Alain Goriely

Enhanced Photoluminescence and Solar Cell Performance via Lewis Base Passivation of Organic–Inorganic Lead Halide Perovskites

Organic-inorganic metal halide perovskites have recently emerged as a top contender to be used as an absorber material in highly efficient, low-cost photovoltaic devices. Solution-processed semiconductors tend to have a high density of defect states and exhibit a large degree of electronic disorder. Perovskites appear to go against this trend, and despite relatively little knowledge of the impact of electronic defects, certified solar-to-electrical power conversion efficiencies of up to 17.9% have been achieved. Here, through treatment of the crystal surfaces with the Lewis bases thiophene and pyridine, we demonstrate significantly reduced nonradiative electron-hole recombination within the CH(3)NH(3)PbI(3-x)Cl(x) perovskite, achieving photoluminescence lifetimes which are enhanced by nearly an order of magnitude, up to 2 μs. We propose that this is due to the electronic passivation of under-coordinated Pb atoms within the crystal. Through this method of Lewis base passivation, we achieve power conversion efficiencies for solution-processed planar heterojunction solar cells enhanced from 13% for the untreated solar cells to 15.3% and 16.5% for the thiophene and pyridine-treated solar cells, respectively.

High-quality bulk hybrid perovskite single crystals within minutes by inverse temperature crystallization

Single crystals of methylammonium lead trihalide perovskites (MAPbX3; MA=CH3NH3+, X=Br− or I−) have shown remarkably low trap density and charge transport properties; however, growth of such high-quality semiconductors is a time-consuming process. Here we present a rapid crystal growth process to obtain MAPbX3 single crystals, an order of magnitude faster than previous reports. The process is based on our observation of the substantial decrease of MAPbX3 solubility, in certain solvents, at elevated temperatures. The crystals can be both size- and shape-controlled by manipulating the different crystallization parameters. Despite the rapidity of the method, the grown crystals exhibit transport properties and trap densities comparable to the highest quality MAPbX3 reported to date. The phenomenon of inverse or retrograde solubility and its correlated inverse temperature crystallization strategy present a major step forward for advancing the field on perovskite crystallization.

Mechanics of the brain: perspectives, challenges, and opportunities

The human brain is the continuous subject of extensive investigation aimed at understanding its behavior and function. Despite a clear evidence that mechanical factors play an important role in regulating brain activity, current research efforts focus mainly on the biochemical or electrophysiological activity of the brain. Here, we show that classical mechanical concepts including deformations, stretch, strain, strain rate, pressure, and stress play a crucial role in modulating both brain form and brain function. This opinion piece synthesizes expertise in applied mathematics, solid and fluid mechanics, biomechanics, experimentation, material sciences, neuropathology, and neurosurgery to address today’s open questions at the forefront of neuromechanics. We critically review the current literature and discuss challenges related to neurodevelopment, cerebral edema, lissencephaly, polymicrogyria, hydrocephaly, craniectomy, spinal cord injury, tumor growth, traumatic brain injury, and shaken baby syndrome. The multi-disciplinary analysis of these various phenomena and pathologies presents new opportunities and suggests that mechanical modeling is a central tool to bridge the scales by synthesizing information from the molecular via the cellular and tissue all the way to the organ level.

Nonlinear dynamics of filaments I. Dynamical instabilities

Abstract The Kirchhoff model provides a well-established mathematical framework to study, both computationaly and theoretically, the dynamics of thin filaments within the approximations of linear elasticity theory. The study of static solutions to these equations has a long history and the usual approach to describing their instabilities is to study the time-dependent version of the Kirchhoff model in the Euler angle frame. Here we study the linear stability of the full, time-independent, equations by introducing a new are length preserving perturbation scheme. As an application, we consider the instabilities of various stationary solutions, such as the planar ring and straight rod, subjected to twisting perturbations. This scheme gives a direct proof of the existence of dynamical instabilities and provides the selection mechanism for the shape of unstable filaments.

