Highly efficient orange and warm white phosphorescent OLEDs based on a host material with a carbazole-fluorenyl hybrid

Article abstract of DOI:10.1002/asia.201400129

A new carbazole-fluorenyl hybrid compound, 3,3′(2,7-di(naphthaline-2- yl)-9H-fluorene-9,9-diyl)bis(9-phenyl-9H-carbazole) (NFBC) was synthesized and characterized. The compound exhibits blue-violet emission both in solution and in film, with peaks centere

Full text of DOI:10.1002/asia.201400129

COMMUNICATION
DOI: 10.1002/asia.201400129
Highly Efficient Orange and Warm White Phosphorescent OLEDs Based on
a Host Material with a Carbazole–Fluorenyl Hybrid
Xiaoyang Du,[a, b] Yun Huang,[a] Silu Tao,*[a] Xiaoxia Yang,[a] Chuan Wu,[b] Huaixin Wei,[b]
Mei-Yee Chan,[c] Vivian Wing-Wah Yam,[c] and Chun-Sing Lee*[b]
Abstract: A new carbazole–fluorenyl hybrid compound, 3,3’-
ACHTUNGTRENNUNG(2,7-di(naphthaline-2-yl)-9H-fluorene-9,9-diyl)bis(9-phenyl-
ciency (IQE) of about 25% due to singlet–triplet statistics.[2]
By making full use of both the singlet and the triplet states,
phosphorescent OLEDs (PHOLEDs) could theoretically
achieve an IQE of 100%.[3] With the rapid development of
OLEDs, highly efficient red and green PHOLEDs with
a long lifetime have been demonstrated.[4,5] However, there
are still several aspects that need to be improved in order to
meet the demands of commercialization. Firstly, the perfor-
mance of blue phosphorescent emitters is still considerably
lower than those of their red and green counterparts.[6] Fur-
thermore, phosphorescent emitters have often a poor life-
time, which is another obstacle for their commercialization.
To address the inadequacy of blue phosphors, fluorescence/
phosphorescence (F/P) hybrid OLEDs using blue fluores-
cent but red and green phosphorescent emitters have been
proposed.[7] On the other hand, the significant efficiency
roll-off in PHOLEDs is another challenge for its application
in solid-state lighting. Many researchers have paid signifi-
cant efforts into this issue and have demonstrated some ef-
fective strategies such as using dual-emitting layers (D-
EMLs),[8] a co-host system, and new host materials.[9,10]
9H-carbazole) (NFBC) was synthesized and characterized.
The compound exhibits blue-violet emission both in solution
and in film, with peaks centered at 404 and 420 nm. In addi-
tion to the application as a blue emitter, NFBC is demon-
strated to be a good host for phosphorescent dopants. By
doping IrACHTUNGTRENNUNG(2-phq)3 in NFBC, a highly efficient orange organic
light-emitting diode (OLED) with a maximum efficiency of
32 cdAÀ1 (26.5 LmWÀ1) was obtained. Unlike most phos-
phorescent OLEDs, the device prepared in our study shows
little efficiency roll-off at high brightness and maintains cur-
rent efficiencies of 31.9 and 26.8 cdAÀ1 at a luminance of
1000 and 10000 cdmÀ2, respectively. By using NFBC simul-
taneously as a blue fluorescence emitter and as a host for
a phosphorescent dopant, a warm white OLED with a maxi-
mum efficiency of 22.9 LmWÀ1 (21.9 cdAÀ1) was also ob-
tained.
