Colloidal nanocrystals of lead halide perovskites (LHP NCs) have a long history. They were initially conceived in Pb-doped CsBr crystals and films 30 years ago. About a decade ago, they were produced as colloids in apolar solvents. Importantly, the fast structural dynamics, essentially entropically stabilized lattice, do not seem to be detrimental to exhibiting the textbook optical quality of a semiconductor. To date, they have become the most widely researched quantum dot material. They have challenged the ethos of this field in nearly every aspect. For instance, they are bright emitters without ever being coated with an epitaxial shell. They are the first colloidal quantum dot (QD) material that exhibited excitonic coherence on a timescale comparable to their radiative rates. They are the first colloidal QD material demonstrating collective and hence accelerated radiative decay of tens of picoseconds – superfluorescence - in the periodic ensembles (known as NC superlattices). LHP NCs exhibit a giant oscillator strength effect, allowing extremely fast emission rates (lifetimes down to 60ps) at large NC sizes, and at a single particle level in the single-photon emission regime – single-photon superradiance. The exciton fine structure of LHP NCs is readily engineerable through the shape anisotropy. We also find that, by simple proximity to highly chiral plasmonic nanostructures, the otherwise linearly polarized emission becomes fully chiral; the transcribed chirality is also manifested in their absorption (dichroism), making them the first fully chiral single-photon emitters. We will review the diverse opportunities that LHP NCs increasingly offer as classical and quantum light sources. The presentation will encompass the work of my interdisciplinary team and diverse international collaborators, whose names will be appropriately mentioned in the presentation and footnotes.

[1] V. Morad, A. Stelmakh, M. Svyrydenko, L.G. Feld, S.C. Boehme, M. Aebli, J. Affolter, C.J. Kaul, N.J. Schrenker, S. Bals, Y. Sahin, D.N. Dirin, I. Cherniukh, G. Raino, A. Baumketner, M.V. Kovalenko Nature, 2024, 626, 542–548

[2] C. Zhu, S.C. Boehme, L.G. Feld, A. Moskalenko, D.N. Dirin, R.F. Mahrt, T. Stöferle, M.I. Bodnarchuk, A.L. Efros, P.C. Sercel, M.V. Kovalenko, G. Rainò. Nature, 2024, 626, 535–541

[3] I. Cherniukh, G. Rainò, T. Stöferle, M. Burian, A. Travesset, D. Naumenko, H. Amenitsch, R. Erni, R.F. Mahrt, M.I. Bodnarchuk & M.V. Kovalenko.  Nature 2021, 593, 535–542

[5] T.Kim, R. M. Kim, J. H. Han, M. Svyrydenko, M. Bodnarchuk, G.Raino, K. T. Nam, M. V. Kovalenko et  al. submitted

High environmental stability and surprisingly high efficiency of solar cells based on 2D perovskites have renewed interest in these materials. These natural quantum wells consist of planes of metal-halide octahedra, separated by organic spacers. The unique synergy of soft lattice and opto-electronic properties are often invoked to explain superior characteristic of perovskites materials in applications. At the same time such unique synergy creates fascinating playground for exciton physics which challenges our understanding of this elementary excitation. I will demonstrate that even after decade of intense investigation the notation” unique” so often used in case of perovskites deserves serious scrutiny.

I will explore the excitonic landscape in 2D semiconductors. First, I will highlight the controversy surrounding the unexpectedly high light emission efficiency of this material and show that it can be explained by the interplay between phonons and the exciton fine structure. I will demonstrate that the soft lattice can suppress relaxation of excitons to dark state making 2D perovskites great light emitters. Moreover, I will discuss the exciton fine structure measured for multiple 2D layered perovskites characterized by a different lattice distortions imposed by organic spacers. Surprisingly, it has a non-trivial impact on the exchange interaction allowing the energy spacing between dark and bright excitons to be tuned. This tuning knob, not available in classic semiconductors, makes 2D perovskites a unique material system where the exciton manifold can be controlled via the steric effect. Finally, I will demonstrate alternative approach to the injection of spin-polarized carriers in 2D perovskites bu building Van der Waals heterostructures. 

The benefits of vacuum processed perovskite solar cells will be discussed. Co-sublimation of perovskite precursors leads to high efficiency solar cells albeit with somewhat low open circuit voltages. Using a home build setup to probe the photoluminescence of the perovskite films while they are grown on a rotating substrate in a high vacuum chamber allows us to see surprising evolution of the photoluminescence.

We will show the effect of seed-layers on the evolution of the photoluminescence of the perovskite film as well as the addition of passivating agents. 

This will be shown for perovskites with different compositions and film thicknesses. The best perovskite are used to prepare thin film solar cells reaching 23 % power conversion efficiency.

This work is done by the postdocs Vladimir Held and Yunseong Chin 

Metal halide perovskite solar cells have recently achieved an efficiency breakthrough of 26.7%, surpassing that of monocrystalline silicon photovoltaics. This remarkable result was only possible due to precise control and engineering of morphology, interfaces, defects, the use of multiple cations at the perovskite A-site, such as Cs⁺, MA⁺ (methylammonium), FA⁺ (formamidinium), and the incorporation of additives to enhance crystallization, among other strategies.

