The current design of high performing metal halide perovskite solar cells consists of multilayer device architecture with glass / transparent conducting oxide (TCO) / electron or hole-transport layer [E(H)TL] / perovskite / H(E)TL / electrode. This stacked configuration leads to efficient charge extraction but also introduces physico-chemical interactions at the buried interfaces that strongly influence device operational stability through a myriad of degradation pathways. In n-i-p architecture, 2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9’-spirobifluorene (spiro-OMeTAD) is still widely employed as HTL. A thorough characterization of the physico-chemical dynamical processes taking place in HTLs is important to further design strategies aiming robust devices. In this talk, we provide our understanding of physico-chemical processes taking place in spiro-OMeTAD including ion migration, redox reactions, dopant instability, and interfacial energy level alignments.^1-3 The p-i-n architecture has also gained increasing attention due to its improved performance. In this inverted structure, the HTL (or hole-selective contact) is implemented using self-assembled monolayer (SAM) strategy. Recent advances in SAMs highlight the importance of molecular design principles including dipole engineering and anchoring/terminal groups. In the second part, this talk emphasizes the roles that HTLs play in p-i-n architectures, considering charge-transport, interfacial chemistry, and operational stability.^4,5

Photoconversion devices have advanced rapidly in recent years, yet emerging technologies such as perovskite solar cells, tandem solar cells, and photoelectrochemical cells still face performance and stability challenges that limit their adoption. Understanding the internal mechanisms governing device operation is essential to identify and overcome these limitations.
In this presentation, I will highlight the power of modulated, or small-perturbation, techniques to probe these mechanisms. Impedance spectroscopy (IS) provides rich in-operando information but has inherent limitations. These can be overcome by combining IS with intensity-modulated photocurrent (IMPS) and photovoltage (IMVS) spectroscopies. A unified analysis of all three techniques reveals the underlying working processes, enables the precise selection of an equivalent circuit for simultaneous analysis of the three spectra, allowing more accurate parameter extraction, and provides access to additional metrics, such as carrier separation efficiency. Applications to various photoconversion devices will be presented[1-3].
Additionally, intensity-modulated photoluminescence spectroscopy (IMPLS) extends these concepts to fully optical, contact-free measurements, revealing slow ionic processes in perovskites[4-5]. Together, these techniques form a versatile toolkit for understanding and optimizing next-generation photoconversion devices.

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. ^1-3

Understanding the photocurrent in a solar cell is a matter of looking at the competition between charge carrier collection and recombination. In perovskite solar cells, these processes are affected by the redistribution of mobile ions during current-voltage scans and degradation, leading to the welk-known hysteresis, so-called ion-induced losses, and slow transients [1].

In this talk, spectrally resolved measurements together with device simulations are presented to better understand the loss mechanisms related to photocurrent. The approach exploits different penetration depths of the light due to the absorption coefficient of the perovskite varying with wavelength. Preconditioning samples under various conditions such as bias voltage and then "freezing" the ion distribution by cooling, allows for a measurement of the external quantum efficiency (EQE) under a stable ion distribution. Subsequent EQEs then monitor the effect of ion redistribution on the spectral shape of the EQE and thus depth-dependent charge collection probability [2].

This method is applied to Carbon-based mesoscopic solar cells with titania and zirconia scaffolds and to SAM-based pin solar cells. Together with the simulations, it provides explanations of certain shapes of the hysteresis such as inverted hysteresis or current overshoots ("bump") in the current-voltage curve [3]. The overall methodology might become a powerful tool to investigate underlying causes of device degradation upon ageing.

Perovskite photovoltaics have reached early stage of commercial deployment in a remarkably short period of maturation, driven by rapid advances in power conversion efficiency over the past decade. Despite these impressive achievements, the long-term reliability and stability of perovskite solar cells (PSCs) remain major challenges, largely due to ion migration. Ion motion affects the performance and operational stability of PSCs, as well as tandem and multijunction device architectures. Different transient experimental techniques have been used to better understand the puzzling performance of ion migration in PSCs [1,2].

In this work, we first investigate ion migration using two complementary spectroscopic techniques [3-6]: operando X-ray photoelectron spectroscopy (op-XPS) and impedance spectroscopy (IS). Op-XPS on lateral PSCs enables probing of device performance and interfacial electronic structure changes induced by ionic motion [4,5]. Second, IS measurements are combined with drift–diffusion (DD) simulations to provide a detailed characterization of device operation and ionic transport. This coupled IS–DD framework is applied to NiOx-based PSCs with different interfacial passivation schemes [6]. The simulations allow the interpretation of IS spectra under varying illumination intensities and during bias-stress tests, revealing that interfacial modification can reduce electron mobility in NiOx and ion mobility within the perovskite absorber.

Together, these combined results enhance our understanding of ion migration and its impact on PSC performance and operational stability.

