A novel tetraphenylsilane-phenanthroimidazole hybrid host material for highly efficient blue fluorescent, green and red phosphorescent OLEDs

Article abstract of DOI:10.1039/c5tc00333d

A novel organosilane compound, bis(4-(1-phenylphenanthro[9,10-d]imidazol-2-yl)phenyl)diphenylsilane (Si(PPI)₂), has been designed and synthesized. It has a high thermal decomposition temperature of 528°C and is able to form an amorphous glass w

Full text of DOI:10.1039/c5tc00333d

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Materials Chemistry C
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A novel tetraphenylsilane–phenanthroimidazole
hybrid host material for highly efficient blue
fluorescent, green and red phosphorescent
OLEDs†
Cite this:DOI: 10.1039/c5tc00333d
Dong Liu, Mingxu Du, Dong Chen, Kaiqi Ye, Zuolun Zhang,* Yu Liu* and Yue Wang
A novel organosilane compound, bis(4-(1-phenylphenanthro[9,10-d]imidazol-2-yl)phenyl)diphenylsilane
(Si(PPI)2), has been designed and synthesized. It has a high thermal decomposition temperature of
528 1C and is able to form an amorphous glass with a high glass-transition temperature of 178 1C. In
addition, it possesses high singlet and triplet energies and displays efficient energy transfer to the
selected blue fluorescent and green and red phosphorescent dopants when used as a host material.
Electrochemical measurements and single-carrier devices indicate that Si(PPI)2 is a bipolar transport
material, allowing the injection and transport of both electrons and holes. By using (Si(PPI)2) as a host,
high-performance fluorescent blue (FB) and phosphorescent green (PG) and red (PR) OLEDs with a uniform
and simple device configuration have been achieved. These OLEDs exhibit very high peak external quantum
efficiency (EQE) and peak power efficiency (PE), i.e. 6.1% and 8.0 lm Wꢀ1 for FB, 19.2% and 51.1 lm Wꢀ1 for PG
and 12.0% and 15.6 lm Wꢀ1 for PR. Moreover, the high-level EQE of 4.0, 19.1 and 10.6% and PE of 3.0, 41.6
Received 3rd February 2015,
Accepted 23rd March 2015
and 7.5 lm Wꢀ1 can be maintained by FB, PG and PR, respectively, at the practical luminance of 100 cd mꢀ2
.
DOI: 10.1039/c5tc00333d
Furthermore, the emission colors of these OLEDs remain almost unchanged within the whole range of driving
voltages. Importantly, the blue OLED displays a pure blue emission (CIE: 0.18, 0.17).
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following intersystem crossing from the conductive host into
the blue emissive triplet state represent another drawback.13
Introduction
Organic light-emitting diodes (OLEDs) have attracted considerable Considering these issues regarding the blue PhOLEDs, adopt-
scientific and industrial interest since the pioneering work by ing the stable blue fluorescent emitters to realize high-quality
Tang et al. in 1987.1 Generally, phosphorescent OLEDs (PhOLEDs), blue EL emission is a complementary effective strategy and
in which the emitting layers (EML) are normally constructed by becomes again an active research area.14–17 On the other hand,
doping the phosphors into charge-transport hosts, have much blue, green and red emitters often require different hosts to
higher electroluminescence (EL) efficiency than the conventional achieve highly efficient energy transfer from host to emitter.18–22
fluorescent OLEDs, due to the radiative harvesting of both electro- Therefore, the full-color and white OLEDs were often established
generated singlet and triplet excitons.2,3 Recently, green and red based on a complex material system and, correspondingly, high-
PhOLEDs with 100% internal quantum efficiency have been cost organic syntheses. In this sense, achieving high perfor-
reported,4–7 but high performance blue PhOLEDs have not yet mance blue, green and red electroluminescence through a
been achieved, which are subject to a sky-blue emission with the simple material system with the aim to reduce the production
Commission International de I’ Eclarirage (CIE) coordinates of cost of materials and simplify the manufacturing process is an
x and/or y Z 0.20 and their instability in the emission color and EL important issue for OLED applications. Given this, many effi-
efficiency.8–12 It has become a bottleneck in the development of cient multi-color and white PhOLEDs based on a universal host
efficient OLEDs in both flat-panel displays and solid-state lighting. have been achieved in the literature.23–26 However, the design
Furthermore, the approximate 0.5–1.0 eV exchange energy losses in and synthesis of an organic molecule that can be employed as a
power efficiency resulting from the energetic relaxation host to realize the efficient blue fluorescent OLEDs as well as
phosphorescent OLEDs with a relatively long-wavelength light,
State Key Laboratory of Supramolecular Structure and Materials,
such as green and red, has not been reported previously, and is
still a considerable challenge.
