Tin Fluoride Research Articles

Synthesis and Ionic Conductivity of K2Bi1-X Sn x F5-X

Introduction In recent years, xEVs have been spreading rapidly. In order to extend the driving range, automotive batteries with high energy density and high safety exceeding lithium-ion batteries are required. Solid-state fluoride batteries (SSFBs), in particular, are currently attracting attention as one of the innovative storage batteries with a high theoretical energy density due to multi-electron reactions. Since organic electrolytes are not used, SSFBs have no risk of leakage and firing and a high level of safety.The fluoride ion is expected to be suitable for ion diffusion at a high rate because it is monovalent and has a small atomic mass. However, the operating temperature of SSFBs is still high at about 150 ˚C. The challenges of SSFBs to lower the operating temperature are especially in electrolytes. Currently, PbSnF4 compounds and tysonite-type fluorides are representative examples of fluoride ionic conductors. However, these electrolytes have issues such as a narrow electrochemical potential window or insufficient ionic conductivity. New fluoride ionic conductors with high ionic conductivity and wide electrochemical potential windows are required.In this study, we focused on K2BiF5-type fluoride with one-dimensional chains of edge-shared BiF7 polyhedra acting as an ionic conductive pathway. Previously we demonstrated that the ionic conductivity of K2-x Rb x BiF5 (0.0 ≤ x ≤ 0.4) increases with increasing Rb content and the activation energy decreases. K2-x Rb x BiF5 with x = 0.4 exhibited the ionic conductivity of 1.0 × 10-5 S-cm-1 at 150°C and the activation energy of 0.68 eV. The expansion of bottle neck size by the substitution of Rb with larger ionic radius than that of K is effective to enhance fluoride ion conductivity. To further improve the ionic conductivity we synthesized Sn-substituted K2BiF5 to introduce fluorine defects and evaluated their ionic conductivity. Experimental K2Bi1-xSnxF5-x with x = 0, 0.05 and 0.10 were synthesized using potassium fluoride KF, bismuth fluoride BiF3, and tin fluoride SnF2 as starting materials. The mechano-chemical treatment was carried out at 600 rpm for 3 - 24 h using a FRITSCH planetary mill with a mill jar and balls made of zirconia. The initial loading of the mixture was 1.5 g. K2Bi1-x Sn x F5 with x = 0, 0.05, and 0.10 were synthesized by mechano-chemical treatment and subsequent heating. The ionic conductivity was evaluated by the AC impedance measurement using Au sputted electrode. Results and discussion The XRD measurements demonstrated that the target phase was obtained after ball-milling for the sample with x = 0 and 0.05, while a small amount of unknown phase was contained. After heating, a single phase of K2Bi1-x Sn x F5 was obtained. For K2Bi1-x Sn x F5 with x = 0.10 K2BiF5 phase did not appear after ball-milling for 3-12 h but appeared after heating at 200 ˚C for 10 h twice. The sample contained a large amount of impurity phase. Considering the lattice parameter of K2Bi1-x Sn x F5, since the Sn2+ ion has a smaller ionic radius than that of the Bi3+ ion, the lattice is expected to shrink with Sn substitution. However, the lattice parameters of a-, b- and c-axes and the lattice volume did not change systematically with Sn content. It would be due to compositional deviation caused by the formation of impurities.The ionic conductivity was evaluated by the AC impedance measurement. With increasing Sn content in the K2Bi1-x Sn x F5, the ionic conductivity was increased and the activation energy was decreased. K2Bi1-x Sn x F5 with x =0.10 showed the ionic conductivity of 3.59× 10-8 S-cm-1 at 25°C and 2.05 × 10-5 S-cm-1 at 150°C. The activation energy was 0.55 eV. They indicate that aliovalent Sn substitution is effective in improving the ionic conductivity of K2BiF5. Acknowledgements This presentation is based on results obtained from a project, JPNP21006, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Advanced Hybrid Polymer/Ceramic PEO-SnF2 Protective Mechanism for Sodium Metal Anodes

