A highly efficient, blue-phosphorescent device based on a wide-bandgap host/FIrpic: Rational design of the carbazole and phosphine oxide moieties on tetraphenylsilane

Article abstract of DOI:10.1002/adfm.201103126

A new series of wide-bandgap materials, 4-dipenylphosphine oxide-4'-9H-carbazol-9-yl-tetraphenylsilane (CSPO), 4-diphenylphosphine oxide-4',4″-di(9H-carbazol-9-yl)-tetraphenylsilane (pDCSPO), 4-diphenylphosphine oxide -4'-[3-(9H-carbazol-9-yl)-carbazole-9-yl]- tetraphenylsilane (DCSPO), 4-diphenylphosphine oxide-4',4″,4″'- tri(9H-carbazol-9-yl)-tetraphenylsilane (pTCSPO) and 4-diphenylphosphine oxide -4'-[3,6-di(9H-carbazol-9-yl)-9H-carbazol-9-yl]-tetraphenylsilane (TCSPO), containing different ratios and linking fashions of p-type carbazole units and n-type phosphine oxide units, are designed and obtained. DCSPO is the best host in FIrpic-doped devices for this series of compounds. By utilizing DCzSi and DPOSi as hole- and electron-transporting layers, a high EQE of 27.5% and a maximum current efficiency of 49.4 cd A^-1 are achieved in the DCSPO/FIrpic doped device. Even at 10 000 cd m^-2, the efficiencies still remain 41.2 cd A^-1 and 23.0%, respectively.

Full text of DOI:10.1002/adfm.201103126

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A Highly Efficient, Blue-Phosphorescent Device Based on
a Wide-Bandgap Host/FIrpic: Rational Design of the
Carbazole and Phosphine Oxide Moieties on
Tetraphenylsilane
He Liu, Gang Cheng, Dehua Hu, Fangzhong Shen, Ying Lv, Guannan Sun, Bing Yang,
Ping Lu,* and Yuguang Ma*
semiconductors because of their intrinsic,
low highest occupied molecular orbital
(HOMO) energy levels and high lowest
unoccupied molecular orbital (LUMO)
di(9H-carbazol-9-yl)-tetraphenylsilane (pDCSPO), 4-diphenylphosphine oxide
energy levels.[3,4] The restriction in the
A new series of wide-bandgap materials, 4-dipenylphosphine oxide-4′-9H-
carbazol-9-yl-tetraphenylsilane (CSPO), 4-diphenylphosphine oxide-4′,4′′-
-4′-[3-(9H-carbazol-9-yl)-carbazole-9-yl]-tetraphenylsilane (DCSPO), 4-diphe-
π-conjugation length results in the reduc-
nylphosphine oxide-4′,4′′,4′′′-tri(9H-carbazol-9-yl)-tetraphenylsilane (pTCSPO)
and 4-diphenylphosphine oxide -4′-[3,6-di(9H-carbazol-9-yl)-9H-carbazol-9-yl]-
tetraphenylsilane (TCSPO), containing different ratios and linking fashions
of p-type carbazole units and n-type phosphine oxide units, are designed and
obtained. DCSPO is the best host in FIrpic-doped devices for this series of
compounds. By utilizing DCzSi and DPOSi as hole- and electron-transporting
layers, a high EQE of 27.5% and a maximum current efficiency of 49.4 cd A−1
are achieved in the DCSPO/FIrpic doped device. Even at 10 000 cd m−2 , the
efficiencies still remain 41.2 cd A−1 and 23.0%, respectively.
tion of the carrier-injection and transport
properties. Thus, it is quite challenging
to design materials capable of giving effi-
cient UV emission and that have balanced
charge-injection properties.
High efficiency in OLEDs is a crucial
factor in promoting their application.
