Functionalized methanofullerenes used as n-type materials in bulk-heterojunction polymer solar cells and in field-effect transistors

Article abstract of DOI:10.1021/ja710621j

The synthesis of two well-solubilized [60]methanofullerene derivatives (p-EHO-PCBM and p-EHO-PCBA) is presented for usage in organic solar cells and in field-effect transistors. The para position of the PCBM's phenyl ring was substituted with a branched alkoxy side chain, which contributes to higher solubility, facilitating synthesis, purification, and processing. We find a small change of the open-circuit voltage (V_oc) as a slight improvement in performance upon application in P3HT/[60]methanofullerene bulk-heterojunction-photovoltaic cells, when compared to PCBM, because of the electron donation of the alkoxy group. In the case of the devices with a TiO_x layer, the best power conversion efficiencies (PCE, η_e) is observed in a layered structure of P3HT/p-EHO-PCBA/ TiO_x (η_e = 2.6%), which slightly exceeds that of P3HT/PCBM/TiO_x (η_e = 2.3%) under conditions reported here. This can be attributed, in part, to the carboxylic acid group in p-EHO-PCBA that leads to an effective interface interaction between the active layer and TiO_x phase. In addition, n-channel organic field-effect transistor (OFET) devices were fabricated with thin films of p-EHO-PCBM and p-EHO-PCBA, respectively cast from solution on SiO₂/Si substrates. The values of field-effect mobility (μ) for p-EHO-PCBM and p-EHO-PCBA are 1 × 10^-2 and 1.6 × 10^-3 cm²/ V·s, respectively. The results in this paper demonstrate the effects of a carboxylic acid group and an electron-donating substituent in [60]methanofullerenes as n-type materials with respect to organic solar cells and OFET applications.

Full text of DOI:10.1021/ja710621j

Published on Web 04/16/2008
Functionalized Methanofullerenes Used as n-Type Materials in
Bulk-Heterojunction Polymer Solar Cells and in Field-Effect
Transistors
Changduk Yang,† Jin Young Kim,‡ Shinuk Cho,‡ Jae Kwan Lee,‡ Alan J. Heeger,‡
and Fred Wudl*,†,‡
Mitsubishi Chemical Center for AdVanced Materials and Department of Chemistry and
Biochemistry and Center for Polymers and Organic Solids, UniVersity of California,
Santa Barbara, California 93106
Received November 27, 2007; E-mail: wudl@chem.ucsb.edu
Abstract: The synthesis of two well-solubilized [60]methanofullerene derivatives (p-EHO-PCBM and p-EHO-
PCBA) is presented for usage in organic solar cells and in field-effect transistors. The para position of the
PCBM’s phenyl ring was substituted with a branched alkoxy side chain, which contributes to higher solubility,
facilitating synthesis, purification, and processing. We find a small change of the open-circuit voltage (Voc)
as a slight improvement in performance upon application in P3HT/[60]methanofullerene bulk-heterojunction-
photovoltaic cells, when compared to PCBM, because of the electron donation of the alkoxy group. In the
case of the devices with a TiOx layer, the best power conversion efficiencies (PCE, ηe) is observed in a
layered structure of P3HT/p-EHO-PCBA/TiOx (ηe ) 2.6%), which slightly exceeds that of P3HT/PCBM/
TiOx (ηe ) 2.3%) under conditions reported here. This can be attributed, in part, to the carboxylic acid
group in p-EHO-PCBA that leads to an effective interface interaction between the active layer and TiOx
phase. In addition, n-channel organic field-effect transistor (OFET) devices were fabricated with thin films
of p-EHO-PCBM and p-EHO-PCBA, respectively cast from solution on SiO2/Si substrates. The values of
field-effect mobility (µ) for p-EHO-PCBM and p-EHO-PCBA are 1 × 10-2 and 1.6 × 10-3 cm2/V·s,
respectively. The results in this paper demonstrate the effects of a carboxylic acid group and an electron-
donating substituent in [60]methanofullerenes as n-type materials with respect to organic solar cells and
OFET applications.
such as (6,6)-phenyl C61-butyric acid methyl ester (PCBM).5,6
Organic solar cells with power conversion efficiencies (PCE)
Introduction
The possibility to use new organic semiconductor materials
in place of silicon wafers in the fabrication of photovoltaic cells
offers the prospects of lower manufacturing costs (printing and
coating technologies using solution-processable materials),
lightweight solar modules, and flexible large-area applications.1
Thus, one of the most promising opportunities in [60]fullerene
research2 involves the potential application, when mixed with
π-conjugated polymers, in mimicking photosynthesis and in
related solar-energy conversion.3,4 The most widely used
configuration of polymer solar cells is the so-called bulk-
heterojunction device, in which the active layer consists of a
blend of electron-donating materials, for example, p-type
conjugated polymers, and an electron-accepting material (n-type)
approaching 5% under AM1.5G illumination have recently been
reported by using a regioregular poly(3-hexylthiophene) (rr-
P3HT)/PCBM blend7–9 and by introducing morphology-improv-
ing procedures on the composite film.10–13 Several factors are
important in further improving the efficiency of an organic solar
cell. Among these, the development of an electron-donating
polymer with low band gap to efficiently absorb light in the
visible area of the solar spectrum and structural manipulation
(5) Hummelen, J. C.; Knight, B. W.; LePeq, F.; Wudl, F.; Yao, J.; Wilkins,
C. L. J. Org. Chem. 1995, 60, 532.
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(7) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.;
Yang, Y. Nat. Mater. 2005, 4, 864.
(8) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. AdV. Funct. Mater.
2005, 15, 1617.