In the face of mounting global energy requirements and pressing environmental issues, the scientific community turned to lithium-metal batteries, celebrated for their superior capacity and efficiency. However, challenges arising from the scarcity and increasing costs of lithium have redirected research efforts towards sodium batteries. Sodium metal anodes, with their remarkable theoretical specific capacity of ~1166 mAh/g, low anode potential of -2.714 V vs standard hydrogen electrode, and cost benefits, are increasingly recognized as promising candidates for conventional energy storage systems. Notwithstanding their advantages, the unstable and fragile solid electrolyte interphase (SEI) due to the spontaneous reaction between metallic Na and a liquid electrolyte remains a significant limitation. As this SEI layer expands, it hinders ion transport, causing increased polarization and diminishing the battery's energy efficiency. Specifically, during the plating process, Na ions are deposited beneath the SEI layer, causing significant expansion. This expansion fractures the fragile SEI, allowing dendrites to grow through these defects. As these dendrites grow, they can pierce the separator, posing a risk of short-circuiting the battery. While numerous studies have proposed designs for protecting sodium metal interphase, to overcome these challenges, this study amalgamates mechano-electrochemical protective strategies to enhance sodium metal anode stability, offering groundbreaking solutions to these challenges.Central to our exploration were tin fluoride (SnF2) and silicon nitride (Si3N4), both established as effective in generating robust artificial SEI layers on the sodium metal surface. These layers, rich in NaF and NaxN respectively, demonstrated a notable improvement in cycling performance, effectively suppressing dendrite growth and enhancing the battery's cycling stability by nearly 3.5 times compared to bare sodium anodes.Building upon these foundational insights, a novel approach was conceptualized: a hybrid protective mechanism combining polyethylene oxide (PEO) with the presence of SnF2 as an additive, is particularly efficacious. This combination was specifically chosen to provide chemical stability, morphological conformality, and mechanical flexibility to accommodate the mechano-electrochemical instability upon sodiation/desodiation processes. Through this novel strategy, we aimed to amalgamate the strengths of both components, ensuring optimal outcomes. This composite artificial SEI under a 0.25 mA/cm2 current density, delivers a cycling performance 10 times superior to bare sodium metal and survived up to 1800 hours of cycling. Impressively, at a high current density of 0.5 mA/cm2, the hybrid system still performed much better than its untreated counterpart and continued cycling for up to 800 hours of cycling. The synergy of PEO and SnF2 remarkably minimized electrolyte decomposition, inhibited dendrite formation, and stabilized Na plating/stripping processes, all of which consolidated its promise for sodium metal battery technology.

Sonochemical synthesis and characterization of SnF2 as a potential fluoride-ion conducting solid electrolyte

Fluoride ion batteries (FIBs) are emerging as a promising alternative to conventional lithium-ion batteries, primarily because of their high theoretical energy density and environmentally sustainable characteristics. For the development of a high-performance solid-state fluoride ion battery, an engineered electrolyte is crucial. As the synthesis technique impacts the phase, microstructure, and morphology of the materials, a unique and facile sonochemical route is employed for the first time herein for synthesizing Tin (II) fluoride as a solid electrolyte. The XRD investigations have indicated the formation of the α-SnF2 phase with a monoclinic structure, and space group C2/c. The particle size of SnF2 synthesized using 225 W and 325 W power of probe sonicator is found to be 2 μm and 0.8 μm respectively, as revealed by Scanning Electron Micrographs. SnF2 prepared with 325 W ultrasonic power has exhibited the maximum ionic conductivity of value 3.4 × 10−4 S/cm at room temperature, with a significant enhancement regarding the values reported so far. The anionic transport number of the electrolyte estimated at room temperature using the combined AC-DC technique is found to be 0.96, indicating fluoride ion conduction.

Synergistic Modulation of Sn-Based Perovskite Solar Cells with Crystallization and Interface Engineering.

A high-quality Sn-based perovskite absorption layer and effective carrier transport are the basis for high-performance Sn-based perovskite solar cells. The suppression of Sn2+ oxidation and rapid crystallization is the key to obtaining high-quality Sn-based perovskite film. And interface engineering is an effective strategy to enhance carrier extraction and transport. In this work, tin fluoride (SnF2) was introduced to the perovskite precursor solution, which can effectively modulate the crystallization and morphology of Sn-based perovskite layer. Furthermore, the hole-transporting layer of PEDOT:PSS was modified with CsI to enhance the hole extraction and transport. As a result, the fabricated inverted Sn-based perovskite solar cells demonstrated a power conversion efficiency of 7.53% with enhanced stability.

