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Towards a Transparent, Highly Conductive
Poly(3,4-ethylenedioxythiophene)**
By Yung-Hoon Ha, Nikolay Nikolov, Steven K. Pollack, John Mastrangelo, Brett D. Martin,
and Ranganathan Shashidhar*
A detailed investigation of the processing parameters influencing the oxidative polymerization of 3,4-ethylenedioxythiophene
(EDOT) and a methanol-substituted derivative (EDOT±CH2OH) was performed with the goal of maximizing the conductivity
of the polymer. We show that the conductivity can be significantly enhanced by varying the monomer, oxidant (iron(III) p-toluenesulfonate (Fe(OTs)3)), weak base (imidazole (Im)), solvent (various alcohols), and solution concentrations. The effect of
each variable on the final materials properties is investigated, and the parameters have been optimized to achieve conductivities as high as 900 S cm±1. Surface resistance below 150 X/& for 80±90 nm thick films with visible-spectrum transparency
exceeding 80 % is achieved. The combination of these properties makes the films highly suitable for numerous device applications.
1. Introduction
A= e c t
The heralded discovery of a polymer which transports
charge[1] has led to unprecedented excitement[2,3] over its possibilities for numerous wide-ranging sets of applications such as
electrochromics,[4] supercapacitors,[5] antistatic and electrostatic coatings,[6] light-emitting diodes,[7±10] photovoltaics,[11,12]
and sensors,[13] in addition to a host of other applications.[14±16]
Most of these applications need materials with low surface
resistances and high optical transparency, and have thus
spurred on numerous research efforts to achieve a combination
of these properties. In general, efforts at improving the conductivity were directed in controlling synthetic conditions,[17,18]
altering the fundamental polymer backbone,[19,20] doping,[21]
and/or functionalizing the backbone with substituent side
groups.[22] For example, significant improvements in the conductivity of the materials were afforded by ªdopingº, where
cations are inserted into the polymer backbone to aid in charge
conduction, and metallic conductivities have been reported.[21]
However, due to the highly conjugated nature of conducting
oligomers/polymers, they are highly absorbing in the neutral
undoped state and even more so in the doped state. The transparency of conducting polymers nominally follows Beer's law:
where A is the total absorption, e is the molecular absorption, c
is the concentration of the absorbing species, and t is the path
length (thickness of the sample). Making thinner films will
result in higher transparency, but generally leads to higher
resistances. Moreover, the upper limit of transparency is dictated by the material itself due to the molecular absorption for
different conducting polymers. Therefore, materials with low
molecular absorption and high conductivity are required for
the desirable combination of high transparency and low resistance.
Among the numerous materials devised, the development of
a polythiophene derivative, poly(3,4-ethylenedioxythiophene)
(PEDOT; see Scheme 1 for structure), has shown significant
promise to meet the challenges of competing properties.[14]
PEDOT can be found in a variety of different forms. The most
widely utilized is the commercialized blend of PEDOT with
poly(styrenesulfonate), with reported film transparencies of
~ 80 % but low conductivities of approximately 10 S cm±1.[14]
Electrochemical polymerization is also widely utilized but typi-
(1)
±
[*] Dr. R. Shashidhar,[+] Y.-H. Ha,[++] N. Nikolov, S. K. Pollack,
J. Mastrangelo, B. D. Martin
U.S. Naval Research Laboratory
Center for Bio/Molecular Science and Engineering
Washington, DC 20375 (USA)
E-mail: rshashidhar@geo-centers.com
[+] Present address: Geo-Centers, Maritime Plaza One, 1201M St. S.E.
Suite #50, Washington, DC 20003, USA.
[++] Present address: Oblon, Spivak et al., 1940 Duke St., Alexandria, VA
22314, USA.
[**] We gratefully acknowledge Dr. Shi-Cheng Tony Wang's helpful and
enthusiastic discussions and support in this work. Y.-H. Ha thanks
the National Research Council for a postdoctoral fellowship. We
gratefully acknowledge Office of Naval Research and DARPA for
financial support.
