Already in the early days of research on metal-halide perovskite solar cells, which dates back a little more than a decade, the ionic conductivity of perovskites was measured and discussed in the context of current-voltage hysteresis, pseudocapacitance, and reversible changes during operation. Since then, mixed electronic-ionic conductivity in these materials has been extensively studied and increasingly linked to effects observed in the long-term operational stability of solar cells.

When it comes to photovoltaic (PV) devices, quantifying the influence of ionic effects in operando is highly challenging for various reasons: 1) A PV device is an active device, with electrons and holes being photogenerated, transported, trapped, and recombined within the mixed conductor. 2) In perovskite PVs, the functionality relies heavily on heterointerfaces with adjacent charge-transport layers that complement the bulk properties. 3) Ionic charges couple to these processes by influencing electronic charge transport and recombination. 4) Ionic species may themselves be affected by illumination and interfacial reactions. 5) The device architecture, comprising several capacitive layers, impedes direct measurements of ion concentrations and mobilities.

In this talk, I will approach this topic by combining device simulation studies with experimental results. Particular focus will be placed on a depth-resolved determination of the collection efficiency of photogenerated charges as influenced by ionic distributions. Experimentally, spatially resolved photocurrent measurements will be employed as a function of prebias voltage [1]. Furthermore, transient measurements used to quantify ion densities will be evaluated. Finally, I will attempt to address the long-standing question of how the perovskite PV device would behave in the absence of ions [2].

Perovskite photovoltaics (PVs) have already demonstrated remarkable efficiency potential at low-cost owing to their excellent optoelectronic properties. However, high density of mobile ionic effects, with various electronic and chemical processes occurring over a broad range of timescales, can significantly affect long-term stability of perovskite. Deconvoluting and understanding these dynamic processes are challenging because they can strongly influence one another. Conventional frequency-domain characterization techniques, such as impedance spectroscopy (IS), intensity-modulated photovoltage spectroscopy (IMVS), and intensity-modulated photocurrent spectroscopy (IMPS), are widely used to investigate these processes¹. However, these techniques are limited to operational devices, and their signals can be influenced by contacts and interfacial effects.

As an alternative, intensity-modulated photoluminescence spectroscopy (IMPLS) has been shown to be a fully optical, contactless technique capable of probing dynamic processes across a broad frequency range². However, a systematic interpretation of IMPLS must be established to unlock its full potential. Therefore, in this work, by correlating IMPLS spectrum with the spectral features of optoelectrical techniques such as IS, IMVS, and IMPS analytically, numerically, and experimentally, we establish a physical framework to interpret IMPLS spectra in complete solar cells. This study reveals that IMPLS can provide the same information as electrical techniques and can reconstruct electrical signals fully optically. Therefore, this work highlights the potential of IMPLS as a powerful optical diagnostic tool for studying dynamic processes and predicting the intrinsic properties of materials by monitoring differential photoluminescence.

Mixed-halide perovskite solar cells (PSCs) are known to be affected by ionic migration, which strongly influences device performance and stability. While ionic transport has been widely studied, the correlation between absorber stoichiometry and the characteristic ionic response remains poorly understood. In this work, we use impedance spectroscopy to systematically investigate how controlled stoichiometric perturbations affect the polarization dynamics of mobile ions in formamidinium–caesium lead mixed-halide perovskite absorbers.

Excess formamidinium halide salts (FAI, FABr, FACl) were introduced into the precursor solution to deliberately disturb absorber stoichiometry. We introduce a new variable, the characteristic ionic response frequency, which is a measure of how fast mobile ions can screen the electric field within the absorber. We find that even minor deviations from ideal stoichiometry lead to a significant increase of the ionic response frequency, suggesting more effective screening of the electric field due to increased ionic conductivity. While we show that an excess is necessary to induce this shift in frequency, the influence of the specific halide appears secondary compared to the overall stoichiometric offset.

To distinguish bulk incorporation from surface-driven effects, we further introduce a partial three-dimensional (3D) to two-dimensional (2D) conversion, therewith inducing surface-confined compositional modifications while largely preserving the pristine bulk absorber. In this case, a shift in ionic response frequency is also observed; however, in contrast to bulk incorporation, the specific halide plays a more pronounced role, linked to ion-exchange between the 2D and 3D layer during film crystallization.

