Mechanosynthesis, Optical, and Morphological Properties of MA, FA, Cs-SnX3 (X = I, Br) and Phase-Pure Mixed-Halide MASnI x Br3 -x Perovskites

Unprecedented Richness of Temperature- and Pressure-Induced Polymorphism in 1D Lead Iodide Perovskite

Inherent features of metal halide perovskites are their softness, complex lattice dynamics, and phase transitions spectacularly tuning their structures and properties. While the structural transformations are well described and classified in 3D perovskites, their 1D analogs are much less understood. Herein, both temperature- and pressure-dependent structural evolutions of a 1D AcaPbI3 perovskitoid incorporating acetamidinium (Aca) cation are examined. The study reveals the existence of nine phases of δ-AcaPbI3, which present the most diverse polymorphic collection among known perovskite materials. Interestingly, temperature- and pressure-triggered phase transitions in the 1D perovskotoid exhibit fundamentally different natures: the thermal transformations are mainly associated with the collective translations of rigid polyanionic units and ordering/disordering dynamics of Aca cations, while the compression primarily affects inorganic polymer chains. Moreover, in the 1-D chains featuring the face-sharing connection mode of the PbI6 octahedra the Pb···Pb distances are significantly shortened compared to the corner-sharing 3D perovskite frameworks, hence operating in the van der Waals territory. Strikingly, a good correlation is found between the Pb···Pb distances and the pressure evolution of the bandgap values in the δ-AcaPbI3, indicating that in 1D perovskitoid structures, the contacts between Pb2+ ions are one of the critical parameters determining their properties.

An intrinsic electrical conductivity study of perovskite powders MAPbX3 (X = I, Br, Cl) to investigate its effect on their photovoltaic performance

An investigation into the intrinsic electrical conductivity of perovskite powders MAPbX3, where X represents iodine (I), bromine (Br), or chlorine (Cl), was conducted to explore its impact on their photovoltaic performance. Results revealed that MAPbCl3 demonstrated light absorption ability in the ultraviolet and visible regions, while MAPbBr3 showed capacity for light absorption at longer wavelengths in the visible spectrum. On the other hand, MAPbI3 exhibited good absorption at longer wavelengths, indicating its ability to absorb light in the near-infrared region. The optical bandgap of each perovskite was determined to be 2.90 eV for MAPbCl3, 2.20 eV for MAPbBr3, and 1.47 eV for MAPbI3. The electrical conductivities of these powders were measured in-plane using the four-probe method and through-plane by electrochemical impedance spectroscopy (EIS). Electrochemical impedance spectroscopy (EIS) studies revealed a significant change in the conductivity of the MAPbI3 perovskite at temperatures between 80 °C and 100 °C. This change could be attributed to structural modifications induced when the temperature exceeds these values. The through-plane conductivity changed from 3 × 10-8 S cm-1 at 60 °C to approximately 6 × 10-5 S cm-1 at 120 °C and around 2 × 10-3 S cm-1 at 200 °C. Meanwhile, the sheet conductivity (in-plane conductivity) measurements performed at ambient temperature reveal that sheet conductivities are 489 × 103 S m-1, 486 × 103 S m-1 and 510 × 103 S m-1 for MAPbBr3, MAPbCl3 and MAPbI3, respectively. This study provides valuable insights for optimizing the performance of perovskite solar cells. Understanding how dopants influence the electrical conductivity and photovoltaic properties of the perovskite material, this work will enable researchers to design and engineer more efficient and stable solar cell devices based on MAPbX3 perovskites.

Tuning the morphology, stability and optical properties of CsSnBr3 nanocrystals through bismuth doping for visible-light-driven applications

