Mechanisms of thin film color electroluminescence

After the establishment of a reasonably detailed understanding of thin film electroluminescence (TFEL) of ZnS:Mn sandwiched between dielectric films, which is commonly addressed as the 'simple model' of TFEL, research activities concentrate now on multi- or full-color displays in thin film technology. A simple extension to other colors than the 'amber' of ZnS:Mn by doping of ZnS with other inner shell emitters. Some promising results with SrS:Ce3+ and CaS:Eu2+, featuring allowed 5d-4f transitions, in the range 0.5. . .1.3 Im/W were reported, but the question on the margins remained, as surprisingly enough the mechanism valid in these compounds seemed to be different from the one studied thoroughly in ZnS:Mn. There is strong evidence of field ionization of the excited Ln ions into the conduction band, giving supposedly rise e.g. to current flow and emission at the trailing edge of the applied driving pulses.