De novo design of silicon-bridged molecule towards a bipolar host: All-phosphor white organic light-emitting devices exhibiting high efficiency and low efficiency roll-off

Article abstract of DOI:10.1002/adma.201002732

A molecular design strategy for a bipolar host material is demonstrated by incorporating both electron donor and electron acceptor into a silicon-bridged structure. A two-color, all phosphor and single-emitting-layer white OLED hosted by the new bipolar c

Full text of DOI:10.1002/adma.201002732

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De Novo Design of Silicon-Bridged Molecule Towards a
Bipolar Host: All-Phosphor White Organic Light-Emitting
Devices Exhibiting High Efficiency and Low Efficiency Roll-Off
By Shaolong Gong, Yonghua Chen, Chuluo Yang,* Cheng Zhong, Jingui Qin,
and Dongge Ma*
White organic light-emitting diodes (WOLEDs) continue to
attract intensive interest due to their applications in full color
flat-panel displays and lighting sources.[1,2] Among various
approaches to realize WOLEDs, adopting all phosphorescent
emitters has great potential because electrophosphorescence
can, in theory, achieve unity internal quantum efficiency
through harvesting both singlet and triplet excitons.[3] All-
phosphor WOLEDs have made considerable progress in recent
years;[4–6] however, they are still a long way from commerciali-
zation. Firstly, a major bottleneck is the lack of highly efficient
and stable blue-emitting electrophosphorescence, which is
indispensable to WOLEDs. Secondly, all-phosphor WOLEDs
usually exhibit significant efficiency roll-off at the luminance
required for practical light sources. Thirdly, the two or multiple
emitting layer (EML) structures widely adopted in WOLEDs
inevitably result in higher complexity, control difficulties and
higher operational voltages during fabrication.[5,7] To address
these issues, elegant materials and elaborate device structures
have to be developed. In terms of material, a bipolar host
capable of hosting blue phosphor can play a vital role in solving
the above-mentioned problems because a bipolar host can
not only broaden the exciton-formation zone – consequently
reducing the efficiency roll-off – but also simplify the device
structure by employing a single emitting layer comprised of a
bipolar host and two or more phosphors. However, the design
of such a host is a significant challenge because three tradeoffs
have to be managed: i) the high enough triplet energy required
to host blue phosphor and bipolar transporting characteristics,
since the p- and n-type segments integrated into the molecule
structure unavoidably lower the band gap of the material due
to intramolecular charge-transfer;[8] ii) the high triplet energy
and minimum energy barrier between the host material and
adjacent charge-transporting layer – a high triplet energy gener-
ally signifies a large electrical band gap, i.e., a low-lying highest
occupied molecular orbital (HOMO) and/or high-lying lowest
occupied molecular orbital (LUMO) level, which are not easily
matched in the energy levels of adjacent layers; and, iii) the
high triplet energy and good morphologically stability needed –
a high triplet energy usually requires limited conjugation length
and molecular size, which makes it difficult for the materials to
acquire morphological stability.[9]
To date, the host materials for blue phosphorescent OLEDs
(PhOLEDs) have mainly been based on carbazole-based com-
pounds.[10] Most recently, diphenylphosphine oxide derivatives
of aromatic molecules have emerged as a new class of bipolar
host materials for blue PhOLEDs.[11] In addition, a series of
tetraarylsiliconcompoundswithultrahighsinglet(around4.5eV)
and triplet (around 3.5 eV) energies as host materials for
deep blue electrophosphorescence have also been reported.[12]
Although the tetraarylsilicon host materials have high triplet
energy levels, they show a poor charge-transporting ability and
a very high hole-injection barrier. The synthetic versatility of
organic materials allows the optimization of their physical prop-
erties, including energy level, charge transport and morpho-
logical stability. In this communication, we report the de novo
design of a silicon-bridged molecule by integrating electron-
donating and -accepting moieties into the tetraarylsilicon skel-
eton. We anticipated that the electron-donating and -accepting
groups would impart the material with bipolar transporting
nature; meanwhile, the whole molecule would maintain rela-
tive high triplet energy owing to silicon-interrupted conjugation
of two segments. The new silicon-bridged compound, com-
bining p-type triphenylamine and n-type benzimidazole groups,
namely p-BISiTPA, shows a high triplet energy level of 2.69 eV.
Hole- and electron-only devices indicate its bipolar transporting
feature. Devices with p-BISiTPA as host show excellent per-
formance, with external quantum efficiencies as high as 16.1%
for blue, 20.4% for orange and 19.1% for white electrophos-
phorescence at a practical brightness of 100 cd m−2 . Even at
a brighter emission of 1000 cd m−2 , the efficiencies were still
14.2%, 18.9% and 17.4%, respectively.
