Establishing charge-transfer excitons in 2D perovskite heterostructures

Tunable Interlayer Delocalization of Excitons in Layered Organic-Inorganic Halide Perovskites

Layered organic-inorganic halide perovskites exhibit remarkable structural and chemical diversity and hold great promise for optoelectronic devices. In these materials, excitons are thought to be strongly confined within the inorganic metal halide layers with interlayer coupling generally suppressed by the organic cations. Here, we present an in-depth study of the energy and spatial distribution of the lowest-energy excitons in layered organic-inorganic halide perovskites from first-principles many-body perturbation theory, within the GW approximation and the Bethe-Salpeter equation. We find that the quasiparticle band structures, linear absorption spectra, and exciton binding energies depend strongly on the distance and the alignment of adjacent metal halide perovskite layers. Furthermore, we show that exciton delocalization can be modulated by tuning the interlayer distance and alignment, both parameters determined by the chemical composition and size of the organic cations. Our calculations establish the general intuition needed to engineer excitonic properties in novel halide perovskite nanostructures.

Dual Emission Bands of a 2D Perovskite Single Crystal with Charge Transfer State Characteristics

Several hybrid halide 2D-perovskite species emit light with an emergent and controversial broadband emission Stokes-shifted down from the narrow band emission. This paper uncovers the sub- and above-bandgap emission and absorption characteristics of PEA₂PbI₄ prepared with gap states introduced during single crystal growth. Here, gap states led to coexistent intrinsic and heterostructured electronic frameworks that are selectively accessible with ultraviolet (UV) and infrared (IR) light, respectively, resulting in the phenomenon of photoluminescence (PL) switching from narrowband green to broadband red. Electron-energy dependent cathodoluminescence shows a relative increase in the broadband red PL intensity as the electron penetration depth increases from 30 nm to 2 μm, confirming the heterostructured framework is formed in the bulk of the crystal. Excitation-emission power slope of 2.5 and up-conversion pump transient absorption (TA) spectra suggest that the IR up-conversion excitation with red photoluminescence, peaked at 655 nm, is a multiphoton process occurring in the heterostructured framework through a nonlinear optical response. The energetic pathways toward the dual emission bands are revealed by pump-probe transient absorption spectroscopy, showing energetically broad gap states with high sensitivity to an IR pump are upconverted and subsequently quickly relax from high to low energy levels within 4 ps. Furthermore, the up-conversion red PL demonstrates a linear polarization with magnetic field effects, thus affirming that the band-like heterostructured framework is crystallographically aligned with characteristics of spatially extended charge-transfer states.

Photogenerated charge transfer in Dion-Jacobson type layered perovskite based on naphthalene diimide

Incorporating organic semiconducting spacer cations into layered lead halide perovskite structures provides a powerful approach to mitigate the typical strong dielectric and quantum confinement effects by inducing charge-transfer between the organic and inorganic layers. Herein we report the synthesis and characterization of thin films of novel DJ-phase organic-inorganic layered perovskite semiconductors using a naphthalene diimide (NDI) based divalent spacer cation, which is shown to accept photogenerated electrons from the inorganic layer. With alkyl chain lengths of 6 carbons, an NDI-based thin film exhibited electron mobility (based on space charge-limited current for quasi-layered 〈n〉 = 5 material) was found to be as high as 0.03 cm² V^-1 s^-1 with no observable trap-filling region suggesting trap passivation by the NDI spacer cation.

Rational adjustment to interfacial interaction with carbonized polymer dots enabling efficient large-area perovskite light-emitting diodes

Film uniformity of solution-processed layers is the cornerstone of large-area perovskite light-emitting diodes, which is often determined by the 'coffee-ring effect'. Here we demonstrate a second factor that cannot be ignored is the solid-liquid interface interaction between substrate and precursor and can be optimized to eliminate rings. A perovskite film with rings can be formed when cations dominate the solid-liquid interface interaction; whereas smooth and homogeneous perovskite emitting layers are generated when anions and anion groups dominate the interaction. This is due to the fact that the type of ions anchored to the substrate can determine how the subsequent film grows. This interfacial interaction is adjusted using carbonized polymer dots, who also orient the perovskite crystals and passivate their buried traps, enabling a 225 mm² large-area perovskite light-emitting diode with a high efficiency of 20.2%.

