Using a new nanoscale structure, Princeton researchers led by electrical engineering professor Stephen Chou increased the brightness and efficiency of light-emitting diodes (LEDs) made of organic materials (flexible carbon-based sheets) by 57%. The researchers also report their method should yield similar improvements in LEDs made in inorganic (silicon-based) materials used most commonly today.
RELATED ARTICLE: Coating that mimics firefly scales boosts LED light extraction
The solution presented by Chou's team is the invention of a nanotechnology structure called PlaCSH (plasmonic cavity with subwavelength hole-array). The researchers reported that PlaCSH increased the efficiency of light extraction to 60%, which is 57% higher than conventional high-end organic LEDs. At the same time, the researchers reported that PlaCSH increased the contrast (clarity in ambient light) by 400%. The higher brightness also relieves the heating problem caused by the light trapped in standard LEDs.
Chou first used the PlaCSH structure on solar cells, which convert light to electricity. In a 2012 paper, he described how the application of PlaCSH resulted in the absorption of as much as 96% of the light striking solar cells' surface and increased the cells' efficiency by 175%. Chou realized that a device that was good at absorbing light from the outside could also be good at radiating light generated inside the device--offering an efficient solution for both light extraction and the reduction of light reflection.
The physics behind PlaCSH are complex, but the structure is relatively simple. PlaCSH has a layer of light-emitting material about 100 nm thick that is placed inside a cavity with one surface made of a thin metal film. The other cavity surface is made of a metal mesh with incredibly small dimensions: it is 15 nm thick; and each wire is about 20 nm in width and 200 nm apart from center to center.
Because PlaCSH works by guiding the light out of the LED, it is able to focus more of the light toward the viewer. The system also replaces the conventional brittle transparent electrode, making it far more flexible than most current displays. Another benefit for manufacturers is cost. The PlaCSH organic LEDs were made by nanoimprint, a technology Chou invented in 1995, which creates nanostructures in a fashion similar to a printing press producing newspapers.
The method also improves the picture clarity of LED displays by 400% compared with conventional approaches. The article published online August 19 in the journal Advanced Functional Materials describes the work.
SOURCE: Princeton University; http://www.princeton.edu/main/news/archive/S41/14/79S63/index.xml?section=topstories
