Light-emitting diodes (LEDs) have now replaced other technologies in most lighting applications. And the organic variety (OLED) is increasingly replacing liquid crystals in the displays of mobile devices, for example. Now LEDs are demonstrating that they can do a whole lot more—in multifunctional displays that supply themselves with electricity.
The fact that light-emitting diodes (LEDs) are not only the most popular light source in the world, but are also sensitive to light, is nothing new. All semiconductor components with a so-called P-N junction react to light with electricity. In the early days of electronics, a bunch of aspiring engineers and electronics enthusiasts scraped the paint off a germanium transistor’s glass housing that had been painted black to get a cheap photo diode. In his well-known “Engineer’s Notebooks” from 1977, Forrest M. Mims refers to the fact that LEDs could also be used as photodiodes. At the time, he described using an LED matrix as a multi-touch field as an application.
Fifty years later, a further development of this “old” principle is getting people’s attention. If it is up to researchers at the University of Illinois, it could be used in the future to charge smartphones via their displays and for touch-free gesture control. Self-charging displays could also transmit data via light (Li-Fi).
Nanorods for self-charging displays
And all that with an array of nanorod LEDs, each smaller than 5 nanometers in diameter. They consist of three different semiconductor materials, one of which emits and absorbs light. The other two control the charge flow through the first semiconductor. It switches between the emitter and the detector using a frequency that is three orders of magnitude faster that the image refresh rate of conventional displays. In other words, it is completely unnoticeable to the human eye.
Besides self charging and gesture control, “simply” reversing the typical LED function would make it possible to implement a number of other applications. For instance, it would make it possible to constantly adapt a tablet’s brightness at the pixel level. Anyone who works outdoors will appreciate that. Portions of the screen that are in the shade could be dimmed down, while the contrast in lighter portions is increased.
It is difficult to say if and when self-charging displays will show up in our everyday lives. Optimizing an electronic component that emits light and converts it into electricity at the same time—precisely opposite characteristics—is certain to come up against physical limits. Right now, the only configuration that works is using red LEDs. That may be because an LED can only “harvest” light whose emission frequency is lower than its own. In other words, an LED that emits red light can convert red, yellow, green or blue light into electricity, but a blue LED can only deal with blue or ultraviolet light. Still, researchers are certain that they have only scratched the surface of possibilities. And no doubt they’re right.