In this online seminar, we want to discuss state of the art metal halides, beyond those of the traditional lead halide perovskites This includes double perovskites (A₂BCX₆), vacancy ordered perovskites (like A₃B₂X₉ and A₂BX₆), lower dimensional metal halides and metal chalcohalides. We will address the current challenges of making these materials, their interesting properties, like self-trapped exciton (STE) emissions, and their use in applications such as lead free metal halide based solar cells. We will also discuss how new chemistry can lead to the discovery of new metal halide structures with different properties to those that are exhibited in traditional lead halide perovskites. Finally, we will address how density-functional theory (DFT) calculations can help us to understand the luminescent properties and defects in these materials, which, in turn can provide a better passivation of defects.

Halide perovskite semiconductors can merge the highly efficient operational principles of conventional inorganic semiconductors with the low‑temperature solution processability of emerging organic and hybrid materials, offering a promising route towards cheaply generating electricity as well as light. Following a surge of interest in this class of materials, research on halide perovskite nanocrystals as well has gathered momentum in the last years. In such a narrow time span, several properties/features of halide perovskite nanocrystals were investigated, among them electroluminescence, lasing, anion-exchange, as well as control of size and shape. While most of the emphasis has been put on CsPbX₃ perovskites, the toxicity of lead has driven the search for non-toxic alternative perovskite (or perovskite-related) NC materials featuring a bright PL emission and, at the same time, a high stability. In this regard, the so-called double perovskite NCs, having chemical formula A⁺₂B⁺B³⁺X₆, have been identified as possible alternative materials, together with various other metal halides, possibly doped with various elements. This brief talk will therefore highlight the research efforts of our group on these materials. Finally, we will introduce a new class of colloidal nanocrystals, made of lead chalcohalides. These are kinetically trapped metastable nanostructures, which in the bulk were only obtained under high-pressure conditions (Pb₄S₃I₂), in traces (Pb₃S₂Cl₂), or that were never obtained at all to date (Pb₄S₃Br₂). Albeit not emissive, they might find applications in photodetectors and solar cells, as shown by preliminary tests in our labs.

Colloidal inorganic metal halide perovskite nanocrystals (NCs) are characterized by a large surface-to-volume ratio that renders them extremely sensible to surface processes. Passivating ligands, employed to stabilize NCs in an organic solvent, play thus an important role in influencing their structure and optoelectronic properties.

A leap forward in solving the above issues is to analyze the surface using first principle simulations, such as Density Functional Theory (DFT). Until now some of the major drawbacks of this approach have been: (i) the size of the system that can be handled that in the best cases is restrained to a few hundredths atoms (i.e. a small sized NC surrounded by short ligands), and (ii) the description of static properties with the absence of dynamic effects.

Here, I show the first multiscale modeling of real sized CsPbBr3 NCs (about 9.0 nm) passivated with oleate ligands and immersed in hexane with a simulation box containing a million of atoms. Molecular dynamics simulations, carried out up to the nanoscale timescale, provide crucial insights on the surface dynamics, and the role of the ligands on the structural optoelectronic properties of these materials.  

Halide perovskites, AMX₃ (A⁺ = Cs, CH₃NH₃, HC(NH₂)₂; M²⁺ = Ge, Sn, Pb and X- = Cl, Br, I), is an emerging class of high-performance semiconductors which operate in the visible and infrared energy range (1.1-3.0 eV). The remarkable physical properties of the halide perovskites stem from their unique electronic structure, which originates from the characteristic arrangement of the ions in the perovskite structure-type. The electronic structure of the halide perovskites is responsible for the observed high absorption coefficients and charge-carrier mobilities, which are comparable to the venerable III-V semiconductors.

A major challenge in halide perovskites relates to the limited perovskite structure formability due to the strict limitations imposed by the tolerance factor (t) which necessitates the halide perovskites to form only when Cs, CH₃NH₃ or HC(NH₂)₂ cations are used as templating A⁺ cations. These restrictions significantly impact the synthetic toolbox for designing new materials with desired target properties. Thus, exploring new synthetic directions that can modify the crystal structure within the perovskite structure limits presents itself as a great challenge in the field. Using small organic cations that can potentially stabilize the perovskite structure, a wide variety of perovskite-related structures can be obtained. Interesting compounds such as perovskite polytypes or unusual metal halide frameworks (so-called “perovskitoids”) and unconventional dimensionally reduced perovskites. The thus obtained compounds are amenable to property tuning governed by specific structural features as these emerge from the three-dimensional connectivity of the octahedral [MX3]- building blocks. Understanding and controlling these structural features in a designed manner will open the way for the discovery of novel perovskite-based semiconductors which may serve as potent substitutes for the currently employed halide perovskite materials.

Lead-free double perovskites such as Cs₂AgBiBr₆ are gaining attention because of their environmental friendliness compared to the lead halide perovskites. The Cs₂BiAgBr₆ QDs exhibit rich excited state dynamics and when adsorbed onto semiconductor oxides they induce ultrafast charge injection process. Whereas the electron transport layer such as TiO₂ or ZnO facilitate transport of electrons to the collecting surface, it can also indirectly induce surface oxidation of QDs if holes are allowed to accumulate. Under steady state photolysis (ambient conditions) the electrons injected into TiO₂ are scavenged by atmospheric oxygen, leaving behind holes which accumulate within the quantum dots (QDs). These accumulated holes further induce oxidation of QDs, resulting in the overall photodegradation of perovskite film. In order to establish their photoactivity, we have probed the excited state behavior of Cs₂AgBiBr₆ nanocrystals and charge injection from their excited state into different metal oxides (TiO₂, ZnO). The electron transfer rate constants determined from ultrafast transient absorption spectroscopy were in the range of 1.2–5.2 × 10¹⁰ s^-1_. Annealed films of Cs₂AgBiBr₆ nanocrystals, when employed as an active layer in solar cell, delivered photocurrent under visible light excitation. Although Cs₂BiAgBr₆ is attractive as lead free perovskite materials, the photovoltaic performance remains rather poor. New strategies are needed to implement these double perovskites in solar cells in a more effective way