Dual optical role of low-index injection layers for efficient polarizer-free high contrast-ratio organic light-emitting diodes.

Polarizer-free high contrast-ratio organic light-emitting diodes (OLEDs) are explored with a structure involving a semi-reflective Cr-based bottom electrode and a dielectric-capped thin Ag top electrode. Their efficiency is shown to be improved significantly with little sacrifice in luminous reflectance by adopting low-refractive-index injection layers that can increase the effective reflectance from the bottom electrode and simultaneously reduce the loss owing to surface plasmon polariton modes. OLEDs employing a low-refractive-index injection layer exhibit improved current efficiency by up to ca. 27.4% than those using index-matched injection layers, with luminous reflectance maintained at as low as 4%.