2D Halide Perovskites Research Articles

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

Two-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.

Uncovering Multiple Intrinsic Chiral Phases in (PEA)2PbI4 Halide Perovskites.

Two-dimensional (2D) halide perovskites offer a unique platform for investigating the ground state of materials possessing significant anharmonicity. In contrast to three-dimensional perovskites, their 2D counterparts offer substantially fewer degrees of freedom, resulting in multiple well-defined crystal structures. In this work, we thoroughly investigate the anharmonic ground state of the benchmark (PEA)2PbI4 compound, using complementary information from low-temperature X-ray diffraction (XRD) and photoluminescence spectroscopy, supported by density functional theory calculations. We extrapolate four crystallographic configurations from low-temperature XRD. These configurations imply that the ground state has an intrinsic disorder stemming from two coexisting chiral sublattices, each with a bioriented organic spacer molecule. We further show evidence that these chiral structures form unevenly populated ground states, portraying uneven anharmonicity, where the state population may be tuned by surface effects. Our results uncover a disordered ground state that may induce intrinsic grain boundaries, which cannot be ignored in practical applications.

Ruddlesden–Popper Perovskite Alloys: Continuous and Discontinuous Tuning of the Electronic Structure

Alloying mixed ratios of elements into the halide perovskite (HP) structure has proven to be an effective method of tuning these materials’ structural and electronic properties for photovoltaic and other optoelectronic applications. However, the standard spectroscopies used to characterize HP alloys such as absorption and photoluminescence are limited in their ability to detect disorder and phase segregation within the structure. Here, we characterize these properties in 2D HP alloys to a greater degree by using electroabsorption spectroscopy to study thin films with mixed-metal (Pb–Sn) and mixed-halide (Br–Cl and Br–I) compositions. The large spectral separation of band-edge states in 2D HPs allow us to detect a coexistence of elemental-rich domains within the Pb–Sn and Br–I alloys. Meanwhile, we find that the Br–Cl alloys exhibit sharper spectral features and a more uniform electroabsorption response indicative of an ordered structure, albeit still not as ordered as their pure Br and Cl counterparts. The band gap energies of the Br–Cl series (PEA2PbBrxCl4 – x) can be continuously tuned between 3.425 and 4.13 eV via the Br:Cl ratio, while the exciton binding energies can be tuned from 349 to 487 meV.

Topotactic, Vapor-Phase, In Situ Monitored Formation of Ultrathin, Phase-Pure 2D-on-3D Halide Perovskite Surfaces.

Two-dimensional (2D) halide perovskites, HaPs, can provide chemical stability to three-dimensional (3D) HaP surfaces, protecting them from exposure to ambient species and from reacting with contacting layers. Both actions occur with 2D HaPs, with the general stoichiometry R2PbI4 (R: long or bulky organic amine) covering the 3D ones. Adding such covering films can also boost power conversion efficiencies of photovoltaic cells by passivating surface/interface trap states. For maximum benefit, we need conformal ultrathin and phase-pure (n = 1) 2D layers to enable efficient tunneling of photogenerated charge carriers through the 2D film barrier. Conformal coverage of ultrathin (<10 nm) R2PbI4 layers on 3D perovskites is challenging with spin coating; even more so is its upscaling for larger-area devices. We report on vapor-phase cation exchange of the 3D surface with the R2PbI4 molecules and real-time in situ growth monitoring by photoluminescence (PL) to determine limits for forming ultrathin 2D layers. We characterize the 2D growth stages, following the changing PL intensity-time profiles, by combining structural, optical, morphological, and compositional characterizations. Moreover, from quantitative X-ray photoelectron spectroscopy (XPS) analysis on 2D/3D bilayer films, we estimate the smallest width of a 2D cover that we can grow to be <5 nm, roughly the limit for efficient tunneling through a (semi)conjugated organic barrier. We also find that, besides protecting the 3D against ambient humidity-induced degradation, the ultrathin 2D-on-3D film also aids self-repair following photodamage.

