Organic-inorganic metal halide perovskites have emerged as attractive materials for solar cells with power-conversion efficiencies of single-junction devices now reaching 28%. Combinatorial optical characterization approaches are vital for probing and analysing such their electronic properties and material stability. We here investigate mixed bromide-iodide perovskites which offer ideal bandgaps for tandem solar cells, but still suffer from light-induced halide segregation, which compromises their operational stability.

We reveal how crystalline film quality and halide segregation are critically affected by bromide fraction x in CH₃NH₃Pb(I_1−xBr_x)₃ through macrostrain and ordered-phase formation [1]. We show that the overall amplitude of phase segregation follows a broadly symmetric distribution in compositional space, maximized near x = 0.5, but the potentially ordered compositions of CH₃NH₃PbIBr₂ and CH₃NH₃PbI₂Br diverge sharply, presenting particularly stable and unstable scenarios, respectively. Notably, halide segregation is shown to occur even below the widely quoted perceived threshold of x = 0.2. Such analysis highlights promising approaches to mitigate halide segregation, through engineering of macrostrained phases and local atomistic ordering.

In addition, we directly probe the impact of halide segregation on charge-carrier dynamics at the interface between a mixed-halide perovskite and charge transport layers by using a free-space synchronous multimodal spectroscopy approach, combining time-resolved microwave conductivity, time-resolved and steady-state photoluminescence [2]. We reveal that charge extraction from such iodide-rich domains is still surprisingly feasible, but competes with enhanced radiative recombination resulting from higher charge concentrations caused by funnelling into these minority phases.

Finally, we demonstrate that the while photoluminescence tracking is universally used to monitor such photoinstability, such data are not good measures of halide segregation [3]. We demonstrate that PL cannot accurately reflect the rate and extent of halide segregation because it is governed by charge funneling to iodide-rich minority domains, which is strongly influenced by additional factors, including luminescence efficiency, band energetics, and charge extraction.

Mixed bromide-iodide perovskites present great potential for optoelectronic devices due to the facile engineering of their optoelectronic properties. However, such mixtures suffer from photo-induced phase segregation when exposed to light (photo de-mixing)¹. While the process is partially reversible (two phases remix back in the dark to the pristine state, dark re-mixing),^{2, 3} this phase instability can potentially lead to unstable optoelectronic properties and device performance, making its understanding essential to progress the field of halide perovskites. Since the observed light induced evolution of different phases involves significant ion transport, clarifying the underlying defect chemical mechanisms involved in the photo de-mixing is critical.⁴

Here, we focused on thin films of 2D Dion-Jacobson mixed halide perovskites (PDMA)Pb(Br0.5I0.5)4 (PDMA: 1,4-phenylenedimethanammonium spacer) as model material to investigate their phase behavior both under light and in the dark using a wide range of experimental techniques.⁵ First, we tracked the compositional evolution in the films during de-mixing and re-mixing by analyzing their time dependent in-situ optical absorption properties. We also simultaneously monitored the conductivity changes during de-mixing, which allows for local probe of the ionic and electronic charge carriers concentration and ion transport through the de-mixed phases. We furthermore take advantage of SEM and TEM to investigate the morphological changes and the nature of the iodide rich and bromide rich phases resulting from phase segregation. Lastly, we propose a model that considers possible opto-ionic effects, which can contribute to the driving force of de-mixing^{6, 7} and should therefore be considered in the overall energy balance of the process, together with the electronic effects discussed in the literature.8 These findings will aid compositional engineering related to halide mixtures to enable optimization of optoelectronic devices as well as the development of other emerging systems exploiting photo de-mixing.

1. E. T. Hoke, D. J. Slotcavage, E. R. Dohner, A. R. Bowring, H. I. Karunadasa and M. D. McGehee, Chem. Sci, 2015, 6, 613–617.

2. Y.-R. Wang, A. Senocrate, M. Mladenović, A. Dučinskas, G. Y. Kim, U. Rothlisberger, J. V. Milić, D. Moia, M. Grätzel and J. Maier, Advanced Energy Materials, 2022, 12, 2200768.

3. P. S. Mathew, J. T. DuBose, J. Cho and P. V. Kamat, ACS Energy Lett., 2021, DOI: 10.1021/acsenergylett.1c01015, 2499–2501.

4. Y.-R. Wang, Dissertation, Stuttgart, Universität Stuttgart, 2023.

5. Y.-R. Wang, M. Mladenović, E. Kotomin, Y. Chao, K. Hahn, J. Lee, W. Sigle, J. V. Milić, P. A. van Aken, U. Rothlisberger, M. Grätzel, D. Moia and M. Joachim, arXiv preprint arXiv:2512.05879, 2025.

