Local Structure of Multinary Hybrid Lead Halide Perovskites Investigated by Nuclear Quadrupole Resonance Spectroscopy

Hybrid organic-inorganic lead halide perovskites have emerged as popular low-cost, solution-processable semiconductors owing to their outstanding optical and electronic characteristics. The structural characterization of these soft solids has proven challenging due to increased structural dynamics and static disorder. Nuclear quadrupole resonance (NQR) may serve as a powerful tool for probing the local structure of the halide anions in bulk and nanocrystalline forms of these perovskites. Here, we characterize the atomic structure of mixed-cation and mixed-anion solid solutions derived from cubic formamidinium lead iodide (FAPbI(3)), typically used in photovoltaics and photodetectors, using I-127 NQR. Even minute quantities of Cs ions on FA sites or Br ions on I sites lead to a multitude of different local environments, readily seen by NQR spectroscopy. At 10% Cs, signatures of Cs-ion clustering arise, pointing to the onset of the phase separation, observed by diffraction methods at higher Cs contents. On the contrary, Br-ion distribution appears homogeneous across all Br-to-I ratios. We anticipate that halide NQR studies will be indispensable for understanding the atomistic structure of diverse multinary metal halides, also in their diverse forms.from single crystals to thin films and nanocrystals.ISSN:0897-475

On the Mechanism of Alkylammonium Ligands Binding to the Surface of CsPbBr3 Nanocrystals

CsPbBr3 nanocrystals (NCs) suffer from instabilities caused by the dynamic and labile nature of both the inorganic core and the organic-inorganic interface. Surface ligand engineering thus remains an imminent research topic. In this study, classical molecular dynamics simulations with an explicit solvent are used to gain insights into the inherent binding properties of three different alkylammonium ligands-primary dodecylammonium (DA), secondary didodecylammonium (DDA), and quaternary dimethyldi-dodecylammonium (DMDDA). Our simulations uncover three main factors that govern the effective ligand-substrate interactions: (i) the ability of the head-group to penetrate into the binding pocket, (ii) the strength of head-group interactions with the polar solvent, and (iii) the higher barrier for ligand adsorption/desorption in the case of multiple alkyl chains. The interplay between these factors causes the following order of the binding free energies: DDA < DA approximate to DMDDA, while surface capping with DDA and DMDDA ligands is additionally stabilized by the kinetic barrier. These findings are in agreement with previous experimental observations and with the results of presented ligand-exchange experiments, wherein DDA is found to loosely bind to the CsPbBr3 surface, while DMDDA capping is more stable than capping with the primary oleylammonium ligand. The presented mechanistic understanding of the ligand-NC interactions will aid in the design of cationic ligands that make perovskite NC surfaces more robust.ISSN:0897-475

Bis(acyl)phosphide - Ambidentate Ligands for the Synthesis of Group 14 and 15 Main Group Element Compounds

The reactivity of the bis(acyl)phosphide ion [P(COR)2]− (BAP−, R=Ph, Mes) with silicon halides SiX4 (X=Cl, Br) and pnictogen chlorides ECl3 (E=As, Sb and Bi) was investigated. The reaction with SiX4 leads to the hexacoordinate silanes SiX2(BAP)2 in which BAP− is coordinated in the chelating κ2-O,O′ mode, analogously to acac−. Unexpectedly, the coordination behaviour of BAP− differs from the one of acac− in the interpnictogen compounds E(BAP)3 (E=As, Sb) in which the formation of E−P bonds is favoured over κ2-O,O′ chelation via the oxygen centres. Finally, the reaction of BiCl3 with three equivalents of Na(BAP) leads to the formation of red, crystalline Bi2(BAP)4, an air stable dibismuthine, as product of a redox reaction.ISSN:2192-650

