Synthesis of oligofluorene modified C60 derivatives for organic solar cell applications

Article abstract of DOI:10.1039/c0jm04506c

New oligofluorene modified C₆₀ derivatives were designed and synthesized via rhodium complex catalyzed direct coupling between C₆₀ and oligofluorene boronic acids in H₂O/1,2-dichlorobenzene (1:4). The product was first purified by silica gel column chromatography and then further purified by semipreparative HPLC equipped with a Buckyprep column using toluene as an eluent. The strong deep-blue fluorescence of the oligofluorene bridge was completely quenched after end-capped with C₆₀, indicating an efficient energy and/or charge transfer between the π-conjugated bridge and the C₆₀ cages. Cyclic voltammetric measurements show that the HOMO and LUMO energy levels of the synthesized C₆₀ derivatives are similar to those of widely used PC₆₁BM. This means that the energy levels of the C₆₀ derivatives mainly depend on the π electron system of the C₆₀ cage. However, the substituents have huge impacts on the other physical properties of the resulting C₆₀ derivatives, such as solubility, crystallinity, and electron mobility. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0jm04506c

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PAPER
Synthesis of oligofluorene modified C60 derivatives for organic solar cell
applications
a
b
a
a
a
a
Jianping Lu, Jianfu Ding, Salima Alem, Salem Wakim, Shing-Chi Tse, Ye Tao, Jacek Stupakc
*
*
and Jianjun Lic
Received 23rd December 2010, Accepted 5th January 2011
DOI: 10.1039/c0jm04506c
New oligofluorene modified C60 derivatives were designed and synthesized via rhodium complex
catalyzed direct coupling between C60 and oligofluorene boronic acids in H2O/1,2-dichlorobenzene
(1 : 4). The product was first purified by silica gel column chromatography and then further purified by
semipreparative HPLC equipped with a Buckyprep column using toluene as an eluent. The strong
deep-blue fluorescence of the oligofluorene bridge was completely quenched after end-capped with C60,
indicating an efficient energy and/or charge transfer between the p-conjugated bridge and the C60 cages.
Cyclic voltammetric measurements show that the HOMO and LUMO energy levels of the synthesized
C60 derivatives are similar to those of widely used PC61BM. This means that the energy levels of the C60
derivatives mainly depend on the p electron system of the C60 cage. However, the substituents have
huge impacts on the other physical properties of the resulting C60 derivatives, such as solubility,
crystallinity, and electron mobility.
chemical reactions have been developed to attach organic func-
Introduction
tional groups to the fullerene skeletons, including cycloaddition
and the addition of nucleophiles and free radicals.6 However, it is
still pretty challenging to achieve selective monoaddition with
wide-range functional group compatibility. Recently, Itami et al.
reported an interesting Rh catalyzed arylation of C60 using
organoboron compounds with good yield and high selectivity.7
Since organoboron compounds are readily prepared and
compatible with a variety of organic functional groups, this
fullerene chemistry opens a new avenue for the versatile func-
tionalization of fullerenes with p-conjugated compounds. This
prompted us to attach oligofluorenes to the C60 molecule as
monodisperse oliogofluorenes possess promising optoelectronic
properties and are widely used as electron-transporting blue
emitters in light-emitting diodes.8 We investigated the effect of
oligofluorene substituents on the energy levels of the resultant
fullerene derivatives. In addition, we also designed and synthe-
sized an oligofluorene bridged C60 dimer with an intention to
lower its LUMO energy level to match with some low band gap
polymers with low-lying LUMO levels (below 3.9 eV).
