Three-dimensional tin halide perovskites are promising materials for photovoltaic applications due to their ability to achieve narrower bandgaps compared to their lead-based counterparts. However, a major challenge in advancing their performance is their intrinsic self-p-doping, which occurs even in the absence of external oxidizing conditions. This native p-type behavior is driven by the unique defect chemistry of the material, leading to reduced minority carrier lifetimes and ultimately limiting charge transfer, extraction, and power conversion efficiency. In this talk, we gain deeper insights into the defect chemistry of tin halide perovskites by combining experimental and computational approaches. Spectroscopic techniques, such as photoluminescence quantum yield and carrier lifetime measurements, are used to evaluate key optoelectronic figures of merit, while density functional theory calculations provided insights into defect formation energies. We first highlight pristine stoichiometric films and explored the role of SnF₂, a commonly used additive in this class of materials, to understand its impact on defect passivation and optoelectronic properties. Then, we extend our studies to compositional engineering, systematically modifying: A-site cations, B-site cations through Sn-Pb mixing with varying ratios, and X-site halides by introducing iodide-bromide mixed systems. Our findings highlight strategies to modulate defect chemistry and charge carrier dynamics, offering pathways to optimize 3D Sn perovskites for efficient photovoltaic applications.

Tin halide perovskites are highly interesting alternatives to lead-based materials for a range of optoelectronic applications. However, their defect content and characteristics, heavily determined by the crystallization process of these materials, has slowed down their development. In this talk, we will cover the main differences in solution properties between lead and tin defining the current limitations of the latter. We will also discuss the actual potential of traditional and novel strategies to deal with this challenge, particularly regarding the solvent nature. Finally, a wide range of solution and thin-film characterization will give us a comprehensive understanding of the key parameters and how to use them to effectively control tin-based perovskite crystallization. This talk aims to provide both an overview and a set of directions to overcome current limitations in tin halide perovskite applications through completely new processing systems. Furthermore, beyond tin-containing perovskites, the fundamental character of this work maked it of high interest also for other similar novel materials under development.

The development of new transparent conducting polymers has gained significant interest due to their potential in various optoelectronic applications. We report new generation of highly transparent bisEDOT-based conducting polymers as an alternative to PEDOT/PSS. The synthetic approach is based on the in situ polymerization of bis-EDOT, a dimer of 3,4-ethylenedioxythiophene, inside polymethylmetacrylate (PMMA) via an oxidative polymerization reaction by Cu(ClO₄)₂. This approach yields a homogeneous transparent conducting polymer with enhanced electrical conductivity, mechanical stability, and excellent film-forming properties, offering significant advantages for high-performance applications in electronic and optoelectronic devices. The bis-EDOT-based conducting polymer can be processed in the form of inks for the formation of layers with thickness control, the possibility of tuning electrical conductivity, high transparency in the visible and near-infrared spectrum, and the possibility of adapting its formulation with various solvents, and additives to be fully compatible with perovskite in manufactured devices. The resulting conducting polymer films exhibit superior charge transport properties and long-term stability, making them ideal candidates for hole transport materials (HTMs) in perovskite-based photovoltaics. Moreover, the simplicity, scalability, and cost-effectiveness of this method make it highly suitable for large-scale production, with applications extending to flexible electronics, organic light-emitting diodes (OLEDs), sensors, and transparent conductive coatings. Overall, this in situ polymerization strategy provides a promising route for fabricating high-performance materials for advanced electronic and energy conversion technologies.

Layered metal-halide perovskites, or two-dimensional perovskites, can be synthesized in solution, and their optical and electronic properties can be tuned by changing their composition. In this talk, I will present a molecular templating method that restricted crystal growth along all crystallographic directions except for [110] and promoted one-dimensional growth out of the inherently layered 2D structures. This templating effect is achieved via introducing directional intermolecular interlayer hydrogen bonding interactions. This approach is widely applicable to synthesize a range of high-quality layered perovskite nanowires with large aspect ratios and tunable organic-inorganic chemical compositions (including Sn and Pb based perovskites, different quantum well thickness, and different halides). These nanowires form exceptionally well-defined and flexible cavities that exhibited a wide range of unusual optical properties beyond those of conventional perovskite nanowires. We observed anisotropic emission polarization, low-loss waveguiding (below 3 decibels per millimeter), and efficient low-threshold light amplification (below 20 microjoules per square centimeter). The 1D-2D mixed dimensional nanostructures provide unprecedented opportunities for next-generation optoelectronics and photonics.

