Thienyl-substituted methanofullerene derivatives for organic photovoltaic cells

Article abstract of DOI:10.1039/b916597e

A series of thienyl-substituted methanofullerenes as electron acceptors for bulk-heterojuction solar cells with poly(3-hexylthiophene) (P3HT) were synthesized and characterized with respect to electrochemical and photophysical properties. The first one-el

Full text of DOI:10.1039/b916597e

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www.rsc.org/materials | Journal of Materials Chemistry
Thienyl-substituted methanofullerene derivatives for organic photovoltaic
cells†
a
Jung Hei Choi, Kyung-In Son, Taehee Kim, Kyungkon Kim, Kei Ohkubo and Shunichi Fukuzumi
b
b
b
a
a
*
*
Received 13th August 2009, Accepted 23rd October 2009
First published as an Advance Article on the web 13th November 2009
DOI: 10.1039/b916597e
A series of thienyl-substituted methanofullerenes as electron acceptors for bulk-heterojuction
solar cells with poly(3-hexylthiophene) (P3HT) were synthesized and characterized with respect
to electrochemical and photophysical properties. The first one-electron reduction potentials of the
higher adducts are shifted toward more negative values by ꢀ100 mV as compared to the
monoadduct. As a result, the solar cells composed of the bisadduct (2) and trisadduct exhibit
a larger open-circuit voltage (Voc) than the solar cell composed of P3HT and [6,6]-phenyl-C61-
butyric acid methyl ester (PCBM), as they have higher LUMO energy levels. Photophysical
studies on spin-coated films by femtosecond laser flash photolysis indicate that ultrafast electron
transfer from the P3HT donor polymer to all of the fullerene derivatives occurs to form the bound
radical pair (BRP) state. No decay of the BRP state of P3HT:thienyl-substituted monoadduct (1)
films was observed in the time range of 3 ns. The AFM investigation on P3HT:1 and P3HT:higher
adduct films after thermal annealing showed fine donor and acceptor domains and larger domains,
respectively. The bulk-heterojunction (BHJ) solar cells based on P3HT:1 exhibited a power
conversion efficiency (PCE) of 3.97%, which is comparable with that of the P3HT:PCBM cell. The
P3HT:2 based cell showed a PCE value of 1.72% with a higher open-circuit voltage of 0.72 V.
acceptor.2d,7 The higher LUMO energy of the acceptor provides
a path towards higher Voc, and therefore, higher PCE in solar
cells.7,8 Several strategies have been developed to improve the
open-circuit voltage of BHJ cells. For example, by placing
electron-donating groups on the phenyl ring of PCBM, it can
raise the LUMO energy level of PCBM, which allows further
optimization of the open-circuit voltage of organic solar
cells.8b,8c Using the PCBM bisadduct8d or endohedral fullerene8e
in BHJ film, its can also achieve higher open-circuit voltage and
PCE. However, the systematic study of polythiophene:PCBM
blend films has received limited attention to date. Studies on the
photophysics and photochemistry of organic blend films to date
have largely focused on polythiophene:fullerene blend films due
to their applicability to solar energy conversion.9 Hummelen
et al. recently reported a device which was fabricated with
a thienyl-substituted fullerene 1 (in Scheme 1) for the purpose
of improving the miscibility with P3HT.10 In order to optimize
the performance of organic solar cells, we need to understand
the process of photoinduced electron transfer in solid-state
films.
Introduction
Bulk-heterojunction (BHJ) solar cells represent a promising
approach in organic solar cells as renewable energy technology
because of their potential of low-cost manufacturing and
producing large-scale flexible devices.1 Various combinations of
donor and acceptor materials have been used to make the active
layer of BHJ solar cells. One of the most efficient BHJ devices is
a blend of poly(3-hexylthiophene) (P3HT) as an electron donor
and a soluble C60 derivative, [6,6]-phenyl-C61-butyric acid methyl
ester (PCBM) as an electron acceptor that showed power
conversion efficiency (PCE) up to 5.5% for single layer devices.2
The efficiency of solar cells based on P3HT:PCBM has reached
a limit by thermal treatment,3 blend composition4 and processing
solvent.5 New materials are desired to achieve a better perfor-
mance for practical applications.6
The performance of polymer solar cells is characterized by
three parameters: open-circuit voltage (Voc), short-circuit current
density (Jsc) and fill-factor (FF). The open-circuit voltage of BHJ
solar cells is related to the energy difference between the highest
occupied molecular orbital (HOMO) of the electron donor and
the lowest unoccupied molecular orbital (LUMO) of the electron
We report herein the synthesis of a series of thienyl-based
methanofullerenes, shown in Scheme 1, a thienyl-substituted
methanofullerene (1), a bisadduct (2) and higher adducts (3 and
4) and also a monoadduct with electron-donating groups on the
thienyl moiety (5), which are employed as electron acceptors to
make BHJ solar cells with P3HT, aiming at improving the
miscibility and the open-circuit voltage in the BHJ solar cells.
