Phonon Mode Transformation Across the Orthohombic-Tetragonal Phase Transition in a Lead Iodide Perovskite CH3NH3PbI3: A Terahertz Time-Domain Spectroscopy Approach

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.

Enhanced cation interaction in perovskites for efficient tandem solar cells with silicon

To achieve the full potential of monolithic perovskite/silicon tandem solar cells, crystal defects and film inhomogeneities in the perovskite top cell must be minimized. We discuss the use of methylenediammonium dichloride as an additive to the perovskite precursor solution, resulting in the incorporation of in situ-formed tetrahydrotriazinium (THTZ-H+) into the perovskite lattice upon film crystallization. The cyclic nature of the THTZ-H+ cation enables a strong interaction with the lead octahedra of the perovskite lattice through the formation of hydrogen bonds with iodide in multiple directions. This structure improves the device power conversion efficiency (PCE) and phase stability of 1.68 electron volts perovskites under prolonged light and heat exposure under 1-sun illumination at 85°C. Monolithic perovskite/silicon tandems incorporating THTZ-H+ in the perovskite photo absorber reached a 33.7% independently certified PCE for a device area of 1 square centimeter.

Real-time detection of aging status of methylammonium lead iodide perovskite thin films by using terahertz time-domain spectroscopy

The inadequate stability of organic-inorganic hybrid perovskites remains a significant barrier to their widespread commercial application in optoelectronic devices. Aging phenomena profoundly affect the optoelectronic performance of perovskite-based devices. In addition to enhancing perovskite stability, the real-time detection of aging status, aimed at monitoring the aging progression, holds paramount importance for both fundamental research and the commercialization of organic-inorganic hybrid perovskites. In this study, the aging status of perovskite was real-time investigated by using terahertz time-domain spectroscopy. Our analysis consistently revealed a gradual decline in the intensity of the absorption peak at 0.968 THz with increasing perovskite aging. Furthermore, a systematic discussion was conducted on the variations in intensity and position of the terahertz absorption peaks as the perovskite aged. These findings facilitate the real-time assessment of perovskite aging, providing a promising method to expedite the commercialization of perovskite-based optoelectronic devices.

Anisotropic Electron-Phonon Interactions in 2D Lead-Halide Perovskites

Two-dimensional (2D) hybrid organic-inorganic metal halide perovskites offer enhanced stability for perovskite-based applications. Their crystal structure's soft and ionic nature gives rise to strong interaction between charge carriers and ionic rearrangements. Here, we investigate the interaction of photogenerated electrons and ionic polarizations in single-crystal 2D perovskite butylammonium lead iodide (BAPI), varying the inorganic lamellae thickness in the 2D single crystals. We determine the directionality of the transition dipole moments (TDMs) of the relevant phonon modes (in the 0.3-3 THz range) by the angle- and polarization-dependent THz transmission measurements. We find a clear anisotropy of the in-plane photoconductivity, with a ∼10% reduction along the axis parallel with the transition dipole moment of the most strongly coupled phonon. Detailed calculations, based on Feynman polaron theory, indicate that the anisotropy originates from directional electron-phonon interactions.

Mechanical Control of Polaritonic States in Lead Halide Perovskite Phonons Strongly Coupled in THz Microcavity

We demonstrate the generation and control of polaritonic states in perovskite phonon polaritons, which are strongly coupled in the middle of a flexible Fabry-Perot cavity. We fabricated flexible perovskite films on a microporous substrate coated with graphene oxide, which led to a virtually free-standing film incorporated into the microcavity. Rabi splitting was observed when the cavity resonance was in tune with that of the phonons. The Rabi splitting energy increased as the film thickness increased, reaching 1.9 meV, which is 2.4-fold higher than the criterion for the strong coupling regime. We obtained dispersion curves for various perovskite film thicknesses exhibiting two polariton branches; clear beats between the two polaritonic branches were observed in the time domain. Flexible cavity devices with perovskite phonons enable macroscopic control over the polaritonic energy states through bending processes, which add an additional degree of freedom in the manipulation of polaritonic devices.

Competition of Carrier Kinetics Contributes to Amplified Spontaneous Emission in Quasi-2D/3D (PBA)2MAn-1PbnBr3n+1 Thin Films under Strip Light Mode

Quasi-2D halide perovskites have potential in lasing due to their amplified spontaneous emission (ASE) properties. The ASE of (PBA)₂MA_n-1Pb_nBr_3n+1 thin films has been confirmed by photoluminescence (PL) testing using stripe light excitation (SLE). The ASE threshold decreases with decreasing environmental temperature (T_E) or increasing number of inorganic layers (n). Using the transient absorption technique, the Auger recombination and the cooling process of the high-activity carrier are accelerated with the decrease of n or T_E. A new ASE mechanism is proposed where high-activity carriers directly emit photons under photon perturbation from adjacent sites, leading to the accumulation and amplification of emitted photons only in the SLE region for ASE to occur. In addition, the reduction of n promotes light scattering between nano-thin layers, which supports a rapid increase in the ASE signal after the ASE threshold is crossed.