In this investigation, we have demonstrated the synthesis of lead-free CsSnBr3 (CSB) and 5 mol% bismuth (Bi) doped CSB (CSB'B) nanocrystals, with a stable cubic perovskite structure following a facile hot injection technique. The Bi substitution in CSB was found to play a vital role in reducing the size of the nanocrystals significantly, from 316 ± 93 to 87 ± 22 nm. Additionally, Bi doping has inhibited the oxidation of Sn2+ of CSB perovskite. A reduction in the optical band gap from 1.89 to 1.73 eV was observed for CSB'B and the PL intensity was quenched due to the introduction of the Bi3+ dopant. To demonstrate one of the visible-light-driven applications of the nanocrystals, photodegradation experiments were carried out as a test case. Interestingly, under UV-vis irradiation, the degradation efficiency of CSB'B was roughly one order lower than that of P25 titania nanoparticles; however, it was almost five times higher when driven by visible light under identical conditions. The water stability of CSB'B perovskite and suppression of the oxidative degradation of Sn were confirmed through XRD and XPS analyses after photocatalysis. Moreover, by employing experimental parameters, DFT-based first-principles calculations were performed, which demonstrated an excellent qualitative agreement between experimental and theoretical outcomes. The as-synthesized Bi-doped CSB might be a stable halide perovskite with potential in visible-light-driven applications.

Tinkering with Mechanochemical Tools for Scale Up

Mechanochemistry provides an environmentally benign platform to develop more sustainable chemical processes by limiting raw materials, energy use, and waste generation while using physically smaller equipment. A continuously growing research community has steadily showcased examples of beneficial mechanochemistry applications at both the laboratory and the preparative scale. In contrast to solution-based chemistry, mechanochemical processes have not yet been standardized, and thus scaling up is still a nascent discipline. The purpose of this Minireview is to highlight similarities, differences and challenges of the various approaches that have been successfully applied for a range of chemical applications at various scales. We hope to provide a discussion starting point for those interested in further developing mechanochemical processes for commercial use and/or industrialisation.

Azetidinium Lead Halide Ruddlesden-Popper Phases

A family of Ruddlesden–Popper (n = 1) layered perovskite-related phases, Az₂PbCl_xBr_4−x with composition 0 ≤ x ≤ 4 were obtained using mechanosynthesis. These compounds are isostructural with K₂NiF₄ and therefore adopt the idealised n = 1 Ruddlesden–Popper structure. A linear variation in unit cell volume as a function of anion average radius is observed. A tunable bandgap is achieved, ranging from 2.81 to 3.43 eV, and the bandgap varies in a second-order polynomial relationship with the halide composition.

NMR spectroscopy probes microstructure, dynamics and doping of metal halide perovskites

Solid-state magic-angle spinning NMR spectroscopy is a powerful technique to probe atomic-level microstructure and structural dynamics in metal halide perovskites. It can be used to measure dopant incorporation, phase segregation, halide mixing, decomposition pathways, passivation mechanisms, short-range and long-range dynamics, and other local properties. This Review describes practical aspects of recording solid-state NMR data on halide perovskites and how these afford unique insights into new compositions, dopants and passivation agents. We discuss the applicability, feasibility and limitations of ¹H, ¹³C, ¹⁵N, ¹⁴N, ¹³³Cs, ⁸⁷Rb, ³⁹K, ²⁰⁷Pb, ¹¹⁹Sn, ¹¹³Cd, ²⁰⁹Bi, ¹¹⁵In, ¹⁹F and ²H NMR in typical experimental scenarios. We highlight the pivotal complementary role of solid-state mechanosynthesis, which enables highly sensitive NMR studies by providing large quantities of high-purity materials of arbitrary complexity and of chemical shifts calculated using density functional theory. We examine the broader impact of solid-state NMR on materials research and how its evolution over seven decades has benefitted structural studies of contemporary materials such as halide perovskites. Finally, we summarize some of the open questions in perovskite optoelectronics that could be addressed using solid-state NMR. We, thereby, hope to stimulate wider use of this technique in materials and optoelectronics research.

Mechanochemical synthesis of pure phase mixed-cation/anion (FAPbI3) x (MAPbBr3)1-x hybrid perovskite materials: compositional engineering and photovoltaic performance

