With the advent of foldable mobile phones, increasing focus has been placed on eco-friendly, high-definition, and energy-saving displays. Displays are primarily used in mobile phones, televisions, and computer monitors and mainly employ liquid crystal displays, quantum dot light-emitting diodes, or organic light-emitting diodes. Furthermore, the demand for large-area displays as a light source for outdoor advertising is increasing. There is also a growing need to use light sources that resemble the geometrical shapes of various products in advertising. A light-emitting device meeting these requirements is typically manufactured using zinc sulfide (ZnS). ZnS belongs to a group of II-V semiconductor materials, that has different bandgaps depending on the growth structure, which fall within the range of 3.5–3.7 eV[1]. In other words, ZnS has significantly wider bandgaps than that of visible light. Therefore, a pure ZnS structure cannot be used as an electronic material for emitting visible light. However, visible light is emitted when an electric field is applied to a doped ZnS phosphor powder, as first observed by Destriau in the mid-1930s. The luminescence resulting from this mechanism is called electroluminescence (EL).

In recent years, EL devices have been used for advertisement lighting that covers large areas that require bright and uniform light emissions. Power supply methods for ZnS light emission can be classified as DC or AC supply methods. However, the alternating current electroluminescence (ACEL) method is more energy-efficient and is preferred to the direct current electroluminescence method[2, 3, 4, 5]. Eco-friendly and highly efficient ACEL displays are in high demand based on the employed ZnS composite, which emits brighter light and allows the color to be controlled.

A study was conducted to control the quantity of dopant material or particle size of ZnS phosphor to develop a highly efficient ACEL device. Additionally, studies have investigated the application of high-voltage electric fields to phosphors, involving the improvement of the electrode shape. Phosphors composed of nanoscale structures were utilized, and these small phosphors exhibited quantum confinement, passivation effects, and high-luminescence properties[6]. Then, a tetrapod-type ZnO whisker was added to a carbon nanotube (CNT) electrode. The combination of ZnS phosphor and polydimethylsiloxane (PDMS) significantly improved the EL emission intensity[7]. In another study, CNTs were mixed with conventional ZnS phosphors; as the surface area between the electrode and phosphor increased, more carrier injections occurred owing to the local electric-field enhancement. Consequently, the increase in the local electric field enhanced EL intensity[8].

In this study, we fabricated EL devices using ZnS:(Cu, Al, Mn) and ZnS:(Cu, Cl) composites acquired from Global Tungsten & Powder. We used commercially available indium tin oxide (ITO) glass, utilized in electrodes in voltage application, as the upper and lower substrates. We washed the ITO glass with acetone, methane, and isopropyl alcohol solution for 10 min. Subsequently, we washed the ITO glass with distilled water and then sprayed N₂ on the ITO electrode to remove any moisture remaining on the substrate. Thereafter, we mixed the ZnS:(Cu, Al, Mn) and ZnS:(Cu, Cl) phosphor mixtures uniformly as per the proportions shown in Table 1 and combined the mixed phosphor and PDMS following a weight ratio of 7:3. The mixture of PDMS and phosphors contained air bubbles from the atmosphere formed during preparation. These bubbles can cause device defects, necessitating their removal from the EL device. After placing the phosphor and PDMS mixture in a desiccator, the air bubbles were removed from the EL device by maintaining several mTorr. We then applied the PDMS and phosphor mixture onto the lower ITO glass via the spin-coating method. After placing the upper ITO glass on the PDMS and phosphor mixture, we applied a constant pressure to maintain uniform thickness. Finally, we placed the device on a hot plate and baked the device at 100 °C for 30 min to complete the fabrication.

Table 1

Phosphor weight of the samples. Phosphor of Sample A comprises ZnS:(Cu, Al, Mn) composite alone. However, ZnS:(Cu, Cl) composites comprise 25% of the total ZnS:(Cu, Al, Mn) weight in Sample B.