Plurality of excitons in Ruddlesden-Popper metal halides and the role of the B-site metal cation

We investigate the effect of metal cation substition on the excitonic structure and dynamics in a prototypical Ruddlesden-Popper metal halide. Through an in-depth spectroscopic and theoretical analysis, we identify the presence of multiple resonances in the optical spectra of a phenethyl ammonium tin iodide, a tin-based RPMH. Based on ab initio calculations, we assign these resonances to distinct exciton series that originate from the splitting of the conduction band due to spin-orbit coupling. While the splitting energy in the tin based system is low enough to enable the observation of the higher lying exciton in the visible-range spectrum of the material, the higher splitting energy in the lead counterpart prevents the emergence of such a feature. We elucidate the critical role played by the higher lying excitonic state in the ultrafast carrier thermalization dynamics.

Light-Emitting Organic Semiconductor-Incorporated Perovskites: Fundamental Properties and Device Applications

Recently, organic semiconductor-incorporated perovskites (OSiPs) have emerged as a new subclass of next-generation organic-inorganic hybrid materials. OSiPs combine the advantages of organic semiconductors, such as large design windows and tunable optoelectronic functionalities, with the excellent charge-transport properties of the inorganic metal-halide counterparts. OSiPs provide a new materials platform for the exploitation of charge and lattice dynamics at the organic-inorganic interfaces for various applications. This Perspective reviews recent achievements in OSiPs highlighting the benefits from organic semiconductor incorporation and elucidates the fundamental light-emitting mechanism, energy transfer, as well as band alignment structures at the organic-inorganic interface. Insights on the emission tunability lead toward a discussion of the potential of OSiPs in light-emitting applications, such as perovskite light-emitting diodes or lasing systems.

Light Emission in 2D Silver Phenylchalcogenolates

Silver phenylselenolate (AgSePh, also known as "mithrene") and silver phenyltellurolate (AgTePh, also known as "tethrene") are two-dimensional (2D) van der Waals semiconductors belonging to an emerging class of hybrid organic-inorganic materials called metal-organic chalcogenolates. Despite having the same crystal structure, AgSePh and AgTePh exhibit a strikingly different excitonic behavior. Whereas AgSePh exhibits narrow, fast luminescence with a minimal Stokes shift, AgTePh exhibits comparatively slow luminescence that is significantly broadened and red-shifted from its absorption minimum. Using time-resolved and temperature-dependent absorption and emission microspectroscopy, combined with subgap photoexcitation studies, we show that exciton dynamics in AgTePh films are dominated by an intrinsic self-trapping behavior, whereas dynamics in AgSePh films are dominated by the interaction of band-edge excitons with a finite number of extrinsic defect/trap states. Density functional theory calculations reveal that AgSePh has simple parabolic band edges with a direct gap at Γ, whereas AgTePh has a saddle point at Γ with a horizontal splitting along the Γ-N₁ direction. The correlation between the unique band structure of AgTePh and exciton self-trapping behavior is unclear, prompting further exploration of excitonic phenomena in this emerging class of hybrid 2D semiconductors.

2D PEA2SnI4 Inkjet-Printed Halide Perovskite LEDs on Rigid and Flexible Substrates

Lead-free PEA₂SnI₄-based perovskite LEDs are successfully inkjet-printed on rigid and flexible substrates. Red-emitting devices (λ_max = 633 nm) exhibit, under ambient conditions, a maximum external quantum efficiency (EQE_max) of 1% with a related brightness of 30 cd/m² at 10 mA/cm².

Degradation mechanisms of perovskite nanocrystals in color-converted InGaN micro-light-emitting diodes

The metal halide perovskite nanocrystals (NCs) have attracted much attention because of their excellent optical properties and potential for application in optoelectronic devices. However, their photo- and thermostability are still practical challenges and need further optimization. Here, we have studied the degradation behaviors of CsPbI₃ NCs utilized as optical conversion layer in InGaN based blue micro-LEDs in situ. Furthermore, the effects of temperature and light irradiation on perovskite NCs were investigated respectively. The results indicate that both blue light irradiation and high temperature can cause the increased nonradiative recombination rate, resulting in the degradation of perovskite NCs and reduction of the photoluminescence quantum yield (PLQY). Especially in high-temperature condition, both the single-exciton nonradiative recombination rate and the biexciton nonradiative recombination rate are increased, causing the significant reduction of PLQY of perovskite NCs in high temperature environment than blue light irradiation. Our work provides a detailed insight about the correlation between the light irradiation and temperature consequences for CsPbI₃ NCs and may help to pave the way toward optoelectronic device applications.

