Dynamic Disorder Dominates Delocalization, Transport, and Recombination in Halide Perovskites

Correlated nanoimaging of structure and dynamics of cation-polaron coupling in hybrid perovskites

Hybrid organic-inorganic perovskites exhibit high photovoltaic performance and other novel photonic functions. While polaron formation is believed to facilitate efficient carrier transport, the elementary processes of the underlying electron-lattice coupling are yet poorly understood because of the multiscale chemical and structural heterogeneities. Here, we resolve in combined ground- and excited-state spatiospectral ultrafast nanoimaging how structural characteristics are related to both molecular cation and polaron dynamics. We use the observed nanoscale spatial variations of the formamidinium (FA) cation transient vibrational blue shifts as a local probe of the nonlocal polaron-cation coupling. From the correlation with nanomovies of the polaron dynamics, we then infer how a softer more polarizable lattice supports stable polarons and longer-lived residual carriers. This, together with a relative intragrain homogeneity in contrast to high intergrain heterogeneity, suggests pathways for improved synthesis and device engineering, and that perovskite photonics performance is still far from any fundamental limits.

Role of A-Site Cation Hydrogen Bonds in Hybrid Organic-Inorganic Perovskites: A Theoretical Insight

Hybrid organic-inorganic halide perovskites (HOIPs) have garnered a significant amount of attention due to their exceptional photoelectric conversion efficiency. However, they still face considerable challenges in large-scale applications, primarily due to their instability. One key factor influencing this instability is the lattice softness attributed to the A-site cations. In this study, we investigated the effects of four different A-site cations (MA, FA, EA, and GA) on the lattice softness of perovskites by using a combination of ab initio molecular dynamics and first-principles calculations. Our results demonstrate that an increase in the number of hydrogen bonds for A-site cations correlates with enhanced lattice and atomic fluctuations, resulting in a reduction in the bulk modulus and an increase in the lattice softness. The strength of hydrogen bonding of the A-site cation increases the rotational energy barrier of the cation, along with the formation energy and kinetic coupling between the A-site cation and the [PbI6]4- octahedron. Consequently, this increases the lifetime of hydrogen bonding and enhances the rigidity of the perovskite lattice. Notably, we found that EA cations, which exhibit stronger hydrogen bonding with fewer total hydrogen bonds, can limit the rotation of the A-site cation, inhibit the rocking motion of the [PbI6]4- octahedron, and thereby increase the rigidity of the inherently soft perovskite lattice, ultimately enhancing the stability of the material. Our findings elucidate the effect of hydrogen bonding in A-site cations on the lattice softness of perovskites, providing valuable theoretical insights for the design of more stable HOIPs.

Surface-binding molecular multipods strengthen the halide perovskite lattice and boost luminescence

Reducing the size of perovskite crystals to confine excitons and passivating surface defects has fueled a significant advance in the luminescence efficiency of perovskite light-emitting diodes (LEDs). However, the persistent gap between the optical limit of electroluminescence efficiency and the photoluminescence efficiency of colloidal perovskite nanocrystals (PeNCs) suggests that defect passivation alone is not sufficient to achieve highly efficient colloidal PeNC-LEDs. Here, we present a materials approach to controlling the dynamic nature of the perovskite surface. Our experimental and theoretical studies reveal that conjugated molecular multipods (CMMs) adsorb onto the perovskite surface by multipodal hydrogen bonding and van der Waals interactions, strengthening the near-surface perovskite lattice and reducing ionic fluctuations which are related to nonradiative recombination. The CMM treatment strengthens the perovskite lattice and suppresses its dynamic disorder, resulting in a near-unity photoluminescence quantum yield of PeNC films and a high external quantum efficiency (26.1%) of PeNC-LED with pure green emission that matches the Rec.2020 color standard for next-generation vivid displays.

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

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

Compression of Organic Molecules Coupled with Hydrogen Bonding Extends the Charge Carrier Lifetime in BA2SnI4

Two-dimensional (2D) metal halide perovskites, such as BA2SnI4 (BA═CH3(CH2)3NH3), exhibit an enhanced charge carrier lifetime in experiments under strain. Experiments suggest that significant compression of the BA molecule, rather than of the inorganic lattice, contributes to this enhancement. To elucidate the underlying physical mechanism, we apply a moderate compressive strain to the entire system and subsequently introduce significant compression to the BA molecules. We then perform ab initio nonadiabatic molecular dynamics simulations of nonradiative electron-hole recombination. We observe that the overall lattice compression reduces atomic motions and decreases nonadiabatic coupling, thereby delaying electron-hole recombination. Additionally, compression of the BA molecules enhances hydrogen bonding between the BA molecules and iodine atoms, which lengthens the Sn-I bonds, distorts the [SnI6]4- octahedra, and suppresses atomic motions further, thus reducing nonadiabatic coupling. Also, the elongated Sn-I bonds and weakened antibonding interactions increase the band gap. Altogether, the compression delays the nonradiative electron-hole recombination by more than a factor of 3. Our simulations provide new and valuable physical insights into how compressive strain, accommodated primarily by the organic ligands, positively influences the optoelectronic properties of 2D layered halide perovskites, offering a promising pathway for further performance improvements.

Experimental evidence for defect tolerance in Pb-halide perovskites

The term defect tolerance (DT) is used often to rationalize the exceptional optoelectronic properties of halide perovskites (HaPs) and their devices. Even though DT lacked direct experimental evidence, it became a "fact" in the field. DT in semiconductors implies that structural defects do not translate to electrical and optical effects (e.g., due to charge trapping), associated with such defects. We present pioneering direct experimental evidence for DT in Pb-HaPs by comparing the structural quality of 2-dimensional (2D), 2D-3D, and 3D Pb-iodide HaP crystals with their optoelectronic characteristics using high-sensitivity methods. Importantly, we get information from the materials' bulk because we sample at least a few hundred nanometers, up to several micrometers, from the sample's surface, which allows for assessing intrinsic bulk (and not only surface-) properties of HaPs. The results point to DT in 3D, 2D-3D, and 2D Pb-HaPs. Overall, our data provide an experimental basis to rationalize DT in Pb-HaPs. These experiments and findings will help the search for and design of materials with real DT.

