Proton Irradiation Effects on the Pyroelectric Properties of P‐Type Bismuth Antimonide/Poly(vinylidene fluoride–trifluoroethylene) Composite Films

Organic pyroelectric materials are widely applied as temperature sensors in wearable electronic devices due to their good biocompatibility and stability. Real‐time monitoring of the physiological state of the human body requires pyroelectric materials with a fast response time and large output voltage. In this study, the pyroelectric characteristics of poly(vinylidene fluoride–trifluoroethylene) (P(VDF–TrFE)) films are improved with the use of commercial inorganic P‐type bismuth antimonide (P‐Bi2Te3) fillers. Composite films with 0.2 wt% P‐Bi2Te3 increase the pyroelectric response time and voltage by improving the thermal diffusivity and enhancing the β‐phase content, respectively. Proton irradiation results in further improvement of the pyroelectric response time from 22 to 0.5 s. The proton irradiation‐induced ionization energy loss improves the conductivity of the composite films, thereby enhancing the pyroelectric response time. These results show that P‐Bi2Te3 doping is beneficial for improving the pyroelectric properties of P(VDF–TrFE) and that proton irradiation is an effective method for further improving the response time of inorganic–organic composite films.


Introduction
The development of flexible pyroelectric materials, which can harvest heat from the human body or from the environment while applied on nonplanar surfaces, has been greatly accelerated by applications such as wearable technology and soft robotics. [1][2][3][4] Recently, temperature sensing has become an important requirement for monitoring body temperature during the coronavirus disease outbreak, requiring the development of faster temperatureresponsive devices to improve detection efficiency. Due to their potential usage in biomedical and industrial applications, polymers such as polyvinylidene fluoride (PVDF) and its copolymer polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) have garnered the most attention among the numerous flexible pyroelectric materials. [5,6] P(VDF-TrFE)'s mechanical attributes, lower cost, lighter weight, and greater stretchability allow it to serve as a viable alternative to pyroelectric ceramics. [5,[7][8][9] However, P(VDF-TrFE) has a slower response time and a lower pyroelectric coefficient than some rigid pyroelectric materials, significantly restricting its use. [10,11] Efforts to develop highperformance pyroelectric devices with high sensitivity and mechanical flexibility for use in flexible sensors are ongoing. There are five phases in P(VDF-TrFE), named α-, β-, γ-, δ-, and εphases, according to differences in macromolecular chain conformation. [12,13] It has been demonstrated that the β-phase in P(VDF-TrFE) is the main ingredient contributing to pyroelectric voltage. [14] Therefore, increasing the β-phase content can effectively improve the pyroelectric properties of P(VDF-TrFE)-based devices; such increases can be accomplished by controlling the annealing time, adding polar solvents, applying polarization, mechanical stretching, and adding ceramics fillers, among other methods. [15][16][17][18][19][20][21] P(VDF-TrFE) films doped with nanoscale fillers are another commonly used method to enhance the bulk pyroelectric properties of the composite films. [22][23][24] Although these methods can successfully enhance the pyroelectric voltage, they offer little enhancement to the pyroelectric response time, which is a crucial characteristic for sensing devices. The electrical time constant, which depends on resistance and capacitance, and the thermal time constant, Organic pyroelectric materials are widely applied as temperature sensors in wearable electronic devices due to their good biocompatibility and stability. Real-time monitoring of the physiological state of the human body requires pyroelectric materials with a fast response time and large output voltage. In this study, the pyroelectric characteristics of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) films are improved with the use of commercial inorganic P-type bismuth antimonide (P-Bi 2 Te 3 ) fillers. Composite films with 0.2 wt% P-Bi 2 Te 3 increase the pyroelectric response time and voltage by improving the thermal diffusivity and enhancing the β-phase content, respectively. Proton irradiation results in further improvement of the pyroelectric response time from 22 to 0.5 s. The proton irradiation-induced ionization energy loss improves the conductivity of the composite films, thereby enhancing the pyroelectric response time. These results show that P-Bi 2 Te 3 doping is beneficial for improving the pyroelectric properties of P(VDF-TrFE) and that proton irradiation is an effective method for further improving the response time of inorganic-organic composite films.
