Fröhlich interaction dominated by a single phonon mode in CsPbBr3

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.

Disentangling the effects of structure and lone-pair electrons in the lattice dynamics of halide perovskites

Halide perovskites show great optoelectronic performance, but their favorable properties are paired with unusually strong anharmonicity. It was proposed that this combination derives from the ns2 electron configuration of octahedral cations and associated pseudo-Jahn-Teller effect. We show that such cations are not a prerequisite for the strong anharmonicity and low-energy lattice dynamics encountered in these materials. We combine X-ray diffraction, infrared and Raman spectroscopies, and molecular dynamics to contrast the lattice dynamics of CsSrBr3 with those of CsPbBr3, two compounds that are structurally similar but with the former lacking ns2 cations with the propensity to form electron lone pairs. We exploit low-frequency diffusive Raman scattering, nominally symmetry-forbidden in the cubic phase, as a fingerprint of anharmonicity and reveal that low-frequency tilting occurs irrespective of octahedral cation electron configuration. This highlights the role of structure in perovskite lattice dynamics, providing design rules for the emerging class of soft perovskite semiconductors.

Lattice dynamics and self-trapped excitons in the Cs2SnBr6double perovskites

Our study delved into the detailed investigation of Cs2SnBr6double perovskites, focusing on their electrical properties, lattice dynamics, and stability. The direct bandgap for Cs2SnBr6was estimated to be at 2.93 eV. One external translational mode of the Cs+lattice withT2gsymmetry and three internal modes of the octahedral withA1g,Eg, andT2gsymmetries are defined by calculated lattice dynamics, experimental micro-Raman scattering. We show a correlation with first-principles calculations, validating using a band-structured electronic approach to understanding the behavior of charge carriers, and electron-phonon interactions in Cs2SnBr6. We propose that electron-vibration interactions result in self-trapped excitons (STEs) displaying significant Stokes shifts (0.508 eV) and broad-spectrum emission. Understanding the behavior of STEs is fundamental for their optoelectronic applications.

Exploring the lead-free halide Cs2MGaBr6 (M = Li, Na) double perovskites for sustainable energy applications

In recent years, there has been a growing emphasis on the exploration of sustainable and eco-friendly materials well-suited for advanced applications in the realms of thermoelectrics and optoelectronics. Lead-free halide double perovskites have emerged as a compelling class of materials in this context. Nevertheless, despite their potential utility, thorough investigations into their thermal transport characteristics remain limited. In this systematic investigation, we employ density functional theory (DFT) and post-DFT techniques to elucidate the essential stability parameters, transport properties, and carrier-lattice interactions of the metal halide-based Cs2MGaBr6 (X = Li, Ga) double perovskites. Our assessment of structural stability involves a meticulous description of stability index parameters and the optimization of pristine structures using the GGA-PBE potential. Additionally, we calibrate the electronic structure while taking spin-orbit coupling (SOC) effects into consideration by using a combination of GGA and GGA + mBJ potentials. Our findings reveal that the TB-mBJ derived band gaps of 1.82 eV and 1.78 eV for Cs2LiGaBr6 and Cs2NaGaBr6 reside within the visible spectrum, prompting further investigation into their thermal transport characteristics. Moreover, we analyze the phonon characteristics and vibrational modes, extending our investigation to examine the electron-phonon coupling strength. The scrutiny of the Fröhlich coupling constant and the Feynman polaron radius unveils a stronger electron-phonon coupling strength. In the domain of thermoelectrics, the significant figure of merit (zT) values of 1.08 and 1.04 for Cs2LiGaBr6 and Cs2NaGaBr6, respectively, emphasize the considerable potential of these materials for deployment in renewable energy applications. Furthermore, our computational investigation into optical properties, including the dielectric constant, optical absorption, and refractive index, demonstrates optimal performance within the visible spectrum. Specifically, elevated absorption coefficient values of 30 × 10 4cm - 1 for Cs2LiGaBr6 and 40 × 10 4cm - 1 for Cs2NaGaBr6 are noted across visible and infrared spectra, highlighting their promising potential in optoelectronic and solar cell technologies.

