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The metal halide (BX 6 ) 4- octahedron, where B represents a metal cation and X represents a halide anion, is regarded as the fundamental structural and functional unit of metal halide perovskites. However, the influence of the way the (BX 6 ) 4- octahedra connect to each other has on the structural stability and optoelectronic properties of metal halide perovskite is still unclear. Here, the octahedral connectivity, including corner-, edge-, and face-sharing, of various Cs x FA 1-x PbI 3 (0 ≤ x ≤ 0.3) perovskite films is tuned and reliably characterized through compositional and additive engineering, and with ultralow-dose transmission electron microscopy. It is found that the overall solar cell device performance, the charge carrier lifetime, the open-circuit voltage, and the current density-voltage hysteresis are all improved when the films consist of corner-sharing octahedra, and non-corner sharing phases are suppressed, even in films with the same chemical composition. Additionally, it is found that the structural, optoelectronic, and device performance stabilities are similarly enhanced when non-corner-sharing connectivities are suppressed. This approach, combining macroscopic device tests and microscopic material characterization, provides a powerful tool enabling a thorough understanding of the impact of octahedral connectivity on device performance, and opens a new parameter space for designing high-performance photovoltaic metal halide perovskite devices., (© 2024 Wiley‐VCH GmbH.)

Transmission electron microscopy (TEM) is a powerful tool for unveiling the structural, compositional, and electronic properties of organic-inorganic hybrid perovskites (OIHPs) at the atomic to micrometer length scales. However, the structural and compositional instability of OIHPs under electron beam radiation results in misunderstandings of the microscopic structure-property-performance relationship in OIHP devices. Here, ultralow dose TEM is utilized to identify the mechanism of the electron-beam-induced changes in OHIPs and clarify the cumulative electron dose thresholds (critical dose) of different commercially interesting state-of-the-art OIHPs, including methylammonium lead iodide (MAPbI 3 ), formamidinium lead iodide (FAPbI 3 ), FA 0.83 Cs 0.17 PbI 3 , FA 0.15 Cs 0.85 PbI 3 , and MAPb 0.5 Sn 0.5 I 3 . The critical dose is related to the composition of the OIHPs, with FA 0.15 Cs 0.85 PbI 3 having the highest critical dose of ≈84 e Å -2 and FA 0.83 Cs 0.17 PbI 3 having the lowest critical dose of ≈4.2 e Å -2 . The electron beam irradiation results in the formation of a superstructure with ordered I and FA vacancies along <110> c , as identified from the three major crystal axes in cubic FAPbI 3 , <100> c , <110> c , and <111> c . The intragrain planar defects in FAPbI 3 are stable, while an obvious modification is observed in FA 0.83 Cs 0.17 PbI 3 under continuous electron beam exposure. This information can serve as a guide for ensuring a reliable understanding of the microstructure of OIHP optoelectronic devices by TEM., (© 2023 Wiley-VCH GmbH.)

Deciphering the atomic and electronic structures of interfaces is key to developing state-of-the-art perovskite semiconductors. However, conventional characterization techniques have limited previous studies mainly to grain-boundary interfaces, whereas the intragrain-interface microstructures and their electronic properties have been much less revealed. Herein using scanning transmission electron microscopy, we resolved the atomic-scale structural information on three prototypical intragrain interfaces, unraveling intriguing features clearly different from those from previous observations based on standalone films or nanomaterial samples. These intragrain interfaces include composition boundaries formed by heterogeneous ion distribution, stacking faults resulted from wrongly stacked crystal planes, and symmetrical twinning boundaries. The atomic-scale imaging of these intragrain interfaces enables us to build unequivocal models for the ab initio calculation of electronic properties. Our results suggest that these structure interfaces are generally electronically benign, whereas their dynamic interaction with point defects can still evoke detrimental effects. This work paves the way toward a more complete fundamental understanding of the microscopic structure-property-performance relationship in metal halide perovskites.

Hybrid organic-inorganic perovskites have high potential as materials for solar energy applications, but their microscopic properties are still not well understood. Atomic-resolution scanning transmission electron microscopy has provided invaluable insights for many crystalline solar cell materials, and we used this method to successfully image formamidinium lead triiodide [CH(NH 2 ) 2 PbI 3 ] thin films with a low dose of electron irradiation. Such images reveal a highly ordered atomic arrangement of sharp grain boundaries and coherent perovskite/PbI 2 interfaces, with a striking absence of long-range disorder in the crystal. We found that beam-induced degradation of the perovskite leads to an initial loss of formamidinium [CH(NH 2 ) 2 + ] ions, leaving behind a partially unoccupied perovskite lattice, which explains the unusual regenerative properties of these materials. We further observed aligned point defects and climb-dissociated dislocations. Our findings thus provide an atomic-level understanding of technologically important lead halide perovskites., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

