Comparison of bipolar hosts and mixed-hosts as host structures for deep-blue phosphorescent organic light emitting diodes

Article abstract of DOI:10.1002/asia.201100596

Mix and match: A bipolar host material with hole-transport and electron-transport units was synthesized as the host material for deep-blue phosphorescent organic light-emitting diodes and was compared with other mixed-host materials with the same core structure. The bipolar host material showed similar current density and quantum efficiency to those of the mixed host.

Full text of DOI:10.1002/asia.201100596

DOI: 10.1002/asia.201100596
Comparison of Bipolar Hosts and Mixed-Hosts as Host Structures for Deep-
Blue Phosphorescent Organic Light Emitting Diodes
Sook Hee Jeong,[a] Chang Woo Seo,[a] Jun Yeob Lee,*[a] Nam Sung Cho,[b]
Jung Keun Kim,[b] and Joong Hwan Yang[b]
In recent years, phosphorescent organic light-emitting
diodes (PHOLEDs) are being actively studied because of
the high quantum efficiency of PHOLEDs. Theoretically,
PHOLEDs can show up to four times higher quantum effi-
ciency than fluorescent organic light-emitting diodes by har-
vesting triplet excitons. There have been several studies re-
porting theoretical maximum external quantum efficiency
over 20% in red, green, and blue PHOLEDs.[1–7]
It is important to balance charge carriers and to confine
triplet excitons in the emitting layer to obtain high quantum
efficiency in PHOLEDs. One effective way of balancing and
confining charge carriers was to adopt a mixed-host emitting
structure in the emitting layer.[8–12] The mixed-host structure
is made up of hole-transport-type and electron-transport-
type host materials. The hole-transport-type host material
provides a path for hole injection and transport, whilst the
electron-transport-type host material is a path for electron
injection and transport. Therefore, both holes and electrons
can be effectively injected and transported using the mixed-
host structure. Several mixed-host emitting structures were
reported to be effective for improving the driving voltage,
quantum efficiency, and efficiency roll-off of red, green, and
blue PHOLEDs.[8–12] However, using the mixed-host emit-
ting structure is complicated because two host materials and
one dopant material should be deposited at the same time.
Therefore, it is better to use one bipolar host material with
hole-transport and electron-transport units in the molecular
structure.[13–19] However, there has been no study correlating
the device performances of the PHOLED with the host
structure.
Herein, we report the use of three dibenzofuran-type host
materials with different functional groups, and the device
performances of the single bipolar host device and mixed-
host device were compared. A dibenzofuran-core-based
hole-transport-type host, electron-transport-type host and
bipolar host with both hole transport and electron transport
functional groups were used. We demonstrated that the
single biplolar host device shows comparable device perfor-
mance to mixed-host device in the deep-blue PHOLEDs. A
high external quantum efficiency close to 20% was achieved
both in the single bipolar host and mixed-host devices.
Three host materials with a dibenzofuran core structure
were used to compare bipolar single-host and mixed-host
structures in deep-blue PHOLEDs because the dibenzofur-
an core has a high triplet energy of 3.14 eV.[20] A dibenzofur-
an-based host with two carbazole units, 2,8-di(9H-carbazol-
9-yl)dibenzoACTHUNTGRNEUNG[b,d]furan (DFCz), was used as the hole-trans-
port-type host material as the carbazole unit has good hole-
transport properties.[21] A dibenzofuran-type host with two
diphenylphosphine oxide units, 2,8-bis(diphenylphosphoryl)-
dibenzoACHTNUTRGNE[NUG b,d]furan (DFPO), was utilized as an electron-trans-
port-type host material as the diphenylphosphine oxide unit
shows strong electron-transport properties.[22] A bipolar host
with the dibenzofuran core, 9-(8-(diphenylphosphoryl)-
[a] S. H. Jeong, C. W. Seo, Prof. J. Y. Lee
Department of Polymer Science and Engineering
Dankook Univeristy
dibenzoACHTNUTRGNE[NUG b,d]furan-2-yl)-9H-carbazole (DFCzPO), was devel-
oped as the bipolar host material with one carbazole unit
and one diphenylphosphine oxide unit. The hole-transport
carbazole and electron-transport diphenylphosphine oxide
groups were combined in the molecular structure to obtain
bipolar charge transport properties. The syntheses of the
DFCz and DFPO have been reported before and the syn-
thesis of the DFCzPO is shown in Scheme 1.[21,22]
Photophysical properties of the DFCzPO host material
were analyzed using UV/Vis and photoluminescence (PL)
spectroscopy. Figure 1 shows UV/Vis and PL spectra of the
126 Jukjeon-dong, Suji-gu, Yongin-si
Gyeonggi-do, 448-701 (Korea)
Fax : (+82)31-8005-3585
E-mail: leej17@dankook.ac.kr
[b] Dr. N. S. Cho, Dr. J. K. Kim, Dr. J. H. Yang
LG Display R&D Center
1007 Deogeun-ri, Wollong-myeon, Paju-si, Gyeonggi-do, 413-811
(Korea)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100596.