A Complete Picture of Cation Dynamics in Hybrid Perovskite Materials from Solid-State NMR Spectroscopy

The organic cations in hybrid organic-inorganic perovskites rotate rapidly inside the cuboctahedral cavities formed by the inorganic lattice, influencing optoelectronic properties. Here, we provide a complete quantitative picture of cation dynamics for formamidinium-based perovskites and mixed-cation compositions, which are the most widely used and promising absorber layers for perovskite solar cells today. We use 2H and 14N quadrupolar solid-state NMR relaxometry under magic-angle spinning to determine the activation energy (Ea) and correlation time (τc) at room temperature for rotation about each principal axis of a series of organic cations. Specifically, we investigate methylammonium (MA+), formamidinium (FA+), and guanidinium (GUA+) cations in current state-of-the-art single- and multi-cation perovskite compositions. We find that MA+, FA+, and GUA+ all have at least one component of rotation that occurs on the picosecond timescale at room temperature, with MA+ and GUA+ also exhibiting faster and slower components, respectively. The cation dynamics depend on the symmetry of the inorganic lattice but are found to be insensitive to the degree of cation substitution. In particular, the FA+ rotation is invariant across all compositions studied here, when sufficiently above the phase transition temperature. We further identify an unusual relaxation mechanism for the 2H of MA+ in mechanosynthesized FAxMA1-xPbI3, which was found to result from physical diffusion to paramagnetic defects. This precise picture of cation dynamics will enable better understanding of the relationship between the organic cations and the optoelectronic properties of perovskites, guiding the design principles for more efficient perovskite solar cells in the future.

Measurement of Optical Properties of CH3NH3PbX3 (X = Br, I) Single Crystals Using Terahertz Time-Domain Spectroscopy

Organometallic lead bromide and iodide perovskite single crystals (PSCs) are potential candidates for terahertz applications. Herein, we performed terahertz time-domain spectroscopy (THz-TDS) in the frequency range of 0.1-3.0 THz on different thicknesses of MAPbBr₃ (0.3, 0.6, and 0.8 mm) and MAPbI₃ (0.6, 0.8, 0.9, 1.3, and 2.3 mm). The measurements were carried out with respect to the position (along the focal area), azimuthal rotation of the PSCs, and incidence angles of the reference THz pulse on the PSCs' surface. Based on the transmitted THz pulses from PSCs from the above measurements, we calculated the real and imaginary parts of the refractive index, dielectric constants, absorption coefficients, and dark conductivity. These optical parameters tend to increase with decreases in the PSCs' thicknesses. The transmission spectra of the terahertz electric field indicate that the measured optical properties do not vary significantly with the position and orientation of PSCs. The real parts of the refractive index and dielectric constants are higher than the imaginary values for both PSCs. On the other hand, a slight blueshift in the optical phonon vibrations corresponding to Pb-Br/I-Pb and Pb-Br/I bonds is observed with an increase in thickness. Interestingly, the phonon vibrations do not vary with the incidence angle of the THz pulses on the same crystal's surface. The optical parameters based on THz-TDS reveal that the PSCs satisfy the requirement for tunable THz devices which need suitable, sensitive, and stable absorption properties between 0.1 and 3 THz.

Nonequilibrium Lattice Dynamics in Photoexcited 2D Perovskites

Interplay between structural and photophysical properties of metal halide perovskites is critical to their utility in optoelectronics, but there is limited understanding of lattice response upon photoexcitation. Here, 2D perovskites butylammonium lead iodide, (BA)₂ PbI₄ , and phenethylammonium lead iodide, (PEA)₂ PbI₄ , are investigated using ultrafast transient X-ray diffraction as a function of optical excitation fluence to discern structural dynamics. Both powder X-ray diffraction and time-resolved photoluminescence linewidths narrow over 1 ns following optical excitation for the fluence range studied, concurrent with slight redshifting of the optical bandgaps. These observations are attributed to transient relaxation and ordering of distorted lead iodide octahedra stimulated mainly by electron-hole pair creation. The c axis expands up to 0.37% over hundreds of picoseconds; reflections sampling the a and b axes undergo one tenth of this expansion with the same timescale. Post-photoexcitation appearance of the (110) reflection in (BA)₂ PbI₄ would suggest a transient phase transition, however, through new single-crystal XRD, reflections are found that violate glide plane conditions in the reported Pbca structure. The static structure space group is reassigned as P2₁ 2₁ 2₁ . With this, a nonequilibrium phase transition is ruled out. These findings offer increased understanding of remarkable lattice response in 2D perovskites upon excitation.

Photoinduced large polaron transport and dynamics in organic-inorganic hybrid lead halide perovskite with terahertz probes

Organic-inorganic hybrid metal halide perovskites (MHPs) have attracted tremendous attention for optoelectronic applications. The long photocarrier lifetime and moderate carrier mobility have been proposed as results of the large polaron formation in MHPs. However, it is challenging to measure the effective mass and carrier scattering parameters of the photogenerated large polarons in the ultrafast carrier recombination dynamics. Here, we show, in a one-step spectroscopic method, that the optical-pump and terahertz-electromagnetic probe (OPTP) technique allows us to access the nature of interplay of photoexcited unbound charge carriers and optical phonons in polycrystalline CH₃NH₃PbI₃ (MAPbI₃) of about 10 μm grain size. Firstly, we demonstrate a direct spectral evidence of the large polarons in polycrystalline MAPbI₃. Using the Drude-Smith-Lorentz model along with the Frӧhlich-type electron-phonon (e-ph) coupling, we determine the effective mass and scattering parameters of photogenerated polaronic carriers. We discover that the resulting moderate polaronic carrier mobility is mainly influenced by the enhanced carrier scattering, rather than the polaron mass enhancement. While, the formation of large polarons in MAPbI₃ polycrystalline grains results in a long charge carrier lifetime at room temperature. Our results provide crucial information about the photo-physics of MAPbI₃ and are indispensable for optoelectronic device development with better performance.

Potassium Iodide Doping Strategy for High-Efficiency Perovskite Solar Cells Revealed by Ultrafast Spectroscopy

Organic-inorganic halide perovskites are promising materials for high-performance photovoltaics. The doping strategy is considered to be an effective method for regulating the performance of perovskite solar cells, yet its efficiency is still far below what has been anticipated. Here, we systematically investigate the regulatory mechanisms of the performance of perovskites by exploiting potassium iodide (KI) doping. We find that the surface states are passivated apart from the modified lattice structure. Most importantly, carrier recombination and transport are regulated by varying two different trap states when doping KI. The corresponding defect penalty can be effectively restrained at an optimal concentration of added KI (5%). A significant increase in the conductivity and radiative efficiency is achieved under such conditions. Our results provide fundamental insights into defect engineering through doping and a promising route toward highly efficient perovskite solar cells.

