Antioxidative Stannous Oxalate Derived Lead‐Free Stable CsSnX 3 (X=Cl, Br, and I) Perovskite Nanocrystals

Recent Progress of Low-Toxicity Poor-Lead All-Inorganic Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have achieved an impressive certified efficiency of 25.7%, which is comparatively higher than that of commercial silicon solar cells (23.3%), showing great potential toward commercialization. However, the low stability and high toxicity due to the presence of volatile organic components and toxic metal lead in the perovskites pose significant challenges. To obtain robust and low-toxicity PSCs, substituting organic cations with pure inorganic cations, and partially or fully replacing the toxic Pb with environmentally benign metals, is one of the promising methods. To date, continuous efforts have been made toward the construction of highly performed low-toxicity inorganic PSCs with astonishing breakthroughs. This review article provides an overview of recent progress in inorganic PSCs in terms of lead-reduced and lead-free compositions. The physical properties of poor-lead all-inorganic perovskites are discussed to unveil the major challenges in this field. Then, it reports notable achievements for the experimental studies to date to figure out feasible methods for efficient and stable poor-lead all-inorganic PSCs. Finally, a discussion of the challenges and prospects for poor-lead all-inorganic PSCs in the future is presented.

Efficient Near-Infrared Luminescence Based on Double Perovskite Cs2SnCl6

Cs₂SnCl₆ double perovskite has attracted wide attention as a promising optoelectronic material because of its better stability and lower toxicity than its lead counterparts. However, pure Cs₂SnCl₆ demonstrates quite poor optical properties, which usually calls for active element doping to realize efficient luminescence. Herein, a facile co-precipitation method was used to synthesize Te⁴⁺ and Er³⁺-co-doped Cs₂SnCl₆ microcrystals. The prepared microcrystals were polyhedral, with a size distribution around 1-3 μm. Highly efficient NIR emissions at 1540 nm and 1562 nm due to Er³⁺ were achieved in doped Cs₂SnCl₆ compounds for the first time. Moreover, the visible luminescence lifetimes of Te⁴⁺/Er³⁺-co-doped Cs₂SnCl₆ decreased with the increase in the Er³⁺ concentration due to the increasing energy transfer efficiency. The strong and multi-wavelength NIR luminescence of Te⁴⁺/Er³⁺-co-doped Cs₂SnCl₆ originates from the 4f→4f transition of Er³⁺, which was sensitized by the spin-orbital allowed ¹S₀→³P₁ transition of Te⁴⁺ through a self-trapped exciton (STE) state. The findings suggest that ns²-metal and lanthanide ion co-doping is a promising method to extend the emission range of Cs₂SnCl₆ materials to the NIR region.

Ultra-stable blue-emitting lead-free double perovskite Cs2SnCl6 nanocrystals enabled by an aqueous synthesis on a microfluidic platform

Blue emitting Sn-based lead-free halide perovskite nanocrystals (NCs) are considered to be a promising material in lighting and displays. However, industrialised fabrication of blue-emitting NCs still remains a significant challenge due to the use of toxic solvents and optical instability, not mentioning in large-scale synthesis. In this work, a green-route synthesis of blue-emitting lead-free halide perovskite Cs₂SnCl₆ powders is developed, in which deionized water with a small amount of inorganic acid is used as the solvent and the synthesis of the Cs₂SnCl₆ powders is achieved on a microfluidic platform. Using the Cs₂SnCl₆ powders, we prepare Cs₂SnCl₆ NCs via an ultrasonication process. Changing the volume ratio of the ligands (oleic acid to oleylamine) can alter the photoluminescence (PL) characteristics of the prepared NCs, including the PL-peak wavelength, PL-peak intensity and quantum yield. The highest photoluminescence quantum yield (PLQY) of 13.4% is achieved by the Cs₂SnCl₆ NCs prepared with the volume ratio of oleic acid to oleylamine of 40 μL to 10 μL. A long-term PL stability test demonstrates that the as-synthesized Cs₂SnCl₆ NCs can retain a stable PLQY over a period of 60 days. This work opens up a new path for a large-scale green-route synthesis of blue-emitting Sn-based lead-free NCs, such as Cs₂SnX₆ (Cl, Br and I), towards their applications in optoelectronics.

