Preparation and Properties of Flexible CuI/Polyvinylpyrrolidone Nanocomposite Thermoelectric Film

: A facile preparation method for ﬂexible p-type CuI/polyvinylpyrrolidone (PVP) nanocomposite thermoelectric (TE) ﬁlm is developed. First, CuI powder was synthesized by a one-pot method; second, PVP was coated in situ with the CuI powder; third, the CuI/PVP nanocomposite ﬁlm was prepared on a nylon membrane by vacuum ﬁltration and then hot-pressing. Transmission electron microscopy (TEM) observation indicates that the ﬁlm consists of CuI nanograins with an average size of ~15 nm and PVP distributed at the inner wall of nanopores and the surface of the CuI nanograins. The composite ﬁlm shows a large Seebeck coefﬁcient of ~605 µ VK − 1 and a power factor of ~8.05 µ Wm − 1 K − 2 at 300 K. The nanocomposite ﬁlm also exhibits excellent ﬂexibility (~96% of initial electrical conductivity retention after being bent 1000 times along a 4 mm radius rod). A single-leg TE module outputs a voltage of ~3.6 mV when the temperature difference is 6 K. This work provides a fast, simple, and environmentally friendly method by which to prepare ﬂexible CuI/PVP nanocomposite TE ﬁlm with a large Seebeck coefﬁcient, which could be used as a wearable sensor.


