Increasing the open circuit voltage of bulk-heterojunction solar cells by raising the LUMO level of the acceptor

Article abstract of DOI:10.1021/ol062666p

We report the synthesis, characterization, and electrochemical properties of ten new fullerene derivatives for usage in organic solar cells. The phenyl ring of PCBM was substituted with electron-donating and electron-withdrawing substituents to study thei

Full text of DOI:10.1021/ol062666p

ORGANIC
LETTERS
2007
Vol. 9, No. 4
551-554
Increasing the Open Circuit Voltage of
Bulk-Heterojunction Solar Cells by
Raising the LUMO Level of the Acceptor
Floris B. Kooistra,† Joop Knol,† Fredrik Kastenberg,† Lacramioara M. Popescu,†
Wiljan J. H. Verhees,‡ Jan M. Kroon,‡ and Jan C. Hummelen*,†
Molecular Electronics, Materials Science CentrePlus, UniVersity of Groningen,
Nijenborgh 4, 9747 AG Groningen, The Netherlands, and Energy Research Centre of
the Netherlands (ECN), Solar Energy, P.O. Box 1, 1755 ZG Petten, The Netherland
j.c.hummelen@rug.nl
Received November 1, 2006
ABSTRACT
We report the synthesis, characterization, and electrochemical properties of ten new fullerene derivatives for usage in organic solar cells. The
phenyl ring of PCBM was substituted with electron-donating and electron-withdrawing substituents to study their influence on the LUMO level
of the parent fullerene. We varied the LUMO level over a range of 86 mV and show a small but significant change of the open circuit voltage
upon application in MDMO−PPV:methanofullerene bulk-heterojunction photovoltaic cells.
The excellent electron-accepting capability of fullerenes has
made them interesting materials for organic solar cells.1
Much work has been done in improving organic solar cells;
efficiencies of more then 2.5% have been reached by using
a MDMO-PPV:[60]PCBM blend (poly[2-methoxy-5-(3,7-
dimethyloctyloxy)-p-phenylenevinylene]:phenyl C61 butyric
methylester).2 We have reported on organic solar cells with
efficiencies of up to 3% using an MDMO-PPV:[70]PCBM
blend, while others obtained higher efficiencies by changing
the donor polymer to regioregular P3HT (poly(3-hexyl-
thiophene)), and by introducing morphology-improving
procedures on the composite film.3 Several factors are of
importance in improving the efficiency of this type of organic
solar cell. It is essential that the conjugated polymer has a
low band gap (<1.8 eV) to efficiently absorb light in the
visible area of the solar spectrum.4 In the case of the
MDMO-PPV:PCBM blend, the solvent from which it is
prepared plays an important role. Spin coating from chlo-
† University of Groningen.
‡ Energy Research Centre of the Netherlands, Solar Energy.
(1) (a) Kroon, J. M.; Wienk, M. M.; Verhees, W. J. H.; Hummelen, J.
C. Thin Solid Films 2002, 403-404, 223. (b) Brabec, C. J.; Zerza, G.;
Cerullo, G.; De Silvestri, S.; Luzzati, S.; Hummelen, J. C.; Sariciftci, N. S.
Chem. Phys. Lett. 2001, 340 (3-4), 232. (c) Yu, G.; Gao, J.; Hummelen,
J. C.; Wudl, F.; Heeger, A. Science 1995, 270, 1789. (d) Sariciftci, N. S.;
Smilowitz, L.; Heeger, A.; Wudl, F. Science 1992, 258, 1474. (e) Brabec,
C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater 2001, 11, 15.
(2) (a) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.;
Fromherz, T.; Hummelen, J. C. Appl. Phys. Lett 2001, 78, 841. (b) Munters,
T.; Martens, T.; Goris, L.; Vrindts, V.; Manca, J.; Lutsen, L.; De Ceuninck,
W.; Vanderzande, D.; De Schepper, L.; Gelan, J.; Sariciftci, N. S.; Brabec,
C. J. Thin Solid Films 2002, 403-404, 247.
(3) (a) Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.;
Hummelen, J. C.; van Hal, P. A.; Janssen, R. A. J. Angew. Chem., Int. Ed.
