Optimising the electroluminescence performance of ZnS: Cu/epoxy composites via exploring the relationship between electroluminescence intensity and filler intrinsic attributes

Abstract

Electroluminescent composites have considerable potential for applications in photoelectric display, electric field measurement and other fields due to their advantages of simple preparation and uniform luminescence. However, high voltage required by the operation of electroluminescent composites hinders their applications in the electric domain, and research on the relationship between electroluminescence principle of composites and filler intrinsic attributes is insufficient. In this paper, the effects of electric field strength, filler mass fraction and voltage frequency on the electroluminescence intensity of ZnS:Cu/epoxy composites are studied, which are majorly used as the luminescent layer of electroluminescent devices. To enhance the electroluminescence intensity of composites, high‐temperature sintered BaTiO₃ and zinc oxide whiskers (ZnOw) particles are introduced in the base matrix. The finite element method is applied to verify that the matrix of high dielectric constant is in favour of improving the electroluminescence intensity of composites. The band structure of the ZnS:Cu particle is calculated based on the density functional theory + U method to analyse the intrinsic relationship between electroluminescence and particle electronic structure. This paper provides a basis for the research of high electroluminescence intensity devices and will be conducive to better understanding on the relationship between electroluminescence principle and filler intrinsic attributes.

INTRODUCTION

With the rapid development of the smart grid, advanced electric field measurement technology is increasingly required, which is one of the most significant segments to ensure the normal operation of power equipment [1, 2]. The amplitude and frequency composition of an electric field becomes more complex accompanied by the introduction of a large number of power electronic devices [3]. Traditional sensors are prone to resonance and need a carrier light source [4, 5], while the sensors based on the electroluminescence effect have the advantages of excellent electrical insulation performance and distributed measurement [6–8]. Electroluminescence refers to a solid luminescence phenomenon, in which composites convert the electric energy into optical energy [9, 10]. Electrons will be accelerated under the action of the electric field, causing the occurrence of collision excitation and ionization [11, 12], and photons are emitted when the electrons transition from the excited state back to the ground state [13–15].

Due to the advantages of uniform luminescence and high efficiency [16–18], research on the preparation of electroluminescence devices has received extensive attention. Jun S [19] et al. used ZnS:Cu/PVB as a luminescent layer and prepared stretchable electroluminescent devices with excellent mechanical properties. However, considering the high voltage required to achieve high electroluminescence intensity, the practical engineering application of electroluminescent devices is limited, and problems such as energy waste will arise [20–22]. Improving the electroluminescence intensity of luminescent layer composites is the key to enhance the performance of electroluminescent devices [23–25]. Jun S Y [26] et al. introduced short carbon nanotubes treated with chlorobenzene to improve the luminance of devices. Gupta [27] et al. introduced rGO particles and found that the luminance of devices with 0.5 wt% rGO particles was the strongest.

It can be visualised that the electroluminescence characteristics of ZnS:Cu particles are related to the quantum process of electron transition [28]. Therefore, it is necessary to study the electroluminescence properties of ZnS:Cu particles from the microscopic point of view, which is inadequate in this area [29]. Density functional theory (DFT), as a powerful tool, is widely used to calculate the band structure of ZnS:Cu particles [30, 31]. Traditional DFT methods including local density approximation and generalised gradient approximation (GGA) cannot describe the electronic properties precisely, leading to the serious underestimation of ZnS:Cu bandgap [32]. By introducing a specific Hubbard parameter U, DFT + U method can accurately correct the delocalisation of d electrons (especially d electrons in transition metals) [33], which has attracted extensive attention. Sharma M et al. [34] calculated the band structure of ZnS particles based on the DFT + U method, and the calculated results are consistent with the theory.

