A new N-fluorenyl carbazole host material: Synthesis, physical properties and applications for highly efficient phosphorescent organic light emitting diodes

Article abstract of DOI:10.1016/j.orgel.2011.02.014

A new N-fluorenyl carbazole material, 9,9′-bis-(9,9-dimethyl-9H- fluoren-2-yl)-9H,9′H-[3,3′]bicarbazolyl (BDFC), was synthesized by bromination, Ullmann and Yamamoto coupling reactions and confirmed using various spectroscopic studies. Thermogravimetric analysis and differential scanning calorimetry studies show the thermal stability (ΔT_5%) of 494.7 °C with high glass transition temperature (T_g) of 177.8 °C. The photophysical and electrochemical studies of BDFC show the photoluminescence at 408 nm and a band gap of 3.01 eV with higher triplet energy of 2.72 eV. The phosphorescent organic light emitting diode using BDFC as host, ITO/di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC)/host: fac-tris(2-phenylpyridine)-iridium [Ir(ppy)₃] (5%)/1,3,5-tris(m- pyrid-3-yl-phenyl)benzene (TmPyPB)/LiF/Al, shows the effective confinement of triplet excitons and efficient energy transfer to the guest emitter in the emissive layer, resulted in the higher device efficiencies of 56.3 cd/A, 18.1% and 21.3 lm/W compared with that (48.1 cd/A, 15.3% and 16.3 lm/W) of device based on (4,4′-N,N′-dicarbazole)biphenyl (CBP) as host. The results show that the new host material BDFC could be useful for the efficient organic light emitting diodes.

Full text of DOI:10.1016/j.orgel.2011.02.014

Organic Electronics 12 (2011) 785–793
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
A new N-fluorenyl carbazole host material: Synthesis, physical
properties and applications for highly efficient phosphorescent organic
light emitting diodes
Min-Gi Shin a,1, K. Thangaraju , Seul-ong Kim , Jong-Won Park , Yun-Hi Kim
⇑
,
Soon-Ki Kwon
a
a,1
a
a
b,
a,
⇑
School of Materials Science, Engineering & Engineering Research Institute (ERI), Gyeongsang National University, Jinju-660 701, South Korea
Department of Chemistry and Research Institute of Natural Sciences (RINS), Gyeongsang National University, Jinju-660 701, South Korea
b
a r t i c l e i n f o
a b s t r a c t
0
0
Article history:
A new N-fluorenyl carbazole material, 9,9 -bis-(9,9-dimethyl-9H-fluoren-2-yl)-9H,9 H-
[
Received 24 November 2010
Received in revised form 14 February 2011
Accepted 15 February 2011
Available online 26 February 2011
0
3,3 ]bicarbazolyl (BDFC), was synthesized by bromination, Ullmann and Yamamoto cou-
pling reactions and confirmed using various spectroscopic studies. Thermogravimetric
analysis and differential scanning calorimetry studies show the thermal stability (
of 494.7 °C with high glass transition temperature (T ) of 177.8 °C. The photophysical
5%
DT )
g
and electrochemical studies of BDFC show the photoluminescence at 408 nm and a band
gap of 3.01 eV with higher triplet energy of 2.72 eV. The phosphorescent organic light
emitting diode using BDFC as host, ITO/di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane
Keywords:
Organic light emitting diodes
N-fluorenyl carbazole derivative
High triplet energy state
High thermal stability
Green electrophosphorescence
High efficiency
(
3
TAPC)/host: fac-tris(2-phenylpyridine)-iridium [Ir(ppy) ] (5%)/1,3,5-tris(m-pyrid-3-yl-
phenyl)benzene (TmPyPB)/LiF/Al, shows the effective confinement of triplet excitons and
efficient energy transfer to the guest emitter in the emissive layer, resulted in the higher
device efficiencies of 56.3 cd/A, 18.1% and 21.3 lm/W compared with that (48.1 cd/A,
0
0
1
5.3% and 16.3 lm/W) of device based on (4,4 -N,N -dicarbazole)biphenyl (CBP) as host.
The results show that the new host material BDFC could be useful for the efficient organic
light emitting diodes.
