Naveed Aziz Khan

High entropy alloys in the form of thin films have been of growing interest in the past few years due to their unique properties such as high corrosion resistance, superior hardness, and high electrical resistivity. We used RF magnetron sputtering to fabricate high entropy alloy thin films of AlCoCrCu0.5FeNi. To tune the microstructure and mechanical properties of the films, three different working pressures of 5, 10, and 15 mTorr were utilized. The films grown at 10 mTorr had the largest grain size with highest surface roughness measured by scanning electron microscope (SEM) and atomic force microscope (AFM), respectively. Energy dispersive spectroscopy (EDS) results show that films grown at lower pressure (5 mTorr) are X-ray amorphous and have significantly higher concentration of aluminium (over 35%) due to the reduced scattering of Al atoms on route to the substrate. The films deposited at 10 mTorr are composed of a mixture of FCC and BCC crystal structures as determined using X-ray diffraction (XRD); have protective surface oxide layers of Al2O3 and Cr2O3, as observed by X-ray photoelectron spectroscopy (XPS); and have high electrical resistivity (over 4500 μΩ-cm) and high hardness (over 13 GPa). This work shows that the deposition pressure is a critical growth parameter that can be used to tune the microstructure and the properties of sputter deposited HEA thin films with potential applications as protective and hard coatings for aerospace and energy applications.

CdTe thin films were deposited on soda lime glass substrates (SLG) by thermal evaporation technique under high
vacuum condition. As-deposited CdTe thin films were subjected to post deposition laser annealing treatment at
three laser output energies of 50, 60, and 70 J/pulse. Laser annealing was employed using the laser beam with
combined wavelengths of 1064 nm and 532 nm, where the laser energy was varied and the oscillator frequency
was kept fixed at 10 Hz. XRD was employed to find the structural properties of the as-deposited and laser
annealed CdTe thin films. Topography and surface morphology of the CdTe thin films were investigated using
AFM and FESEM, respectively. Chemical composition and stoichiometry of the films were analysed by EDX
integrated with FESEM. Electrical properties of the CdTe films were measured using Hall Effect measurement
system and the optical properties of the as-deposited and laser annealed CdTe films were studied by UV–Vis. XRD
analysis showed that as-deposited and laser annealed CdTe thin films had a mixed phase of cubic and hexagonal
structures with the preferential crystal orientation of C (1 1 1) at approximately 2θ=23.80°. CdTe thin films
laser annealed at 60 J/pulse had better crystalline property having minimum internal strain with lower surface
roughness and larger grain size resulting in optimized coalescence. EDX, Hall Effect, and UV–Vis results for the
film laser annealed at 60 J/pulse depicted good compositional stoichiometry, better electrical properties and
optimum optical properties showing the prospects as a potential absorber for CdTe thin film solar cells.

The influence of CdCl2 treatment on the properties of thermally evaporated CdTe thin film was investigated in this analysis to achieve high quality thin films. Thin films of CdTe were deposited on cleaned soda lime glass substrates at room temperature by thermal evaporation technique. Then the samples were treated by CdCl2 and subsequently annealed at annealing temperature of 400ºC for 15 minutes. The structural, optical and electrical properties of the grown samples were investigated through XRD, AFM, UV-Vis spectrometry and Hall-effect measurement analysis. The as-deposited films prepared at 25A were found in polycrystalline form, whereas the films prepared at deposition current of 28A and 30A exhibit cubic crystallinity with (111) preferential orientation around 2θ=23.8º. The crystallinity and the carrier concentration of the films were improved for all the CdCl2 treated films.  The surface roughness of the films was also highly affected by the CdCl2 treatment as it was observed from AFM images. The bandgap has been found around 1.43 eV for the as-deposited films whereas the bandgap decreased to 1.4 eV after CdCl2 treatment. The values of mobility, resistivity and Hall coefficient were observed to decrease after the CdCl2 treatment.

The effects of unintentionally formed n-type transition metal dichalcogenide namely molybdenum telluride (MoTe2) in between Cadmium Telluride (CdTe) absorber layer and Mo back contact is studied from numerical modeling and analysis. The main objective is to analyze the possible effects of n-MoTe2 formation in CdTe thin film solar cell. Energy band line-up of Mo/MoTe2/CdTe interface is investigated in order to explain the interface properties with different parameters. Carrier concentration, bandgap energy, electron affinity and thickness of n-MoTe2 have been varied in the numerical simulation to observe its effects on overall photovoltaic performance. The increase in the carrier concentration and bandgap energy of n-MoTe2 deteriorates the overall performance. This could be attributed to the high value of built-in-potential (Vbi) along with band offset value at nMoTe2/p-CdTe interface, which causes the electrons to be drifted back towards the back contact and results in recombination. Advantageous effects are observed as the electron affinity of n-MoTe2 is increased. This can be explained by the lower value of band offset (∆EC and ∆EV) at n-MoTe2/p-CdTe interface that interrupts the flow of carriers in overall circuit in a moderate way. Numerical results reveal that n-MoTe2 layer thinner than 50 nm affects adversely, possibly due to the shunting.

