High efficiency nondoped deep-blue organic light emitting devices based on imidazole-π-triphenylamine derivatives

Article abstract of DOI:10.1021/cm201789u

High-performance deep-blue emitting phenanthroimidazole derivatives with a structure of donor-linker-acceptor were designed and synthesized. By using different linkers and different linking positions, four deep-blue emitters were obtained and used as emit

Full text of DOI:10.1021/cm201789u

Article
pubs.acs.org/cm
High Efficiency Nondoped Deep-Blue Organic Light Emitting Devices
Based on Imidazole-π-triphenylamine Derivatives
Ying Zhang,†,‡ Shiu-Lun Lai,‡ Qing-Xiao Tong,†,§,* Ming-Fai Lo,‡ Tsz-Wai Ng,‡ Mei-Yee Chan,∥
Zhi-Chun Wen,† Jun He,⊥ Kc-Sham Jeff,⊥ Xiang-Lin Tang,# Wei-Min Liu,# Chi-Chiu Ko,⊥
Peng-Fei Wang,# and Chun-Sing Lee‡,*
†Department of Chemistry, Shantou University, Guangdong 515063, China
‡Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science and ⊥Department of
Biology and Chemistry, City University of Hong Kong, Hong Kong SAR, China
§Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, China
∥Department of Chemistry, The University of Hong Kong, Hong Kong SAR, China
#Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
S
* Supporting Information
ABSTRACT: High-performance deep-blue emitting phenanthroimidazole deriva-
tives with a structure of donor−linker−acceptor were designed and synthesized. By
using different linkers and different linking positions, four deep-blue emitters were
obtained and used as emitters or bifunctional hole-transporting emitters in OLEDs.
Such devices show low turn-on voltages (as low as 2.8 V), high efficiency (2.63 cd/A,
2.53 lm/W, 3.08%), little efficiency roll-off at high current densities, and stable deep-
blue emissions with CIEy < 0.10. Performances are among the best comparing to
recently reported deep-blue emitting devices with similar structures. The results
suggest that the combination of the phenanthroimidazole and the donor−linker−
acceptor structure can be an important approach for developing high performance
deep-blue emitters in particular for lighting applications.
KEYWORDS: high efficiency, nondoped, deep-blue organic light emitting devices
electron-withdrawing moiety for easier electron injection.
However, these materials typically also have deep highest
occupied molecular orbital (HOMO) leading to larger hole−
injection barriers at the hole-transporter/emitter junctions and
thus higher operation voltages as well as lower efficiencies.6,8−18
In fact, deep-blue OLEDs with high efficiency, good color
stability, and low onset/working voltage are rare.
Charge transport and balancing are key factors for obtaining
high device efficiency. Recently some deep-blue emitters were
realized by simultaneously incorporating donor and acceptor
groups into one molecule and proved to be a promising
method for obtaining a high performance deep-blue emit-
ter.7,20−23 In this work, we use the donor−π-acceptor approach
to design three new molecules by using phenanthroimidazole as
the acceptor for its good electron-transporting mobility and
triphenylamine (TPA) as the donor for its good hole transport
mobility and connecting them with different linkers (benzene
or thiophene); TPA-TPI was used here for comparison. By
attaching TPA to different position (1- or 2-) of the imidazole
INTRODUCTION
■
Blue organic light-emitting devices (OLEDs) have attracted
much attention for their importance in both full-color display
and solid-state lighting.1 Given that phosphorescent OLEDs
can theoretically achieve 100% internal quantum efficiency,
many researchers have reported phosphorescent materials with
high efficiency deep-blue emission.2 However, phosphores-
cence OLEDs typically have shorter lifetime and sharper
efficiency roll-off at high brightness. Thus, phosphorescence
and fluorescence OLEDs are often considered to be
complementary and suitable for different applications. On the
other hand, many high performance OLEDs involve doping of
emitters into host materials.3 However, efficacious doping often
requires precise control of doping concentration and inevitably
increases manufacturing cost. It has also been pointed out that
potential phase separation in the dopant−host system can
render energy transfer ineffective.4 For these reasons, non-
doped devices using blue light-emitting fluorescent OLEDs are
still attracting considerable attention.
While many high efficiency blue fluorescent materials have
been reported, many of them give sky-blue emissions instead of
saturated blue with CIEy < 0.10.5 On the other hand, many
saturated blue emitters were designed with incorporation of the
Received: June 23, 2011
Revised: November 29, 2011
Published: November 29, 2011
© 2011 American Chemical Society
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Scheme 1. Synthetic Routes for the TPA−Imidazole Derivatives
ring (acceptor) with different linkers, we investigated their
differences in photophysical properties and effects on orbital
energy levels. The new emitters were found to give deep-blue
emissions with fluorescent quantum yield as high as unity. In
addition, the compounds also have good hole-transporting
properties. OLEDs using these emitters were observed to
deliver high efficiencies of 2.63 cd/A and 2.53 lm/W at deep-
blue color with CIEy coordinate smaller than 0.10. In addition
to low turn-on (e.g., 2.8 V) and operation voltages, the present
devices also show stable emission colors and little efficiency
roll-off at high brightness output. In terms of these performance
parameters, some of the new compounds described here are
among the best nondoped deep-blue emitters reported recently.
