Screening in crystalline liquids protects energetic carriers in hybrid perovskites

Exciton-Phonon-Plasmon Interplay in Hot Carrier Relaxation Dynamics in Perovskite Crystals

Enhancing light-matter interaction in nanostructures using metallic surface plasmons is a dependable route for improving efficiencies in optoelectronic applications. Plasmonic interfaces of organic cation-based halide perovskites that show high quantum efficiency and enhanced carrier lifetimes are seen as a technologically important avenue for new-age photovoltaics and quantum emitters. Here, several interesting multi-particle interplays in hybrid structures of Ga nanodroplets and FAPbBr3 crystals (Ga-NCs) having novel practical applications are reported. In addition to the conventional emission enhancement, a dominant blueshift in the perovskite photoluminescence (PL) is seen in the presence of Ga nanoparticles, which are persistent down to low temperatures. The integrated PL intensity ratio has a non-monotonic temperature dependence indicating a non-trivial exciton-phonon-plasmon interplay. The time-resolved photoluminescence measurements at different excitation wavelengths and transient absorption measurements reveal the strong influence of the Ga nanoparticles on the intrinsic phonon bottleneck typically observed in FAPbBr3 crystals (NCs). Detailed calculations explain the observed results throwing light on the complex interplay of plasmons, excitons, and the phonons in these simple heterojunctions. Ga-supported perovskite nanocrystals with higher quantum yield and ultrafast carrier relaxation pathways are seen to be an exciting system for quantum light emission with facile synthesis techniques.

Correlated Lattice Fluctuations in CsPbBr3 Quantum Dots Give Rise to Long-Lived Electronic Coherence

Electronic coherence is central to numerous areas of science, from quantum biology to quantum materials. In quantum materials, lead-halide perovskite (LHP) quantum dots (QDs) have been shown to support electronic coherence through observation of coherent single-photon emission and superfluorescence arising from spatial coherence at low temperatures. In contrast, direct measurement of temporal coherence between exciton states has been lacking. Here, we employ coherent multi-dimensional spectroscopy to observe an electronic coherence between exciton states in CsPbBr3 QDs that is long-lived at room temperature, surviving nearly three times longer than the electronic dephasing time. This observation of a long-lived electronic coherence at room temperature points to nearly perfectly correlated lattice fluctuations for each excitonic state in the superposition. These experiments reveal that the properties of LHP QDs extend to lattice dynamics that give rise to correlated fluctuations in the basis exciton states, a process that may next be optimized by design.

Unraveling the Nature of Vibrational Dynamics in CsPbI3 by Inelastic Neutron Scattering and Molecular Dynamics Simulations

Cesium lead iodide, CsPbI3, is an optoelectronic material of large interest for various technological applications; however, fundamental questions surrounding the vibrational dynamics of this material, especially regarding its role in structural phase transitions, remain to be elucidated. Here, in a combined variable temperature inelastic neutron scattering (INS) and machine-learning based molecular dynamics (MD) simulation study, we show that the stable phase at room temperature, i.e., the nonperovskite δ-phase, exhibits phonon modes with weak anharmonicity with only a weak temperature dependence from 10 K all the way up to the transition to the cubic perovskite α-phase at approximately 600 K. In contrast, the α-phase features anharmonic and damped vibrational dynamics, mainly associated with overdamped tilting motions of the PbI6 octahedra. Crucially, these overdamped tilting modes, which relate to the tetragonal and orthorhombic distorted perovskite phases (β- and γ-phase, respectively) formed at lower temperatures, stay overdamped by more than 100 K above the respective phase transition. This suggests a flat energy landscape of octahedral tilting motions in α-CsPbI3 and with structural fluctuations on the picosecond time scale with tilting patterns that locally resemble the structure of the β- and γ-phases. The vibrational dynamics of α-CsPbI3 are also characterized by pronounced anharmonic motions with large thermal displacements of the Cs+ ions, but these modes remain underdamped at 600 K.

Multifaceted nature of defect tolerance in halide perovskites and emerging semiconductors

Lead halide perovskites (LHPs) have shot to prominence as efficient energy-conversion materials that can be processed using cost-effective fabrication methods. A reason for their exceptional performance is their crystallographic defect tolerance, enabling long charge-carrier lifetimes despite high defect densities. Achieving defect tolerance in broader classes of materials would impact on the semiconductor industry substantially. Considerable efforts have been made to understand the origins of defect tolerance, so as to design stable and nontoxic alternatives to LHPs. However, understanding defect tolerance in LHPs is far from straightforward. This Review discusses the models proposed for defect tolerance in halide perovskites, evaluating the experimental and theoretical support for these models, as well as their limitations. We also cover attempts to apply these models to identify materials beyond LHPs that could exhibit defect tolerance. Finally, we discuss the experimental methods used to understand defects in mixed ionic-electronic conductors, as well as the important information that is necessary for a deeper understanding, in order to develop improved models that enable the design of defect-tolerant semiconductors.

Giant deformation potential induced small polaron effect in Dion-Jacobson two-dimensional lead halide perovskites

Halide perovskites have attracted substantial attention recently. However, the strong lattice distortion effects in these materials have led to debates regarding the nature of charge carriers. While the behavior of carriers in bulk three-dimensional materials is well-documented, the characteristics of carriers in two-dimensional perovskites remain less well understood. In this study, we provide direct and clear evidence of small polaron formation through transient spectroscopic analysis of deformation potential and dynamic lattice screening. Coherent acoustic phonon wave signals reveal a strong coupling between carriers and lattice degrees of freedom, leading to small polaron formation and a spin lifetime enhancement of up to 10-fold. Utilizing optical Kerr spectroscopy and theoretical modeling, we observed a notably long polarization response time at room temperature, attributed to lattice distortion and small polarons approximately two-unit cells in size. Temperature-dependent coherent phonon dynamics and X-ray diffraction further confirmed the presence of small polarons. This discovery underscores the significance of the cooperative interplay between exciton dynamics and the small polaron field, particularly in influencing the Coulomb exchange interaction of excitons.

Photosensitivity and Carrier Densities of Perovskite Solar Absorbers

Dark and light current-voltage characteristics of perovskite solar absorbers are analyzed in terms of their carrier densities. The analysis reveals p-type large polarons as a dominant carrier type in the investigated perovskite solar cells. The mechanism causing photosensitivity is attributed to the dissociation (and pairing) of bipolarons to large polarons (and vice versa) that are controlled by the internal potential Γ. As an example, the polaron concept is tested for a formamidinium lead triiodide perovskite solar cell. The individual steps of the data analysis are demonstrated and determine the ionicity factor of this perovskite film, quantify the density of the large polarons, and predict the gain and loss of photo-induced carriers. It is deduced that a reversible light-on/off operation can only occur when the bias voltage never exceeds a critical value of the internal potential. The results gained in this study suggest that the novel analysis can be successively applied on different hybrid perovskite materials, too.

