This year marks the first decade of colloidally synthesized lead halide perovskite quantum dots (LHP QDs), defining QDs as size- and shape-uniform ensembles with tunable quantum confinement and single-photon emission. Gradually, during this period, practically the entire compositional within a general formula APbX3 was thoroughly studied, with A being cesium (Cs), methylammonium (MA), formamidinium (FA), and azeridinium (AZ) was produced as high-quality nanocrystals. This journey is, arguably, at its very beginning. The LHP QDs are vastly different from conventional, more covalent semiconductors – they are ionic compounds with much lower formation energies, entropically stabilized, and structurally dynamic. The design of surface capping ligands turned out to be decisive for their stabilization at the nanoscale and for taming their photophysics. Currently, LHP NCs are prototyped as primary green emitters for television displays owing to facile and scalable production, higher emissivity-per-mass under blue excitation, and narrow emission linewidth. Their excitonic characteristics exceed initial expectations in many regards, opening opportunities as quantum light sources. In particular, at cryogenic temperatures, LHP QDs exhibit long excitonic coherence times, which start to match the fast sub-100 ps radiative rates. Both characteristics are optimized, to our surprise, in larger CsPbX3 QDs beyond the quantum confinement, namely, 20-40 nm, owing to the single-photon superradiance effect (giant oscillator strength at the single-exciton per NC regime). Single-component and multicomponent QD superlattices exhibit collective emission, known as superfluorescence, characterized by the oscillating, ultrafast (10-30 ps) radiative decays. This presentation will walk you through both the most essential progress over this first decade, including our current work, and outline future prospects.

Liquid phase exfoliation has been proved to be a cheap, scalable method for the mass production of 2D sheets. This talk will first discuss the galaxy of existent layered materials, with emphasis on synthesis, liquid-phase exfoliation, and characterization, focussing on some key applications recently developed in our laboratories, ranging from energy storage to printed electronics.

We will for example discuss how two-dimensional nanomaterials can be formulated in aqueous and organic viscous inks for conventional slurry castin, as well as extrusion printing, inkjet printing, and aerosoljet 3D printing, and demonstrate direct printing on various substrates

Organic-inorganic metal halide perovskites have emerged as attractive materials for solar cells with power-conversion efficiencies of single-junction devices now exceeding 26%. However, defective interfaces with charge extraction layers, the low hurdle for ionic migration, and the structural flexibility of the perovskite structure still pose both opportunities and challenges to their commercialization in light-harvesting applications. Combinatorial characterization approaches are vital for probing and analysing such instabilities.

We demonstrate a combined modelling and experimental approach[1] towards exploring the effects of energy-level alignment at the interface between wide-bandgap mixed-halide perovskites and charge-extraction layers, which still causes significant losses in solar-cell performance, focusing on FA0.83Cs0.17Pb(I1-xBrx)3 with bromide content x ranging from 0 to 1, and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine| (PTAA). Through a combination of time-resolved photoluminescence spectroscopy and numerical modeling of charge-carrier dynamics[1] we reveal that open-circuit voltage losses associated with a rising energy-level misalignment derive from increasing accumulation of holes in the HOMO of PTAA, which then subsequently recombine non-radiatively across the interface via interfacial defects. These findings highlight the urgent need for tailored charge-extraction materials exhibiting improved energy-level alignment with wide-bandgap mixed-halide perovskites.

We further demonstrate optical-pump THz-probe spectroscopy with controlled intervals of air exposure as an ideal technique to monitor air-induced degradation of optoelectronic parameters such as charge-carrier mobilities and recombination rates in low-bandgap lead-tin iodide perovskites.[2][3] We explore the best choice of A-cation in lead-tin iodide perovskites with intermediate lead-tin ratios and find that air exposure induces hole doping to a similar extent, for methylammonium (MA) formamidinium (FA), FA cesium (Cs) and FA-only cations. However, we find that MAFA-based perovskites are unstable under heat exposure owing to decomposition of MA, and FACs perovskites suffer from A-cation segregation and an accompanying non-perovskite phase formation.[3] Thus we propose that from a stability perspective, efforts should refocus on FASn0.5Pb0.5I3 which minimizes all three effects while maintaining a suitable bandgap for a bottom cell and good performance.