Chem. Asian J. 2011, 6, 2895 – 2898
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2895

COMMUNICATION
tion of the DFCzPO. The
HOMO of the DFCzPO was lo-
calized on the carbazole unit,
whilst the LUMO was localized
on the dibenzofuran unit. As
the carbazole group has elec-
tron-donating character, the
HOMO was concentrated on
the carbazole unit. The elec-
tron-withdrawing character of
the diphenylphosphine oxide in-
duced the localization of the
LUMO on the dibenzofuran.
The simulated HOMO and
LUMO levels of the DFCzPO
Scheme 1. Synthesis of the host material.
were 5.41 eV and 1.39 eV, re-
spectively.
The HOMO level of the DFCzPO was measured using
cyclic voltammetry (CV). The HOMO level of the DFCzPO
was determined from the oxidation curve of the CV spectra
(6.08 eV). The HOMO level of the DFCzPO was similar to
that of the DFCz (6.09 eV) and was much-lower than that
of the DFPO (6.68 eV). As expected from molecular orbital
distribution simulations, the DFCzPO showed a similar
HOMO level to DFCz. The LUMO level was calculated
from the HOMO level and band-gap of the DFCzPO, and
was 2.56 eV.
As the DFCzPO was designed as the bipolar-type host
material, the charge-transport properties of the DFCzPO
were compared with those of hole-transport-type DFCz and
electron-transport-type DFPO. Figure 3 shows the hole-only
and electron-only current-density data of the DFCzPO host
material. The device structure of hole-only device was
indium tin oxide (ITO)/N,N’-diphenyl-N,N’-bis-[4-(phenyl-
Figure 1. UV/Vis, solution PL, solid PL, and low-temperature PL spectra
of DFCzPO.
m-tolyl-amino)-phenyl]-biphenyl-4,4’-diamine
60 nm)/N,N’-di(1-naphthyl)-N,N’-diphenylbenzidine (NPB,
5 nm)/N,N’-dicarbazolyl-3,5-benzene (mCP, 10 nm)/
(DNTPD,
DFCzPO. DFCzPO showed UV/Vis absorption peaks at
325 nm and 339 nm which are assigned to the absorption of
carbazole units. The absorption of the dibenzofuran core
was observed below 300 nm. The band-gap of DFCzPO was
calculated from the edge of the UV/Vis absorption spectra
to be 3.52 eV. The band-gap was similar to that of the DFCz
(3.51 eV) with the carbazole unit because of extended con-
jugation of the dibenzofuran core through the carbazole
unit. A PL emission peak of the DFCzPO was observed at
384 nm. The triplet energy of the DFCzPO was measured
from the low-temperature PL emission spectra and the trip-
let energy of the DFCzPO was 3.00 eV. The DFCzPO
showed high triplet energy (over 2.90 eV) and is suitable as
the host material for deep-blue PHOLEDs.