How spin relaxes and dephases in bulk halide perovskites

Spintronics in halide perovskites has drawn significant attention in recent years, due to their highly tunable spin-orbit fields and intriguing interplay with lattice symmetry. Here, we perform first-principles calculations to determine the spin relaxation time (T1) and ensemble spin dephasing time ([Formula: see text]) in a prototype halide perovskite, CsPbBr3. To accurately capture spin dephasing in external magnetic fields we determine the Landé g-factor from first principles and take it into account in our calculations. These allow us to predict intrinsic spin lifetimes as an upper bound for experiments, identify the dominant spin relaxation pathways, and evaluate the dependence on temperature, external fields, carrier density, and impurities. We find that the Fröhlich interaction that dominates carrier relaxation contributes negligibly to spin relaxation, consistent with the spin-conserving nature of this interaction. Our theoretical approach may lead to new strategies to optimize spin and carrier transport properties.

Pressure influence on excitonic luminescence of CsPbBr3 perovskite

This study investigates the effect of hydrostatic pressure on the luminescence properties of CsPbBr3 single crystals at 12 K. The luminescence at the edge of the band gap reveals a structure attributed to free excitons, phonon replica of the free excitons, and Rashba excitons. Changes in the relative intensity of the free and Rashba excitons were observed with increasing pressure, caused by changes in the probability of nonradiative deexcitation. At pressures around 3 GPa, luminescence completely fades away. The red shift of the energy position of the maximum luminescence of free and Rashba excitons in pressure ranges of 0-1.3 GPa is attributed to the length reduction of Pb-Br bonds in [PbBr6]4- octahedra, while the high-energy shift of the Rashba excitons at pressures above 1.3 GPa is due to [PbBr6]4- octahedra rotation and changes in the Pb-Br-Pb angle.

Coupling to octahedral tilts in halide perovskite nanocrystals induces phonon-mediated attractive interactions between excitons

Understanding the origin of electron-phonon coupling in lead halide perovskites is key to interpreting and leveraging their optical and electronic properties. Here we show that photoexcitation drives a reduction of the lead-halide-lead bond angles, a result of deformation potential coupling to low-energy optical phonons. We accomplish this by performing femtosecond-resolved, optical-pump-electron-diffraction-probe measurements to quantify the lattice reorganization occurring as a result of photoexcitation in nanocrystals of FAPbBr3. Our results indicate a stronger coupling in FAPbBr3 than CsPbBr3. We attribute the enhanced coupling in FAPbBr3 to its disordered crystal structure, which persists down to cryogenic temperatures. We find the reorganizations induced by each exciton in a multi-excitonic state constructively interfere, giving rise to a coupling strength that scales quadratically with the exciton number. This superlinear scaling induces phonon-mediated attractive interactions between excitations in lead halide perovskites.

A High-Rigidity Organic-Inorganic Metal Halide Hybrid Enabling Reversible and Enhanced Self-Trapped Exciton Emission under High Pressure

Zero-dimensional organic-inorganic metal halide hybrids provide ideal bulk-crystal platforms for exploring the pressure engineering of electron-phonon coupling (EPC) and self-trapped exciton (STE) emission at the molecular level. However, the low stiffness of inorganic clusters hinders the reversible tuning of these physical properties. Herein, we designed a Sb3+-doped metal halide with a high emission yield (89.4%) and high bulk modulus (35 GPa) that enables reversible and enhanced STE emission (20-fold) under pressure. The high lattice rigidity originates from the corner-shared cage-structured inorganic tetramers and ring-shaped organic ligands. Further, we reveal that the pressure-enhanced emission regime below 4.5 GPa is owing to the lattice hardening and preferably EPC strength reducing, while the pressure-insensitive emission regime within 4.5-8.5 GPa results from the enhanced intercluster Coulombic attraction force that resists intracluster compression. These results provide insights into the structure-property relation and molecular engineering of zero-dimensional metal halides toward wide-band and pressure-sensitive light sources.

