Fullerene dimers (C60/C70) for energy harvesting

Article abstract of DOI:10.1002/chem.200902039

A new family of fullerenebased compounds, namely, soluble [60]and [70]fullerene homodimers and the [60]/[70]heterodimer linked through 2pyrazolino-pyrrolidino bridges, has been synthesised by simple procedures and in high yield. Electrochemical studies co

Full text of DOI:10.1002/chem.200902039

DOI: 10.1002/chem.200902039
Fullerene Dimers (C60/C70) for Energy Harvesting
Juan L. Delgado,[a, b] Eva Espꢀldora,[a] Moritz Liedtke,[c] Andreas Sperlich,[d]
Daniel Rauh,[c] Andreas Baumann,[d] Carsten Deibel,[d] Vladimir Dyakonov,*[c, d] and
Nazario Martꢀn*[a, b]
Abstract: A new family of fullerene-
based compounds, namely, soluble [60]-
and [70]fullerene homodimers and the
[60]/[70]heterodimer linked through 2-
pyrazolino–pyrrolidino bridges, has
been synthesised by simple procedures
and in high yield. Electrochemical
studies confirm their suitability to act
as electron acceptors in combination
in [70]fullerene-containing dimers rela-
tive to [60]homodimer in solution in
the visible range was observed. Fur-
thermore, in all donor–acceptor blends
studied an efficient charge transfer was
observed by means of photolumines-
cence (PL), photoinduced absorption
and light-induced electron spin reso-
nance spectroscopy. The [70]homodi-
mer was found to be a distinctive spe-
cies, being the strongest PL quencher
and most efficient acceptor with the
longest lifetime of the charge-separated
(polaron) states. As a consequence,
bulk-heterojunction solar cells based
on this novel [70]homodimer blended
with P3HT demonstrated the highest
quantum and power conversion effi-
ciencies of 37 and 1%, respectively,
compared to those of [60]fullerene
dimers.
with
poly(3-hexylthiophene-2,5-diyl)
Keywords: charge transfer · dimeri-
zation · energy conversion · EPR
spectroscopy · fullerenes
(P3HT). Their optical properties in so-
lution and in the solid state were stud-
ied. A significantly stronger absorption
Introduction
Global dependence on fossil fuels is a key issue with impor-
tant consequences in the world nowadays.[1] A reasonable al-
ternative to overcome this need is the use of renewable
energy sources,[2] such as solar energy, which could, in prin-
ciple, fulfil our energy requirements with environmentally
clean procedures and low prices.
[a] Dr. J. L. Delgado, Dr. E. Espꢀldora, Prof. Dr. N. Martꢀn
Departamento de Quꢀmica Orgꢁnica, Facultad de Ciencias Quꢀmicas
Universidad Complutense de Madrid
28040 Madrid (Spain)
Fax : (+34)913-944-103
In this context, the development of photovoltaic (PV) de-
vices based on organic materials, suitable for converting
solar energy into electrical power, is currently a challenging
matter. The mostly efficient organic solar cells to date show
efficiencies in the range of 5–6%[3] and are based on phase-
separated polymer–fullerene blends with donor–acceptor
properties. In such blends, fullerenes and their derivatives
possess important properties, such as small reorganisation
energy, high electron affinity, ability to transport charge and
stability, which makes them the best candidates to act as
electron-acceptor components in bulk-heterojunction (BHJ)
PV devices.[4,5] Typical conjugated-polymer donor counter-
parts are poly(3-hexylthiophene-2,5-diyl) (P3HT), poly[2-
methoxy-5-(3’,7’-dimethyloctyloxyl)]-1,4-phenylenevinylene
(MDMO-PPV) and polyfluorenes as well as their copoly-
mers.[6] Compared with the great variety of conjugated poly-
mers tested, only a few structurally different fullerene deriv-
E-mail: nazmar@quim.ucm.es
[b] Dr. J. L. Delgado, Prof. Dr. N. Martꢀn
IMDEA-Nanociencia, Facultad de Ciencias
Ciudad Universitaria de Cantoblanco
28049 Madrid (Spain)
[c] M. Liedtke, D. Rauh, Prof. Dr. V. Dyakonov
Bavarian Centre for Applied Energy Research (ZAE Bayern)
Am Hubland, 97074 Wꢂrzburg (Germany)
[d] A. Sperlich, A. Baumann, Dr. C. Deibel, Prof. Dr. V. Dyakonov
Experimental Physics VI
Julius-Maximilians-University of Wꢂrzburg
Am Hubland, 97074 Wꢂrzburg (Germany)
Fax : (+49)931-31-83109
E-mail: dyakonov@physik.uni-wuerzburg.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902039. It contains synthetic
procedures, characterisation details for the new compounds, addition-
al electron spin resonance data and PL results concerning the effect
of thermal treatment of the blend films.
