Full Color Perovskite LEDs

A schematic illustration of optical properties of anion-exchanged perovskite nanocrystals. (Source: UNIST)

Perovskite nano­particles is regarded as next-generation of optical materials that can achieve vivid colors even on very large screens. Besides, due to their high color purity and low cost advantages, it has also gained much interests in industry. A recent study, affi­liated with Ulsan National Institute of Science and Tech­nology UNIST has introduced a simple technique to extract the three primary colors from this material.

This break­through has been led by Jin Young Kim in the School of Energy and Chemical Engi­neering at UNIST. In the study, the research team intro­duced a simple technique that freely controls light emitting spectrums by adjusting the anion halides in perovskite materials. The key is to adjust the anion halides by simply dissolving them in solvents to achieve red, blue, and green lights. Appli­cation of this technique to LEDs can result in crystal-clear picture quality.

Perovskite is a semi­conductor material with a special structure, containing metal and halogen elements. The solar cell adopting this material is considered to be the next-generation solar cell candidate because it has high photo­electric efficiency that converts sun light into elec­tricity. This material is also attracting attention as a light emitting device because of its high luminous effi­ciency which turns electricity into light. Perovskite nano­particles are micro­scopic perovskite materials at nano­meter level, which emit different colors depending on the internal halogen element. It is a formula that emits red when it is rich in iodine, green when it is rich in bromine, and blue when it is rich in chlorine.

However, perovskite is highly sensitive, making it difficult to change elements stably. In search for an answer, Kim has deve­loped a simple technique to replace certain elements via solution process. This is a method of inducing element substi­tution, using nonpolar solvent and chemical addi­tives. “In the study, we added a nonpolar solvent, containing iodine (I), bromine (Br) and chlorine (Cl) to a solution of perovskite nano­particles,” says Yung Jin Yoon. Once the reaction takes place, the elements mixed within the nonpolar solvent switches its place with elements in original perovskite, which causes changes in lumi­nescence.

The added chemical additive serve to separate the halogen element present in the nonpolar solvent. As a result, the amount of halogen element in the solution increases, and over time, it is replaced with a halogen element in the conven­tional perovskite. The emission color is deter­mined by the number of elements in the perovskite. The researchers also succeeded in making LEDs with red, blue, and green colors using perovskite nano­particles made with this tech­nology.

Kim Ki-Hwan, a research professor in the Depart­ment of Energy and Chemical Engi­neering, said, “It is stable compared to the existing technology to change the element in the solid perovskite. It can be applied variously to change the element compo­sition in the perovskite material, I hope it will be possible.” “With our simple method, we obtained lumi­nescence covering the entire visible spectrum from 400 to 700 nm,” says Kim. Further­more, saturated and vivid RGB LED devices were success­fully fabricated using the anion-exchanged nano­crystals. (Source: UNIST)

Reference: Y. J. Yoon et al.: Reversible, Full-Color Luminescence by Post-treatment of Perovskite Nanocrystals, Joule, online 6 August 2018; DOI: 10.1016/j.joule.2018.07.012

Link: School of Energy and Chemical Engineering (J. Y. Kim), Ulsan National Institute of Science and Technology UNIST, Ulsan, Republic of Korea

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