Fundamental Band Gap Research Articles

Design of a novel series of small molecule donors for application in organic solar cells

In an effort to study high-efficient electron donor materials for application in organic solar cells (OSC), density functional theory (DFT) calculations were performed to predict a variety of small molecules (SMs) with terminal acceptor-π bridge-central donor-π bridge-terminal acceptor (A-π-D-π-A) framework. An interesting A-π-D-π-A-type SM containing phenothiazine as D unit has been recently synthesized and applied to in the OSC. Motivated by this study, here, we have systematically predicted 10 SM donor materials based on the phenothiazine as the central D unit as well as several A and π units that have been used into materials representing excellent SC as well as charge transport characteristics. Then, the structural, electronic, and optical properties of the predicted SM donors have been investigated. The fundamental band gap energies, orbital spatial distributions, and intrinsic dipole moments were calculated using DFT while the optical band gap energies were obtained using time-dependent density functional theory (TD-DFT). Several SC properties of the predicted SMs were then computed and compared with the available experimental results of OSCs. Our suggested SMs represented the enhancement in the open circuit voltage (VOC) and charge transport properties which may lead to OSCs with improved power conversion efficiency (PCE) compared to the previously synthesized SMs. Moreover, small reorganization energies, large transfer integrals, and high intra-molecular coupling obtained from dense π-stacking give rise to increased electron mobility in the predicted SMs. However, this strategy can be helpful for further improving the performance of SMs in OSCs.

Pressure Effects on the Optical Properties of NdVO4

We report on optical spectroscopic measurements in pure NdVO4 crystals at pressures up to 12 GPa. The influence of pressure on the fundamental absorption band gap and Nd3+ absorption bands has been correlated with structural changes in the crystal. The experiments indicate that a phase transition takes place between 4.7 and 5.4 GPa. We have also determined the pressure dependence of the band-gap and discussed the behavior of the Nd3+ absorption lines under compression. Important changes in the optical properties of NdVO4 occur at the phase transition, which, according to Raman measurements, corresponds to a zircon to monazite phase change. In particular, in these conditions a collapse of the band gap occurs, changing the color of the crystal. The changes are not reversible. The results are analyzed in comparison with those deriving from previous studies on NdVO4 and related vanadates.

Tailoring optical and electrical properties of thin-film coatings based on mixed Hf and Ti oxides for optoelectronic application

In this work multi-magnetron sputtering stand was used for the deposition of the mixed oxides thin films based on HfO2 and TiO2. In order to obtain various material composition the power released to each magnetron (containing metallic hafnium and titanium targets) was precisely selected. Structural, surface, optical, electrical and mechanical properties of as-deposited coatings were analyzed. Depending on the hafnium content in the deposited thin films various types of the microstructure was obtained, i.e. HfO2-monoclinic, amorphous and TiO2-rutile. Increase of Ti content above 28at. % in the as-prepared mixed oxides coatings caused their amorphization. It was found that with an increase of Ti content in prepared coatings their surface roughness and simultaneously water contact angle decreased. Performed measurements of electrical properties revealed that the lowest leakage current density in the range of 10−7–10−8A/cm2 was obtained for amorphous coatings. Moreover, the tailoring of the dielectric constant was possible by a proper selection of material composition and microstructure of the deposited thin films. Average transparency in the visible wavelength region was in the range of ca. 79–86%. The influence of material composition and structure on shifting of the fundamental absorption edge and optical band gap energy was also observed. The refractive index increased with an increase of Ti content, while extinction coefficient was the lowest for amorphous coatings. Additionally, hardness values were dependent on the material composition and optical packing density and were in the range from 7.6GPa to 10.1GPa.

Rigid-band electronic structure of scandium nitride across the n -type to p -type carrier transition regime

Intentional doping and unintentional impurities in intrinsic semiconductors generate carriers that enable device operations. Fundamental to the electronic activity of dopants and impurities is the introduction of defect states inside the forbidden energy gap of semiconductors having shallow and/or deep characteristics, which fundamentally define the ability to engineer its physical properties and associated device performance. Here we demonstrate that unintentional electron doping by oxygen (${\mathrm{O}}_{\mathrm{N}}$) impurities and intentional magnesium hole doping ($\mathrm{M}{\mathrm{g}}_{\mathrm{Sc}}$) in scandium nitride (ScN) do not introduce any defect states inside its fundamental bandgap and that the rigid-band electronic structure remains unchanged. Employing a combination of spectroscopic techniques as well as first-principles density functional theory analysis, we show that the ${\mathrm{O}}_{\mathrm{N}}$ and $\mathrm{M}{\mathrm{g}}_{\mathrm{Sc}}$ defect states in ScN are located inside the bands, which leaves behind the virgin ScN bandgap as well as the valence and conduction band edges that are important for electronic transport. The rigid-band electronic structure of ScN with respect to the electron and hole doping results in high electron and hole concentrations due to the free movement of Fermi level and results in tunable electronic and thermoelectric properties necessary for device applications.

