Geometry Distortion and Small Polaron Binding Energy Changes with Ionic Substitution in Halide Perovskites

Tailoring Polaron Dimensions in Lead-Tin Hybrid Perovskites

Charge carriers in the soft and polar perovskite lattice form so-called polaron quasiparticles, charge carriers dressed with a lattice deformation. The spatial extent of a polaron is governed by the material's electron-phonon interaction strength, which determines charge carrier effective mass, mobility, and the so-called Mott polaron density, that is, the maximum stable density of charge carriers that a perovskite can support. Despite its significance, controlling polaron dimensions has been challenging. Here, experimental substantial tuning of polaron dimensions is reported by lattice engineering, through Pb/Sn substitution in CH3NH3SnxPb1-xI3. The polaron dimension is deduced from the Mott polaron density, which can be composition-tuned over an order of magnitude, while charge carrier mobility occurs through band transport, and remains substantial across all compositions, ranging from 10 s to 100 s cm2 V s-1 at room temperature. The effective modulation of polaron size can be understood by considering the bond asymmetry after carrier injection as well as the random spatial distribution of Pb/Sn ions. This study underscores the potential for tailoring polaron dimensions, which is crucial for optimizing applications prioritizing either high charge carrier density or high mobility.

First-principles study of phase-dependent carrier transport mechanism for MASnI3Sn-based halide perovskite

Organic-inorganic hybrid perovskites have attracted tremendous attentions owing to their excellent properties as next-generation photovoltaic devices. With soft covalent framework, organic-inorganic hybrid perovskites exhibit different phases at different temperatures. The band-edge features of perovskites are mainly contributed by inorganic framework, which means the structural differences between these phases would lead to complex carrier transport. We investigated the carrier transport of Sn-based organic-inorganic hybrid perovskite CH3NH3SnI3(MASnI3), considering acoustic deformation potential scattering, ionized impurity scattering, and polar optical phonon scattering. It is found that the electron mobility of each phase of MASnI3is strongly correlated with the Sn-I-Sn bond angle and there is in-plane/out-of-plane anisotropy. The projected crystal orbital Hamilton population analysis suggested that the tilt and rotation of the [SnI6]4-octahedron influence the Sn(p)-I(p) orbital electron coupling and the electron transport, leading to different band-edge features in multiple phases. The carrier mobility with respect to temperature was further calculated for each phase of MASnI3in respective temperature intervals, showing lower carrier mobility in high temperature. Comparing the contribution of different scattering mechanisms, it was found that the dominant scattering mechanism is polar optical phonon scattering, while multiple scattering mechanisms compete in individual cases.

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.

Interlayer Charge Transport in 2D Lead Halide Perovskites from First Principles

We report on the implementation of a versatile projection-operator diabatization approach to calculate electronic coupling integrals in layered periodic systems. The approach is applied to model charge transport across the saturated organic spacers in two-dimensional (2D) lead halide perovskites. The calculations yield out-of-plane charge transfer rates that decay exponentially with the increasing length of the alkyl chain, range from a few nanoseconds to milliseconds, and are supportive of a hopping mechanism. Most importantly, we show that the charge carriers strongly couple to distortions of the Pb-I framework and that accounting for the associated nonlocal dynamic disorder increases the thermally averaged interlayer rates by a few orders of magnitude compared to the frozen-ion 0 K-optimized structure. Our formalism provides the first comprehensive insight into the role of the organic spacer cation on vertical transport in 2D lead halide perovskites and can be readily extended to functional π-conjugated spacers, where we expect the improved energy alignment with the inorganic layout to speed up the charge transfer between the semiconducting layers.

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.

Phonon-driven intra-exciton Rabi oscillations in CsPbBr3 halide perovskites

Coupling electromagnetic radiation with matter, e.g., by resonant light fields in external optical cavities, is highly promising for tailoring the optoelectronic properties of functional materials on the nanoscale. Here, we demonstrate that even internal fields induced by coherent lattice motions can be used to control the transient excitonic optical response in CsPbBr₃ halide perovskite crystals. Upon resonant photoexcitation, two-dimensional electronic spectroscopy reveals an excitonic peak structure oscillating persistently with a 100-fs period for up to ~2 ps which does not match the frequency of any phonon modes of the crystals. Only at later times, beyond 2 ps, two low-frequency phonons of the lead-bromide lattice dominate the dynamics. We rationalize these findings by an unusual exciton-phonon coupling inducing off-resonant 100-fs Rabi oscillations between 1s and 2p excitons driven by the low-frequency phonons. As such, prevailing models for the electron-phonon coupling in halide perovskites are insufficient to explain these results. We propose the coupling of characteristic low-frequency phonon fields to intra-excitonic transitions in halide perovskites as the key to control the anharmonic response of these materials in order to establish new routes for enhancing their optoelectronic properties.

