Article
pubs.acs.org/Macromolecules
Impact of the Addition of Redox-Active Salts on the Charge
Transport Ability of Radical Polymer Thin Films
Aditya G. Baradwaj,† Si Hui Wong,† Jennifer S. Laster, Adam J. Wingate, Martha E. Hay,
and Bryan W. Boudouris*
School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, Indiana 47907, United States
S Supporting Information
*
ABSTRACT: Radical polymers (i.e., macromolecules composed of a
nonconjugated polymer backbone and with stable radical sites present
on the side chains of the repeat units) can transport charge in the solid
state through oxidation−reduction (redox) reactions that occur
between the electronically localized open-shell pendant groups. As
such, pristine (i.e., not doped) thin films of these functional
macromolecules have electrical conductivity values on the same
order of magnitude as some common electronically active conjugated
polymers. However, unlike the heavily evaluated regime of conjugated
polymer semiconductors, the impact of molecular dopants on the
optical, electrochemical, and solid-state electronic properties of radical
polymers has not been established. Here, we combine a model radical
polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate)
(PTMA), with a small molecule redox-active salt, 4-acetamido-2,2,6,6-tetramethyl-1-oxopiperidinium tetrafluoroborate
(TEMPOnium), in order to elucidate the effect of molecular doping on this emerging class of functional macromolecular
thin films. Note that the TEMPOnium salt was specifically selected because the cation in the salt has a very similar molecular
architecture to that of an oxidized repeat unit of the PTMA polymer. Importantly, we demonstrate that the addition of the
TEMPOnium salt simultaneously alters the electrochemical environment of the thin film without quenching the number of openshell sites present in the PTMA-based composite thin film. This environmental alteration changes the chemical signature of the
PTMA thin films in a manner that modifies the electrical conductivity of the radical polymer-based composites. By decoupling
the ionic and electronic contributions of the observed current passed through the PTMA-based thin films, we are able to establish
how the presence of the redox-active TEMPOnium salts affects both the transient and steady-state transport abilities of doped
radical polymer thin films. Additionally, at an optimal loading (i.e., doping density) of the redox-active salt, the electrical
conductivity of PTMA increased by a factor of 5 relative to that of pristine PTMA. Therefore, these data establish an underlying
mechanism of doping in electronically active radical polymers, and they provide a template by which to guide the design of nextgeneration radical polymer composites.
■
INTRODUCTION
The development of functional macromolecules for organic
electronic applications (e.g., organic photovoltaic devices,
organic field effect transistors, and organic light-emitting
diodes) has expanded rapidly over the past three decades.1−7
In particular, polymeric conductors and semiconductors have
exhibited intriguing charge transport characteristics, and they
have shown great potential for application in myriad electronic
systems.8−10 Elucidating the charge transport means and the
ultimate charge transport ability of these polymers has been
crucial in understanding how the functionality of these systems
can be improved. Significantly, this involves investigating the
charge transport mechanisms in both single polymer systems as
well as probing structure−property relationships in polymer
blends and composites. In fact, numerous polymer composite
systems have shown enhanced properties for optoelectronic
and thermoelectric applications due to these types of
efforts.11−16 However, the mechanism by which these systems
© 2016 American Chemical Society
improve these properties ranges depending on the materials
being utilized. Thus, the systematic development of these
composite systems involves elucidating the electrical, structural,
and morphological interactions between the composite
materials, which are usually coupled. For instance, polymers
blended with carbon nanotubes and inorganic nanoparticles
have the ability to transport charge through a percolating
network within a polymer matrix, leading to high-performance
device responses.13,15,17,18 Furthermore, the addition of
inorganic nanoparticles to polymers has created morphological
changes within the polymer that have been shown to increase
electronic performance.19 Moreover, many of the characteristics
associated with these hybrid systems can be further translated
Received: April 8, 2016
Revised: June 6, 2016
Published: June 22, 2016
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the redox-mediated macroscopic charge transport process. By
using the small molecule analogue of an oxidized (i.e.,
oxoammonium cation-like form) PTMA repeat unit, we
demonstrate that the oxoammonium cations on the TEMPOnium small molecule act as charge transport sites that can
interact with redox-active sites on the polymer chains. We
quantify the molecular interactions within the TEMPOniumdoped PTMA thin films through Fourier transform infrared
(FTIR) and electron paramagnetic resonance (EPR) spectroscopy, and we demonstrate that the addition of TEMPOnium to
the system creates a clear increase in the total number of
cation-functionalized sites within the thin film. Furthermore,
the increase in cation-functionalized sites for charge carrier
transport is reflected in an electrical conductivity enhancement
of these thin films. Importantly, we distinguish the differences
in electronic and ionic conductivity through the utilization of
direct current (dc) bias measurements where the current that is
passed through the film is monitored as a function of time. In
doing so, we illustrate the doping of a radical polymer through
the use of a small molecule redox-active salt. Thus, this effort
establishes a means by which to evaluate how the incorporation
of redox-active salts into radical polymer thin films impacts
charge and ion transport in the amorphous radical polymer
matrix, and it provides a mechanistic description of redox-active
salt doping in radical polymer thin films. In this way, we
anticipate that these findings will provide a platform by which
to improve the electronic performance in the growing field of
solid-state radical polymer conductors.