Organic solar cells based on conjugated polymers and soluble
fullerene derivatives potentially offer low-cost solar electricity by
using spin-coating, ink-jet printing, or roll-to-roll printing tech-
niques to fabricate large-area photovoltaic devices on flexible
and light-weight substrates.1 Recently, significant progress has
been made in this field, and the power conversion efficiencies of
solution-processed bulk heterojunction solar cells have reached
7–8% regime, primarily due to the development of high mobility
low band gap p-type materials2 and better control of the nano-
structured morphology of the interpenetrating electron donor/
acceptor networks by using processing additives.3 In contrast to
the intensive study on p-type materials, the design and synthesis
of new fullerene derivatives for applications in solar cells as
electron acceptors are relatively unexplored with most work
focusing on the modification of PCBM ([6,6]-phenyl C61 butyric
acid methyl ester) derivatives.4
Fullerene derivatives have attracted significant attention as
promising functional components in nanoelectronic devices due
to their high electron affinity and mobility.5 A number of
In this paper, we report on the detailed synthesis and prop-
erties of oligofluorene modified C60 derivatives for potential
applications in organic electronic devices. We found that oligo-
fluorene substituents had little impact on the energy levels of the
C60 derivatives because the conjugation between the oligo-
fluorene bridge and the p-electron system of C60 was disrupted
by a sp3 hybridized carbon. Initial study showed that a power
conversion efficiency of 2.3% was obtained when blending the
synthesized C60 derivative (C60F) 1 with a p-type poly-
(2,7-carbazole) derivative (PCDTBT) as the photovoltaic active
aInstitute for Microstructural Sciences (IMS), National Research Council
of Canada (NRC), 1200 Montreal Road, Ottawa, ON, K1A 0R6, Canada.
E-mail: Jianping.Lu@nrc-cnrc.gc.ca; Ye.Tao@nrc-cnrc.gc.ca; Fax: +1
(613)990-0202; Tel: +1 (613)990-1651
bInstitute for Chemical Process and Environmental Technology (ICPET),
National Research Council of Canada (NRC), 1200 Montreal Road,
Ottawa, ON, K1A 0R6, Canada
cInstitute for Biological Sciences (IBS), National Research Council of
Canada (NRC), 1200 Montreal Road, Ottawa, ON, K1A 0R6, Canada
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layer. The efficiency was further improved to 3.4% by using the
mixture of C60F and PC61BM (1 : 1) as the electron acceptor.
8.02 (d, J ¼ 8Hz, 2H), 7.79 (m, 1H), 7.40–7.26 (m, 3H), 6.49(s,
1H, C60H), 2.26–2.04 (m, 4H), 1.18–0.96 (m, 8H), 0.67 (t, J ¼ 7.0
Hz, 6H). 13C NMR (100 MHz, C6D6, d) 154.58, 153.26, 153.00,
151.43, 148.09, 147.86, 147.63, 147.24, 146.79, 146.72, 146.57,
146.55, 146.26, 146.15, 145.90, 145.85, 145.79, 145.72, 145.05,
144.91, 143.68, 142.97, 142.94, 142.70, 142.37, 142.38, 142.02,
141.93, 141.79, 140.86, 140.61, 136.51, 136.17, 127.59, 127.29,
123.46, 121.97, 121.50, 120.55, 68.70 (C60fluorene), 64.28 (C60H),
55.98, 40.40, 26.71, 23.38, 13.98. MS (MALDI-TOF) calcd for
C81H26: 998.2; found: 998.2. Elemental analysis calcd for C81H26:
C, 97.38; H, 2.62. Found: C, 97.11; H, 2.68%.
Experimental section
Materials and characterization
All reagents and solvents were purchased from Aldrich and
Fisher Chemicals. Anhydrous tetrahydrofuran was distilled over
sodium/benzophenone under Ar prior to use. Poly[N-heptade-
canyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothia-
diazole)] (PCDTBT) was provided by St-Jean Photochemicals,
ꢀ
Quebec,
Canada.
