Group-IIIA element doped BaSnS2 as a high efficiency absorber for intermediate band solar cell from a first-principles insight.

The quest for high-performance solar cell absorbers has garnered significant attention in the field of photovoltaic research in recent years. To overcome the Shockley-Queisser (SQ) limit of ∼31% for single junction solar cell and realize higher power conversion efficiency, the concept of an intermediate band solar cell (IBSC) has been proposed. This involves the incorporation of an intermediate band (IB) to assist the three band-edge absorptions within the single absorber layer. BaSnS2 has an appropriate width of its forbidden gap in order to host an IB. In this work, doping of BaSnS2 was studied based on hybrid functional calculations. The results demonstrated that isolated and half-filled IBs were generated with suitable energy states in the band gap region after group-IIIA element (i.e., Al, Ga, and In) doping at Sn site. The theoretical efficiencies under one sun illumination of 39.0%, 44.3%, and 39.7% were obtained for 25% doping concentration of Al, Ga, and In, respectively; thus, larger than the single-junction SQ-limit. Furthermore, the dopants have lower formation energies when substituting the Sn site compare to occupying the Ba and S sites, and that helps realizing a proper IB with three band-edge absorptions. Therefore, group-IIIA element doped BaSnS2 is proposed as a high-efficiency absorber for IBSC.

Electronic structure, defect properties, and optimization of the band gap of the earth-abundant and low-toxicity photovoltaic absorber Cu3SbS4.

Searching for an earth-abundant and environment-friendly absorber for thin-film solar cells that provides similar power conversion efficiency to CdTe and Cu(In,Ga)Se2 is of great importance for large-scale applications. Success would change the world's solar energy supply to an even more sustainable material resource. In this paper, we have studied by first-principles calculations the electronic structure and defect properties of the promising absorber Cu3SbS4. Its electronic properties, like direct band gap, high absorption coefficient, and light carrier effective masses, satisfy the requirements for an absorber except for its somewhat too small band gap energy. Sulfur and copper vacancies are easily formed defects in Cu3SbS4, where the S vacancy shrinks the band gap and degrades the material. This probably explains the experimental findings of a rather poor device performance. The suitable preparation conditions for Cu3SbS4 as an absorber are anticipated to be Cu-poor, Sb-moderate, and S-rich conditions. Herein, isovalent element alloying is demonstrated to be an effective way to increase the gap energy and thereby improve the material properties.

Experimental and Theoretical Study of Stable and Metastable Phases in Sputtered CuInS2.

The chalcopyrite Cu(In,Ga)S2 has gained renewed interest in recent years due to the potential application in tandem solar cells. In this contribution, a combined theoretical and experimental approach is applied to investigate stable and metastable phases forming in CuInS2 (CIS) thin films. Ab initio calculations are performed to obtain formation energies, X-ray diffraction (XRD) patterns, and Raman spectra of CIS polytypes and related compounds. Multiple CIS structures with zinc-blende and wurtzite-derived lattices are identified and their XRD/Raman patterns are shown to contain overlapping features, which could lead to misidentification. Thin films with compositions from Cu-rich to Cu-poor are synthesized via a two-step approach based on sputtering from binary targets followed by high-temperature sulfurization. It is discovered that several CIS polymorphs are formed when growing the material with this approach. In the Cu-poor material, wurtzite CIS is observed for the first time in sputtered thin films along with chalcopyrite CIS and CuAu-ordered CIS. Once the wurtzite CIS phase has formed, it is difficult to convert into the stable chalcopyrite polymorph. CuIn5 S8 and NaInS2 accommodating In-excess are found alongside the CIS polymorphs. It is argued that the metastable polymorphs are stabilized by off-stoichiometry of the precursors, hence tight composition control is required.

Chemistry of Oxygen Ionosorption on SnO2 Surfaces.

