Synthesis and photovoltaic effects of oligothiophenes incorporated with two [60]fullerenes

Article abstract of DOI:10.1246/cl.2004.654

The triads (nT-2C₆₀) consisting of one oligothiophene and two fullerenes have been synthesized as single-component materials for photovoltaic cells. The sandwich-type cells of a configuration Al/nT-2C₆₀/Au generate much higher photoc

Full text of DOI:10.1246/cl.2004.654

654
Chemistry Letters Vol.33, No.6 (2004)
Synthesis and Photovoltaic Effects of Oligothiophenes Incorporated with Two [60]Fullerenes
Nobukazu Negishi, Kazuo Takimiya, Tetsuo Otsubo,ꢀ Yutaka Harima, and Yoshio Asoꢀy
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527
yThe Institute of Scientific and Industrial Research, Osaka University, CREST, Japan Science and Technology Corporation (JST),
Ibraki, Osaka 567-0047
(Received March 22, 2004; CL-040323)
The triads (nT-2C60) consisting of one oligothiophene and
two fullerenes have been synthesized as single-component mate-
rials for photovoltaic cells. The sandwich-type cells of a config-
uration Al/nT-2C60/Au generate much higher photocurrents
than those based on the oligothiophene-fullerene dyads
(nT-C60).
nylbenzene-dicarbaldehydes 7a-7d, which were prepared from
oligothiophenes 4 (nT)4 by a combination of Stille coupling re-
actions and bromination with NBS. The dialdehydes 7a-7d thus
obtained were treated with [60]fullerene and N-methylglycine
according to Prato’s method5 to smoothly afford the correspond-
ing triads nT-2C60.6
C6H13
S
C6H13
S
i
S
S
H
H
x
H
SnBu3
x
S
S
S
S
Conjugated oligomers incorporated with fullerene are ex-
pected to be promising materials for growing organic solar
cells.1,2 In this regard, we recently discovered that the oligothio-
phene–fullerene dyads 1 (abbreviated as nT-C60) can induce
highly efficient photoinduced charge separation2 and serve as
single-component materials for photovoltaic cells.3 Specifically,
the Al/16T-C60/Au cell showed appreciably high power con-
version efficiency (0.32%) upon illumination from the Al side
with a light of 456 nm. However, illumination from the Au side
generated only a half of the photocurrent at the same wave-
length. This suggests that charge carriers generated close to
the Al side mainly contribute to the photocurrent. In other words,
the oligothiophene moiety constitutes an efficient network for
hole transport in the film, while the fullerenes do not interact
with one another enough to promote electron transport. We have
expected that an increase of the number of fullerenes in the link-
age system would enhance intermolecular interactions among
the fullerenes. The fullerene–oligothiophene–fullerene triads 2
(C60-nT-C60), however, gave no good films because of their
poor solubilities. We have then focused on another type of triads
3 (nT-2C60), where two fullerenes are incorporated at the same
terminal of oligothiophenes. Here we report the synthesis and
photovoltaic properties of nT-2C60 (n ¼ 4, 8, 12, 16).
C6H13
C6H13
4 (nT) x = 1, 2
5a x = 1, 52%
5b x = 2, 55%
5a
ii
C6H13
S
CHO
CHO
S
Br
R
S
S
88%
x
CHO
CHO
C6H13
6
7a R = H, x = 1
8a R = Br, x = 1
7b R = H, x = 2
8b R = Br, x = 2
7c R = H, x = 3
7d R = H, x = 4
NBS, iii 97%
5a, ii 83%
NBS, iii 92%
5a, ii 74%
5b, ii 70%
iv
7a-d
4T-2C , 51%; 8T-2C , 50%; 12T-2C , 49%; 16T-2C60, 38%
60 60 60
Scheme 1. Reagents and conditions: i) 1 equiv. n-BuLi, THF,
ꢁ30 ꢂC, 0.5 h, then n-Bu3SnCl, rt, 12 h; ii) cat. Pd(PPh3)4,
toluene, reflux, 12 h; iii) 1 equiv. NBS, DMF-CS2, rt, 15 h; iv)
C60, N-methylglycine, chlorobenzene, reflux, 1 d.
