Excellent Energy Storage and Photovoltaic Performances in Bi_0.45Na_0.45Ba_0.1TiO₃-Based Lead-Free Ferroelectricity Thin Film

Abstract

Inorganic dielectric films have attracted extensive attention in the field of microelectronic and electrical devices because of their wide operating temperature range, small size, and easy integration. Here, we designed and prepared eco-friendly (1-x)Bi_0.45Na_0.45Ba_0.1TiO₃-xBi(Mg_1/3Nb_2/3)O₃ multifunctional ferroelectric thin films for energy storage and photovoltaic. The results show that Bi(Mg_1/3Nb_2/3)O₃ can effectively improve the energy storage performance. At x = 0.05, the energy storage density and efficiency are as high as 73.1 J/cm³ and 86.2%, respectively, and can operate stably in a wide temperature range. The breakdown field strength of the thin films increased significantly, and the analysis showed that the addition of Bi(Mg_1/3Nb_2/3)O₃ caused a change in the internal conduction mechanism. At the same time, the generation of polar nanoregions increases the relaxation characteristics, thus improving the energy storage properties. In addition, the thin film material also has excellent ferroelectric photovoltaic properties. This work represents a new design paradigm that can serve as an effective strategy for developing advanced multi-functional materials.

1. Introduction

With the increasing demand for renewable energy and the rapid advancements in electrical and electronic technology, the research focus in the energy field has shifted towards developing electrical energy storage devices with high power density, high voltage, low cost, and potential for large-scale applications [1,2,3,4,5]. Dielectric capacitors offer advantages such as higher power density, faster charge and discharge rates, exceptional cycle life, and improved safety, making them suitable for various applications, including green energy storage, pulse power systems, and military facilities. Despite these benefits, dielectric capacitors have limitations in energy storage density, and the evolving trend towards smaller, lighter, and more integrated electronic devices poses challenges for their energy storage performance [6,7,8,9]. Addressing these gaps to enhance energy storage density remains a key research area. The energy storage performance of dielectric capacitors is typically assessed using the following formula [10,11,12]:

$$\mathrm{Energy} \mathrm{storage} \mathrm{density} (W) ={\int }_0^{P_{\max }}\mathit{EdP}$$

(1)

$$\mathrm{Recoverable} \mathrm{energy} \mathrm{storage} \mathrm{density} \left(W_{\mathrm{rec}}\right)={\int }_{P_{\mathrm{r}}}^{P_{\max }}\mathit{EdP}$$

(2)

$$\mathrm{Energy} \mathrm{storage} \mathrm{efficiency} (\eta ) ={\displaystyle \frac{W_{\mathrm{rec}}}{\mathit{W}}} \times 100\%$$

(3)

wherein P_max, P_r, P, and E are the maximum field polarization, residual polarization, polarization, and external electric field, respectively. Increasing the gap between P_max and P_r and enhancing the breakdown strength (BDS) is an effective strategy to maximize energy storage density.

