Local atomic structure often differs from the global average structure as measured with diffraction and yet the local structure has a profound impact on materials functionalities. This structure-function relationship applies in many materials classes, ranging from organics to Li-ion battery cathodes to oxide and halide perovskites. Accurately characterizing this local structure has proven challenging but recent advances in neutron and X-ray diffuse scattering (“between” Bragg peaks) has enabled local structure determination.

In this talk, I will discuss the importance of local structure and how this can be quantified and will demonstrate this for organic-inorganic hybrid halide perovskites [1,2,3]. While the importance of lattice dynamics and dynamical (dis)order have been recognized in these materials, their nature is only poorly known and understood. We used X-ray and neutron diffuse scattering coupled with molecular dynamics to quantify the nature, size, and time scale associated with dynamical local order in CH₃NH₃PbI₃ and CH₃NH₃PbBr₃ perovskites [1] and on [(CHN₂H₄)_xCs_(1-x)]PbBr₃ alloys. We observe that for CH₃NH₃ (MA or methylammonium) A-site cations the nominally cubic perovskite consists of dynamical, two-dimensional (2D) sheets of lower symmetry tetragonal regions of about three nm diameter with several picosecond lifetimes. For (CHN₂H₄)PbBr₃, we observe elliptical, small (ca 3 nm) three dimensional (3D) tetragonal domains, consistent with recent work [2], while Cs has 2D sheet domains [3]. With increasing CHN₂H₄ (FA or formamidinium) on the A-site, creating [FACs]PbBr₃ alloys, we observe a cross-over from the elliptical tetragonal domains to the 2D sheets near about 20% FA with a small region of co-existence. The implications on these dynamical local domains for halide perovskite properties will be discussed.

A thorough understanding of the temperature dependence of semiconductor band gaps is essential for optimizing optoelectronic devices. In this respect, the origin of the pronounced temperature-induced gap bowing observed in low-temperature phases of formamidinium-methylammonium (FA-MA) lead iodide perovskites [1] has remained elusive until now. By combining temperature and pressure-dependent photoluminescence measurements on a series of FA_xMA_1-xPbI₃ mixed-cation single crystals with eleven compositions from 0 to 1, we unravel the origin of this bowing. Both thermal expansion as well as electron-phonon interaction effects are responsible [2,3]. However, the latter is the leading term, driven by the activation of an anomalous electron-phonon coupling mechanism linked to mixed vibrational modes [4], which combine inorganic-cage phonons involving octahedral tilting with low-frequency FA librations, i.e., FA rattler modes. This occurs mainly in a low-temperature (pseudo)tetragonal phase, presumably featuring stripe domains with alternating octahedral tilt patterns for FA concentrations between 20% and 90% [5]. In this way, we have shed light on an intriguing behavior of lead halide perovskites that directly affects their optoelectronic properties.

Two-dimensional lead halide perovskites have emerged as promising materials for photovoltaic and light-emitting applications owing to their excellent environmental stability, tunable quantum confinement, and chemical compatibility with three-dimensional perovskites [1,2]. However, their reduced structural dimensionality gives rise to enhanced excitonic effects and highly anisotropic charge transport, making it essential to understand excitation and charge-carrier diffusion, particularly in the out-of-plane direction. In this presentation, we discuss two complementary studies that establish how thin-film structure governs transport in Ruddlesden–Popper-type lead-iodide 2D perovskites.

First, we demonstrate an effective method for monitoring inter-layer diffusion of photoexcitations in (PEA)₂PbI₄ thin films by tracking time-dependent photoluminescence spectral changes induced by photon reabsorption effects [3]. By selectively exciting the films from either the substrate or air side, we reveal depth-dependent diffusion dynamics across the film profile. Time-dependent diffusion coefficients are extracted using a one-dimensional diffusion model coupled with an interference correction that accounts for refractive-index variations near the strong excitonic resonance. This analysis reveals a low out-of-plane excitation diffusion coefficient of (0.26 ± 0.03) × 10⁻⁴ cm² s⁻¹, corresponding to a diffusion anisotropy of approximately four orders of magnitude [3].

