Flexible Polymer Microcavity LEDs Use Polymer Mirrors
Anne L. Fischer
LED displays are getting larger, brighter, thinner and flexible to the point where they can be shaped and wrapped in unique ways. To do this, the LEDs must be made of a pliable material. Now a group at South China University of Technology in Guangzhou suggests that an all-polymer distributed Bragg reflector made by 3M of St. Paul, Minn., could enable the realization of flexible full-color displays in combination with white polymer LEDs.
The structure of the microcavity polymer LED uses a flexible polymer distributed Bragg reflector thatprovides functionality similar to that of microcavity mirrors.
The investigators fabricated a flexible polymer planar microcavity of a Fabry-Perot resonator using multilayer polymeric interference mirrors as an anode reflector. They used this in combination with a transparent conducting polyaniline film spin-coated from an m-cresol solution as a hole-injection anode. They found that the performance of these polymer LEDs is similar to that of inorganic distributed Bragg reflector microcavity polymer LEDs with regard to their external quantum efficiency, spectral narrowing and angle dependence of resonance peaks.
According to Yong Cao, a professor at the university, the polymer reflector mirrors are key to making this technology possible. However, device efficiencies and emission spectra must be further optimized. He explained that one of the major issues limiting device performance is the low conductivity of the conducting polymer anode layer. Better electrical contact can be achieved by use of a relatively thick polyaniline film.
The research to date is a first step toward printable flexible microcavity devices, Cao said. By using the polymer distributed Bragg reflector, all elements of the device except the metal cathode can be printed without vacuum deposition. The next step is to combine this technology with the printable cathodes developed by the group, in which a metal paste can be printed on top of the emitting layer.
Cao said that, if they can combine these two technologies, they will be able to realize printable microcavity devices on the 3M distributed Bragg reflector. This will open the way to printable large-area polymer multipixel microcavity devices, enabling flexible full-color displays as well as large-area flexible pixelated light sources and detectors.
Applied Physics Letters, Dec. 12, 2005, 243504.
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