The host/phosphorescent-guest doping
strategy is an important approach for
this purpose. In doping systems, a wide-
bandgap host material is a critical factor
for high efficiency in blue OLEDs. In
recent years, tetra-arylsilane derivatives
have emerged as an attractive class of host material, due to
their ultrahigh energy gap and high triplet energy level, and
some of them have achieved a high quantum efficiency in host/
blue-phosphorescent OLEDs. Typical materials are ultrawide
energy gap hosts (UGHs),[4] tirphenyl-(4-(9-phenyl-9H-fluoren-
1. Introduction
Wide-bandgap materials that emit violet or ultraviolet (UV)
light (emission shorter than 400 nm) are of great importance,
since such devices can be used to generate light of all colors,
either by energy transfer or by the irradiation of luminescent
dyes.[1–3] Over the past decade, the development of organic
wide-bandgap materials has been far behind their inorganic
counterparts, which has resulted in a scarcity of UV organic
light-emitting diodes (OLEDs) with high performance. One
reason for this phenomenon is that it is difficult to simulta-
neously inject holes and electrons into wide-bandgap organic
9-yl)phenyl)silane (TPSi-F),[5] 4,4′-bis-triphenylsilanyl-biphenyl
(BSB),[6] 4,4′′-bis(triphenylsilanyl)-(1,1′,4′,1′′)-terphenyl (BST),[6]
(9,9′-dimethylfluoren-2-yl)nSi(phenyl)4-n (SiFln, n = 1, 2, 3,
and 4),[7] 3,5-bis(9-carbazolyl)tetraphenylsilane (SimCP),[8]
and
9-(4-tertbutylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole
(CzSi).[9] Our group has also reported the substitution of tet-
raphenylsilane with carbazole, to obtain a high-triplet-energy
host
material,
bis(4-(9-carbazolyl)phenyl)diphenylsilane
(DPSiCBP),[10] which shows a higher quantum efficiency than
4,4′-bis(9-carbazolyl-)2,2′-biphenyl (CBP) in blue-phosphores-
cent OLEDs. The common character of the hosts mentioned
above is that they predominantly transport holes. A problem
still remains in terms of electron injection and transport. The
phosphine oxide (PO) moiety is known to transport electrons
predominantly as a point of saturation, since first reported by
Sapochak’s group.[11] The study has been intensified by sev-
eral groups, to bond it with biphenyl,[11] fl uorene,[12–14] carba-
zole,[15–19] spirobifluorene,[20] amine,[21,22] dibenzofuran,[23,24]
and dibenzothiophene[19] moieties. The results show that
the PO moiety can lower the LUMO level and increase the
Dr. H. Liu, Dr. D. H. Hu, Dr. F. Z. Shen, Dr. Y. Lv,
G. N. Sun, Prof. B. Yang, Dr. P. Lu, Prof. Y. G. Ma
State Key Laboratory of Supramolecular Structure
and Materials
Jilin University
Changchun, 130012, P. R. China
E-mail: lup@jlu.edu.cn; ygma@jlu.edu.cn
Dr. G. Cheng
State Key Laboratory on Integrated Optoelectronics
College of Electronics Science and Engineering
Jilin University
Changchun, 130012, P. R. China
DOI: 10.1002/adfm.201103126
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N
N
Si
N
O
P
O
P
O
P
Si
N
N
N
Si
N
pTCSPO
pDCSPO
CSPO
N
O
P
O
P
N
Si
N
Si
N
TCSPO
DCSPO
Scheme 1. Chemical structures of the five compounds.
electron-injection property without changing the triplet energy
(ET) of the molecule. Incorporating the PO moiety into carba-
zole-tetraphenylsilane to construct a bipolar-type wide-bandgap
material would be interesting.
Lee’s group reported one compound to connect PO with carba-
zole-substituted tetraphenylsilane (4-((4-(9H-carbazol-9-yl)phenyl)
diphenylsilyl)phenyl)-diphenylphosphine oxide (TSPC) and an
external quantum efficiency (EQE) of 21.8% was obtained when
doping with iridium(III) bis[(5-cyano-4-fluorophenyl)pyridinato-
units was designed as 1:1 for CSPO, 2:1 for pDCSPO, and 3:1
for pTCSPO. The carbazole was also designed to bond with
the tetraphenylsilane, separately in pDCSPO and pTCSPO,
or as dimers and trimers in DCSPO and TCSPO. All of these
materials were synthesized through an Ullmann type reaction
between the respective substituted carbazoles and phosphine
oxide monomers. They were purified by column chromatog-
raphy, followed by recrystallization from a mixture of dichlo-
romethane and ethanol. The chemical structures were fully
characterized by 1H NMR and 13C NMR spectroscopy, mass
spectrometry, and elemental analysis, and they corresponded
well with the expected structures.
N,C2′]picolinate (FCNIrpic).[25] However, a systematic study of
different ratios and various linking fashions of p-type and n-type
units was not carried out. In this paper, we have developed five
wide-bandgap materials: 4-diphenylphosphine oxide-4′-9H-
carbazol-9-yl-tetraphenylsilane (CSPO),
4-diphenylphosphine
(pDCSPO),
2.2. Thermal Properties
oxide-4′,4′′-di(9H-carbazol-9-yl)-tetraphenylsilane
4-diphenylphosphine oxide-4′-[3-(9H-carbazole-9-yl)-9H-carbazol-
High glass-transition temperatures (Tg) above 110 °C were
observed for these materials (Figure 1). We found that the Tg
rose gradually with the increasing number of carbazole units.
