Deuterium isotope effect on bulk heterojunction solar cells. Enhancement of organic photovoltaic performances using monobenzyl substituted deuteriofullerene acceptors

Article abstract of DOI:10.1021/ol4026606

A series of novel monobenzyl-substituted deuteriofullerenes (BnDCs) were synthesized efficiently through Co-catalyzed selective monofunctionalization of C₆₀. Bulk heterojunction solar cells, based on poly(3-hexylthiophene) as the donor and BnDC

Full text of DOI:10.1021/ol4026606

ORGANIC
LETTERS
2013
Vol. 15, No. 22
5674–5677
Deuterium Isotope Effect on Bulk
Heterojunction Solar Cells. Enhancement
of Organic Photovoltaic Performances
Using Monobenzyl Substituted
Deuteriofullerene Acceptors
Shirong Lu,† Tienan Jin,*,† Takeshi Yasuda,‡ Weili Si,† Kazuaki Oniwa,†
Khalid A. Alamry,§ Samia A. Kosa,§ Abdullah Mohamed Asiri,§ Liyuan Han,‡ and
Yoshinori Yamamoto*,†,§
WPI-Advanced Institute for Materials Research (WPI-AIMR), Tohoku University,
Sendai 980-8577, Japan, Photovoltaic Materials Unit, National Institute for Materials
Science, Tsukuba 305-0047, Japan, and Chemistry Department, Faculty of Science, and
Center of Excellence for Advanced Materials Research, King Abdulaziz University, P.O.
Box 80203, Jeddah, Saudi Arabia
yoshi@m.tohoku.ac.jp; tjin@m.tohoku.ac.jp
Received September 13, 2013
ABSTRACT
A series of novel monobenzyl-substituted deuteriofullerenes (BnDCs) were synthesized efficiently through Co-catalyzed selective monofunc-
tionalization of C60. Bulk heterojunction solar cells, based on poly(3-hexylthiophene) as the donor and BnDCs as the acceptors, exhibited higher
photovoltaic performances as compared to the corresponding protonated BnHCs devices.
Functional fullerenes are used as a unique electronic
acceptor for bulk heterojunction (BHJ) polymer solar cells
(PSCs), because they not only maintain the properties of
C60 such as high electron affinity and electron mobility
but also exhibit tunable solubility in organic solvents and
a tunable energy level, as well as the superior packing
arrangement in the solid state by introducing various func-
tional groups on C60 core.1,2 Many efforts on development
of new functional fullerenes have been devoted to improve
the photon-to-current conversion efficiencies. The com-
mercially available [6,6]-phenyl-C61-butyric acid methyl
ester (PC61BM) and its C70 analogue PC71BM exhibit high
solubility and high electron transport ability, which are
used as the most well-studied benchmark acceptors for
testing new donor materials.3 Recently, bisfunctionalized
56π-electron fullerene acceptors with an up-shifted lowest
unoccupied molecular orbital (LUMO), whichis favorable
for increasing the open circuit voltages (Voc) of PSCs, have
been developed to show higher power conversion efficien-
cies (PCEs) than that of PCBM for the PSCs based on
poly(3-hexylthiophene) (P3HT) donor.4 So far, the PSCs
† Tohoku University.
‡ National Institute for Materials Science.
§ King Abdulaziz University.
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r
10.1021/ol4026606
Published on Web 11/01/2013
2013 American Chemical Society

based on blending the newly developed low band gap
donors and PCBM acceptor increased the PCEs up to
7ꢀ8%.5 The development of new fullerene acceptors
toward increasing the PCEs through the combination with
the innovative polymer donors is still highly desirable.