Spectral Fingerprint of Quantum Confinement in Single CsPbBr3 Nanocrystals

Lead halide perovskite nanocrystals are promising materials for classical and quantum light emission. To understand these outstanding properties, a thorough analysis of the band-edge exciton emission is needed, which is not reachable in ensemble and room-temperature studies because of broadening effects. Here, we report on a cryogenic-temperature study of the photoluminescence of single CsPbBr₃ nanocrystals in the intermediate quantum confinement regime. We reveal the size-dependence of the spectral features observed: the bright triplet exciton energy splittings, the trion and biexciton binding energies, and the optical phonon replica spectrum. In addition, we show that bright triplet energy splittings are consistent with a pure exchange model and that the variety of polarization properties and spectra recorded can be rationalized simply by considering the orientation of the emitting dipoles and the populations of the emitting states.

Imaging the Terahertz Nanoscale Conductivity of Polycrystalline CsPbBr3 Perovskite Thin Films

Terahertz (THz) radiation is a valuable tool to investigate the electronic properties of lead halide perovskites (LHPs). However, attaining high-resolution information remains elusive, as the diffraction-limited spatial resolution (∼300 μm) of conventional THz methods prevents a direct analysis of microscopic effects. Here, we employ THz scattering scanning near-field optical microscopy (THz-sSNOM) for nanoscale imaging of cesium lead bromide (CsPbBr₃) thin films down to the single grain level at 600 GHz. Adopting a scattering model, we are able to derive the local THz nanoscale conductivity in a contact-free fashion. Increased THz near-field signals at CsPbBr₃ grain boundaries complemented by correlative transmission electron microscopy-energy-dispersive X-ray spectroscopy elemental analysis point to the formation of halide vacancies (V_Br) and Pb-Pb bonds, which induce charge carrier trapping and can lead to nonradiative recombination. Our study establishes THz-sSNOM as a powerful THz nanoscale analysis platform for thin-film semiconductors such as LHPs.

Strongly Confined CsPbBr3 Quantum Dots as Quantum Emitters and Building Blocks for Rhombic Superlattices

The success of the colloidal semiconductor quantum dots (QDs) field is rooted in the precise synthetic control of QD size, shape, and composition, enabling electronically well-defined functional nanomaterials that foster fundamental science and motivate diverse fields of applications. While the exploitation of the strong confinement regime has been driving commercial and scientific interest in InP or CdSe QDs, such a regime has still not been thoroughly explored and exploited for lead-halide perovskite QDs, mainly due to a so far insufficient chemical stability and size monodispersity of perovskite QDs smaller than about 7 nm. Here, we demonstrate chemically stable strongly confined 5 nm CsPbBr₃ colloidal QDs via a postsynthetic treatment employing didodecyldimethylammonium bromide ligands. The achieved high size monodispersity (7.5% ± 2.0%) and shape-uniformity enables the self-assembly of QD superlattices with exceptional long-range order, uniform thickness, an unusual rhombic packing with an obtuse angle of 104°, and narrow-band cyan emission. The enhanced chemical stability indicates the promise of strongly confined perovskite QDs for solution-processed single-photon sources, with single QDs showcasing a high single-photon purity of 73% and minimal blinking (78% "on" fraction), both at room temperature.

Lattice Dynamics and Electron-Phonon Coupling in Double Perovskite Cs2NaFeCl6

Phonon-phonon and electron/exciton-phonon coupling play a vitally important role in thermal, electronic, as well as optical properties of metal halide perovskites. In this work, we evaluate phonon anharmonicity and coupling between electronic and vibrational excitations in novel double perovskite Cs₂NaFeCl₆ single crystals. By employing comprehensive Raman measurements combined with first-principles theoretical calculations, we identify four Raman-active vibrational modes. Polarization properties of these modes imply Fm3̅m symmetry of the lattice, indicative for on average an ordered distribution of Fe and Na atoms in the lattice. We further show that temperature dependence of the Raman modes, such as changes in the phonon line width and their energies, suggests high phonon anharmonicity, typical for double perovskite materials. Resonant multiphonon Raman scattering reveals the presence of high-lying band states that mediate strong electron-phonon coupling and give rise to intense nA _1g overtones up to the fifth order. Strong electron-phonon coupling in Cs₂NaFeCl₆ is also concluded based on the Urbach tail analysis of the absorption coefficient and the calculated Fröhlich coupling constant. Our results, therefore, suggest significant impacts of phonon-phonon and electron-phonon interactions on electronic properties of Cs₂NaFeCl₆, important for potential applications of this novel material.