Impedance-based techniques are widely used to investigate the behavior of operating perovskite solar cells (PSCs), particularly for assessing ionic migration and its influence on device performance and stability. In this work, we use transient single-frequency capacitance measurements to demonstrate that inter-pixel photo-potential coupling produces long-range in-plane ionic changes that propagate far beyond the illuminated region, affecting neighboring pixels at distances greater than 2500 μm. This finding reveals that ionic redistribution in PSCs can extend over unexpectedly large spatial scales, challenging the typical assumption that ionic motion remains confined to the directly perturbed region. We further apply this mechanism to analyze the impact of incorporating PbS quantum dots as a bulk heterojunction material, showing how their presence modifies the electrical properties and ionic response of the devices. The sensitivity of this method enables us to detect subtle variations in interfacial charge accumulation and ion–electron interactions. Overall, our results highlight photo-potential coupling as a powerful diagnostic tool for probing long-range ionic dynamics in PSCs and for evaluating material modifications.

Interpreting the impedance response of perovskite solar cells (PSCs) is challenging due to coupled ionic and electronic motion, which creates complex, overlapping signals. While drift-diffusion (DD) modelling can extract meaningful physical parameters, its complexity makes direct parameter estimation from experimental data impractical. This work overcomes this limitation by using DD simulations to generate a large synthetic dataset of impedance spectra, which is used to train machine learning (ML) models. For a standard TiO₂/MAPI/spiro cell, a Gradient Boosting Regressor was the most effective at predicting key recombination and ionic parameters. Interpretative analysis revealed that open-circuit measurements best probe recombination losses, while short-circuit conditions are optimal for extracting ionic properties like concentration and mobility. The trained models successfully analyzed experimental data, predicting ion concentrations of (1.3-3.3) × 10¹⁷ cm⁻³ and mobilities of (5-7) × 10⁻¹¹ cm²V⁻¹s⁻¹. This approach demonstrates a viable pathway to accurately derive efficiency-determining physical parameters from impedance measurements in PSCs.

The determination of the energetic depth of a defect is of fundamental importance to identify the severity of its impact on the performance of the solar cell, particularly when characterizing degradation effects as a function of environmental stressors such as light or temperature. In this regard, we develop the theory of the impact of defect-mediated recombination on the measured time constants from optoelectronic small-perturbation measurements, identifying that the trap depth can be qualitatively determined from the slope of the measured time constants versus quasi-Fermi level splitting. Furthermore, we find that the coupling between the geometric capacitance and bulk recombination leads to the slope factors of the time constants being equal to the ideality factors. We apply these findings to experimental data for perovskite solar cells with different bandgaps, confirming the equality between the slope factors and ideality factors, in addition to identifying the existence of shallow or deep defects in different devices.

The impedance spectra of perovskite solar cells frequently exhibit multiple features that are typically modelled by complex equivalent circuits. This approach can lead to the inclusion of circuit elements without a sensible physical interpretation and create confusion where different circuits are adopted to describe similar cells. Spectra showing two distinct features have already been well explained by a drift-diffusion model incorporating a single mobile ionic species but spectra with three features have yet to receive the same treatment and have even been dismissed as anomalous. This omission is rectified here by showing that a third (mid-frequency) impedance feature is a natural consequence of the drift-diffusion model in certain scenarios.

Our comprehensive framework explains the shapes of all previously published spectra, which are classified into six generic types, and approximate solutions to the drift-diffusion equations are obtained in order to illustrate the specific conditions required for each of these types of spectra to be observed. Importantly, it is shown that the shape of each Nyquist plot can be linked to specific processes occurring within a cell, allowing useful information to be extracted by a visual examination of the impedance spectra.

A modified drift-diffusion model, which captures degradation via an increasing recombination rate during the course of characterization experiments, is then used to investigate the effect of device degradation on current−voltage and impedance measurements of perovskite solar cells (PSCs). Numerical solutions of this model are obtained with the open-source drift-diffusion software IonMonger. These show that impedance spectroscopy is significantly more sensitive measure of degradation than current−voltage curves, reliably detecting a power conversion efficiency drop of as little as 0.06% over a 4 h measurement. Furthermore, it is found that fast degradation occurring during impedance spectroscopy can induce loops lying above the axis in the Nyquist plot, the first time this experimentally observed phenomenon has been replicated in a physics-based model.

Ultrathin indium selenide has recently become a promising 2D semiconductor for photodetector devices thanks to its outstanding optical and electronic properties [1, 2]. Nonetheless, InSe free surface is highly sensitive to environment and light exposure, affecting the performance of devices and reducing their lifetimes [3]. Specifically, intrinsic defects, such as selenium vacancies (V_Se) and indium interstitial atoms, do have a major impact on 2D InSe optoelectronic properties either doping the material or creating in-gap trapping states [4]. Therefore, understanding the physical processes affecting surfaces and interfaces can envision valuable strategies for stable 2D InSe optoelectronic device design.

