Benzimidazole/amine-based compounds capable of ambipolar transport for application in single-layer blue-emitting OLEDs and as hosts for phosphorescent emitters

Article abstract of DOI:10.1002/anie.200704113

A bolt from the blue? Compounds like that shown containing benzimidazole and arylamine units exhibit intriguing ambipolar carrier-transport properties and can be used to fabricate single-layer blue-emitting electroluminescent devices with very promising performances (see picture, ITO = indium tin oxide). (Figure Presented)

Full text of DOI:10.1002/anie.200704113

Angewandte
Chemie
DOI: 10.1002/anie.200704113
Electroluminescent Materials
Benzimidazole/Amine-Based Compounds Capable of Ambipolar
Transport for Application in Single-Layer Blue-Emitting OLEDs and
as Hosts for Phosphorescent Emitters**
Mei-Yi Lai, Chih-Hsin Chen, Wei-Sheng Huang, Jiann T. Lin,* Tung-Huei Ke, Li-Yin Chen,
Ming-Han Tsai, and Chung-Chih Wu*
Organic light-emitting diodes (OLEDs) have attracted a
great deal of interest because of their potential applications in
full-color flat-panel displays and lighting sources.[1] Consid-
erable progress has been made on both small-molecule- and
polymer-based OLEDs. Though blue-, green-, and red-
emitting materials are all needed for OLED applications,
currently development of deep-blue emitters with good
stability and high luminescence efficiency is deemed most
critical in effectively reducing the power consumption and
generating emission of different colors (including white light).
In recent years considerable efforts have been shifted to the
development of luminescent transition-metal complexes,
particularly second- and third-row transition metals.[2] As a
result of efficient spin–orbit coupling in these complexes, both
singlet and triplet excitons can be harvested, and theoretically
up to 100% internal quantum efficiencies can be attained.
However, metal complexes are typically crystalline and must
be used as a dopant in an appropriate host. Furthermore, the
host and materials used for carrier transport or carrier
blocking must have sufficiently large triplet (T1) energy to
prevent the loss of triplet excitons from the metal com-
plexes.[3] Therefore, phosphorescence-based devices fre-
quently have complicated structures. Since blue-emitting
phosphorescent materials have relatively a large triplet
energy, it becomes increasingly difficult to find host materials
with a suitably high triplet state.[4] Lastly, there are only few
blue-emitting phosphorescent devices showing Commission
Internationale de l’Eclairage (CIE) y values below 0.2 yet still
with a respectable efficiency for the conversion of electrical
into light energy.[4c,e] Obviously, blue-emitting fluorescent
materials will still play a major role in the foreseeable future,
and it would be useful if a single fluorescent molecule could
be fabricated for application in a single-layer blue-emitting
device. The simplified device structure would also help to
limit the overall cost.
A prerequisite for high-performance single-layer OLEDs
with a single component is to use molecules with high
fluorescent quantum yields and balanced electron and hole
mobilities. Equally important are good film-forming proper-
ties and an energy level matching that of the electrodes.
Ambipolar molecules capable of carrying electrons and holes
are likely to be candidates for single-layer devices. So far,
quite a few ambipolar molecules have been used for
OLEDs.[5] However, most of them could not be fabricated
as single-layer devices or exhibited only low efficiencies.
Recently, a yellow-emitting single-layer device using a boron-
containing material was reported to have good perfor-
mance.[6] We also successfully developed various high-
efficiency single-layer devices, using materials ranging from
yellow-emitting 3-cyano-9-diarylaminocarbazoles[7] to
a
green-emitting dibenzothiophene-S,S-dioxide-based com-
pound containing an arylamine unit.[8] In the first case,
external quantum efficiencies up to 1.4% and current
efficiencies up to 5.1 cdAÀ1 at 100 mAcmÀ2 could be ach-
ieved. In the second case, a high external quantum efficiency
of 3.1% and a current efficiency of 7.5 cdAÀ1 were achieved
at a current density of 100 mAcmÀ2
.
