Electronic States Modulation by Coherent Optical Phonons in 2D Halide Perovskites

Insights Into the Local Heating Effects in Triple-Cation Mixed-Halide Perovskite Cells: Charge Dynamics, Coherent Phonons, Anion Segregation

The behavior of triple-cation mixed halide perovskite solar cells (PSCs) under ultrashort laser pulse irradiation at varying fluences is investigated, with a focus on local heating effects observed in femtosecond transient absorption (TA) studies. The carrier cooling time constant is found to increase from 230 fs at 2 µJ cm⁻2 to 1.3 ps at 2 mJ cm⁻2 while the charge population decay accelerates from tens of nanoseconds to the picosecond range within the same fluence range. At fluences between 0.5 and 5 mJ cm2, distinct oscillations in the TA signal (≈1.1 MHz) reveal the presence of coherent longitudinal acoustic phonons (CLAP). These phonons induce lattice strain propagating at the speed of sound through the perovskite layer and exhibit relatively long damping times. TA spectra further reveal a partially reversible segregation of iodide and bromide anions under pulsed excitation. Interestingly, higher local heating at increased pump fluences slows the segregation process, with time constants extending from 40 min at low fluences to 110 min at high fluences. However, the continuous irradiation results in significantly smaller segregation effects compared to ultrashort pulse irradiation.

Mechanism of Hot-Carrier Photoluminescence in Sn-Based Perovskites

Metal halide perovskites have shown exceptionally slow hot-carrier cooling, which has been attributed to various physical mechanisms without reaching a consensus. Here, experiment and theory are combined to unveil the carrier cooling process in formamidinium (FA) and caesium (Cs) tin triiodide thin films. Through impulsive vibrational spectroscopy and molecular dynamics, much shorter phonon dephasing times of the hybrid perovskite, which accounts for the larger blueshift in the photoluminescence seen at high excitation density for FASnI3 compared to CsSnI3 is reported. Density functional theory investigations reveal that the largest contribution to the blueshift is accounted by a giant, dynamic band-filling effect in Sn-based perovskites, which in turn can explain the cooling disparity with the Pb-based counterparts. Several years after the first experimental observations, here a deeper understanding of the cooling mechanism of these materials is provided. Design principles for hot-carrier materials, which may be useful for future implementations of hot-carrier solar cells are further provided.

Optical signatures of lattice strain in chemically doped colloidal quantum wells

Lattice strain plays a vital role in tailoring the optoelectronic performance of colloidal nanocrystals (NCs) with exotic geometries. Although optical identifications of lattice strain in irregular-shaped NCs or hetero-structured NCs have been well documented, less is known about optical signatures of the sparsely distributed lattice mismatch in chemically-doped NCs. Here, we show that coherent acoustic phonons (CAPs) following bandgap optical excitations in Cu-doped CdSe colloidal quantum wells (CQWs) offer a unique platform for indirectly measuring the dopant-induced lattice strain. By comparing the behavior of CAPs in Cu-doped and undoped CQWs (i.e., vibrational phase/lifetime/amplitude), we have revealed the driving force of CAPs related to the optical screening of lattice strain-induced piezoelectric fields, which thus allows to determine the strain-induced piezoelectric field of ~102 V/m in Cu-doped CdSe CQWs. This work may facilitate a detailed understanding of lattice strain in chemically-doped colloidal NCs, which is a prerequisite for the design of favorable doped colloids in optoelectronics.

Optimizing Sublattice Correlation to Enhance Stability and Charge Carrier Lifetime in Mixed Halide Perovskites

A-site cations in ABX3 metal halide perovskites do not contribute to the frontier electronic states. They influence optoelectronic properties indirectly through interaction with the BX3 sublattice. By systematically investigating correlated motions of Cs cations and the PbX3 lattice (X = Cl, Br, I), we demonstrate that the interaction between the two subsystems depends on electronegativity and size of the X-site anion. The most electronegative Cl halide minimizes thermal atomic fluctuations, favoring optoelectronic performance. CsPbI3 is improved by Cl-doping. Nonadiabatic molecular dynamics simulations demonstrate that charge carrier lifetime is extended by nearly an order of magnitude when atomic fluctuations are minimized, due to reduced electron-vibrational interactions, in agreement with experiments. The detailed atomistic examination of the significant impact of correlated motion of the A-site and BX3 sublattices and its influence on perovskite stability and exciton lifetime offers theoretical guidelines for optimizing perovskite optoelectronic devices.

