Thermoelectric plastics are a class of polymer-based materials that combine the ability to directly convert heat to electricity, and vice versa, with ease of processing.
Molecular
p-doping of the conjugated polymer poly(3-hexylthiophene)
(P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane
(F4TCNQ) is a widely studied model system. Underlying structure–property
relationships are poorly understood because processing and doping
are often carried out simultaneously. Here, we exploit doping from
the vapor phase, which allows us to disentangle the influence of processing
and doping. Through this approach, we are able to establish how the
electrical conductivity varies with regard to a series of predefined
structural parameters. We demonstrate that improving the degree of
solid-state order, which we control through the choice of processing
solvent and regioregularity, strongly increases the electrical conductivity.
As a result, we achieve a value of up to 12.7 S cm–1 for P3HT:F4TCNQ. We determine the F4TCNQ anion concentration and
find that the number of (bound + mobile) charge carriers of about
10–4 mol cm–3 is not influenced
by the degree of solid-state order. Thus, the observed increase in
electrical conductivity by almost 2 orders of magnitude can be attributed
to an increase in charge-carrier mobility to more than 10–1 cm2 V–1 s–1. Surprisingly,
in contrast to charge transport in undoped P3HT, we find that the
molecular weight of the polymer does not strongly influence the electrical
conductivity, which highlights the need for studies that elucidate
structure–property relationships of strongly doped conjugated
polymers.
We
report a record thermoelectric power factor of up to 160 μW
m
–1
K
–2
for the conjugated polymer
poly(3-hexylthiophene) (P3HT). This result is achieved through the
combination of high-temperature rubbing of thin films together with
the use of a large molybdenum dithiolene p-dopant with a high electron
affinity. Comparison of the UV–vis–NIR spectra of the
chemically doped samples to electrochemically oxidized material reveals
an oxidation level of 10%, i.e., one polaron for every 10 repeat units.
The high power factor arises due to an increase in the charge-carrier
mobility and hence electrical conductivity along the rubbing direction.
We conclude that P3HT, with its facile synthesis and outstanding processability,
should not be ruled out as a potential thermoelectric material.
The
thermoelectric power factor of a broad range of organic semiconductors
scales with their electrical conductivity according to a widely obeyed
power law, and therefore, strategies that permit this empirical trend
to be surpassed are highly sought after. Here, tensile drawing of
the conjugated polymer poly(3-hexylthiophene) (P3HT) is employed to
create free-standing films with a high degree of uniaxial alignment.
Along the direction of orientation, sequential doping with a molybdenum
tris(dithiolene) complex leads to a 5-fold enhancement of the power
factor beyond the predicted value, reaching up to 16 μW m–1 K–2 for a conductivity of about
13 S cm–1. Neither stretching nor doping affect
the glass transition temperature of P3HT, giving rise to robust free-standing
materials that are of interest for the design of flexible thermoelectric
devices.
Doping of the conjugated polymer poly(3-hexylthiophene) (P3HT) with the p-dopant 2, 3,5,7,8, is a widely used model system for organic thermoelectrics.We here study how the crystalline order influences the Seebeck coefficient of P3HT films doped with F4TCNQ from the vapour phase, which leads to a similar number of F4TCNQ anions and hence (bound + mobile) charge carriers of about 2 Â 10 À4 mol cm
À3. We find that the Seebeck coefficient first slightly increases with the degree of order, but then again decreases for the most crystalline P3HT films. We assign this behaviour to the introduction of new states in the bandgap due to planarisation of polymer chains, and an increase in the number of mobile charge carriers, respectively. Overall, the Seebeck coefficient varies between about 40 to 60 mV K
À1. In contrast, the electrical conductivity steadily increases with the degree of order, reaching a value of more than 10 S cm
À1, which we explain with the pronounced influence of the semi-crystalline nanostructure on the charge-carrier mobility. Overall, the thermoelectric power factor of F4TCNQ vapour-doped P3HT increases by one order of magnitude, and adopts a value of about 3 mW m À1 K À2 in the case of the highest degree of crystalline order.
The UV-to-IR transitions in p-doped
poly(3-hexylthiophene) (P3HT)
with alkyl side chains and polar polythiophene with tetraethylene
glycol side chains are studied experimentally by means of the absorption
spectroscopy and computationally using density functional theory (DFT)
and tight-binding DFT. The evolution of electronic structure is calculated
as the doping level is varied, while the roles of dopant ions, chain
twisting, and π–π stacking are also considered,
each of these having the effect of broadening the absorption peaks
while not significantly changing their positions. The calculated spectra
are found to be in good agreement with experimental spectra obtained
for the polymers doped with a molybdenum dithiolene complex. As in
other DFT studies of doped conjugated polymers, the electronic structure
and assignment of optical transitions that emerge are qualitatively
different from those obtained through earlier “traditional”
approaches. In particular, the two prominent bands seen for the p-doped
materials are present for both polarons and bipolarons/polaron pairs.
The lowest energy of these transitions is due to excitation from the
valence band to a spin-resolved orbitals located in the gap between
the bands. The higher-energy band is a superposition of excitation
from the valence band to a spin-resolved orbitals in the gap and an
excitation between bands.
The insulation of state-of-the-art extruded highvoltage direct-current (HVDC) power cables is composed of cross-linked low-density polyethylene. Driven by the search for sustainable energy solutions, concepts that improve the ability to withstand high electrical fields and, ultimately, the power transmission efficiency are in high demand. The performance of a HVDC insulation material is limited by its residual electrical conductivity. Here, we demonstrate that the addition of small amounts of high-density polyethylene (HDPE) to a low-density polyethylene (LDPE) resin results in a drastic reduction in DC conductivity. An HDPE content as low as 1 wt % is found to introduce a small population of thicker crystalline lamellae, which are finely distributed throughout the material. The change in nanostructure correlates with a decrease in DC conductivity by approximately 1 order of magnitude to about 10 −15 S m −1 at high electric fields of 30 and 40 kV mm −1 and elevated temperature of 70 °C. This work opens up an alternative design concept for the insulation of HVDC power cables.
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