A team of researchers from the University of California, Berkeley, and the Lawrence Berkeley National Laboratory has developed a bright-light emitting device that is millimeters wide and fully transparent when turned off. The device, described in a paper in the March 26, 2018 issue of the journal Nature Communications, opens the door to invisible displays on walls and windows or in futuristic applications such as light-emitting tattoos.
The new electroluminescent device was developed by University of California, Berkeley Professor Ali Javey and members of his lab.
The light emitting material in this device is a monolayer semiconductor, which is just three atoms thick.
“The material is so thin and flexible that the device can be made transparent and can conform to curved surfaces,” explained Dr. Der-Hsien Lien, a postdoctoral fellow at the University of California, Berkeley.
In 2015, the team published research showing that monolayer semiconductors are capable of emitting bright light, but stopped short of building a light-emitting device.
The new work overcame fundamental barriers in utilizing LED technology on monolayer semiconductors, allowing for such devices to be scaled from sizes smaller than the width of a human hair up to several millimeters. That means that researchers can keep the thickness small, but make the lateral dimensions (width and length) large, so that the light intensity can be high.
Commercial LEDs consist of a semiconductor material that is electrically injected with positive and negative charges, which produce light when they meet.
Typically, two contact points are used in a semiconductor-based light emitting device; one for injecting negatively charged particles and one injecting positively charged particles.
Making contacts that can efficiently inject these charges is a fundamental challenge for LEDs, and it is particularly challenging for monolayer semiconductors since there is so little material to work with.
Professor Javey and colleagues engineered a way to circumvent this challenge by designing a new device that only requires one contact on the semiconductor.
By laying the semiconductor monolayer (MoS2, WS2, MoSe2, and WSe2) on an insulator and placing electrodes on the monolayer and underneath the insulator, they could apply an AC signal across the insulator.
During the moment when the AC signal switches its polarity from positive to negative (and vice versa), both positive and negative charges are present at the same time in the semiconductor, creating light.
The scientists showed that this mechanism works in four different monolayer materials, all of which emit different colors of light.
This device is a proof-of-concept, and much research still remains, primarily to improve efficiency. Measuring its efficiency is not straightforward, but the researchers think it’s about 1% efficient. Commercial LEDs have efficiencies of around 25 to 30%.
“A lot of work remains to be done and a number of challenges need to be overcome to further advance the technology for practical applications,” Professor Javey said.
“However, this is one step forward by presenting a device architecture for easy injection of both charges into monolayer semiconductors.”
Der-Hsien Lien et al. 2018. Large-area and bright pulsed electroluminescence in monolayer semiconductors. Nature Communications 9, article number: 1229; doi: 10.1038/s41467-018-03218-8