Alkyl substituted [6,6]-thienyl-C61-butyric acid methyl esters: Easily accessible acceptor materials for bulk-heterojunction polymer solar cells

Article abstract of DOI:10.1039/b925089a

[6,6]-Thienyl-C₆₁-butyric acid ester derivatives with methyl, hexyl and 2-ethylhexyl at the 5-position of thiophene ring (TCBM-Cn, n represents the number of carbon atom in the alkyl chain) were synthesized. Unlike [6,6]-phenyl-C₆₁-b

Full text of DOI:10.1039/b925089a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Alkyl substituted [6,6]-thienyl-C61-butyric acid methyl esters: easily accessible
acceptor materials for bulk-heterojunction polymer solar cells
a
a
a
a
Huiyu Zhao,ab Xiaoyang Guo,ab Hongkun Tian, Changyin Li, Zhiyuan Xie, Yanhou Geng
*
*
*
and Fosong Wanga
Received 1st December 2009, Accepted 1st February 2010
First published as an Advance Article on the web 1st March 2010
DOI: 10.1039/b925089a
[6,6]-Thienyl-C61-butyric acid ester derivatives with methyl, hexyl and 2-ethylhexyl at the 5-position of
thiophene ring (TCBM-Cn, n represents the number of carbon atom in the alkyl chain) were
synthesized. Unlike [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), these methanofullerene
derivatives ([6,6] adduct) can be directly obtained from the typical diazo addition under mild
conditions, and high temperature isomerization is unnecessary. With a hexyl or 2-ethylhexyl group at
the 5-position of thiophene, the solubility of TCBM-Cn in chlorobenzene is as high as 180 ꢀ
10 mg mlꢁ1. Bulk heterojunction photovoltaic solar cells were fabricated with a device structure of ITO/
poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/poly(3-hexylthiophene)
(P3HT) : TCBM-Cn (1 : 0.8 w/w)/LiF/Al. The device based-on TCBM-C6 exhibited the best
performance with a power conversion efficiency (PCE) of up to 4.26%.
poly(3-hexylthiophene) (P3HT) as the donor.20,24 To further
explore the easily made high performance C60-based acceptors
for BHJ PSCs, herein we synthesized a series of TCBM
analogues with different alkyl chains (methyl, hexyl and ethyl-
hexyl) on the thiophene ring, as shown in Fig. 1. BHJ PSCs based
on the blend of P3HT and these new C60 derivatives exhibited
power conversion efficiencies (PCE) identical to or slightly higher
than those prepared from P3HT and PCBM or TCBM. Most
importantly, these C60 derivatives can be made directly by the
typical diazo addition without high temperature isomerization.
Introduction
Bulk-heterojunction (BHJ) polymer solar cells (PSCs) have
attracted more and more attention as a renewable energy sources
due to their easy and cheap processing.1–5 For this type of device,
a blend of electron-donating materials (p-type conjugated poly-
mers) and an electron-accepting material (n-type materials) is
used as the active layer, and a derivative of buckminsterfullerene
(C60), i.e. [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), is
the most often used electron-accepting material.6 To further
improve the device performance, different C60 derivatives7–20,24
have been synthesized. Particularly, many efforts have been
devoted to modify the PCBM skeleton by introducing substitu-
ents on the phenyl ring,12,15,16 exchanging methyl groups with
long alkyl17 chains, an ethyleneoxy moiety18 or a perfloroalkyl
chain19 to tune the miscibility, thermal properties and energy
levels, and the resulting C60 derivatives have been successfully
used to control the film morphology, raise the open circuit
voltage (Voc), and improve the device stability.
Experimental
Materials
Dichloromethane (CH2Cl2), o-dichlorobenzene (ODCB) and
absolute methanol were subject to stirring with CaO and then
distilled. PCBM with a purity of 99% was purchased from FEM
Technology Co. Inc. Compound 1b with a purity of 99% was
purchased from Pacific Chemsource Inc. Compounds 1c and 1d
were prepared according to ref. 21. P3HT was prepared by the
McCullough method.22 Weight-average molecular weight, poly-
dispersity, and regioregularity are 38 600 g molꢁ1, 1.59 and 97%,
respectively. Other reagents were used as received without
further purification.
Although PCBM is the most popular acceptor materials so far
for BHJ PSCs, it is questionable whether it is the best for polymer
donor materials with the huge diversity of chemical structures.
Therefore, it is still very important to explore new C60 deriva-
tives, particularly those with ease of synthesis and purification. It
is well known that thiophene is an aromatic ring characterized by
easy modification. Recently, a PCBM analogue, i.e. [6,6]-thienyl-
C61-butyric acid methyl ester (TCBM), has been reported to
exhibit device performance close to PCBM with regioregular
aState Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun, 130022, P. R. China. E-mail: yhgeng@ciac.jl.cn; hktian@
ciac.jl.cn; xiezy_n@ciac.jl.cn
bGraduate School of Chinese Academy of Sciences, Beijing, 100049, P. R.
