1,721,004 research outputs found
Methylammonium fragmentation in amines as source of localized trap levels and the healing role of Cl in hybrid lead-iodide perovskites
Full text linkThe resilience to deep traps and localized defect formation is one of the important aspects that qualify a material as a suited photoabsorber in solar cell devices. Here we investigate by ab initio calculations the fundamental physics and chemistry of a number of possible localized defects in hybrid methylammonium lead-iodide perovskites. Our analysis encompasses a number of possible molecular fragments deriving from the dissociation of methylammonium. In particular, we found that in stoichiometric conditions both ammonia and methylamine molecules present lone-pair localized levels well within the perovskite band gap, while the radical cation CH2NH3+ observed by EPR after irradiation injects partially-occupied levels into the band gap but only in p-type conditions. These defects are thus potentially capable of significantly altering absorption and recombination properties. Amazingly, we found that additional interstitial Cl is capable of removing these localized states from the band gap. These results are consistent with the observed improvement of photoabsorption properties due to the Cl inclusion in the solution processing
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
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
Structural and optoelectronic properties of unsaturated ZnO and ZnS nanoclusters
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Journal of Physical Chemistry C
Volume 116, Issue 15, 19 April 2012, Pages 8741-8746
Structural and optoelectronic properties of unsaturated ZnO and ZnS nanoclusters (Article)
Malloci, G.a ,
Chiodo, L.b,
Rubio, A.c,
Mattoni, A.a
a Istituto Officina Dei Materiali (CNRIOM), Unità di Cagliari, Cittadella Universitaria, I-09042 Monserrato (Ca), Italy
b Center for Biomolecular Nanotechnologies UNILE, Istituto Italiano di Tecnologia, European Theoretical Spectroscopy Facility, Via Barsanti, I-73010 Arnesano, Italy
c European Theoretical Spectroscopy Facility (ETSF), Nano-Bio Spectroscopy Group, CFM-CSIC-UPV/EHU-MPC and DIPC, Avenida Tolosa 72, E-20018 San Sebastián, Spain
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Abstract
We report a systematic computational study of the structural and optoelectronic properties of unsaturated ZnO and ZnS nanoclusters with hexagonal prism structure, as a function of length and diameter. We computed the fundamental gap using density functional theory (DFT) in the framework of the ?SCF scheme and the optical gap by means of time-dependent DFT (TDDFT). We found that all ZnO nanostructures transform from wurtzite to graphitic phase. On the contrary, ZnS nanocrystals with diameters above ∼1 nm are found to transform to a zeolite BCT phase. These different structural properties reflect in a very different size dependence of the electronic and optical properties, with a strong discontinuity for ZnS particles. The correlation between morphology and optoelectronic properties is demonstrated by considering models of saturated clusters preserving the wurtzite phase. We additionally compared DFT/TDDFT results with many-body perturbation theory methods showing a general good agreement among the two techniques for this class of nanocrystals of the two material
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
Radiative recombination and photoconversion of methylammonium lead iodide perovskite by first principles: properties of an inorganic semiconductor within a hybrid body
No full textThe excellent photoconversion properties of lead iodide hybrid perovskites, used as absorber in solar cell devices with power conversion efficiencies exceeding 15%, are explained on the basis of ab initio calculated radiative recombination rates and minority carrier lifetimes. Brad ∼ (0.5-1.5) × 10-9 s-1 cm3 and minority lifetime ∼103 ns were obtained for a doping concentration of n ∼ 1015 cm-3 at room temperature. These values, comparable to those of typical optoelectronic semiconductors (e.g., GaAs), reflect the very nature of the perovskite: fully solution-processable owing to its hybrid nature, and yet a truly inorganic semiconductor for with regard to photoconversion properties. Recombination rates are also used to quantitatively describe the maximum limit of power conversion efficiency potentially achievable by these systems, for example, 21% for a 200 nm thick perovskite film and 23% for a 300 nm thickness
Many-Body MYP2 Force-Field: Toward the Crystal Growth Modeling of Hybrid Perovskites
No full textHybrid perovskites are well-known for their optoelectronic and photovoltaic properties. Molecular dynamics simulations allow the study of these soft and ionic crystals by including dynamical effects (e.g., molecular rotations, octahedra tilting, ionic diffusion and hysteresis), yet the high computational cost restricts the use of accurate ab initio forces for bulk or small atomic systems. Hence, great interest exists in the development of classical force-fields for hybrid perovskites of low and linear scaling computational cost, via both empirical methods and machine-learning. This work aims at extending the transferability of our MYP0 model, which has been successfully tailored to methylammonium lead iodide (MAPI) and applied to the study of molecular rotations, vibrations, diffusion of defects, and many other properties. The extended model, named MYP2, improves the description of inorganic or hybrid fragments and the processes of crystal formation while preserving a good description of bulk properties. More importantly, it allows for the direct simulation of the crystal growth of cubic MAPI from deposition of PbI and MAI precursors on the surfaces. Our findings pave the way toward classical force-fields able to model the microstructure evolution of hybrid perovskites and the crystalline synthesis from deposition in vacuo
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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