Adv. Funct. Mater. 2004, 14, No. 6, June
Scheme 1. The chemical structures of 3,4-ethylenedioxythiophene
(EDOT), 2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl methanol (EDOT±
CH2OH) and 3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepin-3-ol (ProDOT).
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cally results in extremely poor transparency and small sample
sizes despite the enhanced conductivity.[23,24] Potentially the
most promising approach is to use the EDOT monomer, which
can be oxidatively polymerized after spin-casting onto various
different substrates. Conductivity values of 300 S cm±1 with
high transparency have been reported.[6] However, a further
improvement is needed for display applications.
Towards this goal, many efforts, ranging from bandgap tuning through the use of substituted or derivatized PEDOT,[25,26]
tailoring doping/dedoping conditions,[27,28] or careful control of
the electrochemical polymerization conditions,[29±31] are currently underway. Surprisingly however, little effort[6,32±34] has
been devoted thus far to manipulating the intrinsic process,
namely oxidative polymerization, by which PEDOT is formed
from the monomer. Also, the effect of derivatized EDOT in
oxidative polymerization has not yet been explored.
The oxidative polymerization of EDOT into a polymer is
depicted in Scheme 2. The process utilizes iron(III) toluenesulfonate (Fe(OTs)3) as a stoichiometric oxidant and imidazole as
a base for the attenuation of the Fe(OTs)3 oxidation potential,
and is analogous to the oxidative polymerization of other conducting polymers. The rate-limiting step in this reaction is most
likely the redox reaction between the FeIII compound and the
monomer. Prior work ascribed the influence of an added base
to a change in the pH of the reaction media and a concomitant
change in the reduction potential of the FeIII/FeII couple.[6]
Furthermore, the Fe(OTs)3 oxidizes the EDOT, transforming
it into a cation radical that dimerizes and is rapidly stabilized
by base-assisted removal of two protons. This base may be
tosylate ion (OTs±) or free amine. Additional Fe(OTs)3 oxidizes the dimers, and chain growth proceeds as a classical step-
polymerization. It also oxidizes the growing chains, leaving the
PEDOT in its doped (conducting) state. However, no detailed
study on the roles of individual components in the formation of
PEDOT has been carried out, and a significant gap exists in
the understanding the effect of numerous variables, such as
base concentration, oxidizer/dopant concentration, types of
solvents, and solution concentration, on the final material's
properties.
In this study we provide an explanation of the variables associated with oxidative polymerization of EDOT and its effect on
the resultant material's properties (conductivity and transparency). Utilizing the understanding of the role of the various
variables in controlling the materials properties, we describe an
optimization process that form PEDOT films with conductivity
of 750 S cm±1 (surface resistance of 270 X/& and 81 % transparency). Furthermore, these studies imply even further enhancement of the properties by the use of methanol-substituted
EDOT. Indeed, we have used this approach to achieve conductivities of 900 S cm±1 (140 X/& surface resistance and 82 %
transparency).
2. Results and Discussion
2.1. Understanding the Oxidative Polymerization of PEDOT
2.1.1. Effect of a Weak Base
To test for the effect of a weak base, imidazole (Im) concentration was varied whilst fixing the molar ratio of the
Fe(OTs)3/EDOT at 2:1. The reactants were dissolved in buta-
Scheme 2. Schematic description of the
oxidative step-growth polymerization of
EDOT into PEDOT. Note the imidazole
acts to reduce the reactivity of Fe(OTs)3
(FeIII) thereby leading to slower polymerization kinetics and lower doping levels.
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nol as a 30 wt.-% solution, which was spin-cast at 1500 rpm.