Interface phenomena and emerging functionalities in ferroelectric oxide based heterostructures

Capitalizing on the nonvolatile, nanoscale controllable polarization, ferroelectric perovskite oxides can be integrated with various functional materials for designing emergent phenomena enabled by charge, lattice, and polar symmetry mediated interfacial coupling, as well as for constructing novel energy-efficient electronics and nanophotonics with programmable functionalities. When prepared in thin film or membrane forms, the ferroelectric instability of these materials is highly susceptible to the interfacial electrostatic and mechanical boundary conditions, resulting in tunable polarization fields and Curie temperatures and domain formation. This review focuses on two types of ferroelectric oxide-based heterostructures: the epitaxial perovskite oxide heterostructures and the ferroelectric oxides interfaced with two-dimensional van der Waals materials. The topics covered include the basic synthesis methods for ferroelectric oxide thin films, membranes, and heterostructures, characterization of their properties, and various emergent phenomena hosted by the heterostructures, including the polarization-controlled metal-insulator transition and magnetic anisotropy, negative capacitance effect, domain-imposed one-dimensional graphene superlattices, programmable second harmonic generation, and interface-enhanced polar alignment and piezoelectric response, as well as their applications in nonvolatile memory, logic, and reconfigurable optical devices. Possible future research directions are also outlined, encompassing the synthesis via remote epitaxy and oxide moiré engineering, incorporation of binary ferroelectric oxides, realization of topological properties, and functional design of oxygen octahedral rotation.

Surface chemistry-engineered perovskite quantum dot photovoltaics

The discovery and synthesis of colloidal quantum dots (QDs) was awarded the Nobel Prize in Chemistry in 2023. Recently, the development of bulk metal halide perovskite semiconductors has generated intense interest in their corresponding perovskite QDs. QDs, more broadly known as nanocrystals, constitute a new class of materials that differ from both molecular and bulk materials. They have rapidly advanced to the forefront of optoelectronic applications owing to their unique size-, composition-, surface- and process-dependent optoelectronic properties. More importantly, their ultrahigh surface-area-to-volume ratio enables various surface chemistry engineering strategies to tune and optimize their optoelectronic properties. Finally, three-dimensional confined QDs, offering nearly perfect photoluminescent quantum yield, slow hot-carrier cooling time, especially their colloidal synthesis and processing using industrially friendly solvents, have revolutionized the fields of electronics, photonics, and optoelectronics. Particularly, in emerging perovskite QD-based PVs, the advancement of surface chemistry has boosted the record power conversion efficiency (PCE) to 19.1% within a five-year period, surpassing all other colloidal QD photovoltaics (PVs). Given the rapid enhancement of device performances, perovskite QD PVs have attracted significant attention. Further study of semiconducting perovskite QDs will lead to advanced surface structures, a deeper understanding of halide perovskites, and enhanced PCE. In this review article, we comprehensively summarize and discuss the emerging perovskite QD PVs, providing insights into the impact of surface chemical design on their electronic coupling, dispersibility, stability and defect passivation. The limitations of current perovskite QDs mainly arise from their "soft" ionic nature and dynamic surface equilibrium, which lead to difficulties in the large-scale synthesis of monodispersed perovskite QDs and conductive inks for high-throughput printing techniques. We present that the development of surface chemistry is becoming a platform for further improving PCE, aiming to reach the 20% milestone. Additionally, we discuss integrating artificial intelligence to facilitate the mass-production of perovskite QDs for large-area, low-cost PV technology, which could help address significant energy challenges.

Domain-Wall Ferroelectric Polarons in a Two-Dimensional Rotor Lattice Model

We demonstrate the formation of ferroelectric domain-wall polarons in a minimal two-dimensional lattice model of electrons interacting with rotating dipoles. Along the domain wall, the rotors polarize in opposite directions, causing the electron to localize along a particular lattice direction. The rotor-electron coupling is identified as the origin of a structural instability in the crystal that leads to the domain-wall formation via a symmetry-breaking process. Our results provide the first theoretical description of ferroelectric polarons, as discussed in the context of soft semiconductors.

Giant Polarizability and Origin of Ferroelectricity in Layered Materials with a Litharge-Type Structural Unit

We discover that a large family of [Pb2F2]- and [Bi2O2]-based mixed-anion materials with a litharge-type structural unit are highly polarizable layered semiconductors in proximity to strain-induced ferroelectricity. First-principles calculations demonstrate that in this family of materials, compounds as diverse as PbFBr, BiOCl, BiCuOSe, Bi2OS2, and Bi5O4S3Cl exhibit static dielectric constants an order of magnitude higher than typical semiconductors. Additionally, they undergo a ferroelectric transition when subjected to a few percent of tensile strain. The ferroelectric transitions of these materials are found to have a universal origin in the strong cross-bandgap hybridization of the cation p orbitals, enabled by the cation 6s2 lone-pair electrons and the litharge-type structure of the [Pb2F2] and [Bi2O2] layers, as demonstrated by the strain-induced ferroelectric transition in the archetypal litharge α-PbO. These results establish materials with a litharge-type structural unit as a large and versatile family of highly polarizable layered semiconductors in proximity to ferroelectricity.

Coupling of electronic transition to ferroelectric order in a 2D semiconductor

A ferroelectric material often exhibits a soft transverse optical (TO) phonon mode which governs its phase transition. Charge coupling to this ferroelectric soft mode may further mediate emergent physical properties, including superconductivity and defect tolerance in semiconductors. However, direct experimental evidence for such coupling is scarce. Here we show that a photogenerated coherent phonon couples strongly to the electronic transition above the bandgap in the van der Waals (vdW) two-dimensional (2D) ferroelectric semiconductor NbOI2. Using terahertz time-domain spectroscopy and first-principles calculations, we identify this mode as the TO phonon responsible for ferroelectric order. This exclusive coupling occurs only with the above-gap electronic transition and is absent in the valence band as revealed by resonant inelastic X-ray scattering. Our findings suggest a new role of the soft TO phonon mode in electronic and optical properties of ferroelectric semiconductors.

