Crystallization of Methylammonium Lead Halide Perovskites by Optical Trapping

Single crystals of organolead halide perovskites attract much attention to electrooptical and photovoltaic applications. They are usually prepared in precursor solutions incubated at controlled temperatures or under optimized vapor atmosphere conditions, and thus, multiple perovskite crystals are nucleated all over the solution. Multiple nucleation of crystals prevents efficient use of precursors in the preferential growth of large single crystals. An innovative approach is presented for spatiotemporally controlled, selective nucleation and growth of single crystals of lead halide perovskites by optical trapping with a focused laser beam. Upon such trapping in unsaturated precursor solutions, nucleation of MAPbX₃ (MA=CH₃NH₃⁺; X=Cl^?, Br^?, or I^?) is induced at the focal spot through increase in the concentration of perovskite precursors in the focal volume. The rate at which the nucleated crystal grows depends upon whether the perovskite absorbs the trapping laser or not. These findings suggest that optical trapping would be useful to prepare various perovskite single crystals and modify their optical and electronic properties; thereby, offering new methods for engineering of perovskite crystals.     

In Situ Ellipsometry Measurements on the Halide Phase Segregation of Mixed Halide Lead Perovskites**

Methylammonium lead iodide bromides MAPb(Br_xI_1-x)₃ are a class of mixed halide lead perovskites, materials that offer high-power conversion efficiencies and bandgap tunability. For these reasons, they are a promising absorber material for future solar cells, although their use is still limited due to several factors. The reversible phase segregation under even low light intensities is one of them, lowering the effective bandgap due to local segregation into iodide-rich and bromide-rich phases. While several studies have been done to illuminate the mechanism and suppression of phase segregation, challenges remain to understand its kinetics. We obtained dynamic ellipsometric measurements from x=0.5 mixed halide lead perovskite thin films protected by a polystyrene layer under green laser light with a power density of ∼11 W/cm². Time constants between 1.7(±0.7)×10⁻³ s⁻¹ for the segregation and 1.5(±0.6)×10⁻⁴ s⁻¹ for recovery were calculated. The phase segregation rate constants are surprisingly two orders of magnitude slower than and the recovery rate is consistent with those measured using photoluminescence methods under similar conditions. These results confirm a concern in the literature about the complexity in the phase separation kinetics measured from photoluminescence. We expect ellipsometry to serve as a complementary technique to other spectroscopies in studying mixed-halide lead perovskites phase segregation in the future.     

Unraveling the Role of Monovalent Halides in Mixed‐Halide Organic–Inorganic Perovskites

The performance of perovskite solar cells is strongly influenced by the composition and microstructure of the perovskite. A recent approach to improve the power conversion efficiencies utilized mixed‐halide perovskites, but the halide ions and their roles were not directly studied. Unraveling their precise location in the perovskite layer is of paramount importance. Here, we investigated four different perovskites by using X‐ray photoelectron spectroscopy, and found that among the three studied mixed‐halide perovskites, CH₃NH₃Pb(I_0.74Br_0.26)₃ and CH₃NH₃PbBr_3?xCl_x show peaks that unambiguously demonstrate the presence of iodide and bromide in the former, and bromide and chloride in the latter. The CH₃NH₃PbI_3?xCl_x perovskite shows anomalous behavior, the iodide content far outweighs that of the chloride; a small proportion of chloride, in all likelihood, resides deep within the TiO₂/absorber layer. Our study reveals that there are many distinguishable structural differences between these perovskites, and that these directly impact the photovoltaic performances.     

Nonradiative Energy Transfer through Distributed Bands in Piezochemically Synthesized Cesium and Formamidinium Lead Halide Perovskites

Repeated absorption of emitted photons, also called photon recycling, in large crystals and thick films of perovskites leads to delayed photoluminescence (PL) and decrease of PL intensity. The role of distinct band gaps, which act as donors and acceptors of energy, and nonradiative energy transfer on such delayed, low intensity emission is yet to be rationalized. Here we report delayed emission by nonradiative energy transfer across a distribution of energy states in close-packed crystallites of cesium lead bromide CsPbBr₃, formamidinium lead bromide FAPbBr₃, or the mixed halide FAPb(BrI)₃ perovskite synthesized in the form of thick pellets by the piezochemical method. The PL lifetime of the bromide-rich domain in the mixed halide pellet is considerably decreased when compared with a pure FAPbBr₃ pellet. Here the domains with higher bromide composition act as the energy donor, whereas the iodide-rich domains are the acceptors. Time-resolved PL measurements of CsPbBr₃, FAPbBr₃, and the mixed halide FAPb(BrI)₃ perovskite pellets help us to clarify the role of nonradiative energy transfer on photon recycling.     