Increasing the amount of Im leads to a dramatic increase in
conductivity and transparency (see Fig. 1).
a)
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b)
Figure 1. Imidazole increases the conductivity of PEDOT (squares), by
simultaneously promoting higher molecular weight chains and preventing
overdoping. Polymerized PEDOT films do not form when more than ~ 2.5
molar ratio of Im/EDOT is added. The conductivity of P(EDOT±CH2OH)
(open circles) decreases above 2.5 molar ratio of Im/EDOT±CH2OH as
Im probably becomes hydrogen bonded to the P(EDOT±CH2OH) backbone and dedopes the polymer. Data on surface resistance, transparency,
and thicknesses for each PEDOT (boxes) and P(EDOT±CH2OH) (circles)
sample are also included in the figure.
de Leeuw et al.[6] attributed the effect of the added amine to
increase in the pH of the reaction media, which in turn reduces
the reactivity of the Fe(OTs)3. Indeed, our cyclic voltammograms show a systematic shift of the formal potentials to more
negative voltages with increasing Im, demonstrating the lowered reactivity of the Fe(OTs)3 (Fig. 2a). This implies that, as
the Im concentration increases, the reduction of FeIII, the critical reaction in the polymerization, becomes increasingly more
difficult. However, the lowered reactivity may not be purely
due to pH changes only. Spectroscopic studies indicate that Im
actually coordinates with the Fe(OTs)3, as a shift in the absorption peak is observed with increasing Im concentration (see
Fig. 2b). Since Fe(OTs)3 is initially hydrated, we interpret this
shift as being due to successive substitution of Im replacing
water or alcohol ligands that are initially present. Thus, we
believe that the lowered reactivity, and thus a decrease in the
polymerization kinetics, may not only be due to the pH
changes induced by the addition of the base as originally postulated by de Leeuw et al., but also due to the coordination of
Fe(OTs)3 with Im around its shell.
This implies that a decrease in polymerization kinetics may
arise because Im prefers to quench a monomer radical versus
an oligomeric radical during the polymerization process. This is
to be expected since the doped PEDOT radical cations are
Adv. Funct. Mater. 2004, 14, No. 6, June
Figure 2. a) The formal potential of Fe(OTs)3 decreases as a function of
Im concentration. The lowered formal potential indicates a decrease in the
reactivity of the Fe(OTs)3 thereby slowing the polymerization kinetics.
b) Peak absorption wavelengths of Fe(OTs)3 shift with the addition of Im
indicating ligation of Im to Fe(OTs)3, which may cause the decrease in
Fe(OTs)3¢s formal potential.
more delocalized in an oligomeric chain and thus have lower
reactivity. Our observation implies that the polymerization
kinetics are skewed toward longer chains with increasing Im
content and Im preferentially quenches the monomeric EDOT
radicals; additionally, the longer oligomers couple with each
other leading to higher molecular weights. The longer chains
allow larger orbital delocalization thus increasing the conductivity, which is demonstrated experimentally. Unfortunately,
the insolubility of the PEDOT in any common organic solvents
preclude the determination of the molecular weight of the samples with any of the techniques available to us. Additionally, it
is important to note that this does not imply the consumption
of the imidazole: namely, quenching of the EDOT radicals creates a positively charged imidazole that in turn reduces the FeII
into the FeIII state, regenerating the initial Im and Fe(OTs)3 in
the FeIII state.
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Another significant factor is that the moderation of the FeIII
activity by the amine probably prevents the chains from
becoming overdoped, as the Fe(OTs)3 not only initiates the
polymerization, but also oxidizes the growing chain (see
Scheme 2). Soaking the conductive (doped) film (286 S cm±1)
in a 2 M imidazole solution for 10 min does in fact lead to a
significant decrease in conductivity (70 S cm±1) and the films
turn violet, which is indicative of dedoping.[35] Mechanistically,
the dedoping reaction must involve electron transfer from Im
to the radical cation in the conducting polymer chain. Therefore, Im also acts as a reducing agent with respect to the doped
polymer to prevent overdoping, which has important consequences for the conductivity and transparency of the resulting
films, and will be explored further in the next section.
