A new benzimidazole/carbazole hybrid bipolar material for highly efficient deep-blue electrofluorescence, yellow-green electrophosphorescence, and two-color-based white OLEDs

Article abstract of DOI:10.1039/c0jm02143a

The bipolar molecule CPhBzIm exhibits an excellent solid state photoluminescence quantum yield (Φ_PL = 69%), triplet energy (E_T = 2.48 eV), and bipolar charge transport ability (μ_h ≈ μ_e ≈ 10^-6-10^-

Full text of DOI:10.1039/c0jm02143a

View Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/materials | Journal of Materials Chemistry
A new benzimidazole/carbazole hybrid bipolar material for highly efficient
deep-blue electrofluorescence, yellow–green electrophosphorescence, and
two-color-based white OLEDs
a
b
a
b
b
b
Wen-Yi Hung,* Liang-Chen Chi, Wei-Jiun Chen, You-Ming Chen, Shu-Hua Chou and Ken-Tsung Wong*
Received 6th July 2010, Accepted 3rd August 2010
DOI: 10.1039/c0jm02143a
The bipolar molecule CPhBzIm exhibits an excellent solid state photoluminescence quantum yield (FPL
¼
ꢀ
6
ꢀ5
6
9%), triplet energy (E
T
¼ 2.48 eV), and bipolar charge transport ability (m
cm V s ). We have used it to fabricate a non-doped deep-blue organic light emitting diode
OLED) exhibiting promising performance [hext ¼ 3%; CIE ¼ (0.16, 0.05)] and to serve as host
material for a yellow–green phosphorescent OLED [hext ¼ 19.2%; CIE ¼ (0.42, 0.56)]. Exploiting
these dual roles, we used CPhBzIm in a simple singly doped, two-color-based white OLED (hext
%; CIE ¼ 0.31, 0.33).
h e
z m z 10 –10
2
ꢀ1 ꢀ1
(
¼
7
Introduction
In this paper, we report the synthesis, characterization, and
various OLED applications of a new bipolar material, CPhBzIm
Organic light emitting diodes (OLEDs) attract considerable
attention because of their great potential applications in flat-
1
panel displays and lighting sources. Among the three prime
(
Scheme 1), comprising two N-phenylbenzimidazole units cova-
lently linked to the C3 and C6 positions of carbazole. We selected
carbazole as the core structure of CPhBzIm for the following
reasons: (1) carbazole is chemically stable itself and can be easily
modified at the 3, 6, or 9 positions. In addition, a phenyl group
attached to the 9-position of carbazole can improve the thermal
colors, many good red and green emitters have been developed to
satisfy the practical requirements for OLEDs. In contrast, the
development of deep-blue emitters that match the National
Television System Committee (NTSC) standard blue with
Commission Internationale de L’ ꢀe clairage (CIE) coordinates of
8
stability and impart good electro-optical properties. (2) The
moderately high oxidative potential of carbazole containing
compounds make them promising as hole transport materials
(
0.14, 0.08) and high electroluminescence (EL) efficiencies
2
remains a challenge that has attracted significant research effort.
A deep-blue emitter would be not only a major constituent in
full-color displays but also the key element for generating white
light in combination with its complementary color. Recently,
a white OLED (WOLED) was fabricated through the novel
concept of combining fluorescent blue emitters with phospho-
(
HTM). In addition, the high triplet energy level of carbazole
makes carbazole-cored materials efficient host materials for
9
phosphors. (3) Because of their planarity and rigidity, carbazole
derivatives normally exhibit relatively intense luminescence,
making it possible to prepare high-performance deep-blue
10
OLEDs. We adopted N-phenylbenzimidazole moieties as elec-
3,4
rescent green and/or red emitters. Through careful control of
the doping concentration of the complementary triplet emitter,
this new strategy allows the efficient application of both the
fluorescence from the host and the complementary phosphores-
cence from the triplet dopant simultaneously to give a white
tron acceptors (A) because such derivatives usually exhibit good
electron transport mobility, which has been reported to connect
with electron donors (D) such as arylamine to form bipolar
11
emitters/hosts. The combination of the various structural
features of the carbazole and benzimidazole units endows
CPhBzIm with several advantageous properties: (1) deep-blue
emissions with high quantum yields (FPL ¼ 69%, as a thin film);
4
emission spectrum. Compared with the widely reported
3a,6
5
stacked, multi-emissive-layer,
7
and multi-doped-layer white
OLEDs, this singly doped emissive layer device significantly
reduces the complexity of the fabrication process, rendering this
new device much more cost-effective and competitive than other
contemporary display technologies. Along this line, a demand
remains for the development of a material possessing a high
triplet energy that can be used as both a deep-blue fluorescent
emitter and a host material for phosphors.