EXPERIMENT AND CHARACTERIZATION
■
All solvents and materials were used as received from commercial
suppliers without further purification. Synthetic routes of the four
compounds are outlined in Scheme 1. 2-(4-Bromophenyl)-1-(4-tert-
butylphenyl)-1H-phenanthro[9,10-d] imidazole (1), 2-(5-bromothio-
phen-2-yl)-1-(4-tert-butyl phenyl)-1H-phenanthro[9,10-d]imidazole
(2), 1-(4-bromophenyl)-2-(4-tert-butylphenyl)-1H-phenanthro[9,10-
d]imidazole (3), and BPA-BPI were prepared according to the
literature.24 Compounds TPA-BPI, TPA-TPI, and PATPA were
prepared through Suzuki coupling reactions.25 Other than the three
new compounds, a reported imidazole-π-triphenylamine derivative
TPA-TPI was also used here for comparison.27
Nuclear magnetic resonance (NMR) spectra were recorded using
CD2Cl2 as solvent with a Varian Gemin-400 or Varian Gemin-300
spectrometer. Mass spectra were recorded on a PE SCIEX API-MS
spectrometer. Single crystal structures of TPA-BPI and BPA-BPI were
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further studied by X-ray diffraction with a Gemini A Ultra X-ray single
crystal diffractometer using graphite monochromatized Mo Kα
radiation (Mo Kα, λ = 0.71073 Å). The crystallographic calculations
were conducted using the SHELXL-97 program. Absorption and
photoluminescence (PL) spectra of the materials were recorded on a
Perkin-Elmer Lambda 2S UV−visible spectrophotometer and Perkin-
Elmer LS50 fluorescence spectrometer, respectively. Fluorescence
quantum yields (Φf) in dichloromethane solution were determined
according to the previous literature26 by a comparative method, using
anthracene as a standard reference with Φf = 0.27 in ethanol.
Thermogravimetric (TGA) measurements were performed on a TA
Instrument TGAQ50 at a heating rate of 10 °C/min under a nitrogen
environment. Differential scanning calorimetric (DSC) measurements
were performed on a TA Instrument DSC2910. The samples were first
heated at a rate of 10 °C/min to melt and then quenched. The glass
transition temperature (Tg) and crystallization temperature (Tc) were
recorded by heating the quenched samples at a heating rate of 10 °C/
min. Ionization potential (IP) of the materials were measured on ITO
glass substrates in a thin-film state via ultraviolet photoelectron
spectroscopy (UPS) in a VG ESCALAB 220i-XL surface analysis
system, while the values of electron affinity (EA) were estimated by
subtracting from IP with the optical band gap (Egap) determined from
the absorption spectra of their solid-state films. Theoretical
calculations of the distributions of HOMO and lowest unoccupied
molecular orbital (LUMO) were performed with the method of
B3LYP/6-31G (d).
was finally obtained. Yield: 2.81 g (88%). mp: 272 °C.1H NMR (400
MHz; CD2Cl2; Me4Si) δH [ppm]: 6.93 (d, J = 8.8 Hz, 2H), 7.06−7.22
(m, 7H), 7.31 (t, J = 7.9 Hz, 5H), 7.43−7.72 (m, 9H), 7.73−7.80 (m,
1H), 8.78 (dd, J = 21.5, 7.9 Hz, 3H). δC (75 MHz, CD2Cl2) 120.99
(s), 121.62 (s), 122.66 (s), 123.34 (s), 123.66−124.92 (m), 124.85 (s),
124.92−125.05 (m), 125.46 (d, J = 18.9 Hz), 126.45 (s), 127.44 (d, J
= 7.9 Hz), 128.31 (s), 128.80−129.92 (m), 129.68−129.92 (m),
130.05 (d, J = 8.1 Hz), 130.38 (s), 139.27 (s), 147.38 (s), 148.54 (s).
MS (ESI+): m/z 538.4 (MH+). Calcd for C39H27N3: 537.22.
N-(4-(4-(1-(4-tert-Butylphenyl)-1H-phenanthro[9,10-d]-
imidazol-2-yl)phenyl-1)phenyl)-N-phenylbenzenamine (TPA-
BPI). A solution of compound 1 (2.38 g, 4.7 mmol), 4-
(diphenylamino)phenylboronic acid (1.74 g, 7.5 mmol), Pd(PPh3)4
(0.29 g, 0.25 mmol), and aqueous Na2CO3 (2 M, 18 mL) in toluene
(36 mL) and ethanol (12 mL) was heated to reflux in an argon
atmosphere for 24 h. The solution was cooled to room temperature
and extracted with dichloromethane. The extracts were dried with
anhydrous Na2SO4 and concentrated by rotary evaporation. The
residue was purified by column chromatography (petroleum ether:
CH2Cl2, 1:1) to obtain the pure product as white powder. Yield: 2.15 g
(68%). mp: 285 °C. 1H NMR (400 MHz; CD2Cl2; Me4Si) δH [ppm]:
1.48 (s, 9H), 7.11 (ddd, J = 24.5, 11.1, 4.8 Hz, 8H), 7.29 (dt, J = 18.6,
8.3 Hz, 6H), 7.47−7.59 (m, 7H), 7.64−7.82 (m, 6H), 8.76 (d, J = 8.1
Hz, 1H),8.79−8.86 (m, 2H). δC (75 MHz; CD2Cl2; Me4Si) δ31.34 (s),
35.14 (s), 121.17 (s), 122.67 (s), 123.38 (d), 123.76 (s), 124.18 (s),
124.87 (d), 125.65 (s), 126.48(s), 127.10−127.86 (m), 128.35 (s),
128.71 (s), 128.73−129.61 (m), 129.76 (s), 133.96 (s), 136.31 (s),
137.52 (s), 140.75 (s), 147.80 (d, J = 3.9 Hz), 150.81 (s),153.72 (s).