College of Chemistry, Jilin University, Changchun 130012, P. R. China.
E-mail: yuliu@jlu.edu.cn, zuolun_zhang@yahoo.com.cn
Organic derivatives and metal complexes of phenanthroimida-
zole (PPI) have been widely utilized as blue fluorescent emitters
† Electronic supplementary information (ESI) available: Photophysical properties
of compound PPI under the same conditions. See DOI: 10.1039/c5tc00333d
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and/or the host materials for highly efficient OLEDs,27–32 due to GC/MS system. Elemental analyses were performed on a flash
their high-energy emission and bipolar transport properties. Addi- EA 1112 elemental analyzer. IR spectra were recorded on a
tionally, tetraarylsilane compounds have attracted a fair amount of Bruker VERTEX 80v spectrometer. UV-vis absorption spectra
attention as organic EL materials, attributed to their wide bandgap were obtained using a Shimadzu UV-2550 UV-vis spectrometer.
and the high thermal stability resulting from their non-conjugated Photoluminescence (PL) spectra were recorded on a Perkin-
tetrahedral molecular geometry.25,33–37 In this contribution, by Elmer LS-55 fluorescence spectrometer using a Xe arc lamp
integrating one tetraphenylsilane and two PPI groups, we have excitation source. Solid state PL efficiencies were measured
prepared bis(4-(1-phenylphenanthro[9,10-d]imidazol-2-yl)phenyl)- using an integrating sphere (C-701, Labsphere Inc.), using a
diphenylsilane (Si(PPI)2) with the expectation that this compound 365 nm Ocean Optics LLS-LED as the excitation source, and the
will have good thermal stability, high-energy emission and good light was introduced into the integrating sphere through an
carrier-transport ability to work as a high-performance host optical fiber. Time-resolved fluorescence spectra were collected
material. We have found that, through doping of fluorescent on an Edinburgh mini-t fluorescence lifetime spectrometer,
blue or phosphorescent green or red emitters into Si(PPI)2, a using an Edinburgh EPL-405 picosecond pulsed diode laser as
series of host–guest systems with efficient energy transfer can be the excitation source. Electrochemical measurements were
realized. They showed very high peak external quantum efficiency performed using a BAS 100 W Bioanalytical electrochemical
(EQE) and power efficiency (PE) values of 5.6% and 8.0 lm Wꢀ1 workstation, using a Pt disk as the working electrode, platinum
for blue emission with the CIE coordinates of (0.18, 0.17), which wire as the auxiliary electrode, and a porous glass wick Ag/Ag+
are among the highest EL efficiencies ever reported for the pure as the pseudo-reference electrode, standardized against ferrocene/
blue fluorescent OLEDs that meet the standard of the CIEx,y ferrocenium. A 0.1 M solution of n-Bu4NPF6 in CH2Cl2 (for
coordinates of x r 0.20 and y r 0.20, as well as 18.8% and oxidation) or DMF (for reduction) was used as the supporting
51.1 lm Wꢀ1 for green emission and 11.9% and 15.6 lm Wꢀ1 for electrolyte. A scan rate of 100 mV sꢀ1 was adopted. All the redox
red emission. They maintain high levels of 3.6, 18.7 and potentials are given against ferrocene/ferrocenium.
10.5% and 3.0, 41.6 and 7.5 lm Wꢀ1 at the practical luminance
of 100 cd mꢀ2 respectively, and their EL emission color remains
Fabrication of the OLEDs and EL measurements
almost unchanged within the whole range of the corresponding The general architecture of the complex multilayer diodes used
driving voltages.
in this study is as follows: the ITO (indium-tin oxide) coated
We will present a comprehensive investigation on Si(PPI)2, glass substrates (20 O squareꢀ1) were pre-cleaned carefully in
which encompasses not only the conventional characterization ethanol, followed by washing in acetone in a soap ultrasonic
for its thermal, photophysical and electrochemical properties, bath and treated by UV/O3 for 2 min. The devices were prepared
but also the emphatically studies on its carrier injection/transport in a vacuum at a base pressure of around 5 ꢁ 10ꢀ6 Torr. All
abilities and EL characteristics as host material.
