Highly efficient deep-blue electrophosphorescence enabled by solution-processed bipolar tetraarylsilane host with both a high triplet energy and a high-lying HOMO level

Article abstract of DOI:10.1002/adma.201102758

Phosphorescent organic light-emitting diodes (PhOLEDs) offer a bright future for the next generation flat-panel displays and lighting sources due to their high quantum efficiency compared with fluorescent OLEDs.^[1,2] To date, green and red PhOL

Full text of DOI:10.1002/adma.201102758

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Highly Efficient Deep-Blue Electrophosphorescence
Enabled by Solution-Processed Bipolar Tetraarylsilane
Host with Both a High Triplet Energy and a High-Lying
HOMO Level
Shaolong Gong, Qiang Fu, Qiang Wang, Chuluo Yang,* Cheng Zhong, Jingui Qin,
a n d Dongge Ma*
Phosphorescent organic light-emitting diodes (PhOLEDs) offer
a bright future for the next generation flat-panel displays and
lighting sources due to their high quantum efficiency compared
with fluorescent OLEDs.[1,2] To date, green and red PhOLEDs
with 100% internal quantum efficiency have been achieved,[3,4]
but highly efficient and stable blue PhOLEDs, especially deep-
blue PhOLEDs, remain to be further developed. Many works
been achieved by complicated device configurations. One of
the reasons is the lack of adequate host materials for deep-blue
phosphor. As an effective host for deep-blue phosphor, its tri-
plet energy (ET) has to be larger than that of FIr6 (2.72 eV)[13] to
maintain effective energy transfer from host to guest, whereas
a high triplet energy generally associates with a low-lying
highest occupied molecular orbital (HOMO) level, which may
cause inefficient hole injection from the indium tin oxide (ITO)
anode. To compensate the scarcity, multiple hole-transporting-
layers (HTLs) with gradient energy pathway for the injection
of holes into the emissive layer have to be adopted.[11,12] This
inevitably increases the complexity of the devices.
on blue PhOLEDs have focused on the blue phosphor of
2
′
iridium(III) bis(4,6-(difluorophenyl)pyridine-N,C ) picolinate
(FIrpic).[5–8] Among these, Kido et al.[8] reported the highest
external quantum efficiencies (EQE) of 26% and 25% at a prac-
tical luminance of 100 and 1000 cd m−2 , respectively. Despite
the efficiency improvement for the FIrpic-based blue PhOLEDs,
their color purity with Commission International de I’Eclairage
(CIE) coordinates of (0.17, 0.34)[5b] is not ideal for the prac-
tical applicability of blue PhOLEDs in full-color displays and
solid lighting. To improve the color quality of blue PhOLEDs,
Thompson and co-workers reported a phosphor of iridium(III)
bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate
(FIr6), and the corresponding devices show deep-blue emis-
sion with CIE coordinates of (0.16, 0.26).[9] Since then, elegant
materials and elaborate device structures have been developed
to improve the performance of FIr6-based devices.[10–12] For
example, Fukagawa et al. reported a highly efficient, low-voltage
deep-blue PhOLED with a maximum EQE of 19% (33 lm W−1 )
by adopting both hole- and electron-transporting hosts in a
double-emission-layer structured device;[11] Xue et al. demon-
strated a maximum EQE of 20% (36 lm W−1 ) by incorporating
a double emissive layer into a p-i-n type cell architecture.[12]
It is noticed that the highly efficient FIr6-based devices have
Solution-processable OLEDs are considered critical to the
realization of low-cost and large-area displays.[14] Obvious
progress for red, green, and sky-blue PhOLEDs by solution
processes has been made.[15–17] However, several challenges
make it difficult to generate a high efficiency, deep-blue phos-
phorescent OLED using a solution process. Primary among
these is the design of a host material that should combine a
high triplet energy (>2.72 eV) and a high-lying HOMO level to
match the Fermi level of poly-(3,4-ethylenedioxythiophene):poly-
(styrenesulfonate) (PEDOT:PSS, 5.1 eV),[18] which is commonly
used as hole-injection material in solution-processed devices.
In this case, the emissive layer, consisting of host and triplet
guest, is usually coated on the PEDOT:PSS layer directly, and
thus one cannot insert several stepped HTLs as in the vacuum-
deposition devices, except for using thermally cross-linkable
hole-transporting materials.[19] Providing that one also expects
the host to possess good film-forming ability and bipolar trans-
porting characteristics, in addition to high ET and a high-lying
HOMO level, it becomes a very big challenge for the search of
the host material for solution-processable deep-blue PhOLEDs.
