Thermal annealing of organic light-emitting diodes based on polymer composites prior to the metal deposition is found to substantially improve the devices' luminescence efficiency. Phase separation between the matrix polymer and the hole conducting low molecular weight component is shown to be the origin of this behavior. We use atomic force microscopy along with selective dissolution of one of the components to establish the domain morphology of the devices. The presence of the metal electrode is shown to significantly influence the phase separation process. Finally, a correlation between the domain morphology and the electrooptical properties is established.