† Mitsubishi Chemical Center for Advanced Materials and Department
of Chemistry and Biochemistry.
(9) Reyes-Reyes, M.; Kim, K.; Carroll, D. L. Appl. Phys. Lett. 2005, 87,
083506.
‡ Center for Polymers and Organic Solids.
(10) Wienk, M. M.; Kroonm, 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.
(1) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. ReV. 2007, 107,
1324.
(2) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992,
258, 1474.
(11) Chirvase, D.; Chiguvare, Z.; Knipper, M.; Parisi, J.; Dyakonov, V.;
Hummelen, J. C. J. Appl. Phys. 2003, 93, 3376.
(12) Camaioni, N.; Ridolfi, G.; Casalbore-Miceli, G.; Possamai, A.;
Maggini, M. AdV. Mater. 2002, 14, 1735.
(3) Brabec, C. J.; Dyakonov, V.; Parisi, J.; Sariciftci, N. S. Organic
PhotoVoltaics. Concepts and Realizations; Springer: Berlin, 2003.
(4) Yu, L.; Gao, L.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science
1995, 270, 1789.
(13) Padinger, F.; Rittberger, R.; Sariciftci, N. S. AdV. Funct. Mater. 2003,
12, 85.
9
6444 J. AM. CHEM. SOC. 2008, 130, 6444–6450
10.1021/ja710621j CCC: $40.75
2008 American Chemical Society

Methanofullerenes in Polymer Solar Cells and FETs
A R T I C L E S
Figure 1. Structure of PCBM, p-EHO-PCBM, and p-EHO-PCBA.
of the acceptor component to optimize the solar cell performance
are important. Therefore, there is still plenty of room to tune
the efficiencies of bulk-heterojunction photovoltaic cells.
Not only have we previously developed the architecture for
high-efficiency polymer photovoltaic cells by using TiOx as an
optical spacer,14 but Fréchet et al.15 have also reported that
carboxylic acid functionalized polythiophene/TiO2 cell enables
improvement of the performance through the interface interac-
tion between the polymer and TiO2. More recently, it was noted
that placing electron-donating substituents on the phenyl ring
of PCBM can raise the lowest unoccupied molecular orbital
(LUMO) of PCBM, which allows further optimization of the
open-circuit voltage (Voc) of polymer/[60]fullerene organic solar
cells.16
In addressing these issues, we directed our attention toward
the synthesis of [60]methanofullerene analogues with the
following points in mind: (i) the introduction of a branched
alkoxy side chain as an electron-donating solubilizer on the
phenylene of PCBM (p-EHO-PCBM; this change was expected
not only to contribute to easier solution processability but also
to potentially increase the open-circuit voltage (Voc)) and (ii)
the availability in p-EHO-PCBA of a carboxylic acid group as
an important functionality for potential interaction with oxides.
This is important because it enables us to study the solar cell
performance with a comparison between a carboxylate ester and
a carboxylic acid in conventional and TiOx layer devices, which,
to the best of our knowledge, has not been investigated so far
(Figure 1).
benzoylbutyric acid (1) was prepared by AlCl3-promoted
Friedel-Crafts acylation of 2-ethylhexyloxy benzene19 with
glutaric anhydride (31%). The benzoylbutyric acid 1 was then
converted to the methyl 4-(2-ethyl-hexyloxy) benzoylbutyrate
(2) in methanol under Fischer esterification conditions (98%).
The synthesis of p-EHO-PCBM was carried out by the
Wudl-Hummelen approach.5 Condensation of 2 with p-
tosylhydrazide in methanol afforded the corresponding p-
tosylhydrazone compound (3), which after sequential treatment
with sodium methoxide in pyridine and [60]fullerene in refluxing
o-dichlorobenzene (o-DCB) gave fullerene derivative 4 in 34%
isolated yield. Although both [6,6]-closed and [5,6]-open isomers
could be expected from the cycloaddition reaction, the high
temperature at which the reaction proceeds allowed the isolation
of the thermodynamically more stable [6,6]-closed isomer,
exclusively, as determined by 13C NMR. The ester was hy-
drolyzed with hydrochloric acid/acetic acid to the corresponding
p-EHO-PCBA (96%). Both [60]methanofullerene analogues (p-
EHO-PCBM and p-EHO-PCBA) were readily soluble in
common organic solvents (THF, dichloromethane, toluene,
o-DCB, etc.).
Electrochemical Properties. The electrochemical properties
of p-EHO-PCBM and p-EHO-PCBA were studied by cyclic
voltammetry (CV) at room temperature in o-DCB solution. The
experiments were carried out by using tetra-n-butylammonium
perchlorate (Bu4NClO4, 0.1 M) as the supporting electrolyte, a
platinum working electrode, a platinum wire as the counter
electrode, and Fc/Fc+ as the internal reference.
Herein, we describe the synthesis and application in bulk-
heterojunction solar cells as well as field-effect transistors
(FETs)17,18 of the above-mentioned novel solution-processable
[60]methanofullerene analogues (p-EHO-PCBM and p-EHO-
PCBA) in combination with P3HT.
Not only does the cyclic voltammogram of p-EHO-PCBM
and p-EHO-PCBA show three reduction waves, but also all
reductions exhibit clear quasireversibility (see Supporting
Information). The CV data are summarized in Table 1, where
E1red, E2red, and E3 reduction potentials (defined as Ered
)
red
0.5(Epa + Epc)) are listed together with [60]fullerene and PCBM
data obtained in this study for comparison. The reduction
potentials of [60]methanofullerene analogues are shifted to more
negative values with respect to C60 because of the decrease of
the number of π-electrons and the release of strain energy.20,21
In the case of p-EHO-PCBM and p-EHO-PCBA, the
reduction values are slightly more negative than those of PCBM.