in high yields.6 We also reported that PSCs based on those
BnHCs as acceptors including 4-MeO-BnHC (9) and
3-CO2Me-BnHC (10) exhibited high PCEs by blending
with P3HT as the donor (Scheme 1).7 Surprisingly, during
our investigation of photovoltaic performances of BnHCs,
we found that PSCs based on the deuterated fullerenes
(BnDCs) bearing deuterium atoms on the fullerene core
and the benzyl moiety displayed a noticeable PCE im-
provement compared to the corresponding protonated
BnHCs-devices. It was noted that the deuterium isotope
effect has been used in organic light-emitting diodes
(OLED) to increase the excited state lifetime of organic
molecules, resulting in the increased external quantum
efficiency as compared to the corresponding protonated
devices, due to the vibronic coupling induced reduction of
the nonradiative decay rate.8 However, to the best of our
knowledge, the deuterium isotope effect on photovoltaic
performance has never been investigated. Herein, we
report the facile synthesis of various novel BnDCs, char-
acterization of photophysical and electrochemical pro-
perties, and their distinguished PSC performances
by blending with P3HT as the donor (Scheme 1). The
deuterated BnDCs exhibited higher PCEs compared to
their corresponding protonated BnHCs. Among them,
the PSC based on the blend of 3D-4-MeO-BnDC
(3)/P3HT showed the highest PCE of 4.16% which is
higher than the PC61BM-based PSC.
Scheme 1. Co-Catalyzed Monobenzylation of C60 for Synthesis
of BnDCs and Structure of Various BnDCs and Their
Corresponding Protonated BnHCsa
BnDCs 1ꢀ8 were prepared in one step following our
previously reported Co-catalyzed selective monobenzyla-
tion method in 46ꢀ56% yields (Scheme 1).6 The reaction
of pristine C60 with various substituted benzyl bromides
(1.5 equiv) was carried out in the presence of CoCl2dppe
(10 mol %), a Mn reductant (3 equiv), and D2O (10 equiv)
in 1,2-dichlorobenzene (ODCB) under an argon atmo-
sphere at rt. The corresponding protonated BnHCs 9 and
10 were prepared under the standard conditions by using
H2O instead of D2O.6 All the products were simply
purified by silica gel chromatography using toluene and
hexane as the eluents. It was noted that BnDCs showed
solubility comparable withBnHCs and PC61BM in chloro-
form, toluene, and ODCB.
a 4-MeO-BnHC (9) and 3-CO2Me-BnHC (10) were prepared by using
H2O (10 equiv) instead of D2O.
Recently, we reported an efficient Co-catalyzed selective
monofunctionalization of C60 with benzyl bromides, pro-
ducing various 2-benzyl-1,2-dihydro[60]fullerenes (BnHCs)
(4) (a) Lenes, M.; Wetzelaer, G. J. A. H.; Kooistra, F. B.; Veenstra,
S. C.; Hummelen, J. C.; Mlom, P. W. M. Adv. Mater. 2008, 20, 2116. (b)
He, Y. J.; Chen, H. Y.; Hou, J. H.; Li, Y. F. J. Am. Chem. Soc. 2010, 132,
1377. (c) Voroshazi, E.; Vasseur, K.; Aernouts, T.; Heremans, P.;
Baumann, A.; Deibel, C.; Xue, X.; Herring, A. J.; Athans, A. J.; Lada,
T. A.; Richter, H.; Rand, B. P. J. Mater. Chem. 2011, 21, 17345. (d)
Cheng, Y. J.; Liao, M. H.; Chang, C. Y.; Kao, W. S.; Wu, C. E.; Hsu,
C. S. Chem. Mater. 2011, 23, 4056. (e) Li, C. Z.; Chien, S. C.; Yip, H. L.;
Chueh, C. C.; Chen, F. C.; Matsuo, Y.; Nakamura, E.; Jen, A. K.-Y.
Chem. Commun. 2011, 47, 10082. (f) Zhang, C.; Chen, S.; Xiao, Z.; Zuo,
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C.-Z.; Tanaka, H.; Nakamura, E. J. Am. Chem. Soc. 2011, 133, 8086. (h)
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2012, 68, 6696. (i) Kim, B.; Lee, J.; Seo, H. H.; Wudl, F.; Park, S. H.;
Yang, C. J. Mater. Chem. 2012, 22, 22958.
