A new multifunctional fluorenyl carbazole hybrid for high performance deep blue fluorescence, orange phosphorescent host and fluorescence/phosphorescence white OLEDs

Article abstract of DOI:10.1016/j.dyepig.2012.12.028

A new multifunctional host material, 3,3′-(2,7-di(quinolin-8-yl)-9H- fluorene-9,9-diyl)bis(9-phenyl-9H-carbazole) has been designed, synthesized and used in high performance OLEDs. This tetrasubstituted fluorene was found to have good morphological and th

Full text of DOI:10.1016/j.dyepig.2012.12.028

Dyes and Pigments 97 (2013) 273e277
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A new multifunctional fluorenyl carbazole hybrid for high
performance deep blue fluorescence, orange phosphorescent host and
fluorescence/phosphorescence white OLEDs
,
,
a
b
b
c,d
Chuan Wu a b, Silu Tao a , Mingming Chen , Hin-Wai Mo , Tsz Wai Ng , Xiaoke Liu
,
*
Xiaohong Zhang c d, Weiming Zhao e, Chun-Sing Lee b
,
,
*
a School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, China
b Center of Super-Diamond and Advanced Films (COSDAF), Department of Physics and Materials Science, City University of Hong Kong, Hong Kong Special Administrative Region,
Kowloon Tong, Hong Kong
c Nano-organic Photoelectronic Laboratory, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
d Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
e Dongguan LITEWELL (OLED) Technology Incorporation, Dongguan 523656, China
a r t i c l e i n f o
a b s t r a c t
A new multifunctional host material, 3,30-(2,7-di(quinolin-8-yl)-9H-fluorene-9,9-diyl)bis(9-phenyl-9H-car-
bazole) has been designed, synthesized and used in high performance OLEDs. This tetrasubstituted fluorene
wasfoundtohavegoodmorphological andthermalstability withahighglasstransitiontemperatureof192 ꢀC
and a decomposition temperature of 480 ꢀC. A non-doped tri-layer blue fluorescent device based on this
tetrasubstituted fluorene shows a deep blue emission with a Commission Internationale de L’Eclairage co-
ordinates of (0.16, 0.10)and a maximum powerefficiencyof1.1 lm/W. An orange phosphorescentdevice using
thistetrasubstituted fluorene asa hostinconjunctionwithan iridium complex asdopantexhibits a maximum
powerefficiencyof21.0 lm/W,whichismuchhigherthanthatofabiscarbazolylbiphenyl-basedcontroldevice
(11.5 lm/W) with the same device structure. Furthermore, by using the tetrasubstituted fluorene as a blue
florescent emitter and a host for phosphorescent dopant, a white emitting OLED with a maximum power
efficiency of 22.5 lm/W and CIE coordinates of (0.38, 0.33) were obtained. These results show that this tet-
rasubstituted fluorene is a versatile multifunctional material for OLED applications.
Article history:
Received 18 October 2012
Received in revised form
27 December 2012
Accepted 27 December 2012
Available online 5 January 2013
Keywords:
Organic light-emitting devices
Deep blue emission
Multifunctional molecule
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
the best and most widely used blue phosphorescent emitter, which
is in fact a blue-green emitter, with two emission peaks centered at
Organic emitters play an important role in high efficiency white
organic light-emitting diodes (WOLEDs), which is important for the
applications in flat-panel displays and solid-state lighting [1].
WOLEDs based on phosphorescent emitters have been shown to
achieve an internal quantum efficiency of 100% as compared to the
theoretical maximum of 25% in fluorescent OLEDs [2]. Never-
theless, WOLEDs based on phosphorescent emitters often suffer
from concentration induced quenching and tripletetriplet annihi-
lation (TTA) at high current densities leading to severe efficiency
roll-off [3]. Meanwhile, a bottleneck for further performance and
stability enhancement in white OLEDs is on the blue phosphor-
escent emitters, whose performance are typically far below those of
472 and 500 nm, respectively. Moreover, the lifetime of FIrPic-
based white OLEDs are often inadequate [5].