Our results reveal two distinct regimes governing the relationship between the ionic response frequency and absorber stoichiometry. In a low-to-moderate ionic density regime, the response frequency is significantly affected by both ionic density and mobility, indicating coupled contributions from ion availability and transport dynamics. However, at higher ionic densities, the response frequency reaches a saturation limit, beyond which further shifting frequencies can only be achieved by increasing ionic mobility. This transition marks a shift from a density- and mobility-controlled regime to one dominated predominantly by ionic mobility.

Temperature-dependent measurements further allow extraction of activation energies associated with ionic transport processes, providing deeper insight into the underlying migration mechanisms. By correlating characteristic ionic response frequencies, activation energies, and absorber stoichiometry, this study highlight viable strategies to tailor ionic dynamics through both bulk composition control and surface-specific modifications, with significant possibilities for improving the stability and performance of perovskite solar cells.

Electrostatics govern both charge extraction and recombination in solar cells and are conventionally discussed in terms of doping densities, built-in fields, and band alignments. In perovskite solar cells, however, additional electrostatic mechanisms can dominate device operation.

Voltage-dependent PL measurements reveal that substantial charge-carrier densities can remain in the perovskite absorber even under external short-circuit conditions, causing recombination losses during extraction and reducing the extracted current.¹ Possible origins include low charge-transport-layer mobilities, extraction barriers, but also electric-field screening by redistributed mobile ionic species of the perovskite layer. Since conventional voltage-dependent PL measurements are typically performed under slow JV-scan conditions, mobile ions can approach a quasi-steady-state distribution and screen the internal field. Fast hysteresis measurements, in contrast, distinguish this ionic steady state from an ion-frozen regime, thereby revealing ion-induced current losses.² Here, I present scan-speed and voltage-dependent PL by recording the PL signal with a photodetector during the fast hysteresis scan. This approach separates electronic extraction losses from ionic current losses associated with field screening, indicating a high density of mobile ions.

Even though lead-halide perovskites exhibit very low equilibrium doping densities in the dark, electrostatic effects may also play an important role in recombination through the charge-neutrality condition. Several studies have suggested that illumination can induce an imbalance between free electron and hole densities, referred to as photodoping.^3–5 Here, I show that the excitation-intensity dependence of steady-state PL in perovskite films yields ideality factors between 1 and 2 across the investigated range. In the absence of contact-induced electrostatics, this behavior is consistent with photodoping mediated by a high density of trap states, leading to an effective ideality factor close to 1.5 over a wide range of excitation conditions.

The simultaneous requirement of high trap-state densities for photodoping and high mobile-ion densities for efficient field screening raises the question of whether these defect populations are physically connected. Do recombination and charge extraction in perovskite solar cells ultimately need to be described within a unified framework based on mobile defect centers?

1.           Akel, S. et al. Adv. Energy Mater. 14, 2401800 (2024).

2.           Thiesbrummel, J. et al. Nat. Energy 9, 664–676 (2024).

3.           Stranks, S. D. et al. Phys. Rev. Appl. 2, 1–8 (2014).

4.           Feldmann, S. et al. Nat. Photonics 14, 123–128 (2020).

5.           Yuan, Y. et al. Nat. Mater. 2024 233 23, 391–397 (2024).

Understanding and Quantification of Ion Migration in Perovskite Solar Cells

Francesco Le Pera^a, Moritz C. Schmidt^a, Agustin O. Alvarez^a, Biruk Alebachew Seid^b, Felix Lang^b, Bruno Ehrler^a

^a AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands

^b Universität Potsdam, Capus Golm, Karl-Liebknecht-Str. 24/25, Germany

Due to their exceptional optoelectronic properties, perovskite solar cells (PSCs) have rapidly achieved power conversion efficiencies exceeding 26%. However, ion migration within the perovskite layer remains a critical bottleneck, degrading the crystal structure and screening the internal electric field upon accumulating at device interfaces. Here, we investigate Thermally Activated Ion Current (TAIC) measurements to quantify mobile ion density in PSCs. In this method, a voltage bias is applied to induce a homogeneous ion distribution across the bulk. The device is then cooled to 175 K using liquid nitrogen to immobilize the ions. Upon removing the bias, the temperature is increased in a controlled manner up to 300 K, allowing the ions to relax back to the interfaces and generate a measurable current transient.

The subsequent decay of this current peak can be governed by two distinct regimes: an ion-limited scenario, where bulk ion depletion stops the current before complete field screening occurs, allowing accurate ion density extraction via current integration, or a field-limited scenario, where severe interfacial ion accumulation screens the internal field, halting further migration and causing an underestimation of the true ion density.

