Anomalous random correlations of force constants on the lattice dynamical properties of disordered Au-Fe alloys

Au-Fe alloys are of immense interest due to their biocompatibility, anomalous hall conductivity, and applications in various medical treatment. However, irrespective of the method of preparation, they often exhibit a high-level of disorder, with properties sensitive to the thermal or magnetic annealing temperatures. We calculate lattice dynamical properties of Au$_{1-x}$Fe$_x$ alloys using density functional theory methods, where, being a multisite property, reliable interatomic force constant (IFC) calculations in disordered alloys remain a challenge. We follow a two fold approach: (1) an accurate IFC calculation in an environment with nominally zero chemical pair correlations to mimic the homogeneously disordered alloy; and (2) a configurational averaging for the desired phonon properties (e.g., dispersion, density of states, and entropy). We find an anomalous change in the IFC's and phonon dispersion (split bands) near $x$=0.19, which is attributed to the local stiffening of the Au-Au bonds when Au is in the vicinity of Fe. Other results based on mechanical and thermo-physical properties reflect a similar anomaly: Phonon entropy, e.g., becomes negative below $x$=0.19, suggesting a tendency for chemical unmixing, reflecting the onset of miscibility gap in the phase diagram. Our results match fairly well with reported data, wherever available

Indirect band gap semiconductors for thin-film photovoltaics: High-throughput calculation of phonon-assisted absorption

Discovery of high-performance materials remains one of the most active areas in photovoltaics (PV) research. Indirect band gap materials form the largest part of the semiconductor chemical space, but predicting their suitability for PV applications from first principles calculations remains challenging. Here we propose a computationally efficient method to account for phonon assisted absorption across the indirect band gap and use it to screen 127 experimentally known binary semiconductors for their potential as thin film PV absorbers. Using screening descriptors for absorption, carrier transport, and nonradiative recombination, we identify 28 potential candidate materials. The list, which contains 20 indirect band gap semiconductors, comprises both well established (3), emerging (16), and previously unexplored (9) absorber materials. Most of the new compounds are anion rich chalcogenides (TiS$_3$, Ga$_2$Te$_5$) and phosphides (PdP$_2$, CdP$_4$, MgP$_4$, BaP$_3$) containing homoelemental bonds, and represent a new frontier in PV materials research. Our work highlights the previously underexplored potential of indirect band gap materials for optoelectronic thin-film technologies

Selenium and the role of defects for photovoltaic applications

We present first principles calculations of the electronic properties of trigonal selenium with emphasis on photovoltaic applications. The band gap and optical absorption spectrum of pristine selenium is calculated from many-body perturbation theory yielding excellent agreement with experiments. We then investigate the role of intrinsic as well as extrinsic defects and estimate the equilibrium concentrations resulting from realistic synthesis conditions. The intrinsic defects are dominated by vacancies, which act as acceptor levels and implies $p$-doping in agreement with previous predictions and measurements, and we show that these do not give rise to significant non-radiative recombination. The charge balance remains dominated by vacancies when extrinsic defects are included, but these may give rise to sizable non-radiative recombination rates, which could severely limit the performance of selenium based solar cells. Our results thus imply that the pollution by external elements is a decisive factor for the photovoltaic efficiency, which will be of crucial importance when considering synthesis conditions for any type of device engineering.Comment: 15 page

GPAW: open Python package for electronic-structure calculations

We review the GPAW open-source Python package for electronic structure calculations. GPAW is based on the projector-augmented wave method and can solve the self-consistent density functional theory (DFT) equations using three different wave-function representations, namely real-space grids, plane waves, and numerical atomic orbitals. The three representations are complementary and mutually independent and can be connected by transformations via the real-space grid. This multi-basis feature renders GPAW highly versatile and unique among similar codes. By virtue of its modular structure, the GPAW code constitutes an ideal platform for implementation of new features and methodologies. Moreover, it is well integrated with the Atomic Simulation Environment (ASE) providing a flexible and dynamic user interface. In addition to ground-state DFT calculations, GPAW supports many-body GW band structures, optical excitations from the Bethe-Salpeter Equation (BSE), variational calculations of excited states in molecules and solids via direct optimization, and real-time propagation of the Kohn-Sham equations within time-dependent DFT. A range of more advanced methods to describe magnetic excitations and non-collinear magnetism in solids are also now available. In addition, GPAW can calculate non-linear optical tensors of solids, charged crystal point defects, and much more. Recently, support of GPU acceleration has been achieved with minor modifications of the GPAW code thanks to the CuPy library. We end the review with an outlook describing some future plans for GPAW

Optoelectronic and transport properties of vacancy-ordered double-perovskite halides:A first-principles study

In the search for stable lead-free perovskites, vacancy-ordered double perovskites (VODPs), A2BX6, have emerged as a promising class of materials for solar harvesting owing to their nontoxicity, better stability, and unique optoelectronic properties. Recently, this class has been explored for a wide range of applications, such as photovoltaics, photodetectors, photocatalysis, and light-emitting diodes. Here, we present the stability and the key physical attributes of a few selected compounds in a systematic manner using state-of-the-art first-principles calculations. A careful structural and stability analysis via simulation of convex hulls and compositional phase diagrams for different structural prototypes discloses 14 stable compounds and one metastable compound in this class. Electronic structure calculations using hybrid functionals reveals that six compounds possess band gaps in the ideal visible region. These six compounds, namely Cs2SnI6, Cs2PdI6, Cs2TeI6, Cs2TiI6, Cs2PtI6, and Cs2PdBr6, show high optical absorption (≈105cm-1), giving rise to high spectroscopic limited maximum efficiency (15-23%) in the thin-film thickness range. Close inspection of the transport properties reveals polar optical phonon scattering to be the dominant mechanism limiting overall mobility. Further analysis of the polaron excitations discloses the possibility of large polaron formation at low to moderate defect concentrations. At high defect concentrations, ionized impurity scattering takes over. Such analysis can be extremely useful for choosing the optimal growth conditions for a given material intended to be used for a desired application. Additionally, a few selected compounds show moderate to high electron mobility values (∼13-63cm2V-1s-1) at room temperature. Overall, the present study paves an important path to help design VODPs as lead-free potential candidates for future optoelectronic applications.</p