Modification of the Poly(bisdodecylquaterthiophene) Structure for High and Predominantly Nonionic Conductivity with Matched Dopants

Article abstract of DOI:10.1021/jacs.7b05300

Four p-type polymers were synthesized by modifying poly(bisdodecylquaterthiophene) (PQT12) to increase oxidizability by p-dopants. A sulfur atom is inserted between the thiophene rings and dodecyl chains, and/or 3,4-ethylenedioxy groups are appended to thiophene rings of PQT12. Doped with NOBF4, PQTS12 (with sulfur in side chains) shows a conductivity of 350 S cm^-1, the highest reported nonionic conductivity among films made from dopant-polymer solutions. Doped with tetrafluorotetracyanoquinodimethane (F4TCNQ), PDTDE12 (with 3,4-ethylenedioxy groups on thiophene rings) shows a conductivity of 140 S cm^-1. The converse combinations of polymer and dopant and formulations using a polymer with both the sulfur and ethylenedioxy modifications showed lower conductivities. The conductivities are stable in air without extrinsic ion contributions associated with PEDOT:PSS that cannot support sustained current or thermoelectric voltage. Efficient charge transfer, tighter ?€-?€ stacking, and strong intermolecular coupling are responsible for the conductivity. Values of nontransient Seebeck coefficient and conductivity agree with empirical modeling for materials with these levels of pure hole conductivity; the power factor compares favorably with prior p-type polymers made by the alternative process of immersion of polymer films into dopant solutions. Models and conductivities point to significant mobility increases induced by dopants on the order of 1-5 cm² V^-1 s^-1, supported by field-effect transistor studies of slightly doped samples. The thermal conductivities were in the range of 0.2-0.5 W m^-1 K^-1, typical for conductive polymers. The results point to further enhancements that could be obtained by increasing doped polymer mobilities.

Full text of DOI:10.1021/jacs.7b05300

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Article
Modification of the Poly(bisdodecylquaterthiophene) (PQT12) Structure
for High and Predominantly Nonionic Conductivity with Matched Dopants
Hui Li, Mallory E. DeCoster, Robert M. Ireland, Jian Song, Patrick E Hopkins, and Howard E. Katz
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b05300 • Publication Date (Web): 24 Jul 2017
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Modification of the Poly(bisdodecylquaterthiophene) (PQT12) Struc-
ture for High and Predominantly Nonionic Conductivity with Matched
Dopants
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Hui Li,† Mallory E. DeCoster,‡ Robert M. Ireland,† Jian Song,† Patrick E. Hopkins‡ and Howard
E. Katz*,†
†Department of Materials Science and Engineering, Johns Hopkins University, 3400 North Charles Street, Balti-
more, Maryland 21218, USA
‡Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia 22904, USA
ABSTRACT: Four p-type polymers were synthesized by modifying poly(bisdodecylquaterthiophene) (PQT12) to increase
oxidizability by p-dopants. A sulfur atom is inserted between the thiophene rings and dodecyl chains, and/or 3,4-
ethylenedioxy groups are appended to thiophene rings of PQT12. Doped with NOBF4, PQTS12 (with sulfur in side chains)
shows conductivity of 350 S cm-1, the highest reported nonionic conductivity among films made from dopant-polymer
solutions. Doped with tetrafluorotetracyanoquinodimethane (F4TCNQ), PDTDE12 (with 3,4-ethylenedioxy groups on
thiophene rings) shows conductivity of 140 S cm-1. The converse combinations of polymer and dopant, and formulations
using a polymer with both the sulfur and ethylenedioxy modifications showed lower conductivities. The conductivities
are stable in air without extrinsic ion contributions associated with PEDOT:PSS and that cannot support sustained cur-
rent or thermoelectric voltage. Efficient charge transfer, tighter π-π stacking and strong intermolecular coupling are re-
sponsible for the conductivity. Values of nontransient Seebeck coefficient and conductivity agree with empirical modeling
for materials with these levels of pure hole conductivity; the power factor compares favorably with prior p-type polymers
made by the alternative process of immersion of polymer films into dopant solutions. Models and conductivities point to
significant mobility increases induced by dopants, on the order of 1-5 cm2 V-1 s-1, supported by field-effect transistor stud-
ies of slightly doped samples. The thermal conductivities were in the range of 0.2 ~ 0.5 W m-1 K-1, typical for conductive
polymers. The results point to further enhancements that could be obtained by increasing doped poly-mer mobilities.
