The photovoltaic performance and long-term stability of metal halide perovskite solar cells (PSC) are critically governed by the energetics and chemistry of their buried interfaces. In particular, hole-selective contacts remain a limiting factor, as their surface reactivity, defect formation pathways, and energy-level alignment with the perovskite absorber strongly influence interfacial recombination and device degradation. Here, I will present a comprehensive investigation of perovskite/HTL interface formation across both inorganic and organic contact strategies, combining soft and hard X-ray photoemission spectroscopy (XPS, HAXPES), ultraviolet photoemission (UPS), and inverse photoemission spectroscopy (IPES).

We begin by examining inorganic NiO hole transport layers in the n-i-p device configuration. Our synchrotron-based HAXPES measurements revealed that the nickel oxide film grown by atomic layer deposition (ALD) directly on top of the perovskite contained large amounts of hydroxide and oxy-hydroxide species and induced the formation of nitrogen containing and lead containing defect states inside the adjacent perovskite. These interfacial reactions were found to degrade the photovoltaic performance. Introducing an organic buffer layer in the form of a 20 nm thick PTAA film between the perovskite and the ALD-NiO suppressed these reactions and improved both device efficiency and operational stability [1].

A complementary study focused on ultraviolet ozone treated nickel oxide and the incorporation of [2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz) as an ultrathin organic interlayer in p-i-n PSCs. Here, we find that the MeO-2PACz interlayer mitigates the formation of interface defects between the NiO film and a halide perovskite layer deposited on top, by passivating the NiO surface, which suppresses the reduction of Ni and restores favorable energetics [2].

We then extend our investigation to 2D/3D perovskite heterostructures used for surface passivation. By tracking the evolution of energetics as a function of 4‐fluoro‐phenethylammonium iodide (4-FPEAI) 2D layer thickness, we show that favorable band alignment is confined to the ultrathin regime: while a monolayer preserves efficient hole extraction, thicker 2D layers induce band-edge shifts of up to 0.2 eV, resulting in detrimental energy barriers. Subsequent deposition of the benchmark organic HTL 2,2',7,7'-Tetra(N,N-di-p-tolyl)amino-9,9-spirobifluorene (spiro-TTB) reveals how these subtle shifts propagate into the CTL/perovskite interface, altering charge-transfer pathways. Supported by DFT calculations, our findings provide direct spectroscopic evidence that the 2D/3D interfacial energetics are highly thickness-dependent.

Understanding charge-carrier collection in metal halide perovskite solar cells (PSCs) is essential to minimizing interfacial losses and achieving high device performance [1]. Despite competitive efficiencies, PSCs lack a simple and straightforward method to quantify interfacial collection losses from steady-state measurements. The presence of mobile ions and field screening complicates the interpretation of steady-state measurements, while advanced transient spectroscopies remain accessible only to specialized laboratories [2].

Here, we introduce a broadly applicable analytical framework to extract spatially resolved charge-collection information from standard internal quantum efficiency (IQE) spectra, without assuming ideal contacts or diffusion-dominated transport. By analyzing the limiting regimes of strong and weak light absorption, we linearize the IQE with respect to the absorption coefficient, enabling direct extraction of physically meaningful quantities: the collection efficiency at the illuminated interface fC(0) and the average collection efficiency across the bulk absorber ⟨fC⟩. We demonstrate the approach experimentally using perovskite devices with and without an electron-transport layer, resolving contact-selectivity limitations and quantifying JSC penalties. The reconstructed IQE spectrum closely matches experimental data, validating the methodology [3].

1. Introduction

This study focuses on the 2D numerical modeling of perovskite-silicon heterojunction (PK-Si) two-terminal (2T) tandem solar cells (see Figure (top)), aiming to enhance the understanding and reliability of characterisation measurements. The approach involves developing a technology computer-assisted design (TCAD) comprehensive model to evaluate the impact of diverse physical phenomena, using Silvaco ©Victory Device. We use the tool to simulate photogeneration and to solve drift-diffusion equations, while the electrochemistry module handles mobile ion transport. The silicon heterojunction (SHJ) model is inspired by previous work on defectivity physics.[1] Among other critical interfaces, modeling the type II heterojunction between the hole transport layer of the top-cell and the n-doped silicon (Si) through transparent conductive oxide is challenging.

