Microporous phase-separated films of polymer blends for enhanced outcoupling of light from OLEDs
Spotlight summary: In recent years, the lighting industry has been undergoing a true historical revolution with the accelerated phasing out of the incandescent bulbs and the widespread adoption of inorganic light-emitting diodes (LEDs). Nevertheless, also widespread is the opinion that sooner or later these new devices will in turn be replaced by organic LEDs (OLEDs), which have a far greater quantum efficiency and are uniquely suited for certain applications like displays, owing to their minimal thickness and large-area emission. (In fact, OLEDs are the first true example of an area emitter). Nevertheless, OLEDs are notoriously plagued by two major drawbacks: their reduced lifetime and the limited brightness they can achieve. A huge research effort has taken place worldwide to address these issues, and the latest laboratory samples have shown dramatic improvements. The real problem now is to find an inexpensive and reliable way to bring these results into mass production.
The article by Liu et al. goes exactly in this direction; it describes a relatively simple technique to improve the OLEDs’ lightextraction efficiency by placing a film with bulk and surface micropores at the air–glass interface. In fact, the typical structure of an OLED, consisting of several layers of organic materials deposited over a flat glass substrate, is intrinsically prone to trap light inside it by total internal reflection (waveguiding). Usually, the best way to cope with this problem is to modify the interface between the air and the glass substrate, for example by corrugating this interface in some way or making an array of microlenses. This approach has the significant advantage of not interfering with the deposition of the organic materials on the other side of the device. However, various techniques for implementing this air–substrate interface modification studied before this work were not well suited for mass production, since they required repeated steps of lithography and curing of the samples. The technique devised by Liu et al. seems to overcome this problem completely: by creation of a microporous film at this critical interface by exploiting the spontaneous polymer phase separation of a blended solution of inexpensive polystyrene and polyethylene glycol during the drying process. The results reported by the authors are impressive, since they claim to have measured an optical coupling efficiency enhancement factor of 60%.
In conclusion, it is difficult to overestimate the importance of this research for the development of inexpensive OLEDs suitable for mass production. The time is coming when we will begin to harvest the huge benefits of this new technology.
-- Gianluca Rotaris
ToC Category: Light-Emitting Diodes
|OCIS Codes:||(080.3630) Geometric optics : Lenses|
|(230.3990) Optical devices : Micro-optical devices|
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