soluble in common organic solvents so that the polymer
INTRODUCTION
is mainly prepared by electrochemical polymerization,
Doped polymeric semiconductors applied to thermoelec-
which results in difficulty controlling the film composi-
trics research have attracted increased attention because
tion, thickness and morphology. These similar drawbacks
they show useful attributes such as low thermal conduc-
exist in several works about other polythiophenes con-
tivity, structural and compositional tunability, and ame-
taining the EDOT subunit synthesized by electrochemical
nability to flexible, printable, large area applications.1,2
polymerization.10-12 On the other hand, the high thermoe-
Poly(3,4-ethylenedioxythiophene) (PEDOT) is among the
lectric efficiency of PEDOT:PSS can be partly attributed
most widely used doped polymers and has been investi-
to an ionic Seebeck effect,13 which is transient and sensit-
gated intensively in the fields of thin-film conductive
coatings and organic electronics because of its environ-
mental stability and wide range of conductivity.3-6 By us-
ing poly(styrenesulfonic acid) (PSS) as the counterion,
PEDOT:PSS shows high conductivity and solution pro-
cessability.7,8 Replacing PSS by small anions, such as
t0sylate (Tos), PEDOT:Tos can reach electrical conductiv-
ity values exceeding 1000 S cm-1 and a high reported
thermoelectric efficiency.9 However, PEDOT itself is in-
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Figure 1. Chemical structures of polymers and dopants and their HOMO and LUMO energy levels, respectively.
ive to humidity, making the output power unstable and
complicating the data analysis.14,15 Most importantly, the
ionic transport contributes an unsustainable current to an
external circuit, which is detrimental to the operation.
For instance, doping this polymer with a weaker acceptor
(F6TCNNQ), the conductivity decreases to 2 S cm-1 be-
cause of a slightly negative offset between the HOMO
level of the polymer and the LUMO level of the dopant.32
The further improvement of performance will require
more rational molecular design and understanding of
structure-property relationships beyond PEDOT and
PBTTT.
The thermoelectric figure of merit, ZT=σS2T/λ, is used
to estimate the performance of thermoelectric material,
where σ and λ are electrical and thermal conductivity,
respectively, S is Seebeck coefficient and T is absolute
temperature. Lacking precise data on λ, thermoelectric
performance is often assessed via a parameter called the
power factor (PF) which equals σS2.16,17 Chemical doping
in solution is a key means of increasing the carrier con-
centration and mobility of the active layer, while main-
taining processing simplicity.18 Besides the highest report-
ed performance of PEDOT:PSS processed via aqueous
suspensions, high power factors were obtained by im-
mersing polymer films into dopant solutions19-21 or expos-
ing films to dopant vapors.22-25 For instance, immersing a
P3HT film in a solvent containing ferric triflimide pro-
vides a conductivity of 82 S cm-1 and PF of 20 μW m-1 K-2.20
To expand our knowledge of the relationship between
polymer structure and conductivity, we synthesized four
conductive polymers, including two new EDOT copoly-
mers. Taking the prototypical polymer PQT12 as structur-
al reference, we introduce sulfur atoms into the side
chains to get poly(bisdodecylthioquaterthiophene)
(PQTS12) and/or ethylenedioxy group modifying the thi-
ophene rings to obtain biEDOT copolymers (PDTDE12
and PDTDES12). All structures are shown in Figure 1. The
introduction of sulfur atoms or the ethylenedioxy groups
into the PQT12 raises the HOMO level of the new poly-
mers and endows them with a larger driving force to
achieve charge transfer between polymer and p-dopants.