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FULL PAPER
atives have been synthesised and used in the preparation of
BHJ-PV devices so far.[7] To date, [6,6]-phenyl C61 butyric
acid methyl ester (PCBM), first synthesised by Hummelen
and Wudl in 1995,[8] remains the fullerene derivative most
employed in the preparation of BHJ-PV devices. Based on
this methanofullerene, which has been intensively studied
worldwide for more than a decade, reproducible power con-
version efficiencies (PCEs) in the range of 5% under air
mass 1.5 (AM1.5) conditions at 100 mWcmꢀ2 have been re-
ported.[9]
Belonging to the same class of methanofullerenes, 1,1-
bis(4,4’-dodecyloxyphenyl)-(5,6) C61, diphenylmethanofuller-
ene (DPM-12) was another acceptor system showing a
promising PCE of 2.3%.[10] A recently reported naphtha-
lene-fused fullerene derivative has been proved to show per-
formances (4.5%) similar to those achieved with PCBM.[11]
Interestingly, it has recently been found that fullerene deriv-
atives with more negative reduction potentials (poorer elec-
tron acceptors) than the parent PCBM give rise to higher
values of the open-circuit voltage (VOC), thus resulting in
higher efficiencies in the PV device.[12]
Among other parameters, the ability of the conjugated
polymer and the fullerene derivative to harvest sunlight is
essential to obtain a good performance of the PV device.
However, [60]fullerene derivatives display a low absorption
in the visible region, due to the high degree of symmetry
(Ih) of the molecule, thus limiting the utilisation of solar
light in the BHJ-PV device. Therefore, the search for new
fullerene-based materials able to collect sunlight more effi-
ciently remains an important challenge to be addressed.
[70]PCBM came into play recently due to its improved opti-
cal absorption.[13] Stimulated by the idea of combining good
electron-accepting and light-harvesting properties, we now
introduce a new class of fullerene-based materials constitut-
ed by two covalently connected fullerene units (C60 and/or
C70). The resulting dimers, endowed with a solubilising alkyl
chain, exhibit increased light-harvesting properties due to
the presence of two units of identical (4, 6) or different (5)
fullerenes linked through a 2-pyrazoline moiety and a pyrro-
lidine ring (Figure 1).
[70]fullerene homodimers remains a much less explored
field. Furthermore, the fullerene dimers are known to exhib-
it poor solubility in common organic solvents, which has pre-
vented their use in optoelectronic devices to a great
extent.[14] Their PV applications are, however, of interest be-
cause they are expected to improve the molar absorptivity,
due to the presence of two fullerenes and/or to the better
light-absorbing properties of C70 relative to C60. Herein, we
report the electrochemical and photophysical properties of
these new homo- (4: C60–C60; 6: C70–C70) and heterodimers
(5: C60–C70) as well as of their blends with P3HT. We estab-
lished excellent film-forming properties for all blends due to
the high solubility of the dimers in common organic sol-
vents. Photoinduced electron transfer was found to occur in
all BHJs studied and was most efficient in P3HT/C70–C70.
Preliminary tests of BHJ-PV devices built with C70–C70/
P3HT blends demonstrated an external quantum efficiency
(EQE) of 37% as well as non-optimised PCE in the range
of 1%. All three types of solar cells show a short-circuit cur-
rent density of 5, 4.3 and 3.4 mAcmꢀ2 for P3HT/C70–C70,
P3HT/C60–C60 and P3HT/C60–C70, respectively. The reduced
PV performance of the heterodimer can be explained by in-
efficient exciton dissociation in the BHJ. Finally, we provide
the first experimental identification of the negative polarons
(radical anions) localised on C70–C70 and C60–C70 dimer
cages in the P3HT composites by using a light-induced elec-
tron spin resonance (LESR) technique. Interestingly, in C60–
C70 dimers we were able to distinguish between electrons on
the C60 cage and the C70 cage. This opens up a completely
new possibility to study electron–electron interactions in
well-defined molecular systems.
Results and Discussion
Synthesis: The preparation of C60-containing dimers 4 and 5
was carried out in a single step, starting from our recently
described
formyl-containing
building
block
(7)
(Scheme 1).[15] This aldehyde was reacted with N-octylgly-
cine and an excess of [60]- and [70]fullerene to prevent the
intramolecular cycloaddition of the in situ generated azome-
thyne ylide on the fullerene sphere in 7. The respective
dimers were obtained in high yields and easily isolated by
column chromatography due
A variety of [60]fullerene dimers bearing different con-
necting bridges have been described so far.[14] However, the
preparation of [60]/[70]fullerene heterodimers and
to their high solubility in
carbon disulfide. The structure
and purity of each compound
were confirmed by different
spectroscopic
techniques,
which allowed detection of a
mixture of isomers for hetero-
dimer 5, as well as a small
amount of 4 resulting from the
low C60 impurity concentration
in the commercial C70 used for
the synthesis. Monofunctionali-
sation of C70 preferentially
Figure 1. [60]- and [70]fullerene homodimers (4, 6) and heterodimer (5). Dimers 5 and 6 are obtained as a mix-
ture of isomers.
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N. Martꢀn, V. Dyakonov et al.
