Adnan Ali

The role of intrinsic vacancy defects in the electronic and magnetic properties of Sr3SnO: a first-principles study

In this study, we use first-principles electronic structure calculations to investigate the structural, electronic and magnetic properties of pristine and intrinsic vacancy containing Sr3SnO inverse perovskite. The thermodynamic stability diagram of Sr3SnO is computed which provides useful information regarding stable synthesis of this material. The limits of atomic chemical potentials of Sr, Sn and O are obtained to determine the relative stability of the formation of metal-atom and oxygen vacancies in Sr3SnO. The DFT calculations reveal Sr vacancy to be the most stable form of vacancy defect under O-rich conditions, while O and Sn vacancies are found to have low formation energies under O-poor and Sn-poor conditions, respectively. It is concluded that Sr and O vacancy containing Sr3SnO is non-magnetic, while Sn vacancy containing Sr3SnO gives rise to stable ferromagnetism. The electronic properties of pristine and Sn, O, and Sr deficient Sr3SnO are discussed. The Sn vacancy containing Sr3SnO, displays stable ferromagnetism which originates from spin-polarization of partially filled Sr dangling bonds with a predominant Sr-4d character. Our results explain the origins of experimentally observed room temperature ferromagnetism in non-stoichiometric Sr3SnO.

Phase transformation from cubic ZnS to hexagonal ZnO by thermal annealing

We have investigated the mechanism of phase transformation from ZnS to hexagonal ZnO by high-temperature thermal annealing. The ZnS thin films were grown on Si (001) substrate by thermal evaporation system using ZnS powder as source material. The grown films were annealed at different temperatures and characterized by X-ray diffraction (XRD), photoluminescence (PL), four-point probe, scanning electron microscope (SEM) and energy dispersive X-ray diffraction (EDX). The results demonstrated that as-deposited ZnS film has mixed phases but high-temperature annealing leads to transition from ZnS to ZnO. The observed result can be explained as a two-step process: (1) high-energy O atoms replaced S atoms in lattice during annealing process, and (2) S atoms diffused into substrate and/or diffused out of the sample. The dissociation energy of ZnS calculated from the Arrhenius plot of 1000/T versus log (resistivity) was found to be 3.1 eV. PL spectra of as-grown sample exhibits a characteristic green emission at 2.4 eV of ZnS but annealed samples consist of band-to-band and defect emission of ZnO at 3.29 eV and 2.5 eV respectively. SEM and EDX measurements were additionally performed to strengthen the argument.

On structural and electrical characterization of n-ZnO/p-Si single heterojunction solar cell

We report structural and electrical characterizations of zinc oxide films grown on sapphire and silicon substrates. The zinc oxide works as an active n-layer of a heterojunction solar cell as well as antireflection coating. The deposition parameters of zinc oxide have been optimized using RF sputtering. The x-ray diffraction and scanning electron microscopy confirm that optimized deposition temperature and RF power are >300 °C and 180 W respectively. The films grown on textured silicon substrates have lower crystallinity as compared to those grown on sapphire. The pn junction formation between zinc oxide and silicon has been confirmed by current-voltage and suns-VOC measurements. The initial solar cell device has been fabricated using silver as front and aluminum as back contact.

Computer modeling of n-ZnO/p-Si single heterojunction bifacial solar Cell

Bifacial solar cell offers the advantage of harvesting the sunlight more effectively and produce extra power from albedo in addition to slightly lower module temperature. Silicon bifacial cells have been studied and commercially produced with doped back surface field (BSF) in conjunction with dielectric passivation. In this study detailed computer modeling of ZnO based bifacial solar cell is performed to critically investigate the potential doped BSF (DBSF)/dielectric passivation and charged passivated BSF (CPBSF). In both structures n-ZnO is employed as the emitter because of its intrinsic n-type conductivity and high transparency. Simulation results show that the bifacial n-ZnO/p-Si solar cells are insensitive to the emitter layer thickness up to 1μm compared to conventional Si solar cell with doped emitter. Furthermore, emitter doping concentration of ZnO in the ZnO/p-Si cell shows stable efficiency up to 1019 cm-3 and drops rapidly for higher doping concentration. The efficiency of the two structures - DBSF and CPBSF with ZnO emitter are similar for base thickness of 140 μm having low carrier lifetime. However, the CPBSF structure exhibits higher efficiency than the DBSF when the carrier lifetime is high. Also, even higher efficiency is obtained with a charge density of 1012 cm2 leading to ~15 mV higher Voc than DBSF counterpart.

PC1D analysis of thin-film crystalline Si x Ge 1−x /Si solar cells

Inability to achieve complete optical absorption in thin-film crystalline Si (TF c-Si) solar cells fundamentally limits efficiency. Therefore, materials with high absorption across the entire solar spectrum are highly desirable. In this study, several TF solar cell configurations based on Silicon Germanium (Si_1-xGe_x) alloys have been investigated. With the help of PC1D software, TF c-Si solar cells with varying composition of Si_1-xGe_x layers with x in the range of 4 % to 50 % have been modeled. In all cases, Si_1-xGe_x solar cells performed better than c-Si TF solar cells; the optimized Ge concentration was determined to be 20%. For example, in case of 10-μm thick TF solar cell, crystalline Si/Si_1-xGe_x exhibits simulated efficiency of ~ 22 % in contrast with ~ 17 % for same thickness c-Si solar cell. Optimization of the alloy composition, doping concentration of the Si_1-xGe_x, thickness of the alloy layer and placement of the layer in the cell structure will be reported at the conference.

Optical characterization of Si x Ge 1−x films grown on nanostructured Si substrates

High quality Ge and SixGe1-x films grown on Si substrates are attractive for a wide range of applications in optics, optoelectronics, and high efficiency solar cells. In this study, heteroepitaxial growth of Ge on nanostructured Si surfaces has been investigated. Thermally evaporated amorphous Ge films are vacuum-deposited and crystallized by thermal annealing at 1000 °C. Scanning electron microscope (SEM), spectroscopy (RS), infrared (IR) transmission, and Raman methods are used to characterize amorphous and crystalline Ge films. SEM analysis reveals presence of dominant features including cracks, microscopic roughness, and islands. RS exhibits strong multiple peaks attributed to crystalline structures related to Si-Ge at ~ 444 cm-1 and Ge at 300 cm-1; narrow and stronger peaks are observed in thermally annealed films. A comparison of IR transmission measurements in 900-1700-nm spectral range shows that amorphous film absorption is significantly higher than that of crystalline films consistent with respective bandgaps. A more detailed analysis including EDX and XRD measurements will be presented at the conference.