Determination of the carrier temperature in weakly confined semiconductor nanocrystals using time-resolved optical spectroscopy

Many applications of nanocrystals rely on their use in light detection and emission. In recent years, nanocrystals with more relaxed carrier confinement, including so-called 'bulk' and 2D implementations, have made their stake. In such systems, the charge carriers generated after (photo-)excitation are spread over a semi-continuous density of states, behaviour controlled by the carrier temperature T. Current established methods to measure this dynamically changing temperature include transient absorption and luminescence spectroscopy, yet they very often fail to agree on the exact temperature leading to contradicting reports. Here, we show through a combined side-by-side experimental and theoretical study on state-of-art II-VI and perovskite nanocrystals under weak confinement that only transient PL can yield unambiguously the correct T. In particular, temperatures extracted from TA are heavily affected by the effective masses of the electron and hole bands involved leading to overestimations. Our results pave a way to a more robust extraction of carrier temperature and will help to consolidate ensuing structure-property relations derived from it.

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.

Band Gap Narrowing in Lead-Halide Perovskites by Dynamic Defect Self-Doping for Enhanced Light Absorption and Energy Upconversion

Metal halide perovskites (MHP) have attracted great attention in the photovoltaic industry due to their high and rapidly rising power conversion efficiencies, currently over 25%. However, hybrid organic-inorganic MHPs are inherently chemically unstable, limiting their application. All-inorganic MHPs perovskites, such as CsPbI3, have many merits, but their stable conversion efficiency is lower, around 18%, due to a larger band gap causing a mismatch with the solar spectrum. Choosing α-CsPbI3 as a prototypical system, we demonstrate a new general concept of dynamic defects that fluctuate between deep and shallow states, and increase the range of absorbed solar photons, without accelerating the nonradiative electron-hole recombination. In their deeper energy state, the defects narrow the band gap and allow the harvesting of light with longer wavelengths. Fluctuating to shallower energies, the defects allow the escape of photogenerated charges into bands, enabling charge transport and resulting in the defect-mediated upconversion of thermal energy into electricity. Defect covalency and participation of low-frequency anharmonic vibrations decouple trapped charges from free charge carriers, minimizing nonradiative charge carrier losses. Our findings demonstrate that defect covalency and defect dynamics are unique and important properties of MHPs, and can be used to optimize MHPs for efficient solar energy harvesting and optoelectronic applications.

Photoinduced Fröhlich Interaction-Driven Distinct Electron- and Hole-Polaron Behaviors in Hybrid Organic-Inorganic Perovskites by Ultrafast Terahertz Probes

The formation of large polarons resulting from the Fröhlich coupling of photogenerated carriers with the polarized crystal lattice is considered crucial in shaping the outstanding optoelectronic properties in hybrid organic-inorganic perovskite crystals. Until now, the initial polaron dynamics after photoexcitation have remained elusive in the hybrid perovskite system. Here, based on the terahertz time-domain spectroscopy and optical-pump terahertz probe, we access the nature of interplay between photoexcited unbound charge carriers and optical phonons in MAPbBr3 within the initial 5 ps after excitation and have demonstrated the simultaneous existence of both electron- and hole-polarons, together with the photogenerated carrier dynamic process. Two resonant peaks in the frequency-dependent photoconductivity are interpreted by the Drude-Smith-Lorentz model along with the ab initio excitation calculation, revealing that the electron-/hole-polaron is related to the vibration modes of the stretched/contracted Pb-Br bond. The red /blue shift of the corresponding peaks as the fingerprints of electron-/hole-polaron provides a channel for observing their dynamic behavior. Different from polarons with long lifetime (>300 ps) in single-crystalline grains, we observed in thin films the transient process from the formation to the dissociation of polarons occurring at timescales within ∼5 ps, resulting from the Mott phase transition for carriers at high concentrations. Moreover, the observation of the polaron dynamic process of the virtual state-assisted band gap transition (800 nm excitation) further reveals the competition of carriers cooling and polaron formation with photocarrier density. Our observations demonstrate a strategy for direct observation and manipulation of bipolar polaron transport in hybrid perovskites.

The Hot Phonon Bottleneck Effect in Metal Halide Perovskites

The hot phonon bottleneck (HPB) effect has been proposed as one of the main phenomena behind the slow cooling in metal halide perovskites. Even though consensus has been reached regarding its existence, open questions remain concerning the HPB's specific applicability and potential regarding hot carrier solar cell (HCSC) applications. We present a full investigation using ensemble Monte Carlo simulations of the HPB effect in metal halide perovskites (MHP). After describing the HPB effect in detail, we quantify how the HPB effect can extend carrier cooling times by orders of magnitude. We show how the HPB effect depends on carrier concentration, longitudinal optical (LO) phonon lifetime, and LO phonon frequency and connect these findings to how MHPs should be tuned concretely. Using ensemble Monte Carlo simulations, we can accurately model the interplay between carrier-phonon and carrier-carrier interactions up to high carrier density, yielding precise predictions regarding the HPB effect. This study provides important insights into the governing dynamics behind the HPB effect and shows how cooling times can be extended far beyond the phonon lifetime. Furthermore, it contributes to the discussion on cooling times in MHPs and their suitability for HCSC applications.

Disruption of the Conjugated π-Electron System of Graphene Oxides Diminishes Their Microwave Reactivity

Graphenes and graphene-based adsorbents have the potential to be thermally regenerated by microwave irradiation due to their electronic mobility and propensity to absorb microwaves. This article investigates the effect of oxidation on their ability to heat during microwave irradiation in conjunction with their ability to adsorb a polycyclic aromatic hydrocarbon. For this, a series of graphene oxides (GOs) were synthesized, and their chemical properties and surface structures were analyzed systematically. As the oxidation levels increased, the microwave reactivity of GOs decreased notably. This was attributed to the disruption of the sp2-hybridized basal plane despite the introduction of polar oxygen-containing functional groups. The findings of this work indicated the role of the conjugated π-electron system on microwave reactivity, possibly posing a juxtaposition with the influence of polar C-O bonds on dielectric reactivity. In addition, the adsorption of the model compound decreased by oxidation, confirming the decrease in π-π electron donor-acceptor interactions and the increase in the formation of water clusters around oxygen-containing functional groups. This study provides the first mechanistic insight into the relationship between the conjugated π-electron network of graphenes and their microwave reactivity. It paves the way for utilizing microwave irradiation to regenerate spent graphenic adsorbents for water treatment.