which depends on thermal capacity and thermal conductivity, are what determine the response time of pyroelectric detectors. [25,26] The poor thermal conductivity of P(VDF-TrFE) films contributes to its relatively long pyroelectric response time. In addition, the pyroelectric ceramics currently in widespread use exhibit high thermal time constants and poor thermal conductivity; thus, offering little help in enhancing pyroelectric response times. It is therefore crucial to choose fillers carefully so that they can improve both heat conductivity and the pyroelectric coefficient. Among such potential fillers, the thermoelectric material with good thermal conductivity can enhance the pyroelectric voltage by increasing the content of β-phase in P(VDF-TrFE) film related to the surface charge-induced nucleation. [27] Those works mainly focus on the influence of thermoelectric fillers on the pyroelectric voltage, while influence on the response time is not studied. Commonly, the thermal conductivity of thermoelectric material is larger than that of P(VDF-TrFE). [28][29][30] It means that doping a P(VDF-TrFE) film with thermoelectric fillers should increase its thermal conductivity, thereby producing a rapid thermal-release electrical response that shortens the pyroelectric response time. Therefore, the thermoelectric fillers are chosen in this work.
On the other hand, ion irradiation is a widely used method in the field of polymers. [31,32] Ion irradiation with controllable energy and fluence is an effective method to alter the properties of various polymers and improve their application range by introducing defects so as to modify their crystal structures. This process causes nuclear energy loss and electron energy loss, which stimulate the formation and quenching of nanocrystals. [33][34][35][36] High-energy particle irradiation is a particularly useful technique for enhancing the pyroelectric performance of P(VDF-TrFE). Various energies of gamma rays, electrons, and ions have been used to irradiate P(VDF-TrFE) in studies of the behavior of P(VDF-TrFE) and its impact on P(VDF-TrFE) pyroelectric performance and properties such as dielectricity, ferroelectricity, and electrocaloricity. [37][38][39][40][41] In such bombardment, proton irradiation exhibits strong penetrating power due to its light weight, small band point, and low energy transfer efficiency. Entering the interior of the material more easily than other radiation types, proton irradiation induces cross-linking reactions within the polymer, altering its physicochemical properties. [42] However, such irradiation-induced changes in the pyroelectric properties of P(VDF-TrFE) with thermoelectric fillers have never before been reported in the literature. Previous literature has merely showed that defects can be produced by proton irradiation in thermoelectric materials and that these defects improve its properties. [43] These findings suggest that the structural modification of thermoelectric-based inorganic-organic composites can be accomplished effectively through proton irradiation of thermoelectric material. Therefore, proton irradiation was introduced in the present study into P(VDF-TrFE) film with thermoelectric fillers to modify their pyroelectric properties.
In this work, the impact of proton irradiation on pyroelectric properties was studied in P(VDF-TrFE) film with bismuth antimonide (Bi 2 Te 3 ) fillers by blending commercial P-type bismuth antimonide (P-Bi 2 Te 3 ) powder into P(VDF-TrFE). It was found that the introduction of P-Bi 2 Te 3 fillers resulted in an increase in the β-phase content of P-Bi 2 Te 3 /P(VDF-TrFE) composite films, enhancing the magnitude of the pyroelectric voltage. In addition, it was discovered that P-Bi 2 Te 3 fillers can shorten the pyroelectric response time of composite films by boosting the thermal diffusivity and thereby lowering the associated thermal constant. The response time of pyroelectric of P-Bi 2 Te 3 /P(VDF-TrFE) composite films was further improved by an order of magnitude after 20 MeV proton irradiation due to the ionization energy loss introduced by proton irradiation. The method proposed in this work for improving the pyroelectric responses of inorganic-organic composite films show potential for a variety of applications for wearable electronics and pyroelectric detection.