The electrochemical and photophysical properties of the new
fullerene derivatives as well as the morphology and performance
of the P3HT:thienyl-substituted methanofullerene BHJ solar
cells are reported in detail.
aDepartment of Material and Life Science, Division of Advanced Science
and Biotechnology, Graduate School of Engineering, Osaka University,
SORST, Japan Science and Technology Agency (JST), Suita, Osaka,
565-0871, Japan. E-mail: fukuzumi@chem.eng.osaka-u.ac.jp; Fax: +81 6
6879 7370; Tel: +81 6 6879 7368
bSolar cell Research Center, Korea Institute of Science and Technology,
P.O. Box 131, Cheongryang, Seoul, 130-650, Korea. E-mail: kimkk@
kist.re.kr; Tel: +82 2 958 5362
† Electronic supplementary information (ESI) available: Fig. S1–S6. See
DOI: 10.1039/b916597e
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films were spin-coated with chlorobenzene solution of the
P3HT:fullerene derivative (1 : 0.8 w/w) on glass using a spin
coater (MIKASA spin coater MS-A100).
Syntheisis
(3-Methoxycarbonyl)propyl-1-thienyl-[6,6]-methanofullerene
(1). A mixture of 8 (190 mg, 0.50 mmol), sodium methoxide
(27 mg, 0.50 mmol), and dry pyridine (5 mL) was placed under
nitrogen and stirred at room temperature for 30 min. To the
mixture, a solution of C60 (300 mg, 0.42 mmol) in o-dichloro-
benzene (25 mL) was added, and the homogeneous reaction
mixture was stirred at 90 ꢃC under nitrogen overnight. The
solution was heated to reflux, and the reaction was allowed to
continue overnight again. The resulting mixture was purified by
chromatography on silica gel (toluene : hexane ¼ 1 : 1) to give
202 mg of 1. Yield: 53%. 1H NMR (CDCl3, 300 MHz) d 7.50 (dd,
J ¼ 3.6 Hz, 1.2 Hz, 1H), 7.48 (dd, J ¼ 5.4 Hz, 1.2 Hz, 1H), 7.14
(dd, J ¼ 5.4 Hz, 3.6 Hz, 1H), 3.69 (s, 1H), 2.96 (t, J ¼ 7.5 Hz, 2H),
2.58 (t, J ¼ 7.5 Hz, 2H), 2.18 (quin, J ¼ 7.5 Hz, 2H). 13C NMR
(CDCl3, 75 MHz) d 173.47 (C]O), 145.67, 145.20, 145.06,
144.80, 144.64, 144.15, 143.79, 143.03, 142.13, 140.72, 139.11,
138.14, 126.18, 79.88 (bridgehead C), 51.72 (OCH3), 45.73
(bridge C), 33.95, 33.66, 22.47. MALDI-TOF MS: m/z 916.38
(C70H12O2S requires 916.06). Elemental analysis. calcd (%): C
91.69, H 1.32, S 3.50; found C 91.19, H 1.87, S 3.55.
Scheme 1 Chemical structures of P3HT, PCBM and thienyl-substituted
methanofullerenes 1–5.
Experimental
Chemicals and measurements
Chemicals were purchased from commercial sources and used
without further purification unless otherwise noted. Fullerene C60
(frontier carbon cooperation, Japan), PEDOT:PSS (poly-
(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate), H. C.
Starck), P3HT (Aldrich) and PCBM (nano-C) were purchased.
All solvents were purified and freshly distilled prior to use
according to literature procedures. 1H and 13C NMR spectra were
recorded on a JEOL AL-300 spectrometer and referenced to the
solvent peak. MALDI-TOF mass spectra were obtained in linear
negative mode using a Kratos Compact MALDI I (Shimadzu),
with dithranol as matrix. Elemental analyses of carbon,
hydrogen, sulfur and nitrogen were carried out with an Elementar
Vario EL III microanalyser. The electrochemical measurements
were performed at 298 K using a BAS 100 W electrochemical
analyzer in deaerated benzonitrile solution containing 0.1 M
TBAPF6 (tetra-n-butylammonium hexafluorophosphate) as the
supporting electrolyte, with Ag/AgNO3 (1.0 ꢁ 10ꢂ2 M) as
a reference electrode and a Pt working electrode and a Pt wire as
a counter electrode. Absorption spectra were recorded as solution
in chloroform by using a Jasco V-570 spectrophotometer in the
range of 250 to 800 nm at room temperature.
