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

Odd-Even Alkyl Chain Effects on the Structure and Charge Carrier Transport of Two-Dimensional Sn-Based Perovskite Semiconductors

Oscillations in the chemical or physical properties of materials, composed of an odd or even number of connected repeating methylene units, are a well-known phenomenon in organic chemistry and materials science. So far, such behavior has not been reported for the important class of materials, perovskite semiconductors. This work reports a distinct odd-even oscillation of the molecular structure and charge carrier transport properties of phenylalkylammonium two-dimensional (2D) Sn-based perovskites in which the alkyl chains in the phenylalkylammonium cations contain varying odd and even carbon numbers. Density functional theory calculations and grazing-incidence wide-angle X-ray scattering characterization reveal that perovskites with organic ligands containing an alkyl chain with an odd number of carbon atoms display a disordered crystal lattice and tilted inorganic octahedra accompanied by reduced mobilities. In contrast, perovskites with cations of an even number of carbon atoms in the alkyl chain form more ordered crystal structures, resulting in improved charge carrier mobilities. Our findings disclose the importance of minor changes in the molecular conformation of organic cations have an effect on morphology, photophysical properties, and charge carrier transport of 2D layered perovskites, showcasing alkyl chain engineering of organic cations to control key properties, of layered perovskite semiconductors.

Phonon Properties and Lattice Dynamics of Two- and Tri-Layered Lead Iodide Perovskites Comprising Butylammonium and Methylammonium Cations-Temperature-Dependent Raman Studies

Hybrid lead iodide perovskites are promising photovoltaic and light-emitting materials. Extant literature data on the key optoelectronic and luminescent properties of hybrid perovskites indicate that these properties are affected by electron-phonon coupling, the dynamics of the organic cations, and the degree of lattice distortion. We report temperature-dependent Raman studies of BA2MAPb2I7 and BA2MA2Pb3I10 (BA = butylammonium; MA = methylammonium), which undergo two structural phase transitions. Raman data obtained in broad temperature (360-80 K) and wavenumber (1800-10 cm-1) ranges show that ordering of BA+ cations triggers the higher temperature phase transition, whereas freezing of MA+ dynamics occurs below 200 K, leading to the onset of the low-temperature phase transition. This ordering is associated with significant deformation of the inorganic sublattice, as evidenced by changes observed in the lattice mode region. Our results show, therefore, that Raman spectroscopy is a very valuable tool for monitoring the separate dynamics of different organic cations in perovskites, comprising "perovskitizer" and interlayer cations.

Orientation of Thiocyanate Ions Tuning the Electron-Phonon Interactions in Pseudohalide Perovskites

Although the importance of electron-phonon interactions on the optoelectronic properties of perovskites has been well documented, the structural origin of electron-phonon interactions remains largely unexplored. In this study, using pseudohalide perovskites Cs2Pb(SCN)2I2(1-x)Br2x as a model, we have revealed how the orientation of SCN- anions tunes the electron-phonon interactions and the effective charge-carrier mobility by utilizing femtosecond sum frequency generation vibrational spectroscopy, supplemented by photoluminescence spectroscopy and femtosecond optical-pump terahertz-probe spectroscopy. The coupling between neighboring SCN- anions decreases as the Br content (x) increases but does not have a significant effect on the electron-phonon interactions. In contrast, the orientation angle of SCN- anions has a strong correlation with the electron-phonon interaction and effective charge-carrier mobility, that is, a more parallel orientation of SCN- anions leads to a higher electron-phonon interaction and lower effective charge-carrier mobility. This finding provides a molecule-level understanding of the inorganic lattice structure in tuning electron-phonon interactions and may offer valuable guidance for optimizing the optoelectronic properties of perovskites.

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.

