[70] Fullerene-based materials for organic solar cells

Article abstract of DOI:10.1002/cssc.201000246

The synthesis, characterization and photovoltaic study of two novel derivatives of [70]fullerene, phenyl-C₇₁-propionic acid propyl ester ([70]PCPP) and phenyl-C₇₁-propionic acid butyl ester ([70]PCPB), are reported. [70]PCPP and [70]PCPB outperform the conventional material (6,6)-phenyl-C₇₁-butyric acid methyl ester ([70]PCBM) in solar cells based on poly(2-methoxy-5-{3′,7′-dimethyloctyloxy}-p-phenylene vinylene) (MDMO-PPV) as a donor polymer using chlorobenzene (CB) or dichlorobenzene (DCB) as solvents. AFM data suggest that improvement of the device efficiency should be attributed to the increased phase compatibility between the novel C₇₀ derivatives and the polymer matrix. [70]PCPP and [70]PCBM showed more or less equally high performances in solar cells comprising poly(3-hexylthiophene) (P3HT) as a donor polymer. Optical modeling revealed that the application of [70]fullerene derivatives as acceptor materials in P3HT-based bulk heterojunction solar cells might give approximately 10 % higher short circuit current densities than using C₆₀-based materials such as [60]PCBM. The high solubility of [70]PCPP and [70]PCPB and their good compatibility with the donor polymers suggest these fullerene derivatives as promising electron acceptor materials for use in efficient bulk heterojunction organic solar cells. She sells C cells: Novel highly soluble derivatives of [70]fullerene show promising efficiencies in organic bulk heterojunction solar cells comprising the polymers MDMO-PPV and P3HT as electron donors. A clear correlation is revealed between the degree of phase separation in the fullerene/polymer blends (average cluster size) and their photovoltaic performance.

Full text of DOI:10.1002/cssc.201000246

DOI: 10.1002/cssc.201000246
[70]Fullerene-Based Materials for Organic Solar Cells
Pavel A. Troshin,*[a] Harald Hoppe,[b] Alexander S. Peregudov,[c] Martin Egginger,[d]
Sviatoslav Shokhovets,[b] Gerhard Gobsch,[b] N. Serdar Sariciftci,[d] and Vladimir F. Razumov[a]
The synthesis, characterization and photovoltaic study of two
novel derivatives of [70]fullerene, phenyl-C71-propionic acid
propyl ester ([70]PCPP) and phenyl-C71-propionic acid butyl
ester ([70]PCPB), are reported. [70]PCPP and [70]PCPB outper-
form the conventional material (6,6)-phenyl-C71-butyric acid
methyl ester ([70]PCBM) in solar cells based on poly(2-me-
thoxy-5-{3’,7’-dimethyloctyloxy}-p-phenylene vinylene) (MDMO-
PPV) as a donor polymer using chlorobenzene (CB) or dichloro-
benzene (DCB) as solvents. AFM data suggest that improve-
ment of the device efficiency should be attributed to the in-
creased phase compatibility between the novel C70 derivatives
and the polymer matrix. [70]PCPP and [70]PCBM showed more
or less equally high performances in solar cells comprising
poly(3-hexylthiophene) (P3HT) as a donor polymer. Optical
modeling revealed that the application of [70]fullerene deriva-
tives as acceptor materials in P3HT-based bulk heterojunction
solar cells might give approximately 10% higher short circuit
current densities than using C60-based materials such as
[60]PCBM. The high solubility of [70]PCPP and [70]PCPB and
their good compatibility with the donor polymers suggest
these fullerene derivatives as promising electron acceptor ma-
terials for use in efficient bulk heterojunction organic solar
cells.
