High-performance blue and green electrophosphorescence achieved by using carbazole-containing bipolar tetraarylsilanes as host materials

Article abstract of DOI:10.1039/c1jm11208b

A series of carbazole-containing tetraarylsilane compounds, namely p-BISiPCz (1), m-BISiPCz (2), p-OXDSiPCz (3) and m-OXDSiPCz (4) were designed and synthesized by incorporating electron-donating carbazole and electron-accepting benzimidazole or oxadiazol

Full text of DOI:10.1039/c1jm11208b

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PAPER
High-performance blue and green electrophosphorescence achieved by using
carbazole-containing bipolar tetraarylsilanes as host materials†
a
b
a
a
a
b
Shaolong Gong, Yonghua Chen, Xin Zhang, Peijun Cai, Cheng Zhong, Dongge Ma, Jingui Qina
*
a
*
and Chuluo Yang
Received 21st March 2011, Accepted 19th May 2011
DOI: 10.1039/c1jm11208b
A series of carbazole-containing tetraarylsilane compounds, namely p-BISiPCz (1), m-BISiPCz (2),
p-OXDSiPCz (3) and m-OXDSiPCz (4) were designed and synthesized by incorporating electron-
donating carbazole and electron-accepting benzimidazole or oxadiazole into one molecule via a silicon-
bridge linkage mode. Their thermal, photophysical and electrochemical properties can be finely tuned
through the different groups and linking topologies. The di-para-position compounds 1 and 3 display
higher glass transition temperatures and slightly lower triplet energies than their di-meta-position
isomers 2 and 4, respectively. The four compounds exhibit similar HOMO levels (5.60–5.63 eV), while
the LUMO level of 3 (2.36 eV) is slightly lower than that of 4 (2.28 eV). The silicon-interrupted
conjugation of the electron-donating and electron-accepting segments endows these materials with
relative high triplet energies, good thermal and morphological stability, and bipolar transporting
ability. For FIrpic-based blue PhOLEDs, the di-meta-position compounds 2 and 4 display better device
performances than their di-para-position analogues 1 and 3, respectively. Device B using 2 as the host
exhibits the best performance with a maximum current efficiency of 29.3 cd Aꢀ1, a maximum power
efficiency of 19.8 lm Wꢀ1, and a maximum external quantum efficiency of 11.4%. Green phosphorescent
devices using (ppy)2Ir(acac) as guest and 1–4 as hosts show excellent EL performances with maximum
external quantum efficiencies of 18.3–22.2%. Remarkably, device H hosted by 4 still exhibits an
external quantum efficiency of 19.4% at the extremely high luminance of 10 000 cd mꢀ2. These
efficiencies are significantly higher than those of blue and green control devices using mCP as host,
respectively.
desirable that a host material should have a high enough triplet
Introduction
energy for effective energy transfer to the guest,8,9 good carrier
transport properties for a balanced recombination of carriers in
the emitting layer,10,11 and high thermal and morphological
stability to extend the device’s operational lifetime.12,13 Recently,
green and red PhOLEDs with 100% internal quantum efficiency
have been reported,14–18 but highly efficient and stable blue
PhOLEDs remain to be developed because of the lack of suitable
host materials simultaneously meeting aforementioned
requirements.
