Methanofullerenes used as electron acceptors in polymer photovoltaic devices

Article abstract of DOI:10.1021/jp048890i

A series of [6,6]-phenyl C₆₁-butyric acid esters, including methanofullerene [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) with different alkyl chain lengths (C₁-C₁₆) was synthesized from the reaction of C₆₀ and alkyl 4-benzoylbutyrate p-tosylhydrazone in the presence of sodium methylate. The solubility of C ₆₀ derivatives in organic solvents increased with the increase in the length of alkyl substitutions. Photovoltaic cells with these derivatives were fabricated with the structure of ITO/PEDT/MEH-PPV + C₆₀ derivatives/ Ba/Al. Device performances with such PCBM analogues were investigated and discussed in terms of Donor/ Acceptor (D/A) phase separation and mobility of acceptor phase. The results clearly indicate that both interfacial properties of the two phases (donor and acceptor) and mobility of electrons and holes within corresponding phases play an important role in the efficiencies of PV cells. This study revealed that methanofullerenes [6,6]-phenyl C₆₁-butyric acid butyl ester, PCBB, possesses better photosensitivity than the PCBM, a widely investigated and well-recognized C₆₀ derivative for polymer PV cells The energy conversion efficiency reaches 2.84% for PCBB under AM1.5 illumination (78.2 mW/cm²), but 2.0% for PCBM fabricated in the same conditions.

Full text of DOI:10.1021/jp048890i

J. Phys. Chem. B 2004, 108, 11921-11926
11921
Methanofullerenes Used as Electron Acceptors in Polymer Photovoltaic Devices
Liping Zheng, Qingmei Zhou, Xianyu Deng, Min Yuan, Gang Yu,† and Yong Cao*
Institute of Polymer Optoelectronic Materials and DeVices, South China UniVersity of Technology,
Key Laboratory of Specially Functional Materials and AdVanced Manufacturing Technology,
Guangzhou 510640, China
ReceiVed: March 12, 2004; In Final Form: May 21, 2004
A series of [6,6]-phenyl C61-butyric acid esters, including methanofullerene [6,6]-phenyl C61-butyric acid
methyl ester (PCBM) with different alkyl chain lengths (C1-C16) was synthesized from the reaction of C60
and alkyl 4-benzoylbutyrate p-tosylhydrazone in the presence of sodium methylate. The solubility of C60
derivatives in organic solvents increased with the increase in the length of alkyl substitutions. Photovoltaic
cells with these derivatives were fabricated with the structure of ITO/PEDT/MEH-PPV + C60 derivatives/
Ba/Al. Device performances with such PCBM analogues were investigated and discussed in terms of Donor/
Acceptor (D/A) phase separation and mobility of acceptor phase. The results clearly indicate that both interfacial
properties of the two phases (donor and acceptor) and mobility of electrons and holes within corresponding
phases play an important role in the efficiencies of PV cells. This study revealed that methanofullerenes
[6,6]-phenyl C61-butyric acid butyl ester, PCBB, possesses better photosensitivity than the PCBM, a widely
investigated and well-recognized C60 derivative for polymer PV cells The energy conversion efficiency reaches
2.84% for PCBB under AM1.5 illumination (78.2 mW/cm2), but 2.0% for PCBM fabricated in the same
conditions.
I. Introduction
SCHEME 1: Molecular Structures of PCBM and C60
Derivatives Used as Acceptor in This Study
The idea of making thin-film photovoltaic devices from
electroactive polymers and organic materials has recently
attracted much attention due to the possibility of creating
extremely light, cheap, flexible solar cells and photodetectors.
