Silyl substituted methanofullerenes as electron acceptors in organic photovoltaic cells

Article abstract of DOI:10.1080/15421401003604203

The electron-accepting capability of fullerenes has made them interesting materials for organic photovoltaic cells (OPVCs). In this study, we propose the new methanofullerene derivatives incorporated with trimethylsilyl moiety to increase the solubility without electron transporting property decrease. Methanofullerene acceptor materials having trimethylsilyl substituent (PCB-Sis) were synthesized from the reaction of fullerene with trimethylsilyl substituted hydrazide type derivatives, electrochemical propertie was examined by cyclicvoltametry. OPVCs fabricated by using the mixture of P3HT and mono-adducts of PCB-Sis (Compound 7-9) as active layer showed 2.82% of power conversion efficiency under 100 mW/cm² AM1.5 simulated light after annealing at 150C for 10min. Also, we have synthesized bis-adduct type PCB-Sis as moderate yield and obtained increased Voc about 0.5-0.14V from P3HT and bis-adduct of PCB-Sis (Compound 10-12) based OPVCs. Unfortunately, these bis-adducts show slightly lower power conversion efficiency compare to mono-adducts due to the numerous region-isomers of bis-adducts. Although it was revealed our new PCB-Sis were candidates for acceptor materials in OPVCs.

Full text of DOI:10.1080/15421401003604203

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Silyl Substituted Methanofullerenes
as Electron Acceptors in Organic
Photovoltaic Cells
Mina Jeon a , Jaewook Jung a , Jaemin Lee a , Jongsun Lim a , Won
Seok Shin a , Sang-Jin Moon a , Changjin Lee a & Sung Cheol Yoon a
a Advanced Materials Division, Korea Research Institute of Chemical
Technology , Daejeon, Korea
Published online: 19 Apr 2010.
To cite this article: Mina Jeon , Jaewook Jung , Jaemin Lee , Jongsun Lim , Won Seok Shin , Sang-
Jin Moon , Changjin Lee & Sung Cheol Yoon (2010) Silyl Substituted Methanofullerenes as Electron
Acceptors in Organic Photovoltaic Cells, Molecular Crystals and Liquid Crystals, 519:1, 266-275, DOI:
10.1080/15421401003604203
To link to this article: http://dx.doi.org/10.1080/15421401003604203
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Mol. Cryst. Liq. Cryst., Vol. 519: pp. 266–275, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421401003604203
Silyl Substituted Methanofullerenes as Electron
Acceptors in Organic Photovoltaic Cells
MINA JEON, JAEWOOK JUNG, JAEMIN LEE,
JONGSUN LIM, WON SEOK SHIN, SANG-JIN MOON,
CHANGJIN LEE, AND SUNG CHEOL YOON
Advanced Materials Division, Korea Research Institute of Chemical
Technology, Daejeon, Korea
The electron-accepting capability of fullerenes has made them interesting materials
for organic photovoltaic cells (OPVCs). In this study, we propose the new metha-
nofullerene derivatives incorporated with trimethylsilyl moiety to increase the solu-
bility without electron transporting property decrease. Methanofullerene acceptor
materials having trimethylsilyl substituent (PCB-Sis) were synthesized from the
reaction of fullerene with trimethylsilyl substituted hydrazide type derivatives, elec-
trochemical propertie was examined by cyclicvoltametry. OPVCs fabricated by
using the mixture of P3HT and mono-adducts of PCB-Sis (Compound 7–9) as
active layer showed 2.82% of power conversion efficiency under 100 mW=cm2
AM1.5 simulated light after annealing at 150ꢀC for 10 min. Also, we have synthe-
sized bis-adduct type PCB-Sis as moderate yield and obtained increased Voc about
0.5–0.14 V from P3HT and bis-adduct of PCB-Sis (Compound 10–12) based
OPVCs. Unfortunately, these bis-adducts show slightly lower power conversion
efficiency compare to mono-adducts due to the numerous region-isomers of bis-
adducts. Although it was revealed our new PCB-Sis were candidates for acceptor
materials in OPVCs.