Despite the achievements having been made so far, few studies concern about the relationship between the electroluminescence mechanism and filler intrinsic attributes, and the research on how to improve the electroluminescence intensity of composites is also insufficient. In this paper, we adopt a simple solution-processing method to prepare ZnS:Cu/epoxy composites, which is majorly applied to the luminescent layer of the devices. The effects of electric field strength, filler mass fraction and frequency on the electroluminescence intensity of composites have been studied. High temperature-sintered barium titanate (BaTiO₃) and Zinc oxide whisker (ZnOw) particles have been introduced to improve the dielectric constant of the matrix [35–37], and consequently improve the electroluminescence intensity of composites. The finite element method is used to prove the positive effect of the increased dielectric constant of matrix on the enhanced electroluminescence intensity. The density functional theory + U method is adopted to analyse the electroluminescence characteristics of ZnS:Cu/epoxy composites from the microscopic point of view. The specific research framework is shown in Figure 1.

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MATERIALS AND METHODS

Materials and sample preparation

The raw materials used in the experiments are listed in Supporting Information. Composites with different filler mass fractions and different introduced ceramic fillers are prepared for the subsequent electroluminescence tests. The experimental flow chart is shown in Figure 2a.

• (1)

  The required epoxy resin is poured into a beaker with the curing agent added in a ratio of 100:85. The flask is heated in a vacuum oven at 60°C, so that the viscosity of epoxy resin can be slightly reduced and the fluidity of the matrix can be increased.

• (2)

  The fillers are weighed according to the required proportion and added to the epoxy resin mixture. The mixture is poured into a three-necked flask and placed in an oil bath with the temperature controlled at 60°C and the speed of 300 r/min. The mixture is stirred for 1 h. When the matrix and fillers are fully mixed, accelerant is added in a ratio of 100:1 and stirred for 10 min.

• (3)

  The mixed solution is poured into a dry beaker and put into a vacuum drying oven at 60°C for several vacuumising operations until there are no bubbles on the surface.

• (4)

  The mould sprayed with the release agent is put into the blast dryer to preheat, and the blast dryer is preheated at 100°C for 10 min. Then the vacuumised mixed solution is poured into the mould and dried in the oven again. The oven temperature is set to 100°C for 4 h and then raised to 150°C for 10 h to prepare the required composites.

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Experimental platform

The schematic and practical diagrams of the experimental platform are shown in Figure 2b–c. High voltage AC power, voltage probe and oscilloscope are used for electroluminescence tests. Spectral luminance metre is used to test the electroluminescence intensity of composites.

Microstructure and luminescence analysis

Scanning electron microscopy is used to observe the morphology of ZnS:Cu, BaTiO₃ and ZnOw particles, and the overall distribution of particles in ZnS:Cu/epoxy, ZnS:Cu/BaTiO₃/epoxy and ZnS:Cu/ZnOw/epoxy composites. As shown in Figure 3a–c, ZnS:Cu particles are irregularly granular and the particle size is on the order of micron grade. BaTiO₃ particles clump together and the particle size is much smaller than that of ZnS:Cu particles. ZnOw particles have distinct acicular structure. It can be seen from Figure 3d–f that the filler particles evenly distribute in the matrix, the interface bonding strength between particles and the matrix is excellent, and no obvious defects and voids are observed. The three small figures embedded in the upper right corners are ZnS, BaTiO₃ and ZnOw particles, respectively. BaTiO₃ with small particle size and ZnOw with needle shape help to fill the gaps between ZnS:Cu particles, thus forming a denser luminescent layer.

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The samples are all wafers with a diameter of 70 mm and thickness of 1.5 mm. In the dark situation, the voltage is applied after the luminance of samples is completely attenuated. After a period of pressurisation, it is found that the luminescence phenomenon first occurs near the electrode, and it is a distinct green light as shown in Figure 3g. The Commission Internationale de L'Eclairage (CIE) coordinate (X, Y) is used to quantitatively represent the luminescent colours [38, 39], where X represents the red component and Y represents the green component. As shown in Figure 3h, all the monochromatic light lies on the tongue curve, and the specific location of the colour light emitted by the sample is marked as (0.2413, 0.5915), which is within the range of green light. A fluorescence spectrophotometer is used for spectral tests as shown in Figure 3i. It is found that when excited by 375 nm light, the strongest emission peak of ZnS:Cu is located at 533 nm, which is in the range of green light band. A UV-Vis absorption spectrometer is used to test the UV-Vis diffuse reflection spectrum of ZnS:Cu particles. The data interval is set to 1.0 nm and the scan speed is set to medium speed.