We further utilize a combination of ultra-low frequency Raman and infrared terahertz time-domain spectroscopies to provide a systematic examination[4] of the ultra-low frequency vibrational response for a wide range of metal-halide semiconductors: FAPbI3, MAPbIxBr3–x, CsPbBr3, PbI2, Cs2AgBiBr6, Cu2AgBiI6, and AgI. We examine the cause of a frequently reported “central Raman peak” and rule out extrinsic defects, octahedral tilting, cation lone pairs, and “liquid-like” Boson peaks as causes.[4][5] Instead, we propose that the central Raman response results from an interplay of the significant broadening of Raman-active, low-energy phonon modes that are strongly amplified by a population component from Bose–Einstein statistics toward low frequency.[4] These findings elucidate the complexities of light interactions with low-energy lattice vibrations in soft metal-halide semiconductors emerging for photovoltaic applications.

The charge carrier lifetime is a widely used metric for characterizing recombination in semiconductors. However, in the case of lead halide perovskites, there is considerable ambiguity surrounding reported lifetime values and their interpretation. This raises two fundamental questions: how should the lifetime be measured, and more importantly, what does it truly represent?

In theory and experiment, a multitude of different “lifetimes” can be defined, most of which are functions of the carrier density instead of constant. In doped semiconductors, mechanisms like detrapping and photodoping due to trapping can often be ignored, allowing different lifetime definitions to converge, so that no distinction is necessary. Additionally, their values are simplified to constants in many situations, streamlining the interpretation. None of these simplifications can be done for semiconductors with low intrinsic doping concentrations such as lead halide perovskites.

This presentation will explore the differences between the various definitions of time scales in theory and photoluminescence experiments and their connection to each other in intrinsic semiconductors. We provide experimental evidence indicating that high densities of shallow traps dominate transient photoluminescence decays in lead halide perovskites. Furthermore, we will clarify how these observations contrast with lifetimes derived from steady-state measurements, offering insights into the complexity of recombination dynamics in these materials.

Two-dimensional hybrid perovskites are a highly intriguing class of materials, composed of alternating inorganic and organic molecular layers. Their reduced dimensionality combined with weak dielectric screening leads to the formation of tightly bound excitons that efficiently absorb and emit radiation. A central questions for excitons in perovskites from the perspectives of both fundamental physics and applications is their mobility. In addition, the flexibility of the material design allows for the integration of a variety of functional compounds including chiral molecules to enable polarization control of the optical response. In this talk I will focus on the transport of optically detected excitons in 2D perovskites via transient, ultrafast microscopy, featuring different regimes of propagation featuring free and localized states. I will show how the exciton transport is directly monitored using optical means, how it depends on conditions favoring either free particle diffusion or hopping, as well as how non-equilibrium populations lead to intriguing time-dependent dynamics. Finally, exciton propagation in chiral 2D perovskites will be demonstrated and its key relationship to the disorder discussed.

Quantum dots (QDs) offer unique physical properties and novel application possibilities like single-photon emitters for quantum technologies. While strongly-confined II-VI and III-V QDs have been studied extensively, their complex valence band structure often limits clear observations of individual transitions. In recently emerged lead-halide perovskites, band-degeneracies are absent around the bandgap reducing the complexity of optical spectra. These QDs exhibit several distinct absorption resonances,[1] which can be assigned to excitons confined with respect to their center-of-mass motion within the weak confinement regime. [2] The well-defined excitonic energy landscape with significant confinement energies of these QDs suggests a high potential for light amplification. However, there is still debate on the nature of gain in perovskite QDs, which has been attributed to different origins such as biexcitons, trions, and single excitons. Here we study amplified spontaneous emission and optical gain of monodisperse spherical CsPbBr₃ QDs and conclusively assign the gain to biexcitons.[3] This is based on the gain threshold and its spectral position which we study via femtosecond transient absorption spectroscopy. Furthermore, the optical gain vanishes within 30 ps, matching the biexciton lifetime, demonstrating the strong correlation to the biexciton population. By identifying the intrinsic mechanism of optical gain in CsPbBr₃ QDs and its limiting factors, our findings show the direction for future work on optimizing their gain threshold and lifetime.

In order to implement quantum dots (QDs) into most optoelectronic applications, from photovoltaic to light emitting devices, one needs to organize them into an interconnected array in the shape of a QD film. In this configuration a certain degree of electronic coupling among QDs is achieved, allowing for charge injection/extraction, at the expense of a reduction of the quantum confinement that characterizes these structures.

In this presentation we will discuss recent results where carrier cooling and recombination have been studied in QD solids as a function of connectivity. In order to exert control on electronic coupling among QDs we vary their average separation through their concentration when grown within the pores of nanoporous metal-oxide matrices. [1] We perform a thorough photophysical characterization comprising linear spectroscopy, PL quantum yield and ultrafast spectroscopic measurements while tuning the average QD separation and observe a transition in carrier recombination from isolated nanostructures to interconnected ones. The effects on global emission properties in terms of isolated and bulk-like behavior are pointed out in detail. [2]

Similar studies in the initial instants after carrier photo-excitation allow us to unveil the role of electronic coupling in the intra-band cooling of excited carriers. A retardation of the cooling dynamics is observed as we reduce QD connectivity and associated with carrier reheating taking place as a consequence of Auger-like processes. [3]

Thus a broad picture is provided on the fate of photoexcited carriers in QD films as a function of connectivity that can help to build an intuition when planning the use of these structures in devices operating in different carrier density regimes.     