Bisadduct (2). 1H NMR (CDCl3, 300 MHz) d 7.78–6.90 (br. m,
6H), 3.78–3.57 (multiple singlets, 6H), 3.20–1.95 (br. m, 12H). 13
C
NMR (CDCl3, 75 MHz) d 173.42 (C]O), 150.38–131.08, 126.25,
125.96, 125.75, 80.58, 80.56, 80.42, 80.14, 79.84, 79.32, 79.08, 78.77,
78.70, 78.47, 51.71 (OCH3), 51.51 (OCH3), 47.56, 47.32, 44.81,
44.78. 44.73, 44.72, 44.62, 44.51, 43.14, 43.11, 43.01, 42.99, 42.90,
34.42, 34.18, 34.03, 33.77, 33.60, 33.54, 22.30. MALDI-TOF MS:
m/z 1112.59 (C80H24O4S2 requires 1112.11). Elemental analysis.
calcd (%): C 86.32, H 2.17, S 5.76; found C 85.98, H 2.52, S 5.96.
Trisadduct (3). 1H NMR (CDCl3, 300 MHz) d 7.53–6.92 (br. m,
9H), 3.78–3.54 (multiple singlets, 9H), 3.20–1.50 (br. m, 18H). 13
C
NMR (CDCl3, 75 MHz) d 173.58 (C]O), 173.56 (C]O), 147.63–
139.43, 131.88, 131.57, 131.44, 131.22, 126.80 ꢂ 125.20, 51.75
(OCH3), 51.70 (OCH3), 51.52 (OCH3), 34.43, 34.30, 34. 11, 33.95,
33.66, 33.60, 33.39, 33.11, 22.56, 22.36, 22.22. MALDI-TOF MS:
m/z 1308.37 (C90H36O6S3 requires 1308.17). Elemental analysis.
calcd (%): C 82.55, H 2.77, S 7.35; found C 82.15, H 3.13, S 7.19.
Femtosecond transient absorption spectroscopy experiments
were conducted using an ultrafast source: Integra-C (Quantronix
Corp.), an optical parametric amplifier: TOPAS (Light
Conversion Ltd.) and a commercially available optical detection
system: Helios provided by Ultrafast Systems LLC. The source
for the pump and probe pulses were derived from the funda-
mental output of Integra-C (780 nm, 2 mJ per pulse and fwhm ¼
130 fs) at a repetition rate of 1 kHz. 75% of the fundamental
output of the laser was introduced into TOPAS which has optical
frequency mixers resulting in tuneable range from 285 to
1660 nm, while the rest of the output was used for white light
generation. Typically, 2500 excitation pulses were averaged for
5 s to obtain the transient spectrum at a set delay time. Kinetic
traces at appropriate wavelengths were assembled from the time-
resolved spectral data. All measurements were conducted at
room temperature. The differential absorption spectra were
recorded using spin-coated films in each laser excitation. The
1
Tetrakisadduct (4). H NMR (CDCl3, 300 MHz) d 7.80–6.90
(br. m, 12H), 3.82–3.51 (multiple singlets, 12H), 3.20–1.70 (br. m,
24H). 13C NMR (CDCl3, 75 MHz) d 173.49 (C]O), 147.89–
139.11, 131.40, 131.23, 130.95, 130.83, 126.33, 126.07, 125.83,
125.33, 125.26, 52.16 (OCH3), 51.88 (OCH3), 51.70 (OCH3),
51.64 (OCH3), 51.52 (OCH3), 34.41, 33.62, 32.95, 22.49, 22.23.
MALDI-TOF MS: m/z 1505.19 (C100H48O8S4 requires 1504.22).
Elemental analysis. calcd (%): C 79.77, H 3.21, S 8.52; found C
79.77, H, 3.28, S 8.25.
1-(3-Methoxycarbonyl)propyl-1-(3,4-dimethoxythien-2-yl)-[6,6]-
methanofullerene (5). This compound was prepared by the
1
procedure described for 1 using compound 9. Yield: 72%. H
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NMR (CDCl3, 300 MHz) d 6.34 (s, 1H), 4.02 (s, 3H), 3.92 (s, 3H),
3.69 (s, 3H), 2.90 (t, J ¼ 7.2 Hz, 2H), 2.59 (t, J ¼ 7.2 Hz, 2H), 2.27
(qn, J ¼ 7.2 Hz, 2H). 13C NMR (CDCl3, 75 MHz) d 173.62
(C]O), 150.44, 146.11, 145.97, 145.18, 145.14, 145.11, 145.08,
144.77, 144.70, 144.60, 144.56, 144.45, 143.85, 143.76, 143.01,
142.97, 142.88, 142.35, 142.17, 142.09, 140.91, 96.03 (bridgehead
C), 60.15 (Th-OCH3), 57.22 (Th-OCH3), 51.65 (OCH3), 43.59
(bridge C), 33.78, 32.88, 22.42. MALDI-TOF MS: m/z 976.38
(C72H16O4S requires 976.08). Elemental analysis. calcd (%): C
88.52, H 1.65, S 3.28; found C 88.32, H 1.85, S 3.34.
143.93, 135.80, 129.49, 129.34, 128.05, 97.68, 60.76, 57.15, 52.29,
32.12, 26.68, 21.57, 20.82.
