Solution-processible fluorinated carbazole derivative for phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1246/cl.2008.294

A fluorinated carbazole, 9,9′-[2,2′-bis(trifluoromethyl) biphenyl-4,4′-diyl]bis(9H-carbazole) (6FCBP) was prepared, which exhibited better solubility compared to the well-known hole-transporting material 4,4′-bis(carbazol-9-yl)biphenyl (CBP). 6FCBP was employed to fabricate OLEDs by spin coating or vacuum deposition with excellent performance. Copyright

Full text of DOI:10.1246/cl.2008.294

294
Chemistry Letters Vol.37, No.3 (2008)
Solution-processible Fluorinated Carbazole Derivative
for Phosphorescent Organic Light-emitting Diodes
Ziyi Ge,1 Teruaki Hayakawa,1 Shinji Ando,1 Mitsuru Ueda,1 Toshiyuki Akiike,2 Hidetoshi Miyamoto,2
Toru Kajita,2 and Masa-aki Kakimotoꢀ1
1Department of Organic & Polymeric Materials, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552
2Display Research Laboratories, JSR Corporation, 100 Kawajiri-cho, Yokkaichi 510-8552
(Received December 21, 2007; CL-071418; E-mail: mkakimot@o.cc.titech.ac.jp)
A
fluorinated carbazole, 9,90-[2,20-bis(trifluoromethyl)-
methyl)biphenyl (6FDI) was prepared in diazotation followed by
iodination from 2,20-bis(trifluoromethyl)biphenyl-4,40-diamine.
6FCBP was synthesized in an Ullmann reaction starting from
6FDI and carbazole catalyzed by copper/K2CO3 in o-dichloro-
benzene. The structures of 6FDI8 and 6FCBP9 were identified
by IR and NMR spectra.
biphenyl-4,40-diyl]bis(9H-carbazole) (6FCBP) was prepared,
which exhibited better solubility compared to the well-known
hole-transporting
(CBP). 6FCBP was employed to fabricate OLEDs by spin
coating or vacuum deposition with excellent performance.
material
4,40-bis(carbazol-9-yl)biphenyl
The thermal properties of 6FCBP were investigated by dif-
ferential scanning calorimetry (DSC) and thermal gravimetric
analysis (TGA) measurements. As shown in Figure 1, 6FCBP
was thermally stable up to 350 ꢁC with the thermal decomposi-
tion temperature (Td) of 363 ꢁC in nitrogen. By examination
of DSC, 6FCBP exhibits a glass-transition temperature (Tg) of
101 ꢁC and a melting point of 232 ꢁC, suggesting it has a good
thermal stability.
Figure 2 outlines the UV spectra, in which the absorption
centered at 291 nm originates from the ꢀ–ꢀꢀ transition. The
experimental values of HOMO levels were determined with a
Riken AC-2 photoemission spectrometer (PES), and those of
LUMO (S1) and the triplet energy levels (T1) were estimated
from the UV and phosphorescent spectra, respectively. The
HOMO and LUMO levels of 6FCBP were determined to be
5.98 and 2.41 eV, respectively with the energy gap of HOMO–
LUMO of 3.57 eV. The triplet energy level of 6FCBP is higher
than that of fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) by
2.91 to 2.44 eV, indicating that 6FCBP is suitable to be used
as the host material for Ir(ppy)3 due to the efficient prevention
of back energy transfer from the guest to the host.10
6FCBP can readily dissolve in common organic solvents,
such as chloroform, 1,2-dichloroethane, THF, chlorobenzene,
and anisole owing to the incorporation of trifluoromethyl groups
with large free volume, which enables the preparation of
homogeneous thin films by spin coating.