Application of perovskites in bioimaging: the state-of-the-art and future developments

BACKGROUND

Recently, the development of perovskite-based nanocrystals for sustainable applications in bioimaging and clinical diagnostics have become a very active area of research. From 2D hybrid to zero-dimensional quantum dots (QDs), perovskites along with a variety of characteristic features, specifically non-linear optoelectronics properties, have attracted enormous research attention. These characteristics can be tuned by the type of cations or anions and their ratio used in host perovskites. Carrier doping and chemical modifications are additional alternatives to control optical and magnetism in radiodiagnostics.

AREA COVERED

This review begins by explaining the physical phenomena associated with luminescence or optical features of novel perovskites in diagnostic applications. Moreover, reported oxide, halide, doped, and QDs-based nanoprobes were elaborated. At last, the need for novel perovskite development, for example, persistent luminescent and low cytotoxicity is discussed, and the futuristic perspective of perovskites in clinical diagnostics with real-time demonstration is explained.

EXPERT OPINION

Our article concludes that hybrid perovskites, including metal-free, core-shell nanocomposites-based, and alloy-based perovskites, exhibit tunable bandgap and high photoluminescence quantum yields which ultimately result in high optical features. However, given limited understanding of ion transport mechanisms and dependency on environmental conditions of the perovskites, more research is needed.

Giant tunable Rashba spin splitting in two-dimensional polar perovskites TlSnX3 (X = Cl, Br, I)

The electronic structures and Rashba effect of two-dimensional polar tetragonal perovskites TlSnX₃ (X = Cl, Br, I) are investigated by first-principles density functional theory, and intrinsic Rashba effects are found around the Γ point. In particular, TlSnI₃ has the largest Rashba constant of 1.072 eV Å^-1. Additionally, TlSnBr₃ and TlSnI₃ respond strongly to the applied electric field, and the electric field responsivity of TlSnI₃ can reach 0.790 e Ų. Therefore, due to the large Rashba constants and strong electric field responses, these 2D polar perovskites are promising semiconductor materials with short channel lengths. The nano-scale short spin coherence length can keep the spin coherence of spin FETs, which is superior to the traditional 3D micron spin FETs, and will show a broad application prospect in the Rashba semiconductor field.

Stimulating and Manipulating Robust Circularly Polarized Photoluminescence in Achiral Hybrid Perovskites

Circularly polarized light (CPL) is essential for optoelectronic and chiro-spintronic applications. Hybrid perovskites, as star optoelectronic materials, have demonstrated CPL activity, which is, however, mostly limited to chiral perovskites. Here, we develop a simple, general, and efficient strategy to stimulate CPL activity in achiral perovskites, which possess rich species, efficient luminescence, and tunable bandgaps. With the formation of van der Waals heterojunctions between chiral and achiral perovskites, a nonequilibrium spin population and thus CPL activity are realized in achiral perovskites by receiving spin-polarized electrons from chiral perovskites. The polarization degree of room-temperature CPL in achiral perovskites is at least one order of magnitude higher than in chiral ones. The CPL polarization degree and emission wavelengths of achiral perovskites can be flexibly designed by tuning chemical compositions, operating temperature, or excitation wavelengths. We anticipate that unlimited types of achiral perovskites can be endowed with CPL activity, benefiting their applications in integrated CPL sources and detectors.

