Soluble narrow band gap and blue propylenedioxythiophene-cyanovinylene polymers as multifunctional materials for photovoltaic and electrochromic applications

Article abstract of DOI:10.1021/ja061274a

A family of soluble narrow band gap donor-acceptor conjugated polymers based on dioxythiophenes and cyanovinylenes is reported. The polymers were synthesized using Knoevenagel polycondensation or Yamamoto coupling polymerizations to yield polymers with molecular weights on the order of 10 000-20 000 g/mol, which possess solubility in common organic solvents. Thin film optical measurements revealed narrow band gaps of 1.5-1.8 eV, which gives the polymers a strong overlap of the solar spectrum. The energetic positions of the band edges were determined by cyclic voltammetry and differential pulse voltammetry and demonstrate that the polymers are both air stable and show a strong propensity for photoinduced charge transfer to fullerene acceptors. Such measurements also suggest that the polymers can be both p- and n-type doped, which is supported by spectroelectrochemical results. These polymers have been investigated as electron donors in photovoltaic devices in combination with PCBM ([6,6]-phenyl C₆₁-butyric acid methyl ester) as an electron acceptor based on the near ideal band structures designed into the polymers. Efficiencies as high as 0.2% (AM1.5) with short circuit current densities as high as 1.2-1.3 mA/cm² have been observed in polymer/PCBM (1:4 by weight) devices and external quantum efficiencies of more than 10% have been observed at wavelengths longer than 600 nm. The electrochromic properties of the narrow band gap polymers are also of interest as the polymers show three accessible color states changing from an absorptive blue or purple in the neutral state to a transmissive sky-blue or gray in the oxidized and reduced forms. The wide electrochemical range of electrochromic activity coupled with the strong observed changes in transmissivity between oxidation states makes these materials potentially interesting for application to electrochromic displays.

Full text of DOI:10.1021/ja061274a

Published on Web 09/12/2006
Soluble Narrow Band Gap and Blue
Propylenedioxythiophene-Cyanovinylene Polymers as
Multifunctional Materials for Photovoltaic and Electrochromic
Applications
Barry C. Thompson,† Young-Gi Kim,† Tracy D. McCarley,‡ and John R. Reynolds*,†
Contribution from The George and Josephine Butler Polymer Research Laboratory and the
Center for Macromolecular Science and Engineering, Department of Chemistry, UniVersity of
Florida, GainesVille, Florida 32611-7200, and Department of Chemistry, Louisiana State
UniVersity, Baton Rouge, Louisiana 70803-1804
Received February 22, 2006; E-mail: reynolds@chem.ufl.edu
Abstract: A family of soluble narrow band gap donor-acceptor conjugated polymers based on dioxy-
thiophenes and cyanovinylenes is reported. The polymers were synthesized using Knoevenagel polycon-
densation or Yamamoto coupling polymerizations to yield polymers with molecular weights on the order of
10 000-20 000 g/mol, which possess solubility in common organic solvents. Thin film optical measurements
revealed narrow band gaps of 1.5-1.8 eV, which gives the polymers a strong overlap of the solar spectrum.
The energetic positions of the band edges were determined by cyclic voltammetry and differential pulse
voltammetry and demonstrate that the polymers are both air stable and show a strong propensity for
photoinduced charge transfer to fullerene acceptors. Such measurements also suggest that the polymers
can be both p- and n-type doped, which is supported by spectroelectrochemical results. These polymers
have been investigated as electron donors in photovoltaic devices in combination with PCBM ([6,6]-phenyl
C61-butyric acid methyl ester) as an electron acceptor based on the near ideal band structures designed
into the polymers. Efficiencies as high as 0.2% (AM1.5) with short circuit current densities as high as 1.2-
1.3 mA/cm2 have been observed in polymer/PCBM (1:4 by weight) devices and external quantum efficiencies
of more than 10% have been observed at wavelengths longer than 600 nm. The electrochromic properties
of the narrow band gap polymers are also of interest as the polymers show three accessible color states
changing from an absorptive blue or purple in the neutral state to a transmissive sky-blue or gray in the
oxidized and reduced forms. The wide electrochemical range of electrochromic activity coupled with the
strong observed changes in transmissivity between oxidation states makes these materials potentially
interesting for application to electrochromic displays.
Introduction
any application, it is the ability to tailor the electronic and
material properties of conjugated polymers via the organic
Conjugated polymers are currently the subject of an intense
research effort, primarily focused on the development of
electronic and electrochemical devices such as light emitting
diodes (LEDs),1 photovoltaic devices (PVDs),2 field effect
transistors (FETs),3 and electrochromic devices (ECDs).4 For
structure that makes this class of polymer the focus of such a
broad research effort. For device applications, it is especially
important to control the electronic band structure of the polymer
to achieve a band gap of the desired magnitude and frontier
orbitals, HOMO (highest occupied molecular orbital) and
LUMO (lowest unoccupied molecular orbital) (or band edges),
of the proper energies. Developing methodologies to attain such
† University of Florida.
‡ Louisiana State University.
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Kohler, A.; Friend, R. H.; Williams, C. K.; Holmes, A. B. J. Am. Chem.
Soc. 2006, 128, 6647-6656. (b) Niu, Y.-H.; Jen, A. K.-Y. Shu, C. J. Phys.
Chem. B 2006, 110, 6010-6014. (c) Huang, Q.; Evmenenko, G. A.; Dutta,
P.; Lee, P.; Armstrong, N. R.; Marks, T. J. J. Am. Chem. Soc. 2005, 127,
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J. AM. CHEM. SOC. 2006, 128, 12714-12725
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Multifunctional Materials for Applications
A R T I C L E S
precise control over the polymer electronic structure is a key
aspect for the advancement of the field.5 Equally important is
the control of a materials physical properties. The development
of novel polymers that are soluble in common organic solvents
and processable by industrially relevant techniques, such as spin-
coating, spray-coating, screen-printing, inkjet-printing, or roller
coating, is necessary for the practical application of conjugated
polymers. Such processable polymers with tailored electronic
structures also possess the possibility to be multifunctional
materials that can find application in numerous device archi-
tectures.