Herein, we report blue-emitting fluorescent molecules for
high-performance single-layer devices. To the best of our
knowledge, no single-layer blue-emitting devices using small
molecules as the sole component have ever been reported to
have acceptable efficiencies. In designing a blue-emitting
molecule, one should avoid an extended conjugated p system
and as well as strong charge-transfer character in the
molecular skeleton. Moreover, the molecule should possess
segments capable of effectively transporting electrons and
holes suitable for the single-component single-layer devices.
In view of the good electron mobility ( ꢀ 10À5 cm2 VÀ1 sÀ1) of
1,3,5-tris(phenyl-2-benzimidazolyl)benzene (TPBI)[9] and
[*] M.-Y. Lai, Dr. C.-H. Chen, W.-S. Huang, Dr. J. T. Lin
Institute of Chemistry, Academia Sinica
Taipei, 115 Taiwan
and Department of Chemistry, National Central University
Chungli, 320 Taiwan
Fax: +(2)27831237
E-mail: jtlin@chem.sinica.edu.tw
T.-H. Ke, L.-Y. Chen, M.-H. Tsai, Dr. C.-C. Wu
Department of Electrical Engineering
Graduate Institute of Electro-optical Engineering
and Graduate Institute of Electronics Engineering
National Taiwan University, Taipei, 106 Taiwan
E-mail: chungwu@cc.ee.ntu.edu.tw
good
hole
mobility
of
arylamines
( ꢀ 10À3–
10À5 cm2 VÀ1 sÀ1),[10] we decided to link an arylamine unit
and a (phenyl-2-benzimidazolyl)benzene unit with a fluorene
spacer that may also prevent strong charge transfer from the
amine to imidazole.
[**] We thank the Academia Sinica, National Taiwan University, and the
National Science Council for supportingthis work.
Scheme 1 illustrates the synthesis of the new compounds 1
and 2 examined in this study. Standard procedures[11] were
followed to construct benzimidazoles from N-phenyl-o-phe-
Supportinginformation for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Table 1: Physical data of the compounds.
1
2
Tm/Tg/Td [8C][a]
284/104/373
396/170/467
309, 373
428 (76)
457 (72)
2.3 (2.2)
413 (70)
5.21/2.21
labs [nm] EA[b]
306, 370
428 (89)
466 (80)
2.3 (2.2)
386 (91)
5.19/2.20
lem [nm] (Ff [%]) toluene[c]
lem [nm] (Ff [%]) film[c]
ET [eV][d]
Eox (DEp) [mV][e]
HOMO/LUMO [eV][f]
À1
[a] The heatingand coolingrates were 10Kmin
and 30KminÀ1
,
respectively. Tm: meltingpoint; Tg: glass transition temperature; Td:
decomposition temperature. [b] labs: absorption maximum. EA=ethyl
acetate. [c] lem: emission maximum. Measurement of quantum yields
(Ff) in solution and in thin films are described in Ref. [14]. [d] Triplet
energy measured in toluene at 77 K (and measured in film). [e] Eox
=
1/2(Epa + Epc), DEp =EpaÀEpc , where Epa and Epc are anodic and cathodic
potentials, respectively. Measured in CH2Cl2. All the potentials are
reported relative to ferrocene, which was used as the internal standard in
each experiment. Ferrocene oxidation potential was located at +272 mV
relative to the Ag/AgNO3 nonaqueous reference electrode. [f] The
HOMO and LUMO energies were determined from cyclic voltammetry
and absorption data.
Devices of different structures were fabricated for elec-
troluminescence (EL) studies: Device A: indium tin oxide
(ITO)/1 or 2 (80 nm)/LiF (1 nm)/Al (150 nm); Device B:
ITO/1,4-bis[(1-naphthylphenyl)amino]biphenyl
(NPB,
40 nm)/1 or 2 (40 nm)/LiF (1 nm)/Al (150 nm); Device C:
ITO/1 (40 nm)/1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene
(TPBI, 40 nm)/LiF (1 nm)/Al (150 nm). 1,4-Bis[(1-naphthyl-
phenyl)amino]biphenyl)[16] and 1,3,5-tris(N-phenylbenz-
imidazol-2-yl)benzene[17] were used as the hole- and the
electron-transporting materials, respectively, while LiFand Al
were used as the electron-injection layer and the cathode. All
devices are blue-emitting, and representative EL spectra are
shown in Figure 1. The device performances are summarized
in Table 2. Current–luminance (I–L) characteristics for rep-
resentative devices are shown in Figure 2. It is worth noting
that the unoptimized single-layer devices A show very
Scheme 1. Synthesis of compounds 1 and 2.
nylenediamine and fluorene-containing aromatic aldehydes.
