Scaling law for excitons in 2D perovskite quantum wells

Circularly Polarized Photoluminescence from Chiral Perovskite Thin Films at Room Temperature

Hybrid organic-inorganic perovskites allow the synthesis of high-quality, nanostructured semiconducting films via easily accessible solution-based techniques. This has allowed tremendous development in optoelectronic applications, primarily solar cells and light-emitting diodes. Allowed by the ease of access to nanostructure, chirality has recently been introduced in semiconducting perovskites as a promising way to obtain advanced control of charge and spin and for developing circularly polarized light sources. Circular polarization of photoluminescence (CPL) is a powerful tool to probe the electronic structure of materials. However, CPL in chiral perovskites has been scarcely investigated, and a study in bulk thin films and at room temperature is still missing. In this work, we fabricate bromine-based chiral perovskites by using a bulky chiral organic cation mixed with CsBr, resulting in Ruddlesden-Popper perovskite thin films. We measure CPL on these films at room temperature and, by using unpolarized photoexcitation, we record a degree of circular polarization of photoluminescence in the order of 10^-3 and provide a full spectral characterization of CPL. Our results show that chirality is imparted on the electronic structure of the semiconductor; we hypothesize that the excess in polarization of emitted light originates from the charge in the photogenerated Wannier exciton describing an orbit in a symmetry-broken environment. Furthermore, our experiments allow the direct measurement of the magnetic dipole moment of the optical transition, which we estimate to be ≥0.1 μ_B. Finally, we discuss the implications of our findings on the development of chiral semiconducting perovskites as sources of circularly polarized light.

Terahertz Excitonics in Carbon Nanotubes: Exciton Autoionization and Multiplication

Excitons play major roles in optical processes in modern semiconductors, such as single-wall carbon nanotubes (CNTs), transition metal dichalcogenides, and 2D perovskite quantum wells. They possess extremely large binding energies (>100 meV), dominating absorption and emission spectra even at high temperatures. The large binding energies imply that they are stable, that is, hard to ionize, rendering them seemingly unsuited for optoelectronic devices that require mobile charge carriers, especially terahertz emitters and solar cells. Here, we have conducted terahertz emission and photocurrent studies on films of aligned single-chirality semiconducting CNTs and find that excitons autoionize, i.e., spontaneously dissociate into electrons and holes. This process naturally occurs ultrafast (<1 ps) while conserving energy and momentum. The created carriers can then be accelerated to emit a burst of terahertz radiation when a dc bias is applied, with promising efficiency in comparison to standard GaAs-based emitters. Furthermore, at high bias, the accelerated carriers acquire high enough kinetic energy to create secondary excitons through impact exciton generation, again in a fully energy and momentum conserving fashion. This exciton multiplication process leads to a nonlinear photocurrent increase as a function of bias. Our theoretical simulations based on nonequilibrium Boltzmann transport equations, taking into account all possible scattering pathways and a realistic band structure, reproduce all of our experimental data semiquantitatively. These results not only elucidate the momentum-dependent ultrafast dynamics of excitons and carriers in CNTs but also suggest promising routes toward terahertz excitonics despite the orders-of-magnitude mismatch between the exciton binding energies and the terahertz photon energies.

Exciton Polarons in Two-Dimensional Hybrid Metal-Halide Perovskites

While polarons, charges bound to a lattice deformation induced by electron-phonon coupling, are primary photoexcitations in bulk metal-halide hybrid organic-inorganic perovskites (HOIPs), excitons, Coulomb-bound electron-hole pairs, are the stable quasi-particles in their two-dimensional (2D) analogues. However, are polaronic effects consequential for excitons in 2D-HOIPs? We argue that they are manifested intrinsically in the exciton spectral structure, which is composed of multiple nondegenerate resonances with constant interpeak energy spacing. We highlight population and dephasing dynamics that point to the apparently deterministic role of polaronic effects. We contend that an interplay of long-range and short-range exciton-lattice couplings gives rise to exciton polarons, which fundamentally establishes their effective mass and radius and, consequently, their quantum dynamics. Finally, we highlight opportunities for the community to develop the rigorous description of exciton polarons in 2D-HOIPs to advance their fundamental understanding as model systems for condensed-phase materials with strong lattice-mediated correlations.

Extremely Long Spin Lifetime of Light-Emitting States in Quasi-2D Perovskites through Orbit-Orbit Interaction

This paper reports an extremely long spin relaxation time of optically polarized light-emitting states at room temperature in quasi-2D perovskites [(PEA)₂(MA)₄Pb₅Br₁₆ with n = 5], when the long-range orbit-orbit interaction between excited states is developed through orbital polarization. Our studies found that the quasi-2D perovskite [(PEA)₂(MA)₄Pb₅Br₁₆ with n = 5] demonstrates a long-range orbit-orbit interaction between excited states to conserve the spins of optically polarized light-emitting states, identified by the positive change on photoluminescence intensity (+ΔPL) in steady state upon switching the photoexcitation from linear to circular polarization. Meanwhile, the PL circular polarization (σ⁺σ⁺ - σ⁺σ^-) can maintain in nanosecond under fixed photoexcitation (σ⁺). In contrast, the 2D/3D mixed perovskite (n > 5) shows a short-range orbit-orbit interaction between excited states through orbital magnetic dipoles, identified by the -ΔPL by switching from linear to circular photoexcitation. At the same time, the spin lifetime of light-emitting states becomes undetectable.

Directional Light Emission from Layered Metal Halide Perovskite Crystals

Metal halide perovskites are being increasingly explored for use in light-emitting diodes (LEDs), with achievements in efficiency and brightness charted across the spectrum. One path to further boosting the fraction of useful photons generated by injected electrical charges will be to tailor the emission patterns of devices. Here we investigate directional emission from layered metal halide perovskites. We quantify the proportion of in-plane versus out-of-plane transition dipole components for a suite of layered perovskites. We find that certain perovskite single crystals have highly anisotropic emissions and up to 90% of their transition dipole in-plane. For thin films, emission anisotropy increases as the nominal layer thickness decreases and is generally greater with butylammonium cations than with phenethylammonium cations. Numerical simulations reveal that anisotropic emission from layered perovskites in thin-film LEDs may lead to external quantum efficiencies of 45%, an absolute gain of 13% over equivalent films with isotropic emitters.