with poly(arylene methine)9 and poly(thieno[3,4b]thiophene),10
polymers. A third route is based on the control of regioregularity
in polythiophenes. In this case, band gaps as low as 1.4 eV
have been realized for the case of regioregular poly(3-alkox-
ythiophenes).11 A more general route is the donor-acceptor
(DA) approach,5b,6c which has proven especially successful for
the development of electropolymerizable polymers with band
gaps as low as 0.3-0.5 eV.12 Several types of soluble narrow
band gap DA polymers have been reported, especially for use
in PVDs.13 Band gaps as low as 1.3 eV have been reported for
alternating copolymers of dialkyl fluorene and bis-thienylthi-
adiazoloquinoxaline14 and 1.5 eV for alternating copolymers of
bis-thienyl-N-alkylpyrroles and benzothiadiazole.15 Another class
of DA polymers are based on cyanovinylene (CNV) as an
acceptor used in concert with electron-rich aromatics. Band gaps
as low as 1.1 eV have been reported for electropolymerized
bis-(3,4-ethylenedioxythiophene)-CNV (BEDOT-CNV) poly-
mers.16 The primary route to soluble CNV polymers has been
through the Knoevenagel polycondensation polymerization.17
In the case of CN-PPV (Figure 1), a band gap of 2.1 eV is
observed when dialkoxybenzene is used as the donor. This band
gap is not reduced relative to MEH-PPV based on the relatively
weak donor nature of dialkoxybenzene, although replacing one
or both dialkoxybenzenes with alkylthiophenes has led to band
gaps of 1.8 and 1.6 eV, respectively.18 Soluble, narrow band
gap CNV polymers based on 3,4-ethylenedioxythiophene (EDOT)
with band gaps as low as 1.6 eV have also been realized by
oxidative polymerization of bis-heterocycle-CNV monomers.19
The major advantage of the DA approach is that proper choice
of the donor and acceptor groups allows one to select the
approximate HOMO and LUMO energies of the resulting
polymer. This is especially attractive for the development of
An especially interesting class of conjugated polymer are the
soluble, narrow band gap polymers. Such polymers offer a broad
range of attractive features for use in a variety of device
architectures, such as a strong overlap with the solar spectrum
(PVDs), ease of oxidation and reduction (ECDs) as well as the
potential for high electron and hole mobility (PVDs, TFTs, and
LEDs), and a potentially transparent oxidized state (ECDs).
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development of narrow band gap polymers,5b,6 soluble and
processable systems are rare. Four main approaches toward the
development of soluble, narrow band gap polymers dominate
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of poly(thienylenevinylene) polymers and analogues,7 for which
band gaps of 1.6-1.8 eV are commonly observed. The second
approach is based on decreasing the band gap by increasing
the quinoidal character of the polymer backbone, and this
approach is primarily based on poly(isothionaphthene),8 along
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A R T I C L E S
Thompson et al.
development of an “ideal” narrow band gap donor polymer has
been published. An ideal donor polymer can be defined as a
polymer with the minimal band gap allowed by frontier orbital
requirements for air stability and charge transfer to a given
acceptor. The polymer must also exhibit suitably high hole
mobility to work effectively in concert with the acceptor for
charge transport in a device.25 Our approach to an ideal donor,
communicated recently,26,27 is outlined below. Here, optimiza-
tion of hole mobility is considered a second level of polymer
modification in structures that exhibit an ideal electronic
structure. Figure 1 illustrates the band structure of several
polymers including MEH-PPV and an ideal donor relative to
PCBM. To arrive at the ideal donor band structure, it is first
desired that a polymer be air stable (i.e., resistant to oxidation)
to allow ease of handling and processing by having a fairly
low-lying HOMO (∼5.2 eV or lower,28 assuming that the energy
level of SCE is 4.7 eV20 below the vacuum level). Additionally,
a donor polymer intended for use with a fullerene acceptor
should be capable of electron transfer to the fullerene upon
photoexcitation. As an approximation, electron transfer is
assumed possible if the band offset between the donor LUMO
and the acceptor LUMO is greater than the exciton binding
energy, estimated to be 0.4-0.5 eV,29 although LUMO offsets
of as little as 0.3 eV have been shown sufficient for photoin-
duced electron transfer.30 Thus, the donor polymer should have
a LUMO energy of 3.8 eV or above, for the case of a PCBM
acceptor (LUMO ) 4.2 eV). To transform MEH-PPV into an
ideal donor for PCBM, the LUMO must be lowered from 3.2
to ∼3.8 eV. The ideal donor polymer, with a LUMO of 3.8 eV
and a HOMO of 5.2 eV, will allow a minimal band gap (∼1.4
eV), without appreciably compromising the Voc of the PVD (the
Figure 1. Band diagram for an ideal donor polymer for PCBM. Also shown
are donors MEH-PPV, CN-PPV, and PProDOT-Hx2. Dashed lines indicate
the thresholds for air stability (5.2 eV) and effective charge transfer to PCBM
(3.8 eV). Note that these values are all approximate as some values come
from the literature and the method of determining HOMO-LUMO energies
varies from sample to sample. In all cases, orbital energies are given based
on the assumption that the energy of SCE is 4.7 eV vs vacuum,20 and Fc/
Fc+ is +0.380 V vs SCE21 (i.e., 5.1 eV relative to vacuum).
polymers for electronic device applications, such as PVDs. At
the present time, conjugated polymer-based PVDs are able to
give efficiencies of 2.5-5.0%.22 The best results are achieved
with so-called bulk-heterojunction devices based on dialkoxy-
PPVs (MEH-PPV or MDMO-PPV) or poly(3-alkylthiophenes)
(P3AT) as donors in combination with soluble fullerene
acceptors, most notably PCBM.23 By optimizing film morphol-
ogy and composition as well as device construction (choice of
electrodes, buffer layers, etc.), efficiency appears to peak at
∼5%. One deficiency in the operation of PPV or P3AT devices
is the limitation induced by the relatively high polymer band
gaps of 2.2 and 2.0 eV, respectively. Although the peak of
photon flux from the sun occurs at ∼1.8 eV (700 nm), such
polymers do not have an appreciable absorbance in this
wavelength range. To increase the number of photons absorbed
at peak solar wavelengths, it is necessary to shift the absorbance
spectrum of the donor polymer toward the red end of the visible
spectrum, and thus, the synthesis of new, narrow band gap
polymers is required.24
V
oc is thought to be related to the energetic difference between
the donor HOMO and the acceptor LUMO, although the work
functions of the electrodes do play a role24).
As one avenue toward the conversion of dialkoxy-PPV into
an ideal donor, the replacement of vinylene linkages with
cyanovinylene linkages is known to lower both the LUMO and
the HOMO by ∼0.5 eV, as seen for CN-PPV in Figure 1. The
incorporation of an electron-rich moiety in place of a dialkoxy-
benzene should cause the HOMO of the polymer to be raised
relative to that of CN-PPV while relying on the stronger
donor-acceptor interaction to concomitantly compress the band
gap and approximate the ideal donor. An advantage of this
approach is that little effect on the LUMO energy of CN-PPV
is expected upon increasing the electron richness of the
backbone, as detailed work by our group has shown that in a
homologous series of donor-acceptor CNV polymers, the donor
strength controls the HOMO energy without significantly
affecting the acceptor determined LUMO energy.16 As CN-
PPV has a LUMO of ∼3.8 eV, this route appears promising.
However, possessing a narrow band gap is not sufficient to
ensure that a polymer will give optimal or ideal performance
in a solar cell, as the positions of the frontier orbitals are also
critical to ensure effective charge transfer and collection. To
the best of our knowledge, no systematic approach for the
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Biele, M.; Brabec, C. J. Chem. Mater. 2005, 17, 2175-2180. (c) Kim, Y.;
Choulis, S. A.; Nelson, J.; Bradley, D. D. C.; Cook, S.; Durrant, J. R. Appl.
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F.; Fromherz, T.; Hummelen, J. C. Appl. Phys. Lett. 2001, 78, 841-843.
