Halide perovskite semiconductors can merge the highly efficient operational principles of conventional inorganic semiconductors with the low‑temperature solution processability of emerging organic and hybrid materials, offering a promising route towards cheaply generating electricity as well as light. Following a surge of interest in this class of materials, research on colloidal halide perovskite nanocrystals (NCs) has gathered momentum in the last decade. This talk will highlight several findings of our group on their synthesis, for example our recent studies on the influence of various factors on the growth of perovskite NCs, which can lead to the formation of NCs with peculiar shapes (for example hollow structures) and to NC heterostructures by promoting/suppressing the heterogenous nucleation of selected materials. We will highlight several cases of core-shell heterostructures prepared by partial halide or B cation exchange. Detailed structural and optical investigations of these heterostructures will be covered, and their potential in a set of photocatalytic reactions and in light emitting devices will be discussed.

Low-dimensional (LD) organic metal halide hybrids (OMHHs) have recently emerged as a versatile class of solution-processable semiconductors with highly tunable structures and optoelectronic properties. Their unique combination of structurally isolated metal halide building blocks and functional organic components enables precise control of exciton dynamics, charge transport, and defect chemistry, offering exciting opportunities for next-generation radiation detection technologies. In this talk, I will present our recent advances in the design, synthesis, and photophysical understanding of LD OMHHs for both X-ray scintillation and direct X-ray detection. For scintillation, I will discuss molecular and structural engineering strategies that enable high photoluminescence quantum efficiencies, efficient X-ray-to-visible energy conversion, fast radioluminescence, and excellent environmental stability. Particular emphasis will be placed on zero-dimensional (0D) and one-dimensional (1D) OMHHs, as well as solution-processed amorphous hybrid films that combine outstanding scintillation performance with scalable, low-cost fabrication for high-resolution X-ray imaging and ultrahigh-dose-rate radiation dosimetry. I will also highlight the development of semiconducting LD OMHHs for direct X-ray detection. By exploiting molecular sensitization, reduced ionic migration, and tailored electronic structures, these materials exhibit high carrier transport, low dark current, excellent operational stability, high sensitivity, and low detection limits. Recent advances demonstrate that rational molecular design enables environmentally friendly hybrid semiconductors capable of competing with conventional inorganic detector materials while offering substantially greater synthetic flexibility and processability.

Colloidal perovskite nanocrystals have emerged as building blocks for light-coupled nanomaterials. As the structure of their ordered assemblies (superlattices) becomes better understood, the next challenge is to use this knowledge to engineer functionality, and eventually, some use. In this talk, I will discuss CsPbBr₃ perovskite nanocrystal superlattices as structurally coherent, yet soft and disorder-rich, materials whose properties are shaped by surfaces, ligands, and interfaces.

Perovskite nanocrystal superlattices are unusually ordered colloidal nanomaterials. Structural coherence, normally associated with epitaxial materials and single crystals, is a feature of perovskite nanocrystal superlattices, including those made from cubes and nanoplatelets. Combined with quantitative diffraction analysis, this coherence provides access to interparticle spacing, average nanocrystal size, disorder, surface passivation, and information about ligands located between nanocrystals. 

This structural understanding opens new directions. First, improved self-assembly and superlattice formation enable structural characterization at the single-superlattice level, using X-ray diffraction to extract information beyond what is accessible in ensemble-averaged experiments. Second, by taking advantage of mild solvent evaporation nanocrystal assembly, colloidal superlattices can be combined with two-dimensional metal halides to form heterostructures with directed energy transfer.

Metal-halide perovskites (MHPs) and “perovskite-inspired materials” (PIMs) have recently emerged as versatile materials for solar cells and photocatalytic applications. Ultrafast optical probes of photoconductivity dynamics are particularly useful here, uncovering the generation, localisation and ultimate recombination of charge carriers following photon absorption.

