Development of microelectronic integrated technology resulted in increase in the heat flux density, which seriously affects the performance and service life of the electronic devices^[1,2]. Therefore, how to effectively take away the harmful waste heat poses great challenges. Among various heat dissipation techniques^[3], thermoelectric (TE) cooling based on Peltier effect of TE materials has attracted increasing attention due to a series of unique advantages such as no noise, no pollution, no moving parts, rapid cooling, simple operation, high reliability, and lightweight, which is expected to solve the problem of heat dissipation of electronic devices with high heat flux^[4,5,6]. However, most of the commercial TE cooling devices adopt a vertical design, in which the heat flows along the longitudinal direction^[7,8,9]. These TE cooling devices are composed of bulk P and N type thermoelectric legs connected electrically in series through metallic electrodes and then sandwiched between two electrically insulated and thermally conductive ceramic plates. Thus this structure has poor flexibility and is very difficult to realize dimensionally match between TE devices and electronic components^[10]. However, the cooling performance deteriorated seriously when the device is downscaled to micro level. While for another type of TE cooling devices using planar design, in which the heat flow is parallel to the substrate, their relatively long TE legs can be fabricated from TE films which are beneficial to flexibility, miniaturization and relatively large temperature difference.

Bi₂Te₃-based alloys (Bi_xSb_2-xTe₃ and Bi₂Te_3-xSe_x) are customarily regarded as the representative of V₂VI₃ TE materials, and have been commercialized and widely used for refrigeration and energy conversion applications in the low-temperature range^[11,12]. Although great progress was made in developing bulk Bi₂Te₃ materials with high performance, performance of Bi₂Te₃-based films is still unsatisfactory. As a simple and efficient process to prepare TE films^{[13,14,15,16]}, the bottleneck of printing lies in the deterioration of electrical transport properties which results from low density and various structural defects such as holes and cracks^[17]. Several strategies were employed to enhance the electrical transport properties of the printed TE films, such as cold isostatic pressing^[18,19], addition of sintering aids^[20], and high-temperature sintering to remove the insulating binders^[21]. However, realization of high-performance flexible TE cooling devices is still frustrated by the poor electrical transport properties.

In previous work^[22], Bi_0.5Sb_1.5Te₃/epoxy flexible thick films with (000l) preferential orientation were prepared on polyimide substrates by combination of brush-printing and hot-pressing curing processes, which greatly improved the electrical transport properties of the composite thick films. However, brush-printing process is difficult to realize mass production due to its poor controllability. In this work, a series of Bi_0.5Sb_1.5Te₃/epoxy composite flexible thermoelectric thick films were successfully prepared on polyimide substrates by screen printing, and the electrical transport properties of the thick films were further improved by optimizing the content of Bi_0.5Sb_1.5Te₃ powder.

The preparation of thermoelectric slurries refers to the method reported in former work^[22]. Thermoelectric slurry was prepared by using diglycidyl ether of bisphenol-F epoxy resin as adhesives, methylhexahydrophthalic anhydride as hardener, 2-ethyl-4-methylimidazole (2E4MI) as anhydride accelerator and Bi_0.5Sb_1.5Te₃ as thermoelectric component. Firstly, Bi_0.5Sb_1.5Te₃ ingots were crushed and pulverized with a planetary ball mill at a speed of 200 r/min for 24 h under the protection of argon, then dried in vacuum oven at 60 ℃ for 2 h to get thermoelectric powder. Secondly, epoxy resin, hardener, catalyst and diluent were mixed to form the epoxy system mixture.

The obtained thermoelectric slurries were printed on the pre-cleaned polyimide (PI) substrates by screen printing to form thick films. The films were dried in vacuum at 100 ℃ for 60 min, and then cured at 300 ℃ for 4 h in hot-pressing apparatus. To further reveal the impact of the content of Bi_0.5Sb_1.5Te₃ on the electrical properties of the composite films, the slurries were prepared with different content of Bi_0.5Sb_1.5Te₃ powder (x=6.5, x=7, x=8, x=9 and x=10) where x represents the mass ratio of Bi_0.5Sb_1.5Te₃ to epoxy system (epoxy resin, hardener and catalyst).

The phase constituents of all thick films were determined by X-ray diffraction (XRD, PANalytical χ’ Pert PRO) using Cu K_α radiation (λ=0.15418 nm). Microstructures were examined by a field emission scanning electron microscope (FESEM, Zeiss ULTRA-PLUS-43-13). The density of the thick films was measured with the Archimedes method. The Hall coefficient (R_H), carrier concentration (n), mobility (μ_H) and electrical conductivity (σ) of the thick films were tested by the HL5500 Hall effect test system at room temperature. The in-plane electrical conductivity (σ) and Seebeck coefficient (α) were measured with the standard four-probe method (Sinkuriko, ZEM-3) in He atmosphere. The measurement error for σ and α is ±5%. The bending tests of the thick films were performed using a homemade bending test apparatus. The temperature distribution of the TE leg was measured under different applied currents (I) by visual infrared thermometer (FLUKE, TI 400).

XRD patterns of these composite films are displayed in Fig. 1. All diffraction peaks can be indexed to the standard diffraction data of Bi_0.5Sb_1.5Te₃ (JCPDS 49- 1713), indicating that these films are composed of single- phase Bi_0.5Sb_1.5Te₃.