Keywords Acceptor; C60; fullerene; n-type; OPVCs; organic photovoltaic cells;
organic semiconductor; organic solar cells; silyl
Introduction
Recently, organic photovoltaic cells (OPVCs) have attracted much attention, due to
their ease of fabrication, potential for low-cost production and easy methods to pro-
duce energy from light [1]. For OPVCs, the open circuit voltage (Voc), the
short-circuit current density (Jsc) and the fill factor (FF) are basic and important
parameters, which can affect the efficiency directly. Several research groups have
reported that such solution-processed organic photovoltaic cells can achieve power
conversion efficiencies under standard illumination conditions (AM 1.5G, 100
mW=cm2) in the 4–5% range with poly(3-hexylthiophene) (P3HT) [2–5] representing
the cost efficient OPVCs reported to date. Nevertheless, there is still plenty of room
Address correspondence to Sung Cheol Yoon, Korea Research Institute of Chemical
Technology, Jang-dong 100, Yusung-gu Daejeon 305-600, Korea. E-mail: yoonsch@krict.
re.kr
266

Silyl Substituted Methanofullerenes as Electron Acceptors
267
to improve the efficiency of this type of bulk-heterojunction photovoltaic cell.
Recently the necessity of improving the Voc has attracted much attention [6]. There
is a need for optimizing the electronic match between the donor and acceptor
component, in order to minimize unnecessary internal loss of Voc. Brabec et al.
reported, within certain limits with respect to the electrodes’ workfunction (i.e., as
long as there are ohmic contacts at both electrodes), the Voc of a bulk-
heterojunction PV cell scales linearly with the decrease of the first reduction potential
of the acceptor [7,8]. Later, others confirmed the related relationship, between the
Voc and the first oxidation potential of the donor material [9]. Hence, there is, within
certain limits, a linear relationship between the HOMO level of donor and the
LUMO level of acceptor energy difference and the Voc of the bulk-heterojunction
device. Raising the LUMO of the acceptor, on the other hand, will directly result
in a higher Voc without affecting the absorption of the cell. It has been shown that
the second approach is theoretically more beneficial for a single-layer solar cell, and
results in an estimated efficiency of 8.4% when the LUMO offset is reduced to 0.5 eV
[10]. To improve Voc, it is necessary to decrease the HOMO energy of donor and
increase the LUMO energy of the Acceptor.
During the past several years, we have already reported lots of fullerene type
acceptor materials for organic photovoltaic cells to improve the OPV performance
[11–13]. Now, we propose the new methanofullerene derivatives incorporated with
trimethylsilyl moiety which are mono- and bis-adducts to increase the solubility with-
out electron accepting performance decrease, as a new fullerene-based n-type semi-
conductor material. In addition, we expected that our bis-adducts can improve the
power conversion efficiency due to the LUMO level increase.
Experimental
Materials
P3HT was purchased from Rieke Metals and C60 were purchased from Bucky USA.
Other chemicals including solvents and reagents were purchased from Aldrich
Chemicals and used without further purification.
Syntheses of PCB-Sis
Methyl 5-(4-(Trimethylsilyl)phenyl)5-oxopentanate (Compound 1). A solution of
1-bromo-4-(trimethylsilyl)benzene (5.52 g, 24.1 mmol) in dry THF (100 ml) was
placed under nitrogen. Under stirring, a solution of n-BuLi (15 ml, 1.6 N in hexane,
24 mmol) was added dropwise while the mixture was kept at ꢁ78ꢀC. The resulting
homogeneous solution was allowed to warm up to ꢁ55ꢀC in 30 min. Copper(I)
iodide (2.38 g, 12.49 mmol) was then added at once, resulting in a creamy brown
solution. The mixture was allowed to warm up to ꢁ30ꢀC in 20 min. The obtained
dark brown slurry was stirred for another 30 min, after which methyl 5-chloro-5-
oxopentanoate (1.66 g, 10.1 mmol) was added, resulting in a green mixture. The
mixture was allowed to warm up to 0ꢀC in 1 h. Dry methanol (4 ml) was added,
resulting in some precipitation. Aqueous ammonium chloride (50 ml) was added to
the reaction mixture. The mixture was then filtered through celite. And the filtrate
separated two layers of blue and green. The aqueous (blue) layer was extracted
with ether (3 ꢂ 50 ml). The combined organic extracts were washed with water

268
M. Jeon et al.
(100 ml) and dried over anhydrous magnesium sulfate and concentrated in vacuo
yielding a greenish liquid. The crude compound 1 was purified by column
chromatography using the ethyl acetate and hexane to produce transparent oil.