In this work, we designed a new blue-violet emitter, 3,3’-
Introduction
ACHTUNGERTN(NUNG 2,7-di(naphthaline-2-yl)-9H-fluorene-9,9-diyl)bis(9-phenyl-
9H-carbazole) (NFBC). An OLED using NFBC as a non-
doped fluorescent emitter gives blue-violet emission with
a peak at 428 nm and CIE coordinates of (0.16, 0.05). An
orange OLED with a maximum efficiency of 32 cdAÀ1
(26.5 LmWÀ1) has been obtained by doping tris(2-phenyl-
Organic light-emitting diodes (OLEDs) have important ap-
plications in high-efficiency full-color flat panel displays and
solid-state lighting.[1] OLEDs based on fluorescent emitters
are theoretically capped to have an internal quantum effi-
quinoline)iridium [IrACHTNUGTRNEUNG(2-phq)3] into NFBC. It is interesting
that the device presents an extremely low efficiency roll-off
and maintains efficiencies of 31.9 and 26.8 cdAÀ1 at a lumi-
nance of 1000 and 10000 cdmÀ2 respectively. These perfor-
mance parameters are much better than those of most re-
ported orange or red/orange phosphorescent OLEDs. Fur-
thermore, by combining the blue-violet fluorescence with
the orange phosphorescent emission, an efficient warm
white F/P OLED has been obtained with a maximum effi-
ciency of 22.9 LmWÀ1 (21.9 cdAÀ1).
[a] X. Du, Y. Huang, Dr. S. Tao, X. Yang
School of Optoelectronic Information
University of Electronic Science and Technology of China (UESTC)
Chengdu 610054 (China)
Fax : (+86)28-83201745
E-mail: silutao@uestc.edu.cn
[b] X. Du, C. Wu, H. Wei, Prof. C.-S. Lee
Center of Super-Diamond and Advanced Films (COSDAF) & De-
partment of Physics and Materials Science
City University of Hong Kong
Hong Kong Special Administrative Region (China)
Fax : (+852)27847826
E-mail: apcslee@cityu.edu.hk
Results and Discussions
[c] Dr. M.-Y. Chan, Dr. V. W.-W. Yam
Department of Chemistry
Scheme 1 depicts the synthetic routes and chemical structure
of
3,3’ACTHUNGTERNNU(G 2,7-di(naphthaline-2-yl)-9H-fluorene-9,9-diyl)bis(9-
The University of Hong Kong
Hong Kong Special Administrative Region (China)
phenyl-9H-carbazole) (NFBC). The molecular structure of
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Chun-Sing Lee, Silu Tao et al.
1
NFBC was confirmed by H-NMR spectroscopy, elemental
furan (2-MeTHF) solution measured at 77 K. This triplet
energy is high enough to sensitize a common orange dopant
(such as IrACHTUNGRTNEUNG(2-phq)3, 2.0 eV). Key physical data of NFBC are
listed in Table 1. Owing to its highly efficient blue-violet
emission and high triplet level, NFBC should be a promising
material as a blue-violet emitter and a host for OLED appli-
cations.
analysis, and mass spectrometry. Thermal properties of
NFBC were studied by thermogravimetric analysis (TGA)
and differential scanning calorimetry (DSC). NFBC shows
a high thermal decomposition temperature (Td, correspond-
ing to a weight loss of 5%) of 4848C and a high glass transi-
tion temperature (Tg) of 1838C (Figure 1).
Table 1. Key physical data of NFBC.
Tg
Td
lem, max (film/solu- HOMO LUMO Eg
F (film/sol-
[8C] [8C] tion) [nm][a]
[eV]
[eV]
[eV] ution)[b]
183 484 420/404
5.9
2.9
3.0 0.58/0.5
[a] Emission maxima of NFBC in film and in CH2Cl2 solution. [b] Fluo-
rescence quantum yield of NFBC in film and tetrahydrofuran (THF) so-
lution using 2-aminopyridine (F=0.6) as a standard.
To investigate the blue-violet emission property of NFBC,
a non-doped device with a configuration of ITO/NPB
(40 nm)/TCTA (10 nm)/NFBC (20 nm)/TPBi (30 nm)/LiF
(1 nm)/Al (80 nm) was fabricated. In the device, 4,4’-bis[N-
(1-naphthyl)-N-phenylamino]biphenyl (NPB) is the hole-
transporting layer (HTL); 4,4’,4’’-tris(N-carbazolyl)triphe-
nylamine (TCTA) is used as the electron-blocking layer
Figure 1. TGA and DSC (inset) measurements of NFBC.
