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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Anomalous thickness-dependent electrical conductivity in van der Waals layered transition metal halide, Nb 3 Cl 8

Understanding the electronic transport properties of layered, van der Waals transition metal halides (TMHs) and chalcogenides is a highly active research topic today. Of particular interest is the evolution of those properties with changing thickness as the 2D limit is approached. Here, we present the electrical conductivity of exfoliated single crystals of the TMH, cluster magnet, Nb 3 Cl 8 , over a wide range of thicknesses both with and without hexagonal boron nitride (hBN) encapsulation. The conductivity is found to increase by more than three orders of magnitude when the thickness is decreased from 280 µm to 5 nm, at 300 K. At low temperatures and below ~50 nm, the conductance becomes thickness independent, implying surface conduction is dominating. Temperature dependent conductivity measurements indicate Nb 3 Cl 8 is an insulator, however, the effective activation energy decreases from a bulk value of 310 meV to 140 meV by 5 nm. X-ray photoelectron spectroscopy (XPS) shows mild surface oxidation in devices without hBN capping, however, no significant difference in transport is observed when compared to the capped devices, implying the thickness dependent transport behavior is intrinsic to the material. A conduction mechanism comprised of a higher conductivity surface channel in parallel with a lower conductivity interlayer channel is discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

High-Voltage, Intermediate-Temperature, Fe- and Al-Mixed Metal Halide Molten Salt for Molten Sodium Battery Energy Storage

An inorganic Fe and Al halide-based, low-temperature molten salt catholyte is described, which, when paired with a molten sodium anode, has high operating potentials rivaling those of Li ion batteries. The newly developed catholyte consists of metal halides FeCl 3 /FeCl 2 –AlCl 3 –NaCl and is intended to cycle between Fe 3+ /Fe 2+ redox couples in the molten salt. The multicomponent molten salt was initially evaluated for phase behavior and basic electrochemical behavior before full battery testing. The assembled battery, utilizing a 20:35:45 (FeCl 3 :AlCl 3 :NaCl) composition, with a 50.83 Ah/kg theoretical gravimetric capacity and a specific energy of 176.95 Wh/kg, was cycled at variable depths of discharge (DoD) and current densities to determine its cycling efficiencies and limitations. In conclusion, preliminary cycling tests showed two operational potential regimes, with higher potential, 3.91 V (vs Na/Na + ), at low DoD and lower potential, 3.39 V (vs Na/Na + ), at high DoD with excellent energy efficiencies and cycling behavior under both regimes.

Aluminum↗
DOE OSTI2021

Optically Induced Static Magnetization in Metal Halide Perovskite for Spin‐Related Optoelectronics

Abstract Understanding the feasibility to couple semiconducting and magnetic properties in metal halide perovskites through interface design opens new opportunities for creating the next generation spin‐related optoelectronics. In this work, a fundamentally new phenomenon of optically induced magnetization achieved by coupling photoexcited orbital magnetic dipoles with magnetic spins at perovskite/ferromagnetic interface is discovered. The depth‐sensitive polarized neutron reflectometry combined with in situ photoexcitation setup, constitutes key evidence of this novel effect. It is demonstrated that a circularly polarized photoexcitation induces a stable magnetization signal within the depth up to 7.5 nm into the surface of high‐quality perovskite (MAPbBr 3 ) film underneath a ferromagnetic cobalt layer at room temperature. In contrast, a linearly polarized light does not induce any detectable magnetization in the MAPbBr 3 . The observation reveals that photoexcited orbital magnetic dipoles at the surface of perovskite are coupled with the spins of the ferromagnetic atoms at the interface, leading to an optically induced magnetization within the perovskite’s surface. The finding demonstrates that perovskite semiconductor can be bridged with magnetism through optically controllable method at room temperature in this heterojunction design. This provides the new concept of utilizing spin and orbital degrees of freedom in new‐generation spin‐related optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2022

X-ray ptychographic tomography reveals buried 3D structural defects in metal halide perovskites

For the first of its kind, X-ray ptychographic imaging is utilized for probing metal halide perovskite crystals, revealing various 3D structural defects fully or partially buried underneath the perovskite surface. Here, this work demonstrates a powerful, non-intrusive 3D tomographic characterization method to illuminate the complete microstructure of perovskites for advanced energy applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Resolving spatial and energetic distributions of trap states in metal halide perovskite solar cells

We report the profiling of spatial and energetic distributions of trap states in metal halide perovskite single-crystalline and polycrystalline solar cells. The trap densities in single crystals varied by five orders of magnitude, with a lowest value of 2 × 10 11 per cubic centimeter and most of the deep traps located at crystal surfaces. The charge trap densities of all depths of the interfaces of the polycrystalline films were one to two orders of magnitude greater than that of the film interior, and the trap density at the film interior was still two to three orders of magnitude greater than that in high-quality single crystals. Suprisingly, after surface passivation, most deep traps were detected near the interface of perovskites and hole transport layers, where a large density of nanocrystals were embedded, limiting the efficiency of solar cells.

