Metal-halide perovskites have emerged as highly promising materials for next-generation optoelectronic devices including light-emitting diodes (LEDs) and solar cells. However, perovskites inherently suffer from internal defects, which can quench charge carriers through non-radiative recombination, thereby degrading their optoelectronic properties. Moreover, defect migration can lead to issues such as current-voltage hysteresis and luminance overshooting, compromising device efficiency and stability. To overcome these challenges, various defect passivation strategies have been explored, such as the incorporation of organic additives and the application of low-dimensional perovskites.

Here, we introduce a distinct approach involving the 6H polytype of FAPbI₃, which is chemically identical to its cubic counterpart (3C), to enable highly effective defect engineering at the interface with cubic FAPbI₃ while maintaining material homogeneity and crystallinity. The use of hetero-polytypic(3C-6H) perovskite film resulted in a power conversion efficiency (PCE) of 21.92% (certified PCE: 21.44%) for a solar module along with enhanced long-term stability. Furthermore, we highlight the detrimental impact of shallow-level defects, particularly iodide vacancy (V_I⁺), which have conventionally been regarded as benign. We believe that our findings pave the way for the employment of perovskite polytypes in practice to achieve high-performing perovskite optoelectronic devices, while also elucidating the necessity of engineering shallow-level defects.

Mixed-halide perovskite materials are exceptional candidates for multijunction solar cells to surpass the detailed-balance limit. However, perovskite solar cells (PSC) still suffer performance losses due to nonradiative charge recombination, which is induced by electronic defects (traps). A common technique to assess nonradiative losses in semiconductors is photoluminescence (PL) spectroscopy. Using transient PL (tr-PL) measurements, we study the time-resolved recombination decay, providing insight into charge recombination kinetics. In metal-halide perovskites, these transients exhibit extremely long decay times dominated by trapping and de-trapping at shallow defects. [1]

Here, we investigate shallow defects in metal-halide perovskite partial solar cell stacks and examine how surface treatments modify their energy and density using low-temperature transient PL spectroscopy. At lower temperatures, PL decay is dominated by radiative recombination, with longer decay times suggesting reduced nonradiative losses. Fitting the temperature-dependent tr-PL indicates that, upon cooling, shallow traps appear less deep and eventually merge into the valence or conduction band. From these low-temperature transients, we extract activation energies and construct a shallow trap density of states (tDOS), visualizing defect distributions within the perovskite film. We extend this approach to films treated with choline chloride, fullerene C₆₀, and choline chloride combined with C₆₀, showing that these treatments alter the shallow tDOS and energy compared to untreated films, demonstrating that surface treatments influence shallow as well as deep defects.

Our results show that low-temperature tr-PL enables experimental probing of shallow defect densities and energies, providing a better understanding of shallow defects in metal-halide perovskite films and informing passivation strategies and charge transport layer design for higher-efficiency PSCs.

Perovskites have shown a high potential for their implementation in both monolithic and multijunction solar cells. Due to the tunability of their bandgap, as well as current matching capabilities, they are a good match as high bandgap material in combination with crystalline silicon. Despite the promising power conversion efficiencies (PCE) reached for the current state of the art, degradation and defects due to interfaces are still a challenge for potential commercialization. A deep understanding of the defect mechanisms therefore plays a crucial role. A powerful method to probe defect densities and activation energies for holes and electrons separately, is given by using a metal-insulator-semiconductor (MIS) setup for thermally stimulated current (TSC) measurements, where the trap filling is achieved using electrical pumping. In comparison to classical TSC measurements this approach 1. mitigates recombination and 2. doesn't rely on the use of diode-like samples to probe the individual charge carriers, allowing a unipolar evaluation for both electron- and hole-type traps using the same sample, and thus lowering the impact of unwanted interface effects on the signal interpretation [1].

Here, we report the results of charge carrier selective MIS TSC measurements on state-of-the-art triple-cation mixed halide (TCMH) perovskites with a bandgap of 1.68 eV, suitable for inverted (p-i-n) structured perovskite solar cells (PSCs) for potential use in a Si tandem configuration. The MIS TSC measurements were done using TCMH perovskite thin films enabling PSCs with a PCE of up to 19.6% for film thicknesses of one micrometer [2]. The measurements unveil the activation energies of different defects for both, electrons and holes, for bulk and surface states in the pristine perovskite. Furthermore, the effect of degradation is investigated. For this, samples are stored under different conditions to gain insight into their respective influence. The storing conditions for this experiment are ambient, both under natural light illumination and without, under the same type of illumination using protective nitrogen atmosphere and in a glovebox environment. The results for the measurements show the effect of the respective influence by the evolution of defect densities and activation energies.

Since the first demonstration of efficient halide perovskite solar cells, there has been sustained and growing research interest in this class of materials. With facile deposition processes and excellent optoelectronic properties, these materials have found applications not only in photovoltaics, but in a myriad of optoelectronic devices. While research into halide perovskites for light emission and X-ray detection is just beginning to surge, perovskites are most well known for their remarkable PV performance, achieving certified power conversion efficiencies over 26% in single junction devices. Despite their truly impressive device performance, these materials have not yet reached their true potential. One potential hurdle to this is an incomplete understanding of the crystallisation dynamics and interfaces in halide perovskite thin-films, which can result in marked changes to the long-term stability and performance of these devices. In this talk, I will discuss kinetic approaches to fabricating improved stability halide perovskite thin-films and devices of a variety of bandgaps. Importantly, I will discuss how utilising various kinetic pathways enable us to stabilise conventionally thermodynamically unstable phases, opening up new pathways to perovskite-based tandem devices.

The tin halide perovskites are emerging as promising non-toxic photo-absorbers in solar cell devices, with a state-of-the-art power conversion efficiency (PCE) of ~17%.^1,2 Despite these developments, the intrinsic phenomena particularly Tin oxidation (Sn⁺² to Sn⁺⁴) followed by self p-doping, remain an Achille’s heel in the advancement of lead-free perovskite solar cells.³ The theoretical calculations suggest that tin oxidation is considerably futile in bulk, whereas it is energetically favoured at a non-passivated perovskite surface.⁴ We present robust strategies for surface passivation of the tin halide perovskites to gridlock the unwanted oxidation. In the talk, we shall highlight the recent understanding of solution chemistry and defect chemistry of tin perovskite solar cells.

References:

1. He, D. et al. Nat. Nanotechnol. (2025).
2. A.Abate, ACS Energy Lett. 8 (2023) 1896.
3. Hao, F., et al.,  Adv. Energy Mater., 13, (2023)2300696
4. D. Ricciarelli et al., ACS Energy Lett. 5 (2020) 2787.