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.

Thermal Disorder-Induced Strain and Carrier Localization Activate Reverse Halide Segregation

The reversal of halide ions is studied under various conditions. However, the underlying mechanism of heat-induced reversal remains unclear. This work finds that dynamic disorder-induced localization of self-trapped polarons and thermal disorder-induced strain (TDIS) can be co-acting drivers of reverse segregation. Localization of polarons results in an order of magnitude decrease in excess carrier density (polaron population), causing a reduced impact of the light-induced strain (LIS - responsible for segregation) on the perovskite framework. Meanwhile, exposing the lattice to TDIS exceeding the LIS can eliminate the photoexcitation-induced strain gradient, as thermal fluctuations of the lattice can mask the LIS strain. Under continuous 0.1 W cm⁻2 illumination (upon segregation), the strain disorder is estimated to be 0.14%, while at 80 °C under dark conditions, the strain is 0.23%. However, in situ heating of the segregated film to 80 °C under continuous illumination (upon reversal) increases the total strain disorder to 0.25%, where TDIS is likely to have a dominant contribution. Therefore, the contribution of entropy to the system's free energy is likely to dominate, respectively. Various temperature-dependent in situ measurements and simulations further support the results. These findings highlight the importance of strain homogenization for designing stable perovskites under real-world operating conditions.

Gain enhancement of perovskite nanosheets by a patterned waveguide: excitation and temperature dependence of gain saturation

Optical gain enhancement of two-dimensional CsPbBr3 nanosheets was studied when the amplified spontaneous emission is guided by a patterned structure of polyurethane-acrylate. Given the uncertainties and pitfalls in retrieving a gain coefficient from the variable stripe length method, a gain contour [Formula: see text] was obtained in the plane of spectrum energy (ℏω) and stripe length (x), whereby an average gain was obtained, and gain saturation was analysed. Excitation and temperature dependence of the gain contour show that the waveguide enhances both gain and thermal stability due to the increased optical confinement and heat dissipation, and the gain origins were attributed to the two-dimensional excitons and the localized states.

Influence of X-Ray Irradiation During Photoemission Studies on Halide Perovskite-Based Devices

Metal halide perovskites (MHPs) are semiconductors with promising application in optoelectronic devices, particularly, in solar cell technologies. The chemical and electronic properties of MHPs at the surface and interfaces with adjacent layers dictate charge transfer within stacked devices and ultimately the efficiency of the latter. X-ray photoelectron spectroscopy is a powerful tool to characterize these material properties. However, the X-ray radiation itself can potentially affect the MHP and therefore jeopardize the reliability of the obtained information. In this work, the effect of X-ray irradiation is assessed on Cs0.05 MA0.15 FA0.8 Pb(I0.85 Br0.15 )3  (MA for CH3 NH3 , and FA for CH2 (NH2 )2 ) MHP thin-film samples in a half-cell device. There is a comparison of measurements acquired with synchrotron radiation and a conventional laboratory source for different times. Changes in composition and core levels binding energies are observed in both cases, indicating a modification of the chemical and electronic properties. The results suggest that changes observed over minutes with highly brilliant synchrotron radiation are likely occurring over hours when working with a lab-based source providing a lower photon flux. The possible degradation pathways are discussed, supported by steady-state photoluminescence analysis. The work stresses the importance of beam effect assessment at the beginning of XPS experiments of MHP samples.

Synergy of 3D and 2D Perovskites for Durable, Efficient Solar Cells and Beyond

Three-dimensional (3D) organic-inorganic lead halide perovskites have emerged in the past few years as a promising material for low-cost, high-efficiency optoelectronic devices. Spurred by this recent interest, several subclasses of halide perovskites such as two-dimensional (2D) halide perovskites have begun to play a significant role in advancing the fundamental understanding of the structural, chemical, and physical properties of halide perovskites, which are technologically relevant. While the chemistry of these 2D materials is similar to that of the 3D halide perovskites, their layered structure with a hybrid organic-inorganic interface induces new emergent properties that can significantly or sometimes subtly be important. Synergistic properties can be realized in systems that combine different materials exhibiting different dimensionalities by exploiting their intrinsic compatibility. In many cases, the weaknesses of each material can be alleviated in heteroarchitectures. For example, 3D-2D halide perovskites can demonstrate novel behavior that neither material would be capable of separately. This review describes how the structural differences between 3D halide perovskites and 2D halide perovskites give rise to their disparate materials properties, discusses strategies for realizing mixed-dimensional systems of various architectures through solution-processing techniques, and presents a comprehensive outlook for the use of 3D-2D systems in solar cells. Finally, we investigate applications of 3D-2D systems beyond photovoltaics and offer our perspective on mixed-dimensional perovskite systems as semiconductor materials with unrivaled tunability, efficiency, and technologically relevant durability.

