Effective Mass Of Charge Carriers Research Articles

2D lead free Ruddlesden-Popper phase perovskites as efficient photovoltaic materials: A first-principles investigation

Though lead halide perovskites hold impressive optoelectronic properties, because of the bottleneck of lead toxicity, searching for the lead free perovskites is highly demanding for their commercial applications in the optoelectronic devices. Alongside of the lead free perovskites, two dimensional (2D) organic–inorganic hybrid halide perovskites (OIHHPs) are getting great attention as photovoltaic materials due to their air stability over the three dimensional(3D) counterpart and high performance. Herein, we have investigated recently synthesized lead free 2D Ruddlesden-Popper (RP) phase OIHHPs (X-PEA)2SnI4 (PEA=phenylethylammonium and X=H, para-F, meta-F, ortho-F) along with (PEA)2Sn1−xGexI4 where x = 0.5 and 1. Here studied all perovskites are direct band gap semiconductors with band gap in the solar energy region with high absorption coefficients. We have calculated the charge carrier effective masses along X–Γ–X and Y–Γ–Y and finally we have estimated maximum theoretical photoconversion efficiency (PCE) for all which reaches up to 25.58% making them suitable for their applications in high performance photovolatic devices.

Theoretical exploration of the optoelectronic properties of InAsNBi/InAs heterostructures for infrared applications: A multi-band k·p approach

We have investigated numerous electronic and optical aspects of the InAsNBi material, considering a multi-band k·p Hamiltonian. We have scrutinised electronic band dispersion, changes in spin–orbit splitting energy (so), charge carrier effective mass, band offset, and optical gain variations of InAsNBi and Quantum Well (QW) structures lattice-matched to InAs. The band gap of InAs0.9209N0.0291Bi0.05 is observed to be 131 meV under lattice-matched conditions, and the operating wavelength equivalent to this bandgap is 9.46 μm, which corresponds to the atmospheric Long Wavelength Infrared (LWIR) communication window of 8–12 μm. In order to tune the bandgap of a quantum confined heterostructure over a wide range, strain analysis plays a pivotal role. While compressive strain of 0.7% offers a band gap of 96 meV, tensile strain of 1.16% generates a band gap to 178 meV, which is almost equivalent to the room temperature bandgap of InSb. Under the influence of strain, spin-orbit splitting energy (Δso) spans the range from 0.31 eV to 0.45 eV. In addition to this, the inclusion of N and Bi into InAs reduces the effective mass of electrons to 0.01343 m0, which is ∼0.5 times of InAs. We have also computed the band structure of the InAsNBi/InAs QW heterostructure and found that under the influence of tensile strain with N = 2.5% and Bi = 1%, there is a changeover from Type-I to Type-II heterostructure. Indeed, InAsNBi can be a potential material for realizing various optoelectronic applications such as atmospheric LWIR communication, intermediate band solar cells, and night vision cameras.

A study of 2H and 1T phases of Janus monolayers and their van der Waals heterostructure with black phosphorene for optoelectronic and thermoelectric applications

Pristine Janus monolayers (MXY; M ​= ​Mo, W and X/Y = S, Se, Te) in 2H and 1T phases and their corresponding vdW heterostructures with black phosphorene are studied under density functional theory calculations. Phonon spectrum confirms dynamic stability of all monolayers in both patterns except MoSSe, WSSe, WSeTe and WSTe in 1T layout. Direct energy gap is observed for MoSSe, MoSeTe, WSSe, and WSeTe in 2H arrangement. In 1T phase, MoSSe, WSSe and their respective heterostructures exhibit metallic attributes and are suitable for high speed supercapacitors. Remnant heterostructures are indirect band gap semiconductors in both phases. PDOS affirms type-I band alignment of BlackP-MoSSe and BlackP-MoSeTe in 2H and BlackP-WSTe in 1T phase that can be well employed for future IR detectors. Moreover, globally desirable type-II band association is investigated for remaining heterostructures rendering exclusive laser, optoelectronic, photovoltaic and sensing applications. Each system manifests incredible effective masses of charge carriers. Heterostructures present a significant potential drop and work function (Φ). Also, every single heterostructure demonstrates noteworthy charge transfer which is accountable for electrostatic as well as vdW interaction at interface. Furthermore, sophisticated optical properties of all heterostructures are achieved by estimating absorption spectra. Optical responses suggest their eminent optoelectronic device applications like: telecommunication laser, optical fiber, photo diodes and solar cells. In addition, a comprehensive insight into thermoelectric properties of all the systems illustrates their ability for thermoelectric generator. Likewise, these materials are suitable alternative for non-toxic and inexpensive thermoelectric devices for waste heat recovery at ambient temperature.

