Changes in the external quantum efficiency of bilayer organic light emitting devices with layer length have been measured for devices of configuration ITO\TPD\Alq\Mg:Ag with the Alq length varying between 25-200nm. It has been independently concluded for similar devices that the thickness of the Alq layer can be optimised with regard to the external quantum efficiency. However, our simulations of the internal quantum efficiency of this structure with an electrical transport model predict that the internal quantum efficiency is invariant with respect to the Alq layer thickness. We deduce that optical microcavity effects cause the variation in external quantum efficiency. These microcavity effects alter the external efficiency through optical interference and through altering the singlet exciton density profile. A combined electrical-optical model based on our electrical transport model and an optical model has been used to calculate the external efficiency for these devices. We find a clear variation in efficiency with layer thickness, matching the experimentally observed trends.