Bis-cinnolines as n-type semiconducting material with high electron mobility and thermal stability and their application in organic photovoltaic cells

Article abstract of DOI:10.1002/asia.201100234

Several bis-cinnoline derivatives were found to act as a new class of n-type amorphous organic material. They exhibit high electron-drift mobility up to the order of 10^-3 cm²V^-1 s^-1 and high thermal stability, a

Full text of DOI:10.1002/asia.201100234

DOI: 10.1002/asia.201100234
Bis-Cinnolines as n-Type Semiconducting Material with High Electron
Mobility and Thermal Stability and their Application in Organic Photovoltaic
Cells
Hayato Tsuji,*[a] Yuki Yokoi,[a] Yoshiharu Sato,[b] Hideyuki Tanaka,[a] and
Eiichi Nakamura*[a, b]
Dedicated to Professor Eun Lee on the occasion of his retirement and 65th birthday
Organic photovoltaic (OPV) devices[1–4] are attracting
much attention as a new source of renewable energy. A
major chemical interest in this area of research lies in the
development of new organic semiconductors. Molecular
design endows in an organic compound either a hole-trans-
porting (p-type), electron-transporting (n-type), or ambipo-
lar property through the incorporation of heteroatoms and
new conjugation systems to the basic carbon framework.[5–8]
The design of p-type semiconductors usually consists of in-
stalling electron-donating amino substituents on an aromatic
system to raise the HOMO level. We recently discovered
the p-type semiconductor properties of benzodifurans, and
found it interesting that furans have received much less at-
tention than thiophene and pyrrole semiconductors. This ob-
Scheme 1. Structures of bis-cinnolines used in this study.
servation led us to explore a variety of aromatic rings bear-
ing electron-deficient heteroatoms that may generally serve
as n-type materials, which are less easy to design, and hence
less-developed than the p-type materials.[5,6,9–15] Herein, we
report that a naphthalene-type molecule bearing two nitro-
gen atoms called cinnoline (Scheme 1) shows good n-type
characteristics together with favorable thermal and morpho-
logical stabilities and high electron mobility (up to the order
of 10À3 cm2 VÀ1 sÀ1), the highest value among known amor-
phous organic materials. Cinnolines have received some at-
tention in drug discovery research,[16] but so far none at all
in organic electronics research. We demonstrate their utility
as a cathode buffer material in organic thin-layer OPV devi-
ces by taking advantage of their low LUMO character and
potential ability to coordinate to metal surface.
[a] Dr. H. Tsuji, Y. Yokoi, Dr. H. Tanaka, Prof. Dr. E. Nakamura
Department of Chemistry
To obtain quick and efficient access to a variety of bis-cin-
noline derivatives to be screened for device applications, we
have developed a modular synthetic approach (Scheme 2).
Thus, the synthesized 8-bromocinnoline 1[17] is a key module
that could be coupled with either para- or meta-benzenedi-
boronic acid by a Suzuki–Miyaura coupling reaction to
obtain p8-BCB or m8-BCB as yellow solids in 72% or 38%
yield, respectively. The nickel-catalyzed homocoupling reac-
tion[18] of 1 afforded the 8,8’-bis-cinnoline (8-BC) in 31%
yield. Similarly, p4-BCB, a positional isomer of p8-BCB, was
isolated from 4-bromocinnoline 2 in 50% yield as a yellow
solid.
School of Science, The University of Tokyo
7-3-1, Hongo, Bunkyo-ku
113-0033 Tokyo (Japan)
Fax : (+81)3-5800-6889
E-mail: nakamura@chem.s.u-tokyo.ac.jp
tsuji@chem.s.u-tokyo.ac.jp
[b] Dr. Y. Sato, Prof. Dr. E. Nakamura
Nakamura Functional Carbon Cluster Project, ERATO
Japan Science and Technology Agency
7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100234.
Chem. Asian J. 2011, 6, 2005 – 2008
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2005