(
2) high morphological and thermal stability [glass transition
ꢁ
temperature (T
erties (m z m
energy (E
g
) ¼ 170 C]; (3) bipolar carrier transport prop-
ꢀ6
ꢀ5
2
ꢀ1 ꢀ1
e
h
z 10 –10 cm V s ); and (4) suitable triplet
T
¼ 2.48 eV), making it a suitable host for green
phosphors. Non-doped deep-blue OLEDs incorporating
CPhBzIm as the emitter exhibit a high value of hext of 3% with
CIE coordinates of (0.16, 0.05), which are very close to the NTSC
blue standard. Furthermore, CPhBzIm functions as an excellent
2
host material for the yellow-green-emitting dopant Ir(pbi) -
12
a
Institute of Optoelectronic Sciences, National Taiwan Ocean University,
Keelung, 202, Taiwan. E-mail: wenhung@mail.ntou.edu.tw; Fax: +886 2
4634360; Tel: +886 2 24622192 ext. 6718
(acac), providing high-performance OLEDs exhibiting CIE ¼
ꢀ1
(
0.42, 0.56) and a high value of h of 19.2% (62 cd A ) at 120 cd
ext
2
b
ꢀ
2
m . The two CIE coordinates indicate that WOLEDs may be
accessible by appropriate color mixing of blue light from
Department of Chemistry, National Taiwan University, Taipei, 106,
Taiwan. E-mail: kenwong@ntu.edu.tw; Fax: +886 2 33661667; Tel:
+
886 2 33661665
CPhBzIm and yellow-green light from Ir(pbi)
2
(acac). Through
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 10113–10119 | 10113

View Online
Scheme 1 Synthetic pathways toward bipolar host CPhBzIm and new hole-transporting material DTAF.
simple control over the dopant concentration (0.1 wt%), we
achieved a two-color-based WOLED that exhibited EL effi-
ꢀ1
ꢀ1
ciencies of 7% (15.5 cd A ) and 12.8 lm W with CIE coordi-
nates of (0.31, 0.33).
Results and discussion
Scheme 1 displays our synthetic route towards CPhBzIm. We
13
synthesized the benzimidazole derivative 2 from the reaction of
N-phenyl-o-phenylenediamine with 4-bromobenzoyl chloride,
which we generated from p-bromobenzoic acid (1) in situ. Suzuki
coupling of 2-(4-bromophenyl)-1-phenylbenzimidazole (2) and
14
the carbazole-based boronic ester 3 gave CPhBzIm in 94%
yield. A new fluorene-based hole-transporting material DTAF
used in this work was also synthesized in an efficient way from 9-
fluorenone with ditolylphenyl amine in the presence of Eaton’s
reagent.
The molecular configuration of CPhBzIm provided it with
high thermal stability, as indicated by a high decomposition
ꢁ
temperature (T
d
¼ 480 C, corresponding to 5% weight loss
during thermogravimetric analysis) and a rather high glass-
ꢁ
transition temperature [T ¼ 170 C, determined using differ-
g
ential scanning calorimetry (DSC)]. As a result, CPhBzIm forms
homogeneous and stable amorphous films upon thermal evap-
oration.
Fig. 1 (a) Room-temperature absorption (UV–Vis) and emission (PL)
2 2
spectra of CPhBzIm in CH Cl solutions and as neat films and the cor-
Fig. 1(a) presents the UV–Vis absorption and photo-
luminescence (PL) spectra of CPhBzIm in dilute solution
responding phosphorescence (Phos) spectrum recorded from an EtOH
solution at 77 K. (b) The emission spectra of CPhBzIm in various
solvents.
(
2 2
CH Cl ) and as a neat film on a quartz plate. The absorption
1
0114 | J. Mater. Chem., 2010, 20, 10113–10119
This journal is ª The Royal Society of Chemistry 2010

View Online
maximum of CPhBzIm in dilute solution, centered at 317 nm, is
similar to that provided by its thin film (322 nm), implying that
no significant intermolecular interactions occur in the ground
state. However, as indicated in Fig. 1(b), the emission of
CPhBzIm exhibits evident solvatochromism, implying the
occurrence of charge transfer upon photo-excitation. To gain
more insight into the electronic structure of CPhBzIm, density
function theory (DFT) calculations were performed at a B3LYP/
6
-31G(d) level for the geometry optimization. As shown in Fig. 2,
the HOMO of CPhBzIm is mainly populated over the carbazole
and phenyl benzimidazole with considerable contribution from
the former, whereas the LUMO has a sizable contribution from
the phenyl benzimidazole fragments. Examination of the frontier
orbitals of CPhBzIm suggests that the HOMO–LUMO excita-
tion would shift the electron density distribution from the central
carbazole to the phenyl benzimidazole moieties, leading to
a polarized excited state which agrees with our observation of
red-shifted emission in polar solvents.
Fig. 3 Cyclic voltammogram of CPhBzIm.
