Effect of driving method on the degradation of organic light emitting diodes

Lifetime testing results are reported for organic light emitting diodes (OLEDs) having the structure ITO (anode)/NN-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-diphenyl-4,4'-diamine (TPD)/tris-(8-hydroxyquinoline)aluminum (AIQ)/Al (cathode) and operated using dc and pulsed waveforms for comparison. In ambient atmosphere non-encapsulated devices show a lifetime of about 70 h in pulsed operation at an initial luminance of 500 cd/m(2), almost four times longer than in dc operation. A fast initial decay of luminance is observed for dc operation. It is most probably due to a combination of Joule heating and mobile ionic impurities migration within the OLED structure under the continuous forward electric field. We study also the transient electroluminescence (EL) and report the observation of a short duration spike in the leading edge of the EL response. Its origin is attributed to the temporal shift of the recombination zone in the AIQ layer from the bulk to the interface with TPD subsequent the application of a voltage step. (C) 2003 Elsevier Science B.V. All rights reserved.