Recent research demonstrates that the photoresponse of a homo
semiconductor polymer can be further enhanced by being
blended with another polymer or an organic molecule with
different electronic structures.1-4 Photoinduced charge transfer
(CT) and subsequent quick charge separation occur in such a
blend; that is, the electrons are energetically liable to move into
the phase with greater electron affinity (acceptor phase, such
as C60 or PCBM), while the photoinduced holes are energetically
liable to move into the polymer phase with smaller ionization
potential (donor phase, such as poly(2-methoxy-5-(2′-ethylhex-
oxy)-p-phenylene (MEH-PPV)). The photoinduced CT sepa-
rates electrons and holes and prevents them from early
recombination. By carefully controlling the morphology of the
D and A phases, one can achieve high interfacial area within a
‘bulk D/A heterojunction’ material. A bicontinuous network
morphology provides pathways with the ability to collect the
separated carriers at external electrodes. Thus, phase behavior
of D and A components is extremely important to the realization
of efficient photovoltaic cells and efficient photodetectors with
high carrier collection efficiency.5-7 Recently, Sariciftci and co-
workers8-10 have demonstrated that the power conversion
efficiency of bulk-heterojunction plastic solar cells from the
blend of a conjugated polymer, poly(2-methoxy-5-(3′,7′-di-
methyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) and
PCBM can be raised to 2.5% under AM1.5 irradiation by
manipulating the solid-state morphology (nanostructure) of the
blend. They pointed out that the severalfold enhancement in
the short-circuit current of photovoltaic devices most possibly
originated from enhanced interfacial contacts between the donor
and the acceptor. It was also shown that significant improvement
in cell efficiency of devices with PCBM as the acceptor phase
in comparison with unsubstituted C60 is closely related to the
improvement in phase separation of MEH-PPV and PCBM.5,7
In the previous paper, we demonstrated that photosensitivity
reaches a maximum at a weight ratio of PCBM to MEH-PPV
equal to 4.13 Device performance deteriorates when the PCBM/
MEHPPV ratio is beyond 4. This effect was attributed to
reduction of film quality and phase structure of the D/A blend.
Aiming at improving the phase behavior of the polymer PV
cells, in this paper we synthesized a series of new C60
derivatives; PCBM analogues [6,6]-phenyl C61-butyric acid ester
with alkyl chains length C4-C16 as acceptor (Scheme 1).
Through systematic investigation of chain length influence on
the PV cells performance, we found out that, with alkyl length
equal to 12 carbon atoms, performance of the PV cells shows
only a slight dependence on the length of alkyl chain substitu-
* E-mail: poycao@scut.edu.cn.
† Dupont Displays.
10.1021/jp048890i CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/10/2004

11922 J. Phys. Chem. B, Vol. 108, No. 32, 2004
Zheng et al.
tion. Among them, methanofullerenes [6,6]-phenyl C61-butyric
acid butyl ester(PCBB) shows better device performance than
the PCBM, a widely investigated and the best C60 derivative-
type acceptor having been reported so far for polymer PV cells.
The energy conversion efficiency reaches 2.84% for PCBB
under AM1.5 illumination (78.2 mW/cm2), compared with 2.0%
for methanofullerenes [6,6]-phenyl C61-butyric acid methyl ester,
PCBM, fabricated at the same conditions. Once the alkyl group
becomes the C16H33, the device performance deteriorates
significantly. These results clearly indicate that both the
interfacial properties of two phases (donor and acceptor) and
the mobility of electrons and holes within corresponding phases
play an important role in the efficiency of the PV cells.
II. Experimental Section
Details of synthesis of methanofullerene derivatives (3a-f)
spectroscopic and analytical data can be found in the Supporting
Information.
Cyclic Voltammetry (CV). The electrochemical measure-
ments of the organic molecule were performed in an electrolyte
consisting of 0.1 mol/L tetrabutylammonium hexafluorophos-
phate (TBAPF6) dissolved in 1,2-dichlorobenzene solution. In
each case, a glassy carbon was used as the working electrode,
platinum wire was used as a counter electrode, and a saturated
calomel electrode was used as a quasi-reference electrode. The
absorption spectrum was taken by an HP 8453 spectrophotom-
eter.
Figure 1. Cyclic Voltammograms of C60 derivatives at 50 mv/s in
1,2-dichlorobenzene containing 0.1M (n-Bu)4NPF6.
butyric acid esters were synthesized according to Scheme S2
by the diazomethane route reported previously by Hummelen
et al.11 All the synthesized fullerene derivatives have been
refluxed in 1,2-dichlorobenzene for more than 8 h. The signals
for the fullerene-sp2 carbons in 13C NMR spectrum indicate Cs-
symmetry. The UV-vis absorbed at 700 nm specific to [6,6]
addition in C60. Matrix-assisted laser desorption ionization time-
of-flight mass spectrometry (MALDI-TOF-MS) gave the M-,
according to the calculated value of m/z of all the synthesized
C60 derivatives. The FTIR spectrum showed absorption features
at 521 cm-1, indicative of the fullerene core. The synthesized
compounds’ full and abbreviated names are listed below:
3a: [6,6]-phenyl C61-butyric acid methyl ester, PCBM.
