Bifluorene compounds containing carbazole and/or diphenylamine groups and their bipolar charge transport properties in organic light emitting devices

Article abstract of DOI:10.1016/j.orgel.2010.01.010

Novel bipolar bifluorene compounds containing carbazole and/or diphenylamine groups were synthesized by C-C and C-N coupling using a palladium catalyst. The ionization potentials of the compounds reflected the electron withdrawing or donating nature of the substituents. The charge transport properties of these compounds were evaluated from time-of-flight transient photocurrent measurements. Large transient currents (10^-4-10^-3 cm²/Vs) based on holes and electrons were observed. Hole or electron-only devices containing these compounds with p-type doping with MoO₃ and n-type with Cs showed Ohmic current density-voltage characteristics. Organic light emitting devices with homo-junction structures containing the bifluorene compounds (ITO/bifluorene:MoO₃ (50 mol%, 20 nm)/bifluorene (10 nm)/bifluorene:rubrene (60 nm)/bifluorene (10 nm)/bifluorene:Cs (30 wt.%, 20 nm)/Al) exhibited external quantum efficiencies of 1.5-2.0%.

Full text of DOI:10.1016/j.orgel.2010.01.010

Organic Electronics 11 (2010) 717–723
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Bifluorene compounds containing carbazole and/or diphenylamine
groups and their bipolar charge transport properties
in organic light emitting devices
*
*
Keiji Noine, Yong-Jin Pu , Ken-ichi Nakayama, Junji Kido
Department of Organic Device Engineering, Yamagata University, Yonezawa, Yamagata 992-8510, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel bipolar bifluorene compounds containing carbazole and/or diphenylamine groups
were synthesized by C–C and C–N coupling using a palladium catalyst. The ionization
potentials of the compounds reflected the electron withdrawing or donating nature of
the substituents. The charge transport properties of these compounds were evaluated
from time-of-flight transient photocurrent measurements. Large transient currents
(10ꢀ4–10ꢀ3 cm2/Vs) based on holes and electrons were observed. Hole or electron-only
devices containing these compounds with p-type doping with MoO3 and n-type with
Cs showed Ohmic current density–voltage characteristics. Organic light emitting devices
with homo-junction structures containing the bifluorene compounds (ITO/bifluo-
rene:MoO3 (50 mol%, 20 nm)/bifluorene (10 nm)/bifluorene:rubrene (60 nm)/bifluorene
(10 nm)/bifluorene:Cs (30 wt.%, 20 nm)/Al) exhibited external quantum efficiencies of
1.5–2.0%.
Received 6 November 2009
Accepted 8 January 2010
Available online 14 January 2010
Keywords:
Organic light emitting diode
Bipolar
Fluorene
Time-of-flight
Charge mobility
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
diphenylamine, 7,70-bis-(N,N0-carbazolyl)-9,9,90,90-tetra-
ethyl-2,20-bifluorene (BCzBF), 7,70-bis-(N,N0-diphenyl-
There has been a growing interest in organic light emit-
ting devices (OLEDs) because they are expected to be the
next generation in energy efficient flat panel displays and
illumination light sources [1,2]. The charge balance of
holes to electrons in the emitting layer is one of the impor-
tant factors determining device efficiency. To ensure a
well-balanced charge ratio, the host material in an emit-
ting layer has to be bipolar, possessing both hole and elec-
tron transporting ability. The charge mobilities need to be
as high as possible to reduce the driving voltage of the
device. Recently host materials showing bipolar charge
transport properties have been reported by several groups
[3–10]. In this paper, we report the synthesis of novel
bifluorene compounds incorporating carbazole and/or
amino)-9,9, 90,90-tetraethyl-2,20-bifluorene (BDABF) and
7-(N-carbazole)-70-(N-diphenylamine)-9,9,90,90-tetraethyl-
2,20-bifluorene (CzDABF). The charge mobilities of these
compounds were assessed using time-of-flight (TOF) tech-
niques. Carbazole groups are expected to endow the com-
pounds with bipolar properties. However, the ionization
potential (Ip) of a carbazole derivative should be large, like
that of 4,40,N,N0-diphenylcarbazole (CBP) (6.1 eV) [9]. This
will cause a large energy barrier to hole injection from
the anode which increases the driving voltage. CzDABF
and BDABF which contain diphenylamine as an electron
donating group were designed to decrease Ip. These bipolar
materials were incorporated in OLEDs with a homo-junc-
tion structure [10,11] using p-type and n-type chemical
doping [12–14]. The homo-junction device structure is
effective at reducing the number of organic/organic inter-
faces, increasing the driving voltage and side reactions
through accumulated charges [11].