Impedance Spectroscopy of Metal Halide Perovskite Solar Cells from the Perspective of Equivalent Circuits

Impedance spectroscopy (IS) provides a detailed understanding of the dynamic phenomena underlying the operation of photovoltaic and optoelectronic devices. Here we provide a broad summary of the application of IS to metal halide perovskite materials, solar cells, electrooptic and memory devices. IS has been widely used to characterize perovskite solar cells, but the variability of samples and the presence of coupled ionic-electronic effects form a complex problem that has not been fully solved yet. We summarize the understanding that has been obtained so far, the basic methods and models, as well as the challenging points still present in this research field. Our approach emphasizes the importance of the equivalent circuit for monitoring the parameters that describe the response and providing a physical interpretation. We discuss the possibilities of models from the general perspective of solar cell behavior, and we describe the specific aspects and properties of the metal halide perovskites. We analyze the impact of the ionic effects and the memory effects, and we describe the combination of light-modulated techniques such as intensity modulated photocurrent spectroscopy (IMPS) for obtaining more detailed information in complex cases. The transformation of the frequency to time domain is discussed for the consistent interpretation of time transient techniques and the prediction of features of current-voltage hysteresis. We discuss in detail the stability issues and the occurrence of transformations of the sample coupled to the measurements.

A monoclinic semiorganic molecular crystal GUHP for terahertz photonics and optoelectronics

In this paper we describe the properties of the crystal of guanylurea hydrogen phosphite (NH\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2CNHCO(NH\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2)H\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2PO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_3$$\end{document}3 (GUHP) and propose its application in terahertz photonics and optoelectronics. GUHP crystal has a wide window of transparency and a high optical threshold in the visible and NIR spectral regions and narrow absorption bands in the terahertz frequency range. The spectral characteristics of absorption and refraction in the THz range were found to be strongly dependent on crystal temperature and orientation. Computer simulations made it possible to link the nature of the resonant response of the medium at THz frequencies with the molecular structure of the crystal, in particular, with intermolecular hydrogen bonds and the layered structure of the lattice. The possibility of application of the crystal under study for the conversion of femtosecond laser radiation from visible an NIR to terahertz range was demonstrated. It was shown that dispersion properties of the crystal allow the generation of narrow band terahertz radiation, whose spectral properties are determined by conditions close to phase matching. The properties of the generated terahertz radiation under various temperatures suggest the possibility of phonon mechanism of enhancement for nonlinear susceptibility of the second order.

Liquid-phase sintering of lead halide perovskites and metal-organic framework glasses

[Figure: see text].

Low-field onset of Wannier-Stark localization in a polycrystalline hybrid organic inorganic perovskite

Methylammonium lead iodide perovskite (MAPbI₃) is renowned for an impressive power conversion efficiency rise and cost-effective fabrication for photovoltaics. In this work, we demonstrate that polycrystalline MAPbI₃s undergo drastic changes in optical properties at moderate field strengths with an ultrafast response time, via transient Wannier Stark localization. The distinct band structure of this material - the large lattice periodicity, the narrow electronic energy bandwidths, and the coincidence of these two along the same high-symmetry direction - enables relatively weak fields to bring this material into the Wannier Stark regime. Its polycrystalline nature is not detrimental to the optical switching performance of the material, since the least dispersive direction of the band structure dominates the contribution to the optical response, which favors low-cost fabrication. Together with the outstanding photophysical properties of MAPbI₃, this finding highlights the great potential of this material in ultrafast light modulation and novel photonic applications.

Terahertz Wave Absorption Property of all Mixed Organic-Inorganic Hybrid Perovskite Thin Film MA(Sn, Pb)(Br, I)3 Fabricated by Sequential Vacuum Evaporation Method

All mixed hybrid perovskite (MA(Sn, Pb)(Br,I)3) thin film was fabricated by sequential vacuum evaporation method. To optimize the first layer with PbBr2 and SnI2, we performed different annealing treatments. Further, MA(Sn, Pb)(Br, I)3 thin film was synthesized on the optimized first layer by evaporating MAI and post-annealing. The formed hybrid perovskite thin film exhibited absorptions at 1.0 and 1.7 THz with small absorbance (<10%). Moreover, no chemical and structural defect-incorporated absorption was found. In this study, the possibility of changing terahertz absorption frequency through the mixture of metal cations (Sn+ and Pb+) and halogen anions (Br- and I-) was verified.

Multipulse Terahertz Spectroscopy Unveils Hot Polaron Photoconductivity Dynamics in Metal-Halide Perovskites

Hot carriers in metal-halide perovskites (MHPs) present a foundation for understanding carrier-phonon coupling in the materials as well as the prospective development of high-performance hot carrier photovoltaics. While the carrier population dynamics during cooling have been scrutinized, the evolution of the hot carrier properties, namely mobility, remains largely unexplored. Here we introduce novel ultrafast visible pump-infrared push-terahertz probe spectroscopy to monitor the real-time conductivity dynamics of cooling carriers in methylammonium lead iodide. We find a decrease in mobility upon optically re-exciting the carriers, as expected for band transport. Surprisingly, the conductivity recovery is incommensurate with the hot carrier population dynamics measured by infrared probe and exhibits a negligible dependence on the hot carrier density. Our results reveal the importance of localized lattice heating toward the hot carrier mobility. This collective polaron-lattice phenomenon may contribute to the unusual photophysics of MHPs and should be accounted for in hot carrier devices.