Scheme 1. Synthesis of dimer compounds 4 and 5.
yields C(1)–C(2) (type a) adducts,[16] with the C(5)–C(6)
(type b) adduct being the second most favoured compound.
In the case of 1,3-cycloaddition of azomethine ylides to C70,
ꢀ
the addition of the dipole to the C(7) C(21) bond also takes
place as a less-favoured process.[17] Therefore, considering
our Cs-symmetrical azomethine ylide, six possible isomers
can be generated through the reaction of the dipole with
C70. Indeed, compound 5 is obtained as a mixture of isomers
as observed by 1H NMR spectroscopy (see the Supporting
Information); four isomers are formed in different propor-
tions (39, 30, 22 and 9% by 1H NMR) together with a small
quantity of compound 4 (3% by HPLC), due to the above-
mentioned presence of C60 in the commercial C70. To date
only a small number of C60–C70 heterodimers have been de-
scribed,[18] some of them prepared by a mechanochemical
solid-state reaction.[19] Furthermore, their low solubility in
common organic solvents has usually prevented their use in
further applications. In our case, the presence of the octyl
chain on the pyrrolidine nitrogen atom in 4–6 allowed us to
obtain stable solutions of the dimers in organic solvents and,
consequently, facilitate their characterisation as well as their
incorporation into BHJ-PV devices.
Scheme 2. Synthesis of dimer compound 6.
It is worth mentioning that in the above search for new
materials, this new family of compounds represents the first
example of covalently bonded pyrrolidino-pyrazolino-fuller-
ene dimers. Although both fullerene families, pyrrolidinoful-
lerenes[21] and pyrazolinofullerenes,[23] are well known in
fullerene chemistry and have been extensively studied, to
the best of our knowledge this is the first report on mole-
cules containing both organic fragments and, therefore, their
properties and applications are still unexplored.
The synthesis of C70 homodimer 6 was accomplished in a
two-step procedure from derivative 8,[20] which was oxidised
with selenium dioxide according to our previously described
methodology[15] to afford the new aldehyde 9 in very good
yield (78%). This formyl-bearing [70]fullerene was then re-
acted following the methodology of Prato et al.[21] in the
presence of N-octylglycine and an excess of [70]fullerene to
Optical properties: Bridging two fullerenes together may
have several consequences for the photophysics of the poly-
mer–dimer blends. They are of intrinsic nature, for example
due to enhanced molecular absorptivity of C70-based dimers,
but are also extrinsic by influencing the blend morphology.
The optical absorption spectra of compounds 4–6 in toluene
solution are shown in Figure 2. Compound 4 displays a typi-
cal [60]fullerene derivative absorption pattern, showing
weak absorption in the visible region. In the UV region, the
values of molar absorptivity for this dimer (4) are larger by
approximately a factor of 2 than those observed for precur-
sor 7. In the case of heterodimer 5, characteristic bands for
both [60]fullerene precursor 7 (287, 325 nm) and [70]fuller-
ene (397, 468, 544 nm) are observed, which comprise a su-
perimposition of two components. For the [70]homodimer 6,
representative absorption features of [70]fullerenes are seen.
Moreover, as can be seen in Figure 2, the molar absorptivity
values for these bands are approximately double that ob-
served for compound 5, which contains one [70]fullerene
unit only. As a consequence of the [70]fullerene-related op-
tical absorption in the visible range, solutions of compound
afford compound
6 in a relatively good yield (52%;
Scheme 2). Dimer 6 is formed of a mixture of isomers, the
composition of which was studied by high-pressure liquid
chromatography (HPLC). Interestingly, precursor
9 is
formed by three aldehyde–C70 isomers (9a: 32%, 9b: 38%,
9c: 30% by 1H NMR)[22] and a small amount of 7 resulting
from the C70 sample used, which contains around 2% of
pristine C60 (MER Corporation). When this mixture is react-
ed with N-octylglycine and C70, 18 possible C70–C70 isomers
can be formed in addition to six possible C60–C70 isomers, as
well as three additional C70–C60 dimers, very small amounts
of which can be formed, if any. This complex isomeric mix-
ture was detected by HPLC (toluene, 1 mLminꢀ1) and as-
signed by studying its UV/Vis spectra, although it was not
possible to isolate the components by using chromatograph-
ic techniques. Therefore, dimer 6 as a mixture of C70–C70
(96% by HPLC) and C70–C60/C60–C70 isomers (4% by
HPLC; see the Supporting Information) was used in these
studies.
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Fullerene Dimers for Energy Harvesting
FULL PAPER
combination with our novel dimers. Note that the absorp-
tion spectra of P3HT and [70]fullerenes in the visible range
are strongly overlapping.
Photoluminescence (PL) measurements performed on the
same systems in the solid state are shown in Figure 4. PL
originates from the radiative recombination of singlet exci-
*
Figure 2. Molar absorption coefficient spectra for dimers 4 ( ), 5 (&) and
6 ( ) in toluene solution at 1ꢄ10ꢀ5 m.
~
5 display a characteristic red-brown colour, which is even
darker for the dimer 6 (see the Supporting Information).