The dimensionality of perovskite materials can be readily tuned by selecting appropriate A-site cations and adjusting the stoichiometry, enabling the formation of structures ranging from zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), to three-dimensional (3D). This tunability broadens the applicability of perovskites in a wide range of optoelectronic devices.

In this presentation, we summarize key results obtained through in situ experiments that probe the formation dynamics and crystallization of 2D perovskite materials incorporating various organic cations, along with their stability profiles. The dynamics of structure and interface formation in solution or solid state, and their thermal stability and aggregation behavior were investigated via in situ techniques. These included time-resolved grazing incidence wide-angle X-ray scattering (GIWAXS), high-resolution XRD, in situ TEM, and in situ photoluminescence (PL) spectroscopy, conducted at the Brazilian Synchrotron Light Laboratory, Lawrence Berkeley National Laboratory, and other research facilities.

Additionally, we present our most recent findings on the spatial mapping of 2D nanostructures using cathodoluminescence coupled with scanning electron microscopy (SEM), as well as the role of organic cation chain length in crystallization dynamics, defect passivation, and carrier transport. These properties were probed using advanced characterization techniques such as nanoscale X-ray diffraction and AFM-based infrared spectroscopy.

Two-dimensional lead halide perovskites (2DHPs) with a general structural formula A2A'(n-1)PbnX3n+1 (A = bulky spacer, A'= Small monovalent cation, X = I, Br, Cl) have emerged as a promising class of materials for next-generation optoelectronic devices. Suppressed photoinduced halide-ion segregation (PHS) was reported in 2D mixed halide perovskites (2DMHPs) due to suppressed ion migration, and strong excitonic effects resulting from this unique layered structure. However, 2DMHPs remain a major argument in both theoretical estimation and experimental observations. Notably, all previous reports of PHS studies in MH2DHPs are based on polycrystalline thin films[1, 2], which could be an important reason for inconsistent results due to the big variations in defect density. There is a lack of observation for the formation of iodide-rich domains under photoexcitation using photoluminescence tools, which is a signature feature in 3D PHS studies [3, 4].

Here, we demonstrate that BA2PbBrxI4-x (BA = butylammonium, x=1, 2, 3) 2DMHP single crystals have highly ordered halide stacking preference with bromide and iodide exclusively occupying B and T-sites respectively in thermal equilibrium. With hyperspectral PL and absorption imaging methods, we directly reveal the formation of a new phase upon exposure to above-bandgap excitations, which can be attributed to a photo-induced halide switching process, whereas T-site iodide switches its position with B-site bromide with local PbX64- octahedral. No PHS induced I-rich domains are observed in both PL and absorption mappings in those single-crystalline 2DMHPs, which is due to such photoisomerization does not involve any multiple unit cell mass transfer. A schematic and photoluminescence imaging of this photoisomerization of 2DMHP are presented in Figure 1. We conducted temperature dependent single crystal X-ray diffraction (SCXRD) and powder XRD (PXRD) with in situ photoexcitation measurements to reveal change in lattice constant during photoisomerization. We find that the halide switching results in an expansion of in-plane lattice constant while a reduction of interlayer distance. The new photo-switched structure and its dark form are chemically inequivalent structural isomers which exhibit distinctively different physical properties. The ability to alter local halide distributions with light while not causing phase inhomogeneity could be a key to enable a new pathway for in quantum technologies, optical switching and memory applications.

Phase change materials (PCMs) have a sizeable latent heat associated with a first-order phase transition (solid-solid, solid-liquid), which renders them excellent candidates for thermal management applications, such as thermal energy storage and solid-state heat pumps and air- conditioners.[1] Among different PCMs, those undergoing solid-solid transitions are the most desirable due to the absence of fluid leakages.[2] To be implemented in most thermal management technologies, solid-solid PCMs should display an elusive combination of i) high thermal conductivity (>10¹ W m^-1 K^-1), which allows a rapid thermal exchange, and ii) high latent heat (> 10² J g^-1), which minimizes the amount used in the devices.[3] Additionally, the phase change should occur at the desired temperature. Solid-solid PCMs displaying such a combination of properties have not been realized so far.

Two-dimensional (2D) halide perovskites are receiving renewed attention as an emerging class of solid-solid PCMs. Because the two sublattices are individually tunable, it has been proposed that these materials could exhibit a combination of high latent heat and thermal conductivity. In this talk, we will discuss our advancements in achieving Cu-based perovskites with the formula (C_nH_2n+1NH₃)₂CuX₄ (X = Cl^–, Br^–), with even n = 16–22 that show a remarkable latent heat (67 J g^-1) at relatively high temperatures (80–120 C). Additionally, we will show our recent efforts to increase the electrical and thermal conductivity of halide perovskite via internal charge-transfer doping.

Perovskite solar cells (PSCs) offer high power conversion efficiency and low production costs [1], but their commercial potential is still constrained by insufficient long-term stability. The incorporation of two-dimensional (2D) cationic layers onto three-dimensional (3D) perovskite frameworks has emerged as a promising strategy to mitigate instability in perovskite solar cells (PSCs) [2]. While this approach enhances moisture resistance and suppresses interfacial recombination, the relationship between spacer chemistry, concentration, and phase behavior remains not fully understood [3]. In this work, we present a systematic comparison of two alkylammonium spacers—butylammonium iodide (BAI) and butyl-1,4-diammonium diiodide (BDAI₂)—which form Ruddlesden–Popper (RP) and Dion–Jacobson (DJ) 2D phases, respectively, onto CH₃NH₃PbI₃ 3D perovskite.

By varying spacer concentration under ambient conditions, we accessed distinct structural regimes. At high concentrations ([BAI] = 50 mmol L⁻¹ and [BDAI₂] = 5 mmol L⁻¹), it is observed that BAI facilitated the evolution from n = 1 to n = 2 RP phases, suggesting dynamic structural reorganization, while using BDAI2 DJ structures showed rapid degradation without observable phase progression. At lower concentrations, both spacers acted primarily as passivation agents, but only BAI significantly enhanced environmental stability without disrupting charge transport.

Characterizations via UV–vis, XRD, SEM, c-AFM, PL, and EIS demonstrated a strong correlation between dimensionality, interfacial conductivity, and long-term device performance. Notably, DJ-phase samples exhibited promising initial optoelectronic properties but suffered from severe degradation due to humidity.

Our findings highlight critical trade-offs between rigidity and ionic migration in hybrid perovskite systems. This study offers insights into interface engineering and dimensional control, paving the way for the design of robust, high-efficiency perovskite solar cells (PSCs) with improved operational lifetimes.

Photocatalytic hydrogen production using 2D Ruddlesden-Popper tin iodide perovskites emerges as a promising approach for sustainable energy conversion. However, a key challenge associated with these materials is their susceptibility to degradation due to the oxidation of tin and iodide. In this study, microcrystals of 4-fluorophenethylammonium tin iodide perovskite were synthesized in a mixture of hydroiodic acid (HI) and water, demonstrating long-term photostability and robust photocatalytic hydrogen production via HI splitting. Intermittent light irradiation was found to enhance hydrogen evolution by promoting more efficient charge separation and reducing the accumulation of trapped charge carriers that would otherwise lead to recombination. Notably, reused samples exhibited improved photocatalytic performance over time. Furthermore, degraded samples could be easily regenerated through a simple chemical treatment, restoring their hydrogen production capability. In addition, the use of Sn-based perovskite solar cells as photocathodes in aquous solution for hydrogen generation will be also reported, highlighting the interest of Pb-free perovskites for photocatlytic applications.

Polycrystalline perovskite thin films are consistently associated with high performance optoelectronic devices. The main challenges currently under the spotlight lie with the stability and reliability of these devices, requiring a better understanding of the thin film surfaces and interfaces. However, very little is known about them. Given the soft nature of these materials, there is a struggle to directly assign the origin of the electronic features and to correlate them to the chemical nature of the surfaces. Here, by exploiting a multimodal approach, we measure for the first time, at sub-micron scales, the diffraction patterns of the thin film grain surface, which provide the material fingerprint, and its related electronic properties. Armed with this knowledge, we are eventually able to unambiguously assign the origin of the photoemission spectra for a wide library of metal halide perovskites. In lead halide perovskites, we identified their photoemission spectra to be the convolution of three different spectra, that of pristine perovskite, lead halide inclusions, and metallic lead. In particular, metallic lead is identified to be the origin of the often-reported mid-gap photoemissive states, generally assigned to deep electronic states within the halide perovskite semiconductor band gap. The chemical composition of the metal halides, the photo-degradation of the perovskites under visible light, and the quality of the precursors heavily define the presence of such states. Overall, the achievement of such understanding elucidates the origin of carrier loss, photo-degradation, and sample reproducibility, aiding in the targeted improvement of the stability and reliability of these metal halide perovskites and their associated optoelectronic devices.

Mixed-metal halide perovskites, with the general formula ABX₃ - where A is a cation (methylammonium, formamidinium, Cs), B is a metal (Sn, Pb) and X is an anion (I, Br, Cl) – are integral to all-perovskite tandem solar cells. Although groundbreaking research has pushed efficiencies to 30.1% so far, this is still just a fraction of fundamental limits (>40%). Device parameters such as the open-circuit voltage (V_OC) and fill-factor (FF) trace back directly back to the photoluminescence quantum yield.[1] This picture is complicated in Sn-containing perovskites given that the presence of Sn⁴⁺ impurities in the precursor solution leads to both the introduction of background hole dopants (serving to increase the PLQY) and non-radiative recombination centers (serving to decrease the PLQY). [3] Nevertheless, these fundamental relationships make PL microscopy a powerful technique for understanding mixed-metal halide perovskites - particularly when correlated with other compositional, structural, or topographical imaging modes.

To probe the compositional origins of photophysical heterogeneity in halide perovskites, we correlate steady-state & time-resolved, wide-field PL microscopy with nanoprobe X-ray fluorescence (n-XRF). We find that what appear to be micron sized grains in atomic force microscopy images are in fact highly heterogeneous in Sn/Pb composition. From PL microscopy, we observe that Sn-rich regions are red-shifted and higher intensity in steady-state PL images. Time-resolved photoluminescence microscopy shows that while the initial photocarrier distribution is homogeneous, these carriers funnel into more localized regions. Finally, the combination of time-resolved and steady-state photoluminescence microscopy allows us to obtain the spatial dopant density on the microscale.[3] Together, these results suggest that Sn-rich regions have a higher dopant density and play a role as recombination centres. We expect these results to be informative for the design of next-generation dopant management strategies in mixed-metal halide perovskites to supress, or even harness, self-doping.

[1] Giles E. Eperon, Maximilian T. Hörantner & Henry J. Snaith, Metal halide perovskite tandem and multiple-junction photovoltaics, Nature Reviews Chemistry, 2017, 1, 0095

[2] Kunal Datta, Junke Wang, Dong Zhang, Valerio Zardetto, Willemijn H. M. Remmerswaal, Christ H. L. Weijtens, Martijn M. Wienk, René A. J. Janssen, Monolithic All‐Perovskite Tandem Solar Cells with Minimized Optical and Energetic Losses, Advanced Materials, 2021, 34 (11), 2110053

[3] Robert J. E. Westbrook, Margherita Taddei, Rajiv Giridharagopal, Meihuizi Jiang, Shaun M. Gallagher, Kathryn N. Guye, Aaron I. Warga, Saif A. Haque & David S. Ginger, Local Background Hole Density Drives Non-Radiative Recombination in Tin Halide Perovskites, ACS Energy Lett., 2024, 9 (2), 732-739

Mobile ions are a key contributor to the degradation and instability of metal halide perovskite solar cells. Accurately quantifying their properties—such as density, mobility, and activation energy—is essential for understanding and mitigating their impact on device performance. While electrical techniques like impedance spectroscopy, fast current-voltage scans, and transient measurements are commonly used for this purpose, their interpretation often relies on assumptions about the device, and they always measure the whole stack at once. As a result, reported values for ion-related parameters vary widely across the literature.

In this talk, I will highlight the limitations of relying on individual electrical techniques and demonstrate how a combined approach—integrating multiple electrical measurements with drift-diffusion simulations—provides a more reliable and comprehensive picture of mobile ion behavior.[1] I will show how this approach can restrict the parameter space enough to quantify ionic properties. Additionally, I will discuss methods that combine electrical and optical perturbations to understand ion migration.

Vapor-phase deposition techniques are widely used in the semiconductor industry. In the case of metal halide perovskites (MHPs), vapor deposition is also being explored as a solvent-free method, avoiding the use of toxic solvents. However, the complex compositions of MHPs, often involving precursors with vastly different volatilities, require strategies where each precursor is evaporated independently or alternative approaches that enable out-of-equilibrium vaporization of all precursors simultaneously. Among physical vapor deposition (PVD) methods, pulsed laser deposition (PLD) stands out as a promising yet underexplored technique for MHPs. PLD utilizes laser energy to eject material from a target through both thermal and non-thermal processes. This enables high versatility in target composition, allowing the deposition of complex thin films from a single-source target. In this presentation, we highlight recent advances in the PLD of MHPs. We discuss methods enabling compositional flexibility, ranging from inorganic to hybrid compounds, as well as optimization strategies for polymorph control. These include the transition from polycrystalline to epitaxial monocrystalline layers, the development of 2D structures, and the fabrication of porous scaffolds for hybrid vapor-vapor or vapor-solution growth. We also address the challenges of PLD and present approaches for scaling up this method. Furthermore, we explore how insights gained from PLD can be transferred to more industry-standard techniques such as sputtering deposition.

Ref.

https://doi.org/10.1021/acsenergylett.4c01466

Metal iodide thin films have gained a lot of attention during the recent decade. Although their applications cover a variety of technological fields, majority of the research is motivated by photovoltaics. Halide perovskites are the most studied metal iodides for photovoltaics. Halide perovskite solar cells are made from abundant and low-cost materials, yet they show high solar conversion efficiencies. Unfortunately, some of the halide perovskites show poor chemical and thermal stability. In addition, the presence of lead in the best-performing materials causes concern. These issues have recently drawn attention to other types of materials including Ag₂BiI₅ and Cs₃Bi₂I₉, for example. Halide perovskites are expected to find their main application in tandem solar cells with silicon, and the same may be true also for Ag₂BiI₅, Cs₃Bi₂I₉ and related materials.

Large-scale applications of halide perovskite and other metal iodide thin films require scalable and well-controllable deposition methods. The currently used methods are simple and low-cost but are difficult to scale up for industrial mass production of solar cells. Atomic layer deposition (ALD) is well known for its unique controllability and excellent scalability and has therefore a lot to give also in the field of metal iodide films. We have developed, as the first team in the world, ALD processes for various metal iodides. We started by developing processes for the binary iodides PbI₂ [1], CsI [2], and SnI₂ [3]. All these processes use metal silylamides as the metal precursors and SnI₄ as the iodine precursor. The binary processes can be combined to make more complex materials: so far we have made the inorganic halide perovskites CsPbI₃ [2] and CsSnI₃ [3] by combining CsI with PbI₂ and SnI₂, respectively. CH₃NH₃PbI₃ can be prepared as well by exposing PbI₂ to CH₃NH₃I vapor [1]. We recently designed a new iodine source that produces anhydrous HI vapor on-site and overcomes thus the limitations of SnI₄ such as high cost and tin contamination in the deposited films. We have demonstrated the feasibility of the source by depositing CsI.