* Corresponding authors. Tel.: +81 238 26 3052; fax: +81 238 26 3412.
E-mail addresses: pu@yz.yamagata-u.ac.jp (Y.-J. Pu), kid@yz.yamagata-u.
ac.jp (J. Kido).
1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2010.01.010

718
K. Noine et al. / Organic Electronics 11 (2010) 717–723
g, 1.24 mmol) and sodium tert-butoxide (1.49 g, 23.4
2. Experimental
mmol) in toluene (60 ml) was stirred under reflux for
24 h under a nitrogen atmosphere. After cooling to room
temperature, the mixture was poured into water and then
extracted with toluene. The combined organic phases were
washed with brine and dried over MgSO4. The product was
purified by column chromatography on silica gel (chloro-
form/hexane = 1/3), affording 3 (2.09 g, 57.6%) as a white
2.1. Syntheses and characterization of compounds
Unless otherwise noted, all chemicals were obtained
from commercial suppliers and used without further puri-
fication. 1H NMR spectra were measured in deuterated sol-
vents on a JEOL ECX 400 MHz spectrometer. Elemental
analyses were carried out by the Elemental Analysis Ser-
vice, Yamagata University, Japan. Thermal gravimetric
analysis (TGA) was performed on Seiko SII EXSTAR 6000
and TGA/DTA 6200 analyzers. Differential Scanning Calo-
rimetry (DSC) was performed on a Perkin–Elmer Diamond
DSC calorimeter. Ionization potentials were measured with
a photoelectron spectrometer surface analyzer (RIKEN KEI-
KI AC-3). UV–visible absorption spectra were recorded of
solutions in chloroform or films on quartz with a Shimadzu
UV-3150 spectrometer. PL spectra were recorded on a Jo-
bin Yvon Fluoromax-2 fluorometer. PL quantum efficien-
cies were measured on a Hamamatsu C9920-01 integral
sphere system under nitrogen.
Compound 2: A mixture of 2,7-dibromo-9,9-diethylflu-
orene (1) (22.8 g, 60.0 mmol), carbazole (5.01 g, 30.0
mmol), copper (11.4 g, 180 mmol) and potassium carbon-
ate (25.2 g, 180 mmol) in o-dichlorobenzene (200 ml)
was stirred under reflux for 24 h under a nitrogen atmo-
sphere. After cooling to room temperature, the mixture
was poured into water and then extracted with toluene.
The combined organic phases were washed with brine
and dried over MgSO4. The crude mixture was purified by
column chromatography on silica gel (chloroform/hex-
ane = 1/3) affording 2 (8.61 g, 61.6%) as a white solid. 1H
NMR (400 Mz, CDCl3, ppm) d 0.44 (6H, t, J = 7.4 Hz),
2.02–2.07 (4H, m), 7.26–7.32 (2H, m), 7.39–7.45 (4H, m),
7.49–7.55 (4H, m), 7.64 (1H, d, J = 8.8 Hz), 7.89 (1H, d,
J = 8.8 Hz), 8.16 (1H, d, J = 7.8 Hz).
solid. 1H NMR (400 Mz, CDCl3, ppm)
d 0.35 (6H, t,
J = 6.5 Hz), 1.9 (4H, t, J = 7.8 Hz), 7.00 (3H, t, J = 7.3 Hz),
7.06 (1H, s), 7.10 (4H, d, J = 7.3 Hz), 7.21–7.27 (4H, m),
7.39–7.47 (3H, m), 7.52 (1H, d, J = 8.2 Hz).
BDABF: A mixture of 3 (1.20 g, 2.56 mmol), bis(pinacola-
to)diboron (0.541 g, 2.13 mmol), PdCl2(dppf) (60.0 mg,
0.0768 mmol) and potassium acetate (0.627 g, 6.39 mmol)
in DMSO (60 ml) was stirred at 80 °C for 5 h under a
nitrogen atmosphere. After cooling the solution to room
temperature, 3 (0.80 g, 1.70 mmol), PdCl2(dppf) (0.04 g,
0.0510 mmol) and 2 M aqueous potassium carbonate
(5.5 ml, 5.0 eq) were added and the mixture was stirred
at 80 °C under nitrogen overnight. The mixture was cooled
to room temperature and then the product was extracted
with chloroform and washed with water, brine and dried
over MgSO4. The product was purified by column chroma-
tography on silica gel (chloroform/hexane = 1/2), giving
BDABF (0.880 g, 53.2%) as a yellow solid. The product
was further purified by train sublimation. Yield: 0.801 g,
47.9%. 1H NMR (400 Mz, CDCl3, ppm) d 0.42 (12H, t,
J = 7.32 Hz), 1.9–2.1 (8H, m), 7.00–7.07 (6H, m), 7.13
(10H, d, J = 8.1 Hz), 7.23–7.28 (8H, m), 7.55–7.69 (8H, m).
