Organic–Inorganic Perovskites: Structural Versatility for Functional Materials Design

Exploration of organic–inorganic hybrid perovskites for surface-enhanced infrared spectroscopy of small molecules

Organic–inorganic hybrid perovskites allow the infrared absorption of small molecules to be efficiently enhanced.

KLi3Fe(C2O4)3: a perovskite-like compound with 1 : 3 ordering at both A and B sites

KLi₃Fe(C₂O₄)₃ adopts a perovskite-like structure with a unique 1 : 3 cation ordering pattern at both A and B sites.

Structure-Band Gap Relationships in Hexagonal Polytypes and Low-Dimensional Structures of Hybrid Tin Iodide Perovskites

The present study deals with the structural characterization and classification of the novel compounds 1-8 into perovskite subclasses and proceeds in extracting the structure-band gap relationships between them. The compounds were obtained from the employment of small, 3-5-atom-wide organic ammonium ions seeking to discover new perovskite-like compounds. The compounds reported here adopt unique or rare structure types akin to the prototype structure perovskite. When trimethylammonium (TMA) was employed, we obtained TMASnI₃ (1), which is our reference compound for a "perovskitoid" structure of face-sharing octahedra. The compounds EASnI₃ (2b), GASnI₃ (3a), ACASnI₃ (4), and IMSnI₃ (5) obtained from the use of ethylammonium (EA), guanidinium (GA), acetamidinium (ACA), and imidazolium (IM) cations, respectively, represent the first entries of the so-called "hexagonal perovskite polytypes" in the hybrid halide perovskite library. The hexagonal perovskites define a new family of hybrid halide perovskites with a crystal structure that emerges from a blend of corner- and face-sharing octahedral connections in various proportions. The small organic cations can also stabilize a second structural type characterized by a crystal lattice with reduced dimensionality. These compounds include the two-dimensional (2D) perovskites GA₂SnI₄ (3b) and IPA₃Sn₂I₇ (6b) and the one-dimensional (1D) perovskite IPA₃SnI₅ (6a). The known 2D perovskite BA₂MASn₂I₇ (7) and the related all-inorganic 1D perovskite "RbSnF₂I" (8) have also been synthesized. All compounds have been identified as medium-to-wide-band-gap semiconductors in the range of E_g = 1.90-2.40 eV, with the band gap progressively decreasing with increased corner-sharing functionality and increased torsion angle in the octahedral connectivity.

Structural Stability, Vibrational Properties, and Photoluminescence in CsSnI3 Perovskite upon the Addition of SnF2

The CsSnI₃ perovskite and the corresponding SnF₂-containing material with nominal composition CsSnI_2.95F_0.05 were synthesized by solid-state reactions and structurally characterized by powder X-ray diffraction. Both materials undergo rapid phase transformation upon exposure to air from the black orthorhombic phase (B-γ-CsSnI₃) to the yellow orthorhombic phase (Y-CsSnI₃), followed by irreversible oxidation into Cs₂SnI₆ within several hours. The phase transition occurs at a significantly lower rate in the SnF₂-containing material rather than in the pure perovskite. The high hole-carrier concentration of the materials prohibits the detection of Raman signals for B-γ-CsSnI₃ and induces a very strong plasmonic reflectance in the far-IR. In contrast, far-IR phonon bands and a rich Raman spectrum are observed for the Y-CsSnI₃ modification below 140 cm^-1 with weak frequency shift gradients versus temperatures between -95 and +170 °C. Above 170 °C, the signal is lost due to B-α-CsSnI₃ re-formation. The photoluminescence spectra exhibit residual blue shifts and broadening as a sign of structural transformation initiation.

Electron-Phonon Scattering in Atomically Thin 2D Perovskites

Two-dimensional (2D) atomically thin perovskites with strongly bound excitons are highly promising for optoelectronic applications. However, the nature of nonradiative processes that limit the photoluminescence (PL) efficiency remains elusive. Here, we present time-resolved and temperature-dependent PL studies to systematically address the intrinsic exciton relaxation pathways in layered (C₄H₉NH₃)₂(CH₃NH₃)_n-1Pb_nI_3n+1 (n = 1, 2, 3) structures. Our results show that scatterings via deformation potential by acoustic and homopolar optical phonons are the main scattering mechanisms for excitons in ultrathin single exfoliated flakes, exhibiting a T^γ (γ = 1.3 to 1.9) temperature dependence for scattering rates. We attribute the absence of polar optical phonon and defect scattering to efficient screening of Coulomb potential, similar to what has been observed in 3D perovskites. These results establish an understanding of the origins of nonradiative pathways and provide guidelines for optimizing PL efficiencies of atomically thin 2D perovskites.