6. G. Y. Kim, A. Senocrate, Y.-R. Wang, D. Moia and J. Maier, Angewandte Chemie International Edition, 2021, 60, 820–826.

7. Y.-R. Wang, G. Y. Kim, E. Kotomin, D. Moia and J. Maier, JPhys Energy, 2022, 4, 011001.

8. S. Draguta, O. Sharia, S. J. Yoon, M. C. Brennan, Y. V. Morozov, J. S. Manser, P. V. Kamat, W. F. Schneider and M. Kuno, Nat Commun, 2017, 8, 200.

The ongoing development of new-generation optoelectronic structures requires a continuous improvement of experimental methods used to characterise the physical properties of constituent materials in multi-layer devices. The traditional approach to obtaining absorption-related features in the solid state, intended to reveal the band structure in semiconductors, often suffers from a limited sensitivity imposed by a strong background signal and light scattering. Subtle changes in the sub-band-gap spectral region of semiconductors contain crucial information about defect activity and temperature-activated effects in the tails of the density of states. Extracting parameters such as the Urbach energy, representing the energetic disorder in thin-film materials, calls for alternative tools overcoming the optical detection constraints.

This talk will showcase the recent advancements in photothermal spectroscopy applied to investigate optical absorbance in a wide range of organic and hybrid semiconductors. Detection schemes based on converting the modulated light beam energy into heat oscillations overcome the sensitivity limits, enabling accurate probing of optical transitions characterised by low oscillator strength, preserving high dynamic range in layers as thin as 1 – 10 nm. Specific benefits of two selected experimental methods, photoacoustic spectroscopy (PAS) and photothermal deflection spectroscopy (PDS), will be presented in relation to the physical parameters of thin films crucial for the fabrication of device stacks [1,2]. The absorption edge sharpness, quantified by the Urbach energy, is compared against external quantum efficiency spectra on fully assembled solar cell and light-emitting diode structures utilising lead-halide perovskites and their low-dimensional counterparts as the active materials. Photothermal spectroscopy, due to its insensitivity to light scattering, also overcomes limitations commonly found in the characterisation of solution-processed materials at low concentrations. The obtained results provide a deeper insight into the fundamental properties of emerging material platforms, serving as a predictor of energy conversion efficiency in thin-film optoelectronic devices [3]. The talk concludes by presenting prospects for non-contact studies of thermal transport in solid state by ultrafast optical thermometry.

In this talk, I will present recent results and insights into the factors governing structure-property relationships in 3D bulk halide perovskites. A unifying theme is that these materials host dynamic, short-range octahedral tilt order, a local structure distinct from the average crystallographic phase that varies with the A-site cation and with thermal history.

I will first present Brillouin spectroscopy that reveals differences in the acoustic phonons of MA- and FA-based perovskites. We observe asymmetric Brillouin spectra, a signature of piezoelectric coupling between longitudinal acoustic (LA) phonons and the dynamic fluctuations of octahedral tilts. Complementary electric-field-dependent single crystal measurements show a corresponding contrast between the two cations, with a piezoelectric response present in MA but absent in FA, consistent with this coupling.

I will then introduce our 4D time-resolved hyperspectral photoluminescence imaging, which provides simultaneous temporal and spectral resolution at the diffraction limit. This allows us to identify charge transfer from the band edges to sub-gap states in thin CsPbBr3 single crystals in regions under tensile strain, providing a unified link between local structural fluctuations, nanoscale strain and macroscopic properties. Using the same technique, we follow the evolution of photoluminescence near phase transitions and find it to be strongly thermal-history dependent, revealing a further intrinsic instability. Together, these results establish local structural dynamics and thermal history as central, and potentially tunable, degrees of freedom in halide perovskites.

Understanding the fundamental properties of halide perovskites requires a microscopic description of structural dynamics and excess-charge behavior at the local scale. Building a unified microscopic picture is still challenging because structural dynamics and the behavior of excess charges vary strongly with chemistry, phase, dimensionality, and local environment. In this talk, I will present examples from our recent work on the role of local structural dynamics in phase behavior, vibrational response, and charge localization. I will first discuss the local structural response of FAPbI₃ surfaces to molecular adsorbates. Surface templating has been proposed experimentally as a route to stabilize selected perovskite phases, and we use atomistic simulations to test this picture at the local scale. I will then show how machine-learned interatomic potentials can be used to connect structural dynamics across different perovskite chemistries with infrared and Raman spectra. Finally, I will discuss charge localization in lead-free halide perovskites, focusing on how chemistry and dimensionality affect the stability of localized charges and the associated lattice distortions.

