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 and the effects of using different sized A-cations and mixed Pb-Sn compositions as there is limited understanding of the impact of Sn substitution on the ion dynamics of Pb halide perovskites; (ii) Insights into passivating perovskites with molecular amino-silane compounds including surface interactions of additives; here we find that strong binding of amino-silanes 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); C. Kamaraki et al., Adv. Energy Mater., 14, 2302916 (2024).

Tin halide perovskites (chemical formula ASnX₃, with A=methylammonium MA⁺, formamidinium FA⁺ or cesium Cs⁺ and X= iodide I^- and/or bromide Br^-) have emerged as promising materials for next-generation photovoltaic and optoelectronic applications. Despite the promise, their power conversion efficiency has not yet matched that of lead-based solar cells (>25%). The soft nature of the tin-halide lattice and the facile oxidation of tin(II) to tin(IV) result in high probability of defect formation and increased background doping levels, thereby leading to significant carrier recombination and low power conversion efficiencies in devices.[1] Recent research has focused on understanding and enhancing carrier dynamics, particularly examining the effects of electronic doping and defect states on the quality and optoelectronic properties of the semiconductor. Optimizing doping levels and minimizing defect concentrations are critical for improving device performance.

By combining simulations with spectroscopy measurements, the role of defects in polycrystalline thin films of tin-based perovskites has been rationalized. Through chemical composition tuning, the chemistry of defects can be altered. Very high background doping limits carrier dynamics due to enhanced recombination of carriers through Auger processes. [2] Electronic doping can be controlled by adding excess tin in the precursor solution, such as extra SnF₂, or by partially substituting iodine with bromine. Furthermore, fluoride can passivate Sn surface defects,[2] while halide alloying tailors the bandgap of the material and improves structural stability.[3] Importantly, both single-halide and mixed-halide tin perovskites exhibit excellent photostability.[4-5] The central cation does not directly affect electronic doping, it significantly influences the morphology of the polycrystalline film, which in turn affects mobility and conductivity.

Integrating these findings provides a comprehensive understanding of how doping, defect states, and chemical composition interact to influence the performance of tin halide perovskites. Leveraging these insights can pave the way for the development of highly efficient and stable perovskite-based devices, advancing the field of renewable energy and optoelectronics.

 Lead halide perovskite (HaPs) shows fast technological progress in photovoltaic and other devices due to their attractive optoelectronic properties. However, the stability of perovskite films remains a major roadblock to their practical implementation which creates concern and calls for advanced characterization methods. The intrinsic self-healing (SH) property of halide perovskites, owing to their strong lattice dynamics, offers a potential solution to this issue. SH means that a material can recover from damage autonomously. Till today, though the mechanism(s) of SH in HaPs and the experimental and material parameters and properties for it are still mostly a black box.

We will show how the SH dynamics can be measured by combining Fluorescence Recovery After Photobleaching (FRAP) and PL imaging, for MAPbI₃ (MAPI) and γ-CsPbI₃ (CsPI) [1-2].  FRAP quantifies fluorescence recovery after photobleaching (by a high-intensity laser source) in a particular region of interest (ROI). Here we use FRAP to resolve the diffusion kinetics of SH-related species.  Specifically, we damaged at different power densities for 3 seconds at 8 different ROIs. For both CsPI and MAPI, we observe a combination of fast (few seconds or less) and slow (tens of minutes) kinetics, both at the illuminated spot and in regions of the film surrounding it. In the periphery of the excitation spot, CsPI shows photo-darkening and MAPI shows photo-brightening, immediately following damage. During the self-healing process, of the directly illuminated spot, MAPI peripheral fluorescence returns to its initial level, whereas CsPI exhibits gradual photo-brightening to above the original level.

We attribute these spatio-temporal effects to a combination of fast carrier diffusion following photoexcitation and slower ionic diffusion that kick in at later stages.  The role of carrier migration in emission dynamics following photoexcitation has generally been neglected but may be an important factor in experiments studying the system response under illumination. Assessment of the diffusion coefficients for the slower process from the self-healing rate is commensurate with halide ion diffusion. This is partially supported by Raman spectroscopy of damaged films. Overall, our work provides visual evidence for the spatiotemporal dynamics of photo-damage and SH of HaPs, an understanding of which will aid the development of long-term device applications.