to systems consisting of solely organic materials, which are of
interest due to their earth-abundant and nontoxic natures.
In fully organic systems, polymers that have been strategically
blended with other macromolecules or organic small molecules
have shown improvements in the charge transport ability (i.e.,
higher conductivity values) relative to the pristine (i.e., single
component) polymers. In fact, three of the most common
examples of the molecular doping of conjugated polymers are
as follows. First, poly(styrenesulfonate) (PSS) has been used as
a macromolecular dopant in the oxidation of many conjugated
polymer conductors. The most frequently implemented PSSdoped system is when PSS is combined with poly(3,4-ethylene
dioxythiophene) (PEDOT) to form a highly conductive
dispersion, which has been used for widespread applications.10,20−23 Second, and in a similar manner, desirable
electronic characteristics have been observed by doping
PEDOT with the small molecules (e.g., salts containing the
tosylate anion) as well.24,25 Third, 2,3,5,6-tetrafluoro-7,7,8,8tetracyanoquinodimethane (F4-TCNQ) has been used as a
dopant for another common polythiophene derivative, poly(3hexylthiophene) (P3HT), and the resulting interaction between
the dopant within the polymer matrix creates an increase in
electrical conductivity of the system.26−28 Varying the
molecular dopants in these multicomponent systems results
in differing nanostructural and electronic relationships, thereby
making the underlying charge transport process easier to probe;
however, much of the research in these areas has focused on
conjugated polymers and small molecules. Here, we extend
these analyses to an emerging materials class, radical polymers,
in order to begin to establish a paradigm for the doping of
nonconjugated polymer conductors.
Radical polymers are a group of conducting polymers that
have shown charge transfer characteristics both in solution and
in the solid state.29−34 Unlike traditional semiconducting and
conducting polymeric systems, radical polymers typically have
completely nonconjugated backbones and contain open-shell
chemistries as the pendant groups of their macromolecular
backbones. Moreover, radical polymers transport charge
through a reversible oxidation−reduction (redox) reaction
where charges are able to hop from radical site to radical site.
While the solid-state charge transport characteristics of a model
pristine radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy
methacrylate) (PTMA), have been quantified,32,35−39 little has
been done to assess how small molecules can be incorporated
into the polymer thin films in order to manipulate the charge
transport (e.g., the macroscopic electrical conductivity) in a
controlled manner as has been done with common conjugated
polymers. This could be a critical handle by which to tune the
properties of the polymer as the oxidation state of the
functionalized site has been shown to contribute to the
electrical conductivity of PTMA.40,41 Specifically, by creating a
mixture of neutral radical sites and oxidized oxoammonium
cation sites, a change in the electrical conductivity created a
fundamental change in the charge transport ability in this
previous work. However, because continually oxidizing the
polymer leads to the creation of nonfunctional sites, the charge
transport interaction between radical and cation sites could not
be fully profiled.40
Here, we overcome this previous limitation in order to
establish how the addition of a redox-active small molecule salt,
4-acetamido-2,2,6,6-tetramethyl-1-oxopiperidinium tetrafluoroborate (TEMPOnium), to pristine PTMA causes alterations to
the electrochemical compositions of the thin films and, thus,
■
EXPERIMENTAL METHODS
Materials. All chemicals were used as received from Sigma-Aldrich
unless otherwise noted. The small molecule salt dopant, 4-acetamido2,2,6,6-tetramethyl-1-oxopiperidinium tetrafluoroborate (TEMPOnium), and 2,2,6,6-tetramethyl-4-piperidinyl methacrylate (TMPM)
were purchased from TCI America. The chemical structures of both
molecules are inset in Figure 1a. Poly(3,4-ethylene dioxythiophene)
doped with poly(styrenesulfonate) (PEDOT:PSS) was purchased
from Clevios. PTMA (Mn = 11.3 kg mol−1 and Đ = 1.4, as measured
using size-exclusion chromatography against polystyrene (PS) standards) was synthesized and characterized as reported previously.40
The synthesis of the particular polymer, generated through a
