Improved thermoelectric performance in n-type flexible Bi 2 Se 3+x /PVDF composite films Improved thermoelectric performance in n-type Bi 2 Se 3+x films.

Bismuth selenide materials (Bi 2 Se 3 ) have high performance around room temperature, demonstrating potential in thermoelectric applications. Presently, most vacuum preparation techniques used to fabricate the film materials, such as magnetron sputtering and molecular beam epitaxy, usually require complex and expensive equipment. This limits the practical applications of flexible thermoelectric films. Here, we prepared Bi 2 Se 3+x nanoplate/ polyvinylidene fluoride composite films with good flexibility using a facile chemical reaction method. Their thermoelectric performance and microstructures were systematically studied. The composite films exhibit a highly preferred orientation along (015). The carrier concentration and mobility were optimized by adding excessive element Se, eventually leading to an improvement in thermoelectric performance. The optimized power factor is 5.2 μ W/K 2 m at 300 K. Furthermore, the performance remains stable after 2500 bending cycles at a radius of 1 cm, suggesting promising applications in wearable/portable electronics.


INTRODUCTION
Recently, wearable/portable electronic devices are ubiquitous, bringing great convenience to our lives [1][2][3] . However, they are usually dependent on the power from traditional chemical batteries with a finite lifetime and requiring periodic recharging, thus limiting their further popularity [4][5][6] . Thermoelectric materials, as one of the most competitive energy materials, can generate electricity from heat and realize the direct conversion between heat and electricity, showing a good potential in flexible electronics [7][8][9][10] . Generally, the energy conversion efficiency of thermoelectric modules depends on the dimensionless thermoelectric figure of merit: zT, zT = S 2 σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, T is the absolute temperature, and S 2 σ is the power factor [11][12][13] . Bi 2 Se 3 is a type of V 2 VI 3 semiconductor material with a layered crystal structure, which exhibits excellent thermoelectric performance around room temperature [14] . With the rapid development of wearable/ portable electronics, thermoelectric films considering Bi 2 Te 3 -based compounds have also been a hot area of research in recent years [15,16] . Currently, there are many methods to fabricate the films, including top-down and bottom-up processes [17] . Top-down processes use mechanical peeling or liquid phase peeling from the block to the sheet structure [18] . Ambrosi et al. [19] proposed a simple and rapid electrochemical approach to exfoliate natural Bi 2 Se 3 and Bi 2 Te 3 crystals in aqueous media to obtain single/few-layer nanoplates. However, this has a limitation on the dimension of the exfoliation materials, which also lacks good stability. Thus, it is challenging to obtain large-area films [20,21] . On the other hand, classic bottom-up methods include chemical vapor deposition [22] , magnetron sputtering [23,24] , molecular beam epitaxy [25] , etc. These methods usually require high vacuum or complex equipment, largely limiting the practical applications. For example, thin films of Bi 2 Te 2.7 Se 0.3 were deposited on a SiO 2 -coated silicon (SiO 2 /Si) substrate using a pulsed laser deposition system [26] , where the base pressure of 3.8 × 10 -6 Torr and preheating temperature in the range of 20-800 °C are required. Fortunately, the chemical solution method can avoid this limitation as well as make it easier to prepare large-area films [27] . Masood et al. [28] used hydrazine hydrate to realize a redox reaction and synthesized Bi 2 Te 3 nano-sticks.
In this study, we adopted a facile chemical solution method to prepare flexible Bi 2 Se 3+x /PVDF composite films. The reaction equipment is easy to assemble. It is emphasized that this chemical solution method is different from the previous reports [29,30] , where the reaction system ensures an inert atmosphere by circulating high-purity nitrogen. Ethylene glycol (EG) is used as a reducing agent, which is less toxic and safer than hydrazine hydrate. The whole process happens under the protection of high-purity nitrogen (N 2 ). PVDF is used to buffer the deformation and protect the matrix materials from destruction; it also regulates the thermoelectric performance by introducing organic-inorganic interfaces. Besides, the thickness and area of the flexible Bi 2 Se 3+x /PVDF composite films can be effectively adjusted by controlling the solution concentration and the substrate size. The composite films show a good flexibility and an improved thermoelectric performance, achieving a power factor of 5.2 μW/K 2 m at 300 K.