Strain-Driven Solid-Solid Crystal Conversion in Chiral Hybrid Pseudo-Perovskites with Paramagnetic-to-Ferromagnetic Transition

Hybrid organic-inorganic perovskites (HOIPs) are promising stimuli-responsive materials (SPMs) owing to their molecular softness and tailorable structural dimensionality. The design of mechanically responsive HOIPs requires an in-depth understanding of how lattice strain induces intermolecular rearrangement that impacts physical properties. While chirality transfer from an organic cation to an inorganic lattice is known to influence chiral-optical properties, its effect on strain-induced phase conversion has not been explored. As opposed to achiral or racemic organic cations, chiral organic cations can potentially afford a new dimension in strain-responsive structural change. Herein, we demonstrate that mechanical strain induces a solid phase crystal conversion in chiral halide pseudo-perovskite single crystals (R/S)-(FE)₂CuCl₄ (FE = (4-Fluorophenyl)ethylamine) from a 0D isolated CuCl₄ tetrahedral to 1D corner-sharing CuFCl₅ octahedral framework via the incorporation of Cu···F interaction and N-H···F hydrogen bonding. This strain-induced crystal-to-crystal conversion involves the connection of neighboring 0D CuCl₄ tetrahedra via Cu²⁺-Cl^--Cu²⁺ linkages as well as the incorporation of a F-terminated organic cation as one of the X atoms in BX₆ octahedra, leading to a reduced band gap and paramagnetic-to-ferromagnetic conversion. Control experiments using nonchiral or racemic perovskite analogs show the absence of such solid phase conversion. To demonstrate pressure-sensitive properties, the 0D phase is dispersed in water-soluble poly(vinyl alcohol) (PVA) polymer, which can be applied to a large-scale pressure-induced array display on fibrous Spandex substrates via a screen-printing method.

Improved Defect Tolerance and Charge Carrier Lifetime in Tin-Lead Mixed Perovskites: Ab Initio Quantum Dynamics

Simulations by nonadiabatic (NA) molecular dynamics demonstrate that mixing tin with lead in CH₃NH₃PbI₃ can passivate the midgap state created by an interstitial iodine (I_i) via the imposed compressive strain and upshifted valence band maximum, reduce NA coupling by decreasing electron-hole wave functions overlap, and shortens pure-dephasing time by introducing high-frequency phonon modes. Thus, the charge carrier lifetime extends to 3.6 ns due to the significantly reduced nonradiative electron-hole recombination, which is an order of magnitude longer than the I_i-containing CH₃NH₃PbI₃, over 2.5 times longer than the pristine CH₃NH₃PbI₃ (1.4 ns), and even 1.7 times longer than the tin-lead mixed perovskite without the I_i defects (2.1 ns). Tin-lead alloying simultaneously increases the I_i defect formation energy to 0.402 eV from 0.179 eV in CH₃NH₃PbI₃, which effectively enhances defect tolerance by reducing the defect concentration. The study reveals the factors controlling the enhanced performance of tin-lead mixed perovskite solar cells.

Dimensional Control of Highly Anisotropic and Transparent Conductive Coordination Polymers for Solution-Processable Large-Scale 2D Sheets

Controlling the dimensional aspect of conductive coordination polymers is currently a key scientific interest. Herein, solution-based dimension control strategies are proposed for copper chloride thiourea (CuCl-TU) coordination polymers that enable centimeter-scale, 2D nanosheet formation for use as transparent electrodes. Despite the wide bandgap of CuCl-TU polymers (4.33 eV), through polaron-mediated electron transfer, the electrical conductivity of the 2D sheet at room temperature is able to reach 4.45 S cm^-1 without intentional doping. This leads to a highly anisotropic electronic conductivity of up to the order of ≈10³ differences, depending on the material orientation. Furthermore, by substituting alternative thiourea candidates, it is demonstrated that it is possible to predesign CuCl-TU structures with the desired functionality, stability, and porosity through dimensional control. These findings provide a blueprint to design next-generation transparent conducting materials that can operate at room temperature, thereby expanding their applicability to different fields.