Disorder to order: how halide mixing in MAPbI3-xBrx perovskites restricts MA dynamics

Mixed-halide lead perovskites are of particular interest for the design of tandem solar cells currently reaching record efficiencies. While halide phase segregation upon illumination of mixed perovskites is extensively studied, the effect of halide disorder on A cation dynamics is not well understood, despite its importance for charge carrier diffusion and lifetime. Here, we study the methylammonium (MA) reorientational dynamics in mixed halide MAPbI_3-xBr_x perovskites by a combined approach of experimental solid-state NMR spectroscopy and molecular dynamics (MD) simulations based on machine-learning force-fields (MLFF). ²⁰⁷Pb NMR spectra indicate the halides are randomly distributed over their lattice positions, whereas PXRD measurements show that all mixed MAPbI_3-xBr_x samples are cubic. The experimental ¹⁴N spectra and ¹H double-quantum (DQ) NMR data reveal anisotropic MA reorientations depending on the halide composition and thus associated disorder in the inorganic sublattice. MD calculations allow us to correlate these experimental results to restrictions of MA dynamics due to preferred MA orientations in their local Pb₈I_12-nBr_n "cages". Based on the experimental and simulated results, we develop a phenomenological model that correlates the ¹H dipolar coupling and thus the MA dynamics with the local composition and reproduces the experimental data over the whole composition range. We show that the dominant interaction between the MA cations and the Pb-X lattice that influences the cation dynamics is the local electrostatic potential being inhomogeneous in mixed halide systems. As such, we generate a fundamental understanding of the predominant interaction between the MA cations and the inorganic sublattice, as well as MA dynamics in asymmetric halide coordinations.

Nanoscale heterogeneity of ultrafast many-body carrier dynamics in triple cation perovskites

In high fluence applications of lead halide perovskites for light-emitting diodes and lasers, multi-polaron interactions and associated Auger recombination limit the device performance. However, the relationship of the ultrafast and strongly lattice coupled carrier dynamics to nanoscale heterogeneities has remained elusive. Here, in ultrafast visible-pump infrared-probe nano-imaging of the photoinduced carrier dynamics in triple cation perovskite films, a ~20 % variation in sub-ns relaxation dynamics with spatial disorder on tens to hundreds of nanometer is resolved. We attribute the non-uniform relaxation dynamics to the heterogeneous evolution of polaron delocalization and increasing scattering time. The initial high-density excitation results in faster relaxation due to strong many-body interactions, followed by extended carrier lifetimes at lower densities. These results point towards the missing link between the optoelectronic heterogeneity and associated carrier dynamics to guide synthesis and device engineering for improved perovskites device performance.

The optoelectronic performance of lead halide perovskite in highfluence applications are hindered by heterogeneous multi-polaron interactions in the nanoscale. Here, Nishda et al. spatially resolve sub-ns relaxation dynamics on the nanometer scale by ultrafast infrared pumpprobe nanoimaging.

Structural Disorder in Higher-Temperature Phases Increases Charge Carrier Lifetimes in Metal Halide Perovskites

Solar cells and optoelectronic devices are exposed to heat that degrades performance. Therefore, elucidating temperature-dependent charge carrier dynamics is essential for device optimization. Charge carrier lifetimes decrease with temperature in conventional semiconductors. The opposite, anomalous trend is observed in some experiments performed with MAPbI₃ (MA = CH₃NH₃⁺) and other metal halide perovskites. Using ab initio quantum dynamics simulation, we establish the atomic mechanisms responsible for nonradiative electron-hole recombination in orthorhombic-, tetragonal-, and cubic MAPbI₃. We demonstrate that structural disorder arising from the phase transitions is as important as the disorder due to heating in the same phase. The carrier lifetimes grow both with increasing temperature in the same phase and upon transition to the higher-temperature phases. The increased lifetime is rationalized by structural disorder that induces partial charge localization, decreases nonadiabatic coupling, and shortens quantum coherence. Inelastic and elastic electron-vibrational interactions exhibit opposite dependence on temperature and phase. The partial disorder and localization arise from thermal motions of both the inorganic lattice and the organic cations and depend significantly on the phase. The structural deformations induced by thermal fluctuations and phase transitions are on the same order as deformations induced by defects, and hence, thermal disorder plays a very important role. Since charge localization increases carrier lifetimes but inhibits transport, an optimal regime maximizing carrier diffusion can be designed, depending on phase, temperature, material morphology, and device architecture. The atomistic mechanisms responsible for the enhanced carrier lifetimes at elevated temperatures provide guidelines for the design of improved solar energy and optoelectronic materials.

Suppression of Phase Transitions in Perovskite Thin Films through Cryogenic Electron Beam Irradiation

Organic-inorganic hybrid perovskites (OIHPs) with superior optoelectronic properties have emerged as revolutionary semiconductor materials for diverse applications. A fundamental understanding of the interplay between the microscopic molecular-level structure and the macroscopic optoelectronic properties is essential to boost device performance toward theoretical limits. Here, we reveal the critical role of CH₃NH₃⁺ (MA) in the regulation of the physicochemical and optoelectronic properties of a MAPbI₃ film irradiated by an electron beam at 130 K. The order-to-disorder transformation of the MA cation not only leads to a notably enhanced photoluminescence emission but also results in the suppression of the orthorhombic phase down to 85 K. Taking advantage of the regulation of MA cation dynamics, we demonstrate a perovskite photodetector with 100% photocurrent enhancement and long-term stability exceeding one month. Our study provides a powerful tool for regulating the optoelectronic properties and stabilities of perovskites and highlights potential opportunities related to the organic cation in OIHPs.

Dangling Octahedra Enable Edge States in 2D Lead Halide Perovskites

The structural reconstruction at the crystal layer edges of 2D lead halide perovskites (LHPs) leads to unique edge states (ES), which are manifested by prolonged carrier lifetime and reduced emission energy. These special ES can effectively enhance the optoelectronic performance of devices, but their intrinsic origin and working mechanism remain elusive. Here it is demonstrated that the ES of a family of 2D Ruddlesden-Popper LHPs [BA₂ CsPb₂ Br₇ , BA₂ MAPb₂ Br₇ , and BA₂ MA₂ Pb₃ Br₁₀ (BA = butylammonium; MA = methylammonium)] arise from the rotational symmetry elevation of the PbBr₆ octahedra dangling at the crystal layer edges. These dangling octahedra give rise to localized electronic states that enable an effective transport of electrons from the interior to layer edges, and the population of electrons in both the interior states and the ES can be manipulated via controlling the external fields. Moreover, the abundant phonons, activated by the dangling octahedra, can interact with electrons to facilitate radiative recombination, counterintuitive to the suppressive role commonly observed in conventional semiconductors. This work unveils the intrinsic atomistic and electronic origins of ES in 2D LHPs, which can stimulate the exploration of ES-based exotic optoelectronic properties and the corresponding design of high-performance devices for these emergent low-dimensional semiconductors.