Tin halide perovskites (THP) are one of the most promising and less toxic alternatives to lead-based perovskites. These materials have gained increasing interest in recent decades due to their exceptional optoelectronic properties, structure tunability and straightforward processing, that when included in solar cells allow the obtainment of performances up to silicon-based ones. Despite the outstanding properties of THPs, one of the main disadvantages of this class of materials is the high concentration of defects associated to Sn (II) instability and intrinsic tendency to oxidise over time. The consequent self p-doping of the perovskite leads to increased free-charge recombination rate and reduced efficiency of solar cells. Engineering the surface characteristics of tin perovskites is one of the key strategies to address both stability and performance enhancement of solar cells, an example is the use of additives, such as tin halides or hydrazine, the latter being a potent reducing agent able to compensate for tin vacancies.

Most of the explored strategies are solvent-based. Herein, we investigated for the first time an innovative use of plasma as a solvent-free, reproducible and scalable approach, ^1,2 to gently modify the perovskite surface and reduce surface defects. A preliminary study focused on nitrogen-based plasma treatment, applied to a DMSO-free FASnI₃ perovskite surface ³. The mild nature of the plasma process enabled subtle surface modifications, effectively suppressing the natural tendency of tin (II) to oxidize, as confirmed by the XPS analysis performed on aged films. Subsequently, we extended this approach to a FASnI_2.7Br0.₃ perovskite, employing a plasma generated from a mixture of N₂ and H₂ gases. Owing to the reducing character of hydrogen-based plasma, we observed a notable enhancement in device performance, accompanied by increased photoluminescence and reduced non-radiative recombination. The reactive hydrogen species generated within the plasma interact with the perovskite surface, mitigating carrier losses associated with self-doping, thereby contributing to improved device efficiency.

These studies establish the basis for a novel application of plasma technology to enhance tin-based perovskite solar cells, offering an approach that is not only effective but also readily scalable for industrial implementation.

Over relatively short period of time perovskite solar cell (PSC) efficiency skyrocketed from 3.8 % to 27 % in terms of power conversion efficiency. Despite these gains in performance stability issues still persist holding the technology back from wide scale commercial application. Recently more and more articles are being published demonstrating increased longevity of devices through perovskite surface defect passivation, however most of the materials used for passivation appear once and then disappear into the scientific void never to be heard from again. Furthermore, application of said passivators varies in deposition conditions, perovskite composition and device architecture without any clear indication why the conditions were chosen as they were. In this work four carbazole based quaternary ammonium salts and their effect on perovskite photophysical and structural properties are studied while also varying deposition conditions for the purpose of perovskite surface passivation are evaluated. In order to better understand the interactions between perovskite surface and materials used for passivation it is necessary to discuss about the discrepancies that arise between photoluminescence, time resolved photoluminescence, x-ray photoelectron spectroscopy and liquid state nuclear magnetic resonance measurements which might lead to misinterpretation on the effectiveness of the material used for passivation.

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. Additionally, I will introduce a complementary optical method that enables the quantification of ion migration in perovskite layers before contact deposition, offering new opportunities for studying ion dynamics in half-stacks and individual films.

Hybrid organic-inorganic perovskites tend to undergo several structural transformations that can be caused by temperature, humidity and light. One of the striking is light-induced halide phase segregation in mixed lead-halide perovskites, i.e., formation of I-rich and Br-rich domains by the illumination with visible light [1]. This effect influences the homogeneity of the chemical composition of the mixed perovskite phase, influence the band gaps and therefore it has strong impact on the perovskite-based photovoltaic devices. Several microscopic mechanisms have been proposed to explain this effect, but the full understanding is yet to be obtained [2].

In this work, we used X-ray photon correlation spectroscopy (XPCS) [3], to perform time-resolved studies of the light-induced phase segregation in (CH₃NH₃)PbBr_1.8I_1.2 [4] and track the formation of pure (CH₃NH₃)PbBrI₃ and (CH₃NH₃)PbI₃. This was done by observing the (001) diffraction peak from the cubic perovskite lattice and, specifically, quantifying the dynamics of coherent speckles originating from domains with different halide concentration.

We observe that the phase segregation is characterized by three distinct time scales corresponding to the rapid formation of small seeds of the I-rich phase, fluctuations of the ion distribution around the quasi-equilibrium state (dynamics), and a directional drift of the ions within the crystal grains (kinetics). We also investigate a series of samples with interstitials and vacancies in the halide sublattice [5] and observe the influence of defects in the halide sublattice on phase separation. We find that samples with interstitial halides exhibit slower phase separation as samples with vacancies in the halide sublattice.

Mixed halide perovskites have emerged as leading contenders for multi-junction solar cells, thanks to their tunable band gaps—controlled by halide composition—and excellent optoelectronic properties. However, their potential for application suffers from phase instability. In compounds like MAPbI_3-xBr_x, exposure to light drives halide segregation leading to the formation of iodine- and bromine-rich domains that ultimately compromise device performance. Despite extensive research, the mechanisms behind this light-induced instability remain poorly understood, as they are shaped by a complex interplay of a multitude of different factors.

In this study, we synthesized a suite of MAPbI_1.5Br_1.5 powders via mechanochemical routes[1] using different synthesis strategies. Additionally, we prepared mixed halide perovskite powders incorporating varying amounts of the ionic liquid BMIMBF₄, which has been shown to act as a passivating agent[2]. We characterized the structural and optical properties of the samples using a range of complementary methods, such as SEM, X-ray diffraction (XRD), time-resolved photoluminescence (TRPL) as well as solid-state NMR spectroscopy. The results showed that the different MAPbI_1.5Br_1.5 perovskite powders exhibit varying crystallite sizes and defect densities but otherwise similar properties.

To investigate phase segregation behavior, we performed in-situ XRD measurements under illumination. We find that the segregation kinetics are primarily governed by the defect density, whereas the extent of segregation, reflecting the thermodynamic constraints of the process, is dictated by crystallite size. In particular, we observed less segregation in powders with small crystallites, and we predict that segregation even completely ceases below a threshold of around 10-15 nm. The recovery behavior of the segregated samples in the dark, was governed by different factors than the segregation process. Specifically, the presence of BMIMBF₄ decreases the segregation rate, yet enhances (re-)mixing rates.

Together, these findings provide more profound experimental insight on the kinetics and thermodynamics of halide segregation and its correlation to material structure, which might offer a pathway toward more stable perovskite materials for photovoltaic applications in the future.

