Excitons in 2D Organic–Inorganic Halide Perovskites

Phonon Directionality Impacts Electron-Phonon Coupling and Polarization of the Band-Edge Emission in Two-Dimensional Metal Halide Perovskites

Two-dimensional metal halide perovskites are highly versatile for light-driven applications due to their exceptional variety in material composition, which can be exploited for the tunability of mechanical and optoelectronic properties. The band-edge emission is defined by the structure and composition of both organic and inorganic layers, and electron-phonon coupling plays a crucial role in the recombination dynamics. However, the nature of the electron-phonon coupling and what kind of phonons are involved are still under debate. Here we investigate the emission, reflectance, and phonon response from single two-dimensional lead iodide microcrystals with angle-resolved polarized spectroscopy. We find an intricate dependence of the emission polarization with the vibrational directionality in the materials, which reveals that several bands of low-frequency phonons with nonorthogonal directionality contribute to the band-edge emission. Such complex electron-phonon coupling requires adequate models to predict the thermal broadening of the emission and provides opportunities to design polarization properties.

Interplay of Phonon Directionality and Emission Polarization in Two-Dimensional Layered Metal Halide Perovskites

ConspectusLayered metal halide perovskites represent a natural quantum well system for charge carriers that provides rich physics, and the organic encapsulation of the inorganic metal halide layers not only increases their stability in devices but also provides an immense freedom to design their functionality. Intriguingly, these organic moieties strongly impact the optical, electrical, and mechanical properties, not only through their dielectric, elastic, and chemical properties but also because of induced mechanical distortions in the inorganic lattice. This tunability makes two-dimensional layered perovskites (2DLPs) highly attractive as light emitters. Common consensus is that exciton-phonon coupling plays an important role in radiative recombination. For bulk and some two-dimensional (2D) materials, the band edge emission broadening can be described by the classic models for polar inorganic semiconductors, while for the temperature dependence of the self-trapped exciton emission, an analysis developed for color centers has been successfully applied. For many 2DLPs these approaches do not work because of the complexity of their vibrational spectra. However, their emission is still strongly determined by phonons, and therefore, an adequate understanding of the electron-phonon coupling needs to be developed.With polarized and angle-resolved Raman spectroscopy studies on single 2DLP flakes based on different ammonium molecules as organic cations, in 2020 we revealed very rich phonon spectra in the low-frequency regime. Although the phonon bands at low frequency can generally be attributed to the vibrations of the inorganic lattice, we found very different responses by only changing the type of organic cations. In addition, the intensity of the different phonon modes depended strongly on the angle of the linearly polarized excitation beam with respect to the in-plane axes of the octahedron lattice. In 2022, we mapped this angular dependence of the phonon modes, which allowed identification of the directionality of the different lattice vibrations. By correlating the phonon spectra with the temperature-dependent emission for a set of 2DLPs that featured very different self-trapped exciton (STE) emission, we demonstrated that the exciton relaxation cannot be related to coupling with a single (longitudinal-optical) phonon band and that several phonon bands should be involved in the emission process. To gain insights into the exciton-phonon coupling effects on the band edge emission, we performed both angle-resolved polarized emission and Raman spectroscopy on single 2D lead iodide perovskite microcrystals. These experiments revealed the impact of the organic cations on the linear polarization of the emission and corroborated that multiple phonon bands should be involved in the radiative recombination process. Analysis of the temperature-dependent line width broadening of the band edge emission showed that for many systems, the behavior cannot be described by assuming the involvement of only one phonon mode in the electron-phonon coupling process. Our studies revealed a wealth of highly directional low-frequency phonons in 2DLPs from which several bands are involved in the emission process, which leads to diverse optical and vibrational properties depending on the type of organic cation in the material.

Bluish-white Light-emitting 2D Sheets of Lead-free Perovskite Cesium Titanium Bromide (CsTiBr3) by a Two-stage Deposition Technique

Bluish-white light-emitting materials are commonly used in LED lighting because they produce natural-looking light. Here we report the photoluminescent emission (PL) of novel, two-dimensional lead-free CsTiBr3 perovskite prepared via a two-stage deposition process. The formation of two-dimensional nanosheets of CsTiBr3 perovskite is confirmed by XRD, EDAX, and FESEM analysis. The height of the cesium bromide thin film substrate from the titanium bromide vapor source plays an important role in the formation of two-dimensional CsTiBr3. The CsTiBr3 perovskite nanosheets exhibit unique exciton- luminescence at 440 nm and self-trapped exciton emission at 595 nm which are the characteristics of two-dimensional halide structure, along with the band-to-band emission at 400 nm at an excitation wavelength of 340 nm. The resulting bluish-white light PL emission makes two-dimensional CsTiBr3 perovskite an alternative material to the traditional lead-based perovskite in LEDs, display technology, solid-state lighting, and various optoelectronic devices, addressing environmental concerns.

Unravelling Structural, Optical, and Band Alignment Properties of Mixed Pb-Sn Metal-Halide Quasi-2D Ruddlesden-Popper Perovskites

Lead-tin (Pb-Sn) mixed-halide perovskites show potential for single-junction and tandem solar cells due to their adjustable band gaps, flexible composition, and superior environmental stability compared to three-dimensional (3D) perovskites. However, they have lower power conversion efficiencies. Understanding band alignment and charge carrier dynamics is essential for enhancing photovoltaic performance. In this view, herein we have prepared thin films of mixed Pb-Sn-based two dimensional (2D) Ruddlesden-Popper (RP) perovskites BA2FA(Pb1-xSnx)2I7 using a solution-based method. XRD study revealed the formation of orthorhombic phases for pristine (BA2FAPb2I7) and mixed Pb-Sn perovskite thin films. UV-vis analysis showed that different n = 2 and n = 3 phases are present in the pristine sample. In contrast, Pb-Sn-doped samples showed no signature of other phases with a prominent red-shift in the visible spectral region. Cyclic voltammetry showed peaks for electron transfers at the band edges. Additionally, electrochemical and optical band gap matching was observed, along with decreased peak intensity due to less reactant and altered electrolyte-perovskite interface stability. Density functional theory (DFT) calculations revealed that the reduced band gap is due to the alteration of electrostatic interactions and charge distribution within the lattice upon Sn substitution. Low-temperature PL analysis provided insights into charge carrier dynamics with Sn substitution and suggested the suppression of higher n phases and self-trapped excitons/carriers in mixed Pb-Sn quasi-2D RP perovskite thin films. This study sheds light on the electron transfer phenomena between TiO2 and SnO2 layers by estimating band offsets from valence band maximum (VBM) and conduction band minimum (CBM), which is crucial for future applications in fabricating stable and efficient 2D-Pb-Sn mixed perovskites for optoelectronic applications.

