Transient electroluminescence from single- and double-layer light-emitting diodes (LEDs) based on polymer blends

The transient behavior of the electroluminescence (EL) from single- and double-layer light-emitting diodes (LEDs) has been measured upon application of a pulse voltage. The active materials were tri(methoxystilbene)amine (MSA) and an oxadiazole (PBD) dispersed in polysulfone (PSu) and polystyrene (PS), respectively. In double-layer devices onset of EL occurs after a delay time t(d) that reflects accumulation of a space charge layer at the internal interface. The rise time reflects build-up of the electron density in the recombination zone. In singlelayer LEDs t(d) is governed by build-up of a space charge near the aluminum cathode followed by enhanced electron injection. The rise time is controlled by electron motion into the bulk of the sample.