Introduction
Owing to the excessive pursuit of industrialization, the global energy crisis is becoming more and more serious, and the use of fossil fuels has also caused serious harm to the environment, such as global warming, acid rain, pollution of the soil environment, and animal extinction.Therefore, it is urgent to explore sustainable new energy to replace traditional fossil fuels [1,2].Thermoelectric (TE) material is a kind of material with wide application prospects.It can convert heat energy into electrical energy directly by the transport of carriers within solids under a low-temperature difference [3].The performance of TE materials is evaluated by a dimensionless figure of merit, ZT = S 2 σTk −1 , where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, k is the total thermal conductivity, and S 2 σ is called the power factor (PF).A good TE material should have a high electrical conductivity, a large Seebeck coefficient, and a low thermal conductivity.However, these three parameters are strongly coupled to each other and hence it is a great challenge to obtain a high ZT value.Various approaches have been chosen to improve TE performance, such as doping, nanostructuring, annealing, alloying, and so on, and much progress has been made in several material systems [4], such as SnSe [5,6], Cu 2 Se [7,8], and GeTe [9,10].
In recent years, the demand for wearable electronic devices has explosively increased.But most of the devices are powered by traditional batteries, which need to be replaced or recharged frequently.And this may limit further development of the wearable devices [11,12].Recently, wearable sensors, which can be installed or worn on the human body to transmit, sense, and process information, have been widely used in consumer electronics, fitness, and health fields.The wearable sensors play an important role, especially phases of CuI, and the typical Seebeck coefficient of γ-CuI at 300 K is ~237 µVK −1 (at a hole concentration of 10 20 cm −3 ).Since then, the TE properties of CuI have attracted increasing attention, especially CuI films [41][42][43].For example, Murmu et al. [43] deposited CuI films using an ion beam sputtering under a high vacuum at an ambient temperature and annealed the resulting CuI films in a quartz tube filled with argon.The electrical conductivity of the deposited CuI film is ~21.9Scm −1 and the Seebek coefficient is ~264.7 µVK −1 .After annealing at 250 • C, the electrical conductivity of the film decreased to 5.1 Scm −1 , which is 76% of that before annealing, while the Seebek coefficient increased to 789.5 µVK −1 , which is ~3 times as high as that before annealing, and the final PF was 740.9 µWm −1 K −2 .Optical transparency of the CuI film in the visible range is found to be 60~85%.However, the film is inflexible.CuI film has been deposited on polyethylene terephthalate (PET) substrate by magnetron sputtering [44], varying the iodine partial pressure in the sputter chamber during the magnetron sputtering to control the hole concentration of the CuI films.The obtained CuI film has a Seebeck coefficient of ~172 µVK −1 , a PF ~359 µWm −1 K −2 , and ZT ~0.21 at room temperature, which is attributed to a combined effect of the heavy element iodine and strong phonon scattering.With a compressive bending angle up to 90 • , the change in the internal resistance is <3%, and, after repeatedly bending up to 400 cycles, the change in the internal resistance is <0.2%.A prototype of a CuI-based transparent TE module shows good mechanical flexibility with a high power density of ~2.4 mWcm −2 at a temperature difference (∆T) of 50 K, which shows the great potential of the CuI film as a transparent flexible TE material, and the power density of the CuI film is comparable with those of Bi 2 Te 3 /Sb 2 Te 3 -based devices.Klochko et al. [45] synthesized CuI film on a nanocellulose (NC) substrate via a successive ionic layer adsorption and reaction (SILAR) method.The CuI/NC films obtained by repeating 25-40 SILAR cycles consist of cubic (111)-oriented γ-CuI crystals with faceted surfaces of 200-300 nm.The CuI/NC films have a PF of ~140 µWm −1 K −2 at 333 K.When the temperature difference is 40 K, the CuI/NC-based single TE leg can generate a voltage of 3.5 mV and a maximum power of 3.8 nW.However, these methods, including pulsed laser deposition [43], sputtering [44,46], and SILAR [45,47], are expensive and time-consuming.
Our group recently developed a method for the synthesis of Ag 2 Se nanowires combined with vacuum filtration and hot-pressing to prepare flexible Ag 2 Se film on nylon.This Ag 2 Se film has PF ~987 µWm −1 K −2 at room temperature, and it has good flexibility [48].Based on this process, our group has successfully prepared various kinds of flexible Ag 2 Sebased composite TE film [49][50][51][52][53][54], such as polyvinylpyrrolidone (PVP)/Ag 2 Se composite film, starting from Ag 2 Se nanostructures coated with PVP, the PF of which can reach ~1910 µWm −1 K −2 (corresponding ZT ~1.1) at room temperature [49].The PVP (molecular formula being (C 6 H 9 NO) n ) acts as a great additive for improving flexibility (a 5.5% decrease in PF after being bent along a rod with a radius of 4 mm 1000 times) and the PVP-coated Ag 2 Se nanostructures can disperse more evenly in ethanol leading to a more homogeneous film.This is because PVP is a non-ionic surfactant that has good solubility in water and most organic solvents and can well disperse inorganic nanoparticles due to forming a uniform PVP coating nanolayer on the nanoparticles.PVP has high solubility in both polar and non-polar solvents, biocompatibility, good complexation, good adhesion, low toxicity, and good spinnability [55].Although PVP is insulating, which will have a negative effect on electrical conductivity, it also has extremely low thermal conductivity; if only a very small amount of PVP is added, the direct effect of adding PVP on the thermoelectric performance of the composite film could be neglected.
In our previous work [56], we reported the preparation and TE properties of CuI film, but the film was porous and hence with ultralow electrical conductivity and low stability.Hence, in this work, PVP is used to improve the flexibility, electrical conductivity, and stability of the CuI film.PVP-coated CuI powder was prepared in situ by a one-pot method at room temperature, and then flexible CuI/PVP film was prepared on a nylon membrane combined with vacuum filtration and hot-pressing.The TE properties and flexibility of the CuI/PVP nanocomposite films were investigated.In addition, a flexible single-leg TE module was fabricated with the nanocomposite film, and its touch tests were investigated.