Therefore, a promising alternative approach for highly efficient
WOLEDs is to use a fluorescent/phosphorescent (F/P) hybrid system
[6]. Combining the blue fluorescent emission with phosphorescent
emissions of complimentary colors, highly efficient WOLEDs can
be achieved. The crucial advantage of a F/P structure is to avoid using
the unstable blue phosphorescent emitters, consequently, a long
lifetime of the WOLEDs can be obtained [7]. A promising strategy for
high performance F/P WOLEDs is to employ a multifunctional host
material with a suitable triplet energy for sensitizing red and/or
green phosphorescent dyes, good charge transporting properties for
low operation voltage and high efficiency blue fluorescence [8].
In this paper, a new multifunctional host, 3,30-(2,7-di(quinolin-
8-yl)-9H-fluorene-9,9-diyl)bis(9-phenyl-9H-carbazole) (DFBC) has
been designed, synthesized and characterized. DFBC contains a fluo-
renyl core linked with two 9-phenylcarbazole groups and two
green and yellow phosphorescent emitters [4]. Iridium(III) bis
0
[(4,6-difluorophenyl)-pyridinato-N,C2 ] picolinate (FIrPic) remains
* Corresponding author.
E-mail addresses: silutao@uestc.edu.cn (S. Tao), apcslee@cityu.edu.hk (C.-S. Lee).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.12.028

274
C. Wu et al. / Dyes and Pigments 97 (2013) 273e277
quinoline moieties. The bulky quinoline and 9-phenylcarbazole
groups increase the steric hindrance of the molecular and endow
the compound withgood thermalstability[9]. Asexpected, DFBCwas
found to have good morphological and thermal stability with a high
glass transition temperature (Tg) of 192 ꢀC and decomposition tem-
perature (Td) of 480 ꢀC. The quinoline and 9-phenylcarbazole groups
also gave the compound with good charge transport properties. It
was found that a non-doped tri-layer blue fluorescent device based
on DFBC showed deep blue emission with CIE coordinates of (0.16,
0.10) and a maximum power efficiency of 1.1 lm/W. An orange
phosphorescent device using DFBC as host and Ir(2-phq)3 as dopant
exhibited a maximum power efficiency of 21.0 lm/W, which is much
higher than that of a CBP-based device (11.5 lm/W) with the same
device structure. Furthermore, a white F/P OLED shows a maximum
power efficiency of 22.5 lm/Wand CIE coordinates of (0.38, 0.33). The
results indicate that DFBC can be a multifunctional host material for
OLED applications.
measured in a VG ESCALAB 220i-XL ultrahigh vacuum (UHV) surface
analysis system). The lowest unoccupied molecular orbital (LUMO)
value was determined from the difference between the HOMO en-
ergy and the energy gap determined from the optical absorption
edge. Decomposition temperature (Td) was measured by thermog-
ravimetric analysis (TGA) measurements using TA instruments Q50
TGA at a heating rate of 10 ꢀC/min under nitrogen flow. Glass tran-
sition temperature (Tg) was measured with a differential scanning
calorimeter (DSC) at a heating rate of 10 ꢀC/min.
2.3. Device fabrication
Indium tin oxide (ITO) coated glass substrates with a sheet
resistance of 30
U
squareꢁ1 were used as anodes for OLEDs. The
substrates were patterned by traditional lithography, washed with
cleaning agent (DECON 90), rinsed with deionized water and dried
in an oven at 120 ꢀC. The ITO substrates were then treated with
ultraviolet-ozone before loading into a deposition chamber. All
organic layers and cathode were sequentially deposited onto the ITO
substrates by thermal evaporation at a pressure of about 10ꢁ6 Torr.
Current densityevoltageeluminance (JeVeL) characteristics, CIE
coordinates, and electroluminescent (EL) spectra of the devices were
measured with a programmable Keithley model 237 source meter
and a Photoresearch PR650 photometer under ambient atmosphere.
