Silicon-based material with spiro-annulated fluorene/triphenylamine as host and exciton-blocking layer for blue electrophosphorescent devices

Article abstract of DOI:10.1002/chem.201301106

A novel silicon-based compound, 10-phenyl-2′-(triphenylsilyl)-10H- spiro[acridine-9,9′-fluorene] (SSTF), with spiro structure has been designed, synthesized, and characterized. Its thermal, electronic absorption, and photoluminescence properties were studied. Its energy levels make it suitable as a host material or exciton-blocking material in blue phosphorescent organic light-emitting diodes (PhOLEDs). Accordingly, blue-emitting devices with iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C²′] picolinate (FIrpic) as phosphorescent dopant have been fabricated and show high efficiency with low roll-off. In particular, 44.0 cd A^-1 (41.3 lm W^-1) at 100 cd m^-2 and 41.9 cd A^-1 (32.9 lm W^-1) at 1000 cd m^-2 were achieved when SSTF was used as host material; 28.1 lm W^-1 at 100 cd m^-2 and 20.6 lm W^-1 at 1000 cd m^-2 were achieved when SSTF was used as exciton-blocking layer. All of the results are superior to those of the reference devices and show the potential applicability and versatility of SSTF in blue PhOLEDs. Into the blue: Blue phosphorescent material 10-phenyl-2′-(triphenylsilyl)-10H-spiro[acridine-9,9′-fluorene] (SSTF) has been synthesized. The triphenylsilyl side group imparts good thermal stability. The energy levels of SSTF make it suitable as a host material or exciton-blocking layer in blue phosphorescent organic light-emitting diodes (see figure; PE=power efficiency). Copyright

Full text of DOI:10.1002/chem.201301106

FULL PAPER
Phosphorescence
H. Chen, Z.-Q. Jiang,* C.-H. Gao,
M.-F. Xu, S.-C. Dong, L.-S. Cui,
S.-J. Ji,* L.-S. Liao ........... &&&& — &&&&
Silicon-Based Material with Spiro-
Annulated Fluorene/Triphenylamine
as Host and Exciton-Blocking Layer
for Blue Electrophosphorescent
Devices
Into the blue: Blue phosphorescent
material 10-phenyl-2’-(triphenylsilyl)-
10H-spiro[acridine-9,9’-fluorene]
(SSTF) has been synthesized. The tri-
phenylsilyl side group imparts good
thermal stability. The energy levels of
SSTF make it suitable as a host mate-
rial or exciton-blocking layer in blue
phosphorescent organic light-emitting
diodes (see figure; PE=power effi-
ciency).
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DOI: 10.1002/chem.201301106
Silicon-Based Material with Spiro-Annulated Fluorene/Triphenylamine as
Host and Exciton-Blocking Layer for Blue Electrophosphorescent Devices
Hua Chen,[a] Zuo-Quan Jiang,*[b] Chun-Hong Gao,[b] Mei-Feng Xu,[b]
Shou-Cheng Dong,[b] Lin-Song Cui,[b] Shun-Jun Ji,*[a] and Liang-Sheng Liao[b]
Abstract: A novel silicon-based com-
pound, 10-phenyl-2’-(triphenylsilyl)-
10H-spiro[acridine-9,9’-fluorene]
ting devices with iridium
difluorophenyl)-pyridinato-N,C2’]pico-
ACHTUNGTRNElNUNG inate (FIrpic) as phosphorescent
dopant have been fabricated and show
high efficiency with low roll-off.
In particular, 44.0 cdAÀ1 (41.3 lmWÀ1)
(III) bis[(4,6-
at
100 cdmÀ2
and
41.9 cdAÀ1
(32.9 lmWÀ1) at 1000 cdmÀ2 were ach-
ieved when SSTF was used as host ma-
terial; 28.1 lmWÀ1 at 100 cdmÀ2 and
20.6 lmWÀ1 at 1000 cdmÀ2 were ach-
ieved when SSTF was used as exciton-
blocking layer. All of the results are su-
perior to those of the reference devices
and show the potential applicability
and versatility of SSTF in blue PhO-
LEDs.