Ionosorbed oxygen is the key player in reactions on metal-oxide surfaces. This is particularly evident for chemiresistive gas sensors, which operate by modulating the conductivity of active materials through the formation/removal of surface O-related acceptors. Strikingly though, the exact type of species behind the sensing response remains obscure even for the most common material systems. The paradigm for ab initio modeling to date has been centered around charge-neutral surface species, ignoring the fact that molecular adsorbates are required to ionize to induce the sensing response. Herein, we resolve this inconsistency by carrying out a careful analysis of all charged O-related species on three naturally occurring surfaces of SnO2. We reveal that two types of surface acceptors can form spontaneously upon the adsorption of atmospheric oxygen: (i) superoxide O2- on the (110) and the (101) surfaces and (ii) doubly ionized O2- on the (100) facet, with the previous experimental evidence pointing to the latter as the source of sensing response. This species has a unique geometry involving a large displacement of surface Sn, forcing it to attain the coordination resembling that of Sn2+ in SnO, which seems necessary to stabilize O2- and activate metal-oxide surfaces for gas sensing.

Stoner Ferromagnetism in Hole-Doped CuMIIIAO2 with MIIIA = Al, Ga, and In.

Using density functional theory calculations, we examine the effect of hole doping on the magnetic and electronic properties of CuMIIIAO2, with MIIIA = Al, Ga, and In. CuMIIIAO2 nonmagnetic semiconductors switch to ferromagnetic half-metals upon hole doping. For CuAlO2, the nonmagnetic-to-ferromagnetic transition occurs for hole densities of ∼7 × 1019/cm3. Ferromagnetism arises from an exchange splitting of the electronic states at the valence band edge, and it can be attributed to the high-lying Cu-d states. Hole doping induced by cation vacancies and substitutional divalent dopants is also investigated. Interestingly, both vacancies and nonmagnetic divalent dopants result in the emergence of ferromagnetism.

Alkali Dispersion in (Ag,Cu)(In,Ga)Se2 Thin Film Solar Cells-Insight from Theory and Experiment.

Silver alloying of Cu(In,Ga)Se2 absorbers for thin film photovoltaics offers improvements in open-circuit voltage, especially when combined with optimal alkali-treatments and certain Ga concentrations. The relationship between alkali distribution in the absorber and Ag alloying is investigated here, combining experimental and theoretical studies. Atom probe tomography analysis is implemented to quantify the local composition in grain interiors and at grain boundaries. The Na concentration in the bulk increases up to ∼60 ppm for [Ag]/([Ag] + [Cu]) = 0.2 compared to ∼20 ppm for films without Ag and up to ∼200 ppm for [Ag]/([Ag] + [Cu]) = 1.0. First-principles calculations were employed to evaluate the formation energies of alkali-on-group-I defects (where group-I refers to Ag and Cu) in (Ag,Cu)(In,Ga)Se2 as a function of the Ag and Ga contents. The computational results demonstrate strong agreement with the nanoscale analysis results, revealing a clear trend of increased alkali bulk solubility with the Ag concentration. The present study, therefore, provides a more nuanced understanding of the role of Ag in the enhanced performance of the respective photovoltaic devices.

Full-Spectrum High-Resolution Modeling of the Dielectric Function of Water.

In view of the vital role of water, exact knowledge of its dielectric function over a large frequency range is important. We report on currently available measurements of the dielectric function of water at room temperature (25 °C) across the full spectrum: microwave, IR, UV, and X-ray (up to 100 eV). We parameterize the complex dielectric function of water with two Debye (microwave) oscillators and high resolution of IR and UV/X-ray oscillators. We also report dielectric parameters for ice-cold water with a microwave/IR spectrum measured at 0.4 °C, while taking the UV spectrum at 25 °C (assuming negligible temperature dependence in UV). We employ van der Waals dispersion interactions to contrast our model of ice-cold water with earlier models. Air bubbles in water and dissolved gas molecules show attraction toward interfaces rather than repulsion. The van der Waals interaction promotes complete freezing rather than supporting a thin layer of water on ice. We infer that premelting is driven by charge and ion adsorption. Density-based extrapolation from warm to cold water of the dielectric function is satisfactory in microwave but poor (40% error) at IR frequencies.

Carrier-mediated ferromagnetism in two-dimensional PtS2.

