Kobe University's Associate Professor Tachikawa Takashi and Dr. Karimata Izuru have succeeded in completely substituting the halide ions of perovskite nanocrystals while maintaining their morphology and light-emitting efficiency.
Furthermore, by using techniques like single-particle photoluminescence imaging, the researchers were able to understand the momentary changes in light emission and the crystal structure, which enabled them to develop a principle for controlling ion composition.
Hybrid organic-inorganic perovskites, such as organic lead halide perovskites (for example, CH3NH3PbX3 (X = Cl, Br, I)), have been receiving attention as a promising material for efficient solar cells. Furthermore, the color of the light that they emit can be controlled by altering the type and composition of the halide ions. Consequently, it is hoped that hybrid organic-inorganic perovskites can be applied to light-emitting devices such as displays and lasers.
However, the halide ions inside the crystals are known to move around even at room temperature, and this high flexibility causes issues such as reductions in both synthesis reproducibility and device durability.
In this study, the researchers used a custom-made flow reactor to precisely control the exchange reaction between the CH3NH3PbI3 nanocrystals and Br- ions in solution. This enabled them to successfully convert the nanocrystals into CH3NH3PbBr3 nanocrystals while maintaining their morphology and light-emitting efficiency.
It is important to know what kind of reaction will occur inside the crystals in order to develop synthesis techniques. To understand this, the researchers used a fluorescence microscope to observe how each individual nanocrystal was reacting. From this observation, they understood that once the red light emitted by the CH3NH3PbI3 had completely disappeared, the green light originating from the CH3NH3PbBr3 was suddenly generated after an interval of 10s to 100s of seconds. Based on the results of structural analysis using an x-ray beam, it was revealed that Br- ions replaced I- ions inside the crystal structure while a bromide-rich layer formed on the surface. Afterwards, the bromide on the surface layer gradually moved into the inner regions.
It is believed that the red light emissions became unobservable because the inner regions of the crystal structure were partially disordered during the ion transition, which led to the loss of energy necessary for light emission. Subsequently, CH3NH3PbBr3 crystal nuclei formed inside the nanocrystal particle and a cooperative transition to the green light generating state occurred.
From these results, it can be said that temporally separating the crystal structure transitions and the subsequent restructuring (that occurs on a nanometer scale) is one of the keys to the successful, precise synthesis of organic lead halide perovskites.
The structural transformation process observed in perovskite nanocrystals in this study is thought to be related to all modes of nanomaterial synthesis that are based on ion exchange, therefore future research could hopefully illuminate the underlying mechanism. Although researchers have a negative impression of organic halide perovskites' flexibility, it is hoped that this characteristic could be exploited and applied to the development of new materials and devices that can react to the environment and external stimuli.
