Synthesis of novel cyanine-fullerene dyads for photovoltaic devices

Article abstract of DOI:10.1039/b413557c

Two novel cyanine-fullerene dyads have been synthesized by means of the Prato reaction from the corresponding cyanine-aldehyde, N-methylglycine and C₆₀. As well as a thorough characterization using MS-TOF, ¹H-NMR, ¹³C-NMR

Full text of DOI:10.1039/b413557c

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Synthesis of novel cyanine–fullerene dyads for photovoltaic devices
Fanshun Meng,a Jianli Hua,a Kongchang Chen,a He Tian,*a Libero Zuppirolib and Frank Nu¨esch*c
Received 2nd September 2004, Accepted 30th November 2004
First published as an Advance Article on the web 10th January 2005
DOI: 10.1039/b413557c
Two novel cyanine–fullerene dyads have been synthesized by means of the Prato reaction from
the corresponding cyanine-aldehyde, N-methylglycine and C60. As well as a thorough
1
characterization using MS-TOF, H-NMR, 13C-NMR and elemental analysis, the optical and
electrochemical properties of these two dyads were investigated. The fluorescence of the cyanine–
fullerene dyads was quenched efficiently as compared to the pristine cyanine, which we have
attributed to photo-induced electron transfer from the cyanine to C60. The redox potentials
obtained by cyclic voltammetry provide evidence that this electron transfer is energetically
favorable. Single and double layer photovoltaic devices were fabricated to evaluate the
photovoltaic properties of the dyad. The energetic conversion efficiency of the double layer device
exceeds 0.1% under 3.1 mW cm22 white light irradiation. The results show that cyanine–fullerene
dyads are promising materials for photovoltaic devices.
cyanines have received only little attention in the field of
1. Introduction
photovoltaics. So far, cyanines have proven to exhibit high
The field of organic photovoltaics has attracted considerable
reliability as spectral sensitizers in photography, optical
attention because of its low cost and simple, large-area
recording media in optical disks, laser dyes, photorefractive
materials, and fluorescent biosensors.8 Only recently, cyanine
manufacturing techniques. Although the overall conversion
efficiency is still low as compared to inorganic photovoltaics,
dyes have also been used as highly efficient sensitizers in
Gra¨tzel type solar cells.9 In previous work, we have
the recent developments in the field of organic photovoltaic
devices are most promising. Since the discovery of ultrafast
investigated the use of cyanine dyes to build thin film
photo-induced electron transfer from conjugated polymers to
heterojunction photovoltaic devices by the spin coating
buckminsterfullerene (C60)1 and the introduction of the ‘‘bulk
heterojunction’’ concept,2 the efficiency of polymer photo-
voltaic devices has been improved steadily in recent years.3 The
conversion efficiency of polymer solar cells was raised to 3.5%
based on P3HT and PCBM.3e At the same time, the efficiency
of heterojunction thin-film photovoltaics made from small
molecules was also improved quickly and a value as high as
4.2% was achieved with a copper phthalocyanine/C60 double
layer solar cell.4 These significant improvements could be
achieved by both the development of advanced materials and
the introduction of novel device concepts and architectures.
One of the key problems for boosting the photovoltaic
efficiency is the dissociation of the exciton into a positive
and negative charge carrier. Excitons are formed upon photon
absorption and are able to diffuse in the organic material until
decay or dissociation by a charge transfer process occurs. In
order to develop materials with high exciton dissociation
efficiency, molecules were designed where C60 is covalently
linked to the light-absorbing chromophore. Not only dyads but
also polymers and oligomers were studied in this respect.5–7
Indeed, the ‘‘in phase’’ between absorption and photocurrent
spectrum indicates that excitons are dissociated throughout the
whole organic film and not just at a heterointerface. The recent
work by Fukuzumi shows that the efficiency based on devices
using porphyrin–fullerene dyads can reach nearly 1% for
optimized devices.7g Among the different classes of dyes,
method.10 Among the various advantages cyanines offer are
the high extinction coefficients, the ease of solution processing
as well as their advantageous electrochemical properties.
In this paper, we describe the synthesis as well as the optical
and electrochemical properties of two novel cyanine–fullerene
dyads 1a and 1b (see Fig. 1) and the preliminary investigation
of photovoltaic devices based on the latter.
2. Experimental
UV-Vis spectra were recorded on a Cary 50 Bio UV-Visible
Spectrophotometer, while fluorescence spectra were measured
1
using a FluoroMax-3. H-NMR and 13C-NMR spectra were
recorded on a Bruker AM-500 spectrometer with tetramethyl-
silane (TMS) as internal reference. TOF-MS spectra were
recorded on a VG12-250. Elemental analysis data were
obtained on a Perkin Elmer 240c instrument. Cyclic voltam-
metry measurements were performed on
Instruments VersaStat II with three-electrode cell in
dichloromethane solution at a scan rate of 100 mV s21
a
EG&G
a
.
The supporting electrolyte was tetrabutylammonium perchlo-
rate (0.1 M), the working electrode was glassy carbon,
the counter electrode was Pt and the reference electrode was
Ag/AgCl. C60 was purchased from SES Research Inc.
