A team at the Okinawa Institute of Science and Technology (OIST) has introduced a glow-in-the-dark material fabrication method that it believes could reduce reliance on inorganic crystals derived from rare-earth materials. The team used the method to generate glow-in-the-dark effects using readily available organic materials. “Not only are organic materials much more available and easier to work with than inorganic materials, but 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 Chihaya Adachi, director of the Center for Organic Photonics and Electronics Research (OPERA) at Kyushu University. “Another important application is their potential use in bioimaging, which could have a myriad of benefits for health science.” Glow-in-the-dark materials are used worldwide for emergency signs, watches, and paint. Courtesy of OIST. In 2017, the team demonstrated for the first time that two organic materials could create a glow-in-the-dark effect. However, the effect was almost 100× weaker than inorganic methods. To demonstrate the method, the team used ultraviolet light to generate emissions. The effect could only be seen in a dark room, and the samples could not be exposed to oxygen. In the current work, the researchers progressed from a method that used two components to a method that uses three components, as well as different molecules. The results showed emissions lasting for more than an hour at room temperature — a tenfold improvement from the previous work. “It is a four-stage process to create the glow-in-the-dark effect — charge transfer, separation, recombination, and, finally, emissions,” said Ryota Kabe, who leads OIST’s Organic Optoelectronics Unit. “Within the molecules, electrons are nestled in holes. An important part of the process is separating the electrons from the holes. When the two come back together, it generates the glow.” By tweaking the emission mechanism and the molecules used, researchers improved the performance of organic glow-in-the-dark materials by tenfold. The resulting emissions lasted for over one hour in air at room temperature. Courtesy of OIST. In the team’s previous work, when the organic materials were energized by light, electrons would be transferred from a molecule dubbed the electron donor to a molecule dubbed the electron acceptor. However, an issue arose when the electron acceptor couldn’t store anymore electrons. When the electrons returned to the donor, this recombination created the glow effect, but because the number of stored electrons was limited, the glow was not as strong and rapidly faded away. In the current work, the researchers used molecules that ensured that the holes moved rather than the electrons. This hole diffusion system reduced the probability of the molecules reacting with the air, thereby ensuring that the samples could still glow while exposed to oxygen. Second, the researchers used a hole trapper, which kept the electron and the hole separated for longer and allowed more holes to build up. The addition of the hole trapper additionally increased the resulting emissions period. Finally, the team used molecules that required less energy to move between the different steps of the process. This ensured that the whole process required less energy and allowed the emissions to be generated in visible light, rather than just ultraviolet light. According to Kabe, the organic molecules now work in air, though their performance remains weak. The team will continue to tune the emissions until they are on par with those produced by the inorganic crystals, Kabe said. The research was published in Nature Materials (www.doi.org/10.1038/s41563-021-01150-9).