Results and Discussion
A schematic illustration to the enhancement of the content of β phase in P(VDF-TrFE) by P-Bi 2 Te 3 is shown in the Figure 1a (left), schematic diagram of proton irradiation on the composite film (middle of Figure 1a), and the schematic illustration to the ionization effect and displacement damage effect in the P-Bi 2 Te 3 /P(VDF-TrFE) after the proton irradiation (right of Figure 1a). The pyroelectric response curves for P(VDF-TrFE) with various mass fractions of P-Bi 2 Te 3 were determined. The schematic of the pyroelectric devices is shown in Figure S1, Supporting Information. Figure 1b shows the pyroelectric response curves of intrinsic P(VDF-TrFE) and P-Bi 2 Te 3 / P(VDF-TrFE) composite film under a temperature difference of 10 K. The spontaneous polarization of P(VDF-TrFE) weakens as the film temperature rises over time (dT/dt > 0), increasing the dipole oscillation. As a result, the pyroelectric voltage starts to fall after reaching its highest value during the heating process, as shown in Figure S2, Supporting Information. As the temperature of the P(VDF-TrFE) film decreases (dT/dt < 0), the spontaneous polarization of P(VDF-TrFE) rises, and the pyroelectric output reverse voltage increases, in agreement with previous studies. [44,45] The pyroelectric response times of intrinsic P(VDF-TrFE) and P-Bi 2 Te 3 /P(VDF-TrFE) composite film are shown in Figure 1c. To reduce the error, the confidence interval method was used to intercept the lowest and highest points of 10-90% data; and then, the rise time and fall time were obtained by subtraction. [46][47][48] The pyroelectric response times of P(VDF-TrFE) composite films doped with P-Bi 2 Te 3 were shorter than the response time of intrinsic P(VDF-TrFE) film (34 s); for example, the response time of 0.2 wt% P-Bi 2 Te 3 / P(VDF-TrFE) composite film was 22.3 s. When the mass fraction of P-Bi 2 Te 3 was increased to 1 wt%, the response time of the composite film increased somewhat but remained shorter than that of intrinsic P(VDF-TrFE). As shown in Figure 1d, the magnitude of pyroelectric voltage was higher for a composite film with a 0.2% P-Bi 2 Te 3 mass fraction than for any other composite film or intrinsic P(VDF-TrFE). In order to study the potential response capability and linearity of pyroelectric device applications, a temperature difference varying from 1 to 10 K was applied in the intrinsic P(VDF-TrFE) film and the 0.2 wt% P-Bi 2 Te 3 /P(VDF-TrFE) composite film to obtain the peak value of pyroelectric voltage, as shown in Figure 1e. Both the intrinsic P(VDF-TrFE) film and the 0.2 wt% P-Bi 2 Te 3 /P(VDF-TrFE) composite film prepared in this work showed good linearity.
In order to clarify the mechanism whereby P-Bi 2 Te 3 fillers affect the pyroelectric response time and the magnitude of the pyroelectric voltage of P-Bi 2 Te 3 /P(VDF-TrFE) composite films, we tested intrinsic P(VDF-TrFE) films and P-Bi 2 Te 3 / P(VDF-TrFE) composite films in detail. In pyroelectric devices, the response time depends on the thermal time constant and the electrical time constant. The thermal time constant , where H is the thermal capacity and G T is the thermal conductivity. [49] The specific thermal capacities of P-Bi 2 Te 3 and P(VDF-TrFE) are 154 J kg −1 K −1 and 1170 J kg −1 K −1 , respectively. [50,51] The bulk thermal capacity of each composite film was obtained by calculating a weighted average according to the mass ratio of P-Bi 2 Te 3 and P(VDF-TrFE), with results shown in Figure 2a. The bulk thermal capacity is lower for composite films with higher P-Bi 2 Te 3 content. The thermals conductivity is obtained by the thermal diffusivity, density, and specific heat capacity. P-Bi 2 Te 3 -induced enhancement of bulk thermal diffusivity in composite films can be explained by the much higher thermal diffusivity of P-Bi 2 Te 3 (1.5 mm 2 s −1 ) [52] compared to that of intrinsic P(VDF-TrFE) film (0.63 mm 2 s −1 ). Figure 2b indicates that the bulk thermal conductivity is higher for composite