Poly(styrene sulfonate) doped poly(3,4-ethylenedioxythio-
phene) or PEDOT:PSS was purchased from Bayer AG,
Germany. 4-(2-Hydroxyethoxy)benzaldehyde (2),11 5-car-
boxyl-1-ethyl-2,3,3-trimethyl-3H-indolium iodide (4),12 2-(1-
ethyl-3,3-dimethylindolin-2-ylidene)acetaldehyde (5a),13 and
*tianhe@ecust.edu.cn (He Tian)
frank.nueesch@empa.ch (Frank Nu¨esch)
This journal is ß The Royal Society of Chemistry 2005
J. Mater. Chem., 2005, 15, 979–986 | 979

View Article Online
Fig. 1 The structures of the cyanine–fullerene dyads.
2-(3-ethyl-1,1-dimethylbenz[e]indolin-2-ylidene)acetaldehyde (5b)13
5-Carboxyl-2-[3-(1,3-dihydro-1,1-dimethyl-3-ethyl-
were synthesized according to reported procedures.
2H-benz[e]indol-2-ylidene)propenyl]-3,3-dimethyl-1-ethyl-
3H-indolium perchlorate (6b). Analogous to the synthesis of 6a,
but 2-(3-ethyl-1,1-dimethylbenz[e]indolin-2-ylidene)acetalde-
hyde (5b) was used instead of 2-(1-ethyl-3,3-dimethylindolin-
2-ylidene)acetaldehyde (5a). The yield was 75%. 1H-NMR
(CDCl3, 500 MHz): d 1.34 (t, 3H, –CH2CH3), 1.41 (t, 3H,
–CH2CH3), 1.76 (s, 6H, –C(CH3)2), 1.99 (s, 6H, –C(CH3)2),
4.16 (m, 2H, –CH2CH3), 4.37 (m, 2H, –CH2CH3), 6.54 (d, 1H,
J = 13.06 Hz, –CHLCH–CHL), 6.72 (d, 1H, J = 13.76 Hz,
–CHLCH–CHL), 7.50 (d, 1H, J = 8.41 Hz, Ar–H), 7.58 (t, 1H,
Ar–H), 7.69 (t, 1H, Ar–H), 7.87 (d, 1H, J = 8.96 Hz, Ar–H),
8.03 (d, 1H, J = 8.28 Hz, Ar–H), 8.10 (d, 1H, J = 9.56 Hz,
Ar–H), 8.17 (t, 2H, Ar–H), 8.36 (d, 1H, J = 8.54 Hz, Ar–H),
8.50 (t, 1H, –CHLCH–CHL), 13.00 (s, br, 1H, –COOH).
TOF-MS (ES+): 479.2 (M 2 ClO42, 100%), 480.2 (M +
1 2 ClO42, 35%).
Synthesis of cyanine–fullerene dyads
5-Carboxyl-2-[3-(1,3-dihydro-3,3-dimethyl-1-ethyl-2H-
indol-2-ylidene)propenyl]-3,3-dimethyl-1-ethyl-3H-indolium
perchlorate (6a).
A mixture of 5-carboxyl-1-ethyl-2,3,3-
trimethyl-3H-indolium iodide (4, 3.59 g, 10 mmol), 2-(1-
ethyl-3,3-dimethylindolin-2-ylidene)acetaldehyde (5a, 2.14 g,
10 mmol) and acetic anhydride (30 mL) was heated for 1 hour
at 100–110 uC. The reaction mixture was poured into water
(200 mL) and the solution was stirred at room temperature
until a precipitate was formed. The solid was filtered and
washed twice with water (200 mL) under stirring. The solid
was filtered and dried under vacuum. The crude product was
dissolved in ethanol at reflux temperature and 6.1 g sodium
perchlorate hydrate was added. The solution was refluxed for
0.5 h and then cooled down overnight. The precipitated solid
was recrystallized again from ethanol to give 3.8 g (72%) 6a.
Mp 178–180 uC. 1H-NMR (CDCl3, 500 MHz): d 1.31–1.37
(m, 6H, –CH2CH3, –CH2CH3), 1.72 (s, 12H, –C(CH3)2,
–C(CH3)2), 4.16 (m, 2H, –CH2CH3), 4.23 (m, 2H,
–CH2CH3), 6.54 (d, 1H, J = 13.22 Hz, –CHLCH–CHL), 6.67
(d, 1H, J = 13.64 Hz, –CHLCH–CHL), 7.37 (t, 1H, Ar–H),
7.48–7.51 (m, 2H, Ar–H), 7.56 (d, 1H, J = 7.98 Hz, Ar–H),
7.69 (d, 1H, J = 7.34 Hz, Ar–H), 8.03 (d, 1H, J = 8.56 Hz,
5-[2-(4-Formylbenzoxy)ethoxycarbonyl]-2-[3-(1,3-dihydro-
3,3-dimethyl-1-ethyl-2H-indol-2-ylidene)propenyl]-3,3-dimethyl-
1-ethyl-3H-indolium perchlorate (7a).