Recent advances in two-dimensional perovskite materials for light-emitting diodes

Light-emitting diodes (LEDs) are an indispensable part of our daily life. After being studied for a few decades, this field still has some room for improvement. In this regard, perovskite materials may take the leading role. In recent years, LEDs have become a most explored topic, owing to their various applications in photodetectors, solar cells, lasers, and so on. Noticeably, they exhibit significant characteristics in developing LEDs. The luminous efficiency of LEDs can be significantly enhanced by the combination of a poor illumination LED with low-dimensional perovskite. In 2014, the first perovskite-based LED was illuminated at room temperature. Furthermore, two-dimensional (2D) perovskites have enriched this field because of their optical and electronic properties and comparatively high stability in ambient conditions. Recent and relevant advancements in LEDs using low-dimensional perovskites including zero-dimensional to three-dimensional materials is reported. The major focus of this article is based on the 2D perovskites and their heterostructures (i.e., a combination of 2D perovskites with transition metal dichalcogenides, graphene, and hexagonal boron nitride). In comparison to 2D perovskites, heterostructures exhibit more potential for application in LEDs. State-of-the-art perovskite-based LEDs, current challenges, and prospects are also discussed.

Temperature-dependent excited states for detecting reversible phase transitions in 2D lead(II) iodide perovskites

Significant interest exists in water-tolerant 2D lead iodide perovskites owing to their stability and proven potential in photovoltaic and photonic applications. These materials have solid-state phase transitions that are accessible below 100 °C. Here, the study witnesses the multiple phase transitions of the last members of a series of organic-inorganic hybrid materials, [(CnH2n+1NH3)2PbI4], with even n as n = 14, 16, and 18, once again. By employing temperature-dependent steady-state photoluminescence (PL) and temperature-dependent time-resolved photoluminescence (TRPL) spectroscopy in the temperature range of -18 to +90 °C and at -196 °C, we explore the thermal responses of these materials. The investigation reveals reversible phase transitions occurring between room temperature (RT) and elevated temperatures, impacting the optical properties and emitting colors of the perovskite compounds. The longer the alkyl chain, the higher the phase transition temperature, attributed to increased conformational disorder and enhanced perovskite symmetry. The decay constants for all compounds are very close in value, which confirms the underlying excited-state dynamics, pointing to contributions primarily from inorganic components across different phases. We anticipate that our results on the detection of phase transitions in 2D perovskites will not only motivate the use of these techniques for detecting phase transitions but also would help to understand their excited states in more details to selectively use them for solar cell and next-generation display technologies.

Is Dielectric Mismatch Actually Important in 2D Perovskites?

Understanding and controlling exciton properties are important for the design of 2D semiconductors, such as monolayer transition metal dichalcogenides (TMDCs) and 2D halide perovskites (HPs). This paper demonstrates that the widespread strategy used for the exciton engineering of 2D HPs, based on dielectric mismatch, is flawed since dielectric mismatch has very little correlation with exciton properties. For monolayer TMDCs, however, the dielectric mismatch is shown to be more important.

Experimental evidence for defect tolerance in Pb-halide perovskites

The term defect tolerance (DT) is used often to rationalize the exceptional optoelectronic properties of halide perovskites (HaPs) and their devices. Even though DT lacked direct experimental evidence, it became a "fact" in the field. DT in semiconductors implies that structural defects do not translate to electrical and optical effects (e.g., due to charge trapping), associated with such defects. We present pioneering direct experimental evidence for DT in Pb-HaPs by comparing the structural quality of 2-dimensional (2D), 2D-3D, and 3D Pb-iodide HaP crystals with their optoelectronic characteristics using high-sensitivity methods. Importantly, we get information from the materials' bulk because we sample at least a few hundred nanometers, up to several micrometers, from the sample's surface, which allows for assessing intrinsic bulk (and not only surface-) properties of HaPs. The results point to DT in 3D, 2D-3D, and 2D Pb-HaPs. Overall, our data provide an experimental basis to rationalize DT in Pb-HaPs. These experiments and findings will help the search for and design of materials with real DT.

Exciton engineering of 2D Ruddlesden-Popper perovskites by synergistically tuning the intra and interlayer structures

Designing two-dimensional halide perovskites for high-performance optoelectronic applications requires deep understanding of the structure-property relationship that governs their excitonic behaviors. However, a design framework that considers both intra and interlayer structures modified by the A-site and spacer cations, respectively, has not been developed. Here, we use pressure to synergistically tune the intra and interlayer structures and uncover the structural modulations that result in improved optoelectronic performance. Under applied pressure, (BA)2(GA)Pb2I7 exhibits a 72-fold boost of photoluminescence and 10-fold increase of photoconductivity. Based on the observed structural change, we introduce a structural descriptor χ that describes both the intra and interlayer characteristics and establish a general quantitative relationship between χ and photoluminescence quantum yield: smaller χ correlates with minimized trapped excitons and more efficient emission from free excitons. Building on this principle, we design a perovskite (CMA)2(FA)Pb2I7 that exhibits a small χ and an impressive photoluminescence quantum yield of 59.3%.

Ferroics in Hybrid Organic-Inorganic Perovskites: Fundamentals, Design Strategies, and Implementation

Hybrid organic-inorganic perovskites (HOIPs) afford highly versatile structure design and lattice dimensionalities; thus, they are actively researched as material platforms for the tailoring of ferroic behaviors. Unlike single-phase organic or inorganic materials, the interlayer coupling between organic and inorganic components in HOIPs allows the modification of strain and symmetry by chirality transfer or lattice distortion, thereby enabling the coexistence of ferroic orders. This review focuses on the principles for engineering one or multiple ferroic orders in HOIPs, and the conditions for achieving multiferroicity and magnetoelectric properties. The prospects of multilevel ferroic modulation, chiral spin textures, and spin orbitronics in HOIPs are also presented.

Superior Phonon-Limited Exciton Mobility in Lead-Free Two-Dimensional Perovskites

Tin-based two-dimensional (2D) perovskites are emerging as lead-free alternatives in halide perovskite materials, yet their exciton dynamics and transport remain less understood due to defect scattering. Addressing this, we employed temperature-dependent transient photoluminescence (PL) microscopy to investigate intrinsic exciton transport in three structurally analogous Sn- and Pb-based 2D perovskites. Employing conjugated ligands, we synthesized high-quality crystals with enhanced phase stability at various temperatures. Our results revealed phonon-limited exciton transport in Sn perovskites, with diffusion constants increasing from 0.2 cm2 s-1 at room temperature to 0.6 cm2 s-1 at 40 K, and a narrowing PL line width. Notably, Sn-based perovskites exhibited greater exciton mobility than their Pb-based equivalents, which is attributed to lighter effective masses. Thermally activated optical phonon scattering was observed in Sn-based compounds but was absent in Pb-based materials. These findings, supported by molecular dynamics simulations, demonstrate that the phonon scattering mechanism in Sn-based halide perovskites can be distinct from their Pb counterparts.

