Time-resolved electroluminescence from single and bilayer LEDs based upon substituted poly-arylenevinylenes

Light emitting diodes (LEDs) have been fabricated on the basis of poly-arylenevinylenes substituted at the vinylene double bond and blended with either polycarbonate (PC) or polyvinylcarbazole (PVK). Contacts were indium-tin-oxide (ITO) and aluminum (Al). The transient behavior of both single layer as well as bilayer devices with oxadiazole dispersed in polystyrene being the second (cathodic) layer was studied upon application of rectangular voltage pulses. The external quantum efficiency of single layer and bilayer devices is of order 10(-2)% and 1%, respectively. Detailed studies were performed on Poly[1,4-phenylene-1,2-di(4-phenoxyphenyl)vinylene] (DPOP) because this system showed evidence for a change of the j(E) characteristic from being space charge limited to injection limited upon replacing PVK as blend material by PC. This illustrates the profound effect the binder material can have on injection as well as charge transport in a LED based upon polymer blends. In the single layer LED delayed onset of the light emission reflects the transit time of injected holes, while in bilayer LED it is determined by the time minority carriers (electrons) need to reach the internal interface.