Vapour based deposition techniques are highly interesting for perovskite solar cell fabrication. We investigated and quantified the reactant diffusion in sequentially deposited FAPbI3 perovskite (PVK) layers. Therefore, we created a model to determine the diffusion coefficient of FAI in a sequential deposited PbI2-FAI stack, in which a perovskite film grows during the necessary annealing step. Additionally, not only the 3D FAPbI3 was observed, but also lower dimensional perovskites (LDP) FAxPbI(2+x). The main measurement technique of our model is an in situ XRD set up. With the measured FAI and PVK intensity transients we are able to calculate the underlying diffusion coefficient for an isothermal annealing experiment. We performed this experiment with different annealing temperatures, so that the activation energy was determined. The model simulates FAI concentration gradients inside the PVK layer and calculates the corresponding integrated intensities for FAI and PVK. By comparing the simulated integrated intensities with the measured one, the best fitting diffusion coefficient is determined. Via Arrhenius plot we calculated the activation energy and preexponential factor as 0.83 eV and 2.37 cm2/s, respectively. This model lays the theoretical basis for quantifying multiple PVK systems. For example, the model will allow us to calculate the effect on reactant diffusion by adding caesium (Cs) or chloride (Cl), which is known from literature to increase diffusibility.

Among the various deposition techniques available for perovskite thin-film fabrication, thermal evaporation stands out for its ability to produce uniform and high quality films with precise control over thickness and composition. However, it is known that this method comes with its challenges, such as the decomposition of organic components and limited control over morphology. While most studies on thermal evaporation focus on pure iodide systems, like MAPbI₃ and FAPbI₃, the challenges related to mix-halide perovskites, which are crucial for bandgap tuning and device stability, remain elusive. These challenges are further compounded by the critical role of substrates, which play a critical role in determining the morphology, crystallinity, and optoelectronic properties of the resulting films.

For this purpose, we systematically investigated the influence of various substrates, such as PTAA, NiOx, PEDOT:PSS and the self-assembled monolayer (SAM) MeO-2PACz, on the formation of thermally evaporated FAPbI₁Br₂ perovskite films with thicknesses ranging from 3 nm to 200 nm. Characterization techniques including X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and  scanning electron microscopy (SEM) were utilized to analyze the surface properties and the morphology of the evaporated films. Bulk properties, like crystal structures and optical absorption characteristics of the films, are investigated using X-ray diffraction (XRD) and UV-vis spectroscopy.

Our results reveal distinct substrate-dependent effects on the formation and composition of resulting perovskite thin films. For instance, on PEDOT:PSS  and NiOx we initially only observe FAPbI₃ formation for low coverages, and only after the deposition of ~45 nm the bromide becomes incorporated and the desired FAPbI₁Br₂ forms. This delayed incorporation is attributed to the formation of volatile bromine species, such as HBr or Br₂, during co-evaporation, triggered by the interaction of the precursors with the substrate surface at the interface. In contrast, on PTAA substrates, bromide is incorporated right away, forming the intended FAPbI₁Br₂ composition, which indicates the absence of reactions leading to volatile bromine species. Maybe the most surprising results were obtained for the SAM substrates, which are commonly and successfully employed in solution-processed perovskite devices. Here, no bromide incorporation was observed for any layer thickness up to 200 nm and overall the perovskite formation was hindered. This highlights the significant challenges SAM layers may present in thermal evaporation and that the choice of substrate can influence the film growth not only at the interface but also across device-relevant thicknesses.

In conclusion, this study provides valuable insight into the critical role of the chosen substrate in the thermal evaporation of FAPbI₁Br₂ perovskite thin-film, with the challenges of achieving consistent halide incorporation being particularly evident through the formation of volatile bromide species. Understanding the relationship between substrate properties and perovskite film characteristics is crucial for optimizing device performance and advancing the development of efficient and stable perovskite solar cells.

Photoluminescence (PL) based methods are powerful tools to investigate and understand the different physical mechanisms governing the performance of perovskite devices. In this contribution, we highlight the different methods that can be derived from bias-dependent absolute calibrated hyperspectral imaging and time-resolved PL and V_OC decay. 

First, we discuss the absolute calibration of a hyperspectral imaging tool that features LED-based illumination. This enables spatially resolved extraction of photoluminescence quantum yield, quasi-Fermi level splitting (QFLS) and implied V_OC. Next, with the ability to apply a bias-voltage PL, measurements can be performed also under short-circuit conditions and the calculation of a charge extraction coefficient is possible. Measurements at varying voltages result in reconstructed local implied J(V) curves. Further, illumination intensity variations yield both suns-V_OC and local suns-PL curves, such that losses from interfaces and from transport can be disentangled. Measurements at different temperature levels finally reveal complex spatial variations in recombination phenomena.  Overall, by assessing the QFLS, charge extraction quality, and reconstructed JV characteristics, we are able to gain insights on the essential solar cell parameters, V_OC, J_SC, and FF,  with microscopic spatial resolution.

Additionally, we performed also time-resolved PL measurements. The setup has two distinct features: First, the illumination is quasi-continuous and has a rapid shut-off capability, allowing for PL decay measurements from a defined illumination level. Next, the V_OC decay can be tracked parallel to the PL signal. From the direct comparison, the impact of ion movements can be derived.

Applied to both pristine and especially samples subjected to accelerated aging, the combination of this powerful set of tools allows for the investigation of the fundamental causes of performance limitations.

The functionality of organic-inorganic perovskites in large area electronics depends on the quality of the molecular organization and thin film morphology. Therefore, control of nucleation and growth is important to reduce structural defects and domain boundaries in the perovskite film to ensure unhindered carrier transport and high device performance. Grain engineering controlled by film deposition parameters and the chemical structure of organic cations are novel approaches to improve charge carrier transport and understand the relationship between crystallization and ions migration. First, a hot casting method to effectively modulate grain size and grain boundary number to improve in-plane carrier transport in perovskite films is discussed. Based on the obtained results a close correlation between grain boundary density, carrier transport and ion migration will be presented.[1] Secondly, a distinct odd-even effect in 2D Sn-based perovskite semiconductors is presented for the first time by incorporating phenylalkylammonium-based organic cations with different alkyl side chain lengths with odd and even carbon atoms.[2, 3] An odd-even oscillation of the charge carrier transport is revealed by optically pumped terahertz spectroscopy and transistor devices. Combining density functional theory calculations and simulated grazing-incidence wide-angle X-ray scattering, we show that the organic ligands with odd carbon atoms are featured with disordered crystal lattice and tilted inorganic octahedron leading to larger effective mass and thus inferior charge mobilities compared to the perovskites with an even carbon number of the organic cations

Two-dimensional (2D) halide perovskites have garnered significant attention for their ability to enhance both the performance and stability of perovskite devices. While changes in band gap and crystal structure in 2D halide perovskites are widely agreed upon, understanding energy level positions and band alignments at the interfaces remains elusive. To bridge this knowledge gap, we present a systematic investigation of alkylammonium-based Ruddlesden–Popper perovskites (n = 1, A'2PbI4) with varying alkyl chain lengths of spacer cations (from propylammonium, C3, to decylammonium, C10), using X-ray diffraction (XRD), UV-vis spectroscopy, and ultraviolet photoelectron spectroscopy (UPS).