The performance and stability of metal halide perovskite solar cells (PSC) are ultimately defined by the complex electronic structure and chemistry of their buried interfaces. Despite rapid progress in device engineering, a fundamental understanding of how these interfaces form, transform, and govern charge transfer across organic and inorganic charge transport layers remains incomplete. Addressing this challenge requires a methodological shift toward comprehensive interface analytics that can correlate chemistry, energetics, and device functionality across multiple length and time scales.

In this talk, I present our approach to interface characterization in PSC, combining complementary spectroscopic and microscopic probes that span occupied and unoccupied electronic states as well as spatially resolved electrostatics. In particular, we use direct and inverse photoemission spectroscopy (UPS/XPS and IPES), hard X-ray photoemission spectroscopy (HAXPES), and operando techniques such as photoluminescence mapping (PL), Kelvin probe force microscopy (KPFM), and time-resolved spectroscopy to build a consistent picture of interfacial energetics and defect formation.

Rather than focusing on isolated material systems, this framework highlights how different interlayers, ranging from inorganic oxides to organic self-assembled monolayers and 2D/3D perovskite passivation layers,^1,2 can be understood within a common analytical language. By probing how interfacial dipoles, band alignment, and chemical reactivity plays into layer deposition and device operation, we reveal general design rules for controlling charge extraction and suppressing recombination at buried interfaces.

This multi-modal perspective establishes interface analytics as a central tool for next-generation perovskite photovoltaics, enabling not only the diagnosis of degradation pathways but also the rational design of stable, energetically aligned contacts. Ultimately, bridging spectroscopy, microscopy, and operando measurements provides a pathway toward predictive control of interface physics in complex hybrid semiconductor systems.

1. N. Mallik, et al. EES Solar 2025, DOI: 10.1039/D5EL00044K

2. J. Hajhemati et al. ACS Appl. Mater. Interfaces 2025, DOI: 10.1021/acsami.5c18456

Halide perovskites are widely celebrated for their exceptional optoelectronic properties, yet their most distinctive semiconductor trait may be their chemically and ionically dynamic nature. Soft lattices, mobile ionic species, low defect formation energies, and strong coupling to external stimuli are routinely associated with instability, hysteresis, and variability, but these same phenomena become a powerful design resource when properly understood and controlled.
This talk explores how chemical dynamics can be deliberately steered across different halide perovskite platforms and device contexts. In tin-based perovskites, molecular additives demonstrate how targeted chemistry can regulate ionic processes at the precursor stage, reshaping crystallization and suppressing oxidative degradation. Strikingly, the same ionic landscape that threatens stability also enables spontaneous performance recovery under realistic stress conditions, a self-healing behavior rooted in the material’s dynamic character. Beyond photovoltaics, the coupling between ion migration, defect chemistry, and electrochemical response underpins resistive switching in perovskite memristors, revealing that the very dynamics complicating solar cell operation are precisely what enables computing-oriented functionality.
A central challenge across all these contexts is disentangling the contributions of bulk transport, interfacial charge accumulation, and ionic redistribution, processes that overlap in both time and frequency domains. Operando characterization combining impedance spectroscopy and luminescence analysis provides a powerful diagnostic framework to address this. Impedance spectra, interpreted through physically grounded equivalent circuit models, resolve the frequency-dependent response of selective contacts and heterojunction interfaces, revealing how ion accumulation modulates band bending, recombination kinetics, and extraction barriers under working conditions. Coupled with photoluminescence and electroluminescence, this approach quantifies non-radiative losses at specific interfaces and distinguishes reversible ionic rearrangements from irreversible degradation pathways in real operating conditions.
Together, these results reframe chemical dynamics not as a nuisance to be suppressed, but as a material property to be engineered, pointing toward halide perovskite devices that are simultaneously efficient, self-healing, and multifunctional