High-performance tin perovskite transistors through formate pseudohalide engineering

The lack of high-performance p-type semiconducting materials hinders the integration of complementary metal-oxide semiconductors with well-established n-type metal-oxide counterparts. Although tin halide perovskites are promising p-type material candidates, their practical implementation is hindered by excessive hole concentrations and difficulties in precisely controlling crystallization, which leads to poor device performance and yield. In this paper, we propose a formate pseudohalide engineering method to overcome these issues and demonstrate high-performance tin perovskite thin-film transistors (TFTs). The incorporation of formate anion greatly suppresses the vacancy defects at the surfaces of the perovskite films with an increase in crystallinity and grain size. This reduces the hole concentration and eliminates the dependence on the addition of excessive tin fluoride for hole suppression. Hence, high-performance TFTs with a high average field-effect hole mobility of 57.34 cm2 V−1 s−1 and on/off current ratios surpassing 108 can be achieved, approaching p-channel low-temperature polysilicon devices.

Hot-Carrier Cooling Regulation for Mixed Sn-Pb Perovskite Solar Cells.

The rapid relaxation of hot carriers leads to energy loss in the form of heat and consequently restricts the theoretical efficiency of single-junction solar cells; However, this issue has not received much attention in tin-lead perovskites solar cells. Herein, tin(II) oxalate (SnC2O4) is introduced into tin-lead perovskite precursor solution to regulate hot-carrier cooling dynamics. The addition of SnC2O4 increases the length of carrier diffusion, extends the lifetime of carriers, and simultaneously slows down the cooling rate of carriers. Furthermore, SnC2O4 can bond with uncoordinated Sn2+ and Pb2+ ions to regulate the crystallization of perovskite and enable large grains. The strongly reducing properties of the C2O4 2- can inhibit the oxidation of Sn2+ to Sn4+ and minimize the formation of Sn vacancies in the resulting perovskite films. Additionally, as a substitute for tin(II) fluoride, the introduction of SnC2O4 avoids the carrier transport issues caused by the aggregation of F- ions at the interface. As a result, the SnC2O4-treated Sn-Pb cells show a champion efficiency of 23.36%, as well as 27.56% for the all-perovskite tandem solar cells. Moreover, the SnC2O4-treated devices show excellent long-term stability. This finding is expected to pave the way toward stable and highly efficient all-perovskite tandem solar cells.

Na4Sn4(C2O4)3F6 and NaSnC2O4F·H2O: two tin(II) fluoride oxalates display large birefringence.

Birefringent materials play an important role in laser techniques as an essential part of optical devices. Therefore, the exploration of high-performance birefringent materials has been a central focus of researchers. Herein, two tin(II) fluoride oxalates Na4Sn4(C2O4)3F6 and NaSnC2O4F·H2O were gained by the combination of birefringence-active groups of Sn2+ with stereochemically active lone pairs and planar π-conjugated [C2O4]2- groups. These groups assemble into low-dimensional structures of 0D [C2O4F4]6- clusters and 1D [SnC2O4F]∞- chains in Na4Sn4(C2O4)3F6, and double [Sn2(C2O4)2F2]∞2- chains in NaSnC2O4F·H2O, which gives rise to the large birefringence of 0.160@546 nm and 0.189@546 nm, respectively. Detailed structure-property analysis and theoretical calculations indicate that strong optical anisotropy can be induced by the rational arrangement of the Sn2+-polyhedra and [C2O4]2- groups.

New type of tin(IV) complex based turn-on fluorescent chemosensor for fluoride ion recognition: elucidating the effect of molecular structure on sensing property.