Exciton-Polaron in a Quasi-One-Dimensional Chain of Hexyl-Diammonium-BiI5 Octahedra

Lower-dimensional organic-inorganic hybrid perovskite materials promise to revolutionize the optoelectronics industry due to the tremendous possibilities of exotic control on excitonic properties driven via quantum confinement. Flexible organic cations acting as spacers and stabilizers enhance electron-phonon couplings, further amplifying the potential for modular light-matter interactions in these materials. Herein we unravel the nature of excitons in a quasi-1D chain of corner-sharing bismuth iodide octahedra with an intrinsic quantum well structure stabilized by a hexyl-diammonium cation. Using broadband femtosecond impulsive Raman spectroscopy and detailed electronic structure calculations, we directly quantify the exciton lifetime along with the electron-phonon coupling constants to fully describe the excitation as an exciton-polaron. We find ∼30 times larger electron-phonon couplings beyond the standard 2D-hybrid perovskite materials along with picosecond time-scale decoherences, thereby shedding light for the first time on the immense potential of these 1D perovskite analogues for developing novel materials for efficient light-conversion technologies.

Slow Dephasing of Coherent Optical Phonons in Two-Dimensional Lead Organic Chalcogenides

Hybrid organic-inorganic semiconductors with strong electron-phonon interactions provide a programmable platform for developing a variety of electronic, optoelectronic, and quantum materials by controlling these interactions. However, in current hybrid semiconductors such as halide perovskites, anharmonic vibrations with rapid dephasing hinder the ability to coherently manipulate phonons. Here, we report the observation of long-lived coherent phonons in lead organic chalcogenides (LOCs), a new family of hybrid two-dimensional semiconductors. These materials feature harmonic phonon dynamics despite distorted lattices, combining long phonon dephasing times with tunable semiconducting properties. A dephasing time -up to 75 ps at 10 K, with up to ∼500 cycles of phonon oscillation between scattering events, was observed, corresponding to a dimensionless harmonicity parameter that is more than an order of magnitude larger than that of halide perovskites. The phonon dephasing time is significantly influenced by anharmonicity and centrosymmetry, both of which can be tuned through the design of the organic ligands enabled by the direct bonding between the organic and inorganic motifs. This research opens new opportunities for the manipulation of electronic properties with coherent phonons in hybrid semiconductors.

The Role of Cations in Coherent Phonon Properties in Dion-Jacobson Type 2D Hybrid Organic-Inorganic Perovskites

2D hybrid organic-inorganic perovskites (HOIPs) have great prospects in new-generation photovoltaic and luminescence applications. The complex interaction between exciton and lattice motions plays an important role in determining the exciton properties of 2D HOIPs. However, the exploration of the regulation mechanism of phonons and the exciton-phonon (ex-ph) interaction is still very limited, especially for Dion-Jacobson (DJ) type 2D HOIPs. Here, we investigate three DJ-type 2D HOIPs with different cations, (3AMP)PbI4, (4AMP)PbI4, and (1,6-HDA)PbI4, using temperature-dependent steady-state spectra and time-resolved spectra. It is found that the subtle difference of functional group position in cations and the difference between ring and straight carbon chains have a profound effect on the structural distortion, the ex-ph coupling strength, and even the band gap variation trend with temperature in the DJ type 2D HOIP system. The study provides new insights into how to manipulate ex-ph interactions by targeting ligand modifications to 2D perovskite structures.

Persistent Exciton Dressed by Weak Polaronic Effect in Rigid and Harmonic Lattice Dion-Jacobson 2D Perovskites

The emerging two-dimensional (2D) Dion-Jacobson (DJ) perovskites with bidentate ligands have attracted significant attention due to enhanced structural stability compared with conventional Ruddlesden-Popper (RP) perovskites with monodentate ligands linked by van der Waals interactions. However, how the pure chemical bond lattice interacts with excited state excitons and its impact on the exciton nature and dynamics in 2D DJ-perovskites, particularly in comparison to RP-perovskites, remains unexplored. Herein, by a combined spectroscopy study on excitonic and structural dynamics, we reveal a persistent exciton dressed by a weak polaronic effect in DJ-perovskite due to their rigid and harmonic lattice, in striking contrast to significantly screened exciton polaron observed in RP-perovskites. Despite the similar exciton binding energy (∼0.3 eV) in both n = 1 DJ- and RP-perovskites with near-identical crystal structure, photoexcitation results in a slightly screened exciton with minimal structural relaxation and a retained binding energy of ∼0.29 eV in DJ-perovskites but strongly screened exciton polaron with a binding energy of ∼0.13 eV in RP-perovskites. Structural dynamics further highlight the rigid and harmonic lattice motion in DJ-perovskites, as opposed to the thermally activated anharmonic lattice in RP-perovskites, arising from their distinct bonding modes. Our study offers insights into modulating excited state properties in 2D perovskites, simulating the rational design of hybrid semiconductors with tailored properties and functionalities.