Second, we extend this dynamic photon reabsorption approach to a systematic series of 2D perovskite thin films incorporating non-conjugated alkylammonium spacer cations with chain lengths from three to eight carbon atoms. Pronounced odd–even effects are observed in absorption coefficients, photoluminescence energies and lifetimes, out-of-plane excitation diffusion, and in-plane charge-carrier mobility measured by optical pump–terahertz probe spectroscopy. Grazing-incidence wide-angle X-ray scattering reveals that these transport trends arise from cation-controlled nanostructural orientation: even-numbered alkyl spacers promote highly ordered lead-iodide planes lying within the film plane, whereas odd-numbered spacers induce more disordered stacking. Furthermore, the observed 1/d² dependence of inter-layer diffusion on the inter-plane distance d indicates that Förster resonance energy transfer underpins excitation transport between lead-iodide layers. Together, these findings establish a direct structure–transport relationship in 2D perovskite thin films and provide design guidelines for anisotropic optoelectronic devices [4].

While the optoelectronic properties of metal halide perovskites resemble traditional semiconductors, like III-Vs and Si, their soft and dynamic crystal structure confers an assortment of complex thermally-activated properties. This includes ion (or defect) transport and polaron formation, local polar fluctuations and complex thermal-phase relations. Underpinning these interesting properties is the tendency of perovskites to exhibit fluctuations in their local crystal structure and strong lattice anharmonicity – deviations from the harmonic approximation for oscillating atoms. This is because there exists both a “flexible” network of corner-sharing metal-halide octahedra, alongside an ionic sublattice of caged cations, with varying degrees of freedom and interactions. At elevated temperatures, a static picture of the perovskite solar cell structure simply breaks down and one must account for microscopic anharmonic lattice dynamics to understand its macroscopic, bulk physical properties. In this contribution, we aim to connect the atomistic origins of anharmonic lattice dynamics in perovskites with their macroscopic properties, including optoelectronic and thermo-mechanical properties. For example, we assess how the degree of anharmonicity varies across temperature, composition, and phase, and highlight the emergence of corresponding macroscopic changes and phase boundaries. By linking fundamental physics to device-level challenges, we provide a framework for engineering high-performance perovskite absorbers. We anticipate that providing a clear perspective for these topics will help deepen our knowledge of the nature of ionic semiconductors in general.

A detailed picture of atomic structure and dynamics in materials is needed to refine microscopic theories of transport and thermodynamics, to design next-generation energy materials or computing devices. For instance, phonons impact numerous functional behaviors, ranging from structural transitions and optoelectronic properties of metal halide perovskites (MHPs) to ferroelectric properties or transport properties in thermoelectrics  and solid-state electrolytes. Significant progress has been achieved in understanding simple crystal structures, but disorder and complex temporal and spatial correlations of ions remain challenging to probe and rationalize. Isolating spontaneous off-centerings, subunit distortions or tilts, compositional fluctuations, or soft phonon modes – and their couplings – requires careful measurements and detailed modeling.

This presentation will highlight our investigations of the unusual atomic dynamics and structural fluctuations in MHPs through inelastic neutron scattering (INS), inelastic x-ray scattering (IXS), diffuse neutron/x-ray scattering measurements and comprehensive first-principles and machine-learning augmented simulations. Through INS/IXS and diffuse scattering measurements, we found systematic quasi-elastic diffuse scattering rods in reciprocal space in all MHP compounds investigated -- both single and double-perovskite halides-- in their cubic or tetragonal phases, which originate from fluctuating planar domains featuring correlated tilts of halide octahedra. The diffuse rods exhibit a slow, overdamped dynamic response with a characteristic time of a few picoseconds, modulated across Q space, and reflecting the strong lattice anharmonicity of the inorganic framework. These collective dynamic fluctuations are traced to strongly anharmonic phonon modes featuring an overdamped character, for wavevectors along the edges of the Brillouin zone (single perovskite setting). These results offer valuable insights into the unusual anharmonic atomic dynamics and intricate correlated structural distortions in MHPs, which will be critical for rationalizing and further tailoring their thermal and optoelectronic properties.

