Device model investigation of bilayer organic light emitting diodes

Organic materials that have desirable luminescence properties, such as a fa vorable emission spectrum and high luminescence efficiency, are not necessa rily suitable for single layer organic light-emitting diodes (LEDs) because the material may have unequal carrier mobilities or contact limited inject ion properties. As a result, single layer LEDs made from such organic mater ials are inefficient. In this article, we present device model calculations of single layer and bilayer organic LED characteristics that demonstrate t he improvements in device performance that can occur in bilayer devices. We first consider an organic material where the mobilities of the electrons a nd holes are significantly different. The role of the bilayer structure in this case is to move the recombination away from the electrode that injects the low mobility carrier. We then consider an organic material with equal electron and hole mobilities but where it is not possible to make a good co ntact for one carrier type, say electrons. The role of a bilayer structure in this case is to prevent the holes from traversing the device without rec ombining. In both cases, single layer device limitations can be overcome by employing a two organic layer structure. The results are discussed using t he calculated spatial variation of the carrier densities, electric field, a nd recombination rate density in the structures. (C) 2000 American Institut e of Physics. [S0021-8979(00)06504-X].