This could be attributed to the inductive effect of the alkoxy
group on the redox behavior. It was reported that, within similar
Results and Discussion
The synthetic approach to p-ethylhexyloxy-substituted
[60]methanofullerene derivatives (p-EHO-PCBM and p-EHO-
PCBA) is depicted in Scheme 1. The 4-(2-ethyl-hexyloxy)
(14) Kim, J. Y.; Kim, S. H.; Lee, H.-H.; Lee, K.; Ma, W.; Gong, X.; Heeger,
A. J. AdV. Mater. 2006, 18, 572.
(15) Liu, J.; Kadnikova, E. N.; Liu, Y.; McGehee, M. D.; Fréchet, M. J.
J. Am. Chem. Soc. 2004, 126, 9486.
(16) Kooistra, F. B.; Knol, J.; Kastenberg, F.; Popescu, L. M.; Verhees,
W. J. H.; Kroon, J. M.; Hummelen, J. C. Org. Lett. 2007, 9, 551.
(17) Nakamura, J.; Murata, K.; Takahashi, K. Appl. Phys. Lett. 2005, 87,
132105.
(19) Kim, S. W.; Shim, S. C.; Jung, B. J.; Shim, H. K. Polymer 2002, 43,
4297.
(18) Shibao, M.; Morita, T.; Takahashi, K.; Kaneto, K. Jpn. J. Appl. Phys.
2007, 46, L123.
(20) Haddon, R. C. Science 1993, 261, 545.
(21) Lamparth, I.; Hirsch, A. J. Chem. Soc., Chem. Commun. 1994, 1727.
9
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A R T I C L E S
Yang et al.
Scheme 1. Synthesis of [60]Methanofullerene Derivatives (p-EHO-PCBM and p-EHO-PCBA)
Table 1. Electrochemical Dataa
Polymer Solar Cells. The polymer solar cells had a layered
structure of glass/PEDOT:PSS/P3HT/[60]methanofullerene de-
rivatives (PCBM, p-EHO-PCBM, and p-EHO-PCBA) blend/
Al. The active layer was P3HT/[60]methanofullerenes respec-
tively at 1:1 weight ratio. The blends were spin-coated from a
solution containing the P3HT/[60]methanofullerenes in chlo-
roform onto an ITO/glass substrate covered by a PEDOT:PSS
(Baytron P) layer with a thickness of ∼100 nm that had
previously been coated from aqueous solution.23 In the case of
the device with a TiOx layer, the TiOx precursor solution was
applied in air on top of the P3HT/[60]methanofullerenes
composition. Subsequently, a thin layer of aluminum was
thermally deposited under vacuum. All data were obtained under
white light AM1.5G illumination from a calibrated solar
simulator with irradiation intensity of 100 mW/cm2.
The device performance of a solar cell is determined by the
open-circuit voltage (Voc), short-circuit current density (Jsc), and
fill factor (FF). The PCE (ηe) of a solar cell is given as ηe )
100(JscVocFF)/Pinc, where Pinc is the intensity of incident light.
Higher values of those three parameters yield larger light-to-
electricity PCE. The postproduction annealing at high temper-
ature (∼150 °C) was performed because it has been proven to
be a very efficient method for improving the electrical charac-
teristics of P3HT/PCBM-based solar cells.8,9,13
compound
E1
E2
E3
red
red
red
p-EHO-PCBM
p-EHO-PCBA
PCBM
-1.171
-1.181
-1.163
-1.077
-1.544
-1.565
-1.538
-1.456
-2.048
-2.087
-2.040
-1.914
C60
a Experimental conditions: values for 0.5(Epa + Epc) in V vs Fc/Fc+;
10-4 to 10-3 mol/L o-DCB solution; Bu4NClO4 (0.1 M) as supporting
electrolyte; Pt wire as counter electrode; 50 mV/s scan rate.
structure, the inductive effect by attached organic groups is the
most important factor to determine the redox properties of
organofullerene derivatives.22 Furthermore, it has recently been
proven that alkoxy groups on the phenyl ring of PCBM can
lead to a decrease of the reduction potential.16 A slightly
decreased reduction potential of p-EHO-PCBA versus p-EHO-
PCBM was also observed. As a result, it is expected that the
LUMO levels of p-EHO-PCBM and p-EHO-PCBA are
somewhat raised in comparison with the LUMO level of PCBM.
Film Morphology Study. The physical morphology of the thin
films of the three blends of P3HT/PCBM, P3HT/p-EHO-
PCBM, and P3HT/p-EHO-PCBA, observed by atomic force
microscopy (AFM), are shown in Figure 2. All three spin cast
films have smooth surfaces and remarkably similar features. The
formation of voids, typically attributed to incompatibility
between donor and acceptor components, is not observed. The
rms roughness is 10.4 nm for P3HT/PCBM, 17.4 nm for P3HT/
p-EHO-PCBM, and 13.0 nm for P3HT/p-EHO-PCBA. One
possible conclusion is that the bulky solubilizing groups in
p-EHO-PCBM and p-EHO-PCBA induce formation of a
slightly larger feature size, when compared to PCBM, however
without significant change in morphology.
Representative characteristics of the solar cells are listed in
Table 2. The current-voltage curves of the solar cells based
on the three blends P3HT/PCBM, P3HT/p-EHO-PCBM, and
P3HT/p-EHO-PCBA are shown in Figure 3. The cell based
on P3HT/PCBM serves as a standard reference for the com-
parison of the efficiencies with [60]methanofullerene analogues
and has a short-circuit current density (Jsc) of 6.98 mA/cm2, an
open-circuit voltage (Voc) of 0.60 V, and a fill factor (FF) of
(22) Suzuki, T.; Maruyama, Y.; Akasaka, T.; Ando, W.; Kobayashi, K.;
(23) Al-Ibrahim, M.; Ambacher, O.; Sensfuss, S.; Gobsch, G. Appl. Phys.
