Princeton researchers have refined the manufacturing of light sources made with crystalline substances known as perovskites, a more efficient and potentially lower-cost alternative to materials used in LEDs found on store shelves (Photos by Sameer Khan/Fotobuddy).Perovskite materials have optoelectronic properties that are promising for cheap, easy to make light-emitting diodes (LEDs). But for perovskite devices to emit light efficiently, electrons and holes must be confined within small regions to drive the radiative recombination necessary for light emission.
Researchers from Princeton University believe the answer could lie in forming small grains in a perovskite layer when an LED is fabricated [Xiao et al., Nature Photonics (2017), doi: 10.1038/nphoton.2016.269].
“We knew that, as a direct band gap semiconductor, many metal halide perovskites have significant potential as LEDs,” explains Barry P. Rand, who led the effort. “Success hinges on preparing thin, smooth, and pinhole-free films of perovskites with well-passivated crystallites.”
But left unfettered perovskite crystallizes rapidly, forming grains up to hundreds of nanometers in size. To create smaller, nanosized grains, Rand and his team came up with a simple means of limiting the growth of perovskite crystals.
During the room-temperature solution processing of perovskite thin-film layers, long-chain organic molecules (n-butylammonium halides, or BAX, where X is I or Br) are introduced into the mixture to impede the growth of the crystal grains and reduce the film roughness.
“This strategy works quite well and has allowed us to make devices with external quantum efficiency (EQE) of approximately 10% for bromide (green) and iodide (red/near-infrared) perovskites,” says Rand.
The addition of long-chain ammonium halides has other benefits too: the overall reproducibility, performance, and long-term stability of perovskite LEDs are all improved. Perovskite LEDs prepared without long-chain ammonium halides see efficiency decrease substantially in just a few days. By contrast, I- and Br-perovskite LEDs show no degradation in performance after months of storage.
The researchers believe that their approach could provide a general means of preparing efficient, stable perovskite LEDs and other optoelectronic devices.
“We have determined our approach to be quite general, having applied this processing paradigm to both hybrid organic and fully inorganic metal halide perovskites,” says Rand.
This means that the team has been able to demonstrate both efficient green emitting devices and red/near-infrared emitters.
“That could make these devices useful for displays, where their narrowband emission can act as saturated color pixels, as well as in solid-state lighting applications, where the ability to make multiple colors enables the mixing of those colors to produce white light,” he explains.
This article was originally published in Nano Today (2017), doi: 10.1016/j.nantod.2017.02.002.