Ó 2011 Elsevier B.V. All rights reserved.
1
. Introduction
The organic light emitting diodes (OLEDs) have been at-
electroluminescent (EL) efficiency due to the harvesting
of both singlet- and triplet-excitons for the light emission
[3,4]. Hence, the tremendous efforts have recently been
made in the development of highly efficient phosphores-
cent organic light emitting diodes (PHOLEDs) [5–10]. In
PHOLEDs, phosphorescent materials are often doped into
a host matrix as guest triplet emitters, in order to reduce
aggregation quenching and triplet–triplet annihilation.
For the efficient electrophosphorescence from the triplet
guests in a host matrix, HOMO–LUMO (HOMO: highest
occupied molecular orbital; LUMO: lowest unoccupied
tracted very much attention in the recent past for the next-
generation flat-panel displays, solid state lighting and
other applications due to its self-emission, high luminous
efficiency, high contrast ratio, fast response time, and wide
color gamut [1,2]. In particular, studies with respect to
phosphorescent materials have been an essential focus
on the OLED research, which efficiently improve the
g
molecular orbital) energy gap (E ) and the triplet energy le-
⇑
Corresponding authors. Tel.: +82 55 751 5296 (Y.-H. Kim), tel.: +82 55
51 5296; fax: +82 753 6311 (S.-K. Kwon).
E-mail addresses: ykim@gnu.ac.kr (Y.-H. Kim), skwon@gnu.ac.kr (S.-K.
vel of the host material must be higher than those of the
guest to facilitate exothermic energy transfer from the host
to guest and to prohibit reverse energy transfer from the
guest back to the host (i.e., to effectively confine triplet
7
Kwon).
1
These authors are equally contributed to this work.
1
566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2011.02.014

7
86
M.-G. Shin et al. / Organic Electronics 12 (2011) 785–793
excitons to the guest molecules) [11–15]. The host materi-
als for electrophosphorescence reported thus far are lim-
ited to a few functionalities, including carbazoles and
phenylsilanes [13–16]. The carbazole derivatives, which
possess sufficiently large triplet energies and carrier trans-
port properties, are widely used as host materials in
PHOLEDs [17–19]. Examples of these materials include
the samples. All other compounds were used as received.
All the materials used in the phosphorescent organic light
emitting diodes were purchased from Lumtec corp.,
Taiwan.
2.2. Synthesis
0
0
0
0
4
,4 -bis(9-carbazolyl)-2,2 -biphenyl (CBP), 1,3-bis(9-car-
2.2.1. Synthesis of 9-(9 ,9 -dimethyl-9H-fluoren-2-yl)-9H-
carbazole (1)
bazolyl)benzene (mCP) and 3,5-bis(9-carbazolyl)tetra-
phenylsilane (SimCP). The most commonly used host
material CBP has been shown useful for green- and red-
light-emitting PHOLEDs [20–26]. However, CBP has several
A mixture of activated copper powder (11.4 g, 0.120
mol),
2 3
K CO (16.53 g, 0.120 mol), 9H-carbazole (20 g,
0
0
0.120 mol), 2-bromo-9 ,9 -dimethyl-9H-fluorene (35.94 g,
0.132 mol)and18-crown-6(4.74 g, 0.0179 mol)indichloro-
benzene (200 mL) was heated to 200 °C for 30 h. After that,
the reaction mixture was filtered with methylene chloride
(MC). After dichlorobenzene was removed by vacuum
distillation, the crude product was purified by column chro-
matography(using MC/hexane: 1/5 as eluent) and recrystal-
g
problems such as low T and low triplet energy state which
deteriorate efficiency and longevity of the device. It has
been reported that fluorene derivatives have been impor-
tant materials for OLEDs because of their high quantum
yield of photoluminescence (PL), electroluminescence
(
EL) efficiencies and high thermal stability [27–29]. How-
1
ever, it has been reported that the extension of the carba-
zole conjugation, which decreases the triplet energy of the
carbazole as in the case of CBP, can be minimized by alter-
ing the linkage group to carbazole unit, while also maxi-
mizing the thermal stability of the material [18,30]. Thus,
the research directed towards the design and synthesis of
new carbazole derivatives with high triplet energy and
thermal stability has not been limited [31,32].
lization with ethanol. Yield: 38.33 g (89.15%). H NMR
3
(300 MHz, CDCl ) d: 8.21–8.18 (d, 2H), 7.96–7.93 (d, 1H),
7.84–7.81 (t, 1H), 7.65–7.64 (d, 1H), 7.58–7.54 (dd, 1H),
7.51–7.39 (m, 7H), 7.35–7.30 (m, 2H), 1.58–1.55 (d, 6H).