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Y.; Yu, L.; Wu, Y.; Li, G. Nat. Photonics 2009, 3, 649. (b) Liang, Y.; Xu,
Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. Adv. Mater. 2010,
ꢀ
22, E135. (c) Chu, T.-Y.; Lu, J.; Beaupre, S.; Zhang, Y.; Pouliot, J.-R.;
Figure 1. (a) UVꢀvis absorption in chloroform. (b) Cyclic voltam-
mograms in ODCB solution with Bu4NBF4 as a supporting
electrolyte (vs Ag/AgCl).
Wakim, S.; Zhou, J.; Leclerc, M.; Li, Z.; Ding, J.; Tao, Y. J. Am. Chem.
Soc. 2011, 133, 4250. (d) Zhou, H.; Yang, L.; Stuart, A. C.; Price, S. C.;
Liu, S.; You, W. Angew. Chem., Int. Ed. 2011, 50, 2995. (e) Price, S. C.;
Stuart, A. C.; Yang, L.; Zhou, H.; You, W. J. Am. Chem. Soc. 2011, 133,
4625. (f) Son, H. J.; Wang, W.; Xu, T.; Liang, Y.; Wu, Y.; Li, G.; Yu, L.
J. Am. Chem. Soc. 2011, 133, 1885. (g) He, Z.; Zhong, C.; Huang, X.;
Wong, W.-Y.; Wu, H.; Chen, L.; Su, S.; Cao, Y. Adv. Mater. 2011, 23,
4636. (h) Huo, L.; Zhang, S.; Guo, X.; Xu, X.; Li, Y.; Hou, J. Angew.
Chem., Int. Ed. 2011, 50, 9697.
The UVꢀvis absorption of BnDCs and BnHCs as well
as the reference PC61BM were measured in chloroform
(Figure 1a and Figure S1 in the Supporting Information (SI)).
Org. Lett., Vol. 15, No. 22, 2013
5675

BnDCs devices were fabricated. PSCs utilizing other
BnDCs such as 2ꢀ6 with deuterium atoms at the benzylic
position or on the phenyl ring or on the methoxy group
also showed higher PCEs (3.28% to 4.16%) than that of the
protonated 4-MeO-BnHC (9). Among them, 3D-MeO-
BnDC (3) showed considerable improvement of the PCEs
up to 4.16% with Jsc = 11.10 mA cmꢀ2, Voc = 0.62 V, and
FF = 60%, which accounts for 73% improvement com-
pared to the corresponding protonated 4-MeO-BnHC (9)
device. It should be noted that the devices using 2D-MeO-
BnDC (2), 3D-MeO-BnDC (3), and 8D-MeO-BnDC (6) as
acceptors, respectively, displayed higher PCEs as compared
to the PC61BM device.
A similar deuterium effect was also observed for the
devices using a series of 3-CO2Me-BnDCs. The PCEs of
D-3-CO2Me-BnDC (7) and 2D-CO2Me-BnDC (8) with an
ester group on the phenyl ring are 3.75% and 3.63%,
respectively, which are higher than that of the correspond-
ing protonated 3-CO2Me-BnHC (10) (2.74%) (Table 2
and Figure S2, SI). However, it should be noted that this
deuterium effect on photovoltaic performance might not
always hold for various fullerene acceptors. For example,
we found that the devices using PC61BM and 5D-PC61BM
showed almost similar PCEs of 3.78% and 3.66%,
respectively (Table 2 and Figure S3, SI). It was noted
that every acceptor combined with the P3HT donor
was evaluated at least 3 times using the same fabrica-
tion method and the PSCs based on BnDCs showed a
much higher average PCE than that of the protonated
BnHCs, indicating high reliability of device performances
(Table S1, SI).
