Sabine Körbel

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

We 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.

Stability and electronic properties of new inorganic perovskites from high-throughput ab initio calculations

Using a high-throughput approach based on density functional theory, we perform an extensive study of possible ABX3 perovskites, where X is a non-metal and A and B span a large portion of the periodic table. We calculate the ternary phase diagram for each composition and we discuss the thermodynamic stability of perovskite phases. We find a large number of ABX3 perovskites, which are still absent from available databases, and which are stable with respect to decomposition into known ternary, binary or elementary phases. For these structures, we then calculate electronic band gaps, hole effective masses, and the spontaneous ferroelectric and magnetic polarization, which are relevant material properties for a number of specific applications in photovoltaics, transparent contacts, piezoelectrics, and magnetoelectrics. Some of our novel perovskites exhibit promising properties for applications.

Interactions of defect complexes and domain walls in CuO-doped ferroelectric (K,Na)NbO3

“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 − V O • • ) ′ and ( V O • • − Cu ′ ′ ′ Nb − V O • • ) • . Concerning the interplay of these defect complexes with the piezoelectric materials properties, the trimeric ( V O • • − Cu ′ ′ ′ Nb − V O • • ) • defect complex primarily has an elastic dipole moment and thus is proposed to impact the electromechanical properties, whereas the dimeric ( Cu ′ ′ ′ Nb − V O • • ) ′ 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.”

Influence of the A/B Stoichiometry on Defect Structure, Sintering, and Microstructure in Undoped and Cu-Doped KNN

Development of ceramics based on the alkaline niobate (KNN) system is one of the major lines of current research pointing to substitution of the lead containing ferroelectrics by lead‐free materials. Sodium potassium niobate (K0.5Na0.5)NbO3 is a prototype material of lead‐free alkaline‐transition metal ferroelectrics with \({\rm A}^{1+}{\rm B}^{5+}{\rm O}_3^{2-}\) perovskite structure. Processing procedures for KNN‐based ceramics are however challenging due to the hygroscopic behavior of sodium‐ and potassium carbonates and the evaporation of alkalines at the elevated processing temperatures, which make it difficult to control the stoichiometry of the ceramics. Alkaline (A‐site) or niobium (B‐site) excess results in pronounced qualitative differences of the microstructure in KNN ceramics.

Research Update: Stable single-phase Zn-rich Cu2ZnSnSe4 through In doping

Alloying in the system Cu2ZnSnSe4–CuInSe2–ZnSe (CZTISe) is investigated experimentally and theoretically. The goal is to distinguish single-phase and multi-phase regions within the Cu2ZnSnSe4-2CuInSe2-4ZnSe pseudo-ternary phase diagram. CZTISe thin films are prepared by co-evaporation of the chemical elements and are investigated in real-time during growth using in situ angle dispersive X-ray diffraction. The focus is mainly on thin films along the Cu2ZnSnSe4–2CuInSe2 isopleth with small ZnSe addition as well as on films along the Cu2ZnSnSe4-4ZnSe isopleth with small CuInSe2 addition. For both cases, ab initio calculations with density-functional theory are performed to estimate the stability of the alloy with respect to the formation of secondary phases. Both in experiment and calculation, we find a surprisingly large single-phase region in the Cu2ZnSnSe4 corner of the pseudo-ternary phase diagram slightly off the Cu2ZnSnSe4-4ZnSe isopleth. This may help avoiding secondary phase formation under Zn-rich c...

Stable hybrid organic–inorganic halide perovskites for photovoltaics from ab initio high-throughput calculations

Hybrid perovskites, such as methylammonium lead iodide, have revolutionized research on solar cells in the past few years. Well known instability and toxicity issues restrain however the large-scale application of these perovskites in commercial photovoltaic technology. It is therefore the most urgent task to find a way to chemically stabilize these and other lead-free perovskites, preserving at the same time their excellent absorption and charge-transport properties. The obvious route to follow is chemical substitution. In this work we screen the periodic table of elements for hybrid organic–inorganic halide perovskites, using high-throughput density-functional theory calculations. We consider compounds with the composition A+B2+X3−, where A is a molecular organic cation, X is a halogen, and B is a divalent element. For the molecular cation, we vary the molecule size from sulfonium (H3S, very small) to tert-butylammonium (C4NH12, very large). All thermodynamically stable hybrid perovskites are then further characterized by calculating their band gaps and effective masses, to identify the most promising candidates for further experimental and theoretical characterization. We find that the substitution of the organic molecule is the most promising way to enhance thermodynamic stability, while there is no optimal replacement for lead or Sn, unless one considers partial substitution or alloying.