A Soluble Donor-Acceptor Double-Cable Polymer: Polythiophene with Pendant Fullerenes

Article abstract of DOI:10.1007/s00706-002-0555-y

We report the synthesis and the spectroscopic properties of a novel soluble thiophene-based copolymer carrying pendant fullerene moieties (donor-acceptor double-cable polymer). Photoinduced absorption (PIA) experiments on spin cast films reveal a photoind

Full text of DOI:10.1007/s00706-002-0555-y

Monatshefte f€ur Chemie 134, 519–527 (2003)
DOI 10.1007/s00706-002-0555-y
A Soluble Donor–Acceptor Double-Cable
Polymer: Polythiophene with Pendant
Fullerenes
Antonio Cravino1;ꢀ, Gerald Zerza1, Michele Maggini2,
Stefania Bucella2, Mattias Svensson3, Mats R. Andersson3,
Helmut Neugebauer1, Christoph J. Brabec1, and N. Serdar Sariciftci1
1
Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler
University Linz, A-4040 Linz, Austria
2
Department of Organic Chemistry, University of Padova, I-35131 Padova,
Italy
3
Organic Chemistry and Polymer Technology, Chalmers University of Technology,
SE-412 96 Goteborg, Sweden
Received August 16, 2002; accepted (revised) August 26, 2002
Published online February 20, 2003 # Springer-Verlag 2003
Summary. We report the synthesis and the spectroscopic properties of a novel soluble thiophene-
based copolymer carrying pendant fullerene moieties (donor–acceptor double-cable polymer). Photo-
induced absorption (PIA) experiments on spin cast films reveal a photoinduced electron transfer
from the polythiophene chain onto the fullerene moieties. The performance of photovoltaic devices
produced with this copolymer is also discussed.
Keywords. Polythiophenes; Fullerenes; Double-cable polymers; Photoinduced spectroscopy;
Organic photovoltaics.
Introduction
Organic photovoltaics based on photoinduced electron transfer from conjugated
polymers to fullerenes is the focus point of an interdisciplinary research [1]. In
state-of-the-art devices a ‘‘bulk-heterojunction’’ architecture is used, where the
photoactive layer consists of a blend of a conjugated polymer as electron donor
(hole conducting material) and a fullerene as electron acceptor (electron conduct-
ing material) [2–4]. Such devices reach power conversion efficiencies (AM1.5)
higher than 3%, but there is still quite some potential for improvements concern-
ing, e.g., the film morphology [4, 5]. In fact, the different solubilities of the two
ꢀ
Corresponding author. E-mail: antonio.cravino@jku.at

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A. Cravino et al.
components in the organic solvents used for the film casting and their problematic
solid-state miscibility cause morphologic problems, like phase separation and ful-
lerene clustering. These effects influence the photoinduced charge generation as
well as the charge carriers mobility in the blend and thus, the overall efficiency of
the photovoltaic device [4, 6]. As an attempt to overcome the problem of phase
separation, polymeric systems in which a number of electron acceptor moieties are
covalently tethered to a conjugated chain have been proposed [7–13]. Such an
arrangement, called ‘‘double-cable’’, guarantees a more homogeneous distribution
of electron donor and electron acceptor domains as compared to composite ‘‘bulk-
heterojunctions’’. In addition, the interface between donor and acceptor domains
should be maximised. Electropolymerised polythiophenes carrying fulleropyrroli-
dine- or tetracyanoanthraquinodimethane-type moieties have been presented in
recent publications [10, 11]. Although thin films of these materials allowed the
first observation of photoinduced electron transfer in double-cable polymers, film
imperfections such as pinholes produced during the electrochemical preparation,
make the use of the novel materials in photovoltaic devices rather difficult. In
addition, the electrochemically prepared double-cable polymers were insoluble.
Solubility is an important prerequisite for an easy preparation of high quality
low-cost films for photovoltaic and optoelectronic applications. Recently, soluble
double-cable polymers and their implementation in photodiode devices started to
appear in literature [7–9]. Among them, a soluble polythiophene-based copolymer
bearing fulleropyrrolidine moieties has been reported by Zhang et al. [9]. This
soluble double-cable polymer has been used to fabricate photodiodes which were
characterised under monochromatic illumination. Here, we detail its synthesis, its
photophysical properties, and its photovoltaic performance determined in solar
illumination simulated conditions.
Results and Discussion
Polymer 1 (Fig. 1) is a random copolymer with repeating units of 3-(40-(100,400,700-
trioxaoctyl)phenyl)thiophene (8) [14] and of a thiophene-substituted fulleropyrrol-
idine 7 (Scheme 1). Poly(3-(40-(100,400,700-trioxaoctyl)phenyl)thiophene) (PEOPT)
(2) [14] (Fig. 1) served as reference for photoinduced absorption (PIA) measure-
ments and solar cell devices.
Scheme 1 outlines the synthetic strategy towards double-cable 1. Fulleropyr-
rolidine 7 was synthesised in five steps starting from commercially available
4-octylphenol and 1,8-dibromooctane that in 2-butanol and K2CO3 at reflux tempera-
ture afforded bromoderivative 3 in 66% isolated yield. Compound 3 was brominated
with N-bromosuccinimide (NBS) in DMF for 24 h at room temperature, affording 4
in 97% isolated yield, which was used in a palladium-catalysed coupling [15] with
3-thiopheneboronic acid [16] in dimethoxyethane (DME) affording 5 in 40% isolated
yield. Reaction of 5 and 4-hydroxybenzaldehyde in the presence of K2CO3 and
acetone at reflux temperature gave thiophenealdehyde 6 in 80% yield. Aldehyde 6
was then condensed with sarcosine in the presence of C60 affording 7 in 52% isolated
yield [17]. All spectroscopic and analytical data were consistent with the proposed
molecular structures. Polymer 1 was prepared by mixing monomers 7 and 8 in a 1:9
ratio in CHCl3 followed by addition of FeCl3 over a period of 5 hours [14]. The
resulting material was carefully dedoped and Soxhlet extracted with diethyl ether