A novel type of chemosensor based on tin(IV) complexes incorporating hydroxyquinoline derivatives has been designed and investigated for selectively detecting fluoride ions. Sn(meq)2Cl2 (meq = 2-methyl-8-quinolinol) (complex 1) exhibits a significant enhancement in luminescence upon the introduction of fluoride ions. This enhancement greatly surpasses that observed with Snq2Cl2 and Sn(dmqo)2Cl2 (q = 8-hydroxyquinnoline; dmqo = 5,7-dimethyl-8-quinolinol). Furthermore, complex 1 displays excellent sensitivity and selectivity for fluoride detection in comparison to halides and other anions. As a result, complex 1 serves as an outstanding turn-on fluorescent chemosensor, effectively sensing fluoride ions. The Benesi-Hilderbrand method and Job's plot confirmed that complex 1 associates with F- in a 1 : 2 binding stoichiometry. Also, complex 1 exhibited a large binding constant (pKb = 10.4 M-2) and a low detection limit (100 nM). To gain a deeper insight into the photophysical properties and the underlying mechanism governing the formation of the tin(IV) fluoride complex via halide exchange, we successfully synthesized partially fluorinated Sn(meq)2F0.67Cl1.33 (2) and fully fluorinated Sn(meq)2F2 (3), all of which were characterized through computational studies, thereby elucidating their photophysical properties. DFT studies reveal that converting Sn(meq)2Cl2 to Sn(meq)2F2, an endergonic process, leads to greater stability due to reducing steric hindrance about the metal center. Furthermore, the fluorinated complex significantly increases dipole moment, resulting in high affinity toward the F- ion.

Chiral amino acid-templated tin fluorides tailoring nonlinear optical properties, birefringence, and photoluminescence.

In this study, we successfully synthesized two types of new chiral amino acid-templated tin fluoride crystals: (R)-[(C8H10NO3)2]Sn(IV)F6, (S)-[(C8H10NO3)2]Sn(IV)F6, (R)-[C8H10NO3]Sn(II)F3, and (S)-[C8H10NO3]Sn(II)F3, employing a slow evaporation method. The crystal structures of Sn(IV)-compounds were determined to belong to the noncentrosymmetric (NCS) nonpolar space group, P21212. Conversely, the structures of Sn(II)-compounds were found to crystallize in the NCS polar space group, P21, as revealed by single-crystal X-ray diffraction analysis. Remarkably, Sn(IV)-compounds exhibited a larger birefringence (0.328@546.1 nm), attributed to the well-stacked arrangement of planar π-conjugated benzene rings along the b-axis. The ability of tin(IV) fluorides to form more hydrogen bonds with ligands increased the probability of π-π interactions between benzene rings, enabling the growth of centimeter-sized crystals in Sn(IV)-compounds. In contrast, Sn(II)-compounds displayed a stronger second-harmonic generation (SHG) response (0.85 × KDP) than Sn(IV)-compounds (0.46 × KDP). This enhanced SHG response in Sn(II)-compounds was attributed to the increased dipole moments resulting from the presence of lone pairs. Additionally, Sn(II)-compounds exhibited photoluminescent properties due to the transition from the metal-to-ligand charge transfer state, facilitated by the presence of the lone pairs.

Co-treatment of an inorganic additive SnF2 and an organic additive P(VDF-TrFE) for improving the performances of MASnBr3 films

Non-toxic tin (Sn)-based perovskite is one of the most promising substitutes for toxic lead (Pb)-based perovskite. However, it is difficult to prepare well due to its instability and self-oxidation. In this study, co-adding suitable inorganic tin (II) fluoride (SnF2) and organic poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] in the Sn-based precursor solution was applied for the first time to obtain the high-quality methylammonium tin bromide (MASnBr3) perovskite film. The structural, morphological, and optical analyses discussed in this article indicate that simply adding inorganic SnF2 is insufficient to obtain MASnBr3 perovskite films with high crystallinity, uniform surface, and good optical properties. However, the SnF2 and P(VDF-TrFE) co-additives help to cause a significant decrease in morphological defects, an increase in absorption, and an enhancement in photoluminescence (PL). The X-ray diffraction (XRD) peak positions do not differ regardless of the type of additives, but the peak intensities are changed depending on the amount of the additives. It is exciting that the device fabricated using the co-added MASnBr3 perovskite films has an increased current density and a lower turn-on voltage.