Using first principles calculations based on density functional theory, we study the impact of hole doping on the magnetic and electronic properties of two dimensional PtS2. Although 2D PtS2 is intrinsically non-magnetic, a stable ferromagnetic phase is found for a wide range of hole densities, owing to the so-called Stoner instabilities. Besides spontaneous magnetization, half-metallicity is additionally observed. The majority and minority spin states exhibit insulating and metallic nature, respectively, allowing a fully polarized spin transport in 2D PtS2. Lastly, hole doping resulting from substitutional doping is investigated. For As-doped PtS2 shallow spin-polarized states close to the valence band edge are observed, and among all studied group-V dopants, As replacing S, is the most promising one to induce p-type conductivity and a subsequent ferromagnetic order in PtS2.

Chemical stability of Ca3Co4-x O9+δ /CaMnO3-δ p-n junction for oxide-based thermoelectric generators.

An all-oxide thermoelectric generator for high-temperature operation depends on a low electrical resistance of the direct p-n junction. Ca3Co4-x O9+δ and CaMnO3-δ exhibit p-type and n-type electronic conductivity, respectively, and the interface between these compounds is the material system investigated here. The effect of heat treatment (at 900 °C for 10 h in air) on the phase and element distribution within this p-n junction was characterized using advanced transmission electron microscopy combined with X-ray diffraction. The heat treatment resulted in counter diffusion of Ca, Mn and Co cations across the junction, and subsequent formation of a Ca3Co1+y Mn1-y O6 interlayer, in addition to precipitation of Co-oxide, and accompanying diffusion and redistribution of Ca across the junction. The Co/Mn ratio in Ca3Co1+y Mn1-y O6 varies and is close to 1 (y = 0) at the Ca3Co1+y Mn1-y O6-CaMnO3-δ boundary. The existence of a wide homogeneity range of 0 ≤ y ≤ 1 for Ca3Co1+y Mn1-y O6 is corroborated with density functional theory (DFT) calculations showing a small negative mixing energy in the whole range.

Impact of effective polarisability models on the near-field interaction of dissolved greenhouse gases at ice and air interfaces.

We present a theory for Casimir-Polder forces acting on greenhouse gas molecules dissolved in a thin water film. Such a nano-sized film has been predicted to arise on the surface of melting ice as stabilized by repulsive Lifshitz forces. We show that different models for the effective polarisability of greenhouse gas molecules in water lead to different predictions for how Casimir-Polder forces influence their extractions from the melting ice surface. For instance, in the most intricate model of a finite-sized molecule inside a cavity, dispersion potentials push the methane molecules towards the ice surface whereas the oxygen typically will be attracted towards the closest interface (ice or air). Previous models for effective polarisability had suggested that O2 would also be pushed towards the ice surface. Release of greenhouse gas molecules from the surface of melting ice can potentially influence climate greenhouse effects. With this model, we show that some molecules cannot escape from water as single molecules. Due to the contradiction of the results and the escape dynamics of gases from water, we extended the models to describe bubble filled with several molecules increasing their buoyancy force.

Energy, Phonon, and Dynamic Stability Criteria of Two-Dimensional Materials.

First-principles calculations have become a powerful tool to exclude the Edisonian approach in search of novel two-dimensional (2D) materials. However, no universal first-principles criteria to examine the realizability of hypothetical 2D materials have been established in the literature yet. Because of this, and as the calculations are always performed in an artificial simulation environment, one can unintentionally study compounds that do not exist in experiments. Although investigations of physics and chemistry of unrealizable materials can provide some fundamental knowledge, the discussion of their applications can mislead experimentalists for years and increase the gap between experimental and theoretical research. By analyzing energy convex hull, phonon spectra, and structure evolution during ab initio molecular dynamics simulations for a range of synthesized and recently proposed 2D materials, we construct energy, phonon, and dynamic stability filters that need to be satisfied before proposing novel 2D compounds. We demonstrate the power of the suggested filters for several selected 2D systems, revealing that some of them cannot be ever realized experimentally.

Suppression of surfaces states at cubic perovskite (001) surfaces by CO2 adsorption.

