Photoinduced charge transfer between tetracyano-anthraquino-dimethane derivatives and conjugated polymers for photovoltaics

Article abstract of DOI:10.1021/jp000729u

The photoinduced charge transfer between tetracyano-anthraquino-dimethane (TCAQ) derivatives and poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) has been studied by photoinduced absorption (PIA) spectroscopy in the VIS and IR spectral region and by light induced electron spin resonance (LESR). Studies on three different TCAQ derivatives with different side chain substitutions are reported. Among them a supermolecule with TCAQ attached to a fullerene have been synthesized to serve as tandem electron acceptor. The photoinduced absorption in the Vis/near-IR range shows a broad plateau around 1.8 eV followed by two peaks at 1.35 and 1.24 eV with an additional broad feature below 0.5 eV for all three acceptors in composite with MDMO-PPV. All PIA features have a power law excitation intensity dependence with an exponent close to 0.5 as expected for bimolecular kinetics. LESR studies show one absorption at a g-factor of 2.0028 with a width (peak-to-peak) of 3.5 G, originating from TCAQ anion radical and the polymer cation radical. Evidence for the formation of a C₆₀ anion was found in the case of electron transfer from the MDMO-PPV polymer to the TCAQ-C₆₀ acceptor dyade from these LESR studies. Finally bulk-heterojunction type thin film structures of MDMO-PPV as donor and the TCAQ derivatives as acceptor have been fabricated and tested as photovoltaic devices.

Full text of DOI:10.1021/jp000729u

J. Phys. Chem. A 2000, 104, 8315-8322
8315
Photoinduced Charge Transfer between Tetracyano-Anthraquino-Dimethane Derivatives
and Conjugated Polymers for Photovoltaics
Gerald Zerza,*,† Markus C. Scharber, Christroph J. Brabec, and N. Serdar Sariciftci
Physical Chemistry, Johannes Kepler UniVersity Linz, Altenbergerstrasse 69, A-4040 Linz, Austria
Rafael Go´mez, Jose´ L. Segura, and Nazario Mart´ın*,‡
Departamento de Qu´ımica Orga´nica, Facultad de Quimica, UniVersidad Complutense, 28040 Madrid, Spain
Vojislav I. Srdanov
Institute for Polymers and Organic Solids, UniVersity of California, Santa Barbara, California 93106
ReceiVed: February 24, 2000; In Final Form: May 28, 2000
The photoinduced charge transfer between tetracyano-anthraquino-dimethane (TCAQ) derivatives and poly-
[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) has been studied by photo-
induced absorption (PIA) spectroscopy in the VIS and IR spectral region and by light induced electron spin
resonance (LESR). Studies on three different TCAQ derivatives with different side chain substitutions are
reported. Among them a supermolecule with TCAQ attached to a fullerene have been synthesized to serve as
tandem electron acceptor. The photoinduced absorption in the Vis/near-IR range shows a broad plateau around
1.8 eV followed by two peaks at 1.35 and 1.24 eV with an additional broad feature below 0.5 eV for all three
acceptors in composite with MDMO-PPV. All PIA features have a power law excitation intensity dependence
with an exponent close to 0.5 as expected for bimolecular kinetics. LESR studies show one absorption at a
g-factor of 2.0028 with a width (peak-to-peak) of 3.5 G, originating from TCAQ anion radical and the polymer
cation radical. Evidence for the formation of a C60 anion was found in the case of electron transfer from the
MDMO-PPV polymer to the TCAQ-C60 acceptor dyade from these LESR studies. Finally bulk-heterojunction
type thin film structures of MDMO-PPV as donor and the TCAQ derivatives as acceptor have been fabricated
and tested as photovoltaic devices.
1. Introduction
to the fullerene molecules as well as formation of charged
species in solvents with high electron affinity.1 Replacing the
Light induced charge transfer from conjugated polymers and
their oligomers to different tetracyano-p-quinodimethane (TCNQ)
derivatives and fullerenes has been studied extensively.1-4 From
timeresolved photoinduced absorption (PIA) studies it is well-
known that the charge-transfer process from a conjugated
polymer to C60 takes place on an ultrafast time scale (<100fs).2
On the other hand the back electron transfer is remarkably slow;
lifetimes of the order of milliseconds can be observed for the
charge separated states in such systems. This very efficient
charge separation has been utilized in polymeric photovoltaic
devices.5-11
Photoexcitation of oligothiophenes in liquid or solid solutions
across the π-π* energy gap produces a metastable triplet state
by intersystem crossing from the excited singlet manifold.12-17
These triplet states were identified by their dipole allowed T1-
T2 transition, which exhibit a red shift with increasing chain
length.17 The lifetimes of these triplet states are of the order of
10-100 ms. Electron spin resonance studies of end-capped
oligothiophenes in toluene matrices give an unambiguous proof
of the triplet character of these excited states.18 Steady-state
photoinduced absorption (PIA) studies on mixtures of oligo-
thiophenes with fullerenes in solutions at room temperature have
shown both triplet-triplet energy transfer from the thiophenes
acceptor by tetracyanoethylene (TCNE), which has a triplet state
higher in energy than the triplet of the oligomer, favored the
electron transfer from the triplet state of the oligothiophene to
TCNE.
Various PIA studies on thin films of pristine conjugated
polymers have also shown the formation of the triplet state after
photoexcitation. In composite films of these polymers mixed
with electron accepting molecules the triplet absorption is
quenched due to charge transfer. Absorption detected magnetic
resonance (ADMR) experiments on films of pristine poly[2-
methoxy-5-2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-
PPV) have shown that the origin of the photoinduced absorption
in the range between 1.1 and 1.6 eV is due to excited triplet
states (spin 1), whereas mixed films of MEH-PPV and C60
show in the same energetic region features due to charge
separated states of spin 1/2.19 Light induced electron spin
resonance (LESR) studies on mixed films of poly[2-methoxy-
5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-
PPV) and a C60 derivative 1-(3-methoxycarbonyl)-propyl-1-
1-phenyl-(6,6)C61 (PCBM) showed two light induced ESR lines,
one with a g-factor of 2.0025 being due to the polymer radical
cation and the other one with a g-factor 1.9995 originating from
the fullerene radical anion.20 The authors observed the saturation
of the polymer-line already at low microwave powers (<1 mW),
whereas they could not see any saturation effects on the PCBM
† E-mail: Gerald.Zerza@jk.uni-linz.ac.at.
‡ E-mail: Nazmar@eucmax.sim.ucm.es.