[1] C. Mao et al. "Correlated dynamic disorder, octahedral tilts, and acoustic phonon softening in CsSnBr3 and CsPbBr3", Physical Review Materials 9, 065401 (2025)
[2] X. He et al. "Multiple Lattice Instabilities and Complex Ground State in Cs2AgBiBr6", PRX Energy 3, 013014 (2024)
[3] T. Lanigan-Atkins et al. "Two-dimensional overdamped fluctuations of the soft perovskite lattice in CsPbBr3", Nature Materials 20, 977–983 (2021)

Surfaces and interfaces play a central role in the stability and performance of halide perovskite optoelectronic devices. However, most first-principles studies of cubic inorganic halide perovskite surfaces still rely on ideal high-symmetry structures. These models neglect the local positional disorder that is intrinsic to these soft and anharmonic materials. Here, we show that local disorder is not only important for bulk properties [1–2], but also has a sizable effect on surface stability and surface electronic properties.

We investigate cubic CsBX₃ perovskites (B = Sn or Pb and X = I, Br, or Cl) using density functional theory (DFT). Locally disordered (polymorphous) bulk configurations were generated using the Anharmonic Special Displacement Method proposed by Zacharias et al. [2]. Polymorphous surface slabs were then constructed by repeating the polymorphous bulk supercells along the (001) direction. To reduce self-interaction errors and improve the calculated electronic properties, we applied the DFT-1/2 quasiparticle correction method [3].

For the surfaces, local disorder lowers the surface energy, indicating that polymorphous surfaces are thermodynamically more favorable than their high-symmetry counterparts. It also shifts the absolute valence-band maximum (VBM) downward with respect to the vacuum level. This shift originates from disorder-induced B-X bond elongation and tilting of the BX₆ octahedra. The resulting structural distortions stabilize the bulk VBM and modify the surface dipole, leading to a lower absolute VBM.

The calculated VBM trends are strongly affected by the level of theory. Semilocal DFT gives an incorrect trend across the halide series, while DFT-1/2 recovers the trend observed experimentally [4].  Moreover, polymorphous structures give VBM values that are closer to experimental measurements than those obtained from ideal monomorphous structures. These results demonstrate that the polymorphous nature of halide perovskites must be included to describe their surface electronic properties realistically. Our findings identify local disorder as a key design parameter for tuning surface energetics, interfacial band alignment, and device performance.

In the progress towards all-perovskite tandem solar cells, significant effort is focused on optimizing half-tin-half-lead perovskites. However, their structure-property relationships remain underexplored, largely due to the difficulty in probing short- versus long-range ordering of tin and lead atoms in the lattice. The tin and lead arrangement has been suggested to impact defect density, oxidation resistance, and energy disorder, and has recently become a particularly hot topic, as many record-efficiency tin-lead solar cells have been reported to achieve high performance by mitigating clustering of lead and tin.^[1-6] It thus deserves more attention. We investigate the presence of larger-scale lead and tin clusters in CsSn_xPb_1-xI₃ through energy-dispersive X-ray spectroscopy. Under a spatial resolution of ~120 nm, no significant heterogeneity is observed. To probe short-range order in the atomic lattice, we employ solid-state nuclear magnetic resonance (NMR) across the full compositional series from CsPbI₃ to CsSnI₃. ¹³³Cs NMR indicates the formation of a fully mixed random solid solution, with neither nanoclustering nor global Pb-Sn ordering. Finally, we perform atomic-resolution scanning transmission electron microscopy (STEM) and electron diffraction on CsSn_0.5Pb_0.5I₃ to discuss the absence of clustering and the potential presence of local regions of ordering in the Pb & Sn lattice sites.

Two-dimensional metal halide perovskites (2D-MHPs) are chemically and structurally diverse semiconducting materials with promising applications in photovoltaics and optoelectronics. The understanding of charge and heat transport is crucial for improving the performance and stability of devices made from these materials. I will discuss our recent efforts on developing time-resolved optical spectroscopy and imaging techniques for characterizing the anisotropic flow of heat in these hierarchically architectured materials, as well as heat dissipation in MHP-based films and devices that feature defects, grain boundaries, and cracks. I will then demonstrate how we might turn the generally poor thermal transport property of these strongly excitonic materials into a desirable characteristic, where we employ these materials for thermal-type, all-optical photodetection of long-wavelength light covering the short-wave infrared and the long-wave infrared. If time permits, I will also present our efforts on spatiotemporally resolving phase transitions in 2D- and 3D-MHPs with time-resolved optical microscopy at the intrinsic time and length-scales.