The linking fashion also had an effect on the Tg. DCSPO and
9-yl]-tetraphenylsilane (DCSPO), 4-diphenylphosphine oxide-
4′,4′′,4′′′-tri(9H-carbazol-9-yl)-tetraphenylsilane (pTCSPO), and
4-diphenylphosphine
oxide-4′-[(3,6-di(9H-carbazol-9-yl)-9H-
carbazol-9-yl]-tetraphenylsilane (TCSPO), taking tetraphenylsilane
as the core, functionalized with different ratios of p-type carbazole
and n-type PO units, to pursue a balanced charge-injecting and
-transporting ability. We found that a carbazole dimer coupled
with a PO moiety in DCSPO is the best choice for this series of
materials. A high current efficiency of 49.4 cd A−1 and an EQE of
110 oC
CSPO
124 oC
pDCSPO
and 27.5%, with Commission Internationale de l′Eclairage (CIE)
coordinates of (0.15, 0.28), was demonstrated by using DCSPO
as host in bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)
iridium(III) (FIrpic)-doped OLEDs: this is the best efficiency
value among the blue OLEDs reported in the literature.
140 oC
DCSPO
173 oC
pTCSPO
184 oC
TCSPO
2. Results and Discussion
2.1. Synthesis and Characterization
60
120
180
240
300
Temperature (oC)
All of the compounds in this work had a tetraphenylsilane
core (Scheme 1). The ratio of p-type carbazole and n-type PO
Figure 1. DSC data recorded at a heating rate of 10 K min−1
.
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a
CSPO
CSPO
b
pDCSPO
DCSPO
pTCSPO
TCSPO
pDCSPO
DCSPO
pTCSPO
TCSPO
300
350
400
450
500
300
330
360
390
Wavelength (nm)
Wavelength(nm)
409nm
417nm
409nm
409nm
d
c
CSPO
CSPO
pDCSPO
pTCSPO
DCSPO
TCSPO
429nm
409nm
400
450
500
550
400
440
480
520
560
Wavelength(nm)
Wavelength(nm)
Figure 2. a,b) Absorption (a) and photoluminescence (PL) (b) spectra in THF solution at 10−5 M. c,d) PL at 77 K in frozen THF.
TCSPO both showed a Tg more than 10 °C higher than that
of pDCSPO and pTCSPO, indicating dimer and trimer car-
bazole groups are more stable than those solely connected in
pDCSPO and pTCSPO. They also exhibited a high thermal-
decomposition temperature (Td), ranging from 420 °C to 530 °C
(Figure S1, Supporting Information). All of the five materials
had better thermal properties than commonly used hosts, such
as 1,3-bis(9-carbazolyl)benzene (mCP). The introduction of
the carbazole and PO groups improved the thermal stability of
the tetra-arylsilanes greatly, which is highly important for their
application in devices.
spectra (Figure 2b) showed the same phenomena as the absorp-
tion spectra. The further red shift of the signal for the DCSPO
and TCSPO indicates a larger conjugation. To evaluate the ET
of these materials, the emission at low temperature was also
measured. The ET ranged from 2.89 eV to 3.02 eV (Figure 2c,d),
which is higher than that of FIrpic (ET = 2.67 eV), indicating
that the confinement of the triplet excitation on the dye was
guaranteed. The photophysical data are summarized in Table 1.
2.4. Electrochemical Properties
Similar phenomena caused by the different linking fashions
were also observed in the electrochemical properties during
2.3. Photophysical Properties
The different linking fashions led to different bandgaps and
conjugation lengths for the molecules. For example, CSPO,
pDCSPO, and pTCSPO showed the same absorption and emis-
sion spectra in tetrahydrofuran (THF) because of the same
building fragments (Figure 2a). The maximum absorption peak
at 293 nm and the absorption band from 324 nm to 344 nm are
attributed to the π–π* transition of the carbazole. As compared,
DCSPO and TCSPO showed a red shift and a broadening band
in the absorption spectrum, which was due to the enlarged
conjugation through the substituted dimer and timer carba-
zole unit. The optical-energy band gaps (Eg), calculated from
the onset of absorption, are 3.56 eV for CSPO, pDCSPO, and
pTCSPO, and 3.38 eV for DCSPO and TCSPO. The emission
T a b le 1 . The photophysical and thermal properties.