2. Experiment
2.1. Material synthesis
All solvents and raw materials were used as received from
commercial suppliers without further purification.
2.1.1. 3,30-(2,7-dibromo-9H-fluorene-9,9-diyl)bis(9-phenyl-9H-
carbazole) (compound 1)
3. Results and discussions
A mixture of 2,7-dibromo-9H-fluoren-9-one (1.02 g), 9-phen
ylcarbazole (12.2 g) and methanesulfonic acid (1 mL) was heated
at 140 ꢀC under a nitrogen atmosphere for 10 h. After cooling to
room temperature, the mixture was poured into an excess of sodium
carbonate solution for removing the superfluous acid, then extrac-
ted with dichloromethane. Evaporation of the solvent, followed by
column chromatography on silica gel with petroleum ether-
dichloromethane gave a white product. Yield: 1.74 g (72%).
The synthetic route to DFBC is shown in Scheme 1. The molec-
ular structure of DFBC was confirmed by 1H nuclear magnetic
resonance, mass spectrometry and elemental analysis. The PL
quantum efficiency in CH2Cl2 solution was measured with a Perki-
nElmer LS 50B Luminescence Spectrometer to be 0.73, by using
9,10-diphenyl-anthracence (
f
¼ 0.9) as a standard. DFBC is ther-
mally stable up to 480 ꢀC (Td, corresponding to 5% weight loss), as
shown by its decomposition temperature (Td) determined from its
TGA measurement (Fig. 1). DSC results show that DFBC has a glass
transition temperature (Tg) of 192 ꢀC. These results indicate that
DFBC has good thermal stability which is attributed to the
increased steric hindrance and molecular non-planarity from the
two bulky 9-phenylcarbazole groups and two quinoline groups
with the fluorenyl core at the 2- and 7-positions.
2.1.2. 3,30-(2, 7-di(quinolin-8-yl)-9H-fluorene-9,9-diyl)bis(9-
phenyl-9H-carbazole) (DFBC)
Compound 1: (0.81 g), 8-quinolineboronic acid (0.433 g),
Pd(PPh3)4 (0.1 g), aqueous Na2CO3 (2 M, 20 mL), toluene (16 mL),
ethanol (20 mL) were mixed in a flask. The mixture was degassed
several times and then heated under refluxed at 100 ꢀC under
a nitrogen atmosphere for 24 h. The mixture was cooled to room
temperature and extracted with dichloromethane (70 mL, 2 times).
The extracts were dissolved in acetone (30 mL) and then dried
with rotary evaporation. The residue was purified by column
chromatography (petroleum ether:CH2Cl2, 3:1) on silica gel. The
product was then further purified by recrystallization with petro-
leum ether and CH2Cl2 to obtain a pure product as a white solid.
Yield: 0.56 g (62%). 1H nuclear magnetic resonance: (300 MHz,
Fig. 2 depicts photoluminescence (PL) spectra of DFBC in dilute
dichloromethane solution and as a solid film. The solution and the
N
Br
Br
Br
Br
Methanesulfonic acid
°C,
CDCl3,) d: 7.13e7.26 (m, 3H), 7.30e7.42 (m, 9H), 7.45e7.57 (m, 11H),
N
N
N
O
2
7.64e7.67 (m, 1H), 7.77e7.82 (m, 3H),7.85e7.88 (t, 2H), 7.92e7.94
(d, 2H, J ¼ 5.76), 7.96e8.08 (m, 5H). 8.14e8.18 (m, 2H), 8.21e8.22
(s, 1H), 8.29e8.30 (s, 1H), 8.86e8.88 (d, 2H, J ¼ 7.38); Mass Spec-
trometry (ESIþ): m/z 903.3 (MHþ), Calcd for C67H42N4: 903.1; and
elemental analysis: Anal. Calcd. For C67H42N4 C: 89.11%, H: 4.69%,
N: 6.20 %. Found: C: 88.78%, H: 4.71%, N: 6.05%.