We propose an efficient solution and present one of the first 2D large-scale detailed model of PK-Si cells, while former ones were constrained to unidimensional, simplified simulations.[2,3]

2. Novelties and preliminary results

Building the perovskite (PK) top-cell involves numerous, hardly experimentally accessible parameters (e.g., carrier mobilities, ion densities and diffusivities, Shockley-Reed-Hall (SRH) recombination rates, ...): this, and the lack of standard materials, hinder an easy setting of the simulation. Our solution to calibrate the present model is to use time-resolved photoluminescence (TRPL) measurements on the full cell: these measurements are directly linked to the material properties and interfaces of the active PK region. We represent such a measurement result in the Figure (middle) in blue, along with a simulation result after machine learning (ML) calibration (red). By performing numerous 2D-TCAD simulations with varying parameters, we could train a ML regression model to predict the distance between the simulated and experimental TRPL curves. This allowed to converge toward a set of parameters yielding the simulation of the Figure. The parameter values resulting from the fitting procedure are shown in the middle inset.

As stated above, an important innovation of the present work is the 2D engineering of the TCAD model, including an electrode (see Figure, bottom). This opens the way to investigate the impact of inhomogeneities in real-world systems. Indeed, in the process of cell fabrication, defects may occur (device handling, layer growth, ...). Their impact on cell efficiency might be dramatic since locally the junction separating holes and electrons might not be present anymore, creating a ”shunt” in the device. Also, defects in the bulk of the PK absorber might act as deleterious recombination centers. The present demonstration features a defect in the PK with a hundredfold lower SRH lifetime, simulating for example a highly defective PK grain. Its width was swept to yield the bottom inset in the Figure. Interestingly, Voc and Jsc seem not to be affected by these defects, contrarily to the FF that dramatically drops when the width increases from 1 to 300 μm. On the Figure (bottom), the extracted SRH recombination rate proves that losses occur at this defect, rather close to the front surface.

3. Perspectives

As a conclusion, the present 2D-TCAD-based simulation framework helps us improve our understanding of the realistic device under operation. Future work will focus on extending the TCAD model to include additional defects. Subsequently, a sensitivity analysis of the different types of defects will be possible.

Solar cells have a great potential in replacing fossil fuels in electricity generation, if requirements of low production costs can be met. In the last years, lead halide perovskites have drastically changed the solar cell research field due to their ease of synthesis and high power conversion efficiencies, which now reach over 25%. The future success of these developments crucially depends on understanding the details charge separation, charge transport and charge recombination at the interfaces between the different layers in a solar cell as well as what parameters limit solar cell stability. X-ray based techniques such as photoelectron spectroscopy (PES) are powerful tools for obtaining electronic structure information of materials at an atomic level.

In this presentation, I will show how we have used photoelectron spectroscopy to gain insights into the interface between the perovskite, and the electron transport layers in a p-i-n structure (C₆₀, BCP and silver). These studies are carried out by in-vacuum cleaving of perovskite single crystals and therefore obtaining a clean crystalline perovskite surfaces [1]. We then follow the details of band alignment of C₆₀ on this surface through sequential evaporation of the electron transport material. We show that this interface is chemically stable but shows downwards band bending towards the perovskite, which is unfavourable for charge extraction in the solar cell. This band bending can be counteracted upon addition of BCP. However, upon evaporation of the back contact silver, we find that chemical reactions occur at the interface with perovskites ions migrating towards the silver contact.   

Hot carrier solar cells have gained increasing attention due to their potential for achieving high-efficiency solar cells. With phonon regulation in perovskite materials, the hot carrier lifetime and initial carrier temperature can be significantly enhanced. Therefore, exploring methods to regulate electron-phonon coupling in perovskite materials may be an effective approach to advancing hot carrier strategy. By introducing various alkyl chains, the hot carrier lifetime can be improved. Using ultrafast transient spectroscopy, the relaxation and surface recombination kinetics of hot carriers can be measured. According to the transient kinetic results, perovskite materials with the highest entropy A-site cations exhibit the highest initial carrier temperature, longest carrier relaxation time, and slowest hot carrier relaxation rate. As the entropy of the A-site cations increases, perovskite materials show a trend of increasing initial carrier temperature and phonon emission time. This behavior can be attributed to the reduced electron-phonon coupling strength caused by the entropy increase of the A-site cations. Our findings demonstrate the potential of using high-entropy cations to drive hot carrier perovskite solar cells.

