Correlated disorder substrate-integrated nanodisk scatterers for light extraction in organic light emitting diodes

Abstract

A major loss mechanism in organic light emitting diodes (OLEDs) is the coupling of the emitter molecule light field to waveguide modes in the OLED thin film stack. In this work, a disordered 2D array of TiO2 nanodisk scatterers is integrated into the OLED substrate to enable efficient light extraction from these waveguide modes. Fabrication of the nanodisks is based on a bottom-up, colloidal lithography technique and subsequent pattern transfer into high refractive index TiO2 via reactive ion etching. The substrates are completed by spin-coating a polymer planarization layer before applying the OLED thin film stack. This ensures reproducible optoelectronic properties of the OLED through leaving the electrically active layers planar. Simultaneously, the nanodisks in close vicinity to the thin film stack ensure efficient out-of-plane scattering of waveguide modes. In a monochromatic OLED (center wavelength λ0 = 520 nm), a 44.2%rel increase in external quantum efficiency is achieved in comparison to a device without scattering structure. An in-depth numerical analysis reveals that this significant enhancement is only partly due to the out-coupling of waveguide modes. Additional enhancement is suspected to result from out-coupling of substrate modes through scattering by the nanodisks. Further improvements to the scattering structure are numerically evaluated.