organics were thermally evaporated at a rate of 1.0 Å sꢀ1 at a
base pressure of around 3.5 ꢁ 10ꢀ4 Pa. A LiF layer (0.5 nm) was
deposited at a rate of 0.2 Å sꢀ1. The finishing Al electrode
(cathode) was deposited at a rate of 10 Å sꢀ1 in another
chamber. The thicknesses of the organic materials and the
cathode layers were controlled using a quartz crystal thickness
Experimental section
General information
Auxiliary materials, such as 1,4-bis[(1-naphthylphenyl) amino]- monitor. The electrical characteristics of the devices were
biphenyl (NPB), 4,40,400-tri(N-carbazolyl) triphenylamine (TCTA), measured using a Keithley 2400 source meter. The EL spectra
1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi), and the and luminance of the devices were obtained on a PR650
series of emitting fluorescent/phosphorescent materials includ- spectrometer. All the device fabrication and device character-
ing An(PPI)2,38 Ir(ppy)3,3 and (bt)2Ir(dipba)39 (Fig. 5) for fabricat- ization steps were carried out at room temperature under
ing the OLEDs were prepared according to literature procedures ambient laboratory conditions. Current–voltage characteristics
and purified further by vacuum sublimation prior to use. Start- of single-carrier devices were measured using the same semi-
ing materials for the synthesis were obtained from commercial conductor parameter analyzer as for OLED devices.
suppliers and used without further purification. Anhydrous THF
Synthesis
was distilled with sodium benzophenone ketyl under a nitrogen
atmosphere and degassed using the freeze–pump–thaw method. The synthetic route to Si(PPI)2 is shown in Scheme 1.
All glassware, syringes, magnetic stirring bars and needles were
Di(4-formylphenyl)-diphenylsilane40. n-BuLi (1.6 M in hexane,
dried in a convection oven for at least 4 hours. Reactions were 20 mL, 32 mmol) was added slowly to a THF (120 mL) solution of
monitored using thin layer chromatography (TLC). Commercial 4-bromobenzaldehyde dimethyl acetal (7.40 g, 32 mmol) at ꢀ78 1C,
TLC plates (Silica gel 60 F254, Merck Co.) were developed and and then the mixture was stirred at this temperature for 1 h. After
the spots were seen under UV light at 254 and 365 nm. Silica dichlorodiphenylsilane (1.52 mL, 12.5 mmol) was added, the
column chromatography was done using silica gel 60 G (particle reaction mixture was stirred at ꢀ78 1C for 1 h, slowly warmed to
1
size 5–40 mm, Merck Co.). H NMR spectra were recorded on a r.t., stirred for 12 h at r.t., quenched with 2 N HCl and extracted
Bruker AVANCE 300 MHz spectrometer with tetramethylsilane with ether. The organic phase was washed with brine and dried
as the internal standard. Mass spectra were measured on a over MgSO4, and then the solvent was evaporated under reduced
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Scheme 1 Synthetic procedure and structures of Si(PPI)2.
pressure. To the obtained residue, acetic acid (10 mL) and H2O
(3 mL) were added, and the obtained mixture was stirred for 3 h at
r.t., poured into saturated aqueous NaHCO3 and extracted with
ether. The extracts were washed with brine, dried over MgSO4,
filtered and concentrated under vacuum. The obtained residue was
purified using column chromatography (silica gel, hexanes/EtOAc
4 : 1) to give the product as a white solid (2.85 g, 58%). H NMR
(300 MHz; CDCl3; Me4Si): d 10.01 (s, 2H), 7.89 (d, J = 8.0 Hz, 4H),
7.74 (d, J = 8.0 Hz, 4H), 7.56–7.40 (m, 10H).
Bis(4-(1-phenylphenanthro[9,10-d]imidazol-2-yl)phenyl) diphenyl-
silane (Si(PPI)2). A mixture of 9,10-phenanthrenequinone (312 mg,
1.5 mmol), di(4-formylphenyl)-diphenylsilane (294 mg, 0.75 mmol),
aniline (0.55 mL), ammonium acetate (1.16 g, 15 mmol) and glacial
acetic acid (30 mL) was refluxed for 24 h. After being cooled to room
temperature, the reaction mixture was poured into stirred methanol.
