Optical signatures of defects in BiFeO3_3

Optical absorption in rhombohedral BiFeO$_3$ starts at photon energies below the photoemission band gap of $\approx$ 3 eV calculated from first principles. A shoulder at the absorption onset has so far been attributed to low-lying electronic transitions or to oxygen vacancies. In this work optical spectra are calculated ab initio to determine the nature of the optical transitions near the absorption onset of pristine BiFeO$_3$, the effect of electron-hole interaction, and the spectroscopic signatures of typical defects, i.e. doping (excess electrons or holes), intrinsic defects (oxygen and bismuth vacancies), and low-energy structural defects (ferroelectric domain walls)

Electron trapping by neutral pristine ferroelectric domain walls in BiFeO3_3

First-principles calculations for pristine neutral ferroelectric domain walls in BiFeO$_3$ reveal that excess electrons are selectively trapped by the domain walls, while holes are only weakly attracted. Such trapped excess electrons may be responsible for the thermally activated electrical conductivity at domain walls observed in experiments. In the case of a periodic array of domain walls, the trapped excess electrons create a zigzag potential, whose amplitude depends on the electron concentration in the material and the domain-wall distance. The potential is asymmetric for 71{\deg} and 109{\deg} domain walls. This could modify the open-circuit voltage in a solar cell and hence influence the photoelectric effect in BiFeO$_3$

Optical properties of Cu-chalcogenide photovoltaic absorbers from self-consistent GW and the Bethe-Salpeter equation

International audienceSelf-consistent GW calculations and the solution of the Bethe-Salpeter equation are to date the best available approaches to simulate electronic excitations in a vast class of materials, ranging from molecules to solids. However, up to now numerical instabilities made it impossible to use these techniques to calculate optical absorption spectra of the best-known thin-film absorbers for solar cells: Cu(In,Ga)(S,Se) 2 chalcopyrites and Cu 2 ZnSn(S,Se) 4 kesterites/stannites. We show here how to solve this problem using a finite-difference method in k space to evaluate the otherwise diverging dipole matrix elements, obtaining an excellent agreement with experiments. Having established the validity of this approach, we use it then to calculate the optical response of the less studied, but promising, Cu 2 ZnGe(S,Se) 4 compounds, opening the way to predictive calculations of still unknown materials

Zig-zag charged domain walls in ferroelectric PbTiO3_3

We report a theoretical investigation of a charged 180$^\circ$ domain wall in ferroelectric PbTiO$_3$, compensated by randomly distributed immobile charge defects. For this we utilize atomistic shell-model simulations and continuous phase-field simulations in the framework of the Ginzburg-Landau-Devonshire model. We predict that domain walls form a zig-zag pattern and we discuss its properties in a broad interval of compensation-region widths, ranging from a couple to over a hundred nanometers

Transition between large and small electron polaron at neutral ferroelectric domain walls in BiFeO3_3

Ferroelectric domain walls are planes within an insulating material that can accumulate and conduct charge carriers, hence the interaction of the domain walls with the charge carriers can be important for photovoltaic and other electronic applications. By means of first principles calculations we predict a transition from a large two-dimensional electron polaron to a small polaron at the domain walls at a critical electron density, with polaron signatures in optical absorption and photoluminescence. We find that large and small polarons at the domain walls create different absorption peaks within the band gap that are not present in the case of pristine domain walls. These are an extended Drude peak in the case of large electron or hole polarons and a narrow mid-gap peak in the case of the small electron polaron.Comment: The main finding of the article, the transition between a large and a small electron polaron as a function of the polaron density, is an artefact, which resulted from applying an unsuitable methodology for modeling diluted polarons. This affects results depicted in Figs. 3, 4, 5, and

Size-dependent optical absorption of Cu2ZnSn(Se,S)4 quantum dot sensitizers from ab initio many-body methods

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Interactions of defect complexes and domain walls in CuO-doped ferroelectric (K,Na)NbO3123

“Lead-free” piezoelectric sodium potassium niobate has been studied with respect to its defect structure when doping with CuO. The results indicate that two kinds of mutually compensating charged defect complexes are formed, (Cu′′′Nb−VO••)′ and (VO••−Cu′′′Nb−VO••)•. Concerning the interplay of these defect complexes with the piezoelectric materials properties, the trimeric (VO••−Cu′′′Nb−VO••)• defect complex primarily has an elastic dipole moment and thus is proposed to impact the electromechanical properties, whereas the dimeric (Cu′′′Nb−VO••)′ defect possesses an electric dipole moment in addition to an elastic distortion. Both types of defect complexes can impede domain-wall motion and may contribute to ferroelectric “hardening.

Benchmark Many-Body GW and Bethe–Salpeter Calculations for Small Transition Metal Molecules

International audienceWe study the electronic and optical properties of 39 small molecules containing transition metal atoms and 7 others related to quantum-dots for photovoltaics. We explore in particular the merits of the many-body GW formalism, as compared to the ΔSCF approach within density functional theory, in the description of the ionization energy and electronic affinity. Mean average errors of 0.2–0.3 eV with respect to experiment are found when using the PBE0 functional for ΔSCF and as a starting point for GW. The effect of partial self-consistency at the GW level is explored. Further, for optical excitations, the Bethe–Salpeter formalism is found to offer similar accuracy as time-dependent DFT-based methods with the hybrid PBE0 functional, with mean average discrepancies of about 0.3 and 0.2 eV, respectively, as compared to available experimental data. Our calculations validate the accuracy of the parameter-free GW and Bethe–Salpeter formalisms for this class of systems, opening the way to the study of large clusters containing transition metal atoms of interest for photovoltaic applications