Greatly Enhanced Thermoelectric Performance of Flexible Cu2−xS Composite Film on Nylon by Se Doping

In this work, flexible Cu2−xS films on nylon membranes are prepared by combining a simple hydrothermal synthesis and vacuum filtration followed by hot pressing. The films consist of Cu2S and Cu1.96S two phases with grain sizes from nano to submicron. Doping Se on the S site not only increases the Cu1.96S content in the Cu2−xS to increase carrier concentration but also modifies electronic structure, thereby greatly improves the electrical properties of the Cu2−xS. Specifically, an optimal composite film with a nominal composition of Cu2−xS0.98Se0.02 exhibits a high power factor of ~150.1 μW m−1 K−2 at 300 K, which increases by ~138% compared to that of the pristine Cu2−xS film. Meanwhile, the composite film shows outstanding flexibility (~97.2% of the original electrical conductivity is maintained after 1500 bending cycles with a bending radius of 4 mm). A four-leg flexible thermoelectric (TE) generator assembled with the optimal film generates a maximum power of 329.6 nW (corresponding power density of 1.70 W m−2) at a temperature difference of 31.1 K. This work provides a simple route to the preparation of high TE performance Cu2−xS-based films.


Introduction
Flexible thermoelectrics (TEs) can be used in self-powered technologies and hold promise in wearable sensors and electronics for health and environment monitoring, which has attracted more and more attention in recent years [1][2][3].Flexible TE generators (f-TEGs), which are easily bent to well fit the curved skin surface of the human body, can utilize temperature difference (∆T) between the human body and environment based on the Seebeck effect [4].In addition, this kind of generator has advantages that traditional generators do not have: no moving parts, no noise, no pollution, and maintenance-free.In the past decade, a variety of flexible TE materials has been developed, some of which have been assembled into f-TEGs that have demonstrated the ability to generate electricity at the nW or even µW level and to power some body sensors [5,6].To evaluate the energy conversion efficiency of a material, the dimensionless figure of merit called ZT is introduced and quantified by ZT = α 2 σT/κ, where α is the Seebeck coefficient, σ is the electrical conductivity, κ is thermal conductivity, and T is absolute temperature [7].Usually, α 2 σ, called power factor (PF), is used to evaluate the performance of TE films [8].According to the definition, a high PF and a low κ are indispensable for a good TE material.
Flexible TE films need to have both good flexibility and TE performance.Depending on the presence or absence of a substrate, they can be categorized into self-supporting films or films on flexible substrates [9].The former mainly focuses on conducting polymers and their composite materials [10,11], which possess the benefits of low weight, inherent low κ, and intrinsically high flexibility [12], but with less favorable TE performance compared to inorganic TE materials.The latter, using flexible substrates to support inorganic materials, can take advantage of the high PF of inorganic TE materials and the high flexibility of organic materials [13].Bi 2 Te 3 is the best TE material near room temperature Nanomaterials 2024, 14, 950 2 of 12 (RT).Bi 0.4 Sb 1.6 Te 3 /Te particles were deposited on a Kapton surface by screen printing and then heat treated, and the flexible BiSbTe-based film exhibited an ultrahigh PF of ~3000 µW m −1 K −2 at RT [14].In addition, Ag 2 Se is a promising alternative to Bi 2 Te 3 , with a PF up to 3520 µW m −1 K −2 for Ag 2 Se bulks at 300 K [15].In 2019, our group prepared the flexible Ag 2 Se film on a nylon membrane by vacuum-assisted filtration and hot pressing.The film demonstrated a PF of 987 µW m −1 K −2 and good flexibility at RT [16].Since then, a lot of investigations have been conducted on flexible Ag 2 Se film [17][18][19][20][21]. Recently, Liu et al. [22] prepared Ag/Ag 2 Se composite film on a nylon membrane by adjusting the nominal ratios of Ag/Se based on a one-pot method, and the film showed a high PF of ~2275 µW m −1 K −2 at RT.Nevertheless, low reserves, high cost, and toxicity for the elements Te and Se limit the application of Bi 2 Te 3 and Ag 2 Se.