At present, commercially available dielectric energy storage devices are mainly made of organic polymer materials, but such capacitors are often not resistant to high temperatures [13]. In contrast, inorganic ceramic-based dielectric materials have better temperature stability, making them suitable for a wide temperature range [14,15]. Additionally, ceramic-based dielectric materials have high dielectric constant and polarization strength, which can achieve significant energy storage density even under low electric field conditions [16,17,18]. Among them, ceramic films have a small thickness and can achieve high electric field strength and energy storage density at very low voltage levels. These materials are lightweight, highly integrated, and show great potential for various applications. Lead-containing ceramics generally exhibit good energy storage properties by reducing grain size and constructing field-induced phase transition with higher releasable energy density (W_rec) and energy storage efficiency (η) [19]. However, lead can cause environmental pollution problems and damage human health, so it is very necessary to develop alternative lead-free energy storage ceramics. Lead-free ferroelectric thin film materials, such as sodium bismuth titanate-based (Na_0.5Bi_0.5TiO₃, NBT) ferroelectric thin films, have garnered attention due to their unique diffusion phase transition behavior and relaxation ferroelectric properties [20,21,22]. Despite advantages like high dielectric constant, large P_max (>40 μC/cm²), and high Curie temperature (approximately 320 °C), pure NBT faces limitations such as high P_r (approximately 38 μC/cm²), large coercive field (approximately 70 kV/cm), and low BDS (<100 kV/cm), hindering its energy storage density and efficiency improvements. These constraints restrict its widespread application in energy storage and development. In response to the above problems, researchers have improved the energy storage performance of NBT films by inducing or enhancing relaxation and designing material structure. For example, Liu et al. prepared lead-free thin film (0.94-x)Bi_0.5Na_0.5TiO₃-0.06BaTiO₃-xSrTiO₃ (referred to as BNT-BT-xST, x = 0, 0.05, 0.10, 0.15, 0.20) using a sol-gel/spin-coating method and studied the effect of the introduction of strontium on the microstructure, dielectric properties, and energy storage density of the film [23]. The results show that the addition of strontium improves the maximum polarization and dielectric constant of the film. The film has a high maximum recyclable energy storage density of 22.5 J/cm³ and a dielectric constant of 1120 at 1 kHz. Chen et al. conducted component exploration of (1-x)Bi_0.5(Na_0.8K_0.2)_0.5TiO₃-xSrZrO₃ (BNKT-100xSZ) thin film [24]. When x = 0.15, the BNKT-SZ thin film has an energy storage density of 34.69 J/cm³. In addition, the BNKT-15SZ film has good thermal stability over a wide temperature range of 30–100 °C. Therefore, it is effective to modify NBT-based thin film materials using specific components of doped solid solutions.

In this work, based on (Bi_0.5Na_0.5)TiO₃ ceramic film, Ba²⁺ ions were doped at the A-site, and a solid solution was performed with the second component Bi(Mg_1/3Nb_2/3)O₃ to optimize the properties of the films. We systematically studied the phase structure, microstructure, dielectric properties, and energy storage properties of Bi_0.45Na_0.45Ba_0.1TiO₃-xBi(Mg_1/3Nb_2/3)O₃ films. The results show that the introduction of Bi(Mg_1/3Nb_2/3)O₃ significantly improves the breakdown strength and relaxation properties of films, thereby enhancing its energy storage properties. In addition, the film material not only demonstrates outstanding temperature stability but also showcases excellent photovoltaic and photodetection properties. These qualities are essential to ensure the reliable operation of the equipment in practical applications. For energy storage applications, this material can be further made into multi-layer ceramic capacitors (MLCC). MLCC is ubiquitous in modern electronic devices, such as hybrid cars and advanced medical equipment. In photovoltaic applications, photodetectors made mainly of such materials can be used in a variety of fields, such as sensing, imaging, and night vision.

2. Materials and Methods

The (1-x)Bi_0.45Na_0.45Ba_0.1TiO₃-xBi(Mg_1/3Nb_2/3)O₃ (BNBT-xBMN, x = 0, 0.025, 0.050, and 0.075) film were prepared on Pt/Si substrate via a sol-gel method. PVD technique was used to deposit Pt on Si substrate to obtain the bottom electrode. Bismuth nitrate pentahydrate (99%, Aladdin reagent Co., Ltd., Shanghai, China) sodium acetate trihydrate (99%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), tetrabutyl titanate (98%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), barium acetate (99%, Aladdin reagent Co., Ltd., Shanghai, China) magnesium acetate tetrahydrate (99%, Aladdin reagent Co., Ltd., Shanghai, China) ammonium niobate oxalate hydrate (99.99%, Aladdin reagent Co., Ltd., Shanghai, China) were selected as the raw materials. Glacial acetic acid (99.5%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), acetylacetone (99%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), and distilled water were used as the solvents. According to the stoichiometric ratio, two precursor solutions of BNBT and BMN were prepared by mixing the above raw materials and then mixed separately according to the ratio. To compensate for volatilization during annealing, an excess of 10 mol% sodium and bismuth was used. The preparation steps of the BNBT precursor solution were as follows. First, the distilled water and glacial acetic acid were heated to 80 °C; then, sodium acetate trihydrate, bismuth nitrate, and barium acetate were added and fully dissolved. After natural cooling to room temperature, acetylacetone was added and stirred for 20 min. Finally, tetrabutyl titanate was added and stirred for 20 min. The volume ratio of glacial acetic acid, distilled water, and acetylacetone is 4:1:1. BMN precursor solution was prepared by the same method and then mixed with BNBT precursor solution in proportion. Formamide, lactic acid, and polyvinyl pyrrolidone in a ratio of 1:1:1 were added to the mixed solution to increase viscosity and avoid cracks after film formation. The final concentration of the solution is 0.5 mol/L. Then, the resulting BNBT-xBMN precursor solutions were spin-coated on the Pt/Si layer at 3000 rpms for 30 s. After each coating, the wet layer was dried (150 °C, 3 min), pyrolyzed (410 °C, 10 min), and crystallized (700 °C, 3 min) by using a high-temperature tubular furnace. The above steps are repeated until the film reaches the desired thickness. Finally, the sample was annealed in a tube furnace at 700 °C to crystallize.