2003, 42, 3371. (b) Chirvase, D.; Chiguvare, Z.; Knipper, M.; Parisi, J.;
Dyakonov, V.; Hummelen, J. C. J. Appl. Phys. 2003, 93, 3376. (c) Camaioni,
N.; Ridolfi, G.; Casalbore-Miceli, G.; Possamai, A.; Maggini, M. AdV. Mater
2002, 14, 1735. (d) Schillinsky, P.; Waldauf, C.; Brabec, C. J. Appl. Phys.
Lett. 2002, 81, 3885. (e) Padinger, F.; Rittberger, R.; Sariciftci, N. S. AdV.
Funct. Mater. 2003, 13, 85. (f) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.;
Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864.
(4) Winder, C.; Matt, G.; Hummelen, J. C.; Janssen, R. A. J.; Sariciftci,
N. S.; Brabec, C. J. Thin Solid Films 2002, 403-404, 373.
10.1021/ol062666p CCC: $37.00
© 2007 American Chemical Society
Published on Web 01/25/2007

robenzene instead of from toluene led to a threefold increase
in the efficiency of the solar cell.2a Nevertheless, there is
still plenty of room to improve the efficiency of this type of
bulk-heterojunction photovoltaic cell. Recently the necessity
of improving the Voc has attracted much attention.5 There is
a need for optimizing the electronic match between the donor
and acceptor component, in order to minimize unnecessary
internal loss of open circuit voltage. We have previously
shown that, within certain limits with respect to the elec-
trodes’ workfunction (i.e., as long as there are ohmic contacts
at both electrodes), the Voc of a bulk-heterojunction PV cell
scales linearly with the decrease of the first reduction
potential of the acceptor.6 Later, others confirmed the related
relationship, now between the Voc and the first oxidation
potential of the donor material.5a,b Hence, there is, within
certain limits,7 a linear relationship between the donor-
HOMO and acceptor-LUMO energy difference and the Voc
of the bulk-heterojunction device. Consequently, the upper
limit for the Voc of bulk-heterojunction solar cells is
determined by the energy difference of the HOMO of the
electron donor and the LUMO of the electron acceptor. It
should be noted at this point that the fact8 that for any given
donor-acceptor couple as active layer material, minimizing
the difference in workfunction of the two metal electrodes
leads to a lowering of the Voc, (i.e., now entering the situation
where the Fermi levels of one or both electrodes are not
pinned to the relevant molecular orbital energies, leading to
a (also Metal Insulater Metal (MIM) type) device with non-
ohmic contacts), is not in conflict with the above-mentioned
linear relationship between the orbital levels and the Voc.
Since the LUMO levels in MDMO-PPV and [60]PCBM
(1; Figure 1) are estimated to be at 2.8 and 3.7 eV below
raised substantially before the efficiency of the forward
electron transfer from MDMO-PPV to the acceptor is
lowered to a significant extent. (In the extreme case, if the
two LUMO levels, involved in the photoinduced electron
transfer, become too close in energy, the driving force would
be lost.) State-of-the-art MDMO-PPV:[60]PCBM cells
show Voc values up to 850 mV, while the (donor) HOMO-
(acceptor) LUMO gap of this pair is around 1.3 eV. This
Voc value might be close to what can be maximally expected
from this pair, since a loss of 0.2 V at each electrode is
assumed to happen in these specific devices.7b In other words,
we estimate that the Voc of MDMO-PPV:methano[60]-
fullerene cells could, in principle, be increased to ∼1.3 V
(i.e., by ∼50%), upon diminishing the donor-acceptor
LUMO-LUMO gap from 0.9 to 0.5 eV, which would still
allow for efficient charge generation. An increased Voc raises
the efficiency of the solar cells even in a more than linear
way by increasing the fill factor.8 The main challenge,
however, is how to alter the methanofullerene by means of
substituent effects in such a way that the LUMO is
significantly raised, still fully located on the fullerene moiety,
and without introducing concomitant intramolecular photo-
induced electron transfer (or hole transfer) from an electron-
rich addend moiety to the fullerene moiety. This process was
previously observed by Verhoeven et al. in a similar
compound, a 4-dialkylaminophenyl-substituted fulleropyr-
rolidine.9
In this paper, we report on our efforts to influence the
LUMO level of PCBM by placing electron-donating (meth-
oxy and methyl thioether) and electron-withdrawing (fluo-
rine) substituents on the phenyl ring of PCBM.