Metal-halide perovskites have emerged as a versatile materials platform for next-generation electronic and optoelectronic technologies, enabling applications that extend far beyond photovoltaics. In this talk, I will present recent advances in understanding the switching behavior of perovskite memristors and demonstrate how they can be integrated directly into solar cells to address critical challenges in real-world operation.
The first part of the presentation focuses on the fundamental mechanisms governing resistive switching in perovskite memristors [1]. Through detailed device investigations, we show that switching is not mediated by distributed conductive nanofilaments, as commonly assumed, but instead occurs within highly localized regions that form during initial device operation. By introducing a light-assisted patterning approach to precisely define these regions, we eliminate the need for an energy-intensive electroforming step and reveal that switching takes place at the boundaries of these regions. This understanding enables perovskite memristors with low operating voltages, extremely low leakage currents, long data-retention times, and stable performance over millions of switching cycles.
Building on these insights, the second part of the talk demonstrates how memristive functionality can be leveraged to solve one of the key reliability challenges facing perovskite solar cells: instability under reverse-bias conditions caused by partial shading of series-connected soalr cells and modules. We introduce the Memsol concept, a solar cell with an integrated memristor that shares the perovskite absorber and electrodes while acting simultaneously as a self-regulating protection and bypass element [2]. Under reverse-bias conditions, the integrated memristor automatically switches into a low-resistance state, protecting the solar cell from degradation; under normal operating conditions, it returns to a high-resistance state, preserving full photovoltaic performance. Reverse-bias and shading experiments, including demonstrations on multi-cell strings, confirm robust and autonomous operation without the need for external bypass components.
Together, these results demonstrate how advances in the understanding and control of resistive switching in perovskites can enable new device concepts that combine electronic and photovoltaic functionality. The presented work connects fundamental studies of memristive behavior with practical applications in solar-cell technology and highlights the broader potential of perovskite materials for integrated electronic systems.

Tin (Sn)-based halide perovskites have garnered significant attention as promising lead-free alternatives for p-type channels in thin-film transistors (TFTs). However, compared to their lead (Pb) counterparts, they still suffer from poor electrical properties due to intrinsic defect states and limited crystallinity. While conventional electronic doping via substitutional impurities is a powerful tool in sillicon (Si)-based semiconductors, replicating this systematic approach in perovskites is highly challenging due to the strong charge-compensating behaviors of intrinsic defects. Although organic modification offers a potential pathway for electronic tunability, the precise interfacial interactions between organic moieties and perovskites remain largely unexplored.

To address this challenge and establish a reproducible doping framework, we report a robust surface p-doping strategy by introducing a thiophene-containing polymer onto quasi-two-dimensional (2D) tin perovskites. The Sn ions in the perovskite framework effectively interact with the sulfur atoms in the thiophene moieties at the organic–inorganic interface. This targeted interaction induces efficient hole generation, effectively modulating the electronic structure and charge carrier density while overcoming the intrinsic transport bottlenecks of quasi-2D multiple quantum wells. Consequently, the doped quasi-2D tin perovskite transistors exhibit a remarkable field-effect mobility of 53.57 cm² V^-1 s^-1, representing a nearly 7.5-fold increase over the 7.16 cm² V^-1 s^-1 observed in the undoped control device. Additionally, the devices achieve an outstanding on/off current ratio exceeding 10⁷ and superior operational stability. These findings demonstrate that tailored interfacial molecular chemistry provides an effective, experimentally validated route to achieving stable, high-mobility, and lead-free perovskite electronics.

Field effect transport in perovskite devices has been conventionally challenging due to the undesirable ionic defect migration and dipolar disorder. We address both these challenges through suitable compositional engineering in methylammonium free based perovskites to realize high performance n-type and p-type perovskite FETs. We observe that incorporation of Sn based perovskite minimizes the ion-migration to an extent that clean hysteresis free field effect transport is realized with mobility values of > 5 cm²/Vs. Microscopic characterization brings out the role of Sn-vacancies which compensates the ionic defects to realize such high perfromance transistor. Next, we will discuss the possibility of utilizing compositional engineering through nanoscale facets in n-type FAPbI₃ . This is particularly interesting in the context of realizing phase-pure FAPbI₃ for field effect devices. We demonstrate compositional engineering of FAPbI3 using facet-rich nanocrystals to realize n-type transport > 1 cm²/Vs. These devices demonstrate exceptional operational bias stress stability with a marginal threshold voltage shift (∆V_th) ~ 0.7 V for 10 hours of continuous operation which quickly recovers back to pristine condition. Moreover the devices also exhibit > 1500 hours of ambient stability and > 8500 hours of performance retention under nitrogen atmosphere making them one of the champion n-type perovskite compositions till date in terms of stable devices for practical applications. Interestingly, inorporation of faceted CsPbBr3  nanocrystals into FAPbI3 modulates the temperature dependent transport to activated behaviour which is generally not observed in 3D Pb-based perovskite compositions indicating a resilence of gate induced ionic defect migration. By integrating electronic/ionic transport studies along with terahertz time-domain spectroscopy, spetroscopic, and structure–property correlations, we establish the microscopic mechanism behind these observed enhancement in transport properties.

