Color in Nanocrystalline LEDs Can Be Tuned

A novel, economical way to tune the color of LEDs by altering the semiconductor nanocrystals used to make them could be applicable for industrial-scale production.

The color of some LEDs can be tuned by modifying the size of the semiconductor crystals that compose the light-emitting layer. In crystals that are only a few nanometers in size, quantum mechanical effects can affect the LED. Quantum-confined metal halide perovskites offer narrowband, color-pure emitters.

Researchers at Ludwig Maximilian University of Munich, in collaboration with researchers at the University of Linz, have developed a method for producing semiconducting nanocrystals, using perovskite, whose size and shape they can control. The crystals are extremely stable, creating LEDs that exhibit high color fidelity.

Researchers used nanoporous silicon and alumina thin films as templates for the growth of the perovskite nanocrystals. The growth of the perovskite crystals was confined directly within the device-relevant architectures, yielding perovskite nanocrystals a few nanometers in size.

By reducing the pore size, researchers found significantly blue-shifted photoluminescence emission in the nanocrystals. They surmised that confining perovskite nanocrystals within porous oxide thin films drastically increased photoluminescence stability because the templates served to encapsulate the perovskite.

Researchers quantified the template-induced size of the perovskite crystals in nanoporous silicon using microfocus high-energy x-ray depth profiling in transmission geometry, and verified the growth of perovskite nanocrystals throughout the entire thickness of the nanoporous films.

“Optimal measurements of the size of the crystals were obtained using a fine beam of high-energy X-radiation at the Deutsche Elektronen-Synchrotron (DESY) in Hamburg,” said researcher Bert Nickel.

A thin wafer, only a few nanometers thick and patterned like a waffle, was used as a template. The depressions in the wafer served as tiny reaction vessels, whose shape and volume ultimately determined the final size of the nanocrystals.

The wafers were produced by means of an economical electrochemical process and, according to the researchers, could be fashioned directly into LEDs.

“Our nanostructure oxide layers also prevent contact between the semiconductor crystals and deleterious environmental factors such as free oxygen and water, which would otherwise limit the working lifetime of the LEDs,” said researcher Martin Kaltenbrunner. “In the next step, we want to enhance the efficiency of these diodes further, and explore their potential for use in other applications, such as flexible displays.”

The research was published in Science Advances (doi: 10.1126/sciadv.1700738).