Reducing Dielectric Confinement Effect Enhances Carrier Separation in Two-Dimensional Hybrid Perovskite Photocatalysts

Two-dimensional organic-inorganic hybrid perovskites (OIHPs) with alternating structure of the organic and inorganic layers have a natural quantum well structure. The difference of dielectric constants between organic and inorganic layers in this structure results in the enhancement of dielectric confinement effect, which exhibits a large exciton binding energy and hinders the separation of electron-hole pairs. Herein, a strategy to reduce the dielectric confinement effect by narrowing the dielectric difference between organic amine molecule and [PbBr6]4- octahedron is put forward. The Ethanolamine (EOA) contains hydroxyl groups, resulting in the positive and negative charge centers of O and H non-overlapping, which generated a larger polarity and dielectric constant. The reduced dielectric constant produces a smaller exciton binding energy (71.03 meV) of (C2H7NO)2PbBr4 ((EOA)2PbBr4) than (C8H11N)2PbBr4 ((PEA)2PbBr4 (156.07 meV), and promotes the dissociation of electrons and holes. The increasing of lifetime of photogenerated carrier in (EOA)2PbBr4 are proved by femtosecond transient absorption spectra. Density functional theory (DFT) calculations have also indicated that the small energy shift of the total density of states (DOS) between the C/H/N and the Pb/Br in (EOA)2PbBr4 favors the separation of electrons and holes. In addition, this work demonstrates the application of (PEA)2PbBr4 and (EOA)2PbBr4 in the field of photocatalytic CO2 reduction.

Controlling Ultrafast Magnetization Dynamics via Coherent Phonon Excitation in a Ferromagnet Monolayer

Exploring ultrafast magnetization control in 2D magnets via laser pulses is established, yet the interplay between spin dynamics and the lattice remains underexplored. Utilizing real-time time-dependent density functional theory (rt-TDDFT) coupled with Ehrenfest dynamics and nonadiabatic molecular dynamics (NAMD) simulations, we systematically investigate the laser-induced spin-nuclei dynamics with pre-excited A1g and E2g coherent phonons in the 2D ferromagnet Fe3GeTe2 (FGT) monolayer. Selective pre-excitation of coherent phonons under ultrafast laser irradiation significantly alters the local spin moment of FGT, consequently inducing additional spin loss attributed to the nuclear motion-induced asymmetric interatomic charge transfer. Excited spin-resolved charge undergoes a bidirectional spin-flip between spin-down and spin-up states, characterized by a subtle change in the spin moment within approximately 100 fs, followed by unidirectional spin-flip, which will further contribute to the spin moment loss of FGT within tens of picoseconds. Our results shed light on the coupling of coherent phonons with magnetization dynamics in 2D limit.

Elucidating phonon dephasing mechanisms in layered perovskites with coherent Raman spectroscopies

Organic-inorganic hybrid perovskite quantum wells exhibit electronic structures with properties intermediate between those of inorganic semiconductors and molecular crystals. In these systems, periodic layers of organic spacer molecules occupy the interstitial spaces between perovskite sheets, thereby confining electronic excitations to two dimensions. Here, we investigate spectroscopic line broadening mechanisms for phonons coupled to excitons in lead-iodide layered perovskites with phenyl ethyl ammonium (PEA) and azobenzene ethyl ammonium (AzoEA) spacer cations. Using a modified Elliot line shape analysis for the absorbance and photoluminescence spectra, polaron binding energies of 11.2 and 17.5 meV are calculated for (PEA)2PbI4 and (AzoEA)2PbI4, respectively. To determine whether the polaron stabilization processes influence the dephasing mechanisms of coupled phonons, five-pulse coherent Raman spectroscopies are applied to the two systems under electronically resonant conditions. The prominence of inhomogeneous line broadening mechanisms detected in (AzoEA)2PbI4 suggests that thermal fluctuations involving the deformable organic phase broaden the distributions of phonon frequencies within the quantum wells. In addition, our data indicate that polaron stabilization primarily involves photoinduced reorganization of the organic phases for both systems, whereas the impulsively excited phonons represent less than 10% of the total polaron binding energy. The signal generation mechanisms associated with our fifth-order coherent Raman experiments are explored with a perturbative model in which cumulant expansions are used to account for time-coincident vibrational dephasing and polaron stabilization processes.

Modulating Carrier Dynamics in PdSe2: The Role of Pressure in Electronic and Phononic Interactions

PdSe2 is a puckered transition metal dichalcogenide that has been reported to undergo a two-dimensional to three-dimensional structural transition under pressure. Here, we investigated the electronic and phononic evolution of PdSe2 under high pressure using pump-probe spectroscopy. We observed the electronic intraband and interband transitions occurring in the d orbitals of Pd, revealing the disappearance of the Jahn-Teller effect under high pressure. Furthermore, we found that the decay rates of interband recombination and intraband relaxation lifetimes change at 3 and 7 GPa, respectively. First-principles calculations suggest that the bandgap closure slows the decay rate of interband recombination after 3 GPa, while the saturation of phonon-phonon scattering is the main reason for the relatively constant intraband relaxation lifetime. Our work provides a novel perspective for understanding the evolution of the electron and modulation of the carrier dynamics by phonons under pressure.