Phosphorescent organic light-emitting diodes (PhOLEDs) have
attracted intense interest because they can approach 100%
internal quantum efficiency by utilizing both singlet and triplet
excitons.1–5 To achieve highly efficient electrophosphorescence,
phosphorescent emitters of heavy-metal complexes are usually
doped into a suitable host material by reducing competitive
factors such as concentration quenching and triplet–triplet
annihilation.6 Therefore, the design and synthesis of host mate-
rials are vital to the realization of efficient PhOLEDs.7 It is
Carbazole-based compounds have been widely utilized as the
host materials for blue PhOLEDs.19–22 Additionally, tetraar-
ylsilane compounds with ultrahigh singlet (ꢁ4.5 eV) and triplet
(ꢁ3.5 eV) energies have been reported as host materials for deep
blue electrophosphorescence, but their poor charge-transporting
ability and very high hole-injection barrier hamper their further
usage as host materials.23 To reduce the hole-injection barrier,
p-type tetraarylsilane compounds by introducing carbazole
moiety have been recently reported as hosts for blue
PhOLEDs.24,25 However, problems still remain in terms of low
efficiency and large efficiency roll-off due to the imbalance of
aDepartment of Chemistry, Hubei Key Lab of Organic and Polymeric
Optoelectronic Materials, Wuhan University, Wuhan, 430072, People’s
Republic of China. E-mail: clyang@whu.edu.cn
bState Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun, 130022, People’s Republic of China. E-mail: mdg1014@ciac.
jl.cn
† Electronic supplementary information (ESI) available: Current
density-voltage-brightness characteristics and current efficiency and
power efficiency versus current density curves for mCP-based blue and
green control devices. See DOI: 10.1039/c1jm11208b
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carrier injection and transport. To address this issue, a bipolar
host can play an important role because it could facilitate the
balanced transport of both hole and electron, and broaden the
exciton-formation zone, consequently improving the device
performance and reducing efficiency roll-off.26–28 In this study,
electronic structures were calculated at the s-HCTHhyb/6-311+
+G(d,p) level.32 Molecular orbitals were visualized using
Gaussview.
Device fabrication and measurement
we designed and synthesized
a series of tetraarylsilane
compounds by integrating electron-donating carbazole and
electron-accepting benzimidazole or oxadiazole into one mole-
cule. We anticipate that the electron-donating and electron-
accepting moieties would impart the material with both bipolar
transporting characteristics and relative high triplet energies.
Meanwhile, different groups and linking modes of the electron-
donor and electron-acceptor would finely tune their photo-
physical and electrochemical properties, and consequently
modulate their triplet energies and HOMO/LUMO levels to
some extent. We will present a comprehensive investigation that
encompasses the thermal, photophysical, and electrochemical
properties of the compounds as well as the theoretical modeling,
and demonstrate the applicability of these materials as hosts for
blue and green PhOLEDs.
The hole-injection material MoO3, hole-transporting material
1,4-bis[(1-naphthylphenyl)amino]biphenyl (NPB), electron/
exciton-blocking material 4,40,400-tri(N-carbazolyl)triphenyl-
amine (TCTA) and electron-transporting material 1,3,5-tris(N-
phenylbenzimidazol-2-yl)benzene (TPBI) were commercially
available. Commercial ITO (indium tin oxide) coated glass with
sheet resistance of 10 U per square was used as the starting
substrate. Before device fabrication, the ITO glass substrates
were pre-cleaned carefully and treated by oxygen plasma for
2 min. Then the sample was transferred to the deposition
system. MoO3 (10 nm) was firstly deposited to ITO substrate,
followed by NPB (80 nm), TCTA (5 nm), emissive layer, and
TPBI (40 nm). Finally, a cathode composed of lithium fluoride
(1 nm) and aluminum (100 nm) was sequentially deposited onto
the substrate in the vacuum of 10ꢀ6 Torr. The L-V-J of the
devices was measured with a Keithley 2400 Source meter and
a Keithley 2000 Source multimeter equipped with a calibrated
silicon photodiode. The EL spectra were measured by JY
SPEX CCD3000 spectrometer. The EQE values were calculated
according to previously reported methods.33 All measurements
were carried out at room temperature under ambient
conditions.