Colloidal CsPbX3 Nanocrystals with Thin Metal Oxide Gel Coatings

Lead halide perovskite (LHP) nanocrystals (NCs) have gathered much attention as light-emitting materials, particularly owing to their excellent color purity, band gap tunability, high photoluminescence quantum yield (PLQY), low cost, and scalable synthesis. To enhance the stability of LHP NCs, bulky strongly bound organic ligands are commonly employed, which counteract the extraction of charge carriers from the NCs and hinder their use as photoconductive materials and photocatalysts. Replacing these ligands with a thin coating is a complex challenge due to the highly dynamic ionic lattice, which is vulnerable to the commonly employed coating precursors and solvents. In this work, we demonstrate thin (<1 nm) metal oxide gel coatings through non-hydrolytic sol-gel reactions. The coated NCs are readily dispersible and highly stable in short-chain alcohols while remaining monodisperse and exhibiting high PLQY (70-90%). We show the successful coating of NCs in a wide range of sizes (5-14 nm) and halide compositions. Alumina-gel-coated NCs were chosen for an in-depth analysis, and the versatility of the approach is demonstrated by employing zirconia-and titaniabased coatings. Compact films of the alumina-gel-coated NCs exhibit electronic and excitonic coupling between the NCs, leading to two orders of magnitude longer photoluminescence lifetimes (400-700 ns) compared to NCs in solution or their organically capped counterparts. This makes these NCs highly suited for applications where charge carrier delocalization or extraction is essential for performance.ISSN:0897-475

Pressure‐Induced Perovskite‐to‐non‐Perovskite Phase Transition in CsPbBr3

The expanding range of optoelectronic applications of lead‐halide perovskites requires their production in diverse forms (single crystals, thin‐ and thick‐films or even nanocrystals), motivating the development of diverse materials processing and deposition routes that are specifically suited for these structurally soft, low‐melting semiconductors. Pressure‐assisted deposition of compact pellets or thick‐films are gaining popularity, necessitating studies on the pressure effects on the atomic structure and properties of the resulting material. Herein we report the phase transformation in bulk polycrystalline cesium lead bromide from its three‐dimensional perovskite phase (γ‐CsPbBr3) into the one‐dimensional polymorph (δ‐CsPbBr3) upon application of hydrostatic pressure (0.35 GPa). δ‐CsPbBr3 is characterized by a wide bandgap of 2.9 eV and broadband yellow luminescence at 585 nm (2.1 eV) originating from self‐trapped excitons. The formation of δ‐CsPbBr3 was confirmed and characterized by Raman spectroscopy, 207Pb and 133Cs solid‐state nuclear magnetic resonance, X‐ray diffraction, absorption spectroscopy, and temperature‐dependent and time‐resolved photoluminescence spectroscopy. No such phase transition was observed in colloidal CsPbBr3 nanocrystals.ISSN:0018-019XISSN:1522-267

Lead-Halide Scalar Couplings in 207Pb NMR of APbX3 Perovskites (A = Cs, Methylammonium, Formamidinium; X = Cl, Br, I)

Understanding the structure and dynamics of newcomer optoelectronic materials - lead halide perovskites APbX3 [A = Cs, methylammonium (CH3NH3+, MA), formamidinium (CH(NH2)2+, FA); X = Cl, Br, I] - has been a major research thrust. In this work, new insights could be gained by using 207Pb solid-state nuclear magnetic resonance (NMR) spectroscopy at variable temperatures between 100 and 300 K. The existence of scalar couplings 1JPb-Cl of ca. 400 Hz and 1JPb-Br of ca. 2.3 kHz could be confirmed for MAPbX3 and CsPbX3. Diverse and fast structure dynamics, including rotations of A-cations, harmonic and anharmonic vibrations of the lead-halide framework and ionic mobility, affect the resolution of the coupling pattern. 207Pb NMR can therefore be used to detect the structural disorder and phase transitions. Furthermore, by comparing bulk and nanocrystalline CsPbBr3 a greater structural disorder of the PbBr6-octahedra had been confirmed in a nanoscale counterpart, not readily captured by diffraction-based techniques.ISSN:2045-232