DFCzPO, or DFCz (30 nm)/DNTPD (5 nm)/Al and the
device structure of the electron-only device was ITO/diphe-
nylphosphine oxide-4-(triphenylsilyl)phenyl (TSPO1, 5 nm)/
DFCzPO or DFPO (30 nm)/TSPO1 (25 nm)/LiF/Al. The
hole-current density of the DFCzPO was compared with
that of the hole-transport-type DFCz, whilst the electron-
current density of the DFCzPO was compared with that of
the DFPO. The bipolar-type DFCzPO showed a little higher
hole-current density than the DFCz and comparable elec-
tron-current density to the DFPO. This result confirms that
the DFCzPO shows bipolar charge-transport properties
Molecular simulation was carried out to study the highest
occupied molecular orbital (HOMO) and the lowest unoccu-
pied molecular orbital (LUMO) distribution of the
DFCzPO. Density functional theory calculation of the com-
pounds was carried out using the Gaussian 09 program and
the nonlocal density functional of Beckeꢁs 3-parameters em-
ploying Lee–Yang–Parr functional (B3LYP) with 6-31G*
basis sets. Figure 2 shows the HOMO and LUMO distribu-
Figure 2. HOMO and LUMO distribution of DFCzPO.
2896
www.chemasianj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2011, 6, 2895 – 2898

Bipolar Hosts and Mixed-Hosts as Host Structures for Deep-Blue Phosphorescent OLEDs
tages investigated although the current density and lumi-
nance were a little higher in the single-host device than in
the mixed-host device at high voltage. This result indicates
that the single bipolar host is similar to the mixed host in
terms of charge density and exciton formation. The current
density is generally determined by the hole and electron
density in the device. More hole and electron injection into
the emitting layer usually increases the current density.
Therefore, the similar current density of the single-host
device and the mixed-host device implies similar charge-in-
jection ability of the two different host systems. The basic
concept of the mixed-host structure is to inject holes and
electrons through a hole-transport-type DFCz host and an
electron-transport-type DFPO host, respectively. The prob-
lem of poor electron transport of the DFCz host and poor
hole transport of DFPO host can be solved by mixing two
host materials. The single-host material, DFCzPO, possesses
the hole-transport carbazole unit in the DFCz and the elec-
tron-transport diphenylphosphine oxide unit in the DFPO.
Therefore, it can have both hole-transport and electron-
transport properties. The charge transport must be compara-
ble in both cases because the hole-transport and electron-
transport groups are the same in the single host and the
mixed host, as shown in the molecular orbital diagram.
Quantum-efficiency–luminance curves of the single-host
and mixed-host devices are shown in Figure 5. The quantum
efficiency of the two devices showed little difference over
the measured luminance range although the quantum effi-
ciency of the single-host device was a little higher than that
of the mixed-host device. The maximum quantum efficiency
and quantum efficiency at 1,000 cdmÀ2 of the single-host
device were 21.4% and 17.5%, respectively, whilst those of
the mixed-host device were 20.3% and 16.7%, respectively.
A high quantum efficiency of over 20% was achieved both
in the single-host and mixed-host devices. The high quantum
efficiency of the two devices originated from the balanced
charge density in the emitting layer. In the mixed-host
device, both holes and electrons were effectively injected
from the charge-transport layer through the hole-transport-
type DFCz and electron-transport-type DFPO host materi-
Figure 3. Hole only and electron only device data of DFCzPO.
owing to the hole-transport carbazole and electron-transport
diphenylphosphine oxide groups. Considering that the
mixed-host device shows the hole-transport properties of
the hole-transport-type host and electron-transport proper-
ties of the electron-transport-type host, the bipolar
DFCzPO host is comparable to the mixed-host in terms of
hole- and electron-transport properties.
