Efficiency enhancement in blue phosphorescent organic light emitting diode with silver nanoparticles prepared by plasma-assisted hot-filament evaporation as an external light-extraction layer

Abstract

The uniform growth of silver (Ag) nanoparticles in large areas is important for improving the flat panel devices, mainly organic light emitting diodes (OLED). In this work, two dimensional (2D) arrays of Ag nanoparticles were grown on glass surface substrates via plasma-assisted hot-filament evaporation technique at different plasma environments: no plasma, plasma cleaning (PC), plasma deposition (PD) and PC&PD, which were utilized as an external light-extraction layer for blue phosphorescent OLED device. The surface morphology of the Ag nanoparticles revealed the formation of spherical-like shapes of uniform size which ranged from 7 to 14 nm at different plasma conditions. X-ray diffraction showed that the nanoparticles were mainly single crystals and their crystallinity was significantly enhanced by employing plasma process in PC&PD. The localized surface plasmon resonance peaks of Ag nanoparticles were located at around 380–430 nm which suggests a dependency on diameter and interparticle spacing. The variation of refractive index of Ag nanoparticles 2D array in the visible wavelength from 1.9 to 2.4 plays a key role in high performance OLED. The light-extraction of the OLED device with Ag nanoparticles at PC&PD condition appeared to increase by 82.5% the luminance efficiency compared to that of a reference device without the Ag nanoparticle layer. The effects of the plasma environment on the morphological, structural and optical properties of Ag nanoparticles layer are discussed in the following sections by taking into consideration the effects of glass modification by these Ag nanoparticles on OLED performance.