Fluorinated Artificial Solid-Electrolyte-Interphase Layer for Long-Life Sodium Metal Batteries.

Sodium metal batteries have garnered significant attention due to their high theoretical specific capacity, cost effectiveness, and abundant availability. However, the propensity for dendritic sodium formation, stemming from the highly reactive nature of the sodium metal surface, poses safety concerns, and the uncontrollable formation of the solid-electrolyte interphase (SEI) leads to large cell impedance and battery failures. In this study, we present a novel approach where we have successfully developed a stable fluorinated artificial SEI layer on the sodium metal surface by employing various weight percentages of tin fluoride in a dimethyl carbonate solution, utilizing a convenient, cost-effective, and single-step method. The resulting fluoride-rich protective layer effectively stabilized the Na metal surfaces and significantly enhanced cycling stability. The engineered artificial SEI layer demonstrated an enhanced lifetime of Na metal symmetric cells of over 3.5 times, over 700 h at the current density of 0.25 mA/cm2, in cycling performance compared to the untreated sodium, which is attributed to the suppression of dendrite formation and the reduction of undesired SEI formation during high-current cycling.

Design on modified glass fiber separator by ball-milled tin fluoride particles for Zn metal anodes with high reversibility

Design on modified glass fiber separator by ball-milled tin fluoride particles for Zn metal anodes with high reversibility

Hybrid cathodes of fluoride-ion batteries with carbon nanotubes

Solid-state fluoride-ion galvanic cells (metallic Ce anode, La1-xBaxF3-x (x ≈ 0.05) electrolyte) with carbon components in the cathode material – carbon nanotubes and nanocomposites based on them – have been created and studied. The investigations were carried out by means of cyclic voltammetry, impedance spectroscopy and electron microscopy. Tin fluoride SnF2 was used as a basic cathode material. The basic cathode was modified with additives of carbon nanotubes, which channels were filled with SnF2, PbSnF4 and their eutectic composition. Introduction of these additives lead to the charge-discharge currents improvement. The maximum value of open-circuit voltage (OCV ∼ 1.9 V) is observed with adding into the cathode material the nanocomposite of carbon nanotubes filled with the eutectic composition. Carbon nanotubes filled with tin fluoride SnF2@SWCNT proved to be the best additive in the cathode material. The sample has the biggest current density compared with the other samples (∼1.3 A/m2 at 180 °C, which is ∼4 times higher than that when using the basic cathode). The OCV of the galvanic cell with SnF2@SWCNT additive in the cathode material is stable within the investigated temperature range from 25 to 180 °C. For other galvanic cells temperature dependencies of OCV are decreasing. Results of the direct and alternating current investigations of the solid electrolyte in Ce/electrolyte/Ce symmetric cell show good converging. Direct current internal resistance of the solid electrolyte is defined by grain boundaries resistance. The impedance of a galvanic cell is determined by the composition and structure of SnF2 cathode material as the component of the cell with the highest resistance.

Aldiminium Cations as Countercations to Discrete Main Group Fluoroanions.

The reactions of group 14 tetrafluorides (SiF4, GeF4, and SnF4) and group 15 pentafluorides (PF5, AsF5, and SbF5) with the CAAC-based trifluoride reagent [MeCAACH][F(HF)2] led to the isolation of salts containing discrete 5- or 6-coordinated fluoroanions. The syntheses of [MeCAACH][SiF5], [MeCAACH][GeF5], [MeCAACH][(THF)SnF5], and the structurally related [MeCAACH][(dioxane)SnF5], [MeCAACH][PF6], [MeCAACH][AsF6], and [MeCAACH][SbF6] are effective, selective and in high yield. All compounds were characterized by X-ray single-crystal structure analysis, NMR and Raman spectroscopy. It is worth noting that the synthesized [MeCAACH][GeF5] is a rare example of a structurally characterized compound with discrete [GeF5]- anion, while [MeCAACH][(THF)SnF5] and [MeCAACH][(dioxane)SnF5] represent the first compounds with discrete octahedrally coordinated tin fluoride anions with incorporated solvent molecules. Finally, the aldiminium-based cation [MeCAACH]+ proved to be suitable for the stabilization of rare discrete main group fluoride anions.