a)
a)
a)
b)
Absmax
[nm]
PLmax
[nm]
Eg
ET
Tgc)
[°C]
Tdd)
[°C]
[eV]
3.56
3.56
3.35
3.56
3.38
[eV]
3.02
3.02
2.97
3.02
2.89
CSPO
293, 325, 340 345, 360
293, 325, 340 345, 361
110
124
140
173
184
420
433
420
529
512
pDCSPO
DCSPO
pTCSPO
TCSPO
293, 342
293, 325, 340 345, 360
293, 342 382
370
a)Measured in THF solution at 10−5 M; b)Measured in frozen THF; c)Obtained from
DSC measurements; d)Obtained from TGA measurements.
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T a b le 2 . HOMO and LUMO energy level of the five materials.
TCSPO
HOMOa) LUMOa)
Eg
HOMOb) LUMOb)
Eg
a)
b)
[eV]
[eV]
[eV]
3.35
3.36
3.24
3.37
3.26
[eV]
[eV]
[eV]
4.35
4.27
4.05
4.31
3.98
CSPO
−5.60
−5.59
−5.43
−5.57
−5.46
−2.25
−2.20
−2.19
−2.20
−2.20
−5.40
−5.39
−5.16
−5.48
−5.20
−1.05
−1.12
−1.11
−1.17
−1.22
pTCSPO
DCSPO
pDCSPO
CSPO
pDCSPO
DCSPO
pTCSPO
TCSPO
a)Determined from the onset of the oxidation/reduction voltages; b)Calculated from
the theoretical simulations.
of the CSPO, pDCSPO, DCSPO, and pTCSPO were completely
separated, which resulted from the disruption effect of the tet-
raphenylsilane between the electron-rich and electron-deficient
moieties. Taking DCSPO as an example (Figure 4), the HOMO
level was located on the carbazole dimer units. The LUMO
level was distributed on the benzene ring adjacent to the PO
moiety. Interestingly, the HOMO level of TCSPO was the same
as that of the other four materials. However, the LUMO+1 level,
instead of the LUMO level, was distributed on the benzene ring
close to PO group. The data are shown in Table 2.
-3
-2
0.0
0.5
1.0
1.5
2.0
+
V (v.s.Ag/Ag )
Figure 3. The CV curves in acetonitrile/CH2Cl2.
cyclic voltammetry (CV) measurements (Figure 3). In the
cathodic scan, CSPO, pDCSPO, and pTCSPO underwent the
same oxidation processes, originating from their p-type sole
carbazole unit. Thus, they exhibited similar HOMO energy
levels of about –5.60 eV. For the DCSPO and the TCSPO, the
carbazole was designed as a dimer and a trimer, which made
oxidation occur more easily, resulting in higher HOMO levels
of about –5.43 eV, which is 0.17 eV higher than that of CSPO,
pDCSPO, and pTCSPO. The LUMO levels of the five mate-
rials were identical, all being about –2.20 eV, caused by the
same reduction PO unit. As compared, the LUMO energy level
of pDCSPO was 0.2 eV lower than that of the compound we
reported previously, DPSiCBP,[10] which had the same carbazole
group, but was without the PO units. This result indicates that,
on the one hand, PO can indeed lower the LUMO energy level;
on the other hand, dimer and trimer carbazole groups can
increase the HOMO energy level.
2.6. Electroluminescence Properties
To evaluate the effects of the different ratios and linking fash-
ions of the D-A group in the blue-phosphorescent OLEDs,
doped FIrpic devices were evaluated. Devices A-E were fabri-
cated with the structure of indium tin oxide (ITO)/poly(3,4-ethyl-
enedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/N,N′-
di(1-naphthyl)-N,N,-diphenylbenzidine (NPB) (80 nm)/mCP
(30 nm)/host:FIrpic (30 nm, 16 wt%)/1,3,5-tri(1-phenyl-1H-
benzimidazol-2-yl)benzene (TPBI) (50 nm)/LiF (0.5 nm)/
Al (100 nm). NPB and TPBi were used as hole- and electron-
transporting layers, and mCP was used as an exciton-blocking
layer. The energy levels of the correlative materials used in the
devices are depicted in Figure 5a .