1
3
3
C NMR (300 MHz, CDCl ) d: 155.16, 153.85, 141.03,
138.49, 138.45, 136.62, 127.60, 127.25, 125.96, 125.87,
123.38, 122.77, 121.46, 121.09, 120.36, 120.20, 119.89,
+
109.87, 47.16, 27.16. M : 359.
Thus, we designed highly twisted N-fluorenyl carbazole
as a new host material. The introduction of fluorene unit,
possessing high photoluminescence and electrolumines-
cence efficiencies, to carbazole unit is to inhibit the exten-
sion of conjugation and improve the thermal stability of
the material, which is suitable for host in the efficient elec-
troluminescent devices. Moreover, the highly twisted
structure due to coupling of N-fluorenylated carbazole
would increase its glass transition temperature and molec-
ular rigidity as well as inhibited intermolecular interaction.
In the present study, we report on the synthesis and char-
acterization of a new carbazole derivative BDFC in which
fluorene-coupled at 3-position of carbazole unit. A new
N-fluorenyl carbazole material BDFC shows the higher
0
0
2.2.2. Synthesis of 3-bromo-9-(9 ,9 -dimethyl-9H-fluoren-2-
yl)-9H-carbazole (2)
N-Bromosuccinimide (13.37 g, 0.097 mol) was added to
0
0
the solution of 9-(9 ,9 -dimethyl-9H-fluoren-2-yl)-9H-car-
bazole (1) (35 g, 0.097 mol) and silica-gel (18 g) in methy-
lene chloride (400 mL). The reaction mixture was stirred at
room temperature. Before extraction with water and
Methylene chloride, the reaction mixture was filtered with
Methylene chloride. The mixture of reaction was purified
by column chromatography (using MC/hexane: 1/5 as elu-
ent) and recrystallization with ethanol. Yield: 40.59 g
(95.10%). H NMR (300 MHz, CDCl
8.14–8.12 (d, 1H), 7.96–7.93 (d, 1H), 7.60–7.59 (d, 1H),
7.53–7.52 (d, 1H), 7.51–7.50 (t, 3H), 7.47–7.33 (m, 6H),
1.57–1.56 (d, 6H). C NMR (300 MHz, CDCl ) d: 155.56,
3
1
3
), d: 8.29–8.28 (d, 1H),
thermal stability (
D
T5%) of 494.7 °C (CBP: 447 °C) with high
1
3
glass transition temperature (T
g
) of 177.8 °C (CBP: 62 °C)
and a band energy gap of 3.01 eV with high triplet energy
of 2.72 eV (CBP: 2.56 eV) for use as host material in the
efficient organic light emitting diodes. We fabricate
green-emitting phosphorescent organic light emitting
diodes using the new BDFC as host and Ir(ppy) as guest
emitter and report its higher device performances com-
pared with that of the device based on widely used CBP
host.
153.82, 141.36, 139.71, 138.83, 138.27, 136.08, 128.63,
127.72, 127.27, 126.68, 125.79, 125.08, 123.06, 122.77,
122.27, 121.35, 121.18, 120.52, 120.29, 120.24, 112.61,
+
111.37, 110.09, 47.18, 27.13. M : 439.
0
2.2.3. Synthesis of 9,9 -bis-(9,9-dimethyl-9H-fluoren-2-yl)-
0
0
9H,9 H-[3,3 ]bicarbazolyl (BDFC)
mixture of bis(cyclooctadiene)nickel(0) (2.85 g,
0.37 mmol) and 2,2-bipyridyl (1.86 g, 11.92 mmol) in
A
1
dimethylformamide (40 mL) and toluene (160 mL) was
2
. Experimental
heated to 80 °C. After 30 min, cyclooctadiene (0.93 g,
0
0
8
.64 mmol) and 3-bromo-9-(9 ,9 -dimethyl-9H-fluoren-2-
2
.1. Materials
yl)-9H-carbazole (2) (4 g, 9.12 mmol) in toluene (40 mL)
were added to the reaction mixture and stirred for 144 h.