By using first-principles approach, the interaction of CO2 with (001) surfaces of six cubic ABO3 perovskites (A = Ba, Sr and B = Ti, Zr, Hf) is studied in detail. We show that CO2 adsorption results in the formation of highly stable CO3-like complexes with similar geometries for all investigated compounds. This reaction leads to the suppression of the surfaces states, opening the band gaps of the slab systems up to the corresponding bulk energy limits. For most AO-terminated ABO3(001) perovskite surfaces, a CO2 coverage of 0.25 was found to be sufficient to fully suppress the surface states, whereas the same effect can only be achieved at 0.50 CO2 coverage for the BO2-terminated surfaces. The largest band gap modulation among the AO-terminated surfaces was found for SrHfO3(001) and BaHfO3(001), whereas the most profound effect among the BO2-terminated surfaces was identified for SrTiO3(001) and BaTiO3(001). Based on these results and considering practical difficulties associated with measuring conductivity of highly resistive materials, TiO2-terminated SrTiO3(001) and BaTiO3(001) were identified as the most prospective candidates for chemiresistive CO2 sensing applications.

Orientational Dependence of the van der Waals Interactions for Finite-Sized Particles.

Dispersion forces, especially van der Waals forces as interactions between neutral and polarizable particles act at small distances between two objects. Their theoretical origin lies in the electromagnetic interaction between induced dipole moments caused by the vacuum fluctuations of the ground-state electromagnetic field. The resulting theory well describes the experimental situation in the limit of the point dipole assumption. At smaller distances, where the finite size of the particles has to be taken into account, this description fails and has to be corrected by higher orders of the multipole expansion, such as quadrupole moments and so on. With respect to the complexity of the spatial properties of the particles this task requires a considerable effort. In order to describe the van der Waals interaction between such particles, we apply the established method of a spatially spread out polarizability distribution to approximate the higher orders of the multipole expansion. We thereby construct an effective theory for effects from anisotropy and finite size on the van der Waals potential.

Distance-Dependent Sign Reversal in the Casimir-Lifshitz Torque.

The Casimir-Lifshitz torque between two biaxially polarizable anisotropic planar slabs is shown to exhibit a nontrivial sign reversal in its rotational sense. The critical distance a_{c} between the slabs that marks this reversal is characterized by the frequency ω_{c}∼c/2a_{c} at which the in-planar polarizabilities along the two principal axes are equal. The two materials seek to align their principal axes of polarizabilities in one direction below a_{c}, while above a_{c} their axes try to align rotated perpendicular relative to their previous minimum energy orientation. The sign reversal disappears in the nonretarded limit. Our perturbative result, derived for the case when the differences in the relative polarizabilities are small, matches excellently with the exact theory for uniaxial materials. We illustrate our results for black phosphorus and phosphorene.

Tailoring electronic properties of multilayer phosphorene by siliconization.

Controlling the thickness dependence of electronic properties for two-dimensional (2d) materials is among the primary goals for their large-scale applications. Herein, employing a first-principles computational approach, we predict that Si interaction with multilayer phosphorene (2d-P) can result in the formation of highly stable 2d-SiP and 2d-SiP2 compounds with a weak interlayer interaction. Our analysis demonstrates that these systems are semiconductors with band gap energies that can be governed by varying the thicknesses and stacking arrangements. Specifically, the siliconization of phosphorene allows designing 2d-SiPx materials with a significantly weaker thickness dependence of electronic properties than that in 2d-P and to develop ways for their tailoring. We also reveal the spatial dependence of electronic properties for 2d-SiPx highlighting the difference in the effective band gaps for different layers. Particularly, our results show that the central layers in the multilayer 2d systems determine their overall electronic properties, while the role of the outermost layers is noticeably smaller.

Effective Polarizability Models.

Theories for the effective polarizability of a small particle in a medium are presented using different levels of approximation: we consider the virtual cavity, real cavity, and the hard-sphere models as well as a continuous interpolation of the latter two. We present the respective hard-sphere and cavity radii as obtained from density-functional simulations as well as the resulting effective polarizabilities at discrete Matsubara frequencies. This enables us to account for macroscopic media in van der Waals interactions between molecules in water and their Casimir-Polder interaction with an interface.