There is recent evidence that, upon polaron formation, excitonic interactions in bulk lead halide perovskites experience different dielectric screening at short and long distances.[1,2] Above  the polaron radius, static dielectric screening prevails. Under the polaron radius, dynamic screening prevails instead.

Here we investigate how such non-hydrogenic interactions affect the electronic structure of excitons, trions and biexcitons in confined halide perovskites: nanoplatelets and nanocrystals. To do so we develop a theoretical model which accounts for the main physical factors in these systems: (i) quantum confinement, through k·p theory, (ii) dielectric confinement, through image charge methods, (iii) electronic correlations, through a variational Quantum Monte Carlo method, and (iv) polaronic interactions, through Haken-like potentials.

We show that trions are formed by an exciton plus an excess carrier orbiting at longer distances, which reveals a significant ionic character of the bond. Biexcitons are formed by two excitons with strong intra-exciton interactions, and weaker inter-exciton ones. Polaronic interactions lead to large binding energies for excitons, trions and biexcitons. These are in good agreement with experimental measurements of nanoplatelets and Ruddlesden-Popper perovskites.[3-5] For nanocrystals, however, biexciton binding energies are systematically smaller than experimental values. This suggests excitons and biexcitons polarize the lattice differently.

Perovskite nanocrystals exhibit exceptional optical properties, including ultrawide color tunability, high quantum efficiency and coherent single-photon emission. [1]
This renders them optimal candidates for optoelectronic devices, yet a comprehensive understanding of their optical characteristics is crucial.

Spectroscopy on single perovskite crystals helps to reveal their excitonic fine structure, which in thin films is concealed by inter-particle phenomena.
While photoluminescence (PL) studies have been conducted on single perovskite nanocubes, their wavelength-dependent absorption cross section has not been reported so far.
Since PL emission always originates from the energetically lowest state of the material, obtaining absorption data can contribute to a more complete picture of the energetic landscape.

The lack of these absorption measurements arises from the inherent difficulty in measuring the marginal absorption of such nanoscale samples.
To overcome this challenge we use an optical resonator, specifically a high-finesse microcavity [2], in which the light passes through the sample a thousand of times, thereby enhancing its absorption to a measurable amount.

This enables us to conduct ultra-sensitive and spectrally resolved measurements of the absorption cross section of perovskite nanocubes at room temperature.
The combination of this novel technique with PL and scanning electron microscopy (SEM) provides unique insight into the size- and morphology dependence of the energetic structure of perovskite nanocubes. Moreover, the degradation of the nanocrystals can be monitored by time-dependent changes in the spectral absorption signal.
An imminent replication of our measurements at cryogenic temperatures is expected to provide further understanding of the material’s optical properties.
The samples are provided by the groups of Prof. Urban and Prof. Kovalenko.

The findings subsume in a series of hyperspectral absorption measurements that were previously conducted on carbon nanotubes and defects in a two-dimensional material [3].
The progress towards routine measurements of samples on the nanoscale demonstrate that cavity-enhanced absorption spectroscopy has the potential to become a standard tool for the characterization of perovskite nanocrystals and other nanoscale samples, similar to the measurement of PL.

     Lead-based mixed-halide perovskites have a number of uses in solar and in light emitting applications. Unfortuantely, they exhibit ionic instabilities under illumination. Chief among them is photosegregation, a phenomenon first reported by McGehee in 2015. Since then, photosegregation has beceome well established as the light-driven segregation of iodine and bromine anions in mixed-halide lattices into iodine-rich and bromine-rich inclusions of the alloy. The phenomenon is reversible under darkness and segregated anions can remix.

     Multiple models have been developed to explain photosegregation. Among them are thermodynamic, polaron, chemical, and trap-related models. While models explain photosegregation to varying extents only a band gap thermodynamic model has explained in a quantitative manner, observations of excitation intensity (I_exc)-dependent terminal halide stoichiometries and photoegregation I_exc thresholds.

     One observation that has not been explained by any model though is the recent observation by Bach and co-workers of light-driven anion remixing in methylammonium lead iodide/bromide microcrystals. Hindering a deeper understanding of this photoresponse has been the fact that the Bach report is the only report of persistent photoremixing in a mixed-halide perovskite.