China
Fig. 1 Chemical structures of TCBM, TCBM-C1, TCBM-C6 and
TCBM-C2,6.
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5-[5-Alkyl-(2-thienyl)]-5-oxo-pentanoic acids (2a–d)25
chromatography on silica gel with petroleum ether : ethyl acetate
(9 : 1) as eluent.
AlCl3 (132 mmol) was added in parts to glutaric anhydride
(60 mmol) and 2-alkylthiophene (60 mmol) in CH2Cl2 (120 ml)
with cooling in an ice-water bath. The mixture was allowed to
warm to room temperature slowly, stirred for 5 h and then
quenched with 2M HCl for extraction with CH2Cl2. The organic
extracts were washed with brine, and dried with MgSO4. The
residue was purified by column chromatography on silica gel
with ethyl acetate as eluent.
4a (yield 68%); 1H NMR (300 MHz, CDCl3): d (ppm) 8.98 (s,
1H), 7.91 (d, J ¼ 8.4 Hz, 2H), 7.30 (d, J ¼ 1.2 Hz, 1H), 7.29 (d, J
¼ 8.4 Hz, 2H), 7.18 (dd, J ¼ 1.2 Hz, J ¼ 1.2 Hz, 1H), 6.97 (dd, J
¼ 3.9 Hz, J ¼ 3.6 Hz, 1H), 3.80 (s, 3H), 2.61 (t, J ¼ 7.8 Hz, 2H),
2.40 (s, 3H), 2.31 (t, J ¼ 6.0 Hz, 2H), 1.68–1.78 (m, 2H).
4b (yield 70%); 1H NMR (300 MHz, CDCl3): d (ppm) 8.84 (s,
1H), 7.90 (d, J ¼ 7.8 Hz, 2H), 7.30 (d, J ¼ 7.8 Hz, 2H), 6.97 (d, J
¼ 2.7 Hz, 1H), 6.61 (d, J ¼ 2.7 Hz, 1H), 3.78 (s, 3H), 2.56 (t, J ¼
7.5 Hz, 2H), 2.45 (s, 3H), 2.40 (s, 3H), 2.30 (t, J ¼ 7.5 Hz, 2H),
1.59–1.71 (m, 2H).
4c (yield 56%); 1H NMR (300 MHz, CDCl3): d (ppm) 8.82 (s,
1H), 7.91 (d, J ¼ 8.4 Hz, 2H), 7.29 (d, J ¼ 8.4 Hz, 2H), 6.99 (d, J
¼ 3.6 Hz, 1H), 6.63 (d, J ¼ 3.6 Hz, 1H), 3.78 (s, 3H), 2.75 (t, J ¼
7.5 Hz, 2H), 2.56 (t, J ¼ 7.5 Hz, 2H), 2.40 (s, 3H), 2.30 (t, J ¼ 6.0
Hz, 2H), 1.63–1.74 (m, 2H), 1.31–1.39 (m, 8H), 0.87–0.92 (m,
3H).
1
2a (Yield 60%); H NMR (300 MHz, CDCl3): d (ppm) 7.71
(dd, J ¼ 0.9 Hz, J ¼ 0.9 Hz, 1H), 7.64 (dd, J ¼ 0.9 Hz, J ¼ 0.9 Hz,
1H), 7.14 (dd, J ¼ 3.9 Hz, J ¼ 3.9 Hz, 1H), 3.02 (t, J ¼ 7.2 Hz,
2H), 2.51 (t, J ¼ 7.2 Hz, 2H), 2.04–2.14 (m, 2H).
2b (Yield 72%); 1H NMR (300 MHz, CDCl3): d (ppm) 7.54 (d,
J ¼ 3.3 Hz, 1H), 6.79 (d, J ¼ 3.3 Hz, 1H), 2.95 (t, J ¼ 6.9 Hz, 2H),
2.53 (s, 3H), 2.49 (t, J ¼ 6.9 Hz, 2H), 2.02–2.11 (m, 2H).
2c (Yield 86%); 1H NMR (300 MHz, CDCl3): d (ppm) 7.56 (d,
J ¼ 3.9 Hz, 1H), 6.79 (d, J ¼ 3.9 Hz, 1H), 3.01 (t, J ¼ 7.5 Hz, 2H),
2.77 (t, J ¼ 7.2 Hz, 2H), 2.50 (t, J ¼ 7.5 Hz, 2H), 2.06–2.08 (m,
2H), 1.30–1.38 (m, 2H), 1.30–1.38 (m, 8H), 0.87–0.92 (m, 3H).