Device fabrication
Organic BHJ solar cells were fabricated on patterned ITO
(indium tin oxide)-coated glass substrates as the anode. The ITO/
glass substrates were first cleaned in ultrasonic baths of isopropyl
alcohol, acetone and isopropyl alcohol. After the ITO/glass
ꢃ
substrate was dried in an oven at 80 C for 10 min, they were
treated with UV-ozone for 20 min. The surface of the ITO
substrate was modified by spin-coating conducting PEDOT:PSS
(CLEVIOSꢀ PH500) with a thickness of around 40 nm, fol-
lowed by baking at 120 ꢃC for 10 min in an oven. The P3HT
concentration is 12 mg mLꢂ1, while P3HT:PCBM (1 : 0.8 w/w),
P3HT:1 (1 : 1), P3HT:2 (1 : 1.2), P3HT:3 (1 : 1.4), P3HT:4
(1 : 1.6) and P3HT:5 (1 : 1.1) were blended together and dis-
solved in chlorobenzene. The active layers were obtained by spin
coating the blend solutions, and thermal annealing at 128 ꢃC for
10 min. The thickness of the active layers was around 130 nm, as
measured with an Alpha-step IQ. The cathode consists of 100 nm
of aluminium, and was thermally evaporated on top of the
polymer film through the use of a shadow mask to define an
active area of 0.12 cm2 under a base pressure of 1 ꢁ 10ꢂ6 Torr
(1 Torr ¼ 133.32 Pa).
Methyl 5-(2-thienyl)-5-oxopentanoate (6). 11 SnCl4 (5.94 mL in
dichloromethane, 5.94 mmol) was added to a mixture of thio-
phene (0.5 g, 5.94 mmol) and methyl 5-chloro-5-oxoꢃvalerate
(0.82 mL, 5.94 mmol) in benzene (10 mL) cooled to 0 C. The
reaction mixture was stirred for 1 h at 0 ꢃC. After addition of ice
and dilution with dichloromethane, the mixture was washed
successively with water, dried over Na2SO4 and concentrated in
vacuo. The residue was purified by chromatography on silica gel
(ethyl acetate : hexane ¼ 1 : 2) to give 1.00 g of 6. Yield: 79%. 1H
NMR (CDCl3, 300 MHz) d 7.72 (dd, J ¼ 3.9 Hz, J ¼ 1.2 Hz, 1H),
7.62 (dd, J ¼ 4.8 Hz, J ¼ 1.2 Hz, 1H), 7.11 (dd, J ¼ 4.8 Hz,
J ¼ 3.9 Hz, 1H), 3.67 (s, 3H), 2.98 (t, J ¼ 7.2 Hz, 2H), 2.43 (t, J ¼
7.2 Hz, 2H), 2.06 (qn, J ¼ 7.2 Hz, 2H).
The current density–voltage curves were obtained using
a Keithley 2400 source-measure unit. The photocurrent was
measured under illumination using a Newport class-A 150 mW
cmꢂ2 solar simulator (AM 1.5 G) and the light intensity was
calibrated with an NREL-calibrated Si solar cell with KG-1 filter
to give approximately one sun light intensity. Incident photon-
to-current conversion efficiency (IPCE) was measured in the
range 300 to 800 nm using a specially designed IPCE system
(PV Measurements, Inc.). A 75 W xenon lamp was used as a light
source for the generating monochromatic beam. A calibration
was performed using a silicon photodiode, which was calibrated
using the NIST-calibrated photodiode G425 as a standard, and
IPCE values were collected under bias light at a low chopping
speed of 10 Hz. The morphology of the P3HT:PCBM and P3HT:
fullerene 1–5 films was determined using AFM (Asylum
Research MFP-3D) operating in a contact mode, under ambient
conditions.
Methyl 5-(3,4-dimethoxythien-2-yl)-5-oxopentanoate (7). This
compound was prepared by the procedure described for 6 using
1
3,4-dimethoxythiophene instead of thiophene. Yield: 72%. H
NMR (CDCl3, 300 MHz) d 6.52 (s, 1H), 4.02 (s, 3H), 3.84 (s, 3H),
3.66 (s, 3H), 2.97 (t, J ¼ 7.2 Hz, 2H), 2.42 (t, J ¼ 7.2 Hz, 2H), 2.02
(qn, J ¼ 7.2 Hz, 2H). 13C NMR (CDCl3, 75 MHz) d 192.46
(C]O), 173.67 (C]O), 150.56, 149.44, 125.97, 104.26, 60.34,
57.38, 51.43, 39.86, 33.19, 19.28.
Methyl 5-(2-thienyl)-5-oxopentanoate p-tosylhydrazone (8). A
mixture of the methyl ester 6 (1.0 g, 4.71 mmol) and p-toluene-
sulfonyl hydrazide (1.05 g, 5.65 mmol) in MeOH (7 mL) was
stirred and refluxed for 13 h. The mixture was then extracted into
dichloromethane, washed with water and dried over Na2SO4.