reversible addition−fragmentation chain transfer (RAFT) polymerization mechanism, used in this effort is as follows. In a 100 mL vessel
that contained a Teflon-coated magnetic stir bar, 5 g of 2,2,6,6tetramethyl-4-piperidinyl methacrylate (TMPM, 22.2 mmol), 75 mg of
2-phenyl-2-propylbenzodithioate (0.276 mmol), and 9 mg of 2,2′azobis(2-methylpropylnitrile) (AIBN, 0.5055 mmol) were dissolved in
75 mL of toluene. The reaction solution was degassed via three
freeze−pump−thaw cycles and was then heated to 75 °C. The reaction
was allowed to proceed for 24 h. After cooling the polymerization
solution, the polymer was precipitated into cold hexanes, filtered, and
dried under reduced pressure overnight to generate the PTMPMRAFT polymer precursor. The chain transfer terminus associated with
the RAFT polymerization was removed by reacting the parent polymer
with excess AIBN (75:1 molar excess of AIBN to end groups). The
removal of the chain transfer terminus is necessary before oxidation as
attempting to oxidize the parent polymer with the chain transfer
terminus intact results in an insoluble product. In a 100 mL vessel
containing a Teflon-coated magnetic stir bar, 2.5 g of PTMPM-RAFT
(Mn = 11.3 kg mol−1) and 1.72 g of AIBN were dissolved in 60 mL of
toluene. The reaction solution was degassed via three freeze−pump−
thaw cycles and then heated to 75 °C. The reaction was allowed to
proceed for 24 h. Then, the polymer was precipitated into cold
hexanes, filtered, and dried under reduced pressure overnight. This
PTMPM polymer was subsequently oxidized to PTMA by reacting the
polymer with m-chloroperoxybenzoic acid (mCPBA) in anhydrous
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DMF (2 vol %) and benzene (98 vol %) solvent mixture with a
concentration of 0.2 mg of polymer per 1 mL of solvent. Cyclic
voltammetry data of the neat TEMPOnium and TEMPOnium-doped
PTMA solutions (25 mg total) in a 300 mL acetonitrile solution,
containing 0.05 M tetrabutylammonium tetrafluoroborate as the
supporting electrolyte, were obtained in a three-electrode electrochemical cell with a platinum mesh counter electrode. The potential
was swept between −0.5 V ≤ V ≤ +1.2 V versus an Ag/AgCl
(saturated KCl) reference electrode at a scan rate of 100 mV s−1 using
a Princeton Applied Research potentiostat.
Solid-State Thin Film Fabrication and Electrical Characterization. Tin-doped indium oxide (ITO) substrates (Delta Technologies, Inc.) were cleaned in acetone, chloroform, and isopropyl
alcohol by sonication for 10 min each in a sequential manner.
Subsequently, 0.2 mL of an aqueous suspension of PEDOT:PSS (1.75
wt % solids content) were spun-coat onto the cleaned ITO substrates
at a rate of 2000 rpm for 60 s to create a ∼100 nm thick layer. A strip
of PEDOT:PSS on the edge of the substrate was removed with water
using a cotton applicator to expose the bottom ITO contact. These
substrates were then baked at 120 °C for 20 min to remove any
residual water. Solutions containing PTMA and TEMPOnium (with a
total solids mass of 5 mg) were dissolved in 1 mL of DMF and stirred
at room temperature for 0.5 h. The solutions contained TEMPOnium
added in 0.5 mg increments, with the highest doped solution having
2.5 mg of both PTMA and TEMPOnium. A similar fabrication
procedure was used when fabricating TEMPOnium-doped PTMPM
devices and PTMA devices doped with tetrabutylammonium
tetrafluoroborate for control experiments. All of the results shown
below are converted into a molar fraction by using the molecular
weight of a repeat unit of PTMA and the molecular weight of
TEMPOnium. That is the molar basis is the ratio of the moles of
TEMPOnium to the total moles of TEMPOnium and PTMA repeat
units in the blended material. These percentage values are those listed
in all figures of the article.
After ensuring complete dissolution of the polymer species in
solution, 0.1 mL of the solution was cast on the ITO-PEDOT:PSS
substrates, which were held at a temperature of 140 °C. After baking
the films for 5 min, heat was removed from the films, and they were
allowed to cool to ∼50 °C before the substrate was removed from the
hot plate. This yielded transparent films with a total film thickness of
∼800 nm. After coating of the PTMA thin films, the substrates were
brought inside of an inert atmosphere glovebox. Housed inside of the
glovebox was a thermal evaporator, and this instrument was used to
deposit 100 nm of gold into patterned arrays on top of the PTMA
films using a shadow mask under reduced pressure (P ∼ 10−5 bar).
