OLED Encapsulation Strategies
Cover glass for rigid OLED devices: As noted above, glass encapsulation strategies are the most prevalent today, and the dominant suppliers are Corning and DuPont.
The majority of current OLEDs, especially bottom-emitting structures, are encapsulated using a cover glass, desiccant (or getter), and an edge-sealing expoy in a batch process strategy that works especially well for small displays like those used in cell phones, etc. First, in these smaller devices, the high barrier properties required are not so difficult to achieve, and the product lifetimes of many of these smaller OLEDs are relatively short anyway. NanoMarkets believes that even for larger, non-flexible OLEDs, glass/getter/epoxy encapsulation is likely to continue to be a leading choice for OLEDs.
• This approach will benefit from its “tried and true” status, and without the need for flexibility or conformability, there are few compelling reasons to consider other options, especially if the glass/getter/epoxy suppliers can demonstrate reduction in processing costs and complexity.
• While these arguments may be difficult to uphold for very large size panels and high throughput manufacturing, this situation will not be the case for manufacturing of smaller and mid-sized panels used in smartphones, netbooks, tablets, etc.
Flexible glass?: For bottom-emission OLEDs, the emergence of viable flexible glass substrates that are compatible with R2R processing cannot be ignored. Both Asahi Glass and Corning have announced the development of 0.1-mm thick flexible glass rolls.
If the firms can also demonstrate ways to include adhesive and desiccant for top-emission structures, they could successfully overcome the biggest impediment to using glass for encapsulation, namely the cost.
Multilayer barrier films: The most common non-glass approach to OLED encapsulation is the use of a multilayer barrier film of alternating polymer and oxide layers deposited by physical or chemical vapor deposition (PVD or CVD):
• Multilayer barriers – whether in the form of pre-formed film laminates or monolithic thin films deposited directly onto the OLED devices – have been touted as having the potential to greatly reduce costs compared to rigid cover glass designs through use of cheaper materials, thinner layers, and easier processing.
• Monolithic encapsulation has been more prevalent than laminates, since monolithic encapsulation of the OLED devices minimizes (but does not always eliminate) the need for adhesives and getters to improve edge imperviousness. Unfortunately, in reality the cost advantages have been elusive, often because so many layers are needed to achieve the required barrier performance that any processing cost benefits are lost.
The fundamental problem with multilayer barriers is that defects such as pinholes, cracks and grain boundaries are common in the thin oxide films used, especially when fabricated onto plastic substrates.
To solve this problem, multilayer barrier technologies have focused on using alternating organic and inorganic multilayers, or dyads, which stagger the defects in adjacent layers. This creates a “torturous path” for water and oxygen molecules; the idea is to make the pathway long enough so that water and oxygen simply will not be able to make it to the sensitive OLED stack.