In a typical procedure, PVP-coated CuI powder was prepared as follows: first, 1.3288 g of Cu(NO 3 ) 2 •3H 2 O and 2g of PEG were dissolved in 50 mL ethanol to form solution A and 0.83 g of KI and 2g of PEG were dissolved in 50 mL ethanol to form solution B. Second, a certain amount of PVP (0.01 g, 0.03 g or 0.05 g) and 100 µL N 2 H 4 •H 2 O were added to solution B with stirring.Third, solution B was slowly added into solution A with continuous stirring for 20 min.Finally, PVP-coated CuI powder was collected by centrifugation and washed with ethanol and deionized water (DI) several times.
The PVP-coated CuI powder was ultrasonically dispersed in ethanol for 5 min, and then deposited on a porous nylon membrane via vacuum-assisted filtration.The asprepared film was dried in a vacuum oven for 24 h at 65 • C then hot-pressed at 230 • C and 1 MPa for 30 min to obtain CuI/PVP composite film.Figure 1 shows a schematic diagram of the preparation of the CuI/PVP composite film Schematic illustration of CuI grain growth with PVP when hot-pressing is illustrated in Figure S8.To study the effect of the content of PVP on CuI, PVP content was adjusted from 0.01 and 0.03 to 0.05 g; for simplicity, the corresponding powders were named P1-powder, P2-powder, and P3-powder, respectively, and the corresponding films were named P1-film, P2-film, and P3-film, respectively.Pure CuI powder and film were named P0-powder and P0-film, respectively.
membrane combined with vacuum filtration and hot-pressing.The TE properties a ibility of the CuI/PVP nanocomposite films were investigated.In addition, a flexi gle-leg TE module was fabricated with the nanocomposite film, and its touch tes investigated.
In a typical procedure, PVP-coated CuI powder was prepared as follows: first g of Cu(NO3)2•3H2O and 2g of PEG were dissolved in 50 mL ethanol to form sol and 0.83 g of KI and 2g of PEG were dissolved in 50 mL ethanol to form solution B. S a certain amount of PVP (0.01 g, 0.03 g or 0.05 g) and 100 µL N2H4•H2O were ad solution B with stirring.Third, solution B was slowly added into solution A with uous stirring for 20 min.Finally, PVP-coated CuI powder was collected by centrif and washed with ethanol and deionized water (DI) several times.
The PVP-coated CuI powder was ultrasonically dispersed in ethanol for 5 m then deposited on a porous nylon membrane via vacuum-assisted filtration.The pared film was dried in a vacuum oven for 24 h at 65 °C then hot-pressed at 230 ° MPa for 30 min to obtain CuI/PVP composite film.Figure 1 shows a schematic diag the preparation of the CuI/PVP composite film Schematic illustration of CuI grain with PVP when hot-pressing is illustrated in Figure S8.To study the effect of the of PVP on CuI, PVP content was adjusted from 0.01 and 0.03 to 0.05 g; for simplic corresponding powders were named P1-powder, P2-powder, and P3-powder, tively, and the corresponding films were named P1-film, P2-film, and P3-film, tively.Pure CuI powder and film were named P0-powder and P0-film, respective A flexible single-leg TE module was fabricated by first sticking a strip (20 m mm) of the CuI/PVP composite film onto polyimide (PI) substrate with a doubl adhesive tape; then, Ag paste (SPI# 04998-AB) was painted onto two ends of the connect with conducting wires.A finger touch test on a single-leg TE module wa by a self-made experimental instrument (see Figure S8).As Figure S8 shows, in the leg TE module, heat transfers from the hot side to the cold side along the length d of the module.We used an automatic temperature controlling system to heat a plate and one end of the module was put on the plate as the hot side (T + ΔT).Th A flexible single-leg TE module was fabricated by first sticking a strip (20 mm × 5 mm) of the CuI/PVP composite film onto polyimide (PI) substrate with a double-faced adhesive tape; then, Ag paste (SPI# 04998-AB) was painted onto two ends of the strip to connect with conducting wires.A finger touch test on a single-leg TE module was tested by a self-made experimental instrument (see Figure S8).As Figure S8 shows, in the single-leg TE module, heat transfers from the hot side to the cold side along the length direction of the module.We used an automatic temperature controlling system to heat a copper plate and one end of the module was put on the plate as the hot side (T + ∆T).The other end of the module was put in the air as the cold side (T).Then, we collected the output voltage at a particular temperature difference.
The electrical conductivity at room temperature was measured using a four-probe technical machine, with the help of a Hall effect measurement system (Ecopia HMS-3000).The Seebeck coefficient at room temperature was measured by the slope of the linear relationship between the thermal electromotive force and temperature difference (∆T) between two ends on one side of each film.The temperature-dependent TE properties were measured by a Cryoall CTA-3 instrument in He atmosphere, with an instrument test error of ±5% for both electrical conductivity and Seebeck coefficient.The flexibility of composite film was tested by measuring the electrical conductivity after bending the film along a rod with a radius of 4 mm at different times.
The crystallinity and phase composition of the CuI powder and films were examined by X-ray diffraction (XRD) (Bruker D8 Advance).All XRD measurements were recorded in the 2θ range of 10-90 • at a scanning speed of 5 • /min.The thickness of the film was around 10.87 µm, which was determined by field-emission scanning electron microscopy (FESEM, FEI Nova NanoSEM 450) observation of the cross-section of the film.The FESEM observation was also used to examine the surface morphology of CuI powder and CuI/PVP films.High-resolution transmission electron microscopy (HRTEM) (JEM-2100F) was used to observe the CuI powder.A transmission electron microscope (TEM, Titan Themis G2 60-300, Thermo Fisher Scientific, Shanghai, China) was used to investigate the internal details of the film.