[*] S. Gong, Prof. C. Yang, C. Zhong, Prof. J. Qin
Department of Chemistry
Hubei Key Lab on Organic and Polymeric Optoelectronic Materials
Wuhan University
Wuhan 430072 (PR China)
E-mail: clyang@whu.edu.cn
The bipolar molecule p-BISiTPA was synthesized in two
steps (Scheme 1). The electron-withdrawing benzimidazole
moiety was first introduced into the silicon-containing skel-
eton by a cyclization of N-phenyl-o-phenylenediamine and
4-[(4-bromophenyl)(diphenyl)silyl]benzoyl chloride. Then, the
electron-donating diphenylamine segment was connected to
the skeleton through the palladium-catalyzed C–N coupling
Y. Chen, Prof. D. Ma
State Key Laboratory of Polymer Physics and Chemistry
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences
Changchun 130022 (PR China)
E-mail: mdg1014@ciac.jl.cn
DOI: 10.1002/adma.201002732
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N
N
1) THF/NEt3
HN
Br
Si
COCl +
H2N
Br
Si
2) HOAc, reflux
-BISi-Br
p
Pd(OAc) / BuONa/( Bu) PHBF
2 t
N
N
t
3
4
N
Si
diphenylamine toluene, reflux
p-BISiTPA
Scheme 1. Synthesis of p-BISiTPA.
group cannot be observed,[14] indicating the disruption of the
π -conjugation between the two parts owing to the silicon-bridged
linkage mode. The triplet energy of the compound was deter-
mined to be 2.69 eV by the highest energy vibronic sub-band
of the phosphorescence spectrum in 2-methyltetrahydrofuran
at 77 K, which is higher than that of blue phosphor iridium(III)
bis(4,6-(difluorophenyl)pyridine-N,C2’) picolinate (FIrpic,
2.65 eV).[15]
The electrochemical properties of p-BISiTPA were investi-
gated by cyclic voltammetry (CV). The compound exhibits one
quasi-reversible, one-electron oxidation process (see Figure S1
in the Supporting Information), which can be assigned to the
oxidation of arylamine moiety. The HOMO energy level deter-
mined from the onset of the oxidation potential is 5.27 eV with
regard to the energy level of ferrocene (4.8 eV below vacuum),
which is much higher than the approximately 7.0 eV of tetraar-
ylsilicon compounds. The HOMO level of the compound is even
higher than that of most widely used hole transporting material
1,4-bis[(1-naphthylphenyl)amino]biphenyl (NPB, 5.4 eV),[16]
suggesting no hole–injection barriers from NPB to the
compound.
reaction of the intermediate p-BISi–Br with diphenylamine.[13]
The chemical structure of p-BISiTPA was fully characterized by
1H NMR and 13C NMR spectroscopies, mass spectrometry and
elemental analysis.
The good thermal stability of p-BISiTPA is indicated by the
high decomposition temperatures (Td, corresponding to 5%
weight loss) of 405 °C in the thermogravimetric analysis, and
the glass transition temperature (Tg) of 102 °C through differ-
ential scanning calorimetry (DSC), which is substantially higher
than those of carbazole-based 1,3-bis(9-carbazolyl)benzene
(mCP, 60 °C)[10a] and tetraarylsilicon compounds (26–53 °C).[12]
The high Tg and Td values of p-BISiTPA can improve the film
morphology and reduce the possibility of phase separation
upon heating.
The absorption and fluorescence spectra of p-BISiTPA are
shown in Figure 1. The absorption peak at 308 nm (logε =
4.9) can be attributed to the π–π∗ transition. It is noteworthy
that charge transfer absorption from the electron-donating
triphenylamine to the electron-withdrawing benzimidazole
To gain insight into the electronic state of the new com-
pound, DFT calculations were performed. The calculated
HOMO and LUMO values are 5.22 and 1.78 eV, respectively.