Compared with Te and Se, S has the advantage of abundant reserve, low toxicity, and low cost.Among TE sulfides, Cu 2−x S (0 ≤ x ≤ 0.25), which is a candidate for "phononliquid electron-crystal", possesses a low lattice thermal conductivity [23,24] and has become a hot research topic in the TE community.Cu 2−x S shows very complicated low temperature crystal structures, and the TE performance is sensitive to x (Cu deficiency).With the increase in x, Cu 2−x S (such as Cu 1.96 S, Cu 1.8 S) demonstrates an increased σ and a decreased α.Typically, the stoichiometric Cu 2 S has a high α of ~300 µV K −1 but a low σ (≤10 S cm −1 ) at RT, which limits its TE application.In order to improve the σ of Cu 2 S, many efforts have been made by doping and phase tuning.Wang et al. [25] optimized the hole concentrations by tuning the chemical bonding in Mn-doped Cu 2 S. The σ of optimal sample increased from ~50 to ~400 S cm −1 at 323 K.However, the sample with an optimal σ exhibited a PF of ~100 µW m −1 K −2 at 323 K, which was not significantly enhanced due to the large reduction in α compared to the pristine sample.Yao et al. [26] prepared Cu 2 S 1−x Se x bulks by mechanical alloying and then spark plasma sintering (SPS), and they found that Se doping could increase carrier concentrations by narrowing the band gap.With α maintained at a proper value (above 100 µV K −1 ), Cu 2 S 0.85 Se 0.15 showed an enhanced PF of 125 µW m −1 K −2 at 323 K. Phase tuning of Cu 2−x S has also attracted much attention recently.It is of interest to note that Cu 1.96 S possesses higher σ than Cu 2 S [27].If the fraction of Cu 1.96 S in Cu 2−x S can be adjusted, Cu 2−x S would obtain an enhanced σ compared to Cu 2 S. Yang et al. [28] achieved the modulation of Cu 1.96 S content in Cu 2−x S composite by adjusting the molar ratio of raw materials.Benefiting from the introduction of Cu 1.96 S, the σ was increased from ~10 S cm −1 to 230 S cm −1 at 300 K. Nevertheless, the α dropped to ~50 µV K −1 at RT, resulting in the PF of Cu 2−x S composite at 300 K (~50 µW m −1 K −2 ) lower than that of pristine Cu 2 S. Recently, Yue et al. [29] synthesized a Cu 2−x S micro/nanocomposite by a hydrothermal method combined with SPS.And the σ reached ~1050 S cm −1 at 323 K due to the synergistic effect of the increased Cu 1.96 S content and the introduction of nanostructures.As a consequence, the micro/nano Cu 2−x S bulks presented a large PF of ~300 µW m −1 K −2 at 323 K.
In this work, we synthesized a series of Se-doped Cu 2−x S powders based on a simple hydrothermal method and prepared Cu 2−x S 1−y Se y films on a nylon membrane by vacuumassisted filtration and hot pressing.By adjusting the nominal content of Se, the electrical properties of Cu 2−x S films were improved.An optimized PF of 150.1 µW m −1 K −2 was obtained from the Cu 2−x S 0.98 Se 0.02 film at RT.The flexibility of the film and output performance of an assembled four-leg f-TEG were studied.

Materials and Methods
Cu 2−x S 1−y Se y (y = 0,0.01,0.02,0.03)powders were synthesized by a facile hydrothermal method based on our recent work [30].Subsequently, the dried powders were dispersed in ethanol, then deposited on the nylon membranes by vacuum-assisted filtration, and finally hot pressed at 270 • C and 1 MPa for 30 min.It should be noted that the Se content (y) is a nominal one.The preparation process of the Cu 2−x S 1−y Se y composite film on a nylon membrane is shown in Figure S1 in Supplementary Materials.Details of raw materials and experiments are given in Note S1 of Supplementary Materials.The assembly process of the f-TEG is described in Note S2.The details of various characterizations and related measurements for the films and f-TEG are given in Note S3.

Results and Discussion
All the Cu 2−x S 1−y Se y (y = 0,0.01,0.02,0.03)powders have similar diffraction patterns, corresponding well to tetragonal Cu 2 S (T-Cu 2 S, PDF#72-1072) and Cu 1.96 S (T-Cu 1.96 S, PDF#29-0578), shown in Figure S2a.When the doping content (y) increases to 3%, additional peaks that belong to Cu 1.8 S (PDF#24-0061) appear.Since Cu 2−x S has different crystal structures, it is difficult to synthesize a pure Cu 2 S phase by the hydrothermal method [31].The Cu 2−x S 1−y Se y powders consist of particles with sizes between 100 and 500 nm (see Figure S2b).