The high reactivity without imidazole has another consequence. It leads to thicker films as oligomers (or polymers)
may begin forming in solution prior to spin-casting. In fact, the
reactivity of the Fe(OTs)3 is so high that the polymerization initiates (as evidenced by the change in film color toward a bluish
hue) within 60 s, even at room temperature, after spin-casting
without the presence of the imidazole. This has a detrimental
effect on the transparency, as thicker samples lead to higher absorption and hence lower transparency. Additionally, the excessively fast polymerization kinetics also cause prepolymerized
nanoscopic droplets to precipitate onto the substrate, causing
high surface roughness. In contrast, the presence of the amine
moderator preserves the EDOT in monomeric form after spincasting, and the polymerization at elevated temperature after
spin-casting leads to smoother surface morphology with increasing connectivity between the domains (see Fig. 3). This
not only leads to enhanced conductivity, but also enables
achieving a smoother surface morphology which is particularly
important for display devices.
We have thus demonstrated that imidazole serves three major roles in this system:
d it retards the polymerization kinetics by reducing the reactivity of Fe(OTs)3 either by coordination and/or lowering
of the pH,
d it promotes higher molecular weight polymeric/oligomeric
chains, and
it prevents the polymer from becoming overdoped.
Thus, the presence of the imidazole increases transparency
(by decreasing the path length and reducing the surface roughness) and the conductivity (by promoting higher molecular
weights and preventing overdoping).
d
2.1.2. Effect of Fe(OTs)3
To further illustrate the concept of overdoping, the Fe(OTs)3
concentration was varied whilst fixing the Im/EDOT ratio at
2:1. Again, a 30 wt.-% butanol solution was employed using a
1500 rpm spin speed. Samples containing a 1.5 molar ratio of
Fe(OTs)3 did not polymerize, potentially due to a sub-stoichiometric quantity of the oxidizing agent or to the relative excess
of the Im, which lowers the polymerization reactivity as discussed in the previous section. Increasing the Fe(OTs)3 content
leads to polymerizable films, and a decrease in conductivity
and transparency are observed (see Fig. 4). Both of these effects can be attributed to overdoping. The conductivity decrease is probably due to immobilized charge carriers, while
the transparency decrease is due to thicker film formation
caused by faster polymerization kinetics. In addition, the transparency decrease could also be due to a higher number of absorbing moieties as will be illustrated next.
The dedoped sample from the previous section (recall the
conductivity dropped to 70 S cm±1) was soaked in a saturated
Fe(OTs)3 solution, and the conductivity increased to approximately 130 S cm±1 with a transparency of 72 %, which are both
lower values than for the starting material (280 S cm±1 and
80 % transparency). These experiments confirm that although
the Fe(OTs)3 is required for polymerization to occur and to
dope the growing polymer chains, excess amounts of Fe(OTs)3
should be avoided to prevent overdoping, which decreases the
transparency and the conductivity.
2.1.3. Effect of the Solvent
The choice of solvent may also affect the final properties.
Alcoholic solvents with higher boiling points should remain in
Figure 3. AFM image showing the drastic differences in the surface roughness of sample
containing zero (left) and two (right) molar
ratio of Im where the Fe(OTs)3/EDOT ratio
was fixed at 2:1. The reduction of polymerization kinetics induced by the addition of Im
leads to significantly smoother surface roughness.
