Molecular modification of 2,7-diphenyl[1]benzothieno[3,2-b]benzothiophene (DPh-BTBT) with diarylamino substituents: From crystalline order to amorphous state in evaporated thin films

Article abstract of DOI:10.1246/cl.2009.420

Introduction of diarylamino substituents on 2,7-dipheny][1]-benzothieno[3, 2-b]benzothiophene (DPh-BTBT) caused morphological change in the evaporated thin-film state: from highly crystalline films with edge-on molecular orientation for DPh-BTBT to amorph

Full text of DOI:10.1246/cl.2009.420

420
Chemistry Letters Vol.38, No.5 (2009)
Molecular Modification of 2,7-Diphenyl[1]benzothieno[3,2-b]benzothiophene (DPh-BTBT)
with Diarylamino Substituents: From Crystalline Order to Amorphous State
in Evaporated Thin Films
Takafumi Izawa,1 Hiroki Mori,1 Yusuke Shinmura,1 Masahito Iwatani,1 Eigo Miyazaki,1
Kazuo Takimiya,ꢀ1;2 Hsio-Wen Hung,3 Masayuki Yahiro,3 and Chihaya Adachi3
1Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527
2Institute for Advanced Materials Research, Hiroshima University, Higashi-Hiroshima 739-8530
3Center for Future Chemistry, Kyushu University, Fukuoka 819-0395
(Received January 30, 2009; CL-090110; E-mail: ktakimi@hiroshima-u.ac.jp)
1
Introduction of diarylamino substituents on 2,7-diphenyl[1]-
Ar
(HO) B
N
2
benzothieno[3,2-b]benzothiophene (DPh-BTBT) caused mor-
phological change in the evaporated thin-film state: from highly
crystalline films with edge-on molecular orientation for DPh-
BTBT to amorphous films for the diarylamino derivatives. The
former was unsuitable as a hole-transporting layer in organic
light-emitting diodes (OLEDs), whereas the latter acted as a
superior hole-transporting layer in multilayered OLEDs.
S
2
Ar
I
I
S
Pd cat.
1
Ar1, Ar2 = Ph
2
3
83%
89%
1
Ar
Ar1= Ph,
S
N
2
2
1
Ar2 = 1-Naphthyl
Ar
Ar
Ar
N
S
Ar1, Ar2
=
4
64%
Recent prevailing interest in organic electronics has encour-
aged synthetic chemists to develop new organic semiconductors
as an active material for various electronic device applications
such as organic light-emitting diodes (OLEDs), organic field-
effect transistors (OFETs), and photovoltaic cells.1 One of such
organic semiconductors recently developed is 2,7-diphenyl[1]-
benzothieno[3,2-b]benzothiophene (DPh-BTBT, Figure 1) that
gives high-performance OFETs with field-effect mobility
(ꢀFET) higher than 1.0 cm2 Vꢁ1 sꢁ1.2 Not only the high mobility,
but also its improved air-stability and ready availability3 make
DPh-BTBT an attractive material for other organic electronic
devices, e.g. OLEDs or photovoltaic cells.
Preliminary experiments using DPh-BTBT as the hole-
transporting material (HTM) in multi-layered OLED devices
(ITO/DPh-BTBT/Alq3/LiF/Al) (Figure S1) however, gave dis-
couraging results: most of the fabricated devices showed negli-
gible emission. The reason for the poor results seemed to be
two fold: one is energetic mismatching of the highest occupied
molecular orbital (HOMO) of DPh-BTBT (ꢂ5:6 eV below the
vacuum level) with the work function of ITO electrode
(ꢂ5:0 eV). The other, presumably the major one, is unfavorable
molecular orientation of DPh-BTBT in the thin film state as
evidenced by an X-ray diffraction measurement (XRD) of the
evaporated thin film on the ITO substrate (Figure S2). The film
has a crystalline phase with edge-on molecular orientation sim-
ilar to DPh-BTBT thin film on Si/SiO2 substrates. Such molecu-
lar orientation in the crystalline film is unsuitable in OLED
devices, although it is optimal for high-performance OFET
devices.
Scheme 1. Synthesis of DPh-BTBT derivatives.
employed in the development of HTMs, because the groups can
raise the HOMO energy level and promote the formation of an
amorphous phase in the thin film state.4 In this communication,
we report the synthesis of diarylamino derivatives of DPh-BTBT
2–4 (Scheme 1) and their characterization as an active electronic
material in OFET and OLED devices.
