A structural evaluation of ZnS-based electroluminescent phosphors and devices: a comparison with performance

A number of ZnS phosphor powders, both commercial and experimental, have been characterized by transmission electron microscopy (TEM). The microstructure of the phosphors has been compared with the light output (intensity and spectra) obtained when incorporated into an ac-driven electroluminescent test lamp. No evidence has been found in any samples examined by TEM of Cu2S precipitates. It was also found that the brightest emission is obtained when the phosphor particles have a very heavily multiply twinned structure together with a rough profile, and least output when they show the cubic phase. Examination of the electroluminescence output of individual phosphor grains indicates that emission is mostly from material close to the surface and suggests that field emission occurs from the surface which is aided by a marked surface roughness. Finally, the microstructure of a commercial ac powder phosphor sheet device was examined and also found to contain ZnS in the same heavily twinned form