Static Contributions Research Articles

(Invited) Order, Disorder, and Energy Loss in Organic Photovoltaics

We will discuss our recent work to better understand the role of charge transfer (CT) states in organic solar cell function. For one, we have been exploring organic semiconductor-based thin films that feature crystalline grains of up to 1 mm in extent, termed microcrystalline films. We have found that CT states incorporating these long-range-ordered films can be highly delocalized, contributing to noticeably lower energy losses. In another system, we are studying donor-acceptor pairs that feature very high optical gaps (>3 eV) but relatively small frontier orbital energy offset (<1 eV). Such interfaces present multiple CT states that reveal new insight about photocurrent generation and nonradiative recombination at donor-acceptor interfaces. Finally, we have developed a framework to discriminate between dynamic and static disorder contributions. Both are shown to contribute, and the relative contribution depends on materials choice and temperature.

Contingent Reasoning and Dynamic Public Goods Provision

Contributions toward public goods often reveal information that is useful to others considering their own contributions. This experiment compares static and dynamic contribution decisions to determine how contingent reasoning differs in dynamic decisions where equilibrium requires understanding how future information can inform about prior events. This identifies partially cursed individuals who can only extract partial information from contingent events, others who are better at extracting information from past rather than future or concurrent events, and Nash players who effectively perform contingent thinking. Contrary to equilibrium, the dynamic provision mechanism does not lead to lower contributions than the static mechanism. (JEL C92, D71, D82, D91, H41)

A Low-Frequency Stable, Excitation Agnostic Discretization of the Right-Hand Side for the Electric Field Integral Equation on Multiply-Connected Geometries

In order to accurately compute scattered and radiated fields in the presence of arbitrary excitations, a low-frequency stable discretization of the right-hand side (RHS) of a quasi-Helmholtz preconditioned electric field integral equation (EFIE) on multiply-connected geometries is introduced, which avoids an ad-hoc extraction of the static contribution of the RHS when tested with solenoidal functions. To obtain an excitation agnostic approach, our approach generalizes a technique to multiply-connected geometries where the testing of the RHS with loop functions is replaced by a testing of the normal component of the magnetic field with a scalar function. To this end, we leverage orientable global loop functions that are formed by a chain of Rao-Wilton-Glisson (RWG) functions around the holes and handles of the geometry, for which we introduce cap surfaces that allow to uniquely define a suitable scalar function. We show that this approach works with open and closed, orientable and non-orientable geometries. The numerical results demonstrate the effectiveness of this approach.

The Analytical Application of Quenching Phenomena of CdTe Quantum Dot Nanoparticles

This paper is devoted to the synthesis and application of CdTe quantum dot (QD) nanoparticles covered with organic ligands containing a thiol group, mostly mercaptopropionic acid (MPA) and glutathione (GSH). The simple one-step synthetic procedure was optimized to prepare greater quantities of nanoparticles for analytical purposes. The prepared CdTe QD nanoparticles were characterized by various analytical techniques, and their interaction with some metal ions (Cu(II), Pb(II), and Hg(II)) was studied by using luminescence spectroscopy in both steady-state and time-resolved modes. The mathematical analysis of the quenching effect of Cu(II) ions on the luminescence of CdTe QD nanoparticles shows that the static contribution is mostly responsible for the overall effect, but experimental conditions, such as pH, ionic strength, or the concentration of nanoparticles in aqueous solution, could also be important. The presence of metal ions in the form of a metal complex species could play an important role, and this phenomenon could be used to tune the selectivity of the quenching process. These findings have been utilized for the development of an analytical procedure for the detection and quantitative analysis of Cu(II) and Pb(II) ions in environmental water samples. In practice, this procedure could be easily implemented in a microplate format to increase throughput.

Fractional Maxwell viscoelastic model to explain dynamic magneto-viscoelastic properties of an isotropic magnetorheological elastomer containing flake-shaped magnetic particles

ABSTRACT A modified fractional Maxwell viscoelastic model was developed for the dynamic magneto-viscoelastic behavior of an isotropic magnetorheological elastomer (MRE) having flake-shaped iron particles. This model can capture the low-frequency and high magnetic field static friction contribution with frequency, both in storage and loss modulus. The magnetic dipole-dipole interaction model is used to find the influence of the external magnetic field on viscoelastic behavior. The variation of field-induced modulus with the magnetic field is well described using the Langevin function. The predicted and experimental measured storage and loss modulus agree with the present model with an accuracy of ≥99%. The present model is compared with the results of spherical particle-based MRE.

