Local degradation of electroluminescent emission centers in ZnS:Cu,Cl phosphors

ZnS:Cu contains needle-shaped, CuS conducting nano-precipitates along the [111] planes which amplify the applied electric fields near the tips when the field is reversed, enabling AC electroluminescence for image thick devices at AC voltages of image. Unfortunately, the degradation process is poorly understood; previous studies using photodiode measurements were only able to determine the overall device degradation. Here, we analyze many individual emission centers using time-lapse microscope images of several degrading devices. We find that most of the individual emission centers have large, sudden, discrete drops in luminosity and many have a luminosity that actually increases significantly, minutes to hours after a drop. This suggests that there are two primary degradation mechanisms: gradual multi-step diffusion of single-atom trap states away from the high-field region (causing the gradual decrease in luminosity) and the sudden break-up and reformation of complex trap states within the high field region (causing the sudden decreases and increases, respectively).