Study of Thermalization Mechanisms of Hot Carriers in BABr-Added MAPbBr3 for the Top Layer of Four-Junction Solar Cells

The hot carrier multi-junction solar cell (HCMJC) is an advanced-concept solar cell with a theoretical efficiency greater than 65%. It combines the advantages of hot carrier solar cells and multi-junction solar cells with higher power conversion efficiency (PCE). The thermalization coefficient (Qth) has been shown to slow down by an order of magnitude in low-dimensional structures, which will significantly improve PCE. However, there have been no studies calculating the Qth of MAPbBr3 quantum dots so far. In this work, the Qth values of MAPbBr3 quantum dots and after BABr addition were calculated based on power-dependent steady-state photoluminescence (PD-SSPL). Their peak positions in PD-SSPL increased from 2.37 to 2.71 eV after adding BABr. The fitting shows that, after adding BABr, the Qth decreased from 2.64 ± 0.29 mW·K-1·cm-2 to 2.36 ± 0.25 mW·K-1·cm-2, indicating a lower relaxation rate. This is because BABr passivates surface defects, slowing down the carrier thermalization process. This work lays the foundation for the theoretical framework combining perovskite materials, which suggests that the appropriate passivation of BABr has the potential to further reduce Qth and make MAPbBr3 QDs with BABr modified more suitable as the top absorption layer of HCMJCs.

Anomalous Polarons in Two-Dimensional Organometallic Perovskite Ferroelectric

The concept of ferroelectric polarons is proposed to partially explain the exceptional optoelectronic properties observed in lead halide perovskites (LHPs). It is intriguing but unclear how this proposal, which involves local or transient polarizations, applies in general to 2D LHPs with long-range ferroelectricity. Here, this work presents a pioneering time-domain experimental investigation of polarons in ferroelectric (IA)2(MA)2Pb3Br10 (IMPB; IA is isoamylammonium and MA is methylammonium) using transient absorption spectroscopy. Compared to non-ferroelectric LHPs, IMPB exhibits several distinct polaronic properties closely associated with macroscopic polarizations of ferroelectricity, including a prolonged polaron formation time (≈1.1 ps), a Stark splitting of the bleaching (≈63 meV), and a giant polaron Mott density (≈7.6 × 1018 cm-3). These findings broaden the realm of 2D polaron systems and reveal the decisive role of static/unidirectional polarizations on polaron physics in 2D LHPs.

Enhancing Extraction and Suppressing Cooling of Hot Electrons in Lead Halide Perovskites by Dipolar Surface Passivation

Slowing hot carrier (HC) cooling and improving HC extraction are considered two pivotal factors for enhancing power conversion efficiency in emerging HC photovoltaic applications of perovskites and other materials. Employing ab initio quantum dynamics simulations, we demonstrate the simultaneous slow cooling and efficient extraction of hot electrons at the C60/CsPbI3 interface through dipolar surface passivation with phenethylammonium and 4-fluorophenethylammonium ligands. The passivation effectively suppresses I-Pb lattice vibrations, weakens the hot electron-phonon interaction in CsPbI3, and thus slows down the HC cooling. At the same time, the dipolar surface passivation elevates the LUMO + 1 state in C60 and reduces the energy gap for HC extraction. Concurrently, higher-frequency vibrations of the dipolar layer enhance the coupling between C60 and CsPbI3, promoting efficient HC extraction further. These phenomena are intensified with increased polarity of the dipolar layer. Furthermore, we find that dipolar passivation has the opposite influence on cold electron collection at the band edge, underscoring the fact that the observed improvement in photovoltaic performance stems preferentially from the effective utilization of HCs rather than cold electrons. The work provides a new strategy for achieving high-performance HC perovskite solar cells.

Observation of Hot Carrier Localization Affected by A Cations in Hybrid Perovskites

Organic-inorganic lead halide perovskites (OLHPs) have demonstrated exceptional properties in high-performance photoelectric devices. However, the impact of A-site cations, specifically formamidinium and methylammonium (MA), on the optoelectronic properties of OLHPs, particularly in the context of hot carrier utilization, remains a topic of debate. In this study, we propose a method for characterizing hot carrier transportation by measuring the hot carrier mobility and momentum-dependent transient photocurrent influenced by A-site cations in OLHPs. Our findings reveal that the direction of photon drag current is reversed upon substitution of the MA cation, suggesting the strong localization of hot carriers by the MA cation dipole. Furthermore, the correlation between the hot carrier photoconductivity and the electronic structure in different A-site cation samples indicates that hot carrier mobility in OLHPs can be reduced by >50% due to the influence of A-site cations.

Fate of Optical Excitons in FAPbI3 Nanocube Superlattices

Understanding the nature of the photoexcitation and ultrafast charge dynamics pathways in organic halide perovskite nanocubes and their aggregation into superlattices is key for potential applications as tunable light emitters, photon-harvesting materials, and light-amplification systems. In this work, we apply two-dimensional coherent electronic spectroscopy (2DES) to track in real time the formation of near-infrared optical excitons and their ultrafast relaxation in CH(NH2)2PbI3 nanocube superlattices. Our results unveil that the coherent ultrafast dynamics is limited by the combination of the inherent short exciton decay time (≃40 fs) and the dephasing due to the coupling with selective optical phonon modes at higher temperatures. On the picosecond time scale, we observe the progressive formation of long-lived localized trap states. The analysis of the temperature dependence of the excitonic intrinsic line width, as extracted by the antidiagonal components of the 2D spectra, unveils a dramatic change of the excitonic coherence time across the cubic to tetragonal structural transition. Our results offer a new way to control and enhance the ultrafast coherent dynamics of photocarrier generation in hybrid halide perovskite synthetic solids.

Unraveling the excitonics of light emission from metal-halide perovskite quantum dots

Metal halide semicondictor perovskites have been under intense investigation for their promise in light absorptive applications like photovoltaics. They have more recently experienced interest for their promise in light emissive applications. A key aspect of perovskites is their glassy, ionic lattice that exhibits dynamical disorder. One possible result of this dynamical disorder is their strong coupling between electronic and lattice degrees of freedom which may confer remarkable properties for light emission such as defect tolerance. How does the system, comprised of excitons, couple to the bath, comprised of lattice modes? How does this system-bath interaction give rise to novel light emissive properties and how do these properties give insight into the nature of these materials? We review recent work from this group in which time-resolved photoluminescence spectroscopy is used to reveal such insights. Based upon a fast time resolution of 3 ps, energy resolution, and temperature dependence, a wide variety of insights are gleaned. These insights include: lattice contributions to the emission linewidths, multiexciton formation, hot carrier cooling, excitonic fine structure, single dot superradiance, and a breakdown of the Condon approximation, all due to complex structural dynamics in these materials.

