A single cis-2 regioisomer of ethylene-tethered indene dimer-fullerene adduct as an electron-acceptor in polymer solar cells

Article abstract of DOI:10.1039/c5cc01712b

A pure cis-2 isomer of indene dimer-based fullerene bis-adduct has been synthesized, isolated and applied to bulk heterojunction solar cells for the first time. The device based on the cis-2 isomer and poly(3-hexylthiophene) exhibited a power conversion efficiency of 2.8%, which is higher than those of the reference devices.

Full text of DOI:10.1039/c5cc01712b

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A single cis-2 regioisomer of ethylene-tethered
indene dimer–fullerene adduct as an electron-
acceptor in polymer solar cells†
Cite this:DOI: 10.1039/c5cc01712b
Received 26th February 2015,
Accepted 18th March 2015
a
a
a
b
Ran Tao, Tomokazu Umeyama, Tomohiro Higashino, Tomoyuki Koganezawa
ac
and Hiroshi Imahori*
DOI: 10.1039/c5cc01712b
www.rsc.org/chemcomm
4
A pure cis-2 isomer of indene dimer-based fullerene bis-adduct has acceptors is the Diels–Alder bis-adduct of indene, IC BA. The
6
0
been synthesized, isolated and applied to bulk heterojunction solar optimized PSC device based on P3HT and IC BA achieved a
6
0
cells for the first time. The device based on the cis-2 isomer and high VOC of 0.89 V as well as a high power conversion efficiency
4
b
poly(3-hexylthiophene) exhibited a power conversion efficiency of (PCE) of 7.5%.
2
.8%, which is higher than those of the reference devices.
The high-lying LUMO level and the facile synthesis are great
advantages of fullerene bis-adducts, whereas the coexistence of
Fullerene derivatives, organic materials with unique large multiple regioisomers in the bis-adduct is a great threat to short
p-conjugation spheres, have been widely used as electron- circuit current density (JSC) and fill factor (FF) values, considering
acceptors in bulk heterojunction (BHJ) polymer solar cells (PSCs) that molecular packing and arrangement of fullerene acceptors
1
due to their high electron deficiency and electron mobility. As in the BHJ structure have a large influence on charge separation
5
the open circuit voltage (VOC) of a BHJ PSC device is dependent and electron-transporting processes. Inherently, a C bis-adduct
60
on the energy difference between the highest occupied mole- with the same two symmetrical substituents has eight regio-
cular orbital (HOMO) of the donor and the lowest unoccupied isomers: trans-1, trans-2, trans-3, trans-4, e, cis-1, cis-2, and cis-3
molecular orbital (LUMO) of the acceptor, a higher LUMO level (Fig. 1a). It was very recently that the regioisomer effects on the
2
6–8
of a fullerene acceptor leads to a higher VOC. The p-conjugation PSC device performances were investigated in detail. In particular,
size is the major determinant of the LUMO level of fullerene we successfully separated trans-1, trans-2, trans-3, trans-4, e, and a
derivatives. Taking [6,6]-phenyl–C –butyric acid methyl ester mixture of cis-2 and cis-3 isomers of alkoxycarbonyl-substituted
6
1
(
[60]PCBM) as the representative example, the 58-p-conjugation dihydronaphthyl-based [60]fullerene bis-adducts (NCBA) by a
6
is dwindled to 56-p-conjugation as covalently tethering the same preparative HPLC method. PSCs based on the regioisomerically
substituent to [60]PCBM. Concomitantly, the LUMO level of the separated NCBA generally showed superior device performances
bis-adduct is elevated by 0.1 eV, and therefore leads to a 20% to those based on the NCBA regioisomer mixtures. It should be
increase in VOC (from 0.61 V to 0.73 V) of the corresponding PSCs noted, however, that the mixture of cis-2 and cis-3 isomers could
with poly(3-hexylthiophene) (P3HT) as a donor. The second not be separated by the special HPLC method and even cis-1 was
functionalization on the fullerene sphere reduces the p-conjugation not generated due to the steric hindrance. As such, cis-isomers
3
6
size and the electron delocalization region, rendering the electron of fullerene bis-adducts are yet to be isolated for PSC applica-
acceptors of the fullerene bis-adduct weaker than their mono-adduct tions, although the less steric hindrance of the cis-isomers would
analogues. The most successful example for fullerene bis-adduct be favorable for their more close packing in the BHJ film and
8
superior photovoltaic properties.
