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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 1,333 records · Page 74
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Syntheses, structures, and 1 H, 13 C{ 1 H} and 119 Sn{ 1 H} NMR chemical shifts of a family of trimethyltin alkoxide, amide, halide and cyclopentadienyl compounds

The synthesis and full characterization, including Nuclear Magnetic Resonance (NMR) data ( 1 H, 13 C{ 1 H} and 119 Sn{ 1 H}), for a series of Me 3 SnX (X = O-2,6-tBu 2 C 6 H 3 (1), (Me 3 Sn)N(2,6- i Pr 2 C 6 H 3 ) (3), NH-2,4,6- t Bu 3 C 6 H 2 (4), N(SiMe 3 ) 2 (5), NEt 2 , C 5 Me 5 (6), Cl, Br, I, and SnMe 3 ) compounds in benzene-d 6 , toluene-d 8 , dichloromethane-d 2 , chloroform-d 1 , acetonitrile-d 3 , and tetrahydrofuran-d 8 are reported. The X-ray crystal structures of Me 3 Sn(O-2,6- t Bu 2 C 6 H 3 ) (1), Me 3 Sn(O-2,6- i Pr 2 C 6 H 3 ) (2), and (Me 3 Sn)(NH-2,4,6- t Bu 3 C 6 H 2 ) (4) are also presented. As a result, these compiled data complement existing literature data and ease the characterization of these compounds by routine NMR experiments.

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DOE OSTI2016

Guest-tuned spin crossover in flexible supramolecular assemblies templated by a halide (Cl - , Br - or I - )

Ligand 1,3-bis(3-(pyridin-2-yl)-1H-pyrazol-5-yl)benzene, L, forms mononuclear spin crossover complexes [FeL 3 ] 2+ with pendant arms that cause them to dimerize through numerous intermolecular interactions forming supramolecular (X@[FeL 3 ] 2 ) 3+ cations. Finally, hey have the flexibility to encapsulate Cl - , Br - or I - , which allow tuning the magnetic properties, in the solid state and in solution.

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DOE OSTI2016

Grain boundary dominated ion migration in polycrystalline organic–inorganic halide perovskite films

The efficiency of perovskite solar cells is approaching that of single-crystalline silicon solar cells despite the presence of large grain boundary (GB) area in the polycrystalline thin films. Here, by using a combination of nanoscopic and macroscopic level measurements, we show that the ion migration in polycrystalline perovskites is dominated through GBs. Conducting atomic force microscopy measurements reveal much stronger hysteresis both for photocurrent and dark-current at the GBs than on the grains interiors, which can be explained by faster ion migration at the GBs. The dramatically enhanced ion migration results in a redistribution of ions along the GBs after electric poling, in contrast to the intact grain area. The perovskite single-crystal devices without GBs show negligible current hysteresis and no ion-migration signal. Furthermore, the discovery of dominating ion migration through GBs in perovskites can lead to broad applications in many types of devices including photovoltaics, memristors, and ion batteries.

14 SOLAR ENERGY↗
DOE OSTI2016

Structural and Chemical Evolution of Methylammonium Lead Halide Perovskites during Thermal Processing from Solution

Following the prominent success of CH3NH3PbI3 in photovoltaics and other optoelectronic applications, focus has been placed on better understanding perovskite crystallization from precursor and intermediate phases in order to facilitate improved crystallinity often desirable for advancing optoelectronic properties. Understanding of stability and degradation is also of critical importance as these materials seek commercial applications. In this study, we investigate the evolution of perovskites formed from targeted precursor chemistries by correlating in situ temperature-dependent X-ray diffraction, thermogravimetric analysis, and mass spectral analysis of the evolved species. This suite of analyses reveals important precursor composition-induced variations in the processes underpinning perovskite formation and degradation. The addition of Cl- leads to widely different precursor evolution and perovskite formation kinetics, and results in significant changes to the degradation mechanism, including suppression of crystalline PbI2 formation and modification of the thermal stability of the perovskite phase. This work highlights the role of perovskite precursor chemistry in both its formation and degradation.

