Physics

Carbon electrode-based perovskite solar cells (PSCs) with low-cost and long-term stability have been recognized as a competitive candidate toward future practical applications. However, energy level mismatch and ineffective hole extraction at the carbon electrode/perovskite interface limit device performance. Herein, we develop a low-cost carbon-based electrode that utilizes a cheap small-molecule semiconductor copper phthalocyanine (CuPc) as both the interface modifier and dopant. The resultant planar PSC yields a power conversion efficiency of 14.8%, ∼30% higher than that based on the bare carbon electrode. This is due to higher work function and better hole extraction properties of the CuPc-modified carbon electrodes. The simple modification process of the carbon electrode has potential applications for large-scale fabrication. We further applied such electrodes in large-area solar modules and flexible solar cells, demonstrating their capability of upscaling and flexibility.

All-inorganic halide perovskites are promising materials for optoelectronic applications. The surface or interface structure of the perovskites plays a crucial role in determining the optoelectronic conversion efficiency, as well as the material stability. A thorough understanding of surface atomic structures of the inorganic perovskites and their contributions to their optoelectronic properties and stability is lacking. Here we show a scanning tunneling microscopy investigation on the atomic and electronic structure of CsPbBr 3 perovskite. Two different surface structures with a stripe and an armchair domain are identified, which originates from a complex interplay between Cs cations and Br anions. Our findings are further supported and correlated with density functional theory calculations and photoemission spectroscopy measurements. The stability evaluation of photovoltaic devices indicates a higher stability for CsPbBr 3 in comparison with MAPbBr 3 , which is closely related to the low volatility of Cs from the perovskite surface.

This paper presents a formulation of a general form of an equation for pressure using thermodynamic principles. The motivation for this is in large part due to the need for a pressure equation for smoothed particle hydrodynamics, SPH, that takes into account the role of entropy. This is necessary because the use of physical and artificial viscosity leads to an increase in entropy.

A method to couple interparticle contact models with Stokesian dynamics (SD) is introduced to simulate colloidal aggregates under flow conditions. The contact model mimics both the elastic and plastic behavior of the cohesive connections between particles within clusters. Owing to this, clusters can maintain their structures under low stress while restructuring or even breakage may occur under sufficiently high stress conditions. SD is an efficient method to deal with the long-ranged and many-body nature of hydrodynamic interactions for low Reynolds number flows. By using such a coupled model, the restructuring of colloidal aggregates under shear flows with stepwise increasing shear rates was studied. Irreversible compaction occurs due to the increase of hydrodynamic stress on clusters. Results show that the greater part of the fractal clusters are compacted to rod-shaped packed structures, while the others show isotropic compaction.

Aggregate Colloid Uniform and shear flow Low Reynolds number Stokes flow Comparison of methods Simulation FEM LBM SD a b s t r a c t Dilute suspensions of colloidal aggregates of two different types are investigated numerically by three different methods; Stokesian dynamics (SD), the finite element method (FEM) and the lattice Boltzmann method (LBM). The Navier-Stokes equations are solved in the zero-Reynolds-number limit, and the fluid forces acting on particles within the aggregates as well as the aggregates' influence on the fluid flow are evaluated by the discrete methods. Two fluid-flow profiles are considered; uniform plug flow and linear shear flow. The paper compares three methods in their own typical simulation environment. Thus, the results may provide practical guidelines on which methods to chose for certain accuracy requirements.

We have developed a novel computational scheme that allows direct numerical simulation of the mechanical behavior of sticky granular matter under stress. We present here the general method, with particular emphasis on the particle features at the nanometric scale. It is demonstrated that, although sticky granular material is quite complex and is a good example of a challenging computational problem (it is a dynamical problem, with irreversibility, self-organization and dissipation), its main features may be reproduced on the basis of rather simple numerical model, and a small number of physical parameters. This allows precise analysis of the possible deformation processes in soft materials submitted to mechanical stress. This results in direct relationship between the macroscopic rheology of these pastes and local interactions between the particles.

