Simple Color Tuning of LED Bulbs

A new technique – the result of an inter­national colla­boration of scientists from Lehigh University, West Chester University, Osaka Univer­sity and the Uni­versity of Amsterdam – could pave the way for monolithic inte­gration for simple color tuning of a light bulb, according to Volkmar Dierolf, Chair of Lehigh’s Department of Physics, who worked on the project. “This work could make it possible to tune between bright white and more comfortable warmer colors in commercial LEDs,” says Dierolf.

A GaN:Eu LED, which can be tuned from red-yellow due to red and green light mixing from different Eu states. Below a GaN:Eu LED with additionally added Si/Mg, which adds blue emission. (Source: West Chester U.)

The team demonstrated the possi­bility of color tuning Gallium Nitride (GaN)-based GaN LEDs simply by changing the time sequence at which the operation current is provided to the device. Light-emitting diodes or LEDs are semi­conductor devices that emit light when an electric current is passed through it. Notably, the technique is compatible with current LEDs that are at the core of commercial solid state LED lighting.

In today’s active LED displays, different colors are produced by three to four indi­vidual LEDs that are placed close to each other and create the different funda­mental colors needed to produce the full color spectrum. “We demonstrate that this can be achieved by a single LED,” says Dierolf. “We show that is possible to attain red, green and blue emissions originating from just one GaN LED-structure that uses doping with a single type of rare earth ion, Europium (Eu). Using inten­tional co-doping and energy-transfer engineering, we show that all three primary colors can emit due to emission ori­ginating from two different excited states of the same Eu3+ ion (~620 nm and ~545nm) mixed with near band edge emission from GaN centered at ~430nm. The inten­sity ratios of these transi­tions can be controlled by choosing the current injection conditions such as injection current density and duty cycle under pulsed current injection.” In other words, the team achieved color-tunability in a single GaN-based LED through the mani­pulation of the emission properties of an atomic-type dopant.

Mitchell pointed out that “The main idea of this work – the simultaneous active exploi­tation of multiple excited states of the same dopant – is not limited to the GaN:Eu system, but is more general. 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.” According to Dierolf, this research may benefit those who are looking for more comfortable “warmer” white light from LEDs.

“It could pave the way for monolithic inte­gration for simple color tuning of a light bulb,” adds Dierolf. “It would also be beneficial for micro-LED displays, since it allows for higher density of pixels.” The materials used in previous research on color tunable LEDs did not allow for easy inte­gration with current LED tech­nology, he adds. This work is compatible with current GaN-based LEDs that are at the core of commercial solid state LED lighting. (Source: Lehigh U.)

Reference: B. Mitchell et al.: Color-Tunablility in GaN LEDs Based on Atomic Emission Manipulation under Current Injection, ACS Phot., online 17 April 2019; DOI: 10.1021/acsphotonics.8b01461

Link: Deming Lewis Lab., Dept. of Physics, Lehigh University, Bethlehem, USA

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