Structural Diversity and Magnetic Properties of Hybrid Ruthenium Halide Perovskites and Related Compounds

There has been a great deal of recent interest in extended compounds containing Ru³⁺ and Ru⁴⁺ in light of their range of unusual physical properties. Many of these properties are displayed in compounds with the perovskite and related structures. Here we report an array of structurally diverse hybrid ruthenium halide perovskites and related compounds: MA₂RuX₆ (X=Cl or Br), MA₂MRuX₆ (M=Na, K or Ag; X=Cl or Br) and MA₃Ru₂X₉ (X=Br) based upon the use of methylammonium (MA=CH₃NH₃⁺) on the perovskite A site. The compounds MA₂RuX₆ with Ru⁴⁺ crystallize in the trigonal space group and can be described as vacancy‐ordered double‐perovskites. The ordered compounds MA₂MRuX₆ with M⁺ and Ru³⁺ crystallize in a structure related to BaNiO₃ with alternating MX₆ and RuX₆ face‐shared octahedra forming linear chains in the trigonal space group. The compound MA₃Ru₂Br₉ crystallizes in the orthorhombic Cmcm space group and displays pairs of face‐sharing octahedra forming isolated Ru₂Br₉ moieties with very short Ru–Ru contacts of 2.789 Å. The structural details, including the role of hydrogen bonding and dimensionality, as well as the optical and magnetic properties of these compounds are described. The magnetic behavior of all three classes of compounds is influenced by spin–orbit coupling and their temperature‐dependent behavior has been compared with the predictions of the appropriate Kotani models.     

Design Principles of Large Cation Incorporation in Halide Perovskites

Perovskites have stood out as excellent photoactive materials with high efficiencies and stabilities, achieved via cation mixing techniques. Overcoming challenges to the stabilization of Perovskite solar cells calls for the development of design principles of large cation incorporation in halide perovskite to accelerate the discovery of optimal stable compositions. Large fluorinated organic cations incorporation is an attractive method for enhancing the intrinsic stability of halide perovskites due to their high dipole moment and moisture-resistant nature. However, a fluorinated cation has a larger ionic size than its non-fluorinated counterpart, falling within the upper boundary of the mixed-cation incorporation. Here, we report on the intrinsic stability of mixed Methylammonium (MA) lead halides at different concentrations of large cation incorporation, namely, ehtylammonium (EA; [CH₃CH₂NH₃]⁺) and 2-fluoroethylammonium (FEA; [CH₂FCH₂NH₃]⁺). Density functional theory (DFT) calculations of the enthalpy of the mixing and analysis of the perovskite structural features enable us to narrow down the compositional search domain for EA and FEA cations around concentrations that preserve the perovskite structure while pointing towards the maximal stability. This work paves the way to developing design principles of a large cation mixture guided by data analysis of DFT data. Finally, we present the automated search of the minimum enthalpy of mixing by implementing Bayesian optimization over the compositional search domain. We introduce and validate an automated workflow designed to accelerate the compositional search, enabling researchers to cut down the computational expense and bias to search for optimal compositions.     

Controlled Growth of Hybrid Halide Perovskites by Crown Ether Complexation for Perovskite Solar Cells

Host-guest complexation has demonstrated potential for controlling hybrid organic-inorganic metal halide perovskite materials. In particular, crown ethers have been used due to their capacity to interact with metal cations (e. g., Pb²⁺) and small organic cations (e. g., methylammonium (MA)), which can affect hybrid perovskite materials and their solar cells. However, this strategy has been underexploited in perovskite photovoltaics, and the underlying mechanisms are not well understood. In this study, we investigate the influence of 15-crown-5 ( 15C5 ) and its benzannulated derivative (benzo-15-crown-5, B15C5 ), as well as amino-functionalized analogues (15-crown-5)-2-methylamine, 2A-15C5 , and 4′-aminobenzo-15-crown-5, 4A-B15C5 , on MAPbI₃ perovskite crystallization and inverted solar cell performance. We demonstrate the propensity of crown ether modulators to interact with Pb²⁺ cations at the perovskite interface by density functional theory calculations. This has been shown to facilitate oriented crystal growth and homogeneous film formation, as revealed by X-ray diffraction analysis complemented by scanning electron microscopy. As a result, we demonstrate an increase in the power conversion efficiency of the solar cells of interest to advancing hybrid photovoltaics.     