Directional Anisotropy of the Vibrational Modes in 2D-Layered Perovskites

The vibrational modes in organic/inorganic layered perovskites are of fundamental importance for their optoelectronic properties. The hierarchical architecture of the Ruddlesden-Popper phase of these materials allows for distinct directionality of the vibrational modes with respect to the main axes of the pseudocubic lattice in the octahedral plane. Here, we study the directionality of the fundamental phonon modes in single exfoliated Ruddlesden-Popper perovskite flakes with polarized Raman spectroscopy at ultralow frequencies. A wealth of Raman bands is distinguished in the range from 15 to 150 cm^-1 (2-15 meV), whose features depend on the organic cation species, on temperature, and on the direction of the linear polarization of the incident light. By controlling the angle of the linear polarization of the excitation laser with respect to the in-plane axes of the octahedral layer, we gain detailed information on the symmetry of the vibrational modes. The choice of two different organic moieties, phenethylammonium (PEA) and butylammonium (BA), allows us to discern the influence of the linker molecules, evidencing strong anisotropy of the vibrations for the (PEA)₂PbBr₄ samples. Temperature-dependent Raman measurements reveal that the broad phonon bands observed at room temperature consist of a series of sharp modes and that such mode splitting strongly differs for the different organic moieties and vibrational bands. Softer molecules such as BA result in lower vibrational frequencies and splitting into fewer modes, while more rigid molecules such as PEA lead to higher frequency oscillations and larger number of Raman peaks at low temperature. Interestingly, in distinct bands the number of peaks in the Raman bands is doubled for the rigid PEA compared to the soft BA linkers. Our work shows that the coupling to specific vibrational modes can be controlled by the incident light polarization and choice of the organic moiety, which could be exploited for tailoring exciton-phonon interaction, and for optical switching of the optoelectronic properties of such 2D layered materials.

Overcoming the exciton binding energy in two-dimensional perovskite nanoplatelets by attachment of conjugated organic chromophores

In this work we demonstrate a novel approach to achieve efficient charge separation in dimensionally and dielectrically confined two-dimensional perovskite materials. Two-dimensional perovskites generally exhibit large exciton binding energies that limit their application in optoelectronic devices that require charge separation such as solar cells, photo-detectors and in photo-catalysis. Here, we show that by incorporating a strongly electron accepting moiety, perylene diimide organic chromophores, on the surface of the two-dimensional perovskite nanoplatelets it is possible to achieve efficient formation of mobile free charge carriers. These free charge carriers are generated with ten times higher yield and lifetimes of tens of microseconds, which is two orders of magnitude longer than without the peryline diimide acceptor. This opens a novel synergistic approach, where the inorganic perovskite layers are combined with functional organic chromophores in the same material to tune the properties for specific applications.

Optoelectronic Properties of Two-Dimensional Bromide Perovskites: Influences of Spacer Cations

Two-dimensional (2D) halide perovskites have displayed unique emission properties, making them potential candidates for next-generation light-emitting devices. Here, we combine nonadiabatic molecular dynamics and time-domain density functional theory to investigate the fundamental mechanisms of carrier recombination processes. Considering monolayer bromide perovskites with dissimilar organic spacer molecules, n-butylammonium (BA) and phenylethylammonium (PEA) cations, we find a strong correlation between temperature-induced structural fluctuations and nonradiative carrier recombination rates in these materials. The more flexible geometry of (BA)₂PbBr₄ compared to that of (PEA)₂PbBr₄, results in faster electron-hole recombination and shorter carrier lifetime, diminishing the photoluminescence quantum yield for softer 2D perovskites. Reduced structural fluctuations in relatively rigid (PEA)₂PbBr₄ not only indicate of a longer carrier lifetime but also suggest a narrower emission line width, implying a higher purity of the emitted light. Our ab initio modeling of excited state properties in 2D perovskites conveys material designing strategies to fine-tune perovskite emissions for solid-state lighting applications.

Band Alignment in Two-Dimensional Halide Perovskite Heterostructures: Type I or Type II?

Formation of heterostructures is often inevitable in two-dimensional (2D) halide perovskites and band alignment in 2D perovskite heterostructures is of central importance to their applications. However, controversies abound in literature on the band alignment of the 2D perovskite heterostructures. While external factors have been sought to reconcile the controversies, we show that the 2D perovskite heterostructures are in fact intrinsically prone to band "misalignment", driven by thermal fluctuations. Owing to the "softness" of inorganic layers in the perovskites, electron-phonon coupling at room temperature could be strong enough to override band offsets at zero temperature, leading to oscillatory band alignment between type-I and type-II at 300 K. We further demonstrate that by tuning the inorganic layers, one can increase the band offsets and stabilize the band alignment, paving the way for optoelectronic applications of the 2D perovskite heterostructures.

Frenkel-Holstein Hamiltonian applied to absorption spectra of quaterthiophene-based 2D hybrid organic-inorganic perovskites

For the prototypical two-dimensional hybrid organic-inorganic perovskites (2D HOIPs) (AE4T)PbX₄ (X = Cl, Br, and I), we demonstrate that the Frenkel-Holstein Hamiltonian (FHH) can be applied to describe the absorption spectrum arising from the organic component. We first model the spectra using only the four nearest neighbor couplings between translationally inequivalent molecules in the organic herringbone lattice as fitting parameters in the FHH. We next use linear-response time-dependent density functional theory (LR-TDDFT) to calculate molecular transition densities, from which extended excitonic couplings are evaluated based on the atomic positions within the 2D HOIPs. We find that both approaches reproduce the experimentally observed spectra, including changes in their shape and peak positions. The spectral changes are correlated with a decrease in excitonic coupling from X = Cl to X = I. Importantly, the LR-TDDFT-based approach with extended excitonic couplings not only gives better agreement with the experimental absorption line shape than the approach using a restricted set of fitted parameters but also allows us to relate the changes in excitonic coupling to the underlying geometry. We accordingly find that the decrease in excitonic coupling from X = Cl to Br to I is due to an increase in molecular separation, which in turn can be related to the increasing Pb-X bond length from Cl to I. Our research opens up a potential pathway to predicting optoelectronic properties of new 2D HOIPs from ab initio calculations and to gain insight into structural relations from 2D HOIP absorption spectra.

Ultrastable Laurionite Spontaneously Encapsulates Reduced-dimensional Lead Halide Perovskites

Reduced dimensional lead halide perovskites (RDPs) have attracted great research interest in diverse optical and optoelectronic fields. However, their poor stability is one of the most challenging obstacles prohibiting them from practical applications. Here, we reveal that ultrastable laurionite-type Pb(OH)Br can spontaneously encapsulate the RDPs in their formation solution without introducing any additional chemicals, forming RDP@Pb(OH)Br core-shell microparticles. Interestingly, the number of the perovskite layers within the RDPs can be conveniently and precisely controlled by varying the amount of CsBr introduced into the reaction solution. A single RDP@Pb(OH)Br core-shell microparticle composed of RDP nanocrystals with different numbers of perovskite layers can be also prepared, showing different colors under different light excitations. More interestingly, barcoded RDP@Pb(OH)Br microparticles with different parts emitting different lights can also be prepared. The morphology of the emitting microstructures can be conveniently manipulated. The RDP@Pb(OH)Br microparticles demonstrate outstanding environmental, chemical, thermal, and optical stability, as well as strong resistance to anion exchange processes. This study not only deepens our understanding of the reaction processes in the extensively used saturation recrystallization method but also points out that it is highly possible to dramatically improve the performance of the optoelectronic devices through manipulating the spontaneous formation process of Pb(OH)Br.