The precipitate was filtered, washed with methanol and dried to give
the crude product, which was further purified by vacuum sublima-
tion to provide the desired compound as a white powder (414 mg,
60%). 1H NMR (300 MHz, CDCl3, ppm): d 8.89 (d, J = 7.2 Hz, 2H),
8.77 (d, J = 8.4 Hz, 2H), 8.71 (d, J = 8.1 Hz, 2H), 7.75 (t, J = 7.5 Hz, 2H),
7.69–7.61 (m, 12H), 7.56–7.43 (m, 16H), 7.37 (t, J = 7.2 Hz, 4H),
7.29–7.24 (m, 2H), 7.17–7.14 (d, J = 8.1 Hz, 2H). 13C NMR (75 MHz,
CDCl3, ppm): d 120.87, 122.84, 123.07, 124.09, 124.93, 125.66,
126.25, 127.15, 127.27, 127.91, 128.15, 128.33, 128.38, 128.48,
128.60, 128.67, 128.88, 129.07, 129.11, 129.38, 129.42, 129.53,
129.74, 129.95, 130.09, 130.20, 133.03, 133.53, 136.19, 136.38,
137.96, 138.71, 138.77. Ms m/z: 919.9 [M]+ (calcd: 920.3). Anal. calcd
(%) for C66H44N4Si: C, 86.05; H, 4.81; N, 6.08. Found: C, 86.38; H,
4.73; N, 6.05. IR (KBr, n, cmꢀ1): n = 3092, 3062, 3010, 1599, 1486,
1455, 1425, 1378, 1296, 1234, 1183, 1152, 1106, 1019, 957, 839, 804,
752, 696, 568, 506.
Fig. 1 TGA thermogram of Si(PPI)2. Inset: DSC thermogram.
device operation. According to the DSC analysis, Si(PPI)2 has a high
melting point of 385 1C (Fig. 1, inset). More importantly, it has the
ability to form an amorphous glass with a high glass-transition
temperature of 178 1C during the second-heating scan, which is
beneficial for the formation of homogeneous and amorphous films
upon thermal evaporation, as well as to keep the stability of the
film and to decrease the phase separation of the host–guest system
when Si(PPI)2 is used as a host material.
1
Photophysical properties
Fig. 2a displays the UV-vis absorption and photoluminescence
(PL) spectra of Si(PPI)2 in dichloromethane solution (5 ꢁ 10ꢀ5 M)
at room temperature (298 K) and its phosphorescence spectrum
in frozen 2-MeTHF (5 ꢁ 10ꢀ5 M) at 77 K. Si(PPI)2 exhibits intense
deep-blue emissions in dichloromethane solution as well as in
the solid film. An(PPI)2 is considered to be an appropriate blue
dopant for our system because of its preferred pure-blue fluores-
cence and, importantly, a large overlap between its absorption
spectrum and the solid-state emission spectrum of Si(PPI)2,
which may lead to efficient energy transfer from Si(PPI)2 to
An(PPI)2. In the low-temperature (77 K) emission spectrum of
Si(PPI)2 (Fig. 2a), a phosphorescence band showing vibrational
structures can be observed. By using the highest-energy vibronic
sub-band, the triplet energy (ET) of Si(PPI)2 is estimated to be
2.55 eV, which is clearly lower than that of the classical blue
phosphor bis[2-(4,6-difluorophenyl)pyridinato-N,C20](picolinato)-
iridium(III) (FIrpic) (B2.65 eV)12 and is not high enough to
sensitize this blue phosphor, leading to incomplete host–dopant
energy transfer accompanied by the poor EL performance
(see the ESI†). However, this ET level of Si(PPI)2 is sufficiently
Results and discussion
Thermal properties
The thermal properties of Si(PPI)2 were investigated using higher than those of most green and red phosphorescence
thermogravimetric analysis (TGA) and differential scanning emitters. This result suggests that, besides the singlet–singlet
¨
calorimetry (DSC) under a nitrogen atmosphere at a heating (Forster and/or Dexter) energy transfer, a triplet–triplet Dexter
rate of 10 1C minꢀ1. The compound exhibits very good thermal energy transfer can be expected when doping appropriate green
stability, as evidenced by its high decomposition temperature and red emitters into Si(PPI)2. The classical green phosphor
(corresponding to 5% weight loss) of 528 1C in the TGA Ir(ppy)3 and a high-performance red phosphor (bt)2Ir(dipba)
thermogram (Fig. 1). This will prevent the compound from were selected as the phosphorescent dopants in this study.