An electrical conductivity of 350 S cm-1 was obtained by
doping PQTS12 with NOBF4, which is the highest value
attained for single solution-deposited p-type materials
without ionic conductivity, while high electrical conduc-
tivity of 140 S cm-1 was obtained by doping PDTDE12 with
F4TCNQ. This result suggests that the structural differ-
ences between the substituents (sulfur atoms or eth-
ylenedioxy group) incorporated onto PQT12 is responsible
for the higher performance with different dopants. Com-
bining the results of doping level determinations and
morphology observations, obtained by absorption and
grazing incidence X-ray scattering, respectively, we postu-
late that NOBF4 is prone to interact with sulfur in side
chains without unduly disturbing the packing of the pol-
ymer backbone, while π-conjugated F4TCNQ mainly im-
pacts the packing of polymer backbone and side chains,
simultaneously. Furthermore, no ionic contribution to
Seebeck effect is observed in the doped polymers with
Evaporating
F4TCNQ
or
(tridecafluoro-1,1,2,2-
tetrahydrooctyl)trichlorosilane (FTS) on top of a PBTTT14
film yields high σ over 200 S cm-1 and a large power factor
of ∼ 100 μW m-1 K-2.22,24 All-solution drop-casting or spin-
coating methods are easier to implement for flexible or
large-area devices and increase the accuracy with which
the doping level can be controlled. For instance, doped
with the strong acceptor F4TCNQ, σ of P3HT and PBTTT
can be improved from 10-6 S cm-1 for pure polymers up to
1-4 S cm-1.26-28 Kang et al. reported a significant increase of
the conductivity by further doping of PEDOT:PSS with a
small amount of F4TCNQ.29 Very recently, Müller et al.
reported an oligo ethylene glycol functionalized polythio-
phene doped with F4TCNQ resulting in an electrical con-
ductivity of up to 100 S cm-1.30 Donor-acceptor (D-A) pol-
ymers with high mobility are also developed as all-
solution thermoelectric materials. Recently, a diketo-
pyrrolopyrrole-based polymer doped by a strong acceptor
(CN6-CP) demonstrated a highest σ of 70 S cm-1.31 Howev-
er, the comparatively low-lying highest occupied molecu-
high conductivities, which is unlike PEDOT materials.33
The relationship of σ and S is in agreement with Chabin-
yc’s empirical model of extensive prior thermoelectric
lar orbit (HOMO) level of D-A polymers requires new
dopants with much lower lowest unoccupied molecular
orbit (LUMO) levels to achieve high doping efficiency.
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UV-Vis-NIR absorption spectra were employed to verify
data23 and fits the high performance part of a more de-
tailed model most recently reported by Snyder.15
the formation of charge transfer complexes of the four
polymers with different dopants (Figure 2). Doped films
were prepared by combining separate polymer solution
and NOBF4 solution or F4TCNQ solution according to
the desired doping concentration and then drop casting
the mixture onto a glass substrate. The molar ratio of all
doped polymers mentioned below is 0.5 unless indicated
otherwise, meaning one NOBF4 or F4TCNQ molecule is
added per two repeat units of polymer (one repeat unit
contains four thiophene rings). For the four pure poly-
mers, the film absorption ranges are between 400 ~ 700
nm, and the finer absorption peaks of PQT12 and PQTS12
(Figure 2a) compared to PDTDE12 and PDTDES12 (Figure
2b) indicate that more ordered structures exist in PQT12
and PQTS12 films. The charge transfer occurs with the
addition of the dopant because the neutral state of poly-
mers is bleached with simultaneous occurrence of new
absorption bands around 700 nm and > 1200 nm originat-
ing from polaronic transitions. Furthermore, negatively
charged F4TCNQ molecules are observed at 410 nm, 765
nm and 870 nm.41 The overlap of absorption between the
cation of the polymer
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RESULTS AND DISCUSSION
PQT12 and PQTS12 were synthesized via Stille coupling
while PDTDE12 and PDTDES12 were synthesized by using
a direct C-H coupling reaction34-36 that avoids using toxic
stannylated compounds. The detailed synthesis proce-
dures can be found in the Supporting Information. This
method was investigated to link EDOT subunits with oth-
er comonomers.37-39 By using this method, Imae et al. syn-
thesized an EDOT copolymer and reported the highest
conductivity of 10 S cm-1 via in situ electrochemical meas-
urements.37 To obtain soluble polymers, monomers 1 and
2 were functionalized with long alkyl chains. Monomer 3
was synthesized via previously reported methodologies.40
The molecular weights of the two biEDOT-based poly-
mers were controlled by adjusting amounts of Pd catalyst,
volumes of solvent, and in particular, the reaction time.