(SSTF), with spiro structure has been
designed, synthesized, and character-
ized. Its thermal, electronic absorption,
and photoluminescence properties
were studied. Its energy levels make it
suitable as a host material or exciton-
blocking material in blue phosphores-
cent organic light-emitting diodes
(PhOLEDs). Accordingly, blue-emit-
Keywords: electrophosphores-
cence · host–guest systems · lumi-
nescence · organic light-emitting di-
odes · spiro compounds
Introduction
tion and triplet–polaron annihilation of triplet-state excitons
at high concentration, which originates from its intrinsic
long lifetime of phosphorescence. An effective way to solve
the problem is by adopting a host matrix to separate the
metal complex.[5–10] Therefore, synthesizing good host mate-
rials is another important issue as well as seeking good guest
emitters.[11–22]
A rapid growth of organic light-emitting diode (OLED)
technology, commercialized in small flat-panel displays, cell
phones, cameras, and solid-state lighting, has been witnessed
in recent years.[1,2] Meanwhile, fundamental organic materi-
als research on OLEDs has also been dramatically devel-
oped to meet the daily-increasing demand. In the develop-
ment of OLED materials, phosphorescent materials have at-
tracted tremendous attention because of their capability of
harvesting both the singlet and triplet excitons, since the
pioneering work in 1998.[3,4] The large spin–orbital coupling
of the heavy metals (Ir, Pt, or Os etc.) leads to an efficient
intersystem crossing and thus to a theoretical internal quan-
tum efficiency as high as 100%. The high-efficiency emitter,
however, cannot be utilized directly as a single emitting
layer because of its almost inevitable triplet–triplet annihila-
The first principle for designing good host materials is im-
peding reverse energy transfer from the guest to the host
and confining the exciton in the emitting layer.[23] Under this
guideline, many materials have been prepared and one of
the prominent classes is silicon-based compounds. In 2004,
Ren et al. first reported a series of host materials, including
1,4-bis(triphenylsilyl)benzene (UGH) (Scheme 1), with ul-
trawide energy gap;[24] in 2008, Lin et al. prepared bis-styryl-
benzene (BSB) with the aim to increase the conductivity of
UGH.[25] Note that these types of materials could also easily
fulfill the second principle for designing host materials:
forming stable amorphous films.[26] This property guarantees
that the emitter stays uniformly diluted in the matrix to min-
imize the effect of concentration quenching. As a bulky
group, tetraphenylsilane could play a major role in avoiding
strong intermolecular aggregation.[27] The third principle for
designing good host materials is a suitable HOMO and
LUMO for charge transfer and injection. Due to the high
injection barrier in UGH and BSB resulting from their deep
HOMO levels, the performances are relatively low though
they can meet the aforementioned two principles. The solu-
tion is to introduce electron-active moieties, and in this way,
many host materials have been developed in the past few
years.
[a] H. Chen, Prof. Dr. S.-J. Ji
Key Laboratory of Organic Synthesis of Jiangsu Province
College of Chemistry, Chemical Engineering, and Materials Science
Soochow University, Suzhou 215123 (P. R. China)
Fax : (+86) 0512-65880307
E-mail: shunjun@suda.edu.cn
[b] Prof. Dr. Z.-Q. Jiang, C.-H. Gao, M.-F. Xu, S.-C. Dong, L.-S. Cui,
Prof. Dr. L.-S. Liao
Jiangsu Key Laboratory for
Carbon-Based Functional Materials & Devices
Institute of Functional Nano & Soft Materials (FUNSOM)
Soochow University, Suzhou 215123 (P. R. China)
Fax : (+86) 0512-65882846
E-mail: zqjiang@suda.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301106.
