In experiments performed by Kang Wang, a postdoctoral researcher in chemical engineering, he demonstrated that a rationally designed organic conjugated ligand enables efficient and bright red-emitting perovskite LEDs. The figure 0~90 indicates that the phenyl-phenyl dihedral angle is 90 degrees, which is critical for improving the phase purity of perovskite film as well as the LED device performances. This research represents the most efficient perovskite LED devices in the red color region.

Light-emitting diodes (LEDs) are semiconductor devices that have gained tremendous popularity over the last two decades due to their energy-efficient sources of lighting. They can be found almost everywhere in daily life, ranging from traffic lights to automobile brake and turn signal lights. Compared to incandescent sources, conventional LED technology has desirable advantages, including lower power consumption, longer life, broad color tunability, smaller size and faster switching speed.

Letian Dou, Charles Davidson Associate Professor of Chemical Engineering

Further advancing LED capabilities, a Purdue group in the Davidson School of Chemical Engineering has discovered that LEDs based on halide perovskites — direct band gap semiconductors — can produce more vivid, colorful and brighter images on the display panels of devices such as cell phones and TVs. Led by Letian Dou, Charles Davidson Associate Professor of Chemical Engineering, the team has published its work in Nature Communications. Their research represents the most efficient perovskite LED devices in the red color region.

On the downside, perovskite materials are less stable and can degrade quickly. Further, device efficiency has not been fully optimized to compete with conventional LEDs.

“Our work aims to resolve these critical issues,” said Dou, who conceived the idea, supervised the project and provided funding support.

Dou’s postdoctoral researcher, Kang Wang, was instrumental in the process. He synthesized the organic ligands and incorporated them into the hybrid perovskite thin films to improve the phase purity, reduce defect density and improve stability. Then, he clarified the effect of ligands on the crystallization mechanism of the perovskite films by combining a series of in-situ and ex-situ characterizations. Finally, he fabricated very efficient LED devices based on the optimal ligand and film and conducted data analysis.

“We achieved efficient and stable deep-red LEDs with a peak external quantum efficiency of 26.3%, which is the highest efficiency for red perovskite LEDs. The stability results are also better than the perovskite LEDs based on similar materials,” Wang said. “In addition, our devices exhibited wide wavelength tunability and improved spectral and phase stability compared with existing perovskite LEDs.”

Fundamentally, these discoveries provide critical insights into the molecular design and crystallization kinetics of low-dimensional perovskite semiconductors, which is critical for light-emitting devices.

Kang Wang

“Realistically, the molecular engineering strategy shown in this work could be expanded to other perovskite-based optoelectronic devices, and it also suggests that there is a bright future in transforming perovskite LED technology into real-world applications,” Dou said.

Brett Savoie, a fellow Charles Davidson Associate Professor of Chemical Engineering, assisted with the project, conducting molecular dynamics simulations and data analysis to support Wang’s findings. Alexandra Boltasseva, the Ron and Dotty Garvin Tonjes Professor of Electrical and Computer Engineering with a courtesy appointment in materials engineering, conducted optical constant measurements for the team’s LED devices. Libai Huang, professor in the Department of Chemistry, carried out ultrafast spectroscopy measurements on the perovskite films.

“We will keep pushing the efficiency and stability of LEDs to a commercialization level — for example, an external quantum efficiency of 30% and half-life of 10,000 hours,” Dou said. “These are the basic requirements for LED devices in practical use. We are very optimistic about soon achieving this goal by combining our rationally designed organic ligands with halide perovskite semiconductors. We will soon extend our strategy to make efficient green and blue LEDs to realize full-color displays. We have obtained some preliminary results, which makes me feel we are on track.”

Wang described Nature Communications as a high-impact journal with a wide reach.

“It only publishes significant breakthroughs that can attract broad interest in the related field. Having our work published there may help secure more external funding support and increase the impact of the Purdue research community,” he said.