     We have recently found that pulsed laser irradiation of photosegregated mixed-halide perovskite films can induce robust and reproducible, persistent photoremixing. This opens the door to furthering our understanding of mixed-halide photoinstabilities since it allows for the first time, concerted studies of photosegregation and photoremixing on the same system. More importantly, it provides an important test for any model seeking to self-consistently rationalize mixed-halide perovskite photoinstabilities.

Efficiency and emission rate are two traditionally conflicting parameters in radiation detection, and achieving their simultaneous maximization could significantly advance ultrafast time-of-flight (ToF) technologies. In this study, we demonstrate that this goal is attainable by harnessing the giant oscillator strength (GOS) inherent to weakly confined perovskite nanocrystals, which enables superradiant scintillation under mildly cryogenic conditions that align seamlessly with ToF technologies. Importantly, we show that the radiative acceleration due to GOS encompasses both single and multiple exciton dynamics arising from ionizing interactions, further enhanced by suppressed non-radiative losses and Auger recombination at 80 K. The outcome is ultrafast scintillation with 420 ps lifetime and light yield of ~10’000 photons/MeV for diluted NC solutions, all without non-radiative losses. Temperature-dependent light-guiding experiments on test-bed nanocomposite scintillators finally indicate that the light-transport capability remains unaffected by the accumulation of band-edge oscillator strength due to GOS. These findings suggest a promising pathway toward developing ultrafast nanotech scintillators with optimized light output and timing performance.

Colloidal lead halide perovskite nanocrystals (NCs) are very promising materials for next-generation light-emitting devices due to their large gap tunability and high emission quantum yield. At ambient conditions, metal halide perovskites, whether in bulk or nanocrystalline form, exhibit a fairly linear increase in fundamental gap with increasing temperature, when crystallized in a tetragonal or cubic phase [1]. This also holds for CsPbBr₃ NCs, although they presumably crystallize in an orthorhombic phase. Strikingly, the chlorine counterpart exhibits a sign reversal in the temperature slope of the fundamental gap [2]. We combined temperature and pressure-dependent photoluminescence (PL) measurements of colloidal CsPb(Br_1-xCl_x)₃ mixed-anion NCs to unravel the origin of such a reversal. The NC composition was varied continuously by ionic exchange in the primal colloidal solution. Careful analysis of the PL data allowed us to disentangle the effects of thermal expansion and electron-phonon interaction on the variation of the gap with temperature. We show that, concomitant with a transition into an orthorhombic phase, occurring for Cl contents exceeding ca. 40%, the electron-phonon interaction undergoes a sudden and radical change in sign and magnitude. In contrast, thermal expansion effects remain the same. Based on recent observations in mixed-cation Cs_xMA_1-xPbI₃ single crystals with low Cs content [3], we interpret such behavior as due to the activation of an anomalous electron-phonon coupling mechanism linked to lattice anharmonicities induced by coupling with Cs rattling modes. This takes place in the shrunken cage voids of the orthorhombic NC lattice for high Cl concentrations. In this way, we have clarified a puzzling result about metal halide perovskite NCs, providing valuable insights into the role of A-site cation dynamics in their optoelectronic properties.

Lead halide perovskites nanocrystals (PNCs), prized for their solution processability, strong light-matter interaction, tunable and high radioluminescence within visible range, are emerging high-atomic number scintillating materials for next generation scintillator/photodetectors for ionizing radiation detection. However, the challenge of embedding NCs into optical-grade nanocomposites without degrading their optical properties continues to hinder their widespread adoption. Moreover, fundamental aspects of the scintillation mechanisms remain insufficiently understood, leaving the scientific community without well-defined fabrication protocols or rational design guidelines for fully harnessing their potential. Recently, innovative strategies for embedding PNCs in impermeable host matrices have been developed [1], effectively preserving their luminescence properties in harsh environments while preventing Pb dispersion, thereby enabling their safe use in biological applications. Furthermore, studies have identified key parameters in the design of scintillation detectors at the isolated NC level, providing critical insights for optimizing their performance [2]. We investigate the embedding of PNCs within mesoporous silica particles, enabling improved stability and controlled dispersion in non-scintillating solvents. Our findings suggest that localized PNC concentration can trigger cascading scintillation, where secondary excitation enhances emission efficiency. Furthermore, this approach mitigates lead dispersion while maintaining strong radioluminescence, addressing key concerns in high-loading nanocomposite systems. This study advances the understanding of nanoscale scintillation mechanisms and provides insights into designing next-generation PNC-based scintillation detectors.

