2D Halide Perovskites Research Articles

Tailoring the Fabrication Method of Dion–Jacobson 2D Halide Perovskites toward Highly Crystalline and Oriented Films (Adv. Funct. Mater. 49/2023)

Tailoring the Fabrication Method of Dion–Jacobson 2D Halide Perovskites toward Highly Crystalline and Oriented Films (Adv. Funct. Mater. 49/2023)

Occurrence of robust memristive behavior for low-power transient resistive switching and photo-responsive neuromorphic computing in low-dimensional perovskite

Recent progress on the memristive devices owing to the multi-functional approach have surged the exploration of new functional materials. Here, we have incorporated two-dimensional (2D) halide perovskite in memristive device. The Pb-free Cs3Bi2I9 perovskite active layer was fabricated to obtain the ITO/Cs3Bi2I9/Pt structure. The device shows typical bipolar resistance switching behavior with Off state resistance in TΩ scale. Due to the higher work function of the top Pt electrode, the barrier height at the Pt/Cs3Bi2I9 junction is higher and the observed device conductance is low in the Off state. Enhanced memory parameters such as endurance (>103 cycles) and retention (104 s) for multiple conductance states is observed. In the pulse modulated synaptic characterization, the device emulated essential post-synaptic plasticity, short-term and long-term potentiation with power consumption in femto Joule. The classical conditioning learning was mimicked by the device by electric and optical stimulation procuring basic associative training. The non-toxicity of the perovskite material has been successfully utilized for the transient memristor application. The robust in-memory photonic memristive property of the device along with water stimulated transient behavior demonstrates the significance of the proposed device in multi-dimensional field of electronic device.

Observation on structural and optical features of new nanostructured lead-free methylammonium zinc or cobalt iodide perovskites for solar cells applications

The toxicity of lead-based halide perovskites has become a significant drawback to be employed in optoelectronic devices. Therefore, developing other environmentally friendly candidates with tunable optoelectronic properties for highly efficient solar cells is indispensable. Lead-free perovskite solar cells (PSCs) are promising to have a crucial role in large-scale commercial non-toxic photovoltaic devices. Here, the microstructure and optoelectronic properties of 2D halide perovskites without pb (CH3NH3)2BI4 (where B = Zn or Co) have been investigated for use in solar cells. The synthesized samples are characterized by X-ray diffraction (XRD), Raman spectroscopy, FT-IR, FESEM, and TEM. The variation in the optical and photoluminescence (PL) is recognized. The results indicate that (CH3NH3)2ZnI4 and (CH3NH3)2CoI4 crystals demonstrate a wide band gap of about 2.42 and 1.87 eV, respectively. A comparative study is presented for the optical properties of Zn- versus Co-based perovskites. It is noticed that Co is a better candidate than Zn to be a good replacement choice for Pb as Co-containing compounds have lower optical bandgap than Zn-containing compounds. PCBM is employed as a hole transport material, and PEDOT:PSS as an electron transport layer. The p-i-n PSCs are fabricated, and the electrical parameters are measured, obtaining power conversion efficiencies (PCE) of 0.73 and 2.45% for (CH3NH3)2ZnI4 and (CH3NH3)2CoI4, respectively. This work opens the door for further investigations to increase the PCE of both devices.

Measuring the Exciton Binding Energy: Learning from a Decade of Measurements on Halide Perovskites and Transition Metal Dichalcogenides

AbstractThe exciton binding energy (Eb) is a key parameter that governs the physics of many optoelectronic devices. At their best, trustworthy and precise measurements of Eb challenge theoreticians to refine models, are a driving force in advancing the understanding of a material system, and lead to efficient device design. At their worst, inaccurate Eb measurements lead theoreticians astray, sow confusion within the research community, and hinder device improvements by leading to poor designs. This review article seeks to highlight the pros and cons of different measurement techniques used to determine Eb, namely, temperature‐dependent photoluminescence, resolving Rydberg states, electroabsorption, magnetoabsorption, scanning tunneling spectroscopy, and fitting the optical absorption. Due to numerous conflicting Eb values reported for halide perovskites (HP) and transition metal dichalcogenides (TMDC) monolayers, an emphasis is placed on highlighting these measurements in an attempt to reconcile the variance between different measurement techniques. It is argued that the experiments with the clearest indicators are in agreement on the following values: ≈350–450 meV for TMDC monolayers between SiO2 and vacuum, ≈150–200 meV for hBN‐encapsulated TMDC monolayers, ≈200–300 meV for common lead‐iodide 2D HPs, and ≈10 meV for methylammonium lead iodide.