10.1021/jp000729u CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/12/2000

8316 J. Phys. Chem. A, Vol. 104, No. 35, 2000
Zerza et al.
SCHEME 1: Synthesis and Chemical Structures of the Three TCAQ-type Electron Acceptors (a) 6a,b and (b) 9
ESR-line up to 200 mW. They concluded that this is due to
different spin-lattice relaxation times of the two spins on the
polymer and the fullerene.
in photovoltaic energy conversion and might candidate for
substitution of the fullerenes commonly used in these de-
vices.21,22
In another study different TCNQ derivative acceptors were
tested with respect to their charge-transfer efficiency to conju-
gated polymers.4 Both the amount of luminescence quenching
and the PIA signal of the charged states were investigated for
two conjugated polymers (MEH-PPV and poly[3-(2-(3-meth-
ylbutoxy)ethyl) thiophene] (P3MBET)) and TCNQ derivatives
as electron acceptors with different electron affinity. The light
induced electron transfer was observed to depend on the electron
affinity of the acceptor. Tetracyano-anthraquino-dimethane
(TCAQ) was found to be a more efficient electron acceptor than
TCNQ despite its lower electron affinity.
2. Experimental Section
2.1. Preparation of the TCAQ Derivatives. All melting
points were measured with a electrothermal melting-point
apparatus and are uncorrected. FTIR spectra were recorded as
KBr pellets with a Shimadzu FTIR 8300 spectrometer. UV-
vis spectra were recorded with a Hewlett-Packard-8452A
1
instrument. 13C- and H NMR spectra were recorded with a
1
Bruker AC-200 spectrometer (200 MHz and 50 MHz for H
and 13C respectively). Chemical shifts are given as δ values
(internal standard: TMS). MS were recorded with a Finnigan
8430 (70 eV) spectrometer. Cyclic voltammetric measurements
were performed with an EG & PAR Versastat potentiostat using
250 Electrochemical Analysis software. A Metrohm 6.0840.C10
glassy carbon electrode was used as indicator electrode in
voltammetric studies. 2-(Hidroxymethyl)-9,10-anthraquinone
and 2-carboxy-9,10-anthraquinone are commercially available
and were used without further purification.
In this work TCAQ derivatives with alkyl side chain
substitutions were prepared, one of them attached to a C60
molecule forming the acceptor dyade 9 as shown in Scheme 1.
The alkyl side chains should provide better solubility in organic
solvents and also prevent phase separation between polymer
and the electron acceptor. The photoinduced charge transfer from
poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene-
vinylene] (MDMO-PPV) to these acceptors was studied using
PIA in the VIS/NIR and IR range as well as LESR. This
photoinduced charge transfer in composite films has been
utilized to fabricate bulkheterojunction photovoltaic devices. The
results show that these materials have encouraging performance
The preparation of the novel acceptor molecules was carried
out by reaction of malonitrile with the respective quinones (5a,b)
using Lehnert’s reagent.23 The starting quinones were in turn
prepared in two steps from the commercially available 2-car-
boxy-9,10-anthraquinone (1) by reaction with thionyl chloride

Charge Transfer between TCAQ and MDMO-PPV
J. Phys. Chem. A, Vol. 104, No. 35, 2000 8317
and further reaction of the formed acid chloride (3) with the
respective alcohol (4) to afford compounds 5a,b bearing long
alkyl chains (Scheme 1).
On the other hand, fulleropyrrolidine 9 was prepared from
the previously reported 2-formil-11,11,12,12-tetracyano-
anthraquinodimethane (7)24 and the polyether containing glycine
(8)25 by reaction with C60 in hot toluene. Organofullerene 9 is
obtained by cycloaddition reaction of the in situ formed
azomethyne ylide to C60 according to Prato’s procedure.26
Synthesis of 2-Alkoxycarbonyl-9,10-anthraquinones (5). Gen-
eral Procedure. Ten milliliters (140 mmol) of thionyl chloride
(2) and one gram (4 mmol) of 1,4-anthraquinone-2-carboxylic
acid (1) were refluxed under argon for 6 h (Scheme 1). After
this time, the reaction was allowed to cool at room temperature
and the excess of thionyl chloride was removed under vacuum
to yield the corresponding acid chloride 3 which was used in
the next synthetic step without further purification.
Five milliliters (40 mmol) of the corresponding alcohol 4 were
added to the above obtained acid chloride 3 and afterward 0.8
mL (10 mmol) of dry pyridine were added dropwise and the
mixture was heated at 100 °C for 5 h. After this time, the
solution was cooled in an ice bath and then 10 mL of a 1 M
HCl solution were carefully added. The mixture was extracted
several times with ethyl ether; the combined organic layers were
washed first with saturated NaHCO3 solution and finally with
water. After drying the organic layer with magnesium sulfate,
the diethyl ether was removed under vacuum and the excess of
alcohol was eliminated by vacuum distillation. The residue
obtained was purified by flash chromatography (silica gel,
hexane:ethyl acetate).
Figure 1. Cyclic voltammogram of 6a,b in dichloromethane and of 9
in toluene:acetonitrile 4:1 with tetrabutylammonium perchlorate as
electrolyte and glassy carbon as working electrode and SCE as reference
at room temperature.
odimethane (6a). The crude product obtained following the
above general procedure was purified by crystallization on
acetonitrile to afford the pure product in a 48% yield, mp 296-
1
298 °C. H NMR (CDCl3, 200 MHz) δ 8.84 (s, 1H), 8.34-
8.15 (m, 4H), 7.75-7.66 (m, 2H), 5.08 (q, 1H), 1.74-1.55 (m,
4H), 1.38-1.18 (m, 2H), 0.93-0.78 (m, 6H). 13C NMR (CDCl3,
50 MHz) δ 163.3, 159.2, 134.3, 133.3, 133.0, 132.7, 132.6,
130.4, 130.1, 130.0, 128.3, 127.8, 127.7, 127.6, 112.8, 112.6,
77.8, 35.6, 26.9, 18.5, 13.9, 9.53. FTIR (KBr) ν 2966, 2933,
2228, 1716, 1570, 1458, 1267, 1205, 1107, 758 cm-1. MS m/z
(%I) 406 (4), 331 (33), 306 (100), 278 (12), 251 (12), 224 (7),
207 (15). Cyclic voltammetry (CH2Cl2): working electrode,
glassy carbon; reference electrode, calomel; supporting elec-
trode, Bu4NClO4. E1/2 ) -0.291 V (2e). UV-vis (CH2Cl2)
red
λ(nm) 236, 290, 312, 348.
