Zhao Kui is conducting the experiment of preparing perovskite thin film by air coating. Photo courtesy of the project team
If the perovskite battery needs to be put into practical use, its key part, the perovskite material film, must be large enough in area and good in quality, so as to ensure that the photoelectric energy conversion efficiency of the battery is sufficiently high, which requires "The time is right for everyone."
The perovskite solar cell is too hot.
Since its discovery in 2009, the "prodigy" has been making great strides, constantly refreshing the photoelectric conversion efficiency record. Coupled with its simple assembly process and low cost, it is used by scientists as the best substitute for silicon-based solar cells that have been industrialized.
However, the currently reported high-efficiency perovskite batteries are very susceptible to temperature, humidity and ultraviolet light, and their unstable performance makes it difficult to obtain large-scale preparation and application. However, what is surprising is that recently, the team of Liu Shengzhong, a researcher at the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, and Zhao Kui, a professor at Shaanxi Normal University, achieved large-scale preparation of all-inorganic perovskite batteries in air.
What means did they use to "subdue" the "urchin"? A study published recently in "Joule" gives the answer.
Urgently need "time and place to benefit people"
If the perovskite battery needs to be put into practical use, its key part, the perovskite material film, must have a large enough area and good quality, so as to ensure that the battery's photoelectric energy conversion efficiency is sufficiently high. In the words of the corresponding author of the paper, Zhao Kui, "the right place and the right people need peace."
Here, "human harmony" refers to the material element of the perovskite battery, and "natural and favorable conditions" refers to the environment in which the battery is prepared.
In the preliminary work, the team studied the crystalline phase transition process of organic-inorganic hybrid perovskite printing in air, and found that the growth mode and phase transition mechanism of perovskite during printing are the key factors affecting the quality of large-area films factor. Through effective phase change control, the team finally achieved the preparation of high-quality large-area organic-inorganic hybrid perovskite thin films and high-efficiency solar cells, but the key thermal stability issues have not yet been resolved.
If you want to stabilize the "urchin", you have to change from the battery itself. Scientists have found that replacing the organic part of perovskite with inorganic elements and preparing all-inorganic perovskite is an effective strategy.
"All-inorganic perovskites CsPbI2Br and CsPbI3 have good thermal stability and photoelectric properties, and are not easily decomposed even if they withstand several Baidu high temperatures. They are a class of all-inorganic materials with development potential." Zhao Kui told the China Science Journal .
However, changing the environment is not that simple. In an uncontrolled air environment, the phase transitions of the CsPbI2Br and CsPbI3 thin films, which are no longer stable, are still easy to occur. In Other words, the preparation of all-inorganic perovskite batteries can only be limited to an inert gas (N2) environment, and so far there has been no report of printing large-area solar cells in air.
The reason for this is that the key to water vapor in the air is the inhibition of the crystallization process of the perovskite precursor, which leads to the fact that the perovskite precursor cannot effectively grow into a crystal phase structure with photoelectric activity. At the same time, there are theoretical research gaps in the complex hydrodynamics and crystalline phase transition behavior during printing preparation, which leads to no experience in printing and preparing high-quality all-inorganic perovskite thin films.
Is there a way to adjust the "feng shui", so that the printed all-inorganic perovskite battery can run efficiently and stably? Researchers have begun a intensive preparation.
Adjust "Feng Shui" to make the film more stable
Fan Yuanyuan, the first author of the thesis and a graduate student of Shaanxi Normal University, told the China Science Journal that the experiment used a doctor blade coating method compatible with large-scale industrial printing production to prepare CsPbI2Br film in air.
The doctor blade coating method, that is, the method of dragging the solution on the substrate with a doctor blade to quickly move the film, compared with the spin coating method of preparing a small area in the laboratory, the doctor blade coating method can greatly save the cost of raw materials and is more suitable for Qualcomm Preparation of large-area films.
In response to the problem of "unfavorable feng shui", the experimenters carried out a study on the morphology of fluid mechanics. The study found that increasing the substrate temperature can accelerate fluid drying kinetics, thereby reducing the effect of water vapor in the air on the precursor chemical reaction. Even so, researchers need to control another key factor: Bénard-Marangoni convection. The high temperature substrate will cause a temperature gradient difference inside the solution, which leads to the discontinuous deposition of solutes and destroys the continuity of the film morphology.
Therefore, only when the temperature is moderate can the negative effects of water vapor on the crystallization and the negative effects of Bénard-Marangoni convection on the film morphology be avoided.
Based on the National Laboratory's synchrotron radiation platform, the team also performed in-situ analysis and kinetic adjustments on the crystallization phase transition mechanism during the printing process, and found that the in-situ chemical reaction of the perovskite precursor was not "one-stop", and "gradual crystallization" ": Mr. is CsPbBr3, then I ion gradually participates, and eventually grows to CsPbI2Br. When the precursor gradually crystallizes during the solution film formation, the ion mobility is strong, which is conducive to the formation of high-quality perovskite thin films. "This is a new discovery that has never been reported in previous studies." Fan Yuanyuan said.
Finally, the study achieved the printing preparation of α-phase CsPbI2Br large area film. Compared with the film prepared by spin coating in the glove box, the crystallinity and stability of the newly printed film are significantly improved.
Achieve high-quality all-inorganic perovskite batteries
What is a high-quality perovskite film? It is not only about its morphology, but also its crystallinity, defect density and grain boundaries.
The film prepared this time has an area of ​​up to 70 square centimeters. The film is uniform, free of hole defects, high crystallinity, stable α phase, long carrier life and few defects. This is also the first time that scientists have realized the printing and preparation of high-efficiency all-inorganic perovskite solar cells in large areas in the air.
In Liu Shengzhong's view, there are two scientific breakthroughs in this research.
One is to reveal the influence of hydrodynamic behavior on the perovskite film during printing. The perovskite material itself is sensitive to air humidity. By increasing the substrate temperature, the environmental adaptability of the preparation process of the perovskite layer is improved, and the limitation of the preparation process itself on the environmental conditions is reduced.
The second is to reveal the crystallization phase transformation mechanism of perovskite in the printing process. The precursor reacts directly to the perovskite phase after skipping the solvated mesophase, and there is "gradual crystallization" in this process. The "gradual crystallization" during solution film formation is more conducive to the formation of thin films with high crystallinity and low defect density.
It is understood that the newly developed fully inorganic perovskite CsPbI3 solar cell currently has a photoelectric conversion rate of approximately 15%. "Although it is lower than the organic-inorganic hybrid perovskite solar cell, it has a good market prospect because it solves the major defect of poor thermal stability." Zhao Kui said.
He said that based on theoretical guidance, the team will continue to explore the all-print preparation of all-inorganic perovskite CsPbI3 solar cells with higher efficiency in air. "By addressing the key scientific issues affecting the solidification and crystallization of perovskite solutions, we will provide important experience for printing and preparing high-efficiency perovskite solar cells and provide more references for the commercial manufacture of perovskite photovoltaic devices."
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