The reaction mixture in HCl and Dioxane was stirred for
15 min and washed with HCl. The reaction mixture was
All reagents and solvents were purchased from Aldrich
Chemical Co. and Fluka. All the solvents were freshly dis-
tilled over appropriate drying reagents before use. The
2 2
purified by column chromatography (using CH Cl /hexane:
spectroscopic grade CHCl
UV–visible absorption and photoluminescence spectra of
3
(Aldrich) was used to measure
1/10 as eluent) and recrystallization with methylene chlo-
ride and ethanol. Yield: 2.53 g (77.35%). FT-IR (KBr): 3043,

M.-G. Shin et al. / Organic Electronics 12 (2011) 785–793
787
1
2
8
954. H NMR (300 MHz, CDCl
3
) d: 8.53–8.52 (s, 1H), 8.28–
out using a Genesis II FT-IR spectrometer. A Jeol JMS-700
high-resolution mass spectrometer (HRMS) was used to
obtain the mass spectra of the samples. Thermogravimetric
analysis (TGA) was performed under nitrogen using a TA
instrument 2025 thermogravimetric analyzer. The sample
was heated from 50 °C to 800 °C with a heating rate of
10 °C/min. Differential scanning calorimeter (DSC) studies
were carried out under nitrogen using a TA instrument
2100 differential scanning calorimeter. The sample was
heated from 30 °C to 300 °C with a heating rate of 10 °C/
min. UV–visible absorption and photoluminescence (PL)
studies at room temperature were carried out using
Perkin Elmer LAMBDA-900 UV/VIS/NIR spectrometer and
LS-50B luminescence spectrophotometer, respectively
and the phosphorescence spectrum was recorded from
the delayed emission of BDFC at 77 K. Cyclic voltammo-
gram (CV) of the sample was recorded using a Epsilon E3
.27 (d, 1H), 8.00–7.97 (d, 1H), 7.86–7.83 (m, 2H), 7.71
s, 1H), 7.64–7.59 (m, 2H), 7.53–7.43 (m, 6H), 1.61–1.60
d, 6H).
41.50, 140.17, 138.50, 138.46, 136.70, 134.37, 127.61,
27.26, 126.10, 125.87, 125.81, 124.02, 123.62, 122.78,
21.39, 121.14, 120.48, 120.21, 120.00, 118.94, 110.15,
40 2
10.01, 47.19, 27.18. HRMS: calcd. for C54H N : 716.3191,
found: 716.3192.
(
(
1
1
1
1
1
3
3
C NMR (300 MHz, CDCl ) d: 155.50, 153.86,
2
.3. Measurements
1H nuclear magnetic resonance (NMR) spectra were re-
corded using DRX 300 MHz Bruker spectrometer and
chemical shifts in spectra were reported in parts per mil-
lion (ppm) units with tetramethylsilane as internal stan-
dard. Infrared (IR) studies of the samples were carried
Fig. 1. The synthetic scheme of BDFC.
5
0
5
-
-10
-15
-20
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers
Fig. 2. FT-IR spectrum of BDFC.

7
88
M.-G. Shin et al. / Organic Electronics 12 (2011) 785–793
Fig. 3. The molecular structure (determined by Spartan08 software) of BDFC.
Fig. 4. The UV–visible absorption, photoluminescence (at room temper-
ature) and phosphorescence (at 77 K) spectra of BDFC in toluene
1 ꢀ 10ꢁ M).