Boosting exciton mobility approaching Mott-Ioffe-Regel limit in Ruddlesden-Popper perovskites by anchoring the organic cation

Exciton transport in two-dimensional Ruddlesden-Popper perovskite plays a pivotal role for their optoelectronic performance. However, a clear photophysical picture of exciton transport is still lacking due to strong confinement effects and intricate exciton-phonon interactions in an organic-inorganic hybrid lattice. Herein, we present a systematical study on exciton transport in (BA)2(MA)n-1PbnI3n+1 Ruddlesden-Popper perovskites using time-resolved photoluminescence microscopy. We reveal that the free exciton mobilities in exfoliated thin flakes can be improved from around 8 cm2 V-1 s-1 to 280 cm2V-1s-1 by anchoring the soft butyl ammonium cation with a polymethyl methacrylate network at the surface. The mobility of the latter is close to the theoretical limit of Mott-Ioffe-Regel criterion. Combining optical measurements and theoretical studies, it is unveiled that the polymethyl methacrylate network significantly improve the lattice rigidity resulting in the decrease of deformation potential scattering and lattice fluctuation at the surface few layers. Our work elucidates the origin of high exciton mobility in Ruddlesden-Popper perovskites and opens up avenues to regulate exciton transport in two-dimensional materials.

Halide Ion Mixing across Colloidal 2D Ruddlesden-Popper Perovskites: Implication of Spacer Ligand on Mixing Kinetics

Halide ion exchange seen in metal halide perovskites provide a substantial opportunity to control their halide composition and corresponding optoelectronic properties. Halide ion mixing across colloidal 3D perovskite nanocrystals have been extensively studied while the mixing within colloidal 2D counterparts remain underexplored. In this study, the halide ion exchange kinetics across colloidally stable 2D Ruddlesden-Popper layered bromide (Br) and iodide (I) perovskites using two different spacer ligands such as aromatic phenethylammonium (PEA) versus linear butyammonium (BA) is demonstrated. The halide exchange kinetic rate constant (k), as determined by tracking time-dependent absorbance changes, indicates that Br/I halide mixing in 2D PEA-based perovskites (2.7 × 10-3 min-1 ) occurs at an order of magnitude slower than in 2D BA-based perovskites (3.3 × 10-2 min-1 ). Concentration (≈1 mM to 100 mM) and temperature-dependent (50 to 80 °C) kinetic studies further allow for the determination of activation barrier for halide ion mixing across the 2D layered perovskites with 75.2 ± 4.4 kJ mol-1 (2D PEA) and 57.8 ± 7.8 kJ mol-1 (2D BA), respectively. The activation energy reveals that the type of spacer cations plays a crucial role in controlling the halide ion mobility and halide stability due mainly to the internal ligand chemical interaction within 2D structures.

Tuning Structure and Excitonic Properties of 2D Ruddlesden-Popper Germanium, Tin, and Lead Iodide Perovskites via Interplay between Cations

The compositional tunability of 2D metal halide perovskites enables exploration of diverse semiconducting materials with different structural features. However, rationally tuning the 2D perovskite structures to target physical properties for specific applications remains challenging, especially for lead-free perovskites. Here, we study the effect of the interplay of the B-site (Ge, Sn, and Pb), A-site (cesium, methylammonium, and formamidinium), and spacer cations on the structure and optical properties of a new series of 2D Ruddlesden-Popper perovskites using the previously unreported spacer cation 4-bromo-2-fluorobenzylammonium (4Br2FBZ). We report eight new crystal structures and study the consequence of varying the B-site (Pb, Sn, Ge) and dimension (n = 1, 2, vs 3D). Dimension strongly influences local distortion and structural symmetry, and the increased octahedral tilting and lone pair effects in Ge perovskites lead to a polar n = 2 perovskite that exhibits second harmonic generation, (4Br2FBZ)2(Cs)Ge2I7. In contrast, the analogous Sn and Pb perovskites remain centrosymmetric, but the B-site metal influences the photoluminescence properties. The Pb perovskites exhibit broad, defect-mediated emission at low temperature, whereas the Sn perovskites show purely excitonic emission over the entire temperature range, but the carrier recombination dynamics depend on dimensionality and dark excitonic states. Wholistic understanding of these differences that arise based on cations and dimensionality can guide the rational materials design of 2D perovskites for targeting physical properties for optoelectronic applications based on the interplay of cations and the connectivity of the inorganic framework.

Insights into the Growth Orientation and Phase Stability of Chemical-Vapor-Deposited Two-Dimensional Hybrid Halide Perovskite Films

Chemical vapor deposition (CVD) offers a large-area, scalable, and conformal growth of perovskite thin films without the use of solvents. Low-dimensional organic-inorganic halide perovskites, with alternating layers of organic spacer groups and inorganic perovskite layers, are promising for enhancing the stability of optoelectronic devices. Moreover, their multiple quantum-well structures provide a powerful platform for tuning excitonic physics. In this work, we show that the CVD process is conducive to the growth of 2D hybrid halide perovskite films. Using butylammonium (BA) and phenylethylammonium (PEA) cations, the growth parameters of BA2PbI4 and PEA2PbI4 and mixed halide perovskite films were first optimized. These films are characterized by well-defined grain boundaries and display characteristic absorption and emission features of the 2D quantum wells. X-ray diffraction (XRD) and a noninteger dimensionality model of the absorption spectrum provide insights into the orientation of the crystalline planes. Unlike BA2PbI4, temperature-dependent photoluminescence measurements from PEA2PbI4 show a single excitonic peak throughout the temperature range from 20 to 350 K, highlighting the lack of defect states. These results further corroborate the temperature-dependent synchrotron-based XRD results. Furthermore, the nonlinear optical properties of the CVD-grown perovskite films are investigated, and a high third harmonic generation efficiency is observed.

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.

Dimension Tuning of All-Inorganic Ag-Based Metal Halides by Solvent Engineering

Dimension growth of metal halides is important for its properties and applications. However, such dimension control of the metal halides is rarely reported in the literature and the growth mechanism is not clear yet. A minute difference of solvent properties can tremendously alter the process of nucleation and growth of crystals. Herein, an intriguing phenomenon of dimension tuning for Ag-based metal halides is reported. The 1D Cs2 AgCl3 crystals can be obtained in pure DMF while the 2D CsAgCl2 crystals are obtained in pure DMSO. Both exhibit bright yellow emission, which are derived from self-trapping excitons (STEs). The photoluminescence quantum yield (PLQY) of Cs2 AgCl3 (1D) and CsAgCl2 (2D) are 28.46 % and 20.61 %, respectively. In order to understand the mechanism of the dimension change, additional solvents (N,N-dimethylacetamide, DMAC, 1,3-Dimethyl-Tetrahydropyrimidin-2(1H)-one, DMPU) are also selected to process the precursor for crystal growth. By comparing the functional group, dielectric constant, and donor number among the four solvents, we find the donor number plays the predominant role in nucleation process for Cs2 AgCl3 and CsAgCl2 . This research reveals the relationship between coordination ability of the solvent and the dimension of metal halides.