Through subtraction of the optical energy gap (E ¼ 3.12 eV)
g
from the HOMO energy level, we calculated the energy level of
the lowest unoccupied molecular orbital (LUMO) to be ꢀ2.37
eV.
We used the time-of-flight (TOF) transient-photocurrent
technique at room temperature to determine the carrier mobil-
ities of CPhBzIm. Fig. 4(a) and (b) present the representative
TOF transient for holes and electrons, respectively, of CPhBzIm,
revealing dispersive transport behavior. To determine the carrier
CPhBzIm exhibits deep-blue emissions, with PL peaks
centered at 412 nm in dilute solution and 426 nm in its thin film.
The PL quantum yield (FPL) of CPhBzIm, determined using an
integrating sphere (Hamamatsu C9920), reached as high as 98%
15
in CH Cl and 69% in its thin film. Thus, it appears that the
2
2
mobilities, the carrier transit time, t , can be evaluated from the
T
non-coplanar conformation of CPhBzIm that prevents close
molecular packing in the solid state also effectively inhibits
excimer formation and fluorescence quenching. Such a high PL
quantum yield in the solid state suggested that CPhBzIm could
be used as an efficient deep-blue emitter in OLEDs. From the
highest energy peak of the phosphorescent emission in EtOH at
7
T
7 K, we estimated the triplet energy (E ) of CPhBzIm to be 2.48
eV. The extended p-conjugation, via 3,6-substitution on the
central carbazole unit, results in CPhBzIm having a lower value
9
a
of E relative to that of CzSi (E ¼ 3.02 eV). The value of E of
T
T
T
CPhBzIm is, however, still suitable for it to serve as a host
material for green phosphorescent OLEDs (PhOLEDs).
Fig. 3 reveals the bipolar electrochemical character of
CPhBzIm, as evidenced using cyclic voltammetry (CV). We
observed one reversible oxidation potential at 1.24 V and one
quasi-reversible reduction potential at ꢀ2.06 V (vs. Ag/AgCl).
More importantly, no electropolymerization occurred during
multiple cycles of CV scanning. Thus, the appending of the
electron-accepting N-phenylbenzimidazole units onto the
carbazole core renders CPhBzIm with promising electrochemical
stability and bipolar characteristics. Using atmospheric photo-
electron spectroscopy, we estimated the energy level of the
highest occupied molecular orbital (HOMO) to be ꢀ5.49 eV.
Fig. 4 Typical transient photocurrent signals for CPhBzIm (thickness:
ꢀ1
5
1
.56 mm) at E ¼ 5.7 ꢂ 10 V cm : (a) hole; (b) electron. Insets: Double-
Fig. 2 Frontier molecular orbitals (HOMO and LUMO) of CPhBzIm
logarithmic plots of (a) and (b). (c) Electron and hole mobilities plotted
1
/2
calculated with DFT on a B3LYP/6-31G(d) level.
with respect to E .
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 10113–10119 | 10115

View Online
intersection point of two asymptotes in the double-logarithmic
representation of the TOF transient. Fig. 4(c) displays the field
dependence of the carrier mobility of CPhBzIm; the hole
ꢀ
6
ꢀ5
2
ꢀ1 ꢀ1
mobility ranged from 3.4 ꢂ 10 to 1.3 ꢂ 10 cm V
fields varying from 2.3 ꢂ 10 to 5.7 ꢂ 10 V cm ; the electron
for
s
for
5
5
ꢀ1
ꢀ6
ꢀ6
2
ꢀ1 ꢀ1
mobility ranged from 2.8 ꢂ 10 to 4.2 ꢂ 10 cm V
s
5
5
ꢀ1
fields varying from 4.9 ꢂ 10 to 6.4 ꢂ 10 V cm . The ability of
CPhBzIm to conduct both electrons and holes is important for
maintaining charge balance in the emissive layer (EML), in turn
generating broader recombination zones and steering the elec-
trons and holes away from the interfaces with the charge-trans-
port layers to suppress emissive exciton quenching.
Because of its high solid state PL quantum yield, CPhBzIm can
be applied to fabricate non-doped deep-blue-emitting devices. In
addition, the high triplet energy allows CPhBzIm to serve as
a host material for green PhOLEDs. Hence, we fabricated
devices incorporating CPhBzIm layers doped with various
concentrations of Ir(pbi)
and white light emissions from a simple three-layer configura-
tion: indium tin oxide (ITO)/poly(3,4-ethylene-
2
(acac) to realize blue, yellow–green,
dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (30 nm)/
DTAF (25 nm)/CPhBzIm doped with x% (pbi) Ir(acac) (25 nm)/
2
TPBI (50 nm)/LiF (0.5 nm)/Al (100 nm). Here, we used DTAF as
the hole-transporting layer (HTL) because it possesses an
appropriate HOMO energy level (–5.22 eV) to provide a low hole
injection barrier, a low-lying LUMO energy level (–1.75 eV) to
Fig. 6 (a) I–V–L characteristics and (b) EL efficiency plotted with
respect to brightness for CPhBzIm doped with various concentrations of
block the injected electrons, and a high triplet energy level (E
.87 eV) to confine triplet excitons within the EML (see the
schematic energy level diagram Fig. 5). We used the complex
T
¼
2
Ir(pbi) (acac).