Electric breakdown testing

In the room temperature, the breakdown strength tester HCDJC-50 kV is used for the electric breakdown test. We apply pressure at 2 kV/s until the sample is punctured. Ten samples are tested in each group for subsequent analysis.

RESULT AND DISCUSSION

Relationship between electroluminescence intensity and electric field strength and filler mass fraction

ZnS:Cu/epoxy composites with filler mass fraction of 55 wt%, 60 wt%, 65 wt%, 70 wt% and 75 wt% are prepared to study the relationship between electroluminescence intensity and electric field strength. Considering that the sample emits obvious green light under the action of the electric field. The proportion of green in the three primary colours (G) is taken as the characteristic value to represent the electroluminescence intensity of composites in this paper, which is specified as equation (1). 1 $$\text{Electroluminescence}\,\text{intensity}\,=\,\frac{{G}_{\text{sample}}}{{G}_{\mathrm{max}}}\,(\text{pu})$$ Where G_sample represents the proportion of green in the three primary colours of the composites, G_max represents the max green ratio, which is equal to 100. For the purpose of improving the accuracy and universality of the test results, four symmetrical positions are selected for each sample to test their electroluminescence intensity, respectively. Then, the average value of the four points is calculated and adopted as the final electroluminescence intensity of the sample.

In order to ensure the safety of experiments, the breakdown electric field strength of composites under the AC electric field is tested first. Weibull distribution [40, 41] is used to process the breakdown electric field strength data and shown in equation (2). 2 $$P(x)=1-\mathrm{exp}\left[-{\left(\frac{x}{{E}_{0}}\right)}^{\beta }\right]$$ where x is the independent variable, representing the breakdown electric field strength measured in the experiment; β is the shape parameter, representing the dispersion degree of data; and E₀ is the scale parameter, representing the breakdown electric field strength when the breakdown possibility is 63.2%, namely the average breakdown electric field strength.

The least square method and Ross failure probability distribution function are used to solve the correlation coefficient, and the correlation formula is as follows.

The Ross failure probability distribution function is shown in equation (3). 3 $$P(i,n)=\frac{i-0.44}{n+0.25}\times 100\%$$ where n represents the total number of test points, and i represents the ith test point.

The breakdown electric field strength data are arranged in an increasing order and the correlation coefficients are calculated by the least square method of linear fitting as shown in equation (4). 4 $$\left\{\begin{array}{c}{X}_{i}=\mathrm{lg}\hspace*{.5em}{E}_{i}\\ {Y}_{i}=\mathrm{lg}[-\mathrm{ln}(1-P(i,n))]\end{array}\right.$$

The experimental data processing results are shown in Figure 4a.

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It is found that the breakdown electric field strength of composites is all above 10 kV/mm. With the increase of filler mass fraction, the breakdown electric field strength of composites first increases and then decreases. When the mass fraction of ZnS:Cu particles reaches 65 wt%, the breakdown electric field strength of composites has a maximum value of 21.91 kV/mm. The minimum excitation electric field strength required for luminescence is also tested, and it is found that the initial excitation electric field strength of composites is no greater than 2 kV/mm. The comparison of the minimum excitation electric field strength required for luminescence and breakdown electric field strength is shown in Figure 4b. The breakdown electric field strength is much stronger than the minimum excitation electric field strength, which not only ensures safety, but also has practical application value. Thus, in the subsequent experiments, the pressurisation time is controlled for 10 min, and the electroluminescence intensity of composites with different filler mass fractions is tested under the electric field strength of 2, 3, 4, 5 and 6 kV/mm, respectively.