(i) Reyes-Reyes, M.; Kim, K.; Carroll, D. L. Appl. Phys. Lett. 2005, 87,
083506. (j) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery,
K.; Yang, Y. Nat. Mater. 2005, 4, 864-868. (k) Ma, W.; Yang, C.; Gong,
X.; Lee, K.; Heeger, A. J. AdV. Funct. Mater. 2005, 15, 1617-1622.
(23) Hummelen, J. C.; Knight, B. W.; Lepec, F.; Wudl, F. J. Org. Chem. 1995,
60, 532-538.
(25) Melzer, C.; Koop, E. J.; Mihailetchi, V. D.; Blom, P. W. M. AdV. Funct.
Mater. 2004, 14, 865-870.
(26) Thompson, B. C.; Kim, Y. G.; Reynolds, J. R. Macromolecules 2005, 38,
5359-5362.
(27) Kim, Y.-G.; Thompson, B. C.; Ananthakrishnan, N.; Padmanaban, G.;
Ramakrishnan, S.; Reynolds, J. R. J. Mater. Res. 2005, 20, 3188-3198.
(28) de Leeuw, D. M.; Simenon, M. M. J.; Brown, A. R.; Einerhard, R. E. F.
Synth. Met. 1997, 87, 53-59.
(29) (a) Arkhipov, V. I.; Bassler, H. Phys. Stat. Sol. 2004, 201, 1152-1187.
(b) Meskers, S. C. J.; Hubner, J.; Oestreich, M.; Bassler, H. J. Phys. Chem.
B 2001, 105, 9139-9149.
(30) (a) Brabec, C. J.; Winder, C.; Sariciftci, N. S.; Hummelen, J. C.; Dhanabalan,
A.; van Hal, P. A.; Janssen, R. A. J. AdV. Funct. Mater. 2002, 12, 709-
712. (b) Winder, C.; Matt, G.; Hummelen, J. C.; Janssen, R. A. J.; Sariciftci,
N. S.; Brabec, C. J. Thin Solid Films 2002, 403-404, 373-379.
(24) Winder, C.; Sariciftci, N. S. J. Mater. Chem. 2004, 14, 1077-1086.
9
12716 J. AM. CHEM. SOC. VOL. 128, NO. 39, 2006

Multifunctional Materials for Applications
A R T I C L E S
Scheme 1
the reaction was run at reflux in a 1:1 mixture of THF and tert-
butyl alcohol with at least 1.1 equiv of tBuOK per -CH2CN
group, as has been suggested to be the most effective conditions
in the literature.34 It should be noted that this is not a classical
A-A + B-B polycondensation as the bis-acetonitrile com-
pound must be deprotonated to be activated for polymerization.
The second anion on the compound will not form prior to the
first condensation due to the elevated pKa after deprotonation,
which will then cause the polymerization to most likely proceed
through coupling of dimer units.
For the preparation of monomers for Yamamoto polymeri-
zation or electropolymerization (Scheme 2), the cyanovinylene
moiety is set in place by a Knoevenagel reaction prior to
polymerization. Synthesis of monoaldehyde precursors 6 and
11 was achieved in an analogous fashion to the dialdehydes 3
and 4 via a lithiation route, whereas monoacetonitrile 5 was
synthesized by a Negishi coupling with bromoacetonitrile,
similar to that previously described.16 With the bis-heterocycle-
CNV monomers (7, 9, 12), electropolymerization could directly
be achieved (described in the Supporting Information). Prior to
Yamamoto coupling polymerization, the bromination of 7 to
yield monomer 8 was carried out in DMF with NBS, as has
been shown to be an effective method for the bromination of
electron-rich heterocycles.35 Yamamoto coupling polymerization
was selected for 8 on the basis of the extremely mild reaction
conditions, which provide compatibility with the nitrile func-
tionality.36
Although CNV is typically a structural motif used in electron
accepting polymers, CNV polymers have been used as donors
in PCBM-based photovoltaic devices, albeit with limited suc-
cess.31 In the design of the ideal donor using CN-PPV as the
parent structure, our primary choice for the electron-rich donor
moiety used for replacing dialkoxybenzene is a dialkylated
derivative of poly(3,4-propylenedioxythiophene) (ProDOT-R2).32
The band structure of PProDOT-Hx2 is shown in Figure 1, and
the high lying HOMO is indicative of the electron-rich nature
of polymers incorporating dioxythiophene (XDOT) moieties.
To establish the utility of the CN-PPV motif as the platform
for an ideal donor for photovoltaics, here we present a family
of soluble, narrow band gap cyanovinylene polymers (shown
in Schemes 1 and 2), in which the donor strength and sequence
distribution is varied to determine the effect of the donor-
acceptor interaction on the electronic properties of the polymers.
Such polymers are potentially useful for numerous other
applications in addition to photovoltaics, and the electrochromic
properties are investigated as proof of the multifunctional nature
of this class of polymers.
Polymer Characterization. For the soluble polymers, struc-
1
tures are supported by H NMR, IR, elemental analysis, and
MALDI-MS (full characterization is found in the Supporting
1
Information). All the polymers showed H NMR spectra that
were consistent with data reported in the literature for the parent
CN-PPV polymer.37 For the Knoevenagel polymers, analysis
by IR spectroscopy showed the disappearance of the saturated
nitrile stretch at ∼2250 cm-1 in the monomer and the appear-
ance of a peak at ∼2210 cm-1, characteristic of a cyanovinylene
moiety. Analysis by MALDI-MS served to support the proposed
structures and Figure 2 shows the spectra for PProDOT-Hx2:
CN-PPV and PBProDOT-Hx2:CNV as representative ex-
amples. Figure 2a shows the mass spectrum measured for
PProDOT-Hx2:CN-PPV, using terthiophene as an electron-
transfer MALDI matrix,38 confirming the presence of polymer
chains with masses up to nearly 30,000 amu. The spacing
between the major peaks is ∼1738 amu, which corresponds to
the calculated molecular weight of two repeat units of the
polymer (four condensation reactions), although it is unclear
why this is the dominant series in the spectrum. Notice in the
inset in Figure 2a that the peaks at m/z 8693 and 10 432
correspond to n ) 10 and n ) 12, respectively, with an
additional mass loss of 18, which is observed throughout the
dominant series in the spectrum. Small molecules are sometimes
Results
Polymer Synthesis. The polymers described here were
synthesized by three different polymerization routes: Knoev-
enagel polycondensation, Yamamoto polymerization, and oxida-
tive electropolymerization. Scheme 1 illustrates the synthesis
of the polymers realized by Knoevenagel polycondensation.
Phenylene diacetonitrile 1 was synthesized by bromomethylation
of dialkoxybenzene (all synthetic procedures are in the Sup-
porting Information section) followed by substitution with
sodium cyanide or by direct conversion of terephthalaldehyde
2 to the diacetonitrile by reaction with tosyl methyl isocyanide
(TosMIC).33 For alkylthiophene and dioxythiophene monomers,
synthesis of diacetonitriles was precluded by their relative
instability. Dialdehyde monomers 3 and 4 were readily obtained
by lithiation followed by reaction with DMF. For the Knoev-
enagel polycondensation, the best results were achieved when
(34) Boucard, V.; Ades, D. Siove, A. Romero, D.; Schaer, M.; Zuppiroli, L.