À
The palladium-catalyzed C N coupling reaction developed
by Hartwig et al.[12] was then adopted to incorporate the
arylamine moiety. The compounds were isolated in good
yields ( ꢀ 70%).
The thermal properties of compounds 1 and 2 were
determined by differential scanning calorimetry (DSC) and
thermogravimetric analysis (TGA) (Table 1). Though the
compounds are crystalline, they readily form a glass from
their melts upon fast cooling, and the glassy state persists in
the subsequent heating cycles. Replacement of fluorene by
spirobifluorene[13] increases the glass transition temperature
by ꢀ 608C (2 vs. 1). The thermal decomposition temperatures
of the compounds are 3738C (1) and 4678C (2).
promising performances: external quantum efficiency (hext
)
2.4%, maximum power efficiency (hp,max) 1.2 lmWÀ1, current
efficiency (hc) 2.4 cdAÀ1, luminance (L) 2378 cdmÀ2, and CIE
indices [x,y] (0.15, 0.12) at 100 mAcmÀ2 for 1, and respective
values of 1.4%, 0.9 lmWÀ1, 1.8 cdAÀ1, 1750 cdmÀ2, and (0.15,
0.15) at 100 mAcmÀ2 for 2. The Commission Internationale
de l’Eclairage (CIE) chromaticity coordinates of the two
devices are shown in Figure 3. The performance of the single-
layer device of 1 compares favorably with those of non-doped
blue-emitting devices reported in literature.[18] Among the
devices A–Cfor the same compound, device B has the best
performance, but device A gives performances similar to
those of device B. No discernible light emission was detected
for device Cmade with compound 2. Device B with 1 appears
to have the best performance of all the blue-emitting devices
reported here: 2.8%, 1.9 lmWÀ1, 2.6 cdAÀ1, and 2626 cdmÀ2
at 100 mAcmÀ2.
Photophysical properties of compounds 1 and 2 are also
presented in Table 1. The absorption bands in the region
around 370 nm are attributed to p–p* transitions of the
conjugated aromatic segments. The compounds are blue-
emitting with very high solution quantum yields (> 70% in
toluene and in the film).[14] Each compound displays a quasi-
reversible wave in its cyclic voltammagram (Table 1), which is
attributable to the oxidation of the arylamine unit. No
reduction waves were detected. The HOMO energy levels
of the compounds were calculated from cyclic voltammetry
and by comparison with ferrocene.[15] These together with
absorption spectra were then used to obtain the LUMO
energy levels (Table 1).
The triplet energies of 1 and 2 were measured to be 2.2
and 2.3 eV in the film state and in toluene solution,
respectively.[19] We thus estimate that they may act as
appropriate hosts for orange/red-emitting phosphorescent
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Figure 2. Plot of luminance versus current density for devices A and C.
Figure 1. The EL spectra of the devices, in which, for example, “1,
device A” denotes device A made with compound 1. Note the EL
spectra of devices B and C made with compounds 1 and 2 are
essentially the same as those of device A made with 1 and 2,
respectively (not shown here).
