Synthesis and characterization of a poly(1,3-dithienylisothianaphthene) derivative for bulk heterojunction photovoltaic cells

Article abstract of DOI:10.1021/jp013073t

The synthesis of a poly(1,3-dithienylisothianaphthene) (PDTITN) derivative obtained via oxidative polymerization of 5,6-dichloro-l,2-bis(3a?2-dodecylthienyl)isothianaphthene is described. PDTITN exhibits a band gap of 1.80-1.85 eV. The redox properties of PDTITN were characterized using cyclic voltammetry and spectroelectrochemisty. Photoexcitation of PDTITN results in photoluminescence (PL) in the near-IR region and the formation of a triplet state. In the presence of a methanofullerene (PCBM) as an electron acceptor, PL and triplet formation of PDTITN are quenched. In photovoltaic devices using blends of the polymer with PCBM, the observed incident-photon-to-collected-electron efficiency (IPCE) up to 24% at 400 nm and the 3 orders of magnitude increase of short circuit current as compared to the polymer alone prove the photoactivity of the PDTITN/PCBM blend in the device.

Full text of DOI:10.1021/jp013073t

11106
J. Phys. Chem. B 2001, 105, 11106-11113
Synthesis and Characterization of a Poly(1,3-dithienylisothianaphthene) Derivative for Bulk
Heterojunction Photovoltaic Cells
D. L. Vangeneugden and D. J. M. Vanderzande
Laboratory of Organic and Polymer Chemistry, Institute for Material Research,
DiVision Chemistry, UniVersity Campus, Limburg UniVersity, B-3590 Diepenbeek, Belgium
J. Salbeck†
FB 6 Chemistry-Geography, Electrochemistry and Optoelectronic Materials,
Gerhard-Mercator-UniVersity Duisburg, Lotharstr. 1, D-47048 Duisburg, Germany
P. A. van Hal and R. A. J. Janssen*
Laboratory of Macromolecular and Organic Chemistry, EindhoVen UniVersity of Technology,
P.O. Box. 513, 5600 MB EindhoVen, The Netherlands
J. C. Hummelen
Stratingh Institute and MSC, UniVersity of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
C. J. Brabec, S. E. Shaheen, and N. S. Sariciftci
Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler UniVersity Linz,
Altenbergerstr. 69, A-4040 Linz, Austria
ReceiVed: August 9, 2001
The synthesis of a poly(1,3-dithienylisothianaphthene) (PDTITN) derivative obtained via oxidative polym-
erization of 5,6-dichloro-1,2-bis(3′-dodecylthienyl)isothianaphthene is described. PDTITN exhibits a band
gap of 1.80-1.85 eV. The redox properties of PDTITN were characterized using cyclic voltammetry and
spectroelectrochemisty. Photoexcitation of PDTITN results in photoluminescence (PL) in the near-IR region
and the formation of a triplet state. In the presence of a methanofullerene (PCBM) as an electron acceptor,
PL and triplet formation of PDTITN are quenched. In photovoltaic devices using blends of the polymer with
PCBM, the observed incident-photon-to-collected-electron efficiency (IPCE) up to 24% at 400 nm and the 3
orders of magnitude increase of short circuit current as compared to the polymer alone prove the photoactivity
of the PDTITN/PCBM blend in the device.
Introduction
tovoltaic cells exhibit optical band gaps of Eg ) 1.9-2.2 eV.
Hence, the use of low band gap (Eg < 1.8 eV) polymers1
shall allow for an improved overlap of the polymer absorption
spectrum with the solar emission and an enhancement in photon
harvesting.
0,11
Composite films of semiconducting polymers as donors and
molecular electron acceptors such as fullerenes have been used
to construct “bulk-heterojunction” polymer photovoltaic cells.
1-5
The primary step in these devices is a fast photoinduced charge-
transfer reaction at the donor-acceptor interface, which results
in a metastable charge-separated state.6 The nanoscopic inter-
penetrating network of the two constituents in a bulk hetero-
junction photovoltaic cell ensures a large interfacial area and,
thus, an efficient charge generation. Photovoltaic power conver-
sion efficiencies (ηe) of 2.5% under AM1.5 illumination have
recently been reported for a bulk heterojunction solar cell
consisting of a poly(2,5-dialkoxy-1,4-phenylene vinylene) as a
Here we describe a novel conjugated polymer (PDTITN,
Scheme 1) with a poly(1,3-dithienylisothianaphthene) back-
bone. This polymer can be regarded as a regular copolymer
of poly(3-alkylthiophene) (P3AT) and poly(isothianaphthene)
(PITN). Although P3ATs have a band gap at 1.9-2.0 eV,
PITN possesses a considerably lower band gap at 1.1 eV.1
By combining the structural characteristics of P3AT and PITN
in the novel copolymer PDTITN, we have designed a ma-
terial that possesses an intermediate optical band gap (Eg )
∼1.8 eV).
-8
2-16
9
donor and a methanofullerene as acceptor. Further improvement
of these devices may be achieved by optimizing the absorption
of light and the selective transport of charges.
We have investigated the electrochemical and photophysical
properties of PDTITN as is and in combination with a meth-
At present, the mostly used materials such as substituted poly-
(p-phenylene vinylene)s and polythiophenes in polymer pho-
17
anofullerene derivative (PCBM, Chart 1) as an electron accep-
tor using photoinduced absorption spectroscopy (PIA). Both,
the high IPCE value (24%) and the 3 orders of magnitude in-
crease of short circuit current in the composite films prove the
*
To whom correspondence should be addressed.
FB 18 Physics, Macromolecular Chemistry and Molecular Materials,
†
University Kassel, Heinrich-Plett-Str. 40, D-34109 Kassel, Germany
1
0.1021/jp013073t CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/19/2001

Synthesis of PDTITN Derivative
J. Phys. Chem. B, Vol. 105, No. 45, 2001 11107
SCHEME 1
CHART 1
afforded the 2-iodo-3-dodecylthiophene in a yield of 74%.20
Ring closure of diketone 6 with Lawesson’s reagent resulted in
5,6-dichloro-1,3-bis(3′-dodecylthienyl)-isothianaphthene 7. The
polymerization of 7 was achieved by using anhydrous FeCl3 at
2
1,22
room temperature under nitrogen atmosphere.