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Blue Electrophosphorescent Devices
FULL PAPER
spiro[acridine-9,9’-fluorene] (SSTF) with an asymmetric con-
formation. As a result, efficient blue phosphorescent devices
employing the new host materials have been fabricated. The
device with BSSTF as host shows a maximum current effi-
ciency of 41 cdAÀ1 (35.2 lmWÀ1). This performance is quite
good relative to previous work. Additionally, the device
with SSTF as host exhibits an even lower turn-on voltage of
3.3 V and higher maximum current efficiency of 44.0 cdAÀ1
(41.5 lmWÀ1). Remarkably, a rather low roll-off value of
4.8% is observed in this device at a practical luminance of
1000 cdmÀ2. To the best of our knowledge, this is among the
highest performances for iridiumACTHNUGRTNEUNG(III) bis[(4,6-difluorophen-
yl)-pyridinato-N,C2’]picolinate (FIrpic)-based phosphores-
cent OLEDs (PhOLEDs) accompanied by such low-efficien-
cy roll-off. Furthermore, we found that the SSTF can also
act as a good exciton-blocking material in PhOLEDs and
we compared it with the traditional exciton materials (1,1-
bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC), 4,4’,4’’-
tris(N-carbazolyl)triphenylamine (TCTA), and mCP) to
reveal its versatile and superior application in this research
area.
Scheme 1. Chemical structures of materials employed.
For example, Kang et al. synthesized 9-(4-triphenylsilanyl-
(1,1’,4,1’’)-terphenyl-4’’-yl)-9H-carbazole by replacing one
triphenylsilyl group of BSB with N-carbazole, but its triplet
energy drops steeply to 2.4 eV, which means it is unsuitable
for blue phosphorescence.[28] Yeh et al. added a triphenylsilyl
group to the conventional blue host N,N’-dicarbazolyl-3,5-
benzene (mCP) (e.g., SimCP) to improve its thermal stabili-
ty and achieved a power efficiency of 11.9 lmWÀ1.[29] Tsai
et al. developed 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-
9H-carbazole by appending two triphenylsilyl groups on the
para positions of phenyl-9H-carbazole, which achieved a
power efficiency of 26.7 lmWÀ1.[30] Han et al. developed a
series of tetrahedral silicon motifs with carbazole, and the
best performance was 13.6 lmWÀ1.[31] Jiang et al. proposed
that the bridged triphenylamine may be a good candidate
for blue host materials, and the afforded bridged triphenyl-
amine triphenylsilane (BTPASi) with two triphenylsilyl
groups showed a power efficiency of 35 lmWÀ1.[32] Leung
et al. used electron-deficient oxadiazole containing silane to
construct an n-type host, and the maximum power efficiency
was 31.4 lmWÀ1.[33] Gong et al. used the tetraphenylsilane as
a bridge to link diphenylamine and benzimidazole groups
(e.g., p-BISiTPA), and this material displayed a power effi-
ciency of 26.1 lmWÀ1 and low-efficiency roll-off.[34]
Results and Discussion
Synthesis: The synthetic route is outlined briefly in
Scheme 2. According to the literature, we first synthesized
the precursors 1 and 2 by treating bromotriphenylamine
Herein, we describe a successful combination of a high
triplet energy moiety based on the BSB molecule and the
triphenylamine moiety through spiro-annulated linkage to
give
10-phenyl-2’,7’-bis(triphenylsilyl)-10H-spiro[acridine-
Scheme 2. Synthetic route of materials employed.
9,9’-fluorene] (BSSTF; Scheme 1). This structure possesses
the triphenylamine as hole-transport group with high triplet
energy[35] and the 3D cardo moiety to improve thermal sta-
bility to form the amorphous glassy state;[36–38] the nonconju-
gated expansion in size retained the high triplet energy of
BSB. In general, we suppose that these aspects will comply
well with the requirements of good host materials. After this
initial molecular design, we speculated that it might not be
necessary to append two triphenylsilyl groups on BSSTF
and we subsequently subtracted one of them to obtain a
monosubstituted analogue 10-phenyl-2’-(triphenylsilyl)-10H-
with nBuLi at À788C, then adding 2-bromo-9,9’-spirobifluo-
rene or 2,7-dibromo-9,9’-spirobifluorene, following Friedel–
Crafts cyclization to afford good yields.[39] Subsequently, the
novel materials BSSTF and SSTF were synthesized from
these precursors by a one-step reaction through lithium–hal-
ogen exchange then quenching with triphenylchlorosilane.