Synthesis of new DPh-BTBT derivatives 2–4 was easily
achieved by the palladium-catalyzed Suzuki–Miyaura coupling
between 2,7-diiodo[1]benzothieno[3,2-b]benzothiophene (1)2
and the corresponding boronic acids, 4-(N,N-diphenylamino)-
phenyl boronic acid for 2,5 4-[N-(1-naphthyl)-N-phenylamino]-
phenyl boronic acid for 3,6 and 4-(9-carbazolyl)phenyl boronic
acid for 4,7 respectively (Scheme 1).8,9
Table 1 lists their oxidation potentials (Eox), estimated
HOMO energy levels,10,11 energy gaps estimated from the ab-
sorption edge in dichloromethane solution, and glass-transition
temperatures (Tg) together with those of a representative
HTM, N,N0-diphenyl-N,N0-bis(1-naphthyl)-1,10-biphenyl-4,40-
diamine (ꢁ-NPD) for comparison.12
Physical vapor deposition of 2–4 gave homogenous thin
films on Si/SiO2 or ITO substrates. XRDs of evaporated thin
films of 2–4 on both substrates showed no peak, indicating that
Table 1. Some molecular properties of 2–4
Egc/eV
Eoxa/V
HOMOb/eV
Tgd/ꢃC
2
3
4
+0.39
+0.41
+0.73
+0.26
5.2
5.2
5.5
5.1
2.88
2.88
3.10
123
140
—
95
With these preliminary results, we planned to chemically
modify DPh-BTBT with diarylamino substituents that are often
e
ꢁ-NPD
S
aOxidation onset vs. Fc/Fcþ. bEstimated from the oxidation onsets.
cEstimated from absorption edges in solution UV–vis spectra.
dDetermined with DSC traces. eTg was not confirmed on the
DSC trace.
S
Figure 1. Molecular structure of DPh-BTBT.
Copyright ꢀ 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.5 (2009)
421
Table 2. Some device characteristics extracted from OFETs
and OLEDs based on 2–4
(a)
(b)
10-7
0.0003
0.00025
0.0002
0.00015
0.0001
5 10-5
-1.4 10-7
-1.2 10-7
-1 10-7
-8 10-8
-6 10-8
-4 10-8
-2 10-8
0
Vg
= -80 V
OFET
OLED
b
10-8
10-9
a
ꢁ1
c
ꢀFET
/cm2 Vꢁ1 sꢁ1
LJ
V1000
Vg
= -70 V
1000
Ion/off
/cd Aꢁ1
/V
Vg
=
-60
-50
V
10-10
10-11
10-12
2
3
4
2:0 ꢄ 10ꢁ4
1:1 ꢄ 10ꢁ4
2:8 ꢄ 10ꢁ5
1:4 ꢄ 10ꢁ4
5 ꢄ 103
103
2.44
1.92
0.53
2.38
7.5
8.4
9.4
8.5
Vg
=
V
103
Vg
Vg
=
=
-40
-30
V
V
ꢁ-NPD
5 ꢄ 103
0
0
-10
-20
-30
-40
-50
-60
-70
-80
-60
-40
-20
/ V
0
20
aExtracted from the saturation regime. bCurrent efficiency at
1000 cd mꢁ2. Driving voltage at 1000 cd mꢁ2
Vd / V
V
g
c
.
Figure 2. Output (a) and transfer (b) curves of 2-based OFETs.
devices showed a longer lifetime than that of the standard
OLEDs using ꢁ-NPD as HTM (Figure S9).
formation of amorphous thin film was highly likely (Figure S6).
These results strongly implied that introduction of the diaryl-
amino substituents deters the edge-on molecular orientation
and is effective to change the thin-film morphology from crystal-
line order to an amorphous state.
In summary we synthesized a series of diarylamino deriva-
tives of DPh-BTBT 2–4 via the Suzuki–Miyaura coupling. Intro-
duction of the diarylamino groups on DPh-BTBT induced mor-
phological change in the vapor-deposited thin-films. In accord-
ance with the amorphous nature of 2–4 thin films, ꢀFETs evalu-
ated using the OFET configuration were significantly lower than
that of DPh-BTBT. In turn, the capability of 2–4 forming amor-
phous thin films makes them useful as HTM in multilayered
OLEDs, and thus examined OLEDs showed comparable per-
formances with a standard OLEDs using ꢁ-NPD as HTM.
