Pyridine-modified acridine-based bipolar host material for green phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1002/asia.201101030

A bipolar host material with a phenyl-modified acridine core and a pyridine unit in the molecular structure was synthesized as the host material for green phosphorescent organic light-emitting diodes (PHOLEDs). A high triplet energy of 2.76 eV was obtained from the host and a high quantum efficiency of 13.5 % was achieved in green PHOLEDs using the acridine-based host material.

Full text of DOI:10.1002/asia.201101030

DOI: 10.1002/asia.201101030
Pyridine-Modified Acridine-Based Bipolar Host Material for Green
Phosphorescent Organic Light-Emitting Diodes
Mounggon Kim and Jun Yeob Lee*[a]
In phosphorescent organic light-emitting diodes (PHO-
LEDs) it is important to balance holes and electrons in the
light-emitting layer to obtain a high external quantum effi-
ciency. In general, the charge balance in the emitting layer
depends on the charge transport properties of the host ma-
terials and charge injection from charge transport layers to
the emitting layer. Therefore, bipolar host materials with
balanced hole and electron transport properties have been
typically used as the host materials to improve the external
quantum efficiency of PHOLEDs.[1–10]
to the phenyl-modified acridine core to develop a bipolar
host material.
The phenyl-modified acridine core was synthesized by the
reaction of 1,2-dibromobenzene with diphenylamine fol-
lowed by lithiation using n-butyllithium and a ring-closing
reaction with 4,4’-dibromobenzophenone.[11] The resulting
brominated acridine intermediate was modified with pyri-
dine. The route for the synthesis of 10-phenyl-9,9-bis(4-(pyr-
idin-3-yl)phenyl)-9,10-dihydroacridine (PBPPA) is shown in
Scheme 1.
Bipolar host materials generally contain both hole and
electron transport units in their molecular structure. Carba-
zole and aromatic amine moieties have been used as hole
transport units,[1–10] while triazine,[1] pyridine,[2,3] phosphine
oxide,[4–7] imidazole[8,9], and oxadiazole[10] units have been in-
troduced as electron transport units. Various host materials
have been synthesized by combining hole transport units
and electron transport units, and yielded high external quan-
tum efficiencies in PHOLEDs. However, further design and
synthesis of new hole transport moieties are required to de-
velop improved bipolar host materials for PHOLEDs.
In this work, a phenyl-modified acridine-based hole trans-
port core structure was synthesized, and a bipolar host ma-
terial with the acridine core and a pyridine unit was devel-
oped. We demonstrate that the phenyl-modified acridine-
type host material shows a balanced charge transport and
a high quantum efficiency of 13.5%.
The phenyl-modified acridine was designed as the hole
transport-type high triplet energy core structure. All phenyl
groups in the acridine core were connected to the amine
unit, and two phenyl groups were linked through sp3 carbon
linkage. Therefore, the conjugation of the triphenylamine
group is not extended and the acridine can have the high
triplet energy of triphenylamine. In addition, the rotation of
the two phenyl units is limited by the sp3 carbon linkage,
thus increasing the glass transition temperature of the acri-
dine-based materials. As the acridine is a hole transport-
type core, an electron transport-type pyridine was attached
The photophysical properties of PBPPA were analyzed
using UV/Vis and photoluminescence (PL) spectroscopy.
UV/Vis absorption, solution PL, and low-temperature PL
spectra of PBPPA are presented in Figure 1. PBPPA showed
absorption of the acridine core and phenylpyridine between
250 nm and 340 nm. As the conjugation length of the acri-
dine was the same as that of triphenylamine, PBPPA exhib-
ited a similar p–p* absorption peak to that of triphenyla-
mine.[12] The band gap of PBPPA was calculated as 3.58 eV
from the absorption edge of the UV/Vis absorption peak.
The PL emission of PBPPA was centered at 400 nm. Low-
temperature PL measurement was carried out to obtain the
triplet energy of PBPPA; the obtained value from the first
emission peak was 2.76 eV. The high triplet energy of the
PBPPA is due to the limited conjugation in the phenyl-
modified acridine structure by the sp3 carbon linkage.
PBPPA also showed a high glass transition temperature of
1258C due to the rigidity of the phenyl-modified acridine
core.
The highest occupied molecular orbital (HOMO) of
PBPPA was measured using cyclic voltammetry, and the
lowest unoccupied molecular orbital (LUMO) was calculat-
ed from the HOMO and the band gap. The HOMO of
PBPPA was 5.97 eV and the LUMO was 2.39 eV.
Molecular simulation of PBPPA was carried out to study
the HOMO and LUMO distribution. Density functional
theory calculations of the compounds were carried out using
Gaussian 03 and the nonlocal density functional of Beckeꢀs
3-parameters
employing
Lee–Yang–Parr
functional
(B3LYP) with 6-31G* basis sets.[13] Figure 2 shows the
HOMO and LUMO distribution of PBPPA. The HOMO of
PBPPA was localized on the acridine core, while the LUMO
of PBPPA was dispersed over the pyridine unit. As the
HOMO and LUMO were distributed on the hole transport
acridine and electron transport pyridine, respectively, bipo-
lar charge transport properties were expected from PBPPA.
[a] M. Kim, Prof. J. Y. Lee
Department of Polymer Science and Engineering
Dankook University
126 Jukjeon-dong, Suji-gu, Yongin-si, Gyeonggi-do, 448-701 (Korea)
Fax : (+82)31-8005-3585
E-mail: leej17@dankook.ac.kr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201101030.
Chem. Asian J. 2012, 7, 899 – 902
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COMMUNICATION
Scheme 1. Synthesis of PBPPA.
Figure 3. Current density–voltage curves of hole-only and electron-only
devices.
Figure 1. UV/Vis absorption, solution PL, and low-temperature PL spec-
tra of PBPPA.
erty of PBPPA. As explained in the molecular simulation re-
sults, the bipolar charge transport property of PBPPA was
induced by the hole transport-type acridine core and elec-
tron transport-type pyridine unit. The hole mobility of
PBPPA was 8.1ꢂ10À6 cm2 VÀ1 sÀ1.
As PBPPA showed high triplet energy for use as the host
for green PHOLEDs, green PHOLEDs with the PBPPA
host were fabricated. The doping concentration of PBPPA
was changed to optimize the performance of PBPPA devi-
ces. Figure 4 shows current density–voltage and luminance–
voltage curves of PBPPA devices at different doping con-
centrations of green-emitting iridium
ACHTUNGTRENNUNG
dine) (Ir(ppy)3). The doping concentration of IrCAHTUNGTRENNUNG
G
Figure 2. Calculated HOMO and LUMO of PBPPA.
changed from 3% to 20%. The current density of green
PHOLEDs was increased with an increase in the doping
concentration. The high current density at high doping con-
To confirm the bipolar charge transport properties of
PBPPA, hole-only and electron-only devices were fabricat-
ed. The structure of the hole-only device was indium tin
oxide
(ITO)/N,N’-diphenyl-N,N’-bis-[4-(phenyl-m-tolyl-
amino)-phenyl]-biphenyl-4, 4’-diamine (DNTPD, 60 nm)/
N,N’-di(1-naphthyl)-N,N’-diphenylbenzidine (NPB, 5 nm)/
PBPPA (30 nm)/DNTPD (5 nm)/Al, and the device struc-
ture of the electron-only device was ITO/diphenylphosphine
oxide-4-(triphenylsilyl)phenyl
(TSPO1,
5 nm)/PBPPA
(30 nm)/TSPO1 (25 nm)/LiF/Al. Figure 3 shows the device
data of hole-only and electron-only devices. The current
density of the hole-only device was higher than that of the
electron-only device over the entire voltage range. However,
the difference in current density was less than an order of
magnitude, thus indicating a bipolar charge transport prop-
Figure 4. Current density–voltage–luminance curves of PBPPA devices at
different IrACHTNUTRGNEUNG(ppy)3 doping concentrations.
900
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Chem. Asian J. 2012, 7, 899 – 902