Macromolecules 1999, 32, 4729-4731.
(31) Zerza, G.; Rothler, B.; Sariciftci, N. S.; Gomez, R.; Segura, J. L.; Martin,
N. J. Phys. Chem. B 2001, 105, 4099-4104.
(35) Mitchell, R. H.; Lai, Y.-H.; Williams, R. V. J. Org. Chem. 1979, 44, 4733-
4735.
(32) (a) Reeves, B. D.; Grenier, C. R. G.; Argun, A. A.; Cirpan, A.; McCarley,
T. D.; Reynolds, J. R. Macromolecules 2004, 37, 7559-7569. (b) Welsh,
D. M.; Kleoppner, L. J.; Madrigal, L.; Pinto, M. R.; Thompson, B. C.;
Schanze, K. S.; Abboud, K. A.; Powell, D.; Reynolds, J. R. Macromolecules
2002, 35, 6517-6525.
(36) (a) Yamamoto, T. Synlett 2003 425-450. (b) Yamamoto, T. Macromol.
Rapid Comm. 2002, 23, 583-606.
(37) Chen, S.-A.; Chang, E.-C. Macromolecules 1998, 31, 4899-4907.
(38) (a) McCarley, T. D.; Noble, C. O.; DuBois, C. J.; McCarley, R. L.
Macromolecules 2001, 34, 7999-8004. (b) McCarley, T. D.; McCarley,
R. L.; Limbach, P. A. Anal. Chem. 1998, 70, 4376-4379.
(33) van Leusen, A. M.; Oomkes, P. G. Synth. Comm. 1980, 10, 399-403.
9
J. AM. CHEM. SOC. VOL. 128, NO. 39, 2006 12717

A R T I C L E S
Scheme 2
Thompson et al.
lost under the relatively harsh conditions of MALDI-Qq-TOF,
although the precise reason for this mass loss is not clearly
understood.
polymer chains with H/H, H/Br, and Br/Br end groups,
respectively. In this case, values of 3481, 3560, and 3640 amu
are observed, in excellent agreement with the calculated values.
Molecular weights were further evaluated by GPC, and Table
1 lists the values estimated by GPC vs polystyrene standards
in THF and CHCl3. Importantly, all polymers show number
average molecular weights (Mn) above 10 000 g/mol relative
to polystyrene, and with the exception of Th-CN-PPV, the
polydispersities are reasonably narrow. For PBProDOT-Hx2:
CNV, chloroform did not serve as a suitable solvent for GPC.
To investigate the effect of molecular weight on spectral
properties, the variation in polymer UV-visible absorbance with
elution time was monitored with an inline photodiode array
detector during GPC analysis (in CHCl3), for the Knoevenagel
polymers (see Supporting Information). This experiment not
only allows one to monitor how the polymer electronic
properties vary with molecular weight in solution, it gives a
direct means to assess the molecular weight at which electronic
properties saturate. PProDOT-Hx2:CN-PPV shows an absor-
bance maximum at ∼630 nm for fractions with molecular
weights >16 000 g/mol, whereas at a molecular weight of 6000
g/mol, a blue-shifted λmax is found at 573 nm, which indicates
that the electronic properties of the polymer are no longer
saturated for this polymer which has ∼14 aromatic rings. At
this low degree of polymerization, the absorption profile of the
polymer also changes, showing a single peak, rather than the
two peaks observed for the higher molecular weight fractions
(see also Figure 3b). For Th-CN-PPV the spectrum extracted
at the elution peak (14 900 g/mol) shows a maximum at 504
nm, and higher molecular weight fractions show maxima at 513
and 509 nm, whereas the fraction with a molecular weight of
6400 shows a maximum at 496 nm. The relative independence
of absorption maximum and curve shape on molecular weight
suggest that the optical properties appear to be saturated even
at low molecular weights for Th-CN-PPV. Similar behavior
is observed for CN-PPV, as the absorption spectra do not show
any significant deviation in absorbance maximum or curve shape
for molecular weights between 41 000 and 6200 g/mol.
Figure 2b shows the MALDI-MS spectrum for PBProDOT-
Hx2:CNV in a terthiophene matrix and masses of greater than
10 000 amu are observed. Peak spacing follows a regular pattern
with peaks appearing at intervals of 696 amu, which corresponds
to the repeat unit molecular weight of the polymer. The inset
of Figure 2b also shows that MALDI reveals information about
the end groups in the polymer. For n ) 5, the molecular weights
are calculated to be 3482.3, 3561.2, and 3640.1 g/mol for
Polymer Optical Properties. Figure 3 shows a comparison
of the solution absorption spectra of the soluble ProDOT
Figure 2. MALDI MS for (a) PProDOT-Hx2:CN-PPV and (b) PBPro-
DOT-Hx2:CNV. In both cases, terthiophene was used as the matrix.
9
12718 J. AM. CHEM. SOC. VOL. 128, NO. 39, 2006

Multifunctional Materials for Applications
A R T I C L E S
a
Table 1. GPC Estimated Polymer Molecular Weights in THF and CHCl3
polymer (GPC solvent)
Mn (g/mol)
Mw (g/mol)
PDI
λabs (nm)
λF (nm) (φF)
PProDOT-Hx2:CN-PPV (THF)
PProDOT-Hx2:CN-PPV (CHCl3)
Th-CN-PPV (THF)
17 400
14 700
25 500
12 500
13 700
10 900
14 300
26 300
28 700
102 500
53 000
29 900
24 400
40 000
1.51
1.95
4.02
4.25
2.17
2.24
2.80
627
660 (0.11)b
680 (0.036)c
557 (0.050)c
723 (0.033)b
522
463
610
Th-CN-PPV (CHCl3)
CN-PPV (THF)
CN-PPV (CHCl3)
PBProDOT-Hx2:CNV (THF)
a Also shown are polymer absorption and emission data collected in toluene solution. b Zinc phthalocyanine in pyridine, λex ) 610 nm, φF ) 0.30. c Zinc
tetraphenylporphyrin in toluene, λex ) 420 nm, φF ) 0.033.
soluble polymers in toluene solution along with the emission
quantum yield.
To optically evaluate the magnitude of the band gap, the thin
film absorbance is measured as shown in Figure 4 for either
spin cast films or electrodeposited and charge neutralized films.
Observe that the optical band gaps range from 2.1 to 1.5 eV,
with the parent CN-PPV possessing the largest band gap.