1
N
N
B
HO
OH
2.2. Characterization
CH3CH2OH, Na2CO3 (2M), toluene,
Pd(pph3)4, reflux under N2
N
N
Optical absorption and fluorescence spectra were recorded
respectively with a Perkin Elmer Lambda 2S UVeVis spectropho-
tometer and a Perkin Elmer LS50B Luminescence spectrophotometer.
The highest occupied molecular orbital (HOMO) value of DFBC
was measured via ultraviolet photoelectron spectroscopy (UPS:
DFBC
Scheme 1. Synthetic route of DFBC.

C. Wu et al. / Dyes and Pigments 97 (2013) 273e277
275
1.0
0.8
0.6
0.4
0.2
448 nm (0.16, 0.10)
L (cd/m2)
101
102
103
0.0
300
400
500
600
700
800
Wavelength (nm)
Fig. 1. TGA and DSC (inset) curves of DFBC.
Fig. 3. EL spectra of the blue device under different luminance.
solid film of DFBC exhibit a similar deep blue emission peaked at
416 and 427 nm, respectively. A low-temperature PL spectrum of
DFBC in a 2-methyltetrahydrofuran (2-MeTHF) solution at 77 K is
also shown in Fig. 2. The triplet energy of DFBC is determined to be
2.33 eV from the highest energy phosphorescent emission peak
which was located at about 532 nm. The triplet energy level of
DFBC is considerably higher than those of other commonly used
blue fluorescent materials and phosphorescent orange/red emitters
(tris(2-phenylquinoline)iridium(III) Ir(2-phq)3, 2.0 eV) [10]. HOMO
and LUMO energy levels of DFBC were determined to be ꢁ5.8 eV
and ꢁ2.5 eV by UPS and optical absorption measurements. Incor-
poration of the hole-transporting 9-phenylcarbazole moiety and
the electron-transporting quinoline moiety in DFBC is expected to
endow DFBC with good charge transporting properties for both
holes and electrons. Owing to the deep blue emission, the high
triplet energy level and possibly good charge transporting prop-
erties, DFBC is potentially a promising multifunctional host mate-
rial for applications in F/P OLEDs.
To further characterize the blue emission properties of DFBC, an
OLED was fabricated by using DFBC as the fluorescent emitter with
a device structure of ITO/NPB (50 nm)/DFBC (20 nm)/TPBi(25 nm)/LiF
(1 nm)/Al (100 nm). In the device, 4,40-bis[N-(1-naphthyl)-N-phenyl
amino]biphenyl(NPB)isusedasthehole-transportinglayer(HTL)and
2,20,200-(1,3,5-benzinetriyl) tris (1-phenyl-1-H-benzimidazole) (TPBi)
[11] is used as the electron-transporting layer (ETL). ITO and LiF/Al
were used as the anode and the cathode, respectively. Fig. 3 shows the
EL spectra of the device at different brightness. The devices show deep
blue emission with the emission peak centered at 448 nm and CIE
coordinates of (0.16, 0.10). Similarity of the PL and the EL spectra
confirms that the EL emission is from the singlet excited state of DFBC.
EL spectra of the device are almost identical over a wide brightness
range from 10 to 1000 cd/m2. Fig. 4 shows JeVeL characteristics of the
device. The turn-on voltage (defined as the driving voltage required to
give a brightness of 1 cd/m2) of the device is only 3.1 V and the device
achieves a brightness of over 5000 cd/m2 at 9.5 V. The maximum ef-
ficiency of the device is 1.6 cd/A (1.1 lm/W). Key performance data of
the device is listed in Table 1.
Due to the high triplet energy level (higher than that of phos-
phorescent orange emitter tris(2-phenylquinoline)iridium (III)
(Ir(2-phq)3, DFBC can be used as a host matrix for orange phosphors.