The XRD results indeed confirm the expected increase in d-spacing with the alkyl chain length, while (UPS) measurements show consistent changes in the density of states (DOS) depending on the length of these spacer cations. These variations can be well explained by comparison to DOS calculations done by density functional theory (DFT) when considering the low probing depth of UPS. Surprisingly, the ionization energy (i.e., VB position) remains nearly constant across all samples, however, a slight odd-even variation is observed in UPS data. The extracted ionization energy values are only marginally larger than what we find for 3D MAPbI3, suggesting that the increase in band gap for 2D perovskites seems predominantly due to an upward shift in the conduction band.

The mentioned odd-even effect is even more pronounced when investigating the optical band gaps. Perovskites with odd-numbered alkyl chains exhibit blue-shifted absorption onset compared to their even-numbered counterparts by up to 90 meV. DFT simulations reveal that this effect arises from distortions in the Pb-I-Pb bond angles within the inorganic framework, influenced by the packing efficiency of the alkyl chains. Specifically, odd-numbered cations lead to larger structural distortions, resulting in higher bond angle deviations and larger band gaps.

In conclusion, our study reveals that the observed variations in optical properties stem from an odd-even effect driven by the packing of spacer cations, rather than changes in interplane distance.

Metal halide perovskite semiconductors have captured significant interest in the thin-film optoelectronics community, with impressive performance demonstrated in both solar cells and light-emitting devices. But when it comes to explaining behavior that defies conventional semiconductor physics, we often fall back on the catch-all term “ion migration.”

To make real progress in understanding device operation and degradation, we need to look more closely at what happens at the interfaces between halide perovskites and other materials. Our work shows that these materials exhibit not just mixed ionic-electronic transport, but also significant chemical reactivity — including redox (electron transfer) and acid-base (proton transfer) reactions. These processes contribute to phenomena such as metal contact corrosion, ITO etching, iodine diffusion into hole transport layers, halide phase segregation, and even gold migration.

By bringing together insights from both semiconductor device physics and electrochemistry, we can move beyond the limitations of “ion migration” and toward a more complete understanding of these materials.

Perovskite solar cells are a promising technology for next-generation photovoltaics due to their astonishing values of power conversion efficiency. However, long-term stability remains a critical challenge for the implementation and commercialization of this star photovoltaic material. This study introduces an innovative approach to rapidly evaluate the degradation mechanisms of perovskite solar cells using small amplitude time transient techniques,^1,2 supported by Impedance Spectroscopy.³ By analyzing the evolution of the current responses during a voltage sweep under operational conditions (classical method to estimate the device efficiency), our methodology provides comprehensive insights into the effects of ageing and performance variations via physics-based equivalent circuits.^1,2,4 The proposed framework enables rapid and accurate assessment of efficiency and stability metrics, potentially offering a valuable tool for regenerating the perovskite samples (reversible degradation) or optimizing device design during fabrication processes (irreversible ageing). Our work contributes to bridging the gap between laboratory-scale research and large-scale deployment of perovskite photovoltaic technology.

This work focuses on the determination of defects in perovskite solar cells, a crucial aspect for enhancing device performance and reliability. Capacitance-voltage (C(V)) measurements and Mott-Schottky analysis are often employed to determine doping densities in devices. However, geometric capacitance, which depends inversely on the thickness of the device, makes it impossible to accurately measure charge density in thin-film devices. [1] To overcome this limitation, we investigate single-crystal devices with thicknesses ranging from 20 to 80 µm. Further, we are investigating thin film devices, with a lateral contact arrangement with 100 µm distance between the electrodes. Our methodology incorporates both classical steady-state C(V) and pulsed C(V), which is a new type of measurement that avoids the influence of mobile ions. Additionally, drift diffusion simulations with the software Setfos are employed to find out what could cause a measured charge density in a C(V) measurement. The combination of experiments and simulation helps us to understand how defects and mobile ions impact the overall performance of perovskite solar cells.

Perovskite solar cells have entered a stage where market introduction is within reach. For successful large-scale commercialization, precise control over the quality of the perovskite material is crucial. To achieve this, the research community currently relies heavily on laboratory experience, engineering solutions, and other heuristic approaches. One of the central challenges remains the controlled crystallization of perovskite thin films. Various strategies, such as anti-solvent techniques and additive engineering, are widely used to increase process control. While these approaches have demonstrably influenced the morphology and quality of the resulting layers, the fundamental mechanisms governing the crystallization process are still the subject of vigorous debate. A frequently cited hypothesis is that crystallization additives mediate the heterogeneous nucleation, where solvate complexes are expected to form the colloidal seeds for crystallization.[1,2] However, direct and unambiguous insights linking the complex formation in precursor inks to the final perovskite structure remain elusive.

In this work, we present compelling evidence that the decisive influence of typical crystallization additives is not the nucleation phase. Instead, they promote coarsening grain growth by enhancing ion mobility across grain boundaries. Our conclusions are drawn from a comprehensive study that integrates ex-situ and in-situ characterization techniques, device performance analysis, phase-field simulations, and density functional theory (DFT) calculations. This multi-faceted approach allows us to propose a general mechanism that holds true across a variety of additives and perovskite compositions.

Starting from the precursor ink, we monitor the evolution of colloidal species using NMR, UV–vis spectroscopy, and electrochemical conductivity measurements. We then investigate the perovskite formation dynamics via in-situ GIWAXS, supported by phase-field simulations that model the crystal growth process. These analyses provide the first clear evidence that grain coarsening, rather than nucleation, is the dominant mechanism shaping the final film morphology. To identify the mechanism underlying the enhanced coarsening rate, we employ solid-state NMR, which reveals that additives significantly increase ion mobility across gain boundaries. Complementary techniques, including UPS, FTIR, and DFT calculations, reveal that this enhanced mobility likely arises from the interaction of additives with the grain boundaries. Upon annealing, weakly bound additives detach, opening up defect states, that act as ion channels. In some cases, additives can also serve as ion shuttles, further facilitating ion transport across grain boundaries.

We validate the generalizability of our proposed mechanism by applying it to various perovskite formulations and additive types. Moreover, we demonstrate that the same underlying principles can explain the effects of post-processing techniques, where an additive may be processed on top of an already crystallized perovskite film. Indeed surprising similarities exist to the thermal hot-pressing process, where ion mobility is increased through elevated temperatures.[3]

Our finding provides a decisive piece that complements the nucleation theory for perovskite thin film fabrication and which bridges the gap between the precursor phase and the final film. Thereby, we take a crucial step beyond heuristics and pave the way for revised additive and crystallization engineering.