Phosphorescent OLED devices using 6FCBP as the host
were fabricated by spin-coating and vacuum deposition
with the structure of ITO/PEDOT (20 nm)/6FCBP + 6% mol
Ir(ppy)3 (50 nm)/TPBI (30 nm)/Cs:BCP(1:1) (20 nm)/Al(100
Considerable progress has been achieved in organic light-
emitting diodes (OLEDs) since the promising work by Tang
and VanSlyke in 1987.1 So far, OLEDs have been successfully
employed in commercial fields such as full-color displays and
solid light sources, whereas they are limited by the low fabrica-
tion yield down to 50% resulting from the complicated multi-
layer thermal evaporation techniques.2 Of the high-performance
OLEDs reported to date, most were fabricated by multi-layer
vacuum deposition even up to six organic layers.3 Development
of solution-processible fabrication such as spin coating or ink-jet
printing in OLEDs will offer a promising alternative to vacuum
deposition techniques, if high efficiency can also be simultane-
ously provided.4,5 4,40-Bis(carbazol-9-yl)biphenyl (CBP) is a
well-known hole-transporting material and has been extensively
employed in OLED due to its high hole-transporting mobility
in addition to the good thermal properties.6 However, it can only
be fabricated by thermal evaporation due to the low solubility
arising from the rigid structure. Further, it exhibits undesirable
crystallinity which possibly causes phase separation or aggrega-
tion in the layers. Hence, there is a requirement to modify its
structure without obviously decreasing its good properties.
In this letter, we reported a hole-transporting molecule, 9,90-
[2,20-bis(trifluoromethyl)biphenyl-4,40-diyl]bis(9H-carbazole)
(6FCBP), incorporating two trifluoromethyl groups at the bi-
phenyl unit in CBP. Trifluoromethyl group has been widely in-
troduced in molecules to improve the solubility arising from
the hydrophobicity and large free volume.7
6FCBP was synthesized according to the synthetic proce-
dure as shown in Scheme 1. First, 4,40-diiodo-2,20-bis(trifluoro-
(a)
(b)
CF3
CF3
0
-20
-1000
-2000
-3000
-4000
-5000
-6000
-7000
H2SO4/NaNO2
NaI/I2
Tg: 101 o
C
H2N
NH2
I
I
-40
F3C
F3C
6FDI
-60
CF3
CF3
F3C
-80
m.p: 232 o
C
Cu/K2CO3
N
N
-100
NH + I
I
80 120 160 200 240 280
o-Dichlorobenzene
100 200 300 400 500 600
Temperature/oC
Temperature/oC
F3C
6FDI
6FCBP
Figure 1. Thermal properties of 6FCBP: (a) TGA curve, (b)
DSC curve.
Scheme 1. Synthetic procedure of 6FCBP.
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.3 (2008)
295
Table 1. EL performance of the devices
Device Fabrication Vona/V Lmaxb/cd mꢂ2 ꢂc,maxc/cd Aꢂ1
3.0
EL
100
80
60
40
20
0
Dep
2.5
2.0
1.5
1.0
0.5
0.0
6FCBP
6FCBP
CBP
Spn
Dep
Dep
5.1
9.9
5.3
4900
14000
14500
2.6
10.4
12.9
Spn
UV
aTurn-on voltage. bMaximum luminance. cMaximum current
efficiency.
In summary, we have successfully optimized the solubility
of CBP by incorporating two trifluoromethyl groups at the
biphenyl unit and developed a fluorinated carbazole molecule
of 6FCBP, which exhibits desirable solubility in common sol-
vents allowing a solution-processed fabrication. The spin-coated
6FCBP device doped with Ir(ppy)3 as the guest exhibits a good
performance, while the vacuum-deposited one shows an en-
hanced maximum luminescence and current efficiency up to
14000 cd/m2 and 10.4 cd/A, respectively, which is comparable
with the similar CBP-based device.
335
250 300 350 400 450 500 550 600 650 700 750 800
Wavenumbers/nm
Figure 2. UV absorption spectra of 6FCBP (in THF) and EL
spectra of the spin-coated (Spn) and vacuum-deposited (Dep)
devices.
16000
15000
References and Notes
1
Spn
Dep
CBP device
14000
13000
12000
11000
10000
9000
8000
7000
6000
5000
4000
3000
2000
1000
0
C. W. Tang, S. A. VanSlyke, Appl. Phys. Lett. 1987, 51,
913.
2
C.-L. Ho, W.-Y. Wong, G.-J. Zhou, B. Yao, Z. Xie, L. Wang,
Adv. Funct. Mater. 2007, 17, 2925.
3
4
Y. Agata, H. Shimizu, J. Kido, Chem. Lett. 2007, 36, 316.
J.-A. Cheng, C. H. Chin, C. H. Liao, Chem. Mater. 2004, 16,
2862.