Strongly Anharmonic Octahedral Tilting in Two-Dimensional Hybrid Halide Perovskites

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

Terahertz Analysis of CH3NH3PbI3 Perovskites Associated with Graphene and Silver Nanowire Electrodes

In order to investigate the thermal and chemical (in)stabilities of MAPbI₃ incorporated with graphene and silver nanowire (AgNW) electrodes, we employed the terahertz (THz) time-domain spectroscopy, which has a unique ability to deliver the information of electrical properties and the intermolecular bonding and crystalline nature of materials. In in situ THz spectroscopy of MAPbI₃, we observed a slight blue-shift in frequency of the 2 THz phonon mode as temperatures increase across the tetragonal-cubic structural phase transition. For MAPbI₃ with the graphene top electrode, no noticeable frequency shift is observed until the temperature reaches the maximum operating temperature of solar cells (85 °C). Phonon frequency shift is sensitive to the strain-induced tilt of PbI₆ octahedra and our results indicate that graphene forms a stable interface with MAPbI₃ and is also effective in suppression of the undesirable phase transition. Meanwhile, for MAPbI₃ coupled with the AgNW bottom electrode, the THz conductivity was found to be as low as that of the MAPbI₃ single layer, attributed to the chemical reaction between Ag atoms and iodide ions. The THz conductivity is greatly increased when an ultrathin Al₂O₃ interlayer is introduced to cover the AgNW network via the atomic layer deposition (ALD) method. ALD of Al₂O₃ on the AgNW surfaces at low temperature guarantees a conformal coating, which strongly affects the ohmic contacts between the NWs. Our results demonstrate the advantage of THz spectroscopy for the comprehensive analysis of thermal and chemical stabilities of perovskites associated with the electrode materials.

Enhancing the Hot-Phonon Bottleneck Effect in a Metal Halide Perovskite by Terahertz Phonon Excitation

We investigate the impact of phonon excitations on the photoexcited carrier dynamics in a lead-halide perovskite CH_{3}NH_{3}PbI_{3}, which hosts unique low-energy phonons that can be directly excited by terahertz pulses. Our time-resolved photoluminescence measurements reveal that strong terahertz excitation prolongs the cooling time of hot carriers, providing direct evidence for the hot-phonon bottleneck effect. In contrast to the previous studies where phonons are treated as a passive heat bath, our results demonstrate that phonon excitation can significantly perturb the carrier relaxation dynamics in halide perovskites through the coupling between transverse- and longitudinal-optical phonons.

Phonon, thermal, and thermo-optical properties of halide perovskites

Metal halide perovskites are semiconductors with many fascinating characteristics and their widespread use in optoelectronic devices has been expected. High-quality thin films and single crystals can be fabricated by simple chemical solution processes and their fundamental electrical, optical, and thermal properties can be changed significantly by compositional substitution, in particular halogen ions. In this perspective, we provide an overview of phonon and thermal properties of metal halide perovskites, which play a decisive role in determining device performance. After a brief introduction to fundamental material properties, longitudinal-optical phonons and unusual thermal properties of metal halide perovskites are discussed. Remarkably, they possess very low thermal conductivities and very large thermal expansion coefficients despite their crystalline nature. In line with these discussions, we present optical properties governed by the strong electron-phonon interactions and the unusual thermal properties. By showing their unique thermo-optic responses and novel application examples, we highlight some aspects of the unusual thermal properties.

Hot Carrier Relaxation in CsPbBr3-Based Perovskites: A Polaron Perspective

Long-standing interpretations for the exceptional photovoltaic and optoelectronic properties showcased by the perovskite family pertain to the underlying complicated interplay of polaron formation and hot carrier cooling. This Perspective primarily focuses on reassessing the existing status of polaron studies conducted on CsPbBr₃-based systems in particular, in the framework of transient absorption investigations. The role of the key aspect that is ultimately accountable for deciding the fate of polaron formation, i.e., the carrier-longitudinal optical phonon coupling, has been comprehensively evaluated in terms of diverse factors which affect this Fröhlich interaction-mediated coupling. The study provides a detailed discussion regarding the alterations in lattice polarity, surrounding dielectric medium, lattice temperature, and system dimensionality which can influence the charge screening extent and thereby the polaron formation. Such studies concerning strategies for achieving easily attainable modulations in polaron formation in CsPbBr₃-based systems are highly relevant for technological advancement.

Phonon-Polaritons in Lead Halide Perovskite Film Hybridized with THz Metamaterials

In this work, we demonstrated a phonon-polariton in the terahertz (THz) frequency range, generated in a crystallized lead halide perovskite film coated on metamaterials. When the metamaterial resonance was in tune with the phonon resonance of the perovskite film, Rabi splitting occurred due to the strong coupling between the resonances. The Rabi splitting energy was about 1.1 meV, which is larger than the metamaterial and phonon resonance line widths; the interaction potential estimation confirmed that the strong coupling regime was reached successfully. We were able to tune the polaritonic branches by varying the metamaterial resonance, thereby obtaining the dispersion curve with a clear anticrossing behavior. Additionally, we performed in situ THz spectroscopy as we annealed the perovskite film and studied the Rabi splitting as a function of the films' crystallization coverage. The Rabi splitting versus crystallization volume fraction exhibited a unique power-law scaling, depending on the crystal growth dimensions.

Optical control of terahertz plasmon-induced transparency based on hybrid CsPbBr3 quantum dot metasurfaces

Incorporating photosensitive material into structured metamaterials explores opportunities for dynamical operation across the terahertz functional devices, enabled by the efficient interaction between light and matter. In this work, the CsPbBr₃ quantum dots are incorporated into the metasurfaces, realizing the active control of the plasmon-induced transparency. In the experiment, the normalized modulation depth of transparency effect is up to 74%. Rigorous numerical and theoretical simulations verify that the variation of dynamic physical process is associated with the charge storage capacity in the capacitive metasurface. An observed phase advance and group delay indicate the hybrid metasurface is useful for slow light application. In addition, the simple process provides a convenient way for the development of terahertz functional devices.