Elastic, electronic, optical and thermoelectric properties of the novel Zintl-phase Ba2ZnP2

We report and discuss the results of a detailed first-principles calculations of the structural, elastic, electronic, optical and thermoelectric properties of the new Zintl phase dibarium zinc diphosphide Ba2ZnP2. The calculated structural parameters using the GGA-PBEsol functional are in excellent agreement with the available experimental counterparts. From the monocrystalline elastic constants numerically estimated through the stress-strain technique, a set of related properties, viz., mechanical stability, elastic anisotropy, brittle/ductile character, anisotropic sound velocities, polycrystalline elastic moduli, including isotropic bulk modulus, shear modulus, Young's modulus, Poisson's ratio, average sound velocity and Debye temperature, are deduced. The electronic and optical properties are investigated through the state-of-the art FP-(L)APW + lo method with the accurate TB-mBJ potential. Ba2ZnP2 is an indirect semiconductor with a gap of 1.24 eV. The charge-carrier effective masses are calculated. The valence band maximum is less dispersive than the conduction band minimum. The microscopic origins of the electronic states composing the energy bands are determined via the PDOS diagrams. Topological analysis of the charge density shows that a covalent character is dominantly ruling the Zn–P bond inside the block ZnP4, while an ionic bonding is mainly ruling the bond between the cation Ba and the polyanion ZnP4. Frequency-dependent macroscopic linear optical functions are predicted in a wide energy range 0–30 eV. Within the visible spectra, the calculated magnitude of the absorption coefficient, reflectivity and refractive index are in the ranges ∼4−35×104cm−1, 29−36% and 3.18−3.47, respectively. The semi-classical Boltzmann transport theory within the constant relaxation time approximation is used to study the thermoelectric properties. The title compound has a figure of merit of ∼1.77 at 300 K, which makes it a potential candidate for thermoelectric applications.

Antiperovskite materials as promising candidates for efficient tandem photovoltaics: First-principles investigation

In the past few years, the perovskite tandem solar cells show commendable developments in achieving the power conversion efficiency of 30% which is relatively higher than that produced by the single -junction solar cells. But, to the best of our knowledge, there is no literature available regarding the antiperovskite based tandem solar cells. This is the first study reporting the applicability of some predicted antiperovskite compounds Mg3PN, Ca3PN, Sr3PN, Ba3PN, BaSr2PN, Ba1.5Sr1.5 PN, and Ba2SrPN with suitable properties for using as the best absorbing layers in higher efficient tandem solar cell. These seven antiperovskite compounds attain direct band gap nature with band gap values ranging from 1.1 eV to 2.3 eV. The absorption coefficient, optical conductivity and extinction coefficients are found relatively higher in the visible region. Besides, low reflectivity is also analysed in the visible region for the compounds. Low charge carrier effective masses and low recombination rate between the charge carriers can also be observed in the seven materials and thus boost up the carrier mobility and electrical conductivity. Besides, the excitonic binding energy values are also found to be very small for all the predicted compounds. Since all the compounds stabilize in the same crystal structure, the lattice mismatch between each cell will become so less which in turn will be beneficial for making the tandem solar cell. So, it is more suitable to use these materials as the absorbing layers in tandem solar cells apart from use them in single junction photovoltaics since the power conversion efficiency will be more for multijunction/tandem solar cells.