White CsPbBr3: Characterizing the One‐Dimensional Cesium Lead Bromide Polymorph

Inorganic lead halide perovskites have gained immense scientific interest for optoelectronic applications. In this work, we present a one‐dimensional polymorph of cesium lead bromide (δ‐CsPbBr3) synthesized through a simple anion‐exchange reaction, wherein distorted edge‐sharing PbBr6 octahedra form 1D chains isolated by Cs ions. δ‐CsPbBr3 was characterized by Raman spectroscopy, X‐ray diffraction, 207Pb and 133Cs solid‐state NMR, and by optical emission and absorption spectroscopies. This non‐perovskite material irreversibly transforms into the well‐known three‐dimensional perovskite phase (γ‐CsPbBr3) upon heating to above 151 °C. The indirect bandgap was determined by absorption measurements and calculation to be 2.9 eV. δ‐CsPbBr3 exhibits broadband yellow photoluminescence with a quantum yield of 3.2 %±0.2 % at room temperature and 95 %±5 % at 77 K, and this emission is attributed to the recombination of self‐trapped excitons. This study emphasizes that the metastable δ‐CsPbBr3 may be a persistent, concomitant phase in Cs−Pb‐Br‐containing materials systems, such as those used in solar cells and LEDs, and it showcases the characterization tools used for its detection.ISSN:0018-019XISSN:1522-267

White CsPbBr 3

Inorganic lead halide perovskites have gained immense scientific interest for optoelectronic applications. In this work, we present a one‐dimensional polymorph of cesium lead bromide (δ‐CsPbBr3) synthesized through a simple anion‐exchange reaction, wherein distorted edge‐sharing PbBr6 octahedra form 1D chains isolated by Cs ions. δ‐CsPbBr3 was characterized by Raman spectroscopy, X‐ray diffraction, 207Pb and 133Cs solid‐state NMR, and by optical emission and absorption spectroscopies. This non‐perovskite material irreversibly transforms into the well‐known three‐dimensional perovskite phase (γ‐CsPbBr3) upon heating to above 151 °C. The indirect bandgap was determined by absorption measurements and calculation to be 2.9 eV. δ‐CsPbBr3 exhibits broadband yellow photoluminescence with a quantum yield of 3.2 %±0.2 % at room temperature and 95 %±5 % at 77 K, and this emission is attributed to the recombination of self‐trapped excitons. This study emphasizes that the metastable δ‐CsPbBr3 may be a persistent, concomitant phase in Cs−Pb‐Br‐containing materials systems, such as those used in solar cells and LEDs, and it showcases the characterization tools used for its detection.ISSN:0018-019XISSN:1522-267

The Rb7Bi3−3xSb3xCl16 Family: A Fully Inorganic Solid Solution with Room‐Temperature Luminescent Members

Low‐dimensional ns2‐metal halide compounds have received immense attention for applications in solid‐state lighting, optical thermometry and thermography, and scintillation. However, these are based primarily on the combination of organic cations with toxic Pb2+ or unstable Sn2+, and a stable inorganic luminescent material has yet to be found. Here, the zero‐dimensional Rb7Sb3Cl16 phase, comprised of isolated [SbCl6]3− octahedra and edge‐sharing [Sb2Cl10]4− dimers, shows room‐temperature photoluminescence (RT PL) centered at 560 nm with a quantum yield of 3.8±0.2 % at 296 K (99.4 % at 77 K). The temperature‐dependent PL lifetime rivals that of previous low‐dimensional materials with a specific temperature sensitivity above 0.06 K−1 at RT, making it an excellent thermometric material. Utilizing both DFT and chemical substitution with Bi3+ in the Rb7Bi3−3x Sb3x Cl16 (x ≤1) family, we present the edge‐shared [Sb2Cl10]4− dimer as a design principle for Sb‐based luminescent materials.ISSN:1433-7851ISSN:1521-3773ISSN:0570-083

Guanidinium-Formamidinium Lead Iodide: A Layered Perovskite-Related Compound with Red Luminescence at Room Temperature

Two-dimensional hybrid organic–inorganic lead halides perovskite-type compounds have attracted immense scientific interest due to their remarkable optoelectronic properties and tailorable crystal structures. In this work, we present a new layered hybrid lead halide, namely [CH(NH2)2][C(NH2)3]PbI4, wherein puckered lead-iodide layers are separated by two small and stable organic cations: formamidinium, CH(NH2)2+, and guanidinium, C(NH2)3+. This perovskite is thermally stable up to 255 °C, exhibits room-temperature photoluminescence in the red region with a quantum yield of 3.5%, and is photoconductive. This study highlights a vast structural diversity that exists in the compositional space typically used in perovskite photovoltaics.ISSN:0002-7863ISSN:1520-512