The solution-based fabrication of reproducible, high-quality lead iodide perovskite films demands a detailed understanding of the crystallization dynamics, which is mainly determined by the perovskite precursor solution and its processing conditions. We conducted a systematic in situ study during the critical phase before the nucleation in solution to elucidate the formation dynamics of lead iodide perovskite films. Using UV absorption spectroscopy during spin coating allows us to track the evolution of iodoplumbate complexes present in the precursor solution. We find that prior to film formation, a novel absorption signature at 3.15 eV arises. We attribute this to the emergence of a PbI₂-DMF solvated (PDS) phase. The amount of PDS phase is closely connected to the concentration of the solution layer during spin coating. We also propose that PDS clusters are a predecessor of crystalline perovskite phases and act as nucleation seeds in the precursor solution. In this way, our work provides insights into the early stages of perovskite crystallization.

abricating thick (1000 nm) solution-processed perovskite layers is expected to increase the efficiency of carbon-contact-based solar cells compared to thinner (500 nm) films. However, increasing only the deposited layer thickness often results in buried voids inside the dry film. This is detrimental to the efficiency of the device. Recently, we have developed a theoretical framework based on Phase Field simulations[1]. It is capable of describing the main physical processes determining the morphology: evaporation, diffusion, spontaneous nucleation, crystal growth, and advection[2]. With the help of the simulations, it is possible to explain why voids form in the film. The crystals nucleate at random spots inside the liquid film. The movement of the condensed-vapor interface, due to evaporation, leads to an agglomeration of the crystals at the film surface. The crystals block further evaporation and the remaining solvent is the origin of the buried voids inside the dry film. We explain how adding seeds on the substrate before coating the thick film can prevent this. In this case, processing conditions have to be modified compared to standard operating procedures for thin films. The theoretical expectations can be verified experimentally, leading to a performance improvement of the devices.

The surface topography of the lead halide perovskite layer is a crucial aspect that influences the performance of perovskite solar cells (PSCs). In this work, two different quenching approaches for the crystallization of wide-bandgap perovskite films are investigated: antisolvent quenching and the gas quenching. Both methods, aimed at removing the solvent of the precursor solution and initiate the perovskite nucleation, differ mechanistically and result in different rates of crystallization, which cause surface topographical irregularities, in the form of elongated and elevated structures on the films, termed “wrinkles”. This study shows antisolvent-quenched perovskite films exhibit a higher density of wrinkles than the gas-quenched counterparts. Pinholes were found along the wrinkles, thus a higher density of wrinkles leads to more pinholes and to more defective surface topography. The wrinkles also make the surface rougher, hindering a homogeneous contact with the adjacent layer and reducing the overall performance of the solar cells. By comparing the two different quenching methods, we obtain insight into the formation of the wrinkles and their effects on the optoelectronic properties of the perovskite films. We identify the gas quenching method as a way to reduce the wrinkle density to achieve better photovoltaic performance in comparison to the antisolvent method.

Understanding the photocurrent in a solar cell is a matter of looking at the competition between charge carrier collection and recombination. In perovskite solar cells, these processes are affected by the redistribution of mobile ions during current-voltage scans and degradation, leading to the welk-known hysteresis [1], so-called ion-induced losses [2], and slow transients.

In this talk, spectrally resolved measurements together with device simulations are presented to better understand the loss mechanisms related to photocurrent [3]. The approach exploits different penetration depths of the light due to the absorption coefficient of the perovskite varying with wavelength. Preconditioning samples under various conditions such as bias voltage and then "freezing" the ion distribution by cooling, allows for a measurement of the external quantum efficiency (EQE) under a stable ion distribution. Subsequent EQEs then monitor the effect of ion redistribution on the spectral shape of the EQE and thus depth-dependent charge collection probability.

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

Further improvements of perovskite solar cells (PSCs) require a deeper understanding of their main power loss mechanisms, namely inefficient charge carrier extraction through the low-mobility transport layers and non-radiative recombination mediated by defects at the interfaces of the device. The response of these processes are generally difficult to isolate in typical characterization measurements, due to overlapping signals from several other mechanisms. Therefore, advanced models are required that discriminate between these processes and allow determining the key device parameters associated with these loss mechanisms. In this regard, we develop an advanced model of the perovskite solar cell that explicitly accounts for the non-ideal charge carrier extraction due to the transport layers, and its impact on recombination in the perovskite absorber. This model is applied to the case of small-perturbation optoelectronic measurements in the frequency domain (intensity-modulated photocurrent (IMPS) and photovoltage (IIMVS) spectroscopy) that are widely used to characterize PSCs. We show the predicted evolution of the typical spectra as a function of DC parameters such as light intensity and external voltage, while also identifying the underlying mechanisms that generate the characteristic time constants observed in these measurements. We further incorporate the developed model with the diffusion-recombination model in a unified framework, identifying that the transport layers behave as a voltage-dependent resistance and that an equivalent circuit can only be defined at open-circuit conditions. These insights are applied to experimental IMPS and IMVS measurements of a perovskite solar cell, identifying the significant impact of the transport layers on charge extraction and recombination in the device.

Understanding charge carrier transport in halide perovskites is crucial for optimizing their performance in solar energy conversion applications. However, modeling transport in this material class is challenging due to strong anharmonic nuclear dynamics and dynamic disorder [1]. To address these effects, we construct a time-dependent, real-space hopping model using molecular dynamics trajectories, parametrized with hybrid density functional theory, which enables accurate predictions of electron and hole mobilities in MAPbI₃ and MAPbBr₃ [2, 3]. By tracking the dynamics of orbital occupation configurations, we directly link the time-resolved electronic structure to the transport behavior in the two halide perovskites. Our analysis uncovers three transport channels, each corresponding to a bond parameter in the real-space model, which together constitute the primary microscopic mechanisms governing charge mobility in these materials. In particular, the transport channel associated with the ppπ bond represents a critical bottleneck for charge carrier transport, which is modulated by the halide spin-orbit coupling, pointing to the difference between electron and hole transport in halide perovskites [4].

Despite its critical importance for solar cell device efficiency, the determination of the charge carrier
lifetime under steady state operation in perovskites is still a challenge: commonly used methods such
as time-resolved photoluminescence (TRPL) or intensity-modulated photovoltage spectroscopy (IMVS),
open-circuit voltage decay or transient photovoltage have their drawbacks. While TRPL is applicable to
thin-films as well as full devices it requires a) a precise calibration to get a 1 sun equivalent lifetime and
b) results in a lifetime that might be influenced by the (partial) filling of shallow traps [1]. The voltage based
techniques require the presence of contacts and are therefore only applicable to devices.
In this work we demonstrate the application of intensity modulated photoluminescence spectroscopy
(IMPLS), also called modulated photoluminescence (mPL) [2] or Quasi-Steady-State Photoluminescence
(QSSPL) [3] to halide perovskites. Similarly to IMVS the illumination of the sample is sinusoidally modulated
around a working point. This results in a modulation of the charge carrier density and therefore
of the PL signal (and in full devices of the photovoltage). The evaluation of the frequency-dependent
PL response allows for the determination of the charge carrier lifetime. Although this approach is well
known its application to perovskites has been limited [4],[5]. We apply this method to both halide perovskite
thin films and devices and suggest the use of a simple theoretical model for its evaluation. Albeit their
striking resemblance, IMPLS and IMVS, to our knowledge, have never been compared directly. We address
this omission by performing the two methods back to back and demonstrating the consistency of
the corresponding results.

