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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 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Point Defects and Grain Boundaries in Rotationally Commensurate MoS 2 on Epitaxial Graphene

With reduced degrees of freedom, structural defects are expected to play a greater role in two-dimensional materials in comparison to their bulk counterparts. In particular, mechanical strength, electronic properties, and chemical reactivity are strongly affected by crystal imperfections in the atomically thin limit. Here, ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS) are employed to interrogate point and line defects in monolayer MoS 2 grown on epitaxial graphene (EG) at the atomic scale. Five types of point defects are observed with the majority species showing apparent structures that are consistent with vacancy and interstitial models. The total defect density is observed to be lower than MoS 2 grown on other substrates and is likely attributed to the van der Waals epitaxy of MoS 2 on EG. Grain boundaries (GBs) with 30° and 60° tilt angles resulting from the rotational commensurability of MoS 2 on EG are more easily resolved by STM than atomic force microscopy at similar scales due to the enhanced contrast from their distinct electronic states. For example, band gap reduction to ~0.8 and ~0.5 eV is observed with STS for 30° and 60° GBs, respectively. In addition, atomic resolution STM images of these GBs are found to agree well with proposed structure models. In conclusion, this work offers quantitative insight into the structure and properties of common defects in MoS 2 and suggests pathways for tailoring the performance of MoS 2 /graphene heterostructures via defect engineering.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Optical-Vibrational Properties of the Cs 2 SnX 6 (X = Cl, Br, I) Defect Perovskites and Hole-Transport Efficiency in Dye-Sensitized Solar Cells

We report the vibrational and optical properties of the ‘defect’ perovskites Cs 2 SnX 6 (X = Cl, Br, I) as well as their use as hole-transporting materials (HTMs) in solar cells. All three air-stable compounds were characterized using powder X-ray diffraction and Rietveld refinement. Far-IR reflectance, Raman, and UV–vis spectroscopy as well as electronic band structure calculations show that the compounds are direct band gap semiconductors with a pronounced effect of the halogen atom on the size of the energy gap and the vibrational frequencies. Scanning electron microscopy and atomic force microscopy confirmed that the morphology of the perovskite films deposited from $N,N$-dimethylformamide solutions on TiO 2 substrates also strongly depends on the chemical composition of the materials. The Cs 2 SnX 6 perovskites were introduced as hole-transporting materials in dye-sensitized solar cells, based on mesoporous titania electrodes sensitized with various organic and metal–organic dyes. The solar cells based on Cs 2 SnI 6 HTM and the Z907 dye performed best with a maximum power conversion efficiency of 4.23% at 1 sun illumination. The higher performance of Cs 2 SnI 6 is attributed to efficient charge transport in the bulk material and hole extraction at the perovskite-Pt interface, as evidenced by electrochemical impedance spectroscopy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Overcoming Short-Circuit in Lead-Free CH 3 NH 3 SnI 3 Perovskite Solar Cells via Kinetically Controlled Gas–Solid Reaction Film Fabrication Process

The development of Sn-based perovskite solar cells has been challenging because devices often show short-circuit behavior due to poor morphologies and undesired electrical properties of the thin films. A low-temperature vapor-assisted solution process (LT-VASP) has been employed as a novel kinetically controlled gas–solid reaction film fabrication method to prepare lead-free CH 3 NH 3 SnI 3 thin films. We show that the solid SnI 2 substrate temperature is the key parameter in achieving perovskite films with high surface coverage and excellent uniformity. The resulting high-quality CH 3 NH 3 SnI 3 films allow the successful fabrication of solar cells with drastically improved reproducibility, reaching an efficiency of 1.86%. Furthermore, our Kelvin probe studies show the VASP films have a doping level lower than that of films prepared from the conventional one-step method, effectively lowering the film conductivity. Above all, with (LT)-VASP, the short-circuit behavior often obtained from the conventional one-step-fabricated Sn-based perovskite devices has been overcome. Finally, this study facilitates the path to more successful Sn-perovskite photovoltaic research.

14 SOLAR ENERGY↗
DOE OSTI2017

Buta-1,3-diyne-Based π-Conjugated Polymers for Organic Transistors and Solar Cells

We report the synthesis and characterization of new alkyl-substituted 1,4-di(thiophen-2-yl)buta-1,3-diyne (R-DTB) donor building blocks, based on the -C≡C-C≡C- conjugative pathway, and their incorporation with thienyl-diketopyrrolopyrrole (R'-TDPP) acceptor units into π-conjugated PTDPP-DTB polymers (P1-P4). The solubility of the new polymers strongly depends on the DTB and DPP solubilizing (R and R', respectively) substituents. Thus, solution processable and high molecular weight PDPP-DTB polymers are achieved for P3 (R = n-C12H25, R' = 2- butyloctyl) and P4 (R = 2-ethylhexyl, R' = 2-butyloctyl). Systematic studies of P3 and P4 physicochemical properties are carried using optical spectroscopy, cyclic voltammetry, and thermal analysis, revealing characteristic features of the dialkynyl motif. For the first time, optoelectronic devices (OFETs, OPVs) are fabricated with 1,3-butadiyne containing organic semiconductors. OFET hole mobilities and record OPV power conversion efficiencies for acetylenic organic materials approach 0.1 cm2/(V s) and 4%, respectively, which can be understood from detailed thin-film morphology and microstructural characterization using AFM, TEM, XRD, and GIWAXS methodologies. Importantly, DTB-based polymers (P3 and P4) exhibit, in addition to stabilization of frontier molecular orbitals and to -C≡C-C≡C- relief of steric torsions, discrete morphological pliability through thermal annealing and processing additives. The advantageous materials properties and preliminary device performance reported here demonstrate the promise of 1,3-butadiyne-based semiconducting polymers.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Electrocatalytic Water Oxidation by a Copper(II) Complex of an Oxidation-Resistant Ligand

The copper(II) complex Cu(pyalk) 2 (pyalk = 2-pyridyl-2-propanoate) is a robust homogeneous water-oxidation electrocatalyst under basic conditions (pH > 10.4). Water oxidation occurs at a relatively low overpotential for copper of 520–580 mV with a turnover frequency of ~0.7 s –1 . Controlled potential electrolysis experiments over 12 h at 1.1 V vs NHE resulted in the formation of >30 catalytic turnovers of O 2 with only ~20% catalyst degradation. The robustness of the catalyst under fairly harsh conditions and the low overpotential further highlight the oxidation resistance and strong donor character of pyalk.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Performance Enhancement of Lead-Free Tin-Based Perovskite Solar Cells with Reducing Atmosphere-Assisted Dispersible Additive

Sn-based halide perovskite materials have attracted tremendous attention and have been employed successfully in solar cells. However, their high conductivities resulting from the unstable divalent Sn state in the structure cause poor device performance and poor reproducibility. Herein, we used excess tin iodide (SnI 2 ) in Sn-based halide perovskite solar cells (ASnI 3 , A = Cs, methylammonium and formamidinium tin iodide as the representative light absorbers), combined with a reducing atmosphere to stabilize the Sn 2+ state. Excess SnI 2 can disperse uniformly into the perovskite films and functions as a compensator as well as a suppressor of Sn 2+ vacancies thereby effectively reducing the p-type conductivity. This process significantly improved the solar-cell performances of all the ASnI3 materials on mesoporous TiO 2 . Optimized CsSnI3 devices achieved a maximum power conversion efficiency of 4.81%, which is the highest among all inorganic Pb-free perovskite solar cells to date.

14 SOLAR ENERGY↗