The observed enhancement in the absorption from 4 to 6 is
a consequence of the presence of the [70]fullerene unit and
may become very important for improving the efficiency of
PV devices.
UV/Vis absorption spectra of solid blends are shown in
Figure 3. The 4–6 dimers blended with P3HT (1:1 ratio)
have in general the same spectral features as P3HT/
[60]PCBM blend. On the other hand, the absorption curves
for new dimer blends are redshifted by 25 nm with respect
to P3HT/[60]PCBM. The observed shift may result not only
from the carbon chains bridging the two fullerene molecules,
but also from the effect of better crystallisation of P3HT in
Figure 4. PL of blends of P3HT and fullerene dimers 4–6, magnified by a
factor of 5, in comparison to the PL of pristine P3HT (a) at T=30 K.
Excitation was with a 514 nm laser. The weak PL signal of P3HT/C70–C70
indicates an efficient charge-carrier separation comparable to that of
P3HT/[60]PCBM.
tons formed on P3HT chains. Laser light of wavelength
514 nm was used for the initial singlet excitation. The PL
quenching may be due to the following effects: 1) photoin-
duced charge transfer between polymer and fullerene;
2) resonance energy transfer between species in the excited
and ground states due to overlap of emission and absorption
spectra; and iii) structural modification upon blending and
thermal treatment of the sample, which affects the self-
quenching of the initial excitation (see the Supporting Infor-
mation). For the occurrence of charge transfer, the binding
energy of the photogenerated exciton corrected for Cou-
lomb attraction between separated charges should be small-
er than the difference in the electron affinities of the partici-
pating donor and acceptor species. The PL studies alone are
not sufficient to justify one or another process. Strong PL
quenching may, however, serve as an indicator for charge-
carrier transfer between donor and acceptor in blends, for
which additional experiments are needed. According to
cyclic voltammetry (CV) studies (see below), electron trans-
fer between P3HT and dimers 4–6 is energetically possible.
As follows from Figure 4, the PL quenching is observed in
all compounds; however, only the [70]homodimer quenches
the luminescence in P3HT as strongly as [60]PCBM. We
also note the same vibronic structure of the residual PL in
the blends, which is a sign of no additional interaction influ-
encing the elementary excitation process. The difference in
Figure 3. Absorption coefficients of blends of the fullerene dimers 4–6
with P3HT as indicated in the legend. The AM1.5 solar spectrum and the
absorption of P3HT/[60]PCBM are shown for reference. All coefficients
are normalised to that of P3HT/[60]PCBM at 500 nm; the normalisation
factors are given in parentheses.
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N. Martꢀn, V. Dyakonov et al.
Table 1. Redox potentials (OSWV) of dimers 4–6, aldehydes 7 and 9, C60
PL quenching for different dimers is quite obvious. The
strongest residual PL (the weakest quenching) is found in
the P3HT/C60–C70 sample. As we shall see later, this is re-
flected in the charge-generation efficiency and may be due
to different phase separation (demixing) in the studied films.
and C70.[a]
Compound
E1
E2
E3
E4
red
red
red
red
4
5
6
7
9
C60
C70
ꢀ0.72 ꢀ0.90 ꢀ1.17 ꢀ1.32 ꢀ1.57 ꢀ1.80 ꢀ1.96 ꢀ2.22
ꢀ0.73 ꢀ0.90 ꢀ1.16 ꢀ1.31 ꢀ1.61 ꢀ1.80 ꢀ2.24
–
ꢀ0.72 ꢀ0.87 ꢀ1.14 ꢀ1.30 ꢀ1.55 ꢀ1.77 ꢀ2.07 ꢀ2.23
ꢀ0.75
ꢀ0.75
ꢀ0.78
ꢀ0.77
ꢀ1.15
ꢀ1.11
ꢀ1.19
ꢀ1.15
ꢀ1.61
ꢀ1.56
ꢀ1.66
ꢀ1.56
ꢀ2.06
ꢀ1.91
ꢀ2.11
ꢀ1.97
Electrochemistry: The redox behaviour of dimers 4–6 was
studied by Osteryoung square-wave voltammetry (OSWV),
as shown in Figure 5. It is mainly characterised by the ap-
[a] V vs. Ag/AgNO3; glassy carbon electrode as the working electrode;
0.1m Bu4N+PF6ꢀ; ODCB/MeCN (4:1); scan rate=100 mVsꢀ1
.
the reduction of the p-nitrophenyl moiety, which is consis-
tent with related systems.[23d]
Comparison of the first reduction potentials of dimers 4
and 5 and aldehyde 7 reveals the existence of a weak elec-
tronic interaction in the ground state. Thus, when the formyl
group is replaced by the pyrrolidinofullerene moiety, the
first reduction potential of the 2-pyrazolinofullerene unit is
shifted by 20 to 30 mV to more positive values. A similar be-
haviour is observed for dimer 6, which shows reduction po-
tentials 30 mV more positive than that observed for 9 which
bears a formyl substituent. Nevertheless, according to the
observed data, we can conclude that both 2-pyrazolinofuller-
ene and pyrrolidinofullerene behave almost independently
in the ground state.