To interpret these mechanisms, we employ drift-diffusion simulations. Our modeling dictates that devices should transition into the field-limited regime at ion densities exceeding 10¹⁷ cm^-3. Paradoxically, experimental TAIC measurements yield apparent ion densities on the order of 10¹⁸ cm^-3. To resolve this discrepancy, we evaluate TAIC profiles as a function of the initial voltage bias. Simulations reveal that increasing the initial bias yields two distinct behavioral regimes contingent on the total mobile ion concentration, elucidating how screening effects distort density extraction. Finally, we experimentally validate these simulation trends using stable triple-cation PSCs with a Cs_0.05(MA_0.05FA_0.95)_0.95Pb(I_0.95Br_0.05) composition, establishing TAIC as a powerful diagnostic tool for optimizing perovskite photovoltaics.

Metal halide perovskites are attracting increasing interest for radiation detection owing to their outstanding optoelectronic properties, solution processability, and compatibility with flexible device architectures. Beyond their technological potential, these materials provide a unique platform for investigating the interplay between electronic transport, ionic motion, and defect-mediated processes under high carrier injection conditions. Understanding these coupled phenomena is essential not only for improving detector performance but also for establishing general design principles applicable across perovskite-based electronic and optoelectronic devices.

In this talk, I will present recent advances in the investigation of transport mechanisms in low-dimensional and mixed-dimensional metal halide perovskites, with particular emphasis on the role of defects, trap-assisted transport, and carrier dynamics in determining device response and operational stability. Flexible photoconductive detectors based on layered low-dimensional perovskite thin films serve as model systems to probe charge transport under excitation conditions spanning UV illumination, X-rays, γ-rays, and proton beams. Although these materials exhibit excellent sensitivity and long-term stability, a universal sublinear dependence of the photocurrent on the carrier generation rate reveals the presence of complex defect-assisted photoconductive transport mechanisms.

Experimental investigations combined with physical modeling demonstrate that the detector response is governed by the interplay between carrier trapping, detrapping, and recombination through multiple trap distributions with different activation energies. The analysis identifies the injected charge density as a universal parameter describing transport over a broad range of irradiation conditions and highlights how defect occupation controls photoconductive gain and dynamic response. The intrinsic structural robustness of layered perovskites, together with their enhanced environmental stability, further contributes to reliable operation under prolonged irradiation.

Overall, these results highlight the central role of defect-mediated charge dynamics in determining the performance and operational stability of perovskite radiation detectors, providing valuable insights for the broader development of stable perovskite-based optoelectronic devices.

The optimal performance of both photovoltaic and electroluminescent devices necessitates minimal nonradiative recombination losses. In perovskites, such losses are significantly influenced by carrier traps associated with mobile ions and vacancies, resulting in I–V hysteresis in solar cells and affecting the performance of perovskite-based optoelectronic devices, including photodetectors and LEDs.[1][2] Ideally in a perovskite LED (PeLED) operation, there are cations at the hole injection layer (HIL)/perovskite interface and anions at the electron injection layer (EIL)/perovskite interface, and upon the application of a forward bias, there is a redistribution of the ions at opposite interfaces under sufficient time scales. This consequently alters the energy landscape in a manner that promotes increased radiative recombination in the bulk.[3] In this work, we study ion dynamics at both the HIL and EIL interfaces along with the bulk of quasi-2D and 3D PeLEDs with time-resolved electroluminescence (TrEL) measurements, where we observed that with shorter pulse periods and longer pulse durations, the electroluminescence of quasi-2D PeLEDs underwent significant degradation compared to their 3D counterpart, which showed an opposite trend under similar conditions. We attribute this to the screening of the field at the HIL and EIL interfaces in the quasi-2D PeLEDs due to the presence of bulky spacer cations of the 2D perovskite lattice, which hamper the ionic redistribution at interfaces [4], thus mitigating the radiative recombination in the bulk. The presence of a large number of grain boundaries in quasi-2D PeLEDs is another factor contributing to strong ionic migration pathways under pulsed operations in their bulk. [5] In this work we corroborate the TrEL results with cross-section Kelvin probe force microscopy (KPFM) analysis and conductive atomic force microscopy (cAFM) studies performed under similar pulsed operations to study the ionic dynamics at the interface and the bulk. We also plan to investigate qualitatively the ionic species, mainly uncoordinated lead (Pb_i) and lead vacancies (V_Pb) formed under such pulsed operations through Deep Level Transient Spectroscopy (DLTS) under similar pulsed conditions implemented in TrEL measurements. Finally, to mitigate these field-induced traps, targeted bulk and interface passivation strategies will be deployed, incorporating novel charged dopants to passivate ionic species, suppress non-radiative recombination centers, and stabilize the quasi-2D PeLED lattice.

