Getting better control of the light emitted from organic LEDs (OLEDs) could lead to faster links between the Internet and mobile devices, according to a Scottish researcher.
Anyone who's tried to use the Wi-Fi on a crowded airplane or a packed hotel conference room knows it can be maddeningly slow; there usually isn't enough bandwidth to go around. Some researchers, notably Harold Haas, head of the mobile communications group at the University of Edinburgh, have proposed an alternate system—Li-Fi—which rapidly flickers room lighting to send signals. To get even more bandwidth out of such a system, it would help if there were an easy way to break the light up into different colors, using individual wavelengths to send different signals.
"We're running out of radio frequency bandwidth, with WiFi, computers, and all our phones," says Ifor Samuel, who heads the organic semiconductor optoelectronics group at the University of St. Andrews in Scotland. He described a method his team has developed for making patterned OLEDs for a LiFi system at the fall meeting of the Materials Research Society (MRS) in Boston in December.
Samuel, along with Haas and other colleagues from Oxford, Cambridge, and the University of Strathclyde, are pursuing what they call ultra-parallel visible light communications, under a four-year, €4.6 million grant from the UK government. The idea, he explained to the MRS audience, is that the signal would be created by high-speed CMOS chips that alter the blue light coming from an array of small, nitride-based LEDs. OLEDs on top of the LEDs would act as a color conversion layer, multiplexing the signals into other colors.
Because OLEDs are malleable, it would be easy to imprint a diffraction grating into them. Such gratings could control the direction in which the signal was sent. That could be useful, Samuel says, in slower-speed communications where controlling direction is more desirable than a high data rate, to provide increased security or reduce power consumption.
To that end, his team has developed gratings with periods comparable in size to the wavelengths of visible light—about 300 to 400 nanometers. One method for making such gratings is called solvent-assisted micromolding, which would allow a grating to be built directly into an OLED. The polymer that makes up the OLED is coated with a solvent, then pressed against a mold containing the grating pattern. The solvent causes the polymer to swell while it dissolves away a thin layer. Once the solvent dries, the now-patterned light-emitting polymer can be lifted away from the mold and used to construct an OLED.
Another technique they've used is nanoimprint lithography, which involves creating a stamp, pressing it into a photoresist, and then using ultraviolet light to expose the photoresist in particular areas, forming the grating.
"The key thing is we have simple techniques for making the very small microstructures we need" to design directional OLED emitters, Samuel says.
One problem is that OLEDs emit a fairly broad spectrum of light, and different wavelengths will pass through the grating at different angles, forming a rainbow. To minimize this, the team searched for OLEDs with very narrow emission characteristics. OLEDS constructed with the rare earth element europium offer narrow emission, but they aren't very efficient. Samuel says his team has managed to raise the efficiency—the amount of input energy that comes out as light—to 4.3 percent. Another option is to add quantum dots, which have narrow emission spectra, as a color conversion layer in the OLED, Samuel says. The underlying OLED would cause the dots to emit the desired color of light.
"It's not so easy to have narrow-linewidth OLEDs," he admits.
Still, he argues, the work could help make Li-Fi a success. The ability to control the output of OLEDs might have applications in medicine as well. Controlling the phase and the wavefront of light would allow researchers to adjust the depth to which it penetrates tissue, which could aid both in diagnostics and in treatments that, for example, use light to activate a drug.