Our most recent efforts are directed towards Ag₂BiI₅, Cs₃Bi₂I₉ and related materials. As the first steps, we have developed ALD processes for the silver halides AgCl, AgBr and AgI. We are currently working on an ALD process for bismuth iodide BiI₃. Also, the first experiments aiming to combine AgI and BiI₃ to ternary iodides are underway.

Halide perovskite (HaP)-silicon tandem solar cells received considerable interest from the scientific community, which resulted in a substantial increase in power conversion efficiency (PCE) up to 34.9 %,[1] considerably higher than the individual single-junction cells. The increase in efficiency has led to an enhanced interest in the industrialization of this technology. However, the most efficient devices are fabricated using deposition techniques that are not compatible with large scale production, making the technology transfer more challenging. Co-evaporation is an up-scalable technique, which offers good thickness control and conformal coverage of micro-size pyramidal textures on silicon bottom cells,[2] making it particularly suitable for HaP-silicon tandem devices. It was shown that the growth of co-evaporated HaP films can be modified using seed layers [3,4] or by rinsing the self-assembled monolayer (SAM) hole-extraction layer (HTL) interface with ethanol.[5,6] Still, it remains unclear how these modifications influence the early HaP film formation. Previous studies have shown that incomplete SAM coverage can lead to variations in HaP film quality and, ultimately, to degraded device performance.[7] In the present study, we use synchrotron-based surface-sensitive techniques, in particular x-ray photoemission electron microscopy (XPEEM) and infrared scattering type scanning near-field optical microscopy (IR s-SNOM). Both XPEEM and IR s-SNOM techniques enable experiments with a lateral resolution of 20 nm, allowing us to analyze the influence of SAM inhomogeneities and CsCl seed layer on the composition of 5 and 20 nm thick co-evaporated FA_0.8Cs_0.2PbI_2.7Br_0.3 (FA⁺ = formamidinium cation, C₆H₅N₂⁺) HaP films. The measurements are complemented with cathodoluminescence, photoluminescence, and x-ray diffraction analyses of both thin (5, 20 nm) and thick (100, 500 nm) films, as well as the characterization of the buried interface by film delamination. We use a model system to study the influence of inhomogeneities in [2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz) SAM coverage where we identified regions with monolayer MeO-2PACz, as well as regions with thicker MeO-2PACz layer containing unbound molecules. The incorporation of FA⁺ is impeded in regions with a monolayer MeO-2PACz, unless a CsCl seed layer is employed. We show how inhomogeneities in MeO-2PACz typically occur when spin-coating SAM on textured silicon bottom cells, and their influence on HaP formation can be mitigated using CsCl seed layer, allowing for ~30 % (certified) efficient HaP-silicon tandem solar cells, the highest value for fully vacuum-processed HaP films to the best of our knowledge. Our study provides a deep understanding of co-evaporated HaP film formation at the nanoscale, allowing for future evidence-based buried interface optimization.

The black-phase cesium lead iodide (CsPbI₃) perovskite is a highly promising material for next-generation optoelectronic devices due to its optimal bandgap, high carrier mobility, and excellent light absorption properties. However, its practical application remains significantly limited by its structural instability under ambient conditions, where it tends to rapidly convert into the non-perovskite yellow δ-phase. Various strategies have been explored to stabilize the black phase, including A-site and X-site doping, surface passivation, laser writing, and strain engineering. While these approaches have shown partial success, they also present significant limitations. A-site doping often involves volatile organic cations, which can introduce chemical instability [1]. X-site halide substitution (e.g., Br^-, Cl^-) tends to alter the bandgap [2], which is undesirable for certain optoelectronic applications. Additionally, methods such as surface passivation [3] and laser-induced [4] stabilization can create by-products or defects that impair charge transport, limiting device performance.

Among the emerging approaches, strain engineering has demonstrated significant potential for stabilizing the black phase. Interface strain, typically induced by annealing thin films on substrates with differing thermal expansion coefficients, has been shown to suppress the phase transition to the yellow phase [5]. However, this effect is confined to the interface region and effective only in ultrathin films (<100 nm), which are insufficient for high-performance optoelectronic devices like solar cells and photodetectors that require thicker active layers (300–500 nm).

In this work, we propose a novel approach to induce bulk-localized nanoscale strain through partial B-site substitution in CsPbI₃. By incorporating a small amount of dopant cations at the Pb²⁺ site, we introduce localized lattice distortions throughout the bulk of the film. This nanoscale strain reduces the spontaneous orthorhombic distortion of the crystal lattice and increases the Pb–I–Pb bond angle, both of which are critical factors in stabilizing the black perovskite phase. Unlike interface-limited strain methods, our strategy enables stabilization throughout the entire film thickness, thereby overcoming the primary limitation of previous strain engineering techniques.

