Optically detected magnetic resonance study of efficient two-layer conjugated polymer light-emitting diodes

We report the results of electroluminescence-detected magnetic resonance (ELDMR) measurements on efficient two-layer polymer light-emitting diodes (LED's) comprising a hole-transporting layer of poly(p-phenylenevinylene) (PPV) and an emissive layer of a cyano-substituted PPV derivative (CN-PPV). By comparing the ELDMR and conductivity-detected magnetic resonance (CDMR) results with photoluminescence-detected magnetic resonance on films of the polymers making up the LED, we are able to identify the components of the ELDMR and CDMR signals with processes occurring in either the PPV or the CN-PPV layers. We are, thus, able to establish the presence of triplet excitons in the CN-PPV layer. We also reveal the importance of bipolaron formation in the PPV layer, and discuss the assignment of the other ELDMR and CDMR features.