Figure 1 displays the XRD patterns of the Cu 2−x S 1−y Se y (y = 0,0.01,0.02,0.03)films.As shown in Figure 1a, the diffraction peaks of all films can be indexed to monoclinic Cu 2 S (M-Cu 2 S, PDF#33-0390) and T-Cu 1.96 S. The T-Cu 2 S converts to the M-Cu 2 S during hot pressing, which is consistent with our previous report [30].With the increase in Se content, the peaks corresponding to the T-Cu 1.96 S (marked with the light gray box) become stronger, indicating that the fraction of the T-Cu 1.96 S increases.To discuss the effect of Se doping on the content of the T-Cu 1.96 S, we calculated the ratio of the T-Cu 1.96 S and M-Cu 2 S (abbreviated as T:M) according to the following equation [32]: where I is the integral intensity of different diffractions.The ratio of the above two phases can be quantified in the 2θ = 32-39.5• region.The peaks corresponding to (103) and (104) planes are the strongest in the T-Cu 1.96 S. The planes of (034) and (204) belong to the M-Cu 2 S.These peaks are all highlighted in Figure 1b-d, where the orange and cyan fitting lines correspond to the diffractions of T-Cu 1.96 S and M-Cu 2 S, respectively.Herein, the results based on Equation (1) for all samples are listed in Table 1, and it can be seen that the value of T:M increases from 0.39:1 at y = 0 to 0.85:1 at y = 0.03, indicating that the T-Cu 1.96 S content increases with Se doping.Figure 1e shows that the strongest peak of the films shifts to lower angles with x increasing from 0 to 0.03, which is due to Se 2− having a larger ionic radius (1.98 Å) than S 2− (1.84 Å).It is evident that Se doping will lead to the expansion of the Cu 2 S lattice.In order to obtain a better understanding of the composition and elemental valence of the Cu 2−x S 1−y Se y films, XPS measurements were carried out for the Cu 2−x S 0.98 Se 0.02 film (see Figure S3).The high-resolution spectrum of Cu 2p demonstrates two strong peaks at 932.7, 952.5 eV corresponding to Cu + [33].The weak split peaks at about 934.8 eV (Cu 2p 3/2 ), 954.9 eV (Cu 2p 1/2 ), and the satellite peaks are attributed to Cu 2+ , which is due to the presence of Cu 1.96 S [34,35].Two characteristic peaks of S 2p 3/2 and S 2p 1/2 are located at 161.8 and 163.0 eV, respectively.Deconvolution of the Se 3d peaks at 54.0 and 55.1 eV suggests that Se has been successfully doped into the Cu 2−x S films as a divalent ion [36,37].In particular, the XPS spectra of Cu 2p (Figure S4) show that the binding energy of the peaks shifts toward lower values and the proportion of Cu 2+ increases with increasing y, indicating the formation of more Cu 1.96 S [38], which agrees well with our calculation results in Table 1.
lines correspond to the diffractions of T-Cu1.96S and M-Cu2S, respectively.Herein, the results based on Equation ( 1) for all samples are listed in Table 1, and it can be seen that the value of T:M increases from 0.39:1 at y = 0 to 0.85:1 at y = 0.03, indicating that the T-Cu1.96Scontent increases with Se doping.Figure 1e shows that the strongest peak of the films shifts to lower angles with x increasing from 0 to 0.03, which is due to Se 2− having a larger ionic radius (1.98 Å) than S 2− (1.84 Å).It is evident that Se doping will lead to the expansion of the Cu2S lattice.Figure 2 shows the SEM images for the Cu 2−x S 1−y Se y films.The grain boundaries are obscure, which suggests that the films have been sintered only to some extent.The average grain size for Se-doped films lies at 190-250 nm, which is larger than that of the Cu 2−x S film (~136 nm).It is consistent with the phenomena observed in Se-doped Cu 2 S bulk [26].The grain growth is facilitated by the large diffusion rate and small diffusion activation energy of Se in the substitution solid solution [39].The distribution of elements in the Se-doped Cu  In order to obtain a better understanding of the composition and elemental valence of the Cu2−xS1−ySey films, XPS measurements were carried out for the Cu2−xS0.98Se0.02film (see Figure S3).The high-resolution spectrum of Cu 2p demonstrates two strong peaks at 932.7, 952.5 eV corresponding to Cu + [33].The weak split peaks at about 934.8 eV (Cu 2p3/2), 954.9 eV (Cu 2p1/2), and the satellite peaks are attributed to Cu 2+ , which is due to the presence of Cu1.96S [34,35].Two characteristic peaks of S 2p3/2 and S 2p1/2 are located at 161.8 and 163.0 eV, respectively.Deconvolution of the Se 3d peaks at 54.0 and 55.1 eV suggests that Se has been successfully doped into the Cu2−xS films as a divalent ion [36,37].In particular, the XPS spectra of Cu 2p (Figure S4) show that the binding energy of the peaks shifts toward lower values and the proportion of Cu 2+ increases with increasing y, indicating the formation of more Cu1.96S[38], which agrees well with our calculation results in Table 1.
