Secondary Light Output from ZnS:Mn Thin Film Electroluminescent Devices

Abstract

Substantial insights into the processes responsible for light emission from ac thin-film electroluminescent (EL) devices can be deduced from the time dependences of the EL light output (emission waveforms). Under steady-state ac drive, differences in phosphor materials are exhibited not only by different emission decay times, but also by the number of emission peaks per period. Specifically under bipolar square pulse drive, ZnS:Mn devices normally exhibit light output only at the leading edges of the voltage pulses, but alkaline earth sulfide devices show both leading and trailing edge (secondary) light outputs. These differences stem from different light generation mechanisms [1, 2]. Secondary light output is generally attributed to trap-controlled recombination of the transferred charge with excited activators. The absence of secondary light output in ZnS:Mn, which was also noted for ZnS:Mn EL powders, in contrast to ZnS phosphors with other activators [3], is ascribed to the specific nature of the Mn activator that allows only localized transitions within the luminescence center [3]. However, the specific light generation mechanism in ZnS:Mn does not rule out the appearance of secondary (trailing edge) light. Here we will both report its observation, and discuss the underlying cause and implications, going beyond the explanation of a previous observation [4].