Charge Carrier Recombination Dynamics of Two-Dimensional Lead Halide Perovskites

Two-dimensional (2D) lead halide perovskites with better chemical stability and tunable dimensionality offer new opportunities to design optoelectronic devices. We have probed the transient absorption behavior of 2D lead halide (bromide and iodide) perovskites of different dimensionality, prepared by varying the ratio of methylammonium:phenylethylammonium cation. With decreasing dimensionality (n = ∞ → 1), we observe a blue shift in transient absorption bleach in agreement with the trend observed with the shift in the excitonic peak. The lifetime of the charge carriers decreased with decreasing layer thickness. The dependence of charge carrier lifetime on the 2D layers as well as the halide ion composition shows the dominance of excitonic binding energy on the charge carrier recombination in 2D perovskites. The excited-state behavior of 2D perovskites discussed in this study shows the need to modulate the layer dimensionality to obtain desired optoelectronic properties.

Structure-Electronic Property Relationships of 2D Ruddlesden-Popper Tin- and Lead-based Iodide Perovskites

Two-dimensional (2D) halide perovskites are receiving considerable attention for applications in photovoltaics, largely due to their versatile composition and superior environmental stability over three-dimensional (3D) perovskites, but show much lower power conversion efficiencies. Hence, further understanding of the structure-property relationships of these 2D materials is crucial for improving their photovoltaic performance. Here, we investigate by means of first-principles calculations the structural and electronic properties of 2D lead and tin Ruddlesden-Popper perovskites with general formula (BA)₂A_n-1B_nI_3n+1, where BA is the butylammonium organic spacer, A is either methylammonium (MA) or formamidinium (FA) cations, B represents Sn or Pb atoms, and n is the number of layers (n = 1, 2, 3, and 4). We show that the band gap progressively increases as the number of layers decreases in both Sn- and Pb-based materials. Through substituting MA by FA cations, the band gap slightly opens in the Sn systems and narrows in the Pb systems. The electron and hole carriers show small effective masses, which are lower than those of the corresponding 3D perovskites, suggesting high carrier mobilities. The structural distortion associated with the orientation of the MA or FA cations in the inorganic layers is found to be the driving force for the induced Rashba spin-splitting bands in the systems with more than one layer. From band alignment diagrams, the transfer process of the charge carriers in the 2D perovskites is found to be from smaller to higher number of layers n for electrons and oppositely for holes, in excellent agreement with experimental studies. We also find that, when interfaced with 3D analogues, the 2D perovskites could function as hole transport materials.

The strain effects in 2D hybrid organic-inorganic perovskite microplates: bandgap, anisotropy and stability

Strain engineering provides an efficient strategy to modulate the fundamental properties of semiconducting structures for use in functional electronic and optoelectronic devices. Here, we report on how the strain affects the bandgap, optical anisotropy and stability of two-dimensional (2D) perovskite (BA)₂(MA)_n-1Pb_nI_3n+1 (n = 1-3) microplates, using photoluminescence spectroscopy. Upon applying external strain, the bandgap decreases at a rate of -5.60/-2.74/-1.38 meV per % for n = 1, 2, and 3 2D perovskites, respectively. This change of the bandgap can be ascribed to the distortion of the octahedra (Pb-I bond contraction) in 2D perovskites, supported by a study on emission anisotropy, which increases with the increase of strain. In addition, the external strain can significantly deteriorate the stability of 2D perovskites due to the strain induced distortion which would make the penetration of moisture and oxygen into the perovskite microplates easier, resulting in much faster degradation rates. Our findings not only provide insights into the design and optimization of functional devices, but also provide a new approach to improve the stability of 2D perovskite based devices.

Interfacial Electromechanics Predicts Phase Behavior of 2D Hybrid Halide Perovskites

Quasi-two-dimensional (2D) mixed-cation hybrid halide perovskites (A'₂A_N-1M_NX_3N+1; A' = large organic molecule with cationic group, A = [Cs⁺, CH₃NH₃⁺, HC(NH₂)₂⁺], M = [Pb, Sn, Ge], X = [I^-, Br^-, Cl^-]) have rapidly emerged as candidates to improve the structural stability and device lifetime of 3D perovskite semiconductor devices under operating conditions. The addition of the large A' cation to the traditional AMX₃ structure introduces several synthetic degrees of freedom and breaks M-X bonds, giving rise to peculiar critical phase behavior in the phase space of these complex materials. In this work, we propose a thermodynamic model parametrized by first-principles calculations to generate the phase diagram of 2D and quasi-2D perovskites (q-2DPKs) based on the mechanics and electrostatics of the interface between the A' cations and the metal halide octahedral network. Focusing on the most commonly studied methylammonium lead iodide system where A' is n-butylammonium (BA; CH₃(CH₂)₃NH₃⁺), we find that the apparent difficulty in synthesizing phase-pure samples with a stoichiometric index N > 5 can be attributed to the energetic competition between repulsion of opposing interfacial dipole layers and mechanical relaxation induced by interfacial stress. Our model shows quantitative agreement with experimental observations of the maximum phase-pure stoichiometric index (N_crit) and explains the nonmonotonic evolution of the lattice parameters with increasing stoichiometric index (N). This model is generalizable to the entire family of q-2DPKs and can guide the design of photovoltaic and optical materials that combine the structural stability of the q-2DPKs while retaining the charge carrier properties of their 3D counterparts.

Origin of Rashba Spin-Orbit Coupling in 2D and 3D Lead Iodide Perovskites

We studied spin dynamics of charge carriers in the superlattice-like Ruddlesden-Popper hybrid lead iodide perovskite semiconductors, 2D (BA)₂(MA)Pb₂I₇ (with MA = CH₃NH₃, and BA = CH₃(CH₂)₃NH₃), and 3D MAPbI₃ using the magnetic field effect (MFE) on conductivity and electroluminescence in their light emitting diodes (LEDs) at cryogenic temperatures. The semiconductors with distinct structural/bulk inversion symmetry breaking, when combined with colossal intrinsic spin-orbit coupling (SOC), theoretically give rise to giant Rashba-type SOC. We found that the magneto-conductance (MC) magnitude increases monotonically with the emission intensity and saturates at ≈0.05% and 0.11% for the MAPbI₃ and (BA)₂(MA)Pb₂I₇, respectively. The magneto-electroluminescence (MEL) response with similar line shapes as the MC response has a significantly larger magnitude, and essentially stays constant at ≈0.22% and ≈0.20% for MAPbI₃ and (BA)₂(MA)Pb₂I₇, respectively. The sign and magnitude of the MC and MEL responses can be quantitatively explained in the framework of the Δg-based excitonic model using rate equations. Remarkably, the width of the MEL response in those materials linearly increases with increasing the applied electric field, where the Rashba coefficient in (BA)₂(MA)Pb₂I₇ is estimated to be about 7 times larger than that in MAPbI₃. Our studies might have significant impact on future development of electrically-controlled spin logic devices via Rashba-like effects.