Figure 2 shows the SEM images for the Cu2−xS1−ySey films.The grain boundaries are obscure, which suggests that the films have been sintered only to some extent.The average grain size for Se-doped films lies at 190-250 nm, which is larger than that of the Cu2−xS film (~136 nm).It is consistent with the phenomena observed in Se-doped Cu2S bulk [26].The grain growth is facilitated by the large diffusion rate and small diffusion activation energy of Se in the substitution solid solution [39].The distribution of elements in the Sedoped Cu2−xS film is further examined by elemental mapping shown in Figure S5.The elements of Cu, S, and Se are evenly distributed.As an example, the internal microstructure of the Cu 2−x S 0.98 Se 0.02 film was observed by TEM, and the results are presented in Figure 3.As Figure 3a,b shows, the film contains submicron grains (above 100 nm) and significant number of nanograins with size of 20-100 nm.The size of these nanograins is smaller than that of the Cu 2−x S powder (see Figure S2), indicating the powder underwent a melting and recrystallization process.A clear grain boundary (GB) can be seen between two grains in the high-resolution TEM (HRTEM) (Figure 3b).The two grains form a continuous GB, corresponding to the (101) plane of Cu 2 S, with a misalignment between two (101) planes of about 15 • .The continuous GB favors the carrier transport.Figure 3c is an enlarged image of the blue square marked in Figure 3a, showing a typical triangular GB with different orientations of the zone axis.And the three grains (grains A, B, and C) are well bonded.The lattice spacing of grain A and grain B is about 0.755 and 0.326 nm, corresponding to the (101) plane of Cu 2 S and the (012) plane of Cu 1.96 S, respectively, which also indicates the coexistence of Cu 2 S and Cu 1.96 S in the Cu 2−x S 0.98 Se 0.02 film.Moreover, Figure 3d is an enlarged TEM image of the area marked by the red square in Figure 3a, indicating that the film contains nanograins.Figure 3e is an enlarged image of the pink square marked in Figure 3d, which contains three grains (grains D, E, and F) forming a triangle boundary.It can be seen from Figure 3f that grain D looks to have a very wide lattice spacing.However, the HRTEM image (inset of Figure 3f) reveals that there exist two additional lattice planes marked by green and brown in between the planes marked by yellow.The lattice spacing of grain E is ~0.As an example, the internal microstructure of the Cu2−xS0.98Se0.02film was observed by TEM, and the results are presented in Figure 3.As Figure 3a,b shows, the film contains submicron grains (above 100 nm) and significant number of nanograins with size of 20-100 nm.The size of these nanograins is smaller than that of the Cu2−xS powder (see Figure S2), indicating the powder underwent a melting and recrystallization process.A clear grain boundary (GB) can be seen between two grains in the high-resolution TEM (HRTEM) (Figure 3b).The two grains form a continuous GB, corresponding to the (101) plane of Cu2S, with a misalignment between two (101) planes of about 15°.The continuous GB favors the carrier transport.Figure 3c is an enlarged image of the blue square marked in Figure 3a, showing a typical triangular GB with different orientations of the zone axis.And the three grains (grains A, B, and C) are well bonded.The lattice spacing of grain A and grain B is about 0.755 and 0.326 nm, corresponding to the (101) plane of Cu2S and the (012 ) plane of Cu1.96S, respectively, which also indicates the coexistence of Cu2S and Cu1.96S in the Cu2−xS0.98Se0.02film.Moreover, Figure 3d is an enlarged TEM image of the area marked by the red square in Figure 3a, indicating that the film contains nanograins.Figure 3e is an enlarged image of the pink square marked in Figure 3d, which contains three grains (grains D, E, and F) forming a triangle boundary.It can be seen from Figure 3f that grain D looks to have a very wide lattice spacing.However, the HRTEM image (inset of Figure 3f) reveals that there exist two additional lattice planes marked by green and brown in between the planes marked by yellow.The lattice spacing of grain E is ~0.274 nm, corresponding to the (103) plane of Cu1.96S.A typical high-angle annular dark field (HAADF) image and corresponding EDS images are shown in Figure 3g, which indicates that Cu, S, and Se are homogeneously distributed in the Cu2−xS1−ySey film.The temperature dependence