High-efficiency nondoped deep-blue-emitting organic electroluminescent device

Article abstract of DOI:10.1021/cm100100w

A new blue emitter, 9,9-bis-(3- (9-phenyl-carbazoyl))-2,7-dipyrenylfluorene (DCDPF), has been synthesized and characterized. Organic light-emitting devices (OLEDs) using DCDPF as a non-doped emitter exhibits deep-blue emission with a peak at 458 nm and CI

Full text of DOI:10.1021/cm100100w

2138 Chem. Mater. 2010, 22, 2138–2141
DOI:10.1021/cm100100w
High-Efficiency Nondoped Deep-Blue-Emitting Organic
Electroluminescent Device
Silu Tao,*,†,‡ Yechun Zhou,‡ Chun-Sing Lee,‡ Xiaohong Zhang,*,§ and Shuit-Tong Lee*,‡
†School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC),
Chengdu 610054, China, ‡Center of Super-Diamond and Advanced Films (COSDAF), and Department of
Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, China, and
§
Nano-organic Photoelectronic Laboratory and Key Laboratory of Photochemical Conversion and
Optoelectronic Materials, Technical Institute of Physics and Chemistry,
Chinese Academy of Sciences, Beijing, 100190, China
Received January 12, 2010
A new blue emitter, 9,9-bis-(3- (9-phenyl-carbazoyl))-2,7-dipyrenylfluorene (DCDPF), has been
synthesized and characterized. Organic light-emitting devices (OLEDs) using DCDPF as a non-
doped emitter exhibits deep-blue emission with a peak at 458 nm and CIE coordinates of (0.15, 0.15).
The maximum efficiency of the device is 4.4 cd/A (3.1 lm/W). The results suggest that the introduction
of carbazole units at the 9-position of fluorene provides an effective way to suppress molecular
aggregation which would cause red shift in emission.
Introduction
two peaks at 472 and 500 nm, respectively.8,9 The color
purity of the OLEDs based on FIrPic is thus compro-
mised. Moreover, it is known that FIrPic-based devices
are accompanied by fast degradation which means a short
lifetime.10 Till now, deep blue phosphorescent emitters
with a long lifetimes have been scarce. Therefore, blue fluo-
rescent emitters should still play a key role in the future
applications of OLEDs.
The color purity and efficiency of most blue OLEDs are
often mutually compromised. While many blue fluores-
cent emitters have been reported, materials emitting in the
deep-blue region with high efficiency are still rare.11-21
Thus, there is still much need for improvements in both
efficiency and color purity of blue OLEDs, especially with
an undoped device structure.
Since the pioneering work on organic light-emitting
devices (OLEDs) by the Kodak group, extensive research
has been carried out to bring OLEDs into commercial
applications in flat-panel displays and solid-state light-
ings.1,2 To produce high-efficiency OLEDs, a key approach
is to develop high-performance electroluminescent mate-
rials including RGB (red, green, blue) emitters. Many new
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demands of full-color displays. Phosphorescent OLEDs
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cially iridium (Ir) complexes, are of great interest because
of the feasibility to achieve an internal quantum efficiency
up to 100%.3 Performances of green and red-emitting
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are few alternatives of blue phosphorescent emitters.
However, FIrPic is in fact a blue-green emitter, with
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*Corresponding author. Fax: þ852-27844696 (S.T.L.); þ86-10-62554670
(X.H.Z.);þ86-28-83201745(S.L.T.). E-mail: apannale@cityu.edu.hk (S.T.L.);
xhzhang@mail.ipc.ac.cn (X.H.Z.); silutao@uestc.edu.cn (S.L.T.).
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r
pubs.acs.org/cm
Published on Web 01/29/2010
2010 American Chemical Society