14 SOLAR ENERGY↗
DOE OSTI2022

Spatiotemporal Route to Understanding Metal Halide Perovskitoid Crystallization

Here, a spatiotemporal experimental route is reported for the antisolvent vapor diffusion crystal growth of metal halide perovskitoids. A computational analysis combining automated image capture and diffusion modeling enables the determination of the critical concentrations required for nucleation and crystal growth from a single experiment. Five different solvent systems and ten distinct organic ammonium iodide salts were investigated with lead iodide, from which nine previously unreported compounds were discovered. Automated image capture of the mother liquor and antisolvent vials was used to determine changes in solution meniscus positions and detect the nucleation event location. Matching the observations to a numerical solution of Fick's second law diffusion model enables the calculation of reactant, solvent, and antisolvent concentrations at both the time and position of the first stable nucleation and crystal growth. A machine learning model was trained on the resulting data, and it reveals solvent- and amine-specific crystallization tendencies. Solvent systems that interact more weakly with dissolved lead species promote crystallization, while those with stronger interactions can prevent crystallization through increased solubilities. Organic amines that interact more strongly with inorganic components and exhibit greater rigidity are more likely to be incorporated into crystalline products.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Unlocking the Dynamics of Ion Migration and Voltage Bias Stress Effects through Crystallite Engineering in Metal Halide Perovskites

Understanding the interplay between crystal engineering and the coupled electronic-ionic charge transport properties of metal halide perovskites remains a critical issue in the field. In this work, we developed an experimental approach to tune the crystallite orientation of methylammonium lead iodide (CH 3 NH 3 PbI 3 ) while maintaining their overall crystal structure. This approach allows us to selectively manipulate crystallite orientations to control out-ofplane ion migration and mitigate voltage bias stress effects in CH 3 NH 3 PbI 3 thin films. By employing advanced diffraction and spectroscopic techniques, we achieved a comprehensive characterization of the anisotropic crystallite properties in CH 3 NH 3 PbI 3 thin films with distinct preferred orientations. Our findings reveal that specific crystallite orientations, particularly those that limit halide ion migration pathways along the (200) crystallographic plane, significantly suppress out-of-plane ion migration. This suppression reduces hysteresis and alleviates voltage bias stress effects in CH 3 NH 3 PbI 3 solar cells, ultimately enhancing device stability and performance. These insights not only deepen our understanding of the relationship between crystallite orientation and device functionality but also highlight a promising strategy for regulating ion migration in MHP-based devices. This approach holds significant potential for advancing the stability and efficiency of perovskite solar cells and extending its applicability to other optoelectronic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Scalable Approaches to Address Thermomechanical and Chemical Instabilities in Metal Halide Perovskites

We will discuss our group’s recent efforts to address the thermomechanical and chemical instabilities of metal halide perovskites through multiple approaches. These approaches cover several length scales and aim to provide better understanding of behavior in all stages of processing—i.e., precursor behavior in inks, growth behavior of films during printing, and resulting optoelectronic properties and robustness of perovskite films and devices. We will highlight our efforts toward scalable printing of high-quality perovskite using the method recently invented by our group restricted area printing by ink drawing (RAPID). Additionally, we will discuss how molecular additives in precursor inks can influence the stability of the inks and resultant films. Finally, we will also describe our recent progress in addressing the poor mechanical toughness of perovskite using multifunctional nanofiber reinforcements. Together, all of these concepts are complimentary and compatible with scalable printing of perovskites and a necessary step towards developing a viable technology with long operational lifetimes.

Printz, Adam↗
DOE OSTI2020

Method to synthesize metal halide perovskite particles with high luminescence and stability

The invention provides a convenient solution-based synthesis for metal halide perovskite particles. This method requires only mild conditions and can be easily scaled up. A ligand of a proper molecule size and an optimized precursor ratio are important to obtain pure phase particles. As an example, lead bromide perovskite particles exhibited a narrow monochromatic band gap, which can be tuned by partial halide substitution, and PLQY up to 75% making this material an excellent candidate for light emission and display applications.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Organic Cation Influence on Organic–Inorganic Thermal Equilibration within 2D Metal Halide Perovskites