2d (Yield 52%); 1H NMR (300 MHz, CDCl3): d (ppm) 7.56 (d,
J ¼ 3.9 Hz, 1H), 6.79 (d, J ¼ 3.9 Hz, 1H), 2.96 (t, J ¼ 6.9 Hz, 2H),
2.77 (d, J ¼ 6.6 Hz, 2H), 2.50 (t, J ¼ 6.9 Hz, 2H), 2.05–2.10 (m,
2H), 1.56–1.66 (m, 1H), 1.27–1.35 (m, 8H), 0.86–0.91 (m, 6H).
4d (yield 75%); 1H NMR (300 MHz, CDCl3): d (ppm) 8.86 (s,
1H), 7.91 (d, J ¼ 8.4 Hz, 2H), 7.29 (d, J ¼ 8.1 Hz, 2H), 6.99 (d, J
¼ 3.6 Hz, 1H), 6.61 (d, J ¼ 3.6 Hz, 1H), 3.78 (s, 3H), 2.69 (d, J ¼
6.6 Hz, 2H), 2.56 (t, J ¼ 8.1 Hz, 2H), 2.40 (s, 3H), 2.31 (t, J ¼ 8.1
Hz, 2H), 1.62–1.80 (m, 2H), 1.54–1.64 (m, 1H), 1.28–1.35 (m,
8H), 0.86–0.91 (m, 6H).
Methyl 5-[5-alkyl-(2-thienyl)]-5-oxo-pentanoate (3a–d)
[6,6]-Thienyl-C61-butyric acid methyl ester (TCBM)
A solution of 2a–d (36 mmol) in MeOH (70 ml) with a catalytic
amount of hydrochloric acid was refluxed for 5 h. The mixture
was extracted with CH2Cl2. The organic extracts were washed
with brine, and dried with MgSO4. The residue was purified by
column chromatography on silica gel with petroleum ether-
: ethyl acetate (6 : 1) as eluent.
3a (yield 90%); 1H NMR (300 MHz, CDCl3): d (ppm) 7.71 (dd,
J ¼ 0.9 Hz, J ¼ 0.9 Hz, 1H), 7.64 (dd, J ¼ 0.9 Hz, J ¼ 0.9 Hz,
1H), 7.14 (dd, J ¼ 3.9 Hz, J ¼ 3.9 Hz, 1H), 3.69 (s, 3H), 2.99 (t, J
¼ 7.2 Hz, 2H), 2.45 (t, J ¼ 7.2 Hz, 2H), 2.03–2.13 (m, 2H).
3b (yield 95%); 1H NMR (300 MHz, CDCl3): d (ppm) 7.54 (d, J
¼ 3.3 Hz, 1H), 6.79 (d, J ¼ 3.3 Hz, 1H), 3.68 (s, 3H), 2.91 (t, J ¼
3.6 Hz, 2H), 2.53 (s, 3H), 2.43 (t, J ¼ 3.6 Hz, 2H), 2.03–2.08 (m,
2H).
3c (yield 96%); 1H NMR (300 MHz, CDCl3): d (ppm) 7.56 (d, J
¼ 3.9 Hz, 1H), 6.79 (d, J ¼ 3.9 Hz, 1H), 3.69 (s, 3H), 2.98 (t, J ¼
7.5 Hz, 2H), 2.71 (t, J ¼ 7.2 Hz, 2H), 2.50 (t, J ¼ 7.5 Hz, 2H),
2.05–2.07 (m, 2H).
3d (yield 95%); 1H NMR (300 MHz, CDCl3): d (ppm) 7.56 (d, J
¼ 3.9 Hz, 1H), 6.79 (d, J ¼ 3.9 Hz, 1H), 3.69 (s, 3H), 2.93 (t, J ¼
6.9 Hz, 2H), 2.71 (t, J ¼ 6.6 Hz, 2H), 2.49 (t, J ¼ 6.9 Hz, 2H),
2.04–2.09 (m, 2H), 1.55–1.65 (m, 1H), 1.28–1.36 (m, 2H), 0.87–
0.92 (m, 2H).
