Dixit, Ambesh

We investigated a single heterojunction solar cell with stannite phase Cu2HgSnS4 as a new potential absorber material using the hybrid density functional theory for materials parameter and macroscopic device simulation studies for the photovoltaic response. The lattice constants are optimized using PBEsol and GGA exchange–correlation approach with mBJ parameterization and considering spin–orbit coupling effect on heavy element Hg, while strong correlation of Cu and Hg 3d electrons are taken into account by Hubbard parameter U = 0.52 Ry. The computed bandgap is ~1.33 eV. The effective mass of electrons and holes in the respective band edges (electron in conduction band and the hole in the valence band) is 0.25 m0 and 0.91 m0, respectively. Further, the computed materials optoelectronic parameters are used to optimize the device performance by introducing a variation in minority carrier lifetime, defect concentration in Cu2HgSnS4 absorber, Cu2HgSnS4/CdS interface, and the absorber thickness to achieve a realistic photovoltaic response. The optimal conversion efficiency is 11.6% after taking more realistic parameters in the considered single-junction solar cell. However, the maximum photovoltaic response >17% can be achieved by controlling absorber and interface defects together with optimal carrier concentration.

The irradiation effect of high energy silver ion (Ag8+) has been investigated on ZnO thin films to understand their impact on structural, microstructural, optical, electronic and magnetic properties. The RF sputtered pristine ZnO/Si thin films are polycrystalline and highly textured along (0 0 2) plane. The irradiated ZnO/Si samples showed enhanced structural and microstructural defects with irradiation time. The lattice parameters have reduced for irradiated ZnO thin films, suggesting structural deformation. The irradiation has resulted into enhanced defect density and strain in these thin films. A weak room temperature ferromagnetic moment (3.24 × 10−4 emu cm−2) has been observed, which has reduced drastically for Ag8+ irradiated samples up to 1.05 × 10−4 emu cm−2. The observed reduction in saturation magnetization has been attributed to the enhanced microstructural defects, lowering the free charge carriers and increasing the recombination centers simultaneously.

Mössbauer analysis is carried out on CZTS samples, subjected to a low temperature processing at 300 °C (S1) and high temperature processing at 550 °C under sulfur environment (S2). Loss of Sn is observed in sample S2 due to high temperature thermal treatment. The isomer shifts obtained in the Mössbauer spectra confirms the existence of Sn at its 4 + valence state in both the samples. Relatively high quadriple splitting value is observed in S1 with respect to S2, suggesting dislocations and crystal distortion present in S1, which are reduced drastically by high temperature annealed S2 sample. The fabricated solar cell with S1 and S2 absorbers showed significant improvement in efficiency from ∼0.145% to ∼1%. The presence of excess Sn in S1 allows enhanced recombination and the diode ideality factor shows larger value of 4.23 compared to 2.17 in case of S2. The experiments also validate the fact that S1 with Sn rich configuration shows lower acceptor carrier concentration as compared to S2 because of enhanced compensating defects in S1.

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.

We studied the influence of various point defects such as vacancy, antisite and impurities at different ionic sites in CdO monolayer through the electronic and magnetic properties using density functional theory. Vacancy defect such as Cd, O, Cd/O, antisite defects Cd ↔ O and extrinsic B, C, and N impurity elements are considered in CdO monolayer. The thermodynamic stability of pristine CdO monolayer is studied using phonon band dispersion and that of CdO monolayer with defect is studied in terms of the formation energy. The oxygen vacancy defect OV introduces the indirect band gap of 1.4 eV while Cd/OV showed the metallic behavior. Further, CdV and Cd/OV vacancy defects showed the onset of ferromagnetism with 2.0 μΒ and 1.21 μΒ magnetic moment, respectively. In contrast, the direct band gap nature of CdO monolayer is persistent against antisite defects and also no magnetic behavior is observed. The extrinsic B and C doped CdO monolayer showed half-metallic behavior, whereas N doped CdO monolayer showed semiconducting nature. These extrinsic dopants showed the onset of ferromagnetic ordering with 0.98μΒ, 1.99 μΒ and 0.99 μΒ magnetic moments for BO, CO, and NO defects in CdO monolayer.