Exciton polaron formation and hot-carrier relaxation in rigid Dion-Jacobson-type two-dimensional perovskites

The efficiency of two-dimensional Dion-Jacobson-type materials relies on the complex interplay between electronic and lattice dynamics; however, questions remain about the functional role of exciton-phonon interactions. Here we establish the robust polaronic nature of the excitons in these materials at room temperature by combining ultrafast spectroscopy and electronic structure calculations. We show that polaronic distortion is associated with low-frequency (30-60 cm-1) lead iodide octahedral lattice motions. More importantly, we discover how targeted ligand modification of this two-dimensional perovskite structure manipulates exciton-phonon coupling, exciton polaron population and carrier cooling. At high excitation density, stronger exciton-phonon coupling increases the hot-carrier lifetime, forming a hot-phonon bottleneck. Our study provides detailed insight into the exciton-phonon coupling and its role in carrier cooling in two-dimensional perovskites relevant for developing emerging hybrid semiconductor materials with tailored properties.

Polarization dependent exciton-plasmon coupling in PEA2PbI4/Al and its application to perovskite solar cell

This paper reports the strong coupling between Al nanostructure and two-dimensional (2D) layered perovskite PEA2PbI4 (PEPI) films. The high exciton binding energy of 118 meV and long carrier lifetime of 216 ps are characterized from the 2D PEA2PbI4 film, which indicates that the excitons in perovskite are robust and can couple to metal plasmons. The ordinary and extraordinary optical dispersions are revealed from the anisotropic 2D perovskite. The transmission spectra of PEA2PbI4/Al nanoparticle arrays are simulated under different polarization excitations, and the typical anti-crossing behaviors originating from exciton-plasmon strong coupling are demonstrated. We found that compared with transverse magnetic (TM) polarization, transverse electric (TE) polarization excitation is more conducive to the realization of exciton-plasmon coupling with a larger Rabi splitting. Furthermore, the PEA2PbI4/Al nanoparticle arrays are proposed, which present polarization-dependent local electrical field enhancement due to the exciton-local surface plasmon polariton coupling. Additionally, it is noticed that the proposed plasmonic structure increases the photo-generation rate inside the active material with improved current density. Therefore, the 2D proposed plasmonic design increases the power conversion efficiency (PCE) with an enhancement of 3.3% and 1.3% relative to the planar structures for TE and TM polarizations, respectively. This study provides a deeper understanding of polarized exciton-plasmon coupling properties, promoting the development of the field of plasmon and providing guidance for the design and preparation of efficient optoelectronic devices.

Organic and inorganic sublattice coupling in two-dimensional lead halide perovskites

Two-dimensional layered organic-inorganic halide perovskites have successfully spread to diverse optoelectronic applications. Nevertheless, there remain gaps in our understanding of the interactions between organic and inorganic sublattices that form the foundation of their remarkable properties. Here, we examine these interactions using pump-probe spectroscopy and ab initio molecular dynamics simulations. Unlike off-resonant pumping, resonant excitation of the organic sublattice alters both the electronic and lattice degrees of freedom within the inorganic sublattice, indicating the existence of electronic coupling. Theoretical simulations verify that the reduced bandgap is likely due to the enhanced distortion index of the inorganic octahedra. Further evidence of the mechanical coupling between these two sublattices is revealed through the slow heat transfer process, where the resultant lattice tensile strain launches coherent longitudinal acoustic phonons. Our findings explicate the intimate electronic and mechanical couplings between the organic and inorganic sublattices, crucial for tailoring the optoelectronic properties of two-dimensional halide perovskites.

Energy Transfer-Dominated Quasi-2D Blue Perovskite Light-Emitting Diodes

The bromide-chloride mixed quasi-two-dimensional (2D) perovskite, with a natural quantum well structure and tunable exciton binding energy, has gained significant attention for high-performance blue perovskite light-emitting diodes (PeLEDs). However, the relative importance of having a low trap state density or efficient exciton transfer for high-efficiency electroluminescence (EL) performance remains elusive. Here, two molecules with the benzoic acid group, sodium 4-fluorobenzoate (SFB) and 3,5-dibromobenzoic acid (DBA), are used to modulate the phase distribution and trap state to explore the effect between energy transfer and defect passivation. As a result, when the n = 1 phase is inhibited in both films, the DBA@SFB-modified perovskite films achieve a higher photoluminescence quantum yield (PLQY) than the SFB-modified perovskite films due to effective defect passivation. However, DBA@SFB-modified PeLEDs exhibit lower external quantum efficiency (EQE) compared to SFB-modified PeLEDs due to the poor exciton transfer between the low-dimensional phase. This demonstrates that passivation strategies may enhance photoluminescence through reducing nonradiative recombination, but the effect of phase distribution is pivotal for EL performance by efficient energy transfer in quasi-2D perovskites. Femtosecond time-resolved transient absorption measurements confirm the fastest carrier dynamics in SFB-modified perovskite films, further corroborating the above result. This work provides useful information about phase modulation and defect passivation for high-efficiency blue quasi-2D PeLEDs.