TE parameters for the Cu 2−x S 1−y Se y (y = 0,0.01,0.02,0.03)films are presented in Figure 4.As the temperature rises, the σ for all samples exhibits the same trend with a turning point at about 350 K in Figure 4a, which corresponds to the phase transition of Cu 2−x S. With the increase in Se content, the σ at RT enhances from 8.5 S cm −1 at y = 0 to 93.0 S cm −1 at y = 0.03.The Hall effect measurement results shown in Figure S6a reveal that the carrier concentration (n) increases from 12.2 × 10 20 to 38.6 × 10 20 cm −3 at 300 K with increasing Se content.Additionally, the mobility (µ) first increases from 2.91 cm 2 V −1 s −1 at y = 0 to 5.95 cm 2 V −1 s −1 at y = 0.01 and then decreases further with increasing y.The synchronous increase in n and µ after Se doping is beneficial for the enhancement in σ at RT.To obtain further insight into the mechanism of enhanced σ after Se doping, the activation energy of electrical resistivity (E a ) was estimated by the following Arrhenius equation [40]: where ρ is electrical resistivity, ρ 0 is the temperature-dependent constant, and k B is the Boltzmann constant.The plot of ln ρ vs. 1/T in the range of 300 K < T < 423 K has two linear portions with different slopes corresponding to E a1 and E a2 .As shown in Figure 4b and Table S1, E a1 and E a2 decrease with increasing y, indicating that the Fermi level approaches the valance band and the band structure has been altered [40,41], which contributes to the electrical conduction.To sum up, we deduce that the increase in σ can be attributed to the increased content of Cu 1.96 S, which possesses a higher Cu vacancy concentration, and is also simultaneously affected by the movement of the Fermi level.The variation in band structure induced by Se doping can also influence the µ of Cu 2−x S, which will be discussed hereinafter.The α is positive for all samples over the measured temperature range in Figure 4c, demonstrating a p-type conduction of the Cu 2−x S materials [42].It increases with the rising temperature, which is insensitive to the phase transition from a monoclinic to hexagonal structure [25] unlike σ.In addition, with increasing y, α exhibits the opposite trend to σ: it decreases from 271.8 to 116.2 µV K −1 at RT.We calculated the carrier effective mass (m*) using a single parabolic band (SPB) model based on the measured α and n values.Figure S6b shows the α vs. n curves called Pisarenko plots for the Cu 2−x S 1−y Se y films at RT (more details can be found in Note S4 of Supplementary Materials).The m* decreases after doping while slightly increases with increasing y, which reflects the variation in the band structure.Since µ is inversely proportional to m*, µ = qτ/m * (q is the electric charge and τ is the carrier scattering time), the reduced m* after Se doping favors carrier transport, thereby increasing µ.Additionally, the m* of the T-Cu 1.96 S is higher than that of the M-Cu 2 S [33], which is the reason for the increase in m* from 3.09 m e at y = 0.01 to 3.39 m e at y = 0.03.
The Hall effect measurement results of the Cu 2−x S 0.98 Se 0.02 film show that n and µ first increase and then decrease with the increase in temperature (see Figure 4d), which is related to the phase transition and consistent with the change in σ.The variation in α with temperature is a comprehensive effect of n, m*, and temperature.[25,26,29,43] and flexible films [30,44,45] at room temperature.
The Hall effect measurement results of the Cu2−xS0.98Se0.02film show that n and μ first increase and then decrease with the increase in temperature (see Figure 4d), which is related to the phase transition and consistent with the change in σ.The variation in α with temperature is a comprehensive effect of n, m*, and temperature.
Owing to enhanced σ (69.4 S cm −1 ) and high α (147.0 μV K −1 ), the Cu2−xS0.98Se0.02film exhibits a maximum PF of 150.1 μW m −1 K −2 at RT, which is ~138% higher than that of the undoped Cu2−xS film.The PF increases to 244.5 μW m −1 K −2 at 403 K. Figure 4f shows a comparison of the PF values at RT of our work and reported Cu2S-based bulks [25,26,29,43] and flexible films [30,44,45].The present PF value is outstanding compared to that of the reported Cu2S-based flexible films and comparable with that of Cu2S1−xSex bulk (PF = 115.2μW m −1 K −2 ) [26].However, the value is still lower than that of micro/nano Cu2−xS bulk [29], which is mainly due to the higher density of the latter.
