Aging Characteristics of Electroluminescent Phosphors

The brightness, B, of electroluminescent light sources is observed to decay gradually as a function of operating time, t, according to the empirical formula, B=B0/(1+t/tc), where B0 is the initial brightness and tc is the time to half-brightness. At 60 cps the values of tc range in the hundreds to thousands of hours, while at 5000 cps tc is observed to be less than 100 hr for the particular phosphor used. The value of tc depends slightly, if at all, on voltage; but a very significant change in the voltage dependence of brightness occurs during the aging process. The results are interpreted on the basis of the Piper-Williams theory and are thought to be consistent with a field-induced migration of the deep donor impurity.