Ultrathin carbon nanotube with single, double, and triple bonds

A metastable carbon nanotube with single, double, and triple bonds has been predicted from abinitio simulation. It results from the relaxation of an ideal carbon nanotube with chirality (2,1), without any potential barrier between the ideal nanotube and the new structure. Ten-membered carbon rings are formed by breaking carbon bonds between adjacent hexagons; eight-membered rings, already present in the ideal structure, become the smallest rings. This structure is stable in molecular dynamics simulations at temperatures up to 1000K. Raman, infrared, and optical absorption spectra are simulated to allow its identification in the laboratory. The structure can be described as a double helical chain with alternating single, double, and triple bonds, where the chains are bridged by single bondsThis work was supported by Grants No. SB2010-0119 (MEC), No. CTQ2010-19232 (MICIN), and No. A1/035856/11 (AECID

Atomic scale model and electronic structure of Cu2_2O/CH3_3NH3_3PbI3_3 interfaces in perovskite solar cells

Cuprous oxide has been conceived as a potential alternative to traditional organic hole transport layers in hybrid halide perovskite-based solar cells. Device simulations predict record efficiencies using this semiconductor, but experimental results do not yet show this trend. More detailed knowledge about the Cu$_2$O/perovskite interface is mandatory to improve the photoconversion efficiency. Using density functional theory calculations, here we study the interfaces of CH$_3$NH$_3$PbI$_3$ with Cu$_2$O to assess their influence on device performance. Several atomistic models of these interfaces are provided for the first time, considering different compositions of the interface atomic planes. The interface electronic properties are discussed on the basis of the optimal theoretical situation, but in connection with the experimental realizations and device simulations. It is shown that the formation of vacancies in the Cu$_2$O terminating planes is essential to eliminate dangling bonds and trap states. The four interface models that fulfill this condition present a band alignment favorable for photovoltaic conversion. Energy of adhesion, and charge transfer across the interfaces are also studied. The termination of CH$_3$NH$_3$PbI$_3$ in PbI$_2$ atomic planes seems optimal to maximize the photoconversion efficiency.Comment: 16 pages; 8 figures. Submitted to ACS Applied Materials & Interfaces. Published after changes not included her

Nurses' perceptions of aids and obstacles to the provision of optimal end of life care in ICU

Contains fulltext : 172380.pdf (publisher's version ) (Open Access

Alloy [FA,Cs]PbI₃ perovskite surfaces. The role of surface cesium composition in stability and tolerance to defect formation

Halide-perovskite alloys that include cesium have achieved records of stability and efficiency in solar cells. Controlling the surface composition, defects, and electronic properties guarantees interface stability and improves performance. By using density functional theory and molecular dynamic simulations, we analyzed which surface compositions of the formamidinium (FA) and cesium (Cs) lead iodide perovskite FA₁₋ₓCsₓPbl₃ with 25 and 50% of Cs become more stable than pure perovskites. Structural and electronic properties and tolerance to defect formation were also evaluated. Surface energy calculations show that only the alloys with 25% Cs and F Al-enriched surfaces are more stable than pure FAPbl₃ ones. The most stable alloy surface shows electronic energy levels similar to the FAPbl₃ perovskite, suggesting that this alloy may also be efficient for charge transport in the cell. However, the presence of Cs on the alloy surface, although low, favors the formation of FAI vacancies, which is detrimental to the stability of the perovskite. These results suggest evaluating FA₁₋ₓCsₓPbl₃ alloys with small Cs compositions to mitigate the formation of defects or using a passivation scherne. This study delivers valuable information for efficiency device improvement from the perspective of interface stability

Effective Interfaces between Fullerene Derivatives and CH₃ NH₃ PbI₃ to Improve Perovskite Solar Cell Performance