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Figure 4. The conductivity of PEDOT (squares) decreases with higher
solution concentration of Fe(OTs)3 due to overdoping induced by excess
Fe(OTs)3. The conductivity of P(EDOT±CH2OH) (open circles) reach a
maximum as hydrogen-bonded Im dedopes at low concentrations of
Fe(OTs)3, but eventually overdopes with excess Fe(OTs)3. The measured
surface resistance, transparency, and thicknesses for each PEDOT (boxes)
and P(EDOT±CH2OH) (circles) sample are included in the figure.
the sample for a longer time during polymerization at 110 C
and thus should lead to thinner (more transparent) films as the
reactant concentration is kept relatively low during polymerization. Alternatively, the viscosity increase observed with
higher boiling point solvents may retard polymerization kinetics. This implies a competition between these two factors in
the case of lower boiling point solventsÐalthough the polymerization kinetics are faster (promoting longer chain formation),
a high solvent evaporation rate may kinetically trap the polymerization at an early stage. The solvent was varied while fixing the Im/Fe(OTs)3/EDOT at a 2:2:1 molar ratio. Prior to
spin-casting, a gradual color change with time (over ~ 1 h) is
observed in solutions formed from the lower boiling point
methanol, whereas solutions formed with the higher boiling
point pentanol did not exhibit any noticeable color changes.
This suggests EDOT oligomerization occurs in the lower viscosity solvents as discussed.
Another interesting feature is that nearly constant conductivities are observed for all solvents despite the increase in film
thickness for samples polymerized with lower boiling point solvents (see Fig. 5). This suggests that the two competing factors
discussed previously are surprisingly balanced. Namely, the
thickness is higher in lower boiling point solvents because more
EDOT units are oligomerized in solution prior to spin-casting
and attached to the surface, but the rapid evaporation rate of
the solvent kinetically restricted the growing polymers to a certain chain length. This implies that if a higher probability of
oligomer formation can be induced at fixed Im/Fe(OTs)3/
EDOT ratios, longer polymers should form, allowing higher
conductivities to be realized.
Adv. Funct. Mater. 2004, 14, No. 6, June
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Y.-H. Ha et al./Towards a Transparent, Highly Conductive Poly(3,4-ethylenedioxythiophene)
Figure 5. The PEDOT conductivity (squares) remains approximately constant, P(EDOT±CH2OH) conductivity (open circles) decreases with higher
boiling point solvents. The higher polymerization kinetics is surprisingly
balanced by the evaporation rate for different boiling point solvents for the
PEDOT. Thus the conductivity remains constant but the thickness increases with decreasing boiling point solvents. Hydrogen bonding again
plays a role for the P(EDOT±CH2OH), decreasing the conductivity with
longer chain solvents, which prevents efficient packing. The measured surface resistance, transparency, and thicknesses for each PEDOT (boxes)
and P(EDOT±CH2OH) (circles) sample are included in the figure.
2.1.4. Effect of Solution Concentration
In light of the evidence from the previous section, the solution concentration and spin speed was varied by 15±45 %, using
2-methoxyethanol as the solvent. This effectively alters the
overall reactant concentration drastically. Increase in the overall reactant concentration leads to a higher probability (rate)
of chain propagation reaction, presumably leading to higher
molecular weight chains and to higher conductivities. Additionally, thicker films will result due to higher viscosity, which
decreases the transparency according to Beer's Law.
Figure 6 shows that lowest solution concentration (15 %)
leads to extremely thin films where the conductivity decreases
with decreasing thickness. However, the conductivity is approximately 300 S cm±1 and 500 S cm±1 for all thicknesses measured at 30 % and 45 % solution concentration, respectively.
The linear relationship of transparency with thickness again
confirms that the film absorption follows Beer's Law, i.e., depends on film thickness only. On the other hand, the differences in the observed conductivity can be attributed to the
higher molecular weights, as discussed previously, or to a different packing scheme. However, electron diffraction studies on
these systems using transmission electron microscopy (TEM;
not shown) indicate that there is no difference in the packing
scheme of the chains due to different solution concentrations.
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Figure 6. The variation of solution concentration (in methoxy ethanol) at,
15 % (squares), 30 % (circles), and 45 % (triangles), leads to different values in conductivity (solid symbols) and transparency (open symbols). The
transparency follows Beer's Law for all concentrations studied. Furthermore, higher solution concentration leads to higher conductivities presumably due to higher molecular weight chains that allow longer delocalization of the conducting orbitals.