Exploring, Identifying, and Removing the Efficiency-Limiting Factor of Mixed-Dimensional 2D/3D Perovskite Solar Cells

ConspectusThree-dimensional (3D) halide perovskite (HP) solar cells have been thriving as promising postsilicon photovoltaic systems. However, despite the decency of efficiency, they suffer from poor stability. Partial dimensionality reduction from 3D to 2D was found to significantly meliorate the instability, thus mixed-dimensional 2D/3D HP solar cells have been expected to combine favorable durability and high efficiency. Nevertheless, their power conversion efficiency (PCE) does not live up to the expectation, hardly exceeding 19%, in sharp contrast with the ∼26% benchmark for pure 3D HP solar cells. The low PCE primarily arises from the restricted charge transport of the mixed-phasic 2D/3D HP layer. Understanding its photophysical dynamics, including its nanoscopic phase distribution and interphase carrier transfer kinetics, is essential for fathoming the underlying restriction mechanism. This Account outlines the three historical photophysical models of the mixed-phasic 2D/3D HP layer (denoted as models I, II, and III hereafter). Model I opines (i) a gradual dimensionality transition in the axial direction and (ii) a type II band alignment between 2D and 3D HP phases, hence favorably driving global carrier separation. Model II takes the view that (i) 2D HP fragments are interspersed in the 3D HP matrix with a macroscopic concentration variation in the axial direction and (ii) 2D and 3D HP phases instead form a type I band alignment. Photoexcitations would rapidly transfer from wide-band-gap 2D HPs to narrow-band-gap 3D HPs, which then serve as the charge transport network. Model II is currently the most widely accepted. We are one of the earliest groups to unveil the ultrafast interphase energy-transfer process. Recently, we further amended the photophysical model to consider also (i) an interspersing pattern of phase distribution but (ii) the 2D/3D HP heterojunction to be a p-n heterojunction with built-in potential. Anomalously, the built-in potential of the 2D/3D HP heterojunction increases upon photoexcitation. Therefore, local 3D/2D/3D misalignments would severely impede charge transport due to carrier blocking or trapping. Contrary to models I and II which hold 2D HP fragments as the culprit, model III rather suspects the 2D/3D HP interface for blunting the charge transport. This insight also rationalizes the distinct photovoltaic performances of the mixed-dimensional 2D/3D configuration and the 2D-on-3D bilayer configuration. To extinguish the detrimental 2D/3D HP interface, our group also developed an approach to alloy the multiphasic 2D/3D HP assembly into phase-pure intermediates. The accompanying challenges that are coming are also discussed.

Contact Engineering and In Situ Formation of 2D Perovskite Via Solid‐Phase Growth for Efficient Hole‐Transport‐Layer‐Free Perovskite Solar Cells

Carbon‐based perovskite solar cells (C‐PSCs) without hole transport layer have piqued the interest of researchers due to their low‐cost fabrication processes and exceptional stability. However, the efficiency of C‐PSCs lags far behind that of state‐of‐the‐art metal‐based devices, owing to an energy‐level mismatch and poor interfacial contact at the interface between carbon electrode and perovskite layer. Herein, a simple method for performing a solid‐phase reaction at the perovskite/carbon interface using a hot‐pressing process is presented. Unlike the solution processes commonly used in PSCs to obtain 2D halide perovskite layers, butylammonium iodide (BAI) is introduced into carbon electrodes, which are then hot pressed onto 3D perovskite in this work. During hot pressing, BAI in the carbon electrode reacts with the 3D perovskite layer to form a 2D perovskite layer at the perovskite/carbon interface through a solid‐phase reaction, resulting in a contiguous contact perovskite/carbon interface and better energy‐level alignment. Charge recombination at the perovskite/carbon interface is greatly reduced thanks to the seamless contact and the in situ formed 2D perovskite layer as passivation and electron blocking layer. This strategy results in a high power conversion efficiency of up to 16.86% and significantly improved the device stability for C‐PSCs.