Probing charge-carrier motion in highly anisotropic semiconductors poses particular challenges. We examine such charge transport in layered, two-dimensional metal halide perovskites (2DPs),[1,2,3,4] whose electronic landscape is moderated through quantum confinement. We examine the effects of the high anisotropy of transport in thin films comprising layers that are highly oriented either parallel or perpendicular to the substrate plane [1]. We further utilise a powerful non-contact technique to assess the degree of transport anisotropy in 2DP films [2], based on time-dependent photon reabsorption effects and THz conductivity probes [2-4]. We show that (PEA)₂PbI₄ exhibits strongly suppressed out-of-plane diffusion coefficients of 0.26x10⁻⁴ cm² s⁻¹, consistent with a diffusion anisotropy of about 4 orders of magnitude [2]. We further investigate the effect of spacer cation length on the electronic and optical properties of (Cx)₂PbI₄ 2DPs, using Cx alkylammonium cations of varying carbon chain lengths. We reveal that such films exhibit pronounced odd–even dependence on the carbon number in both optical and transport properties, including absorption coefficients, photoluminescence energies and lifetimes, and excitation diffusion dynamics [3,4]. Out-of-plane diffusion of photoexcitations displays an opposite odd–even trend to in-plane charge-carrier mobility, causing a pronounced odd–even modulation of the thin-film mobility anisotropy [4].

We further report on charge-carrier conduction in new metal-halide based PIMs, which have been shown to exhibit dynamic transitions from large to small polaronic states [5,6,7]. We show that in Mixed-metal chalcohalides (A₂BCh₂X₃) charge-carrier localisation can be overcome through judicious chemical substitution substitution of the M(II) cation on the A-site [7]. We show that a shift in lattice symmetry from the lower-symmetry monoclinic P21/c phase in Pb₂SbS₂I₃ to thehigher-symmetry orthorhombic Cmcm phase in Sn₂SbS₂I₃ has a crucial effect on such charge self-localisation, which is observed only for Pb₂SbS₂I₃, whereas Sn₂SbS₂I₃ maintains a longer-lived nanosecond photoconductivity. We attribute this observation to the higher electronic dimensionality of the Cmcm Sn₂SbS₂I₃ structure, influenced by its more symmetric lattice. These findings establish a direct link between structural and optoelectronic properties in metal chalcohalides, demonstrating how facile chemical tuning can be harnessed to overcome charge-carrier localisation in PIM absorbers for solar energy harvesting.

The local energy landscape in semiconductors is essential for controlling charge-carrier and exciton transport in optoelectronic devices.[1] In two-dimensional metal halide perovskites (2DLPs), semiconducting metal-halide layers are separated by bulky organic spacer cations that largely suppress out-of-plane transport, making vertical heterostructures less effective for engineering charge and energy flow.[2] In contrast, lateral heterojunctions enable in-plane control of the composition while preserving efficient transport within the inorganic framework, providing an attractive platform for band gap engineering. However, general synthetic strategies for the deterministic fabrication of such heterostructures remain elusive.

Here, we introduce a versatile one-pot sequential recrystallization strategy for the fabrication of lateral 2DLP heterostructures.[3] The method exploits differences in solubility and crystallization kinetics, while the sequence, timing, and composition of precursor injections determine the resulting heterostructure architecture. This approach enables the formation of heterojunctions between materials with different halides and metal cations and can be extended to multiple heterojunctions incorporating up to three different halides. Furthermore, we identify two distinct heterostructure morphologies, core-frame and triptych-shaped, that arise from different crystallographic orientations of the core crystal, leading to facet-selective growth of the surrounding phase. The resulting heterostructures exhibit crystalline junctions, tunable multicolour emission, and optical and electronic coupling across the interfaces. Beyond demonstrating a broad range of heterostructure architectures in 2DLPs, this sequential recrystallization strategy has the potential to provide a general route for band-gap engineering in solution-processable low-dimensional semiconductors.