Replacing one dialkoxybenzene in CN-PPV with an alkyl
thiophene in the case of Th-CN-PPV results in a decrease in
the optical band gap from 2.1 to 1.8 eV as has been observed
previously for a closely related polymer structure18 due to the
increased donor-acceptor interaction with the more electron-
rich alkylthiophene. The propylenedioxythiophene iterations of
CN-PPV result in similar decreases in the optical band gap as
observed for the cases of PProDOT-Hx2:CN-PPV, PBPro-
DOT-Hx2:CN-PPV, and PBProDOT-Hx2:CNV with band
gaps of 1.7, 1.6, and 1.5 eV, respectively. Moving to the even
more electron-rich EDOT based polymer PBEDOT:CN-PPV
does not further decrease the optical band gap beyond the
observed value of 1.5 eV. Notice that in the case of the
electrodeposited films (Figure 4d-f) that the residual absorbance
at long wavelengths is attributed to scattering induced by the
rough ITO substrate. The thin film absorption coefficients at
λ
max of the polymers were estimated to be on the order of 2 ×
104 cm-1 in all cases, calculated using thicknesses measured
by profilometry. This absorption coefficient indicates that the
polymers are absorbing strongly at low energies and compares
favorably with the high absorption coefficients reported for
many conjugated polymers.2h
Polymer Electrochemistry. As a more complete means of
mapping the band structures of the polymers and gaining a
deeper understanding of the electrochromic and photovoltaic
properties, electrochemical measurements were employed to
estimate the magnitude of the band gap and the absolute energies
of the HOMO and LUMO levels. Here, cyclic voltammetry (CV)
and differential pulse voltammetry (DPV) were employed for
the analysis of the polymer thin films which were either
electrodeposited or solution cast on Pt. Results for all polymers
are presented in the Supporting Information, and all electro-
chemical results are summarized in Table 2. In most cases, the
electrochemically determined band gaps were found to be larger
than the optically determined band gaps and the band gaps
determined by CV were found to be larger than those estimated
from DPV. The lack of precise agreement between band gap
values determined by these three methods has been observed
previously with CNV polymers.16
Figure 3. Solution absorbance of CN-PPV analogues in toluene. (a) CN-
PPV, (b) PProDOT-Hx2:CN-PPV, (c) PBProDOT-Hx2:CNV.
containing polymers relative to CN-PPV in toluene along with
a photograph of each solution. It can be seen that with the
polymers based on the strongest donor-acceptor interaction, a
deep blue is achieved due to the red-shifted absorbance, which
is in stark contrast to the orange-yellow CN-PPV solution.
Table 1 lists the maxima for absorption and emission of all the
Close examination of the results presented in Table 2 and
summarized pictorially in Figure 5 demonstrates the structure-
property relationships evident in this polymer family. First, as
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J. AM. CHEM. SOC. VOL. 128, NO. 39, 2006 12719

A R T I C L E S
Thompson et al.
Figure 4. Normalized thin-film optical absorbance of CN-PPV analogues with their respective band gaps as estimated from the onset of the π-π* transition.
(a) Solution cast CN-PPV, (b) solution cast Th-CN-PPV, (c) solution cast PProDOT-Hx2:CN-PPV, (d) electrodeposited PBProDOT-Hx2:CNV, (e)
electrodeposited PBProDOT-Hx2:CN-PPV, and (f) electrodeposited PBEDOT:CN-PPV.
Table 2. Comparison of Electrochemically and Optically Determined Polymer Band Structuresa
Eox onset
(CV)
HOMO
(CV)
eV
Ered onset
(CV)
LUMO
(CV)
eV
Eg
(CV)
eV
Eox onset
(DPV)
V
HOMO
(DPV)
eV
Ered onset
(DPV)
V
LUMO
(DPV)
eV
Eg
(DPV)
eV
Eg
(optical)
eV
polymer
V
V
CN-PPV
1.0
0.9
0.7
0.3
0.3
0.3
-0.3
6.1
6.0
5.8
5.4
5.4
5.4
4.8
-1.7
-1.6
-1.6
-1.5
-1.5
-1.7
-1.5
3.4
3.5
3.5
3.6
3.6
3.4
3.6
2.7
2.5
2.3
1.8
1.8
2.0
1.2
1.0
0.9
0.6
0.2
0.3
0.3
-0.4
6.1
6.0
5.7
5.3
5.4
5.4
4.7
-1.7
-1.5
-
-1.6
-1.5
-1.6
-1.4
3.4
3.6
-
3.5
3.6
3.5
3.7
2.7
2.4
-
1.8
1.8
1.9
1.0
2.1
1.8
1.7
1.5
1.5
1.6
1.5
Th-CN-PPV
PProDOT-Hx2:CN-PPV
PBProDOT-Hx2:CNV (echem)
PBProDOT-Hx2:CNV (drop cast)
PBProDOT-Hx2:CN-PPV
PBEDOT:CN-PPV
a All potentials are reported vs. Fc/Fc+ and all HOMO and LUMO energies are derived from the electrochemical data based on the assumption that the
Fc/Fc+ redox couple is 5.1 eV relative to vacuum.
all of the polymers are donor-acceptor polymers in which the
acceptor is CNV, the LUMO energy is relatively constant across
the series, as expected. The measured LUMO energies for all
the polymers fall between 3.4 and 3.7 eV, considering both CV
and DPV measurements. As the proportion of electron-rich
heterocycle incorporated into the polymer is increased, the
HOMO is observed to undergo a concomitant increase in energy
level. Looking at the HOMO energies, the value stays essentially
constant for Th-CN-PPV relative to CN-PPV at ∼6 eV,
despite the addition of the more electron-rich 3-alkylthiophene.
Comparing PProDOT-Hx2:CN-PPV with its direct analogue
Th-CN-PPV, the HOMO energy increases from 6 to 5.7 eV
upon replacing alkylthiophene with the more electron-rich
ProDOT-Hx2. There is even a more marked change when
progressing from PProDOT-Hx2:CN-PPV to PBProDOT-
Hx2:CN-PPV in which the HOMO energy increases from 5.7
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Multifunctional Materials for Applications
A R T I C L E S
Figure 5. Summary of polymer band structures incorporating optical and electrochemical data. Shaded areas represent the differences between optically and
electrochemically determined band gaps, with the optical band gap centered around the baricenter of the overall band gap.
to 5.4 eV based on the equivalent of substituting a BiProDOT-
Hx2 moiety into the repeat unit. The HOMO energy of
PBProDOT-Hx2:CNV remains at essentially the same level
as PBProDOT-Hx2:CN-PPV, as the HOMO appears to be
determined by the electron-rich ProDOT in either case, sug-
gesting that a limiting value for the band gap of polymers based
on ProDOT as the donor and CNV as the acceptor is reached
at 1.5-1.6 eV. A similar band gap limiting phenomena was
previously observed for polymers based on EDOT and CNV in
which the inherent HOMO energy of EDOT and LUMO energy
of CNV could lead to a donor-acceptor interaction strong
enough to reduce the band gap to a value no lower than 1.1
eV.16 Interestingly, PBEDOT:CN-PPV is the only polymer
of this family to have a HOMO energy above 5.2 eV, at ∼4.9
eV. This is further evidence that EDOT is a stronger donor than
ProDOT. As discussed earlier, PBProDOT-Hx2:CN-PPV and
PBEDOT:CN-PPV have similar optical band gaps (1.6 and
1.5 eV repectively), however the effect of the powerful donor
nature of EDOT is clearly observable when examining the
HOMO energy measured electrochemically. It is evident that
PBEDOT:CN-PPV is 0.6-0.7 V easier to oxidize than
PBProDOT-Hx2:CN-PPV. As such, one can conclude that
ProDOT is more effective than EDOT for the synthesis of
soluble narrow band gap polymers that are air stable. The
accessibility of symmetrically substituted and easy to handle
ProDOT derivatives with solubilizing alkyl chains is another
advantage of ProDOT over EDOT. Overall, PBProDOT-Hx2:
CN-PPV and PBProDOT-Hx2:CNV come the closest to
approaching the ideal electron donor (Figure 1) for PCBM.
