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Wang et al. Soft Sci 2023;3:34 https://dx.doi.org/10.20517/ss.2023.25 Page 21 of 26

n-type Ag Se/Ag/CuAgSe ternary films with a recorded power factor of 1,593.9 μW/mK 2[122] . The bending
2
test shows that the hybrid film has excellent flexibility, and the power factor only decreases by 10% after
bending 1,000 times along the rod with a radius of 4 mm. Optimized carrier transport and the interfacial
energy filtering effect of the composite contribute to the ultrahigh power factor. They also prepared
PEDOT/Ag Se/CuAgSe (1,603 μW/mK 2[123] ), Ag Se/Ag/PEDOT (1,442 μW/mK 2[124] ), Ag Se/Se/Polypyrrole
2
2
2
(2,240 μW/mK 2[125] ) composite films for flexible TEGs. These results are several orders of magnitudes higher
than those of Ag Se-based composite films prepared from other methods, such as Ag Se NW/PEDOT:PSS
2
2
composite films prepared from a drop-casting method with a maximum room-temperature power factor of
178.59 µW/mK 2[126] ; and Ag Se NW/PEDOT:PSS composite film prepared from a digital light processing-
2
based 3D printing method with a power factor of 81.94 µW/mK 2[127] , indicating the significant advantages of
vacuum filtration method for the preparation of flexible TE films.
CONCLUSION AND OUTLOOK
Recent years have seen significant progress in using vacuum filtration to advance flexible TE films
scientifically and technologically. The current review article highlights the construction of conducting
polymer-based, carbon nanoparticle-based, inorganic, two-dimensional materials, and ternary composites,
respectively. Despite the recent success in this research area, many critical issues must be addressed.

(1) Currently, the filter membranes used in the filtration method are insulating. On the one hand, they are
embedded in the TE film as a flexible skeleton. On the other hand, their insulation affects the overall TE
performance, considering that TE materials are the properties of bulk materials. In the future, we could
design a filter membrane with high conductivity to provide electrical conductivity further to enhance TE
performance while achieving flexible support. Even once the organic TE filter membrane is developed, the
composite TE film can be directly prepared through vacuum filtration.

(2) Although some materials can detangle from the filter to form self-supporting films, this is limited to
thick films at the micron scale. Is it possible to transfer nano-thickness films such as PEDOT:PSS from the
filter membrane substrate to organic optoelectronic devices in the future? Most PEDOT:PSS hole transport
layers in organic optoelectronic devices are prepared by spin coating or drip coating methods, which are still
difficult to remove PSS. Since the filtration method we discussed above can effectively filter out much
PSS-assisted conduction, it is only a step short of transfer to the actual application in organic photoelectric
devices. If a breakthrough is made, there will be excellent prospects.

(3) The future of wearable applications will require flexibility, stretchability, and anti-fatigue stability. TE
films prepared based on this method have achieved the goal of flexibility, but there are few studies on
stretchability, fatigue stability, etc. In the future, improving both the substrate and the material may be
possible, such as finding a stretchable substrate or further enhancing the binding force between the
substrate and the film. Only in this way can the material prepared by the vacuum filtration method realize
the leap from flexibility to stretchability and be closer to practical application.

(4) The integrated use of multifunctional equipment is the future development trend of intelligent
equipment. For example, combining TE devices and supercapacitors can enable the integration of functions
from power generation to power storage. The combination of TE and electrochromic devices allows the
device to achieve the role of color change in the environment without a power supply. Suppose the process
of preparing TE flexible films by vacuum filtration can be extended to the preparation process of other
functional films. The difficulty and cost of preparing multifunctional integrated devices will be significantly
simplified in that case. There have been reports on the preparation of flexible supercapacitor films by the