Interrelationship of bonding strength with structural stability of ternary oxide phases of MgSnO3: A first-principles study

We have studied the Ilmenite, Perovskite and LiNbO3 type crystal structures of ternary oxide MgSnO3 by first-principles methods using density functional theory (DFT) and beyond. We conclude that MgSnO3 in LiNbO3 type is both mechanically and dynamically stable, whereas Ilmenite and Perovskite crystal structures are mechanically stable but dynamically unstable. Vibrational stability in MgSnO3 warrants some distortion in the octahedra caused due to the strength of bonding between the atom pairs Sn–O in LiNbO3 type crystal structure. Similarly, Ilmenite and Perovskite crystal structures require a higher number of Mg–Sn and (Mg–Mg, Sn–Sn, O–O and Mg–O) bonds respectively. Ilmenite and LiNbO3 type crystal structures can be used as window layers featuring a large bandgap of 5.22 eV and 3.88 eV respectively, along with a lower absorption coefficient and reflectivity. Likewise, Perovskite crystal structure with a bandgap of 2.55 eV can be deployed as an absorber layer that traps green light of the solar irradiation in tandem solar cells. Perovskite crystal structure has the lowest charge carrier effective masses among all the structures in MgSnO3. LiNbO3 type crystal structure shows a high hardness of 56.5 GPa. It should be tested experimentally in applications requiring super-hard materials.

Multi-ion scattering of charged carriers by ionized impurities in heavily doped semiconductors: From bulk to nanowires

Analytical expressions for the low-field mobility in heavily doped 3D, 2D, and 1D semiconductor structures are obtained using the quantum-kinetic approach. The study takes into account the multi-ion (M-ion) scattering of charge carriers by ionized impurities. The calculated dependences of the carrier mobility on doping concentration are compared with experiment in the heavily doped bulk materials (3D) Si, InP, GaAs, n-In0.49Ga0.51P, in heavily doped In0.15Ga0.85As quantum wells and InN nanowires, respectively. When calculating mobility in n-Si, the anisotropic effective masses of electrons in the valleys are taken into account. We explain the difference in the electron mobility of n-Si bulk crystals heavily doped by phosphorus and arsenic in the framework of the M-ion scattering model, which considers the scattering of electrons by interaction potentials with two characteristic lengths: the screening length and the effective radius of the doping ion. The number of ions M participating in the scattering process depends on the effective masses of charge carriers. For the light carriers with effective masses m<0.1m0 (m0 is the free electron mass), the two-ion (M = 2) scattering is more probable. For carriers with higher effective masses, three- and four-ion scattering is relevant.

Understanding the Photocatalytic Activity of La5Ti2AgS5O7 and La5Ti2CuS5O7 for Green Hydrogen Production: Computational Insights.

Green production of hydrogen is possible with photocatalytic water splitting, where hydrogen is produced while water is reduced by using energy derived from light. In this study, density functional theory (DFT) is employed to gain insights into the photocatalytic performance of La5Ti2AgS5O7 and La5Ti2CuS5O7—two emerging candidate materials for water splitting. The electronic structure of both bulk materials was calculated by using hybrid DFT, which indicated the band gaps and charge carrier effective masses are suitable for photocatalytic water splitting. Notably, the unique one-dimensional octahedral TiOxS6–x and tetragonal MS4 channels formed provide a structural separation for photoexcited charge carriers which should inhibit charge recombination. Band alignments of surfaces that appear on the Wulff constructions of 12 nonpolar symmetric surface slabs were calculated by using hybrid DFT for each of the materials. All surfaces of La5Ti2AgS5O7 have band edge positions suitable for hydrogen evolution; however, the small overpotentials on the largest facets likely decrease the photocatalytic activity. In La5Ti2CuS5O7, 72% of the surface area can support oxygen evolution thermodynamically and kinetically. Based on their similar electronic structures, La5Ti2AgS5O7 and La5Ti2CuS5O7 could be effectively employed in Z-scheme photocatalytic water splitting.

Band structure modulation by methoxy-functionalization of graphene nanoribbons

Graphene nanoribbons functionalized with methoxy groups were synthesized. Theoretical studies predicted reduced bandgap and effective mass of charge carriers. THz spectroscopy revealed ∼25% enhancement of the photoconductivity.