Lead halide perovskite nanocrystals (LHP NCs) [1] - the latest generation of colloidal quantum dots (QDs) - possess dynamic, entropically stabilized soft lattices and electronically benign surfaces that, remarkably, do not compromise their textbook semiconductor optical quality. The compositional diversity of APbX₃ comprises cesium ions on A-site as the only ones forming fully inorganic perovskite lattice of this kind, whereas a range of organic cations readily form this lattice, methylammonium (MA), formamidinium (FA), aziridinium (AZ) [2-4]. The surface chemistry of perovskite QDs is paramount for instilling (photo)chemical stability. We will review four distinct cases of capping ligands, based on their binding tightness and dynamicity, emphasizing synthetic phospholipids as the best-performing [5]. Structurally dynamic FA/MA/AZ-based QDs, stabilized with zwitterions, turned out to be near-perfect (blinking free, saturable, high single-photon purity) single-photon sources (incoherent) at room temperatures [6], as attributed to the dynamic wavefunction localization. Beyond photophysics, LHP QDs have recently proven to be efficient photocatalysts, mediating organic redox transformations that remain inaccessible to conventional photocatalysts [7], and which properties are governed by the dynamic surface-ligand interface [8]. We then demonstrate observation of a single photocatalyst via micro-photoluminescence measurements of its single-photon emission [9], addressing static and temporal heterogeneity in photocatalytic function. The presentation will summarize the contributions of my interdisciplinary team and our international collaborators, whose names will be acknowledged in the presentation and accompanying notes.

1. L. Protesescu et al. Nano letters 2015, 15, 3692–3696

2. Q. Akkermann et al. Science, 2022, 377, 1406-1412

3. V. Morad et al. J. Am. Chem. Soc., 2025, 147, 8, 6795–6804

4. M. Bodnarchuk et al. ACS Nano, 2024, 18, 7, 5684–5697

5. V. Morad et al. Nature, 2024, 626, 542–548

6. L. Feld et al. Nature Comm., 2026, 17, 1974.

7. V. M. Amberg et al. J. Am. Chem. Soc., 2025, 147, 10, 8548–8558

8. Y. Sahin et al. Nano Lett., 2026, 26, 9, 3107–3116

9. L. Feld et al. 2026 in revision, https://doi.org/10.26434/chemrxiv.10001851/v1

With the advent of metal halide perovskites into the world of emerging sustainablesemiconductors, a host of previously unprecedented applications has materialised. Aside from their ragingsuccess in photovoltaics, perovskites as applied to light emitting diodes (LEDs) and displays have gainedparticular interest due to their extremely facile bandgap tunability, directional and narrow-linewidth emissioncharacteristics, high brightness, superior efficiencies and colour purity among other desirable features.However, most of the world records in terms of device efficiency and stability have been achieved on lab-scalepixels processed using solution-based techniques, primarily spin-coating. However, with its inherentadvantages of scalability, reproducibility and precise thickness control, vacuum-based thermal evaporationprovides an edge over solution-processing for all optoelectronic applications.

Following recent advancements in vacuum thermal evaporation of perovskite solar cells in our group, we haveemployed this technique to fabricate highly luminescent red-emitting perovskite films for efficient andoperationally stable light emitting diodes (LEDs). By optimizing the growth conditions, we have been able toachieve previously unreported photoluminescence quantum efficiencies (PLQE) close to 20% under 1-sun-equivalent conditions for red emitters. Moreover, no sign of unwanted halide segregation has been observedunder continuous illumination, thereby resulting in stable PL emissions in the wavelength range of 630-640 nm(within CIE pure-red range). To understand the effect of deposition conditions on the resulting optoelectronicproperties of evaporated perovskites, a range of characterization including intensity-dependent PLQE, fluence-dependent TRPL, THz spectroscopy and transient photoconductivity measurements have also beenconducted. Furthermore, X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy(UPS) have been employed to obtain information on the chemical composition and electronic properties of theevaporated films. To gain deeper insight into the microscopic origins of device performance, we employphotoluminescence (PL) imaging to directly probe static and dynamic defect populations within the perovskitefilms. These measurements reveal pronounced spatial heterogeneity and provide insight into carrierrecombination pathways that remain poorly understood in vacuum-deposited perovskite emitters. Theoptimized LEDs exhibit negligible electroluminescence peak shifts with increasing drive voltage, indicative ofsuppressed phase segregation and spectral instability, while operational lifetimes (T₈₀) of several minutes areachieved under continuous operation. To further elucidate degradation mechanisms, fresh and aged devicestacks are investigated using a combination of STEM-EDX, ToF-SIMS, and PL microscopy. This multimodalanalysis tracks changes in structure, morphology, composition, and defect distributions throughout themultilayer architecture, enabling identification of the dominant degradation pathways responsible forperformance losses under sustained current-bias stress. Finally, by extensive screening of the charge injectionlayers, we have been able to demonstrate red LEDs with champion external quantum efficiency (EQE) ~5% and turn-on voltage~ 3 V which exhibit unprecedented high luminance of > 3000 cd/m².

