Pyridoindole derivative as electron transporting host material for efficient deep-blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1002/adma.201001221

An electron-transporting host material suitable for deep-blue phosphorescent organic light-emitting diodes has been developed. The material exhibits electron-transport characteristics, high triplet energy, and high thermal stability. It is designed by int

Full text of DOI:10.1002/adma.201001221

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Pyridoindole Derivative as Electron Transporting Host
Material for Efficient Deep-blue Phosphorescent Organic
Light-emitting Diodes
By Hirohiko Fukagawa,* Norimasa Yokoyama, Shiro Irisa, and Shizuo Tokito
Organic light-emitting diodes (OLEDs) are very attractive for
full-color flat-panel displays and lighting applications. OLED
characteristics, such as device stability and the power efficiency,
must be improved for them to become practical. Low-power-
consumption OLEDs can be fabricated by i) improving the
current efficiency and ii) reducing the driving voltage. There
have been many reports on phosphorescent OLEDs (POLEDs)
because their emission efficiencies are higher than that of con-
ventional fluorescent OLEDs.[1–3] For fluorescent OLEDs, a 25%
internal quantum efficiency can be obtained from 25% of sin-
glet spin states since the singlet and triplet excitons are gener-
ated at a ratio of about 1:3 under electrical excitation. On the
other hand, POLEDs show an internal quantum efficiency of
about 100% since the remaining 75% of triplet spin states can
also emit light.[2] In fact, a near theoretical limitation of green-
light emission was obtained using fac-tris(2-phenylpyridinato)
iridium(III) [Ir(ppy)3] as the phosphorescent emitter.[4] By using
phosphorescent dyes, red and green POLEDs with higher power
efficiencies have already been reported.[4–6] Compared to the
higher power efficiency of red and green POLEDs previously
reported, the power efficiency of blue POLEDs is poor, espe-
cially in deep-blue POLEDs. Blue light is not only one of the
three primary colors from which white light can be obtained,[7]
but it can also generate other low-energy emissions, such as
green and red, by using a color-change medium. The external
quantum efficiency (EQE) of blue POLEDs, which consists of
iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2′]picolinate
(FIrpic) and/or iridium(III) bis(4′,6′-difluorophenylpyridinato)
tetrakis(1-pyrazolyl)borate (FIr6), have been improved by
synthesizing host materials with a high triplet energy (ET) to
confine the triplet excitons.[8]−[17] The host material, which
possesses high carrier mobility and high ET, is essential for
achieving low-power-consuming blue POLEDs. In addition, an
effort should be made to reduce the operating voltage by using
efficient charge-transporting materials. There have been many
reports on carbazole derivatives, which possess sufficiently
high ET and hole-transporting properties, suitable for deep-blue
POLEDs.[10,11,13,15,17] To improve efficiency, the development of
an efficient electron-transporting host or a blocking layer is still
a major objective. The electron transporting host mainly used
in deep-blue POLEDs is p-bis(triphenylsilyly)benzene (UGH2)
becauseofitslargeET.[9,12,17–19]However,theelectron-transporting
property of UGH2 is relatively poor since it has no electron-
transporting substituent. As a result, almost all deep-blue POLEDs
using UGH2 require a high driving voltage, and their power
efficiency becomes less efficient. Thus, a substituent similar to
carbazole, which combines sufficiently high ET and electron-
transporting characteristics, is necessary to improve the power
efficiency of deep-blue POLEDs.
In this communication, a new electron transporting host for
use in high-efficiency low-voltage blue POLEDs, 2,2-bis(4-pyri-
doindolyl-9-lyphenyl)adamantane (Ad-Pd), is presented. The
newly proposed pyridoindole substituent has an electron-trans-
porting pyridine part to provide both high ET and electron-
transporting characteristics.[20,21] It has been demonstrated that
POLEDs using Ad-Pd as the electron-transporting host mate-
rial and FIr6 as the deep-blue phosphorescent guest exhibit low
driving voltages of 5, 6.4, and 8.2 V for current densities of 1, 10,
and 100 mA cm−2 , respectively. These driving voltages are sig-
nificantly lower than those of conventional deep-blue POLEDs
using UGH2, which require 7.6, 9.6, and 12.4 V, respectively,
to reach the corresponding current densities. With such low
driving voltages, the maximum power efficiency of FIr6-based
POLEDs using Ad-Pd is dramatically enhanced to 33 lm W−1 .