2-(1′-Ethylbutoxycarbonyl)-9,10-anthraquinone (5a). By fol-
lowing the general procedure and using 3-hexanol as the alcohol,
2-(Octyloxycarbonyl)-11,11,12,12-tetracyanoanthraquino-
dimethane (6b). The crude product obtained following the above
general procedure was purified by flash chromatography (silica
gel, hexane:ethyl acetate) to afford the pure product in a 75%
1
5a was obtained in a 80% yield, mp 69-71 °C. H NMR
(CDCl3, 200 MHz) δ 8.86 (s, 1H), 8.40-8.21 (m, 4H), 7.81-
7.72 (m, 2H), 5.10 (q, 1H), 1.72-1.28 (m, 6H), 0.94-0.81 (m,
6H). 13C NMR (CDCl3, 50 MHz) δ 182.5, 182.3, 164.8, 135.9,
135.8, 134.5, 134.4, 134.3, 133.5, 133.4, 133.3, 128.4, 127.4,
127.4, 127.3, 77.2, 36.3, 27.7, 18.3, 13.9, 9.3 ppm. FTIR (KBr)
ν 2962, 2935, 1718, 1680, 1591, 1460, 1330, 1269, 1246, 1171,
1
yield, mp 86-90 °C. H NMR (CDCl3, 200 MHz). δ ) 8.83
(s, 1H), 8.33-8.17 (m, 4H), 7.75-7.66 (m, 2H), 4.32 (t, 2H),
1.73 (q, 2H), 1.23 (s, 10H), 0.82 (t, 3H). 13C NMR (CDCl3, 50
MHz). δ ) 163.8, 158.2, 134.0, 133.3, 132.9, 132.7, 132.6,
130.4, 130.0, 129.8, 128.3, 127.7, 127.6, 112.6, 66.4, 31.7, 29.1,
29.0, 28.5, 25.8, 22.5, 14.0. FTIR (KBr) ν 2926, 2855, 2228,
1722, 1560, 1466, 1329, 1294, 1203, 1178, 1128, 771, 692 cm-1.
MS m/z (%I) 460 (5), 434 (16), 348 (28), 331 (38), 323 (100),
306 (17), 279 (34), 251 (27), 224 (15), 112 (32), 84 (45), 69
(69), 55 (63). Cyclic voltammetry (CH2Cl2): working electrode,
glassy carbon; reference electrode, calomel; supporting elec-
933, 702 cm-1
.
2-(Octyloxycarbonyl)-9,10-anthraquinone (5b). By following
the general procedure and using n-octanol as the alcohol, 5b
was obtained in a 80% yield, mp 82 °C. 1H NMR (CDCl3, 200
MHz) δ 8.87 (s, 1H), 8.39-8.22 (m, 4H), 7.81-7.72 (m, 2H),
4.33 (t, 2H), 1.75 (q, 2H), 1.24 (s, 10H), 0.81 (t, 3H). 13C NMR
(CDCl3, 50 MHz) δ 182.5, 182.3, 165.0, 135.9, 135.5, 134.5,
134.4, 134.3, 133.5, 133.4, 133.3, 128.5, 127.5, 127.4, 127.3,
66.0, 31.7, 29.2, 29.1, 28.6, 25.9, 22.6, 14.0. FTIR (KBr) ν 2951,
2932, 2856, 1724, 1672, 1591, 1475, 1277, 1244, 1173, 949,
trode, Bu4NClO4. E1/2 ) -0.293 V (2e). UV-vis (CH2Cl2)
red
λ(nm) 236, 290, 312, 350.
Synthesis of N-(3,6,9-Trioxadecyl)-2-[2′-(11,11,12,12-tetra-
cyanoanthra-quinodimethane)] pyrrolidino[3,4:1,2][60]fullerene
(9). To a solution of 144 mg (0.19 mmol) of C60 in 78 mL of
toluene were added 63 mg (0.19 mmol) of the TCAQ-
carboxaldehyde 723 and 208 mg (1 mmol) of N-(3,6,9-trioxa-
decyl)glycine (8).24 The mixture was heated to reflux for 48 h,
brought to room temperature, poured on top of a silica gel
column, and eluted with toluene-ethyl acetate to obtain a black
solid which was washed with hexane and diethyl ether (Scheme
708 cm-1
.
Synthesis of 2-alkoxycarbonyl-9,10-anthraquinodimethanes
(6). General Procedure. To a refluxing solution of 1.8 mol of
the corresponding ester (5) in dry chloroform were added, under
argon atmosphere, 297 mg (4.5 mmol) of malononitrile, 0.5 mL
of TiCl4 (4.6 mmol), and 0.7 mL (8.75 mmol) of dry pyridine.
The reaction was kept under reflux for 72 h, and every 24 h
similar amounts of malononitrile, pyridine, and titanium tetra-
chloride were added. After this time the reaction was allowed
to cool at room temperature and then it was poured on a water-
ice mixture. The mixture was extracted twice with chloroform;
the combined organic layers were washed with water and dried
over magnesium sulfate. After removing the solvent under
vacuum, the corresponding TCAQ derivative was obtained.
2-(1′-Ethylbutoxycarbonyl)-11,11,12,12-tetracyanoanthraquin-
1
1b). Yield 25%. H NMR (CDCl3, 200 MHz) δ 8.73 (s, 1H),
8.19-8.34 (m, 3H), 7.78-7.69 (m, 2H), 7.39-7.37 (s, 1H),
5.36 (s, 1H), 5.28 (d, 1H), 4.39 (d, 1H), 3.79-3.55 (m, 10H),
3.36 (s, 3H). 13C NMR (CDCl3, 50 MHz) δ 159.7, 153.8, 152.2,
147.4, 147.3, 146.2, 146.1, 146.0, 145.8, 145.5, 145.2, 146.2,
144.8, 144.5, 144.3, 143.5, 143.5, 143.3, 143.2, 142.6, 142.2,
142.0, 141.8, 140.3, 139.4, 136.3, 135.5, 133.5, 133.1, 132.5,
132.5, 130.1, 129.0, 128.2, 128.1, 127.8, 127.7, 113.0, 83.3,

8318 J. Phys. Chem. A, Vol. 104, No. 35, 2000
Zerza et al.