Table 1. Electrochemical Reduction Potentials and LUMO
Energy Levels of BnDCs, BnHCs and PC61BMa,b
compd
E1/21/V
E1/22/V
E1/23/V
LUMO/eV
1
ꢀ0.60
ꢀ0.59
ꢀ0.58
ꢀ0.59
ꢀ0.59
ꢀ0.59
ꢀ0.60
ꢀ0.60
ꢀ0.60
ꢀ0.61
ꢀ0.58
ꢀ0.99
ꢀ0.99
ꢀ0.98
ꢀ0.98
ꢀ0.98
ꢀ0.99
ꢀ0.99
ꢀ0.99
ꢀ0.99
ꢀ0.99
ꢀ0.98
ꢀ1.56
ꢀ1.54
ꢀ1.53
ꢀ1.52
ꢀ1.55
ꢀ1.55
ꢀ1.45
ꢀ1.54
ꢀ1.56
ꢀ1.46
ꢀ1.48
ꢀ3.56
ꢀ3.57
ꢀ3.58
ꢀ3.57
ꢀ3.57
ꢀ3.57
ꢀ3.56
ꢀ3.56
ꢀ3.56
ꢀ3.55
ꢀ3.58
2
3
4
5
6
7
8
9
10
PC61BM
a Potential values are versus Ag/AgCl reference electrode; reduction
potential of ferrocene (0.64 V) is versus Ag/AgCl. b The LUMO energy
levels were estimated using the following equation: LUMO = ꢀ(4.80 þ
1
E1/2 ꢀ 0.64) eV.
BnDCs and their corresponding BnHCs show similar
absorption spectra despite the different functional groups
on the phenyl ring, and the different position and number
of deuterium atoms, while they exhibit slightly enhanced
absorption compared to that of PC61BM in the regions
280ꢀ330 nm and 400ꢀ500 nm. The reduction potentials
were measured by cyclic voltammetry (CV) in ODCB,
and the LUMO energy levels were estimated by the
first reduction potentials (E1/21) (Figure 1b, Table 1,
and Figure S1, SI). Both BnDCs and BnHCs having an
electron-donating (1ꢀ6 and 9) and an electron-withdraw-
ing group (7, 8, and 10) showed similar potentials
to PC61BM. The LUMO energy levels are estimated to
be in the range ꢀ3.56 eV to ꢀ3.58 eV, comparable with
PC61BM (ꢀ3.58 eV), which are expected to be applicable
to acceptors in PSCs. We concluded that the introduction of
deuterium atoms does not affect the absorption and energy
levels of BnDCs compared to the protonated BnHCs.
PSCs based on blending new BnDCs acceptors and the
P3HT donor were fabricated with a thickness of about 200
nm. The photovoltaic characterizations for ITO/PEDOT:
PSS (40 nm)/P3HT:D-BnDCs (w/w = 1/1)/LiF (1 nm)/Al
(80 nm) under illumination with 100 mW cm2 of AM 1.5
are shown in Table 2. Previously, we reported that the PSC
device based on the protonated 4-MeO-BnHC (9) blended
Table 2. PSC Performances Based on BnDCs and BnHCs
Acceptors with P3HT Donor (w/w = 1/1)a
acceptor
Jsc [mA cmꢀ2
]
Voc [V]
FF [%]
PCE [%]
1
2
3
4
5
6
7
8
9
10.08
10.75
11.10
9.16
0.62
0.61
0.62
0.60
0.60
0.61
0.60
0.62
0.60
0.61
0.60
0.58
57.4
59.5
60.3
59.5
58.3
57.6
58.2
58.2
48.4
46.6
64.0
61.5
3.63
3.93
4.16
3.28
3.48
4.03
3.75
3.63
2.40
2.74
3.78
3.66
10.04
11.44
10.72
10.13
8.24
10
9.66
with P3HT exhibited a moderate PCE of 2.40% (Jsc
=
8.24 mA cmꢀ2, Voc = 0.60 V, and FF = 48%).7 It was
interesting that when the D-4-MeO-BnDC (1) having a
deuterium atom on the C60 cage was employed as the
acceptor under the same device conditions, a much higher
PCE of 3.63% (Jsc = 10.08 mA cmꢀ2, Voc = 0.62 V, and
FF = 57%) was achieved. In order to further investigate
the deuterium effect on PSC performances, a series of
PC61BM
5D-PC61BM
9.86
10.29
a Blend film was prepared using P3HT (20 mg) and BnDCs (20 mg) in
1,2-dichlorobenzene (1 mL); annealing temperature is 110 °C (10 min).
Voc: open-circuit voltage; Jsc: short-circuit current density; FF: fill factor.
