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Shen et al. Soft Sci 2023;3:20 https://dx.doi.org/10.20517/ss.2023.10 Page 9 of 14

[Figure 6B and Table 1]. The PBEDs films exhibited deep highest occupied molecular orbital (HOMO)
levels, with onset oxidation and reduction potentials estimated to be less than -6 eV, which is significantly
[3]
deeper than PEDOT's -5.1 eV . This difference could be attributed to the more rigid structure of PBED
precursors, resulting in fewer repeating units compared to PEDOT [50-53] . Furthermore, the UV-vis
absorption spectra of PBEDs films after de-doping with hydrazine hydrated ethanol solution were analyzed
and presented in Supplementary Figure 8. The absorption spectra of PBEDs showed strong absorption in
the 400-700 nm region. Notably, the absorption spectrum of P(BED-T) exhibited a blue shift phenomenon,
while P(BED-TT)'s had a slight red shift. The difference in the maximum absorption peak position of the
PBEDs film may be due to the variation in intermolecular π-π stacking interaction between these
[54]
polymers . Additionally, the optical bandgap of PBEDs was estimated by absorption edge and presented in
Table 1. The results showed that the optical band gap of the PBEDs film was very small, ranging between
1.73-1.78 eV, demonstrating that the incorporation of thienyl groups minimally affected the energy band of
PBEDs.

Figure 7 illustrates the density functional theory (DFT) calculation of the BEDs trimer, revealing that the
lowest occupied molecular orbital (LUMO) level of the polymer experiences a significant impact upon the
incorporation of thienyl groups, leading to an increase in its electronic affinity. The results indicate that the
introduction of thienyl groups can reduce the intramolecular charge density, resulting in a decrease in the
effective doping degree with dopant molecules. In Figure 8, the incorporation of Th results in a reduction of
the distortion angle between conjugate planes from 1.25° to 0.62° and 0.57° by utilizing non-covalent forces
between O and S atoms. Furthermore, a distinct twist angle of the conjugated plane is observed, with the
embedded TT structure resulting in a larger change in the twist angle compared to the introduction of the
Th structure. This could be due to the rigid structure and steric hindrance of the TT structure, which affects
the overall conformation of the molecule. This phenomenon is consistent with the precursor E
ox
electrochemical characterization and validates the results of GIWAXS characterization. Notably, the
electron cloud of the polymer is mainly distributed on the conjugated ring. However, upon introducing
thienyl groups, the distribution of electron clouds on the conjugated main chain of the molecule becomes
relatively uneven and aggregates towards the center when compared to PBED. This effect is more
pronounced in P(BED-TT).

The TE properties of PBEDs films were measured using a standard four-point probe technique. As shown
in Figure 9A, the embedding of the Th structure had a relatively small impact on the σ of the polymer film
compared to the PBED film. The GIWAXS and HRTEM characterization of the films suggest that the slight
decrease in film σ may be attributed to the ordered stacking of P(BED-T) molecules, which promotes carrier
mobility. On the other hand, the embedding of the TT structure in the P(BED-TT) film resulted in a
significant decrease in its σ due to the absence of apparent long-range ordered stacking of molecules. The
embedding of the Th and TT structures resulted in a decrease in the proportion of EDOT units in the
polymer molecules, which reduced the charge density on the polymer molecules, leading to a lower HOMO
energy level and unfavorable effective doping. Consequently, the S of P(BED-T) and P(BED-TT) showed a
significant increase, approximately four times that of the S of the PBED film.

To further optimize the TE performance of P(BED-T) films, we annealed the polymer films to release
intermolecular stress and promote the free stretching of polymer and dopant molecules, thereby further
optimizing the ordered stacking of polymer molecules. Figure 9B illustrates the impact of different
annealing durations on the TE performance of the films. The TE performance of the films was optimized
-2
-1
after annealing for 10 minutes at 120 ºC in air, reaching 14.9 μW·m ·K . This represents a nearly 50%
increase in the PF compared to before annealing. Surprisingly, high temperature annealing in air led to a