Enhanced light emission from top-emitting organic light-emitting diodes by optimizing surface plasmon polariton losses

TL;DR

This paper demonstrates a 20% increase in light extraction efficiency in top-emitting OLEDs by using a low refractive index hole transport layer to reduce surface plasmon polariton losses, enabling further improvements with additional extraction techniques.

Contribution

It introduces a method to enhance OLED light outcoupling by optimizing the HTL material's refractive index to mitigate surface plasmon losses, applicable to various emitter orientations.

Findings

Enhanced light extraction factor of 1.2 achieved.

Low refractive index HTL reduces surface plasmon polariton losses.

Potential for further enhancement with micro lenses or gratings.

Abstract

We demonstrate enhanced light extraction for monochrome top-emitting organic light-emitting diodes (OLEDs). The enhancement by a factor of 1.2 compared to a reference sample is caused by the use of a hole transport layer (HTL) material possessing a low refractive index (1.52). The low refractive index reduces the in-plane wave vector of the surface plasmon polariton (SPP) excited at the interface between the bottom opaque metallic electrode (anode) and the HTL. The shift of the SPP dispersion relation decreases the power dissipated into lost evanescent excitations and thus increases the outcoupling efficiency, although the SPP remains constant in intensity. The proposed method is suitable for emitter materials owning isotropic orientation of the transition dipole moments as well as anisotropic, preferentially horizontal orientation, resulting in comparable enhancement factors. Furthermore, for sufficiently low refractive indices of the HTL material, the SPP can be modeled as a propagating plane wave within other organic materials in the optical microcavity. Thus, by applying further extraction methods, such as micro lenses or Bragg gratings, it would become feasible to obtain even higher enhancements of the light extraction.