Lead halide perovskites exhibit a unique combination of defect tolerance, long carrier lifetimes, and solution processability, enabling rapid advances in photovoltaic and optoelectronic device performance. Yet, despite power conversion efficiencies now exceeding 26%, fundamental questions remain regarding the role of doping and defect-mediated electronic processes in these materials. A particularly intriguing but underexplored phenomenon is photodoping—the illumination-induced imbalance of electron and hole populations caused by preferential trapping of one carrier type in shallow, localized states. While early studies proposed photodoping as a key feature of halide perovskites, its quantification has been hindered by the difficulty of experimentally distinguishing electron and hole densities, as most optical probes are sensitive only to their sum or product. In this work, we demonstrate that both steady-state and transient photoluminescence (PL) provide accessible, powerful routes to identifying and characterizing photodoping in perovskite thin films. We show that the excitation-intensity dependence of steady-state PL directly reflects changes in charge neutrality conditions and yields modified ideality factors, including extended regions where n_id ≈ 1.5—behavior not captured by classical trap-assisted recombination models. By incorporating photodoping into recombination frameworks, we connect these optical signatures to the presence and density of shallow traps that may contribute simultaneously to defect tolerance and recombination asymmetries. Complementarily, we use high dynamic range transient PL measurements, enabled by ultra-low repetition rate excitation and gated CCD detection, to probe power-law decay dynamics spanning microseconds to beyond 100 μs [1]. By using particularly low repetition rates, we extract decay times over a large range of injection conditions, providing an increased amount of confidence in the inferred defect parameters.

Halide perovskites (PVK) can be used as absorber material in solar cells because of their high-power conversion efficiency, low-cost and potential industry-scalable technology. Anyway, together with lab-to-fab scaling, their long-term stability due to air, humidity and temperature sensitivity still stands as a major factor preventing industrialization. The need to understand and mitigate these ageing processes brought us to develop a fast and reliable experimental photoluminescence (PL) imaging protocol to investigate the PVK fragile structure. We illustrate a method through which spectral and time-resolved fluorescence parameters can be compared and correlated on operando characterization of PVK in different relative humidity (RH) and temperature (T). In this context, machine learning denoising was used to accelerate and improve data acquisition and treatment.     

Studying photovoltaic materials with photoluminescence imaging techniques, especially at 1-SUN equivalent generation, can be very hard because of poor local signal-to-noise ratio (SNR). In order to do not excessively extend the acquisition times, and reliably record the evolution in the material’s properties during operando experiments, we choose to shorten the measurement time-window and use a machine learning algorithm to ameliorate the images in post-processing.

We customed a new learning image restoration Noise2Noise (N2N) algorithm [2] to denoise multidimensional datasets, in which the loss function is zero-shot trained on a physics-based model for time-resolved fluorescence imaging (TR-FLIM) [3]. The main advantage of N2N with respect to other algorithms lies in its unsupervised learning framework, which makes it particularly well-suited to complex, real-world image denoising situations when no ground truth is accessible. With this approach, we performed in-situ TR-FLIM data acquisitions on halide PVK thin films under controlled humidity or temperature conditions (RH 85% or T 65 °C). Specifically, we tested half-cells of double-cation PVK (FA_0.83Cs_0.17Pb(I_0.83Br_0.17)₃) deposited by slot die, with or without self-assembled monolayer (MeO₂PACz) in its bulk. Finally, having acquired hyperspectral datasets on the samples at the beginning and at the end of the aging period, we were able to correlate the quasi-Fermi level splitting (QFLS) and lifetime images over the same illuminated area, for both fresh and aged materials.