MS (ESI+): m/z 670.6 (MH+). Calcd for C49H39N3: 669.31.
N-(4-(4-(2-(4-tert-Butylphenyl)-1H-phenanthro[9,10-d]-
imidazol-1-yl)phenyl-1)phenyl)-N-phenylbenzenamine
(PATPA). Using a similar approach for TPA-BPI by Suzuki coupling
reaction, white powder was finally obtained. Yield: 2.54 g (76%). mp:
306 °C. 1H NMR (300 MHz; CD2Cl2; Me4Si) δH [ppm]: 1.31 (s, 9H),
7.06−7.23 (m, 7H), 7.29−7.39 (m, 8H), 7.53 (ddd, J = 8.4, 5.8, 2.5
Hz, 1H), 7.61 (dd, J = 8.5, 2.1 Hz, 4H), 7.69 (dd, J = 8.7, 2.0 Hz, 3H),
7.72−7.80 (m, 2H), 7.87 (d, J = 8.6 Hz, 2H), 8.72−8.82 (m, 3H). δC
(75 MHz, CD2Cl2) 31.10 (s), 121.11 (d), 122.70 (d), 123.48 (dd),
124.00−124.62 (m), 124.78−125.71 (m), 126.16 (d), 126.64 (s),
127.48 (d), 128.22 (dd), 129.04−129.43 (m), 129.60 (d), 130.23 (d),
130.84 (s), 131.66 (d), 132.44 (s), 133.10 (s), 137.56 (d), 139.04 (s),
141.12 (s), 142.05 (s), 147.71 (s), 148.39 (d), 151.13 (s), 152.26 (s).
TOF-MS (EI+): Found: 669.27 (M+). Calcd for C49H39N3: 669.31.
N-(4-(5-(1-(4-tert-Butylphenyl)-1H-phenanthro[9,10-d]-
imidazol-2-yl)thiophen-2-yl)phenyl)-N-phenylbenzenamine
(TPA-TPI). TPA-TPI was synthesized using an approach similar to that
in our previous work.27Yield: 2.3 g (99%). mp: 276 °C. 1H NMR (400
MHz; CD2Cl2; Me4Si) δH [ppm]: 1.53 (s, 9H), 6.64 (d, J = 4.0 Hz,
1H), 7.02−7.17 (m, 9H), 7.23 (d, J = 7.3 Hz, 1H), 7.27−7.37 (m,
5H), 7.42−7.65 (m, 5H), 7.69 (dd, J = 11.1, 4.2 Hz, 1H), 7.77 (t, J =
7.4 Hz, 3H), 8.61−8.93 (m, 3H); δC (50 MHz; CDCl3; Me4Si) 31.52
(s), 35.19 (s), 120.64 (s), 122.51 (s), 123.17 (dd), 124.08 (s), 124.74
(d), 125.60 (s), 126.41 (d), 127.09 (d), 127.80 (d), 127.50 (s), 128.29
(d), 128.70 (s), 129.15 (s), 129.38 (s), 131.41 (s), 135.68 (s), 137.59
(s), 145.78 (s), 146.12 (s), 147.40 (s), 147.61 (s), 154.15 (s). MS
(ESI+): m/z 676.6 (MH+). Calcd for C47H37N3S: 675.27.
SYNTHESIS
■
2-(4-Bromophenyl)-1-(4-tert-butylphenyl)-1H-phenanthro-
[9,10-d]imidazole (1). The product was prepared by refluxing 9,10-
phenanthrenequinone (2.12 g, 10 mmol), 4-bromobenzaldehyde (1.86
g, 10 mmol), 4-tert-butylbenzenamine (1.92 mL, 12 mmol), and
ammonium acetate (9.49 g, 122.3 mmol) in glacial acetic acid (50 mL)
for 24 h under an argon atmosphere. After cooling to room
temperature, a pale yellow mixture was obtained and poured into a
methanol solution under stirring. The separated solid was filtered off,
washed with methanol, and dried to give a pale yellow solid. The solid
was purified by column chromatography (petroleum ether: CH2Cl2,
1:1) on silica gel. A white powder was finally obtained after it was
stirred in refluxing ethanol, subsequently filtered, and dried in vacuum.
Yield: 4.03 g (79%). 1H NMR (300 MHz; CD2Cl2; Me4Si) δH [ppm]:
1.47 (s, 9H), 7.16−7.83 (m, 13H), 8.76 (dd, J = 15.2, 8.2 Hz, 3H). δC
(75 MHz; CD2Cl2; Me4Si) 31.32 (s), 35.15 (s), 121.18 (s), 122.62 (s),
123.30 (d), 124.20 (s), 125.16 (s), 125.77 (s), 126.53 (s), 127.46 (d),
128.49 (d), 129.33 (s), 130.05 (s), 130.91 (s), 131.47 (s), 135.98 (s),
137.46 (s), 149.88 (s), 153.90 (s). MS (ESI+): m/z 505.5 (MH+).
Calcd for C31H25BrN2: 504.12.