Composite	Sample A	Sample B
ZnS:(Cu, Al, Mn) (%)	100	75
ZnS:(Cu, Cl) (%)	0	25

Figure 1 shows the cross-sectional view of the fabricated EL device. AC bias was supplied by an external power source using the upper and lower ITO electrodes. Figure 2 shows scanning electron microscopy (SEM) images of the ZnS phosphors mixed with PDMS. The ZnS was less than 30 microns in size, and most of the ZnS was filled with PDMS. Figure 3 depicts the energy dispersive spectrometry (EDS) measurement of the ZnS composite of Sample A, circled in the figure. The EDS analysis shows the main peaks of Zn and S; Al and Mn components, used as dopants, were also observed. However, Cu peaks were not observed, contrary to our expectations. We believe that this was caused by maintaining a relatively low doping state, compared to other components. In addition, we observed C, O, and Si components. These elements are considered to be the residues of the PDMS constituents distributed on the phosphor surfaces.

Figure 1. (Color online) Schematic of light emission from the mixed ZnS composite and PDMS with applied AC bias.

Figure 2. (Color online) SEM image of mixed ZnS: composite and PDMS.

Figure 3. (Color online) EDS measurement of ZnS composite of Sample A.

In this study, an AC source (APS-7050, GW Instek Co.) was used to emit light in the EL device, which was capable of supplying a specific applied voltage and frequency based on the experimental conditions. We received light emitted from the EL device at the same distance using an optical fiber. We analyzed the light transmitted by the optical fiber using a spectrometer (TM-UV/VIS C10082CA) to measure the intensity of light according to the wavelength.

Figure 4(a) depicts the measured intensity according to the wavelength when a frequency of 100–500 Hz is applied to the device at the same applied bias (200 V_rms) at room temperature. Spectra with maximum intensity values of approximately 502 and 582 nm were observed when a voltage of 200 V_rms and a frequency of 100 Hz were applied. As can be seen, as the frequency increases, the maximum value of intensity shifts towards the blue wavelength. However, the second maximum value measured at 582 nm only increased the intensity value at the same wavelength without being affected by the applied frequency. Figure 4(b) shows the deconvolution performed using Origin when a voltage of 200 V_rms and a frequency of 300 Hz were applied. The first decomposition peak near 446 nm (2.78 eV) could be attributed to the donor-acceptor emission (V_s → CuZn - Cu_i) owing to the trap state emission caused by native zinc vacancy[9, 10]. The second peak showing the maximum de-convoluted component near approximately 485 nm (2.56 eV) could be attributed to the transition between the conduction band of ZnS and the t₂ level of excited Cu²⁺ located in the ZnS band gap[11]. The third peak of the de-convoluted spectrum at 522 nm (2.37 eV) occurred possibly owing to a donor-acceptor recombination process from Al in the Zn site to Cu in the Zn site[9]. Finally, the fourth de-convoluted peak near 582 nm (2.13 eV) was associated with the radiation transition of electrons from ⁴T₁ to ⁶A₁ of Mn²⁺ ions[11]. Figure 4(c) displays the results of deconvolution performed using Origin for the measurement when a voltage of 200 V_rms and a frequency of 500 Hz were applied. The peaks at 582 nm indicate the emission characteristics at the same wavelength (i.e., at 582 nm), irrespective of the increase in frequency. On the contrary, we observed that at the wavelength band below 500 nm, the emission characteristics of the short wavelength acted as the dominant mechanism as the frequency increased. These emission peak shifts to the short wavelength (i.e., higher energy) could be attributed to the increase in excitation energy with input frequency based on the donor-acceptor (D-A) pair emission theory[12, 13].

Figure 4. (Color online) (a) Measured intensity according to the wavelength when a frequency within the range of 100–500 Hz is applied. (b) and (c) Deconvolution graph for different values of applied voltage and frequency.

