Increasing demand for metal halide perovskites in applications beyond photovoltaics (e.g., hard radiation detection) necessitates hundreds of microns to millimeters thick active layers for optimal performance. This review critically assesses low‐temperature, pressure‐assisted ceramic processing as a pivotal technique to fabricate dense, polycrystalline wafers with precisely controlled macroscopic dimensions as well as microstructural properties. Exploiting the soft ionic bonding and subsequent plasticity of metal halide perovskites, pressure‐assisted ceramic processing can achieve rapid densification at drastically reduced temperatures (≲100 °C) compared to common oxide ceramics (≳1000 °C). As a direct result, metal halide perovskite ceramics can be manufactured using cost‐effective pressure‐assisted ceramic processing methods (e.g., low‐temperature uniaxial pressing). However, a definitive understanding of the intricate relationship between sintering parameters (temperature, pressure, and powder characteristics) and the resulting microstructural attributes (pore/grain size and distribution, relative density, porosity, and crystallinity) and optoelectronic responses remains a knowledge gap. To metal halide perovskite experts, the aim is to emphasize the considerable advantages of pressure‐assisted ceramic processing, and to expert ceramists, the aim is to demonstrate that metal halide perovskites offer a compelling material class for established ceramic processing techniques. Bridging the knowledge between these two distinct areas of expertise will help drive transformative progress in metal halide perovskite‐based technologies.

Lanthanide-activated CsPbX3 perovskites (Ln3+:CsPbCl3) hold immense promise for the development of next-generation solid-state lasers. Realizing their full potential hinges on achieving controlled lanthanide concentrations with homogenous distribution throughout the perovskite host-a considerable material processing challenge that hinders widespread application. This work introduces a robust, scalable mechanochemical synthesis for producing both singly doped Ln3+:CsPbCl3 (Ln3+ = Pr3+, Nd3+, Ho3+, Er3+, Yb3+) as well as co-doped Ln3+(I)/Ln3+(II):CsPbCl3 (Ln3+(I)/Ln3+(II) = Nd3+/Yb3+, Ho3+/Pr3+, Er3+/Pr3+) microcrystalline powders. Complementary structural characterization techniques (e.g., X-ray diffraction, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, and X-ray fluorescence) directly quantify doping concentration and confirm homogeneous Ln3+ distribution throughout the powders. Detailed photoluminescence (PL), PL excitation, and PL lifetime measurements reveal distinct emissive and absorptive states in the visible to short-wave infrared (IR) stemming from the dopants. Melt-grown crystals, derived from mechanochemically prepared powders, exhibit enhanced PL lifetimes compared to their source powders and are transparent at IR emission wavelengths. Overall, this mechanochemical approach allows for considerable control over dopant incorporation, yielding powders that are well-suited for the melt-growth of rare-earth-doped single crystals and the future development of halide perovskite-based solid-state lasers.

In this report, we utilize room-temperature uniaxial pressing at applied loads achievable with low-cost, laboratory-scale presses to fabricate freestanding CH3NH3PbX3 (X-=Br- ,Cl-) polycrystalline ceramics with millimeter thicknesses and optical transparency up to ~70% in the infrared. As-fabricated perovskite ceramics can be produced with desirable form factors (i.e., size, shape, and thickness) and high quality surfaces without any post-processing (e.g., cutting or polishing). We additionally expect this method to be broadly applicable to a large swath of metal halide perovskites and not just the compositions shown here. Highly scalable methods to produce polycrystalline lead halide perovskite ceramics will enable key advancements in critical perovskite-based technologies (e.g., direct X-ray/-ray detectors, scintillators, nonlinear optics). In addition to ceramic fabrication, we analyze microstructure—optical property relationships through detailed experiments (e.g., transmission measurements, electron microscopy, X-ray tomography, optical profilometry, etc.) as well as modelling based on Mie light scattering theory. In tandem, experiments and modelling illustrate the effects of scattering sources on transparency and reveal microstructural parameters necessary to attain near optimal transparency in perovskite polycrystalline ceramics.