A mixture of 4a (313 mg, 0.83 mmol), sodium methoxide (49 mg,
0.91 mmol), and dry pyridine (15 ml) was stirred at room
temperature for 30 min. Then a solution of C60 (590 mg,
0.83 mmol) in ODCB (60 ml) was added, and the homogeneous
reaction mixture was stirred at 75 ꢂC under argon for 24 h. The
solvent was evaporated at reduced pressure, and the residue was
purified by column chromatography on silica gel with toluene as
eluent. The product was precipitated with MeOH with a yield of
1
40% (300 mg). H NMR (300 MHz, CDCl3): d (ppm) 7.47–7.50
(m, 2H), 7.13–7.15 (m, 1H), 3.69 (s, 3H), 2.96 (t, J ¼ 4.8 Hz, 2H),
2.58 (t, J ¼ 7.2 Hz, 2H), 2.22–2.26 (m, 2H). 13C NMR (150 MHz,
CDCl3): d (ppm) 173.45 (CO2CH3), 148.30, 147.49, 145.69,
145.25, 145.22, 145.49, 145.15, 145.08, 144.83, 144.73, 144.67,
144.60, 144.52, 144.18, 143.82, 143.80, 143.12, 143.06, 143.02,
142.95, 142.21, 142.16, 142.14, 140.97, 140.75, 139.45, 138.18,
131.96, 126.24, 126.19, 79.92 (bridgehead C), 51.69, 45.76, 33.98,
33.67, 22.48. Anal. Calcd for C70H12O2S (%): C, 91.69; H, 1.32.
Found (%): C, 91.52; H, 1.10.
5-Methyl-[6,6]-thienyl-C61-butyric acid methyl ester (TCBM-C1)
TCBM-C1 was synthesized following the procedure for prepa-
ration of TCBM with a yield of 40% (380 mg). 1H NMR
(300 MHz, CDCl3): d (ppm) 7.26 (d, J ¼ 4.8 Hz, 1H), 6.76 (d, J ¼
4.8 Hz, 1H), 3.69 (s, 3H), 2.92 (t, J ¼ 7.2 Hz, 2H), 2.59 (s, 3H),
2.57 (t, J ¼ 7.2 Hz, 2H), 2.19–2.29 (m, 2H). 13C NMR (150 MHz,
CDCl3): d (ppm) 173.52 (CO2CH3), 148.47, 147.62, 145.73,
145.24, 145.20, 145.13, 144.82, 144.73 144.65, 144.58, 144.48,
144.16, 143.82, 143.79, 143.10, 143.05, 143.01, 142.93, 142.91,
142.25, 142.17, 142.12, 140.94, 140.82, 140.72, 138.25, 138.17,
136.41, 132.02, 124.24, 80.03 (bridgehead C), 51.67, 46.10, 33.84,
Methyl 5-[5-alkyl-(2-thienyl)]-5-oxo-pentanoate-p-tosylhydrazo-
ne (4a–d)
A mixture of 3a–d (30 mmol), p-toluene-sulfonyl hydrazide
(33 mmol), and MeOH (60 ml) with a catalytic amount of
CH3COOH was refluxed for 17 h. The mixture was extracted
with CH2Cl2. The organic extracts were washed with brine, and
dried with MgSO4. The residue was purified by column
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33.70, 22.51. Anal. Calcd for C71H14O2S (%): C, 91.60; H, 1.52.
Found (%): C, 90.37; H, 1.31.
Photovoltaic device fabrication and characterization
The PSCs were fabricated with the device structure of ITO/
poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PED-
OT:PSS)/P3HT : TCBM-Cn (1 : 0.8 w/w)/LiF/Al. The ITO glass
was precleaned and modified by a 40 nm PEDOT:PSS (Baytron
P4083) layer which was spin-coated from a PEDOT-PSS
aqueous solution on the ITO substrate, and was dried subse-
5-Hexyl-[6,6]-thienyl-C61-butyric acid methyl ester (TCBM-C6)
TCBM-C6 was synthesized following the procedure for prepa-
ration of TCBM with a yield of 46% (490 mg). 1H NMR
(300 MHz, CDCl3): d (ppm) 7.26 (d, J ¼ 4.5 Hz, 1H), 6.76 (d, J ¼
4.5 Hz, 1H), 3.69 (s, 3H), 2.87–2.95 (m, 4H), 2.58 (t, J ¼ 7.2 Hz,
2H), 2.21–2.29 (m, 2H), 1.72–1.79 (m, 2H), 1.35–1.43 (m, 2H),
0.90 (s, 3H). 13C NMR (150 MHz, CDCl3): d (ppm) 173.53
(CO2CH3), 148.54, 147.66, 147.03, 145.75, 145.24, 145.20,
145.13, 144.82, 144.73, 144.64, 144.57, 144.48, 144.14, 143.82,
143.79, 143.10, 143.05, 143.00, 142.93, 142.90, 142.26, 142.17,
142.12, 140.93, 140.70, 138.25, 138.11, 136.00, 131.77, 122.83,
80.10 (bridgehead C), 51.66, 46.21, 33.81, 33.70, 31.52, 31.36,
30.40, 28.85, 22.56, 22.52, 14.06. Anal. Calcd for C76H24O2S (%):
C, 91.18; H, 2.42. Found (%): C, 91.00; H, 2.19.