COMMUNICATION
The first reduction potentials,
obtained from the DPV meas-
urements, were À2.03, À2.11,
À2.05, and À2.08 V (vs. Fc/Fc+)
for p8-BCB, m8-BCB, 8-BC,
and p4-BCB, respectively. The
estimated electron affinities
(EA) were 2.77, 2.69, 2.75, and
2.72 eV,[19] thus indicating that
the more-conjugated p8-BCB
shows a higher EA than the
other derivatives. UV/Vis ab-
sorption measurements (see
the Supporting Information,
Figure S3) indicated that these
bis-cinnolines absorb little visi-
ble light, the absorption onset
wavelengths ranging from 431
to 451 nm. Based on this opti-
cal gap, the ionization poten-
tials of p8-BCB, m8-BCB, 8-
BC, and p4-BCB were estimat-
ed to be 5.52, 5.45, 5.56, and
5.60 eV, respectively, which are
high enough to exhibit a hole-
Scheme 2. A modular synthetic approach to a variety of bis-cinnoline derivatives.
blocking capability.
The bis-cinnoline derivatives
Electrochemical and physical property measurements re-
vealed the n-type semiconductor properties of these bis-cin-
noline compounds. The data are summarized in Table 1. The
show electron mobility, as determined by the time-of-flight
(TOF) measurement for an amorphous film with microme-
ter thickness prepared by vacuum deposition (Figure 1).
These compounds showed dispersive electron-transient-cur-
rent signals, and negative dependence upon the applied elec-
tric field. No hole-transient-current signal was observed.
The electron mobilities of these bis-cinnolines were 2ꢁ10À3–
Table 1. Electrochemical and physical properties of the bis-cinnolines.
p8-BCB
m8-BCB
À2.11
8-BC
p4-BCB
Ered [V][a]
À2.03,
À2.16
2.77
À2.05
À2.08,
À2.17
2.72
EA [eV][b]
lonset [nm][c]
IP [eV][d]
2.69
448
2.75
441
451
431
5.52
5.45
5.56
5.60
[e]
me [cm2VÀ1 sÀ1
Tm [8C][g]
]
3ꢁ10À3
331–332
460
5ꢁ10À3[f]
338–339
433
2ꢁ10À3
380–381
409
2ꢁ10À3
296–297
390
Td [8C][h]
[a] Reduction potential by DPV measurements in THF. THF=tetrahy-
drofuran. [b] Electron affinity estimated from the DPV measurement.
[c] UV/Vis absorption-onset wavelength in CH2Cl2. [d] Ionization poten-
tial estimated from the EA and the optical gap. [e] Electron drift mobili-
ty measured by the TOF method using vacuum-deposited films at room
temperature at an electric field of 2.5ꢁ105 VcmÀ1 unless otherwise noted.
[f] At an electric field of 2.0ꢁ105 VcmÀ1
weight-loss temperature by TGA.
. [g] Melting range. [h] 5%
cyclic voltammograms of p8-BCB and p4-BCB in tetrahy-
drofuran showed a pseudo-reversible reduction wave, whilst
that of m8-BCB showed a reversible first reduction wave
(see the Supporting Information, Figure S1). The differential
pulse voltammetry (DPV) measurements showed that p8-
BCB and p4-BCB exhibited two-step reduction, whilst m8-
BCB and 8-BC exhibited one reduction peak within the sol-
vent window (see the Supporting Information, Figure S2).
Figure 1. Electron drift mobility of amorphous film of p4-BCB as a func-
tion of a square root of the applied field strength.
2006
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Chem. Asian J. 2011, 6, 2005 – 2008