The N2N denoising algorithm enabled us to resolute the micrometer-scale dynamics of spatial heterogeneity in PVK degradation and helped in distinguishing the different external stimuli response obtained from the various tested compositions. Moreover, it was crucial to unlock spectral-time resolved parameters correlation after aging.

In conclusion, this work stands as further proof of how unsupervised denoising algorithms like N2N make fast high-resolution imaging of halide perovskites possible. [4] Using AI to access physical/chemical properties of these materials under realistic conditions, not only makes us a step closer to resolve their long-term stability issues, but also paves the way for studying other sensitive materials in a wide range of applications.

Reducing nonradiative recombination is crucial for minimizing voltage losses in metal-halide perovskite solar cells and achieving high power conversion efficiencies. Photoluminescence spectroscopy on complete or partial perovskite solar cell stacks is often used to quantify and disentangle bulk and interface contributions to the nonradiative losses. Accurately determining the intrinsic loss in a perovskite layer is key to analyzing the origins of nonradiative recombination and developing defect engineering strategies. We study perovskite films on glass and indium tin oxide-covered glass substrates, functionalized with a range of different molecules, using absolute and transient photoluminescence. We find that grafting these substrates with 1,6-hexylenediphosphonic acid effectively reduces the nonradiative losses in pristine perovskite films for a series of perovskite semiconductors with widely different bandgaps.^[1] The results suggest that perovskites processed on HDPA-functionalized substrates suffer the least from nonradiative recombination and thus approach the properties of a defect-free semiconductor.

Still, remaining shallow defects dominate charge recombination in metal-halide perovskites. \ Shallow defects can be modified using perovskite bulk additives or top surface treatments and can impact efficiency. We studied the shallow defect properties in metal-halide perovskite films with passivation and electron transport layers on top. By measuring transient photoluminescence (tr-PL), we confirm that the tr-PL decay is dominated by shallow defects. Interestingly, lowering the temperature changes the trap energy landscape, making the shallow defects shallower until they vanish into the conduction or valance bands at cryogenic temperature. This result is corroborated by an increase in PL quantum yield and is explained by noting that the shallow defects are intrinsic to the perovskite and formed in a thermally activated process.  Choline chloride as passivating agent, C₆₀ as electron transport layer, or a combination of both on top of the perovskite can induce significant nonradiative recombination losses at room temperature, but cooling to cryogenic temperature makes the charge recombination dynamics almost identical across all treatments. At low temperature nonradiative recombination losses become less dominant because interfacial recombination velocities are reduced. The results provide a new insight into perovskite shallow defects and recombination losses and can help us to better understand the effect of surface treatments on shallow defect properties.

Lead halide perovskites are well known for their very large optical absorption coefficient in the visible spectrum, enabling thin film solar cells, few hundreds of nanometres in thickness, to fully absorb sunlight. Quantum mechanical unitarity of the absorption process ensures that the reciprocal process, optical emission, has an equally large matrix element and therefore is also very efficient, a feature that is known as reciprocity and that ensures that the best possible solar cell materials are also the best possible light emitting ones.

I will present evidence that light-matter is strong enough to give rise to polariton states, with a distinctive dispersion, even in unpatterned perovskite thin films, without external cavities or light confinement structures. Since correlated electron-hole pairs are the majority optical excitations in 3D hybrid perovskite thin films at room temperature, such polaritons are of a novel kind, not exciton-polaritons, but more appropriately band-edge polaritons. Not having to depend on the stability of bound excitons, band-edge polaritons survive at very large excitation densities.

Ultrafast optical spectroscopy experiments demonstrate that band edge polaritons even undergo Bose-Einstein condensation at the threshold for stimulated emission. Such findings open new avenues for perovskite optoelectronics thanks to the peculiar properties of polaritons: firstly, polariton lasers are potentially thresholdless because they do not require inversion nor optical gain; second, polaritons propagate at large speed in planar waveguides, and may be applied to integrated photonic circuits; third, and most important, polaritons inherit the strong nonlinearities from their matter component, so that efficient quantum gates may be realized.