The residue was suspended in acetone. The solution was filtered
and the residue was collected, providing 8 as a white solid. Yield:
92%. 1H NMR (CDCl3, 300 MHz) d 8.96 (s, 1H), 7.89 (d, J ¼ 8.4
Hz, 2H), 7.31–7.26 (m, 3H), 7.16 (dd, J ¼ 3.6 Hz, J ¼ 1.2 Hz,
1H), 6.95 (dd, J ¼ 5.1 Hz, J ¼ 3.6 Hz, 1H), 3.77 (s, 3H), 2.59 (t, J
¼ 7. 8Hz, 2H), 2.38 (s, 3H), 2.30 (t, J ¼ 7.8 Hz, 2H), 1.86–1.68
(m, 2H). 13C NMR (CDCl3, 75 MHz) d 174.55, 150.28, 143.76,
142.04, 135.61, 129.38, 128.11, 128.01, 127.09, 126.33, 52.23,
32.01, 26.64, 21.53, 21.10. Elemental analysis. calcd (%) for
C17H20N2O4S2: C 53.66, H 5.30, N 7.36, S 16.85; found C 53.60,
H 5.22, N 7.63, S 16.41.
Results and discussion
Synthesis
The synthetic approach to thienyl-substituted methanfullerenes
1–5 is depicted in Scheme 2. The thiophene derivatives 6 and 7
were acylated by using tin(IV) chloride with methyl 5-chloro-5-
oxovalerate. The synthesis of methanofullerenes 1–5 was carried
out according to Hummelen–Wudl synthetic routes.12 Conden-
sation of 6 and 7 with p-tosylhydrazide in methanol gave the
corresponding p-tosylhydrazone 8 and 9. A mixture of 8, sodium
methoxide, pyridine and C60 in o-dichlorobenzene was stirred
Methyl 5-(3,4-dimethoxythien-2-yl)-5-oxopentanoate p-tosyl-
hydrazone (9). This compound was prepared by the procedure
described for 8 using methyl 5-(3,4-dimethoxythien-2-yl)-5-oxo-
1
ꢃ
pentanoate 7. Yield: 63%. H NMR (CDCl3, 300 MHz) d 9.23
overnight at 90 C under nitrogen. The solution was heated to
(s, 1H), 8.01 (d, J ¼ 8.4 Hz, 2H), 7.43 (d, J ¼ 8.4 Hz, 2H), 6.28
(s, 1H), 3.98 (s, 3H), 3.94 (s, 3H), 3.92 (s, 3H), 2.84 (t, J ¼ 7. 8 Hz,
2H), 2.55 (s, 3H), 2.42 (t, J ¼ 7.8 Hz, 2H), 1.96 ꢂ 1.78 (m, 2H).
13C NMR (CDCl3, 75 MHz) d 174.88, 155.83, 150.58, 145.83,
reflux, and the reaction was allowed to continue overnight again.
The resulting mixture was purified through silica gel column
chromatography with toluene as eluent to give unreacted C60
(11%), monoadduct 1 (53%) and bisadduct 2 (18%). Bis or higher
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Scheme 2 Synthetic route to methanofullerenes 1 and 5.
adducts 2, 3 and 4 were synthesized by the procedure described
for 1 using excess p-tosylhydrazone 8 and the yield of bisadduct
2, trisadduct 3 and tetrakisadduct 4 was 13%, 26% and 56%. The
products were further purified by reprecipitation from methanol.
The bisadduct 2, trisadduct 3 and tetrakisadduct 4 consist of
a number of regioisomers.
The obtained methanofullerenes 1–5 were characterized by
MALDI-TOF mass spectrometry (Fig. S1, ESI†), elemental
analysis, 1H and 13C NMR spectroscopy (Fig. S2, ESI†).
Although both [6,6]methanofullerene and [5,6]fulleroid isomers
could be expected from the cycloaddition reaction, the temper-
ature as high as 180 ꢃC at which the reaction proceeds converted
the [5,6]-open isomer into the thermodynamically more stable
[6,6]-closed isomer.12 These isomers are clearly distinguishable
by the 13C NMR chemical shifts of bridging units. The 13C
NMR spectrum of fullerene 1 and 5 exhibited bridgehead
carbon at 79.88 ppm and 96.03 ppm and bridge carbon at
45.73 ppm and 43.59 ppm. These results are clearly suggestive of
a methanofullerene structure with a 6,6 junction. The carbon
peaks of higher adducts 2, 3, and 4, with the exclusion of ten
peaks of the bisadduct bridgehead carbon, showed the multiple
signals.
Fig. 1 (a) Cyclic voltammograms and (b) differential pulse voltammo-
grams of PCBM and 1–5 in benzonitrile containing 0.1M TBAPF6.
Photophysical properties
The UV-vis absorption properties of C60, PCBM and fullerene
derivatives 1–5 in chloroform solution are shown in Fig. 2a. The
UV-vis spectra of PCBM and 1 in solution are almost identical,
with a peak at 328 nm and 432 nm and broad band at 490 nm,
respectively. The absorption features of monosubstituted
fullerene derivatives, PCBM, 1 and 5 show a distinguishable
sharp band at 432 nm, respectively, which reflects the partially
broken symmetry of the fullerene core relative to pristine C60.
