efficient.
Experimental Section
Synthesis: Synthesis method of DCzDCN is described in supporting
information.
Lifetime of the DCzDCN:4CzIPN and DCzDCN:Ir(ppy)3
devices were measured to study the stability of the DCzDCN
host material. CBP was used as the standard host mate-
rial and the device structure was ITO (120 nm)/ N,N′-
Device Fabrication and Measurements: The device structure of TADF
and phosphorescent OLEDs was ITO (50 nm)/PEDOT:PSS (60 nm)/
TAPC (20 nm)/mCP (10 nm)/DCzDCN:4CzIPN or DCzDCN:Ir(ppy)3
(25 nm)/TSPO1 (35 nm)/LiF (1 nm)/Al (200 nm). Doping
concentrations of 4CzIPN and Ir(ppy)3 were 3% and 5%. CBP was used
as the standard host for 4CzIPN and Ir(ppy)3. Device structure of hole
only device was ITO (50 nm)/PEDOT:PSS (60 nm)/TAPC (20 nm)/mCP
(10 nm)/DCzDCN (25 nm)/TAPC (5 nm)/Al and that of electron only
device was ITO (50 nm)/Ca (5 nm)/DCzDCN (25 nm)/TSPO1 (35 nm)/
LiF (1 nm)/Al (200 nm). The device performances of the fabricated
devices were measured using Keithley 2400 source measurement unit
and CS1000 spectroradiometer after encapsulating the OLEDs using a
glass lid sealed with epoxy adhesive. Lifetime measurement was carried
out at 1000 cd/m2 at a constant current mode.
diphenyl-N,N′-bis-[4-(phenyl-m-tolylamino)-
phenyl]-
biphenyl-4,40-diamine (60 nm)/N,N′-di(1-naphthyl)-N,N′-
diphenylbenzidine(30 nm)/emitting layer (25 nm)/LG201
(35 nm)/LiF (1 nm)/Al (200 nm). DCzDCN:4CzIPN,
DCzDCN:Ir(ppy)3, CBP:4CzIPN, and CBP:Ir(ppy)3 were emit-
ting layers and the doping concentration of the emitter was
15%. Figure 4 shows lifetime curves of the green TADF and
phosphorescent OLEDs measured at 1000 cd/m2. The lifetime
of the DCzDCN devices was much longer than that of the CBP
devices. The DCzDCN:4CzIPN device showed long lifetime of
200 h up to 90% of initial luminance and the DCzDCN:Ir(ppy)3
device exhibited lifetime of 220 h at the same condition. The
CBP:4CzIPN and CBP:Ir(ppy)3 devices showed lifetime of only
10.2 h and 53 h. There are several factors for the long lifetime of
the DCzDCN devices, but charge confinement, high glass tran-
sition temperature, low singlet energy and little charge accu-
mulation at the interface due to good charge transport prop-
erties and charge injection properties of DCzDCN seemed to
contribute to the improved lifetime. Charges are effectively con-
fined in the emitting layer due to large energy barrier for hole
and electron leakage, which suppresses degradation of hole and
electron transport materials. High glass transition temperature
has also positive effect on the lifetime because crystallization
of emitting layer can be avoided. Low singlet energy of 2.88 eV
of DCzDCN is also helpful to increase the lifetime due to less
stress of the host material during light emission process. In
addition, little charge accumulation at the interface between
emitting layer and charge transport layers reduces exciton-
polaron quenching, resulting in long lifetime. The lifetime of
the TADF device was comparable to that of the phosphorescent
device. Therefore, it can be presumed from this result that the
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
This research was supported by Basic Science Research Program
through the National Research Foundation of Korea(NRF) funded by the
Ministry of Education, Science and Technology(2013R1A1A2007991) and
Ministry of Science, ICT and futurew Planning (2013R1A2A2A010674).
Received: January 22, 2014
Revised: March 6, 2014
Published online: April 6, 2014
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