Understanding Phase Transformation in Crystalline Ge Anodes for Li-Ion Batteries

Lithium-ion batteries using germanium as the anode material are attracting attention because of their high-capacity, higher conductivity, and lithium-ion diffusivity relative to silicon. Despite recent studies on Ge electrode reactions, there is still limited understanding of the reaction mechanisms governing crystalline Ge and the transformations into intermediate amorphous phases that form during the electrochemical charge and discharge process. In this work, we carry out in operando X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) studies on Ge anodes during the initial cycles to better understand these processes. These two probes track both crystalline (XRD) and amorphous (XAS) phase transformations with potential, which allows detailed information on the Ge anode to be obtained. We find that crystalline Ge lithiates inhomogeneously, first forming amorphous Li₉Ge₄ during the beginning stage of lithiation, followed by the conversion of the remaining crystalline Ge to amorphous Ge. The lithiation of amorphous Ge then forms amorphous Li_<i>x</i>Ge, which are then further lithiated to form crystalline Li₁₅Ge₄. During delithiation, crystalline Li₁₅Ge₄ transforms directly into a heterogeneous mix of amorphous Li_<i>x</i>Ge, which eventually form amorphous Ge, and interestingly, no amorphous Li₉Ge₄ are detected. Both our in operando XRD and XAS results present new insights into the reaction mechanism of Ge as anodes in LIBs, and demonstrate the importance of correlating electrochemical results with in operando studies

Chlorine in PbCl₂‑Derived Hybrid-Perovskite Solar Absorbers

Chlorine in PbCl₂‑Derived Hybrid-Perovskite Solar Absorber

Degree of Orientation in Liquid Crystalline Elastomers Defines the Magnitude and Rate of Actuation

The anisotropy of liquid crystalline elastomers (LCEs) is derived from the interaction-facilitated orientation of the molecular constituents. Here, we correlate the thermomechanical response of a series of LCEs subjected to mechanical alignment to measurements of the Hermans orientation parameter. The LCEs were systematically prepared with varying concentrations of liquid crystalline mesogens, which affects the relative degree of achievable order. These compositions were subject to varying degrees of mechanical alignment to prepare LCEs with orientations that span a wide range of orientation parameters. The stimuli-response of the LCEs indicates that the liquid crystalline content defines the temperature of actuation, whereas the orientation parameter of the LCE is intricately correlated to both the total actuation strain of the LCE as well as the rate of thermomechanical response

Effect of Surfactant Concentration and Aggregation on the Growth Kinetics of Nickel Nanoparticles

The effect of trioctylphosphine (TOP) concentration on the growth of nickel nanoparticles is studied using <i>in situ</i> synchrotron small-angle X-ray scattering. The growth kinetics are fitted using a two-step nucleation and autocatalytic growth model. TOP acts as a nucleating agent and then acts as an inhibitor against rapid particle growth. Increasing the TOP concentration results in smaller nanoparticles. Once there is a critical concentration of nickel particles above a certain size, they start to aggregate. This results in a broadening of the particle size distribution at later times due to particles on the outside of the aggregates continuing to grow, while those on the inside cease to grow as the nickel precursor is locally depleted

Mechanism of Tin Oxidation and Stabilization by Lead Substitution in Tin Halide Perovskites

The recent development of efficient binary tin- and lead-based metal halide perovskite solar cells has enabled the development of all-perovskite tandem solar cells, which offer a unique opportunity to deliver high performance at low cost. Tin halide perovskites, however, are prone to oxidation, where the Sn²⁺ cations oxidize to Sn⁴⁺ upon air exposure. Here, we identify reaction products and elucidate the oxidation mechanism of both ASnI₃ and ASn_0.5Pb_0.5I₃ (where A can be made of methylammonium, formamidinium, cesium, or a combination of these) perovskites and find that substituting lead onto the B site fundamentally changes the oxidation mechanism of tin-based metal halide perovskites to make them more stable than would be expected by simply considering the decrease in tin content. This work provides guidelines for developing stable small band gap materials that could be used in all-perovskite tandems

Evolution of Iodoplumbate Complexes in Methylammonium Lead Iodide Perovskite Precursor Solutions

Here we investigate the local structure present in single-step precursor solutions of methylammonium lead iodide (MAPbI₃) perovskite as a function of organic and inorganic precursor ratio, as well as with hydriodic acid (HI), using X-ray absorption spectroscopy. An excess of organic precursor as well as the use of HI as a processing additive has been shown to lead to the formation of smooth, continuous, pinhole free MAPbI₃ films, whereas films produced from precursor solutions containing molar equivalents of methylammonium iodide (MAI) and PbI₂ lead to the formation of a discontinuous, needlelike morphology. We now show that as the amount of excess MAI in the precursor solution is increased, the iodide coordination of iodoplumbate complexes present in solution increases. The use of HI results in a similar increase in iodide coordination. We therefore offer insight into how solution chemistry can be used to control MAPbI₃ thin film morphology by revealing a strong correlation between the lead coordination chemistry in precursor solutions and the surface coverage and morphology of the resulting MAPbI₃ film