Figure 4c shows the relationship between electroluminescence intensity and filler mass fraction under different electric field strength at 50 Hz. It can be seen that with the increase of filler mass fraction, the electroluminescence intensity of composites shows an overall upward trend, and when the electric field strength reaches 3 kV/mm, the upward trend is more obvious. Especially, when the electric field strength goes up to 4 kV/mm, the electroluminescence intensity of composites with 75 wt% filler mass fraction is 147.4% than that of composites with 55 wt% filler mass fraction. This can be explained that with the increase of the filler mass fraction, the number of luminescence centres increases, and more electrons collide with the luminescent centres, thus enhancing the electroluminescence intensity of composites. The saturation is achieved when the filler concentration reaches 75 wt%, and the composite could not be poured with further addition of ZnS:Cu particles. The relationship between electroluminescence intensity and electric field strength with different filler mass fraction at 50 Hz is shown in Figure 4d. With the increase of electric field strength, the electroluminescence intensity of composites increases gradually. This is due to that with the increase of electric field strength, on the one hand, the electrons attain more energy, which is favourable for the occurrence of electron collision and ionization, thus doubling the number of electrons. On the other hand, more electrons collide with the luminescence centre, which is in favour of the excitation and de-excitation of electrons to produce photons, and ultimately improves the electroluminescence intensity. When the mass fraction of the filler is low, the electroluminescence intensity of composites increases more with the increase of electric field strength, especially for the composites with a filler mass fraction of 55 wt%. When the electric field strength increases from 2 kV/mm to 6 kV/mm, the electroluminescence intensity of composites with 55 wt% filler mass fraction increases by 125.1%, and the electroluminescence intensity of composites with filler mass fraction of 60 wt% and 65 wt% increases almost linearly with the increase of electric field strength.

Figure 4e shows the luminescence photographs of composites with different filler mass fractions (55 wt%, 60 wt%, 65 wt%, 70 wt% and 75 wt%) under the electric field strength of 5 kV/mm. Due to the randomness of electron migration path and filler distribution [42], the luminescence region is not always symmetrical. It can be found that with the increase of the filler mass fraction, the luminous range of composites becomes larger due to the increase of the probability of electron collision with luminescence centre at each position [43]. Figure 4f shows the luminescence photographs of 65 wt% ZnS:Cu/epoxy composites under different electric field strength (2, 3, 4, 5 and 6 kV/mm). When the applied electric field strength is 2 kV/mm, the luminance of composite is very low and only the circle near the electrode emits light. With the increase of applied electric field strength, the luminous range becomes larger and the emitted green light is more obvious.

In order to further study the relationship between electroluminescence characteristics and electronic structure of ZnS:Cu particles, DFT calculation is carried out based on the plane wave pseudopotential method to optimise the geometric configuration of ZnS:Cu particles [44, 45], and the band structure of ZnS:Cu particles under different electric field strength is compared.

In cubic ZnS, the Zn atom is located at (0, 0, 0) and S atom is located at (0.25, 0.25, 0.25), which has a face-centred cubic crystal structure [46]. Each Zn atom is surrounded by four S atoms, and each S atom is surrounded by four Zn atoms, which can exist stably in nature. Firstly, a 2 × 2 × 1 supercell is established on the basis of the ZnS protocell. The ZnS supercell contains 32 atoms (16 Zn atoms and 16 S atoms). When considering doping, the most likely doping model in ZnS is the substitution doping of the Cu atom instead of the Zn atom, both Cu atom and Zn atom vacancy at the gap position cannot exist stably [47, 48]. Therefore, the following calculation only considers the form of substitution doping in which a Cu atom replaces a Zn atom. The model is shown as Figure 5a.

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Geometric optimization should be carried out before calculation. The commutative correlation function is approximated by GGA's (Generalised gradient approximations) PBE (Perdew–Burke–Ernzerhof) functional, and the GGA + U method is applied to introduce a specific Hubbard potential to correct the problem of bandgap underestimation in the GGA method [34, 45]. The supersoft pseudopotential is used to describe the interaction between valence electrons and the real ion.