The pattern of the top Au contacts and the bottom ITO contact
resulted in a total device area of 0.06 cm2. All film thicknesses were
determined using a KLA Tencor profilometer. The current−voltage
response of the thin films was evaluated using a Keithley 2400
sourcemeter and recorded using an in-house LabView code. The
current−voltage data were obtained by sweeping voltages across −3 V
≤ V ≤ +3 V at a rate of 1.5 V s−1 and measuring the current between
the top gold electrode and grounded bottom ITO electrode.
Additional sweep rates were probed in order to confirm the
consistency of the electrical measurements. The current as a function
of time data for the PTMA-based thin films were also obtained using
the Keithley 2400 sourcemeter using a constant voltage of +2.0 V. The
data were collected at a rate of 10 s per data point over a time span of
48 h for the longer time studies and at 1 s per data point for the
shorter time studies. Temperature-dependent conductivity experiments were performed between a temperature range of 100 K ≤ T ≤
350 K at intervals of 25 K under vacuum using a Lakeshore Model
TTPX vacuum probe station. The sample was left to equilibrate at
each temperature for 20 min prior to data acquisition. The current−
voltage data were obtained in these measurements in the same manner
as those specified above.
Figure 1. FTIR spectra of TEMPOnium-loaded PTMA thin films. As
the loading of TEMPOnium is increased, a clear signal at υ ≈ 1540
cm−1 increases in intensity, and this signal is associated with the +N
O chemistry. Additionally, the relative intensity of the signal
corresponding to the nitroxide radical (υ ≈ 1460 cm−1) does not
decrease dramatically as TEMPOnium is added, indicating that the
composite film contains roughly the same amount of radical species as
a pristine PTMA thin film. The signal observed in the doped PTMAbased thin films at υ ≈ 1650 cm−1, which is not seen in the pristine
PTMA, is observed in a thin film of the pristine TEMPOnium salt
(Figure S1). The chemical structures of both PTMA and the
TEMPOnium salt are inset into the figure.
dichloromethane (DCM). In this reaction, 1.5 g of PTMPM (6.7
mmol of repeat units) and 3 g of mCPBA (17 mmol) were separately
dissolved in 15 and 30 mL of dichloromethane, respectively. The two
solutions were combined in a 100 mL vessel that contained a Tefloncoated magnetic stir bar, and the reaction was allowed to proceed at
room temperature for 3 h. The polymer solution was then washed with
an aqueous sodium carbonate solution (20 wt %) three times. The
organic fraction was collected, precipitated into hexanes, filtered, and
dried under reduced pressure overnight.
Molecular Characterization. Size exclusion chromatography
(SEC) results were obtained using a Hewlett-Packard 1260 Infinity
series equipped with three PLgel 5 μm MIXED-C columns. The SEC
was calibrated using polystyrene standards (Agilent Easi Cal) ranging
from 1 to 200 kg mol−1. Tetrahydrofuran (THF) flowing at a rate of 1
mL min−1 and at a temperature of 40 °C was used as the mobile phase.
Fourier transform infrared (FTIR) spectroscopy data were collected
using a Thermo-Nicolet Nexus FTIR on a diamond substrate using a
deuterated triglycine sulfate (DTGS) KBr detector and a KBr beam
splitter. Experiments were performed under a nitrogen purge. The data
were collected in 36 scans between 800 cm−1 ≤ υ ≤ 3500 cm−1. The
samples for the FTIR data acquisition were cast on glass substrates,
using the conditions listed below for the deposition of the films onto
PEDOT:PSS. Then, they were placed on top of the diamond (i.e., with
the PTMA interface that was away from the glass substrate touching
the diamond) to acquire the FTIR spectra. Solution ultraviolet−visible
(UV−vis) light absorption spectroscopy experiments were conducted
using a Cary 60 spectrometer where dimethylformamide (DMF)
served as the blank, and the solution absorption was monitored
between wavelengths of 200 nm ≤ λ ≤ 800 nm. The total
concentration of each solution was 2.5 mg of solids content per 1
mL of DMF. Electron paramagnetic resonance (EPR) spectroscopy
data were obtained using a Bruker EPR-EMX spectrometer using a
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■
RESULTS AND DISCUSSION
As expected, the addition of the TEMPOnium salt into the
PTMA thin films increases the number of oxoammonium
cations present in the polymer thin film. Specifically, as shown
in the FTIR spectrum of the pristine TEMPOnium salt (Figure
S1), the signal at υ ≈ 1540 cm−1 corresponds well with the
oxoammonium cation, and the FTIR spectra of the pristine and
doped PTMA thin films show the increasing presence of the
oxoammonium cation (Figure 1).40 Additionally, the FTIR
spectra do not display any additional peaks that are not already