Further improvements in the performance of perovskite solar cells requires a deep understanding of their device physics and the different power loss mechanisms. In the literature, defect densities at the perovskite/transport layer interfaces, ion-mediated non-radiative recombination and imperfect charge extraction have been cited as the main sources of these power losses. However, the magnitudes of the parameters of these loss mechanisms, such as defect densities, ion densities, capture coefficients and time constants/lifetimes are not well known. This is due to the difficulty of discriminating between multiple mechanisms that respond simultaneously in a typical characterization measurement, in addition to the lack of optoelectronic models describing these loss mechanisms clearly. In this talk, I will provide improved data analysis methods for typical optoelectronic methods (both in the time and frequency domain) used to characterize perovskite solar cells, to accurately discriminate between the loss mechanisms and determine their parameters.

In the case of capacitance measurements to determine defect/doping/ion densities, I will show that the capacitance response of the perovskite layer is hidden by the response of the electrodes and the resistive transport layers. This effect dominates the response in several capacitance measurements reported in literature, leading to the calculated defect densities and related parameters largely being artefacts of measurement. Analytical resolution limits are derived for these techniques to distinguish a real defect response from a measurement artefact. Finally, I will show experimental measurements on single crystal devices that overcome these limits and allow identifying the actual defect density in the device.

I will furthermore develop an optoelectronic model that explicitly accounts for the extraction of charge carriers through the resistive transport layers, and apply it to the analysis of small perturbation methods in the time and frequency domain. This model predicts the existence of a time constant for charge carrier extraction, in additional to bulk and electrode-mediated recombination time constants. By developing modified transfer functions in the frequency domain, the charge extraction and recombination time constants can be accurately extracted using these methods. I will also develop a figure of merit that determines the efficiency of charge extraction in the perovskite solar cell. This figure of merit depends on the time constants of charge extraction and recombination, and can be calculated at each bias point of the current-voltage curve. Figure of merit values between 0.7–0.95 at or close to the 1 sun open-circuit voltage are obtained experimentally, indicating a significant electric field exists in the transport layers in these conditions.

Understanding the atomic-scale crystallographic properties of photovoltaic semiconductor materials such as silicon, GaAs, and CdTe has been essential in their development from interesting materials to large-scale energy conversion industries. However, studying photoactive hybrid perovskites by transmission electron microscopy (TEM) has proved particularly challenging due to the large electron energies typically employed in these studies. [1,2] In particular, the very close structural relationship between a number of crystallographic orientations of the pristine perovskite and lead iodide has resulted in severe ambiguity in the interpretation of EM-derived information, severely impeding the advance of atomic resolution understanding of the materials.

In this talk, I will outline how to reliably study hybrid organic-inorganic perovskite materials using electron microscopy. With the ability to image the pristine phase of these beam-sensitive materials, we are able to obtain highly localised crystallographic information about technologically relevant materials. Using low-dose selected area electron diffraction, I will show how mixing the archetypal CH(NH2)2PbI3 (FAPbI3) and CH3NH3PbI3 (MAPbI3­) improves solar cell device performance through the elimination of twin domains and stacking faults. [3]

Using a careful low-dose scanning TEM (STEM) protocol, we are also able to image these materials in their thin-film form with atomic resolution. [4] Our images enable a wide range of previously undescribed phenomena to be observed, including a remarkably highly ordered atomic arrangement of sharp grain boundaries and coherent perovskite/PbI2 interfaces, with a striking absence of long-range disorder in the crystal. These findings explain why inter-grain interfaces are not necessarily detrimental to perovskite solar cell performance, in contrast to what is commonly observed for other polycrystalline semiconductors. Additionally, we observe aligned point defects and dislocations that we identify to be climb-dissociated, and confirm the room-temperature phase of CH(NH2)2PbI3 to be cubic. We further demonstrate that degradation of the perovskite under electron irradiation leads to an initial loss of CH(NH2)2+ ions, leaving behind a partially unoccupied, but structurally intact, perovskite lattice, explaining the unusual regenerative properties of partly degraded perovskite films. Our findings thus provide a significant shift in our atomic-level understanding of this technologically important class of lead-halide perovskites.

