1,720,983 research outputs found
Atomistic investigation of the solubility of 3-alkylthiophene polymers in tetrahydrofuran solvent
No full textWe study the solubility properties of regioregular oligo(3-alkylthiophene)s in tetrahydrofuran solvent as a function of their alkyl chains length by an atomistic investigation based on model potential molecular dynamics. We make use of the Flory-Huggins theory that is typically used to study the miscibility of macromolecules and that is here applied for the first time to study the solubility of conjugated conducting polymers in a typical organic solvent. The properties of the isolated solvent and polymer are correctly reproduced, and the calculated solubilities of the oligo(3-alkylthiophene)s in tetrahydrofuran as a function of their side chains lengths are in agreement with available experimental data. Present investigation shows that the atomistic approach based on molecular dynamics is a powerful tool to study the solubility of alkylthiophenes in molecular solvents
Modeling hybrid perovskites by molecular dynamics
No full textThe topical review describes the recent progress in the modeling of hybrid perovskites by molecular dynamics simulations. Hybrid perovskites and in particular methylammonium lead halide (MAPI) have a tremendous technological relevance representing the fastest-advancing solar material to date. They also represent the paradigm of an organic-inorganic crystalline material with some conceptual peculiarities: an inorganic semiconductor for what concerns the electronic and absorption properties with a hybrid and solution processable organic-inorganic body. After briefly explaining the basic concepts of ab initio and classical molecular dynamics, the model potential recently developed for hybrid perovskites is described together with its physical motivation as a simple ionic model able to reproduce the main dynamical properties of the material. Advantages and limits of the two strategies (either ab initio or classical) are discussed in comparison with the time and length scales (from pico to microsecond scale) necessary to comprehensively study the relevant properties of hybrid perovskites from molecular reorientations to electrocaloric effects. The state-of-the-art of the molecular dynamics modeling of hybrid perovskites is reviewed by focusing on a selection of showcase applications of methylammonium lead halide: molecular cations disorder; temperature evolution of vibrations; thermally activated defects diffusion; thermal transport. We finally discuss the perspectives in the modeling of hybrid perovskites by molecular dynamics
Electronic properties of hybrid Zinc Oxide - Oligothiophene nanostructures
No full textUsing density functional theory in combination with model potential molecular dynamics, we study hybrid systems consisting of oligothiophene molecules with increasing chain length (two, four, and six rings) adsorbed onto a ZnO nanoparticle model. We investigate the energetics of adhesion and the morphological features at the curved interface. We compute the energy-level alignment taking many body effects into account within the ΔSCF approach. Our results show that, as a consequence of the local curvature of the interface, the electronic coupling between the organic and inorganic component affects the energy-level alignment in all systems, making it less favorable for charge separation. In particular, the energy-level alignment for sexithiophene on the ZnO curved nanoparticle does not lead to a type-II junction with staggered band gaps, contrary to what was recently found for sexithiophene on a flat (101̅0) ZnO surface. Although the limited size (and hence the large curvature) of the nanoparticle does not allow us to make a general statement, this indicates a trend that is valid for systems in which quantum confinement effects are important. As a side result of our study, we propose a simple practical model to predict the energy-level alignment in hybrid systems, which gives consistent results compared to ΔSCF
On the Development of a Classical Interatomic Potential for MAPbBr3
No full textWe develop a classical interatomic potential for MAPBr. The model belongs to the class of MYP force-fields for hybrid perovskites based on two-body Buckhingam-Coulomb and dispersive terms to describe organic-inorganic interactions and already successfully applied to MAPI. The model calibration is based on a simplified procedure able to extend one existing parameterization to a different halide by suitable scaling of selected subgroups of parameters. The main static and dynamical properties of MAPBr are well reproduced by the developed model: the lattice constant, cohesive energy curve, bulk modulus, energy barriers for cation rotations (both static and dynamic), the phase transition temperatures and structural parameters evolution with temperature. The model also provides valid relationship between MAPBr and MAPI: MAPBr has shorter lattice constant, higher cohesive energy, lower phase transition temperatures, and lager anisotropy in orthorhombic phase. The good comparison extends also to the vibrational properties at finite temperatures that have been benchmarked on experimental and DFT results. The developed MAPBr model is further used to calculate the MA dynamics in MAPBr at room temperature finding a reorientation time of ~3ps in good agreement with experimental data. Present work represents an important step towards the large-scale atomistic modeling of MAPBr and the development of a general class of force fields for hybrid perovskites
Poly(3-hexylthiophene) adhesion on Zinc Oxide Nanoneedles