In this report, we utilize room-temperature uniaxial pressing at applied loads achievable with low-cost, laboratory-scale presses to fabricate freestanding CH3NH3PbX3 (X-=Br- ,Cl-) polycrystalline ceramics with millimeter thicknesses and optical transparency up to ~70% in the infrared. As-fabricated perovskite ceramics can be produced with desirable form factors (i.e., size, shape, and thickness) and high quality surfaces without any post-processing (e.g., cutting or polishing). We additionally expect this method to be broadly applicable to a large swath of metal halide perovskites and not just the compositions shown here. Highly scalable methods to produce polycrystalline lead halide perovskite ceramics will enable key advancements in critical perovskite-based technologies (e.g., direct X-ray/-ray detectors, scintillators, nonlinear optics). In addition to ceramic fabrication, we analyze microstructure—optical property relationships through detailed experiments (e.g., transmission measurements, electron microscopy, X-ray tomography, optical profilometry, etc.) as well as modelling based on Mie light scattering theory. In tandem, experiments and modelling illustrate the effects of scattering sources on transparency and reveal microstructural parameters necessary to attain near optimal transparency in perovskite polycrystalline ceramics.

Lead halide perovskites are notorious for water-sensitivity and low hardness. Consequently, polishing CsPbBr3 crystals to achieve high-quality surfaces is challenging. We present a breakthrough mechanical polishing methodology tailored to the specific needs of these soft, moisture-sensitive semiconductors. Three-dimensional optical surface profiles over 1 mm2 areas demonstrate high-quality surfaces with root-mean-square roughness values (< 10 nm) that are unparalleled for melt-grown CsPbBr3. We additionally delve into the polished wafers’ fundamental optical constants and introduce an anti-reflection coating method, setting new standards for short-wave infrared transparency in CsPbBr3. These pivotal processing guidelines pave the way for advancing halide perovskite applications beyond academic curiosity.

Hybrid organic–inorganic hybrid perovskite (OIP) nanocrystals have gained considerable excitement due to high photoluminescence (PL) quantum yields, bandgap tunability, and narrow band emission, which are essential for photovoltaic devices, light emitting diodes (LEDs), and optical displays. While researchers have designed numerous ways to synthesize OIP nanomaterials, there is still a need to explore faster, cheaper, and scalable methods of making stable, highly performing nanomaterials for device applications. Polymers are commonly used to encapsulate OIP nanomaterials, yielding enhancements in long-term stability as well as improved PL properties. However, the exact impact of polymer chemical composition on perovskite nanocrystal growth and material properties is still unknown. Here, we reveal how polymer chemical composition directly modulates the formation of perovskite composite materials with ∼75 wt% perovskite with respect to polymer and the optical properties during a one-step, co-precipitation synthesis procedure. Specifically, a series of polymers were explored, poly(styrene) (PS), poly(4-vinylpyridine) (P4VP), poly(ethyleneimine) (PEI), poly(ethylene oxide) (PEO), poly(vinylpyrrolidone) (PVP), and poly(methyl methacrylate) (PMMA), to compare the structure and optical properties of the resulting OIP materials. Polymers with nitrogen-containing functional groups, such as amides, pyridine, and amines, are shown to preferentially bind to and passivate perovskite surfaces, acting as polymer macroligands. Nitrogen atoms in the polymer coordinate with under-coordinated lead ions on the perovskite surface, passivating surface defects and leading to an enhancement in the optical properties. Polymer macroligands also promote nanocrystal formation in a similar method as prototypical surface-active ligands used in nanocrystal syntheses. This work uncovers design rules for creating composite materials exhibiting desired nanostructures and enhanced optical properties for future OIP devices through the use of polymer macroligands.

Liquid phase exfoliation of non-van der Waals materials has generated renewed interest in fundamental optical and electronic materials discovery and processing. However, such approaches can limit access to novel two-dimensional materials due to the chemistry of exfoliation and processing conditions employed (e.g., processing temperature, mechanical energy input, volatile organic compounds, and sensitive redox chemistries). Here, we demonstrate the exfoliation of bulk hematite (α-Fe2O3) powder using a mild bath sonication methodology in liquid monomer media to form stable colloidal dispersions with quasi-two-dimensional hematene nanoflakes. These colloidal dispersions were further processed to form hematene poly(methyl methacrylate) matrix composite substrates.

