Quantum confinement and dielectric profiles of colloidal nanoplatelets of halide inorganic and hybrid organic-inorganic perovskites

International audienceQuantum confinement as well as high frequency ε∞ and static εs dielectric profiles are described for nanoplatelets of halide inorganic perovskites CsPbX3 (X = I, Br, Cl) and hybrid organic-inorganic perovskites (HOP) in two-dimensional (2D) and three-dimensional (3D) structures. 3D HOP are currently being sought for their impressive photovoltaic ability. Prior to this sudden popularity, 2D HOP materials were driving intense activity in the field of optoelectronics. Such developments have been enriched by the recent ability to synthesize colloidal nanostructures of controlled size of 2D and 3D HOP. This raises the need to achieve a thorough description of the electronic structure and dielectric properties of these systems. In this work, we go beyond the abrupt dielectric interface model and reach atomic scale description. We examine the influence of the nature of the halogen and of the cation on the band structure and dielectric constants. Similarly, we survey the effect of dimensionality and shape of the perovskite. In agreement with recent experimental results, we show an increase of the band gap and a decrease of ε∞ when the size of a nanoplatelet reduces. By inspecting 2D HOP, we find that it cannot be described as a simple superposition of independent inorganic and organic layers. Finally, the dramatic impact of ionic contributions on the dielectric constant εs is analysed

Importance of Vacancies and Doping in the Hole-Transporting Nickel Oxide Interface with Halide Perovskites

International audienceNickel oxide (NiO) is a commonly used contact material for a variety of thin-film optoelectronic technologies based on organic or hybrid materials. In such setups, interfaces play a crucial role as they can reduce, if not kill, the device performances by bringing additional traps or energy barriers, hindering the extraction of charge carriers from the active layer. Here, we computationally examine a prototype halide perovskite architecture, NiO/MAPbI (MA = CHNH), that has shown excellent photovoltaic performance and, in particular, a large open-circuit voltage. We show that efficient hole collection is achieved only when considering the role of vacancies induced by standard material deposition techniques. Specifically, Ni vacancies lead to nearly perfect valence band energy level alignment between the active layer and the contact material. Finally, we show how Li doping greatly improves the performances of the device and further propose alternative dopants. Our results suggest the high tunability of NiO interfaces for the design of optimized optoelectronic devices far beyond that of halide perovskites

Interplay between Electronic, Magnetic, and Transport Properties in Metal Organic–Radical Frameworks

International audienceDevelopment of modern electronic and spintronic technologies depends in large part on the ability to design materials exhibiting switchable magnetic and electrical properties. Here, motivated by the successful demonstration of reversible redox switching of magnetic order and electrical conductivity in 2dimensional metal-organic frameworks (MOFs) based on benzoquinoid linkers, we perform hybrid density functional theory calculations to investigate this phenomenon at the atomistic level. Electronic, magnetic and charge transport properties have been systematically investigated for oxidized and reduced forms of Mn and Fe benzoquinoid frameworks (i.e., (Me 4 N) 2 [Mn 2 L 3 ], (Me 4 N) 2 [Fe 2 L 3 ] and Na 3 (Me 4 N) 2 [Mn 2 L 3 ], Na(Me 4 N) 2 [Fe 2 L 3 ], respectively with deprotonated chloranilic acid as L). We demonstrate that the experimentally observed large increase in electronic conductivity upon ligand-centered reduction in the Mn MOF (10 9 S•cm −1), is due to cooperative effects arising from band gap reduction and the presence of electrons with lower effective mass. Superior conductivity (by at least 3 orders of magnitude) of the redox pair of the Fe benzoquinoid framework as compared to the Mn analog stems from similar factors and, notably, a large increase in electron delocalization for the reduced Fe compound

Rashba and Dresselhaus Couplings in Halide Perovskites: Accomplishments and Opportunities for Spintronics and Spin-Orbitronics

International audienceIn halide hybrid organic-inorganic perovskites (HOP), spin-orbit coupling (SOC) presents a well-documented large influence on band structure. However, SOC may also present more exotic effects, such as Rashba and Dresselhaus couplings. In this perspective, we start by recalling the main features of this effect and what makes HOP materials ideal candidates for the generation and tuning of spin-states. Then, we detail the main spectroscopy techniques able to characterize these effects and their application to HOP. Finally, we discuss potential applications in spintronics and in spin-orbitronics in those non-magnetic systems, that would complete the skill set of HOP and perpetuate its ride on the crest of the wave of popularity started with optoelectronics and photovoltaics