3b: [6,6]-phenyl C61-butyric acid butyl ester, PCBB.
The typical device structure used in this study was a sandwich
structure with ITO/PEDOT as a hole-collecting electrode and
Ba/Al as an electron-collecting electrode. In the structure of
the devices, the active layer has a thickness of 100 nm, as
determined by surface profilometer (Tencor, ALFA-Step 500).
Indium/tin oxide (ITO) was used as the anode and a 50-nm-
thick poly (ethylene dioxythiophene)/polystyrenesulfonic acid
(PEDT-PSS, Batron-P, Bayer AG) layer was incorporated
between the ITO and the active layer to reduce device leakage.
Organic molecules as acceptor were dissolved in p-xylene or
THF solution. MEHPPV was dissolved in a THF/p-xylene
mixture (2:8) solution. Both solutions were mixed to give the
calculated weight ratio of donor material to acceptor material.
The blend solution was spincoated onto the top of PEDT-PSS.
Finally, 3-5 nm of barium followed by 150-nm-thick Al layers
were thermally evaporated onto the top of the photoactive
polymer blend. The deposition rates and the thickness of the
evaporation layers were monitored by a thickness/rate meter
(Sycon). The deposition rates for barium and aluminum were
usually 0.01-0.02 nm/s and 1-2 nm/s, respectively. The
crossing area between the cathode and the anode define the
sensing area. Unless otherwise specified, an 0.15-cm2 active
area was typically used in this study. All the fabrication steps
except spincasting the PEDT:PSS layer were carried out in a
nitrogen glovebox. The IV characteristics in the dark and under
illumination were measured with a Keithley 236 source-measure
unit. Photocurrent was measured under a solar simulator with
AM1.5 illumination (78.2 mW/cm2). The spectral response was
measured with a commercial photomodulation spectroscopic
setup including a Xenon lamp, an optical chopper, a mono-
chromator, and lock-in amplifier operated by PC computer
(Merlin, Oriel). A calibrated Si photodiode was used as a
standard in determination of photosensitivity.
3c: [6,6]-phenyl C61-butyric acid (1-methyl)heptyl ester,
PCBMH.
3d: [6,6]-phenyl C61-butyric acid octyl ester, PCBO.
3e: [6,6]-phenyl C61-butyric acid dodecyl ester, PCBD.
3f: [6,6]-phenyl C61-butyric acid cetyl ester, PCBC.
High molecular weight poly(2-methoxy, 5-(2′-ethylhexyloxy)-
1,4-phenylene vinylene) (MEH-PPV) in this study was syn-
thesized according to the process described by Wudl et al.12
Electrochemical and Optical Properties of C60 Derivatives.
The electrochemical properties of the C60 derivatives were
studied by cyclic voltammetry at a room temperature in 1,2-
dichlorobenzene solution using TBAPF6 (0.1 M) as the sup-
porting electrolyte. 1,2-dichlorobenzene was used as the solvent
for all the C60 derivatives that have very good solubility in this
solvent. The concentration of C60 derivatives (3a-f) ranges from
10-4 to 10-3 M. Graphite working electrodes, Pt counter
electrodes, and SCE as a reference electrodes were used. All
six C60 derivatives (3a-f) show similar cyclic voltammograms
with three quasireversible reduction waves in the potential
ranging from 0.0V to -1.8V. To get clear and distinguished
curves we choose only three representative curves in Figure 1.
More detailed data are listed in Table 1 where E1red, E2red, and
E3red reduction potential (defined as the onset of each reduction
wave) are listed along with C60 and PCBM data obtained in
this study and also from the references.9 Results in Figure 1
and in Table 1 show that all substituted PCBM have almost
identical reduction potential, which indicates that substitution
of alkyl chain length in the butyric esters does not affect the
reduction potential of C60 derivatives.
III. Results and Discussion
Materials. Synthetic route and abbreviations for C60 deriva-
tives are shown in Schemes 1 and S2. The [6,6]-phenyl C61-

Methanofullerenes Used as Electron Acceptors
J. Phys. Chem. B, Vol. 108, No. 32, 2004 11923
Figure 2. Normalized UV-vis absorption spectra of (a) C60 derivatives in toluene; (b) C60 derivatives/MEHPPV blends (weight ratio equal to 4)
solid thin film (film thickness in range 80-110 nm).
pristine C60. From the UV-visible absorption, we conclude that
there is no dramatic change in the band structure for compounds
3a-f.