A systemic study on Thallium based 3D halide perovskite with enhanced figure of merit

First principles calculations are carried out to explore structural, elastic, electronic, charge density and thermoelectric properties of TlZnX3 (X = F, Cl, Br and I) 3D halide cubic perovskites within the Generalized Gradient Approximation (GGA) through Quantum Espresso code. The elastic coefficients (C11, C12 and C44) and elastic modulii such as Young’s modulus, bulk modulus, shear modulus reveals that stiffness is higher in TlZnF3. Additionally using Pugh’s ratio and Cauchy’s pressure TlZnF3 is classified as ductile whereas TlZnX3 (X = Cl, Br and I) are classified as brittle materials. Phonon dispersion curve proves the dynamic stability of TlZnF3. The band structures of TlZnX3 (X = F, Cl and Br) and TlZnI3 shows semiconducting and metallic nature respectively. Due to replacement of halide ion the indirect band gap of TlZnX3 (X = F, Cl and Br) decreases from 3.82 eV, 1.39 eV and 0.37 eV as the size of the anion increases. Using the Boltztrap code, thermoelectric parameters are analysed at temperatures of 400 K, 600 K and 800 K and chemical potential provides insight about optimal dopant. Large Seebeck Coefficient combined with ultra low thermal conductivity results in a high figure of merit for TlZnF3 that satisfies industry standards (ZT ≈ 1). Together, these outcomes highlights that these Thallium based halide perovskite are reliable materials for high temperature thermoelectric devices.

Unveiling the Impact of Organic Spacer Cations on Auger Recombination in Layered Halide Perovskites

AbstractA library of large organic cation spacers is available for engineering the performance of layered two‐dimensional (2D) halide perovskite devices. Despite extensive photophysics studies, there remains a research gap over the structure‐function relations in 2D perovskites, especially the underlying factors influencing the Auger recombination (AR) process. Herein, the contributions of exciton binding energy, exciton‐phonon coupling, and defects/film morphology to the AR process in 2D perovskites are examined. Phenyl‐alkyl‐ammonium cations with different lengths of attached alkyl groups, commonly used in blue light‐emitting diodes, are investigated. The findings reveal an order of magnitude higher threshold carrier density for the AR onset as well as a reduced AR in cations with longer alkyl chain length. Although possessing similar exciton binding energies, the exciton‐phonon coupling strength is found to play a major role in reducing the AR rate, with a smaller contribution from the defect states/film morphology. The findings can help provide further guidance on organic spacer cation engineering for highly efficient 2D perovskite light emitters.

Quantum confinement effect on the electronic and optical properties of two-dimensional halide perovskites

Two-dimensional (2D) halide perovskites have garnered significant attention due to their inherent stability, excellent optoelectronic properties, and versatile structural diversity, making them promising for solar cell applications. Here, by using the first-principles calculation based on density functional theory, the structural and optoelectronic properties of 2D halide perovskites are investigated. Results reveal a clear relationship in structural and optoelectronic properties. The band gaps of 2D halide perovskites decrease with the sheet of inorganic (or organic) layers increasing (or decreasing). Calculated charge density at the band edges and averaged electrostatic potential explain this structure-properties relationship comes from quantum confinement effect induced by the unique quantum-well-like structure. This theoretical work provides comprehensive understanding of the optoelectronic properties and a guideline for designing new 2D halide perovskite with desired optoelectronic properties.

Trap or Triplet? Excited-State Interactions in 2D Perovskite Colloids with Chromophoric Cations.