81.6, 77.2, 75.8, 72.0, 70.6, 69.3, 67.5, 59.1, 52.4. FTIR(KBr)
ν 2950, 2878, 2223, 1198, 1105, 762, 726 cm-1. Cyclic
voltammetry (toluene:acetonitrile 4:1): working electrode,
glassy carbon; reference electrode, calomel; supporting elec-
Figure 7 in section 3.7). After drying of the PEDOT film, the
first layer was cast by doctor blading from a heated solution of
MDMO-PPV in toluene. In the second process step, the
acceptor molecule (9, 6b, PCBM) was cast from toluene
solution. The concentration of the acceptor molecules in solution
was varied between 0.25 and 2 wt %. All devices were produced
on transparent ITO coated polyester (PET) substrates with 4
cm by 6 cm by doctor blading technique. One substrate carried
36 devices with an active area of 7.5 mm2 each. The counter
electrode was in all cases thermally evaporated aluminum. I/V
curves were recorded with a Keithley SMU 2400, typically by
averaging over 80 measurements for one point. Illumination was
provided by white light from a Halogen lamp with 60 mW/
cm2.
trode, Bu4NClO4. E1red ) -0.40 V (2e), E2red ) -0.65 V, E3
red
) -1.03 V, E4red ) -1.66 V, E5red ) -2.17 V. UV-vis (CH2-
Cl2) λ(nm) 240, 262, 274, 288, 316, 440.
2.2. Experimental Setup for Charge-Transfer Studies. The
MDMO-PPV polymer was delivered by Covion, Inc., and used
as received. For LESR studies samples were prepared as films
on a KBr powder. A weight ratio of 1:1 between polymer and
the acceptor material was dissolved in toluene in a concentration
of 1 mg/mL. The solution was dropped on KBr powder and
dried under vacuum before being filled in the ESR-tubes. These
samples were placed in the high-Q-cavity of a X-band ESR
spectrometer and illuminated with 488 nm light of an Ar-ion
laser. The samples were irradiated during the run of the ESR
spectra and light off spectra were taking directly afterward to
be subtracted from the former one to compensate for the
persistent charges produced in the samples. For PIA studies in
the VIS/NIR spectral region the samples were prepared as
solution cast films on glass substrates using a weight ratio of
4:1 between the polymer and the electron acceptors (6a,b and
9 in Scheme 1) and a concentration of 1 mg/ml sample in
toluene. PIA spectra were taken using an Ar-ion laser at 488
nm as a pump (typically 40 mW on a 4 mm diameter spot).
Using mechanical modulation of the pump beam at 132 Hz,
the changes in the white light (120W tungsten-halogen lamp)
probe beam transmission (-∆T) were detected after dispersion
with a 0.3 m monochromator in the range from 0.55 to 2.1 eV
with a Si-InGaAsSb sandwich detector and recorded phase
sensitively with a dual-phase lock-in amplifier. The probe light
transmission (T) was recorded separately using the same chopper
frequency. From this the PIA spectra (-∆T/T ≈ ∆Rd) are
obtained after correction for the sample luminescence and
normalization on the probe light transmission.
3. Results and Discussion
3.1. Synthesis. In contrast to previously reported TCNQ
derivatives,27 the novel compounds prepared exhibit reasonably
good solubility in common organic solvents due to the presence
of the alkyl chains, thus allowing a full spectroscopic and
electrochemical characterization. The UV-vis spectra suggest
that these molecules are severely distorted from planarity as
has been confirmed by X-ray data for the related unsubstituted
11,11,12,12-tetracyanoanthraquinodimethane (TCAQ).28 This
fact is also supported by the relatively high value for the
stretching vibration of the cyano groups which appear at 2230
cm-1 in the IR spectra. In addition, compound 9 shows a typical
weak absorption band at 440 nm in the UV-vis spectrum,
similar to most dihydrofullerenes. The 1H NMR spectrum of 9
shows, in addition to the expected aromatic signals (see
Experimental Section), the presence of the pyrrolidine protons
at δ 4.39 and 5.28 as doublets (J ) 9.9 Hz; geminal hydrogens)
and δ 5.12 (CH). The 13CNMR spectrum shows the signals at
δ 83.3, 77.3, 70.0, and 69.2 for the sp3 carbons of the pyrrolidine
ring and those at the 6,6-ring junction of the C60 cage.
3.2. Electrochemistry. The redox properties of the novel
acceptors were studied by cyclic voltammetry at room temper-
ature in dichloromethane for 6a,b and in toluene 4:acetonitrile
1 for 9 using tetrabutylammonium perchlorate as the supporting
electrolyte, glassy carbon as working electrode, and SCE as
reference.
Infrared activated vibrational (IRAV) spectra are taken using
a Bruker IFS 66/S spectrometer with a liquid nitrogen cooled
MCT detector. The samples were prepared by drop casting from
the mixed polymer/acceptor molecules solution on pressed KBr
pellets. Spectra were recorded by 200 accumulations of 10 dark
and 10 illuminated cycles. All measurements for LESR, PIA-
VIS/NIR and IRAV spectra are done at liquid nitrogen tem-
perature and under vacuum better than 10-5 mbar.
Similarly to the parent TCAQ,29 substituted derivatives 6a,b
show a single-wave, two-electron reduction to the dianion. It
has been previously demonstrated that the negative charges in
the radical anion and dianion species are mainly located at the
two cyanomethylene groups.30
Photovoltaic devices were produced by sensitizing MDMO-
PPV with 9 and 6b respectively (Scheme 1). Results were
compared to reference photovoltaic devices with an active layer
consisting either of pristine MDMO-PPV or MDMO-PPV
sensitized with 1-(3-methoxycarbonyl)-propyl-1-1phenyl-(6,6)-
C61 (PCBM). Preliminary studies showed strong phase separa-
tion of TCAQ with MDMO-PPV when spin cast from one
solution. These films did not give photovoltaic devices due to
large pinholes. Therefore, we used a special production tech-
nique, the “bilayer diffusion technique” to produce photovoltaic
devices. In this technique, the donor polymer is cast in a first
step to give a pinhole free, thin film. In a second step, the
acceptor molecule is cast from a solvent or a mixture of solvents
that dissolves and/or swells the polymer partially but not
completely. During the drying step the acceptor molecules can
diffuse into the polymer matrix. Even in the case of a phase
separation, this method allows to produce films to fabricate
devices. The indium tin oxide (ITO) substrates were covered
by a film of poly(3,4-ethylenedioxythiophene)-poly-styrene-
sulfonate (PEDOT) from Bayer AG by doctor blading (see also
Compounds 6a,b showed the presence of only one reduction
wave (6a, E1/2red ) -0.291 V; 6b, E1/2red ) -0.293 V) involving
a two-electron process to form the respective dianion species.