EXPERIMENTAL
Bi 2 Se 3+x /PVDF thermoelectric films were prepared by the chemical solution method. The whole reaction process is shown in Figure 1A. Firstly, according to the chemical composition of the Bi 2 Se 3+x /PVDF thermoelectric films (x = 0, 0.2, 0.3, 0.4), analytically pure sodium selenite (Na 2 SeO 3 , 99%, SIGMA-ALDRICH), bismuth nitrate [Bi (NO 3 ) 3 , 98.0%, SIGMA], and ethylene glycol (EG, 99+%, Alfa Aesar) were mixed in a three-necked flask. High-purity nitrogen (N 2 ) was circulated in the reaction device for 20 min at a high speed to remove the air, ensuring an inert atmosphere. Then, the raw materials were reacted at 200 °C for 5 h to obtain Bi 2 Se 3+x powder. After the solution cooled to room temperature, the powder was precipitated using ethanol. The obtained pure Bi 2 Se 3+x powder and polyvinylidene fluoride powder (PVDF, ALDRICH) (2:1 ratio) were dissolved in dimethylformamide (DMF), obtaining a homogeneous suspension by ultrasound for 3 h. Finally, dripping the suspension on glass substrates and baking at 80 °C for 10-12 h achieved the composite films.
The excessive Se in powder is indeterminate and active [32][33][34] , which is easily oxidized to form selenium oxide during the fabrication of the films, as shown in Equation (2.4).
The phase compositions of the samples were characterized by X-ray diffraction (XRD, Bruker D8 four-circle diffractometer) using Cu-Kα radiation. The morphology and element content of the samples were analyzed by scanning electron microscope (SEM, ZEISS SIGMA, FEI-Siron) and inductively coupled plasma mass spectrometry (ICP-MS, Agilent 725ES and Agilent 5110), respectively. The spectra of the elements were obtained by X-ray photoelectron spectroscopy analysis (XPS, Thermo ESCALAB 250XI). The transmission electron microscopy (TEM, FEI/Tecnai G2 F20S-TWIN TMP TEM) technique was adopted to further analyze the microstructures of films.
The electrical conductivity and Seebeck coefficient were measured simultaneously in a helium atmosphere by an MRS-3 measurement system (thermoeletric measurement system, Joule Yacht). The roomtemperature Hall carrier concentration and mobility were obtained by a Hall measurement (HMS-7000).

RESULTS AND DISCUSSION
Characterization of Bi 2 Se 3+x /PVDF composite films XRD measurement of Bi 2 Se 3+x /PVDF thermoelectric composite films (x = 0, 0.2, 0.3, 0.4) was performed, as shown in Figure 2A and B. The diffraction peaks are consistent with the standard Bi 2 Se 3 card (PDF #33-0214), indicating that the main crystal phase is Bi 2 Se 3 . For these composite films, the three main diffraction peaks are (015), (006), and (1,0,10), where (015) possesses the highest intensity among all these peaks, indicating that most Bi 2 Se 3 grains grow along the (015) direction. It should be noted that the phase structures are not changed due to the increasing Se content in the films. Additionally, some diffraction peaks of the SeO 3 phase can also be observed at ~26.7°, although they are not found in the powder  Figure 2C shows the XPS spectrum of Bi 2 Se 3.2 powder, which is in good agreement with the standard binding energy cards. The two prominent peaks at 54.63 and 55.51 eV correspond to Se3d 5/2 and Se3d 3/2 , respectively, which indicates the existence of Se 2in the films. Besides, there is a peak with low intensity at 56.95 eV [ Figure 2C, Supplementary Table 1], which can be attributed to the elemental Se. Combining with the results of XRD and XPS, we can conclude that there is some Se in the synthesized Bi 2 Se 3+x powder, which is easy to be oxidized in the fabrication process of Bi 2 Se 3+x /PVDF composite films due to the high reactivity, eventually resulting in the formation of SeO 3 .
We also analyzed the element content in the Bi 2 Se 3+x (x = 0, 0.2, 0.3, 0.4) samples by ICP-MS, as shown in Figure 2D. As x increases, the measured Se/Bi values are slightly higher than the theoretical values, although they show a consistent changing trend. The reasons can be attributed to the following two aspects: (1) according to the results of XRD and XPS, there is some Se in the Bi 2 Se 3+x powder, which did not react with Bi ions; and (2) the unreacted Bi ions are removed in the process of precipitation Bi 2 Se 3+x powder, resulting in the decreasing Bi content in the materials [Supplementary Figure 2]. Figure 3 show the surface morphology of composite films. All the composite films are formed by hexagonal sheets, which are connected by PVDF. By adjusting the Bi 2 Se 3+x /PVDF:DMF ratio and the solution volume dripped on the glass substrates, composite films with different thicknesses can be prepared. Figure 3B shows the thickness of the Bi 2 Se 3.2 /PVDF composite films, around 28.83 μm. The TEM technique was adopted to further analyze the microstructures, as shown in Figure 3C-E. The Bi 2 Se 3 hexagonal sheet was obviously observed [ Figure 3C], consistent with the results of SEM. Figure 3D  is the HRTEM image of the region in Figure 3C, where the measured lattice spacing values are 0.304 nm, corresponding to (015) and crystal planes of Bi 2 Se 3 , respectively. It should be noted that it is challenging to distinguish specific crystal plane [i.e., (015) or ] due to the same lattice spacing. The selected area electron diffraction (SAED) further verifies the crystal structure of Bi 2 Se 3 [ Figure 3E]. However, the Se phase can also be observed, which has a different crystal structure and is separated from Bi 2 Se 3 . The lattice spacing values of the Se crystals are 0.294 and 0.329 nm, which correspond to the and (141) crystal planes, respectively. In this case, the Bi 2 Se 3 /Se heterostructure is formed. Furthermore, there are many heterostructure interfaces between Bi 2 Se 3 nanosheets and PVDF, all of which can adjust the carrier transportation, leading to the improved thermoelectric performance of the films. Additionally, the amorphous material shown in Figure 3D is primarily caused by the bombardment of the high-speed moving electron beam in the TEM measurement.