Control of Crystal Symmetry Breaking with Halogen-Substituted Benzylammonium in Layered Hybrid Metal-Halide Perovskites

Layered hybrid metal-halide perovskites with non-centrosymmetric crystal structure are predicted to show spin-selective band splitting from Rashba effects. Thus, fabrication of metal-halide perovskites with defined crystal symmetry is desired to control the spin-splitting in their electronic states. Here, we report the influence of halogen para-substituents on the crystal structure of benzylammonium lead iodide perovskites (4-XC₆H₄CH₂NH₃)₂PbI₄ (X = H, F, Cl, Br). Using X-ray diffraction and second-harmonic generation, we study structure and symmetry of single-crystal and thin-film samples. We report that introduction of a halogen atom lowers the crystal symmetry such that the chlorine- and bromine-substituted structures are non-centrosymmetric. The differences can be attributed to the nature of the intermolecular interactions between the organic molecules. We calculate electronic band structures and find good control of Rashba splittings. Our results present a facile approach to tailor hybrid layered metal halide perovskites with potential for spintronic and nonlinear optical applications.

Understanding of perovskite crystal growth and film formation in scalable deposition processes

Hybrid organic-inorganic perovskite photovoltaics (PSCs) have attracted significant attention during the past decade. Despite the stellar rise of laboratory-scale PSC devices, which have reached a certified efficiency over 25% to date, there is still a large efficiency gap when transiting from small-area devices to large-area solar modules. Efficiency losses would inevitably arise from the great challenges of homogeneous coating of large-area high quality perovskite films. To address this problem, we provide an in-depth understanding of the perovskite nucleation and crystal growth kinetics, including the LaMer and Ostwald ripening models, which advises us that fast nucleation and slow crystallization are essential factors in forming high-quality perovskite films. Based on these cognitions, a variety of thin film engineering approaches will be introduced, including the anti-solvent, gas-assisted and solvent annealing treatments, Lewis acid-base adduct incorporation, etc., which are able to regulate the nucleation and crystallization steps. Upscaling the photovoltaic devices is the following step. We summarize the currently developed scalable deposition technologies, including spray coating, slot-die coating, doctor blading, inkjet printing and vapour-assisted deposition. These are more appealing approaches for scalable fabrication of perovskite films than the spin coating method, in terms of lower material/solution waste, more homogeneous thin film coating over a large area, and better morphological control of the film. The working principles of these techniques will be provided, which direct us that the physical properties of the precursor solutions and surface characteristics/temperature of the substrate are both dominating factors influencing the film morphology. Optimization of the perovskite crystallization and film formation process will be subsequently summarized from these aspects. Additionally, we also highlight the significance of perovskite stability, as it is the last puzzle to realize the practical applications of PSCs. Recent efforts towards improving the stability of PSC devices to environmental factors are discussed in this part. In general, this review, comprising the mechanistic analysis of perovskite film formation, thin film engineering, scalable deposition technologies and device stability, provides a comprehensive overview of the current challenges and opportunities in the field of PSCs, aiming to promote the future development of cost-effective up-scale fabrication of highly efficient and ultra-stable PSCs for practical applications.

Non-Preheating Processed Quasi-2D Perovskites for Efficient and Stable Solar Cells

Although the hot-casting (HC) technique is prevalent in developing preferred crystal orientation of quasi-2D perovskite films, the difficulty of accurately controlling the thermal homogeneity of substrate is unfavorable for the reproducibility of device fabrication. Herein, a facile and effective non-preheating (NP) film-casting method is proposed to realize highly oriented quasi-2D perovskite films by replacing the butylammonium (BA⁺ ) spacer partially with methylammonium (MA⁺ ) cation as (BA)_{2- x} (MA)_{3+ x} Pb₄ I₁₃ (x = 0, 0.2, 0.4, and 0.6). At the optimal x-value of 0.4, the resultant quasi-2D perovskite film possesses highly orientated crystals, associated with a dense morphology and uniform grain-size distribution. Consequently, the (BA)_1.6 (MA)_3.4 Pb₄ I₁₃ -based solar cells yield champion efficiencies of 15.44% with NP processing and 16.29% with HC processing, respectively. As expected, the HC-processed device shows a poor performance reproducibility compared with that of the NP film-casting method. Moreover, the unsealed device (x = 0.4) displays a better moisture stability with respect to the x = 0 stored in a 65% ± 5% relative humility chamber.

Anisotropy of Excitons in Two-Dimensional Perovskite Crystals

Two-dimensional (2D) perovskites show great potential for optoelectronic applications due to their bandgap tunability, extremely large excition binding energy, and large crystal anisotropy compared with their three-dimensional counterparts. To fully explore exciton-based applications and improve their performance, it is essential to understand the exciton behavior in 2D perovskites. Here, we investigate exciton anisotropy within the crystallographic plane and cross plane of (C₄H₉NH₃)₂PbI₄ 2D perovskite crystals by polarization-resolved photoluminescence, reflection, and photoconductivity studies. We observe a polarization-dependent emission evolution and an enhanced self-trapped exciton emission with an oblique incident excitation from the cross plane. Furthermore, the anisotropy of excitons in (C₄H₉NH₃)₂PbI₄ 2D perovskite crystals is identified by polarization-resolved photoluminescence and photoconductivity measurement, and a completely opposite polarization-dependent behavior was observed for free excitons and self-trapped excitons. We attribute this different anisotropy to the existence of out-of-plane excitons and different optical selection rule for free excitons and self-trapped excitons. Our findings will shed light on designing and improving the performance of exciton-based optoelectronic devices in 2D perovskites.