Understanding how mobile ions redistribute within metal halide perovskites is essential for interpreting device stability and quantitative ion-density measurements. While most studies focus on ionic transport along the device stack, lateral ion motion and edge effects remain relatively unexplored, despite their strong relevance and demonstrated impact [1]. Especially in small-area, lab-scale photovoltaic devices, lateral migration can also distort ion density measurements. Furthermore, lateral migration can significantly impact device stability and ageing.

In this talk, I will discuss side collection of ions at device edges and its consequences both for measured ion densities as well as long-term material stability. I will also investigate the impact of device area and shadow masking on device stability. Experimental results are complemented with 2D drift-diffusion simulations performed using ChargeTransport [2].

Finally, I will discuss how the impact of field screening due to mobile ions in textured perovskite solar cells differs from that in planar devices, and whether this might open up opportunities to engineer devices with suppressed ion-induced current losses.

The transition toward sustainable solar energy conversion requires the development of efficient, stable and non-toxic light-harvesting materials. Heavy-pnictogen-based semiconductors, particularly bismuth- and antimony-derived perovskite-inspired materials (PIMs), have emerged as promising alternatives to lead-based systems, combining defect-tolerant electronic structures with improved environmental compatibility and aqueous stability.^1,2

In this contribution, we present a unified materials and device perspective on heavy-pnictogen derivatives for both photovoltaics and photoelectrochemistry. We focus on two complementary material classes: Ag–Bi chalcogenides (e.g., AgBiS₂) and Sb-based halide PIMs (e.g., Cs₃Sb₂I₉–xClₓ), processed via scalable techniques such as ultrasonic spray coating. These systems enable low-cost, air-compatible fabrication while maintaining strong optical absorption and tunable bandgaps across the visible and near-infrared spectrum.

Overall, we will show that this work establishes heavy-pnictogen derivatives as a versatile platform bridging photovoltaics and photoelectrochemistry, where control over disorder, dimensionality, transport and interfaces is key to unlocking their full potential for sustainable solar-to-chemical and solar-to-electrical energy conversion.

References:

1. F. Schmitz, T. Gatti et al. Heavy pnictogens-based perovskite-inspired materials: Sustainable light-harvesters for indoor photovoltaics APL Energy 1, 021502 (2023)

2. I. Poli, T. Gatti et al. Lead-free perovskites and derivatives for photogeneration: a roadmap to sustainable approaches for photovoltaics and photo(electro)catalysis J. Phys. Energy 8, 011501 (2026)

The rapid advance of artificial intelligence demands energy-efficient hardware beyond traditional von Neumann architectures, positioning memristors as promising candidates to emulate biological synapses. Metal halide perovskites (MHPs) are promising candidates for memristive applications owing to their mixed ionic-electronic conductivity, yet their inherent instability and tendency toward abrupt resistive switching remain critical barriers to neuromorphic implementations. Here, we present a nanocomposite strategy in which MAPbBr₃ nanocrystals are synthesized in situ within a nickel acetate matrix through a one-step, annealing-free, and glovebox-free process, enabling ambient-stable memristive devices. By modulating the perovskite volume fraction, the ionic-electronic dynamics are tuned, inducing a transition from abrupt, non-volatile digital switching in polycrystalline MAPbBr₃ to gradual, volatile analog switching in the nanocomposite. Impedance spectroscopy and photophysical analyses attribute this transition to a shift from ion migration-driven switching in the bulk material to interfacial charge trapping and detrapping in the nanocomposite. The resulting volatile memristors emulate key synaptic functionalities, including short- and long-term plasticity, paired-pulse facilitation, and spike-dependent weight modulation, with stable performance over 1000 cycles under ambient conditions. Exploited in a reservoir computing framework, the nanocomposite memristor achieves ~89% classification accuracy on noisy digit patterns with a linear readout, rising to 98% with a deeper network. This scalable, solution-processed approach offers a viable route to analog memristors for neuromorphic and edge computing.