Two-dimensional inorganic–organic lead halide perovskites exhibit tunable optoelectronic properties that are dictated by alternating layers of metal halide octahedra and organic cations. In such hybrid materials, vibrational coupling and thermalization between the low mass, insulating, organic cation spacers, and high mass inorganic octahedra remains poorly understood. Here, in this study, using femtosecond infrared-pump electronic-probe (IPEP) spectroscopy, we investigate the role of organic cation identity regarding the kinetics of vibrational energy exchange and sublattice mechanical coupling. Linear aliphatic cation-containing 2D perovskites with 4, 6, or 8 carbons (butylammonium = BA, hexylammonium = HA, and octylammonium = OA) were produced as thin films and mid-infrared pump pulses and then selectively excited organic cation stretch vibrations. Thermal energy introduction was evaluated via visible-wavelength optical probing that conveys inorganic octahedra response, owing to changes in electronic absorption. Multiple distinct spectral shifts appear upon excitation of the organic cations. Following an initial optical Stark shift caused by pump–probe temporal overlap, a redshift occurs within the first 10 ps that we attribute to compression of the octahedra by the expanded organic layers. Subsequently, a blueshift of the bandgap occurs commensurate with equilibration of both sublattices at an elevated temperature with a time constant of 21.8 to 31.6 ps depending on the spacer identity, with thermal energy transfer slowing by nearly 50% for the longest linker. By examining the effect that altered cation identity has on vibrational energy exchange, this work begins to offer routes to tune nonequilibrium response as well as provides insight into the fundamental impacts of organic spacers on thermal energy exchange in perovskite materials.

Peifer, Shoshanna E. [Northwestern Univ., Evanston↗
DOE OSTI2024

Flexible Soft X-Ray Image Sensors based on Metal Halide Perovskites With High Quantum Efficiency

Soft X-ray imaging is a powerful tool to explore the structure of cells, probe material with nanometer resolution, and investigate the energetic phenomena in the universe. Conventional soft X-ray image sensors are by and large Si-based charge coupled devices that suffer from low frame rates, complex fabrication processes, mechanical inflexibility, and required cooling below -60 °C. Here, a soft X-ray photodiode is reported based on low-cost metal halide perovskite with comparable performance to commercial Si-based device. Nanothrough network electrode minimized the optical loss due to the shadowing of insensitive layers, while a multidimensional perovskite heterojunction is generated to reduce the photo-generated carrier loss. Further, this strategy promoted a record quantum efficiency of 8 × 10 3 % without cooling, several orders of magnitude greater than the previously achieved. Flexible and curved soft X-ray imaging arrays are fabricated based on this high-performance device structure, demonstrating stable soft X-ray response and sharp imaging capabilities. This work highlights the low-cost and efficient perovskite photodiode as a strong candidate for the next-generation soft X-ray image sensors.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Subsurface Characteristics of Metal-Halide Perovskites Polished by an Argon Ion Beam

Focused ion beam (FIB) techniques have been frequently used to section metal-halide perovskites for microstructural investigations. However, the ion beams directly irradiating the sample surface may alter its properties far different from those of pristine, potentially leading to modified deterioration mechanisms under aging stressors. Here, we combine complementary approaches to measure the subsurface characteristics of polished perovskites and identify the chemical species responsible for the measured properties. Analysis of the experimental results in conjunction with Monte Carlo simulations indicates that atomic displacements and local heating occur in the first ≈15 nm of the subsurface of methylammonium lead iodide (MAPbI 3 ) by glancing-angle Ar + beam irradiation (4 kV at 3°). The lead-rich, iodine-deficient surface promotes rapid phase segregation under thermal aging conditions. On the other hand, despite the subsurface modification, our experiments confirm that the rest of the MAPbI 3 bulk retains the material integrity. Our observation supports that polished perovskites could serve in studying the properties of bulk or buried junctions far away from the altered subsurface with care.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Control of light, spin and charge with chiral metal halide semiconductors

We report the relationship between the structural asymmetry and optoelectronic properties of functional materials is an active area of research. The movement of charges through an oriented chiral medium depends on the spin configuration of the charges, and such systems can be used to control spin populations without magnetic components - termed the chiral-induced spin selectivity (CISS) effect. CISS has mainly been studied in chiral organic molecules and their assemblies. Semiconductors are non-magnetic extended systems that allow for the control of charge transport, as well as the absorption and emission of light. Therefore, introducing chirality into semiconductors would enable control over charge, spin and light without magnetic components. Chiral metal halide semiconductors (MHSs) are hybrid organic-inorganic materials that combine the properties of small chiral organic molecules with those of extended inorganic semiconductors. Reports of CISS in chiral MHSs have resulted in breakthroughs in our understanding of CISS and in the realization of spin-dependent optoelectronic properties. This Review examines the fundamentals and applications of CISS in chiral MHSs. The structural diversity and key structure-property relationships, such as chiral transfer from the organic to the inorganic components, are summarized. With a focus on the underlying chemistry and physics, the control of spin, light and charge in these semiconductors is explored.