The differences of the ratios and linking fashions of the p-type
and n-type units led to totally different device performances. As
Lee reported a 21.8% EQE using CSPO as the host, we found
that the 1:1 ratio of the carbazole and PO moiety for the host is
not the best choice for blue OLEDs. Device C, hosted by DCSPO
(carbazole dimer and PO moiety), showed the best perform-
ance. The maximum current efficiency and EQE could reach
24.6 cd A−1 and 11.2%, respectively (Figure 6) .
2.5. Theoretical Calculations
To understand the electronic properties further, density func-
tional theory (DFT) calculations were carried out with the geom-
etry optimized at the B3LYP/6-31G* level. The distributions of
the front molecular orbital are shown in Figure 4 and Figure S2
(Supporting Information); the HOMO and LUMO distributions
The EQE roll-off of device C was only 3.5%
at 100 cd m−2 and 7.1% at 1000 cd m−2 . We
think that the HOMO level of the DCSPO was
about 0.2 eV higher than that of CSPO, which
may have resulted in a more-balanced ratio of
holes and electrons injected into the emissive
layer and improved the device performance.
The efficiency of device E (TCSPO) decreased
a little, compared with device C, which means
the carbazole dimer was the best couple for
Figure 4. Simulation of front molecular orbital of DCSPO through DFT calculation.
the PO unit.
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demonstrates the same result as device B.
The coupling of the carbazole dimer with
PO would be the best choice for FIrpic for
this series of materials. The data are listed in
Table 3.
To optimize the performance of DCSPO
in phosphorescent OLEDs further, we
changed the hole and electron injection
layers. We synthesized 4,4′-di(9H-car-
bazol-9-yl)-tetraphenylsilane (DCzSi) and
4,4′-bis(diphenylphosphine oxide)-tetraphe-
nylsilane (DPOSi), to use them as hole- and
electron-transporting layers, respectively.
They had a similar chemical structure as the
host (DCSPO) in the emissive layer (EL), indi-
cating the better compatibility of hole-trans-
porting layer (HTL), electron-transporting
layer (ETL), and EL. Additionally, the large
Figure 5. a) Energy-level diagrams for devices A–E. b) Energy-level diagram for device I and the
structure of DCzSi and DPOSi.
Although the 2:1 ratio of the donor and acceptor in DCSPO
showed a good device performance, pDCSPO, which also had
the same 2:1 ratio of donor and acceptor but a different linking
position, did not show satisfactory device performance. The
J–V and efficiency curves illustrated that device B, hosted by
pDCSPO, had a higher current density and a lower efficiency
than device A, indicating that device B had unbalanced hole-
and electron-injection and transport properties. Considering
the structures of these two materials, we think the hole injec-
tion and transport of device B was much more than that of elec-
trons. The further decrease of efficiency in device D (pTCSPO)
Eg (3.52 eV for DCzSi and 4.32 eV for DPOSi) and high ET
(3.02eV for DCzSi and 3.40 eV for DPOSi) could block nearly
all of the excitons in the EL, which ensured the high efficiency
of the device. Devices F–I, with structures of ITO/PEDOT:PSS/
HTL(40 nm)/DCSPO:FIrpic (30 nm, 8 wt%)/ETL (30 nm)/
LiF (0.8 nm)/Al (100 nm), were fabricated, and the details are
shown in Figure 7 and Table 4. Device F and H had a higher
current density but a lower efficiency than those of devices G
and I, indicating the carriers were confined well by applying
DPOSi as the ETL. According to the energy diagram shown
in Figure 5b, there was almost no injection barrier for holes
15
600
b
a
500
A host: CSPO
10
B host: pDCSPO
400
C host: DCSPO
D host: pTCSPO
300
200
100
0
E host: TCSPO
5
0.1
1
10
10
12
14
16
18
Current density (mA/cm2)
Voltage (V)
Figure 6. a) Current-density–voltage characteristics for devices A–E. b) Curves of external quantum efficiency versus current density for devices A–E.
T a b le 3 . Device performance of devices A–E: Von = turn - on voltage; brightness = maximum luminance; CE = current efficiency; PE = power efficiency;
EQE = external quantum efficiency.
Device
Von
(V)
Brightness
CEmax
PEmax
EQEmax
[%]
CEa)
PEa)
EQEa)
[%]
CEb)
PEb)
EQEb)
[%]
[cd m−2
11 668
20 328
29 909
17 199
23 229
]
[cd A − 1
]
[lm W − 1
]
[cd A − 1
]
[lm W − 1
]
[cd A − 1
]
[lm W − 1
]
A
B
C
D
E
3.4
3.6
3.6
4.0
3.6
19.6
16.1
24.6
12.0
22.3
9.1
7.7
9.9
5.1
9.4
9.1
7.6
19.3
15.7
23.9
12.0
22.3
7.7
6.2
9.6
4.6
8.8
8.8
7.4
14.8
13.0
22.9
10.1
19.4
4.3
3.7
6.9
2.8
5.6
7.0
6.1
11.2
5.5
10.8
5.4
10.4
4.6
10.0
10.0
8.7
a)Value at 100 cd m− 2 b)Value at 1000 cd m− 2
; .