Compared with BSSTF, the SSTF has a smaller molecular
size and may be more suitable for vapor deposition in
device fabrication. Their chemical structures were fully char-
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Z.-Q. Jiang, S.-J. Ji et al.
1
acterized by H and 13C NMR spectroscopy, mass spectrom-
5.33 eV for SSTF, which were quite different from the re-
sults from UPS (Figure S9, Supporting Information).
etry, and elemental analysis.
Thermal properties: We investigated the thermal stabilities
of the compounds by thermogravimetric analysis (TGA)
and differential scanning calorimetry (DSC) in a nitrogen
atmosphere at a scanning rate of 108CminÀ1. Both the mate-
rials exhibited high thermal decomposition temperatures
(Td, corresponding to 5% weight loss, see Figure 1): 4588C
Photophysical properties: BSSTF and SSTF also show
almost the same electronic absorption, for example, 280 nm
for BSSTF and 273 nm for SSTF (Figure 2), thus indicating
Figure 2. UV/Vis absorption and PL spectra measured in dichlorome-
thane solution at 10À5 m; phosphorescence (Phos) spectra measured in
frozen 2-methyltetrahydrofuran matrix at 77 K.
Figure 1. DSC traces recorded at a heating rate of 108CminÀ1
.
that the absorption could be attributed to the p–p* transi-
tion of their common spiro-triphenylamine/fluorene core.
Their photoluminescence (PL) spectra in CH2Cl2 solution
depict that the BSSTF has a larger Stokes shift than SSTF,
which may account for the vibration relaxation of the two
triphenylsilyl side groups of BSSTF. The phosphorescence
spectra of BSSTF and SSTF (Figure 2) were measured in a
frozen 2-methyltetrahydrofuran (2-MeTHF) matrix at 77 K.
The energy gaps determined by the onset of UV/Vis spectra
and the triplet energies calculated from the first peak of the
phosphorescence spectra indicate that the SSTF has a slight-
ly higher singlet energy gap (3.81>3.78 eV) and triplet
energy (2.81>2.76 eV). Their geometrical and electronic
properties were further studied by density functional theory
(DFT) calculations using the B3LYP hybrid functional.
From the computer simulation, we found that the triphenyl-
silyl group exerted little effect on the distributions on
HOMOs, LUMOs, and localized spin densities of the T1
states for BSSTF and SSTF; the tendency of the calculated
results also correlated well with the measurements (Figur-
es S10 and S11, Supporting Information). These data, as
well as the electron affinities (EAs) calculated from the op-
tical bandgaps and the IPs measured by UPS, are summar-
ized in Table 1.
for BSSTF and 3278C for SSTF. Their glass transition tem-
peratures Tg were observed at 1348C for BSSTF and 1098C
for SSTF, which are substantially higher than those of tradi-
tional host materials, such as 4,4’-bis(9-carbazolyl)biphenyl
(CBP) (628C) and mCP (608C). As we know, a bigger ana-
logue always results in relatively higher Td and Tg values,
and these results are not beyond our preliminary design.
The rigid spiro structure of the new materials always leads
to good thermal characteristics, and the triphenylsilyl side
group can effectively avoid strong intermolecular packing to
form stable amorphous films.
Ultraviolet photoelectron spectroscopy: The ionization po-
tentials (IPs) of the host materials were measured by using
ultraviolet photoelectron spectroscopy (UPS). This techni-
que determines the HOMO energy level in organic thin
films by bombarding the sample with UV photons and
measuring the kinetic energies of the ejected valence elec-
trons. Thus, the HOMO values obtained by this technique
are very precise relative to the more commonly used techni-
que of cyclic voltammetry (CV), which requires the use of
approximations and has a high degree of error associated
with the measurements (>0.1 eV). The IPs for BSSTF and
SSTF are 5.67 and 5.66 eV, respectively. These values are
nearly identical, which means that the difference of substitu-
tion paths did not affect the electron distribution of the
HOMOs, because the HOMOs are mainly distributed on
the triphenylamine parts as expected. For comparison, we
also tested their electrochemical behavior by CV. The same
oxidation onsets also determined that they have similar
HOMO energy levels, but values of 5.35 eV for BSSTF and
Electroluminescence performance: In light of the high trip-
let energies of the novel materials, blue electrophosphores-
cent devices with BSSTF and SSTF as host materials and
mCP as reference were fabricated. The device structure was
ITO/HAT-CN (10 nm)/TAPC (45 nm)/host FIrpic 15 wt%
(20 nm)/TmPyPB (40 nm)/Liq (2 nm)/Al (100 nm), in which
ITO=indium tin oxide, HAT-CN=dipyrazino[2,3-f:2’,3’-
h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile,
TmPyPB=
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Blue Electrophosphorescent Devices
FULL PAPER
Table 1. Physical properties of BSSTF and SSTF.
