S. V. Ghaisas

Photovoltaic, I-V and C-V Characteristics of SnO2/SiO2/a-Si:H/mc-Si:H Structures

Structures of the type SnO2/SiO2/a-Si:H/mc-Si:H/Al and SnO2/SiO2/a-Si:H/mc-Si:H/a-Si:H/mc-Si:H/ were fabricated and studied for their I-V, C-V and photovoltaic characteristics. Results indicate that the structures behave like two diodes formed in the back-to-bak geometry. Furthermore, from the I-V studies under illumination, it is observed that a sufficient drift field exists in the structure without intentional doping of Si-based layers to yield a gain in current under illumination of about 104.

Diffusion of Cd and Te adatoms on CdTe(111) surfaces: A computational study using density functional theory

CdTe is one of the most promising semiconductor for thin-film based solar cells. Here we report a computational study of Cd and Te adatom diffusion on the CdTe (111) A-type (Cd terminated) and B-type (Te terminated) surfaces and their migration paths. The atomic and electronic structure calculations are performed under the DFT formalism and climbing Nudge Elastic Band (cNEB) method has been applied to evaluate the potential barrier of the Te and Cd diffusion. In general the minimum energy site on the surface is labeled as Aa site. In case of Te and Cd on B-type surface, the sub-surface site (a site just below the top surface) is very close in energy to the A site. This is responsible for the subsurface accumulation of adatoms and therefore, expected to influence the defect formation during growth. The diffusion process of adatoms is considered from Aa (occupied) to Aa (empty) site at the nearest distance. We have explored three possible migration paths for the adatom diffusion. The adatom surface interact...

A computational ab initio study of surface diffusion of sulfur on the CdTe (111) surface

In order to discern the formation of epitaxial growth of CdS shell over CdTe nanocrystals, kinetics related to the initial stages of the growth of CdS on CdTe is investigated using ab-initio methods. We report diffusion of sulfur adatom on the CdTe (111) A-type (Cd-terminated) and B-type (Te-terminated) surfaces within the density functional theory (DFT). The barriers are computed by applying the climbing Nudge Elastic Band (c-NEB) method. From the results surface hopping emerges as the major mode of diffusion. In addition, there is a distinct contribution from kick-out type diffusion in which a CdTe surface atom is kicked out from its position and is replaced by the diffusing sulfur atom. Also, surface vacancy substitution contributes to the concomitant dynamics. There are sites on the B- type surface that are competitively close in terms of the binding energy to the lowest energy site of epitaxy on the surface. The kick-out process is more likely for B-type surface where a Te atom of the surface is disp...

Paramagnetic Silicon Nanoparticles Without Magnetic Ion Doping: An Ab-Initio Study Prediction

Finite magnetic moments in Si-H nanoparticles (NPs) without magnetic ion doping were observed due to the presence of dangling bonds. Ab-initio methods were employed to investigate the electronic and magnetic properties of such Si-H NPs. It is shown that Si10H15, Si10H14, Si10H13 and the corresponding large size NPs Si20H35, Si20H34 and Si20H33 exhibit 1, 2 and 3 μB magnetic moments, respectively with a relatively large exchange splitting. It is proposed that paramagnetic materials can be formed by assembling such NPs. Appropriate combinations of these NPs with other NPs or molecules can protect the dangling bond and preserve the magnetic moment.The stability of these paramagnetic Si-H NPs in the presence of water and oxygen is also discussed.