decomposing during the processes of vacuum deposition and The PL spectra of the thin films prepared by doping respectively
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and LUMO energy levels were estimated to be ca. ꢀ5.6 eV and
ꢀ2.2 eV respectively from the onsets of the oxidation and
reduction potentials with regard to the energy level of ferrocene
(ꢀ4.8 eV41 below vacuum). This HOMO level is much higher than
those (Bꢀ7.0 eV) of previously reported tetraarylsilane-based series
of UGH host materials,33 reflecting a better hole-injection property
of Si(PPI)2 due to the derivatization by the PPI group. Moreover, the
HOMO level of Si(PPI)2 is even higher than that of the classic
fluorescent host material 4,40-N,N0-dicarbazolylbiphenyl (CBP)
(Bꢀ6.0 eV) and matches well with the most widely used hole-
transporting material NPB (Bꢀ5.4 eV) and electron-blocking
material TCTA (ꢀ5.7 eV), implying little or no hole-injection barrier
from NPB or TCTA to this compound.
To gain more insight into the electronic structure of Si(PPI)2,
DFT calculations (B3LYP/6-31G(d)) were carried out. The HOMO
Fig. 2 (a) UV-vis absorption and PL spectra of Si(PPI)2 (H) and An(PPI)2
(B) measured in dichloromethane solution (5 ꢁ 10ꢀ5 M) at room tempera-
ture and the phosphorescence spectrum of Si(PPI)2 in 2-MeTHF solution
(5 ꢁ 10ꢀ5 M) at 77 K. (b) PL spectra of the neat films of Si(PPI)2 (H), blue
fluorophore (B), green (G) and red (R) phosphors and these dopant
emitters doped in Si(PPI)2 (H) thin films at 10 wt% doping concentration.
10 wt% of the selected blue, green and red dopants into Si(PPI)2
are measured to confirm the efficient energy transfer. As can be
seen from Fig. 2b, all these doped films show an emission very
close to those of the neat-films of the respective dopants, while
the emission of Si(PPI)2 cannot be observed. They exhibit the PL
quantum efficiency values of 8 ꢂ 1%, 96 ꢂ 3% and 24 ꢂ 2%
respectively, and their corresponding transient PL decay curves
show almost single-exponential decay with the lifetimes of 3 ns,
0.97 ms and 0.71 ms at room temperature (see ESI† file), indicat-
ing the complete energy transfer from the host (Si(PPI)2) to the
three dopants. Therefore, these host–dopant systems are
expected to have the potential to realize high-performance OLED
devices. The slightly red-shifted emission from the doped films
to the neat films of corresponding dopants should be related to
the tighter packing between the dopant molecules in the neat films.
Electrochemical properties and theoretical calculations
Fig. 3 (a) Cyclic voltammogram of Si(PPI)2 in CH2Cl2 for oxidation and
DMF for reduction. (b) Contour plots of HOMO and LUMO in Si(PPI)2 using
DFT calculations.
Cyclic voltammetry (CV) was performed to investigate the
electrochemical properties of Si(PPI)2 (Fig. 3a). The HOMO
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is mainly distributed on one of the two phenanthroimidazole Characterization of fluorescent/phosphorescent OLEDs
groups, with a small contribution from the C6H4 group attached
According to the above studies, Si(PPI)2 could be a good host
to it while the LUMO is localized partly on the other phenan-
material for both fluorescent and phosphorescent OLEDs. To
throimidazole group and mainly on the C6H4 group between
evaluate its practical utility, a series of OLEDs with a uniform
this phenanthroimidazole group and the Si atom (Fig. 3b).