Organic–inorganic hybrid perovskites have emerged as promising light harvesting materials for many optoelectronic devices. Here, we present a facile mechanochemical synthesis (MCS) route for the preparation of a series of pure phase mixed-cation/anion (FAPbI₃)_x(MAPbBr₃)_1−x (0 ≤ x ≤ 1) hybrid perovskite materials for high-efficiency thin-film perovskite solar cells (PSCs). The use of (α-FAPbI₃)_0.95(MAPbBr₃)_0.05 perovskite prepared by MCS for the thin-film PSCs achieves a maximum PCE of 15.9% from a current–voltage (J–V) scan, which stabilises at 15.4% after 120 s of the maximum power point output. Furthermore, PSCs based on (KPbI₃)_0.05(FAPbI₃)_0.9(MAPbBr₃)_0.05 perovskite prepared by MCS exhibit higher photovoltaic performance and lower hysteresis compared with (α-FAPbI₃)_0.95(MAPbBr₃)_0.05, with a maximum PCE of 16.7%. These results indicate that the use of mechanochemically synthesised perovskites provides a promising strategy for high performance PSCs and superior control in optoelectronic properties, leading to improved control in fabrication approaches and facilitating the development of efficient and stable PSCs in the future.

Pure phase mixed-cation/anion (α-FAPbI₃)x(MAPbBr₃)1−x (0 ≤ x ≤ 1) hybrid perovskites are efficiently prepared via MCS, and the band gaps can be tuned easily. PSCs based on 5% K-doped perovskite exhibit low I–V hysteresis, with a maximum PCE of 16.7%.

Halide Mixing and Phase Segregation in Cs2AgBiX6 (X = Cl, Br, and I) Double Perovskites from Cesium-133 Solid-State NMR and Optical Spectroscopy

All-inorganic double perovskites (elpasolites) are a promising potential alternatives to lead halide perovskites in optoelectronic applications. Although halide mixing is a well-established strategy for band gap tuning, little is known about halide mixing and phase segregation phenomena in double perovskites. Here, we synthesize a wide range of single- and mixed-halide Cs₂AgBiX₆ (X = Cl, Br, and I) double perovskites using mechanosynthesis and probe their atomic-level microstructure using ¹³³Cs solid-state MAS NMR. We show that mixed Cl/Br materials form pure phases for any Cl/Br ratio while Cl/I and Br/I mixing is only possible within a narrow range of halide ratios (<3 mol % I) and leads to a complex mixture of products for higher ratios. We characterize the optical properties of the resulting materials and show that halide mixing does not lead to an appreciable tunability of the PL emission. We find that iodide incorporation is particularly pernicious in that it quenches the PL emission intensity and radiative charge carrier lifetimes for iodide ratios as low as 0.3 mol %. Our study shows that solid-state NMR, in conjunction with optical spectroscopies, provides a comprehensive understanding of the structure-activity relationships, halide mixing, and phase segregation phenomena in Cs₂AgBiX₆ (X = Cl, Br, and I) double perovskites.

Recent Advancements and Challenges for Low-Toxicity Perovskite Materials

Lead-based organic-inorganic hybrid perovskite materials have been developed for advanced optoelectronic applications. However, the toxicity of lead and the chemical instability of lead-based perovskite materials have so far been demonstrated to be an overwhelming challenge. The discovery of perovskite materials based on low-toxicity elements, such as Sn, Bi, Sb, Ge, and Cu, with superior optoelectronic properties provides alternative approaches to realize high-performance perovskite optoelectronics. In this review, recent advances in the aspects of low-toxicity perovskite solar cells, photodetectors, light-emitting diodes, and thermoelectric devices are highlighted. The antioxidation stability of metal cation and the crystallization process of the low-toxicity perovskite materials are discussed. In the last part, the outlook toward addressing various issues requiring further attention in the development of low-toxicity perovskite materials is outlined.

Local Structure and Dynamics in Methylammonium, Formamidinium, and Cesium Tin(II) Mixed-Halide Perovskites from 119Sn Solid-State NMR