Work up
involved Soxhlet extraction, using methanol and acetone to
remove oligomers and dissolving the residue in chloroform. The
chloroform solution was evaporated to dryness in vacuo, and
the material was considered as the soluble polymer fraction
(60%) which was used for further studies. The combined meth-
anol and acetone extracts were evaporated, redissolved in chloro-
form, and washed with diluted HCl (1 N) to remove the iron
salts. The organic layer was dried with MgSO4 and evaporated
to yield the oligomeric fraction (34%).
enhanced photoactivity of the PDTITN/PCBM blend inside the
device.
Characterization of PDTITN. GPC measurements, per-
formed in THF relative to narrow low molecular weight
polystyrene standards, indicated a limited molecular weight of
Mw ) 6511 g/mole and a polydispersity index of PDI ) 3.47.
For poly(3-alkylthiophene)s, obtained by oxidative polymeri-
zation of 3-alkylthiophenes with FeCl3, molecular weights
ranging from 30 000 to 300 000 g/mole with PDI from 1.3 to 5
Results
Synthesis of PDTITN. The synthesis of the poly(1,3-
dithienylisothianaphthene) (PDTITN) derivative is outlined in
Scheme 1. The synthesis of the monomer starts from 4,5-
dichlorophthalic acid 1, which is heated with two equivalents
of PCl5 to afford 4,5-dichlorophthalic acid chloride 2. Compound
2
3
have been reported. However, when bithiophene or ter-
thiophene derivatives are oxidatively polymerized, chemically
or electrochemically, considerable lower molecular weights are
2
was subsequently allowed to react with mercaptopyridine to
obtain 4,5-dichloro-1,2-di(S-(2-pyridyl))-benzenedithioate 3.
This dithioester is not stable in solution and slowly isomerizes
into 3,3-di(S-pyridyl)-3H-5,6-dichloro-isobenzofuran-1-one 4.
This isomerization process has been previously encountered for
2
1,23-25
obtained.
The reduced Mw is explained by the reduced
reactivity extended monomers compared with thiophenes, and
this because of the stabilization of the corresponding radical
18
10
related compounds. The isomerization is considerably acceler-
ated upon heating and when an excess of mercaptopyridine is
present, 4 is further transformed into 3-(S-pyridyl)-3H-5,6-
dichloroisobenzofuran-1-one 5 and pyridyl disulfide through a
radical scavenging process. The isomerization can be limited
by adding 2 to a cold solution of mercaptopyridine and trieth-
ylamine (TEA) in THF and quenching the reaction immediately
with diluted aqueous HCl (1 M). Because the lactones 4 and 5
are soluble in diethyl ether but the dithioester 3 is not, the desired
product can be separated from the side products by recrystal-
lization from dichloromethane/diethyl ether. The yield of this
reaction is about 80%. 4,5-Dichloro-1,2-bis(3′-dodecylthienoyl)-
benzene 6 was obtained using the Grignard reagent of 2-iodo-
cations involved in the polymerization. Side chain effects can
play an important role in this context. Electron withdrawing
groups will generally increase the reactivity, whereas electron-
donating groups will stabilize the radical cations. For the latter,
this causes a lowering of the monomer and oligomer reactivities
and hence results in a decrease of the molecular weight of the
2
6-29
corresponding polymers.
The thermal stability of the polymer was investigated by
thermogravimetric analysis (TGA). Measurements were per-
formed under a constant argon flow with a heating rate of 10
°C/min ranging from room temperature to 900 °C. The decomp-
osition temperatures, as determined from the maximum of the
differential thermogram (TDTG) and as the temperature at which
5% of the original weight was lost (T5%), are 442 and 338 °C,
respectively.
3-dodecylthiophene in a yield of 66%. The 3-dodecylthiophene
used in this reaction was synthesized via Grignard coupling
reaction of 3-bromothiophene with dodecylmagnesium bromide
One of the difficulties of the FeCl3 polymerization is the re-
1
9
30
by NiCl2(dppp), analogously to literature procedures. Subse-
quent iodination with iodine and mercuric oxide in benzene
moval of the iron salts after polymerization. Often large
excesses of oxidant are needed, resulting in substantial amounts

11108 J. Phys. Chem. B, Vol. 105, No. 45, 2001
Vangeneugden et al.
Figure 1. Absorption spectrum of PDTITN on quartz.
Figure 3. Spectroelectrochemical measurements of PDTITN on ITO
in acetonitrile (0.1 M TBAH). Gradual reduction from +600 to 0 mV
+
vs Fc/Fc .
Figure 4. PIA and PL spectra of PDTITN on quartz. Data were
recorded at 80 K by excitation at 2.54 eV (488 nm, 25 mW) and a
modulation frequency of ω ) 275 Hz.
Figure 2. Cyclic voltammogram (five consecutive cycles, 10 mV/s)
of PDTITN on Pt in acetonitrile (0.1 M TBAH). Potential versus
+
Fc/Fc .
TABLE 1: Results of Fitting Intensity Dependence (-∆T ∝
r
-x
I ) and Frequency Dependence (-∆T ∝ ω , Figure 5) for
of iron salts to be removed to obtain pure polymeric material.
Soxhlet extraction with MeOH removes most of the doping by
iron salts but small amounts may remain present in the
PIA Bands of PDTITN
position (eV)
R
x
position (eV)
R
x
0
1
1.4
.4
.2
0.6
0.8
0.8
-0.4
-0.1
-0.1
1.8
2.1
0.7
0.8
-0.2
-0.3
2
3
material. Atomic absorption spectroscopy (AAS) measure-
ments indicated that the residual iron content is highest for the
insoluble polymer fraction (0.1-3%), whereas lower amounts
are detected for the oligomeric and soluble polymer fractions.
For the present polymer, the residual iron content is 0.10%.
The absorption spectra of thin films of PDTITN differ slightly
from film to film. The onset of the absorption is at about
The absorption spectra recorded for a thin film of PDTITN
on quartz after chemical doping using nitrosonium tetrafluo-
roborate agree well with those observed by spectroelectrochem-
istry under high oxidation conditions.
6
4
70-690 nm (1.80-1.85 eV), with a broad maximum at about
58-468 nm (2.65-2.71 eV; Figure 1).
Photoexcited Spectroscopy. The CW-modulated photoin-
duced absorption (PIA) spectrum of a thin film of PDTITN
(Figure 4) at 80 K reveals a number of optical signatures. A
small peak is found at 0.3 eV, and a much stronger broad peak
is present starting at 1.2 eV and extending to 1.8 eV. Bleaching
transitions are found at 2.0 (shoulder) and 2.1 eV (peak). In
addition to the PIA, there is a weak, but clear, signature of
photoluminescence (PL). The PL peaks are at 1.67 and 1.53
eV. The excitation pump intensity dependence of the PIA peaks
Cyclic Voltammetry and Spectroelectrochemistry. For
cyclic voltammetry (CV), a thin film of PDTITN was deposited
on the Pt working electrode by dip coating. Figure 2 displays
the cyclic voltammograms of five consecutive scans, whereas
no further changes in the CV occurred after the second scan.