Ultrafast transient infrared spectroscopy for probing trapping states in hybrid perovskite films

Studying the charge dynamics of perovskite materials is a crucial step to understand the outstanding performance of these materials in various fields. Herein, we utilize transient absorption in the mid-infrared region, where solely electron signatures in the conduction bands are monitored without external contributions from other dynamical species. Within the measured range of 4000 nm to 6000 nm (2500-1666 cm-1), the recombination and the trapping processes of the excited carriers could be easily monitored. Moreover, we reveal that within this spectral region the trapping process could be distinguished from recombination process, in which the iodide-based films show more tendencies to trap the excited electrons in comparison to the bromide-based derivatives. The trapping process was assigned due to the emission released in the mid-infrared region, while the traditional band-gap recombination process did not show such process. Various parameters have been tested such as film composition, excitation dependence and the probing wavelength. This study opens new frontiers for the transient mid-infrared absorption to assign the trapping process in perovskite films both qualitatively and quantitatively, along with the potential applications of perovskite films in the mid-IR region.

Toward Continuous-Wave Pumped Metal Halide Perovskite Lasers: Strategies and Challenges

Reliable and efficient continuous-wave (CW) lasers have been intensively pursued in the field of optoelectronic integrated circuits. Metal perovskites have emerged as promising gain materials for solution-processed laser diodes. Recently, the performance of CW perovskite lasers has been improved with the optimization of material and device levels. Nevertheless, the realization of CW pumped perovskite lasers is still hampered by thermal runaway, unwanted parasitic species, and poor long-term stability. This review starts with the charge carrier recombination dynamics and fundamentals of CW lasing in perovskites. We examine the potential strategies that can be used to improve the performance of perovskite CW lasers from the materials to device levels. We also propose the open challenges and future opportunities in developing high-performance and stable CW pumped perovskite lasers.

Transversal Halide Motion Intensifies Band-To-Band Transitions in Halide Perovskites

Despite their puzzling vibrational characteristics that include strong signatures of anharmonicity and thermal disorder already around room temperature, halide perovskites (HaPs) exhibit favorable optoelectronic properties for applications in photovoltaics and beyond. Whether these vibrational properties are advantageous or detrimental to their optoelectronic properties remains, however, an important open question. Here, this issue is addressed by investigation of the finite-temperature optoelectronic properties in the prototypical cubic CsPbBr3 , using first-principles molecular dynamics based on density-functional theory. It is shown that the dynamic flexibility associated with HaPs enables the so-called transversality, which manifests as a preference for large halide displacements perpendicular to the Pb-Br-Pb bonding axis. The authors find that transversality is concurrent with vibrational anharmonicity and leads to a rapid rise in the joint density of states, which is favorable for photovoltaics since this implies sharp optical absorption profiles. These findings are contrasted to the case of PbTe, a material that shares several key properties with CsPbBr3 but cannot exhibit any transversality and, hence, is found to exhibit much wider band-edge distributions. The authors conclude that the dynamic structural flexibility in HaPs and their unusual vibrational characteristics might not just be a mere coincidence, but play active roles in establishing their favorable optoelectronic properties.

Ultrafast infrared nano-imaging of far-from-equilibrium carrier and vibrational dynamics

Ultrafast infrared nano-imaging has demonstrated access to ultrafast carrier dynamics on the nanoscale in semiconductor, correlated-electron, or polaritonic materials. However, mostly limited to short-lived transient states, the contrast obtained has remained insufficient to probe important long-lived excitations, which arise from many-body interactions induced by strong perturbation among carriers, lattice phonons, or molecular vibrations. Here, we demonstrate ultrafast infrared nano-imaging based on excitation modulation and sideband detection to characterize electron and vibration dynamics with nano- to micro-second lifetimes. As an exemplary application to quantum materials, in phase-resolved ultrafast nano-imaging of the photoinduced insulator-to-metal transition in vanadium dioxide, a distinct transient nano-domain behavior is quantified. In another application to lead halide perovskites, transient vibrational nano-FTIR spatially resolves the excited-state polaron-cation coupling underlying the photovoltaic response. These examples show how heterodyne pump-probe nano-spectroscopy with low-repetition excitation extends ultrafast infrared nano-imaging to probe elementary processes in quantum and molecular materials in space and time.

Photo-induced enhancement of lattice fluctuations in metal-halide perovskites

The optoelectronic properties of metal-halide perovskites (MHPs) are affected by lattice fluctuations. Using ultrafast pump-probe spectroscopy, we demonstrate that in state-of-the-art mixed-cation MHPs ultrafast photo-induced bandgap narrowing occurs with a linear to super-linear dependence on the excited carrier density ranging from 10¹⁷ cm^-3 to above 10¹⁸ cm^-3. Time-domain terahertz spectroscopy reveals carrier localization increases with carrier density. Both observations, the anomalous dependence of the bandgap narrowing and the increased carrier localization can be rationalized by photo-induced lattice fluctuations. The magnitude of the photo-induced lattice fluctuations depends on the intrinsic instability of the MHP lattice. Our findings provide insight into ultrafast processes in MHPs following photoexcitation and thus help to develop a concise picture of the ultrafast photophysics of this important class of emerging semiconductors.