The role of grain boundary scattering in reducing the thermal conductivity of polycrystalline XNiSn (X = Hf, Zr, Ti) half-Heusler alloys.

Thermoelectric application of half-Heusler compounds suffers from their fairly high thermal conductivities. Insight into how effective various scattering mechanisms are in reducing the thermal conductivity of fabricated XNiSn compounds (X = Hf, Zr, Ti, and mixtures thereof) is therefore crucial. Here, we show that such insight can be obtained through a concerted theory-experiment comparison of how the lattice thermal conductivity κ Lat(T) depends on temperature and crystallite size. Comparing theory and experiment for a range of Hf0.5Zr0.5NiSn and ZrNiSn samples reported in the literature and in the present paper revealed that grain boundary scattering plays the most important role in bringing down κ Lat, in particular so for unmixed compounds. Our concerted analysis approach was corroborated by a good qualitative agreement between the measured and calculated κ Lat of polycrystalline samples, where the experimental average crystallite size was used as an input parameter for the calculations. The calculations were based on the Boltzmann transport equation and ab initio density functional theory. Our analysis explains the significant variation of reported κ Lat of nominally identical XNiSn samples, and is expected to provide valuable insights into the dominant scattering mechanisms even for other materials.

Reducing the Charge Carrier Transport Barrier in Functionally Layer-Graded Electrodes.

Lithium-ion batteries (LIBs) are primary energy storage devices to power consumer electronics and electric vehicles, but their capacity is dramatically decreased at ultrahigh charging/discharging rates. This mainly originates from a high Li-ion/electron transport barrier within a traditional electrode, resulting in reaction polarization issues. To address this limitation, a functionally layer-graded electrode was designed and fabricated to decrease the charge carrier transport barrier within the electrode. As a proof-of-concept, functionally layer-graded electrodes composing of TiO2 (B) and reduced graphene oxide (RGO) exhibit a remarkable capacity of 128 mAh g-1 at a high charging/discharging rate of 20 C (6.7 A g-1 ), which is much higher than that of a traditionally homogeneous electrode (74 mAh g-1 ) with the same composition. This is evidenced by the improvement of effective Li ion diffusivity as well as electronic conductivity in the functionally layer-graded electrodes.

Band gap modulation of SrTiO3 upon CO2 adsorption.

Herein, CO2 chemisorption on SrTiO3(001) surfaces is studied using ab initio calculations to establish new chemical sensing mechanisms. It was found that CO2 adsorption opens the band gap of the material. However, the mechanisms are different: the CO2 adsorption on the TiO2-terminated surface neutralizes the surface states at the valence band (VB) maximum, whereas for the SrO-terminated surface it suppresses the conduction band (CB) minimum. For the TiO2-terminated surface, the effect is explained by the passivation of dangling bonds, whereas for the SrO-terminated surface, the suppression is caused by surface relaxation. Modulation of the VB states implies a more direct change in charge distribution, and thus, the induced change in the band gap is more prominent at the TiO2 termination. Further, it has been shown that both CO2 adsorption energy and surface band gap are strongly dependent on CO2 coverage, suggesting that the observed effect can be utilized in sensing applications for a wide range of CO2 concentrations.

Stability and electronic properties of phosphorene oxides: from 0-dimensional to amorphous 2-dimensional structures.

Combining the screening by first-principles calculations and Born-Oppenheimer molecular dynamics simulations, we fully reconsider phosphorene oxidation and the formation of low-dimensional phosphorus oxides (PxOy). It is found that the previously reported 2-dimensional PxOy (2d-PxOy) structures cannot provide a full understanding of 2d-PxOy properties. We show that the P-O interaction can result in highly stable 0d-PxOy and 2d-PxOy structures with close energetics, but a noticeable difference in band-gap energies. Here, the possibility of the formation of amorphous 2d-PxOy structures and their unique electronic properties are also studied in detail.