The optical band gap (1.80-1.85 eV) as observed from the onset
in Figure 1 does not match the difference of oxidation and
reduction potentials determined form electrochemistry, even for
the onset values (0.45 + 1.93 ) 2.38 eV).
R
follows a power law (-∆T ∝ I ) with exponents R considerably
higher than 0.5 (Table 1), indicating monomolecular decay
processes (-∆T ∝ I) dominate the relaxation. The modulation
frequency dependences displayed in Figure 5 are numerically
31-36
By means of spectroelectrochemical (SPEL) measurements,
the spectral changes upon electrochemical doping of the
PDTITN films were investigated. Spectra recorded at intermedi-
ate doping levels are gathered in Figure 3. At high doping levels,
two broad near-IR bands at 923 (1.34) and 1460 nm (0.85 eV)
are dominating. These in the gap absorptions are generally
indicative for polaron/bipolaron type relaxations of the polymer
electronic structure upon doping.
-
x
summarized in Table 1 by fitting a power-law (-∆T ∝ ω ) to
the experimental curves.
Photoexcited spectroscopy on a composite film of PDTITN
and PCBM (1:1 by weight) measured under the same conditions
reveals that the spectrum of the blend is completely different
from that of the individual compounds (Figure 6), whereas the

Synthesis of PDTITN Derivative
J. Phys. Chem. B, Vol. 105, No. 45, 2001 11109
Figure 5. Semilogarithmic plot of the PIA signals of PDTITN as a
function of the modulation frequency ω. Data were recorded at 80 K
by excitation at 2.54 eV (488 nm, 25 mW). Solid lines represent fits to
-
x
a power law (-∆T ∝ ω , Table 1).
Figure 8. Semilogarithmic plot of the I/V characteristics of three
devices under illumination with 80 mW/cm from a solar simulator.
2
Pristine devices from PDTITN (circles) were spun from a PMMA/
PDTITN (2:3) solution to a thickness of ∼90 nm, whereas PCBM
(triangles) was spun directly from solution to a comparable thickness.
The thickness of the device from PMMA/PDTITN/PCBM (1:2:6)
(
squares) was ∼100 nm as determined by alpha stepper measurements.
The measurement temperature was 50 °C.
TABLE 3: Photovoltaic Performance of Devices of PMMA/
a
PDTITN (2:3), PCBM, and PMMA/PDTITN/PCBM (1:2:6)
V
mV)
oc
I
sc
FF
(%)
AM1.5 η
e
2
(
(µA/cm )
(%)
PDTITN
PCBM
PDTITN/PCBM
360
565
880
21
77
1128
24
38
25
0.0023
0.021
0.31
a
Figure 6. PIA spectrum of PDTITN/PCBM (1:1 w/w) composite film
on quartz. Data were recorded at 80 K by excitation at 2.54 eV (488
nm, 25 mW) and a modulation frequency of ω ) 275 Hz.
Data were recorded under illumination with a solar simulator with
2
80 mW/cm .
poly(methyl methacrylate) (PMMA) as a host matrix to improve
the film forming properties. Figure 8 shows the I/V curve of a
solar cell with an active layer consisting of a spin cast composite
film of PMMA/PDTITN/PCBM (1:2:6 w/w/w) compared to the
performance of devices from pristine PCBM and from PMMA/
PDTITN (2:3 w/w). The photovoltaic characteristics of the
devices (energy conversion efficiency ηe, fill factor FF, and
incident photon to current efficiency IPCE) are given in Table
3
. The device parameters were calculated using the following
equations:
Figure 7. Schematic layout of the photodiodes used.
2
V [V]I [A/cm ]FF
oc
sc
r
η
e
[%] )
(1)
(2)
(3)
TABLE 2: Results of Intensity Dependence (-∆T ∝ I ) PIA
for Bands of PDTITN/PCBM
2
P [W/cm ]
in
position (eV)
R
position (eV)
R
V I
p p
0
1
.5
.5
0.5
0.7
1.8
2.3
0.8
0.8
FF )
V I
oc sc
photoluminescence is almost completely quenched. The ob-
served excitation intensity dependence of the PIA band scan
be fitted to power law, and the results are compiled in Table 2.
The photoexcitation pump intensity dependence as well as the
observed lifetime are different for 0.5 eV band. The 0.5 eV
peak corresponds to a long-lived photoexcitation with bimo-
lecular relaxation, whereas the other bands in the PIA spectrum
correspond to shorter-lived species.
Photovoltaic Cells. Photovoltaic cells consisted of an active
layer sandwiched between an ITO front electrode covered with
a PEDOT:PSS conducting polymer layer and an Al back
electrode (Figure 7). For devices containing PDTITN, we used
2
1
240I [µA/cm ]
sc
IPCE [%] )
2
λ[nm]I[W/m ]
in which Voc is the open circuit voltage, Isc is the short-circuit
current, λ is the excitation wavelength, I is the monochromatic
light intensity, Pin is the intensity of AM1.5 light from a solar
simulator, and Vp and Ip are the voltage and current at the
maximum power point of the I/V curve.
Devices containing PDTITN showed a relative low fill factor
(FF ) 0.25) and rather low or even negligible rectification. This
was also found for the PMMA/PDTITN device. Conversely,

11110 J. Phys. Chem. B, Vol. 105, No. 45, 2001
Vangeneugden et al.
Figure 10. Schematic drawing of the potentials inside a donor/acceptor
bulk heterojunction solar cell. EFD and EFA are the quasi-Fermi levels
Figure 9. (a) Percentage of absorbed photons in thin films spun from
solutions of PMMA/PDTITN (2:3) and PMMA/PDTITN/PCBM (1:2:
of the donor and the acceptor, respectively; E
C V
and E are the
6
), respectively. Films were spun from the same solutions as the
conduction and the valence band of the single materials.
corresponding devices characterized in Figure 8 on cleaned glass slides,
and absorption measurements were performed in transmission geometry.
possibilities to explain these differences: either electron transfer
from the polymer to the methanofullerene or, alternatively,
energy transfer. What is the nature of the peak at 1.5 eV in the
PDTITN/PCBM film? One possibility is to assign this band to
a triplet-triplet absorption of the PCBM. In solution, the triplet
spectrum of PCBM gives a triplet-triplet absorption at 1.7 eV
(b) Incident photons to converted electrons efficiency (IPCE) of a
PMMA/PDTITN/PCBM (1:2:6) device, recorded at room temperature.
the PCBM diode shows a higher fill factor (FF ) 0.38) and
more than 3 orders of rectification at (2 V. Additionally, the
electron injection at forward bias (+2 V) is more than 1 order
of magnitude higher compared to that of the PMMA/PDTITN/
PCBM device and more than 3 orders of magnitude higher
compared to that of the PMMA/PDTITN device.
