Page 243 - Read Online

Girase et al. Energy Mater. 2025, 5, 500132 https://dx.doi.org/10.20517/energymater.2025.14 Page 13 of 33

[22]
after doping . As a result, there has been growing interest in n-type materials and dopants, leading to
significant advancements in the development of new n-type TE materials. The designed strategy for good
n-type TE materials requires careful tuning of their lowest unoccupied molecular orbital (LUMO) energy
levels. A reduced LUMO facilitates thermodynamically favorable electron transfer from the dopant to the
polymer and thereby enhances n-doping efficiency [125,126] and ambient stability because they resist quenching
by environmental oxygen, water, or trap states [127,128] . The electronic nature of the polymer backbone also
plays a very significant role in establishing the extent of polaron or bipolaron delocalization with doping .
[129]
[130]
A-A type backbones tend to have more charge delocalization than D-A systems . Moreover, good mixing
allows the dopant molecules to be well dispersed in the polymer matrix without aggregating and ensures
[131]
doping effectiveness without interfering with charge mobility . Close and well-ordered packing of
polymer backbones is crucial for the formation of an ideal morphology when doped. In such structures,
molecular dopants tend to localize within the amorphous regions, where efficient electron transfer is
possible, and the crystalline domains remain comparatively unperturbed for efficient charge transport [101,132] .
Additionally, face-on and edge-on mixed stacking arrangements have been demonstrated to trade off
doping efficiency and charge transport via improved dopant accommodation [133,134] . Therefore, the
solid-state packing and backbone alignment optimization is a critical approach to realize high σ and
comprehensive TE performance in n-type CPs.

Currently, various kinds of n-type building blocks are used for the development of n-type OTE materials,
s u c h a s n a p h t h a l e n e d i i m i d e (NDI) [135,136] , e r y l e n e d i i m i d e (PDI) , e n z o d i f u r a n d i o n e
[130]
b
p
polyphenylenevinylene (BDOPV) [137,138] , and bithiophene diimide (BTI) [139,140] . Among these, the most used
electron-deficient building block for n-type OTE is NDI, and its representative compound would be
-1
-2
[141]
N2200 . The compound has an σ of about 10 S cm and a PF of about 10 μW m K -2[135] . In other words,
-1
-3
it is of the utmost importance for the production of n-type OTE materials in order to develop new
electron-deficient blocks. DPP has also attracted much attention as versatile unit for n-type semiconducting
polymer due to its very strong electron-accepting character, planarity, and ease of synthesis for two
decades [29,50,53,142] . However, the presence of the electron-rich thiophene side groups in DPP leads to high
frontier molecular orbitals (FMOs) energy levels and consequently renders DPP-based copolymers to often
display p-type or ambipolar transport properties, which severely limits their assessments as effective n-type
[143]
materials in OTEs . Connecting of two thiophene units with a central DPP core generates the 1,4-
diketopyrrole[3,4-c]pyrrole dithiophene (DBT), which exhibits ambipolar nature and demonstrates
2
-1
exceptionally high hole mobility 10 cm V s -1[144] . To further improve the electron mobility (μ ) of DPP-
e
based polymers, Sun et al. synthesized DBPy by substituting the electron-rich thiophene by the more
electron-deficient pyridine. The copolymer of DBPy and bithiophene (PDBPyBT) exhibited significantly
improved μ of 6.30 cm V s . Such enhancements are crucial for the optimization of the performance of n-
2
-1 -1
e
type materials utilized in TE devices . The potential application of DPP-based polymers for n-type OTE
[144]
devices, first developed by Yang et al., was demonstrated in 2018. They synthesized two polymers, namely
Poly[2,5-bis(2-octyldodecyl)-3,6-di(pyridin-2-yl)-pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dionealt-(E)-2,2’-
(ethene-1,2-diylbis(3,4-difluorothiophene-5,2-diyl)) (PDPF) and Poly[2,5-bis(2-octyldodecyl)-3,6-
di(pyridin-2-yl)-pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dionealt-(E)-2,2’-(ethene-1,2-diylbis(thiophene-5,2-
diyl))] (PDPH), with and without fluorine substituents possessing electron-withdrawing capability
respectively . Figure 5 displays the chemical structures of several DPP-based n-type D-A polymers and
[133]
Table 3 summarizes their optimal doped TE performance. The lower energy of LUMO (E LUMO ) of PDPF,
which contained fluorine, was measured at -4.11 eV, while that of PDPH was close to -3.93 eV. This means
that PDPF has more affinity to electrons than PDPH, an important factor for n-type performance in
thermoelectrics. PDPH shows nonuniform doping, with areas of the film exhibiting variable doping
concentrations that may affect its overall performance. In contrast, PDPF has a more uniform distribution
of dopants due to the structural binodal backbones, which is believed to allow multiple packing