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

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

Spatial Heterogeneity of Biexcitons in Two-Dimensional Ruddlesden-Popper Lead Iodide Perovskites

Quantum confinement in two-dimensional (2D) Ruddlesden-Popper (RP) perovskites leads to the formation of stable quasi-particles, including excitons and biexcitons, the latter of which may enable lasing in these materials. Due to their hybrid organic-inorganic structures and the solution phase synthesis, microcrystals of 2D RP perovskites can be quite heterogeneous, with variations in excitonic and biexcitonic properties between crystals from the same synthesis and even within individual crystals. Here, we employ one- and two-quantum two-dimensional white-light microscopy to systematically study the spatial variations of excitons and biexcitons in microcrystals of a series of 2D RP perovskites BA2MAn-1PbnI3n+1 (n = 2-4, BA= butylammonium, MA = methylammonium). We find that the average biexciton binding energy of around 60 meV is essentially independent of the perovskite layer thickness (n). We also resolve spatial variations of the exciton and biexciton energies on micron length scales within individual crystals. By comparing the one-quantum and two-quantum spectra at each pixel, we conclude that biexcitons are more sensitive to their environments than excitons. These results shed new light on the ways disorder can modify the energetic landscape of excitons and biexcitons in RP perovskites and how biexcitons can be used as a sensitive probe of the microscopic environment of a semiconductor.

Discovery of enhanced lattice dynamics in a single-layered hybrid perovskite

Layered hybrid perovskites exhibit emergent physical properties and exceptional functional performances, but the coexistence of lattice order and structural disorder severely hinders our understanding of these materials. One unsolved problem regards how the lattice dynamics are affected by the dimensional engineering of the inorganic frameworks and their interaction with the molecular moieties. Here, we address this question by using a combination of spontaneous Raman scattering, terahertz spectroscopy, and molecular dynamics simulations. This approach reveals the structural dynamics in and out of equilibrium and provides unexpected observables that differentiate single- and double-layered perovskites. While no distinct vibrational coherence is observed in double-layered perovskites, an off-resonant terahertz pulse can drive a long-lived coherent phonon mode in the single-layered system. This difference highlights the dramatic change in the lattice environment as the dimension is reduced, and the findings pave the way for ultrafast structural engineering and high-speed optical modulators based on layered perovskites.

Ultrafast imaging of polariton propagation and interactions

Semiconductor excitations can hybridize with cavity photons to form exciton-polaritons (EPs) with remarkable properties, including light-like energy flow combined with matter-like interactions. To fully harness these properties, EPs must retain ballistic, coherent transport despite matter-mediated interactions with lattice phonons. Here we develop a nonlinear momentum-resolved optical approach that directly images EPs in real space on femtosecond scales in a range of polaritonic architectures. We focus our analysis on EP propagation in layered halide perovskite microcavities. We reveal that EP-phonon interactions lead to a large renormalization of EP velocities at high excitonic fractions at room temperature. Despite these strong EP-phonon interactions, ballistic transport is maintained for up to half-exciton EPs, in agreement with quantum simulations of dynamic disorder shielding through light-matter hybridization. Above 50% excitonic character, rapid decoherence leads to diffusive transport. Our work provides a general framework to precisely balance EP coherence, velocity, and nonlinear interactions.

1D Hybrid Semiconductor Silver 2,6-Difluorophenylselenolate

Organic-inorganic hybrid materials present new opportunities for creating low-dimensional structures with unique light-matter interaction. In this work, we report a chemically robust yellow emissive one-dimensional (1D) semiconductor, silver 2,6-difluorophenylselenolate─AgSePhF₂(2,6), a new member of the broader class of hybrid low-dimensional semiconductors, metal-organic chalcogenolates. While silver phenylselenolate (AgSePh) crystallizes as a two-dimensional (2D) van der Waals semiconductor, introduction of fluorine atoms at the (2,6) position of the phenyl ring induces a structural transition from 2D sheets to 1D chains. Density functional theory calculations reveal that AgSePhF₂ (2,6) has strongly dispersive conduction and valence bands along the 1D crystal axis. Visible photoluminescence centered around λ_p ≈ 570 nm at room temperature exhibits both prompt (110 ps) and delayed (36 ns) components. The absorption spectrum exhibits excitonic resonances characteristic of low-dimensional hybrid semiconductors, with an exciton binding energy of approximately 170 meV as determined by temperature-dependent photoluminescence. The discovery of an emissive 1D silver organoselenolate highlights the structural and compositional richness of the chalcogenolate material family and provides new insights for molecular engineering of low-dimensional hybrid organic-inorganic semiconductors.

Connecting the dots for fundamental understanding of structure-photophysics-property relationships of COFs, MOFs, and perovskites using a Multiparticle Holstein Formalism

Photoactive organic and hybrid organic-inorganic materials such as conjugated polymers, covalent organic frameworks (COFs), metal-organic frameworks (MOFs), and layered perovskites, display intriguing photophysical signatures upon interaction with light. Elucidating structure-photophysics-property relationships across a broad range of functional materials is nontrivial and requires our fundamental understanding of the intricate interplay among excitons (electron-hole pair), polarons (charges), bipolarons, phonons (vibrations), inter-layer stacking interactions, and different forms of structural and conformational defects. In parallel with electronic structure modeling and data-driven science that are actively pursued to successfully accelerate materials discovery, an accurate, computationally inexpensive, and physically-motivated theoretical model, which consistently makes quantitative connections with conceptually complicated experimental observations, is equally important. Within this context, the first part of this perspective highlights a unified theoretical framework in which the electronic coupling as well as the local coupling between the electronic and nuclear degrees of freedom can be efficiently described for a broad range of quasiparticles with similarly structured Holstein-style vibronic Hamiltonians. The second part of this perspective discusses excitonic and polaronic photophysical signatures in polymers, COFs, MOFs, and perovskites, and attempts to bridge the gap between different research fields using a common theoretical construct - the Multiparticle Holstein Formalism. We envision that the synergistic integration of state-of-the-art computational approaches with the Multiparticle Holstein Formalism will help identify and establish new, transformative design strategies that will guide the synthesis and characterization of next-generation energy materials optimized for a broad range of optoelectronic, spintronic, and photonic applications.

Spacer-Dependent and Pressure-Tuned Structures and Optoelectronic Properties of 2D Hybrid Halide Perovskites

Compared with their 3D counterparts, 2D hybrid organic-inorganic halide perovskites (HOIPs) exhibit enhanced chemical stabilities and superior optoelectronic properties, which can be further tuned by the application of external pressure. Here, we report the first high-pressure study on CMA₂PbI₄ (CMA = cylcohexanemethylammonium), a 2D HOIP with a soft organic spacer cation containing a flexible cyclohexyl ring, using UV-visible absorption, photoluminescence (PL) and vibrational spectroscopy, and synchrotron X-ray microdiffraction, all aided with density functional theory (DFT) calculations. Substantial anisotropic compression behavior is observed, as characterized by unprecedented negative linear compressibility along the b axis. Moreover, the pressure dependence of optoelectronic properties is found to be in strong contrast with those of 2D HOIPs with rigid spacer cations. DFT calculations help to understand the compression mechanisms that lead to pressure-induced bandgap narrowing. These findings highlight the important role of soft spacer cations in the pressure-tuned optoelectronic properties and provide guidance to the design of new 2D HOIPs.

