Further breakthroughs in halide perovskite solar cells require advances in new compositions and underpinning materials science. Indeed, a deeper understanding of these complex hybrid perovskite materials requires atomic-scale characterization of their transport, electronic and stability behaviour. This presentation will describe recent combined modelling and experimental studies on metal halide perovskites [1,2] in two fundamental areas related to improving operational stability in optoelectronic devices: (i) iodide ion transport properties and the effects of mixed Pb-Sn compositions, as there is limited understanding of the impact of Sn substitution on the ion dynamics of halide perovskites; (ii) insights into passivating perovskites with molecular additives including surface interactions and dynamics; here we find strong binding of certain molecular passivators at undercoordinated surface Pb ions adjacent to iodide vacancies and thereby promoting surface passivation.

[1] Y.H. Lin, M.S. Islam, H.J. Snaith et al., Science, 384, 767 (2024); R. Wang, B. Saunders et al., Energy Environ. Sci., 16, 2646 (2023).

[2] K. Dey, M.S. Islam, S.D. Stranks et al., Energy Environ. Sci., 17, 760 (2024); A.N. Arber et al., Chem. Mater., 37, 12, 4416 (2025)

A drift-diffusion model is developed to calculate the spectral response, current-voltage characteristics and conversion efficiency of a thin film metal halide perovskite-silicon tandem solar cell. The model is based on solving carrier continuity partial differential equations to obtain the spatial distribution and time evolution of electron and hole densities, coupled with Poisson’s equation in order to get the electric field and potential influence on carrier drift considering the doped charges in layers of the device.

The photogeneration of carriers is dependent upon the absorption coefficient and normal incident reflectivity. Drift and diffusion currents correspond to the flow of carriers due to the electric field and concentration gradient, which depend on carrier mobilities, recombination mechanisms both at the surface and within the device [1-3], and absorption-reflection coefficients derived from the dielectric function of the perovskite and silicon materials [4-6], with temperature effects arising from the variation of energy levels due to lattice expansion and contraction [7].

The structure of the modeled two-terminal solar cell consists of spiro-OMeTAD/CH₃NH₄PbI₃/SnO₂ layers in tandem over a p⁺⁺/n/ n⁺⁺ silicon cell. Ohmic contacts are assumed at the front and back of the device with corresponding boundary conditions for carrier densities and potential. Results have been compared to similar perovskite-silicon tandem solar cells with high conversion efficiency [8-11].

The equations were numerically solved employing both finite difference and finite element discretization methods using MATLAB and COMSOL Multiphysics software.

For performance enhancement, the solar cell quantum efficiency, fill factor, conversion efficiency and open circuit voltage have been evaluated for different values of structural parameters such as layer depth and doping, as well as operating variables such as temperature and photon energy, in order to decrease charge accumulation and carrier recombination and approach the Shockley-Queisser efficiency limit.

Improving the photocatalytic performance remains a key challenge in perovskite nanocrystal-based systems.[1] One effective strategy is to introduce a semiconductor-semiconductor electronic junction, designed to promote charge separation at the interface and increase the accessibility of photogenerated carriers.[2] Colloidal nanocrystal heterostructures, that are nanoparticles composed of at least two distinct materials sharing an interface, emerge in this context as highly promising solution. Indeed, proper band alignment at the heterojunction can suppress charge recombination, thus improving the overall efficiency of the photocatalytic process.[3]

To this end, we optimized the synthesis of a family of perovskite-chalcohalide heterostructures based on CsPbX₃/Pb₄S₃X₂ (X= Cl, Br, I).[4] These semiconductor-semiconductor junctions have recently attracted considerable attention as they can effectively convert sunlight into electron-hole pairs, which are then separated through the electronic structure of the material. By combining absorption and ultraviolet photoelectron spectroscopies, we characterized the band alignments of these materials, which can be tuned through the halide composition. This compositional control allows for a variety of different band configurations, suitable for many photocatalytic reactions. 