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300
250
200
150
100
50
F
G
H
I
a
b
100
10
1000
1
100
10
1
0.1
400
500
600
700
Wavelength (nm)
0.01
1E-3
0
1
10
100
1000
10000
4
5
6
7
8
9
10
11
12
Brightness (cd/m2)
Voltage (V)
Figure 7. a ) J–V curves for devices F–I. Inset: EL spectra of devices F–I at 8 V. b) Curves of current efficiency and EQE versus brightness for devices F–I.
T a b le 4 . Details of devices F–I.
DSC analysis was carried out using a NETZSCH
(DSC-204) instrument at 10 K min−1 while flushing
with nitrogen. Electrochemical measurements were
performed using a BAS 100W Bioanalytical System: a
glass-carbon disk electrode was used as the working
electrode, a Pt wire as the counter electrode, Ag/Ag+
as the reference electrode and Bu4NPF6 (0.1 M) in
N,N-dimethylformamide (DMF) or CH2Cl2 as the
electrolyte. The devices were fabricated by vacuum
evaporation. The ITO-coated glass substrates were
cleaned in an ultrasonic bath with toluene, acetone,
ethanol, and deionized water, respectively; they
Device
HTL
ETL
Von Brightness
CEmax
PEmax
CIE
[x, y]
[V]
3.5
3.5
6.0
6.0
[cd m− 2
24 000
12 000
12 500
11 000
]
[cd A− 1
]
[lm W− 1
]
F
NPB (30 nm)/mCP (10 nm)
NPB (30 nm)/mCP (10 nm)
DCzSi
TPBi
DPOSi
TPBi
21.9
37.2
12.6
49.4
13.8
22.1
5.5
0.15, 0.28
0.14, 0.28
0.15, 0.28
0.15, 0.28
G
H
I
DCzSi
DPOSi
20.5
were then dried with nitrogen and were finally irradiated in a UV-ozone
chamber. The organic materials were sequentially deposited onto the
cleaned ITO glass substrates through thermal evaporation. The layer
thickness of the deposited material was monitored in situ using an
oscillating-quartz thickness monitor. Finally a LiF buffer layer and an Al
cathode were deposited onto the organic films. The background pressure
of the chamber was less than 10−6 Torr during the deposition process.
The luminance–current characteristics of devices A to E were measured
using a PR650 spectroscan spectrometer and those of devices F to I were
measured using a PR655 spectroscan spectrometer. The current–voltage
characteristics were studied using a Keithley 2400 sourcemeter. All of
the device measurements were carried out at room temperature under
ambient conditions. The EQE was calculated according to a method
reported previously by Okamoto et al.[26]
and electrons from the DCzSi and DPOSi to the EL in device I.
The maximum current efficiency and EQE of device I reached
49.4 cd A−1 and 27.5%, respectively. Even at 10 000 cd m−2 , the
current efficiency and EQE remained at 41.2 cd A−1 and 23.0%,
respectively. Employing full wide-bandgap materials and a
double blocking device structure are effective ways to obtain a
high efficiency. This is the best device performance based on
FIrpic.
3. Conclusions
In summary, we have designed different ratios and linking
fashions of p-type carbazole and n-type PO moieties on tetra-
phenylsilane to pursue balanced charge injection and transport.
The dimer and trimer carbazole-linking fashions could induce a
longer conjugation length and increase the HOMO energy level.
More balanced charge injection and transport was realized in
DCSPO with dimer-carbazole/PO-substituted tetraphenylsilane.
By applying DCzSi and DPOSi as an HTL and an ETL, respec-
tively, the maximum current efficiency and EQE of devices based
on DCSPO could reach 49.4 cd A−1 and 27.5%, respectively, with
very low roll-off. The results indicate that PO and carbazole-
substituted tetraphenylsilane can be utilized to construct wide-
bandgap materials with good semiconducting properties.
