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Wang et al. Soft Sci 2023;3:34 https://dx.doi.org/10.20517/ss.2023.25 Page 15 of 26

Figure 11. (A) XRD pattern of the Ag Se film; (B) Low magnification FESEM image of Ag Se film; (C) High magnification FESEM image
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of Ag Se film; (D) Overview high-angle annular dark field-STEM image; (E) Typical STEM image; (F) FFT image corresponding to (E);
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(G) Temperature dependence of Seebeck coefficient, electrical conductivity, and power factor for the Ag Se film. Reproduced with
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[92] ©
permission from Ref . Copyright 2019. Springer Nature. FESEM: Field emission scanning electron microscopy; FFT: fast Fourier
transform; STEM: scanning transmission electron microscopy; XRD: X-ray diffraction analysis.
conductivity was retained after 1,000 bending cycles around an 8-mm diameter rod. This excellent flexibility
came from the synergetic effect of the nylon membrane and the Ag Se film intertwined with numerous high
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aspect-ratio Ag Se grains. Suitable element doping treatment improves the TE performance of Ag Se .
[93]
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Therefore, they studied the influence of different elements (including Ga, S) doping on the TE properties of
flexible Ag Se films [94,95] .
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To solve the high porosity and control the growth direction of the as-prepared Ag Se film, they optimized
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the synthesis temperature of Ag Se NWs from room temperature to 40 °C. They kept the other process
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conditions the same as Refs [92,96] . The morphology synthesized Ag Se changed from uniform NWs to
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multisized nanostructures, leading to a more compact film (~90% relative density) after hot pressing with a
suppressed growth direction along the (00l). Finally, the power factor of the optimized Ag Se film increased
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to 1,882 μW/mK at 300 K, which is nearly twice as large as the previous report. However, the high density
limited the flexibility of the film, and the power factor decreased by 16.1% after bending 1,500 times due to
the break of some weak-bonded Ag Se grain boundaries. To solve the problem, they introduced PVP as a
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binder to hinge rigid Ag Se micro grains to improve the flexibility and take full advantage of the intrinsic
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[97]
low thermal conductivity of PVP to increase the ZT value of the Ag Se film . The microstructure
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