Conventional spectroscopic techniques often provide limited insight into the complex carrier dynamics of multilayer semiconductor devices under operational conditions. Although time-resolved photoluminescence (TRPL) measurement is widely used to monitor carrier dynamics in a perovskite film, interpreting carrier dynamics under operating conditions remains a critical challenge due to competing processes, including surface and bulk recombination, carrier transfer to the charge transport layers (CTL), carrier trapping and detrapping, capacitive charging and discharging. Understanding the interplay among these processes is crucial for optimizing charge collection, minimizing recombination losses, and enhancing photodetector device performance. In this work, we have investigated operando carrier dynamics of a 2D Ruddlesden-Popper mixed-halide perovskite (Cs₂PbBr₂I₂) photodetector using simultaneous TRPL and photocurrent measurements under pulse laser and LED illumination with varied wavelengths. A pulse laser is used as an excitation source for TRPL, while a CW LED with different wavelengths is used to modify the carrier population and trap occupancy during device operation. The impact of CTLs (TiO₂ and CuI) on carrier extraction or interfacial transport was initially confirmed by PL and TRPL measurements, which exhibited strong quenching, indicating efficient carrier transfer through the transport layers. The operando measurements reveal that photocurrent and decay lifetime are strongly dependent on the wavelength and intensity of the background illumination, demonstrating that trap occupancy and interfacial charge extraction affect the measured carrier dynamics and device performance. Furthermore, bias-dependent TRPL measurements under short-circuit and forward bias also show a significant decrease in the carrier lifetime, accompanied by an increase in photocurrent, indicating improved charge extraction in the device. These observations underscore the role of carrier trapping, detrapping, recombination, and carrier extraction processes in an operating device. This work demonstrates operando TRPL as a powerful tool for unraveling the interplay between carrier recombination, trapping, and charge extraction in multilayer perovskite photodetectors, providing a more reliable interpretation of carrier dynamics in a working device.

Single molecule spectroscopy has tremendously impacted every field in which the technique was applied, ranging from catalysis over plasmonics, polymer physics, biophysics to cell biology and DNA sequencing. Furthermore, single molecule techniques have allowed researchers to push the resolution of fluorescence microscopy past the diffraction limit, based on characteristic single molecule intermittence of fluorescence. In this presentation, I will give an overview of how recent single molecule experiments and development of new microscopy modalities in my laboratory have been impacted by material science and how these experiments have been driving developments in material science, with a focus on our perovskite research. 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-function relationship in these materials, especially in operational devices like solar cells, where traditional methods such as scanning electron microscopy fall short due to the layered structure. Moreover, electron and x-ray-based analytical techniques are often invasive altering the material properties.

To address these challenges, we developed Correlation Clustering Imaging (CLIM), a novel noninvasive method that utilizes photoluminescence fluctuations to reveal contrasts associated with defect dynamics in semiconductor materials. 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. Statistical analysis of these intensity fluctuations provides insights into the type of metastable defects responsible for fluctuating non-radiative recombination processes.

The insights gained from CLIM contribute to a deeper understanding of device efficiency, structure, and degradation, which are crucial for the rational engineering of the next generation of devices. The broad applicability of CLIM, requiring only a standard wide-field microscope and our user-friendly, open-source algorithm, positions it as an important new tool for material chemists, engineers, and device scientists [1,2].

Inorganic halide perovskites present unique light-dependent characteristics, enabling their implementation as active components within switchable photonic devices. We have experimentally demonstrated optical switchability in the mid-infrared (MIR) transmission from 3–20 μm as a direct result of the material’s lattice distortion, which creates polaron-like states [1]. We measured up to 10% changes in transmission, with modulation times <1 millisecond, and multi-state response. Moreover, we demonstrate transient, lattice distortions in single crystal MAPbBr3, FAPbBr3, and CsPbBr3 in response to above-bandgap light excitation. Using an X-ray probe, we unravel fully reversible and hysteresis-free photoinduced lattice deformation [2]. CsPbBr3 shows the highest resilience against lattice deformation with a 0.062% change in its out-of-plane lattice parameter. Yet, the organic cations led to more significant yet elastic distortion, with MAPbBr3 exhibiting up to 0.3% change. Overall, our results establish halide perovskites as potential building blocks for photostriction and switchable photonic devices.

 [1] Science Advances, in press (2026).

 [2] Adv. Materials, online (2026). DOI: 10.1002/adma.202521800

Photoluminescence of metal halide perovskite semiconductors exhibits highly complex temporal dynamics under time-modulated excitation. The characteristic timescales range from nanoseconds to hours and even days, depending on the underlying photo-induced processes. Fast dynamics are governed by charge trapping, detrapping, and recombination, whereas slower dynamics originate from ion migration and defect chemistry.

