Dendritic oligothiophene bearing perylene bis(dicarboximide) groups as an active material for photovoltaic device

Article abstract of DOI:10.1039/b819008a

A dendritic oligothiophene containing perylene bis(dicarboximide) units was synthesized and its electronic properties and photovoltaic performance were investigated. The Royal Society of Chemistry.

Full text of DOI:10.1039/b819008a

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Dendritic oligothiophene bearing perylene bis(dicarboximide) groups
as an active material for photovoltaic devicew
Yutaka Ie, Toshihiko Uto, Nobuhiro Yamamoto and Yoshio Aso*
Received (in Cambridge, UK) 27th October 2008, Accepted 4th December 2008
First published as an Advance Article on the web 13th January 2009
DOI: 10.1039/b819008a
A dendritic oligothiophene containing perylene bis(dicarboximide)
units was synthesized and its electronic properties and photo-
voltaic performance were investigated.
spectroscopy, and elemental analysis. Detailed synthesis and
characterizations are described in the ESI.w
The cyclic voltammograms of both 1 and 2 are shown in
Fig. 2. The voltammogram of 1 exhibited two reversible waves
corresponding to successive reduction of the PDI units at the
half-wave potentials of À1.12 and À1.36 V and irreversible
broad waves with the first peak potential of +0.24 V corres-
ponding to oxidation of the oligothiophene units. The redox
waves of 2 were observed at almost the same potentials as
those of 1, indicating that the HOMO and LUMO levels of
these compounds are similar. From the redox potentials, the
HOMO–LUMO energy gap of 1 can be estimated around
1.36 eV.
Photovoltaic devices based on organic semiconductor materials
have attracted research interest due to their advantages of low
production cost, large-area printing, and flexible films.1 Among
them, the active organic layers composed by blends of electron-
donating and electron-accepting materials, so-called bulk
heterojunction (BHJ) type, have been extensively investigated
in terms of the ease of fabrication.1a,2 For further developing
this BHJ concept to induce efficient photoinduced charge
separation and to improve phase segregation in the active layer,
the potential of p-conjugated systems linked covalently with
electron-accepting units have also been demonstrated by us and
others.3–7 Recently, we have reported the synthesis of highly
branched dendritic oligothiophenes, their strong self-association
behaviours, and the application to p-channel OFET devices.8
Then we have reported the synthesis of dendritic oligo-
thiophene/[60]fullerene linkage molecules, which exhibit
ambipolar characteristics on FET devices.9 Motivated by these
results, our attention has focused on the utilization of
alkylated perylene bis(dicarboximide) (PDI),10,11 which has a
large molecular-absorption coefficient in the visible region, as
a soluble electron-accepting unit, and we have anticipated that
its combination with the dendritic oligothiophene will be
promising materials for the active component of organic
photovoltaic devices.6,7 In this communication, we report the
synthesis, properties, and photovoltaic performances of
dendritic oligothiophene/PDI linkage molecule 1 (Fig. 1).
The syntheses of 1 and 2 are shown in Scheme 1. Since
we could not apply conversion reactions via lithiation to
PDI-attached compounds due to the presence of carboximide
groups, precursors 5 and 6 were prepared directly by
molar-ratio-controlled Stille cross-coupling reactions. Finally,
the Stille coupling reaction between 4 and 6 afforded the target
compound 1 in 80% yield. The reference linear compound 2
was synthesized by Pd-catalyzed homo-coupling of 5.12
Compounds 1 and 2 were soluble in common organic solvents
such as chloroform and o-dichlorobenzene, which allowed us
to use preparative gel-permeation liquid chromatography
(GPLC) for the purification. All of the new compounds
were fully characterized by MALDI-TOF MS, 1H NMR
To gain insight into the electronic structure of 1, the mole-
cular orbitals were calculated by hybrid density functional
theory (B3LYP) with 6-31G basis set as implemented in the
Gaussian 03 program. The HOMO of 1 spreads over two arms
of the dendritic oligothiophene, whereas the LUMO is perfectly
related to the electron-accepting PDI moiety (Fig. S1, ESIw).
This result clearly indicates that these two chromophores
scarcely conjugate with each other owing to twisted conforma-
tions between them. The HOMO–LUMO energy gap is