Mobile ions in metal halide perovskites are known to negatively impact the intrinsic stability of perovskite-based solar cells (PSCs). Under illumination, mobile ions can migrate along grain boundaries and accumulate at perovskite interfaces, where they create metastable trap states, and in some cases, penetrate into charge transport layers, ultimately contributing to device failure ^1,2.

 In this presentation, we quantitatively investigate photo-induced aging and self-healing in PSCs under light/dark cycling. Under prolonged illumination, the device degrades as ion density and ionic loss increase.³ However, when the device is allowed to rest in the dark ⁴, part of this ionic loss is recovered, leading to reduced ion density and partial performance restoration. The incomplete restoration of power conversion efficiency (PCE) indicates the simultaneous presence of reversible and irreversible degradation mechanisms, which progressively accumulate over successive cycles. Consequently, degradation accumulates over repeated light/dark cycles, representing a key factor that limits the operational stability of PSCs.

By directly comparing a stable and a poorly stable system, we observe that, while degradation and subsequent recovery are clearly observable in the poorly stable device, the stable device exhibits gradual performance improvement during successive cycles. This contrasting behavior highlights the different metastability characteristics of the two device types. Moreover, we investigate the link between PSC recovery and temperature. We further identify perovskite composition as a key factor governing device self-healing during dark resting. Complementary current density-voltage (J-V), fast hysteresis (FH), and bias-assisted charge extraction (BACE) analyses confirm these findings.

While degradation is generally considered unavoidable in PSCs, understanding the intrinsic self-repair capability of perovskites and learning how to control it to slow the efficiency loss process offers a promising step towards achieving stable perovskite solar cells. Together, these findings highlight light/dark cycling-based test protocols as a transformative approach for understanding and controlling degradation, offering new pathways toward extending the lifetime of PSCs.

Perovskite solar cells are making their way to the market despite intrinsic instability issues remain. Also, replacing lead by less toxic elements remains a major challenge, with tin likely being the only suitable alternative.  While lead-based perovskites are affected by instability related to iodide oxidation, tine based materials are plagued by stability of Sn(IV) phases, which are related to the lower oxidation potential of tin compared to lead. A related phenomenon is the stability of tin vacancies, which introduce significant p-doping in tin-halide perovskites, while their lead-based counterpart are essentially intrinsic semiconductors. Defect activity clearly controls doping and could also contribute to the instability towards Sn(IV) phases. Controlling doping and defect activity thus represents a pathway towards obtaining stable perovskites with optimal optoelectronic properties. The different defect activity of tin- and lead-based materials is at the origin of their respective thermal and phot-induced degradation phenomena, including halide demixing and loss of I₂ in lead-halide perovskites. 

Here we present results of advanced modelling studies on the defect mediated degradation pathways of prototypical lead- and tin-based materials. We show that iodine chemistry dominates lead-based perovskites while Sn-vacancies are central in promoting both material p-doping and formation of Sn(IV) phases. Interestingly, while p-doping dominates in the bulk, Sn oxidation is only favoured at surfaces or grain boundaries. Thus achieving uniform thin films coupled with proper surface passivation strategies represent a pathway towards achieving more stable THP-based devices. We further unveil the key factors determining the stability of mixed-halide THPs against photoinduced halide segregation phenomena. Molecular and ionic strategies to mitigate p-doping in THPs are also presented.

Metal halide perovskites are mixed ionic–electronic semiconductors in which vacancy-like defects formed during crystallization can influence phase stability, hysteresis, and degradation. Here I will discuss flash infrared annealing (FIRA) as a sub-second processing and diagnostic platform to access non-equilibrium defect landscapes in formamidinium lead iodide (FAPbI3) thin films [1–3].

FIRA replaces conventional minute-scale thermal annealing by millisecond-to-sub-second infrared pulses, allowing the perovskite conversion pathway to be driven far from quasi-equilibrium. Using high-speed optical imaging, X-ray diffraction/GIWAXS, positron annihilation lifetime spectroscopy (PALS), and first-principles calculations, we correlate crystallization history with alpha-phase retention and vacancy-like positron trapping signatures. In our latest FAPbI3 series, chemically and thermally modified films show a clear hierarchy in the PALS long-lifetime intensity ratio, with I2/I1 increasing from Cs-modified and fast-ramp FAPbI3 (~0.16–0.19) to TEMPO-modified films (~0.33) and long-ramp references (~0.47). First-principles positron calculations give a bulk lifetime close to the experimental short component and support the assignment of the long component to vacancy-like trapping environments, consistent with previous PALS/DFT studies of vacancy defects in lead halide perovskites [4]. These trends co-vary with the alpha-phase fraction obtained from XRD screening, indicating that the annealing pathway leaves a measurable defect fingerprint relevant to ionic stability.