A Review on Cs-Based Pb-Free Double Halide Perovskites: From Theoretical and Experimental Studies to Doping and Applications

Despite the progressive enhancement in the flexibility of Pb-based perovskites for optoelectronic applications, regrettably, they are facing two main challenges; (1) instability, which originates from using organic components in the perovskite structure, and (2) toxicity due to Pb. Therefore, new, stable non-toxic perovskite materials are demanded to overcome these drawbacks. The research community has been working on a wide variety of Pb-free perovskites with different molecular formulas and dimensionality. A variety of Pb-free halide double perovskites have been widely explored by different research groups in search for stable, non-toxic double perovskite material. Especially, Cs-based Pb-free halide double perovskite has been in focus recently. Herein, we present a review of theoretical and experimental research on Cs-based Pb-free double halide perovskites of structural formulas Cs₂M⁺M³⁺X6 (M⁺ = Ag⁺, Na⁺, In⁺ etc.; M³⁺= Bi³⁺, In³⁺, Sb³⁺; X = Cl⁻, Br⁻, I¯) and Cs₂M⁴⁺X₆ (M⁴⁺ = Ti⁴⁺, Sn⁴⁺, Au⁴⁺ etc.). We also present the challenges faced by these perovskite compounds and their current applications especially in photovoltaics alongside the effect of metal dopants on their performance.     

Small‐Band‐Gap Halide Double Perovskites

Despite their compositional versatility, most halide double perovskites feature large band gaps. Herein, we describe a strategy for achieving small band gaps in this family of materials. The new double perovskites Cs₂AgTlX₆ (X=Cl ( 1 ) and Br ( 2 )) have direct band gaps of 2.0 and 0.95 eV, respectively, which are approximately 1 eV lower than those of analogous perovskites. To our knowledge, compound 2 displays the lowest band gap for any known halide perovskite. Unlike in A^IB^IIX₃ perovskites, the band‐gap transition in A^I₂BB′X₆ double perovskites can show substantial metal‐to‐metal charge‐transfer character. This band‐edge orbital composition is used to achieve small band gaps through the selection of energetically aligned B‐ and B′‐site metal frontier orbitals. Calculations reveal a shallow, symmetry‐forbidden region at the band edges for 1 , which results in long (μs) microwave conductivity lifetimes. We further describe a facile self‐doping reaction in 2 through Br₂ loss at ambient conditions.     

Lights and Shadows of DMSO as Solvent for Tin Halide Perovskites

In 2020 dimethyl sulfoxide (DMSO), the ever-present solvent for tin halide perovskites, was identified as an oxidant for Sn^II. Nonetheless, alternatives are lacking and few efforts have been devoted to replacing it. To understand this trend it is indispensable to learn the importance of DMSO on the development of tin halide perovskites. Its unique properties have allowed processing compact thin-films to be integrated into tin perovskite solar cells. Creative approaches for controlling the perovskite crystallization or increasing its stability to oxidation have been developed relying on DMSO-based inks. However, increasingly sophisticated strategies appear to lead the field to a plateau of power conversion efficiency in the range of 10–15 %. And, while DMSO-based formulations have performed in encouraging means so far, we should also start considering their potential limitations. In this concept article, we discuss the benefits and limitations of DMSO-based tin perovskite processing.     

Recent Advances and Optoelectronic Applications of Lead-Free Halide Double Perovskites

Organic–inorganic metal halide perovskites (most notably CH₃NH₃PbI₃) have demonstrated remarkable physical attributes for photovoltaic and diverse optoelectronic applications, whereas concerns about toxicity owing to the use of lead in the chemical composition still motivate further exploration of new, nontoxic candidates. Lead-free halide double perovskites (HDPs), designed by the rational chemical substitution of Pb²⁺ with other nontoxic candidate elements, have recently attracted interest as a fascinating alternative to their Pb-based counterparts. Herein, recent advances in crystal structures, physical properties, and versatile optoelectronic applications of lead-free HDPs, such as solar cells, photodetectors, X-ray detectors, and light-emitting diodes, are reviewed. Perspectives to improve the physical and photoelectric properties of existing HDP materials are also discussed and will favor future development of new, lead-free HDP candidates.     