seen within the pristine PTMA or pristine TEMPOnium films,
demonstrating that the interactions between the PTMA and the
TEMPOnium within the films are relatively stable and
nonreactive, in the absence of an applied potential and in
ambient conditions. That is the magnitude of the nitroxide
radical peak at υ ≈ 1460 cm−1 (scaled to the loading of the
PTMA in the films) remains relatively constant across a range
of TEMPOnium loadings. This implies that the addition of the
TEMPOnium to the system does not decrease the overall
radical density within the composite thin film although it does
not necessarily ensure that there were no oxidation−reduction
reactions occurring between the repeat units of the PTMA
chains and the TEMPOnium salts in solution (vide inf ra). To
probe this initial observation more deeply, solution EPR spectra
(dissolved in the same mass ratios as that of the thin films used
in the FTIR spectroscopy measurements) show that the
absorption intensities corresponding to the radical density
remain constant with increasing TEMPOnium loadings (Figure
S2a,b). It must be noted that the EPR data and a CV spectrum
of 100% TEMPOnium (Figure S3) indicate that the
commercially available TEMPOnium contains a non-negligible
amount of radical sites. Furthermore, in order to decouple any
electron exchange effects the TEMPOnium could have with
PTMA, a separate EPR analysis was performed where PTMA
was selectively precipitated from an initial solution containing
both dissolved PTMA and TEMPOnium (Figure S4). This
illustrates that the reprecipitated PTMA does show some
hyperfine splitting due to decreased spin exchange interactions
from redox reactions occurring between the TEMPOnium and
PTMA (i.e., it is likely that some of the PTMA repeat units
were oxidized to positively charged species and some of the
TEMPOnium cation species were reduced to TEMPO-like
units). However, the integrated EPR intensity values indicate
the radical density of the overall system does not significantly
change. So while there is some electron exchange between
PTMA and TEMPOnium, it does not impact the total radical
content of the system. This important result demonstrates that
the addition of the TEMPOnium salt to the PTMA does
impact the electrochemical nature of the main chain of the
radical polymers, which, in turn, should impact the solid-state
electrical conductivity.
Furthermore, the addition of the TEMPOnium salt to PTMA
creates a clear oxoammonium cation absorbance signal in the
ultraviolet−visible (UV−vis) light absorption spectra of
solutions containing the small molecule mixed with PTMA.
This again indicates that the oxoammonium cation and
nitroxide radical functionalities remain conserved when
combined in solution (Figure 2). In fact, as the loading of
TEMPOnium is increased, the peak absorbance of the solutions
increases at lower wavelengths, away from where PTMA
typically absorbs at λ ≈ 450 nm.36 This shift corresponds more
closely to the absorption peak of the TEMPOnium small
Figure 2. UV−vis light absorption spectra illustrating the increasing
oxoammonium cation presence as the TEMPOnium loading is
increased. The increased light absorbance at lower wavelengths (λ <
450 nm) for the samples containing higher TEMPOnium loadings
indicates increasing presence of the oxoammonium cation.
molecule itself and illustrates the presence of the increasing
TEMPOnium within the system.
As the TEMPOnium salt is added at higher loadings within
the PTMA thin films, there is a change in the redox behavior of
the system. Specifically, the initial onset oxidation and
reduction peaks of the pristine PTMA appear at Vox ≈ +0.82
V and Vred ≈ +0.68 V (vs Ag/AgCl), respectively, and they
change minimally with the addition of the TEMPOnium salt at
8% loading (Figure 3). As the loading of TEMPOnium is
Figure 3. Cyclic voltammetry data of PTMA solutions in 300 mL of
acetonitrile with representative loadings of the TEMPOnium salt.
Manipulating the TEMPOnium loading does not change the onset
oxidation peak but shifts the reduction peak at higher loadings.
increased up to 44%, the reduction peak shifts to Vred ≈ +0.60
V. In this way, there is a slight alteration with respect to the
electrochemical environment which is experienced by the
PTMA upon the addition of large amounts of TEMPOnium.
This shift is also independent of the reduction peak exhibited
by a 100% TEMPOnium sample (Figure S3). This is a rather
expected change, as increasing the amount of oxoammonium
cation sites should lead to a lowered energy required for
reduction of the system while not affecting the oxidation energy
of the composite. This is because a larger fraction of the thin
films have species that can be reduced but not oxidized (i.e., the
oxoammonium cation species of the TEMPOnium dopants).