3
9
with a shoulder at 1.5 eV. The lowest neutral excited state in
a PDTITN/PCBM blend is probably the triplet state of PCBM,
estimated to be at about E* ) 1.50-1.60 eV above the ground
state, based on comparison with C60 and N-methyl-fullero-
Mixing PCBM with PDTITN increases the short circuit
current of the device compared to the value for the PMMA/
PDTITN device by 1 order of magnitude. The overall power
conversion efficiency of the PMMA/PDTITN/PCBM device was
measured to be 0.3% under AM1.5 simulated light with 80 mW/
4
0-42
pyrrolidine.
The reduction potential (AA) of PCBM in
+
dichloromethane is -1.0 V against Fc/Fc . With the lowest-
energy excited state at E* ) 1.50-1.60 eV, the oxidation
potential should be less than approximately 1.55 - 1.0 ) +0.55
+
2
eV against Fc/Fc . From the oxidation potential measured for
cm , whereas the pristine PCBM and the PMMA/PDTITN
PDTITN, the onset value that is found is +0.56 V in the first
cycle and +0.45 V in subsequent cycles, i.e., just on the
borderline. This simple calculation, not taking into account the
Coulombic effects, suggests that the charge-transferred state will
not be stabilized energetically and may efficiently relax by a
fast back transfer.
In the solar cell devices, the situation is different. Upon
applying a top and a bottom metal contact with different work
functions, as it is the case for the photovoltaic devices, an
internal electrical field may be generated. An upper limit for
this field inside the two-component system is given by the built-
in potential of the system divided by the thickness of the active
layer. For photovoltaic devices, the built-in potential may be
estimated from the open circuit voltage (i.e., the difference of
the quasi-Fermi levels). Although this approximation is only
valid for low temperatures, the error is usually small (i.e.,
<10%). By assuming further that the field distribution is
homogeneous and by neglecting interface effects (non rectifying
metal contacts in the sense of a space charge region), the internal
devices show 0.021 and 0.0023% efficiency, respectively.
Figure 9a shows the absorption spectrum of spin cast films
from the polymer and the polymer/methanofullerene embedded
in PMMA. Films were cast on glass from the same solutions
as those used for device production. In Figure 9b, the incident
photon to converted electron efficiency for the PMMA/PDTITN/
37
PCBM device is plotted.
The highest IPCE value, almost 24%, was observed in the
high-energy region around 400 nm. The monochromatic quan-
tum efficiency of the device at 400 nm is calculated with 5.3%,
assuming a Voc of 880 mV, a FF of 0.25, and an IPCE of 24%.
Discussion
Although it is clear from the PIA experiments described above
that there are major differences between the photoexcitations
generated in pure PDTITN and those generated in the PDTITN/
PCBM blend, the actual interpretation is less straightforward.
The PIA spectrum of the pure PDTITN has strong similarities
to the PIA spectrum of poly(3-hexylthiophene).38 The most
likely explanation is that the strong transition in the region from
electrical field (E ) in the device consisting of an PFTITN/
bi
PCBM active layer, embedded between ITO/PEDOT and Al
electrodes, may be estimated by the following equation:
1
.1 to 1.9 eV corresponds to a triplet-triplet absorption. The
lifetime of this photoexcited triplet state is less than 100 µs.
The intensity exponents (R ) 0.8) for the most prominent peaks
(1.2 and 1.4 eV) are fully consistent with this assignment. A
E ) V /d
(4)
bi
oc
considerable quenching of the PDTITN fluorescence occurs in
the PDTITN/PCBM blend, and the PIA spectrum has a blue
shifted maximum at 1.5 eV instead of 1.2 eV compared to that
of the pure PDTITN. In principle, one could envisage two
where d is the thickness of the active layer. A schematic drawing
of the relevant potentials is given in Figure 10. From Figure
10, it is also clear that the region over which the electrical field
is developed may be smaller than the device thickness. With

Synthesis of PDTITN Derivative
J. Phys. Chem. B, Vol. 105, No. 45, 2001 11111
an Voc of 880 mV and a thickness of 100 nm, the internal field
reference electrode connected to an EG&G Princeton poten-
tiostat/galvanostat model 273. The measurements were per-
formed under nitrogen in dried and degassed acetonitrile and
dichloromethane solutions. Tetrabutylammonium hexafluoro-
phosphate (TBAH) was used as inert electrolyte in a concentra-
tion of 0.1 M. After each measurement, an internal calibration
5
for the PDTITN/PCBM devices is estimated by 8.8 × 10 V/cm.
Fields of this strength are well-known to trigger photoinduced
charge transfer reactions which would be otherwise thermody-
4
3,44
namically limited.
The occurrence of photoinduced charge transfer between
PDTITN and PCBM inside the photovoltaic devices is evidenced
by the huge increase in short circuit current for this device
compared to the devices with single-component active layers
as denoted in Table 3. Further evidence for photoinduced charge
transfer comes from evaluation of the spectrally resolved
photocurrent. Both the high IPCE value (24%) as well as the
shape of the IPCE curve versus wavelength prove the photo-
activity of the two components inside the device.
+
versus the Fc/Fc couple was performed, and all potentials are
+
given relative to Fc/Fc . Cyclic voltammograms of the polymer
were taken from films deposited on the surface of the working
electrode by means of dip coating from a dichloromethane
solution. These measurements were performed in an acetonitrile
electrolyte solution, generally at a scan rate of 10 mV/s.
Spectroelectrochemistry. A thin-layer spectroelectrochemi-
4
5
cal cell was used. The thin-layer cell contained a transparent
ITO working electrode, an Ag/AgCl pseudo-reference electrode,
and a gold-coated stainless steel counter electrode. As for the
Experimental Section
+
cyclic voltammetry, all potentials are given relative to Fc/Fc .
