The influence of source powder composition on the electroluminescence of Ca1−xSrxS:Eu thin films

CaS:Eu has been studied as a red emitter for AC thin film electroluminescent (ACTFEL) devices in the beginning of the 1990s. This phosphor has not yet been used for practical display purposes because of the limited luminance and the slow response of the devices. Part of the CaS:Eu emission spectrum is situated at wavelengths longer than 650 nm where the eye sensitivity is very low. The partial replacement of the Ca-ions by Sr-ions causes a shift of the Eu emission to shorter wavelengths and thus an increase in the apparent luminous efficiency. To obtain a high luminance and a good temporal response, very homogeneous, crystalline and pure Ca1−xSrxS:Eu thin films were found to be necessary. Several powder preparation techniques were applied to mix CaS and SrS powders on a microscopic scale, in order to achieve a single Ca1−xSrxS:Eu phase. The overall phase distribution was determined by θ−2θ X-ray diffraction (XRD) measurements and by assessing the photoluminescent properties of the Eu-activated powders. Scanning electron microscopy (SEM) images were useful in the determination of the grain size and the porosity of the material. The combination with energy-dispersive X-ray analysis (EDX) allowed mapping of the microscopic distribution of Ca and Sr. A single-phase of Ca1−xSrxS:Eu could be obtained using wet chemical synthesis, which considerably improved the luminance of the corresponding ACTFEL devices.