Halide perovskites are exciting materials for next-generation optoelectronic devices including solar cells, LEDs and X-ray detectors and they show intriguing photophysical behaviour. Here, I will outline our group's recent results on the photophysics of perovskites on different length scales. This work includes nanoscale quantum behaviour at low temperature in bulk thin films, recombination seen deep within, and at interfaces of, single crystals in X-ray detector devices, charge transfer pathways in 2D perovskites with electroactive molecules and charge and energy transfer at the interface between high quality 2D/3D heterojunctions. I will show how the multimodal toolboxes can shed important light on these processes by connecting the photophysical findings directly with structural, chemical and morphological properties. This information sheds light on sources of unwanted non-radiative recombination processes, and opportunities for further functionality tuning through different compositions, structures and heterojunctions. These findings are only possible when considering physically realistic models, and such models and appropriate fitting approaches are discussed in detail.

Metal halide perovskites have attracted remarkable interest as promising materials for optoelectronic applications. A key factor underpinning their performance is the behavior of photogenerated excess charges. These highly polarizable materials exhibit strong charge-lattice interactions, which can result in diverse forms of charge localization, including polarons, bipolarons, and self-trapped excitons (STEs). Such localized states play an important role in defining the optoelectronic properties of perovskites, influencing charge transport, recombination, and light emission. In this talk, I will present a series of studies that investigate the formation, stability, and dynamics of these charge-localized states, employing advanced computational approaches such as non-empirical hybrid functionals and time-dependent density functional theory. I will discuss examples from single lead- and tin-based perovskites as well as double halide perovskites. By connecting computational predictions with experimental findings, I will illustrate how polarons, bipolarons, and STEs manifest in halide perovskites and their implications for optoelectronic and photonic properties.

Unlike conventional semiconductor materials, metal halide perovskites (MHP) possess soft and ionic crystal structures leading to several unique features like facile ion migration, self-healing, elasticity, and memory. Within this dynamic system, external stimuli like high photon doses, electron beams, electrical bias, and mechanical stress induce structural changes and alter associated optoelectronic properties. Therefore, it is crucial to investigate the structure-photophysics relationship in these materials, especially in operational devices like solar cells, where traditional methods such as scanning electron microscopy fall short due to the layered structure. Moreover, electron and x-ray-based analytical techniques are often invasive altering the material properties.

To address these challenges, we developed Correlation Clustering Imaging (CLIM), a novel noninvasive method that utilizes photoluminescence fluctuations to reveal contrasts associated with defect dynamics in semiconductor materials. In films, CLIM images show the polycrystalline grain structure ideally correlating with electron microscopy images. Analysis of photoluminescence fluctuations suggests the presence of one type of metastable defect dominating the fluctuations. Although the relative amplitude of PL fluctuations is small in films, it is significantly larger in solar cells under short-circuit conditions. The correlated regions within devices are notably larger (up to 10 micrometers) than those in films (up to 2 micrometers). We propose that the regions resolved by CLIM in solar cells possess a common pool of charge extraction channels, which fluctuate and cause PL to vary. Since photoluminescence fluctuations report on the dynamics of non-radiative recombination processes, CLIM offers insights into structural and functional aspects related to carrier transport, ion migration, defect formation and annihilation, and recombination losses, which are crucial for the rational engineering of the next generation of devices.

CLIM provides imaging contrasts based on properties which were never used before for optical non-invasive imaging of luminescent materials and devices based on them in operando. The broad applicability of CLIM, requiring only a standard wide-field microscope and our user-friendly, open-source algorithm, positions it as an important new tool for material chemists, engineers, and device scientists.

Charge carrier recombination models based on fluence dependent time resolved photoluminescence (TRPL) datasets can provide crucial insights into the optoelectronic properties of semiconducting materials. In the case of halide perovskites, the validity of the assumed recombination model is however often challenged [1], where an ongoing debate exists on the number and type of defect populations to consider, such that a multitude of defect populations need to be assumed to reproduce experimental data even of a canonical perovskite thin film [2]. In systems with enhanced complexity, such as perovskite quantum dot solids presenting pronounced diffusion processes [3], a modelling with common recombination models fails and an alternative approach as well as a validation protocol of the assumed models is required.

Herein, we present a novel TRPL data representation method that allows to (1) extract recombination constants from complex material systems beyond common recombination models and (2) simultaneously evaluate the validity of assumed processes in the different recombination models (such as the presence and type of defects) [4]. To do so, a method that processes time-resolved datasets to absolute carrier density-resolved datasets is developed. By analyzing different derivative representations, monomolecular and bimolecular recombination processes are graphically isolated, where a rigorous comparison of experimental and simulated datasets allows their clear association to different recombination processes as well as to equilibrium and out-of-equilibrium processes. This enables, among others, a clear distinction between shallow and deep trap contributions. The proposed analysis method is applied to different systems and the extracted recombination constants are validated through simultaneously acquired fluence dependent PLQY curves.

The proposed approach provides thus a broadly applicable analysis tool that allows to differentiate between different recombination and charge transfer processes and helps to identify the dominant contributions in a given semiconducting system, with largely varying defect populations and diffusion pathways.

[1] Kiligaridis, A., Frantsuzov, P. A., Yangui, A., Seth, S., Li, J., An, Q., ... & Scheblykin, I. G. (2021). Are Shockley-Read-Hall and ABC models valid for lead halide perovskites?. Nature communications, 12(1), 3329.

[2] Yuan, Y., Yan, G., Dreessen, C., Rudolph, T., Hülsbeck, M., Klingebiel, B., ... & Kirchartz, T. (2024). Shallow defects and variable photoluminescence decay times up to 280 µs in triple-cation perovskites. Nature Materials, 23(3), 391-397.

[3] Tiede, D. O., Romero-Pérez, C., Koch, K. A., Ucer, K. B., Calvo, M. E., Srimath Kandada, A. R., ... & Míguez, H. (2024). Effect of Connectivity on the Carrier Transport and Recombination Dynamics of Perovskite Quantum-Dot Networks. ACS nano, 18(3), 2325-2334.

[4] Tiede, D. O. et al – in preparation

Metal-halide perovskite (MHP) semiconductors are highly relevant candidates for the fabrication of next generation solar cells thanks to their very good opto-electronic properties as well as ease of processability. That said, considerable challenges are still ahead for MHP solar cells: Indeed, under continuous external stressors, such as heat, moisture, bias and light, MHP materials suffer from various (irr)reversible chemical reactions [1, 2, 8]. These phenomena impede devices from attaining the technological readiness levels required for deployment. Therefore, it is crucial to understand the initial stages of perovskite material evolution under external stressors in order to mitigate long term degradation of devices.

As part of long-term degradation of solar cell devices, bulk absorber stability under continuous illumination is needed in order to conserve appropriate device function. Stable bulk properties would also avoid second hand reactions at interfaces with available decomposition products of the bulk [3].

Striking evidence for absorber instability under continuous irradiation has been previously described, where steady state photoluminescence (PL) either increases or decreases, depending on conditions of continuous illumination [4,5,6] or quasi-continuous illumination [7]. Additionally, these reports also highlight the crucial role of the sample environment on PL evolution under continuous illumination [8]. For all these works, the common and favored explanation for PL evolution under continuous illumination is the introduction and/or annihilation of new trapping sites which respectively causes a drop or increase in PL intensity. Though correlations to kinetic models or DFT calculations have been shown previously [7], experimental evidence for the effect of newly introduced trapping sites on the dominating recombination mechanisms at play is lacking. This is where time resolved photoluminescence (trPL) is a very useful tool: it can be used to identify what type of recombination mechanism is dominating from carrier-density dependent lifetimes, using so called differential lifetime plots [9].

In this presentation, we will focus on methylammonium-lead-iodide (MAPI) thin films and the (ir)reversible introduction of traps triggered by quasi-continuous illumination. For this, we develop a method that uses long pulses of light in combination with trPL counting schemes, calling it long pulsed trPL (LP-trPL). From the method, we observe the inclusion of long lived and non-deep trapping sites due to continuous illumination. The data also suggests a highly asymmetric mechanism of trap formation, where trap annihilation is much slower than observed formation. We conclude that previously described mechanisms of iodine outgassing [7] is compatible with the observed shallow nature of traps introduced as well as the asymmetric process of formation/annihilation.

Lead halide perovskites, known for their excellent optoelectronic properties, hold significant industrial promise across applications such as solar cells, photocatalysis, and radiation detection. However, their instability and toxicity remain critical challenges. As a promising alternative, halide double perovskites, that substitute lead with elements like silver and bismuth, offer greater stability and reduced toxicity. Here, I will show how the optical, mechanical and toxicity characteristics of double perovskites can be tuned by compositional engineering. By employing a solvent-free, mechanochemical synthesis method, we circumvent common issues such as poor precursor solubility and the formation of unwanted side phases. Using synchrotron radiation, we tracked the formation mechanisms during the mechanochemical synthesis of Cs2Ag[BiM]Br6 (M = Sb, In, or Fe), identifying new intermediate phases and gaining valuable insights into the reaction kinetics.