Experimental section
General information
1H NMR and 13C NMR spectra were measured on
a MERCURY-VX300 spectrometer. Elemental analyses of
carbon, hydrogen, and nitrogen were performed on a Vario EL
III microanalyzer. LC-Mass spectra were measured on a Waters
ZQ-Mass ESI. UV-Vis absorption spectra were recorded on
a Shimadzu UV-2500 recording spectrophotometer. PL spectra
were recorded on a Hitachi F-4500 fluorescence spectropho-
tometer. Differential scanning calorimetry (DSC) was performed
on a NETZSCH DSC 200 PC unit at a heating rate of
10 ꢂC minꢀ1 from 20 to 300 ꢂC under argon. The glass transition
temperature (Tg) was determined from the second heating scan.
Thermogravimetric analysis (TGA) was undertaken with
a NETZSCH STA 449C instrument. The thermal stability of the
samples under a nitrogen atmosphere was determined by
measuring theirꢂweight loss while heating at a rate of 10 ꢂC minꢀ1
from 25 to 800 C. Cyclic voltammetry (CV) was carried out in
nitrogen-purged dichloromethane (oxidation scan) at room
temperature with a CHI voltammetric analyzer. Tetrabuty-
lammonium hexafluorophosphate (TBAPF6) (0.1 M) was used as
the supporting electrolyte. The conventional three-electrode
configuration consists of a platinum working electrode, a plat-
inum wire auxiliary electrode, and an Ag wire pseudo-reference
electrode with ferrocenium-ferrocene (Fc+/Fc) as the internal
standard. Cyclic voltammograms were obtained at scan rate of
100 mV sꢀ1. The onset potential was determined from the inter-
section of two tangents drawn at the rising and background
current of the cyclic voltammogram.
Synthesis of materials
2-{4-[(4-bromophenyl)(diphenyl)silyl]phenyl}-1-phenyl-1H-
benz -imidazole, 2-{3-[(3-bromophenyl)(diphenyl)silyl]phenyl}-1-
phenyl-1H-benzimidazole, 2-{4-[(4-bromophenyl)(diphenyl)silyl] -
phenyl}-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, 2-{3-[(3-bromo-
phenyl)(diphenyl)silyl]phenyl}-5-(4-tert-butylphenyl)-1,3,4-oxadi-
azole were prepared according to the reported procedures.34,35
9-(4-{diphenyl[4-(1-phenyl-1H-benzimidazol-2-yl)phenyl]silyl}
phenyl)-9H-carbazole (1). A mixture of 2-{4-[(4-bromophenyl)
(diphenyl)silyl]phenyl}-1-phenyl-1H-benzimidazole (1.82 g,
3.00 mmol), carbazole (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 mixture was extracted with CH2Cl2, and
the organic layer was 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
1
by vol.) as the eluent to give a white powder. Yield: 82%. H
NMR (300 MHz, CDCl3) d [ppm]: 8.15 (d, J ¼ 7.8 Hz, 2H), 7.91
(d, J ¼ 8.1 Hz, 1H), 7.76 (d, J ¼ 8.1 Hz, 2H), 7.67–7.59 (m, 10H),
7.53–7.49 (m, 6H), 7.47–7.41 (m, 7H), 7.39–7.36 (m, 3H), 7.32–
7.26 (m, 4H). 13C NMR (75 MHz, CDCl3) d [ppm]: 152.24,
143.15, 140.73, 139.38, 138.11, 137.56, 137.12, 136.64, 136.57,
136.11, 133.64, 133.24, 131.45, 130.16, 128.97, 128.36, 127.65,
126.37, 126.20, 123.78, 123.38, 120.56, 120.37, 120.13, 110.77,
110.12. MS (ESI): m/z 694 (M+ + H). Anal. calcd for C49H35N3Si
(%): C 84.81, H 5.08, N 6.06; found: C 84.98, H 4.75, N 5.96.
