Design of fulleropyrrolidine derivatives as an acceptor molecule in a thin layer organic solar cell

Article abstract of DOI:10.1039/c0jm01565b

A systematic study on the design of fulleropyrrolidine derivatives as the acceptor of photovoltaic cells has been carried out using poly(3-hexylthiophene) (P3HT) as the model base polymer. It was found that N-methoxyethoxyethyl-2-(2- methoxyphenyl)fulleropyrrolidine worked as a good acceptor partner with P3HT and a high power conversion efficiency (PCE) (3.44%) was obtained; this is superior to that of the P3HT polymer including methyl [6,6]-phenyl-C61-butylate ([C60]-PCBM) under the same experimental conditions.

Full text of DOI:10.1039/c0jm01565b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Design of fulleropyrrolidine derivatives as an acceptor molecule in a thin layer
organic solar cell†
Kei Matsumoto,a Kohji Hashimoto,a Masaya Kamo,a Yasunori Uetani,b Shuichi Hayase,a Motoi Kawatsuraa
a
*
and Toshiyuki Itoh
Received 22nd May 2010, Accepted 23rd August 2010
DOI: 10.1039/c0jm01565b
A systematic study on the design of fulleropyrrolidine derivatives as the acceptor of photovoltaic cells
has been carried out using poly(3-hexylthiophene) (P3HT) as the model base polymer. It was found that
N-methoxyethoxyethyl-2-(2-methoxyphenyl)fulleropyrrolidine worked as a good acceptor partner with
P3HT and a high power conversion efficiency (PCE) (3.44%) was obtained; this is superior to that of the
P3HT polymer including methyl [6,6]-phenyl-C61-butylate ([C60]-PCBM) under the same
experimental conditions.
acceptor source of polymer solar cells and reported that PCE
Introduction
values of the solar cells remained at an insufficient level.16–23
However, we recognized that no systematic study had been carried
out on the design of fulleropyrrolidines as an acceptor partner in
polymer solar cells to date.
Extensive studies have been made regarding the applications of
fullerene derivatives in the field of material sciences.1 Among
such studies, the development of fullerene-based polymer solar
cells has attracted significant interest in recent years, because
fullerenes have become recognized as excellent electron acceptors
due to their unique p-electron system, excited state electronic
properties, and absorption spectra extending into most of the
visible region.2,3 It is well known that the fullerene generally has
a poor solubility in conventional organic solvents and this makes it
difficult to prepare fullerene-based electric devices using econom-
ical techniques such as the spin-coating procedure. Therefore, the
development of stable fullerene derivatives that show a high power
conversion efficiency (PCE) with a sufficient affinity toward or-
ganic solvents is desired. Methyl [6,6]-phenyl-C61-butylate ([C60]-
PCBM)4 is known to be the best blending material among the
fullerene derivatives as the acceptor with polythiophene which
is a donor partner in polymer solar cells (Fig. 1).5–15 Hummelen
and co-workers synthesised [C60]-PCBM derivatives substituting
electron-donating or electron-withdrawing groups on the phenyl
ring and revealed that the substituent influenced the LUMO level
of the parent fullerene; they found that differences of the LUMO
level due to the substituent were small but caused a significant
change of the open circuit voltage of the resulting solar cell.14
Troshin, Hoppe, and co-workers also synthesized various types of
methanofullerene derivatives and established that [C60]-PCBM
was the best acceptor towards P3HT polymer.15
We recently reported the design of fulleropyrrolidine–imida-
zolium hybrids that have a high solubility in various types of
organic solvents and even in water.24 Since the key intermediate
of our fulleropyrrolidine–imidazolium hybrid was N-methoxy-
ethoxyethyl-2-(4-(1H-imidazol-1-yl)phenyl)fulleropyrrolidine
(FP-PhIm)24 as shown in Fig. 1, we had a large quantity of this
compound on hand. Therefore, we attempted to fabricate
a polymer solar cell using FP-PhIm as the acceptor molecule
by blending in a thin film composed of poly-3-hexylthiophene
(P3HT). Fabrication of the film using the spin-coating method
was very easily accomplished due to the highly soluble nature
of the FP-PhIm in organic solvents, however, the cell showed
no characteristics as a solar cell. Since it was anticipated that
the imidazole moiety installed at the para-position of the
benzene ring might inhibit the smooth electron transfer
between the fullerene molecules, we next prepared the P3HT
We were fascinated by the fulleropyrrolidines from the stand-
point of their stable nature under atmospheric conditions and ease
of producing various types of analogues.1a Several examples have
been reported concerning the use of fulleropyrrolidines as the
aDepartment of Chemistry and Biotechnology, Graduate School of
Engineering, Tottori University, 4-101 Koyama-minami, Tottori, 680-8552,
Japan. E-mail: mailto:titoh@chem.tottori-u.ac.jp; titoh@chem.tottori-u.ac.