We demonstrate that the optimized B-site substitution by Sn²⁺, Bi³⁺, and Cd²⁺ (up to 20%) does not significantly alter the optical bandgap of CsPbI₃, preserving its desirable optoelectronic properties. Furthermore, this method enhances phase stability under ambient conditions without compromising film quality or charge transport. Devices fabricated using this strain-stabilized CsPbI₃ exhibit superior performance, as evidenced by enhanced photodetector metrics, including higher responsivity, faster response times, and improved operational stability.

This study establishes a alternative route to achieving long-term black-phase stability in thicker perovskite films by leveraging atomic-level strain engineering via targeted B-site doping. Our findings provide a pathway toward the practical deployment of CsPbI₃-based optoelectronic devices with improved reliability and performance.

Quantum cutting represents a transformative strategy to mitigate thermalization losses that typically occur when high-energy photons are absorbed by semiconductors.[1,2] Recent advances have extended this concept from rare-earth doped crystals to semiconductor–rare-earth hybrid systems, particularly those utilizing halide perovskite absorbers,[2] thereby exploiting their exceptional optoelectronic properties.

In this study, we focus on Ytterbium (Yb)-doped CsPb(Cl_1-xBr_x)₃, a metal halide perovskite that absorbs in the visible and exhibits intense near-infrared (NIR) photoluminescence (PL) — a clear signature of quantum cutting. We first optimized the composition of Yb-doped CsPb(Cl_1-xBr_x)₃ by tuning both the Br content (x = 0.2 - 0.65) and [Yb:Cs] ratio (0.4 - 1.2). We observe a gradual blue shift of the absorption peak as the Br content decreases, and a red shift as the Yb concentration increases. Upon absorption of photons with wavelength < 500 nm, the doped perovskite converts the absorbed energy into NIR photons. The NIR PL signal appears at ~985 nm (1.26 eV), characteristic of Yb³⁺ ²F₅/₂ → ²F₇/₂ f-f transitions, confirming the occurrence of quantum cutting. The highest PL intensity is observed for (Cl_0.6Br_0.4) and a [Yb:Cs] ratio of 0.6. The NIR emission energy is slightly higher than the energy gap of Sn–Pb based perovskites (MA(Pb_1-ySn_y)I₃), which exhibit an optical bandgap around 1.21 eV when y = 0.65 - 0.85. This spectral alignment would be critical for enabling efficient energy transfer between the quantum cutting layer and the absorber layer.

Both doped and undoped perovskite films are synthesized following a double-step deposition from solution.[2] We use a suite of advanced spectroscopic techniques, including Rutherford backscattering spectrometry (RBS), X-ray photoelectron spectroscopy (XPS), PL, ultraviolet photoelectron and inverse photoemission spectroscopies (UPS/IPES), to systematically investigate the elemental composition and electronic structure of Yb-doped CsPb(Cl_0.6Br_0.4)₃. RBS and XPS depth profiling provides insights into the composition and elemental distribution of the films. Both techniques reveal an Yb enrichment near the surface of the film, corroborated by PL measurements, which show a stronger NIR 985 nm emission when the film is illuminated from the surface side than from the bottom side. We discuss the potential integration of Yb-doped CsPb(Cl_0.6Br_0.4)₃ with Sn–Pb based perovskite absorbers, offering a pathway to surpass conventional efficiency limits while providing a cost-effective strategy for enhanced energy conversion.

Lead (Pb) based perovskites with the general formula ABX₃ have attracted significant attention in providing desirable optoelectronic devices, such as solar cells, light-emitting diodes (LEDs), and photocatalysis. However, the high toxicity and low stability of Pb are a big concern for health and the environment, as well as commercialization, leading to extensive research to replace lead with less toxic elements. Recent works have been exhibiting tin (Sn²⁺) as a promising alternative due to comparable ionic radius and in a group electronic configuration to Pb⁺. Nevertheless, Sn²⁺ is prone to oxidation, easily transforming into Sn⁴⁺ in ambient conditions, which creates a destabilized perovskite structure and significantly reduces its functional performance. To overcome this issue, this work explores an alternative strategy by inserting the Sn²⁺ octahedron into a layered double perovskite (LDP) with the formula Cs₄M(II)M(III)₂Cl₁₂. In this LDP structure, Sn²⁺ is placed on the M(II) site, while the M(III) site is occupied by different trivalent metal cations such as In³⁺ or Sb³⁺. The two M(III) octahedrons in a unit cell serve as top and bottom protective layers, effectively suppressing the oxidation sensitivity of Sn²⁺ by minimizing the contact with any degradation factors, such as oxygen and water molecules leads to the formation of Sn⁴⁺.

Our x-ray diffraction (XRD) and elemental analysis confirm the successful formation of LDP Cs₄M(II)M(III)₂Cl₁₂ nanocrystals (NCs) via the modified hot injection method, showing well-defined and stable crystal structures. The as-synthesized perovskite NCs demonstrate a morphology of hexagonal nanoplatelets with diverse size distributions based on the selected M (III) cations. Specifically, In-based nanoplatelets show an average size of 41.4 nm, while Sb cases exhibit average sizes of 58.8 nm. 