Polaronic Signatures in Doped and Undoped Cesium Lead Halide Perovskite Nanocrystals through a Photoinduced Raman Mode

Lead halide perovskites (LHPs) are promising candidates for photovoltaic applications as they exhibit large carrier diffusion lengths and long carrier lifetimes among many other interesting properties. One of the widely accepted mechanisms for these properties is polaron formation, which is mainly driven by octahedral distortions of the inorganic framework. Since structure modifications of the framework largely affect associated distortions, we investigated Mn-doped and undoped CsPbX₃ (where X = Cl, Br, Cl/Br) using a local probe via micro-Raman spectroscopy and density functional theory (DFT) calculations for polaron formation. Our results highlight a new vibrational lattice mode at 132 cm^-1 due to polaronic distortion upon photoinduction. From the DFT studies, we have shown that the polaronic states are dominated by the B-site cation in the perovskite structure, but it is the strong covalent overlap of the halide which determines its stability. This elucidation to map polaronic signatures with excellent spatial resolution using traditional Raman spectroscopy can be used as a simple tool to understand the structural changes and their impacted electronic properties and thus design superior devices using its in situ applications.

Impact of the polar optical phonon and alloy scattering on the charge-carrier mobilities of FA0.83Cs0.17Pb(I1-xBrx)3 hybrid perovskites

Lead mixed-halide perovskites are promising absorption materials that are suitable for applications in tandem solar cells using existing silicon technology. Charge-carrier mobility is an important factor that affects the performance of tandem solar cells. However, a detailed understanding of the fundamental mechanisms of lead mixed-halide perovskites remains elusive. Here, we used LO (longitudinal optical) phonons and alloy scattering to the elucidate charge-carrier mobilities in the FA_0.83Cs_0.17Pb(I_1-xBr_x)₃ hybrid perovskite system. It was found that these scattering mechanisms provided very good quantitative agreement with the experimental results, between 11-40 cm² V^-1 s^-1. Our findings provide new insights into charge transport scattering in lead mixed-halide hybrid perovskites and pave the way toward design of novel semiconductor alloys for solar cell applications.

Traversing Excitonic and Ionic Landscapes: Reduced-Dimensionality-Inspired Design of Organometal Halide Semiconductors for Energy Applications

At the very heart of the global semiconductor industry lies the omnipresent push for new materials discovery. New materials constantly rise and fall out of fashion in the scientific literature, with those passing an initial phase of research scrutiny becoming hotbeds of characterization and optimization efforts. Yet, innumerable hours of painstaking research have been devoted to materials that have ultimately fallen by the wayside after crossing over an indefinable threshold, whereupon historical optimism is met with newfound skepticism. Materials have to perform well, and they have to do it quickly. In the past decade, metal-halide perovskites (MHPs) have garnered widespread attention. The hegemonic view in both academic and industrial circles is that these materials could be engineered to meet the demands of the semiconductor industry. Their promise as inexpensive solar cell devices is highly attractive, and it has been nothing short of remarkable that efficiencies have risen from 3.8% in 2009 to more than 25.5% in 2021. Moreover, MHPs are poised to be revolutionary materials in more ways than one. The highest MHP LED efficiency was recently reported (23.4%), and MHPs have demonstrated promise in photodetectors, memristors, and transistors. However, the many excellent properties of MHPs are contrasted by longstanding stability and reproducibility limitations that have hindered their commercialization. Overcoming the limitations of MHPs is ultimately a materials engineering problem, which should be solved by mapping more precise relationships between structure, composition, and device performance. In 1958, Francis Crick famously developed the central dogma of molecular biology which describes the unidirectional flow of information in biological systems. In the words of Crick, "nature has devised a unique instrument in which an underlying simplicity is used to express great subtlety and versatility." In this Account, taking inspiration from the hierarchical organization of nature, we describe a hierarchical approach to materials engineering of organic metal-halide semiconductors. We demonstrate that organo-metal halide semiconductors' dimensionality, composition, and morphology dictate their optoelectronic properties and can be exploited in defining more explicit relationships between structure and function. Here, we traverse three-dimensional (3D), two-dimensional (2D), and one-dimensional (1D) organo-metal halide semiconductors, detailing the morphological and compositional differences in each and the implications that can be drawn within each domain on the engineering process. Control over ion migration pathways via morphology engineering as well as control over charge formation in organic-inorganic semiconductors is demonstrated. Fundamental insights into the amount of static and dynamic disorder in the MHP lattice are provided, which can be continuously tuned as a function of composition and morphology. Using electroabsorption spectroscopy on 2D MHPs, a disorder-induced dipole moment in the exciton proportional to the summed value of static and dynamic disorder is measured. Spectroscopic isolation of exciton features in 2D MHP electroabsorption spectra allows us to obtain precise, model-independent measurements of exciton binding energies to study the effect of chemical substitutions, such as Sn²⁺ → Pb²⁺, on the value of the exciton binding energy. Finally, we conclude that this multidimensional platform, with the aid of machine learning and robotics, will be foundational in accurately predicting structure-property-device relationships in organo-metal halide semiconductors in the future.

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.

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

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

Bimolecular Generation of Excitonic Luminescence from Dark Photoexcitations in Ruddlesden-Popper Hybrid Metal-Halide Perovskites

The nature of photoexcitations in Ruddlesden-Popper (RP) hybrid metal halide perovskites is still under debate. While the high exciton binding energy in the hundreds of millielectronvolts indicates excitons as the primary photoexcitations, recent reports found evidence for dark, Coulombically screened populations, which form via strong coupling of excitons and the atomic lattice. Here, we use time-resolved mid-infrared spectroscopy to gain insights into the nature and recombination of such dark excited states in (BA)₂(MA)_n-1Pb_nI_3n+1 (n = 1,2,3) via their intraband electronic absorption. In stark contrast to results in the bulk perovskites, all samples exhibit a broad, unstructured mid-IR photoinduced absorbance with no infrared activated modes, independent of excitonic confinement. Further, the recombination dynamics are dominated by a bimolecular process. In combination with steady-state photoluminescence experiments, we conclude that screened, dark photoexcitations act as a population reservoir in the RP hybrid perovskites, from which nongeminate formation of bright excitons precedes generation of photoluminescence.

