image: a Operation diagram of the HAAPP strategy; b Digital photographs of the sample in each step; c Mechanism description about the growth processes of the HAAPP strategy; d LaMer model for description of the crystal growth processes; Digital photographs of the samples e once adding concentrated HCl and f after shaking for ~5 min; g Comparison of products with (left) and without (right) UV irradiation during synthesis; Digital photographs of the products obtained by mass preparation under h room light and i 365 nm UV light view more
Credit: by Huanxin Yang, Xiangxiang Chen, Yiyue Chu, Changjiu Sun, Haolin Lu, Mingjian Yuan, Yuhai Zhang, Guankui Long, Libing Zhang and Xiyan Li
In recent years, lead-free perovskite materials, represented by Cs2NaInCl6, have achieved a rapid breakthrough of luminous quantum yield from zero to near-unity by Ag/Na and In/Bi alloying schemes. Thanks to the self-trapping exciton, Cs2Na1-xAgxIn1-yBiyCl6 usually exhibit eyes-friendly luminescence behavior of full coverage of visible spectrum, thus attracts people's attention. Moreover, the warm-white light emission of a single component avoids the problem of light reabsorption among multiple components, which is easy to occur in the previous "blue + yellow" or "blue + green + red" schemes. These remarkable characteristics make lead-free perovskites highly commercially useful in scenarios such as LEDs lighting or LCD white-light backboards.
On the basis of remarkable luminous quantum yield, it is a prerequisite for commercialization to seek a synthesis strategy that can simultaneously achieve multiple ideal conditions such as mass production, rapid reaction rate, eco-friendliness, low cost and thermal- or pressure-free, etc. Traditional high temperature solid-state reaction and hydrothermal methods generally show unsafe high reaction temperature (or high pressure) and hour-scale time cost. In contrast, the recrystallization method based on liquid phase reaction generally exhibit more rapid nucleation growth. Unfortunately, it is limited by the solubility of raw materials (such as Ag sources, etc.), which may lead to a large amount of solvent consumption, increasing the cost of preparation. Therefore, to explore a synthetic strategy that can meet the above requirements has become one of the difficult problems to realize the commercialization of lead-free perovskite materials.
In a new paper published in Light: Science & Applications, a team of scientists, led by Prof. Xiyan Li from Nankai University and co-workers have proposed a universal HAAPP strategy for quick and mass preparation for lead-free perovskite microcrystals. Interestingly, they ignored the solubility and prepared 1 mmol double perovskite microcrystals by using only 1-2 mL concentrated HCl. The novel HAAPP strategy could accomplish the industrialization goals of thermal-, pressure-free, eco-friendliness, short-time, low-cost and high product yield (~90%), simultaneously. In addition to Cs2Na1-xAgxIn1-yBiyCl6, the proposed HAAPP strategy can be used to prepare other lead-free perovskites with different structures, such as Cs2ZrCl6, CsMnCl3, Cs4MnBi2Cl12 and Cs2InCl5·H2O, etc. More intriguingly, this strategy can be further extended to Br- or I-based perovskites, such as Cs2AgBiBr6 or Cs3Bi2I9. The HAAPP strategy with wide applicability is expected to provide a reliable reference for the futural preparation of new materials. These scientists summarize the HAAPP strategy:
“We design the HAAPP strategy for three purposes in one: (1) to provide a rapid mass production scheme for the industrialization of lead-free perovskite materials; (2) to provide a safe, reliable and widely supported method for scientific research; (3) to provide a new train of thought about the mechanism understanding of the conventional hydrothermal and recrystallization methods.”
“In our HAAPP strategy, the continuous crystallization accompanies the gradual release of free ions from raw materials with low solubility, promoting the chemical equilibrium of the reversible reaction to the free ions’ direction continuously until the raw materials are exhausted. This unique powder-to-powder transition will provide a new train of thought about the mechanism understanding of the conventional recrystallization method, that is, complete dissolution of the raw materials seems not strictly necessary.”
“The concentrated hydrochloric acid, as the only solvent used, exhibits multiple functions in our HAAPP strategy, which are summarized as follows:
1) Provide a liquid-state environment for rapid reaction;
2) Provide a poor solubility for products to improve the chemical yield;
3) Improve the luminescence efficiency by anion passivation;
4) Guide the product growth direction to ensure the pure phase of products.
The HAAPP strategy proposed in this paper can simultaneously meet the ideal preparation conditions of mass production, rapid response, eco-friendliness, low cost, thermal- and pressure-free, etc., which provides industrialization foundation for the application of lead-free perovskite in LEDs lighting or LCD backlight plate. Various ions-doping for luminous regulation also indicates that the HAAPP strategy is expected to become another new preparation scheme after solid-state, hydrothermal and recrystallization methods. In addition, the induction of reaction mechanism and the summary of HCl functions will provide a powerful reference for the specific growth process analysis of hydrothermal method.” The scientists forecast.
Journal
Light Science & Applications