The science and mechanics of adhesion: An industrial view.

Undoubtedly, adhesion is one of the broadest terms in science and technology used to describe several bulk and interface related phenomena. While the thermodynamic work of adhesion is determined by contacting surfaces and their intrinsic surface energetics, it is important to understand how adhesive properties of materials are additionally governed and amplified by their dissipative rate processes in the bulk or near the interface as they go through large strains and deformation. Systematic review of the literature showed that the involved interfacial mechanisms were grouped into several categories ranging from micromechanical interlocking to molecular interdiffusion of surface constituents, a characteristic of most polymeric systems. This paper addressed the static and dynamic contributions to the adhesion energy and discussed its relation to microstructure and surface architecture in pressure sensitive and fracture in structural adhesives. While the focus was on industrial view of adhesion, parallels in adhesive dentistry were given where connections between adhesion, boundary geometry/compliance, shrinkage stress, material model, joint design, retention, and interfacial curing were made. Adhesion science and mechanics are complex multi-disciplinary fields involving surfaces, substrates, and loading system involving a broad range of mechanisms applicable to dentistry.

Dynamic Effects on Hole Transport in Amorphous Organic Semiconductors: a Combined QM/MM and kMC Study.

Small-molecule-based amorphous organic semiconductors (OSCs) are essential components of organic photovoltaics and organic light-emitting diodes. The charge carrier mobility of these materials is an integral and limiting factor in regard to their performance. Integrated computational models for the hole mobility, taking into account structural disorder in systems of several thousand molecules, have been the object of research in the past. Due to static and dynamic contributions to the total structural disorder, efficient strategies to sample the charge transfer parameters become necessary. In this paper, we investigate the impact of structural disorder in amorphous OSCs on the transfer parameters and charge mobilities in different materials. We present a sampling strategy for incorporating static and dynamic structural disorder which are based on QM/MM methods using semiempirical Hamiltonians and extensive MD sampling. We show how the disorder affects the distributions of HOMO energies and intermolecular couplings and validate the results using kinetic Monte Carlo simulations of the mobility. We find that dynamic disorder causes an order of magnitude difference in the calculated mobility between morphologies of the same material. Our method allows the sampling of disorder in HOMO energies and couplings, and the statistical analysis enables us to characterize the relevant time scales on which charge transfer occurs in these complex materials. The findings presented here shed light on the interplay of the fluctuating amorphous matrix with charge carrier transport and aid in the development of a better understanding of these complex processes.

Three-level homogeneous model for the study of heat transfer mechanism in metallic nanofluids

To explain heat conduction mechanism in nanofluids, different existing models have been analysed, but none of these can predict exact variation of thermal conductivity with volume concentration. In present study, it is found that the static and dynamic contributions of nanoparticles in the base fluid are responsible for its conductivity. Unfortunately, these both aspects have not been included in any of the existing model. We have modified the Xuan model including effect of shape and size of nanoparticles (static contribution) and allocation of nanoparticles, temperature, junction layer, congregate pattern, and Brownian diffusion (dynamic contribution) to propose a new model which can predict the exact mechanism of thermal conductivity of nfs. We have tested the proposed model to compute the thermal conductivity of Al2O3, ZnO, Ag, CuO, Co3O4 and Fe2O3 based nanofluids at different volume concentrations. Computed results using our proposed model are in very good agreement with experimental data.

The Electronic Disorder Landscape of Mixed Halide Perovskites.

Band gap tunability of lead mixed halide perovskites makes them promising candidates for various applications in optoelectronics. Here we use the localization landscape theory to reveal that the static disorder due to iodide:bromide compositional alloying contributes at most 3 meV to the Urbach energy. Our modeling reveals that the reason for this small contribution is due to the small effective masses in perovskites, resulting in a natural length scale of around 20 nm for the "effective confining potential" for electrons and holes, with short-range potential fluctuations smoothed out. The increase in Urbach energy across the compositional range agrees well with our optical absorption measurements. We model systems of sizes up to 80 nm in three dimensions, allowing us to accurately reproduce the experimentally observed absorption spectra of perovskites with halide segregation. Our results suggest that we should look beyond static contribution and focus on the dynamic temperature dependent contribution to the Urbach energy.

