Optical properties of inorganic powder EL device with a multifunctional dielectric layer

UC phosphors were fabricated by the solid state reaction method, with BaTiO₃ used as a host material. BaTiO₃, Yb₂O₃, and Er₂O₃ powders (Kojundo Chemical laboratory Co., Ltd.) were mixed together in an alumina crucible. The mixed powders were then heated for 4 h at temperatures of 1200 °C, 1300 °C, 1400 °C, and 1450 °C. After returning to room temperature, the powders were again heated for 4 h at the same temperatures. We compared the samples heated at 1400 °C. We determined that BaTiO₃: Yb³⁺, Er³⁺ was the ideal UC phosphor to be used as the dielectric layer of the inorganic powder EL device (Fig. 1).

Zoom In Zoom Out Reset image size

First, a ZnS-based phosphor (Sylvania GG45) and high-dielectric polymer (Shin-Etsu Chemical CR-S, CR-V) were mixed together to produce a liquid. The liquid was then coated on a glass substrate with an ITO electrode using the spin coating method. The coating was heated and dried to form the inorganic powder EL device phosphor layer on a glass substrate.

Then, a mixture of BaTiO₃: Yb³⁺, Er³⁺ and high-dielectric polymer was deposited onto the phosphor layer with the spin coating method. Finally, an Al electrode was deposited onto the dielectric layer using the resistance heating evaporation method. In descending order, the device structure consists of the Al electrode, the BaTiO₃: Yb³⁺, Er³⁺ dielectric layer, the phosphor layer, the ITO electrode, and the glass substrate.

First, we analyzed the photoluminescence (PL) spectrum and the crystal structure of BaTiO₃: Yb³⁺, Er³⁺ using an X-ray diffraction (XRD). An NIR laser beam was focused on the glass substrate side of the inorganic powder EL device while applying 1.8 kHz of AC voltage. The PL spectrum, luminance, and chromaticity were measured during this process.

The PL spectrum of the UC phosphor (BaTiO₃: Yb³⁺, Er³⁺) at various temperatures is shown in Fig. 2. A 200 mW, 900 nm NIR laser was used in the experiments. The sample heated at 1200 °C showed slight luminescence around 650 nm. However, the samples heated at 1300 °C and higher showed higher luminescence near 550 nm (⁴H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2) and 650 nm (⁴F_9/2 → ⁴I_15/2). The sample heated at 1450 °C showed the highest intensity of luminescence.

Zoom In Zoom Out Reset image size

XRD measurement results of BaTiO₃: Yb³⁺, Er³⁺ at various temperatures are shown in Fig. 3. The lines in Fig. 3 show the temperatures of 1200 °C, 1300 °C, 1400 °C, and 1450 °C. The results were compared with the powder diffraction data file to analyze the compound and crystal structures. The standard patterns are shown at the bottom of Fig. 3 for BaTiO₃ (01-070-9164), Er₂Ti₂O₇ (01-073-1647), Yb₂Ti₂O₇ (01-074-9560), Er₂O₃ (00-008-0050), and Yb₂O₃ (01-077-0455). Since the peak positions of Er₂Ti₂O₇ and Yb₂Ti₂O₇ are so similar, it is difficult to determine whether the compound is Er₂Ti₂O₇ or Yb₂Ti₂O₇. Therefore, we use the name RE₂Ti₂O₇ as a generic term for Er₂Ti₂O₇ and Yb₂Ti₂O₇.

Zoom In Zoom Out Reset image size

As shown in Fig. 2, a highly luminescent green light is emitted from the UC phosphor (BaTiO₃: Yb³⁺, Er³⁺) heated once at 1450 °C. However, it is not practical to heat the UC phosphor at this temperature, as it may break the alumina crucible. Therefore, in our experiment, we focus more on the UC phosphor heated at 1450 °C, which has a relatively high emission intensity when heated once. In order to increase its emission intensity even further, we heated the UC phosphor once again at 1400 °C for 4 h.

According to our analysis of the samples that were heated once, Re₂Ti₂O₇ is only present in the sample heated at 1450 °C. BaTiO₃ is found in every sample, whereas Er₂O₇ and Yb₂O₃ are found in every sample except for the 1450 °C sample. RE₂Ti₂O₇ is also produced at 1450 °C and higher.

The PL spectrum of BaTiO₃: Yb³⁺, Er³⁺ heated twice is shown in Fig. 4. The emission intensity is low when the sample is heated once, and the integrated intensities of 550 nm (⁴H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2) and 650 nm (⁴F_9/2 → ⁴I_15/2) are similar. Since green and red components are present in the spectrum, the sample emitted a yellowish green light. On the other hand, the emission intensity of the sample heated twice increased the most at 550 nm and emitted a clear green light. Therefore, as the alumina crucible is less likely to break, it is most effective to heat the sample twice at 1400 °C to produce a clear green light. Figure 5 shows the XRD measurement results of BaTiO₃: Yb³⁺, Er³⁺ heated twice. BaTiO₃ peaks were observed both times the sample was heated, which means that BaTiO₃ exists regardless of the number of times the sample is heated.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Furthermore, slight peaks of RE₂Ti₂O₇ were observed in the sample heated twice. As previously stated, among the samples heated once, RE₂Ti₂O₇ is only found in the 1450 °C sample. However, when heated twice, RE₂Ti₂O₇ is also found in the 1400 °C sample.