The main peak at 500 nm of the monoadduct 1 is slightly blue-
shifted to about 480 nm in the bisadducts 2 and the trisadduct 3.
The UV-vis absorption spectra of fullerene 1–5 and PCBM with
P3HT in the thin film show very similar shape (Fig. 2b). Only the
absorption of the P3HT:bisadduct 2 film and the P3HT:tri-
sadduct 3 film around 520 nm increased. From the UV-vis
absorption, we conclude that there is no distinct change for
fullerene 1–5 compared to other methanofullerenes.
Electrochemical properties
The electrochemical properties of fullerene derivatives 1–5 were
investigated by cyclic voltammetry (CV) and differential pulse
voltammetry (DPV) at room temperature in benzonitrile solution
containing 0.1 M TBAPF6. From Fig. 1, cyclic voltammograms
of compound 1, 5, PCBM and C60 show three reversible reduc-
tion waves, but compound 2 and 3 show only two reversible
reduction waves. The reduction potentials for 1–5, are listed in
Table 1 along with PCBM and C60 as reference compounds. In
the case of dimethoxy-substituted fullerene 5, the reduction
values are slightly more negative than those of compound 1. This
could be attributed the inductive effect of the methoxy group on
the thiophene ring. It was reported that the alkoxy group of the
phenyl ring of PCBM can decrease the reduction potential.8b The
first reduction potentials of the higher adducts 2–4 are shifted
toward more negative values of ꢀ100 mV with respect to mon-
oadduct 1 because of fewer p electrons and release of strain
energy.13 As a result, it is estimated that the LUMO levels of
methanofullerenes 2–5 are raised in comparison with mono-
adduct 1 and PCBM.
To understand the excited-state interaction of P3HT polymer
with fullerenes 1–5, we studied the photophysical behaviour in
spin-coated films by femtosecond laser flash photolysis. Femto-
second laser excitation at 450 nm of a P3HT:1 film results in the
appearance of a transient absorption band in the NIR region at
1 ps, as shown as a black dot in Fig. 3a, which is due to the singlet
excited state of P3HT.8e The transient absorption band of 1P3HT*
disappears, accompanied by the appearance of a new broad
absorption at around 1000 nm due to P3HT positive polaron
(P3HTc+)9a,14 and the methanofullerene radical anion15 at 1000 ps
(grey dot in Fig. 3a). This suggests that ultrafast electron transfer
from P3HT to the fullerene derivative occurs to form the bound
radical pair (BRP) state9a (P3HTc+/1cꢂ) in the thin film.
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Table 1 Electrochemical and photophysical data of fullerene derivatives 1–5
CVa
DPV
c
d
1/2
E1
1/2
E2
1/2
Compound
E3
E1
E2
E3
kET /sꢂ1
kBET /sꢂ1
1
2
3
4
ꢂ0.57
ꢂ0.66
ꢂ0.79
ꢂ1.19b
ꢂ0.58
ꢂ0.57
ꢂ0.49
ꢂ0.98
ꢂ1.06
ꢂ1.10
ꢂ1.51
ꢂ0.51
ꢂ0.63
ꢂ0.76
ꢂ0.90
ꢂ0.54
ꢂ0.52
ꢂ0.42
ꢂ0.91
ꢂ1.04
ꢂ1.12
ꢂ1.31
ꢂ0.96
ꢂ0.93
ꢂ0.84
ꢂ1.46
ꢂ1.68
ꢂ1.86
1.0 ꢁ 1012
8.2 ꢁ 1011
6.4 ꢁ 1011
4.4 ꢁ 1011
2.9 ꢁ 1011
5.0 ꢁ 1011
2.8 ꢁ 108
1.6 ꢁ 108
3.0 ꢁ 108
3.3 ꢁ 108
3.3 ꢁ 108
3.1 ꢁ 108
ꢂ1.71b
5
ꢂ1.07
ꢂ0.98
ꢂ0.83
ꢂ1.51
ꢂ1.51
ꢂ1.38
ꢂ1.49
ꢂ1.46
ꢂ1.33
PCBM
C60
a
Experimental conditions: values for (Epa + Epc)/2 in V vs. SCE in benzonitrile (10ꢂ3 mol dmꢂ3) with TBAPF6 (0.1 mol dmꢂ3) as supporting electrolyte;
100 mV sꢂ1 scan rate. b Irreversible peak potential (Eredpc). c Rate constants for electron transfer from 1P3HT* to C60 in P3HT:fullerene 1–5 films. d Rate
constants for back electron transfer in P3HT:fullerene 1–5 films.
Fig. 2 UV-vis absorption spectra of (a) C60, PCBM and fullerene
derivatives 1–5 in CHCl3 and (b) PCBM, and fullerenes 1–5 with P3HT in
the thin film.
Similar transient absorption spectra were also observed in the
case of P3HT:2, P3HT:5 and P3HT:PCBM8e films as shown in
Fig. S3, ESI†.