Results
X-ray diffraction (XRD) patterns of the P1-film and P1-powder are in shown Figure 2a.As seen, all peaks are indexed to the γ-CuI with no impurity peaks [57].The CuI thin film exhibits strong peaks at (111), (220), and (311) planes.Note that the (220) and (311) peaks of the P1-film are stronger than those of P1-powder, showing that hot-pressing improves the crystallinity of the CuI.No XRD peaks of PVP are detected, which may be because of its small content and amorphous characteristic [49].Figure 2b and Figure S1a show scanning electron microscopy (SEM) images of P1 and P0-powder.From the SEM images, P0-powder consists of numerous nanoparticles and thin slices, and P1-powder is composed of many slices with an average size of ~1 µm.It was noticed that adding PVP changes the morphology of the CuI powder.Figure 2c shows a TEM image of the P1-powder.There is an amorphous layer with an average thickness of ~20 nm uniformly coated on the surface of the CuI powder.Considering that a small content of PVP is added in situ when synthesizing CuI and that no impurity peaks (Figure 2a) are detected, we think that the amorphous layer is PVP. Figure 2d and Figure S1c show typical surface SEM images of the P1-film and the CuI film reported in our previous work [56].By comparing Figure 2d with Figure S1c, the P1-film is denser.This is probably because the PVP-coated CuI powder can disperse more evenly in ethanol to avoid agglomeration, forming a more uniform film after vacuum filtration.Figure 2e is a cross-sectional SEM image of the P1-film, which also shows that the film is dense with a small amount of nanopores.
Figure 3 shows TEM and high-resolution TEM (HRTEM) images of the P1-film.The P1-film consists of nanograins with an average size of ~15 nm. Figure 3b is an enlarged image of the red box in (a), and Figure 3c is an enlarged image of the green box in (a). Figure 3b,c shows that CuI nanograins are connected by an amorphous PVP phase and there exists a small number of nanopores (~5-10 nm).The insets of Figure 3c show inverse fast Fourier transform (IFFT) images corresponding to grains A and C. Notably, the crystallinity of the CuI grains is good.Figure 3d displays an HRTEM image of another area, which shows several connected grains coated with an amorphous PVP layer.Figure 3e shows a semi-coherent GB.The insets of Figure 3e show the corresponding IFFT and fast Fourier transform (FFT) images, indicating that the lattice spacings of regions A and B are both 0.349 nm, corresponding to the (111) plane of CuI. Figure 3f displays a twin boundary, which can significantly reduce the thermal conductivity and has little effect on the electrical conductivity [58].Figure 3g shows an HRTEM image containing several grains.Figure 3h,i shows enlarged images of the orange and blue boxes marked in (g). Figure 3h shows a typical triangular GB, indicating the three grains (grains A, B, and C) are well-sintered together.Figure 3i  Figure 3 shows TEM and high-resolution TEM (HRTEM) images of the P1-film.The P1-film consists of nanograins with an average size of ~15 nm. Figure 3b is an enlarged image of the red box in (a), and Figure 3c is an enlarged image of the green box in (a). Figure 3b,c shows that CuI nanograins are connected by an amorphous PVP phase and there exists a small number of nanopores (~5-10 nm).The insets of Figure 3c show inverse fast Fourier transform (IFFT) images corresponding to grains A and C. Notably, the crystallinity of the CuI grains is good.Figure 3d displays an HRTEM image of another area, which shows several connected grains coated with an amorphous PVP layer.Figure 3e shows a semi-coherent GB.The insets of Figure 3e show the corresponding IFFT and fast Fourier transform (FFT) images, indicating that the lattice spacings of regions A and B are both 0.349 nm, corresponding to the (111) plane of CuI. Figure 3f displays a twin boundary, which can significantly reduce the thermal conductivity and has little effect on the electrical conductivity [58].Figure 3g shows an HRTEM image containing several grains.Figure 3h,i shows enlarged images of the orange and blue boxes marked in (g). Figure 3h shows a typical triangular GB, indicating the three grains (grains A, B, and C) are wellsintered together.Figure 3i   The TE performances of P0-film, P1-film, P2-film, and P3-film at room temperature are shown in Table 1.It can be seen from Table 1 that, with increasing PVP content, the Seebeck coefficient increases from 522 to 605 µVK −1 and then gradually decreases, which indicates that the films are p-type conductors.The variation in electrical conductivity is consistent with the variation of the Seebeck coefficient, which may be related to the content of PVP.Compared with the P0-film, the electrical conductivity and Seebeck coefficient of the P1-film are larger.There are two opposing effects of adding PVP.One is that PVP is an insulating polymer, which leads to the degradation of the electrical conductivity of the composite film.The other is that adding PVP makes the composite film more uniform and denser, which is beneficial for increasing the electrical conductivity.For the P1-film, the amount of PVP added is very small and the latter effect of increasing electrical conductivity is more significant, but for the P2-film and P3-film, the amount of PVP added is more, and hence the former effect of decreasing electrical conductivity is more significant.However, the Seebeck coefficient is not sensitive to the density.Dun et al. [59] studied Cu 0.1 Bi 2 Se 3 nanoplatelet/polyvinylidene fluoride (PVDF) composite films.They think that there could be an energy filtration effect at the Cu 0.1 Bi 2 Se 3 /PVDF interface, leading to an increase in the Seebeck coefficient.However, because PVDF is an insulator, the energy barrier between PVDF and Cu 0.1 Bi 2 Se 3 could be so high that no carriers can surmount it.In the present case, PVP is also an insulator; hence, the reason for the P1-film having an increased Seebeck coefficient value is not clear yet.Consequently, P1-film exhibits an enhanced PF of ~8.05 µWm −1 K −2 at room temperature.Therefore, PVP should not be