9,9-Dibutylfluorene-2-boronic
acid,
Synthesis of 2,20-(perfluoro-1,4-phenylene)bis(9,9-
dioctylfluorene) 2
and 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctyl-
fluorene were prepared according to our previous procedures.9
The rhodium catalyst, bis(acetonitrile)(cyclo-octa-1,5-diene)-
rhodium tetrafluoroborate ([Rh(COD)(MeCN)2]BF4), was
synthesized according to literature procedures.10 NMR spectra
were recorded on a 400 MHz Varian Unity Inova spectrometer
using tetramethylsilane as internal references. UV-vis absorption
spectra were recorded on a Varian Cary 50 spectrophotometer
and fluorescence measurements were carried out on a Spex
Fluorolog 3 spectrometer. The differential scanning calorimetry
(DSC) analysis of fullerene derivatives was performed under
a nitrogen atmosphere on a TA Instruments DSC 2920 at heating
A 100 mL flask was charged with 2-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-9,9-dioctylfluorene (4.20 g, 8.13 mmol) and
1,4-dibromo-2,3,5,6-tetrafluorobenzene (1.00 g, 3.25 mmol). The
flask was evacuated and filled with argon 3 times to remove
traces of air. Pd(PPh3)4 (75 mg) was added into a nitrogen-filled
glovebox. The flask was equipped with a condenser and again
evacuated and filled with argon 2 times. Then, a degassed
mixture of indoloantricaprylylmethylammonium chloride
(Aliquatꢀ 336) (several drops), 2 M K2CO3 aqueous solution
(16 mL), and toluene (35 mL) was added via a syringe. The
reaction flask was flushed with argon once more. Then, the
reaction mixture was heated to reflux with stirring and kept at
this temperature for 30 h. After cooling down, the organic layer
was separated, and the aqueous layer was extracted with hexanes
twice. Then, the organic layers were combined and solvent-
stripped. The crude product was purified by column chroma-
tography using 2% ethyl acetate in hexanes as the eluent. The
purified product crystallized when left in refrigerator overnight.
Finally, 2.82 g of white crystals were obtained in 93.7% yield.
1H NMR (400 MHz, CDCl3, d) 7.82 (dd, J1 ¼ 8 Hz, J2 ¼ 0.4 Hz,
2H), 7.78–7.72 (m, 2H), 7.52–7.46 (m, 4H), 7.38–7.32 (m, 6H),
2.04–1.94 (m, 8H), 1.25–1.00 (m, 40H), 0.80 (t, J ¼ 7.2 Hz, 12H),
0.75–0.60 (m, 8H). 19F NMR (376 MHz, CDCl3, d) ꢁ144.79.
rates of 10 C minꢁ1. High performance liquid chromatography
ꢀ
(HPLC) analysis was performed on a Waters model 515 equip-
ped with a Cosmosil Buckyprep column (10.0 mm ꢂ 250 mm,
Nacalai USA) using toluene as an eluent.
Synthesis of 1-(9,9-dibutylfluoren-2-yl)-1,9-dihydro[60]fullerene
1 (C60F)
To a 100 mL two-necked flask equipped with a short condenser
were added C60 (216 mg, 0.3 mmol) and 9,9-dibutylfluorene-2-
boronic acid (66 mg, 0.2 mmol). The flask was evacuated and
filled with argon 3 times to remove traces of air. Rh(COD)-
(MeCN)2]BF4 (22 mg, 0.058 mmol) was then added into
a nitrogen-filled glovebox. In another 100 mL flask, 1,2-dichlo-
robenzene (35 mL) and water (9 mL) were added. The resulting
solvent mixture was degassed and then transferred to the reaction
flask via a syringe. The reaction flask was again evacuated and
filled with argon once more. The reaction mixture was then
Synthesis of 7,70-(perfluoro-1,4-phenylene)bis(2-bromo-9,9-
dioctylfluorene) 3
Compound 2 (2.00 g, 2.16 mmol) was dissolved in 10 mL CH2Cl2
containing 5 mg of iodine. Then, Br2 (0.25 mL, 4.75 mmol) was
added via a syringe. The reaction solution was stirred at room
temperature overnight. The reaction was then quenched with
Na2S2O3 aqueous solution. The water layer was extracted with
CH2Cl2 (2 ꢂ 10 mL), and the organic layers were combined and
dried over MgSO4. The crude product was purified by crystalli-
zation from 1 : 1 hexanes/ethanol to give 2.15 g of white crys-
ꢀ
heated to 60 C under argon. After stirring for 5 h, the second