A mixture of 6a
(1.43 g, 2.71 mmol), 4-(2-hydroxyethoxy)benzaldehyde (2,
0.5 g, 3.01 mmol), 1,3-dicyclohexylcarbodiimide (DCC, 1.4 g,
6.78 mmol), 4-(dimethylamino)pyridine (DMAP, 66 mg,
0.54 mmol) and dichloromethane (75 mL) was stirred at room
temperature for 24 hours. The resulting precipitate of
dicyclohexylurea was filtered and the filtrate was washed
with 1 M HCl, saturated NaCO3 solution and water. The
dichloromethane layer was dried over MgSO4, and the
solvent was evaporated. The residue was purified by column
Ar–H), 8.14 (s, 1H, Ar–H), 8.38 (t, 1H, –CHLCH–CHL), 13.05
2
(s, br, 1H, –COOH). TOF-MS (ES+): 429.2 (M 2 ClO4
,
100%), 430.2 (M + 1 2 ClO42, 45%).
980 | J. Mater. Chem., 2005, 15, 979–986
This journal is ß The Royal Society of Chemistry 2005

View Article Online
chromatography (silica gel, dichloromethane–methanol 30 : 1)
to afford 1.43 g (78%) of 7a. Mp 103–105 uC. 1H-NMR (CDCl3,
500 MHz): d 1.46 (t, 3H, –CH2CH3), 1.51 (t, 3H, –CH2CH3),
1.72 (s, 6H, –C(CH3)2), 1.74 (s, 6H, –C(CH3)2), 4.20 (m, 2H,
–CH2CH3), 4.30 (m, 2H, –CH2CH3), 4.44 (t, 2H, –CH2CH2–),
4.74 (t, 2H, –CH2CH2–), 6.85 (d, 1H, J = 13.16 Hz, –CHLCH–
CHL), 6.98 (d, 1H, J = 13.72Hz, –CHLCH–CHL), 7.07–7.11 (m,
3H, Ar–H), 7.22 (d, 1H, J = 7.93 Hz, Ar–H), 7.33 (t, 1H, Ar–H),
7.41–7.46 (m, 2H, Ar–H), 7.87 (d, 2H, J = 8.68 Hz, Ar–H), 7.97
(s, 1H, Ar–H), 8.12 (d, 1H, J = 8.84 Hz, Ar–H), 8.43 (t, 1H,
–CHLCH–CHL), 9.91 (s, 1H, –CHO). TOF-MS (ES+): 577.3
(M 2 ClO42, 100%), 578.3 (M + 1 2 ClO42, 60%).
145.99, 146.15, 146.25, 146.34, 151.33, 153.57, 158.57,
2
172.78, 175.07. TOF-MS (ES+): 1324.6 (M
2 ClO4 ,
99.7%), 1325.6 (M + 1 2 ClO42, 100%), 1326.6 (M + 2 2
ClO42, 25.4%). Anal. Calcd for C99H46ClN3O7: C 83.45, H
3.25, N 2.95%. Found: C 83.34, H 3.26, N 2.97%.
Cyanine–fullerene dyad 1b. Mp 272 uC (dec). 1H-NMR
(CDCl3): d 1.25 (t, 3H, –CH2CH3), 1.46 (t, 3H, –CH2CH3),
1.76 (s, 6H, –C(CH3)2), 2.03 (s, 6H, –C(CH3)2), 2.80 (s, 3H,
N–CH3), 4.12 (m, 2H, –CH2CH3), 4.26 (d, 1H, J = 9.29 Hz,
Pyrrolid–H), 4.35 (t, 2H, –CH2CH2–), 4.43 (m, 2H,
–CH2CH3), 4.70 (t, 2H, –CH2CH2–), 4.90 (s, 1H, Pyrrolid–
H), 4.99 (d, 1H, J = 9.38 Hz, Pyrrolid–H), 6.87 (d, 2H, J =
13.12 Hz, –CHLCH–CHL), 7.01–7.08 (m, 4H, Ar–H), 7.45 (d,
1H, J = 8.86 Hz, Ar–H), 7.54 (t, 1H, Ar–H), 7.66 (t, 1H,
Ar–H), 7.76 (s, br, 1H, Ar–H), 7.99–8.02 (m, 3H, Ar–H), 8.11–
8.14 (m, 2H, Ar–H), 8.52 (t, 1H, –CHLCH–CHL). 13C-NMR
(CDCl3, 100 MHz): d 12.41, 13.16, 14.12, 27.60, 28.25, 39.60,
39.98, 40.51, 46.75, 48.24, 51.33, 63.55, 66.02, 68.98, 69.96,
83.08, 104.10, 105.59, 109.71, 110.91, 122.01, 123.54, 125.69,
128.05, 130.29, 131.13, 142.07, 142.13, 142.26, 142.56, 145.19,
145.21, 145.28, 145.31, 145.54, 145.93, 146.10, 146.17, 146.20,
146.28, 146.33, 150.36, 153.56, 158.55, 172.17, 176.74. TOF-
MS (ES+): 1374.7 (M 2 ClO42, 100%), 1375.7 (M + 1 2
ClO42, 99.4%), 1376.7 (M + 2 2 ClO42, 11.9%). Anal. Calcd
for C103H48ClN3O7: C 83.87, H 3.28, N 2.85%. Found: C
83.92, H 3.27, N 2.87%.