A Soluble Donor–Acceptor Double-Cable Polymer
521
Fig. 1. Structure of polymer 1 (n ¼ 93, m ¼ 7) and polymer 2 (PEOPT)
Scheme 1. (a) 1,8-dibromooctane, K2CO3, 2-butanol, reflux, 24 h, 66%; (b) NBS, DMF, RT, 20 h,
97%; (c) 3-thiopheneboronic acid, tetrakis(triphenylphosphine)palladium(0), NaHCO3 (1 M), DME,
reflux, 10 h, 40%; (d) 4-hydroxybenzaldehyde, K2CO3, acetone, reflux, 36 h, 80%; (e) N-methylgly-
cine, C60, chlorobenzene, reflux, 5 h, 52%; (f) CHCl3, FeCl3 (6 equiv.) (0.05 M), 5 h, 36%
to remove unreacted monomers and low molecular weight oligomers. The molecular
weight of the copolymer, isolated in 36% yield, was Mn ¼ 28 000, Mw ¼ 48 000,
determined by size exclusion chromatograpy using polystyrene standards. The

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A. Cravino et al.
Fig. 2. Photoinduced absorption spectra of double-cable 1 (solid line) and reference polymer 2
(dashed line) excited at 476 nm, T ¼ 100 K
1H NMR spectrum of 1 showed that approximately 7% of 7 was incorporated in the
copolymer. Monomer 8, as well as polythiophene 2 (PEOPT), were prepared as
described in Ref. [14].
Thin films of polymer 1 and 2 were prepared by spin casting from chloroform
solutions. Both 1 and 2 form orange films that can be converted upon heating to a
blue, more ordered form [18, 19]. However, due to the procedure used for the
photovoltaic device preparation, the experiments presented here were carried out
only on the orange forms of double-cable 1 and reference polymer 2.
Figure 2 shows the PIA spectra of films of 1 and reference 2. Polymer 2 may be
identified through a single peak in the PIA spectrum at 1.28 eV. As this feature
shows a monomolecular recombination kinetics and no accompanying polaronic
peak at lower energies is observed, it can be assigned to a neutral long lived
excitation, i.e. a triplet state. On the other hand, the PIA spectrum of double-cable
1 shows two features, one at 1.42 eV and one below 0.6 eV, typical for charged
excitations in conjugated polymers. Both features may be assigned to the absorp-
tion of radical-cations (positive polarons) on the polythiophene backbone produced
by photoinduced electron transfer from the latter onto the pendant fullerenes. The
observed square root excitation intensity dependence of the PIA signals is due to
the bimolecular recombination kinetics of these charged species. Very similar PIA
spectra have been observed for blends of the phenyl substituted polythiophene 2
mixed with fullerene derivatives [19]. The PIA spectrum of double-cable 1 also
resembles the one of the electropolymerised double-cable polythiophene described
in Ref. [10]. These results prove the photoinduced electron transfer from the poly-
mer backbone to the pendant fullerenes in this novel soluble double-cable material.
Figure 3 compares the short circuit current=open circuit voltage characteristic
curves for devices made of double-cable 1 to devices made using reference poly-
mer 2. A drop of the open circuit voltage (Voc) from about 1 V for the reference
polymer 2 to a Voc of ca. 0.5 V for the double-cable 1 is observed. However, for the
devices made using 1, the short circuit current (Isc) as well as the photocurrent are
increased by a factor of ꢁ3, indicating the more efficient charge generation as well
as the enhanced electron transport properties of the double-cable polymer com-
pared to the reference material. The absolute Isc values and the low rectification of