In our recent work, we demonstrated a bipolar silicon-
bridged host material that has a triplet energy of 2.69 eV and
a HOMO level of 5.27 eV.[20] The HOMO value is close to that
of PEDOT:PSS, but the triplet energy is not high enough to
host deep-blue phosphor FIr6. Here, we successfully design a
new bipolar tetraarylsilane derivative with both a sufficiently
high triplet energy (2.93 eV) and a high-lying HOMO level
(5.28 eV) by incorporating diphenylamine and 1,2,4-triazole
units into a silicon-bridged framework. Additionally, the Si
compounds with multiple arylsilane groups as well as the
rigid 1,2,4-triazole unit would enhance the likelihood that the
material forms morphologically stable amorphous films. As a
S. Gong, Prof. C. Yang, Dr. C. Zhong, Prof. J. Qin
Department of Chemistry
Hubei Key Lab on Organic and Polymeric Optoelectronic Materials
Wuhan University
Wuhan 430072, P. R. China
E-mail: clyang@whu.edu.cn
Q. Fu, Dr. Q. Wang,Prof. D. Ma
State Key Laboratory of Polymer Physics and Chemistry
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences
Changchun 130022, P. R. China
E-mail: mdg1014@ciac.jl.cn
DOI: 10.1002/adma.201102758
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result, an efficient deep-blue phosphorescent device employing
the new host material via a solution process has been realized
with the maximum current efficiency of 12.5 cd A−1 (6.2 lm W−1 ,
6.3%). Remarkably, the current efficiency still remains as high
as 11.4 cd A−1 with a low roll-off value of 8.8% at the practical
brightness of 1000 cd m−2 . To the best of our knowledge,
this is the first report of a small-molecule-based, solution-
processed, FIr6-based PhOLED. Additionally, the performance
is the highest ever reported for solution-processed FIr6-based
devices. Furthermore, the new host material is also applicable
for the widely used sky-blue emitter-FIrpic, and the solution-
processed device has achieved a maximum current efficiency of
23.7 cd A−1 (11.8 lm W−1 , 11.2%), which is among the highest
for FIrpic-based PhOLEDs by solution processes ever reported.
The new tetraarylsilane derivative, namely {4-[{4-[5-(4-tert-
butylphenyl)-4-phenyl-4H-1,2,4-triazol-3-yl]phenyl}(diphenyl)-
silyl]phenyl}diphenylamine (p-TAZSiTPA, the inset of Figure 1) ,
was synthesized in two steps. The electron-withdrawing 1,2,4-
triazole moiety and the electron-donating diphenylamine seg-
ment were sequentially introduced to the tetraphenylsilane
skeleton (Scheme S1, Supporting Information). The chemical
1
structure was fully characterized by H NMR and 13C NMR
spectroscopy, mass spectrometry, and elemental analysis.
The absorption and fluorescence spectra of p-TAZSiTPA
are shown in Figure 1. The triplet energy of the compound
was determined to be 2.93 eV by the highest-energy vibronic
sub-band of the phosphorescence spectrum in 2-methyltet-
rahydrofuran at 77 K, which is substantially higher than that of
deep-blue phosphor FIr6 (2.72 eV).
The good thermal stability of p-TAZSiTPA is indicated by
the high decomposition temperatures (Td, corresponding to 5%
weight loss) of 421 °C in the thermogravimetric analysis (TGA),
and the glass transition temperature (Tg) of 119 °C through dif-
ferential scanning calorimetry (DSC) (Figure S1, Supporting
Information), which is much higher than those of previously
reported tetraarylsilane compounds, such as the UGH (ultra-
high energy gap hosts) series (26–53 °C).[13] The high Tg value of
p-TAZSiTPA should benefit the stability of film morphology. As
Figure 2. AFM topographic images (angled and top views) of the solution-
processed p-TAZSiTPA films doped with a) 10 wt% FIr6 and b) 15 wt%
FIrpic after drying at 80 °C under nitrogen atmosphere.
shown in Figure 2, the atomic force microscopy (AFM) images
of solution-processed p-TAZSiTPA films doped with FIr6 or
FIrpic display the smooth and homogeneous film morpholo-
gies, with small values of root-mean-square (RMS) roughness
of 0.36 and 0.42 nm, respectively. We believe that the silicon tet-
rahedral configuration[21] and the rigid 1,2,4-triazole unit could
provide a stabilizing environment for dopants and keep films
integrity throughout the entire fabrication process.
The electrochemical properties of p-TAZSiTPA were probed
using cyclic voltammetry (CV). The compound exhibits one
quasi-reversible, one-electron oxidation process (Figure S2, Sup-
porting Information). The HOMO energy level evaluated from
the onset of the oxidation potential is 5.28 eV with regard to the
energy level of ferrocene (4.8 eV below vacuum). This HOMO
level is consistent with the calculated HOMO value (5.31 eV)
from density functional theory (DFT) calculations (Figure S3,
Supporting Information). The matched HOMO level with
PEDOT:PSS (5.1 eV) should result in a low hole-injection
barrier from PEDOT:PSS to the host.