5-[2-(4-Formylbenzoxy)ethoxycarbonyl]-2-[3-(1,3-dihydro-
1,1-dimethyl-3-ethyl-2H-benz[e]indol-2-ylidene)propenyl]-3,3-
dimethyl-1-ethyl-3H-indolium perchlorate (7b). Analogous to
the synthesis of 7a, but 6b was used in place of 6a. The yield
was 81%. 1H-NMR (CDCl3, 500 MHz): d 1.47 (t, 3H,
–CH2CH3), 1.57 (t, 3H, –CH2CH3), 1.76 (s, 6H, –C(CH3)2),
2.03 (s, 6H, –C(CH3)2), 4.20 (m, 2H, –CH2CH3), 4.41–4.57 (m,
4H, –CH2CH3, –CH2CH2–), 4.74 (t, 2H, –CH2CH2–), 6.86 (d,
1H, J = 13.11 Hz, –CHLCH–CHL), 7.03 (d, 1H, J = 13.83 Hz,
–CHLCH–CHL), 7.09 (d, 3H, J = 8.56 Hz, Ar–H), 7.47 (d, 1H,
J = 8.84 Hz, Ar–H), 7.54 (t, 1H, Ar–H), 7.64 (t, 1H, Ar–H),
7.88 (d, 2H, J = 8.63 Hz, Ar–H), 8.00 (t, 3H, Ar–H), 8.13 (t,
2H, Ar–H), 8.54 (t, 1H, –CHLCH–CHL), 9.92 (s, 1H, –CHO).
TOF-MS (ES+): 627.3 (M 2 ClO42, 100%), 628.4 (M + 1 2
ClO42, 40%).
N-Methyl-2-[4-(2-hydroxyethoxy)phenyl]-3,4-fulleropyrroli-
dine (3). 1H-NMR (CDCl3): d 2.79 (s, 3H, N–CH3), 3.96 (m, 2H,
–CH2CH2–), 4.10 (t, 2H, –CH2CH2–), 4.25 (d, 1H, J = 9.36 Hz,
Pyrrolid–H), 4.89 (s, 1H, Pyrrolid–H), 4.98 (d, 1H, J = 9.41 Hz,
Pyrrolid–H), 6.98 (d, 2H, U˛ = 8.47 Hz, Ar–H), 7.17 (t, 2H, Ar–H).
General procedure for the synthesis of the dyads (1a, 1b) and
fulleropyrrolidines (3)
A mixture of 0.69 mmol of aldehyde (7a, 7b and 2), 0.69 mmol
C60, 5.62 mmol N-methylglycine and 200 mL chlorobenzene
was refluxed under an atmosphere of argon for 20 h. Most of
the solution was evaporated under reduced pressure and the
residue was purified by column chromatography (silica gel).
Elution with toluene afforded the unreacted C60, and further
elution with dichloromethane–acetone (20 : 1, for 1a and 1b) or
toluene–ethyl acetate (15 : 1, for 3) afforded the desired
fraction. After washing with methanol, the desired product 1a,
1b and 3 was obtained with a yield of 38–42%.
Preparation and characterization of photovoltaic devices
A layer of PEDOT:PSS was deposited by spin coating on
precleaned ITO glass and then dried at 80 uC under vacuum.
The cyanine–fullerene dyad (60 nm) was spin coated on top of
PEDOT from chlorobenzene solution, heated to 35 uC under
high vacuum and cooled down to room temperature. For
double layer devices, a layer of C60 (40 nm) was deposited
under high vacuum (,1026 Torr). Finally, aluminium (75 nm)
was deposited under high vacuum to form the cathode. The
thicknesses of the films were measured using a Tencor Alpha-
Step 550 Surface Profilometer. The photocurrent action
spectrum was measured using monochromatic light from a
JY HR 460 monochromator with a tungsten lamp source. The
I–V curves were measured using a Keithley 236 source meter.
White light from a tungsten halogen lamp was used to irradiate
the device from the ITO side. The light intensity was varied
using a Neutral Density (ND) filter. All measurements were
performed in a nitrogen atmosphere.
Cyanine–fullerene dyad 1a. Mp 259 uC (dec). 1H-NMR
(CDCl3): d 1.45 (t, 3H, –CH2CH3), 1.51 (t, 3H, –CH2CH3),
1.72 (s, 6H, –C(CH3)2), 1.73 (s, 6H, –C(CH3)2), 2.79 (s, 3H,
N–CH3), 4.19 (m, 2H, –CH2CH3), 4.25 (d, 1H, J = 9.35 Hz,
Pyrrolid–H), 4.30 (m, 2H, –CH2CH3), 4.34 (t, 2H, –CH2CH2–),
4.70 (t, 2H, –CH2CH2–), 4.90 (s, 1H, Pyrrolid–H), 4.98 (d,
1H, J = 9.42 Hz, Pyrrolid–H), 6.87 (d, 2H, J = 13.19 Hz,
–CHLCH–CHL), 7.00 (m, 3H, Ar–H), 7.08 (d, 1H, J = 8.34 Hz,
Ar–H), 7.22 (d, 1H, J = 6.68 Hz, Ar–H), 7.32 (t, 1H, Ar–H),
7.40–7.47 (m, 2H, Ar–H), 7.68 (s, br, 1H, Ar–H), 8.01 (s, 1H,
Ar–H), 8.12 (d, 1H, J = 8.23 Hz, Ar–H), 8.42 (t, 1H, –CHLCH–
CHL). 13C-NMR (CDCl3, 100 MHz): d 12.43, 12.81, 27.97,
28.24, 39.75, 39.98, 40.31, 48.34, 49.50, 63.60, 66.04, 69.02,
70.03, 83.14, 96.15, 104.51, 106.05, 109.85, 111.48, 122.24,
123.56, 126.15, 129.12, 131.55, 141.15, 141.51, 142.08, 142.11,
142.14, 142.17, 142.21, 142.29, 142.62, 145.36, 145.50, 145.58,
3. Results and discussion
Synthesis of the cyanine–fullerene dyads
The route for the synthesis of cyanine–fullerene dyads is shown
in Fig. 2. The asymmetric cyanine dye (6a–b) was obtained
This journal is ß The Royal Society of Chemistry 2005
J. Mater. Chem., 2005, 15, 979–986 | 981

View Article Online
Fig. 2 The synthetic routes of cyanine–fullerene dyads.