jp; Fax: +81 857 31 5259; Tel: +81 857 31 5259
bSumitomo Chemical Co., Ltd., 6 Kitahara, Tsukuba, 300-3294, Japan;
Fax: +81 29 864 4748; Tel: +81 29 864 4305
† Electronic supplementary information (ESI) available: Synthetic details
of fulleropyrrolidine derivatives 1b–1z and results of DFT calculation of
1a, 1p, 1q, 1r, 1x, 1y and 2. See DOI: 10.1039/c0jm01565b
Fig. 1 Fullerene derivatives as the acceptor molecule blended in the
polymer of poly-3-hexylthiophene (P3HT) for polymer solar cell.
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polymer that included the simple N-methoxyethoxyethyl-2-
phenylfulleropyrrolidine (1a) as an acceptor partner (Fig. 1).
To our satisfaction, it was found that the polymer did, in fact,
work as a solar cell. Although the solar cell performance of the
polymer using 1a was less than 3%, the result strongly
encouraged us to investigate the possibility of full-
eropyrrolidine derivatives as an acceptor partner with P3HT
for the polymer solar cell. We now report that several P3TH
polymers blended with a fulleropyrrolidine as acceptor func-
tion as effective solar cells that show high PCE values which
are superior to those of the combination of [C60]-PCBM and
P3HT polymer under the same experimental conditions.
(1H, m), 7.94 (1H, d, J ¼ 7.8 Hz); 13C NMR (125 MHz, ppm,
CDCl3) d 52.19, 54.71, 58.77, 67.45, 69.17, 70.49, 70.57, 71.96,
73.92, 75.24, 76.75, 110.67, 121.10, 125.42, 128.80, 129.74, 134.32,
135.79, 136.21, 136.31, 139.09, 139.17, 139.86, 139.96, 141.27,
141.42, 141.52, 141.69, 141.78, 141.88, 141.96, 142.00, 142.05,
142.27, 142.33, 142.36, 142.71, 142.76, 144.08, 144.13, 144.29,
144.77, 144.94, 144.97, 145.03, 145.28, 145.44, 145.59, 145.74,
145.81, 145.86, 145.90, 145.94, 146.25, 146.47, 146.96, 153.86,
153.91, 154.76, 156.72, 157.79; IR (Neat, cmꢀ1) 2864, 2827, 1489,
1460, 1425, 1284, 1244, 1179, 1109, 1048, 1026, 754, 729, 527;
MALDI-TOF-MS (matrix: SA) found 971.1526 (calcd for
C74H21NO3, exact mass: 971.1521%). For the synthesis of the
fulleropyrrolidine derivatives 1b–1z, see the ESI†.
Experimental
Computational methodologies
General
The DFT calculations of the fulleropyrrolidine derivatives, 1a,
1p, 1q, 1r, 1x, and 2, were performed at the B3LYP/6-31G* level
of theory. All of the calculations were carried out using the
Gaussian 03 suite of programs.26
Photovoltaic devices were prepared by spin-coating the fullero-
pyrrolidine-polymer blends from chlorobenzene onto an indium
tin oxide (ITO) glass electrode as follows: to a P3HT (1.0 wt%)
solution of chlorobenzene were added fulleropyrrolidine 1 (equal
weight vs. P3HT) and silica gel (1.0 wt% vs. P3HT solution), then
the mixture was stirred for 12 h at ambient temperature. It was
then filtered through a Teflon (0.2 mm) filter. The resulting solu-
tion was applied to the surface of an ITO plate by the spin-coating
method at a thickness of ca. 100 nm and the surface was washed
with acetone and irradiated under UV light and ozone gas for
20 min to decompose the impurities. After drying under vacuum
for 20 min, the resulting plate was placed in a vacuum chamber
and the surface was coated with the electrode layers of lithium
fluoride (LiF) (4 nm) and aluminium (100 nm) by evaporation at
10ꢀ4 Pa at rt. We placed the glass plate on the resulting film and
these plates were firmly fixed using a bonding agent under an
argon atmosphere to produce the solar cell. The PCE values were
obtained using the solar simulator OTENTO-SUN II (AM1.5G,
100 mW cmꢀ2).