The physical origin of the exceptional optoelectronic properties of halide perovskites has been the subject of intensive investigations. Various microscopic mechanisms have been proposed to explain the defect tolerance of these materials. These mechanisms are often assumed to be linked to the soft and polar lattice of HPs. However, further experimental investigations at the microscale are necessary to clarify the interactions between defects, the lattice, and charge carriers. HPs single crystals present a very low defect density and superior optoelectronic properties compared to polycrystalline thin films.[1][2] They represent the best platform for studying the intrinsic properties of HPs.[3][4] At very low temperatures, the presence of defect states associated with energy levels within the bandgap can be evidenced directly through their photoluminescence.

Low-temperature luminescence spectroscopy of high-quality single crystals revealed the presence of ultra-sharp emission lines (< 500 µeV at 4 K). A detailed analysis based on steady-state and time-resolved spectroscopy shows that these lines can be assigned to donor-acceptor pairs (DAP). Photoluminescence excitation spectroscopy reveals the existence of a highly efficient excitation pathway for these defect states, located below the bandgap and the free excitonic transition. The ultra-sharp emission lines present significant temporal fluctuations in their emission energy. Statistical analysis of the spectral diffusion (SD) revealed the existence of photoinduced spectral jumps. The spectral fluctuations present a transition from a Gaussian to a Lorentzian distribution with increasing excitation power. Remarkably, the lack of antibunching demonstrates that the emission originates from an ensemble of defect states. Additionally, synchronicity is observed in the SD of multiple lines, suggesting a coupling of an ensemble of defect states with slow correlated lattice deformations. These results provide new insights into the nature of defect states and their interactions with the lattice.

Thin films of lead halide perovskites find applications in photovoltaics, photodetection, and other areas of optoelectronics. One interesting research direction focuses on photoluminescent films that exhibit energy transfer and exotic optical phenomena such as amplified spontaneous emission, lasing, and superfluorescence. Regarding these photoluminescence phenomena, thin-film compositions of methylammonium lead iodide (MAPI) and mixed-dimensional 2D–3D phenethylammonium cesium lead bromide (PEA:Cs)PbBr3 have emerged as popular systems for studying radiative photophysics as a function of composition, phase purity, temperature, and excitation conditions. Given the dynamic nature of lead halide perovskite structure and energy flow, correlating spectroscopic observables with structural changes on similar time scales is of fundamental interest both for understanding mechanisms and for identifying ways to engineer these materials for optimal performance. In this talk, I will discuss our efforts to correlate time-resolved optical spectroscopy with time-resolved diffraction of MAPI and (PEA:Cs)PbBr3 thin films, in order to gain insight into mechanisms governing photoluminescence, energy transfer, and potentially superfluorescence at cryogenic temperatures and under high excitation conditions.

Pseudo-perovskites[1] is a rapidly expanding materials family, derived from metal-halide perovskites. Compared to their predecessors, copper-halide pseudo-perovskites are less toxic and possess an excellent stability under ambient conditions. These properties paired with high photoluminescence quantum yield (PLQY)[2] and a trap-state resistant emission mechanism (self-trapped-exciton emission) made this class of materials popular in optoelectronic applications like photodetectors, LEDs, X-ray and ionizing radiation detection. The cost-effective and diverse synthetic methods allow their preparation in various forms, from single crystals to polycrystalline thin films and even nanocrystals. A few years ago, only inorganic pseudo-perovskites were prevalent in the literature with different stoichiometries (e.g., CsCuX, RbCuX, X=Cl, Br, I). As organic-inorganic hybrid pseudo-perovskites have recently emerged, an even more rapid expansion of the available compositions can be envisioned.
Their optoelectronic properties (e.g., large Stokes shift, high PLQY, long PL lifetime) can be related to a self-trapped exciton emission mechanism, which results from the distortion of their soft crystal lattice. After excitation, different electronic states can be formed, where excitons can be trapped. From these lower energy states, the radiative recombination of trapped exciton results in a bright PL emission. To better understand the electronic and band structure differences between different stoichiometries, easily accessible and fast methods are necessary to determine the midgap states within the band gap. By understanding the band structure, we can transfer this knowledge to optoelectronic device design.
In my presentation, the band structure mapping of different pseudo-perovskite thin films will be discussed, as determined by spectroelectrochemical and optical techniques. We investigated fully inorganic (Cs₃Cu₂I₅) and hybrid ((Gua)₃Cu₂I₅, where Gua = Guanidinium)[3] compositions, synthesized by a spray coating method. For the spectroelectrochemical measurements, the steady-state PL signal was monitored as a function of the applied potential. This method enabled the mapping of the band structure of these materials. The in-depth understanding of the band structure was achieved by analyzing the wavelength-dependent photoluminescence lifetime.