Lett. 2005, 86, 201120.
Nagase, S. J. Am. Chem. Soc. 1994, 116, 1359.
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Methanofullerenes in Polymer Solar Cells and FETs
A R T I C L E S
Figure 2. AFM topography images (5 µm × 5 µm) of films cast from chloroform solutions with (a) P3HT/PCBM, (b) P3HT/p-EHO-PCBM, and (c)
P3HT/p-EHO-PCBA.
Table 2. Characteristics of Bulk-Heterojunction Polymer Solar
Cellsa
ratio (1:1)
Jsc (mA/cm2)
Voc (V)
FF
ηe (%)
P3HT/PCBM
P3HT/p-EHO-PCBM
P3HT/p-EHO-PCBA
6.98
4.43
5.83
0.60
0.65
0.64
0.43
0.35
0.46
1.81
1.01
1.73
a Short-circuit current density (Jsc), open-circuit voltage (Voc), fill
factor (FF), PCE (ηe) at P3HT as the electron donor/various acceptors
(PCBM, p-EHO-PCBM, and p-EHO-PCBA) at weight ratio (1:1 w/w).
The device performance was consistent and reproducible (see Ex-
perimental Section).
Figure 4. J-V characteristics of polymer solar cells with the TiOx optical
spacer under AM1.5G illumination from a calibrated solar simulator with
an intensity of 100 mW/cm2. Inset is a schematic of device architecture.
the observed Voc.24 In the case of both P3HT/p-EHO-PCBM
and P3HT/p-EHO-PCBA, the significantly lower Jsc values lead
to an overall decrease in PCE (ηe). This may be due to one of
two factors: the blend layers are not uniform and changed into
an undesirable nanomorphology because of the branched
substituent, or the genetic character of alkoxy group causes a
negative effect on the device. This assumption is substantiated
by an AFM morphology study, in which the roughness of P3HT/
PCBM appears better and smoother than those of P3HT/p-EHO-
PCBM and P3HT/p-EHO-PCBA. Although the efficiencies of
both P3HT/p-EHO-PCBM and P3HT/p-EHO-PCBA still can
and need to be further improved with respect to high-end
applications, the preliminary results can be comparable with
that of the optimized high-performance polymer solar cell based
on P3HT/PCBM.
We have recently demonstrated that introducing a TiOx optical
spacer to polymer photovoltaic cells results in PCEs that are
increased by approximately 50%, compared to those of similar
devices fabricated without the optical spacer.14 To access optical
interference effect on P3HT/p-EHO-PCBM and P3HT/p-EHO-
PCBA, the devices with TiOx as a spacer between the active
layer and the Al electrode were fabricated (Figure 4), as
described in the Experimental Section.
Figure 3. J-Vcharacteristics of polymer solar cells under AM1.5G
illumination from a calibrated solar simulator with an intensity of 100 mW/
cm2. Inset is a schematic of device architecture.
43%. Under the same white light illumination, the P3HT/p-
EHO-PCBM-based cell exhibits a Jsc of 4.43 mA/cm2, an open-
circuit voltage (Voc) of 0.65 V, and a fill factor (FF) of 35%. Its
PCE (ηe) decreases to 0.80% in comparison with that of P3HT/
PCBM, used as a reference. In the case of the solar cell made
from P3HT/p-EHO-PCBA, the PCE (ηe) increases to 0.73%,
a substantial improvement when compared to that of P3HT/p-
EHO-PCBM but still slightly lower than that of P3HT/PCBM.
As expected, the Voc values of both P3HT/p-EHO-PCBM
and P3HT/p-EHO-PCBA are slightly improved in comparison
with that of P3HT/PCBM because of the lower first potential
reductions. This notion is supported by the linear correlation
of the first reduction potential of [60]fullerene acceptors and
(24) Brabec, C. J.; Cravino, A.; Meissner, D.; Sariciftci, N. S.; Fromherz,
T.; Minse, M.; Sanchez, L.; Hummelen, J. C. AdV. Funct. Mater. 2001,
11, 374.
9
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A R T I C L E S
Yang et al.
Table 3. Characteristics of Bulk-Heterojunction Polymer Solar
PCBA, p-EHO-PCBM, and PCBM, respectively. Such dete-
rioration of the electron mobility in both p-EHO-PCBM and
p-EHO-PCBA OFETs can be largely attributed to the bulkiness
of the substituent that can suppress desirable ordering such as
π-π intermolecular overlaps between C60 moieties in the solid
state. Among them, the lowest mobility value is observed in
the p-EHO-PCBA OFET. This result currently lacks a theoreti-
cal explanation but can be due to the nature of the carboxylic
acid group as an electron-trapping site. Further studies are
required to assess more precisely the correlation between the
size of substituents and/or the various functional groups and
the mobility of an OFET. However, these mobilities are
comparable to the typical values of the fullerene-based OFETs
fabricated by vapor deposition under ultrahigh vacuum (µ )
8.0 × 10-2 and 2.0 × 10-3 cm2/V·s for C60 and C70,
respectively).27,28 In the case reported here, the soluble [60]meth-
anofullerene-based devices were fabricated by spin coating,
which is easier and more cost-effective compared with vacuum
deposition.