Results and discussion
We focused our investigation on the characterization of the
polymers that included the N-methoxyethoxyethyl-2-(2-aryl)-
fulleropyrrolidine derivatives as the acceptor partner with P3HT,
because these types of compounds have a good affinity toward
various types of organic solvents. Fulleropyrrolidine derivatives
were prepared following the method developed by Prato and
Maggini.1a N-Methoxyethoxyethyl glycine24,25 was treated with
the C60 fullerene in the presence of an aldehyde in chlorobenzene
and the mixture was heated at 130 ꢁC for 3 h. Purification of the
desired compounds was easily accomplished by silica-gel flash
column chromatography or silica-gel thin layer chromatography;
the desired mono adduct 1 was thus obtained in acceptable
yields from 40% to 78%. The photovoltaic cells were fabricated
by spin-coating the blended film from a dichlorobenzene solu-
tion of the fulleropyrrolidine/polymer and the PCE values were
obtained using a solar simulator (AM1.5G, 100 mW cmꢀ2). The
obtained characteristic current–voltage parameters are summa-
rized in Table 1.
Materials
The [C60]-fullerene was purchased from Frontier Carbon(nanom
purple ST-A) and P3HT from Aldrich. The silica gel was
purchased from Wako Pure Chemical Industry, Ltd. (Wakogel
C-300, 45–75 mm).
We first investigated the characteristic current–voltage param-
eters of polymer solar cells composed of three isomers 1b, 1c, and
1d (entries 2–4). The power conversion efficiency (PCE) signifi-
cantly depended on the position of the methyl group on the
benzene ring and the best PCE was recorded for compound 1c,
which has a methyl group at the meta-position of the phenyl
group; the PCE is close to that of [C60]-PCBM.
N-Methoxyethoxyethyl-2-(2-methoxyphenyl) fulleropyrrolidine
(1p)
A solution of C60 (500 mg, 0.69 mmol), [2-(2-methoxyethoxy)-
ethylamino]acetic acid24,25 (177 mg, 1.0 mmol), and 2-methoxy-
benzaldehyde (188 mg, 1.38 mmol) in chlorobenzene (100 mL) was
stirred for 3 h at 130 ꢁC under argon. The solvent was evaporated
under reduced pressure and the residue was purified by flash
chromatography (toluene, then CS2/ethyl acetate ¼ 10 : 1)
affording the product 1p (351 mg, 0.36 mmol) as a dark brown
solid in 52% yield, and the unreacted fullerene (174 mg) was
Since it was anticipated that an electron-withdrawing group
may have a favourable effect on the acceptor functionality, we
next prepared the chlorine-, bromine-, or fluorine-substituted
compounds and tested their PCEs. As can be seen in Table 1 for
entries 5–13, the ortho-substituted compounds produced better
results than the corresponding meta- or para-substituted
compounds; in particular, a slightly better PCE compared to that
of [C60]-PCBM was recorded for the fluorine-substituted full-
eropyrrolidine 1k (entry 11). On the other hand, lower PCE
values were obtained for compounds 1n or 1o which have a very
strong electron-withdrawing CF3 group (entries 14 and 15).