batch of 9,9-dibutylfluorene-2-boronic acid (40 mg, 0.12 mmol)
in degassed 1,2-dichlorobenzene (4 mL) was added. The reaction
mixture was further stirred for another 5 h. After cooling down,
the brown reaction mixture was passed through a thin layer of
Celite and washed with toluene (100 mL). The organic layer was
separated, dried over MgSO4, and concentrated under reduced
pressure. The crude product was first purified by a silica gel
column using 1 : 4 toluene/hexanes as the eluent to remove
unreacted C60 and excess boronic acid. Further purification by
semipreparative HPLC equipped with a Buckyprep column
using toluene as an eluent afforded 0.15 g of brown powder in
50% yield. HPLC analysis using benzene as an eluent indicated
that the purified product still contained about 5% isomer, which
can be completely removed by running a second HPLC purifi-
cation with benzene as the eluent. 1H NMR (400 MHz, C6D6, d)
8.64 (d, J ¼ 1.6 Hz, 1H), 8.39 (dd, J1 ¼ 8.0 Hz, J2 ¼ 1.6 Hz, 1H),
1
talline powder in 91% yield. H NMR (400 MHz, C6D6, d) 7.79
(d, J1 ¼ 7.6 Hz, 2H), 7.61 (d, J ¼ 8.8 Hz, 2H), 7.52–7.45 (m, 8H),
2.04–1.88 (m, 8H), 1.25–1.00 (m, 40H), 0.81 (t, J ¼ 7.6 Hz, 12H),
0.75–0.60 (m, 8H). 19F NMR (376 MHz, CDCl3, d) ꢁ144.65.
Synthesis of 7,70-(perfluoro-1,4-phenylene)bis(9,9-
dioctylfluorene-2,7-diyl)diboronic acid 4
Compound 3 (1.00 g, 0.92 mmol) was dissolved in anhydrous
THF (25 mL) under argon, and cooled to ꢁ78 ꢀC. n-Butyl
4954 | J. Mater. Chem., 2011, 21, 4953–4960
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lithium solution (2.5 M, 0.85 mL) was then added. The solution
immediately turned yellow. After stirring for a while, some
yellow solids precipitated out. The paste was further stirred at
oven for 15 min. Thereafter, the ITO-coated slides were spin-
coated (6000 rpm for 60 seconds) with a filtered (0.45 mm NYL w/
GMF syringe filter) aqueous suspension of poly(ethylene dioxy-
thiophene) doped with polystyrene sulfonic acid, PEDOT:PSS
(Baytron P, H. C. Starck), and then baked at 120 ꢀC under N2 for
2 h. The resulting thin PEDOT:PSS layer was about 50 nm thick.
A dichlorobenzene solution of PCDTBT and C60 derivative 1
(1 : 2 by weight) was stirred at 80 ꢀC overnight. After cooling
down, it was spun-cast on top of PEDOT–PSS at 1000 rpm for
60 s. The resulting film (ꢃ100 nm in thickness) was kept in a Petri
dish overnight and then transferred to an evaporation chamber.
The device fabrication was completed by the vacuum deposition
of LiF (1 nm) and Al cathode (120 nm). The solar cells with no
protective encapsulation were subsequently tested in air under
ꢀ
ꢁ78 C for 3 h. Then triisopropyl borate (0.63 mL, 2.75 mmol)
was added via a syringe. After stirring for 15 min, the solids
disappeared to form a light brown solution. The solution was
allowed to warm to room temperature slowly and further stirred
overnight. HCl aqueous solution (1 M, 50 mL) was then added.
The aqueous layer was extracted with ether (3 ꢂ 25 mL). The
organic layers were combined and concentrated. The crude
product was purified by column chromatography first using 10%
acetone in dichloromethane to remove monoboronic acid and
other byproducts and then using pure acetone to elute out the
1
desired product as white powder (0.53 g, 57%). H NMR (400
MHz, CD3COCD3/D2O, d) 8.0–7.92 (m, 4H), 7.89 (d, J ¼ 6.8 Hz,
2H), 7.81 (d, J ¼ 6.8 Hz, 2H), 7.61 (s, 2H), 7.53 (d, J ¼ 7.6 Hz,
2H), 2.14–1.98 (m, 8H), 1.25–0.96 (m, 40H), 0.73 (t, J ¼ 6.0 Hz,
12H), 0.68–0.56 (m, 8H). 19F NMR (376 MHz, CD3COCD3/
D2O, d) ꢁ145.86.
simulated air mass (AM) 1.5 solar irradiation (100 mW cmꢁ2
,
Sciencetech Inc., Model SF150). Current–voltage (I–V) charac-
teristics were recorded using a computer-controlled Keithley
2400 source meter. The light intensity was calibrated with
a power meter (Gentec Solo PE Laser Power & Energy Meter).