Because it is hard to peel off the film from the nylon membrane without destroying it, the κ of the Cu2−xS0.98Se0.02film was not measured.However, we believe that the κ of this film is low for the following reasons: (1) Cu2S has a very low κ of 0.45 W m −1 K −1 [23].(2) The film contains grains with sizes from nano to submicron, grain boundaries, and heterointerfaces between Cu1.96S and Cu2S grains, which can scatter phonons to lower the κ.
Figure 5a displays the flexibility test result for the Cu2−xS1−ySey (y = 0 and 0.02) films.After 1000 and 1500 bending cycles under a bending radius of 4 mm, the σ of the Cu2−xS0.98Se0.02film can retain 97.7% and 97.2% of the initial σ (σ0), respectively, which are higher than that of the Cu2−xS film.To understand the enhancement for flexibility, we observed the Cu2−xS film and Cu2−xS0.98Se0.02film both having a similar thickness (~10 μm) by SEM.SEM images (see Figure S7) show that the Cu2−xS0.98Se0.02film contains fewer pores and is denser than the Cu2−xS film.Cracks are generated at pores and fewer pores contribute to the better flexibility.Compared to the σ/σ0 results of reported Cu2X (X = S, Se) films [30,[44][45][46] and other films (Bi2Te3 [14], Ag2Se [17,47]) under the same test conditions (bending radius of 4 mm, bending cycles of 1000 times), the Cu2−xS0.98Se0.02film exhibits outstanding flexibility, which is mainly due to the excellent flexibility of nylon and the improvement in the density of the film.Owing to enhanced σ (69.4 S cm −1 ) and high α (147.0 µV K −1 ), the Cu 2−x S 0.98 Se 0.02 film exhibits a maximum PF of 150.1 µW m −1 K −2 at RT, which is ~138% higher than that of the undoped Cu 2−x S film.The PF increases to 244.5 µW m −1 K −2 at 403 K. Figure 4f shows a comparison of the PF values at RT of our work and reported Cu 2 S-based bulks [25,26,29,43] and flexible films [30,44,45].The present PF value is outstanding compared to that of the reported Cu 2 S-based flexible films and comparable with that of Cu 2 S 1−x Se x bulk (PF = 115.2µW m −1 K −2 ) [26].However, the value is still lower than that of micro/nano Cu 2−x S bulk [29], which is mainly due to the higher density of the latter.
Because it is hard to peel off the film from the nylon membrane without destroying it, the κ of the Cu 2−x S 0.98 Se 0.02 film was not measured.However, we believe that the κ of this film is low for the following reasons: (1) Cu 2 S has a very low κ of 0.45 W m −1 K −1 [23].
(2) The film contains grains with sizes from nano to submicron, grain boundaries, and heterointerfaces between Cu 1.96 S and Cu 2 S grains, which can scatter phonons to lower the κ.
Figure 5a displays the flexibility test result for the Cu 2−x S 1−y Se y (y = 0 and 0.02) films.After 1000 and 1500 bending cycles under a bending radius of 4 mm, the σ of the Cu 2−x S 0.98 Se 0.02 film can retain 97.7% and 97.2% of the initial σ (σ 0 ), respectively, which are higher than that of the Cu 2−x S film.To understand the enhancement for flexibility, we observed the Cu 2−x S film and Cu 2−x S 0.98 Se 0.02 film both having a similar thickness (~10 µm) by SEM.SEM images (see Figure S7) show that the Cu 2−x S 0.98 Se 0.02 film contains fewer pores and is denser than the Cu 2−x S film.Cracks are generated at pores and fewer pores contribute to the better flexibility.Compared to the σ/σ 0 results of reported Cu 2 X (X = S, Se) films [30,[44][45][46] and other films (Bi 2 Te 3 [14], Ag 2 Se [17,47]) under the same test conditions (bending radius of 4 mm, bending cycles of 1000 times), the Cu 2−x S 0.98 Se 0.02 film exhibits outstanding flexibility, which is mainly due to the excellent flexibility of nylon and the improvement in the density of the film.[14,17,30,[44][45][46][47] with a bending radius of 4 mm for 1000 times.