Photoinduced absorption: Quasi-steady-state photoinduced
absorption (PIA) is a method to probe long-living excited
states in conjugated polymers and their blends.[25] In pure
P3HT, the singlet excitons are clearly seen as a dominant
peak at 1.03 eV (Figure 6).[26] Once blended with a strong
Figure 5. a) OSWV of dimer 4 in o-dichlorobenzene/acetonitrile (ODCB/
MeCN, 4:1). b) Cyclic voltammogram of dimer 4 with reduction waves as
indicated.
pearance of two sets of reduction waves owing to the pres-
ence of two distinct pyrrolidino- and 2-pyrazolinofullerene
units. The [60]- or [70]-2-pyrazolinofullerene moieties dis-
play quasi-reversible reduction waves with potentials 50–
70 mV, that is, more positive than those of the parent C70 or
C60 due to the electron-withdrawing effect of the pyrazoline
ring.[20] In contrast, [60]- or [70]pyrrolidino fragments show
reduction potentials shifted to more negative values com-
pared to those of the parent fullerenes, as a consequence of
the saturation of a double bond on the fullerene sphere,
which raises the LUMO energy level (Table 1).[24] Com-
pounds 4–6 and aldehydes 7 and 9 displayed additional non-
reversible CV waves around 1.63–1.65 V corresponding to
Figure 6. PIA signals of the homo- and heterodimers blended with P3HT,
as indicated in the legend. The low-energy and high-energy polaron sig-
nals at 0.3 and 1.2 eV, respectively, are seen in the blends P3HT/dimers
4–6. Inset: Temperature dependences of PIA peaks in C70–C70/P3HT
blend. The 0.9 eV peak—negative polaron on C70 homodimer—is visible
only at low temperatures.
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Fullerene Dimers for Energy Harvesting
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Table 2. PIA lifetimes and a factors for three P3HT/dimer blends for the different excited states at T=30, 80,
200 K and room temperature. All PIA bands, except that at 1.03 eV in P3HT/C60–C70, show the same trend of
decreasing t and a with increasing temperature. The 1.03 eV state in P3HT/C60–C70 shows a temperature-inde-
pendent a. Da is ꢁ0.01 for all values.
electron acceptor, for example
[60]PCBM, absorption bands
of radical cations (positive po-
larons) can be seen in the PIA
spectrum at 0.3 and 1.2 eV cor-
responding to low- and high-
energy peaks, respectively. The
PIA spectra for P3HT blended
with dimers 4–6 are shown in
Figure 6. In all three blends,
the 0.3 and 1.2 eV polaron
peaks are present. The spec-
trum of P3HT/C60–C60 looks
nearly identical to that of
P3HT/PCBM. In contrast,
P3HT/C70–C70 reveals an addi-
T
30 K
80 K
200 K
RT
Blends
Peaks [eV]
t [ms]
a
t [ms]
a
t [ms]
a
t [ms]
a
P3HT/C60–C60
1.23
0.30
1.23
1.03
0.30
1.23
0.90
1.21
1.23
0.30
89
94
74
56
0.72
0.72
0.73
0.69
0.73
0.71
0.72
0.71
0.80
0.80
59
63
54
58
54
85
77
80
41
41
0.69
0.69
0.72
0.69
0.72
0.69
0.71
0.69
0.76
0.77
9
8
9
0.70
0.79
0.77
1
2
2
0.59
0.58
0.68
P3HT/C60–C70
P3HT/C70–C70
P3HT/PCBM
80
9
27
0.78
0.63
2
9
0.70
0.61
110
117
117
85
27
7
8
0.63
0.80
0.85
10
2
6
0.60
0.69
0.34
187
tional PIA peak at 0.9 eV, which is strongly temperature de-
pendent (see inset to Figure 6) and can only be detected at
low temperatures. Its origin is unknown and will be dis-
cussed later. In P3HT/C60–C70, a shoulder at 1.03 eV shows
up. Taking the PIA spectrum of the pure P3HT into ac-
count, we conclude that the shoulder at 1.03 eV may be due
to either incomplete dissociation of singlet excitons at low
temperatures, or to other excited states. Possible candidates
are triplet excitons and/or charge-transfer excitons, also
called polaron pairs. To be consistent with the observation
of a stronger PL found in the same dimer blends, the recom-
bination of these excited states should lead to the formation
of singlet excitons and thus to a stronger PL in C60–C70 (see
Figure 4). Furthermore, the lifetime of singlet excitons, trip-
let excitons and polaron pairs is expected to be very differ-
ent.
P3HT/C60–C70 blend are lower than those for other peaks.
