Contrast-enhancement in black dielectric electroluminescent devices

A high contrast electroluminescent (EL) device structure is presented. The diffuse luminous reflectivity from the metal/dielectric/phosphor/indium-tin-oxide/glass EL device structure is ∼3%. A Eu-doped GaN phosphor is used to demonstrate the contrast-enhanced operation. Low reflectivity is achieved by inserting a light-absorbing black thick-film BaTiO₃ layer between the phosphor and the rear metal electrode. In addition to providing contrast enhancement, the opaque thick dielectric film exhibits capacitance and high voltage reliability (40 nF/cm², dielectric constant ε_d ∼ 500-1000, breakdown field E_d,br ∼ 0.1-0.4 MV/cm) similar to that of the highest performance transparent thin-film dielectrics. An EL device luminance of only 20 cd/m² is sufficient for a display contrast ratio of ∼10:1 under 140 lux indoor ambient lighting (illumination). Under sunlight illumination of 100000 lux, a display contrast ratio of >3:1 is expected with application of additional contrast enhancement techniques.