Article
In this paper, we report the synthesis and characteri-
Chem. Mater., Vol. 22, No. 6, 2010 2139
Scheme 1. Synthetic Route of DCDPF
zation of a new compound, 9, 9-bis-(3-(9-phenyl-carba-
zoyl))-2,7-dipyrenylfluorene (DCDPF). Although the
carbazole/fluorene hybrids have been reported as host
materials for phosphorescent OLEDs,22 the reports
about carbazole/fluorene/pyrene hybrids being used as
deep-blue emitters are still absent. Our results suggest
that introduction of the carbazole units at the 9-position
of fluorene is an effective way to avoid molecular aggre-
gation of the molecules and the associated red shift in
emission. Moreover, the carbazole units also improve
hole injection and transport properties. OLEDs based
on DCDPF have been fabricated in a nondoped device
structure, which exhibits deep-blue emission with a
peak at 458 nm and CIE coordinates of (0.15, 0.15).
The maximum efficiency of the blue OLED is 4.4 cd/A
(3.1 lm/W), which is among the best values ever reported
for nondoped deep blue fluorescence OLEDs.
OLEDs were fabricated by vacuum deposition on ITO glass
substrates with a sheet resistance of 30Ω/square. Before deposi-
tion, the ITO substrate was carefully cleaned, then dried in an
oven at 120 °C, and finally treated with UV-ozone then loaded
into a deposition chamber. The devices were fabricated by
evaporating organic layers onto the ITO substrate sequentially
Experimental Details
Material Synthesis. Compound 1, 9,9-Bis-(3-(9-phenyl-carba-
zoyl))-2,7-dibromofluorene (1). A mixture of 2,7-dibromofluore-
none, (1.01 g, 3 mmol), 9-phenyl-carbazole (12.1 g, 50 mmol), and
methane sulfonic acid (0.3g, 3 mmol) was heated at 140 °C under a
nitrogen atmosphere for 10 h. After being cooled, the mixture was
extracted with dichloromethane (CH2Cl2). The organic solution
was washed with sodium carbonate solution and water and then
dried with MgSO4. Evaporation of the solvent followed by column
chromatography on silica gel with petroleum ether-dichloromethane
gave a white product. Yield: 1.52 g (63%); MS (m/z): 806 (Mþ).
9,9-Bis-(3-(9-phenyl-carbazoyl))-2,7-dipyrenylfluorene (DCDPF).
Compound 1 (0.81 g, 1 mmol) and 1-pyrenyl boronic acid (0.75 g,
3 mmol), Pd (PPh3)4 (0.2 mmol), aqueous Na2CO3 (2.0 M,
4 mL), ethanol (2 mL), and toluene (10 mL) were mixed in a
flask. The mixture was degassed and then refluxed for 24 h under
a nitrogen atmosphere. After being cooled, the solvent was
evaporated under vacuum and the product was extracted with
dichloromethane (CH2Cl2). The CH2Cl2 solution was washed
with water and dried with MgSO4. Evaporation of the solvent,
followed by column chromatography on silica gel with petro-
leum ether-dichloromethane gave a white product. The product
was recrystallized from hexane-chloroform mixture to give the
pure product. Yield: 0.81 g (77%); MS (m/z): 1048 (Mþ).
1HNMR (CDCl3, 300 MHz): δ: 7.21(s, 2H), 7.32-7.42(m,
8H), 7.54-7.58 (m, 10H), 7.77-7.79 (d, 2H), 7.87-7.90 (d, 2H),
7.96-8.01 (t, 4H), 8.04-8.08 (m, 10H), 8.13-8.22 (m, 6H), 8.27
(s, 2H), 8.32-8.36 (d, 2H); MS (m/z): 1048 (Mþ). Anal. Calcd
For C81H48N2: C, 92.72; H, 4.61; N, 2.67%. Found: C, 92.70; H,
4.60; N, 2.48%.
˚
at an evaporation rate of 2-4 A/s and a pressure better than 5 ꢀ
10-6 mbar. The Mg:Ag alloy cathode was prepared by coeva-
poration of Mg and Ag at a volume ratio of 10:1. EL spectra and
CIE color coordinates were measured with a Spectrascan PR650
photometer and the current-voltage-luminescence character-
istics were measured with a computer-controlled Keithley 236
SourceMeter under ambient atmosphere.
Results and Discussion
The molecular structure and synthetic route of
9,9-bis-(3-(9-phenyl-carbazoyl))-2,7-dipyrenylfluorene
(DCDPF) are shown in Scheme 1. The chemical struc-
1
ture of DCDPF was confirmed by H nuclear magnetic
resonance, mass spectrometry, and elemental analysis.
DCDPF is thermally stable up to 450 °C, as shown by
its decomposition temperature determined from its Thermo-
gravimetric Analyzer (TGA) measurement. No glass-
transition temperature (Tg) can be observed in its dif-
ferential scanning calorimertry (DSC) measurement. It
indicates that DCDPF has good thermal stability. Figure 1
shows the absorption and fluorescence spectra of DCDPF
in dilute dichloromethane solution and in solid film. The
absorption spectrum in solution shows the characteristic
vibration pattern of isolated fluorene and pyrene units.
The photoluminescence (PL) spectrum of DCDPF in
solution shows a deep blue emission peak centered at
423 nm. The fluorescence quantum yield of DCDPF in
CH2Cl2 solution was measured to be 0.63 by calibrating
against 9, 10-diphenylanthracene (φf ≈ 0.90) as a stan-
dard. The PL spectra in solution and in film are similar
except for a red shift of 30 nm in the solid state.
Measurements and OLEDs Fabrication. Absorption and
fluorescence spectra were recorded respectively with a Perkin-
Elmer Lambda 2S UV-vis spectrophotometer and a Perkin-
Elmer LS50B Luminescence spectrophotometer. The highest
occupied molecular orbitals (HOMO) values were measured dir-
ectly by ultraviolet photoelectron spectroscopy (UPS), whereas
the lowest unoccupied molecular orbital (LUMO) values were
determined from the HOMO and positions of the lowest energy
absorption edge of the UV absorption spectra.
To investigate the potential applications of DCDPF in
OLEDs, a nondoped device with a typical three-layer
structure of ITO/NPB (50 nm)/DCDPF (20 nm)/TPBI
(30 nm)/LiF (0.5 nm)/MgAg (100 nm) was fabricated.
(22) Shih, P. I.; Chiang, C. L.; Dixit, A. K.; Chen, C. K.; Yuan, M. C.;
Lee, R. Y.; Chen, C. T.; Diau, E. W. G.; Shu, C. F. Org. Lett. 2006,
8, 2799.

