Commercially available LEDs are made of direct band gap materials, mainly combinations of gallium. Silicon hitherto was considered unfit for use in LEDs since it only can generate light in the red and near-infrared spectrum. The researchers from Karlsruhe and Toronto have changed this perspective. The extremely tiny silicon nanocrystals they created (one to three nanometers in size) emit light. The light color depends on the size of the crystals: The bigger the crystal the longer the wavelength of the light emitted.
The research team around Uli Lemmer and professor Annie K. Powell (Karlsruhe) and professor Geoffrey A. Ozin (Toronto) also developed a process to separate the nanocrystals by size. This enables them to build tiny LEDs with specific colors and combine several LEDs to multi-color arrays. "The innovation is the controlled production of silicon LEDs emitting various colors", said doctoral candidate Florian Maier-Flaig from the Karlsruhe School of Optics and Photonics who contributed to the research activities. The architecture of such SiLEDs resemble OLEDs in that stacks of these solid state devices are contacted and powered in parallel. They so far however are unable to generate white light. White light theoretically could be generated by mixing different colors, but so far no blue-light emitter has been developed.
Image: SiLEDs offer huge potential for lighting and active screen applications. The contacts are made of organic materials.
The SiLEDs feature a surprising long-term stability which has not been achieved before, Maier-Flaig said. This property is a consequence of the scientists’ ability to separate the objects by size – no relatively large objects no longer can cause electric shortcuts. In addition, the SiLEDs they create feature a very high homogeneity of the light-emitting surfaces.
The researcher also were pleased by their SiLED’s relatively high efficiency of about 1 percent. If one compares this value with conventional LEDs this does not seem to be very impressive, but there is no physical limit to the efficiency, explained Maier-Flaig.
Another advantage of the silicon nanocrystals is that they don’t contain toxic heavy metals. Unlike cadmium selenide, cadmium sulphide and lead sulphide used by other research groups, silicon nanocrystals do not contain any toxicity.
The SiLEDs are still far from being commercialized. The Karlsruhe researchers see potential technical applications in area lighting – for example screen backlighting. In order to bring this technology closer to a commercial use the research teams announced to continue their activities with partners.