Linear and nonlinear optical probing of various excitons in 2D inorganic-organic hybrid structures

Nonlinear optical properties, such as two-(or multi-) photon absorption (2PA), are of special interest for technologically important applications in fast optical switching, in vivo imaging and so on. Highly intense infrared ultrashort pulses probe deep into samples and reveal several underlying structural perturbations (inter-layer distortions, intra-layer crumpling) and also provide information about new excited states and their relaxation. Naturally self-assembled inorganic-organic multiple quantum wells (IO-MQWs) show utility from room-temperature exciton emission features (binding energies ~200–250 meV). These Mott type excitons are highly sensitive to the self-assembly process, inorganic network distortions, thickness and inter-layer distortions of these soft two-dimensional (2D) and weak van der Waal layered hybrids. We demonstrate strong room-temperature nonlinear excitation intensity dependent two-photon absorption induced exciton photoluminescence (2PA-PL) from these IO-MQWs, excited by infrared femtosecond laser pulses. Strongly confined excitons show distinctly different one- and two-photon excited photoluminescence energies: from free-excitons (2.41 eV) coupled to the perfectly aligned MQWs and from energy down-shifted excitons (2.33 eV) that originate from the locally crumpled layered architecture. High intensity femtosecond induced PL from one-photon absorption (1PA-PL) suggests saturation of absorption and exciton-exciton annihilation, with typical reduction in PL radiative relaxation times from 270 ps to 190 ps upon increasing excitation intensities. From a wide range of IR excitation tuning, the origin of 2PA-PL excitation is suggested to arise from exciton dark states which extend below the bandgap. Observed two-photon absorption coefficients (β ~75 cm/GW) and two-photon excitation cross-sections (η₂σ₂ ~ 110GM), further support the evidence for 2PA excitation origin. Both 1PA- and 2PA-PL spatial mappings over large areas of single crystal platelets demonstrate the co-existence of both free and deep-level crumpled excitons with some traces of defect-induced trap state emission. We conclude that the two-photon absorption induced PL is highly sensitive to the self-assembly process of few to many mono layers, the crystal packing and deep level defects. This study paves a way to tailor the nonlinear properties of many 2D material classes. Our results thus open new avenues for exploring fundamental phenomena and novel optoelectronic applications using layered inorganic-organic and other metal organic frameworks.

Alkali Cation Doping for Improving the Structural Stability of 2D Perovskite in 3D/2D PSCs

3D/2D hybrid perovskite systems have been intensively investigated to improve the stability of perovskite solar cells (PSCs), whereas undesired crystallization of 2D perovskite during the film formation process could undermine the structural stability of 2D perovskite materials, which causes serious hysteresis of PSCs after aging. This issue is, however, rarely studied. The stability study for 3D/2D hybrid systems to date is all under the one-direction scan, and the lack of detailed information on the hysteresis after aging compromises the credibility of the stability results. In this work, by correlating the hysteresis of the hybrid PSCs with the 2D crystal structure, we find that the prompt 2D perovskite formation process easily induces numerous crystal imperfections and structural defects. These defects are susceptible to humidity attack and decompose the 2D perovskite to insulating long-chain cations and 3D perovskite, which hinder charge transfer or generate charge accumulation. Therefore, a large hysteresis is exhibited after aging the 3D/2D hybrid PSCs in an ambient environment, even though the reverse-scan power conversion efficiency (PCE) is found to be well-preserved. To address this issue, alkali cations, K⁺ and Rb⁺, are introduced into the 2D perovskite to exquisitely modulate the crystal formation, which gives rise to a higher crystallinity of 2D perovskite and a better film morphology with fewer defects. We achieved PCE beyond 21% due to the preferable charge transfer process and reduced nonradiative recombination losses. The structural features also bring about impressive moisture stability, which results in the corresponding PSCs retaining 93% of its initial PCE and negligible hysteresis after aging in an ambient atmosphere for 1200 h.

Transition Dipole Moments of n = 1, 2, and 3 Perovskite Quantum Wells from the Optical Stark Effect and Many-Body Perturbation Theory

Metal halide perovskite quantum wells (PQWs) are quantum and dielectrically confined materials exhibiting strongly bound excitons. The exciton transition dipole moment dictates absorption strength and influences interwell coupling in dipole-mediated energy transfer, a process that influences the performance of PQW optoelectronic devices. Here we use transient reflectance spectroscopy with circularly polarized laser pulses to investigate the optical Stark effect in dimensionally pure single crystals of n = 1, 2, and 3 Ruddlesden-Popper PQWs. From these measurements, we extract in-plane transition dipole moments of 11.1 (±0.4), 9.6 (±0.6) and 13.0 (±0.8) D for n = 1, 2 and 3, respectively. We corroborate our experimental results with density functional and many-body perturbation theory calculations, finding that the nature of band edge orbitals and exciton wave function delocalization depends on the PQW "odd-even" symmetry. This accounts for the nonmonotonic relationship between transition dipole moment and PQW dimensionality in the n = 1-3 range.

Inducing Charge Separation in Solid-State Two-Dimensional Hybrid Perovskites through the Incorporation of Organic Charge-Transfer Complexes

Two-dimensional (2D) hybrid perovskites make up an emerging class of materials for optoelectronic applications in which inorganic octahedral layers are separated by nonconductive large organic cations. This leads to a high-dimensional and dielectric confinement and hence a high exciton binding energy, which severely limits their application in devices in which charge carrier separation is required. In this work, we achieve improved charge separation by replacing nonconductive organic cations with organic charge-transfer complexes consisting of a pyrene donor and a tetracyanoquinodimethane acceptor. Steady-state absorption measurements show that these materials exhibit optical features that match with the absorption of the organic charge-transfer complexes. Using microwave conductivity and femtosecond transient absorption, we show that photoexcitation of these charge-transfer states leads to long-lived mobile charges in the inorganic layers. While the efficiency of charge separation is relatively low, these experiments demonstrate that it is possible to induce charge separation in solid-state 2D perovskites by engineering the organic layer.

Bright magnetic dipole radiation from two-dimensional lead-halide perovskites

Light-matter interactions in semiconductors are uniformly treated within the electric dipole approximation; multipolar interactions are considered "forbidden." We experimentally demonstrate that this approximation inadequately describes light emission in two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs), solution processable semiconductors with promising optoelectronic properties. By exploiting the highly oriented crystal structure, we use energy-momentum spectroscopies to demonstrate that an exciton-like sideband in 2D HOIPs exhibits a multipolar radiation pattern with highly directed emission. Electromagnetic and quantum-mechanical analyses indicate that this emission originates from an out-of-plane magnetic dipole transition arising from the 2D character of electronic states. Symmetry arguments and temperature-dependent measurements suggest a dynamic symmetry-breaking mechanism that is active over a broad temperature range. These results challenge the paradigm of electric dipole-dominated light-matter interactions in optoelectronic materials, provide new perspectives on the origins of unexpected sideband emission in HOIPs, and tease the possibility of metamaterial-like scattering phenomena at the quantum-mechanical level.