Ad-Pd was synthesized from the reactions outlined in
Scheme 1. 2,2-Bis(4-aminophenyl)adamantane 1 was synthe-
sized by adapting the dehydration method.[22] Reaction of 2-
adamantanone with aniline and aniline hydrogen chloride
under reflux produced the bis-aminophenyl product 1. The bis-
iodophenyl product 2 was synthesized using the Sandmeyer
reaction in which aromatic amino compounds are converted
into halogeno benzene derivatives. Nucleophilic aromatic sub-
stitution of 2 with 5H-Pyrido[4,3-b]indole in the presence of Cu
furnished the desired product Ad-Pd. Results from 1H NMR
analyses were consistent with the proposed structure. Ad-Pd
shows excellent thermal properties with a glass-transition tem-
perature (Tg) of 181 °C and a melting point of 362 °C (both
determined by differential scanning calorimetry (DSC)).
∗
[ ] Dr. H. Fukagawa, Dr. S. Tokito
Japan Broadcasting Corporation (NHK)
Science and Technology Research Laboratories
1–10-11, Kinuta, Setagaya-ku, Tokyo 157–8510 (Japan)
E-mail: fukagawa.h-fe@nhk.or.jp
The physical properties of Ad-Pd and other organic materials
used in this study are summarized in Table 1. The optical band
gap (Eg) of Ad-Pd was estimated from a cutoff wavelength of
absorption peak (340 nm). The triplet energy of Ad-Pd, esti-
mated from a low-temperature photoluminescence spectrum,
was about 2.97 eV, which is larger than that of the guest mole-
cule FIr6. Thus, the triplet energy transfer from FIr6 to Ad-Pd is
N. Yokoyama, S. Irisa
Hodogaya Chemical Co. Ltd.
45, Miyukigaoka, Tsukuba, Ibaraki 305–0841 (Japan)
DOI: 10.1002/adma.201001221
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The applicability of Ad-Pd for deep-blue
POLEDs was investigated in a double-emission
layer (DEL)-structured device using FIr6 as
the emitter. It has been reported that the DEL
structure, which consists of two emitting
layers using hole- and electron-transporting
hosts, is useful in achieving highly efficient
POLEDs.[17,23] The optimized DEL device,
defined as DEL device 1, had the structure
ITO/ PEDOT:PSS (35 nm)/TAPC (40 nm)/
Ad-Cz:5wt%FIr6 (7 nm)/Ad-Pd:10wt%FIr6
(15 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100
nm) (TAPC: 1,1-bis[(di-4-tolylamino)phenyl]
cyclohexane; Ad-Cz: 2,2-bis(4-carbazolyl-9-
lyphenyl)adamantane). The Commission
Internationale de L’Eclairage (CIE) coor-
dinates of DEL device 1 at 1 mA cm−2 are
(0.16, 0.28). We compared the device charac-
teristics of DEL device 1 that utilized Ad-Pd
with that of a previously reported DEL device
(DEL device 2), which had the structure ITO/
Scheme 1. Synthesis of Ad-Pd.
PEDOT:PSS (35 nm)/α-NPD (40 nm)/Ad-
completely suppressed, and the energy is confined in FIr6. The
ionization potential (IP) of Ad-Pd was estimated to be 6.1 eV
from the spectroscopic measurement of the photoemission in
air (AC-3, Rikenkeiki). By subtracting Eg from IP, the electron
affinity (EA) of Ad-Pd was estimated to be 2.5 eV.
