Abstract
Bragg gratings incorporated into organic light-emitting diodes (OLEDs) establish a coupling between waveguide modes and useful light (leaky modes). Here we demonstrate that the net coupling direction depends on the OLED stack design. We fabricated two different device structures with gold Bragg gratings. Angle resolved electroluminescence spectra were recorded. For the first device peaks of enhanced emission due to the Bragg grating are observed corresponding to a net energy transfer in direction of the leaky modes. The second device, on the other hand, exhibits dips in the emission spectrum. This reversed direction of energy transfer from the leaky modes to the waveguide modes is explained considering transfer matrix simulations of modal intensity distributions and device emission simulations. An OLED efficiency enhancement is only achieved, if the waveguide mode extraction is dominant.
Highlights
In the last two decades, organic light-emitting diodes (OLEDs) were the subject of intensive studies [1,2,3]
We investigated the interplay between waveguide modes and leaky modes considering two different polymer OLEDs with gold gratings
The experimental findings were compared to simulations of the waveguide modes and optical device emission, which allows for explaining the experimental findings
Summary
In the last two decades, organic light-emitting diodes (OLEDs) were the subject of intensive studies [1,2,3]. About 50% is trapped in waveguide modes in anode and organic layers and surface plasmon polaritons (SPPs) at the cathode/organic interface Another 30% of the light is reflected at the substrate/air interface due to total internal reflection. Extraction elements must be inside or close to the OLED thin film stack and have to be processable on large-scales to be suitable for commercial applications In this context, waveguide mode extraction by Bragg scattering at grating structures [11,17,18,19] (in the following referred to as Bragg gratings) and scattering at micro structures [20,21] has been extensively studied. By varying the gold grating thickness and the Super Yellow emitter layer thickness, we realized a second device design, which exhibits energy transfer from leaky modes to the fundamental TE waveguide mode and a reduced overall efficiency. The comparison of simulation and experimental results allows for identification of the processes and parameters important for efficient waveguide mode extraction
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