a
Department of Molecular Engineering, Graduate School of Engineering,
Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan
b
Japan Synchrotron Radiation Research Institute, 1-1-1, Kouto, Sayo-cho, Sayo-gun,
Hyogo 679-5198, Japan
c
Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University,
Nishikyo-ku, Kyoto 615-8510, Japan
†
Electronic supplementary information (ESI) available: Preparation and char-
acterization of BIEC, device fabrication procedure, absorption, packing structure,
and electrochemical and thermal properties of BIEC, and optical properties of
P3HT:BIEC composite films. CCDC 1050695. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c5cc01712b
Fig. 1 (a) Nomenclature of fullerene bis-adducts and molecular structures
of (b) cis-2-BIEC, (c) bis-BIEC (regioisomer mixture) and (d) mono-BIEC.
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Herein, we report the synthesis, isolation, and application of a Table 1 Reduction potentials and LUMO energy levels
pure cis-isomer of a fullerene bis-adduct as an electron-acceptor in
PSC for the first time. Because controlled bis-functionalization of C60
is crucial to produce the sterically-hindered cis-isomers and avoid
a
a
a
b
Acceptor
E
1
/V
E
2
/V
E
3
/V
LUMO/eV
LUMO /eV
cis-2-BIEC
bis-BIEC
mono-BIEC
À1.10
À1.10
À0.94
À0.90
À1.49
À1.49
À1.33
À1.29
À2.11
À2.11
À1.88
À1.80
À3.50
À3.50
À3.66
À3.70
À2.82
—
the tedious HPLC separations, we conducted tether-directed remote
À3.07
9
functionalization. Ethylene-tethered indene dimer (1,2-bis(3- [60]PCBM
À3.09
indenyl)ethane, BIE) was chosen as a reactant to synthesize the
a
+
b
3
Values are versus Ag/Ag (0.01 M AgNO ). Calculated by DFT using
cis-2 isomer efficiently without forming relatively remote sub- B3LYP/6-31G*.
stituent patterns, i.e., e and trans isomers (Fig. 1a). The single
regioisomer of cis-2 BIE–C60 adduct (cis-2-BIEC, Fig. 1b) could
Electrochemical properties of the BIE–C60 adducts and [60]PCBM
as a reference were investigated by cyclic voltammetry and
differential pulse voltammetry (Fig. S4, ESI†). Table 1 lists the
be isolated and used in PSC devices with P3HT.
The detailed synthetic procedure is described in the experi-
mental section of the ESI.† Briefly, the Diels–Alder reaction between
1 2 3
first (E ), second (E ), and third (E ) reduction potentials
4a
BIE and C60 was conducted by refluxing 1,2,4-trichlorobenzene.
together with the experimental and theoretical LUMO energy
levels. As expected, the experimental LUMO levels of cis-2-BIEC
and regioisomer mixture bis-BIEC are 0.16–0.20 eV higher than
those of mono-BIEC and [60]PCBM, which would be favorable
for the application as an acceptor in PSCs to improve VOC. The
difference in the LUMO levels of cis-2-BIEC, mono-BIEC and
This reaction yielded four fractions, i.e., unreacted C60, a BIE–C60
adduct with one unreacted indene unit (mono-BIEC, Fig. 1d), a
regioisomer mixture of BIE–C60 adducts with no unreacted indene
units (bis-BIEC, Fig. 1c) and multiple-BIE–C60 adducts (multi-BIEC),
which were eluted in this order by silica gel column chromato-
graphy with hexane/CS (2/1). Further purification of the bis-BIEC
2
[60]PCBM is consistent with that of the corresponding theoretical
regioisomer mixture by HPLC (Fig. S1, ESI†) afforded cis-2-BIEC in
an overall moderate yield (48%). Meanwhile, solvent-free Diels–
Alder reaction of C60 with BIE in the melted state under vacuum at
values (Table 1). Thermal properties of cis-2-BIEC and mono-BIEC
were evaluated by thermal gravimetric analysis (TGA). They display
high thermal stability with onset points of steep weight losses at ca.
10
2
40 1C was also performed. In contrast, this reaction yielded only
three fractions in silica gel column chromatography, i.e., unreacted
60, cis-2-BIEC and multi-BIEC, without forming mono-BIEC and
350 1C and 310 1C, respectively (Fig. S5, ESI†). Differential scanning
calorimetry (DSC) measurements revealed melting points of 154 1C
for cis-2-BIEC and 167 1C for mono-BIEC (Fig. S6, ESI†).
C
other regioisomers of bis-BIEC. Although the total yield is relatively
low (10%), the regioisomerically pure target molecule cis-2-BIEC
was obtained without the HPLC separation.
PSC devices with a configuration of ITO/PEDOT:PSS/
P3HT:fullerene/Ca/Al were fabricated to evaluate the photo-
voltaic performances of the BIE–C60 adducts. The detailed device
fabrication procedure is described in the experimental section of
the ESI.† As shown in Fig. 3a and Table 2, the PSC based on
P3HT:cis-2-BIEC reveals a significantly higher PCE of 2.8% than
P3HT:bis-BIEC (1.8%) and P3HT:mono-BIEC (1.6%). The photo-
voltaic performance of the P3HT:cis-2-BIEC-based device is com-
parable with that of the P3HT:[60]PCBM-based one (2.6%) that
The structures of cis-2-BIEC and mono-BIEC were fully char-
1
13
acterized by H and C-NMR, high-resolution mass spectroscopy
the experimental section in the ESI†), and UV-vis absorption
(
1
1
1
spectroscopy (Fig. S2, ESI†). In the H NMR spectrum of cis-2-
BIEC, characteristic peaks of intact indene units, i.e., protons at 1
and 2 positions of the five-membered ring, are absent as both
indene moieties attached to C , whereas the spectrum of mono-
60
13
was fabricated under the same conditions. As expected, the VOC
BIEC exhibits the corresponding peaks at 6.39 and 4.91 ppm.
Furthermore, the structure of cis-2-BIEC was unequivocally identified
by X-ray crystallography (Fig. 2a) with the two methylene bridges of
the indene addends protruding on the same side. The packing
diagrams of crystalline (Fig. 2b and Fig. S3, ESI†) show arranged
rows of contacted cis-2-BIEC molecule pairs with the solvent tetra-
chloroethane in the gaps between the rows.‡
values (0.80 V) of the PSCs with cis-2-BIEC and bis-BIEC are
higher than those (0.61 V) with mono-BIEC and [60]PCBM owing
to the elevated LUMO levels by bis-functionalization. In contrast,
the BIE–C60 adduct-based devices have lower JSC values than the
12
[60]PCBM-based one, which should be ascribed to the rather
Fig. 3 (a) Current density–voltage curves and (b) photocurrent action spectra
of PSC devices based on P3HT:cis-2-BIEC (red), P3HT:bis-BIEC (black), and
P3HT:mono-BIEC (blue). The convolution of the spectral response in (b) with
the photon flux of the AM 1.5G spectrum provided the estimated JSC values of
Fig. 2 (a) X-ray crystal structure of cis-2-BIEC obtained from C
ellipsoids represent 50% probability. Solvent molecules are omitted for clarity.
b) Packing structure with solvent molecules along the a-axis.
2 2 4
D Cl . Thermal
À2
(
6.83, 4.99, and 5.24 mA cm , respectively.
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a
Table
2
Device characteristics of PSCs under AM1.5 conditions (87%) 4 P3HT:bis-BIEC (84%) (Fig. S9, ESI†), i.e., the generated
À2
(100 mW cm
)
excitons reach the P3HT–fullerene interface most efficiently in
P3HT:cis-2-BIEC with the smallest domain size. The P3HT:cis-2-BIEC
film would be suitable for forming more effective bicontinuous
donor–acceptor networks, giving rise to the highest JSC and PCE
À2
Acceptor
J
SC/mA cm
VOC/V
FF
PCE/%
cis-2-BIEC
bis-BIEC
mono-BIEC
6.60 Æ 0.30
4.70 Æ 0.35
5.15 Æ 0.08
7.50 Æ 0.54
0.80 Æ 0.03
0.80 Æ 0.01
0.61 Æ 0.01
0.61 Æ 0.01
0.53 Æ 0.04
0.47 Æ 0.03
0.51 Æ 0.02
0.57 Æ 0.02
2.8 Æ 0.2
1.8 Æ 0.2
1.6 Æ 0.1 in the corresponding three devices.
