Ideal Band Research Articles

The difference on the physical properties between CsPbX3 and Cs2PbX6: A comparative study

A complete theoretical investigation is performed to study the physical properties of all-inorganic halide perovskites CsPbX3 and Cs2PbX6 (X ​= ​Cl, Br, I). The various physical properties are compared and discussed. The results show that the bond length Pb−X of Cs2PbX6 is shorter than that of CsPbX3. Six compounds are ductile materials and mechanically stable. According to the phonon spectrum calculations, Cs2PbCl6 and Cs2PbBr6 are dynamically stable, while Cs2PbI6 is unstable. The band gap of CsPbX3 is larger than that of Cs2PbX6. The strong spin−orbit coupling effect is found for CsPbX3, while it has little influence on the band structure of Cs2PbX6. The mobility between the electron and hole varies from CsPbX3 to Cs2PbX6. The different in physical properties mainly results from the different valence states of lead. The results further indicate that Cs2PbBr6 has an ideal band gap value and shows a stronger light absorption than that of CsPbI3 between 450 and 800 ​nm, which is expected to be a promising candidate for perovskite solar cells. Therefore, our study can provide further interest for exploring the potential of all-inorganic stable lead-based double perovskites in optoelectronic devices.

First-principles study of a new BP2 two-dimensional material

Two-dimensional materials have a wide range of applications in many aspects due to their unique properties. Here we carry out a detailed structural search and design of the BP2 using the first principles method, and find a new PMM2 sheet. The analysis of the phonon dispersive curves shows that the 2D PMM2 is dynamic stable. The study of molecular dynamics shows that the 2D PMM2 can be stable under high temperature, even at 600 K. Most importantly, when a suitable strain is applied, the structure can exhibit other electronic properties such as direct band gap semiconductor. In addition, the small strain can tune the band gap value of the PMM2 structure to around 1.4 eV, which is very close to the ideal band gap of solar materials. Therefore, the 2D PMM2 may have potential applications in the field of photovoltaic materials.

Broadband Achromatic and Polarization Insensitive Focused Optical Vortex Generator Based on Metasurface Consisting of Anisotropic Nanostructures

Optical vortex (OV) beams are ideal carriers for high-capacity optical communication, and long-wave infrared (LWIR) is the ideal wavelength band for long-range optical communication. Here, we propose a method for the achromatic generation of focused optical vortex (FOV) beams with arbitrary homogenous polarization states in the LWIR using a single all-germanium metasurface. The chromatic aberration and polarization sensitivity are eliminated by superimposing a polarization-insensitive geometric phase and a dispersion-engineered dynamic phase. The method is validated using two broadband FOV generators with the same diameter and focal length but different topological charges. The results indicate that the FOV generators are broadband achromatic and polarization insensitive. The proposed method may pave the way for chip-scale optical communication devices.

First principles study of electronic, optical, and thermoelectric properties of K2Pd (Cl/Br)6 for solar cells and renewable energy

The vacancy-ordered double perovskites are extensively studied for solar cells and thermoelectric characteristics. Herein, the electronic, mechanical, optical, and transport characteristics of K2Pd(Cl/Br)6 are addressed comprehensively. The structural, and dynamic stabilities are confirmed by tolerance factors (0.93, 0.95) and the positive frequencies of phonon band structures. The criteria for mechanical stability and elastic parameters ensure their stable, anisotropic, and ductile nature. The computation of band structures (BS) and density of states (DOS) show band gaps 2.0 eV, and 1.3 eV for K2PdCl6, and K2PdBr6, respectively. The optical properties are calculated by dielectric constants, absorption coefficient, and their dependent parameters. The K2PdBr6 has an ideal absorption band for visible light solar cells. The thermoelectric performance has been analyzed by figure of merit (ZT). The positive Seebeck coefficient p-type nature with significantly high ZT (0.73, 0.74) at room temperature.