characterized with TOF-MS, 1H-NMR, 13C-NMR and ele-
through reaction of indolium iodide (4) with the corresponding
aldehyde (5a–b) in acetic anhydride at 100–110 uC. The crude
product obtained in the first step was refluxed with sodium
perchlorate in ethanol to give the carboxyl acid cyanine (6a–b).
This approach avoids the production of symmetric cyanines,
allowing easy purification by recrystallization in ethanol as well
as a high yield. The coupling of carboxyl acid cyanine (6a–b)
with 4-(2-hydroxyethoxy)benzaldehyde 2 was carried out
in the presence of 1,3-dicyclohexylcarbodiimide (DCC) and
4-(dimethylamino)pyridine (DMAP) as catalyst in dichloro-
methane at room temperature in high yield. After completion
of the reaction, the precipitated dicyclohexylurea was
removed by filtration. The solution was washed with diluted
HCl solution followed by aq. Na2CO3 and water, and then
purified by column chromatography to give the cyanine-
aldehyde (7a–b).
mental analysis.
Optical and electrochemical properties
Fig. 3 shows the absorption and fluorescence spectra of the
cyanine–fullerene dyads (1a–b), pristine cyanine (6a–b) and the
fulleropyrrolidine (3) in dichloromethane solution. Their
respective optical data are shown in Table 1. In CH2Cl2
solution, the dyads (1a–b) exhibit maximum absorption at
556 nm and 580 nm with vibrational shoulder peaks at 524 nm
and 546 nm, which is similar to the corresponding pristine
cyanines (6a–b) in the visible region. The maximum absorption
of 1b is red shifted by 24 nm compared with 1a due to the
longer conjugation length. In the UV region, the absorption of
the dyads is stronger than that of the corresponding pristine
cyanine due to the presence of C60. From the insert figures of
Fig. 3, we can see that the dyads (1a–b) show weak absorption
at about 703 nm, which is the characteristic absorption of
fulleropyrrolidine. The absorption spectra of the dyads (1a–b)
are close to the superposition of the spectra of fulleropyrro-
lidine (3) and pristine cyanine (6a–b). This result means that
the electronic properties of the cyanine do not change after
connection with fullerene and charge transfer does not occur
between the donor and the acceptor in their ground state.
Under excitation of the maximum absorption wavelength,
the cyanine dyes (6a–b) show relatively strong fluorescence
The cyanine–fullerene dyads (1a–b) and fulleropyrrolidine
(3) were synthesized by the Prato reaction14 from the
corresponding aldehyde (7a–b and 2), N-methylglycine and
C60 in chlorobenzene. The final products were isolated by
column chromatography with a yield of 38–42%. The cyanine–
fullerene dyads (1a–1b) can also be synthesized by the coupling
of carboxylic acid cyanine (6a–b) with 3 in the presence of
DCC and DMAP in dichloromethane. However, the difficulty
of removing dicyclohexylurea from the final products reduces
the yield and purity of the cyanine–fullerene dyads. The
structure and purity of the cyanine–fullerene dyads were
982 | J. Mater. Chem., 2005, 15, 979–986
This journal is ß The Royal Society of Chemistry 2005

View Article Online
Fig. 3 The absorption and fluorescence spectra of cyanine–fullerene dyads (1a–b), pristine cyanine (6a–b) and fulleropyrrolidine (3) (1025 M) in
CH2Cl2 solution. The insets show the enlarged absorption spectra in the range of 600–800 nm.
Table 1 The optical and electrochemical properties of fullerene-cyanine dyads (1a and 1b), fulleropyrrolidine (3) and pristine cyanine (6a and 6b)
labs
Compound nm
/
e/1025
lfl/
max
M21 cm21 nm Eox/V (Epc, Epa
)
Ered1/V (Epc, Epa
)
Ered2/V (Epc, Epa
)
Ered3/V (Epc, Epa)
3
—
—
—
—
578 1.27 (1.335, 1.195) 20.88 (20.935, 20.82)
20.62 (20.67, 20.57)
21.00 (21.055, 20.95)
—
—
—
6a
1a
6b
1b
a
555
556
579
580
1.514
1.469
1.144
1.170
578 1.27 (1.325, 1.21)
601 1.20 (1.25, 1.145)
600 1.21 (1.265, 1.145) 20.62 (20.665,20.58)
20.61 (20.665,20.575) 20.89 (20.935, 20.85) 21.01 (21.065, 20.96)
20.88 (20.93, 20.83)
— —
20.91 (20.945, 20.88) 21.01 (21.065, 20.96)
Note: The reduction or oxidation potential for each couple peaks are calculated according to E = (Epc + Epa)/2, where Epc is the cathodic
peak potential and Epa is the anodic peak potential.