The band structure of ZnS:Cu particles without the electric field is calculated first as shown in Figure 5b. In practical studies, we are more concerned with the band distribution near the Fermi level, so the figures only show the electron energy distribution in the valence band and conduction band near the Fermi level. The calculated bandgap of ZnS:Cu particles is 2.831 eV; the lowest point of conduction band and the highest point of valence band are both at the same K-point (G), indicating the direct bandgap semiconductor material of ZnS:Cu. In order to further study the electronic structure of ZnS:Cu particles, density of states (DOS) of Zn, S and Cu atoms are calculated as shown in Figure 5c. It can be seen that the conduction band of 3–6 eV mainly comes from the contribution of Zn 4s and 3p states, and the upper valence band region located in the energy range of −6–0 eV mainly relates to the contribution of S 3p and Cu 3d states. The hybridisation of S 3p and Cu 3d broadens the valence band of ZnS:Cu and forms a p-d hybridisation peak, which corresponds to a shallow acceptor level. Electrons in the valence band can first jump to this impurity level, and then to the conduction band, which is beneficial to the absorption of visible light of ZnS. UV-Vis the diffuse reflection spectrum of ZnS:Cu particles which is tested to compare with the calculated value as shown in Figure 5d. The bandgap is determined by Tauc plot from the analysis of the absorption spectrum in Figure 5e [49] as shown in equation (5). 5 $$\alpha hv={\alpha }_{0}{\left(hv-{E}_{\mathrm{g}}\right)}^{n}$$ where hν represents the energy of incident photons, and E_g is the value of bandgap corresponding to the transitions. α₀ is a constant which depends on the transition probability. The obtained bandgap of ZnS:Cu is 2.809 eV, which is basically consistent with the tested value, verifying the reliability of our calculated model.

The bandgap of ZnS:Cu particles under different electric field strength is shown in Table 1.

TABLE 1 The calculated bandgap of ZnS:Cu particles under different electric field strengths

Electric field strength (kV/mm)	0	5	10	50	100
Bandgap of ZnS:Cu particle (eV)	2.831	2.829	2.816	2.764	2.682

ZnS:Cu particle has a narrower bandgap when an electric field is applied, which may be due to that the energy band has a tendency to expand and moves slightly in the direction of the Fermi level with the increase of electric field strength, leading to the decrease of bandgap, and when the electric field strength exceeds 10 kV/mm, this phenomenon is more obvious. The reduction of bandgap is beneficial to the transfer of electrons from occupied orbit to vacant orbit and the emission of photons, which contributes to the enhancement of composite electroluminescence intensity.

Research on improving the electroluminescence intensity of composites

The low luminance of composites impedes the practical implementation. To enhance the performance of ZnS:Cu/epoxy composites and expand the scope of practical engineering application, studies on how to improve the electroluminescence intensity of composites have been carried out. The dielectric constant of the matrix can be improved by introducing ceramic fillers, so that the electric field can be concentrated near the phosphor particles [8]. This contributes to the occurrence of collision excitation and ionization of electrons [50], and also the reduction of interface refractive index difference between the matrix and the phosphor particles, which makes it easier for the light to pass through the matrix, and consequently enhances the electroluminescence intensity.

The BaTiO₃ particle sintered at high temperature has the advantages of high dielectric constant and low price, which is widely used in the electric field [51, 52]. The ZnOw particle is a wide bandgap semiconductor with high exciton binding energy, which has great application potential in the field of optoelectronics [53]. According to the results in the previous sections, when the filler mass fraction is low, the electroluminescence intensity of composites cannot meet expectations, while as the mass fraction of fillers increases to 75 wt%, the agglomeration risk will increase, which damages the electrical and mechanical properties of composites. The filler mass fraction of 65 wt% reaches the optimal trade-off. The breakdown electric field strength is higher than that in other cases, and the electroluminescence intensity is also relatively high. Therefore, the mass fraction of ZnS:Cu fillers is controlled at 65 wt%, 1 wt% and 5 wt% BaTiO₃ and ZnOw particles are introduced, respectively, to study the effects of the introduction of ceramic fillers on the electroluminescence intensity of composites.

The breakdown electric field strength of composites is tested first, and the results are shown in Figure 6a. With the increase of the mass fraction of ceramic fillers, the breakdown electric field strength decreases gradually. Compared with BaTiO₃ introduced composites, the breakdown electric field strength decreases more obviously after the introduction of ZnOw particles, which may be related to the irregular shape of ZnOw particles [54]. Especially after the introduction of 5 wt% ZnOw particles, the breakdown electric field strength reduces to 7.72 kV/mm, which is 35.25% of the pure ZnS:Cu/epoxy composites. The low breakdown electric field strength of composites is not conducive to the subsequent research and practical applications. Therefore, only 1 wt% BaTiO₃, 5 wt% BaTiO₃ and 1 wt% ZnOw particles are selected to study the effects of their introduction on the electroluminescence intensity of composites, expressed as ZnS:Cu-BaTiO₃-1, ZnS:Cu-BaTiO₃-5 and ZnS:Cu-ZnOw-1, respectively.