Dynamic nanodomains in lead halide perovskites directly influence their macroscopic optoelectronic properties [1]. Consequently, achieving full control over their structural properties is of utmost importance. Using single crystal X-ray diffuse scattering, we demonstrate that compositional engineering at the A and X sites, combined with halide alloying, enables a diverse array of local structural landscapes, effectively quenching the local structure or eliminating or reducing its density. We employ a phenomenological model for local octahedral tilting to identify the symmetries of the local structure across various compositions. Machine-learning-assisted molecular dynamics simulations show excellent agreement with the phenomenological model and experimental diffuse scattering data, further validating our conclusions. We further show that cooling rates play a crucial role, allowing control over not only the sequence of phase transitions, but also the underlying local structure. These findings highlight strategies to tune the macroscopic parameters of nominally cubic perovskites to achieve desirable optoelectronic properties.

The performance of perovskite solar cells is limited by non-radiative recombination which occurs both in the bulk and at the interfaces with other layers in solar cell devices. These losses are due to structural and chemical defects and heterogeneities, and/or improper energetic alignment [1]. A commonly used strategy to overcome defects at interfaces and improve energetic alignment is molecular passivation. Recently, amino-silane molecules have been demonstrated to be effective molecular passivation for perovskite solar cells with a range of bandgaps (~1.6-1.8 eV). These molecules work by passivating defects subsequently resulting in solar cells with significantly boosted efficiency [2,3,4].

Scanning probe microscopy is a valuable tool for understanding the nano- and micro-scale properties of perovskite semiconductors and their solar cells. Kelvin Probe Force Microscopy (KPFM) can be particularly powerful due to its capability to map the surface photovoltage temporally and spatially which can provide valuable information about defect densities, chemical stabilisation, and electrical heterogeneities [5]. 
In this work we use KPFM to investigate the interaction of AEAPTMS (C₉H₂₃NO₃Si) with a 1.6 eV Cs_0.13FA_0.87Pb(I_0.9Br_0.1)₃ perovskite correlating its impact on structural and electrical heterogeneity spatially. Using temporal KPFM measurements under white light illumination we examine the impact of AEAPTMS passivation and show that it works to reduce both the heterogeneity in the surface photovoltage and the time taken for it to stabilise. We also examine this passivation molecule on the surface potential at the grain boundaries and interiors. In unpassivated perovskite thin films there is often a significant difference between the surface photovoltage at grain boundaries compared to that measured within the bulk of the grain. Prior to illumination we observe that the AEAPTMS passivation results in a significant decrease in the difference in surface photovoltage between the grain boundaries and interior (relative to an unpassivated perovskite). Under illumination this effect is maintained, indicating that the passivation reduces heterogeneity subsequently decreasing band bending at grain boundaries. Combining temporal and spatial measurements we show the dual passivation effect of this amino silane molecule in both reducing defect densities and homogenising the energetic landscape. Furthermore we will highlight some areas of best practise when undertaking KPFM measurements to ensure the extraction of quantitative data at high resolution.

The compositional and process flexibility of halide perovskites is fundamentally attractive for discovering new materials and novel functionality. However, the same flexibility also gives rise to operational instability due to phase and structural defect formation. The final film is a result of a complex evolution from solution through crystallization and grain grwoth with exquisite process sensitivity and contains significant heterogeneities at the nanoscale. In this talk, I will focus on our efforts to understand and reign in the substantial microscopic complexities in halide perovskites for solar energy conversion and the design of perovskite processing for stability. I will highlight in situ X-ray microscopy as a unique lens we use to investigate the relationship between structural templates, crystallization pathways, nanoscale chemistry and the functional photovoltaic properties. By assembling correlative microscopy data at the nanoscale in unadulterated film and device stacks, the X-ray microscopy provides foundational insights regarding crystallization pathways to reduce structural defects and improve optoelectronic operational stability.

