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Materials for energy

Towards a multi-scale approach to the simulation of silicon hetero-junction solar cells
PV The silicon hetero-junction (SHJ) technology holds the current efficiency record of 25.6% for silicon-based single junction solar cells and shows great potential to become a future industrial standard for high-efficiency crystalline silicon (c-Si) cells. One of the main advantages of this concept over other wafer based silicon technologies are the very high open-circuit voltages that can be achieved thanks to the passivation of contacts by thin films of hydrogenated amorphous silicon (a-Si:H). The a-Si:H/c-Si interface, while central to the technology, is still not fully understood in terms of transport and recombination across this nanoscale region, especially concerning the role of the different localized tail and defect states in the a-Si:H and at the a-Si:H/c-Si interface and of the band offsets and band bending induced by the heterostructure potential and the large doping, respectively. For instance, a consistent microscopic picture of transport and recombination processes with treatment of thermal and tunneling mechanisms on equal footing is lacking. On the other hand, there are new SHJ device architectures like thin wafers with light trapping structures or interdigitated back contact (IBC) cells, which define additional requirements on the modelling approach concerning the integration of 3D optical and electrical simulations.
  • U. Aeberhard, P. Czaja, M. Ermes, B. E. Pieters, G. Chistiakova, K. Bittkau, A. Richter, K. Ding, Forschungszentrum Julich, Germany
  • S. Giusepponi, M. Celino, ENEA
H-abstraction from methane in H-ZSM5 zeolite with extra-framework O atom: dft and d-dft in comparison
PV The interaction of methane with an extra-framework oxygen atom in acidic Zeolite (H-ZSM5) porous substrate has been investigated by means of Density Functional Theory plus Dispersion energy calculations and reaction path has been obtained exploiting Climbing Image Nudged Elastic Band method (c-NEB). Zeolite was modelled by its crystallographic structure subject to periodic boundary condition. The reaction path for the H-abstraction reaction of methane, in presence of an open shell oxygen atom within zeolite along the (010) straight channel, leads to the formation of a slightly distorted H2O water molecule and CH3 radical and proceeds with a small barrier. When dispersion energy is applied the barrier disappears revealing known flaws of the approach adopted. These in bulk calculations support anyway the interesting idea that open shell systems, involving an extra cage Oxygen atom, favour the H-abstraction from small hydrocarbons within acidic zeolite channels.
  • A. Palma, CNR
Hydrogen storage in nanotubes
SWNT_Li_doping The recent discovery of the enhanced hydrogen adsorption in Li doped single walled carbon nanotubes is revised. The adsorption isotherms have been calculated by means of grand canonical monte carlo simulations using accurate parameters obtained by fitting ab-initio data. In the pressure range explored and for the tube bundles geometry chosen it is shown that, contrarily to recent analogous studies, the adsorption is still below the target fixed by the United States Department of Energy.
  • S. Mirabella and G. Zollo, Univ. La Sapienza

The effects of vacancies in the mechanical properties of tungsten: an ab-initio study
The ideal tensile strength of bcc crystal tungsten in presence of mono and divacancy has been investigated by using first-principles total energy method based on the density functional theory. Crystalline tungsten containing vacancies (concentration of vacancies about 2% and 4%) has been characterized in terms of structural and mechanical properties. The maximum tensile stress required to reach elastic instability under increasing load has been further computed.
  • Simone Giusepponi and Massimo Celino, ENEA
Molecular dynamics of ionic self-diffusion at an MgO grain boundary
Wurtzite The characterization of self-diffusion in MgO grain boundaries is a materials science problem of general interest, being relevant to the stability and reactivity of MgO layers in artificial nanostructures as well as to the understanding of mass transport and morphological evolution in polycrystalline metal oxides which are employed in many technological applications. In addition, atomic transport in MgO is a key factor to describe the rheology of the Earth's lower mantle. In this work, we tackle the problem using a classical molecular dynamics model and finite-temperature simulations. To this purpose, we first design a stable grain boundary structure, which is meant to be representative of general internal interfaces in nanocrystalline MgO. The Mg and O self-diffusion coefficients along this grain boundary are then determined as a function of temperature by calculating the mean-square ionic displacement in the boundary region. Two different diffusion regimes at low and high temperature are identified, allowing to obtain the relevant activation enthalpies for migration from the temperature dependance of the diffusion coefficients. Our results prove that Mg diffusion along MgO grain boundaries is sufficiently fast to explain the recently reported development of MgO hollow structures during repeated hydrogen sorption cycles in Mg/MgO nanoparticles.
  • F. Landuzzi, L. Pasquini, Univ. Bologna

Quantum dots:
Wurtzite First-principle molecular dynamics characterization of CdS nanostructures for optoelectronic applications. The CdS cluster has been carefully characterized structurally at several temperatures up to T= 600 K. At the temperature of 340 K atomic diffusion on the surface allows the onset of a new stable atomic configuration. Surface configurations and electronic properties are computed to characterize the phase transformation.
  • Emiliano Burresi, ENEA Faenza