The external quantum efficiency (EQE) was performed using
a Jobin–Yvon Triax 180 spectrometer, a Jobin–Yvon xenon light
source, a Merlin lock-in amplifier, a calibrated Si UV detector,
and an SR570 low noise current amplifier.
Synthesis of 7,70-(perfluoro-1,4-phenylene)bis(9,9-
dioctylfluorene-2,7-diyl)bis(1,9-dihydro[60]fullerene) 5
To a 100 mL two-necked flask equipped with a short condenser
were added C60 (120 mg, 0.167 mmol) and diboronic acid 4 (76
mg, 0.075 mmol). The flask was evacuated and filled with argon
3 times to remove traces of air. Rh(COD)(MeCN)2]BF4 (11 mg,
0.029 mmol) was then added into a nitrogen-filled glovebox. In
another 100 mL flask, 1,2-dichlorobenzene (24 mL) and water
(6 mL) were added. After degassing, the resulting solvent mixture
was transferred to the reaction flask via a syringe. The reaction
flask was again evacuated and filled with argon one more time.
The reaction mixture was then heated to 60 ꢀC under argon and
stirred at this temperature for 1 day. Then THF (2 mL) was
added. The reaction mixture was further stirred for another day.
After cooling down, the reaction mixture was diluted with
chlorobenzene (30 mL), passed through a thin layer of Celite,
and washed with toluene (50 mL). The organic layer was sepa-
rated, dried over MgSO4, and concentrated under reduced
pressure. The crude product was first purified by a silica gel
column chromatography to remove unreacted C60 and diboronic
acid. Further purification by semipreparative HPLC using
toluene as an eluent afforded 56 mg of brown powder in 31%
Results and discussion
Synthesis
We first examined the Rh catalyzed C60 arylation by reacting C60
with 9,9-dibutylfluorene-2-boronic acid in H2O/1,2-dichloro-
benzene (1 : 4) under Ar according to Itami’s procedure,7 as
shown in Scheme 1. The catalyst used was bis(acetonitrile)(cyclo-
octa-1,5-diene)rhodium
tetrafluoroborate
([Rh(COD)-
(MeCN)2]BF4). The reaction was followed by HPLC analysis on
a Cosmosil Buckyprep column using toluene as the eluent. It was
found that besides our desired product, multi-substituted C60
derivatives were also formed, leading to a low reaction yield
(ꢃ25%). Since C60 dissolved into dichlorobenzene slowly and
9,9-dibutylfluorene-2-boronic acid was quite soluble in dichlo-
robenzene, we envisioned that in the beginning 9,9-dibutyl-
fluorene-2-boronic acid was in large excess in solution with
respect to the amount of the dissolved C60. This could result in
the formation of multi-substituted C60 derivatives. In addition,
as boronic acid was over-consumed in the beginning, the reaction
system lacks boronic acid in the later stage. As a result, some C60
remained unreacted. Our hypothesis was supported by the fact
1
yield. H NMR (400 MHz, 1,2-C6D4Cl2, d) 8.81 (d, J ¼ 1.6 Hz,
2H), 8.65 (dd, J1 ¼ 7.6 Hz, J2 ¼ 1.6 Hz, 2H), 8.35 (d, J ¼ 8.0 Hz,
2H), 8.20 (d, J ¼ 8.0 Hz, 2H), 7.99 (s, 2H), 7.85 (d, J ¼ 8.0 Hz,
2H), 6.87 (s, 1H, C60H), 2.60–2.40 (m, 8H), 1.35–1.10 (m, 48H),
0.89 (t, J ¼ 6.8 Hz, 12H). 19F NMR (376 MHz, 1,2-C6D4Cl2, d)
ꢁ143.95. MS (MALDI-TOF) calcd for C184H82F4: 2366.6;
found: 2366.8. Elemental analysis calcd for C184H82F4: C, 93.30;
H, 3.49. Found: C, 92.62; H, 3.71%.