Chiral hybrid perovskites have emerged as a highly promising platform for exploring the interplay between spin, lattice, and optical degrees of freedom, owing to their intrinsically noncentrosymmetric crystal structures, strong spin-orbit coupling, and soft, dynamically responsive lattices. In these materials, chirality can give rise to unusual spin-selective optical responses and enhanced coupling between electronic and structural excitations, making them especially attractive for studying nonequilibrium phenomena in soft quantum materials. In this work, we investigate ultrafast spin and phonon transduction in chiral hybrid perovskites, with particular emphasis on how photoexcitation initiates coupled spin-lattice dynamics on femtosecond to picosecond timescales.

Using ultrafast spectroscopic techniques, we track the generation, evolution, and decay of coherent phonons following optical excitation, and examine how these lattice vibrations interact with spin-polarized electronic excitations. Our measurements reveal that photoexcitation does not simply perturb the electronic system in isolation, but instead drives a strongly coupled response in which vibrational and spin degrees of freedom evolve together. The observed dynamics provide evidence for efficient transduction between spin and phonon channels, likely mediated by the chiral crystal environment, spin-orbit interactions, and strong electron-phonon coupling inherent to the material system. These results suggest that lattice motion can serve as an effective conduit for manipulating spin information, opening a route toward dynamic control of spin states through structural degrees of freedom.

More broadly, these findings point to a new strategy for engineering ultrafast functionality in soft quantum materials, where chirality and lattice dynamics can be leveraged to control spin behavior on extremely short timescales. Such control may enable future advances in ultrafast spintronics, chiroptical switching, and hybrid optoelectronic devices that exploit the intimate coupling between light, spin, and lattice motion.

Polaron formation is one of the central processes by which an electronic excitation becomes dressed by a polarizable lattice. Although polarons are usually treated as quasiparticles with defined energies and couplings, their formation is intrinsically dynamical: the lattice polarization must reorganize in time, creating a new many-body state of the material. In this talk, I will describe how coherent multidimensional spectroscopy can directly resolve this birth process in lead-halide perovskite quantum dots.

Using phase-resolved ultrafast spectroscopy with femtosecond time resolution, we show that the anti-diagonal linewidth and inter-excitonic coupling features provide a direct experimental measure of the evolving polaron order parameter. In contrast to conventional II–VI quantum dots such as CdSe, where exciton–phonon coupling is dominated by underdamped normal modes, perovskite quantum dots exhibit overdamped, liquid-like lattice response. The resulting spectral diffusion and delayed growth of coherent inter-excitonic structure reveal the formation of a Landau polaron on a ~100–200 fs timescale.

I will then discuss the quantum-optical consequences of this dynamically generated quasiparticle state. The same polaronic reorganization that dresses the exciton also reshapes the excitonic Hilbert space, enabling delayed electronic coherence, decoherence-protected subspaces, and cooperative optical responses such as superabsorption and time-reversed superfluorescence at elevated temperatures. These results suggest that lead-halide perovskite quantum dots are not simply colloidal quantum objects with strong exciton–phonon coupling, but quantum materials in which collective lattice polarization generates new optical and many-body functionality in real time.

Halide perovskites are often described through their average crystal structures, although many of their important properties arise from fluctuations that are absent from this static picture. In this talk, I will discuss how atomistic simulation can resolve their structural dynamics and connect them to electronic behavior. First-principles molecular dynamics provides microscopic insight into many important effects. These include octahedral tilting, local distortions, and can also account for their temperature dependence. However, this approach is restricted to relatively small systems and short trajectories. Machine-learning force fields overcome much of this limitation and make it possible to sample realistic structural ensembles at substantially larger scales. Structural dynamics alone, however, do not provide the electronic information needed to predict optoelectronic properties. I will therefore introduce HAMSTER, a physics-informed Hamiltonian-learning approach that combines an approximate physical model with a machine-learned corrections for fluctuating atomic environments. With only modest first-principles input, our HAMSTER model remains transferable across temperatures and compositions and can treat systems containing tens of thousands of atoms. This combination of learned atomic dynamics and learned electronic Hamiltonians offers a route toward interpretable finite-temperature predictions for structurally complex halide perovskites and other materials.