Advances and Promises of Layered Halide Hybrid Perovskite Semiconductors

Layered halide hybrid organic-inorganic perovskites (HOP) have been the subject of intense investigation before the rise of three-dimensional (3D) HOP and their impressive performance in solar cells. Recently, layered HOP have also been proposed as attractive alternatives for photostable solar cells and revisited for light-emitting devices. In this review, we combine classical solid-state physics concepts with simulation tools based on density functional theory to overview the main features of the optoelectronic properties of layered HOP. A detailed comparison between layered and 3D HOP is performed to highlight differences and similarities. In the same way as the cubic phase was established for 3D HOP, here we introduce the tetragonal phase with D_4h symmetry as the reference phase for 2D monolayered HOP. It allows for detailed analysis of the spin-orbit coupling effects and structural transitions with corresponding electronic band folding. We further investigate the effects of octahedral tilting on the band gap, loss of inversion symmetry and possible Rashba effect, quantum confinement, and dielectric confinement related to the organic barrier, up to excitonic properties. Altogether, this paper aims to provide an interpretive and predictive framework for 3D and 2D layered HOP optoelectronic properties.

Surface and Interface Aspects of Organometal Halide Perovskite Materials and Solar Cells

The current challenges (e.g., stability, hysteresis, etc.) in organometal halide perovskite solar cell research are closely correlated with surfaces and interfaces. For instance, efficient generation of charges, extraction, and transport with minimum recombination through interlayer interfaces is crucial to attain high-efficiency solar cell devices. Furthermore, intralayer interfaces may be present in the form of grain boundaries within a film composed of the same material, for example, a polycrystalline perovskite layer. The adjacent grains may assume different crystal orientations and/or have different chemical compositions, which impacts charge excitation and dynamics and thereby the overall solar cell performance. In this Perspective, we present case studies to demonstrate (1) how surfaces and interfaces can impact material properties and device performance and (2) how these issues can be investigated by surface science techniques, such as scanning probe microscopy, photoelectron spectroscopy, and so forth. We end this Perspective by outlining the future research directions based on the reported results as well as the new trends in the field.

N-Methylformamide as a Source of Methylammonium Ions in the Synthesis of Lead Halide Perovskite Nanocrystals and Bulk Crystals

We report chemical routes for the synthesis of both nanocrystals and bulk crystals of methylammonium (MA) lead halide perovskites employing N-methylformamide (NMF) as a source of MA ions. Colloidal nanocrystals were prepared by a transamidation reaction between NMF and an alkyl amine (oleylamine). The nanocrystals showed photoluminescence quantum yields reaching 74% for MAPbBr₃ and 60% for MAPbI₃. Bulk crystals were grown at room temperature, with no need for an antisolvent, by the acid hydrolysis of NMF. Important advantages of using NMF instead of MA salts are that the syntheses involve fewer steps and less toxic and less expensive chemicals.

High brightness formamidinium lead bromide perovskite nanocrystal light emitting devices

Formamidinium lead halide (FAPbX₃) has attracted greater attention and is more prominent recently in photovoltaic devices due to its broad absorption and higher thermal stability in comparison to more popular methylammonium lead halide MAPbX₃. Herein, a simple and highly reproducible room temperature synthesis of device grade high quality formamidinium lead bromide CH(NH₂)₂PbBr₃ (FAPbBr₃) colloidal nanocrystals (NC) having high photoluminescence quantum efficiency (PLQE) of 55–65% is reported. In addition, we demonstrate high brightness perovskite light emitting device (Pe-LED) with these FAPbBr₃ perovskite NC thin film using 2,2′,2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) commonly known as TPBi and 4,6-Bis(3,5-di(pyridin-3-yl)phenyl)-2-methylpyrimidine (B3PYMPM) as electron transport layers (ETL). The Pe-LED device with B3PYMPM as ETL has bright electroluminescence of up to 2714 cd/m², while the Pe-LED device with TPBi as ETL has higher peak luminous efficiency of 6.4 cd/A and peak luminous power efficiency of 5.7 lm/W. To our knowledge this is the first report on high brightness light emitting device based on CH(NH₂)₂PbBr₃ widely known as FAPbBr₃ nanocrystals in literature.

The Bright Side of Perovskites

Incubating in the rise of perovskite photovoltaic era, the advances in material design encourage further promising optoelectronic exploitations. Here, we evaluate halide perovskite envisioning light-emitting applications, with a particular focus to the role that this material can effectively play in the field, discussing advantages and limitations with respect to state of art competing players. Specific benefits derive from the use of low dimensional and nanostructured perovskites, marginally exploited in photovoltaic devices, allowing for a tuning of the excited states properties and for the obtainment of intrinsic resonating structures. Thanks to these unique properties, halide perovskite ensure a great potential for the development of high-power applications, such as lighting and lasing.