Large amplitude oscillatory shear (LAOS) behavior of chocolates of different compositions

The complex rheological response of chocolates of different compositions was analyzed in depth from stationary shear flow curves and large amplitude oscillatory shear tests. The samples have a wide range of cocoa content that controls the rheological behavior characterized by a yielding response. The strain amplitude at which the chocolates changed from elastic-dominated to viscous-dominated behavior ranged from 0.2 to 0.4%. In this region, the stress response to the applied sinusoidally oscillating shear strain showed transient overshoots whose values were similar to yield stress calculated using the Casson equation in continuous flow. The nonlinear response at the steady-state was analyzed. The Q0 nonlinearity parameter, obtained from Fourier Transform analysis, increases as the concentration of the solids conferring bimodality to the particle size distribution increases. In the study of intracycle nonlinearities, the analysis of the third-order Chebyshev elastic and viscous coefficients showed that the sample which contains the highest amount of solids different from cocoa exhibited the lowest strain-stiffening and the highest shear-thinning behaviors. Stress bifurcation analysis identified the beginning of the yielding transition and evidenced a nonlinear solid-like behavior before the solid-liquid transition. Analysis of Lissajous–Bowditch curves allowed the static and dynamic contributions of the yielding transition to be identified, and the full Sequence of Physical Process quantitative analysis captures the time-dependent nonlinear response of chocolates over the history of applied deformations.

Exploring the interaction of sesamol as an antilung cancer compound with albumin through spectroscopic and bioinformatic analyses and the mechanism of anticancer effect

Sesamol has moved into biomedical research in recent years. However, its interactions with blood proteins and cancer cells have not been fully explored. Therefore, we aimed to investigate the interaction of sesamol with human serum albumin (HSA), A549 human nonsmall cell lung cancer (NSCLC) cell line, and Raw 264.7 macrophage. The interaction of HSA with sesamol was explored via application of fluorescence and circular dichroism (CD) spectroscopy studies as well as molecular docking analysis. Then, the cytotoxic effects of sesamol on A549 lung cancer cells and Raw 264.7 macrophages were evaluated by qPCR analysis. It was found that sesamol spontaneously (ΔG˚=-45.89 kJ/mol) binds with HSA having a high affinity (log K b = 8.05, n = 1.70, T = 298 K ) and form a static complex trough contribution of hydrogen bonds and van der Waals interactions (ΔH˚=-409.43 kJ/mol, TΔS˚=-363.54 kJ/mol) which was supported by molecular docking study. Furthermore, by using CD and synchronous fluorescence spectroscopy analyses it was found that sesamol induced some minor secondary and tertiary structural changes, respectively in HSA structure. Cellular assays displayed that sesamol triggered selective cytotoxicity against A549 lung cancer cells through regulation of intrinsic apoptosis pathway mediated by mitigation of mitochondrial membrane potential, elevation of ROS generation, downregulation of Bax, and up regulation of caspase-9, −3. In conclusion, it was found that sesamol could show high affinity with HSA and mediate intrinsic apoptosis pathway through ROS generation in the A549 lung cancer cell lines. These data indicate that the biochemical and anticancer mechanisms of sesamol can be further investigated in future studies to integrate it in the biomedical platforms.

Superconductivity in black phosphorus and the role of dynamical screening

Simple cubic phosphorus exhibits superconductivity with a maximum $T_c$ of up to 12 K under pressure. The pressure dependence of $T_c$ cannot be consistently explained with a simple electron-phonon mechanism, which has stimulated investigations into the role of electronic correlations and plasmonic contributions. Here, we solve the gap equation of density functional theory for superconductors using different electron-electron and electron-phonon contributions to the kernel. We find that the phonon contribution alone yields an overestimation of $T_c$, while the addition of the static electronic contribution results in an underestimation. Taking into account the full frequency dependence of the screened interaction, the one-shot $GW$ approximation predicts $T_c$ values in good agreement with the experiments in the pressure range appropriate for the cubic phase. We also explore the use of quasi-particle bands in the calculation of the electronic and phononic kernels, and show that this modification significantly improves $T_c$ in the high-pressure region.

Unraveling Urbach Tail Effects in High-Performance Organic Photovoltaics: Dynamic vs Static Disorder

The origin of Urbach energy (EU) in organic semiconductors and its effect on photovoltaic properties remain a topic of intense interest. In this letter, we demonstrate quantitative information on the EU value in emerging Y-series molecules by an in-depth analysis of the line shape of the temperature-dependent quantum efficiency spectra. We found that the static disorder (EU(0)), which is dominated by the conformational uniformity in Y-series acceptors, contributes 10–25 meV to the total Urbach energy. Particularly, this static contribution in organic solar cells (OSCs) is much higher than those (EU(0) ≈ 3–6 meV) in inorganic/hybrid counterparts, such as CH3NH3PbI3 perovskite, crystalline silicon (c-Si), gallium nitride (GaN), indium phosphide (InP), and gallium arsenide (GaAs). More importantly, we establish clear correlations between the static disorder and photovoltaic performance and open-circuit voltage loss. These results suggest that suppressing the static disorder via rational molecular design is clearly a path for achieving higher performance.