added too much in the synthesis process.In this work, the CuI/PVP mass ratio of ~0.0095:1 was the best, so P1-film was chosen for further study.The TE performances of P0-film, P1-film, P2-film, and P3-film at room tempera are shown in Table 1.It can be seen from Table 1 that, with increasing PVP content, Seebeck coefficient increases from 522 to 605 µVK −1 and then gradually decreases, wh indicates that the films are p-type conductors.The variation in electrical conductivit consistent with the variation of the Seebeck coefficient, which may be related to the c tent of PVP.Compared with the P0-film, the electrical conductivity and Seebeck co cient of the P1-film are larger.There are two opposing effects of adding PVP.One is PVP is an insulating polymer, which leads to the degradation of the electrical conducti of the composite film.The other is that adding PVP makes the composite film more form and denser, which is beneficial for increasing the electrical conductivity.For the film, the amount of PVP added is very small and the latter effect of increasing electr conductivity is more significant, but for the P2-film and P3-film, the amount of PVP ad is more, and hence the former effect of decreasing electrical conductivity is more sig cant.However, the Seebeck coefficient is not sensitive to the density.Dun et al. [59] s ied Cu0.1Bi2Se3 nanoplatelet/polyvinylidene fluoride (PVDF) composite films.They th that there could be an energy filtration effect at the Cu0.1Bi2Se3 /PVDF interface, leadin an increase in the Seebeck coefficient.However, because PVDF is an insulator, the ene barrier between PVDF and Cu0.1Bi2Se3 could be so high that no carriers can surmoun In the present case, PVP is also an insulator; hence, the reason for the P1-film having  Figure 4a shows the TE performance of the P1-film from 20 • C to 105 • C.Over the entire temperature range, the Seebeck coefficient value remains about ~600 µVK −1 as the temperature increases.The electrical conductivity is ~0.22 Scm −1 at room temperature, and it tends to increase slightly before 90 • C and then drops gradually.Finally, the PF of the P1-film increases gradually with the temperature.At room temperature, P1-film exhibits a PF of ~8.05 µWm −1 K −2 .The PF of the P1-film is relatively low; however, it is higher than that of a CuI/amorphous carbon composite pellet, which exhibits a PF of ~7.5 µWm −1 K −2 (corresponding electrical conductivity of ~0.5 Scm −1 and Seebeck coefficients of ~390 µVK −1 ); the PF of the composite pellet markedly decreases to a range of 0.119 to 0.637 µVK −1 after a two-to twelve-cycle measurement from room temperature (RT) to 200 • C [60], which is somewhat lower than that of CuI/bacterial cellulose nanofiber (BCNF) composite film with a PF of ~24.6 µWm −1 K −2 at RT [61] and Cu 1.75 Te/polyvinylidene fluoride (PVDF) composite film with a PF of 23 µWm −1 K −2 at RT [62].Nevertheless, the component Te of the latter is toxic and expensive.
cients of ~390 µVK −1 ); the PF of the composite pellet markedly decreases to a range of 0.119 to 0.637 µVK −1 after a two-to twelve-cycle measurement from room temperature (RT) to 200 °C [60], which is somewhat lower than that of CuI/bacterial cellulose nanofiber (BCNF) composite film with a PF of ~24.6 µWm −1 K −2 at RT [61] and Cu1.75Te/polyvinylidene fluoride (PVDF) composite film with a PF of 23 µWm −1 K −2 at RT [62].Nevertheless, the component Te of the latter is toxic and expensive.
To study the thermal stability of P1-film, we tested temperature-dependent TE properties (the electrical conductivity and Seebeck coefficient) of the same film twice in succession, and the results are shown in Figures 4b and S4.Note that both parameters are almost the same during the two tests, suggesting that the prepared films have much better thermal stability below 100 °C. Figure 5a is a photo of the P1-film flexibility test.Figure 5b shows the flexibility test result.It indicates that, after being bent around a rod with a radius of 4 mm 1000 times, the electrical conductivity of P1-film decreases by only 4%, which is better than most of the reported flexible TE materials.
Compared with the CuI film reported in our previous work [56], the P1-film has better flexibility, higher electrical conductivity, and higher thermal stability, mainly because CuI nanograins in the film are sintered together and the adhesive effect of PVP.
Figure 5c,d shows the results of one-cycle and five-cycle touch tests on a single-leg TE module, respectively.In Figure 5c, note that when this module touched the heating plate, it immediately generated a voltage of ~1.5 mV in 0.5 s, reached a maximum voltage To study the thermal stability of P1-film, we tested temperature-dependent TE properties (the electrical conductivity and Seebeck coefficient) of the same film twice in succession, and the results are shown in Figures 4b and S4.Note that both parameters are almost the same during the two tests, suggesting that the prepared films have much better thermal stability below 100 • C.
Figure 5a is a photo of the P1-film flexibility test.Figure 5b shows the flexibility test result.It indicates that, after being bent around a rod with a radius of 4 mm 1000 times, the electrical conductivity of P1-film decreases by only 4%, which is better than most of the reported flexible TE materials.Before moving the module from the heating plate, there was a slight increase in voltage, which may have been because the contact between the heating plate and the module enhanced somewhat before the module was moved away.It is indicated that the temperature difference (∆T) between the heating plate and room temperature (29 °C) is 6 K.The Seebeck coefficient can be evaluated at about ~600 µVK −1 using the formula S = V/∆T, which indicates the voltage touch test results are reliable.Figure 5d shows the result for multiple cycles.The peak voltage was somewhat different for each touch, which should be because the contact between the heating plate and the module was slightly different each time.The touch test shows that flexible PVP/CuI composite films have great potential prospects in wearable sensors.