Computational details
The geometric and electronic properties were calculated with
the Gaussian 09 program package.29 The calculation was
optimized by means of the B3LYP (Becke three parameters
hybrid functional with Lee-Yang-Perdew correlation func-
tionals) with the 6-31G(d) atomic basis set.30,31 Then the
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9-(3-{diphenyl[3-(1-phenyl-1H-benzimidazol-2-yl)phenyl]silyl}
phenyl)-9H-carbazole (2). Prepared as a white solid according to
a similar procedure to 1, from 2-{3-[(3-bromophenyl)(diphenyl)silyl]
(ESI): m/z702 (M+ + H). Anal. calcd for C48H39N3OSi (%): C 82.13, H
5.60, N 5.99; found: C 82.09, H 5.44, N 5.86.
1
phenyl}-1-phenyl-1H-benzimidazo-le. Yield: 78%. H NMR (300
9-{3-[{3-[5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]phenyl} -
(diphenyl)silyl]phenyl}-9H-carbazole (4). Prepared as a white solid
according to a similar procedure to 1, from 2-{3-[(3-bromo-
phenyl)(diphenyl)silyl]phenyl}-5-(4-tert-butylphenyl)-1,3,4-oxa-
diazole. Yield: 66%. 1H NMR (300 MHz, CDCl3) d [ppm]: 8.41 (s,
1H), 8.22 (d, J ¼ 7.5 Hz, 1H), 8.10 (d, J ¼ 7.8 Hz, 2H), 7.97 (d, J ¼
8.4 Hz, 2H), 7.82–7.77 (m, 2H), 7.67–7.64 (m, 6H), 7.59–7.54 (m,
2H), 7.52–7.39 (m, 9H), 7.36–7.31 (m, 2H), 7.24–7.22 (m, 3H),
1.36 (s, 9H). 13C NMR (75 MHz, CDCl3) d [ppm]: 164.99, 164.56,
155.55, 140.87, 139.73, 137.82, 136.61, 136.52, 135.83, 135.33,
134.68, 133.04, 130.46, 129.94, 129.01, 128.54, 127.03, 126.28,
126.19, 124.17, 123.67, 121.29, 120.51, 120.27, 109.97, 35.30,
31.37. MS (ESI): m/z 702 (M+ + H). Anal. calcd for C48H39N3OSi
(%): C 82.13, H 5.60, N 5.99; found: C 81.87, H 5.31, N 5.97.
MHz, CDCl3) d [ppm]: 8.11 (d, J ¼ 7.2 Hz, 2H), 7.95 (d, J ¼ 7.2 Hz,
1H), 7.87 (d, J ¼ 7.8 Hz, 1H), 7.62–7.56 (m, 5H), 7.49–7.42 (m, 7H),
7.38–7.30 (m, 8H), 7.24–7.16 (m, 6H), 7.07–7.04 (m, 5H). 13C NMR
(75 MHz, CDCl3) d [ppm]: 152.26, 142.81, 140.43, 137.15, 136.92,
136.86, 136.45, 136.34, 136.17, 134.83, 134.19, 133.79, 132.77, 131.20,
129.78, 129.62, 129.46, 129.33, 128.22, 128.01, 127.95, 127.78, 126.89,
125.78, 123.24, 122.91, 120.16, 119.89, 119.70, 110.35, 109.59. MS
(ESI): m/z 694 (M+ + H). Anal. calcd for C49H35N3Si (%): C 84.81,
H 5.08, N 6.06; found: C 85.01, H 4.93, N 5.71.
