Glow-in-the-dark materials are used worldwide for emergency signs, watches and paint. This useful feature fuels a global market worth approximately US$400 million. But the inorganic crystals that are currently needed to generate this ability at a high level of performance require rare earth metals and manufacturing temperatures above 1000 degrees Celsius.
Now, writing in Natural materialsresearchers from Okinawa Institute of Science and Technology Graduate University (OIST) and Kyushu University, both in Japan, have developed a method to generate light that glows in the dark using the most organic materials more readily available.
“Not only are organic materials much more available and easier to work with than inorganic materials, they are also soluble, which has the potential to diversify and expand the use of glow-in-the-dark objects, as the characteristic could be added to inks, films and textiles”, said Professor Chihaya Adachi, director of the Organic Photonics and Electronics Research Center (OPERA) at Kyushu University. “Another important application is their potential use in bioimaging, which could have myriad benefits for health sciences.”
In 2017, researchers showed, for the first time, that two organic materials could create a phosphorescent effect. It was a great success and published in Nature. However, the performance was almost 100 times lower than with the inorganic varieties. In fact, the researchers had to use ultraviolet light to generate the emissions, had to go into a dark room to see the light, and couldn’t expose the samples to oxygen.
Now the researchers got better results by switching from a two-component method to a three-component method and changing the molecules they were using. The result was emissions that lasted over an hour at room temperature, a tenfold improvement over previous work.
“It’s a four-step process to create the phosphorescent effect; charge transfer, separation, recombination and, finally, emissions,” explained Professor Ryota Kabe, who leads OIST’s Organic Optoelectronics Unit. “Within molecules, electrons are nested in holes. An important part of the process is separating the electrons from the holes. When the two come together, it generates the glow.”
In previous research, when organic materials were energized by light, electrons were transferred from a molecule called an electron donor to a molecule called an electron acceptor. However, a problem was caused because the electron acceptor could not store many electrons. When the electrons returned to the donor, this recombination created the glow effect, but since the number of stored electrons was limited, the glow was not strong and quickly faded.
However, in this new work, the researchers did several things differently. First, they used molecules that ensured the holes were the things that moved rather than the electrons. This hole diffusion system reduced the likelihood of molecules reacting with air, ensuring that the samples would glow when exposed to oxygen.
Second, the researchers added a hole trap to the third component, which kept the electron and hole separated longer, allowing more holes to accumulate and increasing the resulting emission period. And finally, they used molecules that required less energy to move between the different stages of the process, ensuring that the whole process consumed less energy and allowing the emissions to be generated in visible light, rather than in ultraviolet light.
“By refining the method, we managed to improve the performance of organic molecules by ten times compared to previous work,‘ Professor Kabe concluded. “Organic molecules now work in air, although performance is still low. We will continue to work to adjust emissions until they are on par with those produced by inorganic crystals.”