Proper derivation of CH3NH3PbX3 (MAPbX3; where X = Cl−, Br−, I−) optical constants is a critical step toward the development of high‐performance perovskite devices. To date, the optical dispersions at all wavelengths have been inconsistently characterized by under‐approximating or omitting anomalous spectral features. Herein, a rigorous optical dispersion data analysis of single‐crystal MAPbBr3 involving variable‐angle spectroscopic ellipsometry data appended with transmission intensity data is presented. This approach yields a more robust derivation of the refractive index and extinction coefficient for both anomalous (absorptance) and normal (no absorptance) optical dispersion regimes. Using the derived optical constants, illustrative modeled perovskite solar cell device designs are presented in relation to nonrealistic designs prepared using representative optical constants reported in the literature. In comparison, the derived optical constants enables the modeling of layer thicknesses to maximize absorption by the active layer (MAPbBr3) and minimize parasitic optical absorptance by the nonactive layers at broad angles of incidence (≈0°–70°). This robust derivation of MAPbBr3 optical constants is expected to impact the optical dispersion data analysis of all perovskite analogs and expedite targeted development of, for example, solar cell, light‐emitting diode, photo‐ and X‐ray/γ‐ray detector, and laser system technologies.

Colloidal mixed halide perovskite nanocrystals (NCs) irreversibly degrade when exposed to ultraviolet-visible irradiation. Here, mixed halide perovskite NC photolysis is tracked via mass spectrometry, electron microscopy, and photoluminescence. The data shows continuous wave ultraviolet-visible irradiation causes the heavier halides within the alloy to sublimate. This ultimately transforms CsPb(I1-xBrx)3 and CsPb(Cl1-xBrx)3 (x approximate to 0.50) NCs into CsPbBr3 and CsPbCl3 NCs, respectively. Time-resolved mass spectrometry demonstrates real-time desorption of volatile halide species (e.g., I2(g)/HI(g)) during irradiation. Energy-dispersive X-ray spectroscopy confirms near complete expulsion of I- from CsPb(I1-xBrx)3 and Br- from CsPb(Cl1-xBrx)3 NCs. Electron diffraction and cathodoluminescence establish lattice contractions and emission blueshifts consistent with formation of single halide perovskites from parent mixed halide alloys. Finally, increasing photolysis rates at higher temperatures follow an Arrhenius relationship with an effective activation energy of similar to 62 kJ mol-1 for CsPb(I1-xBrx)3 NCs (x approximate to 0.50). Altogether, this work provides important insight into the photolysis of colloidal perovskite NC alloys.

Journal Article STEM-EELS Exploration of Beam-Sensitive Perovskite Nanocrystals Get access Brittany Ford, Brittany Ford Department of Materials Science & Engineering, The Ohio State University, Columbus, OH, United States Corresponding author: ford.1027@osu.edu Search for other works by this author on: Oxford Academic Google Scholar David W McComb, David W McComb Department of Materials Science & Engineering, The Ohio State University, Columbus, OH, United StatesCenter for Electron Microscopy and Analysis, The Ohio State University, Columbus, OH, United States Search for other works by this author on: Oxford Academic Google Scholar Michael Brennan, Michael Brennan Air Force Research Laboratory, WPAFB, OH, United States Search for other works by this author on: Oxford Academic Google Scholar Tod Grusenmeyer Tod Grusenmeyer Air Force Research Laboratory, WPAFB, OH, United States Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 28, Issue S1, 1 August 2022, Pages 2160–2161, https://doi.org/10.1017/S1431927622008352 Published: 01 August 2022

Accurate measurements of semiconductor nanocrystal (NC) emission quantum yields (QYs) are critical to condensed phase optical refrigeration. Of particular relevance to measuring NC QYs is a longstanding debate as to whether an excitation energy-dependent (EED) QY exists. Various reports indicate existence of NC EED QYs, suggesting that the phenomenon is linked to specific ensemble properties. We therefore investigate here the existence of EED QYs in two NC systems (CsPbBr3 and CdSe) that are possible candidates for use in optical refrigeration. The influence of NC size, size-distribution, surface ligand, and as-made emission QYs are investigated. Existence of EED QYs is assessed using two approaches (an absolute approach using an integrating sphere and a relative approach involving excitation spectroscopy). Altogether, our results show no evidence of EED QYs across samples. This suggests that parameters beyond those mentioned above are responsible for observations of NC EED QYs.

