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Person# Ariadni Boziki

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Related publications (7)

Density-functional theory (DFT) is a computational quantum mechanical modelling method used in physics, chemistry and materials science to investigate the electronic structure (or nuclear structure) (principally the ground state) of many-body systems, in particular atoms, molecules, and the condensed phases. Using this theory, the properties of a many-electron system can be determined by using functionals, i.e. functions of another function. In the case of DFT, these are functionals of the spatially dependent electron density.

A perovskite solar cell (PSC) is a type of solar cell that includes a perovskite-structured compound, most commonly a hybrid organic–inorganic lead or tin halide-based material as the light-harvesting active layer. Perovskite materials, such as methylammonium lead halides and all-inorganic cesium lead halide, are cheap to produce and simple to manufacture. Solar-cell efficiencies of laboratory-scale devices using these materials have increased from 3.8% in 2009 to 25.

Molecular dynamics (MD) is a computer simulation method for analyzing the physical movements of atoms and molecules. The atoms and molecules are allowed to interact for a fixed period of time, giving a view of the dynamic "evolution" of the system. In the most common version, the trajectories of atoms and molecules are determined by numerically solving Newton's equations of motion for a system of interacting particles, where forces between the particles and their potential energies are often calculated using interatomic potentials or molecular mechanical force fields.

Ursula Röthlisberger, Marko Mladenovic, Ariadni Boziki

Lead halide perovskites with mixtures of monovalent cations have attracted wide attention due to the possibility of preferentially stabilizing the perovskite phase with respect to photovoltaically less suitable competing phases. Here, we present a theoretical analysis and interpretation of the phase stability of binary (CH6N3)(x)HC(NH2)(2)PbI3= GUA(x)FA((1-x))PbI(3)and ternary Cs(y)GUA(x)FA((1-y-x))PbI(3)mixtures. We first estimate if such mixtures are stable and if they lead to a stabilization of the perovskite phase based on static Density Functional Theory (DFT) calculations. In order to investigate the finite temperature stability of the phases, we also employ first-principles molecular dynamics (MD) simulations. It turns out that in contrast to the FA(+)-rich case of FA/Cs mixtures, although mixing of FA/GUA is possible, it is not sufficient to stabilize the perovskite phase at room temperature. In contrast, stable ternary mixtures that contain 17% of Cs(+)can be formed that lead to a preferential stabilization of the perovskite phase. In such a way, the enthalpic destabilization due to the introduction of a too large/too small cation that lies outside the Goldschmidt tolerance range can be (partially) compensated through the introduction of a third cation with complementary size. This allows to suggest a new design principle for the preparation of stable perovskite structures at room temperature with cations that lie outside the Goldschmidt range through mixtures with size-complementary cations in such a way that the effective average cation radius of the mixture lies within the stability range.

Ursula Röthlisberger, Ariadni Boziki, Mohammad Ibrahim Dar, Gwénolé Jean Jacopin

CsPbBr3 has received wide attention due to its superior emission yield and better thermal stability compared to other organic-inorganic lead halide perovskites. In this study, through an interplay of theory and experiments, we investigate the molecular origin of the asymmetric low-temperature photoluminescence spectra of CsPbBr3. We conclude that the origin of this phenomenon lies in a local dipole moment (and the induced Stark effect) due to the preferential localization of Cs+ in either of two off-center positions of the empty space between the surrounding PbBr6 octahedra. With increasing temperature, Cs+ ions are gradually occupying positions closer and closer to the center of the cavities. The gradual loss of ordering in the Cs+ position with increasing temperature is the driving force for the formation of tetragonal-like arrangements within the orthorhombic lattice.

2021Michael Graetzel, David Lyndon Emsley, Ursula Röthlisberger, Ariadni Boziki, Simone Meloni, Dominik Józef Kubicki, Aditya Mishra

Mixed cation perovskites, [HC(NH2)2]xCs(1-x)PbI3, (FAxCs(1-x)PbI3) with x=0.8 achieve high solar cell power conversion efficiencies (PCEs) while exhibiting long-term stability under ambient conditions. In this work, we perform density functional theory (DFT) calculations, first-principles molecular dynamics (MD) simulations, solid-state nuclear magnetic resonance (NMR) and X-ray powder diffraction (XRD) measurements aimed at investigating the possible phase stability of Cs+-rich FAxCs(1-x)PbI3, (0≤x≤0.5) mixed-cation materials as potential candidates for tandem solar cell applications. Estimations of the free energy of the mixtures with respect to the pure compounds together with calculations of the relative phase stability at 0 K and at finite temperature show that although the mixtures can form, the δ phase remains the thermodynamically most stable phase at room temperature. This is fully corroborated by solid-state NMR and XRD measurements and is in contrast to FA+-rich Cs/FA mixtures where small additions of Cs+ are sufficient to stabilize the perovskite phase at ambient conditions. The atomistic origin for this contrasting behavior arises from an energetic destabilization of the perovskite phase on the one hand caused by the incorporation of a large cation (FA+) into the relatively small host lattice of γ-CsPbI3 and on the other hand is induced by the lower degree of distortion of the host lattice. These observations allow us to propose a new design principle for the preferential stabilization of the perovskite phase over the competing δ phase.