2-(5-Bromothiophen-2-yl)-1-(4-tert-butylphenyl)-1H-
phenanthro[9,10-d]imidazole (2). The synthesis procedure is
according to our previous work.27 Yield: 8 g (78%). H NMR (400
1
MHz; CD2Cl2; Me4Si) δH [ppm]: 1.52 (s, 9H), 6.50 (d, J = 4 Hz, 1H),
6.89 (d, J = 4 Hz, 1H), 7.20 (dd, J = 8.3, 0.8 Hz, 1H), 7.25−7.34 (m,
1H), 7.49−7.59 (m, 3H), 7.64−7.81 (m, 4H), 8.70−8.81 (m, 3H). MS
(ESI+): m/z 511.4 (MH+). Calcd for C29H23BrN2S: 510.08.
1-(4-Bromophenyl)-2-(4-tert-butylphenyl)-1H-phenanthro-
[9,10-d]imidazole (3). Using a similar approach for 1 by replacing 4-
tert-butylbenzenamine and 4-bromobenzaldehyde with 4-bromoben-
zenamine and 4-tert-butylbenzaldehyde, a white powder was finally
obtained. Yield: 5.08 g (67%).1H NMR (400 MHz; CD2Cl2; Me4Si)
δH [ppm]: 1.33 (s, 9H), 7.23 (d, J = 8.2 Hz, 1H), 7.30−7.59 (m, 8H),
7.63−7.82 (m, 4H), 8.77 (dd, J = 17.3, 8.1 Hz, 3H). δC (75 MHz,
CD2Cl2) 31.10 (s), 34.81 (s), 72.35 (s), 120.92 (s), 122.70 (s), 123.26
(d), 123.96 (s), 124.32 (s), 125.09 (s), 125.48 (s), 125.77 (s), 126.63
(s), 127.30−128.36 (m), 128.37 (s), 129.18 (d), 131.05 (s), 133.63
(s), 137.94 (d), 138.38−138.75 (m), 151.08 (s), 152.46 (s). MS
(ESI+): m/z 505.1 (MH+). Calcd for C31H25BrN2: 504.12.
RESULTS AND DISCUSSION
■
X-ray Crystal Structures. For the molecule structure
determination, single crystals of TPA-BPI and BPA-BPI were
grown by layering of n-hexane onto a dichloromethane solution
of the compounds. Molecule structures of BPA-BPI and TPA-
BPI (CCDC numbers are 840393 and 840394, respectively) are
depicted in Figure 1. The benzene ring/t-Bu-benzene ring was
highly twisted about the phenanthroimidazole plane with
dihedral angles of 73° and 82° for BPA-BPI and TPA-BPI,
respectively. These are expected to help suppress fluorescence
quenching caused by aggregation in the solid state. The BPA
moiety in BPA-BPI and the TPA moiety in TPA-BPI also
N-Phenyl-N-(4-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-
yl)phenyl)benzenamine (BPA-BPI). Using a similar approach for 1
by replacing 4-tert-butylbenzenamine and 4-bromobenzaldehyde with
aniline and 4-(diphenylamino)benzaldehyde, a light yellow powder
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Figure 1. Molecule structures of TPA-BPI and BPA-BPI.
twisted about the phenanthroimidazole plane with dihedral
angles of 29° and 43°, respectively. These would reduce the
extent of conjugation in the molecules and thus the amount of
charge transfer from the donor (i.e., BPA or TPA) to the
acceptor (phenanthroimidazole).27 The nonplanarity and the
reduced extent of conjugation are beneficial for maintaining the
short wavelength emission in solid state.28
Figure 2. TGA curves for TPA-BPI, BPA-BPI, PATPA, and TPA-TPI.
Thermal Properties. The thermal properties of the four
compounds were investigated with TGA and DSC under a
nitrogen atmosphere, and their thermal data are summarized in
Table 1. With the introduction of the rigid phenanthro-
backbone and the TPA moiety, all compounds exhibit good
thermal stability and film-forming ability. As shown in Figure 2,
decomposition temperatures (Td), defined as the temperature
at which the materials show a 5% weight loss, were measured to
be 420, 375, 423, and 440 °C for TPA-BPI, BPA-BPI, PATPA,
and TPA-TPI, respectively. Figure 3 shows DSC curves of the
materials. Melting temperatures (Tm) of the materials were
observed upon heating up to 272−306 °C. The glass-transition
temperatures (Tg) of BPA-BPI, PATPA, and TPA-TPI were
found to be 124, 141, and 121 °C, respectively. It is worth
noting that, for TPA-BPI, no obvious Tg could be detected, and
upon second heating, a crystallization temperature (Tc) of 201
°C was observed.
Figure 3. DSC curves for TPA-BPI, BPA-BPI, PATPA, and TPA-TPI.