Phosphorescent OLEDs were fabricated by using DFBC as host and
(Ir(2-phq)3) as orange dopant with a device structure of ITO/NPB
(40 nm)/DFBC: 2% Ir(2-phq)3 (30 nm)/TPBi (40 nm)/LiF (1 nm)/Al
(100 nm).Forcomparison, acontroldevicewasalsopreparedwiththe
commonly used host material 4,40-bis(N-carbazolyl)-1,10-biphenyl
(CBP) with the same structure except with an optimized doping
concentration of 4%. Fig. 5 gives the JeVeL characteristic of the DFBC
based orange device. The turn-on voltage of the DFBC based device
(3.3 V) is lower than that of CBP-based device (3.6 V), which is due to
the relatively low-lying HOMO energy level of CBP (w6.1 eV for CBP
[12], 5.8 eV for DFBC). The maximum power efficiency of the DFBC
based device is 21.0 lm/W, which is much higher than that of the
CBP-based device(11.5 lm/W). The results demonstrate that DFBCcan
be a good host for orange phosphorescent dopants.
PL in solution at 293K
PL in film at 293K
Phosphorescence at 77K
1.0
0.8
0.6
0.4
0.2
0.0
600
103
500
102
400
300
101
200
100
100
300
400
500
600
700
10-1
wavelength (nm)
2
3
4
5
6
7
8
9
10
Voltage (V)
Fig. 2. Photoluminescence (PL) spectra of DFBC in dichloromethane solution and as
solid film, as well as low-temperature PL spectra of DFBC in a 2-methyltetrahydrofuran
(2-MeTHF) solution at 77 K.
Fig. 4. Current densityevoltageeluminance (JeVeL) characteristic of the blue device.

276
C. Wu et al. / Dyes and Pigments 97 (2013) 273e277
Table 1
Characteristics of the blue, orange and white OLEDs.
Type
Conc. hc
hp
Von V@1000 cd/m2 Emission
CIE[x,y]e
(%)
(cd/A) (lm/w) (V) (V)
peak (nm)
Bluea
e
1.6
20.1
15.8
21.5
1.1
21.0
11.5
22.5
3.1 5.3
3.3 5.8
3.6 5.9
3.1 4.6
448
584
584
0.16, 0.10
0.55, 0.42
0.56, 0.43
DFBC-Ob 2.0
CBP-Oc
Whited
4.0
e
584/512/448 0.41, 0.35
hc, hp are the maximum current efficiency and power efficiency, respectively. Von is
turn-on voltage (required to reach the luminance of 1 cd/m2).
a
Blue OLED: ITO/NPB (50 nm)/DFBC (20 nm)/TPBi (25 nm)/LiF (1 nm)/Al
(100 nm).
b
DFBC-O: ITO/NPB (40 nm)/2% Ir(2-phq)3:DFBC(30 nm)/TPBi (40 nm)/LiF
(1 nm)/Al(100 nm).
c
CBP-O: ITO/NPB (40 nm)/4% Ir(2-phq)3:CBP(30 nm)/TPBi (40 nm)/LiF
(1 nm)/Al(100 nm).
d
White OLED: ITO/NPB (40 nm)/DFBC: 2% Ir(2-phq)3 (10 nm)/DFBC (15 nm)/TPBi: 7%
Ir(ppy)3 (15 nm)/TPBi (20 nm)/LiF (1 nm)/Al (100 nm).
e
The value taken at L ¼ 1000 cd/m2.
Fig. 6. An energy level diagram of the white OLED.