A Soluble Donor–Acceptor Double-Cable Polymer
523
Fig. 3. I=V curves of photovoltaic devices with double-cable 1 (circles), double-cable 1 with
additional fullerene (weight ratio 1:3) (triangles) and the reference polymer 2 (squares) as
active materials
these double-cable based devices reveal that the fullerene concentration of one
fullerene-bearing unit for ten repeating polymer units is below the percolation
threshold for electron transport in bulk heterojunction solar cells [20]. Attaching
a higher percentage of fullerenes to the backbone, although it has been difficult up
to now due to solubility problems, may result in improved transport properties. For
this reason we additionally mixed the fullerene derivative [6,6]-phenyl C61-butyric
acid methyl ester (PCBM) [21] with the double-cable polymer and investigated
the performance of devices fabricated using 1:PCBM ¼ 1:3 composites (Fig. 3).
Results show the potential in these materials to match the efficiency of composites
with the added functionality that these non-composite double-cables can be pro-
cessed into other hosts, stretch oriented, and may be further functionalised for self-
assembly.
In conclusion, we synthesised a soluble electron donor–acceptor double-cable
based on phenol-substituted-thiophene=phenol-substituted-thiophene-fulleropyr-
rolidine units. The photoinduced charge transfer from the polythiophene backbone
to pendant fullerenes was observed in PIA spectra and further proven in photovoltaic
devices produced from this double-cable. The relative low photocurrent shows that
the fullerene concentration of this donor–acceptor double-cable polymer is below
the percolation threshold for electron transport. Increase in fullerene concentration
of the double-cable is predicted to enhance the photovoltaic performance.
Experimental
General
NMR spectra were recorded on a Varian 300 spectrometer, on a Bruker AF 200 spectrometer, and on a
1
Bruker AF 250 MHz spectrometer. Tetramethylsilane was used as an internal standard for H and
13C NMR. FTIR spectra of compounds 6 and 7 were recorded on a Perkin-Elmer 1720 X spectro-
photometer. UV-VIS absorption spectra were recorded on a Perkin-Elmer Lambda 5 spectropho-
tometer. Matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF
MS) was performed on a mass spectrometer ReflexTM (Bruker) in positive linear mode at 15 KV
acceleration voltage, using 2,5-dihydroxybenzoic acid as matrix. HRM spectra were recorded using a