Terahertz radiation encompasses a wide band of the electromagnetic spectrum, spanning from microwaves to infrared light, and is a particularly powerful tool for both fundamental scientific research and applications such as security screening, communications, quality control, and medical imaging. Considerable information can be conveyed by the full polarization state of terahertz light, yet to date, most time-domain terahertz detectors are sensitive to just one polarization component. Here we demonstrate a nanotechnology-based semiconductor detector using cross-nanowire networks that records the full polarization state of terahertz pulses. The monolithic device allows simultaneous measurements of the orthogonal components of the terahertz electric field vector without cross-talk. Furthermore, we demonstrate the capabilities of the detector for the study of metamaterials., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

Metal halide perovskite semiconductors have the potential to enable low-cost, flexible, and efficient solar cells for a wide range of applications. Physical vapor deposition by co-evaporation of precursors is a method that results in very smooth and pinhole-free perovskite thin films and allows excellent control over film thickness and composition. However, for a deposition method to become industrially scalable, reproducible process control and high device yields are essential. Unfortunately, to date, the control and reproducibility of evaporating organic precursors such as methylammonium iodide (MAI) have proved extremely challenging. We show that the established method of controlling the evaporation rate of MAI with quartz microbalances (QMBs) is critically sensitive to the concentration of the impurities MAH 2 PO 3 and MAH 2 PO 2 that are usually present in MAI after synthesis. Therefore, controlling the deposition rate of MAI with QMBs is unreliable since the concentration of such impurities typically varies from one batch of MAI to another and even during the course of a deposition. However once reliable control of MAI deposition is achieved, we find that the presence of precursor impurities during perovskite deposition does not degrade the solar cell performance. Our results indicate that as long as precursor deposition rates are well controlled, physical vapor deposition will allow high solar cell device yields even if the purity of precursors changes from one run to another.

Organic-inorganic hybrid perovskites, such as CH 3 NH 3 PbI 3, have shown highly promising photovoltaic performance. Electron microscopy (EM) is a powerful tool for studying the crystallography, morphology, interfaces, lattice defects, composition, and charge carrier collection and recombination properties at the nanoscale. Here, the sensitivity of CH 3 NH 3 PbI 3 to electron beam irradiation is examined. CH 3 NH 3 PbI 3 undergoes continuous structural and compositional changes with increasing electron dose, with the total dose, rather than dose rate, being the key operative parameter. Importantly, the first structural change is subtle and easily missed and occurs after an electron dose significantly smaller than that typically applied in conventional EM techniques. The electron dose conditions under which these structural changes occur are identified. With appropriate dose-minimization techniques, electron diffraction patterns can be obtained from pristine material consistent with the tetragonal CH 3 NH 3 PbI 3 phases determined by X-ray diffraction. Radiation damage incurred at liquid nitrogen temperatures and using Ga + irradiation in a focused ion beam instrument are also examined. Finally, some simple guidelines for how to minimize electron-beam-induced artifacts when using EM to study hybrid perovskite materials are provided., (© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Organo-lead halide perovskites (OHPs) have recently emerged as a new class of exceptional optoelectronic materials, which may find use in many applications, including solar cells, light emitting diodes, and photodetectors. More complex applications, such as lasers and electro-optic modulators, require the use of monocrystalline perovskite materials to reach their ultimate performance levels. Conventional methods for forming single crystals of OHPs like methylammonium lead bromide (MAPbBr 3 ) afford limited control over the product morphology, rendering the assembly of defined microcavity nanostructures difficult. We overcame this by synthesizing for the first time (MA)[PbBr 3 ]⋅DMF (1), and demonstrating its facile transformation into monocrystalline MAPbBr 3 microplatelets. The MAPbBr 3 microplatelets were tailored into waveguide based photonic devices, of which an ultra-low propagation loss of 0.04 dB μm -1 for a propagation distance of 100 μm was demonstrated. An efficient active electro-optical modulator (AEOM) consisting of a MAPbBr 3 non-linear arc waveguide was demonstrated, exhibiting a 98.4 % PL intensity modulation with an external voltage of 45 V. This novel synthetic approach, as well as the demonstration of effective waveguiding, will pave the way for developing a wide range of photonic devices based on organo-lead halide perovskites., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Organic-inorganic hybrid perovskites are exciting candidates for next-generation solar cells, with CH 3 NH 3 PbI 3 being one of the most widely studied. While there have been intense efforts to fabricate and optimize photovoltaic devices using CH 3 NH 3 PbI 3 , critical questions remain regarding the crystal structure that governs its unique properties of the hybrid perovskite material. Here we report unambiguous evidence for crystallographic twin domains in tetragonal CH 3 NH 3 PbI 3 , observed using low-dose transmission electron microscopy and selected area electron diffraction. The domains are around 100-300 nm wide, which disappear/reappear above/below the tetragonal-to-cubic phase transition temperature (approximate 57 °C) in a reversible process that often 'memorizes' the scale and orientation of the domains. Since these domains exist within the operational temperature range of solar cells, and have dimensions comparable to the thickness of typical CH 3 NH 3 PbI 3 films in the solar cells, understanding the twin geometry and orientation is essential for further improving perovskite solar cells.