DFT based studies on the structural, electronic and optical properties of LiNbO3 using some hybrid techniques

Structural, electronic, and optical properties of ferroelectric material, Lithium niobate (LiNbO3) are explored and studied using two different techniques namely, OLCAO-MGGA-TB09+c and OLCAO-GGA-PBES + U under the framework of density functional theory (DFT). The electronic properties such as band diagram, the effective mass of charge carriers, the total density of state (TDOS), and partial density of state (PDOS) are investigated in depth. Band gap values obtained using MGGA-TB09+c is 3.79 eV whereas, it is 3.78 eV for GGA-PBES + U. Furthermore, several optical properties such as dielectric constant, reflectivity, refractive index, optical conductivity, loss function, and absorption coefficient have also been extracted. It is seen that the results so obtained are consistent with experimental data which shows the soundness of our calculations. From the statistical analysis, it is found that both the techniques provide very good results on electronic and optical properties of LiNbO3 compared to existing computational work as both give an approximation of experimental data. The methods presented here will be useful for researchers to get accurate results on structural, electronic, and optical properties of other materials.

Electronic structure and stability of Cs2TiX6 and Cs2ZrX6 (X = Br, I) vacancy ordered double perovskites

Vacancy ordered halide perovskites have been extensively investigated as promising lead-free alternatives to halide perovskites for various opto-electronic applications. Among these, Cs2TiBr6 has been reported as a stable absorber with interesting electronic and optical properties, such as a bandgap in the visible, and long carrier diffusion lengths. Yet, a thorough theoretical analysis of the exhibited properties is still missing in order to further assess its application potential from a material's design point of view. In this Letter, we perform a detailed analysis for the established Ti-based compounds and investigate the less-known materials based on Zr. We discuss in detail their electronic properties and band symmetries, highlight the similarity between the materials in terms of properties, and reveal limits for tuning electronic and optical properties within this family of vacancy ordered double perovskites that share the same electron configuration. We also show the challenges to compute accurate and meaningful quasi-particle corrections at the GW level. Furthermore, we address their chemical stability against different decomposition reaction pathways, identifying stable regions for the formation of all materials, while probing their mechanical stability employing phonon calculations. We predict that Cs2ZrI6, a material practically unexplored to date, shall exhibit a quasi-direct electronic bandgap well within the visible range, the smallest charge carrier effective masses within the Cs2BX6 (B = Ti, Zr; X = Br, I) compounds, and a good chemical stability.

Improving the Thermoelectric Properties of the Material and Increasing the Efficiency of Conversion Processes

The area of practical application of thermoelectric materials depends on the value of the thermoelectric figure of merit. The use of semiconductor materials makes it possible to realize the conditions under which the ratio of their parameters ensures the achievement of high values of thermoelectric figure of merit. The achievement of the maximum thermoelectric figure of merit causes an increase in the efficiency of conversion processes due to the improvement of the thermoelectric properties of the material. The position of the maximum value of the thermoelectric figure of merit is predetermined by the scattering parameters and the ratio of the mobilities and effective masses of charge carriers. The nature of the change in electrical conductivity is determined by the behavior of the concentration of charge carriers. Thermal conductivity, like electrical conductivity, is proportional to the concentration of electrons and the mean free path. An increase in thermoelectric efficiency is achieved by optimizing thermoelectric parameters by doping and improving the properties of com¬pounds, which leads to an optimization of the concentration of charge carriers, a change in the density of states, and a decrease in the phonon component of thermal conductivity. The improvement of the thermoelectric properties of the material and the increase in the efficiency of the conversion processes are provided at a certain concentration of charge carriers, which corresponds to the optimal value.