We find that mechanochemical synthesis is a successful approach to make compounds that were not accessible via solution-based synthesis routes, such as Cs2AgBi0.5In0.5Br6, and Cs2AgBi1-xFexBr6. Hence, this solvent-free approach enables tuning of the absorption onset across the entire visible spectrum of light. In addition, high-pressure synchrotron-based X-ray diffraction (XRD) experiments revealed that the mechanical properties of these materials varies significantly with chemical composition. The improved understanding of the mechanochemical formation of alloyed-AgBi double perovskites opens new pathways for designing materials with tailored optical and mechanical properties, advancing their potential in sustainable energy applications.

Non-equilibrium Bose-Einstein condensates of exciton-polaritons in metal halide perovskites present a versatile platform for fundamental and applied research. The transition to spontaneous coherence is widely believed to be driven by intricate many-body interactions between polaritons and the optically inaccessible reservoir of background excitons. To elucidate these interactions, we employ advanced optical spectroscopy techniques to probe the coherent dynamics of polaritons and identify the critical factors—ranging from material properties to multi-particle interactions—that determine the condensation threshold.

Focusing on two-dimensional Ruddlesden-Popper (RP) metal-halide semiconductors, we uncover key processes that hinder polariton condensation.  Compared to the bare semiconductor, we find enhanced nonlinear exciton-exciton interactions in the microcavity environment. This is accompanied by ultrafast, parametric polariton scattering that prevents effective thermalization of sufficient population into the lowest-energy polariton state to enable condensation. Furthermore, we detect distinct spectral signatures of multi-particle correlations between polaritons and multiple excitonic transitions characteristic of RP perovskites.

Our findings suggest that the complex scattering landscape between the exciton reservoir and polaritons imposes significant limitations on polariton condensation in these materials. To address this, we propose strategies for reducing competing scattering pathways by systematically tuning the exciton fine structure via targeted organic cation substitutions. Finally, we discuss the broader implications of these results for highly polar materials, where lattice-mediated multi-particle correlations play a pivotal role in shaping many-body dynamics.

Hybrid halide perovskites are a novel class of semiconductor materials with promising and versatile optoelectronic properties enabled by their chemically adjustable structures and dimensionality. The diversity in the metal ions, halide anions, and organic spacers enables a wide range of materials with highly tunable properties and variable dimensionalities. These materials are studied for various applications, such as photovoltaics, detectors, and light-emitting devices. The chemical and structural agility of halide perovskites, enabling the adjustment of the optical and electronic properties for a desired application, is significant. In our research, we seek to gain additional information regarding the correlation between structure and composition to optoelectronic properties in low-dimensionality halide perovskites.

Specifically, we study low-dimensionality hybrid halide perovskites that exhibit broad-spectrum, white-light emission at room temperature, associated with self-trapped excitons (STE). These compounds are ideal candidates for illumination applications. We study the correlation between structural and chemical motifs of low-dimensionality halide perovskites and their STE emission.

In this research, we have studied how exchanging the halide anions while maintaining the structure affects the STE properties. We have focused on a unique 1D perovskite structure based on edge-sharing dimers, exhibiting strong, broad emission with PLQY of approximately 40%. By changing the halide from I to Br and Cl, we observe an increase in the bandgap energy, as expected. However, the broad emission shows an anti-correlated behavior, resulting in red-shifted broad emission for the Cl sample, with a significantly larger stokes shift. We further study how mixing Br and Cl in a single structure affects the broad emission properties and how different synthetic approaches can be utilized to fabricate these compounds.

To gain additional information regarding the STE properties with different compositions we have studied the temperature-dependent photoluminescence of these compounds. We utilize combined spectrally and temporally resolved photoluminescence measurements, allowing us to study the transition from band-edge to STE emission upon excitation.  We observed how this transition, along with additional properties of the STE emission, evolved with temperature and composition.

Two-dimensional (2D) perovskites are of great scientific interest as they have exceptional optoelectronic properties and are easily tunable in their bandgap, making them viable to be used for a variety of optoelectronic devices, such as LEDs, solar cells and photodetectors. 2D perovskites have a multi-quantum well structure, with alternating layers of insulating organic spacer molecules and inorganic perovskite layers, with varying thickness given by the integer n. As a result, they have narrow excitonic features and large exciton binding energies of up to several hundred meV. This begs the question of how the excitonic dissociation in 2D perovskites works, as they show efficient charge carrier separation, despite the large excitonic binding energy.