Movement of energy within light-harvesting assemblies is typically carried out with separately synthesized donor and acceptor species, which are then brought together to induce an interaction. Recently, two-dimensional (2D) lead halide perovskites have gained interest for their ability to accommodate and assemble chromophoric molecules within their lattice, creating hybrid organic-inorganic compositions. Using a combination of steady-state and time-resolved absorption and emission spectroscopy, we have now succeeded in establishing the competition between energy transfer and charge trapping in 2D halide perovskite colloids containing naphthalene-derived cations (i.e., NEA2PbX4, where NEA = naphthylethylamine). The presence of room-temperature triplet emission from the naphthalene moiety depends on the ratio of bromide to iodide in the lead halide sublattice (i.e., x in NEA2Pb(Br1-xIx)4), with only bromide-rich compositions showing sensitized emission. Photoluminescence lifetime measurements of the sensitized naphthalene reveal the formation of the naphthalene triplet excimer at room temperature. From transient absorption measurements, we find the rate constant of triplet energy transfer (kEnT) to be on the order of ∼109 s-1. At low temperatures (77 K) a new broad emission feature arising from trap states is observed in all samples ranging from pure bromide to pure iodide composition. These results reveal the interplay between sensitized triplet energy transfer and charge trapping in 2D lead halide perovskites, highlighting the need to carefully parse contributions from competing de-excitation pathways for optoelectronic applications.

Emerging Two-Dimensional Organic Semiconductor-Incorporated Perovskites─A Fascinating Family of Hybrid Electronic Materials.

Halide perovskites have attracted a great amount of attention owing to their unique materials chemistry, excellent electronic properties, and low-cost manufacturing. Two-dimensional (2D) halide perovskites, originating from three-dimensional (3D) perovskite structures, are structurally more diverse and therefore create functional possibilities beyond 3D perovskites. The much less restrictive size constraints on the organic component of these hybrid materials particularly provide an exciting platform for designing unprecedented materials and functionalities at the molecular level. In this Perspective, we discuss the concept and recent development of a sub-class of 2D perovskites, namely, organic semiconductor-incorporated perovskites (OSiPs). OSiPs combine the electronic functionality of organic semiconductors with the soft and dynamic halide perovskite lattice, offering opportunities for tailoring the energy landscape, lattice and carrier dynamics, and electron/ion transport properties for various fundamental studies, as well as device applications. Specifically, we summarize recent advances in the design, synthesis, and structural analysis of OSiPs with various organic conjugated moieties as well as the application of OSiPs in photovoltaics, light-emitting devices, and transistors. Lastly, challenges and further opportunities for OSiPs in molecular design, integration of novel functionality, film quality, and stability issues are addressed.

Spontaneous Polarization Induced Optical Responses in a Two-Dimensional Ferroelectric Halide Perovskite.

Two-dimensional (2D) halide perovskites exhibit unique structural and optical properties because large organic molecular cations distort the perovskite structure and the excitons confined in the 2D layers are stable. Here, we report the temperature dependences of the absorption spectra, second harmonic generation (SHG) intensity, and lattice constants of 2D perovskite (BA)2(EA)2Pb3I10 single crystals, where BA is n-butylammonium and EA is ethylammonium. We found that the Urbach tail of the absorption spectrum significantly changes at around 200 K and that the change is correlated with the SHG intensity and the in-plane lattice distortion. We concluded that a random distribution of spontaneous polarizations in the ferroelectric phase modifies the linewidth of the band-edge exciton transition and is the cause of the anomalous temperature dependence of the steepness parameter of the Urbach tail.

Flexible array of high performance and stable formamidinium-based low-n 2D halide perovskite photodetectors for optical imaging