Charge carrier dynamics and transient spectral evolutions in lead halide perovskites

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

Insights to Carrier-Phonon Interactions in Lead Halide Perovskites via Multi-Pulse Manipulation

A fundamental understanding of the hot-carrier dynamics in halide perovskites is crucial for unlocking their prospects for next generation photovoltaics. Presently, a coherent picture of the hot carrier cooling process remains patchy due to temporally overlapping contributions from many-body interactions, multi-bands, band gap renormalization, Burstein-Moss shift etc. Pump-push-probe (PPP) spectroscopy recently emerges as a powerful tool complementing the ubiquitous pump-probe (PP) spectroscopy in the study of hot-carrier dynamics. However, limited information from PPP on the initial excitation density and carrier temperature curtails its full potential. Herein, this work bridges this gap in PPP with a unified model that retrieves these essential hot carrier metrics like initial carrier density and carrier temperature under the push conditions, thus permitting direct comparison with traditional PP spectroscopy. These results are well-fitted by the phonon bottleneck model, from which the longitudinal optical phonon scattering time τLO , for MAPbBr3 and MAPbI3 halide perovskite thin film samples are determined to be 240 ± 10 and 370 ± 10 fs, respectively.

Unveiling Charge-Transfer Dynamics at Singlet Fission Layer/Hybrid Perovskite Interface

Singlet fission (SF) materials have been applied in various types of solar cells to pursue higher power conversion efficiency (PCE) beyond the Shockley-Queisser (SQ) limit. SF implementation in perovskite solar cells has not been successfully realized yet due to the insufficient understanding of the SF/perovskite heterojunctions. In this work, we attempt to elucidate the charge dynamics of an SF/perovskite system by incorporating a well-known SF molecule, TIPS-pentacene, and a triple-cation perovskite Cs0.05(FA0.85MA0.15)0.95PbI2.55Br0.45, owing to their well-matched energy structures. The transient absorption spectra and kinetic fitting plots suggest an electron-transfer process from the triplet state of TIPS-pentacene to perovskite in the picosecond regime, which increases the carrier density by 20% in the perovskite layer. This work confirms the existence of an electron-transfer process between the SF material and perovskite, providing a pathway to SF-enhanced perovskite solar cells.

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.

Classification of perovskite structural types with dynamical octahedral tilting

Perovskites ABX₃ with delocalized positions of the X atoms represent a distinct class of dynamically distorted structures with peculiar structural relations and physical properties. The delocalization originates from atoms crossing shallow barriers of the potential energy surface. Quantum mechanically, they can be treated similar to light atoms in diffusive states. Many of these perovskite structures are widely used functional materials thanks to their particular physical properties, such as superconductivity, ferroelectricity and photo-activity. A number of these properties are related to static or dynamic motion of octahedral units. Yet, a full understanding of the relationships between perovskite crystal structure, chemical bonding and physical properties is currently missing. Several studies indicate the existence of dynamic disorder generated by anharmonic motion of octahedral units, e.g. in halide perovskite structures. To simplify structural analysis of such systems we derive a set of space groups for simple perovskites ABX₃ with dynamical octahedral tilting. The derived space groups extend the well established space group tables for static tiltings by Glazer [Acta Cryst. B (1972). 28, 3384-3392], Aleksandrov [Ferroelectrics (1976). 24, 801-805] and Howard & Stokes [Acta Cryst. B (1998). 54, 782-789]. Ubiquity of dynamical tilting is demonstrated by an analysis of the structural data for perovskites reported in recent scientific publications and the signature of dynamic tilting in the corresponding structures is discussed, which can be summarized as follows: (a) volume increase upon a lowering of temperature, (b) apparent distortion of octahedra (where Jahn-Teller distortions can be ruled out), (c) mismatch between observed instantaneous symmetry and average symmetry, (d) deviation of the experimental space group from the theoretically predicted structures for static tilting, (e) inconsistency of lattice parameters with those suggested by the theory of static tilts, and (f) large displacement parameters for atoms at the X and B sites. Finally, the possible influence of dynamic disorder on the physical properties of halide perovskites is discussed.