Halide perovskites have rapidly emerged as a leading class of semiconductors for low-cost, high-efficiency optoelectronic technologies. However, their solution-based synthesis inherently produces a rich landscape of micro- and nanoscale inhomogeneities whose influence on device performance and long-term reliability remains insufficiently understood.

In this talk, I will present our recent efforts to unravel how structural motifs, chemical composition, and local photophysical behaviour are interconnected at the nanoscale, leveraging state-of-the-art multimodal imaging methods. We will show how synchrotron nanoprobe and optical spectroscopic methods, when correlated at the nanoscale, can explain the compromise between high-performing, defect-tolerant perovskite devices and irreversible degradation.[1,2] We will comment on the implications of these findings for the optical properties of halide perovskites, and how we can use them for the design of LEDs and photodetectors with new functionalities,[3,4] highlighting emerging opportunities for tailoring emission pathways, controlling carrier dynamics, and engineering stability through rational nanoscale design principles.

Metal halide perovskites (MHP) are promising semiconductors for the next generation of optoelectronic devices. Due to their tunable bandgap, they fulfil the requirement for various applications, including solar cells, light-emitting diodes, lasers and photodetectors. Unfortunately, MHPs still lack of long-term stability which hampers their commercial application. Degradation occurs due to their instability under environmental triggers, such as moisture and oxygen. Thus, the combination of in situ and ex situ TEM techniques can provide valuable insights about the underlying degradation mechanisms to extend the working lifetime of perovskites. Another challenge for TEM investigations of MHPs is their high electron beam sensitivity. Therefore, we develop low-dose protocols using 4D STEM. Hence, we are able to investigate the MHP without degradation due to Pb-cluster formation or amorphization and quantitatively interpret the STEM images using statistical parameter estimation theory.

Via low-dose 4D STEM measurements we investigated FAPbBr₃ nanocrystals (NCs), which are of great interest for green light-emitting diodes. To retrieve phase contrast imaging from 4D STEM datasets, we utilize the capacity of machine learning. In the phase contrast reconstructions, the different atomic columns in the perovskite structure could be detected, and it was also possible to clearly distinguish the light FA cations and the Br atomic columns from the image intensity. Furthermore, we quantified the observed elongation of the projections of the Br atomic columns, suggesting an alternation in the position of the Br atoms perpendicular to the Pb−Br−Pb bonds. Together with molecular dynamics simulations, these results remarkably reveal local distortions in an on-average cubic structure [1,2].

We also investigated CsPbI₃, whose photoactive phases spontaneously convert into a non-photoactive yellow orthorhombic δ-phase under ambient conditions. This transformation results in a significant increase in bandgap and a loss of photoactive functionality. We studied the impact of Zn²⁺ and Cd²⁺ dopants on the phase stability of CsPbI₃ NCs, emphasizing the formation of Ruddlesden–Popper (RP) planar defects. Via TEM we followed the temporal evolution of the phase transformation, where black-phase NCs agglomerate and form elongated microtubes with a yellow-phase crystal structure. Our observations demonstrate that doped samples are significantly more stable, while the dopants are key factors in the formation of the RP-like defects with specific atomic arrangements [3].

Additionally, we investigate II–VI semiconductor nanoplatelets (NPLs), in particular CdSe NPLs. It has been shown that via ligand exchange a high circular dichroism (CD) can be achieved [4]. The chiral morphology of helical NPLs was analyzed via electron tomography. Furthermore, exchanging the native acetate ligands with tartrate ligands results in a flat morphology and an orthorhombic distortion of the lattice, which we investigated via local nano-beam 4D STEM diffraction patterns.

To fully exploit the potential of perovskite technology for energy generation, it is crucial to perform comprehensive diagnostics of the dynamic evolution of the optical, structural, and electrical properties under varying temperatures, environmental conditions, illumination levels, electrical bias, over time.