We observed the spectrum for Sample B for a voltage of 200 V_rms and a frequency of 100–500 Hz, as shown in Fig. 5(a). In this sample, the intermixed phosphor composites, ZnS:(Cu, Cl), contained 25% of the total weight of the phosphor. The activated wavelength was determined by varying the concentration of the activator dopant (Cu) and by applying a treatment method of ZnS after the phosphor formation[11]. The spectral characteristics comprised a main peak and a sub-peak. At 100 Hz, the sub-peaks were indistinguishable, but at 500 Hz, the peaks were entirely distinct. In addition, the spectrum of the blue light band became more active as the frequency increased. The full width at half maximum was measured to be approximately 108 nm. As the frequency increased by five times, the corresponding maximum intensity of the spectrum increased by approximately 3.4 times. Figure 5(b) shows the deconvolution result of Fig. 5(a) at 200 V_rms and 500 Hz. The center wavelength band of Peak C of the de-convoluted graph corresponded to approximately 500 nm (2.48 eV). This band corresponded to the green emission in the ZnS:(Cu, Cl) doped sample, and it was the luminescence property expressed by the recombination process from the shallow donor level to the t₂ level of the Cu²⁺ ions below the conduction band.

Figure 6(a) shows the peak intensity recorded while increasing the input bias from 100 to 300 V_rms for Samples A and B with a constant input frequency of 400 Hz. The peak intensity of Sample B was 1.7 times higher than that of Sample A at 300 V_rms and 400 Hz. Figure 6(b) displays the peak intensity values for Samples A and B, which depend on the applied frequency under the same bias (200 V_rms). The peak intensity of Sample B was also 1.7 times higher than that of Sample A at 200 V_rms and 500 Hz. Therefore, when the ZnS:(Cu, Cl) composite is added to the ZnS:(Cu, Al, Mn) composite, the luminous efficiency of the EL device increases.

Figure 6. (Color online) Peak intensity variation for Samples A and B. (a) corresponds to increased input bias from 100 to 300 V_rms. (b) corresponds to a different applied frequency from 100 to 500 Hz.

Figure 7(a) represents the International Commission on Illumination (CIE) color coordinates of the emission spectrum when the applied bias increases from 100 to 300 V_rms under the same input frequency (400 Hz) for the two samples. The change in the CIE color coordinates varied within 0.01 for both the x and y values. Therefore, these samples were considered to have a relatively small color change with the increase in input bias. Figure 7(b) shows the CIE color coordinates of the emission spectrum when the input frequency increases from 100 to 500 Hz under the same applied bias (200 V_rms) for Samples A and B. The CIE color coordinates of Samples A and B obtained at 100 Hz were (0.30,0.40) and (0.24,0.42), respectively. At 500 Hz, these values changed to (0.28,0.31) and (0.23,0.32), respectively. The x values of the CIE color coordinates decreased by 0.02 and 0.01 for Samples A and B, respectively, whereas the y values decreased by 0.09 and 0.10, respectively. Therefore, when the ZnS:(Cu, Cl) composite is used instead of the ZnS:(Cu, Al, Mn) composite alone, the color can be easily changed while achieving brighter emitted light, depending on the degree of increase in the frequency.

Figure 7. (Color online) CIE color coordinate of the emission spectrum for the two samples. (a) Applied bias change from 100 to 300 V_rms. (b) Input frequency variation from 100 to 500 Hz under the same applied bias (200 V_rms) for Samples A and B.

We fabricated two types of EL devices: a composite of ZnS:(Cu, Al, Mn) alone and a ZnS:(Cu, Al, Mn) composite mixed with a ZnS:(Cu, Cl) composite amounting to 25% of the total weight of the composites. After the ZnS phosphor was mixed with PDMS, the mixture was spin-coated on the lower electrode. Thereafter, the EL device was fabricated by covering the upper electrode using ITO glass. The change in CIE color coordinates varied within the range of 0.01 for both x and y values when the applied bias was increased from 100 to 300 V_rms under a constant input frequency of 400 Hz for the two samples. Consequently, these two samples showed only a slight change in color owing to the change in input bias. However, the CIE coordinates of Samples A and B at 100 Hz were (0.30,0.40) and (0.24,0.42), respectively, which changed to (0.28,0.31) and (0.23,0.32), respectively, at 500 Hz. Therefore, the ZnS:(Cu, Al, Mn) EL device mixed with the ZnS:(Cu, Cl) composite can be applied as an optical illumination source for large display areas, which require an active color change based on the change in input frequency.

This research was partially supported by the Chonnam National University Research Program.