Heat treatment at the optimum condition on MEH-PPV OLED device with single-layer structure was performed. The surface morphology of the Al back electrode of the device after being operated at 12.0V is observed by Metallovert. It is found that the bubbles and black spots on the Al electrode surface was reduced and the luminescence performance of the device was improved obviously after the heat treatment. Compared with the unannealed device

the turn-on voltage of the annealed one is reduced by 2.0V

the maximum EL intensity is increased by ten times

the relative energy efficiency is improved by ten times at the same drive voltage

the half decay time is increased by 12.7 times. We analysised the reason of this result and considered that this is mainly because the bubbles and black spots caused by the evolved gases which were probably produced by the electrolysis of water coming from the humid air were reduced effectively by heat treatment

on the other hand

heat treatment also changes the characteristics of the interface between the light emitting polymer layer and the cathode by promoting the formation of the C-Al or C-O-Al chemical bond

this can enhance the interfacial adhesion and as a result

the electron injection is improved.