ciency of 17.4%. On the other
hand, it is noticeable that the
driving voltages of Device 2
(3.7 V) and Device 3 (3.3 V)
met the requirement for por-
[a]
[a]
[b]
[b]
[c]
[d]
Tg
Td
lmax,abs
lmax,f
lph
Eg
ET/IP/EA[e]
[eV]
ET/IP/EA[f]
[eV]
[8C]
[8C]
[nm]
[nm]
[nm]
[eV]
BSSTF
SSTF
134
109
458
327
292, 319
285, 313
437
413
449
440
3.78
3.81
2.76/5.67/1.89
2.81/5.66/1.85
2.92/5.28/1.37
2.98/5.31/1.31
[a] Tg: glass transition temperature, Td: decomposition temperature; measured by DSC and TGA. [b] Meas-
ured in dichloromethane solution at room temperature. [c] Measured in 2-MeTHF glass matrix at 77 K. [d] Eg:
the bandgap energies were estimated from the optical absorption edges of UV/Vis absorption spectra. [e] ET:
the triplet energies were estimated from the onset peak of the phosphorescence spectra; IP and EA were cal-
culated from UPS and the optical bandgaps from the absorption spectra. [f] ET, IP, and EA were calculated
from DFT simulations.
table
display
(<4 V
at
100 cdmÀ2).
In
addition,
Device 3 presented not only
the highest efficiency and the
lowest driving voltage of all
the devices, but also a very low
efficiency roll-off in device op-
1,3,5-tri(m-pyrid-3-yl-phenyl)benzene), and Liq=8-hydroxy-
quinolatolithium. HAT-CN and Liq were used as hole-injec-
tion and electron-injection layers, respectively, TAPC was
used as hole-transporting layer, and TmPyPB served as elec-
tron-transporting layer; the emitting material consisted of
FIrpic doped in host materials mCP (Device 1), BSSTF
(Device 2), or SSTF (Device 3). All the devices showed
identical spectra from the guest FIrpic without any emission
from the host and/or adjacent layer, thus indicating that the
excitons were entirely confined in the emitting layer. All
device characteristics are summarized in Table 2. The refer-
ence Device 1 obtained a maximum current efficiency of
34.5 cdAÀ1, a maximum power efficiency of 23.2 lmWÀ1,
and a maximum external quantum efficiency of 13.7% with
turn-on voltage of 4.2 V at 100 cdmÀ2 (Figures 3 and 4).
Both Device 2 and Device 3 exhibited better performance
than Device 1. For example, Device 2 achieved a maximum
current efficiency of 41.0 cdAÀ1, a maximum power efficien-
cy of 35.2 lmWÀ1, and a maximum external quantum effi-
eration. The maximum current efficiency of Device 3 was
44.0 cdAÀ1, corresponding to
a
power efficiency of
41.5 lmWÀ1 and an external quantum efficiency of 18.7%;
the current efficiency roll-off, notably, was just 4.8% at a lu-
minance of 1000 cdmÀ2 (Figure 3). In general, the perform-
ance (41.9 cdAÀ1 and 32.9 lmWÀ1) and the efficiency roll-off
Figure 4. Current density and luminance versus voltage for Devices 1–3.
(4.8%) of Device 3 at the practical lighting brightness of
1000 cdmÀ2 are, to the best of our knowledge, the best
among the blue PhOLEDs with FIrpic as guest emitter.