To evaluate the bipolar character of p-TAZSiTPA, a hole-
only device with the structure of Al/MoO3 (5 nm)/p-TAZSiTPA
(200 nm)/MoO3 (5 nm)/Al and an electron-only device with the
configuration of Al/LiF (1 nm)/p-TAZSiTPA (200 nm)/LiF (1 nm)/
Al were fabricated. The current density versus voltage curves show
that the electron current density is comparable to the hole cur-
rent density in p-TAZSiTPA (Figure S4, Supporting Information),
indicating the bipolar transporting property of p-TAZSiTPA.
The high triplet energy, the high-lying HOMO level, the
bipolar transporting character, and the good film-forming ability
for the new compound make it a good candidate as a solution-
processed host for deep-blue PhOLEDs. We initially fabricated the
FIr6-based blue phosphorescent device (device A) with the device
architecture of ITO/PEDOT:PSS (40 nm)/p-TAZSiTPA:FIr6
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
N
N
N
Si
N
-TAZSiTPA
p
Abs
Fl at RT
Ph at 77K
300
400
500
600
700
Wavelength (nm)
Figure 1. The absorption and emission spectra of p-TAZSiTPA in hexane
at 5 × 10−6 M at room temperature, and phosphorescence spectrum of
p-TAZSiTPA in 2-methyltetrahydrofuran at 77 K. Inset: the molecular
structure of p-TAZSiTPA.
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the electroluminescence (EL) spectrum originated from the triplet
emission of FIr6 with CIE coordinates of (0.16, 0.27), indicative
of efficient energy transfer from the host p-TAZSiTPA to FIr6.
The deep-blue phosphorescent device achieved a maximum cur-
rent efficiency (ηc,max) of 12.5 cd A−1 , a maximum power effi-
ciency (ηp,max) of 6.2 lm W−1 , and a maximum external quantum
efficiency (ηext,max) of 6.3% (Figure 3). These values are over two
times higher than those of the solution-processed polymer device
using poly(9-vinylcarbazole) (PVK) as host (ηc,max of 3.66 cd A−1
and ηext,max of 1.48%).[22] It is worth noting that the current effi-
ciency still remains as high as 11.4 cd A−1 with a low roll-off
value of 8.8% at the brightness of 1000 cd m−2 . To the best of our
knowledge, this is the first report of small-molecule-based deep-
blue PhOLED by solution-processed emissive layer and the device
performance is the highest ever reported for solution-processed
FIr6-based devices.
a)
160
Device A
Device B
104
103
102
101
100
140
120
100
80
LiF/Al
TPBI (30 nm)
Tm3PyPB (5 nm)
EML (40 nm)
PEDOT (40 nm)
ITO Glass
60
40
20
0
2
4
6
8
10
Voltage (V)
b)
To evaluate the utility of the compound as host material for
other blue phosphorescent emitters, we then fabricated the
device B with the same configuration as device A, with the
exception of using 15 wt% FIrpic as the dopant. The FIrpic-
based device achieved ηc,max of 23.7 cd A−1 , ηp,max of 11.8 lm W−1
and ηext,max of 11.2% (Figure 3), respectively, with CIE coordi-
nates of (0.15, 0.32) at 7 V. Moreover, the device showed rather
low efficiency roll-off; at the luminance of 1000 cd m−2 , ηc was
still as high as 23.1 cd A−1 with a very low roll-off value of 2.5%.
The performance of the device B is among the highest for
solution-processed FIrpic-based PhOLEDs ever reported.[17,23]
The high efficiencies and low efficiency roll-off at high lumi-
nance for blue and deep-blue PhOLEDs can be attributed to
the superiority of the host p-TAZSiTPA: the high triplet energy
ensures efficient energy transfer from the host to the phosphor
and triplet exciton confinement on the phosphor; the high-lying
HOMO level facilitates efficient hole injection to the emitting
layer; and bipolar feature renders balanced charge fluxes and a
broad distribution of recombination region within the emitting
layer.
In summary, we have developed a new tetraarylsilane deriva-
tive by incorporating an electron-donor diphenylamine group
and an electron-acceptor 1,2,4-triazole unit into the silicon-
bridged structure. Such molecular design endows the material
with a high triplet energy and a high-lying HOMO level simul-
taneously. Along with the good film-forming ability and bipolar
property, the new compound enables a solution-processed deep-
blue PhOLED with high efficiency and low efficiency roll-off.
Further application of the new compound as a host material in
solution-processable white PhOLEDs is in progress.