Stable Lead-Free Tin Halide Perovskite with Operational Stability >1200 h by Suppressing Tin(II) Oxidation

Sn-based perovskites are the most promising alternative materials for Pb-based perovskites to address the toxicity problem of lead. However, the development of Sn^II -based perovskites has been hindered by their extreme instability. Here, we synthesized efficient and stable lead-free Cs₄ SnBr₆ perovskite by using SnF₂ as tin source instead of easily oxidized SnBr₂ . The SnF₂ configures a fluorine-rich environment, which can not only suppress the oxidation of Sn²⁺ in the synthesis, but also construct chemically stable Sn-F coordination to hinder the electron transfer from Sn²⁺ to oxygen within the long-term operation process. The SnF₂ -derived Cs₄ SnBr₆ perovskite shows a high photoluminescence quantum yield of 62.8 %, and excellent stability against oxygen, moisture, and light radiation for 1200 h, representing one of the most stable lead-free perovskites. The results pave a new pathway to enhance the optical properties and stability of lead-free perovskite for high-performance light emitters.

Alcohol-Stable Perovskite Nanocrystals and Their In Situ Capsulation with Polystyrene

In recent years, lead halide perovskite nanocrystals (PNCs) have presented potential scalable applications in all fields due to their outstanding properties. However, most commonly used PNCs capped with oleic acid (OA) and oleylamine (OAm) suffer from bad stability in polar solutions and thus require various surface protections with organic or inorganic materials. Encapsulation with highly hydrophobic polystyrene (PS) is one of the most efficient ways to protect PNCs; however, the presently used swelling-shrinking strategy faces several challenges, such as weak interaction between PS chains and the surface ligands in nonpolar media causing a low encapsulation efficiency, and serious aggregation of PS particles during the shrinkage process leading to very different particle sizes. Herein, alcohol-stable polyacrylic acid-capped CsPbBr3 PNCs (i.e., PAA-PNCs) are first synthesized and then in situ encapsulated with PS shells by polymerizing styrene monomer on the PNC surfaces in a polar organic solvent (e.g., ethanol). The in situ PS-encapsulated PAA-PNCs (i.e., PAA-PNCs@iPS) exhibit outstanding monodispersity, remarkable water, heat, and UV stability, high fluorescence activity, and color purity. The unique synthesis strategy and good performances of PAA-PNCs@iPS will boost the applications of PNCs in LEDs, biological imaging, and chemosensing.

Interfacial electronic properties of metal/CsSnBr3heterojunctions

All-inorganic lead-free perovskite CsSnBr₃, has been proved good stability and optoelectronic properties in theory and experiment. However, the interfacial electronic properties of metal/CsSnBr₃are still unclear in electronic devices. Herein, we systematically investigate the interfacial properties of metal electrodes (Al, Ag and Au) and CsSnBr₃with different atomic terminals (SnBr₂-T and CsBr-T) through the first-principles calculation. SnBr₂-T and CsBr-T have various contact types and Schottky barriers due to their different interaction strengths with metals. In particular, the moderate interlayer coupling strength with Al leads to the ultra-low Schottky barrier and tunneling barrier, which makes Al possess the best contact performance among the studied metals. Furthermore, the external electric field can be effective in regulating the Schottky barrier and realizing the Ohmic contact. These findings provide useful guidance for the design of perovskite-based nanoelectronic devices with high performance.

Lead-free CsSnCl3 perovskite nanocrystals: rapid synthesis, experimental characterization and DFT simulations