Supramolecular Coordination of Pb2+ Defects in Hybrid Lead Halide Perovskite Films Using Truxene Derivatives as Lewis Base Interlayers

Truxene derivatives, due to their molecular structure and properties, are good candidates for the passivation of defects when deposited onto hybrid lead halide perovskite thin films. Moreover, their semiconductor characteristics can be tailored through the modification of their chemical structure, which allows-upon light irradiation- the interfacial charge transfer between the perovskite film and the truxene molecules. In this work, we analysed the use of the molecules as surface passivation agents and their use in complete functional solar cells. We observed that these molecules reduce the non-radiative carrier recombination dynamics in the perovskite thin film through the supramolecular complex formation between the Truxene molecule and the Pb²⁺ defects at the perovskite surface. Interestingly, this supramolecular complexation neither affect the carrier recombination kinetics nor the carriers collection but induced noticeable hysteresis on the photocurrent vs voltage curves of the solar cells under 1 sun illumination.     

The Role of Surface Termination in Halide Perovskites for Efficient Photocatalytic Synthesis

Halide perovskites have received attention in the field of photocatalysis owing to their excellent optoelectronic properties. However, the semiconductor properties of halide perovskite surfaces and the influence on photocatalytic performance have not been systematically clarified. Now, the conversion of triose (such as 1,3‐dihydroxyacetone (DHA)) is employed as a model reaction to explore the surface termination of MAPbI₃. By rational design of the surface termination for MAPbI₃, the production rate of butyl lactate is substantially improved to 7719 μg g^?1 cat. h^?1 under visible‐light illumination. The MAI‐terminated MAPbI₃ surface governs the photocatalytic performance. Specially, MAI‐terminated surface is susceptible to iodide oxidation, which thus promotes the exposure of Pb^II as active sites for this photocatalysis process. Moreover, MAI‐termination induces a p‐doping effect near the surface for MAPbI₃, which facilitates carrier transport and thus photosynthesis.     

Aqueous Synthesis of Methylammonium Lead Halide Perovskite Nanocrystals

Methylammonium lead halide perovskite nanocrystals offer attractive optoelectronic properties but suffer from fast degradation in the presence of water. In contradiction to this observation, we demonstrate the possibility of a direct aqueous synthesis of CH₃NH₃PbX₃ (X=Br or Cl/Br) nanocrystals through the reaction between the lead halide complex and methylamine when the pH is maintained in the range of 0–5. Under these synthetic conditions, the positively charged surface of the perovskite nanocrystals and the proper ionic balance help to prevent their decomposition in water. Additional surface capping with organic amine ligands further improves the photoluminescence quantum yield of the perovskite nanocrystals to values close to 40 %, ensures their stability under ambient conditions for several months, and their photoluminescence performance under continuous 0.1 W mm^?2 405 nm light irradiation for over 250 hours.     

Halide Chemistry in Tin Perovskite Optoelectronics: Bottlenecks and Opportunities

Tin halide perovskites (Sn HaPs) are the top lead-free choice for perovskite optoelectronics, but the oxidation of perovskite Sn²⁺ to Sn⁴⁺ remains a key challenge. However, the role of inconspicuous chemical processes remains underexplored. Specifically, the halide component in Sn HaPs (typically iodide) has been shown to play a key role in dictating device performance and stability due to its high reactivity. Here we describe the impact of native halide chemistry on Sn HaPs. Specifically, molecular halogen formation in Sn HaPs and its influence on degradation is reviewed, emphasising the benefits of iodide substitution for improving stability. Next, the ecological impact of halide products of Sn HaP degradation and its mitigation are considered. The development of visible Sn HaP emitters via halide tuning is also summarised. Lastly, halide defect management and interfacial engineering for Sn HaP devices are discussed. These insights will inspire efficient and robust Sn HaP optoelectronics.     

