Breaking: Scientists Develop The First Color-Tunable LED

A Holy Grail in the production of next-generation displays is the need for LEDs that can be induced to change their color in response to shifting electrical inputs. Despite the obvious utility of such a device, scientists have been widely stumped by this problem. This is no longer the case with scientists last week finally reporting in Nature Communications that a color shifting LED has been developed using everyone’s favorite wonder material, graphene. A revolution in display technology is imminent.

Color-Changing LED: changing the applied voltage lets this light switch from red to blue and every color in between. Image reproduced with permission from Nature Comm. (2015) 6, 7767.Color-Changing LED: changing the applied voltage lets this light switch from red to blue and every color in between. Image reproduced with permission from Nature Comm. (2015) 6, 7767.

Light emitting diodes (LEDs) are a ubiquitous component in today’s semiconductor technology allowing, among other things, the development of high-performance communication, low-cost lighting and smart displays. Typically, they emit a single color which is predefined during fabrication by the materials employed. This is a relatively inefficient system as the color must be engineering by complicated synthetic means and each color of LED requires its own production line. For this reason, considerable research has been put into finding a way to make them color-tunable, but always with minimal success. Researchers have never even managed to make one that can switch back and forth between just two colors. That is part of why last week’s announcement is so remarkable – not only does the new graphene-based LED change colors, it is able to emit across almost the entire visible range from purple to red.

So how were scientists able to achieve such a feat? The key insight was the recognition that a single form of graphene did not have the required properties, instead a blend of two forms would be necessary. Indeed, there is no single form of graphene that emits any light at all. The two forms chosen were graphene oxide and reduced graphene oxide. Neither will emit light on its own, but for two opposite reasons: graphene oxide possesses too large an energy gap, and reduced graphene oxide possesses zero energy gap. When the two are put together, a region at the interface is formed with varying middle-of-the-road energy gap sizes that allow the emission of varying wavelengths of light.

Naturally, the next step is to grasp the commercial potential of this invention. The authors of the study are careful to note that this publication represents the very first breakthrough in this area and that subsequent studies must follow to optimize the design. There are two major obstacles that have to be overcome to produce commercially viable versions.  Firstly, the devices display a relatively low efficiency – a lot of electricity is needed to generate a useful amount of light. Secondly, the lifetime of the devices is quite short due to instability in air. Both of these are already being tackled by the researchers who have plenty of previous work to draw on as these same shortcomings were observed in early single-color LEDs as well.

Flexible and bright: though there remains room for improvement, the devices are already very bright and flexible -- two properties that will prove valuable for up-and-coming electronics. Image reproduced with permission from Nature Comm (2015) 6, 7767.Flexible and bright: though there remains room for improvement, the devices are already very bright and flexible -- two properties that will prove valuable for up-and-coming electronics. Image reproduced with permission from Nature Comm (2015) 6, 7767.

Despite the challenges, the advantages and potential market share make color-tunable graphene LEDs well worth the effort. Along with their ability to shift color, they are also thin, flexible, bright, and made of relatively inexpensive materials. They can “enable the realization of flexible display technologies that can cover the entire visible spectrum” says lead study author Professor Tian-Ling Ren at Tsinghua University in Beijing who also suggests that it may only take a few years to progress from prototype to commercialization due to the low cost and simplicity of their fabrication. Having written repeatedly now about the inevitable demand for flexible electronics, it is always exciting to see new technologies emerge to meet this need.

Via Phys.org and Nature Communications.