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VG ZabSpec with EI as ionization method. The purity of fulleropyrrolidine 7 was also checked
by HPLC using an analytical Phenomenex Luna SiO2 column (250Â 4.6 mm) and toluene:ethyl
acetate ¼ 97:3. 1 ml=min, as eluent. The elution was monitored with a spectrophotometric detector
at 340 nm. Elemental analysis of 1–7 were found to agree favourably with the calculated values.
4-(80-Bromooctyloxy)octylbenzene (3, C22H37BrO)
5 g of 4-octylphenol (0.024 mol), 19.8g of 1,8-dibromooctane (0.073mol), and 23.5g of dry K2CO3
(0.17 mol) were mixed in 100 cm3 of 2-BuOH. The reaction mixture was stirred under reflux for 24h.
After cooling to room temperature, 100 cm3 of H2O were added, followed by 100 cm3 of n-pentane.
The water phase was removed and the organic phase was washed with H2O, 2 M HCl, 2 M NaOH and
H2O subsequently. After drying (NaSO4), the solvent was removed in vacuo and the crude product
was distilled in a Kugelrohr apparatus. The distillate was chromatographed on silica using n-
pentane:CH2Cl2 ¼ 1:1 as eluent to yield 7.06g (66%) of a colorless oil. 1H NMR (300MHz, CDCl3):
ꢀ ¼ 7.07 (d, J ¼ 8.7Hz, 2H), 6.81 (d, J ¼ 8.7 Hz, 2H), 3.92 (t, J ¼ 6.6Hz, 2H), 3.41 (t, J ¼ 6.9 Hz, 2H),
2.53 (t, J ¼ 7.6 Hz, 2H), 1.90–1.70 (m, 4H), 1.6–1.2 (m, 20H), 0.85 (t, J ¼ 7.2 Hz, 3H)ppm; 13C NMR
(62.5MHz, CDCl3): ꢀ ¼ 157.3, 137.3, 135.1, 129.4, 114.5, 68.1, 35.3, 34.2, 33.0, 32.1, 32.0, 29.7, 29.3,
29.4, 28.9, 28.3, 26.2, 22.9, 14.3ppm.
3-Bromo-4-(80-bromooctyloxy)octylbenzene (4, C22H36Br2O)
A solution of NBS (3.3g, 0.019 mol) in 10 cm3 of DMF was added dropwise to 7.06 g of 3 (0.018mol)
in 80cm3 of DMF. The reaction mixture was stirred, in dark, at room temperature for 20h. n-Pentane
was added and the organic phase was washed with 2 M HCl, 10% Na2S2O5 solution, and H2O. Drying
(Na2SO4) and evaporation of the solvent afforded the crude product, which was purified by flash
chromatography (silica gel, petroleum ether 40–60ꢂC:ethyl acetate ¼ 20:1 as eluent) to yield 8.8 g
1
(97%) of the title product as colourless oil. H NMR (300MHz, CDCl3): ꢀ ¼ 7.35 (s, 2H), 7.02 (dd,
J ¼ 2.8 Hz, J ¼ 8.3 Hz, 1H), 6.79 (dd, J ¼ 2.5 Hz, J ¼ 8.3 Hz, 1H), 3.99 (t, J ¼ 6.6 Hz, 2H), 3.41 (t,
J ¼ 6.9 Hz, 2H), 2.51 (t, J ¼ 7.6Hz, 2H), 1.9–1.8 (m, 4H), 1.6–1.2 (m, 20H), 0.88 (t, 3H) ppm;
13C NMR (62.5 MHz, CDCl3) ꢀ ¼ 153.3, 136.8, 133.3, 128.4, 113.4, 112.2, 81.6, 69.5, 35.0, 33.0,
33.0, 32.1, 31.8, 29.7, 29.5, 28.9, 28.3, 26.1, 22.9, 14.3ppm.
3-(20-(800-Bromooctyloxy)-50-octylphenyl)thiophene (5, C26H39BrOS)
4 (8.8g, 16.8 mmol) and 0.214g of tetrakis(triphenylphosphine)palladium(0) (0.19 mmol) were mixed
in 80 cm3 of DME and stirred for 10 min. 3-Thiopheneboronic acid (3.31 g, 0.026mol), dissolved in
15cm3 of DME, was added, immediately followed by 50cm3 of an aqueous 2 M NaHCO3 solution.
The reaction mixture was refluxed for 16h under N2 and vigorous stirring. The DME was evaporated
and the residue was extracted with diethyl ether. The ether phase was washed with H2O, dried
(Na2SO4), and the solvent was evaporated. The residue was purified by flash chromatography on silica
with n-pentane as eluent to provide 4.85g (55%) of the product as colorless oil. 1H NMR (300 MHz,
CDCl3): ꢀ ¼ 7.61 (dd, J ¼ 1.2 Hz, J ¼ 2.4 Hz, 1H), 7.46 (dd, J ¼ 1.2 Hz, J ¼ 5.1 Hz, 1H), 7.31 (m, 2H),
7.04 (dd, J ¼ 8.7 Hz, J ¼ 2.6 Hz, 1H), 6.87 (d, J ¼ 8.7 Hz, 1H), 3.98 (t, J ¼ 6.6Hz, 2H), 3.41
(t, J ¼ 6.9 Hz, 2H), 2.57 (t, J ¼ 7.6 Hz, 2H), 2.9–2.7 (m, 4H), 1.65–1.30 (m, 20H), 0.88 (t,
J ¼ 7.3 Hz, 3H) ppm; 13C NMR (62.5 MHz, CDCl3): ꢀ ¼ 154.2, 138.9, 135.3, 130.0, 128.8, 128.1,
125.1, 124.3, 123.1, 112.6, 68.7, 35.4, 34.2, 33.0, 32.1, 29.7, 29.6, 29.3, 28.3, 26.3, 22.9, 14.3 ppm.
4-((8-(4-Octyl-2-(3-thienyl)phenoxy)octyl)oxy)benzaldehyde (6, C33H44O3S)
A mixture of 600 mg of 5 (1.3mmol), 190 mg of 4-hydroxybenzaldehyde (1.6mmol), 290 mg of
K2CO3 (5.2mmol), and 20 cm3 of acetone was stirred at reflux temperature for 36h. After cooling,

