NEWS | Jan. 31, 2018

Materials Research Team Lights the Way for More Efficient LEDs

By Daniel Parry

WASHINGTON — Researchers at the U.S. Naval Research Laboratory (NRL) Center for Computational Materials Science, working with an international team of physicists, have revealed that nanocrystals made of cesium lead halide perovskites (CsPbX3), is the first discovered material which the ground exciton state is "bright,” making it an attractive candidate for more efficient solid-state lasers and light emitting diodes (LEDs).

“The discovery of such material, and understanding of the nature of the existence of the ground bright exciton, open the way for the discovery of other semiconductor structures with bright ground excitons,” said Dr. Alexander Efros, research physicist, NRL. “An optically active bright exciton in this material emits light much faster than in conventional light emitting materials and enables larger power, lower energy use, and faster switching for communication and sensors.”

The work, which was partially sponsored by the Office of Naval Research through a program managed by Dr. Chagaan Baatar, studied lead halide perovskites with three different compositions, including chlorine, bromine, and iodine. Nanocrystals made of these compounds and their alloys can be tuned to emit light at wavelengths that span the entire visible range, while retaining the fast light emission that gives them their superior performance.

Semiconductors emit light when bound pairs of electrons and holes, known as excitons, recombine in a process called radiative decay.

“In all known semiconductors and semiconductor nanostructures, the lowest energy state for a bound electron-hole pair is a ‘dark’ state,” said Efros. “This means the material emits light slowly and weakly.”

Because in perovskite nanocrystals the lowest energy exciton is bright, the time it takes for the electron and hole to recombine and emit light, known as their radiative lifetime, is 20 times faster than conventional materials at room temperature and 1000 times faster at cryogenic temperatures.

It is known, that quantum-dot based LEDs, or QLEDs, can suffer from “droop,” or reduced efficiency, at high pumping intensity due to processes that dissipate the energy of excitons before they have time to emit light. The decreased radiative lifetime should make it possible for LEDs based on these perovskites to use all of the energy input to create light before it is dissipated through slower processes.

“The increased rate of light emission of these materials holds great promise for various technological applications that rely on LEDs and lasers,” Efros said. “In principle, the 20 times shorter lifetime could therefore lead to 20 times more intense LEDs and lasers.” The power of a laser depends on the gain of the material it is made of, and this gain is proportional to the radiative emission rate.

Communication in free space using visible light, which makes it possible to transmit information in tight beams for long distances without fiber optic or copper cables, would also benefit from the increased light emission rates. “The maximum bandwidth of the communication system is limited by the rate at which the LEDs can turn on and off, and the shorter radiative lifetime translates directly into faster switching and therefore a higher data transmission rate,” says Efros.

The success of this work was due to a close collaboration between several experimental groups in Zurich, Switzerland and U.S. theoreticians. The industrial, academic and laboratory research team that contributed equally to this publication include: Michael A. Becker, Thilo Stöferle, Rainer F. Mahrt and Gabriele Rainò from IBM Research, Zurich, Switzerland; David J. Norris from Optical Materials Engineering Laboratory, ETH Zurich, Zurich, Switzerland; Roman Vaxenburg and Andrew Shabaev from Computational Materials Science Center, George Mason University, Fairfax, Virginia; Georgian Nedelcu and Maksym V. Kovalenko from Institute of Inorganic Chemistry, Department of Chemistry and Applied Bioscience, ETH Zurich, Zurich Switzerland and Laboratory of Thin Films and Photovoltaics, Empa – Swiss Federal Laboratories for Materials Science and Technology, Dübendorf, Switzerland; Peter C. Sercel from T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California; Michael J. Mehl from U.S. Naval Academy, Annapolis, Maryland; and John G. Michopoulos, Samuel G. Lambrakos, Noam Bernstein and John L. Lyons in addition to Alexander Efros from the Center for Computational Materials Science, Naval Research Laboratory, Washington, D.C.

Full details of this research entitled “Bright triplet excitons in cesium lead halide perovskites,” can be found in the January 11, 2018 edition of Nature (doi:10.1038/nature25147).