5
(
at room temperature in a 0.1 M solution of tetrabutylam-
monium perchlorate (Bu NClO ) in acetonitrile under
4
4
nitrogen environment at a scan rate of 50 mV/s. The plati-
num rod was used as working and the counter electrode,
and a Ag/AgNO electrode was used as the reference elec-
3
trode. The mobility of BDFC molecule was determined
from the Time-of-Flight (ToF) experiment. For ToF experi-
ment, a semitransparent aluminum (Al) layer of 20 nm
thickness was deposited on the glass substrate as bottom
electrode by thermal evaporation process. A thick BDFC
Fig. 5. (a) Thermogravimetric analysis (TGA) and (b) differential scanning
calorimetry (DSC) studies of BDFC. Inset shows the glass transition
film of 1 lm thickness was drop-casted on the bottom
electrode using the BDFC solution (3 wt.% in toluene) by
vacuum drying at 60 °C for 24 h. The thickness and flatness
of drop-cast BDFC film were measured using an alpha-
stepper (Alpha step 500, Tencor). The device for ToF exper-
iment was completed by depositing a thin Al film of 20 nm
thickness on the above prepared BDFC film for the top elec-
trode. The ToF experiment was performed in a vacuum
cryogenic chamber by illuminating the sample with N la-
2
ser (337 nm) pulses of duration 6 ns under a dc bias for the
ToF photocurrent transients [33].
g
temperature (T ) of BDFC.
2.4. Device fabrication
The phosphorescent organic light emitting diodes were
fabricated as shown in Fig. 6. The device structure was as
follows: indium-tin-oxide (ITO)/1,1-bis[di-4-tolylami-
no]phenyl]cyclohexane (TAPC) as the hole transport layer
(HTL) (30 nm)/emissive layer (EML) (30 nm)/1,3,5-tris
(m-pyrid-3-yl-phenyl)benzene (TmPyPB) as the electron

M.-G. Shin et al. / Organic Electronics 12 (2011) 785–793
789
transport layer (ETL) (30 nm)/LiF (1 nm)/Al (120 nm). The
was prepared as depicted in Fig. 1. The BDFC was synthe-
sized via manifold chemical reactions such as bromination,
Ullmann and Yamamoto coupling reactions as described
below. The compound 1 was obtained by Ullmann coupling
0
0
(
9
4,4 -N, N -dicarbazole)biphenyl (CBP) host for device A or
,9 -Bis-(9,9-dimethyl-9H-fluoren-2-yl)-9H,9 H-[3,3 ]bicar-
0
0
0
bazolyl (BDFC) host for the device B doped with 5%
green-emitting phosphorescent dopant, fac-tris(2-phenyl-
0
0
reaction of 9H-carbazole and 2-bromo-9 ,9 -dimethyl-
9H-fluorene using activated copper powder, K CO and
3
pyridine)iridium [Ir(ppy) ] was used as emissive layer
2
3
(
EML). The CBP host based device was fabricated for the
18-crown-6. BDFC was prepared by Yamamoto coupling
comparison of the device performances with that of BDFC
host based device. The patterned ITO (anode) coated glass
substrates were ultrasonically cleaned in acetone for
reaction of compound 2 obtained from mono-bromination
of compound 1. The structure of BDFC was confirmed by H
1
NMR, IR and HRMS. In the IR spectrum, we confirmed the
structure from disappearing of aromatic C–Br stretching
1
0 min followed by isopropyl alcohol, dried in each step
ꢁ1
at 90 °C for 10 min and UV treated for 10 min before load-
ing into the deposition chamber for the device fabrication.
All organic layers were deposited by thermal evaporation
at 1060 cm (Fig. 2). The angle of 126° between carbazole
and fluorene units in the highly twisted structure of BDFC
molecule was determined by Spartan08 software as shown
in Fig. 3.
ꢁ6
process under a vacuum of ꢂ5 ꢀ 10 Torr and LiF as elec-
tron injection layer and Al cathode were deposited without
breaking the chamber vacuum. The fabricated devices
were transferred into the glove-box for the encapsulation
of the devices with moisture getter using UV curable
epoxy. The current density–voltage (J–V) characteristics
of all devices were measured using Keithley 236 source
meter. The electroluminescence and luminescence charac-
teristics of the devices were recorded using Spectrascan
PR655 spectroradiometer under ambient condition.
