Researchers from various universities and laboratories around the world have recently melted metal-organic frameworks (MOFs) and mixed them with perovskites to yield highly stable luminescent composites. The mixtures reportedly resist exposure to heat, air, and humidity.
Lead-halide perovskites, such as cesium lead iodide, naturally exhibit photoluminescence, says chemist Thomas D. Bennett of Cambridge University and the paper's lead author. 'However, this light-emitting phase is only stable at high temperatures, and its effects disappear when the material cools down,' he adds. In the new work, researchers preserved this photoluminescence using MOF glasses to trap this metastable phase at room temperature and, at the same time, encapsulate and protect the perovskite.
'The [MOF] glass matrix stabilizes the luminescent phase of perovskites,' explains Sebastian Henke, a MOF expert at the Technical University of Dortmund. For the first time, MOF glasses 'encapsulate a very important class of [perovskite] nanoparticles, . . . protecting [them] from degradation by heat and solvents,' he adds.
'The MOFs form a protective layer by introducing stabilizing interactions,' explains Christine Luscombe, a materials expert at the Okinawa Institute of Science and Technology, who says the materials' stability is 'amazing'
The researchers heated the MOF to 304 Â°C to form a glass, then sintered it with the perovskite until the two materials combined into a composite. Within this blend, the MOF interacts with the perovskite structure, thus preventing the transformation to the non-emitting phase. Photospectrometry experiments showed that luminescence at room temperature is two orders of magnitude higher than in unprotected perovskites. Moreover, durability tests showed the light-emitting devices resist over 10,000 h of water immersion and 650 days stored at ambient conditions.
Researchers tuned the emission color by varying the perovskite's composition. 'This is a well-known effect: tweaking the proportion of halogens in the structure yields different luminescence,' Bennett explains. This could open the door to using the new materials in LEDs and other optoelectronic devices, although there are still challenges to overcome. 'MOF glasses [require] relatively high temperatures and inert atmosphere,' explains Henke.
So far, MOF glasses have found applications in batteries, gas separation and water treatment. 'This new combination with perovskites opens new possibilities,' Bennett says. 'Stabilizing their light-emitting phase at room temperature will foster uses beyond photovoltaics and energy harvesting,' he concludes.