Wudl et al. have found that para substituents on the phenyl
rings in diphenylmethano[60]fullerenes (3; Figure 1) have a
negligible effect on the reduction potential of the fullerene
moiety.10 Subsequently, it was claimed that in spirometha-
nofullerenes a better interaction between the aromatic sub-
stituent and the fullerene cage occurred through “pericon-
jugation”. The difference in the first reduction potential
between a spirofluorene derivative and the corresponding bis-
4-dialkylamino derivative (4; Figure 1) was found to be ∼70
meV.
In contrast to their two approaches we aimed for a possible
direct through space effect of a substituent itself on the
carbon cage. We hypothesized that an electron-donating
substituent on the phenyl ring of PCBM should be at closest
proximity to the fullerene cage when connected to the
2-position of the phenyl ring. Subsequent alkoxy or thioether
groups on the phenyl ring could then add to the effect of the
Figure 1. Structures of [60]PCBM (1), 4-N,N-dialkylamino[60]-
PCBM (2), diphenylmethano[60]fullerene (3), bis-4-N,N-dialkyl-
amino-diphenylmethano[60]fullerene (4) and substituted PCBM
compounds (5): R ) 4-OMe (a), 3,4-OMe (b), 2,3,4-OMe (c),
2-OMe (d), 2,5-OMe (e), 2,4,6-OMe (f), 3,4-methylenedioxy (g),
2-SMe (h), 4-SMe (i), and pentafluoro (j).
the vacuum level, respectively1e (i.e., a 0.9 eV difference!),
we expect that the LUMO level of the acceptor can still be
(7) That is: as long as the Fermi levels of the electrodes are pinned to
the energie levels of the relevant molecular orbitals; i.e., in a device with
ohmic contacts. See also: (a) Frohne, H.; Shaheen, S. E.; Brabec, C. J.;
Mu¨ller, D. C.; Sariciftci, N. S.;Meerholz, K. ChemPhysChem 2002, 795.
(b) Mihailetchi, V. D.; Blom, P. W. M.; Hummelen, J. C.; Rispens, M. T.
J. Appl. Phys. 2003, 94, 6849.
(5) (a) Gadisa, A.; Svensson, M.; Andersson, M. R.; Ingana¨s, O. Appl.
Phys. Lett. 2004, 84, 1609. (b) Scharber, M. C.; Mu¨hlbacher, D.; Koppe,
M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. J. AdV. Mater. 2006,
18, 789. (c) Mutolo, K. L.; Mayo, E. I.; Rand, B. P.; Forrest, S. R.;
Thompson, M. E. J. Am. Chem. Soc. 2006, 128, 8108.
(8) Mihailetchi, V. D.; Koster, L. J. A.; Blom, P. W. M. Appl. Phys.
Lett. 2004, 85, 970.
(6) (a) Brabec, C. J.; Cravino, A.; Meissner, D.; Sariciftci, N. S.;
Fromherz, T.; Rispens, M. T.; Sanchez, L.; Hummelen, J. C. AdV. Funct.
Mater. 2001, 11, 374. (b) Brabec, J. C.; Cravino, A.; Meissner, D.; Sariciftci,
N. S.; Fromherz, T.; Rispens, M. T.; Sanchez, L.; Hummelen, J. C. Thin
Solid Films 2002, 403-404, 368.
(9) Williams, R. M.; Zwier, J. M.; Verhoeven, J. W. J. Am. Chem. Soc.
1995, 117, 4093.
(10) Eiermann, M.; Haddon, R. C.; Knight, B.; Li, Q. C.; Maggini, M.;
Martin, N.; Ohno, T.; Prato, M.; Suzuki, T.; Wudl, F. Angew. Chem., Int.
Ed. 1995, 34, 1591.
