Color-Tuning Technique Uses a Single LED

In active LED displays, different colors are produced by three to four individual LEDs that are placed close to each other to create the different colors needed to make up the full color spectrum. An international research team has demonstrated that this result could be achieved using a single LED.

Scientists from Lehigh University, West Chester University, Osaka University, and the University of Amsterdam tuned the color of gallium nitride (GaN)-based LEDs by manipulating the emission properties of an atomic-type dopant, europium (Eu). Using intentional codoping and energy-transfer engineering, the team attained all three primary colors, due to an emission originating from two different excited states of the same Eu³⁺ ion mixed with near-band-edge emission from GaN centered at about 430 nm. The researchers said that intensity ratios of these transitions could be controlled by choosing the current injection conditions, such as injection current density.

The top row is a GaN:Eu LED, which can be tuned from red-yellow due to red and green light mixing from different Eu states. The middle and bottom rows are of a GaN:Eu LED with added Mg/Si, which adds blue emission. Each picture is under a different current injection/filtering condition. Courtesy of West Chester University.

Professor Brandon Mitchell said that the experimental findings were not limited to the GaN:Eu system. “The presented results could open up a whole new field of tunable emission of colors from a single dopant in semiconductors, which can be reached by simple injection current tuning,” he said.

The research could benefit those who are looking for more comfortable white light from LEDs. “This work could make it possible to tune between bright white and more comfortable warmer colors in commercial LEDs,” professor Volkmar Dierolf said.

The team believes that its work could pave the way for monolithic integration for simple color tuning of a light bulb. “It would also be beneficial for microLED displays, since it allows for higher density of pixels,” Dierolf said.

According to the team, in previous research on color-tunable LEDs, the materials used did not allow for easy integration with current LED technology. The technique used by the team from Lehigh et al. is compatible with the GaN-based LEDs that are used in today’s commercial solid-state LED lighting.

The research was published in ACS Photonics (http://dx.doi.org/10.1021/acsphotonics.8b01461).