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Self-Assembling Particles Could Produce Lower Cost LEDs

Photonics Spectra
Mar 2017
PRINCETON, N.J. — Self-assembling nanoscale perovskite — crystalline substances — particles could be a more efficient and lower-cost alternative material for LEDs.

Princeton engineering researchers have developed a novel self-assembling technique, where in the past the inability to create uniform and bright nanoparticle perovskite films limited their potential.

New type of LED made with crystalline perovskites.
A new type of LED is made with crystalline substances known as perovskites. Courtesy of Sameer A. Khan/Fotobuddy.

Barry Rand, an assistant professor of electrical engineering and assistant professor at the Andlinger Center for Energy and the Environment at Princeton, said “Our new technique allows these nanoparticles to self-assemble to create ultra-fine grained films, an advance in fabrication that makes perovskite LEDs look more like a viable alternative to existing technologies.”

LEDs emit light when voltage is applied across the LED. When the light is turned on, electrical current forces electrons from the negative side of the diode to the positive side. This releases energy in the form of light. LEDs operate best when this current can be strictly controlled. Rand and his team have created a device with thin nanoparticle-based films that allows just that.

Rand's team and others researchers are exploring perovskites as a potential lower-cost alternative to gallium nitride (GaN) and other materials used in LED manufacturing. Lower-cost LEDs would speed the acceptance of the bulbs, reducing energy use and environmental impacts.

Perovskites can be either superconductive or semiconductive, depending on their structure; this makes them promising materials for use in electrical devices. Potentially, they could also serve as a replacement for silicon in solar panels, as they are cheaper to manufacture yet offer equal efficiency as some silicon-based solar cells.

Hybrid organic-inorganic perovskite layers are fabricated by dissolving perovskite precursors in a solution containing a metal halide and an organic ammonium halide. It is a relatively cheap and simple process that could offer an inexpensive alternative to LEDs based on silicon and other materials.

However, while the resulting semiconductor films could emit light in vivid colors, the crystals forming the molecular structure of the films were too large, which made them inefficient and unstable.

Rand and his team made some changes to the process using an additional type of organic ammonium halide, and in particular a long-chain ammonium halide, adding it to the perovskite solution during production. This dramatically constrained the formation of crystals in the film. The resulting crystallites were much smaller (around 5-10 nanometers across) than those generated with previous methods, and the halide perovskite films were far thinner and smoother.

This led to better external quantum efficiency, meaning the LEDs emitted more photons per number of electrons entering the device. The films were also more stable that those produced by other methods.

Russell Holmes, a professor of materials science and engineering at the University of Minnesota, said the Princeton research brings perovskite-based LEDs closer to commercialization.

"Their ability to control the processing of the perovskite generated ultra-flat, nano-crystalline thin films suitable for high efficiency devices," said Holmes. "This elegant and general processing scheme will likely have broad application to other perovskite active materials and device platforms."

This advance could speed the use of perovskite technologies in commercial applications such as lighting, lasers and television and computer screens. The research finding have been reported in the journal Nature Photonics (doi:10.1038/nphoton.2016.269).

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