films with higher P-Bi 2 Te 3 content. Notably, the bulk thermal time constants for P-Bi 2 Te 3 /P(VDF-TrFE) composite films were lower than that for intrinsic P(VDF-TrFE) films according to the above equation. As a result, the pyroelectric response time of the composite film with 0.2 wt% P-Bi 2 Te 3 (22.3 s) was shorter than that of the intrinsic P(VDF-TrFE) film. This result confirms that the P-Bi 2 Te 3 doping of P(VDF-TrFE) can shorten the pyroelectric response times of composite films by reducing the thermal constant. However, the pyroelectric response times of composite films are actually longer for composite films with higher P-Bi 2 Te 3 content. To explain this phenomenon, we next explore changes in the β-phase content of the composite films. Figure 2c shows X-ray diffraction (XRD) images of P-Bi 2 Te 3 /P(VDF-TrFE) composite films and intrinsic P(VDF-TrFE) film as a function of the P-Bi 2 Te 3 mass fraction. The results indicate the existence of both P-Bi 2 Te 3 fillers and polar β-phase in composite films. To clarify the influence of P-Bi 2 Te 3 fillers on β-phase content, the β-phase (110) peak and P-Bi 2 Te 3 (006) peak are shown in Figure 2d as functions of the P-Bi 2 Te 3 www.advelectronicmat.de mass fraction. As expected, the P-Bi 2 Te 3 (006) peak intensity increased as the P-Bi 2 Te 3 mass fraction increased. While the addition of small amounts of P-Bi 2 Te 3 fillers promoted β-phase formation in composite films, β-phase content began to decrease as the P-Bi 2 Te 3 mass fraction increased beyond a certain threshold. We speculate that the doping effect of P-Bi 2 Te 3 is optimal at 0.2 wt% P-Bi 2 Te 3 , with additional doping inhibiting β-phase crystallization. The effects of P-Bi 2 Te 3 doping on P(VDF-TrFE) composite film structure were studied by Fourier-transform infrared (FTIR) spectrometry, as shown in Figure 2e. The β-phase peaks for P(VDF-TrFE) were 838, 874, 1070, 1167, and 1230 cm −1 . The α-phase peaks for P(VDF-TrFE) were 614, 764, and 1400 cm −1 , in agreement with the literature. [53] In addition, the relative β-phase content was calculated according to the equations proposed by Gregorio and Cestarizz. [54] The relative β-phase content for each fractionally doped composite film, as shown in Figure 2f, was determined by substituting the α-phase absorption intensity at 764 cm −1 and the β-phase absorption intensity at 838 cm −1 into crystallinity calculations. The proportion of P(VDF-TrFE) manifesting as β-phase was very high (85.5%) at a 0.2% P-Bi 2 Te 3 mass fraction, gradually decreasing to 59% as the P-Bi 2 Te 3 mass fraction increased to 1 wt%. The effects of P-Bi 2 Te 3 content on the crystallization capacities of the composite films are explained as follows. In the annealing stage, P-Bi 2 Te 3 fillers in P(VDF-TrFE) can effectively alter the polymer's crystallization rate, which is related to its crystallization energy. The lattice transformation in P(VDF-TrFE) requires heat energy, and the greater the heat energy, the more likely the occurrence of the transformation. α-phase P(VDF-TrFE) could transform to β-phase under annealing at high-temperature. Due to the poor thermal conductivity of the polymer, substantial heat accumulated in the film after crystallization in the annealing stage; such heat can easily cause recrystallization. At Figure 2. a) Thermal conductivity and b) thermal capacity for composite films of P-type bismuth antimonide (P-Bi 2 Te 3 ) and poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) vary with the mass fraction of P-Bi 2 Te 3 ; c) X-ray diffraction (XRD) images of P-Bi 2 Te 3 /P(VDF-TrFE) composite films with various mass fractions of P-Bi 2 Te 3 ; d) (110) and (006) peak intensities as functions of the mass fraction of P-Bi 2 Te 3 in the composite film; e) Fourier-transform infrared (FTIR) spectroscopy of P-Bi 2 Te 3 /P(VDF-TrFE) composite films with various mass fractions of P-Bi 2 Te 3 ; f) β-phase content of P-Bi 2 Te 3 as a function of the mass fraction of P-Bi 2 Te 3 in the composite film.
