Electron-accepting π-conjugated systems based on cyclic imide and cyano-substituted benzothiadiazole for non-fullerene organic photovoltaics

Article abstract of DOI:10.1246/cl.150072

To develop organic semiconductors for the application to acceptor materials in organic photovoltaics, we synthesized new π-conjugated compounds consisting of cyclic imide and cyano-substituted benzothiadiazole. Photophysical and electrochemical properties

Full text of DOI:10.1246/cl.150072

CL-150072
Received: January 25, 2015 | Accepted: February 7, 2015 | Web Released: February 17, 2015
Electron-accepting π-Conjugated Systems Based on Cyclic Imide and Cyano-substituted
Benzothiadiazole for Non-fullerene Organic Photovoltaics
Yutaka Ie,*1,2 Seihou Jinnai,1 Makoto Karakawa,1 and Yoshio Aso*1
1The Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047
2JST-PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 333-0012
(E-mail: yutakaie@sanken.osaka-u.ac.jp)
To develop organic semiconductors for the application to
acceptor materials in organic photovoltaics, we synthesized new
π-conjugated compounds consisting of cyclic imide and cyano-
substituted benzothiadiazole. Photophysical and electrochemical
properties as well as semiconducting performance of these
compounds were investigated.
compounds PhPI-BTCN, NpPI-BTCN, F4PI-BTCN, and F4(PI-
BTCN)2. As shown in Figure S1 (see the Supporting Informa-
tion (SI)), density functional theory (DFT) calculations at the
B3LYP/6-31G(d,p) level predict that both the highest occupied
molecular orbital (HOMO) and LUMO of PhPI-BTCN are
delocalized over the whole conjugated backbone.8 On the other
hand, the introduction of naphthalene or fluorene on the imide
nitrogen makes a large contribution to the HOMOs of these
units, whereas the LUMOs remain delocalized along the
backbone. In this paper, synthesis, properties, and OPV perform-
ance of these compounds are reported.
Synthesis of the target compounds is shown in Scheme 1.
We utilized Pd-catalyzed Sonogashira-Hagihara coupling for
the construction of the π-conjugated framework. Phthalimide
derivatives 1a-1e were synthesized using a standard trimethyl-
silyl ethynylation technique. The trimethylsilyl groups of 1a-1e
were removed to give 2a-2e, which were then utilized in a cross-
coupling reaction with 7-bromo-2,1,3-benzothiadiazole-4-car-
bonitrile (3)9 to give MHPI-BTCN, PhPI-BTCN, NpPI-BTCN,
F4PI-BTCN, and F4(PI-BTCN)2 in good yields. The chemical
structures of these compounds were proved by NMR and MS.
Unexpectedly, NpPI-BTCN was found to have a limited
solubility of less than 10 mg mL¹1 in common organic solvents,
which prevented OPV device fabrication.
Research on organic photovoltaics (OPVs) using π-conju-
gated systems as semiconducting materials has attracted in-
creasing attention in recent years because OPVs have the
advantage of lightness, flexibility, low cost, and ease of
processing.1,2 The use of both hole-transporting (donor) and
electron-transporting (acceptor) semiconductors is essential for
the construction of the active layer in OPVs. To increase the
interfacial area for efficient charge separation, the architecture
of the active layer is based on the bulk-heterojunction (BHJ)
structure with a three-dimensional interpenetrating network of
these materials.3 Poly(3-hexylthiophene) (P3HT) and [6,6]-
phenyl-C61-butyric acid methyl ester (PC61BM) have been
utilized as typical donor and acceptor materials, respectively,
in such systems.4 Significant progress has been achieved for
the donor materials, and low-band-gap compounds have been
recognized as a rational molecular design.5 On the other hand,
methanofullerene derivatives represented by PC61BM have faced
a few disadvantages such as weak absorption in the visible
region, high cost of production, and difficulty of purification.
Thus, the development of non-fullerene acceptor materials based
on π-conjugated systems remains a critical challenge.6
The UV-vis absorption spectra of MHPI-BTCN, PhPI-
BTCN, NpPI-BTCN, F4PI-BTCN, and F4(PI-BTCN)2 in CHCl3
solutions are shown in Figure 2. These spectra showed similar
shapes with distinct absorption bands at 300-370 and 400-
480 nm. As summarized in Table 1, optical energy gaps
Recently, we reported that a new triadic π-conjugated
system containing benzothiadiazole as a central unit and cyclic
imide as a terminal unit possesses electron-accepting character-
istics with the lowest unoccupied molecular orbital (LUMO)
energy level of ¹3.32 eV.7 This compound showed OPV
characteristics in combination with P3HT.7 For further tuning
of the LUMO energy level and molecular orbital distribution, we
designed a new π-conjugated compound MHPI-BTCN by direct
connection of cyano group with benzothiadiazole (Figure 1).
Furthermore, to investigate the influence of the substituent on
the nitrogen atom in the imide group, we also designed new
S
N
N
Br
N
O
R
(43%)
(60%)
MHPI-BTCN
PhPI-BTCN
3
N
[Pd(PPh3)4], CuI
X
NpPI-BTCN (95%)
(57%)
1a, 2a : R = 1-methylhexyl
1b, 2b : R = phenyl
1c, 2c : R = naphthyl
O
toluene/NEt3, reflux
F4PI-BTCN
1a-1d X = TMS
K2CO3
THF/MeOH, r.t.
2a-2d
X = H
1d, 2d : R = dibutylfluorenyl
O
H9C4C4H9
O
N
N
C5H11
X
X
O
R =
S
CH3
MHPI-BTCN
N
N
O
O
R
PhPI-BTCN NpPI-BTCN
C4H9 H9C4 C4H9
N
1e X = TMS
2e
K2CO3
H9C4
N
THF/MeOH, r.t.
O
X = H
3, [Pd(PPh3)4], CuI
R =
2e
F4(PI-BTCN)
2 (66%)
F4(PI-BTCN)2
toluene/NEt3, reflux
F4PI-BTCN
Figure 1. Chemical structures.
Scheme 1. Synthesis of target compounds.
694 | Chem. Lett. 2015, 44, 694–696 | doi:10.1246/cl.150072
© 2015 The Chemical Society of Japan