As a proof-of-concept, we evaluated the heterostructures performances in the photooxidative coupling of p-substituted thiophenols [5] under visible-light irradiation, at room temperature, in air and in the absence of a sacrificial electron donor. Our CsPbBr₃/Pb₄S₃Br₂ heterostructures achieved up to 94 % selectivity toward disulfide formation in 90 minutes when using p-OCH₃ thiophenol. We also propose a plausible mechanism for this reaction, based on experiments with several scavenger species such as 1,4 benzoquinone (superoxide anion scavenger), N,N-diisopropylethylamine (hole scavenger) and the radical trapping agent 2,2′,6,6′-tetramethylpiperidine-1-oxyl (TEMPO). These results highlight the crucial role of the type-II heterojunction in promoting charge separation and the efficient electron delocalization across semiconductor domains. This synergistic behavior enhances the overall reduction capability of the heterostructures nanoparticles, thus improving their photocatalytic performances. These promising results pave the way to further photocatalytic investigations and are a significant first step towards the real-world application of these complex yet fascinating heterostructures.
 

Hybrid AMX₃ perovskites (A=Cs, CH₃NH₃; M=Sn, Pb; X=halide) have in the last years revolutionized the scenario of photovoltaic technologies. Despite the extremely fast progress, the materials electronic properties which are key to the performance are relatively little understood. We developed an effective GW method incorporating spin-orbit coupling [1] which allows us to accurately model the electronic, optical and transport properties of halide perovskites, opening the way to new materials design. In parallel, a series of different strategies will be reported to increase the device stability and efficiency.[2] While instability in aqueous environment has long impeded employment of metal halide perovskites for heterogeneous photocatalysis, recent reports have shown that some particular tin halide perovskites (THPs) can be water-stable and active in photocatalytic hydrogen production. To unravel the mechanistic details underlying the photocatalytic activity of THPs, we compare the reactivity of the water-stable and active DMASnBr₃ (DMA = dimethylammonium) perovskite against prototypical MASnI₃ and MASnBr₃ compounds (MA = methylammonium), employing advanced electronic–structure calculations. We find that the binding energy of electron polarons at the surface of THPs, driven by the conduction band energetics, is cardinal for photocatalytic hydrogen reduction.[3] In this framework, the interplay between the A-site cation and halogen is found to play a key role in defining the photoreactivity of the material by tuning the perovskite electronic energy levels. Our study, by elucidating the key steps of the reaction, may assist the development of more stable and efficient materials for photocatalytic hydrogen reduction. We report a report is made on a composite system including a double perovskite, Cs₂AgBiCl₆/g-C₃N₄, used in parallel for solar-driven hydrogen generation and nitrogen reduction, quantified by a rigorous analytical approach. [4] Finally, a new approach for enantioselective synthesis has been reported with chiral perovskite catalyst. The overall picture of our theoretical investigations underlines a crucial role of computational investigation, casting the possibility of performing predictive modeling simulations, in which the properties of a given system are simulated even before the materials laboratory synthesis and characterization. At the same time, computer simulations are shown to offer the required atomistic insight into hitherto inaccessible experimental observables.

The dual electronic-ionic nature of perovskite materials has greatly complicated the characterisation of perovskite devices; impacting almost all 'standard' electrochemical characterisation techniques. For example, when ions move on the timescale of current-voltage measurements, they can act to modify carrier recombination rates and carrier extraction, influencing the shape of the response. Ions can also modify fast measurements, where the ‘frozen in’ ion distribution impacts the electronic response of the device. It is widely assumed in the literature that the presence of these mobile ions is bad for e.g. perovskite solar cells, whether leading to field screening that lowers efficiency or contributing to material degradation.  In this talk I will consider whether ions are always bad for our devices, or whether they can also be beneficial. I will introduce our electrochemical measurements where we use ions as diagnostic probes inside perovskite solar cells. Finally, the talk will look at whether we can design devices differently to better take advantage of the intrinsic ionic properties of perovskite materials.

