6714
J. Am. Chem. Soc. 2001, 123, 6714-6715
Here we report the synthesis of a novel processable π-conju-
gated polymer 3 (Scheme 1) with covalently linked methano-
fullerenes using a palladium-catalyzed cross-coupling reaction to
overcome these limitations. We demonstrate that photoexcitation
of 3 results in an electron-transfer reaction from the conjugated
backbone to the pendant C60 moiety and that 3 can be applied
via spin coating to form the active layer of the first polymer solar
cell based on a covalently linked donor-acceptor bulk-hetero-
junction.
Photoinduced Electron Transfer and Photovoltaic
Devices of a Conjugated Polymer with Pendant
Fullerenes
Alicia Marcos Ramos,† Minze T. Rispens,‡
Jeroen K. J. van Duren,† Jan C. Hummelen,*,‡ and
Rene´ A. J. Janssen*,†
Laboratory of Macromolecular and Organic Chemistry
EindhoVen UniVersity of Technology
P.O. Box 513 5600 MB EindhoVen, The Netherlands
Stratingh Institute and MSC, UniVersity of Groningen
Nijenborgh 4, 9747 AG Groningen, The Netherlands
Scheme 1
ReceiVed February 1, 2001
Photoinduced electron transfer from a donor to an acceptor is
widely studied to mimic the natural photosynthetic reaction center
and to investigate the prospects of molecular materials in
photovoltaic energy conversion.1 Promising conversion efficien-
cies have been obtained in so-called bulk-heterojunction solar cells
in which the active layer is a composite film of a conjugated
donor polymer and an acceptor polymer or a fullerene derivative.2,3
In these blends charges are preferentially formed at the donor-
acceptor interface, and intimate mixing of donor and acceptor is
therefore beneficial for charge generation. For efficient transport
of the positive charge carriers through the donor phase and of
electrons via the acceptor phase to the electrodes, a phase-
segregated bicontinuous network is required. A convenient route
to obtain a predefined nanoscopic phase-segregated network is
linking donor and acceptor via a covalent bond.4,5 We intend to
achieve the desired nanoscopic bicontinuous networks by syn-
thesizing π-conjugated polymers with pendant fullerenes. The
preparation of well-defined polymers incorporating fullerenes has
remained a challenge over the years, and only a few synthetic
routes have ensured full structural homogeneity of the final
polymer.6 Conjugated polymers incorporating fullerenes have
previously been prepared by electrochemical polymerization of
oligothiophene-fullerene dyads or by grafting C60 on precursor
polymers, but these materials have not been incorporated in
electrooptical devices.7
Polymer 3 was synthesized via a palladium-catalyzed cross-
coupling reaction of diiodobenzene 1, carrying a pendant metha-
nofullerene, and oligo(p-phenylene vinylene) 2, end-capped with
two reactive ethynylenes, under inert conditions in 1,2-dichlo-
robenzene/triethylamine (7:3 v/v) (Scheme 1). This polymerization
can be performed under mild conditions and is one of the reactions
developed in recent years for the synthesis of electronic polymer
materials, which ensures the alternation of the two monomers.8
The synthetic details of the preparation and characterization of
the two monomers are provided in the Supporting Information.
The presence of both double and triple bonds in the backbone
make 3 a hybrid polymer of poly(p-phenylene vinylene) and poly-
(p-phenylene ethynylene), similar to those recently reported.9
The polymerization reaction was followed by absorption
spectroscopy, as a red-shift in time of the π-π* transition of the
polymer with respect to that of monomer 2 (λmax ) 428 nm).
The three hexyloxy chains ensure that 1 is a highly soluble C60
derivative, but its reactivity toward 2 under these conditions is
less than that of 1,4-diiodo-2,5-bis(2′-ethylhexyloxy)benzene (4).
Reaction of 2 and 4 was performed to obtain polymer 5, which
is similar to 3 but lacks the pendant methanofullerenes and is
used as a reference in the spectroscopic studies described below.
After 24 h of polymerization, the effective conjugation length of
† Eindhoven University of Technology.
‡ University of Groningen.
(1) (a) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183. (b) Granstro¨m, M.;
Petrisch, K.; Arias, A. C.; Lux, A.; Lux, M.; Andersson. M. R.; Friend, R. H.
Nature 1998, 395, 257.
(2) (a) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.;
Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498. (b) Yu,
G.; Gao, Y.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270,
1789.