Metal halide perovskites are attracting increasing attention as photodetectors (PDs) materials due to their excellent photoelectric properties. However, integrating perovskite to realize high-resolution perovskite pixel arrays remains critical to enable practical devices. Herein, we demonstrate a high-responsivity and stable 10 × 10 flexible PDs array based on formamidinium-based quasi-2D Ruddlesden-Popper (BA)2FAPb2I7 (n = 2) (BA= butylammonium, C4H9NH3+) perovskite elements for image sensing. The high-performance (BA)2FAPb2I7 arrays are synthesized on a polyethylene terephthalate (PET) flexible substrate, on a large scale, high quality, and controllable manner by introducing Au nanoparticles to enhance film quality upon the annealing process and recrystallization by applying formamidinium chloride (FACl) post-treatment to reduce perovskite film defects, combined with an innovative SiO2-assisted hydrophilic and hydrophobic surface treatment strategy. The prepared flexible PDs arrays show excellent optoelectronic properties with high responsivity (5.9 A W−1), a detectivity of 8.2 × 1012 Jones, large on/off current ratio (up to 3 × 104), ultimate response speed (12.8 μs/10.1 μs), and wide spectral response range. In addition, the device shows outstanding mechanical stability under severe bending (θ = 120°) and excellent folding durability after thousands of bending cycles. It also exhibits excellent environmental stability with only a slight degradation of photoresponse after 100 days of storage under the ambient environment. Finally, the integrated flexible PDs arrays can be used as an effective visible light image sensor with good spatial resolution, paving a multifunctional and competitive way for future wearable sensors and artificial vision systems. One-sentence summaryLow-dimensional halide perovskite layers pixelized on a polyethylene terephthalate (PET) flexible substrate provide high performance photodetectors.

Two-Dimensional Halide Pb-Perovskite-Double Perovskite Epitaxial Heterostructures.

Epitaxial heterostructures of two-dimensional (2D) halide perovskites offer a new platform for studying intriguing structural, optical, and electronic properties. However, difficulties with the stability of Pb- and Sn-based heterostructures have repeatedly slowed the progress. Recently, Pb-free halide double perovskites are gaining a lot of attention due to their superior stability and greater chemical diversity, but they have not been successfully incorporated into epitaxial heterostructures for further investigation. Here, we report epitaxial core-shell heterostructures via growing Pb-free double perovskites (involving combinations of Ag(I)-Bi(III), Ag-Sb, Ag-In, Na-Bi, Na-Sb, and Na-In) around Pb perovskite 2D crystals. Distinct from Pb-Pb and Pb-Sn perovskite heterostructures, growths of the Pb-free shell at 45° on the (100) surface of the lead perovskite core are observed in all Pb-free cases. The in-depth structural analysis carried out with electron diffraction unequivocally demonstrates the growth of the Pb-free shell along the [110] direction of the Pb perovskite, which is likely due to the relatively lower surface energy of the (110) surface. Furthermore, an investigation of anionic interdiffusion across heterostructure interfaces under the influence of heat was carried out. Interestingly, halide anion diffusion in the Pb-free 2D perovskites is found to be significantly suppressed as compared to Pb-based 2D perovskites. The great structural tunability and excellent stability of Pb-free perovskite heterostructures may find uses in electronic and optoelectronic devices in the near future.

Halide Ion Migration in 2D Metal Halide Perovskites

Two-dimensional (2D) halide perovskites have drawn attention for their unique optoelectronic properties. Interestingly, Ruddlesden-Popper 2D mixed-halide perovskite films with spacer cations such as butylammonium and Dion-Jacobson 2D mixed-halide perovskite films with 1,4-phenylenedimethanammonium undergo halide segregation under irradiation. Anions become mobile under steady state irradiation to form I-rich and Br-rich domains. Upon ceasing illumination, film reverts to their original, mixed-halide compositions, establishing the reversibility of the phase segregation phenomenon. However, if films are in contact with dichloromethane, light irradiation causes the sequential expulsion of iodide and bromide and introduces irreversible changes to 2D films. The sequential disappearance of I- and Br- from pristine films (BA2Pb2Br4, and BA2Pb2I4) under photoirradiation, as observed from variances in expulsion rates, reflect differences in halide ion mobilities in these films. The impact of anion migration, leading to I- and Br- expulsion into solvents, within the context of 2D perovskite photostability will be discussed.