Two-dimensional (2D) layered hybrid organic-inorganic perovskites (HOIPs) have emerged as promising materials for optoelectronic devices such as solar cells, LEDs, lasers, and photodetectors. Compared to their three-dimensional counterparts, 2D HOIPs offer superior environmental stability and remarkable structural tunability. Their assembly is largely dictated by the choice of organic ammonium cations, which traditionally have little direct impact on optical or electronic properties. However, incorporating electroactive cations has recently enabled hybrids with extended absorption, improved out-of-plane charge transport, and lower exciton binding energies.[1]

In 2023, we demonstrated that carbazole-based ammonium cations with varying alkyl spacer lengths (Cz-x) can tune the properties of 2D lead iodide HOIPs. Shorter spacers enhanced electronic coupling between organic and inorganic layers, and light-induced charge transfer was observed for all tested lengths (three, four, and five carbons). The shortest spacer (Cz-3) exhibited a distinct interlayer charge-transfer state and delivered the highest out-of-plane mobility, outperforming a reference system based on phenylethylammonium (PEA).[2]

Building on this, our recent work investigates how molecular design shapes the optical and electronic behavior of low-dimensional HOIPs. We compared two hybrids that share a carbazole-inspired (dibenzocarbazole) cation but differ in the connectivity of their lead iodide framework (corner vs. edge-sharing). These structural variations shift energy alignment between organic and inorganic states, altering charge-transfer dynamics and enabling new pathways.[3] In parallel, we resolved crystal structures of 2D HOIPs with pyrene-based electroactive cations and, through combined experimental-computational analysis, revealed that interlayer electronic coupling is highly sensitive to the orientation of the organic core relative to the inorganic lattice.[4]

Tin halide perovskites (THP) are one of the most promising and less toxic alternatives to lead-based perovskite solar cells. These materials have gained increasing interest in recent decades due to their exceptional optoelectronic properties and related tunability, straightforward processing and high efficiencies, approachable to silicon solar cell’s ones. Despite the outstanding properties of THPs, one of the main disadvantages of this class of materials is the high concentration of defects present in the bulk, due to Sn (II) instability and intrinsic tendency to oxidise over time. The consequent self p-doping of the perovskite leads to increased free-charge recombination rate and reduced efficiency of solar cells. Engineering the surface passivation of tin perovskites is one of the key strategies to address both stability and performance enhancement of solar cells, for example by using additives, such as tin halides or hydrazine, which is a potent reducing agent able to compensate for tin vacancies.

Among the various strategies explored, mainly solvent-based, we investigated for the first time an innovative use of plasma as a solvent-free, reproducible and scalable approach, 1,2 to gently modify the perovskite surface and reduce surface defects. A preliminary study focused on nitrogen-based plasma treatment, applied to a DMSO-free FASnI3 perovskite surface 3. The mild nature of the plasma process enabled subtle surface modifications, effectively suppressing the natural tendency of tin (II) to oxidize, as confirmed by the XPS analysis performed on aged films. Subsequently, we extended this approach to a FASnI2.7Br0.3 perovskite, employing a plasma generated from a mixture of N2 and H2 gases. Owing to the reducing character of hydrogen-based plasma, we observed a notable enhancement in device performance, accompanied by increased photoluminescence and reduced non-radiative recombination. The reactive hydrogen species generated within the plasma interact with the perovskite surface, mitigating carrier losses associated with self-doping, thereby contributing to improved device efficiency. 4

These studies establish the basis for a novel application of plasma technology to enhance tin-based perovskite solar cells, offering an approach that is not only effective but also readily scalable for industrial implementation.

Armenise, V., Covella, S., Fracassi, F., Colella, S. & Listorti, A. Plasma‐Based Technologies for Halide Perovskite Photovoltaics. Solar RRL (2024) doi:10.1002/solr.202400178.

Perrotta, A. et al. Plasma-Driven Atomic-Scale Tuning of Metal Halide Perovskite Surfaces: Rationale and Photovoltaic Application. Solar RRL (2023) doi:10.1002/solr.202300345.

Covella, S. et al. Plasma-Based Modification of Tin Halide Perovskite Interfaces for Photovoltaic Applications. ACS Appl Mater Interfaces (2024) doi:10.1021/acsami.4c09637.

Covella, S. et al. manuscript in preparation.

Hybrid organic-inorganic halide perovskite materials have become one of the leading semiconductors in optoelectronics and the emerging field of optoionics. However, their instability under device operating conditions, such as voltage bias and light, limits their practical applications.[1] Moreover, these materials primarily contain toxic lead, which can be detrimental to the environment.[2–4] These critical setbacks can be overcome by incorporating tailored organic moieties into lead-free halide perovskite frameworks to form hybrid low-dimensional or layered perovskite architectures that are more resilient under operating conditions.[2–4] We rely on supramolecular engineering to develop a new generation of such lead-free halide perovskite materials, including mixed-dimensional tin-based perovskites[2–3] and layered double perovskite analogues,[4] as well as metal-free perovskite alternatives.[5] Finally, we explore their structural and opto(electro)ionic characteristics to demonstrate the utility in photovoltaics and neuromorphic systems toward more sustainable technologies.