Figure 2 shows the UV/Vis absorption and photolumines-
cence (PL) spectra of NFBC recorded at room temperature
from a dilute CH2Cl2 solution as well as a neat film deposit-
ed on a quartz substrate. The PL spectra of NFBC show
(EBL),
while
1,3,5-tris(N-phenylbenzimidazol)benzene
(TPBi) works as an electron-transporting layer (ETL) as
well as a hole-blocking layer (HBL) and NFBC is the emit-
ting layer (EML). As shown in Figure 3, the device gives
Figure 2. Absorption and photoluminescence spectra of NFBC.
Figure 3. EL spectra at different luminance and J–V–L curve (inset) of
the blue-violet device.
a blue-violet emission with maxima at 404 and 420 nm, re-
spectively, in solution and in the solid film. Remarkably, the
compound exhibits a narrow emission with full width at half
maximum (FWHM) of about 40 nm in both spectra, which
is helpful to obtain a saturated color in OLEDs. The fluores-
cence quantum yield (F) of NFBC in tetrahydrofuran
(THF) solution was measured to be 0.5 by using 2-amino-
pyridine (F=0.6) as a standard, while that in the solid film
was measured to be 0.58. NFBC has a HOMO level of
5.9 eV, as determined by UV photoelectron spectroscopy
(UPS), and a LUMO level of 2.9 eV, which was obtained by
subtracting the HOMO value with its optical gap of 3.0 eV.
The triplet energy level of NFBC was determined to be
2.32 eV from its PL characteristics in a 2-methyltetrahydro-
blue-violet emission with a maximum at 428 nm with CIE
coordinates of (0.16, 0.05), which is shifted slightly over the
brightness range of 10–1000 cdmÀ2. The current density–
voltage–luminescence (J–V–L) characteristics are shown in
the inset. The device shows a turn-on voltage (defined as
the driving voltage at a brightness of 1 cdmÀ2) of 3.4 V.
Figure 4 shows plots of the current efficiency and the exter-
nal quantum efficiency as a function of luminance of the
blue-violet device. The obtained maximum external quan-
tum efficiency (hext, max) of 2.8% and the maximum current
efficiency of 0.8 cdAÀ1 (0.6 LmWÀ1) at a luminance of
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Chun-Sing Lee, Silu Tao et al.
Figure 4. Current efficiency–luminance–EQE curve of the blue-violet
device.
700 cdmÀ2 with a mild efficiency roll-off is comparable with
those of reported devices with CIEy<0.055.[12]
Figure 5. EL spectra at different luminance and J–V–L characteristics of
the orange device.
To evaluate the potential of NFBC as a host material for
phosphorescent devices, a device using Ir
orange dopant was fabricated with a configuration of ITO/
NPB (30 nm)/TCTA (10 nm)/NFBC:2%Ir(2-phq)3 (30 nm)/
ACHTUNGTERNUN(NG 2-Phq)3 as an
AHCTUNGTRENNUNG
TPBi (40 nm)/LiF (1 nm)/Al (80 nm). Figure 5 shows the EL
spectra and the current density–voltage–luminance (J–V–L)
characteristics (inset) of the device. The device has a turn-
on voltage of 3.7 V and shows an orange emission with
a maximum at 588 nm. The emission spectrum remains es-
sentially unchanged over a wide range of brightness, thus
suggesting that the exciton generation zone has been suc-
cessfully confined. This is attributed to the high triplet
energy levels of TCTA (2.76 eV) and TPBi (2.6 eV).