For n>1 2D perovskites, so called edge states have been observed along the edges of single crystal flakes and the grain boundaries of thin films. The excitons dissociate into these edge states resulting in an efficient charge carrier separation. They show a red-shifted photoluminescence (PL), which dominates the PL signal in perovskite thin films. In n=1 2D perovskites, these states have not been observed. However, previously we demonstrated that even in n=1 2D perovskites efficient charge carrier separation takes place, enabling a 9 % external quantum efficiency in a photoconducting photodetector. [1]

In this work we provide insights into the process of charge carrier separation in n=1 2D perovskites using spatially resolved photocurrent measurements. We correlate photocurrent and PL, while exciting locally with a focused laser beam with a resolution of less than 1 µm. We observed higher photocurrents along the grain boundaries compared to the inside of the grains of butylammonium lead iodide (BA₂PbI₄). At the same time, the PL is less bright at the grain boundaries with the peak maximum being shifted slightly red and the peak being broader. This supports the hypothesis that edge states are existent within n=1 2D perovskites, even though they are not as easily observable in PL as in n>1 2D perovskites, and that they are crucial for the charge carrier separation. With our contribution we hope to shed light on the exciton dissociation in n=1 2D perovskites with the aim to improve device efficiency for those materials. 

High environmental stability and surprisingly high efficiency of solar cells based on 2D perovskites have renewed interest in these materials. These natural quantum wells consist of planes of metal-halide octahedra, separated by organic spacers. The unique synergy of soft lattice and opto-electronic properties are often invoked to explain superior characteristic of perovskites materials in applications. At the same time such unique synergy creates fascinating playground for exciton physics which challenges our understanding of this elementary excitation. I will demonstrate that even after decade of intense investigation the notation” unique” so often used in case of perovskites deserves serious scrutiny.

I will explore the excitonic landscape in 2D semiconductors. First, I will highlight the controversy surrounding the unexpectedly high light emission efficiency of this material and show that it can be explained by the interplay between phonons and the exciton fine structure. I will demonstrate that the soft lattice can suppress relaxation of excitons to dark state making 2D perovskites great light emitters. Moreover, I will discuss the exciton fine structure measured for multiple 2D layered perovskites characterized by a different lattice distortions imposed by organic spacers. Surprisingly, it has a non-trivial impact on the exchange interaction allowing the energy spacing between dark and bright excitons to be tuned. This tuning knob, not available in classic semiconductors, makes 2D perovskites a unique material system where the exciton manifold can be controlled via the steric effect. Finally, I will demonstrate the first experimental evidences of polaron formation in the optical spectra of these materials.

Halide perovskites are fascinating semiconductors for light-emitting applications. Compared to conventional inorganic covalent semiconductors like silicon or GaAs, perovskites are structurally soft and often more disordered. Understanding the consequences of this remains a key challenge for commercialization but offers also opportunities for tailoring properties to target applications.

Here I will present our recent mechanistic insights from spectroscopy on the role of composition, doping and dimensionality to control light emission through localization effects in these materials.

I will talk about charge carrier accumulation in mixed-halide 3D systems, compare the photophysics of 2D Pb with that of new 2D germanium perovskites, and, time permitting, will introduce a new synthesis route to ambient doping of 0D nanocrystals giving for the first time access to strongly confined, transition-metal doped perovskite nanocrystals, with profound consequences for the light emitting properties of the resulting materials.

Overall I hope to highlight the promise the vast tunability of this material class holds for next-generation optoelectronic applications, once their working mechanisms are better understood.

The interest in lead halide perovskite materials has increased over the last decade. Understanding the fundamental photo-physical and chemical nature of these materials is of great interest as they have great potential in the application in solar cells or light emitting devices (LEDs). Therefore, lead halide perovskite nanocrystals (NCs) will be the most relevant material. Their optical tunability by anion exchange, high photoluminescence quantum yield and their defect tolerance make them outstanding. However, there is still lack of understanding the chemical nature surrounding the inorganic NC. Therefore, it is necessary to investigate how the electronic structure of this type of NCs can be influenced by the organics and, ultimately, the devices can be improved.

In this work, we investigate cinnamic acid covered CsPbBr₃ NCs to study their absolute band edge position by spectroelectrochemistry (SEC). Kroupa et al. have previously a shift of the valence band of PbS nanoparticles by introducing electron withdrawing or donating functional groups to cinnamic acids.[1] Here, we can also observe a shift of the valence band. In contrast to PbS, electron withdrawing ligands facilitate the oxidation of NCs compared to electron donating ligands. A shift of 0.34 V can be extracted from SEC in photoluminescence. Combining this result with theory, the earlier oxidation can be explained by a shift of the HOMO to the cinnamic acid derivative. Depending on the strength of the electronic properties of the aromatic ligand, the HOMO is more or less located on the ligand, whereas this is not the case for PbS.[1] This result (of the easier oxidation of CACF₃ covered NCs) is supported by data from LEDs. In particular, focusing on the hole transport layer gives insight into the charge injection depending on the ligand surrounding the NCs.