However, the change in peak location minimally changes the
translated singularly occupied molecular orbital (SOMO)
energy level of the system.35 Thus, the increasing TEMPOnium
loading does not create any additional energy barriers relative
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sweep rate was fast enough to make ionic contributions
negligible, as has been demonstrated previously.44
In order to confirm that the increase in electrical conductivity
was due to interaction between the TEMPOnium and PTMA
repeat units specifically, control experiments were performed.
The use of TEMPOnium as a dopant for the poor conducting
closed-shell precursor to PTMA, PTMPM, showed no
enhanced electronic performance (Figure S5). The (relatively
low) conductivity of the TEMPOnium loaded PTMPM is
marginally different (1.5-fold increase) relative to the pristine
PTMPM film. This is significantly lower than the 5-fold
increase observed for the PTMA and TEMPOnium blend.
Furthermore, the absolute conductivity of this blend is much
lower than that of the radical polymer-based system. Moreover,
the use of a different salt that lacked the TEMPOnium
functionality, tetrabutylammonium tetrafluoroborate, as a
dopant for PTMA also showed no trend in electronic
performance (Figure S6). In fact, the conductivity of these
two systems (tetrafluoroborate salt doped PTMA and pristine
PTMA) differs by less than 5%. This indicates that the increase
in electrical conductivity when using TEMPOnium as a dopant
is due to the redox interaction between the TEMPOnium and
PTMA.
Additionally, the conductivity extracted from current−
voltage data shows no dependence on voltage sweep rate
(Figure S7), if sufficiently fast biasing sweeps are employed.
This illustrates the consistency of the device performance
regardless of speed of voltage bias switching examined in this
effort, and it highlights that the main contribution to the
increase in conductivity of these doped thin films is the
electrical conductivity and not the ionic conductivity.
While continually increasing the loading of TEMPOnium
within the PTMA thin films will necessarily add more cationfunctionalized charge transport sites, the trend of increasing the
conductivity of the system does not continue. In fact, increasing
levels of TEMPOnium begin to cause decreases in the electrical
conductivity of the thin films, and these values eventually fall to
levels below that of pristine PTMA. As seen in Figure 5, this
decrease in conductivity can be attributed to a drastic decrease
in film quality, which has been seen numerous times previously
to what is observed in pristine PTMA with respect to charge
injection and extraction when the PTMA thin films are tested
with respect to their electrical conductivity in experiments
described below. That is, this SOMO energy level of 5.2 eV
matches very well with the work functions of both Au and
PEDOT:PSS. The minimal energy difference between these
levels suggests that an ohmic contact will be made with both
PEDOT:PSS and Au, allowing for thin film devices with these
electrodes to be fabricated and characterized in a facile manner.
Conversely, a quantifiable change in the solid-state electrical
conductivity of these thin films is observed by varying the
amount of TEMPOnium in the film (Figure 4). Importantly,
Figure 4. Relative conductivity of PTMA thin films as a function of
TEMPOnium loading. The geometry used for the conductivity
experiments was one that used PEDOT:PSS coated onto ITO and
Au as the two contacts of the thin film as the work functions of these
two materials have useful electronic alignment with the transport level
of PTMA (i.e., an ohmic-like contact is made). The maximum in
electrical conductivity occurs at a TEMPOnium loading of 8% (on a
molar basis) to produce a material that has a 5-fold increase in
electrical conductivity relative to pristine PTMA.
the addition of the small molecule TEMPOnium to the PTMA
thin film increases the conductivity of the thin film by a factor
of 5 at the optimized loading condition (i.e., 8% TEMPOnium). This increase in electrical conductivity is a result of the
additional cation sites that can participate in the charge
transport redox reaction as well as the contributions that exist
from the counteranion stabilizing these cation sites. The
inclusion of these electron-deficient nitrogen cation sites
increases the overall hole density within the system, thereby
increasing the charge transport ability of the radical polymer. As
each cation salt is added, the ratio of radical to cation species is
changed. In turn, this could promote the charge transfer
reactions throughout the system due to the increased presence
of an electron-deficient species acting as acceptor sites for the
radical sites. Furthermore, by adding the small molecule salt,
the intermolecular packing within the film changes. It is
possible that this could decrease the site-to-site distance for a
charge hopping event, on average, and this has been
computationally predicted to improve the charge transport
ability of PTMA.42,43 From a macroscopic perspective, this
allows for more charges to travel across the thin film from one
electrode to the other without being localized to a redox site
that is isolated from other redox sites for a significant period of
time (i.e., a deep trap site). Note that all of the samples were
transiently biased at a sweep rate of 3 V s−1 to ensure that the
Figure 5. Top-down optical microscopy images of (a) pristine (0%
TEMPOnium) PTMA, (b) 8% TEMPOnium-loaded PTMA, (c) 17%
TEMPOnium-loaded PTMA, and (d) 26% TEMPOnium-loaded
PTMA thin films. All loading values are in terms of molar percentages.