Elemental Anal. Calcd for C58H52N2: C, 89.65; H, 6.75; N,
3.61. Found C, 89.65; H, 6.82; N, 3.48%.
Compound 4: n-Butyl lithium (1.6 M, 49.3 ml,
78.9 mmol) was added to a stirred solution of 1 (30.0 g,
78.9 mmol) in dry THF (500 ml) under a nitrogen atmo-
sphere at ꢀ78 °C. After stirring for 0.5 h, trimethylchlorosi-
lane (11.1 ml, 94.7 mmol) was added. The mixture was
warmed to room temperature, then the product was ex-
tracted with chloroform and washed with water, brine
and dried over MgSO4. The crude mixture was purified by
column chromatography on silica gel (hexane) to give 4
(27.6 g, 93.7%) as a colorless oil. 1H NMR (400 Mz, CDCl3,
ppm) d 0.28–0.35 (15H, m), 1.95–2.06 (4H, m), 7.41–7.46
(4H, m), 7.48 (1H, d, J = 7.5 Hz), 7.56 (1H, d, J = 8.7 Hz),
7.65 (1H, d, J = 7.3 Hz).
Compound 5: n-Butyl lithium (1.6 M, 55.3 ml, 88.7
mmol) was added to a stirred solution of 1 (27.5 g,
73.8 mmol) in dry THF (500 ml) under a nitrogen atmo-
sphere at ꢀ78 °C. After stirring for 0.5 h, trimethylchloro-
silane (27.2 ml, 133 mmol) was added. The mixture was
cooled to room temperature, then the product was ex-
tracted with chloroform and washed with water, brine
and dried over MgSO4. The crude mixture was purified
by column chromatography on silica gel (hexane) fol-
lowed by recrystallization from toluene/ethanol = 1/4 to
give 5 (21.9 g, 52.0 mmol) as a white solid. 1H NMR
(400 Mz, CDCl3, ppm) d 0.26–0.32 (15H, m), 1.4 (12H, s),
2.0–2.1 (4H, m), 7.45 (1H, s), 7.49 (1H, d, J = 7.3 Hz),
7.70 (2H, d, J = 7.3 Hz), 7.75 (4H, s), 7.8 (1H, d, J =
7.3 Hz).
BCzBF: A mixture of 2 (4.20 g, 9.00 mmol), bis(pinacola-
to)diboron (2.00 g, 15.0 mmol), PdCl2(dppf) (0.183 g,
0.225 mmol), and potassium acetate (2.21 g, 22.5 mmol)
in DMSO (350 ml) was stirred at 80 °C for 8 h under a nitro-
gen atmosphere. After cooling the solution to room tem-
perature,
2 (3.80 g, 6.00 mmol), PdCl2(dppf) (0.183 g,
0.225 mmol) and 2 M aqueous potassium carbonate
(10 ml, 5.0 eq) were added and the mixture was stirred at
80 °C under nitrogen overnight. The mixture was cooled
to room temperature and then the product was extracted
with chloroform and washed with water, brine and dried
over MgSO4. The crude mixture was purified by column
chromatography on silica gel (chloroform/hexane = 1/2),
giving BCzBF as a yellow solid. The product was further
purified by train sublimation. Yield: 2.08 g, 35.4%. 1H
NMR (400 Mz, CDCl3, ppm) d 0.53 (12H, t, J = 7.4 Hz),
2.08–2.23 (8H, m), 7.30–7.34 (6H, m), 7.42–7.46 (6H, m),
7.53–7.59 (4H, m), 7.69 (2H, s), 7.75 (2H, d, J = 7.4 Hz),
7.75 (2H, d, J = 8.1 Hz), 7.96 (2H, d, J = 7.5 Hz), 8.18 (4H,
d, J = 8.0 Hz). Elemental Anal. Calcd for C58H48N2: C,
90.12; H, 6.26; N, 3.62. Found C, 89.93; H, 6.26; N, 3.56%.
