Effects of ion-irradiation on light emitting MEH-PPV and PDPB polymers

The effects of ion-irradiation on light emitting poly [2-methoxy-5-(2'-ethy l-hexyloxy), p-phenylene vinylene] (MEH-PPV) and poly (2,5-dihexyloxy-1,4-p henylene-alt-4,4'-biphenylene) (PDPB) materials are investigated. Argon and nitrogen ions are used for ion-irradiation, and the energy of ion-irradiat ion is varied from 300 eV to 1 keV. The flux: of irradiated ions is changed from 1x10(13) to 1x10(17) ions/cm(2) at a fixed ion beam current (=0.2 mu A). The intensities of the PL spectra of the samples vary with the ion dose , the kind of ion, and the energy of the ion beam, while the positions of m ain peaks in the PL spectra are not shifted. We observe that there is no si gnificant difference between the current vs voltage characteristics of ligh t emitting diodes (LEDs) before and after treatment with ion-irradiation. T he Variation of the surface morphology of the PDPB samples due to ion-irrad iation is investigated through atomic force microscope (AFM) images. The pe aks on the surface of the pristine PDPB sample are smooth, and the spatial density of the peaks is relatively low. The peaks become sharper with incre asing ion dose, which indicates an enhancement of the roughness of the poly mer surface. The external quantum efficiency of the LED devices as a functi on of the current density decreases, but is more stable with changing curre nt density after treatment with ion-irradiation. We analyze that the stabil ity of the quantum efficiency of the LED devices with ion-irradiated sample s might be due to the roughness and the chemical change in the polymer surf ace and to the increase of the interface area between the polymer and the A l electrode.