www.advelectronicmat.de the annealing temperature, the potential energy of the β-phase was 1.26 kJ mol −1 higher than that of the α-phase; the β-phase, when left in a continuous high-temperature state, thus easily transforms into the α-phase under annealing conditions. [55][56][57] When P-Bi 2 Te 3 powder was added to the P(VDF-TrFE), P-Bi 2 Te 3 fillers with high thermal conductivity were able to boost the rate of heat transfer by allowing the polymer near the filler to conduct heat more quickly than elsewhere. [58] This process resulted in temperature differences within the film, leading to the accumulation of spongy crystals around the filler, as seen in the cross-section image of the composite film in Figure S3a, Supporting Information, as well as in the schematic of the β-phase formation process shown in Figure S3b, Supporting Information. Therefore, P(VDF-TrFE) with 0.2 wt% P-Bi 2 Te showed a larger pyroelectric voltage and a shorter pyroelectric response time than intrinsic P(VDF-TrFE) film or 0.1 wt% P-Bi 2 Te composite films. As for films with higher P-Bi 2 Te 3 mass fractions, β-phase content in the composite film decreased to 60.1% as the P-Bi 2 Te 3 mass fraction increased beyond 0.2 wt%. Considering that the high density of Bi 2 Te 3 caused it to tend to aggregate and deposit mostly at the bottom of the substrate, [59] it is inferred that the aggregation of P-Bi 2 Te 3 filler is responsible for the decline in β-phase fraction with increasing P-Bi 2 Te 3 mass fraction. Note that in the relation between β-phase fraction and pyroelectric properties, the pyroelectric voltage decreasing with increasing P-Bi 2 Te 3 filler content beyond the 0.2 wt% threshold was related to changes in the β-phase fraction in the composite film. When P-Bi 2 Te 3 fillers were added into P(VDF-TrFE), β-phase content in the composite changed as did the thermal conductivity. The synergy of these two effects resulted in a longer pyroelectric response time for composite films with higher filler content beyond the 0.2 wt% threshold. However, the pyroelectric response times of composite films were faster than those of intrinsic P(VDF-TrFE) film when the P-Bi 2 Te 3 was doped into it.
Next, the pyroelectric properties of intrinsic P(VDF-TrFE) film and P-Bi 2 Te 3 /P(VDF-TrFE) composite films irradiated with protons at an energy of 20 MeV were studied. The pyroelectric voltages of intrinsic P(VDF-TrFE) film and P-Bi 2 Te 3 / P(VDF-TrFE) composite films after irradiation were determined, with the resulting pyroelectric response curves shown in Figure 3a. The response times of intrinsic P(VDF-TrFE) film and P-Bi 2 Te 3 /P(VDF-TrFE) composite films were substantially lower after proton irradiation than before. The response time of intrinsic P(VDF-TrFE) film after irradiation was reduced from 34 to 1.8 s. After proton irradiation, the response time of 0.2 wt% P-Bi 2 Te 3 /P(VDF-TrFE) composite films was shortened to 0.5 s, while its pyroelectric voltage was larger than the intrinsic P(VDF-TrFE) film. The response times of composite films with various P-Bi 2 Te 3 mass fractions were extracted by intercepting the 10-90% confidence interval, allowing the fitting of a curve of response time versus P-Bi 2 Te 3 mass fraction, as shown in Figure 3b. When the mass fraction of P-Bi 2 Te 3 fillers in the composite film increased from 0.2 wt% to 1 wt%, Figure 3. a) Post-irradiation response curves versus P-Bi 2 Te 3 mass fraction for intrinsic poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) film and composite films of P-type bismuth antimonide (P-Bi 2 Te 3 ) and P(VDF-TrFE); b) pyroelectric response times of irradiated intrinsic P(VDF-TrFE) film and P-Bi 2 Te 3 /P(VDF-TrFE) composite films with various P-Bi 2 Te 3 mass fractions. The inset shows the differential multiples of response time before and after irradiation; c) resistance-capacitance (RC) time constant versus P-Bi 2 Te 3 mass fraction before and after irradiation; d) ionization energy loss of (IEL) proton and recoil in P-Bi 2 Te 3 /P(VDF-TrFE); e) non-ionization energy loss (NIEL) of proton and recoil in P-Bi 2 Te 3 /P(VDF-TrFE).