and simple configuration of [ITO/NPB (30 nm)/TCTA (5 nm)/
The almost complete spatial separation of HOMO and LUMO
EML (25 nm)/TPBi (20 nm)/LiF (1 nm)/Al (100 nm)] were
energy levels suggests that the HOMO–LUMO excitation would
fabricated, and the energy diagram of the materials used in
shift the electron density distribution from one side of the
the EL devices is shown in Fig. 5. NPB was used as the hole-
Si(PPI)2 molecule as the donor to the other side as the acceptor,
transporting material (HTL), TCTA was used as the triplet
leading to a polarized excited state. It is more important that
exciton-blocking layer in the PhOLEDs, and TPBi was used as
such separation of HOMO and LUMO can provide hole- and
the electron transport/hole-blocking layer (ETL/HBL). The films
electron-transporting channels respectively, where holes and
of An(PPI)2, Ir(ppy)3 and (bt)2Ir(dipba) doped in Si(PPI)2 at the
electrons can realize intermolecular hopping smoothly along
concentration of 10 wt% were adopted as the EMLs to fabricate
their respective conducting pathways. Thus, this is likely to
the fluorescent blue device FB, the phosphorescent green and
result in this compound possessing bipolar charge transporting
red devices PG and PR respectively. The device FB exhibits pure
ability.42–46
blue emissions (CIE: 0.18, 0.17) with the emission maximum at
452 nm at the luminance of 1000 cd mꢀ2 (Fig. 6), which
Charge carrier injection and transport properties
remains almost unchanged over a wide range of driving vol-
tages from 3.5 V to 12 V. The EL spectrum is consistent with the
PL spectrum of An(PPI)2, suggesting that the blue EL emission
results from the intrinsic emission of An(PPI)2. On the other
hand, the devices PG and PR present bright green and saturated
red emissions with the maximum peaks at 516 and 612 nm and
CIEx,y coordinates of (0.32, 0.61) and (0.64, 0.36) respectively at
the luminance of 1000 cd mꢀ2, and no additional emission
from the host was observed.
The current density–voltage–luminance (J–V–L) characteris-
tics of these OLEDs are shown in Fig. 7a. All these devices
displayed low turn-on voltages of 3.1–3.4 V (Table 1), and the
corresponding current density and luminance exhibited sus-
tained increase with increasing the driving voltage. The driving
voltages at the practical luminance of 100 cd mꢀ2 are 5.6 V, 4.7
and 6.2 V, respectively. At driving voltages of 12.6, 9.1 and
13.5 V for FB, PG and PR respectively, very high luminance
values of 410 000 cd mꢀ2 were obtained. The quickly increas-
ing luminance and current density indicate a good carrier
To further understand both hole and electron injection/
transport properties of Si(PPI)2, two single-carrier devices were
fabricated using the following configurations: ITO/NPB (10 nm)/
Si(PPI)2 (30 nm)/NPB (10 nm)/Al (100 nm) (hole-only device)
and ITO/TPBi (10 nm)/Si(PPI)2 (30 nm)/TPBi (10 nm)/LiF
(1 nm)/Al (100 nm) (electron-only device). NPB and TPBi layers
are used to prevent electron and hole injection from the
cathode and anode, respectively.47–49 Fig. 4 shows the current
density versus voltage curves of both devices, respectively. It is
obvious that both devices can conduct significant current,
indicating that Si(PPI)2 is capable of transporting both elec-
trons and holes, and exhibits its bipolar transporting nature,
which is effective for balancing holes and electrons in the
emitting layer (EML) based on Si(PPI)2. The substantially lower
electron current density than the hole curve at the same voltage
level should be attributed to the high-lying LUMO energy level
of Si(PPI)2, which means a certain injection barrier (B0.5 eV)
from ETL.32
Fig. 4 Current density versus voltage characteristics of the hole-only and
Fig. 5 Energy level diagram of the materials used and the chemical structures
electron-only devices for Si(PPI)2.
of the emitting materials.
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Table 1 Electroluminescence properties of the devicesa
Lmax/cd mꢀ2
Device Von/V (V at Lmax
PEb/
EL emission
)
EQEb/%
lm Wꢀ1
peak, CIE (x,y)c
FB
PG
PR
3.1
3.4
3.3
15 000
(13.5)
60 470
(12.5)
12 330
(14.0)
6.1, 4.0, 1.9 8.0, 3.0,
0.9
19.2, 19.1, 51.1, 41.6, 516,
17.8 26.5 (0.32, 0.61)
12.0, 10.6, 15.6, 7.5, 612,
7.3 3.3 (0.64, 0.36)
452,
(0.18, 0.17)
a
Abbreviation: Von: turn-on voltage (recorded at 1 cd mꢀ2). Lmax: maximum
luminance. EQE: external quantum efficiency. PE: power efficiency. b In the
order of maximum, then values at 100 and 1000 cd mꢀ2
.
