Accuracy of Hybrid Functionals with Non-Self-Consistent Kohn-Sham Orbitals for Predicting the Properties of Semiconductors

Atomic insights into the interaction of N2, CO2, NH3, NO, and NO2 gas molecules with Zn2(V, Nb, Ta)N3 ternary nitride monolayers

The search for promising carrier blocking layer materials with high stability, including resistance to surface inhibition by environmental molecules that cause a drop in carrier mobility, is critical for the production of tandem solar cells. Based on density functional theory calculations, the reaction of atmospheric gases, including N2, CO2, NH3, NO, and NO2, with three promising Zn2(V, Nb, Ta)N3 monolayers is discovered. The results suggest the chemical adsorption of NH3 and physical adsorption of NO and NO2. In addition, the Zn2(V, Nb, Ta)N3 monolayers are characterized by a weak bonding with N2 and CO2. Charge redistribution is found at the interface between the monolayers and NH3, NO and NO2 molecules, leading to the formation of a local surface dipole that affects the functionality of the Zn2(V, Nb, Ta)N3 monolayers. The Zn2VN3 monolayer is less reactive with atmospheric gases and thus is the most promising for application in tandem solar cells. Notably, the revealed nontrivial behavior of the Zn2(V, Nb, Ta)N3 monolayers towards N-containing gases makes them promising for application in gas sensing. Specifically, the Zn2TaN3 monolayer is the most promising for application in molecular sensing due to its high reversibility and distinguished interaction with NH3, NO, and NO2 gases.

Limits to Hole Mobility and Doping in Copper Iodide

Over one hundred years have passed since the discovery of the p-type transparent conducting material copper iodide, predating the concept of the "electron-hole" itself. Supercentenarian status notwithstanding, little is understood about the charge transport mechanisms in CuI. Herein, a variety of modeling techniques are used to investigate the charge transport properties of CuI, and limitations to the hole mobility over experimentally achievable carrier concentrations are discussed. Poor dielectric response is responsible for extensive scattering from ionized impurities at degenerately doped carrier concentrations, while phonon scattering is found to dominate at lower carrier concentrations. A phonon-limited hole mobility of 162 cm2 V-1 s-1 is predicted at room temperature. The simulated charge transport properties for CuI are compared to existing experimental data, and the implications for future device performance are discussed. In addition to charge transport calculations, the defect chemistry of CuI is investigated with hybrid functionals, revealing that reasonably localized holes from the copper vacancy are the predominant source of charge carriers. The chalcogens S and Se are investigated as extrinsic dopants, where it is found that despite relatively low defect formation energies, they are unlikely to act as efficient electron acceptors due to the strong localization of holes and subsequent deep transition levels.

Improving Results by Improving Densities: Density-Corrected Density Functional Theory

Density functional theory (DFT) calculations have become widespread in both chemistry and materials, because they usually provide useful accuracy at much lower computational cost than wavefunction-based methods. All practical DFT calculations require an approximation to the unknown exchange-correlation energy, which is then used self-consistently in the Kohn-Sham scheme to produce an approximate energy from an approximate density. Density-corrected DFT is simply the study of the relative contributions to the total energy error. In the vast majority of DFT calculations, the error due to the approximate density is negligible. But with certain classes of functionals applied to certain classes of problems, the density error is sufficiently large as to contribute to the energy noticeably, and its removal leads to much better results. These problems include reaction barriers, torsional barriers involving π-conjugation, halogen bonds, radicals and anions, most stretched bonds, etc. In all such cases, use of a more accurate density significantly improves performance, and often the simple expedient of using the Hartree-Fock density is enough. This Perspective explains what DC-DFT is, where it is likely to improve results, and how DC-DFT can produce more accurate functionals. We also outline challenges and prospects for the field.

Phase stability of the tin monochalcogenides SnS and SnSe: a quasi-harmonic lattice-dynamics study

The tin monochalcogenides SnS and SnSe adopt four different crystal structures, viz. orthorhombic Pnma and Cmcm and cubic rocksalt and π-cubic (P2₁3) phases, each of which has optimal properties for a range of potential applications. This rich phase space makes it challenging to identify the conditions under which the different phases are obtained. We have performed first-principles quasi-harmonic lattice-dynamics calculations to assess the relative stabilities of the four phases of SnS and SnSe. We investigate dynamical stability through the presence or absence of imaginary modes in the phonon dispersion curves, and we compute Helmholtz and Gibbs free energies to evaluate the thermodynamic stability. We also consider applied pressures up to 15 GPa to obtain simulated temperature-pressure phase diagrams. Finally, the relationships between the orthorhombic crystal phases are investigated by explicitly mapping the potential-energy surfaces along the imaginary harmonic phonon modes in the Cmcm phase, and the relationships between the cubic phases are found by transition-state modelling using the climbing-image nudged elastic-band method.

Cost-Effective High-Throughput Calculation Based on Hybrid Density Functional Theory: Application to Cubic, Double, and Vacancy-Ordered Halide Perovskites

Hybrid density functional theory calculations are commonly used to investigate the electronic structure of semiconductor materials but have not been ideal for high-throughput calculations due to heavy computation costs. We developed a computational approach to obtain the electronic band gap cost-effectively by employing not only non-self-consistent field calculation methods but also sparse k-point meshes for the Fock exchange potential. The benchmark calculation showed that our method is at least 30 times faster than the conventional hybrid density functional theory calculation to quickly screen materials. The band gaps of 290 materials in 5 different structures including cubic, double, and vacancy-ordered perovskites were obtained. The physical properties of Cs₂WCl₆ and Cs₂NaInBr₆, screened for optoelectronic applications, were in good agreement with the experiment.

Al-Balushi R, Rather JA, Munam A, Ilmi R, Raithby PR, Zhang Y, Fu Y, Xie Z, Chen S, Islam SM, Wong WY, Skelton JM, Khan MS. Synthesis, characterization, and optoelectronic properties of phenothiazine-based organic co-poly-ynes

We present the synthesis of seven new organic co-poly-ynes P1–P7 incorporating phenothiazine (PTZ) motif and evaluate their optoelectronic properties and performance in polymer light-emitting diodes (PLEDs) and polymer solar cells (PSCs).

Density Functional Theory for Molecule-Metal Surface Reactions: When Does the Generalized Gradient Approximation Get It Right, and What to Do If It Does Not

While density functional theory (DFT) is perhaps the most used electronic structure theory in chemistry, many of its practical aspects remain poorly understood. For instance, DFT at the generalized gradient approximation (GGA) tends to fail miserably at describing gas-phase reaction barriers, while it performs surprisingly well for many molecule-metal surface reactions. GGA-DFT also fails for many systems in the latter category, and up to now it has not been clear when one may expect it to work. We show that GGA-DFT tends to work if the difference between the work function of the metal and the molecule's electron affinity is greater than ∼7 eV and to fail if this difference is smaller, with sticking of O₂ on Al(111) being a spectacular example. Using dynamics calculations we show that, for this system, the DFT problem may be solved as done for gas-phase reactions, i.e., by resorting to hybrid functionals, but using screening at long-range to obtain a correct description of the metal. Our results suggest the GGA error in the O₂ + Al(111) barrier height to be functional driven. Our results also suggest the possibility to compute potential energy surfaces for the difficult-to-treat systems with computationally cheap nonself-consistent calculations in which a hybrid functional is applied to a GGA density.

Segregation tendencies of transition-metal dopants in wide band gap semiconductor nanowires

The segregation tendency of an impurity in a semiconductor nanowire can be tuned by adjusting the Fermi level position.