The crystal structure and microstructure of the films were analyzed by X-ray diffractometer (XRD Bruker D8 Advance diffractometer, Germany) and scanning electron microscope (FE-SEM, Zeiss GeminiSEM 300, Germany), respectively. The polarization electric field (P-E) hysteresis loops and leakage current density-electric field (J-E) curves were performed through ferroelectric test system (Radiant Technologies, Inc., Albuquerque, NM, USA). The energy storage performance of the film was calculated by P-E loops. The dielectric properties were investigated by Agilent E4980A LCR analyzer. The evolution of microstructure, domain structure, and the piezoelectric response of the films were measured by piezoelectric force microscopy (PFM, Bruker, Icon, Germany). The photovoltaic performance of the films was measured by using electrometers (Keithley, 6517B, USA) and a solar simulator (NBeT, Solar500, China).

3. Results and Discussion

The XRD pattern of BNBT-xBMN grown on the Pt/Si substrate measured at room temperature is shown in Figure 1a; all the samples show a typical perovskite structure. The diffraction peak of the second phase can be observed near 36°. JADE software (MDI jade 6) analysis indicates that this phase is most likely NbO, whose ICSD number is 61634. Compared with the x = 0 sample, the (200) peak of the samples doped with BMN is obviously shifted, which is due to the lattice distortion caused by the introduction of atoms with different ionic radii at both A and B positions. Figure 1b shows the surface microstructure of the sample with x = 0.050, and the grains are evenly distributed and closely arranged. The smaller grains increase the number of grain boundaries, which will increase the channel for carrier transport, helping to improve photovoltaic performance. The illustration in Figure 1b shows the morphology of the sample cross-section. The boundary between the dielectric layer and the substrate is obvious; the dielectric layer is uniform and continuous, and the thickness is about 200 nm. The other components also exhibit dense microstructure and smaller grains, as shown in Figure S1.

Figure 2a illustrates P-E loops measured for the BNBT-xBMN thin films under a critical electric field at a frequency of 10 Hz. Obviously, the x = 0 sample is broken down at a lower electric field and exhibits a large P_r and coercive field (E_c), resulting in a fatter ring of the P-E loops. With the increase in BMN doping, the breakdown electric field of the film sample obviously increases, the P_r decreases, and the shape of the curve gradually becomes thin. But, the introduction of BMN also caused a decrease in the P_max. The W_rec and η corresponding to Figure 2a were calculated, and their change curves were shown in Figure 2b. It can be seen that with the increase in BMN doping, W_rec increases first and then decreases, while η keeps increasing. Finally, it showed the best energy storage performance in the x = 0.050 sample. The P-E loops of x = 0.050 sample measured under different electric fields are shown in Figure 2c. Its maximum electric field is up to 2500 kV/cm, and due to relaxation properties, the curve always remains thin as the electric field changes. By calculation (Figure 2d), W_rec keeps increasing as the electric field increases, and η is the opposite. The W_rec and η of the x = 0.050 sample changed from 6.1 J/cm³ and 88.7% at 625 kV/cm to 73.1 J/cm³ and 86.2 % at 2500 kV/cm.