Anion Doping Delays Nonradiative Electron-Hole Recombination in Cs-Based All-Inorganic Perovskites: Time Domain ab Initio Analysis

Using time-domain density functional theory combined with nonadiabatic (NA) molecular dynamics, we demonstrate that composition engineering of the X-site anions has a strong influence on the nonradiative electron-hole recombination and thermodynamic stability of cesium-based all-inorganic perovskites. Partial substitution of iodine(I) with bromine (Br) and acetate (Ac) anions reduces the NA electron-vibrational coupling by minimizing the overlap between the electron and hole wave functions and suppressing atomic fluctuations. The doping also widens the energy gap to further reduce the NA coupling and to enhance the open-circuit voltage of perovskite solar cells. These factors increase the charge carrier lifetime by an order of magnitude and improve structural stability in the series CsPbI_1.88BrAc_0.12 > CsPbI₂Br > CsPbI₃. The fundamental atomistic insights into the influence of anion doping on the photophysical properties of the all-inorganic lead halide perovskites guide the design of efficient optoelectronic materials.

Control of Polaronic Behavior and Carrier Lifetimes via Metal and Anion Alloying in Chalcogenide Perovskites

Transition-metal perovskite chalcogenides (TMPCs) have emerged as lead-free alternatives to lead-halide perovskites and have been currently of increasing interest for optoelectronic applications because of their suitable band gaps, high carrier mobility, strong light absorption, and high stability. Here, we systematically report a study on the effects of Ti- and Se-alloying strategies on polaron behavior and carrier lifetimes in nonradiative recombination. Although such alloying can effectively tune the band gap of BaZrS₃, we observe localized small polaron formation upon Ti alloying and large polarons generating in Se alloying. Ti-alloying strengthens the electron-phonon coupling, leading to a reduced carrier lifetime. Remarkably, Se-alloying weakens the electron-phonon coupling and prolongs the nonradiative electron-hole recombination lifetime by up to 60% compared to that in pristine BaZrS₃ material. The simulations rationalize the difference in carrier lifetimes in TMPC alloys and provide guidelines for further improvements in TMPC-based photoelectronic devices.

Correlated organic-inorganic motion enhances stability and charge carrier lifetime in mixed halide perovskites

Organic cations are believed to have little influence on the charge carrier lifetime in hybrid organic-inorganic perovskites. Experiments defy this expectation. We consider formamidinium lead iodide (FAPbI₃) doping with and without Br as two prototypical systems, and perform ab initio time-domain nonadiabatic (NA) molecular dynamics simulations to investigate nonradiative electron-hole recombination. The simulations demonstrate that correlated organic-inorganic motion stabilizes the lattice and inhibits nonradiative charge recombination in FAPbI₃ upon Br doping. Br doping suppresses the rotation of FA and the vibrations of both organic and inorganic components, and leads to hole localization and the extent of localization is enhanced upon thermal impact, notably reducing the NA coupling by decreasing the overlap between the electron and hole wave functions. Doping also slightly increases the bandgap for further decreasing NA coupling and enhances the open-circuit voltage of perovskite solar cells. The small NA coupling and large bandgap beat the slow coherence loss, delaying electron-hole recombination and extending the charge carrier lifetime to 1.5 ns in Br-doped FAPbI₃, which is on the order of 1.1 ns in pristine FAPbI₃. The obtained time scales are in good agreement with experiments. Multiple phonon modes, including those of both the inorganic and organic components, couple to the electronic subsystem and accommodate the excess electronic energy lost during nonradiative charge recombination. The study reveals the unexpected atomistic mechanisms for the reduction of electron-hole recombination upon Br doping, rationalizes the experiments, and advances our understanding of the excited-state dynamics of perovskite solar cells.

Unlocking surface octahedral tilt in two-dimensional Ruddlesden-Popper perovskites

Molecularly soft organic-inorganic hybrid perovskites are susceptible to dynamic instabilities of the lattice called octahedral tilt, which directly impacts their carrier transport and exciton-phonon coupling. Although the structural phase transitions associated with octahedral tilt has been extensively studied in 3D hybrid halide perovskites, its impact in hybrid 2D perovskites is not well understood. Here, we used scanning tunneling microscopy (STM) to directly visualize surface octahedral tilt in freshly exfoliated 2D Ruddlesden-Popper perovskites (RPPs) across the homologous series, whereby the steric hindrance imposed by long organic cations is unlocked by exfoliation. The experimentally determined octahedral tilts from n = 1 to n = 4 RPPs from STM images are found to agree very well with out-of-plane surface octahedral tilts predicted by density functional theory calculations. The surface-enhanced octahedral tilt is correlated to excitonic redshift observed in photoluminescence (PL), and it enhances inversion asymmetry normal to the direction of quantum well and promotes Rashba spin splitting for n > 1.