This work positions FIRA not only as a scalable low-thermal-budget fabrication route, but also as a controlled perturbation method to study how crystallization kinetics, additive chemistry, and thermal history define vacancy populations that can later mediate ion migration, hysteresis, and degradation [5,6].

Aluminum oxide is known to suppress ion migration, improve crystal quality, and scavenge I₂, thereby enhancing perovskite solar‑cell stability, which is crucial for upscaling and large‑scale production. We investigate the impact of blade‑coated Al₂O₃ nanoparticles on coupled electronic and ionic dynamics in inverted perovskite solar cells with the baseline architecture ITO/NiOx/SAM/C_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃/PEAI/C₆₀/BCP/Ag. Al₂O₃ is introduced in three processing configurations: as a separate nanoparticle layer beneath the self‑assembled monolayer (SAM), as a nanoparticle layer on top of the SAM, and as a SAM-Al₂O₃ nanoparticle blend deposited in a single step. These routes are specifically chosen to be compatible with fast, large‑area fabrication and to reduce the need for conventional layer‑by‑layer deposition, thereby making interface engineering more suitable for industrial‑scale processing.

To disentangle how Al₂O₃ nanoparticles influence transport and stability, we combine impedance spectroscopy and fast hysteresis measurements with time‑correlated single‑photon counting in the picosecond regime and femtosecond transient absorption spectroscopy. Impedance and hysteresis measurements probe how different Al₂O₃ configurations affect interfacial recombination resistance, capacitive behavior, and ion migration pathways, while time-resolved optical techniques elucidate changes in carrier lifetimes, interfacial extraction dynamics, and field-induced trapping, thereby allowing us to correlate microscopic kinetics with macroscopic device metrics.

Overall, SAM-Al₂O₃ nano‑architectures in this ITO/NiO_x‑based stack appear to offer a promising pathway toward durable, high‑efficiency perovskite photovoltaics that can be manufactured using scalable blade‑coating processes. Data collection and analysis are ongoing; preliminary and final results will be presented at the conference.

Small changes in composition can strongly affect how perovskite materials behave under light. Mixed halide lead perovskites display light-induced halide migration, leading to iodide clustering and redshifted emission, often called photosegregation, that is reversible in the dark. This effect is usually explained by the high diffusivity of halide ions at room temperature. We have recently shown that these light-induced spectral changes are even more pronounced and highly dynamic in very bromide rich samples doped with a small amount of iodide.

In this work, we ask whether a similar effect can also occur in the less mobile part of the perovskite: the metal cations. We study mixed lead-tin perovskites doped with low percentages of Sn. We find that these materials also show highly dynamic photoluminescence emission spectra, suggesting that Sn-rich and Pb-rich regions can form under illumination. Our results demonstrate that light-driven restructuring in perovskites is not limited to halides, but can also involve the metal ions. This suggests that light could be used as a tool to actively reshape materials and control their properties. This could enable new ways to “write” functional structures into materials using light, instead of fabricating them beforehand.

Perovskite devices involve processes that occur on multiple characteristic timescales, including electronic and ionic characteristic times, and across different physical regions, including the bulk, interfaces, and contacts. A key challenge is that standard measurements often mix these contributions, making it difficult to identify the origin of performance limitations.
Impedance spectroscopy (IS), the most established frequency-domain technique, enables detailed in-operando insight, particularly by separating processes based on their characteristic times.[1, 2] However, linking these dynamics to specific physical origins, such as electronic or ionic transport, bulk or interfacial recombination, or contact effects, remains challenging. I will show how combining IS with intensity-modulated photocurrent (IMPS) and photovoltage (IMVS) enables the identification of the dominant limiting mechanisms in different perovskite solar cells.[3-5]
I will also present recent results on intensity-modulated photoluminescence spectroscopy (IMPLS), a fully optical technique. I will show representative perovskite responses, discuss their analysis, and highlight the information that can be extracted. By combining IMPLS with electrical modulated techniques, we demonstrate how additional insight into underlying mechanisms can be obtained beyond electrical measurements alone. In particular, IMPLS provides a contact-free way to probe internal limiting processes in perovskite materials and devices.[6, 7]
 

The performance and stability of perovskite solar cells are strongly influenced by the intricate coupling between electronic charge transport and ionic migration. This interplay governs slow dynamical responses, hysteresis, and memory effects that emerge during device operation and degradation. In this work, we apply a dynamic model that simultaneously accounts for electronic processes and ionic redistribution, enabling a detailed interpretation of these phenomena.