ꢂ
quently at 120 C for 30 min. The active layer of the blend of
P3HT and TCBM-Cn was prepared by spin-coating the chlo-
robenzene solution of the P3HT and TCBM-Cn (1 : 0.8, w/w)
with the P3HT concentration of 10 mg mlꢁ1 on the ITO/
PEDOT:PSS electrode. The thickness of the active layer was
controlled to ca. 115–125 nm by adjusting the rotating speed of
the spin-coater. Then the LiF/Al cathode was deposited at
a vacuum level of 4 ꢃ 10ꢁ4 Pa. The thicknesses of the LiF and Al
layers are 1 and 200 nm, respectively. The devices were annealed
ꢂ
at 160 C for 10 min in the glove-box. Eight devices were fabri-
cated per material to ensure the reproducibility of results. The
effective area of the unit cell is 12 mm2. The current–voltage
(I–V) measurement of the devices was conducted on a computer-
controlled Keithley 236 Source Measure Unit. A xenon lamp
(500 W) was used as the white light source, and the optical power
at the sample was 100 mW cmꢁ2. The external quantum efficiency
(EQE) was measured using a Model SR830 DSP Lock-in
Amplifier coupled with a SBP500 monochromator, a DSC102
date acquisition system and a Model SR540 chopper controller.
The light intensity at each wavelength was calibrated with
a standard single-crystal Si photodiode.
5-(2-Ethylhexyl)-[6,6]-thienyl-C61-butyric acid methyl ester (TC-
BM-C2,6)
TCBM-C2,6 was synthesized following the procedure for
1
preparation of TCBM with a yield of 49% (500 mg). H NMR
(300 MHz, CDCl3): d (ppm) 7.26 (d, J ¼ 4.5 Hz, 1H), 6.76 (d, J ¼
4.5 Hz, 1H), 3.69 (s, 3H), 2.90–2.95 (m, 2H), 2.80–2.85 (m, 2H),
2.80–2.85 (m, 2H), 2.58 (t, J ¼ 4.5 Hz, 2H), 2.22–2.29 (m, 2H),
1.64–1.66 (m, 1H), 1.31–1.43 (m, 8H), 0.88–0.96 (m, 6H). 13C
NMR (150 MHz, CDCl3): d (ppm) 173.50 (CO2CH3), 148.58,
147.66, 145.76, 145.24, 145.19, 145.12, 144.74, 144.71, 144.64,
144.57, 144.48, 144.13, 143.82, 143.79, 143.10, 143.07, 143.04,
143.00, 142.92, 142.90, 142.23, 142.20, 142.18, 142.12, 140.93,
140.70, 138.22, 138.13, 136.25, 131.75, 123.96, 80.12, 80.10
(bridgehead C), 51.65, 46.21, 41.55, 34.53, 33.75, 33.70, 31.45,
28.85, 25.93, 22.94, 22.52, 14.11, 11.00. Anal. Calcd for
C71H14O2S (%): C, 91.03; H, 2.74. Found (%): C, 90.69; H, 2.54.
Results and discussion
Synthesis and characterization
As shown in Scheme 1, TCBM derivatives, i.e. TCBM-C1,
TCBM-C6 and TCBM-C2,6 with methyl, hexyl and 2-ethylhexyl
groups at the 5-position of the thiophene ring, were synthesized
following the procedure for preparation of PCBM and
TCBM,6a,20 but without high temperature isomerization. TCBM
was also synthesized for comparison. First, 2a–d were prepared
from 1a–d by Friedel–Crafts acylation25 in yields of 52 to 86%.
Compounds 3a–d were prepared from 2a–d by esterification.
They were reacted with p-toluenesulfonyl hydrazide in methanol
under reflux to afford compounds 4a–d. For preparation of the
final compounds, C60 was dissolved in dried ODCB by ultrasonic
method, and the resulting solution was added to the solution of 4
and sodium methoxide in pyridine. Unlike the preparation of
PCBM, the successive reaction at 75 ꢂC for 24 h directly afforded
[6,6] isomers instead of the mixture of [5,6] and [6,6] adducts
withꢂout further isomerization at high temperature, such as
180 C typically for PCBM-type compounds as reported in the
literature.6a,12,16–18 The resulting TCBM-Cn were purified by
column chromatography on silica gel with toluene as eluent to
give TCBM, TCBM-C1, TCBM-C6 and TCBM-C2,6 in yields of
40, 40, 46 and 49%, respectively. The enhanced yields for TCBM-
C6 and TCBM-C2,6 are attributed to their high solubility, which
makes the compounds easily recovered from chromatography.
Actually, the solubility of TCBM and TCBM-C1 in chloroben-
zene is 50 ꢀ 5 mg mlꢁ1, lower than 80 ꢀ 5 mg mlꢁ1 of PCBM.
