Improvement of EL efficiency in polymer light-emitting diodes by heat treatments

Efficiency of polymer light-emitting diodes (LEDs) would depend upon the heat treatment condition and procedure greatly. We have tried thermal annealing at the temperature above Tg of the emissive polymer, poly[2-methoxy-5-(2′-ethly-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV), before and after Al deposition, respectively, in ITO/MEH-PPV/Al structure. The anomalous semiconducting behavior of an unannealed device has been screened out by appropriate heat treatments. It is remarkable that the annealing treatment after Al deposition makes the EL efficiency enhanced by two orders and reduces the operating voltage about 40%. The high temperature annealing after Al deposition enhances the interface adhesion between the emissive polymer and the Al cathode, and might form an interfacial polymer layer with wide band gap, which seems to play a critical role to improve the efficiency of EL devices.