Octahedral Distortion and Excitonic Behavior of Cs3Bi2Br9 Halide Perovskite at Low Temperature

The metal halide ionic octahedron, represented as [MX6]n− (M = metal cation, X = halide anion), serves as the basic structural unit in halide perovskites and plays a crucial role in determining their optoelectronic and chemical properties. Thus, it is possible to correlate the responses of metal halide perovskites to various environmental stimuli with the dynamic behaviors of the [MX6]n− octahedra. In this study, with the temperature-dependent single-crystal X ray diffraction (SCXRD) measurements on Cs3Bi2Br9 2D halide perovskites, we can identify two classes of distortions through the lowering of temperature: intraoctahedral distortion, which is the off-centering of Bi3+ cation within a [BiBr6]3– octahedron due to the Bi3+ 6s2 lone pair electrons, and interoctahedral distortion, which is the collective misalignments among the [BiBr6]3– building blocks. Free exciton (FE) and self-trapped exciton (STE) models are used to study the relationship between the distortion of octahedra in Cs3Bi2Br9 and the corresponding changes in its optoelectronic properties, which transform from dominating blue emission above 100 K to red emission at 4 K. This work provides new insights into the excitonic behaviors of perovskites and suggests a possibility that we can design and rationalize the optical properties of halide perovskites by regulating the environmental stimuli based on the knowledge of behaviors of the individual [MX6]n− building blocks.

X‐Ray Quantum Cutting Scintillator Based on CsPbClxBr3−x:Yb3+ Single Crystals

AbstractMetal‐halide perovskites, especially for 3D halide perovskite single crystals (SCs), are emerging as the promising scintillators. However, the strong self‐absorption and low light yield of these perovskites restrict their practical applications. Quantum cutting can convert a high energy photon into multiple low energy photons, endowing a large Stokes shift and high photoluminescent quantum yield (PLQY). Here, this work reports a novel quantum cutting material, CsPbClxBr3−x:Yb3+ perovskite SCs, with the PLQY of 149% and a Stokes shift &gt;550 nm, successfully conquering the issues of poor PLQY induced by the thermal quenching and the large self‐absorption in perovskite SCs. Benefiting from the extremely high PLQY and negligible self‐absorption effect, the first high‐performance quantum cutting scintillators based on CsPbClxBr3−x:Yb3+ perovskite SCs are developed, exhibiting high transmittance, large X‐ray absorption/conversion efficiency, outstanding light yield of ≈1.12 × 105 photons MeV−1, and a detection limit as low as 176.5 nGyair s−1. Finally, a single‐pixel X‐ray imaging is achieved by integrating CsPbCl3:Yb3+ SCs with silicon photomultiplier. This work sheds light on exploring highly competitive scintillators beyond the scope of traditional perovskites.

Navigating the Site-Distinct Energy Conversion Properties of Perovskite Quantum Wells

Two-dimensional (2D) halide perovskites (HPs) are self-assembled multi-quantum wells with excitonic peculiarities. They afford favorable moisture and photothermal stability compared to traditional 3D counterparts. However, the energy conversion properties of 2D HPs are highly site-distinct, and they are found to have complicated origins that are associated with diverse local exciton relaxation dynamics, hence causing researchers severe bewilderment when targeting customized applications. This Review intends to offer a thorough survey on this topic. We first introduce the crystallographic structures of 2D HPs and how they render a quantum-well electronic structure. Subsequently, we elaborate on the exciton transport mechanisms in 2D HPs to interpret how excitons arrive at various local sites. Then we concretely introduce the diversiform local/bulk exciton relaxation dynamics that give rise to the wealthy energy conversion attributes. Finally, challenges and opportunities for better understanding and manipulating the exciton dynamics of this promising class of materials are discussed.