PQT12 and PQTS12 have good solubility in chloroform
and chlorobenzene while PDTDE12 and PDTDES12 show
limited solubility in chloroform but good solubility in
chlorobenzene and dichlorobenzene. The molecular
weights are summarized in Table 1. The molecular
weights of PDTDE12 and PDTDES12 might be underesti-
mated due to high-molecular-weight polymer filtered out
of THF prior to testing by gel permeation chromatog-
raphy. However, all thin-film devices were prepared with
chlorobenzene solution, which weakens the effect of DP
difference. Also, as seen later, mobility was not apparent-
ly correlated with polymer molecular weight. Compared
with PQT12, the influence of sulfurs on HOMO energy
level of PQTS12 is not pronounced, but the introduction
of ethylenedioxy groups significantly raises the HOMO
energy levels of PDTDE12 and PDTDES12 (Figure S1). The
detailed calculations of HOMO energy levels of polymers
and LUMO energy level of NOBF4 are described in SI.
Employing DFT calculation (Figure S2, left column), we
find the HOMOs of PDTDE12 and PDTDES12 can be delo-
calized on the oxygen atoms of ethylenedioxy group,
which make these two polymers easily oxidized.
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Table 1. Summary of the Molecular Weights and Elec-
trochemical Properties of Four Polymers
b
Polymer
Mn
PDI
DPa
Eox
HOMO
(eV)
(kDa)
(V)
PQT12
PQTS12
10.7
30.9
7.5
1.17
1.81
1.32
1.09
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42
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19
0.45
0.40
0.19
0.03
-5.09
-5.04
-4.83
-4.67
PDTDE12
PDTDES12
16.4
aDegree of polymerization. bOnset oxidation potential.
Figure 2. Thickness-normalized absorption spectra of un-
doped and doped polymers. All films were annealed at 120 oC
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in nitrogen for 10 min. (a) PQT12 and PQTS12; (b) PDTDE12
PDTDES12/NOBF4 shows the highest ratio of AI/AII, the
1
2
and PDTDES12.
electrical conductivity of the film is still low. That means
that even if every dopant molecule undergoes a charge
and anion of F4TCNQ makes the features difficult to dis-
tinguish. Therefore, we use the ratio of integrated area
between Band I (700 ~ 1200 nm) and Band II (400 ~ 700
nm) as an indication that greater AI/AII implies a higher
doping level (Table S1). According to the values of AI/AII,
we can reach the conclusions that: 1) For a certain poly-
mer, NOBF4 gives a higher ratio than that of F4TCNQ,
resulting from higher oxidizing activity of NOBF4; 2) For
all NOBF4-doped polymers, the ratio of AI/AII increases
with the rising HOMO level of the polymers, indicating
that efficient doping can be attributed to the large driving
force of charge transfer; 3) For F4TCNQ-doped polymers,
the ratio of AI/AII also increases with the rising HOMO
level of the polymers, but PDTDES12 seems to be an ex-
ception. Ultraviolet photoemission spectroscopy (UPS)
was employed to investigate the electronic properties of
undoped and doped polymers. As shown in Figure S3,
compared to undoped polymers, the HOMO edges of
doped PDTDE12 and PDTDES12 shift obviously toward
the Fermi level (EF = 0 eV), together with the cutoffs, as
expected for p-doping.42 Although it is difficult to obtain
the UPS spectra of pure PQT12 and PQTS12 because of
photohole accumulation shifting and distorting the spec-
trum of these relatively insulating polymers,43 the larger
shifts of doped PDTDE12 and PDTDES12 are observed
than those of doped PQT12 and PQTS12 indicating a high-
er doping level exists in doped PDTDE12 and PDTDES12,
which is consistent with the result of absorption spectra.
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Figure 3. Thermoelectric properties of NOBF4-doped
PQTS12 (a) and F4TCNQ-doped PDTDE12 (b) at different
molar doping ratios.