Perovskites are highly versatile and promising materials for energy and sensing applications, and beyond. They are often referred to as “chameleon materials” as their properties can be tailored via compositional sub­stitution in the ABO₃ or ABX₃ structure [1]. Due to their potential in an extremely wide range of applications, from hydrogen or energy storage, in particular for lithium- and sodium-ion batteries, to catalysis, solar cells, sensors, thermochromic [2] and optoelectronic devices for displays, LEDs and radiation detection, perovskites are also synthesized in several forms, from single bulk crystals to polycrystalline films, dispersed and assembled nanocrystals, down to quantum dots, also with different structural dimensionality [3-7]. By scaling crystal size down to the nano-range, unique size-dependent properties and much larger surface-to-volume ratios arise, with consequently improved specific performances. Structure stability is a key point for perovskite design, also requiring careful optimization of microstructural and morphological parameters; low structural dimensionality e.g. can lead to improved stability as well, along with device flexibility. As a result, research on perovskite materials is a hot current topic and suitable characterization techniques and approaches are needed to assess structure-property relationships, particularly concerning nanostructures, taking into account the key constraint of rapid screening and short path from preparation to characterization laboratories before material degradation. Micro-diffraction, in particular for single crystal investigation, is typically available at synchrotron beamlines and is needful for structural studies whenever large crystals are not available; on the other hand, high intensity  X-rays with short-wavelength available at synchrotron beamlines can be necessary to study e.g. large perovskite crystals including heavy atoms, such as Pb, causing significant X-ray absorption. Advanced X-ray Scattering techniques in the small and wide angle ranges (SAXS and WAXS) are nowadays available in specialized laboratories thanks to high brilliance micro-sources and high-performance detectors (https://www.ic.cnr.it/laboratorio/xmi-lb/; https://www.itaca-sb.it/biosaxs/). Several experimental strategies are exploited, including Scanning Microscopy with scattering/absorption contrast, or Grazing Incidence geometry (GISAXS/GIWAXS), to address a large variety of studies requiring multiscale structural characterization. Representative case studies relevant to halide perovskite characterization will be shown, giving an insight into the effectiveness of state-of-the-art laboratory facilities combined with in-house developed software tools [8], as ready-to-use screening methods for self-consistent investigations as well as for preliminary tests to design further in-situ and operando experiments at large-scale facilities.

All-inorganic perovskite solar cells (PSCs) are promising for their superior thermal stability, yet their performance is often limited by solvent trapping and inefficient crystallization. Here, we present a co-solvent engineering strategy in which dimethyl sulfoxide (DMSO) is partially replaced with acetonitrile (AcN) in the precursor solution. We propose that this binary solvent system promotes the formation of intermediate coordination complexes, enabling more effective removal of residual DMSO during the crystallization process. A range of characterization techniques is employed to elucidate the solvent interactions and crystallization behavior, revealing improved film formation and structural quality. Consequently, CsPbI₂Br perovskite solar cells fabricated with an optimized DMSO:AcN ratio exhibit enhanced morphology, reduced trap-state density, and an increase in power conversion efficiency from 12.7% to 14.0%. This work demonstrates a straightforward and scalable approach to achieving more uniform and reproducible perovskite thin films, representing an important step toward their practical and industrial deployment.

The enthusiasm of the scientific community towards identifying alternative materials for photovoltaics has led to the quick development of halide perovskites. Yet, they lack stability upon exposure to outdoor, environmental conditions. Because this burgeoning class of material entails a colossal chemical composition space, machine learning (ML) is very suitable to replace the conventional trial-and-error approach used in their characterization [1,2]. I will discuss how we have implemented ML algorithms varying from echo state networks to statistical models to classify and predict physical properties such as hole transport layer electrical conductivity and halide perovskite photoluminescence response [3,4]. Through automated, in situ optical measurements, we were able to predict the response of these materials for >50 hours, with >90% accuracy [5]. Moreover, we established hidden correlations between composition and thermal stability [6]. Our high-throughput measurements and ML-supported analyses validate the potential of ML to forecast perovskites’ response with a variety of chemical compositions [7,8].  

References:

[1] J. M. Howard, E. M. Tennyson, B. R. A. Neves, M. S. Leite. Machine Learning for Perovskites’ Reap-Rest-Recovery Cycle. Joule 3, 325 (2019)

[2] M. Srivastava, J. M. Howard, T. Gong, M. R. S. Dias, and M. S. Leite. Machine Learning Roadmap for Perovskite Photovoltaics. J. Phys. Chem. Lett. 12, 7866 (2021).

[3] J. M. Howard, K. Palm, Q. Wang, E. Lee, A. Abate, J. N. Munday, and M. S. Leite. Water-Induced and Wavelength-Dependent Light Absorption and Emission Dynamics in Triple-Cation Halide Perovskites. Advanced Optical Materials 9, 2100710 (2021).