The model reproduces characteristic impedance features observed experimentally, including both capacitive and inductive behavior. In particular, the appearance of an inductive loop is explained as a form of chemical inductance, arising from delayed feedback between ion migration and electronic recombination or transport. This mechanism parallels the nonlinear, history-dependent responses reported in perovskite memristors, where coupled ionic–electronic dynamics produce pronounced hysteresis and memory effects.

By integrating impedance spectroscopy with time-domain transient measurements, we extract distinct time constants associated with charge accumulation, ion migration, and carrier recombination, and we monitor their evolution during degradation. Two main degradation pathways are identified: (i) reduced charge collection, which limits photocurrent, and (ii) increased recombination, which lowers photovoltage.

Correlating these dynamical electrical signatures with physical mechanisms allows us to build a comprehensive framework linking hysteresis, chemical inductance, and degradation in perovskite solar cells. This approach establishes a powerful diagnostic strategy to elucidate and mitigate performance losses in emerging perovskite optoelectronic technologies.

The instability of halide perovskite optoelectronic devices remains a bottleneck for long-term operation, largely due to persistent defect formation and associated non-radiative (NR) losses. However, unlike conventional semiconductors, MHPs possess an inherent self-healing (SH) ability that allows spontaneous defect repair without external intervention[1–5], which presents a pathway toward resilient, low-maintenance devices; however, its practical realization hinges on deciphering and regulating the microscopic origins of SH. The underlying microscopic mechanisms remain poorly understood, particularly the role of interfacial chemistry on trap dynamics and healing kinetics. Here, I will discuss SH and defect evolution in triple-cation mixed-halide (TCMH) perovskite films and their device-relevant charge-transport-layer heterostructures subjected to photo-induced damage. Using correlation clustering imaging (CLIM) [6], our recently developed local functional imaging tool, I will show how we map spatiotemporal photoluminescence heterogeneity to track defect dynamics in pristine and hetero-structure films (Figure 1). Our results demonstrate that the chemical nature of charge-transport layers modulates trap activity, healing kinetics, and halide redistribution, with hetero-structures exhibiting faster recovery than pristine films, a boon for device resilience. These findings provide new insights into the dynamic interactions among defects, interfaces, and ion migration and establish a framework for the rational design of durable, next-generation perovskite optoelectronic devices"

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 surface-sensitive 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 device properties.

To address these challenges, we developed Correlation Clustering Imaging (CLIM)^1,2, a novel noninvasive method that utilizes photoluminescence fluctuations to reveal contrasts associated with defect dynamics in semiconductor materials. CLIM images of perovskite thin films show one-to-one matching with the grains in SEM images captured at the same locations. Particularly noteworthy is the application of CLIM to high-efficiency photovoltaic devices, uncovering previously unnoticed photoluminescence intensity fluctuations that strongly depend on the device's operational regime. CLIM coupled with quasi-Fermi level splitting (QFLS) mapping provides rationale for the evolving landscape of device performance through mechanistic understanding of charge carrier dynamics.

These findings deepen our understanding of device efficiency, structure, and degradation — knowledge critical for the rational design of next-generation devices. Requiring only a standard wide-field microscope and an open-source, user-friendly algorithm, CLIM is broadly applicable and poised to become an important tool for materials chemists, engineers, and device scientists alike.

Since 2013, metal halide perovskites have been studied as a promising new material for X-ray detection in medical imaging. [1] Their use as thick layers (>100 µm) in direct X-ray detection devices could provide spatial resolution and sensitivity superiors to that of indirect scintillator-based detectors [1,2] and would allow for better medical diagnostic and reduced exposure dose for the patient. The all-inorganic CsPbBr₃ perovskite demonstrates an average high atomic number, providing good X-ray absorption. Moreover, CsPbBr₃ shows improved chemical stability compared to hybrid halides perovskites [3]. However, further insight into the physical and chemical properties of CsPbBr₃ is still needed to improve device performance and reproducibility. In particular, unintentional extrinsic doping coming from the working atmosphere. To this end, thick layers of polycrystalline films obtained by Close Space Sublimation [4] were exposed to different atmospheric conditions in order to asses their influence on the detector performances.