9-{4-[{4-[5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]phenyl} -
(diphenyl)silyl]phenyl}-9H-carbazole (3). Prepared as a white solid
according to a similar procedure to 1, from 2-{4-[(4-bromophenyl)-
(diphenyl)silyl]phenyl}-5-(4-tert-butylphenyl)-1,3,4-oxadiazole. Yield:
69%. 1H NMR (300 MHz, CDCl3)d[ppm]: 8.21–8.14 (m, 4H), 8.07 (d,
J ¼ 8.4 Hz, 2H), 7.84–7.81 (m, 4H), 7.68–7.63 (m, 6H), 7.58–7.43 (m,
12H), 7.33–7.30 (m, 2H), 1.37 (s, 9H). 13C NMR (75 MHz, CDCl3)
d [ppm]: 165.02, 164.51, 155.60, 140.70, 139.56, 139.13, 138.11, 137.22,
136.63, 133.32, 132.88, 130.39, 128.49, 127.03, 126.48, 126.39, 126.31,
126.21, 125.40, 123.79, 121.31, 120.56, 120.41, 110.09, 35.29, 31.35. MS
Results and discussion
Synthesis and characterization
The synthetic routes and chemical structures of the compounds,
9-(4-{diphenyl[4-(1-phenyl-1H-benzimidazol-2-yl)phenyl]silyl}-
Scheme 1 Synthesis and molecular structures of 1–4.
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Table 1 Physical data of 1–4
phenyl)-9H-carbazole (p-BISiPCz, 1), 9-(3-{diphenyl[3-(1-
phenyl-1H-benzimidazol-2-yl)phenyl]silyl}phenyl)-9H-carbazole
(m-BISiPCz, 2), 9-{4-[{4-[5-(4-tert-butylphenyl)-1,3,4-oxadiazol-
1
2
3
4
Tg/Td/ꢂC
120/425
294
350/364
5.61/NA
97/358
292
350/365
5.60/NA
120/420
293
349/364
5.63/2.36
97/397
292
350/364
5.63/2.28
2-yl]phenyl}(diphenyl)silyl]phenyl}-9H-carbazole
(p-OXD-
SiPCz, 3) and 9-{3-[{3-[5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-
yl]phenyl}(diphenyl)silyl]phenyl}-9H-carbazole (m-OXDSiPCz,
4), are depicted in Scheme 1. The electron-withdrawing benz-
imidazole or oxadiazole moiety was firstly introduced into the
silicon-containing skeleton, and then the electron-donating
carbazole segment was connected to the skeleton through the
palladium-catalyzed C–N coupling reaction of the corresponding
bromide precursors with diphenylamine in good yields
(66–82%).36 Detailed synthetic procedures are presented in the
Experimental section. For 1 and 3, the electron-donor and
acceptor units are connected to the di-para-position of the
silicon-containing skeleton; while for 2 and 4, the electron-donor
and acceptor units are connected to the di-meta-position of the
skeleton. The chemical structures of all of the compounds were
labsa/nm
lem, maxa/nm
HOMO/LUMOexp
eV
b
/
c
HOMO/LUMOcal
eV
/
5.55/1.88
5.41/1.78
5.63/2.26
5.39/2.17
ETd/eV
2.68
2.72
2.69
2.72
a
Measured in toluene. b Estimated from the onset of oxidation/reduction
c
potentials. Obtained from DFT calculation.
methyltetrahydrofuran at 77 K.
d
Measured in 2-
which are substantially higher than those of tetraarylsilane
compounds (26–53 ꢂC),23 indicating that the introduction of
benzimidazole or oxadiazole moiety improves their morpholog-
ical stability. Additionally, 2 and 4 with a di-meta-position
linkage show significantly lower Tg values than 1 and 3 with
a di-para-position connection, which can be rationalized by the
greater energetic disorder in the meta derivatives.37
1
fully characterized by H NMR and 13C NMR, mass spectrom-
etry and elemental analysis.
Thermal properties
The thermal properties of the compounds were investigated by
thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) (Fig. 1 and Table 1). The four compounds
exhibit high thermal decomposition temperatures (Td, corre-
Photophysical properties
Fig. 2 shows the electronic absorption and fluorescence spectra
of the compounds in dilute toluene solution. The absorption
peaks around 293 nm for all compounds are consistent with the
carbazole-centered n–p* transition. In addition, no charge
transfer absorption band from the electron-donating carbazole
to the electron-withdrawing benzimidazole or oxadiazole group
can be observed,38,39 indicating the disruption of the p-conju-
gation between the two parts owing to the silicon-bridge linkage
mode.