Halide perovskites doped with magnetic impurities (such as the transition metals Mn2+, Co2+, Ni2+ are being explored for a wide range of applications beyond photovoltaics, such as spintronic devices, stable light-emitting diodes, single-photon emitters, and magneto-optical devices. However, despite several recent studies, there is no consensus on whether the doped magnetic ions will predominantly replace the octahedral B-site metal via substitution or reside at interstitial defect sites. Here, by performing correlated nanoscale X-ray microscopy, spatially and temporally resolved photoluminescence measurements, and magnetic force microscopy on the inorganic 2D perovskite Cs2PbI2Cl2, we show that doping Mn2+ into the structure results in a lattice expansion. The observed lattice expansion contrasts with the predicted contraction expected to arise from the B-site metal substitution, thus implying that Mn(2+ )does not replace the Pb2+ sites. Photoluminescence and electron paramagnetic resonance measurements confirm the presence of Mn(2+ )in the lattice, while correlated nano-XRD and X-ray fluorescence track the local strain and chemical composition. Density functional theory calculations predict that Mn2+ atoms reside at the interstitial sites between two octahedra in the triangle formed by one Cl- and two I- atoms, which results in a locally expanded structure. These measurements show the fate of the transition metal dopants, the local structure, and optical emission when they are doped at dilute concentrations into a wide band gap semiconductor.

Alloyed lead halide perovskites have taken a dominant role in the quest for third generation solar cells. This is due to optimal light-harvesting properties, which can be tuned across the visible spectrum by mixing halide (X = Cl-, Br-, and (I)-) anions and A(+) cations (A(+) = FA(+) MA(+), and Cs+). Durability issues related to ion movement within the perovskite lattice, however, impede large-scale commercialization. Unifonn|y mixed halide perovskites [e.g., APb(I-1-Br-x(x))(3)perovskites] reversibly segregate into narrow bandgap l-rich and wide bandgap Br-rich domains during continuous visible illumination. Subsequent I-rich domains reduce local open circuit voltages and decrease mixed halide perovskite solar cell power conversion efficiencies. In this review, we assess the known effects of halide segregation on the structural and optical properties of mixed halide materials, discuss ongoing research to suppress the phenomenon, and provide a mechanistic overview of its underlying origins.

Light-induced halide segregation is an intrinsic instability of mixed-halide perovskites, which complicates their successful use in tandem solar cells. Methods to suppress this phenomenon remain elusive because of its ambiguous origin. Beal et al. demonstrate that photostability is not an exclusive property of perovskite crystal structure and instead highlight the need for a microscopic understanding of the phenomenon.

Light-induced halide segregation is an intrinsic instability of mixed-halide perovskites, which complicates their successful use in tandem solar cells. Methods to suppress this phenomenon remain elusive because of its ambiguous origin. Beal et al. demonstrate that photostability is not an exclusive property of perovskite crystal structure and instead highlight the need for a microscopic understanding of the phenomenon.

Size-dependent photoluminescence Stokes shifts (ΔEs) universally exist in CsPbX3 (X = Cl-, Br-, or I-) perovskite nanocrystals (NCs). ΔEs values, which range from ∼15 to 100 meV for NCs with average edge lengths (l) from approximately 13 to 3 nm, are halide-dependent such that ΔEs(CsPbI3) > ΔEs(CsPbBr3) ≳ ΔEs(CsPbCl3). Observed size-dependent Stokes shifts are not artifacts of ensemble size distributions as demonstrated through measurements of single CsPbBr3 NC Stokes shifts (⟨ΔEs⟩ = 42 ± 5 meV), which are in near quantitative agreement with associated ensemble (l = 6.8 ± 0.8 nm) ΔEs values (ΔEs ≈ 50 meV). Transient differential absorption measurements additionally illustrate no significant spectral dynamics on the picosecond time scale that would contribute to ΔEs. This excludes polaron formation as being responsible for ΔEs. Altogether, the results point to an origin for ΔEs, intrinsic to the size-dependent electronic properties of individual perovskite NCs.