Revealing the stability and optoelectronic properties of novel nitride and phosphide semiconductors: A DFT prediction

Nitride semiconductors have attracted considerable attention in recent years. CaZn2N2 and SrZn2N2 are the promising materials for optoelectronic applications. In order to explore more novel semiconductors, a detailed theoretical study has been performed to investigate the structural, electronic, and optical properties of AB2×2 (A = Ca, Sr, Ba; B = Mg, Zn, Cd; X = N, P) and their alloys. The calculated band gaps of nitride compounds with the HSE06 functional are consistent well with the reported experimental values. Most compounds are direct bandgap semiconductors and they show tunable band gaps. Nine compounds show suitable direct band gaps with the values of 1.4–1.8 eV. In addition, the stability of four alloying compounds is confirmed by different theoretical methods. Moreover, these phosphide compounds and their alloys also show good carrier mobility. Importantly, Sr(Zn0.5Cd0.5)2P2 contains less toxic Cd element, and it has an ideal band gap of 1.38 eV and high absorption coefficient, which is the most promising material for optoelectronic applications. The present study has indicated the potential suitability of phosphide semiconductors for solar cell applications.

In Situ Formation of δ-FAPbI3 at the Perovskite/Carbon Interface for Enhanced Photovoltage of Printable Mesoscopic Perovskite Solar Cells

Formamidinium lead triiodide (FAPbI3) perovskite has attracted intensive research attention due to its ideal band gap and low defect density for photovoltaic applications. Particularly, the existence of non-photoactive δ-phase FAPbI3 has been considered detrimental to regular-structured perovskite solar cells (PSCs). Here, in hole-conductor-free triple-mesoscopic PSCs, we observe that α-phase FAPbI3 transforms to δ-phase FAPbI3 at the perovskite/carbon interface and in situ forms an α/δ-phase junction when the as-fabricated cells are exposed to humid atmosphere. The α/δ-phase junction shows a favorable band alignment and significantly suppresses the charge recombination at the interface. By controlling the relative humidity of the atmosphere, we fabricate FAPbI3-based cells that deliver a champion efficiency of 17.11% with an enhanced open-circuit voltage (VOC) of 1020 mV. This work demonstrates the potential of δ-phase FAPbI3 for benefiting the device performance of PSCs and proposes the concept of constructing perovskite-based junctions at the interface between a perovskite-absorbing layer and charge-transporting layers for enhancing the VOC of PSCs.

Continuous Oxygen Vacancy Gradient in TiO2 Photoelectrodes by a Photoelectrochemical‐Driven “Self‐Purification” Process

AbstractOxygen vacancies have been treated as an important material engineering tool to enhance catalytic performance; for instance, oxygen vacancies suppress charge recombination at the Schottky interface, and thus, the photocurrent can be improved. In this regard, the gradient distribution of oxygen vacancies in n‐type metal oxides produces the ideal band structure for minimizing e−/h+ recombination by efficient hole extraction; however, its achievement remains a daunting challenge. Here, a photoelectrochemical (PEC)‐driven “self‐purification” process is suggested, which can effectively generate a gradient distribution of oxygen vacancies in the thickness range of ≈9.5 nm. As a result, a charge transport efficiency of >95% can be achieved by efficient hole migration from the photoanode to the electrolyte. This unique protocol is expected to provide an advanced metal oxide photocatalyst and photoelectrochemical electrode that exhibit superior photocatalytic performance with enhanced charge separation.

Theoretical study on ferroelectric nitrides with super-wurtzite structures for solar energy conversion applications.

Polarized structured nitride semiconductors are attractive due to their unique and environment-friendly electronic properties. The stability, ferroelectricity and photocatalytic and photovoltaic properties of super-wurtzite Mg2XN3 (X = Bi, Mo, Nb, Sb, Ta, Tc and W) were determined based on first principles calculations in this study. The calculated results indicate that Mg2XN3 (X = Sb, Ta, Bi and Nb) are stable polar nitrides by phonon frequencies, elastic coefficients and ferroelectric analysis. Mg2XN3 (X = Sb, Ta and Nb) with large ferroelectric polarization strength could absorb ultraviolet light to promote photocatalytic water splitting for hydrogen production. Mg2BiN3 is a new excellent photovoltaic candidate due to its ideal energy band, high electron mobility, high absorption coefficient and large ferroelectric polarization strength.

