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Page 12 of 33 Girase et al. Energy Mater. 2025, 5, 500132 https://dx.doi.org/10.20517/energymater.2025.14

dopant, the resulting TE performance based on P29DPP-BTOM was impressive, with a maximum σ
-1
-1
-1
2
reaching 242.4 S cm and a charge carrier mobility of 0.18 cm V s when compared to 298 S cm and
-1
-1 -1
2.44 cm V s in the case of P29DPP-BT. Further comparison reveals that P29DPP-BTOM exhibited an
2
-1
-2
even larger S, thereby providing a PF of 195.1 μW m K and that of a doped P29DPP-BT being only
158.3 μW m K . Additionally, doping with F4TCNQ only worked for P29DPP-BTOM, although the
-2
-1
-1
maximum σ was lower (88.9 S cm ), thereby highlighting the stronger doping efficiency of FeCl 3 [112] . Both
F4TCNQ and FeCl doped P29DPP-BTOM films exhibit highly similar crystalline structures, which
3
indicates that dopant size and structure did not affect film microstructure. This is due to the fact that the
extensive lamellar spacing and sufficient free volume between alkyl side chains allow for successful
accommodation of the dopant. Thus, lower oxidation strength in the F4TCNQ-doped film is the reason for
its lower conductivity, not differences in crystallinity. Furthermore, the impact of electronic structures in
D-A conjugated polymers on their TE properties was systematically investigated by synthesizing a series of
random D-A copolymers with similar crystalline structures. These copolymers were designed by varying the
ratio of two building blocks: the donor DPP-TVT and the acceptor DPP-CNTVT, with DPP-CNTVT
incorporating an electron-withdrawing cyano group on its vinylene moiety . The doped copolymers were
[113]
then studied for their doping behavior and thermoelectric properties, with p-type doping using FeCl and
3

n-type d o p i n g u s i n g 4-(2,3-dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine
(N-DMBI). The incorporation of DPP-TVT raised the HOMO levels as well as the doping efficiency with
FeCl . The PF and the µ were found to be maximum with the copolymer with the lowest amount of DPP-
3
-2
CNTVT (CN1), which gave a maximum PF value of 79.8 μW m K . The decrease in PF with higher DPP-
-1
CNTVT content was attributed to polaron localization caused by the acceptor’s electronic structure. Single
polymer with both thermoelectric properties and stretchability for self-powered wearable electronics is
challenging. One straightforward way to develop intrinsically stretchable OTE (IS-OTE) polymers is
through the incorporation of aliphatic breakers into the polymer’s backbone [114-116] . However, this usually
decreases charge carrier mobility. An alternative would be to increase amorphous regions in the polymer by
incorporating fused-ring conjugated breakers, which might benefit the dissipation of stress during
mechanical deformation [117,118] . By applying the above strategy, Tseng et al. synthesized novel IS-OTE
polymers by introducing a fused DITT unit into the polymer backbone. Nevertheless, the rigid coplanar
structure of DITT ensures efficient charge transport across amorphous domains of the polymer. Varying the
amounts of DITT in its copolymer with respect to quantity of DPP will balance the amorphous and
crystalline regions and render stretchable polymers such as these with enhanced TE performance. The
copolymer exhibited high PF of 12.5 μW m K achieved by DITT30, with an impressive crack onset strain
-1
-2
of over 100%, maintaining 80% of its original PF after 200 stretch-release cycles. This work has shown the
first case of an IS-OTE polymer capable of bright potential applications in the field of wearable
electronics .
[119]

Diketopyrrolopyrrole-based n-type thermoelectric polymers
Even though significant amount of research has been carried out on p-type TE materials and dopants in
recent decades, studies of their n-type counterparts have significantly lagged behind those of p-type
materials due to a variety of intrinsic and extrinsic limitations. One of the fundamental issues is the
relatively low electron affinity (EA) of most conjugated polymers, which makes efficient and stable n-type
doping difficult . Additionally, n-type dopants tend to be more air-sensitive, chemically labile, or have
[120]
harsh doping conditions such as high temperature or vacuum treatment that prevent practical use and
scalability [121-123] . Furthermore, poor miscibility and limited dopant-polymer backbone compatibility often
result in inhomogeneous doping, phase segregation, or inefficient charge transfer, resulting in reduced
[124]
conductivity . Some p-type materials have reached σ of over 1000 S cm -1[20] . With the increasing demand
for high-performance OTE materials, the development of n-type CPs has been one of the major focuses of
-1
research; most n-type materials have low σ, and only a few of them have reached σ values above 1 S cm