Compound 9 showed, in addition to the reduction wave of
the TCAQ moiety at E1 ) -0.40 V (2e-), the presence of
red
four quasi-reversible reduction waves at E2red ) -0.65 V; E3
red
) -1.03 V; E4 ) -1.66 V; E5 ) -2.17 V, (Figure 1)
red
red
similarly to that found for the parent unsubstituted C60 [-0.60,
-1.00, -1.52, -2.04 V]. It is interesting to note that the first
reduction potential of 9 appears cathodically shifted in com-
parison with 6a,b which can be accounted for by the electronic
effect of the alkoxycarbonyl group in 6a,b. In addition, the
reduction potentials of 9 corresponding to the C60 moiety are
slightly shifted to more negative values related to C60. This fact
has been explained by the saturation of a double bond of the
C60 core which raises the LUMO energy,31 and is in good
agreement with that found for other fulleropyrrolidines.32
3.3. Light Induced ESR. No ESR signals could be detected
in the pristine MDMO-PPV films neither before nor during

Charge Transfer between TCAQ and MDMO-PPV
J. Phys. Chem. A, Vol. 104, No. 35, 2000 8319
Figure 2. LESR signals of (a) MDMO-PPV:9 and (b) MDMO-PPV:
6b composite films (weight ratio 1:1) on KBr powder. Illumination at
488 nm, microwave power of 0.2 mW at 100 K.
illumination with 488 nm light. Samples of the pure electron
acceptors 9 and 6b (in Scheme 1) showed a very weak LESR
signal consisting of one line with a g-value 2.0028. Figure 2
shows the LESR spectra of thin films of MDMO-PPV mixed
with 9 and 6b. The estimated weight ratio between polymer
and acceptor is 1:1. For both samples an ESR line with ∆HP-P
3.5 G is observed. The g-factor has been determined to be
2.0028(3). Switching off the exciting light causes a slow
decrease of the so-called “prompt” ESR signal until it stabilizes
at a constant value after some 100 s. This so-called “persistent”
LESR signal, about one-third of the intensity of the original
LESR signal, vanishes only after heating the sample to room
temperature. Laser power dependent measurements of the LESR
signal show an increase of the LESR signal with laser intensity
following a power law with an exponent close to 0.2 and 0.4
for 9 and 6b composites, respectively. For excitation intensity
dependence34,35 Dyakonov et al. observed a near square root
dependence of the LESR signal on the pump power for
MDMO-PPV:PCBM composites suggesting bimolecular re-
combination.20 In this work for both TCAQ electron acceptors
(9, 6b) exponents smaller than 0.5 have been observed.36
From ESR studies it is known that the g-factor of both the
TCAQ anion (2.0027(1)) as well as this of the MDMO-PPV
polymer cation (positive polaron) (2.0025) are in the range
reported here (2.0028(3)).20,33 So both radicals of the electron
donor as well as of the electron acceptor could contribute to
the observed LESR signal. Janssen et al. published single broad
line LESR spectra of oligothiophenes in solutions containing
TCNE as acceptor.1
For samples containing the molecule 9 (Scheme 1), which
combines the two strong electron acceptors TCAQ and C60, an
additional shoulder like feature is observed in the LESR
spectrum at higher fields (Figure 2b) corresponding to a g-factor
close to 2.000. For C60 and C60 derivative anion radicals, a
g-factor close to 2.000 is observed.20 According to the work of
Janssen et al. C60 is more effective photoinduced electron
acceptor than TCAQ or TCNQ,4 despite the higher electron
affinity of TCAQ or TCNQ. Following our LESR observations
the dominant final state of the electron transferred from the
polymer to the electron acceptor dyade can be the TCAQ unit
(dominant LESR line) with a small fullerene anion formation
(weak LESR shoulder). From these results it seems that both
parts of the acceptor dyade are involved in the charge transfer.
A stepwise charge transfer, donor f acceptor 1 (fullerene) f
Figure 3. Photoinduced absorption spectra of (a) pristine MDMO-
PPV film and composite (b) MDMO-PPV:6a, (c) MDMO-PPV:9,
(d) MDMO-PPV:6b films. Excitation at 488 nm with 40 mW and
132 Hz modulation, T ) 100K.
acceptor 2 (TCAQ), cannot be excluded by these results. Time-
resolved LESR or PIA studies are required to distinguish
between direct and stepwise charge transfer.
3.4. Photoinduced Absorption (PIA) at VIS/NIR Region.
The photoinduced absorption spectra for a pristine MDMO-
PPV film as well as for composite films of MDMO-PPV with
the three different TCAQ derivatives are shown in Figure 3. In
the case of the pristine MDMO-PPV film a single excited state
absorption line at 1.36 eV is observed (Figure 3a). This
photoinduced absorption feature is generally associated with the
triplet state.4 Several new PIA features appear in the composite
films with 6a, 9 and 6b presented in Figure 3b-d. A plateau
between 1.9 and 1.4 eV is followed by two closely spaced peaks
at 1.24 and 1.33 eV and an additional broad absorption feature
lying below 0.5 eV. For all three electron acceptors used, the
shape of the PIA spectra is quite similar (Figure 3 b-d). Janssen
et al. reported on comparable PIA features for films of MEH-
PPV mixed with different TCNQ derivatives.4 Besides a
somewhat higher intensity for the samples containing the
fullerene-derivatized supermolecule (Figure 3c), no differences
could be observed in the PIA spectra using the three TCAQ
type electron acceptors (Figure 3b-d).
The excitation intensity dependencies of the PIA signals are
shown in Figure 4. These measurements can give insight into

8320 J. Phys. Chem. A, Vol. 104, No. 35, 2000
Zerza et al.
Figure 4. Laser power dependence of the photoinduced absorption in
Figure 3 for a modulation frequency of 132 Hz: (9) pristine MDMO-
PPV (at 1.36 eV), (1) MDMO-PPV:6a (at 1.24 eV), (b) MDMO-
PPV:9 (at 1.24 eV), (2) MDMO-PPV:6b (at 1.24 eV). The solid lines
show fits to a power law dependence.