The image shows the film coated on both the ITO-PEDOT:PSS and
glass-PEDOT:PSS interfaces. Both pristine PTMA and 8% TEMPOnium-loaded PTMA films have minimal physical defects, and they
appear relatively smooth. The 17% TEMPOnium-loaded PTMA film
and the 26% TEMPOnium-loaded film contain clear physical defects
on both the glass and ITO surfaces. Thus, the additional amount of
small molecule seems to hinder quality film formation.
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for the solution casting of small molecule organics.45,46
Specifically, the optical microscopy images of pristine PTMA
(Figure 5a) and 8% TEMPOnium-loaded PTMA films (Figure
5b) exhibit minimal physical defects and appear fairly smooth
to the eye. This correlates well with the increasing electrical
conductivity up to this loading of small molecule. However, by
increasing the TEMPOnium loading to just 17%, clear optical
defects begin to form in the film (Figure 5c). The optical
microscopy images shown indicate that the quality of the tested
thin films are especially poor in the region where the film rests
on the glass/PEDOT:PSS surface. At a loading of 26%, the thin
film has heavily deteriorated, greatly inhibiting the peak
conductivity of the system (Figure 5d). This is not surprising
as traditionally small molecules have been known to have
distinct film formation issues when solution processed.46−48
Furthermore, a casted film of pristine TEMPOnium also
displays numerous physical defects (Figure S8). Ultimately,
while the addition of charge transporting sites should lead to
more defect-free transport, processing of the film does not
allow for confirmation of this effect in macroscopic devices.
Therefore, it is noted that extreme care must be implemented
when balancing the redox-active properties of molecular
dopants with the formation of continuous thin films in solidstate radical polymer applications.
In order to illustrate the effects of the contributions to the
current flowing through the system from the cation and anion
species present in the film, the current was measured over a
constant voltage for two separate time periods, as this strategy
has been effectively used for ion-containing conjugated
semiconducting polymers previously.49,50 The initial temporal
response (Figure 6) illustrates a sharp increase in current in the
indicative of just the electronic contributions to the overall
conductivity. After ∼4 h, the current values at each doping level
have stabilized, and these current values remain stable. It is
noted that even after this extended time frame, the overall
current within the TEMPOnium loaded films is larger than that
found in the pristine PTMA film, which is consistent with the
rapid scanning results of Figure 4.
A distinct lack of temperature dependence with respect to
the conductivity of the pristine and doped PTMA thin films
further demonstrates the minimal ion contribution to the
overall conductivity of the system. Figure 7 illustrates the
Figure 7. Conductivity measured as a function of temperature for 8%
TEMPOnium (blue) and 0% TEMPOnium (black) thin films. There
is no temperature dependence for the TEMPOnium-loaded devices
and minimal temperature dependence for the pristine PTMA films.
This implies that the ionic contribution to the overall conductivity is
very small and that the redox reaction dictating charge transport is not
limited within this temperature range. These data are consistent with a
rapid redox electron transfer reaction within the film that is limited by
the site-to-site proximity of the relatively localized redox-active sites.
temperature effect on conductivity of an 8% TEMPOnium
doped PTMA film and a pristine PTMA film between 100 and
350 K. This behavior would indicate that even at low
temperatures where ion movement should be nearly nonexistent, the electrical conductivity is maintained. Thus, the
charge transport is occurring exclusively through charge
hopping via redox exchange reactions, and this redox reaction
is not limited by the temperature of the system within this
range. Therefore, the loading of TEMPOnium redox-active
sites into the pristine PTMA thin films does alter the redox
reaction, which leads to the improved electronic conductivity of
the radical polymer thin films.
Thus, the mechanism of transport in these TEMPOniumdoped PTMA films can be viewed in a manner somewhat akin
to the electron donor−electron acceptor systems implemented
commonly in the conjugated polymer literature. That is the
redox-active oxoammonium cation serves as an electrondeficient species within the thin film. As charges are injected
into the system and the redox charge hopping transport begins,
the oxoammonium cation sites facilitate the one electron redox
reaction by preferentially accepting a charge carrier from either
the electrodes or from a neighboring redox-active site.