Cesium-mediated electron redistribution and electron-electron interaction in high-pressure metallic CsPbI3

Electron-phonon coupling was believed to govern the carrier transport in halide perovskites and related phases. Here we demonstrate that electron-electron interaction enhanced by Cs-involved electron redistribution plays a direct and prominent role in the low-temperature electrical transport of compressed CsPbI3 and renders Fermi liquid (FL)-like behavior. By compressing δ-CsPbI3 to 80 GPa, an insulator-semimetal-metal transition occurs, concomitant with the completion of a slow structural transition from the one-dimensional Pnma (δ) phase to a three-dimensional Pmn21 (ε) phase. Deviation from FL behavior is observed upon CsPbI3 entering the metallic ε phase, which progressively evolves into a FL-like state at 186 GPa. First-principles density functional theory calculations reveal that the enhanced electron-electron coupling results from the sudden increase of the 5d state occupation in Cs and I atoms. Our study presents a promising strategy of cationic manipulation for tuning the electronic structure and carrier scattering of halide perovskites at high pressure.

High-Frequency Sheet Conductance of Nanolayered WS2 Crystals for Two-Dimensional Nanodevices

Time-resolved terahertz (THz) spectroscopy is a powerful technique for the determination of charge transport properties in photoexcited semiconductors. However, the relatively long wavelengths of THz radiation and the diffraction limit imposed by optical imaging systems reduce the applicability of THz spectroscopy to large samples with dimensions in the millimeter to centimeter range. Exploiting THz near-field spectroscopy, we present the first time-resolved THz measurements on a single exfoliated 2D nanolayered crystal of a transition metal dichalcogenide (WS₂). The high spatial resolution of THz near-field spectroscopy enables mapping of the sheet conductance for an increasing number of atomic layers. The single-crystalline structure of the nanolayered crystal allows for the direct observation of low-energy phonon modes, which are present in all thicknesses, coupling with free carriers. Density functional theory calculations show that the phonon mode corresponds to the breathing mode between atomic layers in the weakly bonded van der Waals layers, which can be strongly influenced by substrate-induced strain. The non-invasive and high-resolution mapping technique of carrier dynamics in nanolayered crystals by time-resolved THz time domain spectroscopy enables possibilities for the investigation of the relation between phonons and charge transport in nanoscale semiconductors for applications in two-dimensional nanodevices.

Charge Carriers Trapping by the Full-Configuration Defects in Metal Halide Perovskites Quantum Dots

Metal halide perovskites quantum dots (MHPQDs) have aroused enormous interest in the photovoltaic and photoelectric disciplines because of their marvelous properties and size characteristics. However, one of the key problems of how to systematically analyze charge carriers trapped by defects is still a challenging task. Here, we study multiphonon processes of the charge carrier trapping by various defects in MHPQDs based on the well-known Huang-Rhys model, in which a method of a full-configuration defect, including different defect species with variable depth and lattice relaxation strength, is developed by introducing a localization parameter in the quantum defect model. With the help of this method, these fast trapping channels for charge carriers transferring from the quantum dot ground state to different defects are found. Furthermore, the dependence of the trapping time on the radius of quantum dot, the defect depth, and temperature is given. These results not only enrich the knowledge of charge carrier trapping processes by defects, but also bring light to the designs of MHPQDs-based photovoltaic and photoelectric devices.

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.

Phase segregation in mixed-halide perovskites affects charge-carrier dynamics while preserving mobility

Mixed halide perovskites can provide optimal bandgaps for tandem solar cells which are key to improved cost-efficiencies, but can still suffer from detrimental illumination-induced phase segregation. Here we employ optical-pump terahertz-probe spectroscopy to investigate the impact of halide segregation on the charge-carrier dynamics and transport properties of mixed halide perovskite films. We reveal that, surprisingly, halide segregation results in negligible impact to the THz charge-carrier mobilities, and that charge carriers within the I-rich phase are not strongly localised. We further demonstrate enhanced lattice anharmonicity in the segregated I-rich domains, which is likely to support ionic migration. These phonon anharmonicity effects also serve as evidence of a remarkably fast, picosecond charge funnelling into the narrow-bandgap I-rich domains. Our analysis demonstrates how minimal structural transformations during phase segregation have a dramatic effect on the charge-carrier dynamics as a result of charge funnelling. We suggest that because such enhanced recombination is radiative, performance losses may be mitigated by deployment of careful light management strategies in solar cells.