Figure 6. Photoinduced IRAV measurements of (a) MDMO-PPV:9
(solid line) and (b) MDMO-PPV:6b (solid line) composite films on
KBr. The dotted lines in (a) and (b) show for comparison the IRAV
signals for the pristine polymer film. Excitation with 488 nm at T )
100K.
increase by 2 orders of magnitude (Figure 5), so approximately
with 1/ω0.5. Also shown in Figure 5 are fits of the frequency
dependence of the PIA signals (at 1.24 eV) to a bimolecular
recombination kinetics as described by Dellepiane et al.34
However, only a fit with multiple time constants (see eq 2)
between 100 µs and 30 ms achieved good agreement between
the experimental results and the bimolecular model as shown
in eq 2:
Figure 5. Modulation frequency dependence of the photoinduced
absorption signal in Figure 3 for 40 mW laser power. (9) pristine
MDMO-PPV (at 1.36 eV), (b) MDMO-PPV:9 (at 1.24 eV), (1)
MDMO-PPV:6a (at 1.24 eV), (2) MDMO-PPV:6b (at 1.24 eV).
Dashed lines show fit to monomolecular and bimolecular recombination
behavior (see text).
Ri tanh Ri
∆T
,
i ) 1..3
(2)
∑
Ri + tanh Ri
i
the relaxation kinetics involved in the photoexcited processes.
For monomolecular kinetics one predicts a linear increase of
the photoproduct with the excitation intensity and for a
bimolecular recombination in the low modulation frequency
limit (ωτ , 1) a square root dependence.35,38 For the PIA signals
of these donor/acceptor composite films a power law depen-
dence with an exponent close to 0.5 can be extracted from Figure
4. For the pristine MDMO-PPV films on the other hand the
observed dependence of the PIA signal on the excitation power
does not follow a simple power law (Figure 4). It resembles
rather the behavior of a saturated absorption.
where ∆T is the steady-state PIA signal and Ri ) π/(ωτi) with
the modulation frequency ω of the exciting laser.
Janssen et al. reported on a similar frequency dependence of
their PIA spectra of MEH-PPV films with different TCNQ
derivative acceptors.4 Botta et al. observed a 1/ω decay of the
PIA signal for long-lived charged excitations of polymers in
solutions, with only one lifetime involved.35,38 On the other hand,
the 1/ω0.5 dependence of the PIA signal in polymer films, similar
as in our case, was attributed to a broad distribution of different
lifetimes of trapped charge carriers.
Also a modulation frequency ω dependent measurement of
the PIA signal can give insight into the mechanisms involved
in the kinetics of the photoexcited states and give an estimate
on the lifetime of the photoproducts.34,35 Figure 5 shows a
comparison of the PIA signals for the pristine MDMO-PPV
film (at 1.36 eV) and the mixed films (at 1.24 eV) as a function
of the chopping frequency. For the case of pristine MDMO-
PPV the signal stays nearly constant up to frequencies of 800
Hz until it decreases remarkably. A fit assuming monomolecular
kinetics as given in equation 1
3.5. Infrared Activated Vibrational (IRAV) Spectra. A
further indication for the charge transfer from the MDMO-
PPV to the TCAQ type electron acceptors is by light induced
FT IR spectroscopy.39 The light induced IRAV spectra of
pristine MDMO-PPV and MDMO-PPV with 9 and 6b, taken
under 488 nm light illumination, are shown in Figure 6. For
the pristine polymer no detectable light induced features appear
(dotted line in Figure 6). By adding the electron acceptor to
the polymer the PIA results in a broad electronic subgap
absorption at 3000 cm-1 (0.37 eV) and strong infrared active
vibrational bands around 1490 cm-1 (0.185 eV) and 1000 cm-1
(0.125 eV) emerge in Figure 6a,b. These bands corresponds to
Raman active modes of the neutral polymer, which become
infrared active for the charged species.39 The broad unstructured
absorption around 3000 cm-1 corresponds to the continuation
of the low energy polaron feature (<0.5 eV) shown in Figure
3b-d. Brabec et al. reported on IRAV measurements on MEH-
PPV with the fullerene derivative PCBM in polystyrene matrices
1
∆T
,
i ) 1..2
(1)
∑
i
1 + ω2τ 2
x
i
gives good results if at least two lifetime components τi of the
order of 40 and 200 µs are assumed.34 For the composite samples
the signal decays by 1 order of magnitude for a frequency

Charge Transfer between TCAQ and MDMO-PPV
J. Phys. Chem. A, Vol. 104, No. 35, 2000 8321
TABLE 1: Luminescence Quantum Efficiencies of Pristine
and Composite Polymer Films Measured with an Integrating
Sphere Setup for 488 nm Excitation at Room Temperature
MDMO-
PPV pristine
MDMO-
PPV:PCBM
MDMO-
PPV: 9
MDMO-
PPV:6b
2.6%
0.3%
0.3%
0.9%
with similar results.40 Unsubsituted PPV shows light induced
IR-absorptions at 1470, 1398, 1274, and 1100 cm-1 41
For
Figure 7. Schematic setup of a plastic solar cell device. polyester
(PET), indium tin oxide (ITO), poly(3,4-ethylenedioxythiophene)-poly-
styrenesulfonate (PEDOT).
.
another substituted PPV derivative, IRAV bands at 1485, 1217,
1086, and 1028 cm-1 have been published.41
From excitation intensity dependent measurements of the IR
absorption at 1000 and 3000 cm-1 (Figure 6) a sublinear increase
with an exponent considerably smaller than 0.5 is observed like
in the LESR measurements. Cha et al. showed, from their
photoinduced infrared absorption studies on various conjugated
polymers, that a bimolecular recombination of polarons formed
from polaron pairs can be responsible for such an power law
intensity dependence with an exponent smaller than 0.5.36
According to their calculations in the limit of high excitation
intensities a power law dependence with an exponent of 0.25
should be observed.
3.6. Luminescence Quenching. To compare the efficiencies
for the charge transfer of the different TCAQ acceptors and the
fullerene derivative PCBM, the luminescence quantum ef-
ficiency of the pristine polymer sample and the mixed polymer/
acceptor samples have been determined using an integrating
sphere arrangement.42,43 For all samples a relatively low electron
acceptor concentration of 0.1 mol % was used. Table 1 shows
a comparison of the luminescence quantum efficiencies for the
MDMO-PPV samples with three different electron acceptors
mixed inside and excited at 488 nm. The optical density of the
samples at the excitation wavelength was determined to be close
to 1. The luminescence quantum efficiency of the pristine
MDMO-PPV sample is of the order of 3%. Adding 0.1 mol
% of PCBM or 9 to the MDMO-PPV quenches the lumines-
cence by a factor of 9, whereas using the same amount of 6b
as acceptor only reduces the MDMO-PPV luminescence by a
factor of 3 (Table 1). So there is evidence that those electron
acceptors containing the C60 unit (PCBM and 9) induce a more
efficient luminescence quenching as compared to the pure
TCAQ derivative acceptors. Janssen et al. report on a about 4
times higher quenching rate of the MEH-PPV luminescence
by C60 compared to TCAQ.4 Moreover for the case of the
“electron acceptor dyade” 9 composed of a fullerene and a
TCAQ unit there seems to be some evidence for an additional
influence of the C60 unit on the electron accepting properties
of the whole compound. Though a change in morphology and
phase separation of the sample in going from a TCAQ-type to
a fullerene-type electron acceptors may also strongly influence
the amount of luminescence quenching.