To further understand the difference in the device per-
formance with BSSTF or SSTF as host material, we fabricat-
ed hole-only devices with a structure of ITO/MoO3 (10 nm)/
host (100 nm)/MoO3 (10 nm)/Al (100 nm). The MoO3 layer
was utilized to prevent electron injection from the cathode,
which could be ascribed to the large electron-injection bar-
rier between the MoO3 (LUMO=2.3 eV) and Al (4.3 eV)
layers. From this measurement, we found that the SSTF pre-
sented the highest hole mobility among the three host mate-
rials (Figure 5). On comparing BSSTF with SSTF, the bigger
interference by the more substituted triphenylsilyl groups of
Figure 3. Current efficiency and power efficiency versus luminance for
Devices 1–3.
Table 2. Electroluminescence characteristics of devices 1–3.
[a]
[b]
[c]
[d]
Host
Von
hc.max
[cdAÀ1
hc@100 cdmÀ2
[cdAÀ1
hc@1000 cdmÀ2
[cdAÀ1
hp.max
[lmWÀ1
hp@100 cdmÀ2
[lmWÀ1
hp@1000 cdmÀ2
[lmWÀ1
hext.max
CIE[e]
(x,y)
[V]
G
]
G
]
R
]
G
]
N
]
G
]
[%]
ACHTUNGTRENNUNG
mCP
BSSTF
SSTF
4.2
3.7
3.3
34.5
41.0
44.0
34.3
40.9
44.0
32.1
38.0
41.9
23.2
35.2
41.5
22.7
34.9
41.3
17.1
25.5
32.9
13.7
17.4
18.7
0.167, 0.366
0.162, 0.364
0.161, 0.364
[a] Turn-on voltage at 100 cdmÀ2. [b] Maximum current efficiency. [c] Maximum power efficiency. [d] Maximum external quantum efficiency. [e] Commis-
sion Internationale de lꢁꢂclairage coordinates measured at 5 mAcmÀ2
.
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Z.-Q. Jiang, S.-J. Ji et al.
Figure 6. Current efficiency and power efficiency versus luminance for
devices with different EBLs.
Figure 5. Current density–voltage (J–V) curves for the hole-only devices
(ITO/MoO3 (10 nm)/mCP, BSSTF or SSTF (100 nm)/MoO3 (10 nm)/Al
(100 nm).
mCP:FIrpic 8 wt% (20 nm)/TmPyPB (40 nm)/Liq (2 nm)/Al
(100 nm), in which EBL=TAPC, TCTA, mCP, or SSTF, to
evaluate their performance. Satisfactorily, the SSTF-based
device also showed the best performance in this kind of blue
phosphorescent device with a maximum current efficiency
of 36.5 cdAÀ1, a maximum power efficiency of 29.1 lmWÀ1,
and a maximum external quantum efficiency of 15.1%
(Figure 6, Table 3). Especially at an applied brightness of
1000 cdmÀ2, the SSTF-based device showed a superior per-
formance to that of the others (Figure 7). The power effi-
ciency of SSTF presented 42 and 60% improvement relative
to that of TAPC and mCP, respectively, and had more than
twice the power efficiency of TCTA.
BSSTF may hamper the carrier hopping from one molecule
to a neighboring molecule and thus lead to lower mobility.
The tendency for hole mobility agreed well with the corre-
sponding device performance. We also made electron-only
devices with a structure of ITO/TmPyPB (20 nm)/mCP,
BSSTF or SSTF (100 nm)/TmPyPB (20 nm)/Liq (2 nm)/Al
(100 nm), and the hole-only current densities (J) of SSTF
and BSSTF were about two orders of magnitude bigger than
the electron-only J values, which indicated negligible elec-
tron transport in SSTF and BSSTF (see the Supporting In-
formation). Considering that BSSTF and SSTF possessed
very similar HOMO/LUMO levels and triplet energies, the
higher hole mobility may account for the lower driving volt-
age and better performance of the SSTF-based device.