A Soluble Donor–Acceptor Double-Cable Polymer
525
the suspension was filtered and the acetone was removed under reduced pressure. The residue, treated
with 10cm3 of brine, was extracted twice with CH2Cl2. The combined organic layers were dried
(MgSO4), the solvent was evaporated, and the residue was purified by flash column chromatography
(SiO2, petroleum ether:ethyl acetate ¼ 9:1) giving 532mg of 6 (81%) as a pale, oily compound. TLC
(petroleum ether:ethyl acetate ¼ 9:1): Rf ¼ 0.64; IR (KBr): ꢁꢀ ¼ 3039, 2941, 2921, 2869, 2849, 1703,
1602, 1508, 1477, 1309, 1259, 1238, 1214, 1156, 1036cmÀ 1; 1H NMR (200MHz, CDCl3): ꢀ ¼ 9.88
(s, 1H), 7.82 (d, J ¼ 9 Hz, 2H), 7.61 (dd, J ¼ 1.2 Hz, J ¼ 3 Hz, 1H), 7.46 (dd, J ¼ 1.2 Hz, J ¼ 5 Hz, 1H),
7.31 (m, 2H), 7.05 (dd, J ¼ 2 Hz, J ¼ 7.4 Hz, 1H), 6.99 (d, J ¼ 9 Hz, 2H), 6.88 (d, J ¼ 9 Hz, 1H), 4.00
(m, 4H), 2.57 (t, J ¼ 7.6Hz, 2H), 1.84–0.70 (m, 27H) ppm; 13C NMR (62.5MHz, CDCl3): ꢀ ¼ 190.8,
164.2, 154.0, 138.6, 135.0, 132.0, 129.7, 128.6, 127.9, 124.8, 124.0, 122.8, 114.7, 112.4, 68.5, 68.4,
35.1, 31.9, 31.8, 29.5, 29.3, 29.2, 29.0, 25.9, 22.7ppm.
Fulleropyrrolidine-thiophene 7 (C95H49O2NS)
A mixture of 250 mg of aldehyde 6 (0.48 mmol), 690 mg of C60 (0.96 mmol), and 85 mg of N-
methylglycine (0.96 mmol) in 300 cm3 of chlorobenzene was heated to reflux temperature for 5 h.
After cooling to room temperature, the mixture was concentrated under reduced pressure to about
30cm3 and loaded on top of a SiO2 chromatography column. Elution with toluene:petroleum
ether ¼ 6:4 gave first unreacted C60 and then 640 mg of fulleropyrrolidine 7 (52%) as a brown solid.
IR (KBr): ꢁꢀ ¼ 3238, 2923, 1639, 1617, 1247, 620 cmÀ 1; UV-VIS (CH2Cl2): ꢂ(") ¼ 232 (106154), 256
(122733), 306 (42227), 327 (32767), 430 (4069), 700 (870) nm (dm3 molÀ 1 cmÀ 1); 1H NMR
(250 MHz, CDCl3=CS2): ꢀ ¼ 7.62 (m, 2H), 7.47 (dd, J ¼ 1.2 Hz, J ¼ 3 Hz, 1H), 7.32 (dd, J ¼ 1.2 Hz,
J ¼ 5 Hz, 1H), 7.16 (m, 2H), 6.94 (dd, J ¼ 2 Hz, J ¼ 8.2 Hz, 1H), 6.85 (d, J ¼ 9 Hz, 2H), 6.75 (d,
J ¼ 9 Hz, 1H), 4.93 (d, J ¼ 9.5 Hz, 1H), 4.84 (s, 1H), 4.22 (d, J ¼ 9.5Hz, 1H), 3.94 (m, 4H), 2.76 (s,
3H), 2.52 (t, J ¼ 7.5, 2H), 1.88–0.87 (m, 27H)ppm; 13C NMR (62.5 MHz, CDCl3=CS2): ꢀ ¼ 14.2,
22.8, 26.1, 26.2, 29.3, 29.4, 29.6, 31.8, 32.0, 35.2, 39.9, 67.7, 68.3, 68.7, 69.8, 83.0, 112.2, 114.4,
122.8, 123.9, 124.6, 127.8, 128.4, 129.6, 130.2, 134.7, 135.6, 136.4, 136.6, 138.4, 139.4, 139.8, 140.0,
140.02, 141.4, 141.5, 141.7, 141.8, 141.9, 141.9, 142.1, 142.4, 142.5, 142.8, 143.0, 144.2, 144.5, 144.5,
145.0, 145.1, 145.3, 145.4, 145.4, 145.6, 145.8, 146.0, 146.0, 146.2, 146.2, 146.3, 146.6, 147.1, 153.5,
153.0, 156.2, 159.0ppm. MS (MALDI-TOF): m=z ¼ 1268 (Mþ H)þ .
Polymerization
To a solution of 0.395 g of (3-(40-(100, 400, 700-trioxaoctyl)phenyl)thiophene) (1.42 mmol) and 0.200 g of
7 (0.157mmol) in 12cm3 of CHCl3 a slurry of 1.54 g of FeCl3 (9.5mmol) in 20cm3 of CHCl3 was
added over 5 h at room temperature. The polymer was obtained as a dark solid after precipitation from
methanol. The polymer was carefully dedoped by conc. NH4OH followed by 0.05M EDTA, then lower
molecular weight and irregular compounds were removed by Soxhlet extraction with diethyl ether. To
remove any remaining monomer, the precipitated polymer was slurried in toluene at room temperature
and filtered to provide 222 mg (36 w=w%) of pure polymer 1. The purity was checked by size exclusion
chromatography, to establish that no monomers were remaining in the polymer.
Photoinduced Absorption
PIA spectra of the polymer films were taken using the 476nm line of an Arþ laser beam as an
excitation source, with a typical intensity of 40mW on a 4 mm diameter spot. The pump beam was
modulated mechanically at a frequency of 210Hz and the changes in the white light (120W tungsten-
halogen lamp) probe beam transmission ( À ꢁT) were detected, after dispersion with a 0.3 m mono-
chromator, in the range from 0.55 to 2.15 eV by a Si–InGaAsSb sandwich detector. The detector signals
were recorded phase sensitively with a dual-phase lock-in amplifier. The probe light transmission