However, the solubility of TCBM-C6 and TCBM-C2,6 is as high
Measurements
1H NMR and 13C NMR spectra were recorded on a Bruker AV
300 spectrometer at 300 MHz and a Bruker AV 600 spectrometer
at 600 MHz. Elemental analysis was performed on a varioEL
elemental analysis system. UV-Vis-NIR spectra were recorded
on a Shimadzu UV-3600 UV-Vis-NIR spectrometer. Cyclic
voltammetry (CV) was performed on a CHI660a electrochemical
analyzer with a three-electrode cell in a solution of 0.1 M tetra-
butylammonium hexafluorophosphate (Bu4NPF6) in CH2Cl2 at
a scan rate of 100 mV sꢁ1. A platinum disc electrode with
a diameter of 10 mm, a Pt wire, and a saturated calomel electrode
(SCE) were used as the working electrode, the counter electrode,
and the reference electrode, respectively. The potential was
calibrated against the ferrocene/ferrocenium (Fc/Fc+) couple.
The lowest unoccupied molecular orbital (LUMO) energy levels
onset
red
were estimated by the equations: LUMO ¼ ꢁ(4.80 + E ) eV.26
Transmission electron microscopy (TEM) was performed on
a JEOL JEM-1011 transmission electron microscope operated at
an acceleration voltage of 100 kV.
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Scheme 1 Synthesis of fullerene derivatives. (i) glutaric anhydride, AlCl3, CH2Cl2, 0 ꢂC; (ii) CH3OH, HCl, reflux; (iii) p-toluenesulfonyl hydrazide
(TsNHNH2), CH3OH, CH3COOH, 75 ꢂC; (iv) C60, CH3ONa, ODCB, pyridine, 75 ꢂC.
as 180 ꢀ 10 mg mlꢁ1. This high solubility of TCBM-C6 and
TCBM-C2,6 is promising for the device fabrication.
Electrochemical properties
Electrochemical properties of TCBM-Cn were studied by cyclic
voltammetry (CV) at a scan rate of 100 mV sꢁ1 with CH2Cl2 and
Bu4NPF6 as the solvent and supporting electrolyte, respectively.
As shown in Fig. 3 and Table 1, all C60 derivatives including
TCBM and PCBM have a similar CV scans with three quasi-
reversible redox waves in the potential ranging from 0 to ꢁ2.0 V.
The structures of TCBM-Cn were validated by NMR spec-
1
troscopy and elemental analysis. Both H NMR and 13C NMR
spectra indicate their [6,6] structural features. Fig. 2 shows the 1H
NMR spectra of TCBM-C1 and 13C NMR spectra of TCBM-
1
C2,6 as an example. As reported in the literature,6a,23 the H
NMR signals of ArC(CH2)3COOMe are different for [5,6] and
[6,6] isomers in terms of chemical shift. The presence of three sets
of signals at ꢄ2.9, 2.6 and 2.2 ppm assigned to the aforemen-
tioned methylene groups in 1H NMR spectrum (Fig. 2a) together
with the 13C signals at ꢄ80 ppm attributed to the bridgehead
carbons in 13C NMR spectrum (Fig. 2b) clearly indicate the
formation of the neat methanofullerene structure, that is the [6,6]
isomer.
The first half-wave potentials for all C60 derivatives (Ered1) are
½
around ꢁ1.07 V vs. Fc/Fc+, corresponding to the lowest unoc-
cupied molecular orbital (LUMO) level of ꢁ3.77 eV (Table 1).
This indicates that the introduction of an alkyl chain onto the
thiophene ring does not affect the LUMO level of TCBM
derivatives.
Photovoltaic properties
BHJ PSCs with the device structure of ITO/PEDOT:PSS/
P3HT : TCBM-Cn (1 : 0.8, w/w)/LiF/Al were fabricated for
evaluation of the properties of the new acceptors. Control
devices based-on P3HT/PCBM or TCBM were also prepared
under identical conditions for comparison. As shown in Table 2
and Fig. 4, the performance of the control devices are well
Fig. 2 1H NMR spectrum of TCBM-C1 and 13C NMR spectrum of
TCBM-C2,6 in CDCl3 at room temperature.
Fig. 3 Cyclic voltammogram scans of PCBM, TCBM and TCBM-Cn.