As all three host materials showed high triplet energies of
over 2.90 eV, a deep-blue-emitting dopant, FCNIrpic, was
doped in the single bipolar host and the mixed host. Two
deep-blue PHOLEDs with a single bipolar host or a mixed
host were fabricated to compare the device performances of
the two different host systems. DFCz, DFPO, and DFCzPO
were tested as the host materials for blue PHOLEDs. The
device structure of the blue PHOLEDs was ITO (150 nm)/
DNTPD (60 nm)/NPB (5 nm)/mCP (10 nm)/DFCz:DFPO
(50:50) or DFCzPO:bis((3,5-difluoro-4-cyanophenyl)pyri-
dine) iridium picolinate (FCNIrpic) (30 nm, 3%)/TSPO1
(20 nm)/LiFACHTUNGTRENNUNG(1 nm)/AlACHTUTGNREN(NUGN 200 nm). The doping concentration of
the FCNIrpic was 3% in all devices. In the mixed-host
device, the hole-transport-type DFCz and electron-trans-
port-type DFPO were mixed and the relative ratio of the
DFCz to DFPO was 50:50. Figure 4 shows the current-densi-
ty-voltage–luminance curves of the single-host and mixed-
host devices. The single-host and the mixed-host devices
showed similar current density and luminance over all vol-
Figure 4. Current density/voltage/luminance curves of DFCzPO and
DFCz/DFPO (50:50) mixed-host devices.
Figure 5. Quantum efficiency/luminance curves of DFCzPO and DFCz/
DFPO (50:50) mixed-host devices.
Chem. Asian J. 2011, 6, 2895 – 2898
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
2897

J. Y. Lee et al.
COMMUNICATION
Acknowledgements
This work was supported by GRRC program of Gyeonggi Province
(GRRC-Dankook-C01 Host development for high efficiency blue phos-
phorescent organic light emitting diode) and LG display.
Keywords: bipolar compounds · high efficiency · host-guest
systems · OLEDs · phosphorescence
[1] L. Xiao, S.-J. Su, Y. Agata, H. Lan, J. Kido, Adv. Mater. 2009, 21,
1271.
[2] S. J. Su, T. Chiba, T. Takeda, J. Kido, Adv. Mater. 2008, 20, 2125.
[3] H. Sasabe, E. Gonmori, T. Chiba, Y. J. Li, D. Tanaka, S. J. Su, T.
Takeda, Y. J. Pu, K. Nakayama, J. Kido, Chem. Mater. 2008, 20,
5951.
[4] N. Chopra, J. Lee, Y. Zheng, S. H. Eom, J. Xue, F. So, Appl. Phys.
Lett. 2008, 93, 143307.
[5] S. O. Jeong, S. E. Jang, H. S. Son, J. Y. Lee, Adv. Mater. 2010, 22,
1346.
Figure 6. Electroluminescence spectra of DFCzPO and DFCz/DFPO
(50:50) mixed-host devices.
[6] S.-J. Su, D. Tanaka, Y. J. Li, H. Sasabe, T. Takeda, J. Kido, Org. Lett.
2008, 10, 941.
als, resulting in balanced hole- and electron density in the
emitting layer. Similarly, both holes and electrons were ef-
fectively injected and transported through the hole-transport
carbazole and electron-transport diphenylphosphine oxide
units, leading to charge balance in the emitting layer. There-
fore, both the single bipolar host and the mixed-host
worked comparably in the deep-blue PHOLEDs. The maxi-
mum current efficiency and current efficiency at 1000 cdmÀ2
of the DFCzPO device were 26.4 cdAÀ1 and 21.5 cdAÀ1, re-
spectively. The maximum power efficiency and power effi-
ciency at 1000 cdmÀ2 of the DFCzPO device were
22.6 LmWÀ1 and 10.0 LmWÀ1, respectively.
[7] R. Meerheim, S. Scholz, S. Olthof, G. Schwartz, S. Reineke, K.
Walzer, K. Leo, Appl. Phys. Lett. 2008, 104, 14510.
[8] S. H. Kim, J. Jang, J. Y. Lee, Appl. Phys. Lett. 2007, 91, 083511.