Additionally, it is possible that the insertion of the small
molecule redox-active salt changes the intermolecular packing
within the thin film, thereby decreasing the site to site distance
between redox-active sites. If the TEMPOnium is in fact acting
as an electron acceptor that enables the redox charge transport,
we would anticipate that increasing the cation salt concen-
Figure 6. Current response as a function of time for an ITOPEDOT:PSS-TEMPOnium doped PTMA-Au thin film held at a
constant bias of V = +2.0 V for a 10 s time frame. The initial increase
in current is due to the movement of the counteranion BF4− stabilizing
the TEMPOnium as well as the cation salt itself. As expected, the
highest concentration of ions shows the largest initial current.
first 10 s at a constant applied dc bias of V = +2.0 V. This
increase in current can be attributed to the initial movement of
ions within the system.49,50 Of course, this does not occur in
the pristine PTMA film, where there is a very small amount
(due to residual synthetic impurities) of mobile ions present.
Once this mobilization and polarization of the ions have
occurred in the TEMPOnium-loaded PTMA thin films, the
current begins to decrease over the course of a few hours under
the constantly applied bias (Figure S9). This decrease in
current can be attributed to diminishing effects from the ionic
current. Thus, the steady-state current that is reached is
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DOI: 10.1021/acs.macromol.6b00730
Macromolecules 2016, 49, 4784−4791
Macromolecules
■
tration to higher doping levels would increase the overall
conductivity of the system. This was clearly not the case, as
increasing the TEMPOnium molar concentration diminished
the conductivity of the thin films due to a great decrease in film
quality. Thus, we believe that using alternative redox-active
materials, especially those that are polymeric in nature and form
high-quality films, to serve as dopants should similarly lead to
an increase in the electrical performance of radical polymers,
assuming that the redox-active material and radical polymer can
form well-defined, defect-free thin films.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail [email protected] (B.W.B.).
Author Contributions
†
A.G.B. and S.H.W. contributed equally to this work.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We deeply thank the Air Force Office of Scientific Research
(AFOSR) for the support of the majority of this work through
the Organic Materials Chemistry Program (Grant FA9550-151-0449, Program Manager: Dr. Charles Lee). The contributions
of A.J.W. were supported by the National Science Foundation
(NSF) CAREER Award through the Polymers Program
(Award 1554957, Program Manager: Dr. Andrew Lovinger),
and we gratefully acknowledge this support as well. J.S.L.
appreciatively acknowledges the NSF for support through the
Graduate Research Fellowship Program (Award DGE1333468).
CONCLUSIONS
The addition of the redox-active TEMPOnium salt to PTMA
thin films was found to increase the concentration of
oxoammonium cations participating in redox-facilitated charge
transport. However, the addition of the salt did not diminish
the overall radical density of the system. An optimal loading of
8% of TEMPOnium (on a molar basis) increased the
conductivity of the film by a factor of 5 relative to a pristine
PTMA film. This increase in conductivity is attributed to
additional redox-active oxoammonium species facilitating the
redox charge transport as an electron acceptor. Unfortunately,
larger loadings of the TEMPOnium salt resulted in decreased
macroscopic electrical conductivity due to clear physical defects
in the thin films; these defects were associated with the poor
film-forming properties of the small molecule additive. The
contributions of the counteranion stabilizing the cation in the
TEMPOnium salt were illustrated through the measurement of
current in the thin films over an extended period of time.
Furthermore, no dependence of the conductivity on temperature is observed for the TEMPOnium doped films. We
anticipate that the doping of radical polymers with a different
redox-active salt that can form defect-free films would result in
a larger increase in electronic performance. As such, these
results provide a basis for improving the electronic behavior of
radical polymers through an understanding of the role redoxactive salts and oxidation state play in charge transport within
radical polymer thin films.
■
Article
■
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ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macromol.6b00730.
FTIR spectrum of a pristine TEMPOnium thin film, EPR
data of PTMA loaded with TEMPOnium, EPR data
demonstrating the radical density of PTMA loaded with
TEMPOnium, CV data of a 100% TEMPOnium sample,
EPR data comparing pure PTMA and reprecipitated
PTMA, current−voltage data illustrating the effect of
doping PTMPM with TEMPOnium, current−voltage
data demonstrating the effect of doping PTMA with
tetrabutylammonium tetrafluoroborate, current−voltage
data taken at varying sweep rates to indicate the lack of
dependence of current−voltage behavior on sweep rate,
optical microscopy image of a 100% TEMPOnium film,
current as a function of time at a constant applied bias,
SEM images of pristine PTMA and TEMPOnium-loaded
PTMA thin films (PDF)
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Macromolecules 2016, 49, 4784−4791
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