14 SOLAR ENERGY↗
DOE OSTI2017

Interpretation of inverted photocurrent transients in organic lead halide perovskite solar cells: proof of the field screening by mobile ions and determination of the space charge layer widths

In Methyl Ammonium Lead Iodide (MAPI) perovskite solar cells, screening of the built-in field by mobile ions has been proposed as part of the cause of the large hysteresis observed in the current/voltage scans in many cells. Here, we show that photocurrent transients measured immediately (e.g. 100 μs) after a voltage step can provide direct evidence that this field screening exists. Just after a step to forward bias, the photocurrent transients are reversed in sign (i.e. inverted), and the magnitude of the inverted transients can be used to find an upper bound on the width of the space charge layers adjacent to the electrodes. This in turn provides a lower bound on the mobile charge concentration, which we find to be ≳1 x 10 17 cm -3 . Using a new photocurrent transient experiment, we show that the space charge layer thickness remains approximately constant as a function of bias, as expected for mobile ions in a solid electrolyte. We also discuss additional characteristics of the inverted photocurrent transients that imply either an unusually stable deep trapping, or a photo effect on the mobile ion conductivity.

14 SOLAR ENERGY↗
DOE OSTI2017

Shallow trapping vs. deep polarons in a hybrid lead halide perovskite, CH 3 NH 3 PbI 3

There has been considerable speculation over the nature of charge carriers in organic–inorganic hybrid perovskites, i.e., whether they are free and band-like, or they are prone to self-trapping via short range deformation potentials. Unusually long minority-carrier diffusion lengths and moderate-to-low mobilities, together with relatively few deep defects add to their intrigue. Here we implement density functional methods to investigate the room-temperature, tetragonal phase of CH 3 NH 3 PbI 3 . We compare charge localization behavior at shallow levels and associated lattice relaxation versus those at deep polaronic states. The shallow level originates from screened Coulomb interaction between the perturbed host and an excited electron or hole. The host lattice has a tendency towards forming these shallow traps where the electron or hole is localized not too far from the band edge. In contrast, there is a considerable potential barrier that must be overcome in order to initiate polaronic hole trapping. The formation of a hole polaron (I 2 - center) involves strong lattice relaxation, including large off-center displacement of the organic cation, CH 3 NH 3 + . This type of deep polaron is energetically unfavorable, and active shallow traps are expected to shape the carrier dynamics in this material.

Source record↗
DOE OSTI2018

Temperature dependence of the spin state and geometry in tricobalt paddlewheel complexes with halide axial ligands

Trinuclear cobalt paddlewheel complexes, [Co 3 (dpa) 4 X 2 ] (dpa = the anion of 2,2'-dipyridylamine, X = Cl–, Br–, –NCS–, –CN–, (NC) 2 N–), are known to demonstrate a thermally-induced spin-crossover (SCO). Despite a wealth of structural and magnetic information about such complexes, the role of the axial ligand on the characteristic SCO temperature (T 1/2 ) remains ambiguous. The situation is complicated by the observation that the solid state geometry of the complexes, symmetric or unsymmetric, with respect to the central cobalt ion, also appears to influence the SCO behavior. In order to seek trends in the relationship between the nature of the axial ligand, geometry and magnetic properties, we have prepared the first examples of tricobalt paddlewheel complexes with axial fluorido and iodido ligands, as well as two new chlorido and bromido solvates. Their SCO properties are discussed in the context of an examination of previously reported chlorido and bromido adducts. The main conclusions are: (1) T 1/2 values follow the trend I– < Br– ≈ Cl– < F–; (2) while the molecular geometry is predominantly guided by crystal packing for the Cl–, Br– and I– derivatives, the presence of an axial fluoride may favor a more symmetric core; (3) the magnetic characterization of a second example of an unsymmetric complex supports the observation that they display dramatically lower T 1/2 values than their symmetric analogues; and (4) SCO in crystallographically symmetric compounds apparently occurs without loss of molecular or crystallographic symmetry, while a gradual geometric transformation linking the temperature dependence of quasi-symmetric to unsymmetric in crystallographically unconstrained compounds was found.

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DOE OSTI2018

Antisolvent processing of lead halide perovskite thin films studied by in situ X-ray diffraction

Here, whe conversion mechanism from the precursor ink to the perovskite film using antisolvent-induced crystallization has been studied using in situ X-ray diffraction during blade coating and antisolvent deposition. We study various solvent systems forming methylammonium lead iodide perovskite. We find that it is critical to add the antisolvent before the formation of a crystalline intermediate phase. Compared to thermal crystallization, the antisolvent-induced crystallization alters the conversion route. Instead of heterogeneous nucleation and conversion through a crystalline intermediate phase, antisolvent-assisted annealing leads to a rapid direct crystallization, resulting in highly regular smooth films.

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