Solid-state mechanochemical synthesis of multinary metal halide semiconductors for optoelectronics: From powder to thin film

En la última década, las perovskitas de haluro de plomo, así como otros haluros de metales múltiples, incluidas las alternativas sin plomo, han demostrado ser materiales prometedores para su uso en optoelectrónica. Por lo tanto, se buscan activamente nuevas formas de producir semiconductores de alta pureza a gran escala. Por tanto, el objetivo principal de esta tesis doctoral es el desarrollo de perovskitas y semiconductores relacionados utilizando métodos sin disolventes. Además, con la perspectiva del uso de dichos materiales en optoelectrónica a escala industrial, el foco está puesto en trabajar con materiales benignos para el desarrollo de alternativas a las perovskitas tóxicas. La síntesis mecanoquímica ha surgido recientemente como un método muy conveniente y confiable para obtener perovskitas de haluro de plomo de alta calidad, así como otros haluros metálicos multinarios sin plomo. Por lo tanto, esta tesis contribuye a un estudio de materiales para el desarrollo de perovskitas y materiales relacionados a través de una ruta mecanoquímica libre de solventes y la investigación de sus características fundamentales. Además, con el interés en la implementación de estos semiconductores de alta pureza en optoelectrónica, además de un estudio de materiales, esta tesis contribuye a la investigación de una forma novedosa para la fabricación de perovskitas de película delgada a través de la deposición al vacío de una sola fuente de perovskita sintetizada mecanoquímicamente. polvos. Como tal, este trabajo allana el camino para una nueva forma tanto de comprender la formación de perovskitas como de una forma alternativa para el desarrollo de películas delgadas con la perspectiva de la fabricación de aplicaciones en el campo de la optoelectrónica.In the past decade, lead halide perovskites, as well as other multinary metal halides – including lead-free alternatives – have shown to be promising materials for their use in optoelectronics. Therefore, new ways of producing high-purity semiconductors in large scale are actively sought after. Hence, the main aim of this doctoral thesis is the development of perovskites and related semiconductors using solvent-free methods. Furthermore, with the prospect of the use of such materials in optoelectronics on an industrial scale, the focus is put on working with benign materials for the development of alternatives to toxic perovskites. Mechanochemical synthesis has recently emerged as a highly convenient and reliable method to obtain high-quality lead halide perovskites, as well as other lead-free multinary metal halides. Hence, this thesis contributes to a material study for the development of perovskites and related materials via a solvent-free mechanochemical route and the investigation of the fundamental characteristics thereof. Furthermore, with the interest in the implementation of these high-purity semiconductors in optoelectronics – besides a material study – this thesis contributes to the investigation of a novel manner for the fabrication of thin film perovskites via single-source vacuum deposition of mechanochemically-synthesized perovskite powders. As such, this work paves the way to a new manner for both understanding the formation of perovskites, as well as an alternative way for thin film development with the prospect of the fabrication of applications in the field of optoelectronics

Making by Grinding: Mechanochemistry Boosts the Development of Halide Perovskites and Other Multinary Metal Halides

Mechanochemical synthesis has recently emerged as a promising route for the synthesis of functional lead halide perovskites as well as other (lead‐free) metal halides. Mechanochemical synthesis presents several advantages with regards to more commonly used solution‐based processes such as an inherent lower toxicity by avoiding organic solvents and a finer control over stoichiometry of the final products. The ease of implementation, either through the use of a simple mortar and pestle or with an electrically powered ball‐mill, and low amount of side products make mechanochemical synthesis appealing for upscaling the production of halide perovskites. Due to the defect tolerance of lead halide perovskites, they are ideally suited to be prepared by this solvent‐free method. However, the implementation of these semiconductors in high‐efficiency optoelectronic devices requires the transformation of synthesized powder into smooth thin films where still some hurdles remain to be cleared

Single-Source Vacuum Deposition of Mechanosynthesized Inorganic Halide Perovskites

Fully inorganic cesium lead halide perovskite thin films were prepared by an easy, fast and dry process based on single-source vacuum deposition. We investigated the structural and optical characteristics of the so-formed films as a function of chemical composition (chloride, bromide and iodide films were formed), post-deposition thermal annealing, as well as previous mechanosynthesis of perovskite powders. We found out that the CsPbX3 perovskite was preferentially formed for the smaller halides and favored by previous ball-milling of CsX and PbX2 precursors. When bigger halides were used and/or CsX and PbX2 precursors were simply mixed without previous mechanosynthesis, PbX2-rich compounds such as CsPb2X5 were preferentially formed in the thin films

Tunable Wide‐Bandgap Monohalide Perovskites

Herein the mechanochemical synthesis of inorganic as well as hybrid organic-inorganic monohalide perovskites with tunable bandgaps is reported. It is shown that the bandgap bowing known for iodide mixed Sn-Pb perovskites is also present in the pure bromide analogous. This results in technologically very interesting materials with bandgaps in the range of 1.7-1.9 eV. Similar bandgap perovskites are typically achieved by mixing two halides that are prone to segregate over time. This limits the achievable open circuit voltage. For monohalide perovskites this problem is eliminated, making these materials especially promising wide bandgap absorbers for tandem solar cells

Mechanochemical synthesis of inorganic halide perovskites: evolution of phase-purity, morphology, and photoluminescence