The electronic absorption spectra of nT-2C60 in o-dichloro-
benzene consist of a superposition of a strong band of the oligo-
thiophene chromophore at 400–600 nm and a strong band of the
fullerene at around 330 nm tailing up to 700 nm (Figure 1). Ap-
parently, there is no interaction between the two chromophores
in the ground state. On the other hand, the fluorescence spectra
showed marked reduction of the oligothiophene emission com-
pared to the corresponding oligothiophene (nT), when the
oligothiophene chromophore was excited. This indicates
efficient intramolecular electron transfer from the photoexcited
oligothiophene moiety to the pendant fullerene, as observed
for the dyads nT-C60.2 In addition, it is worth noting that thanks
to the increased number of the pendant fullerenes, the triads
nT-2C60 undergo stronger quenching than nT-C60.
CH3
C6H13
N
S
S
H
S
S
x
C6H13
1 (nT-C60) x = 1 − 4
CH3
N
CH3
N
C6H13
S
S
S
S
x
C6H13
2 (C60-nT-C60) x = 1 − 4
CH3
N
The sandwich devices of Al/nT-2C60/Au were fabricated
ꢀ
as follows. A semitransparent Al electrode (100 A) was first vac-
uum-deposited on a glass substrate. Then a thin film of nT-2C60
C6H13
S
S
H
S
S
ꢀ
(2500–3000 A) was prepared by spin-coating of a chloroform so-
x
C6H13
N
ꢀ
lution. Finally a Au electrode (100 A) was vacuum-deposited.
CH3
Upon illumination from the Al side with 10-mWcmꢁ2 monochro-
matic light, the photovoltaic cells of 12T-2C60 and 16T-2C60
showed marked photocurrents. The action spectra are in strict ac-
cordance with the absorption spectra of the oligothiophene
3 (nT-2C60) x = 1 − 4
Scheme 1 outlines the synthetic route of a homologous ser-
ies of the triads nT-2C60. The key precursors are the oligothie-
Copyright ꢀ 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.6 (2004)
655
15
enes, as compared to the previous dyads nT-C60.
This work was partially supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
16T-2C60
12T-2C60
8T-2C60
4T-2C60
10
5
References and Notes
ε
1
a) J.-F. Nierengarten, J.-F. Eckert, J.-F. Nicoud, L. Ouali, V. Krasnikov,
and G. Hadziioannou, Chem. Commun., 1999, 617. b) J.-F. Eckert, J.-F.
Nicoud, J.-F. Nierengarten, S.-G. Liu, L. Echegoyen, F. Barigelletti, N.
Armaroli, L. Ouali, V. Krasnikov, and G. Hadziioannou, J. Am. Chem.
Soc., 122, 7467 (2000). c) E. Peeters, P. A. van Hal, J. Knol, C. J. Brabec,
N. S. Sariciftci, J. C. Hummelen, and R. A. J. Janssen, J. Phys. Chem. B,
104, 10174 (2000). d) T. Gu, D. Tsamouras, C. Melzer, V. Krasnikov,
J.-P. Gisselbrecht, M. Gross, G. Hadziioannou, and J.-F. Nierengarten,
ChemPhysChem, 2002, 124. e) D. M. Guldi, C. Luo, A. Swartz, R.
0
300
400
500
600
700
800
wavelength / nm
Figure 1. Electronic absorption spectra of nT-2C60 in o-dichlo-
robenzene.
´
´
Gomez, J. L. Segura, N. Martın, C. Brabec, and N. S. Sariciftci,
J. Org. Chem., 67, 1141 (2002). f) N. Armaroli, G. Accorsi, J.-P.
Gisselbrecht, M. Gross, V. Krasnikov, D. Tsamouras, G. Hadziioannou,
chromophores (Figure 2). The photocurrents increase with the
chain extension of the oligothiophene, similar to the case of
the nT-C60 series. The cells of 4T-2C60 and 8T-2C60 showed
no photocurrents, being attributable to appreciable defects in
their spin-coated films.