(3) Shaheen, S. E.; Brabec, J. C.; Padinger, F.; Fromherz, T.; Hummelen,
J. C. Sariciftci, N. S. Appl. Phys. Lett. 2001, 78, 841.
3, as inferred from λmax ) 468 nm, approaches that of 5 (λmax
474 nm) (Figure 1a).
)
(4) (a) Nierengarten, J.-F.; Eckert, J.-F.; Nicoud, J.-F.; Ouali, L.; Krashikov,
V. V.; Hadziioannou, G. Chem. Commun. 1999, 617. (b) Stalmach, U.; de
Boer, B.; Videlot, C.; van Hutten, P. F.; Hadziioannou, G. J. Am. Chem. Soc.
2000, 122, 5464. (c) Eckert, J.-F.; Nicoud, J.-F.; Nierengarten, J.-F.; Liu, S.-
G.; Echegoyen, L.; Barigelletti, F.; Armaroli, N.; Ouali, L.; Krasnikov, V.;
Hadziioannou, G. J. Am. Chem. Soc. 2000, 122, 7467.
(5) Peeters, E.; Van Hal, P. A.; Knol, J.; Brabec, C. J.; Sariciftci, N. S.;
Hummelen, J. C.; Janssen, R. A. J. J. Phys. Chem. B 2000, 122, 10174.
(6) (a) Shi, S.; Khemani, K. C.; Li, C.; Wudl, F. J. Am. Chem. Soc. 1992,
114, 10656. (b) Sun, Y.-P.; Liu, B.; Moton, D. K. Chem. Commun. 1996,
2699. (c) Zhang, N.; Schricker, S. R.; Wudl, F.; Prato, M.; Maggini, M.;
Scorrano, G. Chem. Mater. 1995, 7, 441. (d) Gu¨gel, A.; Belik, P.; Walter,
M.; Kraus, A.; Harth, E.; Wagner, M.; Spickermann, J.; Mu¨llen, K.
Tetrahedron 1996, 52, 5007. (e) Kraus, A.; Mu¨llen, K. Macromolecules 1999,
32, 4241. (f) Ilhan, F.; Rotello, V. M. J. Org. Chem. 1999, 64, 1455. (g)
Xiao, L.; Shimotani, H.; Ozawa, M.; Li, J.; Dragoe, N.; Saigo, K.; Kitazawa,
K. J. Polym. Sci. A 1999, 37, 3632. (h) Sun, Y.-P.; Lawson, G. E.; Huang,
W.; Wright, A. D.; Moton, D. K. Macromolecules 1999, 32, 8747. (i) Okamura,
H.; Miyazono, K.; Minoda, M.; Komatsu, K.; Fukuda, T.; Miyamoto, T. J.
Polym. Sci., Part A: Polym. Chem. 2000, 38, 3578.
The molecular weight of the polymers determined by size-
exclusion chromatography (SEC, Figure 1b) shows that 3 (Mw
)16.2 kg/mol, PDI ) 2.82) has a lower degree of polymerization
than 5 (Mw )35.8 kg/mol, PDI ) 2.32), consistent with the small
6 nm hypsochromic shift. The difference in SEC molecular
(7) (a) Benincori, T.; Brenna, E.; Sannicolo´, F.; Trimarco, L.; Zotti, G.;
Sozzani, P. Angew. Chem., Int. Ed. Engl. 1996, 35, 648. (b) Ferraris, J. P.;
Yassar, A.; Loveday, D. C.; Hmyene, M. Opt. Mater. 1998, 9, 34. (c) Cravino,
A.; Zerza, G.; Maggini, M.; Bucella, S.; Svensson, M.; Andersson, M. R.;
Neugebauer, H.; Sariciftci, N. S. Chem. Commun. 2000, 2487.
(8) (a) Weder, C.; Wrighton, M. S. Macromolecules 1996, 29, 5157. (b)
Yamamoto, T.; Honda, K.; Ooba, N.; Tomaru, S. Macromolecules 1998, 31,
7. (c) Moroni, M.; Le Moigne, J.; Luzatti, S. Macromolecules 1994, 27, 562.
(9) Brizius, G.; Pschirer, N. G.; Steffen, W.; Stitzer, K.; Zur Loye, H.-C.;
Bunz, U. H. F. J. Am. Chem. Soc. 2000, 122, 12435.
10.1021/ja015614y CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/14/2001