Figure 6 shows efficiencies of the device at different bright-
ness outputs. The device has a maximum current efficiency
of 32 cdAÀ1, a maximum power efficiency of 26.5 LmWÀ1,
and a maximum external quantum efficiency of 15.3%. It is
noteworthy that the device shows a small efficiency roll-off
and maintains current efficiencies of 31.6, 31.9, and
26.8 cdAÀ1, respectively at 100, 1000, and 10000 cdmÀ2. This
may be attributed to the sufficient triplet energy level of
NFBC resulting in a complete energy transfer from NFBC
Figure 6. Current efficiency–luminance–power efficiency characteristics
of the orange device.
Considering its blue-violet fluorescence and good perfor-
mance as a host material, the merits of NFBC were exploit-
ed in F/P hybrid white OLEDs (WOLEDs) with configura-
to Ir
(2-phq)3. The high emission efficiency and mild efficien-
tions of: device I: ITO/NPB (30 nm)/NFBC: 2%Ir
(5 nm)/NFBC (15 nm)/TPBi: 8%Ir(ppy)3 (20 nm)/TPBi
(15 nm)/LiF (1 nm)/Al (80 nm) and device II: ITO/NPB
(30 nm)/NFBC: 2%Ir(2-phq)3 (5 nm)/NFBC (20 nm)/TPBi:
ACHTUNGTRENNUNG(2-phq)3
cy roll-off of this device are comparable to the best values
reported for orange or orange-red phosphorescent OLEDs
with similar device structures (Table 2).[13]
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
Table 2. Summary of the emission characteristics of representative orange OLEDs.
[b]
[c]
d
e
Emission
layer
EQE[a]
[%]
lem, max [nm]
hc.max
[cdAÀ1
hc.100
[cdAÀ1
hc.1000
[cdAÀ1
hc.10000
[cdAÀ1
]
CIE coordinates
Ref.
G
]
E
]
N
]
R
NFBC:Ir
DADBT:Ir
BPPI:Ir(2-phq)3
DFBC:Ir(2-phq)3
CBP:(pbi)2Ir(biq)
CBP:Ir(TBT)2A(acac)
CBP:(impy)2Ir(acac)
OTPD:x-emitter
Bebq2:Ir(phq)2acac
G
15.3
16.7
12.4
–
588
584
581
584
586
588
568
570
605
32
31.6
31.8
22.5
–
20
15.8
32
31.9
29.6
23
26.8
–
21.5
–
9.2
–
10.5
–
(0.57,0.42)
(0.56,0.43)
(0.61,0.39)
(0.55,0.42)
(0.53,0.46)
(0.58,0.41)
(0.52,0.48)
(0.55,0.44)
(0.62,0.37)
This work[h]
[13a][h]
[13b][h]
[11a][h]
[13c][p]
A
32.7
23.9
20.1
20.2
15.8
34.2
18.4
26.5
E
G
–
G
N
–
17.5
10.9
25
15
20.53
G
G
9.1
12.7
–
[13d][p]
[13e][p]
[13f][p]
E
N
18.4
26
N
21.0
17.6
[13g][h]
[a] Maximum external quantum efficiency. [b] Maximum current efficiency. [c,d,e] Current efficiency at 100 cdmÀ2, 1000 cdmÀ2, and 10000 cdmÀ2, respec-
tively. [h] New host materials for orange OLEDs. [p] New dopant for orange OLEDs.
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Chun-Sing Lee, Silu Tao et al.
8%Ir
Here, Ir
(ppy)3) were used as orange and green phosphorescent emit-
ters, respectively. Due to the high triplet energy of Ir(ppy)3
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
(2.40 eV), TPBi was used as the host. Firstly, TPBi has
a high enough triplet energy of 2.74 eV when compared
with that of IrACHTUNGTRENNUNG(ppy)3, which is beneficial for energy transfer.
