In solar cells, the cheap, easy to make materials called perovskites are adept at turning photons into electricity. Now, perovskites are turning the tables, converting electrons into light with an efficiency on par with that of the commercial organic light-emitting diodes (LEDs) found in cellphones and flat screen TVs. And in a glimpse of how they might one day be harnessed, researchers reported last week in Science Advances that they’ve used a 3D printer to pattern perovskites for use in full-color displays.
“It’s a fantastic result, and quite inspirational,” says Richard Friend, a physicist at the University of Cambridge in the United Kingdom whose team created the first perovskite LED in 2014. The result raises hopes that the computer screens and giant displays of the future will consist of these cheap crystalline substances, made from common ingredients. Friend cautions, however, that the new perovskite displays aren’t yet commercially viable.
The materials in current semiconductor LEDs, including the organic versions, require processing at high temperatures in vacuum chambers to ensure the resulting semiconductors are pristine. By contrast, perovskites can be prepared simply by mixing their chemical components in solution at room temperature. Only a brief heat treatment is needed to crystallize them. And even though the perovskite crystals end up with imperfections, these defects typically don’t destroy the materials’ ability to emit light.
In most perovskite LEDs, electrodes sandwiching the light-emitting material deliver charges—negatively charged electrons and positively charged electron vacancies. When the charges meet at the center of the sandwich, electrons fill the vacancies and give up a bit of their energy as a photon of light.
The color of the photon depends on the perovskite’s chemical constituents, enabling researchers to tune the color by changing the perovskite’s recipe. The Cambridge group’s first perovskite LEDs glowed near-infrared, red, or green, depending on their makeup. Since then, the team and other groups have made a full spectrum of colors.
The earliest perovskite LEDs converted only 0.76% of electrons into photons. That’s because electrical charges moving through the material got stuck at the boundaries between the myriad crystallites making up the material. But numerous teams have overcome that hurdle. Late last year in Nature Photonics, for example, Friend’s group reported that by adding a light-emitting polymer layer that helps steer charges around the surface defects, it had made red perovskite LEDs with an efficiency of 20.1%.
A team led by chemist Edward Sargent at the University of Toronto in Canada took a different approach last year, spiking its perovskite recipe with an additive that formed crystalline shells around the perovskite crystallites. The shells blocked the defects from trapping charges, resulting in a green perovskite LED with 20.3% efficiency, the team reported in Nature. That remains well below the efficiency of many inorganic LEDs, but is probably good enough for some applications.
Researchers led by Feng Gao, a physicist at Link?ping University in Sweden, reported online on 25 March in Nature Photonics that they developed yet another way to tackle the defect problem. They targeted the tendency of lead ions at the edges of perovskite crystallites to trap passing electrons. With an additive that bound to the lead, they reduced the ions’ hunger for electrons and created a near-infrared LED that had 21.6% efficiency.
The pace of improvements in the past 5 years has been “quite exceptional,” Friend says. Still, none of the perovskite devices survives more than about 50 hours, well below the estimated 10,000 hours needed for commercial use. Just why the perovskite crystals fall apart after a few dozen hours isn’t clear, Gao says. But short lifetimes also plagued early organic LEDs, he notes. And perovskite solar cell–makers have largely solved similar longevity issues by protecting their devices from air and humidity. “I’m optimistic this area can also develop quickly, and perovskite LEDs can improve,” Gao says.
If they do, the latest work from researchers led by Jennifer Lewis, a materials scientist at Harvard University, could point to new strategies for constructing displays. Lewis and her colleagues used a 3D printer to arrange tiny, wire-shaped perovskite structures in multicolor displays. As the “ink” carrying the nanowires passed through the printer nozzle, shear forces aligned them, Lewis says. The common orientation of the nanowires gave light from each LED a single preferred oscillation, or polarization.
For their prototype displays, Lewis’s team didn’t wire each LED to electrodes; instead, the researchers exposed the entire display to ultraviolet (UV) light. Like an applied electric voltage, the UV light kicks electrons out of their normal state, allowing them to move. Then, they can recombine with vacancies and emit visible light. But because the emitted light was polarized, Lewis and her colleagues could use polarizing filters to control it.
In one example, the researchers used three different perovskite formulations to create displays in which each pixel contained a red, green, and blue spot side by side, with the orientation of the nanowires in each spot offset by 60°. By rotating a polarizing filter, the researchers could mix colors or isolate a single color.
Plenty of hurdles remain for perovskite LEDs, Sargent says. But he adds, “This work jumps ahead 10 years in the future and shows what cool things we can do.”