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.

Area-Independence of the Biexciton Oscillator Strength in CdSe Colloidal Nanoplatelets

Colloidal CdSe nanoplatelets (NPLs) are unique systems to study two-dimensional excitons and excitonic complexes. However, while absorption and emission of photons through exciton formation and recombination have been extensively quantified, few studies have addressed the exciton-biexciton transition. Here, we use cross-polarized pump-probe spectroscopy to measure the absorption coefficient spectrum of this transition and determine the biexciton oscillator strength (f_BX). We show that f_BX is independent of the NPL area and that the concomitant biexciton area (S_BX) agrees with predictions of a short-range interaction model. Moreover, we show that f_BX is comparable to the oscillator strength of forming localized excitons at room temperature while being unaffected itself by center-of-mass localization. These results confirm the relevance of biexcitons for light-matter interaction in NPLs. Moreover, the quantification of the exciton-biexciton transition introduced here will enable researchers to rank 2D materials by the strength of this transition and to compare experimental results with theoretical predictions.

Spontaneous Formation of a Ligand-Based 2D Capping Layer on the Surface of Quasi-2D Perovskite Films

Two-dimensional (2D) Ruddlesden-Popper phase perovskites (RPPs) are attracting growing attention for photovoltaic applications due to their enhanced stability compared to three-dimensional (3D) perovskites. The superior tolerance of 2D RPPs films to moisture and oxygen is mainly attributed to the hydrophobic nature of the introduced long-chain spacer cations (ligands). In this work, it is revealed that a thin capping layer, consisting of self-assembled butylammonium ligands, is spontaneously formed on the top surface of a quasi-2D perovskite film prepared by conventional one-step hot casting. Based on morphological and crystallographic analyses of both the top/bottom surfaces and the interior of quasi-2D perovskite films, the formation process of the 2D capping layer and the assembly of RPPs, comprising both large and small slab thickness (large-n, small-n), is elucidated. The vertical orientation of RPPs that is required for sufficient charge transport for 2D perovskite solar cells (PSCs) is further verified. We propose that the surface capping layer is directly responsible for the long-term stability of 2D PSCs. This work provides detailed insight into the microstructure of quasi-2D RPPs films that should assist the development of strategies for unlocking the full potential of 2D perovskites for high-performance PSCs and other solid-state electronic devices.

Effect of Dimensionality on Photoluminescence and Dielectric Properties of Imidazolium Lead Bromides

Hybrid organic–inorganic lead halide perovskites have emerged as promising materials for various applications, including solar cells, light-emitting devices, dielectrics, and optical switches. In this work, we report the synthesis, crystal structures, and linear and nonlinear optical as well as dielectric properties of three imidazolium lead bromides, IMPbBr₃, IM₂PbBr₄, and IM₃PbBr₅ (IM⁺ = imidazolium). We show that these compounds exhibit three distinct structure types. IMPbBr₃ crystallizes in the 4H-hexagonal perovskite structure with face- and corner-shared PbBr₆ octahedra (space group P6₃/mmc at 295 K), IM₂PbBr₄ adopts a one-dimensional (1D) double-chain structure with edge-shared octahedra (space group P1̅ at 295 K), while IM₃PbBr₅ crystallizes in the 1D single-chain structure with corner-shared PbBr₆ octahedra (space group P1̅ at 295 K). All compounds exhibit two structural phase transitions, and the lowest temperature phases of IMPbBr₃ and IM₃PbBr₅ are noncentrosymmetric (space groups Pna2₁ at 190 K and P1 at 100 K, respectively), as confirmed by measurements of second-harmonic generation (SHG) activity. X-ray diffraction and thermal and Raman studies demonstrate that the phase transitions feature an order–disorder mechanism. The only exception is the isostructural P1̅ to P1̅ phase transition at 141 K in IM₂PbBr₄, which is of a displacive type. Dielectric studies reveal that IMPbBr₃ is a switchable dielectric material, whereas IM₃PbBr₅ is an improper ferroelectric. All compounds exhibit broadband, highly shifted Stokes emissions. Features of these emissions, i.e., band gap and excitonic absorption, are discussed in relation to the different structures of each composition.

Three imidazolium lead bromides of various chemical compositions and crystal structures display broadband photoluminescence that can be tuned from bluish-green to orange. All compounds exhibit two structural phase transitions, which lead to interesting optical and electrical properties such SHG activity, ferroelectricity, or dielectric switching.

Origin and physical effects of edge states in two-dimensional Ruddlesden-Popper perovskites

The edge region of two-dimensional (2D) Ruddlesden-Popper (RP) perovskites exhibits anomalous properties from the bulk region, including low energy emission and superior capability of dissociating exciton, which is highly beneficial for the optoelectronic devices like solar cells and photodetectors, denoted as “edge states”. In this review, we introduce the recent progress on the edge states that have been focused on the origin and the optoelectronic properties of edge states in 2D RP perovskites. By providing theoretical frameworks and experimental observations, we elucidate the origin of the edge states from two aspects, intrinsic electronic properties and extrinsic structure distortions. Besides, we introduce the physical properties of the edge states and current debating on this topic. Finally, we present an outlook on future research about the edge states of 2D RP perovskites.

Multifunctional and Transformative Metaphotonics with Emerging Materials

Future technologies underpinning multifunctional physical and chemical systems and compact biological sensors will rely on densely packed transformative and tunable circuitry employing nanophotonics. For many years, plasmonics was considered as the only available platform for subwavelength optics, but the recently emerged field of resonant metaphotonics may provide a versatile practical platform for nanoscale science by employing resonances in high-index dielectric nanoparticles and metasurfaces. Here, we discuss the recently emerged field of metaphotonics and describe its connection to material science and chemistry. For tunabilty, metaphotonics employs a variety of the recently highlighted materials such as polymers, perovskites, transition metal dichalcogenides, and phase change materials. This allows to achieve diverse functionalities of metasystems and metasurfaces for efficient spatial and temporal control of light by employing multipolar resonances and the physics of bound states in the continuum. We anticipate expanding applications of these concepts in nanolasers, tunable metadevices, metachemistry, as well as a design of a new generation of chemical and biological ultracompact sensing devices.

