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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 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

In-situ Grazing-Incidence Wide-Angle Scattering Reveals Mechanisms for Phase Distribution and Disorientation in 2D Halide Perovskite Films

Two-dimensional (2D) hybrid halide perovskites with the formula (A') 2 (A) n-1 Pb n I 3n+1 have remarkable stability and promising efficiency in photovoltaic and optoelectronic devices, yet fundamental understanding of film formation, key to optimizing these devices, is lacking. In this study, in-situ grazing-incidence wide-angle X-ray scattering (GIWAXS) is used to monitor film formation during spin-coating. This elucidates the general film formation mechanism of 2D halide perovskites during one-step spin-coating. There are three stages of film formation: sol-gel, oriented 3D, and 2D. Three precursor phases form during the sol-gel stage and transform to perovskite, first giving a highly oriented 3D-like phase at the air/liquid interface followed by subsequent nucleations forming slightly less oriented 2D perovskite. Furthermore, heating before crystallization leads to fewer nucleations and faster removal of the precursors, improving orientation. This outlines the primary causes of phase distribution and perpendicular orientation in 2D perovskite films and paves the way for rationally designed film fabrication techniques.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Perspectives on intrinsic toughening strategies and passivation of perovskite films with organic additives

We report on the inclusion of 5-aminovaleric acid (5-AVA)—a bulky, long-chained organic molecule—as an additive to enhance the mechanical integrity of hybrid perovskite films. We studied a range of organic cation additives and found that 5-AVA was the most effective at improving cohesion energy, a key metric of mechanical reliability. MAPbI 3 films reinforced with 5-AVA added in 5% concentration increased cohesion energy 12-fold from 0.53 J/m 2 to 6.04 J/m 2 , an effect which is attributed to increased plasticity and crack deflection around grain boundaries of the additive-containing perovskite. The addition of 5-AVA also improves V oc in perovskite solar cells and carrier lifetimes with a minimal decrease in PCE, attributed to passivation reducing defect and trap densities. A Tauc plot analysis of the bandgap shows that 5-AVA increases the band gap of the perovskite, correlating with reduced film stress compared to MAPbI 3 . Finally, the usage of 5-AVA improves the intrinsic thermomechanical reliability, but this improvement comes at the penalty of slightly reduced device efficiency due to reduced charge extraction from the presence of the bulky, insulating organic additive.

14 SOLAR ENERGY↗
DOE OSTI2023

Support Effect Studied on Thin-Film Perovskites

Some functional oxides exhibit strong support effects but typically have low surface areas and stabilities that negate the promotional effects. The primary goal of our work in the past three years was to develop high-surface-area versions of these supports by Atomic Layer Deposition onto stable supports, then use these to understand how the oxide supports affect the metal catalytic properties. The primary application targeted for these materials was methane oxidation and related reactions for emissions control. Systems of particular interest were supported, precious metals on thin-film perovskites in order to develop “intelligent” catalysts in which the metal can reversibly ex-solve from perovskite lattices. This following report lists the papers that were published with the support of this DOE grant, together with a brief summary of the results from each paper.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Growth-Controlled Broad Emission in Phase-Pure Two-Dimensional Hybrid Perovskite Films

Two-dimensional hybrid metal halide perovskites (2D perovskites) are attractive for light-emitting devices and other applications because their emission is tunable across the visible spectrum. The emission profile of 2D perovskites can be broadened via a variety of mechanisms and is further complicated by the presence of impurities. Here, the challenge of making phase-pure films in Ruddlesden–Popper phases [(A') 2 (A) n–1 B n X 3n+1 structure] is overcome by using a single A/A'-site cation, ethylammonium (EA), whose optimal size also prohibits the formation of off-target phases. In the (EA) 2 (EA) n–1 Pb n Br 3n+1 family, the low-energy, broad emission observed in bulk crystals is reduced in spin-cast, polycrystalline films. This decrease in broad emission, attributed to phonon-mediated processes, is correlated with the strain in polycrystalline films that is observed by X-ray scattering. Photothermal deflection spectroscopy shows that strain also increases the electronic disorder near the free exciton absorbance. Broad emission in films can be recovered by slowing growth kinetics, which removes the strain acquired from spin-casting and increases the domain size. Furthermore, these results help extend the utility of 2D perovskites by suggesting design rules for the growth of thin films with the targeted phase and emission.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Spin switchable optical phenomena in Rashba band structures through intersystem crossing in momentum space in solution-processing 2D-superlattice perovskite film