The halide perovskite solar cell absorbers require an appropriate band gap, high light absorption, high charge carrier mobility, long charge carrier lifetime and a work function that matches available selective transport layers. In this presentation we will discuss the different contributions of the constituents of the respective crystal to the dielectric constant of the absorber material. The dielectric response strongly affects the charge carrier lifetime and the screening of defect states. We will consider a number of materials and our understanding of the effects six years ago and now. Cases of 3D, 2D, 1D and 0D octahedra connectivities will be compared. New molecular cations will be incorporated. The apparent conflict between the terminology of dielectric and ferroelectric effects will be consolidated. The influence of unpaired electrons in heavy ions and the dynamics of the molecular cations will be compared. Structural phase transitions associated with ordering processes will be discussed. 

Lead halide perovskites represent a versatile platform for studying spin phenomena in semiconductors, owing to their tunable crystal symmetry, strong spin–orbit coupling, and long spin lifetimes [1]. We overview our experimental studies of spin dependent properties and spin dynamics in the perovskite crystals and nanocrystals. We investigate exciton and carrier spin polarization at cryogenic temperatures in bulk perovskite crystals with near-cubic (FA₀.₉Cs₀.₁PbI₂.₈Br₀.₂, FAPbBr₃) and orthorhombic (MAPbI₃, CsPbBr₃) symmetries. A high degree of optical orientation up to 85% is observed, indicating strong spin selectivity and efficient spin initialization under circularly polarized excitation [2,3,4].

In FA₀.₉Cs₀.₁PbI₂.₈Br₀.₂, we explore the spin orientation of localized electrons and holes using polarized photoluminescence and time-resolved differential reflectivity. At 1.6 K, continuous-wave excitation yields optical orientation degrees of 6% and 2% for electrons and holes, respectively. These contributions are clearly distinguished from excitonic signals by analyzing the Hanle effect in transverse magnetic fields and the polarization recovery effect in longitudinal fields [5].

The spin orientation remains stable to excitation energy detunings up to 0.25 eV and decreases gradually up to 0.9 eV, indicating inefficient spin relaxation during carrier energy relaxation. Across all studied symmetries, the absence of spin relaxation acceleration suggests suppression of the Dyakonov–Perel mechanism and no evidence of Dresselhaus–Rashba spin splitting, implying preserved spatial inversion symmetry. Coherent spin quantum beats and electron-hole spin correlations further support these conclusions [2].

Even with additional symmetry reduction in CsPbI₃ nanocrystals due to spatial confinement [6], spin relaxation mechanisms associated with inversion symmetry breaking are not activated. Instead, exciton spin polarization is primarily governed by exchange interactions within the exciton.

Furthermore, spin relaxation of localized carriers is driven by hyperfine interactions with nuclear spins. Dynamic nuclear spin polarization is observed, with Overhauser fields reaching +4 mT for electrons and −76 mT for holes [5]. This pronounced asymmetry highlights a unique characteristic of lead halide perovskites, where hole–nuclear spin coupling significantly exceeds that of electrons, in a contrast to most conventional semiconductors.

These findings demonstrate the intrinsic robustness of spin polarization and coherence in lead halide perovskites. Their ability to maintain spin orientation under structural and energetic perturbations makes them highly promising candidates for spintronic applications, where optical control and long spin lifetimes are essential.

Quantum technologic and spintronic applications require reliable semiconducting materials that enable a significant, long-living spin polarization of electronic excitations and offer the ability to manipulate it optically in an external field. Due to the specifics of band structure and remarkable spin-dependent properties, the lead halide perovskite semiconductors are suitable candidates for that. They have sufficiently long spin relaxation and spin coherence times, which exceed the typical repetition period of 13.2 ns of the mode-locked pulsed laser operating at 76 MHz. Photogenerated spin polarization and spin coherence do not fully relax between the laser pulses and can be accelerated or decelerated by the following pulses bringing rise to several spin accumulation effects for electron and holes that we present here:

(i) Resonant spin amplification.

(ii) Spin mode locking.

(ii) Polarization recovery.

(iii) Spin inertia.

Their experimental appearance are demonstrated in MAPbI₃ and (FA,Cs)Pb(I,Br)₃ single crystals [1,2] and CsPb(Cl,Br)₃ nanocrystals [3]. For that we use cryogenic temperature of 1.6 K and magnetic fields up to 3 Tesla, as well as time-resolved Faraday/Kerr rotation technique to generate and detect carrier spin dynamics.

We investigate coherent exciton dynamics in CsPbBr₃ nanocrystal (NC) samples with sizes ranging from 8 to 28 nm, employing transient four-wave mixing (FWM) and time-resolved photoluminescence (TRPL) spectroscopy at 2 K. Due to inhomogeneous broadening of optical transitions in the NC ensemble, photon echoes (PE) are generated in the FWM signal. We perform two- and three-pulse PE measurements under resonant excitation to evaluate the exciton coherence time T₂ and population relaxation time T₁, respectively. We find that T₂ is limited by T₁, following the relation T₂ = 2T₁. This indicates that exciton coherence is governed by escape (energy relaxation) dynamics or exciton recombination, while elastic scattering is negligible at low temperature.

The coherent dynamics addressed in the low-energy tail of the absorption spectrum are attributed to zero-phonon excitons, which show the longest relaxation time T₁ compared to excitation at higher photon energies. For different samples with increasing NC size, this time decreases from 100 to 20 ps and is consistent with exciton lifetimes evaluated from the photoluminescence decay in TRPL experiments. The strong size dependence of the lifetime is attributed to an increase in exciton oscillator strength, implying radiative recombination with time constant τ_rad.

At higher photon energies within the same ensemble, exciton decay processes are accelerated due to energy relaxation via emission of optical phonons and exciton-polaron formation. The exciton-polaron formation is quantified via the decay rate γ_ε = 1/T₁ − 1/τ_rad, which increases with photon energy. We show that T₂ and T₁ exhibit a consistent dependence on NC size (comparing different ensembles) and on excitation energy (within each ensemble), with the longest coherence times observed near the low-energy tail of the PE absorption spectrum.

Our results provide detailed insight into the size and energy dependence of exciton relaxation processes in perovskite NCs. In particular, we show that the homogeneous linewidth of low-energy zero-phonon excitons is determined primarily by the radiative lifetime (ℏ/τ_rad), which may enable future applications in quantum communication.