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the response time of the composite film gradually lengthened from 0.5 to 3.8 s. In general, the response time of composite films after proton irradiation was about one order of magnitude lower after irradiation than before. When the proton was implanted into the Bi 2 Te 3 particle, the defects were induced through the displacement damage effect related to the nonionization energy loss (Figure 3e). A nonuniform distribution of charge is generated in Bi 2 Te 3 through the proton irradiation implanted H + ions, and act as adsorption and nucleation sites. Those sites provide the position for the attachment of P(VDF-TrFE) chains, and enhance the β-phase content. The enhancement of β-phase fraction based on this mechanism was reported in PVDF film with others fillers. [27,60] However, the total crystallinity in P-Bi 2 Te 3 /P(VDF-TrFE) composite films decreased due to the cross-linking of molecular chain in P(VDF-TrFE) film, and made the pyroelectric voltage of irradiated composite films smaller than that before unirradiated. Next, the resistancecapacitance (RC) time constants of the pyroelectric composite films were measured using an oscilloscope because the electric time constant determines the limiting velocity of pyroelectric response to a certain extent. [30] The selected metric was the time required for a capacitor to be charged to 63.2% of its maximum voltage from initial 0 through a resistor, or for a capacitor to be discharged to 36.8% of its initial voltage through the same resistor. Charging curves for various pyroelectric film devices before and after irradiation are shown in Figure S4, Supporting Information. The RC time constant of each device was obtained by intercepting time, as shown in Figure 3c. The larger the RC time constant, the longer the charge and discharge time and the longer the transition process. This framework is consistent with our observed change trend in pyroelectric response time as measured experimentally. Each RC time constant after irradiation was lower than before; the 40.1 ms RC time constant for the composite film with 0.2 wt% P-Bi 2 Te 3 filler was lower than that for intrinsic P(VDF-TrFE) film (43.8 ms) or for P-Bi 2 Te 3 /P(VDF-TrFE) composite films of greater or lesser P-Bi 2 Te 3 content. This finding further reveals that proton irradiation and P-Bi 2 Te 3 fillers are effective ways to enhance the response times of pyroelectric materials. It is noted that the response time of irradiation P-Bi 2 Te 3 /P(VDF-TrFE) composite films increases with increasing of P-Bi 2 Te 3 fillers content. In ultra-fast pyroelectric sensors, the RC time constant is the dominant temporal factor for the response time. [25] In the irradiation P-Bi 2 Te 3 /P(VDF-TrFE) composite films, the tendency of RC constant and response time as a function of the content of fillers are the same, in which the magnitude of the electrical time constant is smaller than that of the response time. This confirmed that the pyroelectric response time of the irradiation film is determined by the RC constant. Therefore, the RC time constant increases with increasing of fillers content, and causes the longer responses.
A Geant4 software simulation was then used to calculate the ionization range/depth of P-Bi 2 Te 3 /P(VDF-TrFE) composite films by proton irradiation at 20 MeV. A schematic of the simulation is shown Figure S5, Supporting Information. The incident trajectory of proton radiation at 20 MeV is denoted by the red line in Figure S5, Supporting Information. The blue lines represent the trajectories of protons before injection into the composite films, while the red lines represent the trajectories of electrons. The interactions of protons with electrons outside the nucleus cause ionization energy loss during proton incidence, and the proton impact causes the displacement of atoms in the material, causing non-ionization energy loss. The ionization energy loss and non-ionization energy loss in P-Bi 2 Te 3 /P(VDF-TrFE) composite films, as induced by protons and recoil, respectively, are shown in Figures 3d and 3e, respectively. In general, atoms of relatively higher mass show relatively stronger ability to stop protons, and when protons interact with P-Bi 2 Te 3 during transport, more ionization energy is lost. As shown in Figure 3d, the presence of P-Bi 2 Te 3 resulted in higher ionization energy losses at 0-0.5 and 16.5-20.5 µm. The ionization energy loss and non-ionization energy loss induced by incident protons were mainly concentrated near the proton track, and the ionization energy loss at a given incidence depth was at least four orders of magnitude greater than the non-ionization energy loss.