The temperature-dependent ε_r and tanδ of the BNBT-xBMN thin films at 10 kHz are depicted in Figure 3. It can be seen that the ε_r of the x = 0 samples increases sharply with the increase in temperature in the lower temperature region and shows an obvious inflection point near 150 °C, and the amplitude of change slows down. With the increase in BMN content, the magnitude of ε_r changes gradually decreases, and the inflection point around 150 °C becomes increasingly blurred. This suggests that the introduction of BMN leads to a decrease in polar coupling and an increase in relaxation behavior [25]. The dielectric loss also shows a similar phenomenon with the change in temperature. With the increase in BMN, the change of tanδ slows down, and the dielectric loss decreases overall.

Figure 4a,d and Figure S2a,d display atomic force microscope (AFM) images of BNBT-xBMN films. The arithmetic average roughness (R_a) of each component film was calculated using NanoScope Analysis software (NanoScope Analysis 2.0), resulting in values of 4.29 nm, 4.48 nm, 4.11 nm, and 4.46 nm. Notably, the BMN-doped film with x = 0.050 exhibited the smallest R_a value, suggesting an optimal solid solution ratio between BNBT and BMN. This particular film sample demonstrated superior grain growth and surface flatness. On the other hand, Figure 4b,e and Figure S2b,e show out-of-plane pressure power microscopy (PFM) images of BNBT-xBMN thin films, with different colors indicating various phase directions [26,27]. The x = 0 sample (Figure 4b) exhibited a distinct electric domain structure, indicative of a long-range ordered ferroelectric state. As the BMN content increased, the domain size reduced, and the number of domains multiplied, leading to the formation of polar nanoregions (PNRs). At x = 0.050 (Figure 4e), no continuous large-sized ferroelectric domains were observed, emphasizing the presence of PNRs as a key structural characteristic of relaxation ferroelectrics. The evolution of the domain structure clearly illustrates that the addition of BMN induces a transition from a ferroelectric to a relaxation state. The addition of BMN causes local lattice distortion and fluctuation in charge distribution, leading to the development of a random electric field internally. This electric field can either pin or distort domain walls, facilitating the formation of PNRs. With an increase in BNZ content, the local electric field strength rises, resulting in the creation of more PNRs and ultimately enhancing the relaxation of the system. To better understand the behavior of ferroelectric domains and PNRs, the phase and amplitude of the piezoelectric response were measured in selected areas of all samples as a function of applied DC voltage, as illustrated in Figure 4c,f and Figure S2c,f. The phase-voltage loop shows that at x = 0, the loop appears nearly square with evident electrical hysteresis. As BMN content increases, the square shape gradually diminishes, transforming into an approximate single line shape at x = 0.050, indicating the gradual formation of the PNRs region and a decrease in hysteresis. This suggests that PNRs are more easily influenced by external electric fields compared to traditional ferroelectric domains. Furthermore, the amplitude–voltage diagram at x = 0 displays a classic butterfly curve, demonstrating clear ferroelectric behavior. As BMN content increases, the butterfly curve becomes narrower, indicating heightened relaxation behavior [28]. Overall, the introduction of BMN leads to the emergence of PNRs, triggering the transition from a ferroelectric state to a relaxed state.