and pristine cyanine (6a–b). This means that the covalent
linkage between the donor and the acceptor has not
significantly altered the energy levels of cyanine and C60. The
lowest unoccupied molecular orbital (LUMO) of the cyanine is
about 0.3 eV higher than that of fulleropyrrolidine, thus
photo-induced electron transfer from the cyanine dye to the
fullerene is energetically favorable. The strong fluorescence
quenching in cyanine–fullerene dyads 1a and 1b is due to
photo-induced electron transfer from the cyanine to C60.
with maxima at 578 nm and 601 nm. Their fluorescence spectra
are the mirror image of the absorption spectra. However, the
fluorescence of the dyads (1a–b) is efficiently quenched
compared to the pristine cyanine dyes with efficiencies of
about 95% and 91%, which indicates the possibility of photo-
induced electron transfer from the cyanine dyes to C60.
The reduction and oxidation (redox) potentials of the
cyanine–fullerene dyads (1a–b), pristine cyanine (6a–b) and
fulleropyrrolidine (3) were determined by cyclic voltammetry
(Fig. 4) and the data are shown in Table 1. The reduction or
oxidation potential for each couple peaks are calculated
according to E = (Epc + Epa)/2, where Epc is the cathodic
peak potential and Epa is the anodic peak potential.15
Photovoltaic properties
In order to investigate the photovoltaic properties of the
dyads, we chose cyanine–fullerene dyad 1b because of its good
film-forming properties. Single layer devices with architecture
(ITO/PEDOT:PSS/1b/Al) were fabricated. The absorption and
IPCE spectra of the device are shown in Fig. 5. The film made
from dyad 1b shows two obvious absorption peaks at 558 nm
and 600 nm when spin coated from chlorobenzene solution,
which we have attributed to the main absorption band and
corresponding vibration band, respectively. Compared to the
absorption in dichloromethane solution, the absorption peaks
of the films are red-shifted by 11 nm and 20 nm, respectively.
The relative intensity and the width of the peaks in the film are
also different from those in dichloromethane solution. The
difference between the solution and film spectra is due to a
combination of solid state effects, amongst which the
formation of small cyanine aggregates as well as the polarity
of the organic solid are likely to play a dominant role.
In the range of 21.5 to 1.5 V, fulleropyrrolidine 3 shows
two reversible reduction peaks at 20.62 V and 21.00 V. The
pristine cyanine dye, 6a (or 6b), shows one quasi-reversible
oxidation peak at 1.27 V (1.20 V for 6b) and one quasi-
reversible reduction peak at 20.88 V (20.88 V for 6b), while
the dyad 1a (or 1b) shows three quasi-reversible reduction
peaks at 20.61 V, 20.89 V and 21.01 V (20.62 V, 20.91 V
and 21.01 V for 1b), as well as one oxidation peak at 1.27 V
(1.21 V for 1b). The cyanine–fullerene dyad (1a–b) and cyanine
(6a–b) also exhibit shoulders in the oxidation region that are
different from the main oxidation peaks and the reason for this
is not clear yet.
On comparing these data, it appears that the redox poten-
tials obtained for cyanine–fullerene dyads (1a–b) are precisely
a combination of the values measured for fulleropyrrolidine (3)
This journal is ß The Royal Society of Chemistry 2005
J. Mater. Chem., 2005, 15, 979–986 | 983

View Article Online
Fig. 4 The cyclic voltammetry of fulleropyrrolidines (3), cyanine (6a–b) and cyanine–fullerene dyad (1a–b).
that the device does not have short circuit problem. When
the irradiation intensity was increased from 0.31 mW cm22 to
310 mW cm22, the open circuit voltages (Voc) of the device
increase from 0.18 V to 0.33 V, but the short circuit current
increases in a sub-linear way. Clearly, the photo-current
density in the device is not proportional to the irradiation
intensity. The fill factor and conversion efficiency of the
single layer devices is extremely low. Such behavior is usually
found when the photodiode has a high series resistance. This
resistance could been introduced at the aluminium cathode
interface due to thermal degradation of the cyanine during
thermal evaporation of the metal electrode. In order to
avoid this problem, we have added a 40 nm thick C60 layer
between the cathode and the cyanine–dyad layer such that
Fig. 5 The IPCE spectrum of the single layer device (ITO/
PEDOT:PSS/1b/Al) and the absorption spectrum of dyad 1b in the
film.
The IPCE spectrum of the single layer device shows a ‘‘in
phase’’ correlation to the absorption spectrum of the cyanine,
indicating that charge carriers are generated within the bulk of
the film under excitation by light. The maximum IPCE value
of the device reaches 6%, which is higher than that of our
previous cyanine/C60 two layer device.10 The increased
photocurrent yield in the blue spectral region is due to
photo-carriers generated by C60.16 The small IPCE feature at
about 750 nm is assigned to the absorption of fulleropyrro-
lidine, as the cyanine has no absorption in this region.
The current–voltage (I–V) characteristics for the single
layer devices in the dark and under irradiation at different
intensities are shown in Fig. 6. A tungsten lamp was used as
white light source. The device shows good diode behavior in
the dark with rectification ratios of 287 at ¡1 V, which implies
Fig. 6 Current–voltage characteristics under different white light
irradiation intensities for the single layer device (ITO/PEDOT:PSS/1b/
Al). The inset in the figure shows the current–voltage characteristics in
the dark.