Introduction
Intensive worldwide research in the field of organic solar cells
has resulted in the development of novel highly efficient mate-
rial combinations and laboratory-scale devices with certified
power conversion efficiencies of 6–7%.[1–3] Highly efficient solu-
tion-based technologies applied to the fullerene/polymer
blends should allow for industrial production of cheap organic
solar cell modules.[4–6] The feasibility of this approach was dem-
onstrated by Konarka Technologies, who launched the first
production facility in the USA in 2009.[7] Recent estimations
suggest that electricity generation at the cost of 5–10 cents
per kWh is quite achievable using organic solar batteries.[8]
Intensive research during last few years was focused almost
entirely on the design of novel low band-gap conjugated poly-
mers.[9–11] However, few reports clearly demonstrated that full-
erene component also has to be optimized to reach maximal
performance.[12–20] A recent review addressed the structural
design of fullerene-based acceptor materials for organic solar
cells.[21] Another important issue is that almost all known low
band-gap polymers yield high power conversion efficiencies in
solar cells only when combined with (6,6)-phenyl-C71-butyric
acid methyl ester ([70]PCBM).[2,3,22–26] Fortunately, the cost of
pristine C70 has decreased in recent years and is now 300–
sides [70]PCBM were applied in organic photovoltaics.[31] We
recently demonstrated that the physical properties and, in par-
ticular, solubility of the fullerene-based component has a
strong influence on the performance of polymer–fullerene
solar cells.[16,17] In particular, fullerene derivative with matching
solubility should be provided for every novel low band gap
polymer to give maximal photovoltaic performance of the
blend. Therefore, a family of [70]fullerene derivatives with vari-
able solubilities and other physical properties should be de-
signed and applied in studies of novel low band-gap copoly-
mers.
Herein we report preparation and investigation of two novel
[70]fullerene derivatives, phenyl-C71-propionic acid propyl ester
([70]PCPP) and phenyl-C71-propionic acid butyl ester
([70]PCPB), following the nomenclature introduced by Humme-
[a] P. A. Troshin, V. F. Razumov
Institute of Problems of Chemical Physics of Russian Academy of Sciences
Semenov Prospect 1, Chernogolovka, Moscow region, 142432 (Russia)
Fax: (+7)496-522-3507
E-mail: troshin2003@inbox.ru
[b] H. Hoppe, S. Shokhovets, G. Gobsch
Ilmenau University of Technology, Institute of Physics
Weimarer Str. 32, 98693 Ilmenau (Germany)
500 USDgÀ1 [27]
Taking into account the low thickness of the
.
light-harvesting layer in organic solar cells (100–200 nm), the
cost of the materials, if they remain as expensive as [70]fuller-
ene derivatives, should not be a crucial issue for their success-
ful commercialization.
[c] A. S. Peregudov
A. N. Nesmeyanov Institute of Organoelement Compounds
1 Vavylova St. 28, B-334, Moscow, 119991 (Russia)
[d] M. Egginger, N. S. Sariciftci
Linz Institute for Organic Solar Cells (LIOS), Johannes Kepler University Linz
Altenbergerstrasse 69, 4040 Linz (Austria)
Surprisingly, the range of alternative [70]fullerene derivatives
used in organic solar cells is very limited. To our knowledge,
only BTPF70[28,29] and some C70-based fullerene dimers[30] be-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201000246.
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119

P. A. Troshin et al.
len et al. for [70]PCBM.[31] The designed compounds incorpo-
rate longer alkyl groups at the ester functionality (propyl and
butyl instead of methyl in PCBM), which should improve the
compatibility of these fullerene derivatives with conjugated
polymers that are decorated with long solubilizing alkyl side
chains. Owing to such structural modifications, [70]PCPP and
[70]PCPB might be considered as promising acceptor materials
for investigation in polymer–fullerene bulk heterojunction solar
cells.
Results and Discussion
Synthesis
Both [70]PCPP and [70]PCPB were synthesized using a modi-
fied Hummelen–Wudl procedure, first applied for the synthesis
of [60]PCBM and [70]PCBM (Scheme 1).[32] Benzoylpropionic
Figure 1. a) Molecular structures of the commercially available materials
used in this work; b) schematic layout of organic bulk heterojunction solar
cell.