Especially for the case of PBProDOT-Hx2-CNV, the band
edges almost perfectly overlap the defined energies for the
HOMO and LUMO of the ideal polymer.
offer a direct means of evaluating the electrochromic properties
of a conjugated polymer. On a deeper level, such measurements
ascertain the relative donor or acceptor character of a polymer
based on the charge-carrier induced spectral changes that occur
upon oxidation and/or reduction. Figure 6 shows the oxidative
spectroelectrochemistry for PProDOT-Hx2:CN-PPV and PB-
ProDOT-Hx2:CNV films. In Figure 6a, it can be seen that in
the neutral form of PProDOT-Hx2:CN-PPV (+0.15 V vs. Fc/
Fc+), a strong absorption is observed, centered at 600 nm, that
corresponds to the π-π* transition of the polymer and imparts
a deep blue color to the film. Upon increasing the potential in
a stepwise fashion (50 mV steps), the π-π* transition is
observed to bleach with concomitant formation of lower energy
charge-carrier associated peaks in the near-infrared. The neutral
polymer’s optical spectrum is invariant from +0.15 to +0.45
V, followed by a decrease in the π-π* transition at +0.5 V
(as indicated in Figure 6a), and the film is fully oxidized by
+0.85 V.
During this process, the polymer is undergoing p-type doping,
in which the polymer is converted to a more conductive, doped
form. Throughout this process, the polymer thin film is observed
to become gradually more transparent and, at +0.85 V the
polymer is a transmissive sky blue. The color change observed
here is similar to that observed in PEDOT, but the applied
potentials required to affect bleaching are considerably higher,
as the spectrum of PEDOT is observed to evolve at potentials
as low as -0.4 V vs. Fc/Fc+.39 From an electrochromic point
of view, PProDOT-Hx2:CN-PPV is an organic soluble form
of PEDOT that has an air stable neutral form. The relatively
weak near-infrared absorbance in the oxidized polymer is
perhaps evidence that relatively few charge carriers are induced
in the polymer backbone upon p-doping.
Polymer Spectroelectrochemistry and Colorimetry. From
an application standpoint, spectroelectrochemical measurements
(39) Kumar, A.; Welsh, D. M.; Morvant, M. C.; Piroux, F.; Abboud, K. A.;
Reynolds, J. R. Chem. Mater. 1998, 10, 896-902.
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J. AM. CHEM. SOC. VOL. 128, NO. 39, 2006 12721

A R T I C L E S
Thompson et al.
Figure 7. Reductive spectroelectrochemistry of PProDOT-Hx2:CN-PPV.
No data is shown between 860 and 950 nm as the detectors do not cover
this wavelength range. Potentials were increased in 100 mV steps and all
potentials are reported vs Fc/Fc+. The supporting electrolyte consisted of
0.1 M TBAPF6/acetonitrile. Note that the data between 1125 and 1200 nm
as well as 1325-1425 nm (dashed lines) has been removed due to overtones
that appear to be attributed to acetonitrile, which appear when using a fiber
optic spectrometer.
changes are reflected in reversible color changes in the films,
and thus considering both the oxidative and reductive spectro-
electrochemical results, a continuum of color states is observed
for each polymer throughout the electroactive range. The range
of colors attainable from each polymer has been quantified by
colorimetric analysis,40 and the specific color states for the
oxidized, neutral, and reduced forms of each polymer are listed
in the form of L*a*b* coordinates in the Supporting Information.
In general, the polymer films are highly colored in the absorptive
neutral form, ranging from blue to orange. Upon oxidation and
reduction, the films become more transmissive across the visible
region and are typically a color-neutral gray. For the electropo-
lymerized polymer PBProDOT-Hx2:CN-PPV, films thick
enough for spectroelectrochemistry and colorimetry were not
formed, precluding the collection of data.
The question of interest is whether the above spectral changes
upon reduction can be attributed to a true n-type doping or not.
Previous work in our group has indicated that simple electro-
chemical reduction in conjugated polymers cannot be directly
attributed to an n-type doping processes.16,41 For a polymer to
be considered n-doped, not only must the polymer show a
reduction, but the polymer must show evidence for charge carrier
formation on reduction, as can be assessed by spectroelectro-
chemistry or in-situ conductivity measurements. The strong
increase in the near-infrared absorption of the polymers upon
reduction suggests that a true n-type doping process is occurring.
Previous work in our group16 on electropolymerizable cyanovi-
nylene polymers has relied on in situ conductivity measurements
to establish n-doping and these measurements showed that the
conductivity in the polymers is best attributed to a redox or
hopping type conductivity upon reduction as the carrier mobility
is likely rather low. From the practical standpoint of application
of the polymers as electron donors in solar cells, the main
interest in the spectroelectrochemical results presented here is
Figure 6. Spectroelectrochemistry of (a) PProDOT-Hx2:CN-PPV and
(b) PBProDOT-Hx2:CNV. Polymer films were cast from dichloromethane
solution (1% w/w) onto ITO coated glass. All potentials are reported vs
Fc/Fc+. The supporting electrolyte consisted of 0.1 M TBAP/acetonitrile.
In both cases, the potential was increased in 50 mV steps. Note that the
fully neutral spectrum for PBProDOT-Hx2-CNV is only attainable by
reducing the film with hydrazine, indicating that once the film has been
cycled, it is difficult to fully neutralize the polymer electrochemically.
Figure 6b shows the spectroelectrochemical series for PB-
ProDOT-Hx2:CNV. Here, the neutral polymer is also deep blue
and upon oxidation the visible absorption is gradually bleached,
resulting in a polymer that is transmissive sky blue in the
oxidized form. The two main differences observed for PBPro-
DOT-Hx2:CNV relative to PProDOT-Hx2:CN-PPV are a
lower switching potential at 0.0 V (as opposed to 0.5 V) and a
much more intense near-infrared absorption induced upon
doping. The lower switching potential is expected based on the
results presented in Table 2, which indicate a higher lying
HOMO for PBProDOT-Hx2:CNV. The much more intense
absorption band in the near-infrared suggests that a higher
doping level is achieved in PBProDOT-Hx2:CNV relative to
PProDOT-Hx2:CN-PPV. This phenomenon could be due to
some inherent property of the two films or it could simply be
attributed to the fact that PBProDOT-Hx2:CNV is stable over
a wider range of potentials anodic of the onset of oxidation,
possibly allowing higher doping levels.