2
performance (hext) of device A remained as 2.4% (8.1 V). Thus,
the high PL quantum efficiency and balanced charge mobilities
make CPhBzIm a promising candidate for realizing fairly good
deep-blue light emitting devices. In device B, CPhBzIm func-
12
(
pbi)
2
Ir(acac) as the yellow-green dopant and TPBI as an
16
electron-transport and hole-blocking layer.
Fig. 6 presents the current density–voltage–luminance (I–V–L)
characteristics and EL efficiencies of devices prepared at various
doping concentrations. When we increased the concentration of
the phosphor, the hole-injection efficiency increased, presumably
because of direct charge injection from the DTAF (HTL) into
2
tioned as the host for the yellow–green dopant (pbi) Ir(acac); the
optimal performance occurred at a doping concentration of 10
wt%. Device B featured a relatively low turn-on voltage of 2 V
ꢀ2
ꢀ2
and a maximum brightness of 96 000 cd m at 1014 mA cm at
2
Ir(pbi) (acac). Fig. 7 displays EL spectra and CIE coordinates of
1
1 V, with CIE coordinates of (0.42, 0.56). The maximum values
ꢀ1
these OLEDs; Table 1 summarizes the device performances. All
devices displayed low turn-on voltages in the range 2–2.5 V. The
non-dopant device A exhibited the pure blue emission [CIE (0.16,
ꢀ1
of hext [19.2% (62 cd A )] and h
ꢀ2
p
(62 lm W ) were achieved at
a brightness of 120 cd m . Even at a practical brightness of 1000
ꢀ1
ꢀ2
cd m , the EL efficiencies remained at 18.5% (57.4 cd A ) and
ꢀ1
0
.05)] of CPhBzIm, almost matching the standard blue point
3
5 ml W , with only a limited external quantum efficiency roll-
recommended by the NTSC. Its maximum brightness was 4600
off. We believe that the balanced electron and hole transporting
ꢀ
2
cd m at 12 V; its maximum external quantum efficiency (hext
)
ꢀ
2
was 3%. Even at a practical brightness of 1000 cd m , the
Fig. 7 EL spectra and CIE1931 coordinates of devices prepared at
Fig. 5 Schematic energy levels diagram of device.
various doping concentrations.
1
0116 | J. Mater. Chem., 2010, 20, 10113–10119
This journal is ª The Royal Society of Chemistry 2010

View Online
Table 1 Electroluminescence data of the devices prepared at various doping concentrations
a
ꢀ2
ꢀ2
ꢀ1
ꢀ1
L ¼ 1000 cd mꢀ2[V, %] CIE1931 [x, y]
Dopant [wt %]
V
on /V
L
max/cd m
Imax/mA cm
max. hext/% max. h
l
/cd A
p
max h /lm W
A
B
C
0
10
0.1
2.5
2
2.5
4 600 (12 V)
96 000 (11 V)
26 500 (11.5 V) 1130
1013
1014
3
19.2
7
1.6
62
15.5
1.07
62
12.8
8.1, 2.4
5.2, 18.5
7, 5.1
0.16, 0.05
0.42, 0.56
0.31, 0.33
a
defined as the voltage at which the EL is rapidly enhanced.
behavior of CPhBzIm was responsible for these excellent device
characteristics.
cost of fabricating efficient OLEDs for use in displays and
lighting applications.