Mixing of Azetidinium in Formamidinium Tin Triiodide Perovskite Solar Cells for Enhanced Photovoltaic Performance and High Stability in Air

Overcoming the issue of the stability of tin-based perovskites is a major challenge for the commercial development of lead-free perovskite solar cells. To attack this problem, a new organic cation, azetidinium (AZ), is incorporated into the crystal structure of formamidinium tin triiodide (FASnI₃ ) to form the mixed-cation perovskite AZ_x FA_1-x SnI₃ . As AZ has a similar size to FA but a larger dipole moment, hybrid AZ_x FA_1-x SnI₃ films exhibit variation in optical and electronic properties on increasing the proportion of AZ. Trifluoromethylbenzene (CF₃ C₆ H₅ ) serves as antisolvent to fabricate smooth and uniform perovskite films for the devices with an inverted planar heterojunction structure. The device performance is optimized to produce the greatest efficiency at x=0.15 (AZ15), for which a power conversion efficiency of 9.6 % is obtained when the unencapsulated AZ15 device is stored in air for 100 h. Moreover, the device retains 90 % of its initial efficiency for over 15 days. The significant performance and stability of this device reveal that the concept of mixed cations is a promising approach to stabilize tin-based perovskite solar cells for future commercialization.

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.

DC bias electric field effects on ac electrical conductivity of MAPbI3suggesting intrinsic changes on structure and charge carrier dynamics

Methylammonium lead iodide (MAPbI₃) emerges as a promising halide perovskite material for the next generation of solar cells due to its high efficiency and flexibility in material growth. Despite intensive studies of their optical and electronic properties in the past ten years, there are no reports on dc bias electric field effects on conductivity in a wide temperature range. In this work, we report the combined effects of frequency, temperature, and dc bias electric field on the ac conductivity of MAPbI₃. We found that the results of dc bias electric fields are very contrasting in the tetragonal and cubic phases. In the tetragonal phase, sufficiently high dc bias electric fields induce a conductivity peak appearance ∼290 K well evidenced at frequencies higher than 100 kHz. Excluding possible degradation and extrinsic factors, we propose that this peak suggests a ferroelectric-like transition. In the absence of a dc bias electric field, the ac conductivity in the tetragonal phase increases with temperature while decreases with temperature in the cubic phase. Also, ac activation energies for tetragonal and cubic phases were found to be inversely and directly proportional to the dc bias electric field, respectively. This behavior was attributed to the ionic conduction, possibly of MA⁺and I^-ions, for the tetragonal phase. As for the cubic phase, the ac conduction dynamics appear to be metallic-like, which seems to change to a polaronic-controlled charge transport to increased dc bias electric fields.

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

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

Significant Enhancement of Thermoelectric Properties of PTB7 Conducting Polymer by Mixed-Solvent Approach

The thermoelectric properties of poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}), commonly known as PTB7 conducting polymer, were investigated for the first time by Rastegaralam et al. in 2017 [ Crystals 2017, 7, 292]. PTB7 showed higher electrical conductivity or Seebeck coefficient (or even both) and, hence, a higher thermoelectric power factor than a variety of organic semiconductors. Therefore, it is worth working more on this semiconductor to improve its thermoelectric figure of merit. In this work, for the first time the effect of cosolvents on the thermoelectric properties of PTB7 is investigated. PTB7 conducting polymer dissolved in chlorobenzene was treated with different solvents: N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), acetonitrile (AC), and 1,2-dichloroethane (DCE). Upon addition of DMF, DMSO, and NMP, a significant enhancement in the electrical conductivity of the samples accompanied by a reduction in the Seebeck coefficient occurred as a result of doping, while the use of AC and DCE led to simultaneous enhancement in the electrical conductivity and the Seebeck coefficient by increasing the mobility. The dopants used in this work are inexpensive, are easily available, and do not need to perform any synthesis process. The highest estimated figure of merit value obtained in this work without optimization is 0.1, which is of the highest values for organic thermoelectric materials.

Influence of Ligand Structure on Excited State Surface Chemistry of Lead Sulfide Quantum Dots

The ligand-nanocrystal boundaries of colloidal quantum dots (QDs) mediate the primary energy and electron transfer processes that underpin photochemical and photocatalytic transformations at their surfaces. We use mid-infrared transient absorption spectroscopy to reveal the influence that ligand structure and bonding to nanocrystal surfaces have on the changes of the excited state surface chemistry of this boundary in PbS QDs and the corresponding impact on charge transfer processes between nanocrystals. We demonstrate that oleate ligands undergo marked changes in their bonding to surfaces in the excitonic excited states of the nanocrystals, indicating that oleate passivated PbS surfaces undergo significant structural changes following photoexcitation. These changes can impact the surface mobility of the ligands and the ability of redox shuttles to approach the nanocrystal surfaces to undergo charge transfer in photocatalytic reactions. In contrast, markedly different transient vibrational features are observed in iodide/mercaptoproprionic acid passivated PbS QD films that result from charge transfer between neighboring nanocrystals and localization of holes at the nanocrystal surfaces near MPA ligands. This ability to distinguish the influence that excitonic excited states vs charge transfer processes have on the surface chemistry of the ligand-nanocrystal boundary lays the groundwork for exploration of how this boundary can be understood and controlled for the design of nanocrystalline materials tailored for specific applications in solar energy harvesting and photocatalytic reactions.