Fig. 2 shows the cross-sectional FESEM images of these composite films. The thicknesses of these films are approximate 20 μm. It is apparent that there are organic residues and pores in all the films. As the content of Bi_0.5Sb_1.5Te₃ powder increased from x=6.5 to x=8, the organic residues were reduced. However, with x further increasing, the pores were obviously increased. Thus the film prepared with x=8 has the most compact microstructure, which could be also identified from the variation of film densities (Table 1).

The room-temperature electrical transport properties of these composite films are listed in Table 1. The positive Hall coefficient R_H indicated that most of carriers were holes. The p-type conduction character was consistent with the corresponding bulk materials^[23]. Variation of carrier concentration n with x can be explained by the difference of film density induced by evolution of microstructure. It is noteworthy that the Hall mobility μ_H of thick films increased by 30% from 45.15 cm²·V^-1·s^-1 for x=6.5 to 58.85 cm²·V^-1·s^-1 for x=8. The enhancement of μ_H is attributed to the improved electrical contact between Bi_0.5Sb_1.5Te₃ powders. However, with x further increasing, the Hall mobility decreased. This is due to the loose microstructure caused by relatively low content of epoxy resin. The variation trend of electrical conductivity σ is consistent with μ_H.

Fig. 3 shows the temperature dependence of electrical conductivity σ, Seebeck coefficient α and power factor α²σ of these composite films. σ decreased as the test temperature increases, similar with the metallic transport behavior of the bulk Bi_0.5Sb_1.5Te₃ materials^[24]. It is worth noting that σ increased significantly with the content of Bi_0.5Sb_1.5Te₃ powder, x increasing from 6.5 to 8 and then decreased as the x was further increased. The improved σ can be explained by the reduced organics and improved grain contact as shown in Fig. 2, which led to greater mobility. When x further increases, the amount of epoxy resin is not enough to effectively adhere the grains, which leads to loose microstructure. Obviously, the deteriorated microstructure must cause decreased μ_H and σ. As shown in Fig. 3(b), α almost remains constant with x increasing. Thus, the power factor α²σ increased significantly with the content of Bi_0.5Sb_1.5Te₃ powder, x increasing from 6.5 to 8 and then decreased as the x was further increased. When x=8, the optimized power factor of thick films reached 1.12 mW·m^-1·K^-2. Table 2 summarizes the electrical conductivity σ and power factor α²σ of Bi₂Te₃-based composite films in this work and literature^{[10,14,17,20,22,25]}. As shown in Table 2, the optimized electrical conductivity and power factor of the as-prepared composite films are much higher than all of the reported composite films, showing increments of 33% and 331% as compared with the power factor of our previous work^[22] and Bi₂Te₃+1%Se/epoxy composite films^[20], respectively.

Fig. 4 shows the variation rates of resistance (∆R/R₀) for the TE leg prepared by the optimal content of Bi_0.5Sb_1.5Te₃ powder before and after anti-bending tests with different bending radius and bending cycles. As shown in Fig. 4(a), the resistance variation rates are almost 0 when the bending radius gradually decreases from 90 mm to 20 mm, and rapidly increases when the bending radius is less than 10 mm. As shown in Fig. 4(b), the resistance variation rates are less than 5% within 3000 bending cycles when the bending radius is 20 mm. The anti-bending test results indicate the as-prepared films have potential application in flexible TE devices.

Fig. 5 shows the temperature distribution of the TE leg prepared by the optimal content of Bi_0.5Sb_1.5Te₃ powder. As shown in Fig. 5, the TE leg could establish a hot end and a cold end when applied different currents. The temperature difference (DT) between the hot end (T_h) and cold end (T_c) increases from 4.2 ℃ to 7.8 ℃ with the current increasing from 0.01 A to 0.05 A, showing potential application in planar cooling field. However, both the temperatures of hot end and cold end gradually increases with the current increasing due to Joule heat effect induced by the internal resistance of the TE leg. According to the thermodynamic analysis of TE cooling device^[26,27], there are several different effects for the thermal energy in the device, such as thermal energy from Peltier effect, thermal conduction energy from Fourier effect, and Joule thermal energy from the internal resistance of the device.When the thermal energy from Peltier effect is greater than those from Fourier effect and Joule heat effect, T_c decreases. Thus, the research in next step will focus on decrease of the internal resistance of TE legs and the device structure design.

A series of flexible Bi_0.5Sb_1.5Te₃/epoxy composite thermoelectric thick films were successfully prepared on polyimide substrates by screen printing. It is discovered that the content of Bi_0.5Sb_1.5Te₃ powder plays a vital role in improving the microstructure and electrical transport properties. The results indicate that as the content of Bi_0.5Sb_1.5Te₃ powder increases, the electrical conductivity firstly increases and then decreases with the Seebeck coefficient remaining almost constant, and the power factor follows the same trend as the electrical conductivity. The optimized power factor reaches 1.12 mW·K^-2·m^-1, increased by 33% as compared with our previous work. The anti-bending test indicates that the resistance of the thick films remains unchanged when the bending radius is over 20 mm and slightly increases within 3000 bending cycles when the bending radius is 20 mm, implying that the as-prepared films have potential application in flexible thermoelectric devices. The cooling performance of a single TE leg was evaluated by applying different current. A temperature difference from 4.2 to 7.8 ℃ was observed under working current from 0.01 A to 0.05 A, showing potential application in planar cooling field.