Polaron transport in hybrid CH3NH3PbI3 perovskite thin films

A comprehensive study of the transport properties of a prototypical CH3NH3PbI3 thin film is presented. The polaron-longitudinal optical (LO) phonon scattering mechanism, based on Low-Pines's polaron mobility, was studied to elucidate the charge-carrier mobility. We found that the calculated mobilities showed very good quantitative agreement with the experimental data measured in thin film samples using photoconductivity techniques. In THz mobility, the calculated results yielded room-temperature (RT) mobilities of ∼650 cm2 V-1 s-1 (single crystal) and ∼220 cm2 V-1 s-1 (disordered thin film) at a low quantum yield (φ) and 32 cm2 V-1 s-1 (high-quality thin film) at φ = 1. The dynamic disorder due to organic reorientation was included in the calculations. Its effect provided a power law mobility of μ ∝ Tm and satisfactorily supported temperature-dependent mobility over the temperature range of 80-370 K. In the orthorhombic and tetragonal phases, the charge-carrier mobilities with dynamic disorder were approximately 47% and 22% lower than those obtained from phases without dynamic disorder. The RT mobility was 26 cm2 V-1 s-1 at φ = 1. In the low-temperature orthorhombic phase, the structural phase transition was considered. The mobility followed a power law with m = -1.7. In the tetragonal and cubic phases, the mobility also followed a power law, but with m = -1.1, which is an intermediate range in optical phonon scattering. When combined with recent theoretical analysis, we also found three limitations of power law mobility with exponents between -0.46 and -1.1 for polaron-LO phonon scattering, -1.2 and -1.6 for bare carrier-LO phonon scattering, and -1.7 and -2.0 for carrier scattering off optical phonons and lattice fluctuations. This work not only provides a description of temperature-dependent mobility in CH3NH3PbI3 thin films, but also gives new insights into THz photoconductivity and the relationship between LO phonon scattering and power law mobility.

Ultrafast charge carrier dynamics in quantum confined 2D perovskite

We studied the charge carrier dynamics in 2D perovskite NBT₂PbI₄ by ultrafast optical pump-THz probe spectroscopy. We observed a few ps long relaxation dynamics that can be ascribed to the band to band carrier recombination, in the absence of any contribution from many-body and trap assisted processes. The transient conductivity spectra show that the polaron dynamics is strongly modulated by the presence of a rich exciton population. The polarization field resulting from the exciton formation acts as the source of a restoring force that localizes polarons. This is revealed by the presence of a negative imaginary conductivity. Our results show that the dynamics of excitons in 2D perovskites at room temperature can be detected by monitoring their effect on the conductivity of the photoinduced polaronic carrier.

Halide Perovskites: Thermal Transport and Prospects for Thermoelectricity

The recent re-emergence of halide perovskites has received escalating interest for optoelectronic applications. In addition to photovoltaics, the multifunctional nature of halide perovskites has led to diverse applications. The ultralow thermal conductivity coupled with decent mobility and charge carrier tunability led to the prediction of halide perovskites as a possible contender for future thermoelectrics. Herein, recent advances in thermal transport of halide perovskites and their potentials and challenges for thermoelectrics are reviewed. An overview of the phonon behavior in halide perovskites, as well as the compositional dependency is analyzed. Understanding thermal transport and knowing the thermal conductivity value is crucial for creating effective heat dissipation schemes and determining other thermal-related properties like thermo-optic coefficients, hot-carrier cooling, and thermoelectric efficiency. Recent works on halide perovskite-based thermoelectrics together with theoretical predictions for their future viability are highlighted. Also, progress on modulating halide perovskite-based thermoelectric properties using light and chemical doping is discussed. Finally, strategies to overcome the limiting factors in halide perovskite thermoelectrics and their prospects are emphasized.

Strong Linear Correlation between CH3NH2 Molecular Defect and THz-Wave Absorption in CH3NH3PbI3 Hybrid Perovskite Thin Film

To control the density of a CH₃NH₂ molecular defect, which strongly contributed to a significant THz-wave absorption property in the CH₃NH₃PbI₃ hybrid perovskite thin film formed by the sequential vacuum evaporation method, we performed post-annealing processes with various temperatures and times. In the thin film after post-annealing at 110 °C for 45 min, the density of the CH₃NH₂ molecular defect was minimized, and CH₃NH₃I and PbI₂ disappeared in the thin film after the post-annealing process at 150 °C for 30 min. However, the density of the CH₃NH₂ molecular defect increased. Moreover, the THz-wave absorption property for each thin film was obtained using a THz time-domain spectroscopy to understand the correlation between the density of a molecular defect and the THz-wave oscillation strength at 1.6 THz, which originated in the molecular defect-incorporated hybrid perovskite structure. There is a strong linear correlation between the oscillator strength of a significant THz-wave absorption at 1.6 THz and the CH₃NH₂ molecular defect density.

Temperature and Gate Dependence of Carrier Diffusion in Single Crystal Methylammonium Lead Iodide Perovskite Microstructures

We investigate temperature-dependent photogenerated carrier diffusion in single-crystal methylammonium lead iodide microstuctures via scanning photocurrent microscopy. Carrier diffusion lengths increased abruptly across the tetragonal to orthorhombic phase transition and reached 200 ± 50 μm at 80 K. In combination with the microsecond carrier lifetime measured by a transient photocurrent method, an enormous carrier mobility value of 3 × 10⁴ cm²/V s was extracted at 80 K. The observed highly nonlocal photocurrent and the rapid increase of the carrier diffusion length at low temperatures can be understood by the formation and efficient transport of free excitons in the orthorhombic phase as a result of reduced optical phonon scattering due to the dipolar nature of the excitons. Carrier diffusion lengths were tuned by a factor of 8 by gate voltage and increased with increasing majority carrier (electron) concentration, consistent with the exciton model.