Investigation of the Structural, Electronic and Mechanical Properties of Type-VIII Ba8Si46 Clathrate under High-Pressure through First-Principles

By means of density functional theory (DFT) calculations, we have systematically investigated the effect of hydrostatic pressure on the structural, electronic and elastic properties of barium-doped silicon clathrate Ba8Si46 in the type-VIII structure (α phase). Physical properties are calculated under different conditions of pressure (0 GPa to 45 GPa) using the GGA-PBE functional. Those calculations have been performed using the Cambridge serial total energy package CASTEP within the Materials Studio package. Electronic properties have shown that type-VIII Ba8Si46 has metal-like properties with a fundamental bandgap of 1 eV. Under pressure the fundamental bandgap increases slightly and the positions of the valence band maximum VBM and the conduction band minimum CBM remain unchanged when pressure is changed from 0 to 25 GPa, but when pressure is set to 30 GPa the CBM is transited from its initial position (Γ-H segment) towards the Γ(0,0,0) point, this indicates that the bandgap nature of this material can be tuned through strain engineering. We found that the compound is mechanically stable under the pressure range and might be unstable or collapse when pressure exceeds 44 GPa. These findings need to be confirmed experimentally through synthesis, a comparison with the type-I and the guest-free counterparts has exhibited promising features for the type-VIII Ba8Si46.

First-Principles Study of Structural and Electronic Properties of MoS1.5Se0.5 Alloy

The effects of relative positions of Se atoms in a monomolecular layer of MoS[Formula: see text]Se[Formula: see text] have been studied. It is demonstrated that the distribution of Se atoms between top and bottom chalcogen planes is most energetically favorable. For a more probable distribution of Se atoms this monolayer alloy is a direct semiconductor with the fundamental bandgap of 2.35[Formula: see text]eV. We have also evaluated the optical band gaps of the alloy at 77[Formula: see text]K (1.86[Formula: see text]eV) and room temperature (1.80[Formula: see text]eV), which are in a good agreement with the experimentally measured bandgap of 1.79[Formula: see text]eV.

Optical properties of PVT grown bromoaluminium phthalocyanine nanostructures using UV–visible–NIR spectroscopy

In this paper, we have studied the optical properties of bromoaluminium phthalocyanine (BrAlPc) nanostructures prepared by physical vapor phase transport (PVT) using UV–visible–NIR spectroscopy. Field emission scanning electron microscopy (FESEM) images show that different nanostructures namely nanocorals, nanorods and nanothistles have been obtained at varied source-substrate distances. By analyzing the spectrophotometric measurements of the absorbance, transmittance and reflectance spectra in the range of 300–2500 nm the optical characteristics, such as the optical absorption, Urbach energy, extinction coefficient, refractive index, and dielectric characteristics as a function of incident photon energy have been evaluated. We have found that the values of fundamental optical band gap and the energy of trap levels of all nanostructures have no remarkable changes. It is seen that the Urbach energy of BrAlPc nanorods is higher than nanocorals and nanothistles indicating higher structural disorder. Moreover, the occurrence of doublet in the Q-band region reveals that the BrAlPc nanocorals and nanorods are in $$\alpha$$ -phase while the BrAlPc nanothistles is in $$\beta$$ -phase. Moreover, different dispersion and absorption parameters have been estimated for different BrAlPc nanostructures. The obtained optical results for novel grown BrAlPc by PVT method may be of interest for practical applications.

Single-crystalline Cu2O thin films of optical quality as obtained by the oxidation of single-crystal Cu thin films at low temperature

