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2024/02/18 #Research
Process Engineering in Metal Halide Perovskites
Process Engineering in Metal Halide Perovskites
What is metal halide perovskite?
What is metal halide perovskite?
Metal halide perovskites are a class of crystalline materials that have gained significant attention for their application in solar cells and other optoelectronic devices. They have the general formula ABX₃, where 'A' is a cation (often an organic molecule like methylammonium (CH₃NH₃⁺) or an inorganic cation like cesium), 'B' is a metal cation (typically lead (Pb) or tin (Sn)), and 'X' is a halide anion (chlorine (Cl), bromine (Br), or iodine (I)).
Applications
Applications
The most anticipated application of perovskite is in solar cells. Its efficiency in converting light to electricity is comparable to that of commercial silicon solar cells. Furthermore, perovskite solar cells can be manufactured using a method similar to printing ink on paper, enabling the creation of light and flexible solar cells. Leveraging this characteristic, it becomes possible to install solar cells in locations where traditional solar cells could not be placed due to issues like load-bearing (e.g., on the walls of buildings). While there remain challenges related to durability, currently, many companies are advancing research and development towards their commercialization. Besides solar cells, applications in LEDs and X-ray detectors are also anticipated (details).
Photo credit: Saidaminov Lab
Materials Engineering –What do you produce?–
Materials Engineering –What do you produce?–
Perovskite, when observed as a microcrystalline structure, appears to be the same material throughout. However, by changing its composition or shape, significant differences in the properties of the material as a macro substance can be observed. It is possible to control various properties, such as the color of the material, its luminescence, its ability to absorb light, and its conductivity to electricity.
One of the representative strategies in materials engineering is to alter the composition of materials. For perovskites, represented by the general formula ABX₃, this means using different atoms or molecules in each lattice site. For instance, it is known that by changing the halogen composition, the emission wavelength of the material can be controlled across nearly the entire visible spectrum. The photo on the right demonstrates the change in color when the halogen composition is varied in CsPbX₃ nanocrystals (Nano Lett. 2015, 15, 6, 3692–3696).
Altering the shape of materials is also a key aspect of materials engineering. The four photos shown below all feature the material composition CsPbBr₃, but their shapes vary from left to right as quantum dots, thin films, thick films, and single crystals, respectively. It's a bit of a crude analogy, but you can think of it as if you have the same type of Lego blocks, and the way you combine them results in completely different finished products.
Quantum dots refer to semiconductor crystals about 10 nm in diameter, also known as nanocrystals. The physical properties of semiconductors change significantly when the crystal size is reduced to this extent, resulting in very strong luminescence. Therefore, research on quantum dots as luminescent materials for applications such as LEDs and scintillators is actively pursued. In cases like solar cells or photodetectors, where the goal is to absorb light and convert it into electricity, it's necessary to consider the required thickness of perovskite for absorption when designing the material. For perovskites, visible light can be sufficiently absorbed with a thickness of less than 1 µm, making thin-film applications popular. For the fabrication of radiation detectors that absorb X-rays or gamma rays, thick films with a thickness of over 100 µm or single crystals in the millimeter size are used
Process Engineering –How do you produce?–
Process Engineering –How do you produce?–
So far, we have explained how the properties of perovskite materials can greatly vary depending on their composition and shape. Now, we will focus on the crucial question of "how are they made?" Even just for the fabrication of thin films, a variety of methods have been proposed, including spin coating, dip coating, inkjet printing, and vacuum deposition. Each method has its advantages and disadvantages, and it's important to choose the process that best suits the objective. Furthermore, even within the same process, changing the conditions for forming perovskite can naturally lead to differences in the quality of the resulting film. Therefore, understanding the characteristics and mechanisms of the process well, to enable the synthesis of materials that meet specific objectives, is crucial.
My research has focused on "how to make perovskite materials? How can high-quality ones be fabricated?"
Photo by Yuki Haruta @ Kyoto University
Perovskite Film Formation (2017-2022)
ーMist Deposition Methodー
Perovskite Film Formation (2017-2022)
ーMist Deposition Methodー
Perovskite is a material that is expected to have applications in solar cells and X-ray detectors, but for practical use, it is necessary to establish a process that can reproducibly produce it in practical-level sizes with high-quality. Dr. Haruta has been working on the deposition of various perovskite materials using a coating technique called mist deposition, and has been engaged in elucidating their growth mechanisms.
Photo by Yuki Haruta @ University of Victoria
Synthesis of Perovskite Single Crystals (2022-2024)
ーFeedback-controlled growth systemー
Synthesis of Perovskite Single Crystals (2022-2024)
ーFeedback-controlled growth systemー
Single crystals, compared to polycrystalline thin films, have a lower density of defects and sufficient thickness, making them subjects of research for applications as detectors for X-rays and gamma rays. In the University of Victoria, I worked on developing more innovative single-crystal synthesis methods under Makhsud I. Saidaminov, who developed the Inverse Temperature Crystallization (ITC) method, the most commonly used method for synthesizing perovskite single crystals.
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