The spectral changes observed upon reduction were also
measured for the family of polymers under an oxygen and water
free environment in an argon-filled glovebox. Figure 7 shows
the reductive spectroelectrochemical series for PProDOT-Hx2:
CN-PPV (spectra for Th-CN-PPV, CN-PPV, and PBE-
DOT:CN-PPV are shown in the Supporting Information),
which showed the most pronounced spectral changes upon
reduction. In all cases, at increasingly cathodic potentials, the
neutral π-π* transition is observed to bleach at the expense of
lower energy charge-carrier associated transitions. These spectral
(40) Thompson, B. C.; Schottland, P.; Zong, K.; Reynolds, J. R. Chem. Mater.
2000, 12, 1563-1571.
(41) (a) DuBois, C. J.; Reynolds, J. R. AdV. Mater. 2002, 14, 1844-1846. (b)
DuBois, C. J.; Abboud, K. A.; Reynolds, J. R. J. Phys. Chem. B 2004,
108, 8550-8557.
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Multifunctional Materials for Applications
A R T I C L E S
that all of the polymers show a strong donor character, as they
are readily p-type doped.
Polymer Photovoltaic Devices. The polymers described in
the previous sections were designed to function as electron
donors for a fullerene acceptor in a photovoltaic device. Before
incorporating a polymer into a device with PCBM, it is
interesting to examine the energetic relationship between the
two components in thin films. Proof for charge transfer can be
established using a variety of techniques such as light induced
electron spin resonance (LESR), photoinduced absorption (PIA),
and time-resolved transient absorption (TA).24 Another simpler
technique is photoluminescence (PL) quenching, which was
utilized for all the soluble polymers presented here. With the
exception of PBProDOT-Hx2:CNV, all of the soluble polymers
showed photoluminescence in thin films. The PL quenching
behavior for PProDOT-Hx2:CN-PPV has been previously
reported26 and it was observed that greater than 95% of the PL
was quenched in thin film blends with PCBM. Similar results
were observed for Th-CN-PPV and CN-PPV (see Support-
ing Information).
Figure 8. Photocurrent action spectra for a PProDOT-Hx2:CN-PPV/
PCBM solar cell based on a 1:4 blend (w:w) of polymer and PCBM. Black
squares represent the IPCE value at the given wavelength. The solid curve
is the absorption spectra of a pristine polymer film shown as reference.
Figure 8 shows the IPCE of a PProDOT-Hx2:CN-PPV/
PCBM (1:4) solar cell in which the active layer was spin coated
from dichlorobenzene (30 mg/mL). The IPCE data indicates
that PProDOT-Hx2:CN-PPV is acting as a photoexcited
electron donor and is thus the major contributor to the
photocurrent in the device. The slight blue-shift in the IPCE
relative to the polymer absorption spectra is a consequence of
blending the polymer with PCBM. The IPCE data shown in
Figure 8 is for a representative device, but the observed spectral
dependence of the photocurrent is reproducible from device to
device. For this same type of device, an AM1.5 efficiency of
0.15% and a Jsc of ∼1.3 mA/cm2 were measured, which are
reasonable values for an unoptimized device. The low FF of
only 0.25 is indicative of an unoptimized device with a high
series resistance (see Supporting Information for an I-V
characteristic).
Bulk heterojunction photovoltaic devices were constructed
based on 1:4 (w:w) blends of each soluble polymer with PCBM.
In all cases, blends were spin coated from dichlorobenzene and
the chosen weight ratio of 1:4 is based on studies in our labs
and others that suggests that this weight ratio is the most
effectiveforproducingefficientdeviceswithvinylenepolymers.22g,h
The measurement of the incident photon to current efficiency
(IPCE) or external quantum efficiency in preliminary devices
is crucial, as it can offer strong proof of electron transfer from
the polymer to PCBM through a photocurrent action spectrum
that strongly parallels the polymer absorption spectrum. It should
be noted that these experiments were done under conditions
that would allow the fundamental contribution of each polymer
to the device to be compared to the others in the family, and as
of yet, we have not focused on the practical aspect of optimizing
device engineering.
The surface morphology of PProDOT-Hx2:CN-PPV/PCBM
(1:4) blends spin coated from 30 mg/mL solutions (dichlo-
robenzene) was studied on PEDOT-PSS coated glass slides by
atomic force microscopy (AFM). The surface roughness is
calculated to be only 1 nm, indicating a very smooth surface
and on the micron scale the films do not appear to be phase
separated. This material shows a superior film quality relative
to the devices based on Th-CN-PPV and PCBM, which could
help to explain the improved performance of PProDOT-Hx2:
CN-PPV devices relative to Th-CN-PPV devices. Films
were 50 nm thick as measured by profilometry.
As with PProDOT-Hx2:CN-PPV, devices based on PB-
ProDOT-Hx2:CNV/PCBM (1:4) gave reasonable photovoltaic
performance. A measured FF of 0.32 indicates that the devices
are operating as somewhat more ideal diodes than seen with
the devices based on PProDOT-Hx2:CN-PPV. Again, a Jsc
of ∼1.2 mA/cm2 and a power conversion efficiency of 0.21%
are reasonable values for an unoptimized device. The IPCE data
for these PBProDOT-Hx2:CNV devices were also measured
(see Supporting Information) and values of greater than 15%
are observed at wavelengths in the UV region of the spectrum.
The photocurrent does mirror the polymer absorption near the
maximum absorption of the polymer; however, the IPCE at that
point is only ∼2%. Such behavior indicates the polymer is not
Measurement of IPCE for Th-CN-PPV/PCBM (1:4) solar
cells (see Supporting Information) in which the active layer was
spin coated from dichlorobenzene (30 mg/mL) indicates that
the photocurrent matches the polymer absorption spectrum very
closely; however, the peak efficiency of only ∼1% indicates
that the overall process of photogeneration and collection of
charge carriers is not efficient in the devices constructed under
the employed conditions. Importantly though, it does appear
that the photoexcited polymer is a contributor to the photocurrent
in the device. The diode characteristics show an open circuit
voltage (Voc) of ∼0.45 V with a short circuit current density
(Jsc) of 0.1 mA/cm2 and a fill factor (FF) of 0.25 for an overall
AM1.5 power conversion efficiency (ηeff) of only 0.01%.
AFM studies of the blends of Th-CN-PPV and PCBM spin
coated from dichlorobenzene on PEDOT-PSS coated glass
revealed that the films had a surface roughness of 3 nm (the
film thickness was approximately 50 nm as established by
profilometry). However, there appear to be fissures in the surface
(see Supporting Information) and it is evident that the film
quality in the blends of Th-CN-PPV and PCBM is less than
optimal. Optimization based on solvent choice, solution con-
centration, etc., could possibly lead to an improvement in film
quality and perhaps device performance.
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A R T I C L E S
Thompson et al.
acting primarily as a photoexcited electron donor for PCBM.
Instead, the IPCE data appears to suggest that PCBM is the
primary contributor to the photocurrent. A mechanism based
on electron transfer from the HOMO of the polymer to the
HOMO of photoexcited PCBM may be predominating in this
case. Referring to Figure 5, it is clear that the HOMO of
PBProDOT-Hx2:CNV is higher than the HOMO of PProDOT-
Hx2:CN-PPV, which may help to drive this inverse mode of
solar cell operation in the former case relative to the latter case.