Synthesis, investigation of the properties and evaluation the stability of the mix halogen perovskite (Cl, Br, I) and fabrication of perovskite solar cells

Perovskite CH3NH3PbI3 has been attracted significant attention because of its high absorption coefficient, the ideal band gap energy (Eg =1.5 eV) and the low cost of synthesis. However, CH3NH3PbI3 had an intrinsic issue regarding the long-term stability due to the unstable bond of Pb‒I. In addition, the CH3NH3PbI3 cubic structure had easily been occurred in the phase transition. The chloride or bromide ion, a high electronegativity, was used to exchange for I- in CH3NH3PbI3 to create the mixed halide perovskites, CH3NH3PbI3-xBrx (x = 0.5‒1.5) and CH3NH3PbI1,8Cl1,2. The structure of materials was identified by XRD. The stability of CH3NH3PbI3-xBrx or CH3NH3PbI3-xClx was investigated during 290 hours in an environmental chamber (80 % humidity, 40 °C). The band gap of perovskites could be tunned by controlling the Cl-, Br- or I- ratio. The result revealed that the half‒life of CH3NH3PbI3-xBrx and CH3NH3PbI3-xClx were higher from 2 to 4 times than that of CH3NH3PbI3. Perovskite solar cells based-on CH3NH3PbI2,5Br0,5 and CH3NH3PbI1,8Cl1,2 were fabricated by using spin-dip coating 2 steps process possessing the maximum efficiency of 11.4 % (Jsc = 0.99 mA.cm-2, Voc = 1.05 V) and 12.3 % (Jsc = 1.14 mA.cm-2, Voc = 1.01 V) under AM 1.5 simulated sunlight of 1,000 W.m-2 light intensity

Every Lateral Band is the Kernel of an Orthogonally Additive Operator

{In this paper we continue a study of relationships between the lateral partial order $\sqsubseteq$ in a vector lattice (the relation $x \sqsubseteq y$ means that $x$ is a fragment of $y$) and the theory of orthogonally additive operators on vector lattices. It was shown in~\cite{pMPP} that the concepts of lateral ideal and lateral band play the same important role in the theory of orthogonally additive operators as ideals and bands play in the theory for linear operators in vector lattices. We show that, for a vector lattice $E$ and a lateral band $G$ of~$E$, there exists a vector lattice~$F$ and a positive, disjointness preserving orthogonally additive operator $T \colon E \to F$ such that ${\rm ker} \, T = G$. As a consequence, we partially resolve the following open problem suggested in \cite{pMPP}: Are there a vector lattice~$E$ and a lateral ideal in $E$ which is not equal to the kernel of any positive orthogonally additive operator $T\colon E\to F$ for any vector lattice $F$?

Emerging Surface, Bulk, and Interface Engineering Strategies on BiVO4 for Photoelectrochemical Water Splitting

The photoelectrochemical (PEC) cell that collects and stores abundant sunlight to hydrogen fuel promises a clean and renewable pathway for future energy needs and challenges. Monoclinic bismuth vanadate (BiVO4 ), having an earth-abundancy, nontoxicity, suitable optical absorption, and an ideal n-type band position, has been in the limelight for decades. BiVO4 is a potential photoanode candidate due to its favorable outstanding features like moderate bandgap, visible light activity, better chemical stability, and cost-effective synthesis methods. However, BiVO4 suffers from rapid recombination of photogenerated charge carriers that have impeded further improvements of its PEC performances and stability. This review presents a close look at the emerging surface, bulk, and interface engineering strategies on BiVO4 photoanode. First, an effective approach of surface functionalization via different cocatalysts to improve the surface kinetics of BiVO4 is discussed. Second, state-of-the-art methodologies such as nanostructuring, defect engineering, and doping to further enhance light absorption and photogenerated charge transport in bulk BiVO4 are reviewed. Third, interface engineering via heterostructuring to improve charge separation is introduced. Lastly, perspectives on the foremost challenges and some motivating outlooks to encourage the future research progress in this emerging frontier are offered.