The Interplay of Organic Spacers and Small Cations for Efficient Dion–Jacobson Perovskite Solar Cells

2D halide perovskites have displayed wide tunability in structure and physicochemical properties through the diversity and versatility of interlayer spacers. However, the interaction between the large spacer and small cation inside the inorganic framework has not been elucidated yet. Herein, the impact of small‐organic cation on the crystallization kinetics, carrier behavior, and the structure distortion of the low‐dimensional Dion–Jacobson (DJ) perovskite materials and solar cells is systematically studied. The formamidium (FA)‐based DJ film exhibits higher crystal orientation and larger crystal domain induced by slower crystallization as compared to methylammonium‐based one, accompanied with lower trap density, more efficient carrier transfer, and higher photovoltaic performance. As a result, the BDAFA2Pb3I10 perovskite solar cell delivers a champion power conversion efficiency up to 15.34%, which is the highest among FA‐based small‐n DJ series. Theoretical simulation results uncover that the smaller tilting and higher rigidity of the octahedra in the FA‐based DJ systems render more outstanding environmental stability, leading to well‐maintained 85% of its efficiency after prolonged storing time of 1020 h at 65 °C.

The rise of quasi-2D Dion-Jacobson perovskites for photovoltaics.

With the advance of nanotechnology, the past couple of years have witnessed the fast development of quasi two-dimensional (2D) halide perovskites, which exhibit outstanding long-term stability against moisture and heat, compared with their three-dimensional (3D) counterparts. As one of the most common structures in 2D halide perovskites, quasi-2D Dion-Jacobson (DJ) perovskites show multiple-quantum-well structures with n layers of [BX6]4- octahedral inorganic sheets sandwiched by two layers of diammonium spacers, thus exhibiting superior structural stability due to the elimination of van der Waals gaps. Thanks to the achievement of high power conversion efficiency accompanied by impressive stability, quasi-2D DJ perovskite solar cells (PSCs) have recently drawn extensive attention in the field. This review first introduces the fundamental understanding of quasi-2D DJ halide perovskites, including their superior stability, high exciton binding energy, and compositional flexibility and tunable properties. We then summarize detailed strategies to prepare high-quality quasi-2D DJ perovskites for PSCs, encompassing compositional engineering, solvent engineering, additive addition, and annealing processes. Moreover, the surface/interface modification and 2D-3D hybrid perovskite heterojunction are also discussed, for providing strategies to optimize the fabrication of quasi-2D DJ PSCs. Lastly, current challenges and perspectives toward the future development of quasi-2D DJ perovskites for photovoltaics are outlined.

Surface termination and strain-induced modulation of the structure and electronic properties in 2D perovskites (Cs2BCl4 & CsB2Cl5, B = Pb, Sn): a first-principles study.

Two-dimensional (2D) halide perovskites have demonstrated impressive long-term stability and superior device performance as compared to their three-dimensional (3D) counterparts. The potential of 2D halide perovskites for advanced photovoltaic applications can be enhanced by an understanding of how external factors like strain could be used to tune their optoelectronic properties. This study explores the effects of biaxial strain on the structure and electronic transport properties of 2D halide perovskites, focusing on the lowest energy (001) surfaces of (Cs2BCl4 and CsB2Cl5, B = Pb or Sn) with CsCl and BCl2 terminations. Using first-principles calculations, we find that the lower energy CsCl terminated surface, resulting in Cs2BCl4, couples strongly with biaxial strain. This termination shows bandgap modulations from approximately 1.5 eV to 1.8 eV for Cs2PbCl4 and 1.2 eV to 1.5 eV for Cs2SnCl4 with biaxial strain. Within the acoustic deformation potential theory, we compute hole mobilities, and find substantial enhancements of approximately 80% for Pb-based and 50% for Sn-based systems, thereby emphasizing the potential of strain engineering to further optimize charge transport properties in 2D halide perovskites.