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

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

Picosecond Charge Localization Dynamics in CH3NH3PbI3 Perovskite Probed by Infrared-Activated Vibrations

Hybrid metal halide perovskites exhibit well-defined semiconducting properties and efficient optoelectronic performance considering their soft crystal structure and low-energy lattice motions. The response of such a crystal lattice to light-induced charges is a fundamental question, for which experimental insight into ultrafast time scales is still sought. Here, we use infrared-activated vibrations (IRAV) of the organic components within the hybrid perovskite lattice as a sensitive probe for local structural reorganizations after photoexcitation, with femtosecond resolution. We find that the IRAV signal response shows a delayed rise of about 3 ps and subsequent decay of pronounced monomolecular character, distinguishing it from absorption associated with free carriers. We interpret our results as a two-step carrier localization process. Initially, carriers localize transiently in local energy minima formed by lattice fluctuations. A subpopulation of these can then fall into deeper trapped states over picoseconds, likely due to local reorganization of the organic molecules surrounding the carriers.

Dielectric polarization effect and transient relaxation in FAPbBr3 films before and after PMMA passivation

In organic-inorganic hybrid ionic lead halide perovskites with a naturally arranged layered structure, the dielectric polarization effect caused by the dielectric mismatch between the organic and inorganic layers takes effect in their optical responses. But this effect has received little attention. Here we used infrared transient spectroscopy to study FAPbBr3 perovskite polycrystalline films before and after PMMA film passivation and found that there is a dielectric polarization effect at the interface between the organic cation layer and the inorganic lattice layer inside the perovskite lattice, and also at the interface between the PMMA film and perovskite film. Due to the dielectric polarization effect and the spatial confinement of the surface electronic (or polaron) state, the luminescence intensity of the passivated perovskite film is significantly enhanced, and the exciton lifetime is greatly increased. Dielectric polarization enhances their efficient transient absorption (TA) and leads to the intramolecular vibration frequency red-shifts, which exhibited the combined relaxation kinetics of the large polaron with dielectric polarization in the perovskite film. Dielectric polarization between the internal lattice and the nanocrystal surface of the perovskite film shows different relaxation processes. The polarization-dependent TA spectrum reveals that the dielectric polarization field causes light-induced anisotropy by changing the chemical bond configurations. These direct TA experimental observations help us to understand the influence of the dielectric polarization effect on the electronic state in various organic-inorganic nanocomposite perovskites.

Materials, photophysics and device engineering of perovskite light-emitting diodes

Here we provide a comprehensive review of a newly developed lighting technology based on metal halide perovskites (i.e. perovskite light-emitting diodes) encompassing the research endeavours into materials, photophysics and device engineering. At the outset we survey the basic perovskite structures and their various dimensions (namely three-, two- and zero-dimensional perovskites), and demonstrate how the compositional engineering of these structures affects the perovskite light-emitting properties. Next, we turn to the physics underpinning photo- and electroluminescence in these materials through their connection to the fundamental excited states, energy/charge transport processes and radiative and non-radiative decay mechanisms. In the remainder of the review, we focus on the engineering of perovskite light-emitting diodes, including the history of their development as well as an extensive analysis of contemporary strategies for boosting device performance. Key concepts include balancing the electron/hole injection, suppression of parasitic carrier losses, improvement of the photoluminescence quantum yield and enhancement of the light extraction. Overall, this review reflects the current paradigm for perovskite lighting, and is intended to serve as a foundation to materials and device scientists newly working in this field.

Photoinduced Dynamics of Charge Carriers in Metal Halide Perovskites from an Atomistic Perspective

Perovskite solar cells have attracted intense attention over the past decade because of their low cost, abundant raw materials, and rapidly growing power conversion efficiency (PCE). However, nonradiative charge carrier losses still constitute a major factor limiting the PCE to well below the Shockley-Queisser limit. This Perspective summarizes recent atomistic quantum dynamics studies on the photoinduced excited-state processes in metal halide perovskites (MHPs), including both hybrid organic-inorganic and all-inorganic MHPs and three- and two-dimensional MHPs. The simulations, performed using a combination of time-domain ab initio density functional theory and nonadiabatic molecular dynamics, allow emphasis on various intrinsic and extrinsic features, such as components, structure, dimensionality and interface engineering, control and exposure to various environmental factors, defects, surfaces, and their passivation. The detailed atomistic simulations advance our understanding of electron-vibrational dynamics in MHPs and provide valuable guidelines for enhancing the performance of perovskite solar cells.

Dimensionality engineering of metal halide perovskites

Metal halide perovskites are a class of materials that are ideal for photodetectors and solar cells due to their excellent optoelectronic properties. Their low-cost and low temperature synthesis have made them attractive for extensive research aimed at revolutionizing the semiconductor industry. The rich chemistry of metal halide perovskites allows compositional engineering resulting in facile tuning of the desired optoelectronic properties. Moreover, using different experimental synthesis and deposition techniques such as solution processing, chemical vapor deposition and hot-injection methods, the dimensionality of the perovskites can be altered from 3D to 0D, each structure opening a new realm of applications due to their unique properties. Dimensionality engineering includes both morphological engineering-reducing the thickness of 3D perovskite into atomically thin films-and molecular engineering-incorporating long-chain organic cations into the perovskite mixture and changing the composition at the molecular level. The optoelectronic properties of the perovskite structure including its band gap, binding energy and carrier mobility depend on both its composition and dimensionality. The plethora of different photodetectors and solar cells that have been made with different compositions and dimensions of perovskite will be reviewed here. We will conclude our review by discussing the kinetics and dynamics of different dimensionalities, their inherent stability and toxicity issues, and how reaching similar performance to 3D in lower dimensionalities and their large-scale deployment can be achieved.