Hence, this confirms that the molecular weight is indeed the
dominant factor in improving the conductivity of these materials.
Furthermore, the conductivity most likely follows the classical relationship (low conductivity at very thin films but saturates to a constant value after a finite thickness is reached),[36]
and each solution concentration is likely to exhibit a qualitatively similar, but quantitatively different trend. Unfortunately,
our experimental conditions simply do not allow us to access
the full range of thicknesses to clearly elucidate this for each
concentration.
2.1.5. Optimization of Properties
Having thus arrived at a clear understanding of the role of
the various variables associated with oxidative polymerization
in the overall materials properties of PEDOT, we can achieve
an optimized combination of properties. For this purpose, we
have set the Im/Fe(OTs)3/EDOT ratio as 2:1.75:1 using pentanol as the solvent at a 60 % solution concentration. Results obtained under these conditions show a dramatic improvement in
conductivity and transparency (see Fig. 7) reaching 750 S cm±1.
2.2. Further Improvements Utilizing EDOT±CH2OH
Our studies on PEDOT, which involve maximizing the chain
length with a controlled level of doping, suggest that PEDOTs
that can stay in solution longer before deposition as a film
should lead to higher molecular weights and thus higher conductivities. As a test of this hypothesis, we have carried out a
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Figure 7. The design and fabrication of highly transparent and conductive
films is shown utilizing the principles learned in this study. Conductivity
for PEDOT (squares) using 60 % pentanol solution at Im/Fe(OTs)3/
EDOT = 2:1.75:1 resulted in 750 S cm±1 exhibiting 84 % transparency at
300 X/&. Conductivity for P(EDOT±CH2OH) (circles) using 60 % methanol solution at Im/Fe(OTs)3/EDOT±CH2OH = 2.5:2:1 resulted in
900 S cm±1 exhibiting 82 % transparency at 140 X/&.
similar set of studies using the EDOT±CH2OH monomers.
Results of these studies are discussed and the results are compared with films formed from EDOT monomers.
First, the molar concentration of imidazole was increased
whilst fixing the Fe(OTs)3/EDOT±CH2OH molar ratio at 2:1
using a 30 % butanol solution. As shown in Figure 1 (open circles), the conductivity reaches a maximum at a ratio of approximately 2.5. These results show that the introduction of an
alcoholic side group allows use of significantly higher Im concentrations as compared to unpolymerized samples of PEDOT
when the Im molar ratio exceeded two. This is most probably
due to the hydrogen bonding of the Im to the ±OH side group
on the PEDOT backbone. Thus, the coordination of the Im
with Fe(OTs)3 saturates and the excess Im becomes hydrogen
bonded to the ±OH side groups in the PEDOT backbone, lowering the conductivity as shown. This implies that the
P(EDOT±CH2OH) system should be more susceptible to
dedoping reactions as some Im may be hydrogen bonded to
the backbone. As shown in Figure 4 (open circles), with Im/
EDOT±CH2OH at a 2.5:1 molar ratio and a 30 % butanol solution concentration, an increase in the Fe(OTs)3 molar concentration leads to a maximum in the conductivity, whereas
PEDOT conductivities exhibited a linear decrease under these
conditions. This is consistent with the argument of Im becoming hydrogen bonded to the P(EDOT±CH2OH), which causes
dedoping, but with increasing Fe(OTs)3 concentration, the system eventually overdopes again.
