CL-150216
Received: March 10, 2015 | Accepted: April 12, 2015 | Web Released: April 17, 2015
Synthesis of Novel PushPull Chromophores based on N-Ethylcarbazole
for Vacuum Deposition Processed Organic Photovoltaics
Takahiro Kono,1,2 Yosei Shibata,* Zhiping Wang, Tetsuhiko Miyadera, and Yuji Yoshida*1
1
1
1,3
1
Research Center for Photovoltaic Technologies, National Institute of Advanced Industrial Science and Technology (AIST),
Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565
2
Center for Energy and Environmental Science, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567
3
PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012
(
E-mail: yosei.shibata@aist.go.jp, yuji.yoshida@aist.go.jp)
Two novel p-type materials, Cz(Et)ThOM and Cz(Et)OM®
respectively with and without a thiophene π-spacing unit®have
been synthesized for vacuum-deposited organic photovoltaics
NC
NC
N
CN
N
S
CN
O
(OPVs). The power conversion efficiencies of planar hetero-
O
junction OPVs of the former are clearly higher than that of the
latter. Further improvement of the device with Cz(Et)ThOM was
obtained by employing bulk heterojunction structure and post-
annealing. The structural analysis of thin films by synchrotron
X-ray diffraction and atomic force microscopy shows that this
improvement is due to Cz(Et)ThOM becoming highly ordered
during annealing.
1
: Cz(Et)ThOM
2: Cz(Et)OM
Figure 1. The molecular structure of Cz(Et)ThOM and Cz(Et)OM.
power conversion efficiency of up to 9% in dye-sensitized solar
cells with the MK dye
9
11
2
¹1 ¹1
and a hole mobility of 0.1 cm V
s
1
2
in indolocarbazole organic field effect transistors. The elec-
tron-donating property and steric planar structure of the carba-
zole unit clearly contributed to these results. However, a smaller
structure would be preferable for OPV applications to confer
the thermal stability required for vacuum deposition. In small
π-conjugated molecules, the intramolecular interactions between
strong donor and acceptor units can narrow the HOMOLUMO
gap and afford long-wavelength absorption, while a strong
acceptor unit also provides deep HOMO and LUMO levels.
Herein, we report the synthesis of, and the optical and
electrochemical properties of the novel pushpull chromophores,
Cz(Et)ThOM and Cz(Et)OM (hereafter referred to as com-
pounds 1 and 2; Figure 1). The performance of planar hetero-
junction OPV devices prepared with compound 1 being clearly
superior, a more focused optimization was performed, revealing
substantial improvements after preparation of a bulk hetero-
junction (BHJ: donoracceptor blend) structure and thermal
treatment. This paper focuses on the relationship between the
OPV performance and crystallinity of Cz(Et)ThOM films, as
revealed by synchrotron X-ray diffraction.
Vacuum deposition has recently attracted much attention as
a potential means to realize tandem-structured organic photo-
voltaic (OPV) devices with a wide absorbance for sunlight.
1
,2
One of the advantages of organic semiconductor materials in
this context is that the absorption range is controllable from
ultraviolet/visible to near-infrared wavelengths. High-perform-
ance vacuum-deposited OPVs need to be achieved with small
molecules. Therefore, to obtain enhanced performance, novel
electron donor materials in OPVs are required with high molar
absorption coefficients (¾) for long wavelengths. For OPV
applications, electron donor materials should have narrow
energy gaps between their highest occupied and lowest un-
occupied molecular orbitals (HOMO and LUMO, respectively).
On the other hand, a deep HOMO level is also important
because the open-circuit voltage (V ) in OPV devices strongly
oc
depends on the difference between the HOMO of the donor and
3
the LUMO of the acceptor. Furthermore, in view of vacuum
fabrication, the thermal stability of the material is crucial. To
date, however, the literature on small molecules for vacuum-
deposited OPV devices is sparse.
The synthesis of these compounds is described in the
Supporting Information. Ultravioletvisible absorption spectra
were recorded to investigate their photophysical and electro-
chemical properties, with absorption maxima and ¾ in a tetra-
To synthesize small molecules that satisfy the above-
mentioned requirements, we focused on pushpull chromo-
phores with an asymmetric structure. Pushpull chromophores
based on a donorπ-spaceracceptor (DπA) structure offer
potentially high ¾ values, making them attractive candidates for
high-performance OPV applications.4 In addition, their light
absorption properties and HOMOLUMO levels are readily
controllable via structural modifications of the electron donor
and acceptor moieties. In spite of these advantages, however,
an optimal molecular design based on pushpull chromophores
with a π-spacer for OPV devices has not been firmly established
till date. As donor and acceptor skeletons, respectively, we chose
N-ethylcarbazole and 2-(3-oxoindene)malononitrile, the latter
possessing two cyano and one ketone substituents with strong
electron-accepting properties. The carbazole unit affords high
¹
1
¹1
hydrofuran solution observed at 572 nm (¾ = 45400 M cm
)
¹
1
¹1
and 514 nm (¾ = 33900 M cm ) for compounds 1 and 2,
respectively. The molar absorbance coefficient of compound 1 is
higher than that of 2 and its absorption peak is red-shifted
compared with 2. These results are ascribed to the extended
π-conjugation that results from the insertion of the electron-
donating thiophene ring. The HOMO level energies of thin
films of the two materials were measured by photoelectron
yield spectroscopy, giving 5.78 and 5.94 eV, respectively. This
suggests that OPV devices prepared with compound 1 or 2 as the
8
donor material will have a large V , since as mentioned above,
oc
this strongly depends on the difference between the HOMO and
© 2015 The Chemical Society of Japan