Cation Migration in Physically Paired 2D and 3D Lead Halide Perovskite Films

Abstract2D/3D interfaces provide long‐term stability for the operation of metal halide perovskite solar cells. However, the cation migration under heat and light can disturb the interface and create a graded cation interface. This study has now probed the cation migration by physically pairing X2PbI4 (X = butylammonium BA, oleylammonium OA, or phenethylammonium PEA) 2D film and (CH3NH3, MA)PbI3 3D film at different temperatures and recording changes in the absorption and emission spectra. The migration of the methyl ammonium cation toward the 2D film slowly transforms the n = 1 layered phase into n = 2 and 3 layered phases. The 3D film, on the other hand, exhibits relatively small changes, as the inclusion of spacer cation has little effect on its phase. The apparent activation energy determined from the temperature‐dependent cation migration kinetics (Ea = 29– 50 kJ mol−1) indicates alkyl ammonium cations such as butyl ammonium migrate more readily into the 3D layer than aromatic cations such as PEA. The ease of migration of spacer cations between physically paired films suggests that the varied composition at the interface should be considered while evaluating the effectiveness of the 2D/3D design strategy for improving the performance of perovskite solar cells.

Room-Temperature Solvent Evaporation Induced Crystallization: A General Strategy for Growth of Halide Perovskite Single Crystals by Applying the Le Chatelier's Principle.

The growth of high-quality halide perovskite single crystals is imperative to study their intrinsic physical properties and to realize high-performance optoelectronic devices. Here, a room-temperature solvent evaporation-induced crystallization (RTSEIC) method is reported based on Le Chatelier's principle, which provides a general strategy to grow halide perovskite single crystals including 3D, 2D, 1D, and 0D, and either hybrid or all-inorganic halide perovskites. Taking 2D n-BA2 PbBr4 (n-BA = butylammonium) as an example, the room-temperature crystallization kinetics is demonstrated. The centimeter-sized n-BA2 PbBr4 single crystals exhibit an extremely small full width at half maximum (FWHM) of 0.024° in (0 0 2) plane rocking curve and a small trap density of 2.74× 1010 cm-3 . The superior crystalline quality endows the n-BA2 PbBr4 single crystal ultraviolet photodetectors with recorded performance among reported n-BA2 PbBr4 ultraviolet photodetectors, demonstrating a detectivity reaching 1.8× 1013 Jones, a fast response time of 55 µs and a high on-off ratio of 104 . The low-cost, simple, general, and efficient RTSEIC method is anticipated to promote the blossoming of halide perovskites single crystals.

Built-in Electric Field in Quasi-2D CsPbI3 Perovskites Using High-Polarized Zwitterionic Spacer for Enhanced Charge Separation/Transport.

Two-dimensional (2D) halide perovskites are promising candidates for the fabrication of stable and high-efficiency solar cells. However, the low power conversion efficiency (PCE) of cell devices using 2D perovskites is attributed to reduced charge transport caused by poor organic barrier conductivity. In this study, we propose the use of a high-polarized organic zwitterionic spacer, p-aminobenzoic acid (PABA), to construct novel quasi-2D perovskite structures with enhanced self-driven charge separation and transfer. The NH3+ and COO- groups in PABA generate an aligned electric field, promoting carrier separation and aggregation on the opposite edges of the inorganic layer. This enables efficient in-plane transportation along the inorganic layer. Additionally, PABA intercalated quasi-2D perovskite exhibits improved stability compared with counterparts with diamine cation spacers due to the strong interaction between -COO- and inorganic layers. Our findings suggest that high-polarized organic zwitterionic spacers, with NH3+ and COO- functionality, hold promise for stable and efficient quasi-2D perovskite solar cells.

Formamidine Engineering the Lattice Distortion of Chiral Halide Perovskites for Efficient Blue Circularly Polarized Emission

AbstractChiral halide perovskites have emerged as promising circularly polarized light sources due to their great potential applications in 3D displays, optical data storage, and optical communication devices. However, to date, chiral halide perovskites with efficient blue circularly polarized luminescence (CPL) are scarcely unearthed. Herein, through formamidine engineering to decrease lattice distortion, the first new types of chiral corrugated 2D halide perovskites are presented (R/S)‐(BPEA)FAPbBr4·H2O (2‐R/S) (BPEA = 4‐bromophenylethylaminium, FA = formamidinium). Interestingly, driven by the incorporation of FA organic cations, the chiral perovskite layer realizes the transformation from (100)‐oriented (R/S)‐(BPEA)2PbBr4·H2O (1‐R/S) to (110)‐oriented 2‐R/S accompanying with reduced lattice distortion. More importantly, (110)‐oriented chiral perovskites 2‐R exhibit remarkable blue CPL with large asymmetry factors (glum =8.4 × 10−3). This work offers an exciting approach to realizing efficient blue CPL and promotes their further applications in photoelectronic devices.