The thermoelectric parameters σ, S and PF were meas-
ured under the same molar ratio (0.5) of dopant molecule
to repeat unit (Figure S4). The data are summarized in
Table S2. The value of σ was measured via a four point
probe method. The Seebeck coefficient was obtained by
linear fitting of a data series taken by imposing tempera-
ture differences across the samples and measuring the
thermovoltages. Thin films were drop cast from solution
mixtures (see Supporting Information). For the NOBF4-
doped polymers, PQTS12 shows the highest conductivity
of up to 350 S cm-1. For F4TCNQ-doped polymers,
PDTDE12 gives the highest conductivity of 65 S cm-1. We
adjust the ratio of dopant for these two combinations to
further optimize the performance. Figure 3 shows the
device parameters as a function of molar doping ratio of
NOBF4 or F4TCNQ. The σ values of films change dramat-
ically with increasing the concentration of dopants into
polymers. For PQTS12, the addition of NOBF4 with molar
ratio of 0.5 results in the highest conductivity and an op-
timized PF of 8 μW m-1 K-2. An increase of σ, from 65 S
cm-1 to 140 S cm-1, is observed after a 2-fold increase in
molar doping ratio from 0.5 to 1.1 for F4TCNQ doped
PDTDE12. The best balance between the σ and S gives a
high power factor of 11 μW m-1 K-2 for PDTDE12 at a molar
ratio of 0.85. Combined with the data from absorption
spectra, we find that the conductivities show no propor-
tional correlation with the increase of AI/AII. Even though
transfer process within the polymer, but not all these
charge carrier pairs contribute to electrical conductivity.44
Therefore, the high conductivity of PQTS12/NOBF4 and
PDTDE12/F4TCNQ cannot only be attributed to the high
doping level. The microstructure of the doped films may
give the information how these charge carrier pairs are
available for electrical conduction.
The molecular packing of pure polymers and doped
polymers was examined by using two-dimensional graz-
ing incidence X-ray scattering (GIXRS). As shown in Fig-
ure 4a-f, PQT12, PQTS12 and corresponding doped films
were compared. Both the pure polymer and doped poly-
mer show multi-ordered diffractions representative of
lamellar structure (qz = 0.25~1.5 Å-1) and π-π stacking
structure (qxy ≈ 1.7 Å-1), indicating the predominant edge-
on orientation in all films. Doped with NOBF4, PQTS12
shows the fifth order of diffraction along qz axis and clear-
er π-stacking peak along qxy axis compared with NOBF4-
doped PQT12. In contrast, F4TCNQ-doped PQT12 show
higher crystallinity because of the appearance of the fifth
order of diffraction and absence of off-axis scattering.
Figure 4 (g) and 4 (h) show the line cuts along qz and qxy
axis, respectively. The data are shown in Table S3 in the
Supporting Information. After doping, the lamellar spac-
ing increases but π-π stacking distance decreases for
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doped polymers compared with the pure polymers. Fur-
ther, the lamellar spacing of NOBF4-doped PQTS12 has a
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significant increase (Δ = 3.3 Å) and the film shows short-
est π-π stacking of 3.53 Å (qxy = 1.78 Å-1). This means the
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Figure 4. 2D-GIXRS images of the PQT12 film (a), PQTS12 film (b), PQT12/NOBF4 film (c), PQTS12/NOBF4 film (d),
PQT12/F4TCNQ film (e) and PQTS12/F4TCNQ film (f). Out-of-plane (g) and in-plane (h) GIXRS patterns of the PQT12 film,
PQTS12 film and doped films.
addition of NOBF4 increases the lamellar packing dis-
tance and causes the intense packing of backbones which
induces strong coupling.45 The addition of F4TCNQ also
increases the distance of lamellar structure, however, the
scattering itself appears at qxy = 0.81 Å-1 in the in-plane
pattern of doped-PQTS12.
PDTDES12 show a distortion along their backbone due to
the steric effect of the ethylenedioxy group.46 Thus, the
insertion of sulfur does not affect the planarity of PQT12
and leads to the highly ordered packing structure in
PQTS12 films. However, the introduction of the ethylene-
dioxy group into PQT12 changes the steric configuration
of the backbone. PDTDE12 and PDTDES12 show similar
changes after doping with NOBF4, that is, the lamellar
distance increases and π-π stacking becomes intense. Ad-
ditionally, the lamellar distance of PDTDES12 doped with
NOBF4 significantly increases (Δ = 4.0 Å) compared with
PDTDE12 (Δ = 2.0 Å) (Table S4). This confirms that the
NOBF4 preferentially interacts with sulfur atoms on the
side chains. As shown in Figure 5 (g) and 5 (h), F4TCNQ-
doped polymers show indistinct peaks around 1.6 ~ 1.8 Å-1
Different from the ordered structure of PQT12 and
PQTS12, there are some large arcing diffractions indicat-
ing disoriented fractions in PDTDE12 and PDTDES12 film
(Figure 5). Meanwhile, edge-on and face-on orientations
are simultaneously observed in pure PDTDES12 (Figure
5b). Quantum-chemical calculations (B3LYP/6-31G) were
performed to predict the optimized configuration of four
polymer backbones (Figure S2, right column). PQT12 and
PQTS12 possess a planar backbone while PDTDE12 and
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along the qxy axis. However, the crystallinity of the lamel-
lar structure is significantly improved for F4TCNQ-doped
PDTDE12 and the lamellar distance slightly decreases
compared with pure PDTDE12, indicating the ethylenedi-
oxy group probably draws F4TCNQ closer to the polymer
backbone and away from the side chains. For F4TCNQ-
Page 6 of 11
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Figure 5. 2D-GIXRS images of the PDTDE12 film (a), PDTDES12 film (b), PDTDE12/NOBF4 film (c), PDTDES12/NOBF4 film (d),
PDTDE12/F4TCNQ film (e) and PDTDES12/F4TCNQ film (f). Out-of-plane (g) and in-plane (h) GIXRS patterns of the PDTDE12
film, PDTDES12 film and doped films.
doped PDTDES12, the existence of sulfur in the side
chains leads F4TCNQ to unduly disturb the polymer
packing, and the scattering from isolated domains of neu-
tral F4TCNQ indicates reduced miscibility.
conjugated backbone packing closer, which hinders the
charge transport between lamellar layers but enhances
the charge transport along the direction of π-π stacking.47
Therefore, the microstructure of PQTS12/NOBF4 film is
beneficial for efficient transport of holes. Although the
addition of F4TCNQ is prone to disturb the packing of
According to the diffraction information above, we
propose a packing geometry of polymers with different
dopants, as shown in Figure 6. The insertion of sulfur
promotes NOBF4 to interact with side chains without
interfering with the packing of backbone while the eth-
ylenedioxy group brings F4TCNQ closer to the polymer
backbone. The addition of NOBF4 into the polymer in-
duces a more significant increase of the lamellar distance
of polymer chains than that of F4TCNQ, but makes the
polymers and increase the proportion of disordered struc-
ture, the lamellar structures of PDTDE12 is negligibly af-
fected and the higher conductivity can be improved by
heavily doping with much more F4TCNQ molecules.
There is no observation of two separate narrow peaks
between qxy = 1.3 ~ 1.8 Å-1 as observed in PBTTT/F4TCNQ
film,28 so we cannot confirm that the F4TCNQ and repeat
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polymer backbone order in a cofacial arrangement. But
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the observation of scattering from F4TCNQ domain and
the existence of weak scattering between qxy = 1.3 ~ 1.6 Å-1
provide evidence that the packing structure is somewhat
similar to P3HT/F4TCNQ blends.27
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The surface morphologies were investigated by atomic
force microscopy (AFM) (Figure S5 and Figure S6). Both
pure PQT12 and PQTS12 film show crystalline domains
after slow evaporation of solvent due to the high crystal-
linity of the polymer backbone. Doping with NOBF4, the
roughness significantly increases indicating the molecular
structure changes upon doping. This is consistent with
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Figure 6. Illustration of doping process and packing models doped by NOBF4 and F4TCNQ, respectively.
the result that these polymers show relatively higher elec-
trical conductivity when doping with NOBF4 due to
strong coupling between π-conjugated backbones. Fur-
ther, the amount and size of crystalline domains in
PQTS12/NOBF4 significantly increase compared with
other combinations. Except for PDTDE12/F4TCNQ, the
addition of F4TCNQ makes negligible difference to the
roughness of films suggesting this dopant may not con-
tribute to the optimization of backbone packing.