Compound 3: A mixture of 1 (5.89 g, 15.5 mmol),
diphenylamine (1.31 g, 7.75 mmol), Pd(OAc)2 (34.8 mg,
0.155 mmol), 1,10-bis(diphenylphosphino)ferrocene (0.687

K. Noine et al. / Organic Electronics 11 (2010) 717–723
719
Compound 6: A mixture of 2 (5.68 g, 12.2 mmol), 5
(6.14 g, 14.6 mmol), Pd(PPh3)4 (0.28 g, 0.244 mmol) and
2 M aqueous potassium carbonate (5.00 g, 36.6 mmol) in
toluene (60 ml) and ethanol (30 ml) was stirred under re-
flux for 24 h under nitrogen. After cooling to room temper-
ature, the mixture was poured into water and then
extracted with toluene. The combined organic phases were
washed with brine and dried over MgSO4. The crude mix-
ture was purified by column chromatography on silica
gel (chloroform/hexane = 2/3), affording 6 (6.32 g, 76.1%)
as a white solid. 1H NMR (400 Mz, CDCl3, ppm) d 0.33
(9H, s), 0.42 (6H, t, J = 7.3 Hz), 0.51 (6H, t, J = 7.4 Hz), 2.0–
2.2 (8H, m), 7.28–7.35 (2H, m), 7.44–7.56 (8H, m), 7.63–
7.74 (5H, m), 7.80 (1H, d, J = 7.8 Hz), 7.86 (1H, d,
J = 8.2 Hz), 7.95 (1H, d, J = 7.8 Hz), 8.19 (2H, d, J = 7.8 Hz).
Compound 7: A mixture of 6 (6.50 g, 9.55 mmol) and io-
dine monochloride (1 M in CH2Cl2, 10.5 ml, 10.5 mmol) in
CH2Cl2 (80 ml) was stirred at 0 °C for 1 h under nitrogen.
After cooling to room temperature, the mixture was
poured into water and then extracted with toluene. The
combined organic phases were washed with brine and
dried over MgSO4. The crude mixture was purified by col-
umn chromatography on silica gel (chloroform/hexane = 1/
3), affording 7 (4.60 g, 65.6%) as a white solid. 1H NMR
(400 Mz, CDCl3, ppm) d 0.40 (6H, t, J = 7.8 Hz), 0.51 (6H, t,
J = 7.8 Hz), 2.0–2.2 (8H, m), 7.28–7.35 (2H, m), 7.44 (4H,
d, J = 4.2 Hz), 7.50 (1H, d, J = 3.2 Hz), 7.53–7.71 (8H, m),
7.76 (1H, d, J = 7.8 Hz), 7.77 (1H, d, J = 7.8 Hz), 7.85 (1H,
d, J = 7.8 Hz), 7.95 (2H, d, J = 7.8 Hz).
CzDABF: A mixture of 7 (3.50 g, 4.77 mmol), diphenyl-
amine (1.21 g, 7.16 mmol), Pd2(dba)3 (34.9 mg, 0.0382
mmol), tri(tert-butyl)phosphine (0.0114 ml, 0.0477 mmol)
and sodium tert-butoxide (0.688 g, 7.16 mmol) in toluene
(60 ml) was stirred under reflux for 24 h under nitrogen.
After cooling to room temperature the mixture was poured
into water and then extracted with toluene. The combined
organic phases were washed with brine and dried over
MgSO4. The crude mixture was purified by column chroma-
tography on silica gel (chloroform/hexane = 1/2), affording
CzDABF (2.17 g, 58.7%) as a white solid. The product was fur-
ther purified by train sublimation. Yield: 0.801 g, 51.7%. 1H
NMR (400 Mz, CDCl3, ppm) d 0.44 (6H, t, J = 7.0 Hz), 0.51
(6H, t, J = 6.8 Hz), 1.9–2.2 (8H, m), 7.00–7.07 (3H, m), 7.14
(5H, d, J = 7.2 Hz), 7.25–7.30 (6H, m), 7.44 (4H, d,
J = 3.6 Hz), 7.53–7.72 (8H, m), 7.85 (1H, d, J = 8.2 Hz), 7.94
(1H, d, J = 7.8 Hz), 8.17 (2H, d, J = 7.8 Hz). Elemental Anal.