Scheme 1. Synthesis of [70]PCPP and [70]PCPB.
tion of the solar irradiation. Nevertheless, MDMO-PPV has been
investigated in great detail as a model system. Most important-
ly, the advantages of [70]fullerene derivatives over the respec-
tive [60]fullerene analogues were first demonstrated in solar
cells using MDMO-PPV as a donor polymer.[31]
acid was used as a precursor due to its much lower price and
higher availability in comparison with the benzoylbutyric acid
used for preparation of PCBM. Derivatives of [70]fullerene were
obtained as typical mixtures of 3 isomers where content of the
major 1,2-addition product (8,25 according to new IUPAC no-
menclature) was in the range of 83–88%. Careful column chro-
matography on silica with toluene/hexane eluent allowed iso-
lation of [70]PCBM, [70]PCPP, and [70]PCPB with isomeric puri-
ties as high as 90–95%. These values were supported by
1H NMR spectroscopy and HPLC. Molecular structures of the
new compounds were also confirmed by 13C NMR spectrosco-
py, absorption spectroscopy (absorption features of 1,2-addi-
tion product are quite characteristic) and chemical analysis.
We fabricated photovoltaic cells using MDMO-PPV as poly-
mer component combined with one of the C70 derivatives,
[70]PCPP or [70]PCPB, or with the reference compound
[70]PCBM (Table 1). Photoactive fullerene derivative/MDMO-
PPV blends were casted from chlorobenzene (CB) or from 1,2-
dichlorobenzene (DCB) solvents. The power conversion effi-
ciencies of organic solar cells fabricated using DCB as a solvent
were significantly higher than those of the cells processed with
CB. This solvent effect was reported previously for [70]PCBM/
MDMO-PPV blends.
For both CB-cast and DCB-cast cells, power conversion effi-
ciency increases in the order [70]PCBM<[70]PCPPꢀ[70]PCPB.
This is particularly illustrated by the I–V curves for the devices
produced using DCB as solvent (Figure 2).
Solar cells with MDMO-PPV
Poly(2-methoxy-5-{3’,7’-dimethyloctyloxy}-p-phenylene
vinyl-
ene) (MDMO-PPV; Figure 1) was one of the first conjugated
polymers successfully applied as an electron-donor material for
fabrication of bulk heterojunction solar cells. The photovoltaic
potential of MDMO-PPV is very limited due to its narrow ab-
sorption spectrum, which allows collection of only a small frac-
To explain these results, we investigated the topology of the
fullerene derivative/polymer blends by atomic force microsco-
py (AFM; Figure 3). First we studied a series of the films depos-
ited from CB (Figure 3, left column). [70]PCBM/MDMO-PPV
blend gave very inhomogeneous films due to a large-scale
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[70]Fullerene-Based Organic Solar Cells
Table 1. Power conversion efficiencies of photovoltaic cells based on dif-
ferent [70]fullerene derivatives and MDMO-PPV.
Blend
VOC [V] ISC [mAcmÀ2
]
FF
h [%]
Cast from chlorobenzene
[60]PCBM/MDMO-PPV (4:1 w/w)
[70]PCBM/MDMO-PPV (5:1 w/w)
[70]PCPP/MDMO-PPV (5:1 w/w)
[70]PCPB/MDMO-PPV (5:1 w/w)
0.8
0.8
0.8
0.78
4.4
1.3
3.0
3.8
0.56 2.0
0.53 0.5
0.44 1.0
0.46 1.4
Cast from 1,2-dichlorobenzene
[70]PCBM/MDMO-PPV (5:1 w/w)
[70]PCPP/MDMO-PPV (5:1 w/w)
[70PCPB/MDMO-PPV (5:1 w/w)
0.74
0.8
0.8
5.5
7.1
7.6
0.53
0.52
0.5
2.2
2.8
3.1
Figure 3. AFM images of the [70]fullerene derivative/MDMO-PPV blend films
processed from CB (left column) and DCB (right column).
Figure 2. Dark and light-on (AM 1.5, 100 mWcmÀ2) I/V curves of solar cells
comprising MDMO-PPV and different [70]fullerene derivatives processed
from 1,2-dichlorobenzene solutions.
phase separation. This results in the appearance of large is-
lands in the polymer matrix presumably corresponding to the
clusters of pure fullerene derivative of fullerene-rich domains.