Synthesis
of
4-Diphenylphosphine
oxide-4′-9H-carbazol-9-yl-
tetraphenylsilane (CSPO): A mixture of 4-bromo-4′-diphenylphosphine
oxide-tetraphenylsilane (1.23 g, 2.00 mmol), carbazole (368 mg,
2.20 mmol), CuI (38 mg, 0.20 mmol), K3PO4 (1.06 g, 5.00 mmol), and
( )-trans-1,2-diamino-cyclohexane (280 mg, 0.20 mmol) was added to
1,4-dioxane (5 ml), then refluxed under nitrogen for 24 h. After cooling,
the reaction mixture was quenched with (NH4)2CO3 solution and
extracted with CH2Cl2, then dried over anhydrous MgSO4. After removal
of the solvent, the residue was purified by column chromatography
on silica gel using ethyl acetate/petroleum (1:2 v/v) as the eluent,
followed by recrystallizing with CH2Cl2 and ethanol to give a white
1
powder. Yield: 60%; H NMR (500 MHz, CDCl3, δ): 7.287–7.316 (2H, t,
J = 7.324, Ar H), 7.402–7.513 (14H, m, Ar H), 7.547–7.577 (2H, t, J =
7.324, Ar H), 7.613–7.640 (6H, t, J = 6.104, J = 7.324), 7.689–7.793 (10H,
m, Ar H), 8.13–8.159 (2H, d, J = 7.629, Ar H); 13C NMR (125 MHz,
CDCl3, δ): 140.55, 139.33, 137.90, 134.45, 133.63, 133.00 132.60, 132.20,
132.13, 131.77, 131.36, 131.30, 130.16, 128.65, 128.56, 128.23, 126.28,
126.00, 123.59, 120.37, 120.18, 109.90; MALDI-TOF-MS (M) (m/z):
703.2 [M + H]+; Anal. calcd for C48H36NOPSi: C 82.14, H 5.17, N 2.00;
found: C 81.99, H 5.27, N 2.00.
4. Experimental Section
Measurements: The 1H and 13C NMR spectra were recorded using a
Bruker AVANCE 500 spectrometer at 500 MHz and 125 MHz respectively,
at 298 K using CDCl3 as the solvent and tetramethylsilane (TMS) as the
internal standard. UV–vis and fluorescence spectra were recorded using,
respectively, a Shimadzu UV-3100 spectrophotometer and a Shimadzu
RF-5301PC spectrophotometer, and using 1 cm-path-length quartz cells.
Synthesis of 4-Diphenylphosphine oxide-4′,4′′-di(9H-carbazol-9-yl)-
tetraphenylsilane (pDCSPO): pDCSPO was prepared according to a
similar procedure to CSPO using 4,4′-dibromo-4′′-diphenylphosphine
oxide-tetraphenylsilane to replace the 4-bromo-4′-diphenylphosphine
oxide-tetraphenylsilane. Yield: 70%; 1H NMR (500 MHz, CDCl3, δ):
©
6
wileyonlinelibrary.com
2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Funct. Mater. 2012,
DOI: 10.1002/adfm.201103126

www.afm-journal.de
www.MaterialsViews.com
Education of China (Grant No. 20070183202), the Ministry of Science
and Technology of China (Grant No. 2009CB623605), and PCSIRT.
8.162–8.147 (4H, d, J = 7.935, Ar H), 7.874–7.825 (5H, m, Ar H), 7.789–
7.675 (12H, m, Ar H), 7.583–7.469 (14H, m, Ar H), 7.439–7.408 (4H, t, J
= 7.019, J = 8.240, Ar H), 7.321–7.291 (4H, t, J = 7.324, J = 7.629, Ar H);
13C NMR (125 MHz, CDCl3, δ): 140.54, 139.58, 138.99, 137.95, 136.45,
134.76, 133.94, 132.70, 132.16, 131.71, 131.52, 131.46, 130.40, 128.70,
128.61, 128.42, 126.43, 126.05, 123.64, 120.42, 120.27, 109.91; MALDI-
TOF-MS (M) (m/z): 868.8 [M + H]+; Anal. calcd for C60H43N2OPSi: C
83.11, H 5.00, N 3.23; found: C 81.89, H 5.10, N 3.26.