Tailoring the Fabrication Method of Dion–Jacobson 2D Halide Perovskites toward Highly Crystalline and Oriented Films

AbstractHalide perovskites are potential next‐generation optoelectronic devices. However, the film quality of this charming material fabricated by the conventional spin‐coating method is far from satisfactory, significantly affecting the optoelectronic devices' performance. Here, one facile slow‐evaporating solvent (SE) method is demonstrated to synthesize high‐quality organic–inorganic halide perovskite films. Compared with the conventional spin coating method, the films fabricated by this SE method show much higher crystallinity, oriented lattice, smoother surface morphology, and lower trap density. Besides, the photodetector manufactured by the SE method‐based film also performs much better than the ones by the spin‐coating method. Importantly, this universal method can be applied to different organic–inorganic halide perovskites, such as Dion–Jacobson (DJ) type and Ruddlesden–Popper type 2D halide perovskites and conventional 3D halide perovskites. This work gives an effective solution to improve the quality of the DJ‐type halide perovskites, which endows the halide perovskites with a more practical chance to be commercialized cosmically.

Synergy of 3D and 2D Perovskites for Durable, Efficient Solar Cells and Beyond.

Three-dimensional (3D) organic-inorganic lead halide perovskites have emerged in the past few years as a promising material for low-cost, high-efficiency optoelectronic devices. Spurred by this recent interest, several subclasses of halide perovskites such as two-dimensional (2D) halide perovskites have begun to play a significant role in advancing the fundamental understanding of the structural, chemical, and physical properties of halide perovskites, which are technologically relevant. While the chemistry of these 2D materials is similar to that of the 3D halide perovskites, their layered structure with a hybrid organic-inorganic interface induces new emergent properties that can significantly or sometimes subtly be important. Synergistic properties can be realized in systems that combine different materials exhibiting different dimensionalities by exploiting their intrinsic compatibility. In many cases, the weaknesses of each material can be alleviated in heteroarchitectures. For example, 3D-2D halide perovskites can demonstrate novel behavior that neither material would be capable of separately. This review describes how the structural differences between 3D halide perovskites and 2D halide perovskites give rise to their disparate materials properties, discusses strategies for realizing mixed-dimensional systems of various architectures through solution-processing techniques, and presents a comprehensive outlook for the use of 3D-2D systems in solar cells. Finally, we investigate applications of 3D-2D systems beyond photovoltaics and offer our perspective on mixed-dimensional perovskite systems as semiconductor materials with unrivaled tunability, efficiency, and technologically relevant durability.

Design Rules for Two‐Dimensional Organic Semiconductor‐Incorporated Perovskites (OSiP) Gleaned from Thousands of Simulated Structures**

AbstractTwo‐dimensional (2D) halide perovskites are an attractive class of hybrid perovskites that have additional optoelectronic tunability due to their accommodation of relatively large organic ligands. Nevertheless, contemporary ligand design depends on either expensive trial‐and‐error testing of whether a ligand can be integrated within the lattice or conservative heuristics that unduly limit the scope of ligand chemistries. Here, the structural determinants of stable ligand incorporation within Ruddlesden‐Popper (RP) phase perovskites are established by molecular dynamics (MD) simulations of over ten‐thousand RP‐phase perovskites and training of machine learning classifiers capable of predicting structural stability based solely on generalizable ligand features. The simulation results show near‐perfect predictions of positive and negative literature examples, predict trade‐offs between several ligand features and stability, and ultimately predict an inexhaustibly large 2D‐compatible ligand design‐space.