Ternary Hybrid Perovskite Solid Solution Single Crystals: Growth,Composition Determination and Phase Stability in Highly Moist Atmosphere

Ternary hybrid perovskite solid solutions have shown superior optoelectronic properties and better stability than their ABX₃ simple perovskite counterparts under ambient conditions. However, crystal growth and identification of the accurate composition of these complex crystalline compounds remain challenging, and their stability under extreme conditions such as in highly moist atmosphere is unknown. Herein, large-size (up to 2 cm) single crystals of ternary perovskite 0.80FAPbI₃ ⋅ x′FAPbBr₃ ⋅ y′CsPbI₃ (x′+y′=0.20) are grown. An elemental analysis method based on wavelength dispersive X-ray fluorescence is proposed to determine their accurate compositions. Among these single crystals, the composition with y′=0.12 shows the best moisture stability at 90 % relative humidity for 15 days. Other components with richer or poorer Cs⁺ ions undergo different phase segregation behaviours. The performance and stability of photodetectors based on these single crystals are tested. This work offers a deeper insight into phase stability of ternary hybrid perovskite solid solution crystals in highly moist atmosphere.     

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.     

Preparation and characterization of a bis thiourea sodium iodide (BTSI)

A semi-organic nonlinear optical single crystal of bis thiourea sodium iodide (BTSI) has been successfully grown from aqueous solution using the slow evaporation solvent technique (SEST) at room temperature. Obtained crystals using the SEST method were characterized by using different characterization techniques. Structural studies of the grown crystals have been carried out by single-crystal XRD to confirm the crystal system and functional groups by FT-IR spectroscopy. Single-crystal XRD reveals orthorhombic structure of semi-organic BTSI single crystals and its unit cell parameters. Metal complex coordination of the single crystal is studied by FT-IR spectroscopy. The optical absorption study revealed excellent optical transparency of BTSI crystal in the entire visible region with a sharp lower cutoff wavelength 298 nm. The energy band gap of BTSI is found to be 4.16 eV. Thermal stability and thermal decomposition of BTSI single crystals were investigated by TGA–DTA and DSC analysis. The surface appearance of BTSI crystals by scanning electron microscopy reveals the formation of layer growth pattern. The structural perfection and growth features of the grown crystal were analyzed by wet chemical etching studies. The above studies reveal the effect of incorporation of sodium iodide into the lattice of thiourea crystals. The as-grown BTSI single crystals can be used as a potential candidate for NLO material as well as in electronic and optoelectronic devices.     

Rapid Growth of Halide Perovskite Single Crystals: From Methods to Optimization Control

Metal halide perovskites are emerging as new generation optoelectronic materials due to their high carrier mobility, long carrier diffusion length and large light absorption coefficient, which have broad applications in solar cell, light‐emitting diode, laser, photodetector and transistors. Perovskite single crystal is an ideal platform for discerning the intrinsic properties of these materials. In some cases, perovskite single crystals are better candidates to gain high performance optoelectronics. However, the growth of perovskite single crystals is time and cost consuming, which has an obvious disadvantage for device exploration. Therefore, fast growth technique is highly desirable in not only promoting the use of perovskites in commercial applications but also facilitating deep physical investigation of the materials. In this review, we summarize thoroughly the development of fast growth of the halide perovskites single crystal. Specifically, we highlight the progress of rapid growth techniques with emphasis on the optimization control.     

Understanding the Impact of Bismuth Heterovalent Doping on the Structural and Photophysical Properties of CH3NH3PbBr3 Halide Perovskite Crystals with Near-IR Photoluminescence

A comprehensive study unveiling the impact of heterovalent doping with Bi³⁺ on the structural, semiconductive, and photoluminescent properties of a single crystal of lead halide perovskites (CH₃NH₃PbBr₃) is presented. As indicated by single-crystal XRD, a perfect cubic structure in Bi³⁺-doped CH₃NH₃PbBr₃ crystals is maintained in association with a slight lattice contraction. Time-resolved and power-dependent photoluminescence (PL) spectroscopy illustrates a progressively quenched PL of visible emission, alongside the appearance of a new PL signal in the near-infrared (NIR) regime, which is likely to be due to energy transfer to the Bi sites. These optical characteristics indicate the role of Bi³⁺ dopants as nonradiative recombination centers, which explains the observed transition from bimolecular recombination in pristine CH₃NH₃PbBr₃ to a dominant trap-assisted monomolecular recombination with Bi³⁺ doping. Electrically, it is found that the mobility in pristine perovskite crystals can be boosted with a low Bi³⁺ concentration, which may be related to a trap-filling mechanism. Aided by temperature (T)-dependent measurements, two temperature regimes are observed in association with different activation energies (E_a) for electrical conductivity. The reduction of E_a at lower T may be ascribed to suppression of ionic conduction induced by doping. The modified electrical properties and NIR emission with the control of Bi³⁺ concentration shed light on the opportunity to apply heterovalent doping of perovskite single crystals for NIR optoelectronic applications.     

Unveiling the Photo- and Thermal-Stability of Cesium Lead Halide Perovskite Nanocrystals

Lead halide perovskites possess unique characteristics that are well-suited for optoelectronic and energy capture devices, however, concerns about their long-term stability remain. Limited stability is often linked to the methylammonium cation, and all-inorganic CsPbX₃ (X=Cl, Br, I) perovskite nanocrystals have been reported with improved stability. In this work, the photostability and thermal stability properties of CsPbX₃ (X=Cl, Br, I) nanocrystals were investigated by means of electron microscopy, X-ray diffraction, thermogravimetric analysis coupled with FTIR (TGA-FTIR), ensemble and single particle spectral characterization. CsPbBr₃ was found to be stable under 1-sun illumination for 16 h in ambient conditions, although single crystal luminescence analysis after illumination using a solar simulator indicates that the luminescence states are changing over time. CsPbBr₃ was also stable to heating to 250 °C. Large CsPbI₃ crystals (34±5 nm) were shown to be the least stable composition under the same conditions as both XRD reflections and Raman bands diminish under irradiation; and with heating the γ (black) phase reverts to the non-luminescent δ phase. Smaller CsPbI₃ nanocrystals (14±2 nm) purified by a different washing strategy exhibited improved photostability with no evidence of crystal growth but were still thermally unstable. Both CsPbCl₃ and CsPbBr₃ show crystal growth under irradiation or heat, likely with a preferential orientation based on XRD patterns. TGA-FTIR revealed nanocrystal mass loss was only from liberation and subsequent degradation of surface ligands. Encapsulation or other protective strategies should be employed for long-term stability of these materials under conditions of high irradiance or temperature.     

Templated‐Assembly of CsPbBr3 Perovskite Nanocrystals into 2D Photonic Supercrystals with Amplified Spontaneous Emission

Perovskite nanocrystals (NCs) have revolutionized optoelectronic devices because of their versatile optical properties. However, controlling and extending these functionalities often requires a light‐management strategy involving additional processing steps. Herein, we introduce a simple approach to shape perovskite nanocrystals (NC) into photonic architectures that provide light management by directly shaping the active material. Pre‐patterned polydimethylsiloxane (PDMS) templates are used for the template‐induced self‐assembly of 10 nm CsPbBr₃ perovskite NC colloids into large area (1 cm²) 2D photonic crystals with tunable lattice spacing, ranging from 400 nm up to several microns. The photonic crystal arrangement facilitates efficient light coupling to the nanocrystal layer, thereby increasing the electric field intensity within the perovskite film. As a result, CsPbBr₃ 2D photonic crystals show amplified spontaneous emission (ASE) under lower optical excitation fluences in the near‐IR, in contrast to equivalent flat NC films prepared using the same colloidal ink. This improvement is attributed to the enhanced multi‐photon absorption caused by light trapping in the photonic crystal.     

Monodisperse Adducts-Induced Homogeneous Nucleation Towards High-Quality Tin-Based Perovskite Film

The classic solvent system can't sufficiently separate one-dimensional edge-sharing SnI₂ crystals in solution, which severely restricts the fabrication of high-quality tin-based perovskite film. Herein, a strong Lewis base (hexamethylphosphoramide, HMPA) has been introduced to coordinate Sn²⁺ to modulate solvation behaviours on perovskite precursor and regulate crystallization kinetics. The large molecular volume of HMPA and stronger bind energy of SnI₂ ⋅ 2HMPA (−0.595 eV vs −0.118 eV for SnI₂ ⋅ 2DMSO) change the solvation structure of SnI₂ from edge-sharing cluster to monodisperse adduct, which contributes to uniform nucleation sites and prolongs crystal growth process. Delightfully, a fully-covered perovskite film is formed on the large-area substrate and tin-based perovskite solar cells processed with HMPA exhibit an excellent efficiency of 13.46 %. This research provides novel insights and directions for the solution preparation of smooth and uniform large-area tin-based perovskite film.     

Single-crystal growth of Tl2Ru2O7 pyrochlore using high-pressure and flux method

Single crystals of the thallium ruthenium pyrochlore have been grown by flux method under high oxygen pressure. The growth conditions were determined by direct observations using in situ powder X-ray diffraction (XRD) method under high pressure and high temperature. The crystals were grown using NaCl-KCl flux at 1350 °C and B₂O₃ flux at 1150 °C. High growth temperature of 1350 °C for the NaCl-KCl flux caused Pt contamination from the crucible and oxygen deficiency for the crystals obtained. The crystal growth using B₂O₃ flux proceeded at lower temperature by grain growth with material transfer through B₂O₃. The crystal obtained was characterized by single-crystal XRD method, and was found to have a stoichiometric composition, Tl₂Ru₂O_7−δ (δ=0), with a structural phase transition around 120 K. The grain growth technique with B₂O₃ is efficient for high-temperature single-crystal growth under high pressure.     

Synthesis and properties of lead-free formamidinium bismuth bromide perovskites

In the past few years, Pb-free metal halide perovskites have been recognized as a promising material for various optoelectronic applications because of some of their unique features, such as direct and tunable bandgap, visible light emission, narrow emission spectra, lower toxicity level, and easy solution processability. Recently, several Bi-based perovskite-like single crystals (SCs) and nanocrystals (NCs) were reported, which are mostly suffering from their poor structural stability and lower emission intensity. Here, we report the growth of millimeter-sized formamidinium bismuth bromide (FA₃Bi₂Br₉) perovskite SCs via slow solvent evaporation method. They crystallized into a trigonal crystal structure and exhibit an indirect bandgap of 2.71 eV. These results are supported by the first-principle density-functional theory studies. We have also synthesized nanometer-sized spherical blue-emitting FA₃Bi₂Br₉ NCs by solvent ligand-assisted reprecipitation method and achieved a maximum photoluminescence quantum yield of 22%. We observe that the addition of excess ligands into the FA₃Bi₂Br₉ NCs solution before the purification step significantly improves the optical and colloidal stability of the NCs.     

Visualizing Phase Segregation in Mixed‐Halide Perovskite Single Crystals

Mixed organolead halide perovskites (MOHPs), CH₃NH₃Pb(Br_xI_1?x)₃, have been shown to undergo phase segregation into iodide‐rich domains under illumination, which presents a major challenge to their development for photovoltaic and light‐emitting devices. Recent work suggested that phase‐segregated domains are localized at crystal boundaries, driving investigations into the role of edge structure and the growth of larger crystals with reduced surface area. Herein, a method for growing large (30×30×1 μm³) monocrystalline MAPb(Br_xI_1?x)₃ single crystals is presented. The direct visualization of the growth of nanocluster‐like I‐rich domains throughout the entire crystal revealed that grain boundaries are not required for this transformation. Narrowband fluorescence imaging and time‐resolved spectroscopy provided new insight into the nature of the phase‐segregated domains and the collective impact on the optoelectronic properties.     

Polarization-Dependent Photoluminescence of a Highly (100)-Oriented Perovskite Film

Lead halide perovskite is one of the attractive functional materials owing to its outstanding opto-electronic properties, which have been addressed in numerous studies. This study aims to clarify the link between the growth pattern and the charge carrier related properties for the highly oriented perovskite film along the [100] direction. For this purpose, a CH₃NH₃PbI₃ thin film mainly grown along the [100] direction was fabricated and subjected to spectroscopic analysis to understand the basic optoelectronic features of the oriented film. A perovskite film with random growth was also fabricated for comparison. In particular, results from excitation polarization photoluminescence spectroscopy (ExPPS) revealed that the orientation of transition dipole moment, which is relevant to the anisotropic property of the film, is attributed to the growth direction of the perovskite film. This study suggests that the absorption anisotropy can affect the anisotropy in properties of the perovskite device. Furthermore, photodetectors with the perovskite films were fabricated to investigate the effect of growth direction on the photodetector performances, revealing that a photodetector with the oriented perovskite film showed larger photoresponses. In order to provide an explanation for such result, we performed a PL lifetime imaging study of the oriented and randomly grown perovskite films.     

How Chemical Bonding Impacts Halide Perovskite Nanocrystals Growth to Bulk Films: Implication of Pb−X Bond on Growth Kinetics

Lead halide perovskites nanocrystals have emerged as a leading candidate in perovskite solar cells and light-emitting diodes. Given their favorable, tunable optoelectronic properties through modifying the size of nanocrystals, it is imperative to understand and control the growth of lead halide perovskite nanocrystals. However, during the nanocrystal growth into bulk films, the effect of halide bonding on growth kinetics remains elusive. To understand how a chemical bonding of Pb−X (covalency and ionicity) impact on growth of nanocrystals, we have examined two different halide perovskite nanocrystals of CsPbCl₃ (more ionic) and CsPbI₃ (more covalent) derived from the same parent CsPbBr₃ nanocrystals. Tracking the growth of nanocrystals by monitoring the spectral features of bulk peaks (at 445 nm for Cl and at 650 nm for I) enables us to determine the growth activation energy to be 92 kJ/mol (for CsPbCl₃) versus 71 kJ/mol (for CsPbI₃). The electronegativity of halides in Pb−X bonds governs the bond strength (150–240 kJ/mol), characteristics of bonding (ionic versus covalent), and growth kinetics and resulting activation energies. A fundamental understanding of Pb−X bonding provides a significant insight into controlling the size of the perovskite nanocrystals with more desired optoelectronic properties.     

Synthesis of Lead‐free CsGeI3 Perovskite Colloidal Nanocrystals and Electron Beam‐induced Transformations

Colloidal nanocrystals (NCs) of metal halide perovskite have recently aroused great research interest, due to their remarkable optical and electronic properties. We report a solution synthesis of a new member in this category, that is, all‐inorganic lead‐free cesium germanium iodine (CsGeI₃) perovskite NCs. These CsGeI₃ colloidal NCs are confirmed to adopt a rhombohedral structure. Moreover, the electron beam‐induced transformations of these lead‐free perovskite NCs have been investigated for the first time. The fracture of single‐crystalline CsGeI₃ nanocubes occurs first, followed by the emergence and growth of cesium iodine (CsI) single crystals and the final fragmentation into small debris with random orientations. Notably, the electron‐reduced Ge species in CsGeI₃ nanocubes exhibit a distinctive transformation path, compared to heavier Pb atoms in lead halide perovskite NCs.     

Aqueous Chemical Solution Deposition of Functional Double Perovskite Epitaxial Thin Films

Double perovskite structure (A₂BB′O₆) oxides exhibit a breadth of multifunctional properties with a huge potential range of applications in fields as diverse as spintronics, magneto-optic devices, or catalysis, and most of these applications require the use of thin films and heterostructures. Chemical solution deposition techniques are appearing as a very promising methodology to achieve epitaxial oxide thin films combining high performance with high throughput and low cost. In addition, the physical properties of these materials are strongly dependent on the ordered arrangement of cations in the double perovskite structure. Thus, promoting spontaneous cationic ordering has become a relevant issue. In this work, our recent achievements by using polymer-assisted deposition (PAD) of environmentally friendly, water-based solutions for the growth of epitaxial ferromagnetic insulating double perovskite La₂CoMnO₆ and La₂NiMnO₆ thin films on SrTiO₃ and LaAlO₃ single-crystal substrates are presented. It is shown that the particular crystallization and growth process conditions of PAD (very slow rate, close to thermodynamic equilibrium conditions) promote high crystallinity and quality of the films, as well as favors spontaneous B-site cationic ordering.     

Centimeter-Sized Single Crystal of Two-Dimensional Halide Perovskites Incorporating Straight-Chain Symmetric Diammonium Ion for X-Ray Detection

Two-dimensional (2D) AA′_n−1M_nX_3n+1 type halide perovskites incorporating straight-chain symmetric diammonium cations define a new type of structure, but their optoelectronic properties are largely unexplored. Reported here is the synthesis of a centimeter-sized AA′_n−1M_nX_3n+1 type perovskite, BDAPbI₄ (BDA=NH₃C₄H₈NH₃), single crystal and its charge-transport properties under X-ray excitation. The crystal shows a staggered configuration of the [PbI₆]⁴⁻ layers, a band gap of 2.37 eV, and a low trap density of 3.1×10⁹ cm⁻³. The single-crystal X-ray detector exhibits an excellent sensitivity of 242 μC Gy_air⁻¹ cm⁻² under the 10 V bias (0.31 V μm⁻¹), a detection limit as low as 430 nGy_air s⁻¹, ultrastable response current, a stable baseline with the lowest dark current drift of 6.06×10⁻⁹ nA cm⁻¹ s⁻¹ V⁻¹, and rapid response time of τ_rise=7.3 ms and τ_fall=22.5 ms. These crystals are promising candidates for the next generation of optoelectronic devices.     