Cz:3wt%FIr6 (35 nm)/UGH2:3wt%FIr6 (5 nm)/TPBI (40 nm)/
LiF (1 nm)/Al (100 nm).[17] The main difference between the two
DEL devices is the electron-transporting host, Ad-Pd or UGH2,
for the second emitting layer. The current density–driving
voltage–luminance characteristics of DEL devices 1 and 2 are
shown in Figure 2a. The turn-on voltage, defined as the bias
required to attain a brightness of 0.1 cd m−2 , is 3.2 and 4.4 V
for DEL devices 1 and 2, respectively. DEL device 1 required
lower driving voltages of 5.0, 6.4, and 8.2 V for current densities
of 1, 10, and 100 mA cm−2 , respectively, compared to voltages
of 7.6, 9.6, and 12.4 V for the same current densities for DEL
device 2. Figure 2b shows the EQE and power efficiency of each
device. A maximum EQE of 19% was obtained in DEL device 1,
which is higher than that of DEL device 2. Because of the low
driving voltage, the power efficiency of DEL device 1 dramatically
The electron transport properties of Ad-Pd thin films and
UGH2 thin films were compared in an electron-only device,
as shown in Figure 1. The structure of the electron-only device
was ITO/PEDOT:PSS (35 nm)/BCP (10 nm)/Ad-Pd or UGH2
(x nm, x = 0 or 15 or 30)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm)
(ITO: indium tin oxide; PEDOT:PSS: poly(3,4-ethylenediox-
ythiophene)/poly(styrene sulfonic acid); BCP: 2,9-dimethyl-4,
7-diphenyl-1,10-phenanthroline; TPBI: 1,3,5-tris(N-phenylbenz-
imidazol-2-yl)benzene). The driving voltage for obtaining a cur-
rent density of 10−2 mA cm−2 of the device changed by inserting
an Ad-Pd layer, while it did not change by inserting UGH2. The
change in driving voltage upon inserting Ad-Pd is attributed
to the energy difference of the lowest unoccupied molecular
orbital (LUMO) between TPBI and Ad-Pd, as summarized in
Table 1. The current density of UGH2 decreased rapidly with
the increase in film thickness, while no significant change was
observed in Ad-Pd. These results indicate that the electron-
injection barrier at the Ad-Pd/TPBI interface is higher than
that at the UGH2/TPBI interface, and the electron-transporting
ability of Ad-Pd is higher than that of UGH2.
T a b le 1 . Physical Properties of organic materials used for devices.
Organic material
TAPC [19]
Eg [eV]
3.5
IP [eV]
5.5
EA [eV]
2.0
ET [eV]
2.87
2.88
2.97
2.72
2.58
3.50
Ad-Cz [17]
Ad-Pd [this work]
FIr6 [9]
3.2
5.8
2.6
3.6
6.1
2.5
3.0
6.1
3.1
Figure 1. Current density–voltage characteristics of electron-dominant
devices: ITO/PEDOT:PSS (35 nm)/BCP (10 nm)/Ad-Pd or UGH2 (x nm,
x = 0, 15, 30)/TPBI (40 nm)/LiF/Al.
TPBI
3.5
6.3
2.8
UGH2 [9]
4.4
7.2
2.8
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the injected holes and electrons transported within the Ad-Pd
layer, and excited triplet states of FIr6 were generated in the
Ad-Pd layer. Thus, the difference in power efficiency between
DEL device 1 and the POLED using UGH2, DEL device 2, is
caused by the difference in the electron transportability of each
electron-transporting host material. The results of our experi-
ment clearly show that the pyridoindole substituent is useful
for developing an electron-transporting host with a high ET as
well as carbazole substituent, which is useful for synthesizing
a hole-transporting host with a high ET. We will apply pyridoin-
dole derivatives, such as Ad-Pd, to p–i–n structured devices[24]
to improve power efficiency.
In summary, we proposed a pyridoindole derivative of Ad-Pd,
which possesses good electron transportability and a high ET,
as a host material for deep-blue POLEDs. Its application in a
highly efficient, low-voltage POLED has been demonstrated.