The Universal Growth of Ultrathin Perovskite Single Crystals

Perovskites have engaged significant attention owing to rich species and remarkable physical properties as well as optoelectronic applications. Compared to bulk counterparts, ultrathin perovskites exhibit more available compositions due to the breaking of bulk lattice limitation. Coupled with crystal lattice relaxation and quantum confinement, infinite intriguing properties of ultrathin perovskites deserve to be explored. Developing ultrathin perovskites with alterable composition and structure is a necessity to fully explore this versatile family. Herein, a universal strategy is conceived via constructing oriented solvent microenvironment induced by the interfacial electric field originated from the charge separation between solid and liquid phases, which is conducive to controlling the precursor distribution and makes crystals preferentially nucleate and grow in the preferentially lateral mode. From layered to nonlayered, organic to inorganic, and toxic to low-toxic lead-free perovskite, a full-range synthesis is achieved of ultrathin perovskites. This work opens up opportunities both for ultrathin perovskite exploration through compositional engineering and for device miniaturization in energy conversion applications.

High-Performance Tin-Lead Mixed-Perovskite Solar Cells with Vertical Compositional Gradient

Sn-Pb mixed perovskites with bandgaps in the range of 1.1-1.4 eV are ideal candidates for single-junction solar cells to approach the Shockley-Queisser limit. However, the efficiency and stability of Sn-Pb mixed-perovskite solar cells (PSCs) still lag far behind those of Pb-based counterparts due to the easy oxidation of Sn²⁺ . Here, a reducing agent 4-hydrazinobenzoic acid is introduced as an additive along with SnF₂ to suppress the oxidation of Sn²⁺ . Meanwhile, a vertical Pb/Sn compositional gradient is formed spontaneously after an antisolvent treatment due to different solubility and crystallization kinetics of Sn- and Pb-based perovskites and it can be finely tuned by controlling the antisolvent temperature. Because the band structure of a perovskite is dependent on its composition, graded vertical heterojunctions are constructed in the perovskite films with a compositional gradient, which can enhance photocarrier separation and suppress carrier recombination in the resultant PSCs. Under optimal fabrication conditions, the Sn-Pb mixed PSCs show power conversion efficiency up to 22% along with excellent stability during light soaking.

Structural Disorder in Layered Hybrid Halide Perovskites: Types of Stacking Faults, Influence on Optical Properties and Their Suppression by Crystallization Engineering

Layered hybrid halide perovskites (LHHPs) are an emerging type of semiconductor with a set of unique optoelectronic properties. However, the solution processing of high-quality LHHPs films with desired optical properties and phase composition is a challenging task, possibly due to the structural disorder in the LHHP phase. Nevertheless, there is still a lack of experimental evidence and understanding of the nature of the structural disorder in LHHPs and its influence on the optical properties of the material. In the current work, using 2D perovskites (C4H9NH3)2(CH3NH3)n-1PbnI3n+1 (further BA2MAn-1PbnI3n+1) with n = 1-4 as a model system, we demonstrate that deviations in LHHPs optical properties and X-ray diffraction occur due to the presence of continuous defects-Stacking Faults (SFs). Upon analyzing the experimental data and modeled XRD patterns of a possible set of stacking faults (SFs) in the BA2MAPb2I7 phase, we uncover the most plausible type of SFs, featured by the thickness variation within one perovskite slab. We also demonstrate the successful suppression of SFs formation by simple addition of BAI excess into BA2MAn-1PbnI3n+1 solutions.

2D Nanomaterials for Effective Energy Scavenging

The development of a nation is deeply related to its energy consumption. 2D nanomaterials have become a spotlight for energy harvesting applications from the small-scale of low-power electronics to a large-scale for industry-level applications, such as self-powered sensor devices, environmental monitoring, and large-scale power generation. Scientists from around the world are working to utilize their engrossing properties to overcome the challenges in material selection and fabrication technologies for compact energy scavenging devices to replace batteries and traditional power sources. In this review, the variety of techniques for scavenging energies from sustainable sources such as solar, air, waste heat, and surrounding mechanical forces are discussed that exploit the fascinating properties of 2D nanomaterials. In addition, practical applications of these fabricated power generating devices and their performance as an alternative to conventional power supplies are discussed with the future pertinence to solve the energy problems in various fields and applications.