To verify the potential application of the Cu2−xS0.98Se0.02film, a four-leg f-TEG was assembled (see the details on the fabrication process in Note S2 of the Supplementary Materials).The internal resistance (Rin) of the whole device is measured to be ~254 Ω.According to the resistance (R1~236.4Ω) of the four legs calculated from the σ of the corresponding film, the contact resistance (Rc) is 17.6 Ω if the resistance of the metal electrode is neglected.
Figure 6a shows the open-circuit voltage (Voc) generated by the f-TEG at different ΔTs.Voc exhibits a primary linear relationship with ΔT, corresponding to the equation Voc=N|α|ΔT (N is the number of legs).When the ΔT is 21.8 and 31.1 K, Voc is measured to be 12.95 and 18.31 mV, respectively.Furthermore, the f-TEG was connected into a circuit, as shown in the inset of Figure 6a, to obtain the output characteristics.Figure 6b records the output voltage (Vout) and power (Pout) vs. current by adjusting load resistance (Rload) under different ΔTs.Obviously, the measured Vout is inversely proportional to the current, and the calculated Pout displays a tendency to first increase and then decrease with the increasing current.When ΔT of 21.8 and 31.1 K is applied, the maximum power (Pmax) generated by the device reaches 160.0 and 329.6 nW, respectively, with a Rload of about 240 Ω.According to the following equation: where Rex is the external resistance including Rload, and the resistance of the variable resistor box and the ammeter (R2, measured to be 15.7 Ω), Pmax can be obtained when Rex equals Rin, which is close to our measurement.To verify the potential application of the Cu 2−x S 0.98 Se 0.02 film, a four-leg f-TEG was assembled (see the details on the fabrication process in Note S2 of the Supplementary Materials).The internal resistance (R in ) of the whole device is measured to be ~254 Ω.According to the resistance (R 1 ~236.4Ω) of the four legs calculated from the σ of the corresponding film, the contact resistance (R c ) is 17.6 Ω if the resistance of the metal electrode is neglected.
Figure 6a shows the open-circuit voltage (V oc ) generated by the f-TEG at different ∆Ts.V oc exhibits a primary linear relationship with ∆T, corresponding to the equation V oc = N|α|∆T (N is the number of legs).When the ∆T is 21.8 and 31.1 K, V oc is measured to be 12.95 and 18.31 mV, respectively.Furthermore, the f-TEG was connected into a circuit, as shown in the inset of Figure 6a, to obtain the output characteristics.Figure 6b records the output voltage (V out ) and power (P out ) vs. current by adjusting load resistance (R load ) under different ∆Ts.Obviously, the measured V out is inversely proportional to the current, and the calculated P out displays a tendency to first increase and then decrease with the increasing current.When ∆T of 21.8 and 31.1 K is applied, the maximum power (P max ) generated by the device reaches 160.0 and 329.6 nW, respectively, with a R load of about 240 Ω.According to the following equation: where R ex is the external resistance including R load , and the resistance of the variable resistor box and the ammeter (R 2 , measured to be 15.7 Ω), P max can be obtained when R ex equals R in , which is close to our measurement.The maximum power density (PD max = P max /N•A, where A is the cross-sectional area of one leg) of the f-TEG is 1.70 W m −2 under a ∆T of 31.1 K. To facilitate the comparison of the output performance of different f-TEGs, the normalized PD max (PD max L/∆T 2 , where L is the length of one leg) is estimated to be ~35.15µW m −1 K −2 .As shown in Table S2, this work exhibits better output performance compared with reported Cu x A (A = S, Te)-based f-TEGs.However, the value of PD max L/ ∆T 2 is inferior to that of reported Cu 2 Se-based f-TEGs.Figure 6c displays a digital photograph of the f-TEGs attached to a beaker half-filled with warm water to generate power.The f-TEG can generate a voltage of 4.7 mV under a ∆T of 6.9 K between ambient environment and warm water, indicating that the f-TEG assembled is feasible as an energy supply device for wearable electronics.The maximum power density (PDmax=Pmax/N•A, where A is the cross-sectional area of one leg) of the f-TEG is 1.70 W m −2 under a ΔT of 31.1 K. To facilitate the comparison of the output performance of different f-TEGs, the normalized PDmax (PDmaxL/ΔT 2 , where L is the length of one leg) is estimated to be ~35.15μW m −1 K −2 .As shown in Table S2, this work exhibits better output performance compared with reported CuxA (A = S, Te)-based f-TEGs.However, the value of PDmaxL/ΔT 2 is inferior to that of reported Cu2Se-based f-TEGs.Figure 6c displays a digital photograph of the f-TEGs attached to a beaker halffilled with warm water to generate power.The f-TEG can generate a voltage of 4.7 mV under a ΔT of 6.9 K between ambient environment and warm water, indicating that the f-TEG assembled is feasible as an energy supply device for wearable electronics.