The UV–visible absorption, photoluminescence and
phosphorescence (at 77 K) spectra of BDFC in toluene
ꢁ5
(1 ꢀ 10 M) are shown in Fig. 4. From the UV–visible
absorption spectrum of BDFC, the absorption bands be-
tween 294 nm and 330 nm are assigned for S
0 2
to S and
S
0
to S electronic transitions of the BDFC molecule [30].
1
BDFC showed the photoluminescence (PL) at 408 nm in
toluene, indicating the emission from its lowest exited
state. The phosphorescence spectrum of BDFC was ob-
tained from the delayed emission of BDFC in toluene
ꢁ5
3
. Results and discussion
The new N-fluorenyl carbazole material, 9,9 -bis-(9,9-
(1 ꢀ 10 M) at 77 K. From the phosphorescence spectrum
of BDFC molecule, the triplet energy (T ) was estimated
from the highest energy peak (455 nm) to be 2.72 eV,
which is higher than that (T : 2.56 eV) of CBP host
19,34]. The mobility of BDFC was determined from the
1
0
0
0
dimethyl-9H-fluoren-2-yl)-9H,9 H-[3,3 ]bicarbazolyl (BDFC),
1
[
Time-of-Flight (ToF) experiment. It was calculated using
2
d
the equation
l
=
Vt, where d is the thickness of the film, V
is the applied dc bias voltage and t is the measured transit
time [33]. The ToF mobility of BDFC host was found to be
ꢁ
4
2
3
.4 ꢀ 10 cm /Vs at bias voltage of 28.26 mV and transit
time of 10.45 s. The thermal properties of BDFC sample
were determined by differential scanning calorimeter
DSC) and thermogravimetric analysis (TGA) measure-
ments. From the DSC data (Fig. 5b), the glass transition
temperature (T ) was observed at 177.8 °C, which is much
higher than that (62 °C) of CBP, and the melting tempera-
ture (T ) was observed at 259.62 °C as a sharp endother-
l
(
g
m
mic peak [19]. In TGA curve of BDFC (Fig. 5a), 5% weight
loss was observed at 494.7 °C, showing the higher thermal
stability compared with that (447 °C) of CBP. The higher
Fig. 6. Cyclic voltammogram of BDFC.
Table 1
The physical properties of BDFC.
Absorptiona
Emission
b
c
d/T
(°C)
e/T
f
g
h
k
abs
k
PL
k
Phos
T
d
g
m
HOMO
(eV)
LUMO
(eV)
E
g
T
1
(
nm)
(nm)
(nm) (at 77 K)
(eV)
(eV)
2
94,305,330
408
455
494.7/177.8/259.6
5.38
2.37
3.01
2.72
a
b
c
The major UV–visible absorption band peaks are listed.
Photoluminescence (PL) maximum measured in tolune at room temperature.
The highest energy peak of the phosphorescence spectrum in tolune at 77 K.
d
e
f
T
T
T
E
T
d
– Temperature corresponding to a 5% weight loss in thermogravimetric analysis.
– Glass transition temperature from differential scanning calorimetry (DSC) studies.
– Melting temperature from DSC studies.
g
m
g
g
h
– Band gap energy calculated from the equation E
g
= hc/k = 1241/k, where k is the edge wavelength (nm) of UV–visible absorption spectrum.
1
– Triplet energy is estimated from the higher energy peak of the phosphorescence spectrum.

7
90
M.-G. Shin et al. / Organic Electronics 12 (2011) 785–793
triplet energy and thermal stability of BDFC compared with
CBP reveal the decreased conjugation of carbazole unit by
fluorene unit coupled to it [18,30]. The electrochemical
properties of BDFC sample was investigated by cyclic vol-
tammetric (CV) studies. The CV curve of BDFC is shown
in Fig. 6. It shows the reversible oxidation potential charac-
terized by Eonset value at 0.88 V. The estimated ionization
potential (IP = Eonset + 4.54 = HOMO level) was 5.38 eV.