[
60]PCBM
2.6 Æ 0.2
In conclusion, the synthesis and isolation of a regioisomerically
a
The photovoltaic parameters were averaged from more than ten pure cis-2 1,2-bis(3-indenyl)ethane–C60 adduct, cis-2-BIEC, have
independent solar cells.
been achieved by the tether-directed functionalization method.
In the BHJ PSC devices with P3HT, cis-2-BIEC showed the PCE of
2.8% with the high VOC of 0.80 V, outperforming the devices with
the corresponding regioisomer mixture (bis-BIEC) and the BIE–C60
adduct with one unreacted indene unit (mono-BIEC). These results
demonstrate that cis-2 isomers of fullerene bis-adducts are highly
promising as excellent electron-acceptors in BHJ PSC devices.
Because the rather bulky ethylene-tethered indene addend may
lessen the fullerene–fullerene contact and decrease JSC in PSCs,
molecular design targeted at the smallest possible addend with
cis-configurations is currently under investigation.
This work is supported by New Energy and Industrial
Technology Development Organization (NEDO) and Grand-in-
Aid (No. 25220801 to H.I.). The synchrotron radiation experi-
ments were performed at the BU46XU of SPring-8 with the
approval of the Japan Synchrotron Radiation Research Institute
8
c,14
sterically-bulky BIE addend.
the cis-2-BIEC-based device (6.60 mA cm ) is still increased
significantly compared to the bis-BIEC -device (4.70 mA cm
and P3HT:mono-BIEC-based device (5.15 mA cm ). Consistently,
the incident photon-to-current efficiency (IPCE) value of the cis-2-
BIEC-based device was higher than those of bis-BIEC- and mono-
BIEC-based ones (Fig. 3b), despite the analogous light-harvesting
properties of the blend films (Fig. S7, ESI†). Electron mobilities
of the composite films P3HT:cis-2-BIEC, P3HT:bis-BIEC and
P3HT:mono-BIEC, which were estimated by the space charge-
Note here that the JSC value of
À2
À2
)
À2
6b
À5
À5
limited current (SCLC) method, are 3.5 Â 10 , 2.5 Â 10 and
À5
2
À1 À1
2
.7 Â 10 cm V
s
respectively. They are in accord with the
J
SC
values, suggesting that the regioisomerically pure, relatively
compact structure of cis-2-BIEC formed a well-packed structure
in the composite film as illustrated in Fig. 2b, thereby leading to
more efficient charge transportation and the higher JSC value
(JASRI) (Proposal No. 2014B1616 and 2014B1596).
6
Notes and references
than bis-BIEC and mono-BIEC.
The isomer effect of the BIE–C60 adducts on the molecular ‡ Crystallographic data for cis-2-BIEC: C80H18ÁC H Cl , M = 1146.78,
2
2
4
r
orthorhombic, Pbca (No. 61), a = 17.0287(17) Å, b = 18.1277(17) Å,
c = 29.437(3) Å, V = 9086.8(15) Å , rcalcd = 1.677 g cm , Z = 8,
ordering of P3HT in the composite films was assessed by grazing
incidence wide-angle X-ray scattering (GIWAXS) (Fig. S8, ESI†). The
3
À3
7
1 139 reflections measured, 10 374 unique (Rint = 0.0341), R = 0.0495
1
GIWAXS profiles from the P3HT:cis-2-BIEC, P3HT:bis-BIEC, and [I 4 2s(I)], wR = 0.1372 (all data), GOF = 1.048. CCDC 1050695.
2
P3HT:mono-BIEC films exhibit out-of-plane (l00) and in-plane (010)
1
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structure oriented edge-on with the substrate irrespective of the
fullerene isomers. Meanwhile, to shed light on the relationship
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2
3
(
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4
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Fig. 4 Tapping-mode atomic force micrographs of (a) P3HT:cis-2-BIEC,
(
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(
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1
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À1
1
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