Phase segregation in mixed halide perovskite is known to alter the optoelectronic properties, but how it affects charge carriers is not clear. Here, the authors use THz spectroscopy to reveal that high carrier mobilities are well preserved, while recombination dynamics is affected by charge funnelling upon segregation.

Energy Resonance Transfer between Quantum Defects in Metal Halide Perovskites

Quantum defects have been shown to play an essential role in nonradiative recombination in metal halide perovskites (MHPs). Nonetheless, the processes of charge transfer assisted by defects are still ambiguous. Herein, we theoretically study the nonradiative multiphonon processes among different types of quantum defects in MHPs using Markvart's model for the induced mechanisms of electron-electron and electron-phonon interactions. We find that the charge carrier can transfer between the neighboring levels of the same type of shallow defects by multiphonon processes, but it will be distinctly suppressed with an increase in the defect depth. For the nonradiation multiphonon transitions between donor- and acceptor-like defects, the processes are very fast and not sensitive to the defect depth, which provides a possible explanation for the phenomenon of blinking of photoluminescence spectra. We also discuss the temperature dependence of these multiphonon processes and find that their variational trends depend on the comparison of the Huang-Rhys factor with the emitted phonon number. These theoretical results not only fill some of the gaps in defect-assisted nonradiative processes in the perovskite materials but also provide deeper physical insights into producing higher-performance perovskite-based devices.

The necessity of periodic boundary conditions for the accurate calculation of crystalline terahertz spectra

Terahertz vibrational spectroscopy has emerged as a powerful spectroscopic technique, providing valuable information regarding long-range interactions - and associated collective dynamics - occurring in solids. However, the terahertz sciences are relatively nascent, and there have been significant advances over the last several decades that have profoundly influenced the interpretation and assignment of experimental terahertz spectra. Specifically, because there do not exist any functional group or material-specific terahertz transitions, it is not possible to interpret experimental spectra without additional analysis, specifically, computational simulations. Over the years simulations utilizing periodic boundary conditions have proven to be most successful for reproducing experimental terahertz dynamics, due to the ability of the calculations to accurately take long-range forces into account. On the other hand, there are numerous reports in the literature that utilize gas phase cluster geometries, to varying levels of apparent success. This perspective will provide a concise introduction into the terahertz sciences, specifically terahertz spectroscopy, followed by an evaluation of gas phase and periodic simulations for the assignment of crystalline terahertz spectra, highlighting potential pitfalls and good practice for future endeavors.

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.

Ultrafast Photoconductivity and Terahertz Vibrational Dynamics in Double-Helix SnIP Nanowires

Tin iodide phosphide (SnIP), an inorganic double-helix material, is a quasi-1D van der Waals semiconductor that shows promise in photocatalysis and flexible electronics. However, the understanding of the fundamental photophysics and charge transport dynamics of this new material is limited. Here, time-resolved terahertz (THz) spectroscopy is used to probe the transient photoconductivity of SnIP nanowire films and measure the carrier mobility. With insight into the highly anisotropic electronic structure from quantum chemical calculations, an electron mobility as high as 280 cm²   V^-1 s^-1 along the double-helix axis and a hole mobility of 238 cm²   V^-1 s^-1 perpendicular to the double-helix axis are detected. Additionally, infrared-active (IR-active) THz vibrational modes are measured, which shows excellent agreement with first-principles calculations, and an ultrafast photoexcitation-induced charge redistribution is observed that reduces the amplitude of a twisting mode of the outer SnI helix on picosecond timescales. Finally, it is shown that the carrier lifetime and mobility are limited by a trap density greater than 10¹⁸  cm^-3 . The results provide insight into the optical excitation and relaxation pathways of SnIP and demonstrate a remarkably high carrier mobility for such a soft and flexible material, suggesting that it could be ideally suited for flexible electronics applications.