First-principles study of electric field and strain modulation in GaS-BSe vdW heterostructured bilayer for bandstructure engineering

Inspired by previous studies on the electronic properties of the two-dimensional (2D) hexagonal structure group III-VI binary monolayers, this work further explores possible heterostructures and their potential applications. People have been focusing on transition metal dichalcogenides (TMDCs) based materials for a long time, while the low mobility greatly hinders their further application. After years of exploration and research, the related composite forms and synthesis methods for constructing a heterostructure with ideal band alignment have been well developed. Based on the density functional theory (DFT), we here combine monolayer GaS and BSe through vdW interaction to obtain a new heterostructure form with well-defined type-II alignment. This work proves for the first time that GaS-BSe heterostructures can fully meet all ideal criteria and has a better mobility than pristine structures. We found that the applied electric field can adjust the band alignment between type-I and type-II, while the applied strain can maintain two heterostructures within a relatively wide type-II range, thereby effectively separating the light-induced carriers in space. The tensile strain applied structure can promote light absorption within the solar spectrum. When an external strain is applied, the predicted efficiency of s-GaS can be as high as 21%. This work provides a promising route to design new vdW heterostructures based on group III-VI monolayers and s-GaS with high flexibility and tunability, and is a potential candidate for new electronic devices and photocatalysts.

Spin qubit based on the nitrogen-vacancy center analog in a diamond-like compound C3BN

The nitrogen-vacancy (NV) center in diamond plays important roles in emerging quantum technologies. Currently available methods to fabricate the NV center often involve complex processes such as N implantation. By contrast, in a diamond-like compound C3BN, creating a boron (B) vacancy immediately leads to an NV-center analog. We use the strongly constrained and appropriately normed semilocal density functional—this functional leads to nearly the same zero-phonon line (ZPL) energy as the experiment and as obtained from the more time-consuming hybrid density functional calculations—to explore the potential of this NV-center analog as a novel spin qubit for applications in quantum information processing. We show that the NV-center analog in C3BN possesses many similar properties to the NV center in diamond including a wide bandgap, weak spin–orbit coupling, an energetically stable negatively charged state, a highly localized spin density, a paramagnetic triplet ground state, and strong hyperfine interactions, which are the properties that make the NV center in diamond stand out as a suitable quantum bit (qubit). We also predict that the NV-center analog in C3BN exhibits two ZPL energies that correspond to longer wavelengths close to the ideal telecommunication band for quantum communications. C3BN studied here represents only one example of A3XY (A: group IV element; X/Y: group III/V elements) compounds. We expect many other compounds of this family to have similar NV-center analogs with a wide range of ZPL energies and functional properties, promising to be the new hosts of qubits for quantum technology applications. Furthermore, A3XY compounds often contain group IV elements such as silicon and germanium, so they are compatible with sophisticated semiconductor processing techniques. Our work opens up ample opportunities toward scalable qubit host materials and novel quantum devices.