[4] J. M. Howard, Q. Wang, M. Srivastava, T. Gong, E. Lee, A. Abate, and M. S. Leite. Quantitative Predictions of Moisture-Driven Photo-Emission Dynamics in Metal Halide Perovskites via Machine Learning. J. Phys. Chem. Letters 13, 2254 (2022). 

[5] M. Srivastava, A. R. Hering, Y. An, J.-P. Correa-Baena, and M. S. Leite. Machine Learning Enables Prediction of Halide Perovskites’ Optical Behavior with >90% Accuracy. ACS Energy Letters 8, 1716 (2023).

[6] A. R. Hering, M. Dubey, E. Hosseini, M. Srivastava, Y. An, J.-P. Correa-Baena, H. Homayoun, and M. S. Leite. Machine Learning Reveals Composition Dependent Thermal Stability in Halide Perovskites. In review (2025).

[7] A. R, Hering, M. Dubey, and M. S. Leite. Emerging Opportunities for Hybrid Perovskite Solar Cells using Machine Learning. APL Energy 1, 020901 (2023).

[8] A. R, Hering, C. Sutter-Fella, and M. S. Leite. An AI-Accelerated Pathway for Stable and Reproducible Halide Perovskites. Submitted (2025). 

Hybrid organic–inorganic metal halide perovskites (HOIPs) have attracted significant attention due to their structural and chemical versatility that offers an ideal platform to engineer not only their optoelectronic behavior [1], but also their magnetic characteristics, making them promising candidates for spintronic applications [2-5]. Hybrid double perovskites and magnetic doped halide perovskites have provided an additional way to tune the properties of HOIPs [6-8]. Through the combination of trivalent metal cations, such as In³⁺/Ru³⁺/Fe³⁺/Mo³⁺, with monovalent cation M⁺, researchers have been able to tune the magnetism in hybrid metal halides. However, the influence of isovalent metal cations with unpaired spins has been scarcely investigated [9]. In this work, we studied three different doped compounds, (PEA)₂M¹_1−xM²_xCl₄ (M = Cu²⁺, Mn²⁺, Co²⁺; PEA = phenethylammonium). In each system we explore the incorporation of two different metallic cations with different spin-configuration in order to study the effect of the dopant on the final magnetic and optical properties of the material. The compounds have been synthetized by common chemical approaches and optimized protocols led to single-phase crystals for low dopant amount (x < 0.1). Optically, all the doped systems changed in color and band-gap when compared to the undoped compound, which also has an additional effect on the photoluminescence emission of Mn²⁺ based HOIPs. In terms of magnetic properties, although the antiferromagnetic behavior in (PEA)₂Mn_1−xCo_xCl₄ is kept for all the compositions studied, the doping promotes changes in the Neel temperature from 45 K to 62 K and even the disappearance of magnetic phenomena, such as spin flop and spin canting. On the opposite, doping has negligible effects on (PEA)₂Mn_1−xCu_xCl₄ system, which behaves as canted antiferromagnet, and (PEA)₂Cu_1−xCo_xCl₄, which presents ferromagnetic properties. Therefore, our work proves that transition metal doping is a promising strategy to modify the optical and magnetic properties of layered HOIPs, providing an approach to create tailored 2D magnets for future spintronic applications. Furthermore, our results emphasize the importance of the proper selection of the host crystal and guest dopant to alter the 2D magnetism [10].