These islands are represented by bright, grain-like features in
Figure 2. The size of these islands became dramatically re-
duced when [70]PCBM was replaced with [70]PCPP and then
even further reduced when [70]PCPB was used (Figure 3). All
blends cast from DCB were much smoother than the CB-pro-
cessed systems. Nevertheless, [70]PCBM/MDMO-PPV blend
showed grainy topology, suggesting significant phase separa-
tion in the blend. However, any detectable phase segregation
disappears when [70]PCBM is replaced with [70]PCPP and
[70]PCPB (Figure 3).
Figure 4. Dependence of the short-circuit current density (ISC) and power
conversion efficiency (h) of the solar cells on the size of the clusters (grains)
determined from the AFM images.
These results suggest that there is clear correlation between
the scale of the phase separation in the blends and the photo-
voltaic performance of these blends. To visualize this effect, we
estimated average size of the clusters (grains) in the AFM
images of all composites. Afterwards, we plotted short-circuit
current density (ISC) and power conversion efficiency (h) of the
solar cells against the size of the clusters (grains) determined
from the AFM images (Figure 4).
in MDMO-PPV-based solar cells. The influence of the cluster
size on the performance of the MDMO-PPV/PCBM blends in or-
ganic solar cells and underlying physical processes have been
considered in great detail recently.[32] Figure 4 shows that
[70]PCPP and [70]PCPB perform better than the reference
[70]PCBM due to the improved compatibility of these fullerene
derivatives with MDMO-PPV. We believe that such compatibili-
ty comes from longer alkyl substituents at the carboxyl group
(propyl and butyl) in [70]PCPP and [70]PCPB molecules capable
of favorable intermolecular interactions with the polymer side
Indeed, both photovoltaic parameters monotonously de-
crease with increase in the cluster size, which is a good illustra-
tion of the morphology control over the device performance
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121

P. A. Troshin et al.
chains. Thus, we may conclude that derivatives of [70]fullerene
designed in this work might be considered as promising elec-
tron-acceptor materials for bulk heterojunction organic solar
cells.
Solar cells with poly(3-hexylthiophene)
Poly(3-hexylthiophene) (P3HT) is another model conjugated
polymer that has been well studied and is widely used in the
ongoing research in the field of organic photovoltaics. There-
fore, we investigated [70]PCPP, [70]PCPB, and reference com-
pounds [70]PCBM and [60]PCBM as electron-acceptor materials
in combination with P3HT used as the electron-donor compo-
nent (Table 2).
Table 2. Power conversion efficiencies of photovoltaic cells based on dif-
ferent fullerene derivatives and P3HT.
Blend
VOC [V]
ISC [mAcmÀ2
]
FF
h [%]
[60]PCBM/P3HT (7:12 w/w)
[70]PCBM/P3HT (2:3 w/w)
[70]PCPP/P3HT (2:3 w/w)
[70]PCPB/P3HT (2:3 w/w)
0.62
0.62
0.66
0.60
9.8
11.5
11.1
7.9
0.59
0.56
0.49
0.37
3.6
4.0
3.6
1.8
[70]PCPP gives comparably high performances with the ref-
erences [60]PCBM and [70]PCBM (Table 2). In contrast, solar
cells comprising [70]PCPB/P3HT blends yielded much lower
power-conversion efficiency. The reason for such behavior
might be in the active layer morphology. Indeed, AFM images
revealed that [70]PCPB/P3HT composite films are very smooth
without any detectable characteristic features suggesting
phase separation (Figure 5). Therefore we may assume that
[70]PCPB is fairly well mixable with P3HT. This mixability results
in a low degree of phase separation and thereby bad percola-
tion. Therefore, photogenerated charge carriers in the
[70]PCPB/P3HT blend cannot be transported efficiently to the
electrodes and recombine instead. Similar conclusions have
been reported for a family of diphenylmethanofullerenes ap-
plied as electron acceptors in organic solar cells.[33,34] [70]PCPP/
P3HT and [70]PCBM/P3HT blends have coarser film topology
providing optimal phase separation required for efficient pho-
tovoltaic operation (Table 2).
Figure 5. AFM images of the fullerene derivative/P3HT blend films as cast
(left column) and after thermal annealing at 1658C for 5 min (right column).