Lead-Free Inverted Planar Formamidinium Tin Triiodide Perovskite Solar Cells Achieving Power Conversion Efficiencies up to 6.22

Efficient lead (Pb)-free inverted planar formamidinium tin triiodide (FASnI₃ ) perovskite solar cells (PVSCs) are demonstrated. Our FASnI₃ PVSCs achieved average power conversion efficiencies (PCEs) of 5.41% ± 0.46% and a maximum PCE of 6.22% under forward voltage scan. The PVSCs exhibit small photocurrent-voltage hysteresis and high reproducibility. The champion cell shows a steady-state efficiency of ≈6.00% for over 100 s.

The Additive Coordination Effect on Hybrids Perovskite Crystallization and High-Performance Solar Cell

The coordination effects of additives during perovskite crystal growth are investigated, and a novel technique to fabricate high-quality perovskite thin films by introduction of weak coordination additives (e.g., acetonitrile) in the precursors is demonstrated.

Benign-by-Design Solventless Mechanochemical Synthesis of Three-, Two-, and One-Dimensional Hybrid Perovskites

Organic-inorganic hybrid perovskites have attracted significant attention owing to their extraordinary optoelectronic properties with applications in the fields of solar energy, lighting, photodetectors, and lasers. The rational design of these hybrid materials is a key factor in the optimization of their performance in perovskite-based devices. Herein, a mechanochemical approach is proposed as a highly efficient, simple, and reproducible method for the preparation of four types of hybrid perovskites, which were obtained in large amounts as polycrystalline powders with high purity and excellent optoelectronics properties. Two archetypal three-dimensional (3D) perovskites (MAPbI₃ and FAPbI₃ ) were synthesized, together with a bidimensional (2D) perovskite (Gua₂ PbI₄ ) and a "double-chain" one-dimensional (1D) perovskite (GuaPbI₃ ), whose structure was elucidated by X-ray diffraction.

Enhanced Optical and Electrical Properties of Polymer-Assisted All-Inorganic Perovskites for Light-Emitting Diodes

Highly bright light-emitting diodes based on solution-processed all-inorganic perovskite thin film are demonstrated. The cesium lead bromide (CsPbBr₃ ) created using a new poly(ethylene oxide)-additive spin-coating method exhibits photoluminescence quantum yield up to 60% and excellent uniformity of electrical current distribution. Using the smooth CsPbBr₃ films as emitting layers, green perovskite-based light-emitting diodes (PeLEDs) exhibit electroluminescent brightness and efficiency above 53 000 cd m^-2 and 4%: a new benchmark of device performance for all-inorganic PeLEDs.

Polar Nature of (CH3NH3)3Bi2I9 Perovskite-Like Hybrids

High-quality single crystals of perovskite-like (CH₃NH₃)₃Bi₂I₉ hybrids have been synthesized, using a layered-solution crystal-growth technique. The large dielectric constant is strongly affected by the polar ordering of its constituents. Progressive dipolar ordering of the methylammonium cation upon cooling below 300 K gradually converts the hexagonal structure (space group P6₃/mmc) into a monoclinic phase (C2/c) at 160 K. A well-pronounced, ferrielectric phase transition at 143 K is governed by in-plane ordering of the bismuth lone pair that breaks inversion symmetry and results in a polar phase (space group P2₁). The dielectric constant is markedly higher in the C2/c phase above this transition. Here, the bismuth lone pair is disordered in-plane, allowing the polarizability to be substantially enhanced. Density functional theory calculations estimate a large ferroelectric polarization of 7.94 μC/cm² along the polar axis in the P2₁ phase. The calculated polarization has almost equal contributions of the methylammonium and Bi³⁺ lone pair, which are fairly decoupled.

Color-Pure Violet-Light-Emitting Diodes Based on Layered Lead Halide Perovskite Nanoplates

Violet electroluminescence is rare in both inorganic and organic light-emitting diodes (LEDs). Low-cost and room-temperature solution-processed lead halide perovskites with high-efficiency and color-tunable photoluminescence are promising for LEDs. Here, we report room-temperature color-pure violet LEDs based on a two-dimensional lead halide perovskite material, namely, 2-phenylethylammonium (C6H5CH2CH2NH3(+), PEA) lead bromide [(PEA)2PbBr4]. The natural quantum confinement of two-dimensional layered perovskite (PEA)2PbBr4 allows for photoluminescence of shorter wavelength (410 nm) than its three-dimensional counterpart. By converting as-deposited polycrystalline thin films to micrometer-sized (PEA)2PbBr4 nanoplates using solvent vapor annealing, we successfully integrated this layered perovskite material into LEDs and achieved efficient room-temperature violet electroluminescence at 410 nm with a narrow bandwidth. This conversion to nanoplates significantly enhanced the crystallinity and photophysical properties of the (PEA)2PbBr4 samples and the external quantum efficiency of the violet LED. The solvent vapor annealing method reported herein can be generally applied to other perovskite materials to increase their grain size and, ultimately, improve the performance of optoelectronic devices based on perovskite materials.

Inorganic–organic hybrid white light phosphors

This feature article reviews the development of inorganic–organic hybrid white-light phosphors for the applications on light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs).