Metal halide perovskite solar cells have become a viable option for future renewable energy. Record single and tandem junction all-perovskite solar cells already provide power efficiencies of over ~26% and ~30%, respectively. The next target in photovoltaic energy conversion can possibly be met by developing all-perovskite multi-junction solar cells. These require highly efficient and stable perovskite sub-cells with bandgaps in the range between 1.2 and 2.3 eV. Especially for narrow and wide bandgap perovskites challenges remain in reducing the energy loss between bandgap and open-circuit voltage. Guided by ultra-sensitive photocurrent spectroscopy, absolute and time-resolved photoluminescence spectroscopy, and in combination with bulk and interface passivation strategies, it is possible to reduce non-radiative losses in each of the bandgap regions and achieve open-circuit voltages that approach and sometimes exceed 90% of the detailed balance limit. By monolithically stacking multiple perovskite sub cells with complementary bandgap using recombination junctions designed to provide near-zero electrical and optical losses, it is possible to fabricate monolithic multi-junction configurations with high power conversion efficiencies.

The presentation will review some recent results on the effect of carrier-lattice coupling and lattice matching on the optoelectronic properties of halide perovskites and their heterostructures and nanostructures. The early theoretical prediction and experimental demonstration of screening of electron-hole interactions by charge carrier-lattice coupling in 3D perovskites, was crucial to explain why photocarrier collection is possible at RT and how solar cell architectures could evolve toward much thicker active layers. The presentation will introduce recent methodological developments for the description of electron-coupling in the high temperature regime where lattice fluctuations are dominated by slow structural relaxations and strong anharmonicity. For the low-temperature regime, the importance of excitonic polarons will be stressed and new empirical and semi-empirical approaches will be described to account for the complexity of the carrier-lattice coupling in halide perovskites. Finally, the concept of lattice-matching will be presented and its predictive power will be demonstrated through a few experimental examples related to 2D/3D and multilayered halide perovskite heterostructures.  

Perovskite solar cells have demonstrated impressive efficiencies and affordable manufacturing costs, able to either compete with silicon solar cells or enhance them with tandem designs. [1] However, a well-known challenge with perovskite solar cells is their limited lifetime. The short life-cycle is further compounded by a lack of clear recycling strategies for perovskite solar cells. [1] Our goal in this study is to address the twin problem of short lifetime and lack of recycling methodology by demonstrating a way to revive degraded perovskite solar cells with green solution-based recycling technique.

Typically, the degradation of perovskite solar cells is caused by the deterioration of the perovskite layer itself. [2] The idea behind perovskite solar cell revival is to remove the degraded perovskite layer with a suitable solvent, in a way that the surrounding scaffold structure remains intact. The perovskite layer is then regenerated by infiltrating the scaffold with a fresh perovskite solution and annealing to remanufacture the cell. The main challenges in the revival process is thorough removal of degraded perovskite without damaging the scaffolding structure and ensuring sufficient revival rate.

We used perovskite solar cells with carbon electrodes due to their high stability and scalability. [3] The recycling was performed with green and non-toxic γ-Valerolactone (GVL) solvent [4] to maximise the sustainability of the process. Our preliminary results show that full 100 % revival rate is possible in champion cases but the technique requires further optimisation to reach these results consistently. Another important observation is that incorporating zirconia nanoparticles into the solar cell scaffold improves its robustness during degraded perovskite flushing. By embracing circular economy techniques, we can strive for more sustainable PV technologies.

Perovskite solar cells (PSCs) exhibit excellent efficiencies, but face challenges related to interfacial and environmental stability.¹ Here, we present two quantum dot (QD)-based strategies to address these limitations. In the first, CdS QDs are introduced as an interfacial layer between SnO₂ and the perovskite absorber. This approach reduces surface oxygen vacancies and hydroxyl groups, as confirmed by XPS, while Kelvin probe force microscopy reveals enhanced surface potential uniformity. Perovskite films grown on CdS-passivated SnO₂ show larger grain sizes and reduced PL intensity, suggesting improved charge extraction. Time-resolved PL confirms a significant increase in electron transfer rate leading to ~15% higher device efficiency. Previously, we also investigated halide exchange at the heterojunction between perovskite QDs and 3D perovskite films using in situ photoluminescence.² By extracting the activation energy of the Br-to-I exchange, we demonstrate its role in defect passivation and suppression of bimolecular recombination. QDs also enable favorable energy level alignment, enhancing hole extraction and stability under thermal stress. Their hydrophobic ligands further protect the perovskite against moisture ingress. In summary QD-based interfacial engineering strategies can improve both performance and durability of PSCs.

Hybrid organic inorganic perovskites (HOIPs) have started to emerge as leading materials for optoelectronic technologies including solar cells, photodetectors, lasers, and light‐emitting diodes. Within this family, two-dimensional (2D) layered HOIPs have attracted growing attention in recent years owing to their intrinsically superior environmental stability compared to most 3D HOIPs, as well as their remarkable structural and compositional tunability. The choice of organic ammonium cations, drawn from a vast library of candidate molecules, governs the assembly of these hybrid materials. Typically, this organic component does not directly influence the hybrid’s optical or electronic behavior; however, the incorporation of so-called electroactive organic cations has begun to receive considerable research interest.[1] For example, through the incorporation of such tailored organic cations, 2D HOIPs with an extended absorption spectrum, enhanced (out-of-plane) charge carrier transport, and reduced exciton binding energy have been obtained.

In 2023, we [2] showed that carbazole-based organic ammonium cations with different alkyl spacer lengths can be used to tune the optical and electronic properties of 2D lead iodide HOIPs. With decreasing spacer length, there was evidence for enhanced electronic coupling between the organic and inorganic layers. For all spacer lengths (3, 4, and 5 carbon spacers), light-induced charge transfer from the organic to the inorganic layer was detected. Specifically for the carbazole with the shortest spacer (Cz-3), an organic-inorganic (interlayer) charge transfer state was observed. The out-of-plane charge carrier transport was enhanced for all the carbazole-containing 2D HOIPs compared to that of the reference 2D HOIP containing an electronically inactive phenylethylammonium (PEA) cation, with the 2D HOIP based on Cz-3 possessing the highest charge carrier mobility.

In recent research, we build further on this work to gain a deeper understanding of the influence of molecular design on the optical and electronic properties of low-dimensional HOIPs. We compare the properties of two low-dimensional hybrids containing the same carbazole-inspired organic cation but with a different lead iodide inorganic framework. Depending on the connectivity of the octahedra, differences in the photoinduced charge transfer dynamics between the organic and inorganic layers are obtained, and other transfer pathways become available because of changes in relative energy alignment between organic and inorganic states. In other recent work, we studied 2D HOIPs containing electroactive organic cations for which we were able to determine the crystal structures to deduce detailed structure-property relationships. In a combined experimental-computational study, we show that the organic-inorganic interlayer electronic coupling is highly sensitive to the orientation of the organic core with respect to the inorganic framework.

With the advent of metal halide perovskites into the world of emerging sustainable semiconductors, a host of previously unprecedented applications has materialised. Aside from their raging success in photovoltaics, perovskites as applied to light emitting diodes (LEDs) and displays have gained particular interest due to their extremely facile bandgap tunability, directional and narrow-linewidth emission characteristics, high brightness, superior efficiencies and colour purity among other desirable features. However, most of the world records in terms of device efficiency and stability have been achieved on lab-scale pixels processed using solution-based techniques, primarily spin-coating. However, with its inherent advantages of scalability, reproducibility and precise thickness control, vacuum-based thermal evaporation provides an edge over solution-processing for all optoelectronic applications. Thus, as we move towards integrating sustainable semiconductors in consumer electronics, it is important to optimise thermal evaporation-based device fabrication, especially for large-area, flexible and niche applications for light sources and displays.

Following the recent advancements in vacuum-evaporated perovskite solar cells in our group, we have employed this technique to fabricate highly luminescent all-inorganic CsPbI₂Br perovskite films for red light emitting diodes (LEDs). By optimizing the growth conditions, we have been able to achieve unprecedented photoluminescence quantum efficiencies (PLQE) close to 20% under 1-sun equivalent conditions. Moreover, no sign of unwanted halide segregation has been observed under continuous illumination, thereby resulting in a stable PL emission in the wavelength range of 630-640 nm (pure-red emission). To understand the effect of deposition conditions on the resulting optoelectronic properties of evaporated perovskites, a range of fundamental characterization including intensity-dependent PLQE, fluence-dependent TRPL, widefield hyperspectral imaging, temperature-dependent PL, THz spectroscopy and transient photoconductivity measurements have also been conducted. Furthermore, X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) has been employed to obtain information on the chemical composition and electronic properties of the evaporated films. There is very little existing information on the structure-property relationships in evaporated films tailored to luminescence. To our knowledge, our work is the first of its kind to take into account various factors related to the unique morphology of these films and to study their concomittant effect on their optoelectronic behaviours. Finally, by extensive screening of the charge injection layers, we have been able to demonstrate proof-of-concept LEDs with external quantum efficiency >3% with turn-on voltage ~ 3V, which is a world record for evaporated red perovskite LEDs as of now.

Overshoot pulses of perovskite light-emitting diodes reveal two their operation regimes controlled by variations of driving voltage.

Rokas Gegevičius¹, Ignas Ledzinskas¹, Jevgenij Chmeliov¹, Iakov Goldberg², Robert Gehlhaar² Karim Elkhouly² and Vidmantas Gulbinas¹

¹Center for Physical Sciences and Technology, Sauletekio Ave. 3, LT-10257 Vilnius, Lithuania;

²IMEC, 3001 Leuven, Belgium;

In some applications, perovskite light-emitting diodes (PeLEDs) are expected to operate in pulsed mode. The generation of high-intensity light pulses requires substantial electrical pumping power, which can lead to deterioration of PeLED performance, its degradation or even damage. Contrarily, PeLEDs operating in a non-conventional regime, based on the so-called overshoot effect enables the generation of short, high-intensity, perfectly electrically synchronisable optical pulses while maintaining relatively low electrical pumping power. Here we demonstrate the generation of overshoot pulses (OSP) by FAPI PeLEDs with different perovskite layer thickness and analyse their dependence on alternating driving voltage and temperature.   The intensity and shape of the OSPs are determined not only by the voltage and duration of the pump pulses, but also by the offset voltage applied between the pump pulses, as well as the afterpulse voltage applied immediately after the end of the pump pulse. The offset voltage determines the distribution of the mobile ions, strongly affects the strength and spatial distribution of the internal electric field during the pump pulse action and thus is a crucial parameter determining evolution of the conventional electroluminescence intensity and generation of the OSPs. Meanwhile, the afterpulse voltage controls the intensity and duration of the OSPs.  The intensity of the OSPs increases strongly at temperatures below ~ 200 K. Mathematical modelling makes it possible to reproduce the electroluminescence dynamics and identifies two distinct PeLED operation regimes: one that facilitates OSP generation and another that prevents it.

Perovskite nanocrystals for bright and stable PeLEDs and LED arrays

Sarika Kumari,¹  Rafael S. Sánchez,¹ and Iván Mora-Seró^1, *

¹ Institute of Advanced Materials (INAM), Universitat Jaume I, 12006 Castelló, Spain.

Corresponding authors: (rasanche@uji.es, sero@uji.es)

Perovskite materials have been exploited for their applications in light emitting devices for several years since their first application in 2014¹. All inorganic PeNCs have been considered suitable materials for the light emitting diodes because of their properties like high colour purity, high PLQY, high carrier mobility and direct bandgap. High brightness is achieved by these colloidal Perovskite nanocrystals. We have aimed to fabricate the PeLEDs array for writing different characters by using the different PeLEDs pixels on the substrate. The PeLEDs were fabricated with high brightness up to 96,720 cd/m², T₅₀ of 90 min with 23 % EQE. In this work, we have used our already established synthesis protocol of high purity CsPbBr₃ NCs and the device fabrication process with architecture of ITO/PEDOT: PSS/Poly-TPD/CsPbBr₃ NCs/POT2T (40 nm) /LiF (1.0 nm)/Al (100 nm) is described in this report². 

The LED array devices containing 64 pixels by using the substrates of the area of 25 mm*25 mm as shown in Fig 1. In this stage, we have encountered the problem of turning up of the random pixels when trying to turn on one pixel which means that the device pixels were uncontrollable.

Lead-free halide perovskite quantum dots (QDs), including Sn-based and double perovskite QDs, have gained increasing attention as environmentally benign alternatives to lead-based perovskites for optoelectronic applications. These materials offer tunable bandgaps, strong light absorption, and excellent solution processability. However, achieving high crystal quality, defect tolerance, and efficient charge transport remains critical to unlocking their full potential.

In our recent studies, we have successfully synthesized phase-stable, low-defect Sn-based, Sn–Pb alloyed, and double perovskite QDs with enhanced photoluminescence quantum yields (PL QYs) and long carrier lifetimes [1–6]. Strategies such as Sn(IV) suppression and metal ion doping effectively minimized trap-mediated recombination and lattice distortion. Ultrafast transient absorption (TA) and time-resovled photoluminescence (TRPL) further revealed negligible electron or hole trapping, consistent with enhanced PL QY in optimized samples.

Focusing on double perovskite QDs, we developed Sb³⁺/Mn²⁺ co-doped Cs₂NaInCl₆ systems that exhibit efficient broadband white-light emission via self-trapped excitons. This co-doping approach not only induces white emission but also suppresses cation disorder, leading to PL QYs approaching 100%. To improve device performance, we replaced long-chain ligands with short-chain alternatives[8], which increased film conductivity by nearly 20-fold and reduced the hole-injection barrier by 0.4 eV. These improvements enabled light-emitting diodes (LEDs) with an external quantum efficiency (EQE) of 0.86%—the highest reported to date for double perovskite QD-based LEDs.

In this talk, we will present our recent progress in the synthesis, surface passivation, and photophysical characterization of lead-free perovskite QDs, and discuss their promising applications in optoelectronic devices such as LEDs. Our findings offer new insights into the design principles for achieving high-efficiency, stable, and environmentally friendly perovskite-based devices.

The discussed work deals with the control and characterization of spin degrees of freedom of photo-generated carriers in halide perovskite materials via magnetic doping. The materials under consideration, CsPbBr3 and PEA2PbI4, include the diluted concentration of Ni2+ or Mn2+ ions at a metal substitution position. Magnetic doping was implemented extensively in colloidal quantum dots, but with a limited way in the perovskite semiconductors, wherein the latter possesses other significant properties (e.g., dopant-carrier spin-exchange interactions, a large g-factor and extension of spin relaxation time).

Our work reports a thorough investigation of spin degrees of freedom in the mentioned materials, monitored by magneto-photoluminescence and optically detected magnetic resonance (ODMR) spectroscopies, which provide a significant information on exact location of host carriers and dopants, as well as examine interactions between them.

specifically, one project focused on a thorough investigation of the influence of Ni2+ dopants on the optical and magneto-optical properties of CsPbBr3 nano-cubes. The study implemented methodologies that are applied to halide perovskites for the first time, like steady-state and transient optically detected magnetic resonance (ODMR) spectroscopy, leading to significant advances that address long-standing debates in this field: (a) A direct identification of defect centers, in conflict with the widely assumed defect-free behavior of halide perovskites; (b) Direct observation of spin-exchange interactions between dopant unpaired electrons and photo-generated carriers, which has not been resolved previously. The extracted physical parameters from the ODMR experiments included: g-factors and their anisotropy, spin exchange interactions, angular momentum, carrier-dopant coupling constants, radiative and spin-lattice relaxation times. A second project focused on the influence of Mn2+ dopants in 2D MA2PbI4 and PEA2PbI4 single crystals, implementing the mentioned magneto-optically spectroscopies, focusing on the added values of the dopant to the so-called Rashba effect (an ongoing project).

The spin properties mentioned here undoubtedly can play an important role in the development of new spin-based technologies.

Exciton-polaritons are solid-state quasi-particles presenting properties lying in between light and matter such as non-linear interactions (from the excitonic Coulomb repulsion), fast propagation (arising from the photonic component). The interest of polaritons¹ spans from the fundamentals of correlated light-matter interaction to emergent applications such as ultra-thin lasers, optical amplifiers, logic gates and even emulators of lattice-like Hamiltonians.

We implement a monolithic microcavity composed of two mirrors, a bottom distributed Bragg reflector made of 10 alternating SiO₂ and TiO₂ layer pairs, and a top silver mirror of ~20 nm thickness. Between them, we deposit 2D layered phenethylammonium (PEA₂PbI₄) perovskites of ~100 nm thickness. As a cavity spacer, we use PMMA polymer, whose thickness is controllably tuned to bring the photonic and excitonic modes in energy resonance. The vertical design of the planar microcavity system is guided by transfer matrix method simulations. We measure the dispersion relation of polaritons under white light (left-hand side of Fig. 1 panels) and weak, non-resonant driving for three exciton-photon energy detunings (right-hand side of Fig. 1 panels)  resulting from the different PMMA thicknesses.

Interestingly, under circularly polarized, non-resonant (3.06 eV) CW laser driving, the lower polariton branch emission follows the circular pump polarization. Under the same excitation conditions, the exciton does not exhibit a circular degree of polarization, due to the slower decay of bare excitons, as opposed to exciton-polaritons. Finally, photostability experiments for long exposure excitation time scales (min) also reveal that the polariton emission is more resilient than bare excitons, indicating that strong coupling serves as a protection for excitons².

Screening the Local Structural Space in Lead Halide Perovskites 
 
Milos Dubajic, Samuel D. Stranks
 
 
 
Recent investigations have revealed that halide perovskites are best described at equilibrium as containing short-lived (picosecond), nanoscale (a few unit cells) domains of lower symmetry than the bulk average structure, where the corner-sharing lead halide octahedra are tilted relative to each other. In this study, we employ Brillouin spectroscopy together with X-ray diffuse scattering to systematically screen the local structural space across various perovskite compositions. We demonstrate that, in selected compositions, finite polarization vector fields are generated in the vicinity of these nanodomains. Furthermore, by controlling the temperature, we modulate both the size and the dynamics of the nanodomains, a change that is directly observable as a softening of the elastic stiffness tensor upon cooling. Our precise determination of the symmetry and shape of these nanodomains across different average phases highlights the tunability achieved through A-site cation and halide anion substitutions. Given the strong correlation between these local structures and the macroscopic performance of these materials, our findings pave the way for the future tuning of optoelectronic properties in lead halide perovskite devices.

Hybrid organic-inorganic perovskites (HOIPs) are a highly promising class of materials for advanced applications in nonlinear optics, especially for second-harmonic generation (SHG), a process that occurs only in materials with a noncentrosymmetric crystal structure. The breaking of inversion symmetry is not only crucial for SHG but also enables other important functionalities, including ferroelectricity and the bulk photovoltaic effect (BPVE). The BPVE can produce ultrafast, dissipation-less photocurrents without the need for heterostructures or interfaces, making these materials highly attractive for next-generation photovoltaics and self-powered photodetectors

A sure way to achieve noncentrosymmetry is through the incorporation of homochiral organic ligands, which inherently forces the material to crystallize in one of the chiral Sohncke space groups. While this is a guaranteed route to an SHG-active material, it limits the structural possibilities to chiral space groups, among which only a part is polar. Thus, non-chiral acentric structures can crystallize in a broader range of noncentrosymmetric space groups, potentially offering greater chance for e.g. ferroelectric properties.

This presentation will provide an overview of rational design strategies to engineer SHG-active hybrid perovskites and key techniques and challenges for comprehensive characterization of their temperature-dependent SHG responses. We will focus on crystal engineering techniques that induce noncentrosymmetry in achiral systems. Ligand halogenation, also known as halogen engineering, has proven to be a potent tool for inducing polar distortions and symmetry breaking, often through the formation of halogen bonds.[1] The introduction of specific organic cations, such as methylhydrazinium (MHy⁺), while being a nonchiral molecule, represents another powerful approach known to promote the formation of noncentrosymmetric strcture in lead halide perovskites.[2-4] These organic cations can induce structural distortions and break inversion symmetry through their unique geometric and electronic properties.

Through a series of case studies, this talk will demonstrate how these synthetic approaches can be leveraged to design and obtain novel hybrid perovskites with tailored SHG activity, feature multinoncentrosymmetry (the presence of multiple distinct temperature-dependent noncentrosymmetric crystal phases within a single material system) and other noncentrosymmetry-induced functionalities. The combination of rational crystal engineering strategies and comprehensive variable-temperature characterization provides a pathway toward developing next-generation nonlinear optical materials with enhanced performance characteristics.

Understanding ionic dynamics in halide perovskites is critical for improving the performance and stability of devices, including solar cells, X-ray detectors, and memristors [1-3]. Conventional frequency-domain optoelectrical techniques often suffer from contact-related effects and interfacial recombination, which can obscure the signatures of ionic dynamics [4-6]. Optical approaches such as time-resolved photoluminescence can avoid these limitations, but are rarely used to study ionic responses due to the difficulty of separating overlapping contributions with similar timescales in the time domain [7].

In this talk, we present intensity-modulated photoluminescence spectroscopy (IMPLS) as a fully optical method to probe dynamic behavior across a broad range of timescales. By analyzing the phase and amplitude of the PL response as a function of the modulated excitation frequency, IMPLS enables the identification of distinct mechanisms based on their characteristic times. We demonstrate its use on halide perovskite films and compare the results to standard optoelectronic techniques [8]. This approach provides new insights into slow processes such as ion migration and defect dynamics, and opens possibilities for broader material and device characterization.

The efficiency of halide perovskite solar cells has been continuously rising over the past decade to values above 26%. Future technological development will have to deal with issues of device stability but also thrive to further minimize efficiency-limiting loss processes in the bulk and at interfaces within the cell stack. The identification and understanding of electrical losses will require the ability to characterize solar cells and multilayer stacks with a variety of steady-state, time-domain and frequency-domain techniques that are sensitive to the transport and recombination of charge carriers. Especially, time- and frequency-domain techniques offer a large amount of information on dynamic processes in the solar cell, while posing a substantial challenge in terms of the complexity of data analysis.¹ Here, I discuss our recent work related to transient photoluminescence (TPL) applied to halide perovskites. I show that by using extremely low repetition rates and a gated CCD camera, we can obtain high dynamic range TPL data with continuously changing decay times that exceed 100µs.^2-3 Furthermore, I show that by changing the repetition rate, basically any decay time can be extracted from one sample, whereby the extracted decay time is approximately the inverse repetition rate. I explain why this is the case both mathematically and physically. Further, I present recent results on the determination of diffusion lengths as a function of injection level⁴ using the reabsorption effect.

In the recent years, chiral hybrid organic-inorganic perovskites (HOIPs) have gained a huge interest for applications in optoelectronics, spintronics, photodetection, energy harvesting and beyond, allowing for the absorption and subsequent emission of polarized light with enhanced tunability across the electromagnetic spectrum [1,2]. So far the research has widely centered on low dimensional systems, such as 2D and quasi-2D HOIPs, with fewer examples of 1D and 0D ones, demonstrating significant chiroptoelectronic and spin-polarization features. However, expanding the corner-sharing interconnection of the inorganic motif to the three dimensions is highly demanded for practical applications where a isotropic charge transport is demanded, since the organic layers usually behave as dielectrics in low-dimensional systems [3]. However, the steric constrains imposed by the bulky chiral cations usually prevent the accordance with the Goldschmidt tolerance factor.
In this scenario, we have developed novel chiral HOIPs derivatives displaying a 3D corner-sharing octahedral interconnection closely resembling that of prototypical perovskites [4]. This architecture is attained by integrating the relatively small ditopic cation R/S-3-aminoquinuclidine (R/S-3AQ) yielding the (R/S-3AQ)Pb₂Br₆ materials, featuring a direct bandgap and a isotropic electronic band structure in agreement with a 3D delocalized excitation, in stark contrast with the Ruddlesden-Popper counterpart (R/S-3AQ)₂PbBr₄·2Br showing typical 2D characteristics. The experimentally determined chiral anisotropy factor aligns well with theoretical predictions based on first-principles calculations for this type of chiral structure, and a pronounced Rashba-type spin splitting is detected in the conduction band, expected for a non-centrosymmetric semiconductor and driven by the combined effects of spin-orbit coupling and structural chirality. A reduced exciton binding energy was determined in the 3D material, index of a more stable excitonic population and favorable for an increased charge transport. Conductivity and spin-relaxation measurements are currently in progress to further assess the influence of these electronic properties on the material's potential for optoelectronic and spintronic applications. Thanks to their structural chirality and broad chemical tunability, these 3D chiral HOIP derivatives emerge as a promising foundation for the development of next-generation nonlinear functional materials.

Zero-dimensional copper halides Cs₃Cu₂X₅ (X = Cl, Br, I) are promising materials for optoelectronic applications due to their high photoluminescence efficiency, stability, and large Stokes shifts [1]. In this work, we use density functional theory to uncover the chemical bonding origin of the Stokes shift in these materials.

Upon excitation, the [Cu₂X₅]³⁻ cluster undergoes strong local distortions, including shortened Cu–Cu and Cu–X bonds. These structural changes are driven by the formation of a self-trapped exciton, where a hole localizes on Cu(d) orbitals [2-3]. Analysis of the electronic structure and -pCOHP reveals reduced antibonding interactions and enhanced bonding character in the excited state, stabilizing the distorted geometry.

Our results establish a direct link between orbital-specific hole localization, bonding rearrangement, and the resulting Stokes shift. This provides a fundamental understanding of the excitation mechanism in Cs₃Cu₂X₅ and offers design principles for tuning optical properties in 0D copper halides.

Synthesizing perovskite quantum dots (PQDs) within nanoporous matrices offers a promising alternative to traditional colloidal methods [1]. These matrices feature a controllable network of voids, acting as nanoreactors in which precursor solutions can be infiltrated. Further thermal or chemical treatments convert them into nanocrystals. This confinement enables precise control over PQD dimensions, size distribution, and crystallinity without requiring stabilizing ligands.

This talk highlights the method's potential to enhance PQD stability and functionality and to study fundamental properties of both individual QDs and QD networks. Key advantages include: (i) Stabilization of metastable phases, like the α-phase of CsPbI₃ even at room temperature [2]; (ii) Control over the environmental responsiveness [3]; (iii) High optical performance, with quantum yields exceeding 85% for specific compositions such as FAPbBr₃ [4]; (iv) Versatility for low-dimensional Structures, as the approach extends to fabricating low-dimensional perovskite structures, enabling remarkable blue photoluminescence with quantum yields surpassing 40% [5] or the formation of exciton-polaritons; Efficient charge transport within the embedded QD network, which depends critically on achieving PQD interconnectivity to enable dot-to-dot charge transfer, central for successful device operation [6].

Mechanochemistry has a great potential for the solvent-free synthesis of complex multinary metal halides. [1, 2] Indeed, typical bottlenecks related to the different solubilities of metal halide precursors and the difficulty in ensuring a precise stoichiometry in final crystals when utilizing solution processes can be overcome by dry mechanochemical synthesis. Herein I will show how this strategy can be employed to achieve high-entropy 3D halide perovskites for the first time. [3]

More precisely, we have achieved substantial simultaneous alloying in three of the four sublattices of halide double perovskites with general formula Cs₂(B1:B2)(C1:C2)(X1:X2)₆ by means of ball-milling and thermal annealing. This leads to highly relevant properties such as pronounced bandgap bowing and full visible light absorption even for pure-chloride compositions.

The fundamental processes involved in such high-entropy alloying are revealed through a combination of detailed structural characterization and DFT calculations, highlighting the different ion-exchange kinetics and how these are linked to defect formation energies in different phases.

Eventually, I will show how these materials can be implemented as photoelectrocatalysts in the highly-sought after oxygen evolution reaction which is a key process in many energy-related applications.

Two-dimensional perovskite materials have attracted increasing interest as active materials in photonic and optoelectronic applications, primarily due to their intriguing optical and electronic properties. Among their most striking properties is pronounced exciton-phonon coupling, which is believed to be relevant to a variety of key (opto)electronic parameters, including carrier mobility, photo- and electroluminescence, and exciton binding energies. However, the interplay between the structure of the perovskite and electron-phonon coupling effects remains poorly understood; understanding the relationship between these effects is therefore essential to the continued development of high-performance perovskite materials.

To this end, we employ advanced optical methods to investigate a series of 2D Ruddleston-Popper perovskites, employing derivatives of the chiral methylbenzylammonium (MBA) molecule as organic spacer ligands. By fine-tuning the structure of the MBA derivatives—leveraging both substitution and stereochemical effects—we demonstrate control over the strength of electron-phonon coupling. We link these observations to the mechanical properties of the inorganic lattice, which are modulated by the organic spacer ligands. 

Our results reveal a clear pathway to control the dynamics of the inorganic lattice of 2D perovskites through molecular design of the organic spacer ligand, and further highlight the importance of stereochemical and enantiomeric effects in controlling electron-phonon coupling in these materials.

Halide perovslkites are well known for their excellent optoelectronic properties that has enabled them to create record breaking performances in areas of solar harvesting, light emitting diodes and detectors. In this talk we will focuss on the ionic effects within this family of semiconductors which is normally as its weakness. The talk will cover our efforts to modulate and utilise the ionic properties of halide perovskites. 

The significant ionic activity in these materials enables their use in memory devices, where the interplay between ionic and electronic transport gives rise to resistive switching behavior. These distinctive characteristics make halide perovskites highly promising for neuromorphic applications. Their strong light absorption and coupled ionic–electronic transport allow them to be stimulated by both electrical and optical inputs. In this presentation, I will discuss various approaches to modulate the material compositions, interfaces to tailor memristive behavior in a wide variety of architectures including memristive PV devices as well as perovskite-based LEDs with integrated memory functions. These devices can emulate a wide variety of functions such as human visual processing, enabling functions such as contrast enhancement, feature extraction, and other forms of sensory pre-processing. Finally, I will also touch on how perovskite-based memory devices can be engineered to emulate artificial neurons, paving the way for highly integrated and multifunctional neuromorphic architectures.

The dual electronic-ionic nature of perovskite solar cells has complicated the interpretation of almost all the standard PV 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. On the flip side, the ions can act as probes, giving us useful information about how well a cell is operating.

One technique we have used is impedance spectroscopy, a very common characterisation technique. The Nyquist plots measured for PSCs show a wide variety of different shapes, and many different interpretations of these spectra can be found in the literature. We recently showed that all of these experimentally observed shapes can be reproduced by a standard three layer drift diffusion model with a single mobile ion species, without the need to invoke any exotic physics within the device. The low frequency regime contains a wealth of information about the internal workings of the cell that can be obtained purely from shape recognition of the Nyquist plot, without any modelling expertise. This presentation will cover our recent work measuring and modelling a wide variety of perovskite solar cells (PSCs) and using ion migration to diagnose device physics.

Tin-based halide perovskites promise lead-free optoelectronic devices, but surface degradation remains the critical barrier to commercialization. Here, we combine density functional theory calculations with thermodynamic modeling to reveal the atomic-scale mechanisms governing stability in the 2D hybrid perovskite 4FPSI. By systematically investigating point defects including vacancies and interstitials in both in-plane and out-of-plane configurations, we uncover how defect spatial distribution controls surface reactivity and device performance.