Photophysics of 2D Organic-Inorganic Hybrid Lead Halide Perovskites: Progress, Debates, and Challenges

2D organic-inorganic hybrid Ruddlesden-Popper perovskites (RPPs) have recently attracted increasing attention due to their excellent environmental stability, high degree of electronic tunability, and natural multiquantum-well structures. Although there is a rapid development of photoelectronic applications in solar cells, photodetectors, light emitting diodes (LEDs), and lasers based on 2D RPPs, the state-of-the-art performance is far inferior to that of the existing devices because of the limited understanding on fundamental physics, especially special photophysics in carrier dynamics, excitonic fine structures, excitonic quasiparticles, and spin-related effect. Thus, there is still plenty of room to improve the performances of photoelectronic devices based on 2D RPPs by enhancing knowledge on fundamental photophysics. This review highlights the special photophysics of 2D RPPs that is fundamentally different from the conventional 3D congeners. It also provides the most recent progress, debates, challenges, prospects, and in-depth understanding of photophysics in 2D perovskites, which is significant for not only boosting performance of solar cells, LEDs, photodetectors, but also future development of applications in lasers, spintronics, quantum information, and integrated photonic chips.

Tunable Rashba Spin Splitting in Two-Dimensional Polar Perovskites

Two-dimensional (2D) Rashba semiconductors with structure inversion asymmetry and a spin-orbit coupling (SOC) effect show promising applications in nanospintronics, such as spin field effect transistors (FETs). Here, we systematically investigate the electronic structures and Rashba effect of 2D polar perovskites ABX₃ (A = Cs⁺ or Rb⁺; B = Pb²⁺ or Sn²⁺; X = Cl, Br, or I) by first-principles density functional theory calculations. We demonstrate that, except for the cubic case, 2D polar perovskites from tetragonal and orthorhombic three-dimensional (3D) bulks exhibit a strong intrinsic Rashba effect around the Γ point, due to their structure inversion asymmetry and the strong SOC effect of heavy atoms. In particular, 2D orthorhombic RbSnI₃ shows the largest Rashba constant of 1.176 eV Å among these polar perovskites, which is comparable to that of 3D bulk perovskites previously reported in experiments and theory. Furthermore, several 2D polar perovskites also show a strong electric field response. In particular, 2D tetragonal RbPbI₃ and tetragonal CsPbI₃ have strong electric field responses of >0.5 e Ų. Therefore, 2D polar perovskites as promising Rashba semiconductors possess large Rashba constants and strong electric field responses, resulting in a short spin channel length of tens of nanometers to preserve the spin coherence in spin FETs, superior to conventional 3D micrometer spin FETs.

Quantifying Exciton Heterogeneities in Mixed-Phase Organometal Halide Multiple Quantum Wells via Stark Spectroscopy Studies

Solution-processable two-dimensional (2D) organic-inorganic hybrid perovskite (OIHP) quantum wells naturally self-assemble through weak van der Waals forces. In this study, we investigate the structural and optoelectronic properties of 2D-layered butylammonium (C₄H₉NH₃⁺, BA⁺) methylammonium (CH₃NH₃⁺, MA) lead iodide, (BA)₂(MA)_n-1Pb_nI_3n+1 quantum wells with varying n from 1 to 4. Through conventional structural characterization, (BA)₂(MA)_n-1Pb_nI_3n+1 thin films showcase high-quality phase (n) purity. However, while investigating the optoelectronic properties, it is clear that these van der Waals heterostructures consist of multiple quantum well thicknesses coexisting within a single thin film. We utilized electroabsorption spectroscopy and Liptay theory to develop an analytical tool capable of deconvoluting the excitonic features that arise from different quantum well thicknesses (n) in (BA)₂(MA)_n-1Pb_nI_3n+1 thin films. To obtain a quantitative assessment of exciton heterogeneities within a thin film comprising multiple quantum well structures, exciton resonances quantified by absorption spectroscopy were modeled as Gaussian features to yield various theory-generated electroabsorption spectra, which were then fit to our experimental electroabsorption features. In addition to identifying the quantum well heterostructures present within a thin film, this novel analytical tool provides powerful insights into the exact exciton composition and can be utilized to analyze the optoelectronic properties of many other mixed-phase quantum well heterostructures beyond those formed by OIHPs. Our findings may help in designing more efficient and reproducible light-emitting diodes based on 2D mixed-phase metal-organic multiple quantum wells.