ꢂ
sponding to 5% weight loss) in the range of 358–425 C. Their
glass-transition temperatures (Tg) range from 97 to 120 ꢂC,
All of the compounds show near-ultraviolet emissions at about
350 nm with a side peak at about 364 nm in dilute toluene
solution. Their phosphorescence spectra were measured in
a frozen 2-methyltetrahydrofuran matrix at 77 K (Fig. 3), and
the triplet energy levels were estimated from the highest-energy
vibronic sub-band of the phosphorescence spectra. All the
compounds show higher triplet energy levels (2.68 eV for 1,
2.72 eV for 2, 2.69 eV for 3, and 2.72 eV for 4) than that of blue
Fig. 1 a) TGA thermograms of 1–4 recorded at a heating rate of 10 ꢂC
Fig. 2 UV-vis absorption and PL spectra of 1–4 in toluene solution at
5 ꢃ 10ꢀ6 M.
minꢀ1. b) DSC traces of 1–4 recorded at a heating rate of 10 ꢂC minꢀ1
.
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sweeping in THF, 3 and 4 exhibit reversible reduction waves
(Fig. 4b), arising from the reduction of the n-type oxadiazole
segments;16 while 1 and 2 exhibit no reduction wave owing to the
benzimidazole structure.38,42 The HOMO/LUMO energy levels
were estimated from the onsets of the oxidation and reduction
potentials with regard to the energy level of ferrocene (4.8 eV
below vacuum), respectively (Table 1). The four compounds
exhibit similar HOMO levels (5.60–5.63 eV), which are much
higher than ꢁ7.0 eV of the tetraarylsilicon compounds previ-
ously reported.23 The LUMO level of 4 (2.28 eV) is slightly higher
than that of 3 (2.36 eV), which could be attributed to the different
linking modes.
Theoretical calculations
Fig. 3 The phosphorescence spectra of 1–4 in a frozen 2-methylte-
trahydrofuran matrix at 77 K.
To understand the structure-property relationship of the
compounds at the molecular level, the geometry was optimized at
the B3LYP/6-31G(d) level, and the electronic properties of the
compounds were calculated at s-HCTHhyb/6-311++G(d,p) level
(for details, see the Experimental Section†). According to density
functional theory (DFT) calculations, the HOMO orbitals are
mainly located on the electron-donating carbazole moiety, while
the LUMO orbitals are mainly distributed on the electron-
accepting benzimidazole or oxadiazole moiety (Fig. 5). All of the
compounds have almost complete separation of the HOMO and
LUMO at the hole- and electron-transporting moieties,
0
phosphor iridium(III) bis(4,6-(difluorophenyl)pyridine-N,C2 )
picolinate (FIrpic, 2.65 eV),8 which implies they could serve as
host materials for the blue triplet emitter.
Electrochemical properties
Cyclic voltammetry (CV) were performed to investigate the
electrochemical properties of the compounds (Fig. 4). During the
anodic scan in dichloromethane, the four compounds all exhibit
irreversible oxidation waves like most carbazole derivatives due
to the instability of their radical cations.40,41 Upon cathodic
Fig. 4 Cyclic voltammograms of 1–4 in a) CH2Cl2 for oxidation and b)
Fig. 5 Calculated spatial distributions of the HOMO and LUMO levels
of 1–4.
THF for reduction.
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respectively, which can be rationalized by the disruption of the
p-conjugation between the electron-donating and electron-
accepting moiety. The complete separation is preferable for
efficient hole- and electron-transporting properties, and the
prevention of reverse energy transfer.43 The calculated HOMO/
LUMO values are in the range of 5.39–5.63/1.78–2.26 eV, which
correlate with the experimental data (Table 1).