Ion migration represents an intrinsic instability of metal halide perovskite solar cells. Here we show that triple-cation FAxMAyCs1–x–yPbI3 [FA+ = (NH2)2CH+, MA+ = CH3NH3+] active layers with mixed orthorhombic, post-perovskite (δortho-CsPbI3), and cubic perovskite (α) phases (i.e., α/δ-phase FAxMAyCs1–x–yPbI3) exhibit improved cation stability against applied bias relative to pure α-phase perovskites (i.e., FA0.85Cs0.15PbI3 and FA0.76MA0.15Cs0.09PbI3). Infrared photothermal heterodyne imaging and time-of-flight secondary ion mass spectrometry are used to visualize exclusive α-phase perovskite lateral device A+ cation accumulation (depletion) at perovskite negative (positive) electrode interfaces. The resulting compositional heterogeneities lead to degradation. Operational stability testing of solar cells reveals similar degradation behavior; α/δ-phase FAxMAyCs1–x–yPbI3 lateral devices/solar cells, by contrast, show improved stabilities. Enhanced α/δ-FAxMAyCs1–x–yPbI3 stability is rationalized by δortho-phase inclusions, acting as barriers through which A+ cations do not easily migrate. This study thus provides new insights into cation migration in FAxMAyCs1–x–yPbI3 perovskites and suggests a materials design strategy toward suppressing cation instabilities in hybrid perovskites.

Perovskite nanocrystal superlattices (NC SLs) are the nearest real-world approximations to monodisperse NC ensembles. NC SLs thus represent ideal model systems for evaluating the optical and structural stability of CsPb(I1-xBrx)(3) NCs at a macroscopic level. Here, photoinduced changes to CsPb(I1-xBrx)(3) NC SLs (0 < x < 1.0) are probed via in situ photoluminescence, X-ray diffraction, and electron microscopy. We find that prolonged (similar to 10-20 h) ultraviolet-visible irradiation causes irreversible PL blueshifts, photobrightening, and crystal structure contractions. These changes stem from gradual photoinduced I-2 sublimation, which transforms CsPb(I1-xBrx)(3) into CsPbBr3. Despite eliminating half of the initial halides from individual CsPb(I0.53Br0.47)(3) particles, NCs within SLs remarkably preserve their initial crystallinity, cuboidal shapes, edge lengths, and size distributions. This work illustrates compositional control toward generating precisely engineered perovskite NC SLs. It also highlights iodide photo-oxidation as a hurdle that must be overcome if mixed halide perovskite nanomaterials are to be applied beyond fundamental studies.

Solution-processed mixed halide perovskites are excellent materials for multijunction solar cells. Unfortunately, light-induced halide phase segregation has prevented their effective integration into working devices. In this study, we rationalize and quantify anion photosegregation in stoichiometric and halide-deficient MAPb(I1-xBrx)(3) thin films through kinetic Monte Carlo simulations and complementary optical measurements. Our study reveals that segregation rates are dictated by halide vacancy hopping barriers and are modulated by vacancy concentrations. The simulations further suggest that near-ubiquitous emission energies, which converge on that for MAPb(I0.8Br0.2)(3) (i.e., x approximate to 0.2) following photosegregation, arise from the existence of kinetically trapped Br- within nucleated I-rich domains. An established photosegregation excitation intensity threshold is independent of the number of vacancies and instead depends critically on parameters such as carrier diffusion length, lifetime, and bandgap tunability. The study thus sheds new light on important parameters that define halide photosegregation and presents opportunities for controlling the phenomenon.