1
Compound 1 (Yield: 55%): H NMR (CDCl3, ppm) d 7.92–7.90 (d, J ¼ 8.1, 2H,
ArH), 7.62–7.59 (d, J ¼ 8.1, 2H, ArH), 3.67 (3H, s, CH3), 3.07–3.02 (t, J ¼ 7.2, 2H,
CH2), 2.47–2.42 (t, J ¼ 7.2, 2H, CH2), 2.09–2.04 (t, J ¼ 7.2, 2H, CH2), 0.28 (9H, s,
CH3).
3-(Trimethylsilyl)alkyl 5-Oxo-5-phenylpentanoate (Compound 2 and 3). Diethyl
azodicarboxylate (DEAD) (9.37 mmol) was added dropwise into a solution of
trimethylsilyl ethanol (7.81 mmol), triphenylphosphine (9.37 mmol), and benzoyl
butyric acid (7.81 mmol) in THF (50 mL) under nitrogen atmosphere. The reaction
mixture was stirred at room temperature for 24 hr. After filtration, extraction was
carried out with use of ethyl acetate. The combined organic layers were washed
with water (3 ꢂ 100 ml) and then dried over anhydrous magnesium sulfate. The
crude compound 2 or 3 was purified by column chromatography using the
mixture of ethyl acetate and hexane to produce transparent oil.
1
Compound 2 (Yield: 60%): H NMR (CDCl3, ppm) d 7.94–7.91 (d, J ¼ 9.0, 2H,
ArH), 7.51–7.39 (m, 2H, ArH), 4.17–4.11 (t, J ¼ 8.4, 2H, CH2), 3.04–2.99 (t, J ¼ 7.2,
2H, CH2), 2.38–2.36 (t, J ¼ 6.9, 2H, CH2), 2.05–2.01 (m, 2H, CH2), 0.97–0.92 (t,
J ¼ 9.0, 2H, CH2), 0.001 (s, 9H, CH3).
1
Compound 3 (Yield: 65%): H NMR (CDCl3, ppm) d 8.02–7.99 (d, J ¼ 8.7, 2H,
ArH), 7.60–7.47 (m, 3H, ArH), 4.10–4.06 (t, J ¼ 6.9, 2H, CH2), 3.13–3.08 (t, J ¼ 7.2,
2H, CH2), 2.51–2.46 (t, J ¼ 7.2, 2H, CH2), 2.15–2.10 (t, J ¼ 7.2, 2H, CH2), 1.68–1.62
(m, 3H, CH3), 0.56–0.50 (m, 2H, CH2), 0.04 (9H, s, CH3).
General Procedure of Benzoyl Butyrate P-Tosylhydrazone (Compound 4–6). One of
Compound 1, 2, and 3 (5.88 mmol) was mixed with p-touenesulfonyl hydrazide
(6.47 mmol) in 30 ml MeOH. The mixture was stirred and refluxed for 5 hours.
The mixture was left without heating for 4 hours and then cooled to ꢁ10ꢀC.
Compound 4, 5, or 6 was crystallized from the mixture and collected by filtration
and washed with chilled MeOH.
Compound 4 (Yield: 65%): 1H NMR (CDCl3, ppm) d 9.13 (s, 1H, NH), 7.72–7.69
(d, J ¼ 8.4, 2H, ArSO2ꢁ), 7.45–7.42 (d, J ¼ 8.1, 2H, ArH), 7.29–7.26 (d, J ¼ 8.1, 2H,
ArH), 7.08–7.05 (J ¼ 8.4, 2H, ArSO2ꢁ), 3.51 (3H, s, CH3), 2.46–2.41 (t, J ¼ 8.1, 2H,
CH2), 2.15 (s, 3H, CH3), 2.11–2.06 (m, 2H, CH2), 1.50–1.45 (m, 2H, CH2), 0.03 (9H,
s, CH3).