Compared to conventional lead-based perovskite, these Sn-based LDP nanocrystals reveal significant enhancements in the structural stability. The long-term stability test upon the tracking of XRD patterns reveals that these NCs could retain their original structural integrity for more than 300 days, a remarkable improvement over Pb or other Sn(II)-based perovskite, which latter case normally degrades even within hours. In addition, the optical stability of the LDP NCs, as regularly measured by UV-visible absorption spectroscopy, maintained the original feature for over 40 days, indicating a hint of their potential for stable optoelectronic applications.

Besides, this nanomaterial demonstrates tunable electronic properties by substituting distinct trivalent metal cations at the M(III) site, which means the bandgap could be adjusted. For instance, the presence of In³⁺ shows a relatively wide bandgap at 3.54 eV, indicating a good fit for a wide scale of bandgap semiconductors. Interestingly, once Sb³⁺ is placed at the M(III) site, the bandgap could be narrowed to 1.89 eV. This characteristic of tunable bandgap is considered a good platform for customizing the material’s properties for corresponding needs. We further evaluated the photoelectrochemical performance through chronoamperometry (I-t curves) under 1 sun illumination. The samples exhibited rapid photo responses upon light switching on/off, with steady-state photocurrent densities of 35.31 µA cm⁻² and 49.66 µA cm⁻², respectively.

In summary, this work demonstrates a novel Sn(II)-based LDP NCs with different trivalent metal ions at the M(III) site, showing significantly enhanced structural stability and tunable optical properties, all which make them highly versatile for a wide range of application in future, such as solar cells, light-emitting devices, and photocatalysis.

The ideality factor n_id quantifies the deviation of the current-voltage characteristics of a solar cell from the ideal diode equation, excluding the influence of series and shunt resistances. It can be influenced by two effects: the scaling of the electron and hole density with injection level and the scaling of the dominant recombination mechanism with the electron and hole density. In an extrinsically-doped semiconductor in low-level injection (e.g. n << p = N_A), the majority carrier density is independent of the injection level which leads to an ideality factor of n_id = 1, independent of the dominant recombination mechanism. In the case of an intrinsic semiconductor (i.e. high-level injection, n = p), the ideality factor serves as tool to distinguish between recombination mechanisms: n_id = 1 is typically associated with radiative recombination or recombination via shallow traps; n_id = 2 with recombination through deep trap states; n_id = 2/3 with Auger recombination; and interface recombination can span a range of values depending on injection conditions and transport asymmetries.

Lead halide perovskites neither perfectly fall in the doped nor the intrinsic category. They have very low doping densities in the dark but show photodoping effects under illumination due to trapping of charge carriers. This situation is uncommon in semiconductor physics and requires innovative ways of analyzing and understanding charge carrier dynamics in general and ideality factors in particular.

Here, we discuss how photodoping in otherwise intrinsic semiconductors affects the ideality factor. At low injection levels, the trap occupation is limited by minority carrier detrapping. The trap behaves as a shallow trap yielding an ideality factor of n_id = 1, just as in the intrinsic case. At higher injection levels however, we find that the trap occupation is now limited by the majority carrier trapping, interestingly leading to an ideality factor of n_id = 1.5. At very high injection levels, the electron and hole density surpass the trap density, leading to the intrinsic case of a deep trap with n_id = 2. We point out that the ideality factor of n_id = 1.5 is a normal outcome of SRH theory and does not require interface recombination. We discuss the consequence of this effect on the photoluminescence quantum yield in steady state and on the transient photoluminescence behavior and compare the theory to measurements on perovskite thin films.

Solution-processable semiconductors like halide perovskites and certain molecules are promising for next-generation spin-optoelectronic applications [1]. Yet, we don’t fully understand what governs spin and light polarization in these materials, and even less how these are affected by chirality [2].

In this talk, I will give an overview of our recent efforts to understand the spin-optoelectronic performance of these materials through time-, space- and polarization-resolved spectroscopy and microscopy.

For investigating halide perovskite films, we pushed broadband circular dichroism to diffraction-limited spatial and 15 fs time resolution for creating a spin cinematography technique to witness the ultrafast formation of spin domains due to local symmetry breaking and spin-momentum locking [3].

I will then briefly explain the fundamentals and artefacts involved in measuring circularly polarized luminescence reliably and introduce an open-access methodology and code to do so [4]. Finally, I will show our most recent development of a transient sensitive broadband full-Stokes spectroscopy with unprecedented time- and polarization resolution to track the emergence of chiral light emission [5].

[1] Nature Reviews Materials 8, 365 (2023).

[2] Nature Reviews Chemistry 9, 208 (2025).

[3] Nature Materials 22, 977 (2023).

[4] Advanced Materials 35, 2302279 (2023).

[5] Nature, https://doi.org/10.1038/s41586-025-09197-3 (2025).

Halide perovskites are a uniquely versatile class of materials, exhibiting highly tunable photonic, electronic, and spin-dependent properties. Their soft, ionically conductive lattice and broad chemical flexibility allow for precise control over structure and composition, making them promising candidates for applications ranging from solar energy conversion and light-emitting devices to spintronics and quantum information technologies. These materials offer a compelling platform for designing multifunctional, adaptive interfaces, and for integrating advanced physical phenomena into scalable device architectures.