I will discuss these dynamic processes across the entire range of timescales, with particular emphasis on photodoping and on halide segregation in mixed-halide perovskites. The presentation will focus on steady-state and time-resolved spectroscopic methods employing pulse bursts and even more complex excitation sequences recently developed in our laboratory.

This unusual photoluminescence response makes perovskites well suited to the definition of a memlumor—a luminophore with memory—an elementary building block for photonic information processing, including neuromorphic computing.[1] The photoluminescence quantum yield of a memlumor "remembers" the history of previous optical excitation through photo-induced changes in the parameters governing its photophysics and photochemistry. I will discuss memory effects arising from photodoping in CsPbBr₃ and related materials,[2,3] demonstrate their application to the recognition of 5-bit temporal pulse sequences,[4] and present preliminary results on controlling photo-induced halide segregation in mixed-halide perovskites using tailored excitation protocols.

Understanding nonradiative recombination in metal halide perovskites is essential for accurately describing their photophysics and efficiency limits. While first-order trap-assisted recombination is well established, the existence and origin of second-order nonradiative recombination have remained controversial because similar experimental observations can be explained by multiple competing mechanisms[1,2].

Here, we provide direct experimental evidence for second-order nonradiative recombination in triple-cation perovskite thin films and demonstrate that it is consistently explained by an Auger-assisted trapping mechanism. We combine excitation-fluence-dependent transient and steady-state photoluminescence measurements over a wide carrier density range with a unified kinetic model that quantitatively reproduces both datasets using a single set of parameters[3]. This approach enables the separation of the contributions from shallow traps, deep traps, and Auger-assisted trapping.

Our results resolve a long-standing ambiguity in the interpretation of carrier recombination dynamics and show that neglecting Auger-assisted trapping leads to an overestimation of the intrinsic bimolecular radiative recombination coefficient and an inaccurate assessment of theoretical efficiency limits. The identified recombination pathway is particularly relevant under practical solar-cell operating conditions, including one-sun illumination, where it contributes significantly to nonradiative losses.

These findings provide new insight into the recombination physics of triple-cation perovskites and establish a robust framework for identifying dominant recombination pathways, enabling more accurate modelling of perovskite photovoltaic and light-emitting devices.

Two-dimensional (2D) Ruddlesden–Popper metal halide perovskites exhibit one of the most intriguing optical responses among layered semiconductors. Their spectra frequently display multiple sidebands, broad quasi-plateaus, and pronounced thickness-dependent features that challenge straightforward excitonic interpretations. Here, we critically reassess the optical response of 2D perovskites by examining the intertwined roles of electronic structure, exciton fine structure, exciton–phonon coupling, and photonic effects. We show that the exceptionally large excitonic oscillator strength and high refractive index of these materials naturally give rise to polaritonic stop bands and interference phenomena that can dominate reflection, transmission, and absorption spectra, even in nominally freestanding crystals. These photonic contributions, often overlooked, substantially reshape spectral line shapes and complicate the identification of distinct excitonic resonances, calling for a more integrated electronic–photonic framework for interpreting the optical response of two-dimensional perovskites.

In the second part of my talk, I will revisit the lattice dynamics of 2D layered perovskites and show that Raman active modes, involve substantial motion of the organic sublattice. This finding challenges the conventional assignment of low-frequency modes to predominantly inorganic framework motion and instead reveals their intrinsically hybrid organic–inorganic character. Moreover, these modes can be selectively engineered through chemical substitution of the organic spacer, establishing an additional degree of freedom for controlling lattice dynamics in 2D perovskites. We next probe the lattice response under electronic excitation and identify signatures consistent with polaron formation. These polarons are hybrid in nature: their stabilization requires the coupled response of both organic and inorganic sublattices. Such coupling provides a route to tune the polaronic state through hybrid phonon modes. This hybrid polarons shape the linear optical response of layered 2D perovskites

Lead-halide perovskites have revolutionized optoelectronics owing to their remarkable defect tolerance and efficient charge transport. In contrast, lead-free perovskite-inspired materials (PIMs) often exhibit substantial structural disorder, deep defects, and pronounced charge localization, factors that continue to limit their performance in conventional photovoltaic applications.¹

This talk explores how defects and disorder shape the properties of pnictogen-based PIMs. I will discuss strategies to understand and control defect landscapes through compositional engineering, including Sb–Bi alloying² and mixed-halide approaches,³ and their impact on carrier dynamics and device performance.