Compound 5 (Yield: 85%): 1H NMR (CDCl3, ppm) d 9.30 (s, 1H, NH), 7.93–7.90
(d, J ¼ 9.0, 2H, ArSO2ꢁ), 7.66–7.63 (m, 2H, ArH), 7.35–7.32 (m, 3H, ArH),
7.31–7.28 (d, J ¼ 9.0, ArSO2ꢁ), 4.33–4.27 (t, J ¼ 9.0, 2H, CH2), 2.66–2.60 (t,
J ¼ 9.0, 2H, CH2), 2.40 (s, 3H, CH3), 2.30–2.26 (t, J ¼ 6.0, 2H, CH2), 1.72–1.63
(m, 2H, CH2), 1.07–1.01 (t, J ¼ 9.0, 2H, CH2), 0.001 (s, 9H, CH3).
Compound 6 (Yield: 95%): 1H NMR (CDCl3, ppm) d 9.30 (s, 1H, NH), 7.93–7.90
(d, J ¼ 8.4, 2H, ArSO2ꢁ), 7.66–7.63 (m, 2H, ArH), 7.35–7.32 (m, 3H, ArH),
7.30–7.27 (d, J ¼ 8.4, ArSO2ꢁ), 4.18–4.13 (t, J ¼ 7.2, 2H, CH2), 2.66–2.61 (t,
J ¼ 7.8, 2H, CH2), 2.40 (s, 3H, CH3), 2.33–2.29 (t, J ¼ 7.8, 2H, CH2), 1.71–1.66
(m, 4H, CH2), 1.07–1.01 (t, J ¼ 8.7, 2H, CH2), 0.02 (s, 9H, CH3).
General Procedure of PCB-Sis (Benzoyl C61 Butyric Acid Trimethylsilyl Alkyl Ether,
Compound 7–12). One of p-tosylhydrazone compounds 4, 5, and 6 (7.81 mmol) was

Silyl Substituted Methanofullerenes as Electron Acceptors
269
dissolved in 10 mL of dry pyridine in a dried two-necked flask provided with N2
inlet, a thermometer, and a magnetic stirring bar. Then, NaOMe (8.12 mmol) was
added, and the mixture was stirred during 15 min. A solution of C60 (3.51 mmol) in
o-DCB (100 mL) was added, and the homogeneous reaction mixture was stirred
overnight at 70ꢀC under argon atmosphere. The solution was heated to reflux, and
the reaction was allowed to continue overnight again. Bis-adducts of PCB-Sis are
normally obtained as by-product of the preparation of mono-adduct of PCB-Sis.
The crude compound was purified and separated by column chromatography using
the mixture of ethyl acetate and hexane. The solution was concentrated in vacuo,
redissolved in a minimal amount of dichloromethane, 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:Hexane, 5:1% v=v).
Compound 7 (Yield: 25%): 1H NMR (CDCl3, ppm) d 7.89–7.86 (d, J ¼ 8.1,
2H, ArH), 7.68–7.66 (d, J ¼ 8.1, 2H, ArH), 3.68 (s, 3H, CH3), 2.92–2.89
(t, J ¼ 8.1, 2H, CH2), 2.55–2.50 (t, J ¼ 7.5, 2H, CH2), 2.21–2.15 (m, 2H, CH2),
0.35 (s, 9H, CH3).
13C NMR (CDCl3, 75.49 MHz): d 174.50, 149.82, 146.91, 146.21, 146.05, 145.80,
145.68, 145.52, 145.44, 145.02, 144.78, 144.08, 143.94, 143.15, 141.99, 139.03, 138.75,
134.37, 132.26, 102.43, 80.90, 52.97, 52.65, 34.92, 34.72, 23.40, 0.02.
1
Compound 8 (Yield: 40%): H NMR (CDCl3, 300 MHz) d 7.94–7.92 (d, J ¼ 6.0,
2H, ArH), 7.57–7.47 (m, 3H, ArH), 4.19–4.14 (d, J ¼ 9.0, 2H, CH2), 2.94–2.88
(t, J ¼ 6.0, 2H, CH2), 2.52–2.47 (t, J ¼ 7.4, 2H, CH2), 2.21–2.16 (m, 2H, CH2),
1.00–0.94 (t, J ¼ 8.4, 2H, CH2), 0.04 (s, 9H, CH3).