Direct deposition of lead sulfide nano-optoelectronic films on polymer substrate

Nanocrystals have exhibited unique optoelectronic properties and demonstrated a wide range of applications in light-emitting devices, semiconductor devices and solar cell devices. However, previous studies usually deposit nanocrystal films on traditional rigid substrates, limiting their applications in large-scale, direct-deposited flexible device fabrication processes, such as roll-to-roll printing process. Here, we report a direct deposition method for lead sulfide (PbS) nanocrystal films on flexible polymer substrates. By adding triethanolamine-coordinated Pb precursors to the reaction system to enhance the adhesion to the substrate and controlling the precursor ratios, we obtained high-quality flexible PbS films. The film is composed of octahedral PbS nanocrystals with preferred (111) orientation. The optical band gap of the nanocrystal films can be tuned from 1.32 eV to 1.60 eV by adjusting the ratio of the precursors, and an ideal band gap of 1.4 eV for single-junction solar cells was also obtained when Pb to S ratio reaches 1.4:1. More importantly, the flexible PbS films exhibit high charge carrier mobility of up to 25 cm2 V[Formula: see text] s[Formula: see text], which is comparable to that of PbS films grown on traditional rigid substrates (e.g. silicon wafers or glasses). To our knowledge, this mobility is also the highest for PbS fabricated directly on flexible substrates. Our study provides a new approach for the preparation of low-cost and roll-to-roll processable nanocrystal films for future flexible optoelectronic devices.

Promising Lead-Free Double-Perovskite Photovoltaic Materials Cs2MM′Br6 (M = Cu, Ag, and Au; M′ = Ga, In, Sb, and Bi) with an Ideal Band Gap and High Power Conversion Efficiency

Promising Lead-Free Double-Perovskite Photovoltaic Materials Cs₂MM′Br₆ (M = Cu, Ag, and Au; M′ = Ga, In, Sb, and Bi) with an Ideal Band Gap and High Power Conversion Efficiency

Silver-assisted optimization of band gap gradient structure of Cu(In,Ga)Se2 solar cells via SCAPS

Recently, silver (Ag) substitution for copper (Cu) into Cu(In,Ga)Se2 (CIGS) films has attracted widespread attention due to the lower synthesis temperature and the enlarge band gap. In this paper, SCAPS simulation is employed to investigate the influence of Ag doped CIGS absorber layer on the lattice defect and band gap gradient. First, the impact of the [Ga]/([In] + [Ga]) (GGI) and [Ag]/([Ag] + [Cu]) (AAC) on the defect concentrations is calculated systematically. It is demonstrate that the compression (GGI > 0.25) is more likely to cause the tetragonal distortion compared with the tension (GGI < 0.25) on the unit cell. And appropriate substitution of Cu with Ag can reduce the defect concentrations. Further investigations on the variation of the lattice structure indicate the ideal band gap can be enlarged to the range of about 1.20–1.29 eV by silver. With the ideal lattice structure, the efficiency of solar cells has a relative increase of about 48 % compared with that of the reference solar cell. This study offers a new sight for the incorporation of Ag into CIGS films from the perspective of lattice structure, which should be further improve device performance of CIGS solar cells at low temperature deposition process.

Heteroepitaxial growth of β-Ga2O3 thin films on c-plane sapphire substrates with β-(AlxGa1-x)2O3 intermediate buffer layer by mist-CVD method

As a new-type of ultra-wide bandgap semiconductor, β-Ga2O3 has received widespread attention due to its great potential applications in high-power devices and deep ultraviolet photodetectors. However, the conventional material epitaxial growth methods (such as MBE, MOCVD and HVPE) are always based on the expensive vacuum equipment and time-consuming. In this work, a low-cost and non-vacuum mist-CVD method is used to grow β-Ga2O3 films heteroepitaxially on sapphire substrates. In order to improve the quality of β-Ga2O3 films, the β-(AlxGa1-x)2O3 is first grown on sapphire substrate and used as the intermediate buffer layer to epitaxially grow β-Ga2O3 film, which can decrease the lattice mismatch and improve the β-Ga2O3 quality. A series of characterization techniques are used to characterize the properties of samples. The results of XPS and UV-Vis tests show that the aluminum composition of β-(AlxGa1-x)2O3 films grown at 850 °C and 900 °C are 0.24 and 0.43, respectively, and the corresponding band gap is 5.36 eV and 5.55 eV respectively. After using β-(AlxGa1-x)2O3 as the intermediate buffer layer, the FWHM of β-Ga2O3 film is 1.131°, the RMS roughness is 3.04 nm, and it has an ideal band gap (4.91 eV), which demonstrates the obvious improvement compared to the sample without the intermediate buffer layer. This shows that β-(AlxGa1-x)2O3 as an intermediate buffer layer can significantly improve the quality of heteroepitaxially grown β-Ga2O3 films.