Measured
c
at 1000 cd mꢀ2
.
power efficiency (PE) of these devices plotted with respect to the
luminance are shown in Fig. 7b, and the EL performance data
are summarized in Table 1. The blue device FB exhibited the
maximum EQE and PE of 6.1% and 8.0 lm Wꢀ1 respectively. To
the best of our knowledge, these values are among the highest
EL efficiencies ever reported for the pure blue fluorescent
OLEDs that meet the standard of the CIEx,y coordinates of
x r 0.20 and y r 0.20.50–56 Notably, the EQE and PE values of
FB at the brightness of 100 cd mꢀ2, which is the practical level
used in the fields of display or lighting, remain as high as
4.0% and 3.0 lm Wꢀ1 respectively. They are much higher than
those of the reference devices adopting the conventional host
materials CBP or N,N0-dicarbazolyl-3,5-benzene (mCP) with the
low peak EQE and PE o 1% and 2 lm Wꢀ1 (see the ESI† file).
Furthermore, the green and red PhOLEDs PG and PR exhibited
rather high peak EL efficiency values of 19.2, 12.0% for EQE
and 51.1, 15.6 lm Wꢀ1 for PE. Although these EL efficiencies
showed a certain roll-off, the EQE values maintained the levels
Fig. 6 EL spectra of devices FB, PG and PR at the luminance of 1000 cd mꢀ2
.
of 17.8% for PG and 7.3% for PR at a luminance of 1000 cd mꢀ2
,
and were as high as 14.2, 4.6% even when the luminance
reached as high as 10 000 cd mꢀ2. Obviously, in terms of the
performance, these two devices are comparable with the highly
efficient green and red PhOLEDs reported to date.4–7 Such high
and stable EL performance should be ascribed to the balanced
carrier injection/transportability of Si(PPI)2 as implied in
single-carrier devices, which result in a broad distribution of
recombination regions within the corresponding EMLs,57,58
and correspondingly, a low probability of triplet–triplet anni-
hilation that usually causes an efficiency roll-off at high current
densities for the PhOLEDs. Charge confinement in the EML is
another key factor for the high EL efficiency. There is a large
energy barrier of 0.6 eV for hole leakage from Si(PPI)2 to TPBi
together with the unipolar property of NPB for electron leakage
from Si(PPI)2 to NPB, respectively. Therefore, holes and elec-
trons can be effectively confined inside the EML, being crucial
in achieving high efficiency and low roll-off OLEDs. Besides, on
the premise that the ET (B2.55 eV) of (Si(PPI)2) is high enough
for working as a host, but not much higher than those of the
green (Ir(ppy)3, ET: B2.4 eV) and red ((bt)2Ir(dipba), ET: B2.1 eV)
phosphorescent dopants, the energy loss during the host-to-
Fig. 7 Current density–voltage–luminance (J–V–L) curves (a) and power
efficiency (PE)–luminance–external quantum efficiency (EQE) curves (b) of
devices FB, PG and PR.
injection and transport property of the host (Si(PPI)2), which is dopant energy transfer process can be reduced as far as possible.
consistent with our results obtained from the single-carrier It is notable that the excellent performances of all our blue, green
devices given above. The external quantum efficiency (EQE) and and red OLEDs were obtained from the same device configuration
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7 T. Peng, G. M. Li, K. Q. Ye, C. G. Wang, S. S. Zhao, Y. Liu,
Z. M. Hou and Y. Wang, J. Mater. Chem. C, 2013, 1, 2920.
8 F.-M. Hsu, C.-H. Chien, C.-F. Shu, C.-H. Lai, C.-C. Hsieh,
K.-W. Wang and P.-T. Chou, Adv. Funct. Mater., 2009, 19, 2834.
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through adopting the same host material. The simple material
system and the easily controlled fabrication process are of
significance and importance for reducing the cost and enhancing
the process stability in commercial mass production.