In summary, CV and absorption spectra data for these C60
derivatives indicate that there should be no dramatic change in
energy level alignment upon exchanging each other as the
acceptor phase in photovoltaic devices with MEHPPV as donor
phase. This characteristic gives us the possibility to investigate
phase compatibility without a change in energy level alignment.
Photovoltaic Properties. Photovoltaic cells with a 100-nm
thick composite film (weight ratio of methanofullerene:MEH-
PPV ) 4:1 in all cases except indicated otherwise) as the active
layer were fabricated by spincoating from chloroform/p-xylene
(1:1 volume ratio) solution on the top of pre-patterned ITO
substrate covered by 100 nm of poly(3,4-ethylene-dioxythio-
phene):poly (styrene sulfuric acid) (PEDOT: PSS, Baytron P
4083, Bayer AG). A thin layer (3-5 nm) of Ba metal was
deposited on the top of the active layer, followed by 200 nm of
Figure 3. Photosensitivity of devices of ITO/PEDT/MEHPPV+C60
the Al layer by thermal deposition under vacuum of (1-2) ×
derivatives (1:4 wt ratio)/Ba/Al at 0V bias-voltage.
10-4 Pa. As we previously reported, a thin layer of Ba inserted
between the active layer and the Al cathode improves energy
conversion efficiency, serving as an exciton-blocking layer.12
Salaneck14 reported that a Ca layer deposited in a low-vacuum
evaporating chamber (ca. 1 × 10-4 Pa) was actually partially
oxidized. Thus, BaO (or most possibly, nonstoichiometric
Ba1+xO1-x) inserted under a negative electrode plays a similar
role as an LiF layer on the top of a active layer reported
previously by Brabec et al.15 Device characteristics were
measured with a commercial photomodulation spectroscopic
setup (Model Merlin, Oriel) including a Xenon lamp, an optical
chopper, a monochromator, and lock-in amplifier operated by
PC computer. A calibrated Si photodiode was used as a standard
in the determination of photosensitivity. Figure 3 compares
photosensitivity for devices fabricated from methanofullerenes
with different alkyl length as electron acceptors, where the
weight ratio of methanofullerenes to MEH-PPV was kept a
constant of 4:1. As can be seen from Figure 3, the spectral
response is similar for each methanofullerene and is roughly
consistent with the absorption curve of the 4:1 blend (Figure
2b). Maximum photosensitivity for all methanofullerene/MEH-
PPV blends was estimated at around 515 nm.13 The magnitude
of the photosensitivity of PCBB (3b) is slightly larger than that
for PCBM (3a) blends and then slightly decreases with
increasing alkyl chain length.
TABLE 1: Reduction Potentials (V vs SCE) of C60 and
Fullerene Derivativesa
compound
E1
E2
E3
red
red
red
C60
-0.55
-0.64
-0.63
-0.67
-0.67
-0.64
-0.65
-0.60
-0.69
-0.95
-1.05
-1.05
-1.08
-1.16
-1.09
-1.09
-1.01
-1.09
-1.40
-1.56
-1.55
-1.59
-1.69
-1.57
-1.62
-1.46
-1.57
PCBM (3a)
PCBB (3b)
PCBMH (3c)
PCBO (3d)
PCBD (3e)
PCBC (3f)
C60 (ref 11)
PCBM (ref 12)
a Experimental conditions: 10-4 to 10-3 mol‚L-1 in 1,2-dichloroben-
zene solution; TBAPF6 (0.1M) as supporting electrolyte; Graphite
working electrodes, Pt as counter electrodes and SCE as reference
electrodes
Absorption spectra obtained from HP 8453 UV-Vis Spec-
trophotometer are consistent with CV data. Figure 2a shows
the UV-visible absorption of 3a-f in comparison with that of
pristine C60 in p-xylene solution. It is noted that the absorption
profile for the derivatives 3b-f is very close to that of PCBM
(3a). These compounds all feature a small peak at 700 nm,
specific to [6,6] addition in C60. The main peak at 515 nm in
PCBM (3a) is slightly blue-shifted to 500 nm in 3b-f.