Polaron states of the full-configuration defects in metal halide perovskites

The systematical analysis for varieties of defects with different depths and lattice relaxation strengths in metal halide perovskites (MHPs) is a challenging task. Here, we study the energy shifts of the full-configuration defects due to the polaron effect based on the all-coupling variational method in MHPs, where these polaron states are formed stemming from different defect species coupling with the longitudinal optical phonon modes via Fro¨hlich mechanism. We find that the polaron effect results in defect levels varying from tens to several hundreds of meV, which are very close to the correction of defect levels due to the defect-polaron effect, especially for these defects migration proved in the recent experiments in MHPs. These results provide the significant enlightenment not only for analyzing the radiation and non-radiation processes of carriers mediated by defects, but also for optimizing defect effect in the photovoltaic and photoelectric devices based on MHPs materials.

Investigation of Two Photon Absorption of Ligand-Modified CsPbBr3 Quantum Dots

The characteristics and application of nonlinear absorption from CsPbBr₃ QDs film with the ligand-modified strategy have been investigated in this work. By means of a near-infrared fs Ti:sapphire laser as a light source, the up-conversion emission of CsPbBr₃ QDs film of around 518 nm revealed a quadratic increase with the pump intensity. Through the temperature-dependent up-conversion emission, we obtained the binding energy and longitudinal optical (LO) phonon energy of CsPbBr₃ QDs film of around 58.1 and 61.2 meV, respectively. Due to more active thermal coupling between the excited electron or hot phonon effect, the photon decay trace under two-photon excitation was prolonged at higher temperatures. The ligand-modified CsPbBr₃ QDs film exhibits a relatively large TPA coefficient of around 28.6 cm/GW by the open aperture Z-scan measurement, and it has been demonstrated as a promising nonlinear medium to obtain the pulsewidth of ultrafast lasers.

Self-trapped excitons in soft semiconductors

Self-trapped excitons (STEs) have attracted tremendous attention due to their intriguing properties and potential optoelectronic applications. STEs are formed from the lattice distortion induced by the strong electron (exciton)-phonon coupling in soft semiconductors upon photoexcitation, which features in broadband photoluminescence (PL) emission spectra with a large Stokes shift. Recently, significant progress has been achieved in this field but many remain challenges that need to be solved, including the understanding of the underlying physical mechanism, tuning of the performance, and device applications. Along these lines, for the first time, systematic experimental characterizations and advanced theoretical calculations are presented in this review to shed light on the physical mechanism. The possibility of tuning the STEs through multiple degrees of freedom is also presented, along with an overview of the STE-based emerged applications and future research perspectives.

Probing the Disorder Inside the Cubic Unit Cell of Halide Perovskites from First-Principles

Strong deviations in the finite temperature atomic structure of halide perovskites from their average geometry can have profound impacts on optoelectronic and other device-relevant properties. Detailed mechanistic understandings of these structural fluctuations and their consequences remain, however, limited by the experimental and theoretical challenges involved in characterizing strongly anharmonic vibrational characteristics and their impact on other properties. We overcome some of these challenges by a theoretical characterization of the vibrational interactions that occur among the atoms in the prototypical cubic CsPbBr3. Our investigation based on first-principles molecular dynamics calculations finds that the motions of neighboring Cs-Br atoms interlock, which appears as the most likely Cs-Br distance being significantly shorter than what is inferred from an ideal cubic structure. This form of dynamic Cs-Br coupling coincides with very shallow dynamic potential wells for Br motions that occur across a locally and dynamically disordered energy landscape. We reveal an interesting dynamic coupling mechanism among the atoms within the nominal unit cell of cubic CsPbBr3 and quantify the important local structural fluctuations on an atomic scale.