Considering its high hole-transporting mobility, wide trip-
let energy level, suitable HOMO/LUMO levels, and good
thermal stability, SSTF could also act as an exciton-blocking
layer (EBL) for which the traditional materials, such as
TAPC, TCTA, and mCP, are always used in blue phosphor-
escent devices. In this kind of device structure, the EBL is
used to prevent excitons diffusing out of the emitting layer
and energy back-transfer from the triplet level of FIrpic to
the adjacent hole-transporting layer, typically if the hole-
transporting material features a low triplet energy (such as
the ubiquitous material 4,4’-bis[N-(1-naphthyl)-N-phenyl-
ACHTUNGTRENNUNGamino]biphenyl (NPB)). The fundamental features of SSTF
are similar to those of commonly used exciton-blocking
materials, so we fabricated a prototype device with the
structure ITO/HAT-CN (10 nm)/NPBACTHNUTRGNEUNG(80 nm)/EBL (15 nm)/
Figure 7. Current density and luminance versus voltage for devices with
different EBLs.
Table 3. Electroluminescence characteristics of the devices with different EBLs.
[a]
[b]
[c]
[d]
Host
Von
hc.max
[cdAÀ1
hc@100 cdmÀ2
[cdAÀ1
hc@1000 cdmÀ2
[cdAÀ1
hp.max
[lmWÀ1
hp@100 cdmÀ2
[lmWÀ1
hp@1000 cdmÀ2
[lmWÀ1
hext.max
CIE[e]
(x,y)
[V]
G
]
G
]
R
]
G
]
N
]
G
]
[%]
ACHTUNGTRENNUNG
TAPC
TCTA
mCP
4.5
5.4
5.6
4.2
30.5
23.0
32.0
36.5
29.8
22.5
31.7
36.4
26.4
20.6
29.6
34.5
23.2
14.5
18.8
29.1
21.2
13.3
17.9
28.1
14.5
9.2
12.9
20.6
13.1
9.8
13.6
15.1
0.168, 0.355
0.169, 0.356
0.169, 0.361
0.173, 0.377
SSTF
[a] Turn-on voltage at 100 cdmÀ2. [b] Maximum current efficiency. [c] Maximum power efficiency. [d] Maximum external quantum efficiency. [e] Commis-
sion Internationale de lꢁꢂclairage coordinates measured at 5 mAcmÀ2
.
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Blue Electrophosphorescent Devices
FULL PAPER
triphenylsilane (2.60 g, 5.5 mmol) were employed. Petroleum ether/di-
chloromethane (6:1, v/v) was used as eluent for column chromatography.
The final product was
Conclusion
a
white powder (1.98 g, 74.3%). 1H NMR
We have synthesized and characterized two materials,
BSSTF and SSTF, and their morphological stability, triplet
energy (ET), and HOMO/LUMO energy levels have been
investigated. On application in blue phosphorescent devices,
the asymmetric material SSTF showed better performance
with a power efficiency of 41.5 lmWÀ1 and a current effi-
ciency of 44 cdAÀ1 with efficiency roll-off of 4.8% at the
practical brightness of 1000 cdmÀ2. These values are among
the highest levels in the scientific literature and represent
the successful design of the silicon-based host material to
date. We also fabricated another kind of blue PhOLED with
SSTF as EBL. It is encouraging to find that its performance
is superior to those of traditional materials, such as TAPC,
TCTA, and mCP. Our results showed that SSTF may be a
promising candidate for high-efficiency blue PhOLEDs.
(400 MHz, CDCl3): d=6.25–6.28 (m, 2H), 6.48–6.51 (m, 2H), 6.57–6.61
(m, 2H), 6.89–6.93 (m, 2H), 7.08–7.11 (m, 2H), 7.26–7.39 (m, 11H),
7.45–7.56 (m, 11H), 7.75 (d, J=7.5 Hz, 1H), 7.80–7.82 ppm (m, 2H);
13C NMR (100 MHz, CDCl3): d=155.46, 155.36, 141.45, 140.78, 140.00,
139.68, 136.34, 135.26, 134.24, 133.82, 133.48, 131.08, 130.88, 129.47,
128.77, 127.72, 126.99, 125.97, 125.00, 120.43, 120.25, 119.54, 114.47,
57.07 ppm; MS: m/z: 665.37; elemental analysis calcd (%) for C49H35NSi:
C 88.38, H 5.30, N 2.10; found: C 88.25, H 5.30, N 2.30.