Moreover, HOMO and LUMO levels distribute over aryamine
and benzimidazole segments, respectively (See Figure S2 in the
Supporting Information). Complete separation is preferable for
efficient hole- and electron-transporting properties, and the pre-
vention of reverse energy transfer.[17]
To evaluate the bipolar character of p-BISiTPA, we fabricated
a hole-only device having the structure ITO/MoO3 (10 nm)/
NPB (80 nm)/4,4’,4’’-tri(N-carbazolyl)triphenylamine (TCTA,
5 nm)/p-BISiTPA (20 nm)/1,3,5-tris(N-phenylbenzimidazol-
2-yl)benzene (TPBI, 40 nm)/MoO3 (40 nm)/Al (100 nm) and
an electron-only device having the configuration ITO/TPBI
(40 nm)/NPB (80 nm)/TCTA (5 nm)/p-BISiTPA (20 nm)/TPBI
(40 nm)/LiF (1 nm)/Al (100 nm). MoO3 and TPBI layers were
used to prevent electron- and hole-injection from the cathode
and anode, respectively. The current density versus voltage
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
Abs
PL at RT
Ph at 77K
300
400
500
600
Wavelength (nm)
Figure 1. The absorption and emission spectra of p-BISiTPA in toluene
at 5 × 10−6 M at room temperature, and phosphorescence spectrum of
p-BISiTPA in 2-methyltetrahydrofuran at 77 K.
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T a b le 1 . Electroluminescence Characteristics of the Devices.[a]
Lmax [cd m− 2 ] (V at Lmax [V])
17 798 (14.1)
ηc [b] [cd A−1
]
η
p [b] [lm W−1
]
η
ext [b] [%]
Device
Von [V]
3.5
CIE [c] [x, y]
B
35.1, 35.1, 31.1
57.8, 57.3, 53.2
51.8, 51.7, 47.3
26.1, 24.5, 15.5
51.9, 43.9, 29.3
42.7, 37.8, 25.2
16.1, 16.1, 14.2
20.5, 20.4, 18.9
19.1, 19.1, 17.4
0.18, 0.32
0.52, 0.48
0.38, 0.44
O
W
3.1
47 756 (13.1)
3.1
45 398 (14.5)
[a] Abbreviations: Von: turn-on voltage; Lmax: maximum luminance; V: voltage; ηext: external quantum efficiency; ηc: current efficiency; ηp: power efficiency; CIE [x, y]:
Commission International de I’Eclairage coordinates; [b] Order of measured value: maximum, then values at 100 and 1000 cd m−2 ; [c] Measured at 7 V.
are significantly reduced compared to our previously reported
results using similar device structures having the conventional
host material mCP, in which the external quantum efficiency
reached a maximum value of 19.3% at a very low luminance
of 7 cd m−2 ;[20b] at a luminance of 1000 cd m−2 , ηext dropped
to 12.3% with a roll-off value of 36.3%, and at a luminance of
10 000 cd m−2 , ηext declined to only 6.3%. The high efficiencies
and low efficiency roll-off at high luminance for device W can
be attributed to the use of the bipolar host p-BISiTPA, which
may have resulted in balanced charge fluxes and a broad dis-
tribution of recombination region within the emitting layer.
Additionally, the white light are quite stable, and the Commis-
sion International de I’Eclairage (CIE) coordinates of the device
remain almost invariable when the luminance going from
4 774 cd m−2 (0.37, 0.43) to 24 295 cd m−2 (0.36, 0.43) (inset of
Figure 2). The color-rendering index (CRI) of 67 for the current
WOLED is not high due to the two-color system.
In summary, we have developed a new molecular design
strategy for bipolar host material by incorporating both electron
donor and electron acceptor into the silicon-bridged structure.
The new silicon-bridged compound p-BISiTPA finds a balance
between the high triplet energy and bipolar transporting nature,
carrier-injection barrier, plus morphological stability. The new
compound was demonstrated to be an efficient host for blue
and orange phosphors. A two-color, all-phosphor and single-
emitting-layer WOLED hosted by p-BISiTPA was fabricated
curves show that the electron-current density is slightly higher
than the hole-current density in p-BISiTPA (see Figure S3 in the
Supporting Information), which indicates the bipolar charge-
transport nature of p-BISiTPA.
We initially examined the performance of FIrpic-based blue
phosphorescent device (device B) with the structure ITO/MoO3
(10 nm)/NPB (80 nm)/TCTA (5 nm)/p-BISiTPA: FIrpic
(8 wt.-%, 20 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm). NPB and
TPBI were used as the hole- and electron-transporting layers,
respectively. TCTA was used as the electron/exciton-blocking
layer. MoO3 and LiF served as the hole- and electron-injecting
layers, respectively; FIrpic doped in p-BISiTPA was used as
the emitting layer. The blue phosphorescent device achieved
a maximum current efficiency (ηc, max) of 35.1 cd A−1 , a max-
imum power efficiency (ηp, max) of 26.1 lm W−1 and a maximum
external quantum efficiency (ηext, max) of 16.1% (see Figure S4
in the Supporting Information and Table 1). It is worth noting
that the external quantum efficiencies still remain as high as
14.2% at a brightness of 1000 cd m−2 . These values are inferior
to the best performance reported for FIrpic-based devices[11,18]
presumably because the triplet energy of p-BISiTPA is not high
enough relative to FIrpic. However, this is a worthy sacrifice to
acquire a compromise among the triplet energy, bipolar trans-
porting nature and carrier–injection barrier.