calculated at 1.45 eV, which is in qualitative agreement with
the electrochemical result.
The UV-Vis absorption spectrum of 1 in chloroform is shown
in Fig. 3(a) together with normalized that of reference 2. Both
spectra displayed a broad p–p* transition band of the oligo-
thiophene moiety in the visible region and sharp transition
bands typical of the PDI moiety at 460, 490 and 530 nm. These
absorption features are interpreted as the superposition of those
of dendritic oligothiophene and PDI derivative 3 as shown in
Fig. S2, ESI,w reflecting little interaction between these two
The Institute of Scientific and Industrial Research, Osaka University,
Ibaraki, Osaka, 567-0047, Japan. E-mail: aso@sanken.osaka-u.ac.jp
w Electronic supplementary information (ESI) available: Details of
synthesis, characterization, UV-Vis spectra of films, computational
method, device fabrication. See DOI: 10.1039/b819008a
Fig. 1 Chemical structures of 1 and 2.
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Fig. 4 UV-Vis spectra of (a) 1 and (b) 2 in a chloroform solution
(red) and in a film (blue).
structural organization in the film state. This behaviour in a
condensed state has been supported by MALDI-TOF mass
spectrometry of 1, which showed obvious peaks assignable
to aggregated monocationic species up to 3-mer as shown in
Fig. S5 (ESIw).
Scheme 1 Synthesis of 1 and 2.
Photovoltaic devices were fabricated on an indium tin oxide
(ITO) coated glass substrate with a 140-nm organic layer spin-
coated from the chloroform solution of 1 or 2 followed by
vacuum deposition of an Al electrode on top of the active
layer.13 As shown in Fig. 5, the action spectra of these devices
upon illumination with 10-mW cmÀ2 monochromatic light are
in good accordance with the absorption spectra in the
films (Fig. 4), and the photocurrent of the ITO/1/Al device
is dramatically larger than that of the ITO/2/Al device.
This result clearly shows the importance of the dendritic
structure in the donor part to effective photocurrent genera-
tion. The current–voltage characteristics of these devices are
shown in Fig. 5(b), and the photovoltaic parameters are
listed in Table 1.14 The device based on 1 shows an incident
photon-to-current efficiency (IPCE) of 5.5% and an overall
power conversion efficiency (Z) of 0.25% under mono-
chromatic light, and its performance is apparently far superior
in all parameters compared to the device using 2. This
improvement of photovoltaic performance can be explained
by the aggregation behaviour of the dendritic structure,
which convincingly leads to construct carrier-transporting
pathways.8,9
Fig. 2 Cyclic voltammograms of 1 and 2 in o-dichlorobenzene–CH3CN
(9 : 1) containing 0.1 M TBAPF6, scan rate of 100 mV sÀ1
.
Fig. 3 (a) UV-Vis spectra of 1 and 2 in CHCl3. (b) Emission spectra
of 1, 2 and 3 in CHCl3with excitation at 490 nm.
units in the ground state. On the other hand, the fluorescent
emission of 1 and 2 was quenched strongly compared with the
case of 3 (Fig. 3(b)). This result indicates that efficient photo-
induced intramolecular electron transfer from the oligothio-
phene to the PDI unit takes place. As shown in Fig. 4, the
film-state UV-vis spectra of 1 and 2 showed red-shifts of the
oligothiophene absorption with broadening of the PDI absorp-
tions compared to their solution spectra, which suggests a
Fig. 5 Photovoltaic performances of ITO/1 or 2/Al devices: (a) photo-
current action spectra irradiated with 10 mW cmÀ2 light through the
ITO electrode and (b) I–V characteristics in the dark and under
illumination.
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Table 1 Photovoltaic performances of 1 and 2 as active layersa
Compd
Jsc/nA mÀ2
linc/nm
IPCE (%)
Voc/V
FF
Z (%)
1
2
204
25
470
460
5.5
0.67
0.46
0.22
0.27
0.26
0.25
0.014
a
Upon illumination with 10-mW cmÀ2 monochromatic light.
In summary, the dendritic oligothiophene bearing PDI units
at the terminal positions has been synthesized, and its proper-
ties and photovoltaic characteristics were revealed. Compari-
son between the devices based on 1 and 2 clearly demonstrates
that the structural modification of p-conjugated system has a
major impact on the performance. Our finding provides a
strategy for the development of new materials in photovoltaic
device application. Further study for the elucidation of
structure–photovoltaic property relationship will be reported
in due course.
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