| Alternating-current thin-film electroluminescent (ACTFEL) phosphors have been studied and improved with respect to brightness, color and efficiency. It is shown that ex-situ co-doping of the sputter deposited ZnS:Mn active layer with K and Cl results in 53% improvement in brightness, 62% improvement in efficiency, and better 100-hour accelerated aging stability. This improvement was demonstrated to result from a 75% increase in excitation efficiency for conduction electrons, combined with a small decrease in both light outcoupling and non-radiative recombination. By studying the electrical behavior of the co-doped devices as compared to the undoped devices, it was determined that there is a reduced amount of static space charge in the films, resulting in a larger average field, increased excitation efficiency, and increased charge multiplication. The reduced space charge is attributed to the addition of charge compensating zinc vacancy-chlorine complexes and isolated chlorine point defects, which are acceptor and donor defects, respectively, and the reduction of zinc vacancy deep hole traps. It is postulated that there is sufficient electron multiplication or donor ionization to create a situation in which the current limit is set by the phosphor resistance rather than a capacitance or density of states.; Thin film Zn2GeO4:Mn phosphor layers were evaluated as green-emitting ACTFEL devices. Devices with Zn2GeO 4:Mn magnetron sputter deposited films exhibited brightness and efficiency values of 74 cd/m2 and 0.19 lm/W, respectively at 40 V above threshold and 60 Hz. Poor outcoupling efficiency suggests an internal efficiency that is much higher. Optical spectroscopy reveals efficient charge transfer from the lattice and a 615 meV sub-band gap defect state to the Mn 2+ ion. Unique transient optical and electrical behavior under EL excitation is observed. A phenomenological model is presented to explain the behavior. The unique aspects of the model include a resistance-capacitance limit on charge injection, efficient charge injection for only one polarity, abnormally large space charge density for one polarity that allows efficient electron-hole pair recombination in low electric field regions, and near-resonant energy transfer to and from the Mn2+ excited state. |