General Remarks and Instrumentation. Unless stated
otherwise, commercial products were of analytical grade and
used without further purification. Solvents were dried and
distilled prior to use. THF and Et2O were distilled over sodium/
benzophenone, and CHCl3, CH2Cl2, and DMSO were dried over
molecular sieves (4 Å) before distillation. Reactions were carried
out under inert (N2) atmosphere. Column chromatography was
carried out over silica gel (Merck Kieselgel 60) using eluents
The electrodes were connected to an EG&G Princeton scanning
potentiostat model 362, and spectra were recorded with a Perkin-
Elmer Lambda 9 UV-Vis-near-IR spectrophotometer. Mea-
surements were performed on thin polymer films prepared by
drop coating on the ITO working electrode. 0.1 M TBAH
acetonitrile was used as electrolyte solution.
Chemical Doping of Films. Films of PDTITN were drop-
cast from o-dichlorobenzene on quartz and subsequently dip-
doped (oxidized) in acetonitrile containing nitrosonium tetra-
fluoroborate.
1
indicated. H NMR spectra were recorded in CDCl3 at 400 MHz
on a VARIAN Inova spectrometer using a 5 mm probe.
Chemical shifts (δ) in ppm are reported relative to tetrameth-
ylsilane (TMS) and were determined relative to the residual
nondeuterated solvent absorption (CHCl3). The chemical shift
of CHCl3 is, according to convention, situated at 7.24 ppm
Photoinduced Absorption Spectroscopy. PIA spectra were
recorded between 0.25 and 3.5 eV by exciting with a mechani-
cally modulated cw Spectra Physics Ar-ion laser (488 nm, 275
Hz) pump beam and monitoring the resulting change in
transmission of a tungsten-halogen probe light through the
sample (∆T) with a phase sensitive lock-in amplifier after
dispersion by a triple grating Oriel 257 monochromator and
detection, using Si, InGaAs, and cooled InSb detectors. The
pump power incident on the sample was 25 mW with a beam
diameter of 2 mm. The photoinduced absorption, -∆T/T ≈
∆Rd, was directly calculated from the change in transmission
after correction for the photoluminescence, which was recorded
in a separate experiment. Photoinduced absorption spectra and
photoluminescence spectra were recorded with the pump beam
in an almost parallel direction to the probe beam. Samples were
held at 80 K in an inert nitrogen atmosphere using an Oxford
Optistat continuous flow cryostat; during measurements, the
temperature was kept constant within (0.1 K. Films for PIA
were prepared as follows. PDTITN was prepared by drop casting
from o-dichlorobenzene solution onto a quartz substrate, which
was preheated to 100 °C to enhance evaporation of the solvent.
The resulting film had an optical density varying from 0.8 to
1.2 over the film. A film of PCBM was prepared by drop casting
from o-dichlorobenzene on quartz. The film had an optical
density of OD ) 0.8 at 488 nm. A small peak at 711 nm (1.74
eV), characteristic for PCBM, marks the onset of the absorption
spectrum. A composite film of PDTITN and PCBM 1:1 by
weight was prepared from an o-dichlorobenzene solution by
drop casting. The optical density varied between 2.0 and 2.7
over the film. The onset of the absorption is at about 705 nm
(∼1.76 eV, a PCBM absorption peak), with a maximum at about
460 nm (2.70 eV, a PDTITN absorption peak).
13
relative to tetramethylsilane (TMS). The C NMR experiments
were recorded at 100 MHz on the same spectrometer using a 5
mm broadband probe. Chemical shifts were determined relative
13
to the C resonance shift of CHCl3 (77.0 ppm relative to TMS).
Chemical shifts could be assigned on the basis of the shift
values, coupling patterns, integration, attached proton test (ATP),
1
1
and two-dimensional techniques as COSY ( H- H), HETCOR
1
13
1
3
(
H- C) optimized for JCH ) 8 Hz (direct) and JCH )
1
3
13
1
40 Hz (long range), and INADEQUATE ( C- C) opti-
1
mized for JCC ) 48 Hz. Direct insert probe mass spectrometry
DIP-MS) analyses were carried out on a Finnigan TSQ 70.
(
Analyses were carried out in the electron impact mode (EI,
electron energy of 70 eV) and the chemical ionization mode
(CI, 3000-4000 mTorr i-butane). GC-MS measurements (Gas
Chromatography Mass Spectrometry) were performed using a
Thermo Quest Voyager GC/MS apparatus. Fourier transform
infra red spectroscopy (FT-IR) was performed on a Perkin-Elmer
1
600 FT-IR apparatus. Melting points were determined on an
Electrothermal 9100. Gel permeation chromatography (GPC)
measurements were performed on a Spectra Physics Spectra
Series P100 equipped with a PLgel 5 µ column (Polymer Labs)
and a refractive index RI-71 detector (Shodex). Molecular
weights were determined in THF solution relative to narrow
distribution low molecular weight polystyrene standards (Poly-
mer Labs). Thermogravimetric analysis (TGA) was performed
on a TA Instrument 951 thermogravimetric analyzer under a
continuous argon-flow of 80 mL/min. Residual iron contents
were determined by atomic absorption spectroscopy (AAS) on
a Perkin-Elmer 1100 B flame atomic absorption spectropho-
tometer. UV-vis-near-IR spectra were recorded on a Perkin-
Elmer Lambda 900 spectrophotometer.
Production and Measurement of Photovoltaic Devices.
ITO/glass substrates were cleaned in ultrasonic baths of acetone,
methanol, and 2-propanol, followed by oxygen plasma treatment.
Poly(ethylenedioxythiophene) doped with poly(styrenesulfonic
acid) (PEDOT:PSS, Bayer AG) was spin-coated to a thickness
Cyclic Voltammetry. The electrochemical setup consisted
of a Metrohm cell equipped with a Pt disk working electrode,
a glassy carbon counter electrode and a Ag/AgCl or SCE

1
1112 J. Phys. Chem. B, Vol. 105, No. 45, 2001
Vangeneugden et al.
of 100 nm on top of the ITO from an aqueous solution. For the
solar cells reported here, poly(methyl methacrylate) (PMMA)
was used as host matrix because the molar mass of the PDTITN
was too low to form homogeneous pinhole free thin films by
solution casting. The weight ratio of the photoactive layer de-
posited by spin casting was PMMA:PDTITN:PCBM ) 1:2:6.
