Curved lighting surfaces
The “Holy Grail” of OLED features is their potential to be fabricated on foils. Such OLED lighting sheets have a special appeal due to new design options – curved lighting surfaces and flexibility – as well as low weight and being unbreakable. The creation of flexible OLED lighting devices however poses some additional major technological challenges compared to common glass substrates. These include the flexibility of encapsulation, the choice of substrate material and the technology of transparent electrodes.
As OLEDs are extremely sensitive to oxygen and water, they have to be encapsulated and sealed against ambient atmosphere. For rigid OLED devices on glass this is easily achieved by covering the OLED with an appropriate glass or metal lid. For the application in flexible OLEDs thin film encapsulation (TFE) is a kind of pre-requisite for adequate hermetic sealing. TFE typically consists of one or more nano-/micrometer thin oxidic or nitridic layers which are often applied by chemical vapor deposition methods (CVD) or similar. Oxides and nitrides are commonly rather brittle, but layers that are thin enough still allow bending down to radii of few centimeters.
Flexible OLED devices have already been demonstrated on both plastics and metal foils having both advantages and disadvantages. On the one hand, transparent plastics like PET or PEN have the advantage of enabling the standard bottom emitter OLED architecture emitting light through the substrate which is well-established by OLEDs on glass. Furthermore, the use of transparent plastic foils would allow for OLED devices which are both transparent and flexible. Their drawback is the poor intrinsic encapsulation behavior.
On the other hand, metal substrates are robust and temperature-stable, intrinsically hermetic to oxygen and water, and good thermal conductors. However, often planarization layers are required due to the metal foil roughness. Metal foils are not transparent, meaning so-called top emitter OLED architectures have to be applied. These are device setups where the light is emitted from the substrate through a transparent top electrode.
On glass, indium tin oxide (ITO) is the common transparent electrode material choice for OLED lighting devices. For flexible OLEDs it is not ideal as it requires high temperature processing for best performance, and here are physical limitations when depositing it onto the organic layer stack where it may cause harm. Also a plastic substrate consists of organic material and it must be checked for compatibility with high temperature processes. Additionally, ITO itself is rather brittle. Thin metal electrodes (TME) are considered to be an alternative without these issues. Low temperature TME electrodes made from ductile metals should be more resistant to bending stress than ITO. TME's pronounced microcavity effects are however challenging for achieving white illumination quality light.
The current challenges for mechanically stable encapsulation techniques, substrate choice and electrode technologies still require some research and development before bringing flexible OLEDs to industrialization.
Finally, it should be noted that the approaches described above do not enable free 3D surfaces. The foils can only be bended into one direction at each single of their surface, enabling so-called 2.5D structures.