Calcd for C58H50N2: C, 89.88; H, 6.50; N, 3.61. Found C,
89.75; H, 6.51; N, 3.58%.
3 surface profilometer. The EL spectra were measured on
a Hamamatsu PMA-11 photonic multichannel analyzer.
The current–voltage (I–V) characteristics and luminance
of the devices were measured using a Keithley 2400 Source
Meter and Konica Minolta CS-200 chromameter, respec-
tively. External quantum efficiencies were calculated
assuming a Lambertian emission pattern and considering
all spectral features in the visible region.
3. Results and discussion
Scheme 1 shows the synthetic routes to the carbazole
and/or diphenylamine substituted-difluorene derivatives.
Monocarbazolyl compound 2 was synthesized by an Ull-
mann coupling reaction between 1 and carbazole. After
synthesis of the boronic ester derivative of 2 by the reac-
tion of bis-(pinacolato)diboron and 1 with a palladium cat-
alyst, dicarbazolyl compound BCzBF was synthesized
successively in the same pot, via palladium-catalyzed Su-
zuki coupling of 2 with the boronic ester derivative of 2.
3 was synthesized by palladium-catalyzed C–N coupling
between 1 and diphenylamine. BDABF was synthesized in
the same manner as BCzBF. In the synthesis of CzDABF,
the coupling reaction between 2 and the boronic ester of
3 and the coupling reaction between 3 and boronic ester
2 gave a small amount of BCzBF or BDABF, respectively,
as byproducts because of a homocoupling side reaction.
These byproducts are difficult to remove from CzDABF be-
cause of their similar structure and polarity. Therefore,
CzDABF was synthesized using a different route. 6 was ob-
tained from a coupling reaction between 2 and 5, then re-
acted with ICl to form 7. Finally, a diphenylamine group
was introduced to 7, giving CzDABF.
TGA showed that the decomposition temperatures (Td)
with loss of 5 wt.% of the compounds were over 400 °C.
The glass transition temperatures (Tg) of BDABF and
CzDABF determined by DSC were higher than 100 °C. On
the other hand, a Tg for BCzBF, which has a symmetrical
structure and rigid carbazole substituents, was not ob-
served in our experiment, probably because of its high
crystallinity.
The UV absorption and PL spectra of the compounds
were measured in the solution state and in the film state
on a quartz substrate (Table 1). Absorption and PL maxima
exhibited red shifts as the number of diphenylamine
groups increased, suggesting that the electron donating
diphenylamine group reduced the HOMO (highest occu-
pied molecular orbital)–LUMO (lowest unoccupied molec-
ular orbital) gap of the compounds. The ionization
potentials (Ip) of the compounds were measured by ultra-
violet photoelectron yield spectroscopy under atmospheric
conditions. The Ip increased as the number of diphenyl-
amine groups increased because diphenylamine moieties
are better at donating electron density than carbazole moi-
eties, which increases the HOMO energy level. Tentative
electron affinity (Ea) values were estimated from the differ-
ence between Ip and the optical energy gap obtained from
the edge of the UV absorption of the films. As the number
of diphenylamine groups increased, the Ea of the com-
pounds decreased and the Ip increased. DFT calculations
2.2. Device fabrication and characterization
Light emitting devices were fabricated on indium tin
oxide (ITO) coated glass substrates, which were prepared
by ultrasonication sequentially in detergent, methanol,
2-propanol and acetone, then exposed to UV-ozone under
ambient conditions for 15 min. MoO3, Cs, rubrene and the
bipolar materials were deposited by thermal evaporation
at less than 1 ꢁ 10ꢀ4 Pa. Finally, the aluminum cathode
was deposited through a shadow mask at less than 1 ꢁ
10ꢀ4 Pa. The active area of each device is 2 mm ꢁ 2 mm.
