How to understand if a microstructure is a pure crystal or polycrystalline grain of perovskite?
The main difference between crystal growth and grain growth during thin film coating of perovskite (MAPI) is that crystal growth involves the formation of a single crystal in the entire thin film, while grain growth involves the formation of multiple individual grains that are spread throughout the thin film. The size and shape of the grains are determined by various factors, such as deposition rate, substrate temperature, and other processing parameters.
if a microstructure is a pure crystal or polycrystalline grain of perovskite by analyzing its crystalline structures. Pure crystals will typically have well-defined crystallographic planes, while polycrystalline grains usually appear as separate, distinct, or randomly oriented regions of crystalline material. Additionally, you may be able to distinguish between the two types of microstructures based on their optical properties, such as color and reflectivity.
Two effective methods to distinguished whether a microstructure is a pure crystal or polycrystalline grain of perovskite are x-ray diffraction and Raman scattering. (e.g., see X-ray diffraction and Raman spectroscopy for lead halide perovskites - ScienceDirect). For example, Raman spectra of crystalline samples show a symmetric peak, while polycrystalline structure shows asymmetric peak with Raman peak being extended towards the lower energy phonon energy (e.g., see Raman line profile in polycrystalline silicon: Journal of Applied Physics: Vol 86, No 8 (scitation.org).
Basically, you asked two questions: I will reply in two steps.
1. Crystal growth and grain growth are two different processes that can occur during the thin film coating of perovskite materials, such as methylammonium lead iodide (MAPbI3 or MAPI).
Crystal growth refers to the process by which atoms or molecules arrange themselves in a repeating, three-dimensional pattern to form a crystal. In the case of MAPI, this process occurs as the precursor solution is deposited on a substrate and subsequently heated. The formation of a single crystal structure is desirable for MAPI thin films as it leads to high performance in solar cells.
Grain growth, on the other hand, refers to the process by which the size of individual crystals within a polycrystalline material increases. In the case of MAPI, this process can occur as the deposited film is annealed at high temperatures. Large grains lead to an increase in the efficiency of solar cells. However, large grains can also lead to a decrease in the open circuit voltage, which is a parameter that affects the performance of the solar cell.
Therefore, controlling the crystal growth and grain growth is important for obtaining high-performance perovskite thin films. Researchers have been able to improve the performance of the solar cells by using methods that can control the crystal growth and grain growth, such as using a low-temperature annealing process, controlling the precursor solution, or using a substrate with a specific surface morphology.
2. There are several methods that can be used to determine whether a microstructure is a pure crystal or a polycrystalline grain of a perovskite material. Some of the most common methods include:
It's worth to mention that these methods are not mutually exclusive and it's common to use a combination of them to ensure the identification of the microstructure. Also, the interpretation of the results of these methods is not always straightforward and it's recommended to have a good understanding of the material and the measurement technique.
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