Our calculations identify two competing degradation pathways. Sn(II) oxidation to Sn(IV) via O₂ exposure dominates under ambient conditions, while I⁻/I₃⁻ redox processes prevail in solution environments. Surface stoichiometry critically determines defect energetics. Sn-poor terminations generate deep acceptor V_Sn traps within the bandgap, whereas Sn-rich surfaces promote interstitial formation that disrupts the SnI₆ octahedral framework. Organic cation coverage provides crucial protection: complete 4F-PEA⁺ layers create substantial water diffusion barriers, but partial coverage exposes reactive Sn sites with significantly lower activation energies.

Most remarkably, we discover intrinsic self-healing through surface reconstruction and interstitial migration. This finding explains experimentally observed performance recovery after rest periods and fundamentally reframes our understanding of tin perovskite stability. The dynamic equilibrium positions Sn(IV) as both a degradation product and a healing intermediate. Our mechanistic insights reveal that surface engineering, not bulk composition, holds the key to stable lead-free photovoltaics. These results provide clear design principles for defect passivation and interface optimization in next-generation solar cells.

Thin film transistors (TFTs) built using lead-tin perovskite have gained immense attention due to its excellent device mobilities along with high on-off ratios, which makes such TFTs perfect devices for switches and digital applications. However, facile oxidation of tin (Sn) from +2 to +4 state leads to undesired doping, resulting in a loss of channel modulation. In this work, the influence of A-site cation on the performance and stability of TFT is evaluated. We report an ambient stable (RH: >70%, RT: 25°c) TFT with a threshold voltage (V_th) of 4.7 V and an on-off ratio of nearly 10⁶ ; stable for an hour of exposure without encapsulation. The stability of the devices were evaluated by observing the shift in transfer characteristics of the control and target composition systems as shown in TOC Figure. The gate modulation in control device is lost within 5 minutes of air exposure (TOC Figure (a)) while target device shows gate modulation even after an hour of air exposure (TOC Figure (b)). This stability is attributed to the substitution of appropriate A-site cation, which may have led to increased defect formation energy and thus lowered oxidation and doping, as evidenced by XPS and Hall measurements, respectively. Devices with such low V_th and high on-off ratios help in realizing circuits with lower operating power and well-defined, wide range on-off states.

Tin-based perovskite solar cells (Sn-PSCs) represent a promising alternative to their lead-based counterparts, offering reduced toxicity and appealing optoelectronic properties. However, their widespread application remains limited by poor operational stability, mainly driven by the oxidation of Sn²⁺ to Sn⁴⁺ under ambient conditions[1]. In this work, we explore the dynamic behavior of Sn-PSCs during operation, focusing on the interplay between degradation and self-healing processes. By incorporating thiophene-2-ethylammonium iodide (TEAI) as an additive in FASnI₃ devices, we observe a spontaneous recovery of photovoltaic performance under continuous light and ambient exposure in unencapsulated conditions.

To gain deeper insight into the underlying mechanisms, we combine electrical and optical characterization techniques, including impedance spectroscopy and photoluminescence measurements. This talk will focus on the processes governing this unusual behavior and will discuss the possible chemical pathways involved in the observed self-healing effect, aiming to contribute to a better understanding of stability challenges in lead-free perovskite solar cells.

Trap states in 2D metal-halide perovskites strongly affect optoelectronic performance [1]. Understanding and distinguishing between the different types of energy carriers, such as excitons, free carriers, and trap-mediated states is essential, as these species govern charge carrier dynamics and determine emission efficiency [2]. Power-dependent studies, when combined with lifetime and spectrally resolved photoluminescence measurements, are key tools to disentangle the contributions of each carrier type. While excitons and free carriers are associated with radiative recombination and efficient light emission, trap states introduce non-radiative losses and reduce device performance. A clear spatial and spectral separation of these carriers is therefore critical for targeted material optimization and precise control of optoelectronic properties [3].

Our study focuses on two-dimensional metal-halide perovskite flakes, which exhibit strong excitonic emission and pronounced carrier trapping phenomena, making them ideal systems for investigating power-dependent spectral and lifetime dynamics. The micron-sized flakes are synthesized via solution-based methods and exfoliated onto transparent substrates for optical characterization.

We combine hyperspectral and Fluorescence Lifetime Imaging Microscopy with direct spatial visualization of carrier dynamics. By systematically evaluating the power dependence of the spectral, temporal, and spatial characteristics of the optical excited state, we can map spectral shifts, intensity changes, and the emergence of distinct emissive components with high fidelity. Backed up by advanced numerical modeling, this multimodal approach enables the precise localization and mapping of charge carrier species in perovskite materials at the micrometer scale. Such fine-grained control is essential to probe and decouple overlapping excitonic, free carrier, and trap-state contributions [2]. This capability is critical for guiding synthesis strategies and improving the optoelectronic quality of low-dimensional perovskites.