Tuning Soft Perovskites by Alloying: Phase Boundaries, Halide Ordering, and Vacancy Migration

Perovskites with the general composition ABX3 are among the most versatile functional materials known, with applications spanning photovoltaics, solid-state electrolytes, and ferroelectrics. A unifying feature is the presence of soft lattice dynamics — low-energy structural distortions that mediate phase transitions and couple strongly to electronic, optical, and transport properties. Alloying on the A, B, or X site provides a powerful handle for tuning these dynamics, but it simultaneously introduces chemical disorder and competing structural motifs whose interplay is often poorly understood.

I will first present an analysis of the phase diagram of the mixed A-site system MA1-xFAxPbI3, modeled as a random alloy, obtained using a machine-learned interatomic potential (MLIP) based on the neuroevolution potential framework. The results reveal a morphotropic phase boundary (MPB) at approximately 27% FA content, delineating the transition between out-of-phase and in-phase octahedral tilt patterns, where the free-energy landscapes of the underlying phonon modes become nearly degenerate. Density functional theory calculations show that band edge fluctuations peak near the MPB, indicating enhanced electron–phonon coupling and dynamic disorder. By demonstrating that phonon overdamping serves as a hallmark of the MPB, this study informs design principles for stable, high-performance perovskite solar cells.

Extending this perspective to inorganic mixed halide perovskites, whose thermodynamic phase behavior is central to compositional design and device stability, I will discuss results in which halide ordering is sampled explicitly — in contrast to the random-mixing treatment above — through combined Monte Carlo and molecular dynamics simulations with MLIPs trained on density functional theory data, capturing both configurational and vibrational degrees of freedom across the CsxRb1–xPbBr3yI3-3y, CsxRb1–xPbBr3yCl3-3y, and CsxRb1–xPbCl3yI3-3y systems. All three exhibit a miscibility gap whose extent correlates with halide ion size mismatch. Outside this gap, all systems show a tendency toward layered halide ordering. In CsPbBr3yI3-3y, this ordering occurs in a device-relevant temperature regime and shifts structural transition temperatures by up to 100 K relative to randomly mixed structures, accounting for the experimentally observed two-regime composition dependence. Introducing Rb on the A-site weakens halide ordering, eliminates the two-regime behavior, and narrows the miscibility gap, establishing halide ordering as a key determinant of structural phase stability. Building on this picture, I will further show how crystal phase, halide composition, and A-site alloying jointly govern vacancy-mediated halide diffusion in these systems.

Hybrid organic-inorganic lead halide perovskites have emerged as promising materials for solar cells, attracting significant research interest over the past decade. A key requirement for fabricating efficient and stable perovskite solar cells (PSCs) is to minimise non-radiative recombination losses, such as trap-mediated and interfacial recombination, so that, under open-circuit conditions, recombination is dominated by the radiative channel.^[1] To monitor the charge transfer dynamics in perovskites, transient spectroscopy techniques, such as transient photoluminescence (TRPL), are widely employed. However, the analysis of the TRPL decays becomes increasingly complex as additional layers are deposited on top of the perovskite. For example, passivating interlayers or charge transport layers can introduce additional midgap states or give rise to asymmetric trapping, further complicating the physical processes taking place following illumination. Moreover, the quantitative analysis of TRPL traces is currently performed by applying biexponential fitting functions or the ABC model, both of which fail to capture the full physical picture in the nanosecond-to-microsecond time regime, as well as the complicated recombination processes governing the perovskite bulk and its interfaces.[^2,3] It is therefore crucial to ascribe physical meaning to the observed transients in order to properly describe the underlying dynamics in a quantitative and analytical manner. ^[4]

Herein, we propose combining multi-parameter recombination models ^[5] with a Bayesian inference framework ^[6] to analyse TRPL decays in perovskite thin films and half-stacks with solar cell-relevant architecture. The Bayesian framework, built on a Markov-Chain Monte-Carlo (MCMC) sampler, explores the highly correlated, multi-dimensional parameter space of the proposed physical model and is applied directly to experimental data. We show that incorporating multimodal data into the Bayesian inference pipeline, specifically nanosecond transient absorption spectroscopy (nsTAS) and photoluminescence quantum yield (PLQY), along with TRPL, better constrains the parameter space and yields a more accurate quantitative description. We further apply this methodology to elucidate the effect of various interlayers on recombination dynamics following photoexcitation, distinguishing between chemical and field-effect passivation mechanisms, and between non-radiative recombination and charge extraction. This establishes a robust protocol for systematically guiding future material and device development.