Predicting 195Pt NMR Chemical Shifts in Water-Soluble Inorganic/Organometallic Complexes with a Fast and Simple Protocol Combining Semiempirical Modeling and Machine Learning

Water-soluble Pt complexes are the key components in medicinal chemistry and catalysis. The well-known cisplatin family of anticancer drugs and industrial hydrosylilation catalysts are two leading examples. On the molecular level, the activity mechanisms of such complexes mostly involve changes in the Pt coordination sphere. Using ¹⁹⁵Pt NMR spectroscopy for operando monitoring would be a valuable tool for uncovering the activity mechanisms; however, reliable approaches for the rapid correlation of Pt complex structure with ¹⁹⁵Pt chemical shifts are very challenging and not available for everyday research practice. While NMR shielding is a response property, molecular 3D structure determines NMR spectra, as widely known, which allows us to build up 3D structure to ¹⁹⁵Pt chemical shift correlations. Accordingly, we present a new workflow for the determination of lowest-energy configurational/conformational isomers based on the GFN2-xTB semiempirical method and prediction of corresponding chemical shifts with a Machine Learning (ML) model tuned for Pt complexes. The workflow was designed for the prediction of ¹⁹⁵Pt chemical shifts of water-soluble Pt(II) and Pt(IV) anionic, neutral, and cationic complexes with halide, NO₂⁻, (di)amino, and (di)carboxylate ligands with chemical shift values ranging from −6293 to 7090 ppm. The model offered an accuracy (normalized root-mean-square deviation/RMSD) of 1.08 %/145.02 ppm on the held-out test set.     

Coordination Chemistry Dictates the Structural Defects in Lead Halide Perovskites

We show the influence of species present in precursor solution during formation of lead halide perovskite materials on the structural defects of the films. The coordination of lead by competing solvent molecules and iodide ions dictate the type of complexes present in the films. Depending on the processing conditions all PbIS₅⁺, PbI₂S_4, PbI₃S₃^?, PbI₄S₂^2?, PbI₅S₂^3?, PbI₆^4?and 1D (Pb₂I₄)_n chains are observed by absorption measurements. Different parameters are studied such as polarity of the solvent, concentration of iodide ions, concentration of solvent molecules and temperature. It is concluded that strongly coordinating solvents will preferentially form species with a low number of iodide ions and less coordinative solvents generate high concentration of PbI₆^?. We furthermore propose that all these plumbate ions may act as structural defects determining electronic properties of the photovoltaic films.     

Porous and Water Stable 2D Hybrid Metal Halide with Broad Light Emission and Selective H2O Vapor Sorption

In this work we report a strategy for generating porosity in hybrid metal halide materials using molecular cages that serve as both structure-directing agents and counter-cations. Reaction of the [2.2.2] cryptand (DHS) linker with Pb^II in acidic media gave rise to the first porous and water-stable 2D metal halide semiconductor (DHS)₂Pb₅Br₁₄. The corresponding material is stable in water for a year, while gas and vapor-sorption studies revealed that it can selectively and reversibly adsorb H₂O and D₂O at room temperature (RT). Solid-state NMR measurements and DFT calculations verified the incorporation of H₂O and D₂O in the organic linker cavities and shed light on their molecular configuration. In addition to porosity, the material exhibits broad light emission centered at 617 nm with a full width at half-maximum (FWHM) of 284 nm (0.96 eV). The recorded water stability is unparalleled for hybrid metal halide and perovskite materials, while the generation of porosity opens new pathways towards unexplored applications (e.g. solid-state batteries) for this class of hybrid semiconductors.     

金属卤化物钙钛矿纳米晶高效发光二极管的制备与器件性能优化

金属卤化物钙钛矿作为一类新型的离子型直接带隙半导体材料在电致发光二极管(LED)中有着重要应用前景. 但实现其应用的前提在于金属卤化物钙钛矿材料需要保持高的发光效率和好的稳定性. 为了提高金属卤化物钙钛矿作为LED发光层的激子结合效率, 从而提升其发光效率, 设计和合成金属卤化物钙钛矿纳米晶材料是一个有效途径. 目前, 基于纳米晶材料设计的金属卤化物钙钛矿LED在绿光和红光(包括近红外光)范围已经展现了高的发光亮度和外量子效率(EQE), 其中最高EQE已经超过了20%, 但其稳定性仍无法满足器件应用的要求. 此外, 更值得关注且更重要的是, 蓝光钙钛矿LED的发光亮度和EQE目前仍然不高. 如何制备高效、 稳定的金属卤化物钙钛矿纳米晶LED, 特别是蓝光LED, 是一个具有重大应用前景且具有挑战性的课题. 本文重点介绍了金属卤化物钙钛矿纳米发光层的结构设计和合成方法及金属卤化物钙钛矿LED的研究进展, 分析了金属卤化物钙钛矿LED不稳定的原因, 并对金属卤化物钙钛矿LED研究面临的挑战和未来发展方向进行了总结与展望.