Mounggon Kim and Jun Yeob Lee
centrations is due to a reduced hopping distance. Holes and
electrons are transported through both host and dopant ma-
terials. The hopping distance between dopant materials is
shortened at high doping concentrations, resulting in better
charge transport at high doping concentrations. The lumi-
nance was also high at high doping concentrations due to
a high current density.
Quantum efficiency–luminance curves of PBPPA devices
are shown in Figure 5. The quantum efficiency gradually in-
creased with an increase in the doping concentration up to
10% and then decreased at a 20% doping concentration.
Thus, the optimum doping concentration of PBPPA PHO-
LEDs was 10%. The maximum quantum efficiency of green
PHOLED with 10% doping concentration was 13.5%, and
the quantum efficiency at 1000 cdmÀ2 was 13.2%. As both
holes and electrons are transported by host and dopant ma-
terials, the charge balance in the emitting layer was greatly
dependent on the doping concentration. Although the quan-
tum efficiency was optimal at 10% doping concentration,
the efficiency roll-off of PHOLEDs was minimized at 20%
doping concentration. The quantum efficiency of PBPPA
device was higher than that of the common 4,4’-di(9H-car-
bazol-9-yl)biphenyl (CBP) device.
Figure 6. Electroluminescence spectra of PBPPA devices at different Ir-
AHCTUNRTGEG(NNNU ppy)3 doping concentrations.
Experimental Section
The synthetic procedure and characterization of PBPPA as well as the
device fabrication process are described in the Supporting Information.
Keywords: acridine · bipolar host · electron transport
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Received: December 19, 2011
Revised: February 3, 2012
Published online: March 12, 2012
902
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ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 899 – 902