Halide Perovskites have emerged as a revolutionary class of semiconductors, garnering intensive interest due to their exceptional and tunable optoelectronic properties, including high photoluminescence quantum yield (PLQY), high carrier mobility, and bandgap tunability across the visible spectrum. This versatility is demonstrated across various morphologies, nanocrystals (NCs), powders, thin films, and large single crystals, each offering unique advantages for specific applications. This presentation provides a comprehensive investigation into the controlled synthesis and advanced applications of halide perovskite materials, specially focusing in Pb-free systems. We show examples of the use of perovskite nanocrystal for the preparation of Light Emitting Diodes (LEDs), The synthesis of powder as precursors for thin film formation and for application in LEDs, we will also investigate chiral perovskites. Single crystals have been synthesized by hydrothermal methodology with a without confined work. Overall, this study highlights the structural and compositional engineering required to exploit the full potential of halide perovskites, from nanoscopic to macroscopic forms, as a fundamental material system poised to drive breakthroughs in efficient illumination, coherent light generation, and sustainable chemical synthesis.

Silver bismuth iodobismuthates have emerged as promising lead-free, environmentally friendly materials for photovoltaic applications. Within the AgI–BiI₃–CuI compositional space, Cu₂AgBiI₆ (CABI) is particularly attractive due to its high absorption coefficient, relatively low exciton binding energy, and wide direct bandgap, making it suitable for tandem solar cells and indoor photovoltaics. The electronic properties of CABI are strongly influenced by its lattice structure, which can be modified by secondary phases formed during synthesis. In this study, we employ halide engineering through chlorine incorporation to enhance both the photovoltaic performance and stability of CABI. We investigate the relationship between anisotropic lattice modifications, film morphology, and optoelectronic properties in CABI-Cl. Mixed-halide CABI solar cells exhibit higher photocurrents under both 1 sun AM1.5 illumination and 1000 lux white LED conditions. Notably, while the open-circuit voltage (V_oc) under 1 sun decreases with increasing chlorine content, it improves under indoor lighting conditions. Impedance spectroscopy reveals that these effects are linked to changes in recombination dynamics, shunt resistance, and interfacial charge transfer. Furthermore, in situ X-ray diffraction coupled with maximum power point tracking demonstrates that CABI-Cl retains its crystal integrity over time, whereas pristine CABI undergoes degradation and phase segregation. These results provide new insights into the impact of halide engineering on CABI, highlighting its potential for stable and efficient indoor photovoltaic applications.

Lead-free perovskite-inspired materials are emerging as attractive absorbers for sustainable photovoltaics in outdoor and indoor Internet-of-Things applications (1-2). Among them, Cs₂AgBi₂I₉ is a particularly promising candidate owing to its wide bandgap (~1.78 eV), strong blue-rich absorption, low exciton binding energy (~40 meV), weak electron–phonon coupling and large-polaron formation, which together favor efficient charge generation, transport and defect tolerance (1). Here we show that targeted hole-transport-layer (HTL) engineering is key to unlocking the photovoltaic potential of Cs₂AgBi₂I₉ under both 1-sun and indoor white LED (WLED) illumination.

We study mesoscopic n–i–p devices with the architecture glass/FTO/c-TiO₂/mp-TiO₂/ Cs₂AgBi₂I₉/HTL/Au and compare the conventional small-molecule HTL Spiro-OMeTAD with conjugated polymer HTLs, focusing on a fluorinated benzothiadiazole-based polymer (FBT, PPDT2FBT). FBT provides more favourable energetic alignment with the Cs₂AgBi₂I₉ valence band, as well as improved film coverage and interfacial contact. Consequently, FBT-based devices exhibit enhanced short-circuit current density, open-circuit voltage and fill factor compared with Spiro-OMeTAD- and PCPDTBT-based references. The champion FBT device delivers a power conversion efficiency (PCE) of 3.96% under AM 1.5G illumination, representing a substantial advance over earlier Cs₂AgBi₂I₉ solar cells without HTL optimisation.