Particular attention will be given to recent results showing that structural disorder can induce local symmetry breaking and enhance second harmonic generation, illustrating how phenomena often regarded as detrimental can generate new functionality.⁴ By connecting defects, disorder, electronic structure, microstructure and charge transport across multiple material families, a common framework emerges for understanding both the limitations and opportunities of lead-free semiconductors.⁵

Finally, I will show how these insights have contributed to the development of high-performance lead-free indoor photovoltaics^6,7 while motivating applications beyond photovoltaics. The results suggest that the future of pnictogen-based perovskite-inspired semiconductors may lie not in replicating lead-halide perovskites, but in exploiting the unique properties that arise from their structural complexity.

Photoluminescence (PL) of self-trapped excitons (STE) is often observed in two dimensional hybrid organic-inorganic perovskites. STE emission is characterized by a very large Stokes shift (200-800 meV) and quite large activation energy of ~50-120 meV. The near-band emission (NBE) may be dominated by free exciton emission or can be more complex and may consist of defect-bound excitons and excitons consisting of large polarons especially under hydrostatic pressure [1]. Due to the high softness of organic-inorganic halide perovskites, their properties are often studied by PL method under high hydrostatic pressure, but the effect of pressure on the activation energy of STE and NBE has not been intensively studied due to measurement challenges (i.e., PL measurements at different temperatures under isobaric conditions). We have recently performed such measurements for the Ruddlesden–Popper perovskite ACE₂PbBr₄ [2], and are currently performing them for other perovskites. In this work, we will present how the activation energy of STE depends on the hydrostatic pressure for ACE₂PbBr₄. By increasing the pressure to 2.6 GPa, the activation energy was observed to decrease from ~100 meV to ~25 meV. The dependence of NBE on pressure has been observed to be more complex and will also be discussed in this presentation. To explain the changes in the activation energy and spectral position of STE and NBE, we used a configuration diagram commonly used to explain STE and free exciton emission in organic-inorganic perovskites. This diagram showed that a blue shift of STE and a red shift of NBE lead to a decrease in the STE activation energy. Similar studies are being conducted for other organic-inorganic halide perovskites and will also be presented in this work.

The emergence of two-dimensional (2D) materials has created new opportunities for engineering electronic, optical, and structural properties at the atomic scale. Among these materials, organic–inorganic hybrid perovskites (HOIPs) have attracted significant attention due to their unique combination of inorganic framework functionality and organic molecular tunability.

In contrast to their three-dimensional counterparts, 2D HOIPs can intrinsically exhibit enhanced quantum confinement, reduced dielectric screening, and highly tunable excitonic behavior, enabling superior performance in a range of optoelectronic applications. Although a lot of reported 2D HOIPs rely on the 2D confinement by organic-inorganic layer distinction, they are physically “bulk” in nature with multiple layers stacked. In this work, we explore the potential of isolated ultrathin 2D HOIPs, even to the monolayer regime achieved directly during the synthesis [1], as substitutes for conventional “bulk” 2D HOIPs.

We present a Copper-Manganese-Chloride (Cu-Mn-Cl) double perovskite where Phenethylamine (PEA) is the spacer, with varying ratio of Cu: Mn (PEA₂Cu_xMn_1-xCl₄). Starting from the pure Mn HOIP(x=0), the x is varied till pure Cu (x=1) HOIP is realized. The varying ratio varies the number of inorganic octahedra in the 2D layer with Cu (and Mn) in the center and thereby the band structure and optical properties. Investigations using temperature-dependent Raman spectroscopy, photoluminescence (PL), and time-resolved photoluminescence (TRPL) provided valuable insights into the optical properties of the samples and their evolution with varying Cu:Mn ratios [2]. Interestingly, low-temperature Raman measurements revealed well-resolved vibrational spectra with distinct signatures of the Cu:Mn ratio, whereas the low-temperature PL emission energy exhibited only weak dependence on the composition. However, as the temperature increased to room temperature, the evolution of the emission energy followed distinct trends, suggesting changes in the underlying emission mechanism. These observations were further corroborated by the TRPL measurements. In addition, it shows potential for further tuning by varying the number of layers, applying pressure, introducing an electric field etc. Hence, we will present an in-depth study towards its potential as a dynamically tunable ultrathin system.

Our results demonstrate that ultrathin 2D HOIPs offer significant advantages over bulk materials, by compatibility with flexible substrates, and opportunities for device miniaturization. These characteristics position them as promising candidates for next-generation photovoltaics, LEDs, photodetectors, and integrated nanoelectronics. The perspective presented contributes to the broader understanding of structure to property relationships in low dimensional hybrid materials and supports their development as versatile systems for future sustainable technologies.

[1] Kalyanasundaram et al (Manuscript under preparation)

[2] Gopalakrishnan et al (Manuscript under preparation)