An n-Type Semiconducting Material with Applications in Organic Photovoltaic Cells
5ꢁ10À3 cm2VÀ1 sÀ1, which are among the highest values for
amorphous organic materials that have ever been reported.
The high thermal stability of the bis-cinnolines was shown
by differential scanning calorimetry (DSC) and thermogravi-
metric analysis (TGA). The DSC trace (see the Supporting
Information, Figure S4) shows clear glass transitions for p8-
BCB and m8-BCB at 150 and 1468C, respectively, which are
high enough for device applications. The TGA analysis (see
the Supporting Information, Figure S5) showed
a 5%
weight loss (Td) at 460, 433, 409, and 3908C for p8-BCB,
m8-BCB, 8-BC, and p4-BCB, respectively.
The bis(8-cinnoline) derivatives serve as an excellent cath-
ode buffer layer in an OPV device as tested for the solu-
tion-processable pin-heterojunction device using benzopor-
phyrin (BP) and a bis(triorganosilylmethyl)fullerene.[20] The
configuration of the device is as follows: ITO/PEDOT:PSS/
BP/BP:SIMEF-2:SIMEF-2/cathode
buffer/Al,
where
SIMEF-2 refers to bis{dimethyl(ortho-anisyl)silylme-
AHCTUNGTRENNUNG
thyl}[60]fullerene, and the cathode buffer layer (6–8 nm)
and the aluminum electrode (80 nm) were deposited in
vacuum. The thickness of the buffer layer was not opti-
mized. We also fabricated a similar device, as a reference,
composed of a popular material, BCP,[21] as the cathode
buffer layer. The whole device was annealed at 808C, and
the annealing improved the performance of these cells.
The photovoltaic characteristics of these devices were de-
termined under irradiation with 100 mWcmÀ2 incident light,
using an AM 1.5G filter. The data are summarized in
Figure 2 and Table 2. The devices composed of the bis(8-cin-
noline)s showed higher JSC values than that of the BCP-
based reference cell and comparable VOC and FF to those of
it. In total, the PCE values of the cells using p8-BCB [4.0-
A
ACHTUNGTRENNUNG
(Æ0.34)%] are higher than that using BCP [3.5
ACHTUNGTRENNUNG
This is due to the high shunt resistance of these 8-cinnoline
layers, as reflected by no or very little reverse-bias current
(leak current) under dark conditions (Figure 2b). The cell
composed of p4-BCB results in a much lower PCE value
[1.9
N
VOC value [0.46
A
formation leads to a shift of vacuum level at the interface
between p4-BCB and the aluminium electrode.[22] This shift
contributes to effective carrier extraction for the electron, as
supported by the comparable Jsc values with the other cells.
Figure 2. J–V plots of OPV devices a) under irradiation with the AM
1.5G filter and b) in the dark.
Table 2. Summary of the photovoltaic characteristics of OPV devices involving
the bis-cinnolines as a cathode buffer layer, together with those of a BCP-based
cell for a reference.
However, unfortunately, a carrier can be injected from the
electrodes through a small barrier by applying a voltage, as
supported by the dark current characteristics, which raises
an interesting issue in molecular design to achieving effi-
cient carrier delivery at the organic/metal interface.
In conclusion, bis-cinnolines were found to serve as suita-
ble cathode buffer materials, featuring high electron-drift
mobility, high thermal stability and coordination ability.
These new bis-cinnolines form a uniform thin film, and
some show high shunt resistance, which contribute to reduce
p8-BCB
m8-BCB
(Æ0.01) 0.77(Æ0.01) 0.77
(Æ0.12) 9.31(Æ0.16) 8.30
8-BC
(Æ0.01) 0.46
(Æ0.06) 8.35
p4-BCB
(Æ0.00) 0.80
(Æ0.28) 7.82
BCP
(Æ0.00)
(Æ0.38)
VOC [V][a]
JSC
0.77
8.84
G
G
N
N
ACHTUNGTRENNUNG
R
R
ACHTUNGTRENNUNG
[b]
[mAcmÀ2
FF[c]
]
0.59
A
A
E
E
G
E
PCE [%][d] 4.0
G
N
N
ACHTUNGTRENNUNG
A
E
E
N
ACHTUNGTRENNUNG
[a] Open-circuit voltage. [b] Short-circuit current density. [c] Fill factor.
[d] Power-conversion efficiency. Maximum efficiency is shown in the parenthe-
ses.
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2007

H. Tsuji, E. Nakamura et al.
COMMUNICATION
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8-Bromo-3,4-diphenylcinnoline (240 mg, 0.665 mmol), 1,4-phenylenedi-
boronic acid (54.4 mg, 0.328 mmol), PdACHTNUTRGNEUNG(PPh3)4 (75.8 mg, 0.0656 mmol),
and Na2CO3 (153 mg, 1.44 mmol) were added to degassed H2O
(0.769 mL), EtOH (0.56 mL), and toluene (1.75 mL). The mixture was
stirred at 1208C for 18 h. After cooling to room temperature, the reaction
was quenched with saturated aqueous NH4Cl. The mixture was filtered
and washed several time with dichloromethane to give the title com-
pound as a yellow solid (72.2 g). The filtrate was extracted with chloro-
form. The combined organic layers were dried over MgSO4 and concen-
trated in vacuo. Recrystallization from toluene afforded the title com-
pound as a yellow solid (78.0 mg), (total 150 mg, 0.235 mmol, 72%). Mp:
338–3398C. 1H NMR (500 MHz, CDCl3): d=7.26–7.30 (m, 6H, ArH),
7.33–7.35 (m, 4H, ArH), 7.45–7.47 (m, 6H, ArH), 7.53–7.55 (m, 4H,
ArH), 7.77–7.81 (m, 4H, ArH), 7.96–7.98 (m, 4H, ArH), 8.05 ppm (s,
4H, ArH). 13C NMR (125 MHz, CDCl3): d=125.0, 126.0, 127.9, 128.0,
128.4, 128.7, 130.50, 130.55, 130.64, 131.2, 132.8, 134.6, 137.6, 137.7, 141.5,
147.2, 152.6 ppm. MS (APCI+): 639 [M+1]. Anal: calcd for C48H52N2:
C 86.49, H 4.73, N 8.77; found: C 86.49, H 4.73, N 8.77.
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[19] Electron affinity (EA) was estimated according to the following
equation: EA=4.80+Ered, where Ered refers to the reduction poten-
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We thank MEXT (KAKENHI for E.N., No. 22000008, H.T., No.
20685005) and the Global COE Program for Chemistry Innovation. This
work was partly supported by the Strategic Promotion of Innovative
R&D, JST.
Keywords: cinnolines · electron mobility · fullerenes · n-
type semiconductors · organic solar cells
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Received: March 4, 2011
Published online: June 21, 2011
2008
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Chem. Asian J. 2011, 6, 2005 – 2008