Optical Properties. Photophysical properties of the four
compounds were investigated by measuring the PL spectra in
both dichloromethane solutions and solid-state thin films on
quartz substrates. Key photophysical data are summarized in
Table 1. As can be seen in Figure 4, all compounds exhibit
broad structureless emission peaks ranging from 418 to 487
nm. The short emission wavelengths are attributed to the
limited donor/acceptor charge transfer due to the nonplanarity
and small extent of conjugation in the molecules.28 It should be
noted that all compounds show negligibly small peak shifts in
their absorption and PL spectra from dichloromethane
solutions to solid states. This indicates that the highly twisted
substituent on the 1-imidazole position as well as the
noncoplanar TPA moiety can effectively suppress the
intermolecular π−π stacking in the solid state. For the
absorption spectra, it can be seen that all compounds show
similar absorption bands at a wavelength of approximately 260
nm, which can be attributed to the π−π* transition of their
Figure 4. UV absorption and PL spectra of TPA-BPI, BPA-BPI,
PATPA, and TPA-TPI in CH2Cl2 solution (empty) and solid-state
thin film (filled).
common thiophene ring and benzene ring.29 The longer
wavelength absorption bands vary in the range of 332−372 nm,
which might be originated from the π−π* transition from the
substituent on the 2-imidazole position to the phenanthroimi-
dazole acceptor. On the other hand, it is worth noting that the
PL spectrum of TPA-TPI is more red-shifted compared with
those of other three compounds. It may be attributed to the
Table 1. Key Physical Properties of the Four Compounds
solutionAbs solutionPL
filmAbs
(nm)
filmPL
(nm)
HOMO (eV)
LUMO (eV)
energy gap (eV)
b
Compounds Tm/Tg/Td (°C)
(nm)
(nm)
Φf
[optical/calculated]
[optical/calculated]
[optical/calculated]
a
TPA-BPI
BPA-BPI
PATPA
285/201 /420
272/124/375
306/141/423
276/121/440
256/361
257/356
265/332
256/400
442
419
426
468
∼1
∼1
260/372
263/369
268/351
262/409
436
−5.26/−4.86
−5.31/−4.78
−5.54/−5.08
−5.00/−4.71
−2.36/−1.2
−2.35/−0.99
−2.39/-1.2
2.9/3.66
428
2.96/3.79
3.15/3.88
2.59/3.29
418
TPA-TPI
0.65
469/487
−2.41/−1.42
a
b
Crystallization temperature. Fluorescent quantum yield in solution measured with respect to anthracene.
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lone pair electron on the thiophene group that requires a lower
energy for the n−π* transition.9,29 In addition, the PL spectrum
of TPA-BPI shows obvious solvatochromic shift, in which its
emission color is red-shifted with increasing solvent polarity
(Figure 5). This indicates the occurrence of charge transfer
upon photoexcitation.30
moiety contributes less to the HOMO energy in PATPA. In
fact, the HOMO level of PATPA was measured by UPS to be
5.54 eV, which is much larger than those of the other three
compounds ranging from 5.00 to 5.31 eV. These results also
suggest that the modification at the 1-position imidazole does
not have significant effect on the HOMO energy of the
compounds. The LUMO levels mostly populate on the
imidazole ring and the linker between the donor and the
acceptor. It is noted that the LUMO and HOMO both locate
on the imidazole ring; on one hand it would contribute for the
balanced charge transfer as the emitter in OLED, but on the
other hand it could facilitate the charge transfer process upon
excitation, which was also confirmed by the solvatochromic
phenomenon observed in the solvent with a different polarity.
Electroluminescent (EL) Properties. EL properties of the
four compounds were studied by using them as the emitters in
trilayer OLEDs (anode/HTL/one of the four compounds as
emitter/ETL/cathode) or the hole-transporting emitter in
bilayer OLEDs (anode/one of the four compounds/ETL/
cathode). Using the four materials, a total of 15 devices were
fabricated. In these devices, ITO is used as the anode; NPB is
used as the hole-transporting layer (HTL); TPBI, BPhen, or
Alq3 are used as the electron-transporting layer (ETL); and
LiF/Al or LiF/Mg:Ag are used as the cathode. Detailed
structures and key performance parameters of the 15 OLEDs
and a reference NPB/Alq3 device are summarized in Table 2.
Figure 5. PL spectra of TPA-BPI dissolved in different solvents.
Theoretical Calculation. To gain insight into the
electronic structures of the compounds, a DFT calculation
was performed at the B3LYP/6-31G (d) level. As is shown in
Figure 6, the HOMO levels mainly populate on the
phenanthroimidazole moiety for all compounds and on the
TPA moiety on all compounds except for PATPA. In other
words, compared with the other three compounds, the TPA
Figure 6. Spatial distributions of frontier orbitals of the PATPA, BPA-BPI, TPA-BPI, and TPA-TPI.