Inverted hybrid perovskite solar cells using fullerene derivatives as an electron transport layer show high energy photoconversion efficiency and improvements in stability. In practice, a wide variety of fullerene derivative functional groups have been proposed, but there is still no clear understanding of the influence of this structure on solar cell behavior. Using density functional theory calculations, we study the conditions that allow the transport of electrons without energetic barriers in the interface formed between the surfaces of CH₃ NH₃ PbI₃ and the derivatives of fulleropyrrolidine and PCBM. Representative atomistic models of the interfaces are provided, and the self-consistent electronic structures obtained with hybrid functionals were analyzed. It is shown that only the perovskite surface terminated in a layer rich in methylammonium iodide offers electron transport without energy barriers for fullerene derivatives. Moreover, the lead iodide (PbI₂)-terminated surface is not passivated with fullerene derivatives. The surface state disappears if the PbI₂-terminated surface is treated with ammonium salts or zwitterionic compounds, such as methylammonium chloride and sulfamic acid. Therefore, these modified surfaces favor the performance of the solar cells if the interfaces remain aligned, without barriers, for the transport of electrons. Our study offers these interface models to contribute to the optimal design of perovskite solar cells

Ferroelectric domains may Lead to two-dimensional confinement of holes, but not of electrons, in CH3NH3PbI3 perovskite

We investigate the possibility that formation of ferroelectric domains in CH3NH3PbI3 can separate the diffusion pathways of electrons and holes. This hypothesis has been proposed to explain the large recombination time and the remarkable performance of solar cells of hybrid perovskites. We find that a twodimensional hole confinement in CH3NH3PbI3 is possible under room-temperatureconditions. Our models of the tetragonal phase show that the alignment of dipole layers of organic cations induces the confinement of holes but not of electrons. This behavior does not change even when the strength of the ordered dipoles is varied. The confinement of holes is favored by asymmetric deformation of the inorganic framework triggered by its interaction with the organic cations. However, the lattice distortions counteract the effect of the oriented organic dipoles, preventing the localization of electrons

Methodological Issues in First-Principle Calculations of CH₃NH₃PbI₃ Perovskite Surfaces: Quantum Confinement and Thermal Motion

Characterization and control of surfaces and interfaces are critical for photovoltaic and photocatalytic applications. In this work, we propose CH₃NH₃PbI₃ (MAPI) perovskite slab models whose energy levels, free of quantum confinement, explicitly consider the spin–orbit coupling and thermal motion. We detail methodological tools based on the density functional theory that allow achieving these models at an affordable computational cost, and analytical corrections are proposed to correct these effects in other systems. The electronic state energies with respect to the vacuum of the static MAPI surface models, terminated in PbI₂ and MAI atomic layers, are in agreement with the experimental data. The PbI₂ terminated slab has in-gap surface states, which are independent of the thickness of the slab and also of the orientation of the cation on the surface. The surface states are not useful for alignments in photovoltaic devices, while they could be useful for photocatalytic reactions. The energy levels calculated for the MAI-terminated surface coincide with the widely used values to estimate the MAPI alignment with the charge transport materials, i.e., −5.4 and −3.9 eV for valence band maximum and conduction band minimum, respectively. Our study offers these slab models to provide guidelines for optimal interface engineering

Mixed-anion mixed-cation perovskite (FAPbI₃)₀ꓸ₈₇₅ (MAPbBr₃)₀ꓸ₁₂₅: an ab initio molecular dynamics study

Mixed-anion mixed-cation perovskites with (FAPbI₃)₁-­ₓ(MAPbBr₃)ₓ composition have allowed record efficiencies in photovoltaic solar cells, but their atomic-scale behaviour is not well understood yet, in part because their theoretical modelling requires consideration of complex and interrelated dynamic and disordering effects. We present here an ab initio molecular dynamics investigation of the structural, thermodynamic, and electronic properties of the (FAPbI₃)₀ꓸ₈₇₅ (MAPbBr₃)₀ꓸ₁₂₅ perovskite. A special quasirandom structure is proposed to mimic the disorder of both the molecular cations and the halide anions, in a stoichiometry that is close to that of one of today's most efficient perovskite solar cells. We show that the rotation of the organic cations is more strongly hindered in the mixed structure in comparison with the pure compounds. Our analysis suggests that this mixed perovskite is thermodynamically stable against phase separation despite the endothermic mixing enthalpy, due to the large configurational entropy. The electronic properties are investigated by hybrid density functional calculations including spin–orbit coupling in carefully selected representative configurations extracted from the molecular dynamics. Our model, that is validated here against experimental information, provides a more sophisticated understanding of the interplay between dynamic and disordering effects in this important family of photovoltaic materials