Mono-ketone modified C60s for acceptor materials in organic photovoltaic cells

Article abstract of DOI:10.1080/15421400802330689

We designed and synthesized methanofullerene derivatives with several types of thiophenyl mono-ketone for application in organic photovoltaic cells (OPVCs) as new accepter materials. The various mono-ketones having thiophene rings were substituted onto fullerene through application of Hirch-Bingel reaction under mild condition to compete with donor materials especially P3HT [Poly(3-hexylthiophene)] for nano-networking bulk-heterojunction structure where efficient light-induced charge separation is enable by a large-area donor-acceptor interface. The newly synthesized methanofullerene derivatives were characterized through ¹H-NMR and FAB Mass spectroscopy. The compounds were also applied in OPV cell devices with P3HT as a donor material and measured electrochemical the property and power conversion efficiency.

Full text of DOI:10.1080/15421400802330689

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Mono-Ketone Modified C s for
60
Acceptor Materials in Organic
Photovoltaic Cells
a b
a
a
Hak Sung Lee
, Sung Cheol Yoon , Jongsun Lim
a
a
,
a
Myongsoo Lee & Changjin Lee
Advances Materials Division, Korea Research
Institute of Chemical Technology (KRICT) , Daejeon ,
Korea
b
Department of Chemistry , Yonsei University ,
Seoul , Korea
Published online: 31 Aug 2012.
To cite this article: Hak Sung Lee , Sung Cheol Yoon , Jongsun Lim , Myongsoo Lee
&
Changjin Lee (2008) Mono-Ketone Modified C s for Acceptor Materials in Organic
60
Photovoltaic Cells, Molecular Crystals and Liquid Crystals, 492:1, 293/[657]-302/
666], DOI: 10.1080/15421400802330689
[
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Mol. Cryst. Liq. Cryst., Vol. 492, pp. 293=[657]–302=[666], 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802330689
Mono-Ketone Modified C s for Acceptor Materials
in Organic Photovoltaic Cells
60
1
,2
1
1
Hak Sung Lee , Sung Cheol Yoon , Jongsun Lim ,
1
1
Myongsoo Lee , and Changjin Lee
1
Advances Materials Division, Korea Research Institute of Chemical
Technology (KRICT), Daejeon, Korea
Department of Chemistry, Yonsei University, Seoul, Korea
2
We designed and synthesized methanofullerene derivatives with several types of
thiophenyl mono-ketone for application in organic photovoltaic cells (OPVCs) as
new accepter materials. The various mono-ketones having thiophene rings were
substituted onto fullerene through application of Hirch-Bingel reaction under mild
condition to compete with donor materials especially P3HT [Poly(3-hexylthio-
phene)] for nano-networking bulk-heterojunction structure where efficient light-
induced charge separation is enable by a large-area donor-acceptor interface.
The newly synthesized methanofullerene derivatives were characterized through
1
H-NMR and FAB Mass spectroscopy. The compounds were also applied in OPV
cell devices with P3HT as a donor material and measured electrochemical the
property and power conversion efficiency.
Keywords: acceptor material; fullerene derivatives; n-type semiconductor; organic
photovoltaic cells; organic solar cells
INTRODUCTION
Recently, the organic photovoltaic cells (OPVCs) using the mixture of
conjugated polymeric materials and donor materials have much
attracted due to the feasibility of low cost production and easy manu-
facturing by solution process [1–2]. Especially, the organic ‘bulk hetero-
junction’ structure as an active layer, where efficient light-induced
This study was supported by a grant from the Fundamental R&D Program for Core
Technology of Materials funded by the Ministry of Commerce, Industry and Energy,
Republic of Korea.
Address correspondence to Changjin Lee, Advanced Materials Division, Korea
Research Institute of Chemical Technology, P.O. Box 107, Sinseongno 19, Yuseong-gu,
Daejeon 305-600, Korea. E-mail: cjlee@krict.re.kr
293=[657]

