As its name implies, “The Drawing Board” column, each month, often discusses innovations that look highly promising in the laboratory but aren’t quite ready for prime time. On occasion, that requires perusing abstracts from scientific journals and browsing through a few Ph.D. dissertations.
Many of these innovations never make it to commercial production; others represent an entirely new direction, and still others promise to breathe new life into mature technologies.
Light-emitting diodes (LEDs) are a good example. They’ve been around for decades: the first mass-produced red LEDs appeared in 1968. Today LEDs are available with outputs that run from near-infrared to ultraviolet.
Despite the maturity of the basic technology, there’s still a steady stream of incremental improvements in efficiency, thermal management, and aging, among others. New technologies appear regularly, too: organic LEDs (OLEDs), for example, have been grabbing a lot of attention over the last couple of years.
Looking down the road, LEDs incorporating quantum-dot technology (QD-LEDs) might well be the next big thing in LED development.
A quantum dot (QD) is a semiconductor structure with a diameter of only a few nanometers. When a quantum dot absorbs a photon, it generates an electron-hole pair that recombines to emit a new photon.
The light color of a standard LED depends on the bandgap of the semiconductor used, but the color of a QD photon depends both on the semiconductor and the size of the quantum dot. As shown in Figure 2, bigger dots emit longer wavelengths, close to red (620-750 nm); smaller dots emit shorter wavelengths, closer to the violet end (380-450 nm) of the spectrum. The emission spectrum has a very narrow band: the full width at half maximum (FWHM)—the bandwidth at which the output power reaches half its maximum value—is less than 30 nm. As a result, the colors appear extremely bright with vivid hues.
Incidentally, so-called “QLED” TVs have been around since last year, but these don’t use QD-LEDs, which are electro-emissive: the quantum dots are stimulated to emit photons by electrons. A QLED TV is a photo-emissive device that uses a blue LED backlight combined with a quantum-dot filter for red and green. The QD conversion is highly efficient (much more than 90 percent), but overall efficiency suffers as many photons are lost between the backlight and the QD filter.
To create a quantum dot with a specific size, manufacturers adjust the temperatures and the timing of the chemical reactions during production. Quantum dots originally used cadmium selenide (CdSe), but cadmium is classified as a hazardous substance under the European RoHS directive, so more recent devices use indium phosphide. Researchers are investigating other cadmium-free materials including graphene and perovskite, but that work is in the early stages.
Current QD-LED challenges include susceptibility to high temperature, moisture, and humidity. The efficiency of electro-emissive QD pixels must also improve. QD leader Nanosys publicly demonstrated red, green, and blue emissive QD pixels for the first time at the 2018 SID/DSCC Business Conference. At that time, the external quantum efficiency (EQE)—the ratio of the number of photons emitted from the LED to the number of electrons passing through the device—for all three colors was 10 percent, compared to 90 percent for photo-emissive QDs.
When these problems have been solved, what will binge-watchers have to look forward to? A display based on quantum-dot electro-luminescence (QDEL) technology (Figure 1) will be fully emissive with no backlight, LCD layer, or color filters. It will be cheap to manufacture because the quantum dots will be applied as a liquid layer at room temperature (think ink-jet printing) and self-organize as they dry. It also will be highly-efficient, extremely thin, feature a wide viewing angle, and display a broader range of colors than current technologies.
We won’t have to wait too long—the first QDEL displays are expected in the next two to four years.
All in all, it looks like the future for LEDs is, um, bright. Feel free to groan.