Sub-angstrom noninvasive imaging of atomic arrangement in 2D hybrid perovskites

Noninvasive imaging of the atomic arrangement in two-dimensional (2D) Ruddlesden-Popper hybrid perovskites (RPPs) is challenging because of the insulating nature and softness of the organic layers. Here, we demonstrate a sub-angstrom resolution imaging of both soft organic layers and inorganic framework in a prototypical 2D lead-halide RPP crystal via combined tip-functionalized scanning tunneling microscopy (STM) and noncontact atomic force microscopy (ncAFM) corroborated by theoretical simulations. STM measurements unveil the atomic reconstruction of the inorganic lead-halide lattice and overall twin-domain composition of the RPP crystal, while ncAFM measurements with a CO-tip enable nonperturbative visualization of the cooperative reordering of surface organic cations driven by their hydrogen bonding interactions with the inorganic lattice. Moreover, such a joint technique also allows for the atomic-scale imaging of the electrostatic potential variation across the twin-domain walls, revealing alternating quasi-1D electron and hole channels at neighboring twin boundaries, which may influence in-plane exciton transport and dissociation.

Effects of Electron-Phonon Coupling and Spin-Spin Coupling on the Photoluminescence of Low-Dimensional Metal Halides

Low-dimensional metal halides (LDMHs), as a derivative of three-dimensional lead halide perovskites, have attracted much attention because of their unique crystal structures and fascinating photonic properties. The simple synthesis and rich photonic properties of LDMHs make them striking candidates for the development of lighting, photodetectors, biological imaging, etc. Although many novel LDMHs have been achieved with strong electron-phonon coupling related to their self-trapped excitons (STEs) and excellent optical responses, transition-metal halides or doped halides have not been covered in regard to their rich spin characteristics. In this Perspective, we aim to deeply understand the role of electron-phonon coupling and STEs with magnetic coupling effects in regulating the optical properties of LDMHs and try to provide a novel way or a series of novel systems for the realization of next-generation high-performance luminescent materials with spin-coupling-involved photonics. Finally, an outlook toward potential challenges and applications of such ionic semiconducting LDMHs is also presented.

Morphological Control of 2D Hybrid Organic-Inorganic Semiconductor AgSePh

Silver phenylselenolate (AgSePh) is a hybrid organic-inorganic two-dimensional (2D) semiconductor exhibiting narrow blue emission, in-plane anisotropy, and large exciton binding energy. Here, we show that the addition of carefully chosen solvent vapors during the chemical transformation of metallic silver to AgSePh allows for control over the size and orientation of AgSePh crystals. By testing 28 solvent vapors (with different polarities, boiling points, and functional groups), we controlled the resulting crystal size from <200 nm up to a few μm. Furthermore, choice of solvent vapor can substantially improve the orientational homogeneity of 2D crystals with respect to the substrate. In particular, solvents known to form complexes with silver ions, such as dimethyl sulfoxide (DMSO), led to the largest lateral crystal dimensions and parallel crystal orientation. We perform systematic optical and electrical characterizations on DMSO vapor-grown AgSePh films demonstrating improved crystalline quality, lower defect densities, higher photoconductivity, lower dark conductivity, suppression of ionic migration, and reduced midgap photoluminescence at low temperature. Overall, this work provides a strategy for realizing AgSePh films with improved optical properties and reveals the roles of solvent vapors on the chemical transformation of metallic silver.

Photophysics of Two-Dimensional Semiconducting Organic-Inorganic Metal-Halide Perovskites

Two-dimensional organic-inorganic hybrid perovskites (2DHPs) consist of alternating anionic metal-halide and cationic organic layers. They have widely tunable structural and optical properties. We review the role of the organic cation in defining the structural and optical properties of 2DHPs through example lead iodide 2DHPs. Even though excitons reside in the metal-halide layers, the organic and inorganic frameworks cannot be separated-they must be considered as a single unit to fully understand the photophysics of 2DHPs. We correlate cation-induced distortion and disorder in the inorganic lattice with the resulting optical properties. We also discuss the role of the cation in creating and altering the discrete excitonic structure that appears at cryogenic temperatures in some 2DHPs, including the cation-dependent presence of hot-exciton photoluminescence. We conclude our review with an outlook for 2DHPs, highlighting existing gaps in fundamental knowledge as well as potential future applications. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 73 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Halide Mixing Inhibits Exciton Transport in Two-dimensional Perovskites Despite Phase Purity

Halide mixing is one of the most powerful techniques to tune the optical bandgap of metal-halide perovskites. However, halide mixing has commonly been observed to result in phase segregation, which reduces excited-state transport and limits device performance. While the current emphasis lies on the development of strategies to prevent phase segregation, it remains unclear how halide mixing may affect excited-state transport even if phase purity is maintained. Here, we study exciton transport in phase pure mixed-halide 2D perovskites of (PEA)2Pb(I1-x Br x )4. Using transient photoluminescence microscopy, we show that, despite phase purity, halide mixing inhibits exciton transport. We find a significant reduction even for relatively low alloying concentrations. By performing Brownian dynamics simulations, we are able to reproduce our experimental results and attribute the decrease in diffusivity to the energetically disordered potential landscape that arises due to the intrinsic random distribution of alloying sites.

Shedding light on the energy applications of emerging 2D hybrid organic-inorganic halide perovskites

Unique performance of the hybrid organic-inorganic halide perovskites (HOIPs) has attracted great attention because of their continuous exploration and breakthrough in a multitude of energy-related applications. However, the instability and lead-induced toxicity that arise in bulk perovskites are the two major challenges that impede their future commercialization process. To find a solution, a series of two-dimensional HOIPs (2D HOIPs) are investigated to prolong the device lifetime with highly efficient photoelectric conversion and energy storage. Herein, the recent advances of 2D HOIPs and their structural derivatives for the energy realms are summarized and discussed. The basic understanding of crystal structures, physicochemical properties, and growth mechanisms is presented. In addition, the current challenges and future directions to provide a roadmap for the development of next generation 2D HOIPs are prospected

Exciton-Phonon Coupling and Low Energy Emission in 2D and Quasi-2D BA2MAn-1PbnI3n+1 Thin Films with Improved Phase Purity

Phonon scattering with photogenerated excitons and free charges greatly affects optoelectronic properties of metal halide perovskites and governs their emission line width. Benefiting from the improved phase purity, we are able to analyze exciton-phonon coupling in 2D and quasi-2D BA₂MA_n-1Pb_nI_3n+1 (n = 1-3) thin films using temperature-dependent photoluminescence (PL) spectroscopy. The layer thickness (n value) dependent coupling of free excitons with both acoustic and longitudinal optical (LO) phonons was extracted quantitatively by fitting the temperature-dependent PL line width and band gap. The low energy emissive signatures below free excitons at low temperature might belong to the emission of self-trapped excitons and bounded excitons in structural defects. Our findings provide a systematic picture for the layer thickness (n value) dependent exciton-phonon coupling in 2D and quasi-2D perovskite thin films and could be helpful for improving the optoelectronic performance of devices made by Ruddlesden-Popper perovskite thin films.