2
94=[658]
H. S. Lee et al.
charge separation is capable by a large-area donor-acceptor interface
3]. It is known that the achieving high performance in organic solar cell
critically depends on increased interface between a conjugated polymer
donor) and a methanofullerene derivative (acceptor), also the formation
of networking path for charge transport [4]. Poly(3-hexylthiophene)
P3HT) has widely used as a donor promising material for OPV cells
[
(
(
due to the relatively low band gap and high charge carrier mobility.
Furthermore, optical and electro property of P3HT can easily controlled
by thermal treatment, changing solvent, evaporation conditions. The
highest efficiency of OPVCs have been reported around 5% using blends
of P3HT and 6,6-Phenyl C61 butyric acid methyl ester (PCBM) after
thermal annealing process. However, the device performance critically
depends on the properties of donor=acceptor materials and the condition
of device fabrication [5]. This variability is believed to be influenced on
the self-organizing properties of P3HT, which means that both optical
and electronic properties are sensitive to the molecular packing. But,
the relationship between the molecular properties of the blended
materials and the device performance has not been investigated clearly.
In our previous paper, we described the feasibility of newly synthe-
sized 1,3-diketone modified C60s as acceptor materials in OPVCs and
this OPVC performed over 3.4% power conversion efficiency under
AM 1.5 condition [6]. Here, we are focused on the feasibility of new
methanofullerene derivatives with various mono-ketones as acceptor
materials in OPVCs through Hirch-Bingel reaction which has been
known promising method of synthesizing methanofullerene derivative
under mild condition.
EXPERIMENTAL
Materials
Thiophene, bithiophene, hexanoyl chloride, octanoyl chloride, N-
bromosuccinimide (NBS), n-butyllithium (2.5 M solution in hexane),
magnesium, carbon tetrabromide (CBr ), 1,8-diazabicylo[5.4.0]undec-
4
7
-ene (DBU), and other reagents were purchased from Aldrich
Chemicals and used without further purification. P3HT [Poly(3-
hexylthiophene)] was purchased from Rieke Metals.
Synthesis of Mono-ketone Modified Methanofullerene
Derivatives
General Procedure
0
2
-Hexylthiophene, 2-hexyl-5-5 -bithiophene, 1-(thiophen-2-yl)octan-1-one,
0
1-(2,2 -bithiophen-5-yl)octan-1-one, 1-(5-hexylthiophen-2-yl)hexan-1-one,

Mono-Ketone Modified C60
s
295=[659]
SCHEME 1 Synthesis of methanofullerene derivatives containing thiophenyl
ꢀ
group. Reaction conditions: (i) NBS, DMF, r.t., 5 h; (ii) Mg, THF, 0 C, 1 h and
4
r.t., 3 h; (iii) o-DCB, CBr , DBU, r.t., 2 h.
0
0
and 1-(5 -hexyl-2,2 -bithiophen-5-yl)hexan-1-one were synthesized in
the similar methods described in the literature [7–8]. Fullerene C60
(
1.0 mmol) was stirred overnight in o-dichlorobenzene solution. And
then, 1.2 equiv. of mono-ketone and 1.2 equiv. of carbon tetrabromide
were added into the reaction mixture. After further stirring for 2 h, 2
equiv. of 1,8-diazabicylo[5.4.0]undec-7-ene (DBU) was slowly added
to the reaction mixture. The reaction mixture was kept stirring for
2
h. Products were purified through column chromatography with
a mixture of hexane and methylene chloride (3:1 by volume) and recrys-
tallization in hexane to obtain as brown solid.
Synthesis of Compound 1 (Yield: 37%)
1
H-NMR (300 MHz, CDCl ): d 8.41-8.39 (d, J ¼ 3.0 Hz, 1H, thiophe-
3
nyl), 7.84-7.83 (d, J ¼ 1.5, 1H, thiophenyl), 7.34-7.31 (t, J ¼ 3.9,
1
H, thiophenyl), 3.09-3.04 (t, J ¼ 7.8, 2H, CH ), 2.04-1.93 (m, 2H,
2
CH ), 1.37-1.25 (m, 6H, CH ), 0.93-0.86 (t, 3H, CH ) ppm. FAB-
MS: 929 (M ).
2
2
3
þ
Synthesis of Compound 2 (Yield: 35%)
1
H-NMR (300 MHz, CDCl ): d 8.24-8.23 (d, J ¼ 3.0 Hz, 1H, thiophenyl),
3
6
2
.99-6.98 (d, J ¼ 3.0 Hz, 1H, thiophenyl), 3.08-3.02 (t, J ¼ 8.1 Hz,
H, CH ), 2.96-2.87 (t, J ¼ 12 Hz, 2H, CH ), 1.96-1.75 (m, 4H,
2
2
CH ), 1.35-1.25 (m,8H, CH ), 1.04-0.90 (m,6H, CH ) ppm; FAB-MS:
2
3
3
þ
1
011 (M ).