Contrasting Ultra-Low Frequency Raman and Infrared Modes in Emerging Metal Halides for Photovoltaics

Lattice dynamics are critical to photovoltaic material performance, governing dynamic disorder, hot-carrier cooling, charge-carrier recombination, and transport. Soft metal-halide perovskites exhibit particularly intriguing dynamics, with Raman spectra exhibiting an unusually broad low-frequency response whose origin is still much debated. Here, we utilize ultra-low frequency Raman and infrared terahertz time-domain spectroscopies to provide a systematic examination of the vibrational response for a wide range of metal-halide semiconductors: FAPbI3, MAPbI x Br3-x , CsPbBr3, PbI2, Cs2AgBiBr6, Cu2AgBiI6, and AgI. We rule out extrinsic defects, octahedral tilting, cation lone pairs, and "liquid-like" Boson peaks as causes of the debated central Raman peak. Instead, we propose that the central Raman response results from an interplay of the significant broadening of Raman-active, low-energy phonon modes that are strongly amplified by a population component from Bose-Einstein statistics toward low frequency. These findings elucidate the complexities of light interactions with low-energy lattice vibrations in soft metal-halide semiconductors emerging for photovoltaic applications.

Chirality of CsPbBr3 Nanocrystals with Varying Dimensions in the Presence of Chiral Molecules

Chiral lead halide perovskite (LHP) nanocrystals (NCs) have been attracting considerable interest for circularly polarized luminescence (CPL)-based optoelectronic applications. This study combined experimental and computational analyses to investigate how the dimensionality of 3D (cubic) to 0D (quantum dots) influences the tunable chiral emission of CsPbBr3 LHP NCs. The circular dichroism (CD) spectra have a significant blue shift from 508 to 406 nm. The dissymmetry factors for CD (gCD) change from ±2.5 × 10-3 to ±7.5 × 10-3 as dimensionality varies from 3D to 0D in the presence of the chiral molecule (cyclohexylethylamine, CHEA). A significant luminescence dissymmetry factor (glum) of ±5.6 × 10-4 is observed in the 0D CsPbBr3 NCs. Theoretical calculations using structural distortion parameters, the extent of charge transfer, and electrostatic potential profiles have revealed that the most significant enhancement of the chirality transfer occurs from the CHEA molecules to 0D NCs, and the order of chirality transfer from CHEA to CsPbBr3 NCs is 0D (quantum dots) > 2D (nanoplatelet) > 3D (cubic).

Hole Localization in Bulk and 2D Lead-Halide Perovskites Studied by Time-Resolved Infrared Spectroscopy

Scattering and localization dynamics of charge carriers in the soft lattice of lead-halide perovskites impact polaron formation and recombination, which are key mechanisms of material function in optoelectronic devices. In this study, we probe the photoinduced lattice and carrier dynamics in perovskite thin films (CsFAPbX3, X = I, Br) using time-resolved infrared spectroscopy. We examine the CN stretching mode of formamidinium (FA) cations located within the lead-halide octahedra of the perovskite structure. Our investigation reveals the formation of an infrared mode due to spatial symmetry breaking within a hundred picoseconds in 3D perovskites. Experiments at cryogenic temperatures show much-reduced carrier localization, in agreement with a localization mechanism that is driven by the dynamic disorder. We extend our analysis to 2D perovskites, where the precise nature of charge carriers is uncertain. Remarkably, the signatures of charge localization we found in bulk perovskites are not observed for 2D Ruddlesden-Popper perovskites ((HexA)2FAPb2I7). This observation implies that the previously reported stabilization of free charge carriers in these materials follows different mechanisms than polaron formation in bulk perovskites. Through the exploration of heterostructures with electron/hole excess, we provide evidence that holes drive the formation of the emerging infrared mode.

Pulse-doubling perovskite nanowire lasers enabled by phonon-assisted multistep energy funneling

Laser pulse multiplication from an optical gain medium has shown great potential in miniaturizing integrated optoelectronic devices. Perovskite multiple quantum wells (MQWs) structures have recently been recognized as an effective gain media capable of doubling laser pulses that do not rely on external optical equipment. Although the light amplifications enabled with pulse doubling are reported based on the perovskite MQWs thin films, the micro-nanolasers possessed a specific cavity for laser pulse multiplication and their corresponding intrinsic laser dynamics are still inadequate. Herein, a single-mode double-pulsed nanolaser from self-assembled perovskite MQWs nanowires is realized, exhibiting a pulse duration of 28 ps and pulse interval of 22 ps based on single femtosecond laser pulse excitation. It is established that the continuous energy building up within a certain timescale is essential for the multiple population inversion in the gain medium, which arises from the slowing carrier localization process owning to the stronger exciton-phonon coupling in the smaller-n QWs. Therefore, the double-pulsed lasing is achieved from one fast energy funnel process from the adjacent small-n QWs to gain active region and another slow process from the spatially separated ones. This report may shed new light on the intrinsic energy relaxation mechanism and boost the further development of perovskite multiple-pulse lasers.

The Impact of Partial Carrier Confinement on Stimulated Emission in Strongly Confined Perovskite Nanocrystals

Semiconductor lead halide perovskites are excellent candidates for realizing low threshold light amplification due to their tunable and highly efficient luminescence, ease of processing, and strong light-matter interactions. However, most studies on optical gain have addressed bulk films, nanowires, or nanocrystals that exhibit little or no size quantization. Here, we show by means of a multitude of optical spectroscopy methods that small CsPbBr3 nanocrystals (NCs) exhibit a progressive red shift of the band-edge transition upon addition of electron-hole pairs, at least one carrier of which occupies a 2-fold degenerate, delocalized state in agreement with strong confinement. We demonstrate that this combination results in a threshold for biexciton gain, well below the limit of one electron-hole pair on average per NC. On the other hand, both the luminescent lifetime and the optical Stark effect of 4.7 nm CsPbBr3 NCs indicate that the oscillator strength of the band-edge transition is considerably smaller than expected from the band-edge absorption. We assign this discrepancy to a mixed confinement regime, with one delocalized and one localized charge carrier, and show that the concomitant reduction of the oscillator strength for stimulated emission accounts for the surprisingly small material gain observed in small NCs. The conclusion of mixed confinement aligns with studies reporting small and large polarons for holes and electrons in lead halide perovskite nanocrystals, respectively, and creates opportunities for understanding multiexciton photophysics in confined perovskite materials.

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.