Cadmium Telluride (CdTe) thin films were deposited on borosilicate glass substrates by Close Spaced Sublimation (CSS) technique at a pressure of 1.5 Torr in Ar gas ambient. The samples were prepared at source temperature of 625ºC and substrate temperature of 595ºC, respectively. The role of various deposition times has been explored with the aim of investigating the impacts on structural, topographical, morphological and electrical properties of CdTe thin films. The crystalline structure, surface topology, surface morphology and electrical properties of the films were determined by using X-ray diffraction (XRD), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and Hall Effect measurement, respectively. XRD patterns reveal that the CdTe films show polycrystalline nature with more than one diffraction peaks corresponding to the (111) cub , (220) cub and(311) cub reflection planes at 2θ=23.76º, 2θ=39.30º and 2θ=46.42º, respectively. Variations in the deposition time are attributed for the deviations in the crystallinity of the CSS grown CdTe thin films. Significant changes were also observed in the film's surface roughness. FESEM images illustrate that the surface morphology and the average grain size of the films are highly dependent on the deposition time. A particular structure and surface morphology were observed in FESEM images for all films. The carrier concentration, mobility, resistivity and Hall coefficients were calculated. Bulk carrier density was in the order around 10 15 cm-3. Therefore, CdTe films possess higher potential to be used in CdS/CdTe thin film solar cells.

—Laser annealing of CdTe thin films with two different wavelengths has been studied in this work. The CdTe thin films were grown by thermal evaporation at a deposition current of 28A and then subjected to post deposition laser annealing at two different wavelengths of 532nm (green) and 1064nm + 532nm (infrared + green). The other parameters like laser output energy, stage velocity and pulse repetition rate were kept fixed. The analyses were carried out using XRD, AFM, UVVis and Hall Effect Measurement system. XRD showed polycrystalline nature for all the films. AFM revealed that laser annealing didn’t change the ‘Sq’ roughness of the films significantly. The UV-Vis analysis depicted significant changes in band gap for both the laser annealed films, ‘T1’ and ‘T2’ on the other hand bulk concentration changed slightly upon laser annealing. FESEM images revealed the change in grain size when laser annealing was done on the CdTe thin films.

Firstly, Cadmium Telluride (CdTe) thin films have been deposited on cleaned soda lime glass substrates at 300°C by using the RF magnetron sputtering technique. After that, Cu thin film was deposited for 5 minutes at 200°C on top of CdCl2 treated CdTe thin films by sputtering. Subsequently, CdTe and Cu stacks were annealed at 400°C for 15 minutes, 20 minutes and 25 minutes in a vacuum furnace. The influence of different annealing times on the structural, topographical and electrical properties of Cu sputtered CdTe thin films were then examined by XRD, AFM and Hall Effect measurement, respectively. XRD patterns reveal that, one CdTe peak corresponding to the (111)cub reflection planes at 2ș=23.8º and another low intensity Cu2Te peak representing (200)hex hexagonal reflection planes at around 2ș=24.8º were found for all the annealing times. Surface roughness and topography were viewed from the AFM images. Noteworthy changes were observed in the films surface roughness due to the different annealing times. The surface roughness values imply rising trend for lower annealing times. Bulk carrier density was in the order of 1018cm-3. The highest carrier concentration of 7.1x1018cm-3 was achieved for the films annealed for 15 min.

Zinc Sulphide (ZnS) is a promising candidate to be an alternative buffer layer to the commonly used cadmium sulphide (CdS) in CZTS solar cells. In this study, buffer layer parameters like layer thickness and buffer layer bandgap have been investigated by Analysis of Microelectronic and Photonic Structures (AMPS-1D) to find out the higher conversion efficiency. A promising result has been achieved with an efficiency of 14.49% (with Voc = 0.81 V, Jsc = 28.85 mA/cm2 and Fill factor = 67.5) by using ZnS as a buffer layer. It is also found that the high efficiency of CZTS absorber layer thickness is between 2 µm and 4 µm. From the simulation results, it is revealed that higher efficiency can be achieved for the buffer layer bandgap around 3.10 eV - 3.25 eV. This result can be explained by the practical work as the bandgap of ZnS is largely dependent on the preparation conditions and stoichiometry. In conclusion, numerous influences of buffer layer are investigated in CZTS solar cell that can lead to the fabrication of high efficiency devices.

The efficiency of CdTe based solar cell can be increased using ternary CdZnTe material as absorber layer. Cd1-xZnxTe has tunable bandgap depending on the composition. In this work the bandgap of CdZnTe layer (1.57 eV) which is in the optimum range, can be achieved with Zn composition of x=0.1. First the carrier concentration of absorber layer in the baseline case is increased then the thicknesses of absorber layer and window layer in the conventional baseline case are reduced and optimized. Finally an optimized cell structure is proposed. After optimization, the total thickness of the baseline case cell is reduced by factor four and results high efficiency. The cell structure in both baseline case and modified cell is: (SnO2/CdS/CdZnTe/Back Contact), however some material parameters are different. The performance parameters are found better in the optimized cell structure. We also investigated the effect of ZnO buffer layer and the operating temperature on the performance parameters.