Figure 2. UV-vis absorption spectra of MHPI-BTCN (black), PhPI-
BTCN (red), NpPI-BTCN (orange), F4PI-BTCN (blue), and F4(PI-
BTCN)2 (green) in CHCl3 solution.
Figure 3. Cyclic voltammograms of MHPI-BTCN (black), PhPI-
BTCN (red), NpPI-BTCN (orange), F4PI-BTCN (blue), and F4(PI-
BTCN)2 (green) in o-DCB/CH3CN containing 0.1 M TBAPF6.
Table 1. Photophysical and electrochemical data
onset
opt
-
¦Eg
Ered
ELUMO
/eVd
1/2
Compds
/nma
/eVb
/Vc
MHPI-BTCN
PhPI-BTCN
NpPI-BTCN
F4PI-BTCN
F4(PI-BTCN)2
418
418
416
420
422
2.97
2.97
2.98
2.95
2.94
¹1.32, ¹1.78 ¹3.48
¹1.33, ¹1.72 ¹3.47
¹1.32, ¹1.71 ¹3.48
¹1.32, ¹1.71 ¹3.48
¹1.32, ¹1.72 ¹3.48
aIn CHCl3. bDetermined by the onset of UV-vis absorption
c
spectra. In o-DCB/CH3CN, 0.1 M TBAPF6, V vs. Fc/Fc+.
dDetermined from the first Ered on the assumption that the
1/2
Fc/Fc+ level is ¹4.8 eV vs. vacuum.
Figure 4. Photoluminescence spectra of P3HT (black dashed line),
P3HT:MHPI-BTCN (black), P3HT:PhPI-BTCN (red), P3HT:F4PI-
BTCN (blue), and P3HT:F4(PI-BTCN)2 (green) films excited at
absorption maximum.
(¦Egopt’s) estimated from the onset of absorption spectra are
found to be in the narrow range of 2.94-2.98 eV, contrary to
our assumption that the localized distribution of HOMOs for
NpPI-BTCN, F4PI-BTCN, and F4(PI-BTCN)2 gives rise to an
intramolecular charge-transfer transition and thus to the decrease
of ¦Egopt’s. According to the time-dependent (TD) DFT
calculations, the longer-wavelength-region band of these com-
pounds is assigned to (HOMO¹1)-LUMO transition (see the
SI). On the other hand, the oscillator strengths of low-energy
HOMO-LUMO transitions are very weak, resulting in the
difficulty of detecting visible difference of HOMO-LUMO gaps
by the ¦Egopt’s.
To investigate the electrochemical properties, cyclic voltam-
metry (CV) and differential pulse voltammetry (DPV) measure-
ments were carried out in o-dichlorobenzene (o-DCB)/CH3CN
(5/1) containing 0.1 M tetrabutylammonium hexafluorophos-
phate (TBAPF6), and the ferrocene/ferrocenium (Fc/Fc+) redox
couple was used as an internal standard for the calibration of the
potentials. As shown in Figure 3, all the compounds showed
two reversible reduction waves. Each reduction wave of MHPI-
BTCN, PhPI-BTCN, NpPI-BTCN, and F4PI-BTCN consists of
a one-electron process, while that of F4(PI-BTCN)2 indicates
a two-electron transfer (Figure S3). The half-wave reduction
potentials (Ered1/2) were observed at almost the same potentials
for all the compounds, indicating that the LUMO levels are
almost identical. These results indicate little electronic commu-
nication between the π-conjugated backbone and the substituents
on the nitrogen atom of phthalimide. From the first Ered1/2 and the
equation ELUMO = ¹ (Ered1/2 + 4.8),10 the LUMO energy levels
(ELUMO) of these compounds were estimated to be ca. ¹3.48 eV.
Photoluminescence spectra of P3HT:acceptor blend films
along with P3HT pristine film are shown in Figure 4, and the
corresponding absorption spectra of blend films are shown in
Figure S4. Photoluminescence of P3HT:MHPI-BTCN, P3HT:
F4PI-BTCN, and P3HT:F4(PI-BTCN)2 blend films were com-
pletely quenched in comparison with the distinct fluorescence of
P3HT film, which implies the occurrence of efficient photo-
induced charge transfer from P3HT to the acceptors. Thus, these
materials having low-lying LUMO energy levels can function as
acceptors in organic solar cells. On the other hand, P3HT:PhPI-
BTCN blend films showed slightly retained fluorescence from
P3HT, indicating reduced efficiency of charge transfer compared
to other blend films.
To investigate the photovoltaic performance of MHPI-
BTCN, PhPI-BTCN, F4PI-BTCN, and F4(PI-BTCN)2, inverted-
BHJ solar cells11 were fabricated using P3HT as the electron
donor, with device structures consisting of glass/indium tin
oxide (ITO)/ZnO/active layer/MoO3/Al. The fabrication con-
ditions of the active BHJ layer were optimized by varying the
blend composition, processing solvent, and thermal annealing
temperature, and we found that the optimized active layer could
be prepared by spin-coating from chloroform solutions of the
P3HT/acceptor blend (1:2 weight ratio). Thermal annealing at
100 °C was effective in the blends based on PhPI-BTCN, F4PI-
BTCN, and F4(PI-BTCN)2. Under air mass 1.5 global (AM
1.5 G)-simulated solar illumination with an irradiation intensity
of 100 mW cm¹2, these devices showed typical OPV character-
Chem. Lett. 2015, 44, 694–696 | doi:10.1246/cl.150072
© 2015 The Chemical Society of Japan | 695

