OLED-based windows: lit inside, coloured outside
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.
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.
“The relative light enhancement mainly depends on the overlap of the reflection spectrum (λmirror) of the dielectric mirror and the emission spectrum of the OLED. If the reflection spectrum does not fully overlap with the emission spectrum, slight variations in the colour perception will occur. If the viewer faces the dielectric mirror (bottom view), losses in luminance as well as changes in colour will be observed since the dielectric mirror modifies the emission spectrum by for example narrowing the emission line”, the researchers write.
The perceived colour on the outside varied from the original yellow light (for λmirror outside the visible region) to purple (λmirror at 550nm), dark (λmirror at 590nm), or light blue (λmirror at 640, 660, or 680nm).
(a) Top view when the OLED is switched off. (b) bottom-view (or outside view in a window application) when the OLED is switched on.
Here, the spectral width as well as the height of the reflection peak of λmirror depends on the refractive indices of the material, the number of deposited layers, and the angle of incidence, they report. According to their calculations, other colours such as green, red, and orange could also be realized providing the right dielectric mirrors were designed.
What’s more, because the colour of the OLED in its non-illuminated state is modified by the white light illumination from the outside (λmirror varying upon the angle of incidence), such a semi-transparent OLED window could bring dynamic colour changes of building facades throughout the day.
They concluded their study by noting that both the dielectric mirror and the OLED can be fully printed, making this concept applicable for upscaling.
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