Furthermore, TPBi has good electron-transporting and hole-
blocking properties, and thus the use of TPBi as the host is
helpful for charge transport and recombination. The undop-
ed NFBC layer was used for blue fluorescent emission. The
structure and energy level diagram of the device are shown
schematically in Figure 7. Figure 8 shows the EL spectra of
Figure 9. Current efficiency–luminance plot and J–V–L curve (inset) of
devices I and II.
ficiency of 21.9 cdAÀ1, and an external quantum efficiency
of 8.2%, which is comparable or better than that of many
reported F/P hybrid WOLEDs.[14] To further tune the color
coordinates of the device, we increased the thickness of the
NFBC layer. The optimized device (device II) shows CIE
coordinates of (0.36, 0.34), which is very close to the stan-
dard white emission of (0.33, 0.33). The maximum efficiency
values of the device are 13.4 LmWÀ1, 14.9 cdAÀ1, and 5.7%
(hext, max).
Figure 7. The device structure of the WOLED and energy levels of the
materials.
Conclusions
In summary, we synthesized and characterized a new carba-
zole-fluorenyl-based compound, that is 3,3’ACTHNUGRTENUNG(2,7-di(naphtha-
line-2-yl)-9H-fluorene-9,9-diyl)bis(9-phenyl-9H-carbazole)
(NFBC), which possesses a high glass transition temperature
(Tg) of 1838C and a decomposition temperature (Td) of
4848C. The compound shows blue-violet emissions both in
solution and in a film, with peaks centered at 404 and
420 nm. An OLED using NFBC as a non-doped fluorescent
emitter gives blue-violet emission with a maximum at
428 nm and CIE coordinates of (0.16, 0.05). Moreover,
highly efficient orange emission has been achieved by using
NFBC as the host doped with IrACHTNUGTRENUNG(2-phq)3, with a maximum
efficiency of 32 cdAÀ1 (26.5 LmWÀ1). It is noteworthy that
the device presents an extremely low efficiency roll-off, with
efficiencies of 31.6, 31.9, and 26.8 cdAÀ1, respectively, at100,
1000, and 10000 cdmÀ2, which is much better than that of
most orange or red/orange phosphorescent OLEDs reported
recently. Furthermore, an efficient warm white emission has
been obtained with a maximum efficiency of 22.9 LmWÀ1
(21.9 cdAÀ1) with the F/P device structure. These results
suggest that NFBC is a promising multifunctional material
for high-performance OLED applications.
Figure 8. EL spectra of a) device I and b) device II at different lumi-
nance.
the two devices at different luminance. As the luminance in-
creases, CIE coordinates of the devices are slightly blue-
shifted, which is attributed to a shift of the exciton genera-
tion zone. Figure 9 shows the corresponding current efficien-
cy–luminance–power efficiency plots and J–V–L curves
(inset) of devices I and II. Device I has a lower turn-on volt-
age of 3.1 V than that of device II (3.7 V). Device I shows
a warm white emission with CIE coordinates of (0.45, 0.40)
and a maximum power efficiency of 22.9 LmWÀ1, current ef-
Chem. Asian J. 2014, 9, 1500 – 1505
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Chun-Sing Lee, Silu Tao et al.
Device Fabrication and Measurements
Experimental Section
Indium tin oxide (ITO)-coated glass with a sheet resistance of 15 W per
square were used as substrates. Before device fabrication, the ITO glass
substrates were washed with isopropyl alcohol and deionized water, dried
in an oven at 1208C for 2 h, and then treated with UV light-ozone for
30 min before they were loaded into a vacuum deposition chamber with
a base vacuum <10À6 Torr. Organic layers were successively deposited
onto the ITO glass substrates with a rate of 1–2 ꢁsÀ1. An electron-inject-
ing LiF layer and an Al cathode were deposited with rates of 0.1 and
10 ꢁsÀ1, respectively. Electroluminance (EL) spectra, CIE coordinates,
and current density–voltage–luminance (J–V–L) characteristics were
measured with a computer-controlled Keithley 2400 source meter and
a Spectrascan PR650 photometer under ambient atmosphere.