Unveiling the Nature of Light-Triggered Hole Traps in Lead Halide Perovskites: A Study with Time-Dependent Density Functional Theory

Structural variations of lead halide perovskites (LHPs) upon light illumination play an important role in their photovoltaic applications. However, density functional theory (DFT)-based simulations have often been performed to unveil the nature of defects in LHPs without light illumination. So far, the nature of traps in LHPs triggered by the light remains largely unexplored. In this work, hole traps induced by the halogen interstitial in LHPs are studied by combining DFT and time-dependent DFT approaches, the latter of which treats electron-hole and electron-nuclei interactions on the same footing. Both a semilocal exchange functional and hybrid functional are adopted to relax the ground-state and excited-state geometries followed by the calculations of energy levels of hole traps. The effect of the self-interaction corrections on the light-triggered geometric deformation and the electronic structure of hole traps is analyzed. Relaxation energies that correspond to the light-triggered geometric deformation are also calculated with different functionals. The relationship between the hole traps and light-triggered geometric variations are then explored.

Defect Tolerance Mechanism Revealed! Influence of Polaron Occupied Surface Trap States on CsPbBr3 Nanocrystal Photoluminescence: Ab Initio Excited-State Dynamics

Lead halide perovskite (LHP) nanocrystals (NCs) show exceptional defect tolerance which has been attributed to their unique electronic structure, where defect energy levels are not introduced inside the fundamental bandgap, and the role of polarons in screening charge carriers from defects. Here, we use ab initio atomistic simulations to explore the interplay between various surface chemistries (A = Cs⁺, R'NH₃⁺; X = Br^-, RCOO^-) used to passivate a CsPbBr₃ NC surface and their impact on the ground-state (GS) and excited-state (ES) photophysical properties. We investigate pristine fully passivated surfaces and A-X vacancy defects that reflect chemical reactions A⁺ + X^- → AX on the surface, which result in ligand desorption. For each surface configuration, calculations are performed in the GS and lowest ES (L-ES) electronic configurations, approximating polaron formation after photoexcitation. For models with A-X surface vacancies, we find that localized electron surface trap (ST) states emerge ∼100-400 meV below the pristine S_e band in the L-ES configuration due to polaronic nuclear reorganization. Surprisingly, these trap states contribute relatively bright S_h → ST spectral features. To test if these surface trap states remain bright in a dynamic (thermal) situation we implement excited-state molecular dynamics simulations. It is found that the surface defected model shows an enhanced nonradiative recombination rate which reduces the photoluminescence quantum yield (PLQY) from 95% for the pristine surface to 75%. This is accompanied by an order of magnitude reduction in PL intensity and a red shift of the transition energy. This study provides more evidence of the defect tolerance of LHP NCs along with evidence of surface trap states contributing to efficient photoluminescence. The observation of relatively bright surface trap states could provide insight into photophysical phenomena, such as size-dependent stretched-exponential photoluminescence decay and Stokes shifts.

Charge Carrier Diffusion Dynamics in Multisized Quaternary Alkylammonium-Capped CsPbBr3 Perovskite Nanocrystal Solids

CsPbBr3 quantum dots (QDs) are promising candidates for optoelectronic devices. The substitution of oleic acid (OA) and oleylamine (OLA) capping agents with a quaternary alkylammonium such as di-dodecyl dimethyl ammonium bromide (DDAB) has shown an increase in external quantum efficiency (EQE) from 0.19% (OA/OLA) to 13.4% (DDAB) in LED devices. The device performance significantly depends on both the diffusion length and the mobility of photoexcited charge carriers in QD solids. Therefore, we investigated the charge carrier transport dynamics in DDAB-capped CsPbBr3 QD solids by constructing a bi-sized QD mixture film. Charge carrier diffusion can be monitored by quantitatively varying the ratio between two sizes of QDs, which varies the mean free path of the carriers in each QD cluster. Excited-state dynamics of the QD solids obtained from ultrafast transient absorption spectroscopy reveals that the photogenerated electrons and holes are difficult to diffuse among small-sized QDs (4 nm) due to the strong quantum confinement. On the other hand, both photoinduced electrons and holes in large-sized QDs (10 nm) would diffuse toward the interface with the small-sized QDs, followed by a recombination process. Combining the carrier diffusion study with a Monte Carlo simulation on the QD assembly in the mixture films, we can calculate the diffusion lengths of charge carriers to be ∼239 ± 16 nm in 10 nm CsPbBr3 QDs and the mobility values of electrons and holes to be 2.1 (± 0.1) and 0.69 (± 0.03) cm2/V s, respectively. Both parameters indicate an efficient charge carrier transport in DDAB-capped QD films, which rationalized the perfect performance of their LED device application.