Acknowledgements
We acknowledge financial support from the Natural Science Foundation
of China (Nos.61036009, 61177016, 21172162, 21202114, and
21161160446), the National High-Tech Research Development Program
(No. 2011AA03A110), and the Natural Science Foundation of Jiangsu
Province (No. BK2010003).
Experimental Section
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Device fabrication and measurement: OLED devices were fabricated on
0.7-mm-thick glass substrates precoated with a transparent ITO conduc-
tive layer with a thickness of approximately 100 nm and a sheet resist-
ance of approximately 30 W per square. The substrates, after being
cleaned, dried, and treated by UV/ozone successively, were transferred
into a vacuum chamber for sequential deposition of all organic and metal
layers on top of the substrates in
a vacuum of approximately 2ꢃ
10À6 Torr. The deposition rates and doping concentrations of materials
were controlled and measured in situ by using calibrated thickness moni-
tors. After the deposition of all layers, four identical OLEDs were
formed on each of the substrates, and each OLED had an emission area
of 0.1 cm2. The electroluminescence characteristics of all the devices were
evaluated by using a Keithley 2400 SourceMeter constant-current source
and a Photo Research SpectraScan PR 655 photometer at room tempera-
ture. The external quantum efficiency values were calculated according
to previously reported methods. All measurements were carried out at
room temperature under ambient conditions.
Synthesis of materials: Compound 1 or 2 was dissolved in THF under
argon and cooled to À788C. n-Butyllithium was added dropwise to the
solution under vigorous stirring. After reaction at À788C for 1 h, a solu-
tion of chlorotriphenylsilane in THF was added dropwise. The resulting
mixture was stirred at À788C for 1 h, and then gradually warmed to
room temperature. After stirring overnight, water (5 mL) was added.
Then the mixture was evaporated, extracted with dichloromethane, and
washed with water successively. The organic layer was collected, dried
with Na2SO4, filtered, and evaporated. The crude product was purified by
column chromatography on silica gel.
BSSTF
(10-phenyl-2’,7’-bis(triphenylsilyl)-10H-spiro[acridine-9,9’-fluo-
rene]): Compound 1 (1.70 g, 3 mmol), n-butyllithium (2.4m, 9 mmol), and
chlorotriphenylsilane (3.54 g, 12 mmol) were employed. Petroleum ether/
ethyl acetate (6:1, v/v) was used as eluent for column chromatography.
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The final product was
a
white powder (1.78 g, 64.1%). 1H NMR
(400 MHz, CDCl3): d=7.90 (s, 2H), 7.77 (d, J=7.6 Hz, 2H), 7.55–7.52 (s,
1H), 7.51–7.45 (m, 13H), 7.44–7.39 (m, 4H), 7.37 (dt, J=2.7, 1.8 Hz,
5H), 7.29 (t, J=7.2 Hz, 12H), 6.91 (dd, J=8.5, 7.1, 1.6 Hz, 2H), 6.75 (dd,
J=6.4, 3.1 Hz, 2H), 6.68–6.60 (m, 2H), 6.57 (dd, J=7.7, 1.5 Hz, 2H),
6.18 ppm (d, J=7.6 Hz, 2H); 13C NMR (100 MHz, CDCl3): d=154.54,
141.58, 140.56, 140.47, 136.37, 135.80, 134.44, 134.21, 133.73, 130.99,
130.77, 129.51, 127.92, 127.82, 127.03, 126.47, 125.08, 120.34, 119.86,
114.39, 57.24 ppm; MS: m/z: 923.34; elemental analysis calcd (%) for
C67H49NSi2: C 87.06, H 5.34, N 1.52; found: C 87.00, H 5.35, N 1.77.
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(10-phenyl-2’-(triphenylsilyl)-10H-spiro[acridine-9,9’-fluorene]):
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Chem. Eur. J. 2013, 00, 0 – 0
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Received: March 22, 2013
Published online: && &&, 0000
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