Table 2: Electroluminescent data of the devices.[a]
1
2
V
L
on [V]
2.9; 2.7; 5.0
16178 (13.5);
21663 (14.0);
5713 (20.5)
452, 456, 446
0.15, 0.12;
2.9; 2.7; 3.1
10011 (13.5);
10418 (14.0);
79750 (21.5)
458; 454; 572
0.15, 0.15;
max [cdmÀ2
]
(V at Lmax, V)
Figure 3. CIE coordinates for devices A made with compounds 1 and 2
(the cross denotes 475 nm).
lem [nm]
CIE [x,y]
0.14, 0.11;
0.15, 0.12;
0.15, 0.12
0.53, 0.47
fwhm [nm]
hext,max [%]
60; 62; 64
68; 62; 76
biphenyl (CBP) as the host.[20] The EL spectrum and the I–L
characteristics of device D are shown in Figure 1 and Figure 2,
respectively, and relevant performance parameters for the
device are summarized in Table 2.
2.5; 3.0; 2.0
2.0; 2.3; 0.7
2.5; 2.8; 2.2
2378; 2626; 1125
2.4; 2.8; 1.2
1.2; 1.9; 0.24
2.4; 2.6; 1.2
1.4; 1.6; 7.8
1.1; 1.6; 7.5
1.8; 1.7; 22
1750; 1432; 19637
1.4; 1.4; 6.9
0.9; 0.9; 4.4
1.8; 1.4; 20
hp,max [lmWÀ1
]
hc,max [cdAÀ1
]
L [cdmÀ2 [b]
hext [%][b]
hp [lmWÀ1 [b]
hc [cdAÀ1 [b]
]
The carrier mobilities for the amorphous compounds 1
and 2 were measured by the time-of-flight (TOF) transient
photocurrent technique[21] in vacuum at room temperature.
Both compounds exhibit nondispersive electron- and hole-
transport characteristics as indicated by the TOF transients.
The field-dependence of mobilities determined from the TOF
transients (Figure 3) follows the nearly universal Poole–
Frenkel relationship: maexp(bE1/2), where b is the Poole–
Frenkel factor.[22] The electron mobilities ( ꢀ 10À5 cm2 VÀ1 sÀ1)
for both compounds are comparable to those reported for
commonly used the electron-transporting materials TPBI and
tris(8-hydroxyquinoline) aluminum (Alq3).[23] In contrast, the
hole mobilities ( ꢀ 10À5 cm2 VÀ1 sÀ1) for 1 and 2 are lower than
that of the typical HTL material NPB ( ꢀ 10À3 cm2 VÀ1 sÀ1).[24]
Compound 1 has slightly higher mobility for electrons than
for holes, and the reverse trend was found in 2 (Figure 4). The
rather balanced and effective bipolar carrier transport (with
hole and electron mobilities both in the 10À5 cm2 Vs range) is
further proof of the successful single-layer devices in this
study.
]
]
[a] The measured values are given in order of the devices A, B, and C for 1
and A, B, and D for 2. Abbreviations: Lmax: maximum luminance; Von
turn-on voltage; V: voltage; lem emission wavelength; CIE [x,y]:
:
:
Commission Internationale de l’Eclairage coordinates; fwhm: full
width at half maximum; hext,max: maximum external quantum efficiency;
hp,max: maximum power efficiency; hc,max: maximum current efficiency; L:
luminance; hext: external quantum efficiency; hp: power efficiency; hc:
current efficiency. Von was obtained from the x-intercept of log(lumi-
nance) vs applied voltage plot. [b] Measured at a current density of
100 mAcmÀ2
.
materials. Indeed, we successfully used compound 2 as the
host for an orange-yellow phosphorescent iridium complex,
[(fbi)2Ir(acac)]
(fbi = 2-(9,9-diethyl-9H-fluoren-2-yl)-1-
phenyl-1H-benzimidazole; acac = acetylacetonate).[20] The
single-layer device D (ITO/2 with 5% [(fbi)2Ir(acac]
(80 nm)/LiF (1 nm)/Al (150 nm)) has efficiencies (6.9%,
4.4 lmWÀ1, 20 cdAÀ1, 19637 cdmÀ2 at 100 mAcmÀ2) rivaling
those of the multilayer device with 4,4’-N,N’-dicarbazole-
In summary, we have developed a convenient synthesis of
highly emissive compounds containing benzimidazole and
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Received: September 6, 2007
Published online: December 3, 2007
Keywords: benzimidazole · electroluminescence ·
.
electron transport · hole transport · light-emitting diodes
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