Fine-Tuning Crystal Structures of Lead Bromide Perovskite Nanocrystals through Trace Cadmium(II) Doping for Efficient Color-Saturated Green LEDs

Decreasing perovskite nanocrystal size increases radiative recombination due to the quantum confinement effect, but also increases the Auger recombination rate which leads to carrier imbalance in the emitting layers of electroluminescent devices. Here, we overcome this trade-off by increasing the exciton effective mass without affecting the size, which is realized through the trace Cd2+ doping of formamidinium lead bromide perovskite nanocrystals. We observe an ~2.7 times increase in the exciton binding energy benefiting from a slight distortion of the [BX6]4- octahedra caused by doping in the case of that the Auger recombination rate is almost unchanged. As a result, bright color-saturated green emitting perovskite nanocrystals with a photoluminescence quantum yield of 96 % are obtained. Cd2+ doping also shifts up the energy levels of the nanocrystals, relative to the Fermi level so that heavily n-doped emitters convert into only slightly n-doped ones; this boosts the charge injection efficiency of the corresponding light-emitting diodes. The light-emitting devices based on those nanocrystals reached a high external quantum efficiency of 29.4 % corresponding to a current efficiency of 123 cd A-1, and showed dramatically improved device lifetime, with a narrow bandwidth of 22 nm and Commission Internationale de I'Eclairage coordinates of (0.20, 0.76) for color-saturated green emission for the electroluminescence peak centered at 534 nm, thus being fully compliant with the latest standard for wide color gamut displays.

Polaronic Mass Enhancement and Polaronic Excitons in Metal Halide Perovskites

In metal halide perovskites, the complex dielectric screening together with low energy of phonon modes leads to non-negligible Fröhlich coupling. While this feature of perovskites has already been used to explain some of the puzzling aspects of carrier transport in these materials, the possible impact of polaronic effects on the optical response, especially excitonic properties, is much less explored. Here, with the use of magneto-optical spectroscopy, we revealed the non-hydrogenic character of the excitons in metal halide perovskites, resulting from the pronounced Fröhlich coupling. Our results can be well described by the polaronic-exciton picture where electron and hole interactions are no longer described by a Coulomb potential. Furthermore, we show experimental evidence that the carrier-phonon interaction leads to the enhancement of the carrier's effective mass. Notably, our measurements reveal a pronounced temperature dependence of the carrier's effective mass, which we attribute to a band structure renormalization induced by the population of low-energy phonon modes. This interpretation finds support in our first-principles calculations.

Intraband Cooling and Auger Recombination in Weakly to Strongly Quantum-Confined CsPbBr3 Perovskite Nanocrystals

Semiconductor nanocrystals (NCs) with size-tuned energy gaps present unique and desirable properties for optoelectronic applications. Recent synthetic advancements offer routes to spheroidal CsPbBr3 perovskite NCs in the strong quantum confinement regime with narrow size dispersion. Using tunable femtosecond laser pulses, we examine intraband carrier relaxation using transient absorption spectroscopy and show that, across the transition from weak to strong confinement, hot carrier lifetime increases compared to larger bulk-like particles. However, further increases of confinement subsequently lead to a reduction of the hot carrier lifetime and increase of the non-radiative Auger recombination rate. Finally, we show that hot carrier lifetimes increase as a function of excess energy above the band gap less sensitively under high confinement in comparison to the bulk. Understanding such unique trends is important for maximizing hot carrier lifetimes for use in next-generation hot carrier devices as well as evaluating the transition from weak to strong confinement.

Topological polarons in halide perovskites

Halide perovskites emerged as a revolutionary family of high-quality semiconductors for solar energy harvesting and energy-efficient lighting. There is mounting evidence that the exceptional optoelectronic properties of these materials could stem from unconventional electron-phonon couplings, and it has been suggested that the formation of polarons and self-trapped excitons could be key to understanding such properties. By performing first-principles simulations across the length scales, here we show that halide perovskites harbor a uniquely rich variety of polaronic species, including small polarons, large polarons, and charge density waves, and we explain a variety of experimental observations. We find that these emergent quasiparticles support topologically nontrivial phonon fields with quantized topological charge, making them nonmagnetic analog of the helical Bloch points found in magnetic skyrmion lattices.

Weakly Confined Organic-Inorganic Halide Perovskite Quantum Dots as High-Purity Room-Temperature Single Photon Sources

Colloidal perovskite quantum dots (PQDs) have emerged as highly promising single photon emitters for quantum information applications. Presently, most strategies have focused on leveraging quantum confinement to increase the nonradiative Auger recombination (AR) rate to enhance single-photon (SP) purity in all-inorganic CsPbBr3 QDs. However, this also increases the fluorescence intermittency. Achieving high SP purity and blinking mitigation simultaneously remains a significant challenge. Here, we transcend this limitation with room-temperature synthesized weakly confined hybrid organic-inorganic perovskite (HOIP) QDs. Superior single photon purity with a low g(2)(0) < 0.07 ± 0.03 and a nearly blinking-free behavior (ON-state fraction >95%) in 11 nm FAPbBr3 QDs are achieved at room temperature, attributed to their long exciton lifetimes (τX) and short biexciton lifetimes (τXX). The significance of the organic A-cation is further validated using the mixed-cation FAxCs1-xPbBr3. Theoretical calculations utilizing a combination of the Bethe-Salpeter (BSE) and k·p approaches point toward the modulation of the dielectric constants by the organic cations. Importantly, our findings provide valuable insights into an additional lever for engineering facile-synthesized room-temperature PQD single photon sources.

Dynamic Structural Evolution and Dual Emission Behavior in Hybrid Organic Lead Bromide Perovskites

The optoelectronic properties of organic lead halide perovskites (OLHPs) strongly depend on their underlying crystal symmetry and dynamics. Here, we exploit temperature-dependent synchrotron powder X-ray diffraction and temperature-dependent photoluminescence to investigate how the subtle structural changes happening in the pure and mixed A-site cation MA1-xFAxPbBr3 (x = 0, 0.5, and 1) systems influences their optoelectronic properties. Diffraction investigations reveal a cubic structure at high temperatures and tetragonal and orthorhombic structures with octahedral distortion at low temperatures. Steady state photoluminescence and time correlated single photon counting study reveals that the dual emission behavior of these OLHPs is due to the direct-indirect band formation. In the orthorhombic phase of MAPbBr3, the indirect band is dominated by self-trapped exciton (STE) emission due to the higher-order lattice distortions of PbBr6 octahedra. Our findings provide a comprehensive explanation of the dual emission behavior of OLHPs while also providing a rationale for previous experimental observations.

Persistent enhancement of exciton diffusivity in CsPbBr3 nanocrystal solids

In semiconductors, exciton or charge carrier diffusivity is typically described as an inherent material property. Here, we show that the transport of excitons among CsPbBr3 perovskite nanocrystals (NCs) depends markedly on how recently those NCs were occupied by a previous exciton. Using transient photoluminescence microscopy, we observe a striking dependence of the apparent exciton diffusivity on excitation laser power that does not arise from nonlinear exciton-exciton interactions or thermal heating. We interpret our observations with a model in which excitons cause NCs to transition to a long-lived metastable configuration that markedly increases exciton transport. The exciton diffusivity observed here (>0.15 square centimeters per second) is considerably higher than that observed in other NC systems, revealing unusually strong excitonic coupling between NCs. The finding of a persistent enhancement in excitonic coupling may help explain other photophysical behaviors observed in CsPbBr3 NCs, such as superfluorescence, and inform the design of optoelectronic devices.