The rapid growth of artificial intelligence demands new computing hardware capable of overcoming the energy and data-transfer limitations of conventional von Neumann architectures. Memristive devices based on mixed ionic-electronic materials have emerged as promising candidates for neuromorphic and in-memory computing by simultaneously storing and processing information. Among them, halide perovskites offer unique advantages owing to their low ion migration barriers, tunable optoelectronic properties, and solution-processability. However, the instability and toxicity of lead-based perovskites remain major obstacles for practical implementation.[1] Therefore, we develop lead-free two-dimensional halide layered double perovskites (LDPs) as a new platform for memristive and neuromorphic devices. The materials are based on Ruddlesden-Popper (BNA)4AgBiX8) and Dion-Jacobson (PDMA)2AgBiX8 (X = I or Br, BNA = benzylammonoium, PDMA = 1,4-phenyldimethylammonium) compositions synthesized through a combined mechanochemical and solution-processing strategy, enabling phase-pure thin films with controlled crystallization and orientation.[2] The influence of supramolecular spacer design, halide composition, and film processing on structural, optical, and electrical properties will be discussed.[3] The LDPs are integrated into three complementary device platforms: asymmetric metal-insulator-metal (MIM) memristors, photovoltaic full-stack devices, and crossbar architectures. The MIM devices exhibit filamentary resistive switching governed by Ag migration and halide-vacancy dynamics, delivering high ON/OFF ratio along with reliable endurance and retention. The photovoltaic-inspired full-stack architecture further demonstrates the possibility of self-powered memristive operation, where the built-in electric field assists ionic transport and switching while electron- and hole-transport layers regulate carrier injection, suppress leakage currents, and improve switching linearity and energy efficiency.[4] Finally, crossbar devices exhibit artificial synaptic behaviors, representing a significant advancement in the development of environmentally benign memristive materials and providing a versatile platform for next-generation low-power neuromorphic hardware and self-powered artificial intelligence systems.

Halide perovskites have significantly transformed the field of optoelectronics, enabling substantial advances in established technologies such as solar cells and light-emitting diodes (LEDs), as well as in emerging devices including memristors. A defining characteristic of these materials is their propensity for ionic migration. Although ion transport is typically regarded as a detrimental effect that contributes to the performance degradation of solar cells and LEDs, it is fundamentally the mechanism that facilitates resistive switching in memristive devices. Consequently, a comprehensive understanding of ionic transport and strategies for its precise control are critical for the rational design of high-performance devices.

This presentation will examine in detail the mechanisms of ionic migration in halide perovskites and outline approaches for modulating this behavior. To expedite progress in this area, high-throughput characterization methodologies have been developed to systematically evaluate ionic transport, thereby enabling precise control over ion dynamics and the realization of functional memristors across diverse applications. A range of device architectures and operational mechanisms—including switchable photovoltaics, photovoltaics exhibiting persistent photovoltage, bipolar photoconductor switching, color-tunable LEDs, and self-rectifying memristors—will be discussed as platforms for achieving novel functionalities. These advances further enable the development of sophisticated hardware security technologies, such as Physical Unclonable Functions (PUFs), as well as advanced neuromorphic systems, including in-sensor and reservoir computing.

Perovskite-based full-colour display technologies remain challenging due to the slower development of stable and efficient blue-emitting perovskites compared to their red and green counterparts. While compositional engineering can induce blue shifts, it often compromises stability, promotes ion migration, and increases defect sensitivity [1]. Multiple quantum wells (MQWs) offer an alternative design strategy in which emission can be tuned through structural confinement and interfacial electronic effects rather than relying solely on chemical modification [2].

In this work, we combine density functional theory (DFT) calculations with experimental characterization to investigate the effects of Rb⁺ substitution and quantum confinement in CsPbBr₃/TPBi MQW heterostructures. Experimental results show that reducing the well thickness and increasing the Rb content progressively widen the bandgap, shifting the emission from the green region near 520 nm in bulk CsPbBr₃ to the blue region near 490 nm in 3 nm Rb_0.37Cs_0.63PbBr₃ wells, in agreement with the DFT calculations. DFT further reveals that Rb substitution has only a minor influence on the TPBi interface, with interfacial charge transfer remaining on the order of 10^-2 e and band bending limited to the meV scale, while also preserving the type-I band alignment favourable for radiative recombination. At the same time, Rb incorporation induces local lattice distortions and electronic perturbations that reduce the transition dipole strength, consistent with the observed suppression of photoluminescence. These adverse effects can be mitigated within the MQW architecture, where stronger confinement and increased well-barrier periodicity help recover emission intensity.

Overall, this combined experimental and DFT study establishes cation-doped MQW heterostructures as a promising platform for blue emission in halide perovskites. By enabling tunability via well thickness, layer number, and composition, these findings provide design principles for developing stable and efficient blue perovskite light emitters for future optoelectronic applications.