A detailed understanding of perovskite behaviour begins at the material level, where the perovskite is examined independently from its integration into a device. Using in-situ spectroscopic ellipsometry, different kind of perovskite materials such as lead halide (e.g. CsPbI₃) [1,2,3] and lead-free perovskites (e.g. FASnI₃) [4,5] have been investigated over the time under precisely controlled light, temperature and environmental conditions. Tracking the evolution of their dielectric function enables the sensitive detection of the earliest signs of instability and phase transformation, signals that static measurements do not reveal. In-situ TEM and XRD further expand the description of the material’s behaviour by providing direct structural insight, allowing the optical signatures identified by ellipsometry to be cross-correlated with the corresponding crystallographic evolution. Together, these insights offer a clearer picture of the intrinsic stability of different perovskite compositions and their response to external stress, enabling more reliable predictions of material durability under operating conditions.

A broader understanding emerges when these materials are examined within complete, working devices. Once incorporated into an operating architecture, perovskites interact with interfaces, electric fields, and illumination in ways that fundamentally reshape their behavior. Here, in-operando characterization represents a natural and necessary evolution from in-situ methods. Through the combined use of X-ray diffraction, photoluminescence tracking, and real-time electrical measurements, the structural, optical, and photovoltaic evolution of mixed-halide perovskite absorbers have been monitored in real time under operational conditions [6]. Bandgap shift, ions redistribution, and lattice responses emerge simultaneously with changes in device performance. This dynamic, multi-channel view reveals mechanisms that remain hidden when materials are characterized independently from device operation, allowing to separate reversible behaviors from the earliest signatures of irreversible degradation, and to understand how composition or interface engineering can drive devices toward more resilient working operation. In addition, it allows to disentangle mutually correlated effects of light, bias and temperature on perovskite materials.  

Linking structural, optical, and electrical evolution across scales reveals how these materials truly operate in real conditions and provides the basis for predicting and controlling degradation mechanisms. This dynamic perspective is essential for advancing perovskite materials and architectures toward reliable, long-term deployment in next-generation optoelectronic technologies.

Hybrid halide perovskites have rapidly established themselves as a leading thin-film photovoltaic technology. In barely a decade, the hybrid organic-inorganic halide perovskite solar cell achieved to compete with all mature crystalline technologies, by reaching a certified 27.0 % power conversion efficiency (PCE) on cells and 20.6 % PCE on small modules.¹ Perovskite’s strength stem from their remarkable opto-electronic properties. However, the technology still requires significant attentions regarding stability, in particular rapid structural and electronic degradation can be engendered when exposed to various external stressors (temperature2-3, humidity4-6, light7-8, electrical bias9). 

To cope with the long-term stability issue, it is a paramount to precisely understand the multiple degradation pathways of the perovskite upon and during the external stressing. To this end, in situ or operando characterization techniques are central tools. In this communication, we will be discussing the degradation of different perovskite composition on the basis of humidity or temperature-controlled in situ x-ray diffraction and corroborated with in situ electron spin resonance spectroscopy and in situ transmission electron microscopy. For example, one key finding which we will discuss is that α-FAPbI₃ degradation is substantially accelerated when temperature is combined to illumination and when it is interfaced with the extraction layers, and, second the existence of a temperature gap region which takes place only under illumination involving an intermediate stage between the thermal-induced perovskite degradation and the formation of PbI₂ by-product.¹⁰

References:

(1)          NREL, PV research. Best Research Cell Efficiency Chart. https://www.nrel.gov/pv/cell-efficiency.html (accessed 2024-12-17).

(2)          Ava, T. T.; Al Mamun, A.; Marsillac, S.; Namkoong, G. Applied Sciences 2019, 9 (1), 188.

(3)          Ma, L.; Guo, D.; Li, M.; Wang, C.; Zhou, Z.; Zhao, X.; Zhang, F.; Ao, Z.; Nie, Z. Chem. Mater. 2019, 31 (20), 8515–8522

(4)          Lin, Z.; Zhang, Y.; Gao, M.; Steele, J. A.; Louisia, S.; Yu, S.; Quan, L. N.; Lin, C.-K.; Limmer, D. T.; Yang, P. Matter 2021, 4 (7), 2392–2402

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