Long-range exciton transport and slow annihilation in two-dimensional hybrid perovskites

Two-dimensional hybrid organic-inorganic perovskites with strongly bound excitons and tunable structures are desirable for optoelectronic applications. Exciton transport and annihilation are two key processes in determining device efficiencies; however, a thorough understanding of these processes is hindered by that annihilation rates are often convoluted with exciton diffusion constants. Here we employ transient absorption microscopy to disentangle quantum-well-thickness-dependent exciton diffusion and annihilation in two-dimensional perovskites, unraveling the key role of electron-hole interactions and dielectric screening. The exciton diffusion constant is found to increase with quantum-well thickness, ranging from 0.06 ± 0.03 to 0.34 ± 0.03 cm² s⁻¹, which leads to long-range exciton diffusion over hundreds of nanometers. The exciton annihilation rates are more than one order of magnitude lower than those found in the monolayers of transition metal dichalcogenides. The combination of long-range exciton transport and slow annihilation highlights the unique attributes of two-dimensional perovskites as an exciting class of optoelectronic materials.

Two-dimensional hybrid perovskites are promising excitonic materials; however, there currently lacks understanding on exciton diffusion and annihilation. Here Deng et al. employ transient absorption microscopy to disentangle quantum-well-thickness-dependent exciton transport and annihilation in these materials.

Templated growth of oriented layered hybrid perovskites on 3D-like perovskites

The manipulation of crystal orientation from the thermodynamic equilibrium states is desired in layered hybrid perovskite films to direct charge transport and enhance the perovskite devices performance. Here we report a templated growth mechanism of layered perovskites from 3D-like perovskites which can be a general design rule to align layered perovskites along the out-of-plane direction in films made by both spin-coating and scalable blading process. The method involves suppressing the nucleation of both layered and 3D perovskites inside the perovskite solution using additional ammonium halide salts, which forces the film formation starts from solution surface. The fast drying of solvent at liquid surface leaves 3D-like perovskites which surprisingly templates the growth of layered perovskites, enabled by the periodic corner-sharing octahedra networks on the surface of 3D-like perovskites. This discovery provides deep insights into the nucleation behavior of octahedra-array-based perovskite materials, representing a general strategy to manipulate the orientation of layered perovskites.

The orientation of layered perovskites plays a crucial role in their charge transport behavior and hence, the efficiency of related solar cells. Here, the authors find that preformed 3D-like perovskites can efficiently template the growth of layered perovskites and determine their orientation.

Fluorination of Organic Spacer Impacts on the Structural and Optical Response of 2D Perovskites

Low-dimensional hybrid perovskites have triggered significant research interest due to their intrinsically tunable optoelectronic properties and technologically relevant material stability. In particular, the role of the organic spacer on the inherent structural and optical features in two-dimensional (2D) perovskites is paramount for material optimization. To obtain a deeper understanding of the relationship between spacers and the corresponding 2D perovskite film properties, we explore the influence of the partial substitution of hydrogen atoms by fluorine in an alkylammonium organic cation, resulting in (Lc)₂PbI₄ and (Lf)₂PbI₄ 2D perovskites, respectively. Consequently, optical analysis reveals a clear 0.2 eV blue-shift in the excitonic position at room temperature. This result can be mainly attributed to a band gap opening, with negligible effects on the exciton binding energy. According to Density Functional Theory (DFT) calculations, the band gap increases due to a larger distortion of the structure that decreases the atomic overlap of the wavefunctions and correspondingly bandwidth of the valence and conduction bands. In addition, fluorination impacts the structural rigidity of the 2D perovskite, resulting in a stable structure at room temperature and the absence of phase transitions at a low temperature, in contrast to the widely reported polymorphism in some non-fluorinated materials that exhibit such a phase transition. This indicates that a small perturbation in the material structure can strongly influence the overall structural stability and related phase transition of 2D perovskites, making them more robust to any phase change. This work provides key information on how the fluorine content in organic spacer influence the structural distortion of 2D perovskites and their optical properties which possess remarkable importance for future optoelectronic applications, for instance in the field of light-emitting devices or sensors.

Large-Scale Growth of Ultrathin Low-Dimensional Perovskite Nanosheets for High-Detectivity Photodetectors

Low-dimensional organic-inorganic hybrid perovskites have demonstrated to be promising semiconductor materials due to their unique optoelectronic properties, however, the controllable growth of high-quality ultrathin 2D perovskites with large lateral dimension still faces great challenges. Herein, we report the controllable growth of large-scale ultrathin 2D (C₆H₅(CH₂)₃NH₃)₃Pb₂I₇ ((PPA)₃Pb₂I₇) perovskite nanosheets (NSs) using a facile antisolvent-assisted crystallization approach under mild condition. As a result, the well-defined regular-shaped (PPA)₃Pb₂I₇ NSs, with the largest lateral size over 100 μm, have been successfully synthesized, which is more than several ten times larger than that of other 2D perovskite NSs previously reported. Moreover, the thickness of the achieved 2D perovskite NSs can be well-tuned by altering the concentration of the precursor solution, with the smallest thickness down to ∼4.7 nm. More importantly, the photodetectors based on the high-quality (PPA)₃Pb₂I₇ perovskites exhibit fascinating performance, including an extremely low dark current (∼1.5 pA), fast response/recovery rate (∼850/780 μs), and high detectivity (∼1.2 × 10¹⁰ Jones). This work provides a simple and promising strategy to controllably grow large-scale and ultrathin 2D perovskite NSs for low-cost and high-performance optoelectronic devices.

Stacking of Layered Halide Perovskite from Incorporating a Diammonium Cation into Three-Dimensional Perovskites

Quasi two-dimensional (2D) layered perovskites have been emerging as promising candidates for photovoltaic cells because they exhibit intrinsic stability and a higher tunability of optical properties compared to three-dimensional (3D) perovskites. However, since most 2D perovskites have bulkier groups as an organic space group, they will inevitably have a van der Waals gap between the inorganic layers and their crystal growth directions orient in a lateral direction. It also interrupts carrier transport across the conducting inorganic layer in the solar cell. Here, we presents the new homologous 2D layered perovskites, (HA)(A)_n-1Pb_nI_3n+1, where HA stands for the histammonium ((C₅N₃H₁₁)²⁺) as a diammonium cation and A stands for methylammonium (CH₃NH₃⁺) or formammonium (HC(NH₂)₂⁺). Since the ditopic HA has a diammoinium cation, it connects the inorganic slabs stacked in the vertical direction. The inorganic layer is stacked on the other layer to form a layered structure, which results in rigid and stable structures. These materials (1.64 eV for (HA)(FA)_n-1Pb_nI_3n+1 and 1.80 eV for (HA)(MA)_n-1Pb_nI_3n+1) have significantly lower band gaps than those of HAPbI₄ (2.20 eV). Compared to the pure 2D and 3D perovskites, these perovskites have a longer electron lifetime due to the vertical crystal structure and show improved environmental stability for perovskite solar cell application.