Fast growth rates of polyethylene single crystals grown at high temperatures and their relevance to crystallization theories

The rates of growth of polyethylene single crystals grown from dilute solution in hexadecane and tetradecanol have been measured over the temperature range T_c = 98–120°C by following the change in turbidity during crystallization of a suspension of crystals of known shape and final size. The rates decrease similarly with T_c in each solvent, but for a given supercooling crystals grow much faster in tetradecanol where the corresponding crystallization temperature is higher. Similarly, the rates are much higher in hexadecane than those previously reported from xylene at equivalent supercoolings but lower T_c. Changes in the corresponding crystal morphologies as T_c is raised are quantified in terms of the axial ratio and the degree of curvature of the nominally {100} faces, both of which increase with T_c. The results can be interpreted as showing a transition from regime I to regime II growth in both solvents, which agrees both qualitatively and quantitatively with the predictions of the nucleation-based kinetic theories. Such a transition has never before been reported for solution crystallization. Using this analysis, reasonable values are obtained for the crystal side-surface energy σ of 7.4–7.5 erg cm^?2 and for the regime I substrate length L of 0.14 μm. No correlation is found between crystal morphology and growth rate and there are no discontinuous changes in morphology at the proposed transition points. The occurrence of curved crystal edges raises the fundamental issue of how to reconcile noncrystallographic growth surfaces with nucleation-controlled growth. A new approach to polymer crystal growth based on equilibrium surface roughening, which does not require nucleation, is therefore very pertinent in this respect and this is discussed.     

Ambient Air Temperature Assisted Crystallization for Inorganic CsPbI2Br Perovskite Solar Cells

Inorganic cesium lead halide perovskites, as alternative light absorbers for organic–inorganic hybrid perovskite solar cells, have attracted more and more attention due to their superb thermal stability for photovoltaic applications. However, the humid air instability of CsPbI₂Br perovskite solar cells (PSCs) hinders their further development. The optoelectronic properties of CsPbI₂Br films are closely related to the quality of films, so preparing high-quality perovskite films is crucial for fabricating high-performance PSCs. For the first time, we demonstrate that the regulation of ambient temperature of the dry air in the glovebox is able to control the growth of CsPbI₂Br crystals and further optimize the morphology of CsPbI₂Br film. Through controlling the ambient air temperature assisted crystallization, high-quality CsPbI₂Br films are obtained, with advantages such as larger crystalline grains, negligible crystal boundaries, absence of pinholes, lower defect density, and faster carrier mobility. Accordingly, the PSCs based on as-prepared CsPbI₂Br film achieve a power conversion efficiency of 15.5% (the maximum stabilized power output of 15.02%). Moreover, the optimized CsPbI₂Br films show excellent robustness against moisture and oxygen and maintain the photovoltaic dark phase after 3 h aging in an air atmosphere at room temperature and 35% relative humidity (R.H.). In comparison, the pristine films are completely converted to the yellow phase in 1.5 h.     

Cesium Lead Halide Perovskite Nanostructures: Tunable Morphology and Halide Composition

Inorganic halide perovskite (CsPbX₃) nanostructures have gained considerable interest in recent years owing to their enhanced stability and optoelectronic applications. Recent developments in the synthesis of nanostructures are reviewed. The impact of the precursor and ligand nature, temperature and growth time on the morphology and shape tuning of CsPbX₃ nanostructures is described in relation to their optical properties. The presynthetic and postsynthetic anion exchange strategies to retain pre‐existing crystal phase and shape are discussed in this minireview.     

Improved Performance of All-Inorganic Perovskite Light-emitting Diodes via Nanostructured Stamp Imprinting

Halide perovskites are emerging emitters with excellent optoelectronic properties. Contrary to the large grain fabrication goal in perovskite solar cells, perovskite light-emitting diodes (PeLEDs) based on small grain enable efficient radiative recombination because of relatively higher charge carrier densities due to spatial confinement. However, achieving small-sized grain growth with superior crystal quality and film morphology remains a challenge. In this work, we demonstrated a nanostructured stamp thermal imprinting strategy to boost the surface coverage and improve the crystalline quality of CsPbBr₃ film, particularly confine the grain size, leading to the improvement of luminance and efficiency of PeLEDs. We improved the thermal imprinting process utilizing the nanostructured stamp to selectively manipulate the nucleation and growth in the nanoscale region and acquire small-sized grain accompanied by improved crystal quality and surface morphology of the film. By optimizing the imprinting pressure and the period of the nanostructures, appropriate grain size, high surface coverage, small surface roughness and improved crystallization could be achieved synchronously. Finally, the maximum luminance and efficiency of PeLEDs achieved by nanostructured stamp imprinting with a period of 320 nm are 67600 cd/m² and 16.36 cd/A, respectively. This corresponds to improvements of 123 % in luminance and 100 % in efficiency, compared to that of PeLEDs without the imprinting.