The continuous development in smart devices and microsystems for the control of industrial processes, biomedical sensors and instruments, visible and NIR light communications, as many other applications, is triggering new demands for photonic chips. Metal halide perovskites (MHPs) can be a good solution, because of their good optoelectronic properties and tolerance against crystalline defects, other than low-cost processing and low CO2 footprint. In the present talk the optical properties of several 2D MHPs of formula ABX3 (A = organic cation, B = Pb, Sn, X = I) have been investigated and will be presented in this talk.

First of all, basic optical properties of Pb-perovskites, as PEA2PbI4 (and higher order Ruddlesden-Popper phases), will be presented, both in the case of polycrystalline thin films and nanoflakes with lateral size greater than 10 µm. Moreover, these nanoflakes can be the base of micrometric photodevices by using Pt-prepatterned Si/SiO2 substrates with channel lengths in the range 2-10 µm. Measured photocurrent is highly dependent on the thickness flake due to the great absorption coefficient and negligible carrier transport in the vertical direction. Interestingly, in the case of few layer nanoflakes, photocurrents from 10 pA to 100 nA can be measured in the range 10 pW to more than 500 nW.

Sn-perovskites are also very interesting 2D semiconductors, because they are non-toxic alternatives of Pb-perovskites for applications as photodevices. However, the use of Sn-perovskites still suffers from very low stability and most of the synthesis, fabrication and/or characterization work must be done under inert atmosphere or vacuum, even if antioxidative synthetic routes can be followed for reducing the negative effect of ambient conditions. 2D tin-perovskites, TEA2SnI4, are gaining more stability and resistance to ambient condition, whose deposition is possible by scalable solution processing techniques as Inkjet-printing. Room- and low-temperature excitonic PL and charge carrier recombination dynamics in (TEA)2SnI4 thin films were studied and demonstrated two excitonic optical transitions. The analysis of micro-PL measurements suggests that the low-energy emission line is associated to the volume of perovskite grains (platelets), while the high-energy excitonic transition seems to be originated at the platelet edges (as identified in the biggest ones). Photoconductive detectors based on TEA2SnI4 inkjet-printed films were also studied after encapsulation. High electrical (dark currents as low as » 10 - 20 nA at 10 V of bias voltage) and electro-optical parameters (responsivities in the range 1-20 A/W) were obtained for these photodevices under ambient conditions over several weeks.

Metal halide perovskite (MHP) semiconductors have garnered significant research attention due to their exceptional physical properties, including long charge carrier diffusion lengths and high absorption coefficients. Furthermore, their synthesis is relatively straightforward, requiring mild conditions and scalable methods that utilize non-critical metals. These characteristics position MHPs as promising, cost-efficient materials for a broad range of applications in photovoltaics, photocatalysis, optoelectronics, and beyond.

Our research group is focused on synthesizing metal halide perovskites from macroscopic structures to nanocrystals (NCs), employing a variety of chemical routes, including traditional wet chemistry and in-situ synthesis. This talk presents a novel strategy for the in-situ synthesis of perovskite NCs embedded in a nanocomposite, achieving outstanding optical properties and enhanced stability.

Hybrid materials of this nature have attracted increasing interest over the past decade as they mitigate NC aggregation and improve long-term stability. By embedding NCs into suitable matrices, it is possible to engineer multifunctional materials that combine optical, mechanical, electrical, thermal, electrochemical, or photocatalytic properties within a single system.

Over the past two years, we have developed an innovative in-situ synthesis method for various perovskite NCs with different compositions and dimensionalities. This approach involves the incorporation of MHPs into a metal- organic matrix (e.g., nickel acetate, magnesium acetate) to produce nanocomposite thin films via solution processing under ambient conditions, eliminating the need for an inert atmosphere (N₂ glovebox) [1,2,3,4].