Femtosecond Exciton and Carrier Relaxation Dynamics of Two-Dimensional (2D) and Quasi-2D Tin Perovskites

The dynamics of exciton and free-carrier relaxation of low-dimensional tin iodide perovskites, BA₂FA_n-1Sn_nI_3n+1, where n = 1 (N1), 2 (N2), 5 (N5), and 10 (N10), were investigated with femtosecond transient absorption spectra (TAS). The absorption and photoluminescence spectra of N1 and N2 show exciton characteristics due to quantum confinement, whereas N5 and N10 display a free-carrier nature, the same as for bulk three-dimensional (3D) films. The TAS profiles were fitted according to a global kinetic model with three time coefficients representing the interactions of biexcitons, trions, and excitons for N1 and N2 and hot carriers, cold carriers, and shallow trap carriers for N5 and N10. The carrier relaxation dynamics of N5 and N10 were similar to those of 3D FASnI₃ except for the absence of surface recombination in the deep-trap states due to passivation of the grain surfaces by the long alkyl chain for these quasi-2D samples (N5/N10 vs 3D).

Size and Quality Enhancement of 2D Semiconducting Metal-Organic Chalcogenolates by Amine Addition

The use of two-dimensional (2D) materials in next-generation technologies is often limited by small lateral size and/or crystal defects. Here, we introduce a simple chemical strategy to improve the size and overall quality of 2D metal-organic chalcogenolates (MOCs), a new class of hybrid organic-inorganic 2D semiconductors that can exhibit in-plane anisotropy and blue luminescence. By inducing the formation of silver-amine complexes during a solution growth method, we increase the average size of silver phenylselenolate (AgSePh) microcrystals from <5 μm to >1 mm, while simultaneously extending the photoluminescence lifetime and suppressing mid-gap emission. Mechanistic studies using ⁷⁷Se NMR suggest dual roles for the amine in promoting the formation of a key reactive intermediate and slowing down the final conversion to AgSePh. Finally, we show that amine addition is generalizable to the synthesis of other 2D MOCs, as demonstrated by the growth of single crystals of silver 4-methylphenylselenolate (AgSePhMe), a novel member of the 2D MOC family.

Traversing Excitonic and Ionic Landscapes: Reduced-Dimensionality-Inspired Design of Organometal Halide Semiconductors for Energy Applications

At the very heart of the global semiconductor industry lies the omnipresent push for new materials discovery. New materials constantly rise and fall out of fashion in the scientific literature, with those passing an initial phase of research scrutiny becoming hotbeds of characterization and optimization efforts. Yet, innumerable hours of painstaking research have been devoted to materials that have ultimately fallen by the wayside after crossing over an indefinable threshold, whereupon historical optimism is met with newfound skepticism. Materials have to perform well, and they have to do it quickly. In the past decade, metal-halide perovskites (MHPs) have garnered widespread attention. The hegemonic view in both academic and industrial circles is that these materials could be engineered to meet the demands of the semiconductor industry. Their promise as inexpensive solar cell devices is highly attractive, and it has been nothing short of remarkable that efficiencies have risen from 3.8% in 2009 to more than 25.5% in 2021. Moreover, MHPs are poised to be revolutionary materials in more ways than one. The highest MHP LED efficiency was recently reported (23.4%), and MHPs have demonstrated promise in photodetectors, memristors, and transistors. However, the many excellent properties of MHPs are contrasted by longstanding stability and reproducibility limitations that have hindered their commercialization. Overcoming the limitations of MHPs is ultimately a materials engineering problem, which should be solved by mapping more precise relationships between structure, composition, and device performance. In 1958, Francis Crick famously developed the central dogma of molecular biology which describes the unidirectional flow of information in biological systems. In the words of Crick, "nature has devised a unique instrument in which an underlying simplicity is used to express great subtlety and versatility." In this Account, taking inspiration from the hierarchical organization of nature, we describe a hierarchical approach to materials engineering of organic metal-halide semiconductors. We demonstrate that organo-metal halide semiconductors' dimensionality, composition, and morphology dictate their optoelectronic properties and can be exploited in defining more explicit relationships between structure and function. Here, we traverse three-dimensional (3D), two-dimensional (2D), and one-dimensional (1D) organo-metal halide semiconductors, detailing the morphological and compositional differences in each and the implications that can be drawn within each domain on the engineering process. Control over ion migration pathways via morphology engineering as well as control over charge formation in organic-inorganic semiconductors is demonstrated. Fundamental insights into the amount of static and dynamic disorder in the MHP lattice are provided, which can be continuously tuned as a function of composition and morphology. Using electroabsorption spectroscopy on 2D MHPs, a disorder-induced dipole moment in the exciton proportional to the summed value of static and dynamic disorder is measured. Spectroscopic isolation of exciton features in 2D MHP electroabsorption spectra allows us to obtain precise, model-independent measurements of exciton binding energies to study the effect of chemical substitutions, such as Sn²⁺ → Pb²⁺, on the value of the exciton binding energy. Finally, we conclude that this multidimensional platform, with the aid of machine learning and robotics, will be foundational in accurately predicting structure-property-device relationships in organo-metal halide semiconductors in the future.

Photoinduced Halide Segregation in Ruddlesden-Popper 2D Mixed Halide Perovskite Films

2D lead halide perovskites, which exhibit bandgap tunability and increased chemical stability, have been found to be useful for designing optoelectronic devices. Reducing dimensionality with decreasing number of layers (n = 10-1) also imparts resistance to light-induced ion migration as seen from the halide ion segregation and dark recovery in mixed halide (Br:I = 50:50) perovskite films. The light-induced halide ion segregation efficiency, as determined from difference absorbance spectra, decreases from 20% to <1% as the dimensionality is decreased for 2D perovskite film from n = 10 to 1. The segregation rate constant (k_segregation ), which decreases from 5.9 × 10^-3  s^-1 (n = 10) to 3.6 × 10^-4  s^-1 (n = 1), correlates well with nearly an order of magnitude decrease observed in charge-carrier lifetime (τ_average  = 233 ps for n = 10 vs τ_avg  = 27 ps for n = 1). The tightly bound excitons in 2D perovskites make charge separation less probable, which in turn decreases the halide mobility and resulting phase segregation. The importance of controlling the dimensionality of the 2D architecture in suppressing halide ion mobility is discussed.

Advances of Nonlinear Photonics in Low-Dimensional Halide Perovskites

Hybrid halide perovskites emerging as a highly promising class of functional materials for semiconductor optoelectronic applications have drawn great attention from worldwide researchers. In the past few years, prominent nonlinear optical properties have been demonstrated in perovskite bulk structures indicating their bright prospect in the field of nonlinear optics (NLO). Following the surge of 3D perovskites, more recently, the low-dimensional perovskites (LDPs) materials ranging from two-, one-, to zero-dimension such as quantum-wells or colloidal nanostructures have displayed unexpectedly attractive NLO response due to the strong quantum confinement, remarkable exciton effect, and structural diversity. In this perspective, the current state of the art is reviewed in the field of NLO for LDP materials. The relationship between confinement effect and NLO is analyzed systematically to give a comprehensive understanding of the function of dimension reduction. Furthermore, future directions and challenges toward the improvement of the NLO in LDP materials are discussed to provide an outlook in this rapidly developing field.