Finally, I will show how we can use gentle conductive AFM to probe the IV-characteristics of mixed halide perovskite thin films very locally, showing how we can control the local hysteresis behaviour by controlling the presence of extended intragrain defects using additives. This allows us to highlight the influence of ion migration on the optoelectronic properties of perovskite thin films with very high resolution. [5]

Our findings thus provide a significant shift in our atomic-level understanding of this technologically important class of lead-halide perovskites.

Ion migration is usually viewed as detrimental to the performance of perovskite solar cells. In particular, the high mobility of the halide anions can initiate phase separation and other material changes which lead to bulk and interface degradation and loss of cell efficiency.

On the other hand, it is becoming clear that if ion mediated degradation can be prevented, then there are many benefits to the presence of mobile ions in perovskites.

Recently it has been demonstrated computationally and experimentally that mobile ions improve the performance of perovskite cells with non-ideal interfacial band offsets [1][2]. The interplay between ion location and charge recombination can also be used to gain key information about the band off-sets in fully operational devices [3]. The ion mediated low frequency impedance response can tell us whether a cell is electron or hole recombination limited and indicate which interface is limiting cell performance [4]. In this presentation the benefits and disadvantages of mobile ions will be discussed; with a particular focus on the key information the ionic response can give us in common characterisation techniques such as JV curves and impedance spectroscopy.

Hybrid organic inorganic metal halide perovskites (MHPs) denote a family of compound semiconductors, which established a novel class of optoelectronics, most prominently known for the perovskite solar cell. While the power conversion efficiency of these photovoltaic devices saw a steep rise in the past decade, tailoring the interfaces between the MHP film and charge transport layer became the major control lever to enhance performance and stability. The use of photoemission spectroscopy to analyze the chemical and electronic properties of these interfaces has been challenging due to many possible chemical reactions at the buried interfaces.¹ However, the formation of a stable interface between the perovskite film and adjacent functional layers is critical to enable effective protection coatings.

Here, I will discuss the use of synchrotron- and lab-based X-ray photoelectron spectroscopy (XPS) experiments to address the particular chemistry of MHP interfaces to adjacent oxide charge transport and pre-encapsulation layers (CTL). At the example of SnO₂ and NiO layers grown by atomic layer deposition (ALD) on top of a double cation mixed halide perovskite film investigated by hard X-ray photoelectron spectroscopy (HAXPES), we find evidence for the formation of new chemical species and changes in the energy level alignment at the interface detrimental to cell performance.²

I will conclude with a general discussion on the use of PES methods for the analysis of MHP layers and in particular the effect of irradiation-induced damage via synchrotron and lab-based X-ray sources,³ which we also use to track unique physicochemical phenomena such as stimulated self-healing in formamidinium lead bromide.⁴

Hybrid metal halide perovskites have given rise to some of the most exciting developments in optoelectronics over the last decade, and 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, charge-carrier mobilities, and charge-phonon coupling in perovskite semiconductors. I will discuss the impact of heterogeneities and low dimensionality, and the peculiar softness of the lattice. Our work reveals band transport with high in-plane mobilities that give rise to efficient long-range conductivity in 2D perovskites. We show how the organic cation moderates the coupling of charge carriers to optical phonon modes, impacting the charge-carrier mobilities. Furthermore, we demonstrate a new experiment for simultaneously recording the terahertz and optical transmission transients, thus allowing us to monitor the exciton formation dynamics over the picosecond timescale. The observed dynamics reveal 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. Our findings provide new insights into the temperature-dependent interplay of exciton and free charge carriers in 2D Ruddlesden-Popper perovskites. Furthermore, the sustained free charge-carrier population and high mobilities revealed by this work demonstrate the potential of these semiconductors for applications that require efficient charge transport, such as solar cells, transistors, and electrically driven light sources.