The effective lifetime t of polaron states (0.3 and 1.2 eV)
strongly decreases from 90 ms (T=30 K) to 1 ms (room tem-
perature). The same holds true for the PIA band at 0.9 eV,
whereas the lifetime of the exciton peak at 1.03 eV (around
60 ms) is unaffected by temperature. Interestingly, in the
whole temperature range available, the excited states in
P3HT/C70–C70 blends have the longest lifetimes relative to
other dimer blends. The assignment of PIA absorption
bands at 0.9 and 1.03 eV to particular states, polarons, polar-
on pairs and singlet (or triplet) excitons is not completely
reliable. According to the long lifetime, the singlet exciton
origin of the 1.03 eV peak can be excluded. Instead, the trip-
let exciton and charge-transfer exciton scenarios should be
further assessed. To identify them unambiguously, spin-sen-
sitive techniques will be necessary, which are presented in
the following section.
To investigate these new PIA peaks in more detail, fre-
quency-dependent PIA measurements were performed. The
modulation (on–off) frequency of the excitation laser light
was varied in a broad frequency range. The PIA response R
on the modulated excitation was described as suggested by
Epshtein et al. [Eq. (1)]:[27]
Electron spin resonance: ESR measurements on pure P3HT
and on pure fullerene dimers in the dark and under illumi-
nation show no signal except for compound 4. In the latter,
a small light-induced signal of the C60-related radical anion
was detected (not shown), which indicates the presence of
minor impurities.
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Rðw ¼ 0Þ
The ESR signals (Figure 7) found in the 1:1 blends of
P3HT/[60]PCBM and in P3HT blended with compound 4
(C60–C60) show nearly the same signal shape and g values,
which is a sign of efficient electron transfer between P3HT
and the C60–C60 dimer. Although the ESR spectrum of
blends of P3HT and heterodimers C60–C70 shows nearly the
same spectral shape as those for P3HT/C60–C60 and P3HT/
[60]PCBM, the ESR peak corresponding to the C60 radial
anion is significantly smaller than the radical cation peak
(P3HT peak). Even more dramatic is the modification of
the ESR spectrum in the P3HT/C70–C70 blends, as there is
only one strong signal detectable (Figure 7). A closer look
at this ESR spectrum reveals a shoulder at larger g factor
(lower magnetic field). We have observed the underlying
signal (shoulder) in several [70]fullerene-based blends,
which is due to radical anions formed during the charge-
transfer process.[28] An example of such a C70-related ESR
R ¼
ð1Þ
a
1 þ ðiwtÞ
where w is the modulation (chopper) frequency, RACTHNUGRTNEUNG(w=0) is
the PIA signal at w=0, t is the effective (mean) lifetime, i is
the imaginary unit, and a is a constant indicating the domi-
nant recombination process, with a=1 indicating pure
monoACHTUNGTRENNUNGmolecular recombination. We fitted the experimental
data with Equation (1), and the parameters a and t for the
polaron peaks at 0.3 and 1.2 eV and for the yet unidentified
peaks at 1.03 and 0.9 eV are summarised in Table 2. (Due to
a strong temperature dependence of the PIA signals at 0.9
and 1.03 eV, measurements were not possible at 200 K and
at room temperature.) All PIA bands respond on the modu-
lation frequency with a fractional power law dependence
with a between 0.59 and 0.79, and nearly a constant at low
temperatures. The a values for the 1.03 eV peak in the
Chem. Eur. J. 2009, 15, 13474 – 13482
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www.chemeurj.org
13479

N. Martꢀn, V. Dyakonov et al.
Figure 7. LESR signals in P3HT/fullerene blends at T=120 K. The
curves are normalised to the g=2.003 peak of P3HT/[60]PCBM. Where-
as the positive polaron on P3HT (low-field signal at g=2.002) is present
in all blends, the negative polaron (high-field signals at g=2.000) is only
seen in [60]PCBM or in C60–C60 and C60–C70. At low magnetic field, a
shoulder at g=2.005 due to the negative polaron on C70–C70 can be seen
in the P3HT/C70–C70 blends.
Figure 8. EQE of BHJ solar cells fabricated with P3HT blended with
dimers 4–6 in 2:1 weight ratio.
1.5:1, 1:1, 1:2 and 1:4; however, the 2:1 blend ratio, that is,
two parts of P3HT and one part of acceptor, was found to
yield solar cells with the highest efficiencies. The corre-
sponding J–V curves are shown in Figure 9. As can be seen,
spectrum in P3HT blended with the commercial [70]PCBM
is shown in the Supporting Information. We speculate, that
this ESR signal is of the same origin as the 0.9 eV PIA peak
found in the P3HT/C70–C70 PIA signals (Figure 6, inset),
namely negative polarons on the C70–C70 dimer.
From LESR measurements, it can be seen that the radical
cation localised on the polymer backbone is not influenced
by the choice of the acceptor molecule. In other words, all
three compounds 4–6 act as acceptor when blended with
P3HT. Also note that in compound 5, radical anions (nega-
tive polarons) located on C60 and C70 cages can be observed
simultaneously. This opens up a new interesting outlook of
using fullerene heterodimers as model systems for studying
spin–spin interactions.
Solar cell performance: The EQE and the current–voltage
(J–V) characteristics were measured for all dimer blends.