Conclusions
In summary, we have developed a novel tetraphenylsilane–
phenanthroimidazole hybrid compound, Si(PPI)2, which can
act as a host that realizes the highly efficient blue fluorescent
OLEDs as well as green and red phosphorescent OLEDs by
adopting a uniform and simple device configuration. The pure-
blue device with the CIE coordinates of (0.18, 0.17) exhibited
13 Q. Wang, C.-L. Ho, Y. B. Zhao, D. G. Ma, W.-Y. Wong and
L. X. Wang, Org. Electron., 2010, 11, 238.
14 T. Peng, K. Q. Ye, Y. Liu, L. Wang, Y. Wu and Y. Wang, Org.
Electron., 2011, 12, 1914.
the maximum EQE and PE values of 6.1% and 8.0 lm Wꢀ1
,
respectively, which are among the highest EL efficiencies ever
reported for pure-blue devices. Furthermore, the EL perfor-
mance of our green and red phosphorescent devices is also
comparable with that of the most efficient green and red
phosphorescent OLEDs reported to date. The high device
performance is due to the high thermal stability, wide bandgap,
efficient energy transfer and the balanced carrier-transport
abilities of Si(PPI)2. Our results indicate that Si(PPI)2 is a rare
and excellent common host, which may have abilities to simplify
the fabrication process and to reduce the material cost in com-
mercial mass production of high-performance multi-color OLEDs.
Moreover, Si(PPI)2 will also enable us to design the structurally
simple three-primary-color fluorescent/phosphorescent hybrid
white OLEDs, which are important and attractive for full-color
display and white lighting. Further application of Si(PPI)2 in white
OLEDs is ongoing in our lab.
15 T. Peng, G. F. Li, Y. Liu, Y. Wu, K. Q. Ye, D. D. Yao, Y. Yuan,
Z. M. Hou and Y. Wang, Org. Electron., 2011, 12, 1068.
16 Y. Zhang, S. L. Lai, Q. X. Tong, M. F. Lo, T. W. Ng, M. Y. Chan,
Z. C. Wen, J. He, K. S. Jeff, X. L. Tang, W. M. Liu, C. C. Ko,
P. F. Wang and C. S. Lee, Chem. Mater., 2012, 24, 61.
17 W. Li, D. Liu, F. Shen, D. Ma, Z. Wang, T. Feng, Y. Xu,
B. Yang and Y. Ma, Adv. Funct. Mater., 2012, 22, 2797.
18 Z. Q. Gao, M. M. Luo, X. H. Sun, H. L. Tam, M. S. Wong,
B. X. Mi, P. F. Xia, K. W. Cheah and C. H. Chen, Adv. Mater.,
2009, 21, 688.
19 H. Fukagawa, N. Yokoyama, S. Irisa and S. Tokito, Adv.
Mater., 2010, 22, 4775.
20 Z. Jiang, T. Ye, C. Yang, D. Yang, M. Zhu, C. Zhong, J. Qin
and D. Ma, Chem. Mater., 2011, 23, 771.
21 K. S. Yook and J. Y. Lee, Adv. Mater., 2012, 24, 3169.
22 S. Y. Shao, J. Q. Ding, T. L. Ye, Z. Y. Xie, L. X. Wang,
X. B. Jing and F. S. Wang, Adv. Mater., 2011, 23, 3570.
23 Q. Wang, J. Q. Ding, D. G. Ma, Y. X. Cheng, L. X. Wang,
X. B. Jing and F. S. Wang, Adv. Funct. Mater., 2009, 19, 84.
24 H.-H. Chou and C.-H. Cheng, Adv. Mater., 2010, 22, 2468.
25 S. Gong, Y. Chen, J. Luo, C. Yang, C. Zhong, J. Qin and
D. Ma, Adv. Funct. Mater., 2011, 21, 1168.
Acknowledgements
Dr D. Liu and M. Du contributed equally to the work reported in
this article. This work was supported by the National Basic
Research Program of China (2013CB834805), the National
Natural Science Foundation of China (51173064, 91333201
and 51373062) and the Program for Chang Jiang Scholars and
Innovative Research Team in University (No. IRT13018).
26 Y.-L. Chang, S. Yin, Z. Wang, M. G. Helander, J. Qiu, L. Chai,
Z. Liu, G. D. Scholes and Z. Lu, Adv. Funct. Mater., 2013,
23, 705.
27 Y. Yuan, D. Li, X. Zhang, X. Zhao, Y. Liu, J. Zhang and
Y. Wang, New J. Chem., 2011, 35, 1534.
28 Y. Zhang, S. L. Lai, Q. X. Tong, M. F. Lo, T. W. Ng,
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