Benign Effects of Twin Boundaries on Charge Carrier Lifetime in Metal Halide Perovskites by a Time-Domain Study

Experiments show that two-dimensional twin boundaries (TBs) defects are benign to the excited-state lifetime of metal halide perovskites and solar cells performance. However, the mechanism remains unclear. By performing nonadiabatic (NA) molecular dynamics simulations on FAPbI₃ (FA= HC(NH₂)₂⁺), we demonstrate that TBs increase the bandgap without introducing midgap states, promote charge separation by localizing electrons and holes that reduce NA coupling and accelerate the loss of coherence, slowing nonradiative electron-hole recombination by a factor of 2.3 compared to pristine FAPbI₃, which occurs within sub-10 ns and agrees well with the experiment. Raising the temperature shortens the coherence time and reduces the NA coupling by increasing the charge localization due to the enhanced distortions of inorganic Pb-I lattice, making the recombination even slower. Our study rationalizes the positive influence of TBs and temperature on perovskite charge dynamics and emphasizes the roles played by the charge localization and quantum coherence.

Polarons and Charge Localization in Metal-Halide Semiconductors for Photovoltaic and Light-Emitting Devices

Metal-halide semiconductors have shown excellent performance in optoelectronic applications such as solar cells, light-emitting diodes, and detectors. In this review the role of charge-lattice interactions and polaron formation in a wide range of these promising materials, including perovskites, double perovskites, Ruddlesden-Popper layered perovskites, nanocrystals, vacancy-ordered, and other novel structures, is summarized. The formation of Fröhlich-type "large" polarons in archetypal bulk metal-halide ABX₃ perovskites and its dependence on A-cation, B-metal, and X-halide composition, which is now relatively well understood, are discussed. It is found that, for nanostructured and novel metal-halide materials, a larger variation in the strengths of polaronic effects is reported across the literature, potentially deriving from variations in potential barriers and the presence of interfaces at which lattice relaxation may be enhanced. Such findings are further discussed in the context of different experimental approaches used to explore polaronic effects, cautioning that firm conclusions are often hampered by the presence of alternate processes and interactions giving rise to similar experimental signatures. Overall, a complete understanding of polaronic effects will prove essential given their direct influence on optoelectronic properties such as charge-carrier mobilities and emission spectra, which are critical to the performance of energy and optoelectronic applications.

Tunable Broad Light Emission from 3D "Hollow" Bromide Perovskites through Defect Engineering

Hybrid halide perovskites consisting of corner-sharing metal halide octahedra and small cuboctahedral cages filled with counter cations have proven to be prominent candidates for many high-performance optoelectronic devices. The stability limits of their three-dimensional perovskite framework are defined by the size range of the cations present in the cages of the structure. In some cases, the stability of the perovskite-type structure can be extended even when the counterions violate the size and shape requirements, as is the case in the so-called "hollow" perovskites. In this work, we engineered a new family of 3D highly defective yet crystalline "hollow" bromide perovskites with general formula (FA)_1-x(en)_x(Pb)_1-0.7x(Br)_3-0.4x (FA = formamidinium (FA⁺), en = ethylenediammonium (en²⁺), x = 0-0.44). Pair distribution function analysis shed light on the local structural coherence, revealing a wide distribution of Pb-Pb distances in the crystal structure as a consequence of the Pb/Br-deficient nature and en inclusion in the lattice. By manipulating the number of Pb/Br vacancies, we finely tune the optical properties of the pristine FAPbBr₃ by blue shifting the band gap from 2.20 to 2.60 eV for the x = 0.42 en sample. A most unexpected outcome was that at x> 0.33 en incorporation, the material exhibits strong broad light emission (1% photoluminescence quantum yield (PLQY)) that is maintained after exposure to air for more than a year. This is the first example of strong broad light emission from a 3D hybrid halide perovskite, demonstrating that meticulous defect engineering is an excellent tool for customizing the optical properties of these semiconductors.