Modification of SnF2 cathode material of a fluoride-ion electrochemical cell with carbon additives

This article presents studies of carbon additives (graphite, thermally expended graphite (TEG), expandable graphite (EG), graphene, single-walled carbon nanotubes (SWCNT), nanocomposite SnF2@SWCNT) in the cathode material (SnF2) of solid-state fluoride-ion galvanic cells. The cells are made in the form of multilayer pellets: metallic cerium anode, tysonite solid solution of barium fluoride in the lanthanium fluoride La1−xBaxF3−x (x ≈ 0.05) electrolyte, SnF2 cathode with carbon additive. The pellets are investigated by scanning electron microscopy, cyclic voltammetry, impedance spectroscopy. Carbon additives are characterized by Raman spectroscopy. The original galvanic cell with SnF2 cathode without additives has open-circuit voltage (OCV) 1.69 V (at room temperature) and ∼1.46 V (at 180 °C). The galvanic cell with EG additive (mechanical mixture) has OCV 2.45 V at room temperature. The introduction of a carbon additive into the tin fluoride melt, followed by mechanical grinding to create a cathode material, reduces the current density. The use of SnF2@SWCNT nanocomposite increases the current density (1.295 A/m2) by more than 2.5 times compared to pure nanotubes. The current density of FIB with SnF2@SWCNT exceeds the values obtained with other carbon additives in the cathode material.

Revealing the Role of Tin Fluoride Additive in Narrow Bandgap Pb-Sn Perovskites for Highly Efficient Flexible All-Perovskite Tandem Cells.

Tin fluoride (SnF2) is an indispensable additive for high-efficiency Pb-Sn perovskite solar cells (PSCs). However, the spatial distribution of SnF2 in the perovskite absorber is seldom investigated while essential for a comprehensive understanding of the exact role of the SnF2 additive. Herein, we revealed the spatial distribution of the SnF2 additive and made structure-optoelectronic properties-flexible photovoltaic performance correlation. We observed the chemical transformation of SnF2 to a fluorinated oxy-phase on the Pb-Sn perovskite film surface due to its rapid oxidation. In addition, at the buried perovskite interface, we detected and visualized the accumulation of F- ions. We found that the photoluminescence quantum yield of Pb-Sn perovskite reached the highest value with 10 mol % SnF2 in the precursor solution. When integrating the optimized absorber in flexible devices, we obtained the flexible Pb-Sn perovskite narrow bandgap (1.24 eV) solar cells with an efficiency of 18.5% and demonstrated 23.1% efficient flexible four-terminal all-perovskite tandem cells.

Antimony fluoride (SbF3): A potent hole suppressor for tin(II)‐halide perovskite devices

AbstractTin (Sn2+)‐based halide perovskites have been developed as the most promising alternatives to their toxic Pb‐based counterparts in optoelectronic devices. However, the facile tin vacancy formation and easy oxidization characteristics make Sn2+‐based perovskites highly p‐doped with excessive hole concentrations, which significantly hinder their applications. Herein, we demonstrate a potent hole inhibitor of antimony fluoride (SbF3), which possesses a higher hole‐suppression capability than conventional tin fluoride (SnF2). A small amount of SbF3 allows a wide range of hole‐density modulation with no or less SnF2 addition, thus mitigating the negative effects of using only SnF2. A SnF2/SbF3 co‐additive approach was further developed to achieve high‐performance Sn2+ perovskite thin‐film transistors operated in the enhancement mode with a five‐fold enhancement of the field‐effect mobility and improved operational stability compared to using only SnF2. We expect that the SbF3 hole suppressor and co‐additive approach can provide opportunities for the development of high‐efficiency Sn2+‐perovskite optoelectronic devices.image

Formation and structural characterization of the basic tin(II) fluoride, Sn9F13O(OH)3 ⋅ 2H2O, containing the unprecedented [Sn4O(OH)3]3+ cage‐ion