Chemicals and Instrumentation
All reactions were carried out under a nitrogen atmosphere and all pow-
ders and solvents were used as received from the commercial suppliers
without further purification. The final products were confirmed by using
1H-NMR spectroscopy, elemental analysis (EA), and mass spectrometry
(MS). The glass transition temperature (Tg) was measured by differential
scanning calorimetry (DSC) using a PerkinElmer Pyris DSC6 system op-
erated at a heating rate of 108CminÀ1. The decomposition temperature
(Td) was measured by thermogravimetric analysis (TGA) on a TA SDT
Q600 instrument at a heating rate of 108CminÀ1 under a nitrogen flow.
Optical absorption and emission spectra were recorded with a Hitachi U-
3010 UV/Vis spectrophotometer and a Hitachi F-4500 fluorescence spec-
trophotometer, respectively. The highest occupied molecular orbital
(HOMO) energy level was measured using ultraviolet photoelectron
spectroscopy (UPS). The lowest unoccupied molecular orbital (LUMO)
energy level was determined from the difference between the HOMO
energy level and the energy gap (Eg) obtained from the optical absorp-
tion edge.
Acknowledgements
C.S.L. would like to acknowledge a grant from the Research Grants
Council of the Hong Kong Special Administrative Region, China (Proj-
ect No. T23-713/11); S.L.T would like to acknowledge the National Natu-
ral Science Foundation of China (NSFC Grant 51373029, 21002011) and
the “Program for New Century Excellent Talents in the University of the
Chinese Ministry of Education (NCET-11-0067)”.
Synthesis
3,3’-(2,7-Dibromo-9H-fluorene-9,9-diyl)bis(9-phenyl-9Hcarbazole) (1, see
Scheme 1) was synthesized according to previously reported methods.[11]
Keywords: blue fluorescent host · doping · fluorescence ·
OLEDs · phosphorescence · warm white emission
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Scheme 1. Synthesis of NFBC.
3,3’ACHTUNGTRENNUNG(2,7-Di(naphthaline-2-yl)-9H-fluorene-9,9-diyl)bis(9-phenyl-9H-
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carbazole) (NFBC)
NFBC was obtained through a classical Suzuki cross-coupling reaction:
Compound 1 (0.81 g, 1 mmol), 2-naphthaleneboronic acid (0.43 g,
2.5 mmol), [PdACHTUNGTRENNUNG(PPh3)4] (0.1 g, 0.087 mmol), aqueous Na2CO3 (2.0m,
20 mL), ethanol (15 mL), and toluene (20 mL) were mixed in a flask. The
mixture was degassed three times and then heated under reflux at 1008C
for 24 h under a nitrogen atmosphere. After cooling of the reaction mix-
ture to room temperature, the solvent was evaporated. The resulting resi-
due was then extracted with CH2Cl2, washed with water, and dried over
MgSO4. The crude product was purified by column chromatography on
silica gel (petroleum ether/CH2Cl2, 3:1). The product was then further
purified by recrystallization with petroleum ether and CH2Cl2 to give
NFBC as
a
white solid. Yield: 0.58 g (65%); 1H-NMR (CDCl3,
300 MHz): d=7.14–7.25 (t, 2H), 7.33–7.42 (m, 6H), 7.43–7.64 (m, 16H),
7.66–7.78 (t, 4H), 7.83–7.93 (t, 4H), 7.93–7.98 (t, 4H), 7.98–8.07 (d,2H),
8.12–8.17 (s, 2H), 8.17–8.24 (s, 2H), 8.24–8.35 ppm (s, 2H); MS (ESI+):
m/z 900 (M+); elemental anal. calcd (%) for C69H44N2: C 92.00, H 4.89,
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Received: January 28, 2014
Revised: March 5, 2014
Published online: April 22, 2014
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