Device A
10
10
1.0
1
0.8
0.6
0.4
0.2
0.0
1
0.1
0.01
400
500
600
700
Wavelength (nm)
0.1
1
10
100
1000
Luminance (cd/m2)
c)
100
10
1
Device B
10
1.0
0.8
0.6
0.4
0.2
0.0
1
0.1
0.01
400
500
600
700
Wavelength (nm)
0.1
1
10
100
1000
10000
Luminance (cd/m2)
Figure 3. a) Current density and luminance versus driving voltage character-
istics for devices A (FIr6-based device, circles) and B (FIrpic-based device,
triangles). Inset: device structure for devices A and B. b) Current efficiency
and power efficiency versus luminance curves for device A. Inset: the normal-
ized electroluminescence (EL) spectra of device A at a driving voltage of 7 V.
c) Current efficiency and power efficiency versus luminance curves for device
B. Inset: the normalized EL spectra of device B at a driving voltage of 7 V.
Experimental Section
Device Fabrication and Measurement: The hole-injection material,
PEDOT:PSS, and electron-transporting material, TPBI, were commercially
available. Commercial ITO-coated glass with a sheet resistance of 10 Ω
square−1 was used as the starting substrates. Before device fabrication,
the ITO glass substrates were precleaned carefully and treated by oxygen
plasma for 2 min. PEDOT:PSS was spin-coated to smooth the ITO surface
and promote hole injection, and then the emissive layer (EML) was spin-
coated from chlorobenzene solution. The samples were annealed at 80 °C
for 30 min to remove residual solvent. The thickness of the EML was
about 40 nm. Finally, a hole/exciton-blocking layer of Tm3PyPB (5 nm),
(10 wt%, 40 nm)/1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (Tm3PyPB,
5
nm)/1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene
(TPBI,
30 nm)/LiF (1 nm)/Al (100 nm). Tm3PyPB[7b] and TPBI were
used as the hole/exciton-blocking and electron-transporting layers,
respectively; PEDOT:PSS and LiF served as hole- and electron-
injecting layers, respectively; and FIr6 doped in p-TAZSiTPA was
used as the emitting layer. As depicted in the inset of Figure 3b ,
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an electron-transporting layer of TPBI (30 nm), and a cathode composed
of lithium fluoride (1 nm) and aluminum (100 nm) were sequentially
deposited onto the substrate by vacuum deposition in a vacuum with
a pressure of 10−6 Torr. The current density–voltage–luminance (J–V–L)
measurements of the devices were recorded with a Keithley 2400 Source
meter and a Keithley 2000 Source multimeter equipped with a calibrated
silicon photodiode. The EL spectra were measured using a JY SPEX
CCD3000 spectrometer. The EQE values were calculated according to
previously reported methods.[24] All measurements were carried out at
room temperature under ambient conditions.
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Synthesis of {4-[{4-[5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazol-
3-yl]phenyl}(diphenyl)silyl]phenyl}diphenylamine (p-TAZSiTPA): A mixture
of p-TAZSi-Br (2.07 g, 3.00 mmol), diphenylamine (0.54 g, 3.20 mmol),
Pd(OAc)2 (13 mg, 0.06 mmol), tBuONa (0.35 g, 3.60 mmol), and
(tBu)3PHBF4 (52 mg, 0.18 mmol) in 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/CH2Cl2 (1:15 by vol) as the eluent to
1
give a white powder. Yield: 45%. H NMR (300 MHz, CDCl3) δ [ppm]:
7.53–7.48 (m, 6H), 7.46–7.40 (m, 6H), 7.38–7.29 (m, 11H), 7.24–7.19
(m, 6H), 7.13 (d, J = 7.2 Hz, 4H), 7.07-7.00 (m, 4H), 1.28 (s, 9H). 13C
NMR (75 MHz, CDCl3) δ [ppm]: 154.82, 154.46, 152.70, 149.07, 147.11,
137.12, 136.61, 136.23, 135.27, 133.93, 129.93, 129.58, 129.25, 128.21,
127.81, 127.65, 125.30, 125.08, 123.83, 123.42, 121.33, 34.64, 31.06. MS
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Acknowledgements
The authors thank the National Natural Science Foundation of China
(Project Nos. 90922020 and 20621401), the National Basic Research
Program of China (973 Program 2009CB623602 and 2009CB930603), the
National Science Fund for Distinguished Young Scholars, the Program
for Changjiang Scholars and Innovative Research Team in University
(IRT1030), and the Open Research Fund of State Key Laboratory
of Polymer Physics and Chemistry, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences for financial support.
Received: July 18, 2011
Published online: September 30, 2011
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Adv. Mater. 2011, 23, 4956–4959
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