Page 4 - Read Online

Page 2 of 14 Shen et al. Soft Sci 2023;3:20 https://dx.doi.org/10.20517/ss.2023.10

conductivity exhibits a nearly imperceptible decreasing trend due to its highly ordered microstructure. This work
highlights the potential to control the aggregation state of polymer molecules and achieve an approximate
decoupling between the conductivity and thermopower of thermoelectric materials by rationally designing polymer
molecules.

Keywords: Aggregation regulation, precursor structure modification, polythiophenyl derivatives, organic
thermoelectric material

INTRODUCTION
Extensive efforts have been invested in comprehending the correlation between structure and performance,
[1]
which has led to significant advancements in organic π-conjugated materials over the last two decades .
The design of molecular structure plays a crucial role in controlling the morphology and thermoelectric
[2]
(TE) properties of materials, making π-conjugated polymers highly promising for use in TE generators .
Research has shown that the ordered structure of π-conjugated polymers, ranging from the molecular scale
[3-7]
to the macroscale, significantly affects the properties of organic TE materials . Within crystalline domains,
the conjugation of molecular units leads to planar structures that orderly stack against each other (i.e., π
stacking), resulting in close intermolecular contacts at the π-electronic levels, intermolecular electronic
coupling, and effective charge transfer [7-12] . Reasonable structural design of precursor molecules, such as
expanding the conjugated plane and introducing non-covalent interactions to promote self-rigidification of
the conjugated backbone, can achieve control over the microscale ordered structure of polymer molecules
to a certain extent. For instance, non-covalent interactions between atoms such as S, O, F and H can reduce
the twisting angle between conjugated planes, promote ordered stacking of molecules, and optimize the
spatial aggregation state of molecules [13-17] . Additionally, extending the conjugated backbone of the precursor
is viewed as a convenient approach to narrow band gaps and regulate the electrical properties of organic
conjugated materials [12,18-22] .

At present, the substituents of thiophene (Th), thieno[3,2-b]thiophene (TT), and 3,4-
ethylenedioxythiophene (EDOT) are commonly used as building blocks to extend the conjugation plane in
organic electronic devices. TT, which is the simplest fused Th consisting of two annulated Th units, has
more rigid and elongated π-conjugation structures than Th. This makes it a valuable tool for adjusting the
band gap and enhancing intermolecular interactions in solid-state organic materials. Compared to Th, the
presence of the dioxyethylene group in the EDOT structure can increase the charge density on the Th ring.
This is beneficial for lowering the monomer oxidation potential and promoting effective doping of the
polymer molecules. Imae et al. explored the effect of different proportions of EDOT units in polymer
molecules on their electrochemical, optical, and electronic transport properties. They accomplished this by
designing the structures of precursor molecules based on derivatives of polythiophene (PTh) [23,24] . Xue et al.
designed and synthesized a series of binary or ternary precursors using components such as EDOT, Th, and
TT. They studied the electrochemical and electrochromic properties of their monomers and polymers [23-27] .
Poly(3,4-ethylenedioxythiophene) (PEDOT) is widely recognized as a leading organic TE material due to its
[28]
environmental stability, broad conductivity (σ) range, and ability to be processed in solution . However,
the inadequate thermopower (S) of PEDOT has long impeded its widespread adoption as an organic TE
material in the market [29-31] . In contrast, the Th and their derivatives generally have a higher S and a lower σ
for PEDOT. It is important to note that the morphology and properties of the polymer film can vary
significantly depending on the specific preparation techniques employed. Compared to the conventional
chemical oxidation method, electrochemical polymerization produces PEDOT with a more uniform and
continuous structure, resulting in an improvement of its σ by an order of magnitude [32-33] . Xiong et al. have
demonstrated that well-ordered thin films of Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)