The electro-optical behaviour of organic light emitting diode devices (OLEDs) is greatlyinfluenced by the morphology of the films. A major parameter is due to the important rolethat the morphology of the active organic thin films plays in the phenomena that lead tolight emission. For vacuum-grown OLEDs, the morphology of the specific thin films canbe varied by modification of the deposition conditions. We have assessed themethod (ultrahigh-vacuum organic molecular beam deposition) and conditions(variation of the deposition rate) for electro-emission (EL) optimization in a standardα-NPB(N,N′-bis-(1-naphthyl)-N,N′ diphenyl-1,1′ biphenyl-4-4′ diamine)/Alq3 (tris-(8-hydroxyquinoline) aluminium) vacuum-grown OLED device. Thebest EL performances have been obtained for OLEDs made in ultrahigh vacuum with theAlq3 layer deposited with a differential deposition rate ranging from . The results are consistent with a model of different Alq3 morphologies, allowing efficientcharge injection at the metal/organic interface, and of the minimization of grain boundariesat the electron–hole recombination interface, allowing efficient radiative excitonic decay. Atthe same time, with the objective of controlling and stabilizing the morphology changesand stabilizing the charge transport over a long OLED operating time, we have studied theeffect of thermal annealing processing in the standard current behaviour of OLEDs.The large current fluctuations typically observed for standard vacuum-grownOLEDs have been smeared out and kept constant over a long operating time by thegiven thermal annealing conditions. The results are interpreted in terms of thestabilization of intrinsic polymorphism of the organic film’s structure induced bythermal energy and leading the morphology to a lowest-energetic configuration.