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Girase et al. Energy Mater. 2025, 5, 500132 https://dx.doi.org/10.20517/energymater.2025.14 Page 17 of 33

around 21%, exceeding the 19% threshold widely used to describe amorphous polymers, whereas pDSe
shows a lower g value of 17%, indicating some extent of minor semicrystallinity. Further, AFM also supports
these findings, with pDFSe showing smooth and featureless morphology and low RMS roughness of 0.37
nm, compared to the more textured crystalline domains and higher roughness of 1.56 nm in pDSe. The
amorphous nature of pDFSe allows for greater miscibility of dopants and easier accommodation of dopant
ions, which translates to enhanced doping efficiency and potential for enhanced TE performance. As a
result, n-type TE based on pDFSe exhibits excellent electron mobility of 6.15 cm V s , significantly higher
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than 0.77 cm V s measured for the control polymer pDSe (without noncovalently fused-ring structure).
After n-doping, pDFSe shows exceptional conductivity of 62.6 S cm with a maximum PF of 133.1 μW m
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K , observed due to deeper LUMO better electron mobility and improved doping efficiency. The obtained
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result is among the highest for solution-processed n-type polymers. Very recent, Ma et al. designed and
investigated a series of new n-type DPP-based conjugated polymers to study the influence of sp -nitrogen
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(N) introduction into the density of states (DOS) and the thermoelectric properties. Three polymers,
PTh-DPP, PThTz-DPP, and PTz-DPP, with increasing numbers of sp -N atoms within the repeating unit by
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polymerizing thiophene-DPP and 3,6-di(thiazole-5-yl)-DPP-functionalized monomers. These structural
modifications were coupled with the use of amphipathic side chains, which improved solubility and
facilitated favorable morphology for TE devices . Upon increase in the sp -N content, the deepening of
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[151]
the energy levels was observed.. The increased DOS facilitated better doping by offering greater electronic
states accessible and promoting polaron formation as evidenced by greater ultraviolet-visible-near infrared
(UV-vis-NIR) polaron absorption and higher spin densities in electron paramagnetic resonance (EPR)
measurements. PTz-DPP, with the maximum sp -N content, showed the best doping response and showed
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least structural perturbation upon doping, retaining molecular order and intimate π-π stacking (0.36 nm),
which are essential for efficient charge transport. After doping with N-DMBI, PTz-DPP possessed a
superior σ of 63.8 S cm and a PF of 111.8 μW m K , which enabled it to possess a ZT of 0.46, placing it
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among the highest-performing n-type organic TE materials. The high TE performance of PTz-DPP was not
only a result of improved doping efficiency but also a result of a change in the mechanism of charge
transport from hopping (seen in PTh-DPP) to more coherent or band-like transport in PTz-DPP, as
determined by Hall-effect measurements and temperature-dependent transport investigations. Moreover,
the increased site density near the Fermi level led to shorter charge hopping distances and increased carrier
mobility. This work highlights the key significance of molecular design specifically the strategic use of
electron-deficient, nitrogen-rich heterocycles in managing the electronic structure and optimizing charge
transport channels in n-type organic TE materials.

MOLECULAR DOPING OF THERMOELECTRIC POLYMERS
Doping is an effective approach to optimize the performance of CP thin films, mainly for thermoelectric
application. With the presence of dopants, there is considerable enhancement in charge carrier
concentration within the polymer matrix, leading to an improvement in the σ, a key component in realizing
high thermoelectric efficiency. Chemical doping and electrochemical doping are the two primary techniques
employed to dope CP thin films. Chemical doping is typically accomplished by introducing small molecular
oxidizing or reducing agents into the CP matrix. The dopants interact with the polymer chains through
charge transfer mechanisms. Good alignment in the energy levels enables the energy exchange to add or
extract electrons, thus altering the electrical properties of the CP. Conversely, electrochemical doping is
performed by casting the CP film on a conductive substrate, which is then immersed in an electrolyte
solution. These two methods represent controllable and effective ways to modulate the electrical properties
of CP thin films, which is important for their applications in high-performance TE devices.