All-inorganic halide perovskite nanoplatelets (PNPls) have recently gained significant attention in the field of materials science. These materials exhibit remarkable optoelectronic properties, making them promising candidates for applications in solar cells, LEDs, and photodetectors. The interest in PNPls stems from their exceptional electronic and optical properties, which can be tuned over a wide range that are deeply intertwined with their unique structural characteristics. Tuning the properties of PNPls has relied at first on changing of the elemental compositional of the perovskite structure, whether the halide part and/or the 1^st cation from organic to inorganic, or between different organic molecules. Over the last few years, studies have shown that optical tuning of PNPls based on the number of monolayers (MLs) is a viable strategy, where the monolayer corresponds to a single microscopic layer of inorganic metal-halide octahedrons surrounded by large organic cations or ligands. While extensive research has explored the optical properties of these systems, the role of phonons (lattice vibrations) in different dimensionalities remains underexplored. This is important for a fundamental understanding of the collective carrier-phonon coupling in the excited states, thermalization process, initial charge separation and final transport that includes the mobility of electrons and holes and their interconnection to the charge carrier-lattice interactions. Since all the above-mentioned phenomena are dimensionality- and phase-related, where the coupling between the excited state and phonons changes dramatically with both the dimensionality and phase, the vibrational properties over different system dimensionalities and/or crystallographic phases should be revealed for a better understanding of the role of phonon in the materials.

In this work, we employed optical and non-resonant Raman spectroscopy for the realization of, firstly, a model for the vibrational properties of 2-6 MLs of CsPbBr₃ NPls in comparison with larger nanocrystals. Our Raman measurements showed that, by systematically varying the number of monolayers of the nanoplatelets, there is distinct changes in the relative intensities of the vibrational modes that are sensitive to the number of monolayers. These observations can be attributed to the quantum confinement effect, which becomes more pronounced as the thickness of the nanoplatelets decreases. In addition, there is strain generated in the materials upon the formation of lower thickness NPls. This can be identified through the shift of Pb-Br vibrational mode. Secondly, we established a model of the vibrational properties over a wide temperature range to identify changes in phase, strain, and anharmonicity for different system dimensionalities. Raman measurements revealed the tetragonal phase formation at room temperature for all prepared nanocrystal systems is dominant with the orthorhombic and cubic phases at low and high temperatures, respectively. This in addition to the phase related and/or the anharmonicity of the shift of phonon modes with temperature. Furthermore, there were changes in the peak intensities’ ratios, which provide valuable insights into the structural variations induced by the number of monolayers.

Strain, phase transitions, anharmonicity, and their dependence on dimensionality are important for electrical and thermal conductivity. Moreover, it helps to clarify the mechanism and pathways of electronic energy into heat in 2D lead halides. Therefore, these results provide an essential initial overview of the crucial vibrational properties of the system, paving the way for a better understanding of carrier-phonon coupling in these materials. The ability to determine these structural parameters via Raman spectroscopy establishes it as an indispensable characterization technique for rapid and accurate analysis of thickness and confinement regimes in perovskite nanoplatelets and suggests its potential for dimensionality analysis in other nanocrystal families.

Three-dimensional (3D) and low-dimensional (LD) perovskite solar cells (PSCs) are among the most promising strategies for achieving highly efficient and stable perovskite solar cells. Despite being popular and effective, the necessity of the surface LD perovskite (LDP) layer remains uncertain [1]. The use of organic salts without forming an LDP raises questions about the need for this capping layer. I will compare recent results obtained with the 3D/LDP configuration to those using surface cation passivation and 3D/LDP bilayers, both of which can achieve the highest efficiency values in perovskite solar cells. This comparison will provide a comprehensive perspective on the benefits of these two strategies, including examples where both approaches have achieved efficiencies exceeding 23%[2,3].

