Controlling perovskite ions’ composition paves the way in which for system functions

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Controlling perovskite ions' composition paves the way for device applications

Hybrid organic-inorganic perovskites (* 1) have received a lot of attention as potential next-generation solar cells and as materials for light-emitting devices.

Kobe University Associate Professor TACHIKAWA Takashi (from Molecular Photoscience Research Center) and Dr. Light emission efficiency.

In addition, using techniques such as single particle photoluminescence imaging, the researchers were able to understand the momentary changes in light emission and crystal structure, which in turn enabled them to develop a principle for controlling ion composition.

It is expected that this research will help enable the synthesis of perovskites of various compositions and advance the development of devices that use them. In addition, it is hoped that the flexibility of perovskite structures can be used to apply them to devices and create new functional materials.

These results were published on October 19, 2020 in the German journal ‘Angewandte Chemie International Edition’.

Research background
Hybrid organic-inorganic perovskites such as organic lead halide perovskites (e.g. CH3NH3PbX3 (X = Cl, Br, I)) have attracted worldwide attention as a promising material for highly efficient solar cells (Figure 1). In addition, the color of the light they emit can be controlled by changing the nature and composition of the halide ions. Hence, it is hoped that hybrid organic-inorganic perovskites can be applied to light-emitting devices such as displays and lasers.

However, it is known that the halide ions in the crystals move even at room temperature, and this high flexibility causes problems such as lowering the reproducibility of synthesis and the durability of the device.

Research methodology
In this study, the researchers used a custom-made flow reactor (* 3) 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 emission efficiency (Figure 2).

It is important to know what kind of reaction is going on in the crystals in order to develop synthetic techniques. To understand this, the researchers used a fluorescence microscope to observe how each individual nanocrystal reacted.

It emerged from this observation that after the red light emitted by CH3NH3PbI3 had completely disappeared, the green light emitted by CH3NH3PbBr3 was suddenly generated after an interval of 10 to 100 seconds (upper part of Figure 2).

Based on the results of structural analysis using an X-ray, it was revealed that Br ions replaced I ions within the crystal structure while a bromide-rich layer was formed on the surface. After that, the bromide on the surface layer gradually moved to the inner areas.

It is assumed that the red light emissions could no longer be observed because the inner areas of the crystal structure were partially disordered during the ion transition, which led to the energy loss required for the light emission (below in Figure 2). Subsequently, CH3NH3PbBr3 crystal nuclei were formed within the nanocrystal particle, and a cooperative transition to the state of green light generation occurred.

From these results it can be deduced that the temporal separation of the crystal structure transitions and the subsequent restructuring (in the nanometer range) is one of the keys to the successful and precise synthesis of organic lead halide perovskites.

Further developments
The structural transformation process observed in perovskite nanocrystals in this study is believed to be related to all modes of nanomaterial synthesis that are based on ion exchange. Hence, hopefully future research could shed light on the underlying mechanism.

Although the researchers have a negative impression of the flexibility of organic halide perovskites, there is hope that this property can be harnessed and used to develop new materials and devices that can respond to the environment and external stimuli.

Research report: “In-situ investigation of the structural transition during the morphology and efficiency-preserving halide exchange on a single perovskite nanocrystal”

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