Cells with the TiOx Optical Spacera
ratio (1:1)
Jsc (mA/cm2)
Voc (V)
FF
ηe (%)
P3HT/PCBM/TiOx
P3HT/p-EHO-PCBM/TiOx
P3HT/p-EHO-PCBA/TiOx
7.64
7.36
7.69
0.60
0.66
0.64
0.50
0.48
0.53
2.29
2.34
2.64
a Short-circuit current density (Jsc), open-circuit voltage (Voc), fill
factor (FF), PCE (ηe) at P3HT as the electron donor/various acceptors
(PCBM, p-EHO-PCBM, and p-EHO-PCBA) at weight ratio (1:1 w/w).
The device performance was consistent and reproducible (see Ex-
perimental Section).
The J-V curves for devices with TiOx layer obtained under
white light illumination (AM1.5 G, 100 mW/cm) are shown in
Figure 4, indicating a considerably improved performance. The
corresponding PCEs (ηe) are 2.29, 2.34, and 2.64% for P3HT/
PCBM, P3HT/p-EHO-PCBM, and P3HT/p-EHO-PCBA, re-
spectively (Table 3). The TiOx optical spacer leads to the overall
increase in PCE (ηe) by creating more photogenerated charge
carriers in the bulk-heterojunction layer, thereby increasing Jsc.
Surprisingly, the TiOx layer is more effective in enhancing the
performance of the new [60]methanofullerene derivatives (p-
EHO-PCBM and p-EHO-PCBA) in comparison to P3HT/
PCBM.
The best performance is observed in the P3HT/p-EHO-
PCBA/TiOx heterojunction solar cell. This positive effect in
photocurrent may be attributed to a combination of the following
factors. The possibility of the chelation of carboxylic acids
(-COOH) in p-EHO-PCBA to the TiOx phase can contribute
to this enhancement by promoting forward interfacial electron
transfer, much in the same manner as in the Grätzel-type solar
cells.25,26 In addition, we cannot rule out that the branched
alkoxy chain on the phenyl ring can produce an improved
interface morphology between the active layer and TiOx phases,
which probably leads to the higher charge carrier mobility within
the devices with TiOx. We are currently studying the trend in
device efficiency for [60]fullerene derivatives with various
substitution patterns, in which energy effects and interaction
effects can be separated, to help us better understand our
observation.
n-Type Organic Field-Effect Transistors. To explore the
electron transport properties of p-EHO-PCBM and p-EHO-
PCBA, field-effect mobilities were investigated through the
fabrication of n-type organic field-effect transistors (OFETs).
All n-type OFETs were prepared on a heavily doped n++ Si
wafer with a 200 nm thick thermally grown SiO2 layer with
top contact geometry as shown in Figure 5a. Ca was chosen as
a metal for the source and drain electrodes to enable easy
injection of electrons, because there is no energy barrier between
the Ca electrode (the workfunction of Ca is 2.87 eV) and the
active layer. A detailed fabrication process is described in the
Experimental Section.
Figure 5b shows the transport characteristics of the n-type
OFETs fabricated with p-EHO-PCBM and p-EHO-PCBA,
compared to the n-type PCBM OFET. All Ids-Vgs curves are
typical of good n-type OFETs, owing to the nature of
[60]fullerene. The linear plot of I1/2 versus Vgs (Figure 5c),
deduced from the measurements of the Ids versus Vgs, gave
electron mobilities of µ1 ) 1.59 × 10-3 cm2/V·s, µ2 ) 1.04 ×
10-2 cm2/V·s, and µ3 ) 2.85 × 10-2 cm2/V·s for the p-EHO-
Conclusion
We have successfully synthesized a series of [60]methanof-
ullerene analogues (p-EHO-PCBM and p-EHO-PCBA). The
electrochemical studies show that both p-EHO-PCBM and
p-EHO-PCBA undergo quasireversible processes. The intro-
duction of a branched alkoxy substituent at the phenyl ring of
[60]methanofullerenes not only renders these materials soluble
and easily processable but also allows to access the influence
of the carboxylic acid group in comparison with the methyl ester
group in photovoltaic cells. Moreover, the alkoxy electron-
donating ability leads to a slightly decreased reduction potential
when compared to PCBM, resulting in somewhat improved Voc
values in the solar cell devices. By utilizing a TiOx layer, the
devices of P3HT/various acceptors (PCBM, p-EHO-PCBM,
and p-EHO-PCBA) were also fabricated, resulting in significant
enhancement of device performance. The corresponding PCEs
(ηe) are 2.29% for PCBM, 2.34% for p-EHO-PCBM, and
2.64% for p-EHO-PCBA. The best solar cell obtained has a
layered configuration of P3HT/p-EHO-PCBA/TiOx, which can
be attributed to the interface interaction between the active layer
and the TiOx phase. In addition, we cannot exclude that the
branched solubilizing groups could tune the morphology of
the active layers/TiOx phase that can be responsible for the
improvement of PCE (ηe).
In conclusion, through the structural manipulation of active
layer materials, the interface interaction between active layer
and TiOx can be influenced, which can potentially enhance the
solar cell performance. To further improve the device perfor-
mance for such a bulk-heterojunction system, it is important to
synthesize an acceptor that has high carrier mobility and good
compatibility with the donor-type conjugated polymer simul-
taneously. The study of MEH-PPV/p-EHO-PCBM and p-EHO-
PCBA as the acceptor is underway to further clarify the positive
effect and the trend in both conventional and TiOx-coated
devices. As a separate work, n-type OFETs with p-EHO-PCBM
and p-EHO-PCBA manufactured from solution have also been
characterized. In a top-contact geometry, electron mobilities in
the saturation regime were found to be µ ) 1.04 × 10-2 cm2/
V · s for p-EHO-PCBM and µ ) 1.59 × 10-3 cm2/V · s for
(25) Grätzel, M. Inorg. Chem. 2005, 44, 6841.