Photovoltaic device fabrication and testing
Glass slides coated with patterned ITO (Colorado Concept
Coatings LLC) were cleaned by sonicating sequentially in
detergent, DI water, acetone, and isopropanol. The active area of
each solar cell device was 5 ꢂ 20 mm2. Immediately prior to
device fabrication, the ITO substrate was treated in a UV-ozone
Scheme 1 Synthesis of a fluorene substituted C60 derivative. Reagents
and conditions: molar ratio: C60/ArB(OH)2/Rh ¼ 1 : 1.1 : 0.2, 60 ꢀC, 10 h
under Ar.
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that the reaction yield was significantly improved from 25% to
50% when 9,9-dibutylfluorene-2-boronic acid was added in two
portions, which was proved by HPLC analysis as shown in
Fig. 1. It is interesting to point out that an isomer (appearing at
7.15 min during HPCL analysis) also formed together with the
desired product (1,2-hydroarylation adduct) as MALDI-TOF
mass spectrometry measurement showed that the shoulder peak
at 7.15 min has the same molecular weight as our target product.
The formation of isomers during this Rh catalyzed C60 arylation
was not reported in the literature. Further purification by HPLC
using benzene as the eluent can completely remove this isomer.
The synthetic route to an oligofluorene bridged C60 dimer 5 is
outlined in Scheme 2. Briefly, oligofluorene 2 was synthesized in
a high yield (93.7%) via a Suzuki coupling reaction between
2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctyl-
fluorene and 1,4-dibromo-2,3,5,6-tetrafluorobenzene. To ensure
complete conversion of the dibromo compound, a 25% excess of
fluorene monoboronate was used. Then, compound 2 was
brominated in CH2Cl2 with Br2 in nearly quantitative yield. The
conversion of compound 3 to diboronic acid 4 was carried out at
Optical properties
The UV-vis absorption spectra of the synthesized C60 derivatives
1 and 5 in chloroform are shown in Fig. 3. For comparison
purpose, the absorption spectrum of widely used PC61BM is also
included in Fig. 3. PC61BM displays an absorption peak at
328 nm, arising from the p–p* transition of the delocalized 58 p
electrons. This absorption band slightly blue-shifts to 311 nm in
compound 1. As to the C60 dimer 5, the absorption band of the
C60 p electron system overlaps with that of the oligofluorene
bridge, appearing at 328 nm. As the oligofluorene itself
(compound 2) shows an absorption maximum at 318 nm in
chloroform solution, we conclude that the interaction in ground
state between the C60 buckyball and the oligofluorene bridge is
weak. This result is not very surprising, considering the conju-
gation between the oligofluorene bridge and the p-electron
system of C60 is disrupted by a sp3 hybridized carbon. However,
the complete quenching of the strong blue fluorescence of the
oligofluorene bridge suggests that energy and/or charge transfer
between the oligofluorene bridge and the C60 buckyball in the
excited state should be quite efficient (see Fig. 4). In addition, as
can be seen from Fig. 3, C60 derivatives 1, 5 and PC61BM have
similar optical band gaps, as calculated from their absorption
edges.
ꢀ
low temperature (ꢁ78 C). Otherwise, the reaction yield would
be low due to side reactions. The Rh catalyzed coupling reaction
between diboronic acid 4 and C60 was carried out first at 60 ꢀC in
H2O/1,2-dichlorobenzene (1 : 4) for 1 day. Then, THF (10%
relative to dichlorobenzene) was added, and the reaction mixture
was further stirred for another 24 h. We found that the addition
of THF to the reaction system was crucial for this reaction,
increasing the reaction yield from ꢃ10% to 31%. The reason why
THF improved the reaction yield is not very clear at this
moment. However, THF helps to dissolve diboronic acid 4 into
the reaction solution. The crude product was isolated by
distilling out the solvents, and purified first by silica gel column
chromatography and then by HPLC using toluene as the eluent.