Gorkov algebraic diagrammatic construction formalism at third order

Background: The Gorkov approach to self-consistent Green's function theory has been formulated by Som\`a, Duguet, and Barbieri in [Phys. Rev. C 84, 064317 (2011)]. Over the past decade, it has become a method of reference for first-principles computations of semimagic nuclear isotopes. The currently available implementation is limited to a second-order self-energy and neglects particle-number nonconserving terms arising from contracting three-particle forces with anomalous propagators. For nuclear physics applications, this is sufficient to address first-order energy differences (i.e., two neutron separation energies, excitation energies of states dominating the one-nucleon spectral function), ground-state radii and moments on an accurate enough basis. However, addressing absolute binding energies, fine spectroscopic details of $N\ifmmode\pm\else\textpm\fi{}1$ particle systems or delicate quantities such as second-order energy differences associated with pairing gaps, requires going to higher truncation orders.Purpose: The formalism is extended to third order in the algebraic diagrammatic construction (ADC) expansion with two-body Hamiltonians.Methods: The expansion of Gorkov propagators in Feynman diagrams is combined with the algebraic diagrammatic construction up to the third order as an organization scheme to generate the Gorkov self-energy.Results: Algebraic expressions for the static and dynamic contributions to the self-energy, along with equations for the matrix elements of the Gorkov eigenvalue problem, are derived. It is first done for a general basis before specifying the set of equations to the case of spherical systems displaying rotational symmetry. Workable approximations to the full self-consistency problem are also elaborated on. The formalism at third order it thus complete for a general two-body Hamiltonian.Conclusions: Working equations for the full Gorkov-ADC(3) are now available for numerical implementation.

Dynamics of Two Ferromagnetic Insulators Coupled by Superconducting Spin Current.

A conventional superconductor sandwiched between two ferromagnets can maintain coherent equilibrium spin current. This spin supercurrent results from the rotation of odd-frequency spin correlations induced in the superconductor by the magnetic proximity effect. In the absence of intrinsic magnetization, the superconductor cannot maintain multiple rotations of the triplet component but instead provides a Josephson type weak link for the spin supercurrent. We determine the analog of the current-phase relation in various circumstances and show how it can be accessed in experiments on dynamic magnetization. In particular, concentrating on the magnetic hysteresis and the ferromagnetic resonance response, we show how the spin supercurrent affects the nonequilibrium dynamics of magnetization which depends on a competition between spin supercurrent mediated static exchange contribution and a dynamic spin pumping contribution. Depending on the outcome of this competition, a mode crossing in the system can either be an avoided crossing or mode locking.

Optical resonances in graded index spheres: A resonant-state-expansion study and analytic approximations

Recent improvements in the resonant-state expansion (RSE), focusing on the static mode contribution, have made it possible to treat transverse-magnetic (TM) modes of a spherically symmetric system with the same efficiency as their transverse-electric (TE) counterparts. We demonstrate here that the efficient inclusion of static modes in the RSE results in its quick convergence to the exact solution regardless of the static mode set used. We then apply the RSE to spherically symmetric systems with continuous radial variations of the permittivity. We show that in TM polarization, the spectral transition from whispering gallery to Fabry-Perot modes is characterized by a peak in the mode losses and an additional mode as compared to TE polarization. Both features are explained quantitatively by the Brewster angle of the surface reflection which occurs in this frequency range. Eliminating the discontinuity at the sphere surface by using linear or quadratic profiles of the permittivity modifies this peak and increases the Fabry-Perot mode losses, in qualitative agreement with a reduced surface reflectivity. These profiles also provide a nearly parabolic confinement for the whispering gallery modes, for which an analytical approximation using the Morse potential is presented. Both profiles result in a reduced TE-TM splitting, which is shown to be further suppressed by choosing a profile radially extending the mode fields. Based on the concepts of ray optics, phase analysis of the secular equation, and effective quantum-mechanical potential for a wave equation, we have further developed a number of useful approximations which shed light on the physical phenomena observed in the spectra of graded-index systems.

