A typical OLED layer system usually consists of layers which are all transparent, except for the cathode which is made from a metal such as aluminum. Therefore, an OLED with a most simple basic structure incorporates a very important design feature. When switched off, it is perceived as a mirror. Given this, a light source turns into a building material exhibiting a second functionality. Switching back and forth between a mirror and an area light source is a tremendous stimulus for designers‘ brains. The potential applications exceed the ideas which come to the mind at first, such as vanity mirrors in bathroom or even automotive environments. A reason for multiple application opportunities lays also in the fact, that – in combination with adequate structuring techniques – just parts of the large mirror can be lit up.
As a technical limit of the mirror technology the angular dependence of the emission colour has to be mentioned which is simply caused by the physics of the device.
The thin layer system of an OLED represents an optical cavity in the direction of the surface normal, i.e. an optical system with a total thickness of some wavelengths of the emitted light and a mirror at each end. Strictly speaking, the OLED is a half-cavity because only the metal cathode acts as a real mirror.
Depending on its thickness and the refractive indices of the materials therein, the cavity shows one or more shallow resonances for light at a certain wavelength. For light rays that leave the device at an angle higher than the normal angle, this resonance condition is shifted to larger wavelengths.
Consequently, the emission color of an OLED may display a pronounced angular dependency which can be minimized by clever engineering of the layer thicknesses and the positions of the emitters with respect to the refl ecting cathode. Furthermore, this means that when generating light within a cavity, there is always an optimal position that provides maximum efficiency for a layer that emits light at a certain wavelength. The optimal position of an emitter can be achieved by tuning the layer thicknesses of the non-emitting layers.
In addition to forming a microcavity, the layer stack also has wave-guiding properties. Light generation in the active layers occurs isotropically, but only a small part, i.e. light with a direction of propagation within the escape cone, which is defined by the condition of the total internal reflection at the interfaces, is actually able to leave the device. A large fraction (about 85 %) is wave-guided in the organic layers as well as the substrate glass and is lost for illumination purposes. One way to overcome this limitation is to add an index-matched layer containing scattering particles to the substrate glass, i.e. a diffuser film. Light traveling in the substrate will enter the diffuser film, alter its direction when hitting a particle and eventually leave the diffuser film at a higher angle. The benefit is a higher extraction efficiency and therefore a higher overall device performance and good color mixing due to statistical scattering of all wavelengths, which results in a much lower angular dependency of the emission color. The only drawback of applying a diffuser film is the loss of the mirror-like appearance when the device is turned off, even if the diffuser film itself has a high-gloss surface. Instead of an external diffusor film on the substrate an internal scattering layer can also be built into the devices which leads to a further enhanced efficiency.