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A. Cravino et al.
(T) was recorded separately using the same chopper frequency. The photoinduced absorption (PIA)
spectra ( À ꢁT=T) were obtained after correction for the sample luminescence and normalization on
the probe light transmission. For the PIA experiments the samples were cooled down to 100 K in a
liquid nitrogen cryostat.
Photovoltaic Devices
Indium tin oxide (ITO) coated glasses (Merck, Darmstadt) were cleaned in an ultrasonic bath with
acetone, then methanol, and finally isopropanol. Poly(ethylenedioxythiophene) doped with polystyrene
sulphonic acid (Baytron, Bayer AG) was spin coated to a thickness of 100nm on top of the ITO. The
photoactive layer consisting of double-cable 1 or its mixtures with PCBM [3] as well as the reference
material 2 were spin coated from 10mg cmÀ 3 CHCl3 solutions to a thickness of less than 100nm. As
electron contact, Al was evaporated on top of the photoactive layer at a vacuum of 10À 4 Pa, and its
average thickness was 80 nm. Current=voltage characterisation of the devices was performed under
illumination from a solar simulator at 800 W mÀ 2
.
Acknowledgements
The authours thank J. C. ‘‘Kees’’ Hummelen for providing PCBM. The European Commission DG XII,
Joule III Programme (Contract No.: JOR3CT980206) and all partners in this European consortium
are also acknowledged. G. Zerza thanks the Austrian Science Fund (FWF Project P12680CHE).
M. Maggini thanks MURST (contract MM03198284).
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