2140 Chem. Mater., Vol. 22, No. 6, 2010
Tao et al.
Figure 3. Current density-voltage-brightness curves ofthe device. Inset
is the energy level of the device.
Figure 1. Absorption and emission spectra of DCDPF in solution and
in film.
Figure 4. Current efficiency-current density-power efficiency charac-
teristics of the device.
Figure 2. EL spectra of a DCDPF-based device emitting at different
brightness levels.
In the device, ITO (indium tin oxide) and Mg:Ag are the
anode and cathode, respectively; 4,40-bis[N- (1-naphthyl)-
N-phenyl amino] biphenyl (NPB) is the hole-transporting
layer (HTL), and 2,20,200- (benzene-1,3,5-triyl)-tris(1-
phenyl-1H- benzimidazole) (TPBI) is the electron-trans-
porting layer (ETL).
Figure 2 shows the electroluminescence (EL) spectra of
the device at different brightness values. The device exhi-
bits deep blue emission with a peak centered at 458 nm
and CIE coordinates of (0.15, 0.15). The full width at half-
maximum (fwhm) of the EL peak is only 71 nm. The EL
spectrum of the device shows little difference (only about
5 nm red-shifted) from the corresponding PL spectrum in
film, indicating that the EL emission originates from the
singlet-excited states of DCDPF. Significantly, at lumi-
nance from 202 to 4165 cd/m2 the spectra of the device are
almost identical and CIE coordinates show little changes
(from (0.154, 0.150) to (0.154, 0.153)). This suggests that
the hole-electron recombination zone is well confined in
the DCDPF layer over the wide range of operating vol-
tage. This can be understood from the appropriate energy
levels of DCDPF with respect to those of the neighboring
layers, which are shown in the inset of Figure 2.
I-V-L characteristics of the DCDPF-based device are
shown in Figure 3. The device shows a turn-on voltage
(at a brightness of 1 cd/m2) of <3.5 V and achieves a
maximum brightness of 7332 cd/m2 at a voltage of 9 V and
a current density of 175 mA/cm2.
Figure 4 shows current efficiency-current density-
power efficiency characteristics of the device. The maxi-
mum current efficiency of the device is 4.4 cd/A (3.1 lm/W).
It is known that the color purity and efficiency of blue
OLEDs are often mutually compromised. Although many
Figure 5. PL of DCDPF and DPF films and EL spectra of their corres-
ponding devices.
blue fluorescent emitters have been reported, high effi-
ciency materials emitting in the deep-blue region are still
rare. Performance of DCDPF device with a maximum
efficiency of 4.4 cd/A (3.1 lm/W) and CIE coordinates of
(0.15, 0.15) are among the best values ever reported
for nondoped deep blue fluorescence OLEDs. Signifi-
cantly, the current efficiency shows only a mild decrease
as the current density increases, and a high efficiency
of 4.2 cd/A can still be obtained at a current density of
175 mA/cm2.
In our previous report, a nondoped OLED using
9,9-dimethyl-2,7-dipyrenylfluorene (DPF) as the host emitter
shows highly efficient blue emission with a peak centered
at 468 nm and CIE coordinates of x = 0.15, y = 0.19.23
The molecular structures of DPF and DCDPF are shown
in the inset of Figure 5. DCDPF has a similar chemical
(23) Tao, S. L.; Zhang, X. H.; Lee, C. S.; Lee, S. T. Appl. Phys. Lett.
2007, 91, 013507.