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.

A Bound Exciton Resonance Modulated by Bulk and Localized Coherent Phonons in Double Perovskites

Optically excited electronic excitations are coupled to the soft and polar halide perovskite lattice, generating coherent phonons after subpicosecond interband laser-excitation. In Ag-based halide double perovskites, Ag-vacancies can bind free excitons, resulting in a pronounced bound exciton resonance. Here, we report the detection of three modulation frequencies corresponding to coherent phonons in Ag-based double perovskite nanocrystals at distinct spectral positions at the bound exciton resonance. Two of them are found in oscillatory spectral shifts of the bound exciton resonance and are identified as Cs- and Br-related bulk phonons. Surprisingly, a third frequency is observed as an intensity modulation. We argue that this amplitude oscillation is a consequence of an optically generated vibronic wave packet localized at a Ag-vacancy. Consequently, the localized coherent phonon modulates the giant oscillator strength of the bound exciton. This optically induced and spatially localized lattice shaking could potentially be useful for initiating photochemical reactions with atomic precision.

Time-Resolved Dynamics of Metal Halide Perovskite under High Pressure: Recent Progress and Challenges

Metal halide perovskites have garnered significant attention in the scientific community for their promising applications in optoelectronic devices. The application of pressure engineering, a viable technique, has played a crucial role in substantially improving the optoelectronic characteristics of perovskites. Despite notable progress in understanding ground-state structural changes under high pressure, a comprehensive exploration of excited-state dynamics influencing luminescence remains incomplete. This Perspective delves into recent advances in time-resolved dynamics studies of photoexcited metal halide perovskites under high pressure. With a focus on the intricate interplay between structural alterations and electronic properties, we investigate electron-phonon interactions, carrier transport mechanisms, and the influential roles of self-trapped excitons (STEs) and coherent phonons in luminescence. However, significant challenges persist, notably the need for more advanced measurement techniques and a deeper understanding of the phenomena induced by high pressure in perovskites.

Carrier Transport in 2D Hybrid Organic-Inorganic Perovskites: The Role of Spacer Molecules

Two-dimensional organic-inorganic hybrid perovskites (2D HOIPs) have been widely used for various optoelectronics applications owing to their excellent photoelectric properties. However, the selection of organic spacer cations is mostly qualitative without quantitative guidance. Meanwhile, the fundamental mechanism of the carrier transport across the organic spacer layer is still unclear. Here, by using the first-principles nonadiabatic molecular dynamics (NAMD) method, we have studied the transport process of excited carriers between 2D HOIPs separated by a spacer cation layer in real time at atomic levels. We find that the excited electrons and holes can transfer from single-inorganic-layer 2D HOIP to bi-inorganic-layer 2D HOIP on a subpicosecond to picosecond scale. Moreover, we have developed a new method to capture the electron-hole interaction in the frame of NAMD. This work provides a promising direction to design new materials toward high-performance optoelectronics.

Small to Large Polaron Behavior Induced by Controlled Interactions in Perovskite Quantum Dot Solids

The polaron is an essential photoexcitation that governs the unique optoelectronic properties of organic-inorganic hybrid halide perovskites, and it has been subject to extensive spectroscopic and theoretical investigation over the past decade. A crucial but underexplored question is how the nature of the photogenerated polarons is impacted by the microscopic perovskite structure and what functional properties this affects. To tackle this question, we chemically tuned the interactions between perovskite quantum dots (QDs) to rationally manipulate the polaron properties. Through a suite of time-resolved spectroscopies, we find that inter-QD interactions open an excited-state channel to form large polaron species, which exhibit enhanced spatial diffusion, slower hot polaron cooling, and a longer intrinsic lifetime. At the same time, polaronic excitons are formed in competition via localized band-edge states, exhibiting strong photoluminescence but are limited by shorter intrinsic lifetimes. This control of polaron type and function through tunable inter-QD interactions not only provides design principles for QD-based materials but also experimentally disentangles polaronic species in hybrid perovskite materials.

