Causes of driving voltage rise in phosphorescent organic light emitting devices during prolonged electrical driving

We studied the driving voltage stability of typical phosphorescent organic light emitting devices (PHOLEDs) based on 4,4'-bis(carbazol-9-yl)biphenyl and Tris(2-phenylpyridine)iridium(III) host: guest system. The results show that the gradual increase in voltage often observed with prolonged electrical driving is mainly governed by the accumulation of holes at the emission layer/hole blocking layer interface. Reducing the build-up of hole space charges in this region, for example, by means of eliminating guest molecules from the vicinity of the interface, leads to a significant improvement in the stability of PHOLED driving voltage. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4764021]