Dry mechanochemical ball-milling of halide precursor salts is a promising route for the synthesis of high-purity halide perovskites in a fast and solvent-free manner. However, there is a lack of information on the process mechanisms, kinetics, and possible side-effects. Here, we investigated in detail the mechanochemical synthesis of fully-inorganic CsPbBr3 by ball-milling of stoichiometric CsBr and PbBr2. Detailed structural, morphological and optical analyses reveal several beneficial and detrimental effects of milling as a function of time. Three stages are identified during the process: (i) at short milling times (t < 5 min) different ternary compounds are formed, including stoichiometric CsPbBr3 as well as Cs4PbBr6, and to a lesser extent, CsPb2Br5. Photoluminescence from "nano" and "bulk" CsPbBr3 species is observed, centered at 525 nm and 545 nm, respectively. (ii) At the optimum time (around 5 min for the present case) the complete transformation of all reactants and byproducts into phase-pure CsPbBr3 has occurred. Photoluminescence corresponds to bulk CsPbBr3; (iii) at much longer milling times (up to 10 hours) eventually smaller quantum-confined CsPbBr3 NCs are exfoliated from the bulk product leading to a broad and blue-shifted emission. At this stage the photoluminescence intensity is strongly reduced which is ascribed to the formation of surface defects induced by ball-milling in dry conditions

Low-dimensional non-toxic A3Bi2X9 compounds synthesized by a dry mechanochemical route with tunable visible photoluminescence at room temperature

We have synthesized fifteen inorganic and hybrid organic-inorganic non-toxic A3Bi2X9 compounds (A = K+, Rb+, Cs+, CH3NH3+ and HC(NH2)2+; X = I−, Br−, Cl−) through dry mechanochemistry. We demonstrate that this synthetic method is very well suited to prepare compounds from poorly soluble precursors, allowing thus the preparation of so far unreported compounds. X-ray diffraction analysis demonstrates the high crystallinity of the so-formed ternary bismuth halides. Furthermore, we show that, through substitution of the A-cation and X-anion, the bandgap of these compounds can be tuned to absorb throughout the whole visible spectrum. As-prepared powders of Cs3Bi2Br9 and Cs3Bi2I9 without any passivating agents show room-temperature photoluminescence covering the visible spectrum from 450 nm to 800 nm, making them especially promising for white-light emission

Mechanochemical Synthesis of Sn(II) and Sn(IV) Iodide Perovskites and Study of Their Structural, Chemical, Thermal, Optical and Electrical Properties

Phase‐pure CsSnI3, FASnI3, Cs(PbSn)I3, FA(PbSn)I3 perovskites (FA = formamidinium = HC(NH2)2+) as well as the analogous so‐called vacancy‐ordered double perovskites Cs2SnI6 and FA2SnI6 are mechanochemically synthesized. The addition of SnF2 is found to be crucial for the synthesis of Cs‐containing perovskites but unnecessary for hybrid ones. All compounds show an absorption onset in the near‐infrared (NIR) region, which makes them especially relevant for photovoltaic applications. The addition of Pb(II) and SnF2 is crucial to improve the electronic properties in 3D Sn(II)‐based perovskites, in particular their charge carriers mobility (≈0.2 cm2 Vs−1) which is enhanced upon reduction of the dark carrier conductivity. Stokes‐shifted photoluminescence is observed on dry powders of Sn(II)‐based perovskites, which makes these materials promising for light‐emitting and sensing applications. Thermal stability of all compounds is examined, revealing no significant degradation up to at least 200 °C. This meets the requirements for standard operation conditions of most optoelectronic devices and is potentially compatible with thermal vacuum deposition of polycrystalline thin films

Incorporation of potassium halides in the mechanosynthesis of inorganic perovskites: feasibility and limitations of ion-replacement and trap passivation

Potassium halides (KX; X = I, Br, or Cl) were incorporated as partial replacements of CsBr in the mechanosynthesis of CsPbBr3. This led to partial substitution of both monovalent ions forming mixed Cs1−xKxPbBr3−yXy perovskites. Longer photoluminescence lifetimes were also observed, possibly linked to the formation of a non-perovskite KPb2X5 passivating layer

Novel thin-film solid nanocomposite electrolyte for lithium-Ion batteries by combined MLD and ALD

This work reports on the first thin-film solid nanocomposite electrolyte (NCE) for lithium-ion batteries made by combining two gas-phase deposition techniques, molecular layer deposition (MLD) and atomic layer deposition (ALD). The advantage of using these techniques for the fabrication of NCEs comes from the ease of integration in thin-film batteries and the possibility to alter the properties of the oxide matrix and of the Li-compound independently. Moreover, the mesoporous oxide matrix based on an MLD and etching process, provides uniform interfaces with continuous conduction paths. An enhancement in Li-ion conductivity of two orders of magnitude, compared to the pure Li-compound, is obtained for an NCE consisting of a mesoporous Al2O3 matrix filled with Li2CO3. The impact of the oxide matrix on the resulting NCE Li-ion conductivity is considerable, showing four orders of magnitude difference between silica and alumina. Finally, integration of the NCE in a thin-film solid-state battery stack utilizing a Li-metal anode is demonstrated with good coulombic efficiency over a broad temperature range. The current findings and approach can expand the possibilities for development of novel thin-film solid-state electrolytes

Novel Thin-Film Solid Nanocomposite Electrolyte for Lithium-Ion Batteries by Combined MLD and ALD

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