Layer thickness calibration was performed using a Dektak

720
K. Noine et al. / Organic Electronics 11 (2010) 717–723
a)
c)
e)
b)
N
N
N
N
Br
2
3
BCzBF
BDABF
d)
f)
N
Br
Br
N
Br
Br
1
O
TMS
B
TMS
O
5
4
g)
h)
i)
N
2 + 5
N
N
N
TMS
I
6
7
CzDABF
Scheme 1. Synthetic route to the bipolar compounds. (a) carbazole, Cu, K2CO3, dichlorobenzene, 180 °C, 20 h, (b) (i) Pd(dppf)Cl2, KOAc, bis(pinacola-
to)diboron, DMF, 80 °C 3 h; (ii) 2, Pd(dppf)Cl2, K2CO3, 80 °C, 20 h, (c) diphenylamine, Pd(OAc)2, 1,10-bis(diphenylphosphino)ferrocene, NaO-t-Bu, toluene,
reflux, 20 h, (d) (i) Pd(dppf)Cl2, KOAc, bis(pinacolato)diboron, DMF, 80 °C 3 h; (ii) 3, Pd(dppf)Cl2, K2CO3, 80 °C 20 h, (e) (i) n-BuLi, ꢀ78 °C 1 h; (ii) (CH3)3SiCl,
ꢀ78 °C to r.t., 20 h, (f) (i) n-BuLi, ꢀ78 °C 1 h; (ii) 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, ꢀ78 °C to r.t., 9 h, (g) Pd(PPh3)4, K2CO3 aq., toluene,
ethanol, reflux, 20 h, (h) ICl, dichloromethane, 0 °C, 1 h, (i) Pd2(dba)3, tri(t-butyl)phosphine, NaO-t-Bu, toluene, reflux, 24 h.
Table 1
Thermal and optoelectrochemical properties and charge mobilities of the bipolar compounds.
Tg (°C) Td5% (°C) UVa
PLa (nm) Ip (eV) Eab (eV)
HOMOc
(eV)
LUMOc
(eV)
lh
le
EOg PT
a
(nm)
(cm2Vꢀ1sꢀ1
)
(cm2 Vꢀ1sꢀ1
)
(eV)
BCzBF
BDABF
CzDABF 126
N.D.
106
477
417
447
350, 297
380, 310
377, 299
425
452
449
5.98
5.57
5.76
2.89
2.69
2.79
3.09
2.88
2.93
5.51
5.16
5.02
1.85
1.72
1.58
2.3 ꢁ 10ꢀ3
1.0 ꢁ 10ꢀ3
1.3 ꢁ 10ꢀ4
1.5 ꢁ 10ꢀ3
N.D.
1.3 ꢁ 10ꢀ3
a
b
c
Neat film.
Ea ¼ Ip ꢀ EOg PT
.
Obtained from DFT calculations using B3LYP 6311+G(d,p)//631G(d).
showed that the HOMO energy levels of BCzBF and BDABF
are delocalized over the whole molecule, while the HOMO
of CzDABF is localized on the diphenylamine and bifluo-
rene units. On the other hand, the LUMOs of all three com-
pounds are localized on the bifluorene unit (Fig. S1). HOMO
and LUMO levels estimated from the calculations were
consistent with the values obtained experimentally for Ip
and Ea.
The charge transport properties of the compounds were
investigated by the TOF transient photocurrent technique
at room temperature (Fig. 1). The carrier transit times were
determined from the folding point in the double-logarith-
mic plots of the data (insets in Fig. 1). The hole mobilities
of all three compounds were high, comparable to the
mobilities of common hole transporting materials [15].
BDABF did not exhibit a photocurrent during measurement
of electron mobility. On the other hand, BCzBF and CzDABF
showed sufficient electron photocurrent to determine their
electron mobilities. The electron mobilities of these two
compounds were as high as their hole mobilities, and
much higher than the electron mobility of 10ꢀ6
–
10ꢀ7 cm2Vꢀ1sꢀ1 of the well-known electron transporting
material, tris-(8-hydroxyquinoline) aluminum (Alq3).
These results demonstrate that the diphenylamine group
increased the hole transport ability of bifluorene, while
the carbazole group gave the compound bipolar charge
transport abilities. The bipolar charge transport properties
of these carbazole compounds are consistent with previ-
ously reported bipolar compounds such as CBP [9].