The benefit of FBT HTLs is even more pronounced under indoor conditions. Under 1000 lux WLED illumination (6500 K), FBT-based devices achieve an indoor PCE close to 9%, demonstrating efficient harvesting of low-intensity blue-rich spectra. High fill factors and open-circuit voltages are retained as the light intensity decreases, underscoring their suitability for practical indoor operation. These efficiencies remain below the spectroscopically limited maximum predicted for Cs₂AgBi₂I₉ under WLED spectra, indicating substantial headroom for further performance gains via combined absorber and interface engineering.

To elucidate the origin of enhanced performance, we employ electrochemical impedance spectroscopy (EIS), transient photovoltage (TPV), transient photocurrent (TPC) and Capacitance-Voltage (CV). EIS shows higher recombination resistance and reduced low-frequency capacitance in FBT-based cells, evidencing suppressed non-radiative recombination and slower ion migration at the Cs₂AgBi₂I₉/HTL interface. TPV and TPC reveal longer carrier lifetimes, faster charge extraction and lower series resistance, while CV confirms improved energetic alignment, a larger built-in potential and reduced interfacial trap density.

Overall, combining the favourable bulk properties of Cs₂AgBi₂I₉ with application-driven HTL engineering enables a clear step-change in performance for lead-free perovskite-inspired photovoltaics. By translating interface design principles from lead-halide perovskites to this bismuth-based analogue, we outline a general blueprint for engineering charge-selective contacts in perovskite-inspired absorbers, with FBT emerging as a powerful HTL platform to bridge materials design and device application and to narrow the gap to the theoretical indoor efficiency limit of this lead-free absorber.

In this talk, we discuss a lead-free photovoltaic absorber candidate, CsBiSCl₂, which has recently attracted attention following a report of >10% power conversion efficiency in solution-processed thin-film solar cells [1]. If repeatable, this would represent a highly promising breakaway material in the field of lead-free photovoltaics. While other works speculated CsBiSCl₂ may adopt a perovskite structure, its true structure remains unverified, and the synthetic route used to produce the reported films has not been verified. We conduct a comprehensive investigation into the Cs–Bi–S–Cl system, integrating global structure search with targeted experimental synthesis to determine whether CsBiSCl₂ is a realistic photovoltaic candidate.

Using ab initio Random Structure Searching accelerated by a bespoke Ephemeral Data Derived Potential, we explore the Cs-Bi-S-Cl potential-energy surface and identify a four-formula-unit CsBiSCl₂ orthorhombic Pnma structure lying 2.4 meV per atom above the convex hull. This phase is dynamically stable and represents an energetically isolated and plausible CsBiSCl₂ polymorph. Perovskite-derived structures (e.g. cubic, tetragonal) were found to be energetically implausible, and simulated diffraction patterns bear little resemblance to those attributed to CsBiSCl₂ in previous reports.

Attempting to repeat the previously reported solution synthesis route for thin-film fabrication, we find that the intermediate “DMABiS₂” precursor does not readily form. Instead, the reaction yields Bi₂S,, alongside persistent iodide residues that cannot be removed through solvent washing. Thin films prepared from this intermediate contain mixtures of Cs₃Bi₂I₉ and Bi₂S₃, and show no evidence of chloride incorporation. Parallel solid-state reactions performed in the absence of iodine likewise fail to produce CsBiSCl₂, instead forming only binary and ternary phases that lie on the convex hull.

Taken together, these results demonstrate that CsBiSCl₂ is difficult to access synthetically, metastable with respect to competing phases, and unlikely to be an effective absorber in a solar cell. We therefore caution against further investigation into this material. This work has recently been published in EES Solar [2]

[1] The Journal of Physical Chemistry Letters 2024 15 (12), 3383-3389 - DOI: 10.1021/acs.jpclett.4c00310

[2] EES Sol., 2025, Advance Article - DOI: 10.1039/D5EL00157A

Hybrid metal halide perovskites are the most promising candidates for high-performance multi-junction solar cells that surpass the fundamental efficiency limits of traditional devices. Furthermore, their bandgap tunability, high radiative quantum yields, and defect tolerance also make them excellent light emitters. Two-dimensional (2D) perovskite semiconductors have promising prospects for enhancing the stability of perovskite-based photovoltaic devices. In addition, these low-dimensional materials with electronic confinement offer further opportunities in light emission and quantum technologies. However, their technological applications still require a comprehensive understanding of the nature of charge carriers and their transport mechanisms.