Reference
[1] Sam Teale, Matteo Degani, Bin Chen, Edward Sargent, Giulia Grancini, Nat Energy, 2024.
[2] M. Degani, Q. An, M. Albaladejo-Siguan, Y. J. Hofstetter, C. Cho, F. Paulus, G. Grancini and Y. Vaynzof, Sci. Adv, 2021, 7, 7930
[3] Matteo Degani, Riccardo Pallotta, Giovanni Pica, Masoud Karimipour, Alessandro Mirabelli, Kyle Frohna  Miguel Anaya, Samuel D. Stranks, Monica Lira Cantù, Giulia Grancini, submitted

To make better electronic and light-based devices using advanced materials, we need to understand how their structure affects their properties. Hybrid organic/inorganic perovskites are promising new materials, which are both soft and ionic, i.e. atoms can wiggle around their equilibrium positions and they are highly charged¹. Transitions of electrons to different bands will distort the crystal lattice, and it will do so along specific vibrational coordinates. While there is consensus that electron-phonon interactions in these materials are crucial in determining their optoelectronic properties, i.e. EPC limits the maximum charge-carrier mobility², it has been extremely challenging to obtain direct- and mode specific information on these interactions.

The big question is: which vibrational modes are coupled how strong to which electrons? And how do we measure this?

Here, we will dive into the world of (multidimensional) THz spectroscopy to answer the above questions. I will highlight our recently developed 2D-electron-phonon-coupling spectroscopy (2D-EPC), which can extract direct information on mode-specific electron-phonon interactions. We benchmarked this technique on prototypical methylammonium lead iodide, and show there is distinct behavior of the coupling of electrons between two different vibrational modes. Temperature dependent experiments allow us to follow the EPC over the tetragonal-to-orthorhombic phase transition, and polarization dependence allows us to study mode-specific and direction-dependent dispersion of this coupling³.

We also varied the A-site cation in three prototypical ‘black-phase’ perovskites: methylammonium (MA) lead iodide, formamidinium (FA) lead iodide and Cesium lead iodide, and demonstrate differences in the coupling between vibrational modes of the inorganic lattice and electrons with different energies.

Lead halide perovskite-based photovoltaics has attracted great attention in the last decade since silicon PV as the dominating technology is approaching its theoretical efficiency limit. Combined in a silicon-perovskite (Si-Pero) tandem solar cell, they promise a substantial leap in power conversion efficiency beyond the single-junction limit without increasing cost significantly. However, realizing high performance in module-sized formats is, in part, held back by the practical challenge of producing uniform high-quality perovskite layers onto large scale silicon bottom cells. Perovskite thin films are highly poly-crystalline and mechanically soft resulting in high defect concentration and inhibiting efficient carrier extraction if not properly controlled.

In pursuit of a non-invasive and robust in-line imaging method that locally resolves the quality of the perovskite thin film absorber processed over a silicon bottom solar cell, we advanced the k-imaging method developed by Hacene et al. [1] also for Si-Pero tandem solar cells. K-imaging is based on intensity dependent perovskite photoluminescence, eliminating the effect from constant optical in- and out-coupling effects. With a basic power law model a single effective parameter k is extracted which reflects the complex superposition of competing recombination processes, i.e. radiative band-to-band, non-radiative trap-assisted bulk (Shockley-Read-Hall) and interface recombination. This parameter k allows for quantitative analysis simplifying the interpretation significantly in contrast to qualitative PL imaging, while barely raising setup complexity and cost.

We show that k-imaging reproducibly identifies general quality discrepancies as well as local inhomogeneities, which clearly correlate with typical defects in the thin film. Further, we proved its applicability regardless of specific perovskite processing techniques and its compatibility with flat as well as industrially relevant textured Silicon bottom cells. Overall, k-imaging is a valuable and unique technique meeting the requirements for efficient optimization in academic research as well as quality assessment in large-scale industrialized production of Si-Pero tandem solar cells.

Despite the remarkable optoelectronic efficiencies of hybrid perovskite based devices heterogeneity in both the performance and stability is observed at the nanoscale [1,2]. Furthermore, improvements in the performance of solar cells, LEDs and X-ray detectors are often achieved through empirical findings, with a full understanding of the underlying structural mechanisms yet to be elucidated. Considering this fact, we present how correlative multimodal microscopy, enabled by computer vision algorithms, has allowed for structural insights into: the intrinsic quantum confinement phenomena observed in FAPbI₃ [3]; and the origins of photo-induced halide segregation observed in mixed halide compositions [4].