Solution-Grown Monocrystalline Hybrid Perovskite Films for Hole-Transporter-Free Solar Cells.

High-quality perovskite monocrystalline films are successfully grown through cavitation-triggered asymmetric crystallization. These films enable a simple cell structure, ITO/CH3 NH3 PbBr3 /Au, with near 100% internal quantum efficiency, promising power conversion efficiencies (PCEs) >5%, and superior stability for prototype cells. Furthermore, the monocrystalline devices using a hole-transporter-free structure yield PCEs ≈6.5%, the highest among other similar-structured CH3 NH3 PbBr3 solar cells to date.

Biomechanical models of hyphal growth in actinomycetes

The tip growth of filamentary actinomycetes is investigated within the framework of large deformation membrane theory in which the cell wall is represented as a growing elastic membrane with geometry-dependent elastic properties. The model exhibits realistic hyphal shapes and indicates a self-similar tip growth mechanism consistent with that observed in experiments. It also demonstrates a simple mechanism for hyphal swelling and beading that is observed in the presence of a lysing agent.

Integrability, partial integrability, and nonintegrability for systems of ordinary differential equations

The integrability of systems of ordinary differential equations with polynomial vector fields is investigated by using the singularity analysis methods. Three types of results are obtained. First, a general relationship between the degrees of first integrals and the so‐called Kowalevskaya exponents is derived. Second, it is shown that all solutions of algebraically integrable systems can be expanded in Puiseux series. Third, a new method to study partially integrable systems is studied. These different aspects allow us to study algorithmically the integrability, partial integrability, and nonintegrability of differential systems.

Towards a classification of Euler-Kirchhoff filaments

Euler–Kirchhoff filaments are solutions of the static Kirchhoff equations for elastic rods with circular cross sections. These equations are known to be formally equivalent to the Euler equations for spinning tops. This equivalence is used to provide a classification of the different shapes a filament can assume. Explicit formulas for the different possible configurations and specific results for interesting particular cases are given. In particular, conditions for which the filament has points of self-intersection, self-tangency, vanishing curvature or when it is closed or localized in space are provided. The average properties of generic filaments are also studied. They are shown to be equivalent to helical filaments on long length scales.

The Nonlinear Dynamics of Filaments

The Kirchhoff equations provide a well-established framework tostudy the statics and dynamics of thin elastic filaments. The study ofstatic solutions to these equations has a long history and provides thebasis for many investigations, both past and present, of theconfigurations taken by filaments subject to various external forces andboundary conditions. Here we review recently developed techniquesinvolving linear and nonlinear analyses that enable one to study, insome detail, the actual dynamics of filament instabilities and thelocalized structures that can ensue. By introducing a novel arc-lengthpreserving perturbation scheme a linear stability analysis can beperformed which, in turn, leads to dispersion relations that provide theselection mechanism for the shape of an unstable filament. Thesedispersion relations provide the starting point for nonlinear analysisand the derivation of new amplitude equations which describe thefilament dynamics above the instability threshold. Here we will mainlybe concerned with the analysis of rods of circular cross-sections andsurvey the behavior of rings, rods, helices and show how these resultslead to a complete dynamical description of filament buckling.

Tissue tension and axial growth of cylindrical structures in plants and elastic tissues

In many cylindrical structures in biology, residual stress fields are created through differential growth. In particular, if the outer and inner layers of a cylinder grow differentially, parts of the cylinder will be in a state of axial compression and other parts will be in tension. These tissue tensions change the overall material properties of the structure. Here, we study the role of tissue tension in the overall rigidity and stability of the cylinder. A detailed analysis, based on nonlinear elasticity, of the effect of tissue tension on the mechanical properties of growing cylinders reveal a subtle interplay between geometry, growth, and nonlinear elastic responses that help understand some of the remarkable properties of stems and other biological tissues.