Charge carrier dynamics and transient spectral evolutions in lead halide perovskites

Lead halide perovskites (LHPs) have emerged as promising materials for solar cell applications due to their unique photophysical properties. Most of the crucial properties related to solar cell performance such as carrier mobility, diffusion length, recombination rates, etc. have been estimated using ultrafast spectroscopic methods. While various methods have been developed to prepare and fabricate high-quality perovskite films for photovoltaic applications, understanding the charge carrier dynamics is also crucial at each stage of the charge generation, cooling, and recombination processes. Using femtosecond (fs) transient absorption (TA) spectroscopy, various stages of charge carrier dynamics in perovskite materials could be monitored. In this article, we focus on some of the recent experimental developments related to charge carrier dynamics in perovskites and discuss the current understanding of (1) exciton dissociation, (2) charge carrier thermalization, (3) hot carrier cooling, and (4) electron-phonon coupling along with some of the crucial spectral emergence in the pump-probe experiments of LHP materials.

Dispersion of the third-order optical nonlinearities in 2D (PEA)2PbI4 perovskite film

We report the wavelength-dependent third-order optical nonlinearity of two-dimensional halide organic-inorganic perovskite (PEA)2PbI4 film experimentally. The high-quality two-dimensional (PEA)2PbI4 film prepared via confinement-assisted drop-casting process exhibits ultrafast optical response and large third-order optical nonlinearities, and the measured nonlinear refractive index is closer to the quantum perturbation model accounting for the excitonic effect. In addition, the wavelength-dependent optical response transition from self-focusing to self-defocusing, saturable absorption to reverse saturable absorption has been observed and investigated. The experimental results confirm the large third-order optical nonlinearities in (PEA)2PbI4 film and may make inroads toward developing cost-effective high-performance optoelectronic devices.

Uncovering the mechanisms of efficient upconversion in two-dimensional perovskites with anti-Stokes shift up to 220 meV

Phonon-assisted photon upconversion holds great potential for numerous applications, e.g., optical refrigeration. However, traditional semiconductors face energy gain limitations due to thermal energy, typically achieving only ~25 milli-electron volts at room temperature. Here, we demonstrate that quasi-two-dimensional perovskites, with a soft hybrid organic-inorganic lattice, can efficiently upconvert photons with an anti-Stokes shift exceeding 200 milli-electron volts. By using microscopic transient absorption measurements and density functional theory calculations, we explicate that the giant energy gain stems from strong lattice fluctuation leading to a picosecond timescale transient band energy renormalization with a large energy variation of around ±180 milli-electron volts at room temperature. The motion of organic molecules drives the deformation of inorganic framework, providing energy and local states necessary for efficient upconversion within a time constant of around 1 ps. These results establish a deep understanding of perovskite-based photon upconversion and offer previously unknown insights into the development of various upconversion applications.

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.

Carriers, Quasi-particles, and Collective Excitations in Halide Perovskites

Halide perovskites (HPs) are potential game-changing materials for a broad spectrum of optoelectronic applications ranging from photovoltaics, light-emitting devices, lasers to radiation detectors, ferroelectrics, thermoelectrics, etc. Underpinning this spectacular expansion is their fascinating photophysics involving a complex interplay of carrier, lattice, and quasi-particle interactions spanning several temporal orders that give rise to their remarkable optical and electronic properties. Herein, we critically examine and distill their dynamical behavior, collective interactions, and underlying mechanisms in conjunction with the experimental approaches. This review aims to provide a unified photophysical picture fundamental to understanding the outstanding light-harvesting and light-emitting properties of HPs. The hotbed of carrier and quasi-particle interactions uncovered in HPs underscores the critical role of ultrafast spectroscopy and fundamental photophysics studies in advancing perovskite optoelectronics.

Strain propagation in layered two-dimensional halide perovskites

Impulsive light excitation presents a powerful tool for investigating the interdependent structural and electronic responses in layered two-dimensional (2D) halide perovskites. However, detailed understanding of the nonlinear lattice dynamics in these soft hybrid materials remains limited. Here, we explicate the intrinsic strain propagation mechanisms in 2D perovskite single crystals using transient reflection spectroscopy. Ultrafast photoexcitation leads to the generation of strain pulses via thermoelastic (TE) stress and deformation potential (DP) interaction whence their detection proceed via Brillouin scattering. Using a two-temperature model together with strain wave propagation, we discern the TE and DP contributions in strain generation. Hot carrier cooling plays a dominant role in effecting the weak modulation amplitude. Out-of-plane lattice stiffness is reduced by the weak van der Waals bond between organic layers, resulting in a slow strain propagation velocity. Our findings inject fresh insights into the basic strain properties of layered perovskites critical for manipulating their functional properties for new applications.