Materials with the perovskite crystal structure have been studied for a wide range of applications, from oxide ferroelectrics to halide-perovskite solar cells. Despite their diverse functionalities, these materials share strong lattice anharmonicity arising from structural instabilities such as polar off-centering and octahedral tilting. These anharmonic distortions strongly influence the low-frequency vibrational response and can be observed in infrared (IR) and Raman spectra. However, low-frequency IR and Raman signatures are typically investigated separately, hindering the development of a unified understanding of vibrational spectroscopy across perovskite classes. In this work, we use machine-learning-based molecular dynamics to simulate the IR and Raman spectra of halide perovskites (CsPbBr₃), halide double perovskites (Cs₂AgAlBr₆ and Cs₂AgBiBr₆), germanium-based perovskites (CsGeBr₃), and oxide perovskites (SrTiO₃ and BaTiO₃). By comparing materials exhibiting polar, tilt-driven, and mixed instabilities, we find that off-centering polar modes lead to low-frequency IR modes and an enhanced static dielectric constant, whereas octahedral tilting is responsible for the formation of a central peak in Raman spectra. In both cases, the low-frequency feature reaches a maximum near phase transitions, providing a direct link between the low-temperature distorted structures and the vibrational signatures in the high-temperature cubic phase. Together, these results establish a unified framework connecting low-frequency IR and Raman spectra to the underlying anharmonic lattice instabilities in perovskites.

This presentation explores the concept of lattice-matching, a well-established principle in the epitaxy of bulk semiconductors, and details its adaptation to halide perovskites. This adaptation involves incorporating a linear-quadratic coupling between strain and octahedral rotations into the mechanical free energy expansion. The utility of this approach is demonstrated through several experimental examples, including: Lattice parameter variations in 2D multilayered perovskites, Thick bilayer heterostructures (both matched and mismatched) combining 2D and 3D materials, Nano-inclusions of 2D perovskites within 3D matrices, Buried 3D perovskite quantum dots embedded in a 3D matrix.

Additionally, the presentation reviews theoretical findings regarding the influence of polymorphism on electronic structure, electron-phonon coupling, and surface properties in halide perovskites. Finally, it demonstrates how tuning transferable atomistic parameters in Density Functional Tight Binding (DFTB) codes offers a robust pathway for calculating the optoelectronic properties of halide perovskites, extending to low-dimensional structures,  complex heterostructures, alloys and perovskitoid compounds.

Lead halide perovskites have received great interest in recent years due to their excellent photovoltaic, optical (linear and nonlinear) and electrical properties. An efficient way to increase structural diversity and tune functional properties of hybrid lead halides is mixing of organic cations. The most famous subclasses of mixed-cation hybrid halide perovskites are multilayered Ruddlesden–Popper (RP, A’₂A_n-1Pb_nX_3n+1) and Dion–Jacobson (DJ, A”A_n-1Pb_nX_3n+1) phases, where the parameter n defines the thickness of corner-sharing PbX₆ octahedral slabs, A’ and A” are large interlayer cations, and A denote small cations located in the perovskite cages [1]. These compounds exhibit improved environmental stability relative to their 3D counterparts and increasing n progressively reduces confinement effects, narrows the band gap, and enhances charge-carrier transport, thereby improving photovoltaic-relevant performance. It is worth noting that these compounds often crystallize in polar structures [2], which is prerequisite for piezo-, pyro- and ferroelectric properties and second-order nonlinear optical properties (e.g. second-harmonic generation, SHG). However, majority of these compounds exhibit in-plane polarization and only a handful of compounds with out-of-plane polarization are known.  Mixing of organic cations may also lead to other type of structures, for instance, 3D perovskitoids such as (c-C3A)₃(MA)₃Pb₅I₁₆ (c-C3A= cyclopropylammonium, MA= methylammonium) or MPDA₂FAPb₄Br₁₃ (MPDA = N-methylpropanediammonium, FA = formamidinium) [3,4].