The effect of the ±OH side group poses another interesting
variable with regards to the alcoholic solvents employedÐthe
possibility of solvent hydrogen bonding with the P(EDOT±
CH2OH). The solvent was varied whilst fixing the Fe(OTs)3/
Im/EDOT±CH2OH molar ratios at 2:2.5:1 at 30 % solution
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concentration. Interestingly, the conductivity decreases with
increasing chain length of the solvent (higher boiling point solvents) while the transparency increases (see Fig. 5, open circles). Presumably the longer alcoholic chains are also hydrogen
bonded to the ±OH side groups, frustrating packing during
polymerization. Although the solvent eventually evaporates,
the chains are still in a frustrated packing scheme. This prevents efficient overlap between the p-orbitals on neighboring
chains, reducing the effective mean conjugation length as the
solvent chains become longer and longer.
Lastly, to maximize the conductivity, we utilized methanol as
the solvent of choice and systematically increased the solution
concentration. In agreement with previous results, the conductivity increases with higher solution concentrations reaching
nearly 900 S cm±1 at 60 % solution concentration (see Fig. 7,
open circles). We believe that the ±OH functionality confers a
higher solubility on higher molecular weight chains in the alcoholic solvent over the PEDOT before forming a film due to the
increased favorable enthalpic interactions.
To illustrate one additional benefit of P(EDOT±CH2OH),
extrapolating the lines in Figure 8 shows that P(EDOT±
CH2OH) exhibits higher transparencies than the PEDOT films
at a fixed thickness. It appears that the OH side groups pro-
of PEDOT and P(EDOT±CH2OH). We have shown that transparency clearly follows Beer's Law and is also partially dependent on doping levels. Furthermore, doping levels control the
degree of conductivity that can be achieved, but more importantly, the key to achieving higher conductivities is to allow the
polymer chains to reach the highest possible degree of polymerization. We demonstrate the highest conductivities thus far
reported for PEDOT by careful control of polymerization conditions (750 S cm±1). Furthermore, we demonstrate an even
higher conductivity and transparency by the use of EDOT±
CH2OH as the monomer. Unprecedented conductivities of
900 S cm±1 are demonstrated, with the added benefit of higher
transparencies as compared to the PEDOT at the same thickness. These studies should provide an invaluable tool for scientists and engineers to tailor a desirable combination of surface
resistance and transparency in numerous conducting polymers
that can be oxidatively polymerized. For example, we have
begun applying this design principle to 2-(2-hydroxyethyl)thiophenes and have observed a very similar trend (surface resistance decreases with increasing concentrations of weak base).
Overall, our study demonstrates the potential for improving
the conductivity using the design principles outlined in this
work on a wide-ranging set of conducting polymer by simply
understanding and controlling the polymerization conditions.
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4. Experimental
Figure 8. Transparency as a function of thickness for PEDOT (filled symbols) and P(EDOT±CH2OH) (open symbols). Note that extrapolation of
the approximate linear functions shows that P(EDOT±CH2OH) is more
transparent at a constant thickness than the PEDOT.
mote either a lower e (molecular absorption) or c (concentration of the absorbing species), as follows from Beer's Law.
Nevertheless, P(EDOT±CH2OH) shows a dramatic improvement in the conductivity and the transparency as compared to
PEDOT.
3. Summary and Conclusions
We have explored a vast parameter space to understand the
role of numerous variables in the overall materials properties
Adv. Funct. Mater. 2004, 14, No. 6, June
Unless specified, all reagents and solvents were purchased from Aldrich Chemical (Milwaukee, WI) and used as supplied. The EDOT
monomer (Baytron M, Bayer) was distilled resulting in a clear colorless liquid. Failure to purify the monomer leads to inferior materials
properties. The alcoholic precursor, 2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl methanol (EDOT±CH2OH), was synthesized according to
Stephan et al. [37]. It should be noted that this procedure leads to a
mixture of approximately 90 % six-membered (EDOT±CH2OH) and
10 % seven-membered ring isomers (ProDOT), as verified through gas
chromatography. This mixture will be referred to as EDOT±CH2OH
for the remainder of the text for simplicity. The chemical structures are
shown in Scheme 1.