The results show that Ad-Pd is a promising host material to
replace the commonly used UGH2, both in terms of power
efficiency and thermal stability. Without a p–i–n structure, the
POLED produced using FIr6 as the guest molecule exhibited
low driving voltages of 5.0, 6.4, and 8.2 V for current densities of
1, 10, and 100 mA cm−2 , respectively. A maximum EQE of 19%
was obtained with a maximum power efficiency of 33 lm W−1 ,
which is comparable with that of previously reported excellent
p–i–n-structured devices. We found that the pyridoindole deriv-
ative can be a suitable electron-transporting host for deep-blue
POLEDs. This finding could provide a useful strategy in the
design of new electron-transporting host materials to satisfy the
special criteria required for deep-blue POLEDs.
Figure 2. (a) Current density (left, black symbol) and luminance (right,
open dot) voltage characteristics of devices 1 and 2. (b) External quantum
efficiency (left, black symbol) and power efficiency (right, open dot)
voltage curves of devices 1 and 2.
Experimental Section
improved up to a value of 23 lm W−1 at 100 cd m−2 and reached
a very high efficiency of 33 lm W−1 . We are convinced that the
low driving voltage and high efficiency of the DEL device using
Ad-Pd is attributed to the higher electron transportability of
Ad-Pd, and the high triplet energy to confine the triplets of FIr6.
Furthermore, we compared the device characteristics of the
DEL device 1 with that of other FIr6-based POLEDs.[19,24] The
power efficiency of DEL device 1 was better than that of the
POLED using UGH2,[19] where the film thickness of the emit-
ting layer is almost the same as that of DEL device 1. Moreover,
the power efficiency and turn-on voltage of DEL device 1 are
comparable to those previously reported for a DEL-structured
POLED with a p–i–n structure by Eom et al.[24] The emitting-
layer material in almost all previously reported FIr6-based
POLEDs was the poor electron-transporting material UGH2,
which resulted in a high driving voltage. It is known that hole
transport in the UGH2:FIr6-emission layer occurs through
FIr6 molecules, and FIr6 acts as an electron trap.[9] Next, we
focused our attention on the emission mechanism in the Ad-
Pd:FIr6-emission layer in DEL device 1. It is likely that a por-
tion of the accumulated holes at the Ad-Cz/Ad-Pd interface is
directly injected into FIr6 in the Ad-Pd layer as in the previ-
ously reported UGH2:FIr6 system since i) the electron mobility
of TPBI (∼1.0 × 10−5 cm2 V−1 s−1 ) is much lower than the hole
mobility of TAPC (∼1.0 × 10−2 cm2 V−1 s−1 ) and ii) the highest
occupied molecular orbital energy of FIr6 is almost the same as
Ad-Pd.[25,26] Therefore, carrier recombination occurred between
1H NMR of Ad-Pd (CDCl3, 600 MHz, δ): 9.4 (2H, s), 8.5 (2H, d, J = 5.5
Hz), 8.2 (2H, d, J = 7.6 Hz), 7.7 (4H, d, J = 8.2 Hz), 7.5–7.4 (8H, m),
7.4–7.3 (4H, m), 2,2 (4H, d, J = 12.4 Hz), 2.0 (2H, s), 2.0 (4H, d, J =
12Hz), 1.9 (4H, d, J = 13.1 Hz), 1.8 (2H, s).
The OLEDs were developed on a glass substrate coated with a
150 nm thick ITO layer. Prior to fabrication of the organic layers, the
substrate was cleaned with ultra-purified water and organic solvents,
and treated with a UV–ozone ambient. To reduce the possibility of
electrical shorts within the device, PEDOT:PSS diluted in water was
spun onto the substrate to form a 35 nm thick PEDOT:PSS layer after
being baked for 1 h at 180 °C. The other organic layers were sequentially
deposited onto the substrate without breaking vacuum at a pressure of
about 10−5 Pa. The devices were encapsulated using a UV–epoxy resin
and a glass cover within a nitrogen atmosphere after cathode formation.
The electroluminesence spectra and luminance were measured with a
spectroradiometer (Minolta CS-1000). A digital source meter (Keithley
2400) and a desktop computer were used to operate the devices. We
assumed that the emission from the OLED was isotropic, such that the
luminance was Lambertian, and calculated the EQE from the luminance,
current density, and EL spectra.
Received: April 5, 2010
Published online: August 27, 2010
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