Celebrating St. Patrick’s Day With A Brand-New Green LED

Title: Material Design of Green-Light-Emitting Semiconductors: Perovskite-Type Sulfide SrHfS3
Authors: Kota Hanzawa, Soshi Iimura, Hidenori Hiramatsu, Hideo Hosono
Publication Info: Journal of the American Chemical Society, 2019, Article ASAP DOI: 10.1021/jacs.8b13622

Light-emitting diodes, or LEDs, are a familiar sight in modern society. They light up everything from your car’s headlights to the display of your TV. Want to light up Niagara Falls in green on St. Patrick’s Day? LEDs are the most efficient way – if you can find some that are green.

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Figure 1. An example of a white LED. Used with permission from Pixabay.

Unlike traditional lightbulbs, LEDs don’t rely on running electricity through a filament to create light. Instead, they rely on a property of semiconductors called electroluminescence. When electric current is run through a semiconductor, the recombination of electrons releases energy in the form of light. The color of light emitted by the LED depends on the type of semiconductor used.

Because LEDs don’t require filaments and don’t generate much heat, they are far more efficient and durable than traditional lightbulbs. However, they also have limitations. One big obstacle to using LEDs is that few materials exist that can emit green light efficiently. That’s where these researchers focused their attention – successfully creating a new, perovskite-based, green-light-emitting LED.

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Figure 2. Typical perovskite crystal lattice of the form A2+B4+X2–3. Red spheres are X atoms, blue spheres are B atoms, and green spheres are A atoms. Reprinted (adapted) with permission from Hanzawa, K. et al. J. Am. Chem. Soc., Article ASAP 2019. Copyright (2019) American Chemical Society.

Perovskite materials, which are semiconductors made from a crystal composed of two cations and one anion, are a popular material for LEDs because the color of light they emit can be tuned by using different cations and anions. Semiconductor LEDs must contain two different types of materials: one with more electrons, called n-type, and one with fewer electrons, called p-type. To emit light, the electrons must recombine across the junction where these two materials meet. Since perovskites can be made as both n-type and p-type materials, they’re a great material for LEDs.

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Figure 3. The approximate color of light emitted by the perovskite LED SrHfS3.

The green-light-emitting perovskite discovered by these researchers is made up of SrHfS3 and contains strontium, hafnium, and sulfur atoms. To make it p-type, the researchers incorporated phosphor atoms into the crystal instead of some of the sulfur atoms; n-type SrHfS3 contains some lanthanide atoms instead of strontium. And most importantly, SrHfS3 emits light strongly at a photon wavelength of 539 nm – resulting in distinctive green light.

As energy efficiency becomes more and more important in the modern age, researchers will need to keep investigating new materials like perovskite LEDs. We can’t let Niagara Falls stay blue on St. Patrick’s Day, after all.

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