Key Takeaways & Executive Findings
- •• Developed a green, low-temperature, ambient-pressure synthesis for lead-free vacancy-ordered double perovskites Cs2SnX6 (X=Cl, Br, I) using ionic liquids and ammonium halides, replacing toxic acids and organic solvents. • The resulting perovskites exhibit high crystallinity, well-defined morphology, and improved thermal stability due to hydrogen bonding interactions and defect passivation from the halogen-rich environment. • The method is versatile, enabling the synthesis of doped crystals such as Bi-doped Cs2SnCl6 with a photoluminescence quantum efficiency of 12.73%, expanding potential optoelectronic applications. • This work provides a scalable and environmentally friendly pathway for producing high-quality lead-free perovskites, addressing toxicity and stability challenges for commercial deployment.
Abstract
Lead-free vacancy-ordered double perovskites have emerged as promising materials for optoelectronic applications due to their environmentally friendly characteristics and exceptional properties. However, conventional synthesis methods often depend on toxic reagents and stringent conditions, limiting their large-scale synthesis and practical application. In this work, an environmentally friendly synthesis route was proposed for preparing vacancy-ordered double perovskites Cs2SnX6 (X = Cl, Br, and I) with high crystallinity under low-temperature and ambient-pressure conditions. This method utilizes ion liquid (i.e., 1-butyl-3-methylimidazolium chloride ([Bmim]Cl), 1-butyl-3-methylimidazolium bromide ([Bmim]Br) and 1-butyl-3-methylimidazolium iodide ([Bmim]I)) in combination with saturated aqueous solutions of ammonium halides as solvents, replacing traditional hydrogen halide acid or polar organic solvents. Experimental and characterization results demonstrate that the Cs2SnX6 (X = Cl, Br, and I) possess high crystallinity, well-defined morphology, and improved thermal stability. These improvements are attributed to the hydrogen bonding interactions between ionic liquids and the perovskite precursors. Additionally, the halogen-rich environment provided by ionic liquids and ammonium halide salts facilitates defect passivation. Furthermore, this method is applicable to the synthesis of doped perovskite crystals, demonstrated by the successful synthesis of Bi-doped Cs2SnCl6 crystals with a photoluminescence quantum efficiency of 12.73%. This study presents a novel strategy for synthesizing high-quality vacancy-ordered double perovskites and their doping or alloyed compounds.
1. Introduction
In recent years, lead halide perovskites have emerged as one of the most competitive semiconductor materials in the optoelectronic field [1–2]. Due to their excellent optoelectronic characteristics, such as high absorption coefficients, high carrier mobility, long carrier diffusion length, and small exciton binding energy, lead halide perovskites have attracted widespread attention for applications including solar cells [3–4], light-emitting diodes [5], lasers [6], scintillators [7], anti-counterfeiting [8], and near-infrared solid-state lighting [9]. Despite significant advancements in the development of lead halide perovskites, their inherent toxicity and intrinsic instability pose major challenges to commercialization [10–12]. On the one hand, the European Union’s Restriction of Hazardous Substances (RoHS) directive limits the lead concentration in electronic products to below 0.1wt% [13–14]. Furthermore, using volatile organic and polar solvents in conventional perovskite synthesis introduces additional risks, including toxicity, flammability, and explosiveness. On the other hand, three-dimensional (3D) lead-based perovskite materials, such as CsPbX3, degrade rapidly under exposure to moisture, oxygen, heat, and light, further limiting their practical application [15]. Therefore, developing environmentally friendly, highly stable, high-performance lead-free perovskite material is important.
To address these issues, researchers have explored alternatives to lead-based perovskites containing other cations [16], such as tin (Sn) [17–18], zirconium (Zr) [19], bismuth (Bi) [20], and copper (Cu) [21]. More research has focused on lead-free perovskites with enhanced structural stability, such as vacancy-ordered double perovskites (Cs2SnCl6 and Cs2ZrCl6 [19,22]). However, the synthesis of lead-free vacancy-ordered double perovskites typically requires strong inorganic acids, such as hydrochloric or hydrobromic acid, under high-temperature and high-pressure conditions [23–26]. Therefore, developing simple and environmentally friendly synthesis methods for vacancy-ordered double perovskites remains a critical issue.
Ionic liquids (ILs) represent a class of ionic compounds with melting points below 100°C. Due to their unique properties, such as odorless, non-polluting, non-flammable, low vapor pressure, and excellent thermal and chemical stability, the ILs serve as ideal alternatives to traditional solvents in green synthesis.
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Yuxin Huang, Yibo Cui, Qipeng Lu, Xin Liu, Lijie Zhu (2025). An environmentally friendly synthesis route: Low-temperature preparation of vacancy-ordered double perovskites Cs2SnX6 (X = Cl, Br, I) via ionic liquid. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-025-3177-8
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Frequently Asked Questions
What are vacancy-ordered double perovskites?
Vacancy-ordered double perovskites are a class of lead-free perovskite materials with a crystal structure where one cation site is vacant, resulting in enhanced stability and optoelectronic properties. Examples include Cs2SnCl6 and Cs2ZrCl6.
Why is the synthesis method described in the paper considered environmentally friendly?
The method uses ionic liquids and ammonium halides as solvents instead of toxic hydrogen halide acids or polar organic solvents. It operates at low temperature and ambient pressure, reducing energy consumption and hazardous waste, thus aligning with green chemistry principles.
What are the key advantages of using ionic liquids in perovskite synthesis?
Ionic liquids provide a halogen-rich environment that facilitates defect passivation, and their hydrogen bonding interactions with precursors improve crystallinity and thermal stability. They are also non-volatile, non-flammable, and recyclable, making them safer and more sustainable.
Can this synthesis method be applied to doped perovskites?
Yes, the method is versatile and can be extended to synthesize doped perovskite crystals. The paper demonstrates successful synthesis of Bi-doped Cs2SnCl6 with a photoluminescence quantum efficiency of 12.73%, indicating potential for tunable optoelectronic properties.
What are the potential applications of the synthesized Cs2SnX6 perovskites?
These lead-free perovskites with high crystallinity and thermal stability are promising for optoelectronic applications such as solar cells, light-emitting diodes, lasers, scintillators, and anti-counterfeiting, offering a safer and more stable alternative to lead-based perovskites.
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