Spin-switchable phenomena are a critical element for the development of spintronic and chiroptic devices. Herein we combine a 2D-superlattice perovskite (4,4-DFPD2PbI4) film with a ferromagnetic cobalt (Co) layer to form a multiferroic perovskite/Co interface, and demonstrate spin-switchable circularly polarized luminescence (CPL) between right-handed σ+ and left-handed σ− polarizations. When the ferromagnetic spins of Co at the Co/perovskite interface are altered between positive and negative magnetic field directions, the CPL from the 2D-superlattice perovskite switches from σ+ to σ− polarization. The magnetic field effects present a unique method to confirm that CPL is generated by the circular-orbital momentum of light-emitting excitons within Rashba band structures, eliminating artifacts involving structural birefringence. Our polarized neutron reflectometry measurements confirm a super long-range spin-orbit interaction occurring in the 2D-superlattice perovskite films. The temperature dependence of spin-switchable phenomenon indicates an extraordinarily long orbital polarization lifetime, reaching microseconds at room temperature and milliseconds at 5 K.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Influence of Methylammonium Chloride on Wide-Bandgap Halide Perovskites Films for Solar Cells

Wide-bandgap perovskites are of paramount importance as the photoactive layer of the top cell in high-efficiency tandem solar cells. Comparably high Br contents are required to widen the perovskite bandgap. However, the increase in Br content causes heterogeneous halide distribution and photoinstability. Here, in this study, the positive effect of the additive methylammonium chloride (MACl) on the optical and electronic properties of Br-rich perovskite, deposited using N-methyl-2-pyrrolidone (NMP) as co-solvent and the gas quenching method, is investigated. Simultaneous in situ grazing-incidence wide-angle X-ray scattering and photoluminescence spectroscopy are used to track the evolution of the structural and optoelectronic properties of the perovskites with different amounts of Br and MACl during the spin-coating and thermal annealing steps. The formation mechanism is elucidated in the presence of MACl. It is observed that chloride ions inhibit the intermediate phases, favoring the formation of a perovskite phase with higher crystallinity. Nano X-ray fluorescence mapping recognizes Br-richer and poorer nanometric domains, whose average sizes reduce for samples with MACl. In conclusion, it is demonstrated that adding MACl affects the formation of wide-bandgap perovskites via destabilization of the intermediate phases and acts on the homogenization of the halide distribution, leading to improved solar cell performances.

14 SOLAR ENERGY↗
DOE OSTI2021

In-situ observation of trapped carriers in organic metal halide perovskite films with ultra-fast temporal and ultra-high energetic resolutions

Abstract We in-situ observe the ultrafast dynamics of trapped carriers in organic methyl ammonium lead halide perovskite thin films by ultrafast photocurrent spectroscopy with a sub-25 picosecond time resolution. Upon ultrafast laser excitation, trapped carriers follow a phonon assisted tunneling mechanism and a hopping transport mechanism along ultra-shallow to shallow trap states ranging from 1.72–11.51 millielectronvolts and is demonstrated by time-dependent and independent activation energies. Using temperature as an energetic ruler, we map trap states with ultra-high energy resolution down to < 0.01 millielectronvolt. In addition to carrier mobility of ~4 cm 2 V −1 s −1 and lifetime of ~1 nanosecond, we validate the above transport mechanisms by highlighting trap state dynamics, including trapping rates, de-trapping rates and trap properties, such as trap density, trap levels, and capture-cross sections. In this work we establish a foundation for trap dynamics in high defect-tolerant perovskites with ultra-fast temporal and ultra-high energetic resolution.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Manufacturing of perovskite films

The present disclosure relates to a mixture that includes a perovskite precursor, a solvent, and an additive that includes at least one of a first amine, a ketone, an aldehyde, a non-nucleophilic sterically hindered base, and/or a halogen-containing compound, where, upon removal of the solvent and the additive, the perovskite precursor is capable of being transformed into a perovskite.

Whitaker, James Bacon↗
DOE OSTI2021

Compositional heterogeneity in Cs y FA 1- y Pb(Br x I 1- x ) 3 perovskite films and its impact on phase behavior

Hybrid organic inorganic lead halide perovskite semiconductors of the form Cs y FA 1-y Pb(Br x I 1-x ) 3 are promising candidate materials for high-efficiency photovoltaics. Notably, cation and anion substitution can be used to tune the band gaps to optimize performance and improve stability. However, multi-component materials can be prone to compositional and structural inhomogeneity and the extent, length scale and impact of this heterogeneity on lead halide perovskite properties are not well understood. Here we use synchrotron X-ray diffraction to probe the evolution of crystal structure across the tetragonal-to-cubic phase transition for a series of Cs y FA 1-y Pb(Br x I 1-x ) 3 thin films with x = 0.05 to 0.3 and y = 0.17 to 0.40. We find that the transition occurs across a broad temperature range of approximately 40 °C, much larger than for pure compounds such as MAPbI 3 and MAPbBr 3 . Finally, we hypothesize that this smearing of the phase transition is due to compositional inhomogeneities that give rise to a distribution of local transition temperatures and we estimate the composition varies by about 10% to 15% with likely greater heterogeneity for the halide anion than the cation. This approach of mapping the transition is a simple and effective method of assessing heterogeneity, enabling evaluation of its impact.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Cation Migration in Physically Paired 2D and 3D Lead Halide Perovskite Films

