The efficiency of the organic light-emitting devices (OLEDs) was enhanced with increasing thickness of a 5,6,11,12-tetraphenylnaphthacene (rubrene) emission layer (EML). The charge trapping and the luminance mechanisms were affected by the thickness of the rubrene layer, regardless of the existence of the barrier layers. The variation in the electroluminescence spectra of devices originated from a variation in the position of the recombination region with increasing thickness of the EML. The enhancement of the luminescence efficiency for devices was attributed to the better charge balance and the optimized recombination region.