In a previous study, proton irradiation was found to induce defects in Bi 2 Te 3 . [43] Ionizing radiation causes oxidation of bismuth and tellurium ions, making the net charge of the system positive. In order to compensate for the charge difference, anti-positional defects form. After proton irradiation, charge imbalance occurs in Bi 2 Te 3 . To compensate for the excess positive charge, bismuth occupies positions in such a way as to form anti-site defects, resulting in decreases in the carrier concentration and electron concentration of P-Bi 2 Te 3 . These decreases, in turn, cause an increase in electrical conductivity; high conductivity affects the rate of charge movement in the composite film, resulting in a faster pyroelectric effect. As shown in Figure S6, Supporting Information, a large number of free charges accumulate near P-Bi 2 Te 3 particles after proton irradiation. When the film is subjected to a temperature difference, the accumulated free charges quickly move to opposite ends of the film. As a result, the shortening of the pyroelectric response time is quite significant for the 0.2 wt% P-Bi 2 Te 3 /P(VDF-TrFE) composite film.
The annealing effect of irradiation on 0.2 wt% P-Bi 2 Te 3 / P(VDF-TrFE) composite films was studied next. The pyroelectric properties of the composite film were tested every 12 h after irradiation. Notably, the voltage magnitude and response time of the film both changed with the annealing time, as shown in Figure 4a,b. As seen in Figure 4a, it can be clearly observed that the response time increases significantly after irradiation, from the unsaturation observed before irradiation to the rapid peak after irradiation when the heating time is fixed as 5 s. When time passed post-irradiation, the response time gradually increased. The stabilities of films subjected to various irradiation annealing times are shown in Figure 4b. After 72 h, the instantaneous response time and quick recovery time of the composite film were 3.2 and 3.3 s, respectively, and the irradiation annealing effect showed good stability of response time. The stability of the composite film remained at a high level after more than 2000 repeated loadings at a temperature difference of 10 K. The 0.2 wt% P-Bi 2 Te 3 /P(VDF-TrFE) composite film was left standing for 3 months before its pyroelectric properties were tested again. The transient response time and rapid recovery time of the 0.2 wt% P-Bi 2 Te 3 /P(VDF-TrFE) composite films were 3.5 and 3.4 s, respectively, indicating good stability of response time. Under each of various temperature gradients www.advelectronicmat.de ranging from 1 to 10 K, the pyroelectric voltage exhibited by composite films showed a decreasing temporal trend over the first 72 h post-irradiation and then reached a stable state, as shown in Figure 4c. As time passed, the ionization damage continued to induce cross-linking reactions in the polymer. The molecular chain in the amorphous region of P(VDF-TrFE) film was usually dominated by cross-linking reactions which were enhanced by ionization energy loss. Therefore, crystalline regions adjacent to the amorphous region also underwent cross-linking reactions. This continuous cross-linking of molecular chains in P(VDF-TrFE) reduced the crystallinity of these films and, in turn, its pyroelectric voltage.
Last, the pyroelectric materials were applied as temperature sensors, with results shown in Figure 5. For temperature differences (∆T) ranging from 1 to 10 K, the pre-irradiation pyroelectric effect showed large pyroelectric voltages, as shown in Figure 5a,b. The sensitivity of the 0.2 wt% P-Bi 2 Te 3 /P(VDF-TrFE) composite film after irradiation (0.59 V/K) was lower than before irradiation (0.62 V/K) due to piezoelectric effects, but the pyroelectric linearity after irradiation (R 2 = 0.9856) was higher than before irradiation (0.9801), as shown in Figure 5c. Under a pyroelectric temperature difference of 10 K after irradiation, a stable continuous output voltage was generated by more than 1000 repeated loadings, just as it was prior to irradiation. This behavior indicates the good irradiation resistance of pyroelectric properties of P-Bi 2 Te 3 /P(VDF-TrFE) composite films (Figure 5d).