Thin films of PEDOT and P(EDOT±CH2OH) (the P denotes polymerized form) were prepared on glass substrates (25 mm ” 25 mm) by
the following procedure. Fe(OTs)3 and Im were dissolved in appropriate solvents (in separate flasks) by heating at 100 C and stirring for
1±2 min. The distilled monomer was then added to the weak base solution and subsequently added to the Fe(OTs)3 solution. The mixture color ranges from clear yellowish to brown depending on the amount of
Fe(OTs)3 present. The mixture was then spin-coated onto glass substrates and polymerization was carried out in an oven at 110 C for
2 min under atmospheric conditions, at which point the films turn sky
blue. The resulting films were thoroughly washed with methanol and
dried with a nitrogen gun.
Surface resistances were measured on a four-point-probe bench (Signatone SYS-301), calibrated against indium tin oxide (ITO) on plastic
reference samples. The thickness of the sample was measured by scanning force microscopy (SPM) (conditions described below) after
making a cut on the film surface with a razor blade to expose the glass
surface. The average of at least five measured values at different locations of the sample film was utilized for both the surface resistance and
thickness values. On average, the error was approximately 10 % from
the measured values. Conductivity values were obtained by the following relationship [38]:
r = 1/(SR t)
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where SR is the surface resistance [X/&] and t is the thickness of the
film [cm]. The transparency of the films was determined using the absorption spectra obtained using the Cary 4G UV-VIS spectrometer and
integrating the curve from 400±700 nm. Transparency through the glass
substrate is used as the reference. Spectroscopic studies of the effects
of Im addition on the absorption spectra of Fe(OTs)3 were also performed on the Cary 4G UV-VIS. Spectra of 1 M Fe(OTs)3 in n-butanol
(1 mL) in a quartz cuvette was initially taken. Spectra were obtained
after each sequential addition of a 2 M Im solution in n-butanol (50 lL
each addition) up to a molar ratio of 0.8:1 Im/Fe(OTs)3.
Electrochemical studies were carried out utilizing a BAS CV-50W
Voltametric Analyzer (Bioanalytical Systems, West Lafayette, IN)
using a standard three-electrode configuration. The working electrode
was a freshly polished platinum disk (0.03 cm2 active area), the counter
electrode was a platinum wire, and the reference electrode was a Ag/
AgCl miniaturized reference electrode (MRE) fabricated according to
literature procedures [39]. Prior to measurement, the working electrode was polished with alumina, sonicated in deionized water, and
rinsed with n-butanol, and the reference electrode was conditioned in
the electrolyte (0.1 LiClO4 in n-butanol) for 10 min. Scans were carried
out from 1.0 V to 0 and back to 1.0 V (relative to MRE) at a scan rate
of 10 mV s±1. The 22.03 mg of Fe(OTs)3 (33.03 lmol) was dissolved in
5 mL of the electrolyte. A cyclic voltammogram (CV) was taken of the
initial solution and then 50±100 lL aliquots of a 0.15 M solution of Im
were added and subsequent CVs obtained.
The surface morphology of the films was obtained with scanning
probe microscopy (SPM) using a Digital Instruments Dimensions 3100.
Silicon cantilevers oscillating at a resonant frequency of approximately
300 kHz were used in the tapping mode (DA/A » 40±70 %, where A is
the free-cantilever amplitude and DA is the amplitude used for the
feedback of the piezo). All images are viewed under height-contrast
mode showing the surface roughness of the samples.
Selected-area diffraction studies were performed on Phillips CM30
TEM operating at 300 keV. The samples were spun cast onto glass substrates as discussed previously and placed in a petri dish. Water was
filled to form a meniscus with the edge of the glass substrate and dilute
HF was dropped into the water. The HF etches the glass at the sample/
substrate interface and the film floats on the water surface after a few
hours. Samples were collected using a 200 mesh copper grid.
Received: September 26, 2003
Final version: November 14, 2003
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