Dixit, Ambesh

Two-dimensional layered transition metal dichalcogenide exhibit important characteristics such as suitable bandgap, high absorption coefficient, and favourable electron transport properties for their uses in nano-electronic such as ultrathin solar cells. We adopted a hybrid simulation approach, where density functional calculations are performed for optoelectronic properties of semiconductor materials and macroscopic device simulation is carried out to evaluate photovoltaic response. We investigated electronic and optical properties of bulk WS2, WSe2 and noticed very high absorption coefficient, making them suitable absorber materials for solar cell. Further, the electronic and optical properties of an ultrathin WSSe Janus layer are investigated using density functional theory and noticed low reflectance and high bandgap, supporting its usefulness as a buffer layer for W(S/Se)2 absorbers. The computed density functional results for W(S/Se)2 and Janus WSSe are used to simulate the photovoltaic response of WSSe/W(S/Se)2 solar cell using macroscopic device simulation. The photovoltaic performance of a single junction solar cell is optimized for W(S/Se)2 absorbers and WSSe Janus buffer materials. The effect of absorber layer thickness, carrier concentration, and contact work function is evaluated to understand the solar cell performance. We noticed that interface recombination speed between absorber and buffer layer and minority carrier lifetime are affecting the solar cell performance. The maximum efficiency of about ~17.73% and 18.87% is noticed for optimized WSSe/WS2 and WSSe/WSe2 solar cell. The present study will provide a new approach to design, develop, and optimize a solar cell and evaluate the impact of different materials parameters on solar cell performance.

The cubic double perovskites A2ZnWO6 (A = Ba, Sr and Ca) are studied to understand the effect of A cation site, using the density functional theory (DFT) based full potential augmented plane wave method (FP-LAPW) with GGA and mBJ exchange correlation potentials. The structural robustness and stability are investigated using the bond lengths and the total energy. The band structure and density of states suggest that all these cubic double perovskites are indirect wide band gap semiconductors. The band gap varies from 3.90 eV (2.97 eV) for Ba2ZnWO6 system to 3.40 eV (2.8 eV) for Ca2ZnWO6 system using mBJ (GGA) exchange correlation potentials. Our studies suggest that A cation site modification has a strong effect on physical and electronic properties, in contrast to the structural robustness. The lattice parameter decreases from 8.19 Å to 7.9 Å from Ba to Ca at alkali cation site and the electronic band gap variation follows the common cation rule. The charge densities show enhanced localization of charges near the zinc and oxygen sites with increasing alkali cation atomic radii. In addition, we discuss the impact of A cation site modification on the dielectric and optical properties for A2ZnWO6 double perovskites.

We investigated the structural, thermodynamic, and optoelectronic properties of InxAl1−xN, InxGa1−xN, and GaxAl1−xN alloys for x = 0.25, 0.50 and 0.75. The optimized lattice constants showed nearly a small deviation trend from Vegard’s law with composition x. The impact of mutual alloying is evaluated in terms of enthalpy and interaction parameters. The calculated electronic band structures and density of states lie in the bandgap ranges from 1.09 eV to 2.72 eV for composition x 0.25 to 0.75. These electronic properties suggested that alloys are suitable bandgap semiconductors with large variations in their bandgap energies for optoelectronic applications. The optical properties are calculated using the dielectric constant and correlated with the calculated electronic band structures. The main reflectivity peak and absorption coefficient showed a significant shift with increasing x. These monolayers’ suitable bandgap and optoelectronic properties make them attractive for optoelectronic applications, including photovoltaics and photodetectors.

We report here on the complex magnetic structure and magnetocapacitance in NiF 2 , a non-oxide multifunctional system. It undergoes an anti-ferromagnetic transition near 68.5 K, superimposed with canted Ni spin driven weak ferromagnetic ordering, followed by a metastable ferromagnetic phase at or below 10 K. Our density functional calculations account for the complex magnetic structure of NiF 2 deduced from the temperature and the field dependent measurements. Near room temperature, NiF 2 exhibits a relatively large dielectric response reaching >10 3 with a low dielectric loss of <0.5 at frequencies >20 Hz. This is attributed to the intrinsic grain contribution in contrast to the grain boundary contribution in most of the known dielectric materials. The response time is 10 μs or more at 280 K. The activation energy for such temperature dependent relaxation is ~500 meV and is the main source for grain contribution. Further, a large negative magneto capacitance >90% is noticed in 1 T magnetic field. We propose that our findings provide a new non-oxide multifunctional NiF 2 , useful for dielectric applications.

Monolayers of transition metal dichalcogenides (TMDs) have been proposed as the next generation electronic materials for nanoscale devices. We evaluated the thermodynamic stability of WS2, Janus WSSe and WSe2 monolayers under biaxial tensile and compressive strain using density functional approach. The phonon band structures of unstrained WS2, WSSe and WSe2 monolayers confirm their thermodynamic stability. The stability of these monolayers is investigated under strain and phonon softening is observed for acoustic and optical mode upto 8% tensile strain. The bond length reduction under compressive strain resulted in out of plane deformation, leading the instability of monolayers under compressive strain. The bond length W-S(Se) and bond angle W-S(Se)-W exhibit significant contribution in coupling strength between transition metal d-orbitals and chalcogen p-orbitals that mediated the electronic properties. The spin orbit coupling showed strong effect on splitting of the valence band for unstrained monolayers. The spin splitting in valence band increases with increasing the tensile strain and decreases with increasing the compressive strain. The effective masses and mobility values are 0.57me, 0.54me, 0.47me; and 0.059, 0.062, 0.072 m2V−1s−1 for unstrained WS2, WSSe and WSe2 monolayers, respectively. The tungsten dx2-y2 orbitals are mainly contributing to the conduction and valence band electronic states in compressive strained monolayers. In contrast under tensile strain tungsten dz2 orbitals contribute to conduction and valence band electronic states, causing the direct to indirect band gap transition in these monolayers. We observed that the impact of tensile strain is more sensitive as compared to that of compressive strain.