Meanwhile, the maximum of this peak appears at 540 nm for
Photovoltaic characteristics were measured under an AM1.5
solar simulator (78.2 mW/cm2). The current vs voltage was

11924 J. Phys. Chem. B, Vol. 108, No. 32, 2004
Zheng et al.
Figure 4. AFM (1 × 1 um2) of a 100-nm thick film of MEHPPV and C60 derivatives: MEHPPV (4:1) blends spin-coated from toluene solution.
TABLE 2: Photovoltaic Characteristics of Methanofullerenes/MEH-PPV (4:1) Blend-Based Photovoltaic Devices under AM1.5
Illumination (78.2 mW/cm2)
active layer
(weight ratio ) 4:1)
ID
I
SC (mA/cm2)
VOC (V)
F.F. (%)
ECE under AM1.5
P*smax (A/W)
λ
Psmax (nm)
3a
3b
3c
3d
3e
3f
PCBM/MEHPPV
PCBB/MEHPPV
PCBMH/MEHPPV
PCBO/MEHPPV
PCBD/MEHPPV
PCBC/MEHPPV
4.61
5.22
3.38
3.48
2.86
0.48
0.85
0.85
0.80
0.80
0.80
0.50
39.9
43.2
42.3
35.8
35.4
34.0
2.00
2.45
1.46
1.27
1.04
0.11
0.12
0.13
0.10
0.09
0.10
0.02
525
525
515
525
525
383
measured with a Keithley 236 source-measure unit. The energy
conversion efficiency ηe, and fill-factor F.F. were calculated
by the following equations:16
PPV ) 4:1 This indicates again that the C4 chain attached to
methanofullerene improves its compatibility with the MEHPPV
phase without significant decrease in electron mobility in the
acceptor phase. We note that the fill factors (F.F.) are very close
for devices with 3a, b, and c but slightly decreases for 3d, e,
and f. The device from 3f also shows much lower short-circuit
current, which is responsible for the decrease in energy
conversion efficiency of factor 10. This indicates that the
mobility of substituted methanofullerene with very long alkyl
substitution like C16H33 might be significantly reduced.
ηe ) F.F. × Isc × Voc/Pin
F.F. ) ImVm/IscVoc
(1)
(2)
where Pin is the incident radiation flux, Isc and Voc are the short-
circuit current and open-circuit voltage, and Im and Vm are the
current and voltage for maximum power output.
Figure 4 compares the AFM height image of pristine
MEHPPV and blend films from methanofullerenes and MEH-
PPV in a weight ratio of 4. It can be seen that, with the increase
of alkyl length, the film homogeneity increases. In contrast to
the relatively smooth surface of pristine MEHPPV, the blend
film of MEHPPV with PCBM (3a) shows large numbers of
voids in the film, which must be related to incompatibility of
The results are listed in Table 2. Under the AM1.5 illumina-
tion (78.2 mW/cm2), all devices made from 3a-e, except 3f,
have the same open-circuit voltage of Voc ) 0.80 V. Generally,
a device from the blend with PCBB (3b) as acceptor shows
better energy conversion efficiency and larger short circuit
current than that of a device with PCBM and other methano-
fullerenes at the same weight ratio of methanofullerene:MEH-

Methanofullerenes Used as Electron Acceptors
J. Phys. Chem. B, Vol. 108, No. 32, 2004 11925
Figure 5. (a) Linear and (b) semilogarithmic plot of the current-voltage characteristics for 3a, 3b, and 3e photovoltaic devices in the dark and
under AM 1.5 white-light illumination.
TABLE 3: Device Performance of PV Cells from Blends of
MEHPPV and PCBM during film formation. With the increasing
of alkyl length in methanofullerenes (Figure 4b-d), the number
PCBB (3b) and MEH-PPV in Different Weight Ratio
weight
ISC
of voids decreases remarkably; its depth also decreases. This
fact unambiguously indicates that the compatibility and film
quality increase with increasing alkyl length. Actually, C60 with
long alkyl substitution can provide a good quality film of
methanofullerenes alone by simple spincoating of the corre-
sponding solution, while PCBM and C60 alone cannot form a
good film without blending with a polymer host. As a result,
we can expect the increase in photosensitivity with the increasing
length of alkyl substitutions in the methanofullerenes, if phase
compatibility is the only issue. Since no remarkable increase
in photosensitivity is observed with increasing the chain length,
except C4 substitution, there should be another important factor
that lowers the photosensitivity performance. The possible
mechanism is that the mobility might decrease with the increase
in alkyl substitution, which can separate C60 cores and increase
barrier height for carrier tunneling between C60 cores.
