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Girase et al. Energy Mater. 2025, 5, 500132 https://dx.doi.org/10.20517/energymater.2025.14 Page 11 of 33

introduction of extended branched 7-decylnonadecyl side chains on DTP donor unit into a DPP-based
[107]
conjugated polymer which was synthesized by Lee et al. . DPP-MeDTP polymer, exhibited high TE
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performance with high σ due to high planarity and crystallinity, leading to a PF as high as 85.2 μW m K
with F4TCNQ doping that turned out to be a multiple of other polymers in the trial. FeCl -doped polymer
3
showed PF of 60.7 μW m K . Although most other dopants can show higher PF with FeCl doping due to
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3
higher doping levels, the F4TCNQ-DPP-MeDTP doped polymer had a high S of 242 μV K , thus giving it a
-1
superior PF. Suh et al. came up with a new approach. They incorporated fused cyclopenta[2,1-b:3,4-
b’]dithiophene (CDT) moieties in the donor part of DPP-based D-A CP, resulting in three new polymers,
namely, PCD-1 with branched side chains on the CDT and DPP units; PCD-2 with linear side chains on the
CDT unit; and PCD-3 with linear side chains on the DPP unit . The highly aggregated polymers, PCD-1
[108]
and PCD-2, primarily formed ion pairs with the F4TCNQ, thus facilitating further aggregation in the thin
film and as a result gave rise to high σ of 0.64 and 3.1 S cm , respectively. Among these, PCD-2 achieved the
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highest PF of 21.8 μW m K . In contrast, PCD-3, which exhibited less aggregation, showed much lower
conductivity (0.02 S cm ) and a PF of 0.47 μW m K , attributed to its lower doping efficiency and the
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-1
formation of Lewis complexes with the dopant. Using the sequential doping technique, the polymer chains
were fully aggregated in the solution, regardless of the presence of dopants. As a result, the PF values of
PCD-1, PCD-2, and PCD-3 became much closer to each other, with values of 9.0, 13.6, and 15.6 μW m K ,
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respectively.
In 2021, Li et al. chose a benzodithiophene (BDT) donor building block whose fused-ring structure causes
high planarity and a narrow band gap, and it possesses excellent electron-donating properties .
[109]
Furthermore, to enhance backbone planarity, the TT unit can also be incorporated into the side
chain to form the BDTTT donor unit and combined it with DPP to generate a two-dimensional (2D)
P(BDTTT-DPP) CP . The resultant 2D polymer P(BDTTT-DPP) possesses a better molecular stacking
[110]
ability than the one-dimensional (1D) P(BDT-DPP) with branched alkoxy chain synthesized for TE
performance comparison. When both compounds were doped with FeCl3, P(BDTTT-DPP) exhibited a
remarkably higher σ of 1.83 S cm , which was 20 times higher than that of the corresponding P(BDT-DPP)
-1
and produced a PF of 6.50 μW m K , 26 times that of P(BDT-DPP). As the temperature ranged, PF
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-1
increased, reaching a maximum value of 10.50 μW m K at 350 K. The significant augmentation in TE
-2
performance was attributed to the enhanced doping level and better π-π stacking in P(BDTTT-DPP), hence
suggesting that incorporating 2D conjugated units into D-A polymers could substantially enhance
thermoelectric properties.
At the same time, another innovative approach was made by Cao et al. to enhance the TE performance by
using D-A copolymers with 1D EDOT framework with 2D BDTTT units, being randomly copolymerized to
combine the high-Seebeck-coefficient of BDTTT with the strong-doping tendency of EDOT . The
[111]
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copolymer P12 demonstrates a superior PF of 12.3 μW m K , which is significantly higher than P10
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-2
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(3.1 μW m K ), with only BDTTT as the donor part, and P01 with PF of 9.5 μW m K , which has only
EDOT as the donor unit.
Significant efforts have been made by Lee et al. to enhance the TE performance. Recently, they have
designed and synthesized methoxy-functionalization donor-acceptor conjugated polymer based on DPP
and bithiophene units and compared the performance between the methoxy-functionalized polymer,
P29DPP-BTOM and its unfunctionalized counterpart, P29DPP-BT. The introduction of methoxy group
increased lamellar stacking distance which enhanced the dopant’s diffusion and consequently its doping
efficiency, notably for the bigger dopants. Hence, the methoxy-functionalized polymer, P29DPP-BTOM,
had increased carrier concentration and mobility when compared with P29DPP-BT. In the case of the FeCl
3