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Table 2. Key Performance Parameters of Devices 1−16
material
8/9
TPA-BPI
BPA-BPI
10/11/12
PATPA
13
TPA-TPI
14/15/16
device
1/2
3/4
5/6/7
Von (V)
λmaxEL
2.8/3.2
2.8/2.8
2.8/2.8/3.5
448/452/452
73/69/75
3.7/3.6
2/2.3/3.5
3.8
424
63
2.6/2.6/2.6
496/496/496
80/80/42
448/448
71/71
448/448
71/71
528/520
428/416/436
65/57/53
fwhm (nm)
CIE
98/96
(0.15, 0.09)
(0.15, 0.09)
(0.15, 0.09)
(0.15, 0.09)
(0.15, 0.09)
(0.15, 0.09)
(0.15, 0.09)
1.83/1.1/0.49
1.58/0.75/0.17
1820/1043/419
(0.32, 0.56)
(0.32, 0.56)
(0.15, 0.05)
(0.15, 0.05)
(0.15, 0.05)
(0.15, 0.06) (0.19, 0.39)
(0.17, 0.38)
(0.18, 0.40)
ηcmax (cd/A)
2.63/2.03
2.53/1.81
2259/1894
2.13/1.46
1.85/1.34
2085/1456
4.1/3.9
0.65/0.45/0.40 0.34
0.68/0.47/0.35 0.24
4.6/1.82/3
max
ηp (lm/W)
2.8/2.5
4.68/1.9/2.9
3222/1517/2465
L (cd/m2) at 100
4069/3807
568/384/353
282
mA/cm2
a
V
(V)
4.6/5.3
5.6/4
4.7/4.4/8.6
7.2/5.7
3.9/3.3
3.6/3.9/7.1
5.3
4.4/4.1/4.2
4/1.72/2.8
a
ηc (cd/A)
2.5 (2.42)/
1.97 (1.90)
2.1 (2.10)/
1.45 (1.46)
1.8/0.97/0.29
0.63/0.43/0.38 0.34
a
ηp (lm/W)
1.7 (1.46)/1.1 (0.97) 1.2 (1.00)/1.1 (0.93) 1.2/0.67/0.16
1.7/1.8
0.55/0.34/0.16 0.2
1.37/0.66/0.85 0.72
2.8/1.32/2
EQEmax (%)
3.08 (2.88)/
2.34 (2.21)
2.62 (2.55)/
1.77 (1.76)
2.14/1.19/0.51
1.30/1.23
1.91/0.75/1.24
TPA-BPI
trilayer devices
1
2
3
4
5
6
7
8
9
ITO/NPB (60 nm)/TPA-BPI (40 nm)/TPBI (20 nm)/LiF (0.5 nm)/Al (80 nm)
ITO/NPB (60 nm)/TPA-BPI (40 nm)/Bphen (20 nm)/LiF (0.5 nm)/Al (80 nm)
ITO/NPB (60 nm)/TPA-BPI (40 nm)/TPBI (20 nm)/LiF (0.5 nm)/Mg:Ag (100 nm)
ITO/NPB (60 nm)/TPA-BPI (40 nm)/Bphen (20 nm)/LiF (0.5 nm)/Mg:Ag (100 nm)
ITO/TPA-BPI (100 nm)/TPBI (20 nm)/LiF (0.5 nm)/Mg:Ag (100 nm)
ITO/TPA-BPI (100 nm)/Bphen (20 nm)/LiF (0.5 nm)/Mg:Ag (100 nm)
ITO/TPA-BPI (120 nm)/LiF (0.5 nm)/Mg:Ag (100 nm)
bilayer devices:
single layer device:
devices with Alq3:
ITO/TPA-BPI (70 nm)/Alq3 (60 nm)/LiF (0.5 nm)/Mg:Ag (100 nm)
ITO/NPB (70 nm)/Alq3 (60 nm)/LiF (0.5 nm)/Mg:Ag (100 nm)
BPA-BPI
10
11
12
ITO/NPB (60 nm)/BPA-BPI (40 nm)/TPBI (20 nm)/LiF (0.5 nm)/Mg:Ag (100 nm)
ITO/NPB (60 nm)/BPA-BPI (40 nm)/Bphen (20 nm)/LiF (0.5 nm)/Mg:Ag (100 nm)
ITO/ITO/BPA-BPI (100 nm)/Bphen (20 nm)/LiF (0.5 nm)/Mg:Ag (100 nm)
PATPA
13
ITO/NPB (60 nm)/PATPA (40 nm)/TPBI (20 nm)/LiF (0.5 nm)/Mg:Ag (100 nm)
TPA-TPI
14
15
16
ITO/NPB (70 nm)/TPA-TPI (40 nm)/Bphen (20 nm)/LiF (0.5 nm)/Al (80 nm)
ITO/NPB (70 nm)/TPA-TPI (40 nm)/TPBI (20 nm)/LiF (0.5 nm)/Mg:Ag (100 nm)
ITO/TPA-TPI (110 nm)/TPBI (20 nm)/LiF (0.5 nm)/Mg:Ag (100 nm)
a
The former data was obtained at 20 mA/cm2; the data in brackets was obtained at high brightness of 1000 cd/m2.
Among the trilayer devices (devices 1−4, 10−16), device 1
based on TPA-BPI gives the best performance in terms of
current, power efficiencies, and external quantum efficiency
(EQE). In particular, maximum current, power, and external
quantum efficiencies (ηc, ηp, EQE) of respectively 2.63 cd/A,
2.53 lm/W, and 3.08% were obtained. The device also has a low
onset voltage (defined as the voltage required to obtain a
luminance of 1 cd/m2) of 2.8 V and low operating voltages
(5.19 and 4.6 V, respectively, to give a current density of 20
mA/cm2 and a luminance of 1000 cd/m2). All the TPA-BPI-
based trilayer devices (devices 1−4) exhibit steady saturated
blue emission of CIE coordinates (0.15, 0.09) (peaked at 448
nm) over a wide range of operation conditions (Figures 7 and
Figure 7. EL spectra for trilayer device 1 at different voltages from
4−9 V.
8).
By comparing to the performance of the bilayer (devices 5−
6) devices using TPA-BPI as both the HTL and the emitter, it
can be seen (Table 1 and Figure 9) that TPA-BPI has good
hole-transporting properties. In fact, the current densities of the
bilayer devices (5 and 6) are slightly higher than those of the
corresponding trilayer devices (3 and 4). For instance, at a
driving voltage of 5 V, the current density of device 5 is 31
mA/cm2, while that of device 3 is 13 mA/cm2. This may be
attributed to the shallow HOMO levels of TPA-BPI (5.26 eV)
comparing to that of NPB (5.40 eV). Device 5 also shows a
reasonably high efficiency (ηc = 1.83 cd/A; ηp = 1.58 lm/W;
EQE = 2.14%), low onset voltage of 2.8 V, and low operation
66
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Chemistry of Materials
Article
Figure 8. CIEx and CIEy at different current densities up to 600
mA/cm2 for the TPA-BPI based devices 1−4.
voltage (20 mA/cm2 at 4.7 V, 5.7 V for 1000 cd/m2),
suggesting that TPA-BPI is also a promising hole-transporting
material. To further confirm the hole-transporting properties of
TPA-BPI, a bilayer device (TPA-BPI/Alq3, device 8) was made
and compared with the standard NPB/Alq3 device (device 9).