The maximum short-circuit photocurrent (ISC) of the Al/
12T-2C60/Au cell is 210 nAꢃcmꢁ2 at 460 nm, and that of the
Al/16T-2C60/Au is 376 nAꢃcmꢁ2 at 466 nm. These values cor-
respond to the incident photon to converted electron ratios
(IPCE) of 14% for 12T-2C60 and 25% for 16T-2C60, which
are nearly double of those previously measured using nT-C60
(Table 1). This result evidently indicates that the triads
nT-2C60 constitute the improved fullerene network for electron
transport. It was found that the monochromatic power conver-
sion efficiency is reasonably high: 0.50% for the Al/12T-
2C60/Au cell and 0.65% for the Al/16T-2C60/Au.
´
M. J. Gomez-Escalonilla, F. Langa, J.-F. Eckert, and J.-F. Nierengarten,
J. Mater. Chem., 12, 2077 (2002).
2
a) M. Fujitsuka, O. Ito, T. Yamashiro, Y. Aso, and T. Otsubo, J. Phys.
Chem. A, 104, 4876 (2000). b) M. Fujitsuka, K. Matsumoto, O. Ito, T.
Yamashiro, Y. Aso, and T. Otsubo, Res. Chem. Intermed., 27, 73
(2001). c) M. Fujitsuka, A. Masuhara, H. Kasai, H. Oikawa, H.
Nakanishi, O. Ito, T. Yamashiro, Y. Aso, and T. Otsubo, J. Phys. Chem.
B, 105, 9930 (2001).
3
4
5
6
N. Negishi, K. Yamada, K. Takimiya, Y. Aso, T. Otsubo, and Y. Harima,
Chem. Lett., 32, 404 (2003)
T. Yamashiro, Y. Aso, T. Otsubo, H. Tang, Y. Harima, and K.
Yamashita, Chem. Lett., 1999, 443.
M. Maggini, G. Scorrano, and M. Prato, J. Am. Chem. Soc., 115, 9798
(1993).
All new compounds were satisfactorily characterized by spectroscopic
measurements and elemental analyses. The triads nT-2C60 are a 1:1
mixture of two diasteromers originated from chiral substitution on the
pyrroridine rings. Selective properties of 4T-2C60: brown powder from
1:9 carbon disulfide–hexane; mp > 300 ꢂC; 1H NMR (CDCl3/CS2)
ꢀ 0.84–0.88 (m, 12H), 1.25–1.34 (m, 24H), 1.58–1.69 (m, 8H), 2.50
and 2.65 (each br, 6H, isomeric), 2.71–2.78 (m, 8H), 4.23 and 4.26 (each
d, J ¼ 10 Hz, 2H, isomeric), 4.94–5.00 (m, 8H), 6.87 (d, J ¼ 5:3 Hz,
2H), 6.94 (d, J ¼ 3:8 Hz, 1H), 6.94 (d, J ¼ 3:8 Hz, 1H), 6.97 (d,
J ¼ 3:8 Hz, 1H), 7.00 (d, J ¼ 3:8 Hz, 1H), 7.04 (d, J ¼ 3:8 Hz, 2H),
7.06 (d, J ¼ 3:8 Hz, 1H), 7.06 (d, J ¼ 3:8 Hz, 1H), 7.11 (d, J ¼
5:3 Hz, 2H), 7.12 and 7.15 (each s, 1H, isomeric), 7.74 (br, 2H); MS
(MALDI-TOF) m=z 2124.8 (Mþ, calcd 2126.3). 8T-2C60: brown pow-
der from 1:9 carbon disulfide–hexane; mp 220 ꢂC (dec.); 1H NMR
(CDCl3/CS2) ꢀ 0.87–0.92 (m, 24H), 1.26–1.42 (m, 48H), 1.62–1.68