Cs2AuBiCl6 is considered to be a potential lead-free double perovskite alternative for perovskite solar cells. Its electronic and optical properties are investigated using density functional theory. The electronic properties of Cs2AuBiCl6 material ensure a bandgap of 1.40 eV (without considering SOC) and 1.12 eV (with SOC) using mBJ exchange-correlation functional, close to the optimal bandgap for solar cell application as per the Shockley–Queisser limit. Optical properties suggest a high absorption coefficient ≈105 cm−1 with low reflectance, making it the optimal absorber material. Furthermore, the photovoltaic performance of Cs2AuBiCl6 based single-junction transparent conducting oxide (TCO)/IDL1/Cs2AuBiCl6/IDL2/Cu2O solar cell is investigated using SCAPS-1D device simulation program. The impact of electron affinity, thickness, carrier concentration, defect density, and interface defect density is examined using interface defect layer (IDL) on the photovoltaic performance. The maximum photoconversion efficiency (PCE) of ≈22.18% is noticed for optimized material's parameters. These studies on TCO/IDL1/Cs2AuBiCl6/IDL2/Cu2O solar cell will provide guidelines for designing and developing an efficient lead-free perovskite-based solar cell as an alternative to conventional halide perovskite materials based solar cell.

Semiconducting Zn chalcogenide monolayers are important members of the 2D family of materials due to their unique electronic properties. In this paper, we focus on strain-modulated electronic properties of monolayers of ZnX, with X being O, S, Se, and Te. ZnO and ZnS monolayers have a hexagonal graphene-like planar structure, while ZnSe and ZnTe monolayers exhibit slightly buckled silicene and germanene-like structures, respectively. Density functional theory calculations find the hexagonal ZnO monolayer to be dynamically stable. However, ZnS, ZnSe, and ZnTe monolayers are predicted to be less stable with small imaginary frequencies. The application of tensile strain to these monolayers, interestingly, yields stability of dynamically less stable structures together with the modification in the nature of the bandgap from direct to indirect. For a tensile strain of about 8%, a closure of the bandgap in ZnTe is predicted with the semiconductor-metal transition. The results, therefore, find strain-induced stability and modification in electronic properties of monolayers of Zn chalcogenides, suggesting the use of these monolayers for novel device applications.

Structural and electronic properties of pristine and transition metal doped ZnS monolayer are investigated within the framework of density functional theory. The pristine ZnS monolayer is showing direct band gap of about 2.8 eV. The investigated transition metal doping showed the transition from non-magnetic semiconductor to a magnetic system e.g. magnetic semiconductor for Co doped ZnS and half metal for Ni doped ZnS monolayers. The Co doped ZnS monolayer showed higher formation energy, confirming the strong bonding than that of Ni doped ZnS monolayer. The electron difference density shows the charge sharing between transition metal (Ni and Co) and S, confirming the covalent bond formation.

Gas sensing mechanism of H2S, NH3, NO2 and NO toxic gases on transition metal dichalcogenides based Janus MoSSe monolayers is investigated using the density functional theory. Three types of defects (i) molybdenum vacancy, (ii) selenium vacancy, and (iii) sulfur/selenium vacancy are considered and their formation energy is computed to predict the stability. We noticed that selenium vacancy is the most stable among other defects. The maximum adsorption energy for H2S, NH3, NO2 and NO molecules on pristine Janus MoSSe monolayer are ~ −0.156 eV, −0.203 eV, −0.252 eV, and −0.117 eV, respectively. NO2 gas molecule dissociates and forms oxygen doped NO adsorption in selenium and sulfur/selenium defect included MoSSe Janus monolayer. The adsorption energy values are ~ −3.360 eV and −3.404 eV for Se and S/Se defects included MoSSe layer, respectively. Further, the adsorption of NO2 molecule induced about 1μB magnetic moment. In contrast, NO molecule showed chemisorption, whereas H2S and NH3 molecules showed physisorption with their adsorption energies in the range of −0.146 to −0.238 eV and − 0.140 to −0.281 eV, respectively. The adsorption of H2S, NH3, NO2 and NO molecule on the pristine and defected monolayers suggest that selenium and sulfur/selenium vacancy defects are more prominent for NO2 and NO gas molecule adsorption.

We computed structural, electronic, and magnetic properties of pristine and both intrinsic/extrinsic point defects included InN monolayer. The stability of the pristine InN monolayer is confirmed through the phonon band dispersion. InN pristine monolayer exhibits an indirect bandgap, 0.61 eV between M and Γ high symmetry points. InV, NV, (In/N)V vacancies, and antisite defects such as In↔N are considered in InN monolayer, together with light elements, Li, Be, B, C O, and F doping at nitrogen (N) atomic site. We calculated the formation energies for understanding the stability of point defect included and doped InN monolayers. The NV included InN monolayer showed the most stable behaviour among structures. Vacancy defects InV, NV, and (In/N)V included monolayers showed 3.00 μB, 0.00 μB, and 2.00 μB magnetic moment, respectively. However, antisite defects included monolayers showed zero magnetic moment. Further, Be and C doped monolayers showed 0.91 μB and 1.09 μB magnetic moment, respectively, whereas the other dopants showed zero magnetic moment. Thus, the onset of magnetism can be realized in InN monolayer by inducing defects with a maximum of 3.00 μB for InV defects.