Fig. 2 shows the proton NMR spectrum of C60 dimer 5 in the
aromatic region and its HPLC curve. Both indicate that a
high-purity product was obtained.
Thermal properties
The differential scanning calorimetry (DSC) analysis reveals that
the prepared fullerene derivatives 1 and 5 are both amorphous
materials with glass transition temperatures at 190.7 and
269.4 ꢀC, respectively. This is in contrast to widely used PC61BM,
which shows a melting peak at 295 ꢀC.4c The oligofluorene bridge
in the compound 5 is a crystalline material with a melting point at
63.7 ꢀC. However, when the oligofluorene is end-capped with two
bulky C60 balls, it cannot form crystalline structures anymore
because of steric hindrance. In the meanwhile, the long rigid
oligofluorene bridge also prevents the C60 moieties from forming
crystals.
Electrochemical properties
To investigate the electrochemical properties of the synthesized
C60 derivatives and compare their LUMO energy levels with
PC61BM, cyclic voltammetry (CV) measurement was carried out
in a three-electrode cell under Ar using 0.1 M Bu4NPF6 in
anhydrous CH3CN/1,2-dichlorobenzene (1 : 4, v/v) as the sup-
porting electrolyte. A Pt disk (1 mm diameter), a Pt wire (0.5 mm
diameter), and a silver wire (2 mm diameter) served as the
working electrode, counter electrode, and quasi-reference
electrode, respectively. The compound was dissolved in the
supporting electrolyte solution. Prior to the CV study, the Ag
quasi-reference electrode was first calibrated using a ferrocene/
ferrocenium (Fc/Fc+) redox couple (0.35 V vs. Ag/AgCl) as an
external standard, and was found to be ꢁ0.02 V vs. SCE.
Therefore, the LUMO energy levels of the C60 derivatives can be
estimated using the following empirical equations, ELUMO
¼
0
0
En + 4.38 eV, respectively, where En is the onset reduction
potential relative to the Ag quasi-reference electrode. Fig. 5
displays the cyclic voltammograms of C60 derivatives 1 and 5.
These two fullerene derivatives underwent reversible cathodic
Fig. 1 HPLC curves of the reaction mixture using toluene as the eluent.
4956 | J. Mater. Chem., 2011, 21, 4953–4960
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Scheme 2 Synthetic route to a p-bridged C60 dimer 5. Reagents and conditions: (i) 1 mol% Pd(PPh3)4, toluene/2 M K2CO3, reflux, 30 h; (ii) 1 equiv. Br2,
RT; (iii) 1.1 equiv BuLi, THF, ꢁ78 ꢀC; 1.5 equiv. B(O-iPr)3, ꢁ78 ꢀC to RT; 1 M HCl; (iv) molar ratio: C60/4/Rh ¼ 2.2 : 1 : 0.4, H2O/1,2-dichlorobenzene/
THF (1 : 4 : 0.4), 60 ꢀC, 2 d under Ar.
Fig. 2 Proton NMR spectrum (aromatic region) of C60 dimer 5 in C6D6. The inset shows its chemical structure and HPLC curve.
reduction with negligible change in the CV curves after successive
multiple potential scans. This indicates that they are good elec-
tron acceptors with high stability for electron injection. It is
interesting to point out that C60 derivatives 1 and 5 have LUMO
energy levels quite similar to that of PC61BM, located at ꢃ3.94
eV. This means that the energy levels of fullerene derivatives
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very deep HOMO energy levels and are hard to be oxidized in the
ground state. In the excited state, however, there is an efficient
charge transfer between the oligofluorene bridge and the two C60
end groups as the LUMO of the oligofluorene is located at 2.79
eV (estimated from the CV measurement), well above the LUMO
energy level of the C60 dimer 5.