The thermoelectric voltages remain constant when meas-
ured in applied temperature intervals, which is different
from the type of transient in ΔV induced by ΔT that had
been previously reported as originating from ions in
PEDOT:PSS.33 Additionally, we tested the stability of de-
vices in air under different humidity (Figure S8). The
conductivity of PQTS12/NOBF4 and PDTDE12/F4TCNQ
did not increase even under the high humidity, and it still
keeps a relatively high value even after one month in air
without any further actions to exclude the exposure of
light or humidity. Furthermore, this molecular doping by
The relationship between S and σ of all doped polymers
fits well with the empirical model reported by Chabinyc
(Figure S7),23 especially for high σ values, which fall on
the highest-performing region of the model plot. This
makes it less likely that the Seebeck coefficient does not
include contributions from ion transport, as is the case for
PEDOT.13 Further evidence against contributions to See-
beck coefficient from mobile ions is shown in Figure 7.
solution processing can avoid de-doping during the
measurement that was reported to occur after vapor dop-
ing.23
To further understand the origins of σ of these poly-
mers, we evaluated the mobility by fabricating thin-film
transistors based on pure polymers and doped polymers.
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The transistors of PQT12 show a hole mobility of 0.027 ±
Page 8 of 11
0.004 cm2 V-1 s-1 and PQTS12 films exhibit a hole mobility
of 0.010 ± 0.002 cm2 V-1 s-1.48 The transfer and output char-
acteristics of pure PDTDE12 and PDTDES12 are shown in
Figure S9. The mobility of PDTDE12 is 2.0×10-3 ± 0.3×10-3
cm2 V-1 s-1 in the saturation regime, based on the meas-
urement of six transistor devices. From an analogous ex-
periment, the mobility of PDTDES12 is 1.5×10-4 ± 0.2×10-4
cm2 V-1 s-1, which is one order lower than that of PDTDE12.
Compared with pure PQT12, the introduction of substitu-
ents decreases the mobility of the polymers. We find that
the variation trend of electrical conductivity of doped
polymers is not consistent with the mobility of pure pol
ymers. This suggests the mobility of pure polymers can-
not represent the charge transport ability of doped films.
The mobility of doped films with high electrical conduc-
tivity cannot be readily determined using transistors be-
cause of significant bulk conduction which impacts the
extraction of accurate carrier mobility. Typical transistor
characteristics could not even be obtained for 2 wt %
F4TCNQ doped PDTDE12 or PDTDES12. Therefore, we
choose two combinations, PQTS12/NOBF4 (3 wt%, molar
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Figure 7. Time dependent thermoelectric voltage response under different temperature gradients ΔT. (a) PQTS12/NOBF4 (mo-
lar ratio = 0.5) (b) PDTDE12/F4TCNQ (molar ratio = 1.1).
ratio = 0.2) and PDTDE12/F4TCNQ (0.5 wt%, molar ratio
= 0.01) to fabricate transistors to test the mobilities (Fig-
ure S10). The mobility of doped PQTS12 (0.045 ± 0.2×10-2
cm2 V-1 s-1) and doped PDTDE12 (0.011 ± 0.2×10-2 cm2 V-1 s-1)
is about five times higher than that of the pure polymer,
respectively. These data indicate the addition of small
amounts of dopant can significantly improve the carrier
density in bulk film. If half of the repeat units of PQTS12
are assumed to be doped (molar ratio is 0.5) in the film of
PQTS12/NOBF4 (the weight of the mixture is 1 g), the
carrier concentration could reach 4.2×1020 cm-3. The carri-
er concentration could reach 5.5×1020 cm-3 in the film of
PDTDE12/F4TCNQ (molar ratio is 1.1) if all repeat units
are assumed to be doped (detailed calculation is included
in SI). According to the equation of σ = neμ, where n is
carrier concentration and e is the fundamental charge, the
mobility μ is up to 5.2 cm2 V-1 s-1 for doped PQTS12 and 1.6
cm2 V-1 s-1 for doped PDTDE12, respectively. This further
means that the high conductivity of PQTS12/NOBF4 can be
partly ascribed to an increase of the charge carrier mobility
accompanying the doping. The estimation of mobility is
supported by Snyder’s model (Figure S11). For pure poly-
mer films with thickness of 100 nm and a gate oxide ca-
pacitance of 10 nF cm-2, the charge density estimated from
the output curves is on the order of 0.01 C cm-3 assuming
that a 5 V gate voltage moves the device from the off to
the on state for PQTS12 film (10 V gate voltage needed for
PDTDE12 film). The mobility would be 0.6 ~ 6 cm2 V-1 s-1
(0.3 ~ 3 cm2 V-1 s-1 for PDTDE12 film) for the σE0 interval
between 3×10-3 to 3×10-2 S cm-1, which indicates that the
estimated mobilities of PQTS12/NOBF4 and highly doped
PDTDE12 samples agree well with this model (red penta-
gram). For other doped films, the points are also in
agreement with the model.