DFT/TDDFT Investigation on donor-acceptor triazole-based copolymers for organic photovoltaics

Using density functional theory (DFT) method, we studied several triazole-based donor-acceptor (DA) π-conjugated molecules and evaluated their lowest unoccupied molecular orbital (LUMO) energies. Our results shows that bithiophene-substituted triazole-based acceptors with short and non-bulky substituents tend to be co-planar and hence exhibit stronger conjugation and electron delocalization, thereby leading to lower orbital energies of LUMO. Introduction of heteroatom such as S and Se within the chemical structure of triazole-based acceptor also significantly reduces the bandgap by lowering the LUMO energies. Among the donor fragments studied, a dithieno [3,2-b:2′,3′-d]phosphole donor fragment (Y4) led to donor-acceptor (DA) pairs with comparatively lower highest occupied molecular orbitals (HOMO, (-5.01 to -4.69 eV) along with higher VOC (>1.29 eV) without much compromise in its bandgap energy. This makes such DA-based polymers promising for bulk heterojunction solar cell applications. TD-DFT calculations were further used to simulate the absorption spectra of the DA monomers and showed that most pairs gave meaningful absorption from 400 to 1100 nm. More interestingly, the addition of long aromatic substituents in the acceptor fragment tends to give broader absorption spectra. Further studies on trimers with DDA repeating unit revealed that copolymers bearing Y4 and a benzo [1,2-d;4,5-d’]bitriazole X1 (or X6) exhibits close to ideal band gap of 1.37 eV (1.36 eV) with favorable open circuit voltage 1.23 eV, though absorption by Y4-Y4-X1 takes place in Infrared scope with higher oscillator strength. All in all, we identified a polymer comprising Y4 donor and X1 acceptor fragment to be a promising material for heterojunction organic solar cell application.

Isolated flat bands in a lattice of interlocking circles

Flat-band physics has attracted much attention in recently years because of its interesting properties and important applications. Some typical lattices have been proposed to generate flat bands, such as Kagome and Lieb lattices. The flat bands in these lattices contact with other bands rather than isolated. However, an ideal flat band should be isolated, because isolation is a prerequisite for a number of important applications. Here, we propose a new lattice that can produce isolated flat bands. The lattice is named as interlocking-circles lattice because its pattern seems like interlocking circles. Moreover, the new lattice is realized in graphene by hydrogenation. In the hydrogenated graphene, there are two nontrivial isolated flat bands appearing around the Fermi level. Upon hole or electron doping, the flat bands split into spin-polarized bands and then result in a ferromagnetic graphene. Our work not only proposes a new type of lattice but also opens a way to find systems with isolated flat bands.

Subband Adaptive Filter in Signal Processing Application

Owing to the powerful digital signal processors and the improvement of advanced edge adaptive algorithms there are an extraordinary number of various applications in which adaptive filters are utilized. Subband adaptive filtering algorithms can build the assembly pace of framework ID undertakings when the info signal is hued. The adaptive filter can filter the dubious noise signal, track the difference in the signal, and consistently change the boundaries to accomplish the ideal filtering impact. Another standardized subband adaptive filtering algorithm has been proposed, whose primary benefit is the lower computational intricacy when contrasted with best in class subband approaches, while keeping up comparable union execution. A connection between the adaptive subband coefficients and the ideal full band move work is determined, and the algorithm is demonstrated to create an asymptotically unprejudiced arrangement. The proficiency of the adaptive filters basically relies upon the plan procedure utilized and the algorithm of variation. The adaptive filters can be analogical plans, digital or blended which show their benefits and inconveniences, for instance, the analogical filters are low power consuming and fast response, however they address balance issues, which influence the activity of the variation algorithm.