Linear viscoelastic properties of ethylene-octene copolymer/nanosilica composites investigated over a broad range of frequencies J. Rheol. 57, 407 (2013) Mechanical characterization and micromechanical modeling of bread dough J. Rheol. 57, 249 (2013) Wall slip and flow of concentrated hard-sphere colloidal suspensions J. Rheol. 56, 1005Rheol. 56, (2012 Using startup of steady shear flow in a sliding plate rheometer to determine material parameters for the purpose of predicting long fiber orientation J. Rheol. 56, 955 Scaling of critical velocity for bubble raft fracture under tension

The hydrodynamic properties of rigid fractal aggregates have been investigated by considering their motion in shear flow in the Stokesian dynamics approach. Due to the high fluid viscosity and small particle inertia of colloidal systems, the total force and torque applied to the aggregate reach equilibrium values in a short time.

Suspensions of rigid aggregates have been investigated by Stokesian dynamics. In our recent work (Seto R, Botet R, Briesen H. Phys Rev E 84:041405, 2011), the motions of freely suspended aggregates in shear flows were determined by considering the force and torque balance, and forces and moments acting on the contact points within aggregates were also evaluated. Here, by comparing the obtained results with a bond strength between particles, the sustainable sizes of the aggregates under shear flows have been estimated, which leads to a viscosity-shear rate relation. Our method allows us to see not only the power-law shear thinning for fractal aggregates but also some deviations due to the finite-size effects.

The presence and the microscopic origin of normal stress differences in dense suspensions under simple shear flows are investigated by means of inertialess particle dynamics simulations, taking into account hydrodynamic lubrication and frictional contact forces. The synergic action of hydrodynamic and contact forces between the suspended particles is found to be the origin of negative contributions to the first normal stress difference $N_{1}$ , whereas positive values of $N_{1}$ observed at higher volume fractions near jamming are due to effects that cannot be accounted for in the hard-sphere limit. Furthermore, we found that the stress anisotropy induced by the planarity of the simple shear flow vanishes as the volume fraction approaches the jamming point for frictionless particles, while it remains finite for the case of frictional particles.

The phenomenon of shear-induced jamming is a factor in the complex rheological behavior of dense suspensions. Such shear-jammed states are fragile, i.e., they are not stable against applied stresses that are incompatible with the stress imposed to create them. This peculiar flow-history dependence of the stress response is due to flow-induced microstructures. To examine jammed states realized under constant shear stress, we perform dynamic simulations of non-Brownian particles with frictional contact forces and hydrodynamic lubrication forces. We find clear signatures that distinguish these fragile states from the more conventional isotropic jammed states.

Particle-based simulations of discontinuous shear thickening (DST) and shear jamming (SJ) suspensions are used to study the role of stress-activated constraints, with an emphasis on resistance to gearlike rolling. Rolling friction decreases the volume fraction required for DST and SJ, in quantitative agreement with reallife suspensions with adhesive surface chemistries and "rough" particle shapes. It sets a distinct structure of the frictional force network compared to only sliding friction, and from a dynamical perspective leads to an increase in the velocity correlation length, in part responsible for the increased viscosity. The physics of rolling friction is thus a key element in achieving a comprehensive understanding of strongly shearthickening materials.

Under an applied traction, highly concentrated suspensions of solid particles in fluids can turn from a state in which they flow to a state in which they counteract the traction as an elastic solid: a shear-jammed state. Remarkably, the suspension can turn back to the flowing state simply by inverting the traction. A tensorial model is presented and tested in paradigmatic cases. We show that, to reproduce the phenomenology of shear jamming in generic geometries, it is necessary to link this effect to the elastic response supported by the suspension microstructure rather than to a divergence of the viscosity.

We introduce a general decomposition of the stress tensor for incompressible fluids in terms of its components on a tensorial basis adapted to the local flow conditions, which include extensional flows, simple shear flows, and any type of mixed flows. Such a basis is determined solely by the symmetric part of the velocity gradient and allows for a straightforward interpretation of the non-Newtonian response in any local flow conditions. In steady homogeneous flows, the material functions that represent the components of the stress on the adapted basis generalize and complete the classical set of viscometric functions used to characterize the response in simple shear flows. Such a general decomposition of the stress is effective in coherently organizing and interpreting rheological data from laboratory measurements and computational studies in non-viscometric steady flows of great importance for practical applications. The decomposition of the stress in terms with clearly distinct roles is also useful in developing constitutive models.