This weight ratio of 1:3 between the donor and the ac-
ceptor was recently found to be favorable for bulk heterojunction
solar cells based on conjugated polymer/fullerene compos-
3-Dodecylthiophene. Dodecylbromide (38.9 g, 0.156 mol)
in diethyl ether (100 mL) is added dropwise to Mg (3.8 g, 0.156
mol) in Et2O (50 mL), activated with a trace of iodine. After
addition, the reaction mixture is refluxed for about 2 h. The
obtained Grignard reagent is allowed to cool and subsequently
added to a cooled mixture (0 °C) of 3-bromothiophene (17.0 g,
0.104 mol) and NiCl (dppp) (70 mg, 1.2 mmol) in Et O (100
2
2
mL). After stirring overnight at room temperature, the reaction
is worked up by the slow addition of water (100 mL). The crude
product is extracted with Et O and dried over MgSO . Purifica-
4
6,47
ites.
Reference devices with pristine polymer and PCBM
2
4
active layers were also prepared. Devices from PDTITN alone,
processed by spin casting, did not form homogeneous and
continuous films and were consequently short-circuited. Blend-
ing PMMA into PDTITN improved the film quality resulting
in a photovoltaic response. However, it cannot be excluded that
the performance of the PMMA/PDTITN device is still limited
by poor film quality. Devices were spin cast from o-dichlo-
robenzene with an average film thickness of ∼90-100 nm as
determined by alpha-stepper measurements. Device fabrication
was performed in a drybox under an argon atmosphere. The Al
top electrode was evaporated thermally with a thickness of ∼80
nm. A schematic layout of the devices is given in Figure 7.
I/V curves were recorded by a Keithley 2400 source meter
tion is performed by means of column chromatography (silica
gel, n-hexane). Yield 20.2 g (77%); IR (NaCl, ν, cm ) 2923,
-
1
2852, 1536, 1465, 1409, 1377, 1152, 1078, 858, 835, 769; MS
+
(EI, m/e) 252 (M ), 111, 98 (-C H ), 97 (-C H ), 85, 55;
1
1
22
11 23
1
H NMR 7.23 (dd, 1H), 6.94 (d, 1H), 6.92 (d, 1H), 2.65 (t,
2H), 1.65 (p, 2H), 1.30 (m, 18H), 0.92 (t, 3H).
-Iodo-3-dodecylthiophene. 3-Dodecylthiophene (14.9 g,
8.5 mmol) and I2 (22.3 g, 87.8 mmol) are dissolved in benzene
2
5
(60 mL) and cooled to 5 °C. Under continuous stirring, HgO
(19.0 g, 87.8 mmol) is added in small portions over about 1 h
until the dark brown-red color disappears and the mixture turns
orange. The formed HgI is filtered off on a Buchner and washed
with Et2O (3 × 20 mL). The combined filtrates are washed with
a Na2S2O3 solution (O.65 N, 2 × 100 mL) and subsequently
with brine (100 mL). After drying over MgSO4, the crude
in inert atmosphere at room temperature under illumination with
2
8
0 mW/cm white light from a Steuernagel solar simulator. No
corrections were made for the spectral mismatch between the
solar simulator and the true AM1.5 spectrum. The measurement
temperature was ∼50 °C. Spectrally resolved photocurrent
measurements were recorded by the lock-in technique, il-
reaction product is purified by means of column chromatography
-1
(silica gel, n-hexane). Yield 16.4 g (74%); IR (KBr, ν, cm )
3
1
2
4
2
103, 3070, 2952, 2914, 2848, 2729, 2676, 1529, 1465, 1397,
375, 1225, 966, 828, 716, 684, 635; MS (EI, m/e) 378 (M ),
+
2
luminating the device with ∼0.2 mW/cm monochromatized
51 (-I), 223 (-C11H23), 137, 111, 97 (-I, -C11H22), 71, 57,
light with a fwhm of ∼4 nm from a Xe arc lamp and white
light background illumination in inert atmosphere. No noticeable
degradation of the devices was observed during the measurement
cycles. Light intensities were measured by a calibrated Si
photodiode.
1
3; H NMR 7.36 (d, 1H), 6.74 (d, 1H), 2.53 (t, 2H), 1.56 (p,
H), 1.26 (m, 18H), 0.88 (t, 3H).
4
,5-Dichloro-1,2-bis(3′-dodecylthienoyl)benzene (6). A so-
lution of 2-iodo-3-dodecylthiophene (16.3 g, 43.1 mmol) in THF
20 mL) is slowly added to Mg (1.1 g, 45.3 mmol) in THF (30
(
mL), activated with a trace of iodine. After 2 h of reflux, the
obtained Grignard reagent is added to a solution of dithioester
Synthesis
3
(7.9 g, 18.7 mmol) in THF (100 mL). The mixture is stirred
4
,5-Dichlorophthalic Acid Chloride (2). Powdered 4,5-
for 30 min after which HCl (10%, 100 mL) is added. Further
work up comprises extraction with Et2O, washing of the
combined extracts with NaOH (10%), NaHCO3 (1 M) and H2O
and drying over MgSO4. The crude product is finally purified
by column chromatography (silica, CHCl3/n-hexane 9/1) and 6
is obtained as a slightly yellow oil. Yield 8.7 g (66%); IR (KBr,
dichlorophthalic acid (20 g, 0.085 mol) and PCl5 (35.5 g, 0.17
mol) are mixed in a reaction flask with a glass rod before heating
to 160 °C. At a temperature of about 60 °C, the reaction starts
quite vigorously and HCl is produced (CAUTION!). The
evolving HCl vapor is directed through a basic solution to
neutralize the acid. After 2 h of heating under reflux and
vigorous stirring, the mixture is allowed to cool. Vacuum
distillation gives 2 as a colorless liquid which crystallizes at
room temperature. Yield 20.36 g (88%); mp 34 °C; bp 160 °C
-1
ν, cm ) 3105, 3077, 3052, 2956, 2921, 2851, 1645, 1579, 1540,
1
521, 1409, 1379, 1284, 1263, 1239, 1133, 970, 846, 721; MS
+
(
EI, m/s) 702 (M ), 451 (-DodTh), 309, 296, 283, 251
+
1
-
1
(DodTh ), 201, 137, 111, 97 (ThCH +), 71, 57, 41; H NMR
(0.9 mbar); IR (KBr, ν, cm ) 3097, 3077, 3021, 1785, 1768,
2
7
.73 (s, 2H), 7.45 (d, 2H), 6.98 (d, 2H), 2.80 (t, 4H), 1.52 (p,
1
735, 1581, 1535, 1460, 1216, 961, 791; MS (EI, m/e) 270
+
4H), 1.23 (m, 36H), 0.86 (t, 6H).
