HIGH-EFFICIENCY SRS,SRSECECL3 BASED THIN-FILM ELECTROLUMINESCENT DEVICES

SrS:CeCl3 thin film electroluminescent devices with different doping levels have been prepared by reactive evaporation method using Sr, S and CeCl3 as source materials. In addition to the usual TCO-coated and thin film insulator covered Corning 7059 glass, conductive Si wafers covered with thermal oxide and CVD Si3N4 insulating layers were in use in order to provide a high temperature stable substrate material. It is shown that a change in CeCl3 doping level from 0.05 to 2 at% results in a shift of the SrS:CeCl3 EL emission spectra from blue to yellowish-white. Highest brightness and efficiency of the fabricated EL devices (600 cd/m2, 0.4 lm/W at 1 kHz sine wave excitation) have been obtained for devices with a respective CeCl3 doping concentration of 0.6 at%. XPS analyses have been employed to determine doping concentrations, chemical valencies of the Ce ions and the chemical bindings in the SrS:CeCl3 films in order to investigate the origin of the shift of EL emission to longer wavelengths for higher doping levels. As a result, concentration dependent changes of the Ce valency have been observed, correlated to EL characteristics. SrSe:CeCl3 based TFEL devices show a blue shift of the emission spectra of about 10 nm due to increased lattice constant of the host material at comparable doping concentrations as in the SrS case. Up to now, brightness and efficiency of these devices are lower in comparison to devices based on SrS films.