(26) Moser, J.; Punchihewa, S.; Infelta, P. P.; Grätzel, M. Langmuir 1991,
7, 3012.
(27) Haddon, R. C. J. Am. Chem. Soc. 1996, 118, 3041.
(28) Haddon, R. C.; Perel, A. S.; Morris, R. C.; Palstra, T. T. M.; Hebard,
A. F.; Fleming, R. M. Appl. Phys. Lett. 1995, 67, 121.
9
6448 J. AM. CHEM. SOC. VOL. 130, NO. 20, 2008

Methanofullerenes in Polymer Solar Cells and FETs
A R T I C L E S
Figure 5. (a) Diagram of n-type OFET structure (L ) 50 µm, W ) 1.5 mm). (b) Transfer characteristics of n-type OFETs. (c) I1/2 versus Vgs plot. 2, PCBM,
b, p-EHO-PCBM, 9, p-EHO-PCBA.
with nitrogen gas. After annealing, the devices were put on a metal
plate and cooled to room temperature before the measurements were
carried out.
p-EHO-PCBA. Even though the OFET parameters are related
to many factors, the present results indicate that the size and
functionality of substituents could eventually lead to high-
mobility fullerene derivatives.
For calibration of the solar simulator, we first carefully minimized
the mismatch of the spectrum (the simulating spectrum) obtained
from the xenon lamp (300 W Oriel) and the solar spectrum by using
an AM1.5G filter. We then calibrated the light intensity by using
calibrated standard silicon solar cells with a proactive window made
from KG5 filter glass traced to the National Renewable Energy
Laboratory. Measurements were done with the solar cells inside
the glovebox by using a high-quality optical fiber to guide the light
from a solar simulator outside the glovebox. Current density-voltage
curves were measured with a Keithley 236 source measurement
unit. The characteristics of the solar cells were optimized by testing
approximately 100 cells with each [60]methanofullerene derivative.
To probe the reproducibility and the trend of the devices with each
compound, all the data were reconfirmed by the optimal 10 devices
and presented in Tables 2 and 3.
OFET Device Preparation and Measurement. All n-type
OFETs were fabricated on heavily doped n-type Si wafers, each
covered with a thermally grown silicon oxide (SiO2) layer with a
thickness of 200 nm. The doped Si wafer acts as the gate electrode,
and the SiO2 layer functions as the gate insulator. The active layer
was deposited by spin-coating at 2500 rpm. All solutions were
prepared at 1 wt% concentration in chlorobenzene. The thickness
of the deposited films was about 60 nm. Prior to deposition of source
and drain electrodes, the films were dried on hot a plate stabilized
at 80 °C for 30 min. All fabrication processes were carried out in
the glovebox filled with nitrogen. Source and drain electrodes using
Ca were deposited by thermal evaporation by using a shadow mask.
The thickness of the source and drain electrodes was 50 nm.
Channel length (L) and channel width (W) were 50 µm and 1.5
mm, respectively. All OFET devices were made in the top-contact
geometry as shown in Figure 5a. Electrical characterization was
performed by using a Keithley semiconductor parametric analyzer
(Keithley 4200) under nitrogen atmosphere. The electron mobility
(µ) was determined by using the following equation in the saturation
regime:
Experimental Section
General. All solvents were purified and freshly distilled prior
to use according to literature procedures. The synthesis of 2-eth-
ylhexyloxy benzene was adapted from literature procedures.19
Commercially available materials were used as received unless
noted. 1H and 13C NMR spectra were recorded on a Varian Mercury
Vx 200 MHz or Varian Unity Inova 500 MHz spectrometer and
referenced to the solvent peak. Mass spectrometry and elemental
analysis were performed by UC Santa Barbara Mass Spectrometry
Laboratory and Elemental Analysis Center. The electrochemical
measurements were carried out in o-DCB solution containing 0.1 M
Bu4NClO4 as the supporting electrolyte, with Ag/AgCl as reference
electrode and a platinum wire as a counter electrode and an internal
ferrocene/ferrocenium standard. Anhydrous o-DCB was obtained
from Aldrich and used as received. The AFM instrumentation
consisted of a Nanoscope III BioScope system (Digital Instruments,
Veeco Metrology Group, Santa Barbara, CA) and standard silicon
tips (type OTESPA-70; L, 160 µm; normal spring constant, 50 N/m;
resonance frequency, 246-282 kHz).
Solar Cell Device Fabrication and Calibration Procedure.
Polymer solar cells were fabricated by using P3HT as the electron
donor and [60]methanofullerene derivatives as the acceptor. The
ITO-coated glass substrates were cleaned subsequently in an
ultrasonic bath with detergent, distilled water, acetone, and isopropyl
alcohol and then dried in an oven at ∼100 °C overnight. Highly
conducting PEDOT:PSS was spin-cast (5000 rpm) with a thickness
of ∼40 nm from aqueous solution after treatment of the ITO
substrates with UV ozone for 40 min. The substrates were dried at
140 °C for 10 min in air and transferred to a nitrogen-filled glovebox
for spin-casting the P3HT/[60]methanofullerene layer. The chlo-
roform solution, comprising P3HT (1.0 wt%) and [60]methanof-
ullerene derivatives (1.0 wt %), was spin-cast at 1200 rpm on top
of the PEDOT:PSS layer. The thickness of the active layer was
∼100 nm. Then, the TiOx (∼30 nm) was spin-cast (4000 rpm) on
top of the P3HT/[60]methanofullerene composite from the 1 wt%
precursor solution and heated at 80 °C for 10 min in air.