Strong Light-Matter Coupling in Lead Halide Perovskite Quantum Dot Solids

Strong coupling between lead halide perovskite materials and optical resonators enables both polaritonic control of the photophysical properties of these emerging semiconductors and the observation of fundamental physical phenomena. However, the difficulty in achieving optical-quality perovskite quantum dot (PQD) films showing well-defined excitonic transitions has prevented the study of strong light-matter coupling in these materials, central to the field of optoelectronics. Herein we demonstrate the formation at room temperature of multiple cavity exciton-polaritons in metallic resonators embedding highly transparent Cesium Lead Bromide quantum dot (CsPbBr3-QD) solids, revealed by a significant reconfiguration of the absorption and emission properties of the system. Our results indicate that the effects of biexciton interaction or large polaron formation, frequently invoked to explain the properties of PQDs, are seemingly absent or compensated by other more conspicuous effects in the CsPbBr3-QD optical cavity. We observe that strong coupling enables a significant reduction of the photoemission line width, as well as the ultrafast modulation of the optical absorption, controllable by means of the excitation fluence. We find that the interplay of the polariton states with the large dark state reservoir plays a decisive role in determining the dynamics of the emission and transient absorption properties of the hybridized light-quantum dot solid system. Our results should serve as the basis for future investigations of PQD solids as polaritonic materials.

Polaron Vibronic Progression Shapes the Optical Response of 2D Perovskites

The optical response of 2D layered perovskites is composed of multiple equally-spaced spectral features, often interpreted as phonon replicas, separated by an energy Δ ≃ 12 - 40 meV, depending upon the compound. Here the authors show that the characteristic energy spacing, seen in both absorption and emission, is correlated with a substantial scattering response above ≃ 200 cm-1 (≃ 25 meV) observed in resonant Raman. This peculiar high-frequency signal, which dominates both Stokes and anti-Stokes regions of the scattering spectra, possesses the characteristic spectral fingerprints of polarons. Notably, its spectral position is shifted away from the Rayleigh line, with a tail on the high energy side. The internal structure of the polaron consists of a series of equidistant signals separated by 25-32 cm-1 (3-4 meV), depending upon the compound, forming a polaron vibronic progression. The observed progression is characterized by a large Huang-Rhys factor (S > 6) for all of the 2D layered perovskites investigated here, indicative of a strong charge carrier - lattice coupling. The polaron binding energy spans a range ≃ 20-35 meV, which is corroborated by the temperature-dependent Raman scattering data. The investigation provides a complete understanding of the optical response of 2D layered perovskites via the direct observation of polaron vibronic progression. The understanding of polaronic effects in perovskites is essential, as it directly influences the suitability of these materials for future opto-electronic applications.

High-Quality Hybrid Perovskite Thin Films by Post-Treatment Technologies in Photovoltaic Applications

Incredible progress in photovoltaic devices based on hybrid perovskite materials has been made in the past few decades, and a record-certified power conversion efficiency (PCE) of over 26% has been achieved in single-junction perovskite solar cells (PSCs). In the fabrication of high-efficiency PSCs, the postprocessing procedures toward perovskites are essential for designing high-quality perovskite thin films; developing efficient and reliable post-treatment techniques is very important to promote the progress of PSCs. Here, recent post-treatment technological reforms toward perovskite thin films are summarized, and the principal functions of the post-treatment strategies on the design of high-quality perovskite films have been thoroughly analyzed by dividing into two categories in this review: thermal annealing (TA)-related technique and TA-free technique. The latest research progress of the above two types of post-treatment techniques is summarized and discussed, focusing on the optimization of postprocessing conditions, the regulation of perovskite qualities, and the enhancement of device performance. Finally, an outlook of the prospect trends and future challenges for the fabrication of the perovskite layer and the production of highly efficient PSCs is given.

In Situ Surface Reconstruction toward Planar Heterojunction for Efficient and Stable FAPbI3 Quantum Dot Solar Cells

Pure-phase α-FAPbI3 quantum dots (QDs) are the focus of an increasing interest in photovoltaics due to their superior ambient stability, large absorption coefficient, and long charge-carrier lifetime. However, the trap states induced by the ligand-exchange process limit the photovoltaic performances. Here, a simple post treatment using methylamine thiocyanate is developed to reconstruct the FAPbI3 -QD film surface, in which a MAPbI3 capping layer with a thickness of 6.2 nm is formed on the film top. This planar perovskite heterojunction leads to a reduced density of trap-states, a decreased band gap, and a facilitated charge carrier transport. As a result, a record high power conversion efficiency (PCE) of 16.23% with negligible hysteresis is achieved for the FAPbI3 QD solar cell, and it retains over 90% of the initial PCE after being stored in ambient environment for 1000 h.

Excitonic Quantum Coherence in Light Emission from CsPbBr3 Metal-Halide Perovskite Nanocrystals

The decay of excited states via radiative and nonradiative paths is well understood in molecules and bulk semiconductors but less so in nanocrystals. Here, we perform time-resolved photoluminescence (t-PL) experiments on CsPbBr3 metal-halide perovskite nanocrystals, with a time resolution of 3 ps, sufficient to observe the decay of both excitons and biexcitons as a function of temperature. The striking result is that the radiative rate constant of the single exciton increases at low temperatures with an exponential functional form, suggesting quantum coherent effects with dephasing at high temperatures. The opposing directions of the radiative and nonradiative decay rate constants enable enhanced brightening of PL from excitons to biexcitons due to quantum effects, promoting a faster approach to the quantum theoretical limits of light emission. Ab initio quantum dynamics simulations reproduce the experimental observations of radiation controlled by quantum spatial coherence enhanced at low temperatures.