It can be seen in Table 2 and Figure 2a that similar Voc
values were observed between BnDCs and the protonated
BnHCs. These results are in agreement with their LUMO
energy levels as shown in Table 1, since it is well-known
that the Voc of PSCs is proportional to the energy gap
between the LUMO of the acceptor and the HOMO of the
donor.9 The photovoltaic performances indicated that the
(6) Lu, S.; Jin, T.; Bao, M.; Yamamoto, Y. J. Am. Chem. Soc. 2011,
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L.; Alamry, K. A. A.; Kosa, S. A.; Asiri, A. M.; Yamamoto, Y.
Tetrahedron 2013, 69, 1302.
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5676
Org. Lett., Vol. 15, No. 22, 2013

enhanced PCEs of BnDC devices are attributed to the
increased Jsc and FF as compared to the protonated
BnHCs devices. The much higher incident photon-to-
current conversion efficiency (IPCE) values of the BnDC
devices than that of the protonated BnHC devices in the
region 350ꢀ650 nm correspond to the higher Jsc values of
BnDC devices (Figure 2b and Figure S2, SI).
Figure 2. (a) Current densityꢀvoltage (IꢀV) curves and (b)
incident photon-to-current conversion efficiency (IPCE).
The homogeneous active layers and obvious nanoscale
phase separation of the BnDCs/P3HT thin films were
observed by atomic force microscopy (AFM) (Figure 3a
and 3b, and Figure S4, SI). For example, the root mean
squares (rms) were 0.78 nm for 3D-4-MeO-BnDC (3) and
5.25 nm for the protonated 4-MeO-BnHC (9), indicating
that the introduction of deuterium atoms influences the
roughness of the blend films. The transmission electron
microscopy (TEM) and electron energy loss spectroscopy
(EELS) mappings of the cross section of BnDC (3) and
BnHC (9) devices, obtained by the ion beam, showed that
the morphology of the BnDC device has a more clear
binary network structure than that of the BnHC device,
indicating a better connection of the donor and acceptor
across the film (Figure 3c and 3d). Taking into considera-
tion the similar energy levels and absorption between
the deuterated BnDCs and the protonated BnHCs, the
increase in PCEs and Jsc for the BnDCs devices might
be ascribed to the optimal interfacial contact between the
polymer donor and fullerene acceptor.
Figure 3. AFM images of P3HT donor with 4-MeO-BnHC (9)
(a) and 3D-4-MeO-BnDC (3) (b); TEM-EELS mappings of
cross section of 4-MeO-BnHC (9) (c) and 3D-4-MeO-BnDC
(3) (d) devices. The bright and dark regions represent the P3HT-
rich domains and fullerene-acceptor-rich domains, respectively.
obviously improved the photovoltaic performances due to
the increased Jsc and FF compared to the protonated
BnHCs by using P3HT as the donor. The highest PCE
was up to 4.16% for the device based on 3D-4-MeO-BnDC
(3) as the acceptor, which is higher than the PC61BM device.
Although further investigation is needed to clarify the
reason for the deuterium effect, it is clear that deuterium
labeling provides a new approach to improve the power
conversion efficiency of PSCs. Further investigation is in
progress.
Acknowledgment. This work was supported by a Scien-
tific Research (B) from Japan Society for Promotion of
Science (JSPS) (No. 25288043), and World Premier Inter-
national Research Center Initiative (WPI), MEXT, Japan,
and the King Abdulaziz University (KAU) under Grant
No. HiCi/28-3-1432.
In conclusion, we have synthesized a new series of
deuterated monobenzyl-substituted deuteriofullerenes
which were successfully applied in bulk heterojunction
polymer solar cells. Partial replacement of a proton for a
deuterium atom in monobenzyl-substituted hydrofullerenes
Supporting Information Available. Experimental pro-
cedures, device fabrication, and characterization. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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Fromherz, T.; Rispens, M. T.; Sanchez, L.; Hummelen, J. C. Adv. Funct.
Mater. 2001, 11, 374. (b) Kooistra, F. B.; Knol, J.; Kastenberg, F.;
Popescu, L. M.; Verhees, W. J. H.; Kroon, J. M.; Hummelen, J. C. Org.
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The authors declare no competing financial interest.
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