The rate of electron transfer was determined from the decay
time profile of 1P3HT* in P3HT:1 film in 20 ps range, as shown in
Fig. 3 (a) Transient absorption spectra of P3HT:1 film obtained by
Fig. 3b. The rate constants obtained are listed in Table 1. The
femtosecond laser flash photolysis taken at 1 and 1000 ps after laser
lifetimes are determined as 1 ps in P3HT:1 film, 1.2 ps in P3HT:2
excitation (450 nm) at 298 K. Time profiles of absorbance at 1000 nm in
film, 1.6 ps in P3HT:3 film, 2.3 ps in P3HT:4 film, 3.4 ps in
the (b) 20 ps and (c) 2000 ps ranges.
P3HT:5 film and 2 ps in P3HT:PCBM film. All films give rise to
the rapid formation of the BRP state9a after laser excitation as
shown in Fig. S4, ESI†. Slow decay of the BRP state was
observed in the time range of 3 ns. The lifetime of the BRP state
was estimated as 3.6 ns in P3HT:1 film, 6.2 ns in P3HT:2 film,
3.3 ns in P3HT:3 film, 3.0 ns in P3HT:4 film, 3.0 ns in P3HT:5
film and 3.2 ns in P3HT:PCBM film. These BRP states of
P3HT:fullerene 1–5 films were found to be stable up to 2000 ps
(Fig. 3c and Fig. S5, ESI†). Thus, fullerenes 1 and 2 can act as
efficient materials for organic solar cells.
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Organic solar cells
(Voc ꢁ Jsc ꢁ FF)/Pin, where Pin is the intensity of incident light
irradiation.
BHJ solar cells with ITO/PEDOT:PSS/P3HT:fullerene deriva-
tives/Al using P3HT as an electron donor and fullerenes 1–5 as
electron acceptors were fabricated and characterized. The weight
ratio between the donor and acceptor was adjusted to have the
same ratio. The weight ratios of the active layers were 1 : 0.8
(w/w) (P3HT:PCBM), 1 : 1 (w/w) (P3HT:1), 1 : 1.2 (w/w)
(P3HT:2), 1 : 1.4 (w/w) (P3HT:3), 1 : 1.6 (w/w) (P3HT:4) and
1 : 1.1 (w/w) (P3HT:5) and were calculated by their molecular
weight. The blends were spin-coated from a chlorobenzene
solution containing the P3HT:fullerene 1–5 on the ITO/glass
substrate covered by a PEDOT:PSS layer, respectively. After
completion of solution processing of the various layers, the
samples were transferred into a thermal vacuum evaporator to
deposit 130 nm of aluminium as the top electrode at 2.1 ꢁ 10ꢂ6
Torr. The deposited Al electrode area defines the active area of
the device as 0.12 cm2. After Al deposition, the device was
annealed at 128 ꢃC by a radiation heater for 10 min in a thermal
vacuum evaporator at the same vacuum level.
Fig. 5 shows the IPCE (incident photon-to-electron conversion
efficiency) plot for the P3HT:PCBM and P3HT: fullerene 1–5
cells. The IPCE plot shows a maximum value of 61.5% and 59.7%
for P3HT:PCBM and P3HT:1 cells, respectively, at a wavelength
of 580 nm. The P3HT:PCBM cell has a Jsc of 10.9 mA cmꢂ2, Voc
of 0.62 V, and FF of 62%. Under the same conditions, the
P3HT:1 cell shows a Jsc of 10.9 mA cmꢂ2, Voc of 0.60 V, and FF
of 61%. The PCE of the P3HT:1 cell is comparable with the
P3HT:PCBM and the value is higher than that previously
reported.10 The enhancement could be ascribed to the different
solvent system and annealing procedure.
Based on the electrochemical data, it is expected that the Voc of
the BHJ solar cell will be increased by replacing the acceptor
from PCBM to bis or higher adducts acceptor due to the
increased energy difference between the HOMO energy of P3HT
and the LUMO energy of the acceptor. The Voc of P3HT:2 cell is
enhanced up to 0.72 V, which is 16% higher than that of
P3HT:PCBM cell. The Voc values of the BHJ solar cell using
higher adducts (P3HT:3 and 4) show similar or smaller Voc
compared to P3HT:PCBM cell. The Jsc values of the BHJ solar
cell of higher adducts (P3HT:2, 3 and 4) are lower than that of
the monoadduct (1) and the Jsc value decreases as the adduct
number increases. For the solar cell based on the P3HT:dime-
thoxy-substituted monoadduct 5 cell, both the Voc and Jsc values
are significantly reduced as compared to that of P3HT:PCBM
cell. The inferior performance of the higher adduct based BHJ
solar cells may be related to the non-optimized BHJ morphology,
which is expected to influence the BHJ solar cell performance.16
The BHJ morphology of the films was investigated by atomic
force microscopy (AFM). Fig. 6 shows the AFM images of the
P3HT:PCBM and P3HT:fullerene 1–5 blend films after thermal
annealing. The scan range of all AFM images was 10 ꢁ 10 mm2
and the Z range was the same. The surface images of the
P3HT:PCBM film and P3HT:1 film show fine features compared
to that of P3HT:2–4 films. The root-mean-square (rms) rough-
ness is 1.1 nm for P3HT:PCBM and P3HT:1, 3.2 nm for P3HT:2,
2.4 nm for P3HT:3, 2.6 nm for P3HT:4, and 0.4 nm for P3HT:5.