Scanning electron diffraction (SED), a variant of 4D-STEM, allows for information on crystallographic phase, orientation, and defects to be uncovered; whereas hyperspectral photoluminescence (PL) mapping provides information on optoelectronic characteristics. The spatial correlation between the two techniques therefore provides a direct link between structure and performance. Similarly to the work of Jones, Osherov and Alsari et al. [5] to find common areas between separate tecniques, gold fiducial markers are synthesized and deposited onto a thin film. Due to the resolution differences between SED (typical probe size ∼5 nm) and hyperspectral PL mapping (resolution diffraction limited to ∼100s of nm) multiple contiguous SED scans are recorded before being stitched together during post processing. To do this robustly a keypoint detection and matching algorithm is applied to virtual bright field images created from the SED data [6]. Once the keypoints are detected the random sample consensus algorithm, or variants thereof, is used to define an affine transform which stitches one image onto the other, thus correcting for differences in rotation, shear, and translation [6]. To finally coregister the hyperspectral PL and stitched SED datasets several options are available, however common approaches operate on the principle of maximising metrics such as the normalised cross correlation or mutual information between datasets [7]. Importantly, as all image transforms are known we can then calculate where each ‘pixel’ from a SED scan corresponds to in the hyperspectral PL data.

Due to the size and complexity of the resulting datasets interpretation of the data is challenging. For instance, if only 50 SED scans are considered, each of which contains 512 x 512 diffraction patterns (typical during one experimental session), this amounts to a total of 13,107,200 individual patterns and hundreds of gigabytes of data. To mitigate this problem and rapidly reduce the dimensionality of the data we have adapted the simple linear iterative clustering (SLIC) algorithm, prevalent in the field of remote sensing [8]. This approach allows us to reduce hundreds of thousands of individual diffraction patterns to ∼100 single crystal patterns obtained by averaging over individual grains of the polycrystalline film. Remarkably, once parallelised, this approach proves exceptionally computationally efficient with typical compute times taking approximately a minute using a standard desktop machine (32Gb RAM, 11th Gen Intel(R) Core(TM) i5-11400 CPU). An automated indexing procedure of the clustered SED patterns can then be employed to obtain phase and orientation maps over large areas (typically ∼20x20 μm) at low computational cost.

This approach has allowed the structural causes of the intrinsic quantum confinement phenomena observed at cryogenic temperatures in FAPbI₃ to be attributed to {111}_c type nanoscale twinning, and an understanding of the structural origins of photo-induced halide segregation to be obtainable. We hope this toolkit, with data analysis pipelines which are open source, enables correlative microscopy experiments with other instruments and on other material systems. Furthermore, we anticipate the insights provided on hybrid perovskites will allow for more directed modifications to the fabrication of devices, thereby accelerating improvements in device efficiency and stability.

Even though the efficiency of perovskite solar cells has increased significantly in recent years, their long-term stability is still poor, hindering commercial applications. One of the main reasons for this poor stability is mobile ionic carriers, which can migrate within the perovskite crystal lattice and accumulate at the adjacent charge transport layers, reducing the extraction efficiency of electronic carriers. To compare strategies to mitigate ion migration, reliable ways of quantifying the density, mobility, and activation energy of mobile ions are necessary. Here, we propose an updated way of characterizing mobile ions based on capacitance transients. In capacitance transients, we generally measure the modulation of the electronic capacitance due to mobile ions drifting through the perovskite. In our updated approach, we first approximate the time-dependent ionic carrier, electronic carrier, and potential distribution within a perovskite solar cell after applying a voltage pulse. Subsequently, we calculate the capacitance from these distributions using a small-signal approximation of the drift-diffusion equations, resulting in capacitance transients. By fitting capacitance transients generated from drift-diffusion simulations, we show that an accurate extraction of the density, mobility, and activation energy of mobile ions within the perovskite is possible. Lastly, we apply the proposed model to measured capacitance transients of p-i-n perovskite solar cells and approximate their ion density, ionic mobility, and activation energy.