The higher HOMO energy level of BDFC can be useful for
the efficient hole-injection and transport. The LUMO en-
ergy level of BDFC was calculated to be 2.37 eV from the
edge wavelength of UV–visible absorption spectrum. BDFC
having the band gap of 3.01 eV with high triplet energy of
3
dopant, fac-tris(2-phenylpyridine)iridium [Ir(ppy) ], as
guest emitter were fabricated as shown in Fig. 7 and char-
acterized. PHOLED based on widely used CBP was fabri-
cated for the comparison. The energy level diagram and
the structure of materials used in the devices are shown
in Fig. 8 [19,34–38]. The PHOLEDs were fabricated using
the following device structure: ITO/1,1-bis[di-4-tolylami-
3
no]phenyl]cyclohexane (TAPC) (30 nm)/host: Ir(ppy) with
optimized concentration of 5% (30 nm)/1,3,5-tris(m-pyrid-
3-yl-phenyl)benzene (TmPyPB) (30 nm)/LiF/Al. The CBP
was used as host in the device A and BDFC host was used
in device B. TAPC with LUMO energy level of 2.0 eV (LUMO:
2.6 eV for CBP; 2.37 eV for BDFC) and higher triplet energy
2
.72 eV could be suitable for use as host material for the
(T
1
) of 2.9 eV (T
1
: 2.56 eV for CBP; 2.72 eV for BDFC;
) was used as hole transport layer
efficient organic light emitting diodes. The thermal, photo-
physical and electrochemical properties of BDFC are listed
in Table 1.
2.46 eV for Ir(ppy)
3
(HTL), while TmPyPB with HOMO energy level of 6.68 eV
(HOMO: 5.9 eV for CBP; 5.38 eV for BDFC) and higher trip-
let energy of 2.78 eV compared with that of host and do-
pants was used as electron transport layer (ETL) in the
devices for the effective confinement of charge carriers
and/or excitons within the emissive layer (EML). Fig. 9
shows the current density–voltage–luminescence (J–V–L)
characteristics of devices A and B. From the J–V–L charac-
teristics of device A, the drive voltages of 5 V for 100 cd/
To investigate the new N-fluorenyl-carbazole material
(
BDFC) as host in organic light emitting diodes (OLEDs),
the phosphorescent organic light emitting diodes (PHOL-
EDs) based on BDFC as host and green phosphorescent
2
2
m
and 8 V for 1000 cd/m were observed and it showed
2
a maximum luminescence of 23,250 cd/m at 16 V. Device
A showed the turn-on voltage (defined as an applied volt-
age required for the luminescence of 1 cd/m ) of 3 V.
2
Fig. 10 shows the current- and external quantum-effi-
ciency characteristics of devices A and B. Device A based
on CBP host showed the maximum current efficiency of
4
8.0 cd/A, external quantum efficiency of 15.3% and power
efficiency of 16.3 lm/W. The electroluminescence spectra
of devices A and B at 10.5 V are shown in Fig. 11. The
device A showed the electroluminescence (EL) emission
Fig. 7. The device structures of the devices A and B.
Fig. 8. The energy level diagram and molecular structures of the materials used in the devices.

M.-G. Shin et al. / Organic Electronics 12 (2011) 785–793
791
Fig. 9. The current density–voltage–luminescence (J–V–L) characteristics
Fig. 11. The electroluminescence spectra of the devices A and B at 10.5 V.
of devices A and B.
adjacent charge transporting layers were observed, indi-
cating the effective confinement of charge carriers and/or
excitons and complete energy transfer to guest emitter in
the EML of the device A.
Device B based on BDFC host showed the drive voltages
2
2
of 8.5 V for 100 cd/m and 13.5 V for 1000 cd/m , and a
2
maximum luminescence of 22,930 cd/m at 20 V as shown
in Fig. 9. The turn-on voltage is increased to 3.5 V com-
pared with that (3 V) of device A. The increased energy bar-
rier of 0.41 eV for electron injection at BDFC/TmPyPB
interface of the device B than that (0.18 eV) of device A
using CBP host resulted in the higher operating voltage
than that of device A. The device B using BDFC host exhib-
ited the higher current efficiency of 56.8 cd/A, external
quantum efficiency of 18.1% and power efficiency of
2
1
1.3 lm/W compared with that (48.1 cd/A, 15.3% and
6.3 lm/W) of device A using CBP host, resulting from the
efficient energy transfer and the higher triplet energy (T
.72 eV) of BDFC host (T : 2.56 eV for CBP). The device B
showed the higher quantum efficiency roll-off (17% at
1
:
2
1
2
8
.3 mA/cm ) for the increasing current densities compared
2
with that (2% at 8.3 mA/cm ) of the device A using CBP
Fig. 10. (a) The current efficiency-current density- and (b) external
quantum efficiency–current density-characteristics of the devices
A and B.