Figure 8. Current-voltage characteristics of different bulk hetero-
junction photovoltaic devices of 0.075 cm2 active area. (a) Under 60
mW/cm2 white light illumination and (b) in the dark. ([) Pristine
MDMO-PPV, (9) MDMO-PPV:PCBM, (b) MDMO-PPV:9, and
(2) MDMO-PPV:6b as photoactive layers.
efficiency of 0.4% for 60 mW/cm2 white light. Using 9 instead
of PCBM reduced the short circuit currents to 8 µA. Under these
illumination conditions Voc was measured to be 480 mV with a
filling factor of 0.3 which results in a power conversion
efficiency of the order of 0.03%. Both, the rectification and the
current under forward bias are lower. It is interesting to note
that the dark and the illuminated I/V curve of the device with
the electron acceptor 9 are rather symmetric around 0 V and
Voc respectively. This symmetry might indicate an ohmic
contribution to the diode, probably due to small shunts as a
consequence of immiscibility between 9 and MDMO-PPV.
Also atomic force microscopy (AFM) studies gave evidence of
phase separation between the MDMO-PPV and 9.
3.7. Photovoltaic Devices. We have fabricated donor/acceptor
interpenetrating network type photovoltaic devices as published
before.8,21,22 Figure 7 describes the setup of the thin film solar
cell devices in a schematic way. Figure 8a,b shows the I/V
curves of photovoltaic devices produced of pristine MDMO-
PPV and MDMO-PPV sensitized with 9, 6b, and PCBM under
illumination with 60 mW/cm2 white light and dark, respectively.
The highest short circuit currents (Isc) are observed for devices
produced from MDMO-PPV with PCBM. The rectification of
these devices is up to 100 at (2 V in the dark. The open circuit
voltage Voc for the device presented is 720 mV, the short circuit
current Isc ) 60 µA for an active area of 0.075 cm2. With a
filling factor of 0.4 this sums up to a power conversion
The photocurrent and the power conversion efficiency of 6b
are clearly less than for 9 by an order of magnitude. The
photophysical investigation presented above gave clear evidence,
that photoinduced charge transfer between MDMO-PPV and
6b occurs. Therefore, the weak photovoltaic performance of
MDMO-PPV with 6b can only be explained by poor charge
carrier transport of the electron via 6b.

8322 J. Phys. Chem. A, Vol. 104, No. 35, 2000
Zerza et al.
(17) Jannsen, R. A. J.; Smilowitz, L.; Sariciftci, N. S.; Moses, D. J.
Chem. Phys. 1994, 101, 1787.
(18) Bennati, M.; Grupp, A.; Ba¨uerle, R.; Mehring, M. Mol. Cryst. Liq.
Cryst. 1994, 256, 751.
(19) Wei, X.; Vardeny, Z. V.; Sariciftci, N. S.; Heeger, A. J. Phys. Ref.
B 1996, 53, 2187.
(20) Dyakonov, V.; Zorinaints, G.; Scharber, M.; Brabec, C. J.; Janssen,
R. A. J.; Hummelen, J. C.; Sariciftci, N. S. Phys. ReV. B 1999, 59, 8019.
(21) Brabec, C. J.; Padinger, F.; Dyakonov, V.; Hummelen, J. C.;
Janssen, R. A. J.; Sariciftci, N. S. In Proceedings of the International
Winterschool on NoVel Materials, Kirchberg am Wechsel, Austria, 1997;
Kuzmany, H., Ed.; World Scientific Publishing Company: River Edge, NJ,
1998; pp 519-522.
(22) (a) Brabec, C. J.; Padinger, F.; Hummelen, J. C.; Sariciftci, N. S.
J. Appl. Phys. 1999, 85, 6866. (b) Gebeyehu, D.; Padinger, F.; Brabec, C.
J.; Fromherz, T.; Hummelen, J. C.; Sariciftci, N. S. International Journal
of Photoenergy 1999, 1, 89-95.
4. Conclusions
Photoinduced charge transfer between the MDMO-PPV
polymer and three TCAQ derivative electron acceptors has been
observed by light induced ESR, photoinduced absorption and
luminescence quenching. From the luminescence quenching data
it can be stated that fullerenes or fullerene containing derivatives,
like the electron acceptor dyade TCAQ-C60, are somewhat more
efficient quenchers than TCAQ type electron acceptors.
The efficiency of photovoltaic devices based on MDMO-
PPV sensitized with TCAQ type acceptors is limited by the
phase separation between the donor and the acceptor. Using
differently substituted TCAQs giving higher compatibility to
PPV type polymers as well as investigations of the photoinduced
charge-transfer properties of TCAQs with other classes of
conjugated polymers, such as polythiophenes, polyfluorenes is
suggested to overcome these phase incompatibility problems.
(23) Lehnert, W. Tetrtahedron Lett. 1970, 4723. Lehnert, W. Synthesis
1974, 667.
(24) Illescas, B.; Mart´ın, N.; Seoane, C. Tetrahedron Lett. 1997, 38,
2015.
(25) da Ros, T.; Prato, M.; Novello, F.; Maggini, M.; Banfi, E. J. Org.
Chem. 1996, 61, 9070.
(26) Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1993, 115,
9798.
(27) (a) Mart´ın, N.; Hanack, M. J. Chem. Soc., Chem. Comm. 1988,
1522; Mart´ın, N.; Behnisch, R.; Hanack, M. J. Org. Chem. 1989, 54, 2563.
(b) De la Cruz, P.; Mart´ın, N.; Miguel, F.; Seoane, C.; Albert, A.; Cano, F.
M.; Gonza´lez, A.; Pingarro´n, J. M. J. Org. Chem. 1992, 57, 6192. (c) Bando,
P.; Mart´ın, N.; Segura, J. L.; Seoane, C.; Ort´ı, E.; Viruela, P. M.; Viruela,
R.; Albert, A.; Cano, F. H. J Org. Chem. 1994, 59, 4618.