This talk will show how time-resolved optical spectroscopy can be employed to investigate charge-carrier dynamics, exciton formation dynamics, transport properties and transfer mechanisms in perovskite semiconductors. We will see how 2D perovskites show band transport with high in-plane mobilities that give rise to efficient long-range conductivity, despite quantum confinement. We show how the organic cation moderates the coupling of charge carriers to optical phonon modes, impacting the charge-carrier mobilities. We demonstrate the exciton formation dynamics over the picosecond timescale using a combination of terahertz and transient absorption spectroscopy, revealing a long-living population of free charge-carriers that greatly surpasses the theoretical predictions of the Saha equation even at temperatures as low as 4K. Finally, we look at charge dynamics in mixed-phase 2D/3D perovskite films, investigating the effect of different organic cations and additives introduced in the fabrication process. Using a combination of experimental investigations and numerical modelling, we show the interplay of trap-assisted recombination and charge transfer dynamics, and investigate the existence of selective hole and electron transfer pathways.

The utilization of a mesoporous material with a narrow pore size distribution constitutes an alternative method to the synthesis of ABX3 quantum dots (QD) due to the ease of processing and the excellent crystallinity and optical properties of the nanocrystals obtained. [1] The ligand-free QDs also possess a pristine surface, which renders them suitable for different functionalization strategies and reaction inside the pores.[2] In this study, we present a pre- and a post-functionalization approach that allowed us to developed light emitting application devices. First, we have synthesized FAPbBr3 QDs within the pore network of high optical quality SiO2 porous films, with the objective of obtaining high optical quality films with strong emission.[3] Subsequent to this, a post-functionalization with PMMA was performed, allowing emitting films with ultrapure colour and PLQY as high as 89% to be obtained. This system was employed for the integration of the components into colour conversion devices. On the other hand, the development of a light-emitting device necessitates an efficient charge transport by percolation through the QD network. In this instance, a pre-functionalization approach was employed, utilizing a combination of linear and branched amines in conjunction with crown ether.[4] This additive approach contributes to a substantial enhancement in device performance, leading to high luminance exceeding and a significant improvement in operational stability.

Two-dimensional lead halide perovskites (2DPs) offer chemical compatibility with three-dimensional perovskites and enhanced stability, which are attractive for applications in photovoltaic and light-emitting devices^[1-2]. However, such lowered structural dimensionality causes increased excitonic effects and highly anisotropic charge-carrier transport^[3-4]. Determining the diffusivity of excitations, in particular for out-of-plane or inter-layer transport, is therefore crucial, yet challenging to achieve. Here, we demonstrate an effective method for monitoring inter-layer diffusion of photoexcitations in (PEA)₂PbI₄ thin films by tracking time-dependent changes in photoluminescence spectra induced by photon reabsorption effects.^[5] Through selective photoexcitation from either substrate- or air-side of the films we reveal differences in diffusion dynamics encountered through the film profile. We extract time-dependent diffusion coefficients from spectral dynamics through a one-dimensional diffusion model coupled with an interference correction for refractive index variations arising from the strong excitonic resonance of 2DPs.^[5] Such analysis, together with structural probes, shows that minute misalignment of 2DPs planes occurs at distances far from the substrate, where efficient in-plane transport consequently overshadows the less efficient out-of-plane transport in the direction perpendicular to the substrate. Through detailed analysis, we determine a low out-of-plane excitation diffusion coefficient of (0.26 ± 0.03) × 10^-4 cm²s^-1, consistent with a diffusion anisotropy of ~4 orders of magnitude^[5].