Properties of Excitons and Photogenerated Charge Carriers in Metal Halide Perovskites

Metal halide perovskites (MHPs) have recently attracted great attention from the scientific community due to their excellent photovoltaic performance as well as their tremendous potential for other optoelectronic applications such as light-emitting diodes, lasers, and photodetectors. Despite the rapid progress in device applications, a solid understanding of the photophysical properties behind the device performance is highly desirable for MHPs. Here, the properties of excitons and photogenerated charge carriers in MHPs are explored. The unique dielectric constant properties, crystal-liquid duality, and fundamental optical processes of MHPs are first discussed. The properties of excitons and related phenomena in MHPs are then detailed, including the exciton binding energy determined by various methods and their influence factors, exciton dynamics, exciton-photon coupling and related applications, and exciton-phonon coupling in MHPs. The properties of photogenerated free charge carriers in MHPs such as the carrier diffusion length, mobility, and recombination are described. Recent progress in various applications is also demonstrated. Finally, a conclusion and perspectives of future studies for MHPs are presented.

2D Perovskites with Giant Excitonic Optical Nonlinearities for High-Performance Sub-Bandgap Photodetection

Two-dimensional (2D) perovskites have proved to be promising semiconductors for photovoltaics, photonics, and optoelectronics. Here, a strategy is presented toward the realization of highly efficient, sub-bandgap photodetection by employing excitonic effects in 2D Ruddlesden-Popper-type halide perovskites (RPPs). On near resonance with 2D excitons, layered RPPs exhibit degenerate two-photon absorption (D-2PA) coefficients as giant as 0.2-0.64 cm MW^{- 1} . 2D RPP-based sub-bandgap photodetectors show excellent detection performance in the near-infrared (NIR): a two-photon-generated current responsivity up to 1.2 × 10⁴ cm² W^-2 s^-1 , two orders of magnitude greater than InAsSbP-pin photodiodes; and a dark current as low as 2 pA at room temperature. More intriguingly, layered-RPP detectors are highly sensitive to the light polarization of incoming photons, showing a considerable anisotropy in their D-2PA coefficients (β_[001] /β_[011] = 2.4, 70% larger than the ratios reported for zinc-blende semiconductors). By controlling the thickness of the inorganic quantum well, it is found that layered RPPs of (C₄ H₉ NH₃ )₂ (CH₃ NH₃ )Pb₂ I₇ can be utilized for three-photon photodetection in the NIR region.

Compositional Control in 2D Perovskites with Alternating Cations in the Interlayer Space for Photovoltaics with Efficiency over 18

2D perovskites stabilized by alternating cations in the interlayer space (ACI) represent a very new entry as highly efficient semiconductors for solar cells approaching 15% power conversion efficiency (PCE). However, further improvements will require understanding of the nature of the films, e.g., the thickness distribution and charge-transfer characteristics of ACI quantum wells (QWs), which are currently unknown. Here, efficient control of the film quality of ACI 2D perovskite (GA)(MA)_n Pb_n I_{3 n +1} (〈n〉 = 3) QWs via incorporation of methylammonium chloride as an additive is demonstrated. The morphological and optoelectronic characterizations unambiguously demonstrate that the additive enables a larger grain size, a smoother surface, and a gradient distribution of QW thickness, which lead to enhanced photocurrent transport/extraction through efficient charge transfer between low-n and high-n QWs and suppressed nonradiative charge recombination. Therefore, the additive-treated ACI perovskite film delivers a champion PCE of 18.48%, far higher than the pristine one (15.79%) due to significant improvements in open-circuit voltage and fill factor. This PCE also stands as the highest value for all reported 2D perovskite solar cells based on the ACI, Ruddlesden-Popper, and Dion-Jacobson families. These findings establish the fundamental guidelines for the compositional control of 2D perovskites for efficient photovoltaics.

Optical Properties of Layered Hybrid Organic-Inorganic Halide Perovskites: A Tight-Binding GW-BSE Study

We present a many-body calculation of the band structure and optical spectrum of the layered hybrid organic-inorganic halide perovskites in the Ruddlesden-Popper phase with the general formula A₂^'A_n-1M_nX_3n+1, where n controls the thickness of the primarily inorganic perovskite layers. We calculate the mean-field band structure with spin-orbit coupling, quasi-particle corrections within the GW approximation, and optical spectra using the Bethe-Salpeter equation. The model is parametrized by first-principles calculations and classical electrostatic screening, enabling an accurate but cost-effective study of large unit cells and corresponding n-dependent properties. A transition of the electronic and optical properties from quasi-two-dimensional behavior to three-dimensional behavior is shown for increasing n, and the nonhydrogenic character of the excitonic Rydberg series is analyzed. For methylammonium lead iodide perovskites with butylammonium spacers, our n-dependent 1s and 2s exciton energy levels are in good agreement with those from recently reported experiments, and the 1s exciton binding energy is calculated to be 302 meV for n = 1, 97 meV for n = 5, and 37 meV for n = ∞ (bulk MAPbI₃). A calculation for an exfoliated n = 1 bilayer predicts a very large 1s exciton binding energy of 444 meV.

Optical deformation potential and self-trapped excitons in 2D hybrid perovskites

Self-trapped excitons (STEs) in two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) emit broadband white light, suggesting the great potential of 2D HOIPs in low-cost lighting and display applications. A prerequisite for understanding STEs' properties is a correct identification of the underlying interaction that leads to the STEs. Here, we show that the long-range polar coupling between electrons and optical phonons is quenched in 2D HOIPs' tightly bound excitons and cannot effect STEs. Rather, the STEs are induced by a short-range optical deformation potential (ODP) arising from the phonon-modulated Pb-X quantum-well thickness. Interaction between transition dipoles in adjacent PbX6 (X = Br or I) octahedra gives rise to highly anisotropic intra- and inter-layer exciton bandwidths. In flat (001) 2D HOIPs, both the ODP and the exciton bandwidths are susceptible to out-of-plane PbX6 tilting but not to the in-plane one, and their interplay can quantitatively account for the observed temperature and structure dependences of luminescence associated with STEs. In corrugated (011) 2D HOIPs, the exciton bandwidth is further reduced and the resultant STEs have a stronger lattice distortion and broader luminescence spectrum. Our results reveal the mechanism of STE formation and suggest ways of tuning STEs and associated broadband luminescence in 2D HOIPs.

