For a bright future in eco-friendly lighting devices, just add dendrimer, cellulose and graphene

July 07, 2023

(Nanowerk News) In research that could lead to a new era in illumination, researchers from Japan and Germany have developed an environmentally friendly light-emitting electrochemical cell using a novel molecule called a dendrimer combined with a biomass-derived electrolyte and graphene-based electrodes. Their findings are published in a journal Advanced Functional Materials (“The Dendri-LEC Family: Building a Bright Future for Dendrimer Emitters in Graphene-Based Light-Emitting Electrochemical Cells”).

Electroluminescence is a phenomenon in which a material emits light in response to a passing electric current. Everything from the screen you are using to read this sentence to the lasers used in state-of-the-art scientific research are the result of the electroluminescence of various materials. Due to its ubiquity and necessity in modern times, it is natural that extensive resources are devoted to research and development to make this technology better.

“One example of an emerging technology is the ‘light-emitting electrochemical cell’ or LEC,” explains Associate Professor Ken Albrecht of the University of Kyushu’s Institute of Chemistry and Materials Engineering and one of the research leaders. “LEC has attracted attention due to its cost advantage over organic light-emitting diodes, or OLEDs. Another reason for LEC’s popularity is its simplified structure.”

OLED devices generally require careful coating of multiple organic films, making them complex and expensive to manufacture. LEC on the other hand can be made with a single layer of organic film mixed with a light emitting material and an electrolyte. The electrodes that connect everything together can even be made from cheap materials unlike the rare or heavy metals used in OLEDs. In addition, the LEC has a lower drive voltage, which means it consumes less energy.

“Our research team has explored new organic materials that can be used in LEC. One of the candidates is a dendrimer,” explained Prof. Rubén D. Costa of the Technical University of Munich, who led the research team in Germany. “These are branched symmetric polymer molecules whose unique structure has led to their use in everything from medicine to sensors, and now in optics.”

Building on their previous work developing dendrimers, the research team set about modifying their material for LEC. Second-generation dendrimer with tert-butyl (left) and methoxy (right) groups applied to a light-emitting electrochemical cell (LEC). The lifetime of LEC devices using dendrimers with hydrophilic methoxy groups (right) is more than 10 times longer than hydrophobic dendrimers. (Image: Kyushu University)

“The dendrimer we developed originally had hydrophobic, or water-repellent, molecular groups. By replacing them with hydrophilic, or water-loving, groups, we found that the lifetime of LEC devices could be extended to more than 1000 hours, more than 10 times that of the original,” explains Albrecht. “What makes it even better is that thanks to our collaboration with Dr Costa’s team, the device is very environmentally friendly.”

For years, Costa’s team in Germany has been working to develop cheaper and more environmentally friendly materials in light-emitting devices. One material they have tried is cellulose acetate, a common organic compound used in everything from clothing fibers to eyeglass frames.

“We used biomass-derived cellulose acetate as the electrolyte in our new LEC devices, and ensured that the devices have the same lifetime,” continues Costa. “In addition, we also found that graphene can also be used as an electrode. This is an important step towards making flexible light-emitting devices using environmentally friendly materials.”

The team explained that while their work is promising, more research is needed before the device can be marketed.

“The tool we are making here only glows yellow, so it needs to be developed to illuminate the three main colors of light: blue, green and red. Luminescence efficiency, how bright the light is, also needs hard work,” concluded Albrecht. “Although thanks to our international collaboration, the future looks bright.”

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