14 SOLAR ENERGY↗
DOE OSTI2020

Voltage bias stress effects in metal halide perovskites are strongly dependent on morphology and ion migration pathways

The solar-to-power power conversion efficiencies (PCEs) of metal halide perovskites (MHP) have improved over the last decade using a wide variety of methods, including composition manipulation, dopant introduction, and interfacial buffers. These methods, however, have taken little regard for the electronic and interfacial effects such alterations may cause within devices under voltage bias stress – a condition required for most device operation. We investigate how halide and cation substitution in MHP structures [specifically, CH 3 NH 3 PbI 2.87 Cl 0.13 and Cs 0.1 (MA 0.17 FA 0.83 ) 0.9 Pb(I 0.83 Br 0.17 ) 3 ] effects the current behavior of devices while under a range of voltage bias stress in both light and dark conditions. Additionally, conducting in depth investigations into the electronic and morphological differences between these two MHP devices, we confirmed their unique voltage bias stress effects are due to intrinsic behavior within the perovskite structure. Importantly, we also determined how crystallite orientation can influence ion migration and therefore alter charge transport and current stability in MHP photovoltaic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2023

How to GIWAXS: Grazing Incidence Wide Angle X‐Ray Scattering Applied to Metal Halide Perovskite Thin Films

Abstract The frequency of reports utilizing synchrotron‐based grazing incident wide angle X‐ray scattering (GIWAXS) to study metal halide perovskite thin films has exploded recently, as this technique has proven invaluable for understanding several structure‐property relationships that fundamentally limit optoelectronic performance. The GIWAXS geometry and temporal resolution are also inherently compatible with in situ and operando setups (including ISOS protocols), and a relatively large halide perovskite research community has deployed GIWAXS to unravel important kinetic and dynamic features in these materials. Considering its rising popularity, the aim here is to accelerate the required learning curve for new experimentalists by clearly detailing the underlying analytical concepts which can be leveraged to maximize GIWAXS studies of polycrystalline thin films and devices. Motivated by the vast range of measurement conditions offered, together with the wide variety of compositions and structural motifs available (i.e., from single‐crystal and polycrystalline systems, to quantum dots and layered superlatices), a comprehensive framework for conducting effective GIWAXS experiments is outlined for different purposes. It is anticipated that providing a clear perspective for this topic will help elevate the quality of future GIWAXS studies—which have become routine—and provide the impetus required to develop novel GIWAXS approaches to resolve unsettled scientific questions.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Recent Progress in Engineering Metal Halide Perovskites for Efficient Visible‐Light‐Driven Photocatalysis

Abstract Artificial photosynthesis has attracted increasing attention due to recent environmental and energy concerns. Metal halide perovskites (MHPs) demonstrating excellent optoelectronic properties have currently emerged as novel and efficient photocatalytic materials. Herein, the structural features of MHPs that are responsible for the photoinduced charge separation and charge migration properties are briefly introduced, and then important and necessary photophysical and photochemical aspects of MHPs related to photoredox catalysis are summarized. Subsequently, the applications of MHPs for solar energy harvesting and photocatalytic conversion, including H 2 evolution, CO 2 reduction, degradation of organic pollutants, and photoredox organic synthesis, are extensively demonstrated, with a focus on strategies for improving the performance (e.g., selectivity, activity, stability, recyclability, and environmental compatibility) of these MHP‐based photocatalytic systems. To conclude, existing challenges and prospects on the future development of MHP‐based materials towards photoredox catalysis applications are detailed.

Han, Chuang↗
DOE OSTI2019

Quasicubic model for metal halide perovskite nanocrystals

In this work, we present an analysis of quantum confinement of carriers and excitons, and exciton fine structure, in metal halide perovskite (MHP) nanocrystals (NCs). Starting with coupled-band k · P theory, we derive a nonparabolic effective mass model for the exciton energies in MHP NCs valid for the full size range from the strong to the weak confinement limits. We illustrate the application of the model to CsPbBr 3 NCs and compare the theory against published absorption data, finding excellent agreement. We then apply the theory of electron-hole exchange, including both short- and long-range exchange interactions, to develop a model for the exciton fine structure. We develop an analytical quasicubic model for the effect of tetragonal and orthorhombic lattice distortions on the exchange-related exciton fine structure in CsPbBr 3 , as well as some hybrid organic MHPs of recent interest, including formamidinium lead bromide (FAPbBr 3 ) and methylammonium lead iodide (MAPbI3). Testing the predictions of the quasicubic model using hybrid density functional theory (DFT) calculations, we find qualitative agreement in tetragonal MHPs but significant disagreement in the orthorhombic modifications. Moreover, the quasicubic model fails to correctly describe the exciton oscillator strength and with it the long-range exchange corrections in these systems. Introducing the effect of NC shape anisotropy and possible Rashba terms into the model, we illustrate the calculation of the exciton fine structure in CsPbBr 3 NCs based on the results of the DFT calculations and examine the effect of Rashba terms and shape anisotropy on the calculated fine structure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