[9] S. H. Kim, J. Jang, K. S. Yook, J. Y. Lee, M. Gong, S. Ryu, G. Chang,
H. J. Chang, J. Appl. Phys. 2008, 103, 054502.
[10] J. Lee, J. I. Lee, J. Y. Lee, H. Y. Chu, Appl. Phys. Lett. 2009, 94,
192205.
[11] M. E. Kondakova, T. D. Pawlik, R. H. Young, D. J. Giesen, D. Y.
Kondakov, C. T. Brown, J. C. Dealton, J. R. Lenhard, K. P. Klubek,
J. Appl. Phys. 2008, 104, 094501.
[12] K. S. Yook, S. O. Jeon, C. W. Joo, J. Y. Lee, J. Ind. Eng. Chem. 2009,
15, 420.
[13] S. O. Jeon, K. S. Yook, C. W. Joo, J. Y. Lee, Adv. Mater. 2010, 22,
1872.
Electroluminescence (EL) spectra of the single-host and
mixed-host devices are also shown in Figure 6. Similar EL
spectra were observed in the two devices with a maximum
emission peak at 457 nm and a shoulder peak at 478 nm. The
color coordinate of the single-host and mixed-host devices
was (0.14,0.18), corresponding to the deep-blue emission
color. No extra peak originated from charge transport layer
was observed in the EL spectra of two devices, thus indicat-
ing an effective charge confinement inside the emitting layer.
In conclusion, the design and synthesis of hole transport,
electron transport, and bipolar host materials revealed that
a single bipolar host can show the same device performan-
ces as the mixed-host device. The current density, lumi-
nance, and quantum efficiency were similar in the deep-blue
PHOLEDs with the single bipolar host and mixed host. A
high quantum efficiency over 20% was achieved both in the
single-bipolar host and mixed-host devices. This approach
proved that a well-designed single bipolar host is compara-
ble to mixed host and can be useful in the future develop-
ment of bipolar host materials for deep-blue PHOLEDs.
[14] S. O. Jeon, K. S. Yook, C. W. Joo, J. Y. Lee, Adv. Funct. Mater. 2009,
19, 3644.
[15] J. Ding, Q. Wang, L. Zhao, D. Ma, L. Wang, X. Jing, F. Wang, J.
Mater. Chem. 2010, 20, 8126.
[16] Y. T. Tao, Q. Wang, C. L. Yang, Q. Wang, Z. Q. Zhang, T. T. Zou,
J. G. Qin, D. G. Ma, Angew. Chem. 2008, 120, 8224; Angew. Chem.
Int. Ed. 2008, 47, 8104.
[17] M. Y. Lai, C. H. Chen, W. S. Huang, J. T. Lin, T. H. Ke, L. Y. Chen,
M. H. Tsai, C. C. Wu, Angew. Chem. 2008, 120, 591; Angew. Chem.
Int. Ed. 2008, 47, 581.
[18] Y. L. Liao, C. Y. Lin, K. T. Wong, T. H. Hou, W. Y. Hung, Org. Lett.
2007, 9, 4511.
[19] Z. Y. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akiike, H. Miyamoto,
T. Kajita, M. Kakimoto, Org. Lett. 2008, 10, 421.
[20] D. Kim, S. Salman, V. Coropceanu, E. Salomon, A. B. Padmaperu-
ma, L. S. Sapochak, A. Kahn, J. L. Bredas, Chem. Mater. 2010, 22,
247.
[21] J. Yin, S. L. Zhang, R. F. Che, Q. D. Ling, W. Huang, Phys. Chem.
Chem. Phys. 2010, 12, 15448.
[22] P. A. Vecchi, A. B. Padmaperuma, H. Qiao, L. S. Sapochak, P. E.
Burrows, Org. Lett. 2006, 8, 4211.
Received: July 5, 2011
Published online: September 2, 2011
Experimental Section
Detailed synthetic procedure and characterization of DFCzPO are de-
scribed in supporting information. All device fabrication process is pre-
sented in the Supporting Information.
2898
www.chemasianj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2011, 6, 2895 – 2898