(m, 16H), 2.50 and 2.66 (each br, 6H, isomeric), 2.71–2.78 (m, 16H),
4.24 and 4.26 (each d, J ¼ 8 Hz, 2H, isomeric), 4.95–5.00 (m, 8H),
6.90 (d, J ¼ 5:2 Hz, 2H), 6.95 (s, 4H), 6.98 (d, J ¼ 3:8 Hz, 3H), 7.00
(d, J ¼ 3:8 Hz, 4H), 7.02 (d, J ¼ 3:8 Hz, 1H), 7.07 (d, J ¼ 3:8 Hz,
3H), 7.08 (d, J ¼ 3:8 Hz, 5H), 7.13 (d, J ¼ 5:2 Hz, 2H), 7.15 and 7.18
(s, 1H, isomeric), 7.74 (br, 2H); MS (MALDI-TOF) m=z 2622.5 (Mþ,
calcd 2622.4). 12T-2C60: brick red powder from 1:9 carbon disulfide–
hexane; mp 180 ꢂC (dec.); 1H NMR (CDCl3/CS2) ꢀ 0.85–0.93 (m,
36H), 1.31–1.40 (m, 72H), 1.60–1.72 (m, 24H), 2.51 and 2.68 (each
br, 6H, isomeric), 2.73–2.78 (m, 24H), 4.24 and 4.26 (each d, J ¼
9 Hz, 2H, isomeric), 4.94–5.01 (m, 8H), 6.89 (d, J ¼ 5:1 Hz, 2H), 6.94
(s, 4H), 6.95 (s, 4H), 6.97 (d, J ¼ 3:8 Hz, 3H), 6.98–7.00 (m, 8H),
7.02 (d, J ¼ 3:8 Hz, 1H), 7.06 (d, J ¼ 3:8 Hz, 3H), 7.08 (d, J ¼
3:8 Hz, 5H), 7.09 (d, J ¼ 3:8 Hz, 4H), 7.13 (d, J ¼ 5:1 Hz, 2H), 7.14
and 7.17 (s, 1H, isomeric), 7.74 (br, 2H); MS (MALDI-TOF) m=z
3119.8 (Mþ, calcd 3119.6). 16T-2C60: brick red powder from 1:9 carbon
disulfide–hexane; mp 160 ꢂC (dec.); 1H NMR (CDCl3/CS2) ꢀ 0.86–0.93
(m, 48H), 1.26–1.42 (m, 96H), 1.60–1.70 (m, 32H), 2.49 and 2.86 (each
br, 6H, isomeric), 2.74–2.78 (m, 32H), 4.24 and 4.26 (each d, J ¼ 9 Hz,
2H, isomeric), 4.93–5.00 (m, 8H), 6.90 (d, J ¼ 5:1 Hz, 2H), 6.95 (s, 4H),
6.96 (s, 8H), 6.98 (d, J ¼ 3:8 Hz, 3H), 6.99 (d, J ¼ 3:8 Hz, 1H), 7.00 (d,
J ¼ 3:8 Hz, 1H), 7.00 (d, J ¼ 3:8 Hz, 5H), 7.01 (d, J ¼ 3:8 Hz, 5H),
7.02 (d, J ¼ 3:8 Hz, 1H), 7.07 (d, J ¼ 3:8 Hz, 3H), 7.08–7.10 (m,
13H), 7.13 (d, J ¼ 5:1 Hz, 2H), 7.14 and 7.17 (each s, 1H, isomeric),
7.45 (br, 2H); MS (MALDI-TOF) m=z 3614.8 (Mþ, calcd 3615.7).
In conclusion, we have developed the triads nT-2C60 as
novel single-component photovoltaic materials. It has turned
out that these triads constitute an improved network for electron
transport owing to the increased number of the pendant fuller-
400
300
16T-2C60
200
100
12T-2C60
0
400
500
600
wavelength / nm
Figure 2. Photocurrent action spectra of the Al/nT-2C60/Au
cells.
Table 1. Photovoltaic properties of nT-2C60 and nT-C60
Comp.
ISC/nAcmꢁ2
ꢁinc/nm IPCE/% VOC/V FF h/%
12T-2C60
16T-2C60
12T-C60
16T-C60
210
376
108
148
460
466
461
456
14
25
0.66 0.36 0.50
0.58 0.30 0.65
7.0
9.7
0.64 0.34 0.32
Published on the web (Advance View) April 26, 2004; DOI 10.1246/cl.2004.654