Static and Dynamic Disorder of Charge Transfer States Probed by Optical Spectroscopy

AbstractSince the key role of charge transfers (CT) states has been identified for organic solar cells (OSCs), research into their properties is a timely topic. Conventionally, their absorption and emission spectra are described in terms of Marcus’ electron transfer theory. This is a single site approach with the essential parameter being the reorganization energy. Thus, it ignores ensemble effects, notably the role of static disorder that is inevitably present in a spin‐coated OSC film. Here time dependent photoluminescence spectroscopy is applied on blends of the polymeric donor MeLPPP with either the non‐fullerene acceptor SF‐PDI2 or with PC61BM within a temperature range from 295 to 5 K. The authors monitor how initially excited singlet states are converted to emissive CT states. Concomitantly, emission from residual singlets on the acceptor is observed rather than hybrid CT‐states. The role of spectral diffusion in this process is discussed. From the temperature and time dependent linewidths of absorption, fluorescence, and CT emission, the static and dynamic contributions to the total disorder are inferred. In both blends, at 295 K, the contribution of static disorder is comparable to the dynamic disorder.

Static and dynamic components of Debye-Waller coefficients in the novel cubic polymorph of low-temperature disordered Cu2ZnSnS4.

Cu2ZnSnS4 (CZTS) is an attractive material for sustainable photovoltaics and thermoelectrics, and several properties originate from its marked polymorphism. High-energy mechanical alloying is found to lead to a disordered phase that possesses a sphalerite-like cubic structure. This is investigated in detail with the aid of laboratory and synchrotron radiation X-ray diffraction, Raman spectroscopy, electron microscopy and ab initio molecular dynamics. The disordered cubic polymorph is preserved below 663 K. With thermal treatments above 663 K, the tetragonal kesterite phase forms, used here as a reference for structural and microstructural features. Particular attention is paid to the stacking arrangement: a significant fraction of twin faults was found in the disordered cubic samples, which then progressively annealed with domain growth and with the transition to the ordered tetragonal phase. This study also focuses on Debye-Waller coefficients, which were found to be considerably larger for the disordered cubic than the tetragonal sample. Indeed, disorder leads to an ∼1 Å2 upward shift through the temperature range 100-700 K, a feature confirmed by ab initio calculations, which points to a particularly high contribution from disordered Sn cations. This supports the general understanding that structural disorder introduces a temperature-independent static contribution to the atomic mean-square displacement. Debye-Waller coefficients are found to be a good measure of this disorder, known to have a critical effect on transport properties.

Investigation of propeller slipstream effects on lateral and directional static stability of transport aircraft

ABSTRACT The aerodynamic characteristics of turboprop aircraft are greatly affected by the running propellers. In this paper, propeller slipstream effects on the static lateral-directional stability characteristics of a typical twin-engine turboprop aircraft with clockwise rotating propellers were investigated through unsteady computational fluid dynamics (CFD) simulations and wind tunnel experiments. The propeller slipstream led to a significant decrease in the lateral static stability contribution of the wing-body-nacelle (WBN), and the most affected wing sections were located near the left boundary of the port propeller slipstream region. Moreover, the influence of the propeller slipstream on the lateral static stability contribution of the vertical tail plane (VTP) significantly altered the rolling moment curve slope of the entire airframe. Due to the effects of the propeller slipstream on the local dynamic pressure and therefore the effectiveness of the VTP, the aircraft showed significantly different directional static stability behaviors between the positive and negative sideslip conditions. Furthermore, it was found that the configurations with clockwise rotating propellers readily experience dramatic losses in directional static stability at large negative sideslip angles.

Analysis of the Urbach tail in cesium lead halide perovskites

The role of structural and dynamical disorder in semiconductors is a topic of fundamental relevance because of its contribution to the spectral line shape of the photoluminescence, and it plays a major role in ruling the carrier transport properties at the band edge. In this regard, a class of semiconductors, i.e., halide perovskites, deeply investigated in the last decade, shows a peculiar degree of disorder, which has only been recently under investigation. The interest to study disorder in halide perovskites is related to the large set of innovative applications of this class of materials, spanning from energy harvesting to high brilliance incoherent and coherent light emitters. In this perspective, we show that quantitative information on the disorder in halide perovskites can be extracted by deep analysis of the photoluminescence in different experimental conditions. Our study, conducted on a large set of samples of a metal halide perovskite, CsPbBr3, prepared with various synthesis/deposition methods, clarifies the relative weight of the static and dynamic contributions. A comparison with theoretical predictions is provided, gaining insights into the exciton/carrier–phonon interaction in metal halide perovskites.