Properties of Excitons and Photogenerated Charge Carriers in Metal Halide Perovskites

Metal halide perovskites (MHPs) have recently attracted great attention from the scientific community due to their excellent photovoltaic performance as well as their tremendous potential for other optoelectronic applications such as light-emitting diodes, lasers, and photodetectors. Despite the rapid progress in device applications, a solid understanding of the photophysical properties behind the device performance is highly desirable for MHPs. Here, the properties of excitons and photogenerated charge carriers in MHPs are explored. The unique dielectric constant properties, crystal-liquid duality, and fundamental optical processes of MHPs are first discussed. The properties of excitons and related phenomena in MHPs are then detailed, including the exciton binding energy determined by various methods and their influence factors, exciton dynamics, exciton-photon coupling and related applications, and exciton-phonon coupling in MHPs. The properties of photogenerated free charge carriers in MHPs such as the carrier diffusion length, mobility, and recombination are described. Recent progress in various applications is also demonstrated. Finally, a conclusion and perspectives of future studies for MHPs are presented.

Monitoring Electron-Phonon Interactions in Lead Halide Perovskites Using Time-Resolved THz Spectroscopy

Lead halide perovskite semiconductors have low-frequency phonon modes within the lead halide sublattice and thus are considered to be soft. The soft lattice is considered to be important in defining their interesting optoelectronic properties. Electron-phonon coupling governs hot-carrier relaxation, carrier mobilities, carrier lifetimes, among other important electronic characteristics. Directly observing the interplay between free charge carriers and phonons can provide details on how phonons impact these properties (e.g., exciton populations and other collective modes). Here, we observe a delicate interplay among carriers, phonons, and excitons in mixed-cation and mixed-halide perovskite films by simultaneously resolving the contribution of charge carriers and phonons in time-resolved terahertz photoconductivity spectra. We are able to observe directly the increase in phonon population during carrier cooling and discuss how thermal equilibrium populations of carriers and phonons modulate the carrier transport properties, as well as reduce the population of carriers within band tails. We are also able to observe directly the formation of free charge carriers when excitons interact with phonons and dissociate and to describe how free carriers and exciton populations exchange through phonon interactions. Finally, we also time-resolve how the carriers are screened via the Coulomb interaction at low and room temperatures. Our studies shed light on how charge carriers interact with the low-energy phonons and discuss implications.

Ultrafast correlated charge and lattice motion in a hybrid metal halide perovskite

Hybrid organic-inorganic halide perovskites have shown remarkable optoelectronic properties, exhibiting an impressive tolerance to defects believed to originate from correlated motion of charge carriers and the polar lattice forming large polarons. Few experimental techniques are capable of directly probing these correlations, requiring simultaneous sub-millielectron volt energy and femtosecond temporal resolution after absorption of a photon. Here, we use time-resolved multi-THz spectroscopy, sensitive to the internal excitations of the polaron, to temporally and energetically resolve the coherent coupling of charges to longitudinal optical phonons in single-crystal CH₃NH₃PbI₃ (MAPI). We observe room temperature intraband quantum beats arising from the coherent displacement of charge from the coupled phonon cloud. Our measurements provide strong evidence for the existence of polarons in MAPI at room temperature, suggesting that electron/hole-phonon coupling is a defining aspect of the hybrid metal-halide perovskites contributing to the protection from scattering and enhanced carrier lifetimes that define their usefulness in devices.

Ultrafast THz Probe of Photoinduced Polarons in Lead-Halide Perovskites

We study the nature of photoexcited charge carriers in CsPbBr_{3} nanocrystal thin films by ultrafast optical pump-THz probe spectroscopy. We observe a deviation from a pure Drude dispersion of the THz dielectric response that is ascribed to the polaronic nature of carriers; a transient blueshift of observed phonon frequencies is indicative of the coupling between photogenerated charges and stretching-bending modes of the deformed inorganic sublattice, as confirmed by DFT calculations.

Significant THz absorption in CH3NH2 molecular defect-incorporated organic-inorganic hybrid perovskite thin film

The valid strong THz absorption at 1.58 THz was probed in the organic-inorganic hybrid perovskite thin film, CH3NH3PbI3, fabricated by sequential vacuum evaporation method. In usual solution-based methods such as 2-step solution and antisolvent, we observed the relatively weak two main absorption peaks at 0.95 and 1.87 THz. The measured absorption spectrum is analyzed by density-functional theory calculations. The modes at 0.95 and 1.87 THz are assigned to the Pb-I vibrations of the inorganic components in the tetragonal phase. By contrast, the origin of the 1.58 THz absorption is due to the structural deformation of Pb-I bonding at the grain boundary incorporated with a CH3NH2 molecular defect.

Phonon coherences reveal the polaronic character of excitons in two-dimensional lead halide perovskites

Hybrid organic-inorganic semiconductors feature complex lattice dynamics due to the ionic character of the crystal and the softness arising from non-covalent bonds between molecular moieties and the inorganic network. Here we establish that such dynamic structural complexity in a prototypical two-dimensional lead iodide perovskite gives rise to the coexistence of diverse excitonic resonances, each with a distinct degree of polaronic character. By means of high-resolution resonant impulsive stimulated Raman spectroscopy, we identify vibrational wavepacket dynamics that evolve along different configurational coordinates for distinct excitons and photocarriers. Employing density functional theory calculations, we assign the observed coherent vibrational modes to various low-frequency (≲50 cm^-1) optical phonons involving motion in the lead iodide layers. We thus conclude that different excitons induce specific lattice reorganizations, which are signatures of polaronic binding. This insight into the energetic/configurational landscape involving globally neutral primary photoexcitations may be relevant to a broader class of emerging hybrid semiconductor materials.