Hot Excitons Cool in Metal Halide Perovskite Nanocrystals as Fast as CdSe Nanocrystals

The idea of phonon bottlenecks has long been pursued in nanoscale materials for their application in hot exciton devices, such as photovoltaics. Decades ago, it was shown that there is no quantum phonon bottleneck in strongly confined quantum dots due to their physics of quantum confinement. More recently, it was proposed that there are hot phonon bottlenecks in metal halide perovskites due to their physics. Recent work has called into question these bottlenecks in metal halide perovskites. Here, we compare hot exciton cooling in a range of sizes of CsPbBr3 nanocrystals from weakly to strongly confined. These results are compared to strongly confined CdSe quantum dots of two sizes and degrees of quantum confinement. CdSe is a model system as a ruler for measuring hot exciton cooling being fast, by virtue of its efficient Auger-assisted processes. By virtue of 3 ps time resolution, the hot exciton photoluminescence can now be directly observed, which is the most direct measure of the presence of hot excitons and their lifetimes. The hot exciton photoluminescence decays on nearly the same 2 ps time scale on both the weakly confined perovskite and the larger CdSe quantum dots, much faster than the 10 ps cooling predicted by transient absorption experiments. The smaller CdSe quantum dot has still faster cooling, as expected from quantum size effects. The quantum dots of perovskites show extremely fast hot exciton cooling, decaying faster than detection limits of <1 ps, even faster than the CdSe system, suggesting the efficiency of Auger processes in these metal halide perovskite nanocrystals and especially in their quantum dot form. These results across a range of sizes of nanocrystals reveal extremely fast hot exciton cooling at high exciton density, independent of composition, but dependent upon size. Hence these metal halide perovskite nanocrystals seem to cool heavily following quantum dot physics.

The persistence of memory in ionic conduction probed by nonlinear optics

Predicting practical rates of transport in condensed phases enables the rational design of materials, devices and processes. This is especially critical to developing low-carbon energy technologies such as rechargeable batteries1-3. For ionic conduction, the collective mechanisms4,5, variation of conductivity with timescales6-8 and confinement9,10, and ambiguity in the phononic origin of translation11,12, call for a direct probe of the fundamental steps of ionic diffusion: ion hops. However, such hops are rare-event large-amplitude translations, and are challenging to excite and detect. Here we use single-cycle terahertz pumps to impulsively trigger ionic hopping in battery solid electrolytes. This is visualized by an induced transient birefringence, enabling direct probing of anisotropy in ionic hopping on the picosecond timescale. The relaxation of the transient signal measures the decay of orientational memory, and the production of entropy in diffusion. We extend experimental results using in silico transient birefringence to identify vibrational attempt frequencies for ion hopping. Using nonlinear optical methods, we probe ion transport at its fastest limit, distinguish correlated conduction mechanisms from a true random walk at the atomic scale, and demonstrate the connection between activated transport and the thermodynamics of information.

Ultrafast vibrational control of organohalide perovskite optoelectronic devices using vibrationally promoted electronic resonance

Vibrational control (VC) of photochemistry through the optical stimulation of structural dynamics is a nascent concept only recently demonstrated for model molecules in solution. Extending VC to state-of-the-art materials may lead to new applications and improved performance for optoelectronic devices. Metal halide perovskites are promising targets for VC due to their mechanical softness and the rich array of vibrational motions of both their inorganic and organic sublattices. Here, we demonstrate the ultrafast VC of FAPbBr3 perovskite solar cells via intramolecular vibrations of the formamidinium cation using spectroscopic techniques based on vibrationally promoted electronic resonance. The observed short (~300 fs) time window of VC highlights the fast dynamics of coupling between the cation and inorganic sublattice. First-principles modelling reveals that this coupling is mediated by hydrogen bonds that modulate both lead halide lattice and electronic states. Cation dynamics modulating this coupling may suppress non-radiative recombination in perovskites, leading to photovoltaics with reduced voltage losses.

Broadband Emission Origin in Metal Halide Perovskites: Are Self-Trapped Excitons or Ions?

It has always been a goal to realize high efficiency and broadband emission in single-component materials. The appearance of metal halide perovskites makes it possible. Their soft lattice characteristics and significant electron-phonon coupling synergistically generate self-trapped excitons (STEs), contributing to a broadband emission with a large Stokes shift. Meanwhile, their structural/compositional diversity provides suitable active sites and coordination environments for doping of ns2 ions, allowing 3 Pn ( n =0,1,2) →1 S0 transitions toward broadband emission. The ns2 ions emission is phenomenologically similar to that of STE emission, hindering in-depth understanding of their emission origin, and leading to failure to meet the design requirements for practical applications. In this scenario, herein, the fundamentals and development of such two emission mechanisms are summarized to establish a clear and comprehensive understanding of the broadband emission phenomenon, which may pave the way to an ideal customization of broadband-emission metal halide perovskites.

Impacts of Exciton Binding Energy and Dielectric Confinement of Layered Lead Halide Perovskites on Carrier Relaxation and Exciton Phonon Interactions

This work highlights the significance of dielectric confinements and exciton binding energy of hybrid layered perovskites (LPs) in controlling the carrier relaxation dynamics of LPs for designing efficient optoelectronic devices. The polarizability of organic spacer cations in LPs modulates the carrier-phonon and carrier-carrier interactions, which eventually control the carrier relaxation dynamics. Here, we have varied the alkyl-ammonium chain length in the LPs to change the dielectric confinement, and the first-principles calculations reveal that the long-chain organic spacer experiences stronger dielectric confinement in comparison to short-chain organic spacer cation-based LPs. Transient absorption spectroscopic analysis suggests that the larger dielectric confinement and higher exciton binding energy exhibit faster carrier relaxation dynamics. The enhanced exciton-phonon interaction leads to faster carrier relaxation dynamics. The much softer phonon modes are responsible for the higher up-conversion of acoustic modes to optical modes, which leads to slower carrier relaxation dynamics in n-butylamine (BA) based LPs.

Sub-Single-Exciton Optical Gain in Lead Halide Perovskite Quantum Dots Revealed by Exciton Polarization Spectroscopy

Optical gain of colloidal quantum dots (QDs) is often attained in the multiexciton regime, which strongly complicates their lasing applications as the gain lifetime is limited by nonradiative Auger recombination occurring typically on the picosecond time scale. In principle, low-threshold gain can be achieved if the gain-active emission has a sizable red shift compared to the absorption. But, this mechanism has been rarely observed in typical QDs featuring small Stokes shift due to their weak electron-phonon coupling. Here, we report the observation of sub-single-exciton gain in CsPbI3 and CsPbBr3 perovskite QDs, which is unequivocally established through pinpointing the stimulated emission and biexciton absorption signatures using polarization-controlled femtosecond transient absorption spectroscopy. The soft lattice of perovskite QDs and hence strong electron-phonon coupling lead to two stimulated emission features from free and self-trapped excitons, respectively. In monodisperse QDs of varying sizes, the Stokes shift of the self-trapped exciton emission is sufficiently large to overcome the biexciton absorption loss and the inhomogeneous line width, enabling optical gain with average exciton occupancy down to <10%.

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.

Alloying metal cations in perovskite nanocrystals is a new route to controlling hot carrier cooling

Hot carrier cooling is slowed down upon alloying tin in lead-halide perovskite nanocrystals through the engineering of carrier-phonon and carrier-defect interactions.