By AFM, a very smooth surface is observed for PBProDOT-
Hx2:CNV blends with PCBM, with a surface roughness
calculated to be less than 1 nm. As with the PProDOT-Hx2:
CN-PPV films discussed above, AFM gives little information
about the precise morphology of the PBProDOT-Hx2:CNV/
PCBM films. However, the relatively smooth and uniform nature
of the film surface suggests that there is no macrophase
separation in the blend.
From a structural point of view, it is the incorporation of
ProDOT as the donor rather than EDOT or thiophene that makes
this unique property set possible. As a stronger donor than
thiophene (or alkylthiophene), ProDOT imparts a stronger
donor-acceptor interaction in combination with acceptors,
resulting in polymers with an enhanced electron-rich character
and narrower band gaps. At the same time ProDOT monomers
substituted with solubilizing groups are symmetrical monomers
that overcome the synthetic challenges of 3-alkylthiophenes and
do not suffer from the unfavorable steric interactions experienced
by 3,4-disubstituted thiophenes. Importantly ProDOT monomers
also show several advantages over EDOT as a donor hetero-
cylcle in donor-acceptor polymers. As shown in Figure 5,
PBEDOT:CN-PPV has a narrower band gap than PBPro-
DOT-Hx2:CN-PPV (1.5 eV vs 1.6 eV optically), but the
incorporation of the stronger EDOT donor results in a HOMO
level of ∼4.9 eV for PBEDOT:CN-PPV, which renders the
polymer unstable to oxidation under ambient conditions, whereas
PBProDOT-Hx2:CN-PPV shows a HOMO of 5.4 eV and is
stable to oxidation in air. Air stability in the neutral form is a
key feature of these materials, which makes them attractive for
application in that the polymers can be processed in air prior to
device fabrication without fear of oxidation. Ease of symmetrical
substitution for solubility in ProDOT is a further advantage
relative to EDOT in the context of serving as a building block
for soluble polymers.
For the PVDs based on the soluble narrow band gap polymers
presented here, it can be observed that only PProDOT-Hx2:
CN-PPV and PBProDOT-Hx2:CNV gave reasonable photo-
voltaic performances in devices based on 1:4 blends of polymer
and PCBM. In both cases however, efficiencies are hampered
by the low Fill Factors, which are indicative of a high resistance
in the films and could point toward low hole mobilities as a
problematic feature of the devices. Optimization of film
composition and morphology and the implementation of so-
called post-production treatment strategies22f such as thermal
and or electrical annealing could serve to optimize the perfor-
mance of the devices. For PProDOT-Hx2:CN-PPV the origin
of the photovoltaic activity is clear as evidenced by the IPCE
shown in Figure 8, although for PBProDOT-Hx2-CNV it is
evident that light absorption by PCBM is the primary source
for photoinduced charge transfer.
As a consequence of the described electronic properties,
ProDOT-CNV polymers show band gaps of 1.5-1.7 eV, making
them near-ideal absorbers of the solar spectrum. Results
presented here for unoptimized photovoltaic devices based on
PProDOT-Hx2:CN-PPV and PBProDOT-Hx2:CNV show
promise for further development. Although AM1.5 efficiencies
of only 0.1-0.2% were observed for these two polymers in 1:4
blends with PCBM, Jsc values larger than 1 mA/cm2 suggest
that optimization of device engineering could yield effective
devices. Further structural optimization of the polymer structures
presented here can also seek to overcome such shortcomings
as the apparent inability of PBProDOT-Hx2:CNV to act as a
photoexcited donor for PCBM. Importantly, the efficiencies
reported here should not be directly compared to the values
approaching 5% that have been obtained for P3HT:PCBM
devices,22i,k as this well established system has been the focus
of intense scrutiny and benefits from years of optimization by
numerous research groups. Instead, the photovoltaic properties
of the polymers presented here should be compared to the
relatively few examples of polymers with band gaps of less than
1.8 eV that have been successfully employed in PCBM-based
solar cells. In this case, only one example exists in which device
efficiencies have reached 1%, as is the case for a complicated
thiophene/pyrrole/benzothiadiazole hybrid polymer (PTPTB, Eg
) 1.6 eV), which gave efficiencies of ∼1% under AM1.5
conditions in 1:3 blends (w:w) with PCBM.30a However a
limitation of PTPTB not suffered by the ProDOT-CNV polymers
is the low molecular weight and limited solubility that is found
with this polymer.15
Summary and Perspective
Here, we have presented a family of soluble dioxythiophene-
cyanovinylene polymers and demonstrated the utility of this
structural platform for generating materials with tailored
electronic properties suitable for a variety of applications such
as photovoltaic and electrochromic devices. Cyanovinylene
polymers based on ProDOT derivatives are attractive targets
because soluble high molecular weight polymers can be realized
through either a Knoevenagel or a Yamamoto polymerization
to give regiosymmetric polymers with well-defined structures.
An advantage of the Knoevenagel route is the robust nature of
this high-yielding reaction that gives high molecular weight
polymers in the neutral form without the use of metal catalysts
that could potentially be trapped in the polymer. On the basis
of the polymer band structures shown in Figure 5, a clear
correlation can be seen between the donor strength and content
in these donor-acceptor polymers and the resulting band
structures of the materials. From an electronic point of view,
these soluble polymers offer several strengths, which include
air-stability (based on a low lying HOMO), a narrow band gap
(based on the donor-acceptor interaction), and a sufficiently
electron-rich structure to allow them to function as donor
materials in solar cells. Relative to the variety of other low band
gap polymers reported in the literature this property set appears
to make these polymers unique.
Beyond the obvious appeal of ProDOT-CNV polymers for
photovoltaic devices, the general strengths of these polymers
make them appealing as multifunctional materials. Specifically,
the electrochromic properties described here illustrate a new
class of soluble electrochromic polymer that is colorless/
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Multifunctional Materials for Applications
A R T I C L E S
transmissive in the oxidized state and colored/absorptive in the
neutral state. These electrochromic properties make this class
of polymer very similar to PEDOT, with the advantage that the
polymers have an air-stable neutral state. Interestingly, the CNV
polymers presented here also have accessible color states in the
reduced form as illustrated by spectroelectrochemistry and
colorimetry. These spectroelectrochemical results also indicate
the possibility that these polymers can exhibit both p- and n-type
doped states, which further increases the range of possible
applications with this class of materials. Future synthetic efforts
and detailed characterization coupled with device optimization
promise to solidify the importance of these ProDOT-CNV
polymers as multifunctional conjugated polymers.
Acknowledgment. We thank the AFOSR (FA955-06-1-0192)
and EIC Laboratories (NASA - NNC05CB23C) for funding
this work. B.C.T. thanks the NSF (NSF Graduate Research
Fellowship) and Eastman Chemical (Eastman Fellowship in
Polymer Science). TDM thanks the NSF for funding (CHE-
0074884). Our thanks to Nisha Ananthakrishnan for her
assistance with the AFM measurements.
Supporting Information Available: Synthetic details and
additional polymer characterization not included in the manu-
script. This material is available free of charge via the Internet
at http://pubs.acs.org.
JA061274A
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