EFDA: European Fusion Development Agreement:
A reliable physical model for Tungsten will be prepared, then total energy calculations will be performed to check if the model can reproduce experimental structural properties. The same procedure will be performed for all the elements that will be considered: Ta, Re, V etc. Ab-initio calculations on the formation enthalpy of the W-Re and W-Ta will be computed at selected values of concentrations. Temperature analysis of the structural properties of the W-Ta and W-V alloys will be performed for the understanding of the phase stability. Moreover irradiated point defects changing the concentrations of alloying elements will be characterized. Radiation damage will be addressed by performing non-equilibrium molecular dynamics simulations of the irradiation of the microstructures. The effect of radiation spectra will be simulated by input distributions of PKA (primary knock-on atom) kinetic energy. Effects of He and H contamination on the microstructure properties (e.g., segregation at grain boundaries and dislocation cores) will be included.
  • Simone Giusepponi and Massimo Celino, ENEA
Lead-bismuth eutectic as coolant nuclear reactors:
LBE Lead-bismuth eutectic (LBE) has been a potential candidate as a coolant material in advanced nuclear reactors design. It is also well known the steel cladding was severely corroded when they are exposed to LBE directly at high temperatures. This crucial issue has presented a critical challenge in the use of LBE and then a full knowledge of corrosion characteristics and how to reduce corrosion are essential for safety of heat transfer systems in reactors. In our previous work we have used MD method to calculate the diffusion coefficient of iron in liquid lead/LBE. In the present work we report the effect of oxygen injection into LBE to reduce the corrosion rate of iron in LBE by using MD method. We predicted the most effective oxygen injection for efficient iron corrosion reduction.
  • Artoto Arkundato, Univ. Bandung, Indonesia
Hydrogen production:
Diffusion of hydrogen and its isotopes in metals is a subject of great interest due to its applications in a large variety of scientific and technological areas. Among the others there is a great technological effort to find the metal that could act as a membrane for hydrogen production. However the task is not easy because the metal suitable for hydrogen production should be able, at the same time, to resist to corrosion, to separate hydrogen from other elements, to have an high diffusion of hydrogen without destroying its lattice and to have an accessible cost of production. Till now only some materials are good candidate to this applications. A numerical approach to the problem of hydrogen production is needed to test quickly different alloys that could fit better to the above prerequisite. A general numerical scheme has been developed in the framework of ab-initio molecular dynamics to design the alloy. This framework has been tested on a crystalline Tantalum because it is seen as a possible material for the realization of metallic thin membranes for Hydrogen separation, with a high resistance to corrosion. Theoretical investigations of the interaction between Hydrogen and Tantalum lattice are useful to test the properties and the resistance of a Tantalum membrane. Investigations of the behaviour of the H atoms in the lattice has been performed, in order to identify the interstitial sites occupied by the Hydrogen atoms, to calculate the formation enthalpy of Ta / H structures at different H concentrations, and to investigate the distortions induced in the lattice by the presence of Hydrogen. Calculations are performed using CPMD code.
  • Roberto Grena and Massimo Celino, ENEA
Hydrogen storage:
MgH2interface Nanostructured magnesium hydride MgH2, prepared by a mechanical milling method, is considered an attractive hydrogen storage material. In particular, MgH2 shows interesting properties such as high H2 gravimetric storage capacity (7.6 wt%), low cost and high abundance. However, this material displays too high temperatures of decomposition, mainly owing to high thermodynamic stability and slow decomposition kinetics, so that several routes have been proposed in order to enhance the reaction and to reduce the decomposition temperature. These processes are based on the introduction of an high density of crystal defects or by ball-milling with intermetallic compounds with lower hydrogen desorption temperature than magnesium. Since the desorption mechanism is strongly influenced by the chemical and mechanical properties at the interface between MgH2 and Mg, a detailed study of this interface is needed. From an experimental point of view there is not a clear evidence of which interfaces are involved in the hydrogen diffusion and which is the atomic dynamics at the interfaces. However, extensive first-principle molecular dynamics simulations of the interface MgH2-Mg give clear indications of both the equilibrium properties and the behaviour of the Mg and H atoms in terms of total energy calculations. The interface and the hydrogen desorption are studied as functions of the temperature. The atomic environment of the Mg atoms at the interface and hydrogen paths for desorption are characterized and studied. Furthermore some indications of the rearrangement of the magnesium atoms after desorption are provided to characterize the phase transition.
Hydrogen desorption from the interface (movie)
  • Simone Giusepponi and Massimo Celino, ENEA
Photovoltaic devices:
Semiempirical and density functional molecular orbital calculations are performed on fullerene derivatives with varying reduction potentials, successfully used as an electron acceptor in bulk heterojunction solar cells. The geometries of all the compounds were optimized with the semiempirical PM3 method. Density functional theory (DFT) single-point calculations, B3LYP/3-21G*, have been carried out with the aim to investigate the energy levels of the frontier orbitals. We have correlated the theoretical lowest unoccupied molecular orbital (LUMO) levels of different fullerenes with the open-circuit voltage of the photovoltaic device based on the blend of poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene (MDMO-PPV) with the acceptor molecules. We have also investigated the influence of new substituents on the LUMO level of the parent fullerene showing the possibility to further increase the open-circuit voltage of the MDMO-PPV:fullerene device.
  • Pasquale Morvillo, ENEA

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