The EQE determines the number of generated charge carri-
ers extracted into the outer circuit relative to the number of
incident photons. Additionally, the short-circuit current den-
sity of the solar cell can be calculated for any illumination
spectrum at a given intensity, in our case for the AM1.5G
solar spectrum at 100 mWcmꢀ2. In J–V measurements, the
intensity of the solar simulator used was adjusted to be
equal to the calculated short-circuit current obtained from
the EQE. This is an indirect method to accurately determine
the main solar-cell parameters, such as short-circuit current
density (JSC), the open-circuit voltage (VOC) and the fill
factor (FF) of the solar cell.
Figure 9. Current–voltage characteristics of P3HT/dimer BHJ solar cells
at room temperature. P=100 mWcmꢀ2
.
the highest short-circuit current density of 5 mAcmꢀ2 was
achieved in P3HT/C70–C70, the values being 4.3 and
3.4 mAcmꢀ2 for P3HT/C60–C60 and P3HT/C60–C70, respec-
tively. Note that the heterodimer blends showed the smallest
EQE and thus the smallest JSC and PCE (Table 3).
Table 3. Characteristic parameters of the P3HT/dimer BHJ solar cell
with weight ratio 2:1: the open-circuit voltage (VOC), short-circuit current
(JSC), fill factor (FF) and calculated PCE.
Figure 8 shows the EQE spectra of BHJ solar cells with
P3HT as electron donor and the three different dimers as
acceptors. The EQE measurements yield values of 37, 31
and 25% for dimers 6, 4 and 5, respectively. The P3HT/
dimer ratio was varied in a broad range of 4:1, 2.5:1, 2:1,
Ratio VOC [mV] JSC [mAcmꢀ2
]
FF [%] PCE [%]
P3HT/C60–C60 2:1
P3HT/C60–C70 2:1
P3HT/C70–C70 2:1
440
430
430
4.27
3.41
5.08
48
46
42
0.91
0.67
0.91
13480
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Chem. Eur. J. 2009, 15, 13474 – 13482

Fullerene Dimers for Energy Harvesting
FULL PAPER
temperature. PL measurements were performed with the same set-up as
that used for PIA.
Conclusion
Photoinduced absorption: For PIA measurements, the samples were
mounted on a helium cold-finger cryostat (20–293 K) and kept under dy-
namic vacuum to avoid photo-oxidation. The excitation source was a me-
A new family of fullerene-based compounds, namely soluble
[60]- and [70]fullerene homo- and heterodimers linked
through 2-pyrazolino–pyrrolidino bridges, has been synthes-
ised. Electrochemical, optical and ESR studies confirmed
their suitability to act as electron acceptors in combination
with regioregular P3HT. Compared to [60]PCBM, used in
our work as a reference, the fullerene dimers exhibit a
larger absorption coefficient thus justifying their use as ab-
sorber in BHJ solar cells. PL studies revealed that P3HT/
C70–C70 blends show efficient PL quenching comparable to
that of the P3HT/[60]PCBM system. The photogenerated
polaron states in P3HT/C70–C70 blends have longer lifetimes
than those of blends with [60]fullerene homo- and hetero-
dimers, as follows from PIA experiments. The new PIA
peak at 0.90 eV in P3HT/C70–C70 blends was identified and
suggested to result from a radical anion. LESR studies on
all blends confirmed this finding, as a radical anion could
solely be identified in C70–C70 dimer-based blends. The
P3HT/C60–C70 blend instead displayed a reduced PL quench-
ing, thus indicating inefficient charge transfer. This blend
shows a shoulder at 1.03 eV in the PIA spectrum, which is
due to unquenched excitons in the blend. We tentatively
assign the 1.03 eV PIA peak to charge-transfer exciton
states; however, triplet excitons could also be responsible.
According to our expectations, the P3HT/C70–C70 blends
should be most promising for BHJ solar cells. Our prelimi-
nary tests demonstrated the non-optimised PCE of about
1% in ITO/PEDOT/PSS/P3HT/C70–C70/Al PV devices (see
Experimental Section). The P3HT/C70–C70-based solar cells
exhibited the most promising EQEs and PCEs, which were
as high as 37 and 0.91%, respectively. The solar cells are by
no means optimised and with a view to obtaining competi-
tive PCEs, the morphology of the blends should be thor-
oughly investigated.
chanically chopped argon-ion laser (Melles Griot) at
a wavelength
514 nm with a power of 68 mW. Additionally, a halogen lamp provided
continuous-wave illumination. Both light sources were focussed onto the
same point of the sample. The transmitted light was collected by large-di-
ameter concave mirrors and focussed into the entrance slit of a corner-
stone monochromator. Depending on the wavelength, detection was pro-
vided by a silicon diode (550–1030 nm) or by a liquid-nitrogen-cooled
InSb detector (1030–5550 nm). Therefore, a broad energy range of 0.23–
2.25 eV (with KBr cryostat windows) was accessible. The signals were re-
corded with a standard phase-sensitive technique synchronised with the
chopping frequency of the laser by using a signal recovery 7265 DSP
lock-in amplifier. Photoinduced changes of the transmission, DT/T, were
monitored as function of the wavelength of the probe light. Frequency-
resolved PIA measurements were carried out by varying the frequency of
the chopper of the argon-ion laser, w/2p, between 56 and 20000 Hz. The
lifetimes of the long-lived (about ms–ms) excitations were deduced by fit-
ting the DT/T(w) dependence.