1
recovered in 35% yield. Rf 0.61 (toluene/methanol ¼ 2/1): H
NMR (500 MHz, ppm, CDCl3) d 2.77–2.82 (1H, m), 3.39 (3H, s),
3.61 (2H, t, J ¼ 4.5 Hz), 3.69 (3H, s), 3.71–3.78 (2H, m), 3.92–4.02
(2H, m), 4.27 (1H, d, J ¼ 9.6 Hz), 5.18 (1H, d, J ¼ 9.6 Hz), 5.73
(1H, s), 6.85 (1H, d, J ¼ 8.2 Hz), 7.01 (2H, t, J ¼ 7.5 Hz), 7.20
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Table 1 Characteristic current–voltage parameters under Standard AM 1.5G conditionsa
Entry
Fullerene derivative Ar
PCE (%)
Jsc/mA cmꢀ2
Voc/V
FF
1
2
3
4
5
6
7
8
C6H5– (1a)
2.81
3.12
3.20
2.97
3.03
2.95
2.94
3.03
2.95
2.87
3.13
3.02
2.98
2.59
2.25
3.44
3.09
3.02
0.093
0.174
2.56
2.64
1.38
3.21
2.70
2.99
2.53
0.849
7.32
7.56
7.54
7.32
7.54
7.65
7.68
7.55
7.74
7.66
7.77
7.84
7.86
7.43
6.79
7.85
7.49
7.34
2.18
2.70
7.71
7.59
5.28
7. 95
7.21
7.51
6.85
2.88
0.599
0.633
0.649
0.634
0.627
0.600
0.591
0.626
0.607
0.597
0.620
0.607
0.581
0.600
0.550
0.660
0.635
0.639
0.190
0.244
0.644
0.605
0.599
0.659
0.637
0.606
0.583
0.562
0.641
0.653
0.654
0.639
0.642
0.642
0.647
0.640
0.627
0.627
0.651
0.635
0.654
0.580
0.602
0.662
0.650
0.645
0.225
0.264
0.516
0.574
0.437
0.613
0.588
0.658
0.633
0.525
2-Me–C6H4– (1b)
3-Me–C6H4– (1c)
4-Me–C6H4– (1d)
2-Cl–C6H4– (1e)
3-Cl–C6H4– (1f)
4-Cl–C6H4– (1g)
2-Br–C6H4– (1h)
3-Br–C6H4– (1i)
4-Br–C6H4– (1j)
2-F–C6H4– (1k)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
3-F–C6H4– (1l)
4-F–C6H4– (1m)
2-CF3–C6H4– (1n)
4-CF3–C6H4– (1o)
2-MeO–C6H4– (1p)
3-MeO–C6H4– (1q)
4-MeOC6H4– (1r)
2,6-Di(MeO)–C6H3– (1s)
2,4,6-Tri(MeO)–C6H2– (1t)
2-(n-HexylO)–C6H4– (1u)
3-(n-HexylO)–C6H4– (1v)
4-(n-HexylO)–C6H4– (1w)
2-Naphthyl– (1x)
1-Naphthyl– (1y)
Thienyl– (1z)
[C60]-PCBM
2
a
Since it was known that PCE values were slightly dependent on the lot of P3HT, we prepared these solar cells using the P3HT polymer having the same
lot number (Aldrich #08510JJ).
The best PCE value (3.44%) with a high Jsc (7.85 mA cmꢀ2) was
obtained when the polymer solar cell was prepared using the 2-
methoxyphenyl-substituted fulleropyrrolidine 1p as the acceptor
(entry 16). A significant reduction in the PCE was recorded using
the di-methoxy or tri-methoxy substituted compounds (entries 19
and 20). The PCEs of the hexyloxy-substituted compounds were
inferior to those of the methoxy-substituted compounds (entries
21–23); we speculated that the long alkyl chain may disturb the
favourable arrangement of these compounds in the P3HT
polymer.
3.38 ꢂ 0.066% and that of the latter was 2.66 ꢂ 0.13%. Although
the performance of the solar cells was somewhat dependent on
the lot of P3HT polymer, the PCE of the P3HT/1p system was
very reproducible and the values were always superior to those of
the P3HT/[C60]-PCBM system under the same experimental
conditions (see Table S1 in ESI†). It should also be noted that 1H
For the naphthyl substituted compounds, 1x and 1y, the 2-
naphthyl-substituted fulleropyrrolidine (1x) acts as a better
acceptor (entries 24 and 25).
Lastly, we tested the thiophene substituted fulleropyrrolidine
(1z) in anticipation of a good affinity of 1z with P3HT. However,
the resulting power was not impressive; although a moderate
PCE was obtained, the polymer showed a low Jsc value (entry
26). It was also confirmed that the presence of an aromatic
substituent on the pyrrolidine ring is essential for the acceptor
functionality because the N-methoxyethoxyethylfulleropyrroli-
dine (2)24,25 showed a very poor PCE (0.849%) along with a low
Jsc value (2.88 mA cmꢀ2) (entry 28).
Fig. 2 shows that the 1p cell has the highest AM 1.5G power
conversion efficiency (PCE) of 3.44%, a short circuit current
density (Jsc) of 7.85 mA cmꢀ2, an open circuit voltage (Voc) of
0.660 V, and a fill factor (FF) of 0.662.
We prepared five sets of samples of the solar cell using two
P3HT polymers with different lot numbers blended with 1p or
[C60]-PCBM. The average PCE value of the former five cells was
Fig. 2 Current density–potential characteristics of 1p and [C60]-PCBM
solar cell devices under illumination by an AM 1.5G solar simulated light
(100 mW cmꢀ2).