AbstractSingle crystals of the title compound Sn9F13O(OH)3 ⋅ 2H2O, 3, along with those of the oxofluoride Sn4OF6, 2, and the fluorophosphate Sn3(PO4)F3, 1, have been obtained in course of an experiment mimicking the reaction of the tooth paste additive SnF2 with Ca3(PO4)2 as dentin surrogate. All three compounds have been characterized by single crystal X‐ray diffraction. While our data on the crystal structures 1 and 2 confirm previous results but improve bond lengths and angles precision significantly, 3 represents the first example of a basic tin(II) fluoride the structure of which was determined. Its crystal structure consists of bilayers held together through weak secondary Sn⋅⋅⋅F‐bonds while their unsymmetrical, protic‐aprotic monolayers are connected with each other on their protic sites via −OH⋅⋅⋅F‐hydrogen bonds. The most prominent building block of the monolayers constitutes the unprecedented, stella octangula like [Sn4O(OH)3]3+ cage‐ion. The coordination spheres of all nine Sn(II) atoms are analyzed with respect to their constitution, geometry and bonding. The observed bond lengths and bond angles of the tin(II) atoms are interpretated in terms of p‐p‐bonding via 2e–2c bonds (first coordination sphere, {SnX3}, trigonal‐pyramidal) and 4e–3c‐bonds (second coordination sphere, {SnX4}/{SnX5}, seesaw/square‐pyramidal). Bond valence calculations have been found to be of limited benefit to predict the oxidation state of Sn(II) especially when Sn−O bonds are present.

Efficient Tin (II) Fluoride‐Free Formamidinium Tin Triiodide Perovskite Solar Cells via Composition and Additive Engineering

Tin halide perovskite solar cells (TPSCs) have attracted extensive attention because of their low toxicity and high theoretical efficiency. However, rapid crystallization, rich defects, and easy oxidation of tin‐based perovskite films seriously restrict solar cell performance. Herein, a composition and additive engineering solution by removing commonly used tin fluoride additive from formamidinium tin triiodide perovskite precursors and replacing it with excess tin (II) iodide and ethylenediamine dihydroiodide (EDAI2) is adopted. The presence of EDAI2 and excess SnI2 and the absence of SnF2 enable tin‐based perovskite films with greatly improved morphology, enhanced crystalline quality, boosted optical properties, and prolonged carrier lifetime. Therefore, the SnF2‐free non‐stoichiometric TPSCs employing the EDAI2 additive and excess SnI2 have increased built‐in potential, lowered dark currents, and reduced carrier recombination. The resulting SnF2‐free and SnI2‐rich TPSCs with EDAI2 addition realize a steady‐state efficiency of 10.0%, which is one of the highest power conversion efficiencies among the SnF2‐free TPSCs, along with considerably improved light stability. This work highlights the crucial roles of additives in TPSCs and provides an effective strategy to fabricate efficient and stable low‐toxic lead‐free perovskite solar cells.

Fast and Highly Sensitive Photodetectors Based on Pb‐Free Sn‐Based Perovskite with Additive Engineering

AbstractOrganic–inorganic halide perovskites have been widely investigated for the fabrication of photodetectors because of their excellent optoelectronic properties. However, the toxicity of Pb‐based perovskites and the lack of manufacturing technology in air limits the commercialization of perovskite‐based photodetectors. Tin halide perovskite (THP) is a promising candidate to replace Pb‐based perovskites due to its low toxicity and potential commercialization. However, THP suffers poor stability in air because Sn2+ is easily oxidized to Sn4+. This “self‐doping” effect not only degrades device performance rapidly, but also makes it difficult to fabricate THP‐based devices in air. Here, Pb‐free Sn‐based 2D perovskite PEA2SnI4 films with tin fluoride (SnF2) additive are prepared in air using a nitrogen quenching hot casting method. The photodetectors with an optimal SnF2 concentration of 20% exhibit a fast response speed of 0.56 ms and an excellent specific detectivity of 6.32 × 1013 Jones, the best combination of THP‐based photodetectors reported hitherto. The SnF2 additive is found to suppress the formation of Sn vacancies and improve the crystallinity of PEA2SnI4 films. The work provides an air‐processing technology of Pb‐free perovskite, which is easy to commercialize, and SnF2 additive engineering offering an effective way to demonstrate high performance photodetectors and image recognition applications.