984 | J. Mater. Chem., 2005, 15, 979–986
This journal is ß The Royal Society of Chemistry 2005

View Article Online
Acknowledgements
This work was supported by the Scientific Committee of
Shanghai, Education Committee of Shanghai and SNSF
(Switzerland). Dr R. Crockett is gratefully acknowledged for
careful reading of the manuscript.
Fanshun Meng,a Jianli Hua,a Kongchang Chen,a He Tian,*a
Libero Zuppirolib and Frank Nu¨esch*c
aLab for Advanced Materials and Institute of Fine Chemicals, East
China University of Science and Technology, Shanghai 200237,
P.R. China. E-mail: tianhe@ecust.edu.cn
bInstitute of Materials, Swiss Federal Institute of Technology EPFL,
CH-1015 Lausanne, Switzerland
cLaboratory for functional polymers, Swiss Federal Laboratories for
Materials Testing and Research—EMPA, CH-8600 Du¨bendorf,
Switzerland. E-mail: frank.nueesch@empa.ch
References
Fig. 7 Current–voltage characteristics under different white light
irradiation intensities for the double layer device (ITO/PEDOT:PSS/
1b/C60/Al). The inset in the figure shows the current–voltage
characteristics in the dark.
1 N. S. Sariciftci, L. Smilowitz, A. J. Heeger and F. Wudl, Science,
1992, 258, 1474.
2 G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger,
Science, 1995, 270, 1789.
3 (a) M. Granstrom, K. Petritsch, A. C. Arias, A. Lux,
M. R. Andersson and R. H. Friend, Nature, 1998, 395, 257; (b)
S. E. Shaheen, C. J. Brabec, N. S. Sariciftci, F. Padinger,
T. Fromherz and J. C. Hummelen, Appl. Phys. Lett., 2001, 78,
841; (c) C. J. Brabec, S. E. Shaheen, C. Winder, N. S. Sariciftci and
P. Denk, Appl. Phys. Lett., 2002, 80, 1288; (d) M. Svensson,
F. L. Zhang, S. C. Veenstra, W. J. H. Verhees, J. C. Hummelen,
J. M. Kroon, O. Inganas and M. R. Andersson, Adv. Mater., 2003,
15, 988; (e) F. Padinger, R. S. Rittberger and N. S. Sariciftci, Adv.
Funct. Mater., 2003, 13, 85; (f) C. Winder and N. S. Sariciftci,
J. Mater. Chem., 2004, 14, 1077.
4 (a) L. Schmidt-Mende, A. Fechtenko¨tter, K. Mu¨llen, E. Moons,
R. H. Friend and J. D. MacKenzie, Science, 2001, 293, 1119; (b)
P. Peumans, V. Bulovic and S. R. Forrest, Appl. Phys. Lett., 2000,
76, 2650; (c) P. Peumans and S. R. Forrest, Appl. Phys. Lett., 2001,
79, 126; (d) A. Yakimov and S. R. Forrest, Appl. Phys. Lett., 2002,
80, 1667; (e) J. Xue, S. Uchida, B. P. Rand and S. R. Forrest, Appl.
Phys. Lett., 2004, 84, 3013.
5 (a) A. M. Ramos, M. T. Rispens, J. K. J. van Duren,
J. C. Hummelen and R. A. J. Janssen, J. Am. Chem. Soc., 2001,
123, 6714; (b) F. L. Zhang, M. Svensson, M. R. Andersson,
M. Maggini, S. Bucella, E. Menna and O. Inganas, Adv. Mater.,
2001, 13, 1871; (c) A. Cravino and N. S. Sariciftci, J. Mater. Chem.,
2002, 12, 1931; (d) A. Cravino, G. Zerza, M. Maggini, S. Bucella,
M. Svensson, M. R. Andersson, H. Neugebauer, C. J. Brabec and
N. S. Sariciftci, Monatsh. Chem., 2003, 134, 519.
6 (a) J.-F. Eckert, J.-F. Nicoud, J.-F. Nierengarten, S.-G. Liu,
L. Echegoyen, F. Barigelletti, N. Armaroli, L. Ouali, V. Krasnikov
and G. Hadziioannou, J. Am. Chem. Soc., 2000, 122, 7467; (b)
E. Peeters, P. A. van Hal, J. Knol, C. J. Brabec, N. S. Sariciftci,
J. C. Hummelen and R.A. J. Janssen, J. Phys. Chem. B, 2000, 104,
10174; (c) A. Dhanabalan, J. Knol, J. C. Hummelen and
R. A. J. Janssen, Synth. Met., 2001, 119, 519; (d) D. Hirayama,
K. Takimiya, Y. Aso, T. Otsubo, T. Hasobe, H. Yamada,
H. Imahori, S. Fukuzumi and Y. Sakata, J. Am. Chem. Soc.,
2002, 124, 532; (e) N. Armaroli, G. Accorsi, J.-P. Gisselbrecht,
M. Gross, V. Krasnikov, D. Tsamouras, G. Hadziioannou,
M. J. Gomez-Escalonilla, F. Langa, J.-F. Eckerte and
J.-F. Nierengarten, J. Mater. Chem., 2002, 12, 2077; (f)
N. Negishi, K. Takimiya, T. Otsubo, Y. Harima and Y. Aso,
Chem. Lett., 2004, 33, 654.
diodes with (ITO/PEDOT:PSS/1b/C60/Al) architecture were
obtained.