A diagnosis approach for semiconductor properties evaluation from ab initio calculations: Ag-based materials investigation

Ag-based semiconductors have been extensively employed as photocatalysts for several decades due to their excellent electronic properties, photochemical properties, and relatively low synthesis cost. In this work, the α-Ag2WO4, β-Ag2WO4, γ-Ag2WO4, α-AgVO3, β-AgVO3, α-Ag2MoO4, β-Ag2MoO4, Ag2CrO4, and Ag3PO4 materials were systematically investigated by DFT/B3LYP calculations. The structural features were described by cations coordination, cluster distortion degree, and structure formers and modifiers. In turn, the evaluation of electronic properties indicates band gap values in the range of 2.54 ​eV–4.32 ​eV. The calculated density of charge carriers shows that the holes are the predominant charge carrier for all evaluated semiconductors, except for α-Ag2MoO4. The effective masses of charge carriers were also computed, and good stability for generated electrons and holes are expected for α- and β-Ag2WO4, α-AgVO3, β-AgVO3, β-Ag2MoO4, Ag2CrO4, and Ag3PO4 materials. Finally, the valence and conduction band edge potentials point to the Ag-based semiconductors as promising candidates for photocatalytic and biocide activities.

Structure-Property Relationships in Transition Metal Dichalcogenide Bilayers under Biaxial Strains.

This paper reports a Density Functional Theory (DFT) investigation of the electron density and optoelectronic properties of two-dimensional (2D) MX2 (M = Mo, W and X = S, Se, Te) subjected to biaxial strains. Upon strains ranging from −4% (compressive strain) to +4% (tensile strain), MX2 bilayers keep the same bandgap type but undergo a non-symmetrical evolution of bandgap energies and corresponding effective masses of charge carriers (m*). Despite a consistency regarding the electronic properties of Mo- and WX2 for a given X, the strain-induced bandgap shrinkage and m* lowering are strong enough to alter the strain-free sequence MTe2, MSe2, MS2, thus tailoring the photovoltaic properties, which are found to be direction dependent. Based on the quantum theory of atoms in molecules, the bond degree (BD) at the bond critical points was determined. Under strain, the X-X BD decreases linearly as X atomic number increases. However, the kinetic energy per electron G/ρ at the bond critical point is independent of strains with the lowest values for X = Te, which can be related to the highest polarizability evidenced from the dielectric properties. A cubic relationship between the absolute BD summation of M-X and X-X bonds and the static relative permittivity was observed. The dominant position of X-X bond participating in this cubic relationship in the absence of strain was substantially reinforced in the presence of strain, yielding the leading role of the X-X bond instead of the M-X one in the photovoltaic response of 2D MX2 material.

Planar heterojunction boosts solar-driven photocatalytic performance and stability of halide perovskite solar photocatalyst cell

The excellent optoelectronic properties of metal halide perovskites (MHPs) have been employed in various photocatalytic applications, but their poor water stability is considered as the main bottleneck for further development. Herein, we protect the light-absorbing CsPbBr3 MHP with a NiOx and TiO2 hole and electron extracting layer. This planar NiOx/CsPbBr3/TiO2 architecture can easily be fabricated through solution-processing. When applied to selective photocatalytic oxidation of benzyl alcohol, this system presents a 7-fold enhancement of photoactivity and an improved stability for over 90 h compared to CsPbBr3 counterpart. Interestingly, we find that trace amounts of water improve photoactivity. Through experimental and theoretical analyses, this improvement could be attributed to water-induced structural reorganization of MHP, leading to improved crystal quality and decreased effective masses of charge carriers. This work indicates planar heterojunction helps improve the photoactivity and stability of MHP photocatalyst, and our findings provide insights into the effect of water on MHPs.

Ab initio prediction of thermoelectric performance of monolayer transition-metal nitride halides MNBr (M = Zr, Hf)

Accurate descriptions of the structure, electronic properties and thermoelectric (TE) properties of transition-metal nitride halides MNBr (M = Zr, Hf) monolayers are calculated using the full-potential linearized augmented plane-wave (FP-LAPW) method within both generalized gradient approximation (GGA) and Tran–Blaha modified Becke–Johnson (TB-mBJ) approximation. The band structure shows that all MNBr monolayers are semiconductors with indirect band gaps. The band gap, calculated by TB-mBJ, is larger than that of GGA for both considered compounds. The charge-carrier effective mass increases when going from ZrNBr to HfNBr because of the extra peak of the density of states profile of monolayer HfNBr in the valence band around −0.5 eV, suggesting that ZrNBr should have higher electrical conductivity and HfNBr should have higher Seebeck coefficient. Meanwhile, the monolayered structure of MNBr has intrinsic low lattice thermal conductivity. The results demonstrate that monolayer MNBr has interesting TE properties with n-type doping.