High-quality, single-crystal-like Cu2O thin films of various thicknesses (10 nm–45 nm) were prepared at a low temperature (150 °C) by controlling layer-by-layer oxidation of wafer-scale Cu thin films sputtered along the (111) direction using a pure single-crystal Cu target. The cross-sectional images of the thin films reveal high crystallinity of Cu2O layers except for 60° twinning in the sequential stacking order as evidenced by high-resolution transmission electron microscopy, which is consistent with the absence of the photoluminescence (PL) signals arising from atomic-scale vacancies. The optical properties of our Cu2O films were investigated using temperature-dependent PL and Raman spectroscopy. All of the Cu2O thin films exhibit characteristic band-to-band transitions together with the series of yellow excitonic transitions slightly below the fundamental bandgap. The spectral locations for the PL are approximately consistent with those for the bulk counterpart. The excellent optical quality of our Cu2O was further demonstrated by significantly reduced quasi-direct transition that occurs at symmetry-breaking crystal imperfection, which relaxes the stringent momentum conservation rule. We identified the three main Raman scattering modes of the Cu2O thin films, where the two forbidden modes of Γ15(1) and Γ12−+Γ25− are resonantly allowed by the proximity of the incident photon energy to the green bandgap. We believe that our synthesis technique can be utilized for the preparation of single-crystal-like metal oxide thin films at low production temperatures with precise thickness control for the development of novel optoelectronic devices and for the exploration of the nanoscale light-matter interaction as well.

Many-Body Quantum Monte Carlo Study of 2D Materials: Cohesion and Band Gap in Single-Layer Phosphorene

Quantum Monte Carlo (QMC) is applied to obtain the fundamental (quasiparticle) electronic band gap, $\Delta_f$, of a semiconducting two-dimensional (2D) phosphorene whose optical and electronic properties fill the void between graphene and 2D transition metal dichalcogenides. Similarly to other 2D materials, the electronic structure of phosphorene is strongly influenced by reduced screening, making it challenging to obtain reliable predictions by single-particle density functional methods. Advanced GW techniques, which include many-body effects as perturbative corrections, are hardly consistent with each other, predicting the band gap of phosphorene with a spread of almost 1 eV, from 1.6 to 2.4 eV. Our QMC results, from infinite periodic superlattices as well as from finite clusters, predict $\Delta_f$ to be about 2.4 eV, indicating that available GW results are systematically underestimating the gap. Using the recently uncovered universal scaling between the exciton binding energy and $\Delta_f$, we predict the optical gap of 1.75 eV that can be directly related to measurements even on encapsulated samples due to its robustness against dielectric environment. The QMC gaps are indeed consistent with recent experiments based on optical absorption and photoluminescence excitation spectroscopy. We also predict the cohesion of phosphorene to be only slightly smaller than that of the bulk crystal. Our investigations not only benchmark GW methods and experiments, but also open the field of 2D electronic structure to computationally intensive but highly predictive QMC methods which include many-body effects such as electronic correlations and van der Waals interactions explicitly.

Density functional theory study explaining the underperformance of copper oxides as photovoltaic absorbers

Most photovoltaic absorbers are identified using the standard Shockley-Queisser selection principle which relies on optimal band-gap values. However, this criterion has been shown to be insufficient, as many materials with appropriate values still perform badly. Here, we have employed calculations based on the density functional theory to assess three copper oxides as potential photovoltaic materials: ${\mathrm{Cu}}_{2}\mathrm{O}$, ${\mathrm{Cu}}_{4}{\mathrm{O}}_{3}$, and $\mathrm{CuO}$. Despite their promising theoretical solar power conversion efficiency of over 20%, experimental values are found to be far lower. Theoretical evaluation of the electronic and optical properties reveals that certain transitions within the band structures are dipole forbidden, whereas the fundamental band gaps of ${\mathrm{Cu}}_{4}{\mathrm{O}}_{3}$ and $\mathrm{CuO}$ are of indirect nature. These findings correlate to the weak and shifted absorption properties found experimentally, which underpin the inefficient light capture by copper oxides. Based on these results and an applied extended selection metric, we can explain why copper oxides are unable to reach the efficiencies previously proposed theoretically and why we need to revise their maximum conversion values.

Band structure and carrier effective masses of boron arsenide: Effects of quasiparticle and spin-orbit coupling corrections

We determine the fundamental electronic and optical properties of the high-thermal-conductivity III–V semiconductor boron arsenide (BAs) using density functional and many body perturbation theory including quasiparticle and spin-orbit coupling corrections. We find that the fundamental bandgap is indirect with a value of 2.049 eV, while the minimum direct gap has a value of 4.135 eV. We calculate the carrier effective masses and report smaller values for the holes than the electrons, indicating higher hole mobility and easier p-type doping. The small difference between the static and high frequency dielectric constants indicates that BAs is only weakly ionic. We also observe that the imaginary part of the dielectric function exhibits a strong absorption peak, which corresponds to parallel bands in the band structure. Our estimated exciton binding energy of 43 meV indicates that excitons are relatively stable against thermal dissociation at room temperature. Our work provides theoretical insights into the fundamental electronic properties of BAs to guide experimental characterization and device applications.