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The results are shown in Figure 6b. Compared with pure ZnS:Cu/epoxy composites, the electroluminescence intensity of composites can be improved by introducing BaTiO₃ and ZnOw particles. The particle size of ZnS:Cu particles is about 20 times than that of BaTiO₃ particles. Therefore, BaTiO₃ particles can fill the gap between ZnS:Cu particles and form a denser luminescent layer. When the mass fraction of introduced BaTiO₃ particles is 1 wt%, the electroluminescence intensity of composites increases most obviously. The electroluminescence intensity of composites with 5 wt% BaTiO₃ particles is lower than that with 1 wt% BaTiO₃ particles, which may be due to that the high concentration of fillers result in the occurrence of agglomeration, and it is not conducive to the collision and excitation of electrons [6, 55]. Moreover, an increase in filling concentration may lead to an increase in defects and leakage current, which produces the blocking effects in the luminescence of composites.

For the purpose of further investigating the fundamental electroluminescence mechanism of ZnS:Cu based composites and the effects of the introduction of ceramic fillers on the luminance of composites, a two-dimensional model is established based on the SEM diagram, and the finite element method is used to calculate the potential and electric field distribution as shown in Figure 7a–b and Figure S1. The relative dielectric constant of the ZnS:Cu particles is set as 9.6 [37], and that of matrix as 3 [56]. The results are shown in Figure 7c–f.

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On account of the irregular shape of ZnS particles, there is a large amount of charge accumulation at the tip, so the electric field strength here is the largest, and the electric field strength inside the matrix is greater than that inside the ZnS particles. Figure 7d–f represent the electric field distribution of the matrix and the ZnS:Cu particles when the dielectric constant of the matrix equals to 3, 5 and 8, respectively. It can be found that with the increase of the dielectric constant of matrix, the electric field strength inside ZnS:Cu particles increases gradually (from dark blue to light blue), indicating that when the dielectric constant of matrix is higher, the electric field strength inside ZnS:Cu particles is higher, which is more conducive to the excitation and ionization of electrons and the improvement of electroluminescence intensity.

The average value of phosphor particle surface electric field strength is denoted as $${E}_{\mathrm{p}}$$, and the average value of matrix surface electric field strength is denoted as $${E}_{\mathrm{M}}$$. The simulation results are shown as Figure 7g. As the dielectric constant of the matrix increases, the electric field strength of the matrix decreases and that of the phosphor particles increases, the gap of electric field strength between the matrix and the phosphor particles becomes smaller and smaller. This also helps to solve the problem of excessively large local electric field strength, so as to improve the breakdown field strength. Moreover, the larger the $${\varepsilon }_{\mathrm{M}}$$ is, the more favourable it is for the electrons to collide with the luminescence centre and improve the luminescence intensity. The measured frequency-dependent dielectric constants for the four kinds of composites are shown in Figure 7h. The dielectric constant of composites increases obviously after adding ceramic fillers. The results verify that the highly polarized matrix by introducing ceramic fillers can concentrate the electric field on phosphor particles and boost the electroluminescence intensity of composites.

Relationship between electroluminescence intensity and voltage frequency

We select four kinds of composites to study the relationship between electroluminescence intensity and voltage frequency, which is 65 wt% ZnS:Cu, 65 wt% ZnS:Cu-BaTiO₃-1, 65 wt% ZnS:Cu-BaTiO₃-5 and 65 wt% ZnS:Cu-ZnOw-1 composite, respectively. Electroluminescence intensity of these composites are studied under 3 kV/mm and 5 kV/mm as the frequency changes from 50 to 450 Hz. The specific results are shown as Figure 8.