We report the successful synthesis of 3D bulk lead-based perovskite NCs with different halides (Cl⁻, Br⁻, I⁻), incorporating both organic (MA⁺, FA⁺) and inorganic monovalent cations (Cs⁺). For specific compositions, we achieve near-unity photoluminescence quantum yield (PLQY). Furthermore, including bulky organic cations such as PEA⁺ and TEA⁺ enables the formation of low-dimensional tin-based perovskites with remarkable stability. In particular, the TEA-Sn molar ratio plays a crucial role in driving the formation and stabilization of 0D Sn-based MHPs, exhibiting exceptional photoluminescence and environmental resilience [5]. Additionally, we can also synthesize Ag- and Bi-based halide double perovskite NCs (i.e. Cs₂AgBiBr₆) with promising potential for catalytic applications. 

This method stands out for its exceptional reproducibility and process reliability under low-demanding fabrication conditions, making it an ideal platform for testing the synthesis of novel perovskites proposed through machine- learning approaches. Its key advantage lies in its high versatility and seamless compatibility with high-throughput roll-to-roll (R2R) printing techniques, enabling the scalable and cost-effective fabrication of large-area, high- performance devices. This approach has already shown a broad range of applications in photocatalysis [4], photovoltaics and light-emitting technologies such as lasing [6], down-conversion, and gas sensing.

Developing high-mobility p-type semiconductors that can be grown using silicon-compatible processes at low temperatures, has remained challenging in the electronics community to integrate complementary electronics with the well-developed n-type counterparts.

This presentation will discuss our recent progress in developing high-performance p-type semiconductors as channel materials for thin film transistors. For the first part of my talk, I will present high-performance tin (Sn²⁺) halide perovskite transistors using high-crystallinity and uniform cesium-tin-triiodide-based semiconducting layers [1.2]. The optimized devices exhibit high field-effect hole mobilities of over 50 cm² V⁻¹ s⁻¹, large current modulation greater than 10⁸, and high operational stability and reproducibility [3]. In addition, we explore triple A-cations of caesium-formamidinium-phenethylammonium to create high-quality cascaded Sn perovskite channel films. As such, the optimized TFTs show record hole mobilities of over 70 cm² V⁻¹ s⁻¹ and on/off current ratios of over 10⁸, comparable to the commercial low-temperature polysilicon technique level.

Next, I present an amorphous p-type oxide semiconductor composed of selenium-alloyed tellurium in a tellurium sub-oxide matrix, demonstrating its utility in high-performance, stable p-channel TFTs, and complementary circuits [4]. Theoretical analysis unveils a delocalized valence band from tellurium 5p bands with shallow acceptor states, enabling excess hole doping and transport. Selenium alloying suppresses hole concentrations and facilitates the p orbital connectivity, realizing high-performance p-channel TFTs with an average field-effect hole mobility of ~15 cm² V^-1 s^-1 and on/off current ratios of 10⁶~10⁷, along with wafer-scale uniformity and long-term stabilities under bias stress and ambient aging.

References

[1] A. Liu, Y.-Y. Noh et al, Nature Electronics 5, 78-83 (2022)

[2] H. Zhu, Y.-Y. Noh et al, Nature Electronics 6, 650-657 (2023)

[3] A. Liu, Y.-Y. Noh et al, Nature Electronics 6, 559-571 (2023)

[4] A. Liu, Y.-Y. Noh et al, Nature, 629, 798–802 (2024)

Low-dimensional (LD) organic metal halide hybrids (OMHHs), comprising diverse organic cations and metal halide anions, represent an emerging class of perovskite-related hybrid materials with exceptional structural and property tunability. By carefully selecting organic and metal halide components, their crystallographic structures can be precisely tailored at the molecular level, with metal halide units forming two-dimensional (2D), one-dimensional (1D), or zero-dimensional (0D) structures. The site isolation and confinement of metal halides by organic cations endow LD OMHHs with unique properties distinct from conventional three-dimensional (3D) metal halide perovskites. For example, 2D and 1D OMHHs have demonstrated broadband white emissions, while 0D OMHHs have achieved near-unity photoluminescence quantum efficiency (PLQE) with tunable emissions spanning blue, green, yellow, orange, and red wavelengths. The true significance of LD OMHHs lies not only in these specific accomplishments but also in their role as a new paradigm in materials design. In this talk, I will present our recent progress in the development and study of LD OMHHs, from synthetic control to device integration. Applications of LD OMHHs in various areas, including optically pumped white LEDs, electroluminescent devices, X-ray scintillators, and direct X-ray detectors will be discussed.