Halide perovskites are exciting new semiconductors that show a great promise in low cost and high-performance optoelectronics devices. However, the poor stability of these materials is limiting their practical use. In this talk, I will present a molecular approach to the synthesis of a new family of hybrid material – Organic Semiconductor-incorporated Perovskite (OSiP), which are structrally more versatile and intrinsically stable. Energy transfer and charge transfer between adjacent organic and inorganic layers are extremely fast and efficient, owing to the atomically flat interface and short interlayer distance. In addition, the rigid conjugated ligand design dramatically enhances their chemical stability, suppresses solid-state ion diffusion, and modulates electron-phonon coupling, making them useful in many applications, particularly solid-state lighting. Using these stable hybrid materials, we demonstrate efficient light emission and amplification in single crystalline nanostructures, epitaxial heterostructures, as well as polycrystalline thin films. 

Inorganic metal halide double perovskites (HDPs) have emerged as promising alternatives to lead-based perovskites, effectively mitigating issues related to toxicity and poor stability. Despite this progress, achieving optimal optical performance remains a significant challenge. While the core@shell heterostructure approach is widely recognized for tailoring optical properties in various nanomaterials, it remains largely unexplored for metal halide double perovskite nanocrystals (NCs). This is primarily due to the intrinsic softness and ionic character of the perovskite lattice, which complicates controlled shell growth. In this work, we report a simple colloidal strategy to grow a lead-free Cs₂NaInCl₆ shell over Cs₂AgBiCl₆ cubic NCs. This heterostructure design results in a tenfold immediate enhancement in photoluminescence quantum yield (PLQY), which continues to improve over time. Optical characterization indicates the emergence of new emissive states likely originating from interfacial alloying, with carrier dynamics further elucidated via time-resolved photoluminescence and transient absorption spectroscopy. This approach offers a compelling route to modulate and enhance the optical behavior of lead-free halide double perovskites through heterostructure engineering.

Lead halide perovskite nanocrystals (NCs) play an important role in future devices since they show a high photoluminescence quantum yield, defect tolerance, and color tunability by their halide composition. These properties can be used in light emitting diodes (LEDs). A current challenge in building efficient LEDs from NCs is their long-term stability and charge carrier imbalance. The effect of charging of NCs can be studied by spectroelectrochemistry (SEC). In combination with photoluminescence (PL), the method allows for precise determination of the valence and conduction band position, the effect of charging on the emission wavelength, and thus decomposition processes can be observed directly.

In this work, we explore p-doping of CsPbBr₃ NCs. First, we investigate the effect of the ligand shell on the electronic structure of the NCs by using different binding motives and different electronic functionalities, e.g. by introducing dipole moments to the surface.[1] In both cases, the absolute band edge position is determined by SEC PL and conclusions are drawn for building LEDs, especially determining the barrier to the hole transport layer.

Second, we present for the first time SEC PL on supercrystals of self-assembled NCs. Supercrystals may find applications as micro-LEDs due to their propensity to exhibit superfluorescence. We monitor the spatially resolved PL of individual supercrystals depending on the applied potential, and we derive how this influences the emission wavelength at different positions of the supercrystal. From this, we can directly observe the start of degradation and the self-repairing in the center.

The study of lead halide perovskite nanocrystal based light-emitting diodes (LEDs) has advanced significantly, with notable improvements in stability and optical properties. However, optimizing charge carrier injection and transport remains a challenge. Efficient electroluminescence requires a balanced transport of both holes and electrons within the emitting material. Here, we investigate cubic CsPbBr₃ nanocrystals passivated with oleylamine and oleic acid, comparing them to ligand-exchanged nanocrystals with didodecyldimethylammonium bromide (DDABr). Nuclear magnetic resonance spectroscopy and transmission electron microscopy confirm successful ligand exchange, revealing reduced ligand coverage in DDABr-treated nanocrystals. Photoelectron spectroscopy, spectroelectrochemistry, and single-carrier devices indicate improved hole injection in DDABr capped nanocrystals. Density functional theory calculations further reveal the influence of ligand type and coverage on energy levels, with oleic acid introducing localized states in native nanocrystals. Additionally, incorporation of a polar electron transport layer (ETL) enhances LED performance by over an order of magnitude in DDABr-capped nanocrystals, driven by improved charge balance arising from the spontaneous orientation polarization (SOP) of the ETL. These findings highlight the critical role of ligand selection, passivation degree, and charge transport control by the adjacent organic transport layers in optimizing LED efficiency.

X-rays, with their high penetration and non-destructive detection capabilities, have brought great convenience to daily life. The demand for affordable and robust detectors for ionizing radiation in medical imaging, non-destructive diagnostic and security control are growing. In recent years, halide-based scintillators have gained significant attention for indirect X-ray sensing and imaging applications due to their superior ability to convert high-energy X-rays into visible light. Among them, lead-based perovskite CsPbBr₃ is one of the most used and promising material. However, its toxicity due to the presence of lead, self-absorption and the requirement for thick films to achieve strong radioluminescence (RL) limit its practicality. Recently, copper-based halides such as CsCu₂I₃ and Cs₃Cu₂I₅ have emerged as promising alternatives. These materials exhibit excellent luminescence properties through electron ionization and fast exciton recombination under X-ray irradiation. Their photoluminescence quantum yields (PLQYs) reach 15.7% and 97.76%, respectively. Additionally, their broad Stokes shifts effectively prevent self-absorption. By employing thermal evaporation techniques, we fabricate high-quality thin films of these copper-based materials. Compared to CsPbBr₃, these films achieve higher spatial resolution and superior RL performance with the same film thickness, making them highly suitable for advanced X-ray imaging applications.

Hybrid metal halide perovskites have emerged as promising materials for X-ray detection due to their scalable, cost-effective, and robust solution growth, combined with their capability to detect single gamma photons under high applied bias voltages. Despite these advantages, their practical application has been limited by rapid degradation under strong electric fields, a consequence of mixed electronic-ionic conduction, which undermines the long-term stability and efficiency of perovskite-based X-ray detectors.

To address this limitation, we previously demonstrated a photovoltaic mode of operation at zero-voltage bias, utilizing thick methylammonium lead iodide (MAPbI₃) single-crystal films (up to 300 µm) grown directly on hole-transporting electrodes through the space-confined inverse temperature crystallization (ITC) method [1]. These devices exhibited near-ideal performance, long-term operational stability, 88% detection efficiency, and a noise equivalent dose of 90 pGyair with 18 keV X-rays. However, we observed that the performance of MAPbI₃ devices deteriorates significantly when the crystal thickness exceeds 200 µm, presenting a challenge for applications that require thicker absorbers to achieve higher detection sensitivity.

Building upon this foundation, we now present [2] advances in compositional engineering that enable nearly 100% charge extraction in perovskite single crystals with thicknesses reaching up to 0.92 mm. These high-quality crystals exhibit significantly prolonged charge carrier lifetimes, enabling efficient X-ray absorption and complete charge collection for both 18 keV and 45 keV photons while maintaining excellent material stability. Remarkably, these devices operate without any external bias, representing a breakthrough in direct X-ray detection technology. In contrast to conventional semiconductor X-ray detectors, which typically require hundreds of volts to establish strong electric fields for efficient charge collection, our perovskite detectors achieve unity charge extraction under zero-bias conditions. This achievement sets a new benchmark, combining high performance, operational simplicity, and enhanced stability.