In-Plane Anisotropic Nonlinear Optical Properties of Two-Dimensional Organic-Inorganic Hybrid Perovskite

Two-dimensional (2D) organic-inorganic hybrid perovskites (OIHPs) with superior nonlinear optical (NLO) properties show great versatility in frequency upconversion applications. Optical anisotropy plays an indispensable role in interpreting the interactions between incoming photons and crystal structure. Recently, the in-plane anisotropic NLO properties of 2D OIHPs have been reported and attracted much attention. However, the structure-related NLO anisotropy of the 2D OIHP framework is not well-established. Here, NLO properties of (C₆H₅(CH₂)₂NH₃)₂PbI₄ (PEPI), (C₆H₁₁NH₃)₂PbI₄ (C6H11), and (C₄H₉NH₃)₂PbI₄ (C4PI) were systematically studied to interrogate the correlation between the in-plane anisotropic NLO responses and its lattice structure. In-plane nonparametric NLO responses, e.g., two-photon photoluminescence (2PPL) and three-photon photoluminescence (3PPL), manifest similar anisotropy configurations for PEPI, C6H11, and C4PI regardless of aromatic, cyclic, or linear organic molecules; however, the anisotropies of THG signals are strongly dependent on the specific crystal structures of the individual flakes, and they are much higher than that of the multiphoton excited photoluminescence.

Strongly Anharmonic Octahedral Tilting in Two-Dimensional Hybrid Halide Perovskites

Recent investigations of two-dimensional (2D) hybrid organic-inorganic halide perovskites (HHPs) indicate that their optical and electronic properties are dominated by strong coupling to thermal fluctuations. While the optical properties of 2D-HHPs have been extensively studied, a comprehensive understanding of electron-phonon interactions is limited because little is known about their structural dynamics. This is partially because the unit cells of 2D-HHPs contain many atoms. Therefore, the thermal fluctuations are complex and difficult to elucidate in detail. To overcome this challenge, we use polarization-orientation Raman spectroscopy and ab initio calculations to compare the structural dynamics of the prototypical 2D-HHPs [(BA)2PbI4 and (PhE)2PbI4] to their three-dimensional (3D) counterpart, MAPbI3. Comparison to the simpler, 3D MAPbI3 crystal shows clear similarities with the structural dynamics of (BA)2PbI4 and (PhE)2PbI4 across a wide temperature range. The analogy between the 3D and 2D crystals allows us to isolate the effect of the organic cation on the structural dynamics of the inorganic scaffold of the 2D-HHPs. Furthermore, using this approach, we uncover the mechanism of the order-disorder phase transition of (BA)2PbI4 (274 K) and show that it involves relaxation of octahedral tilting coupled to anharmonic thermal fluctuations. These anharmonic fluctuations are important because they induce charge carrier localization and affect the optoelectronic performance of these materials.

Vibrational relaxation dynamics in layered perovskite quantum wells

Organic-inorganic layered perovskites, or Ruddlesden-Popper perovskites, are two-dimensional quantum wells with layers of lead-halide octahedra stacked between organic ligand barriers. The combination of their dielectric confinement and ionic sublattice results in excitonic excitations with substantial binding energies that are strongly coupled to the surrounding soft, polar lattice. However, the ligand environment in layered perovskites can significantly alter their optical properties due to the complex dynamic disorder of the soft perovskite lattice. Here, we infer dynamic disorder through phonon dephasing lifetimes initiated by resonant impulsive stimulated Raman photoexcitation followed by transient absorption probing for a variety of ligand substitutions. We demonstrate that vibrational relaxation in layered perovskite formed from flexible alkyl-amines as organic barriers is fast and relatively independent of the lattice temperature. Relaxation in layered perovskites spaced by aromatic amines is slower, although still fast relative to bulk inorganic lead bromide lattices, with a rate that is temperature dependent. Using molecular dynamics simulations, we explain the fast rates of relaxation by quantifying the large anharmonic coupling of the optical modes with the ligand layers and rationalize the temperature independence due to their amorphous packing. This work provides a molecular and time-domain depiction of the relaxation of nascent optical excitations and opens opportunities to understand how they couple to the complex layered perovskite lattice, elucidating design principles for optoelectronic devices.

Multidimensional time-of-flight spectroscopy

Experimental methods based on a wide range of physical principles are used to determine carrier mobilities for light-harvesting materials in photovoltaic cells. For example, in a time-of-flight experiment, a single laser pulse photoexcites the active layer of a device, and the transit time is determined by the arrival of carriers at an acceptor electrode. With inspiration from this conventional approach, we present a multidimensional time-of-flight technique in which carrier transport is tracked with a second intervening laser pulse. Transient populations of separate material components of an active layer may then be established by tuning the wavelengths of the laser pulses into their respective electronic resonances. This experimental technique is demonstrated using photovoltaic cells based on mixtures of organohalide perovskite quantum wells. In these "layered perovskite" systems, charge carriers are funneled between quantum wells with different thicknesses because of staggered band alignments. Multidimensional time-of-flight measurements show that these funneling processes do not support long-range transport because of carrier trapping. Rather, our data suggest that the photocurrent is dominated by processes in which the phases of the thickest quantum wells absorb light and transport carriers without transitions into domains occupied by quantum wells with smaller sizes. These same conclusions cannot be drawn using conventional one-dimensional techniques for measuring carrier mobilities. Advantages and disadvantages of multidimensional time-of-flight experiments are discussed in the context of a model for the signal generation mechanism.

Interlayer Triplet Energy Transfer in Dion-Jacobson Two-Dimensional Lead Halide Perovskites Containing Naphthalene Diammonium Cations

Recently, hybrid perovskites have gained attention as sensitizers for molecular triplet generation. Layered, two-dimensional (2D) perovskites are especially well-suited for this purpose because the triplet donor (inorganic framework) and triplet acceptor (organic layer) are self-assembled into adjacent sheets, so that with the appropriate energetics, triplets can be driven across the interface. Here we examine interlayer energy transfer in a series of mixed-halide Dion-Jacobson 2D perovskites containing divalent naphthalene cations. We find that the sensitized phosphorescence in these compounds is dominated by naphthalene triplet excimer emission, but when the inorganic exciton is tuned near resonance with the naphthalene triplet, naphthalene monomer phosphorescence competes with triplet excimer formation. The interlayer energy-transfer process is further revealed by ultrafast transient absorption spectroscopy through kinetic variations in triplet excimer formation times. Ultimately, gaining control over interlayer interactions in 2D perovskites through cation design will help uncover new functions and applications for these materials.