Subsequently, the device was pumped down in vacuum (<10-6
Torr), and the Al electrode (thickness ∼100 nm) was deposited.
The area of the Al electrode defines the active area of the device
as 4.5 mm2. Thermal annealing was carried out by directly placing
the completed devices on a hot plate at 150 °C in a glovebox filled
Ids ) (WCi ⁄ 2L)µ(Vgs - VT)2
where Ci is the capacitance per unit area of the SiO2 dielectric (Ci
) 15 nF/cm2),and VT is the threshold voltage.
Synthesis of 4-(2-Ethyl-hexyloxy) Benzoylbutyric Acid (1).
Aluminum chloride (41.3 g, 310 mmol) was added in one portion
to glutaric anhydride (27.8 g, 232 mmol) and 2-ethylhexyloxy
benzene (32 g, 155.3 mmol) in 250 mL of dichloromethane with
cooling in an ice-ethanol bath and slowly allowed to warm to room
9
J. AM. CHEM. SOC. VOL. 130, NO. 20, 2008 6449

A R T I C L E S
Yang et al.
temperature. The reaction was stirred overnight and then quenched
with 2 M HCl; the mixture was then extracted into dichloromethane,
washed with brine, and dried. The crude product was chromato-
graphed on silica by using 0-30% ethyl acetate in hexane with
1% acetic acid as eluent and further purified by recrystallization
and the reaction was allowed to continue overnight again. The
resulting mixture was concentrated in vacuo to 100 mL and pre-
eluted with o-DCB (100 mL) and chlorobenzene (200 mL) and
then toluene on SiO2/toluene, 40 × 10 cm. The first fraction,
containing unreacted C60, was collected. After an intermediate
fraction, the fraction containing 4-(2-ethylhexyloxy)-[6,6]-phenyl
C61-butyric acid methyl ester (p-EHO-PCBM) was collected. The
solution was concentrated in vacuo, redissolved in a minimal
amount of toluene, and transferred to a centrifuge tube. The product
was precipitated with MeOH, centrifuged, and decanted. The
product was treated with MeOH several times in the same manner
and washed with a mixture solution (MeOH:diethylether, 1:1% v/v).
Isolated yield ) 3.1 g (34%) as a shiny dark solid. 1H NMR (CDCl3,
500 MHz): δ ppm 7.79 (d, J ) 8.68 Hz, 2H), 7.03 (d, J ) 8.68
Hz, 2H), 3.92 (d, J ) 5.53 Hz, 2H), 3.67 (s, 3H), 2.86 (m, 2H),
2.52 (t, J ) 7.56 Hz, 2H), 2.17 (m, 2H), 1.76 (m, 1H), 1.61-1.30
(m, 8H), 0.97-0.87 (m, 6H). 13C NMR (CDCl3, 125.70 MHz): δ
173.43, 158.93, 148.83, 147.89, 145.77, 145.06, 145.00, 144.90,
144.66, 144.59, 144.53, 144.35, 144.27, 143.87, 143.64, 142.91,
142.86, 142.78, 142.13, 141.99, 140.82, 140.59, 137.87, 137.58,
132.97, 128.05, 114.21, 80.13, 70.42, 51.58, 51.28, 39.38, 33.81,
33.65, 30.48, 29.06, 23.79, 22.99, 14.05, 11.12. FABMS (NBA)
m/z: 1040 ((M + H)+•). Elemental analysis. Calcd for C80H30O3:
C, 92.47; H, 2.91; O, 4.62. Found: C, 92.53; H, 2.74.
1
from hexane. Isolated yield ) 15.5 g (31%) as white crystals. H
NMR (CDCl3, 200 MHz): δ ppm 7.94 (d, J ) 8.85 Hz, 2H), 6.93
(d, J ) 8.85 Hz, 2H), 3.91 (d, J ) 5.68 Hz, 1H), 3.03 (t, J ) 7.16
Hz, 2H), 2.51 (t, J ) 7.08 Hz, 2H), 2.08 (q, J ) 6.88 Hz, 2H),
1.73 (m, 1H), 1.55-1.29 (m, 8H), 0.97-0.81 (m, 6H). 13C NMR
(CDCl3, 50.28 MHz): δ 197.79, 179.12, 163.18, 130.07, 129.29,
113.99, 70.49, 39.04, 36.72, 32.95, 30.21, 28.81, 23.57, 22.79,
19.03, 13.86, 10.87. HRMS (EI) m/z: 320 (M+•). Elemental analysis.
Calcd for C19H28O4: C, 71.22; H, 8.81; O, 19.97. Found: C, 71.49;
H, 8.71.
Synthesis of Methyl 4-(2-Ethyl-hexyloxy) Benzoylbutyrate
(2). A solution of the acid compound 1 (4.0 g, 12.5 mmol) in MeOH
(100 mL) with catalytic amount of sulfuric acid (96%, five drops)
was refluxed for 5 h. The solvent was evaporated, and the residue
was extracted into dichloromethane, washed with brine, and dried.
The crude product was chromatographed on silica by using 0-2%
ethyl acetate in hexane as eluent. Isolated yield ) 4.1 g (98%) as
a light yellow liquid. 1H NMR (CDCl3, 200 MHz): δ ppm 7.92 (d,
J ) 8.95 Hz, 2H), 6.91 (d, J ) 8.95 Hz, 2H), 3.89 (d, J ) 5.68
Hz, 2H), 3.67 (s, 3H), 2.98 (t, J ) 7.17 Hz, 2H), 2.43 (t, J ) 7.17
Hz, 2H), 2.05 (p, J ) 6.90 Hz, 2H), 1.72 (m, 1H), 1.51-1.27 (m,
8H), 0.97-0.83 (m, 6H). 13C NMR (CDCl3, 50.28 MHz): δ 197.30,
173.12, 162.65, 129.59, 128.92, 113.51, 70.01, 50.89, 38.57, 36.40,
32.54, 29.77, 28.37, 23.11, 22.34, 18.89, 13.42, 10.42. HRMS (EI)
m/z: 334 (M+•). Elemental analysis. Calcd for C20H30O4: C, 71.82;
H, 9.04; O, 19.14. Found: C, 71.73; H, 8.72.