Conclusions
In summary, we successfully prepared Cu2−xS1−ySey flexible composite films on a nylon membrane, which contains submicron grains and nanograins.And the composite films possessed more Cu1.96Sby adjusting the nominal amount of Se, which is beneficial to the enhancement of σ.Meanwhile, the calculation for activation energy Ea and the carrier effective mass shows that the electronic structure of Cu2−xS can be modulated efficiently via varying Se content, thereby optimizing the electrical properties.Consequently, the maximum power factor reaches 150.1 μW m −1 K −2 for the Cu2−xS0.98Se0.02film at room temperature, which is approximately 138% higher than that of the pristine Cu2-xS film.In addition, the film possesses superior flexibility: 97.2% of the original electrical conductivity is maintained after 1500 bending cycles with a radius of 4 mm, and it is favorable to application.A four-leg f-TEG assembled with the film can generate a voltage of 18.31 mV

Conclusions
In summary, we successfully prepared Cu 2−x S 1−y Se y flexible composite films on a nylon membrane, which contains submicron grains and nanograins.And the composite films possessed more Cu 1.96 S by adjusting the nominal amount of Se, which is beneficial to the enhancement of σ.Meanwhile, the calculation for activation energy E a and the carrier effective mass shows that the electronic structure of Cu 2−x S can be modulated efficiently via varying Se content, thereby optimizing the electrical properties.Consequently, the maximum power factor reaches 150.1 µW m −1 K −2 for the Cu 2−x S 0.98 Se 0.02 film at room temperature, which is approximately 138% higher than that of the pristine Cu 2−x S film.In addition, the film possesses superior flexibility: 97.2% of the original electrical conductivity is maintained after 1500 bending cycles with a radius of 4 mm, and it is favorable to application.A four-leg f-TEG assembled with the film can generate a voltage of 18.31 mV and a maximum power of 329.6 nW under a temperature difference of 31.1 K. Our work demonstrates that Se doping in Cu 2−x S can be an effective strategy for the modulation of phase composition and band structure, thereby developing low-cost and eco-friendly Cu 2−x S-based flexible TE film with enhanced TE performance.
Figure2shows the SEM images for the Cu 2−x S 1−y Se y films.The grain boundaries are obscure, which suggests that the films have been sintered only to some extent.The average grain size for Se-doped films lies at 190-250 nm, which is larger than that of the Cu 2−x S film (~136 nm).It is consistent with the phenomena observed in Se-doped Cu 2 S bulk[26].The grain growth is facilitated by the large diffusion rate and small diffusion activation energy of Se in the substitution solid solution[39].The distribution of elements in the Se-doped Cu 2−x S film is further examined by elemental mapping shown in FigureS5.The elements of Cu, S, and Se are evenly distributed.

Figure 3 .
Figure 3. TEM analysis of the Cu 2−x S 1−y Se y film (y = 0.02): (a) Typical TEM image.(b-d) Enlarged images of the orange, blue, and red squares marked in (a), respectively.Insets are the fast Fourier

Figure 6 .
Figure 6.Output performance of the four-leg f-TEG assembled with the Cu2−xS0.98Se0.02film: (a) Opencircuit voltage at different ΔT.The inset is a circuit diagram for the output performance test.(b) Output voltage and power vs. current at different ΔT.(c) f-TEG attached to a beaker half-filled with warm water to harvest heat.A voltage of 4.7 mV created by the f-TEG from the ΔT across the water surface in the beaker (the corresponding infrared thermal image is on the right, with the ΔT of 6.9 K).

Figure 6 .
Figure 6.Output performance of the four-leg f-TEG assembled with the Cu 2−x S 0.98 Se 0.02 film: (a) Open-circuit voltage at different ∆T.The inset is a circuit diagram for the output performance test.(b) Output voltage and power vs. current at different ∆T.(c) f-TEG attached to a beaker half-filled with warm water to harvest heat.A voltage of 4.7 mV created by the f-TEG from the ∆T across the water surface in the beaker (the corresponding infrared thermal image is on the right, with the ∆T of 6.9 K).

Table 1 .
The