Electronic structure and optical response for Zn1−xBexSe

Abstract The electronic structures and optical properties of the Zn1−xBexSe semiconductor ternary alloys have been investigated by using the full-potential linear augmented plane wave (FP-LAPW) method. From this study, the energy band gap has a value of about 1.2 eV for zinc-blende ZnSe. However, for Zn0.50Be0.50Se and BeSe, the fundamental band gap energy is found to occur at the highly symmetric X point in the Brillouin zone and has values of about 2.3 eV and 2.8 eV, for Zn0.50Be0.50Se and BeSe, respectively. Optical parameters, such as dielectric constant, refractive index and reflectivity are calculated and analyzed. The results demonstrated that the compounds Zn1−xBexSe have the potential to be used for optoelectronic applications.

Accurate, First-Principle Study of Electronic and Related Properties of the Ground State of Li<sub>2</sub>Se

We present results from ab-initio, self-consistent calculations of electronic and related properties for the ground state of cubic lithium selenide (Li2Se). We employed a local density approximation (LDA) potential and performed computations following the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF). This method verifiably leads to the ground state of materials without employing over-complete basis sets. We present the calculated electronic energies, total and partial densities of states, effective masses, and the bulk modulus. The present calculated band structures show clearly that cubic Li2Se has a direct fundamental energy band gap of 4.065 eV at the Γ point for the room temperature experimental lattice constant of 6.017 A. This result is different from findings of previous density functional theory (DFT) calculations that uniformly reported an indirect band gap, from Γ to X, for Li2Se. We predicted a direct band gap of 4.363 eV, at the computationally determined equilibrium lattice constant of 5.882 A, and a bulk modulus of 35.4 GPa. For the first time known to us, we report calculated electron and hole effective masses for Li2Se. The experimental confirmation of the large, direct gap we found will point to a potential importance of this material for ultraviolet technologies and applications. Due to a lack of experimental results, most of our calculated ones in this paper are predictions for Li2Se.

Effect of the paramagnetic to spin-glass phase transition on the fundamental absorption edge of MnIn2Se4 magnetic semiconducting compound

The temperature dependence of the indirect and direct fundamental band gaps of the layered compound MnIn2Se4,that crystallizes in a rhombohedral defect structure with space group R -3m (D5/3d), was studied by optical absorption spectra. The data were analyzedin terms of current theoretical models that take into account the contribution of antiferromagnetic exchange interaction between spins to the shift of the energy gap EG with T in the vicinity of the critical paramagnetic to spin-glass phase transition. It was established that short- and long-range effect spin correlations dominate the contribution to this shift in the critical region below about 20 K, near the spin-glass freezing temperature Tf, and noncritical one, between about 70 and 160 K, far from Tf, respectively. An intermediate temperature region, compatible with the behaviour expected for a cluster-glass transition where a gradual freezing of the magnetic moments occurs, was also observed.

Approximating Quasiparticle and Excitation Energies from Ground State Generalized Kohn-Sham Calculations.

Quasiparticle energies and fundamental band gaps in particular are critical properties of molecules and materials. It was rigorously established that the generalized Kohn-Sham HOMO and LUMO orbital energies are the chemical potentials of electron removal and addition and thus good approximations to band edges and fundamental gaps from a density functional approximation (DFA) with minimal delocalization error. For other quasiparticle energies, their connection to the generalized Kohn-Sham orbital energies has not been established but remains highly interesting. We provide the comparison of experimental quasiparticle energies for many finite systems with calculations from the GW Green function and localized orbitals scaling correction (LOSC), a recently developed correction to semilocal DFAs, which has minimal delocalization error. Extensive results with over 40 systems clearly show that LOSC orbital energies achieve slightly better accuracy than the GW calculations with little dependence on the semilocal DFA, supporting the use of LOSC DFA orbital energies to predict quasiparticle energies. This also leads to the calculations of excitation energies of the N-electron systems from the ground state DFA calculations of the ( N - 1)-electron systems. Results show good performance with accuracy similar to TDDFT and the delta SCF approach for valence excitations with commonly used DFAs with or without LOSC. For Rydberg states, good accuracy was obtained only with the use of LOSC DFA. This work highlights the pathway to quasiparticle and excitation energies from ground density functional calculations.

Atomic and electronic structure of the Si(331)-(12 1) surface.

