Shaping the Emission Pattern of Organic Light Emitting Diodes by using Plasmonic Ag Nanoparticles Arrays

In this work, we investigate the influence of metallic Ag nanoparticles arrays on the optical and electrical properties of organic light emitting diodes (OLED) with the main objective to select, reshape and improve the emission of the device using the plasmonic effects. The considered structures are regular square arrays of nano-cylinders of 100nm diameter. We study various Ag metallic gratings with periods (p) ranging from 190nm to 480nm by a step of 20nm, fabricated by using electron beam lithography technique. The measured extinction spectra of the samples are analyzed experimentally as well as numerically by using FDTD simulations. For instance, fig. 1.a draws the experimental map of the extinction intensity as a function of the grating period and wavelength using a photo-excitation at a normal incidence. This map gives the dispersion of the plasmonic resonances. From that, two gratings types can be distinguished: the so-called short period gratings for p < 300nm, showing only a near field coupling effect which exhibits a plasmonic response known as the localized surface plasmon resonance (LSPR) effect [2], and the so called long-period gratings that corresponds to the excitation of hybrid resonances [3] showing a dominant far-field coupling effect [4] and called surface lattice resonance (SLR). Fig. 1.b reports the deduced variation of the quality factor of the main two peaks obtained. It indicates that sharp resonances corresponding to high Q quality factors can be obtained in the SLR regime. Therefore, tuning the array parameters offers the possibility to select a specific optical response: near or far field coupling effect.