Phosphorescent OLEDs
To evaluate the utility of these compounds as host materials for
blue phosphor FIrpic, we initially fabricated blue devices A–D
by using 1–4 as the hosts and FIrpic as the guest with the
configurations of ITO/MoO3 (10 nm)/NPB (80 nm)/TCTA
(5 nm)/host 1–4: FIrpic (8%, 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 hole- and elec-
tron-injecting layers, respectively; FIrpic doped in host 1–4 was
used as the emitting layer. Fig. 6 shows the current density-
voltage-brightness (J-V-L) characteristics and efficiency versus
current density curves for the devices, and the electrolumines-
cence (EL) data are summarized in Table 2. Device A hosted by
di-para-position compound 1 achieves a maximum current
efficiency (hc.max) of 22.0 cd Aꢀ1, a maximum power efficiency
(hp.max) of 18.7 lm Wꢀ1 and a maximum external quantum effi-
ciency (hext.max) of 9.3%. In comparison, device B using di-meta-
position 2 as the host exhibits higher efficiencies with hc.max of
29.3 cd Aꢀ1, hp.max of 19.8 lm Wꢀ1, and hext.max of 11.4%, which
could be attributed to the higher triplet energy of 2 (2.72 eV) than
that of 1 (2.68 eV). The same trend can be observed for devices C
and D. Device D hosted by di-meta-position 4 displays
substantially higher efficiencies (hc.max of 19.8 cd Aꢀ1, hp.max of
14.6 lm Wꢀ1, and hext.max of 7.5%) than device C hosted by
di-para-position compound 3 (hc.max of 11.7 cd Aꢀ1, hp.max of
11.1 lm Wꢀ1, and hext.max of 5.3%) (Table 2). The device effi-
ciencies hosted by 1, 2 and 4 are much higher than those of the
blue control device using the conventional host of mCP, which
achieves hc.max of 15.4 cd Aꢀ1, hp.max of 10.4 lm Wꢀ1, and hext.max
of 6.2% (Fig. S1†). It is worth noting that these devices show
a low external quantum efficiency roll-off (Fig. 6b). For example,
when the brightness reaches 1 000 cd mꢀ2, hc still remains as high
as 27.7 cd Aꢀ1 with an efficiency roll-off value of 5.5% for device
B, and 18.5 cd Aꢀ1 with a roll-off value of 6.6% for device D. EL
spectra and the Commission International de I’Eclairage (CIE)
Fig. 6 a) Current density-voltage-brightness characteristics for devices
A–D. b) Current efficiency and power efficiency versus current density
curves for devices A–D. c) EL spectra of devices A–D.
Table 2 Electroluminescence characteristics of the devices.a
CIE (x, y)g
c
d
e
device
EML
Vonb/V
Lmax /cd mꢀ2,V/V
hc.max /cd Aꢀ1
hp.max /lm Wꢀ1
hext.maxf (%)
A
B
C
D
E
F
G
H
1/Ir-B
2/Ir-B
3/Ir-B
4/Ir-B
1/Ir-G
2/Ir-G
3/Ir-G
4/Ir-G
3.7
3.7
3.1
3.5
2.9
3.5
2.7
2.7
19904, 14.1
36035, 14.7
14193, 13.9
21942, 14.5
43533, 10.5
47688, 12.9
48028, 10.5
47987, 10.7
22.0
29.3
11.7
19.8
84.0
69.8
70.6
86.0
18.7
19.8
11.1
14.6
85.1
49.7
74.7
84.3
9.3
11.4
5.3
0.18, 0.37
0.18, 0.40
0.20, 0.43
0.20, 0.42
0.34, 0.62
0.34, 0.62
0.34, 0.62
0.34, 0.62
7.5
22.1
18.3
18.3
22.2
a
Device configuration: ITO/MoO3 (10 nm)/NPB (80 nm)/TCTA (5 nm)/EML (20 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm). b Turn-on voltages at 1 cd
.
c
d
Maximum luminance. Maximum current efficiency. Maximum power efficiency. Maximum external quantum efficiency. Commission
e
f
g
mꢀ2
International de I’Eclairage coordinates measured at 7 V.