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Table 1 Electrochemical data of TCBM-Cn and PCBMa
Compound
Ered1/V
Ered2/V
Ered3/V
LUMO/eV
½
½
½
PCBM
TCBM
TCBM-C1
TCBM-C6
TCBM-C2,6
ꢁ1.07
ꢁ1.07
ꢁ1.07
ꢁ1.07
ꢁ1.06
ꢁ1.46
ꢁ1.46
ꢁ1.45
ꢁ1.44
ꢁ1.44
ꢁ1.96
ꢁ1.96
ꢁ1.95
ꢁ1.95
ꢁ1.95
ꢁ3.75
ꢁ3.76
ꢁ3.77
ꢁ3.77
ꢁ3.77
a
Experiments were carried out in CH2Cl2 solutions (10ꢁ3 M) with
Bu4NPF6 (0.1 M) as the supporting electrolyte at a scan rate of 100
mV sꢁ1. Pt disc, Pt wire and SCE were used as working, counter and
reference electrodes, respectively. Half-wave potentials were reported in
V vs. Fc/Fc+.
Table 2 PSC performance of TCBM-Cn and PCBM under illumination
of 100 mW cmꢁ2 white light.a
Active layer
Jsc/mA cmꢁ2
Voc/V
FF (%)
PCE (%)
P3HT/TCBM
10.33
10.51
10.61
10.53
10.40
0.62
0.60
0.63
0.64
0.62
0.62
0.63
0.64
0.57
0.61
4.00
3.95
4.26
3.84
3.90
P3HT/TCBM-C1
P3HT/TCBM-C6
P3HT/TCBM-C2,6
P3HT/PCBM
a
The thickness of the active layer is in the range of 115 to 125 nm and the
performance data were average values calculated from eight devices for
each material.
Fig. 5 Transmission electron micrographs of P3HT/PCBM (a), P3HT/
TCBM-C6 (b) and P3HT/TCBM-C2,6 (c) films with a thickness of
30 nm. The films were annealed at 160 ꢂC for 10 min.
comparable to the reference report with average PCEs of 3.90
and 4.00% for PCBM and TCBM, respectively.24,27 The devices
of TCBM-C1 show an average PCE of 3.95%. The devices of
TCBM-C2,6 exhibit a slightly lower fill factor (FF) of 0.57,
leading to the lowest PCE of 3.84%. TCBM-C6 shows the best
device performance with a Voc of 0.63 V, a short circuit current
density (Jsc) of 10.61 mA cmꢁ2, and a FF of 0.64, leading to
a PCE of 4.26%. This is different from the results reported by
Troshin et al.,24 who reported that the device efficiencies were
related to the solubilities of C60 derivatives, and the better device
performance was achieved with solubilities of C60 derivatives in
the range of 30 to 80 mg mlꢁ1
.
The films of P3HT/TCBM-Cn with a thickness of ꢄ30 nm
were prepared under conditions identical to the device fabrica-
tion for TEM observation. TEM images of P3HT/PCBM,
P3HT/TCBM-C6 and P3HT/TCBM-C2,6 films are shown in
Fig. 5. All films are uniform with absence of large aggregates,
which is consistent with the good device performance of these
materials.
Fig. 4 (a) J–V curves and (b) EQE spectra of PSCs with P3HT : TCBM-
Cn (1 : 0.8, w/w) as the active layer. J–V curves were in the dark and
under illumination of 100 mW cmꢁ2 white light.
3096 | J. Mater. Chem., 2010, 20, 3092–3097
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´
9 I. Riedel, E. von Hauff, J. Parisi, N. Martın, F. Giacalone and
V. Dyakonov, Adv. Funct. Mater., 2005, 15, 1979.
Conclusions
10 C. Yang, Y. Li, J. Hou, C. He, Z. Tan, B. Fan, Y. Zhou, Q. Sun, Y. Li,
Y. Li and D. Zhu, Polym. Adv. Technol., 2006, 17, 500.
ꢀ
11 S. A. Backer, K. Sivula, D. F. Kavulak and J. M. J. Frechet, Chem.
Mater., 2007, 19, 2927.
12 Y. Zhang, H.-L. Yip, O. Acton, S. K. Hau, F. Huang and
A. K.-Y. Jen, Chem. Mater., 2009, 21, 2598.
13 M. Lenes, S. W. Shelton, A. B. Sieval, D. F. Kronholm,
J. C. Hummelen and P. W. M. Blom, Adv. Funct. Mater., 2009, 19,
3002.
14 T. Niinomi, Y. Matsuo, M. Hashiguchi, Y. Sato and E. Nakamura, J.
Mater. Chem., 2009, 19, 5804.
15 F. B. Kooistra, J. Knol, F. Kastenberg, L. M. Popescu,
W. J. H. Verhees, J. M. Kroon and J. C. Hummelen, Org. Lett.,
2007, 9, 551.
16 C. Yang, J. Y. Kim, S. Cho, J. K. Lee, A. J. Heeger and F. Wudl, J.
Am. Chem. Soc., 2008, 130, 6444.