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Fig. 4 Optimized structure of fulleropyrrolidine derivatives 2 and 1p by
DFT calculation at the B3LYP/6-31G* level of theory.26
Fig. 3 UV-vis spectra of P3HT film blended with [C60]-PCBM and that
blended with 1p (2-MeO).
showed higher LUMO with higher Voc compared to that of para-
methoxy(4-MeO)-substituted [C60]-PCBM.14 Like the PCBM
derivatives, 1p (2-MeO) showed higher LUMO level than did 1r
(4-MeO), and it was found that the Voc of the solar cell is directly
proportional to the LUMO energy of the four blending mate-
rials, 1p, 1q, 1r, and 1a (Fig. 5). On the contrary, no relationship
was found for Voc versus DELUMO among 1x, 1y and 1a: these
three compounds showed similar DELUMO values with different
Voc values.
and 13C NMR analyses confirmed that no decomposition of 1p
took place after storing it for one year under atmospheric
conditions at rt on the bench in the laboratory; fulleropyrrolidine
was indeed a very stable compound.
Fig. 3 shows the UV-vis absorption spectra of the 1p film as
well as the [C60]-PCBM film for comparison. As can be seen,
both films showed a broad p–p* absorption from 300 to 700 nm,
and the lmax of 1p is around 500 nm. In addition, very similar
spectra were obtained from films including compounds 1a to 1z.
Based on a comparison with the devices prepared from [C60]-
PCBM, the open circuit voltage of the 1p polymer increased by
ca. 0.076 V. It is recognized that Voc is generally related to the
difference between the HOMO of the donor (P3HT polymer) and
LUMO of the acceptor (1p).12,14 Therefore, the higher Voc of the
1p film is consistent with the higher LUMO energy levels of the
1p, since the LUMO of PCBM in P3HT is reported to be 4.3 eV
and the LUMO of 1p has been estimated to be 4.25 eV.
We have shown that several fulleropyrrolidine derivatives
work as good acceptor partners with P3HT. Furthermore, it was
found that the PCE of the solar cells depended on the position of
substituent installed on the benzene ring of the blending mate-
rials (Table 1).
We conclude that the PCE of the polymer solar cells is deter-
mined by multiple factors, such as the LUMO energy level of the
acceptor material and structural interaction of the functional
group on the fullerene ring between the donor polymers.
It is well recognized that the acceptor function depends on the
LUMO energy level of the acceptor materials.12 Since ortho-
substituted compounds generally showed higher PCE values
compared to those of the meta- or para-substituted ones, we
conducted DFT calculations of three isomers of (methoxy-
phenyl)fulleropyrrolidines, such as 1p, 1q, 1r, and the reference
compound 2. With the objective of investigating what factor
determines the PCE of the polymer in the structural differences
of the acceptor molecules, calculations were also carried out for
1a, 1x and 1y. The typical optimized structures based on the
DFT calculation of 1p and 2 are shown in Fig. 4. We have
compared Voc versus DELUMO (eV) that shows the difference in
the LUMO energy between the reference compound 2 and that of
each material (see Fig. 5 and Table S2 in ESI†). Hummelen et al.
reported that ortho-methoxy(2-MeO)-substituted [C60]-PCBM
Fig.
5 Relationship between Voc and DELUMO among seven
compounds.
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Conclusion
In summary, we have carried out the rational design of
fulleropyrrolidine derivatives as the acceptor partner with poly-
3-hexylthiophene (P3HT) and established that N-methoxyethox-
yethyl-2-(2-methoxyphenyl)fulleropyrrolidine (1p) might take the
position of [C60]-PCBM. The important advantages of the use of
the fulleropyrrolidine 1p are in its stable nature under ambient
conditions and its ease of preparation; these characteristics make
it possible to reduce the production cost of large-area polymer
solar cells.
It has been established that the band gap of the acceptor
between the LUMO and HOMO strongly depends on the donor
polymer; in fact, several more efficient polymers superior to P3HT
have been developed using [C60]-PCBM as the acceptor.10–15
Therefore, we expect that we will be able to develop efficient donor
polymers which are appropriate for our fulleropyrrolidine deriv-
atives. Further investigation into the development of novel donor
polymers as a partner of our fulleropyrrolidine will allow the
creation of even more efficient solar cells in the near future.
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