13C NMR (CDCl3, 75.49 MHz): d 150.29, 149.29, 147.32, 146.64, 146.47, 146.11,
145.93, 145.45, 144.44, 144.38, 143.65, 143.59, 142.44, 142.20, 139.51, 139.02, 133.55,
129.88, 81.37, 64.20, 53.38, 35.78, 35.17, 23.86, 18.81, 0.04.
1
Compound 9 (Yield: 45%): H NMR (CDCl3, ppm) d 7.94–7.92 (d, J ¼ 6.9, 2H,
ArH), 7.57–7.47 (3H, m, ArH), 4.05–4.00 (t, J ¼ 7.2, 2H, CH2), 2.94–2.89 (t, J ¼ 8.1,
2H, CH2), 2.55–2.50 (t, J ¼ 7.5, 2H, CH2), 2.21–2.16 (m, 2H, CH2), 1.63–1.59 (m,
2H, CH2), 0.51–0.45 (t, J ¼ 8.7, 2H, CH2), 0.004 (s, 9H, CH3).
13C NMR (CDCl3, 75.49 MHz): d 174.79, 150.55, 149.56, 147.60, 146.92, 146.88,
146.82, 146.39, 146.15, 145.49, 144.76, 144.72, 144.66, 143.97, 143.91, 143.84, 139.78,
139.29, 133.82, 130.16, 81.64, 68.98, 53.64, 35.87, 35.42, 24.99, 24.15, 14.22, 0.02.
Compound 10 (Yield: 20%): 1H NMR (CDCl3, ppm) d 8.11–7.54 (m, 4H, ArH),
3.70–3.62 (m, 3H, CH3) 2.77–2.01 (m, 6H, CH2), 0.41–0.27 (m, 9H, CH3).
Compound 11 (Yield: 40%): 1H NMR (CDCl3, ppm) d 8.16–7.33 (m, 5H, ArH),
4.23–4.08 (m, 2H, CH2), 2.78–2.05 (m, 6H, CH2), 1.03–0.89 (m, 2H, CH2), 0.06–0.00
(m, 9H, CH3).
Compound 12 (Yield: 40%): 1H NMR (CDCl3, ppm) d 8.17–7.37 (m, 5H, ArH),
4.10–3.96 (m, 2H, CH2), 2.81–2.05 (m, 6H, CH2), 1.69–1.61 (m, 2H, CH2), 0.51–0.43
(m, 2H, CH2), 0.02–0.00 (m, 9H, CH3).
Electrochemical and Photophysical Properties of PCB-Sis
The cyclicvoltametry (BAS 100) was performed with a solution of tetrabutylammonium
tetrafluoroborate (Bu4NþBF4ꢁ, 0.10M) as the electrolyte and material (10ꢁ3 M) in
o-dichlorobenzene at a scan rate of 100 mV=s at room temperature under argon atmos-
phere. A glassy carbon electrode (0.3mm diameter) was used as the working electrode.

270
M. Jeon et al.
A Pt and an Ag=Agþ electrode were used as the counter electrode and reference
electrode, respectively.
Device Fabrications
The organic photovoltaic cells were built in sandwiched structure between a trans-
parent anode and cathode by spin-coating. The anode consisted of glass substrates
pre-coated with indium tin oxide (ITO), modified by spin-coating PEDOT=PSS
(Bytron P, AI4083) layer (40 nm) followed thermal treatment at 200ꢀC for 5 min
after UV=ozone on the surface of ITO, and then a blend of P3HT and one of new
compound mixture in o-dichlorobenzene was spin-coated. Metal cathode was
˚
deposed by thermal evaporation consisted with LiF and Al approximately 0.5 A
and 130 nm thick. Finally, all devices were encapsulated with glass cap containing
moisture capture materials, calcium oxide. Additional thermal annealing was
proceeded at 150ꢀC for 10 minutes.
J-V Measurements
Photocurrent-voltage curve measurements of each device were performed at room
temperature. The current-voltage (I-V) curves of the devices were obtained using
a Keithley 2400 Source Measure Unit under AM 1.5 simulated light (ORIEL
300 W Xenon lamp). The intensity of light (100 mA=cm2) from the illumination
source was confirmed by using a standard silicon photodiode (BS520, Bunkoh-
Keiki).