(M ), 235 (-Cl), 207 (-COCl), 172 (-Cl, -COCl), 144
1
(-2COCl), 109, 74; H NMR 7.92 (s, 2H).
5,6-Dichloro-1,2-bis(3′-dodecylthienyl)isothianaphthene (7).
A mixture of 4,5-dichloro-1,2-bis(3′-dodecylthienoyl)-benzene
6 (8.45 g, 12.0 mmol) and Lawesson’s reagent (LR) (5.45 g,
13.5 mmol) in CH2Cl2 (200 mL) is stirred at 50 °C. After an
hour, the solvent is evaporated and ethanol (200 mL) is added.
This mixture is refluxed again for 30 min. Subsequently, H2O
(300 mL) is added and the product extracted with CHCl3 (3 ×
100 mL). The combined fractions are washed with NaOH (10%)
and water until neutral. After drying over MgSO4, the solvent
is evaporated and the crude product is purified by column
chromatography (silica, CHCl3). Finally, the product is dissolved
in CHCl3 and precipitated by adding an excess of n-hexane.
After filtration the product is obtained as an orange powder.
4,5-Dichloro-1,2-di(S-(2-pyridinyl))benzenedithioate (3). A
solution of TEA (5 mL) and 2-mercaptopyridine (3.1 g, 27.9
mmol) and THF (50 mL) is stirred for 15 min at 0 °C. To this,
a solution of 2 (3.8 g, 13.9 mmol) in THF (20 mL) is added,
and the reaction is immediately worked up by the addition of
HCl (1%, 200 mL). After extraction with CHCl3, the combined
fractions are washed with NaOH (10%), NaHCO3 (1 M), and
H2O until neutral. Finally, 3 is obtained after drying over MgSO4
and crystallization from CH2Cl2/diethyl ether. Yield 4.7 g (79%);
-
1
mp 119 °C; IR (KBr, ν, cm ) 3077, 3050, 1694, 1673, 1573,
1
4
561, 1539, 1449, 1419, 1212, 961, 808, 772; MS (EI, m/e)
20 (M ), 310 (-SPy), 282 (-COSPy), 172, 144, 109, 78; H
+
1
-1
NMR 8.60 (dq, 2H), 7.93 (s, 2H), 7.72 (m, 4H), 7.29 (m, 2H).
Yield 8.01 g (95%); IR (KBr, ν, cm ) 3105, 3064, 2923, 2852,

Synthesis of PDTITN Derivative
J. Phys. Chem. B, Vol. 105, No. 45, 2001 11113
1
592, 1461, 1436, 1367, 1309, 1099, 993, 871, 834, 720; MS
(9) Shaheen, S. E.; Brabec, J. C.; Padinger, F.; Fromherz, T.;
Hummelen, J. C.; Sariciftci, N. S. Appl. Phys. Lett. 2001, 78, 841.
10) Roncali, J. Chem. ReV. 1997, 97, 173.
11) Pomerantz, M. In Handbook of Conducting Polymers, 2nd ed.;
Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds.; Marcel Dekker
Inc.: New York, 1998; p 277.
+
(
1
EI, m/s) 702 (M ), 547 (-C11H23), 511, 357, 321, 297, 261,
(
(
1
37, 111, 97, 69, 57; H NMR 7.73 (s, 2H, H4), 7.40 (d, 2H,
H5′), 7.07 (d, 2H, H4′), 2.64 (t, 4H, H1′′), 1.60 (p, 4H,
1
3
H2′′), 1.22 (m, 36H, H3′′-H11′′), 0.88 (t, 6H, H12′′);
NMR 142.85 (C3′), 135.02 (C3a), 129.45 (C4′), 129.41 (C5),
26.59 (C2′), 126.49 (C3), 126.40 (C5′), 121.69 (C4), 31.92
C
(
12) Wudl, F.; Kobayashi, M.; Heeger, A. J. J. Org. Chem. 1984, 49,
382.
13) Hoogmartens, I.; Adriaensens, P.; Vanderzande, D.; Gelan, J.;
Quattrocchi, C.; Lazzaroni, R.; Br e´ das, J. L. Macromolecules 1992, 25,
347.
14) Hoogmartens, I.; Adriaensens, P.; Carleer, R.; Vanderzande, D.;
Martens, H.; Gelan, J. Synth. Met. 1992, 51, 219.
(15) Kiebooms, R.; Hoogmartens, I.; Adriaensens, P.; Vanderzande, D.;
Gelan, J. Macromolecules 1995, 28, 4961.
16) Zerbi, G.; Magnoni, M. C.; Hoogmartens, I.; Kiebooms, R.; Carleer,
R. Vanderzande, D.; Gelan, J. AdV. Mater. 1995, 7, 1027.
17) Hummelen, J. C.; Knight, B. W.; LePeq, F.; Wudl, F.; Yao, J.;
Wilkins, C. L. J. Org. Chem. 1995, 60, 532.
18) Kiebooms, R. H. L.; Adriaensens, P. J. A.; Vanderzande, D. J. M.;
Gelan, J. M. J. V. J. Org. Chem. 1997, 62, 1473.
19) Van Pham, C.; Mark, H. B.; Zimmer, H. Synth. Commun. 1986,
3
1
(
(
(
C10′′), 30.89 (C2′′), 29.67, 26.64, 29.56, 29.38 (C5′′), 29.36
C9′′), 29.32 (C3′′), 29.00 (H1′′), 22.69 (C11′′), 14.13 (C12′′);
7
(
UV-vis (λmax (nm), ꢀ) 283 (15 784), 405 (8395); fluorescence
λmax (nm)) 531.
Polymerization.2 Polymerization is performed by adding
(
1,22
(
five equivalents of powdery anhydrous FeCl3 to a 0.07 M
chloroform solution of the monomer. The reaction is carried
out at room temperature and under nitrogen atmosphere, over
(
(
1
2 h. Work up consists of diluting the crude reaction mixture
with chloroform after which methanol is added to precipitate
the polymer. The product is collected in a Soxhlet extraction
case by filtration and extracted with methanol and acetone, with
each solvent until the extracts are completely colorless. The
residue is dissolved in chloroform by stirring at room temper-
ature over 30 min. Insoluble material is removed by filtration,
dried, and considered as the insoluble polymer fraction (6%).
The chloroform solution is evaporated to dryness in vacuo and
the as such obtained material is considered as the soluble
polymer fraction (60%). The combined methanol and acetone
extracts are evaporated, redissolved in chloroform, and washed
with diluted HCl (1 N) to remove the iron salts. The organic
layer is dried with MgSO4 and evaporated to yield the
oligomeric fraction (34%).