2
96=[660]
H. S. Lee et al.
Synthesis of Compound 3 (Yield: 33%)
1
H-NMR (300 MHz, CDCl ): d 8.31-8.30 (d, J ¼ 1.5 Hz, 1H, thiophenyl),
3
7
3
6
.44-7.34 (m, 3H, thiophenyl), 7.12-7.09 (t, J ¼ 3.7, 1H, thiophenyl),
.10-3.05 (t, J ¼ 7.8, 2H, CH ), 2.04-1.93 (m, 2H, CH ), 1.39-1.26 (m,
2
2
þ
H, CH ), 0.93-0.86 (t,3H, CH ) ppm; FAB-MS: 984 (M ).
2
3
Synthesis of Compound 4 (Yield: 39%)
1
H-NMR (300 MHz, CDCl ): d 8.29-8.27 (d, J ¼ 3.0 Hz, 1H, thiophenyl),
3
7
3
1
1
.25-7.19 (m, 2H, thiophenyl), 6.77-7.76 (d, J ¼ 1.9, 1H, thiophenyl),
.10-3.05 (t, J ¼ 9.4, 2H, CH ), 2.85-2.80 (t, J ¼ 7.4, 2H, CH ), 2.02-
2
2
.92 (m, 2H, CH ), 1.72-1.67 (m, 2H, CH ), 1.31-1.25(m, 8H, CH ),
2
2
2
þ
.05-0.83 (t,6H, CH ) ppm; FAB-MS: 1067 (M ).
3
Device Fabrication
A typical OPVCs in this study contained a layer of polymer thin film
between transparent anode (indium tin-oxide, ITO) and a LiF=Al cath-
ode. The active polymeric material was a mixture of each new metha-
nofullerene as an acceptor material and P3HT as a donor material.
To fabricate the organic photovoltaic cells, the patterned ITO glass sub-
strates were cleaned by ultrasonic treatment in detergent, deionized
water, acetone, and isopropyl alcohol, sequentially. After treatment
of UV=ozone on the surface of ITO, PEDOT-PSS (Bytron, Al 4083)
was casted by spin-coating a thin layer (about 40 nm). Post thermal
ꢀ
treatment was followed on the substrates at 200 C for 5 min. And
then, solutions of P3HT blended with each new methanofullerenes
were spin-coated from o-dichlorobenzene (o-DCB) solution on the
PEDOT-PSS layer. Finally, the cathode consisting of LiF and Al layer
was thermally deposited on top of the active layer under vacuum of
ꢁ
7
2
1
0
torr. The active area size of device was measured around 0.04 cm .
Instruments
1
H-NMR spectra were obtained on Bruker DPX-300 spectrometer at
3
1
00 MHz. Cyclic votametry was measured in solution state on Bas
00B electrochemical analyzer at room temperature. UV-Vis absorp-
tion spectra of each compound were taken by Shimadzu UV-2550 spec-
trophotometer. Photocurrent-voltage curve was measured by Keithley
2
400 under AM 1.5 simulated light (ORIEL 300 W Xenon lamp). The
2
light intensity (100 mA=cm ) of the illumination source was confirmed
by using a standard silicon photodiode (BS520, Bunkoh-Keiki).

Mono-Ketone Modified C60
s
297=[661]
RESULTS AND DISCUSSION
Design and Synthesis of Methanofullerene Derivatives
with Various Ketones
To improve the solubility of fullerene and enhance the electron affin-
ity, we have designed methanofullerene derivatives with various
ketones. In addition, we incorporated thiophenyl and bithiophenyl
groups to increase the compatibility with conventional donor polymer,
such as P3HT. The mono-ketone compounds having thiophenyl
group were easily synthesized from the method in the literature
[
7–8]. Compound 1, 2, 3, and 4 were produced as similar procedure
at literature through Hirch-Bingel reaction under mild condition
9–10]. Fullerene C60 (1.0 mmol) was reacted with slightly excess of
[
ketones in the presence of carbon tetrabromide and 2 equiv. of 1,8-
diazabicylo[5.4.0]undec-7-ene (DBU). After work-up, major products
can be purified by column chromatography with the mixture of hexane
and methylene chloride as an eluent. Compound 1, 2, 3, and 4 can be
obtained as yield of 35%, 37%, 33%, and 39%, respectively.
The molecular structure of each compound was characterized by
H NMR and the FAB (fast atom bombardment) mass spectroscopy.
1
Also, we can confirm that all of these compounds have good solubility
in common solvent, such as, o-dichlorobenzene, mono-chlorobenzene,
chloroform, and methylene chloride.
Photophysical and Electrochemical Properties of
Methanofullerene Derivatives with Various Ketones
We have performed the UV absorption experiments. Figure 1 shows
the UV-Visible absorption of each compound in methylene chloride
solution. Each compounds feature two major absorption peaks at 480
and 500 nm, and one small peak at 700 nm (Figure 2). Compound 2
and 4 show red-shifted absorption peaks compare to compound 1 and 3
by several nm. This means that bithiophenyl group of compound 2 and
4
is much more effective electron acceptor than thiophenyl group of com-
pound 1 and 3. From this result, we can control the bandgap of methano-
fullerene by verifying the attached moiety on the fullerene molecules.
Each Compound shows slightly different cyclic voltammograms
with three distinctive quasireversible reduction waves in the potential
ranging from 0.0 V to ꢁ2.0 V. To get clear and distinguished curves,
we choose tree representative curves in Figure 3 and more detailed
data were listed in Table 1. As a result of this experiment, we could
make result that compound 2 and 4 having bithiophenyl group showed
much lower reversible reductive potentials in comparison to compound

2
98=[662]
H. S. Lee et al.
FIGURE 1 Structure of mono-ketone modified C60s.
and 3 by 0.02 V. This means the LUMO energy levels of compound 2
1
and 4 are slightly lower than those of compound 1 and 3, that is to say,
LUMO level of bithiophene substituted compounds (2 and 4) is low-
ered by increasing electron density. We believe that the more fluent
FIGURE 2 UV Absorption Spectra of Compound 1, 2, 3, and 4.