In addition to the relaxation characteristics, the BDS is another important factor affecting the energy storage performance of the dielectric [29]. The leakage current density of a material is closely related to the strength of the BDS, the current densities for BNBT-xBMN thick films are plotted against the applied electric field (J-E) and shown in Figure 5a. Clearly, with the increase in BMN, the leakage current density gradually decreases. When x = 0.075, the leakage current density is the smallest, which corresponds to the changing law of the BDS in Figure 2a. In order to further explore its internal mechanism, the logarithmic plots of J as a function of E for BBT-xBMN films were drawn, and linear fitting was carried out, as shown in Figure 5b. It can be seen that the slope of the x = 0 sample after fitting is always the same, while the thin film after BMN doping has two different fitting results in the lower (below 100 kV/cm) and higher (over 100 kV/cm) electric fields, respectively. At lower electric fields, the slope is approximately 1. At higher electric fields, the slope decreases from 2.16 to 1.39 and gradually approaches 1. Obviously, the curves in the figure can be explained by the space-charge-limited current (SCLC) model [30]. When the slope is near 1, it is Ohmic conduction, while when the slope is near 2, the modified Child’s law conduction is followed [31]. It can be concluded that when the conductivity mechanism in the film is modified by Child’s law conduction, the leakage current density is large, and the film is easy to break down. With the increase in BMN content, the internal conductance model changes to Ohmic conduction, and the conductance mechanism dominated by Ohmic conduction helps to reduce the leakage current density of the film, thereby increasing the breakdown electric field strength.

Temperature reliability is an important index to measure the practical application characteristics of electronic devices [28,32]. Figure 6a plots the unipolar P-E loops of the x = 0.050 film measured at various temperatures under 1000 kV/cm at 10 Hz. As the temperature was elevated from room temperature (~25 °C) to 200 °C, P_max increased gradually, but P-E loops widened slightly. This is due to temperature-induced leakage loss or ionic conduction. As can be seen from Figure 6b by calculating the corresponding energy storage performance, there is a slight decrease in W_rec from 23.6 J/cm³ to 22.4 J/cm³ and a significant reduction in η from 87.4% to 81.8%. Although η attenuates in the high-temperature environment, the relatively stable W_rec can still ensure the normal operation of the device at a high temperature. This proves that the x = 0.050 sample has excellent temperature reliability.

To investigate the photovoltaic (PV) properties of the x = 0.050 sample, the electric current density–voltage (J-V) characteristics were tested in the unpolarized state and polarized state (±80 V). As can be seen from Figure 7a, the as-grown sample did not show obvious photovoltaic properties under illumination. After poling with a voltage of +80 V, a photocurrent density of 1.46 μA/cm² was obtained. After poling with a voltage of −80 V, the direction of the photocurrent reverses, exhibiting the switching effect of ferroelectric photovoltaic. Figure 7b shows that the x = 0.050 sample has extremely high current repeatability and stability under light switching. In order to explore the sensibility of photoresponse in the x = 0.050 sample, the short-circuit current (J_sc) under different light intensities was tested. As shown in Figure 7c, the film has obvious switching effects under different light intensities and has good stability. Under weak light intensity (20 mW/cm²), a photocurrent density of 0.39 μA/cm² was still obtained. The temporal stability and repeatability of J_sc are also important indicators for measuring photodetection devices. As shown in Figure 7d, the stability and repeatability of J_sc within 2000 s under 100 mW/cm² illumination conditions were tested. It can be seen that during the entire test time range, the value of the photocurrent density only decreases slightly, indicating that the photodetector has very stable photoelectric output characteristics.

4. Conclusions

In summary, novel, lead-free (1-x)Bi_0.45Na_0.45Ba_0.1TiO₃-xBi(Mg_1/3Nb_2/3)O₃ thin films were successfully deposited on a Pt/Si substrate by the sol-gel method. The excellent energy storage density of 73.1 J/cm³ and an efficiency of 86.2% were obtained at x = 0.05, especially since the ultra-high energy storage efficiency exceeds that of most of the thin film energy storage capacitors. The results show that the introduction of BMN reduces the domain size and changes the conduction mode, which improves the relaxation characteristics and breakdown strength. In addition, BNBT-0.05BMN exhibits excellent thermal stability over a wide temperature range of 25 °C to 200 °C. The film is also suitable for photovoltaic detection. In addition, the film shows excellent potential for photodetection. This study demonstrates the potential application of BNBT-xBMN films in the field of hybrid electronic devices and provides ideas for the development of advanced multifunctional materials.