In order to evaluate the hole-transporting properties of 2–4
in the thin-film phase, we fabricated their OFET devices with
the top contact configuration (L ¼ 50 mm, W ¼ 1:5 mm). The
output and transfer characteristics of 2-based devices under neg-
ative gate bias are depicted in Figure 2 as a representative. In ac-
cordance with the amorphous nature of the thin film, its ꢀFET
(2:0 ꢄ 10ꢁ4 cm2 Vꢁ1 sꢁ1) was significantly reduced by four
orders of magnitude compared with that of DPh-BTBT-based
OFETs (Table 2). 3-based devices showed ꢀFET in the same
order of magnitude as 2-based ones, while 4-based ones showed
slightly lower ꢀFET. These ꢀFETs are on the other hand, in a sim-
ilar range of reported values for OFETs with an amorphous thin
film-based active channel:13 actually, ꢁ-NPD-based OFETs14
showed almost the same ꢀFET (1:4 ꢄ 10ꢁ4 cm2 Vꢁ1 sꢁ1) when
evaluated using the same device structure under identical condi-
tions.15 These results suggested that the present DPh-BTBT
derivatives, in particular 2 and 3, have a potential as HTM for
OLEDs.
The standard device configuration of OLED (ITO (110 nm)/
HTM (50 nm)/Alq3 (50 nm)/LiF (0.8 nm)/Al (100 nm)) was
employed for evaluating 2–4 as HTM. For each HTM, more than
ten devices were fabricated and evaluated to eliminate device-to-
device deviation. All the devices except for the 4-based devices
showed an emission band centered at around 520 nm, typical
emission from the Alq3 layer (Figure S7). The device character-
istics, current efficiency (LJꢁ11000) and driving voltage (V1000) at
the luminescence of 1000 cd mꢁ2 extracted from typical devices
are listed in Table 2 together with those of OLEDs with ꢁ-
NPD.12 As can be seen from Table 2, new HTMs 2 and 3 give
comparable current efficiency and driving voltage with those
of ꢁ-NPD, whereas 4 gives lower performances. The better per-
formances of 2 and 3 as HTM can be partially attributed to their
higher mobility and better energetic matching of their HOMOs
with the work function of ITO electrode.11
This work was partially supported by Grants-in-Aid for Sci-
entific Research from the MEXT, Japan and by Seeds-Innova-
tion Program (Feasible Study) from JST, Japan. We also thank
Nippon Kayaku Co., Ltd. for research cooperation.
References and Notes
1
a) Organic Electronics, Manufacturing and Applications, ed. by H.
Klauk, Wiley-VCH, Weinheim, 2006. b) See also a special issue on
organic electronics: S. A. Jenekhe, Chem. Mater. 2004, 16, 4381.
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J. Am. Chem. Soc. 2006, 128, 12604.
2
3
4
5
6
DPh-BTBT is commercially available from TCI.
Y. Shirota, J. Mater. Chem. 2000, 10, 1.
Z. H. Li, M. S. Wong, Org. Lett. 2006, 8, 1499.
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8
9
Z. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akiike, H. Miyamoto, T.
Kajita, M. Kakimoto, Chem. Lett. 2008, 37, 262.
All the new BTBT derivatives were characterized by spectroscopic and
combustion elemental analyses. See Supporting Information.
Supporting Information is available electronically on the CSJ-Journal
Web site, http://www.csj.jp/journals/chem-lett/index.html.
´
10 a) J. L. Bredas, R. Silbey, D. S. Boudreaux, R. R. Chance, J. Am. Chem.
Soc. 1983, 105, 6555. b) J. Pommerehne, H. Vestweber, W. Guss, R. F.
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¨
11 Lower HOMO energy level of 4 than those of 2 and 3 is attributed to
weaker electron-donating nature of the carbazole moiety in 4 than
those of diarylamine moieties in 2 and 3.
12 S. A. V. Slyke, C. H. Chen, C. W. Tang, Appl. Phys. Lett. 1996, 69,
2160.
Although the performances of the OLEDs using 2–4 as
HTM are not necessarily far superior to standard OLED devices
with ꢁ-NPD, the present results suggest that new DPh-BTBT de-
rivatives, in particular 2 and 3, showing comparable mobilities
to ꢁ-NPD, have a potential as HTM for practical applications
owing to their better thermal stability as expected from higher
Tgs.4 As a matter of fact, preliminary durability tests of 2-based
13 Y. Shirota, H. Kageyama, Chem. Rev. 2007, 107, 953.
14 Field-effect mobility of a-NPD was reported to be 6:1 ꢄ 10ꢁ5
cm2 Vꢁ1 sꢁ1: T. P. I. Saragi, T. Fuhrmann-Lieker, J. Salbeck, Adv.
Funct. Mater. 2006, 16, 966.
15 We also evaluated hole mobility of 2 using the time-of-flight (TOF)
method. The mobilities obtained by the FET and TOF methods were
consistent with each other. See Supporting Information.
Published on the web (Advance View) March 28, 2009; doi:10.1246/cl.2009.420