Devising OLED-based windows that mostly emit light inside

August 25, 2016 // By Julien Happich
A team of researchers from the Institute of Materials for Electronics and Energy Technology (i-MEET) at the Friedrich-Alexander University (Erlangen-Nuremberg) has found a way to control the light directionality of semi-transparent OLEDs while preserving their transparency in the off-state, for future smart window applications.

Interestingly, the modified OLEDs could serve as windows that mostly emit light towards the inside of a building, while offering a tuneable coloured appearance on the outside for architectural purposes.

One easy way to make an OLED unidirectional would be to block or reflect the light on one side, but this would defeat the purpose of OLED windows. They need to be transparent-enough for incoming daylight when the OLED is switched off, as well as being capable of illuminating the interior of a building at night.

Published in the ACS Photonics journal, their paper "Semi-transparent Organic Light Emitting Diodes with Bi-directionally Controlled Emission" describes how combining a semi-transparent yellow OLED stack with a precisely designed dielectric mirror (another stack of 11 alternating layers of materials with high and low refractive indices), the researchers obtained a semi-transparent OLED whose light directionality was enhanced (on the top side), while the back side of the device (on the outside of a window) could be tuned for different colour perceptions.

Although the first part of their work was theoretical, they validated their results with experimental investigations, combining a yellow OLED (with a transparency of 58.2%) with six different dielectric mirrors configurations. While the yellow colour perception remained the same for an inside observer (top view of the OLED stack), up to 80% of the total emitted light was directed toward the top of the stack. But a bottom view of the stack (looking through the dielectric mirror from what would be the outside of a window), offered different colours depending on the build-up of the dielectric mirror.

Fig. 1: (a) Architecture of the full device including the dielectric mirror. (b) Digital images of the OLED with 15mW cm−2 back illumination (top) and under an applied voltage of 5V.

Here the dielectric mirror (also known as a Bragg mirror) is wavelength-selective at λmirror, and together with the stack's thicknesses of the device itself, acts as a cavity resonator. This explains it can enhance the luminance of the oncoming light it reflects, while selectively narrowing the spectrum of light passing through.