Thermoelectric performance of Bi 2 Se 3+x /PVDF composite films
For the Bi 2 Se 3+x /PVDF composite films, the temperature-dependent thermoelectric performance is demonstrated in Figure 4. Figure 4A shows the negative values of the Seebeck coefficient, indicating n-type conduction behavior. With the increasing temperature up to 300 K, the Seebeck coefficient increases slightly. Then, as the temperature further increases, the value of the Seebeck coefficient decreases. Furthermore, the Seebeck coefficient firstly increases and then decreases with the increasing Se concentration. Theoretically, the Seebeck coefficient depends on the carrier concentration and the scattering mechanism [28,31,35,36] .
where e is the carrier charge, n is the carrier concentration, μ is the carrier mobility, κ B is the Boltzmann constant, h is Planck's constant, and m* is the effective mass of the charge carrier. To further analyze the changing thermoelectric parameters, Hall measurement was performed, as shown in Figure 4D and Table 1.  The electrical conductivities dependent on temperature are shown in Figure 4B. With the increasing temperature, the electrical conductivities of Bi 2 Se 3+x /PVDF composite films increase, indicating the semiconductor conduction behavior. Furthermore, as the Se content increases, the electrical conductivities first increase and then decrease, which reaches the highest values when x = 0.2, about 216.9 S/m at 300 K. It is well known that the electrical conductivity is determined by the carrier concentration and mobility, as expressed by the following equation: Obviously, compared with the composite films without adding extra Se, the carrier concentration and mobility of the Bi 2 Se 3.2 /PVDF composite films are enhanced [ Figure 4D,   The power factor of Bi 2 Se 3+x /PVDF composite films was calculated based on the above-discussed Seebeck coefficient and electrical conductivity, as shown in Figure 4C. The power factor first increases, then reaches the peak value, and finally decreases with the increasing Se content. Doping with a small amount of Se element (x ≤ 0.2) increases the electrical conductivity and Seebeck coefficient. When x = 0.2, the composite films possess the highest power factor among all films, up to 5.2 μW/K 2 m at 300 K, increasing by ~3 times compared with the original sample (x = 0). After further doping with Se element (x > 0.2), it reduces. This suggests that introducing a small amount of Se into matrix materials is an effective strategy to improve the thermoelectric performance of Bi 2 Se 3+x /PVDF composite films. It should be noted that the power factor is lower than that of some reports [29] , which mainly results from the poor vacuum of the reacting device.
Mechanical stability of Bi 2 Se 3+x /PVDF composite films Bi 2 Se 3.2 /PVDF composite thermoelectric films with a dimension of 2.6 cm 2 × 7.6 cm 2 were chosen to evaluate the material flexibility. Figure 5 shows the film resistance dependent on the bending cycles at a bending radius of 1 cm. R 0 and R represent the initial values and measured values of the resistance, respectively. It can be seen that, after 2500 bending cycles, the R/R 0 value of the sample remains almost constant, indicating the good flexibility. This mainly results from the organic PVDF, which can buffer the deformation and protect the matrix materials from destruction, suggesting that Bi 2 Se 3+x /PVDF composite thermoelectric films have potential for dynamic applications.

CONCLUSION
Flexible Bi 2 Se 3+x /PVDF composite films were successfully fabricated by the facile chemical solution method, where the hexagonal Bi 2 Se 3 nanosheets were connected by PVDF. By adjusting the content of Se in the raw material, the carrier concentration and mobility were effectively adjusted, leading to improved thermoelectric properties. Compared with the sample without Se element doping, the optimized power factor of Bi 2 Se 3.2 /PVDF composite film is 5.2 μW/K 2 m at 300 K, which is increased by nearly three times. Furthermore, there is no degradation in performance for the composite films after 2500 bending cycles at a radius of 1 cm, showing the good flexibility and mechanical stability.