Because the line connecting the two CIE coordinates of devices
A and B passed through the white region (Fig. 7), we believed
that WOLEDs would be obtained at an appropriate color mixing
of the blue fluorescence from CPhBzIm with the complementary
Experimental
Synthesis
2
yellow–green phosphorescence from (pbi) Ir(acac) dispersed in
Synthesis of bipolar material CPhBzIm. A mixture of 3,6-
bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9-phenyl-9H-
CPhBzIm. Indeed, controlling the doping concentration at 0.1
wt% allowed us to fabricate a two-color-based WOLED in
a rather simple manner. The white-emitting device C exhibited
a low turn-on voltage of 2.5 V and a maximum brightness of
18
carbazole (3) (1.49 g, 3 mmol) and 2-(4-bromophenyl)-1-phenyl
13
benzoimidazole 2 (2.30 g, 6.6 mmol), Pd(PPh3)4 (173 mg,
0
.15 mmol), K PO (12 mL, 2 M), tri(tert-butyl)phosphine (6 mL,
3 4
ꢀ2
ꢀ2
2
60 500 cd m at 1130 mA cm at 11.5 V. Fig. 7 displays the EL
0
.05 M in toluene, 0.3 mmol) in toluene (30 mL) was refluxed for 2
spectra of device C recorded at various brightnesses. The relative
intensity of the yellow–green emission decreased slightly when
the brightness increased, leading to a slight shift in the CIE
days. After coolingto room temp., the mixture was extracted twice
with CH Cl . The combined organic solution was washed with
brine and dried over MgSO . The product was washed twice with
2
2
ꢀ2
4
coordinates from (0.32, 0.35) at 3620 cd m to (0.30, 0.31) at
hexane and methanol, affording product CPhBzIm as a white
ꢁ
ꢀ2
1
6 650 cd m . This color shift originated from the shifting of the
solid (2.2 g, 94%). mp 208–209 C; IR (KBr) n 3040, 2921, 2850,
recombination zone upon increasing the applied voltage and
from the easier formation of high-energy excitons at higher
ꢀ
1 1
2
358, 2336, 1597, 1499, 1470, 1450, 1378, 1281, 811, 697 cm ; H
NMR (DMSO-d
, 400 MHz) d 8.81 (d, J ¼ 1.6 Hz, 2H), 7.84–7.81
m, 8H), 7.70–7.50 (m, 19H), 7.43 (d, J ¼ 8.4 Hz, 2H), 7.35–7.26
m, 4H), 7.19 (d, J ¼ 7.2 Hz, 2H); C NMR (CD Cl , 100 MHz)
1
7
ꢀ1
6
voltage. The maximum values of hext [7% (15.5 cd A )] and hp
ꢀ2
(
(
ꢀ1
(
12.8 lm W ) were achieved at a brightness of 40 cd m . Even at
13
ꢀ2
2
2
a practical brightness of 1000 cd m , the EL efficiency remained
at 5.1% at 7 V.
d 152.4, 143.5, 142.7, 141.4, 137.9, 137.6, 132.6, 130.3, 130.2, 128.9,
1
1
28.7, 128.0, 127.9, 127.2, 127.1, 125.8, 124.3, 123.5, 123.1, 119.9,
+
19.1, 110.8, 110.7; MS (m/z, FAB ) 780 (39.90); HRMS (m/z,
The structure of this WOLED device—singly doped and
having a single emissive layer—is much simpler than those of
widely reported stacked, multi-emissive-layer, or triple-doped
WOLEDs. Our results indicate that the fabrication of efficient
WOLEDs can be performed in a relatively simple and cost-
effective manner.
+
FAB ) Calcd for C56
H N
37 5
779.3049, found 779.3065; Anal.
Calcd. C, 86.24; H, 4.78; N, 8.98. found C, 86.01; H, 4.89; N, 8.88.
Synthesis of hole-transporting material DTAF. 9H-Fluoren-9-
one (0.18 g, 1 mmol) and 4-methyl-N-phenyl-N-p-tolylaniline
(
683 mg, 2.5 mmol) were dissolved in dichloromethane (1 mL).
19
Under refluxing, Eaton’s reagent (0.3 mL, 4 mmol) was added
drop-after-drop into the solution. Then, the color of the solution
changed to brown. The mixture was stirred for 12 h provided
a greenish suspension. After cooling to ambient temperature, the
precipitate was collected by filtration and washed several times
with methanol, chloroform, and NaOH(aq). The crude product
Conclusions
We have synthesized a novel bipolar material, CPhBzIm,
comprising two electron-accepting N-phenylbenzimidazole units
linked to the C3 and C6 positions of an electron-donating
carbazole unit. The non-coplanar conformation of CPhBzIm
provides steric bulk, resulting in stable amorphous thin films and
pronounced PL quantum efficiency. We applied CPhBzIm to the
fabrication of non-doped deep blue-emitting devices with
promising performance [hext ¼ 3%; CIE ¼ (0.16; 0.05)]; it also
served as a host material, doped with 10 wt% of the yellow–green
2 2
was collected and refluxed in CH Cl for 30 min. The precipitate
was collected to afford DTAF as a white solid (650 mg, 0.92
ꢁ
mmol, 92%). mp: 374 C (DSC); IR (KBr) n 3031, 2920, 2857,
ꢀ1
1610, 1502, 1439, 1321, 1299, 1271, 831, 815, 755, 732, 723 cm ;
1
H NMR (CDCl
3
, 400 MHz) d 7.73 (d, J ¼ 7.2 Hz, 2H), 7.42 (d,
J ¼ 7.6 Hz, 2H), 7.35–7.32 (m, 2H), 7.28–7.24 (m, 2H), 7.02–6.99
(m, 12H), 6.94 (d, J ¼ 8.4 Hz, 8H), 6.83 (d, J ¼ 8.8 Hz, 4H), 2.28
emitter (pbi) Ir(acac), to realize a green PhOLED exhibiting high
2
13
efficiency [hext ¼ 19.2%; CIE ¼ (0.42; 0.56)]. Combining the deep
blue fluorescence of CPhBzIm with the yellow–green phospho-
(s, 12H); C NMR (CDCl , 200 MHz) d 151.7, 146.5, 145.3,
3
140.0, 138.8, 132.3, 129.7, 128.7, 127.5, 127.2, 126.2, 124.5, 121.9,
+
120.0, 96.1, 64.4, 29.7, 20.7; MS (m/z, FAB ) 872 (5.75); MS (m/z,
rescence of 0.1 wt% (pbi) Ir(acac) doped in CPhBzIm, we
2
+
+
obtained an efficient WOLED device [hext ¼ 7%; CIE ¼ (0.31,
44 2
FAB ) 708 (0.09); HRMS (m/z, FAB ) Calcd. for C53H N
0
.33)] that was singly doped and featured a single emissive layer.