552
Org. Lett., Vol. 9, No. 4, 2007

ortho substituent by resonance through the phenyl ring, if at
the right position with respect to the ortho substituent (i.e.,
ortho or para, relative to the ortho substituent). In total, a
series of two isomeric monosubstituted, three isomeric bis-
substituted, two isomeric tris-methoxy-substituted, two iso-
meric mono-methyl thioethers, and a pentafluoro[60]PCBM
derivative was synthesized (see Figure 1 and Scheme 1, 5a-
j).
Table 1. Cyclic Voltammetry Data
compda
E1/2,1,reduction
2-OMe-PCBM
4-OMe-PCBM
2,5-OMe-PCBM
3,4-OMe-PCBM
3,4-methylenedioxy-PCBM
2,3,4-OMe-PCBM
2,4,6-OMe-PCBM
2-SMe-PCBM
∆E (mV)b
-1.104
-1.096
-1.106
-1.099
-1.071
-1.118
-1.128
-1.086
-1.085
-1.042
-1.084
76
76
76
85
59
81
73
73
73
87
83
Scheme 1. Synthetic Overview of All New Fullerenesa
4-SMe-PCBM
F5-PCBM
PCBM
a Experimental conditions: V vs Fc/Fc+, Bu4NPF6 (0.1 M) as the
supporting electrolyte, ODCB/acetonitril (4/1) as the solvent, 10 mV/s scan
rate. b Energy difference between peak potentials (forward and backward
sweep).
Since the first reduction potentials of all but one of the
methoxy-substituted PCBM derivatives are more negative
than the first reduction potential of PCBM, it can be
concluded that the LUMO levels of the fullerenes are indeed
raised slightly, but significantly, with respect to the LUMO
level of PCBM. However, we do not observe a significant
difference between either the first reduction potential of
2-OMe-PCBM and 4-OMe-PCBM (8 mV) or the first
reduction potential of 2,5-OMe-PCBM and 3,4-OMe-PCBM
(7 mV). We therefore conclude that the ortho position of
the phenyl substituent does not result in the expected direct
through space electronic overlap between the oxygen lone
pairs and the fullerene π-system. We do observe a significant
decrease of the first reduction potentials when going from
two to three alkoxy substituents. Hence, it appears that the
number of alkoxy substituents is of importance, rather than
the position of the substituents. Furhermore, we can conclude
that substituting the phenyl ring with methyl-thioether
moieties has no effect on the LUMO level of the parent
fullerene, indicating that their donating power is insufficient.
The electron-withdrawing effect of the fluorine atoms,
however, is clearly visible in the first reduction potential of
F5-PCBM. The fluorescence spectra of all compounds were
qualitatively and quantitatively very similar. Hence, signifi-
cant photoinduced intramolecular charge transfer does not
occur, even in the compounds with the strongest electron-
donating groups (see the Supporting Information).
a Key: (a) R ) 4-OMe, X ) H. (b) R ) 3,4-OMe, X ) H. (c)
R ) 2,3,4-OMe, X ) H. (d) R ) 2-OMe, X ) Br. (e) R ) 2,5-
OMe, X ) H. (f) R ) 2,4,6-OMe, X ) H. (g) R ) 3,4-
methylenedioxy, X ) H. (h) R ) 2-SMe, X ) Br. (i) R ) 4-SMe,
X ) H. (j) R ) pentafluoro, X ) MgBr.
We also aimed for a third tris-methoxy PCBM isomer,
with a 2,3,5-substitution pattern, because that would have
to be the most effective one, according to the above-
mentioned hypothesis. Until now, however, we have been
unable to prepare the correspondingly substituted methyl
benzoylbutyrate derivative.
The 13C NMR spectra of all o-methoxy and o-methyl
thioether substituted phenyl methanofullerene isomers showed
two resonances in the 80 ppm region for the two cyclopropyl
sp3 carbon atoms of the fullerene cage, instead of the usual
one only, clearly indicating a severely hindered rotation of
the phenyl ring when ortho substituted with methoxy or
methyl thioether groups. We therefore conclude that in the
o-methoxy and methyl thioether substituted PCBM deriva-
tives the phenyl rings are, with strong preference, in a
conformation close to horizontal with respect to the fullerene
surface. In principle, this would allow for a good electronic
overlap between the oxygen or sulfur lone pairs and the
fullerene π-system. This should be visible in cyclic volta-
mmetry. A lowering of the first reduction potentials (with
respect to the Fc/Fc+ redox couple) of the ortho-substituted
PCBM derivatives when compared to the PCBM derivatives
without ortho substitution is expected. Furthermore, a dif-
ference between the oxygen and sulfur versions was ex-
pected. The first reduction potentials of all PCBM derivatives
were measured by cyclic voltammetry. The obtained CV data
are presented in Table 1. All reductions were reversible.