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Device fabrication and characterization, thermal conductivi-
To complete the estimate of the figure of merit, ZT, we
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ty measurements, cyclic voltammograms, optimized configu-
ration, thermoelectric parameters, crystallographic parame-
ters, UPS, AFM, fit of models, transistor performance and
synthetic details. This material is available free of charge via
the Internet at http://pubs.acs.org.
measure the thermal conductivities of four doped poly-
mer films: F4TCNQ doped PDTDE12 and PDTDES12 along
with NOBF4 doped PQT12 and PQTS12, using a pump-
probe thermoreflectance-based technique (time domain
thermoreflectance).49-51 In short, this is a non-contact op-
tical technique which uses a modulated laser pulse to
heat the surface of the sample, and then measures the
temporal thermal decay as a function of the change in
reflection of the sample surface. We fit a cylindrically
symmetric, multi-layer Fourier heat conduction model to
the thermal decay curve to back out thermal properties of
interest, such as thermal conductivity. Details of the
measurement, analysis, and uncertainties are discussed
further in the Supporting Information (Figure S12). The
cross plane thermal conductivity of highly F4TCNQ
doped PDTDE12 (molar ratio = 0.85) is 0.232 ± 0.098 W m-
AUTHOR INFORMATION
Corresponding Author
*hekatz@jhu.edu
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
The polymer synthesis and electronic and thermoelectric
property analysis was supported by the Department of Ener-
gy, Office of Basic Energy Sciences, Grant Number DE-FG02-
07ER46465. The thermal conductivity study was supported
the Army Research Office, Grant Number W911NF-16-1-0320
(PEH). The authors gratefully acknowledge Ebuka S. Arinze
and Professor Susanna M. Thon for UV-vis-NIR absorption
measurement. We also thank Dr. Zhengxu Cai for the assis-
tance with GIXRS measurements at Advanced Photon Source
(APS) at the Argonne National Laboratory. We acknowledge
graduate student Michael Barclay and Professor Howard
Fairbrother for acquiring the UPS spectra. We warmly thank
Professor Patty McGuiggan for AFM measurements. H. L.
thanks Dr. Xingang Zhao and Dr. Tejaswini Kale for the sug-
gestions of synthesis. We are grateful to Professor Michael L.
Chabinyc for kind permission to employ Figure S7 and Pro-
fessor G. Jeffrey Snyder for kind permission to use Figure S11.
K-1 with a maximum value of ZT = 0.014, and the value
1
for PQTS12 doped with NOBF4 (molar ratio = 0.5) is 0.461
± 0.152 W m-1 K-1 with a maximum value of ZT = 5.11×10-3.
We note that these thermal conductivity values are in line
with typical thermal conductivities of electrically con-
ducting polymers or PEDOT:PSS.8, 9, 52-55
Compared with the representative examples of p-doped
conjugated polymers summarized in Table S5, the results
reported here demonstrate a significant improvement
upon the electrical conductivity taking into account the
solution mixing of dopant and polymer and characteriza-
tion in air.
CONCLUSION
In conclusion, we have designed and synthesized a series
of p-type polymers via the structural modification of
PQT12. Two EDOT copolymers were obtained via direct
C-H arylation and applied to solution-deposited thin-film
thermoelectrics for the first time. The highest σ up to 350
S cm-1 and 140 S cm-1 were achieved for PQTS12 doped
with NOBF4 and PDTDE12 doped with F4TCNQ, respec-
tively. The electrical conductivity value is comparatively
high among nonionic, polymeric thermoelectric materials
deposited by dopant-polymer solution processing and
exhibits good stability in air on the one month time scale.
The high conductivity mainly depends on the bulk mo-
lecular packing and charge transport in the doped film.
Our result indicates that the mobility of the pure polymer
is not a dominant factor for achieving a high electrical
conductivity, which can be obtained by doping with the
appropriate dopant, because the doping process can
change the packing of bulk film which significantly im-
pacts charge transport. The data fit well with two recent
models of the dependence of the Seebeck coefficient on
electrical conductivity, and point toward further increases
in doped polymer mobilities are a likely route to further
increases in ZT for single phase polymers.
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