In hybrid lead halides, structural distortions of the inorganic framework play a decisive role in governing emission characteristics, excitonic absorption energies, and the emergence of ferroelectricity or SHG activity upon symmetry breaking [5]. Therefore, a comprehensive understanding of the interplay between inorganic layer distortions and organic cation dynamics is essential for rational materials design of these compounds. In this contribution, we report temperature-dependent studies of selected mixed-cation lead halides comprising large mono and diammonium cations as well as small cage cations to elucidate mechanisms of the observed structural phase transitions and relation between structural changes and optoelectronic properties of these compounds.

Polarons are believed to play a key role in determining the optoelectronic properties of halide perovskites, yet their microscopic nature remains poorly understood. First principles calculations are challenging because polarons in these materials arise from complex electron-phonon interactions and may extend over several unit cells, beyond the reach of direct supercell approaches.

In this work, we apply the recently developed ab initio theory of polarons to study polaronic quasiparticles in halide perovskites across length scales, focusing on the lead-free double perovskite Cs2AgBiBr6 as a representative example. Our calculations reveal a rich variety of polaronic species, including large polarons, small polarons, and periodic twist-density waves. These results provide a unified microscopic framework for interpreting carrier localization and photoinduced structural distortions in halide perovskites.

We further find that these emergent quasiparticles support topologically nontrivial displacement fields: small electron polarons carry a finite toroidal moment, while large electron and hole polarons form helical Bloch points with finite helicity, making them non-magnetic analogues of the helical Bloch points found in magnetic skyrmion lattices. These topological polarons give rise to characteristic fingerprints in Huang diffuse scattering, suggesting a route to their detection via ultrafast electron and X-ray scattering experiments.

Metal halide perovskites exhibit remarkable optoelectronic properties despite possessing highly dynamic and locally disordered crystal structures. Understanding how finite-temperature lattice fluctuations influence their electronic and optical behavior remains a central challenge for the field. In this contribution, we present a first-principles framework based on the Special Displacement Method (SDM), which provides a unified description of anharmonic lattice dynamics and electron-phonon coupling within large supercells. By explicitly accounting for thermal atomic displacements and local symmetry breaking, SDM captures the polymorphous nature of perovskites and enables direct access to temperature-dependent electronic structure, optical absorption, carrier transport, and light-emission properties.

Our calculations reveal that local disorder profoundly modifies band-edge states, electron-phonon interactions, and carrier dynamics, often leading to behavior that cannot be explained within conventional average-crystal descriptions. The same framework naturally predicts diffuse scattering patterns arising from finite-temperature atomic correlations, establishing a direct connection between theory and modern X-ray and electron scattering experiments. We discuss how these concepts provide a coherent picture linking dynamic disorder, anharmonicity, and optoelectronic functionality in halide perovskites. Finally, we highlight recent extensions of the methodology to antiperovskites and superionic conductors, demonstrating that local disorder and thermal fluctuations are universal ingredients governing the properties of a broad class of energy and quantum materials.

Understanding carrier dynamics is crucial for developing advanced semiconductor materials and optoelectronic devices across fields: ranging from photovoltaics and light-emitting diodes to sensors and emerging quantum materials. While steady-state photoluminescence (PL) techniques provide valuable information on optical quality, they often fall short in revealing the full picture of how charge carriers recombine, migrate and interact with local defects.

Time-resolved photoluminescence (TRPL) and lifetime imaging techniques offer deeper insights by resolving carrier lifetimes, recombination pathways, and diffusion behavior with high temporal and spatial resolution. Combining TRPL with intensity-dependent measurements help untangle radiative and non-radiative processes and can quantify how excitation conditions influence carrier transport, key parameters for optimizing material design and device performance.

In this contribution, we demonstrate how PicoQuant’s Solira TRPL microscope system enables detailed characterization of carrier diffusion and recombination dynamics in a wide range of semiconductor materials. Using hybrid perovskites as a showcase, given their broad relevance from photovoltaics to photodetectors, we illustrate how time- and space-resolved PL mapping can connect structural heterogeneity to electronic behavior. The same approach is applicable to diverse materials such as III-V semiconductors, 2D materials, quantum dots, and organic semiconductors.

This flexible measurement platform supports researchers aiming to optimize fabrication processes, reveal defect-related loss channels, and better understand fundamental transport mechanisms, all essential steps for advancing next-generation optoelectronic devices and functional materials.