A series of ten photovoltaic devices was fabricated
containing MDMO-PPV as the electron donor and the ten
substituted PCBM derivatives shown in Figure 1, respec-
tively, as the electron-acceptor material. The devices were
made with the following configuration: ITO/PEDOT/
(MDMO-PPV:PCBMX)/LiF/Al. In earlier work it was
shown that for the specific combination of MDMO-PPV
(from Covion) and PCBM, maximum power conversion
efficiencies of 2.5% at standard test conditions (AM1.5, 100
mW/cm2) could be obtained by using a high content of
PCBM (80 wt %) relative to MDMO-PPV and with
chlorobenzene as the solvent for spincoating.
Org. Lett., Vol. 9, No. 4, 2007
553

However, it turned out that most of the new PCBM
acceptor materials were poorly soluble in chlorobenzene, so
that devices could not be made in a proper way from that
solvent. Only two of the PCBM derivatives, i.e., 3,4-OMe-
PCBM and 2,3,4-OMe-PCBM, were soluble enough in
chlorobenzene to make working devices. o-Dichlorobenzene
was therefore chosen as the general processing solvent.
rendered it impossible to construct a representable device.
Similar problems occurred with 2,4,6-OMe-PCBM. We
attribute its low Voc to these insolubility problems, leading
to at best marginally successful bulk-heterojunction morphol-
ogy. When the domain sizes in the blend are too big, fewer
charge carriers are formed, resulting in a decreased splitting
of the quasifermi levels, hence lowering the observed Voc at
the same light intensity.
In Figure 2, the Voc values are plotted against the measured
We would like to stress that the presented values are not
obtained from fully optimized devices since the optimization
of ten new donor acceptor combinations is an extraordinarily
time-consuming process. However, the trend is clearly
visible, indicating that raising the LUMO level of the
acceptor fullerene does give a higher Voc in fullerene:polymer
solar cell devices. Moreover, that trend seems to be linear,
in agreement with previous results.
In conclusion, we have synthesized a series of ten new
PCBM anologues with LUMO levels varying over ∼85 mV,
allowing further optimization of the open circuit voltage of
polymer:fullerene organic solar cells.
Acknowledgment. These investigations were financially
supported by the Dutch Ministries of EZ, O&W, and VROM
through the EET program (EETK97115) and by The
Netherlands’ Agency for Energy and the Environment
(Novem No. 146-120-08-5). F.K. thanks ESA MAP project
AO-99-121 for financial support. We thank Dr. A. B. Sieval
(University of Groningen), Dr. M. T. Rispens (University
of Groningen), Dr. A. England (University of Groningen),
and Prof. Dr. Ir. P. W. M. Blom (University of Groningen)
for useful discussions and suggestions. Dr. M. v. d. Veen
(University of Groningen) is thanked for help with fluores-
cence measurements.
Figure 2. Voc of devices from MDMO-PPV blends vs the first
reduction potental, E1/2,1,red (V vs ferrocence/ferrocence+) of the
PCBM derivatives: squares, from ODCB; circles, from chloroben-
zene. The dotted line is to guide the eye for a 1:1 relationship
between Voc and E1/1
.
red
reduction potentials (from Table 1) of the methoxy-
substituted PCBM derivatives.
In a separate experiment, devices were constructed with
both thioether substituted PCBMs and the pentafluoro
substituted PCBM analogues. We found a slightly lower Voc
value of 4-SMe-PCBM to PCBM (20 mV lower). F5-PCBM
gave a value 40 mV lower than that of PCBM, according to
expectations. The insolubility of 2-SMe-PCBM in ODCB
Supporting Information Available: Experimental pro-
cedures and characterization data for new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL062666P
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Org. Lett., Vol. 9, No. 4, 2007