Isotopic Exchange Extends Charge Carrier Lifetime in Metal Lead Perovskites by Quantum Dynamics Simulations

One may expect that isotopic exchange has no influence on charge carrier lifetime and perovskite solar cell performance because isotopic effects do not affect the fundamental electronic structure of materials. Experiments defy this expectation. By performing nonadiabatic (NA) molecular dynamics simulations, we demonstrate that hydrogen and deuterium exchange significantly enhances the excited-state lifetime and stability of CH₃NH₃PbI₃. Replacing lighter hydrogen with heavier deuterium suppresses the collective motions of organic and inorganic components, thus enhancing lattice stiffness and decreasing the NA coupling. Isotopic exchange further reduces NA coupling by localizing electron wave functions for separation of electrons and holes, which beats the extended coherence time, slowing down nonradiative electron-hole recombination from CH₃ND₃PbI₃ to CD₃ND₃PbI₃ with respect to the pristine system. The unchanged fundamental electronic structure together with the prolonged carrier lifetime and enhanced stability rationalize the improvement of the deuterated CH₃NH₃PbI₃ solar cells. Our work provides valuable insights into isotope effects for the design of high-performance perovskite photovoltaic and optoelectronic devices.

Metal Halide Perovskite Nanorods: Shape Matters

Quasi-1D metal halide perovskite nanorods (NRs) are emerging as a type of materials with remarkable optical and electronic properties. Research into this field is rapidly expanding and growing in the past several years, with significant advances in both mechanistic studies of their growth and widespread possible applications. Here, the recent advances in 1D metal halide perovskite nanocrystals (NCs) are reviewed, with a particular emphasis on NRs. At first, the crystal structures of perovskites are elaborated, which is followed by a review of the major synthetic approaches toward perovskite NRs, such as wet-chemical synthesis, substrate-assisted growth, and anion exchange reactions, and discussion of the growth mechanisms associated with each synthetic method. Then, thermal and aqueous stability and the linear polarized luminescence of perovskite NRs are considered, followed by highlighting their applications in solar cells, light-emitting diodes, photodetectors/phototransistors, and lasers. Finally, challenges and future opportunities in this rapidly developing research area are summarized.

Exciton Polarons in Two-Dimensional Hybrid Metal-Halide Perovskites

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

Polarons in Halide Perovskites: A Perspective

Metal halide perovskites (MHPs) have rapidly emerged as leading contenders in photovoltaic technology and other optoelectronic applications owing to their outstanding optoelectronic properties. After a decade of intense research, an in-depth understanding of the charge carrier transport in MHPs is still an active topic of debate. In this Perspective, we discuss the current state of the field by summarizing the most extensively studied carrier transport mechanisms, such as electron-phonon scattering limited dynamics, ferroelectric effects, Rashba-type band splitting, and polaronic transport. We further extensively discuss the emerging experimental and computational evidence for dominant polaronic carrier dynamics in MHPs. Focusing on both small and large polarons, we explore the fundamental aspects of their motion through the lattice, protecting the photogenerated charge carriers from the recombination process. Finally, we outline different physical and chemical approaches considered recently to study and exploit the polaron transport in MHPs.

Hole Localization Inhibits Charge Recombination in Tin-Lead Mixed Perovskites: Time-Domain ab Initio Analysis

Using time domain density functional theory combined with nonadiabatic molecular dynamics, we demonstrate that the Sn dopants favor forming localized hole states with different extent at low and high doping concentrations, mimicking the small and large polarons, while retain the electron wave functions comparable with the pristine system, leading to nonadiabatic coupling decreasing by a factor of 45% and 38% and bandgap reduction by 0.04 and 0.27 eV, respectively. Furthermore, replacing heavier Pb with lighter Sn increases atomic fluctuations and accelerates loss of quantum coherence, in particular even faster at higher Sn doping concentration. As a result, the interplay among the bandgap, NA coupling, and decoherence time delays the electron-hole recombination by a factor of 3.5 and 1.3 at low and high doping concentration. Our study reveals the atomistic mechanisms of suppressed recombination dependence on Sn doping concentration, providing a new way to design high performance mixed perovskites.