Understanding the Instability of the Halide Perovskite CsPbI3 through Temperature-Dependent Structural Analysis

Despite the tremendous interest in halide perovskite solar cells, the structural reasons that cause the all-inorganic perovskite CsPbI₃ to be unstable at room temperature remain mysterious, especially since many tolerance-factor-based approaches predict CsPbI₃ should be stable as a perovskite. Here single-crystal X-ray diffraction and X-ray pair distribution function (PDF) measurements characterize bulk perovskite CsPbI₃ from 100 to 295 K to elucidate its thermodynamic instability. While Cs occupies a single site from 100 to 150 K, it splits between two sites from 175 to 295 K with the second site having a lower effective coordination number, which, along with other structural parameters, suggests that Cs rattles in its coordination polyhedron. PDF measurements reveal that on the length scale of the unit cell, the PbI octahedra concurrently become greatly distorted, with one of the IPbI angles approaching 82° compared to the ideal 90°. The rattling of Cs, low number of CsI contacts, and high degree of octahedral distortion cause the instability of perovskite-phase CsPbI_3. These results reveal the limitations of tolerance factors in predicting perovskite stability and provide detailed structural information that suggests methods to engineer stable CsPbI₃ -based solar cells.

Universal Size-Dependent Stokes Shifts in Lead Halide Perovskite Nanocrystals

Size-dependent photoluminescence Stokes shifts (ΔE_s) universally exist in CsPbX₃ (X = Cl^-, Br^-, or I^-) perovskite nanocrystals (NCs). ΔE_s values, which range from ∼15 to 100 meV for NCs with average edge lengths (l) from approximately 13 to 3 nm, are halide-dependent such that ΔE_s(CsPbI₃) > ΔE_s(CsPbBr₃) ≳ ΔE_s(CsPbCl₃). Observed size-dependent Stokes shifts are not artifacts of ensemble size distributions as demonstrated through measurements of single CsPbBr₃ NC Stokes shifts (⟨ΔE_s⟩ = 42 ± 5 meV), which are in near quantitative agreement with associated ensemble (l = 6.8 ± 0.8 nm) ΔE_s values (ΔE_s ≈ 50 meV). Transient differential absorption measurements additionally illustrate no significant spectral dynamics on the picosecond time scale that would contribute to ΔE_s. This excludes polaron formation as being responsible for ΔE_s. Altogether, the results point to an origin for ΔE_s, intrinsic to the size-dependent electronic properties of individual perovskite NCs.

Modeling of Electronic Mobilities in Halide Perovskites: Adiabatic Quantum Localization Scenario

The transport properties of MAPbI3 are analyzed within a tight-binding model. We find a strong Fröhlich interaction of electron and holes with the electrostatic potential induced by the longitudinal optical phonon modes. This potential induces a strong scattering and limits the electronic mobilities at room temperature to about 200  cm^{2}/V s. With additional extrinsic disorder, a large fraction of the electrons and holes are localized, but they can diffuse by following nearly adiabatically the evolution of the electrostatic potential. This process of diffusion, at a rate which is given by the lattice dynamics, contributes to the unique electronic properties of this material.

Does Dipolar Motion of Organic Cations Affect Polaron Dynamics and Bimolecular Recombination in Halide Perovskites?

The role of dipolar motion of organic cations in the A-sites of halide perovskites has been debated in an effort to understand why these materials possess such remarkable properties. Here, we show that the dipolar motion of cations such as methylammonium (MA) or formamidinium (FA) versus cesium (Cs) does not influence large polaron binding energies, delocalization lengths, formation times, or bimolecular recombination lifetimes in lead bromide perovskites containing only one type of A-site cation. We directly probe the transient absorption spectra of large polarons throughout the entire mid-infrared and resolve their dynamics on time scales from sub-100 fs to sub-μs using time-resolved mid-infrared spectroscopy. Our findings suggest that the improved optoelectronic properties reported of halide perovskites with mixed A-site cations may result from synergy among the cations and how their mixture modulates the structure and dynamics of the inorganic lattice rather than from the dipolar properties of the cations themselves.

Tailoring Hot Exciton Dynamics in 2D Hybrid Perovskites through Cation Modification

We report a family of two-dimensional hybrid perovskites (2DHPs) based on phenethylammonium lead iodide ((PEA)₂PbI₄) that show complex structure in their low-temperature excitonic absorption and photoluminescence (PL) spectra as well as hot exciton PL. We replace the 2-position (ortho) H on the phenyl group of the PEA cation with F, Cl, or Br to systematically increase the cation's cross-sectional area and mass and study changes in the excitonic structure. These single atom substitutions substantially change the observable number of and spacing between discrete resonances in the excitonic absorption and PL spectra and drastically increase the amount of hot exciton PL that violates Kasha's rule by over an order of magnitude. To fit the progressively larger cations, the inorganic framework distorts and is strained, reducing the Pb-I-Pb bond angles and increasing the 2DHP band gap. Correlation between the 2DHP structure and steady-state and time-resolved spectra suggests the complex structure of resonances arises from one or two manifolds of states, depending on the 2DHP Pb-I-Pb bond angle (as)symmetry, and the resonances within a manifold are regularly spaced with an energy separation that decreases as the mass of the cation increases. The uniform separation between resonances and the dynamics that show excitons can only relax to the next-lowest state are consistent with a vibronic progression caused by a vibrational mode on the cation. These results demonstrate that simple changes to the cation can be used to tailor the properties and dynamics of the confined excitons without directly modifying the inorganic framework.

Dynamic Ligand Surface Chemistry of Excited PbS Quantum Dots

The ligand shell around colloidal quantum dots mediates the electron and energy transfer processes that underpin their use in optoelectronic and photocatalytic applications. Here, we show that the surface chemistry of carboxylate anchoring groups of oleate ligands passivating PbS quantum dots undergoes significant changes when the quantum dots are excited to their excitonic states. We directly probe the changes of surface chemistry using time-resolved mid-infrared spectroscopy that records the evolution of the vibrational frequencies of carboxylate groups following excitation of the electronic states. The data reveal a reduction of the Pb-O coordination of carboxylate anchoring groups to lead atoms at the quantum dot surfaces. The dynamic surface chemistry of the ligands may increase their surface mobility in the excited state and enhance the ability of molecular species to penetrate the ligand shell to undergo energy and charge transfer processes that depend sensitively on distance.