Article
Chem. Mater., Vol. 22, No. 6, 2010 2141
Table 1. Important Data of PL and EL of DPF and DCDPF
PL (nm) (solution)
PL (nm) (film)
fwhm (nm)
EL (nm)
fwhm (nm)
CIE (x, y)
HOMO/LUMO eV
DCDPF
DPF
423
421
453
460
68
81
458
468
71
84
0.15, 0.15
0.15, 0.19
5.9/3.0
5.7/2.7
structure as DPF except for the different substituents at
the 9-position of fluorene. Table 1 lists the key data about
DPF and DCDPF for comparison. The PL spectra of
their films and EL spectra of their devices are also shown
in Figure 5. Although the PL spectra of DCDPF and DPF
in solution are similar, significant difference can be seen in
the PL spectra of their solid films. The PL spectrum of
DCDPF film is much narrower than that of DPF film.
The full width at half-maximum (fwhm) of the PL spec-
trum of DCDPF film is only 68 nm, which is much lower
than that of DPF (81 nm). It indicates that introduction of
the bulky carbazole units at the 9-position of fluorene
would reduce the formation of aggregates emitting at
longer wavelength. This leads to the better blue emission
of DCDPF device than DPF device. The EL spectrum of
DCDPF-based device is thus much narrower and 10 nm
blue-shifted comparing to that of the DPF-based device.
Thus, the CIE coordinates of DCDPF-based device (x=
0.15, y = 0.15) is much better than that of DPF-based
device (x = 0.15, y = 0.19). Due to the compromise
between efficiency and color purity in blue OLEDs,
the efficiency of DCDPF device (4.4 cd/A, 2.3 lm/W) at
20 mA/cm2 is lower than that of DPF device (5.9 cd/A,
2.9 lm/W) after improvement in color purity.
In summary, a new blue emitter, 9,9-bis-(3- (9-phenyl-
carbazoyl))-2, 7-dipyrenylfluorene (DCDPF) has been
synthesized and characterized. OLED based on DCDPF
in a nondoped device structure exhibits deep-blue emis-
sion with a peak centered at 458 nm and CIE coordinates
of (0.15, 0.15). The maximum efficiency of the blue
OLED is 4.4 cd/A (3.1 lm/W). The results indicate that
introducing carbazole units at the 9-position of fluorene is
an efficient means for reducing red shift of the emission
caused by molecular aggregation.
Acknowledgment. This work is supported by Research
Grants Council of HKSAR (No. CityU101508). We thank
the National Natural Science Foundation of China (Grants
50773090 and 50825304) and Beijing Natural Science Founda-
tion (Grant 2072017), P. R. China, for financial support.