Results and Discussion
Syntheses and Characterizations of PCB-Sis
The synthetic scheme of trimethylsilyl-substituted [60]methanofullerene derivatives
(PCB-Sis, compound 7–12) is depicted in Scheme 1. The methyl 5-(4-(trimethylsilyl)-
phenyl)-5-oxopentanate (1) was prepared by Friedel-Crafts acylation of 1-bromo-
4-(trimethylsilyl)-benzene with n-BuLi (55%). The benzoylbutyric acid was then
converted to the compound 2 and 3 in trimethylsilyl ethanol, trimethylsilyl butanol
under Fischer esterification conditions (70%). The syntheses of PCB-Sis (compound
7–9) were carried out by the Wudl-Hummelen approach [14]. Condensation of
compound 1–3 with p-tosylhydrazide in methanol afforded the corresponding
p-tosylhydrazone compound 4–6. After sequential treatment with sodium methoxide
in pyridine and fullerene C60 in refluxing o-dichlorobenzene, mono-adducts of
PCB-Sis (compound 7–9) were obtained with the yield from 30% to 40%.
Although both [6,6] and [5,6] isomers could be expected from the cycloaddition
reaction, the high temperature allowed the isolation of the thermodynamically more
stable [6,6] isomer, exclusively, as determined by 1H-NMR. Synchronously, bis-
adducts (compound 10–12) can be produced with the proceeding of the reaction
(yield: 20–40%), which could be separated using standard column chromatography.
In addition, we can confirm that bis-adducts contain of a number of region-isomers
1
1
by H-NMR and FAB-Mass spectroscopy. The H NMR spectra of mono- (7) and
1
bis TMS-PCBM (10) are shown in Figure 1. From the H NMR measurements, the
methylene protons of substituents show widely split peaks around 0.08–0.14 ppm,

Silyl Substituted Methanofullerenes as Electron Acceptors
271
Scheme 1. Syntheses of PCB-Sis (Compounds 7–12).
Figure 1. 1H NMR spectra of mono- (7) and bis TMS-PCBM (10).

272
M. Jeon et al.
which proved the presence of at least 10 methoxy resonace signals indicating that the
bis-adducts consisted of a much more complex mixture of isomer. In addition, all of
our PCB-Sis (Compound 7–12) show much higher solubility in common organic sol-
vents, such as chloroform, chlorobenzene, o-dichlorobenzene, and so on. Generally,
the purity of organic semiconductor is very important to perform the charge trans-
portation.
Electrochemical Properties of PCB-Sis
The electrochemical properties of PCB-Sis were studied by cyclic voltammetry (CV)
at room temperature in o-DCB solution. Each of new methanofullerenes shows simi-
lar cyclic voltammograms of thee reversible reductive potentials ranged from 0.0 V to
ꢁ2.4 V with PCBM. Tree representative curves were chosen to distinguish clearly
reduction potential as shown in Figure 2 and Table 1.
From these experiments, the first reductive potentials of all mono-adducts of
PCB-Sis (Compound 7–10) show similar with that of PCBM. On the other hand,
the first reductive potentials of bis-adducts (compound 10–12) show much more
negative than that of PCBM. This means that bis-adducts of PCB-Sis have much
higher LUMO energy level compare to mono-adducts of PCB-Sis and PCBM by
about 0.13–0.17 V. It can be concluded that we can obtain highly efficient OPVCs
due to the Voc improvement.
The Performances of Organic Solar Cells
The organic photovoltaic cells were fabricated using one of newly synthesized
PCB-Sis as electron acceptor with conventional donor polymer (regioregular
P3HT) as a donor material. The J-V characteristics of OPVCs are shown in
Figure 3 and Voc, Jsc, FF, and power conversion efficiency (PCE) were summarized
in Table 2. The device performance were measured under 100 mW=cm2 AM 1.5
simulated light illumination after annealing at 150ꢀC for 10 min.
Power conversion efficiency of each device is recorded from 0.57 to 2.82%.
And open circuit voltage (Voc) of each device is recorded from 0.64 to 0.78 V.