peak at 512 nm with shoulder peak at 539 nm with CIE
color coordinates of (0.31, 0.61), originating from Ir(ppy)
dopant [36,39–41]. No emissions from the host and/or
3
Fig. 12. The electroluminescence spectra of the device B at different
voltages.

7
92
M.-G. Shin et al. / Organic Electronics 12 (2011) 785–793
Table 2
The electroluminescence characteristics of the devices.
a
Deviceb
Turn-on
voltage (V)
L
g
g
g
ELc
(nm)
CIE
(x, y)
c
ext
p
2
(cd/m , V)
(cd/A, V)
(%, V)
(lm/W, V)
A
B
3
3.5
23,250, 16.0
22,930, 20.0
48.0, 10.5
56.8, 10.5
15.3, 10.5
18.1, 10.5
16.3, 9.0
21.3, 8.0
512
512
(0.31, 0.61)
(0.30, 0.61)
a
The data for brightness (L), current efficiency (
g
c
), external quantum efficiency (
g
p
ext) and power efficiency (g ) are the maximum values of the
2
corresponding device. The turn-on voltage for all the devices was defined as voltage required for luminescence of 1 cd/m .
b
The device structure is ITO/TAPC (30 nm)/Host: Ir(ppy)
The peak emission wavelength of the EL spectrum of corresponding device.
3
(5%) (30 nm)/TmPyPB (30 nm)/LiF/Al with CBP host for device A and BDFC host for device B.
c
host. The higher efficiency roll-off of the device B can be
explained by the imbalance of charge carriers in the EML
due to the higher HOMO (5.38 eV) and LUMO (2.37 eV)
energy levels of BDFC host than that (HOMO: 5.9 eV;
LUMO: 2.6 eV) of CBP host. In other words, the higher
HOMO energy level of BDFC host in the device reduces
the hole injection barrier, which is much favorable for hole
injection, while the higher LUMO level increases the elec-
tron injection barrier, less favor for election injection, com-
pared with that of CBP host in device and which in turn
makes the charge imbalance in the EML of the device B
based on BDFC, leading to the increased efficiency roll-off
compared with that of device A based on CBP. The device
B showed the characteristic dopant emission at 512 nm
with shoulder peak at 539 nm with CIE color coordinates
of (0.30, 0.61) at 10.5 V (Fig. 11). The device B based on
BDFC host exhibits the EL emission at 512 nm with CIE col-
or coordinates of (0.30, 0.62), (0.30, 0.61), (0.30, 0.61) and
LiF/Al. The device showed the effective confinement of
triplet excitons and efficient energy transfer to the guest
emitter in the EML, resulted in the higher current effi-
ciency of 56.3 cd/A, external quantum efficiency of 18.1%
and power efficiency of 21.3 lm/W compared with that
(48.1 cd/A, 15.3% and 16.3 lm/W) of the device based on
CBP host. The results show that the new host material
BDFC could be useful for the efficient organic light emitting
diodes.
Acknowledgements
This research was financially supported by MKE and
KIAT through the Workforce Development Program in
Strategic Technology, by Strategic Technology under Min-
istry of Knowledge Economy of Korea and by Basic Science
Research Program through the National Research Founda-
tion of Korea (NRF) funded by the Ministry of Education,
Science and Technology (2011-0000310).
(
0.31, 0.61) at 5 V, 10 V, 15 V and 20 V, respectively as
shown in Fig. 12. The small residual emission of device B
around 408 nm at higher operating voltage is attributed
to the BDFC host emission. The device based on BDFC host
showed the effective confinement of triplet excitons and
efficient energy transfer to the guest emitter in the EML
of the device, resulted in the higher device efficiencies
compared with that of CBP based device. The characteris-
tics of the devices A and B are summarized in Table 2.
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