Our group combines first-principles simulations, data-driven modeling, and machine learning to unravel the fundamental structure–property relationships that govern performance, instability, and emergent phenomena in halide perovskites. A major research thrust is the chemistry and dynamics of point defects, which are central to charge recombination, ion migration, and long-term degradation. We investigate defect formation energies, migration barriers, and electrostatic interactions under realistic conditions, and develop mitigation strategies including compositional engineering, strain modulation, and surface passivation.

In parallel, we explore the growing frontier of chiral hybrid perovskites, where the incorporation of chiral organic ligands breaks inversion symmetry and enables spin-selective charge transport via the chiral-induced spin selectivity (CISS) effect. We examine how chirality, spin–orbit coupling, and lattice dynamics together shape chiroptical responses, opening new directions for spin-optoelectronic devices, polarized light detection, and quantum spin filtering.

Together, our work aims to establish a unified, multiscale framework for understanding and engineering halide perovskites as intelligent materials — where light, charge, spin, and lattice degrees of freedom can be co-optimized for next-generation optoelectronic and quantum technologies.

Incorporating chiral ligands into a perovskite structure induces overall chirality, enabling unique properties such as interaction with circularly polarized light [1] and chiral-induced spin selectivity [2]. We investigate the transfer of chirality from chiral organic ligands to the inorganic framework in 2D chiral perovskites with different metal cations, i.e. MBA₂SnI₄ and MBA₂PbI₄. Using structural descriptors, we analyze how the presence of chiral ligands distorts the inorganic layers and how these distortions relate to the overall structural chirality.
By comparing the Sn- and Pb-based perovskites, we find that chirality transfer manifests differently in the two systems [3-4]: it is more pronounced in the asymmetry of Sn–I bonds, while in the Pb structures, it is more evident in the asymmetry of I–Pb-I bond angles. These differences are attributed to the distinct assembly of the chiral cations and the stereochemical differences between Sn and Pb.
Furthermore, we explore the temperature dependence of chirality using molecular dynamics simulations with machine-learned force fields. Our results show that structural chirality decreases with increasing temperature, negating the low-temperature structural differences. This is consistent with previous findings that attribute this loss to the reorientation of the ammonium group that links the ligand to the framework [3].

Chiral organic–inorganic hybrid metal halides are emerging as a promising class of materials for spin-controlled optical and optoelectronic applications. Their lattice chirality can be tuned via the incorporation of enantiopure organic cations.^1-3 However, materials crystallizing in enantiomorphic space groups, where the R- and S-configured structures are non-superimposable mirror images, remain rare. In my talk, I will present novel chiral zero-dimensional (0D) (R-/S-MBA)₂SnBr₆ (MBA = methylbenzylammonium) crystals with enantiomorphic helical space groups P6₅ (M-helix) and P6₁ (P-helix). The helicity of the inorganic framework arises from 60° helical rotational alignment along the six-fold screw axis, stabilized by strong N–H···Br and C–H···π interactions. This cooperative assembly induces long-range chirality in the lattice, supported by DFT calculations showing strong electronic coupling between MBA ligands and SnBr₆²⁻ units. The chiral crystals and thin films exhibit mirrored circular dichroism (CD) spectra with a relatively high g_CD factor of 3.5 × 10⁻². Interestingly, the crystals does not the Cotton effect a rare phenomenon in chiral metal halides. Additionally, the materials demonstrate broadband second harmonic generation (SHG) from 875 to 1200 nm, with a dissymmetry factor (g_CP–SHG) up to 0.44 under 1030 nm excitation. Alloying with Pb induces a structural transition to a non-helical chiral space group (P2₁2₁2₁), highlighting that helicity is intrinsic to specific metal–halide combinations. These results introduce a new family of chiral halides with unique linear and nonlinear optical activity, opening exciting opportunities in spin-optoelectronics and chiroptical photonics.

Keywords:

Chiral metal halides, enantiomorphic space group, helical framework, circular dichroism, second harmonic generation

Chiral two-dimensional (2D) perovskites, derived from hybrid organic-inorganic halide perovskites (HOIPs), offer key advantages for optoelectronics, including defect tolerance, strong light absorption, compositional tunability, and low-cost synthesis. Their 2D structure leads to strong quantum confinement, evident in blue-shifted absorption/emission and elevated exciton binding energy, resulting in excitonic optical and photoluminescence behavior even at room temperature. Incorporating chiral cations distorts the inorganic framework, enabling circular dichroism, circularly polarized photoluminescence, and chiral-induced spin selectivity. These properties are of high interest for optoelectronic and spintronic technologies, including circularly polarized light sources [1], detectors [2], and spin-polarized current control [3]. Since such applications rely on thin films, understanding the impact of grain size on performance is essential.

In this study, we examined the optical properties—absorption, PL, and CD—of (R-3BrPEA)₂PbI₄ thin films based on (1R)-1-(3-bromophenyl)ethanamine [1]. Hypophosphorous acid (HPA) was used as a precursor additive to modulate surface roughness [4], a method previously shown to enhance grain size in MAPbI₃ by adjusting pH, polarity, and surface tension. We demonstrate that varying HPA concentration allows control over surface roughness, film density, and crystalline strain. The results reveal strain as a key determinant of photoluminescence yield, highlighting this approach’s potential for optoelectronic applications.