As shown in the previous Cyclic Voltammetry experiment, we have already con-
firmed that bis-adducts have much higher LUMO level than mono-adducts. How-
ever, devices using bis-PCB-Sis as acceptor materials show lower power conversion
efficiency (1.53%) than OPVCs using mono-PCB-Sis as acceptor materials. We
expected that the regio-isomers of bis-adducts have various electronic states and
this mixture may influence on the electron transporting performance of acceptor
molecules from donor-acceptor junction to electrode. Actually, the lower power
conversion efficiency of OPV using bis-adducts as acceptors is resulted from the
lower current density and fill factor. This means a lot of photo-generated charges
are dissociated in the channel of the n-type materials. Therefore, it is very impor-
tant to purify the acceptor molecules because the purity of bis-adducts is a key
factor to determine the photovoltaic property. And now, we are studying to
develop the purification process of bis-PCB-Sis to increase the power conversion
efficiency in OPVCs.
In conclusion, we are designed and synthesized a series of novel fullerene
derivatives (mono- and bis-PCB-Sis, compound 7–12) in order to minimize the energy
loss in the electron transfer from the donor to the acceptor material in bulk

Silyl Substituted Methanofullerenes as Electron Acceptors
273
Figure 2. Cyclic voltamograms of PCB-Sis and PCBM.
Table 1. Electrochemical properties and energy levels of the
PCB-Sis and PCBM
Compounds
E11=2 reduction (V)
LUMO (eV)
7
8
9
10
11
12
ꢁ0.90
ꢁ0.90
ꢁ0.90
ꢁ1.07
ꢁ1.03
ꢁ1.03
ꢁ0.90
ꢁ3.65
ꢁ3.65
ꢁ3.65
ꢁ3.48
ꢁ3.52
ꢁ3.52
ꢁ3.65
PCBM
ꢃExperimental conditions: Bu4NF4Br (0.1 M) as the supporting electro-
lyte, ODCB as the solvent, 50 mV=s scan rate.
Figure 3. J-V characteristics of ITO=PEDOT:PSS=P3HT:PCB-Sis=LiF=Al after annealing
condition [PCB-Sis: (a) mono-adduct; (b) bis-adduct].

274
M. Jeon et al.
Table 2. Photovoltaic properties of OPVC after annealing at 150ꢀC
for 10 min using one of new compounds as acceptor materials
Acceptors
Voc (V)
Jsc (mA=cm2)
FF (%)
g (%)
7
8
9
10
11
12
0.70
0.64
0.67
0.75
0.78
0.77
7.15
7.11
7.72
2.56
2.27
5.86
40.8
43.9
54.3
29.8
35.5
34.0
2.03
2.01
2.82
0.57
0.58
1.53
ꢃITO=PEDOT-PSS=P3HT:Acceptor=LiF=Al.
ꢃUnder AM 1.5 simulated light illumination (100 mW=cm2).
heterojunction solar cells. The additional functionalization of the fullerene cage in
bis-adduct was shown to have little negative influence on the charge-carrier proper-
ties of the fullerene. As predicted, the higher LUMO resulted in a significantly
enhanced Voc when used in combination with P3HT. Bis-PCB-Sis have higher
LUMO energy level and Voc compared to those of mono-PCB-Sis by about 0.1 V.
The highest power conversion efficiency of 2.82% can be obtained from ITO=PED-
OT:PSS=P3HT:compound 8=LiF=Al device after annealing at 150ꢀC for 10 min.
Even if OPV using bis-PCB-Sis as acceptor materials have advantage of high Voc
value (0.78 V), device performance was slightly lower than the devices using mono-
PCB-Sis as acceptor materials due to the regio-isomerization of bis-adducts. Now,
we are trying to increase the purity of bis-PCB-Sis and additional improvement using
bis-PCB-Sis as acceptor materials will be reported in the end of this year.
Acknowledgment
This work was supported by a grant from the Fundamental R&D Program for Core
Technology of Materials and the Korea Energy Management Corporation
(KEMCO) under the New and Renewable Energy R&D Grant (2008-N-PV08-02)
funded by the Ministry of Knowledge Economy, Republic of Korea.
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