Photovoltaic performance
The PV response of the synthesized C60 derivatives 1 and 5 was
investigated by blending them with PCDTBT at different weight
ratios. PCDTBT was selected as the hole-transporting material
in this study because this polymer is one of the best p-type
materials reported so far for solar cell applications and demon-
strated a power conversion efficiency of 6.0% when blended with
PC71BM.11 Previous studies on this polymer done by us and
other groups have shown that neither thermal annealing nor the
use of 1,8-diiodooctane (a processing additive) can improve the
device performance.11,12 The bulk heterojunction PV cells fabri-
cated in this study had a device structure of ITO/PEDOT–PSS/
PCDTBT:C60 derivatives 1 or 5/LiF (1 nm)/Al (120 nm) with
a PV active area of 1 cm2. The active layer was spin-cast from
dichlorobenzene solution using a slow solvent evaporation
process.13 The weight ratio of PCDTBT to C60 derivatives varies
from 1 : 1 to 1 : 3. Because of the poor solubility of the C60
derivative 5 in dichlorobenzene, it was difficult to get a smooth
and uniform film, and the resulting device had a very low PV
response. For PCDTBT/C60F 1 based solar cells, the best
performance was achieved at 1 : 2 weight ratio, and the power
conversion efficiency reached 2.3% with a Voc of 0.90 V and a Jsc
of 6.4 mA cmꢁ2. Our previous study showed that the PCDTBT/
PC61BM blend gave the best efficiency at the same weight ratio.12
Fig. 6 shows the current–voltage characteristics and external
quantum efficiency (EQE) curves of the fabricated devices. It is
worth pointing out that the short-circuit current measured under
simulated AM 1.5 G solar irradiation at 100 mW cmꢁ2 is the
same as the value calculated from the EQE data and the solar
spectrum. Detailed analysis of the dark I–V curves revealed that
the device series resistance is high (39 U cm2). This can explain the
low fill factor (0.40) of the fabricated solar cells. We conjectured
that the high series resistance may arise from the low electron
mobility of C60F 1 since it is an amorphous material unlike
PC61BM. To confirm this, we fabricated bottom-contact field-
effect transistors using spin-coated neat C60 derivative 1 or
Fig. 3 UV-vis absorption spectra in chloroform of C60 derivatives 1, 5
and PC61BM.
Fig. 4 Fluorescence spectra of C60 dimer 5 and oligofluorene 2.
mainly depend on the p electron system of the C60 skeleton and
the oligofluorene substituents have little effect on the energy
levels. We also carried out anodic oxidation sweeps on the two
C60 derivatives and oligofluorene 2. No anodic oxidation wave
could be observed for all the three compounds up to a potential
of 2.0 V vs. Ag. This suggests that these three compounds have
Fig. 5 Cyclic voltammograms of C60 derivatives 1 and 5 in 0.1 M Bu4NPF6 acetonitrile/DCB (1 : 4) solution at 50 mV sꢁ1
.
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Fig. 6 Current–voltage characteristics (a) and external quantum efficiency (EQE) curves (b) of PCDTBT:C60F 1 (1 : 2) and PCDTBT : C60
1 : PC61BM (1 : 1 : 1) based solar cells. (c) Chemical structure of PCDTBT.
F
PC61BM film as the active layer. The electron mobility was found
to be 4.6 ꢂ 10ꢁ6 and 2.0 ꢂ 10ꢁ3 cm2 Vꢁ1 sꢁ1 for C60F 1 and
PC61BM, respectively. To further improve the device efficiency,
we used the mixture of C60F 1 and PC61BM (1 : 1 by weight) as
the electron acceptor and the weight ratio of PCDTBT to the C60
derivatives was fixed at 1 : 2. An EQE calibrated power
conversion efficiency of 3.4% was achieved, and the series resis-
tance was reduced to 15 U cm2. The interesting thing was that the
Voc was improved to 0.92 V. This result implies that it is possible
to use the mixture of different fullerene derivatives to optimize
the optoelectronic properties and/or nano-scale morphology of
the active layer to enhance the device performance.
support from Sustainable Development Technology of Canada is
greatly appreciated.
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