Coupled Electronic and Anharmonic Structural Dynamics for Carrier Self-Trapping in Photovoltaic Antimony Chalcogenides

V-VI antimony chalcogenide semiconductors have shown exciting potentials for thin film photovoltaic applications. However, their solar cell efficiencies are strongly hampered by anomalously large voltage loss (>0.6 V), whose origin remains controversial so far. Herein, by combining ultrafast pump-probe spectroscopy and density functional theory (DFT) calculation, the coupled electronic and structural dynamics leading to excited state self-trapping in antimony chalcogenides with atomic level characterizations is reported. The electronic dynamics in Sb2 Se3 indicates a ≈20 ps barrierless intrinsic self-trapping, with electron localization and accompanied lattice distortion given by DFT calculations. Furthermore, impulsive vibrational coherences unveil key SbSe vibrational modes and their real-time interplay that drive initial excited state relaxation and energy dissipation toward stabilized small polaron through electron-phonon and subsequent phonon-phonon coupling. This study's findings provide conclusive evidence of carrier self-trapping arising from intrinsic lattice anharmonicity and polaronic effect in antimony chalcogenides and a new understanding on the coupled electronic and structural dynamics for redefining excited state properties in soft semiconductor materials.

Electric Field Modulation of 2D Perovskite Excitonics

Multiquantum-well (MQW) perovskite is one of the forerunners in high-efficiency perovskite LED (PeLEDs) research. Despite the rapid inroads, PeLEDs suffer from the pertinent issue of efficiency decrease with increasing brightness, commonly known as "efficiency roll-off". The underlying mechanisms are presently an open question. Herein, we explicate the E-field effects on the exciton states in the archetypal MQW perovskite (C₆H₅C₂H₄NH₃)₂PbI₄, or PEPI, in a device-like architecture using field-assisted transient spectroscopy and theoretical modeling. The applied E-field results in a complex interplay of spectral blueshifts and enhancement/quenching of the different exciton modes. The former originates from the DC Stark shift, while the latter is attributed to the E-field modulation of the transfer rates between bright/dark exciton modes. Importantly, our findings uncover crucial insights into the photophysical processes under E-field modulation contributing to efficiency roll-off in MQW PeLEDs. Electrical modulation of exciton properties presents exciting possibilities for signal processing devices.

Optically Induced Long-Lived Chirality Memory in the Color-Tunable Chiral Lead-Free Semiconductor (R)/(S)-CHEA4Bi2BrxI10-x (x = 0-10)

Hybrid organic-inorganic networks that incorporate chiral molecules have attracted great attention due to their potential in semiconductor lighting applications and optical communication. Here, we introduce a chiral organic molecule (R)/(S)-1-cyclohexylethylamine (CHEA) into bismuth-based lead-free structures with an edge-sharing octahedral motif, to synthesize chiral lead-free (R)/(S)-CHEA₄Bi₂Br_xI_10-x crystals and thin films. Using single-crystal X-ray diffraction measurements and density functional theory calculations, we identify crystal and electronic band structures. We investigate the materials' optical properties and find circular dichroism, which we tune by the bromide-iodide ratio over a wide wavelength range, from 300 to 500 nm. We further employ transient absorption spectroscopy and time-correlated single photon counting to investigate charge carrier dynamics, which show long-lived excitations with optically induced chirality memory up to tens of nanosecond timescales. Our demonstration of chirality memory in a color-tunable chiral lead-free semiconductor opens a new avenue for the discovery of high-performance, lead-free spintronic materials with chiroptical functionalities.

Phononic Fine-Tuning in a Prototype Two-Dimensional Hybrid Organic-Inorganic Perovskite System

The emerging two-dimensional (2D) lead-halide perovskite materials hold great promise for next-generation photovoltaic and optoelectronic applications, in which phonon engineering plays a crucial role. However, detailed mechanistic exploration related to phonon effects, especially from a dynamics perspective, remains rather limited. Herein, we present a systematic demonstration of phononic fine-tuning in a prototype 2D hybrid organic-inorganic perovskite (HOIP) system, i.e., phenethylammonium lead iodide [(PEA)₂PbI₄] with each hydrogen atom at positions 2 (ortho), 3 (meta), and 4 (para) on the PEA's phenyl group being replaced by a fluorine atom. Through a set of joint observations via ultrafast spectroscopy and temperature-dependent photoluminescence spectroscopy, we reveal that such a fluorination can subtly exert profound impacts on its structural distortion-induced phononic properties, including coherent phonon modes, phonon-phonon/electron-phonon interactions, and the hot-phonon bottleneck effect. This work highlights the significant importance of the atomic-level tailoring of organic cations in low-dimensional HOIP systems, which is usually ignored in conventional notion and practice.