A recent classification of multilayered perovskites as Ruddlesden-Popper, Dion-Jacobson and "Alternative cations in the interlayer" was introduced in relation with the chemistry of the compounds or the crystallographic order along the stacking axis. 2D multilayered perovskites  exhibit  indeed attractive features related to tunable quantum and dielectric confinements, strong lattice anisotropy, more complex combinations of atomic orbitals and lattice dynamics than 3D perovskites, extensive chemical engineering possibilities. But more important for photovoltaic applications, 2D multilayered perovskites have exhibited improved device stability under operation [1]. Combined in quasi-lattice matched [2] 2D/3D thick bilayer structures, excellent solar cell device stability can be achieved. Band alignment calculations nicely explain the difference of performances for n-i-p or p-i-n devices [3]. The lattice mismatch concept [2] can also provide guidance for the choice of the proper 2D/3D combination, with the prospect of using 2D perovskite as a template during the growth of a 3D perovskite, leading also to enhanced stability of 3D-based solar cells. This was demonstrated recently the stabilization of MA-free, solar cells based on pure FAPbI3 [4]

Nanoscale resolved imaging and spectroscopy using scattering-type Scanning Near-field Optical Microscopy (s-SNOM) or tapping AFM-IR (local detection of photothermal expansion) bypasses the diffraction limit of light to achieve a wavelength-independent spatial resolution of < 20 nm in the infrared (IR) frequency range [1,2]. A wide range of analytical capabilities have been demonstrated, e.g. nanoscale chemical mapping and material identification [3], conductivity profiling [4,5], determination of secondary structure of individual proteins [6] and vector field mapping [7], making them a trusted tool for surface analysis in many branches of sciences and technology. Applications are often limited by a lack of suitable light sources, preventing studies of low energy phonons, polaritons, and molecular vibrations. Here we demonstrate s-SNOM and tapping AFM-IR imaging and spectroscopy based on a fully integrated and automated commercial OPO laser source, covering the spectral range 1.4 – 18 μm (ca. 7140 – 550 cm-1) with narrow linewidth < 4 cm-1 in the entire tuning range. Sweeping the laser frequency enables nano-spectroscopy with unprecedented spectral coverage, enabling studies of fundamental molecular resonances and quantum states in the long wavelength IR spectral range, which until now was not possible.

The performance of metal halide perovskite (MHP) thin films in optoelectronic devices is heavily influenced by their final crystal structure and nanomorphology. Therefore, understanding growth processes during deposition and post processing or ageing is important to control the performance in optoelectronic devices. I will show examples of the application of advanced in-situ characterization techniques to gain real-time insights into the formation and evolution of perovskite films. By monitoring the growth dynamics of perovskite thin films during solution processing, we can identify critical factors that govern the nucleation, intermediate phase formation, and final crystallization of the material. In-situ techniques such as grazing incidence wide angle X-ray scattering (GI-XRD), optical microscopy, and photoluminescence (PL) spectroscopy are employed to capture transient phases and intermediate states that are otherwise challenging to detect in ex-situ studies. Machine Learning approaches may help us in future to reduce characterization cost. This research contributes to the broader understanding of MHP thin film fabrication, offering guidelines for the scalable production of high-performance optoelectronic devices.

Stereochemically active lone pair (SCALP) cations are attractive units for realizing optical anisotropy. Antimony(III) chloride perovskites with the SCALP have remained largely unknown to date. We synthesized a new vacancy ordered Cs₃Sb₂Cl₉ perovskite single crystals with SbCl₆ octahedral linkage containing the SCALP. Remarkably, all-inorganic halide perovskite Cs₃Sb₂Cl₉ single crystals exhibit an exceptional birefringence of 0.12 ± 0.01 at 550 nm. The SCALP brings a large local structural distortion of the SbCl₆ octahedra promoting birefringence optical responses in Cs₃Sb₂Cl₉ single crystals. Theoretical calculations reveal that the considerable hybridization of Sb 5s and 5p with Cl 3p states largely contributes to the SCALP. Furthermore, the change in the Sb−Cl−Sb bond angle creates distortion in the SbCl₆ octahedral arrangement in the apical and equatorial directions within the crystal structure incorporating the required anisotropy for the birefringence. This work explores pristine inorganic halide perovskite single crystals as a potential birefringent material with prospects in integrated optical devices.