Fig. 2 shows the current density–voltage (J–V) charac-
teristics of the hole-only devices containing p-doped bipo-
lar compounds (ITO/bifluorene:MoO3 50 mol% (120 nm)/
Au) and undoped bifluorene (ITO/bifluorene (120 nm)/
Au). While the undoped hole-only devices exhibited low
current densities, the current density of the p-doped
hole-only devices increased considerably in an Ohmic
manner. The conductivities of the p-doped devices at
4.0 ꢁ 105 Vcmꢀ1 were 1.5 ꢁ 10ꢀ6 Scmꢀ1, 1.7 ꢁ 10ꢀ6 Scmꢀ1
and 1.1 ꢁ 10ꢀ6 Scmꢀ1 for BCzBF, BDABF and CzDABF,
respectively. Similarly, electron-only devices containing

K. Noine et al. / Organic Electronics 11 (2010) 717–723
721
10-1
10-2
10-3
10-4
a)
b)
10-1
10-2
10-3
10-1
1.5
100
2.5
10-2
10-1
100
time (µs)
time (µs)
0
1
2
3
4
5
0
0.5
1
2
3
time (µs)
time (µs)
c)
10-3
d)
10-4
10-5
10-2
10-1
time (µs)
100
0
0.5
1
1.5
2
2.5
3
3.5
4
0
5
10
15
20
time (µs)
time (µs)
10-1
10-2
f)
e)
10-3
10-4
10-5
10-2
10-3
10-2
10-1
time (µs)
100
10-1
100
time (µs)
101
0
0.2
0.4
time (µs)
0.6
0.8
1
1
3
5
7
9
time (µs)
Fig. 1. Time-of-flight (TOF) transients of the bipolar compounds: (a) BCzBF, hole, E1/2 = 700 (V/cm)1/2, d = 3.0
l
m. (b) BCzBF, electron, E1/2 = 700 (V/cm)1/2
l
m. (e) CzDABF, hole, E1/2 = 700 (V/cm)1/2
,
,
d = 8.0
d = 3.0
l
l
m. (c) BDABF, hole, E1/2 = 700 (V/cm)1/2, d = 3.0
l
m. (d) BDABF, electron, E1/2 = 700 (V/cm)1/2, d = 3.0
m.
m. (f) CzDABF, electron, E1/2 = 700 (V/cm)1/2, d = 3.0
l
n-doped bipolar compounds (ITO/BCP (10 nm)/bifluo-
rene:Cs 30 wt.% (120 nm)/Au) and undoped bipolar com-
pounds (ITO/BCP (10 nm)/bifluorene (100 nm)/Au) were
fabricated. The Cs-doped electron-only devices exhibited
conductivities that were seven orders of magnitude higher
than the undoped electron-only devices. At 4.0 ꢁ
105 Vcmꢀ1, the conductivities of the n-doped electron-only
1.2 ꢁ 10ꢀ6 Scmꢀ1 for BCzBF, BDABF and CzDABF, respec-
tively. These results demonstrate that the bifluorene com-
pounds have small enough Ip to be p-doped and Ea to be n-
doped effectively, in addition to their bipolar charge trans-
port ability in an undoped state.
OLEDs with a homo-junction structure were fabricated
containing BCzBF and CzDABF with the device configura-
tions: device I = ITO/bifluorene:MoO3 (50 mol%, 20 nm)/
devices were 4.7 ꢁ 10ꢀ6 Scmꢀ1
,
1.0 ꢁ 10ꢀ6 Scmꢀ1 and

722
K. Noine et al. / Organic Electronics 11 (2010) 717–723
bifluorene (80 nm)/bifluorene:Cs (30 wt.%, 20 nm)/Al (80
nm) and device II = ITO/bifluorene:MoO3 (50 mol%, 20 nm)/
bifluorene (10 nm)/bifluorene: rubrene (20 wt.%, 60 nm)/
bifluorene (10 nm)/bifluorene: Cs (30 wt.%, 20 nm)/Al
(80 nm). Fig. 4a shows the PL spectra of the bipolar com-
pounds and the EL spectra of the OLED. The EL spectra of de-
vice I did not correspond to the PL spectra of the bipolar
compounds. This might be caused by absorption in the blue
region by a radical species in the doped layers or exciplex
emission in the interface between the doped and undoped
layers. External quantum efficiencies (EQE) of device I were
low at around 0.5% at 100 cd mꢀ2 (Table 2). The low EQE of
these devices were probably caused by the lack of an energy
barrier between the layers making charge recombination
poor and exciton quenching caused by the close proximity
of doped layers to the emission zone. Device II was fabri-
cated with rubrene doped into the emission layer to effi-
ciently trap carriers and undoped bifluorene layers
between the emission layer and the doped layers to prevent
quenching. For device II, only yellow emission derived from
(a)
(b)
103
103
101
101
BCzBF
BCzBF:Cs
BDABF
BDABF:Cs
CzDABF
CzDABF:Cs
10-1
10-3
10-5
10-1
BCzBF
BCzBF:MoO3
10-3
10-5
BDABF
BDABF:MoO3
CzDABF
CzDABF:MoO3
0
1
2
3
4
5
6
0
1
2
3
4
5
6
7
8
Voltage (V)
Voltage (V)
Fig. 2. Current density–voltage characteristics of (a) hole-only devices (ITO/bifluorene or bifluorene:MoO3 (120 nm)/Au) and (b) electron-only devices (ITO/
BCP (10 nm)/bifluorene or bifluorene:Cs (100 nm)/Al).