As shown in Fig. 7, the open circuit voltage of the double
layer device increased from 0.18 V to 0.33 V with increasing
irradiation intensity, which is similar to a single layer device.
However, the short circuit current and fill factor were much
higher than those measured for single layer devices. Under
white light excitation of 310 mW cm22, the short-circuit
current density reaches 1.2 mA cm22 and the conversion
efficiency is 0.041%. At lower light intensity, the efficiency
exceeds 0.1%. We have attributed the better performance of
the double layer device both to the protective effect of the C60
layer during cathode deposition and the advantageous inter-
face formation between C60 and aluminium. The extremely low
open circuit voltage for single and double layer devices can not
be reasonably explained at this stage. Possibly, the formation
of small cyanine aggregates leads to a distribution of energy
levels that reduces the open circuit voltage. Further optimiza-
tion of the device fabrication is needed to improve the open
circuit voltage and short circuit current in order to increase the
efficiency.
4. Conclusion
We have synthesized two novel cyanine–fullerene dyads by
means of the Prato reaction and studied their optical,
electrochemical and photovoltaic properties. The fluorescence
of the cyanine–fullerene dyads was efficiently quenched as
compared to the pristine cyanine. The redox potentials of the
dyads show that photo-induced electron transfer from the
cyanine dye to C60 is energetically favorable. By inserting a C60
layer between the dyad and Al, the short circuit current and fill
factor were significantly improved when compared with the
single layer device. The power conversion efficiency for the
double layer device exceeds 0.1% at low irradiation intensity,
indicating that cyanine–fullerene dyads reported here are
promising materials for photovoltaic devices.
7 (a) M. E. Milanesio, M. Gervaldo, L. A. Otero, L. Sereno,
J. J. Silber and E. N. Durantini, J. Phys. Org. Chem., 2002, 15, 844;
(b) M. Maggini, G. Possamai, E. Menna, G. Scorrano,
N. Camaioni, G. Ridolfi, G. Casalbore-Miceli, L. Franco,
M. Ruzzi and C. Corvaja, Chem. Commun., 2002, 2028; (c)
D. Zhu, Y. Li, S. Wang, Z. Shi, C. Du, S. Xiao, H. Fang and
Y. Zhou, Synth. Met., 2003, 133–134, 679–683; (d) H. Yamada,
H. Imahori, Y. Nishimura, I. Yamazaki, T. K. Ahn, S. K. Kim,
This journal is ß The Royal Society of Chemistry 2005
J. Mater. Chem., 2005, 15, 979–986 | 985

View Article Online
D. Kim and S. Fukuzumi, J. Am. Chem. Soc., 2003, 125, 9129; (e)
M. A. Loi, P. Denk, H. Hoppe, H. Neugebauer, D. Meissner,
C. Winder, C. J. Brabec, N. S. Sariciftci, A. Gouloumis, P. Vazquez
and T. Torres, Synth. Met., 2003, 137, 1491; (f) J. L. Hua,
F. S. Meng, F. Ding, F. Y. Li and H. Tian, J. Mater. Chem., 2004,
14, 1849; (g) H. Imahori and S. Fukuzumi, Adv. Funct. Mater.,
2004, 14, 52.
10 F. S. Meng, K. C. Chen, H. Tian, L. Zuppiroli and F. Nu¨esch,
Appl. Phys. Lett., 2003, 82, 3788.
11 P. C.-M. Mao, J.-F. Mouscadet, H. Leh, C. Auclair and L.-Y. Hsu,
Chem. Pharm. Bull., 2002, 50, 1634.
12 E. Terpetschnig, H. Szmacinski, A. Ozinskas and J. R. Lakowicz,
Anal. Biochem., 1994, 217, 197.
13 J. Wang, W. F. Cao, J. H. Su, H. Tian, Y. H. Huang and Z. R. Sun,
Dyes Pigment, 2003, 57, 171.
14 M. Maggini, G. Scorrano and M. Prato, J. Am. Chem. Soc., 1993,
115, 9798.
15 (a) A. K. Agrawal and S. A. Jenekhe, Chem. Mater., 1996, 8, 579;
(b) T. Yamamoto and B. L. Lee, Macromolecules, 2002, 35, 2993.
16 P. Peumans, A. Yakimov and S. R. Forrest, J. Appl. Phys., 2003,
93, 3693.
8 A. Mishra, R. K. Behera, P. K. Behera, B. K. Mishra and
G. B. Behera, Chem. Rev., 2000, 100, 1973.
9 (a) A. Ehret, L. Stuhl and M. T. Spitler, Electrochim. Acta,
2000, 45, 4553; (b) A. Ehret, L. Stuhl and M. T. Spitler, J. Phys.
Chem., 2001, 105, 9960; (c) K. Sayama, K. Hara, Y. Ohga,
A. Shinpou, S. Suga and H. Arakawa, New J. Chem., 2001, 25,
200.
986 | J. Mater. Chem., 2005, 15, 979–986
This journal is ß The Royal Society of Chemistry 2005