A, Vozzi C, Manna L, Petrozza A, De Angelis F, Stagira S. Large polaron evidence in the ultrafast THz response of Lead-Halide Perovskites

We unveil the large polaron fingerprints in the transient THz dielectric response of lead-halide perovskites. We clarify the mechanism underlying the physics of charge transport of full-inorganic lead-halide perovskites by combining ultrafast Thz spectroscopy with DFT calculations.

How Lattice Dynamics Moderate the Electronic Properties of Metal-Halide Perovskites

Metal-halide perovskites have emerged as highly promising semiconductors with excellent optoelectronic properties. This Perspective outlines how the dynamic response of the ionic lattice affects key electronic properties such as exciton binding energies and charge-carrier mobilities in hybrid perovskites. Such links are shown to derive from the frequency-dependence of the dielectric function, which is governed by contributions from electronic interband transitions, polar vibrations of the metal-halide sublattice, organic cation collective reorientations, and ionic movement. The influence of each of these contributions to charge-carrier screening and carrier-lattice interactions is discussed, which allows for general trends with material composition to be revealed. Overall, this Perspective highlights the challenges and questions arising from the peculiar combination of a soft polar metal-halide sublattice interspersed with rotationally mobile dipolar molecules that is encountered in hybrid metal-halide perovskites.

Longitudinal Optical Phonons Modified by Organic Molecular Cation Motions in Organic-Inorganic Hybrid Perovskites

We performed terahertz time-domain spectroscopy for methylammonium (MA) lead halide perovskite single crystals and characterized the longitudinal optical (LO) phonons directly. We found that the effective LO phonon wave number does not change in the wide temperature range between 10 and 300 K. However, the coupling between MA cation modes and the LO phonon mode derived from lead halide cages induces a mode splitting at low temperatures and a damping of the LO phonon mode at high temperatures. These results influence the interpretation of electron-LO phonon interactions in perovskite semiconductors, as well as the interpretations of mobility, carrier diffusion, and polaron formation.

Impact of the Organic Cation on the Optoelectronic Properties of Formamidinium Lead Triiodide

Metal halide perovskites have proven to be excellent light-harvesting materials in photovoltaic devices whose efficiencies are rapidly improving. Here, we examine the temperature-dependent photon absorption, exciton binding energy, and band gap of FAPbI₃ (thin film) and find remarkably different behavior across the β-γ phase transition compared with MAPbI₃. While MAPbI₃ has shown abrupt changes in the band gap and exciton binding energy, values for FAPbI₃ vary smoothly over a range of 100-160 K in accordance with a more gradual transition. In addition, we find that the charge-carrier mobility in FAPbI₃ exhibits a clear T^-0.5 trend with temperature, in excellent agreement with theoretical predictions that assume electron-phonon interactions to be governed by the Fröhlich mechanism but in contrast to the T^-1.5 dependence previously observed for MAPbI₃. Finally, we directly observe intraexcitonic transitions in FAPbI₃ at low temperature, from which we determine a low exciton binding energy of only 5.3 meV at 10 K.

Phonon mode transformation in size-evolved solution-processed inorganic lead halide perovskite

Recently, lead halide perovskites have attracted significant scientific attention in the fields of photovoltaics, light emitting diodes, lasers, photo-detectors and other optoelectronic functional devices. The most stable crystal form of lead halide perovskites is the cube, including nano-cube and micro-cube, which hold great promise as functional materials due to their combination of unique optoelectronic properties and versatility through colloidal synthesis. Herein, we report the solution-processed synthesis of pure inorganic lead halide nano-cubes- and micro-cubes-based colloidal perovskites. The different size of cubes either into nano-cube or micro-cube are demonstrated that their phonon mode transformation which means the perovskite crystal structure phase change cross the nano-cube to micro-cube. The solution-processed colloidal synthesis method and phonon-mode transformation from nano-cube to micro-cube make pure inorganic lead halide perovskite an ideal platform for fundamental optoelectronic studies and the investigation of functional devices.

Epitaxial strain driven crossover from Drude to Drude-Smith terahertz conductivity dynamics in LaNiO3 thin films

We investigate the hetero-epitaxial strain driven low-energy charge dynamics in compressive and tensile strained LaNiO₃ thin films employing terahertz (THz) time-domain spectroscopy. The complex THz conductivity exhibits a crossover from Drude type metallic behavior for the compressive film to a Drude-Smith type disordered behavior for the tensile film. This demonstration of strain driven crossover in THz conductivity dynamics, while the two films have qualitatively similar dc conductivities, (i) brings out the potential of THz technology in distinguishing between similar dc electronic phases and (ii) suggests that LaNiO₃ under compressive strain is a better candidate for applications as electrodes in oxides electronics.

Polaronic Charge Carrier-Lattice Interactions in Lead Halide Perovskites

Almost ten years after the renaissance of the popular perovskite-type semiconductors based on lead salts with the general formula AMX₃ (A=organic or inorganic cation; M=divalent metal; X=halide), many facets of photophysics continue to puzzle researchers. In this Minireview, light is shed on the low mobilities of charge carriers in lead halide perovskites with special focus on the lattice properties at non-zero temperature. The polar and soft lattice leads to pronounced electron-phonon coupling, limiting carrier mobility and retarding recombination. We propose that the proper picture of excited charge carriers at temperature ranges that are relevant for device operations is that of a polaron, with Fröhlich coupling constants between 1<α<3. Under the aspect of light-emitting diode application, APbX₃ perovskite show moderate second order (bimolecular) recombination rates and high third-order (Auger) rate constants. It has become apparent that this is a direct consequence of the anisotropic polar A-site cation in organic-inorganic hybrid perovskites and might be alleviated by replacing the organic moiety with an isotropic cation.