17 L. Zheng, Q. Zhou, X. Deng, M. Yuan, G. Yu and Y. Cao, J. Phys.
Chem. B, 2004, 108, 11921.
In conclusion, we have demonstrated that [6,6]-thienyl-C61-
butyric acid methyl ester (TCBM) and its alkyl-substituted
derivatives (TCBM-Cn) can be synthesized directly from the
typical diazo addition, and that the additional isomerization at
high temperature is unnecessary. The yields of the TCBM-C6
and TCBM-C2,6 are as high as 46% and 49%, respectively.
Meanwhile, these two C60 derivatives are highly soluble with
a solubility of up to 180 mg mlꢁ1 in chlorobenzene. PSCs of
TCBM-C6 exhibit the best device performance with a PCE of
4.26% (Jsc ¼ 10.61 mA cmꢁ2, Voc ¼ 0.63 V, FF ¼ 0.64). Easy
synthesis, high solubility and excellent device performance
qualify thienyl substituted C60 derivatives as promising alterna-
tives to PCBM, the well known electron-accepting materials for
BHJ PSCs.
18 S. Y. Nam, H. S. Lee, J. Jung, J. Lee, W. S. Shin, S.-J. Moon, C. Lee
and S. C. Yoon, J. Nanosci. Nanotechnol., 2009, 9, 6008.
19 Q. Wei, T. Nishizawa, K. Tajima and K. Hashimoto, Adv. Mater.,
2008, 20, 2211.
20 L. M. Popescu, P. van’t Hof, A. B. Sieval, H. T. Jonkman and
J. C. Hummelen, Appl. Phys. Lett., 2006, 89, 213507.
21 A. J. J. M. van Breemen, P. T. Herwig, C. H. T. Chlon, J. Sweelssen,
H. F. M. Schoo, S. Setayesh, W. M. Hardeman, C. A. Martin,
D. M. de Leeuw, J. J. P. Valeton, C. W. M. Bastiaansen,
D. J. Broer, A. R. Popa-Merticaru and S. C. J. Meskers, J. Am.
Chem. Soc., 2006, 128, 2336–2345.
Acknowledgements
This work is supported by the NSFC (no. 20921061, 20525415
and 50703041), MOST of China (2009CB939702) and Chinese
Academy of Sciences (KJCX2-YW-M11).
Notes and references
1 C. J. Brabec, N. S. Sariciftci and J. C. Hummelen, Adv. Funct. Mater.,
2001, 11, 15.
22 M. Jeffries-El, G. Sauv and R. D. McCullough, Macromolecules,
2005, 38, 10346.
2 K. M. Coakley and M. D. McGehee, Chem. Mater., 2004, 16, 4533.
3 H. Hoppe and N. S. Sariciftci, J. Mater. Chem., 2006, 16, 45.
23 M. Prato, V. Lucchini, M. Maggini, E. Stimpfl, G. Scorrano,
M. Eiermann, T. Suzuki and F. Wudl, J. Am. Chem. Soc., 1993,
115, 8479.
24 P. A. Troshin, H. Hoppe, J. Renz, M. Egginger, J. Y. Mayorova,
A. E. Goryachev, A. S. Peregudov, R. N. Lyubovskaya,
G. Gobsch, N. S. Sariciftci and V. F. Razumov, Adv. Funct. Mater.,
2009, 19, 779.
25 A. De, S. Bhattacharya, S. S. Jash and S. Mukherjee, J. Heterocyclic
Chem., 1992, 29, 1213.
26 A. J. Bard and L. A. Faulkner, Electrochemical Methods-
Fundamentals and Applications, Wiley, New York, 1984.
27 Y. Zhao, Z. Y. Xie, C. Qin, Y. Qu, Y. H. Geng and L. X. Wang, Sol.
Energy Mater. Sol. Cells, 2009, 93, 604.
€
4 S. Gunes, H. Neugebauer and N. S. Sariciftci, Chem. Rev., 2007, 107,
1324.
ꢀ
5 B. C. ThompsonandJ. M. Frechet, Angew. Chem., Int. Ed., 2008, 47, 58.
6 (a) J. C. Hummelen, B. W. Knight, F. LePeq and F. Wudl, J. Org.
Chem., 1995, 60, 532; (b) G. Yu, J. Gao, J. C. Hummelen, F. Wudl
and A. J. Heeger, Science, 1995, 270, 1789.
7 C. J. Brabec, A. Cravino, D. Meissner, N. S. Sariciftci, T. Fromherz,
M. T. Rispens, L. Sanchez and J. C. Hummelen, Adv. Funct. Mater.,
2001, 11, 374.
8 L. Zheng, Q. Zhou, X. Deng, M. Yuan, G. Yu and Y. Cao, Thin Solid
Films, 2005, 489, 251.
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