2D/3D interfaces provide long-term stability for the operation of metal halide perovskite solar cells. However, the cation migration under heat and light can disturb the interface and create a graded cation interface. This study has now probed the cation migration by physically pairing X 2 PbI 4 (X = butylammonium BA, oleylammonium OA, or phenethylammonium PEA) 2D film and (CH 3 NH 3 , MA)PbI 3 3D film at different temperatures and recording changes in the absorption and emission spectra. The migration of the methyl ammonium cation toward the 2D film slowly transforms the n = 1 layered phase into n = 2 and 3 layered phases. The 3D film, on the other hand, exhibits relatively small changes, as the inclusion of spacer cation has little effect on its phase. The apparent activation energy determined from the temperature-dependent cation migration kinetics (E a = 29– 50 kJ mol –1 ) indicates alkyl ammonium cations such as butyl ammonium migrate more readily into the 3D layer than aromatic cations such as PEA. As a result, the ease of migration of spacer cations between physically paired films suggests that the varied composition at the interface should be considered while evaluating the effectiveness of the 2D/3D design strategy for improving the performance of perovskite solar cells.

2D/3D perovskite↗
DOE OSTI2025

Interface Engineering using Vapor Transport Deposited Perovskite Films for Solar Cells

The overarching goal of this project was to demonstrate the viability of vapor transport deposition (VTD) for the fabrication of efficient and stable metal-halide perovskite solar cells. VTD is a solvent-free alternative to conventional solution-processing of the light-absorbing, perovskite active layer. Since solution-processing frequently requires the use of toxic solvents, transition to a dry-processing approach has the potential to remove an obstacle to facile and economically meaningful commercialization. Further, VTD bears similarity to existing industrially-relevant processing approaches used for the production of thin-film CdTe solar modules, suggesting the potential for rapid adoption and transition to industry. The results described within this report offer design rules for the processing of metal-halide perovskites via VTD and new insights into the design of deposition systems to inform future applications.

14 SOLAR ENERGY↗
DOE OSTI2022

Temperature-Dependent Phase Stable Hybrid Halide Perovskite Films by Chemical Vapor Deposition

Methylammonium lead iodide (MAPbI 3-x Cl x ) thin films were synthesized using chemical vapor deposition. Temperature-dependent grazing incidence synchrotron-based X-ray diffraction measurements confirm that the structure of MAP-bI 3-x Cl x , where Cl acts more like a dopant, remains throughout in the tetragonal phase in the temperature range of 20-300 K. Here these studies are further correlated with temperature-dependent photo-luminescence (PL) studies. The PL peak energy monotonically increases with temperature, suggestive of a single-phase behavior. Resistance measurements conducted as a function of temperature show an absence of inflection points, suggesting uniformity in its phase. Between 200 and 325 K, the resistance remains a constant.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Organo-metal halide perovskites films and methods of making the same

An aspect of the present disclosure is a method that includes applying a solution that includes a first solvent, a halogen-containing precursor, and a metal halide to a substrate to form a coating of the solution on the substrate, contacting the coating with a second solvent to form a first plurality of organo-metal halide perovskite crystals on the substrate, and thermally treating the first plurality of organo-metal halide perovskite crystals, such that at least a portion of the first plurality of organo-metal halide perovskite crystals is converted to a second plurality of organo-metal halide perovskite crystals on the substrate. The halogen-containing precursor and the metal halide are present in the solution at a molar ratio of the halogen-containing precursor to the metal halide between about 1.01:1.0 and about 2.0:1.0, and a property of the second plurality of organo-metal halide perovskite crystals is improved relative to a property of the first plurality of organo-metal halide perovskite crystals.

Zhu, Kai↗
DOE OSTI2023

Dark Exciton in 2D Hybrid Halide Perovskite Films Revealed by Magneto-Photoluminescence at High Magnetic Field

Here, a comprehensive study of the exciton fine structure (EFS) is presented in 2D-phenethylammonium lead iodide films using magnetic field-induced polarization of photoluminescence (PL) in both Faraday and Voigt configurations at fields up to 25 Tesla. Three exciton bands are identified in the PL spectrum associated with bound, dark, and bright excitons, respectively. Under a high magnetic field in Faraday/Voigt configuration, large field-induced circular/linear polarization is observed in the PL band related to the dark exciton, which is magnetically activated. Furthermore, it is found that the dark exciton has an anomalous field-induced circular polarization, which cannot be explained by the classical Boltzmann distribution of spin-polarized species. These findings are well explained by an effective mass model that includes exchange terms unique to the monoclinic symmetry as a perturbation of the EFS in the approximate tetragonal symmetry. It is also confirmed that the field-induced linear polarization is sensitive to the monoclinic exchange term, whereas the field-induced circular polarization is immune to such term.

14 SOLAR ENERGY↗