Moisturized 2-dimensional halide perovskite generates a power density of 30 mW cm−3

We report here for the first time on the ionovoltaic effect of 2-dimensional (2D) halide perovskite, which can generate electricity by absorbing moisture.

Airflow-Controlled Crystallization for a Multi-Inch 2D Halide Perovskite Single-Crystal Scintillator for Fast High-Resolution X-ray Imaging

Airflow-Controlled Crystallization for a Multi-Inch 2D Halide Perovskite Single-Crystal Scintillator for Fast High-Resolution X-ray Imaging

Pressure-Induced Emission from All-Inorganic Two-Dimensional Vacancy-Ordered Lead-Free Metal Halide Perovskite Nanocrystals

Although seeking an effective strategy for further improving their optical properties is a great challenge, two-dimensional (2D) halide perovskites have attracted a significant amount of attention because of their performance. In this regard, the pressure-induced emission accompanied by a remarkable pressure-enhanced emission is achieved without a phase transition in 2D vacancy-ordered perovskite Cs3Bi2Cl9 nanocrystals (NCs). Note that the initial Cs3Bi2Cl9 NCs possess extremely strong electron-phonon coupling, leading to the easy annihilation of trapped excitons by the phonon. Upon compression, pressure could effectively suppress phonon-assisted nonradiative decay and give rise to an intriguing emission from "0" to "1". Both the weakened electron-phonon coupling and the relaxed halide octahedral distortion benefiting from the vacancy-ordered structure contributed to the subsequent enhanced emission. This work not only elucidates the underlying photophysical mechanism but also identifies pressure engineering as a robust means for improving their potential applications in environmentally friendly solid-state lighting at extremes.

Unraveling the Defect-Dominated Broadband Emission Mechanisms in (001)-Preferred Two-Dimensional Layered Antimony-Halide Perovskite Film

By adding molar-controlled SbCl3 in a Cs3Sb2Cl9 precursor, we employed a low-temperature solution-processed approach to prepare high-quality (001)-preferred Cs3Sb2Cl9 thin film, which demonstrates a stable defect-dominated broadband emission at room temperature. Density functional theory calculations reveal that the defect emission originates from the donor-acceptor pair (DAP) recombination between chlorine vacancy (VCl) and cesium vacancy (VCs). Furthermore, VCl + VCs DAP is more stable on the (001) surface. The improved film quality and the more stable VCl + VCs DAP increase the activation energy related to defect states, resulting in an enhancement of the defect emission for the high-quality (001)-preferred film. This work provides deep insight into the key role of the (001) surface in defect emission and a feasible strategy to enhance the defect emission in 2D halide perovskites A3B2X9 (A = CH3NH3, Cs, Rb; B = Bi, Sb; X = Cl, Br, I) by control of the thin film preferred orientation.

Spacer Cation Alloying Enables Markedly Improved Chiroptical Properties of Two‐Dimensional Chiral Hybrid Perovskite Nanosheets

AbstractThe combination of unique features of chirality and promising attributes of 2D halide perovskites opens an enticing avenue for the rational design of chiral materials in chiroptoelectronics and spintronics. Despite several impressive approaches available to prepare 2D perovskites with chirality, it poses significant challenges to having a general route to a myriad of high‐quality nanoscale 2D chiral perovskites with tunable and enhanced chiroptical properties. Herein, a robust spacer cation alloying strategy to craft 2D chiral perovskite nanosheets (NSs) is reported, exhibiting a markedly improved circular dichroism signal over the pure chiral cation‐based counterparts. The experimental studies and density functional theory (DFT) modeling revealed that the introduction of achiral cations promoted the defect passivation of NSs via optimization of the binding energy of the cations on the NS surface. More importantly, the compelling effects of spacer cation alloying on the chiroptical properties are readily extended to 2D perovskite NSs with other chiral A‐site cations, lead‐less B‐site cations and X‐site halide anions of interest. This study provides a powerful platform to produce a wide diversity of 2D chiral perovskite nanomaterials and regulate their chiroptical properties via innovative and convenient cation alloying, which may boost the development of chiroptical devices.