Electrical properties such as dark and X-ray photo current were measured under both dry and humid atmosphere as well as in air and inert conditions. It was found that relative humidity is the main factor inducing changes in the material dark current (j_dark) and device sensitivity. Most notably, j_dark was found to increase up to a factor of 100 both in the absence (<0.5 %RH) or excessive presence (>50 %RH) of water. To obtain a deeper understanding of the chemical mechanism at play, steady state, time resolved photoluminescence and X-ray diffraction measurement  under different humidity levels were carried out. We identified two separate mechanisms at the polycrystalline film surface observed below and above 50 % relative humidity.

Mixed metal halide perovskites (AB(XₓY₍₃₋ₓ₎)₃) have attracted significant attention due to their outstanding optoelectronic properties and potential for low-cost device fabrication. However, their practical implementation is limited by ion migration processes that strongly affect stability and performance under operation. One demonstration of this instability is light-induced halide segregation, where initially homogeneous mixed-halide compositions dynamically separate into iodide-rich and bromide-rich domains.

In this work, we demonstrate that photoinduced ionic redistribution, typically viewed as a degradation pathway, can instead be exploited as a functional mechanism for information storage. By tracking the time-dependent photoluminescence (PL) response under femtosecond pulsed excitation, we show that excitation conditions directly govern the kinetics and extent of halide migration. This enables controlled formation of distinct segregation states, each associated with a characteristic PL emission wavelength.

We systematically investigate MAPb(Br₀.₈I₀.₂)₃ and Cs₀.₀₇(FA₀.₈₃MA₀.₁₇)₀.₉₃Pb(Br₀.₅I₀.₅)₃ across a wide excitation parameter space, varying repetition rates from 10 kHz to 20 MHz and average intensities between 9 W/cm² and 1800 W/cm². By doing so, we can access reproducible, input-dependent ionic configurations corresponding to well-defined halide distributions. We demonstrate that this tunable ionic response enables parallel writing of at least nine independent, diffraction-limited, memory channels in a single thin film, with each channel supporting over 100 distinguishable states. This behavior reflects a multi-level encoding scheme far beyond conventional binary systems, corresponding to an effective 7-bit storage capability per diffraction-limited memory channel.

Importantly, the segregated states exhibit long retention times after excitation is removed, indicating metastable ionic configurations that can be read out with temporal delay. Furthermore, we show that thermal activation rapidly restores the homogeneous phase, providing a reliable pathway to erase stored information and reset the system.

Our findings highlight the potential of perovskites for high-density, rewritable optical memory and neuromorphic computing, utilizing their intrinsic dynamic responses for next-generation photonic technologies.

Organic-inorganic hybrid perovskite solar cells (PSCs) have achieved remarkable progress, with certified power conversion efficiencies (PCEs) exceeding 27.3% [1]. A critical factor in this advancement is the engineering of the buried interface between the hole transport layer (HTL) and the perovskite absorber [2]. While self-assembled monolayers (SAMs) such as MeO-2PACz are widely used to tune energy levels and suppress interfacial defects, achieving uniform, stable, and well-anchored coverage on metal oxide substrates like NiOx remains challenging [3].

In this work [4], we present a robust interfacial engineering strategy to regulate the NiOx/MeO-2PACz interface through three distinct post-treatment approaches based on dimethyl sulfoxide (DMSO): (1) simple DMSO rinsing to improve surface homogeneity by removing weakly bound molecules, (2) treatment with a MeO-2PACz-containing DMSO solution to replenish and restore SAM coverage, and (3) the application of an APTES-containing DMSO solution to enhance interfacial adhesion and passivate defects via amino-group functionality. Through atomic force microscopy (AFM) and Raman spectroscopy, we demonstrate that these treatments optimize molecular ordering and foster stronger chemical anchoring to the substrate. Consequently, these modified interfaces facilitate superior perovskite crystallization with enlarged grain sizes and suppressed non-radiative recombination. While the MeO-2PACz/DMSO treatment produced the most homogeneous interface, APTES-modified devices exhibited the highest photovoltaic performance, achieving a PCE of approximately 18.5%, a V_OC of 1.02 V, and an FF of 75%. This result highlights the importance of interfacial defect passivation in addition to morphological optimization for achieving high-performance inverted perovskite solar cells.