Cs₂TiBr₆, a promising lead-free and earth-abundant double perovskite, exhibits excellent photovoltaic and optoelectronic potential due to its 1.8 eV bandgap and theoretical stability under various conditions. Despite its advantages, challenges in conventional synthesis—such as high temperature, pressure, and solubility issues—have limited its practical application. Additionally, its susceptibility to air-induced degradation raises further concerns about its stability.

This work introduces a novel, microwave-assisted synthesis method that significantly reduces time, temperature, pressure, and cost while maintaining structural stability under diverse atmospheric conditions, including air, oxygen, white light, and temperatures exceeding 130°C. A gradual cation exchange process was implemented to enhance stability by substituting Ti⁴⁺ with Sn⁴⁺ in the efficient microwave-assisted synthesis method, developing a double perovskite Cs₂Sn_xTi_1-xBr₆ type. A systematic study of Sn-doping revealed improved air stability for over a week, uniform polygonal crystal morphology, and a slight bandgap broadening.

This efficient and sustainable synthesis approach offers a pathway to more durable and environmentally friendly perovskites, unlocking new opportunities for advanced optical and electronic devices.

Quasi-2D metal halide perovskites (MHPs) employed in photovoltaics involve the use of monoammonium or diammonium spacer cations to form the Ruddlesden-Popper or the Dion-Jacobson phases, respectively.¹ Moreover, the chemical nature of the bulky cations plays a significant role in the optoelectronic properties of the quasi-2D MHPs, controlling the orientation of the inorganic octahedral layers, the interlayer distance and thickness of the inorganic layers, phase distribution and the dielectric constant and dipole moment of the organic barriers which all determines the charge transport in the quasi-2D MHPs.² Moreover, additive engineering and processing strategies can also modulate the charge carrier transport.

We have used a p-conjugated bulky cation, 4,4′-diaminostilbene dihydrochloride (Sb), to synthesize a Dion-Jacobson 2D MHPs that is employed to fabricate photovoltaic devices. The relative ratio of the employed precursors is selected to define the n = 5 phase, (Sb)FA_2.8MA_1.2Pb₅I₁₅. However, a distribution of low dimensional phases is found (n = 1, 2 and 3) although other phases with longer n values are also present. The quasi-2D DJ MHP in the film is highly oriented and the deactivation of the photoexcited charge carriers (mostly excitons) is through a radiative recombination pathway assisted by the quantum confinement in the low dimensional phases. This fast deactivation of the excited state prevents a rapid extraction of the charges from the material and therefore, the short circuit current (J_sc) in the solar devices prepared with this perovskite is low, J_sc = 9.03 mA·cm^-2.

On the contrary, the addition of the MASCN additive to the initial precursor solution modifies the distribution of the phases in the film. Thus, the low dimensional phases are scarce now but phases with higher dimensionality are more present in these films. This is clearly demonstrated by the absence of steady-state and transient absorption features of the low dimensional phases. Moreover, the PL decays display a much longer time constant, which is a hint for a less intense quantum confinement in these samples. The reduction of the defect concentration calculated with the space-charge limited current measurements suggests that the excess of the stilbene derivative is localized in between the perovskite grain passivating the superficial defects and, therefore, reducing the non-radiative recombination pathway.

Scintillating materials aim to detect ionizing radiations and are currently widely used in many detection systems addressing different fields, such as medical imaging, homeland security, high energy physics (HEP) calorimetry, industrial control, and oil drilling exploration. Quality criteria for these materials span over several parameters, three of which are of primary importance: the scintillation yield, the density, and the timing response. In the case of the interaction with a high-energy photon such as X-ray or gamma ray, the time response shows a very complex structure in the multi time-scale regime, making it critical for several applications. Solution-processable perovskite scintillators have been shown to be the solution for the replacements for the current expensive lanthanide scintillators as they share the same or even better properties for state-of-the-art imaging and detection applications. As examples, time-of-flight (TOF) functionalities require time resolution below 100 ps, coincidence techniques often need sub-tens of ns time response [1], counting regime detection prefer sub-μs time response, and afterglow over ms is detrimental for X-ray imaging [2]. Among all perovskite materials, two-dimensional lead halide perovskites have shown remarkable environmental and thermal stability, a large Stokes’ shift, usually coupled with very broad emission compared to their three-dimensional and quantum dot counterparts [3]. Here, we will show the progress for the research towards those applications since the beginning of our activities with perovskite scintillators. Moreover, we will discuss our approaches to tackle problems in some perovskite materials through energy sharing concept and nanophotonic structures. The latter will bring faster and brighter scintillators through Purcell enhancements while we will demonstrate how we can reach this goal through photonic crystals and plasmonic structures [4.5]. Such visions will pave the way towards new research directions and applications on the high-energy physics and nanophotonics interactions.

