Optical and electrical properties of Zn2SiO4:Mn2+ powder electroluminescent device
Introduction
The powder electroluminescence (EL) device consists of a transparent top electrode on a substrate, a phosphor layer and a dielectric layer dispersed in an organic binder and a bottom electrode [1]. The EL phenomenon is attributed to phosphor excitation by hot electrons injected from the dielectric layer to the phosphor layer when a high electric field of order of magnitude of 106 V/cm is applied to the phosphor layer [2]. The most of ZnS-based phosphors are commercially available for powder EL: blue ZnS:Cu, Cl, green ZnS:Cu, Al, and orange ZnS:Cu, Mn [2]. Powder EL devices have non-glare uniform light emission, thin profile of about 200 µm and low power consumption of about 30 W/m2 for a luminance of about 100 cd/m2 [3]. Powder EL devices are used for liquid crystal display (LCD) backlight, and architectural and decorative lighting. One interesting EL application is the display panel for very harsh environments such as space modules and fighting vehicles due to high thermal stability over the temperature range of −50 - +100 °C [1], [3]. Other interesting EL modification is flexible powder EL device based on flexible substrates such as a film of polyethylene terephthalate (PET) coated with indium tin oxide (ITO) or silver nanowires (Ag NWs) [3]. However, the sulfide phosphors used for commercially available EL devices have hydroscopic characteristics and a short-term reliability: the short lifetime of 2500 h at 200 V and 400 Hz in comparison with general fluorescent lamp with lifetime of 10,000 h [1], [4]. Thus, oxide phosphors are required to overcome these inferior characteristics [1], [2], [3]. Using oxide phosphor as the emitting layer of the EL device could overcome this inferiority and satisfy the requirement for long lifetime. Some kinds of phosphors for EL have been reported: doped ZnGa2O4, doped Zn2SiO4, doped (Ca, Ba)TiO3, and doped Y2SiO5 [5], [6], [7], [8], [9], [10], [11]. The green-emissive Zn2SiO4:Mn2+-based thin film EL showed the best luminous efficiency of 0.8 lm/W [12]. Also this phosphor has been known for high efficiency in thin film EL (not powder EL) as well as photoluminescence and cathodoluminescence [13], [14]. In thin film EL device, the hot electron excited luminescent center stops moving at the interface with the insulating layer to form a polarization. The internal polarization field caused from the trapping charges at the interface between the phosphor layer and the insulating layer increases the effective electric field applied to the phosphor layer under AC driving conditions. As a result, luminous efficiency and luminance are increased [1]. Furthermore, it was demonstrated that the emission color was changed through substituting Ge ions for Si ions in Zn2(Si,Ge)O4:Mn2+-based thin film EL device [15]. However, to our knowledge, most Zn2SiO4:Mn2+-based EL devices has focused on the thin film device but there are no papers on Zn2SiO4:Mn2+ phosphor for thick powder EL devices. In this work, the powder EL device using the Zn2SiO4:Mn2+ phosphor was fabricated by screen printing and the optical and electrical properties were investigated. Also the temperature dependence was demonstrated.
Section snippets
Experimental process
Green-emission Zn2SiO4:Mn2+ powders were synthesized through a mixture of ZnO, SiO2 and MnO in 1:1:0.005 mol ratio at 1200 °C for 2 h in a sealed quartz tube. The synthesized powder samples had spherical shapes with a diameter of about 2 µm, as confirmed in Fig. 1, and a single phase of the willemite crystal with a rhombohedral structure (x-ray diffraction pattern not included here). The powder electroluminescence devices were based on the top-emission structure prepared by a screen printing
Results and discussion
The EL spectra of the powder EL device varying from 100 to 300 V for a fixed frequency of 400 Hz are shown in Fig. 2. The top and bottom insets show voltage-dependent EL intensities and turn-on EL image at 300 V, respectively. The peak wavelength is 525 nm and the full width at half maximum (FWHM) is 43 nm. The CIE color coordinate are x = 0.2184 and y = 0.7044. The EL spectra are unchanged with increasing applied voltage. On the contrary, the emission color of the conventional ZnS-based EL device
Conclusion
Green-emission Zn2SiO4:Mn2+ powder phosphor was applied for powder electroluminescence device through screen printing method. Its luminances versus voltages were exponentially increased, and its luminances versus frequencies were linearly increased and then saturated. Its temperature dependence showed the constant behavior at lower temperature, and then the drastic rising pattern up to Curie temperature of the dielectric layer (~ 120 °C). The maximum luminance was 0.96 cd/m2, where the power
Acknowledgement
This work was supported by the Development of R&D Professionals on LED Convergence Lighting for Shipbuilding/Marine Plant and Marine Environments (Project No: N0001363) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).
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