Device 8 shows higher maximum efficiencies (4.1 cd/A, 2.8 lm/
W, 1.3%) than those of device 9 (3.9 cd/A, 2.5 lm/W, 1.23%),
confirming that the TPA-BPI is also a promising hole-
transporting material.
To explore the possible bipolar transporting properties of
TPA-BPI, a single layer device (device 7) with a structure of
ITO/TPA-BPI/LiF/Mg:Ag was also fabricated. However, it
was found that the maximum device efficiencies (i.e., 0.49 cd/A,
0.17 lm/W, 0.51%) are far below those of the bilayer devices
(devices 5 and 6). The poor performance may be due to a large
electron-injection barrier at the TPA-BPI/LiF interface (∼0.54
eV). On the other hand, it is worth noting that all the TPA-BPI
based devices show little efficiency roll-off as current density
increases (Figure 10). Together with their stable emission
color, TPA-BPI is suitable for applications requiring high
brightness deep-blue emission.
To further access the performance of TPA-BPI as a deep-
blue emitter, performance of device 1 (trilayer) and device 5
(bilayer) are compared with those of similarly structured deep-
blue emitting devices reported recently (Table 3).6−19,31−37 It
can be seen that the performance of the TPA-BPI based devices
are among the best in terms of turn-on/operation voltages and
efficiency for nondoped devices with CIEy < 0.10.6−19,31−37
Comparing to TPA-BPI, devices using BPA-BPI or PATPA
as emitters show even more saturated blue emissions.
Particularly, the BPA-BPI based device showed a violet-blue
emission with CIE coordinates of (0.15, 0.05) and a relatively
Figure 10. EQE at different current densities for the TPA-BPI based
devices 1−9.
narrow full width at half-maximum (fwhm) of 65, 57, and 53
nm for trilayer devices 10 and 11 and bilayer device 12,
respectively. For PATPA, the maximum EL peak further blue-
shifted to 424 nm with a fwhm of 63 nm (device 13). This blue
shift on the EL spectra may be due to shorter conjugation
lengths of BPA-BPI and PATPA compared to that of TPA-BPI.
As aforementioned, the highly twisted TPA moiety can
intervene with the conjugation to reduce the extent of charge
transfer from the donor (i.e., BPA) to the acceptor
(phenanthroimidazole), leading to a blue-shifted EL. On the
other hand, performances of the BPA-BPI and PATPA based
devices are not as good as those of the TPA-BPI based devices.
For example, maximum EQE of the trilayer devices based on
BPA-BPI, PATPA, and TPA-BPI (devices 10, 13, and 1) are
1.37, 0.72, and 3.08%, respectively.
The fourth compound TPA-TPI possesses a higher-lying
HOMO energy of 5.00 eV, which is much smaller than those of
other three compounds (5.26, 5.31, and 5.54 eV). It also has a
smaller energy gap of 2.59 eV compared to the values of about
3 eV for the other three compounds. As expected, the device
(device 14) based on TPA-TPI shows higher efficiencies (4.6
cd/A and 4.68 lm/W), while the blue emission is less saturated
(peaked at 496 nm, with CIE coordinates of (0.19, 0.39)). It
was found that TPA-TPI can also act as a hole-transporting
material. In particular, bilayer device 16 (TPA-TPI/TPBI)
shows a better performance in terms of current, power
efficiencies, and EQE (Table 2) than those of trilayer device
15 (NPB/TPA-TPI/TPBI). As shown in Figure 11, at the same
driving voltage, the current density of the bilayer device 16 is
only slightly lower than that of the trilayer device 15, which
Figure 9. Current density (J−V) and luminance (L−V) at different driving voltages for the TPA-BPI based devices 1−7.