Torsional strain engineering of transition metal dichalcogenide nanotubes: an ab initio study

We study the effect of torsional deformations on the electronic properties of single-walled transition metal dichalcogenide (TMD) nanotubes. In particular, considering forty-five select armchair and zigzag TMD nanotubes, we perform symmetry-adapted Kohn–Sham density functional theory calculations to determine the variation in bandgap and effective mass of charge carriers with twist. We find that metallic nanotubes remain so even after deformation, whereas semiconducting nanotubes experience a decrease in bandgap with twist—originally direct bandgaps become indirect—resulting in semiconductor to metal transitions. In addition, the effective mass of holes and electrons continuously decrease and increase with twist, respectively, resulting in n-type to p-type semiconductor transitions. We find that this behavior is likely due to rehybridization of orbitals in the metal and chalcogen atoms, rather than charge transfer between them. Overall, torsional deformations represent a powerful avenue to engineer the electronic properties of semiconducting TMD nanotubes, with applications to devices like sensors and semiconductor switches.

Electronic Structure and Electron-Transport Properties of Three Metal Hexacyanoferrates

Metal hexacyanometallates, or Prussian blue analogs (PBAs), are active materials in important electrochemical technologies, including next-generation sodium- and potassium-ion batteries. They have tunable properties, including reduction potential, ionic conductivity, and color. However, little is known about their electronic conductivities. In this work, we use density-functional theory to model the electronic structure and to explore the likely electron-conduction mechanism in three promising cathodes (manganese, iron, and cobalt hexacyanoferrate) in each of three oxidation states. First, we demonstrate that hybrid functionals reliably reproduce experimentally observed spin configurations and geometric phase changes. We confirm these materials are semiconductors or insulators with band gaps ranging from 1.90 eV up to 4.94 eV. We further identify that for most of the compounds, the electronic band edges originate from carbon-coordinated iron orbitals, suggesting that doping at the carbon-coordinated site may strongly affect carrier conductivity. Finally, we calculate charge-carrier effective masses, which we find are very heavy. This study is an important foundation for making electronic conductivity a tunable PBA material property.

Effect of Tensile Strain on Performance Parameters of Different Structures of MoS2 Monolayer

The present work is based on the computational study of MoS2 monolayer and effect of tensile strain on its atomic level structure. The bandgap for MoS2 monolayer, defected MoS2 monolayer and Silicon-doped monolayer are 1.82 eV (direct bandgap), 0.04 (indirect bandgap) and 1.25 eV (indirect bandgap), respectively. The impact of tensile strain (0-0.7 %) on the bandgap and effective mass of charge carriers of these MoS2 structures has been investigated. The bandgap decrease of 5.76 %, 31.86 % and 6.03 % has been observed in the three structures for biaxial strain while the impact of uniaxial strain is quite low. The impact of higher temperature on the bandgap under biaxial tensile strain has been also analyzed in this paper. These observations are extremely important for 2D material-based research for electronic applications.

Ab-initio investigation on the electronic and thermoelectric properties of new half-Heusler compounds KBiX (X = Ba and Sr)

Electronic structures and thermoelectric (TE) properties of KBiBa and KBiSr half-Heusler compounds are investigated by using the combined framework of first-principles and semi-classical Boltzmann transport theory. Elastic and phonon properties calculations reveal that these compounds are mechanically and dynamically stable. Band structures calculations, using the Tran and Blaha modified Becke–Johnson potential including spin–orbit coupling, show that KBiBa and KBiSr compounds are semiconductors with an indirect bandgap of 0.88 and 0.95 eV respectively. Performing calculations on lattice thermal conductivity, deformation potential, effective mass of charge carriers and their relaxation times allow us to deduce the dependence of the figure of merit ZT on doping concentration and temperature. At 1200 K, KBiBa and KBiSr compounds exhibit a ZT peak value of 2.68 and 1.56 at optimized n-doping values of 2.2 × 1019 and 1.7 × 1019 cm−3, respectively. Considering high ZT values, KBiBa and KBiSr are considered to be ideal TE candidates at high temperature.