Energy Spectrum of Two-Dimensional Excitons in a Nonuniform Dielectric Medium

We demonstrate that, in monolayers (MLs) of semiconducting transition metal dichalcogenides, the s-type Rydberg series of excitonic states follows a simple energy ladder: ε_{n}=-Ry^{*}/(n+δ)^{2}, n=1,2,…, in which Ry^{*} is very close to the Rydberg energy scaled by the dielectric constant of the medium surrounding the ML and by the reduced effective electron-hole mass, whereas the ML polarizability is accounted for only by δ. This is justified by the analysis of experimental data on excitonic resonances, as extracted from magneto-optical measurements of a high-quality WSe_{2} ML encapsulated in hexagonal boron nitride (hBN), and well reproduced with an analytically solvable Schrödinger equation when approximating the electron-hole potential in the form of a modified Kratzer potential. Applying our convention to other MoSe_{2}, WS_{2}, MoS_{2} MLs encapsulated in hBN, we estimate an apparent magnitude of δ for each of the studied structures. Intriguingly, δ is found to be close to zero for WSe_{2} as well as for MoS_{2} monolayers, what implies that the energy ladder of excitonic states in these two-dimensional structures resembles that of Rydberg states of a three-dimensional hydrogen atom.

Layer-Dependent Coherent Acoustic Phonons in Two-Dimensional Ruddlesden-Popper Perovskite Crystals

By combining femtosecond transient reflectance (TR) spectroscopy and density functional theory (DFT) calculations, we reveal the impact of the length of the organic linkers (HOC₂H₄NH₃⁺ and C₆H₅C₂H₄NH₃⁺) and the number of inorganic layers (n = 1-3) on the hot carrier relaxation dynamics and coherent acoustic phonons in 2D Ruddlesden-Popper (RP) perovskites. We find that the interplay between the hot carriers and the coherent longitudinal acoustic phonons (CLAPs) can extend the oscillation of the TR kinetics to nanoseconds, which could lead to the higher thermal conductivities of 2D RP perovskites. Moreover, we find that the frequency of the acoustic phonon oscillation and phonon velocity decreases with the increasing number of layers due to the increased mass of the inorganic layers and reduced electron-phonon coupling. This finding provides new physical insights into how the organic spacers and number of inorganic layers control the overall carrier dynamics of 2D perovskite materials.

Exciton-Exciton Annihilation in Two-Dimensional Halide Perovskites at Room Temperature

Recently, Ruddlesden-Popper 2D perovskite (RPP) solar cells and light-emitting diodes (LEDs) have shown promising efficiencies and improved stability in comparison to 3D halide perovskites. Here, the exciton recombination dynamics is investigated at room temperature in pure-phase RPP crystals (C₆H₅C₂H₄NH₃)₂(CH₃NH₃)_n-1Pb_nI_3n+1 (n = 1, 2, 3, and 4) by time-resolved photoluminescence (TRPL) in a large range of power excitations. As the number of perovskite layers increases, we detect the presence of an increasing fraction of out-of-equilibrium free carriers just after photoexcitation, on a picosecond time scale, while the dynamics is characterized by the recombination of excitons with long lifetime spanning several tens of nanoseconds. At low excitation power, the TRPL decays are nonexponential because of defect-assisted recombination. At high fluence, defects are filled and many-body interactions become important. Similar to other 2D systems, exciton-exciton annihilation (EEA) is then the dominant recombination path in a high-density regime below the Mott transition.

Lasing from Mechanically Exfoliated 2D Homologous Ruddlesden-Popper Perovskite Engineered by Inorganic Layer Thickness

2D Ruddlesden-Popper perovskites (RPPs) have aroused growing attention in light harvesting and emission applications owing to their high environmental stability. Recently, coherent light emission of RPPs was reported, however mostly from inhomologous thin films that involve cascade intercompositional energy transfer. Lasing and fundamental understanding of intrinsic laser dynamics in homologous RPPs free from intercompositional energy transfer is still inadequate. Herein, the lasing and loss mechanisms of homologous 2D (BA)₂ (MA)_{n -1} Pb_n I_{3 n +1} RPP thin flakes mechanically exfoliated from the bulk crystal are reported. Multicolor lasing is achieved from the large-n RPPs (n ≥ 3) in the spectral range of 620-680 nm but not from small-n RPPs (n ≤ 2) even down to 78 K. With decreasing n, the lasing threshold increases significantly and the characteristic temperature decreases as 49, 25, and 20 K for n = 5, 4, and 3, respectively. The n-engineered lasing behaviors are attributed to the stronger Auger recombination and exciton-phonon interaction as a result of the enhanced quantum confinement in the smaller-n perovskites. These results not only advance the fundamental understanding of loss mechanisms in both inhomologous and homologous RPP lasers but also provide insights into developing low-threshold, substrate-free, and multicolor 2D semiconductor microlasers.

Aqueous Synthesis of Low-Dimensional Lead Halide Perovskites for Room-Temperature Circularly Polarized Light Emission and Detection

Low-dimensional lead halide perovskite materials are an emerging class of solution-processable semiconductors with promising potential applications in optoelectronic devices. Unfortunately, it is impossible to synthesize high-crystalline-quality low-dimensional perovskite single crystals without using chemotoxic solutions such as dimethylformamide/dimethyl sulfoxide or applying heating. Herein we report an economical and universal aqueous method to synthesize 2D layered and 1D chain perovskite single crystals at room temperature. The resultant chiral 2D perovskites can efficiently and selectively emit and detect circularly polarized light at room temperature. The as-synthesized 1D perovskite single crystals exhibit strong quantum confinement and enhanced self-trapped states that give efficient warm circularly polarized white-light emission. This aqueous synthetic method is general for other high-quality low-dimensional lead halide perovskite single crystals, and thus our findings would motivate more fundamental investigations on low-dimensional perovskites for potential optoelectronic applications.

Ligand-Induced Surface Charge Density Modulation Generates Local Type-II Band Alignment in Reduced-Dimensional Perovskites

Two-dimensional (2D) and quasi-2D perovskite materials have enabled advances in device performance and stability relevant to a number of optoelectronic applications. However, the alignment among the bands of these variably quantum confined materials remains a controversial topic: there exist multiple experimental reports supporting type-I, and also others supporting type-II, band alignment among the reduced-dimensional grains. Here we report a combined computational and experimental study showing that variable ligand concentration on grain surfaces modulates the surface charge density among neighboring quantum wells. Density functional theory calculations and ultraviolet photoelectron spectroscopy reveal that the effective work function of a given quantum well can be varied by modulating the density of ligands at the interface. These induce type-II interfaces in otherwise type-I aligned materials. By treating 2D perovskite films, we find that the effective work function can indeed be shifted down by up to 1 eV. We corroborate the model via a suite of pump-probe transient absorption experiments: these manifest charge transfer consistent with a modulation in band alignment of at least 200 meV among neighboring grains. The findings shed light on perovskite 2D band alignment and explain contrasting behavior of quasi-2D materials in light-emitting diodes (LEDs) and photovoltaics (PV) in the literature, where materials can exhibit either type-I or type-II interfaces depending on the ligand concentration at neighboring surfaces.