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

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

Lattice Expansion in Hybrid Perovskites: Effect on Optoelectronic Properties and Charge Carrier Dynamics

Hybrid halide perovskites frequently undergo structural expansion due to various stimuli, significantly affecting their electronic properties and in particular their charge carrier dynamics. It is essential to atomistically model how geometric changes modify electronic characteristics that are important for applications such as light harvesting and lighting. Using ab initio simulations, here we investigate the structural dynamics and optoelectronic properties of FAPbI₃ under tensile strain. The applied strain leads to elongation of the Pb-I bonds and a decrease in the level of PbI₆ octahedral tilting, which manifests as blue-shifts in band gaps. Nonadiabatic molecular dynamics simulations further reveal that charge carrier recombination rates moderately decrease in these expanded lattices. The complex influence of lattice dynamics on electron-phonon scattering results in a longer carrier lifetime, which is advantageous for efficient solar cells. By providing detailed information about the structure-property relationships, this work emphasizes the role of controlled lattice expansion in enhancing the electronic functionalities of hybrid perovskites.

Cation Alloying Delocalizes Polarons in Lead Halide Perovskites

Recently, mixed-cation perovskites have promised enhanced performances concerning stability and efficiency in optoelectronic devices. Here, we report a systematic study on the effects of cation alloying on polaronic properties in cation-alloyed perovskites using first principle calculations. We find that cation alloying significantly reduces the polaron binding energies for both electrons and holes compared to pure methylammonium lead iodide (MAPbI₃). This is rationalized in terms of crystal symmetry reduction that causes polarons to be more delocalized. Electron polarons undergo large Jahn-Teller distortions (∼15-30%), whereas hole polarons tend to shrink the lattice by ∼5%. Such different lattice distortion footprints could be utilized to distinguish the type of polarons. Finally, our simulations show that Cs, formamidinium (FA), and MA mixtures can effectively minimize polaron binding energy while weakly affecting band gap, in a good agreement with experimental findings. These modeling results can guide future development of halide perovskite materials compositions for optoelectronic applications.

From Large to Small Polarons in Lead, Tin, and Mixed Lead-Tin Halide Perovskites

The origin of the long carrier lifetime in lead halide perovskites is still under debate, and, among different hypotheses, the formation of large polarons preventing the recombination of charge couples is one of the most fascinating. Using state-of-the art ab initio calculations, we report a systematic study of the polaron formation process in metal halide perovskites, focusing on the influence of the chemical composition of the perovskite on the polaron properties. We examine variations in A-site cations (FA, MA, Cs, and Cs-MA), B-site cations (Pb, Sn, and Pb-Sn), and X-site anions (Br, I). Our study confirms that stronger structural distortions occur for Cs than for MA and FA, with the effect of different A-site cations being almost additive. For the same A cation, bromide features stronger distortions than iodide perovskites. The pure Sn phase has an almost double polaron stabilization energy compared with the pure Pb phase. Surprisingly, the trend of polaron stabilization energy is nonmonotonic in mixed Sn-Pb perovskites, with a maximum for small Sn percentages. Polaron formation is found to be promoted by bond asymmetry, ranging from small to large polarons in mixed Sn-Pb perovskites depending on the relative Sn percentage.

Quasiparticle GW Calculations on Lead-Free Hybrid Germanium Iodide Perovskite CH3NH3GeI3 for Photovoltaic Applications

Lead-free organic-inorganic halide perovskites have gained much attention as nontoxic alternatives to CH3NH3PbI3 in next-generation solar cells. In this study, we have examined the geometric and electronic properties of methylammonium germanium iodide CH3NH3GeI3 using density functional theory. Identifying a suitable functional to accurately model the germanium halide perovskites is crucial to allow the theoretical investigation for tuning the optoelectronic properties. The performance of various functionals (PBE, PBE+D3, PBEsol, PBEsol+D3, HSE06, and HSE06+D3) has been evaluated for modelling the structure and properties. The calculation of electronic properties was further refined by using the quasiparticle GW method on the optimized geometries, and that has an excellent agreement with the experiment. We report from our GW calculations that the characteristic of the density of states for CH3NH3GeI3 resembles the density of states for CH3NH3PbI3 and the effective masses of the charge carriers of CH3NH3GeI3 are comparable to the effective masses of CH3NH3PbI3 as well as silicon used in commercially available solar cells.