Manipulating the Morphology of P3HT–PCBM Bulk Heterojunction Blends with Solvent Vapor Annealing

Using grazing incidence X-ray scattering, we observe the effects of solvent vapors upon the morphology of poly­(3-hexylthiophene)–phenyl-C₆₁-butyric acid methyl ester (P3HT–PCBM) bulk heterojunction thin film blends in real time; allowing us to observe morphological rearrangements that occur during this process as a function of solvent. We detail the swelling of the P3HT crystallites upon the introduction of solvent and the resulting changes in the P3HT crystallite morphology. We also demonstrate the ability for tetrahydrofuran vapor to induce crystallinity in PCBM domains. Additionally, we measure the nanoscale phase segregated domain size as a function of solvent vapor annealing and correlate this to the changes observed in the crystallite morphology of each component. Finally, we discuss the implications of the morphological changes induced by solvent vapor annealing on the device properties of BHJ solar cells

Radiative Thermal Annealing/in Situ X‑ray Diffraction Study of Methylammonium Lead Triiodide: Effect of Antisolvent, Humidity, Annealing Temperature Profile, and Film Substrates

Organic–inorganic hybrid halide perovskites are one of the most promising emerging photovoltaic materials due to their high efficiency and potentially low processing cost. Here, we present a well-controlled, manufacturing relevant annealing method, radiative thermal annealing, for the methylammonium lead triiodide (MAPbI₃) films formed by a solvent engineering process, with dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) as solvent and diethyl ether as the antisolvent. Radiative thermal annealing can produce high quality perovskite films, evidenced by high efficiency solar cell devices (∼18% power conversion efficiency), in a shorter time than the widely used hot plate annealing. Using in situ X-ray diffraction during the radiative annealing, we show that the role of the antisolvent is not to form an important intermediate compound (a PbI₂-MAI-DMSO complex) by washing of the main solvent (DMF), but to achieve a pinhole free, uniform film of MAPbI₃ with minimal intermediate compound. Importantly, we show that having a PbI₂-MAI-DMSO intermediate compound does not guarantee a high quality (pinhole free) perovskite film. We directly show that humidity induces MAPbI₃ to decompose into PbI₂ more rapidly and, as such, negatively impacts the reproducibility of the device performance. The study is extended to reveal the effect of annealing temperature profile and deposition substrate to demonstrate the complexity of perovskite processing parameters. This coupled experimental approach allows a better understanding of the effect of processing protocols, including antisolvent, humidity, and annealing profile, on MAPbI₃ film quality and the resultant solar cell performance

Ultrafast Electron Transfer at Organic Semiconductor Interfaces: Importance of Molecular Orientation

Much is known about the rate of photoexcited charge generation in at organic donor/acceptor (D/A) heterojunctions overaged over all relative arrangements. However, there has been very little experimental work investigating how the photoexcited electron transfer (ET) rate depends on the precise relative molecular orientation between D and A in thin solid films. This is the question that we address in this work. We find that the ET rate depends strongly on the relative molecular arrangement: The interface where the model donor compound copper phthalocyanine is oriented face-on with respect to the fullerene C₆₀ acceptor yields a rate that is approximately 4 times faster than that of the edge-on oriented interface. Our results suggest that the D/A electronic coupling is significantly enhanced in the face-on case, which agrees well with theoretical predictions, underscoring the importance of controlling the relative interfacial molecular orientation

Behaviors of Fe, Zn, and Ga Substitution in CuInS₂ Nanoparticles Probed with Anomalous X‑ray Diffraction

We synthesized CuInS₂ nanoparticles containing up to 20% Fe, Zn, and Ga to study alloying in photovoltaic absorber materials with anomalous X-ray diffraction. The colloidal synthesis allowed for detailed analysis of complex quaternary compounds. Anomalous X-ray diffraction (AXRD) was used to clarify the elemental distribution between phases. Additionally, optical spectroscopy and X-ray diffraction were used to probe the band gap and crystal phase, respectively. Substitution of Zn into wurtzite CuInS₂ produced a controllable increase in the optical band gap, whereas Ga did not substitute into wurtzite CuInS₂, producing no band gap change. Secondary phase precipitation of a chalcopyrite phase was observed with Fe substitution, along with a decrease of the optical band gap. This work demonstrates progress in compositional and structural analysis of quaternary chalcogenide materials using AXRD