Table 2
Turn-on voltage at 1 cd mꢀ2 and efficiencies at 100 cd mꢀ2 and 1000 cd mꢀ2 of OLEDs containing bipolar compounds.
EML
Von (V)
100 cd mꢀ2
Voltage (V)
1000 cd mꢀ2
Voltage (V)
P.E. (lm Wꢀ1
)
C.E. (cd Aꢀ1
)
E.Q.E. (%)
P.E. (lm Wꢀ1
)
C.E. (cd Aꢀ1
)
E.Q.E. (%)
BCzBF
BCzBF:rubrene
CzDABF
3.0
3.8
3.0
3.1
5.5
7.0
5.2
4.6
0.68
2.9
0.76
3.1
1.2
6.5
1.3
4.6
0.56
2.1
0.49
1.5
8.8
10.4
8.2
0.27
1.4
0.32
1.5
0.75
4.5
0.84
3.4
0.37
1.5
0.35
1.1
CzDABF:rubrene
6.9
P.E., power efficiency; C.E., current efficiency; E.Q.E., external quantum efficiency.
105
104
103
102
101
100
10-1
104
(b)
BCzBF
(a)
BCzBF
BCzBF: rubrene (10%)
CzDABF
BCzBF: Rubrene (10%)
CzDABF
103
102
101
100
10-1
10-2
CzDABF: Rubrene (20%)
CzDABF: rubrene (20%)
0
2
4
6
8
10
12
0
2
4
6
8
10
12
Voltage (V)
Voltage (V)
Fig. 3. (a) Current density–voltage and (b) luminance–voltage characteristics of OLEDs.

K. Noine et al. / Organic Electronics 11 (2010) 717–723
723
3
(a)
(b)
BCzBF (PL)
BCzBF
BCzBF: rubrene
CzDABF (PL)
CzDABF
CzDABF: rubrene
BCzBF
1.2
1
BCzBF: Rubene (10%)
CzDABF
2.5
CzDABF: Rubrene (20%)
2
1.5
1
0.8
0.6
0.4
0.2
0
0.5
10-1
100
101
102
103
350 400 450 500 550 600 650 700 750
Wavelength (nm)
Current density (mA/cm2)
Fig. 4. (a) EL spectra and (b) quantum efficiency–current density characteristics of OLEDs containing the bipolar compounds.
rubrene was observed (Fig. 4a). Although the driving volt-
ages increased compared to those of device I because of car-
rier trapping by rubrene (Fig. 3a), device II exhibited higher
EQEs than that of device I (Fig. 4b). The maximum EQE of
1.5% for device II containing CzDABF is consistent with the
theoretical EQE of 1.35% estimated from the PL quantum
yield of 27% of 20 wt.% rubrene in CzDABF. Doping concen-
trations of rubrene of less than 20 wt.% increase the PL quan-
tum yield, but incomplete carrier trapping causes emission
from the bifluorene host and results in lower overall EQE.
The homo-junction structure of the OLEDs was expected to
extendthe lifetimesof thedevices because of a lack of organ-
ic interfaces but the lifetime measurements did not show an
improvement over typical device structures. Further de-
tailed investigations are under way and will be presented
in the future.
homo-junction structure containing these bipolar com-
pounds as the emitting material exhibited poor efficien-
cies. Addition of rubrene as an emitting material into
these devices improved the EQEs to 1.5–2.0%.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.orgel.2010.
01.010.
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Bipolar fluorene compounds were synthesized by palla-
dium-catalyzed C–C and C–N coupling reactions. The en-
ergy levels of these compounds were related to the
number of carbazole and diphenylamine groups. TOF mea-
surements demonstrated that these compounds have bipo-
lar charge transport abilities. Hole mobilities were as high
as that of NPD and electron mobilities were much higher
than that of Alq3. These bifluorene compounds have small
enough Ip to be p-doped and Ea to be n-doped. Hole- and
electron-only devices containing these compounds with
p-type doping with MoO3 and n-type with Cs showed Oh-
mic current density–voltage characteristics. OLEDs with a