708.3504, found 708.3510; Anal. Calcd. C, 89.79; H, 6.26; N,
3.95, found C, 89.38; H, 6.21; N, 4.34.
Our results pave the way toward reducing the complexity and
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 10113–10119 | 10117

View Online
Photophysical measurements
cleaned ultrasonically—sequentially with acetone, methanol, and
deionized water—and then it was treated with UV-ozone. A
hole-injection layer of poly(3,4-ethylenedioxythiophene)-poly(4-
styrenesulfonate) (PEDOT:PSS) was spin-coated onto the
Steady state spectroscopic measurements were conducted both in
solution and solid films prepared by vacuum (2 ꢂ 10ꢀ torr)
deposition on a quartz plate (1.6 ꢂ 1.0 cm). Absorption spectra
were recorded with a U2800A spectrophotometer (Hitachi).
Fluorescence spectra at 300 K and phosphorescent spectra at 77
K were measured on a Hitachi F-4500 spectrophotometer upon
exciting at the absorption maxima. Quantum efficiency
measurements were recorded with an integration sphere coupled
with a photonic multi-channel analyzer (Hamamatsu C9920),
which gave anthracene a quantum yield of 23%. The experi-
mental values of HOMO levels were determined with a Riken
AC-2 photoemission spectrometer (PES), and those of LUMO
levels were estimated by subtracting the optical energy gap from
the measured HOMO.
6
ꢁ
substrates and dried at 130 C for 30 min to remove residual
water. Organic layers were then vacuum deposited at a deposi-
ꢀ1
ꢁ
tion rate of ca. 1–2 A s . Subsequently, LiF was deposited at 0.1
ꢁ
ꢀ1
ꢁ
ꢀ1
A s and then capped with Al (ca. 5 A s ) through shadow
masking without breaking the vacuum. The I–V–L characteris-
tics of the devices were measured simultaneously using a Keithley
6
430 source meter and a Keithley 6487 picoammeter equipped
with a calibration Si-photodiode in a glovebox system. EL
spectra were measured using a photodiode array (Ocean Optics
S2000) with a spectral range from 200 to 850 nm and a resolution
of 2 nm.
Cyclic voltammetry
Acknowledgements
The oxidation potential was determined by cyclic voltammetry
This study was supported financially by the National Science
Council and the Ministry of Economic Affairs of Taiwan.
(
CV) in CH Cl solution (1.0 mM) containing 0.1 M tetra-n-
2 2
6
butylammonium hexafluorophosphate (TBAPF ) as a support-
ꢀ1
ing electrolyte at a scan rate of 100 mV s . The reduction
potential was recorded in THF solution (1.0 mM) containing 0.1
References
4
M tetra-n-butylammonium perchlorate (TBAClO ) as a sup-
1
(a) C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913;
b) M. A. Baldo, M. E. Thompson and S. R. Forrest, Nature, 2000,
403, 750.
(a) Y.-H. Kim, H.-C. Jeong, S.-H. Kim, K. Yang and S.-K. Kwon,
Adv. Funct. Mater., 2005, 15, 1799; (b) Y.-H. Niu, B. Chen, T.-
D. Kim, M. S. Liu and A. K.-Y. Jen, Appl. Phys. Lett., 2004, 85,
ꢀ1
porting electrolyte at a scan rate of 100 mV s . A glassy carbon
electrode and a platinum wire were used as the working and
counter electrodes, respectively. All potentials were recorded
versus Ag/AgCl (saturated) as a reference electrode.