Mono-Ketone Modified C60
s
299=[663]
FIGURE 3 Cyclic voltamograms of compound 1, 2, 3, and 4.
electrons of substituent on fullerene can make the lower LUMO
level, the electron of bithiophenyl group is much richer than that of
thiophenyl. We confirmed the relation of the LUMO level by Cyclic
Voltammetry experiment.
The Performances of Organic Photovoltaic Cells
We fabricated the organic photovoltaic cells using a mixture of P3HT
and one of newly synthesized methanofullerene derivatives. The J-V
characteristics of OPVCs are shown in Figures 4–5 and other perfor-
mances are summarized in Table 2. As shown in Figure 5, OPVCs
using compound 1–4 as acceptor material show good diode behavior
at the range of ꢁ0.5 and ꢁ0.4 V in the dark conditions. In addition,
current density of each device under dark is very low value about
TABLE 1 Electrochemical Properties and Energy Levels of Compound 1, 2,
3
, and 4
1
2
3
ꢂ
LUMO (eV)
Compound
E
red(V)
E
red (V)
E
red(V)
1
2
3
4
ꢁ0.45
ꢁ0.43
ꢁ0.44
ꢁ0.42
ꢁ0.86
ꢁ0.85
ꢁ0.85
ꢁ0.86
ꢁ1.37
ꢁ1.36
ꢁ1.36
ꢁ1.39
ꢁ4.18
ꢁ4.16
ꢁ4.17
ꢁ4.15
ꢂLUMO energy level of PCBM was used as reference as ꢁ 4.2 eV.

3
00=[664]
H. S. Lee et al.
FIGURE 4 J-V characteristics of ITO=PEDOT: PSS=P3HT: Mono-ketone
modified methanofullerene=LiF=Al heterojunction at pristine condition.
ꢁ
6
2
1
0
mA=cm . This low leakage current value proves that the interfa-
cial resistance of our OPVCs is very low and device fabrication skill is
not bad to realize reasonable OPVC.
FIGURE 5 J-V characteristics of ITO=PEDOT: PSS=P3HT: Compound=
ꢀ
LiF=Al after annealing at 170 C for 5 min in a logarithmic scale.

Mono-Ketone Modified C60
s
301=[665]
TABLE 2 Photovoltaic Properties of OPVC at Pristine Condition Using one
of Compound 1, 2, 3, and 4 as Acceptor Materials
P3HT:1
P3HT:2
P3HT:3
P3HT:4
Voc (mV)
Jsc (mA=Cm )
FF (%)
502
3.2
45
505
3.9
44
429
5.0
38
555
4.7
47
2
PCE (%)
0.73
0.87
0.84
1.27
ꢂ
2
Under AM 1.5 simulated light illumination (100 mW=cm ).
The device performances were measured under pristine condition,
that is to say, without thermal treatment. Power conversion efficiency
of each device is recorded from 0.73% to 1.27%. Devices using com-
pound 2 or 4 as acceptor materials show better performance than
those using compound 1 or 3 as acceptor materials. As you can see
in the previous Cyclic Voltammetry experiment, we have already
confirmed that bithiophenyl group has much higher electron attrac-
tion ability than mono-thiophenyl group when they are attached
at fullerene C60s. Now, we can conclude that the number of substi-
tuted thiophene rings is proportionate in the electron affinity of
methanofullerene.
In conclusion, we are designed and synthesized four kinds of mono-
ketone modified fullerenes (compound 1, 2, 3, and 4) having thiophe-
nyl or bithiophenyl substituent. And OPVCs using these compounds
as acceptor materials with P3HT donor shows moderate power conver-
sion efficiency around 1% under AM 1.5 simulated light illumination.
Especially, compound 4 shows highest efficiency as 1.27% under pris-
tine condition. Compound 4 has the highest solubility and lowest
LUMO level among newly synthesized methanofullerene derivatives
in this paper. High solubility and low LUMO level of compound 4
can affect to modify the donor-acceptor bulk-heterojunction and
improve electron attraction ability from P3HT donor. Consequently,
we can confirm that solubility and electron affinity of acceptor mole-
cules are very important factors to increase power conversion
efficiency in OPVCs and we are studying on the improvement of
photovoltaic performance using compound 4.
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