Magnetic Field Effects in X-Ray Damaged NPB and MADN OLEDs

N, N'-di(1-naphthyl)-N, N'-diphenyl-(1,1'-biphenyl)-4, 4'-diamineor 2-methyl-9, 10-bis(naphthalen-2-yl)anthracene-based organic light emitting diode devices, when purposely exposed to X-rays emitted from an evaporation electron gun, reveal interesting new properties in terms of transport, magnetoconductance (MC), as well as spectral changes in their photoluminescence. For devices exhibiting MC, we report magnetic field effects with two distinct regimes. The first one, at low magnetic field, is assigned to hyperfine field effects and is slightly dependent on X-ray exposure. The second one, occurring at larger magnetic field, is due to polaron-triplet exciton coupling and depends on the irradiation dose, as well as on the organic material nature and thickness. We interpret our results in terms of extrinsic trap formation, thanks to X-rays exposure and interactions between polarons and trapped triplet excitons.