[70]PCBM-based blend is higher within the visible region than
that of the [60]PCBM-based blend.
Moreover, as a result of higher absorption coefficient, the
calculated absorbed fraction of the incident light is increased
for the blend comprising [70]PCBM over the entire visible spec-
trum (Figure 7). Consequently a higher short-circuit photocur-
rent is calculated for the [70]PCBM-based blend by folding the
absorbed light fraction with the standard AM 1.5 solar irradia-
tion spectrum, which is in agreement with the experimental
Optical modeling for solar cells based on [70]PCBM/P3HT
blends
For optical modeling of the polymer–fullerene bulk heterojunc-
tions, the complex refractive indices of all device layers were
determined and the transfer matrix formalism was applied for
describing the coherent light propagation within the photovol-
taic device.[35,36,37] The refractive indices of [60]PCBM and
[70]PCBM blended with P3HT were determined by ellipsometry.
From the refractive index, the absorption coefficient is readily
calculated; Figure 6 shows a comparison of the two material
combinations. Clearly, the absorption coefficient of the
Figure 6. Comparison of the absorption coefficients of [70]PCBM and
[60]PCBM-based P3HT blends as used as active layer in the photovoltaic de-
vices.
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[70]Fullerene-Based Organic Solar Cells
ment of the blend morphology and photovoltaic performance
of the MDMO-PPV-based systems. Moreover, P3HT-based devi-
ces also exhibited reasonably good performances. On the basis
of the experimental data and optical modeling results, the de-
signed compounds [70]PCPP and [70]PCPB might be recom-
mended as improved C70-based materials in the study of novel
low-band-gap conjugated polymers.
Experimental Section
Materials
Figure 7. Calculated absorbed fractions of the incident light within the pho-
toactive layer of photovoltaic devices based on [70]PCBM and [60]PCBM in
combination with P3HT. The [70]PCBM-based blend shows an improved ab-
sorption behavior over the [60]PCBM for an active layer thickness of 100 nm.
Polymers MDMO-PPV and P3HT were products of Covion and Rieke
Metals companies, respectively. [60]PCBM was purchased from
Nano-C company. [70]PCBM was prepared according to the
method described in literature.[31] All water-free solvents (chloro-
benzene, 1,2-dichlorobenzene) were purchased from Aldrich and
used as received.
data shown above. Figure 8 displays the result of the calcula-
tion of maximum short-circuit currents—assuming an internal
quantum efficiency of unity—for both blends discussed for a
wide range of active layer thicknesses. From 70 nm layer thick-
ness there is a significant and almost constant offset to higher
densities of short-circuit photocurrent for the [70]PCBM/P3HT
blend comparing to [60]PCBM/P3HT system. Both current–
active layer thickness relations follow the typical wavy trend
arising from interference effects reported previously.[35,37]
Methanofullerenes [70]PCPP and [70]PCPB were prepared from C70
and corresponding tosylhydrazones in complete accordance to the
method reported previously for the preparation of [70]PCBM.[31]
The contents of the major 1,2-isomers were determined from the
1H NMR spectroscopy and HPLC measurements as 90–95%, which
is higher than for [70]PCBM; the remaining 5–10% corresponded
to two possible diastereoisomers of the minor 5,6-adduct. The ob-
tained [70]PCPP and [70]PCPB isomer mixtures were subjected to
spectroscopic characterization and investigation in organic solar
cells without further purification.