Electron spin resonance: To verify the presence of the spin-carrying po-
larons in the polymer–fullerene films after photoexcitation, we used the
ESR technique (Bruker ESR200D). A 150 W halogen lamp guided to the
microwave cavity was used for excitation. All measurements presented
here were performed at 120 K by cooling with a liquid nitrogen flow. A
static magnetic field of 0.33 T was superimposed with the ac magnetic
field at f=100 kHz modulation frequency, which allowed phase-sensitive
lock-in detection. By using the first derivative of the Voigt line shape, the
overlapping ESR signals from negative and positive polarons (radical
anions and radical cations) were deconvoluted. The g factor of the ESR
signals was calibrated for every measurement with a Bruker 035M NMR
gaussmeter and an EIP 28b frequency counter.
Preparation of PV devices: BHJ solar cells were processed by spin-coat-
ing a mixture of P3HT and the fullerene dimers on an oxygen-plasma-
treated indium tin oxide (ITO)-coated glass substrate, coated with a
40 nm layer of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)
(PEDOT/PSS). The PEDOT/PSS layer was treated at 1808C for 10 min
and then transferred to a nitrogen glove box. P3HT and dimers were dis-
solved in ODCB (20 mgmLꢀ1) and blended in various donor-to-acceptor
ratios. The weight ratio was varied in the range from 4:1 to 1:4 for each
of the three dimers. The blend solution was then spin-coated onto the
substrates at 800 rpm for 240 s, which resulted in a layer of thickness
110 nm. The film thickness was measured by a Dektak surface profiler
(Veeco). Before evaporating the 80 nm thick Al cathode, all samples
were thermally treated at 1208C for another 10 min. As a reference ac-
ceptor in BHJs, we used PCBM purchased from Solenne BV.
Experimental Section
PV characterisation: Current–voltage measurements were recorded with
a Keithley 237 SMU instrument. A 300 W xenon lamp was used for illu-
mination. Monochromatic EQE spectra were obtained with a homemade
lock-in set-up equipped with a calibrated photodiode. The light from the
halogen and xenon lamps was spectrally dispersed by a monochromator
with a set of gratings to cover the wavelength region from 250 to
1000 nm. Although the mismatch factor of the xenon lamp to the AM1.5
spectrum was not determined, we used an indirect method to determine
the PCE accurately. The short-circuit current density of the solar cell was
calculated from the EQE for an AM1.5 spectrum at a given light intensi-
ty, for example 1000 Wmꢀ2, and the solar simulator in the current–volt-
age measurements was then adjusted to obtain the short-circuit current
from the solar cell under test.
Sample preparation for optical and ESR spectroscopy: UV/Vis spectra
were obtained in both toluene solutions (at 1ꢄ10ꢀ5 m) and in solid films.
PIA measurements were performed on thin films of pure P3HT and
P3HT blended with fullerene dimers. Regioregular P3HT was purchased
from Rieke Metals and used without further purification. As a reference
acceptor in BHJs, we used PCBM purchased from Solenne BV. All mate-
rials were dissolved in dichlorobenzene at a concentration of 10 mgmLꢀ1
.
The films were deposited onto sapphire substrates by spin-coating and
annealed at 1208C for 10 min. If not stated otherwise, blends with a 1:1
weight ratio were studied. Films were prepared under a nitrogen atmos-
phere in a glove box to prevent ageing effects due to water and oxygen.
The samples for ESR were drop-cast on a flexible substrate, wrapped up
and placed in a sealed quartz tube (Wilmad-LabGlass) in the centre of a
microwave cavity.
Electrochemistry: The electrochemical properties of dimers 4–6 and the
precursor aldehydes 7 and 9 (Schemes 1 and 2) were studied at room
temperature by CV and OSWV in ODCB/CH3CN (4:1) as solvent, with
Acknowledgements
ꢀ
Bu4N+PF6 as the supporting electrolyte.
Optical absorption and photoluminescence: UV/Vis absorption spectra
were recorded with a Perkin–Elmer Lambda 900 spectrometer at room
This work was supported by the MEC of Spain (Ramꢅn y Cajal Program
and projects CTQ2008-00795/BQU and Consolider-Ingenio 2010C-07-
Chem. Eur. J. 2009, 15, 13474 – 13482
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
13481

N. Martꢀn, V. Dyakonov et al.
25200), the CAM (project P-PPQ-000225-0505), the ESF (project 05-
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Bayern is financed by the Bavarian Ministry of Economic Affairs, Infra-
structure, Transport and Technology. We also thank Andreas Baumann
and Daniel Rauh (University of Wꢂrzburg) for the photovoltaic studies.
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Received: July 22, 2009
Published online: November 5, 2009
13482
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Chem. Eur. J. 2009, 15, 13474 – 13482