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.

Efficient phonon cascades in WSe2 monolayers

Energy relaxation of photo-excited charge carriers is of significant fundamental interest and crucial for the performance of monolayer transition metal dichalcogenides in optoelectronics. The primary stages of carrier relaxation affect a plethora of subsequent physical mechanisms. Here we measure light scattering and emission in tungsten diselenide monolayers close to the laser excitation energy (down to ~0.6 meV). We reveal a series of periodic maxima in the hot photoluminescence intensity, stemming from energy states higher than the A-exciton state. We find a period ~15 meV for 7 peaks below (Stokes) and 5 peaks above (anti-Stokes) the laser excitation energy, with a strong temperature dependence. These are assigned to phonon cascades, whereby carriers undergo phonon-induced transitions between real states above the free-carrier gap with a probability of radiative recombination at each step. We infer that intermediate states in the conduction band at the Λ-valley of the Brillouin zone participate in the cascade process of tungsten diselenide monolayers. This provides a fundamental understanding of the first stages of carrier-phonon interaction, useful for optoelectronic applications of layered semiconductors.

Observation of Charge Generation via Photoinduced Stark Effect in Mixed-Cation Lead Bromide Perovskite Thin Films

Extensive transient absorption studies on hybrid organic-inorganic lead halide perovskites have elucidated many optical properties important for their device performance. Despite the enormous progress, the derivative shaped photoinduced absorption feature in transient spectra that is above the bandgap has many explanations, including the photoinduced Stark effect, where the bandgap is blue shifted due to a local electric field generated by charges. In this work, we employ broad band transient absorption and two-dimensional electronic spectroscopy (2DES) to examine the early transient events after photoexcitation of [CH(NH₂)₂]_0.83Cs_0.17PbBr₃ (FA_0.83Cs_0.17PbBr₃). 2DES resolves a photomodulation feature at the excitation energy of the exciton, suggesting the presence of a dipole field created by a polaron pair shifting the exciton transition to higher energies. As this polaron pair dissociates over 200 fs, the exciton transition shifts to higher energies over the same time scale, evidenced by the 2DES diagonal energy spectra. Given that the observations are well explained in terms of the Stark effect, our work provides extra grounds to support the Stark effect assignment of the above-gap photoinduced absorption. Furthermore, our study reports on the time scale of charge generation, contributing to the fundamental understanding of mixed-cation lead bromide perovskite photophysics.

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.

Temperature-Dependent Intraband Relaxation of Hybrid Perovskites

Intraband relaxation in polycrystalline films of hybrid perovskites methylammonium lead tribromide and methylammonium lead triiodide are studied by transient absorption spectroscopy from 80 K to >350 K. This temperature range spans the transitions of these materials from the high-temperature cubic phases, intermediate tetragonal phases, and low-temperature orthorhombic phases. The organic cation undergoes a distinct transition from an ordered lattice in the orthorhombic phase to a plastic crystal in cubic and tetragonal phases, which reportedly influences many optoelectronic properties. The much larger exciton binding energy of orthorhombic MAPbI₃ (compared to cubic or tetragonal phases) or MAPbBr₃ substantially changes the transient spectral responses of the materials by reducing the number of free carriers. However, for these measurements at low fluences, both MAPbBr₃ and MAPbI₃ exhibit subpicosecond intraband relaxation over the entire temperature range studied. Intraband relaxation becomes somewhat faster at higher temperatures, but freezing of organic cations are not accompanied by a discontinuity of the intraband relaxation time. These results suggest that configurational freedom of organic cations does not screen carriers from electron-phonon coupling.