Efficient photosensitized singlet oxygen generation in two-dimensional perovskite nanosheets via energy transfer

Two-dimensional (2D) organic-inorganic hybrid halide perovskites have gained extensive attention in photovoltaics, photodetectors and light-emitting diodes owing to their unique optoelectronic properties and superior stability compared with their three-dimensional (3D) counterparts. However, 2D perovskites have rarely been applied for photochemical conversion applications. Herein, a series of 2D BA2MAn-1PbnBr3n+1 perovskite nanosheets have been synthesized through a facile, ligand-free precipitation (LFP) method at room temperature. Using the specific chemical trapping techniques, we show for the first time that the 2D perovskite nanosheets can effectively generate photosensitized singlet oxygen (1O2) via energy transfer from triplet excitons to molecular oxygen in non-polar solvent. Based on this discovery, the as-prepared BA2PbBr4 and BA2MAPb2Br7 nanosheets successfully catalyze the ENE reaction and 1,2-addition reaction between olefin and singlet oxygen under visible light. Our study is anticipated to develop and demonstrate new mechanisms for 2D halide perovskite materials in photodynamic therapy and photocatalytic applications.

Circularly Polarized Photoluminescence of Chiral 2D Halide Perovskites at Room Temperature.

Chiral halide perovskites have attracted considerable attention because of their chiroptical, second-harmonic generation, and ferroelectricity properties and their potential application in chiroptoelectronics and chiral spintronics. However, the fundamental research of these properties is insufficient. In this work, chiral perovskites were synthesized using precursor solutions with various stoichiometric ratios ⟨n⟩. The chiral perovskite film prepared from the solution with ⟨n⟩ = 1 is composed of (R-/S-/rac-MBA)2PbBr4, whereas the films prepared from the solutions with ⟨n⟩ larger than 1 are a mixture of (R-/S-/rac-MBA)2(CsMA)n-1PbnBr3n+1 with n = 1 and large n values. A photoluminescence quantum yield of approximately 90 was obtained. Symmetric circular dichroism (CD) spectra were observed without an external magnetic field. Under various magnetic fields, magnetic field-induced CD features are superimposed with the intrinsic chirality-induced CD features. For the ⟨n⟩ = 1 chiral perovskite film, the energy level splitting induced by chiral molecules are a few 10 μeV, whereas the energy level splitting induced by magnetic fields are at the range of ∼-250 to ∼250 μeV. Circularly polarized photoluminescence spectra were observed at room temperature and associated with the spin-preserved energy funneling from highly energetic phases to the lower energetic phases.

Reconfiguring band-edge states and charge distribution of organic semiconductor-incorporated 2D perovskites via pressure gating.

Two-dimensional (2D) semiconductor heterostructures are key building blocks for many electronic and optoelectronic devices. Reconfiguring the band-edge states and modulating their interplay with charge carriers at the interface in a continuous manner have long been sought yet are challenging. Here, using organic semiconductor-incorporated 2D halide perovskites as the model system, we realize the manipulation of band-edge states and charge distribution via mechanical-rather than chemical or thermal-regulation. Compression induces band-alignment switching and charge redistribution due to the different pressure responses of organic and inorganic building blocks, giving controllable emission properties of 2D perovskites. We propose and demonstrate a "pressure gating" strategy that enables the control of multiple emission states within a single material. We also reveal that band-alignment transition at the organic-inorganic interface is intrinsically not well resolved at room temperature owing to the thermally activated transfer and shuffling of band-edge carriers. This work provides important fundamental insights into the energetics and carrier dynamics of hybrid semiconductor heterostructures.