Received: December 23, 2011
Published online:
Synthesis of 4-Diphenylphosphine oxide-4′-[3-(9H-carbazole-9-yl)-
9H-carbazole-9-yl]-tetraphenylsilane (DCSPO): DCSPO was prepared
according to a similar procedure to CSPO using 3-(9H-carbazol-9-yl)-
9H-carbazole to replace carbazole. Yield: 55%; 1H NMR (500 MHz,
CDCl3, δ): 8.287–8.283 (1H, s, Ar H), 8.195–8.179 (2H, d, J = 8.240, Ar
H), 8.120–8.105 (1H, d, J = 7.629, Ar H), 7.845–7.828 (2H, d, J = 8.240,
Ar H), 7.781–7.639 (15H, m, Ar H), 7.577–7.387 (19H, m, Ar H), 7.340–
7.283(3H, m, Ar H); 13C NMR (125 MHz, CDCl3, δ): 141.88, 141.28,
139.69, 139.24, 139.04, 138.09, 136.43, 134.52, 133.70, 133.25, 132.91,
132.57, 132.15, 131.74, 131.40, 131.35. 130.23, 130.13, 128.67, 12859,
128.28, 126.76, 126.35, 125.91, 125.56, 124.63, 123.17, 120.65, 120.33,
119.67, 119.54; MALDI-TOF-MS (M) (m/z): 868.6 [M + H]+; Anal. calcd
for C60H43N2OPSi: C 83.11, H 5.00, N 3.23; found: C 81.48, H 5.07, N
3.26.
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Synthesis of 4-Diphenylphosphine oxide-4′,4′′,4′′′-tri(9H-carbazol-9-
yl)-tetraphenylsilane (pTCSPO): pTCSPO was prepared according to a
similar procedure to CSPO using 4,4′,4′′-dibromo-4′′′-diphenylphosphine
oxide-tetraphenylsilane to replace 4-bromo-4′-diphenylphosphine oxide-
tetraphenylsilane. Yield: 63%; 1H NMR (500 MHz, CDCl3, δ): 8.173–
8.157 (6H, d, J = 7.629, Ar H), 7.959–7.914 (8H, m, Ar H), 7.853–7.813
(2H, m, Ar H), 7.782–7.719 (10H, m, Ar H), 7.593–7.478 (12H, m, Ar
H), 7.452–7.421 (6H, t, J = 7.324, J = 7.825, Ar H), 7.331–7.301 (6H,
t, J = 7.324, J = 7.935, Ar H); 13C NMR (125 MHz, CDCl3, δ): 140.92,
140.22, 139.01, 138.39, 136.88, 136.81, 135.61, 134.82, 133.03, 132.59,
132.25, 132.08, 132.03, 129.13, 129.05, 126.98, 126.49, 124.10, 120.86,
120.75; MALDI-TOF-MS (M) (m/z): 1033.8 [M + H]+; Anal. calcd for
C72H50N3OPSi: C 83.79, H 4.88, N 4.07; found: C 82.54, H 4.93, N 3.99.
Synthesis of 4-Diphenylphosphine oxide-4′-[(3,6-di(9H-carbazol-9-
yl)-9H-carbazol-9-yl]-tetraphenylsilane (TCSPO): TCSPO was prepared
according to a similar procedure to CSPO using 3,6-di(9H-carbazol-9-
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1
yl)-9H-carbazole to replace carbazole. Yield: 58%; H NMR (500 MHz,
CDCl3, δ): 8.280–8.276 (2H, s, Ar H), 8.171–8.155 (4H, d, J = 7.324, Ar
H), 7.902–7.885 (2H, d, J = 8.240, Ar H), 7.798–7.704 (12H, m, Ar H),
7.669–7.653 (4H, d, J = 7.935, Ar H), 7.629–7.607 (2H, d, J = 8.850, Ar
H), 7.580–7.455 (12H, m, Ar H), 7.418–7.380 (8H, m, Ar H), 7.298–
7.272 (4H, m, Ar H); 13C NMR (125 MHz, CDCl3, δ): 140.79, 139.58,
139.13, 138.67, 136.84, 134.29, 133.20, 132.88, 132.63, 132.56, 132.04,
131.86, 13178, 131.03, 130.69, 129.09, 129.00, 128.72, 126.82, 126.72,
126.35, 124.62, 123.62, 120.75, 120.17, 111.88, 110.12; MALDI-TOF-MS
(M) (m/z): 1033.8 [M + H]+; Anal. calcd for C72H50N3OPSi: C 83.78, H
4.88, N 4.07; found: C 82.62, H 5.07, N 3.99.
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Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
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Acknowledgements
The authors are grateful for the support from the National Science
Foundation of China (Grant No. 20834006, 21174050), the Ministry of
©
wileyonlinelibrary.com
Adv. Funct. Mater. 2012,
DOI: 10.1002/adfm.201103126
2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
7