DIRECT MEASUREMENT OF THE INTERNAL ELECTRIC-FIELD DISTRIBUTION IN A MULTILAYER ORGANIC LIGHT-EMITTING DIODE

We present a general electroabsorption technique to measure the electric field in each layer of multilayer organic light-emitting diodes. The electroabsorption signal from each layer is identified spectroscopically and measured as a function of de bias. Measurements were made on three layer devices consisting of a hole transport layer, a light-emitting layer, and an electron transport layer. In reverse bias, without significant charge injection, the de electric field is uniform throughout the device. In forward bias, the de electric field is distributed nonuniformly; it is smallest in the light-emitting layer and largest in the hole transport layer. The nonuniform de electric field distribution is caused by the accumulation of electrons (holes) at the interface between the light-emitting layer and the hole (electron) transport layer. The maximum accumulated charge densities an 2X10(12) electrons/cm(2) and 3X10(11) holes/cm(2). These results highlight the carrier blocking role of monopolar transport layers and demonstrate a powerful technique to characterize multilayer organic structures. (C) 1995 American Institute of Physics.