White-emission from ZnS:Eu incorporated in AC-driven electroluminescent devices via ultrasonic spray pyrolysis

Highlights: • White-emitting-AC-thin film electroluminescent device has been achieved using only europium doped ZnS as phosphor layer. • The electroluminescent device with a MISIM architecture was fabricated by low-cost ultrasonic spray pyrolysis technique. • The use of ZrO₂ insulator layers promotes the incorporation of Eu³⁺ and point defects at the phosphor-insulator interfaces. • The white emission comes supposedly from electron-impact excited Eu²⁺, Eu³⁺, and defects at the phosphor-insulator interface. • The white emission of the electroluminescent device is close to that of a standard illuminant. In this work, white-emitting-alternating-current-thin film electroluminescent (w-ACTFEL) devices are demonstrated using europium-doped zinc sulfide (ZnS:Eu) and zirconium oxide (ZrO₂) as the emissive and dielectric layers, respectively. These films were deposited by the ultrasonic spray pyrolysis technique on antimony-doped tin oxide glass substrates, forming a standard metal-insulator-semiconductor-insulator-metal (MISIM) architecture. 10 kHz sinusoidal voltages activated the white-EL of the devices. The colorimetric characteristics were investigated for three amplitudes of the applied voltage. The emission of the devices is made up of wide and narrow bands with peaks corresponding to violet, blue, green and red light, which together produce the resulting white light. According to the colorimetric analysis, this white light is close to the standard D65 CIE illuminant with a minimal dominant blue component. The variation in voltage amplitude induces small changes in the visual characteristics of the EL emission. The white-EL emission of these MISIM devices is attributed to the electron-impact excitation and subsequent relaxation of the excited levels of Eu²⁺ and Eu³⁺ impurities, and defect levels in the sublayer regions adjacent to the ZrO₂–ZnS:Eu interfaces.