Direct observation of mode-specific phonon-band gap coupling in methylammonium lead halide perovskites

Methylammonium lead iodide perovskite is an outstanding semiconductor for photovoltaics. One of its intriguing peculiarities is that the band gap of this perovskite increases with increasing lattice temperature. Despite the presence of various thermally accessible phonon modes in this soft material, the understanding of how precisely these phonons affect macroscopic material properties and lead to the peculiar temperature dependence of the band gap has remained elusive. Here, we report a strong coupling of a single phonon mode at the frequency of ~ 1 THz to the optical band gap by monitoring the transient band edge absorption after ultrafast resonant THz phonon excitation. Excitation of the 1 THz phonon causes a blue shift of the band gap over the temperature range of 185 ~ 300 K. Our results uncover the mode-specific coupling between one phonon and the optical properties, which contributes to the temperature dependence of the gap in the tetragonal phase.Methylammonium lead iodide perovskite, a promising material for efficient photovoltaics, shows a unique temperature dependence of its optical properties. Kim et al. quantify the coupling between the optical gap and a lattice phonon at 1 THz, which favorably contributes to the thermal variation of the gap.

Hybrid Lead Halide Perovskites for Ultrasensitive Photoactive Switching in Terahertz Metamaterial Devices

The recent meteoric rise in the field of photovoltaics with the discovery of highly efficient solar-cell devices is inspired by solution-processed organic-inorganic lead halide perovskites that exhibit unprecedented light-to-electricity conversion efficiencies. The stunning performance of perovskites is attributed to their strong photoresponsive properties that are thoroughly utilized in designing excellent perovskite solar cells, light-emitting diodes, infrared lasers, and ultrafast photodetectors. However, optoelectronic application of halide perovskites in realizing highly efficient subwavelength photonic devices has remained a challenge. Here, the remarkable photoconductivity of organic-inorganic lead halide perovskites is exploited to demonstrate a hybrid perovskite-metamaterial device that shows extremely low power photoswitching of the metamaterial resonances in the terahertz part of the electromagnetic spectrum. Furthermore, a signature of a coupled phonon-metamaterial resonance is observed at higher pump powers, where the Fano resonance amplitude is extremely weak. In addition, a low threshold, dynamic control of the highly confined electric field intensity is also observed in the system, which could tremendously benefit the new generation of subwavelength photonic devices as active sensors, low threshold optically controlled lasers, and active nonlinear devices with enhanced functionalities in the infrared, optical, and the terahertz parts of the electromagnetic spectrum.

Room-Temperature Coherent Optical Phonon in 2D Electronic Spectra of CH3NH3PbI3 Perovskite as a Possible Cooling Bottleneck

A hot phonon bottleneck may be responsible for slow hot carrier cooling in methylammonium lead iodide hybrid perovskite, creating the potential for more efficient hot carrier photovoltaics. In room-temperature 2D electronic spectra near the band edge, we observe amplitude oscillations due to a remarkably long lived 0.9 THz coherent phonon population at room temperature. This phonon (or set of phonons) is assigned to angular distortions of the Pb-I lattice, not coupled to cation rotations. The strong coupling between the electronic transition and the 0.9 THz mode(s), together with relative isolation from other phonon modes, makes it likely to cause a phonon bottleneck. The pump frequency resolution of the 2D spectra also enables independent observation of photoinduced absorptions and bleaches independently and confirms that features due to band gap renormalization are longer-lived than in transient absorption spectra.

Ultrafast terahertz snapshots of excitonic Rydberg states and electronic coherence in an organometal halide perovskite

How photoexcitations evolve into Coulomb-bound electron and hole pairs, called excitons, and unbound charge carriers is a key cross-cutting issue in photovoltaics and optoelectronics. Until now, the initial quantum dynamics following photoexcitation remains elusive in the hybrid perovskite system. Here we reveal excitonic Rydberg states with distinct formation pathways by observing the multiple resonant, internal quantum transitions using ultrafast terahertz quasi-particle transport. Nonequilibrium emergent states evolve with a complex co-existence of excitons, carriers and phonons, where a delayed buildup of excitons under on- and off-resonant pumping conditions allows us to distinguish between the loss of electronic coherence and hot state cooling processes. The nearly ∼1 ps dephasing time, efficient electron scattering with discrete terahertz phonons and intermediate binding energy of ∼13.5 meV in perovskites are distinct from conventional photovoltaic semiconductors. In addition to providing implications for coherent energy conversion, these are potentially relevant to the development of light-harvesting and electron-transport devices.

Positron Annihilation Spectroscopic Investigation on the Origin of Temperature-Dependent Electrical Response in Methylammonium Lead Iodide Perovskite

Organic-inorganic hybrid perovskite has appeared as one of the leading materials for realizing solution-based high-performing optoelectronic devices. The charge transport properties in this class of material are quite intriguing and still need to be carefully investigated. The temperature-dependent electrical property of methylammonium lead iodide (CH₃NH₃PbI₃) has been investigated by employing positron annihilation spectroscopy (PAS), which unambiguously reveals the gradual formation of open volume defects with the enhancement in temperature. The high-temperature ionic conductivity is due to the generation of both cationic (CH₃NH₃⁺) and anionic (I^-) vacancies, possibly because of the elimination of methylammonium iodide (CH₃NH₃I) as identified from the coincidence Doppler broadening (CDB) of the positron annihilation spectroscopy. Further, the evolution of temperature-dependent defect density and corresponding electrical responses has been correlated with the structural phase transitions of CH₃NH₃PbI₃. This is the first ever report of temperature-dependent PAS measurement on hybrid lead halide perovskites to understand the nature and the origin of its electrical characteristics arising due to the variation in temperature.