Progress,highlights and perspectives on NiO in perovskite photovoltaics

The power conversion efficiency (PCE) of NiO based perovskite solar cells has recently hit a record 22.1% with a hybrid organic–inorganic perovskite composition and a PCE above 15% in a fully inorganic configuration was achieved. Moreover, NiO processing is a mature technology, with different industrially attractive processes demonstrated in the last few years. These considerations, along with the excellent stabilities reported, clearly point towards NiO as the most efficient inorganic hole selective layer for lead halide perovskite photovoltaics, which is the topic of this review. NiO optoelectronics is discussed by analysing the different doping mechanisms, with a focus on the case of alkaline and transition metal cation dopants. Doping allows tuning the conductivity and the energy levels of NiO, improving the overall performance and adapting the material to a variety of perovskite compositions. Furthermore, we summarise the main investigations on the NiO/perovskite interface stability. In fact, the surface of NiO is commonly oxidised and reactive with perovskite, also under the effect of light, thermal and electrical stress. Interface engineering strategies should be considered aiming at long term stability and the highest efficiency. Finally, we present the main achievements in flexible, fully printed and lead-free perovskite photovoltaics which employ NiO as a layer and provide our perspective to accelerate the improvement of these technologies. Overall, we show that adequately doped and passivated NiO might be an ideal hole selective layer in every possible application of perovskite solar cells.

The power conversion efficiency of NiO based perovskite solar cells has recently hit a record 22.1%. Here, the main advances are reviewed and the role of NiO in the next breakthroughs is discussed.     

Exceptional lie algebras at the very foundations of space and time

While describing the results of our recent work on exceptional Lie and Jordan algebras, so tightly intertwined in their connection with elementary particles, we will try to stimulate a critical discussion on the nature of spacetime and indicate how these algebraic structures can inspire a new way of going beyond the current knowledge of fundamental physics.     

Boosts in an Arbitrary Direction and Maximal Causal Velocities in a Deformed Minkowski Space

We discuss boosts in a deformed Minkowski space, i.e., a four-dimensional spacetime with metric coefficients depending on nonmetric coordinates (in particular on the energy). The general form of a boost in an arbitrary direction is derived in the case of space anisotropy. Two maximal trivector velocities are mathematically possible, an isotropic and an anisotropic one. However, only the anisotropic velocity has physical meaning, being invariant indeed under deformed boosts.     

Freudenthal duality and generalized special geometry

Freudenthal duality, introduced in Borsten et al. (2009) [1] and defined as an anti-involution on the dyonic charge vector in d=4 space-time dimensions for those dualities admitting a quartic invariant, is proved to be a symmetry not only of the classical Bekenstein-Hawking entropy but also of the critical points of the black hole potential.Furthermore, Freudenthal duality is extended to any generalized special geometry, thus encompassing all N>2 supergravities, as well as N=2 generic special geometry, not necessarily having a coset space structure.     

A kinematical study of Kirchhoff-Rayleigh flow

Summary A method of calculating the separated flow of a viscous fluid is proposed, which allows to split up properly the boundary condition problem from the viscous phenomena. The theory is developed for the flow past a plate and yields wakes of finite extension having an underpressure which depends directly on the amount of vorticity diffusion and dissipation occurring in the fluid. Application of the method to real flows shows good agreement between the calculated and the measured velocity distributions in front of the plate and in the wake.

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Résumé Une méthode de calcul de l'écoulement décollé d'un fluide visqueux est proposée qui permet de séparer clairement le problème aux limites des phénomènes visqueux. La théorie est développée pour l'écoulement autour d'une plaque et donne des sillages de longueur finie ayant une dépression de culot directement dépendante de l'intensité de la diffusion et dissipation de la vorticité se produisant dans le fluide. L'application de la méthode à des écoulements réels montre une bonne concordance entre les répartitions de vitesse calculées et mesurées sur le devant de la plaque et dans le sillage.