The luminances of the inorganic powder EL device, which used BaTiO₃: Yb³⁺, Er³⁺ as the dielectric layer, are shown in Figs. 6(a) and 6(b). Figure 6(a) shows the results of an inorganic powder EL device with the UC phosphor heated once at 1450 °C, and Fig. 6(b) shows the results of an inorganic powder EL device with the UC phosphor heated twice at 1400 °C. In Fig. 6(a), the luminance was 323 cd m⁻² when 150 root mean square voltages (V_RMS), with a frequency of 1.8 kHz, was applied. However, the luminance rose to 356 cd m⁻² when the same amount of voltage was applied while simultaneously performing infrared irradiation.

Zoom In Zoom Out Reset image size

In Fig. 6(b), the luminance was 318 cd m⁻² when 150 V_RMS, with a frequency of 1.8 kHz, was applied. However, the luminance rose to 409 cd m⁻² when the same amount of voltage was applied while simultaneously performing infrared irradiation. In Fig. 6(a), the luminance increased by 33 cd m⁻² when infrared irradiation was performed, while the luminance increased by 91 cd m⁻² in Fig. 6(b). According to these results, the luminance of BaTiO₃: Yb³⁺, Er³⁺ increases about three times more when it is heated twice at 1400 °C compared to when it is heated once at 1450 °C. The emission spectrum of the inorganic powder EL device, which used the twice-heated (1400 °C) UC phosphor of BaTiO₃: Yb³⁺, Er³⁺ as the dielectric layer, is shown in Fig. 7. These results depend on the PL emission intensity of BaTiO₃: Yb³⁺, Er³⁺ used as the dielectric layer.

Zoom In Zoom Out Reset image size

The emission spectrums of the inorganic powder EL device, which used the twice-heated (1400 °C) UC phosphor of BaTiO₃: Yb³⁺, Er³⁺ as the dielectric layer, are shown in Fig. 7.

As shown in Fig. 7(a), the AC voltage changed while the inorganic powder EL device was being irradiated with an NIR laser. Each line in Fig. 7(a) shows the experimental results of luminescence when 0 V_RMS, 50 V_RMS, 100 V_RMS, and 150 V_RMS were applied to the inorganic powder EL device.

Each line in Fig. 7(a) shows the experimental results of 150 V_RMS, 100 V_RMS, 50 V_RMS, 0 V_RMS. When the AC voltage is 0 V_RMS, only the emission spectrum of the UC phosphor can be measured. Additionally, when the AC voltage was increased, the emission spectrum of the inorganic powder EL devices phosphor layer increased. The peak near 510 nm shows the peak of the ZnS-based phosphor, which is used as the phosphor layer for the inorganic powder EL device.

Figure 7(b) shows the results of results of luminescence when the inorganic powder EL device is irradiated with an NIR laser, but without applying AC voltage. Figure 7(b) shows the same line of 0 V_RMS in Fig. 7(a), but the scale of the vertical axis is increased five times. The results of Fig. 7(b) are almost identical to those of Fig. 4, however the measurement conditions are different. In Fig. 4, the simple substance of BaTiO₃: Yb³⁺, Er³⁺ was directly irradiated with NIR laser. On the other hand, In Fig. 7(b), BaTiO₃: Yb³⁺, Er³⁺ was irradiated with NIR laser through the glass substrate, ITO transparent electrode, and phosphor layer, as shown in Fig. 1. Since the ZnS-based phosphors in the phosphor layer do not emit light by NIR excitation, only the peaks were detected from BaTiO₃: Yb³⁺, Er³⁺ in the dielectric layer.

The chromaticity of the inorganic powder EL device, which uses the UC phosphor of BaTiO₃ co-doped with Yb³⁺ and Er³⁺ heated twice at 1400 °C as the dielectric layer. Figure 8(a) shows the results of when only AC voltage was applied to the inorganic powder EL device. Figure 8(b) shows the results of when the AC voltage was applied while simultaneously irradiating the inorganic powder EL device with an NIR laser. In Fig. 8(a), no voltage dependency was observed, and the chromaticity was X = 0.19, Y = 0.42 when the applied AC voltage was both 100 V_RMS and 150 V_RMS. The chromaticity only slightly changed to X = 0.19, Y = 0.42 when the applied voltage was 50 V_RMS, and X = 0.19, Y = 0.41 when the voltage was 100 V_RMS and 150 V_RMS in Fig. 8(a).

Zoom In Zoom Out Reset image size

In Fig. 8(b), the chromaticity was X = 0.30, Y = 0.68 when the applied AC voltage was 0 V_RMS. But the chromaticity changed when the AC voltage was increased, as exemplified by the chromaticity of X = 0.22, Y = 0.48 when the applied AC voltage was 150 V_RMS. The chromaticity changed with an increase of AC voltage due to the emission intensity of the ZnS-based phosphor increasing. We concluded that the chromaticity changed mainly due to the increase of emission intensity of the ZnS-based phosphor. From the results of Figs. 6, 7, and 8, BaTiO₃: Yb³⁺, Er³⁺ can effectively be used as the dielectric layer of the inorganic powder EL device. The possibilities of practical uses have thus been expanded as we could effectively increase luminance and change the chromaticity of the inorganic powder EL device.