In this work, we investigate the operation of 2D InSe nanosheets (20-40 nm) deposited on Pt electrodes. Current-voltage measurements confirmed the formation of a double Schottky junction. Photoconductivity ON/OFF cycles at low light excitation power conducted on pristine samples revealed a trap sensitized response that allows for a high responsivity at the expense of slow rise/decay times. However, after sufficiently high illumination, an effective photopassivation was observed: response times decreased remarkably along with a reduction of dark currents and an enhancement of photocurrent linearity with light power. To gain an understanding of these phenomena, light-assisted Kelvin probe force microscopy (KPFM) maps of the InSe nanosheets have been recorded for different light illumination intensities. We observe a gradual surface photovoltage (SPV) change on pristine samples: initially the SPV is negative, whereas for longer and more powerful exposures SPV becomes positive. This effect is permanent and causes a surface potential increase, around 100 mV. On the other hand, hBN-encapsulated InSe nanosheets show no evolution with illumination power and they are characterized by a fast response, steadier responsivity and negative SPV for all illuminations. Our SPV results are consistent with an increase of n-doping that shifts InSe Fermi energy level, and a transient behavior during irradiation related to band flattening in the air/InSe interface after trap states are deactivated. We concluded that photopassivation is taking place due to V_Se filling with reactive species, mainly oxygen. These processes are inhibited in encapsulated samples, as hBN suppresses oxygen leaking into InSe surface.

Lab-scale devices have consistently showcased the efficiency potential of monolithic 2-terminal (2T) perovskite-silicon tandems, recently surpassing 34%. In addition, the technology has seen successful transfer from lab-scale devices to full-sized cells, with efficiencies of 28.6% on M10 and even commercially available modules reaching efficiencies up to 27%. However, if the perovskite-silicon tandem technology is to lead the efficiency increase of photovoltaic modules beyond 30%, the primary challenge to overcome is their long-term stability.

To test long-term operation performance and degradation including the underlying degradation mechanisms, we developed a bichromatic LED indoor stability testing setup with integrated, in-operando luminescence (PL/EL) imaging capabilities in addition to standard MPP tracking and in-situ I-V measurements. The periodic in-operando measurements provide us with a rich dataset enabling in-depth root cause analysis of degradation mechanisms of perovskite single junction and perovskite-silicon tandem solar cells either under indoor or outdoor operating conditions.

We utilized our monitoring setup to study the effect of individual layers within the single-junction perovskite stack on the long-term stability of the perovskite absorber with a FACs (Fa_0.82Cs_0.17Pb(I_0.83Br_0.17)₃) composition, gradually building towards a full layer stack with additional AlO_x capping as a moisture ingress barrier. We could distinguish three different PL intensity trends correlated with the stack composition, with perovskite absorber alone being the most stable in terms of PL intensity, ETL and HTL adding some dynamics to the initial PL stabilisation and the addition of copper layers leading to a tremendous PL intensity increase and device degradation after a few days. Nevertheless, across all tested samples, the presence of the ETL (C60) within the cell stack significantly promoted the formation of inhomogeneities across the device.

In addition to single junction perovskite devices, the monitoring setup was used for accelerated testing of perovskite-silicon tandem solar cells under cycled perovskite-/silicon-limited operating regimes, reaching t₈₀ of more than 600 hours. Combining integral electrical parameters (MPP, I-V) and bias dependent luminescence images (PL at OC, SC, MPP) we are able to track the local evolution of the perovskite sub-cell degradation, as well as to extract the reversible and irreversible perovskite/tandem degradation rates within the different limitation regimes. Periodic PL images revealed that that the reversible degradation can be mainly attributed to lateral ion migration, whereas the irreversible degradation rate of the perovskite sub-cell is strongly correlated with the excess current density not extracted from the perovskite during silicon limitation.

As Si cells are reaching their fundamental limits, silicon/perovskite tandem solar cells are considered to be the next-generation mainstream technology. These and other perovskite-based tandem solar cells usually rely on wide-bandgap (WBG) mixed halide perovskites. However, these cells are always more or less limited by ion migration and halide segregation (HS), which is a critical factor for operational stability.[1] In this talk, I will discuss the intriguing correlation between HS, ion density, and ionic loss evolution in wide-bandgap perovskite solar cells. This also includes how recovery effects seen in HS segregation relate to a recovery in ion density and ionic losses. Related to this, I will also discuss irreversible ionic losses, which stay “locked-in” after prolonged illumination and are therefore critical for the operational stability. I will then elaborate on strategies to mitigate mobile ion-induced performance degradation. This includes pure-iodine, dimethylammonium (DMA)-stabilized WBG perovskites, which are not only efficient (>22% certified) and stable under maximum power point tracking (T80 lifetimes exceeding 8700 h), but also demonstrate notable performances under combined photothermal stressors (light + 85 °C).