11202 | J. Mater. Chem., 2011, 21, 11197–11204
This journal is ª The Royal Society of Chemistry 2011

View Article Online
coordinates of devices A–D are shown in Fig. 6c. Devices A–D
show the same main peak at 475 nm, but have different intensities
of the shoulder peak at 500 nm, which could be attributed to the
recombination zone shift in devices owing to the different charge
transporting abilities of the hosts 1–4.44,45
and H obtain hc.max as high as 84.0 cd Aꢀ1 (hp.max of 85.1 lm Wꢀ1
,
hext.max of 22.1%) and 86.0 cd Aꢀ1 (hp.max of 84.3 lm Wꢀ1, hext.max
of 22.2%), respectively. These values are substantially
higher than those of the green control device using the host of
mCP (hc.max of 66.8 cd Aꢀ1, hp.max of 59.2 lm Wꢀ1, and hext.max of
17.2%, Fig. S2†). Remarkably, devices E and H still exhibit over
17% external quantum efficiencies (17.8% for device E and 19.4%
To test the applicability of the hosts as a green phosphorescent
emitter, we also fabricated devices E–H by using 1–4 as the hosts
0
and 9% iridium(III) bis(2-phenylpyridinato-N,C2 ) acetylaceto-
for device H) at the extremely high luminance of 10 000 cd mꢀ2
.
nate [(ppy)2Ir(acac)] as the dopant, with an identical device
structure as device A. Fig. 7 shows the J-V-L characteristics and
curves of external quantum efficiency and power efficiency versus
current density. All of the devices show typical green emission
around 530 nm that originates from the guest [(ppy)2Ir(acac)]
(Fig. 7c). High performance of green electrophosphorescence can
be achieved for devices E–H (Table 2). For example, devices E
The high efficiencies and low efficiency roll-off at high luminance
for devices E and H can be attributed to the use of the bipolar
host which may result in balanced charge fluxes and a broad
distribution of recombination region within the emitting layer.28b
Conclusions
In summary, we have developed a series of carbazole-containing
tetraarylsilane compounds by incorporating electron-donating
carbazole and electron-accepting benzimidazole or oxadiazole
into one molecule via a silicon-bridge linkage mode. This design
strategy endows these materials with relative high triplet ener-
gies, high thermal and morphological stability, and bipolar
transporting ability. We have successfully fabricated highly
efficient blue and green electrophosphorescences by using the
tetraarylsilane compounds as host materials. The best perfor-
mance of the blue phosphorescent devices was achieved using 2
as the host, with a maximum current efficiency of 29.3 cd Aꢀ1
,
and a maximum power efficiency of 19.8 lm Wꢀ1; while the best
performance of a green phosphorescent device was achieved
using 4 as the host, with a maximum current efficiency of 86.0 cd
Aꢀ1, and a maximum power efficiency of 84.3 lm Wꢀ1. We also
note that all the devices exhibit low efficiency roll-off at high
luminance. For example, when the brightness reaches 1 000 cd
mꢀ2, the current efficiency of blue phosphorescent device B
hosted by 2 still remains as high as 27.7 cd Aꢀ1 with an efficiency
roll-off value of 5.5%; green phosphorescent device H hosted by
4 still exhibits an external quantum efficiency of 19.4% at the
extremely high luminance of 10 000 cd mꢀ2. This can be attrib-
uted to the advantages of the bipolar tetraarylsilane compounds
as hosts.
Acknowledgements
The authors are grateful to the National Natural Science
Foundation of China (Project Nos. 90922020), and the National
Basic Research Program of China (973 Program 2009CB623602
and 2009CB930603), the Open Research Fund of State Key
Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
and the Fundamental Research Funds for the Central
Universities of China for financial support of this research.
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