Figure 5 shows the linear (Figure 5a) and semilogarithmic
(Figure 5b) plot of the I-V curve of devices from the blend
with 3a, 3b, and 3e in the dark and under AM1.5 illumination
(78.2 mW/cm2). The I/V characteristics of the cells prepared
from 3a, 3b, and 3e show a good diode behavior with a
rectification ratio of ∼2000 at (2 V in the dark. From the
semilogarithmic plot of the I-V, it can be clearly seen that not
only photo current but also dark current decreases with the
increasing length of alkyl substitutions. This fact supports the
speculation about the decrease in carrier mobility with increasing
alkyl chain length in methanofullerenes.
Further evidence for better compatibility between methano-
fullerenes of long alkyl substitutions with MEHPPV can be
obtained from investigation of device performance as function
of blend composition (weight ratio of methanofullerene to
MEH-PPV). As we reported previously,12 the photosensitivity
is very sensitive to the weight ratio of PCBM to MEHPPV in
the blend film. The maximum photosensitivity for the PCBM/
MEHPPV blend is observed when the weight ratio of PCBM/
MEHPPV is equal to 4. With the further increase in PCBM
ratio photosensitivity decays remarkably. In Table 3 we compare
the energy conversion efficiencies (ECE) of devices from the
blend of MEHPPV and PCBB (3b) in different weight ratios.
The results indicate that the ECE reaches a maximum at a weight
ratio of PCBB:MEHPPV ) 32:1. At this weight ratio, energy
conversion efficiency increases from 2.45% for a 4:1 weight
active layer
ratio (mA/cm2) VOC (V) F.F. (%) ECE (%)
PCBB/MEHPPV
PCBB/MEHPPV
PCBB/MEHPPV 16:1
PCBB/MEHPPV 32:1
PCBB/MEHPPV 64:1
4:1
8:1
5.22
5.28
5.01
6.08
5.03
0.85
0.85
0.85
0.85
0.80
43.2
45.2
46.3
43.0
42.0
2.45
2.60
2.52
2.84
2.16
TABLE 4: Device Performance of PV Cells from Blends of
3f and MEH-PPV in Different Weight Ratio
3f:MEH-PPV
weight ratio
ISC
(mA/cm2)
VOC (V)
F.F. (%)
PSa (A/W)
18:1
12:1
8:1
4:1
3:1
2:1
1:1
1.1
1.3
1.0
1.8
1.1
0.9
0.8
0.80
0.75
0.75
0.80
0.75
0.75
0.75
40.337
40.476
39.038
37.415
39.402
38.549
39.325
0.05
0.04
0.05
0.06
0.05
0.06
0.04
a Data at 520 nm.
ratio to 2.84%. The short-circuit current increases slightly from
5.22 to 6.08 mA/cm2. Further increasing the PCBB:MEHPPV
weight ratio to 64:1 leads to decreasing device performance.
Similar results can be obtained for other long-chain alkyl-
substituted methanofullerenes. Table 4 shows the device per-
formance when the weight ratio of 3f:MEH-PPV varies from
1:1 to 18:1. We note from the results in Table 4 that all device
characteristics are almost constant until a weight ratio of 18:1.
Results in Tables 3 and 4 indicate unambiguously the importance
of compatibility of the donor and acceptor phases. Longer alkyl
chain substitution improves compatibility of donor and acceptor
phases. This feature is not only important for device efficiency
but also for long-term stability of such devices. The improved
compatibility of donor and acceptor phase might prevent self-
aggregation of D and A phases in the blend during storage and
operation. Corresponding experiments for the comparison of
shelf and stress life are in progress.