Metal halide perovskites have revolutionized next-generation optoelectronics and photovoltaics due to their exceptional semiconducting properties and low-cost processing.[1] Yet, their widespread deployment remains hindered by long-term instability and the environmental concerns associated with lead-based compositions.[2]To overcome these limitations, low-dimensional (2D) halide perovskites have emerged as promising alternatives based on incorporation of tailored organic spacer cations that enhance structural robustness. Building on this concept, layered double perovskites (LDPs) introduce a lead-free framework based on general formula of S’mAn-1(MIMIII)nX3n+1 (S’ organic spacer cation, A central cation (e.g., Cs+), MI monovalent cation (e.g., Ag+), MIII trivalent cation (e.g., Bi3+, In3+, Cu3+), and X halide anion), forming Ruddelsden-Popper or Dion-Jacobson phases that are comprised of mono (m = 2) or bifunctional (m = 1) organic spacer layers templating inorganic perovskite layers (n).[3] Their properties can be tailored by relying on supramolecular engineering (Figure 1), enabling precise molecular control over their structure and functionality, which remain underexploited.[4] In this work, we develop a new generation of lead-free 2D LDPs through supramolecular design and mechanochemical synthesis, enabling precise molecular control over their structure and functionality. We investigate their structural, electronic, and optical characteristics through a suite of complementary techniques, uncovering key design principles that dictate stability and performance. The versatility of spacer molecules opens pathways not only for more sustainable and stable perovskite photovoltaics, but also for broader optoelectronic applications such as neuromorphic computing.[5]

Despite significant progress, computational materials design still faces major challenges—particularly when simulating advanced and chemically complex materials with the accuracy of density functional theory (DFT) or beyond.^[1] To overcome these limitations, machine learning (ML) methods have gained considerable traction in recent years.

We have developed robust data-generation strategies to support the creation and benchmarking of new ML models.^[2] In this talk, I will highlight methods for large-scale quantum-chemical bonding analysis and workflows for ML interatomic potentials.

Our work demonstrates that quantum-chemical bonding properties can be incorporated into ML models to predict phononic properties.^[3] This approach enables large-scale validation of expected correlations—such as the link between bonding strength and force constants or thermal conductivities.

Furthermore, we have built an automated training framework for machine-learned interatomic potentials (autoplex).^[4] Initial workflows include random structure searches, suitable for general-purpose potentials, as well as specialized workflows targeting ML potentials with accurate phonon properties.

While atomistic simulations are highly effective for certain material properties, others—such as magnetism or synthesizability—remain challenging. In these cases, promising strategies include benchmarking established ab initio methods against chemical heuristics or developing new ML models primarily based on experimental data.^[5,6]

Achieving precise control over the crystalline phases and dimensionality of metal halide semiconductors is essential for optimizing the performance and long-term stability of perovskite-based optoelectronic devices. In this talk, I will present our recent advances in the rational design of 3D, 2D, and mixed 3D/2D metal halide structures, with particular attention to how different phases evolve over time in the presence of a variety of environmental and operational stressors. I will highlight how substrate characteristics and additives can modulate chemistry, structural phase purity, optoelectronic properties of halide perovskite, and how these changes impact device behavior. These insights will be discussed in the context of both solution- and vapor- deposition routes, providing a comparative perspective on phase selective synthesis strategies. Device applications including photovoltaics and exciton polaritons will be discussed in this talk. Overall, our findings offer generalizable guidelines for designing stable, phase-pure perovskite and perovskite-inspired materials for next-generation optoelectronic applications.

Perovskites are a remarkably versatile class of semiconducting materials, of composition ABX3. The material class features a wide variety of compositions that exhibit outstanding optoelectronic or ferroelectric properties. However, they often involve toxic metal ions, limiting their usability due to environmental and safety concerns. Recently, completely metal free perovskites have gathered attention. Rather than a metal, the B-site of the perovskite is taken by ammonium (NH4+), and the A-site by an organic divalent ion. Certain compositions of these materials have been shown to have ferroelectric properties, while their opto-electronic properties are still under investigation. Although metal free perovskites are much more environmentally friendly than their metal-containing counterparts, a new challenge arises: the precursors are not as easily soluble, making the synthesis more difficult.

In this talk, we present a facile mechanosynthesis of DABCONH4X3 (X = Cl, Br, I). Due to the nature of mechanosynthesis, the process is easily scalable and solvent free, avoiding solubility issues, and results in a phase-pure perovskite powder. The properties and quality of the powder is thoroughly analyzed using a wide variety of crystallographic and optical characterization techniques. Our findings highlight the potential of mechanochemistry to overcome the synthesis challenges of metal free perovskites, paving the way for their future application in sustainable (optoelectronic) devices.

