A Rutgers-led team of scientists has developed an eco-friendly, very stable, ultra-bright material and used it to generate deep-blue light (emission at ~450 nm) in a light-emitting diode (LED), an energy-efficient device at the heart of all major lighting systems.
The new copper-iodide hybrid emitter materials are expected to contribute to the advancement of blue LED technologies because of their excellent qualities, according to the scientists who pioneered the discovery. The process that produces the material is described in the science journal Nature.
"Deep-blue LEDs are at the heart of today's energy-efficient lighting technologies," said Jing Li , a Distinguished Professor and Board of Governors Professor of Chemistry and Chemical Biology in the Department of Chemistry and Chemical Biology in the School of Arts and Sciences who leads the study. "However, existing options often present issues with stability, scalability, cost, efficiency or environmental concerns due to the use of toxic components. This new copper-iodide hybrid offers a compelling solution, leveraging its nontoxicity, robustness and high performance."
LEDs are lighting devices that use special materials called semiconductors to turn electricity into light in an efficient and durable way. Blue LEDS were discovered in the early 1990s and earned their discoverers the 2014 Nobel Prize in physics.
Blue LEDs are particularly important because they are used to create white light and are essential for general lighting applications.
Li and her colleagues at Rutgers collaborated with scientists at Brookhaven National Laboratory and four other research teams representing national and international institutions in the effort to work on new materials that would improve upon existing blue LEDs.
The researchers involved in the study found a way to make blue LEDs more efficient and sustainable by using a new type of hybrid material: a combination of copper iodide with organic molecules.
"We wanted to create new kind of materials that give very bright deep-blue light and use them to fabricate LEDs at lower cost than current blue LEDs," Li said.
The new hybrid copper-iodide semiconductor offers a number of advantages over some other materials used in LEDs, scientists said. Lead-halide perovskites, while cost effective, contain lead, which is toxic to humans, as well as have issues with stability, due to their sensitivity to moisture and oxygen. Organic LEDs (OLEDs) are flexible and potentially efficient but may lack structural and spectral stability, meaning they can degrade quickly and lose their color quality over time. Colloidal quantum dots perform well mainly in green and lower-energy LEDs and are often cadmium-based, which may raise toxicity concerns. Phosphorescent organic emitters may be costly and complex to synthesize.
"The new material provides an eco-friendly and stable alternative to what currently exists, addressing some of these issues and may potentially advance LED technology," Li said.
The hybrid copper-iodide material possesses favorable qualities such as a very high photoluminescence quantum yield of about 99.6%, meaning it converts nearly all the photoenergy it receives into blue light. Blue LEDs made from this material have reached a maximum external quantum efficiency (the ratio between the number of emitted photons and number of injected electrons) of 12.6%, among the highest achieved so far for solution-processed deep-blue LEDs.
Not only are these LEDs bright, they also last longer compared with many others. Under normal conditions, they have an operational half-lifetime of about 204 hours, meaning they can keep shining for a good amount of time before their brightness starts to fade. In addition, the material works well in larger-scale applications. The researchers successfully created a larger device that maintains high efficiency, showing that this material has potential to be used in real-world applications.
The secret to the material's impressive performance lies in an innovative technique developed by the scientists called dual interfacial hydrogen-bond passivation. The manufacturing technique significantly boosts the performance of the LEDs four-fold.
"Our processing method minimizes defects that can impede the movement of electric charges at the interface of these hybrid materials," said Kun Zhu, a former graduate student and postdoctoral associate at Rutgers who is now at the Max Planck Institute in Germany and is the paper's first author. "This approach could be a versatile strategy for generating high-performance LEDs."
If the LED can be imagined as a sandwich with different layers, each layer has a specific job, such as emitting light or transporting electrons and holes. Sometimes, the emissive layer doesn't interact perfectly with its interface layers, which can reduce efficiency or shorten lifespan. The technique eliminates such problems by forming hydrogen bonds between the layers to create better connections.
"Overall, this type of new material is paving the way for better, brighter and longer-lasting LEDs," Li said.
Other Rutgers scientists contributing to the study included Deirdre O'Carroll, associate professor, and Nasir Javed, doctoral student, of the Department of Chemistry and Chemical Biology and Department of Materials Science and Engineering; and Sylvie Rangan, assistant research professor, and Leila Kasaei, postdoctoral research associate, of the Department of Physics and Astronomy.
The research was funded by the U.S. Department of Energy.
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