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Figure 8a shows the relationship between electroluminescence intensity and frequency under the electric field strength of 3 kV/mm. It is found that with the increase of frequency, the electroluminescence intensity of composites increases gradually, and the electroluminescence intensity of composites with the introduction of ceramic fillers is always greater than that of the pure ZnS:Cu/epoxy composites. It has the sharpest increase when the frequency increases from 150 to 250 Hz; this is because when the voltage frequency increases, the number of electron accelerations per unit time increases, resulting in an increase in the number of collisions, and eventually improves the electroluminescence intensity. When the frequency increases from 350 to 450 Hz, the amplitude of electroluminescence intensity enhancement is relatively small. This may be due to that electron energy plays a dominant role this time, the increase of frequency will shorten the acceleration time of electrons, and reduce the corresponding electron energy as shown in Figure 9. The electrons cannot jump to the conduction band to release photons with insufficient energy.

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Figure 8b shows the relationship between electroluminescence intensity of composites and voltage frequency under the electric field strength of 5 kV/mm. The variation law is consistent with the trend of composites under 3 kV/mm. The luminescence photographs of 65 wt% ZnS:Cu-BaTiO₃-1 composites at the frequency of 50 Hz, 150 Hz, 250 Hz, 350 and 450 Hz under the electric field strength of 3 kV/mm and 5 kV/mm are shown as Figure 8c–d. It is found that with the increase of frequency, the luminescence range expands and the luminescence phenomenon is more obvious, which is consistent with the above analysis.

CONCLUSION

In summary, we demonstrate a transparent light-emitting composite with electroluminescence property. The effects of electric field strength, filler mass fraction, voltage frequency and the introduction of ceramic fillers on the electroluminescence intensity of composites have been studied. the finite element method is used to discuss the effect of the dielectric constant of the matrix on the electric field distribution in the matrix and fillers, and then the electroluminescence intensity of composites. The density functional theory + U method is applied to analyse the intrinsic relationship between electroluminescence intensity and the microscopic structure of ZnS:Cu. This study provides the basis for optimising the electroluminescence characteristics by understanding the relationship between electroluminescence principle and filler attributes, and analyses the potential application of electroluminescent composites in the electric field measurement field as shown in Supporting information. The specific conclusions are as follows:

• (1)

  When the applied electric field strength is constant, the electroluminescence intensity of composites increases gradually with the increase of the filler mass fraction. This can be explained that with the increase of the filler mass fraction, more luminescent centres are introduced, which is favourable for electron transition to produce photons. When the filler mass fraction is constant, the electroluminescence intensity of composites continuously rises with the enhancement of applied electric field strength. This is due to the higher energy of electrons and the narrower bandgap of ZnS:Cu particles, which are conducive to the electron collision excitation and ionization. As a result, the electroluminescence intensity of composites is enhanced.

• (2)

  The introduction of BaTiO₃ and ZnOw particles is beneficial to the enhancement of electroluminescence intensity of composites. It is studied from two aspects, experiment and simulation that the dielectric constant of the matrix can be enhanced with the introduction of BaTiO₃ and ZnOw particles, leading to the concentration of the electric field near the phosphor particles, which contributes to the excitation and ionization of electrons, thus enhancing the electroluminescence intensity of composites. This positive effect is related to both the type and concentration of the introduced fillers, and it is most obvious with the introduction of 1 wt% BaTiO₃ particles.

• (3)

  The electroluminescence intensity of composites gradually increases with the increase of voltage frequency, and the luminous range has a trend of diffusion according to the photographs. The reasons are as follows: when the frequency of applied voltage rises, the number of electron accelerations per unit time adds, resulting in an increase in the number of electron collisions to excite the luminescence centre, thus improving the electroluminescence intensity. As the frequency constantly increases, the amplitude of electroluminescence intensity enhancement tends to decrease, which may be attributed to the inadequate acceleration time of electrons and the corresponding insufficient energy, which blocks the generation of photons.

ACKNOWLEDGEMENTS

This work was supported by National Natural Science Foundation of China (No.51877082), and Science and Technology Project of State Grid Corporation of China (No.7000-202158440A-0-0-00).

CONFLICT OF INTEREST

The author declares that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.

DATA AVAILABILITY STATEMENT

Data that support the findings of this study are available from the corresponding author upon reasonable request.

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