Next we fabricated an orange phosphorescent device (device
O) with the same configuration as device B except for using
bis[2-(9,9-diethyl-9H-fluoren-2-yl)-1-phenyl-1H-benzoimidazol-
N,C3]-iridium(acetylacetonate) [(fbi)2Ir(acac)] as the dopant. The
device achieved ηc, max of 57.8 cd A−1 , ηp, max of 51.9 lm W−1 and
100
η
ext, max of 20.5%, respectively. Moreover, the device displays low
external quantum efficiency roll-off, with a roll-off value of 7.8%
at a luminance of 1000 cd m−2 . The performance of the device
O is among the highest for orange PhOLEDs ever reported.[19]
Finally, we demonstrated a two-color based phosphorescent
WOLED (device W) with the configuration ITO/MoO3 (10 nm)/
NPB(80nm)/TCTA(5nm)/p-BISiTPA:8wt.-%FIrpic:0.67wt.-%
(fbi)2Ir(acac) (20 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm).
FIrpic and (fbi)2Ir(acac) were co-doped into the bipolar
host p-BISiTPA as the single emitting layer. The device had
ηc, max of 51.8 cd A−1 , ηp, max of 42.7 lm W−1 and ηext, max of
19.1% (Figure 2 and Figure S4 in the Supporting Information).
To the best of our knowledge, the device efficiencies are among
the highest reported for single emitting layer WOLEDs.[20]
Remarkably, the device shows rather low efficiency roll-off:
at a luminance of 1000 cd m−2 , ηext is still as high as 17.4%
with a roll-off value of 8.9%, and at the extremely high lumi-
nance of 10 000 cd m−2 , ηext is still 12.2%. The roll-off values
10
1.6
8 V, (0.37, 0.43) 4774 cd/m2
1.4
10 V, (0.37, 0.43) 11940 cd/m2
1.2
12 V, (0.36, 0.43) 24295 cd/m2
1.0
10
1
0.8
0.6
0.4
0.2
0.0
0.1
400 450 500 550 600 650 700 750
Wavelength (nm)
1
1
10
100
1000 10000
Luminance (cd/m2)
Figure 2. Current efficiency and power efficiency versus luminance
for device W. Inset: the normalized EL spectra of device W at various
voltages.
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Experimental Section
Synthesis of (4-{Diphenyl[4-(1-phenyl-1H-benzimidazol-2-yl)phenyl]silyl}
phenyl)diphenylamine(p-BISiTPA):Amixtureofp-BISi-Br(1.82g,3.00mmol),
diphenylamine (0.54 g, 3.20 mmol), Pd(OAc)2 (13 mg, 0,06 mmol),
tBuONa (0.35 g, 3.60 mmol), (tBu)3PHBF4 (52 mg, 0.18 mmol) and
toluene (20 mL) was refluxed under argon for 18 h. After cooling, the
reaction mixture was extracted with brine and CH2Cl2, and dried over
anhydrous Na2SO4. After removal of the solvent, the residue was purified
by column chromatography on silica gel using ethyl acetate/petroleum
(1:5 v/v) as the eluent to give a white powder. Yield: 72%. 1H NMR
(300 MHz, CDCl3) δ [ppm]: 8.35 (d, J = 8.1 Hz, 1H), 7.80 (d, J = 8.1 Hz, 2H),
7.67–7.57 (m, 6H), 7.52–7.34 (m, 12H), 7.32–7.27 (m, 8H), 7.14 (d, J
= 7.8 Hz, 4H), 7.07–7.00 (m, 4H). 13C NMR (75 MHz, CDCl3) δ [ppm]:
152.28, 149.45, 147.52, 142.69, 137.52, 137.42, 137.16, 137.02, 136.63,
134.35, 130.74, 130.25, 129.93, 129.62, 129.06, 128.82, 128.17, 127.67,
125.44, 123.90, 123.77, 123.54, 121.75, 119.99, 110.84. MS (ESI): m/z
696 [M + H]+. Anal. calcd for C49H37N3Si (%): C 84.57, H 5.36, N 6.04;
found: C 84.72, H 5.12, N 5.95.
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Acknowledgements
We thank the National Natural Science Foundation of China (Project
Nos. 90922020, 50773057, and 20621401), the National Basic Research
Program of China (973 Program 2009CB623602 and 2009CB930603) and
the Fundamental Research Funds for the Central Universities of China
for financial support.
Received: July 29, 2010
Published online: October 22, 2010
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