Organic–inorganic tin(II) halide perovskites have emerged as promising alternatives to lead halide perovskites in optoelectronic applications. While they suffer from considerably poorer performance and stability in comparison to their lead analogues, their performance improvements have so far largely been driven by trial and error efforts due to a critical lack of methods to probe their atomic-level microstructure. Here, we identify the challenges and devise a ¹¹⁹Sn solid-state NMR protocol for the determination of the local structure of mixed-cation and mixed-halide tin(II) halide perovskites as well as their degradation products and related phases. We establish that the longitudinal relaxation of ¹¹⁹Sn can span 6 orders of magnitude in this class of compounds, which makes judicious choice of experimental NMR parameters essential for the reliable detection of various phases. We show that Cl/Br and I/Br mixed-halide perovskites form solid alloys in any ratio, while only limited mixing is possible for I/Cl compositions. We elucidate the degradation pathways of Cs-, MA-, and FA-based tin(II) halides and show that degradation leads to highly disordered, qualitatively similar products, regardless of the A-site cation and halide. We detect the presence of metallic tin among the degradation products, which we suggest could contribute to the previously reported high conductivities in tin(II) halide perovskites. ¹¹⁹Sn NMR chemical shifts are a sensitive probe of the halide coordination environment as well as of the A-site cation composition. Finally, we use variable-temperature multifield relaxation measurements to quantify ion dynamics in MASnBr₃ and establish activation energies for motion and show that this motion leads to spontaneous halide homogenization at room temperature whenever two different pure-halide perovskites are put in physical contact.

Solvent-Free Mechanochemical Synthesis of a Systematic Series of Pure-Phase Mixed-Halide Perovskites MAPb(Ix Br1-x )3 and MAPb(Brx Cl1-x )3 for Continuous Composition and Band-Gap Tuning

Hybrid perovskites have recently received much attention in optoelectronic applications. However, hybrid perovskites are unstable in a humid environment. Mixed halide perovskites (MHPs) show enhanced stability and band-gap tunability upon engineering of their halide composition. Here, MHPs are prepared through a solvent-free mechanochemical synthesis (MCS) route that allows superior control over halide compositions than the solvent synthesis routes (SS). The MCS route eliminates the problem in the preparation of MAPb(I_x Br_1-x )₃ with continuously varying x, while maintaining the material properties and suppressing phase segregation present in SS routes. UV-vis absorption and X-ray diffraction patterns confirm the production of the desired pure-phase MHPs. For MAPb(I_x Br_1-x )₃ (0≤x≤1), with increased ratio of halide (x), the cubic phase gradually transforms into the tetragonal phase and band-gap tunability is accomplished. The MCS route for the preparation of MHPs is a very promising and efficient technique for superior control in optoelectronic properties, leading to improved control in fabrication approaches.

Mechanoperovskites for Photovoltaic Applications: Preparation, Characterization, and Device Fabrication

Hybrid organic-inorganic metal halide perovskites (MHPs) have emerged as excellent absorber materials for next generation solar cells owing to their simple solution-processed synthesis and high efficiency. This breakthrough in photovoltaics along with an accompanying impact in light-emitting applications prompted a renaissance of interest in the broad family of MHPs. Notably, the optoelectronic properties and the photovoltaic parameters of MHPs are highly sensitive to the adopted synthetic strategy. The preparation of MHPs has commonly relied on solution-based methods requiring elevated temperatures for homogeneity of reaction mixtures. While the solution-based approach is relatively versatile, it faces challenges such as limitations in compositional engineering of MHPs or their long-term storage among others. Therefore, there is a continuous great challenge to develop efficient synthetic strategies affording various high-quality MHP materials for numerous technological optoelectronic applications. In the past decade, mechanochemistry has appeared as a green alternative to traditional synthesis. This solid-state, re-emerging efficient synthetic methodology mediated by direct absorption of mechanical energy is growing explosively across organic and inorganic chemistry and materials science. In this Account, we describe our shared interest in the productive use of mechanical force in chemistry of MHPs, as well as assembly of the respective solar cell devices. We highlight the milestones achieved by our groups along with the seminal contributions by other groups. In particular, we demonstrate that mechanochemistry efficiently allows the formation of various phase pure hybrid lead and lead-free halide perovskite compositions (called hereafter "mechanoperovskites"). The progress in solvent-free solid-state synthesis is greatly enhanced by the integration of advanced methods of solid-state analysis like powder X-ray diffraction (pXRD), solid-state nuclear magnetic resonance (ss-NMR) and UV-vis spectroscopies, and we aim to illustrate this ongoing integration through appropriate examples. Furthermore, we show that thin films based on mechanoperovskites have the advantage of providing a higher degree of control of the stoichiometry and higher reproducibility, stability, and material phase purity. The impact of using powdered mechanoperovskite as a precursor for thin film formation on the electrochemical and photovoltaic properties of the solar cells is also discussed. Finally, our view of current challenges and future directions in this emerging interdisciplinary area of research is provided.