Lead-free metal halide perovskites have attracted great attention as light harvesters due to their promising optoelectronic and photovoltaic properties. In this investigation, we have successfully synthesized thermally stable cubic phase cesium tin chloride (CsSnCl₃) perovskite nanocrystals with improved surface morphology by adopting a rapid hot-injection technique. The excellent crystalline quality of these cubic shaped nanocrystals was confirmed by high-resolution transmission electron microscopy imaging. The binding of organic ligands on the surface of the sample was identified and characterized using nuclear magnetic resonance spectroscopy. UV-visible spectroscopy confirmed that the CsSnCl₃ nanocrystals have a direct band gap of ∼2.98 eV, which was further confirmed using steady-state photoluminescence spectroscopy. The band edge positions calculated using the Mulliken electronegativity approach predicted the potential photocatalytic capability of the as-prepared nanocrystals, which was then experimentally corroborated through the photodegradation of rhodamine-B dye under both visible and UV-visible irradiation. Our theoretical calculations employing experimentally obtained structural parameters within the generalized gradient approximation (GGA) and GGA+U methods demonstrated a 90% accurate estimation of the experimentally observed optical band gap when U_eff = 6 eV was considered. The ratio of the effective mass of the hole and electron expressed as was also calculated for U_eff = 6 eV. Based on this theoretical calculation and experimental observation of the photocatalytic performance of CsSnCl₃ nanocrystals, we have proposed a rational interpretation of the "D" value. We think that a "D" value of either much smaller or much larger than 1 is an indication of the low recombination rate of the photogenerated electron-hole pairs and the high photocatalytic efficiency of the photocatalyst. We believe that this comprehensive investigation might be helpful for the large-scale synthesis of thermally stable cubic CsSnCl₃ nanocrystals and also for a greater understanding of their potential in photocatalytic, photovoltaic and other prominent optoelectronic applications.

Phase Engineering of Cesium Manganese Bromides Nanocrystals with Color-Tunable Emission

For display applications, it is highly desirable to obtain tunable red/green/blue emission. However, lead-free perovskite nanocrystals (NCs) generally exhibit broadband emission with poor color purity. Herein, we developed a unique phase transition strategy to engineer the emission color of lead-free cesium manganese bromides NCs and we can achieve a tunable red/green/blue emission with high color purity in these NCs. Such phase transition can be triggered by isopropanol: from one dimensional (1D) CsMnBr₃ NCs (red-color emission) to zero dimensional (0D) Cs₃ MnBr₅ NCs (green-color emission). Furthermore, in a humid environment both 1D CsMnBr₃ NCs and 0D Cs₃ MnBr₅ NCs can be transformed into 0D Cs₂ MnBr₄ ⋅2 H₂ O NCs (blue-color emission). Cs₂ MnBr₄ ⋅2 H₂ O NCs could inversely transform into the mixture of CsMnBr₃ and Cs₃ MnBr₅ phase during the thermal annealing dehydration step. Our work highlights the tunable optical properties in single component NCs via phase engineering and provides a new avenue for future endeavors in light-emitting devices.

Theory-Guided Synthesis of Highly Luminescent Colloidal Cesium Tin Halide Perovskite Nanocrystals

The synthesis of highly luminescent colloidal CsSnX₃ (X = halogen) perovskite nanocrystals (NCs) remains a long-standing challenge due to the lack of a fundamental understanding of how to rationally suppress the formation of structural defects that significantly influence the radiative carrier recombination processes. Here, we develop a theory-guided, general synthetic concept for highly luminescent CsSnX₃ NCs. Guided by density functional theory calculations and molecular dynamics simulations, we predict that, although there is an opposing trend in the chemical potential-dependent formation energies of various defects, highly luminescent CsSnI₃ NCs with narrow emission could be obtained through decreasing the density of tin vacancies. We then develop a colloidal synthesis strategy that allows for rational fine-tuning of the reactant ratio in a wide range but still leads to the formation of CsSnI₃ NCs. By judiciously adopting a tin-rich reaction condition, we obtain narrow-band-emissive CsSnI₃ NCs with a record emission quantum yield of 18.4%, which is over 50 times larger than those previously reported. Systematic surface-state characterizations reveal that these NCs possess a Cs/I-lean surface and are capped with a low density of organic ligands, making them an excellent candidate for optoelectronic devices without any postsynthesis ligand management. We showcase the generalizability of our concept by further demonstrating the synthesis of highly luminescent CsSnI_2.5Br_0.5 and CsSnI_2.25Br_0.75 NCs. Our findings not only highlight the value of computation in guiding the synthesis of high-quality colloidal perovskite NCs but also could stimulate intense efforts on tin-based perovskite NCs and accelerate their potential applications in a range of high-performance optoelectronic devices.