[70]PCPP (45%): 1H NMR (CS2/(CD3)2CO 9:1, 400 MHz): d=0.88 (t,
0.3H), 1.00 (t, 3H), 1.52 (q, 0.2H), 1.67 (q, 2H), 2.39 (m, 0.4H), 2.73
(m, 4H), 3.77 (t, 0.2H), 4.00 (t, 2H), 7.44 (t, 1.1H), 7.52 (t, 2.2H), 7.79
(d, 0.1H), 7.91 ppm (br d, 2H); 13C NMR (CS2/(CD3)2CO 9:1,
150 MHz): d=11.01(CH3), 22.71, 29.81, 30.26, 30.98, 35.19 (meth-
ane bridge C), 66.06 (OCH2), 69.70 (cage sp3 C), 71.75 (cage sp3 C),
128.33, 128.43, 128.72, 129.05, 130.44, 130.63, 130.71, 130.85,
131.62, 132.73, 133.73, 133.92, 136.65, 137.67, 138.61, 139.38,
140.22, 141.05, 141.29, 141.55, 141.77, 142.54, 142.66, 143.27,
143.34, 143.63, 143.70, 143.87, 143.89, 144.08, 144.52, 144.54,
145.48, 145.65, 145.82, 145.91, 146.03, 146.24, 146.78, 146.92,
147.31, 147.38, 147.41, 147.46, 147.65, 147.82, 148.08, 148.28,
148.34, 148.42, 148.46, 148.49, 148.56, 149.03, 149.07, 149.15,
149.28, 149.34, 150.42, 150.47, 150.72, 150.74, 170.74 ppm (COO);
MALDI TOF MS: m/z=1044.10 (calculated m/z=1044.11); UV/Vis
(toluene): l=324, 357, 372, 402, 464, 537 nm; FTIR (KBr): u=459
(w), 534 (m), 546 (w), 578 (m), 643 (w), 674 (m), 699 (m), 726 (w),
744 (w), 752 (w), 761 (w), 783 (w), 795 (m), 1024 (w), 1069 (w), 1135
(m), 1153 (m), 1177 (m), 1220 (w), 1230 (w), 1259 (w), 1290 (w),
1327 (w), 1376 (w), 1392 (w), 1416 (s), 1428 (vs), 1444 (m), 1454
(m), 1493 (w), 1560 (w), 1602 (w), 1636 (w), 1686 (w), 1733 (s), 2850
(w), 2872 (w), 2926 (w), 2959 cmÀ1 (w).
Figure 8. Calculated short-circuit photocurrent for [60]PCBM- and [70]PCBM-
based bulk heterojunctions with P3HT as the donor polymer for various
active layer thicknesses, under the assumption of an internal quantum effi-
ciency of unity. For thicknesses of 70 nm and greater, the [70]PCBM-based
photoactive layer produces 0.5–1 mAcmÀ12 higher short-circuit current den-
sities than [60]PCBM/P3HT.
Conclusions
We have verified experimentally and by modeling that C70 de-
rivatives used as electron-acceptor materials in polymer–fuller-
ene bulk-heterojunction solar cells yield increased photocur-
rents due to an improved combined absorption strength,
when compared to [60]PCBM-based composites.
[70]PCPB (50%): 1H NMR (CS2/(CD3)2CO 9:1, 400 MHZ): d=0.96 (t,
3H), 1.43 (m, 2H), 1.65 (m, 2H), 2.80 (m, 4H), 4.11 (t, 2H), 7.47 (t,
1H), 7.56 (t, 2H), 7.95 ppm(br d, 2H); 13C NMR (CS2/(CD3)2CO 9:1,
150 MHZ): d=14.42 (CH3), 20.00 (CH3CH2), 30.35, 31.08, 31.31,
35.30 (methane bridge C), 64.43 (OÀCH2), 69.81 (cage sp3 C), 71.86
(cage sp3 C), 128.53, 128.82, 130.54, 130.74, 130.79, 130.95, 131.73,
132.83, 134.02, 136.75, 137.58, 137.79, 138.73, 139.49, 140.32,
141.15, 141.38, 141.65, 141.87, 142.64, 142.76, 143.37, 143.44,
143.73, 143.8, 143.97, 144.18, 144.64, 145.60, 145.75, 145.92,
146.02, 146.13, 146.35, 146.89, 147.03, 147.42, 147.48, 147.52,
147.56, 147.76, 147.93, 148.19, 148.39, 148.52, 148.56, 148.67,
We have presented two novel highly soluble derivatives of
[70]fullerene investigated in model photovoltaic systems using
MDMO-PPV and P3HT as donor polymers. It was revealed that
[70]PCPP and [70]PCPB, due to peculiarities of their molecular
structures, have better phase compatibility with the investigat-
ed donor polymers. This effect resulted in a strong improve-
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