     

Measurement and DFT calculation of Fe(cp)(2) redox potential in molecular monolayers covalently bound to H-Si(100)

The electron transfer to self-assembled molecular monolayers carrying a ferrocene (Fc) center, grafted on a flat Si(100) surface, is a recent subject of experimental investigation. We report here the density functional theory (DFT) ab initio calculation of Fc-silicon hybrid redox potentials. The systems were modeled with a slab of H-terminated Si(100) 1 x 1 and 2 x 1 surfaces: geometries were optimized using the ONIOM method, and solute-solvent interactions were included through the polarizable continuum model (PCM) method. Two new routes for Si functionalization with ethyl- (EtFC) and ethynyl-Fc (EFC) differing only in the unsaturation degree of the anchoring arm have been successfully explored, and the redox potential of the resulting hybrids has been measured by cyclic voltammetry: 0.675 and 0.851 V versus NHE for the EtFC and EFC derivatives, respectively. These values, along with the previously measured potential (0.700 V) for the mono-unsaturated derivative, vinyl-Fc, allow the relation between the unsaturation degree and the adduct redox potential to be studied. The comparison among the measured and computed potentials allows one to discriminate between different adduct isomers for the saturated species and more importantly provides strong indications that the carbon-carbon unsaturation initially present in the molecular arm used for anchoring to the surface is preserved upon addition, in contrast with the commonly accepted reaction mechanism.     

Electrochemically deposited ZnO films: an XPS study on the evolution of their surface hydroxide and defect composition upon thermal annealing

Electrodeposition from ZnCl₂ aqueous solution was performed to grow ZnO thin films on the surface of polycrystalline copper plates. Electrochemical parameters for deposition were optimized by means of cyclic voltammetry (CV). The morphology of the deposits was studied via scanning electron microscopy (SEM), and their chemical composition was ascertained by means of X-ray photoelectron spectroscopy (XPS). The effects of changing the deposition bath temperature (T _bath) and the role played by post-deposition treatments, such as temperature and time of annealing in air, were studied. SEM images of freshly deposited vs. annealed samples have shown that in the former case the films display a rough morphology with mixed grain/hexagonal platelets structures and in the latter smaller but more uniformly dispersed cubic grains. T _bath is found to be the key parameter to induce the different morphology in the deposited films, which reflects in a different chemical reactivity of surface species, as found on the basis of the binding energies and relative quantitative ratios between Zn 2p and O 1s peaks. In fact, a higher T _bath favours a more efficient desorption of OH groups upon annealing, the O 1s peak resulting to much more drastically modified oxide/hydroxide intensity ratio with respect to the case of the sample deposited at lower T _bath.     

1,2‐Dimethoxyethane Degradation Thermodynamics in Li−O2 Redox Environments

The reaction thermodynamics of the 1,2‐dimethoxyethane (DME), a model solvent molecule commonly used in electrolytes for Li?O₂ rechargeable batteries, has been studied by first‐principles methods to predict its degradation processes in highly oxidizing environments. In particular, the reactivity of DME towards the superoxide anion O₂^? in oxygen‐poor or oxygen‐rich environments is studied by density functional calculations. Solvation effects are considered by employing a self‐consistent reaction field in a continuum solvation model. The degradation of DME occurs through competitive thermodynamically driven reaction paths that end with the formation of partially oxidized final products such as formaldehyde and methoxyethene in oxygen‐poor environments and methyl oxalate, methyl formate, 1‐formate methyl acetate, methoxy ethanoic methanoic anhydride, and ethylene glycol diformate in oxygen‐rich environments. This chemical reactivity indirectly behaves as an electroactive parasitic process and therefore wastes part of the charge exchanged in Li?O₂ cells upon discharge. This study is the first complete rationale to be reported about the degradation chemistry of DME due to direct interaction with O₂^?/O₂ molecules. These findings pave the way for a rational development of new solvent molecules for Li?O₂ electrolytes.