Evaluation of Leakage Current in 1-D Silicon Dangling-Bond Wire Due to Dopants

International audienceMolecular devices will be contacted by systems with increasingly reduced dimensions. The device will need to be held somehow, possibly on a solid surface, and electronic currents will be addressed to the device via some kind of 1-D interconnect of atomic size. The fact that the device is posed on a surface brings in perturbations and eventually malfunctions of the device. Semiconducting surfaces have been deemed to be good candidates for this molecular technology because they have an electronic gap that prevents current losses from the device plus their surfaces are full of directional chemical bonds that make them ideal to hold a molecular device. However, semiconductors are generally doped, intentionally or unintentionally. Here, we summarized our findings on how destructive the presence of dopants is on the working parameters of a possible device. In fact, instead of a molecular device, we choose an ideal 1-D surface interconnect made out of Si dangling bonds in an otherwise passivated Si(100)-H surface. The current lost into a doped silicon substrate from a surface-supported nanowire is evaluated using transport calculations based on the density functional theory. We considered two concentration limits: either a single-dopant nearby the wire or a massively doped system. Both limits yield qualitatively similar results, stressing the strong perturbation that a single dopant can exert on an atomic-size wire. Our calculations permit us to conclude that n-doped Si will be less leaky than p-doped S

Propriétés optoélectroniques des pérovskites hybrides : quelques résultats théoriques et expérimentaux récents

National audienceA partir de 2012, la thématique des pérovskites hybrides pour le photovoltaïque a explosé au niveau mondial, alors qu’elle était totalement passée sous silence jusqu’à la fin 2011. Cette émergence soudaine a amené le magazine « Science » à classer les cellules solaires à base de pérovskites hybrides dans le « Science journal’s top 10 breakthroughs in 2013 » et le journal "la Recherche" dans les 10 découvertes les plus importantes de l'année 2014. Les rendements photovoltaïques (http://www.nrel.gov/ncpv/images/efficiency_chart.jpg) ont atteint très rapidement 10% (2012), 15% (2013), puis 22% récemment en 2016 dans une filière non répertoriée jusqu’en 2013.L’équipe de simulation associant les équipes FOTON-OHM et ISCR-CTI a poursuivi récemment son travail théorique en 2015-2016, en explorant différents aspects associés aux propriétés optoélectroniques des matériaux 3D et 2D. Des études de spectroscopie en collaboration avec les groupes de Groningen et Stanford, permettent de mieux comprendre les rôles joués par le réseau inorganique et les cations organiques dans les propriétés excitoniques. Les collaborations entamées avec les groupes de Los Alamos, North Western University et Groningen, montrent également que la complexité des processus de génération et de transport des porteurs dans les matériaux 3D et 2D conditionnent les performances des dispositifs

Physical properties and stability of 3D and layered perovskite materials for photovoltaic applications

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Rudorfittes for solar-cell applications: Insights on their structures, electronic and optical properties from rst-principles

International audienc

Does the Rashba splitting in CH 3 NH 3 PbBr 3 arise from 2×2 surface reconstruction?

International audienceAs a result of early theoretical predictions, evidence for the Rashba or Dresselhaus effect in hybrid perovskites has recently attracted several experimental investigations, motivated by possible applications in spin-orbitronics. For instance, a large Rashba splitting has recently been reported for the (001) surface of CH3NH3PbBr3. This effect is forbidden in the bulk material since both low-temperature and room-temperature crystal structures present inversion symmetry. Here we investigate the effects of two (001) nanoscale surface reconstructions of CH3NH3PbBr3 using first-principles approaches based on density functional theory (DFT). The two experimental reconstructions are related to different orientations of MA cations at the surface, defining zigzag and dimer phases. The impact of these structural transformations on their electronic structures is thoroughly investigated. Whereas calculations reveal the occurrence of surface-induced Rashba effect, its amplitude is considerably smaller than the experimentally reported value, in agreement with other experimental investigations and leading to the conclusion that mesoscale surface polar domains and/or surface defects may result from sample preparation

Optoelectronic properties of Halide Perovskite: needs for extremely demanding high performance computing

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