LUMO for F4PI-BTCN and F4(PI-BTCN)2 facilitate the
formation of charge-transfer transition state, which also might
contribute to an efficient charge separation, leading to increasing
these factors. The external quantum efficiency (EQE) action
spectrum shown in Figure 5b, in which this device exhibited a
broad photoresponse range of 300-650 nm, with the maximum
EQE of 20% observed at 540 nm.
In summary, a series of electron-accepting organic semi-
conductor materials based on cyclic imide and cyano-substituted
benzothiadiazole was designed and synthesized. The combina-
tion of electron-deficient aromatic units with cyano group leads
to low-lying LUMO energy levels. These compounds were
successfully used as acceptors for BHJ OPV devices in
combination with P3HT. Substituent groups on the imide
nitrogen markedly affected OPV performance, and the fluorene
substituent contributed to increasing the Jsc and FF, which might
be correlated with the morphology of the blend films and/or
the distributed frontier molecular orbitals. This study shed
light on the fundamental structure-property-device-performance
relationships, leading to further molecular design of acceptor
materials for OPVs.
This work was supported by Japan Science and Technology
Agency (PREST) and Grants-in-Aid for Scientific Research
from the Ministry of Education, Culture, Sports, Science and
Technology (Molecular Architectonics), Japan, and by cooper-
ative Research with Sumitomo Chemical Co., Ltd. We acknowl-
edge Mr. Takeo Makino for the support of synthesis. Thanks are
extended to the Comprehensive Analysis Center (CAC), ISIR,
for assistance in obtaining elemental analyses.
Figure 5. (a) J-V curves of the OPV devices based on blend of
P3HT:MHPI-BTCN (black), P3HT:PhPI-BTCN (red), P3HT:F4PI-
BTCN (blue), and P3HT:F4(PI-BTCN)2 (green) under AM 1.5
illumination (100 mW cm¹2). (b) EQE spectra of OPV devices.
Table 2. Characteristics of OPVs
PCE
/%
Jsc
/mA cm
Voc
/V
Compds
FF
Supporting Information is available electronically on J-STAGE.
References and Notes
¹2
P3HT:MHPI-BTCN
P3HT:PhPI-BTCN
P3HT:F4PI-BTCN
P3HT:F4(PI-BTCN)2
0.19
0.12
0.32
0.28
0.93
1.39
1.45
1.34
0.57
0.23
0.53
0.47
0.36
0.37
0.41
0.45
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696 | Chem. Lett. 2015, 44, 694–696 | doi:10.1246/cl.150072
© 2015 The Chemical Society of Japan