Conclusions
In conclusion, we have prepared flexible CuI/PVP nanocomposite films by a simple method, and this method is environmentally friendly and highly efficient.In addition, Compared with the CuI film reported in our previous work [56], the P1-film has better flexibility, higher electrical conductivity, and higher thermal stability, mainly because CuI nanograins in the film are sintered together and the adhesive effect of PVP.
Figure 5c,d shows the results of one-cycle and five-cycle touch tests on a single-leg TE module, respectively.In Figure 5c, note that when this module touched the heating plate, it immediately generated a voltage of ~1.5 mV in 0.5 s, reached a maximum voltage of about ~3.6 mV in 10 s and then remained the same voltage value.When moving this module away from the heating plate, the voltage quickly decreased to ~0 mV in ~60 s.Before moving the module from the heating plate, there was a slight increase in voltage, which may have been because the contact between the heating plate and the module enhanced somewhat before the module was moved away.It is indicated that the temperature difference (∆T) between the heating plate and room temperature (29 • C) is 6 K.The Seebeck coefficient can be evaluated at about ~600 µVK −1 using the formula S = V/∆T, which indicates the voltage touch test results are reliable.Figure 5d shows the result for multiple cycles.The peak voltage was somewhat different for each touch, which should be because the contact between the heating plate and the module was slightly different each time.The touch test shows that flexible PVP/CuI composite films have great potential prospects in wearable sensors.