(
2
5
433; (c) Z. Q. Gao, Z. H. Li, P. F. Xia, M. S. Wong, K. W. Cheah
and C. H. Chen, Adv. Funct. Mater., 2007, 17, 3194; (d) S. Tang,
M. R. Liu, P. Lu, H. Xia, M. Li, Z. Q. Xie, F. Z. Shen, C. Gu,
H. P. Wang, B. Yang and Y. Ma, Adv. Funct. Mater., 2007, 17,
Time-of-flight (TOF) mobility measurements
Carrier-transport properties were studied in vapor-deposited
2
869; (e) T.-C. Tsai, W.-Y. Hung, L.-C. Chi, K.-T. Wong,
glasses of CPhBzIm by the time-of-flight (TOF) transient
C.-C. Hsieh and P.-T. Chou, Org. Electron., 2009, 10, 158; (f)
C.-H. Chien, C.-K. Chen, F.-M. Hsu, C.-F. Shu, P.-T. Chou and
C.-H. Lai, Adv. Funct. Mater., 2009, 19, 560.
20
photocurrent technique. The samples for the TOF measure-
ment were prepared by vacuum deposition using the structure:
ITO glass/CPhBzIm (1.56 mm)/Al (150 nm), and then placed
inside a cryostat and kept under vacuum. The thickness of
organic film was monitored in situ with a quartz crystal sensor
and calibrated by a thin film thickness measurement (K-MAC
ST2000). A pulsed nitrogen laser was used as the excitation light
source through the transparent electrode (ITO) induced photo-
generation of a thin sheet of excess carriers. Under an applied dc
bias, the transient photocurrent was swept across the bulk of the
organic film toward the collection electrode (Al), and then
recorded with a digital storage oscilloscope. Depending on the
polarity of the applied bias, selected carriers (holes or electrons)
3
(a) Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson and
S. R. Forrest, Nature, 2006, 440, 908; (b) G. Schwartz, M. Pfeiffer,
S. Reineke, K. Walzer and K. Leo, Adv. Mater., 2007, 19, 3672; (c)
H. Kanno, N. C. Giebink, Y. Sun and S. R. Forrest, Appl. Phys.
Lett., 2006, 89, 023503; (d) J. H. Seo, J. H. Park, Y. K. Kim,
J. H. Kim, G. W. Hyung, K. H. Lee and S. S. Yoon, Appl. Phys.
Lett., 2007, 90, 203507; (e) B.-P. Yan, C. C. C. Cheung,
S. C. F. Kui, H.-F. Xiang, V. A. L. Roy, S.-J. Xu and C.-M. Che,
Adv. Mater., 2007, 19, 3599; (f) Y. H. Lee, B.-K. Ju, W. S. Jeon,
J. H. Kwon, O. O. Park, J.-W. Yu and B. D. Chin, Synth. Met.,
2
009, 159, 325.
4 (a) P. Anzenbacher, Jr., V. A. Montes and S. Takizawa, Appl. Phys.
Lett., 2008, 93, 163302; (b) Y. Tao, Q. Wang, Y. Shang, C. Yang,
L. Ao, J. Qin, D. Ma and Z. Shuai, Chem. Commun., 2009, 77.
5
(a) H. Kanno, R. J. Holmes, Y. Sun, S. Kena-Cohen and
S. R. Forrest, Adv. Mater., 2006, 18, 339; (b) F. Guo and D. Ma,
Appl. Phys. Lett., 2005, 87, 173510; (c) C.-C. Chang, J.-F. Chen, S.-
W. Hwang and C. H. Chen, Appl. Phys. Lett., 2005, 87, 253501.
(a) Y.-S. Park, J.-W. Kang, D. M. Kang, J.-W. Park, Y.-H. Kim, S.-
K. Kwon and J.-J. Kim, Adv. Mater., 2008, 20, 1957; (b) C.-H. Chang,
K.-C. Tien, C.-C. Chen, M.-S. Lin, H.-C. Cheng, S.-H. Liu, C.-
C. Wu, J.-Y. Hung, Y.-C. Chiu and Y. Chi, Org. Electron., 2010,
T
are swept across the sample with a transit time of t . With the
applied bias V and the sample thickness D, the applied electric
field E is V/D, and the carrier mobility is then given by m ¼
6
2
D/(t
T
E) ¼ D /(Vt
T T
), in which the carrier transit time, t , can be
extracted from the intersection point of two asymptotes to the
plateau and the tail sections in double-logarithmic plots.
1
1, 412.
7
8
(a) J. Lee, J.-I. Lee, J. Y. Lee and H. Y. Chu, Appl. Phys. Lett., 2009,
94, 193305; (b) Y.-H. Niu, M. S. Liu, J.-W. Ka, J. Bardeker,
M. T. Zin, R. Schofield, Y. Chi and A. K.-Y. Jen, Adv. Mater.,
2007, 19, 300; (c) X. Yang, Z. Wang, S. Madakuni, J. Li and
G. E. Jabbour, Adv. Mater., 2008, 20, 2405.
(a) K. R. J. Thomas, J. T. Lin, Y. T. Tao and C. W. Ko, J. Am. Chem.