(
1
6, 689.
(20) Uhlenbroek, J. H.; Bijloo, J. D. Recl. TraV. Chim. Pays-Bas 1960,
9, 1181.
21) Andreani, F.; Salatelli, E.; Lanzi, M. Polymer 1996, 37, 661.
22) Hanna, R.; Leclerc, M. Chem. Mater. 1996, 8, 1512.
7
(
(
(23) McCullough, R. D. AdV. Mater. 1998, 10, 93.
(24) Kaeriyama, K. In Handbook of Organic ConductiVe Molecules and
Polymers; Nalwa, H. S., Ed.; Wiley: New York, 1996; Vol. 2.
(
25) Konestabo, O. R.; Aasmundtveit, K. E.; Samuelsen, E. J.; Bakken,
E.; Carlsen, P. H. J. Synth. Met. 1997, 84, 589.
(26) Zotti, G.; Gallazzi, M. C.; Zerbi, G.; Meille, S. V. Synth. Met. 1995,
3, 217.
27) Zotti, G.; Marin, R. A.; Gallazzi, M. C. Chem. Mater. 1997, 9,
945.
28) Barbarella, G.; Zambianchi, M.; Di Toro, R.; Colonna, M. J.; Iarossi,
D.; Goldoni, F.; Bongini, A. J. Org. Chem. 1996, 61, 8285.
29) Fr e´ chette, M.; Bellet eˆ te, M.; Bergeron, J.-Y.; Durocher, G.; Leclerc,
M. Macromol. Chem. Phys. 1997, 198, 1709.
30) Abdou, M.; Lu, X.; Xie, Z. W.; Orfino, F.; Deen, M.; Holdcroft,
7
(
2
(
(
Acknowledgment. The work performed in Diepenbeek was
financially supported by the Flemish Government (IWT). The
work in Eindhoven was supported by the Council for Chemical
Sciences of The Netherlands Organization for Scientific Re-
search (CW-NWO) and the Eindhoven University of Technol-
ogy in the PIONIER program (98400). The work in Linz was
performed within the Christian Doppler Society dedicated
laboratory on Plastic Solar Cells funded by the Austrian Ministry
of Economic Affairs and Quantum Solar Energy Linz Ges. m.
b. H. The work was further supported by the “Fonds zur
F o¨ rderung der wissenschaftlichen Forschung“ of Austria (Project
Nos. P-12680-CHE and M539-CHE), the Magistrat Linz, the
Land Ober o¨ sterreich (ETP Wi(Ge)-200513/1), and The Neth-
erlands Organization for Energy & Environment (NOVEM).
(
S. Chem. Mater. 1995, 7, 631.
(31) Doblhofer, K.; Rajeshwar, K. In Handbook of Conducting Polymers,
2
nd ed.; Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds.; Marcel
Dekker: New York, 1998; p 531.
32) Trznadel, M.; Zagorska, M.; Lapkowski, M.; Louarn, G.; Lefrant,
S.; Pron, A. J. Chem. Soc., Faraday Trans. 1996, 92, 1387.
(33) Trznadel, M.; Pron, A.; Zagorska, M.; Pielichowski, C. R. J.
Macromolecules 1998, 31, 5051.
(
(
34) Daub, J.; Feuerer, M.; Mirlach, A.; Salbeck, J. Synth. Met. 1991,
4
1-43, 1551.
(35) Leuninger, J.; Uebe, J.; Salbeck, J.; Gherghel, L.; Wang, C.; M u¨ llen,
K. Synth. Met. 1999, 100, 79.
(
(
36) Buvat, P.; Hourquebie, P. Macromolecules 1997, 30, 2685.
37) Roman, L. S.; Mammo, W.; Pettersson, L. A. A.; Andersson, M.
R.; Ingan a¨ s, O. AdV. Mater. 1998, 10, 774.
(38) Van Hal, P. A.; Christiaans, M. P. T.; Wienk, M. M.; Kroon, J.
M.; Janssen, R. A. J. J. Phys. Chem. B 1999, 103, 4352.
(
39) Janssen, R. A. J.; Hummelen, J. C.; Wudl, F. J. Am. Chem. Soc.
995, 117, 544.
40) Williams, R. M.; Zwier, J. M.; Verhoeven, J. W. J. Am. Chem.
Soc. 1995, 117, 4093.
41) Thomas, K. G.; Biju, V.; George, M. V.; Guldi, D. M.; Kamat, P.
V. J. Phys. Chem. A 1998, 102, 5341.
(42) Guldi, D. M.; Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem.
Soc. 1997, 119, 974.
(43) Borsenberger, P. M.; Weiss, D. S. Organic Photoreceptors for
Xerography; Marcel Dekker: New York, 1998.
(44) Pope, M.; Swenberg, C. E. Electronic Processes in Organic
Crystals; Oxford University Press: Oxford, U.K., 1999; Chapter 13.
(45) Salbeck, J. Anal. Chem. 1993, 65, 2169.
(46) Brabec, C. J.; Padinger, F.; Dyakonov, V.; Hummelen, J. C.;
Janssen, R. A. J.; Sariciftci, N. S. Electronic Properties of Novel Materials-
Progress in Molecular Nanostructures. AIP Conference Proceedings, 1998;
p 519.
References and Notes
1
(
(
1) 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.
2) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science
995, 270, 1789.
(
(
1
(
3) Yu, G.; Heeger, A. J. J. Appl. Phys. 1995, 78, 4510.
(4) Roman, L. S.; Andersson, M. R.; Yohannes, T.; Ingan a¨ s, O. AdV.
Mater. 1997, 9, 1164.
(
5) Granstr o¨ m, M.; Petrisch, K.; Arias, A. C.; Lux, A.; Lux, M.;
Andersson, M. R.; Friend, R. H. Nature 1998, 395, 257.
6) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science
992, 258, 1474.
7) Smilowitz, L.; Sariciftci, N. S.; Wu, R.; Gettinger, C.; Heeger, A.
J.; Wudl, F. Phys. ReV. B 1993, 47, 13835.
8) Meskers, S. C. J.; van Hal, P. A.; Spiering, A. J. H.; Hummelen,
J. C.; van der Meer, A. F. G.; Janssen, R. A. J. Phys. ReV. B. 2000, 61,
917.
(
1
(
(
9
(47) Gao, J.; Hide, F.; Wang, H. Synth. Met. 1997, 84, 979.