(28) Schubert, U.; Hu¨nig, S.; Aumu¨ller, A. Liebigs Ann. Chem. 1985,
1216.
Acknowledgment. This work is supported by the “Fonds
zur Fo¨rderung der wissenschaftlichen Forschung“ of Austria
(Project P-12680-CHE) and was also performed within the
Christian Doppler Foundations dedicated laboratory on Plastic
Solar Cells funded by the Austrian Ministry of Economic Affairs
and Quantum Solar Energy Linz GesmbH. The authors also
gratefully acknowledge the support of the European Community
(Joule III).
(29) (a) Aumu¨ller, A.; Hu¨nig, S. Liebigs Ann. Chem. 1984, 618, Kini,
A. M.; Cowan, D. O.; Gerson, F.; Mo¨ckel, R. J. Am. Chem. Soc. 1985,
107, 556. (b) Two one-electron waves at -0.46 and -0.65 V in acetonitrile
have been also reported for TCAQ. Yamaguchi, S.; Tatemitsu, H.; Sakata,
Y.; Misumi, S. Chem. Lett. 1983, 1229.
(30) (a) Gerson, F.; Heckendorn, R.; Cowan, D. O.; Kini, A. M.;
Maxfield, M. J. Am. Chem. Soc. 1983, 105, 7017. (b) Mart´ın, N.; Seoane,
C. In Handbook of Organic ConductiVe Molecules and Polymers; Nalwa,
H. S., Ed.; John Wiley & Sons Ltd.: New York, 1997; Vol. 1.
(31) (a) Suzuki, T.; Muruyama, Y.; Akasaba, T.; Ando, W.; Kobayashi,
K.; Nagase, S. J. Am. Chem. Soc. 1994, 116, 1359. (b) For a recent review,
see: Echegoyen, L.; Echegoyen, L. E. Acc. Chem. Res. 1998, 31, 593.
(32) Prato, M.; Maggini, M. Acc. Chem. Res. 1998, 31, 519.
(33) Kini, A. M.; Cowan, D. O.; Gerson, F.; Mo¨ckel, R. J. Am. Chem.
Soc. 1985, 107, 556.
(34) Dellepiane, G.; Cuniberti, C.; Comoretto, D.; Musso, G. F.; Figari,
G.; Piaggi, A.; Borghesi, A. Phys. ReV. B 1993, 48, 7850.
(35) Botta, C.; Luzzati, S.; Tubino, R.; Bradley, D. D. C.; Friend, R.
H. Phys. ReV. B 1993, 48, 14809.
(36) Cha, Y.-H.; Furukawa, Y.; Tasumi, M.; Noguchi, T.; Ohnishi, T.
Chem. Phys. Lett 1997, 273, 159.
(37) Fichou, D.; Horowitz, G.; Xu, B.; Garnier, F. Synth. Met. 1990,
39, 243.
(38) Botta, C.; Luzatti, S.; Tubino, R.; Borghesi, A. Phys. ReV. B 1992,
46, 13008.
(39) Ehrenfreund, E.; Vardeny, Z. V.; Brafman, O.; Horowitz, B. Phys.
ReV. B 1987, 36, 1535.
(40) Brabec, C. J.; Dyakonov, V.; Sariciftci, N. S.; Graupner, W.;
Leising, G.; Hummelen, J. C. J. Chem. Phys. 1998, 109, 1185.
(41) Tian, D. B.; Zerbi, G.; Mu¨llen, K. J. Chem. Phys. 1991, 95, 3198.
(42) Park, J. Y.; Heeger, A. J.; Srdanov, V. I. 2000. In press.
(43) Samuel, I. D. W.; Rumbels, G.; Friend, R. H. In Primary
Photoexcitations in Conjugated Polymers: Molecular Excitations Versus
Semiconductor Band Model; N. S. Sariciftci, N. S., Ed.; World Scientific
Publishing Company: River Edge, NJ, 1997.
References and Notes
(1) Janssen, R. A. J.; Moses, D.; Sariciftci, N. S. J. Chem. Phys. 1994,
101, 9519.
(2) (a) Kraabel, B.; Hummelen, J. C.; Vacar, D.; Moses, D.; Sariciftci,
N. S.; Heeger, A. J.; Wudl, F. J. Chem. Phys. 1996, 104, 4267. (b) Brabec,
C. B.; Zerza, G.; Sariciftci, N. S.; Cerullo, G.; De Silvestri, S. Nature. 2000.
Submitted for publication.
(3) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science
1992, 258, 1474.
(4) Janssen, R. A. J.; Christiaans, M. P. T.; Hare, C.; Mart´ın, N.;
Sariciftci, N. S.; Heeger, A. J.; Wudl, F. J. Chem. Phys. 1995, 103, 8840.
(5) Sariciftci, N. S.; Braun, D.; Zhang, C.; Srdanov, V.; Heeger, A. J.;
Stucky, G.; Wudl, F. Appl. Phys. Lett. 1993, 62, 585.
(6) Morita, S.; Zakhidov, A. A.; Yoshino, K. Jpn. J. Appl. Phys. 1993,
32, L1873.
(7) Sariciftci, N. S.; Heeger, A. J.; U.S. Patent 5,331,183, July 19, 1994.
(8) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science
1995, 270, 1789.
(9) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.;
Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498.
(10) Tada, K.; Hosoda, K.; Hirohata, M.; Hidayat, R.; Kawai, T.; Onoda,
M.; Teraguchi, M.; Masuda, T.; Zakhidov, A. A.; Yoshino, K. Synth. Met.
1997, 85, 1305.
(11) Tada, K.; Hidayat, R.; Hirohata, M.; Kawai, T.; Lee, S. B.;
Bakhadirov, I. U.; Zakhidov, A. A.; Yoshino, K. Synth. Met. 1997, 85,
1349.
(12) Wintgens, V.; Valat, P.; Garnier, F. J. Phys. Chem. 1994, 98, 229.
(13) Chosrovian, H.; Grebner, D.; Rentsch, S.; Naarman, H. Synth. Met.
1992, 52, 213.
(14) Lap, D. V.; Grebner, D.; Rentsch, S.; Naarman, H. Chem. Phys.
Lett. 1993, 211, 135.
(15) Chosrovian, H.; Rentsch, S.; Grebner, D.; Dahm, D. U.; Birckner,
E.; Naarman, H. Synth. Met. 1993, 60, 23.
(16) Charra, F.; Fichou, D.; Nunzi, J.-M.; Pfeffer, N. Chem. Phys. Lett.
1992, 192, 566.