The simplicity of this synthesis process and ability to tune the material via impact energy and composition significantly broadens the range of potential applications for these metal free halide perovskites.

In the recent years, the interest in chiral hybrid organic-inorganic metal halides (HOIMHs) for applications such as optoelectronics, spintronics, photodetection, energy harvesting and beyond has significantly increased, due to the promising absorption and subsequent emission of polarized light with enhanced tunability across the electromagnetic spectrum.[1,2] In these compounds, the insertion of chiral molecules inducing crystallization in noncentrosymmetric crystal structures allows for intriguing nonlinear optical properties in addition to the features mentioned above. Despite the significant scientific interest on such topic, several key questions still need to be addressed. On one hand, the research has widely centred on low dimensional systems only, i.e. from 2D to 0D, due to the steric hindrance usually displayed by the chiral organic cations, posing challenges in applications where a isotropic charge transport is demanded since the organic layers usually act as dielectrics.[3] On the other hand, the impact of octahedral distortions on the chiroptical behaviour is not well understood, although it is a crucial parameter for optimizing practical devices.

In this scenario, we have developed novel series of chiral HOIMHs by wisely tuning composition and dimensionality with the aim of unveiling the relationships between crystal structure and optical behaviour. By integrating the relatively narrow ditopic cation R/S-3-aminoquinuclidine (R/S-3AQ), the 3D corner-sharing (R/S-3AQ)Pb₂Br₆ compound has been attained along with its 2D counterpart (R/S-3AQ)₂PbBr₄·2Br, disclosing a delocalized photoexcitation resembling that of 3D materials and promising for charge transport along the three dimensions. Moreover, an increase of the chiral dissymmetry factor has been unveiled by lowering the dimensionality, in line with other corner-sharing 1D and 2D materials obtained in our laboratory. Starting from the 2D compound, the series (R/S-3AQ)₂MBr₄·2Br (M: Pb, Sn, Ge) has been synthesized, unveiling a Ruddlesden-Popper structure in all cases and demonstrating a significant octahedra distortion increase with the trend Pb^II < Sn^II < Ge^II. The same trend holds true for the series (R/S-3APD)PbX₄ - (R/S-3APD)SnX₄ - (R/S-3APD)₂GeX₄·2X (R/S-3APD : R/S-3-aminopiperidine; X: Br, I), where changes in the crystal structure have been observed by tuning the metal center but a corner-sharing connectivity was disclosed in the highly distorted tin- and germanium-based compounds. In contrast, a small distortion index change was reported for the two metal centers in the face-sharing (R/S-AMOL)MI₃ (R/S-AMOL: (2-R/S,2'-R/S)-1,1'-azanediylbis(butan-2-ol); M: Pb^II, Sn^II), indicating the role of octahedra connectivity in the octahedral tilting. With these examples, we aim to rationally investigate the structural factors concurring to optimal chiro-optical responses, providing new tools for the development of next-generation nonlinear functional materials.

Molecular inks, composed of reactive metal complexes, offer a versatile platform for the controlled synthesis of a large library of materials. By controlling the ink composition, a large variety of perovskite nanostructures including 2D Ruddlesden-Popper phases, 3D nanocrystals and epitaxial thin films can be selectively synthesized. Focusing on tin perovskites, due to their excellent optoelectronic properties, the effect of ligand coordination in tin iodide inks was studied using UV-Vis spectroscopy and ¹¹⁹Sn NMR. By tuning the ink composition, the formation of 2D Ruddlesden-Popper nanostructures or 3D FASnI­₃ perovskite nanocrystals could be selectively controlled. In parallel, tin chalcogenide molecular inks derived from reactive tin thiolate complexes were screened as platforms for the synthesis of epitaxial tin chalcogenide perovskite thin films. By coupling ¹¹⁹Sn NMR of the ink solutions with XRD of the annealed inks, the relationship between the tin precursor complex and the resulting nanostructure could be elucidated. Equipped with these insights, we show how the strategic design of tin based molecular inks can be used to synthesize targeted tin perovskite nanostructures, paving the way for next generation materials.