In conclusion, this study introduces a novel approach to perovskite-based X-ray detectors by integrating compositional engineering with advanced solution growth techniques. The resulting devices exhibit superior charge collection efficiency, extended carrier lifetimes, and robust long-term stability, opening new pathways for cost-effective, high-performance X-ray imaging technologies with simplified device architectures.

[1] Sakhatskyi, K.† Turedi, B.†, Bakr, O. M, Kovalenko, M. V. et al. Nature Photonics 2023, 17(6), 510-517.

†equal first-authors

[2] unpublished

Taiwan, situated in the subtropical region, receives abundant sunlight, making it highly suitable for solar photovoltaic development. By 2023, Taiwan had installed over 12 GW of solar capacity. However, due to its limited land area and high population density, Taiwan faces significant challenges in further expanding its solar capacity. It is suggested to actively develop next-generation solar cell technologies, leveraging advanced technology to maximize land use efficiency and to increase the electricity output per unit of land area. These next-generation solar technologies include tandem photovoltaics, solar cells with high-capacity factors and photovoltaic technologies for emerging installation fields.

Perovskite solar cells (PSC), which use organic lead halide perovskite (PVSK) as the absorber layer, have made remarkable progress since 2009. Moreover, PVSK's tunable bandgap and high defect tolerance make it an ideal material for the aforementioned three emerging photovoltaic technologies for Taiwan's needs. However, PSCs still face several challenges, including stability, large-area conversion efficiency, and manufacturability. As noted, maintaining high efficiency at scale is one of the major challenges for PSC commercialization. High efficiency lab devices are usually realized on an active area of far less than 1 cm². However, the materials, fabrication technologies or even device structure reported in those high-efficiency devices maybe not stable or not suitable to be used at large scale. Consequently, maintaining high efficiencies while achieving stability in large-area modules will be a core course for the practical use of PSC. Scaling up engineering is urgently required to accelerate commercial production of PSCs. Aiming for pushing PSC toward commercial applications, we have endeavored ourselves in developing various technologies/materials/processes with controllability and reproducibility. The journey on developing production technologies in our group will be presented. A milestone of perovskite solar module pilot line has been established in late 2022 in NTHU campus.

In this study, we present an optoelectronic simulation study that enhances the understanding of interface engineering of electron and hole transport layers (ETL, HTL) and their impact on the performance of perovskite-based tandem cells. The complexity of the structures, the metastability and the intrinsic perovskite bulk make it difficult to understand and quantify electrical losses of perovskite-based tandem solar cells without simulation. However, in recent years, the predictive power of simulation models has significantly improved through the improvements of models and multiple comparisons with experimental perovskite results [1–4].

At the ETL side, in this case C₆₀/perovskite, we address two important parameters influencing device performance: the quality of chemical passivation, indicated by the surface recombination velocity (S_0,ETL), and the conduction band offset (ΔE_C,ETL) at the perovskite/C₆₀ interface (Fig 1A), which can be optimized experimentally by dipole engineering at the ETL interface. We show that a reduction of the conduction band offset results in an increased ratio of majority to minority charge carriers, suppressing recombination losses at the interface and minimizing selectivity losses (defined as difference between internal and external voltage), within the device (Fig 1B). Notably, we demonstrate that the reduction of the ETL/perovskite conduction band offset not only enhances so-called field-effect passivation, thereby suppressing interface recombination, but also results in an electron accumulation throughout the perovskite absorber, thereby enhancing conductivity and reducing transport losses, which are crucial for improving fill factor (FF) and therefore efficiency.

For the HTL side, we show state-of-the-art modelling of self-assembled monolayers (SAMs) on transparent conductive oxides (TCOs). The perovskite/SAM/TCO interface is investigated and the influence of the SAM dipole moment, TCO doping concentration and TCO electron affinity on the device performance starting from the baseline TCO (here indium tin oxide (ITO)) are studied (Fig 1C). We showcase that the ITO properties cause selectivity losses reflected in a Fermi level gradient towards the ITO/SAM stack which results in an open-circuit voltage of 1245 mV that is significantly lower than the high internal voltage 1323 mV (Fig 1D). We could show both by simulation and experimentally, that by replacing ITO with zinc tin oxide (ZTO) the performance of the perovskite top cell is enhanced, due to lower electron concentration and higher effective work function of the ZTO. This results in an increase of both internal voltage (from 1323 mV to 1339 mV) and external voltage (from 1245 mV to 1302 mV), whereby the selectivity losses are significantly reduced. The effect of field-effect passivation of a C₆₀ or TCO strongly depends on the perovskite bandgaps in the perovskite-based tandem cell. In the conference presentation, the interplay of perovskite bandgap and transport layer stack will be further discussed and the losses quantified.

Overall, this study contributes to an enhanced understanding of interface physics and supports the optimization of perovskite-based tandem solar cells.

Over the past decade, substantial strides have been made in improving the performance of perovskite-based solar cells. Single-junction devices have surpassed the 27% efficiency barrier, while dual-junction all-perovskite tandem solar cells have exceeded 30%, and perovskite–silicon tandems have achieved efficiencies approaching 35%, marked by a series of record-breaking advancements. Our recent research has contributed significantly to this progress, achieving three power conversion efficiency records of 32.5%, 33.2%, and 33.7%. Furthermore, our realistic calculations project achievable efficiencies of up to 37.8%, underscoring the strong potential of our approach. These achievements were made possible by a series of improvements at the interfaces and the bulk of the perovskite. As for the interfaces, we had to solve several issues, such as introducing a dielectric interlayer between perovskite and fullerene contacts to mitigate induced defect states, enhancing the recombination junction through ultrathin indium zinc oxide electrodes, introducing alternative hole selective contacts including polymers, nickel oxide, and self-assembled contacts, and using alternative transparent electrodes. We also discovered that interfaces play a crucial role even during the encapsulation process, as thermomechanical stresses drive the degradation of solar cells, making interfacial strengths critically important. Our recent work has also branched into space applications, where interfacial challenges become even more pronounced due to extreme conditions such as thermal shocks and prolonged exposure to harsh environments. Despite the remarkable progress in perovskite–silicon tandem solar cells, their performance under other extreme space conditions has yet to be fully demonstrated. Towards understanding this, our initial investigations focus on high-efficiency single-junction FAPbI₃-based solar cells, specifically examining the role of interfaces in their thermo-mechanical resilience under the realistic temperature fluctuations experienced in low Earth orbit. In this talk, I will present our systematic strategies to enhance the performance of perovskite–silicon tandem solar cells, as well as our approach to translating these technologies to space applications, with illustrative examples based on single-junction devices.