5
6
7
8
M. C. Gather, A. Kohnen, A. Falcou, H. Becker, K.
¨
Meerholz, Adv. Funct. Mater. 2007, 17, 191.
M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson,
S. R. Forrest, Appl. Phys. Lett. 1999, 75, 4.
Z. Ge, S. Yang, Z. Tao, J. Liu, L. Fan, Polymer 2004, 45,
3627.
0
200
400
600
800
1000
Current Density / mA cm-2
Figure 3. Luminance versus current density characteristics for
the devices.
1
Characterization of 6FDI: H NMR (CDCl3, 300 MHz, ꢁ):
8.05 (s, 2H), 7.89–7.85 (d, 2H), 6.99–6.96 (d, 2H). 13C NMR
(CDCl3, 75 MHz, ꢁ): 139.8, 135.8, 135.0, 132.8, 130.9,
130.5, 130.1, 129.6, 128.0, 124.4, 120.7, 117.0, 93.5. IR
(KBr, cmꢂ1): 1612, 1504, 1449, 1302, 1232, 1178, 1129,
749.
nm). Poly(3,4-ethylenedioxythiophene) (PEDOT) is employed
to promote the hole injection and 1,3,5-tris(N-phenylbenzimida-
zol-2-yl)benzene (TPBI) serves as the electron-transporting and
hole-blocking layer. Cs-doped BCP and Al acts as the composite
cathode while ITO served as the anode. 6FCBP doped with
6% mol Ir(ppy)3 is used as the hole-transporting and emitting
layer. As comparison, CBP-based device with the similar struc-
ture was also fabricated by vacuum deposition. As shown in
Figure 2, the EL spectra of the devices exhibit the typical green
luminescence around 520 nm originated from Ir(ppy)3, implying
the energy of triplet exitons are well transferred from the host of
6FCBP to the guest of Ir(ppy)3.
Figure 3 illustrates the current–luminance (I–L) characteris-
tics of the devices and the results are summarized in Table 1.
Spin-coated 6FCBP device exhibits good performance with the
maximum luminance of 4900 cd/m2 and current efficiency of
2.6 cd/A. The corresponding vacuum-deposited device shows
an improved performance with the maximum luminance and
current efficiency of 14000 cd/m2 and current efficiency of
10.4 cd/A, which may be attributed to the better film formation
resulting from the thermal evaporation relative to spin coating.
The performance of vacuum-deposited 6FCBP-based device is
comparable with that of CBP-based one (14500 cd/m2 and
12.9 cd/A) in a similar structure.
9
Synthesis and characterization of 6FCBP: The mixture of
carbazole (4.013 g, 24 mmol), 6FDI (5.420 g, 10 mmol), Cu
(1.398 g, 22 mmol), K2CO3 (5.528 g, 40 mmol), 18-crown-
6 ether (0.260 g, 1 mmol), and o-dichlorobenzene (50 mL)
was heated at reflux for 24 h, followed by diluted with
THF and filtered through celite. The solvents were removed
in vacuum and the residue was purified by column chroma-
tography on silica gel with hexane as the eluent followed by
recrystallization from hexane/THF to offer a white solid of
6FCBP. Yield: 2.81 g (45%). 1H NMR (CDCl3, 300 MHz,
ꢁ): 8.20–8.16 (d, 4H), 8.06 (s, 2H), 7.89–7.84 (d, 2H),
7.71–7.67 (d, 2H), 7.53–7.45 (m, 8H), 7.39–7.32 (t, 4H).
13C NMR (CDCl3, 75 MHz, ꢁ): 140.3, 138.2, 135.3, 133.3,
131.4, 130.9, 130.6, 129.1, 126.4, 125.3, 124.6, 123.7,
121.6, 120.7, 120.5, 109.5. IR (KBr, cmꢂ1): 1927, 1590,
1471, 1395, 1302, 1248, 1151, 1054, 1000, 994, 885, 831,
684, 549. Anal. Cale for C38H22F6N2: C, 73.54; H, 3.57;
N, 4.51%. Found: C, 73.61; H, 3.51; N, 4.53%.
10 K.-T. Wong, Y.-M. Chen, Y.-T. Lin, H.-C. Su, C.-C. Wu,
Org. Lett. 2005, 7, 5361.
Published on the web (Advance View) February 9, 2008; doi:10.1246/cl.2008.294