IV. Conclusions
A series of PCBM analogues was synthesized. Alkyl groups
of different lengths were introduced into the tail of the
methanofullerenes. The electrochemical and photophysical

11926 J. Phys. Chem. B, Vol. 108, No. 32, 2004
Zheng et al.
properties of the new methanofullerenes were characterized by
using CVs and UV-visible spectroscopy. The CVs of all of
these methanofullerenes show almost the same quasireversible
reduction waves ranging from 0V to -2.0V. All the synthesized
C60 derivatives exhibit similar absorption spectra. Photovoltaic
cells were fabricated from the blend of the obtained methano-
fullerenes with MEHPPV. With the alkyl length equal to 1-12
carbon atoms, the PV cells show only slight dependence on the
length of alkyl chain substitution. The best device performance
has been obtained for PCBB with butyl substitution: the device
with Isc ) 6.08 mA/ cm2, Voc ) 0.85, F.F. ) 43.0%, and ECE
) 2.84% at PCBB:MEHPPV weight ratio equal 32:1, but the
device with Isc ) 4.61 mA/cm2, Voc ) 0.85, F.F. ) 39.9, and
ECE ) 2.00% for the best device with PCBM:MEHPPV )
4:1 under AM1.5 illumination (78.2 mW/cm2). Once the alkyl
group reached C16H33, the device performance deteriorated
significantly. The results clearly indicate that both interfacial
properties of two phases (donor and acceptor) and mobility of
electron and holes within corresponding phases (tunneling
barrier between C60 cores) are important for the efficiencies of
PV cells. To further improve the device performance for such
a bulk p-n junction system, it is important to synthesize the
acceptor that has high carrier mobility and good compatibility
with donor-type conjugated polymer simultaneously.
analytical data. This material is available free of charge via the
Internet at http://pubs.acs.org.
References and Notes
(1) Sariciftfci, N. S.; Smilowits, S. L.; Heeger, A. J.; Wudl, F. Science
1992, 258, 1474.
(2) Yu, G.; Pakbaz, K.; Heeger, A. J. Appl. Phys. Lett. 1994, 64, 3422.
(3) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science
1995, 270, 1789.
(4) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.;
Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dosantos, D. A.; Bredas, J.
L.; Lodlund, M.; Salaneck, W. R. Nature 1999, 397, 121.
(5) Yu, G.; Heeger, A. J. J. Appl. Phys. 1995, 78, 4510.
(6) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.;
Frield, R. H.; Moratti S. C.; Holmes, A. B. Nature 1995, 376, 498.
(7) Yu, G.; Gao, J.; Yang, C. Y.; Heeger, A. J. In Proceedings of SPIE
Photodetectors: Materials and DeVice; Brown, G. J., Razeghi, M., Eds.;
SPIE-The International Society for Optical Engineering: Bellingham,
Washington, 1997; 2999, 306.
(8) Shaheen, S. E.; Brabec, C. J.; Padinger, F.; Fromherz, T.;
Hummelen, J. C.; Sariciftci, N. S. Appl. Phys. Lett. 2001, 78, 841.
(9) 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.
(10) Gebeyehu, D.; Brabec, C. J.; Padinger, F.; Fromherz, T.; Hummelen,
J. C.; Badt, D.; Schindler, H.; Sariciftci, N. S. Synth. Met. 2001, 118, 1.
(11) Hummelen, J. C.; Knight, B. W.; Peq F. L.; Wudl, F. J. Org. Chem.
1995, 60, 532.
(12) Wudl, F.; Khemani, K. C.; Harlev, E.; Ni, Z.; Srdanov, G. Polym.
Mater. Sci. Eng. 1991, 64, 201.
Acknowledgment. Financial support for this work from the
NSFC Project (#50028302) and MOST project (#2002CB613400)
is greatly appreciated. We are also grateful to Mr. Ning Bin
and Prof. Zhi-Xin Guo of Institute of Chemistry, Chinese
Academy of Science, for their help in CV measurement.
(13) Deng, X. Y.; Zheng, L. P.; Mo, Y. Q.; Yu, G.; Yang, W.; Weng,
W. H.; Cao, Y. Chin. J. Polym. Sci. 2001, 19, 567.
(14) Broems, P.; Birgersson, J.; Johansson, N.; Loegdlund, M.; Salaneck,
W. R. Synth. Met. 1995, 74, 1514.
(15) Brabec, C. J.; Shaheen, S. E.; Winder, C.; Sariciftci, N. S. Appl.
Phys. Lett. 2002, 80, 1288.
(16) Yu, G.; Srdanov, G.; Wang, H.; Cao, Y.; Heeger, A. J. Organic
Photovoltaics. In Proceedings of SPIE; SPIE-The International Society for
Optical Engineering: Bellingham, Washington, 2001; 4108, 48.
Supporting Information Available: Synthetic route, syn-
thesis of methanofullerene derivatives (3a-f) spectroscopic and