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′-ethyl-hexyloxy), p-phenylene vinylene] (MEH-PPV) and poly (2,5-dihexyloxy-1,4-phenylene-alt-4,4′-biphenylene) (PDPB) materials are investigated. Argon and nitrogen ions are used for ion-irradiation, and the energy of ion-irradiation is varied from 300 eV to 1 keV. The flux of irradiated ions is changed from 1 × 10¹³ to 1 × 10¹⁷ ions/cm² at a fixed ion beam current (=0.2 μA). 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 main peaks in the PL spectra are not shifted. We observe that there is no significant difference between the current vs voltage characteristics of light emitting diodes (LEDs) before and after treatment with ion-irradiation. The variation of the surface morphology of the PDPB samples due to ion-irradiation is investigated through atomic force microscope (AFM) images. The peaks 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 increasing ion dose, which indicates an enhancement of the roughness of the polymer surface. The external quantum efficiency of the LED devices as a function of the current density decreases, but is more stable with changing current density after treatment with ion-irradiation. We analyze that the stability of the quantum efficiency of the LED devices with ion-irradiated samples might be due to the roughness and the chemical change in the polymer surface and to the increase of the interface area between the polymer and the Al electrode.