Structure and energetics of silicon clusters adsorbed on the Au(111) surface : a first principles study

We report an extensive first-principles investigation of the structure and electronic properties of small Si n (n = 1, 2, 3, 4, and 6) clusters deposited on the Au(111) surface. The calculations were performed using a plane wave based pseudopotential method under the framework of density functional theory. The electron-ion interaction energy has been described using ultrasoft pseudopotentials (USPP) and the spin polarised GGA scheme was used for the exchange correlation energy. The results reveal that Si atom prefers to adsorb on the hcp site of the Au(111) surface with strong binding energy. For Si3, the closed triangle geometry of the bare cluster forms an open triangle by rupturing one of the three Si-Si bonds. Remarkable structural changes in the gas phase geometries were observed from Si4 onwards. For example, while the adsorption of rhombus in parallel and perpendicular orientation to the surface plane remains almost unaltered, the tetrahedral conformation becomes almost flat by the interaction of Au surface. The nature of chemical bonding between Au surface and Si clusters was investigated from the charge distribution analysis and the electronic density of states (EDOS). Based on charge distribution analysis it is found that Si atoms transfer charges to the Au surface. Based on the results it is inferred that Si clusters deposition on the Au surface involves large energy release and would lead to coat the surface by spill over of Si atoms rather than island formation.

a-Si:H passivation layer growth by HWCVD for Si heterojunction solar cells: Critical dependence on substrate temperature

We investigate the passivation properties of the hot-wire CVD deposited thin intrinsic a-Si:H layers grown at different substrate temperatures, Tsub, ranging from 150°C - 250°C, on n type crystalline Si (c-Si) substrates. Highest effective minority carrier lifetime of 288 µs at 1015 cm-3 injection level was obtained on symmetrically passivated c-Si wafers with a-Si:H layers (∼ 9 nm) grown at Tsub = 200°C whereas for the greater or lower Tsub, the respective values of τeff drop. The cross section TEM results revealed mixed phase epitaxy for Tsub = 250°C that apparently indicates its detrimental effect on passivation. The defect densities and Urbach energy values within respective bulk a-Si:H films were also compared which lead us to conclude that there exists a balance between having an electronically superior a-Si:H film and reduced defect densities at the a-Si:H/c-Si interface for effective passivation. Finally, prototype hetero-junction solar cells having structure: ITO (95 nm)/ (p) a-Si:H (10 nm)/ (i) a-Si:H (9 nm)/c-Si (280µm) / (i) a-Si:H (9 nm)/ (n+) Si:H (30 nm)/Al were fabricated. A comparably higher Voc of 0.57 V was found for solar cells grown around Tsub of 175°C and the reasons for the same are discussed.

Low temperature surface passivation of silicon solar cells

Low temperature surface passivation is a process that has a potential to reduce the input energy cost of the solar cell with minimum modification of the manufacturing bed, while keeping the efficiency, and life of the cells within acceptable range of values. In this review, low temperature deposition methods of SiO2, Al2O3, a-Si:H, silicon nano particles (NPs), and organic materials, are considered. Surface recombination velocities, defect densities, stability of these passivating layers are discussed along with the mechanisms of passivation on Si surface.

Relative Localization Prediction in Covalent Clusters : An Ab Initio Theory Driven Quest

A method to estimate the relative binding energies of atom within a covalently bonded cluster is proposed. The method uses projected density of states (PDOS) for the individual atoms in the cluster. A parameter similar to variance is defined over certain region of PDOS. The value is seen to be inversely related to the local binding energy of the atom. The method is applied to Si19O8 and Si19O10 clusters to determine the relatively weakly bound atoms in these clusters. Fragmentation of the clusters using simulated annealing reveals that those atoms are fragmented which are identified to be weakly bound from the proposed method.

Ab-Initio Calculation for the Study of Nano Scale Silicon Based Device Structure

Motivated by the possibility of scaling down of various electronic devices at the nanolevel, we have chosen a simple p-n junction like device in silicon structure. An aggregation of seventy-eight silicon atoms, passivated by oxygen is considered. We compute the electronic structure of such a cluster and then the density of states and the optical spectra for this aggregate are compared with a modified one. The modification is introduced by substituting phosphorus and boron atom within this cluster of silicon atoms in both sides in order to make a p-n junction like situation. A variant of this p-n junction like structure is introduced by adding a layer of oxygen between the phosphorus and boron regions. Comparison of the electronic structures of all these systems reveals several interesting properties.