Soc., 2001, 123, 9404; (b) K. R. J. Thomas, J. T. Lin, Y. T. Tao and
C. W. Ko, Adv. Mater., 2000, 12, 1949.
OLED device fabrications
All chemicals were purified through vacuum sublimation prior to
use. The OLEDs were fabricated through vacuum deposition of
ꢀ6
the materials at 10 torr onto ITO-coated glass substrates
ꢀ1
having a sheet resistance of 15 U sqr . The ITO surface was
1
0118 | J. Mater. Chem., 2010, 20, 10113–10119
This journal is ª The Royal Society of Chemistry 2010

View Online
9
(a) M. H. Tsai, H. W. Lin, H. C. Su, T. H. Ke, C. C. Wu, F. C. Fang,
Y. L. Liao, K. T. Wong and C. I. Wu, Adv. Mater., 2006, 18, 1216; (b)
S.-J. Yeh, M.-F. Wu, C.-T. Chen, Y.-H. Song, Y. Chi, M.-H. Ho,
S.-F. Hsu and C. H. Chen, Adv. Mater., 2008, 17, 285;
Mater., 2008, 20, 2532; (c) S. Takizawa, V. A. Montes and
P. Anzenbacher Jr., Chem. Mater., 2009, 21, 2452.
12 (a) W.-S. Huang, J. T. Lin, C.-H. Chien, Y.-T. Tao, S.-S. Sun and T.-
S. Wen, Chem. Mater., 2004, 16, 2480; (b) W.-S. Huang, J. T. Lin and
H.-C. Lin, Org. Electron., 2008, 9, 557.
(
c) H. Sasabe, Y.-J. Pu, K. Nakayama and J. Kido, Chem.
Commun., 2009, 6655; (d) S.-J. Su, H. Sasabe, T. Takeda and
J. Kido, Chem. Mater., 2008, 20, 1691; (e) S. O. Jeon, K. S. Yook,
C. W. Joo and J. Y. Lee, Adv. Funct. Mater., 2009, 19, 3644.
13 Z. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akiike, H. Miyamoto,
T. Kajita and M.-a. Kakimoto, Adv. Funct. Mater., 2008, 18, 584.
14 K.-T. Wong, T.-C. Chao, L.-C. Chi, Y.-Y. Chu, A. Balaiah,
S.-F. Chiu, Y.-H. Liu and Y. Wang, Org. Lett., 2006, 8, 5033.
15 Y. Kawamura, H. Sasabe and C. Adachi, Jpn. J. Appl. Phys., 2004,
43, 7729.
16 S.-J. Yeh, M.-F. Wu, C.-T. Chen, Y.-H. Song, Y. Chi, M.-H. Ho,
S.-F. Hsu and C. H. Chen, Adv. Mater., 2005, 17, 285.
17 Y. Shao and Y. Yang, Appl. Phys. Lett., 2005, 86, 073510.
18 K.-T. Wong, T.-H. Hung, T.-C. Chao and T.-I. Ho, Tetrahedron
Lett., 2005, 46, 855.
1
0 (a) S. J. Lee, J. S. Park, K. J. Yoon, Y. I. Kim, S. H. Jin, S. K. Kang,
Y. S. Gal, S. Kang, J. Y. Lee, J. W. Kang, S. H. Lee, H. D. Park and
J. J. Kim, Adv. Funct. Mater., 2008, 18, 3922; (b) S.-L. Lin, L.-
H. Chan, R.-H. Lee, M.-Y. Yen, W.-J. Kuo, C.-T. Chen and R.-
J. Jeng, Adv. Mater., 2008, 20, 3947; (c) S. Tao, Y. Zhou, C.-S. Lee,
X. Zhang and S.-T. Lee, Chem. Mater., 2010, 22, 2138; (d) Z. Jiang,
H. Yao, Z. Liu, C. Yang, C. Zhong, J. Qin, G. Yu and Y. Liu, Org.
Lett., 2010, 11, 4132.
11 (a) C.-H. Chen, W.-S. Huang, M.-Y. Lai, W.-C. Tsao, J. T. Lin, Y.-
H. Wu, T.-H. Ke, L.-Y. Chen and C.-C. Wu, Adv. Funct. Mater.,
19 P. E. Eaton, G. R. Carlson and J. T. Lee, J. Org. Chem., 1973, 38,
4071.
2009, 19, 2661; (b) Z. Ge, T. Hayakawa, S. Ando, M. Ueda,
T. Akiike, H. Miyamoto, T. Kajita and M. Kakimoto, Chem.
20 P. M. Borsenberger and D. S. Weiss, Organic Photoreceptors for
Imaging Systems; Marcel Dekker, New York, 1993.
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 10113–10119 | 10119