We report on the investigation of the atomic and electronic structures of a clean Si(331)-(12 × 1) surface using a first-principles approach with both plane wave and strictly localized basis sets. Starting from the surface structure proposed by Zhachuk and Teys [Phys. Rev. B 95, 041412(R) (2017)], we develop significant improvements to the atomic model and localized basis set which are critical for the correct description of the observed bias dependence of scanning tunneling microscopy (STM) images. The size mismatch between the Si pentamers from the surface model and those seen by STM is explained within the context of the Tersoff-Hamann model. The energy barriers that separate different Si(331) buckled configurations were estimated, showing that the surface structure is prone to dynamic buckling at room temperature. It is found that empty electronic states on Si(331) are essentially localized on the pentamers with interstitials and under-coordinated Si sp 2-like atoms between them, while filled electronic states are localized on under-coordinated Si sp 3-like atoms and dimers on trenches. The calculated electronic density of states exhibits two broad peaks in the fundamental bandgap of Si: one near the valence band top and the other near the conduction band bottom. The resulting surface bandgap of 0.58 eV is in an excellent agreement with spectroscopy studies.

Dopant Control of Electron–Hole Recombination in Cesium–Titanium Halide Double Perovskite by Time Domain Ab Initio Simulation: Codoping Supersedes Monodoping

Using nonadiabatic (NA) molecular dynamics combined with time domain density functional theory, we simulate electron-hole recombination in pristine and doped inorganic Pb-free double perovskite Cs2TiBr6. We show that replacing the titanium and/or bromine with silicon and/or chlorine extends the charge carrier lifetime. Importantly, dopants avoid deep traps despite the fact that they do not change the fundamental band gap of Cs2TiBr6, and they decrease the NA electron-phonon coupling and accelerate decoherence arising from the reduced overlap of electron and hole wave functions as well as fast phonon modes induced by light dopants, respectively, suppressing electron-hole recombination. More importantly, codoping can reduce the formation energy of silicon and achieve higher doping concentration, potentially increasing the lifetime further. Our study suggests a rational strategy to reduce energy losses by codoping in design of high-performance all-inorganic Pb-free perovskite solar cells.

Nonlinear optical susceptibility of atomically thin WX2 crystals

We have studied tungsten diselenide (WSe2) and tungsten disulfide (WS2) monolayer materials in second harmonic generation spectroscopy and microscopy experiments. Ultra-broadband continuum pulses served as the fundamental beam while its second harmonic spectrum in the visible and ultraviolet (UV) range was detected and analyzed with a better than 0.3 nm spectral resolution (<2 meV). We provide dispersion data and absolute values for χ(2) for the materials within a photon energy range of 2.3–3.2 eV. Fine spectral features that were detected within the dispersion data for the optical nonlinearities indicate the impact of near bandgap exciton transitions. The fundamental bandgap of 2.35 eV and exciton binding energy of 0.38 eV were determined from the measurements for WS2 monolayers while the corresponding values in WSe2 monolayers were 2.22 eV and 0.71 eV. Ranges for the absolute values of the sheet nonlinearity for WS2 and WSe2 are shown to be 0.58–1.65 × 10−18 m2/V and 0.21–0.92 × 10−18 m2/V, correspondingly.

Low energy excitations in NiO based on a direct Δ-SCF approach

This paper reports calculated energies and electronic structures of nineteen excited states in NiO based on a Δ-SCF approach reported previously for the transitions in NiO and Sr2CuO2Cl2. They are the spin-flip: ; eight : (one electron) 3A2g → 3T2g, (two electron) 3A2g → 3T1g, 3A2g → a1Eg, 3A2g → 1T2g, 3A2g → 1T1g, 3A2g → b1Eg, 3A2g → b1T2g; two O(2p) → Ni(3d); seven Ni(3d) → O(2p) (including two spin-flip) and the O(2s) → Ni(3d) charge transfer excitations, which span an energy range from 0.25 eV to 17.53 eV, and include the fundamental band gap associated with an excitonic O(2p) → Ni(3d) transition at 4.23 eV. These are compared to absorption and emission spectra, and previous calculations. In the case of the O(2p) → Ni(3d) excitations, comparisons are given for gap and Γ-point energies derived from HF, PBE0, HSE06, B3LYP, B1WC, GGA and LDA one-electron approximations. Finally, the directly calculated Stokes shifts and associated luminescence energies for the two O(2p) → Ni(3d) transitions are reported, where the excitonic value is found to be in good agreement with the recently reported experimental value.

Non equilibrium anisotropic excitons in atomically thin ReS2

We present a systematic investigation of the electronic properties of bulk and few layer ReS2 van der Waals crystals using low temperature optical spectroscopy. Weak photoluminescence emission is observed from two non-degenerate band edge excitonic transitions separated by ∼20 meV. The comparable emission intensity of both excitonic transitions is incompatible with a fully thermalized (Boltzmann) distribution of excitons, indicating the hot nature of the emission. While DFT calculations predict bilayer ReS2 to have a direct fundamental band gap, our optical data suggests that the fundamental gap is indirect in all cases.