Conclusion
P-Bi 2 Te 3 /P(VDF-TrFE) composite films with various P-Bi 2 Te 3 mass fractions were prepared. The response time and pyroelectric voltage magnitude were tuned by the P-Bi 2 Te 3 content in the composite. The excellent thermal conductivity of P-Bi 2 Te 3 dramatically shortened the response time of the thermal-release apparatus while improving the crystallinity of P(VDF-TrFE) film in the annealing crystallization stage. A comparison of the thermal time constant of each film before and after doping revealed that response time improvements were driven by the www.advelectronicmat.de thermal conductivity of P-Bi 2 Te 3 particles. The polar β-phase content was enhanced by the formation of cavernous crystals around P-Bi 2 Te 3 fillers due to spatial heterogeneities in temperature conduction rates in the composite film. The pyroelectric response time was shortened by an order of magnitude after proton irradiation. The ionization damage to intrinsic P(VDF-TrFE) films and P-Bi 2 Te 3 /P(VDF-TrFE) composite films associated with 20 MeV proton irradiation reduced film crystallinity. However, the pyroelectric response time was dramatically shortened after proton irradiation due to irradiation effects on the conductivity of P-Bi 2 Te 3 fillers and free charges within the film.
This work provides a method to boost the response time and voltage of pyroeletric devices simultaneously using the thermoelectric fillers. It proves that ion irradiation with controllable energy and fluence can directly optimize the response performance of pyroelectric devices. Hence, this work paves the way for the development of ultra-fast pyroelectric sensors with not only dramatically reduced times to implement sensing and temperature detection but also easy integration into future wearable electronics applications. In addition, this work opens up a new application direction for the ion irradiation technique.
Preparation of Composite Films: P(VDF-TrFE) powder was dissolved in DMSO with each of various mass fractions of Bi 2 Te 3 filler to form a set of P-Bi 2 Te 3 /P(VDF-TrFE) mixed solutions. Each P-Bi 2 Te 3 /P(VDF-TrFE) composite solution was coated onto a glass plate at room temperature, and each solution was uniformly adhered to the glass plate by casting. Each plate was placed in an electric blast dryer at 70 °C for 7 h, and then each P-Bi 2 Te 3 /P(VDF-TrFE) composite film was transferred to a digital vacuum drying oven at 130 °C for 10 h. A high-voltage polarization device was used to polarize each prepared composite film.
Test Method: Thermal diffusivity of the P-Bi 2 Te 3 /P(VDF-TrFE) composite film was obtained by the laser flash method (Netzsch LFA457). The metal electrodes on the upper and lower sides of each P-Bi 2 Te 3 /P(VDF-TrFE) composite film were sputtered using physical vapor deposition (DM200). The experiment was carried out in the Xi'an 200 MeV Proton Application Facility (XiPAF). The proton energy used in this experiment was 20 MeV with flux of 2 × 10 8 p cm −2 s −1 , and the corresponding beam spot area was 1 × 1 cm 2 . The proton beam was incident vertically from the surface of the P(VDF-TrFE) thin film and the fluence was 4 × 10 10 p cm −2 . Thermal temperature differences between 1 and 10 K were applied through a laser heating table (PSU-H-LED). The pyroelectric voltage of the device was measured using an electrometer (KEITHLEY 6514). The temperature difference was controlled by a timing switch baffle. A scanning electron microscope (GeminiSEM 300) was used to study the morphology and determine the elemental compositions of the composite films. X-ray diffraction (XRD) (Rigaku + UltimaIV) was used to study the crystal structures of the composite films. The molecular structures of the films were measured by FTIR (Thermo Scientific Nicolet iS20). The charging and discharging processes of the composite films were measured by oscilloscope. Thermal diffusivity of Figure 5. Pyroelectric response curves for poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) films containing 0.2 wt% P-type bismuth antimonide (P-Bi 2 Te 3 ) a) before and b) after irradiation; c) pyroelectric voltage and d) pyroelectric stability curves for 0.2 wt% P-Bi 2 Te 3 /P(VDF-TrFE) composite films before and after irradiation.
Simulation: Geant4 software (CERN, V10.06) was used to simulate the transport of 20 MeV protons in P-Bi 2 Te 3 /PVDF. In the simulations, the thickness of PVDF was 50 µm, and P-Bi 2 Te 3 was idealized as spheres with diameters distributed uniformly from 2 to 10 µm.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.