This reflects the fact that the domain size in the P3HT: 2–4 BHJ
films is larger than that in the P3HT:PCBM and P3HT:1 BHJ
Fig. 4 shows the typical current density–voltage curves for
the BHJ solar cells under AM 1.5 conditions at 100 mW cmꢂ2
.
Table 2 summarizes the device performance of the BHJ solar
cells. The performance of the BHJ cells was characterized by
three parameters, open-circuit voltage (Voc), short-circuit current
density (Jsc) and fill-factor (FF). The PCE (he) is given as PCE ¼
Fig. 4 Current density–voltage (J–V) characteristics of solar cells
composed of P3HT:PCBM (1 : 0.8), P3HT:1 (1 : 1), P3HT:2 (1 : 1.2),
P3HT:3 (1 : 1.4), P3HT:4 (1 : 1.6) and P3HT:5 (1 : 1.1) under AM 1.5 G
simulated solar illumination (100 mW cmꢂ2).
Table 2 Characteristics of bulk-heterojunction solar cellsa
Active layer (ratio)
Jsc/mA cmꢂ2 Voc/V FF
he (%) Rs/U
P3HT/PCBM (1 : 0.8) 10.9
0.62
0.60
0.72
0.64
0.57
0.53
0.62 4.18
7.35 ꢁ 10
6.59 ꢁ 10
4.36 ꢁ 102
1.81 ꢁ 103
5.90 ꢁ 103
1.42 ꢁ 103
P3HT/1 (1 : 1)
10.9
5.91
1.88
0.48
2.13
0.61 3.97
0.41 1.72
0.28 0.34
0.32 0.09
0.29 0.33
P3HT/2 (1 : 1.2)
P3HT/3 (1 : 1.4)
P3HT/4 (1 : 1.6)
P3HT/5 (1 : 1.1)
a
Short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor
(FF), PCE (he) and series resistance (Rs) at P3HT as an electron donor:
various acceptors (PCBM, 1–5) at weight ratio. The device
performance was consistent and reproducible. The active area is 0.12 cm2.
Fig. 5 IPCE spectra of solar cells composed of P3HT:PCBM (1 : 0.8),
P3HT:1 (1 : 1), P3HT:2 (1 : 1.2), P3HT:3 (1 : 1.4), P3HT:4 (1 : 1.6) and
P3HT:5 (1 : 1.1).
480 | J. Mater. Chem., 2010, 20, 475–482
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open-circuit voltage (Voc) than the P3HT:PCBM cell, as they have
higher LUMO energy levels. The ultrafast electron transfer process
from P3HT donor polymer to the fullerene derivative was observed
by femtosecond measurements in the spin-coated P3HT:1 film. No
decay of the bound radical pair state of P3HT:1 film was observed
in the time range of 3 ns, which act as an efficient material for solar
cells. The P3HT:1 BHJ solar cell shows a Jsc of 10.9 mA cmꢂ2, Voc
of 0.60 V, FF of 61% and PCE of 3.97%. Under the same condi-
tions, the P3HT:2 cell shows a Jsc of 5.91 mA cmꢂ2, Voc of 0.72 V,
FF of 41% and PCE of 1.72%. The higher adduct 3 and 4 and
dimethoxy-substituted monoadduct 5 based devices show low
efficiencies as a result of fast decay of radicals. The AFM image of
the P3HT:1 blend film shows high morphology comparable to
other fullerene derivatives. It is demonstrated that thienyl-
substituted fullerene 1 is one of the promising candidates for
acceptor materials for BHJ solar cells.
Acknowledgements
This work was partially supported by KOSEF/MEST through
WCU project (R31-2008-000-10010-0) and a Grant-in-Aid (Nos.
19205019, 21750146) and a Global COE program, ‘‘the Global
Education and Research Center for Bio-Environmental Chem-
istry’’ from the Ministry of Education, Culture, Sports, Science
and Technology. We thank the KIST internal project under
contract 2E20980 and Renewable Energy Grant from KETEP
(Grant No. 2008NPV08J010000).
Fig. 6 10 ꢁ 10 mm2 AFM height images of solar cells composed of (a)
P3HT:PCBM (1 : 0.8), (b) P3HT:1 (1 : 1), (c) P3HT:2 (1 : 1.2), (d)
P3HT:3 (1 : 1.4), (e) P3HT:4 (1 : 1.6) and (f) P3HT:5 (1 : 1.1).
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