Dependence of material properties and photovoltaic performance of triple cation tin perovskites on the iodide to bromide ratio

Abstract

In this work, the influence of a partial introduction of bromide (x = 0–0.33) into MA0.75FA0.15PEA0.1Sn(BrxI1−x)3 (MA: methylammonium, FA: formamidinium, PEA: phenylethylammonium) triple cation tin perovskite on the material properties and photovoltaic performance is investigated and characterized. The introduction of bromide shifts the optical band gap of the perovskite films from 1.29 eV for the iodide-based perovskite to 1.50 eV for the perovskite with a bromide content of x = 0.33. X-ray diffraction measurements reveal that the size of the unit cell is also gradually reduced based on the incorporation of bromide. Regarding the photovoltaic performance of the perovskite films, it is shown that already small amounts of bromide (x = 0.08) in the perovskite system increase the open circuit voltage, short circuit current density and fill factor. The maximum power conversion efficiency of 4.63% was obtained with a bromide content of x = 0.25, which can be ascribed to the formation of homogeneous thin films in combination with higher values of the open circuit voltage. Upon introduction of a higher amount of bromide (x = 0.33), the perovskite absorber layers form pinholes, thus reducing the overall device performance.

Graphic abstract

High Versatility and Stability of Mechanochemically Synthesized Halide Perovskite Powders for Optoelectronic Devices

We show that mechanochemically synthesized halide perovskite powders from a ball milling approach can be employed to fabricate a variety of lead halide perovskites with exceptional intrinsic stability. Our MAPbI₃ powder exhibits higher thermal stability than conventionally processed thin films, without degradation after more than two and a half years of storage and only negligible degradation after heat treatment at 220 °C for 14 h. We further show facile recovery strategies of nonphase-pure powders by simple remilling or mild heat treatment. Moreover, we demonstrate the mechanochemical synthesis of phase-pure mixed perovskite powders, such as (Cs_0.05FA_0.95PbI₃)_0.85(MAPbBr₃)_0.15, from either the individual metal and organic halides or from readily prepared ternary perovskites, regardless of the precursor phase purity. Adding potassium iodide (KI) to the milling process successfully passivated the powders. We also succeeded in preparing a precursor solution on the basis of the powders and obtained uniform thin films for integration into efficient perovskite solar cells from spin-coating this solution. We find the KI passivation remains in the devices, leading to improved performance and significantly reduced hysteresis. Our work thus demonstrates the potential of mechanochemically synthesized halide perovskite powders for long-time storage and upscaling, further paving the way toward commercialization of perovskite-based optoelectronic devices.

Mechanochemical and slow-chemistry radical transformations: a case of diorganozinc compounds and TEMPO

From the green chemistry perspective, molecular solid-state transformations conducted under mild conditions are of great interest and desirability. However, research in this area lacked popularity in the previous century, and thus progressed slowly. In particular, the application of radical reactions in solid-state chemistry has been hampered by several long-standing challenges that are intrinsically associated with the apparent unpredictable nature of radical chemistry. We present a comparative study of model mechanochemical, slow-chemistry and solution radical reactions between TEMPO and homoleptic organozinc compounds (i.e., di-tert-butylzinc and diphenylzinc). In the case of the tBu2Zn/TEMPO reaction system only a dimeric diamagnetic complex [tBuZn(μ-TEMPO*)]2 is obtained in yields slightly varying with the method chosen. In contrast, when TEMPO is mixed with diphenylzinc in a 2 : 1 molar ratio a novel paramagnetic Lewis acid-base adduct [[Ph2Zn(η1-TEMPO)]·TEMPO] is isolated in high yields regardless of the applied methodology. This adduct is also formed in the slow-chemistry process when TEMPO is gently mixed with Ph2Zn in a 1 : 1 molar ratio and left for two weeks at ambient temperature. Within the next week the reaction mixture gives in high yield a diamagnetic dinuclear compound [PhZn(μ-TEMPO*)][PhZn(μ2-η1:η1-TEMPO*)] and biphenyl. The analogous reaction conducted in toluene results in a much lower conversion rate. The reported results open up a new horizon in molecular solid-state radical transformations.