Taking into account the blue-emission property and the high
performance of DFBC as a host for a phosphorescent dopant,
a white F/P OLED was fabricated with a configuration of ITO/NPB
(40 nm)/DFBC: 2% Ir(2-phq)3 (10 nm)/DFBC (15 nm)/TPBi: 7%
Ir(ppy)3 (15 nm)/TPBi (20 nm)/LiF (1 nm)/Al (100 nm) (Fig. 6). In
the device, tris(2-phenylpyridine)iridium (III) (Ir(ppy)3) is used as
the green emitting material. As Fig. 6 depicts, the energy barrier for
holes to cross the DFBC/TPBi:Ir(ppy)3 interface is 0.4 eV, and the
barrier for electrons to cross the TPBi:Ir(ppy)3/DFBC interface is
0.2 eV. It can be deduced that the main exciton generation area
should be located in the DFBC and TPBi:Ir(ppy)3 interface. Both
singlet and triplet excitons are formed in the interface with a ratio
of 1:3.[13] Since the triplet energy of DFBC (2.33 eV) is higher than
that of Ir(2-phq)3 (2.0 eV) and the triplet excitons have long dif-
fusion lengths (w100 nm) [6a], the triplet excitons formed in DFBC
layer can easily diffuse to the DFBC: Ir(2-phq)3 layer to produce
orange phosphorescence. Singlet excitons formed in DFBC can
generate blue fluorescence through direct recombination and the
triplet excitons formed in the TPBi:Ir(ppy)3 layer can generate
green phosphorescence. Combining the blue fluorescence and or-
ange, green phosphorescence, highly efficient white emission can
be obtained.
1.0
0.8
0.6
0.4
0.2
0.0
Luminance (cd/m2) CIE
1000 (0.45, 0.37)
5000 (0.41, 0.35)
10000 (0.41. 0.35)
300
400
500
600
700
800
Wavelength (nm)
Fig. 7. EL spectra of the white OLED under different luminance.
Fig. 8 shows the current densityevoltageeluminance (JeVeL)
characteristics of the white OLED. The turn-on voltage, voltages at
1000 cd/m2, 10,000 cd/m2 for the device are 3.1, 4.6 and 6.6 V,
respectively. The device shows a maximum efficiency of 22.5 lm/W
and an efficiency of 9.4 lm/Wat a luminance of 1000 cd/m2 (a typical
brightness required for lighting applications).
In summary, a new multifunctional molecule (DFBC) has been
designed, synthesized and characterized for use in high performance
OLEDs. DFBC was found to have good morphological and thermal
stability with a glass transition temperature of 192 ꢀC and a decom-
position temperature of 480 ꢀC. It has been demonstrated that
Fig. 7 shows EL spectra of the optimized white OLED at different
brightness. At the luminance of 5000 and 10,000 cd/m2, the corre-
sponding CIE coordinates are (0.41, 0.35) and (0.38, 0.33), respec-
tively. The changes in the color coordinates is due to a mild increase
in blue and green emission, because the generation of singlet exci-
tons on DFBC starts only above a certain voltage, and upon
increasing the operating voltage, more holes can be injected into the
DFBC/TPBi:Ir(ppy)3 interface resulting in enhanced green emission.
104
DFBC:2%Ir(2-phq)3
300
500
400
300
200
100
0
104
103
102
101
100
103
250
200
102
150
101
100
100
50
0
2
3
4
5
6
7
8
9
10
2
3
4
5
6
7
8
9
10
Voltage (V)
Voltage (V)
Fig. 5. JeVeL characteristic of the DFBC based orange OLED.
Fig. 8. JeVeL characteristic of the optimized white OLED.

C. Wu et al. / Dyes and Pigments 97 (2013) 273e277
277
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Acknowledgment
This work was supported by “the Science and Technology Plan-
ning Project of Guangdong Province, China (2011A091200002)”, S. L.
Tao would like to acknowledge supports from “the National Natural
Science Foundation of China (NSFC Grant 21002011), “Program for
New Century Excellent Talents in the University of the Chinese
Ministry of Education (NCET-11-0067)”, the special fund project
of industry, university and research institute collaboration of
Guangdong province and Ministry of Education of P. R. China
(2011B090400464) and the Science and Technology Program for
Dongguan’s Higher Education, Science and Research, and Health
Care Institutions (2011108101006).
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