Conclusions
In conclusion, we have prepared flexible CuI/PVP nanocomposite films by a simple method, and this method is environmentally friendly and highly efficient.In addition, PVP added in situ enhances the density and hence improves the flexibility, electrical conductivity and stability of the CuI film.TEM observation indicates that the film is composed of CuI nanograins (average size of ~15 nm) and PVP distributed at the surface of the CuI nanograins and the inner wall of nanopores.The optimized film shows a large Seebeck coefficient ~605 µVK −1 and PF ~8.05 µWm −1 K −2 at room temperature, and the film exhibits excellent flexibility (~96% of initial electrical conductivity retention after being bent 1000 times along a 4 mm radius rod).A single-leg TE module outputs a voltage of ~3.6 mV when ∆T is 6 K.This work provides a feasible and simple method for preparing dense and low-cost flexible TE nanocomposite films.

Figure 1 .
Figure 1.Schematic diagram of the preparation process of CuI/PVP composite film.

Figure 1 .
Figure 1.Schematic diagram of the preparation process of CuI/PVP composite film.
Figure3shows TEM and high-resolution TEM (HRTEM) images of the P1-film.The P1-film consists of nanograins with an average size of ~15 nm.Figure3bis an enlarged image of the red box in (a), and Figure3cis an enlarged image of the green box in (a).Figure3b,c shows that CuI nanograins are connected by an amorphous PVP phase and there exists a small number of nanopores (~5-10 nm).The insets of Figure3cshow inverse fast Fourier transform (IFFT) images corresponding to grains A and C. Notably, the crystallinity of the CuI grains is good.Figure3ddisplays an HRTEM image of another area, which shows several connected grains coated with an amorphous PVP layer.Figure3eshows a semi-coherent GB.The insets of Figure3eshow the corresponding IFFT and fast Fourier transform (FFT) images, indicating that the lattice spacings of regions A and B are both 0.349 nm, corresponding to the (111) plane of CuI.Figure3fdisplays a twin boundary, which can significantly reduce the thermal conductivity and has little effect on the electrical conductivity[58].Figure3gshows an HRTEM image containing several grains.Figure3h,i shows enlarged images of the orange and blue boxes marked in (g).Figure3hshows a typical triangular GB, indicating the three grains (grains A, B, and C) are wellsintered together.Figure3idisplays grains A, B, and another grain (grain D).It is deduced from the different contrasts of the grains (FigureS6) that grain D lies beneath grains A and B. Namely, in the P1-film, most CuI grains are sintered, and a small number of CuI grains are bonded by PVP.

Figure 3 .
Figure 3. Microstructure characterization of the P1-film.(a) Overview TEM image of the P1-(b,c) enlarged images of the red and green box in (a), insets in (c) are the corresponding respec IFFT images; (d) A HRTEM image containing several connected grains; (e) enlarged image o pink box in (d), insets are the corresponding IFFT and FFT images; (f) enlarged image of the pu box in (d); (g) an HRTEM image containing several grains; (h,i) enlarged images of the orange blue boxes marked in (g), respectively.

Figure 3 .
Figure 3. Microstructure characterization of the P1-film.(a) Overview TEM image of the P1-film; (b,c) enlarged images of the red and green box in (a), insets in (c) are the corresponding respective IFFT images; (d) A HRTEM image containing several connected grains; (e) enlarged image of the pink box in (d), insets are the corresponding IFFT and FFT images; (f) enlarged image of the purple box in (d); (g) an HRTEM image containing several grains; (h,i) enlarged images of the orange and blue boxes marked in (g), respectively.The uppercased letters in the figure such as A, B, C denote different grains.

Figure 4 .
Figure 4. (a) Temperature−dependent Seebeck coefficient, electrical conductivity, and power factor of the P1−film; (b) temperature−dependent Seebeck coefficient of the P1−film for two heating cycles.

Figure 4 .
Figure 4. (a) Temperature−dependent Seebeck coefficient, electrical conductivity, and power factor of the P1−film; (b) temperature−dependent Seebeck coefficient of the P1−film for two heating cycles.

J
. Compos.Sci.2023, 7, x FOR PEER REVIEW 9 of 12 of about ~3.6 mV in 10 s and then remained the same voltage value.When moving this module away from the heating plate, the voltage quickly decreased to ~0 mV in ~60 s.

Figure 5 .
Figure 5. (a) Digital photo of P1−film flexibility test; (b) flexibility test result of the P1−film; the touch test for one−cycle (c), and five−cycle (d).

Figure 5 .
Figure 5. (a) Digital photo of P1−film flexibility test; (b) flexibility test result of the P1−film; the touch test for one−cycle (c), and five−cycle (d).

Table 1 .
TE performances of the P0-P3 films at room temperature.