Optical control of exciton spin dynamics in layered metal halide perovskites via polaronic state formation

One of the open challenges of spintronics is to control the spin relaxation mechanisms. Layered metal-halide perovskites are an emerging class of semiconductors which possess a soft crystal lattice that strongly couples electronic and vibrational states and show promise for spintronic applications. Here, we investigate the impact of such strong coupling on the spin relaxation of excitons in the layered perovskite BA₂FAPbI₇ using a combination of cryogenic Faraday rotation and transient absorption spectroscopy. We report an unexpected increase of the spin lifetime by two orders of magnitude at 77 K under photoexcitation with photon energy in excess of the exciton absorption peak, and thus demonstrate optical control over the dominant spin relaxation mechanism. We attribute this control to strong coupling between excitons and optically excited phonons, which form polaronic states with reduced electron-hole wave function overlap that protect the exciton spin memory. Our insights highlight the special role of exciton-lattice interactions on the spin physics in the layered perovskites and provide a novel opportunity for optical spin control.

Spintronic devices will require long spin lifetimes, but the effect of exciton-lattice coupling on spin lifetime in metal-halide perovskites is not well understood. Here, the authors find a 100-fold increase in the lifetime of exciton spins in a 2D perovskite by exciting with excess energy, resulting from strong coupling between excitons and optically excited phonons.

Phonon-Mediated Slow Hot Carrier Dynamics in Lead-Free Cs3Bi2I9 Perovskite Single Crystal

In this report, we study the hot carrier cooling mechanism of the Cs3Bi2I9 single crystal by using femtosecond transient reflectance (fs-TR) spectroscopy. We find an unusual slow hot carrier cooling associated with longitudinal optical (LO) and coherent longitudinal acoustic phonons (CLAPs) emission during the deexcitation of the hot carriers. We posit the interplay between the hot-carriers and the LO and CLA phonons in subpicosecond to subnanosecond time scales, respectively, by analyzing the TR kinetics upon perturbation with excess energy. Furthermore, we measured the CLAPs propagation velocity in Cs3Bi2I9, the crystal, ranging from 1820 to 2000 ms-1. The elastic constants and frequency of Brillouin oscillations were estimated as 20.08 GPa and 14.66 GHz, respectively. Our discovery delivers new physical insights into how the hot carriers in Cs3Bi2I9 single crystal are coupled with a crystal lattice that controls the hot carrier dynamics.

Exciton-Phonon Coupling of Chiral One-Dimensional Lead-Free Hybrid Metal Halides at Room Temperature

The interaction between organic cations and inorganic metal halide octahedral units strongly affects the properties of organic-inorganic hybrid metal halides. The "soft" property of the lattice provides the possibility of its strong exciton-phonon interaction. Here we report one-dimensional (1D) lead-free chiral organic-inorganic hybrid metal halide single crystals of (R/S)-methylbenzylamine bismuth iodide (R/S-MBA)₂Bi₂I₈, which exhibits a high level of octahedral bond distortion. The introduction of chiral amines leads to a strong chiroptical response in the range of 200-600 nm. The strong exciton-phonon coupling can be observed through the coherent oscillation spectrum of transient absorption dynamics at room temperature. The coherent phonon oscillation frequencies are ∼97 and ∼130 cm^-1, corresponding to the symmetrical stretching or bending of the Bi-I octahedron. Our work provides new insights for the study of exciton-phonon coupling in 1D chiral hybrid metal halides.

Operando Raman spectroscopic evidence of electron-phonon interactions in NiO/TiO2 pn junction photodetectors

The pn junctions significantly affect the responsivity of photodetectors (PDs). However, the enhancement mechanism of the pn junction is still unclear. Herein, operando Raman spectroscopy was employed to study PDs with NiO/TiO₂ pn junctions composed of p-NiO nanoparticles (NPs) and n-TiO₂ nanotube arrays (TNAs). The results suggest that the built-in potential field of the NiO/TiO₂ interface decreases the charge transfer resistance and changes the vibrational frequency of the phonon modes of TiO₂, which is attributed to the electron-phonon coupling effect. Operando Raman spectroscopy is proved to be a powerful tool for manufacturing highly responsive PDs.

Tuning Dielectric Transitions in Two-Dimensional Organic-Inorganic Hybrid Lead Halide Perovskites

Organic-inorganic hybrid metal halide perovskites possessing unique two-dimensional (2D)-layered structures have been demonstrated with excellent molecular tunability and stability, especially the promising semiconductor properties for solar cell applications. In this work, three 2D lead halide organic-inorganic hybrid perovskites (IAA)₂PbX₄ (IAA = isoamylammonium cation and X = Cl, Br, and I) were synthesized by employing a solution processing method and demonstrate distinct tuning solid-state phase transitions coupled with dielectric responses, as well as light absorption properties. Among the title perovskites, the phase transition temperature decreases gradually, and their band gap also indicates a narrowing trend. The results are mainly derived from slight changes in the crystal structure by halogen regulation. These findings might provide an effective crystal engineering strategy for exploring high-performance functional perovskite materials.