No full textWe study the interface between poly(3-hexylthiophene) and wurtzite ZnO nanoneedles by molecular dynamics simulation. We provide evidence that the polymer is easily adsorbed on the nanostructured surface with the largest binding energy found when the chain is aligned to the needle axis. Helically wrapped polymer configurations are nevertheless possible, and they are found to be metastable with long lifetimes and small polymer mobility on the nanoneedle surface. The wrapped configuration lifetime has been compared with the case of carbon nanotube-based systems at finite temperatures and calculated to be much longer. The dependence of the adhesion energy on the polymer orientation is discussed and explained by a model, including strain, adhesion anisotropy, and nanoneedle shape effects (i.e., the presence of edges between facets)
Understanding the Helical Wrapping of Poly(3-hexylthiophene) on Carbon Nanotubes
No full textWe studied poly(3-hexylthiophene) adhesion on single-walled carbon nanotubes via molecular dynamics simulations. We provide evidence that, in absence of the solvent, the polymer tends to unwrap and align to the nanotube. The helical organization in vacuo is metastable, and its lifetime is a function of the temperature and the polymer length. A kinetic model is here proposed according to which unwrapping is thermally activated with an energy barrier as small as 0.09 eV. Our results at room temperature reproduce the experimentally observed pseudohelical morphology of the polymer and confirm the role of the nanotube chirality. Furthermore, we provide evidence of a strong stabilization effect on the wrapped configuration because of neighboring polymer chains
Bulk Structural and Electronic Properties at the Density Functional Theory and Post-Density Functional Theory Level of Calculation
No full textConversion of solar energy to electrical power using photovoltaic (PV) devices is one of the most important sources of renewable energy. Solar cells based on organic-inorganic perovskites, referred to as perovskite solar cells (PSCs), have recently drawn global interest 166because their power conversion efficiencies (PCEs) have increased dramatically, from 3.8% to more than 20%, over the course of only five to six years (NREL 2016). Specifically, in 2009, Miyasaka et al. used methylammonium lead halide, CH3NH3PbX3 (MAPbX3, where X = Br or I), as a light-absorbing layer in liquid-electrolyte-based dye-sensitized solar cells (DSSCs), and reported a PCE of 3.8% (Kojima et al. 2009). However, this type of PSC had a drawback: instability of the deposited MAPbI3 in the liquid electrolyte. This problem was solved by replacing the liquid electrolyte with solid-state, hole-transporting materials (HTMs), leading to an efficiency as high as 9.7% and devices exhibiting long-term stability (Kim et al. 2012). Up to that point, the PSC architectures were identical to those of conventional DSSCs. Light-harvesting dyes or organic-inorganic perovskite nanoparticles were regarded to play the role of sensitizer, which injects the excited electrons into a -mesoporous-TiO2 (mp-TiO2) scaffold or the holes into the HTM
Tuning the thermal conductivity of methylammonium lead halide by the molecular substructure
No full textBy using state-of-the-art atomistic methods we provide an accurate estimate of thermal conductivity of methylammonium lead halide as a function of sample size and temperature, in agreement with experimental works. We show that the thermal conductivity of methylammonium lead halide is intrinsically low, due to the low sound velocity of the PbI lattice. Furthermore, by selectively analyzing the effect of different molecular degrees of freedom, we clarify the role of the molecular substructure by showing that the internal modes above 150 cm-1 (in addition to rotations) are effective in reducing the thermal conductivity of hybrid perovskites. This analysis suggests strategies to tailor the thermal conductivity by modifying the internal structure of organic cations
Implementation of a "Design of experiments" methodology for the prediction of phototransistor degradation in a space environment
No full textA Design Of Experiments (DOE) methodology was suggested to define an optimized irradiation test plan. In this paper, the proposed test plan was used to model the degradation of the main performances (photo and dark current) of silicon based phototransistors arrays with respect to the Total Ionizing Dose (TID) and to the Displacement Damage Dose (DDD), over a wide range of space-kind environments. The expected performance degradation after an 18-year Low Earth Orbit (LEO) mission was calculated using this model. End-Of-Life (EOL) prediction results were compared to experimental ones obtained on devices irradiated with a proton beam degrader that simulates the 18 year LEO environment. The excellent agreement found between theoretical and experimental data makes this methodology particularly valuable for the space qualification of such devices
Hydrophilicity and Water Contact Angle on Methylammonium Lead Iodide
No full textSurface properties are often assessed with measurements of the contact angle of a water drop. The process is however flawed for the very important class of hybrid perovskite materials, extensively employed in solar cells and optoelectronics research, because they are water soluble and their surface degrades during contact angle measurements. While hybrid perovskites are considered to be highly hydrophilic, a contact angle with water of 83° can be measured, as if they were almost hydrophobic. By combining experiments and simulations, the actual value is explained as the result of the interaction of water with degraded superficial layers that form over sub-millisecond time scale at the water/perovskite interface. The models are validated against contact angle measurements for water on a variety of substrates, and are referenced to with the Young–Dupré relation between liquid–solid adhesion and contact angle. Present work reconciles the hydrophilic nature of methylammonium lead iodide with the apparent hydrophobic behavior in contact angle measurements, proposing a methodology for the study of contact angle on evolving substrates
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