Synthesis of 4-(2-Ethylhexyloxy)-[6,6]-phenyl C61-Butyric
Acid Methyl Ester (p-EHO-PCBA). A sample of p-EHO-PCBM
(1.5 g, 1.44 mmol) was dissolved in chlorobenzene, and to this
solution were added acetic acid (100 mL) and concentrated aqueous
HCl (40 mL). The mixture was heated to reflux overnight. The
solvent was removed in vacuo, and the residue was suspended in
methanol. The precipitate was collected by filtration, washed three
times with methanol and twice with a mixture solution (MeOH:
diethylether, 1:1% v/v), and dried. Isolated yield ) 1.42 g (96%)
Synthesis of Methyl 4-(2-Ethyl-hexyloxy) Benzoylbutyrate
p-Tosylhydrazone (3). A mixture of the methyl ester 2 (3.1 g, 9.26
mmol), p-toluene-sulfonyl hydrazide (2.07 g, 11.12 mmol), and
MeOH (35 mL) with catalytic amount of HCl was stirred and
refluxed for 7 h. The mixture was then extracted into dichlo-
romethane, washed with brine, and dried. The crude product was
chromatographed on silica by using 0-2% acetone in dichlo-
1
as a brown solid. H NMR (CDCl3, 500 MHz): δ ppm 7.79 (d, J
) 8.50 Hz, 2H), 7.03 (d, J ) 8.48 Hz, 2H), 3.91 (d, J ) 5.70 Hz,
2H), 2.95-2.79 (m, 2H), 2.55 (t, J ) 7.31, 2H), 2.19 (m, 2H),
1.77 (m, 1H), 1.60-1.28 (m, 8H), 0.97-0.85 (m, 6H). 13C NMR
(CDCl3, 125.70 MHz): δ 178.61, 159.05, 148.86, 147.90, 145.87,
145.14, 145.11, 144.78, 144.72, 144.69, 144.62, 144.45, 144.37,
143.98, 143.72, 143.06, 142.99, 142.95, 142.89, 142.23, 142.19,
142.10, 140.93, 140.68, 138.00, 137.69, 133.03, 128.10, 114.34,
80.18 (bridge), 70.56, 51.29, 39.48, 33.67, 33.62, 30.57, 29.15,
23.89, 23.08, 22.21, 14.15, 11.12. FABMS (NBA) m/z: 1025 ((M
+ H)+•). Elemental analysis. Calcd for C79H28O3: C, 92.56; H, 2.75;
O, 4.68. Found: C, 92.88; H, 3.18.
1
romethane. Isolated yield ) 4.6 g (98%) as a light yellow oil. H
NMR (CDCl3, 200 MHz): δ ppm 9.02 (s, 1H), 7.92 (d, J ) 8.27
Hz, 2H), 7.60 (d, J ) 8.89 Hz, 2H), 7.29 (d, J ) 9.15 Hz, 2H),
6.85 (d, J ) 8.89 Hz, 2H), 3.84 (d, J ) 5.67 Hz, 2H), 3.79 (s, 3H),
2.73-2.50 (m, 2H), 2.41 (s, 3H), 2.32 (m, 2H), 1.81-1.62 (m,
3H), 1.52-1.27 (m, 8H), 0.97-0.87 (m, 6H). 13C NMR (CDCl3,
50.28 MHz): δ 174.22, 160.12, 153.26, 143.15, 135.56, 128.97,
127.88, 127.47, 127.15, 113.84, 70.04, 51.89, 38.81, 31.64, 29.99,
28.57, 27.25, 23.33, 22.57, 21.12, 20.58, 13.62, 10.61. HRMS (EI)
m/z: 503.25 ((M + H)+•). Elemental analysis. Calcd for
C27H38N2O5S: C, 64.51; H, 7.62; N, 5.57; O, 15.91; S, 6.38. Found:
C, 64.52; H, 7.52; N, 5.60.
Acknowledgment. Synthesis and characterization was supported
by MC-CAM. Device fabrication and measurement was supported
by the Air Force Office of Scientific Research (Charles Lee,
Program Officer) and by the U.S. Department of Energy (A. Kini,
Program Officer).
Synthesis of 4-(2-Ethylhexyloxy)-[6,6]-phenyl C61-Butyric
Acid Methyl Ester (p-EHO-PCBM). A mixture of alkoxy-
substitued benzoyl p-tosylhydrazone (4.31 g, 8.63 mmol), sodium
methoxide (0.485 g, 8.99 mmol), and dry pyridine (50 mL) was
placed under argon and stirred at room temperature for 30 min. To
the mixture, a solution of C60 (5 g, 6.95 mmol) in o-DCB (250
mL) was added, and the homogeneous reaction mixture was stirred
at 70 °C under argon overnight. The solution was heated to reflux,
Supporting Information Available: 1H and 13C NMR spectra
for all new compounds and cyclic voltammograms of C60,
PCBM, p-EHO-PCBM, and p-EHO-PCBA. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA710621J
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6450 J. AM. CHEM. SOC. VOL. 130, NO. 20, 2008