67
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Chemistry of Materials
Article
Table 3. Key Characteristics of Devices 1 and 5 and Other Recently Reported Non-Doped Blue Emitting Devices with CIEy <
0.1
a
b
b
emitter
HOMO
Von (V)
EL (nm)
ηcmax (cd/A)
ηp (lm/W)
CIE 1931 (x, y)
ref
c
h
c
h
TPA-BPI
5.26
5.26
5.55
5.92
6.08
6.0
2.8
2.8
5.77
6.5
4.5
−
448
448
460
449
452
444
438
444
444
442
453
430
444
444
448
439
460
428
−
2.63/2.5 /2.6
2.53/1.7 /2.0
(0.15, 0.09)
(0.15, 0.09)
(0.151, 0.097)
(0.15, 0.09)
(0.14, 0.08)
(0.15, 0.08)
(0.16, 0.097)
(0.154, 0.08)
(0.153, 0.079)
(0.153, 0.08)
(0.154, 0.068)
(0.163, 0.073)
(0.151, 0.085)
(0.149, 0.086)
(0.147, 0.096)
(0.16, 0.08)
(0.15, 0.08)
(0.17, 0.08)
(0.15, 0.08)
(0.16, 0.05)
(0.14, 0.09)
(0.15, 0.09)
(0.15, 0.08)
(0.19, 0.08)
(0.16, 0.07)
(0.156, 0.078)
(0.16, 0.07)
(0.156,0.093)
this work
d
c
c
TPA-BPI
DMIP-2-NA
CzPhB
1.83/1.8
1.58/1.2
this work
6
1.17
3.3
0.64
1.3
7
c
c
OPVs1
1.29
0.66
8
DNBN
1.84
2
1.13
9
f
NATSNA
5.9
−
−
10
11
11
11
12
12
13
13
13
14
15
16
17
17
18
19
31
32
33
34
35
36
37
c
c
4a
5.8
4.1
3.0
4
1.9/1.16
1.56/0.49
c
c
5a
5.89
−
2.8/1.67
2.33/1.05
c
c
MADN
TP-EPY
TP-EIF
1.35/1.08
1.01/0.48
6.03
5.94
5.79
5.83
5.82
5.7
9.2
11.2
3.2
3.0
3.2
3
1.72
0.76
1.79
2.63
2.67
0.86
0.85
0.8
0.65
0.27
1.41
1.96
1.92
0.76
−
g
TBMFA
g
TBDNPA
g
TBMFPA
In2Bt
ADF
c
5.73
5.6
9.5
sFTaz
8.1
6.7
−
−
α,α-MADN
CPhBzIm
Cz-NPh
5.8
0.7
0.3
5.49
5.12
5.4
−
1.6
1.07
1.51
3
456
445
−
1.96
i
h
h
TPIP
3.9
−
−
−
−
4.69/4.67
2.71/2.14
c
BH-1SN
DSX-IF
TFSTPA
o,p-TP-EPY
T3
−
−
5.32
0.61
1.37
0.3
−
414
1
−
1.63
1.17
0.47
−
5.38
−
440
414
−
0.3
5
−
e
c
c
PhQ-CVz
∼3.5
−
b
2.06/1.90
1.77/0.85
POAn
2.5
445
3.2
3.3
(0.15, 0.07)
a
c
Von is onset voltage obtaind at 1 cd/m2. ηcmax and ηpmax are maximum current efficiency and power efficiency, respectively. Obtained at 20 mA/
d
e
f
g
h
i
cm2. Bilayer device. 2TNATA inserted. m-MTDATA inserted. PEDOT:PSS inserted. Obtained at 200 cd/m2. BCP and Alq3 were matched as
electron transport layer.
Figure 11. Current density (J−V) and luminance (L−V) at different
driving voltages for the TPA-TPI based devices 14−16.
uses NPB as the HTL, while the brightness of bilayer device 16
is higher than that of trilayer device 15. Again, efficiencies of
the TPA-TPI based devices only show mild decreases at high
Figure 12. Power efficiency and current efficiency at different current
densities for the TPA-TPI based devices 14−16.
current densities (Figure 12). This suggests that TPA-TPI is
also a promising candidate for a blue emitter.
Reservation of phenanthroimidazole’s deep-blue emission was
By combining the advantages of the phenanthroimidazole
backbone and the donor−π-acceptor structure, a deep-blue
emitter TPA-BPI with high fluorescence efficiency and
appropriate HOMO/LUMO energy levels was obtained.
realized by limiting charge transfer and extension of
conjugation via nonplanarity of the molecule. The linker
between donor and acceptor moieties and the donor position
on the skeleton were considered key factors for obtaining a
68
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Chemistry of Materials
Article
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CONCLUSION
■
In summary, we have designed and synthesized three
phenanthroimidazole-based materials with a donor−π-acceptor
structure by attaching the strong electron-donating moiety of
triphenylamine (TPA) to different positions of the phenanthro-
imidazole with different linkers and investigated their
applications in nondoped OLEDs. The fluorophores show
high quantum yields (up to ∼1) and good thermal stability. A
TPA-BPI trilayer device exhibits high efficiencies of 2.63 cd/A,
2.53 lm/W, and 3.08%, low onset voltage of 2.8 V, and low
operating voltage (i.e., 5.19 V for 20 mA/cm2, 4.6 V for 1000
cd/m2) as well as a deep-blue emission with CIE coordinates of
(0.15, 0.09) and little efficiency roll-off and color change at high
brightness. By reducing the extent of conjugation in BPA-BPI
and PATPA, the EL spectra of their devices are further blue-
shifted to show violet blue emission with CIE coordinates of
(0.15, 0.05) and (0.15, 0.06), respectively. Together with their
good hole-transporting properties, the four new materials are
considered to have good potential for applications in blue
OLEDs requiring operation at high excitation densities.
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ASSOCIATED CONTENT
* Supporting Information
Tables of crystal data, structure solution and refinement, atomic
coordinates, bond lengths and angles, and anisotropic thermal
parameters for TPA-BPI and BPA-BPI and UPS spectra for the
reported materials (PDF). Two X-ray crystallographic file for
TPA-BPI and BPA-BPI (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
ACKNOWLEDGMENTS
■
This work was supported by the Research Grants Council of
the Hong Kong Special Administrative Region, China (Project
No. CityU 101508), National Natural Science Foundation of
China (No. 91027041), and the Key Laboratory of Photo-
chemical Conversion and Optoelectronic Materials, TIPC,
CAS.
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