The efficiencies of tin-lead alloyed perovskite solar cell and lead-free tin perovskite solar cells are now 24-25% and 15-16%, respectively, which are still lower than that of lead perovskite solar cells.  Besides the efficiency enhancement of these tin-based solar cells, stability improvement is another important research theme. In this presentation we focuse on the stability improvement of these tin-based perovskite solar cells. We learnt from the stability study of lead perovskite solar cells that ion-migration has to be suppressed for the improvement of stability. To do this for the tin-lead alloyed perovskite solar cells, Ge ions which finally cover the grain boundary as GeOx, were added to the perovskite layer. In addition, ALD SnOx layer was inserted between the electron-transporting layer (Fullerenes).  The stability of the tin-lead alloyed perovskite solar cells was improved drastically.  The conventional composition consisting of FTO/PEDOT-PSS/SnPb-PVK/PCBM/C60/BCP/Ag degraded to 40% of the initial efficiency after the solar cell was put in the 85 ℃ under N2 atmosphere for 200 h. The efficiency decrease of the improved solar cell was suppressed to around 5% of the initial efficiency after the sample was kept for 1000h in the same condition. It is well-known that PEDOT-PSS frequently employed as the hole-transporting layers damages the tin-based perovskite layer by the proton migration. The proton migration is somehow retarded by the GeOx. The ALD SnOx layer suppressed the iodine migration to the electron-transporting layer and the Ag electrode. In the same way, the thermal stability of the lead-free tin perovskite solar cell was improved.  The efficiency of the conventional solar cells consisting of FTO/PEDOT-PSS/Sn-perovskite/C60/BCP/Ag decreased to 20% of the initial efficiency after the sample was put in the 85 ℃ under N2 atmosphere for 100 h. The decrease of the improved sample was suppressed to 20% after the sample was kept for 400 h in the same condition.  It was proved that the thermal stability of the tin-based perovskite solar cells is improved by suppressing the ion migration, which is similar to that of lead perovskite solar cells.

Tin-based halide perovskites (THPs) have emerged as promising candidates for both photovoltaics and near - IR light emitting applications thanks to their high carrier mobilities, low band gap, ideal for pairing with silicon in tandem solar cells^[1,2]. THPs present highly stable acceptor defects such as Sn vacancies and I interstitials, which result in a permanent population of holes: the material than behaves as a p-doped semiconductor, with self-doping densities as high as 10²² cm^-3 that can negatively impact device performances. As such, it is of critical importance to devise strategies to both quantify and control the dopant hole densities in THPs by compensating the oxidation state of Sn during material fabrication using additives such as SnF₂.

In this context, terahertz (THz) spectroscopy represents a powerful tool to characterize the carrier populations and dynamics in THPs. THz radiation is sensitive to mobile charged carriers^[3], as well as their coupling with lattice phonons^[4]: it can then be used to shed light on charge transport properties in THPs, as low frequency phonons in the THz range have been shown to limit their thermal and electrical conductivity. Moreover, as the doping density affects both transparency to THz radiation and carrier phonon coupling, THz absorption can be used as a sensitive, contactless probe to characterize the self – doping density even in samples where it has been brought down to levels comparable to background carrier densities suitable for device applications (e.g. 10¹⁵ cm^-3), and that could be challenging to characterize by traditional Hall effect measurements. Furthermore, time resolved THz spectroscopy after optical excitation of the material allows to follow the dynamics of photogenerated carriers with ps temporal resolution.

Here we develop a robust technique to study the doping hole density in FACsSnI₃ thin films by analyzing the static THz conductivity response, and characterize ultrafast carrier dynamics using time resolved THz spectroscopy. By supporting our results with DFT calculations, we also investigate the effect of doping concentration and defect states on optical phonon frequencies and their coupling to charge carriers. Finally, the possibility of studying the material polaronic response by combining THz and XUV absorption spectroscopy will be discussed.